Conjugated Naphthalenediimide-Pyridinium Salts in\nDonor–Acceptor Hybrid Materials for Enhanced Photochromic Properties

As\nan emerging class of hybrid materials, donor–acceptor\n(D–A) hybrid crystals with photoactive organic and inorganic\ncomponents have gradually become an ideal platform for photochromic\nmaterials. Wherein the most available organic components are electron-poor\nnaphthalenediimide, pyridinium, and triazine derivatives, inorganic\nunits are electron-rich polyoxometalates and metal halides. Herein,\nwe introduced pyridinium moieties into the naphthalenediimide core\nby conjugated bonds so as to increase the electron deficiency of organic\nspecies for enhanced photochromic properties. Four anion-π interaction\ndirected hybrid crystals, (Me<sub>2</sub>DPNDI)<sub>3</sub>·(PW<sub>12</sub>O<sub>40</sub>)<sub>2</sub> (<b>1</b>), (Et<sub>2</sub>DPNDI)<sub>3</sub>·(DMSO)<sub>2</sub>·(PMo<sub>12</sub>O<sub>40</sub>)<sub>2</sub> (<b>2</b>), (Et<sub>2</sub>DPNDI)<sub>3</sub>·(PW<sub>12</sub>O<sub>40</sub>)<sub>2</sub> (<b>3</b>), and (Et<sub>2</sub>DPNDI)<sub>2</sub>·(SiMo<sub>12</sub>O<sub>40</sub>) (<b>4</b>), have been obtained, which possess reversible,\nnaked-eye perceptible color changes with varying light response rates.\nThese rates are as follows: <b>3</b> > <b>1</b> > <b>4</b> > <b>2</b>. As expected, such D–A hybrid\ncrystals\nwith conjugated naphthalenediimide-pyridinium salts feature both photo-\nand thermally induced electron transfer pathways, along with enhanced\nanion-π interactions. Consequently, they demonstrate a higher\nrate of photochromism compared to those of hybrids containing naphthalenediimide\nor pyridinium derivatives as organic acceptors.