Design and Syntheses of Electron-Transfer Photochromic Metal–Organic Complexes Using Nonphotochromic Ligands: A Model Compound and the Roles of Its Ligands

Regulating the Topologies and Photoresponsive Properties of Lanthanum-Organic Frameworks

Metal-organic frameworks (MOFs) show potential application in many domains, in which photochromic MOFs (PMOFs) have received enormous attention. Researchers mainly utilize photoactive ligands to build PMOFs. Recently, the mixed electron donating and accepting ligands strategies have also been used to construct PMOFs driven by the electron transfer between nonphotochromic moieties. However, the potential interligand competition inhibits the formation of PMOFs. Therefore, the exploration of single-ligand-guided assembly is conductive for building PMOFs. Considering the existing electron accepting and donating role of pyridyl and carboxyl, the pyridinecarboxyate derived from the fusion of pyridyl and carboxyl units may serve as single ligand to yield PMOFs (Figure 1d). In this work, the coordination assembly of bipyridinedicarboxylate (2,2'-bipyridine-4,4'-dicarboxylic acid, H2bpdc; 1,10-phenanthroline-2,9-dicarboxylic acid, H2pda) and LaCl3 generate two PMOFs, [La(bpdc)(H2O)Cl] (1) and [La(pda)(H2O)2Cl]⋅2H2O (2). Both complexes feature dinuclear lanthanum as building blocks with differences in the connecting number of likers, in which 1 has (4,8)-connected topology and 2 exhibits sql topology. Their structural differences result in the diversities of photoresponsive functionalities. Compared with reported PMOFs built from photoactive ligands and mixed ligands, this study provides new available categories of single ligand for generating PMOFs and tuning the structure and photoresponsive properties via ligand substitution and external photostimulus.

Photochromism and single-component white light emission from a metalloviologen complex based on 1,5-naphthyridine

Metalloviologens, as emerging electron-transfer photochromic compounds, have shown intriguing properties such as radiochromism, photochromism and photoconductance. However, only a limited number of them have been reported so far. Exploration of new metalloviologens is strongly desired. Herein, we report a new solvothermally synthesized metalloviologen complex [CdCl2(ND)2]n (1, ND = 1,5-naphthalenes) that exhibits photochromic and intrinsic white light emission properties. Density functional theory calculation results reveal that the photochromism could be assigned to photoinduced electron transfer from chlorine atoms to ND molecules. The photoinduced charge-separated states are heat/air stable, attributed to the delocalization of ND and strong intermolecular π-π interactions. Besides, complex 1 consistently emits intrinsic white light when excited with 340-370 nm UV light, achieving high color rendering index (CRI) values (82.54-94.04). By adjusting the excitation wavelength, both "warm" and "cold" white light emission can be produced, making it suitable for the application of a white light emitting diode (WLED). Thus, this work demonstrates that the ND-based metalloviologen is not only helpful in producing photochromism, but also beneficial for creating white-light emission.

Modulating the structure and photochromic performance of hybrid metal chlorides with nonphotochromic 1,10-phenanthroline and its derivative

Hybrid photochromic materias (HPMs), especially crystalline HPMs (CHPMs), have been widely investigated due to their feasibility in maintaining the advantages of each constituent and genearating captivating photomodulated functionality. Metal-organic complexes (MOCs), as promising candidates for fabricating CHPMs, have attracted the interest of researchers. The molecular predesign of ligands plays a crucial role in yielding MOC-based CHPMs with tunable photochromic functionality. Hitherto, a great majority of CHPMs are driven by photosensitive ligands. However, the complicated synthesis and high cost of photosensitive ligands obviously prevent the macro-synthesis and future application of these CHPMs. Thus, it is indispensable to explore novel branches of CHPMs. Herein, we report a series of photochromic solid materials bearing modulated photochromic properties by hybridizing metal chlorides with a nonphotosensitive coplanar dipyridine unit 1,10-phenanthroline (phen) and its derivative 5-chloro-1,10-phenanthroline (5-Cl-phen). The resulting hybrids, [ZnCl₂(phen)] (1), [CdCl₂(phen)] (2), [PbCl₂(phen)] (3), [ZnCl(H₂O)(5-Cl-phen)₂]Cl·2H₂O (4), [Cd₂Cl₄(5-Cl-phen)₂] (5) and [Pb₂Cl₄(5-Cl-phen)₂] (6), exhibit distinct structures from the isolated molecular complexes (1 and 4) to the hybrid chain (2, 3, 5 and 6) because of the distinct coordination mode of central metal ions and chloride ions. After photo-irradiation with a Xe-lamp, all complexes, as expected, exhibited apparent color change because of the photoinduced electron transfer (ET) between coordinated chloride ions (Cl^-) as electron donors (EDs) and the coordinated coplanar phen and 5-Cl-phen species as electron acceptors (EAs). More importantly, the photochromic performance of the title complexes could be modulated by phen and 5-Cl-phen. This study provides a general and facile way for modulating the structure and photochromic performance of hybrid metal chlorides with phen or phen-based derivatives under the synergy of crystalline engineering strategy and ET mechanism.

