Carbon-Rich Materials


Last updated: August 22, 2016

Selected References

In a highly synthesis-driven program, acetylenic advanced materials are prepared and characterized in interdisciplinary collaboration with the objectives:

Acetylenic scaffolding in the proposed research takes advantage of large libraries of building modules programmed for the generation of desired functions. They include the tetraethynylethenes (TEEs) and the 1,2-diethynylethenes (DEEs) as well as newly developed building blocks including the cyanoethynylethenes (CEEs), 1,1,4,4-tetracyanobutadienes (TCBDs) 1,3-diethynylallenes, di- and tetraethynylated butatrienes, azoacetylenes, and triply differentially protected and, hence, chiral triethynylmethanes. Specific functions targeted with the new scaffolds include strong electronic absorptions extending into the near infrared (low HOMO-LUMO gap), high fluorescence quantum yields, facile electron uptake, high nonlinear optical effects, high two-photon absorption (TPA) cross-sections, and photochemical switching between bistable states. In collaboration with the Nanoscale Science Center in Basel, the functions of many of the produced multinanometer-sized objects are visualized by scanning probe and atomic force microscopies (STM and AFM).


Examples from our recent work include:


Molecular wires. With its linearly conjugated 24 double and 48 triple bonds, the 17.8 nm-long monodisperse 24-mer is the longest linear, fully-conjugated molecular wire without aromatic repeat units. Oligomers of the poly(triacetylene)s to which the 24-mer belongs have been shown to feature outstanding nonlinear-optical properties.

Cyanoethynylethenes (CEEs) and 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs). The cyanoethynylethenes (CEEs, examples: 1a-c), are among the most powerful electron acceptors known. Thus 1b undergoes a reversible 1-electron uptake at E0 = –0.57 V in CH2Cl2 (cyclic voltammetry, potential against the ferrocene/ferricinium couple, which rivals tetracyanoethene (E0 = –0.32 V). Upon attachment of additional N,N-dimethylanilino (DMA) donor groups to the residual acetylenic anchor points in the CEEs, chromophores such as 1c with excellent advanced materials properties are obtained. They feature strong intramolecular charge-transfer from the DMA donors to the strongly electron-accepting acetylenic cores, allow crystalline thin-film preparation by vapor deposition, and high two-photon absorption cross-sections. The third-order nonlinear optical properties of donor-substituted CEEs and also of the recently introduced 1,1,4,4-tetracyanobutadienes (TCBDs) are exceptional (collaboration with Prof. I Biaggio, Lehigh Univ.) and the construction of optoelectronic devices by vapor phase deposition and thin-film formation are intensely pursued. In future work, CEEs are introduced into expanded radialenes and radiaannulenes, as well as into new chromophores displaying high second- and third-order optical nonlinearities. Also, dendrimers with multiple TCBD subunits in the branches are prepared for multi-electron uptake and storage.

With the formal [2+2]cycloaddition of tetracyanoethene to electron-rich alkynes, followed by retro-electrocyclization (CA/CR reaction: cycloaddition-cycloreversion reaction), we have developed an atom-economic, high-yielding reaction which meets all the requirements of a click reaction. We have extended the methodology to the cycloaddition of TCNQ and obtained donor-acceptor chromophores capable of facile electron uptake, thereby qualifying as organic superacceptors. In future work, we are expanding this synthetic methodology to other systems, aiming at stable advanced organic materials. Already, in collaboration with Prof. I Biaggio, amorphous thin films of high optical quality and an off-resonant third-oder optical susceptibility three orders of magnitude higher than that of fused silica have been prepared. The high non-planarity of the push-pull chromophores is at the origin of the amorphous supramolecular assembly in the thin films, which ensures full optical transparency in the telecommunication wavelength range. Such films are now exploited, in collaboration, in optoelectronic devices.

 

New click reactions: the cycloaddition of TNCE and TCNQ to electron-rich alkynes, followed by retro-electrocyclization, provides an efficient entry into new non-planar push-pull chromophores featuring intense bathochromically shifted charge-transfer bands and facile reducibility of the acceptor moiety.

 

Ref.

A dendritic TCBD-system, prepared by an efficient click reaction, which can be reversibly charged with 24 electrons and undergoes 12 reversible oxidations as well. In smaller dendritic systems of this type, radical centers formed by one-electron reduction are found to be fully localized on single TCBD moieties, which opens the way towards new multi-spin systems.

