Triplet Exciton Transfer Research Articles

Visualizing Triplet Energy Transfer in Organic Near-Infrared Phosphorescent Host-Guest Materials.

Limited by the energy gap law, purely organic materials with efficient near-infrared room temperature phosphorescence are rare and difficult to achieve. Additionally, the exciton transition process among different emitting species in host-guest phosphorescent materials remains elusive, presenting a significant academic challenge. Herein, using a modular nonbonding orbital-π bridge-nonbonding orbital (n-π-n) molecular design strategy, we develop a series of heavy atom-free phosphors. Systematic modification of the π-conjugated cores enables the construction of a library with tunable near-infrared phosphorescence from 655 to 710 nm. These phosphors exhibit excellent performance under ambient conditions when dispersed into a 4-bromobenzophenone host matrix, achieving an extended lifetime of 11.25 ms and a maximum phosphorescence efficiency of 4.2 %. Notably, by eliminating the interference from host phosphorescence, the exciton transition process in hybrid materials can be visualized under various excitation conditions. Spectroscopic analysis reveals that the improved phosphorescent performance of the guest originates from the triplet-triplet energy transfer of abundant triplet excitons generated independently by the host, rather than from enhanced intersystem crossing efficiency between the guest singlet state and the host triplet state. The findings provide in-depth insights into constructing novel near-infrared phosphors and exploring emission mechanisms of host-guest materials.

Visualizing Triplet Energy Transfer in Organic Near‐Infrared Phosphorescent Host‐Guest Materials

AbstractLimited by the energy gap law, purely organic materials with efficient near‐infrared room temperature phosphorescence are rare and difficult to achieve. Additionally, the exciton transition process among different emitting species in host–guest phosphorescent materials remains elusive, presenting a significant academic challenge. Herein, using a modular nonbonding orbital‐π bridge‐nonbonding orbital (n‐π‐n) molecular design strategy, we develop a series of heavy atom‐free phosphors. Systematic modification of the π‐conjugated cores enables the construction of a library with tunable near‐infrared phosphorescence from 655 to 710 nm. These phosphors exhibit excellent performance under ambient conditions when dispersed into a 4‐bromobenzophenone host matrix, achieving an extended lifetime of 11.25 ms and a maximum phosphorescence efficiency of 4.2 %. Notably, by eliminating the interference from host phosphorescence, the exciton transition process in hybrid materials can be visualized under various excitation conditions. Spectroscopic analysis reveals that the improved phosphorescent performance of the guest originates from the triplet‐triplet energy transfer of abundant triplet excitons generated independently by the host, rather than from enhanced intersystem crossing efficiency between the guest singlet state and the host triplet state. The findings provide in‐depth insights into constructing novel near‐infrared phosphors and exploring emission mechanisms of host–guest materials.

Understanding and Improving Triplet Exciton Transfer in Sensitized Silicon Solar Cells

Understanding and Improving Triplet Exciton Transfer in Sensitized Silicon Solar Cells

Tert-butyl carbazole modified non-fused ring electron acceptor generating high triplet state energy level for efficient organic solar cell

Two-dimensional side-chain plays a crucial role in the construction of efficient non-fused ring electron acceptors (NFREAs). Herein, we introduce a novel NFREA, PCT-4Cl, featuring a tert-butyl carbazole side-chain, and investigate its optoelectronic properties. Notably, PCT-4Cl exhibits a small singlet-triplet energy gap (ΔEST) of 0.25 eV. Blending PCT-4Cl with host eC9 or C8C8–4Cl acceptor extends the lifetime of singlet excitons and exciton diffusion length in the blend films. Moreover, the high energy level of the triplet state (ET1) in PCT-4Cl facilitates efficient transfer of triplet excitons to eC9 or C8C8–4Cl. The addition of PCT-4Cl to PM6:eC9 also leads to a longer crystallization time, restraining eC9 aggregation and enhancing crystallinity in the ternary blend films. Consequently, power conversion efficiencies (PCEs) of 18.84 % and 15.17 % are achieved based on the optimized ternary devices of PM6:eC9:PCT-4Cl (1:1:0.2, wt./wt./wt.) and PM6:C8C8–4Cl:PCT-4Cl (1:1:0.2, wt./wt./wt.), respectively, surpassing the performance of PM6:eC9 (17.34 %) and PM6:C8C8–4Cl (14.05 %). Furthermore, leveraging 2PACz as a hole transporting layer elevates the PCEs of the ternary OSCs to 19.25 % and 15.53 %. These findings underscore the significant role of the tert-butyl carbazole side-chain in elevating the ET1 of NFREA for efficient charge transfer, as well as its role in modulating the crystallinity, presenting a promising pathway for achieving high performance in both binary and ternary OSCs.

