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

Abstract

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.