Solvatochromism polarity increases

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). 

The influence of octane on solubilization of water and solvatochromic polarity is pronounced as shown in Table HI. Two combinations of octane concentration and Wq were investigated. In both cases, large amounts of water are solubilized at low pressures. The polarities do not change with an increase in pressure as they are already comparable with that of bulk water. At these high concentrations of octane, the fluid is much less compressible than ethane. In the future, we will explore lower concentrations of octane to determine if pressure could be used to tune the polarity in the one phase region. 

In 1951, Brooker suggested for the first time that solvatochromic dyes could be used to obtain measures of solvent polarity. This author constructed the Xr scale on the basis of the solvatochromism of the merocyanine dye (5), the electronic transition of which gives rise to a charge-transfer fi-om flie amine nitrogen to a carboxamide group at the other end of tire molecule. Hence, the excited status is more dipolar than the ground state, and the resulting band is shifted bathochromically as solvent polarity increases. Xr values reflect the position of the maximum of the first band for the chromophore in kcal mol . 

Solvatochromism Shift of UV/Vis absorption wavelength and intensity in the presence of solvents. A hypsochromie (blue) shift increases as solvent polarity increases. The shift in the red direction is called bathochromic. 

We have used different methods to characterize and understand the nature of the ILs in the RMs. The measurement of microenvironments in the RMs is one of them. We have used l-methyl-8-oxyquinolinium betaine (QB) to sense the polar core environment of IL RMs [151]. QB is a useful probe as it is small and very sensitive to different microenvironment properties and it locates exclusively at the RM interface [26, 27]. QB presents two electronic absorption bands [26] the band in the visible region, Bj, arises from the transition from a predominantly dipolar ground state to an excited state of considerably reduced polarity leading to negative solvatochromism with increasing solvent polarity. Similar to the behavior of Ej.p )... 

Dimroth et al. introduced 8 as a solvatochromic probe of solvent polarity having absorption in the visible region it shows the largest solvatochromic shift of any substance yet reported. Ey (30) is calculated with Eq. (8-76), like Z. (The peculiar symbolism arose because compound 8 happened to be No. 30 on the list of substances studied by Dimroth et al.) The shift is hypsochromic as solvent polarity is increased. Table 8-16 gives some Ey (30) values. - (30) is linearly... 

Monte Carlo calculations of interactions of 1 //-bcnzotriazole with water reveal significant electronic polarization of the heterocycle. The dipole moment is increased by 2.89 D for the ground state and 2.75 D for the excited state to the total values of 6.89 and 6.40 D, respectively. Direct measurements of dipole moments in water are not possible, but these numerical results are supported by experimental solvatochromic blue shift of the 7t —> Jt transition <2003IJQ572>. 

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... 

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). 

The first point to be addressed is the increase in dipole moment, Apt, on excitation of PRODAN (Formula in Figure 7.4). The determinations of A/x reported in the literature, apart from one, are based on solvatochromic shifts analyzed with the Lippert-Mataga equation. In the original paper by Weber and Farris Apt was estimated to be 20 D, but this value was later recognized to be overestimated and recalculation led to a value of 8 Da) b). Another study yielded a consistent value of 7 Dc) d). A completely different method based on transient dielectric loss measurement provided a somewhat lower value 4.4-5.0 Dd. From all these results, it can be concluded that the increase in dipole moment on excitation is not responsible for the high sensitivity of PRODAN to solvent polarity. 

A thermochromic shift is the displacement of an absorption or emission band with the temperature of the solvent. These displacements result from the change in solvent polarity with temperature, the general rule being that the polarity decreases as the temperature increases. These shifts are small compared with solvatochromic effects and are unlikely to lead to state inversion (Figure 3.52). 

The electronic absorption, fluorescence and excitation spectra of these compounds indicate the presence of an internal charge transfer (ICT) excited state giving rise to a fluorescence band that displays strong solvatochromism. Both the emission wavelengths and the Stokes shifts increase with solvent polarity, in agreement with a large increase in dipole moment in the excited state. As the chain length increases the... 

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