Intraligand Charge Transfer Transitions Research Articles

Synthesis, characterization, luminescence and NLO properties of heterotrinuclear MII-RuII-MII (M=Ni/Zn) complexes composed of coordination and organometallic sites

A new series of heterotrinuclear complexes of the type [Ru(dppm)2((CCC6H4CCC6H3 (O)HCNC6H4X)Ni(L'))2] (1a-c) and [Ru(dppm)2((CCC6H4CCC6H3(O)HCNC6H4X)Zn(L'))2] (2a-c) were prepared by the reaction of trans-[Ru(dppm)2(CCC6H4CCC6H3(OH)HCNC6H4X)2] with Ni(II)/Zn(II) ions in presence of 8-hydroxyquinoline. (where, X= Cl, Br, I; L'= 8-hydroxyquinoline) All the complexes were characterized by elemental analysis, FT-IR, 1H NMR, ESI-MS, UV–visible and thermogravimetric studies. The electrochemical properties of all the complexes indicate quasireversible redox behaviour corresponding to Ru(II)-Ru(III) and Ni(II)/ Ni(III) for 1a-c and Ru(II)/Ru(III) for 2a-c and are susceptible to variation of substituent group on ligand. All complexes show room temperature luminescence corresponding to π→π* intraligand charge transfer (ILCT) transition. The emission wavelength of the complexes is finely tuned by increasing π-conjugation and variation of substituent groups in the complexes. The second harmonic generation (SHG) efficiency of the complexes was measured by Kurtz-powder technique indicating that all complexes displayed the second harmonic generation (SHG) property.

Synthesis, characterization, and in vitro antibacterial and cytotoxic study of Co(II), Ni(II), Cu(II), and Zn(II) complexes of N-(4-methoxybenzyl) N-(phenylethyl) dithiocarbamate ligand

Four new homoleptic complexes of Co(II) [C-1], Ni(II) [C-2], Cu(II) [C-3], and Zn(II) [C-4] derived from N-(4-methoxybenzyl) N-(phenylethyl) dithiocarbamate ligand (KL) with general formula ML2 were synthesized with good yield and characterized by elemental analysis and different spectroscopic techniques like FTIR, 1H and 13C NMR, HRMS, UV–vis and fluorescence spectroscopy. The crystal structure of the complex C-4 was determined by the single crystal X-ray diffraction technique (SC-XRD), which shows a distorted square pyramidal geometry around the Zn(II) ion. Thermogravimetric studies of all four complexes were performed and found that decomposition occurred in three steps and the final product of the thermal decomposition was metal sulfides. The magnetic properties of cobalt, nickel, and copper complexes were analyzed by a vibrating sample magnetometer (VSM) and the hysteresis of magnetization showed the paramagnetic nature of cobalt and copper complexes while the diamagnetic nature of the nickel complex. Electronic spectral studies of the ligand and all four complexes were recorded in methanolic solution and the spectra show the absorption for the ligand at 301 nm and the absorption band for complexes was observed in the range 265-275 nm which is due to intra-ligand charge transfer (ILCT) transitions. Complexes C-1, C-2, and C-3 showed transitions in the range of 325-445 nm due to ligand-to-metal charge transfer (LMCT) transitions. Anti-bacterial studies of all compounds were performed against Bacillus subtilis, Staphylococcus aureus, Clostridium sporogenes, and Escherichia coli and the result showed that complexes C-2 and C-4 have significant anti-bacterial properties. The cytotoxicity study of all compounds against breast cancer cell line MCF-7 shows that complex C-2 has a potential anti-cancerous property with the least IC50 value against normal cell line HEK-293.

Structures, photophysical properties, and optical sensing of zinc– and copper–salicyaldimine complexes

