Negative solvatochromism

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

When Ajx = /positive solvatochromism resulting in the red shift of transitions otherwise, when A/r is negative, the spectra move to the blue and that is the case of negative solvatochromism. As follows from this expression, the change of dipole moments A/r during electronic transition is a necessary condition for observed solvatochromic shift the more this difference -the stronger is the solvatochromic shift. The second important parameter is the local... 

The pA a shift can be directly measured by the solvatochromic shift of the ultraviolet absorption spectra. For PCP, the p%lir is 5.97 in phosphatidyl choline membranes, and increases up to 6.78 in the negatively charged phosphatidyl glycerol membranes [123], The addition of cholesterol decreases the pATam again slightly in both types of membranes. 

Compounds are called solvatochromic when the location of their absorption (and emission) spectra depend on solvent polarity. A bathochromic (red) shift and a hypsochromic (blue) shift with increasing solvent polarity pertain to positive and negative solvatochromism, respectively. Such shifts of appropriate solvatochromic compounds in solvents of various polarity can be used to construct an empirical polarity scale (Reichardt, 1988 Buncel and Rajagopal, 1990). 

Kosower in 1958 was the first to use solvatochromism as a probe of solvent polarity. The relevant Z-scale is based on the solvatochromic shift of 4-methoxycarbonyl-1-ethylpyridinium iodide (1). Later, Dimroth and Reichardt suggested using betain dyes, whose negative solvatochromism is exceptionally large. In particular, 2,6-... 

It is also worth noting that, due to their dipolar and conjugated donor-acceptor nature, all these amino-terminated Group 6 metaUacumulenes exhibit a strong negative solvatochromic effect and they show significant second-order NLO properties [68]. 

Besides uracil-6-iminophosphorane, the iminophosphorane component was extended to pyrazole 3 and pyrazolon-4-iminophosphoranes 363 (94JOC3985). In its electron distribution, 363 can be compared with uracil 346. With arylisocyanates, pyridine, or y-picoline, zwitterionic pyrazolo [3, 4 4,5]pyrido[6,l-a]pyrimidines (364) are obtained and with isoquinoline, 365 is formed (Scheme 131). Again, both systems show a typical negative solvatochromism (94JOC3985). 

Conversely, a more polar ground state leads to the opposite effect and a hyp-sochromic shift in the spectrum (negative solvatochromism). 

The variations in the absorption energies of various dyes have been used to characterise the polarity of various media and create empirical scales. For this purpose, the most widely used dye is the highly, negatively solvatochromic betaine (1.98), known as Reichardt s dye, whose transition energy f .j,(30) in kcal mol, measured in a particular solvent, characterises the polarity of that solvent. 

For example, dissolving the betaine (1.98) or another negative solvatochromic dye in PMM and spin coating the solution to give a film on a sensor tip provides a product suitable for the analysis, in visible absorption or transmission mode, of alcohols and ethers in gasoline. 

All pseudoazulenes possess an electronic transition in the visible spectral region (Table III). As can be derived from the extinction coefficient, as well as from investigations on solvent dependence, this electronic transition is a nn transition having a low negative solvatochromism that can be measured for some compounds.51 86 145 165 These spectral data correspond to the results of quantum chemical calculations (Section IV,B) and are analogous to those of azulenes. 

The oxidation of meta- and para-substituted anilines with imidazolium fluorochro-mate (IFC)18 and nicotinium dichromate (NDC),19 in several organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in the oxidant and TsOH and is zero order with respect to substrate. A correlation of rate data in different solvents with Kamlet-Taft solvatochromic parameters suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation. The oxidation rates with NDC exhibited negative reaction constants, while the oxidation with IFC did not correlate well with any linear free energy relationships. 

A further key feature is the very strong negative solvatochromism exhibited by these CT bands, as exemplified by Pt(dbbpy)(tdt) in Fig. 11. The shift to higher energy in solvents of increasing polarity is typically indicative of an excited state that is less polar than the ground state. This can be understood... 

These bands show negative solvatochromism as revealed by band shifts to lower energy in less polar solvents [5, 7, 8, 12], The direction of the solvent dependence is associated with a reduced (and reversed) molecular dipole in their MLCT excited states. Emissions from these complexes are typically broad and structureless, and they also often exhibit a rigidochromic effect [7-12], Tables 1 and 2 summarize the luminescence characteristics and environmental effects on absorption and emission maxima for rhenium(I) tricarbonyl diimine complexes. 

In a study of the colored forms 9 in various solvents, an interesting effect on the long-wavelength band is observed. Most of the DHI 7 investigated showed a negative solvatochromic effect. This is consistent with earlier reports on spiropy-rans.8... 

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