Dual fluorescence of 2-(4′-N,N-dimethylaminophenyl)benzoxazole: effect of solvent and pH Original Research Article

The absorption, fluorescence and fluorescence excitation spectra of 2-(4′-N,N-dimethylaminophenyl)benzoxazole have been studied in solvents of different polarity and hydrogen bonding capacity. In all the solvents, except water, dual fluorescence is observed and both fluorescences originate from the same ground state precursor. The normal Stokes shifted band originates from the locally excited state and the large Stokes shifted band is due to the emission from a twisted intramolecular charge transfer (TICT) state. The dual emission observed in non-polar solvents suggests that the energy of the TICT state is lower than that of the locally excited state. Nanosecond time resolved study has shown that the formation of the TICT state is an ultra-fast process. Equilibrium is established between the two states in non-polar and polar/aprotic solvents, whereas in polar/protic solvents, the deactivation of the TICT state to the locally excited state is slow. Two kinds of monocation in cyclohexane, acetonitrile and water as solvents and two kinds of dication (only in aqueous medium) are formed simultaneously. The monocation formed by protonating the –N atom is more stable in non-polar medium and the monocation formed by protonating the –NMe2 group is more stable in polar medium. Semiempirical quantum-mechanical calculations have also been carried out to assign the site of protonation in each case. The prototropic equilibria in the S0 and S1 states are studied in aqueous medium and are discussed.