🔬 mesoscale physics

Coulombic control of charge transfer in luminescent radicals with long-lived quartet states

This study establishes design rules for efficient quartet generation in luminescent radicals by demonstrating that Coulombic tuning of electronic coupling via molecular topology controls charge transfer pathways, enabling high luminescence yields dominated by long-lived quartet states.

Original authors: Lujo Matasovic, Petri Murto, Shilong Yu, Wenzhao Wang, James D. Green, Giacomo Londi, Weixuan Zeng, Laura Brown, William K. Myers, David Beljonne, Yoann Olivier, Feng Li, Hugo Bronstein, Timothy J. H.

Published 2026-06-25

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CC BY 4.0

Original authors: Lujo Matasovic, Petri Murto, Shilong Yu, Wenzhao Wang, James D. Green, Giacomo Londi, Weixuan Zeng, Laura Brown, William K. Myers, David Beljonne, Yoann Olivier, Feng Li, Hugo Bronstein, Timothy J. H. Hele, Richard H. Friend, Sebastian Gorgon

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a tiny, molecular flashlight that doesn’t just shine light, but also holds onto a specific "spin" of energy for a long time. This is useful for future quantum technologies, where we need materials that can interact with light and hold quantum information simultaneously.

This paper is about solving a puzzle: How do you make a molecule that glows brightly and keeps its quantum spin alive for a long time?

Here is the story of how the scientists solved it, explained with some everyday analogies.

The Problem: The "Leaky Bucket"

The scientists were working with molecules that contain a special part called a radical (think of it as a loose electron that wants to pair up) and another part called an acene (a ring-shaped structure that absorbs light).

When you shine light on these molecules, they get excited. Ideally, this excitement should turn into a high-spin state called a Quartet. This Quartet state is valuable because it’s stable and can be manipulated for quantum tasks. Eventually, this Quartet should decay back down, releasing light (glowing) in the process.

However, there was a problem. In previous designs, the molecules had a "leak." The energy would get stuck in a dark, non-glowing trap state (called $^2CT_{Ac}$ ) and just vanish as heat instead of turning into light. It was like pouring water into a bucket with a hole in the bottom—the water (energy) leaked out before you could use it.

The Solution: Changing the "Plumbing" (Topology)

The researchers realized that the shape and connection of the molecule—its topology—were the key to plugging that leak. They tested two main designs:

The "Direct Connect" Design (Strong Coupling):
Imagine the radical and the acene are holding hands directly. Because they are so close, the energy gets confused and falls into the "dark trap" state. The light leaks out quickly. This is like two people standing so close they bump into each other constantly, causing chaos.
The "Bridge" Design (Weak Coupling):
The scientists inserted a carbazole unit between the radical and the acene. Think of this as putting a bridge or a spacer between them. Now, they aren’t holding hands directly; they are connected via a bridge.

The "Coulombic Control" Trick

Why does the bridge help? It comes down to Coulombic interactions (electrical attraction).

In the Direct Design: The electrical attraction between the radical and the acene is strong. This pulls the "dark trap" state down in energy, making it the easiest place for the energy to fall. The energy gets stuck there and dies.
In the Bridge Design: The carbazole spacer increases the distance between the radical and the acene. This weakens the electrical pull. As a result, the "dark trap" state is pushed up in energy. It’s no longer the easiest place to fall. Instead, the energy is forced to stay in the "bright" states that can eventually glow.

It’s like adjusting the height of a slide. In the direct design, the slide drops straight into a pit (the trap). In the bridge design, the pit is raised up, so the slide leads to a safe, glowing platform instead.

The Result: A Bright, Long-Lived Glow

By using the "bridge" design (specifically a molecule called T-3Cz-An), the scientists achieved two big wins:

High Efficiency: 55% of the absorbed light was turned back into emitted light. This is very bright for this type of material.
Long-Lived Quantum State: The glowing came from the Quartet state, and it lasted for microseconds (millionths of a second). In the world of quantum physics, that is a long time. It means the molecule holds onto its quantum "spin" long enough to be useful.

Summary of the Rules

The paper establishes three main rules for designing these future quantum materials:

Don’t let the radical and the light-absorbing part touch directly. Use a spacer (like carbazole) to keep them apart.
Push the "dark trap" state up in energy. You do this by increasing the distance between the parts, which weakens the electrical attraction that causes the leak.
Keep the "bright" states accessible. Ensure the energy can still move between the states needed to create the glow, but avoid the dark traps.

Why It Matters

This work shows that by simply changing how the parts of a molecule are connected (the topology), you can control how energy flows inside it. It’s not just about what atoms you use, but how you arrange them. This gives chemists a new toolkit for building molecular materials that can both glow and hold quantum information, which is a crucial step for future quantum technologies.

