A Combined Experimental and Theoretical Study of Conformational Preferences of Molecular Semiconductors

Structural modules used for assembling molecular semiconductors have typically been chosen to give desirable optical and electronic properties. Growing evidence shows that chemical functionalities should be considered for controlling molecular shape, which is important for function because of its influence on polymer secondary structure, lattice arrangements in crystals, and crystallization tendencies. Using density functional theory (DFT) calculations, followed by a natural bond orbital (NBO) analysis, we examine eight molecular semiconductors with resolved single crystal X-ray structures to understand the features that dominate molecular conformations and ultimately develop practical rules that govern these preferences. All molecules can be described by a D′−A−D−A−D′ architecture and have a 4,4-dimethyl-4H-silolo[3,2-b:4,5-b′]dithiophene (DTS) donor (D) core unit, with [1,2,5]thiadiazolo[3,4-c]pyridine (PT), 5-fluorobenzo[c][1,2,5]thiadiazole (FBT), or benzo[1,2,5]thiadiazole (BT) electron acceptor (A) units, and either thiophene, 5-hexyl-2,2′-bithiophene, or benzofuran electron-donating end-caps (D′). The NBO analysis shows that the energy difference between the two alternative conformations, or rotamers, (ΔErot) is a delicate balance of multiple competing nonbonding interactions that are distributed among many atoms. These interactions include attractive “donor−acceptor” electron sharing, steric repulsion, and electrostatic stabilization or destabilization. A proper grouping of these interactions reveals two primary factors determining ΔErot. The first concerns heteroatoms adjacent to the bonds connecting the structural units, wherein the asymmetric distribution of π-electron density across the link joining the units results in stabilization of one of two rotamers. The second factor arises from electrostatic interactions between close-contact atoms, which may also shift the ΔErot of the two rotamers. When all these constituent interactions cooperate, the dihedral angle is “locked” in a planar conformation with a negligible population of alternative rotamers.