Defect-Assisted Exciton Transfer across the Tetracene-Si(111):H Interface.

Exciton transfers are ubiquitous and extremely important processes, but often poorly understood. A recent example is the triplet exciton transfer in tetracene sensitized silicon solar cells exploited for harvesting high-energy photons. The present abinitio molecular dynamics calculations for tetracene-Si(111):H interfaces show that Si dangling bonds, intuitively expected to hinder the exciton transfer, actually foster it. This suggests that defects and structural imperfections at interfaces may be exploited for excitation transfer.

A perspective on next-generation hyperfluorescent organic light-emitting diodes.

Hyperfluorescence, also known as thermally activated delayed fluorescence (TADF) sensitized fluorescence, is known as a next-generation efficient and innovative process for high-performance organic light-emitting diodes (OLEDs). High external quantum efficiency (EQE) and good color purity are crucial parameters for display applications. Hyperfluorescent OLEDs (HF-OLEDs) take the lead in this respect as they utilize the advantages of both TADF emitters and fluorescent dopants, realizing high EQE with color saturation and long-term stability. Hyperfluorescence is mediated through Förster resonance energy transfer (FRET) from a TADF sensitizer to the final fluorescent emitter. However, competing loss mechanisms such as Dexter energy transfer (DET) of triplet excitons and direct charge trapping on the final emitter need to be mitigated in order to achieve fluorescence emission with high efficiency. Despite tremendous progress, appropriate guidelines and fine optimization are still required to address these loss channels and to improve the device operational lifetime. This perspective aims to provide an overview of the evolution of HF-OLEDs by reviewing both molecular and device design pathways for highly efficient narrowband devices covering all colors of the visible spectrum. Existing challenges and potential solutions, such as molecules with peripheral inert substitution, multi-resonant (MR) TADF emitters as final dopants, and exciplex-sensitized HF-OLEDs, are discussed. Furthermore, the operational device lifetime is reviewed in detail before concluding with suggestions for future device development.

High-Performance Hot-Exciton OLEDs via Fully Harvesting Triplet Excited States from Both the Exciplex Co-Host and the TBRb Emitter.

The high-level reverse intersystem crossing (HL-RISC, T2 → S1 ) process from triplet to singlet exciton, namely the "hot exciton" channel, has recently been demonstrated in the traditional fluorescent emitter of TBRb. Although it is a potential pathway to improve the utilization of non-radiative triplet exciton energy, highly efficient fluorescent organic light emitting diodes (FOLEDs) based on this "hot exciton" channel have not been developed. Herein, high-efficiency and low-efficiency roll-off FOLEDs are achieved through doping TBRb molecules into an energy-level matched exciplex co-host. Combining the low-level RISC (LL-RISC, EX3 → EX1 ) process in the exciplex co-host with the HL-RISC process of hot excitons in TBRb to fully harvest the triplet energy, a record-high external quantum efficiency (EQE) of 20.4% is obtained via a proper Dexter energy transfer of triplet excitons, realizing the efficiency breakthrough from fully fluorescent material-based OLEDs with TBRb as an end emitter. Furthermore, the fingerprint Magneto-electroluminescence (MEL) as a sensitive measuring tool is employed to visualize the "hot exciton" channel in TBRb, which also directly verifies the effective energy confinement and the full utilization of hot excitons. Obviously, this work paves a promising way for further fabricating high-efficiency TBRb-based FOLEDs for lighting and flat-panel display applications.

Pyrene as a Triplet Acceptor Enhancing Triplet Energy Transfer of g-C3N4 in Heterogeneous Photooxidation

Triplet photosensitizers have great potential in selective photooxidation by generating 1O2 through triplet energy transfer. However, the heterogeneous bulk photosensitizers easily trap triplet excitons and inhibit photocatalysis efficiency. Previous work focused on controlling ultra-small size to reduce the transfer distance, but we propose a new strategy using triplet acceptor molecules (e.g., pyrenecarboxylic acid, PCA) with proper energy levels to modify the surface of heterogeneous photosensitizers (e.g., g-C3N4), facilitate the transfer of triplet excitons from the interior to the surface of the catalyst, and enhance catalytic reactions on its surface. In the photooxidation reaction of uric acid (UA), the conversion of UA catalyzed by pg-C3N4/PCA can reach 91.3% at 60 min, higher than that without modification (24.0%). DFT calculations and spectroscopic characterization show that the triplet acceptor in pg-C3N4/PCA could reduce the ΔEST value and grab triplet excitons in g-C3N4, thereby enhancing the 1O2 generation. This work provides another approach for high-efficiency utilization of heterogeneous triplet photosensitizers in wide applications.