Metal–salicyaldimine complexes [M(sal-3-py)2] (1, M = Zn; 2, M = Cu), where Hsal-3-py = 3-(salicylideneimino)pyridine, have been synthesized. Nuclear magnetic resonance (NMR) spectroscopy and mass spectroscopy (MS) indicated that 1 and 2 both have a ML2-type discrete molecular structure in the solution and amorphous phases. Single-crystal X-ray diffraction analysis and X-ray powder diffraction (XRPD), however, proven that 1 and 2 both possess an extended two-dimensional sql layer net in the crystalline phase. The electronic absorption spectra of 1 and 2 in MeOH exhibited intense intraligand charge transfer (ILCT) transitions in the UV region (200–340 nm) and metal–ligand charge transfer (CT) transition in the visible region (380–400 nm). The emission spectra of 1 displayed a cyan fluorescence band at 480 nm in solid-state and a green fluorescence band at 502 nm in MeOH solution. Optical detection of 1 in MeOH toward Cu2+, Al3+, and Cr3+ ions showed remarkable changes in electronic absorption spectra and ON–OFF–ON fluorescence color change from green to blue light emission. The sensing processes involves two stages, i.e., direct central metal displacement for Cu2+ detection and analyte-induced decomplexation for Al3+/Cr3+ detection, and subsequently further recognition through the 3-pyridyl functions of the salicyaldimine ligands. Further, 1 displayed remarkable fluorescence color change from green light in MeOH to cyan light in H2O, being potential candidate as H2O probe in methanol.

Rational design of efficient photosensitizers based on cyclometalated iridium(III) complexes with 2-arylbenzimidazole and aromatic 1,3-diketone ligands.

A judicious selection of substituents in cyclometalating 2-arylbenzimidazoles and an ancillary aromatic 1,3-diketone enabled the creation of heteroleptic iridium(III) complexes demonstrating strong light absorption up to 500 nm (ε ≈ 10 000-12 000 M-1 cm-1). The complexes, which were studied by various spectroscopic techniques, single-crystal X-ray diffraction and cyclic voltammetry, displayed tunable absorption maxima depending on the nature of substituents and their positions. The experimental study was corroborated by quantum chemical calculations, which showed an increased contribution of intraligand charge transfer transitions to the visible light absorption in the case of complexes containing electron-withdrawing substituents in the ligands. Despite being of high intensity, some of these transitions are responsible for the formation of the excited states located at large distances from the 'anchoring' fragment incorporated in the ancillary ligand. In turn, incorporation of electron-donating substituents at the para-position to the Ir-C bonds increases the number of excited states located on the ancillary ligand. The destabilization of the HOMO, which is caused by the increase in the electron-donating ability of the substituents in the metalated rings, translated into negative shifts of the Ir4+/Ir3+ redox potential, affecting, in some cases, the degree of electrochemical reversibility of the complexes. Several complexes having strong light-harvesting characteristics and undergoing reversible oxidation in the appropriate potential range were used for coating the TiO2 photoanodes, which reached an efficiency of 2.15% upon irradiation with the standard AM 1.5 spectrum.

Zinc– and Copper–Salicyaldimine complexes: Simultaneous observation of both metal–ligand coordination and weak CH⋯N contact about a single N-donor and the transmetallation reactions

Two salicyaldimine ligands, HsalCl-3-py and HsalOMe-3-py, functionalized with the pyridyl group on the imine side and their Zn and Cu complexes with a general formula [ML2], where M ​= ​Zn, L ​= ​salCl-3-py (1), salOMe-3-py (2); M ​= ​Cu, L ​= ​salCl-3-py (3), salOMe-3-py (4), have been synthesized and characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS), infrared (IR) spectroscopy, elemental analysis (EA), scanning electron microscope (SEM) and further by single-crystal X-ray diffraction analysis and X-ray powder diffraction (XRPD). The crystal structures of Zn– and Cu–salicyaldimine complexes exhibited that the M(II) center is coordinated by two N,O-chelated salicyaldimine ligands in a square-planar geometry, and two further pyridyl donating groups from two neighboring molecules at the axial positions, resulted in self-aggregation to form a two-dimensional (4,4)-sheet. Interestingly, apart from the strong M–N(pyridyl) coordination bond, the weak CH⋯N(pyridyl) hydrogen bonding interactions are simultaneously formed between the pyridyl N-donor and one pyridyl hydrogen of the equatorial salicyaldimine ligand. The electronic absorption spectra of the Zn– and Cu–salicyaldimine complexes in methanol solutions showed the appearance of a metal-to-ligand charge transfer absorption other than the intraligand charge transfer transitions after the complexation of azomethine (–CN–) with M(II) center. On the other hand, salicyaldimine ligands and Cu–salicyaldimine complexes showed no or weak fluorescence emissions. By contrast, Zn–salicyaldimine complexes emitted intense cyan and green light fluorescence in both solid-state and methanol solutions. Noteworthy, transmetallation of Zn–salicyaldimine complexes with Cu2+ ions would be achieved, producing corresponding Cu–salicyaldimine complexes. This implies the stronger binding ability of Cu2+ than that of Zn2+ to salicyaldimine ligands.

