Exploration of Alkyl Group Effects on the Molecular Packing of 5,15-Disubstituted Tetrabenzoporphyrins toward Efficient Charge-Carrier Transport

The high design flexibility of organic semiconductors
should lead
to diverse and complex electronic functions. However, currently available
high-performance organic semiconductors are limited in variety; most
of p-type materials are based on thienoacenes or related one-dimensionally
(1D) extended π-conjugated systems. In an effort to expand the
diversity of organic semiconductors, we are working on the development
of tetrabenzoporphyrin (BP) derivatives as active-layer components
of organic electronic devices. BP is characterized by its large, rigid
two-dimensionally (2D) extended π-framework with high light
absorptivity and therefore is promising as a core building unit of
organic semiconductors for optoelectronic applications. Herein, we
demonstrate that BP derivatives can afford field-effect hole mobilities
of >4 cm2 V–1 s–1 upon
careful tuning of substituents. Comparative analysis of a series of
5,15-bis­(n-alkyldimethylsilylethynyl)­tetrabenzoporphyrins
reveals that linear alkyl substituents disrupt the π–π
stacking of BP cores, unlike the widely observed “fastener
effect” for 1D extended π-systems. The n-octyl and n-dodecyl groups have the best balance
between high solution processability and minimal π–π
stacking disruption, leading to superior hole mobilities in solution-processed
thin films. The resulting thin films show high thermal stability wherein
the field-effect hole mobility stays above 1 cm2 V–1 s–1 even after heating at 160 °C
in air, reflecting the tight packing of large BP units. These findings
will serve as a good basis for extracting the full potential of 2D
extended π-frameworks and thus for increasing the structural
or functional diversities of high-performance organic semiconductors.