Subpicosecond fluorescence anisotropy measurements are used to characterize the rotational dynamics of coumarin 153 (C153) in 35 common solvents and eight solvent mixtures at room temperature. The rotational anisotropy decays of C153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion. Rotational correlation times are observed to be larger in polar solvents than in nonpolar solvents of the same viscosity. This difference is examined in the context of theories of dielectric friction, which relate the extra friction in polar solute/solvent systems to long-range dipole -dipole interactions. Since the latter interactions have been thoroughly characterized via dynamic Stokes shift measurements for the same solute/solvent combinations studied here, the present data provide a unique opportunity to test general concepts of dielectric friction. Contrary to expectations, the departures from simple hydrodynamic behavior cannot be modeled using only theories of rotational dielectric friction. More important than dielectric friction is the role that the relative solute/solvent size plays in determining the extent of solute -solvent coupling. Once this size dependence is approximately accounted for, the remaining departures from simple hydrodynamic behavior are relatively small in all solvents. In polar aprotic solvents, solvation data indicate that dielectric friction effects should be rather modest (10-20% of the total friction). In these solvents no clear correlation is found between dielectric friction predictions and the observed solute -solvent coupling. However, in normal alcohol solvents the effects of dielectric friction are predicted to be large and well beyond the scatter in the experimental data. No evidence for such an important dielectric friction contribution is observed in these solvents, in spite of the fact that long-time components of the solvation dynamics do appear to be present in the rotational friction.

Rotational Dynamics of Coumarin 153: Time-Dependent Friction, Dielectric Friction, and Other Nonhydrodynamic Effects

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 photochr...

Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution

The fluorescence depolarization dynamics of organic fluorescent dye probes (nile red, cresyl violet, DODCI, rhodamine B, and rhodamine DPPE) were studied in cationic, anionic, and neutral micelles by picosecond time-resolved single-photon-counting technique. The fluorescence anisotropy decay of the dye intercalated inside the micelle is a two-exponential function. The anisotropy decay was interpreted by using a model of rotational (wobbling) and translational diffusion of the dye in the micelle coupled with the rotational motion of the micelle as a whole. The rotational and translational diffusion coefficients of the dye, the order parameter, and the semicone angle for the wobbling diffusion in the micelle were determined. The concept of “microviscosity” in the micelle was critically discussed in the light of the rotational and translational diffusion coefficients and their temperature dependence.

Fluorescence Dynamics of Dye Probes in Micelles

A unified theory of reversible diffusion‐influenced geminate and pseudo‐ first‐order reactions is developed. Explicit results are presented for the time dependence of the fraction of molecules that are dissociated at time t for a variety of initial conditions. To introduce the basic ideas of our approach, an elementary and rather complete treatment of the irreversible reaction between a pair of interacting, spherically symmetric particles is presented. The focus is on deriving relations among survival probabilities and bimolecular time‐dependent rate coefficients for the radiation and absorbing boundary conditions and the asymptotic behavior of these quantities. These relations are then generalized to reversible geminate reactions. For example, it is shown that the separation probability for an initially bound pair satisfies a simple convolution relation involving the survival probability of an irreversibly reacting geminate pair initially at contact. An analytic expression is obtained for this separation probability that is exact for free diffusion and is an accurate approximation for interacting particles. Finally, the Smoluchowski approach to irreversible pseudo‐first‐order reactions is extended to reversible reactions. The analysis is based on the generalization of the convolution relations that are rigorously valid for isolated pairs.