Synthesis, Characterization, and the N Atom Transfer Reactivity of a Nitridochromium(V) Complex Stabilized by a Corrolato Ligand.

Metal complexes bearing nitrido ligands (M≡N) are at the forefront of current scientific research due to their resemblances with the metal complexes involved in the nitrogen fixation reactions. An oxo(corrolato)chromium(V) complex was used as a precursor complex for the facile synthesis of a new nitrido(corrolato)chromium(V) complex. The nitrido(corrolato)chromium(V) complex was characterized by various spectroscopic techniques. Density functional theory (DFT) calculations were performed on the nitrido(corrolato)chromium(V) complex to assign the vibrational and electronic transitions of this complex. The chromium–nitrogen (nitrido) bond distance obtained in the DFT-optimized structure is 1.530 Å and matches well with the earlier reported authentic Cr≡N bond distances obtained from the single-crystal X-ray diffraction data. This nitrido(corrolato)chromium(V) compound exhibited a sharp Soret band at 438 nm and a Q band at 608 nm. DFT calculations deliver that the origin of the bands at 438 and 608 nm is due to the intraligand charge transfer transitions. The nitrido(corrolato)chromium(V) complex showed one reversible oxidation and one reversible reduction couple at +0.53 and −0.06 V, respectively, vs the Ag/AgCl reference electrode. The simulation of the electron paramagnetic resonance data of the nitrido(corrolato)chromium(V) compound provided the following parameters: giso = 1.987, A53Cr = 26 G, and A14N = 2.71 G. From all these analyses, we can conclude that the electronic configuration in the native state of nitrido(corrolato)chromium(V) can be best described as [(cor3–)CrV(N3–)]−. Reactions of nitrido(corrolato)chromium(V) with the chloro(porphyrinato)chromium(III) complex resulted in a complete intermetal N atom transfer reaction between chromium corrole and chromium porphyrin complexes. A second-order rate constant of 4.29 ± 0.10 M–1 s–1 was obtained for this reaction. It was also proposed that this reaction proceeds via a bimetallic μ-nitrido intermediate.

Nickel (II), Copper (II), and Zinc (II) Complexes of N-bis (4-methoxybenzyl) Dithiocarbamate: Synthesis, Characterization Studies, and Evaluation of Antitumor Activity

Three new complexes with the general formula [M(L)2] (L=N-bis(4-methoxybenzyl) dithiocarbamate; M= Ni(II) 1, Cu(II) 2, and Zn(II) 3 ), have been synthesized, and characterized by elemental analysis, IR, UV–visible, 1H, 13C NMR and mass spectrometry. The magnetic behavior of nickel and copper complexes was measured and found to be diamagnetic and paramagnetic, respectively. Complex 3 crystallized in the monoclinic space group P21/c and possessed a centrosymmetric dimeric structure stabilized by weak C-H···S, C-H···π, and C-H…O intermolecular interactions. Electronic absorption spectral studies of the complexes show the absorption band in the range of 270-331 nm, which is due to intraligand charge transfer transitions. Complexes 1 and 2 also show transitions in the range of 325-475 nm due to ligand to metal charge transfer transition. The antitumor activities of the complexes have been screened, and the results obtained show that the metal complexes have significant antitumor activity against Dalton's lymphoma cells in a concentration-dependent manner, and the metal complexes were relatively tolerant to normal human cells with low or marginal toxicity against both lymphocytes and monocytes.

