Solvatochromism bathochromism

The colour of indigo depends dramatically upon its physical state and environment for example, the vapour is red but the colour on the fibre is blue. The marked solvatochromism of indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressive bathochromic shift of the visible absorption band is observed as the solvent polarity... 

The marked changes in the carbonyl IR bands accompanying the solvent variation from tetrahydrofuran to MeCN coincide with the pronounced differences in colour of the solutions. For example, the charge-transfer salt Q+ Co(CO)F is coloured intensely violet in tetrahydrofuran but imperceptibly orange in MeCN at the same concentration. The quantitative effects of such a solvatochromism are indicated by (a) the shifts in the absorption maxima and (b) the diminution in the absorbances at ACT. The concomitant bathochromic shift and hyperchromic increase in the charge-transfer bands follow the sizeable decrease in solvent polarity from acetonitrile to tetrahydrofuran as evaluated by the dielectric constants D = 37.5 and 7.6, respectively (Reichardt, 1988). The same but even more pronounced trend is apparent in passing from butyronitrile, dichloromethane to diethyl ether with D = 26, 9.1 and 4.3, respectively. The marked variation in ACT with solvent polarity parallels the behaviour of the carbonyl IR bands vide supra), and the solvatochromism is thus readily ascribed to the same displacement of the CIP equilibrium (13) and its associated charge-transfer band. As such, the reversible equilibrium between CIP and SSIP is described by (14), where the dissociation constant Kcip applies to a... 

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). 

To estimate how many dye molecules fit into the dendritic micelles, UV-titra-tion experiments have been employed. In comparison with the spectra of a pure pinacyanol chloride solution in water, the peaks of the absorption maxima of the dye in the presence of the dendrimer are shifted bathochromically due to solvatochromic effects, which indicates the incorporation of the dye within the branches of the dendrimer. At dye-to-dendrimer molar ratios higher than 4 1, in addition to the bathochromic shifts, hypsochromically shifted peaks start to appear, indicating that the dendrimer is not incorporating further dyes. We interpret this as an incorporation of up to four dyes within the branches of the dendrimer. This observation correlates with the calculated available space within the dendrimer, obtained from the molecular simulations. Further studies of the interactions of the dyes within the dendritic micelle are in progress. 

When the excited state is more polar than the ground state, its stabilisation is favoured by more polar solvents. There is a decrease in transition energy and a bathochromic shift in the spectrum. (Positive solvatochromism, as shown in Fignre 1.39.)... 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantially less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especially the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromically 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). 

If we compare AX values (AX - XDMSO - Xj0iuene) for the four dyes under study we get 35 nm (1428 cm 1), and 27 nm (1269 cm 1), for the nitro, and methylsulfonyl azobenzene derivatives, respectively, and 22 nm (1077 cm 1), and 7 nm (457 cm1), for the nitro, and methylsulfonyl stilbene derivatives, respectively. Further examination reveals that the difference in AX between the two stilbene derivatives is 15 nm (620 cm 1), while that in the case of the azobenzene derivatives is 8 nm (159 cm1). Thus, two interesting conclusions can be drawn from this data a) the bathochromic shift is not only a function of the donor and acceptor groups, but also of the intermediate -system between them and (b) while the measured hyperpolarizability coefficients for the stilbene and azobenzene sulfonyl derivatives are very similar (see below), their solvatochromism behavior is different, and therefore solvatochromism is not an accurate prediction of p. 

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

These data have been tabulated <66AHC(7)39>. In the parent ion (4) only two absorptions are evident, at 202 and 285 nm. The long wavelength band shows a slight solvatochromic effect. The 3-phenyl and 4-phenyl compounds exhibit two absorptions each, at 287 and 356, and 242 and 345 nm, respectively. Electron releasing substituents in the 3-position provide a greater bathochromic shift than those at the 4-position. 

In figure 5 the observed solvatochromism is shown for the spectroscopic detection of the TNT breakdown product 4-amino-3-nitro-toluene. Increasing pressure (respectively density) effects an increasing solubility of 4-amino-3-nitro-toluene in sc-C02 and a bathochromic shift of the three absorption bands caused by increasing C02 polarity. 

A series of yellow to greenish-blue aziridinyl azo dyes and their azo precursors containing a thienyl coupling moiety (i.e., 263), which have been prepared from 2-aminothiophenes, are relatively bathochromic. From the viewpoint of solvatochromism, a clear contrast existed between A ax values in different solvents thus, a positive solvatochromism was observed in aprotic solvents, whereas a hypsochromic shift was brought about in polar protic solvents <1999DP(40)99>. 

The rate of addition decreases moderately with increasing solvent polarity there is a 35-fold rate deceleration in going from cyclohexane to dimethyl sulfoxide. In polar solvents, the dipolar reactant thiyl radical is more stabilized than the less dipolar activated complex. The stabilization of the thiyl radical by solvation has been proven by its strong positive solvatochromism [i. e. bathochromic shift of Imax with increasing solvent polarity) [576]. Similar solvent effects on rate have been observed in the addition of the 4-aminobenzenethiyl radical to styrene [577]. 

A particularly interesting solvatochromic merocyanine dye is l-methyl-4-[(4-oxocyclohexadienylidene)ethylidene]-l,4-dihydropyridine also called Brooker s merocyanine [48]. First it exhibits a bathochromic and then a hypsochromic shift of the long-wavelength n n absorption band as the solvent polarity inereases [309] cf. also entry 14 in Table 6-1. 

Wetzler and coworkers123 employed 4-aminophthalimide (63) and 4-amino-lV-methyl-phthalimide (64) as solvatochromic (and thermochromic) fluorescent probes in solvent mixtures. A bathochromic shift of the emission spectra was found in mixtures of toluene with ethanol and with acetonitrile123 when the more polar solvent was added to toluene, but raising the temperature causes a relative hypsochromic effect. Mixtures of benzene and acetonitrile were studied by Nevecna and coworkers124 for their polarity by means of the probes 46 and 47 and with respect to the correlation of this with the rate constants of the reaction of triethylamine with ethyl iodide. The fluorescence of the ammonium salt of 4-(l-naphthylsulfonate)aniline (84) in dioxane and water mixtures was studied by Hiittenhain and Balzer125. 

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