Unveiling of a puzzling dual ionic migration in lead- and iodide-deficient halide perovskites (d-HPs) and its impact on solar cell J-V curve hysteresis.

Halide perovskite solar cells are characterized by a hysteresis between current-voltage (J-V) curves recorded on the reverse and on the forward scan directions, and the suppression of this phenomenon has focused great attention. In the present work, it is shown that a special family of 3D perovskites, that are rendered lead -and iodide- deficient (d-HPs) by incorporating large organic cations, are characterized by a large hysteresis. The strategy of passivating defects by K+, which has been successful in reducing the hysteresis of 3D perovskite perovskite solar cells, is inefficient with the d-HPs. By glow discharge optical emission spectroscopy (GD-OES), the existence of the classic iodide migration in these layers is unveiled, which is efficiently blocked by potassium cation insertion. However, it is also shown that it co-exists with the migration of the large organic cations characteristics of d-HPs which are not blocked by the alkali metal ion. The migration of those large cations is intrinsically linked to the special structure of the d-HP. It is suggested that it takes place through channels, present throughout the whole perovskite layer after the substitution of PbI+ units by the large cations, making this phenomenon intrinsic to the original structure of d-HPs.

Perovskite versus Nonperovskite: Modulating the Nature and Optical Properties of One-Dimensional Chiral Lead-Bromide Networks.

In the last 5 years, low-dimensional chiral metal-halide semiconductors have attracted great interest in the generation of chiroptical activity. Among this new family of materials, one-dimensional (1D) networks have appeared as the best candidates for strong circular dichroism (CD) and circularly polarized luminescence (CPL). Here, we present a new family of 1D chiral lead-bromide materials prepared from organic S/R/rac-1-hydroxypropyl-2-ammonium (S/R/rac-HP2A) cations. The presence or absence of polarity in the crystal structure as well as the perovskite or nonperovskite nature of the inorganic network depends on the initial stoichiometry of metal-halide salt and chiral amine during the crystallization. The perovskite-type networks exhibit strong CD and second harmonic generation (SHG) responses, while the nonperovskite compounds show the presence of polymorphism in the crystal phase and weak natural optical activity in the final material. These results underline the impact of synthetic conditions and thin film morphology on the structural and optical properties of metal-halide hybrid networks.

Chirality Versus Symmetry: Electron's Spin Selectivity in Nonpolar Chiral Lead-Bromide Perovskites.

In the last decade, chirality-induced spin selectivity (CISS), the spin-selective electron transport through chiral molecules, has been described in a large range of materials, from insulators to superconductors. Because more experimental studies are desired for the theoretical understanding of the CISS effect, chiral metal-halide semiconductors may contribute to the field thanks to their chiroptical and spintronic properties. In this regard, this work uses new chiral organic cations S-HP1A and R-HP1A (HP1A = 2-hydroxy-propyl-1-ammonium) to prepare 2D chiral halide perovskites (HPs) which crystallize in the enantiomorphic space groups P43 21 2 and P41 21 2, respectively. The fourfold symmetry induces antiferroelectricity along the stacking axis which, combined to incomplete Rashba-like splitting in each individual 2D polar layer, results in rare spin textures in the band structure. As revealed by magnetic conductive-probe atomic force microscopy (AFM) measurements, these materials show CISS effect with partial spin polarization (SP; ±40-45%). This incomplete effect is efficient enough to drive a chiro-spintronic device as demonstrated by the fabrication of spin valve devices with magnetoresistance (MR) responses up to 250 K. Therefore, these stable lead-bromide HP materials not only represent interesting candidates for spintronic applications but also reveal the importance of polar symmetry-breaking topology for spin selectivity.

Chemistry in the Molten State: Opportunities for Designing and Tuning the Emission Properties of Halide Perovskites.

