Thermochromic behavior

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

When COMC II (1995) was published, two general types of thermochromism (reversible) had been recognized for polysilanes on lowering the temperature a gradual bathochromic transition, and an abrupt transition to longer wavelength absorption. From the vast amount of experimental data now accumulated, five different types of thermochromic behavior on cooling can be characterized ... 

Cu(II) Complexes with Diethylethylenediamine. The complexes [Cu(dieten)2]X2, where X is an anion and dieten is the bivalent ligand N, V-diethylethylendiamine, well illustrate the effect of the medium on the extent of cooperativity of the transformation. These systems present interesting properties, and they have been investigated extensively [498-503]. When X is BF4, CIO4, or NO3 the complexes have thermochromic behavior and the color changes from red, at low temperature, to blue in the high-temperature form [499, 504]. 

In solution [M(CO)5 Se—C(Aryl)H ] complexes show a thermochromic behavior. The color of the compounds is caused by a MLCT of d electrons into the LUMO localized mainly in the Se = C(Aryl)H ligand. In rf complexes this transition is at considerably lower energy than in if complexes. The observed color of solutions of [M(CO)5 Se = C(Aryl)H ] therefore depends on the rflif equilibrium, which is temprature dependent. Thus, solutions of, e.g., [W(CO)5 Se = C(Ph)H ] are blue at room temperature and green at -78°C.45... 

An interesting material with both electro- and thermochromism behavior, Lix V02 was evaluated for a "smart window" application (25). Films of Li V02 were prepared by reactive sputtering and annealing an electrolyte of LiQ04 and propylene carbonate. 

With this new information in mind, an interpretation of the thermochromic behavior of polysilanes in solution can be outlined. We can begin with two calibration points (1) The polymers ( -butyl2Si) and ( -pentyl2Si) absorb at 315 nm both in the solid state and in solution both polymers as solids are known to have an all-D conformation, with co 154°. (2) The well-studied polymer ( -hexyl2 Si) , in its low-temperature phase, absorbs at 375 nm and has a (nearly) all-A conformation. Values of A,nax between 315 and 375 nm should correspond to intermediate values of the average torsional angle, ox... 

Following the pioneering work of Hirshberg,[421 the photochromism of spiropyrans has been extensively studied.[43] The photochromic and thermochromic behavior of this class of compounds is due to the interconversion of the closed spiropyran form to the open merocyanine form (Scheme 15). UV irradiation of the closed form 28 results in ring-opening to the zwitterionic form 29, which reverts to the closed form either thermally or on irradiation with visible light. 

The thermochromic behavior, first observed for the classical disilene 9, has also been found for other disilenes. Thus, the Z- and E-forms of 25 exhibit a darkening of their colors with increasing temperature. In the case of some of the tetrasilyldisilenes, this even occurs in solution at room temperature39. 

FIGURE 2. Thermochromic behavior of some polysilanes in hexane solution (a) ( -HexSiMe)n (b) ( -PrSiHexn)n (c) [(n-Hex)2Si]n. Reprinted with permission from Reference 23. Copyright 1999 American Chemical Society... 

In the light of these new findings, an interpretation of the thermochromic behavior of polysilanes in solution can be offered. In Type 1 behavior, gradual ordering of the polysilane chain into a strongly-twisted helical form takes place as the temperature is lowered. The absorption wavelength of ca 315 nm is close to that of the known all-D helices in... 

No diarsines or diphosphines show thermochromic behavior similar to that of the distibines and dibismuthines. For example, in the series of dipnictogen compounds A, B, and C illustrated in Scheme 6, the thermochromic distibines and dibismuthines correspond to nonthermochromic diarsines and diphosphines. Structural data are available to compare the three diarsines 51 (46), 52 (57), and 53 (5S) with the corresponding distibines 22 (25,26), 36 (37), and 30 (22). Diarsines 52 and 53 crystallize in gauche conformations as opposed to the trans-staggered conformation of the distibines. In neither case are there intermolecular As---As contacts shorter than 4 A. However, the lack of conformational correspondence makes any comparison tenuous. 

