Spectroscopy and dynamics of methyl-4-hydroxycinnamate: the influence of isotopic substitution and water complexation

Abstract

High-resolution Resonance Enhanced MultiPhoton Ionization (REMPI) and Laser Induced Fluorescence (LIF) excitation spectra of jet-cooled methyl-4-hydroxycinnamate, methyl-4-OD-cinnamate, and of their water clusters have been recorded. Whereas water complexation leads to significant linewidth narrowing, isotopic substitution does for all practical purposes not influence the excited-state dynamics. In this light, we evaluate two previously proposed decay channels of the photoexcited ππ* state involving the dissociative πσ* state (analogous to phenol) and involving the optically dark nπ* state (as concluded for para-coumaric acid). To come to an unambiguous interpretation of the REMPI studies, it has been necessary to determine ionization thresholds. For methyl-4-hydroxycinnamate and its water cluster values of 8.078 and 7.636 eV have been found. Apart from the electronic excitation studies, IR absorption studies have been performed as well. These studies provide important vibrational markers for the assignment of the various conformations that are present under molecular beam conditions, and offer a direct measure of the influence of hydrogen bonding on the properties of the hydroxyl group.

Excited state dynamics of Photoactive Yellow Protein chromophores elucidated by high-resolution spectroscopy and ab initio calculations

“…Indeed, comparison of the line widths in the LIF spectrum with the line widths in the one-colour RE2PI spectrum gave in the past reason to believe that it is hard to avoid partial saturation in one-colour RE2PI. 28 As will be argued below, the observation that the two-colour signal is much stronger than the one-colour signal provides direct evidence for the conclusion that the electronic state that is excited is not the same state from which ionization takes place.…”

“…In the rst instance one might suspect that this is due to the decrease of the ionization energy of the cluster compared to the bare molecule (0.442 eV), 28 which could lead to a more efficient ionization of the np* state in the cluster. However, experiments in which the decay of the REMPI signal is monitored as a function of the delay between excitation and ionization show a signal that follows the convolution of the time proles of the two lasers and does not give any indication of a long-living state.…”

“…14 The same conclusion was reached for the V 0 (pp*) state of OMpCA. 28 The observed intensity enhancement of transitions in the water cluster has therefore been interpreted in terms of a reduced internal conversion rate to the np* state. Recent ps timeresolved REMPI studies on OMpCA and OMpCA-H 2 O suggest on the other hand an explanation in terms of excited-state isomerisation.…”

“…3b and 3c show 49,50 Careful analysis of these origin transitions show that they are actually composed of two close-lying transitions that we associate with the OH-syn and OH-anti conformations, thereby conrming the conclusions drawn in the LIF hole burning experiments. 28 Compared to the spectra of the bare molecule, the band shape of the origin transitions in the water cluster changes signicantly. Fits with a Gaussian line shape now give better results than ts with a Lorentzian line shape, and the bands have a much smaller FWHM, which appears to be determined by inhomogeneous rotational broadening.…”

“…For excitation of the s-cis conformers at 32710 cm À1 the total energy is therefore barely above (150 cm À1 ) the adiabatic ionization energy (65154 cm À1 ). 28 Aer internal conversion from the V 0 (pp*) state the np* state is populated. For this state we calculate a vertical ionization energy to D 0 of 5.751 eV, which is much higher than the employed 4.041 eV photons.…”