Y. Maximilian Klein

Greasy tails switch 1D-coordination [{Zn2(OAc)4(4′-(4-ROC6H4)-4,2′:6′,4′′-tpy)}n] polymers to discrete [Zn2(OAc)4(4′-(4-ROC6H4)-4,2′:6′,4′′-tpy)2] complexes

The homologous series of 4′-(4-ROC6H4)-4,2′:6′,4′′-tpy ligands with R = Me, Et, nPr, nBu, npentyl, nhexyl, nheptyl, noctyl, nnonyl and ndecyl (1–10, respectively) are reported, including single crystal structures of 6 and 7. Reactions of zinc(II) acetate with 1–10 have been investigated using room temperature crystallization methods (diffusion or layering). For ligands with the shortest alkoxy substituents, 1-dimensional coordination polymers [{Zn2(OAc)4(L)}n] (L = 1, 2 or 3) are formed. In each polymer, the 4′-(4-ROC6H4)-4,2′:6′,4′′-tpy ligands bind zinc through the two outer pyridine donors. The polymer structures are similar with the n-propyl chain adopting a folded conformation in [{Zn2(OAc)4(3)}n] which allows it to fit in the cavity occupied by methyl or ethyl groups in [{Zn2(OAc)4(1)}n] and [{Zn2(OAc)4(2)}n]. Reaction between 5 and Zn(OAc)2·2H2O gives both the coordination polymer [{2Zn2(OAc)4(5)·2H2O}n] and the discrete complex [Zn2(OAc)4(5)2]. Although the zig-zag form of the polymer chain in [{2Zn2(OAc)4(5)·2H2O}n] mimics those in [{Zn2(OAc)4(L)}n] (L = 1, 2 or 3), packing interactions differ and the wider separation of the chains in a sheet results in the incorporation of water molecules in the lattice. π-Stacking between pyridine rings in [{Zn2(OAc)4(L)}n] (L = 1, 2 or 3) produces infinite assemblies in contrast to isolated tetradecker π-stacks in [{2Zn2(OAc)4(5)·2H2O}n]. This assembly is replicated in [{4Zn2(OAc)4(7)·3H2O}n] (n-heptoxy substituents). In contrast, the n-hexoxy-containing coordination polymer crystallizes with acetic acid in the lattice; [{Zn2(OAc)4(6)·MeCO2H}n] consists of zig-zag polymer chains which π-stack in a manner which is unique among the other polymers. Further lengthening of the alkoxy chain favours the formation of [Zn2(OAc)4(L)2] (L = 8, 9 or 10) which are analogues of [Zn2(OAc)4(5)2]. In each, the 4′-(4-ROC6H4)-4,2′:6′,4′′-tpy ligand is monodentate. The alkoxy chains are in extended (or close to extended) conformations and pack into planar sheets with interdigitated chains. Pockets in the sheets are occupied by methyl groups of {Zn2(OAc)4} units in the adjacent sheet in a ball-and-socket assembly motif. The study shows that coordination polymers [{Zn2(OAc)4(L)}n] in which π-stacking are the dominant interactions are favoured for small alkoxy substituents (ligands 1–3); for ligands 8–10, discrete complexes [Zn2(OAc)4(L)2] in which van der Waals interactions dominate are observed. In the intermediate range (ligands 5–7), the preference between the two structure types appears to be marginal.

A 3-dimensional {42·84} lvt net built from a ditopic bis(3,2′:6′,3″-terpyridine) tecton bearing long alkyl tails

Divergent bis(terpyridine) tectons are versatile ligands for the assembly of coordination networks; we demonstrate the assembly of a 3-dimensional {42·84} lvt net (still relatively sparse among 4-connected nets in metal–organic frameworks) from the reaction of 1,4-bis(n-octoxy)-2,5-bis(3,2′:6′,3″-terpyridin-4′-yl)benzene and Co(NCS)2.

