Ligand Charge Transfer Transitions Research Articles

A homochiral Nickel(II) complex [Ni(P'N)2]Cl2: Synthesis, characterization, crystal structure, luminescence, DFT and Hirshfeld surface studies

A new complex ([Ni(P'N)2]Cl2) is synthesized from an anhydrous NiCl2 and a bidentate P'N (S,S)-NH2CHPhCHPhPPh2) ligand with an enantiopure ethylene backbone. The solubility of the complex in methanol aids its crystallization by slow evaporation. A single crystal X-ray diffraction study of the complex reveals a distorted square planar geometry with the phosphinyl groups coordinated in a cis(P,P) configuration. This imposes steric effects that influence the crystal packing of the complex in an orthorhombic unit cell. The complex possesses characteristic metal to ligand charge transfer (MLCT) transition in the UV region with a peak absorption at 294 nm corresponding to 4.22 eV, which correlates with 4.24 eV derived from the DFT calculation of the energy band gap between the highest molecular orbital (HOMO:7.39 eV) and lowest unoccupied molecular orbital (LUMO:3.15 eV) of the complex. With an excitation wavelength of 415 nm (3.00 eV), the complex emits photons with wavelength 488 nm (2.54 eV) in the blue region of the visible spectrum. The circular dichroism spectrum has multiple peaks in the UV region mainly associated with transitions within the chiral array of phenyl groups. The bond lengths, bond angles, and vibrational bands obtained by simulation of the optimized structure of the complex provide analytical details that strongly correlate with the experimental data with adjusted R2 of 0.9826 (bond lengths), 0.9911 (bond angles) and 0.9983 (vibrational bands). The Hirshfeld surface analysis shows that H…H and C…H interactions between phenyl rings of the cations contribute significantly to the crystal packing of the complex.

1,4-diaminophthalazine complexes of Re(CO)3

1,3-Diiminoisoindolines and substituted variants react with hydrazine to produce 1,4-diaminophthalazines. In this paper, we present the chemistry of 1,4-diaminophthalazine and two modified versions with the Re(CO)3 unit. The 1,4-diaminophthalazine ligand forms a bimetallic complex similar to that seen with unmodified phthalazine. In contrast, the semihemiporphyrazine-derived chelating ligands bind Re(CO)3 as neutral bidentate compounds; the freebase pyrazole and indazole modified 1,4-diaminophthalazines exhibit multiple tautomerization states with ionizable protons. Notably, protonation of the meso bridging nitrogen atom reduces the degree of conjugation between the two halves of the chelate, resulting in a diimine-like complex that lacks significant absorption in the visible spectrum due to the abrogation of the low energy metal to ligand charge transfer (MLCT) transition.

Electronic structure that gives rise to the optical properties of zinc, cadmium and silver hexacyanocobaltates(III)

Transition metal hexacyanocobaltates(III) correspond to coordination polymers that present physical properties that can be used for technological applications. In this sense, properties like spin-crossover under different physical stimuli (light, temperature, pression, etc) or the colossal mechanical negative thermal expansion has been reported as astonishing properties of these materials (Goodwin et al., 2008; Goodwin et al., 2008; Avila et al., 2022) [1–3]. In this contribution, the electronic properties of hexacyanocobaltates(III) that contain Zn2+, Cd2+ and Ag1+ as metal ions, with a closed d-shell electronic configuration, are studied by means of combined UV–Vis spectroscopy and ab-initio calculations. The influence of the outer metals (Zn, Cd, Ag) when forming the coordination polymers as well as the effect of the inner octahedral moiety [Co(CN)6]3- have on the macroscopic electronic properties is discussed. Metal to ligand charge transfer transitions that produce the optical behavior are also clarified. Furthermore, the origin of the band gap transitions for Zn and Cd hexacyanocobaltates(III) is reported for the first time and supported by ab-initio calculations. New optical band gap energy values are proposed from a combined Urbach and Tauc analysis. Finally, the effects of metal substitution in Prussian blue analogues and the structural phase shift towards a zeolite-like phase on the band gap are analyzed.

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.

