Ligand-to-ligand Charge Transfer Transition Research Articles

Insight into the Ligand-to-Ligand Charge-Transfer Process in Rare-Earth-Metal Diradical Complexes.

While a ligand-to-ligand charge-transfer (LLCT) process is an important way to understand the interactions between metal-bridged radicals for late-transition-metal complexes, there is little clear and evident observation of the LLCT process for rare-earth-metal complexes. In this work, rare-earth-metal diradical complexes supported by diazabutadiene (DAD) ligands [(DAD)2RE(BH4)] [RE = Yb (1), Sm (2)] were synthesized and studied. The coordination geometries of 1 and 2 are different due to the different ionic radii. Reduction of 1 or 2 generated monoradical complexes, with one of their DAD radical anions being reduced. In all of the complexes, Sm and Yb remain at the 3+ valence state. In their UV-vis spectra, the LLCT transition of 1 could be clearly observed, but complex 2 did not show the same transition. These results could be related to the geometric structures of the complexes as well as exchange coupling between diradicals, thus clearly expanding the model for late-transition-metal-bridged diradicals to rare-earth systems experimentally.

Copper(I) Complexes of N-(2-{[(2E)-2-(4-Nitrobenzylidenyl)Hydrazinyl]Carbonyl}Phenyl)Benzamide and Triphenylphosphine: Synthesis, Characterization and Luminescence Properties.

Copper(I) complexes of the formula [Cu(L)(PPh3)2]X (1-4) (X=Cl(1), ClO4(2), BF4(3) and PF6(4)) [where L=N-(2-{[(2E)-2-(4-nitrobenzylidenyl)hydrazinyl]carbonyl}phenyl)benzamide; PPh3=triphenylphosphine] have been prepared by the condensation of N-[2-(hydrazinocarbonyl)phenyl]benzamide with 4-nitrobenzaldehyde followed by the reaction with CuCl, [Cu(MeCN)4]ClO4, [Cu(MeCN)4]BF4 and [Cu(MeCN)4]PF6 in presence of triphenylphosphine as a coligand. Complexes 1-4 were then characterized by elemental analyses, FTIR, UV-visible and 1H NMR spectroscopy. Mononuclear copper(I) complexes 1-4 were formed with L in its keto form by involvement of azomethine nitrogen and the carbonyl oxygen along with two PPh3 groups. A single crystal X-ray diffraction study of the representative complex [(Cu(L)(PPh3)2]CIO4 (2) reveals a distorted tetrahedral geometry around Cu(I). Crystal data of (2): space group = C2/c, a=42.8596 (9) Å, b=14.6207 (3) Å, c=36.4643 (7) Å, V=20,653.7 (7) Å3, Z=16. Complexes 1-4 exhibit quasireversible redox behaviour corresponding to a Cu(I)/Cu(II) couple. All complexes show blue-green emission as a result of fluorescence from an intra-ligand charge transition (ILCT), ligand to ligand charge transfer transition (LLCT) or mixture of both. Significant increase in size of the counter anion shows marked effect on quantum efficiency and lifetime of the complexes in solution.

Electronic structure, charge transfer character and spectroscopic property of electroluminescent/photoluminescent [ZnL2] (HL = 2-(1H-benzo[d]imidazol-2-yl)-4-bromophenol) studied by density functional theory

In this paper the DFT-/TDDFT-based theoretical calculation results of the electroluminescence/photoluminescence zinc(II) chelate complex [ZnL2] are presented, including the molecular geometry, electronic structure, charge transfer character of absorption and emission, effect of the Br group on the absorption/emission property and colors. The charge transfer character of Uv-vis absorptions, and the nature of electroluminescence/photoluminescence are all due to the π → π* ligand-to-ligand charge transfer transition (LLCT). The significant effect of the substituted Br group on the absorption/emission property and colors, and the Zn ligand bonding property of the main ionic interaction are analyzed and discussed in detail based on PDOS/OPDOS and the Mulliken population analyses. It is concluded that the introduction of substituted groups should significantly affects the electroluminescence/photoluminescence metal chelate complexes, with their absorption/emission property and colors being tuned.

Pt(II) bipyridyl complexes bearing substituted fluorenyl motif on the bipyridyl and acetylide ligands: synthesis, photophysics, and reverse saturable absorption.

