UNIVERSIDADE ESTADUAL DE CAMPINAS
INSTITUTO DE QUÍMICA
LUÍS GUSTAVO TEIXEIRA ALVES DUARTE
EXCITED STATE INTRAMOLECULAR PROTON TRANSFER (ESIPT) BASED FLUORESCENT SENSORS AND WHITE EMITTERS FOR
OPTOELECTRONIC DEVICES
SENSORES FLUORESCENTES E EMISSORES BRANCOS PARA DISPOSITIVOS OPTOELETRÔNICOS BASEADOS NA TRANSFERÊNCIA
PROTÔNICA INTRAMOLECULAR NO ESTADO EXCITADO (TPIEE)
CAMPINAS 2019
LUÍS GUSTAVO TEIXEIRA ALVES DUARTE
EXCITED STATE INTRAMOLECULAR PROTON TRANSFER
(ESIPT) BASED FLUORESCENT SENSORS AND WHITE EMITTERS FOR OPTOELECTRONIC DEVICES
SENSORES FLUORESCENTES E EMISSORES BRANCOS PARA DISPOSITIVOS OPTOELETRÔNICOS BASEADOS NA
TRANSFERÊNCIA PROTÔNICA INTRAMOLECULAR NO ESTADO EXCITADO (TPIEE)
Tese de Doutorado apresentada ao Instituto de Química da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Doutor em Ciências
Doctor´s Thesis presented to the Institute of Chemistry of the University of Campinas as part of the requirements to obtain the title of Doctor in Sciences.
Orientadora: Profa. Dra. Teresa Dib Zambon Atvars Co-orientador: Prof. Dr. Fabiano Severo Rodembusch
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O arquivo digital corresponde à versão final da Tese defendida pelo aluno Luís Gustavo Teixeira Alves Duarte e orientada pela Profa. Dra. Teresa Dib Zambon Atvars.
CAMPINAS 2019
Banca Examinadora
Profa. Dra. Teresa Dib Zambon Atvars (Orientadora)
Prof. Dr. Francisco Eduardo Gontijo Guimarães (USP-São Carlos)
Profa. Dra. Marian Rosaly Davolos (UNESP-Araraquara)
Prof. Dr. Diego Pereira dos Santos (UNICAMP)
Prof. Paulo Cesar de Sousa Filho (UNICAMP)
A Ata da defesa assinada pelos membros da Comissão Examinadora, consta no SIGA/Sistema de Fluxo de Dissertação/Tese e na Secretaria do Programa da Unidade.
Este exemplar corresponde à redação final da Tese de Doutorado defendida pelo aluno Luís Gustavo Teixeira Alves Duarte, aprovada pela Comissão Julgadora em 10 de outubro de 2019.
Acknowledgements
Without a shadow of doubt, I would like to deeply thank my family for all the love and support throughout my life. I am thankful to my parents, Maria Clara Lopes Teixeira and Walter Duarte Filho, my grandparents, Maria Alves Teixeira, Maria Dolores de Jesus Duarte, Walter Duarte, Antônio Lopes Teixeira and my grand grandparent Antônia Rodrigues. These are the people who made me the person I am today. I would like to thank my partner in crime, Ana Paula Ferreira de Oliveira, for her companionship and understanding.
I am grateful for the privilege to learn and develop my curiosity about Chemistry during the years of high school and technical course at the Escola Técnica Estadual Conselheiro Antonio Prado – ETECAP. This is the place where I decided to pursue a carrier as chemist.
I am also thankful to my friends from UNICAMP, for all the moments we had together during these ten years. Special thanks to Dr. José Carlos Germino, Dr. Bruno Zornio, Dr. Miguel Galante, Dr. Willian Dantas, Dr. Marcelo Faleiros, Dr. Guilherme Ferbonink, and Emmanuel Moraes. I consider myself fortunate for having these people around to count with and to have endless discussions about science.
I would like to thank Prof. Dr. Fabiano Rodembusch for accepting me as a student in his lab, and the guys from the Federal University of Rio Grande do Sul – UFRGS for the companionship. Also, I would like to thank Prof. Dr. Richard Weiss for having me in his lab, and the Weiss’ group from Georgetown University for being so receptive.
I especially want to express my gratitude to my supervisor, Prof. Dr. Teresa Dib Zambon Atvars, for her guidance and dedication. It was in her lab that I have learned how to do research and the importance of being ethical at work to achieve my goals.
I am thankful to my committee members, Prof. Dr. Diego Pereira dos Santos, Prof. Dr. Paulo Cesar de Sousa Filho, Prof. Dr. Francisco Eduardo Gontijo Guimarães and Prof. Dr. Marian Rosaly Davolos for their contribution to the improvement of this thesis.
I would like to thank UNICAMP and the Institute of Chemistry for the infrastructure and support in order to develop this project. I am grateful to the staff of CPG-IQ and CGU for all the support and understanding, which made my life way easier.
I would like to thanks the financial support from INEO. I am grateful for the support from CNPq (202888/2018-5).
I am also thankful for the financial assistance from FAPESP (2013/16245-2).
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Finance Code 001.
Resumo
Neste estudo, os efeitos de matrizes poliméricas nas propriedades fotofísicas de corantes reativos à transferência de próton intramolecular no estado excitado (TPIEE) foram avaliados por espectroscopias eletrônicas de absorção, emissão e fluorescência resolvida no tempo. Estes efeitos foram explorados por duas abordagens. Na primeira, polímeros fotoativos baseados no estireno com massas molares entre 1.27x104-4.05x104 g mol-1 foram avaliados em termos de suas propriedades fotoativas relativas às 2-(2’-hidróxifenil)benzazolas ligadas às matrizes poliméricas. Os monômeros benzazólicos apresentaram propriedades no estado excitado dependentes da viscosidade induzida pelos meios poliméricos, em que somente a extensão do processo de TPIEE foi desfavorecida, uma vez que a estabilidade das espécies no estado excitado não variaram, como observado pela análise de seus tempos de vida de fluorescência quando diluídas em solvente orgânico. Experimentos adicionais realizados no solvente aprótico não polar de alta viscosidade Nujol corroboraram essas observações. Predições teóricas também foram realizadas no nível de teoria CAM-B3LYP/6-311+G(d,p) e corroboraram as suposições pela análise das barreiras energéticas relativas à reação no estado excitado e conformeros rotacionais, cujas energias de ativação relativas aos equilíbrios conformacionais no estado excitado aumentaram extremamente em comparação ao estado fundamental; este resultado demonstra que a viscosidade é uma força motriz significante para a TPIEE.
Na segunda abordagem, derivados de 2-(2’-hidróxifenil)benzotiazóis foram diluídos como hóspedes de matrizes poliméricas em concentrações muito baixas para obter informações sobre amino-polisolixanos e a reticulação de suas cadeias pela reação dos polímeros com dióxido de carbono e a consequente produção de carbamatos de amônio. Este método muito simples, elegante e reversível de reticulação resulta no aumento da viscosidade dos polímeros devido às forças intermoleculares mais intensas criadas e a diminuição da basicidade dos grupos pendentes por conta da conversão dos grupos amino, provados pelos derivados de benzotiazol pela alteração do processo de TPIEE e desprotonação/protonação dos corantes no estado fundamental. CAM-B3LYP/6-311++G(d,p) foi aplicado para otimizar as moléculas, prever as propriedades fotofísicas e a reatividade frente à TPIEE.
Posteriormente, um novo sensor de fluorescência seletivo para Cu2+ em solução também
foi desenvolvido por meio do efeito de supressão intensificado por quelação (ESIQ). O composto N,N’-bis(salicilideno)-(2-(3’,4’-diaminofenil)benzotiazol (BTS) demonstra ser altamente reativo à TPIEE, apresentando emissão por fluorescência na região do azul com elevado Deslocamento de Stokes (8794 cm-1). Experimentos de titulação espectroscópica
indicaram que não somente a supressão de fluorescência como também a reação de TPIEE permitiram a detecção à olho nu como um sensor de tira de papel. Este sensor químico mostrou-se mostrou-seletivo para Cu2+ dentre 13 cátions testados. A razão BTS:Cu2+ foi determinada pelos gráficos de Job e Benesi-Hildebrand como 1:1. A constante de formação foi também encontrada como 6.0 x 107 mol-2 L2 em 25oC. Os limites de detecção e quantificação também foram obtidos como 31 ppb e 107 ppb, respectivamente. Além disso, também no contexto de compósitos host-guest, as propriedades fotofísicas do BTS foram determinadas, apresentando uma emissão que cobre toda a faixa do visível. Devido à esta larga banda de emissão, BTS foi testado com sucesso como camada ativa de diodos orgânicos emissores de luz processados em solução com emissão de luz branca. Estes diodos foram preparados usando protocolos baseados em solução com o corante solubilizado em poli(9-vinilcabazol) (PVK). Diferentes razões molares para PVK:BTS foram utilizadas como forma de otimizar a performance dos diodos. A arquitetura otimizada resultou numa luminância de 34 cd m-2 em 13.5 V com coordenadas CIE (0.31, 0.40). Para melhorar os dispositivos baseados no BTS, este foi usado como guest do poli[9,9-dioctilfluorenil-2,7-diil] (PFO). Comparado com o diodo similar em PVK, a performance do diodo PFO:BTS foi ampliada em 50 vezes em termos de luminância, 20 vezes em termos de densidade de corrente e 14 vezes em termos de eficiência de corrente. O diodo produziu uma eficiência de corrente de 0.40 cd A-1 e luminância de 492 cd m-2.
