Christoph Schnedermann

Mode-Specificity of Vibrationally Coherent Internal Conversion in Rhodopsin during the Primary Visual Event

Conical intersections play a crucial role in photochemical processes, but limited experimental information exists on the structural distortions that couple electronic with reactive nuclear motion. Using ultra-broadband and highly time-resolved optical spectroscopy, we follow the evolution of vibrational wavepackets after passage through a conical intersection during the primary visual event, the 11-cis to all-trans photoisomerization of the retinal chromophore in rhodopsin. Comparison of nuclear coherences generated under resonant and off-resonant impulsive excitation conclusively reveals coherent wavepacket motion in the bathorhodopsin photoproduct over the full vibrational manifold. We observe strongly enhanced coherences in low-frequency torsional degrees of freedom over the fingerprint region and almost complete suppression of some hydrogen wagging motion. Our ability to monitor the multidimensional evolution of nuclear wavepackets across multiple electronic states is a general means for studying the structural and dynamic origins of efficient photochemistry and provides critical experimental information for theoretical studies.

Vibrationally coherent crossing and coupling of electronic states during internal conversion in β-carotene.

Coupling of nuclear and electronic degrees of freedom mediates energy flow in molecules after optical excitation. The associated coherent dynamics in polyatomic systems, however, remain experimentally unexplored. Here, we combined transient absorption spectroscopy with electronic population control to reveal nuclear wave packet dynamics during the S2 → S1 internal conversion in β-carotene. We show that passage through a conical intersection is vibrationally coherent and thereby provides direct feedback on the role of different vibrational coordinates in the breakdown of the Born-Oppenheimer approximation.

Principles and Applications of Broadband Impulsive Vibrational Spectroscopy

We present an experimental setup for recording vibrational coherences and thereby Raman spectra of molecules in their ground and excited electronic states over the 50-3000 cm(-1) spectral range using broadband impulsive vibrational spectroscopy. Our approach relies on the combination of a <10 fs excitation pulse with an uncompressed white light continuum probe, which drastically reduces experimental complexity compared to frequency domain based techniques. We discuss the parameters determining vibrational coherence amplitudes, outline how to optimize the experimental setup including approaches aimed at conclusively assigning vibrational coherences to specific electronic states, and provide a clear comparison with existing techniques. To demonstrate the applicability of our spectroscopic approach we conclude with several examples revealing the evolution of vibrational coherence in rhodopsin and β-carotene.

Backbone Modification of Retinal Induces Protein-like Excited State Dynamics in Solution

The drastically different reactivity of the retinal chromophore in solution compared to the protein environment is poorly understood. Here, we show that the addition of a methyl group to the C═C backbone of all-trans retinal protonated Schiff base accelerates the electronic decay in solution making it comparable to the proton pump bacteriorhodopsin. Contrary to the notion that reaction speed and efficiency are linked, we observe a concomitant 50% reduction in the isomerization yield. Our results demonstrate that minimal synthetic engineering of potential energy surfaces based on theoretical predictions can induce drastic changes in electronic dynamics toward those observed in an evolution-optimized protein pocket.

Direct observation of the coherent nuclear response after the absorption of a photon.

How molecules convert light energy to perform a specific transformation is a fundamental question in photophysics. Ultrafast spectroscopy reveals the kinetics associated with electronic energy flow, but little is known about how absorbed photon energy drives nuclear motion. Here we used ultrabroadband transient absorption spectroscopy to monitor coherent vibrational energy flow after photoexcitation of the retinal chromophore. In the proton pump bacteriorhodopsin, we observed coherent activation of hydrogen-out-of-plane wagging and backbone torsional modes that were replaced by unreactive coordinates in the solution environment, concomitant with a deactivation of the reactive relaxation pathway.

Synthetic control of retinal photochemistry and photophysics in solution.

Understanding how molecular structure and environment control energy flow in molecules is a requirement for the efficient design of tailor-made photochemistry. Here, we investigate the tunability of the photochemical and photophysical properties of the retinal-protonated Schiff base chromophore in solution. Replacing the n-butylamine Schiff base normally chosen to mimic the saturated linkage found in nature by aromatic amines results in the reproduction of the opsin shift and complete suppression of all isomerization channels. Modification of retinal by directed addition or removal of backbone substituents tunes the overall photoisomerization yield from 0 to 0.55 and the excited state lifetime from 0.4 to 7 ps and activates previously inaccessible reaction channels to form 7-cis and 13-cis products. We observed a clear correlation between the presence of polarizable backbone substituents and photochemical reactivity. Structural changes that increase reaction speed were found to decrease quantum yields, and vice versa, so that excited state lifetime and efficiency are inversely correlated in contrast to the trends observed when comparing retinal photochemistry in protein and solution environments. Our results suggest a simple model where backbone modifications and Schiff base substituents control barrier heights on the excited-state potential energy surface and therefore determine speed, product distribution, and overall yield of the photochemical process.

