Ratiometric Tuning of Luminescence: Interplay between the Locally Excited and Interligand Charge-Transfer States in Pyrazolate-Based Boron Compounds

In an aim to study the potential application toward organic light-emitting diodes, a series of boron compounds NBN-1 to NBN-5 bearing 1-isoquinolinyl pyrazolate, phenyl, substituted aryl, or fused biphenyl appendages were designed and synthesized. Dual emissions specified as F₁ and F₂ bands were observed for NBN-1 and NBN-2 in various solvents. The F₁ emission features solvent independence and is assigned to the intraligand ππ^∗ transition (i.e., LE state) over the isoquinolinyl pyrazolate moiety, whereas the F₂ band shows significant solvatochromism, which originates from the interligand charge transfer (i.e., CT state) from isoquinolinyl to aryl appendages. In comparison, NBN-3 bearing ortho-methyl on the phenyl appendages (cf. para-methyl for NBN-2) shows only F₁ (LE) emission. In contrast, NBN-4 and NBN-5 with a fused biphenyl-like appendage reveal solely the F₂ (CT) emission. Comprehensive time-resolved fluorescence measurements, in combination with the computational approach, let us propose the occurrence of a through-space, photoinduced electron transfer (PET) from the LE to CT states. Depending on the characteristic of aryl appendages, the energetics between LE and CT states play a key role for the locally excited LE versus interligand CT emission. A pre-equilibrium-type PET for NBN-1 and NBN-2 and hence dual emissions are observed, whereas the energetically unfavorable PET for NBN-3 leads to the LE emission only. The highly exergonic PET for NBN-4 and NBN-5 renders solely the CT emission. This work thus demonstrates a strategy of facile appendage tuning of boron compounds that can afford both the LE and interligand CT emissions spanning over the entire visible spectral region.