Intraligand Charge Transfer Transitions Research Articles

Structural, Spectral, Electric-Field-Induced Second Harmonic, and Theoretical Study of Ni(II), Cu(II), Zn(II), and VO(II) Complexes with [N2O2] Unsymmetrical Schiff Bases of S-Methylisothiosemicarbazide Derivatives

New unsymmetrical [N2O2] tetradentate Schiff base complexes of Ni(II), Cu(II), Zn(II), and VO(II) were synthesized by template condensation of the tetradentate precursor 1-phenylbutane-1,3-dione mono-S-methylisothiosemicarbazone with o-hydroxybenzaldehyde or its 5-phenylazo derivative. They were characterized by elemental analysis, IR, UV-vis, electron spin resonance, and NMR spectroscopy, mass spectrometry, and magnetic measurements. The crystal structures of five of them have been determined by X-ray diffraction using, in some cases, synchrotron radiation. These compounds are characterized by a large thermal stability; their decomposition temperatures range from 240 up to 310 degrees C. Complexes with the phenylazo substituent were found to possess a large second-order nonlinear optical (NLO) response, as determined both by measurements of solution-phase direct current electric-field-induced second harmonic generation and by theoretical time-dependent density functional theory (TDDFT) calculations. The molecular hyperpolarizability was found to decrease in the order Zn(II) > Cu(II) > Ni(II) approximately VO(II). The active role of the metal in determining the NLO properties of the complexes was shown through an analysis of their UV-vis spectra, which revealed the presence of metal-to-ligand (in closed-shell complexes) and ligand-to-metal (in open-shell complexes) charge-transfer bands together with intra-ligand charge-transfer transitions. Assignment of the bands was based on the analysis of the TDDFT computed spectra.

Switching between Ligand‐to‐Ligand Charge‐Transfer, Intraligand Charge‐Transfer, and Metal‐to‐Ligand Charge‐Transfer Excited States in Platinum(II) Terpyridyl Acetylide Complexes Induced by pH Change and Metal Ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2''-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.

Ground and Excited State Resonance Raman Spectra of an Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex: Characterization of Excited States, Determination of Structure and Bonding, and Observation of Metal Cation Release from the Azacrown

A [(bpy)Re(CO)3L+] complex (bpy = 2,2'-bipyridine) in which L contains a phenyl-azacrown ether that is attached to Re via an amidopyridyl linking group has been studied by steady state and nanosecond time-resolved resonance Raman spectroscopy. Vibrational band assignments have been aided by studies of model complexes in which a similar electron-donating dimethylamino group replaces the azacrown or in which an electron-donor group is absent, and by density functional theory calculations. The ground state resonance Raman spectra show nu(bpy) and nu(CO) bands of the (bpy)Re(CO)3 group when excitation is exclusively in resonance with the Re --> bpy metal-to-ligand charge-transfer (MLCT) transition, whereas L ligand bands are dominant when it is in resonance with the strong intra-ligand charge-transfer (ILCT) transition present for L ligands with electron-donor groups. Transient resonance Raman (RR) spectra obtained on single color (385 nm) pulsed excitation of the complexes in which an electron-donor group is absent show bpy*- bands of the MLCT excited state, whereas those of the complexes with electron-donor groups show both bpy*- bands and a down-shifted nu(CO) band that together are characteristic of an L-to-bpy ligand-to-ligand charge-transfer (LLCT) excited state. Samples in which a metal cation (Li+, Na+, Ca2+, Ba2+) is bound to the azacrown in the ground state show bands from both excited states, consistent with a mechanism in which the LLCT state forms after metal cation release from the MLCT state. Nanosecond time-resolved RR spectra from two-color (355 nm pump, 500 nm probe) experiments on the electron-donor systems show L-ligand bands characteristic of the LLCT state; the same bands are observed from samples in which a metal cation is bound to the azacrown in the ground state, and their time dependence is consistent with the proposed mechanism in which the rate constant for ion release in the MLCT state depends on the identity of the metal cation.

