Solvent-induced Stokes shift

Also, the solvent-induced Stokes shift between the absorption and emission first spectral moments is... 

One of the consequences of a nonzero Aao is that the relation between the solvent-induced Stokes shift and the corresponding spectral width (A,v = 0)... 

A simple quantitative treatment of the solvent relaxation-induced Stokes shift is based on a model that assumes that the fluorophore is located in a cavity with radius a in a dipolar medium characterized by the bulk dielectric permittivity e and refractive index n (we should recall that the high-frequency limit of the dielectric permittivity e o equals the square of the refractive index n ). Classical treatment yields the Lippert equation [44] describing the difference, Av = va — vp, between the wavenumbers of the emission and absorption maxima of the fully relaxed system ... 

In calculating solvent effects on excitation energies one has to take into account that normally the excitation process is sufficiently rapid for the Franck-Condon principle to hold, which means that while the induced dipole of the solvent molecule can change in response to the change of solute dipole on excitation, the permanent dipoles cannot—that is, there is negligible dipole reorientation. Such reorientation can, however, occur in the time period between absorption and emission and lead to a large Stokes shift in the emission. (Cf. Section 5.3.1.)... 

Dapoxyl dyes are highly environment-sensitive fluorescent compounds. Their fluorescence in water is very low, but induces a large change of fluorescence intensity, Stokes shifts, and extinction coefficients depending on pH and polarity of solvents. 

In reality there are several molecular vibrations that can couple to an electronic transition although the basic phenomenology is retained, namely the resonant character of the S St) 0-0 transition and the mirror symmetry of the vibronic satellites. In case of a jr—Mt transition the dominant vibrational modes are those of the polymer backbone, notably of the phenyl ring. An example is the absorption and fluorescence of it-conjugated molecules, such as tetracene, in the gas phase [22], In fluid solution there is interaction between the transient dipole of the molecule with the permanent and induced dipoles of the solvent. It gives rise to (i) a bathochromic shift of the spectra, (ii) a Stokes shift between the... 

Solvent polarity and the local environment have profound effects on the emission spectra of polar fluorophores. These effects are the origin of the Stokes shift, which is one of the earliest observations in fluorescence. Emission spectra are easily measured, and as a result, there are num ous publications on emission spectra of fluoropho-res in different solvents and when bound to proteins, membranes, and nucleic acids. One common use of solvent effects is to determine the polarity of the probe binding site on the macromolecule. This is accomplished by comparison of the emission Spectra and/or quantum yields of the fluorophore when it is bound to the macromolecule and when it is dissolved in solvents of different polarity. However, there are many additional instances where solvent effects are used. Suppose a fluorescent ligand binds to a protein. Binding is usually accompanied by a spectral shift due to the different environment for the bound ligand. Alternatively, the ligand may induce a spectral shift in the intrinsic or extrinsic protein fluorescence. Additionally, fluorophores often display spectral shifts when they bind to membranes. 

The salicylideneaniline (SAN) molecule and related molecules are highly usefiil model compounds for the investigation of the role of structural flexibihty in Kght-induced structural transformations. This photochromic transformation is connected with adiabatic ESIPT OH—>NH in the Sj state of the structure (E, Scheme 6), which is the origin of the fluorescence anomalous Stokes shifL The anomalous Stokes-shifted fluorescence and absorption spectral maxima of SAN observed under several conditions are listed in Table 68.9. The anomalous Stokes-shifted emission was found to be observed only in rigid matrices, especially in a glassy solvent at 77 K, as well as in crystals. This implies that the necessary step... 

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