Acceptor Stacks Research Articles

Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination.

We report a nonparallel stacked arrangement of donor–acceptor (D–A) pairs for prolonging the lifetime of photoinduced charge-separated states. Hydrogen–hydrogen steric repulsion in naphthalimide-naphthalene (NIN) dyad destabilizes the planar geometry between the constituent units in solution/ground state. Sterically imposed nonplanar geometry of the dyad allows the access of nonparallel arrangement of the donor and acceptor stacks having triclinic space group in the crystalline state. Antiparallel trajectory of excitons in nonparallel D–A stacks can result in lower probability of geminate charge recombination, upon photoexcitation, thereby resulting in a long-lived charge-separated state. Upon photoexcitation of the NIN dyad, electron transfer from naphthalene to the singlet excited state of naphthalimide moiety results in radical ion pair intermediates that survive >10,000-fold longer in the aggregated state (τcra > 1.2 ns) as compared to that of monomeric dyad (τcrm < 110 fs), monitored using femtosecond transient absorption spectroscopy.

Two Closely Related Organic Charge-Transfer Complexes Based on Tetrathiafulvalene and 9H-fluorenone Derivatives. Competition between Hydrogen Bonding and Stacking Interactions

Two 1:1 charge-transfer organic complexes were formed using tetrathiafulvalene as a donor and a 9H-fluorenone derivative as acceptor: 4,5,7-trinitro-9H-fluoren-9-one-2-carboxylic acid (complex 1) or 4,5,7-trinitro-9H-fluoren-9-one-2-carboxylic acid methyl ester (complex 2). Both systems crystallize with alternated donor and acceptor stacks. However, the crystal structure of 1 is influenced by classical hydrogen bonds involving carboxylic acid groups, which force to arrange acceptors as centrosymmetric dimers in the crystal, via R2 2(8) ring motifs, while such a restriction is no longer present in the case of 2, affording thus a different crystal structure. This main difference is reflected in stacking interactions, and, in turn, in the degree of charge transfer observed in the complexes. The degree of charge transfer, estimated using Raman spectroscopy, is δ1 = 0.07 for 1 and δ2 = 0.14 for 2. It thus seems that, at least for the studied complexes, hydrogen bonding is an unfavorable factor for charge transfer.

Structural Diversity in the Complexes of Trimeric Perfluoro-o-phenylene Mercury with Tetrathia- and Tetramethyltetraselenafulvalene

Five potential charge transfer complexes of trimeric perfluoro-o-phenylene mercury (I) with tetrathiafulvalene (TTF) and tetramethyltetraselenefulvalene (TMTSF) were grown from different solvent mixtures. The adducts (I)2·TTF (1) and I·TTF (2) were grown by slow evaporation from the 1:1 mixture of dichloromethane (CH2Cl2, DCM) and carbon disulfide (CS2). Use of the different 1:1 solvent mixtures of dichloromethane (CH2Cl2, DCM) and dichloroethane (C2H4Cl2, DCE) has led to the crystalline adducts I·TTF (3) and I·TTF·DCE (4). Adduct I.TMTSF (5) was grown by the interface crystallization on the border of two immiscible layers, ethyl acetate, and carbon disulfide. The cocrystals differ by the donor–acceptor ratio, molecular packing, and the solvent inclusion. The components in 1–5 form mixed donor–acceptor stacks. The stacks are stabilized by Hg···S and Hg···C short contacts, while the lateral interactions between stacks include F···F, CH···F, and S/Se···F short contacts.

Control of Luminescence Properties in Naphthalene Diimide-Based Gel with Azodibenzoic Acid by Charge Transfer Interaction

The self-sorting between aromaticcompounds that can serve as electron donors and electronacceptors allows the formation of distinct p- and n-typebundles of fibers that further intermix and generate multiplejunction points for efficient electron transfer and transport ofthe charge carriers. However, a difficulty associated withsimple mixing of such compounds is the possibility of adifferent mode of assembly, i.e., charge-transfer interactionmediated by alternate donor–acceptor stacking. Thus, ex-amples related to self-sorting among donor–acceptor chromo-phores have been limited.