Molecular sensing behavior of di-2-pyridylketone-2-thenoylhydrazone (dpktah) in non-aqueous media

Optical measurements performed on di-2-pyridylketone-2-thenoylhydrazone (dpktah) in dimethylsulfoxide (dmso) and N, N-dimethylformamide (dmf) media disclosed two intraligand charge transfer (ILCT) transitions that are sensitive to changes in their environment. Variation in the chemical surroundings of dpktah demonstrate reversible interconversion between the two conformations (α-dpktah associated with the low-energy electronic transition observed at 400 nm and β-dpktah with the electronic transition observed at 326 nm), and allowed for calculation of their extinction coefficients. Thermo-optical measurements confirmed the reversible interconversion between the α- and β-dpktah, and gave Δ H ø = −39.57, −36.58 kJ mol −1, Δ S ø = −145.37, +108.57 J mol −1, Δ G ø = +3.77, +4.22 kJ mol −1 and K = +0.22, +0.38 at 298.15 K in dmf and dmso, respectively. The high values for the extinction coefficients, and low values for the thermodynamic parameters for the interconversion between the β- and α-forms of dpktah demonstrate the facile interconversion between the two forms and possible use of dpktah in dmf and dmso as a molecular sensor. Group 12 metal dichlorides in concentration as low as 1.00 × 10 −5 M can be detected and determined using dpktah in dmf. 1H NMR spectra measured in different solvents confirmed the sensitivity of dpktah to changes in its surroundings. At low temperatures, the spectral signature of two isomers appeared, and as the temperature increased, the spectral signature of a monomer prevails.

Beyond the active-electron approximation: Origin of ferromagnetic exchange in donor–acceptor heterospin biradicals

The complex, ( NNSQ)CuTp Cum,Me, is comprised of three different spin carrying units. These three S = 1/2 sites are comprised of Cu(II), nitronyl nitroxide ( NN), and 5- tert-butyl-semiquinone ( SQ). Both the Cu(II)– SQ and SQ– NN pairwise interactions are strongly exchange coupled resulting an S T = 3/2 ground state due to ferromagnetic interactions. Low-temperature EPR spectroscopy confirms the S T = 3/2 ground state determined from magnetic susceptibility studies and indicates that the quartet is zero-field split by | D| ∼ 0.5 cm −1. We have probed the origin of the strong ferromagnetic exchange within the NNSQ heterospin biradical ligand by a combination of electronic absorption, and low-temperature magnetic circular dichroism spectroscopies. The origin of the strong intraligand NNSQ ferromagnetic exchange has been suggested to result from an intraligand charge transfer (ILCT) transition transition that possesses appreciable SQ π → NN π* charge transfer character. The magnitude of the ferromagnetic exchange has been related to the energy of this ILCT transition, the magnitude of the NN single site exchange integral, and the magnitude of the electronic coupling matrix element that connects the SQ SOMO with the NN LUMO.

Reaction of BSA, CT-DNA, Adenine and Guanine with [Pt(RaaiR′)Cl2] and Characterization of the Products [Pt(RaaiR′)(NB)(H2O)](PF6)2] [RaaiR′=1-alkyl-2-(arylazo)imidazole

The reaction of [Pt(RaaiR′)(solvent)2]2+ with nucleobases (NB), adenine and guanine was studied and the products [Pt(RaaiR′)(NB)(H2O)](PF6)2RaaiR′ = 1-alkyl-2-(arylazo)imidazole, R′ = Me (1), Et (2); R = H (a),OMe (b), NO2(c)] characterized by i.r, u.v.–vis. and 1H-n.m.r spectroscopy. The solution spectra exhibit metal-to-ligand charge-transfer transitions (MLCT) and intra-ligand charge-transfer transitions. The position and symmetry of the bands depend on the nucleobase and arylazoimidazole. The coordination of the ligand is supported by 1H-n.m.r. spectral data. The redox property has been examined by cyclic voltammetric technique and shows the involvement of the azo group of the chelated ligand, RaaiR′ in the reduction process along with the irreversible reduction of the coordinated nucleobase. Binding with Bovine Serum Albumin (BSA) and Calf Thymus DNA was performed spectrophotometrically in the physiological buffer medium and spectral profile was recorded in 24 h.

