The HOMO Nodal Arrangement in Polychromophoric Molecules and Assemblies Controls the Interchromophoric Electronic Coupling

Control of electronic and exchange coupling by bridge substituents in donor acceptor triads with triptycene bridges

A series of triads, consisting of a triarylamine electron donor and a perylene diimide electron acceptor which were attached to two different wings of a triptycene bridging unit, was investigated concerning the dynamics of photoinduced charge separation and charge recombination processes with a particular focus on the involved spin-chemical aspects. Attaching electron-donating or electron-withdrawing substituents to the third wing of the triptycene bridge allowed tuning the electron transfer processes. These processes were investigated via fs-transient absorption spectroscopy and ns-transient absorption spectroscopy in an external magnetic field. The resulting magnetic field-dependent decay dynamics were analysed and modelled using the stochastic Liouville equation which yielded rate constants for the charge recombination and the exchange energy. In combination with a diabatic rate theory and Anderson's perturbative treatment of the exchange energy, these data gave a complete set of rate constants for charge separation and charge recombination from which the diverse electronic couplings between the involved states were derived. These couplings depend linearly on the inverse energy of virtual triptycene bridge states which allows tuning the electron transfer dynamics by modifying the triptycene bridge.

Structure-Property Relationships for Potential Inversion From Electron Acceptors Based on Thiophene-Fused Triptycene Quinones, 1,4-Diketones and Their Malononitrile Adducts

The synthesis and properties of a series of 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) inspired electron acceptors based on thiophene-fused quinone and triptycene motifs is presented. This has yielded insights into structure-property relationships for establishing and modulating simultaneous two-electron reduction processes in TCAQ analogues. These new compounds were synthesised using a Friedel-Crafts acylation between triptycene and thiophene-3,4-dicarbonyl chloride. Isomeric para-quinones featuring a [c]-fused thiophene on one side and a β,β- or α,β-fused triptycene on the other were isolated alongside a thiophene-3,4-diketone which bears two triptycene fragments. Knoevenagel condensation of these products with malononitrile produced a quinoidal bis(dicyanomethylene), an oxo-dicyanomethylene and an acyclic bis(dicyanomethylene). This series of new electron accepting molecules has been studied using X-ray crystallography and the implications of their 3D structures on NMR and UV/vis absorbance spectroscopy and cyclic voltammetry results have been ascertained with conclusions underpinned by computational methods.

Homoconjugation and Tautomeric Isomerism in Triptycene-Fused Pyridylbenzimidazoles

The facile, metal-free synthesis and characterization of three new series of triptycene-fused pyridylbenzimidazoles are reported; such compounds possess an imidazole moiety fused within the benzene rings of the trypticene and a pyridine ring installed at position 2 of the imidazole rings. The position of the nitrogen atom of the pyridyl moiety linked to position 2 of the fused benzimidazole scaffold is systematically changed from the ortho to para position. The number of substituted blades bearing the pyridyl-substituted fused benzimidazole scaffolds has been increased from one to three. Such a library of compounds allowed us to evaluate the enhancement of two main effects: tautomeric isomerism and homoconjugation. The characteristic dynamic equilibrium between different isomers induced by prototropic tautomerization was examined by 1H nuclear magnetic resonance spectroscopy. By comparison of the photophysical properties of the new compounds with those of classical planar pyridylbenzimidazoles, the presence of the homoconjugation effect between the different triptycene blades was demonstrated. Fine details of the electronic structure of the new derivatives were unraveled by a computational analysis. The novel compounds can be employed for the construction of intriguing self-assembled supramolecular architectures.

A Novel Homoconjugated Propellane Triimide: Synthesis, Structural Analyses, and Gas Separation

Rigid three-dimensional (3D) polycyclic propellanes have garnered interest due to their unique conformational spaces, which display great potential use in selectivity, separation and as models to study through-space electronic interactions. Herein we report the synthesis of a novel rigid propellane, trinaphtho[3.3.3]propellane triimide, which comprises three imide groups embedded on a trinaphtho[3.3.3]propellane. This propellane triimide exhibits large bathochromic shift, amplified molar absorptivity, enhanced fluorescence, and lower reduction potential when compared to the subunits. Computational and experimental studies reveal that the effective through-space π-orbitals interacting (homoconjugation) occurs between the subunits. Single-crystal XRD analysis reveals that the propellane triimide has a highly quasi-D3h symmetric skeleton and readily crystallizes into different superstructures by changing alkyl chains at the imide positions. In particular, the porous 3D superstructure with S-shaped channels is promising for taking up ethane (C2H6) with very good selectivity over ethylene (C2H4), which can purify C2H4 from C2H6/C2H4 in a single separation step. This work showcases a new class of rare 3D polycyclic propellane with intriguing electronic and supramolecular properties.