Adducts of Trimeric Perfluro-o-Phenylene Mercury with TTF or DTT

Since the discovery of metallic conductivity in the TTF-TCNQ complex, the charge transfer (CT) complex materials have been studied for such properties as semiconductivity or superconductivity. The synthesis of new CT materials using trimeric perfluoro-ortho-phenylene mercury (Hg3) as electron acceptor compound was done first by Haneline and Gabaï [1]. A series of studies of CT materials containing Hg3 and tetrathiafulvalene (TTF) or dithieno thiophene (DTT) as an electron donor compounds was carried out by different methods. Co-crystalzation with TTF gave a larger variety of co-crystals than with DTT, producing three polymorphs and one pseudo-polymorph. One pot crystal growth of TTF and Hg3 in a 1:1 mixture of solvents dichloromethane (DCM) and dichloroethane (DCA) has led to the formation of TTF polymorphs 1 and 2 with same 1:1 donor acceptor ratio [Hg3·TTF] as well a pseudopolymorph [Hg3·TTF·DCA] 3 that contains the solvent DCA in the asymmetric unit. Slow evaporation in a 1:1 mixture of dichloromethane and carbon disulfide was used to obtain the previously reported CT [(Hg3)2·TTF] (4)1. By slow evaporation as well in pure dichloromethane was obtained the CT material [Hg3·DTT·DCM] 5. All these five CT co-crystals contain mixed donor acceptor stacks, however only in 3, 4 and 5 these stacks are parallel to each other. In the packing of 1 and 2 these stacks are in herringbone arrangement. Polymorphs 1 and 2 differ in crystal shape and color. Contacts Hg···S shorter than the sum of the Van Der Walls radii were observed in the five CTs materials reported, however only 1, 2, 3, and 5 have extended short donor acceptor molecular associates. Moreover only 3, and 5 has these extended donor acceptor associates along one direction.

Three‐Dimensional Architectures Incorporating Stereoregular Donor–Acceptor Stacks

We report the synthesis of two [2]catenane-containing struts that are composed of a tetracationic cyclophane (TC(4+)) encircling a 1,5-dioxynaphthalene (DNP)-based crown ether, which bears two terphenylene arms. The TC(4+) rings comprise either 1) two bipyridinium (BIPY(2+)) units or 2) a BIPY(2+) and a diazapyrenium (DAP(2+)) unit. These degenerate and nondegenerate catenanes were reacted in the presence of Cu(NO3)2⋅2.5 H2O to yield Cu-paddlewheel-based MOF-1050 and MOF-1051. The solid-state structures of these MOFs reveal that the metal clusters serve to join the heptaphenylene struts into grid-like 2D networks. These 2D sheets are then held together by infinite donor-acceptor stacks involving the [2]catenanes to produce interpenetrated 3D architectures. As a consequence of the planar chirality associated with both the DNP and hydroquinone (HQ) units present in the crown ether, each catenane can exist as four stereoisomers. In the case of the nondegenerate (bistable) catenane, the situation is further complicated by the presence of translational isomers. Upon crystallization, however, only two of the four possible stereoisomers--namely, the enantiomeric RR and SS forms--are observed in the crystals. An additional element of co-conformational selectivity is present in MOF-1051 as a consequence of the substitution of one of the BIPY(2+) units by a DAP(2+) unit: only the translational isomer in which the DAP(2+) unit is encircled by the crown ether is observed. The overall topologies of MOF-1050 and MOF-1051, and the selective formation of stereoisomers and translational isomers during the kinetically driven crystallization, provide evidence that weak noncovalent bonding interactions play a significant role in the assembly of these extended (super)structures.

