Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S₁(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S₁(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S₁(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S₁(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [¹ππ*→¹nπ*→³ππ*→S₀] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate.     

EPR and saturation transfer EPR spectra at high microwave field intensities

The eight unique EPR signals at the first and second harmonies of the Zeeman modulation are sensitive to the very slow rotational diffusion of spin labeled biomolecules when these signals are recorded under conditions of microwave saturation and finite Zeeman modulation frequencies and amplitudes. Such saturation transfer sensitive spectra have been employed to study contractile proteins, hemoproteins, enzymes, etc. When these species or their supramolecular complexes are characterized by correlation times in the range 10^?8 to 10^?3 s. Published computer simulation reproduce quite well spectra at the longer correlation times and the general sensitivity of spectra to changing rotational correlation time; however, agreement between experimental and calculated spectral shapes is poor for rotational correlation time on the order of 10^?7 s and the dependence of experimental spectra upon microwave field intensity is not reproduced. In the present communication we show that the previously reported discrepancy between experimental and calculated spectra is due to the neglect of higher order cou magnetic interactions modulated by the molecular motion and involving the spin-microwave field interaction. When these “pseudodiagonal” terms of the spin density equation are explicitly included, experimental spectral lineshapes, spectral line positions, and the ratios of amplitudes of the various signal components are quantitatively reproduced. Plots of the ratios of the heights of the high and low field spectral extrema suggest a procedure for calibrating microwave field intensities as these ratios are found experimentally and theoretically to be nearly a linear function of microwave field intensity for intensities in the range 0.15 to 0.5 G. The separation of low and high field extrema was observed to increase with increasing microwave field intensity, suggesting the need to carefully consider saturation effects when determining rotational correlation times from this separation.