Abstract— 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 (φF) 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 φF and φT1 (φisc) 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.

A number of n-butylamine Schiff bases of polyenals related to retinals as homologues and analogues, and their protonated forms, have been studied for absorption and emission spectral properties. The polyene Schiff bases exhibit the same general features in their absorption spectra as those of the parallel polyenals except that the lB, t IA, and II* t n singlet transitions are at substantially higher energy in the Schiff bases (the shift being larger for the II* +-n transition). The Schiff bases with short polyene chainlength (n = 2, 3 where n is the number of double bonds including C=N) do not fluoresce or phosphoresce in 3-methylpentane in the temperature range 298-77 K. The Schiff bases with intermediate chainlength (n = 4, 5) show fluorescence at 77 K with intensity strongly dependent on the nature of solvent. The Schiff bases with relatively long chainlength (n = 5-7) show strong or moderately strong fluorescence at 77 K and very weak fluorescence at 298 K (n = 7) with intrinsic radiative lifetimes much longer than those estimated from the oscillator strength of the low-energy, strong absorption band ('Bu +-'A,). A discussion on the possible state order and nature of the fluorescing state of the various polyene Schiff base systems is presented.

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