Singlet Fission Chromophores Research Articles

Exploring optimal multimode vibronic pathways in singlet fission of azaborine analogues of perylene.

The development of new singlet fission chromophores is a vibrant area of research to explore the possibility of efficient photovoltaic devices. Using high-level ab-initio density matrix renormalization group calculations, we present a systematic analysis of BN-doped perylenes for their potential application as singlet fission candidates. Four singlet fission chromophores are identified considering the monomer-based properties and their excitonic characters are further analyzed in a dimer configuration optimized in a six-dimensional space for local maxima of fission rates. Furthermore, a multistate multimode vibronic Hamiltonian is employed to identify intra- and interstate vibrational pathways for excitation energy modulation. Several photophysical properties such as Davydov splitting, activation energy and vibronic admixture of multiexcitonic and charge-transfer states are calculated for physically accessible dimers. The optimal dimer packing results in appropriate vibrational relaxation of singlet fission states and promotes significant population transfer which would be more attenuated without such couplings. This work not only identifies potential singlet fission systems with favorable electronic properties but also highlights the sensitivity of dimer packings with respect to the substitution patterns in singlet fission chromophores.

State-specific solvation for restricted active space spin-flip (RAS-SF) wave functions based on the polarizable continuum formalism.

The restricted active space spin-flip (RAS-SF) formalism is a particular form of single-reference configuration interaction that can describe some forms of strong correlation at a relatively low cost and which has recently been formulated for the description of charge-transfer excited states. Here, we introduce both equilibrium and nonequilibrium versions of a state-specific solvation correction for vertical transition energies computed using RAS-SF wave functions, based on the framework of a polarizable continuum model (PCM). Ground-state polarization is described using the solvent's static dielectric constant and in the nonequilibrium solvation approach that polarization is modified upon vertical excitation using the solvent's optical dielectric constant. Benchmark calculations are reported for well-studied models of photo-induced charge transfer, including naphthalene dimer, C2H4⋯C2F4, pentacene dimer, and perylene diimide (PDI) dimer, several of which are important in organic photovoltaic applications. For the PDI dimer, we demonstrate that the charge-transfer character of the excited states is enhanced in the presence of a low-dielectric medium (static dielectric constant ɛ0 = 3) as compared to a gas-phase calculation (ɛ0 = 1). This stabilizes mechanistic traps for singlet fission and helps to explain experimental singlet fission rates. We also examine the effects of nonequilibrium solvation on charge-separated states in an intramolecular singlet fission chromophore, where we demonstrate that the energetic ordering of the states changes as a function of solvent polarity. The RAS-SF + PCM methodology that is reported here provides a framework to study charge-separated states in solution and in photovoltaic materials.

Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

In the photoluminescence spectra of thin films made of singlet fission (SF) materials emission features that are red-shifted from the free exciton emission are of particular interest. They can be fingerprints of the correlated triplet-pair state and as such offer insights into the mechanisms of the multistep SF process. However, excimer formation or trap-state population can also cause such features and a clear disentanglement of the various contributions can be challenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organic semiconductors to control the polarizability of the molecular environment and, thus, to distinguish between excimer emission and emission from the correlated triplet-pair state. Using time-resolved photoluminescence spectroscopy measurements, we clarify the relation between excimer formation and SF in ADT and find that excimer formation constitutes a parallel relaxation channel for the exciton and neither mediates nor hinders SF.

Rapid Computational Approach Towards Designing Singlet-Fission Chromophores by Tuning the Diradical Character of Heteroatom-Doped Polycyclic Aromatic Hydrocarbons Using the Atom-Specific Fukui Function.

