Synergetic Spin-Crossover and Luminescence in a Fe(II) Complex with Aggregation-Induced Emission and Twisted Intramolecular Charge Transfer

We report the precise design and
synthesis of a fluorescent
ligand
(E)-4-(((2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylene)amino)-N,N-diphenylaniline (bpp-TPA), achieved
through the covalent integration of a fluorophore triphenylamine (TPA)
with the coordination subunit 2,6-bis(pyrazol-1-yl)pyridine (bpp),
which provides a N6 octahedral coordination environment
optimized for Fe(II), allowing us to prepare a mononuclear complex
[Fe(bpp-TPA)2]·(CF3SO3)2, bpp-TPA-Fe. This complex exhibits a reversible
thermally induced spin-state switching with a T1/2 of 311 K. Meanwhile, the fluorescent intensity of bpp-TPA-Fe intensified distinctly upon spin-state conversion,
reaching its maximum emission at 360 K, clearly indicative of a synergistic
coupling between the SCO process and luminescent behavior. Benefiting
from its carefully engineered intramolecular motional dynamics and
donor–acceptor (D–A) molecular architecture, bpp-TPA-Fe simultaneously exhibits pronounced aggregation-induced emission
(AIE) and twisted intramolecular charge transfer (TICT) properties.
Furthermore, large bathochromic shifts in the emission spectra with
the increase in solution polarity are realized in this complex. This
work exemplifies a highly precise molecular design strategy to construct
multifunctional molecular materials with tunable magneto-optical properties,
opening avenues for next-generation smart material applications.