DUAL EMISSION OF 2-AMINO-4-METHYLPYRIMIDINE: A THEORETICAL STUDY

DFT and TD-DFT calculations at the B3LYP/LANL2DZ level of theory are performed in order to explain the origins of dual fluorescence and uncover the relaxation pathways of 2-amino-4-methylpyrimidine (L). The computations are in good agreement with experimental data. A high-energy emission band in the photoluminescence spectrum of 2-amino-4-methylpyrimidine (lambda(max) = 350 nm) is related to the fluorescence of monomer L. Comparison of calculated and experimental IR and absorption spectra reveals that in the solid state L molecules form supramolecular {N-H center dot center dot center dot N}(n) hydrogen bonded associates. The formation of such associates favors the excited state double proton transfer (ESDPT). Calculated 2D potential energy curves for the ground and first singlet excited states of dimeric associates L-2 indicate the presence of two stable forms, L-2-amino and L-2-imino. The low-energy band in the emission spectrum of 2-amino-4-methylpyrimidine (lambda(max) = 445 nm) is probably associated with the fluorescence of the L-2-imino form. Both fluorescence bands have a mixed pi-pi*, n-pi* character as is evidenced by frontier orbitals involved in S-1-S-0 transitions.