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O mecanismo de nucleac¸˜ao e crescimento de part´ıculas em escala nanom´etrica ´e de grande interesse n˜ao s´o dos que querem explor´a-las tecnologicamente como tamb´em do que desejam investigar o efeito de outras mol´eculas coadjuvantes pre- sente no meio reacional. No caso das nanopart´ıculas de azul da Pr´ussia, o meca- nismo de formac¸˜ao existente na s´ıntese usual via pol´ımero/Fe2+ e [Fe(CN)6]3− deve

ser distinto do que ocorre quando o precursor ´e o metalopol´ımero. No primeiro caso o pol´ımero age adsorvendo-se nos n´ucleos em crescimento e no segundo caso o meta- lopol´ımero, al´em dessa func¸˜ao, age na liberac¸˜ao dos blocos de construc¸˜ao Fe(CN)3−5 que reagir˜ao com Fe3+ adicionado.

Um experimento visando sondar o mecanismo de formac¸˜ao de AP a partir do [Fe(CN)5(P4VP)25]3− foi feito titulando-se uma soluc¸˜ao dilu´ıda do metalopol´ımero

com uma soluc¸˜ao concentrada de FeC`3. Como controle, o mesmo experimento foi

feito com o solvente puro e com a P4VP pura. Espectros UV-Vis foram tomados logo ap´os as adic¸˜oes consecutivas das al´ıquotas de FeC`3, onde se observou formac¸˜ao da

banda MMCT na regi˜ao acima de ∼800 nm somente quando havia Fe(CN)3−5 .

Os espectros eletrˆonicos obtidos constam nas Figuras 5.7a-b. A Figura 5.7c exibe a absorbˆancia no m´aximo da banda e sua posic¸˜ao em cm−1 em func¸˜ao da raz˜ao Fe3+/Fe(CN)3−5 . Observa-se que a intensidade da banda deixa de aumentar quando esta raz˜ao atinge o valor de 1,0-1,2, sugerindo descontinuidade na formac¸˜ao do produto AP quando a estequiometria atinge essa faixa. Simultaneamente, a banda MMCT desloca-se para maiores energias conforme aumenta a quantidade de Fe3+ relativa a de Fe(CN)3−5 , indicando que o acoplamento eletrˆonico entre os ´atomos de

(a)

(b)

(c)

Figura 5.7: Espectros UV-Vis do metalopol´ımero [Fe(CN)5(P4VP)25]3−mediante adic¸˜ao de porc¸˜oes de FeC`3

equivalentes a f = Fe3+/Fe(CN)3−5 de 0 a 1(a) e de 0 a 4 (b). Posic¸˜ao νmaxdo m´aximo da banda obtido por

ajuste por gaussiana no intervalo de 12,5-10×103cm−1(800-1000 nm) e o valor da absorbˆancia em νmax(c).

ferro ´e sens´ıvel `a quantidade relativa dos n´ucleos met´alicos na estrutura do composto. Enfim, a investigac¸˜ao do mecanismo que leva `a formac¸˜ao de AP a partir dos me- talopol´ımeros ´e um campo a ser explorado em futuros trabalhos, podendo resultar em uma sequˆencia de reac¸˜oes que culminam na formac¸˜ao do produto, em estimati- vas da composic¸˜ao ou na revelac¸˜ao da influˆencia da raz˜ao py/Fe, por exemplo. Este trabalho se encerra enfocando a viabilidade da produc¸˜ao de estruturas auto-montadas de pol´ımeros de coordenac¸˜ao, cujos blocos de construc¸˜ao s˜ao fornecidos por macro- mol´eculas h´ıbridas planejadas atrav´es da qu´ımica de coordenac¸˜ao.

A reac¸˜ao entre o pol´ımero hidrof´obico poli(4-vinilpiridina) com o complexo hi- drof´ılico pentacianoferrato de s´odio gerou uma macromol´ecula h´ıbrida orgˆanico- inorgˆanica [Fe(CN)5(P4VP)S]3− com composic¸˜ao S controlada experimentalmente

pela raz˜ao monˆomero/pentacianoferrato colocada para reagir, desde que S seja maior que ∼ 3,5 unidades monom´ericas para cada unidade Fe(CN)3−5 . A principal evidˆencia da s´ıntese esteve na transic¸˜ao de mistura bif´asica para homogˆenea observada visual- mente ap´os a adic¸˜ao de uma soluc¸˜ao aquosa do complexo `a uma soluc¸˜ao etan´olica do pol´ımero, revelando assim que a macromol´ecula contendo porc¸˜oes sol´uveis em ´agua (meros complexados, porc¸˜ao inorgˆanica) e outras em etanol (meros 4-vinilpiridina livres, porc¸˜ao orgˆanica) torna-se sol´uvel em misturas ´agua/etanol numa ampla faixa de composic¸˜ao.

