Thermochromic shift

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). 

Figure 3.52 Example of a thermochromic shift of the fluorescence spectrum of an exciplex in a solvent of medium polarity. [The temperature increases from (a) to (d) by 50 °C.]...

When the substituent groups are large (aryl or branched alkyl) normally no long-wavelength thermochromic shift takes place. This behavior is found for arylalkyl-polysilanes, diarylpolysilanes, and (cyclo-HexSiMe) . 

For POD, the peak frequency is close to the 18,800 cm" peak observed for the HT TCDU spectrum. The frequency shift within the phenylurethane series is, therefore, nearly equivalent to that observed when PDAs such as ETCD or TCDU undergo thermochromic or pressure induced changes. A major difference here, however, is the retention of the fairly well defined vibronic structure characteristic of the LT bandshape. Application of the strain hypothesis of the thermochromic shift in PDA spectra would imply that the strain on the polymer spines would increase in the order DDMU DDU < ETCD < HDU < POD < TCDU. This extreme shift of the LT electronic spectra of the phenylurethane substituted PDAs presents a problem, since, if it is strain induced, it has not caused the disappearance of the "fine structure" in the vibronic envelope which occurs when the HT phase is induced by temperature or pressure. However, it has caused essentially the same shift of energy of the pi transition of the spine as observed in the thermochromics. 

POD showed a transition in about the same temperature region as HDU. This sample, however, withered almost immediately after the change. The thermal inertia of the apparatus prevented more refined control of the temperature. DDU displayed a reversible thermochromic shift from 148 to 150 C. It reverted from the HT to the LT form upon cooling. The small amounts of the samples prevented any meaningful DSC measurements. The samples were not checked for hysteresis. 

The shift in the spectral band with temperature (T) in the absorption and emission process results, most commonly, from the temperature dependence of the solvent dielectric constant and the refractive index. The contribution of the solvent effect to the thermochromic shift can therefore be expressed as... 

Ghosh [38,86] has calculated the thermochromic shift of PANI and POT at different temperatures. The exciton peak of PANI was found to show a blue shift almost linearly with increasing temperature in the range 25-55° C. The coefficient of the shift was calculated to be 6.2 cm K for DMF and 11.9 cm K"" for... 

CR Yonker, RD Smith. Thermochromic shifts in supercritical fluids. J Phys Chem 93 1261, 1989. 

R , they could be made soluble in their undoped form, and that they could be doped [264, 566]. P(DiAc)s are typically named on the basis of die substituents R, R used. Some of the most common P(DiAc)s, and several others, are listed in Fig. 13-7 below. Many P(DiAc)s display thermochromism and solvatochromism (color changes based on temperature and solvent composition changes), e.g. yellow to red or blue. [567]. Indeed a thermochromic shift in Raman spectra has been observed. Photocurrent behavior in single crystals of PTS and MADF, two P(DiAc)s witli different substituent groups (Fig. 13-7 is very different [568], demonstrating the strong influence of the substituent groups in this class of CPs. 

The sohd-state, transition-metal example in Table 1 of [(CH2)2NH2]3CuCl illustrates another form of thermochromism the color shifts gradually and continuously because of changes in bandwidth with either heating or cooling (6). It is not unique, as this behavior has been mentioned for the class of... 

Hahde complexes of Cu with nitrogen base ligands are known to exhibit another form of reversible spectral change known as fluorescence thermochromism. The example of Cu4l4(Py)4 from Table 1 is typical and shows red shifting ia the visible emission spectmm while the sample is both cooled and irradiated with a 364 nm ultraviolet source (7). 

Some cyan dyes derived from both naphthols and phenols are reported to show thermochromism, a reversible shift in the dye hue as a function of temperature. This can occur in a negative while prints are being made (65). 

The effects of temperature on the color development of the porous film in chlorobenzene were shown in Table 6 [23]. The coloration was reversible thermochromism. The refractive index of the materials generally decreases as the temperature increases, and the temperature dependence of the liquid is greater than that of the solid. For example, the temperature dependence (A/id/°C) of PVA and chlorobenzene was found to be 3.0 x 10 and 4.5 x 10" at 589.3 nm. Consequently, it is interpreted that the wavelength of the crosspoint between the dispersion curves of PVA and chlorobenzene shifts from the long side to the short side with increasing tem-... 

Experimental results corroborate that shifts of 1.2 eV are always present if any of the variables acting on the electrochemical process are changed the solvent, the salt, or the temperature of work. We cannot attribute the observed shift to solvatochromic, counter-ion-chromic, or thermochromic effects taking place inside the film during oxidation-reduction processes. So, as predicted, these shifts are a consequence of the way the chains store or relax energy through conformational changes stimulated by electrochemical oxidation or reduction, respectively. 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

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. 

Specific properties of polysilanes have been linked to the method of synthesis.35 For example, in the case of anionic polymerization of poly[l-(6-methoxy-hexyl)-l,2,3-trimethyldisilanylene] a new type of chromism was induced in the polysilane film by the difference in the surface properties of substrates and was termed a surface-mediated chromism. The polysilane exhibited thermochromism with an absorption maximum at 306 nm at 23°C, but <15°C a band at 328 nm began to appear. A monolayer of the polysilane was transferred onto both a clean hydrophilic quartz plate and a hydrophobic one treated with hexamethyldisilazane by the vertical dipping method. With the hydrophobic plate, a broad UV absorption at 306 nm is obtained, whereas the absorption on a hydrophilic plate shifts to 322 nm. The conformation of the polysilane is preserved by hydrogen bonding between the silica surface and the ether section of the substituent on the hydrophilic plate. The polysilane is attached to the hydrophobic surface only by van der Waals forces, and this weaker interaction would not sustain the thermodynamically unstable conformational state that is attained on the water surface. 

It is interesting that the emission of the complex is thermochromic and thus the temperature influences emission energies and lifetimes. For instance, two bands at 575 and 518 nm appear at 5 K, but at 300 K the former disappear when temperature increases. This was attributed to luminescent traps. In contrast, the most energetic band shifts to blue under the same conditions it appears at 509 at 40 K and at 483 nm at 360 K. 

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