Finalmente, para ampliar ainda mais as propriedades optoelectrônicas deste salicilideno, BTS foi coordenado ao Zinco (II). Isto foi possível por conta do aumento da estabilidade química e rendimento quântico de emissão causados pelo efeito de planarização do Zinco (II) na estrutura do ligante. Apesar destas características serem facilmente moduladas sinteticamente por pequenas modificações na estrutura do ligante, o uso de complexos de Zinco (II) em dispositivos processados em solução não está em evidência, ainda menos para o caso de bases de Schiff tetradentadas e para a produção de sistemas host-guest pela mistura física de emissores azuis, como derivados de polifluoreno. Então, um compósito host-guest final foi criado como camada ativa usando PFO como host e o novo composto de coordenação Zn(BTS) como guest. As propriedades fotofísicas do Zn(BTS) também foram exploradas em solução e no estado sólido. Além disso, este foi previsto teoricamente usando o conjunto de base e o funcional PBE0/6-311++G(d,p). A performance superior do diodo de PFO:Zn(BTS) é uma consequência do aumento significante da mobilidade de buracos, prevista pelo modelo de Corrente Limitada por Cargas de Armadilha (CLCA). Ademais, por conta da combinação de cores e tirando vantagem da emissão azul do PFO, a emissão do BTS envolvendo o processo
de TPIEE e a emissão verde do Zn(BTS), foi produzida luz branca em diodos de camada única baseados na combinação de emissores de baixo custo.
Abstract
In this study, the effects of polymeric matrixes on the photophysical properties of excited state intramolecular proton transfer (ESIPT) dyes were evaluated by steady-state absorption and emission and time-resolved fluorescence spectroscopies. Such effects were explored by two main approaches. In the first one, photoactive styrene-based polymers with molar masses between 1.27x104-4.05x104 g mol-1 were evaluated for their photophysical properties related to 2-(2’-hydroxyphenyl)benzazoles attached to the polymers backbones. The benzazolic monomers presented excited state properties dependent on the viscosity induced by the polymeric media, wherein only the ESIPT process extension was unfavoured once the species stability in the excited state did not vary, as observed in the analysis of their fluorescence lifetimes when diluted in an organic solvent. Additional experiments performed in the highly viscous non-polar aprotic solvent Nujol corroborate these observations. Theoretical predictions were also performed at the CAM-B3LYP/6-311+G(d,p) level of theory and corroborate our assumptions by the analysis of the energy barriers relative to both the excited state reaction and rotational conformers, wherein the activation energies relative to the conformational balances in the excited state were extremely increased in comparison to those relative to the ground state; this outcome demonstrates that viscosity is a significant driving force to ESIPT.
In the second approach, 2-(2’-hydroxyphenyl)benzothiazole derivatives were diluted as guests of polymeric matrixes in very low concentrations to gain information about amino-polysiloxanes and their chain cross-linking by reacting the polymers with carbon dioxide and the consequent production of ammonium carbamates. This very simple, elegant and reversible method results on the increase of viscosity of the polymers due to the stronger intermolecular forces created, and there is a diminishment of basicity of the pendant groups because of the conversion of the amino groups, probed by the benzothiazole derivatives by altering the ESIPT process and deprotonating/protonating the dyes on the ground state. CAM-B3LYP/6-311++G(d,p) was applied to optimize the molecules, photophysical predictions, and ESIPT reactivity.
Afterward, a novel selective fluorescent sensor for Cu2+ in solution has been developed by means of chelation enhancement quenching effect (CHEQ). The compound N,N’-bis(salycilidene)-(2-(3’,4’-diaminophenyl)benzothiazole (BTS) demonstrates to be highly reactive to ESIPT, presenting fluorescence emission in the blue region with very large Stokes shift (8794 cm-1). Spectroscopic titration experiments indicate that not only the fluorescence quenching but also of the ESIPT reaction allowed naked eye detection as a paper strip-based
sensor. This chemosensor showed to be selective to Cu2+ among 13 other cations. The BTS:Cu2+
ratio was determined by both Job and Benesi-Hildebrand plots as 1:1. The binding constant was also found as 6.0 x 107 mol-2 L2 at 25oC. The detection and quantification limits were also obtained as 31 ppb and 104 ppb, respectively. Furthermore, also in the context of host-guest composites, BTS photophysical properties were evaluated, presenting an emission covering the entire range of the visible spectrum. Due to its broad emission band, BTS was successfully tested as an active layer in solution-processed organic light-emitting diodes with white-light emission. These diodes were prepared using solution-based protocols with the dye solubilized in a poly(9-vinylcarbazole) (PVK) matrix. Different PVK:BTS molar ratios were used to optimize the diode performance. The optimized architecture rendered a luminance of 34 cd m -2 at 13.5 V with CIE coordinates (0.31, 0.40). To improve the devices based on BTS, it was
used as a guest in poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO). Compared to the similar diode with PVK, the performance of the PFO:BTS diode was improved by 50-fold in terms of luminance, 20-fold in terms of current density and 14-fold in terms of current efficiency. The diode rendered a current efficiency of 0.40 cd A-1 and luminance of 492 cd m-2.
Finally, to improve the optoelectronic properties of this salicylidene even further, it was coordinated to Zinc (II). It was possible because of the enhancement of chemical stability and emission quantum yield caused by the Zinc(II) planarization effect on the ligand framework. Although these features are easily tuned synthetically by small modifications of the ligand structures, the use of Zinc(II) complexes in solution-processed devices are not in evidence, even less to the case of tetradentate Schiff bases as ligands and to the production of host-guest systems by their physical mixtures with deep blue emitters, such as polyfluorene derivatives. Hence, we created a final host-guest composite as active layer using PFO as host and the new Zn(II) coordination compound as guest; the best device rendered 2936 cd m-2 with 0.45 cd A-1 only with 0.1 % mol/mol of the guest. The photophysical properties of the Zn(BTS) were explored in solution and on the solid-state. Besides, it was theoretically predicted by DFT/TD-DFT calculations using the basis set and functional PBE0/6-311++G(d,p). We explain this higher performance to the significant enhancement of hole charge mobility, predicted by means of the Trap Charge Limited Current (TCLC) model. Moreover, because of the color combination and taking advantage of the PFO deep-blue emission, BTS emission involving the ESIPT process, and Zn(BTS) green emission, white-light emission was produced in a single layer diode based on different combination of low-cost emitters.
