Flexible dimethylsilylene bridges in silicon quantum dot-anthracene adducts promote triplet energy transfer

Triplet energy transfer (TET) underlies key applications in energy storage, conversion, and utilization such as photovoltaics, photon upconversion, singlet fission, and photocatalysis. Fast and long-distance TET is generally desirable in these applications to enhance performance and limit back transfer. However, conventional TET in the weak coupling regime only occurs over short distances between donor and acceptor as Dexter-type electronic coupling for TET decreases exponentially with increasing separation. One way to achieve long-distance TET is to enhance the electronic coupling between donor and acceptor by designing conjugated linking bridges. Here, we reveal three new silicon quantum dot (Si QD):Anthracene hybrid systems with variable-length -[SiMe₂] _n - (n = 2-4) linkers as bridges to promote long-distance TET. Transient absorption experiments and density functional theory calculations show that electronic coupling in each of these four systems is intermediate between non-conjugated ethyl and π-conjugated vinyl bridges. In addition, the TET rates between Si QDs and anthracene facilitated by -[SiMe₂] _n - (n = 1-4) linkers do not show the expected exponential decay trend with increasing separation. Rather we observe an increase in the rate of TET when n is increased from 2 to 3, which we propose arises from greater bridge chain flexibility that opens access to geometries where the anthracene can directly engage the Si QD surface via through-space van der Waals interactions. By controlling the average number of tethered anthracene transmitters, we are able to optimize the performance of Si QD:Anthracene hybrids as photosensitizers for triplet-triplet annihilation photon upconversion, obtaining efficiencies of 6.2 ± 0.4%, 3.4 ± 0.1%, 4.1 ± 0.2% and 3.9 ± 0.1% (out of 100%), respectively for n = 1-4. This work provides insight into the role that electronic coupling plays in hybrid materials to move triplet excitons across semiconductor junctions, which sheds light on material design principles for applications in optoelectronics and photocatalysis.