Effect of Cross-Conjugation on Derivatives of Benzoisoindigo, an Isoindigo Analogue with an Extended π‑System

The
effect of cross-conjugation on small molecule organic semiconductors
derived from benzoisoindigo, a ring-expanded isoindigo derivative,
is reported. In this work, 5-hexyl-2,2′-bithiophene was substituted
at the cross-conjugated 5,5′-position of benzoisoindigo, yielding
a new organic semiconductor with a donor–acceptor structure.
The optoelectronic properties of this benzoisoindigo derivative were
studied and compared with those of both linearly conjugated and cross-conjugated
isoindigo-based analogues. Extending the conjugation of the electron-deficient
isoindigo through ring fusion did substantially red-shift the absorption
maximum of the highest occupied molecular orbital to lowest unoccupied
molecular orbital transition; however, the cross-conjugation significantly
reduced the oscillator strength. As a result, the photocurrent was
significantly lower for organic photovoltaic devices made with cross-conjugated
materials. This suggests that if synthetic methods to access the linearly
conjugated 7,7′-derivatives can be developed, benzoisoindigo
may be able to serve as a useful electron-deficient subunit in donor–acceptor
systems.