Broadband Photoresponsive Bismuth Halide Hybrid Semiconductors Built with π-Stacked Photoactive Polycyclic Viologen

Photoresponse ranges of commercially prevailing photoelectric semiconductors, typically Si and InGaAs, are far from fully covering the whole solar spectrum (∼295-2500 nm), resulting in insufficient solar energy conversion or narrow wave bands for photoelectric detection. Recent studies have shown that infinite π-aggregation of viologen radicals can provide semiconductors with a photoelectric response range covering the solar spectrum. However, controlled assembly of an infinite π-aggregate is still a great challenge in material design. Through directional self-assembly of electron-transfer photoactive polycyclic ligands, two crystalline inorganic-organic hybrid photochromic viologen-based bismuth halide semiconductors, ((Me)₃pytpy)[BiCl₆]·2H₂O [1; (Me)₃pytpy = N,N',N″-trimethyl-2,4,6-tris(4-pyridyl)pyridine] and ((Me)₃pytpy)[Bi₂Cl₉]·H₂O (2), have been synthesized. They represent the first series of pytpy-based photochromic compounds. After photoinduced coloration, the conductivities of both 1 and 2 increased. The radical products have electron absorption bands in the range of 200-1600 nm, exceeding that of Si. Both the conductivity and the photocurrent intensity of 2 are stronger than those of 1, due to better planarity, tighter π-stacking, and higher degrees of overlap of ((Me)₃pytpy)³⁺ cations. This study not only provides a new design idea for synthesizing radical-based multispectral photoelectric semiconductors but also enriches the family of electron-transfer photochromic compounds.

Concomitant Photoresponsive Chiroptics and Magnetism in Metal-Organic Frameworks at Room Temperature

Stimulus-responsive metal-organic frameworks (MOFs) can be used for designing smart materials. Herein, we report a family of rationally designed MOFs which exhibit photoresponsive chiroptical and magnetic properties at room temperature. In this design, two specific nonphotochromic ligands are selected to construct enantiomeric MOFs, {Cu₂(L-mal)₂(bpy)₂(H₂O)·3H₂O}_n (1) and {Cu₂(D-mal)₂(bpy)₂(H₂O)·3H₂O}_n (2) (mal = malate, bpy = 4, 4′ − bipyridine), which can alter their color, magnetism, and chiroptics concurrently in response to light. Upon UV or visible light irradiation, long-lived bpy⁻ radicals are generated via photoinduced electron transfer (PET) from oxygen atoms of carboxylates and hydroxyl of malates to bpy ligands, giving rise to a 23.7% increase of magnetic susceptibility at room temperature. The participation of the chromophores (-OH and -COO⁻) bound with the chiral carbon during the electron transfer process results in a small dipolar transition; thus, the Cotton effects of the enantiomers are weakened along with a photoinduced color change. This work demonstrates that the simultaneous responses of chirality, optics, and magnetism can be achieved in a single compound at room temperature and may open up a new pathway for designing chiral stimuli-responsive materials.

Controlled Photoinduced Generation of "Visual" Partially and Fully Charge Separated States in Viologen Analogues

Charge-separated states with a lifetime scale of seconds or longer not only favor studies using various steady-state analysis techniques but are important for light-energy conversion and other applications. Through a steric-hindrance-induced method, unprecedented photoinduced generation of a partially charge separated (PCS) state with a lifetime of days has been detected in the "visual" mode during the decay of excited states to a commonly observed fully charge separated (FCS) state for viologen analogues. One pale yellow 4,4'-bipyridine-based metalloviologen compound, with an interannular dihedral angle of 1.84° in 4,4'-bipyridine, directly decays to the purple FCS state after photoexcitation. The other pale yellow compound, with a similar coordination framework but a larger interannular dihedral angle (33.74°), changes first to a yellow PCS state and then relaxes slowly (in the dark in Ar, ca. 2 days; 70 °C in Ar, ca. 1 h) to the purple FCS state. The two-step coloration phenomenon is unprecedented for viologen compounds and their analogues and also rather rare for other photochromic species. EPR and Raman data reveal that photoinduced charge separation first generates univalent zinc and radicals and then the received electron in Zn(I) slowly distributes further to 4,4'-bipyridine. Reduction of π-conjugation and a direct to indirect change in band gap account for the prolongation of the relaxation process and the capture of the PCS state. These findings help to understand and control decay processes of excited states and provide a potential design strategy for multicolor photochromism, light-energy conversion with high efficiency, or other applications.

