Akshay Rao

Photon upconversion in lanthanide-doped nanoparticles provides an ultra-large anti-Stokes shift crucial for biomedical and photonic applications, but conventional architectures are typically limited to excitation wavelengths below 1700 nm. Here, we report efficient, direct 2 μm shortwave infrared (SWIR) upconversion to visible red light (640 nm) using self-sensitized holmium-doped core–shell nanoparticles. By exploiting the long-lived 5I7 and 5I6 intermediate energy states of Ho3+, we achieve high-order 4-photon upconversion with a relative upconversion-to-downconversion power conversion efficiency ratio of 13 ± 6%, approaching half the theoretical maximum for a 4-photon process. Crucially, we unveil two distinct operating regimes dictated by spatial boundary conditions, high-efficiency energy transfer upconversion (ETU) in well-passivated core–shell nanoparticles, and a previously unreported surface-defect-mediated photon-looping/avalanche regime in core-only nanoparticles. Finally, we demonstrate the utility of these optimized core–shell nanoconverters for high-contrast, high-resolution SWIR upconversion bioimaging through highly scattering media using low-cost silicon-based detection.

Triplet excitons possess unique characteristics of long lifetimes and fission (singlet fission, SF)–fusion (triplet–triplet annihilation, TTA) features and hence have attracted considerable attention. Because the singlet–triplet transition is spin forbidden, leading to triplet states being optically “dark”, the investigation of triplet features is usually not easy. The existence of triplet excimer-like excited states (T1 + S0) has been debated over the past four decades. Among TTA and SF materials, bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac) is a prominent candidate since its highly efficient TTA has been demonstrated. However, because photophysical studies of TIPS-Ac are relatively nascent, its characteristics have remained elusive. Herein, we shed light on a novel feature of a triplet related excited state, which we assign as a triplet excimer of TIPS-Ac, in solution using pump–probe transient absorption (TA). This triplet excimer feature in the TA spectrum is observed as a photoinduced absorption (PIA) peak at around 700 nm, following isolated triplet PIA, with triplet sensitization and triplet generation via bimolecular interaction converting singlet to triplet excitons (e.g., SF). Moreover, the kinetics of TTA with the triplet sensitized TIPS-Ac in solution, combined with the triplet excimer feature, suggests that dissociation of the triplet excimer reproduces isolated triplets (T1) at room temperature, allowing for a longer existence of triplet excitons in the system by circumventing the consumption of triplet excitons via TTA (viz., triplet reservoir effects), which could be useful for the long-distance transportation of triplet excitons. This work provides valuable guidance for further advancement of the triplet related reactions, including TTA, SF, and beyond.

Although graphite is the most widely used negative electrode material in lithium-ion batteries ¹ , its lithium insertion processes and associated dynamics, particularly those of the dilute stages, remain poorly understood. A fundamental understanding of how symmetry-breaking phase transitions occur continuously under operating conditions is lacking. Here, using operando optical microscopy, we provide a unified picture of ion intercalation dynamics during the dilute stages of graphite intercalation, showing that the graphitic particles undergo rapid, localized deintercalation–intercalation step events, leading to deintercalation–intercalation of micrometre-sized regions within seconds. These are reminiscent of a phase-transition phenomenon, ‘avalanches’, which occurs in disordered materials, involving step changes in the order parameter due to jumps between multiple metastable states ^2,3 . Using a modified random field Ising model, the avalanches are related to static disorder, which disrupts intercalation dynamics. The model can also account for the apparently continuous transitions between stages and the experimental avalanche statistics. Finally, we develop a methodology to spatio-temporally analyse the sequences of avalanche events, revealing considerable heterogeneous connectivity. Our work highlights the role of local and static disorder in explaining unexpected phase-transition behaviour and provides new tools and concepts for studying layered battery materials.

Organic semiconductors continue to make substantial performance gains from photovoltaics to electronics. However, understanding how differences in solid-state structure give rise to large differences in energy transport properties remains unresolved. We report that microcrystals of two perylene diimide (PDI) derivatives differing only in their terminal groups [cyclohexyl (CH) and 4-heptyl (ST)] have exciton diffusion coefficients differing by more than two orders of magnitude. Applying state-of-the-art techniques for microcrystal structure determination, we report the crystal structures of CH-PDI and two polymorphs of ST-PDI. Scanning electron diffraction reveals a range of crystallographic defects in ST-PDI microcrystals, attributed to polymorph intergrowths, while electron energy loss spectroscopy links these defects to nanoscale electronic structure changes. Computational modeling demonstrates that rotational disorder explains the difference in exciton diffusion coefficients. Our observations establish the importance of defect-induced orientational disorder as a source of extrinsic energetic disorder, highlighting the need for defect management in organic semiconductor technologies.

Organic triplet excitons are of great interest for applications in optoelectronics, photochemistry and theranostics. Due to spin-selection rules, triplets are ‘dark states’, rendering direct photoexcitation from the ground state and efficient phosphorescent emission nearly impossible. Overcoming these spin-dependent limitations is a long-standing challenge. Here we report a universal method to brighten organic triplet excitons by attaching chromophores onto the surface of lanthanide-doped nanocrystals, enabling spin-exchange coupling between the unpaired spins of lanthanide ions and organic molecules. This allows direct photoexcitation of the organic triplets, and room-temperature, nanosecond-timescale, oxygen-insensitive phosphorescence in both solution and film under ambient conditions. Different organic chromophores and lanthanide ions are combined to obtain phosphorescence in the visible and near-infrared range. Compared with common organic phosphorescence, which only exists in crystals or at low temperature, the triplet emission established here does not require crystallization, low temperatures or an inert atmosphere. Our approach could open avenues to the application of room-temperature organic phosphorescence in optoelectronic devices and biological labelling and imaging.

