Metal-to-ligand Charge Transfer Transitions Research Articles

Ruthenium(II) and Osmium(II) Complexes Bearing Bipyridine and the N-Heterocyclic Carbene-Based C^N^C Pincer Ligand: An Experimental and Density Functional Theory Study

Ruthenium(II) and osmium(II) complexes [M(C^N^C)(N^N)L](n+) (L = Cl(-), n = 1; L = CH3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C(1)^N^C(1)) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C(2)^N^C(2)), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), C^N^C, N^N, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M(C^N^C)(N^N)L](n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a dπ(Ru(II)/Os(II)) → π*(N^N) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M(C^N^C)(N^N)L](n+) and not a dπ(Ru(II)/Os(II)) → π*(C^N^C) MLCT assignment. This is in stark contrast to [Ru(tpy)(bpy)Cl](+) and [Os(tpy)(bpy)Cl](+) (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) for which the lowest-energy transitions are assigned as dπ(Ru/Os) → π*(tpy) MLCT transitions. [Ru(II)(C^N^C)(N^N)L](n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their dπ(Ru(II)) → π*(N^N) MLCT bands. The electronic structures for [Ru(C^N^C)(N^N)Cl](0) have also been probed by spectroelectrochemistry, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations, which reveal that the lowest unoccupied molecular orbitals (LUMOs) for [Ru(C^N^C)(N^N)Cl](+) are N^N-based.

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.

Monometallic Osmium(II) Complexes with Bis(N-methylbenzimidazolyl)benzene or -pyridine: A Comparison Study with Ruthenium(II) Analogues

Seven bis-tridentate osmium complexes with Mebib or Mebip (Mebib is the 2-deprotonated form of 1,3-bis(N-methylbenzimidazolyl)benzene and Mebip is bis(N-methylbenzimidazolyl)pyridine) have been prepared, and their electrochemical and spectroscopic properties are compared with ruthenium structural analogues. Among them, four complexes have the [Os(NCN)(NNN)]-type coordination, including [Os(Mebib)(Mebip)](PF6)2 (1(PF6)2), [Os(dpb)(Mebip)](PF6) (2(PF6), dpb is the 2-deprotonated form of 1,3-di(pyrid-2-yl)benzene), [Os(Mebib)(ttpy)](PF6) (3(PF6), ttpy = 4'-tolyl-2,2':6',2"-terpyridine), and [Os(dpb)(ttpy)](PF6) (4(PF6)). The other three complexes are [Os(Mebip)2](PF6)2 (5(PF6)2), [Os(Mebip)(tpy)](PF6)2 (6(PF6)2, tpy = 2,2':6',2"-terpyridine), and [Os(ttpy)2](PF6)2 (7(PF6)2) with the [Os(NNN)(NNN)]-type coordination. Single crystals of 2(PF6) and 6(PF6)2 have been obtained, and their structures are studied by X-ray crystallographic analysis. The Os(II/III) redox potentials of 1(PF6)2 to 7(PF6)2 progressively increase from +0.04, +0.23, +0.24, +0.36, +0.56, +0.79 to +0.94 V vs Ag/AgCl, which are 200-300 mV less positive relative to the Ru(II/III) potentials of their ruthenium counterparts. The highest occupied molecular orbital energy levels of 1(+)-7(2+) are calculated to vary in a descending order. The ruthenium and osmium complexes have singlet metal-to-ligand charge-transfer (MLCT) transitions of similar energies and band shapes, while the osmium complexes display additional (3)MLCT transitions in the lower-energy region. Complexes 6(PF6)2 and 7(PF6)2 emit weakly at 780 and 740 nm, respectively. Complex 1(PF6)2 was synthesized as the oxidized Os(III) salt because of the low Os(II/III) potential. The transformation of 1(2+) to 1(+) by chemical reduction or electrolysis led to the emergence of the (1)MLCT transitions in the visible region.

