SOLID STATE PHOTOCHEMISTRY OF THYMINE, ITS N‐METHYLATED DERIVATIVES AND OROTIC ACIDS IN KBr MATRICES

Photodimerization of a number of 2,4‐diketopyrimidines including thymine, 1 ‐methylthymine, 3‐methylthymine, 1,3‐dimethylthymine, orotic and 5‐methylorotic acids was studied in the solid state in KBr matrices. A technique developed for quantitative measurements of monomer and dimer concentrations under these conditions by means of u.v. and i.r. spectrophotometry is described. The absorption spectra of crystalline diketopyrimidines exhibit, in relation to the solution spectra in water, marked hypochromicity (33–69 per cent) and red shift of the long wavelength edge of the absorption (2000–3000 cm ‐1 ) indicative of resonance intermolecular interaction and mixing of different electronic states. In most cases dimerization appeared the only photochemical reaction and the stereochemical form of main dimer corresponded with the predicted one on the basis of known crystal structure of the monomer. Initial quantum yields of monomer disappearance varied from solid to solid in the range of 0.1‐0.9 M/Einstein being, apparently, to a degree dependent on the separation between nearest‐neighbour C=C bonds. Reaction mechanism is discussed in terms of involvement of singlet excimer states.