Vibronic maxima

Pyrene is a useful probe for studying the polarity of the medium because its fluorescence vibronic structure is very sensitive to the microenvironment [92,93]. In particular, the ratio of the intensities of the first (373 nm) to the third (384 nm) fluorescence vibronic maxima (R = I/III) strongly decreases with the decreasing polarity of the medium. The interaction of pyrene (13) with the three CDs has been thoroughly investigated by photophysical methods. There are considerable discrepancies among the results of the various reports, which are probably related to the very low solubility of 13 in H2O (<5 X 10 M). The low solubility imposes the need for special precautions when preparing the samples [94]. 

Of these four properties, spectral shifts are the most sensitive to environmental changes and also the most readily measured. As a result the majority of investigations into electronic absorption spectral changes resulting from surface adsorption have been confined to measurements of spectral shifts. While the shift of the 0-0 bands is the most meaningful measurement to make, these 0-0 bands are not always discernible, especially when the molecules are adsorbed on polar surfaces, so it has become common practice simply to measure the shift of the absorption maximum. In most cases this measurement would correspond to the shift of the 0-0 band, in others, however, adsorption processes can produce unequal displacement of the ground and excited state potential curves, resulting in a different vibronic band shape. 

In the highly regioregular polymer 522, in its pristine amorphous form, the absorption maximum of spin-cast films is 493 nm (2.86 eV) (94MM6503 01JPC(B)7624). On treating the films with chloroform vapor, the maximum was shifted to 602 nm (2.06 eV), and the spectrum showed a fine vibronic structure with AE = 0.19 eV, typical of a more-planar ordered conformation. 

The temperature dependence of non-radiative transitions, caused by linear diagonal and quadratic non-diagonal vibronic interactions, is also investigated on the basis of the non-perturbative quantum theory. It was found that the usual increase of the transition rate with temperature does not hold near some critical values of the non-diagonal interaction and temperature. At these critical values the rate is high (comparable to the mean phonon frequency) and its temperature dependence has a maximum. The results may be important for understanding the mechanisms of catalysis in chemical reactions. 

This method was used to fit the experimental results shown in Figure 3.38, except that the multichannel WR(r) was substituted for the single-channel one because the highly exergonic recombination excites the vibronic mode (whose frequency go = 43.5 x 10 3 eV and corresponding reorganization energy 0.61 eV were taken from Ref. 187). IET fits well the experimental data at a reasonable L= 1.38 A and other parameters [98] and reinforces the fact of principal importance the very existence of the maximum in Z(D) dependence. 

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