Vibronic fine structure

The M 4 M quadraple bond is retained as shown by v(MM) ca 362 cm . The UV-vis spectra contain intense absorptions for the S ) transition at lower energy than for analogues with simpler ligands. Vibronic fine structure arising from v(MoC) rather than v(MoMo) has also been resolved resonance Raman spectra also indicate an enhancement of the former vibration. These data are consistent with frontier orbital mixing between M2 [5,5 ] and CCR [7r,7r ] which both have r-symmetry. 

Comparison of the UV spectrum of polystyrene in the 2600 A region with that of toluene shows a close relationship in terms of both extinction coefficients and vibronic fine structure. The effect of para substituents is most conveniently characterized by the shift in the band corresponding to the a0-o transition. The comparison of substituent effects on the electronic excited states of thepara substituted polystyrenes parallels those for the corresponding para substituted toluenes. Such a correlation would only be expected if the tr - n transitions were effectively localized within a given pendant group of the polymer system. This conclusion is reinforced by the observation that polystyrene and toluene show similar shake up structure in their ESC A spectra with respect to both band profiles and intensities (when due... 

The magnified spectrum in the region between 22 500 cm -1 and 23 500 cm 1 containing the first of the intense, sharp bands as well as some of the smaller features is shown in Fig. 11. The vibronic fine structure on the most intense band is evident in the expanded spectrum. These vibronic bands are not well resolved, but a regular spacing between the shoulders of 65 cm -1 is found. The vibronic structure in the least intense band in Fig. 11 is even less well resolved, but the same energy spacing is evident. 

UV-Vis absorption spectra show that the polythiophene films exhibit an absorption band at 535 nm, corresponding to the first 7t-7t transition, with a clear vibronic fine structure. This absorption can be attributed to a structure that mainly consists of stacks of nearly coplanar extended chains, with the transition dipole moment oriented along the polymer backbone <2005AM708>. 

The low temperature and high-resolution absorption spectrum of permanganate as recorded by Holt and Ballhausen [12] is shown in Fig. 4. The first allowed band (I) starting at 2.27 eV (18,300 cm 1) has a well-resolved vibronic structure. It is followed by a featureless shoulder (II) at 3.47 eV (28,000 cm-1) and another strong band (III) at 3.99 eV (32,000 cm-1) with a clear vibronic fine structure. We finally have a strong featureless band (IV) at 5.45 eV (43,960 cm-1). 

Neugebauer et al. [160] have recently simulated the absorption spectrum for permanganate by including the vibronic fine structure as shown in Fig. 5. 

The present studies were undertaken as a prelude to an analyses of the electronic Si 7 S0 band spectrum of CB. A simulation of the vibronic fine structure in the jet-cooled ultraviolet system requires a knowledge of the puckering-wagging hypersurfaces of both the ground and excited electronic states as well as the molecular structures. This paper represents the first of a two part spectroscopic study of cyclobutanone and an extension to a recent calculation on the S0 and Si states [7]. 

Probable causes for the multiple emission are (a) vibronic fine structure of the squaraine emission, (b) emission from a relaxed excited state, and (c) emission from an exciplex (with solvent) or the excited state of the squaraine-solvent complex. To differentiate these possibilities, the effects of solvent, temperature, and structural changes on the multiple emission have been studied. Sq4 was chosen as a model for these investigations because of its high solubility in... 

Similar spectral analyses have also been carried out for several more unsymmetrical squaraines. The data consistently suggested that vibronic fine structures are the main contributors to the multiple emissions of these compounds [54]. 

Unfortunately, the experimental information for anthracene is not as rich as for naphthalene. Transitions to the lowest 7r-excited state occur at 1.1 eV and are dipole forbidden. In addition, PES [161,162] predicts transitions at 1.76,2.73, and 2.81 eV. The EA spectrum [166] of anthracene recorded in argon matrices agrees with the absorption maxima in organic glasses [140], and the vibronic fine structure is in accord with the Raman spectrum of polycrystalline anthracene [167] at room temperature. The CASSCF/CASPT2 calculations presented by... 

Distinct polarity effects on absorption were encountered for guest molecules exhibiting polarity-dependent vibronic fine structure in their spectra, as for instance naphthalene [21], whicdi showed a marked enhancement of fine structure in the presence of 0.05-M a-CyD but not in that of 0.01-M fi-CyD (Fig. 10.3.1). In such favorable cases, the stoichiometry of inclusion could be probed using a suitably modified Benesi-HUdebrand treatment based on the absorption of a vibronic subband whose intensity is specifically enhanced by CyD inclusion. This is the case for the 290-nm transition of the La band of naphthalene which is threefold more intense in the presence of a high a-CD concentration than in neat aqueous solution... 

In some cases the electron transfer absorption is composed of a sequence, or a progression of resolved components, rather than a single band. Such vibronic fine structure in electron transfer absorptions or emissions is, in principle, a direct measure of and j 36,42,46,47... 

As depicted in Figure 5.5 PFs in film display an unstructured, long absorption maximum centered at 3.3 eV. The photoluminescence emission spectrum of PFs shows a vibronic fine structure with an energetic spacing of 180 meV (stretching vibration of the C = C-C = C structure of the polymer backbone) with the transition at 2.9 eV yielding a deep blue emission. In dilute solution the spectra are very similar to that of the solid state and only a small bathochromic shift of 20 meV is typically observed for both absorption and emission. 

Recently, Turbiez et al. [27] reported a poly(3,4-ethylenedioxythiophene) (PEDOT) like polymer, poly(3,6-dimethoxythieno[3,2-h]thiophene), which exhibited a discernible vibronic fine structure with max at 592 nm (Figure 11.7) and a band-edge gap of 1.65 eV. The electrochemically synthesized polymer was reported as pale blue in the oxidized form, while the spectrum of the oxidized polymer remained unchanged even after 1 week, indicative of a potential transparent semiconductor like PEDOT. 

Vibronic-coupling theory has been a well established area of research since many years. The basic elements of the theory are the concept of dia-batic electronic states, the normal-mode description of vibrational motion, and the application of symmetry selection rules to derive appropriate model Hamiltonians. The applications of vibronic-coupling theory cover the full range of molecular spectroscopy, including, in particular, optical absorption and emission and photoelectron spectroscopy. Typical spectroscopic phenomena associated with vibronic interactions are the appearance of nominally forbidden electronic bands, the excitation of nontotally symmetric modes, or unusual and complex vibronic fine structures of electronic spectra. A fairly comprehensive and up-to-date exposition of vibronic-coupling theory is provided by the monograph of Bersuker and Polinger. ... 

Aromatic hydrocarbons such as pyrene have also been employed as a luminescence probe of polarity and microviscosity in a variety of organized assemblies (109). Pyrene is a good excimer-forming probe due to the long lifetime of fluorescence and formation of excited-state dimers (excimers) at low concentration. Figure 9 shows an example pyrene luminescence spectrum. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. The vibronic fine structure of the pyrene monomer is sensitive... 

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