Milan Kivala

Surface-supported 2D heterotriangulene polymers

We report on the assembly of tribromo-substituted dimethylmethylene-bridged triphenylamine (heterotriangulene) on Ag(111). Depending on activation temperature, two-dimensional porous metal-coordination or covalent networks are obtained.

Organic super-acceptors with efficient intramolecular charge-transfer interactions by [2+2] cycloadditions of TCNE, TCNQ, and F4-TCNQ to donor-substituted cyanoalkynes.

Rivaling the best one: Thermal [2+2] cycloadditions of TCNE, TCNQ, and F(4)-TCNQ to N,N-dimethylanilino-substituted cyanoalkynes afforded a new class of organic super-acceptors featuring efficient intramolecular charge-transfer interactions. These acceptors rival the acceptor F(4)-TCNQ in the propensity for reversible electron uptake as well as in electron affinity (see figure), which makes them interesting as p-type dopants for potential application in optoelectronic devices.Thermal [2+2] cycloadditions of tetracyanoethene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)-TCNQ) to N,N-dimethylanilino-substituted (DMA-substituted) alkynes bearing either nitrile, dicyanovinyl (DCV; -CH==C(CN)(2)), or tricyanovinyl (TCV; -C(CN)==C(CN)(2)) functionalities, followed by retro-electrocyclization, afforded a new class of stable organic super-acceptors. Despite the nonplanarity of these acceptors, as revealed by X-ray crystallographic analysis and theoretical calculations, efficient intramolecular charge-transfer (CT) interactions between the DMA donors and the CN-containing acceptor moieties are established. The corresponding CT bands appear strongly bathochromically shifted with maxima up to 1120 nm (1.11 eV) accompanied by an end-absorption in the near infrared around 1600 nm (0.78 eV) for F(4)-TCNQ adducts. Electronic absorption spectra of selected acceptors were nicely reproduced by applying the spectroscopy oriented configuration interaction (SORCI) procedure. The electrochemical investigations of these acceptors by cyclic voltammetry (CV) and rotating disc voltammetry (RDV) in CH(2)Cl(2) identified their remarkable propensity for reversible electron uptake rivaling the benchmark compounds TCNQ (E(red,1)=-0.25 V in CH(2)Cl(2) vs. Fc(+)/Fc) and F(4)-TCNQ (E(red,1)=+0.16 V in CH(2)Cl(2) vs. Fc(+)/Fc). Furthermore, the electron-accepting power of these new compounds expressed as adiabatic electron affinity (EA) has been estimated by theoretical calculations and compared to the reference acceptor F(4)-TCNQ, which is used as a p-type dopant in the fabrication of organic light-emitting diodes (OLEDs) and solar cells. A good linear correlation exists between the calculated EAs and the first reduction potentials E(red,1). Despite the substitution with strong DMA donors, the predicted EAs reach the value calculated for F(4)-TCNQ (4.96 eV) in many cases, which makes the new acceptors interesting for potential applications as dopants in organic optoelectronic devices. The first example of a charge-transfer salt between the DMA-substituted TCNQ adduct (E(red,1)=-0.27 V vs. Fc(+)/Fc) and the strong electron donor decamethylferrocene ([FeCp*(2)]; Cp*=pentamethylcyclopentadienide; E(ox,1)=-0.59 V vs. Fc(+)/Fc) is described. Interestingly, the X-ray crystal structure showed that in the solid state the TCNQ moiety in the acceptor underwent reductive sigma-dimerization upon reaction with the donor.

Origin of intense intramolecular charge-transfer interactions in nonplanar push-pull chromophores.

