CMA complexes were first reported in 2017 and quickly attracted attention as TADF emitters because they combine easy synthesis, tunable emission colors, and high internal and external quantum efficiencies with unusually fast radiative decay rates. TADF emitters work by harvesting both singlet and triplet excitons: triplet excitons undergo reverse intersystem crossing (rISC) back to the emissive singlet state, allowing nearly all electrically generated excitons to produce light without relying on scarce and expensive iridium or platinum phosphors. Yet despite their promise, OLED devices built from d10 CMA emitters with genuinely long operational lifetimes have remained extremely rare, and previous work by the same team showed that steric engineering of N-heterocyclic carbene ligands could push device lifetimes to LT95 values of up to 2082 hours for gold emitters and 3582 hours for copper emitters at a luminance of 1000 cd m−2. What was still missing was a rigorous, quantitative explanation of how the metal atom itself governs the emission kinetics.

The new study tackled this question with an unusually deep computational arsenal. Because conventional density functional theory (DFT) calculations are too approximate to capture the subtle energetics of these systems, the researchers combined time-dependent DFT with high-level ab initio methods, including similarity-transformed equation-of-motion domain-based local pair natural orbital coupled-cluster theory (STEOM-DLPNO-CCSD) and the combined DFT/multireference configuration interaction (DFT/MRCI) approach. They supplemented these with a battery of bonding analysis techniques — natural adaptive orbital (NAdO) analysis, extended transition-state natural orbitals for chemical valence (ETS-NOCV) decomposition, and charge decomposition analysis — to dissect, atom by atom and electron by electron, how the metal binds to its carbene and carbazole ligands in both ground and excited states.

The central discovery concerns the metal’s (n+1)p orbitals, which sit just above the filled d shell. In a naive picture, a d10 metal ion has no low-lying empty d orbitals to engage in conventional bonding, so metal–ligand π-interactions with strongly donating ligands such as carbazolide would seem improbable. But the calculations show that mixing between the (n+1)p orbital and the nd orbital polarizes the d orbital’s spatial distribution, enlarging its bonding lobe in one direction and suppressing its antibonding lobe in the other. This (n+1)p–nd hybridization creates genuine metal–ligand π-interactions that persist even in the excited state. Crucially, the degree of hybridization differs systematically across the coinage metals: copper displays the largest (n+1)p orbital population (roughly 0.10–0.14 electrons), gold an intermediate amount, and silver the smallest. This single electronic difference cascades through the entire photophysics of the emitter.

The practical consequence involves molecular rotation. In the excited singlet state, an electron is promoted from the highest occupied molecular orbital — largely localized on the carbazole ligand — to the lowest unoccupied orbital, producing a ligand-to-ligand charge-transfer (LLCT) transition. Because the HOMO is half-occupied in this state, the π-interaction between the metal and the carbazolyl nitrogen is actually enhanced relative to the ground state, and the nitrogen-side π-interaction is weaker than the corresponding metal–carbene carbon π-interaction. As a result, the carbazole and carbene ligands can rotate about the metal–nitrogen bond during the excited-state lifetime, swinging between semi-coplanar and orthogonal conformations. NAdO and ETS-NOCV analyses quantified these interactions, showing, for example, that the Au–N π-interaction stabilizes the excited state of a representative gold emitter by about 10.85 kcal/mol, compared with roughly 26.94 kcal/mol for the Au–C π-interactions — confirming that rotation occurs preferentially at the metal–nitrogen bond.

This rotational flexibility turns out to be the key to solving a longstanding trade-off in TADF design. Emitters need a small singlet–triplet energy gap (∆ES1–T1) to enable efficient reverse intersystem crossing, but the usual strategy for shrinking that gap — spatially separating the donor and acceptor orbitals — simultaneously reduces the oscillator strength of the S1→S0 transition, slowing prompt fluorescence. The study shows that flexible excited-state rotation offers an escape route: semi-coplanar rotamers provide large orbital overlap and a fast radiative rate, while orthogonal rotamers provide a small ∆ES1–T1, and rapid thermal interconversion between them allows the molecule to exploit both regimes simultaneously. Silver, with the weakest M–N π-interaction and the greatest rotational freedom, achieves the highest TADF radiative decay rates; copper, with the strongest π-interaction and stiffest rotation, the lowest. The predicted ordering — Ag > Au > Cu — matches experimental measurements across multiple emitter series and solvents, resolving a question that had persisted since the first CMA emitters were reported.

