Intramolecular Singlet Fission Research Articles

Conformational Planarization versus Singlet Fission: Distinct Excited-State Dynamics of Cyclooctatetraene-Fused Acene Dimers.

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Conformational Planarization versus Singlet Fission: Distinct Excited‐State Dynamics of Cyclooctatetraene‐Fused Acene Dimers

AbstractA set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited‐state dynamics in solution. While the anthracene dimer showed a fast V‐shaped‐to‐planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Pentacene Dimers as a Critical Tool for the Investigation of Intramolecular Singlet Fission.

Singlet fission (SF) involves the spontaneous splitting of a photoexcited singlet state into a pair of triplets, and it holds great promise toward the realization of more efficient solar cells. Although the process of SF has been known since the 1960s, debate regarding the underlying mechanism continues to this day, especially for molecular materials. A number of different chromophores have been synthesized and studied in order to better understand the process of SF. These previous reports have established that pentacene and its derivatives are especially well-suited for the study of SF, since the energetic requirement E(S1 )≥2E(T1 ) is fulfilled rendering the process exothermic and unidirectional. Dimeric pentacene derivatives, in which individual pentacene chromophores are tethered by a "spacer", have emerged as the system of choice toward exploring the mechanism of intramolecular singlet fission (iSF). The dimeric structure, and in particular the spacer, allows for controlling and tuning the distance, geometric relationship, and electronic coupling between the two pentacene moieties. This Minireview describes recent advances using pentacene dimers for the investigation of iSF.

Intramolecular singlet fission in a face-to-face stacked tetracene trimer.

A covalently linked tetracene trimer with a "face-to-face" stacked structure was prepared and its molecular structure is characterized by 1H NMR, MALDI-TOF mass spectroscopy, and elemental analysis. The minimized molecular structure reveals that three tetracene subunits within this trimer adopt a partially "face-to-face" stacked configuration. Its absorption spectrum differs significantly from that of the monomeric and dimeric counterparts in solution due to the strong ground state interactions between the neighboring tetracene subunits. Its fluorescence is almost quenched completely. An ultrafast fluorescence decay component is observed in its fluorescence dynamics, indicating the presence of an ultrafast nonradiative decay channel in the trimer. The nonradiative channel is proved to be intramolecular singlet fission (iSF) by femto-second transient absorption studies. Different from the strongly coupled triplet state observed in the corresponding dimer, weakly coupled triplet states can be formed in this trimer. The triplet quantum yield of trimer 4 can reach up to 126% in solution.

Controlling Long-Lived Triplet Generation from Intramolecular Singlet Fission in the Solid State.

The conjugated polymer poly(benzothiophene dioxide) (PBTDO1) has recently been shown to exhibit efficient intramolecular singlet fission in solution. We investigate the role of intermolecular interactions in triplet separation dynamics after singlet fission. We use transient absorption spectroscopy to determine the singlet fission rate and triplet yield in two polymers differing only by side-chain motif in both solution and the solid state. Whereas solid-state films show singlet fission rates identical to those measured in solution, the average lifetime of the triplet population increases dramatically and is strongly dependent on side-chain identity. These results show that it may be necessary to carefully engineer the solid-state microstructure of these "singlet fission polymers" to produce the long-lived triplets needed to realize efficient photovoltaic devices.

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.

Absence of Intramolecular Singlet Fission in Pentacene-Perylenediimide Heterodimers: The Role of Charge Transfer State.

A new class of donor-acceptor heterodimers based on two singlet fission (SF)-active chromophores, i.e., pentacene (Pc) and perylenediimide (PDI), was developed to investigate the role of charge transfer (CT) state on the excitonic dynamics. The CT state is efficiently generated upon photoexcitation. However, the resulting CT state decays to different energy states depending on the energy levels of the CT state. It undergoes extremely rapid deactivation to the ground state in polar CH2Cl2, whereas it undergoes transformation to a Pc triplet in nonpolar toluene. The efficient triplet generation in toluene is not due to SF but CT-mediated intersystem crossing. In light of the energy landscape, it is suggested that the deep energy level of the CT state relative to that of the triplet pair state makes the CT state actually serve as a trap state that cannot undergoes an intramolecular singlet fission process. These results provide guidance for the design of SF materials and highlight the requisite for more widely applicable design principles.

Two-Photon Absorption in Pentacene Dimers: The Importance of the Spacer Using Upconversion as an Indirect Route to Singlet Fission.

In this proof of concept study, we show that intramolecular singlet fission (iSF) can be initiated from a singlet excited state accessed by two-photon absorption, rather than through a traditional route of direct one-photon excitation (OPE). Thus, iSF in pentacene dimers 2 and 3 is enabled through NIR irradiation at 775 nm, a wavelength where neither dimer exhibits linear absorption of light. The adamantyl and meta-phenylene spacers 2 and 3, respectively, are designed to feature superimposable geometries, which establishes that the electronic coupling between the two pentacenes is the significant structural feature that dictates iSF efficiency.

