Femtosecond vibrational transition‐state dynamics in a chemical reaction

Unprecedentedly Ultrafast Dynamics of Excited States of C═C Photoswitching Molecules in Nanocrystals and Microcrystals

The C═C photoswitching molecules [1,2-di(4-pyridyl)ethylene (DPE), 4-styrylpyridine (SP), and trans-1,2-stilbene (TS)] show favorable photoisomerization characteristics. Although the solid states of photoswitching molecules are usually used in optical devices, their excited state's evolution has been little explored. Here, the excited state's relaxation of DPE, SP, and TS in nanocrystal/microcrystal suspensions as well as in solution phase was studied to uncover the early events of their excited states. The dynamics of nanocrystal/microcrystal suspensions was tremendously accelerated in comparison to the kinetics obtained in the solution for these molecules under excitation. DPE exhibits the slowest decay rate, while SP shows the fastest decay rate in nanocrystal suspensions or solution, suggesting SP may be the best candidate for the photoswitching device. The intermolecular interactions and space restriction of the crystal lead to the acceleration of the excited state's evolution for DPE, SP, and TS. This provides new insight into the design of optical materials.

Femtosecond studies: observing transition states and other ultra-short-lived species

To celebrate 2015 as the 'International Year of Light', this article offers a short survey of the progress made since the award of the Nobel Prize of 1999 to Professor Ahmed Zewail for his pioneering work on taking the timescale for observation of light-induced events down to the femtosecond level. Developments have included the extension of studies (i) to larger molecules, leading up to biological systems; (ii) the increased range of detection methods of transient species from the UV-Vis to the infrared region; (iii) the introduction of Raman spectroscopy to augment IR studies; (iv) examination of combination events to supplement dissociation events; (v) the interrogation of transient structures by X-ray absorption spectroscopy; (vi) the study of reactions taking place at solid surfaces.

Photoisomerization among ring-open merocyanines. I. Reaction dynamics and wave-packet oscillations induced by tunable femtosecond pulses

Upon ultraviolet excitation, photochromic spiropyran compounds can be converted by a ring-opening reaction into merocyanine molecules, which in turn can form several isomers differing by cis and trans configurations in the methine bridge. Whereas the spiropyran-merocyanine conversion reaction of the nitro-substituted indolinobenzopyran 6-nitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6-nitro BIPS) has been studied extensively in theory and experiments, little is known about photoisomerization among the merocyanine isomers. In this article, we employ femtosecond transient absorption spectroscopy with variable excitation wavelengths to investigate the excited-state dynamics of the merocyanine in acetonitrile at room temperature, where exclusively the trans-trans-cis (TTC) and trans-trans-trans (TTT) isomers contribute. No photochemical ring-closure pathways exist for the two isomers. Instead, we found that (18±4)% of excited TTC isomers undergo an ultrafast excited-state cis→trans photoisomerization to TTT within 200 fs, while the excited-state lifetime of TTC molecules that do not isomerize is 35 ps. No photoisomerization was detected for the TTT isomer, which relaxes to the ground state with a lifetime of roughly 160 ps. Moreover, signal oscillations at 170 cm(-1) and 360 cm(-1) were observed, which can be ascribed to excited-state wave-packet dynamics occurring in the course of the TTC→TTT isomerization. The results of high-level time-dependent density functional theory in conjunction with polarizable continuum models are presented in the subsequent article [C. Walter, S. Ruetzel, M. Diekmann, P. Nuernberger, T. Brixner, and B. Engels, J. Chem. Phys. 140, 224311 (2014)].

Competitive direct vs. indirect photochromism dynamics of constrained inverse dithienylethene molecules

Relaxation observed through several parallel pathways from the first excited state to the ground-state in inverse dithienylethene molecules.

