trans-Dichloro-bis-(arylazoimidazole)palladium(II): Synthesis, Structure, Photoisomerization, and DFT Calculation

Chemistry of azo-imine based palladium complexes: a brief review

This paper provides a general overview of palladium complexes incorporating numerous multidentate azo-imine ligands, their structural diversity, synthetic protocol, interesting properties such as redox properties, biological activity and their further application on C – N coupling reactions, C – C cross-coupling reactions (e.g., Suzuki and Heck) and catalytic activity on different organic transformations. A detailed literature survey revealed that no brief review has yet been published on the azo-azomethine-based palladium complex. Therefore, the present review incorporates the research undertaken on the synthetic strategies and coordinating behaviour of various azo-imine ligands with palladium metal centre and a few important properties of metal complexes.

A fluorescence ‘turn-on’ chemodosimeter for the specific detection of Pd2+by a rhodamine appended Schiff base and its application in live cell imaging

Allylether-Rhodamine selectively invites Pd²⁺from mixture of other cations and Pd(0) in H₂O-MeCN medium and exhibits maroon emission due to C(allyl)-O bond cleavage and opening of spirolactam ring. The LOD is 50 nM at pH 7. Imaging Pd²⁺species in living cells under physiological conditions is achieved.

The structure and photophysics of di-iodo-zinc(II) complexes of long alkyl chain substituted imidazolyl motif of arylazoimidazoles and the DFT computation

Distorted tetrahedral structure of [Zn(Haai-C10H21)2I2] (Haai-C10H21, 1-decayl-2-(arylazo)imidazole) has been supported by single crystal X-ray diffraction study. The structures of other complexes, [Zn(Raai-CnH2n+1)2I2] (n=10, 12, 14, 16, 18, 20, 22) have been determined by spectroscopic data (FT-IR, UV-vis, (1)H NMR). The complexes show light induced photoisomerisation, E-to-Z (trans-to-cis) of coordinated, Raai-CnH2n+1. The Z-to-E (cis-to-trans) isomerisation is also carried out by thermal activation route. The quantum yields of the E→Z progression (ϕE→Z) of the complexes are less than that of free ligand, which could be due to increase in molar mass and molar volume of the complexes than that of free ligands. The activation energy (Ea) of Z→E isomerisation of the complexes is is less than that of free ligands. This observation is also consistent with femtosecond transient absorption results which suggests that the E(trans)→Z(cis) isomerization occurs through the motion of pendant NNAr of the molecule. The temporal profiles of free ligand shows three decay processes corresponds to 0.24ps (S2 state) and subsequent decay, 0.85ps of the S1 state and finally 5ps to the hot ground state at 500nm. The complexes also show three decay periods approximately at 0.25ps, 1.3ps and 13ps. The spectral property and photochromic efficiency have been explained by DFT computation of optimized geometry of the complexes.

HOMO-LUMO energy gap control in platinum(II) biphenyl complexes containing 2,2'-bipyridine ligands

A series of platinum(ii) biphenyl 2,2'-bipyridine complexes containing electron-donating and electron-withdrawing moieties on the 4 and 4' positions of the bipyridine ligand exhibit emission from excited states in the 600 nm region of the spectrum upon excitation in the metal-to-ligand charge transfer transition located near 450 nm. These complexes are distorted from planarity based on both single crystal structure determinations and density functional theory (DFT) calculations of isolated molecules in acetonitrile. The DFT also reveals the geometry of the lowest-lying triplet state (LLTS) of each complex that is important for emission behavior. The LLTS are assigned based on the electron spin density distributions and correlated with the singlet excited states to understand the mechanism of electronic excitation and relaxation. Time-dependent DFT calculations are performed to compute the singlet excited state energies of these complexes so as to help interpret their UV-Vis absorption spectra. Computational and experimental results, including absorption and emission energy maxima, electrochemical reduction potentials, LLTS, singlet excited states, and LUMO and HOMO energies, exhibit linear correlations with the Hammett constants for para-substituents σp. These correlations are employed to screen complexes that have not yet been synthesized. The correlation analysis indicates that the electronic structure and the HOMO-LUMO energy gap in Pt(ii) complexes can be effectively controlled using electron-donating and electron-withdrawing moieties covalently bonded to the ligands. The information presented in this paper provides a better understanding of the fundamental electronic and thermodynamic behavior of these complexes and could be used to design systems with specific applications.

