Design of Green-Emitting Salts from Substituted Pyridines: Understanding the Solid-State Photodimerization of trans-1,2-bis(4-pyridyl)ethylene.

Polymorphic salts of trans-1,2-bis(4-pyridyl)ethylene (bpe), 2[bpeH2 ] ⋅ (SO4 )(2HSO4 ) (1) and [bpeH2 ] ⋅ 2HSO4 (2) have been synthesized and their structures determined by X-ray crystallography. The Schmidt postulate predicts that neither of the salts will give rise to photodimerization so they can both potentially be applied as green light emitters. Despite the predictions, 1 undergoes a stereospecific solid-state photodimerization reaction with 100 % yield. This is due to UV induced combination of sliding and pedal-like movement of the pyridyl ring system that influences the alignment of C=C bonds. The sliding motion is restricted in 2. Consequently, the green emission from 1 is completely quenched after photodimerization. It is evident that counter ions play a dominant role in dis- and enabling photodimerization; their degree of protonization and lattice placement are important solvent controlled design parameters towards crystal structures that can act as future light emitters.

Solid-state structural transformations and photoreactivity of 1D-ladder coordination polymers of Pb(II).

An attempt has been made to design double-stranded ladder-like coordination polymers (CPs) of hemidirected Pb(II) . Four CPs, [Pb(µ-bpe)(O2 C-C6 H5 )2 ]⋅2H2 O (1), [Pb2 (µ-bpe)2 (µ-O2 C-C6 H5 )2 (O2 C-C6 H5 )2 ] (2), [Pb2 (µ-bpe)2 (µ-O2 C-p-Tol)2 (O2 C-p-Tol)2 ]⋅ 1.5 H2 O (3) and [Pb2 (µ-bpe)2 (µ-O2 C-m-Tol)2 (O2 C-m-Tol)2 ] (4) (bpe=1,2-bis(4'-pyridyl)ethylene), have been synthesised and investigated for their solid-state photoreactivity. CPs 2-4, having a parallel orientation of bpe molecules in their ladder structures and being bridged by carboxylates, were found to be photoreactive, whereas CP 1 is a linear one-dimensional (1D) CP with guest water molecules aggregating to form a hydrogen-bonded 1D structure. The linear strands of 1 were found to pair up upon eliminating lattice water molecules by heating, which led to the solid-state structural transformation of photostable linear 1D CP 1 into photoreactive ladder CP 2. In the construction of the double-stranded ladder-like structures, the parallel alignment of CC bonds in 2-4 is dictated by the chelating and µ2 -η(2) :η(1) bridging modes of the benzoate and toluate ligands. The role of solvents in the formation of such double-stranded ladder-like structures has also been investigated. A single-crystal-to-single-crystal transformation occurred when 4 was irradiated under UV light to form [Pb2 (rctt-tpcb)(µ-O2 C-m-Tol)2 (O2 C-m-Tol)2 ] (5).

Solid-state photochemical behavior of a triple-stranded ladder coordination polymer.

A triple-stranded ladder coordination polymer, [Pb(3)(bpe)(3)(O(2)CCF(3))(4)(O(2)CCH(3))(2)] [bpe = trans-1,2-bis(4-pyridyl)ethene] undergoes a photochemical [2 + 2] cycloaddition reaction in the solid state to furnish 100% rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb) in two steps via cooperative molecular movements of the partially dimerized products formed initially.

Metal-organic frameworks containing a tetrapyridylcyclobutane ligand derived from isomerization reaction.

The rctt isomer of tetrapyridylcyclobutane prepared from the solid-state [2 + 2] cycloaddition reaction of [bpeH(2)] x 2 CF(3)CO(2) [bpe = 1,2-bis(4-pyridyl)ethene] undergoes isomerization reaction to the rtct isomer in 100% yield, and this tetrahedral tetrapyridyl ligand forms two metal-organic frameworks having ptt and pts topology.

Coordination-Polymeric Nanofibers and their Field-Emission Properties.

The large-scale fabrication of nanofibers of coordination polymers (CPs) is reported for the first time from CP gel precursors by the electrospinning method. In the absence of organic polymeric additives, viscous solutions suitable for drawing fibers have been made by diluting the gel precursors, which are obtained by mixing Ni(OAc)(2) and 4-trifluoromethylbenzoic acid with 4,4'-bis(pyridyl)ethylene (bpe) or 4,4'-bipyridine (bpy) linear spacer ligands in organic solvents. One-dimensional polymeric structures have been proposed for these gels and nanofibers based on the crystal structures of the compounds isolated. These fibers show some interesting field-emission properties. The results described here may well be extended to other one-dimensional polymers, thus opening the door for processing of the coordination polymers.

Photodimerization of a 1D hydrogen-bonded zwitter-ionic lead(II) complex and its isomerization in solution.

A pair of bpe-H(+) ligands in a zwitter-ionic complex undergoes photochemical cycloaddition quantitatively accompanied by proton transfer and the cyclobutane ring isomerizes slowly in solution to two more isomers, catalyzed by CF(3)CO(2)H acid.

Stacking of double bonds for photochemical [2+2] cycloaddition reactions in the solid state.

trans-1,2-Bis(4-pyridyl)ethylene (bpe), containing a C=C bond, has been extensively studied for solid-state photochemical [2+2] cycloaddition reactions, in discrete molecular and metal complexes, hydrogen bonded and coordination polymeric structures. The challenges in orienting a pair or more of bpe molecules in the solid-state using crystal engineering principles, and their photochemical behaviour and implications, based on Schmidt's postulates, are discussed.

Solid-state photochemical [2+2] cycloaddition in a hydrogen-bonded metal complex containing several parallel and crisscross C=C bonds.

One-dimensional hydrogen-bonded complex [Zn(bpe)(2)(H(2)O)(4)](NO(3))(2).8/3 H(2)O.2/3 bpe (1, bpe=4,4'-bipyridylethylene) containing coordination complex cations [Zn(bpe)(2)(H(2)O)(4)](2+) with parallel and crisscross double bonds undergoes photochemical [2+2] cycloaddition in the solid state and produces tetrakis(4-pyridyl)cyclobutane (tpcb) in up to 100 % yield with rctt-tpcb (2a) as major and rtct-tpcb (2b) as minor product. The bpe ligands with crisscross conformation of C=C bonds appear to undergo pedal-like motion prior to photodimerization. Grinding single crystals to powder accelerates the pedal motion of crisscrossed olefins in the bpe ligands to parallel alignment and provides the rctt-cyclobutane stereoisomer 2a quantitatively. In addition, 100 % photodimerization of ground 1 indicates that the free bpe ligands, which are remote from each other in a pool of water, and NO(3)(-) ions moved toward each other to give a mixture of rctt- and rtct-tpcb isomers.