Nuclear dynamics of singlet exciton fission in pentacene single crystals

Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales.

Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield

We report a fully efficient singlet exciton fission material with high ambient chemical stability. 10,21-Bis(triisopropylsilylethynyl)tetrabenzo[a,c,l,n]pentacene (TTBP) combines an acene core with triphenylene wings that protect the formal pentacene from chemical degradation. The electronic energy levels position singlet exciton fission to be endothermic, similar to tetracene despite the triphenylenes. TTBP exhibits rapid early time singlet fission with quantitative yield of triplet pairs within 100 ps followed by thermally activated separation to free triplet excitons over 65 ns. TTBP exhibits high photoluminescence quantum efficiency, close to 100% when dilute and 20% for solid films, arising from triplet-triplet annihilation. In using such a system for exciton multiplication in a solar cell, maximum thermodynamic performance requires radiative decay of the triplet population, observed here as emission from the singlet formed by recombination of triplet pairs. Combining chemical stabilisation with efficient endothermic fission provides a promising avenue towards singlet fission materials for use in photovoltaics.

Ultrafast spectroscopy reveals singlet fission, ionization and excimer formation in perylene film

Singlet exciton fission (SF) is a spin-allowed process whereby two triplet excitons are created from one singlet exciton. This phenomenon can offset UV photon energy losses and enhance the overall efficiency in photovoltaic devices. For this purpose, it requires photostable commercially available SF materials. Excited state dynamics in pure perylene film, ease of commercial production, is studied by time-resolved fluorescence and femtosecond transient absorption techniques under different photoexcitation energies. In film, polycrystalline regions contain perylene in H-type aggregate form. SF takes place from higher excited states of these aggregates in ultrafast time scale < 30 fs, reaching a triplet formation quantum yield of 108%. Moreover, at λex = 450 nm singlet fission was detected as a result of two-quantum absorption. Other competing relaxation channels are excimer (1 ps) and dimer radical cation formation (< 30 fs). Excimer radiatively relaxes within 19 ns and radical cation recombines in 3.2 ns. Besides, exciton self-trapping by crystal lattice distortions occurs within hundreds of picosecond. Our results highlight potential of simple-fabricated perylene films with similar properties as high-cost single crystal in SF based photovoltaic applications.

Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground State and Excited State Aromaticity Rules

Singlet exciton fission photovoltaics requires chromophores with their lowest excited states arranged so that 2<i>E</i>(T₁) < <i>E</i>(S₁) and <i>E</i>(S₁) < <i>E</i>(T₂). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4<i>n</i> rule on excited state aromaticity, combined with Hückel’s 4<i>n</i>+2 rule for ground state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the <i>E</i>(T₁), <i>E</i>(S₁) and <i>E</i>(T₂) of benzene and cyclobutadiene (<b>CBD</b>) as, respectively, excited state antiaromatic and aromatic archetypes, and reveal that <b>CBD </b>fulfils the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned by choice of substituents from Baird-antiaromatic to Baird-aromatic in T₁ and S₁, and from Hückel-aromatic to Hückel-antiaromatic in S₀. The T₁ and S₁ states of most substituted fulvenes (159 of 225) are described by singly excited HOMO→LUMO configurations, providing a rational for the simultaneous tuning of <i>E</i>(T₁) and <i>E</i>(S₁) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (K_H,L), which ideally is constant throughout the compound class, providing a constant D<i>E</i>(S₁-T₁). This leads us to a geometric model for identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated <i>E</i>(T₁) of ~1 eV or higher are identified among benzannelated 4<i>n</i>pi-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird’s 4<i>n</i> and Hückel’s 4<i>n</i>+2 rules, together with substituent effects (electronic and steric) and benzannelation, can be used to tailor new chromophores with potential use in singlet fission photovoltaics.<br>

Strategies for Design of Potential Singlet Fission Chromophores Utilizing a Combination of Ground State and Excited State Aromaticity Rules

Singlet exciton fission photovoltaics requires chromophores with their lowest excited states arranged so that 2<i>E</i>(T₁) < <i>E</i>(S₁) and <i>E</i>(S₁) < <i>E</i>(T₂). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird’s 4<i>n</i> rule on excited state aromaticity, combined with Hückel’s 4<i>n</i>+2 rule for ground state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the <i>E</i>(T₁), <i>E</i>(S₁) and <i>E</i>(T₂) of benzene and cyclobutadiene (<b>CBD</b>) as, respectively, excited state antiaromatic and aromatic archetypes, and reveal that <b>CBD </b>fulfils the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned by choice of substituents from Baird-antiaromatic to Baird-aromatic in T₁ and S₁, and from Hückel-aromatic to Hückel-antiaromatic in S₀. The T₁ and S₁ states of most substituted fulvenes (159 of 225) are described by singly excited HOMO→LUMO configurations, providing a rational for the simultaneous tuning of <i>E</i>(T₁) and <i>E</i>(S₁) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (K_H,L), which ideally is constant throughout the compound class, providing a constant D<i>E</i>(S₁-T₁). This leads us to a geometric model for identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated <i>E</i>(T₁) of ~1 eV or higher are identified among benzannelated 4<i>n</i>pi-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird’s 4<i>n</i> and Hückel’s 4<i>n</i>+2 rules, together with substituent effects (electronic and steric) and benzannelation, can be used to tailor new chromophores with potential use in singlet fission photovoltaics.<br>

Activating intramolecular singlet exciton fission by altering π-bridge flexibility in perylene diimide trimers for organic solar cells

We show via time resolved spectroscopy that triplet formation proceeds via intersystem crossing in a rigid-bridged perylene diimide trimer and via efficient and fast intramolecular singlet exciton fission in the analogous flexible-bridged trimer.