A series of Pt(II) diimine complexes bearing benzothiazolylfluorenyl (BTZ-F8), diphenylaminofluorenyl (NPh2-F8), or naphthalimidylfluorenyl (NI-F8) motifs on the bipyridyl or acetylide ligands (Pt-4-Pt-8), (i.e., {4,4'-bis[7-R1-F8-(≡)n-]bpy}Pt(7-R2-F8- ≡ -)2, where F8 = 9,9'-di(2-ethylhexyl)fluorene, bpy = 2,2'-bipyridine, Pt-4: R1 = R2 = BTZ, n = 0; Pt-5: R1 = BTZ, R2 = NI, n = 0; Pt-6: R1 = R2 = BTZ, n = 1; Pt-7: R1 = BTZ, R2 = NPh2, n = 1; Pt-8: R1 = NPh2, R2 = BTZ, n = 1) were synthesized. Their ground-state and excited-state properties and reverse saturable absorption performances were systematically investigated. The influence of these motifs on the photophysics of the complexes was investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The intense absorption bands below 410 nm for these complexes is assigned to predominantly (1)π,π* transitions localized on either the bipyridine or the acetylide ligands; while the broad low-energy absorption bands between 420 and 575 nm are attributed to essentially (1)MLCT (metal-to-ligand charge transfer)/(1)LLCT (ligand-to-ligand charge transfer) transitions, likely mixed with some (1)ILCT (intraligand charge transfer) transition for Pt-4-Pt-7, and predominantly (1)ILCT transition admixing with minor (1)MLCT/(1)LLCT characters for Pt-8. The different substituents on the acetylide and bipyridyl ligands, and the degrees of π-conjugation in the bipyridyl ligand influence both the (1)π,π* and charge transfer transitions pronouncedly. All complexes are emissive at room temperature. Upon excitation at their respective absorption band maxima, Pt-4, Pt-6, and Pt-8 exhibit acetylide ligand localized (1)π,π* fluorescence and (3)MLCT/(3)LLCT phosphorescence in CH2Cl2, while Pt-5 manifests (1)ILCT fluorescence and (3)ILCT phosphorescence. However, only (1)LLCT fluorescence was observed for Pt-7 at room temperature. The nanosecond transient absorption study was carried out for Pt-4-Pt-8 in CH3CN. Except for Pt-7 that contains NPh2 at the acetylide ligands, Pt-4-Pt-6 and Pt-8 all exhibit weak to moderate excited-state absorption in the visible spectral region. Reverse saturable absorption (RSA) of these complexes was demonstrated at 532 nm using 4.1 ns laser pulses in a 2 mm cuvette. The strength of RSA follows this trend: Pt-4 > Pt-5 > Pt-7 > Pt-6 > Pt-8. Incorporation of electron-donating substituent NPh2 on the bipyridyl ligand significantly decreases the RSA, while shorter π-conjugation in the bipyridyl ligand increases the RSA. Therefore, the substituent at either the acetylide ligands or the bipyridyl ligand could affect the singlet and triplet excited-state characteristics significantly, which strongly influences the RSA efficiency.

Effects of Extended π-Conjugation in Phenanthroline (N∧N) and Phenylpyridine (C∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III) Complexes

