Excellent Semiconductors Based on Tetracenotetracene and Pentacenopentacene: From Stable Closed-Shell to Singlet Open-Shell

Designing
stable open-shell organic materials through the modifications
of the π-topology of molecular organic semiconductors has recently
attracted considerable attention. However, their uses as an active
layer in organic field-effect transistors (OFETs) are very limited,
and the obtained hole and electron charge mobilities are around 10–3 cm2 V–1 s–1. Herein, we disclose the synthesis of two peri-fused materials,
so-called tetracenotetracene (TT) and pentacenopentacene (PP), which
have low band gaps of 1.79 and 1.42 eV, respectively. Their ground
state natures have been investigated by different experiments including
steady state absorption, electron spin resonance, superconducting
quantum interfering device, and variable-temperature NMR along with
DFT calculations. TT and PP have closed-shell and singlet open-shell
structures in their ground state, respectively, and possess high stability.
Their biradical characteristics were found to be 0.50 and 0.64. The
origin of the open-shell character of PP is related to the concomitant
opening of two tetracenes with the recovering of two extra aromatic
sextets and a small HOMO–LUMO energy gap (gap <1.5 eV).
Thanks to the high stability, thin film OFET devices could be fabricated.
In TG-BC configuration PP shows a remarkably high hole mobility of
1.4 cm2 V–1 s–1, while
TT exhibits a hole mobility of 0.77 cm2 V–1 s–1. In the configuration of BG-TC, ambipolar
behaviors for both were obtained with hole and electron mobilities
of 0.21 and 0.01 cm2 V–1 s–1 for PP and 0.14 and 0.006 cm2 V–1 s–1 for TT.