Extending the scope of the carbonyl facilitated triplet excited state towards visible light excitation

Abstract

A series of extended π-conjugated benzophenone analogs was synthesized through a facile Lewis-acid catalyzed Friedel–Crafts reaction in order to exploit the integral triplet state properties of benzophenone. Extending the π-conjugated plane of the phenyl ring of benzophenone allowed tuning of the excitation wavelength from the far-UV end (∼260 nm) to the visible spectrum (∼446 nm). Compared to benzophenone, significant red-shifts in the absorption (up to 450 nm in solution) with high photostability were observed for the synthesized benzophenone analogs. As is evident from the density functional theory calculations, expansion of the ring size of the aromatic part of the benzophenone analogs induces a decrease in the HOMO–LUMO gap. The considerable extension of the electron density to the carbonyl group in the LUMO substantiates the triplet nature associated with the benzophenone analogs. By virtue of the properties of the carbonyl functionality, an apparent increase in the triplet quantum yield (Φ_T = 5.4% to 87.7%) was observed for the benzophenone analogs when compared to the corresponding bare polyaromatic hydrocarbon. The spin orbit coupling was computationally estimated for the benzophenone analogs to propose pathways for the observed intersystem crossing process. The plausibility to photoexcite the aromatic-ring-fused benzophenone frameworks for triplet activation in the visible range opens the door for a new class of materials for photonic application.