Photophysical properties of 1-N-methylamino- and 1-N,N-dimethylamino-9,10-anthraquinone dyes: A comparison with 1-amino-9,10-anthraquinone dye

Photophysical properties of 1-N-methylamino-9,10-anthraquinone (1-MAAQ) and 1-N,N-dimethylamino-9,10-anthraquinone (1-DMAAQ) dyes have been investigated in different solvents and solvent mixtures and the results are compared with those of the lower analogue 1-amino-9,10-anthraquinone (1-AAQ) dye [P. Dahiya, M. Kumbhakar, D.K. Maity, T. Mukherjee, A.B.R. Tripathi, N. Chattopadhyay, H. Pal, J. Photochem. Photobiol. A: Chem. 181 (2006) 338]. Qualitatively, 1-MAAQ behaves similar to 1-AAQ dye, displaying unusual deviations in photophysical properties in nonpolar solvents in comparison to the linear solvent polarity dependent changes in the properties for solvents of moderate to higher polarities. Moreover, the fluorescence decays of the dye show strong temperature-dependence in nonpolar solvents though the decays are temperature independent in solvents of moderate to higher polarities. It is inferred from the present results that 1-MAAQ dye adopts two different structures, one nonplanar structure in nonpolar solvents and the other planar intramolecular charge transfer structure in other solvents of moderate to higher polarities. A support for the above has been obtained from ab initio quantum chemical calculations. It is observed that in absolute term the nonradiative deexcitation channel is more dominating in 1-MAAQ than in 1-AAQ dye and this is attributed to the stronger intramolecular hydrogen bond present in the former dye than the latter. This is also supported by the deuterium isotope effect on the fluorescence decays of 1-MAAQ dye. Unlike 1-AAQ and 1-MAAQ dyes, their higher analogue 1-DMAAQ dye is found to be unusually weak in its fluorescence. This is attributed to the possible distortion in the anthraquinone moiety from planarity due to the steric effect of the bulky 1-N(CH3)2 group, which is also supported by the results from ab initio quantum chemical calculations.