The Modulation Effect of Positional Isomeric Electron Donors on Bidirectional Electron Transfer of Photochromic Cd(II) Coordination Polymers

The design and synthesis of photochromic materials with tunable bidirectional electron transfer (ET) are highly desired for practical applications but remain a great challenge. In this work, three Cd(II) coordination polymers (CPs), {[Cd(pa)(3-abpt)(H₂O)]·3H₂O}_<i>n</i> (<b>1</b>), {[Cd(ip)(3-abpt)(H₂O)]₂·3H₂O}_<i>n</i> (<b>2</b>), and [Cd(tp)(3-abpt)(H₂O)]_<i>n</i> (<b>3</b>), have been synthesized by coordinating positional isomeric benzene dicarbonic acids (H₂pa = phthalic acid, H₂ip = isophthalic acid, and H₂tp = terephthalic acid) and 4-amino-3,5-bis(3-pyridyl)-1,2,4-triazole (3-abpt) with Cd²⁺ ions. All three CPs exhibit reversible ET photochromic behaviors with high photosensitivity. Notably, both forward and reverse ET rates gradually decrease in the order of <b>1</b> > <b>2</b> > <b>3</b>, which could be attributed to the modulating effect of positional isomeric electron donors on the molecular conformation of electron acceptors, the donor-acceptor interfacial contact, and the stability of photogenerated radicals. Interestingly, the reverse ET of <b>1</b> can be realized by linear visible light, indicating that <b>1</b> is a new type of photochromic material with linear-light-controlled bidirectional ET. Furthermore, the photochromic behavior of <b>1</b> is also regulated by the loss of lattice water. Our work shows that the introduction of positional isomeric ligands is an effective molecular design strategy to modulate the bidirectional ET and concomitant photochromic behaviors.