Band analysis, excited-state

Back electron-transfer rate, cyanometallates, 116 Band analysis, excited-state structure, 211,25 Base, effect on doublet excited... 

The lowest excited state of azulene is predicted to possess 2 symmetry, which is in agreement with the result obtained using the symmetry rule. A recent vibrational analysis of the longest wave-length absorption band in the electronic spectrum of azulene indicates that the lowest-excited state would possess C2 symmetry . ... 

Ultrafast ESPT from the neutral form readily explains why excitation into the A and B bands of AvGFP leads to a similar green anionic fluorescence emission [84], Simplistic thermodynamic analysis, by way of the Forster cycle, indicates that the excited state protonation pK.J of the chromophore is lowered by about 9 units as compared to its ground state. However, because the green anionic emission is slightly different when it arises from excitation into band A or band B (Fig. 5) and because these differences are even more pronounced at low temperatures [81, 118], fluorescence after excitation of the neutral A state must occur from an intermediate anionic form I not exactly equivalent to B. State I is usually viewed as an excited anionic chromophore surrounded by an unrelaxed, neutral-like protein conformation. The kinetic and thermodynamic system formed by the respective ground and excited states of A, B, and I is sometimes called the three state model (Fig. 7). 

Unfortunately, the above analysis can never be widely applicable to the determination of excited-state geometries since so few molecules and ions exhibit vibronically structured absorption bands and excitation profiles, even at low temperatures. Moreover, some questions arise as to the possible breakdown of the Condon approximation. Other types of molecule for which similar analyses have been carried out include 3-carotene, carotenoids (9) and certain carotenoproteins such as ovorubin (10). In these cases the excitation profiles of three skeletal a bands are monitored, and estimates for the change in C-C and C=C bonds lengths ( 0.02 A) have been made. 

Little is known about the fluorescence of the chla spectral forms. It was recently suggested, on the basis of gaussian curve analysis combined with band calculations, that each of the spectral forms of PSII antenna has a separate emission, with Stokes shifts between 2nm and 3nm [133]. These values are much smaller than those for chla in non-polar solvents (6-8 nm). This is due to the narrow band widths of the spectral forms, as the shift is determined by the absorption band width for thermally relaxed excited states [157]. The fluorescence rate constants are expected to be rather similar for the different forms as their gaussian band widths are similar [71], It is thought that the fluorescence yields are also probably rather similar as the emission of the sj tral forms is closely approximated by a Boltzmann distribution at room temperature for both LHCII and total PSII antenna [71, 133]. 

It is seen from the above that the energy of the molecule in the excited state must be enough to provide for the energy of dissociation, the kinetic energy of the atoms relative to each other and for the energy which is radiated as continuous spectrum. That a similar process occurs in absorption has been shown by Dieke and Hopfield. The work of Witmer on the analysis of bands in the Lyman region gives as the heat of dissociation a value of 4.34 volts and that found by Dieke and Hopfield was 4.38 volts. 

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

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