Five new iridium(III) complexes (3,8-R-Phen)Ir(2-(3-R′-phenyl)pyridine)2 (1, R = fluoren-2-yl, R′ = H; 2, R = 7-benzothiazolylfluoren-2-yl, R′ = H; 3, R = H, R′ = fluoren-2-yl; 4, R = H, R′ = 7-benzothiazolylfluoren-2-yl; 5, R = R′ = 7-benzothiazolylfluoren-2-yl) with fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituent on the 2-phenylpyridine (C∧N) and/or phenanthroline (N∧N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically via UV–vis absorption, emission, and transient difference absorption spectroscopy. Time-dependent density functional theory (TDDFT) calculations were performed to complement the experimental data and aid in our understanding of the characters of optical transitions. In addition, reverse saturable absorption was demonstrated at 532 nm for all complexes using nanosecond laser pulses. All complexes possess a weak low-energy tail that is attributed to 1,3MLCT (metal-to-ligand charge transfer)/1,3LLCT (ligand-to-ligand charge transfer) transitions. The major absorption bands below 475 nm for 1–5 arise from the 1π,π* and intraligand (1ILCT) transitions within the N∧N or C∧N ligands. Attachment of fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents on N∧N ligand results in stronger red-shifts of the main absorption band to 400 and 408 nm for 1 and 2, respectively, as compared to 321 and 361 nm in complexes 3 and 4 with the same substituents on the C∧N ligands. In contrast, the 1,3MLCT/1,3LLCT transitions in 1 and 2 are just slightly red-shifted as compared to those in 3 and 4. The emission of complexes 1 and 2 is attributed to the N∧N ligand-centered 3π,π* state with some admixture of 3MLCT/3ILCT/3LLCT characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the 3MLCT/3LLCT states. For complex 5, which contains 7-benzothiazolylfluoren-2-yl substituents on both the C∧N and the N∧N ligands, the emission predominantly arises from the 3MLCT/3LLCT states with a small portion of N∧N ligand-localized 3π,π* character. All complexes exhibit broadband triplet excited-state absorption in the visible to the near-IR region, with the major absorption bands bathochromically shifted in 1 and 2 as compared to those in 3 and 4. The stronger excited-state absorption leads to dramatic reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses. The RSA strength decreases as 5 ≈ 2 > 4 ≈ 1 > 3, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground state. Extended π-conjugation in the N∧N ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with π-conjugated substituents only on the C∧N ligands (3 and 4).

Nonlinear Absorbing Cationic Iridium(III) Complexes Bearing Benzothiazolylfluorene Motif on the Bipyridine (N∧N) Ligand: Synthesis, Photophysics and Reverse Saturable Absorption

Four new heteroleptic cationic Ir(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) (1 and 2) and phenylpyridine (C∧N) (3 and 4) ligands are synthesized and characterized. The influence of the position of the substituent and the extent of π-conjugation on the photophysics of these complexes is systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The complexes exhibit ligand-centered (1)π,π* transitions with admixtures of (1)ILCT (π(benzothiazolylfluorene) → π*(bpy)) and (1)MLCT (metal-to-ligand charge transfer) characters below 475 nm, and very weak (1,3)MLCT and (1,3)LLCT (ligand-to-ligand charge transfer) transitions above 475 nm. The emission of these complexes at room temperature in CH2Cl2 solutions is ascribed to be predominantly from the (3)MLCT/(3)LLCT states for 1 and from the (3)π,π* state for 2, while the emitting state of 3 and 4 are assigned to be an admixture of (3)MLCT, (3)LLCT, and (3)π,π* characters. The variations of the photophysical properties of 1-4 are attributed to different degrees of π-conjugation in the bipyridine and phenylpyridine ligands induced by different positions of the benzothiazolylfluorenyl substituents on the bipyridine ligand and different extents of π-conjugation in the phenylpyridine ligands, which alters the energy and lifetime of the lowest singlet and triplet excited states. 1-4 all possess broadband transient absorption (TA) upon nanosecond laser excitation, which extends from the visible to the NIR region. Therefore, 1-4 all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. However, the TA of complexes 1, 2, and 3 are much stronger than that of 4. This feature, combined with the difference in ground-state absorption and triplet excited-state quantum yield, result in the difference in RSA strength, which follows this trend: 1 ≈ 2 ≈ 3 > 4. Therefore, complexes 1-3 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.

Theoretical study on photophysical properties of Pt(II) triarylborons with a 2,2-bpy core derivatives

The photophysical properties of the linear and v shaped Pt(II) triarylborons with a 2,2′-bpy core derivatives have been investigated by density functional theory (DFT) method. The calculated electronic absorption wavelengths are in agreement with experimental ones, which can be described as a mixed transition of intra-ligand charge transfer (ILCT), ligand to ligand charge-transfer (LLCT), and metal-to-ligand charge transfer (MLCT). It is found that the MLCT transition is mainly responsible for the low-energy absorption band with relative smaller oscillator strength, while the high-energy absorption band mainly derives from ILCT and LLCT transition. Moreover, the electron absorption wavelengths are not only dependent on the position of the Ph-BMes2 but also on the electron-accepting ability of the acceptor groups. The first hyperpolarizability values of the v shaped complexes are larger than that of the linear shape complex, which indicates that larger intramolecular charge transfer for the v shaped complexes will come into being under the external electric field. Moreover, these complexes exhibit two-dimensional second-order nonlinear optical (NLO) character. Thus, the studied complexes have a possibility to be excellent second-order NLO materials. Based on the two-level model, the variation of first hyperpolarizabilities of the studied complexes can be explained by the combined effect of the difference between the ground state and excited state dipole moment, the oscillator strength, and the cube of the transition energy.

