Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered perylene-3,4:9,10-bis(dicarboximide)s

High-Performance Near-Infrared Chlorinated Rylenecarboximide Fluorophores via Consecutive C-N and C-C Bond Formation

A new class of near-infrared (NIR) fluorophores, PAI, is obtained by consecutive C-N/C-C bond formation between diphenylamines and 9,10-dibromoperylenecarboximide. Owing to the rigid structure, extended π-conjugation and pronounced push-pull substitution, these fluorophores show emission maxima up to 804 nm and large Stokes shifts. The extraordinarily high fluorescence quantum yields from 47 % to 70 % are attributed to chloro substitution in the bay positions of the perylene core. These characteristics, together with high photostability, qualify them as useful NIR emitters for applications as biomarkers and security inks.

Promoting multiexciton interactions in singlet fission and triplet fusion upconversion dendrimers

Singlet fission and triplet-triplet annihilation upconversion are two multiexciton processes intimately related to the dynamic interaction between one high-lying energy singlet and two low-lying energy triplet excitons. Here, we introduce a series of dendritic macromolecules that serve as platform to study the effect of interchromophore interactions on the dynamics of multiexciton generation and decay as a function of dendrimer generation. The dendrimers (generations 1-4) consist of trimethylolpropane core and 2,2-bis(methylol)propionic acid (bis-MPA) dendrons that provide exponential growth of the branches, leading to a corona decorated with pentacenes for SF or anthracenes for TTA-UC. The findings reveal a trend where a few highly ordered sites emerge as the dendrimer generation grows, dominating the multiexciton dynamics, as deduced from optical spectra, and transient absorption spectroscopy. While the dendritic structures enhance TTA-UC at low annihilator concentrations in the largest dendrimers, the paired chromophore interactions induce a broadened and red-shifted excimer emission. In SF dendrimers of higher generations, the triplet dynamics become increasingly dominated by pairwise sites exhibiting strong coupling (Type II), which can be readily distinguished from sites with weaker coupling (Type I) by their spectral dynamics and decay kinetics.

Leveraging Charge-Transfer Interactions in Through-Space-Coupled Pentacene Dendritic Oligomer for Singlet Exciton Fission

Singlet exciton fission in organic chromophores has received much attention during the past decade. Inspired by numerous spectroscopic studies in the solid state, there have been vigorous efforts to study singlet exciton fission dynamics in covalently bonded oligomers, which aims to investigate underlying mechanisms of this intriguing process in simplified model systems. In terms of through-space orbital interactions, however, most of covalently bonded pentacene oligomers studied so far fall into weakly interacting systems since they manifest chain-like structures based on various (non)conjugated linkers. Therefore, it remains as a compelling question to answer how through-space interactions in the solid state intervene this photophysical process since it is hypersensitive to displacements and orientations between neighboring chromophores. Herein, as one of experimental studies to answer this question, we introduced a tight-packing dendritic structure whose mesityl-pentacene constituents are coupled via moderate through-space orbital interactions. Based on the comparison with a suitably controlled dendritic structure, which is in a weak coupling regime, important mechanistic viewpoints are tackled such as configurational mixings between singlet, charge-transfer, and triplet pair states and the role of chromophore multiplication. We underscore that our through-space-coupled dendritic oligomer in a quasi-intermediate coupling regime provides a hint on the interplay of multiconfigurational excited-states, which might have drawn complexity in singlet exciton fission kinetics throughout numerous solid-state morphologies.

Unveiling Charge-Transfer Dynamics at Singlet Fission Layer/Hybrid Perovskite Interface

Singlet fission (SF) materials have been applied in various types of solar cells to pursue higher power conversion efficiency (PCE) beyond the Shockley-Queisser (SQ) limit. SF implementation in perovskite solar cells has not been successfully realized yet due to the insufficient understanding of the SF/perovskite heterojunctions. In this work, we attempt to elucidate the charge dynamics of an SF/perovskite system by incorporating a well-known SF molecule, TIPS-pentacene, and a triple-cation perovskite Cs0.05(FA0.85MA0.15)0.95PbI2.55Br0.45, owing to their well-matched energy structures. The transient absorption spectra and kinetic fitting plots suggest an electron-transfer process from the triplet state of TIPS-pentacene to perovskite in the picosecond regime, which increases the carrier density by 20% in the perovskite layer. This work confirms the existence of an electron-transfer process between the SF material and perovskite, providing a pathway to SF-enhanced perovskite solar cells.

Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structures

Hybrid structures with an interface between two different materials with properly aligned energy levels facilitate photo-induced charge separation to be exploited in optoelectronic applications. Particularly, the combination of 2D transition metal dichalcogenides (TMDCs) and dye molecules offers strong light-matter interaction, tailorable band level alignments, and high fluorescence quantum yields. In this work, we aim at the charge or energy transfer-related quenching of the fluorescence of the dye perylene orange (PO) when isolated molecules are brought onto monolayer TMDCs via thermal vapor deposition. Here, micro-photoluminescence spectroscopy revealed a strong intensity drop of the PO fluorescence. For the TMDC emission, in contrast, we observed a relative growth of the trion versus exciton contribution. In addition, fluorescence imaging lifetime microscopy quantified the intensity quenching to a factor of about 103 and demonstrated a drastic lifetime reduction from 3 ns to values much shorter than the 100 ps width of the instrument response function. From the ratio of the intensity quenching that is attributed to hole or energy transfer from dye to semiconductor, we deduce a time constant of several picoseconds at most, pointing to an efficient charge separation suitable for optoelectronic devices.

A Paradigm Shift from 2D to 3D: Surface Supramolecular Assemblies and Their Electronic Properties Explored by Scanning Tunneling Microscopy and Spectroscopy

Exploring supramolecular architectures at surfaces plays an increasingly important role in contemporary science, especially for molecular electronics. A paradigm of research interest in this context is shifting from 2D to 3D that is expanding from monolayer, bilayers, to multilayers. Taking advantage of its high-resolution insight into monolayers and a few layers, scanning tunneling microscopy/spectroscopy (STM/STS) turns out a powerful tool for analyzing such thin films on a solid surface. This review summarizes the representative efforts of STM/STS studies of layered supramolecular assemblies and their unique electronic properties, especially at the liquid-solid interface. The superiority of the 3D molecular networks at surfaces is elucidated and an outlook on the challenges that still lie ahead is provided. This review not only highlights the profound progress in 3D supramolecular assemblies but also provides researchers with unusual concepts to design surface supramolecular structures with increasing complexity and desired functionality.

Singlet fission preserves polarisation correlation of excitons

Singlet fission (SF) holds the promise to circumvent the photovoltaic efficiency limit to reach a power-conversion efficiency above 34%. SF of TIPS-pentacene (TIPS-Pn) has been investigated but its mechanism is yet to be well elucidated. Recently, we developed a nanoparticle (NP) system, in which doping of TIPS-Pn in a host matrix yields a range of average intermolecular distances, d, to study the dependence of SF in TIPS-Pn on d. At large d values, where the bimolecular SF process should be unfavourable, a relatively high SF quantum yield (Φ_SF) is still observed, which implies a deviation from a random distribution of TIPS-Pn throughout the NP. Here, using polarisation-sensitive femtosecond time-resolved spectroscopy and Monte Carlo simulations of exciton migration and SF, we quantify the level of clustering of TIPS-Pn in the host matrix, which is responsible for the higher than expected Φ_SF. The experimental data indicate a preservation of polarisation correlation by SF, which is uncommon because energy transfer in amorphous materials tends to result in depolarisation. We show that the preservation of polarisation correlation is due to SF upon exciton migration. Although exciton migration decorrelates polarisation, SF acts to remove decorrelated excitons to give an overall preservation of polarisation correlation.

Perylene Diimide-Fused Dithiophenepyrroles with Different End Groups as Acceptors for Organic Photovoltaics

In this study, we synthesized four new A-DA'D-A acceptors (where A and D represent acceptor and donor chemical units) incorporating perylene diimide units (A') as their core structures and presenting various modes of halogenation and substitution of the functional groups at their end groups (A). In these acceptors, by fusing dithiophenepyrrole (DTP) moieties (D) to the helical perylene diimide dimer (hPDI) to form fused-hPDI (FhPDI) cores, we could increase the D/A' oscillator strength in the cores and, thus, the intensity of intramolecular charge transfer (ICT), thereby enhancing the intensity of the absorption bands. With four different end group units─IC2F, IC2Cl, IO2F, and IO2Cl─tested, each of these acceptor molecules exhibited different optical characteristics. Among all of these systems, the organic photovoltaic device incorporating the polymer PCE10 blended with the acceptor FhPDI-IC2F (1:1.1 wt %) had the highest power conversion efficiency (PCE) of 9.0%; the optimal PCEs of PCE10:FhPDI-IO2F, PCE10:FhPDI-IO2Cl, and PCE10:FhPDI-IC2Cl (1:1.1 wt %) devices were 5.2, 4.7, and 7.7%, respectively. The relatively high PCE of the PCE10:FhPDI-IC2F device resulted primarily from the higher absorption coefficients of the FhPDI-IC2F acceptor, lower energy loss, and more efficient charge transfer; the FhPDI-IC2F system experienced a lower degree of geminate recombination─as a result of improved delocalization of π-electrons along the acceptor unit─relative to that of the other three acceptors systems. Thus, altering the end groups of multichromophoric PDI units can increase the PCEs of devices incorporating PDI-derived materials and might also be a new pathway for the creation of other valuable fused-ring derivatives.

