Anisotropic Multiexciton Quintet and Triplet Dynamics in Singlet Fission via Pulsed Electron Spin Resonance.

The quintet triplet-pair state may be generated upon singlet fission and is a critical intermediate that dictates the fate of excitons, which can be exploited for photovoltaics, information technologies, and biomedical imaging. In this report, we demonstrate that continuous-wave and pulsed electron spin resonance techniques such as phase-inverted echo-amplitude detected nutation (PEANUT), which have emerged as the primary tool for identifying the spin pathways in singlet fission, probe fundamentally different triplet-pair species. We directly observe that the generation rate of high-spin triplet pairs is dependent on the molecular orientation with respect to the static magnetic field. Moreover, we demonstrate that this observation can prevent incorrect analysis of continuous-wave electron spin resonance (cw-ESR) measurements and provide insight into the design of materials to target specific pathways that optimize exciton properties for specific applications.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores.

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Pentacene-Bridge Interactions in an Axially Chiral Binaphthyl Pentacene Dimer.

Molecular chirality can be exploited as a sensitive reporter of the nature of intra- and interchromophore interactions in π-conjugated systems. In this report, we designed an intramolecular singlet fission (iSF)-based pentacene dimer with an axially chiral binaphthyl bridge (2,2'-(2,2'-dimethoxy-[1,1'-binaphthalene]-3,3'-diyl) n-octyl-di-isopropyl silylethynyl dipentacene, BNBP) to utilize its chiroptical response as a marker of iSF chromophore-bridge-chromophore (SFC-ß-SFC) interactions. The axial chirality of the bridge enforces significant one-handed excitonic coupling of the pentacene monomer units; as such, BNBP exhibits significant chiroptical response in the ground and excited states. We analyzed the chiroptical response of BNBP using the exciton coupling method and quadratic response density functional theory calculations to reveal that higher energy singlet transitions in BNBP involve significant delocalization of the electronic density on the bridging binaphthyl group. Our results highlight the promising application of chiroptical techniques to investigate the nature of SFC-ß-SFC interactions that impact singlet fission dynamics.

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.

The Environment-Dependent Behavior of the Blatter Radical at the Metal-Molecule Interface.

Stable organic radicals have potential applications for building organic spintronic devices. To fulfill this potential, the interface between organic radicals and metal electrodes must be well characterized. Here, through a combined effort that includes synthesis, scanning tunneling microscopy, X-ray spectroscopy, and single-molecule conductance measurements, we comprehensively probe the electronic interaction between gold metal electrodes and a benchtop stable radical-the Blatter radical. We find that despite its open-shell character and having a half-filled orbital close to the Fermi level, the radical is stable on a gold substrate under ultrahigh vacuum. We observe a Kondo resonance arising from the radical and spectroscopic signatures of its half-filled orbitals. By contrast, in solution-based single-molecule conductance measurements, the radical character is lost through oxidation with charge transfer occurring from the molecule to metal. Our experiments show that the stability of radical states can be very sensitive to the environment around the molecule.

Persistent Multiexcitons from Polymers with Pendent Pentacenes.

Singlet fission has emerged as a key mechanism of exciton multiplication in organic chromophores, generating two triplet excitons from a single photon. Singlet fission is typically studied in crystalline films or in isolated dimers. Here, we investigate an intermediate regime where through-space interactions mediate singlet fission and triplet pair recombination within isolated polymer chains. Specifically, we investigate how appending pentacenes to a polynorbornene backbone can lead to macromolecules that take advantage of through-space π-π interactions for fast singlet fission and rapid triplet pair dissociation. Singlet fission in these systems is affected by molecular dynamics, and triplet-triplet recombination is a geminate process where the rate of recombination scales with molecular-weight. We find that these pendent pentacene polymers yield free triplets with lifetimes that surpass those of crystalline chromophores in both solution as isolated polymers and in thin films.

Anticipating Acene-Based Chromophore Spectra with Molecular Orbital Arguments.