Material Profiling for Photocrystallography: Relating Single-Crystal Photophysical and Structural Properties of Luminescent Bis-Cyclometalated Iridium-Based Complexes

The photophysical properties of seven luminescent iridium complexes are characterized in their single-crystal form, and the photoactivity is related to their molecular structures. Specifically, solid-state optical emission spectra and associated lifetimes are determined from single crystals of iridium complexes containing three bidentate ligands: two variously substituted 2-phenylbenzothiazoles and either a 2,4-pentadione (acetylacetone) or 2-pyridinecarboxylic (picolinic) acid. All complexes studied exhibit emissive behavior in the solid-state which originates from 3π–π* and metal-to-ligand-charge-transfer (MLCT) electronic transitions; this is supported by density functional theory. Phosphorescence is observed in all cases with microsecond lifetimes, ranging from 0.30 to 2.4 μs at 298 K and 1.4–4.0 μs at 100 K. Structure–property relationships are established which are relevant to the potential solid-state application of this series of luminescent complexes as organic light emitting diodes (OLED) material components. In addition, these materials are assessed for their suitability to time-resolved pump–probe photocrystallography experiments, which will reveal their photoexcited state structure. Accordingly, the design process by which materials are selected and technical parameters are defined for a photocrystallography experiment is illustrated. This family of complexes presents a case study for this photocrystallography material profiling. Results show that the time-resolved photoexcited state structure, featuring the MLCT transition is, in principle at least, viable for two of these complexes.

SERRS Spectra and Excitation Profiles of Ru(II) Polypyridine Complexes Attached to Ag Nanoparticle Aggregates: Structural, Electronic, and Resonance Damping Effects of Chemisorption

Surface enhanced resonance Raman scattering (SERRS) spectra and SERRS excitation profiles of a dicationic Ru(II) bis(2,2′-bipyridine)(4,4′-dicarboxy-2, 2′-bipyridine) (Ru(bpy)2(dcbpy)) complex in systems with Ag nanoparticle (NP) aggregates were obtained and compared to those of a dicationic Ru(II) tris(2,2′-bipyridine) (Ru(bpy)3). SERRS spectra provided evidence for chemisorption of Ru(bpy)2(dcbpy) complex onto Ag NP surface via at least one of the carboxylate groups. SERRS excitation profiles of the spectral bands attributed to the Ru-dcbpy unit of the chemisorbed Ru(bpy)2(dcbpy) complex were found to maximize at 488 nm excitation, while those of the two Ru-bpy units peaked at 458 nm. Comparison of the profiles with the electronic absorption spectrum of free Ru(bpy)2(dcbpy) indicates that chemisorption of the complex causes a red-shift of the Ru(II) → dcbpy MLCT (metal to ligand charge transfer) transition band. The energy decrease of the Ru(II) → dcbpy MLCT transition is explained by an increase of the electron-withdrawing ability of the two COO– groups upon their chemisorption on Ag NP surface. Finally, concentration values of SERRS spectral limits of detection of both complexes in system with uniform morphologies of Ag NP aggregates were determined, yielding the 1 × 10–9 M value for the chemisorbed Ru(bpy)2(dcbpy) and the 1 × 10–12 M value for the electrostatically bonded Ru(bpy)3. The major contribution to this difference is attributed to ca. 500× higher resonance damping in SERRS of the chemisorbed complex in comparison to that in SERRS of the electrostatically bonded one.

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.

Rhenium(I) complexes of readily functionalized bidentate pyridyl-1,2,3-triazole “click” ligands: A systematic synthetic, spectroscopic and computational study

