Excitation-regulated fluorescence and triplet yield in 3-nitrobenzanthrone: Experimental and theoretical insights into excited-state dynamics

Abstract

The development of heavy-atom-free triplet materials has emerged as a major research focus due to their potential for diverse photochemical and photophysical applications. Nitroaromatic compounds represent an important class of such systems, exhibiting ultrafast intersystem crossing (ISC), high triplet yields, and rich triplet-mediated photochemistry. Here, we present a comprehensive investigation of the excited-state dynamics of 3-nitrobenzanthrone (NBT), a polycyclic nitroaromatic chromophore of atmospheric and photochemical relevance. Steady-state and time-resolved emission studies reveal that NBT possesses many closely spaced singlet excited states. The fluorescence predominantly originates from the weakly absorbing S1 state, which was confirmed through enhanced fluorescence yield upon direct excitation of the state. In contrast, triplet formation occurs efficiently from the strongly absorbing S2 state. Singlet-oxygen phosphorescence measurements confirm that the triplet yield also depends on the solvent polarity. Femtosecond transient absorption measurements indicated a sequential S2 (LE) → S2 (R*) → Tn pathway with an ISC time constant of ∼10–17 ps depending upon the solvent polarity. Time-dependent density functional theory and spin–orbit coupling calculations support these findings, identifying S2 (1ππ*) → T2 (3nπ*) as the dominant ISC channel. Collectively, the results establish a dual-state photophysical model, in which the solvent polarity and electronic state character govern fluorescence and triplet efficiency in NBT.