 

A new cascade 5-component reaction yielding unprecedented charge-transfer chromophores


New super-acceptors featuring electron acceptor capabilities similar to TNCE and TNCQ

The chemical space of advanced opto-electronic materials could be greatly expanded by cascade reactions, generating new chromophores, such as a donor-substituted 6,6-dicyanofulvenes with strong bathochromically shifted intramolecular charge-transfer bands.

The UV/Vis/NIR spectra of a donor-substituted 6,6-dicyanofulvene  feature a very intense intramolecular charge-transfer band at λmax = 782 nm, with the end-adsorption expanding to 1100 nm.

Two-dimensional acetylenic all-carbon scaffolds. New expanded dehydroannulenes, expanded radialenes and radiaannulenes are prepared in future work to test the limits of synthetic accessibility and stability of large carbon sheets with multinanometer dimensions. The optoelectronic properties of these shape-persistent chromophores are tuned and their stability greatly enhanced by attachment of lateral N,N-dimethylanilino (DMA) donor groups capable of transferring charges into the strongly electron-accepting acetylenic cores. Recent examples include the expanded radialene 2 with a C60 core whose structure was elucidated by X-ray crystallography and the radiaannulene 3 with an acetylenic C50 core. Radiaannulenes are a new class of macrocyclic chromophores that are hybrids between expanded dehydroannulenes and radialenes. In going work, the radiaannulene scaffolds are expanded to all-carbon cores with more than 100 C-atoms. Recent achievements are giant dehydroannulenes (4) and tetrathiafulvalene (TTF)-fused derivatives with unusual opto-electronic properties (5).

              

 

4 5


Three-dimensional acetylenic all-carbon scaffolding
. Among the targets are the synthesis of helical foldamers and macrocycles derived from optically active 1,3-diethynylallenes. Thus we recently described the synthesis and separation of alleno-acetylenic macrocycles such as 6 and 7, featuring exceptional molecular symmetries. Molecular recognition in the cavities of such macrocycles is currently under investigation.

6 7

Another target is the formation of unprecedented all-carbon cages including expanded cubanes and tetrahedranes (“expanded platonic molecular objects”), expanded radialenes, and a C60 cage with the potential for rearrangement into buckminsterfullerene.


Recently we synthesized the first expanded cubane with a C56 core (8). Formally derived by insertion of buta-1,3-diynediyl moieties into all 12 C–C single bonds of octamethoxycubane, its synthesis proceeds by the formation of corners, then edges, and finally faces as key building blocks and intermediates. In future work, we intend to prepare other series of such unprecedented all-carbon cages to investigate their reactivity, physical properties and the host-guest binding properties of their inner phases. Examples include cubanes and tetrahedranes, prismanes, and a C60 cage with the potential for rearrangement into buckminsterfullerene.

 

8

Chirality and amplification of chirality in carbon-rich pi-systems have attracted particular recent attention and will be a key component of our research.


N
-Arylated dinaphthazepines are potent axial chiral electron donors and we introduced them into our push-pull systems as illustrated by 9 and 10.  These molecular rods are strong helical twisting inducers, capable of switching a nematic liquid crystalline phase into a cholesteric one.  This work, in collaboration with Prof. A. Ferrarini (Padova) and Prof. G. P. Spada (Bologna), will be continued to further improving the helical twisting power and thus the transfer from molecular to macroscopic chirality, based on the take-home lessons from this initial study.

Frank, B. B.; Camafort Blanco, B.; Jakob, S.; Ferroni, F.; Pieraccini, S.; Ferrarini, A.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Spada, G. P.; Diederich, F. “N-Arylated 3,5-Dihydro-4-dinaphtho[2,1-c:1',2'-e]azepines: Axially Chiral Donors With High Helical Twisting Powers For Non-planar Push-Pull Chromophores”, Chem. Eur. J., 2009, 15, 9005-9016.


Whereas the allenoacetylenic macrocycle 6 and the alleno-acetylenic cyclophane 7 were only obtained in racemic form, we recently reported the first enantiomerically pure alleno-acetylenic macrocycles such as 11.  Extraordinary large Cotton effects are measured by circular dichroism spectroscopy, and their magnitude can be rationalized by a unique combination of geometric and electronic properties (collaboration with Prof. N. Berova and Prof. N. Harada from Columbia University).

Alonso-Gómez, J. L.; Rivera-Fuentes, P.; Harada, N.; Berova, N. Diederich, F. “An Enantiomerically Pure Alleno-Acetylenic Macrocycle: Synthesis and Rationalization of Ist Outstanding Chiroptical Response“, Angew. Chem. 2009, 121, 5653-5656; Angew. Chem. Int. Ed. 2009, 48, 5545-5548.