Synthesis, structure determination, NBO analysis and vibrational/electronic spectroscopic study of Iron(II) Bis(diethyldithiocarbamate) [Fe(DDTC)2

This work deals with the synthesis, computational molecular modeling, and vibrational/electronic spectroscopic analysis of the coordination complex [Fe(DDTC)2]. The complex was synthesized according to the guidelines provided by the graphical method. The optimization of the molecular structure was performed using Density Functional Theory with the exchange functional B3LYP and basis set 6–311G(d,p). The experimental FT-IR and Raman spectra of the complex were recorded in the range of 4000–0 cm−1 to correlate them with the calculated spectra. Most of the DFT calculated frequencies agreed with the experimental results, and complete vibrational assignments (including metal-ligand) were made. To investigate the internal electronic mobility of the complex, we performed natural bond orbital analysis (NBO), which provided information regarding intramolecular charge transfer interactions resulting from the overlapping of bonding and antibonding orbitals. The NBO analysis also provided information about the electronic distribution between the HOMO and LUMO orbitals due to charge transfers. We found that the NBO analysis provided the main charge transfers between donor and acceptor atoms, and the HOMO-LUMO energy gap revealed that the complex has high kinetic stability. We also correlated the calculated and experimental UV–Vis spectra to investigate the configurations of several excited states of the complex that involve intra-ligand transitions. The charge transfer and d-d (Laporte Forbidden) bands were assigned. The results also corroborate the existence of several Ligand to Metal Charge Transfer (LMCT) and Metal to Ligand Charge Transfer (MLCT) transitions, as well as d-d transitions. We found that the experimental vibrational spectra agreed with the theoretical ones.

Circularly Polarized Luminescence of Palladium(0) Complexes Bearing Chiral Diphosphines

Four palladium(0) complexes bearing diphosphine ligands were prepared and their chiroptical properties were studied. The structures of two new complexes were elucidated by X-ray crystallography. These complexes show photoluminescence in solution, with quantum yields of 0.4–42%, as well as circular dichroism (CD) at the metal–ligand charge transfer transition bands. The three complexes bearing biaryl phosphine ligands exhibit circularly polarized luminescence of moderate intensities; the sign and the dissymmetry factors of which coincide with the CD spectra.

Charge-Transfer Spectroscopy of Bisaxially Coordinated Iron(II) Phthalocyanines through the Prism of the Lever's EL Parameters Scale, MCD Spectroscopy, and TDDFT Calculations.

The position of the experimentally observed (in the UV-vis and magnetic circular dichroism (MCD) spectra) low-energy metal-to-ligand charge-transfer (MLCT) band in low-spin iron(II) phthalocyanine complexes of general formula PcFeL2, PcFeL'L″, and [PcFeX2]2- (L, L', or L″ are neutral and X- is an anionic axial ligand) was correlated with the Lever's electrochemical EL scale values for the axial ligands. The time-dependent density functional theory (TDDFT)-predicted UV-vis spectra are in very good agreement with the experimental data for all complexes. In the majority of compounds, TDDFT predicts that the first degenerate MLCT band that correlates with the MCD A-term observed between 360 and 480 nm is dominated by an eg (Fe, dπ) → b1u (Pc, π*) single-electron excitation (in traditional D4h point group notation) and agrees well with the previous assignment discussed by Stillman and co-workers[ Inorg. Chem. 1994, 33, 573-583]. The TDDFT calculations also suggest a small energy gap for b1u/b2u (Pc, π*) orbital splitting and closeness of the MLCT1 eg (Fe, dπ) → b1u (Pc, π*) and MLCT2 eg (Fe, dπ) → b2u (Pc, π*) transitions. In the case of the PcFeL2 complexes with phosphines as the axial ligands, additional degenerate charge-transfer transitions were observed between 450 and 500 nm. These transitions are dominated by a2u (Pc + L, π) → eg (Pc, π*) single-electron excitations and are unique for the PcFe(PR3)2 complexes. The energy of the phthalocyanine-based a2u orbital has large axial ligand dependency and is the reason for a large energy deviation for B1 a2u (Pc + L, π) → eg (Pc, π*) transition. The energies of the axial ligand-to-iron, axial ligand-to-phthalocyanine, iron-to-axial ligand, and phthalocyanine-to-axial ligand charge-transfer transitions were discussed on the basis of TDDFT calculations.

X-ray absorption near-edge spectroscopy of antimony complexed with organic molecules: a theoretical interpretation

The shoulder feature for the white line (Sb K-edge) was ascribed to metal–ligand charge transfer transitions from Sb 1s to the unoccupied states of the carbon chains. Longer distance for charge transfer caused lower intensity of the shoulder peak.

