Ultrafast Photoinduced Reactions Research Articles

Simultaneous tracking of ultrafast surface and gas-phase dynamics in solid-gas interfacial reactions.

Real-time detection of intermediate species and final products at the surface and near-surface in interfacial solid-gas reactions is critical for an accurate understanding of heterogeneous reaction mechanisms. In this article, an experimental method that can simultaneously monitor the ultrafast dynamics at the surface and above the surface in photoinduced heterogeneous reactions is presented. This method relies on a combination of mass spectrometry and femtosecond pump-probe spectroscopy. As a model system, the photoinduced reaction of methyl iodide on and above a cerium oxide surface is investigated. The species that are simultaneously detected from the surface and gas-phase present distinct features in the mass spectra, such as a sharp peak followed by an adjacent broad shoulder. The sharp peak is attributed to the species detected from the surface, while the broad shoulder is due to the detection of gas-phase species above the surface, as confirmed by multiple experiments. By monitoring the evolution of the sharp peak and broad shoulder as a function of the pump-probe time delay, transient signals are obtained that describe the ultrafast photoinduced reaction dynamics of methyl iodide on the surface and in the gas-phase. Finally, SimION simulations are performed to confirm the origin of the ions produced on the surface and in the gas-phase.

A molecular movie of ultrafast singlet fission

The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. It is now often possible to identify such processes, but a detailed depiction of the crucial nuclear degrees of freedom involved typically remains elusive. Here, combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states. We assign the tuning and coupling modes, quantifying their relative intensities and contributions, and demonstrate how these modes coherently synchronise to drive the reaction. Our combined experimental and theoretical approach reveals the atomic-scale singlet fission mechanism and can be generalized to other ultrafast photoinduced reactions in complex systems. This will enable mechanistic insight on a detailed structural level, with the ultimate aim to rationally design molecules to maximise the efficiency of photoinduced reactions.

Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution.

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha-Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis-trans isomerizations, respectively.

Ultrafast photo-induced reaction dynamics in bacteriorhodopsin and its Trp mutants

This review paper presents the recent advances made in observing and understanding theultrafast photo-reaction dynamics in retinal proteins, in particular bacteriorhodopsin (bR),with a special emphasis on the retinal–protein interactions and the mechanisms of proteinactivation on a sub-picosecond timescale.We review our latest results obtained on wild-type (wt) bR, and on two tryptophanmutants W86F and W182F, obtained by femtosecond pump–probe experiments. It wasshown that light-induced charge translocations and modifications of the proteinelectrostatics can be monitored by the near-UV differential absorption of Trp86, whichexperiences a linear intra-protein Stark effect. In the same spectral region, non-exponentialwavepacket-like dynamics was found for the formation of the 13-cis retinal isomer in wt-bR.The present paper highlights how this finding is underpinned by the experiments on themutants.New results are presented regarding the reaction dynamics in the Trp mutants, studied inthe vis/near-IR by transient absorption. Interestingly, W86F displays a faster excited statedecay and photoproduct formation than wt-bR. This is tentatively attributed to anall-trans/13-cis ground state mixture known to occur in the light-adapted state in thisspecial mutant, due to increased conformational flexibility of the retinal chromophore.

Direct Probing of Spin State Dynamics Coupled with Electronic and Structural Modifications by Picosecond Time-Resolved XAFS

The first direct observation of the transient spin-state in a disordered magnetic system with time-resolved XAFS is reported. By observing the evolution of the Fe(II) 1s-3d transition, the spin crossover transition from the (1)A(1) low spin state to (5)T(2) high spin state has been directly observed on a picosecond time scale. Moreover, observation of the transient spin state with time-resolved XAFS allows for the investigation of the variations in the electronic state and molecular structure. This unique experimental technique probes the excited states involved in the ultrafast photoinduced reactions in disordered magnetic systems.

Investigations into the dynamics and mechanisms of ultrafast photoinduced reactions taking place in photoresponsive protein nanospaces (PNS)

Investigations into the dynamics and mechanisms of ultrafast photoinduced reactions taking place in photoresponsive protein nanospaces (PNS)

Ultrafast Photoinduced Reaction Dynamics and Coherent Oscillations in Photoresponsive Proteins.

We have studied primary processes of ultrafast photoisomerization reactions of photoactive yellow protein (PYP) and visual rhodopsin (Rh) as well as ultrafast photoinduced electron transfer reactions of flavoproteins (FP) by means of the fs fluorescence up-conversion method. On the basis of these studies, we have examined the effects of the protein nanospace (PNS), where the chromophore is held, on the dynamics and mechanisms of the ultrafast and highly efficient photoinduced reactions of these proteins. In this article, we discuss mainly results of our comparative fluorescence dynamics studies on wild type (w. -t.) PYP, it’s site-directed mutants, analogues with replaced chromophore and FP, including the coherent vibrations in the ultrafast fluorescence decays of w. -t. PYP and mutants in the course of the photoisomerization, ultrafast dynamic Stokes shift of fluorescence of PYP analogues, and ultrafast electron transfer of FP.

Ultrafast photoinduced reaction dynamics of photoactive yellow protein (PYP): observation of coherent oscillations in the femtosecond fluorescence decay dynamics

We have observed for the first time coherent oscillations in the fs fluorescence (Fl) dynamics of wild-type (w-t) photoactive yellow protein (PYP) and its site-directed mutants. Two oscillatory modes coupled with the ultrafast Fl decay due to the twisting of the excited chromophore were identified, a high frequency mode (∼140 cm −1) with 700 fs damping time and a low frequency mode (∼50 cm −1) with 500 fs damping time, respectively. The latter becomes overdamped when the structure of the protein nanospace (PNS) surrounding the chromophore becomes more disordered by mutation. Both modes disappear in the Fl dynamics of denatured PYP. Similarities between PYP and rhodopsins (Rh) are discussed.

Adiabatic approach for ultrafast quantum dynamics mediated by simultaneously active conical intersections

In ultrafast photoinduced reactions often more than one conical intersection is involved. We present an adiabatic approach which allows to treat the evolution towards and the transfer via two simultaneously accessible conical intersections (CoIns). The feasibility of quantum dynamical investigations in the adiabatic representation is demonstrated for a prototype system, namely the photoinduced ring opening of 1,3-cyclohexadiene (CHD). We find a branching of the wavepacket already in the excited state followed by a subsequent branching upon return to the ground state. The latter is different for the two intersections and depends on the location and topology.