Triplet Pair Research Articles

Mono- and Bichromophores Formed from Perylene Monoimide Diesters: Competition between Intramolecular Charge Transfer and Intermolecular Singlet Exciton Fission.

Perylene monoimide diesters and the corresponding phenyl-linked bichromophores are strongly fluorescent in dilute solution with minimal triplet population after relaxation of the initial Franck-Condon state. The monomer forms nonemissive face-to-face dimers in solution, wherein illumination leads to formation of a spin-correlated, triplet pair with a yield of ca. 13% and with a time constant of 4 ± 1 ps. The triplet pair, which is localized on the aggregate, cannot separate and decays with a mean lifetime of 80 ± 10 ps. The relaxed S1 state of the weakly coupled, phenyl-linked bichromophores establishes an equilibrium with an intramolecular charge-transfer state over a hundred picoseconds or so, depending on the solvent and the geometry of the linkage. This equilibrium mixture, being dominated by the relaxed S1 state, decays on the nanosecond time scale in solution at room temperature without implication of a triplet state. Self-association occurs at higher concentration and, for the para-bridged bichromophore, leads to inefficient triplet formation in tetrahydrofuran at room temperature.

The Interplay of Strongly and Weakly Exchange-Coupled Triplet Pairs in Intramolecular Singlet Fission.

Singlet fission (SF) and triplet-triplet annihilation upconversion (TTA-UC) nominally enable the interconversion of higher-energy singlet states with two lower-energy triplet states and vice versa, with both processes having envisaged application for enhanced solar power devices. The mechanism of SF/TTA-UC involves a complex array of different multiexcitonic triplet-pair states that are coupled by the exchange interaction to varying extents. In this work a family of bounded intramolecular SF materials, based upon the chromophore 1,6-diphenyl-1,3,5-hexatriene, were designed and synthesized. Their SF behavior was characterized using fluorescence lifetime, transient absorption, and magnetic field dependence studies. The capacity for the formation of weakly exchange-coupled triplet pairs, and subsequent spin-evolution, is shown to be strongly dependent upon the combined factors of oligomer size and geometry. By contextualizing these results with the wider SF literature, we present a general schematic model for SF/TTA-UC of greater completeness than portrayed elsewhere.

The Acetylene Bridge in Intramolecular Singlet Fission: A Boon or A Nuisance?

Various analogues of the alkylsilylacetylene group are frequently used as auxiliary groups to enhance the solubility and stability of the acene dimer core, widely used as platforms to investigate intramolecular singlet fission (iSF) mechanisms. However, while in the 2,2'-linked dimers they are primarily auxiliary groups, these are essential fragments of the bridging units in 6,6'/5,5'-linked dimers, the two preferred choices for dimerization. The starkly different iSF dynamics observed in the two variants raise the question of what role the acetylene bridges play. Here, we systematically designed a set of (oligo-)para-phenylene bridged 2,2'-linked pentacene dimers with an additional acetylene fragment in the bridging unit to mimic the structure of 6,6'-linked dimers. Contrasting the results with previously reported analogous 2,2'-linked and 6,6'-linked pentacene dimers reveals that the acetylene bridges contribute to significant conformational freedom. This effect provides a mechanism to promote spin evolution within the triplet pair to achieve free triplets but also offers new parasitic pathways for triplet-pair recombination, revealing that this structural motif can be both a boon and a nuisance. Additionally, our analysis reveals that these bridges directly modify the electronic states, highlighting significant pitfalls of the standard chromophore-bridge-chromophore framework used to design and interpret photophysics of iSF materials.

The Acetylene Bridge in Intramolecular Singlet Fission: A Boon or A Nuisance?

