Lei Z. Zhang

Spectroscopic and Theoretical Studies of Quantum and Electronic Confinement Effects in Nanostructured Materials

Nanostructured materials have become the central subject of materials research during the last decade in the past 20th century owing to the novel electronic, optical, and catalytic properties observed in such materials. The unusual properties of these nanostructured materials can be attributed to two main microscopic effects: quantum confinement and electronic confinement. These two effects have dominated the variations of molecular properties in a wide variety of nanostructured materials, ranging from inorganic compounds to organic molecules. The recent advances have focused on host-guest complex systems that have resulted in a deeper understanding of the changes in electronic structures when being confined. In this article we cover some of the key advances in the study of quantum and electronic confinement effects, especially in host-guest systems.

[Zn(BDC)(H2O)2]n: a novel blue luminescent coordination polymer constructed from BDC-bridged 1-D chains via interchain hydrogen bonds (BDC = 1,4-benzenedicarboxylate)

Abstract A novel coordination polymer [Zn(BDC)(H2O)2]n 1 (where BDCxa0=xa01,4-benzenedicarboxylate), has been synthesized and characterized by X-ray diffraction. 1 crystallizes in the monoclinic space group C2/c, a=15.09(2) A, b=5.058(7) A, c=12.196(16) A, β=103.62(2)°, V=904(2) A 3 , Z=4. The most striking feature of 1 is that it consists of a high-dimensional network structure constructed from BDC-bridged 1-D chains via interchain hydrogen bonds. The coordination sphere of the zinc(II) ion is a distorted tetrahedron completed by four oxygen atoms from two water molecules and two BDC ligands. BDC adopts the bis-monodentated (syn–anti) coordination mode linking two adjacent zinc(II) ions. 1 shows strong blue photoluminescence as the result of the fluorescence from the intraligand emission excited state.

Molecular orbital confinement of a Schiff base molecule in the nanoporous channels of MCM-41 host

Abstract A new Shiff base molecule N , N ′ -bis(2-hydroxy-5-methylbenzylidene)-1,2-ethanediamine ( 1 ) has been successfully encapsulated in the nanoporous channels of MCM-41 host. The bathochromic shifts of the 0–0 transitions have been correlated with the reduction of the HOMO–LUMO band gap accompanying by the increased energies of the frontier orbitals. This trend indicates that the molecular orbital confinement of organic molecules is indeed realized in larger cavities, such as nanoporous MCM-41. The variations in the 0–0 transitions observed here have also been confirmed by theoretical calculations.

Encapsulation of a luminescent zinc complex in a nanoporous channel host

A luminescent zinc complex has been encapsulated in the ordered, 3.2xa0nm wide hexagonal channels of a nanoporous MCM-41 host. A marked 19xa0nm blue-shift of the major absorption band maximum is found for the zinc complex in ethanol solution relative to the complex loaded in MCM-41, which is correlated with an increased band gap in the former. A 17xa0nm blue-shift of the emission peak is also observed, which is estimated to be related to the decrease of polarity around the zinc ions.

Electronic properties of an organic molecule within MCM-41 host: a spectroscopic and theoretical study toward elucidating the variation in band gaps of the guest species

Abstract An organic molecule salicylidene-1,2-ethanediamine 1 , has been encapsulated in the nanocavities of MCM-41 and this nanocomposite material has been investigated by X-ray diffraction, absorption and emission spectroscopy. Results from the spectroscopic measurements show that the bathochromic shift of the 0–0 transitions is correlated with the reduction of the HOMO–LUMO band gap accompanying by the energy changes of the frontier orbitals. Theoretical studies indicate that the energy levels of HOMO and LUMO increase when 1 is confined, and the HOMO is more sensitive than the LUMO.

Electronic confinement of organic molecules in confined spaces: A spectroscopic study of Zn(phen)2(NO3)2 loaded MCM-41

Steady-state and dynamic spectroscopic studies have been given in support of the electronic confinement of Zn(phen)2(NO3)2, 1 (phen=1, 10-phenanthroline), within nanoporous MCM-41. The bathochromic shift of the 0-0 transitions, together with the shortening of the excited state lifetimes have been correlated with the reduction of the HOMO-LUMO band gap accompanying by the increased energies of the frontier orbitals. This trend indicates that the electronic confinement of organic molecules is indeed realized in larger cavities, such as nanocavities of MCM-41. The variations in the 0-0 transitions observed here have also been confirmed by theoretical calculations.

