Infrared spectroscopy of conjugated organic molecules under high pressure

Conjugated organic molecules are important components in solid-state optoelectronic devices and laser dyes. Changes in the conformations of these molecules affect the optical emission wavelengths. In order to probe the conformation of conjugated molecules, infrared (IR) spectroscopy was performed on organic solids under high hydrostatic pressures.;We investigated polyphenyl molecules, such as biphenyl, p-terphenyl, and p-quaterphenyl. It was shown that polyphenyl molecules experience a phase transition in which molecules change their conformation from a twisted to a planar structure. This transition manifests itself in dramatic changes in the IR spectra. Beyond a critical pressure, specific IR-active modes abruptly become IR-inactive.;IR spectra have been obtained with a Bomem DA8 Fourier-Transform IR spectrometer in the 500--5000 cm-1 spectral range. Those spectra clearly showed that when the pressure increases above the critical value some IR-peaks completely disappear from the spectrum. The critical pressure was found to range from 0.2 GPa for biphenyl to 0.9 GPa for p-quaterphenyl molecule.;Numerical analysis of conjugated molecules has been done using ab initio calculations. Using the local density approximation, calculations of molecular structure, vibrational frequencies, and IR intensities were performed. The results of these numerical simulations confirmed the experimental observations and allowed us to resolve a controversy about the p-terphenyl structure in the twisted conformation. They also demonstrated that high-pressure IR spectroscopy can be a very sensitive tool in probing molecular structure.;In order to investigate the effect of pressure, as dimensions approach the nano-scale, IR spectroscopy was performed on nm-thick organic films. The special technique called surface-enhanced infrared absorption spectroscopy (SEIRA) has been applied to the investigation of a thin film (30 nm) of p-nitrothiophenol under high pressure. The phenomenon of absorption enhancement in the presence of thin metal film has been used to obtain the absorption spectra of thin layer of p-nitrothiophenol.