Dependence of Photothermal Rate Enhancements of Urethane Formation upon the Nature of Light Exposure and Solution Viscosity

The photothermal effect of nanoparticles naturally results in nanometer scale heat sources. Despite their highly localized nature, these heat sources can be used to drive bulk scale chemical transformations. However, due to the localization in both time and space, it is reasonable to expect that the time scale of heating, as well as transport of reactive species into and out of the heated volumes during this time, might play a role in determining the efficacy of photothermal heating for driving chemical reactions. Herein, we report an investigation into these effects for the reaction between hexamethylene diisocyanate and a series of alcohols to form urethane bonds. The length of photothermal heating is controlled via the duration of light exposure, using either a modulated continuous wave (2 min duty cycle) or a nanosecond pulse (8 ns pulses) laser that deliver nearly the same total energy to the system. Mass transport is controlled by changing the alcohol from butanol to butanediol to a polyester diol, resulting in reaction mixtures that change their viscosity from 1.66 to 206 cSt. We use infrared spectroscopy to follow the urethane production and associated isocyanate consumption. We then fit the course of the reaction to a kinetic model from which we extract rate constants used to quantify the degree of photothermal rate enhancement. For the chemical systems used, we find no significant dependence on viscosity. We also find that, for the light sources used, the average rate enhancement is not significantly affected by the length of light exposure, but the rate of the reaction during the time of exposure increases with larger instantaneous power.