List of Figures
Figure 1. Schematic view of Photophysical/Photochemical deactivation of A¹* by unimolecular processes. The table on the right contains the respective rate constants of each process, with hνabs being
the excitation energy of intensity Ia. kIC is the rate constant of internal conversion, kF is the rate constant
of radiative deactivation by fluorescence, where hνFA is the emission energy. kISC is the rate constant of
intersystem crossing, that results in the formation of the triplet state A³*, kP is the rate constant of
radiative deactivation by phosphorescence, where hνPA is the energy of emission. kTISC is the rate
constant of A³* non-radiative deactivation. kr1 represents the rate constant for the photoreaction evolving
A1* with the formation of the product P¹* and k
r2 the rate constant for the photoreaction evolving A3*
with the formation of the product P3* – the deactivation processes related to the photoproducts can be
described in the same way. ...23 Figure 2. Representation of a photochemical process evolving the A¹ species. Ia is the absorbed radiation,
ka the deactivation rate constant of A¹* without the component related to the formation of Ai*, kb is the
rate constant related Ai* formation. k
c is the deactivation rate constant of Ai* without the component
related to the photochemical process, and kr is the rate constant of the photoreaction. ...24
Figure 3. Representation of (a) an adiabatic photoreaction, (b) a diabatic photoreaction and of (c) a hot-ground-state reaction between ground and excited states.9 The wavy purple arrow corresponds to a
non-radiative deactivation pathway of the reagent before the reaction. ...26 Figure 4. Förster Cycle to a protonation/deprotonation balance of a generic system AH. hνAH and hνA-*
are the energetic differences between electronic ground and excited, ΔH e ΔH* are the enthalpic variations, Ka and Ka* are the equilibrium constants of the reactions on ground and excited states,
respectively. ...27 Figure 5. Chemical equilibria involving both neutral and ionic species of 3-hydroxyflavone (3HF), 4’-Diethyl-3-hydroxyflavone (DEA3HF), and 4’-Fluoro-3-hydroxyflavone. The respective pKa values were also presented along with the HOMO’s and LUMO’s on the right-hand side. ...29 Figure 6. Four-level model for the photochemical cycle of the ESIPT process.39 ...30 Figure 7. Steady-state absorption and emission of HBT in cyclohexane (solid lines) and ethanol (dashed lines).18 ...31 Figure 8. (a) Molecular system studied by Wnuk et al. (b) Fluorescence decays of the system measured at the emission peak of the enol (N*) and keto (T*) species. Instrument Response Function (IRF) is also shown as a blue line.76 ...32 Figure 9. Representation of the two possible ESIPT PES’s. (a) The case of an irreversible reaction, and (b) of reversible reaction. ...33
Figure 10. Structures of common semiconductive polymers (PVK: poly(N-vinyl carbazole); PFO: poly[9,9-dioctylfluorenyl-2,7- diyl]; MEH-PPV: poly(2-methoxy-5(2’-ethyl)-hexoxy-1,4-phenylene vinylene), small organic dyes (TPD: N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine; TPBi: 2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) and coordination compounds (Alq3: tris(8-hydroxyquinoline)aluminum; Znq2: bis(8-hydroxyquinoline)zinc; Irppz3: tris(1-phenykpyrazolato)iridium) used to fabricate OLEDs. ...41 Figure 11. Mechanism of charge injection (holes h+ and electrons e-), transport and recombination of OLEDs. ...41 Figure 12. Correlation between the PL (red line) and EL (black line) of PVK thin-film.147 ...44
Figure 13. Schematic view of electronic states population by photoexcitation (PL) and electroexcitation (EL) mechanisms.147 ...44
Figure 14. (a) Molecular structures of the two PFO chemical structures of different morphologies and their respective (a) absorption and (b) photoluminescence spectra of spin-coated thin films produced
from a chloroform solution.153 ...45
Figure 15. A CIE 1931 diagram showing the pure white light coordinates (0.33,0.33).157 ...46
Figure 16. A few strategies used to the production of WOLEDs.162 ...47
Figure 17. (a) Schematic view of the radiative energy transfer. (b) PFOPen and (c) MDMO-PPV structures. (d) Normalized EL spectrum of the WOLED. (e) The spectral overlap between the PFOFPen EL and MDMO-PPV absorption spectra, and the PL spectrum to the CCL.166...48
Figure 18. Synthetic approach to produce the benzothiazole derivatives 1, 2 and 3. ...56
Figure 19. Synthetic methodology for obtaining BTS (6). ...57
Figure 20. Synthetic procedure for obtaining Zn(BTS). ...59
Figure 21. Representation of the devices architecture, where X = BTS or Zn(BTS). The devices based on PVK:BTS followed the same scheme without using PFO. ...66
Figure 22. (a) Structures of the fluorophoric benzazolylvinylene monomeric dyes VM1 and VM2. (b) Preparation of photoactive polymeric materials PVM1 and PVM2, the symbol “Bz” represents the monomeric dyes. ...69
Figure 23. Normalized electronic absorption (continuous line) and emission (continuous line with open circles) spectra in chloroform solution for VM1, VM2 (10.0 µmol∙L-1,exc=338 nm and exc= 328 nm), PVM1 and PVM2 (5.0 mg∙mL-1, exc=350 nm and exc=325 nm). ...70
Figure 24. Fluorescence decays and IRF function for VM1 (1x10-5 mol L-1) and PVM1 (5.0 mg L-1) in chloroform solution acquired at the respective emission maxima of N* (em=415 and 405 nm) and T* (em=541 and 546 nm) species. ...73
Figure 25. Normalized DRUV (continuous line) and emissions (continuous line with open circles) spectra of (a) VM1 (exc=368 nm) and PVM1 (exc=360 nm), (b) VM2 (exc= 328 nm) and PVM2 (exc=340 nm) in the solid state. ...73
Figure 26. Thin-film normalized electronic absorption (continuous line) and emission (continuous line with open circles) spectra of (a) PVM1 (λexc=244 nm) and PVM2 (λexc=290 nm). ...74
Figure 27. Normalized electronic emissions spectra of VM1 and VM2 using Nujol oil as the solvent. Concentration: 10.0 µmol L-1, exc=338 and 328 nm. ...76
Figure 28. ESIPT photochemical cycle constructed using the frontier molecular orbitals (HOMO/LUMO) of VM1 (left) and VM2 (right). ...77
Figure 29. Potential energy surfaces for both electronic states S0 and S1 of (a) VM1 and (b) VM2 obtained at the CAM-B3LYP/6-311+G(d,p) level. ...78
Figure 30. Relative potential energy surfaces of (a) VM1 and (b) VM2 as a function of the dihedral angle between the benzazole and phenol aromatic rings, predicted at the CAM-B3LYP/6-311+G(d,p) level. ...78
Figure 31. Conformational balances on ground (N) and excited (N*) states of (a) VM1 and (b) VM2. ...79
Figure 32.VM1 and VM2 conformers at θ = 90º between benzazole and phenyl moieties on ground (S0) and excited states (S1), and the respective dihedral angles related to the vinylic substituents. ...80
Figure 33. Chemical reaction representing the synthetic method for amino-polysiloxane cross-linking. ...82
Figure 34. Molecular structures of the studied 2-(2’-hydroxyphenyl)benzothiazole derivatives HBT (1), HBT-4’-NH2 (2) and HBT-5’-NH2 (3). ...83
Figure 35. The photochemical cycle of the ESIPT process to 2-(2’-hydroxyphenyl)benzazole derivatives with proposed enol conformers. X = S, O or NH. ...84
Figure 36. IMHB analysis obtained using 1H NMR. ...85 Figure 37. Relative energy of Normal, Tautomeric and Anionic forms of the benzothiazole derivatives 1, 2 and 3 on ground and excited states versus the orientation polarizability function Δf. ...87 Figure 38. Energy profiles along N-H coordinate for ground S0 and excited states S1 of the benzothiazole
derivatives in heptane, THF, DCM, MeOH and Water at the CAM-B3LYP/6-311++G(d,p) level of theory. ...90 Figure 39. The dependence of (a) activation energy and (b) the reaction free energy as a function of the orientation polarizability function f to the benzothiazole derivatives 1, 2 and 3. ...91 Figure 40. Normalized absorption and emission spectra of the benzothiazole derivatives in (a) DCM, DCM with a few drops of NaH (DCM+NaH) and (b) Psil. ...92 Figure 41. Normalized absorption and emission spectra of the benzothiazole derivatives in (6-7)PSil (a) before and (b) after CO2 addition...95
Figure 42. Normalized absorption and emission spectra of the benzothiazole derivatives in 15PSil (a) before and (b) after CO2 addition...96
Figure 43. Normalized absorption and emission spectra of the benzothiazole derivatives in (2-4)PSilLMW
(a) before and (b) after CO2 addition. ...97