Synthesis of Chiral Pyrene-Based 1,4-Dithiins

The 1,4-dithiin motif is known for its reversible redox properties to generate radical cations and diradical dications and thus is interesting for organic electronic applications. However, examples where this motif is embedded into chiral larger fused aromatic compounds are very rare. Here we describe the syntheses of several structurally related pyrene fused dithiins and their spectroscopic investigations with a focus on tuning circular dichroism, with respect to the g values, depending on their connectivity.

Acene-Extended Triptycenes: Synthesis, Characterization, and Singlet Exciton Fission Properties

Three acene-extended triptycenes, TIPS-Antrip, TIPS-Tetrip, and TIPS-Pentrip, which contain TIPS-ethynyl functionalized anthracene, tetracene, and pentacene as subunits, respectively, are synthesized and characterized. It is found that the optoelectronic properties and crystal packing motifs could be modulated by changing the subunits. A preliminary exploration of the excited-state behavior of these molecules indicates that TIPS-Tetrip and TIPS-Pentrip exhibit intramolecular singlet fission (iSF).

Homoconjugation in triptycenes: an inquiry through photochromism

Stabilization of the fleeting photogenerated o-quinonoid colored intermediate of diphenylpyran-annulated triptycene (Trip-chrom) lends credence to homoconjugation.

Synthesis and Optoelectronic Properties of a Quinoxalino-Phenanthrophenazine (QPP) Extended Tribenzotriquinacene (TBTQ)

A six‐step synthesis towards a tribenzotriquinacene (TBTQ) bearing three quinoxalinophenanthrophenazine (QPP) units is presented. The optoelectronic properties are investigated and the effect of the three‐dimensional arrangement of the individual QPP planes is examined using optical spectroscopy, electrochemical analysis and quantum‐chemical calculations.

The inspiration and challenge for through-space charge transfer architecture: from thermally activated delayed fluorescence to non-linear optical properties

Organic molecules consisting of electron donor (D) and electron acceptor (A) subunits linked by π-conjugated bridges are promising building blocks for thermally activated delayed fluorescence (TADF) and non-linear optics (NLO) materials due to their intramolecular charge transfer (CT) processes in response to external stimuli. According to the electron interaction pattern, the CT process in D-π-A architectures can be divided into two categories, through-bond/-space charge transfer (TB/TSCT). To date, research into the TADF properties of TSCT characteristic molecules has since seen significant growth. In fact, TSCT characteristic materials show great advantages in such NLO responses. In this perspective, we first briefly introduced the basic principles of NLO and TADF effects. Successively, we discuss the influence of TBCT and TSCT patterns on NLO and TADF properties, especially for TSCT characteristic. In the final part, we address the diversity and potential advantages of TSCT characteristic molecules as high-performance NLO materials. With these, it is expected that the greater structural flexibility of spatial conjugation can bring more functionality to NLO materials in the future.

Symmetry Effects in Photoinduced Electron Transfer in Chlorin‐Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD‐DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.

Triptycene End-Capped Benzothienobenzothiophene and Naphthothienobenzothiophene

Previously it was demonstrated that triptycene end-capping can be used as a crystal engineering strategy to direct the packing of quinoxalinophenanthrophenazines (QPPs) towards cofacially stacked π dimers with large molecular overlap resulting in high charge transfer integrals. Remarkably, this packing motif was formed under different crystallization conditions and with a variety of derivatives bearing additional functional groups or aromatic substituents. Benzothienobenzothiophene (BTBT) and its derivatives are known as some of the best performing compounds for organic field-effect transistors. Here, the triptycene end-capping concept is introduced to this class of compounds and polymorphic crystal structures are investigated to evaluate the potential of triptycene end-caps as synthons for crystal engineering.

