Molecular Control of Triplet-Pair Spin Polarization and Its Optoelectronic Magnetic Resonance Probes.

ConspectusPreparing and manipulating pure magnetic states in molecular systems are the key initial requirements for harnessing the power of synthetic chemistry to drive practical quantum sensing and computing technologies. One route for achieving the requisite higher spin states in organic systems exploits the phenomenon of singlet fission, which produces pairs of triplet excited states from initially photoexcited singlets in molecular assemblies with multiple chromophores. The resulting spin states are characterized by total spin (quintet, triplet, or singlet) and its projection onto a specified molecular or magnetic field axis. These excited states are typically highly polarized but exhibit an impure spin population pattern. Herein, we report the prediction and experimental verification of molecular design rules that drive the population of a single pure magnetic state and describe the progress toward its experimental realization.A vital feature of this work is the close partnership among theory, chemical synthesis, and spectroscopy. We begin by presenting our theoretical framework for understanding spin manifold interconversion in singlet fission systems. This theory makes specific testable predictions about the intermolecular structure and orientation relative to an external magnetic field that should lead to pure magnetic state preparation and provides a powerful tool for interpreting magnetic spectra. We then test these predictions through detailed magnetic spectroscopy experiments on a series of new molecular architectures that meet one or more of the identified structural criteria. Many of these architectures rely on the synthesis of molecules with features unique to this effort: rigid bridges between chromophores in dimers, heteroacenes with tailored singlet/triplet-pair energy level matching, or side-group engineering to produce specific crystal structures. The spin evolution of these systems is revealed through our application and development of several magnetic resonance methods, each of which has different sensitivities and relevance in environments relevant to quantum applications.Our theoretical predictions prove to be remarkably consistent with our experimental results, though experimentally meeting all the structural prescriptions demanded by theory for true pure-state preparation remains a challenge. Our magnetic spectra agree with our model of triplet-pair behavior, including funneling of the population to the ms = 0 magnetic sublevel of the quintet under specified conditions in dimers and crystals, showing that this phenomenon is subject to control through molecular design. Moreover, our demonstration of novel and/or highly sensitive detection mechanisms of spin states in singlet fission systems, including photoluminescence (PL), photoinduced absorption (PA), and magnetoconductance (MC), points the way toward both a deeper understanding of how these systems evolve and technologically feasible routes toward experiments at the single-molecule quantum limit that are desirable for computational applications.

Triplet-pair spin signatures from macroscopically aligned heteroacenes in an oriented single crystal.

The photo-driven process of singlet fission generates coupled triplet pairs (TT) with fundamentally intriguing and potentially useful properties. The quintet 5TT0 sublevel is particularly interesting for quantum information because it is highly entangled, is addressable with microwave pulses, and could be detected using optical techniques. Previous theoretical work on a model Hamiltonian and nonadiabatic transition theory, called the JDE model, has determined that this sublevel can be selectively populated if certain conditions are met. Among the most challenging, the molecules within the dimer undergoing singlet fission must have their principal magnetic axes parallel to one another and to an applied Zeeman field. Here, we present time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy of a single crystal sample of a tetracenethiophene compound featuring arrays of dimers aligned in this manner, which were mounted so that the orientation of the field relative to the molecular axes could be controlled. The observed spin sublevel populations in the paired TT and unpaired (T+T) triplets are consistent with predictions from the JDE model, including preferential 5TT0 formation at z ‖ B0, with one caveat-two 5TT spin sublevels have little to no population. This may be due to crossings between the 5TT and 3TT manifolds in the field range investigated by TR-EPR, consistent with the intertriplet exchange energy determined by monitoring photoluminescence at varying magnetic fields.

Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs.