Singlet fission (SF) is proposed as a promising method to circumvent the Shockley-Queisser threshold of single junction photovoltaics. Progress towards realizing efficient SF-based devices has been impeded by the fact that only a handful of molecules and their derivatives practically exhibit efficient SF. In the present work, we demonstrate a TDDFT-based rapid and cost-effective computational approach for designing SF chromophores by doping various atomic sites (substituting carbon atoms) of polycyclic aromatic hydrocarbons with nitrogen, phosphorus, and silicon. We establish a hitherto unexplored, direct correlation between the atom-specific chemical reactivity parameter─Fukui function─of these molecules with their frontier molecular orbital energies, diradical characters, and vertical singlet and triplet excitation energies. These quantitative correlations show exactly opposite trends for nitrogen-doped molecules and phosphorus- or silicon-doped molecules. The doped derivatives that have the Fukui function falling in a range of 0.03-0.14 possess the required intermediate diradical character and suitable singlet-triplet energies to qualify for SF candidature. Our findings enable one, at reasonable computational times and cost, to easily assess the doping criteria and to develop design rules for SF molecules in particular and for diradicaloids in general.

Dicarbonyl anthracenes and phenanthrenes as singlet fission chromophores

Singlet fission is a highly desired process in photovoltaic devices as it can significantly enhance photoelectric conversion efficiency. Exploitation of this process in photovoltaics is hindered by the lack of appropriate chromophores. We used mixed-reference spin-flip time-dependent density functional theory (TDDFT) to investigate five dicarbonyl anthracenes and phenanthrenes, with the purpose to design singlet fission chromophores. Two molecules were found to be promising candidates. For all the dicarbonyl molecules, the oxygen lone pair orbitals were found to be involved in the excited states that are relevant to singlet fission.

Computational design of singlet fission biradicaloid chromophores

We present three different computational designs of potential singlet fission chromophores. In the first two cases, we propose structural modifications of two biradicaloid compounds already known with regard to singlet fission, namely 1,3-diphenyl-isobenzofuran (DPBF) and 2,3-diamino-1,4-benzoquinone (DABQ). In the former case, the introduction of methylene bridges between the phenyl rings and the isobenzofuran core of DPBF leads to an improvement of the singlet fission energetics and confers a higher degree of rigidity to the resulting molecule. In the second case, structural modifications of DABQ are proposed in order to overcome its fast nonradiative excited state decay and to improve its singlet fission energetics. The resulting diamino-fluoroquinone compounds fulfill the desired energy criteria and represent new potential chromophores for singlet fission. Finally, in the third proposed design we exploit the captodative effect to tune the energy levels of small biradicaloid molecules, leading to propose several compounds presented for the first time as suitable chromophores for singlet fission.

Structure and photophysics of rubrene–tetracene blends

The application potential of singlet fission (SF), describing the spontaneous conversion of an excited singlet into two triplets, underlines the necessity to independently control SF rates, energetics and the optical band gap. Heterofission, whereby the singlet splits into triplets on chemically distinct chromophores, is a promising approach to control the above-mentioned parameters, but its details are not yet fully understood. Here, we investigate the photophysics of blends of two prototypical SF chromophores, tetracene (TET) and rubrene (RUB) using time-resolved photoluminescence spectroscopy and time-correlated single photon counting to explore the potential for heterofission in combinations of endothermic SF chromophores.

Design of singlet fission chromophores by the introduction of N-oxyl fragments.

Singlet fission is a highly desired process in organic photovoltaic devices. It splits one singlet exciton into two triplet excitons and enhances the power-conversion efficiency. However, the exploitation of this process in photovoltaic devices is plagued by the small number of singlet fission chromophores. In this work, we designed a series of singlet fission chromophores by introducing N-oxyl fragments into anthracene. The diradical character brought by the N-oxyl fragments and the structural reorganizations in response to excitation to the lowest triplet state allow some of the investigated molecules to satisfy the thermodynamic energy criteria for singlet fission chromophores.

Design of potential singlet fission chromophores based on diketofurofuran: an alternative to diketopyrrolopyrrole

Diketofurofuran derivatives show comparable singlet fission relevant excited state energies with diketopyrrolopyrrole analogues and tetracene.