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer.

We propose design rules for building organic molecular bridges that enable coherent long-distance triplet-exciton transfer. Using these rules, we describe example polychromophoric structures with low inner-sphere exciton reorganization energies, low static and dynamic disorder, and enhanced π-stacking interactions between nearest-neighbor chromophores. These features lead to triplet-exciton eigenstates that are delocalized over several units at room temperature. The use of such bridges in donor–bridge–acceptor assemblies enables fast triplet-exciton transport over very long distances that is rate-limited by the donor–bridge injection and bridge–acceptor trapping rates.

Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

AbstractThe spectral diffusion of singlet and triplet excitons in 9,9‐dioctylfluorene‐based conjugated copolymers is investigated using photoluminescence spectroscopy at both low (5 K) and room temperature (300 K). Inclusion of a N,N‐diphenyl‐4‐(pyridin‐2‐yl)aniline moiety into the polymer backbone allows subsequent cyclometalation with platinum acetylacetonate to increase the spin‐orbit coupling and yield radiative decay from the triplet state. For suitably low fractions (≤5%) of bulky ligand inclusion, cyclometalated or not, the resulting longer sequences of fluorene units are able to adopt the chain‐extended β‐phase conformation. Comparison between the phosphorescence spectral diffusion in glassy‐ and β‐phase Pt‐copolymer samples provide insight into the triplet exciton transfer in more‐ or less‐disordered conjugated polymer films. It is found in the glassy‐phase samples with shorter conjugation lengths that the triplet exciton relaxation becomes frustrated at low temperature due to a freezing out of the thermally activated hops required to move from one conjugated segment to another. In contrast, for films containing β‐phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through intra‐segment motion. This work demonstrates controlled use of changes in molecular conformation to optimize triplet diffusion properties in a member of the widely deployed fluorene‐based conjugated copolymers.

Direct Exciton Harvesting from a Bound Triplet Pair.

Singlet fission is commonly defined as the generation of two triplet excitons from a single absorbed photon. However, ambiguities within this definition arise due to the complexity of the various double triplet states that exist in SF chromophores and the corresponding interconversion processes. To clarify this process, singlet fission is frequently depicted as sequential two-step conversion in which a singlet exciton decays into a bound triplet-pair biexciton state that dissociates into two "free" triplet excitons. However, this model discounts the potential for direct harvesting from the coupled biexciton state. Here, it is demonstrated that individual triplet excitons can be extracted directly from a bound triplet pair. It is demonstrated that due to the requirement for geminate triplet-triplet annihilation in intramolecular singlet fission compounds, unique spectral and kinetic signatures can be used to quantify triplet-pair harvesting yields. An internal quantum efficiency for triplet exciton transfer from the triplet pair of >50%, limited only by the solubility of the compounds is achieved. The harvesting process is not dependent on the net multiplicity of the triplet-pair state, suggesting that an explicit, independent dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.

Fundamental Charge Transfer Dynamics in 2D TMDCs for Use in Novel Heterostructures

Efficient transfer of charge carriers and/or excitons between small organic molecules and two-dimensional (2D) semiconductor interfaces is being explored for applications in photovoltaics and quantum information processing.Interfaces of small molecules and 2D semiconductors such as graphene, nanocrystals, quantum dots and transition metal dichalcogenides (TMDCs) are capable of charge, energy, and spin singlet transfer. The long excited state lifetimes of spin triplet excitons makes triplet exciton transfer across such interfaces advantageous for exciton harvesting and photon upconversion. However, to date, triplet energy transfer between physisorbed small molecules and monolayer TMDCs has only been reported in a single study. To our knowledge, the process where photoexcitation of a TMDC/organic interface yields triplet excitons in the TMDC layer has not been reported. In quantum dot (QD) systems, direct covalent attachment of organic molecules to QD surfaces has shown to facilitate triplet energy transfer across the QD/molecule interface, but this covalent approach has not been widely applied to TMDC/molecule interfaces.Here we present a systematic study of covalently functionalized TMDC/molecule interfaces employing synthetically tailored thiolated acenes. We create tunable amounts of sulfur vacancies, which allows for subsequent covalent acene functionalization. We study the impact of covalent bonding on the charge transfer (CT) dynamics of TMDC/organic interfaces with energy level offsets suitable for charge and energy transfer (ET) as well as triplet acceptance and sensitization. We characterize the interfaces with a variety of steady-state and time-resolved spectroscopic techniques and initial results show success in selective isolation of CT vs ET pathways. Figure 1