Planar push–pull chromophores featuring intense intramolecular charge-transfer (CT) interactions have been extensively studied in view of their potential applications in molecular electronics and optoelectronics. In contrast, only a limited number of nonplanar low-molecular-weight donor– acceptor chromophores has been reported and the impact of nonplanarity on their p-conjugative and optoelectronic properties not been systematically investigated. Nonplanar CT chromophores tend to feature some desirable physical properties compared to their planar counterparts: they are usually more soluble, less aggregating, and more readily sublimable, forming amorphous, rather than crystalline films for potential use in optoelectronic devices. We showed recently that donor-substituted alkynes undergo a formal [2+ 2] cycloaddition with electron-accepting olefins, such as tetracyanoethene (TCNE), 7,7,8,8-tetracyanoquinodimethanes (TCNQs), as well as dicyanovinyl (DCV) and tricyanovinyl derivatives, followed by retroelectrocyclization, under formation of nonplanar push–pull chromophores featuring intense intramolecular CT and high third-order optical nonlinearities. Nonplanar, N,N-dimethylanilino (DMA) donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs), obtained by [2+ 2] cycloaddition of TCNE, produce high-optical quality amorphous films by vapor-phase deposition which in the meanwhile have found first application in silicon-organic-hybrid (SOH) waveguides. A key question raised was the origin of the intense intramolecular CT interactions in these push–pull chromophores, in view of their pronounced nonplanarity which is expected to lead to disruption of donor–acceptor p-conjugation and concomitant reduction in CT efficiency and optical nonlinearity. To investigate the effects of sterically enforced deconjugation on optoelectronic properties, we prepared two new series of push–pull chromophores (1 and 2, and 3–8), by adding TCNE or TCNQ to tetrathiafulvalene (TTF)or ferrocenyl (Fc)-substituted alkynes, respectively. These strong electron donors have found numerous applications in intermolecular and intramolecular CT systems, but have not seen much use for activating alkynes electronically for the cycloaddition of TCNE or TCNQ. TTF-appended TCBD 1 was obtained in 41 % yield as a deep blue solid by reaction of TCNE with TTF-substituted alkyne 9 in 1,2-dichloroethane at 80 8C (Scheme 1). The electronically more activated alkyne 10, with both TTF and DMA donor groups, reacted at room temperature to give 2 in 79 % yield. The ferrocene-substituted expanded TCNQs 3–8 were formed as black metallic-like solids in 41–81 % yield by regioselective cycloaddition between TCNQ and the acetylenic precursors 11–16. With the exception of 3 and 8, these push–pull chromophores are thermally stable up to 300 8C, as revealed by thermal gravimetric analysis (TGA); compound 6 can actually be sublimed without decomposition at about 250 8C/1 10 6 Torr. Single crystals of 1 and 3–5 suitable for X-ray analysis were grown from CH2Cl2/hexane at 15 8C (Figure 1 and Figure S1–S4 in the Supporting Information). Considerable nonplanarity is observed in the TCBD and expanded TCNQ acceptor moieties. Thus, the dihedral angle q (C1-C2-C17C18) between the two DCV planes in 1 (with two independent molecules) is 92.7(2)8. In the expanded TCNQ chromophores 3–5, the dihedral angle q between the DCV and the cyclohexa-2,5-diene-1,4-diylidene moiety changes from 51.9(4)8 (3, torsional angle C1-C2-C3-C4, Figure S2) to 68.6(2)8 (5, C5-C8-C9-C16, Figure S4), and to 94.0(2)8 (4, C1-C12-C13-C14). Sterically enforced p-deconjugation is [a] Dr. S.-i. Kato, Dr. M. Kivala, Dr. W. B. Schweizer, Prof. Dr. F. Diederich Laboratorium f r Organische Chemie, ETH Z rich Hçnggerberg, HCI, 8093 Z rich (Switzerland) Fax: (+41) 44-632-1109 E-mail : diederich@org.chem.ethz.ch [b] Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide Intstitut de Chimie-UMR 7177, C.N.R.S. Universit de Strasbourg 4, rue Blaise Pascal, 67000 Strasbourg (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901630.

Long-range energy transport in single supramolecular nanofibres at room temperature.

Efficient transport of excitation energy over long distances is a key process in light-harvesting systems, as well as in molecular electronics. However, in synthetic disordered organic materials, the exciton diffusion length is typically only around 10 nanometres (refs 4, 5), or about 50 nanometres in exceptional cases, a distance that is largely determined by the probability laws of incoherent exciton hopping. Only for highly ordered organic systems has the transport of excitation energy over macroscopic distances been reported—for example, for triplet excitons in anthracene single crystals at room temperature, as well as along single polydiacetylene chains embedded in their monomer crystalline matrix at cryogenic temperatures (at 10 kelvin, or −263 degrees Celsius). For supramolecular nanostructures, uniaxial long-range transport has not been demonstrated at room temperature. Here we show that individual self-assembled nanofibres with molecular-scale diameter efficiently transport singlet excitons at ambient conditions over more than four micrometres, a distance that is limited only by the fibre length. Our data suggest that this remarkable long-range transport is predominantly coherent. Such coherent long-range transport is achieved by one-dimensional self-assembly of supramolecular building blocks, based on carbonyl-bridged triarylamines, into well defined H-type aggregates (in which individual monomers are aligned cofacially) with substantial electronic interactions. These findings may facilitate the development of organic nanophotonic devices and quantum information technology.

π-Conjugated Heterotriangulene Macrocycles by Solution and Surface-supported Synthesis toward Honeycomb Networks

A comparative analysis between a solution and a surface-mediated synthesis of heterotriangulene macrocycles is reported. The results show a preferential formation of the π-conjugated macrocycles on surface due to two-dimensional confinement. The macrocycle prepared on a several hundred milligram scale by solution chemistry was characterized by single-crystal X-ray analysis and was furthermore extended toward next generation honeycomb species. Investigation of the photophysical and electronic properties together with the good thermal stability revealed the potential of MC6 as hole-transport material for organic electronics.

