Excited state structure and femtosecond ring‐opening dynamics of 1,3‐cyclohexadiene from absolute resonance Raman intensities

Ultrafast Dynamics of a Molecular Switch from Resonance Raman Spectroscopy: Comparing Visible and UV Excitation

Resonance Raman spectroscopy probes the ultrafast dynamics of a diarylethene (DAE) molecular switch following excitation into the first two optical absorption bands. Mode-specific resonance enhancements for Raman excitation at visible (750-560 nm) and near-UV (420-390 nm) wavelengths compared with the calculated and experimental off-resonance Raman spectrum at 785 nm reveal different Franck-Condon active vibrations for the two electronically excited states. The resonance enhancements at visible wavelengths are consistent with initial motion on the first excited-state that promotes the cycloreversion reaction, whereas the enhancements for excitation at near-UV wavelengths highlight motions involving conjugated backbone and phenyl ring stretching modes that are orthogonal to the reaction coordinate. The results support a mechanism involving rapid internal conversion from the higher-lying state followed by cycloreversion on the first excited state. These observations provide new information about the reactivity of DAE derivatives following excitation in the visible and near-UV.

Ultrafast Excited State Dynamics of Spatially Confined Organic Molecules

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene. The free space, i.e., the space that is left following the occupation of the guest within the host, is shown to play a significant role in altering the behavior of guest molecules in the excited state. The results reported here suggest that in addition to weak interactions that are commonly emphasized in supramolecular chemistry, the extent of empty space (i.e., the remaining void space within the capsule) is important in controlling the excited-state behavior of confined molecules on ultrafast time scales. For example, the role of free space in controlling the excited-state dynamics of guest molecules is highlighted by probing the cis-trans isomerization of stilbenes and azobenzenes within the OA capsule. Isomerization of both types of molecule are slowed when they are confined within a small space, with encapsulated azobenzenes taking a different reaction pathway compared to that in solution upon excitation to S₂. In addition to steric constraints, confinement of reactive molecules in a small space helps to override the need for diffusion to bring the reactants together, thus enabling the measurement of processes that occur faster than the time scale for diffusion. The advantages of reducing free space and confining reactive molecules are illustrated by recording unprecedented excimer emission from anthracene and by measuring ultrafast electron transfer rates across the organic molecular wall. By monitoring the translational motion of anthracene pairs in a restricted space, it has been possible to document the pathway undertaken by excited anthracene from inception to the formation of the excimer on the excited-state surface. Similarly, ultrafast electron transfer experiments pursued here have established that the process is not hindered by a molecular wall. Apparently, the electron can cross the OA capsule wall provided the donor and acceptor are in close proximity. Measurements on the ultrafast time scale provide crucial insights for each of the examples presented here, emphasizing the value of both "space" and "time" in controlling and understanding the dynamics of excited molecules.

Ultrafast ring closing of a diarylethene-based photoswitchable nucleoside

Deoxyuridine nucleosides embodied into diarylethenes form an especial class of photoswitchable compounds that are designed to stack and pair with DNA bases. The molecular geometry can be switched between "open" and "closed" isomers by a pericyclic reaction that affects the stability of the surrounding double helix. This potentially enables light-induced control of DNA hybridization at microscopic resolution. Despite its importance for the optimization of DNA photoswitches, the ultrafast photoisomerization mechanism of these diarylethenes is still not well understood. In this work, femtosecond transient absorption spectroscopy is applied to study the ring closing reaction upon UV excitation with 45 fs pulses. Excited-state absorption decays rapidly and gives rise to the UV-Vis difference spectrum of the "closed" form within ≈15 ps. Time constants of 0.09, 0.49 and 6.6 ps characterize the multimodal dynamics, where a swift recurrence in the signal anisotropy indicates transient population of the intermediate 21A-like state.

Cycloreversion dynamics of a photochromic molecular switch via one-photon and sequential two-photon excitation

Ultrafast pump-probe (PP) and pump-repump-probe (PReP) measurements examine the ring-opening reaction of a photochromic molecular switch following excitation to the first and higher excited states. Sequential two-photon excitation is a sensitive probe of the excited-state dynamics, because the secondary excitation maps the progress along the S1 reaction coordinate onto the higher excited states of the molecule. In this contribution, secondary excitation at 800 nm accesses more reactive regions of the excited-state potential energy surfaces than are accessible with direct vertical excitation in the visible or UV. The quantum yield for cycloreversion increases by a factor of 3.5 ± 0.9 compared with one-photon excitation when the delay between the 500 nm pump and 800 nm repump laser pulses increases beyond ~100 fs, in contrast with a slower ~3 ps increase that was previously observed for one-color sequential excitation at 500 nm. The evolution of an excited-state absorption band reveals the dynamics of the higher-lying excited state for both one-photon excitation in the UV and sequential two-photon excitation. The spectroscopy and dynamics of the higher-lying excited state are similar for both excitation pathways, including a lifetime of ~100 fs. The complementary PP and PReP measurements provide a detailed picture of the ultrafast excited-state dynamics, including new insight on the role of excited states above S1 in controlling the photochemical cycloreversion reaction.

