Additive-Induced Supramolecular and Structural Isomerism in Zn(II) MOFs: Tuning Emission from Intraligand Local Excitation to Intermolecular Charge Transfer through π–Stacking Control

Abstract

Additive-directed assembly is a powerful yet underexplored route to supramolecular isomers with tunable metal–organic framework (MOF) photophysics. Three Zn(II) MOFs, [Zn(Hbtzip)2(H2O)2]·4H2O (1), [Zn(Hbtzip)2(H2O)2] (2), and [Zn(btzip)] (3), were hydrothermally synthesized from 4,6-bis(triazol-1-yl)isophthalic acid (H2btzip) using 4,5-dicyanoimidazole, aminoguanidine hydrochloride, and acrylamide as respective additive. 1 and 2 are three-dimensional (3D) supramolecular isomers with identical one-dimensional (1D) chains but different hydrogen-bonds and π–π distances; 3 is a structural isomer with a 3D crb framework. Despite common ligand-centered emission, they emit at 535 nm (1; 31 nm red shift), 426 nm (2; 78 nm blue shift), and 499 nm (3; nearly identical to free ligand). Through Hirshfeld analysis, time-dependent density functional theory (TDDFT), natural transition orbitals, and electron–hole distributions, we show that 1 emits via intraligand local excitation (LE), while 2 and 3 involve intermolecular ligand-to-ligand charge transfer (LLCT). The shorter π–π distance in 2 (3.657 Å) enhances through-space coupling and favors charge transfer; the longer separation in 1 (3.873 Å) weakens it, allowing LE to dominate. This work not only enriches the H2btzip-based MOF family but also establishes a clear structure–property framework linking supramolecular packing to luminescence, offering a versatile strategy for emission-tunable materials through controlled noncovalent interactions.