engleski

VIBRATIONAL SPECTRA AND DFT CALCULATIONS OF PPV-OLIGOMERS

Poly(p-phenylenevinylenes) (PPVs) are known as potential materials for application in electroluminiscence devices. The first three members of the PPV-oligomer family were synthesized and their infrared and Raman spectra recorded. These are: 1,4-distyrylbenzene (DSB), p, p'-distyrylstilbene (DSS) and (1,4-distyryl-4',4''-distyryl)benzene (DDB). The structure of the compounds is not known due to their very poor solubility (for NMR measurements) and nonavailability of single crystals (for X-ray analysis). Molecular geometries were optimized by the HF method with the 6-31G* basis set, giving symmetries belonging to point groups Cs, C2 and C1 for DSB, DSS and DDB, respectively. Calculations of vibrational spectra, including IR intensities were carried out using the DFT method (6-31G* basis set). Vibrational wavenumbers, potential energy distributions and IR intensities are in a fair agreement with our own experimental spectra. The results are compared with the very elaborate calculation of the vibrational spectrum of trans-stilbene (tSB). Whereas in tSB the olefinic C=C stretching is strongly coupled with the C-H in-plane deformation, in the three PPV-oligomers a trend is pronounced towards a "pure" C=C stretching mode.

vibrational analysis; DFT; PPV

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