Excited Electronic States in Isolated

6 Electronic Structure and Energy Transfer in Solid a-Sexithienyl 

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F. M. Bickelhaupt, R. H. Fokkens, L. J. de Koning, N. M. M. Nibbering, E. J. Baerends, S. J. Goede, andF. Bickelhaupt, Int. J. Mass Spectrom. Ion Processes, 103, 157 (1991).Isolated Excited Electronic States in the Unimolecular Gas-Phase Ion Dissociation Processes of the Radical Cations of Isocyanogen and Cyanogen. 

Figure Al.6.24. Schematic representation of a photon echo in an isolated, multilevel molecule, (a) The initial pulse prepares a superposition of ground- and excited-state amplitude, (b) The subsequent motion on the ground and excited electronic states. The ground-state amplitude is shown as stationary (which in general it will not be for strong pulses), while the excited-state amplitude is non-stationary. (c) The second pulse exchanges ground- and excited-state amplitude, (d) Subsequent evolution of the wavepackets on the ground and excited electronic states. Wlien they overlap, an echo occurs (after [40]).

Many molecules which have absorption bands in the wavelength region of existing laser lines can be excited by absorption of laser photons into single isolated rotational-vibrational levels of the electronic ground state 1W>-103) (jn the case of infrared laser lines) or of an excited electronic state (with visible or ultraviolet lines)... 

Fig. 20. A schematic representation of the emission of an isolated large molecule following internal conversion from the second to the first singlet, a" and 6J1 denote the amplitudes of the second singlet and quasi-degenerate vibrational levels of the first singlet, respectively, in the excited molecular state >/in. /v, and m are the corresponding electronic dipole transition matrix elements coupling < >n and as indicated.

As seen in Section 1.6, this approximation holds when the spin multiplet ground state is well isolated from excited electronic states, and ZFS is negligible. If 5 = lh, the g values can easily be measured through EPR spectroscopy and the g directions can be determined by single-crystal EPR measurements. When the latter measurements are not available, sometimes the principal g directions can be guessed from the symmetry of the molecule, and an independent estimate of the pseudocontact shift can still be attempted. 

As is well known, in crystalline solids there may be formed collective electron-excitation states called excitons.8182 Such states are excited only in media with periodic structure and are delocalized over a large volume of atoms (or molecules), their excitation energy being 0.1-0.5 eV lower than the energy of the electron states of isolated molecules that produced them. The nature and spectroscopy of exciton states have been thoroughly studied both experimentally and theoretically. In this section we will... 

Because of periodic symmetry, the electronic excitation states in a molecular crystal are also of collective nature. These are the well-studied exciton states.81 82 Their energy is close to that of discrete electronic states of isolated molecules (4-8 eV), but the excitation envelops a large group of molecules, migrating efficiently up to 100 nm along the crystal.82 In the same manner, because of efficient migration, the excitation of a fragment of a polymer chain rapidly spreads over the whole molecule.37... 

It should be emphasized that solvation of excited electronic states is fundamentally different from the solvation of closed-shell solutes in the electronic ground state. In the latter case, the solute is nonreactive, and solvation does not significantly perturb the electronic structure of the solute. Even in the case of deprotonation of the solute or zwitterion formation, the electronic structure remains closed shell. Electronically excited solutes, on the other hand, are open-shell systems and therefore highly perceptible to perturbation by the solvent environment. Empirical force field models of solute-solvent interactions, which are successfully employed to describe ground-state solvation, cannot reliably account for the effect of solvation on excited states. In the past, the proven concepts of ground-state solvation often have been transferred too uncritically to the description of solvation effects in the excited state. In addition, the spectroscopically detectable excited states are not necessarily the photochemically reactive states, either in the isolated chromophore or in solution. Solvation may bring additional dark and photoreactive states into play. This possibility has hardly been considered hitherto in the interpretation of the experimental data. 

Rapid decay of excited electronic states to the ground state is one extreme case. QET is equally fitted, at least in principle, to cope with the other extreme, namely that fragmentation of excited electronic states is faster than any internal conversion or intersystem crossing. Each isolated electronic state would simply be treated separately from all others, and the internal energy available for reaction [E in eqn. (1)]... 

Studies on eleetron donor-acceptor (EDA) molecules in condensed phases have a long history. Since the EDA interaction was first proposed by Mulliken [64], an appreciable number of reviews on this subject have been published during the latter half of the 20th eentury [65]. Similar studies under isolated jet-cooled conditions started in the 1980s and have supplied detailed information on the static and dynamic characters of excited electronic states responsible for the electron transfer [66]. The present section does not intend to cover all the details of the EDA studies in jet-cooled eonditions but rather focuses on seleeted EDA moleeules consisting of anthracene derivatives. Although limited in scope, the subjeet provides the most essential aspects of the EDA phenomena observed in an isolated system or in solvated clusters. 

In an isolated large molecule, each excited electronic state is coupled to a dense manifold of vibrational levels associated with lower electronic states. This... 

Matrix-isolated molecules exhibit a surprising facility for interelectronic relaxation processes. Vibrational relaxation in excited electronic states is often dominated by interstate cascades involving other electronic states. The rates of the individual steps of such a cascade are modulated by the intramolecular Franck-Condon factors and exhibit qualitatively an exponential dependence on the size of the energy gap expected by multiphonon relaxation theories. 

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