On the ambiguity of 1,3,2-benzodiazaboroles as donor/acceptor functionalities in luminescent molecules

Abstract

A series of 1,3-bis(perfluoroaryl)-2-(hetero)aryl-1,3,2-benzodiazaboroles, 1,3-^FAr₂-2-Ar-1,3,2-N₂BC₆H₄ (Ar = Ph, ^FAr = C₆F₅5; Ar = Ph, ^FAr = 4-C₅F₄N 6; Ar = Ph, ^FAr = 4-NCC₆F₄7; Ar = 2-C₄H₃S, ^FAr = C₆F₅8; Ar = 2-C₄H₃S, ^FAr = 4-C₅F₄N 9; Ar = 2-C₄H₃S, ^FAr = 4-NCC₆F₄10), were synthesised by cyclocondensation of the adducts PhBBr₂·PPh₃ or 2-thienylBBr₂·PPh₃ with N,N′-bis(perfluoroaryl)-o-phenylenediamines in the presence of 2,2,6,6-tetramethylpiperidine. Similar treatments of the PPh₃ adducts of 4-(1′,3′-diethyl-1′,3′,2′-benzodiazaborolyl)-phenyldibromoborane with the corresponding diamines gave rise to the push–pull compounds, C₆H₄(NEt)₂B-1,4-C₆H₄-B(N^FAr)₂C₆H₄ (^FAr = C₆F₅11; 4-C₅F₄N 12) and C₆H₄(NEt)₂B-2,5-C₄H₂S-B(N^FAr)₂C₆H₄ (^FAr = C₆F₅13; 4-C₅F₄N 14). The X-ray structures of 8, 11, 12 and 13 were determined. Electronic structure calculations reveal that the LUMOs are located at the perfluoroaryl groups in 5–14; thus the fluorinated benzodiazaborolyl groups are considered as electron-withdrawing moieties. These moieties differ from alkylated benzodiazaborolyl groups which are regarded as donors. The emission spectra for 5–14 show charge transfer bands with significant solvatochromism and large Stokes shifts (6100–12 500 cm⁻¹ in cyclohexane and 8900–15 900 cm⁻¹ in CH₂Cl₂). The emissions of the benzodiazaboroles, 5–10, arise from a different charge transfer (CT) process to the local charge transfer (LCT) process typically found in many fluorescent benzodiazaboroles. This novel remote charge transfer (RCT) process involving the perfluoroaryl groups is supported by CAM-B3LYP computations. The push–pull systems 11–14 here give fluorescent emissions with moderate to high fluorescence quantum yields (65–97%) that arise from the usual LCT process only.