Recent synthetic studies on the organic molecules tetracene and pentacene have found certain dimers and oligomers to exhibit an intense absorption in the visible region of the spectrum that is not present in the monomer or many previously studied dimers. In this article we combine experimental synthesis with electronic structure theory and spectral computation to show that this absorption arises from an otherwise dark charge-transfer excitation "borrowing intensity" from an intense UV excitation. Further, by characterizing the role of relevant monomer molecular orbitals, we arrive at a design principle that allows us to predict the presence or absence of an additional absorption based on the bonding geometry of the dimer. We find this rule correctly explains the spectra of a wide range of acene derivatives and solves an unexplained structure-spectrum phenomenon first observed over 70 years ago. These results pave the way for the design of highly absorbent chromophores with applications ranging from photovoltaics to liquid crystals.

Realizing the Photoene Reaction with Alkenes under Visible Light Irradiation and Bypassing the Favored [2 + 2]-Photocycloaddition.

The textbook photoreaction between two alkenes is the [2 + 2]-photocycloaddition resulting in functionalized cyclobutanes. Herein, we disclose an unusual reactivity of alkenes that favor photoene reaction over the [2 + 2]-photocycloaddition.

Transposed Paternò-Büchi Reaction.

A complementary strategy of utilizing ππ* excited state of alkene instead of nπ* excited state of the carbonyl chromophore in a "transposed Paternò-Büchi" reaction is evaluated with atropisomeric enamides as the model system. Based on photophysical investigations, the nature of excited states and the reactive pathway was deciphered leading to atropselective reaction. This new concept of switching of excited-state configuration should pave the way to control the stereochemical course of photoreaction due to the orbital approaches required for photochemical reactivity.

Tuning Singlet Fission in π-Bridge-π Chromophores.

We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.

Tale of Twisted Molecules. Atropselective Photoreactions: Taming Light Induced Asymmetric Transformations through Non-biaryl Atropisomers.

Photochemical transformations are a powerful tool in organic synthesis to access structurally complex and diverse synthetic building blocks. However, this great potential remains untapped in the mainstream synthetic community due to the challenges associated with stereocontrol originating from excited state(s). The finite lifetime of an excited state and nearly barrierless subsequent processes present significant challenges in manipulating the stereochemical outcome of a photochemical reaction. Several methodologies were developed to address this bottleneck including photoreactions in confined media and preorganization through noncovalent interactions resulting in stereoenhancement. Yet, stereocontrol in photochemical reactions that happen in solution in the absence of organized assemblies remained largely unaddressed. In an effort to develop a general and reliable methodology, our lab has been exploring non-biaryl atropisomers as an avenue to perform asymmetric phototransformations. Atropisomers are chiral molecules that arise due to the restricted rotation around a single bond (chiral axis) whose energy barrier to rotation is determined by nonbonding interactions (most often by steric hindrance) with appropriate substituents. Thus, atropisomeric substrates are chirally preorganized during the photochemical transformation and translate their chiral information to the expected photoproducts. This strategy, where "axial to point chirality transfer" occurs during the photochemical reaction, is a hybrid of the successful Curran's prochiral auxiliary approach involving atropisomers in thermal reactions and the Havinga's NEER principle (nonequilibrating excited-state rotamers) for photochemical transformations. We have investigated this strategy in order to probe various aspects such as regio-, enantio-, diastereo-, and chemoselectivity in several synthetically useful phototransformations including 6π-photocyclization, 4π-ring closure, Norrish-Yang photoreactions, Paternò-Büchi reaction, and [2 + 2]- and [5 + 2]-photocycloaddition. The investigations detailed in this Account clearly signify the scope of our strategy in accessing chirally enriched products during phototransformations. Simple design modifications such as tailoring the steric handle in atropisomers to hold reactive units resulted in permanently locked/traceless axial chirality in addition to incorporating multiple stereocenters in already complex scaffolds obtained from phototransformation. Further improvements allowed us to employ low energy visible light rather than high energy UV light without compromising the stereoenrichment in the photoproducts. Continued investigations on atropisomeric scaffolds have unraveled new design features, with outcomes that are unique and unprecedented for excited state reactivity. For example, we have established that reactive spin states (singlet or triplet excited state) profoundly influence the stereochemical outcome of an atropselective phototransformation. In general, the photochemistry and photophysics of atropisomeric substrates differ significantly from their achiral counterparts irrespective of having the same chromophore initiating the excited state reactivity. The ability of axially chiral chromophores to impart stereoenrichment in the intramolecular photoreactions appears to be promising. A challenging endeavor for the "axial to point chirality transfer" strategy is to enhance stereoenrichment or alter chemical reactivity in intermolecular photoreactions. Insights gained from our investigations will serve as a platform to venture into more complicated yet fruitful research in terms of broad synthetic utility.