The team then translated this mechanistic insight into practical ligand design rules. Calculations of a series of gold emitters bearing electron-withdrawing groups (cyano, trifluoromethyl) or electron-donating groups (methoxy, tert-butyl) on the carbazole ligand revealed a clean trend: electron-withdrawing substituents reduce the metal’s (n+1)p–nd hybridization, weaken the excited-state M–N π-interaction, increase rotational flexibility, and thereby accelerate TADF. Electron-donating substituents do the opposite. Extending the π-system of the carbazole achieves a similar effect through a different route — the hole migrates onto the extended aromatic system, delocalizing the M–N π-electrons and loosening the nitrogen-side bond. The researchers also identified an experimentally accessible descriptor: the emitter’s oxidation potential, measured by cyclic voltammetry, correlates linearly with the calculated strength of the excited-state π(M…N) interaction, giving chemists a simple electrochemical handle for screening new candidates.

But the story comes with a crucial caveat. Weakening the excited-state M–N π-interaction also weakens the bond itself, making the metal–ligand framework more vulnerable to dissociation under the harsh conditions of electroluminescence. To test this, the team fabricated OLEDs based on four gold emitters with matched device structures: Au-1 and three derivatives carrying electron-withdrawing groups. While all emitted light in nearly the same spectral region — 467 to 469 nm — their operational lifetimes differed dramatically. The parent Au-1 device achieved an LT90 of 1709 hours, whereas devices based on Au-1CN, Au-12CF3, and Au-12CN lasted only 131, 1.01, and 0.37 hours, respectively, at an initial luminance of 1000 cd m−2. Because the substituents sit far from the metal center and exert negligible steric protection, the correlation points squarely at the strength of the excited-state M–N π-interaction as a determinant of device durability.

The implication is a genuine design dilemma: the same electronic tuning that maximizes radiative decay rate and efficiency may simultaneously shorten the operational lifetime of the device. The authors emphasize that many factors govern OLED stability — emission energy, steric shielding, molecular packing, and excited-state bond dissociation among them — but their data establish, for the first time, a clear and quantifiable correlation between excited-state metal–nitrogen π-interaction strength and device longevity in this emitter class. Navigating this trade-off, they suggest, will be essential for realizing the full industrial potential of d10 CMA emitters, particularly for blue and green devices where high-energy emission already stresses molecular stability.

Beyond its immediate practical guidance, the study provides a conceptual reframing of TADF mechanism in metal complexes. Rather than treating spin–orbit coupling and the singlet–triplet gap as the sole arbiters of delayed fluorescence kinetics, the work demonstrates that excited-state conformational dynamics — governed by orbitals that textbook bonding pictures would ignore — can dominate the radiative decay rate. The spin–orbit coupling matrix elements, which follow the order Cu > Au > Ag due to differences in d-orbital participation and heavy-atom effects, turn out to be of secondary importance compared with rotational flexibility. For a field racing to replace precious-metal phosphors with earth-abundant copper and silver emitters in displays and lighting, that insight reframes the design problem entirely: the fastest emitters may be those whose metal–ligand bonds are engineered to be just strong enough to survive, yet just weak enough to let the molecule dance.

The synthetic challenge represented by stable diradicaloids has long captivated chemists, owing to their intriguing electronic structures defined by two unpaired electrons which are typically prone to high reactivity and rapid degradation. Chichibabin diradicaloids, a particular class named after the Russian chemist Aleksei Chichibabin, offer tunable electronic configurations that allow for radical stability when appropriately functionalized. The team’s remarkable success in stabilizing this otherwise elusive molecular entity while preserving a luminescent output that extends deep into the NIR region addresses a formidable hurdle that has limited previous applications.

What sets this diradicaloid apart from conventional fluorophores is its combination of inherent photostability and extended emission wavelength, making it a potent candidate for in vivo imaging. Unlike traditional dyes that suffer from rapid photobleaching and shallow penetration depths in biological tissues, this molecule performs robustly under prolonged excitation with minimal photodegradation. Such characteristics dramatically enhance imaging duration and clarity, critical parameters for real-time monitoring of biological processes at the molecular level.

The underlying photophysical properties stem from the molecule’s distinct electronic structure. The coexistence of diradical character and conjugated π-systems facilitates efficient spin–orbit coupling and intersystem crossing, promoting luminescence in the NIR domain. The researchers employed comprehensive spectroscopic techniques, including absorption and emission spectroscopy as well as electron spin resonance (ESR), to elucidate these properties. Their findings reveal that the diradicaloid maintains a strong luminescent signal in the 700 to 900 nm range, far surpassing the performance of many existing organic NIR fluorophores.

