Browsing by Author "Shirakawa, Masayuki"

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Efficient Molecular Aggregation of Rhodamine 6G and Pseudoisocyanine by Light-Induced Force
(2020-05-01)
Shirakawa, Masayuki
;
Kobayashi, Takayoshi
;
Tokunaga, Eiji
;
電子物理學系
;
Department of Electrophysics
Featured Application One can generate by irradiation of light an exotic aggregation state of molecules that cannot exist in thermal equilibrium. Abstract A highly efficient light-induced aggregation of porphyrin molecules in solution was recently reported for 4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid (TPPS). Here, we demonstrate that rhodamine 6G (R6G) and pseudoisocyanine (PIC) also show efficient light-induced aggregation in unsaturated aqueous solution, being detected with a multichannel lock-in amplifier as the absorbance decrease/increase in the monomers/aggregates, induced by a laser at 633 nm, which is non-resonant off their main monomer absorption wavelengths. The light-induced aggregation states are H-aggregates that are hardly formed in the thermal equilibrium. The similar absorbance changes are absent in the monomer molecules fixed in polyvinyl alcohol (PVA) films. The aggregation efficiency defined as (Delta A/A)/(Utrap/kBT), where A is the absorbance of the monomers, Delta A is the absorbance increase in the aggregates, Utrap is the optical gradient force potential, and kBT is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.
Nonlinear Absorption Spectroscopy of Porphyrin J-aggregates in Aqueous Solution: Evidence for Control of Degree of Association by Light-Induced Force
(2017-04-15)
Shirakawa, Masayuki
;
Nakata, Kazuaki
;
Suzuki, Masaya
;
Kobayashi, Takayoshi
;
Tokunaga, Eiji
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.
Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation
(2019-12-01)
Shirakawa, Masayuki
;
Kobayashi, Takayoshi
;
Tokunaga, Eiji
;
電子物理學系
;
Department of Electrophysics
It has been reported that when irradiated with laser light non-resonant with the main absorption peaks, porphyrin molecules (4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid, TPPS) in an aqueous solution become 10,000 to 100,000 times more efficient in light-induced molecular aggregation than expected from the ratio of gradient force potential to the thermal energy of molecules at room temperature. To determine the mechanism of this phenomenon, experiments on the light-induced aggregation of TPPS in alcohol solutions (methanol, ethanol, and butanol) were performed. In these alcohol solutions, the absorbance change was orders of magnitude smaller than in the aqueous solution. Furthermore, it was found that the absorbance change in the aqueous solution tended to be saturated with the increase of the irradiation intensity, but in the ethanol solution, the absorbance change increased linearly. These results can be qualitatively explained by the model in which intermolecular light-induced interactions between molecules within a close distance among randomly distributed molecules in the laser irradiation volume are highly relevant to the signal intensity. However, conventional dipole-dipole interactions, such as the Keesom interaction, are not quantitatively consistent with the results.