Functionalization of Boron Diiminates with Unique Optical Properties: Multicolor Tuning of Crystallization-Induced Emission and Introduction into the Main Chain of Conjugated Polymers

In this article,
we report the unique optical characteristics of
boron diiminates in the solid states. We synthesized the boron diiminates
exhibiting aggregation-induced emission (AIE). From the series of
optical measurements, it was revealed that the optical properties
in the solid state should be originated from the suppression of the
molecular motions of the boron diiminate units. The emission colors
were modulated by the substitution effects (λPL,crystal = 448–602 nm, λPL,amorphous = 478–645
nm). Strong phosphorescence was observed from some boron diiminates
deriving from the effects of two imine groups. Notably, we found some
of boron diiminates showed crystallization-induced emission (CIE)
properties derived from the packing differences from crystalline to
amorphous states. The 15-fold emission enhancement was observed by
the crystallization (ΦPL,crystal = 0.59, ΦPL,amorphous = 0.04). Next, we conjugated boron diiminates
with fluorene. The synthesized polymers showed good solubility in
the common solvents, film formability, and thermal stability. In addition,
because of the expansion of main-chain conjugation, the peak shifts
to longer wavelength regions were observed in the absorption/emission
spectra of the polymers comparing to those of the corresponding boron
diiminate monomers (λabs = 374–407 nm, λPL = 509–628 nm). Furthermore, the absorption and the
emission intensities were enhanced via the light-harvesting effect
by the conjugation with fluorene. Finally, we also demonstrated the
dynamic reversible alterations of the optical properties of the polymer
thin films by exposing to acidic or basic vapors.