A family of electronically tuned fac-Re(CO)3Cl pyridyl-1,2,3-triazole complexes have been synthesized by refluxing methanol solutions of [Re(CO)5Cl] and the substituted 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine ligands (pytri-R). The resulting rhenium(I) complexes were characterised by elemental analysis, HR-ESMS, IR, 1H and 13C NMR and in one case the molecular structure was confirmed by X-ray crystallography. The electronic structure of this family of fac-[(pytri-R)Re(CO)3Cl] complexes was probed using UV–Vis, Raman and emission spectroscopy and cyclic voltammetry techniques. The complexes show intense absorptions in the visible region, comprising strong π→π∗ and metal-to-ligand charge-transfer (MLCT) transitions, which were modelled using time-dependent density functional theory (TD-DFT). Interestingly, the MLCT transition energy and the emission maxima are unaffected by the nature of the R substituent on the 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine ligand indicating that the 1,2,3-triazoyl unit is acting as an electronic insulator. The emission lifetimes of the complexes are modestly dependent on the nature of the 1,2,3-triazole substituent, with the conjugated complexes displaying longer lifetimes than the non-conjugated ones. The shorter lifetimes for complexes with non-conjugated ligands are attributed to the “free-rotor” effect which allows molecules to relax through non-radiative pathways. In this case, the freely rotating CH2 group located between the triazole and the R group causes the decrease in excited lifetime. The electrochemistry of all examples is defined by irreversible Re oxidation and triazole based ligand reduction processes. The nitro substituted complexes show additional nitrobenzene type reduction features. Similarly, the ferrocenyl substituted complex displays the expected reversible one electron oxidation process. Consistent with the spectroscopic data, the position of the oxidation and reduction processes are essentially unaffected by the electronic nature of the 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine substituent.

Neutral cuprous complexes as ratiometric oxygen gas sensors

Four neutral mononuclear Cu(I) complexes, [Cu(pyin)(PPh(3))(2)] (1a), [Cu(pyin)(DPEphos)] (1b), [Cu(quin)(PPh(3))(2)] (2a) and [Cu(quin)(DPEphos)] (2b) (Hpyin = 2-(2-pyridyl)indole, Hquin = 2-(2-quinolyl)indole and DPEphos = bis(2-(diphenylphosphino)phenyl)ether) have been synthesized. X-Ray crystal structure analysis revealed that the central Cu(I) ion in all complexes is in a distorted tetrahedral coordination environment. All four complexes display the typical metal-to-ligand charge transfer (MLCT) absorption band at 371, 363, 413 and 402 nm, respectively. No emission was observed from any complexes in the solid state due to triplet-triplet annihilation. However, the complexes show unusual dual-emission originating from intraligand charge-transfer (ILCT) and MLCT transitions, when dispersed in a rigid matrix (e.g. PMMA) or in frozen CH(2)Cl(2). The oxidation potential of Cu(I)/Cu(II) in these neutral complexes, ∼0.5 V (vs. Ag/AgCl), is lower than those of cationic Cu(I) complexes. Films containing 10 wt% of these complexes in PMMA shows ratiometric fluorescent oxygen gas sensing property with a response ratio of 0.3-3.2 and response time of 3-4 s. Complex 2b acts as a ratiometric oxygen gas sensor with good reversibility through energy and electron transfer mechanisms under the loss of a counteranion.

Synthesis, structure and oxygen-sensing properties of Iridium(iii)-containing coordination polymers with different cations