Figure 44. Normalized absorption and emission spectra of the benzothiazole derivatives in (2-4)PSilHMW
(a) before and (b) after CO2 addition. ...97
Figure 45. Chemical structures of (a) BTS and (b) Zn(BTS). ...101 Figure 46. (a) Molecular structure of BTS with 50% probability displacement ellipsoids. Intramolecular hydrogen bonds are shown as dashed lines. (b) Parallel alignment of BTS molecules, with plane centroid to plane centroid distances shown as black lines. ...102 Figure 47. Optimized geometries for (a) neutral, (b) tautomeric and (d) deprotonated forms of BTS at the CAM-B3LYP/6-311++G(d,p) level. ...103 Figure 48. (a) Normalized steady-state electronic absorption and PL spectra of BTS in DCM and its solid-state PL (exc = 335 nm). (b) Steady-state PLE vs PL surface of BTS in DCM. Concentration: 8.5
µmol L-1. ...104 Figure 49. Normalized steady-state electronic absorption, PLE and PL spectra of BTS in (a) DCM, (b) pyridine, (c) DCM with trace amounts of TFA. Concentration: 8.5 µmol L-1...105 Figure 50. Relative energy profiles along the O-H coordinate for both electronic states S0 and S1 of the
BTS obtained at CAM-B3LYP/6-311++G(d,p). ...108 Figure 51. Figure S15. Fluorescence decays of BTS in (a) DCM, (b) DCM with trace amounts of TFA and in pyridine (exc= 335.2 nm). Concentration: 8.5 µmol L-1. Emission wavelengths used to record the
decays along to IRF profiles and residual plots are related in the figure. ...109 Figure 52. Figure 4. Normalized TRES of BTS in DCM solution (exc= 335.2 nm) from (a) 0.00 ns to
3.05 ns and (b) from 3.05 ns to 14.00 ns. Concentration: 8.5 µmol L-1. ...110 Figure 53. Normalized electronic UV-Vis absorption and fluorescence spectra of BTS in MeCN/H2O
(7:3 v/v) solution (4.0 µmol L-1, exc = 340 nm). ...112
Figure 54. Histogram of BTS normalized fluorescence response upon the addition of various cations in MeCN/H2O (7:3 v/v) solution where F0 is the normalized emission intensity of BTS and F the emission
after metal addition, always monitored at em = 485 nm. ...113
Figure 55. (a) Electronic absorption spectra dependence on the addition of Cu2+ and (b) Job plot using the normalized absorbance at 334 nm to Cu2+ = 0.00 to 0.88 Eq. BTS concentration: 4.0 mol L-1. 113
Figure 56. Electronic emission spectra dependence on the addition of Cu2+ until 55.0 Eq. of BTS initial
concentration (4.0 mol L-1, exc = 340 nm). The inset presents two cuvettes with and without the
Figure 57. (a) Benesi-Hildebrand plot (𝑹𝟐 = 𝟎. 𝟗𝟖𝟔𝟕𝟏) to the range of concentration of Cu2+ from 7.4
x 10-7 mol L-1 to 8.0 x 10-6 mol L-1 and (b) 𝑨𝑨𝟎𝒗𝒔 [𝑪𝒖𝟐+] to the range of concentration of Cu2+
from 0.74 µmol L-1 to 220 µmol L-1 using the emission intensity at 485 nm. BTS concentration: 4.0 µmol L
-1
. ...115
Figure 58. Chemical mechanism proposed to CHEQ phenomena induced by Cu2+ binding. ...115
Figure 59. Picture of the emission of paper strips after their immersion in the BTS solution (top) and in each salt solution (bottom) with the exposure to UV-lamp light. ...116
Figure 60. (a) Diagram with the frontier energy levels of Al,129 Ca,129 ITO,129 PEDOT:PSS,130 PVK131 and BTS. (b) Normalized EL spectra to BTS in PVK at maximum luminance in molar fraction of 0.5%, 1.0% and 2.5%. (c) CIE 1931 chromaticity coordinates to BTS in [1] DCM solution, [2] solid-state and to all [3]-[5] PVK:BTS WOLEDs. ...117
Figure 61. Optical-electronic properties of the PVK:BTS WOLEDs. (a) Current density vs Voltage, (b) Luminance vs Voltage and (c) Current efficiency vs Voltage. ...119
Figure 62. Diagram with the frontier energy levels of Al,129 Ca,129 ITO,129 PEDOT:PSS,130 PVK,131 PFO α-phase301 and BTSand the respective chromaticity coordinates diagram to the BTS:PFO composites on the molar fractions 0.5%, 1.0% and 2.5% mol/mol according to the Comission Internationale de l’Eclairage (CIE 1931). Normalized EL spectra of PFO and the BTS:PFO composites on the molar fractions 0.5%, 1.0% and 2.5%. ...121
Figure 63. Optical-electronic properties of the PFO:BTS WOLEDs. (a) Current density vs Voltage, (b) Luminance vs Voltage and (c) Current efficiency vs Voltage. ...122
Figure 64. ESIPT cycle induced by electrical field on the OLEDs to BTS. The hole and particle NTOs densities for N* and T* are depicted...124
Figure 65. (a) Electronic absorption spectra dependence on the addition of Zn(II) (black arrow) in DMSO solution (inset: Job plot using the absorbance at 420 nm). (b) Normalized electronic absorption and PL spectra of Zn(BTS) in DMSO solution (orange line) and solid-state (red line). Concentration: 10 mol L-1. exc = 418 nm. ...125
Figure 66. Natural Transition Orbital pairs for the first two excited single states of Zn(BTS). The first state is on the left, the holes are the bottom and particles at the top of the figure. ...126
Figure 67. (a) Diagram with the frontier energy levels of Al129, Ca129, ITO129, PEDOT:PSS130, PVK131, PFO and Zn(BTS). (b) Normalized electroluminescence spectra of Zn(BTS) incorporated in PFO at maximum luminance and respective (c) CIE diagrams in the molar fractions 0.1%, 0.5%, 1.0% and 2.5%. ...128
Figure 68. Optical-electronic properties of the PFO:Zn(BTS) WOLEDs. (a) Current density vs Voltage, (b) Luminance vs Voltage and (c) Current efficiency vs Voltage. ...129
Figure 69. Fluorescence confocal images of Zn(BTS) dispersed into the PFO matrix. exc = 458 nm. ...131
Figure S1. HBT (1) 1H NMR using CDCl 3 as the solvent. ...174
Figure S2. HBT (1) 1H NMR using DMSO-d 6 as the solvent. ...175
Figure S3. HBT (1) 13C NMR using DMSO-d6 as the solvent...176
Figure S4. HBT (1) FTIR spectrum. ...176
Figure S5. HBT-4’-NH2 (2) 1H NMR using CDCl3 as the solvent. ...177
Figure S6. HBT-4’-NH2 (2) 1H NMR using DMSO-d6 as the solvent. ...178
Figure S7. HBT-4’-NH2 (2) 13C NMR using DMSO-d6 as the solvent. ...179
Figure S9. HBT-5’-NH2 (3) 1H NMR using CDCl3 as the solvent. ...180
Figure S10. HBT-5’-NH2 (3) 1H NMR using DMSO-d6 as the solvent. ...181
Figure S11. HBT-5’-NH2 (3) 13C NMR using DMSO-d6 as the solvent. ...182
Figure S12. HBT-5’-NH2 (3) FTIR spectrum. ...182
Figure S 13. BTS 1H NMR spectrum. ...183
Figure S14. BTS 13C NMR spectrum. ...184
Figure S15. BTS FTIR spectrum. ...185
Figure S16. BTS mass spectrum. ...185
Figure S17. Zn(BTS) 1H NMR using DMSO-d 6 as the solvent. ...194
Figure S18. Zn(BTS) 13 C NMR using DMSO-d6 as the solvent. ...195
Figure S19. Zn(BTS) FTIR spectrum. ...196
Figure S20. Zn(BTS) MS spectrum. ...197
Figure S21. TGA analysis. ...198
Figure S22. (a) Cyclic voltammogram of BTS (for clarity, only anodic sweep is shown). Zn(BTS) cyclic voltammograms: (b) oxidation and (c) reduction processes. ...198
Figure S23. Solid-state electronic absorption spectrum of BTS on diffuse reflectance mode. ...199
Figure S24. Fluorescence decays and Instrumental Response Functions (IRF) for VM1 and VM2 (1.10 -5 mol L-1,λ exc = 375 nm) in chloroform solution: (a) λem (N*)= 415 nm, (b) λem (T*)= 541 nm, (c) λem (N*)= 410 nm, (d) λem (T*)= 518 nm. ...200
Figure S25. Fluorescence decays and Instrumental Response Functions (IRF) for PVM1 and PVM2 (5.0 mg L-1,λexc = 375 nm) in chloroform solution: (a) λem (N*)= 405 nm, (b) λem (T*)= 546 nm, (c) λem (N*) = 431 nm, (d) λem (T*)= 517 nm. ...201
Figure S26. Fluorescence decays and Instrumental Response Functions (IRF) for PVM1 and PVM2 (λexc = 375 nm) thin films: (a) λem (N*)= 430 nm, (b) λem (T*)= 545 nm, (c) λem (N*)= 425 nm, (d) λem (T*) = 527 nm. ...202
Figure S27. Fluorescence decays of the benzothiazole derivatives 1, 2 and 3 in (6-7)PSil (a) before and (b) after CO2 addition with residual plots below. ...203
Figure S28. Fluorescence decays of the benzothiazole derivatives 1, 2 and 3 in 15PSil (a) before and (b) after CO2 addition with residual plots below. ...204
Figure S29. Fluorescence decays of the benzothiazole derivatives 1, 2 and 3 in (2-4)PSilLMW (a) before and (b) after CO2 addition with residual plots below. ...205
Figure S30. Fluorescence decays of the benzothiazole derivatives 1, 2 and 3 in (2-4)PSilHMW (a) before and (b) after CO2 addition with residual plots below. ...206
Figure S31. Solid-state electronic emission spectrum of BTS using different excitation wavelengths. ...207
Figure S 32. Electronic absorption spectra of the mixture of BTS (5.0 mmol L-1, MeCN/H2O 7:3 v/v) and 1.0 eq of Cu2+ (5.0 mmol L-1, deionized water) in a cuvette with 5.0 cm of optical-length. ...207
Figure S33. EL dependency with the applied field to the device with 0.5% mol/mol of BTS in PVK. The orange arrow indicates the increase of voltage. ...208