Low-cost photo-switches based on stilbene-appended Zn(II)-terpyridine complexes.

We report herein the synthesis, characterization, photophysics, and photo-isomerization behaviors of three Zn(II)-terpyridine complexes of the type [Zn(tpy-pvp-X)2]2+ (X = H, Me, and NO2) covalently tethered with stilbene moiety. The complexes exhibit absorption bands stretching up to the edge of the visible domain due to ligand → ligand charge transfer (LLCT) transitions and strong emission at room temperature in the visible due to radiative deactivation of 3LLCT state having lifetime within 1.0-3.0ns. The stilbene motifs in the complexes undergo trans to cis isomerization upon irradiating with UV and visible light accompanied by significant alteration of their absorption, emission, and 1H NMR spectral profiles. Apart from the variation of electron donating and electron withdrawing substituent (X), the isomerization studies were also carried out in three different solvents (DCM, MeCN, and DMSO) to further tune their kinetic and thermodynamic parameters. The rate, rate constant and quantum yield of isomerization were estimated in all the solvents. The reverse process (cis to trans) also occurs very slowly on keeping but could be accelerated upon heating. Trans to cis photoisomerization leads to quenching of emission in case of 1 and 2, whereas backward thermal cis to trans conversion leads to restoration of emission. By contrast, for the nitro-derivative (3) forward process induces emission enhancement, while backward process gives rise to emission quenching. In essence, "on-off" and "off-on" emission switching is feasible for 1 and 2, whereas "off-on" and "on-off" emission switching occurs in case of 3. Emission spectral responses upon successive action of photonic and thermal input lead to the fabrication of INHIBIT and IMPLICATION logic gates. DFT and TD-DFT computational investigations were also undertaken to visualize their electronic structures, correct assignment of the spectral bands, and mode of isomerization process.

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.

DPP based dual-sensing probe for the multi-color detection of toxic Co2+/Sn2+ and CN− ions in water: An electronic eye development

A dual-sensing mechanism probe for the multi-color detection of toxic Co2+/Sn2+ and CN− ions in water based on a diketopyrrolopyrrole (DPP) moiety was designed and successfully synthesized. Colorimetric and fluorimetric methods were used to confirm the sensing performance of the probe. Different colors were achieved for the detecting ions Co2+, Sn2+, and CN−. pink for Co2+, red for Sn2+, and colorless for CN−, denoting high selectivity in the developed probe. A dual-sensing mechanism confirmed for the metal ion the sensing is via complexation resulting in color (different) change through metal to ligand charge-transfer transition (MLCT), and for anion (cyanide), it is through addition reaction with a disconnection in intramolecular charge-transfer transition (ICT). Pre-added selected ions to the different water samples effectively detect different colors. We developed an electronic eye (RGB - Arduino device) for the detection of toxic ions effectively.

Tunable magnetic circular dichroism via electrochemically controlled charge-transfer transition in Ru(bpy)32+ aqueous solution

Seeking potential aqueous solution with tunable magnetic properties by applied electrode potentials is an emerging research topic for the integration of aqueous solution into devices. In this work, Ru(bpy)32+, which is widely used for high efficiency electroluminescent and photovoltaic devices, has been demonstrated as a potential liquid electrolyte with tunable magnetic properties by applied electrode potentials evidenced by energy resolved magnetic circular dichroism (MCD) spectroscopy. The MCD signal at 450 nm transforms from paramagnetic to nonmagnetic behavior when the applied electrode potentials are >1.3 V. The transition of the MCD signal from paramagnetic to nonmagnetic behavior is ascribed to the disappearance of metal to ligand charge transfer transition during the electrochemical oxidation process. This work might provide a valuable insight into exploration of functional liquid electrolyte with tunable opto-magnetic properties.

Visible Light Absorption and Long-Lived Excited States in Dinuclear Silver(I) Complexes with Redox-Active Ligands.

Well-defined dinuclear silver(I) complexes have been targeted for applications in catalysis and materials chemistry, and the effect of close silver-silver interactions on electronic structure remains an area of active inquiry. In this study, we describe the synthesis, structure, and photophysical properties of dimeric silver complexes featuring a redox-active naphthyridine diimine ligand. Unusually for silver(I), these complexes display absorption features in the visible region due to metal-metal to ligand charge transfer (MMLCT) transitions, which arise from the combination of close silver-silver interactions and low-lying ligand π* orbitals. The complexes' photophysical properties are explored via a combination of spectroscopic and computational studies, revealing MMLCT excited state lifetimes that exceed 1 μs. These results portend previously unforeseen applications of silver(I) dimers in visible light absorption and excited state reactivity.