Molecular sensing behavior of the first Mn(I)-compound of di-2-pyridylketone- p-nitrophenylhydrazone (dpknph), fac-[Mn(CO) 3(dpknph)Br

The reaction between [Mn(CO) 5Br] and di-2-pyridylketone- p-nitrophenylhydrazone (dpknph) in diethyl ether under ultrasonic conditions gave fac-[Mn(CO) 3(dpknph)Br] in good yield. Optical and thermodynamic measurements on fac-[Mn(CO) 3(dpknph)Br] in non-aqueous polar solvents revealed reversible interconversion between two intense charge transfer absorption bands due to π–π* (dpk), followed by dpk → nitro intraligand charge transfer transition (ILCT), mixed with metal ligand charge transfer transition (MLCT) due to d π ( Mn ) → π ( dpk ) ∗ . In non-polar solvents, a single absorption band appeared. Extinction coefficients of 46 200 ± 2000 and 28 400 ± 2000 M −1 cm −1 were calculated in DMSO for the low- and high-energy electronic states of fac-[Mn(CO) 3(dpknph)Br] using excess NaBF 4. Changes in enthalpy (Δ H ø) of +14.0 and −12.1 kJ mol −1, entropy (Δ S ø) of +28.65 and −64.30 J mol −1 K −1, and free energy (Δ G ø) of +5.48 and +7.08 kJ mol −1 at 298 K were calculated for the interconversion between the high and low energy electronic states of fac-[Mn(CO) 3(dpknph)Br]. These results allow for the use of these systems ( fac-[Mn(CO) 3(dpknph)Br] and surrounding solvent or solute molecules) as optical sensors for a variety of physical and chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00 × 10 −9 M can be detected and determined using fac-[Mn(CO) 3(dpknph)Br] in dmso in the presence and absence of NaBH 4.

Electronic and structural effects on the nonlinear optical behavior in push-pull TTF/tricarbonyl chromiun arene complexes.

A novel D-pi-A system in which tetrathiafulvalene (TTF) and pi-extended TTFs as strong electron donors are covalently connected to a tricarbonyl (eta(6)-arene)chromium complex as the acceptor moiety through a systematically increased conjugated bridge of vinylene units (12a-c, 16a-c) have been synthesized by Wittig-Horner olefination reaction. The electronic spectra as well as the electrochemical data reveal a different behavior of TTF derivatives (12a-c) and of exTTF derivatives (16a-c). Cyclic voltammetry shows the influence of the tricarbonylchromium arene on the oxidation potentials in compounds 12a-c, and no remarkable effect is observed for exTTFs (16a-c). The nonlinear optical properties of 12a-c and 16a-c have been calculated by using the ab initio CPHF/6-31G//B3P86/6-31G model, and the time-dependent density functional theory (TD-DFT) method has been used for the calculation of the electronic transitions. The calculations reveal that an intraligand charge-transfer transition (ILCT) and the metal to ligand charge-transfer transition (MLCT) are responsible for the nonlinear response. In addition, the large angles formed by the ground-state dipole moment and the vectorial hyperpolarizability are responsible for the mubeta values determined experimentally by the EFISH technique.

Effect of the Coordination to the “Os3(CO)11” Cluster Core on the Quadratic Hyperpolarizability oftrans-4-(4‘-X-styryl)pyridines (X = NMe2,t-Bu, CF3) andtrans,trans-4-(4‘-NMe2-phenyl-1,3-butadienyl)pyridine

Coordination to the “Os3(CO)11” cluster core of substituted styrylpyridines such as trans-4-(4‘-NMe2-styryl)pyridine (L1), trans-4-(4‘-t-Bu-styryl)pyridine (L2), trans-4-(4‘-CF3-styryl)pyridine (L3), or trans,trans-4-(4‘-NMe2-phenyl-1,3-butadienyl)pyridine (L4) produces an enhancement of their quadratic hyperpolarizability, βEFISH, measured by the solution-phase dc electric-field-induced second harmonic (EFISH) generation method. This effect is due either to a red-shift of the intraligand charge-transfer (ILCT) transition upon coordination (when the substituent in para position is a strong electron donor) or to a metal-to-ligand charge-transfer (MLCT) transition (when the substituent is a strong electron acceptor). In the latter case the quadratic hyperpolarizability has a negative sign, due to the negative value of Δμeg. Therefore the “Os3(CO)11” cluster core displays an ambivalent acceptor or donor role. Some of the complexes investigated in this study show significant values (between 500 × 10-48 and 900 × 10-48 esu) of the product μβ0.

