The effect of N-methylation on photophysical properties of 3-aminoquinoline.

It has been reported previously that the photophysics of 3-aminoquinoline (3AQ) is governed by a flip-flop motion of its amino group in apolar solvents and intersystem crossing in polar solvents. The nonradiative rates are governed by more than one solvent parameters, like polarity and hydrogen bonding ability of the solvent. So, 3AQ is not a well-behaved probe of any individual solvent parameter. In the present work, 3AQ has been modified synthetically; replacing an amino hydrogen atom with a methyl group and photophysical properties were studied in 22 different apolar, polar-aprotic and polar-protic solvents. It is found that Stokes' shifts and fluorescence quantum yields exceptionally low in apolar solvents as compared to those in other solvents. Such substitution causes a decrease in the extent of the effect of the hydrogen bonding ability of the solvent and that nonradiative rate become a more regular function of polarity. However, the flip-flop appears to continue to be the major nonradiative pathway, as the nonradiative rates decrease with increase in micropolarity.