Conjugated NDI–Donor Polymers: Exploration of Donor Size and Electrostatic Complementarity

Conjugated donor–acceptor copolymers comprised of electron-deficient 1,4,5,8-naphthalenetetracarboxylic diimide (NDI) linked to a series of relatively electron-rich aromatics via ethynyl spacers were synthesized and characterized. While LUMO levels remained constant at −3.75 eV, HOMO levels were sensitive to the relatively electron-rich aromatic donors and systematically tuned from −5.68 to −5.17 eV. Regardless of the electron-rich comonomer, fluorescence and X-ray diffraction data were consistent with the polymer chains being assembled through the stacking of NDI moieties in an offset face-to-face fashion rather than alternating donor–acceptor stacks.

Synthesis and solution-state dynamics of donor–acceptor oligorotaxane foldamers

We describe in detail a strategy for creating foldamers in which interactions between mechanically interlocked components dictate the single-molecule assembly of a folded secondary structure. This unique folding motif is based on a flexible polyether dumbbell bearing 1,5-dioxynaphthalene (DNP) donors, which folds its way through a series of cyclobis(paraquat-p-phenylene) (CBPQT4+) acceptor rings in a serpentine fashion to enable extended donor–acceptor (D–A) stacking between DNP and the electron-poor 4,4′-bipyridinium (BIPY2+) units in CBPQT4+. These oligorotaxanes can be prepared in a wide range of sizes, with molecular weights up to >15 000 Da, on account of novel one-pot reactions we developed to generate the necessary oligo-DNP precursors. The product distributions from the final kinetically controlled stoppering reactions are highly biased towards oligorotaxanes in which approximately half of the DNP units are encircled by rings, a fact which can be rationalized if the dominant solution-state structures of the pseudorotaxane precursors reflect the solid-state superstructures of analogous compounds, which express 50% recognition site occupancy because of their proclivity to pack into continuous D–A–D–A stacks. The presence of well-defined folded structures in solution have been confirmed by 1H NMR spectroscopy in CD3CN. Moreover, we discovered an empirical selection rule forbidding CBPQT4+ rings to occupy adjacent DNP sites, which elegantly explains both the product distributions and the 1H NMR spectra. Depending on their adherence to this selection rule, all of the oligorotaxanes belong to one of three families: whereas ‘Confused’ oligorotaxanes adopt multiple translational isomers that satisfy the rule and ‘Frustrated’ species cannot obey it at all, members of the ‘Happy’ family each express only one rule-compliant ‘Goldilocks’ isomer. The NMR spectra of these oligorotaxanes also shed light on their dynamics; rapid 180° rotations of DNP units cause pairs of heterotopic BIPY2+ protons in the accompanying CBPQT4+ rings to exchange sites, giving rise to time-averaged signals. This process, which we term ‘superrotation’, will apply much more generally to other mechanically interlocked systems.

Density functional theory for the description of charge-transfer processes at TTF/TCNQ interfaces

In the field of organic electronics, a central issue is to assess how the frontier electronic levels of two adjacent organic layers align with respect to one another at the interface. This alignment can be driven by the presence of a partial charge transfer and the formation of an interface dipole; it plays a key role for instance in determining the rates of exciton dissociation or exciton formation in organic solar cells or light-emitting diodes, respectively. Reliably modeling the processes taking place at these interfaces remains a challenge for the computational chemistry community. Here, we review our recent theoretical work on the influence of the choice of density functional theory (DFT) methodology on the description of the charge-transfer character in the ground state of TTF/TCNQ model complexes and interfaces. Starting with the electronic properties of the isolated TTF and TCNQ molecules and then considering the charge transfer and resulting interface dipole in TTF/TCNQ donor–acceptor stacks and bilayers, we examine the impact of the choice of DFT functional in describing the interfacial electronic structure. Finally, we employ computations based on periodic boundary conditions to highlight the impact of depolarization effects on the interfacial dipole moment.

