Thermochromism, reversible poly

The DOUS (density of the valence states) obtained from the calculations was in good agreement with the direct measurements by UPS spectra. By studying temperature-induced changes in the UPS spectra, it can be deduced that the torsion angles of the POPT backbone decrease at elevated temperatures, leading to an increase of crystallinity. This conversion process was found to be irreversible, in contrast to the real thermochromism in poly(3-hexylthio-phene), which is reversible. 

Hirota, N., Hisamatsu, N., Maeda, S., Tsuahara, H., Hyodo, K., 1996. Thermochromism of poly(substituted thiophene) with urethane bond and its application to reversible thermal recording. Synthetic Metals 80, 67. 

Poly(dihexylgermane) and poly(dioctylgermane) are strongly thermo-chromic. For example, a thin film of [nHex2Ge]n absorbs at -11 °C at 370 nm, while at 22 °C the absorption shifts to 337 nm [12], These changes are reversible as a function of temperature. On the other hand, poly(diarylstannane)s do not seem to be thermochromic, although poly(dialkylstannane)s are thermochromic [18-20]. 

Poly(dioctyl)stannane and poly(dihexyl)stannane exhibit a reversible thermochromic behaviour as a result of a phase transition at ca 40 °C. A discolouration of the polymers was observed upon slightly warming above room temperature and variable-temperature UV-vis spectrometry showed a reduction of the >.max values of (n-Oct2Sn) in toluene solution from 384 to 369 nm and of (n-Hex2Sn) from 398 to 382 nm82. On the other hand, however, poly(dibutyl)stannane and poly(diaryl)stannane do not exhibit thermochromic behaviour in the temperature range between —10 to 90 °C and —20 to 90 °C82 100, respectively. 

The poly(3-alkylthiophenes) exhibit a reversible thermochromic change that is due to a transition between low-temperature and high-temperature solid-state structures. The thermochromic mechanism involves the conformation of the alkyl group, which is dependent upon the temperature. At low temperatures the alkyl side chains adopt a fully extended, staggered conformation. As the temperature increases, the population of gauche conformations in the alkyl side chains increases... 

Static and dynamic light scattering measurements have been undertaken on dilute solutions of poly(3-dodecylthiophene) (PDDT) to evaluate its conformation over a range of temperature and to learn whether a reversible thermochromic effect is associated with any conformational change <1996MM933>. 

The poly(dialkyl) stannanes, (Hex2Sn) and (Oct2Sn) exhibit a reversible thermochromic behavior upon warming above room temperature. The A. iax values of these polymers undergo a blue-shift from 384 to 369 nm for poly(di-n-octyl)stannane between 30 0 °C in toluene and from 392 to 382 nm in... 

The reversible nature of thermochromism phenomena distinguishes them from the irreversible phenomena which recently have been reported, for instance in a highly regular poly(3-octylphenyl thiophene) showing an ineversible change of optical absorption when heating the virgin spin-coated film of the polymer [22,23]. While this phenomenon may have the same structural basis [24,25] as reversible thermochromism, we will not dwell further on these phenomena. 

Carpick, R.W., T.M. Mayer, D.Y. Sasaki, and A.R. Bums. 2000. Spectroscopic ellipsometry and fluorescence study of thermochromism in an ultrathin poly(diacetylene) film Reversibility and transition kinetics. Langmuir 16 4639. 

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