Four iridium(III)-containing coordination polymers 1-4 using Ir(ppy)(2)(H(2)dcbpy)PF(6) (L-H(2), ppy = 2-phenylpyridine, H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) as the bridging ligand, [ZnL(2)]·3DMF·5H(2)O (1), [CdL(2)(H(2)O)(2)]·3DMF·6H(2)O (2), [CoL(2)(H(2)O)(2)]·2DMF·8H(2)O (3) and [NiL(2)(H(2)O)(2)]·3DMF·6H(2)O (4), have been synthesized and structurally characterized. The emissions from 1-4 are ascribed to a metal-to-ligand charge transfer transition (MLCT). The absolute emission quantum yields for 1-4 in single crystals were measured in air to be 0.274, 0.193, 0.001 and 0.002, respectively. The noteworthy oxygen-sensing properties of 1-4 as well as L-H(2) in a single crystal were also evaluated. The Stern-Volmer quenching constant, K(SV) values, of 1-4 and L-H(2) can be deduced to be 0.834, 2.820, 1.328, 1.111 and 2.476, respectively. The results show promising K(SV) values (e.g.2) that are competitive or even larger than those of many known Ir-complexes. Moreover, the short response time (e.g. compound 2) and recovery times toward oxygen of 1-4 have been measured in their single crystal forms. The reversibility experiments for 1-4 were carried out for seven repeated cycles. As a result, >75% recovery of intensity for 1 and 2 on each cycle demonstrates a high degree of reproducibility during the sensing process. It should be noted that iridium(III)-containing coordination polymers with high emission intensity and notable oxygen sensing properties are obscure, especially in the single crystal form. This, in combination with its fine reversibility, leads to success in single crystal oxygen recognition based on photoluminescence imaging. The detection limit could be 0.50% for gaseous oxygen. Moreover, the temperature effect of compound 2 in a single crystal upon application as an oxygen sensor was expected.

Influence of TiO2Nanocrystals Fabricating Dye-Sensitized Solar Cell on the Absorption Spectra of N719 Sensitizer

The absorption spectra of N719 sensitizer anchored on the films prepared by TiO2nanocrystals with different morphology and size were investigated for improving the performance of dye-sensitized solar cell (DSC). We find that the morphology and size of TiO2nanocrystals can affect the UV-vis and FT-IR spectra of the sensitizer anchored on their surfaces. In particular, the low-energy metal-to-ligand charge-transfer transitions (MLCT) band in the visible absorption spectra of N719 is strongly affected, and locations of these MLCT bands revealed larger differences. The results indicate that there is a red shift of MLCT band in the spectra obtained by using TiO2nanocrystals with long morphology and large size compared to that in solution. And it produced a larger red-shift on the MLCT band after TiO2nanocrystals with small size mixed with some long nanocrystals. Accordingly, the utilization rate to visible light is increased. This is a reason why the DSC prepared by using such film as a photoelectrode has better performance than before mixing.

Electronic and Structural Properties of Low-lying Excited States of Vitamin B12

Time-dependent density functional theory (TD-DFT) has been applied to explore electronically excited states of vitamin B(12) (cyanocobalamin or CNCbl). To explain why the Co-C bond in CNCbl does not undergo photodissociation under conditions of simple photon excitation, electronically excited states have been computed along the Co-C(CN) stretched coordinate. It was found that the repulsive (3)(σ(Co-C) → σ*(Co-C)) triplet state drops in energy as the Co-C(CN) bond lengthens, but it does not become dissociative. Low-lying excited states were also computed as function of two axial bond lengths. Two energy minima have been located on the S(1)/CNCbl, as well as T(1)/CNCbl, surfaces. The full geometry optimization was carried out for each minimum and electronic properties associated with each optimized structure were analyzed in details. One minimum was described as excitation having mixed ππ*/MLCT (metal-to-ligand charge transfer) character, while the second as ligand-to-metal charge transfer (LMCT) transition. Neither of them, however, can be viewed as pure MLCT or LMCT transitions since additional excitation to or from σ-bonds (SB) of N-Co-C unit have also noticeable contributions. Inclusion of solvent altered the character of one of the excitations from ππ*/MLCT/SBLCT to ππ*/LMCT/LSBCT-type, and therefore, both of them gained significant contribution from LMCT/LSBCT transition. Finally, the nature of S(1) electronic state has been comparatively analyzed in CNCbl and MeCbl cobalamins.

