How Methylation Modifies the Photophysics of the Native All-trans-Retinal Protonated Schiff Base: A CASPT2/MD Study in Gas Phase and in Methanol

Abstract

A comparison between the free-energy surfaces of the all- trans-retinal protonated Schiff base (RPSB) and its 10-methylated derivative in gas phase and methanol solution is performed at CASSCF//CASSCF and CASPT2//CASSCF levels. Solvent effects were included using the average solvent electrostatic potential from molecular dynamics method. This is a QM/MM (quantum mechanics/molecular mechanics) method that makes use of the mean field approximation. It is found that the methyl group bonded to C10 produces noticeable changes in the solution free-energy profile of the S excited state, mainly in the relative stability of the minimum energy conical intersections (MECIs) with respect to the Franck-Condon (FC) point. The conical intersections yielding the 9- cis and 11- cis isomers are stabilized while that yielding the 13- cis isomer is destabilized; in fact, it becomes inaccessible by excitation to S. Furthermore, the planar S minimum is not present in the methylated compound. The solvent notably stabilizes the S excited state at the FC geometry. Therefore, if the S state has an effect on the photoisomerization dynamics, it must be because it permits the RPSB population to branch around the FC point. All these changes combine to speed up the photoisomerization in the 10-methylated compound with respect to the native compound.

Torsions, low-frequency vibrations, and vibration–torsion (“vibtor”) levels in the m-chlorotoluene cation

“…Additionally, the anomalous intensity of the + 30 1 m 2 band when exciting via m 1 and, more generally, the activity of + 30 1 m x bands in the spectra for mFT and mClT, confirm that certain out-of-plane vibrational motions are intricately linked to torsional motion. Overall, we conclude that it is unlikely that one can express the vibtor levels of + 30 1 and + 30 2 as simply products of torsional and vibrational wavefunctions.…”

“…Energy flow in molecules is now generally accepted as being facilitated by the coupling of both methyl torsion and vibrational motions and so is important for understanding the photophysics of molecules. 1,2 A very recent example highlights the role vibrational excitation has in light harvesting. 3 Understanding the processes occurring in complicated molecules is greatly aided by detailed studies on small molecules, and recent examples from our, the Reid and Lawrance groups have looked at toluene, 4,5,6 para-fluorotoluene (pFT) 7,8,9,10,11,12,13,14,15 and para-xylene (pXyl), 10,16,17 using a combination of fluorescence and photoionization spectroscopies.…”

“…+ 30 1 , + 302 appear to have potentials that are less hindered, since the spacings are less than those associated with the vibrational origin. A similar picture was seen for mFT,…”

From a one-mode to a multi-mode understanding of conical intersection mediated ultrafast organic photochemical reactions

“…Indeed, QM/MM simulations of a model all-trans retinal (rPSBAT) demonstrated how methyl substitution increased the ES lifetime, both in gas phase and non-polar solvents. 51 However, Kukura and co-workers experimentally found that photoisomerization of 10-methyl-rPSBAT (methylation at C10 in rPSBAT) in methanol is much faster than rPSBAT, 134 which has been verified using QM/MM simulations in both methanol 135,136 and in a protein environment 99 and was rationalized by a reduction in S 2 /S 1 mixing resulting in steeper ES slopes (similar to the discussion related to Fig. 10), illustrating again that the methyl group is not a pure inertial substituent.…”

Deciphering Methylation Effects on S2(ππ*) Internal Conversion in the Simplest Linear α,β-Unsaturated Carbonyl

“…Furthermore, a recent study on the photexcited dynamics of methylated butadienes highlighted the importance of electronic effects of the weakly electron-donating methyl group, influencing nonradiative decay time scales and branching ratios . Electronic effects of the methylation have also been found to accelerate the photoisomerization of a methylated derivative of all-trans retinal (methylated at the C10 position) in solution. − In light of such developments, we aim to resolve the mechanistic details of S 2 ( ππ *) internal conversion in AC and the role of inertial and electronic effects in its methylated derivatives using nonadiabatic dynamics simulations.…”