The effect of solvent polarity on photo-acoustic spectra of alkyl-cyanobiphenyl derivatives

Photo-acoustic (PA) measurements using the microphone technique were applie d to study the heat generation by nonradiative processes in biphenyl deriva tives solutions. Spectra of 4-ethyl-4'-cyanobiphenyl and 4-penthyl-4'-cyano biphenyl dissolved in polar and non-polar solvents were measured and analys ed by using this technique in addition to optical spectra. The photo-acoust ic spectra obtained were analysed using the one-dimensional PA theory for c ondensed samples. Bathochromic shifts in absorption, luminescence and photo -acoustic spectra, characteristic of the pi-pi* transitions, were observed for both biphenyl derivatives. The PA measurements also indicate that the n onradiative deactivation efficiency for the transition investigated slightl y depends on the solvent's polarity. This fact is discussed in terms of rel axation properties of the excited state.