Singlet fission enables the generation of spin-entangled triplet pairs and has recently emerged as a promising route to generate quintet multiexcitons for quantum technologies. Despite this promise, a key challenge remains: the reliable control of quintet spin-sublevel populations, which is crucial for quantum information and sensing applications. With the aim of addressing this, we study intramolecular singlet fission in two oligomers, with a pair of tetracene and pentacene derivatives respectively, bridged by an anthracene unit. The tetracene oligomer has closer HOMO-HOMO and LUMO-LUMO energy alignment between chromophore and bridging unit compared to the pentacene oligomer, a property known as "bridge resonance", which leads to stronger intertriplet exchange coupling, and hence different quintet formation mechanisms. We employ continuous-microwave and pulsed electron spin resonance spectroscopy to probe quintet spin-sublevel populations in these oligomers. Our findings confirm that the tetracene oligomer undergoes predominantly strong-exchange quintet formation, yielding well-defined, reproducible spin-sublevel populations. On the other hand, the pentacene oligomer's weak-exchange quintet formation result in more disordered and less-predictable sublevel populations. These results demonstrate that the tuning of bridge resonance offers a strategy for controlling spin dynamics in singlet fission systems, paving the way for engineered multiexciton states tailored for quantum computing and spintronic devices.

Postchemical treatment is a major strategy for improving the optical and electronic performance of two-dimensional transition-metal dichalcogenides (TMDs), where surface defects and unintentional doping limit radiative recombination. Despite its broad application, the passivation dynamics in both time and space remain insufficiently understood. Here, we employ real-time, spatially resolved photoluminescence (PL) imaging to track the evolution of PL emission from monolayer MoS₂ during the Li-TFSI treatment. The PL intensity increases gradually over 4.7 min, indicating reaction-limited passivation kinetics. At the same time, the spatial variation in PL becomes more pronounced, revealing position-dependent reactivity and heterogeneous treatment across the flake. Pixel-level PL statistics compared with noise-based simulations confirm that this broadening originates from the intrinsic spatial heterogeneity of the material. These results demonstrate that chemical treatment enhances radiative recombination while simultaneously amplifying pre-existing spatial differences, establishing real-time PL imaging as a sensitive approach to probing heterogeneous surface reactions in 2D semiconductors.

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.

Quantum dot (QD)-organic hybrid materials offer promising spectral conversion platforms for applications such as photoredox catalysis, solar energy conversion, 3D printing, and bioimaging. In spectral upconversion, the overall triplet energy transfer (TET) efficiency remains limited due to inefficient secondary TET that occurs beyond the QD core, i.e., from triplet mediators to annihilators (TET2). Here, we reveal that the hydrocarbon ligands on nanoparticles can remotely govern this external TET2 that occurs entirely outside the core. By shrinking the native oleate ligand shell on PbSe QD sensitizers before attaching triplet mediator ligands, the NIR-to-visible upconversion performance can be significantly improved. Transient absorption spectroscopy confirms that the more compact ligand shell substantially accelerates TET2, boosting the highest transfer efficiency from 59.4% to 93.5%. We propose that the enhanced TET2 stems from shortened mediator-annihilator distances induced by reduced steric hindrance from the shorter, proximal hydrocarbon ligands, as confirmed by molecular dynamics simulations. The strategy proves versatile across multiple upconversion systems, including solid-state films, CdSe QD-based green-to-blue systems, and lanthanide-doped nanoparticle-sensitized hybrids. Furthermore, the same principle remains applicable to molecular singlet fission-based downconversion using QD as photon emitters, raising the highest photon-multiplication efficiency from 132% to 163%. Our work demonstrates that ligand shell proximity can remotely tune TET beyond the nanoparticle core, providing a general route to optimize inorganic-organic hybrid spectral up- and downconverters.

Excited-state charge transfer underpins organic photovoltaics, photocatalysis and photodetection, but is traditionally thought to require large energy offsets and strong donor–acceptor coupling that can limit device performance. Here, we investigate through-space polymer non-fullerene-acceptor based model heterojunctions in which a perylene diimide acceptor is covalently tethered to a low-bandgap polymer donor. These systems feature an exceptionally small energy offset (< 100 meV) between frontier orbitals, with weak donor–acceptor coupling in the Franck–Condon region. We nevertheless achieve a charge-transfer timescale of ~18 fs. This ultrafast charge-transfer is accompanied via the launch of coherent wavepackets along a high-frequency vibrational coordinate (26 fs period) on the non-fullerene acceptor’s potential energy surface. We uncover specific polymer-centered driving vibrational modes that enable such rapid charge-transfer rates, by mixing Frenkel exciton and charge-transfer states following photoexcitation. Our results demonstrate that ultrafast charge-transfer can be achieved—ultimately limited by high-frequency vibrational periods—even in the absence of large energy offsets or strong ground-state coupling.