Regioselectivity and Competition of the Paternò–Büchi Reaction and Triplet–Triplet Energy Transfer between Triplet Benzophenones and Pyrimidines: Control by Triplet Energy Levels

Abstract

The photochemical reaction of a pyrimidine and a ketone occurs either as a Paternò-Büchi (PB) reaction or as energy transfer (ET) from the triplet ketone to the pyrimidine. It is rare for the two types of reactions to occur concurrently, and their competitive mechanism remains unknown. In this work, two classes of products, regioisomeric oxetane(s) (2, 3) from a PB reaction and three isomeric dimers of 5-fluoro-1,3-dimethyl uracil (FDMU) (4-6) from a photosensitized dimerization of FDMU, are obtained through the UV irradiation of FDMU with various benzophenones (BPs). The ratio of the two products (oxetanes to dimers) reveals that the two competitive reactions depend strongly on the triplet energy levels (ET ) of the BPs. The BPs with higher ET values lead to higher proportions of dimers, whereas those with lower ET values give higher proportions of oxetane(s), with the generation of just two regioisomeric oxetanes for the BP with the lowest ET of the eight BPs investigated. The ratio of the two oxetanes (2:3) decreases with the BP ET value. The competitive mechanism for the two types of photochemical reactions is demonstrated through quenching experiments and investigation of temperature effects. Kinetic analysis shows that the rate constants of the two [2+2] photocycloadditions are comparable. Furthermore, in combination with the results of previous studies, we have gained insight into the dependence of the photochemical type and the regioselectivity in the PB reaction on the triplet energy gaps (ΔE) between the pyrimidines and ketones. For ketones with higher ET values than the pyrimidines, the photochemical reaction is a photosensitized dimerization of the pyrimidine. In the opposite case, a PB reaction occurs, and the lower the ET of the ketones, the lower the ratio of oxetanes (2:3). When the ET of values of the ketones are close to those of the pyrimidines, the two reactions occur concurrently, and the higher the ET of the ketones, the higher the proportion of the dimers. The ratio of oxetanes (2:3) decreases with the ET value of the BPs.

The Life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore

“…The PB reaction and TET were also studied by Liu et al using ( E )-2-deoxy-2-(fluoromethylene)­uridine (FDMU or FMdU) and various 4,4′-substituted BPs (BPs). The ratio between the PB reaction and TET-induced FMdU dimerization was measured for each case.…”

Generation of the Thymine Triplet State by Through‐Bond Energy Transfer

“…[13] Surprisingly,d irect evidence supporting the formation of the key species 3 Thy* upon BP photosensitizationh as never been provided. The main reasons for this are as follows:1 )The TTET process has to compete with the more efficient Paternò-Büchir eactionb etween the carbonyl moiety of 3 BP* (np*n ature) and the C5=C6 olefinic region of Thy; [14] 2) the very low absorption coefficient of the Thy T-T absorption band, centered at 370 nm, [13a, 15] and 3) the spectralo verlap between 3 BP* (with two bands peaking at 320 and 530 nm) and 3 Thy* (a broad band centereda t3 70 nm). Therefore, until now,o nly with more energetic triplet ketones, such as acetone, acetophenone, propiophenone, or 1-indanone, hasi tb een possible to prove the sensitization of 3 Thy* by TTET.…”

Transient UV–vis absorption spectroscopic characterisation of 2′-methoxyacetophenone as a DNA photosensitiser

“…2 Aromatic ketones such as benzophenones and acetophenones have been often used as triplet photosensitisers; 1 in particular, they have turned out to be very useful tools to investigate the photosensitisation mechanism responsible for the formation of DNA lesions like oxidative damage or cyclobutane pyrimidine dimers (CPDs). [3][4][5][6][7][8][9] While solar light is essential for life on Earth, its absorption by DNA is in the origin of a number of lesions that, if not properly repaired, may lead to mutagenicity and even to cancer. [10][11][12][13][14][15] The most abundant DNA lesions formed upon direct irradiation are CPDs.…”