Photophysical Properties of Nucleic Acid Components—1. The Pyrimidines: Thymine, Uracil, N, N‐dimethyl Derivatives and Thymidine

Low‐temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φ F ) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar‐aprotic vs polar‐protic solvents. The N, N‐dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti‐tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that 1,3 (n,π*) state(s) has(have) changed order relative to a lowest 1 (π,π*) state as a consequence of both the solvent change and N, N‐dimethyl substitution. The lowest triplet state is assigned as 3 (n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φ F and φ T1 (φ isc ) for nucleic‐acid components is the equilibrium coexistence of H‐bonded and non‐H‐bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic‐acid bases as a function of the nature of the solvent upon the photochemical properties.