Polarization red shift

A most comprehensive discussion of the effect of solvent on spectra has been given by Bayliss and McRae.21 They point out that polarization or dispersion forces are the most general interactions involved in solution and that all solution spectra are subject to a generalized polarization red shift, relative to vapor spectra, due to solvent polarization by the transition dipole. However, these dispersion forces are relatively weak and are easily obscured by the effect of dipole-dipole and dipole-static charge forces in polar, but not highly polarizable, solvents. By applying the Franck-Condon principle, they showed... 

Increase of polarity > Red shift Decrease of polarity => Blue shift... 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

The extinction coefficients of carotenoids have been listed completely bnt solvent effects can shift the absorption patterns. If a colorant molecnle is transferred into a more polar environment, then the absorption will be snbjected to a bathochro-mic (red) shift. If the colorant molecnle is transferred into a more apolar enviromnent, the absorption will be subjected to a hypsochromic (blue) shift. If a carotenoid molecule is transferred from a hexane or ethanol solution into a chloroform solution, the bathochromic shift will be 10 to 20 nm. 

Since CT and -+. transitions are red-shifted (i.e., toward lower energy) in polar solvents such as isopropyl alcohol and n - - w transitions... 

Where the + — terms refer to / an type excitations and the to a n - v type transition. These absorptions occur at longer wavelengths than the related model compounds (benzene and dimethylamine for Michler s ketone), have a high intensity, emax 104 liter/mole-cm, a small singlet-triplet splitting, and undergo a red shift of the absorption on going to a more polar solvent. 

These experiments have firmly established that the red shift of the fluorescence band in polar solutions with the change of excitation quanta energy is caused by inhomogeneous configurational broadening of electronic energy levels. Later, it was found that not only singlet but also triplet states of dyes are broadened... 

The anomalous dual fluorescence emission of p-A V-dimethylamino benzoni-trile (DMABN) in polar solvents was first reported by Ernst Lippert in 1962. Emission spectra of DMABN in solvents of different polarity show a dual emission, where the red-shifted emission is stronger relative to the primary emission when the solvent polarity increases. Furthermore, it can be observed that overall emission intensity is reduced in more polar solvents, but higher solvent viscosity increases the emission intensity. Spectra of DMABN in different solvents are shown in the chapter of Tomin in this book [1]. 

Replacing the phenyl group by a coumarin motif produces an important subclass of molecular rotors, represented by 17, which have found several applications in viscosity and polarity measurements [48]. On the other hand, compound 19 has a red-shifted emission as compared to 18 due to the effective delocalization of the 71-electrons in the presence of the thiophene chromophore [49]. 

The fluorescence and absorption spectra of DTT-A.V-dioxidc 20a with polar covalent bonds was studied in THF, toluene, and decalin. The spectral line and peak energy are almost independent of the solvent polarity. The fluorescence spectra of the decalin and toluene solutions (almost the same polarity) are red-shifted by about 5 nm, with respect to the THF solution of higher polarity. No evident solvatochromism was observed. The absorbance and fluorescence excitation spectra (at the fluorescence peak wavelength) for DTT-3, 3 -dioxide 20a (normalized to peak value) was compared. The fluorescence excitation signal is, in fact, dependent both on the density of the excited state (as the absorbance) and on the efficiency of the relaxation from the excited state of the emitting one <2005PCB6004>. 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

The matrix IR spectra of la and several isotopomers (cU-la, l80-la) reveal details of the electronic structure of the carbene.23 In particular the red-shift of the C=0 stretching vibration (compared to p-benzoquinone) below 1500 cm-1 indicates a substantial contribution of the phenoxyl/phenyl resonance structure to the wave function of la. The C2V symmetry of the carbene was experimentally revealed by measuring the IR dichroism of partially oriented samples of matrix-isolated la. The orientation of la in an argon matrix was achieved by irradiation with linearly polarized light. 

The details of how nitroaromatic explosive molecules interact with the chromo-phores in the polymer matrix requires further study. Initial observations suggest that because nitroaromatic explosive molecules are highly electron-deficient, that chro-mophores have an electron-rich donor and bridge, and that both nitroaromatic explosives and chromophores are highly polar, explosive molecules and chromo-phores have a strong tendency to interact with each other. The interaction between explosives and the polymer takes place in two steps. In the initial step nitroaromatic explosive molecules create a more polar environment around the chromophores. The increased polar environment produces a solvatochromic red-shift of the... 

---