Structural characteristics of eD–eA complexes

Table 4.5 Optimal intermolecular distances (d_opt) and interaction energies (E_int) between BDT-TFQ and PC₇₁BM in the BDT-TFQ–PC₇₁BM complexes¹calculated with different functionals and the 6-31G* basis set.

Orientation of PC₇₁BM

Vertical Horizontal

Functional d_opt(Å) E_int(kJ mol⁻¹) d_opt(Å) E_int(kJ mol⁻¹)

B3LYP 4.0 -1.0 4.0 -1.3

B3LYP-D 3.3 -55.5 3.3 -63.7

ωB97X 3.4 -19.5 3.4 -22.7

ωB97X-D 3.3 -58.0 3.3 -66.1

OT-ωB97X 4.7 -0.5 4.7 -0.9

OT-ωB97X-D 3.4 -42.0 3.3 -47.0

1 The results are only for the complexes, where PC₇₁BM is on the top of the acceptor unit of BDT-TFQ. For the other complexes, see the original Publi- cation II.

Overall, the orientation of PC₇₁BM does not have a significant effect on the optimal intermolecular distances, i.e. they are practically the same whether PC₇₁BM is ver- tically or horizontally oriented with respect to BDT-TFQ, see Table 4.5. However, the (absolute) interaction energies are somewhat larger for the horizontal models.

In Publication II, all the functionals are observed to predict the smallest intermolec- ular distances and largest interaction energies, when PC₇₁BM locates on the top of the quinoxaline acceptor unit of BDT-TFQ. In most cases, the largest distances and smallest interaction energies are obtained when PC₇₁BM is on the top of the donor unit of BDT-TFQ, except for the vertical models, where the smallest interaction en- ergies are observed, when PC₇₁BM is on the top of the thiophene unit. The stronger interaction between the acceptor unit of the D–A copolymer and PC₇₁BM has been observed also by a previous experimental study[186]and it may play a role in the preferred binding sites of some copolymer–fullerene systems. However, this does not apply to all systems, as opposite results have been obtained experimentally[186]

and theoretically[34], as well.

As the intermolecular distances between BDT-TFQ and PC BM has been studied

and polymer–SMA systems have been optimized to obtain their relaxed (GS) struc- tures. Similarly to the BDT-TFQ–PC₇₁BM complexes, several relative orientations of these compounds have been considered to see whether they have some prefer- ences. In the most stable configuration predicted for the polymer–polymer type BDT-TzBI–NDI2OD-T2 complexes, the acceptor units of BDT-TzBI and NDI2OD- T2 prefer to be face-to-face, while their donor units are also face-to-face (i.e. the AA(1) configuration). This supports previous experimental findings for the face-to- face stacking between the similar PTzBI-Si copolymer and P(NDI2OD-T2)[80]. In the alternative configuration (with the energy difference of 0.9 kJ mol⁻¹to the most stable one), the donor and acceptor units of BDT-TzBI and NDI2OD-T2 prefer to be face-to-face, respectively, whereas the acceptor and donor units of BDT-TzBI and NDI2OD-T2 are face-to-face, respectively (the DA(2) configuration). These kind of face-to-face orientations between the copolymers are important for forming strong π-orbital overlap at their interfaces, which will reduce the binding energy of the excitons and promote the free charge carrier generation[7]. The intermolecular dis- tances predicted here between the backbones of BDT-TzBI and NDI2OD-T2 are ca.

3.4–4.0 Å with the average of 3.7 Å, which is in line with the experimentalπ–π stacking distance (3.7 Å) for this system[187].

In the polymer–SMA type complexes, DTB-EF-T–ITIC-4F and BDB-T-2F–ITIC- 2Cl, the electron-withdrawing INCN end groups of the ITIC derivatives have been positioned above different backbone units of DTB-EF-T and BDB-T-2F, as these kinds of relative orientations have been predicted in the MD simulations of other polymer–ITIC systems[16]. This tendency for locating the end-groups of the ITIC on the polymer backbone has been explained by the steric hindrance caused by the hexylphenyl side groups in the IT core unit of ITIC, which prevent the approach of the polymer chains. In Publication IV, the DA configuration of DTB-EF-T–ITIC- 4F, where the electron-withdrawing end-group of ITIC-4F is on the top of the BDT donor unit of DTB-EF-T, is predicted to be the energetically more favorable con- formation than the AA configuration. In the AA configuration, the end-group of ITIC-4F has been positioned initially on the top of the acceptor unit, i.e. one of the substituted thiophenes of DTB-EF-T, but is eventually located above the bond connecting the acceptor unit and the neighboring BDT donor unit after the geom- etry optimization of the complex. In the BDB-T-2F–ITIC-2Cl complexes, the op- posite result has been obtained, namely the AA configuration, where the end-group

of ITIC-2Cl is on the top of the acceptor unit of BDB-T-2F is predicted to be en- ergetically more favorable than the DA configuration, where the end-group is on the top of the donor unit. This is inline with the previous MD simulations of the BDB-T-2F–ITIC systems[16, 19].

Alongside the interaction strength between the compounds, the length and degree of branching of the polymer side chains control the accessibility of the backbone and possible intercalation of fullerene derivatives between the side chains[71, 186].

As the full-length side chains have not been included here, the studied complexes do not take these factors into account, but rather give an idea of the mutual interactions between the eD and eA compounds.