The excited states of a set of popular sunscreen agents (2-hydroxybenzophenone, oxybenzone, and sulisobenzone) are studied by using femto- and nanosecond time-resolved spectroscopy. Upon excitation, the compounds undergo an ultrafast excited-state intramolecular proton transfer (ESIPT) reaction as the major energy-wasting process and the rate constant of this reaction is k=2×10(12) s(-1) . The ESIPT yields a keto conformer that undergoes a fast, picosecond internal conversion decay. However, a photodegradative pathway is a monophotonic HO bond breakage that subsequently leads to trace yields of phenoxyl radicals. Because potentially harmful phenoxyl radicals are formed upon irradiation of sunscreen agents, care should be taken about their reactivity towards biologically relevant compounds.

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In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3 IL aryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO) 3 (L n -κ 2 N )] with 1-pyrenyl-functionalized ligands were found to have predominant 3 IL pyrene / 3 ILCT pyrene→terpy character. The 3 IL/ 3 ILCT nature of the lowest energy excited state of [ReCl(CO) 3 (4′-(1-pyrenyl)-terpy-κ 2 N )] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO) 3 (L n -κ 2 N )] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3 MLCT and 3 IL/ 3 ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO) 3 (4′-(1-naphthyl)-terpy-κ 2 N )] and [ReCl(CO) 3 (4′-(1-pyrenyl)-terpy-κ 2 N )] were determined by femtosecond transient absorption studies.

The early events in the triplet 4-carboxybenzophenone (CB)-induced oxidation of N-acetyl-methionine methyl ester (N-Ac-Met-OCH 3 ) are investigated in aqueous solution. Upon electron transfer from the methionine residue of N-AcMet-OCH 3 to 3 CB*, the resulting sulfur radical cation undergoes further reactions:(1) back electron transfer, (2) escape of the radical ions from the solvent cage, or (3) proton transfer and escape of the radicals. The yields and paths of these reactions are shown to depend strongly on the pH of the solution, and, similar to the previously reported results for dipeptides (Met-Gly and Gly-Met), on the structural nature of the methionine substituents. In the experiments performed in this work, low quencher concentrations were used to avoid formation of intermolecular transients (e.g., dimeric sulfur-centered radical cation (S;S) ? ). Under these experimental conditions, the one-electron oxidized sulfur does not seem to become stabilized in an (S;N) ? three-electron bonded intramolecular complex. The proposed mechanism is further supported by the stable products analysis. A detailed mechanism involving characterization of the transients is discussed and compared to that of methionine and methionine-containing dipeptides (Met-Gly and Gly-Met). Moreover, a newly installed transient absorption laser system is described in details.

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