ChemInform Abstract: Charge-Transfer Absorption in Doped Columnar Liquid Crystals.

The optical properties of doped columnar phases formed either by a 1 : 1 mixture of an electron donor [2,3,6,7,10,11-hexa-(n-pentyloxy)triphenylene] and an electron acceptor (2,4,7-trinitrofluoren-9-one) or by a dimer in which the donor (triphenylene moiety) and the acceptor (trifluorenone moiety) are covalently linked are reported and compared with those of the solutions. In concentrated solutions a charge transfer (CT) absorption is observed for both donor–aceptor pairs corresponding to 2.51 and 2.43 eV, respectively. For the 1 : 1 mixture, CT complex formation in solution is greatly favoured by the presence of molecular aggregates. The polarized absorption spectra obtained with oriented thin films reveal that the charge transfer occurs along the alternate donor–acceptor stacking.

Alternative Donor−Acceptor Stacks from Crown Ethers and Naphthalene Diimide Derivatives: Rapid, Selective Formation from Solution and Solid State Grinding

The atypical 1:2 complexation between an electron-rich crown ether host and electron-deficient naphthalene diimide-based guests led to the formation of alternative donor-acceptor (ADA) stacks. The ADA stacks can be expediently obtained in high yield as polycrystalline aggregates from solution. More remarkably, the high degree of organization has also been realized in a simple solid-to-solid mechanical grinding process. The solid-state structures have been verified by solid-state NMR spectroscopy, single crystal, and powder X-ray diffraction analysis. The current findings not only provide convenient ways of obtaining novel donor-acceptor stacks involving a macrocyclic host but also represent an important step in transferring electroactive host-guest systems from solution to the solid state.

Mono‐ and Bis(tetrathiafulvalene)‐1,3,5‐Triazines as Covalently Linked Donor–Acceptor Systems: Structural, Spectroscopic, and Theoretical Investigations

Reaction of 2,4,6-trichloro-1,3,5-triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono- and bis(TTF)-triazines as new covalently linked (multi)donor-acceptor systems. Single-crystal X-ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)-triazine compound, while mixed TTF-triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time-dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)-triazine is paramagnetic, but no spin-spin exchange interaction could be detected.

Identification of dimerization phase transitions driven by Peierls and other mechanisms

One-dimensional (1D) electronic structure is characteristic of face-to-face stacks of planar π-electron donors and/or acceptors. Dimerization phase transitions at T c < 400 K occur for various stacking motifs. Excitations of the dimerized phase are used to distinguish between dimerization driven by electron–phonon coupling (Peierls) and other mechanisms in correlated systems with spin and charge degrees of freedom.

Peierls Transitions in Ionic Organic Charge-Transfer Crystals with Spin and Charge Degrees of Freedom

The quasi-one-dimensional electronic structure of organic charge-transfer (CT) salts rationalizes Peierls transitions in mixed or segregated stacks of pi-electron donors (D) and acceptors (A). A microscopic Peierls-Hubbard model, HCT, is presented for CT salts with mixed stacks (Drho+Arho-)n and ionicity rho > 0.7. Dimerization opens a Peierls gap that, due to electron correlation, is the singlet-triplet gap, EST. In contrast to spin-Peierls systems, such as Heisenberg spin chains with rho = 1 and TSP < 20 K, Peierls transitions in CT salts with rho < 1 occur at higher TP and involve both spin and charge degrees of freedom. Linear electron-phonon coupling and an adiabatic approximation for a harmonic lattice are used to model the dimerization amplitude deltaT for T < TP, the magnetic (spin) susceptibility chiT, and the relative infrared intensity of totally symmetric molecular modes. Exact thermodynamics of HCT for stacks up to N = 12 sites are applied to two CT salts with TP approximately 50 and 120 K whose magnetism and infrared have not been modeled previously and to CT salts with inaccessibly high TP > 350 K whose description has been difficult. Ionic CT salts are correlated Peierls systems with a degenerate ground state (GS) at T = 0 whose elementary excitations are spin solitons, while dimerized ion-radical stacks that support triplet-spin excitons have nondegenerate GS. In less ionic CT salts, modulation of HCT parameters on cooling or under pressure leads to Peierls and/or neutral-ionic transitions of the GS, without appreciable thermal population of excited states. Correlations change the gap equation that relates EST at T = 0 to TP compared to free electrons, and size convergence is fast in stacks with large delta0 and high TP.

