Investigation of the 1A1 .dblharw. 5T2 intersystem crossing dynamics of an iron(II) spin-crossover complex in the solid state by Moessbauer spectroscopy

Spin crossover in dinuclear iron(II) complexes bridged by bis-bipyridine ligands: dimer effects on electronic structure, spectroscopic properties and spin-state switching

Inspired by the well-studied mononuclear spin crossover compound [Fe(H2B(pz)2)2(bipy)], the bipyridine-based bisbidentate ligands 1,2-di(2,2'-bipyridin-5-yl)ethyne (ac(bipy)2) and 1,4-di(2,2'-bipyridine-5-yl)-3,5-dimethoxybenzene (Ph(OMe)2(bipy)2) are used to bridge two [Fe(H2B(pz)2)2] units, leading to the charge-neutral dinuclear iron(II) compounds [{Fe(H2B(pz)2)2}2 μ-(ac(bipy)2)] (1) and [{Fe(H2B(pz)2)2}2 μ-(Ph(OMe)2(bipy)2)] (2), respectively. The spin-crossover properties of these molecules are investigated by temperature-dependent PPMS measurements, Mössbauer, vibrational and UV/Vis spectroscopy as well as X-ray absorption spectroscopy. While compound 1 undergoes complete SCO with T1/2 = 125 K, an incomplete spin transition is observed for 2 with an inflection point at 152 K and a remaining high-spin fraction of 40% below 65 K. The spin transitions of the dinuclear compounds are also more gradual than for the parent compound [Fe(H2B(pz)2)2(bipy)]. This is attributed to steric hindrance between the molecules, limiting intermolecular interactions such as π-π-stacking.

Iron(ii) complexes of tris(2-pyridylmethyl)amine (TPMA) and neutral bidentate ligands showing thermal- and photo-induced spin crossover

Three new mononuclear Fe(ii) complexes have been prepared and characterized by the combination of tetradentate tris(2-pyridylmethyl)amine (TPMA) with three neutral bidentate ligands, such as ethylenediamine (en), 1,2-diaminopropane (pn) and 2-picolylamine (2-pic), in compounds [FeII(TPMA)(en)](ClO4)2 (1), [FeII(TPMA)(2-pic)](ClO4)2 (2) and [FeII(TPMA)(pn)](ClO4)2 (3). Structural and magnetic characterization demonstrates that the three compounds present a complete SCO behavior. The absence of strong intermolecular interactions and solvent molecules leads to reversible and gradual spin transitions. The different ligands allow tuning T1/2 from 130 K (2) to 325 K (3). The compound with the lowest T1/2 (2) shows the LIESST effect with a TLIESST of 43 K. Interestingly, the use of these relatively small bidentate ligands leads to the crystallization in non-centrosymmetric space groups in contrast with previous studies using other bidentate ligands.

Light-Induced Spin Crossover in an Fe(II) Low-Spin Complex Enabled by Surface Adsorption

Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF₃)₂)₂(phen)] (pypyr = 2-(2'-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex.

Facile Oxidation-Based Synthesis of Sterically Encumbered Four-Coordinate Bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I) and Related Three-Coordinate Copper(I) Complexes

A new oxidation-based synthetic route was developed for synthesis of Cu(I) complexes with weakly coordinating ligands, leading to the synthesis of the elusive [Cu(dtbp)2]+ (dtbp, 2,9-di-tert-butyl-1,10-phenanthroline) complex that may be useful as a sensor or as a dye for dye-sensitized solar cells. An acetone solution of either 1 or 2 equiv of dtbp was added to excess Cu(0) and 1 equiv of AgY (Y is O3SCF3-, BF4-, SbF6-, or B(C6F5)4-) in a nitrogen-filled glove box. Following filtration and evaporation under vacuum, crystallization from CH2Cl2 and hexanes results in X-ray quality crystals of Cu(dtbp)(O3SCF3) (1), Cu(dtbp)(BF4) (2), [Cu(dtbp)(acetone)][SbF6] (3), [Cu(dtbp)2][B(C6F5)4].CH2Cl2 (4.CH2Cl2), [Cu(dtbp)2][BF4].CH2Cl2 (5.CH2Cl2), and [Cu(dtbp)2][SbF6].CH2Cl2 (6.CH2Cl2). Complexes 1-6 were characterized by X-ray crystallography and NMR. The Cu atom in complexes 1-3 exhibited distorted trigonal coordination geometries, reflecting the steric effect of the bulky tert-butyl substituents. The structures of the pseudotetrahedral complexes 4, 4.CH2Cl2, 5.CH2Cl2, and 6.CH2Cl2 revealed the longest average Cu-N distances (2.11 A, 2.11 A, 2.10 A, and 2.11 A, respectively) in this class of compounds-longer by more than three standard deviations from the average.

Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) bis-Phenanthrolines in Solution

Ultrafast excited-state structural dynamics of [Cu(I)(dmp)(2)](+) (dmp = 2,9-dimethyl-1,10-phenanthroline) have been studied to identify structural origins of transient spectroscopic changes during the photoinduced metal-to-ligand charge-transfer (MLCT) transition that induces an electronic configuration change from Cu(I) (3d(10)) to Cu(II) (3d(9)). This study has important connections with the flattening of the Franck-Condon state tetrahedral geometry and the ligation of Cu(II)* with the solvent observed in the thermally equilibrated MLCT state by our previous laser-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) results. To better understand the structural photodynamics of Cu(I) complexes, we have studied both [Cu(I)(dmp)(2)](+) and [Cu(I)(dpp)(2)](+) (dpp = 2,9-diphenyl-1,10-phenanthroline) in solvents with different dielectric constants, viscosities, and thermal diffusivities by transient absorption spectroscopy. The observed spectral dynamics suggest that a solvent-independent inner-sphere relaxation process is occurring despite the large amplitude motions due to the flattening of the tetrahedral coordinated geometry. The singlet fluorescence dynamics of photoexcited [Cu(I)(dmp)(2)](+) were measured in the coordinating solvent acetonitrile, using the fluorescence upconversion method at different emission wavelengths. At the bluest emission wavelengths, a prompt fluorescence lifetime of 77 fs is attributed to the excited-state deactivation processes due to the internal conversion and intersystem crossing at the Franck-Condon state geometry. The differentiation between the prompt fluorescence lifetime with the tetrahedral Franck-Condon geometry and that with the flattened tetrahedral geometry uncovers an unexpected ultrafast flattening process in the MLCT state of [Cu(I)(dmp)(2)](+). These results provide guidance for future X-ray structural studies on ultrafast time scale, as well as for synthesis toward its applications in solar energy conversion.

Molecular Structure and Vibrational Spectra of Spin-Crossover Complexes in Solution and Colloidal Media: Resonance Raman and Time-Resolved Resonance Raman Studies

The spin-crossover system [Fe(btpa)](PF(6))(2) (btpa = N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine) and the predominantly low-spin species [Fe(b(bdpa))](PF(6))(2) ((b(bdpa) = N,N'-bis(benzyl)-N,N'-bis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine) have been characterized by means of X-ray diffraction. The unit cell of [Fe(btpa)](PF(6))(2) contains two crystallographically independent molecules revealing octahedral low-spin and quasi-seven-coordinated high-spin structures. The unit cell of [Fe(b(bdpa))](PF(6))(2) contains two crystallographically independent molecules one of which corresponds to a low-spin structure, while the other reveals a disordering. On the basis of magnetic susceptibility and Mössbauer measurements, it has been proposed that this disorder involves low-spin and high-spin six-coordinated molecules. The structures of [Zn(btpa)](PF(6))(2) and [Ru(btpa)](PF(6))(2) have been determined also. Pulsed laser photoperturbation, coupled here with time-resolved resonance Raman spectroscopy (TR(3)), has been used to investigate, for the first time by this technique, the relaxation dynamics in solution on nanosecond and picosecond time scales of low-spin, LS ((1)A) --> high-spin, HS ((5)T) electronic spin-state crossover in these Fe(II) complexes. For the nanosecond experiments, use of a probe wavelength at 321 nm, falling within the pi-pi transition of the polypyridyl backbone of the ligands, enabled the investigation of vibrational modes of both LS and HS isomers, through coupling to spin-state-dependent angle changes of the backbone. Supplementary investigations of the spin-crossover (SCO) equilibrium in homogeneous solution and in colloidal media assisted the assignment of prominent features in the Raman spectra of the LS and HS isomers. The relaxation data from the nanosecond studies confirm and extend earlier spectrophotometric findings, (Schenker, S.; Stein, P. C.; Wolny, J. A.; Brady, C.; McGarvey, J. J.; Toftlund, H.; Hauser, A. Inorg. Chem. 2001, 40, 134), pointing to biphasic spin-state relaxation in the case of [Fe(btpa)](PF(6))(2) but monophasic in the case of [Fe(b(bdpa))](PF(6))(2). The picosecond results suggest an early process complete in 20 ps or less, which is common to both complexes and possibly includes vibrational relaxation in the initially formed (5)T(2) state.

MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.

Simple Cu(I) complexes with unprecedented excited-state lifetimes

This report describes new, readily accessible copper(I) complexes that can exhibit unusually long-lived, high quantum yield emissions in fluid solution. The complexes are of the form [Cu(NN)(POP)]+ where NN denotes 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (dmp) or 2,9-di-n-butyl-1,10-phenanthroline (dbp) and POP denotes bis[2-(diphenylphosphino)phenyl] ether. Modes of characterization include X-ray crystallography and cyclic voltammetry. The complexes each have a pseudotetrahedral coordination geometry and a Cu(II)/Cu(I) potential upward of +1.2 V vs Ag/AgCl. In room-temperature dichloromethane solution, charge-transfer excited states of the dmp and dbp derivatives exhibit respective emission quantum yields of 0.15 and 0.16 and corresponding excited-state lifetimes of 14.3 and 16.1 mus, respectively. Despite the fact that coordinating solvents usually quench charge-transfer emission from copper systems, the photoexcited dmp (dbp) complex retains a lifetime of 2.4 mus (5.4 mus) in methanol.

Tuning charge recombination rate constants through inner-sphere coordination in a copper(I) donor-acceptor compound

The coordination compounds [Cu(bpy-MV2+)(PPh3)2](PF6)3, where bpy-MV2+ is the 1-(4-(4'-methyl-2,2'-bipyridin-4-yl)butyl)-1'-methyl-4, 4'-bipyridinediium(2+) cation, and [Cu(dmb)(PPh3)2](PF6), where dmb is 4,4'-dimethyl-2,2'-bipyridine, have been prepared and characterized. Visible light (417 nm) excitation of [Cu(bpy-MV2+)(PPh3)2]3+ at room temperature leads to rapid intramolecular electron transfer, kcs > 1 x 10(8) s-1, to form a charge-separated state with an electron localized on the pendant viologen group and a copper(II) metal center, abbreviated [CuII-bpy-MV.+]. This state recombines to ground-state products with first-order rate constants that can be tuned with solvent over a approximately 10(7)-10(5) s-1 range. The activation parameters were determined from temperature-dependent electron-transfer data with Arrhenius analysis. A model is proposed wherein a solvent molecule is coordinated to Cu(II) in the charge-separated state, [(S)CuII-bpy-MV.+]. Visible light excitation of [Cu(dmb)(PPh3)2](PF6) in argon-saturated dichloromethane produces long-lived photoluminescent excited states, tau = 80 ns, that are dynamically quenched by the addition of Lewis basic solvents. The measured quenching constants each correlate well with the lifetime of the charge-separated state measured after excitation of [Cu(bpy-MV2+)(PPh3)2]3+ in the corresponding solvent.

Reductive Quenching of Photoexcited Cu(dipp)(2)(+) and Cu(tptap)(2)(+) by Ferrocenes (dipp = 2,9-Diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-Tetraphenyl-1,4,5,8-tetraazaphenanthrene)

The metal-to-ligand charge-transfer excited states of Cu(NN)(2)(+) systems tend to be good reducing agents but poor oxidants for kinetic and thermodynamic reasons. However, this report demonstrates that reductive electron-transfer quenching is an important pathway for ferrocenes that react with the photoexcited states of Cu(dipp)(2)(+) and Cu(tptap)(2)(+) in methylene chloride (dipp = 2,9-diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-tetraphenyl-1,4,5,8-tetraazaphenanthrene). In the case of the dipp complex the bulky isopropyl substituents inhibit structural relaxation within the excited state, and the self-exchange rate for reductive quenching is quite favorable, k() approximately 2 x 10(8) M(-)(1) s(-)(1). Even in the absence of a significant kinetic barrier to reaction, however, for energetic reasons only extensively methylated ferrocene derivatives with relatively negative reduction potentials are capable of transferring an electron to the excited state. In contrast, every ferrocene derivative investigated, except diacetyl ferrocene, reacts with the charge-transfer excited state of the tptap complex by an electron-transfer mechanism. This is mainly due to a difference in driving force which is about 0.5 V greater for the tptap complex. This system also has a favorable self-exchange rate, k() approximately 5 x 10(7) M(-)(1) s(-)(1), evidently because the juxtapositioned phenyl substituents inhibit low-symmetry distortions within the ground state as well as the excited state. Although energy transfer to ferrocene is also possible, this is a less competitive process with the tptap complex because the zero-zero energy of the reactive excited state is rather low ((3)E(00) approximately 1.7 V).