For the sake of completeness, nickel complexes with bis(thiosquaramide) ligands such as [Ni(sq— S4XI2 (sq = squaric acid) (45) are to be noted. They exhibit thermochromic behavior in pyridine solution. Their dinuclear structure is concluded from elemental analyses and spectroscopic data. Complexes such as [Ni(S20—H)2] serve as stabilizers in color photography (46a). They are assumed to exhibit six-coordinate Ni(II) centers. 

J. F. Verrill and F. Malkin, Thermochromic Behavior of the Ceramic Colour Standard, paper presented at the AIC Silver Jubilee Meeting, Princeton, NJ, June 23-24,1992. 

Several research groups are currently investigating the inherent thermochromism of various polymers. Certain polysiloxanes exhibit reversible thermochromic activity [59, 60]. The thermochromic behavior of these macromolecules is due to order-disorder conformational changes that accompany a particular temperature change. This transition perturbs the electron delocalization of the silicone backbone and results in a shift in the absorption maxima in the UV-visible range. 

Tong X, Cui L, Zhao Y. (2004) Confinement effects on photoahgnment, photochemical phase transition and thermochromic behavior of liquid crystahine azobenzene-containing diblock copolymers. Macromolecules 37 3101-3112... 

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. 

Order-Disorder Transitions. General Features, Experimental data are summarized in Table II, and representative thermochromic behaviors are shown in Figure 2. For the dialkyl-substituted polysilylenes the transition is very sharp, with a barely discernible coexistence region and an approximate isosbestic point. On the other hand, the asymmetrically substituted polymers, except poly(n-dodecylmethylsilylene), display very smooth behavior only in n-hexane solution and a broad but clearly discernible transition in dilute toluene solution. The transition width (ATc) in toluene solution was taken to be the interval between departure from the extrapolated, smooth, high-temperature behavior and the onset of peak absorption wavelength saturation at low temperature. The transition temperature (Tq) is defined arbitrarily as the midpoint of this region. 

Y. Sueishi, M. Ohcho, and N. Nishimura, Kinetic studies of solvent and pressure effects on thermochromic behavior of 6-nitrospiropyran, Bull. Chem. Soc. Jpn. 58, 2608-2613 (1985). 

This chapter is mainly devoted to organic compounds for which the observed reversible color changes (coloring and fading reactions) are due to the effect of temperature, exclusively. The thermochromic behavior of inorganics, organo-metallics, macromolecular systems (such as polythiophenes), or supramolecular systems (such as liquid crystals) and of molecular systems for which the observed thermochromism is due to external intervention (association with another species like a metal ion or a proton) or to modification of the medium by a thermal effect (thermosolvatochromism, for instance) are not reviewed in this chapter. 

Krasieva and co-workers21 mentioned the thermochromic behavior of spiropyrans of the dithiolane series (9, 10). These data confirm that the annellation of the benzopyran moiety favors the thermochromic properties of this class of compounds. 

Most of the known spirofindoline-benzoxazines] and spiro[indoline-naphthoxazines] are certainly thermochromic, but the examples of thermochromic behavior described in the literature for spiro[indoline-naphthoxazines] (29— 35),7 14-18 31 32 spirofindoline-phenanthroxazines] (36 and 37).14 17 spiro[indo-line-phenanthrolinooxazines] (38),13 and spiro[oxazepine-naphthoxazines] (39)33 are few in number. 

The lone pair of the imino nitrogen atom is not conjugated with the pyridine ring, and, as a consequence, the hydrogen bond between the hydroxyl group and the imino nitrogen is strengthened [the mean N-H(-0 distance is 1.8A)]. Therefore, the thermochromic behavior of this class of compounds can be interpreted as due to a shift in the tautomeric equilibrium (Scheme 16). 

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