Ligand-Passivated Eu:Y2O3 Nanocrystals as a Phosphor for White Light Emitting Diodes

Eu(III)-doped Y(2)O(3) nanocrystals are prepared by microwave synthetic methods as spherical 6.4 ± 1.5 nm nanocrystals with a cubic crystal structure. The surface of the nanocrystal is passivated by acetylacetonate (acac) and HDA on the Y exposed facet of the nanocrystal. The presence of acac on the nanocrystal surface gives rise to a strong S(0) → S(1) (π → π*, acac) and acac → Ln(3+) ligand to metal charge transfer (LMCT) transitions at 270 and 370 nm, respectively, in the Eu:Y(2)O(3) nanocrystal. Excitation into the S(0) → S(1) (π → π*) or acac → Ln(3+) LMCT transition leads to the production of white light emission arising from efficient intramolecular energy transfer to the Y(2)O(3) oxygen vacancies and the Eu(III) Judd-Ofelt f-f transitions. The acac passivant is thermally stable below 400 °C, and its presence is evidenced by UV-vis absorption, FT-IR, and NMR measurements. The presence of the low-lying acac levels allows UV LED pumping of the solid phosphor, leading to high quantum efficiency (∼19%) when pumped at 370 nm, high-quality white light color rendering (CIE coordinates 0.33 and 0.35), a high scotopic-to-photopic ratio (S/P = 2.21), and thermal stability. In a LED lighting package luminosities of 100 lm W(-1) were obtained, which are competitive with current commercial lighting technology. The use of the passivant to funnel energy to the lanthanide emitter via a molecular antenna effect represents a new paradigm for designing phosphors for LED-pumped white light.

A New Luminescent Ruthenium(II) Polypyridine‐derived Dipicolylamine Complex as a Sensor for Cu2+ Ions

AbstractA chemo‐sensor [Ru(bpy)2(bpy‐DPF)](PF6)2 (1) (bpy=2,2′‐bipyridine, bpy‐DPF=2,2′‐bipyridyl‐4,4′‐bis(N,N‐di(2‐picolyl))formylamide) for Cu2+ using di(2‐picolyl)amine (DPA) as the recognition group and a ruthenium(II) complex as the reporting group was synthesized and characterized successfully. It demonstrates a high selectivity and efficient signaling behavior only for Cu2+ with obvious red‐shifted MLCT (metal‐to‐ligand charge transfer transitions) absorptions and dramatic fluorescence quenching compared with Zn2+ and other metal ions.

Structure and optical spectra of bis(pyrrol-2-ylmethyleneamine) complexes: A DFT and TDDFT study of the self-assembly complexes of bis(pyrrol-2-ylmethyleneamine) ligands linked by alkyl spacers with Cu(II)

The UV-Vis absorption spectra of self-assembly Cu (II) complexes for bis(pyrrol-2-ylmethyleneamine) linked by alkyl(CH2)n(n = 2,3,4,6) spacers were recorded. The ground-state geometries and electronic structures were investigated by density functional theory employing generalized-gradient approximation Perdew-Burke-Ernzerhof (PBE) and hybrid B3LYP (20% Ex,HF)and PBE0 (25% Ex,HF) functional, in conjugation with four basis set combinations.[BS1:6-31G(d); BS2:Lanl2dz for Cu and 6-31G(d) for C,H,N; BS3:6-31+G(d); BS4:6-311+G(d,p) for C,H,N and Lanl2dz for Cu]. Compared the optimized geometries with XRD structures, it was found that the choice of methods appeared to have a more pronounced effect on structure than that of the basis set. PBE0/BS4 optimized geometries provide the best agreement with XRD experimental results and the mean absolute errors for bond length is 0.018 A and for bond angle is 2.1°. Transition excited states of complexes 1–4 in the gas phase and in CH2ClCH2Cl solution have been studied by the time-dependent density functional theory in conjugated with the Polarizable Continuum Model. Pure PBE and hybrid B3LYP, PBE0 functionals, as well as BS1, BS2, and BS5 [6-311++G (d,p)] were selected. Calculations indicate that PBE functional provides more accurate descriptions on main UV-vis absorptions than B3LYP and PBE0 functionals. The strongest absorption bands located at 338–356 nm are due to intraligand charge transfer (ILCT) transitions, a mixed-character ILCT + metal-ligand charge transfer (MLCT) absorptions are calculated at 280–291 nm, the shoulder bands located at 250–257 nm mainly arise from MLCT or ligand-metal charge transfer transitions. Alkyl spacer does not play a crucial role on the main electron transitions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