Realizing an Aza Paternò-Büchi Reaction.

Intramolecular atropselective aza Paternò-Büchi reaction involving atropisomeric enamide and imine functionalities under sensitized irradiation leads to azetidine products in good yield and selectivity (ee >96 %). A mechanistic model based on detailed photophysical and isomerization kinetic studies is provided that shed light into the reactivity of enamides leading to aza Paternò-Büchi reaction.

Exciton Correlations in Intramolecular Singlet Fission.

We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases.

Intramolecular Singlet Fission in Oligoacene Heterodimers.

We investigate singlet fission (SF) in heterodimers comprising a pentacene unit covalently bonded to another acene as we systematically vary the singlet and triplet pair energies. We find that these energies control the SF process, where dimers undergo SF provided that the resulting triplet pair energy is similar or lower in energy than the singlet state. In these systems the singlet energy is determined by the lower-energy chromophore, and the rate of SF is found to be relatively independent of the driving force. However, triplet pair recombination in these heterodimers follows the energy gap law. The ability to tune the energies of these materials provides a key strategy to study and design new SF materials-an important process for third-generation photovoltaics.

Quantitative Intramolecular Singlet Fission in Bipentacenes.

Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley-Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns.

Tailoring atropisomeric maleimides for stereospecific [2 + 2] photocycloaddition--photochemical and photophysical investigations leading to visible-light photocatalysis.

Atropisomeric maleimides were synthesized and employed for stereospecific [2 + 2] photocycloaddition. Efficient reaction was observed under direct irradiation, triplet-sensitized UV irradiation, and non-metal catalyzed visible-light irradiation, leading to two regioisomeric (exo/endo) photoproducts with complete chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity (ee > 98%) and diastereoselectivity (dr > 99:1%) were observed under the employed reaction conditions and were largely dependent on the substituent on the maleimide double bond but minimally affected by the substituents on the alkenyl tether. On the basis of detailed photophysical studies, the triplet energies of the maleimides were estimated. The triplet lifetimes appeared to be relatively short at room temperature as a result of fast [2 + 2] photocycloaddition. For the visible-light mediated reaction, triplet energy transfer occurred with a rate constant close to the diffusion-limited value. The mechanism was established by generation of singlet oxygen from the excited maleimides. The high selectivity in the photoproduct upon reaction from the triplet excited state was rationalized on the basis of conformational factors as well as the type of diradical intermediate that was preferred during the photoreaction.

Enantiospecific photochemical 6π-ring closure of α-substituted atropisomeric acrylanilides--role of alkali metal ions.

Direct irradiation of atropisomeric α-substituted acrylanilides in the presence of alkali metal ions gave high ee values in the 3,4-dihydro-2-quinolin-2-one photoproduct, while in the absence of alkali metal ions, racemic photoproduct was observed. The heavy atom effect leading to enhanced triplet yields alters the reactive pathway leading to the observed enantioselectivity in the photoproduct.

Enantiospecific photochemical transformations under elevated pressure.

Enantiospecific axial-to-point chiral transfer in light-induced transformations was efficient under elevated pressure at high temperatures. Model photoreactions with atropisomeric compounds showed higher enantioselectivity in the photoproducts under elevated pressure. The ee values in the photoproducts were rationalized based on the increased stability of optically pure atropisomeric compounds at elevated pressure, even at high temperatures.