Vibrational Spectroscopy of a Picosecond, Structurally-Restricted Intermediate Containing a Seven-Membered Ring in the Room-Temperature Photoreaction of an Artificial Rhodopsin

The vibrational degrees of freedom of the only photophysical intermediate formed during the photoreaction of an artificial rhodopsin (Rh) containing a retinal with a seven-membered ring blocking 11-cis isomerization (Rh7.10) is measured via picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). PTR/CARS spectra are recorded with time delays ranging from 0 (8-ps cross correlation time) to 50 ps following the 3-ps, 500-nm excitation of Rh7.10. For time delays between 0 and 15 ps, an intermediate (P7.10) with a vibrational structure distinct from that of the ground electronic state Rh7.10 is observed. Although the formation time of P7.10 cannot be resolved from these PTR/CARS data, it is estimated to be ∼1 ps (i.e., slower than the 200-fs process proposed for native Rh). P7.10 completely reforms Rh7.10 with a decay time estimated to be ∼5−6 ps from the increasing intensities (via amplitudes from third-order, nonlinear susceptibility (χ(3)) analysis) in three major PTR/CARS features (at ...