Electronic spectral diffusion in glasses: The influence of coupling to the medium on experimental observables

Abstract

The theory of electronic dephasing in low temperature glasses is extended to include the possibility that the strength of coupling of the chromophore to the solvent medium depends on the nature of the bath dynamical processes and the nature of the chromophore and, therefore, the chromophore-bath coupling can vary as a function of the rate of the dynamics of the medium. In the context of the sudden jump two-level system ͑TLS͒ model of low temperature glasses, this theory is used to reconcile the apparent contradiction implied by differences observed in spectral diffusion data for cresyl violet and metal-porphyrins in deuterated ethanol glass at 1.5 K. Previously, the coupling strength of a chromophore to the TLS has been assumed to be independent of rate of the transition between TLS states. Within the context of this approximation, spectral diffusion data yield, P i (R), the intrinsic TLS fluctuation rate distribution. With the inclusion of the rate dependent coupling, C(R), it is shown that the spectral diffusion observables actually yield P i (R)C(R). Therefore, the observed lack of spectral diffusion for a particular chromophore over some range of times can imply C(R) is zero rather than the current interpretation that P i (R) is zero. To illustrate the importance of C(R), a hueristic model is analyzed. A fluctuation rate distribution is introduced that consists of the sum of three log-normal functions each associated with a specific class of dynamics occurring over three overlapping ranges of rates. The uncharged and nonpolar metal porphyrins is taken to couple to TLS strain dipoles, while the charged and polar cresyl violet also couples to TLS electric dipoles. By taking one of the types of TLS dynamics to only give rise to electric dipole fluctuations, it is possible to fit all of the experimental data in deuterated ethanol with a single intrinsic distribution of TLS fluctuation rates. This analysis of previously reported data is supported by the presentation of new stimulated photon echo data on both cresyl violet and zinc meso-tetraphenyl porphine in deuterated ethanol.

Theory of Single-Molecule Optical Line-Shape Distributions in Low-Temperature Glasses

“…In addition, perhaps there is a distribution of TLS-chromophore coupling constants, and this distribution is correlated with other distributions, for example, with the distribution of tunneling matrix elements (or flip rates). 142 Perhaps the one-phononassisted tunneling model is not appropriate for all TLSs; flipping of some TLSs might, for example, involve a two-phonon Raman process. 64 Before leaving this section we note that Fleury et al 64 have previously performed a related theoretical analysis of SMS line width distributions.…”

Solvent effects on conformational dynamics of proteins: Cytochrome c in a dried trehalose film

“…If these fields are of dipolar character, a Lorentzian line shape may result under certain constraining conditions. [27][28][29] The initial holes in our experiments are usually well described by Lorentzians so that, within the frame of our diffusion model, the overall line shape of the holes after having acquired some additional width due to spectral diffusion broadening would be a Voigtian. Accordingly, the holes were fitted to Voigtians.…”

Two-pulse photon echoes from zinc-meso-tetraphenylporphine/polymethylmethacrylate are not consistent with the tunneling two-level system model

“…For example, it could be that the chromophore-TLS coupling ''constant'' actually depends on the TLS energy. 20 Since TLS with different energies are activated at different temperatures, this could lead to a chromophore-TLS coupling ''constant'' that depends on temperature, as we found in the analysis herein. the theory assumes the chromophores to be dilute and noninteracting.…”