Thermo- and Solvatochromism

In a solid film, most of the perimidinespirocyclohexadienones and their derivatives are stable in the la ring-closed form. Upon dissolution in nonpolar solvents, (e.g., hydrocarbons or tetrachloromethane), an equilibrium is gradually established between la and the deeply colored lb quinoneimine isomeric form, whose appearance is detected from the characteristic long-wavelength absorption band in the region of 560-630 nm. Some of the perimidinespirocyclohexadienones that exist in a solid as purely spirocyclic compounds, such as 4 (R, R =-CH2CH2 -), 8,9, fully convert to the quinoneimine form when dissolved 

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The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1 ,3 ,3 -trimethyl-6-nitrospiro[l-benzopyran-2,2 -17/-indole] (BSP) and its photo-induced merocyanine isomer (MC) were investigated in phosphonium based ILs by UV-vis absorption spectroscopy and the kinetics and thermodynamics of the BSPMC equilibrium were found to be sensitive to the nature of the anion. For example, the MC Xmax shifted from 560 nm to 578 nm in solutions of [Me(C4H9)3P][tos] and [Ci4H29(C6Hi3)3P][dca], respectively. The BSP isomer was highly favoured at equilibrium in the ILs studied. 

Section II. Some of these attributes include a pronounced thermo- and solvatochromism, thermotropic and lyotropic liquid crystallinity, and structural self-assembly. 

ABSTRACT. The poly(3-alkylthiophenes) represent a new generation of conducting polymers, that exhibit both solution and melt processability. These properties open new possibilities for the utilization of conducting polymers in practical applications. Fur eimore, the solubility of these polymers enables the characterization of the physical and chemical structure in greater detail than hitherto have been possible. In this chq)ter we give a brief review of the synthetic routes to the poly(3-alkylthiophenes) as well as an overview of some of the recent results on the physical and chemical characterization of this class of materials. In diis overview we include results on stability properties, transport properties, thermo- and solvatochromism. We also discuss some aspects of the utilization of the poly(3-dkyltiiiophenes) in plications. 

The chemistry and applications of the colour change grouping, containing all the well-known isms of chromic phenomena, namely photochromism, thermo-chromism, ionochromism, electrochromism and solvatochromism, as well as the lesser-known ones such as tribochromism and vapochromism, are covered in Chapter 1. These chromic phenomena impinge on our everyday life, e.g. in photo-chromic spectacle lens, thermochromic temperature indicators, fax paper, smart windows and mirrors and in visual displays. 

S.-R Heum, M.-S. Hur, P. M. Kazmaier, and E. Buncel, Thermo- and photochromic dyes Indolino-benzospiropyrans. Part 1. UV-VIS spectroscopic studies of 1, .3,.3-spiro(2 //-1 -benzopyran-2,2 -indolines) and the open-chain merocyanine forms solvatochromism and medium effects on spiro ring formation, Can. J. Chem., 69, 1940-1947 (1991). 

The rationale for preparing this hybrid copolymer was to combine the desirable properties of polyaniline with those of polythiophene. For example, polythiophene has demonstrated thermo- and electrochromism, solvatochromism, luminescence, and photoconductivity while polyaniline has demonstrated reversible protonic dupability, excellent redox re-cyclability, and chemical stability. 

Several 2// -chromcnc derivatives exhibit photo-, thermo- or solvatochromic behavior in solution. Specific structural features of the substituted di-phcnylchro-menes CH1-CH4, the dimethyl chromenes CH5, and the spiro[fluorene-benzopyr-ans] SFB1 and SFB2 have been studied.75 77... 

In conclusion, it can be said that the outstanding sensitivity of the spectral absorption and chemical reactivity of our pyridinium-N-phenoxide betaine dyes to small changes in solvent, temperature, pressure, and substituents makes these dyes a very useful class of compounds. They are not only solvatochromic compounds, but exhibit also the phenomena of thermo and piezo-solvatochromism. Their use for setting up different reaction series in order to get Linear Free-Energy Relationships has been demonstrated by the fact that the same betaine dye can be used not only for the introduction of a spectroscopic solvent polarity scale, the so-called E i-scale, but also for the establishment of kinetic and spectroscopic scales of substituents. 

The triaryl compounds 290 (R = Ar) are prepared by condensation of 2-aminophenol with triarylpyrylium salts followed by treatment with alkali. The triphenyl betaine (290 R = Ph) is obtained as a purple solid, mp 165 C (decomp), which shows large thermo/solvatochromic effects. Oxidation of the betaine 290 (R = Ph) with hydrogen peroxide gives the triphenyl-pyridinium-3-olate 291 (R = Ph) (see Section III,A,2) and the pyrrole 292 (R = Ph). The mechanism of this unusual reaction has not yet been Established. 

The thermo-solvatochromism of (44) can be easily observed by means of a betaine solution in ethanol at —75 °C the solution is red-colored, while at +75 °C it is blue-violet, corresponding to absorption maxima of /l ax = 513 and 568 nm, respectively [73]. The reason for this thermo-solvatochromism is the increased stabilization of the dipolar electronic ground state of (44) relative to the less dipolar excited state with decreasing temperature, due to better solute/solvent interactions at low temperature. It can be stated that, the lower the temperature, the higher the t(30) value. 

Laurence et al. [237, 238] have shown that some of the absorption bands of the solvatochromic indicator dyes previously used by Kamlet and Taft to fix the reference lines exhibit a solvent- and temperature-dependent vibrational fine structure, which makes the determination of the band maxima, v ax, difficult. Furthermore, the indicator dyes used are thermo-solvatochromic, i.e. v ax is temperature-dependent. No allowance for this temperature-dependence of the a, / , and n values has been made previously. 

Abstract - Empirical parameters of solvent polarity seem now-days as useful for the prediction of solvent effects as Hammett s substituent constants for the estimation of substituent effects on chemical reactions. The extreme sensitivity of the position of the long-wavelength UV/VIS absorption band of pyridinium-N-phenoxide betaine dyes not only changes in temperature (thermo-solvatochromism), external pressure (piezo-solvatochromism), and the introduction of substituents, but also to small changes in solvent polarity (negative solvatochromism) has been used to establish a comprehensive empirical scale of solvent polarity, called the E.p-scale. In addition, the influence of solvents and substituents on the chemical reactivity of the pyridinium-N-phenoxides has now also been studied. 

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