Various analogues of the alkylsilylacetylene group are frequently used as auxiliary groups to enhance the solubility and stability of the acene dimer core, widely used as platforms to investigate intramolecular singlet fission (iSF) mechanisms. However, while in the 2,2’‐linked dimers they are primarily auxiliary groups, these are essential fragments of the bridging units in 6,6’/5,5’‐linked dimers, the two preferred choices for dimerization. The starkly different iSF dynamics observed in the two variants raise the question of what role the acetylene bridges play. Here, we systematically designed a set of (oligo‐)para‐phenylene bridged 2,2’‐linked pentacene dimers with an additional acetylene fragment in the bridging unit to mimic the structure of 6,6’‐linked dimers. Contrasting the results with previously reported analogous 2,2’‐linked and 6,6’‐linked pentacene dimers reveals that the acetylene bridges contribute to significant conformational freedom. This effect provides a mechanism to promote spin evolution within the triplet pair to achieve free triplets but also offers new parasitic pathways for triplet‐pair recombination, revealing that this structural motif can be both a boon and a nuisance. Additionally, our analysis reveals that these bridges directly modify the electronic states, highlighting significant pitfalls of the standard chromophore‐bridge‐chromophore framework used to design and interpret photophysics of iSF materials.

Configuration Interaction in Frontier Molecular Orbital Basis for Screening the Spin-Correlated, Spatially Separated Triplet Pair State 1(T···T) Formation.

In the theoretical screening of Singlet Fission rates in molecular aggregates, often the frontier molecular orbital model for dimers is employed. However, the dimer approach fails to account for recent experimental findings that suggest singlet fission progresses through a further intermediate state featuring two spatially separated, spin-correlated triplets, specifically a 1(T···T) state. We address this limitation by generalizing the often used frontier molecular orbital model for singlet fission by incorporation of both separated Charge Transfer (C···T) and 1(T···T) states as well as mixed triplet-charge transfer states, delivering analytic expressions for the diabatic matrix elements. Applying the methodology to the perylene diimide trimer, we examine the packing dependence of competing formation pathways of the 1(T···T) state by evaluation of diabatic matrix elements.

Supramolecular scaffold-directed two-dimensional assembly of pentacene into a configuration to facilitate singlet fission.

Molecular assemblies featuring two-dimensionality have attracted increasing attention, whereas such structures are difficult to construct simply relying on spontaneous molecular assembly. Here, we present two-dimensional assemblies of acene chromophores achieved using a tripodal triptycene supramolecular scaffold, which have been shown to exhibit a strong ability to assemble molecular and polymer motifs two-dimensionally. We designed pentacene and anthracene derivatives sandwiched by two triptycene units. These compounds assemble into expected two-dimensional structures, with the pentacene chromophores having both sufficient overlap to cause singlet fission and space for conformational change to facilitate the dissociation of a triplet pair into free triplets, which is not the case for the anthracene analog. Detailed spectroscopic analysis revealed that the pentacene chromophore in the assembly undergoes singlet fission with a quantum yield of 88 ± 5%, giving rise to triplet pairs, from which free triplets are efficiently generated (ΦT = 130 ± 8.8%). This demonstrates the utility of the triptycene-based scaffold to design functional π-electronic molecular assemblies.

Macrocyclic Parallel Dimer Showing Quantum Coherence of Quintet Multiexcitons at Room Temperature.

Singlet fission (SF) is a promising approach in quantum information science because it can generate spin-entangled quintet triplet pairs by photoexcitation independent of temperature. However, it is still challenging to rationally achieve quantum coherence at room temperature, which requires precise control of the orientation and dynamics of triplet pairs. Here we show that the quantum coherence of quintet multiexcitons can be achieved at room temperature by arranging two pentacene chromophores in parallel and in close proximity within a macrocycle. By making dynamic covalent Schiff-base bonds between aldehyde-modified pentacene derivatives, macrocyclic parallel dimer-1 (MPD-1) can be selectively synthesized in a high yield. MPD-1 exhibits fast subpicosecond SF in polystyrene film and generates spin-polarized quintet multiexcitons. Furthermore, the coherence time T2 of the MPD-1 quintet is as long as 648 ns, even at room temperature. This macrocyclic parallel dimer strategy opens up new possibilities for future quantum applications using molecular multilevel qubits.

Interplay of coulomb and exciton-phonon coupling controls singlet fission dynamics in two pentacene polymorphs.

