Lowest excited states of 2-aminopurine

Abstract

Absorption and fluorescene spectra and quantum yields of emission over a wide range of pH and solvent polarity were measured for 2-aminopurine and 2-dimethylaminopurine. An increase in dipole moment and a rise of pK values for N 1(N 3) ring atom protonation in the excited state were demonstrated. Thus, the lowest excited singlet state shows partially the (1, a π ∗ ) configuration. The polarization, lifetimes and the phosphorescence-to-fluorescence ratios indicate that spin-orbit coupling of 1( n, π ∗) and 1(1, a π ∗ ) with the lowest triplet state is the main source of phosphorescence intensity. The matrix element of spin-orbit coupling of 1( n, π ∗) to the T 1 state is about 33 times larger than that of 1(1, a π ∗ ). The temperature- dependent process of 2-aminopurine fluorescence quenching in either is presumably due to the thermal population of the second excited triplets state of ( n, π ∗) configuration, which can approximately be located at 370 cm -1 above the lowest (1, a π ∗ ) singlet.