Controlling Polymorphic Transitions in n‑Type Organic Semiconductor Single Crystals by Alkyl Chain Engineering

Control of polymorphic behavior is
crucial for designing functional
organic semiconductor devices as even a slight structural difference
may translate to dramatically different electronic properties. One
route to controlling structure is through stimulus-induced polymorph
transitions, which allows for switching those electronic properties.
However, despite advances in predicting crystal structures, the molecular
design characteristics governing the polymorphic transition mechanism
remains unknown. Here, we systematically investigate a series of n-type
organic semiconductor molecules based on 2-dimensional quinoidal terthiophene
with varying alkyl side chain lengths to modulate two distinct polymorph
transitions, one cooperative martensitic transition and the other
non-cooperative nucleation and growth transition. In the three molecular
systems, we observe that shortening the alkyl chain past a critical
length suppresses the cooperative polymorph transition by limiting
the alkyl chain conformation change. On the other hand, the nucleation
and growth transition temperature increases as the side chain length
decreases, possibly driven by the increase in the melting point of
the alkyl chains. We also found that tuning the alkyl chain length
modulates the associated quinoidal to aromatic biradical switching
that drives the nucleation and growth transition, suggesting a synergy
between the crystal structure and electronic structure. Ultimately
depending on the exact mechanism of the polymorph transition, adjusting
the alkyl chain length may lead to tuning of the polymorph transition
temperature or suppression of the transition altogether. This offers
a potential molecular design rule to target a particular transition
mechanism based on the desired behavior for the system.