Decorating Metal Nitrate with a Coplanar Bipyridine Moiety: A Simple and General Method for Fabricating Photochromic Complexes

As a significant class of photochromic materials, crystalline hybrid photochromic materials (CHPMs) have attracted widespread attention of researchers because of their possibilities for generating other photoresponsive properties and advantages in understanding the underlying relationship between structure and photoresponsive performance. The predesign of suitable ligands plays a major role in generating desirable CHPMs. Hitherto, most CHPMs have been built from photodeformable or photoresponsive tectons. However, the synthesis of these ligands is usually time-consuming and expensive, and this greatly restricts their large-scale preparation and practical application. Therefore, it is necessary to explore new families of CHPMs besides the existing CHPMs. Herein, a simple and general method for constructing CHPMs by decorating metal nitrate with a coplanar bipyridine moiety, namely 1,10-phenanthroline (phen), is reported. The resulting products exhibit photocoloration in response to Xe-lamp irradiation. The electron transfer (ET) from the coplanar NO₃ ^- species (as π-electron donors, π-EDs) to coplanar phen moieties (as π-electron acceptors, π-EAs) is responsible for the resulting photochromism. The influence of the coordination environment and central metal ion on the photochromism was also studied. This work demonstrates that the introduction of coplanar organic tectons as π-EAs to metal nitrates as π-EDs with the collaboration of ET and coordination-assembly strategies is a simple and general method to manufacture CHPMs.

Electron transfer photochromism of solid-state supramolecules constructed by cucurbit[n]uril (n = 5–8) and 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride

Solid-state supramolecular complexes of a bispyridinium ethylene derivative with cucurbit[n = 5–8]uril exhibit fast responsive photochromic properties, which are caused by the electron transfer from the host to the guest.

Metal-dependent photochromic performance in two isostructural supramolecular chains

The introduction of nonphotochromic coplanar 1,10-phenanthroline into a metal carboxylate system produces two isostructural supramolecular chains with metal-dependent photochromic performance.

Photochromism and photoresponsive luminescence in naphthalenediimide coordination polymers with high thermostability

Two novel photochromic complexes [Zn₂(bpdc)₂(m-DPNDI)₂]·H₂O (1) and [Cd(bpdc)(m-DPNDI)]·H₂O (2) (H₂bpdc = 4,4′-diphenic acid, m-DPNDI = N,N′-bis(3-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide) were prepared through a solvothermal method.

Effects of Different Counter Anions on Solid-State Electron Transfer in Viologen Compounds: Modulation of Color and Piezo- and Photochromic Properties

Charge transfer (CT) and electron transfer (ET) underlie the various applications of viologen compounds. However, the CT and ET in solids are still insufficiently understood and difficult to control, which is challenging but fundamental to the design of multifunctional materials. Our strategy is to attempt a delicate control of CT and ET by subjecting model compounds to pressure. In this work, a series of viologens of the same cation with different anions show anion-dependent color and piezo-/photochromism due to ground-state CT and stimuli-induced ET; the ease of solid-state ET can be tuned in the order of Cl^- > Br^- > I^-/BF₄^-/PF₆^-/ClO₄^-. With in situ high pressure UV-vis experiments, we also observed unexpectedly the pressure-induced solid-state ET in [H₂bcpV]·I₂ besides the pressure modulation of the CT absorption. It proves pressure is a more powerful stimulus than light in tuning solid-state CT and ET.