Triplet Harvesting from Intramolecular Singlet Fission in Polytetracene.

Singlet fission (SF), a promising mechanism of multiple exciton generation, has only recently been engineered as a fast, efficient, intramolecular process (iSF). The challenge now lies in designing and optimizing iSF materials that can be practically applied in high-performance optoelectronic devices. However, most of the reported iSF systems, such as those based on donor-acceptor polymers or pentacene, have low triplet energies, which limits their applications. Tetracene-based materials can overcome significant challenges, as the tetracene triplet state is practically useful, ≈1.2 eV. Here, the synthesis and excited state dynamics of a conjugated tetracene homopolymer are studied. This polymer undergoes ultrafast iSF in solution, generating high-energy triplets on a sub-picosecond time scale. Magnetic-field-dependent photocurrent measurements of polytetracene-based devices demonstrate the first example of iSF-generated triplet extraction in devices, exhibiting the potential of iSF materials for use in next-generation devices.

Correction to "Design Principles of Electronic Couplings for Intramolecular Singlet Fission in Covalently-Linked Systems".

Correction to "Design Principles of Electronic Couplings for Intramolecular Singlet Fission in Covalently-Linked Systems".

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers.

Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wave functions of the optical singlets, the lowest triplet exciton, and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against earlier and very recent experimental transient absorptions in the near- and mid-infrared, we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 21Ag- in linear polyenes.

Effect of Off-Diagonal Exciton-Phonon Coupling on Intramolecular Singlet Fission.

Intramolecular singlet fission (iSF) materials provide remarkable advantages in terms of tunable electronic structures, and quantum chemistry studies have indicated strong electronic coupling modulation by high frequency phonon modes. In this work, we formulate a microscopic model of iSF with simultaneous diagonal and off-diagonal coupling to high-frequency modes. A nonperturbative treatment, the Dirac-Frenkel time-dependent variational approach is adopted using the multiple Davydov trial states. It is shown that both diagonal and off-diagonal coupling can aid efficient singlet fission if excitonic coupling is weak, and fission is only facilitated by diagonal coupling if excitonic coupling is strong. In the presence of off-diagonal coupling, it is found that high frequency modes create additional fission channels for rapid iSF. Results presented here may help provide guiding principles for design of efficient singlet fission materials by directly tuning singlet-triplet interstate coupling.

Distinct properties of the triplet pair state from singlet fission.

Singlet fission, the conversion of a singlet exciton (S1) to two triplets (2 × T1), may increase the solar energy conversion efficiency beyond the Shockley-Queisser limit. This process is believed to involve the correlated triplet pair state 1(TT). Despite extensive research, the nature of the 1(TT) state and its spectroscopic signature remain actively debated. We use an end-connected pentacene dimer (BP0) as a model system and show evidence for a tightly bound 1(TT) state. It is characterized in the near-infrared (IR) region (~1.0 eV) by a distinct excited-state absorption (ESA) spectral feature, which closely resembles that of the S1 state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to 1(TT)→Sn and S1→Sn' transitions; Sn and Sn' likely come from the antisymmetric and symmetric linear combinations, respectively, of the S2 state localized on each pentacene unit in the dimer molecule. The 1(TT)→Sn transition is an indicator of the intertriplet electronic coupling strength, because inserting a phenylene spacer or twisting the dihedral angle between the two pentacene chromophores decreases the intertriplet electronic coupling and diminishes this ESA peak. In addition to spectroscopic signature, the tightly bound 1(TT) state also shows chemical reactivity that is distinctively different from that of an individual T1 state. Using an electron-accepting iron oxide molecular cluster [Fe8O4] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T1 in pentacene but not from the tightly bound 1(TT) state. Thus, reducing intertriplet electronic coupling in 1(TT) via molecular design might be necessary for the efficient harvesting of triplets from intramolecular singlet fission.

Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon.

Singlet fission (SF), in which one singlet exciton (S1 ) splits into two triplets (T1 ) on adjacent molecules through a correlated triplet-pair 1 (TT) state, requires precise but difficult tuning of exciton energetics and intermolecular electronic couplings in the solid state. Antiaromatic 4nπ dibenzopentalenes (DPs) are demonstrated as a new class of single-chromophore-based intramolecular SF materials that exhibit an optically allowed S2 state with E(S2 )>2×E(T1 ) and an optically forbidden S1 state. Ultrafast population transfer from a high-lying S2 state to a 1 (TT) state was observed in monomeric solution of styryl-substituted DP (SDP) on a sub-picosecond timescale. There is evidence of exciton diffusion (ED) of the 1 (TT) state to yield two individual long-lived triplets in SDP thin film. The overall triplet yield via intramolecular SF and subsequent triplet-pair diffusion can be as high as 142±10 % in thin film.

Role of the Dark 2Ag State in Donor-Acceptor Copolymers as a Pathway for Singlet Fission: A DMRG Study.

