The photodimerization of 1.3-dimethyluracil in a variety of organic solvents is described. The reactions lead to the formation of the four possible cyclobutane-type dimers, the relative amounts depending on the solvent employed. The structures and stereochemistry of the dimers are described and a triplet state is proposed as their precursor.

The photomonomerization of the four 1,3-dimethyluracil (DMU) photo dimers in the presence of a variety of photosensitizers is described. The reactions lead to the recovery of DMU in yields of up to 100%. The syn dimers are monomerized faster than the anti ones. An excited charge-transfer complex (exciplex) is proposed as the intermediate in these reactions. Monomerization of the photodimers could also be achieved with AlCla.

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ethanol, 5:5:2 volume ratio) as solvent. The solvent was checked for emission each time a spectrum was recorded. No interference due to solvent was found at any time. All phosphorescence samples were degassed by several freeze-thaw cycles. Phosphorescence intensities with or without quencher were reproducible for different samples prepared from the same solution.Acknowledgment. The authors gratefully acknowledge the support of this research by the National Science Foundation (Grant . Aid in the purchase of the nmr spectrometer used in this work was provided by the NSF via an equipment grant.

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