Figure S34. FEG-SEM cross-section image of a solution-processed PVK|PFO thin film (LPVK ≈ 70 nm and LPFO ≈ 23 nm). ...208
Figure S35. Normalized electronic absorption and PL spectra of Zn(BTS) in THF solution. Concentration: 10 mol L-1. exc = 422 nm. ...209
List of Tables
Table 1. Optical properties of VM1 and VM2 (10.0 µmol L-1) and the respective styrene-based
copolymers in chloroform solution (5.0 mg mL-1) , where abs and em are the absorption and emission
maxima, respectively (nm), SS is the Stokes shift (cm-1) and is the fluorescence lifetime (ns) of the N* and T* forms, with the associated error of ². ...72 Table 2. Optical properties of VM1 and VM2 and the respective styrene-based copolymers in the solid state, where abs and em are the absorption and emission maxima, respectively (nm), and SS is the Stokes
shift (cm-1). ...74 Table 3 Optical properties of thin films from copolymers PVM1 and PVM2 (5.0 mg mL-1) where abs
and em are the absorption and emission maxima, respectively (nm) and is the fluorescence lifetime
(ns) of the N* and T* forms, with the associated error of ²...75
Table 4. Relevant bond lengths (Å), bond angles (°), vertical excitation energies (nm), oscillator strengths (f), proton transfer free energy (Ept/kcal mol-1) and activation energy (Ea/ kcal mol-1) of 1, 2
and 3 on the electronic ground (S0) and excited (S1) states based on DFT and TD-DFT methods at the
CAM-B3LYP/6-311++G(d,p) level of theory. ...86
Table 5. Optimized geometries and frontier molecular orbitals of the benzothiazole derivatives at the CAM-B3LYP/6-311++G(d,p) level of theory using DCM as the solvent. ...88
Table 6. Optical parameters of 1, 2 and 3 in different media (2 x 10-5
mol L-1). abs (nm) and em (nm) are the absorption and emission maxima and SS (cm-1) is the Stokes Shift. ...93
Table 7. Fluorescence lifetimes obtained for 1, 2 and 3 at their emission maxima in the amino-polysiloxanes, where Ai (%) is the pre-exponential factor, i (ns) is the lifetime with respective ²
distribution. Samples were excited at the absorption maxima. When two emission bands were identified, a second decay was acquired. ...99 Table 8. Optical parameters of BTS in all the investigated conditions (8.5 µmol L-1
), where abs (nm)
and em (nm) are the absorption and emission maximum, (x 104 L mol-1 cm-1) is the molar extinction
coefficient, 𝛟𝒐𝒃𝒔 (x 10-2
) is the quantum yield of fluorescence, SS (cm-1) is the Stokes Shift and (ns) is the fluorescence lifetime. ...106 Table 9. Excitation energies (abs) and the respective oscillator strength (f) obtained at
CAM-B3LYP/6-311++G(d,p) level for the neutral and deprotonated species. Electronic densities of main molecular orbitals involved in the transitions are also given. ...107 Table 10. Optical-electronic parameters obtained to the PVK:BTS OLEDs, where EL (nm) is the
electroluminescence maxima, Von (V) is the turn-on voltage, µp is the hole mobility (cm2 V-1 s-1), L (cd
m-2) is the maximum luminance, J (A cm-2) is the current density and (mcd A-1) is the current
efficiency. ...119
Table 11. Optical-electronic parameters obtained to the OLEDs, where EL is the electroluminescence
maxima (nm), Von is the turn-on voltage (V), µp is the hole mobility (cm2 V-1 s-1), L is the maximum
luminance (cd m-2), J is the current density (mA cm-2) and is the current efficiency (cd A-1). ...123
Table 12. Resume of optical parameters of Zn(BTS) in DMSO, THF (10 µmol L-1) and solid-state, and theoretical predictions at the PBE0/6-311++G(d,p) level in PFO, where abs (nm) and em (nm) are the
experimental absorption and emission maximum, x104 L mol-1 cm-1) is the molar extinction coefficient, 𝛟is the quantum yield of fluorescence, SS (cm-1) is the Stokes Shift, (nm) is the calculated excitation energies with oscillator strength f. ...126 Table 13. Optical-electronic parameters obtained to the PFO:Zn(BTS) OLEDs, where EL is the
L is the maximum luminance (cd m-2), J is the current density (A cm-2) and is the current efficiency
(cd A-1). ...129
Table S1. Crystal data and structure refinement for BTS. ...186
Table S2. Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2×103) for BTS. U eq is defined as 1/3 of of the trace of the orthogonalised UIJ tensor. ...187
Table S3. Anisotropic Displacement Parameters (Å2×103) for BTS. The Anisotropic displacement factor exponent takes the form: -2π2[h2a*2U 11+2hka*b*U12+…]. ...188
Table S4. Bond Lengths for BTS. ...189
Table S5. Bond Angles for BTS. ...190
Table S6. Hydrogen Bonds for BTS. ...191
Table S7. Torsion Angles for BTS...191
Table S8. Hydrogen Atom Coordinates (Å×104 ) and Isotropic Displacement Parameters (Å2×103) for BTS. ...193
Table S9. Excitation energies and respective oscillator strengths calculated at the PBE0/6-311++G(d,p) level for BTS on the N form considering PFO effects. ...209
Summary
Chapter 1 ...21
Excited State Intramolecular Proton Transfer process ...21
1.1 Motivation...22
1.2 Photophysical and Photochemical Processes ...22
1.3 Photochemical Processes Rates ...24
1.4 Potential Energy Surfaces ...25
1.5 Proton Transfer on the Excited State ...27
1.5.1 Excited State Proton Transfer ...27
1.5.2. Excited State Intramolecular Proton Transfer ...29
1.5.3. ESIPT mechanism ...30
1.5.4. ESIPT Potential Energy Surfaces ...33
1.5.5. ESIPT Applications ...34
1.5.6. Fluorescent Sensors based on ESIPT reactivity ...34
1.5.7. Optoelectronic devices based on ESIPT reaction ...35
1.6. Final remarks ...36
Chapter 2 ...38
Organic Light-Emitting Diodes ...38
2.1 Overview ...39
2.1 Semiconductive Polymers ...42
2.2 White Organic Light-Emitting Diodes – WOLEDs ...46
2.1. Figures of merit ...48
2.2. Charge carriers mobility ...49
2.6 Final Remarks ...51
2.7 Thesis Outcome ...51
Chapter 3 ...53
Objectives ...53
Experimental Procedures ...55
4.1 Synthesis ...56
4.2 Computational Details ...59
4.3 Steady-State spectroscopic characterization of the benzazole derivatives and photoactive polystyrenes VM1, VM2, PVM1 and PVM2 ...60
4.4. Steady-State spectroscopic characterization of the benzothiazole derivatives 1, 2 and 3 ...61
4.5 Steady-State spectroscopy characterization of BTS and Zn(BTS) ...62
4.6 Time-Resolved Fluorescence ...63
4.7 Cyclic Voltammetry ...64
4.8 Electroluminescence measurements ...65
Chapter 5 ...67
Results and Discussion ...67
5.1 Excited state Intramolecular proton transfer process in benzazole fluorophores tailored by polymeric matrix ...68
5.1.1. Optical Characterization...70
5.1.2 Theoretical predictions ...76
5.1.3. Partial Conclusions ...80
5.2 Benzothiazole derivatives as molecular probes of amino-polysiloxanes ...81
5.2.1 Theoretical Predictions ...83
5.2.2. Optical Characterization ...91
5.2.3. Photophysics in amino-polysiloxanes ...93
5.2.4. Partial Conclusions ...99
5.3. A novel and versatile benzothiazole-salophen reactive to the ESIPT process to white-light generation ...100
5.3.1 Chemical characterization of BTS ...102
5.3.2 Optical Characterization of BTS ...103
5.3.3 Fluorescence Dynamics of BTS in solution ...108
5.3.5 All-solution processed WOLEDs based on BTS: electronic characterization of the diodes
PVK:BTS and PFO:BTS ...116
5.3.4 Chemical characterization of Zn(BTS) ...124
5.3.5 Optical properties of Zn(BTS) ...124
5.3.6 Optoelectronic properties of PFO:Zn(BTS) composites ...127
5.3.7 Partial Conclusions ...131
Chapter 6 ...133
Conclusion ...133
References ...136
Chapter 1
Excited State
Intramolecular Proton
Transfer process
1.1 Motivation
Physicochemical dynamics consequent of radiation absorption on the ultraviolet or visible region (UV/UV-Vis) of the electromagnetic spectrum by organic molecular systems are subject of study in many fields. The development of these systems have countless applications, including the design of optoelectronic devices, sensors, and materials for imaging.1–4
In this sense, this chapter presents some concepts related to Photophysics and Photochemistry of chemical processes kinetics and mechanisms on the electronic excited state. Energetic changes were described by the Förster Theory. It is also described one particular example of excited state reactions, a process studied throughout this thesis: the Excited State Intramolecular Proton Transfer Reaction (ESIPT). Some variables capable of altering such balances will be discussed, along with an explanation of the main technological applications derived from the photoreaction.