Pentacene is an important model organic semiconductor in both the singlet exciton fission (SF) and organic electronics communities. We have investigated the effect of changing crystal structure on the SF process, generating multiple triplet excitons from an initial singlet exciton, and subsequent triplet recombination. Unlike for similar organic semiconductors that have strong SF sensitive to polymorphism, we find almost no quantitative difference between the kinetics of triplet pair (TT) formation in the two dominant polymorphs of pentacene. Both pairwise dimer coupling and momentum-space crystal models predict much faster TT formation from the bright singlet excited state of the Bulk vs ThinFilm polymorph, contrasting with the experiment. GW and Bethe-Salpeter equation calculations, including exciton-phonon coupling, reveal that ultrafast phonon-driven transitions in the ThinFilm polymorph compensate the intrinsically slower purely Coulomb-mediated TT formation channel, rationalizing the similarity in observed rates. Taking into account the influence of subtle structural distinctions on both the direct and phonon-mediated SF pathways reveals a predictive capability to these methods, expected to be applicable to a wide variety of molecular crystals.

Meta-learning triplet contrast network for few-shot text classification

Few-shot text classification (FSTC) strives to predict classes not involved in the training by learning from a few labeled examples. Currently, most tasks construct meta-tasks in a randomized manner that fails to give more priority to hard-to-identify classes and samples. Besides, some tasks incorporated a contrast strategy, but the sample could only be compared to positive or negative examples individually. In this work, we propose a Meta-learning Triplet Contrast Network (Meta-TCN) with bidirectional contrast capability to solve the above problem. Specifically, Meta-TCN uses external knowledge with labeled information as the class examples, which decouples the embedding of prototypes from the support pool. Meanwhile, the class examples combine the support samples to construct triplet pairs used for learning. Unlike previous studies, the model can learn negative and positive knowledge simultaneously, ensuring that understanding is enriched and enhances learning. Further, we improve the shortcomings of randomness in the meta-task construction process by proposing a Dynamic Rate of Change (DRC) sampling strategy. DRC enhances the model’s focus on difficult-to-classify samples. We conducted extensive experiments on six benchmark datasets such as Huffpost and RCV1. Experiments show that the average accuracy of Meta-TCN can achieve state-of-the-art performance in the vast majority of tasks.

Chirality in Singlet Fission: Controlling Singlet Fission in Aqueous Nanoparticles of Tetracenedicarboxylic Acid Ion Pairs.

The singlet fission characteristics of aqueous nanoparticles, self-assembled from ion pairs of tetracene dicarboxylic acid and various amines with or without chirality, are thoroughly investigated. The structure of the ammonium molecule, the counterion, is found to play a decisive role in determining the molecular orientation of the ion pairs and its regularity, spectroscopic properties, the strength of the intermolecular coupling between the tetracene chromophores, and the consequent singlet fission process. Using chiral amines has led to the formation of crystalline nanosheets and efficient singlet fission with a triplet quantum yield as high as 133% ±20% and a rate constant of 6.99×109s-1. The chiral ion pairs also provide a separation channel to free triplets with yields as high as 33% ±10%. In contrast, nanoparticles with achiral counterions do not show singlet fission, which gave low or high fluorescence quantum yields depending on the size of the counterions. The racemic ion pair produces a correlated triplet pair intermediate by singlet fission, but no decorrelation into two free triplets is observed, as triplet-triplet annihilation dominates. The introduction of chirality enables higher control over orientation and singlet fission in self-assembled chromophores. It provides new design guidelines for singlet fission materials.

(Invited) Exciton Dynamics for Spin Qubits

Singlet fission (SF) can generate exchange-coupled triplet pairs. This may lead to realizations of quantum manipulation and quantum sensing using entangled multiple qubits. However, direct manipulation of the quantum coherence has been limited in the triplet pairs. Here we show that the quantum coherence of SF-derived, weakly coupled spin correlated triplet pair (T+T) in biphenyl linked tetracece oligomer systems by using time-resolved pulsed electron paramagnetic resonance method.

Persistence of Spin Coherence in a Crystalline Environment.

