Tuning the Optical Characteristics of Diketopyrrolopyrrole Molecules in the Solid State by Alkyl Side Chains

The
optical properties of two sets of donor–acceptor–donor
molecules with terminal bithiophene donor units and a central diketopyrrolopyrrole
(DPP) acceptor unit are studied. The two sets differ in the alkyl
chains on the DPP, which are either branched at the α-carbon
(3-pentyl) (1–4) or linear (n-hexyl) (5–8). Within
each set, the molecules differ by the absence or presence of n-hexyl chains on the terminal thiophene rings in the 3′,
4′, or 5′ positions. While in solution, the optical
spectra differ only subtly; they differ dramatically in the solid
state. In contrast to 5–8, 1–4 are nonplanar as a consequence of
the sterically demanding 3-pentyl groups, which inhibit π-stacking
of the DPP units. Using the crystal structures of 2 (brick
layer stacking) and 6 (slipped stacking), we quantitatively
explain the solid state absorption spectra. By computing the molecular
transition charge density and solving the dispersion relation, the
optical absorption of the molecules in the crystal is predicted and
in agreement with experiments. For 2, a single resonance
frequency is obtained, while for 6 two transitions are
seen, with the lower-energy transition being less intense. The results
demonstrate how subtle changes in substitution exert large effects
in optical properties.