Insight into luminescent bisazoaromatic CNN pincer palladacycle: synthesis, structure, electrochemistry and some catalytic applications in C–C coupling

The 2-(phenylazo)azobenzene furnished novel palladacycles in excellent yield, which showed luminescence at rt and catalytic activity. The optoelectronic and electrochemical responses were substantiated with DFT and TDDFT.

Syntheses, supramolecular structures, magnetic and photochromic properties of six lanthanide complexes based on the 5-azotetrazolyl salicylic acid ligand

Six lanthanide complexes based on 5-azotetrazolyl salicylic acid were synthesized and their crystal structures, magnetic and photochromic properties were reported.

Spontaneous formation in the dark, and visible light-induced cleavage, of a Ru-S bond in water: a thermodynamic and kinetic study

In this work the thermal and photochemical reactivity of a series of ruthenium complexes [Ru(terpy)(N-N)(L)](X)2 (terpy = 2,2';6',2″-terpyridine, L = 2-(methylthio)ethanol (Hmte) or water, and X is Cl(-) or PF6(-)) with four different bidentate chelates N-N = bpy (2,2'-bipyridine), biq (2,2'-biquinoline), dcbpy (6,6'-dichloro-2,2'-bipyridine), or dmbpy (6,6'-dimethyl-2,2'-bipyridine), is described. For each chelate N-N the thermodynamic constant of the dark equilibrium between the aqua- and Hmte- complexes, the Hmte photosubstitution quantum yield, and the rate constants of the thermal interconversion between the aqua and Hmte complexes were measured at room temperature. By changing the steric hindrance and electronic properties of the spectator N-N ligand along the series bpy, biq, dcbpy, dmbpy the dark reactivity clearly shifts from a nonlabile equilibrium with N-N = bpy to a very labile thermal equilibrium with N-N = dmbpy. According to variable-temperature rate constant measurements in the dark near pH = 7 the activation enthalpies for the thermal substitution of H2O by Hmte are comparable for all ruthenium complexes, whereas the activation entropies are negative for bpy and biq, and positive for dcbpy and dmbpy complexes. These data are indicative of a change in the substitution mechanism, being interchange associative with nonhindered or poorly hindered chelates (bpy, biq), and interchange dissociative for more bulky ligands (dcbpy, dmbpy). For the most labile dmbpy system, the thermal equilibrium is too fast to allow significant modification of the composition of the mixture using light, and for the nonhindered bpy complex the photosubstitution of Hmte by H2O is possible but thermal binding of Hmte to the aqua complex does not occur at room temperature. By contrast, with N-N = biq or dcbpy the thermodynamic and kinetic parameters describing the formation and breakage of the Ru-S bond lie in a range where the bond forms spontaneously in the dark, but is efficiently cleaved under light irradiation. Thus, the ratio between the aqua and Hmte complex in solution can be efficiently controlled at room temperature using visible light irradiation.

Transducer influence of polycyclic aromatic hydrocarbons (PAHs) on photoisomerisation of 1-alkyl-2-(arylazo)imidazoles

The UV light irradiation to a solution of 1-alkyl-2-(arylazo)imidazole (L) in toluene shows transformation of trans to cis configuration about -N=N- bond. The rate of photo-isomerisation and quantum yields are influenced by internal parameters like - nature of substituents, coordination to metal ions, steric and electronic effect, protonation etc. and the external factors like solvent (polarity, viscosity, dipole moment etc.), presence of innocent (to be chemically noninteracting) and noninnocent (chemically interacting) ions or molecules. In this work the influence of polycyclic aromatic hydrocarbons (PAH) such as - napththalene, anthracene, phenanthrene, pyrene - an innocent system, on the photochromic efficiency of L has been examined. The PAHs are π-donor and may form a π-π continuum with L which can significantly affect the motional (vibration, rotation) properties of the analyte and hence the photochromic activity. The rate and quantum yield of trans-to-cis isomerisation of L decrease with increasing [PAH] and also with increasing number of the fused phenyl ring(s) in PAH. Thus the photoisomerisation follows the rate sequence: no PAH>napththalene>anthracene-phenanthrene>pyrene. The reverse change, cis-to-trans is very slow upon light irradiation while appreciably fast in thermal process at dark. The activation energy (E(a)) also decreases with [PAH] and number of fused phenyl rings.

