A theoretical investigation of the physical reason for the very different luminescence properties of the two isomers adenine and 2-aminopurine

The geometry of the ground state and the first singlet excite state of adenine and 2-aminopurine was calculated with three different quantum chemical methods: AM1, CIS/6-31G, and CASSCF/G-31G. Three possible deactivation mechanisms or reactions of excited molecule were considered: the pseudo Jahn-Teller distortion, excited-state tautomerism, and formation of so-called twisted intramolecular charge transfer (TICT) states. Different mechanisms for the nonradiative decay are operative for the two isomers. The geometrically relaxed excited state of adenine has n --> pi* character, while it has pi --> pi* character for 2-aminopurine. The state crossing that occurs during the excited-state relaxation of adenine opens up an effective nonradiative deactivation channel not present for 2-aminopurine. Tautomerism in the excited state might explain the difference in luminescence quantum yield upon DNA binding for 2-aminopurine. The excited state of the 7H tautomer of adenine undergoes a large geometry change during the relaxation, and the final geometry of the n --> pi* state resembles a TICT state, but with only little charge transfer.