Synthesis, spectral, electrochemical and DNA binding properties of ruthenium (III) complexes of Schiff bases containing N,O donor atoms

A new series of octahedral Ru(III) complexes of the type [Ru (Cl)2 (DMAPIMP-X) (EPh3)2]; DMAPIMP={2-[(4-N,N’-dimethylaminophenylimino)-methyl]-4-X-phenol}; X = Cl, Br or I; E = P or As) have been prepared and characterized by spectral, magnetic and cyclic voltammetric studies. The Ru (III) complexes are low spin and paramagnetic with one unpaired electron. These complexes absorb strongly in the visible region owing to the metal to ligand charge transfer (MLCT) transition. The IR spectral data reveals that the monobasic bidentate Schiff base ligands are chelated to ruthenium through the imine nitrogen and the deprotonated phenolic oxygen. The three line e.s.r spectrum of RuIII complexes with gx ≠ gy ≠ gz indicates magnetic anisotropy and an asymmetry in the electronic environment around the Ru atom. All Ru(III) complexes display the redox couple in the range – 0.826 to – 0.971 V. Further more the DNA binding experiment of the complexes were carried out by UV–Vis absorption spectral titration and the binding constant Kb = 1.2 ± 0.3 × 104 M−1 to 4.4 ± 0.3 × 104 M−1 have been found. The hyperchromism with moderate red shift of 2–4 nm, such a small change in λmax is more in keeping with groove binding, leading to small perturbations.

Binding a meridional ligand in a facial geometry: A square peg in a round hole

The bis(pyridylimino)isoindoline (BPI) ligand is a tridentate chelate that binds to metals via a meridional coordination mode. However, when this ligand forms a complex with Re(CO)3, an almost exclusively facial moiety, the BPI ligand deforms to coordinate in a facial mode. We have investigated this deformation via structural and theoretical means, and the non-planar binding mode of the ligand bathochromically shifts the metal to ligand charge transfer (MLCT) transition.

Bichromophoric Properties of Ruthenium(II) Polypyridyl Complexes Bridged by Boron Dipyrromethenes: Synthesis, Electrochemical, Spectroscopic, Computational Evaluation, and Plasmid DNA Photoreactions

Two new π‐extended dipyrrins containing isoquinolpyrrole and phenanthro‐fused pyrrole have been synthesized by solvent free reactions with trifluoroacetic acid (TFA) as a catalyst. The boron‐dipyrrin (BDP) of the isoquinolpyrrole was synthesized by standard procedures followed by synthesis of the bis‐ruthenium(II) BDP analog. The spectroscopic properties of this complex were investigated, showing the typical intra‐ligand charge transfer transitions (ILCT) along with the Ru(dπ) to ligand(π*) metal to ligand charge transfer (MLCT) transitions. An intense transition at 608 nm with molar absorptivity greater than 100,000 m–1 cm–1 associated with the ππ* transition of the BDP‐core is observed. The less stable bis‐RuII BDP complex from the phenanthro‐fused dipyrrin was also synthesized giving the typical spectra associated with RuII‐phenanthroline complexes in addition an intense absorption at 618 nm from ππ* transition of the BDP‐core. The spectroscopic properties of this complex were correlated to density functional theory (DFT) and time‐dependent density functional theory (TDDFT) theoretical calculations. This complex shows interesting dual emission of both the Ru(bpy)2phen and the BDP‐core components at 601 nm and 641 nm respectively upon excitation of the lower energy MLCT transition at 453 nm. In acetonitrile solutions both bis‐RuII‐BDP complexes generate significant singlet oxygen when irradiated with low energy light. In aqueous solutions both complexes are capable of photo‐nicking plasmid DNA when irradiated within the photodynamic therapy (PDT) window with the phenanthrol‐fused bis‐RuII‐BDP complex showing greater photoreactivity.