The cooperative spin-state transition of an iron(iii) compound [FeIII(3-MeO-SalEen)2]PF6: thermal- vs. ultra-fast photo-switching

The switching properties of the spin-transition solid [FeIII(3-MeO-SalEen)2]PF6 (H-3-MeO-SalEen resulting from the condensation of 3-methoxy-substituted salicylaldehyde and N-ethylethylenediamine), exhibiting a first-order transition, were investigated by using temperature and light as stimulation. The structural analysis reveals a first-order isostructural transition occurring with a 3 K width thermal hysteresis (T↓ = 162.5 K, T↑ = 165.5 K) coupled to the magnetic transition. The microscopic origin of this bistable behavior derives from strong coupling between pairs of complexes and the molecular packing. An extensive 3D network of intermolecular interactions, reinforced near the transition temperature, appears as a key feature for the cooperativity. The LMCT (ligand-to-metal charge transfer) transitions, detected by single-crystal transmission measurements, were selected for analyzing the photoswitching process of this cooperative solid. Only optical pump-probe experiments could detect photoinduced low-spin to high-spin conversion because of the too short lived excited state. The spin-state transformation induced by the femtosecond laser flash is very fast: electronic excitation corresponding to the low-spin LMCT transition relaxes toward the transient photoexcited high-spin state within 170 ± 50 fs.

Modulating the Light Switch by 3MLCT-3ππ* State Interconversion

The spectroscopic, electronic, and DNA-binding characteristics of two novel ruthenium complexes based on the dialkynyl ligands 2,3-bis(phenylethynyl)-1,4,8,9-tetraaza-triphenylene (bptt, 1) and 2,3-bis(4-tert-butyl-phenylethynyl)-1,4,8,9-tetraaza-triphenylene (tbptt, 2) have been investigated. Electronic structure calculations of bptt reveal that the frontier molecular orbitals are localized on the pyrazine-dialkynyl portion of the free ligand, a property that is reflected in a red shift of the lowest energy electronic transition (1: λmax = 393 nm) upon substitution at the terminal phenyl groups (2: λmax = 398 nm). Upon coordination to ruthenium, the low-energy ligand-centered transitions of 1 and 2 are retained, and metal-to-ligand charge transfer transitions (MLCT) centered at λmax = 450 nm are observed for [Ru(phen)2bptt]2+(3) and [Ru(phen)2tbptt]2+(4). The photophysical characteristics of 3 and 4 in ethanol closely parallel those observed for [Ru(bpy)3]2+ and [Ru(phen)3]2+, indicating that the MLCT excited state is primarily localized within the [Ru(phen)3]2+ manifold of 3 and 4, and is only sparingly affected by the extended conjugation of the bptt framework. In an aqueous environment, 3 and 4 possess notably small luminescence quantum yields (3: ϕH2O = 0.005, 4: ϕH2O = 0.011) and biexponential decay kinetics (3: τ1 = 40 ns, τ2 = 230 ns; 4: τ1 ∼ 26 ns, τ2 = 150 ns). Addition of CT-DNA to an aqueous solution of 3 causes a significant increase in the luminescence quantum yield (ϕDNA = 0.045), while the quantum yield of 4 is relatively unaffected (ϕDNA = 0.013). The differential behavior demonstrates that tert-butyl substitution on the terminal phenyl groups inhibits the ability of 4 to intercalate with DNA. Such changes in intrinsic luminescence demonstrate that 3 binds to DNA via intercalation (Kb = 3.3 × 104 M−1). The origin of this light switch behavior involves two competing 3MLCT states similar to that of the extensively studied light switch molecule [Ru(phen)2dppz]2+. The solvent- and temperature-dependence of the luminescence of 3 reveal that the extended ligand aromaticity lowers the energy of the 3ππ* excited state into competition with the emitting 3MLCT state. Interconversion between these two states plays a significant role in the observed photophysics and is responsible for the dual emission in aqueous environments.

