Evan R. Darzi

Selective Syntheses of [7]–[12]Cycloparaphenylenes Using Orthogonal Suzuki–Miyaura Cross-Coupling Reactions

The divergent, selective syntheses of [7]-[12]cycloparaphenylenes have been accomplished utilizing sequential, orthogonal Suzuki-Miyaura cross-coupling reactions from two late-stage intermediates. Quantum yields decrease dramatically as cycloparaphenylene size decreases, highlighting the unique photophysical behavior of the smaller cycloparaphenylenes.

Synthesis, Properties, and Design Principles of Donor-Acceptor Nanohoops.

We have synthesized a series of aza[8]cycloparaphenylenes containing one, two, and three nitrogens to probe the impact of nitrogen doping on optoelectronic properties and solid state packing. Alkylation of these azananohoops afforded the first donor–acceptor nanohoops where the phenylene backbone acts as the donor and the pyridinium units act as the acceptor. The impact on the optoelectronic properties was then studied experimentally and computationally to provide new insight into the effect of functionalization on nanohoops properties.

The Effects of Cyclic Conjugation and Bending on the Optoelectronic Properties of Paraphenylenes

Cycloparaphenylenes (CPPs) have optoelectronic properties that are unique when compared to their acyclic oligoparaphenylene counterparts. The synthesis and characterization of two bent heptaphenyl-containing macrocycles has been achieved in order to probe the effects of bending and cyclic conjugation on the properties of the CPPs. The study suggests that both bending and cyclic conjugation play a role in the novel properties of the CPPs.

Carbon Nanohoops: Excited Singlet and Triplet Behavior of Aza[8]CPP and 1,15-Diaza[8]CPP.

The excited state properties of two nitrogen-doped cycloparaphenylene molecules, or carbon nanohoops, have been studied using steady-state and time-resolved absorption and emission spectroscopies. Quantum yield of fluorescence (Φf = 0.11 and 0.13) and intersystem crossing (Φisc = 0.45 and 0.32) were determined for aza[8]CPP and 1,15-diaza[8]CPP, respectively. We also present the proton transfer reaction between trifluoroacetic acid and the nitrogen-doped nanohoops, which resulted in significant modifications to the steady-state absorption and emission spectra as well as the triplet-triplet absorption spectra. From fluorescence quenching data we determine the equilibrium constant for the proton transfer reaction between aza[8]CPP (Keq = 1.39 × 10(-3)) and 1,15-diaza[8]CPP (Keq = 2.79 × 10(-3)) confirming that 1,15-diaza[8]CPP is twice as likely to be protonated at a particular concentration of trifluoroacetic acid.

An Operationally Simple and Mild Oxidative Homocoupling of Aryl Boronic Esters To Access Conformationally Constrained Macrocycles

Constrained macrocyclic scaffolds are recognized as challenging synthetic motifs with few general macrocyclization methods capable of accessing these types of systems. Although palladium catalyzed oxidative homocoupling of aryl boronic acids and esters to biphenyls has been recognized as a common byproduct in Suzuki-Miyaura cross-couplings for decades, this reactivity has not been leveraged for the synthesis of challenging molecules. Here we report an oxidative boronic ester homocoupling reaction as a mild method for the synthesis of strained and conformationally restricted macrocycles. Higher yields and better efficiencies are observed for intramolecular diboronic ester homocouplings when directly compared to the analogous intramolecular Suzuki-Miyaura cross-couplings or reductive Yamamoto homocouplings. Substrates included strained polyphenylene macrocycles, strained cycloalkynes, and a key macrocyclic intermediate toward the synthesis of acerogenin A. Notably, this oxidative homocoupling reaction is performed at room temperature, open to atmosphere, and without the need to rigorously exclude water, thus representing an operationally simple alternative to traditional cross-coupling macrocyclizations. The mechanism of the reaction was investigated indicating that 1-5 nm palladium nanoparticles may serve as the active catalyst.

High pressure chemistry and the mechanochemical polymerization of [5]-cyclo-p-phenylene

Evidence for the surprising formation of polymeric phases under high pressure for conjugated nanohoop molecules was found. This paper represents one of the unique cases, in which the molecular-level effects of pressure in crystalline organic solids is addressed, and provides a general approach based on vibrational Raman spectroscopy combining experiments and computations. In particular, we studied the structural and supramolecular chemistry of the cyclic conjugated nanohoop molecule [5]cyclo-para-phenylene ([5]CPP) under high pressures up to 10 GPa experimentally and up to 20 GPa computationally. The theoretical modeling for periodic crystals predicts good agreements with the experimentally obtained Raman spectra in the molecular phase. In addition, we have discovered two stable polymeric phases that arise in the simulation. The critical pressures in the simulation are too high, but the formation of polymeric phases at high pressures provides a natural explanation for the observed irreversibility of the Raman spectra upon pressure release between 6 and 7 GPa. The geometric parameters show a deformation toward quinonoid structures at high pressures accompanied by other deformations of the [5]CPP nanohoops. The quinonoidization of the benzene rings is linked to the systematic change of the bond length alternation as a function of the pressure, providing a qualitative interpretation of the observed spectral shifts of the molecular phase.

Correction to Carbon Nanohoops: Excited Singlet and Triplet Behavior of [9]- and [12]-Cycloparaphenylene

In the abstract and main text of this manuscript, we inadvertently entered wrong estimates of the rate constants of energy transfer (ket) to biphenyl as well as the rate constants of triplet state quenching by molecular oxygen. The correct ket values for reaction 5 are 1.9 × 10 M−1 s−1 and 1.7 × 10 M−1 s−1 for [9]CPP and [12]CPP, respectively. The corrected rates of triplet state quenching by oxygen are 1.7 × 10 M−1 s−1 and 1.8 × 10 M−1 s−1 for [9]CPP and [12]CPP, respectively. Figure 6 is revised to show these corrected values.