2'-Methoxyacetophenone (2M) presents improved UVA absorption as compared with other acetophenone derivatives. On the basis of transient infrared spectroscopy it has been previously claimed that 2M is an interesting photosensitiser for cyclobutane pyrimidine dimers (CPDs) formation. In the present paper, a complete UV-Vis transient absorption spectroscopic characterisation of this compound is provided, including triplet-triplet spectra, triplet lifetimes and rate constants for quenching of 2M by a dimeric thymine derivative. Furthermore, generation of singlet oxygen has been proven by time-resolved near IR phosphorescence measurements. Overall, the obtained results confirm the potential of 2M as a DNA photosensitiser, not only for CPDs formation, but also for oxidative damage.

Polymethylene linked homo-and hetero-bipyrimidine models have been designed with different C5 substitution and bridge length. Selective irradiation of 2'-methoxyacetophenone (2M), with the bipyrimidine models affords cyclobutane pyrimidine dimers, even in the presence of bulky substituents. Substitution at C5 affects both the triplet energies (E T) of the pyrimidine (Pyr) derivatives and the steric hindrance towards intermolecular energy transfer and intramolecular triplet Pyr* quenching. Photophysical studies showed that alkyl substitution resulted in a significant decrease of the E T value. Quenching of the triplet excited state of 2M by the Pyr derivatives was proven, and their quenching rate constants (k q) established. As a general trend, the thymine-containing compounds showed k q values higher than 10 9 M-1 s-1 while in the uracil and tert-butyluracil analogues k q was markedly lower. These data are explained considering three different scenarios: a) triplet energy transfer is the rate controlling step, b) excited state cyclodimerization is the rate controlling step and, c) the rate controlling step switches along the reaction. Thus, by introducing variations in the substitution at C5, the length of the linking bridge or the substrate concentration it is possible to switch from a process governed by the intrinsic dimerization step to an energy transfer controlled process.

Biphotonic chemistry of pyrimidines is produced using a 355 nm laser and 2-methoxyacetophenone as photosensitiser.