Spectral characteristics of 2-(3′,5′-diaminophenyl)benzothiazole: effects of solvents and acid–base concentrations

Spectral characteristics of 2-(3′,5′-diaminophenyl)benzothiazole (3,5-DAPBT) have been studied in different solvents and H0/pH/H range of −10 to 16. Increase in Stokes shifts observed with increase in polarity of solvents is due to increase in dipole moment (μ) on excitation to first excited singlet (S1) state. Polarization also plays a major role in the increase of excited-state dipole moment (μe). Combining the results observed in absorption, fluorescence and fluorescence excitation spectra and lifetime studies, it is found that (i) MC1 is the only monocationic (MC) species in polar aprotic and polar protic solvents, whereas MC1 and MC3 (Scheme 1) are formed in cyclohexane + trifluoroacetic acid (TFA) medium in the ground (S0) and S1 states; (ii) DC2 is the only dicationic species present in polar protic solvents, whereas DC1 and DC2 (Scheme 1) are dicationic species in non-polar and polar aprotic solvents; (iii) only one kind of tricationic species is present in polar and protic solvents; (iv) monoanion (MA) formed by the deprotonation of any of the NH2 group, is non-fluorescent. Semi-empirical quantum mechanical calculations (AM1) and density functional calculations (DFT) have been performed on all kinds of ionic species. The spectral characteristics have been assigned to various prototropic species combining the experimental and theoretical results.