A New Hybrid Optical Semiconductor Based on Polymeric

A New Hybrid Optical Semiconductor Based on Polymeric Iodoplumbate Co-Templated by Both Organic Cation and Polyiodide Anion Hao-Hong Li, Zhi-Rong Chen,* Li-Chuan Cheng, Ji-Bo Liu, Xiao-Bo Chen, and Jun-Qian Li

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4355–4358

College of Chemistry and Chemical Engineering, Fuzhou UniVersity, Fuzhou, Fujian, 350002, P. R. China ReceiVed August 29, 2008; ReVised Manuscript ReceiVed October 14, 2008

ABSTRACT: A new type of hybrid semiconductor co-templated by both organic cations and inorganic polyiodide anions has been synthesized and structurally characterized. The Z-scan technique indicates that it also exhibits good third-order nonlinear optical activity. The prospect of creating new functional materials with tunable properties provides a great motivation for research on organicinorganic hybrids.1 In the past decade, the functional properties (conductivity, magnetism, ion exchange, catalysis, display, storage technology, and so on) of organic-inorganic hybrid solids have been extensively studied.2 Up to now, the hybrid systems under investigation include metal halides,3 chalcogenides4 and phosphates,5 among which metal-halide base hybrids are the most attractive for their opt-electronic device applications such as semiconductor/insulator, excitonic, third-order nonlinear optical (NLO) and ferroelectric properties.1a,b,6 Among the metal-halide hybrid system, lead(II) halides occupy an important position for their versatile topology structures and enthralling properties. The available structures of lead(II) are sporadic and diverse, exhibiting a wide variety of coordination numbers and stereochemistries with or without the suggestion of a “lone pair” in the coordination sphere.7 Because the template effect of the cations is impacted by the ligands, modification of organic moieties can result in multiplicity structures of Pb/X compounds. So far, much effort has been paid to obtain different dimensions of lead halide polyanions by modifying the organic templates. Their anion structures range from isolated anions8 to infinite chains,9 layered perovskites,10 and cubic perovskite structures.11 Most of structures are characterized by PbI6 octahedra with common faces, edges, or vertexes. Therefore, the modification of iodoplumbate polymers using cationic template has been well-established. Recently, hybrid compounds incorporating novel templates have emerged; these new templates include transition-metal (tm) complexes12 and bifunctional organic cations, for example, (hydroxyethyl)-ammonium [(HO(CH2)2NH3)+] and aminocarboxylic acid [(HO2C(CH2)NH3)+] cations.13 In these hybrid systems, N(O)sH · · · X hydrogen bonds between the inorganic and organic components can be observed frequently. In addition, Mitzi et al. have described the intercalation of neutral solvent molecules into the organic cation in a perovskite inorganic-organic hybrid.14 More recently, Guo et al. introduced the anionic-cationic co-template into the hybrid system to give a new type of perovskite hybrid structure.15 However, the templates in all of these hybrids are limited to transition-metal (tm) complexes, organic cations and organic anions; inorganic anions have scarcely been tried in hybrid structures probably because their negative charge is thought to repel inorganic components and would thus make the structure unstable.16 Some efforts have been paid to overcome this problem.16a,17 We here adopt the strategy proposed by the above relative works to introduce inorganic anions into the hybrid through supramolecular interactions between organic cations and inorganic framework, which features an organic cationic/inorganic anion co-template structure. In detail, * To whom correspondence should be addressed. E-mail: [email protected].

