SPECTRAL HOLE-BURNING AND STARK-EFFECT - FREQUENCY-DEPENDENCE OF THE INDUCED DIPOLE-MOMENT OF A SQUARAINE DYE IN POLYMERS

A study of the effect of an external electric field on spectral holes burnt at different frequencies in the inhomogeneous absorption band of a centrosymmetric squaraine dye, bis[4-(diethylamino)-2-hydroxyphenyl ] squaraine (DEAH), in polymers of different polarity is presented. Av erage matrix induced dipole moment differences of about 1 D and 0.37 D were measured in the directions parallel and perpendicular to the lon g axis of DEAH. In all polymers investigated, the induced dipole momen t difference decreased from the higher to the lower frequencies. Solva tochromic shift measurements were performed in order to elucidate the origin of this effect. The matrix field inducing the dipole moment is also partially responsible for the frequency shift of the absorption o f DEAH. With increasing matrix field, the absorption frequency is shif ted to the blue due to electrostatic interaction with the local dipole s of DEAH. The contribution of the electrostatic interactions to the f requency shift is smaller than the dispersion interactions by two orde rs of magnitude in polystyrene, but increases slightly in more polar p olymers.