Effects of the Separation Distance between Two Triplet States Produced from Intramolecular Singlet Fission on the Two-Electron-Transfer Process

To harvest two triplet excitons of singlet fission (SF) via a two-electron transfer efficiently, the revelation of the key factors that influence the two-electron-transfer process is necessary. Here, by using steady-state and transient absorption/fluorescence spectroscopy, we investigated the two-electron-transfer process from the two triplet excitons of intramolecular SF (iSF) in a series of tetracene oligomers (dimer, trimer, and tetramer) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an electron acceptor in solution. Quantitative two-electron transfer could be conducted for the trimer and tetramer, and the rate for the tetramer is faster than that for the trimer. However, the maximum efficiency of the two-electron transfer in the dimer is relatively low (∼47%). The calculation result of the free energy change (ΔG) of the second-electron transfer for these three compounds (-0.024, -0.061, and -0.074 eV for the dimer, trimer, and tetramer, respectively) is consistent with the experimental observation. The much closer ΔG value to zero for the dimer should be responsible for its low efficiency of the two-electron transfer. Different ΔG values for these three oligomers are attributed to the different Coulomb repulsive energies between the two positive charges generated after the two-electron transfer that is caused by their various intertriplet distances. This result reveals for the first time the important effect of the Coulomb repulsive energy, which depends on the intertriplet distance, on the two-electron transfer process from the two triplet excitons of iSF.

The Role of Crystal Packing on the Optical Response of Trialkyltetrelethynyl Acenes

The electronic and optical responses of an organic semiconductor (OSC) are dictated by the chemistries of the molecular or polymer building blocks and how these chromophores pack in the solid state. Understanding the physicochemical natures of these responses are not only critical for determining OSC performance for a particular application, but the UV/visible optical response may also be of potential use to determine aspects of the molecular-scale solid-state packing for crystal polymorphs or thin-film morphologies that are difficult to determine otherwise. To probe these relationships, we report the quantum-chemical investigation of a series of trialkyltetrelethynyl acenes (tetrel = silicon or germanium) that adopt the brickwork (BW), slip-stack (SS), or herringbone (HB) packing configurations; the π-conjugated backbones considered here are pentacene (PEN) and anthradithiophene (ADT). For comparison, HB-packed (unsubstituted) pentacene is also included. Density functional theory (DFT) and G0W0 (single-shot GW) electronic band structures, G0W0-BSE (Bethe-Salpeter Equation)-derived optical spectra, polarized ϵ2 spectra, and distributions of both singlet and triplet exciton wave functions are reported. Configurational disorder is also considered. Further, we evaluate the probability of singlet fission in these materials through energy conservation relationships.

Molecular motion activated by residual stress in a glassy polymer thin film

The activation, by residual stress, of the fast portion of rotational motion of single fluorescent probe molecules inside a polymer thin film near its glass transition temperature is studied at a single molecular level. Spin-casted poly n-butyl methacrylate thin films without thermal annealing are chosen as the model system and single molecule fluorescence defocused microscopy is adopted as the method. The rotational motion of the probes under residual stress is found to be more activated than that under mere thermal activation, and the kinetic energy exhibits a monotonic increase with the stress strength. A rough linear dependence of rotational kinetic energy at low stress is found, yielding the value of characteristic volume for the residual stress to activate the motion of the probes. The values of the volume are close to the van der Waals volume of the probes, indicating that the activation of the fast dynamics by residual stress is localized. The activation effect is weakened and vanishes at or above the glass transition temperature due to stress relaxation. The effect is also absent at temperatures far below T_g due to the frozen molecular motion with a much higher activation energy.