A series of monolayered lead halide hybrid perovskites (HO2C(CH2)n-1NH3)2PbX4, named (Cn)2PbX4 (n = 4-6, X = Cl, Br), exhibiting a low congruent melting temperature (Tm) (Tm = 130 °C for (C4)2PbBr4), high stability in the molten state, and whitish type emission, are reported. From the synthesis in the molten state, rare solid solutions of mixed organic cations (Cn1-xCn'x)2PbX4 (n, n' = 4-6; X = Cl, Br; 0 ≤ x ≤1) as well as solid solutions of mixed halides (Cn)2Pb(X1-yX'y)4 (n = 4-6; X, X' = Cl, Br; 0 ≤ y ≤1) have been prepared and characterized (thermal behavior, powder X-ray diffraction (PXRD), photoluminescence properties). The impact of substitutions is significant on the thermal properties, lowering the Tm down to 100 °C for (C4)2Pb(Br0.25Cl0.75)4. The emission properties are slightly tuned in the case of mixed organic cation systems, whereas modifications are more dramatic in the case of mixed halide systems, leading to emission properties through the entire visible region. These results illustrate the great opportunities offered by the congruent melting properties of halide perovskites allowing syntheses in the molten state.

New Dication-Based Lead-Deficient 3D MAPbI3 and FAPbI3 "d-HPs" Perovskites with Enhanced Stability.

Toxicity induced by the presence of lead and the rather poor stability of halide perovskite semiconductors represent the major issues for their large-scale application. We previously reported a new family of lead- and iodide-deficient MAPbI3 and FAPbI3 perovskites called d-HPs (for lead- and iodide-deficient halide perovskites) based on two organic cations: hydroxyethylammonium HO-(CH2)2-NH3+ (HEA+) and thioethylammonium HS-(CH2)2-NH3+ (TEA+). In this article, we report the use of an organic dication, 2-hydroxypropane-1,3-diaminium (2-propanol 1,3 diammonium), named PDA2+, to create new 3D d-HPs based on the MAPbI3 and FAPbI3 network with general formulations of (PDA)0,88x(MA)1-0,76x[Pb1-xI3-x] and (PDA)1,11x(FA)1-1,22x[Pb1-xI3-x], respectively. These d-HPs have been successfully synthesized as crystals, powders, and thin films and exhibit improved air stability compared to their reference MAPbI3 and FAPbI3 perovskite counterparts. PDA2+-based deficient MAPbI3 was also tested in operational perovskite solar cells and exhibited an efficiency of 13.0% with enhanced stability.

Exploring Solar Cells Based on Lead- and Iodide-Deficient Halide Perovskite (d-HP) Thin Films.

Perovskite solar cells have become more and more attractive and competitive. However, their toxicity induced by the presence of lead and their rather low stability hinders their potential and future commercialization. Reducing lead content while improving stability then appears as a major axis of development. In the last years, we have reported a new family of perovskite presenting PbI+ unit vacancies inside the lattice caused by the insertion of big organic cations that do not respect the Goldschmidt tolerance factor: hydroxyethylammonium HO-(CH2)2-NH3+ (HEA+) and thioethylammonium HS-(CH2)2-NH3+ (TEA+). These perovskites, named d-HPs for lead and halide-deficient perovskites, present a 3D perovskite corner-shared Pb1-xI3-x network that can be assimilated to a lead-iodide-deficient MAPbI3 or FAPbI3 network. Here, we propose the chemical engineering of both systems for solar cell optimization. For d-MAPbI3-HEA, the power conversion efficiency (PCE) reached 11.47% while displaying enhanced stability and reduced lead content of 13% compared to MAPbI3. On the other hand, d-FAPbI3-TEA delivered a PCE of 8.33% with astounding perovskite film stability compared to classic α-FAPI. The presence of TEA+ within the lattice impedes α-FAPI degradation into yellow δ-FAPbI3 by direct degradation into inactive Pb(OH)I, thus dramatically slowing the aging of d-FAPbI3-TEA perovskite.

Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation.

Lead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent-free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2 (MA)n-1 Pbn I3n+1 series) at the quite high temperature Tm (Tmelting ) of 171 °C. Here, we report on the solvent-free preparation (n=2, 4), thermal behaviour and melt-processed thin films of a series of HP (GABA)2 (MA)n-1 Pbn I3n+1 (n=1, 2, 4; GABA+ : 4-ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid-solid reaction of molten (GABA)2 PbI4 or (GABA)2 (MA)Pb2 I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2 (MA)3 Pb4 I11 (n=4, Tm =185 °C), respectively. Melt-processed thin films of n=1, n=2 and n=4 HP have also been prepared.

