Barrier crossing and solvation dynamics in polar solvents: Photoisomerization of trans‐stilbene and E,E‐diphenylbutadiene in compressed alkanols

Excited State Dynamics in Unidirectional Photochemical Molecular Motors

Unidirectional photochemically driven molecular motors (PMMs) convert the energy of absorbed light into continuous rotational motion. As such they are key components in the design of molecular machines. The prototypical and most widely employed class of PMMs is the overcrowded alkenes, where rotational motion is driven by successive photoisomerization and thermal helix inversion steps. The efficiency of such PMMs depends upon the speed of rotation, determined by the rate of ground state thermal helix inversion, and the quantum yield of photoisomerization, which is dependent on the excited state energy landscape. The former has been optimized by synthetic modification across three generations of overcrowded alkene PMMs. These improvements have often been at the expense of photoisomerization yield, where there remains room for improvement. In this perspective we review the application of ultrafast spectroscopy to characterize the excited state dynamics in PMMs. These measurements lead to a general mechanism for all generations of PMMs, involving subpicosecond decay of a Franck-Condon excited state to populate a dark excited state which decays within picoseconds via conical intersections with the electronic ground state. The model is discussed in the context of excited state dynamics calculations. Studies of PMM photochemical dynamics as a function of solvent suggest exploitation of intramolecular charge transfer and solvent polarity as a route to controlling photoisomerization yield. A test of these ideas for a first generation motor reveals a high degree of solvent control over isomerization yield. These results suggest a pathway to fine control over the performance of future PMMs.

Photophysics of First-Generation Photomolecular Motors: Resolving Roles of Temperature, Friction, and Medium Polarity

Light-driven unidirectional molecular rotary motors have the potential to power molecular machines. Consequently, optimizing their speed and efficiency is an important objective. Here, we investigate factors controlling the photochemical yield of the prototypical unidirectional rotary motor, a sterically overcrowded alkene, through detailed investigation of its excited-state dynamics. An isoviscosity analysis of the ultrafast fluorescence decay data resolves friction from barrier effects and reveals a 3.4 ± 0.5 kJ mol^-1 barrier to excited-state decay in nonpolar media. Extension of this analysis to polar solvents shows that this barrier height is a strong function of medium polarity and that the decay pathway becomes near barrierless in more polar media. Thus, the properties of the medium can be used as a route for controlling the motor's excited-state dynamics. The connection between these dynamics and the quantum yield of photochemical isomerization is probed. The photochemical quantum yield is shown to be a much weaker function of solvent polarity, and the most efficient excited-state decay pathway does not lead to a strongly enhanced quantum yield for isomerization. These results are discussed in terms of the solvent dependence of the complex multidimensional excited-state reaction coordinate.

Impact of kilobar pressures on ultrafast triazene and thiacyanine photodynamics

Application of high hydrostatic pressure leads to changes in (sub)picosecond emission dynamics, depending on the mechanism at work for the photoreaction.

Influence of the hydrogen-bond interactions on the excited-state dynamics of a push–pull azobenzene dye: the case of Methyl Orange

H-bonding with the solvent affects significantly the photoisomerisation of Methyl Orange.

Molecular stirrers in action

A series of first-generation light-driven molecular motors with rigid substituents of varying length was synthesized to act as "molecular stirrers". Their rotary motion was studied by (1)H NMR and UV-vis absorption spectroscopy in a variety of solvents with different polarity and viscosity. Quantitative analyses of kinetic and thermodynamic parameters show that the rotary speed is affected by the rigidity of the substituents and the length of the rigid substituents and that the differences in speed are governed by entropy effects. Most pronounced is the effect of solvent viscosity on the rotary motion when long, rigid substituents are present. The α values obtained by the free volume model, supported by DFT calculations, demonstrate that during the rotary process of the motor, as the rigid substituent becomes longer, an increased rearranging volume is needed, which leads to enhanced solvent displacement and retardation of the motor.

Hydrogen transfer reactions in viscous media — Potential and free energy surfaces in solvent–solute coordinates and their kinetic implications

Two-dimensional potential energy and free energy surfaces are obtained using quantum mechanical and molecular dynamics calculations for four hydrogen transfer reactions in n-hexane solvent: the methyl–methane, n-propyl–n-propane, n-pentyl–n-pentane, and t-butyl–isobutane systems. The resultant surfaces have similar landscapes despite the fact the equilibrated solvent cavities for these systems are notably different in size and shape. The kinetic implications of these landscapes are discussed. The Arrhenius and tunneling kinetics of hydrogen transfer in nonpolar hydrocarbon solute–solvent systems are not expected to show any significant viscosity dependence.

Viscosity dependence of intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane in alkane solvents as a function of temperature

Intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane, (1PC(5)1PC) is studied as a function of temperature in a series of alkane solvents and in toluene, covering a wide range of solvent viscosities η, from 0.2 to 125 cP. The rate constant k(a) of the monomer → excimer reaction is determined from the effectively single exponential monomer fluorescence decays. For the viscosity dependence of k(a) in n-alkanes, the Stokes-Einstein relation k(a) ∼ η(-1.0) does not hold. Instead, k(a) is proportional to η(-α), with α increasing upon cooling, from 0.56 at 85 °C to 0.86 at -30 °C. The activation energy E(a) of excimer formation with 1PC(5)1PC, always larger than the activation energy E(T/η) of solvent viscous flow, decreases when the solvent viscosity becomes smaller, from 20.7 kJ/mol in n-hexadecane to 11.8 kJ/mol in n-butane, approaching a value of 11-12 kJ/mol for the low viscosity solvents. As the excimer formation process depends on the restricted diffusion of the 1PC end groups as well as on the C-O and C-C rotations in the -O(CH(2))(5)O- chain, the limiting barrier of 11-12 kJ/mol is attributed to the activation energy E(c) of the multiple bond rotations. This fractional viscosity dependence (α < 1.0) is caused by the multidimensional character of the barrier crossing in the excimer formation process. This multidimensional character should also be taken into account in investigations of polymers and biological media employing excimer formation.

