THERMOCHROMISM AND SOLVATOCHROMISM

Marked changes in the UV-visible and CD spectra of PAn/HCSA salts have been widely reported by changing the nature of the organic solvent, the solvatochromism being associated with changes in the PAn backbone conformation/structure. How- 

Thyssen, A. Borgerding, A. Schiltze, J.W. Makromol. Chem. Macromol. Symp. 1987, 8 1423. 

Tanaka, J. Mashita, N. Mizoguchi, J. Kume, K. Synthetic Metals 1989, 29 E175. 

MacDiarmid, A.G. Epstein, A.J. Synthetic Metals 1994, 65 103, and references cited therein. 

Gonzalez, I. Munoz, M.E. Santamaria, A. Pomposo, J.A. Grande, H. Rodriguez-Parra, J. Macromol. Rapid Commun. 2002, 23 659. 

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Azomethine ylides such as 412 react with triafulvenes again by analogy with cyclopropenones. (3 + 2) Cycloaddition of the 1,3-dipole to the CVC2 bond and subsequent loss of C02 produces l,4-dihydro-4-methylene-N-alkyl pyridines 559, which as merocyanines show marked solvatochromic and thermochromic effects260. ... 

Solvatochromism and thermochromism are also characteristic of spirooxazines (Scheme 3) <1994RCB780>. The two forms 116 and 117 are in equilibrium in solution and more polar solvents shift the equilibrium more to the colored, acyclic form 117. Higher temperatures have the same effect for both solid spirooxazines and their solutions. A comprehensive review of spirooxazines <2002RCR893> has a collection of the absorption maxima for a large number of spirooxazines and their colored forms, which have their absorption maxima in the visible range at 480-670 nm. 

Regioregular poly(3-alkylthiophene)s have received a lot of attention, especially because of their high electrical conductivities in the doped state, and because of their unusual solvatochromic and thermochromic behavior . Hence, a lot of research has been focused on clarifying the structure of these materials, both in the solid state and in solution. Today, it is agreed that supramolecular aggregation of polythiophene chains plays an important role in their physical properties. 

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. 

Morley. J.O. Padfield. J. Experimental and computational studies on the solvatochromism and thermochromism of 4-pyridiniophenolates. J. Chem. Soc., Perkin Trans. 2 2002. 1698-1707. 

Hinckley, D.A. Seybold, RG. Borris, D.P. Solvatochromism and thermochromism of rhodamine solutions. Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy 1986,42, 747-754. 

Typical examples are solvatochromism and thermochromism. The former chromism is observed upon dissolution of the polymers in solvent. This effect is caused by the localization of the electronic wave function as a result of the disorder introduced by the coiled (random) conformation [48]. These chromisms depend largely upon solvent quality. Choice of suitable side-groups on the polythiophene backbone brings about interesting chromism in the solid form as well. Copolymerization supplies further variations in the arrangement of the side-groups on the backbone and, hence, produees specific effects. 

J. Sunamoto and T. Hamada, Solvatochromism and Thermochromism of Co(II) complexes solubilized in reversible micelles. Bull. Chem. 

One breakthrough in applications of conjugated polymers was the development of soluble polymers, as in the case of polythiophene [75], by substitution with, for example, alkyl side chains [76,77], The polyalkylthio-phenes exhibit properties such as solvatochromism and thermochromism [78]. A large variety of substituted polythiophenes with various band gaps exist [79]. 

Other means to change chemical potential or redox equilibrium of a CP, and hence its color, include dilution in solvents (when the CPs are somewhat soluble) and change of temperature. These two effects, respectively Solvatochromism and Thermochromism, are treated elsewhere in this book. 

Circular dichroism (CD) is a very sensitive spectroscopic tool for probing chain conformations in optically active polymers. For example, with peptides, CD spectroscopy has been widely employed to estimate the proportion of the chain present as the alternative a-helix, P-sheet and random coil conformations. Following our recent discovery of both electrochemical - and chemicaP i routes to chiral PAn.HCSA and related ring-substituted emeraldine salts, we have employed CD spectroscopy extensively to (a) distinguish between "extended coil" and "compact coil" PAn conformations, (b) probe redox and pH switching in PAn, (c) characterize conformational changes in solvatochromism and thermochromism for PAn, and (d) distinguish unequivocally between the conformations/structures of electrochemically and chemically prepared PAn. Similar valuable information on poly-... 

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