Cation sensors containing a (bpy)Re(CO)3 group linked to an azacrown ether via an alkenyl or alkynyl spacer: synthesis, characterisation, and complexation with metal cations in solution.

Two [(bpy)Re(CO)3L]+ complexes (bpy = 2,2'-bipyridine), where L contains an aza-15-crown-5 ether which is linked to Re via an alkenyl- or alkynyl-pyridine spacer, have been synthesised along with model complexes. Solutions of the complexes in acetonitrile have been studied by UV-Vis absorption spectroscopy, and by 1D and 2D 1H NMR spectroscopy. Strong UV-Vis bands, assigned to intraligand charge-transfer transitions localised at the L ligands, blue shift on protonation of the azacrown nitrogen atom or on complexation of alkali-metal (Li+, Na+ and K+) or alkaline-earth metal (Mg2+, Ca2+ and Ba2+) cations to the azacrown; the magnitude of the blue shift is dependent on the cation, with protonation giving the largest shift of ca. 100 nm. Cation binding constants in the range of log K= 1-4 depend strongly on the identity of the metal cation. Protonation or cation complexation causes downfield shifts in the 1H NMR resonances from most of the azacrown and L ligand protons, and their magnitudes correlate with those of the blue shifts in the UV-Vis bands; shifts in the azacrown 1H NMR resonances report on how the different metal cations interact with the macrocycle. UV-Vis and 1H NMR spectra of the free L ligands enable the effect of the Re centre to be assessed. Together, the data indicate that the alkene spacer gives a more responsive sensor than the alkyne spacer by providing stronger electronic communication across the L ligand.

Polymorphic forms of a gold(I) arylacetylide complex with contrasting phosphorescent characteristics.

The spectroscopic properties and crystal structures of the gold(I) arylacetylide complexes [(R(3)P)Au(Ctbd1;CAr)] (R = Cy, Ar = 4-nitrophenyl, 1; 4-trifluoromethylphenyl, 2; pentafluorophenyl, 3; R = Ph, Ar = 4-nitrophenyl, 4) have been examined. The dipole-allowed and -forbidden transitions of 1 (4 in parentheses) at lambda(max) 340 (336) and ca. 485 (470) nm in CH(2)Cl(2) solution at 298 K are assigned to the singlet and triplet intraligand charge transfer (ILCT) transitions of the 4-nitrophenylacetylide moiety, whereas 2 (3 in parentheses) shows localized singlet and triplet acetylenic pipi transitions at lambda(max) 287 (276) and 426 nm, respectively. Two polymorphs of 1 with contrasting phosphorescent characteristics have been identified. At 298 K, the emissive form of 1, as well as 2-4, are highly phosphorescent with peak maximum at 504, 425, 521, and 495 nm, respectively; the other polymorph of 1 is nonemissive at 298 K but emission is detected at 77 K with peak maximum at 486 nm. Crystallographic studies reveal that the major differences between the emissive and nonemissive forms of 1 are the orientations of the molecular dipoles and the dihedral angles between neighboring 4-nitrophenyl moieties. Crystal 2 is isostructural to the nonemissive form of 1, but does not display polymorphism. The molecular planes of two neighboring lumophores are coplanar in the emissive form of 1, parallel in 4, and nearly perpendicular (78.6 degrees ) to each other in the nonemissive form of 1. Both the nature of the excited state and the dihedral angle between adjacent [Au(Ctbd1;CAr)] moieties determine the phosphorescent properties of these molecular crystals.