Dinuclear Pd(II) complexes containing bis-O,N-bidentate Schiff base ligands: Synthesis, characterization, DFT study and application as Suzuki–Miyaura coupling catalysts

The reaction of binucleating Schiff base ligands with [PdCl2(PPh3)2] and [Pd(OAc)2] afforded a new family of binuclear palladium(II) Schiff base complexes of the type [Pd2(L)2] and [Pd2Cl2(PPh3)2(L)]. All the palladium complexes are air stable and fully characterized by elemental analysis, FT-IR, UV–Vis and 1H NMR spectral methods. These palladium(II) Schiff base complexes exhibit characteristic metal to ligand charge transfer (MLCT) transitions. Quantum mechanical calculations were also performed to investigate the electronic structure, assign the vibrational spectra and to determine the nature of the frontier molecular orbitals and absorption spectra of the above complexes. These palladium(II) Schiff base complexes exhibited good catalytic activity in the carbon–carbon Suzuki coupling reaction of different aryl halides using phenyl boronic acid.

Sensing phenomena, extraction and recovery of Cu2+ followed by smart phone application using a luminescent pyrene based chemosensor

A new simple, small molecule hydrazone was synthesized from1:1 condensation reaction of pyrene-1-aldehyde and 2-hydrazinobenzoic acid and was characterized by absorption, emission spectrometry, FTIR NMR and mass studies. It was investigated as a reversible, turn-on luminescent chemosensor for copper ion (Cu2+) in aqueous environment with a ratiometric change in emission pattern which may be explained due to due to intra ligand charge transfer transition (ILCT). The receptor shows a prominent colorimetric change from yellow to reddish brown, in presence of Cu2+ with a detection limit in the 10−8 M range. No significant spectral changes were observed in presence of other environmentally significant metal ions (Hg2+, Pb2+, Cd2+, Ni2+, Co2+, Fe2+, Fe3+, Mn2+, Zn2+, Al3+ and Cr3+). Interestingly in presence of Cu2+, the ligand exhibits reversible change in emission pattern with oxalate (C2O42-) and thus offers an interesting property of molecular ‘INHIBIT’ logic gate with Cu2+ and C2O42-as chemical inputs. In practical application, the receptor was used to detect Cu2+ in real samples by selective two phase liquid-liquid extraction using water-dichloromethane mixture (1:1). The systematic color change during Cu2+ extraction was monitored by a readily-usable smartphone as an analysis tool.

Ligand-Tuneable, Red-Emitting Iridium(III) Complexes for Efficient Triplet-Triplet Annihilation Upconversion Performance.

A series of substituted 2-phenylquinoxaline ligands have been explored to finely tune the visible emission properties of a corresponding set of cationic, cyclometallated iridium(III) complexes. The electronic and redox properties of the complexes were investigated through experimental (including time-resolved luminescence and transient absorption spectroscopy) and theoretical methods. The complexes display absorption and phosphorescent emissions in the visible region that are attributed to metal to ligand charge-transfer transitions. The different substitution patterns of the ligands induce variations in these parameters. Time-dependent DFT studies support these assignments and show that there is likely to be a strong spin-forbidden contribution to the visible absorption bands at λ=500-600 nm. Calculations also reliably predict the magnitude and trends in triplet emitting wavelengths for the series of complexes. The complexes were assessed as potential sensitisers in triplet-triplet annihilation upconversion experiments by using 9,10-diphenylanthracene as the acceptor; the methylated variants performed especially well with impressive upconversion quantum yields of up to 39.3 %.

A Varied Alkyl Chains of Ruthenium(II) Complexes for Dye Sensitized Solar Cells (DSSCs)

Ruthenium(II) complexes were synthesized and characterized for dye-sensitized solar cells (DSSCs) with varied alkyl chains specifically methyl (PC01), hexyl (PC02) and octyl (PC03). The photophysical property was studied by UV-Visible absorption. The results showed that the main absorption peaks were asigned to the metal to ligand charge transfer (MLCT) transition (350-600 nm). The incident photon to current conversion efficiency (IPCE) covered the entire visible spectrum reaching to 18-20% at 520 nm. The DSSC performances were investigated with liquid iodide/tri-iodide electrolyte under standard AM 1.5. PC01-PC03 showed that the power conversion efficiency (h) were obtained at 3.08%, 3.18% and 3.14%, respectively, compared with N719 (7.80%). Interestingly, PC02 and PC03 showed the long term stability with %hlossbetter than the standard up to 1000 h.