Concentration Depolarization Of Fluorescence Research Articles

Excitation energy migration in uniaxially oriented polymer films: A comparison between strongly and weakly organized systems

The mechanism of multistep excitation energy migration in uniaxially oriented polymer films is discussed for strongly and weakly orientating dyes in poly(vinyl alcohol) matrix. The comparison between both types of systems is based on concentration depolarization of fluorescence, Monte-Carlo simulations and linear dichroism data. It is found that the alignment of transition dipole moments of fluorophores in the ordered matrix relative to the direction of polymer stretching exhibits strong effect on the concentration depolarization of fluorescence. In ordered matrices of flavomononucleotide and rhodamine 6G concentration depolarization of fluorescence remains quite strong, whereas for linear carbocyanines it is very weak despite effective energy migration.

Electronic Excitation Energy Migration in Partly Ordered Polymeric Films

Multistep intermolecular energy migration between elongated fluorophores (carbocyanines) in uniaxially oriented polymer films is studied based on fluorescence depolarization and Monte-Carlo simulations. Contrary to disordered systems it is found experimentally that the concentration depolarization of fluorescence in uniaxially oriented films is extremely weak despite effective energy migration. Based on the concentration depolarization experiment in the ordered matrix it is possible to estimate the angle between absorption and fluorescence transition moments of carbocyanines. The values of that angle are very close to those obtained from other methods.

On the mechanism of nonradiative electronic excitation energy transport in two-component systems: Concentration depolarization investigations

The effect of concentration depolarization of donor (D) fluorescence in the presence of an acceptor (A) for two D-A systems differing in critical radii ratio, R 0DA R 0DD , and with constant reduced concentration ratio ( γ D γ A ) was investigated. According to the theory of Loring, Andersen and Fayer, the emission anisotropy measurements confirmed different concentration dependences for both systems at high concentrations. The effect of R 0DA/ R 0DD upon the emission anisotropy changes in the subsystems with weak and strong energy migration, as well as the influence of the correlation effect on the concentration depolarization of fluorescence is discussed. Modern theories of nonradiative excitation energy transport in two-component systems were compared in the light of the experimental data obtained.

Concentration depolarization of fluorescence in the presence of molecular rotation

A theory is presented to describe the observables in fluorescence depolarization experiments when both rotational depolarization and concentration depolarization mechanisms are active. The latter mechanism arises from electronic excitation migration among chromophores and becomes more important as their concentration is increased. We treat the simplest case of randomly distributed chromophores, with fixed centers of mass, undergoing independent rotational Brownian motion. Our formalism reduces to existing theories if either depolarization mechanism is turned off, but makes new predictions in the situation where both are operative. Because electronic excitation transfer and molecular orientation are dynamically correlated, we find that previous ad hoc expressions that neglected such correlations are inappropriate. Approximate expressions for the fluorescence anisotropy are provided that can be tested by time-resolved or photostationary experiments on dye molecules in solution. Extensions of the theory to cases of correlated chromophore positions are straightforward and will allow interpretation of experiments on labeled macromolecular systems.

Polymer microheterogeneity and concentration depolarization of dye fluorescence

Concentration depolarization of fluorescence has been investigated with two dyes embedded in layers of poly(N-vinylcarbazole) or poly(N-epoxypropylcarbazole) of different molecular weight. The spatial distribution of dye molecules in the polymers — in contrast to the case of low-molecular-weight compounds and ethanol-glycerol mixtures—shows substantial microheterogeneity induced by non-equilibrium microheterogeneity of the polymeric matrix.

Concentration depolarization of fluorescence in systems with nonuniformly broadened electronic levels

Concentration depolarization of fluorescence in systems with nonuniformly broadened electronic levels

Concentration depolarization of fluorescence in excitation by intense light fluxes

Concentration depolarization of fluorescence in excitation by intense light fluxes

Time-dependent concentration depolarization of fluorescence

A theory of the time course of fluorescence anisotropy (polarization) after impulsive excitation is presented for viscous solutions of identical molecules. In a point-dipole model of the molecular oscillators, it is shown that the relative anisotropy is a universal function A of the quantity T = c2t/τ, where c is solute concentration (in units of the usual critical concentration), t is time, and τ is actual fluorescence lifetime. The function A(T) is expressed as a contribution A0 from the initially excited dipole plus one from all other dipoles, with the latter shown to be quite small in the region of interest. It is also shown that the representation, A0=Σk=0∞αk Tk/2, is useful for calculations except when T ≫ 1. The kth term in the series corresponds to energy transfer pathways involving k + 1 dipoles. The αk alternate in sign, with α0 = 1 and α1 = − 1.06004 derived analytically; numerically derived values of α2 through α6 are also given. We show that A(T) is the Laplace transform of c2 Ã(c), where Ã(c) is the steady-state anisotropy, and we use that relation to obtain an expansion of Ã(c) in powers of c. The resultant steady-state curve compares favorably with the best previous theoretical curves. The results of the point-dipole model are corrected for the effects of finite molecular volume. Specifically, it is shown that, for an excluded spherical volume v of twice the molecular ``radius,'' the behavior of A(T) for t/τ ≪(v/V0)2 is simply 1 − c(V0/v)(t/τ), which gives a finite decay slope at t = 0 in contrast to the point-dipole case. (V0 is the volume of the sphere of radius R̄0.) In addition to the A(T) theory, a simple derivation is given for the fluorescence-intensity formulas for anisotropy in linearly polarized and unpolarized exciting light. Finally, potential applications of the A(T) theory, particularly to testing the validity of the Förster mechanism in multiple homotransfer, are discussed.

Effects of temperature and solution viscosity on concentration depolarization of fluorescence

Effects of temperature and solution viscosity on concentration depolarization of fluorescence

Theory of polarization quenching by excitation transfer

Abstract An argument is disproved which was supposed by Knox to reveal a serious shortcoming of the idea behind an approximate method of predicting concentration depolarization of fluorescence, due to the present author.