Design Strategies for Luminescent Titanocenes: Improving the Photoluminescence and Photostability of Arylethynyltitanocenes.

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d metals are of interest as possible photocatalysts. CpTi(CPh) (where CPh = phenylethynyl) was reported to be weakly emissive in room-temperature (RT) fluid solution from its phenylethynyl-to-Ti LMCT state but readily photodecomposes. Coordination of CuX between the alkyne ligands to give CpTi(CPh)CuX (X = Cl, Br) has been shown to significantly increase the photostability, but such complexes are not emissive in RT solution.

Role of the Trifluoropropynyl Ligand in Blue-Shifting Charge-Transfer States in Emissive Pt Diimine Complexes and an Investigation into the PMMA-Imposed Rigidoluminescence and Rigidochromism.

Square-planar Pt complexes are of interest as dopants for the emissive layer of organic light-emitting diodes. Herein, the photophysics of three Pt bipyridyl complexes with the strongly e withdrawing, high-field, 3,3,3-trifluoropropynyl ligand has been investigated. One complex, (phbpy)PtCCF (phbpy = 6-phenyl-2,2'-dipyridyl), has also been characterized by single-crystal X-ray diffraction.

Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive / Heterobimetallic Titanocene Tweezer Complexes.

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to metals are of interest as possible photocatalysts due to the lack of deactivating d-d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula CpTi(CAr)·MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl.

Ligand-to-Metal Charge-Transfer Photophysics and Photochemistry of Emissive d Titanocenes: A Spectroscopic and Computational Investigation.

Complexes with ligand-to-metal charge-transfer (LMCT) excited states involving d metals represent a new design for photocatalysts. Herein, the photochemistry and photophysics of d titanocenes of the type CpTi(CR), where CR = ethynylphenyl (CPh), 4-ethynyldimethylaniline (CDMA), or 4-ethynyltriphenylamine (CTPA), have been investigated. CpTi(CPh) and CpTi(CDMA) have also been characterized by single-crystal X-ray diffraction.

Insights into the charge-transfer character of electronic transitions in CpTi(CFc) complexes using solvatochromism, resonance Raman spectroscopy, and TDDFT.

A series of complexes with low-energy FeII to TiIV metal-to-metal charge-transfer (MMCT) transitions, Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and MeOOCCp2Ti(C2Fc)2, was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters - solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) - was performed.

Synthesis and characterization of a butyl ester-substituted titanocene dichloride: CpTiCl.

Bis[η-(-butoxyca-rbonyl)-cyclo-penta-dien-yl]di-chlorido-titanium(IV), [Ti(CHO)Cl], was synthesized from LiCp using TiCl, and was characterized by single-crystal X-ray diffraction and H NMR spectroscopy. The distorted tetra-hedral geometry about the central titanium atom is relatively unchanged compared to CpTiCl. The complex exhibits elongation of the titanium-cyclo-penta-dienyl centroid distances [2.

Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an Fe to Ti Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States.

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising Fe donors and Ti acceptors with the general formula CpTi(CFc) (where Cp = Cp*, Cp, and Cp) and CpTi(CFc)(CR) (where R = Ph or CF) are reported. The transient absorption spectra are consistent with an Fe/Ti metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.

Oxidatively stable ferrocenyl-π-bridge-titanocene D-π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states.

The synthesis, spectroscopic, and electrochemical characterization of oxidatively stable D-π-A compounds of the form (Me2CpC2Fc)2TiCl2 and RCp2Ti(C2Fc)2CuX (where Fc = ferrocenyl) are reported. Oxidative stability enabled by the addition of CuX is evidenced by voltammagrams of the RCp2Ti(C2Fc)2CuX compounds which all display two chemically-reversible 1e- FeIII/II couples, indicative of electronic communication between the Fc- termini. Differential pulse voltammetry (DPV) in CH2Cl2/[n-Bu4N][PF6], demonstrated that the redox potential difference between the two 1e- FeIII/II couples (ΔE1/2) is between 112 mV and 146 mV, being most pronounced with the electron rich Cp*2Ti(C2Fc)2CuBr.

A trans-bidentate bis-pyridinyl ligand with a transition metal hinge.

A titanocene based metalloligand, Cp*Ti(C2-py), was synthesized and coordinated to either Cu(i) or Pd(ii). The metalloligand binds Cu(i) between its alkynes and Pd(ii) between its pyridinyl rings, acting as a trans-bidentate ligand. In order to bind Pd(ii), significant structural rearrangements were necessary, which required the flexibility of the C-Ti-C hinge on the titanocene metalloligand.

Complexes with Tunable Intramolecular Ferrocene to Ti(IV) Electronic Transitions: Models for Solid State Fe(II) to Ti(IV) Charge Transfer.

Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti(IV) has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2.

Trifluoropropynyl as a surrogate for the cyano ligand and intense, room-temperature, metal-centered emission from its Rh(III) complex.

The trifluoropropynyl ligand -C≡CCF(3) was studied as a possible surrogate for the cyano ligand. Complexes of the type trans-[M(cyclam)(C≡CCF(3))(2)]OTf (where M = Cr(3+), Co(3+), and Rh(3+); OTf = trifluoromethanesulfonate) were prepared and then characterized by electronic spectroscopy and by cyclic voltammetry for the Co(3+) complex. The UV-vis spectra for all three bear a remarkable similarity to that of the trans-M(cyclam)(CN)(2)(+) cations.

Quantum dot photoluminescence quenching by Cr(III) complexes. Photosensitized reactions and evidence for a FRET mechanism.