Controlling the Excited-State Reaction Dynamics of a Photochromic Molecular Switch with Sequential Two-Photon Excitation

Sequential two-photon excitation increases the cycloreversion yield of a diarylethene-type photochromic molecular switch compared with one-photon excitation. This letter shows for the first time that an optimal delay of ∼5 ps between primary and secondary excitation events gives the largest enhancement of the ring-closing reaction. Pump-probe (PP) and pump-repump-probe (PReP) measurements also provide detailed new information about the excited-state dynamics. The initially excited molecule must first cross a barrier on the excited-state potential energy surface before secondary excitation enhances the reaction. The PReP experiments demonstrate that the reaction path of a photochromic molecular switch can be selectively controlled through judicious use of time-delayed femtosecond laser pulses.

Optimal control theory--closing the gap between theory and experiment

Optimal control theory and optimal control experiments are state-of-the-art tools to control quantum systems. Both methods have been demonstrated successfully for numerous applications in molecular physics, chemistry and biology. Modulated light pulses could be realized, driving these various control processes. Next to the control efficiency, a key issue is the understanding of the control mechanism. An obvious way is to seek support from theory. However, the underlying search strategies in theory and experiment towards the optimal laser field differ. While the optimal control theory operates in the time domain, optimal control experiments optimize the laser fields in the frequency domain. This also implies that both search procedures experience a different bias and follow different pathways on the search landscape. In this perspective we review our recent developments in optimal control theory and their applications. Especially, we focus on approaches, which close the gap between theory and experiment. To this extent we followed two ways. One uses sophisticated optimization algorithms, which enhance the capabilities of optimal control experiments. The other is to extend and modify the optimal control theory formalism in order to mimic the experimental conditions.

Two-Dimensional Electronic Spectroscopy in the Ultraviolet Wavelength Range

Coherent two-dimensional (2D) spectroscopies conducted at visible and infrared wavelengths are having a transformative impact on the understanding of numerous processes in condensed phases. The extension of 2D spectroscopy to the ultraviolet spectral range (2DUV) must contend with several challenges, including the attainment of adequate laser bandwidth, interferometric phase stability, and the suppression of undesired nonlinearities in the sample medium. Solutions to these problems are motivated by the study of a wide range of biological systems whose lowest-frequency electronic resonances are found in the UV. The development of 2DUV spectroscopy also makes possible the attainment of new insights into elementary chemical reaction dynamics (e.g., electrocyclic ring opening in cycloalkenes). Substantial progress has been made in both the implementation and application of 2DUV spectroscopy in the past several years. In this Perspective, we discuss 2DUV methodology, review recent applications, and speculate on what the future will hold.

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Spectral phase effects on nonlinear resonant photochemistry of 1,3-cyclohexadiene in solution

We have investigated the ring opening of 1,3-cyclohexadiene to form 1,3,5-cis-hexatriene (Z-HT) using optical pulse shaping to enhance multiphoton excitation. A closed-loop learning algorithm was used to search for an optimal spectral phase function, with the effectiveness or fitness of each optical pulse assessed using the UV absorption spectrum. The learning algorithm was able to identify pulses that increased the formation of Z-HT by as much as a factor of 2 and to identify pulse shapes that decreased solvent fragmentation while leaving the formation of Z-HT essentially unaffected. The highest yields of Z-HT did not occur for the highest peak intensity laser pulses. Rather, negative quadratic phase was identified as an important control parameter in the formation of Z-HT.

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 1B2 state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 1B2 state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 1A1 state that has been implicated as an intermediate step between the initially excited 1 1B2 state and the ground electronic state. The rise and decay times for the 2 1A1 state were found to be 55 and 84 fs, respectively.

Solvent Dependent Conformational Relaxation of cis-1,3,5-Hexatriene

Ultrafast transient absorption spectroscopy was used to study the conformational relaxation dynamics of 1,3,5-cis-hexatriene (Z-HT) produced in the photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) in methanol and n-propanol solvents. The results are compared with earlier investigations performed using cyclohexane and hexadecane solvents [Anderson, N. A.; Pullen, S. H.; Walker II, L. A.; Shiang, J. J.; Sension, R. J.; J. Phys. Chem. A 1998, 102, 10588-10598.]. The conformational relaxation between hot cZc-HT, cZt-HT, and tZt-HT, where the labels c and t designate cis and trans configurations about the single bonds, is much faster in alcohol solvents than in alkane solvents. The hot Z-HT produced in the photochemical ring-opening reaction evolves from the conformationally strained cZc-HT form to the more stable cZt-HT form on a time scale of 2 ps in alcohols compared with 6 ps in alkanes. The overall decay of the internal vibrational temperature of the hot Z-HT is faster in alcohols (5-6 ps) than alkanes (12-20 ps) and is weakly dependent on the specific alcohol or alkane solvent. A small population of cZt-HT (5-10%) is trapped as the solute equilibrates with the surrounding solvent following UV excitation of CHD or direct UV excitation of Z-HT. The influence of solvent on conformational relaxation of Z-HT was investigated further by probing the temperature dependence of the decay of this thermally equilibrated cZt-HT population. The apparent barrier for the cZt --> tZt conformational isomerization is lower in alcohols (17.4 kJ/mol) than in alkanes (23.5 kJ/mol). However the equilibrium Arrhenius prefactor (A(h)) is an order of magnitude smaller for alcohols (ca. 4 x 10(12)) than alkanes (ca. 6 x 10(13)) resulting in an absolute rate of decay that is faster in the alkane than in the alcohol solvents. These results are discussed in the context of transition state theory and Kramers' theory for condensed phase reaction dynamics.