Tri(pyridinyl)pyridine Viologen-Based Kagome Dual Coordination Polymer with Selective Chromic Response to Soft X-ray and Volatile Organic Amines

The development of new responsive smart materials has been highly desirable in the recent decade due to growing demand in our daily life, and extended viologen-based coordination polymers are regarded as proper and promising candidates for stimuli-responsive study. A tri(pyridinyl)pyridine viologen-based Kagome dual (kgd) topological coordination polymer, [Mn₃Cl₄(tpptb)₂]·Cl₂·(H₂O)₂, (tpptb = N,N',N″-tri(3-carboxybenzyl)-2,4,6-tri(pyridinium-4-yl)pyridine; 1) has been solvothermally synthesized, which can selectively respond to soft X-ray Al-Kα (λ = 8.357 Å) irradiation but not to UV light and hard X-rays of λ < 1.5418 Å at room temperature. Appealingly, 1 is very sensitive and convenient for the visual detection of various volatile amine vapors, especially ethylamine vapors at a low concentration of 100 ppm, and the vapochromic sample can be recovered after exposure in the air at room temperature. The sequence of amines in vapochromism could be rationalized by combined consideration of vapor pressure, the molecular size, and electron-donor ability of various amine molecules as well as the void spaces of 1. In addition, 1 exhibits an obvious hydrochromic transformation upon heating in the air and an anhydrous atmosphere. Combined XPS and EPR confirmed that these physical and chemical stimuli can cause electron transfer from electron-rich groups to quaternary nitrogen atoms of the ligand to generate charge-separated radicals, leading to soft X-ray-induced photochromic and selective vapochromic behavior of 1. Such behavior indicates that it will become a convenient, recyclable, and practical multifunctional material for chemical and environmental sensing. These results provide an effective avenue for the rational design and synthesis of multifunctional chromic materials for potential use in sensing devices.

Photochromism of metal-organic frameworks based on carbazole-dicarboxylic acid and bipyridine: sensing adjustment by controlling strut-to-strut energy transfer

In this paper, two energy-transfer photochromic metal-organic frameworks (MOFs) {[Zn(L)_0.5(bpy)]·H₂O·DMF}_n (1) and {[Zn(L)_0.5(bpe)]·2H₂O·DMF}_n (2) (H₄L = 9,9'-(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-dicarboxylic acid), bpy = 4,4'-bipyridine, bpe = 4,4'-vinylenedipyridine) were designed and synthesized. Both 1 and 2 showed similar pillared-paddle wheel type frameworks with bpy and bpe as the chromophore, respectively, and L^4- as the antenna-type light harvester, yielding strut-to-strut energy transfer (antenna behavior) within the well-ordered structures. Among them, 1 displayed excellent energy-transfer photochromic behavior under UV light accompanied by color transformation from colorless to purple. In addition, the photochromic behavior of 1 has obvious, fast, controllable and reversible characteristics. On the other hand, 2 showed a different energy-transfer photochromic behavior in the aspects of color changing, gamut, and sensitivity. The variation has been ascribed to the substitution of chromophore bpy in 1 with bpe in 2, which influences the efficiency of energy transfer within the MOFs. Therefore, with the structural diversity and tunability of MOFs, the sensitivity, color, and gamut of energy-transfer of the photochromic MOFs can be tuned by the appropriate choice of the constitutions of MOFs. This work will provide useful guidance for developing novel energy-transfer photochromic MOF materials.

Manipulating On/Off Single-Molecule Magnet Behavior in a Dy(III)-Based Photochromic Complex

Exploitation of room temperature (RT) photochromism and photomagnetism to induce single-molecule magnet (SMM) behavior has potential applications toward optical switches and magnetic memories, and remains a tremendous challenge in the development of new bulk magnets. Herein, a series of chain complexes [Ln₃(H-HEDP)₃(H₂-HEDP)₃]·2H₃-TPT·H₄-HEDP·10H₂O (QDU-1; Ln = Dy (QDU-1(Dy)), Gd (QDU-1(Gd)), and Y (QDU-1(Y)); HEDP = hydroxyethylidene diphosphonate; TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine) were synthesized by solvothermal reactions. All the compounds exhibited reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism. More importantly, the PET process induced significant variations in magnetic interactions for the Dy(III) congener. Strong ferromagnetic coupling with remarkably slow magnetic relaxation without applied dc fields was observed between Dy^III ions and photogenerated O^• radicals, showing SMM behavior after RT illumination. For the first time, we observed the reversible RT photochromism and photomagnetism in the lanthanide-based materials. This work realized the radicals-actuated on/off SMM behavior via RT light irradiation, providing a new strategy for constructing the light-induced SMMs.

A viologen-based multifunctional Eu-MOF: photo/electro-modulated chromism and luminescence

A novel viologen-based multifunctional Eu-MOF integrating photochromism, photomodulated fluorescence, and electrochromic and electroluminochromic properties was investigated.