Scheme 1. Valence Structure of I3- Anion

we use polyiodide and heterocycle organic compounds as templates to synthesize a novel coordination polymer possessing new properties. Herein we report the formation and structure of a new organicbased lead iodide-polyiodide {(MPL-H)6 · I3}(Pb2I9)}n (MPL ) morpholine), together with its third-order NLO property. 1 was prepared by the reaction of PbI2, a small amount of I2 and protonated morpholine in DMF solvent at 65 °C with the pH value being adjusted to 6.0 with HI/DMF solution. The structure of 118 is composed of three structural units: unique one-dimensional (1D) ladder-like chains [Pb2I9]n5n-, organic cationic templates(protonated morpholine) and inorganic anion templates (I3-). Generally, the whole structure of 1 could be described as the formation of lead(II) iodide polymers based on co-templates of organic cations and inorganic polyiodide anions. In the polyanion ([Pb2I9]n5n-), there is only one crystallographically independent lead atom, which is situated in a slightly distorted octahedral coordination environment. The Pb(1)I6 octahedron connects with Pb(1)#4I6 (#4: -x, 1 - y, 1 - z) octahedron in a corner-sharing fashion to give a Pb2I11 dimer, which acts as the building block of the inorganic chain. And furthermore, the Pb2I11 dimer links with adjacent two Pb2I11 building blocks to give a ladder-like chain ([Pb2I9]n5n-) along the a axis (Figure 1). In other words, each step of the ladder is defined by four PbI6 octahedra, giving an eight-membered ring shaped by four Pb atoms and four I atoms. It is worth mentioning that the eightmembered ring is generally planar. A similar ladder-like chain in a Sb/X system with the formula of [Sb2Cl9]n5n- has also be reported by Hall et al.19 Comparably, the iodoplumbate polymer [(H2en)7(C2O4)2]n(Pb4I18)n · 4nH2O templated by a anionic-cationic co-template15 also presents the same stoichiometric ratio of Pb:I ) 2:9; however, their real structure is completely different and could be described as a staircase-like sheet constructed from the cornersharing of Pb4I18 tetramers. We now focus our attention on the structure of polyiodide ion, namely, I3-. Heavy halogens exhibit a strong tendency to form chain-like polyhalonium cations and polyhalide anions, for example, I3+, I3-, I42-, I5+, I5-, I7-, I82-, I9-, I122-, I162-, I164-, etc.17b,20 The versatile structures and charge orientations provide a powerful technique for the topology and property control of hybrid materials. In 1, the I3- anion is linear with the I(3)#2-I(5)-I(3) (#2: -x, -y, -z + 2) angle of 180.000(15)°. Two I-I distances of 2.9380(7) Å indicate a symmetrical I3-. Judging from the bond angle and distance of I3-, we can describe it as molecular triiodide. Hall et

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Figure 1. (a) Fragment of the ladder-like chain [Pb2I9]n5n- in 1 extending along the a axis. (b) Space-filling mode of the [Pb2I9]n5n- chain in 1 viewed down the a axis.

Figure 2. Structure of (MPL-H+)2 · I3- co-template.

al. have deduced a valence structure of I3- anion with ab initio calculations.21 They assigned the I-I bond as a dative bond, rather than a covalent bond (Scheme 1). From this structure, we can deduce that the terminal I could act as a good hydrogen bond acceptor. Weak interactions including the hydrogen bond are very important in supramolecular assembly, through which crystal engineering with designed structures and properties will be achieved. In the title compound, the hydrogen-bond interactions occur among the organic/inorganic moieties, stabilizing the co-template structure. In detail, there are three crystallographically independent protonated morpholine rings in the unit cell. The first protonated morpholine defined by N(1) interacts with the I3- anion template via H-bond N(1)-H(15) · · · I(3)#5 (parameters: H(15) · · · I(3)#5 distance 2.895 Å, N(1)-H(15) · · · I(3)#5 angle: 160.30°, #5: x + 1, y, z - 1) to give a (MPL-H+)2 · I3- co-template (Figure 2). However, the second protonated morpholine shaped by N(2) links with [Pb2I9]n5ninorganic chain via hydrogen bond N(2)-H(12) · · · I(2)#6 (parameters: H(14) · · · I(2)#6 distance 2.849 Å, N(2)-H(14) · · · I(2)#6 angle 153.18°, #6: x, y - 1, z + 1). The third protonated morpholine featured by N(3) just stacks among the [Pb2I9]n5n- inorganic chains without any H-bond interaction with other moieties. Furthermore, upon these hydrogen bonds, three kinds of templates could be classified in this compound: (MPL-H+)2 · I3- co-template, MPLH+ organic template stacking among the inorganic chains and MPLH+ organic template hydrogen-bonding with inorganic chains. The former two kinds of templates combine with inorganic moiety with electrostatic force. The protonation of MPL could be extracted from the account of charge balance. But to our disappointment, the hydrogen of protonated amine could not be found in the Fourier map. The nearest I · · · I distances between [Pb2I9]n5n- chains is 5.237(11) Å, and the nearest I · · · I distance between [Pb2I9]n5n-

Figure 3. Packing diagram of 1 illustrating hydrogen bonding interactions.

