Solvent reorganizational red-edge effect in intramolecular electron transfer.

Abstract

Polar solvents are characterized by statistical distributions of solute-solvent interaction energies that result in inhomogeneous broadening of the solute electronic spectra. This allows photoselection of the high interaction energy part of the distribution by excitation at the red (long-wavelength) edge of the absorption bands. We observe that intramolecular electron transfer in the bianthryl molecule from the locally excited (LE) to the charge-transfer (CT) state, which requires solvent relaxation and does not occur in vitrified polar solutions, is dramatically facilitated in low-temperature propylene glycol glass by the red-edge excitation. This allows one to obtain spectroscopically the pure CT form and observe its dependence upon the relaxational properties of the solvent. A qualitative potential model of this effect is presented.Charge-transfer (CT) reactions in polar solvents are different from those in the gas phase. Their rates depend upon the solvent and in many cases can be correlated with such solvent parameters as electronic and nuclear polarizabilities, dipole moments, and relaxation rates (1-3). One can observe the inhomogeneity of reaction kinetics, which is expected to be very important for reactions on surfaces (4) in microheterogeneous media: micelles, microemulsions, and protein molecules (5, 6). Many important questions, however, remain unresolved. What is the dimensionality of the solvent coordinates? Can the solvents be characterized as homogeneous media and thermal baths only? If the solvent-derived inhomogeneity exists, is it static or dynamic in the sense that averaging of local states and interactions occurs faster or slower than the characteristic time of the reaction? Is the solvation of the reactant always at equilibrium in the course of the reaction? What is the nature of the control of reaction rate by the solvent dynamics? Is it controlled by its relaxational motions towards equilibrium, or is it dependent on frictional resistance of the already equilibrated solvent? The analysis of CT reactions in the excited state proves to be advantageous in resolving these problems for several reasons. There is a possibility of starting the reaction by a light pulse and observing its kinetics in real time (7,8). Similar time-resolved experiments can be used to study dipolerelaxational solvent dynamics (8, 9), and the site selectivity also can be easily achieved by excitation at selective wavelengths within inhomogeneously broadened absorption bands (10-12).Inhomogeneous broadening is known to be a significant contribution to the broadening of electronic spectra in polar solutions due to the distribution of solute-solvent interaction energies of a chromophore in different local environments (6,(10)(11)(12)(13)(14)(15). This results in a number of effects, which can be classified as either static (and observed in the steady-state spectra) or dynamic (and detectable in time-resolved experiments) (16,17). Static effects of inhomogeneous broadening are observed in solid and viscous solutio...

Interplay between excited-state intramolecular proton transfer and charge transfer in flavonols and their use as protein-binding-site fluorescence probes.

“…The concept ofparallel dipole moments between the So and the S; states ofthe flavonols studied is consistent with the fact that the position of the 3-HF PT tautomer fluorescence is Experience in the application of the CT fluorescence of bianthryl (44,45) and also with the PT fluorescence of 4-hydroxy-5-azaphenanthrene (37) in the investigation of different protein binding sites is encouraging. The fluorescence probe DHF combines both the CT and the PT fluorescences in one molecule.…”

The red‐edge effects: 30 years of exploration

“…It was found that different excited-state reactions in the conditions of slow mobility in the environment of the excited species exhibit strong site selectivity. Among these intramolecular reactions are excited-state isomerizations and also electron and proton transfers (23,24) These low-barrier reactions may be considered as analogues of catalytic and biocatalytic ground-state reactions, and this opens the possibility of studying their dependence on the dynamics of the protein matrix on a fast scale of nuclear motions (25).…”

Organization and Dynamics of Membrane Probes and Proteins Utilizing the Red Edge Excitation Shift

“…The essential criteria for the observation of the red edge effect can therefore be summarized as follows: (a) the fluorophore should normally be polar to be able to suitably orient the neighboring solvent molecules in the ground state (however, molecules such as bianthryl which are nonpolar in the ground state and yet could be polar in the excited state due to intramolecular charge transfer reaction are exceptions to this 28,29 ); (b) the "solvent" molecules (such as water, protein backbone, lipid headgroups) surrounding the fluorophore should be polar; (c) the solvent reorientation time around the excited state dipole should be comparable to or longer than the fluorescence lifetime (this is the most important criterion of REES); and (d) there should be a relatively large change in the dipole moment of the fluorophore upon excitation. In general, the dipole moment of molecules increases upon excitation.…”