Way to Highly Emissive Materials: Increase of Rigidity by Introduction of a Furan Moiety in Co-Oligomers

Rigid linear organic co-oligomers are prospective materials for organic optoelectronics. In this work, we explored intramolecular factors affecting the torsional rigidity and its influence on optoelectronic properties of the alternating furan/phenylene and thiophene/phenylene co-oligomers in both ground and first singlet excited states. A furan/phenylene co-oligomer exhibits almost twice as high torsional rigidity than its thiophene analogue. The effect of intramolecular O···H and S···H interactions on torsional barriers was found to be negligible as compared with the conjugation efficiency. The higher torsional rigidity of furan and thiophene co-oligomers has been proven to be reflected in the fine structure of the UV–vis absorption spectrum of the former. The increase of furan co-oligomer rigidity as compared with its thiophene analogue lowers reorganization energy for hole, electron, and exciton transfer. Remarkably the substitution of thiophene by furan lowers by almost 20 times the reorganization energy for exciton transfer. A noteworthy finding was also that in furan co-oligomer the higher rigidity was suggested to hinder “in molecule” photoluminescence quenching due to a possible conical intersection between bright state S 1 and the T 3 excited state. Therefore, tuning of torsional rigidity impacts emission and charge transport properties, being a very powerful tool on the way to high performance emissive organic semiconductors.