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 dν/d P show a linear dependence on hole burning frequency (ν) that can be extrapolated to the frequency ν 0( P ) where no pressure shift occurs. The ν 0( P ) values deviate significantly from the actual 0−0 origins of the nonsolvated chromophores. The dependence of dν/d P on ν can be considerably steeper than the 2-fold isothermal compressibility of the matrix 2β 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 2β T ( 1 / 3 β T and 2 / 3 β 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.