Resolving the Turnover of Temperature Dependence of the Reaction Rate in Barrierless Isomerization

Abstract

In this work, the temperature dependence of the barrierless cis-trans isomerization reaction in the excited state of 1,1′-diethyl-4,4′-cyanine (1144-C) has been investigated. Transient absorption kinetics as well as transient absorption spectra were measured at different temperatures and viscosities, making it possible to obtain dynamic information regarding the isomerization process at constant viscosity. Our experimental results suggest a crossover of the reaction rate from a negative temperature dependence (decreasing relaxation rate with increasing temperature) at low viscosities to a positive temperature dependence at higher viscosities. This crossover of the relaxation rate was observed to occur at ∼17 cP; the range of viscosities studied was from ∼2 to ∼30 cP. We discuss the origin of this turnover behavior in terms of the BFO (Bagchi, Fleming, and Oxtoby) theory and show that the turnover can be explained by this model, as the competition between two processes following the excitation, namely transport of the population on the excited-state surface and internal conversion. We also discuss the validity of using the solvent shear viscosity as a measure of the friction experienced by the isomerizing group. Our results suggest that, at least for 1144-C in short-chain alcohols (methanol to hexanol), the shear viscosity can indeed be used as a reasonable measure of the solvent friction in the barrierless isomerization process.

Femtosecond dynamics of a non-steroidal anti-inflammatory drug (piroxicam) in solution: The involvement of twisting motion

“…From the previous experiment with adding KOH pellet to TAC solution, we concluded that the 50 ps component is due to A, while that of 150 ps is the lifetime of KR. An increase in the solvent viscosity and the involvement of stronger hydrogen-bonding interactions with TAC should modify (increase) the energy barrier of K to give KR, as it has been shown previously [28,29]. In these solvents, the excitation spectra gated at different wavelength of emission do not suggest the presence of another form of PX at the ground state, like an open enol structure lacking the intramolecular hydrogen bond, due to a strong H-bond interaction with the solvent as it happens in other media [30,31].…”

A two-state model of twisted intramolecular charge-transfer in monomethine dyes

“…3,4,6,121 The radiationless decay of monomethine dyes in condensed phase has been studied as an example of a barrierless viscosity-controlled process. 21,28,[39][40][41] Predictions of internal charge-transfer coupled to the twisting motion have been known for some time. 26,31 Theoretical models that have been used to interpret experiments describe the reaction as an overdamped motion on either a parabolic or flat potential surface with position-dependent sinks.…”

“…35 The excited-state twisting process in monomethine systems has been invoked as an example of an environmentcontrolled process with no intrinsic barrier. 21,28,[39][40][41][42][43][44] The dynamics have been described with theoretical models invoking overdamped motion. [40][41][42][43][44][45] Although the coupling of chargetransfer behavior to twisting displacements has been predicted for some time, 26 the theoretical models usually do not explicitly describe this.…”

“…This behavior is predicted by very simple and general theoretical considerations. 31,32 It has not received much attention in the discussions of spectroscopic experiments, wherein it is often assumed that the dynamics can be described by a single effective coordinate [40][41][42][43]45 In order to identify whether twist-dependent charge transfer behavior and pathway bifurcation effects are relevant to understanding the twisting dynamics of monomethines, it is necessary to formulate simple physical models that can describe these phenomena for a general case. This is the point of this paper.…”

Visualizing overdamped wavepacket motion: Excited-state isomerization of pseudocyanine in viscous solvents

“…Previously, it was suggested that the motion from the initially excited Franck-Condon region to the energy minimum on the S 1 excited-state potential is barrierless for 1122C in methanol as well as for 1,1 0 -diethyl-4,4 0 -cyanine (1144C) in several alcohols [3,[32][33][34]. Bulky end groups of these structurally related short-chain cyanine molecules exhibit different spatial trajectories during photoisomerization, undergoing the motion that in turn can be correlated to the specific solvent micro-friction similarly to other cyanines [35].…”