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

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 (kf) of (1.1±0.1)×109 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)×108 s−1 (1.21 ns). However, the obtained kf (1.7±0.1)×109 s−1 for ADMP/acid tautomer of (0.58 ns) in cyclohexane is higher than that of the dimer. The results show that ∼89% molecules form dimers in the ground state and ∼86% of the excited molecules are present as dimers while the rest are present as monomers in 1×10−2 M cyclohexane solution.