Synthesis and properties of new dinuclear organoiron(II) hydrazones combining the potent electron-donating [(η 5-C 5H 4)FeCp] fragment with [CpFe(η 6-arene)] +-type acceptors

The stereoselective condensation reaction between organometallic hydrazine precursors [CpFe(η 6- p-RC 6H 4NHNH 2)] +PF 6 − (R=H, ( 1) +PF 6 −; Me, ( 2) +PF 6 −; MeO, ( 3) +PF 6 −; Cl, ( 4) +PF 6 −) and ( E)-(2-ferrocenylvinyl)-methyl ketone, [( E)-[CpFe(η 5-C 5H 4)CHCHCOMe] ( 5), allowed the synthesis of a family of four new push–pull-type homodinuclear hydrazones complexes formulated as ( E, E)-[CpFe(η 6- p-RC 6H 4)NHNCMeCHCH(η 5-C 5H 4)FeCp] +PF 6 − (R=H, ( 6) +PF 6 −; Me, ( 7) +PF 6 −; MeO, ( 8) +PF 6 −; Cl, ( 9) +PF 6 −). These compounds were fully characterized by a combination of elemental analysis and spectroscopic techniques ( 1H-NMR, IR and UV–Vis), and electrochemical studies in order to prove the π-donor–acceptor interactions between the two metallocenic termini through the asymmetric elongated π-conjugated spacer. Cyclic voltammetry studies reveal an electrochemically reversible one-electron oxidation and an irreversible one-electron reduction step for all the hydrazone derivatives. The redox potentials clearly indicate that the oxidation occurs at the ferrocenyl unit, and the reduction is localized on the mixed sandwich entity. In the electronic absorption spectra, two intense bands are observed in the visible region which undergo positive solvatochromic shifts. The origin of the high-energy absorption band is assumed to be an intraligand charge-transfer (ILCT) transition and the low-energy absorption band is assigned to a metal to ligand charge-transfer (MLCT) transition. Both ILCT and MLCT are shifted to lower energy with increasing spacer length.

Luminescent Nitridoosmium(VI) Complexes with Aryl- and Alkylacetylide Ligands: Spectroscopic Properties and Crystal Structures

A series of air- and moisture-stable nitridoosmium(VI) complexes with arylacetylide ligands, [ n Bu 4 N][OsN(C≡CC 6 H 4 R-p) 4 ] (R = H (1), Et (2), OEt (3), Ph (4)), and the alkyl analogue [ n Bu 4 N] [OsN(C≡C t Bu) 4 ] (5) were synthesized. The structures of1, 2, and 4 were determined by X-ray crystal analyses. The Os≡N (1.65(1) A in 1, 1.620(8) A in 2, and 1.619(6) A in 4) and Os-C (mean 2.03 A) distances are similar to that observed for [OsN(CH 2 SiMe 3 ) 4 ] - , whereas the C≡C distances (mean 1.20 A) correspond to terminal arylacetylide groups. The UV-vis absorption spectra of 1-4 display intense absorptions at 248-324 nm (∈ 5 x 10 4 dm 3 mol - 1 cm - 1 ) that are attributed to overlapping of the [p ϖ *(N 3 - → d ϖ *(d x z , d y z )] and [π → π*(arylacetylides)] transitions. The moderately intense absorptions at λ m a x 403-418 nm (E 10 3 dm 3 mol - 1 cm - 1 ) are assigned to admixture of [(d x y ) 2 → (d x y ) 1 (d ϖ *) 1 ] with intraligand charge-transfer transition of the arylacetylides. The broad structureless solid-state luminescence of 1-5 at 298 (λ m a x 600-632 nm) and 77 K (λ m a x 599-635 nm) are assigned to the triplet 3 [(d x y ) 1 (d ϖ *) 1 ] excited state. The 77 K glassy solutions of 1-5 also exhibit 3 [(d x y ) 1 (d ϖ *) 1 ] emissions at λ m a x 597-628 nm. Like [OsNX 4 ] - (X = Cl, Br), complexes 1-5 are nonemissive in dichloromethane at room temperature. Notably, 1 emits weakly at λ m a x 643 nm in benzene at room temperature, whereas 5 exhibits a more intense and long-lived luminescence at λ m a x 640 nm in diethyl ether. Correspondingly, a well-defined absorption at λ m a x 428 nm in the excitation spectrum of 5 (λ e m 640 nm, concentration 8 x 10 - 4 mol dm - 3 ) is assigned to the 1 [(d x y ) 2 ] → 1 (d x y ) 1 (d ϖ *) 1 ] transition.