chain and I3- anion is 4.51(1) Å, indicating the absence of I · · · I interactions. From the packing diagram (Figure 3), we can see that the inorganic layer induced by the latter two kinds of cations are separated by (MPL-H+)2 · I3- co-templates to present an interesting 3-D network arrangement. This packing mode is similar with organic compound C4N2H12 · NH4Cl3 · H2O and 2-H hexagonal C6N2H14 · NH4Cl3, which exhibit perovskite-like arrangement.22 The highly structural feature of 1 is to incorporate inorganic anions I3- into organic cations MPL-H+ through supramolecular interactions to generate a co-template. Compared with the traditional organic template, the co-template may present different sizes and diversity of shape to modulate the formation of a more diverse configuration of an inorganic component. The work about incorporating other inorganic/organic co-templates is still ongoing. The optical absorption spectrum of 1 has been measured by the diffuse-reflectance experiment. The Eg was obtained by the use of a straightforward extrapolation method.23 As a result, the absorption edge for 1 is 2.43 eV, indicating a semiconductor nature (Figure 4) and exhibiting a 0.13 eV blue shift compared with the measured value of 2.30 eV for bulk PbI2. The studies on coordination polymers are greatly focused on their novel properties. Recently, we found that many coordination

Communications

Crystal Growth & Design, Vol. 8, No. 12, 2008 4357 the observed experimental data. This result suggests that the experimentally detected NLO effects have an effective thirdorder characteristic. The effective nonlinear absorptive index β is derived to be 5.68 × 10-14 cm/W. This value could compared with that of [Pb2I4(dpdo)]n (2.678 × 10-14 cm/W), [PbI2(bipyO2)]n (4.495 × 10-14 cm/W).25 A tendency could be found that the NLO property improves to some extent with the increase of heavy atoms (I3- anions are invited in this case). All these measured values were obtained with 1.0 × 10-4 mol/L DMF solution of the title compound (limited by its solubility). A much larger value may be expected with more concentrated solutions provided the solubility can be increased. In summary, upon the idea of incorporation of organic and inorganic templates into a iodoplumbate polymer, a new 1-D hybrid coordination polymer {(MPL-H)6 · I3}(Pb2I9)}n (MPL ) morpholine) (1) has been synthesized. The iodoplumbate polymer presents a ladder-like chain arrangement. The optical absorption and Z-scan measurements indicate that 1 is a good optical semiconductor.

Figure 4. Optical absorption spectrum of 1.

Acknowledgment. We acknowledge support of this research by Natural Science Fund of Fujian Province (E0710008), Innovation Fund for Young Scientist of Fujian Province (2007F3049), Fund of Education Committee of Fujian Province (JA07018), the Special Foundation for Young Scientists of Fuzhou University (XRC-0644), and Science & Technology Promotion Foundation of Fuzhou University (XJJ-0605).

References

Figure 5. Z-scan data for compound 1 in ca. 1.0 × 10-4 mol/dm-3 DMF solution obtained under an open aperture configuration. The block squares are the experimental data, and the solid curve is the theoretical fit.

polymers exhibit interesting third-order NLO properties24a and optical limiting behaviors in DMF solution.24b-d The most important issue in this field is to certify their optical properties in DMF solution originate from the bulk complexes. Thus, the molecular weight measurement of compound 1 is necessary, which was determined with gel permeation chromatography (Waters Associates model HPLC/GPC 515 liquid chromatograph, equipped with a refractive index detector and ´ı-Styragel columns and calibrated with standard polystyrene) at 40 °C, using DMF as the eluent and a flow rate of 1.0 mL · min-1. The result shows that the number-average molecular weight (Mn) and the weightaverage molecular weight (Mw) are 3936 and 4027, respectively, which are large enough to prove its intactness (or partial intactness) in DMF solution. On the basis of the literature and our experimental facts, we think that the optical behaviors in DMF can represent their bulk properties. The third-order NLO property of compound 1 was determined with pulses of a wavelength of 532 nm, a duration of 8 ns, and the energy of the illumination 2 × 10-4 J by a Z-scan experiment using an openaperture configuration in DMF solution with a concentration of 1.0 × 10-4 mol/L. Figure 5 depicts the NLO absorption property of 1, which clearly illustrates that the absorption increases as the intensity of the incident light rises, with light transmittance (T) being a function of the Z position of the samples.24d A reasonably good fit between the experimental data and the theoretical curves could be obtained. It is clearly that the theoretical curves qualitatively reproduce the general pattern of

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