Excited-State Barrier Controls E ? Z Photoisomerization in p-Hydroxycinnamate Biochromophores

Molecules based on the deprotonated p-hydroxycinnamate moiety are widespread in nature, including serving as UV filters in the leaves of plants and as the biochromophore in photoactive yellow protein. The photophysical behavior of these chromophores is centered around a rapid E -> Z photoisomerization by passage through a conical intersection seam. Here, we use photoisomerization and photodissociation action spectroscopies with deprotonated 4hydroxybenzal acetone (pCK-) to characterize a wavelength-dependent bifurcation between electron autodetachment (spontaneous ejection of an electron from the S1 state because it is situated in the detachment continuum) and E -> Z photoisomerization. While autodetachment occurs across the entire S1(7r7r*) band (370-480 nm), E -* Z photoisomerization occurs only over a blue portion of the band (370-430 nm). No E -> Z photoisomerization is observed when the ketone functional group in pCK- is replaced with an ester or carboxylic acid. The wavelength-dependent bifurcation is consistent with potential energy surface calculations showing that a barrier separates the Franck-Condon region from the E -> Z isomerizing conical intersection. The barrier height, which is substantially higher in the gas phase than in solution, depends on the functional group and governs whether E -> Z photoisomerization occurs more rapidly than autodetachment.

Automated summary

This article investigates the photophysical behavior of molecules based on the deprotonated p-hydroxycinnamate moiety, revealing a wavelength-dependent bifurcation between photoisomerization and electron autodetachment. Potential energy surface calculations indicate the presence of a barrier between the Franck-Condon region and the isomerizing conical intersection.