Beyond imaging, the molecule’s photothermal conversion efficiency opens new therapeutic avenues, particularly for photothermal therapy (PTT). By harnessing the absorbed NIR photons, the diradicaloid transitions to energetically excited states and non-radiatively dissipates energy as heat, sufficient to induce localized hyperthermia — a mode of treatment increasingly favored for minimally invasive cancer interventions. The dual functionality of this compound enables seamless integration of diagnostic imaging and therapeutic action in a single molecular platform, promising more precise and targeted treatments with fewer side effects.

The research team further evaluated the biocompatibility and cellular uptake of the diradicaloid using in vitro models, confirming minimal cytotoxicity and effective internalization in cancerous cells. Fluorescence microscopy analyses demonstrated sharp contrast between targeted malignant tissues versus healthy controls, leveraging the NIR emission for clear visualization. Additionally, photothermal assays under NIR laser irradiation confirmed efficient temperature elevation sufficient to induce cytotoxicity selectively in tumor cells.

One of the most compelling aspects of this study is the strategic molecular design that balances radical stability with optical function. By introducing electron-donating and accepting groups symmetrically along the conjugated backbone, the compound achieves remarkable resilience against oxidative degradation without compromising luminescence. This design principle not only stabilizes the diradical centers but also fine-tunes the energy gaps critical for NIR emission, showcasing the power of molecular engineering in addressing long-standing challenges in materials chemistry.

The implications for clinical translation are profound. NIR fluorescence imaging is already emerging as a pivotal tool in surgical guidance, diagnostic mapping, and real-time monitoring of therapeutic interventions. The advent of a stable luminescent diradicaloid capable of both high-resolution imaging and photothermal therapy can accelerate the development of multifunctional theranostic agents — materials that combine therapy and diagnostics in one entity. Such agents could reduce the need for multiple administration steps, lower systemic toxicity, and enhance patient outcomes.

Moreover, the diradicaloid’s structural tunability paves the way for customization to specific clinical needs. By adjusting the peripheral substituents or conjugation length, the electronic properties and absorption/emission wavelengths can be modulated to target distinct biological windows or to respond to different excitation sources. This flexibility heralds a new class of bespoke organic materials with vast potential across biomedical optics, from cancer treatment and neuroimaging to deep tissue visualization.

An additional advantage resides in the organic nature of the compound, which contrasts with traditional inorganic NIR agents such as quantum dots or rare-earth doped nanoparticles that often raise biocompatibility and environmental concerns. The organic diradicaloid offers the ecosystem-friendly and potentially biodegradable profile demanded by next-generation medical materials, aligning with the increasing emphasis on green chemistry and sustainable biomedical solutions.

The study also advances theoretical understanding of diradical physics in complex conjugated systems, providing valuable insights into the interplay between radical stability, electronic transitions, and photoluminescence. Computational modeling coupled with experimental validation facilitated a comprehensive picture of the electronic landscape, highlighting how the balance of singlet and triplet states can be exploited to optimize both luminescence intensity and photothermal conversion efficacy.

Looking ahead, integration of this diradicaloid into nanoplatforms and delivery vehicles represents a promising avenue to enhance targeting specificity and pharmacokinetics. Encapsulation into liposomes, polymeric micelles, or conjugation with targeting ligands could improve biodistribution and accumulation in diseased tissues, optimizing therapeutic windows while minimizing off-target effects. Such strategies are essential in bridging the gap between molecular innovation and clinical practicality.

In conclusion, the introduction of this efficient luminescent stable Chichibabin diradicaloid marks a major milestone at the intersection of chemical synthesis, photophysics, and biomedicine. Its unique combination of NIR luminescence and photothermal functionality offers a formidable platform for next-generation imaging and therapy applications. By pushing the boundaries of radical stability and optical performance, this work paves the way for safer, more effective, and multifunctional treatments that could ultimately transform patient care paradigms in oncology and beyond.

Subject of Research: Not explicitly provided

Article Title: Not explicitly provided

Article References:
Liu, T., Zhu, Z., Wang, S. et al. Efficient luminescent stable Chichibabin diradicaloid for near-infrared imaging and photothermal therapy. Light Sci Appl 14, 289 (2025). https://doi.org/10.1038/s41377-025-01993-w

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01993-w