1.2 Photophysical and Photochemical Processes
With the absorption of electromagnetic radiation, a system may return to its electronic ground state through the dissipation of the acquired energy,5 which may evolve many different
deactivation paths.6 Regardless of the path, deactivation of excited states can be divided into
photophysical and photochemical processes with intrinsic properties or external perturbations.7 Figure 1 depicts the several processes, along with their respective rates, starting by excitation of A from a singlet electronic ground state A1. The excitation process I
a is assigned
by radiation absorption hνabs that leads the system to its electronic excited state A1*. The
intensity of the absorption is proportional to the concentration of A on the excited state. After excitation, the species may deactivate by non-radiative and/or radiative paths, according to the Jablonski Diagram: vibrational relaxation, internal conversion (IC), intersystem crossing (ISC), fluorescence and phosphorescence decays. Besides, in the occasion of a photochemical process, i.e., when a chemical reaction also takes place, A1* may suffer structural transformations on its
Process Rate Excitation IC Fluorescence ISC Phosphorescence ISCT Reaction S1 Reaction T1 Ia KIC[A1*] kF[A1*] kISC[A1*] kP[A3*] kTISC[A³*] kr1[A1*] kr3[A3*]
Figure 1. Schematic view of Photophysical/Photochemical deactivation of A¹* by unimolecular processes. The table on the right contains the respective rate constants of each process, with hνabs being the excitation energy of intensity Ia. kIC is the rate constant of internal conversion,
kF is the rate constant of radiative deactivation by fluorescence, where hνFA is the emission
energy. kISC is the rate constant of intersystem crossing, that results in the formation of the
triplet state A³*, kP is the rate constant of radiative deactivation by phosphorescence, where hνPA
is the energy of emission. kTISC is the rate constant of A³* non-radiative deactivation. kr1
represents the rate constant for the photoreaction evolving A1* with the formation of the product
P¹* and kr2 the rate constant for the photoreaction evolving A3* with the formation of the
product P3* – the deactivation processes related to the photoproducts can be described in the
same way.
Considering the rate constants in Figure 1, in the absence of photoreaction and quenching processes and under photostationary conditions, deactivation of A1* can be
expressed as
[𝑨𝟏∗] = 𝑰𝒂
𝒌𝑰𝑪+𝒌𝑰𝑺𝑪+𝒌𝑭𝑨 Equation 1
and by the quantum yield of fluorescence ΦF, which is giving by Equation 2.7
𝚽𝑭 = # 𝒆𝒎𝒊𝒕𝒕𝒆𝒅 𝒑𝒉𝒐𝒕𝒐𝒏𝒔 # 𝒂𝒃𝒔𝒐𝒓𝒃𝒆𝒅 𝒑𝒉𝒐𝒕𝒐𝒏𝒔= 𝒌𝑭[𝑨𝟏∗] 𝑰𝒂 = 𝑰𝒂 𝒌𝑰𝑪+𝒌𝑰𝑺𝑪+𝒌𝑭 Equation 2
where the quantum yield of fluorescence ΦF is the ratio between the number of emitted and
of excitation Ia. In practical terms, it is the ratio between kF and the sum of rate constants of all
events (non-radiative and radiative). The inverse of this ratio corresponds to the electronic excited state deactivation lifetime τFA (fluorescence lifetime)
The phosphorescence quantum yield is obtained using the same approach. 1.3 Photochemical Processes Rates
When a photochemical process is present, the rate constant must be necessarily lower or of the same magnitude that the radiative rate constants evolved, and faster than other competitive photophysical processes.5,8 Regarding photochemical processes, their efficiency is
defined by the reaction quantum yield ΦR:
𝚽𝑹 = 𝒓𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝒓𝒂𝒕𝒆 𝑰𝒂 =
𝒅[𝑷] 𝒅𝒕
𝑰𝒂 Equation 3
An important aspect is to identify the electronic level that takes part in the process. As previously explained, photochemical processes can be derived from singlet or triplet states, with the last one being slower. For instance, assuming the transformation Ai* P (Figure 2), where
Ai* is not the initial electronic excited state:
Figure 2. Representation of a photochemical process evolving the A¹ species. Ia is the absorbed
radiation, ka the deactivation rate constant of A¹* without the component related to the
formation of Ai*, kb is the rate constant related Ai* formation. kc is the deactivation rate constant
of Ai* without the component related to the photochemical process, and k
r is the rate constant
of the photoreaction.
[𝑨∗𝟏] = 𝑰𝒂 𝒌𝒂+𝒌𝒃 Equation 4 [𝑨𝟏∗] =𝒌𝒃[𝑨𝟏∗] 𝒌𝒄+𝒌𝒓 = 𝒌𝒃𝑰𝒂 (𝒌𝒄+𝒌𝒓)(𝒌𝒂+𝒌𝒃) Equation 5
Hence, the rate of product P formation, assuming a unimolecular process, will be expressed as follows: 𝒅[𝑷] 𝒅𝒕 = 𝒌𝒓[𝑨 𝒊∗] = 𝒌 𝒓 𝒌𝒃 𝒌𝒄+𝒌𝒓 𝑰𝒂 𝒌𝒂+𝒌𝒃 Equation 6
Thus, the reaction quantum yield is further expressed by:
Φ𝑅 = 𝑘𝑟[𝐴𝑖∗]
𝐼𝑎 =
𝑘𝑟𝑘𝑏
(𝑘𝑐+𝑘𝑟)(𝑘𝑎+𝑘𝑏) = (𝑘𝑟𝜏𝐴
𝑖∗)(𝑘𝑏𝜏𝐴1∗) Equation 7
Equation 7 can be generalized as the product of the reaction rate constant, the lifetime of the reactive state, and the multiplicand of all rate constants and lifetimes from the “j” electronic excited states:
𝚽𝑹 = 𝒌𝒓𝝉𝑨𝒊∗∏ 𝒌𝒋𝝉𝒋 Equation 8
When the reaction occurs on the electronic state reached from the absorbed radiation: Ai* = A¹* e k
r = ϕr/τA¹*. If ϕr = 1, so kr = 1/τA¹*6.