Technical Summary: Coulombic Control of Charge Transfer in Luminescent Radicals with Long-Lived Quartet States

Problem Statement
The development of robust spin-optical interfaces is critical for harnessing quantum resources in materials. While defects in inorganic materials (e.g., diamond) are commonly used, they often suffer from low photoluminescence yields and difficult control over spin-center interactions. Organic molecules offer a tunable alternative, particularly those incorporating stable radicals that support multi-spin interactions via $\pi$ -pathways. Specifically, quartet states ( $S=3/2$ ) formed by the exchange coupling of a triplet spin and a radical doublet spin are promising for coherent manipulation. However, previous high-spin radical compounds have been non-luminescent, limiting their utility. Recent work demonstrated a luminescent pathway in tris(2,4,6-trichlorophenyl)methyl (TTM) radical derivatives, but the structure-property relationships governing efficient quartet generation and emission remained unclear, with only a singular design previously known.

Methodology
The authors synthesized four distinct molecular compounds comprising three building blocks: a TTM radical core (T), a carbazole unit, and an acene unit (anthracene or acridine) with a triplet energy close to the emissive state. The compounds were designed to vary the electronic coupling between the radical and the acene:

Strong Coupling: An-T-1Cz and An-T-3PCz, where the anthracene is connected directly to the radical.
Weak Coupling: T-3Cz-An and T-3Cz-Acr, where the acene is linked to the radical via a carbazole bridge.

The study employed a multi-modal characterization approach:

Optical Spectroscopy: Steady-state and time-resolved photoluminescence (PL) were measured in solution and films to determine quantum yields ( $\phi_{PL}$ ), lifetimes, and activation energies.
Transient Absorption (TA): TA spectroscopy tracked excited-state dynamics across picosecond to microsecond timescales to identify intermediate states and decay pathways.
Electron Spin Resonance (ESR): Transient ESR (trESR) and pulsed ESR were used to confirm the formation of quartet states, determine spin Hamiltonian parameters, and measure phase memory times.
Quantum-Chemical Calculations: Time-dependent density functional theory (TDDFT) calculations were performed to map the energetic landscape of excited states, including local excitations ( $^{2,4}LE_{Ac}$ ) and charge-transfer states ( $^{2}CT_{Cz}$ and $^{2}CT_{Ac}$ ).

Key Contributions and Results
The study establishes design rules for efficient quartet generation in luminescent radicals by controlling the electronic coupling strength between molecular units.

Role of Molecular Topology and Electronic Coupling:
- Compounds with strong radical-acene coupling (An-T-1Cz, An-T-3PCz) exhibited modest photoluminescence quantum yields (3–8%). TA and calculations revealed that the acene-to-radical charge-transfer state ( $^{2}CT_{Ac}$ ) lies below the bright carbazole-to-TTM charge-transfer state ( $^{2}CT_{Cz}$ ). This low-lying $^{2}CT_{Ac}$ state acts as a dark trap, facilitating rapid non-radiative decay to the ground state.
- Compounds with weak radical-acene coupling (T-3Cz-An, T-3Cz-Acr), mediated by the carbazole bridge, achieved significantly higher emission efficiencies (50–55%). In these systems, the $^{2}CT_{Ac}$ state is energetically stabilized above the emissive states due to larger electron-hole separation, preventing it from acting as a non-radiative trap.
Coulombic Tuning of Charge Transfer States:
- The energy of the $^{2}CT_{Ac}$ state is dictated by Coulombic interactions. In weakly coupled systems, the spatial separation between the radical SOMO and acene HOMO minimizes the Coulomb attraction ( $J_{A1,R0}$ ), raising the energy of $^{2}CT_{Ac}$ .
- Solvatochromic studies confirmed that in high-polarity solvents (e.g., acetonitrile), the $^{2}CT_{Ac}$ state is stabilized and becomes the lowest excited state, quenching delayed emission. This confirms that the energetic ordering is sensitive to dielectric environment and electron-hole separation.
Quartet State Dynamics and Yield:
- trESR confirmed the formation of strongly exchange-coupled quartet states ( $^{4}LE_{Ac}$ ) in all emissive compounds.
- T-3Cz-An emerged as the champion compound, achieving a 55% total luminescence yield. Crucially, 94% of the emitting excitons originated from the quartet state at microsecond timescales, with a delayed emission lifetime of 1.03 $\mu$ s in solution and up to 2.3 $\mu$ s in films.
- The activation energy for delayed emission (reverse intersystem crossing, RISC) in T-3Cz-An was determined to be 15 $\pm$ 4 meV, consistent with the thermal energy required for efficient RISC.
- The phase memory time ( $T_m$ ) of the quartet state in T-3Cz-An was measured at 0.90 $\mu$ s at 80 K.

Significance
The paper claims to reveal the central role of molecular topology in luminescent quantum materials. By demonstrating that weak electronic coupling between the radical and acene units—achieved via a carbazole bridge—prevents the formation of a non-radiative trap state ( $^{2}CT_{Ac}$ ), the authors provide a general strategy for designing radicals with high luminescence yields and long-lived quartet states. The work presents a blueprint for compounds with microwave-addressable high-spin states that spontaneously access a delayed, radiative decay channel, marking a step forward in the chemical control of electronic and exchange coupling for molecular quantum technologies.