Self-Aligned Bilayers for Flexible Free-Standing Organic Field-Effect Transistors

Free-standing and flexible field-effect transistors based on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/polystyrene bilayers are obtained by well-controlled phase separation of both components. The phase separation is induced by solvent vapor annealing of initially amorphous blend films, leading to crystallization of TIPS-pentacene as the top layer. The crystallinity and blend morphology strongly depend on the molecular weight of polystyrene, and under optimized conditions, distinct phase separation with a well-defined and trap-free interface between both fractions is achieved. Due to the distinct bilayer morphology, the resulting flexible field-effect transistors reveal similar charge carrier mobilities as rigid devices and additionally pronounced environmental and bias stress stabilities. The performance of the flexible transistors remains stable up to a strain of 1.8%, while above this deformation, a close relation between current and strain is observed that is required for applications in strain sensors.

Visible-Light-Induced Photocatalytic Oxidative Decarboxylation of Cinnamic Acids to 1,2-Diketones

A concerted metallophotoredox catalysis has been realized for the efficient decarboxylative functionalization of α,β-unsaturated carboxylic acids with aryl iodides in the presence of perylene bisimide dye to afford 1,2-diketones.

Anisotropy in fifth-order exciton-exciton-interaction two-dimensional spectroscopy

Exciton-exciton-interaction two-dimensional (EEI2D) spectroscopy is a fifth-order variant of 2D electronic spectroscopy. It can be used to probe biexciton dynamics in molecular systems and to observe exciton diffusion in extended systems such as polymers or light-harvesting complexes. The exciton transport strongly depends on the geometrical and energetic landscape and its perturbations. These can be of both local character, such as molecular orientation and energetic disorder, and long-range character, such as polymer kinks and structural domains. In the present theoretical work, we investigate the anisotropy in EEI2D spectroscopy. We introduce a general approach for how to calculate the anisotropy by using the response-function formalism in an efficient way. In numerical simulations, using a Frenkel exciton model with Redfield-theory dynamics, we demonstrate how the measurement of anisotropy in EEI2D spectroscopy can be used to identify various geometrical effects on exciton transport in dimers and polymers. Investigating a molecular heterodimer as an example, we demonstrate the utility of anisotropy in EEI2D spectroscopy for disentangling dynamic localization and annihilation. We further calculate the annihilation in extended systems such as conjugated polymers. In a polymer, a change in the anisotropy provides a unique signature for exciton transport between differently oriented sections. We analyze three types of geometry variations in polymers: a kink, varying geometric and energetic disorder, and different geometric domains. Our findings underline that employing anisotropy in EEI2D spectroscopy provides a way to distinguish between different geometries and can be used to obtain a better understanding of long-range exciton transport.

Photophysical Properties and Redox Potentials of Photosensitizers for Organic Photoredox Transformations

Photoredox catalysis has proven to be a powerful tool in synthetic organic chemistry. The rational design of photosensitizers with improved photocatalytic performance constitutes a major advancement in photoredox organic transformations. This review summarizes the fundamental ground-state and excited-state photophysical and electrochemical attributes of molecular photosensitizers, which are important determinants of their photocatalytic reactivity.

Photogeneration of Long-Lived Triplet States through Singlet Fission in Lycopene H-Aggregates

Molecular triplet excitons produced through singlet fission (SF) usually have shorter triplet lifetimes due to exciton-exciton recombination and relaxation pathways, thereby resulting in complex device architectures for SF-boosted solar cells. Using broadband transient absorption spectroscopy, we here show that the photoexcitation of nanostructured lycopene H-aggregates at room temperature produces free triplets with an unprecedented 35-fold enhancement in the lifetime compared to those localized on the monomer backbone. The observed rise of a spectrally blue-shifted correlated T-T pair state in ∼19 ps with distinct vibronic features provides the basis for SF-induced triplet generation.

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.

