Excitation energy transport and conformational-librational motion in chains

Abstract

Experimental studies of peptide bonds: Identification of the C 7 eq conformation of the alanine dipeptide analog N-acetyl-alanine N ′ -methylamide from torsion-rotation interactions Incoherent electronic excitation transport ͑EET͒ along chromophore-sites has been studied in the presence of conformational and librational motion by using a rotor chain model as the mobile vehicle. The time-evolution of the electronic site excitation probability vector P exc (t) averaged by conformational-librational dynamic disorder has been calculated by a second-order cumulant expansion ͑CE͒ approach. A master equation for conformer distributions in torsional space has been used ͓G. J. Moro, J. Chem. Phys. 94, 8577 ͑1991͒ and J. Chem. Phys. 97, 5749 ͑1992͔͒ to describe the coupled, motional stochastic dynamics of a rotor chain. Based upon this model, the correlation functions of excitation transfer rates, important for the calculation of the second cumulants of the stochastic excitation transfer rate matrices, have been constructed. A simplified procedure for evaluating the correlation functions for the L steep minima of torsional angle-dependent symmetrical potentials of nearest-neighbor rotor-pairs and for a Gaussian distribution of torsional angles displacement has been carried out. By using analytical fits to the second cumulants, the evolution of excitation energy transfer has been calculated in terms of the averaged site probabilities ͗P exc (t)͘ for chromophores, substituted to the L-fold symmetric rotors. The calculated profiles show a pronounced dependence on the curvature ratio in the saddle point of the rotor pair potential, on the number of the pair potential minima as well as on the average of the mean-square deviation of torsional angles from the stable conformations.

Temperature-Dependent Photophysical Properties of a Liquid-Crystalline Random Copolyester

“…The degree of π-orbital overlap throughout the sample will depend on the distribution of rotational-based planarity between adjacent monomeric subunits. The rate of excitation energy transport due to the distribution of rotational conformations are experimentally expressed in the fluorescence emission via a superposition of exponential decays, the first-order stretched exponential decay, as modeled by Kauffmann et al − …”

“…The degree of π-orbital overlap throughout the sample will depend on the distribution of rotational-based planarity between adjacent monomeric subunits. The rate of excitation energy transport due to the distribution of rotational conformations are experimentally expressed in the fluorescence emission via a superposition of exponential decays, the first-order stretched exponential decay, as modeled by Kauffmann et al [30][31][32][33] Figure 7 clearly delineates the presence of two realms, a temperature-independent and temperature-dependent realm. The transition between the two temperature realms lies in the range of the local HNA torsional motions, which corresponds to the room-temperature β phase transition.…”