Photoisomerization of Stilbene: The Detailed XMCQDPT2 Treatment

We report the detailed XMCQDPT2/cc-pVTZ study of trans-cis photoisomerization in one of the core systems of both experimental and computational photochemistry-the stilbene molecule. For the first time, the potential energy surface (PES) of the S1 state has been directly optimized and scanned using a multistate multiconfiguration second-order perturbation theory. We characterize the trans-stilbene, pyramidalized (phantom), and DHP-cis-stilbene geometric domains of the S1 state and describe their stationary points including the transition states between them, as well as S1/S0 intersections. Also reported are the minima and the activation barriers in the ground state. Our calculations correctly predict the kinetic isotope effect due to H/D exchange at ethylenic hydrogens, the dynamic behavior of excited cis-stilbene, and trans-cis branching ratio after relaxation to S0 through a rather unsymmetric conical intersection. In general, the XMCQDPT2 results confirm the qualitative adequacy of the TDDFT (especially SF-TDDFT) picture of the excited stilbene but also reveal quantitative discrepancies that deserve further exploration.

Transition States, analogues, and drug development

Enzymes achieve their transition states by dynamic conformational searches on the femtosecond to picosecond time scale. Mimics of reactants at enzymatic transition states bind tightly to enzymes by stabilizing the conformation optimized through evolution for transition state formation. Instead of forming the transient transition state geometry, transition state analogues convert the short-lived transition state to a stable thermodynamic state. Enzymatic transition states are understood by combining kinetic isotope effects and computational chemistry. Analogues of the transition state can bind millions of times more tightly than substrates and show promise for drug development for several targets.

Photoisomerization of stilbene: a spin-flip density functional theory approach

The photoisomerization process of 1,2-diphenylethylene (stilbene) is investigated using the spin-flip density functional theory (SFDFT), which has recently been shown to be a promising approach for locating conical intersection (CI) points (Minezawa, N.; Gordon, M. S. J. Phys. Chem. A2009, 113, 12749). The SFDFT method gives valuable insight into twisted stilbene to which the linear response time-dependent DFT approach cannot be applied. In contrast to the previous SFDFT study of ethylene, a distinct twisted minimum is found for stilbene. The optimized structure has a sizable pyramidalization angle and strong ionic character, indicating that a purely twisted geometry is not a true minimum. In addition, the SFDFT approach can successfully locate two CI points: the twisted-pyramidalized CI that is similar to the ethylene counterpart and another CI that possibly lies on the cyclization pathway of cis-stilbene. The mechanisms of the cis--trans isomerization reaction are discussed on the basis of the two-dimensional potential energy surface along the twisting and pyramidalization angles.

Spectroscopic tracking of structural evolution in ultrafast stilbene photoisomerization

Understanding a chemical reaction ultimately requires the knowledge of how each atom in the reactants moves during product formation. Such knowledge is seldom complete and is often limited to an oversimplified reaction coordinate that neglects global motions across the molecular framework. To overcome this limit, we recorded transient impulsive Raman spectra during ultrafast photoisomerization of cis-stilbene in solution. The results demonstrate a gradual frequency shift of a low-frequency spectator vibration, reflecting changes in the restoring force along this coordinate throughout the isomerization. A high-level quantum-chemical calculation reproduces this feature and associates it with a continuous structural change leading to the twisted configuration. This combined spectroscopic and computational approach should be amenable to detailed reaction visualization in other photoisomerizing systems as well.

Isomerization Through Conical Intersections

The standard model for photoinduced cis-trans isomerization about carbon double bonds is framed in terms of two electronic states and a one-dimensional reaction coordinate. We review recent work that suggests that a minimal picture of the reaction mechanism requires the consideration of at least two molecular coordinates and three electronic states. In this chapter, we emphasize the role of conical intersections and charge transfer in the photoisomerization mechanism.

Primary steps of the photoactive yellow protein: isolated chromophore dynamics and protein directed function

The cycle of the photoactive yellow protein (PYP) has been extensively studied, but the dynamics of the isolated chromophore responsible for transduction is unknown. Here, we present real-time observation of the dynamics of the negatively charged chromophore and detection of intermediates along the path of trans-to-cis isomerization using femtosecond mass selection/electron detachment techniques. The results show that the role of the protein environment is not in the first step of double-bond twisting (barrier crossing) but in directing efficient conversion to the cis-structure and in impeding radical formation within the protein.

