The last two decades have witnessed the research activities in the area of coordination polymers (CPs), which are structurally diverse and functionally intriguing materials. In this endeavor, the most exploited ligand has been a structurally rigid N-donor compound having an innocent backbone (incapable of forming hydrogen bond) namely 4,4'-bipyridine. Much has been achieved by exploiting this wonder ligand in the area of CPs. However, the positional isomers such as 3,3'-bipyridine or 4,3'-bipyridine (which understandably induce diverse ligating topology as compared to their more symmetrical 4,4' counterpart) were not exploited in much detail presumably because of the difficulty in their synthetic accessibility. To get access to such N-donor ditopic ligands having diverse ligating topology, much efforts have been focused in the last decade or so to design such positional isomers of 4,4'-bipyridine having a non-innocent backbone (capable of forming hydrogen bond). The principal focus of such studies is to decipher the effect of diverse ligating topology and the non-innocent backbone of the ligands on the overall supramolecular structures and functions of the resultant CPs. This tutorial review aims at highlighting some of the developments of such structurally diverse and functionally intriguing CPs derived from N-donor ditopic ligands having a non-innocent backbone.

A crystal engineering based design strategy for metallogels has been demonstrated in a series of mixed ligand based CuII/CoII coordination polymers derived from two bis-pyridyl-bis-amide ligands namely N,N′-bis-(3-pyridyl)isophthalamide (L1) and N,N′-bis-(3-pyridyl)terephthalamide (L2) and various dicarboxylates. These mixed ligand coordination polymers are shown to form gels as well as X-ray quality crystals under different conditions. The crystal structures of the gel fibers in the xerogels have been determined by using powder and single-crystal X-ray diffraction data. The results support the proof of the concept based on which the metallogels are designed.

A new dibenzo[24]crown-8 derivative (1) was synthesized and functionalized with aromatic moieties such as naphthalene and coumarin units. These two fluorophores are known to form an effective FRET (Forster resonance energy transfer) pair, and this formed the basis for the design of this host crown ether derivative. Results of the steady-state and time-resolved fluorescence studies confirmed the resonance energy transfer between the donor naphthalene moiety and acceptor coumarin fragment, while NMR spectra and computational studies support a folded conformation for the uncomplexed crown ether 1. This was found to form an inclusion complex, a [2]pseudorotaxane type with imidazolium ion derivatives as the guest molecules with varying alkyl chain lengths ([C(4)mim](+) or [C(10)mim](+)). The host crown ether (1) tends to adopt an open conformation on formation of the interwoven inclusion complex (1·[C(4)mim](+) or 1·[C(10)mim](+)). This change in conformation, from the folded to a open one, was predicted by computational as well as (1)H NMR studies and was confirmed by single crystal X-ray structure for one (1·[C(4)mim](+)) of the two inclusion complexes. The increase in the effective distance between the naphthalene and coumarin moieties in the open conformation of these inclusion complexes was also supported by the decrease in the effective FRET process that was operational between naphthalene and coumarin moieties in the free molecule (1). Importantly, this inclusion complex formation was found to be reversible, and in the presence of a stronger base/polar solvent, such as triethyl amine/DMSO, the deprotonation/effective solvation of the cationic imidizolium ions ([C(4)mim](+) or [C(10)mim](+)) resulted in decomplexation or dethreading with restoration of the original emission spectra for 1, which signifies the subsequent increase in the FRET process. Thus we could demonstrate that a molecular folding-unfolding type of movement in the crown ether derivative could be induced by chemical input as an imidazolium ion.

Following the supramolecular synthon approach, a combinatorial library comprising 35 organic salts derived from 7 dicarboxylic acids (malonic-, succinic-, adipic-, L-tartaric-, maleic-, phthalic-, and isophthalicacid) and 5 primaryalkyl amines Me-(CH2)n-NH2 (n = 11-15) was prepared and scanned for gelation. About 66% of the salts in the combinatorial library were found to show moderate to good gelling ability in various polar and nonpolar solvents including commercial fuels such as petrol. The majority of the salts having a rigid, unsaturated anionic backbone (maleate, phthalate, and isophthalate) did not show gelation; only the corresponding hexadecylammonium salts showed gelation. Some of the representative gels were characterized by rheology, small-angle neutron scattering (SANS), optical microscopy (OM), and scanning electron microscopy (SEM). Single-crystal structures of two gelator and two nongelator salts were also discussed in the context of supramolecular synthon and structure-property correlation.

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