Em virtude da compatibilizac¸˜ao de porc¸˜oes de solubilidade distintas numa mesma mol´ecula, as propriedades de cada porc¸˜ao foram alteradas pela quantidade rela- tiva da outra porc¸˜ao. Os resultados dos estudos de solvatocromismo mostram que quanto maior ´e a extens˜ao das porc¸˜oes de cadeia de monˆomeros livres, maior ´e a solvatac¸˜ao do pentacianoferrato por etanol. Em outras palavras, a polaridade do am- biente qu´ımico do complexo ´e modulada pelo pol´ımero.

Amostras contendo grande quantidade de complexo como a [Fe(CN)5(P4VP)5]3−

´agua quando o meio encontra-se pobre em ´agua (XH2O< 0,42 e 0,25 respectivamente,

Figura 4.7). Essa observac¸˜ao foi racionalizada idealizando-se a formac¸˜ao de regi˜oes ricas em complexo e em de ´agua, formando “bols˜oes” hidrof´ılicos. Uma alterac¸˜ao no modelo de solvatac¸˜ao preferencial82 foi proposta, tornando-o capaz de prever a XH2O em que ocorre a invers˜ao pela inserc¸˜ao do parˆametro xH2O(B). Amostras

contendo pequena quantidade de complexo, por outro lado, tem os grupos Fe(CN)3−5 completamente solvatados por etanol em XH2O = 0,42. Abaixo desse valor, os meros

4vpy passam a solvatar preferencialmente o complexo. De um modo geral, dado um par de solventes, a solvatac¸˜ao ´e preferencial pelo menos polar.

A influˆencia do pentacianoferrato nas propriedades do pol´ımero est´a na alterac¸˜ao de sua viscosidade intr´ınseca, que se relaciona com a conformac¸˜ao da macromol´ecula em soluc¸˜ao. Foi observada a tendˆencia de que quanto maior a quantidade do com- plexo coordenado `a cadeia polim´erica, maior sua viscosidade intr´ınseca e consequen- temente, maior ´e o volume do novelo polim´erico em soluc¸˜ao, em virtude majoritaria- mente da repuls˜ao eletrost´atica entre os grupos Fe(CN)3−5 . O volume dos novelos da amostra [Fe(CN)5(P4VP)10]3− chegam a ser 7 vezes maiores que o da P4VP pura.

Em decorrˆencia dessa observac¸˜ao, pode-se afirmar que a densidade de carga na cadeia modula o qu˜ao “aberto” ou “fechado” o novelo polim´erico se encontra, ou seja, modula a acessibilidade de mol´eculas pequenas ao interior do novelo. Este fato, aliado `a modulac¸˜ao da polaridade do ambiente qu´ımico do centro met´alico, pode ter importantes implicac¸˜oes em futuros estudos sobre cat´alise de reac¸˜oes no microambiente polim´erico ou mimese de metaloenzimas, por exemplo.

Variando-se a composic¸˜ao do solvente da amostra [Fe(CN)5(P4VP)25]3−, notou-

Um m´aximo em [η] ´e observado em valores de XH2O intermedi´arios, denotando a

condic¸˜ao de melhor solvatac¸˜ao dos meros complexados e dos meros da cadeia de P4VP original. Apesar da mudanc¸a da conformac¸˜ao da cadeia polim´erica, solvatac¸˜ao preferencial por etanol foi observada em toda a faixa de composic¸˜ao, sem a presenc¸a de um m´aximo. Isto indica que a conformac¸˜ao da cadeia provavelmente n˜ao tem correlac¸˜ao com a polaridade da esfera de solvatac¸˜ao das unidades Fe(CN)3−5 .

Os metalopol´ımeros mostraram-se aplic´aveis na produc¸˜ao de estruturas tipo azul da Pr´ussia, resultando em part´ıculas com elevada estabilidade coloidal e tendˆencia de o tamanho m´edio ser inversamente proporcional `a raz˜ao py/Fe do metalopol´ımero precursor. A energia e a absortividade molar das transic¸˜oes MMCT dependeram da raz˜ao py/Fe, logo, a composic¸˜ao controlada experimentalmente interfere no grau de interac¸˜ao eletrˆonica dos centros FeII e FeIII deste composto de valˆencia mista. Foi proposto um modelo para racionalizar a interac¸˜ao das part´ıculas de AP com mol´eculas adsorvidas com base na alterac¸˜ao da energia da banda MMCT observada. Por fim, a investigac¸˜ao do mecanismo de formac¸˜ao dessas estruturas permitiu estimar uma composic¸˜ao para a amostra Fe[Fe(CN)5(P4VP)25] , que se encontra concordante

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