From Zero- to One-Dimensional, Opportunities and Caveats of Hybrid Iodobismuthates for Optoelectronic Applications.

The association of the electron acceptor 4,4'-amino-bipyridinium (AmV2+) dication and BiI3 in an acidic solution affords three organic-inorganic hybrid materials, (AmV)3(BiI6)2 (1), (AmV)2(Bi4I16) (2), and (AmV)BiI5 (3), whose structures are based on isolated BiI63- and Bi4I164- anion clusters in 1 and 2, respectively, and on a one-dimensional (1D) chain of trans-connected corner-sharing octahedra in 3. In contrast with known methylviologen-based hybrids, these compounds are more soluble in polar solvents, allowing thin film formation by spin-coating. (AmV)BiI5 exhibits a broad absorption band in the visible region leading to an optical bandgap of 1.54 eV and shows a PV effect as demonstrated by a significant open-circuit voltage close to 500 mV. The electronic structure of the three compounds has been investigated using first-principles calculations based on density functional theory (DFT). Unexpectedly, despite the trans-connected corner-shared octahedra, for (AmV)BiI5, the valence state shows no coupling along the wire direction, leading to a high effective mass for holes, while in contrast, the strong coupling between Bi 6px orbitals in the same direction at the conduction band minimum suggests excellent electron transport properties. This contributes to the low current output leading to the low efficiency of perovskite solar cells based on (AmV)BiI5. Further insight is provided for trans- and cis-MI5 1D model structures (M = Bi or Pb) based on DFT investigations.

The Key Role of the Interface in the Highly Sensitive Mechanochromic Luminescence Properties of Hybrid Perovskites.

Hybrid perovskite (HP) materials are of interest in photovoltaics and lighting applications. Here we report that hybrid perovskite composites, as crystallized powders, can behave as intelligent materials showing highly sensitive and reversible mechanochromic luminescence (MCL). Composites consisting of monolayered 2D HP and 3D HP components exhibit reversible tunable color emission upon mechanical strain. The bluish-whitish emission of the 2D HP turns into orange in the composite owing to an energy transfer process. The bright green emission, observed as soon as the composite is slightly crushed, originates from the 3D HP after efficient energy funneling from the multi-layered 2D HP produced at the 2D/3D interface by the mechanical treatment. Besides highlighting the key role of the interfaces in light emission of HP, our findings pave the way for hybrid perovskites as highly sensitive MCL smart materials for mechanosensors, security papers, or optical storage applications.

Hybrid Halide Perovskites: Discussions on Terminology and Materials.

Hybrid halide perovskites (HP) have emerged in the last decade as a new class of semiconductors with superior performances in photovoltaic and electronic devices. The literature about these halide semiconductors is abundant and a lot of names/expressions are used to define networks, structures, or materials. In this context, there is a need to offer some discussions about the relevance of using the word "perovskite" and the associated expressions ("RP" (Ruddlesden-Popper), "DJ" (Dion-Jacobson), "ACI" (alternating cations in the interlayer space), "hexagonal perovskites", "hollow perovskites", "d-HP" (deficient 3D HP),…). Moreover, the description of perovskite networks through elimination/substitution processes from the ABX3 structure will be compared to the known dimensional reduction concept.

Enhanced Stability and Band Gap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration.

We report room-temperature synthesis of lead- and iodide-deficient α-[HC(NH2)2]PbI3 perovskites (abbreviated d-α-FAPI, FA+ = formamidinium), with the general formula (A',FA)1+ x[Pb1- xI3- x] (with A' = hydroxyethylammonium (HEA+) or thioethylammonium (TEA+) cations, 0.04 ≤ x ≤ 0.15). These materials retain a 3D character of their perovskite network despite incorporation of large HEA+ or TEA+ cations, demonstrating that the Goldschmidt tolerance factor can be bypassed. We found that thin films of (TEA,FA)1+ x[Pb1- xI3- x] ( x = 0.04 and 0.13) show exceptional α-phase stability under ambient conditions, 1 order of magnitude higher than α-FAPI and α-(Cs,FA)PbI3 thin films. d-α-FAPI phases are shown to maintain a direct band gap, which increases monotonously for x ranging from 0 up to 0.20, with characteristics of a p-type semiconductor for low concentrations of vacancies ( x ≤ 0.13) and n-type for larger ones. They offer alternatives to reach the methylammonium- and bromine-free stable α-FAPI-type phase and open new avenues in the field of perovskite solar cells, up to band gap tuning desirable for tandem solar cells.

Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors.

Hybrid halide perovskites are now superstar materials leading the field of low-cost thin film photovoltaics technologies. Following the surge for more efficient and stable 3D bulk alloys, multilayered halide perovskites and colloidal perovskite nanostructures appeared in 2016 as viable alternative solutions to this challenge, largely exceeding the original proof of concept made in 2009 and 2014, respectively. This triggered renewed interest in lower-dimensional hybrid halide perovskites and at the same time increasingly more numerous and differentiated applications. The present paper is a review of the past and present literature on both colloidal nanostructures and multilayered compounds, emphasizing that availability of accurate structural information is of dramatic importance to reach a fair understanding of quantum and dielectric confinement effects. Layered halide perovskites occupy a special place in the history of halide perovskites, with a large number of seminal papers in the 1980s and 1990s. In recent years, the rationalization of structure-properties relationship has greatly benefited from new theoretical approaches dedicated to their electronic structures and optoelectronic properties, as well as a growing number of contributions based on modern experimental techniques. This is a necessary step to provide in-depth tools to decipher their extensive chemical engineering possibilities which surpass the ones of their 3D bulk counterparts. Comparisons to classical semiconductor nanostructures and 2D van der Waals heterostructures are also stressed. Since 2015, colloidal nanostructures have undergone a quick development for applications based on light emission. Although intensively studied in the last two years by various spectroscopy techniques, the description of quantum and dielectric confinement effects on their optoelectronic properties is still in its infancy.

Lead- and Iodide-Deficient (CH3 NH3 )PbI3 (d-MAPI): The Bridge between 2D and 3D Hybrid Perovskites.

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH3 NH3 )PbI3 (MAPI). The discovery of a new family of hybrid perovskites called d-MAPI is reported: the association of PbI2 with both methyl ammonium (MA+ ) and hydroxyethyl ammonium (HEA+ ) cations leads to a series of five compounds with general formulation (MA)1-2.48x (HEA)3.48x [Pb1-x I3-x ]. These materials, which are lead- and iodide-deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin-coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA+ , HEA+ , Pb2+ and I- ions.

Porous Coordination Polymer Based on Bipyridinium Carboxylate Linkers with High and Reversible Ammonia Uptake.

The zwitterionic bipyridinium carboxylate ligand 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium (pc1) in the presence of cadmium chloride affords novel porous coordination polymers (PCPs): [Cd4(pc1)3Cl6]·CdCl4·guest (1) crystallizing in the P3̅1c space group. In the structure, [Cd4Cl6(CO2)6] building units are linked together by six pc1 ligands, leading to a 3D high-symmetrical network exhibiting hexagonal channels along the c axis. The walls of this PCP consist of cationic electron-acceptor bipyridinium units. The PCP 1 reversibly adsorbs H2O and CH3OH up to about 0.1 g/g at saturation showing the adsorption isotherms characteristic of a moderately hydrophilic sorbent. Adsorption of ammonia (NH3) follows a different pattern, reaching an exceptional uptake of 0.39 g/g (22.3 mmol/g) after the first adsorption cycle. Although the crystalline structure of 1 collapses after the first adsorption, the solid can be regenerated and maintains the capacity of 0.29 g/g (17 mmol/g) in the following cycles. We found that the high NH3 uptake is due to a combination of pore filling taking place below 150 h·Pa and chemisorption occurring at higher pressures. The latter process was shown to involve two phenomena: (i) coordination of NH3 molecules to Cd(2+) cations as follows from (113)Cd NMR and (ii) strong donor-acceptor interactions between NH3 molecules and pc1 ligands.

Bismuth-Based Coordination Polymers with Efficient Aggregation-Induced Phosphorescence and Reversible Mechanochromic Luminescence.

