Local interfacial eD–eA complexes

In addition to the individual eD and eA compounds presented above, local interfa- cial complexes of some of them were constructed (Table 3.2). In all complexes, the isolated eD and eA compounds were first optimized separately at the levels of the- ory specified below, after which they were used to construct the complexes in the following manner.

In Publication II, the complexes of the polymer–fullerene system BDT-TFQ–PC₇₁BM were constructed from the B3LYP/6-31G** (vacuum) optimized GS geometries of the trimer of PBDT-TFQ and PC₇₁BM (Figure 3.3). To reduce the computational cost, a planarized trimer model of PBDT-TFQ, where the dihedral angles between the donor and acceptor units were constrained to 180°, while optimizing the geom- etry otherwise fully, was employed. Additionally, the hexyloxyphenyl side groups of the quinoxaline acceptor unit were replaced by hydrogens. Six different BDT- TFQ–PC₇₁BM complexes were constructed, where BDT-TFQ was oriented on the

xyplane along thexaxis, while PC₇₁BM was positioned either vertically or hor- izontally (with respect to thexaxis) on the top of donor, thiophene, and acceptor units of BDT-TFQ. The intermolecular distance between the compounds, which was measured between the mass centers of the specific rings in BDT-TFQ and PC₇₁BM (Figures 3.3b and 3.3c), were varied from 3.0 Å to 5.0 Å (or 6.0 Å in some cases) by steps of 0.1 Å in the one-dimensional single point (SP) (rigid) potential energy scans. The following TDDFT calculations were carried out at both the optimized and constant (i.e. 3.5 Å) intermolecular distances to take the possible influence of a varying intermolecular distance on the excited state characteristics into account.

The geometries of these complexes were not optimized.

In Publication III, the complexes of the polymer–fullerene type TQ–PC₇₁BM were built from the B3LYP/6-31G** (vacuum) optimized GS geometries of the oligomers of PTQ and PC₇₁BM (Figure 3.4). Two planar TQ oligomers were considered, where either thiophene (3T4Q) or quinoxaline (3Q4T) was the middlemost unit and the length of the oligomers was increased symmetrically by adding either thiophene or quinoxaline units to the chain ends. In the oligomers, the octyloxyphenyl side groups of the quinoxaline acceptor unit were replaced by hydrogens to ensure the pla-

Table 3.2 Studied eD–eA complexes.¹

Complex (eD–eA) Geometry optimization² Publication

BDT-TFQ–PC₇₁BM no II

TQ–PC₇₁BM no III

BDT-TzBI–NDI2OD-T2 yes IV

DTB-EF-T–ITIC-4F yes IV

BDB-T-2F–ITIC-2Cl yes IV

1 In the complexes, the length of the D–A oligomers wasn=3 for the planarized BDT-TFQ andn=1 for BDT-TzBI, NDI2OD-T2, DTB-EF- T, and BDB-T-2F. The oligomers of TQ were seven units long, including

either three thiophenes and four quinoxalines or three quinoxalines and four thiophenes.

2 The GS geometry of the whole eD–eA complex.

3a

3a

3b (a) 3b

(b) (c)

3a, 3b

Figure 3.3 Studied BDT-TFQ–PC₇₁BM complexes, where PC₇₁BM was positioned (a) vertically (3a) or horizontally (3b) on the top of the acceptor unit of BDT-TFQ by superposing the centroids of the specific heterocycles in (b) BDT-TFQ (only the innermost CRU is presented wholly) and (c) PC₇₁BM. Adapted from [149] with permission from the PCCP Owner Societies.

nar backbones. It should be noted that the planarization of the helical TQ[150]and omission of its side groups might have consequences of the predicted results. How- ever, the focus of this study was more on the multi-state treatment of the electronic couplings than on the correct description of the studied system. Furthermore, these approximations in the models led to the reduced computational cost in the coupling

(a)

(b) d = 3.5 Å ReD-eA

z x

3T4Q-PC BM71

z y

x

3Q4T-PC BM71

z y

x

Figure 3.4 Illustrations of (a) intermolecular distance,d, and effective separation,R_eD–eA, be- tween TQ and PC₇₁BM in the studied TQ–PC₇₁BM complexes, where PC₇₁BM was (b) either above the middle thiophene (3T4Q–PC₇₁BM) or quinoxaline (3Q4T–PC₇₁BM) unit of TQ. Thed andR_eD–eAwere determined between the centers of mass of the specific rings (pink spheres) and those of the compounds (green spheres), respectively. Adapted from [151] with permission from the PCCP Owner Societies.

the fixed intermolecular distance,d, of 3.5 Å between the compounds (Figure 3.4a).

The geometries of these complexes were not optimized.

In Publication IV, the complexes of the studied NF PSC systems, i.e. BDT-TzBI–

NDI2OD-T2, DTB-EF-T–ITIC-4F, and BDB-T-2F–ITIC-2Cl, were built from the OT-ωB97X-D/6-31G** (blend) optimized GS geometries of the eD and eA com- pounds. In the case of the D–A copolymers, the monomer models were mostly em- ployed, except for one complex configuration of BDT-TzBI–NDI2OD-T2, where the dimers were used instead. The eD compounds were oriented on thexyplane along thexaxis. The eA compounds were positioned above the donor, thiophene, and acceptor units of the eD along thexaxis with the initial intermolecular distance of 4 Å measured between the centers of mass of the superposed heterorings in the com-

pounds (see the original Publication IV for further information). The geometries of the complexes were then fully optimized without any constraints. The configura- tions, where the acceptor unit of the eA compound is located closest to the donor unit of the eD compound after the optimization, are referred to as the "DA" configu- rations. Similarly, those, where the acceptor unit of the eA compound is eventually closest to the acceptor unit of the eD compound, are referred to as the "AA" con- figurations. In BDT-TzBI–NDI2OD-T2, two different series were considered: (1), where the donor and acceptor units of both compounds were on the same direction and (2), where the donor and acceptor units of the compounds were on the opposite directions (see Publication IV for more detailed information).