Efficient Molecular Aggregation of Rhodamine 6G and Pseudoisocyanine by Light-Induced Force

Abstract

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 <mml:semantics>(Delta A/A)/(Utrap/kBT)</mml:semantics>, where <mml:semantics>A</mml:semantics> is the absorbance of the monomers, <mml:semantics>Delta A</mml:semantics> is the absorbance increase in the aggregates, <mml:semantics>Utrap</mml:semantics> is the optical gradient force potential, and <mml:semantics>kBT</mml:semantics> is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.