Structural and theoretical studies on copper(II) and zinc(II) complexes with a 9-anthraldehyde-derived thiosemicarbazone

Complexes [Cu(ATPh)2]·H2O (1) and [Zn(ATPh)2] (2) were obtained with 9-anthraldehyde-N(3)-phenyl thiosemicarbazone (HATPh). The g|| ≈ 2.15 and A|| ≈ 180 G EPR parameters of complex (1) are compatible with two nitrogen and two sulfur donor atoms in a nearly square planar geometry. The first excited state of complex (1) is not emissive due to fluorescence quenching by the paramagnetic copper(II) ion. The crystal structure of [Zn(ATPh)2]·2DMF (2a) shows that in 2a the anionic thiosemicarbazone ligands are attached to the metal through the NS chelating system in a distorted tetrahedral arrangement. Computational studies revealed that the S1 → S0 emission of complex (2) is attributed to anthracene, the emission band involving two electronic transitions centered at the anthracene rings. The HOMO-1 → LUMO transition is characterized as a ligand-to-ligand charge transfer (LLCT) between anthracene groups, which contributes with 12% and an intra-ligand charge transfer transition (ILCT) centered in the anthracene group of one of the ligands, which contributes with 77% to the emission band. The emission intensity of complex (2) is lower than that of the free thiosemicarbazone, but the presence of two ligand molecules in the complex increases the fluorescence efficiency and the computed fluorescence rate constant for 2 is ∼ 600-fold higher than that of the free ligand.

In Situ Assembled Polynuclear Zinc Oxo Clusters Using Modified Schiff Bases as Ligands.

A series of different cores and nuclearity zinc metal clusters 1–5 have been synthesized using Zn(ClO4)2·6H2O, Schiff-base primary ligands, and dibenzoyl methane (DBM) or monoethanolamine (MEA) as co-ligand in a room-temperature reaction. The structure of the complexes is characterized using single-crystal X-ray diffraction. Among them, (1) [Zn(L1)(DBM)] is mononuclear; (2) [Zn4(L2)2(DBM)4], (3) [Zn4(L2)4(H2O)2(ClO4)2]·2CH2Cl2, and (4) [Zn4(L3)2(DBM)4] have a cubane core; and (5) [Zn4(L4)4(MEA)2(ClO4)2] has a ladderlike core structure. Compounds 1–5 have also been characterized using UV–vis absorption and emission spectroscopies. For an in-depth understanding of the absorption spectra of 1 and 3, density functional theory (DFT) calculations have been performed, which suggest that the transitions correspond to the π → π* intraligand charge transfer (ILCT) transitions.

In-Depth Studies of Ground- and Excited-State Properties of Re(I) Carbonyl Complexes Bearing 2,2':6',2″-Terpyridine and 2,6-Bis(pyrazin-2-yl)pyridine Coupled with π-Conjugated Aryl Chromophores.

In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3ILaryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-functionalized ligands were found to have predominant 3ILpyrene/3ILCTpyrene→terpy character. The 3IL/3ILCT nature of the lowest energy excited state of [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3MLCT and 3IL/3ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO)3(4′-(1-naphthyl)-terpy-κ2N)] and [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] were determined by femtosecond transient absorption studies.

Three coordination polymers as a multi-responsive luminescent probe for the detection of Fe3+, Cr2O72− and antibiotic in aqueous media

Three coordination polymers (CPs), namely {[Co(L)(dpa)]·H2O}n (1), {[Zn(L)2(dpb)]}n (2) and {[Zn(L)(dpe)]}n (3), have been assembled through a dual-ligand strategy of a semi-rigid dicarboxylic acid (H2L = 1,4-bis(4-phenoxy)benzenedicarboxylic acid) with the help of different N-donor co-ligands (dpa = di(pyridin-4-yl)amine, dpb = 1,4-di(pyridin-4-yl)benzene), dpe = trans-1,2-di(4-pyridyl)ethylene). Single crystal X-ray diffraction analyses reveals that compound 1 is a 4-fold interpenetrating 2D sql network and compound 2 is a 2-fold interpenetrating 2D sql network, although the final structures are different because of various inter-layer stacking modes. Compound 3 is a 8-fold interpenetrating 3D dia framework containing an octahedral cage. A photoluminescence investigation shows that compounds 2 and 3 exhibit strong fluorescence originating from the intra-ligand charge transfer transitions. Benefiting from its chemical stability and luminescent properties, compound 2 possesses multi-functional fluorescent responses toward the Fe3+ ion, the Cr2O72− ion and the NZF antibiotic in aqueous medium. The KSV values of compound 2 are 4.68 × 104 M−1 for the Fe3+ ion, 7.23 × 104 M−1 for the Cr2O72− anion, and 1.03 × 104 M−1 for the NZF antibiotic. Additionally, the possible mechanism for the luminescence quenching has been investigated.