Engineering a triplet exciton enhanced photoluminescence self-assembled nanoprobe containing block copolymer and Tb3+/Ga3+ complex for fluoride ion detection in serum

The fluoride ion (F–) concentration in serum is an important indicator of fluorosis. However, the biofluorescence of serum itself makes the concentration of F– difficult to detect. Herein, a Tb‐1@F127 nanoprobe is synthesized by Flash NanoPrecipitation of Tb3+, Ga3+, organic ligands and block copolymer Pluronic (F127). The absorbance and photoluminescence (PL) of the Tb‐1@F127 nanoprobe are enhanced by the "antenna" effect of the organic ligands and the energy transfer of triplet excitons. The molar extinction coefficient of Tb‐1@F127 nanoprobe is 28,000 times higher than the f-f transfer in free Tb3+. Moreover, the addition of F– blocks the above energy transfer process, resulting in a decrease in the PL intensity of Tb‐1@F127 nanoprobe. In the serum solution, the PL intensity of Tb‐1@F127 nanoprobe has a linear response to the concentration of F– in the range of 0–18.81 ppm, and the detection limit is 0.056 ppm. The Tb‐1@F127 nanoprobe demonstrates high sensitivity and selectivity for the detection of F concentration, and is not affected by biological autofluorescence.

Harvesting the Triplet Excitons of Quasi-Two-Dimensional Perovskite toward Highly Efficient White Light-Emitting Diodes

Utilization of triplet excitons, which generally emit poorly, is always fundamental to realize highly efficient organic light-emitting diodes (LEDs). While triplet harvest and energy transfer via electron exchange between triplet donor and acceptor are fully understood in doped organic phosphorescence and delayed fluorescence systems, the utilization and energy transfer of triplet excitons in quasi-two-dimensional (quasi-2D) perovskite are still ambiguous. Here, we use an orange-phosphorescence-emitting ultrathin organic layer to probe triplet behavior in the sky-blue-emitting quasi-2D perovskite. The delicate white LED architecture enables a carefully tailored Dexter-like energy-transfer mode that largely harvests the triplet excitons in quasi-2D perovskite. Our white organic-inorganic LEDs achieve maximum forward-viewing external quantum efficiency of 8.6% and luminance over 15 000 cd m-2, exhibiting a significant efficiency enhancement versus the corresponding sky-blue perovskite LED (4.6%). The efficient management of energy transfer between excitons in quasi-2D perovskite and Frenkel excitons in the organic layer opens the door to fully utilizing excitons for white organic-inorganic LEDs.

Importance of dynamical electron correlation in diabatic couplings of electron-exchange processes.

We demonstrate the importance of the dynamical electron correlation effect in diabatic couplings of electron-exchange processes in molecular aggregates. To perform a multireference perturbation theory with large active space of molecular aggregates, an efficient low-rank approximation is applied to the complete active space self-consistent field reference functions. It is known that kinetic rates of electron-exchange processes, such as singlet fission, triplet-triplet annihilation, and triplet exciton transfer, are not sufficiently explained by the direct term of the diabatic couplings but efficiently mediated by the low-lying charge transfer states if the two molecules are in close proximity. It is presented in this paper, however, that regardless of the distance of the molecules, the direct term is considerably underestimated by up to three orders of magnitude without the dynamical electron correlation, i.e., the diabatic states expressed in the active space are not adequate to quantitatively reproduce the electron-exchange processes.

Realistic Efficiency Limits for Singlet-Fission Silicon Solar Cells.

