Intramolecular N→B Coordination as a Stabilizing Scaffold for π-Conjugated Radical Anions with Tunable Redox Potentials

Abstract

In this work we report on a preparative strategy that allows the facile tuning of the electron affinities of π-conjugated organic n-type materials via intramolecular N→B coordination: 2fold hydroboration of a pyrazine-derived substrate with 9Hborabicyclo[3.3.1]nonane (9H-BBN) and Piers' borane ((C 6 F 5 ) 2 BH) furnishes the intramolecularly N→B-coordinated compounds PzBBN and PzBPF in high yield. For each borane racemic mixtures of the respective chiral derivatives and achiral meso compounds have been isolated, and their electrochemical, optical, and structural properties have been investigated and complemented by DFT calculations. The boranes exhibit high electron affinities with electrochemical LUMO energy levels of −3.69 and −4.30 eV, respectively. The corresponding radical anions PzBBN •− and PzBPF •− can be reversibly generated and are stable in solution at ambient temperature. Analysis of the radical species by EPR spectroscopy and single-crystal analysis of [PzBPF] •− [CoCp 2 ] + allowed further corroboration of the π-delocalized nature of these radicals.

Stable, π-conjugated radical anions of boron-nitrogen dihydroindeno[1,2-b]fluorenes

“…(Figure 1). The solidstate structure was found to incorporate a completely planar pentacyclic core with a B-N bond distance of 1.635(3) Å, closely comparable to the structure of a related derivative previously reported and slightly shorter than the 1.652(1)Å reported for the less conjugated material IV 37 (Chart 1). It was observed that the nature of the aryl substituents on boron had a significant effect on the electronic and photophysical properties of these frameworks.…”

“…34 Both the Otten 35 and Gilroy 36 groups have explored the chemistry of the radical anions of diaryl and dialkynyl borane complexes of formazanate ligands (III). Most germane to the work reported here, Pammer et al have reported radical anions of the partially saturated BN ladder heterocycles IV 37 in which the SOMO is mostly localized on the core pyrazine moiety of the framework. We have recently reported fully conjugated BN dihydroindeno [1,2-b]fluorenes and evaluated their performance as acceptors in OPVs.…”

“…It is well reported in the literature that modifying substituents on boron have significant influence on the optoelectronic properties. 22,37,43,44 To further examine the experimentally determined electronic properties, DFT (PBE0/Def-TZVP) calculated structures and their frontier orbitals are shown in Figure 3. The orbitals are more extensively delocalized across the planar core of the molecules in comparison to those in Pammer's related, but less conjugated materials IV.…”

Super electron donors derived from diboron

“…3c ). The formation of 9 indicated that the radical anion of pyridine could coordinate with a boron Lewis acid center, 12 and the similar EPR spectrum also implied the structural relevance of the unknown radical species to complex 9 .…”

N → B Ladder Polymers Prepared by Postfunctionalization: Tuning of Electron Affinity and Evaluation as Acceptors in All-Polymer Solar Cells

“…To demonstrate the scope of the postfunctionalization strategy, two substrate polymers (PPz and rr-P4Py) were prepared by polymerization of the respective monomers M1 and M2 (Scheme 1). M1 was prepared by a synthetic route analogous to the preparation of a non-halogenated model system (Scheme 1a; see the Supporting Information for details): 111 Starting from 2bromo-5-chlorobenzaldehyde, the olefinic side chain was introduced by Wittig coupling with the phosphonium salt 2. Selective metalation of the resulting 2-bromo-5-chlorostyrene 4 at the 2-position and 2-fold Negishi coupling to 2,5dibromopyrazine then gave M1 in 82% yield.…”