Pump–probe spectroscopy and circular dichroism of nanocrystalline benzophenone—towards absolute kinetic measurements in solid state photochemical reactions

Abstract

Nanocrystals suspended in water can be used to record steady state and pump-probe absorption spectra, which should be useful for the study of excited states and reactive intermediates in the solid state.

Photochemical reaction mechanisms and kinetics with molecular nanocrystals: surface quenching of triplet benzophenone nanocrystals

“…After showing that the triplet lifetime remains unaltered with the surfactant‐free samples, all other experiments were performed with nanocrystals without surfactant. In a subsequent set of experiments, and in agreement with our previous study,27 the triplet lifetime is not affected by equilibration with air or oxygen. However, the signals of dissolved benzophenone obtained in benzene solutions submitted to analogous treatment were completely quenched.…”

“…It is well known that long‐lived triplets are susceptible to intensity‐dependent triplet–triplet annihilation that occurs when two delocalized excitations, diffusing by energy transfer, converge into neighboring molecules and interact to form one molecule in the ground state and the other one in the singlet excited state 38. Experiments carried out with laser power values ranging between 0.2 and 5.9 mJ confirmed a linear dependence on laser power, suggesting that only single excitations occur within this intensity range 27…”

“…Indeed, a lifetime that is over three orders of magnitude shorter in the nanocrystals, relative to dilute glasses, coupled with a lowered quantum yield of phosphorescence of ca. Φ ≈ 0.001 strongly suggests an efficient quenching mechanism 27…”

“…Knowing that transmission UV–Vis spectroscopy of molecular nanocrystals suspended in water provides a simple means to assess aggregation states in terms of simple spectral shifts,22–26 we assumed that they may also be ideal for the study of reaction kinetics in the solid state. With this in mind, we set out to investigate the triplet state of benzophenone as a test system 27. We have previously shown that nanocrystalline suspensions of organic solids allow for multi‐gram photochemical reactions,28, 29 and that optically dense suspensions can be used to determine the absolute quantum yield of reactions by chemical actinometry.…”

Experimental aspects of solid state circular dichroism

“…[59][60][61][62] As a caution: conventional stirring of a suspension in a cuvette may create vortices with induced supramolecular optical activity as pointed out recently. [63][64][65][66] Another recent application deals with nanocrystalline benzophenone 67 where CD spectra in Nujol mull and in suspension are compared, the experimental part is however not well described.…”