Impact of Electronic Coupling, Symmetry, and Planarization on One- and Two-Photon Properties of Triarylamines with One, Two, or Three Diarylboryl Acceptors

We have performed a study of the one- and two-photon absorption properties of a systematically varied series of triarylamino-compounds with one, two, or three attached diarylborane arms arranged in linear dipolar, bent dipolar, and octupolar geometries. Two-photon fluorescence excitation spectra were measured over a wide spectral range with femtosecond laser pulses. We found that on going from the single-arm to the two- and three-arm systems, the peak in two-photon absorption (2PA) cross-section is suppressed by factors of 3-11 for the lowest excitonic level associated with the electronic coupling of the arms, whereas it is enhanced by factors of 4-8 for the higher excitonic level. These results show that the coupling of arms redistributes the 2PA cross-section between the excitonic levels in a manner that strongly favors the higher-energy excitonic level. The experimental data on one- and two-photon cross-sections, ground- and excited-state transition dipole moments, and permanent dipole moment differences between the ground and the lowest excited states were compared to the results obtained from a simple Frenkel exciton model and from highly correlated quantum-chemical calculations. It has been found that planarization of the structure around the triarylamine moiety leads to a sizable increase in peak 2PA cross-section for the lowest excitonic level of the two-arm system, whereas for the three-arm system, the corresponding peak was weakened and shifted to lower energy. Our studies show the importance of the interarm coupling, number of arms, and structural planarity on both the enhancement and the suppression of two-photon cross-sections in multiarm molecules.

One-electron-reduced and -oxidized stages of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes of different molecular architectures.

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Pathway Complexity in the Enantioselective Self-Assembly of Functional Carbonyl-Bridged Triarylamine Trisamides

Functional supramolecular systems like carbonyl-bridged triarylamine (CBT) trisamides are known for their long-range energy transport at room temperature. Understanding the complex self-assembly processes of this system allows for control over generated structures using controlled supramolecular polymerization. Here, we present two novel CBT trisamides with (S)- or (R)-chiral side chains which show a two-pathway self-assembly behavior in solution. Depending on the thermal profile during the self-assembly process, two different stable states are obtained under otherwise identical conditions. A kinetically trapped state A is reached upon cooling to 7 °C, via a proposed isodesmic process. In addition, there is a thermodynamically stable state B at 7 °C that is induced by first undercooling to -5 °C, via a nucleation-elongation mechanism. In both cases, helical supramolecular aggregates comprising H-aggregated CBTs are formed. Additionally, controlled supramolecular polymerization was achieved by mixing the two different states (A and B) from the same enantiomer, leading to a conversion of the kinetically trapped state to the thermodynamically stable state. This process is highly enantioselective, as no conversion is observed if the two states consist of opposite enantiomers. We thus show the importance and opportunities emerging from understanding the pathway complexity of functional supramolecular systems.

Columnar Self‐Assembly in Electron‐Deficient Heterotriangulenes

A series of soluble carbonyl-bridged heterotriangulenes, in which flexible n-dodecyl chains are attached through different spacers to the planar nitrogen-centered polycyclic core, have been synthesized. The introduction of triisopropylsilylethynyl moieties enabled, for the first time, the characterization of single-crystal columnar packing of a substituted heterotriangulene by X-ray crystallography. Electrochemical studies disclosed the carbonyl-bridged heterotriangulene core as a reasonably strong acceptor for a reversible two-electron transfer. The tendency of substituted heterotriangulenes to self-assemble in solution, on surfaces, and in the bulk appeared to sensitively depend on the nature of the lateral substituents, their steric demand, and the applied solution processing conditions. It can be concluded that 1) additional phenylene moieties between the heterotriangulene core and the n-dodecyl chains facilitate self-assembly by extending the π-conjugated polycyclic disc, 2) the rod-like ethynylene spacers introduce some additional flexibility and hence lower the overall aggregation tendency, and 3) the combination of both features in the phenylene-ethynylene moieties induces thermotropic liquid crystallinity.

Multicomponent Self-Assembly with a Shape-Persistent N-Heterotriangulene Macrocycle on Au(111)

Multicomponent network formation by using a shape-persistent macrocycle (MC6) at the interface between an organic liquid and Au(111) surface is demonstrated. MC6 serves as a versatile building block that can be coadsorbed with a variety of organic molecules based on different types of noncovalent interactions at the liquid-solid interface. Scanning tunneling microscopy (STM) reveals the formation of crystalline bicomponent networks upon codeposition of MC6 with aromatic molecules, such as fullerene (C60) and coronene. Tetracyanoquinodimethane, on the other hand, was found to induce disorder into the MC6 networks by adsorbing on the rim of the macrocycle. Immobilization of MC6 itself was studied in two different noncovalently assembled host networks. MC6 assumed a rather passive role as a guest and simply occupied the host cavities in one network, whereas it induced a structural transition in the other. Finally, the central cavity of MC6 was used to capture C60 in a complex three-component system. Precise immobilization of organic molecules at discrete locations within multicomponent networks, as demonstrated here, constitutes an important step towards bottom-up fabrication of functional surface-based nanostructures.