Size Dependence of Chiroptical Activity in Colloidal Quantum Dots

The synthesis of chiral penicillamine-capped CdS and CdSe quantum dots (QDs) was adjusted to control the size of the nanoparticles. This, together with size separation, allowed for simultaneous tuning of absorption, circular dichroism (CD), and fluorescence on a wide wavelength range. Band edge transitions were accompanied by circular dichroism peaks which red-shifted together with the increase in particle size. The clear correlation between absorption and CD bands as well as between absorption bands and size in semiconductor QDs was used to derive an experimental scaling law for optical activity. The decrease in the intensity of circular dichroism-to-absorption ratio (dissymmetry) with the increase in particle size was stronger than linear, probably exponential. In addition, strong material type dependence was observed. The CD line shape appeared to be sensitive to the nature of the transition and may thus serve as a tool for sorting out the electronic states of the QDs. Fluorescence-detected circular dichroism (FDCD) was introduced as a new probe of optically active fluorescent nanoparticles. The analysis of the size and material dependence of the chiroptical induction effect leads to the conclusion that it is primarily an electronic interaction effect between the adsorbed chiral molecules and the electron-hole states.