Electronic and structural instabilities of mixed-stack organic charge-transfer salts

Electronic (valence) and structural (Peierls) instabilities occur either separately or together in organic charge-transfer (CT) crystals with mixed stacks of donors (D) and acceptors (A). A Peierls–Hubbard model, H CT, with site D, A energies and linear electron–phonon coupling in a harmonic lattice is shown to provide a unified microscopic description and to distinguish between thermal and quantum Peierls transitions. Vibrational, magnetic and structural data are interpreted as thermal Peierls transitions in two ionic CT salts and as dimerized ground states with spin solitons in other salts. Quantum Peierls transitions are identified in largely neutral salts, and a neutral–ionic crossover is discussed in a salt with dipolar disorder that suppresses the Peierls instability. The coincident Peierls and neutral–ionic transition of tetrathiafulvalene-chloranil (TTF-CA) is another special case of H CT.

Spin solitons in organic charge-transfer salts

Organic charge-transfer (CT) salts that crystallize in face-to-face stacks of π-electron donors (D) and acceptors (A) are one-dimensional electronic systems with strong coupling to lattice and molecular vibrations. Peierls–Hubbard models of CT salts resemble π-electron theory of conjugated polymers, although transfer integrals t along mixed DA stacks are considerably smaller. Strong D and A yield Peierls systems with dimerized ground states of ion radicals, D + and A −. Topological spin solitons separate regions of opposite dimerization, similarly to solitons in trans-polyacetylene, but are thermally accessible in ionic CT salts due to small t. Salts with still smaller t have Peierls transitions at T P < 300 K to undimerized stacks. A Peierls–Hubbard model, H CT, describes both types of salts and estimates of t are consistent with T P < 300 K in some salts and spin solitons in others with higher T P. Electron paramagnetic resonance (epr) spectra of single crystals provide direct evidence for spin solitons and one-dimensional electronic states. Spin solitons and H CT resolve longstanding conflicts between vibrational and magnetic data that indicate dimerized stacks in several prototypical CT salts, while structural data point to undimerized stacks with large thermal ellipsoids. The low energy scale, availability of single crystals and diversity of CT salts offer opportunities for detailed modeling.

Charge fluctuations and electron–phonon coupling in organic charge-transfer salts with neutral–ionic and Peierls transitions

The first-order transition of the charge-transfer complex tetrathiafulvalene–chloranil (TTF–CA) is both a neutral–ionic (NIT) and a Peierls transition. In related organic charge-transfer complexes, cooling at ambient pressure increases the ionicity ρ in strikingly different ways, and is sometimes accompanied by a dielectric peak, that we relate to lattice stiffness, to structural and energetic disorder, and to the softening of the Peierls mode in the far-IR. The position operator P for systems with periodic boundary conditions makes possible a systematic treatment of electron–phonon (e–p) interactions in extended donor–acceptor stacks in terms of correlated Peierls–Hubbard models. The IR intensity of the Peierls mode peaks at the Peierls transition at small ρ<1/2 in soft lattices, where the dielectric constant also has a large peak. In dimerized stacks, the IR intensity of totally symmetric (ts), Raman active, molecular vibrations is related to charge fluctuations that modulate site energies. Combination bands of molecular and Peierls modes are identified in regular TTF–CA stacks above T c. Energetic disorder can suppress the Peierls transition and rationalize a continuous crossover from small to large ρ. The TTF–CA scenario of a neutral-regular to ionic-dimerized transition must be broadened considerably in view of charge-transfer salts that dimerize on the neutral side, that become ionic without a structural change, or that show vibrational evidence for dimerization at constant ρ<1.