Effect of PEG-200 and Tween-20 on photoisomerization of 1-alkyl-2-(arylazo)imidazoles in toluene

The photoisomerization of 1-alkyl-2-(arylazo)imidazole, trans-to-cis, has been studied in the matrix of PEG-200 and Tween-20 in toluene medium by UV light irradiation. The trans and cis-isomers have different absorption spectra. The cis-to-trans isomerization proceeds slowly in visible light irradiation while it is appreciably fast in thermal process. The rate of trans-to-cis isomerization is decreased by 30-60% in presence of PEG-200 and Tween-20. The quantum yield of the photoisomerization is also decreased by 35-55% and follows the rate sequence: free state>PEG-200-phase>Tween-20 phase. The activation energy (E(a)) of cis→trans, thermal backward isomerization, is reduced in PEG-200 and Tween-20 phase following free state>PEG-200-phase>Tween-20-phase. The branched polyhydroxo structure of Tween-20 may help to wrap the polar photochrome more efficiently than major ether functionalized PEG-200 and stabilizes trans-isomer.

Photoactive azoimine dyes: 4-(2-pyridylazo)-N,N-diethylaniline and 4-(2-pyridylazo)-N,N-dimethylaniline: computational and experimental investigation

4-(2-Pyridylazo)-N,N-dimethylaniline and 4-(2-pyridylazo)-N,N-diethylaniline, two photoactive azoimine dyes, were prepared from the reaction of 2-aminopyridine with N,N-dialkyl-1,4-nitrosoaniline at room temperature. Structural characterizations of these dyes using single crystal X-ray diffraction, (1)H NMR, elemental analysis, mass spectroscopy and IR spectroscopy have been carried out. The X-ray structure indicates a trans configuration around the azo group. The photochemical behavior of these compounds differs from that of 2-phenylazopyridine, the non-dialkylamino substituent compound. The synthesized compounds show emission spectra at room temperature while 2-phenylazopyridine does not. The excitation spectra of these compounds differ from their absorption spectra which can be explained on the basis of the trans to cis photoisomerization which is supported by the TD-PBE0/6-31G(d,p) calculations. Both oxidation of the dialkylamino substituents (-NR(2); R=-CH(3) and -C(2)H(5)) and reduction of -N=N-/-N=N-(-) and -N=N-(-)/-N=N-(2-) were observed in the cyclic voltammogram indicating a π-acidity of both dyes.

Diarylethene-containing cyclometalated platinum(II) complexes: tunable photochromism via metal coordination and rational ligand design

The synthesis, characterization, electrochemistry, photophysics and photochromic behavior of a new class of cyclometalated platinum(II) complexes [Pt(C(∧)N)(O(∧)O)] (1a-5a and 1b-5b), where C(∧)N is a cyclometalating 2-(2'-thienyl)pyridyl (thpy) or 2-(2'-thienothienyl)pyridyl (tthpy) ligand containing the photochromic dithienylethene (DTE) unit and O(∧)O is a β-diketonato ligand of acetylacetonato (acac) or hexafluoroacetylacetonato (hfac), have been reported. The X-ray crystal structures of five of the complexes have also been determined. The electrochemical studies reveal that the first quasi-reversible reduction couple, and hence the nature of lowest unoccupied molecular orbital (LUMO) of the complexes, is sensitive to the nature of the ancillary O(∧)O ligands. Upon photoexcitation, complexes 1a-3a and 1b-3b exhibit drastic color changes, ascribed to the reversible photochromic behavior, which is found to be sensitive to the substituents on the pyridyl ring and the extent of π-conjugation of the C(∧)N ligand as well as the nature of the ancillary ligand. The thermal bleaching kinetics of complex 1a has been studied in toluene at various temperatures, and the activation barrier for the thermal cycloreversion of the complex has been determined. Density functional theory (DFT) calculations have been performed to provide an insight into the electrochemical, photophysical and photochromic properties.