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

Abstract

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 955, 1235, and 1551 cm-1). The structural changes occurring as P7.10 is formed can be derived from vibrational mode assignments in the PTR/CARS spectra such as the 955-cm-1 band assigned as the HC11C12H hydrogen-out-of-plane (HOOP) mode (shifts to 946 cm-1, increases intensity, and broadens its width in P7.10) and the 1551-cm-1 band assigned as the CC stretching mode (shifts to 1546 cm-1 in P7.10). These PTR/CARS data show that incorporation of an 11-ene, seven-membered ring into the retinal chromophore permits some flexibility for torsional motion around the C11C12 bond and within the seven-membered ring, but does not allow considerable out-of-plane motion near the Schiff base or the β-ionone ring. The χ(3) analysis of these PTR/CARS data demonstrates that a series of structurally-distinct retinals having slightly different C11C12 torsional, out-of-plane motion appear (collectively P7.10). The amplitudes and shifts in frequency of the PTR/CARS vibrational features change on a time scale comparable to that of vibrational relaxation in native Rh. The structural variations observed within P7.10 are located at/near the C11C12 bond (reactive in native RhRT, but nonreactive in R7.10) and the degree of π-electron energy delocalization throughout the retinal changes. This latter phenomenon is reflected in the different electronic phase factor (ϑ) found in the χ(3) analysis of the PTR/CARS data. The relationship(s) of the P7.10 structure to that of the C11C12 reaction coordinates in native Rh is discussed.

Structure, initial excited-state relaxation, and energy storage of rhodopsin resolved at the multiconfigurational perturbation theory level

“…This species cannot be isolated even at low temperature, and it is identified through a red-shifted, 570-nm max a . Furthermore, the excited-state or ground-state nature of photo Rh is still a matter of debate (40)(41)(42). Given the limited knowledge provided by our computations on the structure of the S 1 reaction path and S 0 relaxation path we cannot presently assign such an entity.…”

Excited‐state minima and emission energies of retinal chromophore analogues: Performance of CASSCF and CC2 methods as compared with CASPT2

“…The effect of different substitutions in RPSB polyene chain have been thoroughly investigated in both gas phase and in rhodopsin, where such modified chromophores form pigment analogues, to probe different aspects of the photoisomerization process, such as stereospecificity, excited‐state lifetime or reaction rate. Comparative analysis of photoreactivity effects in native rhodopsin and its pigment analogues has unveiled that the structural modification of RPSB may affect to a different extent the outcome of the photoisomerization (like in (de)methylated RPSBs) or even terminate the whole process (like in RPSB chromophore with C11C12 bond locked by five‐membered ring) . However, in rhodopsin analogue containing RPSB chromophore with C11C12 bond locked by eight‐membered ring isomerization process is much faster than in native pigment .…”

Photoisomerization of Stilbene Dendrimers: The Need for a Volume-conserving Isomerization Mechanism¶

“…Photoisomerization is also related to photobiological phenomena such as vision. Rhodopsin is a chromoprotein which accepts light energy and triggers its information, where the primary process is the isomerization around the C¼ ¼C double bond of retinal chromophore through the excited singlet state (21,22). The primary process is the highly selective 11-cis to all trans isomerization of the retinal chromophore.…”