Reported are quantitative studies of the energy transfer from water-soluble CdSe/ZnS and CdSeS/ZnS core/shell quantum dots (QDs) to the Cr(III) complexes trans-Cr(N(4))(X)(2)(+) (N(4) is a tetraazamacrocycle ligand, X(-) is CN(-), Cl(-), or ONO(-)) in aqueous solution. Variation of N(4), of X(-), and of the QD size and composition allows one to probe the relationship between the emission/absorption overlap integral parameter and the efficiency of the quenching of the QD photoluminescence (PL) by the chromium(III) complexes. Steady-state studies of the QD PL in the presence of different concentrations of trans-Cr(N(4))(X)(2)(+) indicate a clear correlation between quenching efficiency and the overlap integral largely consistent with the predicted behavior of a Förster resonance energy transfer (FRET)-type mechanism.

Synthesis of cis and trans bis-alkynyl complexes of Cr(III) and Rh(III) supported by a tetradentate macrocyclic amine: a spectroscopic investigation of the M(III)-alkynyl interaction.

Alkynyl complexes of the type [M(cyclam)(CCR)(2)]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane; M = Rh(III) or Cr(III); and R = phenyl, 4-methylphenyl, 4-trifluoromethylphenyl, 4-fluorophenyl, 1-naphthalenyl, 9-phenanthrenyl, and cyclohexyl) were prepared in 49% to 93% yield using a one-pot synthesis involving the addition of 2 equiv of RCCH and 4 equiv of BuLi to the appropriate [M(cyclam)(OTf)(2)]OTf complex in THF. The cis and trans isomers of the alkynyl complexes were separated using solubility differences, and the stereochemistry was characterized using infrared spectroscopy of the CH(2) rocking and NH bending region. All of the trans-[M(cyclam)(CCR)(2)]OTf complexes exhibit strong Raman bands between 2071 and 2109 cm(-1), ascribed to ν(s)(C≡C).

Measurement of both the equilibrium constant and rate constant for electronic energy transfer by control of the limiting kinetic regimes.

Electronic energy transfer can fall into two limiting cases. When the rate of the energy transfer back reaction is much faster than relaxation of the acceptor excited state, equilibrium between the donor and acceptor excited states is achieved and only the equilibrium constant for the energy transfer can be measured. When the rate of the back reaction is much slower than relaxation of the acceptor, the energy transfer is irreversible and only the forward rate constant can be measured.

Emissive chromium(III) complexes with substituted arylethynyl ligands.

Arylethynylchromium(III) complexes of the form trans-[Cr(cyclam)(CCC(6)H(4)R)(2)]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane, R = H, CH(3), or CF(3) in the para position, and OTf = trifluoromethanesulfonate) have been prepared and characterized by IR spectroscopy and X-ray diffraction. The complexes are emissive with excited-state lifetimes in a deoxygenated fluid solution between 200 and 300 micros.

Effects of steric constraint on Chromium(III) complexes of tetraazamacrocycles. 3. Insights into the temperature-dependent radiationless deactivation of the 2Eg (Oh) excited state of trans-[Cr(N4)(CN)2]+ complexes.

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C.

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2. Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam.

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-C3-cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane) is reported.

Chemistry of constrained dioxocyclam ligands with Co(III): unusual examples of C-H and C-N bond cleavage.

The reactions between H(2)dc3 and Co(acac)(3) have been studied in the presence and absence of base. In the presence of base, a complex with an intramolecular Co-C bond, Co(dc3-C-(8))(H(2)O), 1, is formed, presumably through heterolytic C-H bond activation. An X-ray crystallographic study demonstrates the presence of a Co-C bond and shows that the diazacyclooctane (daco) subunit adopts the chair-boat conformation with respect to the metal.

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles. Chemistry and excited-state behavior of 1,4-C2-cyclam complexes.

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C(2)-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane is reported.

Solution-state and solid-state structural characterization of complexes of a new macrocyclic ligand containing the 1,5-diazacyclooctane subunit.

The synthesis and characterization of a new constrained tetraazamacrocyclic ligand, 1,4,8,11-tetraazabicyclo[9.3.3]heptadecane (1,11-C(3)-cyclam), is reported.

Electronic energy self-exchange with macrocyclic chromium(III) complexes.

The luminescence lifetimes of N-deuterated Cr(III) complexes of macrocyclic tetraamine ligands, trans-CrN(4)X(2)(n)()(+), are substantially longer than those of their undeuterated counterparts in room temperature solution. Thus, excited-state emission quenching of the longer lived species by the shorter lived species may be studied by analyzing the decay profile following pulsed excitation. Flash photolysis experiments were carried out for three deuterated/undeuterated pairs of trans-CrN(4)X(2)(n)()(+) complexes (where X = CN-, NH(3), and F-).

Catalytic Activation of H(2) and C-H Bonds by Electron-Deficient Ruthenium(II) Porphyrins.

The Ru(2) and RuNi derivatives of 1,8-bis(10,15,20-trimesityl-5-porphyrinato)anthracene-a recently reported cofacial diporphyrin ligand comprising two hindered porphyrins spanned by an anthracene bridge-have been synthesized. Both Ru(2)(DPAHM) and RuNi(DPAHM) are extremely reactive species that apparently contain 14-electron Ru(II) centers and, as is the case for their monoporphyrin analog, (5,10,15,20-tetramesitylporphyrinato)ruthenium [Ru(TMP)], must be rigorously protected from oxygen, nitrogen, and other ligating agents. In addition, these electron-deficient Ru(II) porphyrins all appear to bind aromatic solvents such as benzene and toluene, the weakest ligating solvents in which these Ru(II) porphyrins have been found soluble.