Excited State Processes and Solution Conformation of Dipyrimidine Adducts*

The absorption and emission properties of the photochemically produced dipyrimidine adducts are analyzed at 300 and 77K. Those adducts which have a saturated C(5)—C(6) bond in the pyrimidin‐2,4‐dione (Pyr) ring and a pyrimidin‐2‐one (Pyo) ring behave spectroscopically as a substituted Pyo. However, those consisting of one Pyr and one Pyo moiety can be considered as bichromophoric molecules and their spectral properties can be understood in terms of the relative torsional angle between the two rings. The adduct with the most bulky substituents ortho to the torsional bond bears the largest torsional angle and exhibits relatively independent absorption and emission phenomenon. At the other extreme, those adducts with no substituents at this position exist as almost planar molecules and exhibit considerable overlap of absorption bands as well as room temperature fluorescence which, in certain cases, is characteristic of intramolecular exciplex interaction. Using inter‐ring torsional angles of ortho ‐substituted biphenyl molecules as a basis for comparative calculation, quantitative estimates of the torsional angles in dipyrimidine adducts at 300K have been made.