A Theoretical Investigation of the Physical Reason for the Very Different Luminescence Properties of the Two Isomers Adenine and 2-Aminopurine

摘要

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/6-31G. 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 →π↑(*) character, while it hasπ→π↑(*) 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 →π↑(*) state resembles a TICT state, but with only little charge transfer.