Photochromic inorganic–organic complex derived from low-cost deep eutectic solvents with tunable photocurrent responses and photocatalytic properties

A photochromic inorganic–organic complex |C₁₀H₁₀N₂|[GaF(C₂O₄)₂] derived from low-cost deep-eutectic solvents possesses tunable photocurrent responses and photocatalytic activities.

Coordinate bond- and hydrogen bond-assisted electron transfer strategy towards the generation of photochromic metal phosphites

The introduction of conjugated dipyridine-derivative units into metal phosphite system produces two hybrid zincophosphites driven by the coordinate bond- and hydrogen bond-assisted electron transfer.

Selective chemochromic and chemically-induced photochromic response of a metal–organic framework

The MOF provides unique confined space furnished with electron acceptor sites, and exposure to amines/alcohols causes specific and size-selective direct/UV-assisted color change owing to spontaneous/photoinduced electron transfer.

Photochromic Lanthanide(III) Materials with Ion Sensing Based on Pyridinium Tetrazolate Zwitterion

Lanthanide(III) ion (Ln(III)) sensing has become a major research area owing to its intriguing prospect in clinic, biology, and environmental studies. However, the existing methods have limitations like requirement of expensive instrumentation, long analytical times, and sample pretreatments, revealing the necessity of other methods. In this work, by using N-methyl-4-pyridinium tetrazolate (mptz) zwitterion as an electron acceptor, we obtained several new Ln(III) compounds with electron-transfer (ET) photochromic properties: [Ln(NO₃)₃(H₂O)₄]·mptz [Ln = Sm (1), Eu (2), Gd (3), La (4), Ce (5), Pr (6), Nd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14)]. Notably, different Ln(III) ions in these compounds can be visually identified by their different photoinduced color changes related to the ET process. This work may not only contribute to the more understanding of the structure-photosensitivity relationships of pyridinium-based compounds, but also provide a new approach for Ln(III) ion sensing.

Bifunctional photo- and vapochromic behaviors of a novel porous zwitterionic metal–organic framework

A novel porous zwitterionic metal–organic framework shows photo- and vapochromic behaviors with high sensitivity for sensing NH₃, ethylamine, and n-propylamine.

Three viologen-derived Zn-organic materials: photochromism, photomodulated fluorescence, and inkless and erasable prints

Three zinc–viologen materials exhibit reversible photochromism, photomodulated fluorescence and the capability as inkless and erasable print media.

Photochromism and photomagnetism in crystalline hybrid materials actuated by nonphotochromic units

Using the pillar-layer strategy, two isomorphic crystalline coordination compounds with obvious photochromic and photomagnetic behaviors actuated by nonphotochromic ligands were synthesized.

A novel photochromic hybrid containing trinuclear [Cd3Cl12]6− clusters and protonated tripyridyl-triazines

A novel organic–inorganic hybrid [H₃TPT]₂[Cd₃Cl₁₂] exhibits excellent photochromic performance with ultrafast photoresponsive rate, obvious coloration contrast and high thermal stability.

Proton coupled electron transfer: novel photochromic performance in a host–guest collaborative MOF

Proton coupled electron transfer has been successfully introduced to a host–guest collaborative MOF material, which exhibits novel photochromic properties with reversible, controllable and efficient characteristics.

A series of host–guest coordination polymers containing viologens: syntheses, crystal structures, thermo/photochromism and factors influencing their thermo/photochromic behaviors

Some viologen-containing compounds displaying thermo/photochromic behaviors which can be used as inkless print media and anti-counterfeiting materials have been reported.

Differentiable Detection of Volatile Amines with a Viologen-Derived Metal-Organic Material

The development of selective sensing materials for amine detection has received considerable attentions because amines have high toxicity and exist widely. In this article, we demonstrate for the first time that a degree of discriminative detection of alkylamines can be achieved by a metal-organic coordination material. The material is derived from Cd^II and 4,4'-bipyridinium-1,1'-bis(phenylene-3-carboxylate), shows 1D channels lined with electron-deficient viologen chromophores, and exhibits different colors upon contact with amine vapors of different molecular sizes and types (primary, secondary, and tertiary). The vapochromism is attributable to electron transfer from the amine group to viologen. The discrimination between amines is because the analyte-receptor interactions, which either directly mediate or indirectly affect electron transfer, are influenced by the number of the N-H bonds in the amine molecule, the size of the amine molecule relative to the receptor channel and the steric hindrance for the electron donor-acceptor contacts. The material also shows reversible photo- and hydrochromism owing to stimuli-induced reversible electron transfer. The compound can be deposited in paper simply by spraying the mixture solution of the starting metal salt and the ligand. The paper can be used as portable test strips for visual and differentiable detection of amines and as erasable inkless printing medium.