We also prepared alleno-acetylenic oligomers up to the 16-mer starting from optically pure 1,3-diethynylallenes.  These compounds feature amplification of chirality with increasing length and feature exceptionally intense Cotton effects in the circular dichroism spectra.  We explain these observations with a conformational preference for helical conformations of one chirality sense. The further stabilization of these helices and their communication with helical structures from biology are now the subject of intense investigations.

CD and ORD calculations suggest that enantiomerically pure acyclic alleno-acetylene oligomers adopt helical conformations with one chirality sense, explaining their exceptional chiroptical properties.

Conjugation of the push-pull chromophores from the CA/CR reaction to [60]fullerene led to axially chiral derivatives as a result of hindered rotation about the central CC single bond of the push-pull buta-1,3-dienes.  Their enantiomers could be isolated and the absolute configuration assigned.  Upon heating these derivatives, an unprecedented rearrangement takes place, yielding new fullerene tetrakis-adducts.

Conjugates of fullerenes with push-push pull chromophores from the CA/CR reaction are axially chiral due to hindered rotation about the central CC single bond of the buta-1,3-diene fragment and undergo unprecedented rearrangements upon heating.

We also discovered the acetylene-like (proacetylenic) reactivity of a push-pull butatriene in a formal [2+2]cycloaddition-retroelectrocyclization reaction to give a zwitterion, in which charges are separated through cross-conjugation and stabilized by dimethylaniline (DMA) rings (cation) and cyano groups (allyl anion).  The positive charge on the NMe2 centers of the DMA rings is evidenced by complexation with a tetraphosphonate cavitand (in collaboration with E. Dalcanale, Parma).  In the solid state, a supramolecular polymer forms.  The color of the zwitterion upon complexation with the cavitand in solution changes strongly, suggesting the employment of this host system for the colorimetric detection of other, highly polarized push-pull guest chromophores.


We recently discovered an unprecedented access to substituted tetracenes in two steps, starting from tetraaryl[3]cumulenes.  In the first step, the CA-RE reaction at the proacetylenic central C=C bond of the [3]cumulenes with TCNE, followed by two electrocyclizations of the zwitterionic intermediate and dehydrogenation, provided 5,5,11,11-tetracyano-5,11-dihydrotetracenes.  Formal cyanogen elimination in the second step yielded the functionalized tetracenes with a rubrene-like substitution pattern.  These new chromophores crystallize in ordered π-π-arrays, and are thermally stable, sublimable, and highly fluorescent molecular materials. With their unique structural feature of coaligned neighboring phenyl and cyano substituents on the tetracene ring, they are selective molecular sensors of soft ions, such as Cu+ and Ag+ as revealed in both absorption and emission specroscopic titrations. Recently, the group of Prof. M. R. Wasielewski at Northwestern University showed that thin films of the parent dicyano-diphenyltetracene undergo efficient, exoergic singlet exciton fission with SF rates in the sub-picosecond regime.  This collaboration is continued and the potential of the dicyanodiaryltetracenes for organic photovoltaics applications explore.

The click-reaction between electron-rich, anilino-activated alkynes and electron-deficient olefins, such as TCNE indeed has allowed us to greatly expand the chemical space of push-pull chromophores. An illustration of this statement is given in the following Figure. Many of these compounds feature fascinating optoelectronic properties besides desirable materials properties such as high stability. Substituted pentafulvene chemistry has been particularly rewarding and will be pursued with great intensity in the coming years.

Expanding the chemical space for push–pull chromophores by sequential [2+2] cycloaddition–retroelectrocyclization cascades starting from electron-rich alkynes and TCNE.

With the CA–RE reaction as key transformation in the final step, we were able to prepare the first molecular dyad, closely resembling the original design by Aviram and Ratner, featuring a strong donor (TT), separated by a rigid insulating σ-spacer from a strong extended TCNQ acceptor (ExTCNQ). Langmuir-Blodgett films were prepared in collaboration and showed the asymmetric current-voltage (I–V) curve characteristic for rectification. A large number of controls demonstrated that all three entities of the structure, proposed byAviram-Ratner, are essential for rectification:

In our pursuit for stable, neutral π-extended pentalene derivatives, we prepared three series of bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, by an original, double carbopalladation cascade reaction. A combined experimental/computational study including 1H NMR, NICS-XY-scans (in collaboration), X-ray diffraction, optical spectroscopy, and TD-DFT calculations suggests that the molecular properties of the bispentalenes are dominated by the antiaromatic pentalene subunits, despite the [4n+2] π-electron perimeter of the skeletons: