Solvent-dependent photophysical dynamics of nitro-substituted triphenylamine: Insights from spectroscopies and DFT calculations

Triphenylamine (TPA) derivatives, prized for their electron-donating and hole-transport properties, are pivotal in optoelectronic materials, yet the photophysical behavior of nitro-substituted variants remains underexplored despite their potential in triplet-mediated applications. Herein, we investigate the ground- and excited-state dynamics of 4-bromo-N-(4-bromophenyl)-N-(4-nitrophenyl)aniline (Br-NTPA), a nitro-substituted TPA, using Raman, UV-Vis absorption, fluorescence, and nanosecond transient absorption (ns-TA) spectroscopies across various solvents, complemented by DFT calculations. Raman spectra confirm the ground-state structure, while UV-Vis absorption reveals a solvent-sensitive charge transfer (CT) band at 377-394 nm, red-shifting 17 nm in acetonitrile versus cyclohexane. Fluorescence quantum yields drop from 0.211 in solid powder to <0.001 in acetonitrile, reflecting solvent-driven nonradiative decay, with ns-TA spectra identifying triplet states (TCT and TTICT) with quenching rates of (3.89-6.85) × 10⁹ M⁻¹ s⁻¹, approaching diffusion limits. These results highlight solvent polarity's role in modulating singlet and triplet CT character, with intersystem crossing and structural relaxation dictating outcomes. This integrated study elucidates Br-NTPA's tunable photophysics, offering insights for designing efficient optoelectronic and photochemical materials.