Steady-State Fluorescence Signatures of Intramolecular Singlet Fission from Stochastic Predictions

The advent of new multichromophoric systems capable of undergoing efficient intramolecular singlet fission (iSF) has greatly expanded the range of possible motifs for multiexciton generation approaches for organic light energy harvesting materials. Transient absorption (TA) spectroscopic probes are typically used to characterize singlet fission processes that may place limitations on sensitivity and time resolution on scales comparable to the full lifespan of spin-forbidden triplets and interactions. Here, we investigate the ability of fluorescence-based spectroscopic probes to detect iSF activity in isolated dyads based on large substituted conjugated acenes (e.g., tetracene and pentacene derivatives). Photophysical models are simulated from several iSF-active dyad systems reported in the literature using a stochastic approach to assess the sensitivity of steady-state fluorescence to the presence of triplet excitons. The results demonstrate large fluctuations in expected fluorescence yields with varying excitation rate constants for systems with Φ_iSF > 0.5 (assuming weak interchromophore coupling). Exciton-exciton interactions are also investigated, and we further demonstrate how treating iSF dyads stochastically (i.e., finite number of chromophores) accentuates dependences of photophysical yields with excitation rates. Last, our approach reveals the potential ability of single molecule level fluorescence spectroscopy to detect iSF activity that can aid efforts to design and optimize candidate iSF systems.

Molecular packing-dependent exciton dynamics in functionalized anthradithiophene derivatives: From solutions to crystals

Understanding the impact of inter-molecular orientation on the optical properties of organic semiconductors is important for designing next-generation organic (opto)electronic and photonic devices. However, fundamental aspects of how various features of molecular packing in crystalline systems determine the nature and dynamics of excitons have been a subject of debate. Toward this end, we present a systematic study of how various molecular crystal packing motifs affect the optical properties of a class of high-performance organic semiconductors: functionalized derivatives of fluorinated anthradithiophene. The absorptive and emissive species present in three such derivatives (exhibiting "brickwork," "twisted-columnar," and "sandwich-herringbone" motifs, controlled by the side group R) were analyzed both in solution and in single crystals, using various modalities of optical and photoluminescence spectroscopy, revealing the nature of these excited states. In solution, in the emission band, two states were identified: a Franck-Condon state present at all concentrations and an excimer that emerged at higher concentrations. In single crystal systems, together with ab initio calculations, it was found in the absorptive band that Frenkel and Charge Transfer (CT) excitons mixed due to nonvanishing CT integrals in all derivatives, but the amount of admixture and exciton delocalization depended on the packing, with the "sandwich-herringbone" packing motif least conducive to delocalization. Three emissive species in the crystal phase were also identified: Frenkel excitons, entangled triplet pairs ¹(TT) (which are precursors to forming free triplet states via singlet fission), and self-trapped excitons (STEs, similar in origin to excimers present in concentrated solution). The "twisted-columnar" packing motif was most conducive to the formation of Frenkel excitons delocalized over 4-7 molecules depending on the temperature. These delocalized Frenkel states were dominant across the full temperature range (78 K-293 K), though at lower temperatures, the entangled triplet states and STEs were present. In the derivative with the "brickwork" packing, all three emissive species were observed across the full temperature range and, most notably, the ¹(TT) state was present at room temperature. Finally, the derivative with the "sandwich-herringbone" packing exhibited localized Frenkel excitons and had a strong propensity for self-trapped exciton formation even at higher temperatures. In this derivative, no formation of the ¹(TT) state was observed. The temperature-dependent dynamics of these emissive states are reported, as well as their origin in fundamental inter-molecular interactions.

Near infrared absorption/emission perylenebisimide fluorophores with geometry relaxation-induced large Stokes shift

The dyes (P-1 and P-2) of perylenebisimide (PBI) conjugated with 2-(2-hydroxyphenyl)benzothiazole (HBT) were prepared by Sonogashira coupling reaction. The new compounds have special photophysical properties, such as near infrared absorption/emission and large Stokes shift. The UV-vis absorption (range from 651 nm to 690 nm) and emission wavelength (range from 732 nm to 756 nm) of P-1 and P-2 extend to near infrared range. Importantly, they have much larger Stokes shifts (range from 73 nm to 105 nm) compared with the conventional PBI derivatives, such as 7 (from 19 nm to 65 nm) and 9 (from 81 nm to 86 nm). TD-DFT calculation was used to rationalize UV-vis absorption, emission and especially large Stokes shift from the theoretical point of view. We found geometry relaxation of P-1 and P-2 in the excited state is an important reason for the origin of large Stokes shift besides intramolecular electron transfer (ICT).

The dyes with near infrared absorption/emission and large stokes shifts induced by geometry relaxation were prepared.

Stringing the Perylene Diimide Bow

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red-shifted absorptions but also more facile reductions.

