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b S Supporting Information ' INTRODUCTION Phenylacetylene (PA, ethynylbenzene, melting point À48 °C), a simple aromatic compound with weak hydrogen bond donors (C sp2 ÀH and C sp ÀH) and acceptors (aromatic, CtC) invokes interest because of its three known polymorphs, namely, the Rform reported by Weiss et al., 1 the β-form reported by Katrusiak and co-workers, 2 and the γ-form reported recently by us. 3 These trimorphs are obtained under different experimental conditions, the R with normal cooling of the liquid in a capillary, the β by application of pressure on the liquid in a diamond anvil cell, and the γ by quenching the compressed liquid with liquid N 2 . The Rand β-forms are "nearly isostructural" with the same space group P1, and they share a common packing feature, namely, the CtCÀH 3 3 3 π (ethynyl/phenyl) cyclic tetramer (Scheme 2b,c). The γ-form takes the monoclinic space group C2/c with Z 0 = 6. All three PA polymorphs are characterized by a number of T-shaped CtCÀH 3 3 3 π(ethynyl) and CtCÀH 3 3 3 π(aryl) contacts which are particularly common in this group of compounds. These weak interactions develop supramolecular synthons, 4 for example, the cyclic tetramer found in the Rand β-forms or the CÀH 3 3 3 π(ethynyl) zigzag chain found in the γ-form.Hydrogen-bonded complexes of phenylacetylene with a variety of solvent molecules, such as argon, water, ammonia, alcohols, amines, and N-heterocyclic aromatic molecules, have also been investigated using electronic and vibrational spectroscopic techniques in combination with high-level ab initio and density functional theory calculations. 5 In contrast to other

A fluorescence turn-on receptor based on triazole linked calix[4]arene (L) for selective recognition of Zn(2+) in aqueous-methanolic HEPES buffer has been developed and showed its utility for sensing Zn(2+) in blood serum milieu.

This study presents unambiguous experimental evidence in support of the highly debated "halogen bond donor" character of organic fluorine. Two examples of intermolecular Cl•••F contacts, with F-atom as halogen bond acceptor and donor, have been analyzed by in situ cryocrystallography and theoretical charge density studies.

A third polymorph of phenylacetylene (Z 0 = 6), which is obtained by quenching the liquid, shows some similarities to the two earlier known forms, and the adoption of a high Z 0 value in this crystal form is discussed. This communication reports the third polymorph of phenylacetylene, which has Z 0 =6. Weiss et al. reported the first crystal structure of phenylacetylene (R-form; Z 0 =2.5, P1). 1 The structure contains two ordered full molecules and a half molecule disordered across an inversion center. Of the three symmetry independent ethynyl groups, one of the H-atoms points toward a triple bond and the two others to the π-systems of neighboring aromatic rings. Katrusiak and co-workers isolated a second polymorph (β-form) of the compound by application of pressure on the liquid in a diamond anvil cell. 2 These workers described their structure as being "nearly isostructural" to the R-form. The space group remains the same, but the c parameter is increased by 20% and Z 0 increases concomitantly to 3. The two disordered molecules in the R-form are replaced by two ordered molecules that are related by a center of inversion at a distance of 1.6 A ˚from each of the phenyl rings. All the other molecules are in the same relative orientations in Rand β-forms. The structure of the β-form can be understood as arising from a lengthening of the c-axis in the R-form so that there is enough space created for the disordered molecule to be replaced by two ordered molecules related by an inversion center.The new polymorph of phenylacetylene (γ-form) was obtained by us by quenching the compressed liquid. The neat liquid was taken in a Lindemann capillary tube and sealed. This sealing (from both ends, decreasing the length of the air gap in the capillary) produces the compression. The sealed capillary was maintained in a hot water bath at around 90-95 °C. The capillary was quickly inserted into liquid N 2 . This procedure mostly resulted in a breaking of the capillary. Occasionally, the capillary did not break and the quenched mass inside was almost completely the γ-form. Slight changes in the experimental conditions produced the two other forms. For example, if the capillary (sealed without compression) was heated only until about 60-75 °C in the water bath and then quenched suddenly, the β-form resulted. The R-form was produced when the sample was cooled under controlled conditions without a preheating in the water bath. It was not easy to obtain the γ-form. Careful analysis of the reciprocal lattice images was necessary in order to identify the domains of the new form. The γ-form takes the monoclinic space group C2/c with Z 0 = 6. The crystal packings in all three phenylacetylene polymorphs are characterized by a number of T-shaped tC;H 3 3 3 π (ethynyl) and tC;H 3 3 3 π (aryl) contacts 3-5 (Scheme 1). These contacts are particularly common in this group of compounds. These weak tC;H 3 3 3 π interactions form supramolecular synthons, 6 for example the cyclic tetramer (Scheme 1b) found in the Rand β-forms or the tC;H 3 3 3 π (e...

