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Photophysical properties and photochemistry of various substituted cinnamates and cinnamic acids for ultraviolet B blocking were investigated experimentally and theoretically. This series includes monohydroxy, -nitro, and -fluoro derivatives. The absorption spectra were satisfactorily reproduced by the direct SAC-CI method with respect to the peak position and intensity. The transition character of the low-lying two ππ* and σπ* states for these 18 derivatives was analyzed. The para derivatives have a different transition character of the ππ* transitions compared with those of the ortho and meta derivatives. To elucidate the relaxation mechanism, the emission spectra were observed with oxygen quenching and the photostability was examined experimentally. The calculated radiative lifetimes indicate that the ortho- and meta-substituted derivatives have longer lifetimes for emission than the para derivatives. The potential energy curves of the first and second singlet excited states of the hydroxy derivatives as well as the vertical singlet and triplet transitions were examined to investigate the relaxation qualitatively. The ortho and meta derivatives have an energy barrier or flat surface in S1 resulting in fluorescence, whereas the para derivatives show nonradiative decay without an energy barrier. The para-hydroxy derivative was found to be an excellent UV absorber based on its broad absorption in the UVB/UVA regions, less emission, and higher photostability.

The percentage of failures in late pharmaceutical development due to toxicity has increased dramatically over the last decade or so, resulting in increased demand for new methods to rapidly and reliably predict the toxicity of compounds. In this review we discuss the challenges involved in both the building of in silico models on toxicology endpoints and their practical use in decision making. In particular, we will reflect upon the predictive strength of a number of different in silico models for a range of different endpoints, different approaches used to generate the models or rules, and limitations of the methods and the data used in model generation. Given that there exists no unique definition of a 'good' model, we will furthermore highlight the need to balance model complexity/interpretability with predictability, particularly in light of OECD/REACH guidelines. Special emphasis is put on the data and methods used to generate the in silico toxicology models, and their strengths and weaknesses are discussed. Switching to the applied side, we next review a number of toxicity endpoints, discussing the methods available to predict them and their general level of predictability (which very much depends on the endpoint considered). We conclude that, while in silico toxicology is a valuable tool to drug discovery scientists, much still needs to be done to, firstly, understand more completely the biological mechanisms for toxicity and, secondly, to generate more rapid in vitro models to screen compounds. With this biological understanding, and additional data available, our ability to generate more predictive in silico models should significantly improve in the future.

The absorption and emission spectra of ultraviolet B (UVB) blocking cinnamate derivatives with five different substituted positions were investigated using the symmetry-adapted cluster configuration interaction (SAC-CI) method. This series included cis- and trans-isomers of ortho-, meta-, and para-monomethoxy substituted compounds and 2,4,5-(ortho-, meta-, para-) and 2,4,6-(ortho-, para-) trimethoxy substituted compounds. The ground and excited state geometries were obtained at the B3LYP/6-311G(d) and CIS/D95(d) levels of theory. All the compounds were stable as cis- and trans-isomers in the planar structure in both the S(0) and S(1) states, except the 2,4,6-trimethoxy substituted compound. The SAC-CI/D95(d) calculations reproduced the recently observed absorption and emission spectra satisfactorily. Three low-lying excited states were found to be relevant for the absorption in the UV blocking energy region. The calculated oscillator strengths of the trans-isomers were larger than the respective cis-isomers, which is in good agreement with the experimental data. In the ortho- and meta-monomethoxy compounds, the most intense peak was assigned as the transition from next highest occupied molecular orbital (next HOMO) to lowest unoccupied molecular orbital (LUMO), whereas in the para-monomethoxy compound, it was assigned to the HOMO to LUMO transition. This feature was interpreted as being from the variation of the molecular orbitals (MOs) due to the different substituted positions, and was used to explain the behavior of the excited states of the trimethoxy compounds. The emission from the local minimum in the planar structure was calculated for the cis- and trans-isomers of the five compounds. The relaxation paths which lead to the nonradiative decay were also investigated briefly. Our SAC-CI calculations provide reliable results and a useful insight into the optical properties of these molecules, and therefore, provide a useful tool for developing UVB blocking compounds with regard to the tuning of the photoabsorption.