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 T _1/2 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.