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

The influence of hydrostatic pressure (P) up to 200 bar of gaseous He was investigated on holes burned over the inhomogeneous S-1 <-- S-0 absorption bands of polycyclic hydrocarbons, a polymethine dye, and tetrapyrrolic compounds imbedded in polymer matrices. The pressure shift coefficients d nu/dP show a linear dependence on hole burning frequency (nu) that can be extrapolated 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 nonsolvated chromophores. The dependence of d nu/dP on nu can be considerably steeper than the 2-fold isothermal compressibility of the matrix 2 beta(T), expected for the distance dependence of intermolecular potential r(-6) (e.g. London 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 controversies are rationalized in terms of intermolecular repulsive interactions that have a much steeper distance dependence (r(-12)) than the electrostatic or dispersive forces. The solvent shifts of band maxima, the inhomogeneous bandwidths, and the pressure shifts of spectral holes are discussed in terms of intermolecular interaction mechanisms.