Exploring a new class of singlet fission fluorene derivatives with high-energy triplets.

In this study, we report strong experimental evidence for singlet fission (SF) in a new class of fluorene-based molecules, exhibiting two-branched donor–acceptor structures. The time-resolved spectroscopic results disclose ultrafast formation of a double triplet state (occurring in few picoseconds) and efficient triplet exciton separation (up to 145% triplet yield). The solvent polarity effect and the role of intramolecular charge transfer (ICT) on the SF mechanism have been thoroughly investigated with several advanced spectroscopies. We found that a stronger push–pull character favors SF, as long as the ICT does not act as a trap by opening a competitive pathway. Within the context of other widely-known SF chromophores, the unconventional property of generating high-energy triplet excitons (ca. 2 eV) via SF makes these materials outstanding candidates as photosensitizers for photovoltaic devices.

Light-Induced Charge Accumulation in PTCDI/Pentacene/Ag(111) Heterojunctions

The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA; by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump.

(Invited) Electrochemistry for Singlet Fission

Very recently, substantial attention has been paid to search for suitable chromophores for singlet fission. Singlet fission (SF) is a photophysical process in which an organic chromophore in excited singlet state shares its excitation energy with a neighbouring ground-state chromophore and both are converted into two triplet states. [1,2] In this way, one photon can generate theoretically two electrons which should increase the efficiency of inexpensive single-junction solar cells. This spin-allowed process is very fast (in ps or fs time scale), therefore much slower radiative decay is out of competition. Nevertheless, its detailed mechanism is still unknown. Although up to now, SF has not found its way into practice, the synthetic, photophysical, electrochemical and computational research is running.When looking for new molecules – candidates for efficient SF, the following properties are crucial: their crystalline form and molecular packing, the relative energies of the S0 (ground state), T1 (lowest triplet), and S1 (first excited singlet) states, where energy of the triplet state should be lower than a half of the first excited singlet energy, and the potentials of one-electron reduction and one-electron oxidation. Redox properties of molecules for SF are thus critical for their use, therefore electrochemical approach is necessary.Nowadays two main promising groups of chromophores are investigated for SF: 1) functionalized polyacenes, mostly pentacenes, and 2) organic molecules which are able to form biradicaloids, on which our research is aimed.Derivatives of 1,3-diphenyl-isobenzofuran derivatives (DPIBF) were chosen as a first group of molecules suitable for SF. Our study was focused on a series of fluorinated derivatives of DPIBF where the influence of number and position of fluorine atoms in the molecule on the redox potentials and mechanism is followed. With increasing number of F atoms, two discontinuous changes of behavior (mechanisms) were found [3].The second family of molecules which are currently under our investigation are derivatives of a very stable dye cibalackrot which are based on indigo structure. Our current work is focused on the study of oxidation and reduction of a series of differently substituted compounds.In all cases, electrochemical investigation was performed using classic voltammetric techniques (polarography, CV and RDE) and their combination with in situ UV-vis and EPR spectroscopy. The experimental results are correlated with quantum chemical calculations. Acknowledgement The financial support of the grant 19-22806S (Czech Science Foundation - GAČR) and the institutional support RVO: 61388955 are highly appreciated. The authors are grateful to dr. J. Kaleta and dr. M. Dudic for granting the substances and to prof. J. Michl for valuable discussions.