Photoinduced electron transfer and remarkable enhancement of magnetic susceptibility in bridging pyrazine complexes

Three bridging pyrazine complexes that exhibit both photochromism and photomagnetism were prepared. Photoinduced electron transfer can be realized by constructing a donor–metal–accepter system.

Structure of a Cd(ii) mixed-ligand coordination polymer: single crystalline conductance switch involving photoinduced electron transfer and photocoloration

A novel Cd(ii)-MOF with tunable photophysical behavior has been developed. The control of color, fluorescence emission and conductivity is achieved in a single crystalline conductance switch by incorporating donor and acceptor building blocks into a molecular wire.

Energy-dependent photochromism at room temperature for visually detecting and distinguishing X-rays

A radiochromic material with energy-dependent X-ray-induced photochromism was obtained by incorporating a radiosensitive ligand into an electron-transfer photochromic metalloviologen system.

Impact of Lattice Water on Solid-State Electron Transfer in Viologen Pseudopolymorphs: Modulation of Photo- and Piezochromism

Stimuli-induced solid-state electron transfer (ET) underlies the use of viologen compounds as responsive materials, but unequivocal structure-property correlations for solid-state ET are still lacking. With different pseudopolymorphic solids derived from N,N'-bis(4-carboxylphenyl)viologen ([H₂bcpV]²⁺), here we report a systematic study on photo- and piezochromic properties associated with ET. We show that the higher the water content in the lattice, the less sensitively the compounds respond to light and pressure. It is proposed that the lattice water does not act as an electron donor but serves to change the ET energetics through its unique polarity and hydrogen bonding capability. The impedimental impact of water on solid-state ET of viologen compounds has not yet been recognized and elucidated prior to this work. The study also suggests that pressure is more powerful than light in inducing ET.

Unravelling chromism in metal–organic frameworks

Chromophoric MOFs are reviewed, focussing on those which change colour on application of external stimuli such as heat, pressure, light or chemical environment.

Two new photochromic coordination compounds with nonphotochromic ligands and different metal centers

Two new photochromic compounds were obtained by the different metal joint dots coordinated with nonphotochromic ligands. Compound 2 shows better photosensitivity for the short Φ…Φ distance appeared in 2 with only binuclear Zn as metal joint points.

Photochromic and Nonphotochromic Luminescent Supramolecular Isomers Based on Carboxylate-Functionalized Bipyridinium Ligand: (4,4)-Net versus Interpenetrated (6,3)-Net

Photochromic and nonphotochromic luminescent bipyridinium-based supramolecular isomers [Cd(CPBPY)(BDC)(H2O)]n (1) and {[Cd(CPBPY)(BDC)]·H2O}n (2) (CPBPY = N-(3-carboxyphenyl)-4,4'-bipyridinium, BDC= terephthalate) have been successfully obtained by solvothermal reactions at 100 °C via tuning stoichiometric ratios of starting reagents. Isomer 1 features (4,4)-topological layer constructed by edge-shared Cd2 SBUs and BDC linkers attached by N-pendent CPBPY groups. Isomer 2 has (6,3)-topological layers with Cd atoms as nodes and BDC and double CPBPY as linkers, which are 4-fold interpenetrated into 3D network. Although both 1 and 2 contain bipyridinium ligands, only isomer 1 possesses reversible photochromic behavior with quick-switchable luminescence in the solid state. Compound 2 does not show photochromic behavior even after exposure to UV light for more than 2 h. Photochromism process of 1 originates from photostimulated reduction of CPBPY ligands to generate CPBPY(•-) radicals after irradiation, confirmed by EPR spectra. Careful check on structure reveals that the offset π-π stacking interaction between the pyridine ring of CPBPY and benzene ring of BDC with inter-ring shortest C···C distance of 3.214 Å in 1 is responsible for electron transfer to form the CPBPY(•-) radicals. The speculation is further supported by DFT calculation of frontier orbital matching of electron donor and acceptor. HOMO and HOMO-2 orbitals of BDC involve the carbon atoms of benzene ring while LUMO and LUMO+1 orbitals of CPBPY involve the carbon atoms of pyridinium ring. Importantly, the photoinduced formed CPBPY(•-) radicals in 1 have a long-lived lifetime (at least six months in air and room temperature condition), which is mainly attributed to the close packing mode.