A comprehensive investigation of the solution-phase photophysics of tetracene bis-carboxylic acid [5,12-tetracenepropiolic acid (Tc-DA)] and its related methyl ester [5,12-tetracenepropynoate (Tc-DE)], a non-hydrogen-bonding counterpart, reveals the role of the carboxylic acid moiety in driving molecular aggregation and concomitant excited-state behavior. Low-concentration solutions of Tc-DA exhibit similar properties to the popular 5,12-bis((triisopropylsilyl)ethynl)tetracene, but as the concentration increases, evidence for aggregates that form excimers and a new mixed-state species with charge-transfer (CT) and correlated triplet pair (TT) character is revealed by transient absorption and fluorescence experiments. Aggregates of Tc-DA evolve further with concentration toward an additional phase that is dominated by the mixed CT/TT state which is the only state present in Tc-DE aggregates and can be modulated with the solvent polarity. Computational modeling finds that cofacial arrangement of Tc-DA and Tc-DE subunits is the most stable aggregate structure and this agrees with results from 1H NMR spectroscopy. The calculated spectra of these cofacial dimers replicate the observed broadening in ground-state absorption as well as accurately predict the formation of a near-UV transition associated with a CT between molecular subunits that is unique to the specific aggregate structure. Taken together, the results suggest that the hydrogen bonding between Tc-DA molecules and the associated disruption of hydrogen bonding with solvent produce a regime of dimer-like behavior, absent in Tc-DE, that favors excimers rather than CT/TT mixed states. The control of aggregate size and structure using distinct functional groups, solute concentration, and solvent in tetracene promises new avenues for its use in light-harvesting schemes.

Revealing the Singlet Fission Mechanism for a Silane-Bridged Thienotetracene Dimer.

Tetraceno[2,3-b]thiophene is regarded as a strong candidate for singlet fission-based solar cell applications due to its mixed characteristics of tetracene and pentacene that balance exothermicity and triplet energy. An electronically weakly coupled tetraceno[2,3-b]thiophene dimer (Et2Si(TIPSTT)2) with a single silicon atom bridge has been synthesized, providing a new platform to investigate the singlet fission mechanism involving the two acene chromophores. We study the excited state dynamics of Et2Si(TIPSTT)2 by monitoring the evolution of multiexciton coupled triplet states, 1TT to 5TT to 3TT to T1 + S0, upon photoexcitation with transient absorption, temperature-dependent transient absorption, and transient/pulsed electron paramagnetic resonance spectroscopies. We find that the photoexcited singlet lifetime is 107 ps, with 90% evolving to form the TT state, and the complicated evolution between the multiexciton states is unraveled, which can be an important reference for future efforts toward tetraceno[2,3-b]thiophene-based singlet fission solar cells.

Mediating Photochemical Reaction Rates at Lewis Acidic Rare Earths by Selective Energy Loss to 4f-Electron States.

Manifesting chemical differences in individual rare earth (RE) element complexes is challenging due to the similar sizes of the tripositive cations and the corelike 4f shell. We disclose a new strategy for differentiating between similarly sized Dy3+ and Y3+ ions through a tailored photochemical reaction of their isostructural complexes in which the f-electron states of Dy3+ act as an energy sink. Complexes RE(hfac)3(NMMO)2 (RE = Dy (2-Dy) and Y (2-Y), hfac = hexafluoroacetylacetonate, and NMMO = N-methylmorpholine-N-oxide) showed variable rates of oxygen atom transfer (OAT) to triphenylphosphine under ultraviolet (UV) irradiation, as monitored by 1H and 19F NMR spectroscopies. Ultrafast transient absorption spectroscopy (TAS) identified the excited state(s) responsible for the photochemical OAT reaction or lack thereof. Competing sensitization pathways leading to excited-state deactivation in 2-Dy through energy transfer to the 4f electron manifold ultimately slows the OAT reaction at this metal cation. The measured rate differences between the open-shell Dy3+ and closed-shell Y3+ complexes demonstrate that using established principles of 4f ion sensitization may deliver new, selective modalities for differentiating the RE elements that do not depend on cation size.

Optical readout of singlet fission biexcitons in a heteroacene with photoluminescence detected magnetic resonance.

Molecular spin systems based on photoexcited triplet pairs formed via singlet fission (SF) are attractive as carriers of quantum information because of their potentially pure and controllable spin polarization, but developing systems that offer optical routes to readout as well as initialization is challenging. Herein, we characterize the electron spin magnetic resonance change in the photoluminescence intensity for a tailored organic molecular crystal while sweeping a microwave drive up to 10 GHz in a broadband loop structure. We observe resonant transitions for both triplet and quintet spin sublevel populations showing their optical sensitivity and revealing the zero-field parameters for each. We map the evolution of these spectra in both microwave frequency and magnetic field, producing a pattern of optically detected magnetic resonance (ODMR) peaks. Fits to these data using a suitable model suggest significant spin polarization in this system with orientation selectivity. Unusual excitation intensity dependence is also observed, which inverts the sign of the ODMR signal for the triplet features, but not for the quintet. These observations demonstrate optical detection of the spin sublevel population dictated by SF and intermolecular geometry, and highlight anisotropic and multi-scale dynamics of triplet pairs.

