Abstract
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.
Highlights
Since optical trapping and manipulation of a micro-meter size particle were achieved by A
The irradiation intensity dependence of the absorption change spectra was compared between an aqueous solution and ethanol solution
The absorbance change per TPPS concentration grew larger by one order of magnitude than in the alcohol solutions
Summary
Since optical trapping and manipulation of a micro-meter size particle were achieved by A. There have been reports on the possibility of single molecular trapping at room temperature [10,11] and a manipulation method of single molecules with resonant laser irradiation has been theoretically proposed [12,13,14], stable trapping and manipulation of 1-nm-sized molecules at room temperature via light-gradient force is challenging This is because the optical trapping potential ((1/2)αF2 ) of molecules due to the light-induced force (gradient force) is much smaller than the kinetic energy of the thermal motion of the molecules (KB T) at room temperature due to the tiny polarizability volume for α of the molecules. Recent research has focused on enhancing the laser-induced force
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