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 upon excitation outside the J-band. The fluorescence anisotropy gradually increases from -0.16 upon non-resonant excitation at 532 nm to +0.37 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 fluorescence. The dependence of the anisotropy on excitation frequency can be explained by a chain model with two molecules per unit cell. The transition dipole moment of each molecule is estimated to make an angle of 65-70 degrees with the chain direction.