Fluorescence vibronic coupling

A different example of non-adiabatic effects is found in the absorption spectrum of pyrazine [171,172]. In this spectrum, the, Si state is a weak structured band, whereas the S2 state is an intense broad, fairly featureless band. Importantly, the fluorescence lifetime is seen fo dramatically decrease in fhe energy region of the 82 band. There is thus an efficient nonradiative relaxation path from this state, which results in the broad spectrum. Again, this is due to vibronic coupling between the two states [109,173,174]. 

Karpovich DS, Blanchard GJ (1995) Relating the polarity dependent fluorescence response of pyrene to vibronic coupling. Achieving a fundamental understanding of the py polarity scale. J Phys Chem 99 3951-3958... 

The relative changes in intensity of the vibronic bands in the pyrene fluorescence spectrum has its origin in the extent of vibronic coupling between the weakly allowed first excited state and the strongly allowed second excited state. Dipole-induced dipole interactions between the solvent and pyrene play a major role. The polarity of the solvent determines the extent to which an induced dipole moment is formed by vibrational distortions of the nuclear coordinates of pyrene (Karpovich and Blanchard, 1995). 

Karpovich D. S. and Blanchard G. J. (1995) Relating the Polarity-Dependent Fluorescence Response of Pyrene to Vibronic Coupling. Achieving a Fundamental Understanding of the Py Polarity Scale,... 

The excitation and fluorescence spectra of some crown ether complexes have been interpreted in terms of the molecular point group symmetry and vibronic coupling.452,453 This approach tends to be limited to europium(III) complexes. 

In the case of 5T and 6T, the excited states with B symmetry near 4 eV can be occupied by allowed one-photon excitation from the A ground state. For 3T and 4T, states of A symmetry lie next to the excited state. They need vibronic coupling to be excited. The initially occurring absorption A0 is assumed to start from one of the higher electronic states near 4 eV. Relaxation processes from these states may be responsible for the decay of A0 during the first picosecond and for the delayed increase of stimulated fluorescence of 3T-6T. 

Vibrational analysis of the benzene phosphorescence bands indicates that the radiative activity is induced predominantly by e2g vibrations [155, 156]. A weak but observable activity of b2g vibrations has also been found [156, 155, 157]. By introducing spin-orbit- and vibronic coupling through second order perturbation theory Albrecht [158] showed that the vibronic interaction within the triplet manifold is responsible for the larger part of the phosphorescence intensity. This also follows from comparison of the vibrational structure in phosphorescence and fluorescence spectra [159]. The benzene phosphorescence spectrum in rigid glasses [155] reveals a dominant vibronic activity of... 

Another example of the mirror-image relation between absorption and fluorescence spectra is provided by anthracene (Figure 5.11). In this particular case the situation is complicated by vibronic coupling the L, and Lb bands overlap in the absorption spectrum, and the location of the Lb origin has been inferred from the substituent effects evident from the MCD spectrum (Steiner and Michl, 1978). 

Photophysics of Pyrene on Various Silica Gel Surfaces. Pyrene gives information about its environment via changes in its fluorescence fine structure (14). Typically, five vibronic bands are identified the ratio of the HI band at 392 nm to the I band at 372 nm, the 111/1 ratio, increases in noninteracting (nonpolar) media with a concomitant increase in fluorescence lifetime (15). The versatility of the pyrene probe arises from the forbiddenness of the So —> Si transition any intensity for the transition comes from vibronic coupling with higher excited states (10). Interactive or polar solvents, via their interaction with the arene, increase the intensity of the 0 —> 0 transition in both the absorption and the emission. 

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