Regiocontrolled dimerization of asymmetric diazaheptacene derivatives toward X-shaped porous semiconductors

Conformationally rigid X-shaped PAHs are attracting interest due to their self-assembly into unique networks and as models to study through-space exciton and charge delocalization in one single molecule. We report here the synthesis of X-shaped PAHs by dimerization of diazaheptacene diimides. The diimide groups are employed to effectively direct the self-assembly into antiparallel dimer aggregates, which assist the compounds to undergo a regiocontrolled [4 + 4] dimerization, leading to an X-shaped conformation bearing electron-poor and -rich subunits. The resulting PAHs are found to pack in 2D layers with large open channels and infinite π⋯π arrays. Furthermore, these highly crystalline porous materials serve as electron-transporting materials in OFETs due to the long-range π-stacked arrays in the layers. This work presents a potentially generalizable strategy, which may provide a unique class of porous semiconductors for organic devices, taking advantage of their open channels.

The synthesis of conformationally rigid X-shaped PAHs by regiocontrolled cyclodimerization of diazaheptacene diimides is presented. The resulting porous materials exhibit enhanced semiconducting behaviors with large open channels.

Chiral cyclic [n]spirobifluorenylenes: carbon nanorings consisting of helically arranged quaterphenyl rods illustrating partial units of woven patterns

Chiral cyclic [n]spirobifluorenylenes consisting of helically arranged quaterphenyl rods, illustrating partial units of woven patterns, were designed and synthesized as a new family of carbon nanorings. The synthesis was accomplished by the Ni(0)-mediated Yamamoto-coupling of chiral spirobifluorene building blocks. The structures of the cyclic 3-, 4-, and 5-mers were determined by X-ray crystallographic analysis. These carbon nanorings exhibited a strong violet colored emission with high quantum yields in solution (95%, 93%, and 94% for 3-, 4-, and 5-mer, respectively). Other spectroscopic properties, including their chiroptical properties, were also investigated. The g-values for circularly polarized luminescence were found to be in the order of 10⁻³. Characteristic spiroconjugation induced by multiple (≧3) bifluorenyl units, for example the even-odd effect of the number of units in the matching of the signs of the orbitals, was also indicated by DFT calculations.

Chiral cyclic [n]spirobifluorenylenes consisting of helically arranged quaterphenyl rods, illustrating partial units of woven patterns, were designed and synthesized as a new family of carbon nanorings.

Tuning the Optical Properties of Sulfonylaniline Derivatives: Degeneracy Breaking of Benzene Orbitals and Linkage through Nodal Planes

The orbital degeneracy of benzene rings is resolved by an asymmetric push-pull system in 2,6-bis(methylsulfonyl)aniline (BMeSA), in which the highest occupied molecular orbital (HOMO) is located at the 4-position, while the lowest unoccupied molecular orbital (LUMO) is located at a different position and has a nodal plane through the carbon atoms at the 1- and 4-positions. Therefore, the π-extension of BMeSA at the 4-position reveals a strong overlap in the HOMO and a minimal overlap in the LUMO. Consequently, π-extended BMeSA derivatives exhibit longer absorbance and emission wavelengths in the order of the electron-donating abilities of their substituents at the 4-position, which is based on a decrease in an absolute HOMO-level-dependent HOMO-LUMO gap in accordance with the nodal arrangement. Positive fluorescent solvatochromism with polarity-dependent decrease in fluorescent intensity was also observed. The biaryls exhibited more planar geometries in the excited state than in the ground state. The charge transfer mechanism, which can be described as node-induced intramolecular charge transfer (NICT), differs from the planar intramolecular charge transfer (PICT) and twisted intramolecular charge transfer (TICT).

Coexistence of structurally similar but electronically distinct isomers of delocalized cation radicals as a basis for the development of functional materials

Certain low-symmetry aromatic molecules with a small HOMO/HOMO-1 energy gap might co-exist as electronic isomers in their cation radical states that differ only in bond lengths yet have distinct optical and electronic properties. These electronic isomers are sensitive to the environment and might be used for the development of novel functional materials.