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

In this study, two analogous perylene diimide (PDI) trimers, whose structures show rotatable single bond π-bridge connection (twisted) vs. rigid/fused π-bridge connection (planar), were synthesized and investigated. We show via time resolved spectroscopic measurements how the π-bridge connections in A-π-D-π-A-π-D-π-A multichromophoric PDI systems strongly affect the triplet yield and triplet formation rate. In the planar compound, with stronger intramolecular charge transfer (ICT) character, triplet formation occurs via conventional intersystem crossing. However, clear evidence of efficient and fast intramolecular singlet exciton fission (iSEF) is observed in the twisted trimer compound with weaker ICT character. Multiexciton triplet generation and separation occur in the twisted (flexible-bridged) PDI trimer, where weak coupling among the units is observed as a result of the degenerate double triplet and quintet states, obtained by quantum chemical calculations. The high triplet yield and fast iSEF observed in the twisted compound are due not only to enthalpic viability but also to the significant entropic gain allowed by its trimeric structure. Our results represent a significant step forward in structure-property understanding, and may direct the design of new efficient iSEF materials.

Conversion between triplet pair states is controlled by molecular coupling in pentadithiophene thin films

In singlet fission (SF) the initially formed correlated triplet pair state, 1(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 μs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in 5(TT)0. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence.

Spatially Offset Femtosecond Stimulated Raman Spectroscopy: Observing Exciton Transport through a Vibrational Lens

To design better molecular electronic devices, we need a strong understanding of how charges or excitons propagate, as many efficiency losses arise during transport. Exciton transport has been difficult to study because excitons tend to be short-lived, have short diffusion lengths, and can easily recombine. Here, we debut spatially offset femtosecond stimulated Raman spectroscopy (SO-FSRS), a three-pulse ultrafast microscopy technique. By offsetting the photoexcitation beam, we can monitor Raman spectral changes as a function of both time and position. We used SO-FSRS on 6,13-bis(triisopropylsilylethynyl) pentacene, a well-studied organic semiconductor used in photovoltaics and field-effect transistors. We demonstrated that the fast exciton and free charge carrier transport axes are identical and observed that exciton transport is less anisotropic by a factor of ∼3. SO-FSRS is the first technique that directly tracks molecular structural evolution during exciton transport, which can provide roadmaps for tailor-making molecules for specific electronic devices.

The Photophysical Properties of Triisopropylsilyl-ethynylpentacene—A Molecule with an Unusually Large Singlet-Triplet Energy Gap—In Solution and Solid Phases

The process of singlet-exciton fission (SEF) has attracted much attention of late. One of the most popular SEF compounds is TIPS-pentacene (TIPS-P, where TIPS = triisopropylsilylethynyl) but, despite its extensive use as both a reference and building block, its photophysical properties are not so well established. In particular, the triplet state excitation energy remains uncertain. Here, we report quantitative data and spectral characterization for excited-singlet and -triplet states in dilute solution. The triplet energy is determined to be 7940 ± 1200 cm−1 on the basis of sensitization studies using time-resolved photoacoustic calorimetry. The triplet quantum yield at the limit of low concentration and low laser intensity is only ca. 1%. Self-quenching occurs at high solute concentration where the fluorescence yield and lifetime decrease markedly relative to dilute solution but we were unable to detect excimer emission by steady-state spectroscopy. Short-lived fluorescence, free from excimer emission or phosphorescence, occurs for crystals of TIPS-P, most likely from amorphous domains.

Suppressing triplet decay in quinoidal singlet fission materials: the role of molecular planarity and rigidity

Singlet fission, in which one singlet exciton is split into two triplet excitons, provides the potential to exceed the Shockley-Queisser limit for the power conversion efficiencies of organic solar cells. However, the charge transfer from the triplet state is found to be slow in singlet fission materials, so suppression of the triplet decay is crucial for effective utilization of singlet fission. Here, we first investigated triplet decay for the singlet fission molecular materials of ThBF and TThBF, which are characteristic of twisted and flexible quinoidal backbones. It is found that these compounds show rapid nonradiative decay in the Franck-Condon region and through the T₁/S₀ crossing point. Interestingly, upon locking the backbone twist by methylene, the LThBF and LTThBF compounds exhibit much higher energy barriers from T₁ to the T₁/S₀ crossing point, vanishing spin-orbit couplings, and decreased reorganization energies due to the planar and rigid structures. Consequently, both the triplet decay pathways are effectively suppressed. Our work reveals the importance of molecular planarity and rigidity in suppressing triplet decay and will be very helpful for full utilization of singlet fission in organic photovoltaics.

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.