Ruthenium complexes of 2-[(4-(arylamino)phenyl)azo]pyridine formed via regioselective phenyl ring amination of coordinated 2-(phenylazo)pyridine: isolation of products, X-ray structure, and redox and optical properties.

Aromatic ring amination reactions in the ruthenium complex of 2-(phenylazo)pyridine is described. The substitutionally inert cationic brown complex [Ru(pap)(3)](ClO(4))(2) (1) (pap = 2-(phenylazo)pyridine) reacts smoothly with aromatic amines neat and in the presence of air to produce cationic and intense blue complexes [Ru(HL(2))(3)](ClO(4))(2) (2) (HL(2) = 2-[(4-(arylamino)phenyl)azo]pyridine). These were purified on a preparative TLC plate. The X-ray structure of the new and representative complex 2c has been solved to characterize them. The results are compared with those of the starting complex, [Ru(pap)(3)](ClO(4))(2) (1). The transformation 1 --> 2 involves aromatic ring amination at the para carbon (with respect to the diazo function) of the pendant phenyl rings of all three coordinated pap ligands in 1. The transformation is stereoretentive, and the amination reaction is regioselective. The extended ligand HL(2) coordinates as a bidentate ligand and chelates to ruthenium(II) through the pyridine and one of the azo nitrogens. The amine nitrogen of this bears a hydrogen atom and remains uncoordinated. Similarly, the amination reaction on the mixed-ligand complex [Ru(pap)(bpy)(2)](ClO(4))(2) produces the blue complex [Ru(HL(2))(bpy)(2)](ClO(4))(2) (3) as anticipated. The reactions of [RuCl(2)(dmso)(4)] and [Ru(S)(2)(L)(2)](2+) (dmso = dimethyl sulfoxide, S = labile coordinated solvent, L = 2,2'-bipyridine (bpy) and pap) with the preformed HL(2) ligand have been explored. The structure of the representative complex [RuCl(2)(HL(2a))(2)] (5a) is reported. It has the chlorides in trans configuration while the pyridine as well as azo nitrogens are in cis geometry. Optical spectra and redox properties of the newly synthesized complexes are reported. All the ruthenium complexes of HL(2) are characterized by their intense blue solution colors. The lowest energy transitions in these complexes appear near 600 nm, which have been attributed to intraligand charge-transfer transitions. For example, the lowest energy visible range transition in [Ru(HL(2b))(3)](2+) appears at 602 nm and its intensity is 65 510 M(-1) cm(-1). All the tris chelates show multiple-step electron-transfer processes. In [Ru(HL(2))(3)](2+), six reductions waves constitute the complete electron-transfer series. The electrons are believed to be added successively to the three azo functions. In the mixed-ligand chelates [Ru(HL(2))(pap)(2)](2+) and [Ru(HL(2))(bpy)(2)](2+) the reductions due to HL(2), pap, and bpy are observed.

Metal-promoted aromatic ring amination and deamination reactions at a diazo ligand coordinated to rhodium and ruthenium.

Reactions of MCl(3).3H(2)O (M = Rh and Ru) with the ligand 2-[(2-N-arylamino)phenylazo]pyridine [HL(1); NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(1a)), NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(1b)), and NH(4)C(5)N=NC(6)H(4)N(H)C(5)H(4)N (HL(1c))] in the presence of dilute NEt(3) afforded multiple products. In the case of rhodium, two green compounds, viz. [Rh(L(1))(2)](+) ([2](+)) and [RhCl(pap)(L(1))](+) ([3](+)), where L(1) and pap stand for the conjugate base of [HL(1)] and 2-(phenylazo)pyridine, respectively, were separated on a preparative thin layer chromatographic plate. The reaction of RuCl(3).3H(2)O, on the other hand, produced two brown compounds, viz. [RuCl(HL(1))(L(1))] (4) and [RuCl(pap)(L(1))] (5), respectively, as the major products. The X-ray structures of the representative complexes are reported. Except for complex 2, and 4, the products are formed due to the cleavage of an otherwise unreactive C(phenyl)-N(amino) bond. In complex 4, one of the tridentate ligands (HL(1)) does not use its maximum denticity and coordinates as a neutral bidentate donor. Plausible reasons for the differences in their modes of coordination of the ligands as in 2 and 4 have been discussed. The ligand pap in the cationic mixed ligand complex [3](+) reacts instantaneously with ArNH(2) to produce an ink-blue compound, [RhCl(HL(2))(L(1))](+) ([6](+)) in a high yield. The ligand HL(2) is formed due to regioselective fusion of ArNH(2) residue at the para carbon of the phenyl ring (with respect to the azo fragment) of pap in [3](+). The above complexes are generally intensely colored and show strong absorptions in the visible region, which are assigned to intraligand charge transfer transitions. These complexes undergo multiple and successive one-electron-transfer processes at the cathodic potentials. Electrogenerated cationic complexes of ruthenium(III), [4](+) and [5](+), showed rhombic EPR spectra at 77 K.