Triperyleno[3,3,3]propellane triimides: achieving a new generation of quasi-D 3h symmetric nanostructures in organic electronics

Rigid three-dimensional (3D) polycyclic aromatic hydrocarbons (PAHs), in particular 3D nanographenes, have garnered interest due to their potential use in semiconductor applications and as models to study through-bond and through-space electronic interactions. Herein we report the development of a novel 3D-symmetric rylene imide building block, triperyleno[3,3,3]propellane triimides (6), that possesses three perylene monoimide subunits fused on a propellane. This building block shows several promising characteristics, including high solubility, large π-surfaces, electron-accepting capabilities, and a variety of reactive sites. Further, the building block is compatible with different reactions to readily yield quasi-D 3h symmetric nanostructures (9, 11, and 13) of varied chemistries. For the 3D nanostructures we observed red-shift absorption maxima and amplification of the absorption coefficients when compared to the individual subunits, indicating intramolecular electronic coupling among the subunits. In addition, the microplates of 9 exhibit comparable mobilities in different directions in the range of 10-3 cm2 V-1 s-1, despite the rather limited intermolecular overlap of the π-conjugated moieties. These findings demonstrate that these quasi-D 3h symmetric rylene imides have potential as 3D nanostructures for a range of materials applications, including in organic electronic devices.

Synthesis and Electronic Properties of Triperyleno[3.3.3]Propellanes: Towards Two-Dimensional Electronic Structures

Threefold symmetric triperyleno[3.3.3]propellanes (TPRPs), which are composed of three perylene units peri-fused on [3.3.3]proppellane, have been synthesized and their electronic properties characterized. Unsubstituted TPRP assembles into two-dimensional honeycomb-like networks. TPRP displays a bathochromic-shifted absorption band in its absorption spectrum and shows well-separated one-electron redox waves in a cyclic voltammogram, due to a homoconjugative interaction between perylene moieties. A radical cation species of tert-butyl substituted TPRP shows spin delocalization over the whole of the molecule, which is derived from effective overlap of HOMOs among three perylene moieties.

Game of Frontier Orbitals: A View on the Rational Design of Novel Charge-Transfer Materials

Since the first application of frontier molecular orbitals (FMOs) to rationalize stereospecificity of pericyclic reactions, FMOs have remained at the forefront of chemical theory. Yet, the practical application of FMOs in the rational design and synthesis of novel charge transfer materials remains under-appreciated. In this Perspective, we demonstrate that molecular orbital theory is a powerful and universal tool capable of rationalizing the observed redox/optoelectronic properties of various aromatic hydrocarbons in the context of their application as charge-transfer materials. Importantly, the inspection of FMOs can provide instantaneous insight into the interchromophoric electronic coupling and polaron delocalization in polychromophoric assemblies, and therefore is invaluable for the rational design and synthesis of novel materials with tailored properties.

Isolation of a chiral anthracene cation radical: X-ray crystallography and computational interrogation of its racemization

Chiral cation-radical salts hold significant promise as charge-transfer materials, chiroptical switches, and electron-transfer catalysts for enantioselective synthesis. Herein we demonstrate that the readily-available chiral 9,10-diphenyleanthracene derivative (i.e.^SANT) forms a robust cation radical, whose structure was elucidated by X-ray crystallography and DFT calculations. While ^SANT was observed to racemize on a timescale (t_1/2) of 1.1 hours, a computational conformational search and kinetic analysis of the racemization pathway led us to identify a simple methyl substituted ^SANT derivative, which does not racemize (racemization t_1/2 10¹³-10¹⁷ years).

From Intramolecular (Circular) in an Isolated Molecule to Intermolecular Hole Delocalization in a Two-Dimensional Solid-State Assembly: The Case of Pillarene

To achieve long-range charge transport/separation and, in turn, bolster the efficiency of modern photovoltaic devices, new molecular scaffolds are needed that can self-assemble in two-dimensional (2D) arrays while maintaining both intra- and intermolecular electronic coupling. In an isolated molecule of pillarene, a single hole delocalizes intramolecularly via hopping amongst the circularly arrayed hydroquinone ether rings. The crystallization of pillarene cation radical produces a 2D self-assembly with three intermolecular dimeric (sandwich-like) contacts. Surprisingly, each pillarene in the crystal lattice bears a fractional formal charge of +1.5. This unusual stoichiometry of oxidized pillarene in crystals arises from effective charge distribution within the 2D array via an interplay of intra- and intermolecular electronic couplings. This important finding is expected to help advance the rational design of efficient solid-state materials for long-range charge transfer.