Experimental Evidence for Ultrafast Electronic Relaxation in Molecules, Mediated by Diffuse States

The existence of a mediating state for the ultrafast electronic relaxation in ethylenic-like molecules has been shown by femtosecond electron imaging techniques. This state is of Rydberg character, and its high efficiency is due to its ability to link the electron distributions of the initial and final electronic states.

Excited-State Behavior of trans and cis Isomers of Stilbene and Stiff Stilbene: A TD-DFT Study

The first part of the isomerization path on the two lowest excited states of trans and cis isomers of stilbene and stiff stilbene is investigated by means of TD-PBE0 calculations in the gas phase and in heptane solution. Solvent effects are taken into account by the PCM model. The excited-state optimized structures and the computed absorption and emission frequencies are in good agreement with the available experimental results. In all of the examined compounds, the isomerization process before barrier crossing occurs on the HOMO --> LUMO bright state, whereas the role played by other single-excitation states appears negligible. The relative energy barriers on the isomerization paths are consistent with the experimental excited-state lifetimes, suggesting a unifying picture of the isomerization process in stilbene-like molecules.

Ultrafast Photodissociation Dynamics of Acetone at 195 nm: I. Initial-state, Intermediate, and Product Temporal Evolutions by Femtosecond Mass-Selected Multiphoton Ionization Spectroscopy

The photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm has been studied by using femtosecond pump-probe mass-selected multiphoton ionization spectroscopy. For the first time, the temporal evolutions of the initial state, intermediates, and methyl products were simultaneously measured and analyzed for this reaction to elucidate the complex dynamics. Two mechanisms were considered: (1) the commonly accepted mechanism in which the primary dissociation occurs on the first triplet-state surface, and (2) the recently proposed mechanism in which the primary dissociation takes place on the first singlet-excited-state surface. Our results and analyses supported the validity of the new mechanism. On the other hand, the conventional mechanism was found to be inadequate to describe the observed dynamics. The temporal evolution of methyl products arising from the secondary dissociation of hot acetyl intermediates exhibited a very complex behavior that can be ascribed to the combination of a nonuniform initial vibrational distribution and the competition between dissociation and slow intramolecular vibrational redistribution.

Structure of the conical intersections driving the cis-trans photoisomerization of conjugated molecules

High-level ab initio calculations show that the singlet photochemical cis-trans isomerization of organic molecules under isolated conditions can occur according to two distinct mechanisms. These mechanisms are characterized by the different structures of the conical intersection funnels controlling photoproduct formation. In nonpolar (e.g. hydrocarbon) polyenes the lowest-lying funnel corresponds to a (CH)3 kink with both double and adjacent single bonds twisted, which may initiate hula-twist (HT) isomerization. On the other hand, in polar conjugated systems such as protonated Schiff bases (PSB) the funnel shows a structure with just one twisted double bond. The ground-state relaxation paths departing from the funnels indicate that the HT motion may take place in nonpolar conjugated systems but also that the single-bond twist may be turned back, whereas in free conjugated polar molecules such as PSB a one-bond flip mechanism dominates from the beginning. The available experimental evidence either supports these predictions or is at least consistent with them.

Infrared femtosecond laser pulse induced permanent reduction of Eu3+ to Eu2+ in a fluorozirconate glass

We report what is believed to be the first observation of permanent photoreduction of Eu(3+) to Eu(2+) in transparent and colorless Eu(3+) -doped fluorozirconate glass at room temperature, using an infrared femtosecond laser. Difference absorption and electron-spin-resonance spectra of the glass before and after laser irradiation showed that a portion of the Eu(3+) ions in the focused part of the laser inside the glass were reduced to Eu(2+) ions after laser irradiation. It is suggested that Eu(3+) ions act as electron-trapping centers, whereas active sites in the glass matrix act as hole-trapping centers, leading to the formation of Eu(2+) ions. The observed phenomenon is inferred to be useful in the fabrication of optical memory devices with high storage density and waveguide-type micro-optical devices.