[Na8Zn4(CH3CO2)16·2H2O]n: two-dimensional sheet-like coordination polymer with strong blue emission

Abstract A novel heteronuclear compound, [Na8Zn4(CH3CO2)16·2H2O]n 1, with an unusual two-dimensional condensed polymeric structure was successfully synthesized. 1 crystallizes in the triclinic space group P-1, a=9.3332(19) A, b=10.511(2) A, c=16.152(3) A, α=79.78(3)°, β=83.82(3)°, γ=64.85(3)°, V=1410.6(5) A 3 , Z=2. The most striking feature of 1 is that the bridging acetate ligands show four coordination modes within one compound. The luminescence studies show strong blue photoluminescence as the result of the fluorescence from the intraligand emission excited state.

Theoretical studies of the electronic properties of confined aromatic molecules in support of electronic confinement effect

Theoretical studies of the electronic properties of three confined aromatic molecules—benzene, naphthalene and anthracene—have been presented in support of the electronic confinement effect. The confined space of the cavities has been modeled using a mica sheet with the molecule–surface distance in the range of 1.5–4.0 A. Evidence of the confinement has been revealed by semiempirical calculations, which are theoretically interpreted by means of the Huckel molecular orbital theory. It has been found that the HOMO has been predicted to be more sensitive to the confinement than the LUMO and the overall effect is a reduction on the band gap of the frontier molecular orbitals when the molecule–surface distance is less than ca. 2.5 A. The variations of the frontier orbital energies and band gaps are correlated with the increase of both Coulomb integral, α, and resonance integral, β. The order of magnitude of the energy increment of Δα and Δβ values is evaluated from data of the above semiempirical calculations. It is also found that the confinement effect is associated with the conjugated system of the aromatic molecules. The theoretical evaluations here prove that confining organic molecules in the cavities is sufficient to alter their electronic properties as a consequence of changes in the molecular orbital energies and band gaps.

A novel 4,4′-azopyridine-bridged binuclear zinc complex

A novel binuclear zinc(II) complex [Zn2(azpy)3(2Cl-azpy)2(H2O)6](ClO4)4(azpy)4(H2O)10 (1) (azpy = 4,4′-azopyridine, 2Cl-azpy = (2,6-dichloro)-4,4′-azopyridine) has been synthesized and characterized by single crystal X-ray diffraction, IR and excitation and emission spectra. The Complex 1 crystallizes in the triclinic space group , with a = 10.730(3), b = 15.455(4), c = 18.893(5)u2009Å, α = 73.358(5), β = 87.999(6), γ = 71.865(6)°, V = 2847.7(14)u2009Å3, Z = 1. The azpy ligand links two zinc(II) ions forming a binuclear entity. Each zinc(II) ion lies in a distorted octahedron completed by three oxygen atoms from water molecules and three nitrogen atoms from 4-pyridyl donor groups. Luminescence studies of 1 in aqueous solution show strong blue photoluminescence as the result of fluorescence from an intraligand excited state.

Manganese(II)-phenanthroline-azide compounds: Versatile Precursors as Ligands in Designing Heteropolymetallic Systems

Two novel manganese(II) complexes, [Mn(phen)2N3·H2O]ClO4·H2O and Mn(phen)2(N3)2 have been synthesized by the reaction of Mn(ClO4)2·6H2O and Mn(CH3CO2)2·4H2O with NaN3 and phen in EtOH/H2O solution, respectively (where phen = 1,10-phenanthroline). Their crystal structures have been determined by X-ray diffraction. Both complex molecules have distorted octahedral geometry and two 1,10-phenanthroline molecules chelate to a Mn(II) atom with a cis-configuration. To [Mn(phen)2N3·H2O]ClO4·H2O, one nitrogen atom from an azide anion and one oxygen atom from a water molecule cis-coordinate to the Mn(II) atom while two nitrogen atoms occupy cis positions in Mn(phen)2(N3)2. These complexes are versatile precursors for the design of heteropolymetallic systems.