Towards the rational design of novel charge-transfer materials: biaryls with a dihedral angle-independent hole delocalization mechanism

Herein, we demonstrate a rational design of novel materials using FMO analysis on the example of biaryls with dihedral angle independent mechanism of hole delocalization.

When Substituents Do Not Matter: Frontier Orbitals Explain the Unusually High and Invariant Oxidation Potential in Alkoxy-, Alkyl-, and H-Substituted Iptycenes

Frontier molecular orbitals (FMOs) have played a critical role in predicting reactivity/selectivity of pericyclic reactions. Here we show that the structurally similar iptycene-based hydroquinone ether (HE), that is, ^MeOIpt and ^BOHE/^BHHE, molecules have drastically different ordering of bisallylic and quinoidal FMOs. They are almost degenerate in ^BOHE/^BHHE, while in ^MeOIpt, the bisallylic orbital lies far below the quinoidal HOMO. Oxidation of ^BOHE/^BHHE induces coplanarization of the methoxy group and destabilizes the bisallylic HOMO, leading to a relatively low oxidation potential. In ^MeOIpt, considerable energy must be invested in coplanarization of the methoxy group to bring about orbital swapping, resulting in an oxidation potential higher than that in structurally similar ^BOHE/^BHHE. As the quinoidal HOMO density does not extend to the substituent-bearing carbon in H-, alkyl-, and alkoxy-substituted iptycenes, their redox potentials remain invariant. This case study involving a simple visual inspection of the nodal arrangement as well as energetics of the FMOs and Walsh analysis could serve as a tool for the design of organic molecules with a desired redox potential.

Prolonged Charge Separated States in Twisted Stacks of All-Carbon Donor and Acceptor Chromophores

Twisted donor-on-donor and acceptor-on-acceptor bicontinuous assembly in all-carbon pyren-1-ylaceanthrylene (PA) dyad extends the survival time of the photoinduced radical ion-pair intermediates. Aceanthrylene, a functional analog of C₇₀, acts as a versatile electron acceptor owing to its high electron affinity and visible light absorption. Antithetical trajectories of the excitons in the nonparallel π-ways led to persistent radical ion-pair intermediates in aggregated (τ_cr^A ∼ 1.28 ns) vs monomeric (τ_cr^M ≤ 110 fs) PA dyad as observed using femtosecond transient absorption spectroscopy. Marcus theory of charge transfer rates predicts an ambipolar transport characteristic in crystalline PA, thereby endorsing PA as an all-carbon DA hybrid for nonfullerene photovoltaic applications.

Two's Company, Three's a Crowd: Exciton Localization in Cofacially Arrayed Polyfluorenes

Understanding the mechanisms of long-range energy transfer through polychromophoric assemblies is critically important in photovoltaics and biochemical systems. Using a set of cofacially arrayed polyfluorenes (Fn), we investigate the mechanism of (singlet) exciton delocalization in π-stacked polychromophoric assemblies. Calculations reveal that effective stabilization of an excimeric state requires an ideal sandwich-like arrangement; yet surprisingly, emission spectroscopy indicates that exciton delocalization is limited to only two fluorene units for all n. Herein, we show that delocalization is determined by the interplay between the energetic gain from delocalization, which quickly saturates beyond two units in larger Fn, and an energetic penalty associated with structural reorganization, which increases linearly with n. With these insights, we propose a hopping mechanism for exciton transfer, based upon the presence of multiple excimeric tautomers of similar energy in larger polyfluorenes (n ≥ 4) together with the anticipated low thermal barrier of their interconversion.

A search for blues brothers: X-ray crystallographic/spectroscopic characterization of the tetraarylbenzidine cation radical as a product of aging of solid magic blue

Magic blue (MB+˙ SbCl6− salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in the solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of the main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB+˙ SbCl6− and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. The recognition of this fact led us to the rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’ (BC: Eox1 = 0.78 V vs. Fc/Fc+, λmax(BC+˙) = 805 nm, εmax = 9930 cm−1 M−1), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of the aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC+˙) promise that BC will serve as a ready replacement for MB and an oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations.

Three cation-templated Cu(i) self-assemblies: synthesis, structures, and photocatalytic properties

Cation-templated self-assembly of three Cu(i) hybrids has been achieved. Their optical band gap and photocatalytic degradation properties were also investigated.