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