Chromium complexes of an isomeric N-donor ligand, 2-[(N-arylamino)phenylazo]pyridine: amination reactions, X-ray structure, and redox properties.

The chromium chemistry of two positional isomers of the ligand 2-[(N-arylamino)phenylazo]pyridine (HL(1)and HL(2)) are described. While the ligand HL(1) coordinates as a bischelating tridentate N,N,N-donor, [L(1)](-), with deprotonation of the amine nitrogen, its isomer HL(2) coordinates as a neutral bidentate N,N-donor. The amine nitrogen in this case remains protonated. Thus the reaction of CrCl(3).nH(2)O with HL(1) produced the brown cationic complex, [Cr(L(1))(2)](+), [1](+). The representative X-ray structure of [1a](ClO(4)) is reported. The two azo nitrogens of the anioinc tridentate ligand approach the metal center closest with Cr(1)-N(azo) av 1.862(6) A. There is a significant degree of ligand backbone conjugation in the coordinated ligands, which resulted in shortening of the C-N distances and also in lengthening of the diazo (N=N) distances. Two synthetic approaches for the synthesis of chromium complexes of HL(2) are investigated. The first approach is based on the substitution reaction, wherein all the coordinated CO ligands of Cr(CO)(6) were completely substituted by the three bidentate HL(2) ligands to produce a violet complex [Cr(HL(2))(3)]. The second approach is based on para-amination reaction of coordinated 2-(phenylazo)pyridine (pap). Thus the reaction of an inert complex, [CrCl(2)(pap)(2)], with ArNH(2) yields a mixed ligand complex, [CrCl(2)(pap)(HL(2))], 3. In this reaction one of the two coordinated pap ligands in [CrCl(2)(pap)(2)] undergoes amination at the para carbon (with respect to the diazo function) to yield HL(2) in situ. This metal-promoted transformation is authenticated by the X-ray structure determination of a representative complex, [CrCl(2)(pap)(HL(2a))], 3a. Notable differences in bond distances along the ligand backbones of the two coordinated ligands in 3a indicate different levels of metal-ligand overlap in this complex. All the chromium complexes of HL(2) are characterized by their intense blue-violet color. The frequencies of the visible range transitions in these complexes linearly correlate with the Hammett's substitution constant. Intraligand charge-transfer transitions in the visible region are believed to be responsible for the intense color. Redox properties of all these complexes are reported.

Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene: contributions from intra-ligand charge transfer states.

The photophysical properties of Ru(II) and Re(I) polypyridyl complexes including a bis-bipyridyl pyrene ligand are presented. The complexes ([(bpy)(2)Ru](2)bpb)(4+) and [(CO)(3)ReCl(bpb)] (bpy = 2,2'-bipyridine, bpb = 1,6-bis-(4-(2,2'-bipyrid-yl)-pyrene) were designed with the intent of examining intramolecular energy migration between MLCT states localized on the metal complexes and pyrene-localized (3)(pi-pi) states. Absorption spectroscopy of both complexes containing the bpb ligand reveals that in addition to the MLCT and the pyrene-centered (1)(pi-pi) transitions, a new absorption band is observed near 400 nm for both complexes. Absorption spectral data for the Re(I) complex strongly suggest the presence of a pyrene(pi) to bpy(pi) intraligand charge transfer (ILCT) transition. Emission spectra at room temperature and at 77 K are almost identical for the Ru(II) and Re(I) complexes containing the bpb ligand. The (3)MLCT emission of related bipyridyl compounds lacking the pyrene is observed at higher energy than for the pyrene-containing complexes, ([(bpy)(2)Ru](2)bpb)(4+) and [(CO(3)ReCl(bpb)]. The Ru(II) complex emits at room temperature with a remarkably long lifetime (130 micros in degassed DMSO). This emission is also strongly sensitive to oxygen and is almost entirely quenched in an aerated solution. In addition, excited-state absorption spectra exhibit features not consistent with (3)MLCT or (3)(pi-pi) states of the parent chromophores. The combined characteristics suggest the emission arises from either (3)(pi-pi) or (3)ILCT states or a state with mixed parentage.