Two bismuth coordination polymers (CPs), (TBA)[BiBr4 (bp4mo)] (TBA=tetrabutylammonium) and [BiBr3 (bp4mo)2 ], which are based on the rarely used simple ditopic ligand N-oxide-4,4'-bipyridine (bp4mo), show mechanochromic luminescence (MCL). High solid-state phosphorescence quantum yields of up to 85 % were determined for (TBA)[BiBr4 (bp4mo)] (λem =540 nm). Thorough investigations of the luminescence properties combined with DFT and TDDFT calculations revealed that the emission is due to aggregation-induced phosphorescence (AIP). Upon grinding, both samples became amorphous, and their luminescence changed from yellow to orange and red, respectively. Heating or exposure to water vapor led to the recovery of the initial luminescence. These materials are the first examples of mechanochromic phosphors based on bismuth(III).

Photo- and thermochromic and adsorption properties of porous coordination polymers based on bipyridinium carboxylate ligands.

The zwitterionic bipyridinium carboxylate ligand 1-(4-carboxyphenyl)-4,4'-bipyridinium (hpc1) in the presence of 1,4-benzenedicarboxylate anions (BDC(2-)) and Zn(2+) ions affords three porous coordination polymers (PCPs): [Zn5(hpc1)2(BDC)4(HCO2)2]·2DMF·EtOH·H2O (1), [Zn3(hpc1)(BDC)2(HCO2)(OH)(H2O)]·DMF·EtOH·H2O (2), and [Zn10(hpc1)4(BDC)7(HCO2)2(OH)4(EtOH)2]·3DMF·3H2O (3), with the formate anions resulting from the in situ decomposition of dimethylformamide (DMF) solvent molecules. 1 and 3 are photo- and thermochromic, turning dark green as a result of the formation of bipyridinium radicals, as shown by electron paramagnetic resonance measurements. Particularly, crystals of 3 are very photosensitive, giving an eye-detectable color change upon exposure to the light of the microscope in air within 1-2 min. A very nice and interesting feature is the regular discoloration of crystals from the "edge" to the "core" upon exposition to O2 (reoxidation of organic radicals) due to the diffusion of O2 inside the pores, with this discoloration being slower in an oxygen-poor atmosphere. The formation of organic radicals is explained by an electron transfer from the oxygen atoms of the carboxylate groups to pyridinium cycles. In the structure of 3', [Zn10(hpc1)4(BDC)7(OH)6(H2O)2], resulting from the heating of sample 3 (desolvation and loss of CO molecules due to the decomposition of formate anions), no suitable donor-acceptor interaction is present, and as a consequence, this compound does not exhibit any chromic properties. The presence of permanent porosity in desolvated 1, 2, and 3' is confirmed by methanol adsorption at 25 °C with the adsorbed amount reaching 5 wt % for 1, 10 wt % for 3', and 13 wt % for 2. The incomplete desorption of methanol at 25 °C under vacuum points to strong host-guest interactions.

Aggregation induced phosphorescent N-oxyde-2,2'-bipyridine bismuth complexes and polymorphism-dependent emission.

Unprecedented bismuth complexes, based on the rarely used ditopic ligand N-oxide-2,2'-bipyridine (bp2mo), crystallizing as three polymorphs, α- (), ß- () and γ-[BiBr3(bp2mo)2] (), exhibit phosphorescence with a quantum yield up to 17% for the crystal phase (), while the complex displays a weak fluorescence in solution. A study of the luminescence properties combined with DFT/TDDFT calculations reveals that the lighting phenomenon originated by aggregation induced phosphorescence correlated with the weak intermolecular interactions present in the different crystal phases.

Unprecedented stacking of MV2+ dications and MV˙+ radical cations in the mixed-valence viologen salt (MV)2(BF4)3 (MV = methylviologen).

Using a slow liquid-gas diffusion method, the mixed-valence viologen salt (MV)2(BF4)3 (1) and the radical cation salt (MV)(BF4) (2) are crystallized. Both structures contain regular stacks of MV˙(+) radical cations (2) or alternating MV˙(+) and MV(2+) entities (1). A short intrastack intermolecular separation (3.23 Å) unprecedently reveals strong interactions between MV(2+) and MV˙(+) in 1.

Large spontaneous polarization and clear hysteresis loop of a room-temperature hybrid ferroelectric based on mixed-halide [BiI3Cl2] polar chains and methylviologen dication.

