Phenylurethane PDAs

Pertinent experimental details are given in the following section after which are presented the spectra of a phenylurethane series of PDA crystals. The nature of the coupling between the transitions is pursued using these results. The subsequent section presents the low temperature piezomodulation spectrum for PTS. The nature of the vibronic structure in this spectrum and its relation to the electronic structure is discussed. 

Samples were provided by R. R. Chance then of Allied Corporation. The samples were used without further processing. Both the normal incidence specular reflection (15) and piezomodulated reflection experiments are described elsewhere (16). The size and physical character of the first three members of the phenylurethane series of PDA crystals prevented their investigation with our low temperature apparatus. 

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. 

Since other PDAs with urethane sidechains display thermochromism, the phenylurethane series was investigated for this effect. All were found to be thermochromic. Because of the remote location of the hot stage thermometer from the sample, an accurate transition temperature was not determined but a range could be defined. The color change was determined visually. 

The spectral behavior of the PDA phenylurethanes indicates that understanding of the electronic properties of these systems, and PDAs in general, still requires development. Calculations simulating strain on the backbone have not been able to address the observed distribution of intensity amongst the vibronic levels nor the changes in the vibrational frequencies or their distribution (6-9). While a substantial blue shift has been calculated, it is unclear what the strain should do to the shape of the electronic band envelope. The higher energy spectrum has previously been taken to be that of the HT form. 

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