The effect of dangling -diphenyl rings on the solid-state emission of quinoxaline-based D-A-D molecules

In this paper, we prepared four donor (D)-acceptor (A)-donor (D) compounds utilizing thiophene as D and quinoxaline as A to investigate the efficacy of o -diphenyl side group in preventing aggregation caused quenching (ACQ). The phenyl rings were placed on the quinoxaline core ( QT-Ph ) and further modified at the para position with decyloxy ( QT-Ph-OC10 ), decanoate ( QT-Ph-EstC10 ), and benzoate ( QT-Ph-EstPh ) substituents. From UV-Vis spectroscopy, it was found that in solution the compounds exhibit similar absorption patterns with λ max around 440 nm with the exception of QT-Ph-OC10 which had a shorter λ max at 407 nm. In solution, all four compounds exhibited high fluorescence quantum yields at ca. 55% with emission maxima following the optical HOMO-LUMO gap trend. In the solid-state, all compounds experienced varying degrees of red-shift in their absorption and emission compared to solution. ACQ of 50-60% was observed for QT-Ph-OC10 and QT-Ph-EstPh in the solid-state. However, QT-Ph and QT-Ph-EstC10 showed significantly diminished ACQ and retained high quantum yields of 46% and 44%, respectively. The optimized geometries generated by theoretical calculations at the B3LYP/6-31G* level revealed that the dihedral angles between structural subunits may play a key role in the different degrees of ACQ. Notably, the dihedral angle between the quinoxaline and phenyl side group was found to be ca. 40°. The ester group in QT-Ph-EstC10 was orthogonally arranged to the phenyl ring which may have prevented any significant ACQ. Both QT-Ph and QT-Ph-EstC10 proved to be excellent organogelators in several polar and nonpolar solvents. Polarized optical microscopy and scanning electron microscopy on dried gels revealed the existence of one-dimensional fibers. Finally, their potential as fluorescent acid sensors was investigated. While all the title compounds showed fast emission quenching upon exposure to trifluoroacetic acid (TFA) vapor in the solid-state, only QT-Ph-EstC10 demonstrated fast emission recovery upon removal of TFA. The reversibility was discussed with electrostatic potential energy map. The current study demonstrates not only the utility of dangling o -diphenyl groups in reducing ACQ, but also feasibility of further modification that can tune photophysical and assembling properties. A systematic investigation on the ability of o -diphenyl side group to suppress aggregation caused quenching of thiophene-quinoxaline-thiophene based D-A-D molecules.