Structural investigations on bis-(semicarbazido)dihydrazine nickel(II) complex synthesized by using uracil and hydrazine hydrate

A mixed-ligand bis-(semicarbazido)dihydrazine nickel(II) complex, [Ni(sem)2(Hz)2] (Hsem=semicarbazide, Hz=hydrazine) has been synthesized by unusual ring opening of uracil reacting with hydrazine hydrate in the presence of nickel(II) chloride. The complex was characterized by elemental analyses, IR, UV–Vis spectral studies and magnetic susceptibility measurement. The molecular structure of the complex was finally determined by single crystal X-ray diffraction study. The complex acquires a six coordinate distorted octahedral geometry in which semicarbazide ligand binds through hydrazinic –NH2 and a deprotonated C–O− group whereas, hydrazine coordinates through one of the –NH2 group to nickel(II) centre. The structure was stabilized by the presence of strong inter-molecular hydrogen bonding interactions N–H⋯N and N–H⋯O forming a huge supramolecular network. Further, the structure was satisfactorily modeled by density functional theory calculations (DFT). UV–Vis and IR spectra are analyzed in depth with the help of time dependent-DFT. The theoretical analyses of electronic structure and molecular orbitals have suggested that the high-energy absorption bands in UV–Vis spectrum are mainly π→π∗ ligand-to-ligand charge transfer transitions (LLCT) and lower energy absorption bands are π→(dz2 or dx2−y2, dyz) ligand-to-metal charge transfer transitions (LMCT) in nature.

Synthesis, structures, photoluminescence, and theoretical investigations on two new Zn(II)/Mn(II) complexes with pyridine-2-amidoxine and carboxylate ligands

Two new neutral mononuclear Zn(II) and Mn(II) complexes with pyridine-2-amidoxine and carboxylate ligands, [Zn(paH)2(OAc)2]·2CH3OH (paH = pyridine-2-amidoxine, HOAc = acetic acid) (1), and [Mn(paH)2(OAc)2]·C2H5OH·2H2O (2), have been prepared and characterized structurally by X-ray crystallography. 1 and free paH exhibit photoluminescence at room temperature in solid state, which is rare so fare for metal complexes with oxime-based ligand. The emissions of 1 and free paH arise from the metal-perturbed paH-based π → π* ligand-to-ligand charge transfer transition (LLCT) and π → π* charge transfer transition in nature, respectively, in terms of the density functional theory level calculations and molecular orbital analyses. Two new neutral mononuclear Zn (II) and Mn(II) complexes with pyridine-2-amidoxine and carboxylate ligands, [Zn(paH)2(OAc)2]·2CH3OH (paH = pyridine-2-amidoxine, HOAc = acetic acid) (1), and [Mn(paH)2(OAc)2]·C2H5OH·2H2O (2), have been synthesized and characterized structurally. The rarely reported photoluminescent behavior for oxime-based metal complexes has been observed in solid 1, indicating that such neutral mononuclear oxime-based metal complexes are a new class of potential candidates for photoluminescence and/or electroluminescence devices. The photoluminescent property and spectroscopic shifts have been investigated in detail for 1 and free paH ligand by means of the density functional theory level calculations and molecular orbital analyses. The emission for 1 and free paH ligand, are related to the metal-perturbed paH-based π → π* ligand-to-ligand charge transfer transition (LLCT) and π → π* charge transfer transition in nature, respectively.

