The two clusters at the heart of this work are chemical cousins with a fascinating structural relationship. M44, formally Au24Ag20 and protected by twenty-four 4-tert-butylphenylacetylide ligands and two chlorides, consists of a hollow icosahedral Au12 kernel wrapped in a fullerene-like Ag20 inner shell and capped by twelve exterior gold atoms. M81, with the formula Au43Ag38, is essentially a dimer of M44: two Au12 units fuse through an additional gold atom to form an Au25 kernel, while the two Ag20 shells merge with the loss of two silver atoms to create an Ag38 shell, all further wrapped by eighteen gold atoms and ligands. This face-fusion architecture is distinct from the side-on biicosahedral fusion seen in related clusters such as Au20Ag32 and Au23. Kohn-Sham molecular orbital analysis reveals that M44 behaves as a superatom resembling Au25, whereas M81 shows molecular-like character reminiscent of Au38, giving the researchers a matched pair in which electronic structure alone dictates whether a cluster leans toward photoluminescence or photothermal behavior.

The synthetic route is elegant in its simplicity. A mixture of chloro(dimethylsulfide)gold(I) and silver acetate dissolved in dichloromethane and methanol is combined with the deprotonated acetylene ligand, forming a reddish-yellow Au-Ag-alkynyl polymeric intermediate within minutes. Reduction with tert-butylamine borane, followed by reaction in the dark for one to three days, yields the clusters. Rather than relying on reaction timing and direct crystallization as in earlier work, the team employed thin-layer chromatography to separate the two products, which appear as distinct bands and were unambiguously identified by ultraviolet-visible-near-infrared absorption spectroscopy. Reaction time biases the outcome: quenching at roughly twenty-four hours favors M44, while extending to thirty-six or seventy-two hours promotes M81 growth. This post-synthetic purification strategy makes both clusters accessible from a single pot, a practical advantage for scaling up.

Optically, the monomer-to-dimer transformation produces dramatic consequences. M44 shows absorption peaks at 365 nanometers, convoluted features at 450 and 505 nanometers, and a broad band at 640 nanometers with tailing into the near-infrared. M81 displays additional bands at 640 and 765 nanometers and extends its absorption tail into the NIR-II window beyond 1000 nanometers. Most strikingly, the dimerization boosts the molar extinction coefficient at the long-wavelength band by roughly sixty times, from about 850 to 50,000 per molar per centimeter, consistent with the stronger transition dipole of the rod-like M81 structure. Both clusters emit photoluminescence in the NIR-II region, peaking near 1080 nanometers for M44 and 1320 nanometers for M81, the latter being among the rarest emission depths achieved by atomically precise clusters, previously matched only by gold quantum rods.

The photoluminescence quantum yields, however, are deliberately low: 3 percent for M44 and 1.5 percent for M81 in deuterated chloroform solution. Herein lies the counterintuitive design principle behind the work. Strong electron-phonon coupling quenches radiative emission and instead channels the absorbed photon energy into lattice vibrations, releasing it as heat. Transient absorption spectroscopy confirmed the underlying excited-state dynamics, with approximately one-picosecond components attributed to internal conversion and intersystem crossing, and longer nanosecond components matching the emission lifetimes of roughly 300 nanoseconds for M81. The long-lived triplet states even enable singlet oxygen generation in M44, as confirmed by the diagnostic 1275-nanometer singlet-oxygen phosphorescence signal under oxygen purging, while M81’s emission energy of 0.89 electron volts falls just below the 0.97-electron-volt gap needed for triplet-to-singlet oxygen energy transfer, so no such signal appears.

Systematic photothermal measurements across varying concentrations and optical power densities revealed a crucial and often overlooked trade-off: photothermal conversion efficiency and maximum temperature rise are counteracting parameters. At a modest optical power density of 0.5 watts per square centimeter and low absorbance, M81 achieved its record 91 percent efficiency with a temperature rise of 11.2 degrees Celsius. Pushing the power density to 2.5 watts per square centimeter dropped the efficiency to 38 percent but drove the temperature rise to 35.4 degrees Celsius, reaching a solution temperature of nearly 60 degrees. At higher absorbance, M81 heated toluene to 96.8 degrees Celsius. M44 followed similar trends with a peak efficiency of 69 percent and temperature rises up to 58 degrees. Under 1064-nanometer excitation deep in the NIR-II window, M81 still produced a measurable temperature rise despite weak extinction there, one of the first demonstrations of direct NIR-II photothermy from atomically precise clusters.

