Chromophores electronic structure

Minor modification of the chromophore structure also is likely to result in the least perturbation of chromophore electronic structure so that we do not have to worry about... 

Model analogs of the green type chromophore HBI have been chemically synthe-tized in different forms carrying blocking groups in place of the protein polypeptide chain [21, 24, 68, 69]. However, the covalent structure of HBI does not uniquely define its optical properties, because the molecule undergoes several protonation and conformational equilibria that directly affect its electronic structure. 

Bublitz G, King BA, Boxer SG (1998) Electronic structure of the chromophore in green fluorescent protein (GFP). J Am Chem Soc 120 9370-9371... 

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

Acceptor substituents in nitrophenyl anion-radical frequently form quinoid structures. Donor substituents cannot participate in the formation of such structures. As to electron spectroscopy, it is very sensitive to changes in the electron structure of a chromophore system. The influence of acceptor groups is, therefore, stronger than that of donor groups. If changes in chromophore systems are absent, the method of spectrophotometry remains relatively less informative. 

Further analysis is based on the idea that the characteristic experimental behavior of different classes of compounds and the suitability of those or other models used to describe this behavior is ultimately related to the extent to which the chromophores or electron groups physically present in the molecular system are reflected in these models. It is easy to notice, that the MM methods work well in case of molecules with local bonds designated in Table 1 as valence bonds the QC methods apply both to the valence bonded systems, and for the systems with delocalized bonds (referred as orbital bonds in Table 1). The TMCs of interest, however, not covered either by MM or by standard QC techniques can be physically characterized as those bearing the d-shell chromophore. The magnetic and optical properties characteristic for TMCs are related to d- or /-states of metal ions. The basic features in the electronic structure of TMCs of interest, distinguishing these compounds from others are the following ... 

These properties of the d-shell chromophore (group) prove the necessity of the localized description of d-electrons of transition metal atom in TMCs with explicit account for effects of electron correlations in it. Incidentally, during the time of QC development (more than three quarters of century) there was a period when two directions based on two different approximate descriptions of electronic structure of molecular systems coexisted. This reproduced division of chemistry itself to organic and inorganic and took into account specificity of the molecules related to these classical fields. The organic QC was then limited by the Hiickel method, the elementary version of the HFR MO LCAO method. The description of inorganic compounds — mainly TMCs,— within the QC of that time was based on the crystal field... 

The goal of theory and computer simulation is to predict. S (7) and relate it to solvent and solute properties. In order to accomplish this, it is necessary to determine how the presence of the solvent affects the S0 —> Si electronic transition energy. The usual assumption is that the chromophore undergoes a Franck-Condon transition, i.e., that the transition occurs essentially instantaneously on the time scale of nuclear motions. The time-evolution of the fluorescence Stokes shift is then due the solvent effects on the vertical energy gap between the So and Si solute states. In most models for SD, the time-evolution of the solute electronic structure in response to the changes in solvent environment is not taken into account and one focuses on the portion AE of the energy gap due to nuclear coordinates. 

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. 

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