Lessons from intramolecular singlet fission with covalently bound chromophores.

Molecular dimers, oligomers, and polymers are versatile components in photophysical and optoelectronic architectures that could impact a variety of applications. We present a perspective on such systems in the field of singlet fission, which effectively multiplies excitons and produces a unique excited state species, the triplet pair. The choice of chromophore and the nature of the attachment between units, both geometrical and chemical, play a defining role in the dynamical scheme that evolves upon photoexcitation. Specific final outcomes (e.g., separated and uncorrelated triplet pairs) are being sought through rational design of covalently bound chromophore architectures built with guidance from recent fundamental studies that correlate structure with excited state population flow kinetics.

Sensitizing Singlet Fission with Perovskite Nanocrystals.

The marriage of colloidal semiconductor nanocrystals and functional organic molecules has brought unique opportunities in emerging photonic and optoelectronic applications. Traditional semiconductor nanocrystals have been widely demonstrated to initiate efficient triplet energy transfer at the nanocrystal-acene interface. Herein, we report that unlike conventional semiconductor nanocrystals, lead halide perovskite nanocrystals promote an efficient Dexter-like singlet energy transfer to surface-anchored pentacene molecules rather than triplet energy transfer. Subsequently, molecular pentacene triplets are efficiently generated via singlet fission on the nanocrystal surface. Our demonstrated strategy not only unveils the obscure energy dynamics between perovskite nanocrystal and acenes, but also brings important perspectives of utilizing singlet fission throughout the solar spectrum.

Structure and photophysics of indigoids for singlet fission: Cibalackrot.

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model.

Control of Energy Flow Dynamics between Tetracene Ligands and PbS Quantum Dots by Size Tuning and Ligand Coverage.

We have prepared a series of samples with the ligand 6,13-bistri(iso-propyl)silylethynyl tetracene 2-carboxylic acid (TIPS-Tc-COOH) attached to PbS quantum dot (QD) samples of three different sizes in order to monitor and control the extent and time scales of energy flow after photoexcitation. Fast energy transfer (∼1 ps) to the PbS QD occurs upon direct excitation of the ligand for all samples. The largest size QD maintains the microsecond exciton lifetime characteristic of the as-prepared oleate terminated PbS QDs. However, two smaller QD sizes with lowest exciton energies similar to or larger than the TIPS-Tc-COO- triplet energy undergo energy transfer between QD core and ligand triplet on nanosecond to microsecond timescales. For the intermediate size QDs in particular, energy can be recycled many times between ligand and core, but the triplet remains the dominant excited species at long times, living for ∼3 µs for fully exchanged QDs and up to 30 µs for partial ligand exchange, which is revealed as a method for controlling the triplet lifetime. A unique upconverted luminescence spectrum is observed that results from annihilation of triplets after exclusive excitation of the QD core.

Singlet Fission Involves an Interplay between Energetic Driving Force and Electronic Coupling in Perylenediimide Films.

Due to its ability to offset thermalization losses in photoharvesting systems, singlet fission has become a topic of research interest. During singlet fission, a high energy spin-singlet state in an organic semiconductor divides its energy to form two lower energy spin-triplet excitations on neighboring chromophores. While key insights into mechanisms leading to singlet fission have been gained recently, developing photostable compounds that undergo quantitative singlet fission remains a key challenge. In this report, we explore triplet exciton production via singlet fission in films of perylenediimides, a class of compounds with a long history of use as industrial dyes and pigments due to their photostability. As singlet fission necessitates electron transfer between neighboring molecules, its rate and yield depend sensitively on their local arrangement. By adding different functional groups at their imide positions, we control how perylenediimides pack in the solid state. We find inducing a long axis displacement of ∼3 Šbetween neighboring perylenediimides gives a maximal triplet production yield of 178% with a fission rate of ∼245 ps despite the presence of an activation barrier of ∼190 meV. These findings disagree with Marcus theory predictions for the optimal perylenediimide geometry for singlet fission, but do agree with Redfield theory calculations that allow singlet fission to occur via a charge transfer-mediated superexchange mechanism. Unfortunately, triplets produced by singlet fission are found to decay over tens of nanoseconds. Our results highlight that singlet fission materials must be designed to not only produce triplet excitons but to also facilitate their extraction.

