REORIENTATION OF TRANSITION DIPOLES DURING EXCITON RELAXATION IN J-AGGREGATES PROBED BY FLUORESCENCE ANISOTROPY

A negative fluorescence anisotropy is observed in J-aggregates of two thiacarbocyanine dyes in water-ethylene glycol low-temperature glass u pon excitation outside the J-band. The fluorescence anisotropy gradual ly increases from -0.16 upon non-resonant excitation at 532 nm to +0.3 7 upon excitation at 610 nm (near the maximum of the J-band absorption ). The fluorescence anisotropy is found to vary non-monotonically as a function of the emission wavelength within the J-band of the fluoresc ence. The dependence of the anisotropy on excitation frequency can be explained by a chain model with two molecules per unit cell. The trans ition dipole moment of each molecule is estimated to make an angle of 65-70 degrees with the chain direction.