Five New Pseudopolymorphs of sym-Trinitrobenzene

1,3,5-Trinitrobenzene (TNB) was recrystallized from about 30 solvents and five mixed solvents in around 50 experiments to yield five new solvates. All the solvates lose their solvent readily, and special precautions were required to determine their crystal structures. In all cases, the solvents that gave solvates are electron-rich aromatics leading to the conjecture that π···π donor−acceptor stacking interactions are important. This was confirmed from the crystal structures. Additionally, these structures are composed of hexagonal sheets in which TNB and solvent molecules are linked with weak C−H···O hydrogen bonds. The benzene solvate contains an unusual finite TNB motif not hitherto seen. The toluene and chlorobenzene solvates are not isostructural, and C−H···Cl interactions in the latter are unusually short and linear, accounting thereby for the failure of chloro-methyl structural exchange. The m-xylene and mesitylene solvates are similar to the toluene solvate. This study indicates that the use of terms such as solvate, pseudopolymorph, donor−acceptor complex, and molecular complex is a subjective matter, and also that a better definition for the term pseudopolymorph may be needed, especially because it occurs frequently in the pharmaceutical literature.

Charge transfer salts based on M(dcbdt)2 complexes (M=Au and Ni)

BEDT-TTF [Au(dcbdt) 2 ] and TMTSF [Co(dcbdt) 2 ] (dcbdt = 4,5-dicyanobenzene-1,2-dithiolate) were obtained as small single crystals by electrocrystallisation. BEDT-TTF [Au(dcbdt) 2 ] is triclinic, space group P1, and its structure consists in alternated donor acceptor stacks, responsible for electrical insulating properties. TMTSF [Co(dcbdt) 2 ] presents semiconducting properties with a relatively high electrical conductivity (σ RT ∼10-150 S/cm, E a =78 meV) and its thermopower (S RT ∼ 9 μV/K) has a temperature dependence suggesting a two band conduction mechanism.

Interactions of metalloporphyrins as donors with the electron acceptors C60, tetracyanoquinomethane (TCNQ) and trinitrofluorenylidenemalonitrile

Crystals of C60·PtII(OEP)·2(C6H6), TCNQ·CuII(OEP), TCNQ·H2(OEP), TCNQ·2CuII(OEP), TCNQ·2ZnII(OEP) and TNFM·CoII(OEP) [OEP is the dianion of octaethylporphyrin, TCNQ is 7,7,8,8-tetracyanoquinodimethane, TNFM is (2,4,7-trinitrofluorenylidene)malonitrile] have been obtained by diffusion of a solution of the porphyrin as donor into a solution of the respective acceptor molecule. The structure of C60·PtII(OEP)·2(C6H6) consists of an ordered C60 cage nestled against the platinum porphyrin which makes close face-to-face contact with another PtII(OEP) molecule. In contrast, there are no close face-to-face contacts between porphyrins in the crystal structures of TCNQ·CuII(OEP), TCNQ·H2(OEP), and TNFM·CoII(OEP). These compounds consist of classical donor–acceptor stacks of interleaved porphyrin and TCNQ or TNFM molecules with separations of ca. 3.3 A between adjacent molecules. However with TCNQ·2CuII(OEP) and TCNQ·2ZnII(OEP) the structures involve TCNQ (A) and MII(OEP) (D) molecules that crystallize in stacks with a DDA(DDA)nDDA arrangement. Within these stacks there are pairwise contacts between MII(OEP) molecules and these pairs are compared to those found in C60·PtII(OEP)·2(C6H6) and related fullerene-containing crystals.