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

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 <inline-formula> <math display="inline"> <semantics> <mrow> <mrow> <mo>(</mo> <mrow> <mo>Δ</mo> <mi>A</mi> <mo>/</mo> <mi>A</mi> </mrow> <mo>)</mo> </mrow> <mo>/</mo> <mrow> <mo>(</mo> <mrow> <msub> <mi>U</mi> <mrow> <mi>t</mi> <mi>r</mi> <mi>a</mi> <mi>p</mi> </mrow> </msub> <mo>/</mo> <msub> <mi>k</mi> <mi>B</mi> </msub> <mi>T</mi> </mrow> <mo>)</mo> </mrow> </mrow> </semantics> </math> </inline-formula>, where <inline-formula> <math display="inline"> <semantics> <mi>A</mi> </semantics> </math> </inline-formula> is the absorbance of the monomers, <inline-formula> <math display="inline"> <semantics> <mrow> <mo>Δ</mo> <mi>A</mi> </mrow> </semantics> </math> </inline-formula> is the absorbance increase in the aggregates, <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>U</mi> <mrow> <mi>t</mi> <mi>r</mi> <mi>a</mi> <mi>p</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula> is the optical gradient force potential, and <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>k</mi> <mi>B</mi> </msub> <mi>T</mi> </mrow> </semantics> </math> </inline-formula> is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.