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.