Tuning Vibrational Lifetimes by Chemical Substitution and Impact on Plasmon-Assisted Catalysis

Recent experiments indicate that the NO stretch of 4-nitrobenzenethiol (NBT) attached to gold can be selectively excited via plasmonic excitation, catalyzing the decomposition of NBT. However, the effectiveness of catalysis is limited by intramolecular vibrational redistribution, which depletes the population of the excited NO stretch in a few picoseconds. In this study, vibrational lifetimes of the NO stretch are computed quantum mechanically for 20 substituted NBTs, including 10 chemical groups in the meta and ortho positions, attached to plasmonic nanoparticles. Variation in the lifetime of the NO stretch with chemical substitution arises from the tuning of resonances as well as systematic changes in values of anharmonic constants. The lifetime of the NO stretch of NBT depends largely on resonances involving three other modes. Upon substitution, one of those modes shifts far enough in frequency that it no longer affects the NO stretch lifetime. For substituents in the ortho position, new resonances appear that can shorten the lifetime, but most are detuned, and the coupling is weaker for the same substituents in the meta position. The impact of the longer NO stretch lifetimes found, particularly for substituents in the meta position, on enhancing the population of the NO stretch and catalysis is discussed.