The S1 dynamics of 4- and 5-methylpyrimidine: the role of frame and internal rotor symmetries

The dynamics of the structural isomers 4-methylpyrimidine and 5-methylpyrimidine were studied with single rotorvibronic resolution after excitation of vibronic bands of the S1 electronic state up to an excess energy of ∼ 1000 cm−1. Quantum beats and biexponential decays indicate intermediate case dynamics due to coherent excitation of S1−T1 eigenstates. By comparing the dynamics of the isomers, the effects of frame symmetry on the density of the coupling vibronic T1 states and their nonradiative T1 ∑ S0 intersystem crossing decay rate are manifested. Despite the expected symmetry effects, the internal rotor states of A1 and E symmetry of 4-methylpyrimidine show identical dynamics suggesting that the torsional potentials in the S1 and the T1 state are similar.