1.4 Potential Energy Surfaces
Apart from the kinetics description with the determination of every rate constant, a more accurate description of a balance on the excited state, no matter its nature (physical or chemical) or electronic state, should also account for energetic changes. To excited state balances, a thermodynamic treatment requires the knowledge of the Potential Energy Surface (PES) and at least one electronic excited state. In photochemistry, these information are crucial to the understanding of nuclear motion, hence, of the reaction direction.9 In a PES, each point of the curve provides the nuclear and electronic configurations of the molecule, as well as its energy. As expected, the reaction flows to an energy minimum. There is also the possibility of two different curves of two different electronic states are isoenergetic. In this case an additional deactivation mechanism might take place, which results in the variation of the reaction rate through what is called “Conical Intersection” or “Photochemical Funnel” (Figure 3b).9–11
Förster defined three types of cycles based on their deactivation mechanisms: (a) when the reaction occurs over a single PES, and all surfaces are distant from each other;12 (b) when
there are two or more PES’s that touch themselves on a region, allowing the existence of a Photochemical Funnel;13 (c) when after excitation, there is a deactivation to an excited vibrational state of the electronic ground state and the excess of energy is enough to establish the reaction flow, a process known as hot-ground-state14 reaction (Figure 3).9,11
According to the Franck-Condon Principle, absorption of light is a “vertical” process. Therefore, at the moment of excitation, the molecule geometry is virtually equal to the previous one. However, the system is now driven by a new PES, so when the reaction reaches the state of equilibrium, it will be at an energy minimum on the excited state.11
Figure 3. Representation of (a) an adiabatic photoreaction, (b) a diabatic photoreaction and of (c) a hot-ground-state reaction between ground and excited states.9 The wavy purple arrow
corresponds to a non-radiative deactivation pathway of the reagent before the reaction.
Case (a) is defined as an adiabatic photoreaction because it is related to processes in which small structural changes take place.9 Such classification method might be used to assign
an adiabatic reaction as diabatic when deactivation of the product P on the excited state can also be understood as a jump between PES’s. In other words, there are different ways to understand the mechanism of the same photoreaction. Usually, a photoreaction is classified as adiabatic if it is possible to observe some spectroscopic information from the product via excitation of the reagent.11
1.5 Proton Transfer on the Excited State
1.5.1 Excited State Proton Transfer
Proton transfer reaction is an elementary process with great relevance to nature and technological development.15–17 To balances on the excited state, a proton can be transferred
inter (Excited State Intermolecular Proton Transfer) or intramolecularly (Excited State Intramolecular Proton Transfer).18 Essentially, ESPT process is related to a protonation/deprotonation balance on the excited state. It is described by the Förster Cycle (Figure 4), a thermodynamic cycle used to determine the acidity constant of the excited state (pKa*) starting from the acidity constant on the ground state (pKa) and the energies of the main
electronic transitions related to protonated (AH) and deprotonated (A-) species.18,19
Figure 4. Förster Cycle to a protonation/deprotonation balance of a generic system AH. hνAH
and hνA-* are the energetic differences between electronic ground and excited, ΔH e ΔH* are
the enthalpic variations, Ka and Ka* are the equilibrium constants of the reactions on ground
and excited states, respectively.
This is an example of an adiabatic reaction (Figure 3a), where the geometry changes are very small, so entropic variations from ground to excited states are considered to be close to zero. Hence, the reaction free energy depends mainly on the enthalpy contribution, estimated as the average of absorption and emission maxima from AH and A- and pK
a* is given by:
𝒑𝑲𝒂 − 𝒑𝑲𝒂∗ =𝒉𝝂𝑨𝑯 − 𝒉𝝂𝑨−
𝟐.𝟑𝟎𝟑𝑹𝑻 Equation 9
where: R is the ideal gas constant, and T is the temperature and the difference 𝑝𝐾𝑎 − 𝑝𝐾𝑎∗is related to both the electronic density redistribution and the variation of dipole moments between ground and excited states.20 The Förster cycle allows us to estimate the relative change in the
acidity of a chromophore upon electronic excitation ΔpKa, independent of prior knowledge of
the ground state pKa.
There are several examples of molecules with enormous changes of 𝑝𝐾𝑎 − 𝑝𝐾𝑎∗, as in the case of 1-naphtol and 2-naphtol. Rosenberg et al determined the values of pKa and pKa* to
both molecules using Equation 9 and by spectroscopic titration: to 1-naphtol it was obtained 9.23, 2.10 and 1.84, and to 2-naphtol 9.45, 3.00 and 2.85. This effect is more prominent when the hydroxyl moiety is at the naphthalene short axis.21
The substituent effects were also explored in 2017 by Duarte et al to a series of 3-hydroxyflavones in aqueous media.22 It was chosen three derivatives with complementary effects regarding the substitution of the proton in the position 4’ of the system (Figure 5): a model compound unsubstituted (3HF), one with a diethylamine group (DEA3HF) and another with a fluorine at the position 4’ (F3HF). The pKa of each derivative was obtained using the
absorption spectra in different pH’s and applying the Henderson-Hasselback equation as 9.81, 10.75, and 10.54, respectively (Figure 5). Both substituents (DEA and F) have a donor character that results on the diminishment of the hydroxyl acidity. However, on the excited state the presence of a fluorine atom in the structure causes a decrease of the pKa* value due to its
electron withdrawing effect. In contrast, the presence of an electron donating group, such as the DEA, causes an opposite effect, increasing the pKa* value. pKa* was obtained using the
emission spectra as -1.85, -1.89 and -2.25. The explanation to these values came from the analysis of the frontier molecular orbitals (Highest Occupied Molecular Orbital, HOMO; and Lowest Unoccupied Molecular Orbital, LUMO) of the molecules (Figure 4). As in strong acid media the flavonols are entirely protonated, DEA3HF presents the diethylamino group with a central positively charged nitrogen atom. Hence, the predicted electron donor effect is neutralized assuming that the molecule deprotonates in the excited state similarly to 3HF. That is why their pKa* values are similar. Oppositely, the fluorine is still depicting a withdrawing
Figure 5. Chemical equilibria involving both neutral and ionic species of 3-hydroxyflavone (3HF), 4’-Diethyl-3-hydroxyflavone (DEA3HF), and 4’-Fluoro-3-hydroxyflavone. The respective pKa values were also presented along with the HOMO’s and LUMO’s on the right-hand side.
Despite being extremally helpful, theoretical prediction of pKa* using Equation 9 is an approximation due to the exclusion of entropic variations and the assumption that the rate constant of the forward reaction (deprotonation process) is higher than the rate constant of the backward reaction.19
1.5.2. Excited State Intramolecular Proton Transfer
Among the photoreactions that follow adiabatic potential energy surfaces9,18,20,23, ESIPT
plays a great relevance on the development of fluorescent probes24–26, photonic devices27–32 and applications in catalysis3 and biochemistry.33–36 It evolves a dynamics between a proton donor and a proton acceptor sites present on the same molecular structure, on a mechanism firstly formulated by Förster and Weller37,38. In this model a four-level photochemical cycle describes a phototautomerization process, where N is the normal/initial reactive form, containing both proton donor and acceptor groups. Subsequent excitation to its first electronic excited state S1
leads to electronic density reorganization to produce a locally excited species N* (Figure 6) that can either rapidly deactivate radiatively (normal emission) or can undergo proton transfer on the excited state (ESIPT), producing the tautomeric form T*. Once formed, it can either be
deactivated radiatively to T or perform an electronic excited state equilibrium with N* (N* ⇋ T*). Finally, proton recombination takes place on the S0 (Ground State Intramolecular Proton
Transfer – GSIPT). Hence, the typical reaction is composed of a tautomeric equilibrium triggered by light.
Figure 6. Four-level model for the photochemical cycle of the ESIPT process.39
Common systems undergoing ESIPT are aromatic derivatives able to present a strong intramolecular hydrogen bond (IMHB) between reactive sites.40–43 In general, these molecules contain hydroxyl44,45 or amino42,46,47 moieties as proton donors and carbonyl48–51 or azomethinic nitrogen52 moieties as proton acceptors. Because molecules undergoing ESIPT may depict two emission bands, they are potentially suitable for white-light emission.53,54 In addition, the reaction involving N* and T* has an ultrafast reaction rate constant55 due to a very small energy barrier between the two electronic states. Specific interactions such as intra and intermolecular hydrogen bonds also play important roles in the reaction mechanism.56,57 This work deals specifically with two important classes of ESIPT reactive compounds, 2’-hydroxyphenol substituted benzazoles and salicylidene derivatives.3,37,40–48 Both classes are also widely used in coordination chemistry as ligands with useful optoelectronic properties67–71 and as
chemosensors.72–75
1.5.3. ESIPT mechanism
For being a reversible process, when both reagent and product are bright emissive the ESIPT reaction is a very interesting phenomenon because its features open many characteristics for molecular design and modeling in terms of atomic distances, energy interactions, and reaction rates. The reaction kinetics is frequently explored by ultrafast time-resolved spectroscopic techniques, revealing rate constants that range from femto to picoseconds. For example, depending on the condition, the emission spectra of
2-(2’-hydroxyphenyl)benzothiazole (HBT) can change significantly (Figure 7). In a polar protic media such as ethanol, its emission spectrum is populated by the N* form due to the existence of specific interactions between solute and solvent (intermolecular hydrogen bond). In aprotic solvents of low polarity such as cyclohexane, the IMHB becomes favored, and the extension of the reaction is greater.
