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 ( Δ A / A ) / ( U t r a p / k B T ) , where A is the absorbance of the monomers, Δ A is the absorbance increase in the aggregates, U t r a p is the optical gradient force potential, and k B T is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.
Highlights
The molecular aggregation is a complex phenomenon because aggregate formation is affected by various factors such as molecular concentration [1], pH of solution [2], temperature [3], etc
We previously reported molecular aggregation induced by continuous wave laser irradiation, which is non-resonant from the main absorption peak of porphyrin molecules, 4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid (TPPS) [38,39]
10shorter-wavelength side of the absorption byresonance irradiationbyofirradiation the cw laser where the light-induced force wavelength side of theresonance absorption ofat the cwnm, laser at 633 nm, where the lightshould act on the molecule as an attractive force if it is dominated by the gradient force
Summary
The molecular aggregation is a complex phenomenon because aggregate formation is affected by various factors such as molecular concentration [1], pH of solution [2], temperature [3], etc. There is a collaborative effect among the molecular hydrophobicity or hydrophilicity, and solute-solvent and intra-molecular hydrogen bonding [4,5,6]. This causes various phenomena relevant to aggregation. “J-aggregates” is one of the molecular aggregation phenomena that is accompanied by the red shift in molecular absorption peaks [7,8]. A J-aggregate was first reported by Jelley [9] as a narrow molecular absorption band associated with fluorescence of slightly longer wavelength and named after E.
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