Singlet Fission and Excimer Formation in Disordered Solids of Alkyl-Substituted 1,3-Diphenylisobenzofurans.

We describe the preparation and excited state dynamics of three alkyl derivatives of 1,3-diphenylisobenzofuran (1) in both solutions and thin films. The substitutions are intended to disrupt the slip-stacked packing observed in crystals of 1 while maintaining the favorable energies of singlet and triplet for singlet fission (SF). All substitutions result in films that are largely amorphous as judged by the absence of strong X-ray diffraction peaks. The films of 1 carrying a methyl in the para position of one phenyl ring undergo SF relatively efficiently (≥75% triplet yield, ΦT) but more slowly than thin films of 1. When the methyl is replaced with a t-butyl, kinetic competition in the excited state favors excimer formation rather than SF (ΦT = 55%). When t-Bu groups are placed in both meta positions of the phenyl substituent, SF is slowed further and ΦT = 35%.

Solvent-Controlled Branching of Localized versus Delocalized Singlet Exciton States and Equilibration with Charge Transfer in a Structurally Well-Defined Tetracene Dimer.

A detailed photophysical picture is elaborated for a structurally well-defined and symmetrical bis-tetracene dimer in solution. The molecule was designed for interrogation of the initial photophysical steps (S1 → 1TT) in intramolecular singlet fission (SF). (Triisopropylsilyl)acetylene substituents on the dimer TIPS-BT1 as well as a monomer model TIPS-Tc enable a comparison of photophysical properties, including transient absorption dynamics, as solvent polarity is varied. In nonpolar toluene solutions, TIPS-BT1 decays via radiative and nonradiative pathways to the ground state with no evidence for dynamics related to the initial stages of SF. This contrasts with the behavior of the previously reported unsubstituted dimer BT1 and is likely a consequence of energetic perturbations to the singlet excited-state manifold of TIPS-BT1 by the (trialkylsilyl)acetylene substituents. In polar benzonitrile, two key findings emerge. First, photoexcited TIPS-BT1 shows a bifurcation into both arm-localized (S1-loc) and dimer-delocalized (S1-dim) singlet exciton states. The S1-loc decays to the ground state, and weak temperature dependence of its emissive signatures suggests that once it is formed, it is isolated from S1-dim. Emissive signatures of the S1-dim state, on the other hand, are strongly temperature-dependent, and transient absorption dynamics show that S1-dim equilibrates with an intramolecular charge-transfer state in 50 ps at room temperature. This equilibrium decays to the ground state with little evidence for formation of long-lived triplets nor 1TT. These detailed studies spectrally characterize many of the key states in intramolecular SF in this class of dimers but highlight the need to tune electronic coupling and energetics for the S1 → 1TT photoreaction.

Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran.

Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para'-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such "excitation isomerism" is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly.

Quantum confined electron-phonon interaction in silicon nanocrystals.

We study the micro-Raman spectra of colloidal silicon nanocrystals as a function of size, excitation wavelength, and excitation intensity. We find that the longitudinal optical (LO) phonon spectrum is asymmetrically broadened toward the low energy side and exhibits a dip or antiresonance on the high-energy side, both characteristics of a Fano line shape. The broadening depends on both nanocrystal size and Raman excitation wavelength. We propose that the Fano line shape results from interference of the optical phonon response with a continuum of electronic states that become populated by intraband photoexcitation of carriers. The asymmetry exhibits progressive enhancement with decreasing particle size and with increasing excitation energy for a given particle size. We compare our observations with those reported for p- and n-doped bulk Si, where Fano interference has also been observed, but we find opposite wavelength dependence of the asymmetry for the bulk and nanocrystalline Si. Our results have important implications for potentially controlling carrier energy relaxation channels in strongly confined Si nanocrystals.

Mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran.