Impact of non-equilibrium molecular packings on singlet fission in microcrystals observed using 2D white-light microscopy

Singlet fission, the process of splitting a singlet exciton into two triplet excitons, has been proposed as a mechanism for improving the efficiency of future photovoltaic devices. In organic semiconductors exhibiting singlet fission, the geometric relationship between molecules plays an important role by setting the intermolecular couplings that determine the system energetics. Here, we spatially image TIPS-pentacene microcrystals using ultrafast two-dimensional white-light microscopy and discover a low-energy singlet state sparsely distributed throughout the microcrystals, with higher concentrations at edges and morphological defects. The spectra of these singlet states are consistent with slip-stacked molecular geometries and increased charge-transfer couplings. The picosecond-timescale kinetics of these low-energy singlet states matches that of the correlated triplet-pair state, which we attribute to singlet/triplet-pair interconversion at these sites. Our observations support the conclusion that small populations of geometries with favourable energetics can play outsized roles in singlet fission processes.

Slow charge transfer from pentacene triplet states at the Marcus optimum

Singlet fission promises to surpass the Shockley-Queisser limit for single-junction solar cell efficiency through the production of two electron-hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~10⁷ s^-1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.

Perylene Bisimide Dyes with up to Five Independently Introduced Substituents: Controlling the Functionalization Pattern and Photophysical Properties Using Regiospecific Bay Substitution

We report herein a versatile and user-friendly synthetic methodology based on sequential functionalization that enables the synthesis of previously unknown perylene bisimide (PBI) dyes with up to five different substituents attached to the perylene core (e.g., compound 15). The key to the success of our strategy is a highly efficient regiospecific 7-mono- and 7,12-di-phenoxy bay substitution at the "imide-activated" 7- and 12-bay positions of 1,6,7,12-tetrachloroperylene monoimide diester 1. The facile subsequent conversion of the diester groups into an imide group resulted in novel PBIs (e.g., compound 14) with two phenoxy substituents specifically at the 7- and 12-bay positions. This conversion led to the activation of C-1 and C-6 bay positions, and thereafter, the remaining two chlorine atoms were substituted to obtain tetraphenoxy-PBI (compound 15) that has two different imide and three different bay substituents. The methodology provides excellent control over the functionalization pattern, which enables the synthesis of various regioisomeric pairs bearing the same bay substituents. Another important feature of this strategy is the high sensitivity of HOMO-LUMO energies and photoinduced charge transfer toward sequential functionalization. As a result, systematic fluorescence on-off switching has been demonstrated upon subsequent substitution with the electron-donating 4-methoxyphenoxy substituent.

Polymer additive controlled morphology for high performance organic thin film transistors

Solution-crystallizable small-molecule organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 5,11-bis(triethylgermylethynyl)anthradithiophene (diF-TEG-ADT), 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), and N,N'-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2), demonstrate various practical advantages including high mobility, air stability and solution processibility. In this article, we review various polymer additive based approaches to control the crystal morphology and the resultant charge transport of some bench-mark, high performance, solution crystallizable, small-molecule organic semiconductors. The polymer additives are discussed under the categories of non-conjugated polymers and conjugated polymers. The approaches and structure-performance correlations that we discussed here may be applied far beyond the examples shown in this review and have important implications for high performance organic semiconductors in general.

Improved model on fluorescence decay in singlet fission materials

Singlet fission (SF) materials are a kind of promising material for breaking the solar cell efficiency limit. Here we rebuild the four-electron spin Hamiltonian under our coordinate system and present an improved model described by the population evolution equations on fluorescence decay (FD) dynamics that contain several detailed physical processes. The improved model for total random molecular orientation gives a more consistent fitting on the experimental data [G. B. Piland et al., J. Phys. Chem. C, 2013, 117, 1224] about time-resolved FD of amorphous rubrene thin films in the presence of a strong magnetic field. The fitting can reflect the relative rates of the real physical processes. Further on, our results show two kinds of magnetic field effect for the variety of two molecular relative orientations with respect to each other and the magnetic field by investigating the singlet projection and FD dynamics of the system.