The search for hybrid organic-inorganic materials, which have the great advantage that they can be synthesized at moderate temperature (T < 200 °C), remains a great challenge in the field of ferroelectrics. Here, a room-temperature ferroelectric material with interesting characteristics, (MV)[BiI(3)Cl(2)] (MV(2+) = methylviologen), is reported. Its structure is based on polar inorganic chains resulting from a remarkable Cl/I segregation induced by methylviologen entities, which coincide with the fourfold polar axis of the tetragonal structure. Of great importance is that this room-temperature hybrid ferroelectric displays a clear electrical hysteresis loop with a large spontaneous polarization (>15 µC·cm(-2)).

Photochromism, electrical properties, and structural investigations of a series of hydrated methylviologen halobismuthate hybrids: influence of the anionic oligomer size and iodide doping on the photoinduced properties and on the dehydration process.

Syntheses, X-ray structural analyses, thermal behaviors, photochromism, and electrical properties of a series of methylviologen (MV(2+)) halobismuthate hybrids, namely, (MV)(3)[Bi(4)Cl(18)](H(2)O)(y) (1a, y approximately = 1.7), (MV)(4)[Bi(6)Cl(26)](H(2)O)(y) (2a, y approximately = 1.7), (MV)(4)[Bi(6)Cl(25.6)I(0.4)](H(2)O)(y) (3a, y approximately = 1.5), and (MV)(4)[Bi(6)Cl(24.6)I(1.4)](H(2)O)(y) (4a, y approximately = 1.3), are reported. Because of the thermal effect of a UV lamp or as a result of being heated up to 100 degrees C, all of the above compounds undergo a complete (1a, 2a, and 3a) or a partial (4a) dehydration together, in 2a and 3a, with an impressive structural reorganization involving a 90 degrees rotation of methylviologen dimers and, in 3a, a new Cl/I distribution, finally leading to (MV)(3)[Bi(4)Cl(18)] (1b), (MV)(4)[Bi(6)Cl(26)] (2b), (MV)(4)[Bi(6)Cl(25.6)I(0.4)] (3b), and (MV)(4)[Bi(6)Cl(24.6)I(1.4)](H(2)O)(x) (4a, x approximately = 0.65), respectively. In its turn, 4a (x approximately = 0.65) undergoes an abrupt structural change at 160 degrees C when water molecules are completely removed, leading to (MV)(4)[Bi(6)Cl(24.6)I(1.4)] (4b). Obviously, the two first dehydrated phases can be considered as the n = 2 (1b) and n = 3 (2b) members of the (MV)((2n+2)/2)[Bi(2n)Cl(8n+2)] family, and the ultimate member (n = infinity) with an infinite 1D double-chain inorganic framework, namely, (MV)[Bi(2)Cl(8)], has already been reported. According to the results of structural refinements, some positions of the Cl atoms in the [Bi(6)Cl(26)](8-) anionic cluster of 3a and 4a have been occupied by I atoms, finally leading to iodide-doped materials of the 2a type (percentage of doping: 3a, 1.5%; 4a, 5.4%). Upon UV irradiation, yellow crystals of 2a and 3a (which become 2b and 3b because of the thermal effect of the UV lamp) or yellow crystals of 2b, 3b, and 4a undergo a color change to black crystals (in the case of 2b), as observed in (MV)[Bi(2)Cl(8)], or light-brown crystals (in the cases of 3b and 4a). These photochromic properties are probably due to the photoinduced electron transfer from the anionic part to the methylviologen dications. In contrast, no color change is observed when yellow crystals of 1a or 1b and the iodide-doped (MV)[Bi(2)Cl(8-epsilon)I(epsilon)] material are irradiated. Because the relative positions of methylviologen to the host anionic frameworks are comparable in all structures (the N...Cl distances are about 3.4 A), these results indicate that such kinds of photochemical reactions depend on the dimension of the anionic networks, as well as the iodide doping. The single-crystal electrical conductivity measurements of 2b before and after irradiation were carried out between 150 and 393 K. The results prove that both of them are semiconductors with weak room temperature conductivity and that the band gap of the irradiated crystal (2b, 0.35 eV) is much smaller than that of the original hybrid 2a (1.0 eV).