The solid-state results are where the numbers become genuinely startling. Thin films of the clusters embedded in a polymethyl methacrylate matrix, irradiated at 2.5 watts per square centimeter, reached temperature increases of 226 degrees Celsius for M44 and 155 degrees for M81. Such extreme heating in a solid film is precisely what is needed for energy-harvesting devices, since converting light into electricity requires a large, localized temperature gradient rather than the rapid dissipation that occurs in dilute solutions. Crucially, ultraviolet-visible-near-infrared spectra recorded before and after irradiation in all three media, organic solution, aqueous suspension, and solid film, showed no discernible change, confirming that both clusters survive months of ambient storage and repeated intense heating without degradation. The team also encapsulated both clusters in the FDA-approved poloxamer Pluronic F-127, producing water-soluble supraparticles around 150 nanometers in diameter with photothermal efficiencies near 50 percent in water, a regime well suited to photothermal tumor therapy.

The culmination of the study is the first thermoelectric generator application of atomically precise nanoclusters. The researchers deposited a layer of clusters onto the hot side of a commercial bismuth telluride thermoelectric generator and irradiated it with the same 808-nanometer laser, deliberately omitting any external cooling on the cold side to demonstrate electricity generation under minimal, device-integrated conditions. The Seebeck effect, in which a temperature gradient across a thermoelectric material drives directional charge-carrier diffusion, converted the cluster-generated heat into stable open-circuit voltages of 146 millivolts for M44 and 127 millivolts for M81, sustained over 900 seconds of continuous irradiation. Cycling experiments showed the devices could be switched on and off repeatedly without measurable degradation, and control experiments on bare thermoelectric generators produced no detectable voltage, confirming that the clusters themselves are responsible for the effect. Thermal paste at the interface yielded comparable voltages, indicating that direct cluster deposition already provides efficient thermal coupling.

The broader implications extend well beyond the laboratory laser experiments. More than half of the total solar radiation arrives in the near-infrared, yet conventional solar energy harvesting captures predominantly the ultraviolet and visible portions. The absorption profiles of both clusters overlap substantially with the solar spectrum, suggesting that direct sunlight could in principle drive the same photo-thermo-electric cascade. Encapsulating the clusters within heat-resistant carbon or silica shells could allow operation at even higher temperatures under focused solar illumination. Perhaps most conceptually significant is the reframing of electron-phonon coupling, typically dismissed as an unwanted quenching mechanism, into a deliberate design principle for near-infrared photonic energy harvesting. Key challenges remain, including broadening the absorption further into the shortwave infrared, improving thermal coupling, and demonstrating long-term operational stability, but this work establishes atomically precise nanoclusters as functional device components rather than mere solution-phase chromophores, and points toward a future in which quantum-sized metal clusters help reclaim the invisible half of sunlight.

Subject of Research: Near-infrared photothermal conversion and photo-thermo-electric energy generation using atomically precise bimetallic gold-silver nanoclusters

Article Title: Near‐Unity Photothermal Conversion in Bimetallic M44 and M81 Nanoclusters Enables NIR‐Driven Photo‐Thermo‐Electricity Generation

Article References: Sardar, A., Mazumder, A., Ji, W., He, G., Wang, Y., Luo, L., Chen, S., & Jin, R. (2026). Near‐Unity Photothermal Conversion in Bimetallic M 44 and M 81 Nanoclusters Enables NIR‐Driven Photo‐Thermo‐Electricity Generation. Advanced Science, Article e78061. https://doi.org/10.1002/advs.78061

Image Credits: AI Generated

DOI: 10.1002/advs.78061

Keywords: atomically precise nanoclusters, gold-silver nanoclusters, photothermal conversion, near-infrared, thermoelectric generator, Seebeck effect, NIR-II photoluminescence, solar energy harvesting, electron-phonon coupling, bismuth telluride, photothermal therapy, nanomaterials

At the heart of this research is a specialized organic molecule known as P3TTM, distinguished by possessing an unpaired electron that imparts unique magnetic and electronic properties rarely found in organic materials. This radical feature enables the molecule to engage in electronic interactions analogous to those seen in Mott-Hubbard insulators—a class of materials where electron-electron interactions create distinctive insulating states and complex charge dynamics. The collaboration between the Yusuf Hamied Department of Chemistry and the Department of Physics at Cambridge, led by Professors Hugo Bronstein and Sir Richard Friend respectively, has enabled the synthesis and in-depth exploration of these molecules, revealing their previously concealed capability for efficient charge generation.

