Increased Antiaromaticity through Pentalene Connection in [n]Cyclo-1,5-dibenzopentalenes.

Conjugated nanohoops incorporating nonalternant hydrocarbons have altered optoelectronic properties compared to [n]cycloparaphenylenes or other purely aromatic hoops. We synthesized [n]cyclo-1,5-dibenzopentalenes (n = 4, 5), in which nonalternant dibenzo[a,e]pentalenes are connected through their pentalene units. This leads to an increase in antiaromatic character and low-lying LUMO energies. Calculations show puckered or entangled conformations of the precursor macrocyclic Pt-complexes. Our study proves dibenzopentalene as a versatile nonalternant building block for conjugated nanohoops with modifiable antiaromaticity and optoelectronic properties.

Enantiopure nanohoops through racemic resolution of diketo[n]CPPs by chiral derivatization as precursors to DBP[n]CPPs.

Strained conjugated nanohoops are attractive synthetic targets due to the bending of their π-system, which leads to intriguing optoelectronic properties, among others. By incorporating non-mirror-symmetric aromatic panels, chiral nanohoops can be obtained. We herein present a strategy to enantiopure nanohoops by racemic resolution through chiral derivatization of diketone-embedded hoops. The resulting diketo[n]CPPs (n = 6, 7) contain two stereogenic carbon atoms each and possess high fluorescence quantum yields paired with circularly polarized luminescence. These are versatile precursors to chiral dibenzo[a,e]pentalene-based nanohoops DBP[n]CPPs with antiaromatic character and ambipolar electrochemical behavior. Due to their strained structures the DBP[n]CPPs do not racemize at room temperature, which is supported by high calculated isomerization barriers. X-ray crystallographic investigations on the DBP[n]CPPs and their precursors as well as DFT calculations provide insight into the build-up of strain energy during the synthetic transformations.

[n]Cyclodibenzopentalenes as Antiaromatic Curved Nanocarbons with High Strain and Strong Fullerene Binding.

Conjugated nanohoops provide a platform to study structure-property relationships; they are attractive hosts for supramolecular chemistry as well as promising candidates as new organic materials. We herein present [n]cyclodibenzopentalenes ([n]CDBPs) as antiaromatic analogues of [n]cycloparaphenylenes. Platinum-mediated macrocyclization of dibenzopentalene boronic esters provided the trimer and tetramer with strain energies of up to 80 kcal mol-1. In the solid state, the cylindrical [4]CDBP molecules align to form columnar structures. The larger hoop [4]CDBP binds both fullerenes C60 and C70 with temperature-dependent exchange behavior, providing higher activation energies for the exchange compared to [10]CPP. The antiaromatic character of the [n]CDBPs paired with the cyclic conjugation leads to high HOMO energies and lowered LUMO energies with band gaps below 2 eV. This work presents a new class of the antiaromatic and nonalternant curved nanocarbons with intriguing supramolecular and ambipolar optoelectronic properties.

Chiral Dibenzopentalene-Based Conjugated Nanohoops through Stereoselective Synthesis.

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π-systems. They can possess unexpected optoelectronic properties and their radially oriented π-system makes them attractive for host-guest chemistry. Bending the π-subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel "corner unit" in the synthesis of the hoops. The [6]DBP[4]Ph-hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO-LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane.

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.

Single-molecule conductance of dibenzopentalenes: antiaromaticity and quantum interference.

The effects of antiaromaticity and destructive quantum interference (DQI) are investigated on the charge transport through dibenzo-[a,e]pentalene (DBP). 5,10-Connectivity gives high single-molecule conductance whereas 2,7 gives low conductance due to DQI. Comparison of the 5,10-DBP with phenyl and anthracene analogues yields the trend GDBP ≈ GAnth > GPh, despite the aromatic anthracene having a larger HOMO-LUMO gap than 5,10-DBP. This is explained by unfavourable level alignment for 5,10-DBP.

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero- or Polycyclic Aromatics.

In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size-dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.

Thiocoumarin Caged Nucleotides: Synthetic Access and Their Photophysical Properties.

Photocages have been successfully applied in cellular signaling studies for the controlled release of metabolites with high spatio-temporal resolution. Commonly, coumarin photocages are activated by UV light and the quantum yields of uncaging are relatively low, which can limit their applications in vivo. Here, syntheses, the determination of the photophysical properties, and quantum chemical calculations of 7-diethylamino-4-hydroxymethyl-thiocoumarin (thio-DEACM) and caged adenine nucleotides are reported and compared to the widely used 7-diethylamino-4-hydroxymethyl-coumarin (DEACM) caging group. In this comparison, thio-DEACM stands out as a phosphate cage with improved photophysical properties, such as red-shifted absorption and significantly faster photolysis kinetics.

Synthesis and Properties of Conjugated Nanohoops Incorporating Dibenzo[a,e]pentalenes: [2]DBP[12]CPPs.

Conjugated nanohoops allow studying the effect of cyclic conjugation and bending on the incorporated π-systems. To date, no such system containing antiaromatic units has been reported. We herein present [12]cycloparaphenylenes incorporating two dibenzo[a,e]pentalene units: [2]DBP[12]CPP nanohoops. Dibenzo[a,e]pentalene is a nonalternant hydrocarbon with antiaromatic character. The syntheses and optoelectronic properties of two different [2]DBP[12]CPP nanohoops with electronically modifying substituents are reported, accompanied by TDDFT calculations.

Dibenzo[a,e]pentalenophanes: Bending a Non-Alternant Hydrocarbon.

In cyclophanes, an aromatic moiety is incorporated into a (strained) cyclic structure. Of particular interest as model systems for bent carbon nanostructures are those containing polycyclic aromatic hydrocarbons. Dibenzo[a,e]pentalene (DBP) is a non-alternant polycyclic hydrocarbon with small band gap and tunable optoelectronic properties. However, changing these properties by bending of the DBP structure has yet to be investigated. Herein, we report the synthesis, optoelectronic, and structural properties of (2,7)dibenzo[a,e]pentalenophanes with four different bridge sizes and bending angles of the DBP unit, accompanied by (TD)DFT calculations. The last, strain-inducing dehydration reaction was accomplished by using Burgess' reagent. The HOMO and LUMO levels and the magnetic shielding of protons pointing inside the cyclophane cavity grew stepwise with increasing ring strain. Single-crystal X-ray structures of the smallest three derivatives revealed a near semi-circle and a bend angle of the DBP unit of almost 88° for the smallest derivative. We demonstrated the synthetic versatility of our approach by varying the substituents at the DBP unit, allowing for further tuning of optoelectronic properties. The synthetic strategy presented herein may pave the way for the synthesis of conjugated DBP nanorings.