Synthesis, structure, spectroscopic properties, and theoretical studies of alkaline earth metal complexes of 1,3-bis(carboxymethyl)benzimidazolium

Alkaline earth metal complexes of 1,3-bis(carboxymethyl)benzimidazolium (HL) including [Mg(L) 2(H 2O) 4]·2H 2O ( 1), [Ca(L) 2(H 2O) 4]·2H 2O ( 2) and [Ba(L) 2(H 2O) 2·2H 2O] n ( 3) have been synthesized by a hydrothermal procedure and characterized by X-ray crystallography, spectroscopy and thermoanalysis. The results of X-ray crystallography revealed that complexes 1 and 2 are neutral monomers, while complex 3 is a polymeric species, and the carboxymethyl moiety of the deprotonated ligand (L −) acts as a bridge between metal ions. The spectroscopic properties of the complexes were evaluated through density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations. The results revealed that the absorption peaks in UV–Vis spectra were derived from ligand to ligand charge-transfer transitions (LLCT) and/or (p, π) → ( σ ∗, π ∗) intraligand charge-transfer (ILCT). HL and HL-based complexes displayed similar emission peaks in the near-UV region due to ligand-based LLCT. The results of the Fukui index of the complexes supported LLCT and ILCT mechanisms. In addition, the calculated value of the luminescence oscillator strength displayed a linear relationship with the charge distribution, and the DFT chemical reactivity index, and correlation coefficients ( R 2) of 0.999 and 0.996 were obtained, respectively. Thus, the results revealed that the emission strength of the complex can be controlled by coordinating the ligand to different metal ions.

One novel and unprecedented one-dimensional zinc coordination polymer with a N,O-donor chelating phenolic ligand, 3-n-butyl-2-(2-hydroxyphenyl)-3H-benzimidazole: synthesis, structure, characterization, photoluminescence and theoretical calculations

One novel and unprecedented 1D zinc coordination polymer with an unsymmetrical N,O-donor phenolic ligand, [Zn(bpbm)2]n (1) (Hbpbm·HCl·H2O=3-n-butyl-2-(2-hydroxyphenyl)-3H-benzimidazole hydrochloride hydrate) has been prepared and characterized crystallographically, and the geometric structure and photoluminescent properties, investigated experimentally and theoretically by DFT level. The deprotonated ligands, bpbm−, as μ2-bridging, adopt uncommon (O,N)-bridging poly(oligo)merization coordination mode, and not the common O-bridging poly(oligo)merization coordination mode. The photoluminescent property is assigned to π→π∗ ligand-to-ligand charge transfer transition (LLCT).

Synthesis, X-ray crystal structure, DFT- and TDDFT-based spectroscopic property investigations on a mixed-ligand chromium(III) complex, bis[2-(2-hydroxyphenyl)benzimidazolato]-(1,10-phenanthroline)chromium(III) isophthalate

A mixed-ligand Cr(III) complex with 2-(2-hydroxyphenyl)benzimidazole, 1,10-phenanthroline and isophthalic acid, [Cr(pbm) 2(phen)]X 0.5 ( 1X 0.5) (Hpbm = 2-(2-hydroxyphenyl)benzimidazole; phen = 1,10-phenanthroline; H 2X = isophthalic acid) has been prepared by heating in aqueous solution and characterized, and the geometric structure and spectroscopic properties, investigated experimentally and theoretically by using the density functional theory level (DFT) and the time-dependent density functional theory level (TDDFT). The theoretical–experimental agreement is satisfactory. Further theoretical analyses of electronic structure and molecular orbitals have demonstrated that the low-lying absorption bands in UV–Vis spectrum are mainly π → π∗ ligand-to-ligand charge transfer transition (LLCT) and or π → ( d z 2 - d x 2 - y 2 - d yz ) ligand-to-metal charge transfer transition (LMCT) in nature.

Synthesis, structure, photoluminescence and theoretical studies of an In(III) complex with 2-(2′-hydroxylphenyl)benzoxazole

The synthesis and characterization of [In(pbx) 3] ( 1) (Hpbx = 2-(2′-hydroxylphenyl)benzoxazole) are presented. The ground and low lying excited electronic states in 1 are studied using density functional theory level (DFT). The optimized geometry is compared to the experimentally observed structure. Time-dependent density functional theory level (TDDFT) is employed to investigate the excited singlet states. The calculated energies of the low lying singlet states in 1 are in considerable agreement with the experimental data. All the low lying transitions are categorized as π → π∗ ligand-to-ligand charge transfer transitions (LLCT) in nature. The emissive state of 1 is assigned as a singlet metal-perturbed π → π∗ ligand-to-ligand charge transfer transition (LLCT).

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.