(Invited) Advanced Concepts for Ultra-High Conversion Efficiency of Solar Photons into Photovoltaics and Solar Fuels Based on Semiconductor Nanostructures and Molecular Singlet Fissionajn

In order to utilize solar power for the production of solar electricity and solar fuels on a global scale, it is necessary to develop solar photon conversion systems that have an appropriate combination of very high efficiency (delivered watts/m2) and low capital cost ($/m2). One potential promising approach to attain high conversion efficiencies well above the Shockley-Queisser limit of 33% is to utilize the unique properties of semiconductor nanostructures (viz., quantum dots/rods/films) and singlet fission chromophores to control the relaxation dynamics of photogenerated carriers and photochemical excited states, respectively. The former process creates enhanced photocurrent through efficient photogenerated electron-hole pair multiplication (electron-hole pairs exist as excitons in size-quantized nanostructures), or enhanced photopotential through hot carrier transport, transfer, and extraction processes. The latter process of singlet fission generates two triplets from an excited singlet state. These fundamental dynamics in various bulk and nanoscale semiconductors and in molecular chromophores have been studied using transient absorption, photoluminescence, photocurrent and THz spectroscopy with fs to ns time resolution. The predictions that the generation of more than one electron-hole pair or exciton per absorbed photon would be an efficient process in nanostructures, and via singlet fission, have been confirmed in different classes of materials, molecules, and their architectures in photon conversion devices. Selected aspects of this work will be summarized and recent advances will be discussed, including the very remarkable and beneficial theoretical effects of combining MEG or singlet fission with solar concentration and controlling the thicknesses of the photoactive layers in a two-photon photoelectrosynthesis cell structure for solar fuels.

Singlet Fission in Dideuterated Tetracene and Pentacene

AbstractThe impact of molecular vibrations on singlet fission, which is the spontaneous fission of a singlet exciton into two triplet excitons, is studied using ultrafast optical spectroscopy for the prototypical singlet fission chromophores tetracene and pentacene. We modify the frequency of intramolecular vibrations by deuteration, without impacting thin film structure and molecular arrangement, and study the resulting changes in exo‐ and endothermic singlet fission rates by comparing the deuterated and parent chromophores. We find that changes in the frequency of the C−C deformation modes of Δω=6 cm−1 and the occurrence of C−D vibrational modes do not lead to significant modifications in the singlet fission time constants. We conclude that the changes in the frequency of phonon modes induced by deuteration are too small to significantly impact the electron–phonon coupling that drives the singlet fission process.

High-Lying 31Ag Dark-State-Mediated Singlet Fission.

Singlet fission (SF), the conversion of one high-energy singlet to two low-energy triplets, provides the potential to increase the efficiency of photovoltaic devices. In the SF chromophores with C2h symmetry, exemplified by polyenes, singlet-to-triplet conversion generally involves a low-lying 21Ag dark state, which serves as either a multiexciton (ME) intermediate to promote the SF process or a parasitic trap state to shunt excited-state populations via internal conversion. This controversial behavior calls for a deep understanding of dark-state-related photophysics involving the higher-lying singlet state. However, the optical "dark" and "transient" nature of these dark states and strong correlation feature of double exciton species make their characterization and interpretation challenging from both experimental and computational perspectives. In the present work combining transient spectroscopy and multireference electronic structure calculations (XDW-CASPT2), we addressed a new photophysical model, i.e., a high-lying 31Ag dark-state-mediated ultrafast SF process in the benzodipyrrolidone (BDPP) skeleton. Such a 31Ag dark state with distinctive double excitation character, described as the ME state, could be populated from the initial 11Bu bright state on an ultrafast time scale given the quasi-degeneracy and intersection of the two electronic states. Furthermore, the suitable optical band gap and triplet energy, high triplet yield, and excellent photostability render BDPP a promising SF candidate for photovoltaic devices. These results not only enrich the arsenal of SF materials but also shed new insights into the understanding of dark-state-related photophysics, which could promote the development of new SF-active materials.

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design.

The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S1–T1 energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T1). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin distribution on the chromophore and thereby manipulate the excited state energy levels. Additionally, the improved understanding of the aromatic characters enables us to demonstrate a feasible design strategy to vary the excited state energy levels by tuning the number and nature of Hückel-aromatic units in the excited state. Finally, our study elucidates the complications and pitfalls of the excited state aromaticity and antiaromaticity concepts, highlighting that quantitative results from quantum chemical calculations of various aromaticity indices must be linked with qualitative theoretical analysis of the character of the excited states.