Population-Controlled Impulsive Vibrational Spectroscopy: Background- and Baseline-Free Raman Spectroscopy of Excited Electronic States

We have developed the technique of population-controlled impulsive vibrational spectroscopy (PC-IVS) aimed at providing high-quality, background-free Raman spectra of excited electronic states and their dynamics. Our approach consists of a modified transient absorption experiment using an ultrashort (<10 fs) pump pulse with additional electronic excitation and control pulses. The latter allows for the experimental isolation of excited-state vibrational coherence and, hence, vibrational spectra. We illustrate the capabilities of PC-IVS by reporting the Raman spectra of well-established molecular systems such as the carotenoid astaxanthin and trans-stilbene and present the first excited-state Raman spectra of the retinal protonated Schiff base chromophore in solution. Our approach, illustrated here with impulsive vibrational spectroscopy, is equally applicable to transient and even multidimensional infrared and electronic spectroscopies to experimentally isolate spectroscopic signatures of interest.

Vibronic Dynamics of the Ultrafast all-trans to 13-cis Photoisomerization of Retinal in Channelrhodopsin-1.

Channelrhodopsins are light-gated ion channels with extensive applications in optogenetics. Channelrhodopsin-1 from Chlamydomonas augustae (CaChR1) exhibits a red-shifted absorption spectrum as compared to Channelrhodopsin-2, which is highly beneficial for optogenetic application. The primary event in the photocycle of CaChR1 involves an isomerization of the protein-bound retinal chromophore. Here, we apply highly time-resolved vibronic spectroscopy to reveal the electronic and structural dynamics associated with the first step of the photocycle of CaChR1. We observe vibrationally coherent formation of the P1 intermediate exhibiting a twisted 13-cis retinal with a 110 ± 7 fs time constant. Comparison with low-temperature resonance Raman spectroscopy of the corresponding trapped photoproduct demonstrates that this rapidly formed P1 intermediate is stable for several hundreds of nanoseconds.

Sub-10-fs pulses tunable from 480 to 980 nm from a NOPA pumped by an Yb:KGW source

We describe two noncollinear optical parametric amplifier (NOPA) systems pumped by either the second (515 nm) or the third (343 nm) harmonic from an Yb:KGW source. Pulse durations as short as 6.8 fs are readily obtained by compression with chirped mirrors. The availability of both the second and third harmonics for NOPA pumping allows for gap-free tuning from 520 to 980 nm. The use of an intermediate NOPA to generate seed light at 780 nm extends the tuning range of the third harmonic pumped NOPA toward 450 nm.

Prediction of Electron Densities, the Respective Laplacians, and Ellipticities in Bond-Critical Points of Phenyl−CH−Bonds via Linear Relations to Parameters of Inherently Localized CD Stretching Vibrations and 1H NMR-Shifts

Electron densities rho, the respective laplacians nabla(2)rho, and ellipticities epsilon in bond-critical points (BCPs) are reactivity-determining characteristics according to the theory of atoms in molecules. These quantities are experimentally detectable only for substances in the crystalline state. To facilitate the determination of rho, nabla(2)rho, and epsilon values of BCPs of dissolved or liquid substances, the relations between DFT-calculated rho, nabla(2)rho, and epsilon and DFT-calculated vibrational and (1)H NMR spectroscopic quantities were studied for a set of 18 monosubstituted benzene derivatives. We found that via linear functions of rho, nabla(2)rho, or epsilon reliable predictions of rho, nabla(2)rho, and epsilon are possible, dependent on at least one of the variables vibrational transition energy, IR intensity, Raman activity of an inherently localized CD-stretching vibration, and the (1)H NMR shift. For the determination of rho, nabla(2)rho, and epsilon values in the ph-CH BCPs, the most important variables are the vibrational transition energy of the CD-stretching vibration and the corresponding (1)H NMR shift. The parameters of the functions best suited to predict rho, nabla(2)rho, and epsilon in the certain CH BCPs of the phenyl ring are presented.