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants as well as well-known carcinogens. Therefore, it is important to develop an effective receptor for the detection and quantification of such molecules in solution. In view of this, a 1,3-dinaphthalimide derivative of calix[4]arene (L) has been synthesized and characterized, and the structure has been established by single crystal XRD. In the crystal lattice, intermolecular arm-to-arm π···π overlap dominates and thus L becomes a promising receptor for providing interactions with the aromatic species in solution, which can be monitored by following the changes that occur in its fluorescence and absorption spectra. On the basis of the solution studies carried out with about 17 derivatives of the aromatic guest molecular systems, it may be concluded that the changes that occur in the fluorescence intensity seem to be proportional to the number of aromatic rings present and thus proportional to the extent of π···π interaction present between the naphthalimide moieties and the aromatic portion of the guest molecule. Though the nonaromatic portion of the guest species affects the fluorescence quenching, the trend is still based on the number of rings present in these. Four guest aldehydes are bound to L with K(ass) of 2000-6000 M(-1) and their minimum detection limit is in the range of 8-35 μM. The crystal structure of a naphthaldehyde complex, L.2b, exhibits intermolecular arm-to-arm as well as arm-to-naphthaldehyde π···π interactions. Molecular dynamics studies of L carried out in the presence of aromatic aldehydes under vacuum as well as in acetonitrile resulted in exhibiting interactions observed in the solid state and hence the changes observed in the fluorescence and absorption spectra are attributable for such interactions. Complex formation has also been delineated through ESI MS studies. Thus L is a promising receptor that can recognize PAHs by providing spectral changes proportional to the aromatic conjugation of the guest and the extent of aromatic π···π interactions present between L and the guest.

The terms phase transformation, polymorphism, disorder, isosterism, and isostructuralism are often the keywords used in the design and engineering of molecular crystals. Three benzoylcarvacryl thiourea derivatives with [-NH–C(S)–NH–C(O)-] cores generate molecular crystals, which provide the basis for exploring a common link between the structures related by aforementioned terms. The apparent “origin” of all these structural modifications has been traced to the formation of a planar molecular dimeric chain built with homomeric R2 2(12) and R2 2(8) synthons occurring in tandem, one formed with N–H···O and the other with N–H···S hydrogen bonds.

The work carried out by our research group over the last couple of decades in the context of quantitative crystal engineering involves the analysis of intermolecular interactions such as carbon (tetrel) bonding, pnicogen bonding, chalcogen bonding, and halogen bonding using experimental charge density methodology is reviewed. The focus is to extract electron density distribution in the intermolecular space and to obtain guidelines to evaluate the strength and directionality of such interactions towards the design of molecular crystals with desired properties. Following the early studies on halogen bonding interactions, several “sigma-hole” interaction types with similar electrostatic origins have been explored in recent times for their strength, origin, and structural consequences. These include interactions such as carbon (tetrel) bonding, pnicogen bonding, chalcogen bonding, and halogen bonding. Experimental X-ray charge density analysis has proved to be a powerful tool in unraveling the strength and electronic origin of such interactions, providing insights beyond the theoretical estimates from gas-phase molecular dimer calculations. In this mini-review, we outline some selected contributions from the X-ray charge density studies to the field of non-covalent interactions (NCIs) involving elements of the groups 14–17 of the periodic table. Quantitative insights into the nature of these interactions obtained from the experimental electron density distribution and subsequent topological analysis by the quantum theory of atoms in molecules (QTAIM) have been discussed. A few notable examples of weak interactions have been presented in terms of their experimental charge density features. These examples reveal not only the strength and beauty of X-ray charge density multipole modeling as an advanced structural chemistry tool but also its utility in providing experimental benchmarks for the theoretical studies of weak interactions in crystals.