In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1) we have performed ultrafast transient absorption spectroscopy as a function of sample temperature and excitation fluence on polycrystalline thin films composed of two polymorphs. Our earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepared singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time constant of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is a primary competitor of singlet fission at higher fluence and that fission from higher-lying states can also contribute to the triplet formation process. Measurements employing different excitation energies and sample temperatures reveal the role that trapping processes play in attenuating the triplet quantum yield to produce the complex temperature dependence of the singlet fission yield. The rate constants for singlet fission itself are essentially temperature independent.

The role of chromophore coupling in singlet fission.

Certain organic materials can generate more than one electron-hole pair per absorbed photon, a property that could revolutionize the prospects for solar energy. This process, called singlet fission, is one possible "exciton multiplication" scheme that could be useful in a variety of photovoltaic device designs from dye-sensitized solar cells to solar cell bilayers to bulk heterojunctions. For such applications to be possible, however, singlet fission must occur with near perfect efficiency in compounds that also have other requisite properties such as strong visible light absorption and photostability. Many recent investigations of singlet fission have focused on crystalline polyacenes, which have been known for some time to undergo singlet fission. While these materials have promise, limitations in stability, cost, and performance may hinder practical application of polyacene solar cells, while their complex photophysics may limit our fundamental understanding of singlet fission in crystalline polyacenes. In this Account, we describe rationally designed singlet fission chromophores whose excited state dynamics should be fairly simple and whose coupling can be well controlled through the formation of covalent dimers, aggregates, or polycrystalline films. In principle, investigations of these chromophores should provide the clearest connection to theoretical concepts explaining how an excited state evolves from a singlet (S1) into two triplets (TT). Realizing the promise of efficient singlet fission rests with two tasks: (i) producing an ideal molecular energy level structure and (ii) inducing the correct type and strength of chromophore coupling. In this Account, we offer theoretical guidance for achieving (i) and consider more extensively recent results aimed at (ii). For (i), theoretical guidance suggests that, in addition to alternant hydrocarbons like tetracene and pentacene, biradicals (i.e., molecules with two independent radical centers) may also be used as the basis for designing chromophores with low-lying triplet states such that the energy relationship 2E(T1) ≤ E(S1) is satisfied. Although molecules that do not fulfill this condition can also exhibit singlet fission from a higher lying or vibrationally excited singlet state, fast relaxation processes will likely reduce the singlet fission yield and complicate determination of the singlet fission mechanism. For (ii), once an appropriate chromophore has been chosen, the task of coupling two or more of them together must be done carefully. We discuss three pathways by which a dimer could undergo singlet fission: (1) A direct route in which slipped cofacial stacking is favorable under certain conditions. Cofacial stacking is common in molecular crystals, and it is likely not a coincidence that recent reports of efficient singlet fission involve slipped-stacked molecules in polycrystalline thin films. (2) A mediated route in which S1 interacts with (TT) through a virtual radical cation/anion state, which may be important in some situations. (3) A two-step route (i.e., through a real charge transfer intermediate) which others have suggested theoretically. We present data on 1,3-diphenylisobenzofuran (DPIBF) dimers that are consistent with this pathway. Finally, we review potential solar photoconversion efficiency gains utilizing singlet fission in several contexts.

Coherent exciton delocalization in strongly coupled quantum dot arrays.

Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical transitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron-hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter.

Surface Loading Dictates Triplet Production via Singlet Fission in Anthradithiophene Sensitized TiO2 Films.

Singlet fission, the process of transforming a singlet excited state into two lower energy triplet excited states, is a promising strategy for improving the efficiency of dye-sensitized solar cells. The difficulty in utilizing singlet fission molecules in this architecture is understanding and controlling the orientation of dyes on mesoporous metal oxide surfaces to maximize triplet production and minimize detrimental deactivation pathways, such as electron injection from the singlet or excimer formation. Here, we varied the concentration of loading solutions of two anthradithiophene dyes derivatized with either one or two carboxylic acid groups for binding to a metal oxide surface and studied their photophysics using ultrafast transient absorption spectroscopy. For the single carboxylic acid case, an increase in dye surface coverage led to an increase in apparent triplet excited-state growth via singlet fission, while the same increase in coverage with two carboxylic acids did not. This study represents a step toward controlling the interactions between molecules at mesoporous interfaces.