We analyze quantum interference in the triplet-exciton pair generated by singlet exciton fission in a molecular crystal and introduce transport-induced dephasing (TID) as a key effect that can suppress the expected fluorescence quantum beats when the triplet-exciton wave function can localize on inequivalent sites. TID depends on the triplet-exciton hopping rate between inequivalent sites and on the energy shifts among the stationary states of the entangled triplet pair in different spatial configurations. The theoretical model is confirmed by experiments in rubrene single crystals, where triplet pairs remain entangled for more than 50ns but quantum beats are suppressed by TID within a few nanoseconds when the magnetic field is misaligned by just a few degrees from specific symmetric directions. Our experiments deliver the zero-field parameters for the rubrene molecule in its orthorhombic lattice and information on triplet-exciton transport, in particular, the triplet-exciton hopping rate between inequivalent sites, which we evaluate to be of the order of 150ps in rubrene.

Josephson tunneling controlled by spin-orbit interaction

On the occasion of the 50th anniversary of point-contact spectroscopy, we review and extend some of our recent studies on the effects of spin-orbit interactions located at an electric point contact connecting two bulk superconductors. In particular, we study the effects on the Josephson tunneling of Cooper pairs between the superconductors. We demonstrate that such tunneling forces the electronic spin to fluctuate quantum mechanically, transforming a BCS condensate into a quantum superposition of singlet and triplet pairs. The relative prevalence of those pairs can be controlled electrostatically and mechanically. The singlet pairs contribute to the Josephson charge current flowing through the point contact but not to the Josephson spin current injected into the superconductors. The spin-polarized pairs contribute to the injected Josephson spin current but not to the superconducting charge flow. These results allow for new functionalities of spin-active Josephson junctions.

Magnetic fields reveal signatures of triplet-pair multi-exciton photoluminescence in singlet fission

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton, respectively. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. It is thus attributed to a strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission.

Plasmon-enhanced sensitization of singlet oxygen on silver island films

The influence of the plasmonic effect of silver island films on the absorption, fluorescence and long-term luminescence of rose bengal in polyvinyl butyral films has been studied. Under the plasmon effect, the optical density of the dye increased by 3.3 times, the fluorescence intensity by 8.5 times, and the long-lived fluorescence and phosphorescence by 7.08 times and 10.21, respectively. It is shown that in the plasmon field of metal nanoparticles, both an increase in the excitation rate and an increase in the rates of radiative decay of excited singlet and triplet states occur. Phosphorescence of singlet oxygen with a lifetime of 86.46 μs was observed when excited in the absorption band of rose bengal. Based on the calculation of the quenching constants of rose bengal phosphorescence by molecular oxygen molecules, it is shown that the plasmonic effect enhances the energy transfer from dye triplets to oxygen molecules in collision complexes [ТPS…3  g –] as a result of heteroannihilation. Subsequently, the triplet pair decomposes to form a singlet oxygen molecule and a photosensitizer molecule in the ground state. The plasmonic effect of silver nanoparticles leads to an increase in the generation of singlet oxygen by 4.8 times.

Coherent photoexcitation of entangled triplet pair states.

The functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S1→1TT). The dark state 1TT is typically generated from bright S1 after optical excitation; however, the mechanistic details are hotly debated. Here we report a 1TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S1. Using <10-fs transient absorption spectroscopy and pumping sub-resonantly, 1TT is directly generated from the ground state. Applying this method to a range of pentacene dimers and thin films of various aggregation types, we determine the critical material properties that enable this forbidden pathway. Through a strikingly simple technique, this result opens the door for new mechanistic insights into 1TT and other dark states in organic materials.

Investigation of ultrafast intermediate states during singlet fission in lycopene H-aggregate using femtosecond stimulated Raman spectroscopy.

