5,12-Diacetyl-5,12-dihydroquinoxalino[2,3-b]quinoxalines: Solid-State Fluorescence, AIE Properties, and Orbital Switching by Substituent Effect

The compound 5,12-diacetyl-5,12-dihydroquinoxalino[2,3-b]quinoxaline 1 a and its derivatives were prepared, and their solid- and solution-state spectroscopic properties were studied; 1 a shows stronger fluorescence in solution than in the solid state due to aggregation caused by self-quenching. Phenyl- or alkoxy-substituted derivatives 1 b–d show solid-state fluorescence with moderate quantum yields of about Φ=0.12–0.15, although the corresponding values are 0.01–0.07 in solution. The spectroscopic properties of alkoxy-substituted derivatives were hardly changed compared to 1 a and 1 b, although 1 a and 1 b have similar absorption and fluorescence maxima in solution and in the solid state. DFT calculations indicate that orbital switching occurs between HOMO and HOMO-1 and HOMO-2 due to orbital interactions with introduced substituents. Crystal structure analysis revealed that the molecules have bent structures around tertiary nitrogen atoms and form a characteristic dimeric structure.