Optical Quantum Control of the Electron Transfer Reactions in Protein Flavodoxin

The optical quantum control has been successfully applied in modulating biological processes such as energy transfer and bond isomerization. Among the reactions in realizing biological functions, the electron transfer (ET) process is fundamental; hence, the quantum control over such an ET reaction is of far-reaching significance. Here, we realized optical quantum control over ultrafast ET processes in a protein, flavodoxin, by applying various chirped excitation pulses. We observed the wavepacket dynamics within a dephasing time of less than 1 ps. Within this time window, we found that the ultrafast photoinduced ET reaction can be controlled by different chirped excitations with a rate change by a factor of about 2. Furthermore, the control effect is propagated into the subsequent ultrafast back ET reaction, showing a variation of the BET dynamics with different excitation chirps. The underlying mechanism is the initial wavepacket dynamics; the differently prepared wavepackets with chirped excitation evolve along various pathways, resulting in the changes of ET rates. The successful demonstration of optical quantum control of ultrafast biological ET is significant and opens a new avenue to explore the quantum control of real biological ET reactions.

Optical Control of Crossing the Conical Intersection in β-Carotene

Optical control of dynamic processes has been challenging yet has only been demonstrated in several chemical and biological systems. The control of a reaction passing the widely present conical intersection has not been realized. Here, we modulated the phase of the excitation pulse to control the dynamics of β-carotene through accessing the conical intersection (CI). We observed different dynamics in 110-220 fs into the CI and the consecutive process in 400-600 fs through another CI by various chirped excitation pulses. We successfully controlled those ultrafast wavepacket dynamics passing the CIs on the femtosecond time scales. The method developed here can be used to control a various of ultrafast chemical and biological reactions through the CI(s).

Excited State Vibrational Spectra of All- trans Retinal Derivatives in Solution Revealed By Pump-DFWM Experiments

The ultrafast structural changes during the photoinduced isomerization of the retinal-protonated Schiff base (RPSB) is still a poorly understood aspect in the retinal's photochemistry. In this work, we apply pump-degenerate four-wave mixing (pump-DFWM) to all- trans retinal (ATR) and retinal Schiff bases (RSB) to resolve coherent high- and low-frequency vibrational signatures from excited electronic states. We show that the vibrational spectra of excited singlet states in these samples exhibit pronounced differences compared to the relaxed ground state. Pump-DFWM results indicate three major features for ATR and RSB. (i) Excited state vibrational spectra of ATR and RSB consist predominately of low-frequency modes in the energetic range 100-500 cm^-1. (ii) Excited state vibrational spectra show distinct differences for excitation in specific regions of electronic transitions of excited state absorption and emission. (iii) Low-frequency modes in ATR and RSB are inducible during the entire lifetime of the excited electronic states. This latter effect points to a transient molecular structure that, following initial relaxation between different excited electronic states, does not change anymore over the lifetime of the finally populated excited electronic state.

Understanding/unravelling carotenoid excited singlet states

Carotenoids are essential light-harvesting pigments in natural photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and thus expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet-singlet excitation energy transfer, and carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. The photochemistry and photophysics of carotenoids have often been interpreted by referring to those of simple polyene molecules that do not possess any functional groups. However, this may not always be wise because carotenoids usually have a number of functional groups that induce the variety of photochemical behaviours in them. These differences can also make the interpretation of the singlet excited states of carotenoids very complicated. In this article, we review the properties of the singlet excited states of carotenoids with the aim of producing as coherent a picture as possible of what is currently known and what needs to be learned.

Interfering resonance as an underlying mechanism in the adaptive feedback control of radiationless transitions: Retinal isomerization

Control of molecular processes via adaptive feedback often yields highly structured laser pulses that have eluded physical explanation. By contrast, coherent control approaches propose physically transparent mechanisms but are not readily visible in experimental results. Here, an analysis of a condensed phase adaptive feedback control experiment on retinal isomerization shows that it manifests a quantum interference based coherent control mechanism: control via interfering resonances. The result promises deep insight into the physical basis for the adaptive feedback control of a broad class of bound state processes.

Pulse-train control of photofragmentation at constant field energy

We consider a phaselocked two-pulse sequence applied to photofragmentation in the weak-field limit. The two pulses are not overlapping in time, i.e., the energy of the pulse-train is constant for all time delays. It is shown that the relative yield of excited Br (*) in the nonadiabatic process: I + Br* ← IBr → I + Br, changes as a function of time delay when the two excited wave packets interfere. The underlying mechanisms are analyzed and the change in the branching ratio as a function of time delay is only a reflection of a changing frequency distribution of the pulse train; the branching ratio does not depend on the detailed pulse shape.

A perspective on controlling quantum phenomena

Controlling dynamical processes at the atomic and molecular scales with laser radiation has been a long-standing dream. The Faraday Discussion presented a cross section of the current experimental and theoretical advances as well as the challenges for the field. This paper summarizes the current status of controlling quantum dynamics phenomena and provides a perspective on the future.