The singlet fission process involves the conversion of one singlet excited state into two triplet states, which has significant potential for enhancing the energy utilization efficiency of solar cells. Carotenoid, a typical π conjugated chromophore, exhibits specific aggregate morphologies known to display singlet fission behavior. In this study, we investigate the singlet fission process in lycopene H-aggregates using femtosecond stimulated Raman spectroscopy aided by quantum chemical calculation. The experimental results reveal two reaction pathways that effectively relax the S2 (11Bu+) state populations in lycopene H-aggregates: a monomer-like singlet excited state relaxation pathway through S2 (11Bu+) → 11Bu- → S1 (21Ag-) and a dominant sequential singlet fission reaction pathway involving the S2 (11Bu+) state, followed by S* state, a triplet pair state [1(TT)], eventually leading to a long lifetime triplet state T1. Importantly, the presence of both anionic and cationic fingerprint Raman peaks in the S* state is indicative of a substantial charge-transfer character.

Electronic Couplings for Triplet-Triplet Annihilation Upconversion in Crystal Rubrene.

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process able to repackage two low-frequency photons into light of higher energy. This transformation is typically orchestrated by the electronic degrees of freedom within organic compounds possessing suitable singlet and triplet energies and electronic couplings. In this work, we propose a computational protocol for the assessment of electronic couplings crucial to TTA-UC in molecular materials and apply it to the study of crystal rubrene. Our methodology integrates sophisticated yet computationally affordable approaches to quantify couplings in singlet and triplet energy transfer, the binding of triplet pairs, and the fusion to the singlet exciton. Of particular significance is the role played by charge-transfer states along the b-axis of rubrene crystal, acting as both partial quenchers of singlet energy transfer and mediators of triplet fusion. Our calculations identify the π-stacking direction as holding notable triplet energy transfer couplings, consistent with the experimentally observed anisotropic exciton diffusion. Finally, we have characterized the impact of thermally induced structural distortions, revealing their key role in the viability of triplet fusion and singlet fission. We posit that our approaches are transferable to a broad spectrum of organic molecular materials, offering a feasible means to quantify electronic couplings.

Fabrication of planar halfmetallic ferromagnetic Josephson junctions with long range coupling

Superconducting junctions with a ferromagnet as the weak link, where triplet correlations can transport supercurrents over a substantial distance, have been of long-standing interest. In this work, we study the triplet transport in planar La0.7Sr0.3MnO3 (LSMO) nanowire Josephson junctions with NbTi superconducting contacts. By meticulous ion etching with an artificial Pt hard mask, the NbTi/LSMO bilayer is structured to form an LSMO bridge without damaging its top layer. We observe superconducting (critical) currents of the order of 109 A/m2 in a junction with a length of 1.3 μm, and distinguishing superconducting quantum interference (SQI) patterns when sweeping a magnetic field perpendicular (B⊥) to the plane of the wire or parallel (B∥) to the plane and along the wire. The observed Gaussian-shaped SQI pattern is attributed to the diffusive transport of triplet pairs in the LSMO. Our work demonstrates that combinations of oxide magnets with conventional (s-wave) alloy superconductors can be a promising new route to realizing superconducting spintronics.

Enhanced controllable triplet proximity effect in superconducting spin–orbit coupled spin valves with modified superconductor/ferromagnet interfaces

In a superconductor/ferromagnet hybrid, a magnetically controlled singlet-to-triplet Cooper pair conversion can modulate the superconducting critical temperature. In these triplet superconducting spin valves, such control usually requires inhomogeneous magnetism. However, in the presence of spin–orbit coupling from an interfacial heavy metal layer, the singlet/triplet conversion rate and, thus, critical temperature can be controlled via the magnetization direction of a single homogeneous ferromagnet. Here, we report significantly enhanced controllable pair conversion to a triplet state in a Nb/Pt/Co/Pt superconducting spin valve in which Pt/Co/Pt is homogenously magnetized and proximity-coupled to a superconducting layer of Nb. The Co/Pt interface furthest away from Nb is modified by a sub-nanometer-thick layer of Cu or Au. We argue that the enhancement is most likely associated from an improvement of the Co/Pt interface due to the insertion of Cu and Au layers. Additionally, the higher normalized orbital moments in Au measured using x-ray magnetic circular dichroism shows that increasing spin–orbit coupling enhances the triplet proximity effect—an observation supported by our theoretical calculations. Our results provide a pathway to enhancing triplet pair creation by interface engineering for device development in superspintronics.