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.