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Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light's energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley–Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon's biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 μs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system's utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion. 

The photophysics of silicon quantum dots (QDs) is not well understood despite their potential for many optoelectronic applications. One of the barriers to the study and widespread adoption of Si QDs is the difficulty in functionalizing their surface, to make, for example, a solution-processable electronically-active colloid. While thermal hydrosilylation of Si QDs is widely used, the high temperature typically needed may trigger undesirable side-effects, like uncontrolled polymerization of the terminal alkene. In this contribution, we show that this high-temperature method for installing aromatic and aliphatic ligands on non-thermal plasma-synthesized Si QDs can be replaced with a low-temperature, radical-initiated hydrosilylation method. Materials prepared via this low-temperature route perform similarly to those created via high-temperature thermal hydrosilylation when used in triplet fusion photon upconversion systems, suggesting the utility of low-temperature, radical-initiated methods for creating Si QDs with a range of functional behavior. 

Photon upconversion may have the highest impact in biological applications because incoming photons transparent to tissue can be combined to make visible light useful for photodynamic therapy and imaging. The ability to use semiconductor nanocrystals as light absorbers for photon upconversion is important because their strong absorption profiles are synthetically tunable. In particular, the use of earth‐abundant, environmentally benign silicon quantum dots (QDs) as light absorbers for photon upconversion is very attractive. In this work, the authors demonstrate a general strategy employing both physical and chemical barriers to achieve air‐stable fusion of triplet excitons photosensitized by silicon QDs, crucial to practical applications of photon upconversion. Gel permeation chromatography (GPC) and dynamic light scattering (DLS) show that thermal hydrosilylation critical for colloidal stability and efficient triplet energy transfer creates a polymeric barrier to oxygen. This kinetic barrier to oxygen arises from the presence of cross‐linked surfactants and is complemented by the sacrificial oxidation of silicon QDs itself. Photon upconversion lasted longer than 4 days with quantum yields (QYs) as high as 7.5% (out of a maximum of 50%) using Si QD light absorbers with diphenylanthracene in methyl oleate. Oil‐in‐water micelles are air‐stable for 2 days with absolute upconversion QYs of 5.5%.