The beneficial effects of trifluoromethyl-substituents on the photoconversion efficiency of copper(I) dyes in dye-sensitized solar cells

The synthesis and characterization of [Cu(2)2][PF6] and [Cu(3)2][PF6] in which 2 = 6,6′-bis(trifluoromethyl)-2,2′-bipyridine and 3 = 6-trifluoromethyl-2,2′-bipyridine are reported. The single crystal structure of [Cu(2)2][PF6] confirms that the copper(I) centre is sterically protected by the four CF3 groups in a near regular tetrahedral environment. The Cu+/Cu2+ oxidation potential is shifted from +0.44 to +0.72 V on going from [Cu(1)2][PF6] to [Cu(2)2][PF6] (1 = 6,6′-dimethyl-2,2′-bipyridine), in keeping with the electron-withdrawing effects of the CF3 groups. 1H and 19F NMR spectroscopic data for a CH2Cl2 solution containing [Cu(1)2][PF6] and [Cu(2)2][PF6] demonstrate that the ligands in [Cu(2)2][PF6] remain highly labile. An on-surface procedure was used to assemble the dyes [Cu(4)(2)]+ and [Cu(4)(3)]+ (4 = anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) on TiO2; solid-state absorption spectra show that dye coverage for [Cu(4)(3)][PF6] exceeds that of [Cu(4)(2)][PF6]. The performances of the dyes in dye-sensitized solar cells (DSCs) are compared with that of [Cu(4)(1)]+. DSCs containing the fluorinated ancillary ligands exhibit photoconversion efficiencies of ≈30–34% relative to N719 set at 100%; this is a significant enhancement with respect to DSCs containing [Cu(4)(1)]+ with no fluoro-groups. Density functional theory (DFT) calculations for the heteroleptic dyes demonstrate that the HOMO is stabilized upon introduction of the CF3 groups, but that the compositions of the molecular orbitals in the HOMO–LUMO manifold are little changed. The improved DSC performance is associated with enhanced short-circuit current density (JSC) for [Cu(4)(3)]+ and [Cu(4)(2)]+ versus [Cu(4)(1)]+; this is also seen in high EQEmax values of 46% for [Cu(4)(2)]+ and 51% for [Cu(4)(3)]+ (both at λmax = 480 nm).

Manipulating connecting nodes through remote alkoxy chain variation in coordination networks with 4′-alkoxy-4,2′:6′,4″-terpyridine linkers

The effects of increasing the length of the alkoxy substituent in 4′-alkoxy-4,2′:6′,4′′-terpyridines when they are combined with cadmium(II) nitrate under conditions of room temperature crystallization and in the same cadmium:ligand (1:3) ratio have been investigated. The divergent ligand 4′-n-propoxy-4,2′:6′,4′′-terpyridine (2) reacts with Cd(NO3)2·4H2O to give [{Cd2(NO3)4(2)3}·3CHCl3]n in which the Cd atoms act as 3-connecting nodes and assemble into a (6,3) net with each ligand 2 linking adjacent Cd atoms. One of the three independent n-propoxy groups nestles into a cleft in the next 2-dimensional sheet; this ‘tail-in-pocket’ interaction restricts the length of the alkyl chain that can be accommodated. Replacing the n-propoxy by an n-pentoxy, n-hexoxy or n-heptoxy substituent results in a switch from a (6,3) to (4,4) net; in [{Cd2(NO3)4(3)4}·3CHCl3]n (3 = 4′-n-pentoxy-4,2′:6′,4′′-terpyridine) and [{Cd2(NO3)4(4)4}·CHCl3·MeOH]n (4 = 4′-n-hexoxy-4,2′:6′,4′′-terpyridine), each Cd atom is a 4-connecting node with trans-nitrato ligands, while in [{Cd(NO3)2(5)2}·2MeOH]n (5 = 4′-n-heptoxy-4,2′:6′,4′′-terpyridine) a cis-arrangement of nitrato ligands is observed. The reaction between Cd(NO3)2·4H2O and 4 was also investigated using a 1:1 ratio of reagents; this leads to the assembly of the 1-dimensional ladder [Cd2(NO3)4(MeOH)(4)3]n in which each Cd atom is a 3-connecting node. In each structure, face-to-face π-stacking of the central pyridine rings or of pyridine/phenyl rings of ligands in adjacent sheets or chains is a primary packing interaction; the role of van der Waals interactions as the chain length increases is discussed. Powder diffraction confirmed that each coordination polymer or network characterized by single crystal X-ray crystallography was representative of the bulk sample. The solid-state emission properties of ligands 2, 3 and 4 and their coordination polymers are reported; the blue emission of the free ligands is red-shifted by up to 59 nm upon formation of the coordination networks, and quantum yields are in the range 11–22%.