Structural Diversity of Lanthanide 3-Nitrotrispyrazolylborates: Tunable Nuclearity and Intra-Ligand Charge Transfer Sensitization of Visible and NIR Ln3+ Emission.

Reported are the syntheses, crystal structures, and photophysical properties of 28, novel lanthanide compounds across five structural types, [Ln(3-NO2Tp)2(NO3)] (1-Ln, Ln = La-Tm, except Pm), [Bu4N][Ln(3-NO2Tp)(NO3)3] (2-Ln, Ln = Yb, Lu), [Eu(3-NO2Tp)2Cl(H2O)]·2iPrOH (3-Eu), [{Ln(3-NO2Tp)2}2(μ2-CO3)]·MeOH (4-Ln, Ln = La-Gd, except Pm), and [{Ln(3-NO2Tp)}4(μ2-OMe)6(μ4-O)] (5-Ln, Ln = Pr-Tb, except Pm) with the 3-nitrotrispyrazolylborate (3-NO2Tp-) ligand. The reaction of methanol or isopropanol solutions of LnX3 (X = Cl, NO3) with the tetrabutyl ammonium salt of the flexidentate 3-NO2Tp- ([Bu4N][3-NO2Tp]) yields Ln(3-NO2Tp)x complexes of various nuclearities as either monomers (1-Ln, 2-Ln, 3-Eu), dimers (4-Ln), or tetramers (5-Ln) owing to the efficient conversion of atmospheric CO2 to CO32- (dimers) or ligand controlled solvolysis of lanthanide ions (tetramers). 3-NO2Tp- is an efficient sensitizer for both the visible and near-IR (NIR) emissions of most of the lanthanide series, except thulium. Optical measurements, supported by density functional theory calculations, indicate that the dual visible and NIR Ln3+ emission arises from two intraligand charge transfer (ILCT) transitions of 3-NO2Tp-. This is the first report of lanthanide complexes with a nitro-functionalized pyrazolylborate ligand. The derivatization of the known Tp- ligand results in new coordination chemistry governed by the increased denticity of 3-NO2Tp-, imparting remarkable structural diversity and charge transfer properties to resultant lanthanide complexes.

A mononuclear zinc complex with a diamine: Synthesis, characterization, self assembly, luminescence property and DFT calculations

A zinc complex with a N2O2 donor diamine is synthesized and its fluorescence property is explored. The hydrogen bonding environment in its solid state structure is well supported qualitatively and quantitatively with the help of Reduced Density Gradient (RDG) based NCI (non-covalent interactions) index calculation and AIM analyses. Also, the physical nature of other weak non-covalent interactions in the complex is examined. The absorption and emission properties of the complex are explained using density functional theory (DFT) calculations. Analysis of the natural transition orbitals (NTO) shows that the excited state can be mainly characterized by an intra-ligand charge transfer (ILCT) transition within a diamine from the highest-occupied NTO (hole) to the lowest-unoccupied NTO (electron).

Unsymmetrical salen nickel (II) complex embracing phenol bridge: X-ray structure, redox investigation, computational calculations, antimicrobial and catalytic activities

An exhaustive description of the crystal structure, DFT, TD-DFT and redox properties of a non-symmetrical nickel complex NiLOH, N,N’-2,3-hydroxyphenol -2,3-dyil-bis (salicylideneimine)nickel(II), are reported. The complex crystallizes, in the monoclinic crystal system, with P21/c space group. Oxygen-atom of the phenolic spacer contributes in hydrogen bonding forming a zigzag chain along a axis. Cyclic voltammetric study of the complex revealed a quasi-reversible process Ni(II)/Ni(I) on a glassy carbon electrode at E1/2 = -1371, -1312, -1305 mV/SCE in DMSO, CH3CN and DCM, respectively. From DFT calculations, the electronic properties of NiLOH complex and its reactivity with DNA bases, in polar and non-polar media, shows a good agreement with the experimental results. TD-DFT revealed that the absorption electronic states correspond to a mixture of metal to ligand charge transfer (d→π*) and intra-ligand charge transfer (π→π*) transitions. The NiLOH complex as catalyst proved to be catalytically active in the cyclohexene epoxidation reaction. The ligand and the NiLOH complex were scrutinized for their antibacterial activity in agar medium by the disk diffusion method against some Gram positive and Gram negative strains. Better inhibitory potential towards some examined bacteria was shown.

Ground- and excited-state properties of Re(I) carbonyl complexes – Effect of triimine ligand core and appended heteroaromatic groups

In this work, a series of six rhenium(I) complexes bearing 2,2′:6′,2″-terpyridine (terpy), 2,6-di(thiazol-2-yl)pyridine (dtpy), and 2,6-di(pyrazin-2-yl)pyridine (dppy) with appended quinolin-2-yl and N-ethylcarbazol-3-yl groups were prepared and spectroscopically investigated to evaluate the photophysical consequences of both the trisheterocyclic core (terpy, dtpy and dppy) and the heterocyclic substituent. The [ReCl(CO)3(Ln-κ2N)] complexes are regarded as ideal candidates for getting structure–property relationships, while terpy-like framework represents an excellent structural backbone for structural modifications. The replacement of the peripheral pyridine rings of 2,2′:6′,2″-terpyridine by thiazoles and pyrazines resulted in a significant red-shift of the absorption and emission of [ReCl(CO)3(Ln-κ2N)] due to stabilization of the ligand-centred LUMO orbital. Both quinoline and N-ethylcarbazole are extended π-conjugation organic chromophores, but they differ in electron-donating abilities. The low-energy absorption band of Re(I) complexes with the triimine ligands bearing quinolin-2-yl group was contributed by the metal-to-ligand charge-transfer (MLCT) electronic transitions. The introduction of electron-donating N-ethylcarbazol-3-yl substituent into the triimine acceptor core resulted in the change of the character of the HOMO of Re(I) complexes and a significant increase of molar absorption coefficients of the long-wavelength absorption, which was assigned to a combination of 1MLCT and 1ILCT (intraligand charge-transfer) transitions. Regardless of the appended heteroaromatic group, the emitting excited state of Re(I) terpy-based complexes was demonstrated to have predominant 3MLCT character, as evidenced by comprehensive studies including static and time-resolved emission spectroscopy along with ultrafast transient absorption measurements. The diodes with Re(I) complexes dispersed molecularly in a PVK:PBD matrix were emissive and effects of the complex structure on colour of emitted light and its intensity was pronounced.

A Luminescent Two-Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for the Enhancement of TADF Radiative Decay Rate.

A luminescent bimetallic AuI complex comprised of N-heterocyclic carbene (NHC) and carbazole (Cz) ligands, that is, (NHC')Au(NHC)AuCz has been synthesized and studied. Both carbene ligands in the bimetallic complex act as electron acceptors in tandem to increase the energy separation between the ground and excited state, which is higher than those found in either monometallic analogue, (NHC)AuCz and (NHC')AuCz. A coplanar geometry designed into the tandem complex ensures sufficient electronic coupling between the π-orbitals of the ligands to impart a strong oscillator strength to the singlet intra-ligand charge-transfer (1 ICT) transition. Theoretical modelling indicates that the emissive ICT excited state involves both NHC ligands. The tandem complex gives blue luminescence (λmax =480 nm) with a high photoluminescent quantum yield (ΦPL =0.80) with a short decay lifetime (τ=0.52 μs). Temperature-dependent photophysical studies indicate that emission is via thermally assisted delayed fluorescence (TADF) and give a small singlet-triplet energy difference (ΔEST =50 meV, 400 cm-1 ) consistent with the short TADF lifetime.

Unusual Reactivity and Photophysical Properties of Platinum(II) Pincer Complexes Containing 6,6'‐Diphenyl‐2,2'‐bipyridine Ligands

A series of novel 6,6'‐diphenyl‐2,2'‐bipyridine ligands modified with the substituents at phenyl and bipyridine moieties have been prepared and used in the synthesis of pincer platinum chloride complexes 1–3, [Pt(NNC–R)Cl], where NNC–R – dimethyl 6,6'‐diphenyl‐[2,2'‐bipyridine]‐5,5'‐dicarboxylate (R = H, (1)), dimethyl 6,6'‐bis(4‐fluorophenyl)‐[2,2'‐bipyridine]‐5,5'‐dicarboxylate (R = F (2)), dimethyl 6,6'‐bis(4‐methoxyphenyl)‐[2,2'‐bipyridine]‐5,5'‐dicarboxylate (R = OMe (3)). Reactions of the obtained compounds with triphenylphosphine proceed through unusual pathway, which gives the [Pt(NNC–R)(PPh3)2Cl] products (1a–3a) containing two phosphine ligands, with the chloride retained in coordination sphere and diphenyl‐bipyridine fragment coordinated in η1‐mode through metalation of a pyridine ring. All complexes have been characterized with mass‐spectrometry and NMR spectroscopy, solid‐state structure of 1–3 and 1a, 3a was revealed by using XRD crystallography. The complexes 1–3 are phosphorescent in dichloromethane solution and in solid state. Their photophysical characteristics were determined and analyzed by DFT calculations, which gave assignment of emissive excited state character with the major contribution from the intraligand charge transfer (3ILCT, Ph→bipyridine) and ligand centered (3LC, bipyridine fragment) transitions.

Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes.

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 μs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing 3MLCT and 3ILCT contributions across the series.

Ru(II)-polypyridine complexes with alkynyl Schiff base ligand: influence of π-conjugation, donor/acceptor substituents, and counter anions on electrochemical, luminescence, and catalytic properties

Ru(II)-polypyridine complexes of the general formula [Ru(L1/L2)(phen)2]X2 (1a–6a) and [Ru(L1/L2)(bipy)2]X2 (1b–6b) (where X = ClO4, BF4, PF6; phen = 1,10-phenanthroline, bipy = 2,2′-bipyridine) were prepared by the reaction of [Ru(phen)2Cl2]·2H2O and [Ru(bipy)2Cl2]·2H2O with (E)-5-((4-methoxyphenyl)ethynyl)-N-(pyridin-2-ylmethylene)pyridin-2-amine (L1) and (E)-5-((4-nitrophenyl)ethynyl)-N-(pyridin-2-ylmethylene)pyridine-2-amine (L2) in the presence of NaBF4, NaClO4, and NaPF6. The electrochemical properties of all the complexes indicate reversible redox behavior corresponding to Ru(II)–Ru(III) couple and are susceptible to variation of electron-donating/accepting properties of substituent group on L1 and L2. All complexes showed room temperature luminescence corresponding to π→π* intra-ligand charge-transfer (ILCT) transition with chelation enhanced fluorescence and is finely tuned by increasing π-conjugation, size of counter anions, and variation of substituent group with different electronic effects in the complexes. All the complexes worked as an effective catalyst for the oxidation of benzyl alcohol to corresponding benzaldehyde in good yield at room temperature.

Picolinate-appended tacn complexes for bimodal imaging: Radiolabeling, relaxivity, photophysical and electrochemical studies

Based on our previous works involving two 1,4,7-triazacyclononane (tacn)-based ligands Hno2py1pa (1-Picolinic acid-4,7-bis(pyridin-2-ylmethyl)-1,4,7-triazacyclononane) and Hno1pa (1-Picolinic acid-1,4,7-triazacyclononane), we report here the synthesis of analogues bearing picolinate-based π-conjugated ILCT (Intra-Ligand Charge Transfer) transition antenna (HL1, HL2), using regiospecific N-functionalization of the tacn skeleton and their related transition metal complexes (e.g. Cu2+, Zn2+ and Mn2+). Coordination properties as well as their photophysical and electrochemical properties were investigated in order to quantify the impact of such antenna on the luminescent or relaxometric properties of the complexes. The spectroscopic properties of the targeted ligands and metal complexes have been studied using UV–Vis absorption and fluorescence spectrocopies. While the zinc complex formed with HL1 possesses a moderate quantum yield of 5%, complexation of Cu2+ led to an extinction of the luminescence putatively attributed to a photo-induced electron transfer, as supported by spectroscopic and electrochemical evidences. The [Mn(L2)]+ complex is characterized by a fluorescence quantum yield close to 8% in CH2Cl2. The potential interest of such systems as bimodal probes has been assessed from radiolabeling experiments conducted on HL1 and 64Cu2+ as well as confocal microscopy analyses and from relaxometric studies carried out on the cationic [Mn(L2)]+ complex. These results showed that HL1 can be used for radiolabeling, with a radiochemical conversion of 40% in 15 min at 100 °C. Finally, the relaxivity values obtained for [Mn(L2)]+, r1p = 4.80 mM−1·s−1 and r2p = 8.72 mM−1·s−1, make the Mn(II) complex an ideal candidate as a probe for Magnetic Resonance Imaging.