Results for Pentacene in p-Terphenyl

All the investigated pentacene molecules (from sites 0 ) featured a linear and reversible red shift without additional broadening upon pressure increase [35]. The measured shifts Av of the resonance frequency for 3 different pentacene molecules are shown in Fig. 20. Linear fits to the measured data are also plotted. The numbers near the points of molecule M2 (gray circles) indicate the temporal sequence in which the data were collected. The shifts were completely reversible within the resolution of the experiments. The slope of the linear fits for the five investigated pentacene molecules, spectroscopically located in the red of 0, varied from -0.74 to -1.0 MHz/hPa with an average of -0.9 + 0.1 MHz/hPa. 

A similar behavior is observed for terrylene [37. All 35 investigated terrylene molecules (from the sites X2, X3, X4) featured a linear and reversible red shift. A typical study of a single terrylene molecule is shown in Fig. 21. A molecule spectrally located at 578.82 nm at 1.6 K, was investigated at different pressures from 2000 down to 500 hPa. The baseline of the spectra has been shifted vertically, proportional 

In the framework of solvent shift theories, using a Lennard-Jones type potential for the interaction of a nonpolar dye in a nonpolar matrix, Sesselman et al. [34] developed a simple theory for the interpretation of hole-burning data in the low pressure range ( 20 MPa). In this regime, die pressure shift Av varies linearly with the solvent shift Avs, i.e. the difference between the molecular absorption frequency in the matrix and in vacuum, the local hydrostatic compressibility k and the pressure change Ap  

Using Eq. (10), the measured average shift of -0.9 MHz/hPa and a solvent shift for a pentacene molecule in the center of Oj of -1745 cm (the So( Aig) — Si( B2u) absorption of pentacene in a supersonic jet is at 18628 cm [38]), a compressibility K = 0.086 0.009 GPa is obtained. This value is about hdf of the compressibility of polymers (like polyethylene, polystyrene) at low temperatures. Sesselman et al. [34] measured a spectral shift for pentacene in polymethylmethacrylate (PMMA) of -0.33 + 0.02 cm /MPa ( —0.99 MHz/hPa) using holeburning. The mechanically measured, low temperature compressibility of PMMA is 1.5 times larger than the compressibility we calculated for p-terphenyl. This difference is approximately compensated by the different solvent shifts of pentacene in the two matrices, resulting in a similar pressure shift in Eq. (10). 

The absorption spectra of terrylene in the gas phase are not known. Using the compressibility calculated from the pentacene data, the solvent shift of terrylene in p-terphenyl is estimated to be Avs = 2345 + 411 cm and therefore its vacuum absorption wavelength should be 509 11 nm. The larger solvent and pressure shifts found for terrylene as compared to pentacene seem reasonable, since terrylene is bigger and therefore expected to exhibit larger polarizabilities in its ground and excited state. 

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