Chemical Society Seminar: Mark Wilson - Nanocrystal-sensitized triplet fusion upconversion photochemistry
Zoom link: https://mcgill.zoom.us/j/86201000272?pwd=QUl4U3F6and6M2VVai9OYzJLUWljUT09
Abstract:
The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for volumetric 3D printing, background-free biomedical imaging, and sensitizing silicon focal plane arrays to the short-wave infrared. Our approach uses colloidal quantum dots to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules.1‒3 Once there, pairs of these long-lived excitations can combine via triplet fusion to generate shorter-wavelength fluorescence.
I will discuss how we are advancing triplet-fusion upconversion by using optical spectroscopy to deepen our understanding of nanocrystal synthesis, and probe the movement of energy and charge within and between organic and inorganic semiconductors. For instance, we uncovered that a pre-nucleation cluster intermediate had historically frustrated efforts to synthesize low-dispersity ensembles of small (d<4 nm) PbS nanocrystals, and showed that basic additives can restore one-step growth and yield markedly narrower heterogeneous linewidths in reactions that run to completion.4 We are expanding from this insight to build mechanistic understanding of the synthesis5 and surface6,7 of metal-chalcogenide nanocrystals.
I will focus on how we harnessed this refined synthesis and used ultra-small (d~1.9 nm, hνpeak,abs=2.0 eV) PbS quantum dots to sensitize ‘red-to-blue’ triplet-fusion upconversion in solution.8 We showed that the long (>µs) photoluminescence lifetimes help to overcome a mildly endothermic sensitization scheme that maximizes the anti-Stokes shift (ΔE=1.04 eV) and minimizes non-specific excitation. This architecture facilitated the photo-initiated polymerization of methylmethacrylate using only long-wavelength light (λ: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry.8 However, this performance may have been unanticipated, because steady-state spectroscopy of these (and other9,10) ultra-small nanocrystals implies an unacceptable loss of incident photon energy through excited-state relaxation and/or structural reorganization. However, from the quasi-equilibrium dynamics of triplet energy transfer that we measure, we infer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of properties for upconversion photochemistry, while reinforcing questions regarding the emissive state.
References:
1. Wu, Congreve, MWBW et al. (Bulovic, Bawendi, Baldo) Nature Photon. 10:31 (2016)
2. Huang et al. (Bardeen, Tang) Nano Lett. 15:5552 (2015)
3. Mongin, et al. (Castellano) Science 351(6271):369-372 (2016
4. Green, et al. (MWBW) Chem. Mater. 32(9):4803–4094 (2020)
5. Yarur Villanueva, et al. (MWBW) ACS Nano (2021) 15(11):18085-18099
6. Green, et al. (MWBW) ACS Appl. Nano. Mater. 4(6):5655–5664 (2021)
7. Green, et al. (MWBW) Chem. Mater. 33(23): 9270-9284
8. Imperiale, et al. (MWBW) Chemical Science. 12(42):14111-14120 (2021)
9. Mooney, et al. (Kambhampati), Phys. Rev. B 87(8):1‒5 (2013)
10. Hasham, et al. (MWBW) Submitted
Bio:
Mark W.B. Wilson (he/him) is an Assistant Professor in the Department of Chemistry at the University of Toronto, where his team uses optical spectroscopy to explore, understand, and develop excitonic and nanostructured materials for optoelectronic applications. His first degrees were in Engineering Physics and History at Queen’s University (Kingston, Canada). He next received a PhD in Physics (2012) from the University of Cambridge for his studies of singlet exciton fission under the supervision of Prof. Sir Richard Friend. Then, as a member of the Centre for Excitonics at the Massachusetts Institute of Technology, he pursued postdoctoral studies (2012-2016) into triplet energy transfer between organic molecules and semiconductor nanocrystals with Prof. Moungi Bawendi (Chemistry) and Prof. Marc Baldo (Electrical Engineering), before starting his independent career.