Vibronic bands

This last transition moment integral, if plugged into equation (B 1.1.2). will give the integrated intensity of a vibronic band, i.e. of a transition starting from vibrational state a of electronic state 1 and ending on vibrational level b of electronic state u. 

Equation (B 1.1.8) gives the intensity of one vibronic band in an absorption spectrum. It is also of interest to consider... 

Often it is possible to resolve vibrational structure of electronic transitions. In this section we will briefly review the symmetry selection rules and other factors controlling the intensity of individual vibronic bands. 

If the experunental technique has sufficient resolution, and if the molecule is fairly light, the vibronic bands discussed above will be found to have a fine structure due to transitions among rotational levels in the two states. Even when the individual rotational lines caimot be resolved, the overall shape of the vibronic band will be related to the rotational structure and its analysis may help in identifying the vibronic symmetry. The analysis of the band appearance depends on calculation of the rotational energy levels and on the selection rules and relative intensity of different rotational transitions. These both come from the fonn of the rotational wavefunctions and are treated by angnlar momentum theory. It is not possible to do more than mention a simple example here. 

Similarly, parallel or perpendicular elecfronic or vibronic bands of a symmefric rotor (see Section 6.2.4.2) show widely varying shapes, unlike fhe parallel or perpendicular vibrational bands illusfrafed in Figure 6.28 and 6.29, respectively. 

Figure 6-18. T(, single irystalpolan/ed fluorescence spectra al 4.2 K. The main vibronic bands built On the origin and on the A, B, and C false origins arc indicated. See text for discussion (Section 6.6.1).

Figure 14. Mode selectivity in photodissociation of V (OCO). The ratio of the reactive (VO + CO) to nonreactive (V + CO2) product is measured at the peaks of the vibronic bands labeled in Fig. 13. The data below 16,600 cm is from bands accessed by one-photon excitation data at higher energy was obtained by vibrationally mediated photodissociation exciting the OCO antisymmetric stretch.

Halogen oxide radicals such as CIO and BrO are important reactive intermediates in the catalytic cycles of ozone destruction in the middle and upper stratosphere. The first absorption band CIO(/l211 <— X2 I) starts from 318 nm and has a series of vibronic bands that converge to a broad continuum at wavelengths shorter than 264nm (Fig. 8).98-101 In this continuum region four dissociation pathways are thermodynamically possible,33... 

It is relatively easy to decide which vibronic bands have a common origin. This is accomplished by observing the phosphorescence intensity change of each band upon microwave saturation at a frequency that corresponds to transitions between rz and tx. This is known as phosphorescence-microwave double resonance (PMDR) spectroscopy. These frequencies for 2,3-dichloroquinoxaline are given in Table 6.3. 

Figure 6.1. Jablonski-type diagram for pyrazine. The zero-field splittings (between tx, tV) t2) are not drawn to scale. Spin polarization ( x x x) resulting from the most probable intersystem crossing routes and part of the emission spectrum where different vibronic bands (v = /,/, k) have different zf origins are schematically indicated. (After El-Sayed.(17))...

Symmetry-forbidden transitions. A transition can be forbidden for symmetry reasons. Detailed considerations of symmetry using group theory, and its consequences on transition probabilities, are beyond the scope of this book. It is important to note that a symmetry-forbidden transition can nevertheless be observed because the molecular vibrations cause some departure from perfect symmetry (vibronic coupling). The molar absorption coefficients of these transitions are very small and the corresponding absorption bands exhibit well-defined vibronic bands. This is the case with most n —> n transitions in solvents that cannot form hydrogen bonds (e 100-1000 L mol-1 cm-1). 

For some aromatic hydrocarbons such as naphthalene, anthracene and pery-lene, the absorption and fluorescence spectra exhibit vibrational bands. The energy spacing between the vibrational levels and the Franck-Condon factors (see Chapter 2) that determine the relative intensities of the vibronic bands are similar in So and Si so that the emission spectrum often appears to be symmetrical to the absorption spectrum ( mirror image rule), as illustrated in Figure B3.1. 

The fluorescence spectrum of a compound may be used in some cases for the identification of species, especially when the spectrum exhibits vibronic bands (e.g. in the case of aromatic hydrocarbons), but the spectra of most fluorescent probes (in the condensed phase) exhibit broad bands. 

Many aromatic hydrocarbons such as naphthalene or pyrene can form excimers. The fluorescence band corresponding to an excimer is located at wavelengths higher than that of the monomer and does not show vibronic bands (see Figure 4.6 and the example of pyrene in Figure 4.7). 

Weak coupling leads to minor alterations of the absorption spectrum (hypo-chromism or hyperchromism, Davidov splitting of certain vibronic bands). 

Polarity-induced changes in vibronic bands. The Py scale of polarity... 

In some aromatic molecules that have a high degree of symmetry, i.e. with a minimum D2h symmetry (e.g. benzene, triphenylene, naphthalene, pyrene, coronene), the first singlet absorption (So —> Si) may be symmetry forbidden61 and the corresponding oscillator strength is weak. The intensities of the various forbidden vibronic bands are highly sensitive to solvent polarity (Ham effect). In polar solvents, the intensity of the 0-0 band increases at the expense of the others. 

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). 

Fig. 7.8. Fluorescence spectra of pyrene in hexane, n-butanol, methanol and acetonitrile showing the polarity dependence of vibronic band intensities (excitation wavelength 310 nm) (reproduced with permission from Kalyanasun-daran and Thomas, 1977b).

Tab. 7.4. Solvent dependence of the ratio / //m of the fluorescence intensities of the first and third vibronic bands in the fluorescence spectrum of pyrene.

In contrast, the Py scale, based on the relative intensities of vibronic bands of pyrene, appears to be relatively insensitive to hydrogen bonding ability of solvents. 

Kalyanasundaran K. and Thomas J. K. (1977b) Environmental Effects on Vibronic Band Intensities in Pyrene Monomer Fluorescence and their Application in Studies of Micellar Systems, J. Am. Chem. Soc. 99, 2039-2044. 

Figure B9.3.1 shows the parallelism between the increase in emission spectrum displacement and fluorescence anisotropy observed for the red-edge of most vibronic bands and especially for the 0-0 one. It can be interpreted in terms of inhomogeous spectral broadening due to solvation heterogeneity. The decrease in energy transfer that is observed upon red-edge excitation is evidence that energy hopping is not chaotic but directed toward lower energy chromophores, as in photosynthetic antennae.

A detailed study of the electronic structure and optical properties was published for the spiro derivative of f-Bu-PBD, Spiro-PBD (40) [108]. The vibronic structure of the lowest energy absorption band is well resolved, in solution as well as in the amorphous him. The 0-0 transition is at 351 nm (3.53 eV), the 0-1 and 0-2 vibronic bands that have a higher oscillator strength, are at 336 nm (3.69 eV) and 318 nm (3.90 eV), respectively. The fluorescence spectrum of this compound is symmetrical to the absorption spectrum with a Stokes shift of 43 nm. 

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