Daniel G. Congrave

The characterization of thermally activated delayed fluorescence (TADF) relies on detecting delayed emission following triplet formation. However, triplet population under optical excitation is fundamentally limited by the intrinsic intersystem crossing (ISC) efficiency of the emitter and in direct competition with often desirable singlet radiative decay. Here, we implement a general optical strategy to directly populate the triplet states of TADF chromophores in solution via diffusional triplet energy transfer, circumventing reliance on intrinsic ISC. This approach can enhance delayed-fluorescence detectability by up to 3 orders of magnitude across structurally diverse emitters spanning the blue to near-infrared and disparate emission time scales, converting near-threshold experiments into routine measurements. Through decoupling triplet population from ISC, we reveal previously hidden intrinsic TADF in two state-of-the-art chromophores, leading to a striking conclusion: unimolecular TADF remains highly relevant within multiple high-performing material families where delayed fluorescence─and the resulting high device performance─has previously been attributed solely to intermolecular solid-state effects. Accordingly, through providing an approach to directly interrogate the intrinsic triplet state photophysics of TADF materials, triplet sensitization can enable the identification and understanding of previously inaccessible photophysical behavior and property combinations, providing a foundation for future advances in TADF materials research.

Singlet fission (SF) offers a pathway to surpass the Shockley–Queisser limit by converting one photoexcited singlet exciton into two triplet excitons. However, in molecular assemblies, this process commonly yields a strongly exchange-coupled correlated triplet pair, 1(TT), where intertriplet interactions facilitate geminate recombination and back-transfer to the singlet manifold. Here we implement an energy-gradient strategy in linear diphenylhexatriene (DPH) hetero-oligomers to guide postfission dynamics toward triplet-pair separation while minimizing thermodynamic loss. The hetero-oligomer series is constructed from two DPH-based building blocks with matched chromophore frameworks but offset triplet energies: a TIPS-substituted unit (T-DPH) and an alkyl-substituted unit (P-DPH). By positioning T-DPH units at the termini of the hetero-oligomers, we encode a modest downhill triplet-energy gradient (∼60 meV) that drives the migration of the triplet population away from the initially formed 1(TT) configuration toward the chain periphery. Using femtosecond and nanosecond transient absorption spectroscopy, we track the kinetic evolution from the singlet manifold through 1(TT) formation to a spatially separated, weakly exchange-coupled (T··T) state, and ultimately to free triplets. Magnetic-field-dependent transient absorption and photoluminescence measurements provide supporting evidence for population of a weak-exchange triplet-pair regime. Comparison with a homotrimer (PPP-DPH3) lacking energetic asymmetry directly isolates the role of this design lever, demonstrating that a ∼60 meV offset is sufficient to accelerate 1(TT) → (T··T) separation. Thus, this energy-gradient strategy enhances harvestable free-triplet yield while retaining the high triplet energy required for device-relevant SF optoelectronics and provides a simple and transferable design principle for SF materials that favor productive triplet-pair separation over geminate loss.

Bridge–chromophore resonance governs intramolecular singlet fission: weak LE–CT mixing enables correlated triplet pair ( ¹ TT) formation, while strong LE–CT mixing redirects S ₁ into an exciplex, suppressing singlet fission.

To investigate multichromophore phenomena, progress traditionally relies on model covalent dimers for spectroscopic interrogation. Integrating molecular semiconductors into nucleic acid libraries can enable rapid screening of multichromophore phenomena. Here, we report DNA-directed assembly of up to five π-conjugated chromophores that demonstrate charge separation and electronic delocalization phenomena. We integrate a range of porphyrins and perylene diimides (PDIs)─molecular semiconducting materials widely used in organic electronic devices─in DNA, encoding nearest-neighbor assembly through base-sequence programmed hybridization. In this way, we can assemble multicomponent stacks with tailored electronic properties from a central chromophore-DNA library. This allows dimer and multimer production on demand, within hours, from presynthesized DNA-chromophores for spectroscopic analysis. We demonstrate the library's ability to optimize for charge transfer, computationally prescreening for close π-stacking as a proxy for large orbital overlap and exchange energy. Our modular DNA assembly reveals opportunities for rapid development of simple, bespoke chromophore architectures with stoichiometric chromophore control and ordering.

Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have emerged as popular candidates for the development of blue organic light-emitting diodes (OLEDs), offering narrowband emission, high photoluminescence quantum yields (PLQYs), and the ability to upconvert dark triplet states to bright singlet states. However, their planar polycyclic structures promote detrimental intermolecular interactions in the solid-state which diminish the color purity and introduce nonradiative loss pathways. Furthermore, the intrinsic luminescence of many MR-TADF emitters fails to satisfy the stringent color purity standards required for next-generation display technologies. Here, we synthetically address these issues by covalently encapsulating a blue-shifted MR-TADF emitter within a protective macrocyclic ring. We identify a previously undiscovered utility of macrocyclic encapsulation, whereby it can shield the MR core from the surrounding environment to enhance its radiative rate, PLQY, and reverse intersystem crossing (RISC) efficiency. Only with spectrally resolved transient photoluminescence measurements were we able to identify the weakly emissive aggregate and excimer species, and definitively confirm that the macrocycle suppresses their formation in the solid-state, thereby preserving narrowband deep-blue emission and reducing nonradiative losses. Notably, these performance enhancements were achieved without compromising thermal stability or vacuum-processability. When integrated into an OLED device based on the "hyperfluorescent" strategy, this emitter delivers an exceptional combined maximum external quantum efficiency (EQE) of 33% and (0.146, 0.046) CIE_x,y coordinates with peak emission at 451 nm, satisfying BT.2020 blue color requirement, and significantly outperforming its nonencapsulated analogue. This material represents one of the highest efficiency deep-blue OLEDs to date and therefore establishes macrocyclic encapsulation as a powerful synthetic strategy for unlocking the full potential of MR-TADF materials for next-generation OLEDs.

Control of the molecular configuration at the interface of an organic heterojunction is key to the development of efficient optoelectronic devices. Due to the difficulty in characterizing these buried and (probably) disordered heterointerfaces, the interfacial structure in most systems remains a mystery. Here we demonstrate a synthetic strategy to design and control model interfaces, enabling their detailed study in isolation from the bulk material. This is achieved by the synthesis of a polymer in which a non-fullerene acceptor moiety is covalently bonded to a donor polymer backbone using dual alkyl chain links, constraining the acceptor and donor units in a through space co-facial arrangement. The constrained geometry of the acceptor relative to the electron-rich and -poor moieties in the polymer backbone can be tuned to control the kinetics of charge separation and the energy of the resultant charge-transfer state giving insight into factors that govern charge generation at organic heterojunctions.

Lanthanide-doped nanoparticles (LnNPs) possess unique optical properties and are employed in various optoelectronic and bioimaging applications. One fundamental limitation of LnNPs is their low absorption cross-section. This hurdle can be overcome through surface modification with organic chromophores with large absorption cross-sections. Controlling energy transfer from organic molecules to LnNPs is crucial for creating optically bright systems, yet the mechanisms are not well understood. Using pump-probe spectroscopy, we follow singlet energy transfer (SET) and triplet energy transfer (TET) in systems comprising different length 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives coordinated onto ytterbium and neodymium-doped nanoparticles. Photoexcitation of the ligands forms singlet excitons, some of which convert to triplet excitons via intersystem crossing when coordinated to the LnNPs. The triplet generation rate and yield are strongly distance-dependent. Following their generation, TET occurs from the ligands to the LnNPs, exhibiting an exponential distance dependence, independent of solvent polarity, suggesting a concerted Dexter-type process with a damping coefficient of 0.60 Å^-1. Nevertheless, TET occurs with near-unity efficiency for all BPEA derivatives due to the lack of other triplet deactivation pathways and long intrinsic triplet lifetimes. Thus, we find that close coupling is primarily important to ensure efficient triplet generation rather than efficient TET. Although SET is faster, we find its efficiency to be lower and more strongly distance-dependent than the TET efficiency. Our results present the first direct distance-dependent energy transfer measurements in LnNP@organic nanohybrids and establish the advantage of using the triplet manifold to achieve the most efficient energy transfer and best sensitization of LnNPs with π-conjugated ligands.

Open-shell systems with extensive π-conjugation have fascinating properties due to their narrow bandgaps and spin interactions. In this work, we report neutral open-shell di- and polyradical conjugated materials exhibiting intriguing optical and magnetic properties. Our key design advance is the planarized geometry allowing for greater interaction between adjacent spins. This results in absorption and emission in the near infrared at 803 and 1050 nanometers, respectively, and we demonstrate a unique electronic structure where a bright zwitterionic excited state is the lowest-accessible electronic transition. Electron paramagnetic resonance spectroscopy and superconducting quantum interference device measurements reveal that our materials are open-shell singlets with different degrees of spin interactions, dynamics, and antiferromagnetic properties, which likely contributed to the formation of their emissive zwitterionic singlet excited state and near-infrared emission. In addition, our materials show reversible and stable electrochromic switching with more than 500 cycles, indicating their potential for optoelectronic and electrochemical energy storage applications.

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.