Exploring photon upconversion through advanced triplet sensitization techniques
Imagine a solar panel that could absorb not just the visible light from the sun but also the typically wasted infrared heat, or medical treatments that could use harmless, deep-penetrating near-infrared light to trigger therapeutic reactions in the body.
This isn't science fiction—it's the promising frontier of photon upconversion, a process where low-energy light is transformed into higher-energy light.
At the heart of recent breakthroughs in this field lies a quantum mechanical phenomenon known as the triplet state. For years, efficiently accessing these triplet states has been a major hurdle, causing significant energy loss that limited practical applications 1 .
Materials absorb low-energy photons (e.g., infrared light)
Energy is transferred to long-lived triplet excitons
Two triplet excitons combine to create one high-energy singlet
Upconverted light is emitted at a shorter wavelength
To appreciate these advances, we first need to understand the quantum mechanics of light emission.
When a molecule absorbs light, one of its electrons jumps to a higher energy level, creating an "excited state." This excited state comes in two main flavors, determined by a quantum property called spin:
The spins of the two electrons are paired oppositely. Think of this as a stable, short-lived state that easily emits light (fluorescence) as it returns to normal.
The spins of the electrons are aligned in parallel. This state is metastable and long-lived because the transition back to the ground state is "spin-forbidden" by quantum rules 6 .
The process of moving from a singlet to a triplet state is called intersystem crossing (ISC). For decades, the energy loss during ISC has been a major bottleneck for photon upconversion, particularly a type called triplet-triplet annihilation upconversion (TTA-UC) 1 . In TTA-UC, two molecules in their triplet states collide and "fuse" their energy, creating one molecule in a high-energy singlet state that can then emit upconverted light.
The following table summarizes the three innovative routes that are minimizing energy loss and revolutionizing triplet sensitization.
| Sensitization Route | Core Mechanism | Key Advantage | Example Materials |
|---|---|---|---|
| Thermally Activated Delayed Fluorescence (TADF) Molecules | Minimizes the energy gap between singlet (S₁) and triplet (T₁) states, enabling easy interconversion via Reverse ISC 3 . | Minimal energy loss during triplet state formation, enabling visible-to-UV upconversion 1 . | Organic donor-acceptor molecules . |
| Inorganic Nanocrystals | Possess naturally small exchange splitting, minimizing the energy loss when creating triplet states 1 . | Broad absorption bands, particularly in the near-infrared (NIR) region; high stability 1 . | Lead-free perovskite nanocrystals (e.g., Cs₂ZrCl₆) 2 . |
| Direct Singlet-to-Triplet (S-T) Absorption | Bypasses the singlet state entirely by directly exciting an electron from the ground state (S₀) to the triplet state (T₁) 4 . | Eliminates energy loss from ISC, allowing excitation with longer-wavelength visible light 7 . | Metal complexes (e.g., Osmium), or organic molecules with heavy atoms 1 . |
Minimize energy gap between singlet and triplet states for efficient upconversion.
Offer broad absorption and high stability for NIR upconversion.
Bypasses energy loss pathways for more efficient triplet formation.
One of the most conceptually groundbreaking routes is the direct population of the triplet state. A 2019 study published in Chemical Science provides a perfect example of this approach in action 4 7 .
The researchers aimed to achieve direct S₀→T₁ absorption in purely organic molecules, a process typically so weak it's nearly impossible to observe. Their strategy involved a clever molecular design:
They started with 9-phenyl-9H-carbazole (PhCz), a molecule with a heteroatom (Nitrogen) that promotes spin-orbit coupling.
They introduced bromine (Br) atoms as substituents at different sites on the PhCz molecule, creating derivatives like pBrPhCz, mBrPhCz, and DBrPhCz. Heavy atoms like bromine enhance the interaction between electron spin and orbital motion, making the forbidden singlet-to-triplet transition more likely 7 .
The molecules were studied in their crystalline form. The rigid, aggregated crystal structure suppresses molecular vibrations that would otherwise drain the triplet state's energy, stabilizing it for long-lived afterglow emission.
The experiment yielded remarkable results that underscore the power of direct triplet excitation:
While the original PhCz molecule required UV light (295 nm) for weak afterglow, the brominated crystals showed significantly enhanced afterglow when excited with visible light (400 nm) 7 .
The directly excited triplet states led to an afterglow with a quantum efficiency of up to 9.5%—a dramatic improvement over previous organic afterglow materials and one of the highest reported values at the time 7 .
The afterglow lifetime reached 0.25 seconds, making it easily visible to the naked eye in ambient conditions after the light source was turned off 7 .
This experiment was pivotal because it demonstrated that the spin-forbidden S₀→T₁ transition could be made efficient enough for practical use. By bypassing the traditional ISC pathway, it minimizes energy loss and red-shifts the excitation wavelength into the visible spectrum, a crucial advance for biological and energy applications where deep-penetrating, non-damaging light is essential 7 .
| Compound | Afterglow Lifetime (s) | Afterglow Quantum Efficiency (%) | Excitation Wavelength |
|---|---|---|---|
| PhCz | Shorter, weak emission | Low | Primarily UV (~295 nm) |
| pBrPhCz | 0.20 s | Significantly improved | UV (295 nm) |
| mBrPhCz | 0.25 s | 9.5% | Visible (400 nm) |
Data derived from reference 7
Enhanced solar cells that capture infrared light, breaking theoretical efficiency limits.
Targeted cancer treatments activated by harmless infrared light for deep tissue penetration.
High-resolution imaging that sees deep into tissue with unprecedented clarity.
Advanced displays and optoelectronic devices with improved efficiency and new capabilities.
The move beyond traditional triplet sensitization marks a significant maturation of photonic materials science. The strategic use of TADF molecules, inorganic nanocrystals, and direct singlet-to-triplet absorption is transforming our ability to manipulate light at the most fundamental level.
These approaches, which minimize or even bypass the energy loss inherent in traditional intersystem crossing, are no longer just laboratory curiosities. They are paving the way for real-world technologies: solar cells that break theoretical efficiency limits, targeted cancer therapies activated by harmless infrared light, and high-resolution bioimaging that can see deep into tissue with unprecedented clarity 1 8 . As researchers continue to refine these triplet-harvesting strategies, the future of how we generate, sense, and use light is looking brighter than ever.
References will be added here in the appropriate format.