Speaker
Description
Photoexcitation of donor–acceptor chromophores initiates coupled electronic, structural, vibrational, and solvent dynamics. Here, we investigate the solvent-dependent photophysics of para-nitroaniline (PNA), a prototypical push–pull chromophore, using femtosecond broadband transient absorption spectroscopy following excitation at 400 nm.¹˒² Measurements were performed in water, methanol, dimethyl sulfoxide, acetonitrile, and tetrahydrofuran. In every solvent, photoexcitation produces a prompt ground-state bleach below approximately 400 nm together with a broad positive transient absorption across the visible region. These features are consistent with formation of a charge-transfer-like excited-state population followed by rapid electronic, nuclear, and solvent reorganization. ¹˒² Despite the similar initial response, the subsequent dynamics are strongly solvent dependent. Global analysis reveals a rapid sub-picosecond contribution followed by dominant recovery components of approximately 1.44 ps in water, 4.89 ps in methanol, 5.03 ps in dimethyl sulfoxide, 6.77 ps in tetrahydrofuran, and 7.77 ps in acetonitrile. These timescales describe the coupled evolution of excited-state relaxation, hot-ground-state formation, vibrational redistribution, and solvent-mediated energy dissipation.¹˒³ The long-delay response follows a different solvent trend. No pronounced persistent signal is observed in water or dimethyl sulfoxide, whereas absorption remains beyond the available 7.4 ns window in methanol. In acetonitrile and tetrahydrofuran, long-lived components are resolved with effective decay constants of approximately 187 and 114 ns, respectively, and are predominantly assigned to triplet-state absorption.²˒⁴ Methanol and dimethyl sulfoxide exhibit nearly identical picosecond recovery times but markedly different long-delay behaviour, showing that broadband recovery does not uniquely determine persistent-state formation. These results demonstrate that picosecond recovery and long-lived-state population are related but non-equivalent aspects of PNA relaxation. Their solvent dependence reflects the combined influence of charge-transfer-state stabilization, specific solvation, vibrational-energy dissipation, and branching between internal conversion and the triplet manifold.¹˒²˒⁵