Search

The electrochemical dinitrogen reduction represents an attractive approach of converting N 2 and water into ammonia, while the rational design of catalytic active centers remains challenging. Investigating model molecular catalysts with well-tuned catalytic sites should help to develop a clear structure-activity relationship for electrochemical N 2 reduction. Herein, we designed several polycyclic aromatic hydrocarbon (PAH) molecules with well-defined positions of boron and nitrogen atoms. Theoretical calculations revealed that the boron atoms possess high local positive charge densities as Lewis acid sites, which are beneficial for N 2 adsorption and activation, thus serving as major catalytic active sites for N 2 electrochemical reduction. Furthermore, the close vicinity of two boron atoms can further enhance the local positive density and subsequent catalytic activity. Using the PAH molecule with two boron atoms separated by two carbon atoms (B-2C-B), a high NH 3 production rate of 34.58 μg·h−1·cm−2 and a corresponding Faradaic efficiency (5.86%) were achieved at −0.7 V versus reversible hydrogen electrode, substantially exceeding the other PAHs with single boron or nitrogen-containing molecular structures. [ABSTRACT FROM AUTHOR]

BN‐embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X‐ray crystallography, optical spectroscopy, and cyclic voltammetry. The B−N bonds show delocalized double‐bond characteristics and the conjugation can be extended through the trans‐orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN‐embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN‐embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge‐transfer interactions. [ABSTRACT FROM AUTHOR]

BN‐embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X‐ray crystallography, optical spectroscopy, and cyclic voltammetry. The B−N bonds show delocalized double‐bond characteristics and the conjugation can be extended through the trans‐orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN‐embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN‐embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge‐transfer interactions. [ABSTRACT FROM AUTHOR]

A series of polycyclic aromatic hydrocarbons (PAHs), consisting of two pairs of BN units, have been designed and their synthesis has been achieved by electrophilic C−H borylation. Two conjugation extension directions can be found in these B2N2‐embedded PAHs. The B2N2‐containing backbone with shorter effective conjugation length is isoelectronic with diaryl‐fused anthracene, whereas the second derivative, with longer effective conjugation length, is isoelectronic with bis(trans‐arylvinyl)benzene. By incorporating different aryl groups, i.e. furyl, thienyl, benzo[b]furyl, and benzo[b]thienyl groups, into the two crossed directions of the B2N2‐embedded PAHs, their electronic and optical properties have been comparatively investigated by photophysical, electrochemical, and theoretical approaches. It is found that both the substituents and their conjugation extension directions have significant effects on the aromatic and photophysical properties of the B2N2‐embedded PAHs. The conjugation extension in the shorter backbone is more pronounced on the effective conjugation length than the longer backbone. Moreover, all the B2N2‐embedded PAHs behave as both Lewis acids and Lewis bases, and reversible photoluminescence switching can be observed by simply neutralizing the added Lewis acid or Lewis base. [ABSTRACT FROM AUTHOR]