Solvent-Dependent Excited-State Dynamics of 1-Aminoanthraquinone Revealed by Transient Absorption Spectroscopy

This study investigates hydrogen bonding and intramolecular charge transfer (ICT) dynamics in 1-aminoanthraquinone (1-NH₂-AQ) using transient absorption spectroscopy and quantum chemical calculations. In acetone, excitation at 440 nm (absorption peak: 503 nm) revealed a 23 nm red-shift attributed to π→π* transitions, with excess energy of 2846 cm⁻¹ above the S₀→S₁ transition. Analysis of N─H vibrational modes (3211/3312 cm⁻¹) and fluorescence-absorption energy gaps (2550 cm⁻¹) demonstrated insufficient energy for intramolecular N─H⋯O═C hydrogen bond formation. Solute-solvent hydrogen bonding was also precluded due to incompatible donor-acceptor pairing between 1-NH₂-AQ and acetone. Quantum calculations showed highest occupied molecular orbital (HOMO) electron density localized on the amino group, while lowest unoccupied molecular orbital (LUMO) density was depleted there, confirming charge transfer character. In n-hexane, hydrogen bonding was prohibited by the solvent's nonpolar nature. Notably, a 90° amino group torsion in the S₁ state facilitated ICT from the amino donor to anthraquinone acceptor, with acetone solutions showing a 454 ps fluorescence lifetime. In contrast, n-hexane induced complete fluorescence quenching due to ICT state destabilization. The torsional barrier extended ICT lifetimes beyond typical femtosecond timescales, highlighting the interplay between molecular conformation and charge transfer dynamics.