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Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation

1
Department of Physics, Faculty of science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
2
Advanced Ultrafast Laser Research Centerand Department of Engineering Science, the University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
3
Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2019, 9(24), 5381; https://doi.org/10.3390/app9245381
Received: 7 October 2019 / Revised: 5 December 2019 / Accepted: 6 December 2019 / Published: 9 December 2019
(This article belongs to the Section Applied Physics)
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. View Full-Text
Keywords: light-induced force; gradient force; J-aggregate; porphyrin; aqueous solution; alcohol solution; TPPS; Frenkel exciton; pump-probe spectroscopy; nonlinear absorption spectroscopy light-induced force; gradient force; J-aggregate; porphyrin; aqueous solution; alcohol solution; TPPS; Frenkel exciton; pump-probe spectroscopy; nonlinear absorption spectroscopy
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Shirakawa, M.; Kobayashi, T.; Tokunaga, E. Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation. Appl. Sci. 2019, 9, 5381.

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