Theory of reversible diffusion‐influenced reactions

Investigating mitochondrial redox state using NADH and NADPH autofluorescence

Fluorescence lifetimes and rotational reorientation times for four structurally similar dye molecules—three monocations: cresyl violet, nile blue, and oxazine 720 and one neutral but polar: nile red—have been measured by picosecond time‐resolved fluorescence depolarization spectroscopy using the single‐photon counting technique, in a number of solvents, which included a wide range of alcohols, other hydrogen‐bonding liquids, and a few aprotic liquids. The rotational reorientation of the dye probes (assumed to be oblate ellipsoids) are sought to be explained in terms of the Stokes–Einstein–Debye theory and dielectric friction. The individual contributions to the rotational friction due to the above two factors were calculated using reasonable values for the molecular volume and dipole moment of the solute. The rotational behavior of all the four dyes in amides and aprotic solvents is reasonably well explained in terms of the simple stick hydrodynamic model with the ‘‘molecular volume’’ obtained by using th...

Rotational reorientation dynamics of polar dye molecular probes by picosecond laser spectroscopic technique

Rate coefficients of diffusion‐limited fast reactions of charged reactants are predicted to be time dependent or time independent in different theoretical approaches to solve the Debye–Smoluchowski equation. Different rate coefficient expressions are also predicted in different time domains. Fluorescence quenching experiments using two cationic dyes (rhodamine B and cresyl violet) and four efficient ionic quenchers (iron complexes) in aqueous solutions were carried out to verify the theoretical prediction. A detailed data analysis of the quenched fluorescence decay over a range of concentration of the quenchers supports the prediction that the ‘‘long’’ time rate coefficient is k(t)∼a+bt−1/2. Exact agreement with the theoretical predictions is not obtained in any of the fluorophore–quencher systems described in this work. Cresyl violet–ferrocyanide and cresyl violet–ferricyanide are two systems for which the experimental values of diffusion coefficient D and distance parameter RHN are justifiably close to theoretical expectation.

Diffusion controlled reactions: fluorescence quenching of cationic dyes by charged quenchers

The transient effect in a diffusion limited reaction is described by a time‐dependent rate coefficient: k(t)=a+b exp(c2t)erfc(ct1/2), where a, b, and c are expressed in terms of the diffusion coefficient (D), the encounter distance (R), and the absolute rate coefficient (ka); and for ionic reactions, the Onsager length (rc). Time resolved fluorescence quenching studies on cresyl violet–potassium iodide system in water confirm the validity of the above equation. The values of D and R obtained from the fluorescence quenching studies are in good agreement with the values inferred from other sources, and the value of ka is reasonable.

http://www.cchem.berkeley.edu/grfgrp/Papers/1988%20Periasamy%20JChemPhys89.pdf

Diffusion controlled reactions: Experimental verification of the time‐dependent rate equation

On the origin of heterogeneity of fluorescence decay kinetics of reduced nicotinamide adenine dinucleotide

The non-radiative processes of deactivation from the lowest singlet excited state of aminoanthraquinones have been studied using steady state and time-resolved methods. The fluorescence decay rate constant kf correlates well with the solvent polarity parameter ET(30) in non-hydrogen-bonding solvents. Large deuterium isotope effects in fluorescence lifetimes τf and quantum yields φf are observed in the case of 1-amino and 1-methylamino anthraquinones, where the S1 state is mainly deactivated through internal conversion to the ground state. The temperature dependence of the fluorescence quantum yields of various aminoanthraquinones was also investigated. φf and τf exhibited strong temperature dependences in the case of 1-acetylaminoanthraquinone. In the case of 1-acetylaminoan-thraquinone, intersystem crossing to the triplet state is a major deactivation channel from the S1 and in this derivative a close-lying T2 state seems to be responsible for the high kisc rate.

B. Venkataraman

Papers

Rotational reorientation dynamics of polar dye molecular probes by picosecond laser spectroscopic technique

Diffusion controlled reactions: fluorescence quenching of cationic dyes by charged quenchers

Diffusion controlled reactions: Experimental verification of the time‐dependent rate equation

On the origin of heterogeneity of fluorescence decay kinetics of reduced nicotinamide adenine dinucleotide

The non-radiative processes from the S1 state of aminoanthraquinones: a steady state and time-resolved study