The simultaneous modulation effect of N-substituents on the photochromic and electrochromic properties of naphthalenediimide-based coordination polymers

Abstract

The controllability of the properties of stimulus-responsive chromic materials (SRCM) is of great significance for their practical application. Two two-dimensional (2D) layered coordination polymers (CPs), [Mn(CMNDI)(H₂O)₂] (1) and [Mn(CENDI)(H₂O)₂] (2) (H₂CMNDI = N,N′-bis(carboxymethyl)-naphthalenediimide and H₂CENDI = N,N′-bis(carboxyethyl)-naphthalenediimide), have been synthesized by the combination of H₂CMNDI/H₂CENDI and Mn²⁺ ions. 1 and 2 exhibit entirely different photochromic behaviors, which are reflected in the photoresponsive rate (1 s for 1vs. 10 s for 2), coloration contrast (from yellow to dark brown for 1vs. from light yellow to light brown for 2) and photoresponsive range (from ultraviolet to visible light for 1vs. ultraviolet light for 2). The outstanding photochromic properties of 1 relative to those of 2 should be ascribed to the synergistic effect of dual electron transfer (ET) pathways (intermolecular π–π and intramolecular lone pair–π interactions). Furthermore, the excellent visible light-induced photochromic performance of 1 should be attributed to the extension of the excitonic absorption to the visible light region via charge transfer (CT) caused by strong π–π/lone pair–π interactions. Notably, the regulation of the photoresponsive range is achieved for the first time by simply changing the N-substituents on both sides of NDI cores. Meanwhile, 1 and 2 also exhibit obviously distinguishing electrochemical and electrochromic behaviors, which are mainly reflected in the reduction potential (−0.8 V for 1vs. −0.75 V for 2) and optical contrast ratio (∼6% for 1vs. ∼14% for 2). This work provides an effective strategy for the simultaneous modulation of photochromic and electrochromic behaviors by varying the N-substituents.