Nonlinear Absorption Spectroscopy of Porphyrin J-aggregates in Aqueous Solution: Evidence for Control of Degree of Association by Light-Induced Force

Abstract

Spectroscopic evidence was obtained for molecular aggregation states to be controlled by the irradiation of light, which is off-resonant below the peak absorption energies of both monomers and well-grown J-aggregates. In low (undersaturated)-concentration aqueous solutions of porphyrin molecules (tetraphenyl porphyrin tetrasulfonic acid; TPPS) where the monomer absorbance dominates, irradiation with a 532nm laser induces a decrease in the monomer absorbance and an increase in the aggregate absorbance. The increase in the absorbance of J-aggregates occurs in a broad spectral range associated with the increase in the number of not only variously sized oligomer aggregates but also aggregates structurally different from well-grown stable J-aggregates. In high-concentration solutions where the J-aggregate absorbance dominates, a blue shift of the absorption peak of J-aggregates is induced at the same 532nm irradiation, corresponding to a decrease in the aggregation number or in the association energy. By contrast, for spincoated polymer films of monomers and J-aggregates where molecules are immobile, these features are not observed. It is remarkable that the gradient force potential is smaller by more than seven orders of magnitude than the kinetic energy of the thermal motion of the molecule at room temperature, but the absorption change in solution indicating the increase in the number of aggregates is as large as Delta A similar to 10(-3) in magnitude.