Charge Tunneling and Exciton Dissociation in Photochromic Molecule Bridged Quantum Dot Systems

The inherent limitation of electronic-based computational systems has motivated many to pursue the development of alternative optical-based computing. To achieve functional, high-performance photonic devices, an optical analog to a transistor (an optical switch) must be developed. Photochromic molecules (PCMs) can reversibly change their chemical configuration, and thus their energy levels, between an “open” and a “closed” discrete state upon illumination of certain wavelengths of light. We report a novel material system composed of lead sulfide quantum dots (QDs) bridged with PCMs which exhibits strong, reversible, and tunable optical switching effects. A photoswitch is a device that can reversibly switch between two discrete photoluminescence intensities. Effective and controlled systems with photoswitching properties are desirable in many fields, especially in photonics, where photoswitches can play the role of transistors for optical computing. We construct a NIR photoswitch using charge tunneling between PbS quantum dots through photochromic molecule bridges. By modulating the potential barrier between the QD and PCM through optically induced chemical configuration change of the PCM, we observe dramatic changes in the photoluminescence (PL). We demonstrate a dependence of PL change on the functional group of the diarylethene molecule and the size of the quantum dots. We compare the degree of PL switching to changes in the length and the energy level of the Diarylethene bridge molecules upon configuration change and find that the energy band alignment between the QDs and the PCMs plays a central role in controlling switching. Density Functional Theory (DFT) calculations were also carried out to determine the absorbance levels and estimated energy levels of the PCMs. Further tuning of the PCM’s end group allows for control over the degree of switching. This design for photoswitches offers many advantages for specialized use over other popular mechanisms such as Fourier Resonant Energy Transfer (FRET) or direct charge injection.