Facile transformation of 1-aryltriphenylenes into dibenzo[fg,op]tetracenes by intramolecular Scholl cyclodehydrogenation: synthesis, self-assembly, and charge carrier mobility of large π-extended discogens
The search for new organic semiconductors with enhanced charge transport properties and self-organizing abilities plays a pivotal role in the development of new applications in the emerging field of organic electronics. We have synthesized two series of discotic mesogenic materials derived from hexasubstituted triphenylene, including (i) 1-aryl-2,3,6,7,10,11-hexakis(pentyloxy)triphenylenes (14 new compounds) by a Suzuki cross-coupling reaction between the appropriate 1-bromotriphenylene precursor and various arylboronic acids and (ii) unsymmetrical facial dibenzo[fg,op]tetracene discotic molecules (11 new compounds) by an FeCl3-oxidized cyclodehydrogenation reaction of the former. The mesomorphism has been investigated by polarizing optical microscopy, differential scanning microscopy, and small-angle X-ray scattering. Most aryl-substituted triphenylene derivatives exhibit a single hexagonal columnar mesophase, enantiotropic over small temperature ranges or monotropic, with this low stability being likely attributed to the free-rotating bulky side-on arene group that disrupts a perfect stacking. The corresponding more rigid and flat dibenzo[fg,op]tetracene derivatives also self-organize into a hexagonal columnar mesophase, but with a larger mesophase stability than their parents, and occurring slightly above room temperature. The UV/vis absorption and fluorescence emission spectra have been measured. Tetracenes show stronger photoluminescence than aryltriphenylene in solution, while the reversed is observed in thin films, where a strong excimer emission for one of the polar 1-aryltriphenylenes is observed. The charge carrier mobility of two representative discogens has been measured by the time-of-flight photocurrent technique. The results show that the discogen with a lateral nucleus dipole displays a hole mobility of 10−4 cm2 V−1 s−1 in the mesophase, while the non-polar compound exhibits a hole mobility of 10−2 cm2 V−1 s−1 in its metastable-induced ordered phase. The charge carrier mobility is discussed as a function of the supramolecular organization.