Traditionally, organic semiconductors rely on paired electrons whose interactions with adjacent molecules are weak, limiting their utility in photovoltaic applications. However, the arrangement of radicals in P3TTM molecules facilitates strong inter-molecular electron interactions. According to lead researcher Biwen Li, when these molecules assemble, their unpaired electrons adopt an alternating spin alignment—an up-down pattern indicative of Mott-Hubbard behavior. This unique spin configuration allows for photogenerated electrons to hop between neighboring molecules, effectively separating charges and creating pathways for electrical current.

The team’s experimental efforts culminated in the fabrication of a novel solar cell device composed exclusively of a P3TTM thin film. Remarkably, this organic radical semiconductor demonstrated near-unity charge collection efficiency upon light absorption, indicating that almost every photon incident on the device produces a corresponding electrical charge. Unlike conventional molecular semiconductors, which require interfaces between electron donor and acceptor materials for charge separation, the P3TTM system intrinsically enables energetically favorable electron transfer between identical molecules, circumventing the limitations imposed by interface engineering.

This photoinduced electron transfer is governed by the electrostatic charging energy of the molecules, known as the Hubbard U parameter, which energetically favors the formation of separated positive and negative charges across molecular sites. As an electron absorbs a photon and hops to a neighboring molecule, it creates a negatively charged species (with double electron occupancy) balanced by a positively charged neighbor, forming a stable charge-separated state capable of conducting current. This mechanism is groundbreaking because it removes the conventional necessity for heterojunctions, potentially simplifying the architecture and manufacturing of organic photovoltaic devices.

Central to achieving this finely balanced electronic interplay was the molecular engineering that exquisitely controls the contact and energy landscape between the P3TTM molecules. Dr. Petri Murto’s contributions in the chemistry department enabled tunable molecular designs that optimize inter-molecular electronic coupling and the energy considerations fundamental to Mott-Hubbard physics. These advances not only enhance the basic understanding of quantum interaction in organic radicals but also open the door to scalable, single-material solar technologies that defy the complexity and costs of today’s multi-component systems.

This work carries profound historical and scientific significance, marking a full-circle moment for the physics community. Professor Sir Richard Friend, who has had a personal academic lineage connected to Sir Nevill Mott—the Nobel laureate physicist who laid the conceptual foundations of electron correlations in disordered materials—expressed deep satisfaction. The recognition that Mott’s theoretical insights into electron interactions manifest in these novel organic materials offers a powerful new chapter in both condensed matter physics and applied materials science.

The implications extend beyond academic curiosity. By harnessing the intrinsic photoinduced charge separation enabled by radical organic semiconductors, future solar cells could become remarkably more efficient, less expensive, lighter, and simpler to produce. Eliminating the typical reliance on complex donor-acceptor blends and layered interfaces reduces fabrication steps and materials costs. This advancement could drive a paradigm shift in the design and commercial viability of organic photovoltaic and optoelectronic technologies.

Moreover, the research challenges decades-old assumptions regarding the limitations of organic semiconductors in charge generation efficiency. Where previous designs depended on exciton dissociation at heterojunction interfaces, the P3TTM radical system inherently possesses mechanisms to spontaneously generate free charges within a homogeneous molecular lattice. This self-charge generation contravenes traditional textbook teachings and necessitates an updated theoretical framework for understanding organic semiconductor physics.

The experimental validation of these fundamental phenomena also underscores the vital intersection of chemistry and physics in novel material discovery. Combining synthetic control with physical insight and device engineering has enabled comprehensive exploration, from molecular scale electronic interactions to macroscopic device performance. This interdisciplinary approach exemplifies the future of materials research where collaborative expertise drives innovation.

Looking ahead, the Cambridge team anticipates further refinement of molecular components to enhance stability, scalability, and integration into commercial applications. Extensions of this research may also probe similar radical systems to unlock a deeper reservoir of quantum mechanical behaviors that could revolutionize electronics and energy harvesting technologies. The newfound understanding of Mott-Hubbard physics in organic systems heralds a fertile ground for discovery and development in the years to come.

In conclusion, the revelation that organic radical semiconductors can intrinsically separate charge through Mott-Hubbard interactions is a paradigm-transforming advancement. It breathes new life into organic photovoltaic research, offering a streamlined path to high-efficiency solar energy devices based on single-material architectures. As the world grapples with energy demands and sustainability challenges, these discoveries at the forefront of quantum materials science offer exciting promise for a cleaner, smarter energy future.

Article Title: Intrinsic intermolecular photoinduced charge separation in organic radical semiconductors

News Publication Date: 30-Sep-2025

Web References: http://dx.doi.org/10.1038/s41563-025-02362-z

Image Credits: Biwen Li – Cavendish Laboratory, University of Cambridge

Keywords

Physics, Materials science, Energy, Condensed matter physics