The kinetics and mechanism of the substitution reactions of the aquapentacyanoruthenate(II) ion with naphthalene‐substituted ligands in aqueous medium

AbstractThe kinetics and mechanism of substitution reaction of [Ru(CN)5H2O]3− anion with two naphthalene‐substituted ligands viz. Ln = nitroso‐R‐salt (NRS) and α‐nitroso‐β‐naphthol (αNβN) have been studied spectrophotometrically by monitoring an increase in absorbance at λmax = 525 nm corresponding to metal to ligand charge transfer (MLCT) transitions due to formation of substituted [Ru(CN)5L]n−3 as a function of pH, ionic strength, temperature, a wide range of ligands concentration, and [Ru(CN)5H2O3−] under pseudo‐first‐order conditions. The experimental observation suggests that [Ru(CN)5H2O]3− ion interacts with both ligands, which finally get converted into corresponding, [Ru(CN)5L]n−3 complexes as a final reaction product. The reaction is found to obey first‐order dependence each in [Ru(CN)5H2O3−] and [Ln]. The substituted products, viz. [Ru(CN)5L]n−3, in each case have strong MLCT transitions in visible region. The substitutional lability of [Ru(CN)5H2O]3− has been discussed in terms of electronic effect on the MOH2 bond interactions. The kinetic observation suggests that the complexation reaction of [Ru(CN)5H2O]3− with both the ligands, i.e., NRS and αNβN, follows an ion pair dissociative mechanism. The thermal activation parameters ΔH≠ and ΔS≠ have been calculated using Eyring's equation and provided in support for the proposed mechanistic scheme. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 21–30, 2011

Oligo- and Polyfluorene-Tetheredfac-Ir(ppy)3: Substitution Effects

A set of conjugated oligo- and polyfluorene-tethered fac-Ir(ppy)3 complexes were synthesized. In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine para to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring para to the iridium ion exhibited a particularly long triplet lifetime (>100 μs), indicative of a 3LC excited state. A moderately high quantum yield (∼0.5) was displayed by this 3LC-featured phosphor. DFT calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)3 at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)3 were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance.

A theoretical study on the efficient reversible redox-based switching of the second-order polarizabilities of two-dimensional nonlinear optical-active donor–acceptor phenanthroline-hexamolybdate

The Λ-shaped phenanthroline-hexamolybdate compounds that are based on the reversible Mo-centered redox process were investigated. The attachment of hexamolybdate terminals to phenanthroline by a π-conjugated phenylamine bridge generated the organic ligand-centered or Ni-centered HOMO and the transition metal Mo-centered LUMO. The population in HOMO and LUMO predicted the reversible Mo VI/V redox process and the ligand-to-metal charge transfer (LMCT) to a polyanion acceptor, which consequently evoked a significant second-order nonlinear optical (NLO) response. Moreover, the electron transition of these compounds exhibited a large β zyy tensor along the y-axis, which confirms a promising two-dimensional (2D) character with sizable anisotropy values. Interestingly, the addition of electrons into the high-valence Mo atom in the hexamolybdate acceptor evoked dramatic enhancements in the NLO response for the reduction states in contrast to the response of the corresponding oxidation states. The reduction states in system I exhibited second-order NLO responses about 200 times larger than the oxidation states. In addition, the attachment of the Ni atom in compound IIa red enhanced the NLO response to nearly 1019 times greater than the response of the corresponding oxidation state compound IIa. The Ni atom as the electron donor plays an important role in the major electron transition for the reduction states in system II. Therefore, the NLO response of such compounds can be reversibly switched through the transition metal Mo VI/V redox that is effectively coupled with the LMCT transition. Thus, the NLO activity can be controlled by a one-electron redox process, and the redox-active phenanthroline-hexamolybdate compounds are promising candidates for 2D redox-switching NLO materials in novel optoelectronic applications.