Synthesis, Structure and Photophysical Properties of a New Class of Inherently Chiral Boron(III) Chelates—The tert‐Leucine Complexes

Abstract

A new family of boron(III) chelates is introduced whereby molecular chirality, confirmed by circular dichroism, is imported during synthesis such that isolation of the diastereoisomers does not require separation procedures. The photophysical properties of two members of the family have been examined: the N,O,O‐salicylaldehyde‐based derivative shows pronounced intramolecular charge‐transfer character in fluid solution and is weakly fluorescent, with a large Stokes shift. The corresponding 2‐methylamino‐benzaldehyde‐derived N,N,O‐chelate absorbs and fluoresces in the visible region with a much smaller Stokes shift. Orange fluorescence is also observed for this compound as a cast film. Temperature‐dependence studies show that decay of the fluorescent state is weakly activated but emission is less than quantitative at 77 K. Quite rare for boron(III)‐based chelates, this derivative undergoes intersystem crossing to form a meta‐stable triplet‐excited state. X‐ray crystal structures are reported for both compounds, along with simulated ECD spectra.

Near‐Infrared Circularly Polarised Luminescence from Helically Extended Chiral N,N,O,O‐Boron Chelated Dipyrromethenes

“…To assign the correct configuration ECD spectral calculations were performed using two theoretical levels: B3LYP/6‐311++G(3df,2pd) and CAM‐B3LYP/6‐311++G(3df,2pd). As observed before, the CAM‐B3LYP functional overestimates the excitation energies by about 100 nm [14] . The ECD patterns calculated on both theoretical levels are, nevertheless, very similar (considering the mentioned excitation energy shift) and generally compare well with the experiments, apart from the vibrational signatures.…”

“…As observed before, the CAM-B3LYP functional overestimates the excitation energies by about 100 nm. [14] The ECD patterns calculated on both theoretical levels are, nevertheless, very similar (considering the mentioned excitation energy shift) and generally compare well with the experiments, apart from the vibrational signatures. The vibrational levels could, in principle, be included in the predictions as presented by Ponce-Vargas et al, [15] but are not required for the current chiral centre assignment.…”

“…As observed before, the CAM-B3LYP functional overestimates the excitation energies by about 100 nm. [14] The ECD patterns , [15] but are not required for the current chiral centre assignment.…”

Enantioenriched BoronC,N‐Chelates via Chirality Transfer

“…Moreover, Toyota, [25][26][27] Nowak-Król [28] and others [29][30][31][32] showed that resolution of racemic mixtures of boron complexes via chiral HPLC is possible and give rise to, for example, 3 [25] or 4 [28] in enantiopure form (Schema 1b). Synthetic concepts towards enantioenriched boron atoms involve diastereoselective syntheses [30,[33][34][35][36] as in case of NHC-borane 5 [8] (Scheme 1c) or intramolecular chirality transfer towards tridentate complex 6 [37] (Scheme 1d).…”

Two-photon excitable boron complex based on tridentate imidazo[1,5-a]pyridine ligand for heavy-atom-free mitochondria-targeted photodynamic therapy

“… 4 Various BODIPY analogues based on the dipyrromethene ligand have been developed to achieve the applications mentioned above, whereas the research on the development of novel luminescent boron complexes based on aromatic ligands has emerged to explore and improve the photofunctionalities. For example, some successful examples of rigid four-coordinated boron complexes based on tridentate ligands, which show unique and useful luminescent properties such as self-assembly, 5 aggregation-induced emission 6 and chiroptical properties, 7 have been reported.…”