The mechanism of intramolecular singlet fission in donor-acceptor-type copolymers, especially the role of the dark 2Ag state, is not so clear. In this Letter, the electronic structure of the benzodithiophene (B)-thiophene-1,1-dioxide (TDO) copolymer is calculated by density matrix renormalization group theory with the Pariser-Parr-Pople model. We find that the dark 2Ag state is the lowest singlet excited state and is nearly degenerate with the 1Bu state. So, a fast internal conversion from 1Bu to 2Ag state is highly possible. The 2Ag state has a strong triplet pair character, localized on two neighboring acceptor units, which indicates that it is an intermediate state for the intramolecular singlet fission process. With the increase of the donor-acceptor push-pull strength in our model, this triplet pair character of the 2Ag state becomes more prominent, and meanwhile the binding energy of this coupled triplet pair state decreases, which favors the separation into two uncoupled triplet states. We propose a model in which the competition between the singlet fission process and the nonradiative decay process from the 2Ag state would determine the final quantum yield.

(Invited) Tuning Molecular Singlet Fission – Strong Versus Weak Electronic Coupling

Innovative strategies are necessary to overcome the Shockley-Queisser limit, which places the maximum solar conversion efficiency for a single p-n junction to slightly more than 30%. One such strategy, singlet fission (SF), allows high energy, singlet excited state photons (S1) to be down-converted into up to twice as many low-energy, triplet excited state photons (T1). As such, SF is a fascinating many-electron problem from a physical chemistry perspective. In an optimal SF scenario, a chromophore in a singlet excited state (S1) interacts with a nearby ground state (S0) chromophore to yield up to two, non-interacting T1s. Doubling the number of excited states, thus, goes hand in hand with sacrificing half or more of the overall energy per excited state. The energetic requirement for SF is E(S1) ≥ 2(E(T1)), and the free reaction enthalpy for the S1 ® 2(T1) conversion governs the energy loss in SF. The SF rate depends on the free reaction energy and the electronic coupling matrix element between one chromophore in its S1 and another in its S0 ground state. A strict definition of the SF rate implies the formation rate of the two individual triplets (T1 + T1), not that of the correlated triplet pair, 1(T1T1). In (T1 + T1), the two triplets should have lost electronic coherence owing to interactions with the environment, but can retain spin coherence on much longer timescales. In a nutshell, electronic coupling between the chromophores should be sufficient to drive SF, while at the same time keeping the coupling sufficiently weak to allow spin decoherence leading to two independent triplet states. The systematic variation / comprehensive understanding of electronic coupling between the individual pentacenes constitutes one of the grand challenges in the field of molecular SF, especially when experiments are conducted in solution. By means of tailoring the synthesis of pentacene dimers, trimers, and tetramers we will span our investigations from a weakly coupled, charge transfer to a strongly coupled, charge separated regime. In turn, we are confident to gain control over the rates and yields of intramolecular SF and, subsequent, inter- and intramolecular charge extraction. In particular, we plan to investigate pentacene materials, in which the energy of the charge transfer state and that of the charge separated state is tuned in the range around twice of the triplet excited state, and to explore the impact on the formation of pairs of independent triplets (T1 + T1).

A Covalently Linked Tetracene Trimer: Synthesis and Singlet Exciton Fission Property.

A linear tetracene trimer linked by phenyl groups has been prepared for the first time. The triplet quantum yield formed via intramolecular singlet fission can reach up to 96% in this trimer, which is enhanced significantly compared with that in the dimer. This can be attributed to the stronger electronic coupling between tetracene subunits and more delocalized excitons in the trimer.

Singlet Fission: Progress and Prospects in Solar Cells.

The third generation of photovoltaic technology aims to reduce the fabrication cost and improve the power conversion efficiency (PCE) of solar cells. Singlet fission (SF), an efficient multiple exciton generation (MEG) process in organic semiconductors, is one promising way to surpass the Shockley-Queisser limit of conventional single-junction solar cells. Traditionally, this MEG process has been observed as an intermolecular process in organic materials. The implementation of intermolecular SF in photovoltaic devices has achieved an external quantum efficiency of over 100% and demonstrated significant promise for boosting the PCE of third generation solar cells. More recently, efficient intramolecular SF has been reported. Intramolecular SF materials are modular and have the potential to overcome certain design constraints that intermolecular SF materials possess, which may allow for more facile integration into devices.

Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering.

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.

Through-Linker Intramolecular Singlet Fission: General Mechanism and Designing Small Chromophores

We perform quantum chemistry calculations and quantum dynamics simulations to investigate the covalent linker's through-bond effects in intramolecular singlet fission. A model molecule with two diazadiborine chromophore units and the para-phenylene linker is proposed. A general, step-by-step picture for the conversion from the single- to the multiexcitonic state through the linker is presented. On the basis of the picture, we discuss the triplet-pair delocalization into the linker and design two more chromophores with higher fission efficiency. All three designed chromophores have promising picosecond fission time scales and make good candidates for azaborine synthesis.