Solvent polarity and intramolecular hydrogen bonding effects on the photophysical properties of 1-amino-9,10-anthraquinone dye

Photophysical properties of 1-amino-9,10-anthraquinone (1AAQ) dye have been investigated in different solvents using steady-state and time-resolved fluorescence measurements. The behavior of the dye is different in nonpolar solvents than in other solvents of moderate to higher polarities. In nonpolar solvents, the absorption and fluorescence maxima (ν¯abs and ν¯fl, respectively) are largely blue-shifted and fluorescence quantum yields (Φf) and fluorescence lifetimes (τf) are unusually higher in comparison to those in other solvents. Further, in nonpolar solvents, the fluorescence decay kinetics is strongly temperature-dependent, while decay kinetics is temperature-independent in all other solvents of moderate to higher polarities. The differences in the behavior of 1AAQ dye in nonpolar and other solvents are suggestive of a structural change in the dye in the two sets of solvents. It is inferred that this structural change arises through the 1-NH2 group of the dye. Deuterium isotope effect on the fluorescence decays of the dye in nonpolar and polar solvents also supports the above structural changes. To obtain further supportive evidences for the above structural changes, ab initio quantum chemical studies have been carried out on the ground and excited state structures of the dye under different environmental conditions. Correlating different experimental and theoretical results we infer that in nonpolar solvents 1AAQ dye adopts a nonplanar conformation where as in all other solvents the dye adopts a planar conformation with strong intramolecular charge transfer (ICT) character. Present results also indicate a strong role of intramolecular hydrogen bonding on the deexcitation mechanism of the excited state of the dye both in its nonplanar and planar ICT structures.