The Synthesis and Characterization of Highly Fluorescent Polycyclic Azaborine Chromophores

Abstract

Six new heteroaromatic polycyclic azaborine chromophores were designed, synthesized, and investigated as easily tunable high-luminescent organic materials. The impact of the nitrogen-boron-hydroxy (N-BOH) unit in the azaborines was investigated by comparison with their N-carbonyl analogs. Insertion of the N-B(OH)-C unit into heteroaromatic polycyclic compounds resulted in strong visible absorption and sharp fluorescence with efficient quantum yields. The solid-state fluorescence of the heteroaromatic polycyclic compounds displayed a large Stokes shift compared to being in solution. The large Stokes shifts observed offset the self-quench effect in the solid state.

The nature of multiple boron-nitrogen bonds studied using electron localization function (ELF), electron density (AIM), and natural bond orbital (NBO) methods

“…The last molecule in the series is the abitud molecule. Its structure has been described by Saint-Louis et al [8] who reported a formal single B-N bond as a part of the sixmember ring (see Scheme 1). The molecule contains an unusual N-BOH fragment [8] and intramolecular hydrogen bond, C=O ... H-O.…”

“…In the second part of the research, ten organoboron molecules, i.e., cojwaa [39], cofvuo [47], sictii [58], yecvor [40], axuviy [70], zeypuo [71], bpampb [48], iditas [72], abitud [8], and notlud [73] have been investigated as molecules potentially containing double B=N bonds. Optimized r opt (B,N) bond lengths are between 1.269 Å in cojwaa and 1.455 Å for one of the BN bonds in the ring of iditas.…”

“…In the world of fluorescent organic compounds, incorporation of the B-N bond within the polycyclic aromatic hydrocarbons is a method of rigidifying the compound core, yielding more intense fluorescence, extraordinary thermal and photochemical stability, and high fluorescence quantum yields. For example, Saint-Louis et al [8] when studying polycyclic azaborine chromophores, designed and synthetized the molecule, where the N-C(=O)-N unit has been replaced with the N-B(OH)-C unit. The obtained azaborine molecules displayed higher molar absorption coefficient than their carbonyl analogs and presented higher emission quantum yields when compared with the imide analogs.…”

Polycyclic Aromatic Heterocycles with a Benzo[c][1,2]azaborinine Core

“…The molecule has a crystallographic center of inversion, as does its close structural counterpart 1,5‐dihydroxyanthraquinone ( 8 ) . The most striking comparison of the X‐ray structure of 7a with those of the hydantoin product 1 and a 3‐pyrrolin‐2‐one adduct 9 is the B─O─H···O═C hydrogen bond distance, which at 1.824 Å in 7a is much shorter than that at 2.225 Å in 1 and 2.239 Å in 9 . This is because the C═O acceptor for H‐bonding to the B─OH group is in a six‐membered ring fused to the boron heterocycle in 7a , whereas it is in a five‐membered ring in 1 and 9 .…”

“…Another is the unanticipated production of the bicyclic boron heterocycle 2 from a Horner–Wadsworth–Emmons olefination of 2‐formyl‐4‐methoxyphenylboronic acid and N ‐Cbz protected phosphonoglycine trimethylester, despite running the reaction under Cu‐catalyzed conditions designed to favor indole ring formation . Finally, the condensation of 2‐formylphenylboronic acids with isoindolinones enabled ready access to a collection of fluorescent tetracyclic boron heterocycles like 3 . The goal of the work described herein was to explore and expand the scope of similar reactions of 2‐formylphenylboronic acids with active methylene reagents and, in particular, to take advantage of microwave‐accelerated conditions that could make the production of new boron heterocycles easy and rapid.…”