Syntheses, Crystal Structures and Photophysical Aspects of Discrete and Polymeric Azido‐Bridged Zinc(II) and Cadmium(II) Complexes: Sensing Properties and Structural Resemblance

Abstract

The work in this report presents syntheses, characterization, crystal structures and steady state and time resolved photophysical properties of four compounds of composition [ZnII3L12(N3)4] (1), [CdII3L22(N3)4]n (2), [ZnII2L2(N3)3]n (3) and [CdII2L1(N3)3]n (4), where HL1 and HL2 are the Schiff base ligands, obtained on 1:1 condensation of 3‐ethoxysalicylaldehyde (for HL1)/3‐methoxysalicylaldehyde (for HL2) and N,N,2,2‐tetramethyl‐1,3‐propanediamine. Compound 1 is a discrete trizinc(II) system, while the other three compounds are one‐dimensional coordination polymers, where the building units are, respectively, tricadmium(II), dizinc(II) and dicadmium(II) moieties. The nuclearities of the cores in the ZnII–azido compound from HL1 and the CdII–azido compound from HL2 are similar (1 and 2 are trinuclear) and, similarly, the ZnII–azido compound from HL2 and the CdII–azido compound from HL1 are similar (3 and 4 are dinuclear), revealing an unusual structural resemblance of two metal ions of two different rows in the periodic table as the function of slight change in the ligand periphery (OEt in HL1, OMe in HL2). The steady state/time‐resolved photophysical properties reveal that cadmium(II) enhances slightly but zinc(II) enhances appreciably the fluorescence of the Schiff base ligands HL1 and HL2. It has been found that both HL1 and HL2 are fluorogenic sensors of zinc(II).

Molecular Packing Dependent Solid State Fluorescence Response of Supramolecular Metal–Organic Frameworks: Phenoxo-Bridged Trinuclear Zn(II) Centered Schiff Base Complexes with Halides and Pseudohalides

“…Crystallographic data for the complexes are presented in Table 1. Complexes [1, 2, and 4−6] are isostructural, crystallizing in the monoclinic space group C2/ c, while complex [3], BN(CN)2 crystallizes in monoclinic P2 1 / n space group. In [1, 2, 4−6], there are two Zn(II) centers for one ligand and one anion in the asymmetric unit.…”

“…Luminescent compounds are attracting current research interest because of their vast applications in emitting materials for solid state organic light emitting diodes, light harvesting materials for photocatalysis, lasers, fluorescent sensors for organic or inorganic analyses, and as biological materials with explosive mimics. − Zinc Schiff bases complexes have been extensively studied for their interesting photophysical properties. , Spectroscopic experiments suggest that coordination to a Zn­(II) center would influence the molecular aggregation via noncovalent interactions such as electrostatic interactions, stacking, and hydrogen bonding. By modifying these interactions, assembly of new materials with exciting properties may be generated. − For example, the use of “salen” style Schiff base ligands can lead to a variety of supramolecular assemblies that can determine their spectroscopic and photophysical properties in solution in relation to the nature of molecular aggregation.…”

“…1−4 Zinc Schiff bases complexes have been extensively studied for their interesting photophysical properties. 3,5 Spectroscopic experiments suggest that coordination to a Zn(II) center would influence the molecular aggregation via noncovalent interactions 6 such as electrostatic interactions, stacking, and hydrogen bonding. By modifying these interactions, assembly of new materials with exciting properties may be generated.…”

“…The complexes exhibit an anion dependent solid state fluorescence response, which is explained on the basis of differences in their molecular packing. Among all six complexes, [2] and [3] exhibit high and considerable red-shifted fluorescence response in the solid state. The title complexes are the first example of Schiff base complexes demonstrating intriguing molecular packing dependent emission, which is the main attraction of this paper.…”

“…Fluorescence spectra were recorded on a Horiba spectrofluorometer (Fluoromax). Crystal data for complexes [1], [2], [4], and [6] were collected on an Oxford Diffraction X Caliber EoS diffractometer using graphite monochromatized Mo−Kα radiation at 293 K, and data for complexes [3] and [5] were collected on a Bruker Kappa APEXII using D8 X-ray generator, Cryoflex (model Tr-60) Mo−Kα radiation at 200 K. Time-resolved fluorescence intensity decays were collected using a commercial TCSPC setup (Life Spec II, Edinburgh Instruments, U.K.). All samples were excited at 377 nm, and the full width at half-maxima of the instrument response function was about 100 ps.…”