Bridge Resonance Effects in Singlet Fission.

A major benefit of intramolecular singlet fission (iSF) materials, in which through-bond interactions mediate triplet pair formation, is the ability to control the triplet formation dynamics through molecular engineering. One common design strategy is the use of molecular bridges to mediate interchromophore interactions, decreasing electronic coupling by increasing chromophore-chromophore separation. Here, we report how the judicious choice of aromatic bridges can enhance chromophore-chromophore electronic coupling. This molecular engineering strategy takes advantage of "bridge resonance", in which the frontier orbital energies are nearly degenerate with those of the covalently linked singlet fission chromophores, resulting in fast iSF even at large interchromophore separations. Using transient absorption spectroscopy, we investigate this bridge resonance effect in a series of pentacene and tetracene-bridged dimers, and we find that the rate of triplet formation is enhanced as the bridge orbitals approach resonance. This work highlights the important role of molecular connectivity in controlling the rate of iSF through chemical bonds and establishes critical design principles for future use of iSF materials in optoelectronic devices.

Advances in Singlet Fission Chromophore Design Enabled by Vibrational Spectroscopies

Singlet fission leads to the formation of two separate triplet T1 excitons from an initial singlet S1 exciton through 1(TT) and 1(T...T), multiexcitonic intermediates that retain singlet character....

Anisotropic Triplet Exciton Diffusion in Crystalline Functionalized Pentacene

Exciton multiplication through singlet fission (SF) offers scope for next-generation photovoltaic devices to exceed the Shockley–Queisser limit. Organic SF chromophores typically exhibit significant structural anisotropy in their crystal packing, which can impact exciton transport and influence the design of SF-enhanced devices. An improved understanding of the link between structural anisotropy and exciton diffusion is therefore crucial for developing SF-based photovoltaics. Here, we use femtosecond transient absorption spectroscopy to quantify the anisotropic triplet mobility in 6,13-(triisopropylsilylethynyl)pentacene (TIPS-Pn), a prototypical SF chromophore. Bimolecular triplet–triplet annihilation (TTA) in crystalline TIPS-Pn is well-described by a kinetic model that assumes isotropic, three-dimensional triplet exciton diffusion, but with best-fit parameters that do not correspond to any physical parameters of the material. Kinetic models that assume either one-dimensional or anisotropic three-dimensional exciton diffusion describe the annihilation equally well but yield more physically realistic fit parameters, suggesting that triplet diffusion on the subnanosecond time scale occurs mostly along a single axis of the material. These findings highlight the need to treat parameters obtained from fits of experimental data with models of isotropic diffusion with caution for systems with anisotropic packing such as TIPS-Pn. Diffusion coefficients calculated by density functional theory predict that triplet exciton diffusion occurs predominantly along the crystallographic a-axis, with migration in any other direction through the crystal slower by over an order of magnitude. This anisotropic diffusion suggests that fast, directional exciton transport in layers or films of TIPS-Pn may be achieved by control of the chromophore morphology.

Singlet Heterofission in Tetracene-Pentacene Thin-Film Blends.

Heterofission is a photophysical process of fundamental and applied interest whereby an excited singlet state is converted into two triplets on chemically distinct chromophores. The potential of this process lies in the tuning of both the optical band gap and the splitting between singlet and triplet energies. Herein, we report the time‐domain observation of heterofission in mixed thin films of the prototypical singlet fission chromophores pentacene and tetracene using excitation wavelengths above and below the tetracene band gap. We found a time constant of 26 ps for endothermic heterofission of a singlet exciton on pentacene in blends with low pentacene fractions, which was outcompeted by pentacene homofission for increasing pentacene concentrations. Direct excitation of tetracene lead to fast energy transfer to pentacene and subsequent singlet fission, which prevented homo‐ or heterofission of a singlet exciton on tetracene.