Charge transfer spectra and photoreactivity of ( E)-1-ferrocenyl-2-(1-methyl-4-pyridiniumyl)ethylene cation

The electronic spectrum of ( E)-1-ferrocenyl-2-(1-methyl-4-pyridiniumyl)ethylene ( E-fmpe +) cation shows two long-wavelength absorptions near 570 and 370 nm which are assigned to metal-to-ligand charge transfer (MLCT) and intraligand charge transfer (ILCT) transitions, respectively. The complex displays a strong negative solvatochromism. MLCT as well as ILCT excitation leads to a trans/ cis isomerization at the olefinic double bond. The complex is photochromic since the photoisomerization is thermally reversed. The ion pair E-fmpe +[Fe(CN) 6] 4− shows a photoactive outer-sphere charge transfer absorption at λ max=490 nm.

Quadratic Hyperpolarizability Enhancement ofpara-Substituted Pyridines upon Coordination to Organometallic Moieties: The Ambivalent Donor or Acceptor Role of the Metal

Coordination of para-substituted pyridines 4-X-C5H4N (where X = NMe2, CMe3, H, COMe, CN) to metal carbonyl moieties such as “cis-M(CO)2Cl” (M = Rh(I), Ir(I)) and “fac-Os(CO)3Cl2” produces an enhancement up to about 2 orders of magnitude of the quadratic hyperpolarizability βλ of the free pyridine. This effect is due either to a red shift of the intraligand charge-transfer (ILCT) transition upon coordination (when X is a strong electron donor) or to a significant effect on the metal-to-ligand charge-transfer (MLCT) transitions (when X is a strong electron acceptor). In this latter case the quadratic hyperpolarizability may assume a negative sign, due to the negative value of Δμeg upon excitation. Therefore we confirmed that as already shown by metal carbonyl moieties such as “M(CO)5” (M = Cr, W), a soft metal center displays, from the point of view of the perturbation of the quadratic hyperpolarizability of pyridines, an ambivalent acceptor or donor role. The quadratic hyperpolarizability of complexes of m...

Protonation-State-Dependent Luminescence and Excited-State Electron-Transfer Reactions of 2- and 4-Pyridine (-ium)-Substituted Metallo-1,2-enedithiolates

Luminescence and excited-state electron-transfer reactions of (dppe)Pt{S2C2(2-pyridine)(H)} and (dppe)Pt{S2C2(4-pyridine)(H)} (dppe = diphenyldiphosphinoethane) are enabled by protonation of the appended pyridine, thus serving as a novel means of electronic switching. The neutral complexes have low-lying d-to-d transitions that lead to rapid decay of excited states by nonradiative processes. However, upon protonation, a 1,2-enedithiolate-to-heterocycle π* intraligand charge-transfer transition (ILCT) becomes lower in energy than the d-to-d transition, thus giving rise to emissive 1ILCT* and 3ILCT* excited states for [(dppe)Pt{S2C2(2-pyridinium)(H)}][BF4] and [(dppe)Pt{S2C2(4-pyridinium)(H)}][BF4]. The assignment of these excited states was based on their energies and lifetimes (τ) which range from τ = 3 to 4 ns for the singlet and from τ = 2000 to 7500 ns for the triplet, respectively. Emission quantum yields (φ) increase with solvent polarity and range from φ = 0.0006 to 0.003 for the singlet and from φ ...