Excited-state double-proton transfer of pyrimidines mediated by hydrogen-bonded complexes

The spectroscopy and dynamics of the excited-state double-proton transfer (ESDPT) in 2-amino-4,6-dimethyl pyrimidine (ADMP) and 2-amino-4-methoxy-6-methyl pyrimidine (AMMP) have been studied by means of steady-state and time-resolved measurements. The thermodynamic data indicating that dual hydrogen-bonding formation for ADMP/acid and AMMP/acid complexes are stronger than those obtained from ADMP and AMMP self-association. The fluorescence from the ADMP dimer in cyclohexane decays with rate (k(f)) of (1.1 +/- 0.1) x 10(9) s(-1) (0.9 ns), where the fluorescence from its tautomeric excited state formed by the proton transfer reaction decays with rate of (8.26 +/- 0.2) x 10(8) s(-1) (1.21 ns). However, the obtained k(f) (1.7 +/- 0.1) x 10(9) s(-1) for ADMP/acid tautomer of (0.58 ns) in cyclohexane is higher than that of the dimer. The results show that similar to 89% molecules form dimers in the ground state and similar to 86% of the excited molecules are present as dimers while the rest are present as monomers in 1 x 10(-2) M cyclohexane solution. (C) 2001 Elsevier Science B.V. All rights reserved.