Interplay between Intra‐ and Interligand Charge Transfer with Variation of the Axial N‐Heterocyclic Ligand in Osmium(II) Pyridylpyrazolate Complexes: Extensive Color Tuning by Phosphorescent Solvatochromism

The rational design and syntheses of a new series of Os(II) complexes with formula [Os(fppz)(2)(CO)(L)] (1: L=4-dimethylaminopyridine; 2: L = pyridine; 3: L = 4,4'-bipyridine; 4: L = pyridazine; 5: L = 4-cyanopyridine), bearing two (2-pyridyl)pyrazolate ligands (fppz) together with one carbonyl and one N-heterocyclic ligand at the axial positions are reported. Single-crystal X-ray diffraction studies of, for example, 2 reveal a distorted octahedral geometry in which both fppz ligands reside in the equatorial plane with a trans configuration and adopt a bent arrangement at the metal center with a dihedral angle of approximately 23 degrees , while the carbonyl and pyridine ligands are located at the axial positions. Variation of the axial N-heterocyclic ligand leads to remarkable changes in the photophysical properties as the energy gap and hence the phosphorescence peak wavelength can be tuned. For complexes 1 and 2 the solvent-polarity-independent phosphorescence originates from a combination of intraligand (3)pi-pi* ((3)ILCT) and metal-to-ligand charge transfer transitions ((3)MLCT). In sharp contrast, as supported by cyclic voltammetry measurements and theoretical calculations, complexes 3--5 exhibit mainly ligand-to-ligand charge transfer (LLCT) transitions, resulting in a large dipolar change. The phosphorescence of complexes 3--5 thus exhibits a strong dependence on the polarity of the solvent, being shifted for example, from 560 (in C(6)H(12)) to 665 nm (in CH(3)CN) and from 603 (in C(6)H(12)) to 710 nm (in CH(3)CN) for complexes 3 and 5, respectively. The results clearly demonstrate that a simple, straightforward derivatization of the axial N-heterocyclic ligand drastically alters the excitation properties per se from intraligand charge transfer (ILCT) to LLCT transitions. The latter exhibit remarkable LLCT phosphorescence solvatochromism so that a broad range of color tunability can be achieved.

Arylazoimidazoleplatinum(II) complexes and their dioxolene derivatives: single crystal X-ray structure of (catecholato){1-ethyl-2-( p-tolylazo)-imidazole}platinum(II)

The reaction of 1-ethyl-2-(arylazo)imidazoles (RaaiEt, 1) [R=H ( 1a), p-Me ( 1b), p-Cl ( 1c)] with K 2PtCl 4 in boiling acetonitrile–water (1:1) produces red–brown Pt(RaaiEt)Cl 2 ( 2) complexes. Addition of dioxolene in the presence of Et 3N to chloroform–methanol solution of Pt(RaaiEt)Cl 2 has yielded green-coloured mixed-ligand complexes [Pt(RaaiEt)(O,O)] [O,O=catecholate (cat) ( 3), 4- tert-butylcatecholate (tbcat) ( 4), 3,5-di- tert-butylcatecholate (dtbcat) ( 5), tetrachlorocatecholate (tccat) ( 6)]. The structure of [Pt( p-MeaaiEt)(cat)]·1/2H 2O ( 3b) was confirmed by X-ray diffraction. Electronic spectra exhibit ligand-to-ligand charge transfer transition (LLCT) in the VIS–NIR region; the position and symmetry of the band depend on the substituent type on the dioxolene and arylazoimidazole frames. This is qualitatively assigned as a HOMO(cat)→LUMO(RaaiEt) transition. Cyclic voltammograms of the complexes show four successive redox responses: two couples at positive to saturated calomel electrode (SCE) correspond to catechol to semiquinone and semiquinone to quinone oxidations, and two couples at negative to SCE correspond to the azo reductions. The difference in the potential E 1/2 of the first oxidation and reduction process changes linearly with the LLCT transition.

Photolysis of methylcobalamin. Nature of the reactive excited state

Photolysis of methylcobalamin (Co IIIcorrin(CH 3)L + H 2O + O 2 → Co III(H 2O)L + H 2CO + OH −) shows a pronounced wavelength dependence. It is suggested that the reactive excited state is of the ligand-to-ligand charge transfer (LLCT) type and involves the promotion of an electron from the CoC σ-bond to a π*(corrin) orbital. This LLCT transition mixes with the ππ*(corrin) transitions. Owing to this LLCT contribution, the ππ*-absorption bands are also photoactive but with reduced efficiency.