Ligand-centered Charge Transfer Research Articles

Biological Evaluation of Platinum(II) Sulfonamido Complexes: Synthesis, Characterization, Cytotoxicity, and Biological Imaging.

Platinum-based compounds are actively used in clinical trials as anticancer agents. In this study, two novel platinum complexes, (C1 = [PtCl2(N(SO2quin)dpa)], C2 = [PtCl2(N(SO2azobenz)dpa)]) containing quinoline and azobenzene appended dipicolylamine sulfonamide ligands were synthesized in good yield. The singlet attributable to methylene CH2 protons of the ligands of C1 and C2 appears as two doublets in 1H NMR spectra, which confirms the presence of magnetically nonequivalent protons upon coordination to platinum. Structural data of N(SO2quin)dpa (L1), N(SO2azobenz)dpa (L2) and PtCl2(N(SO2quin)dpa) confirmed the formation of the desired compounds. Time-dependent density functional theory calculations suggested that the excitation of L1 show quin-unit-based π⟶π∗ excitations (i.e., ligand-centered charge transfer, LC), while C1 shows the metal-ligand-to-ligand charge-transfer (MLLCT) character. L1 displays intense fluorescence from the 1LC excited state, while C1 gives phosphorescence from the 3LC state. Mammalian cell toxicity of ligands and complexes was assessed with NCI–H292 nonsmall-cell lung cancer cells. Further, C1 and C2 showed significantly low IC50 values compared with N(SO2azobenz)dpa and PtCl2(N(SO2quin)dpa). Fluorescence imaging data of both ligands and complexes revealed the potential fluorescence activity of these compounds for biological imaging. All four compounds are promising novel candidates that can be further investigated on their usage as potential anticancer agents and cancer cell imaging agents.

Unraveling the Effect of Single Atom Doping on the Carrier Relaxation Dynamics of MAg24n- Nanoclusters.

Precisely doped metal nanoclusters (NCs) are currently emerging nanomaterials for their unique photophysical properties. Here, we report the influence of single atom doping on the excited state relaxation dynamics of a series of MAg24(2,4-Me2PhS)18n- NCs where M is Ag, Au, Pd, and Pt. The NCs with a group 11 metal (Ag and Au) as central atoms exhibit dual emission at NIR and visible range, whereas it shows only NIR emission for group 10 metal (Pd and Pt) doped NCs. Global target analyses of transient absorption (TA) data reveal the three-state relaxation, i.e., initially excited state (Sn), ligand-centered charge transfer (CT) state (SL), and metal-centered lowest excited state (S1). Apart from the HOMO-LUMO (H-L) energy gap, the electron affinity of the central metal atom and rigidity of the NC structural framework influence the relaxation processes of the NCs. The extensive study into the relaxation dynamics will bestow the single atomic level modulation of photophysical properties.

Structure and reversible crystal-to-crystal transformations of a zinc(II) coordination polymer constructed from an imide-based dicarboxylic acid

An imide-based O-donor ligand, 4-carboxyphenyl-1,3-dioxoisoindoline-5-carboxylic acid (H2In-4-ba), and one Zn(II) coordination polymer, [Zn(In-4-ba)(py)2]n (1, py ​= ​pyridine), have been synthesized. Single-crystal X-ray diffraction indicates that 1 has a quasi-31 helical chain structure, which shows one-dimensional to two-dimensional (1D → 2D) extension to be a homochiral layer of consecutively interweaving poly-helices with the same helicity. Coexistence of such homochiral layers in opposite chirality makes individual crystal of 1 to be in a racemic manner. Thermogravimetric (TG) analysis indicated that 1 shows high thermal stability approaching 440 ​°C. Thermal activation of 1 gives a desolvated phase, [Zn(In-4-ba)]n (1′), as supported by TG trace. Of particular note, reversible crystal-to-crystal transformations between solvated 1 and desolvated 1′ have been achieved upon decoordination and recoordination of the py ligands, as evidenced by XRPD patterns, IR spectra, and TG diagrams. The photoluminescence properties of as-synthesized 1, desolvated 1′, and resolvated 1′/py have also been studied. All of them exhibit an emission band centered at 486, 492, and 486 ​nm, respectively, which is originated from the ligand-centered charge transfer. Further, the emission intensity of 1′ is weaker than that of 1 and 1′/py, implying the influence of coordinated py ligands and/or structure regulation on photoluminescence.

Crystal Structures, Optical Properties, and TD-DFT Study of a Zinc(II) Schiff-Base Complex Derived from Salicylaldehyde and N1-(3-aminopropyl)Propane-1,3-Diamine

A zinc(II) complex of the pentadentate Schiff base ligand 2,2′(((azanediylbis(propane-3,1-diyl))bis(azanylylidene))bis(methanylylidene))diphenol was synthesized by reaction of the ligand with Zn(OAc)2. Reaction of salicylaldehyde with N1-(3-aminopropyl)propane-1,3-diamine [HN(C3H6NH2)2] in absolute ethanol was used to prepare the ligand. Non-merohedrally twinned crystals of the complex were characterized by single-crystal XRD and found to be in the monoclinic system, space group P21/n, with the following lattice parameters: a = 14.6594(8) A, b = 11.6908(7) A, c = 23.2789(14) A, and β = 105.520(3)°. Luminescent characteristics of this complex were investigated in the solid state. Computational study of the electronic properties of this complex with the aid of time-dependent density functional theory (TD-DFT) showed good agreement with experimental data and hence consolidated the assignment of the state as ligand centered charge transfer. This complex exhibits a strong fluorescence with a broad blue emission maximum centered at 442 nm.