Cyanoacrylic- and (1-cyanovinyl)phosphonic acid anchoring ligands for application in copper-based dye-sensitized solar cells

The syntheses and characterization of four new anchoring ligands (2–5) for copper(I) bis(diimine) dyes in dye-sensitized solar cells (DSCs) are reported. Ligands 2 and 3 contain a 6,6′-dimethyl-2,2′-bipyridine copper-binding unit, while 4 and 5 contain a 2,9-dimethyl-1,10-phenanthroline unit; 2 and 4 contain cyanoacrylic acid anchoring units, and 3 and 5 possess (1-cyanovinyl)phosphonic acid anchors. The performance of DSCs sensitized by [Cu(Lanchor)(Lancillary)]+ in which Lanchor is 2–5 and Lancillary is either 6,6′-dimethyl-2,2′-bipyridine (6) or 6-trifluoromethyl-2,2′-bipyridine (7) are compared with those of DSCs containing the dyes [Cu(1)(6)]+ or [Cu(1)(7)]+ where anchoring ligand 1 is the previously reported and well-performing ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid). Among dyes incorporating 2–5, the best performing dye contained anchor 3 (6,6′-Me2bpy/(1-cyanovinyl)phosphonic acid combination). The better performances of dyes containing the bpy-based 2 and 3 compared to the phen-based 4 and 5 are rationalized largely in terms of the greater flexibility of the bpy vs. phen unit, allowing dyes containing 2 and 3 to adopt a conformation that leads to better surface coverage on mesoporous TiO2. Replacing 1 by 3 leads to a small gain in the short-circuit current density (JSC), but dyes with anchor 1 (in place of 3) have enhanced open-circuit voltage (VOC). The results of electrochemical impedance spectroscopy (EIS) support the trends found from the J–V measurements. The EIS data for DSCs with dyes containing anchors 3 or 1 are compared; the latter has a higher recombination resistance and chemical capacitance although the former exhibits a lower transport resistance.

Constructing chiral MOFs by functionalizing 4,2':6',4"-terpyridine with long-chain alkoxy domains: rare examples of neb nets†

Reactions of 4′-(4-nalkyloxyphenyl)-4,2′:6′,4′′-terpyridines (alkyl = hexyl or nonyl) with Co(NCS)2 lead to three structurally characterized chiral 3D assemblies which adopt rare neb topologies. For the nhexyl-functionalized ligands, both enantiomorphic lattices of the neb nets (crystallizing in the tetragonal space groups P41212 and P43212, respectively) are presented.

What a difference a tail makes: 2D → 2D parallel interpenetration of sheets to interpenetrated nbo networks using ditopic-4,2′:6′,4′′-terpyridine ligands

Under conditions of crystal growth by layering at room temperature, 1,4-bis(n-hexoxy)-2,5-bis(4,2′:6′,4′′-terpyridin-4′-yl)benzene (4) or 1,4-bis(n-decoxy)-2,5-bis(4,2′:6′,4′′-terpyridin-4′-yl)benzene (5) reacts with ZnCl2 to yield [Zn2Cl4(4)]n or [Zn2Cl4(5)·2MeOH]n. The compounds crystallize in the C2/c space group with 2-dimensional (4,4) nets which interpenetrate in a 2D → 2D parallel manner. The ligands act as planar 4-connecting nodes linked through the zinc(II) centres. In contrast, 1,4-bis(3-phenylpropoxy)-2,5-bis(4,2′:6′,4′′-terpyridin-4′-yl)benzene (6) reacts with ZnBr2 to give [Zn2Br4(6)·H2O]n which crystallizes in the trigonal R space group with a 3D assembly consisting of 2-fold interpenetrating nbo nets. Ligand 6 acts as a 4-connecting node and the zinc(II) centres are linkers. The pendant phenyl rings in 6 lie over the 4,2′:6′,4′′-tpy domains in an adjacent net and the resulting close association of the interpenetrated nets leads to a highly porous network.