Singlet fission is a carrier multiplication mechanism that could make silicon solar cells much more efficient. The singlet-fission process splits one high-energy spin-singlet exciton into two lower-energy spin-triplet excitons. We calculated the efficiency potential of three technologically relevant singlet-fission silicon solar cell implementations. We assume realistic but optimistic parameters for the singlet-fission material and investigate the effect of singlet energy and entropic gain. If the transfer of triplet excitons occurs via charge transfer, the maximum efficiency is 34.6% at a surprisingly small singlet energy of 1.85 eV. For the Dexter-type triplet energy transfer, the maximum efficiency is 32.9% at a singlet energy of 2.15 eV. For Förster resonance energy transfer (FRET), the triplet excitons are first transferred into a quantum dot, from which they then undergo FRET into silicon. For this transfer mechanism, the maximum efficiency is 28.% at a singlet energy of 2.33 eV. We show that the efficiency gain from singlet fission is larger the more efficient the silicon base cell is, which stands in contrast to tandem perovskite–silicon solar cells.

Bidirectional triplet exciton transfer between silicon nanocrystals and perylene.

Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light's energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley–Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon's biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 μs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system's utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion.

Lanthanide-doped inorganic nanoparticles turn molecular triplet excitons bright.

The generation, control and transfer of triplet excitons in molecular and hybrid systems is of great interest owing to their long lifetime and diffusion length in both solid-state and solution phase systems, and to their applications in light emission1, optoelectronics2,3, photon frequency conversion4,5 and photocatalysis6,7. Molecular triplet excitons (bound electron-hole pairs) are 'dark states' because of the forbidden nature of the direct optical transition between the spin-zero ground state and the spin-one triplet levels8. Hence, triplet dynamics are conventionally controlled through heavy-metal-based spin-orbit coupling9-11 or tuning of the singlet-triplet energy splitting12,13 via molecular design. Both these methods place constraints on the range of properties that can be modified and the molecular structures that can be used. Here we demonstrate that it is possible to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic insulating nanoparticles. This allows the classically forbidden transitions from the ground-state singlet to excited-state triplets to gain oscillator strength, enabling triplets to be directly generated on molecules via photon absorption. Photogenerated singlet excitons can be converted to triplet excitons on sub-10-picosecond timescales with unity efficiency by intersystem crossing. Triplet exciton states of the molecules can undergo energy transfer to the lanthanide ions with unity efficiency, which allows us to achieve luminescent harvesting of the dark triplet excitons. Furthermore, we demonstrate that the triplet excitons generated in the lanthanide nanoparticle-molecule hybrid systems by near-infrared photoexcitation can undergo efficient upconversion via a lanthanide-triplet excitation fusion process: this process enables endothermic upconversion and allows efficient upconversion from near-infrared to visible frequencies in the solid state. These results provide a new way to control triplet excitons, which is essential for many fields of optoelectronic and biomedical research.

Evolution from Tunneling to Hopping Mediated Triplet Energy Transfer from Quantum Dots to Molecules.

Efficient energy transfer is particularly important for multiexcitonic processes like singlet fission and photon upconversion. Observation of the transition from short-range tunneling to long-range hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This is firmly supported by steady-state photon upconversion measurements, a direct proxy for the efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon upconversion quantum efficiencies increase again to 0.468% and 0.413%, 1.5-1.6 fold higher than that with 3 phenylene units (using the convention where the maximum upconversion quantum efficiency is 100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first observation of the tunneling-to-hopping transition for long-range triplet energy transfer between nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should further be explored in the context of artificial photosynthesis.

Red-to-blue photon upconversion based on a triplet energy transfer process not retarded but enabled by shell-coated quantum dots.

Charge and/or energy transfer from photoexcited quantum dots (QDs) is often suppressed by a wide-bandgap shell. Here, we report an interesting, counter-intuitive observation that interfacial triplet energy transfer from QDs is not retarded but rather enabled by an insulating shell. Specifically, photoluminescence of red-emitting CdSe QDs could not be quenched by surface-anchored Rhodamine B molecules; in contrast, after ZnS shell coating, their emission was effectively quenched. Time-resolved spectroscopy reveals that the shell eliminates ultrafast hole trapping in the QDs and hence opens up the triplet exciton transfer pathway. The triplet energy of Rhodamine B can be reversely transferred back to QDs by thermal activation, or it can be passed to triplet acceptors in the solution. Capitalizing on the latter, we demonstrate red-to-blue photon upconversion based on QD-sensitized triplet-triplet annihilation with an efficiency of 2.8% and an anti-Stokes shift of 1.13 eV.