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

Three Zn(II) MOFs, [Zn(Hbtzip)₂(H₂O)₂]·4H₂O (<b>1</b>), [Zn(Hbtzip)₂(H₂O)₂] (<b>2</b>), and [Zn(btzip)] (<b>3</b>), were hydrothermally synthesized from 4,6-bis(triazol-1-yl)isophthalic acid (H₂btzip) using 4,5-dicyanoimidazole, aminoguanidine hydrochloride, and acrylamide as respective additive. <b>1</b> and <b>2</b> are three-dimensional (3D) supramolecular isomers with identical one-dimensional (1D) chains but different hydrogen-bonds and π–π distances; <b>3</b> is a structural isomer with a 3D <i><b>crb</b></i> framework.

Despite common ligand-centered emission, they emit at 535 nm (<b>1</b>; 31 nm red shift), 426 nm (<b>2</b>; 78 nm blue shift), and 499 nm (<b>3</b>; nearly identical to free ligand). Through Hirshfeld analysis, time-dependent density functional theory (TDDFT), natural transition orbitals, and electron–hole distributions, we show that <b>1</b> emits via intraligand local excitation (LE), while <b>2</b> and <b>3</b> involve intermolecular ligand-to-ligand charge transfer (LLCT).