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Thermally activated photophysical processes are ubiquitous in numerous organic and metal-organic molecules, leading to chromophores with excited-state properties that can be considered an equilibrium mixture of the available low-lying states. Relative populations of the equilibrated states are governed by temperature. Such molecules have been devised as high quantum yield emitters in modern organic light-emitting diode technology and for deterministic excited-state lifetime control to enhance chemical reactivity in solar energy conversion and photocatalytic schemes. The recent discovery of thermally activated photophysics at CdSe nanocrystal-molecule interfaces enables a new paradigm wherein molecule-quantum dot constructs are used to systematically generate material with predetermined photophysical response and excited-state properties. Semiconductor nanomaterials feature size-tunable energy level engineering, which considerably expands the purview of thermally activated photophysics beyond what is possible using only molecules. This Perspective is intended to provide a nonexhaustive overview of the advances that led to the integration of semiconductor quantum dots in thermally activated delayed photoluminescence (TADPL) schemes and to identify important challenges moving into the future. The initial establishment of excited-state lifetime extension utilizing triplet-triplet excited-state equilibria is detailed. Next, advances involving the rational design of molecules composed of both metal-containing and organic-based chromophores that produce the desired TADPL are described. Finally, the recent introduction of semiconductor nanomaterials into hybrid TADPL constructs is discussed, paving the way toward the realization of fine-tuned deterministic control of excited-state decay. It is envisioned that libraries of synthetically facile composites will be broadly deployed as photosensitizers and light emitters for numerous synthetic and optoelectronic applications in the near future.

The generation and transfer of triplet excitons across the molecular-semiconductor interface represents an important technological breakthrough featuring numerous fundamental scientific questions. This contribution demonstrates curious delayed formation of TIPS-pentacene molecular triplet excitons bound on the surface of PbS nanocrystals mediated through the initial production of a proposed charge transfer intermediate following selective excitation of the PbS quantum dots. Ultrafast UV-vis and near-IR transient absorption spectroscopy was used to track the dynamics of the initial PbS exciton quenching as well as time scale of the formation of molecular triplet excited states that persisted for 10 μs on the PbS surface, enabling subsequent energy and electron transfer reactivity. These results provide the pivotal proof-of-concept that PbS nanocrystals absorbing near-IR radiation can ultimately generate molecular triplets on their surfaces through processes distinct from direct Dexter triplet energy transfer. More broadly, this work establishes that small metal chalcogenide semiconductor nanocrystals interfaced with molecular chromophores exhibit behavior reminiscent of supramolecular chemical systems, a potentially impactful concept for nanoscience.

This article proposes the exploitation of widely available, inexpensive, innocuous "green" liquid polyethylene glycol (PEG) polymers containing the oxygen scavenger oleic acid (OA) as promising media for studying oxygen-sensitive photochemical processes. Here we report the successful application of this media to detailed investigations of triplet-sensitized photochemical upconversion, previously established as being readily poisoned by dissolved oxygen. Three different PEG materials were investigated with increasing molecular weight from 200 to 600 g/mol, coded as PEG-200, PEG-400, and PEG-600. These fluidic polymers facilitate an oxygen-depleted environment in comparison to commonly employed organic solvents while providing high solubility and diffusion for the dissolved chromophores. Moreover, the low oxygen permeation afforded by these PEG solvents allows them to remain deoxygenated in open containers under ambient conditions for extended time periods. OA, 9,10-dimethylanthracene (DMA), and 2,5-dimethylfuran (DMF) are shown to efficiently and quantitatively consume dissolved oxygen in the PEG environment in the presence of the photoactivated triplet sensitizer platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP). Oxygen consumption was directly correlated with systematically increasing sensitizer excited-state lifetimes that eventually reach the same plateau as achieved through extensive N2 sparging. Diffusion-controlled bimolecular triplet-triplet energy transfer quenching between PtTPBP and the acceptor/annihilator 9,10-bisphenylethynylanthracene (BPEA) was observed in all three PEG formulations investigated. Subsequent triplet-triplet annihilation, between triplet excited BPEA acceptors, achieves bright and stable upconverted singlet fluorescence from BPEA with no decrease in intensity over 20 h under ambient conditions. In the champion composition (PEG 200), the upconversion quantum efficiency reached 31% under conditions where triplet-triplet annihilation was maximized. This is in stark contrast for the same upconverting pair measured in toluene under ambient conditions, which rapidly decomposes upon exposure to visible light. To illustrate that these PEG compositions could be translated into a suitable solid-state format, these viscous solutions were embedded in a transparent polyurethane polymer shell yielding a flexible and long-term stable upconverting cell that can be manipulated for possible real-world applications. Although the current investigation focused on photochemical upconversion, the oxygen-depleted environments developed here can be utilized to study a plethora of oxygen-intolerant photochemical reactions.