Photophysical properties of 2-amino-9,10-anthraquinone: evidence for structural changes in the molecule with solvent polarity

Abstract

The photophysical properties of 2-amino-9,10-anthraquinone (2AAQ) have been investigated in different solvents and solvent mixtures and correlated with the Lippert-Mataga solvent polarity parameter, Δf. In the low solvent polarity region with Δf < ca. 0.1, the dye shows unusually high fluorescence quantum yields (Φ_f) and lifetimes (t_f) in comparison to those in other solvents of medium to high polarities. Similarly, the radiative rate constants (k_f) are relatively lower and the non-radiative rate constants (k_nr) are relatively higher in the low polarity solvents in comparison to those in the medium to high polarity solvents. The current results have been rationalized assuming that the dye adopts different structural forms below and above the Δf value of ≈0.1. It is inferred that in the low solvent polarity region the dye exists in a non-planar structure, with its 2-NH₂ plane away from that of the 9,10-anthraquinone moiety in the ground state. In solvents of medium to high polarities, the dye exists in a polar intramolecular charge transfer (ICT) structure, where the amino lone pair of the 2-NH₂ group is in strong resonance with the anthraquinone π-cloud in the ground state. In all the solvents, however the dye is inferred to exist in the ICT structure in its excited (S₁) state. Supportive evidence for the above hypothesis has been obtained from the solvent polarity effect on the Stokes' shifts for the dye. Quantum chemical studies on the structures of 2AAQ dye in the gas phase also give qualitative support for the inferences drawn from the photophysical properties of the dye in different solvents.