Figure 7. Steady-state absorption and emission of HBT in cyclohexane (solid lines) and ethanol (dashed lines).18
Kinetically, for the N* form, the reaction rate constant can be derived from all rate constants relative to the deactivation of this species and the T* form: kRN*, kRT*, knRN*, knRT*
and the actual forward and backward reaction rates k+ and k-. Hence, the next two differential
equations can be written53 𝒅[𝑵∗] 𝒅𝒕
= −(𝒌
𝑹 𝑵∗+ 𝒌
𝒏𝑹 𝑵∗+ 𝒌
+)[𝑵
∗] + 𝒌
−[𝑻
∗]
Equation 10 𝒅[𝑻∗] 𝒅𝒕= −(𝒌
𝑹 𝑻∗+ 𝒌
𝒏𝑹 𝑻∗+ 𝒌
−)[𝑻
∗] + 𝒌
+[𝑵
∗]
Equation 11Taking as boundary conditions that at the initial time t = 0, [N*] = [N*]0 and [T*] = 0,
a possible solution to Equation 10 and 11 are [𝑵∗] = [𝑵∗] 𝟎(𝜶𝟏𝑵∗𝒆 −𝒕 𝝉 𝟏 ⁄ + 𝜶 𝟐 𝑵∗𝒆−𝒕⁄𝝉𝟐) Equation 12 [𝑻∗] = [𝑵∗] 𝟎(𝜶𝟏𝑻∗𝒆 −𝒕𝝉 𝟏 ⁄ + 𝜶 𝟐 𝑻∗𝒆−𝒕⁄𝝉𝟐) Equation 13
Equations 12 and 13 can be directly correlated to decays of excited state species typically obtained from time-resolved emission experiments if the ESIPT process is reversible, where 1 and 2 are the emission lifetimes of N* and T*, respectively. The pre-exponential
factors 𝛼1𝑁∗ and 𝛼
2𝑁∗ dictate the decay (consumption) or the rise (formation) of the species, being defined as follows:
𝜶𝟏𝑵∗ = 𝜸𝑵∗−𝟏 𝝉⁄𝟐 𝟏𝝉 𝟏 ⁄ −𝟏 𝝉⁄𝟐 Equation 14 𝜶𝟐𝑵∗ = 𝟏⁄ −𝜸𝝉𝟏 𝑵∗ 𝟏𝝉 𝟏 ⁄ −𝟏 𝝉⁄𝟐 Equation 15 −𝜶𝟏𝑻∗ = 𝜶𝟐𝑻∗ Equation 16 𝜸𝟏, 𝜸𝟐= 𝝉𝟏−𝟏, 𝝉𝟐−𝟏 = 𝟏 𝟐{(𝜸𝑵∗+𝜸𝑻∗)±[(𝜸𝑵∗−𝜸𝑻∗)𝟐+𝟒𝒌+𝒌−] 𝟏 𝟐} Equation 17 𝜸𝑵∗= 𝒌𝑹𝑵∗+ 𝒌𝒏𝑹𝑵∗ + 𝒌+ Equation 18 𝜸𝑻∗= 𝒌𝑹𝑻∗+ 𝒌 𝒏𝑹 𝑻∗ + 𝒌 − Equation 19
In 2014, Wnuk et al applied this approach to study a benzoxazole derivative (a benzazole from Figure 8).76 According to time-resolved fluorescence up-conversion experiments, k+ and k- were obtained as 100 fs and 38 ps in tetrachloroethene solution. The
fluorescence temporal evolutions of the respective N* and T* species are depicted in Figure 8, where it is possible to identify the characteristics of the decay described above: the enol decay has a short component (N*) that matches with the rising of the keto (T*) decay.
Figure 8. (a) Molecular system studied by Wnuk et al. (b) Fluorescence decays of the system measured at the emission peak of the enol (N*) and keto (T*) species. Instrument Response Function (IRF) is also shown as a blue line.76
It is worthy of mentioning that the absorption spectra of ESIPT reactive systems are only populated by the N form, as the T form is only present on the excited state. Thus, a good alternative to determine if the tautomerization balance is only occurring on the excited state is to record the excitation spectrum.
Electronic effects due to substituent also alter the ESIPT balance in different ways, as demonstrated by Liu et al for a series of 1-hydroxy-11H-benzo[b]fluoren-11-ones with alkyl substitutes in different positions.77 According to the authors, the increase in basicity was correlated with the IMHB strength using the O-H distances predicted theoretically. Finally, the two physico-chemical parameters were well correlated to the ESIPT reaction rates obtained in the way described so far, ranging from 560 fs to 51 ps.
1.5.4. ESIPT Potential Energy Surfaces
The analysis of an ESIPT PES is very useful to understand the emission spectrum, the fluorescence lifetimes, and the reaction rate constant. There are two possible descriptions related to those data, which were detailed by Tomin et al as criteria for irreversible and reversible processes (Figure 9).53 The first one, occurs when the N* species has a big difference in energy compared to T* species, and any external perturbation (temperature, viscosity, solvent polarity) may induce changes of the reaction energy barrier. As a result, the backward rate constant k- will be much slower than k+, so the balance is shifted towards T* that will have a
stronger emission intensity in comparison to N*. In addition, because of the necessity of overcoming an energy barrier, N* species should have enough time to deactivate radiatively.
The second criteria, now for reversible ESIPT process, occurs when N* and T* species are close in energy, lowering the energy barrier and depicting a faster k- (Figure 9b). This
condition allows the system to reach the equilibrium in a rate much faster than the fluorescence lifetimes of both species. Hence, the emission spectrum will be populated by the two tautomeric forms with similar intensities, and external perturbations are expected to alter the emission of both species without affecting the reaction yield.
Figure 9. Representation of the two possible ESIPT PES’s. (a) The case of an irreversible reaction, and (b) of reversible reaction.
1.5.5. ESIPT Applications
The easiness of tuning the ESIPT reaction has led to a series of different technological applications. For example, the rapid conversion to T* is of interest to the production of photostabilizers78 and lasing materials.79 Moreover, the unique spectral sensitivity to the environment enabled the design of fluorescent probes for ions80 and organic molecules,81 photopatterning82 and the possibility of producing white-light emitters rendered the innovative application in optoelectronic devices.83
1.5.6. Fluorescent Sensors based on ESIPT reactivity
One category of sensors is intrinsic fluorescent molecules. This class refers to compounds of natural occurrence, as the green fluorescent protein84 and aromatic amino acids like the tryptophan.8 Other is the extrinsic category, which includes probes that are added to a sample to produce fluorescence when it is nonfluorescent. These molecules might be used as non-invasive alternative of great sensitivity, with spatial and temporal resolution for detecting different parameters as polarity, viscosity, temperature, distances, pH, ionic species and small organic pollutants concentrations. Each parameter might be detected based on a specific property of the fluorophore, altered by the media properties not only in terms of its steady-state emission but also dynamically.8,85 When it comes to ESIPT reactive systems, fluorescence
sensing will be performed mainly by the favoring or extinction of the reaction. As a result, the dual emission profile observed on these molecules is modified, in a sense that the characteristic large Stokes Shift is helpful to avoid inner-filter effects.86
For instance, in 2019, Xie and collaborators developed a salicylidene derivative capable of probe hydrazine and hypochlorite in solution based on different effects.87 Hydrazine (N2H4)
is an industrial material that can be used as catalyst and rocket fuel; hypochlorite (ClO-) is a common disinfector used on daily routine. However, both compounds are toxic to humans, so probing their levels is a necessity. The probe, 4-(1-pyrrolidinyl)-benzenamine salicylaldehyde (PBAS), was a product of a condensation reaction with an iminic group as proton acceptor to the ESIPT process. PBAS spectral changes were: hydrazine acted as a base promoting the hydrolyzes of the probe into the respective reagents. Thus the T* emission at em = 570 nm
vanished, and an emission peak at em = 470 nm appeared. Concomitantly, the addition of ClO
-resulted on the deprotonation of PBAS with the formation of the complex PBAS-ClO-, so the ESIPT process was disrupted, and a residual emission around em = 450 nm was observed,