Discrete Dimers of Redox-Active and Fluorescent Perylene Diimide-Based Rigid Isosceles Triangles in the Solid State

The development of rigid covalent chiroptical organic materials, with multiple, readily available redox states, which exhibit high photoluminescence, is of particular importance in relation to both organic electronics and photonics. The chemically stable, thermally robust, and redox-active perylene diimide (PDI) fluorophores have received ever-increasing attention owing to their excellent fluorescence quantum yields in solution. Planar PDI derivatives, however, generally suffer from aggregation-caused emission quenching in the solid state. Herein, we report on the design and synthesis of two chiral isosceles triangles, wherein one PDI fluorophore and two pyromellitic diimide (PMDI) or naphthalene diimide (NDI) units are arranged in a rigid cyclic triangular geometry. The optical, electronic, and magnetic properties of the rigid isosceles triangles are fully characterized by a combination of optical spectroscopies, X-ray diffraction (XRD), cyclic voltammetry, and computational modeling techniques. Single-crystal XRD analysis shows that both isosceles triangles form discrete, nearly cofacial PDI-PDI π-dimers in the solid state. While the triangles exhibit fluorescence quantum yields of almost unity in solution, the dimers in the solid state exhibit very weak-yet at least an order of magnitude higher-excimer fluorescence yield in comparison with the almost completely quenched fluorescence of a reference PDI. The triangle containing both NDI and PDI subunits shows superior intramolecular energy transfer from the lowest excited singlet state of the NDI to that of the PDI subunit. Cyclic voltammetry suggests that both isosceles triangles exhibit multiple, easily accessible, and reversible redox states. Applications beckon in arenas related to molecular optoelectronic devices.

Electron diffraction tomography and X-ray powder diffraction on photoredox catalyst PDI

The crystal structure of photoredox catalyst PDI-iPr was determined by a combination of electron diffraction tomography and XRPD.

Systematic control of the rate of singlet fission within 6,13-diphenylpentacene aggregates with PbS quantum dot templates

Perturbation of molecular packing and dielectric environment at a quantum dot surface can promote singlet fission in diphenylpentacene aggregates.

One- to Two-Exciton Transitions in Perylene Bisimide Dimer Revealed by Two-Dimensional Electronic Spectroscopy

The excited-state energy levels of molecular dimers and aggregates play a critical role in their photophysical behavior and an understanding of the photodynamics in such structures is important for developing applications such as photovoltaics and optoelectronic devices. Here, exciton transitions in two different covalently bound PBI dimers are studied by two-dimensional electronic spectroscopy (2DES), a powerful spectroscopic method, providing the most complete picture of vibronic transitions in molecular systems. The data are accurately reproduced using the equation of motion-phase matching approach. The unambiguous presence of one-exciton to two-exciton transitions are captured in our results and described in terms of a molecular exciton energy level scheme based on the Kasha model. Furthermore, the results are supported by comparative measurements with the PBI monomer and another dimer in which the interchromophore distance is increased.

Thionated Perylene Diimide-Phenothiazine Dyad: Synthesis, Structure, and Electrochemical Studies

Perylene diimides (PDIs) are promising candidates for n-type semiconductor materials and, thus, for use in organic electronics. Thionation of the imide moiety provides an efficient strategy to control the donor-acceptor gap of these types of compounds, although the degree and selectivity of thionation can be hard to achieve. Through the design of a sterically encumbered PDI-phenothiazine dyad, a previously unattained geminal thionation pattern has been realized, providing the first example of a perylene-monoimide-monothioimide. The electrochemical and solid-state structural properties of this uniquely thionated dyad are reported and compared to those of the nonthionated parent molecule. It is found that thionation enhances the electron affinity of the PDI core, affecting electrochemical and spectroelectochemcial behavior of the dyad without significantly affecting the solid-state packing of the molecules.

The Potential of Singlet Fission Photon Multipliers as an Alternative to Silicon-Based Tandem Solar Cells

Singlet fission, an exciton multiplication process in organic semiconductors that converts one singlet exciton into two triplet excitons, is a promising way to reduce thermalization losses in conventional solar cells. One way to harvest triplet excitons is to transfer their energy into quantum dots, which then emit photons into an underlying solar cell. We simulate the performance potential of such a singlet fission photon multiplier combined with a silicon base cell and compare it to a silicon-based tandem solar cell. We calculate the influence of various loss mechanisms on the performance potential under real-world operation conditions using a variety of silicon base cells with different efficiencies. We find that the photon multiplier is more stable against changes in the solar spectrum than two-terminal tandem solar cells. We furthermore find that, as the efficiency of the silicon base cell increases, the efficiency of the photon multiplier increases at a rate higher than that of the tandem solar cell. For current record silicon solar cells, the photon multiplier has the potential to increase the efficiency by up to 4.2% absolute.