Dynamic solvent effects on the thermal isomerization of zinc dithizonate

The pressure dependence of the rate constant for the thermal isomerization of zinc dithizonate was measured at various temperatures in highly viscous solvents, i.e., glycerol triacetate and 2,4-dicyclohexyl-2-methylpentane, as well as in their chemically similar nonviscous solvents, such as methyl acetate and methylcyclohexane. In the nonviscous solvents, the reaction was slightly retarded upon external pressure suggesting the validity of the transition state theory. On the other hand, in the viscous solvents, the reaction, even at ambient pressure, was strongly retarded by an increase in external pressure suggesting the slow thermal fluctuation of the solvent molecules upon external pressure in highly viscous systems. This made it possible to observe the dynamic solvent effects on the isomerization of zinc dithizonate in viscous solvents. The viscosity dependence of the rate constants was rationalized by the two-dimensional reaction coordinate model.

Vibrational dynamics of carboxylic acid dimers in gas and dilute solution

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl(4)). Ultrafast excitation of the broad O-H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O-H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm(-1)) and the chemical identity of the dimer.

Ab initiotreatment of time-resolved x-ray scattering: Application to the photoisomerization of stilbene

In this work we present a general theoretical outline for calculating time-dependent x-ray scattering signal changes from first principles. We derive a formalism for the description of atom-atom correlation functions as Fourier transforms of quantum-chemically calculated electron densities and show their proportionality to the molecular form factor. The formalism derived in this work is applied to the photoisomerization of stilbene. We can demonstrate that wide-angle x-ray scattering offers a possibility to study the changes in electron densities in nonperiodic complex systems, which renders it a suitable technique for the investigation of (bio)organic systems.

Solvent Effect on the Excited-State Dynamics of Analogues of the Photoactive Yellow Protein Chromophore

We previously reported that two analogues of the Photoactive Yellow Protein chromophore, trans-p-hydroxycinnamic acid (pCA(2-)) and its amide derivative (pCM-) in their deprotonated forms, undergo a trans-cis photoisomerization whereas the thioester derivative, trans-p-hydroxythiophenyl cinnamate (pCT-), does not. pCT- is also the only one to exhibit a short-lived intermediate on its excited-state deactivation pathway. We here further stress the existence of two different relaxation mechanisms for these molecules and examine the reaction coordinates involved. We looked at the effect of the solvent properties (viscosity, polarity, solvation dynamics) on their excited-state relaxation dynamics, probed by ultrafast transient absorption spectroscopy. Sensitivity to the solvent properties is found to be larger for pCT- than for pCA(2-) and pCM-. This difference is considered to reveal that either the relaxation pathway or the reaction coordinate is different for these two classes of analogues. It is also found to be correlated to the electron donor-acceptor character of the molecule. We attribute the excited-state deactivation of analogues bearing a weaker acceptor group, pCA(2-) and pCM-, to a stilbene-like photoisomerization mechanism with the concerted rotation of the ethylenic bond and one adjacent single bond. For pCT-, which contains a stronger acceptor group, we consider a photoisomerization mechanism mainly involving the single torsion of the ethylenic bond. The excited-state deactivation of pCT- would lead to the formation of a ground-state intermediate at the "perp" geometry, which would return to the initial trans conformation without net isomerization.

Fluorescence Excitation Spectrum, Lifetimes and Photoisomerization of Jet-Cooled Conformers of 1,1´-bi(benzocyclobutylidene)

First measurements of fluorescence excitation spectra of the recently synthesized rigid stilbene analogue 1,1´-bi(benzocyclobutylidene) in asupersonic jet expansion show that, in contrast to the parent compound, both the

On the possibility of the Observation of Intramolecular Reaction Dynamics in Liquids by Time-resolved X-ray Scattering

The feasibility of time-resolved structural studies using pulsed X-ray radiation from a 3rd generation synchrotron is discussed and compared with scattering experiments which might be possible with a future free electron laser (X-FEL). As an example, it is shown that the structural changes during the isomerisation process of trans-stilbene into cis-stilbene can be observed using time-resolved X-ray diffraction in the gas-phase as well as in solution.

Magnetic field effect on the exciplex between all-s-trans- 1,4-diphenylbuta- 1,3-diene and 1,4-dicyanobenzene: a comparative study with other alpha,omega-diphenyl polyenes

The exciplex between all-s-trans-1,4-diphenylbuta-1,3-diene and 1,4-dicyanobenzene has been studied by steady state fluorescence along with the magnetic field effect (MFE) and compared with the other alpha,omega-diphenyl polyenes. The exciplex formation and magnetic field effect are dictated by the chain length of the polyene rather than the electronic requirement of these phenomena. The wavelength dependence of the MFE confirms the presence of two different charge-transfer complexes.