Electrons, energy relaxation

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

A many-atom system excited by light or by collisions, such as occurs in the photo-excitation of a molecule adsorbed on a surface or in photosynthesis and vision, leads to energy dissipation on different time scales. A fast dissipation typically occurs due to electronic energy relaxation in the medium, while a slow (delayed) dissipation arises from vibrational energy relaxation. Here we concentrate on localized phenomena where a relatively small number... 

Robinson, G.W., Frosch, R.P. (1962), Theory of Electronic Energy Relaxation in the Solid Phase. J. Client. Pliy.s. 37, 1962. 

Electronic Energy Relaxation in Aromatic Vinyl Homopolymers... 

In this work the main aspect has been concerned with the problem of electronic energy relaxation in polychro-mophoric ensembles of aromatic horaopolymers in dilute, fluid solution of a "good" solvent. In this morphological situation microscopic EET and trapping along the contour of an expanded and mobile coil must be expected to induce rather complex rate processes, as they proceed in typically low-dimensional, nonuniform, and finite-size disordered matter. A macroscopic transport observable, i.e., excimer fluorescence, must be interpreted, therefore, as an ensemble and configurational average over a convolute of individual disordered dynamical systems in a series of sequential relaxation steps. As a consequence, transient fluorescence profiles should exhibit a more complicated behavior, as it can be modelled, on the other hand, on the basis of linear rate equations and multiexponential reconvolution analysis. 

The theory of energy transfer considered in this subsection was used to interpret the experiments with PbSe quantum dots (58) on the size-dependent energy relaxation in a quantum dot. In this paper it was shown that smaller dots have faster relaxation. In the theoretical paper by Hong et al. (59) it was assumed that the above energy transfer from a quantum dot exciton to surface states of the dot is a dominant channel of the electronic energy relaxation. Hong et al. considered in their calculations a spherical quantum dot of radius R and the transfer rate was obtained from the calculation of the power dissipation W on the surface of the quantum dot by the relation... 

Figure 2—36. Electronic energy relaxation through formation of intermediate ionic complexes example of relaxation of an alkaline atom Me( P) — Me( S) by collision with molecular nitrogen N2 and vibrational...

Three modem developments have been produced in the last years that are the key for the comprehension of the photophysics and photochemistry of many chemical and biochemical phenomena (1) rapid advances in quantum-chemical methods allow to study the excited states with high accuracy (2) improved molecular beams techniques permit studies of isolated molecules, despite their sometimes low vapor pressme and propensity for thermal decomposition, and (3) the revolutionary impact that femtosecond laser and multiphoton techniques have had on the study of the electronic energy relaxation processes. Indeed, now it is possible to get information about reaction intermediates at very short times from femtochemical techniques, and, more than ever, the participation of quantum chemistry to interpret such findings has become crucial. A constructive interplay between theory and experiment can provide an insight into the chemistry of the electronic state that cannot be easily derived from the observed spectra alone. 

Theory of electronic energy relaxation in the solid phas. J. Chem. Phys., 37, 1962 Robinson, G.W. and Frosch, R.P. (1963) Electronic excitation transfer and relaxation. J. Chem. Phys., 38, 1187. 

By using the same approach as Giuffrida, Darugar et al. prepared Cu NPs from the Cu(acac)2 precursor and studied time-resolved transient absorption phenomena for nanoparticles of 12 and 30 nm. The conduction electron energy relaxation was found to be faster in the smaller particles, whereas fluorescence showed an opposite trend [151]. 

Darugar, Q., Qian, W., El-Sayed, M.A. and Pileni, M.P. (2006) Size-dependent ultrafast electronic energy relaxation and enhanced fluorescence of copper nanopartides. The Journal of Physical Chemistry B, 110,143-9. 

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