Pressure shift mechanisms of spectral holes in the optical spectra of dyesin polymer host matrices

The influence of hydrostatic pressure (P) up to 200 bar of gaseous He was i nvestigated on holes burned over the inhomogeneous S-1 <-- S-0 absorption b ands of polycyclic hydrocarbons, a polymethine dye, and tetrapyrrolic compo unds imbedded in polymer matrices. The pressure shift coefficients d nu/dP show a linear dependence on hole burning frequency (nu) that can be extrapo lated to the frequency nu(0(P)), where no pressure shift occurs. The nu(0(p )), values deviate significantly from the actual 0-0 origins of the nonsolv ated chromophores. The dependence of d nu/dP on nu can be considerably stee per than the 2-fold isothermal compressibility of the matrix 2 beta(T), exp ected for the distance dependence of intermolecular potential r(-6) (e.g. L ondon forces). Other solvent shift mechanisms, such as linear and quadratic Stark effects in the matrix,cavity field, yield lower slope values than 2 beta(T) (1/3 beta(T) and 2/3 beta(T), respectively). Tentatively, these con troversies are rationalized in terms of intermolecular repulsive interactio ns that have a much steeper distance dependence (r(-12)) than the electrost atic or dispersive forces. The solvent shifts of band maxima, the inhomogen eous bandwidths, and the pressure shifts of spectral holes are discussed in terms of intermolecular interaction mechanisms.