Positional spectral shifts

Figure 3.17 presents ps-TR spectra of the olehnic C=C Raman band region (a) and the low wavenumber anti-Stokes and Stokes region (b) of Si-rra i-stilbene in chloroform solution obtained at selected time delays upto 100 ps. Inspection of Figure 3.17 (a) shows that the Raman bandwidths narrow and the band positions up-shift for the olehnic C=C stretch Raman band over the hrst 20-30 ps. Similarly, the ratios of the Raman intensity in the anti-Stokes and Stokes Raman bands in the low frequency region also vary noticeably in the hrst 20-30 ps. In order to better understand the time-dependent changes in the Raman band positions and anti-Stokes/Stokes intensity ratios, a least squares htting of Lorentzian band shapes to the spectral bands of interest was performed to determine the Raman band positions for the olehnic... 

Initial position of instant spectrum of fluorescence and character of spectral shifts in time depend on the excitation frequency, i.e., inhomogeneous broadening is of dynamic nature as a degree of broadening is maximal at the initial instants of time and decreases with time of emission registration (demonstration in panel d of Fig. 5). 

Contrary to earlier expectations (see Dorfman, 1965), Hentz and Kenney-Wallace (1972, 1974) failed to find any correlation between s and Emax. Actually, there is a better correlation of matrix polarity with the spectral shift from e(to e upon solvation and the time required to reach the equilibrium spectrum (Kevan, 1974). Furthermore, Hentz and Kenney-Wallace point out that emax is smaller f°r alcohols with branched alkyl groups, the spectrum being sensitive to the number, structure, and position of these groups relative to OH. Clearly, a steric effect is called for, and the authors claim that a successful theory must not rely too heavily on continuum interaction as appeared in the earlier theories ofjortner (1959,1964). Instead, the dominant interaction must be of short range, and probably the spectrum is determined by optimum configuration of dipoles within the first solvation shell. 

BODIPY fluorophores are a class of probes based on the fused, multi-ring structure, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (Figure 9.27) (Invitrogen) (U.S. patent 4,774,339). This fundamental molecule can be modified, particularly at its 1, 3, 5, 7, and 8 carbon positions, to produce new fluorophores with different characteristics. The modifications cause spectral shifts in its excitation and emission wavelengths, and can provide sites for chemical coupling to label biomolecules. 

The chromophore environment can affect the spectral position of the absorption and emission bands, the absorption and emission intensity (eM, r), and the fluorescence lifetime as well as the emission anisotropy, e.g., in the case of rigid matrices or hydrogen bonding. Changes in temperature typically result only in small spectral shifts, yet in considerable changes in the fluorescence quantum yield and lifetime. This sensitivity can be favorably exploited for the design of fluorescent sensors and probes [24, 51], though it can unfortunately also hamper quantification from simple measurements of fluorescence intensity [116], The latter can be, e.g., circumvented by ratiometric measurements [24, 115],... 

The results are shown in Figure 2. Spectra are plotted for various values of normalised input intensity yinc = n2 inc which can be considered as the strength of the nonlinearity. A positive nonlinearity, yjnc > 0, that shifts the spectra to the right was used. As expected the nonlinearity (spectral shift) is enhanced with increasing number of the periods. Moreover the structures with 2 and 3 periods exhibit bistable behavior for A, > Aq. In this case bistable switching is controlled by frequency tuning while input intensity is fixed. 

For NP2 and NP3 at pH 7.5, the shift in Soret band positions of the NO complexes for the two oxidation states is somewhat larger—8-10 nm, from 421-423 to 413 nm for the Fe(III) and Fe(II) complexes, respectively (50). However, in contrast to NPl-NO (49) and NP4-NO (50), at pH 5.5 NP2-NO and NP3-NO show very different spectral shifts upon electrochemical reduction, as shown in Fig. 6c for NP3-NO. The Soret band shifts to 395 nm, and both the wavelength maximum and shape of the Soret band are typical of five-coordinate heme-NO centers, including guanylyl cyclase, upon binding NO (53, 54). The reduced forms of both NP2-NO and NP3-NO exhibit similar pH dependence of the absorption spectra, whereas NPl-NO and NP4-NO do not show any pH dependence of their absorption spectra over the pH range 5.5-7.5 (50). 

The opposite direction of spectral shift with solvent polarity is observed for the spiro-oxazine [4] and naphthopyran mero-forms and this is generally accepted to infer a quinoidal HOMO and a zwitterionic LUMO state in their cases. This shift is shown also in Fig. 7b for a naphthopyran TT isomer (CHR2). Of course, H-bonding and other specific interactions will also affect the position of the mero-form spectral maximum. In the case of the substituted BIPS, spiro-oxazine, and CHR2, there is evidence to support their respective assignments to a zwitterionic and quinoidal forms, including x-ray [36,80-85] and NMR [36,55,86-88] data. 

The above features allow one to adjust the modulation parameters for a given scenario to obtain an optimal decrease or increase of R. The PM scheme with a small 0 is preferable near the continuum edge (Figure 4.6d), since it yields a spectral shift in the required direction (positive or negative). The adverse effect of k O peaks in F((a>) then scales as (p and hence can be significantly reduced by decreasing 0. On the other hand, if is near a symmetric peak of G( ), R is reduced more effectively for (p n, as in Refs [40, 41], since the main peaks of Ff co) at o i then shift stronger with than the peak at q = -cp/r for (pC.. ... 

Here the long lasting absorption transients are also well seen. Even a first inspection of the absorption transient in Fig. 2 reveals spectral shifts of the absorption bands throughout the whole time range. They are well resolved in the absorption spectra plotted for specific delay times (Fig. 4a). Here a blue shift with time both of the n-n ( 500 nm) and the n-n ( 350 nm) absorption band is evident. The position of the newly formed n-n band of the trans-molecules (350 nm) is of special interest. It is plotted in Fig. 4b as a function of time delay (filled circles). At later delay times this spectral shift is dominated by a 50 ps kinetic compound. 

In more recent work, Johnston and co-workers (17,18,20,27,32) showed quantitatively that the local fluid density about the solute is greater than the bulk density. In these papers, results were presented for CQ2, C2H4, CF3H, and CF3C1. Local densities were recovered by comparison of the observed spectral shift (or position) to that expected for a homogeneous polarizable dielectric medium. Clustering manifests itself in deviation from the expected linear McRae continuum model (17,18,20,27,32,56,57). These data were subsequently interpreted using an expression derived from Kirkwood-Buff solution theory (20). Detailed theoretical... 

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