Photoabsorption spectra of deprotonated trans p-coumaric acid and two of its methyl substituted derivatives have been studied in gas phase both experimentally and theoretically. We have focused on the spectroscopic effect of the location of the two possible deprotonation sites on the trans p-coumaric acid which originate to either a phenoxide or a carboxylate. Surprisingly, the three chromophores were found to have the same absorption maximum at 430 nm, in spite of having different deprotonation positions. However, the absorption of the chromophore in polar solution is substantially different for the distinct deprotonation locations. We also report on the time scales and pathways of relaxation after photoexcitation for the three photoactive yellow protein chromophore derivatives. As a result of these experiments, we could detect the phenoxide isomer within the deprotonated trans p-coumaric acid in gas phase; however, the occurrence of the carboxylate is uncertain. Several computational methods were used simultaneously to provide insights and assistance in the interpretation of our experimental results. The calculated excitation energies S(0)-S(1) are in good agreement with experiment for those systems having a negative charge on a phenoxide moiety. Although our augmented multiconfigurational quasidegenerate perturbation theory calculations agree with experiment in the description of the absorption spectrum of anions with a carboxylate functional group, there are some puzzling disagreements between experiment and some calculational methods in the description of these systems.

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The very limited class of fluorophores, with a long fluorescence lifetime (>10 ns) and fluorescence beyond 550 nm, has been expanded with two benzo-fused triangulenium derivatives and two cationic [5]-helicene salts. The syntheses of the benzo-bridged dioxa-and diazatriangulenium derivatives (BDOTA + and BDATA + , respectively) required two different synthetic approaches, which reflect the structural and physiochemical impact on the reactivity of [5]-helicenium precursors. Spectroscopic investigations show that the introduction of the benzo bridge into the triangulenium chromophore significantly redshifts the absorption and emission while maintaining fluorescence lifetimes above 10 ns. The combination of a high quantum yield, long fluorescence lifetime, and emission above 600 nm is possible only if the structural aspects of the triangulenium framework are perfectly harmonized to secure a low rate of nonradiative deactivation. The new benzo bridge may be a general motif to obtain red-shifted derivatives of other dye classes.

The importance of the donor-acceptor push-pull system in the photoabsorption of the trans p-coumaric acid, the cofactor within the photoactive yellow protein and other xanthopsins, has been investigated. We recorded gas-phase absorption spectra and performed high-level quantum chemical calculations of three chromophore models, namely, the deprotonated trans ortho-, meta- and para-methyl coumarates. The ortho and para isomers, which have the electron-donating phenoxy oxygen and the electron-withdrawing acyl group in conjugation, present absorptions in the high-energy region of the visible spectrum, that is, in the interval of wavelengths in which the photoactivity of the xanthopsins is observed. On the other hand, the meta isomer, in which the conjugation between the phenoxy and acyl groups is disrupted, exhibits a significantly shifted maximum and presents no absorption in the region from blue to ultraviolet A. It is found that the push-pull system in the trans p-coumaric acid is critical for the wavelength and the intensity of its photoabsorption. Absorption spectra were also measured in methanol and showed an appreciable hypsochromic effect. Linear response calculations within the formalism of the approximate coupled cluster singles and doubles CC2 model and time-dependent DFT using the functional CAM-B3LYP provided insights into the relevant processes of excitation and aided to the interpretation of the experimental results. There is good agreement between theory and experiment in the description of the gas-phase absorption spectra of the considered chromophore models. Differential density plots were used to predict the effect of hydrogen-bonded amino acids to the trans p-coumaric acid on the protein tuning of this chromophore.