Doping effects in p-terphenyl molecular crystals: a study by Raman spectroscopy

The use of doped p-terphenyl crystals as organic charge-transfer agents cal ls for detailed information on their structure. We report results on lattic e (60-150 cm(-1)), torsional (228-230 cm(-1)), inter-ring (1250-1300 cm(-1) ) and intra-ring C-C (1570-1630 cm(-1)) Raman-modes of p-terphenyl in doped crystals with tetracene and terrylene in order to compare them with those of the pure p-terphenyl crystal, looking for possible doping effects upon m olecular conformational changes in the crystal, which greatly influence the excitonic properties. From the studied Raman spectra, we come to the conclusion that within the d oping concentrations under study (less than 0.01%), at corresponding temper atures, there are not any significant spectral changes either in frequencie s or in linewidths of the p-terphenyl Raman spectrum from the pure crystall ine compound to the doped p-terphenyl/tetracene and p-terphenyl/tenylene do ped crystals. The conformational changes in the doped-crystals induced with decreasing temperature seem to be the same as in the undoped crystal, incl uding a non-discontinuous order-disorder phase transition at the same tempe rature. (C) 1999 Elsevier Science B.V. All rights reserved.