Molecular structure dependent convective control in binary solvents: a femtosecond thermal lens study

Using femtosecond time-resolved thermal lens spectroscopy, we demonstrate that the molecular architecture of dyes dictates their interaction strength with binary solvents, thereby controlling convective heat transport. We investigated two structurally distinct dye classes, xanthenes (rhodamine 6G, rhodamine 101) and triarylmethanes (Malachite Green, Brilliant Green), in water-methanol mixtures. Strongly interacting xanthene dyes consistently increased steady-state thermal lens strength across all solvent fractions, which we attribute to dye-solvent cluster formation that enhances local viscosity and suppresses convective fluid motion. Conversely, weakly interacting triarylmethane dyes showed minimal thermal lens enhancement but significantly higher convection amplitudes and signal fluctuations in methanol-dominated regions, indicating they participate freely in solvent flow without forming obstructive clusters. Concentration-dependent measurements (10^-4-10^-3 M) confirm this interaction model. We established a precise mechanism linking molecular structure to dye-solvent coupling strength, which directly modulates convective efficiency and thermal dissipation pathways. This work provides a rational strategy for engineering thermo-optical properties of liquid systems through molecular design.