Facile interconversion of mesitylcopper into a CuMes-Cu2bis(amidinate) triangle and a tetracuprous Möbius strip.

A new flexible bis(amidine) ligand featuring two additional N-donor groups incorporates a defined [CuIMesCu2I]2+ fragment from mesitylcopper into a triangular cluster with a reactive organometallic coordination site. Subtle changes to the reaction protocol result in the formation of an intertwined tetracuprous arrangement that adopts the shape of a Möbius strip.

Electrochemically Assisted Single Crystal Growth of Reduced Preyssler Polyoxometalates Decorated with M2+ (M = Co, Ni) and Cubane-Like Ni4O4 Units.

Polyoxometalates (POMs) are of great interest to the scientific community, and their reduction and nucleation have been well-established by multi-step techniques. The present study develops an electrochemical approach for simultaneous reduction and nucleation of polyoxometalate-containing solids. Herein we report crystal growth of reduced Preyssler polyoxotungstate-based (anionic formula [NaP5W30O110]14-) new crystalline solids made of Preyssler anions interlinked by Co2+ and Ni2+ ions. Crystal nucleation and in situ reduction were achieved at room temperature using a two silver wire electrode setup in various aqueous solutions under constant applied potentials. The POM material was deposited on the cathode, and its structure was characterized by X-ray diffraction techniques. The primary structure type observed involves POMs decorated by disordered Co2+/Ni2+ octahedra and fused into 1-D pillars by additional Co2+/Ni2+ octahedra. A secondary phase was observed in the Ni-based reactions, where reduced Preyssler anions are decorated by Ni4O4 cubane-like units. To understand the electrochemical process, polarization curves of the electrolyte solutions are presented, suggesting an applied potential best suited for crystal growth. The work highlights the effectiveness of an electrochemical pathway where nucleation and simultaneous reduction of POMs can make novel reduced POM solids.

Synthesis, Structures, and Magnetic Investigations of Nickel Phosphates: Ni2(PO4)(OH), Ni7(PO4)3(HPO4)(OH)3, and NaNiPO4 Including Potential Haldane Behavior.

Three novel nickel-phosphate structures are reported, Ni2(PO4)(OH) (I), Ni7(PO4)3(HPO4)(OH)3 (II), and NaNiPO4 (III). Each new system was prepared via a high-temperature hydrothermal synthesis at 600-650 °C. All three compounds are built of quasi-one-dimensional (quasi-1-D) Ni2+ containing chains with varying phosphate bridging modes and were characterized by single crystal X-ray diffraction and magnetic susceptibility. All three compounds display very different magnetic behavior. Anisotropic magnetic data is reported for Ni2(PO4)(OH) (I) exhibiting slow antiferromagnetic ordering in the high-temperature regime with substructures that begin to form below 32 K at different field strengths. These characteristics affirm I as being one of the few Haldane-like material candidates. The Ni7(PO4)3(HPO4)(OH)3 (II) material is a member of the unusual ellenbergerite structural family and displays complex inter- and intrachain magnetic interactions while NaNiPO4 (III) shows antiferromagnetic ordering near 18 K. This magnetic behavior is correlated with their structures.

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters.

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Cycloaddition of Phenyltriazolinedione with Carbazole-Alkynes and Yne-Carbamates to Access Diazacyclobutenes.

Previously, we described the synthesis of stable, bicyclic examples of the rather rare diazacyclobutene (DCB) motif by means of a cycloaddition between triazolinediones and electron-rich thiolated alkynes. Here, we report the investigation of the cycloaddition of triazolinediones with related electron-rich yne-carbamates and carbazole-alkynes. Bicyclic DCBs arising from yne-carbamates were isolated in 8-65% yield, while those arising from carbazole-alkynes were isolated in 28-59% yield. Mechanistic studies and characterization of isolable byproducts shed light on the underlying issues leading to poor to moderate yields.

Comparison of N···I and N···O Halogen Bonds in Organoiodine Cocrystals of Heterocyclic Aromatic Diazine Mono-N-oxides.

A series of cocrystals of halogen bond donors 1,4-diiodotetrafluorobenzene (p-F4DIB) and tetraiodoethylene (TIE) with five aromatic heterocyclic diazine mono-N-oxides based on pyrazine, tetramethylpyrazine, quinoxaline, phenazine, and pyrimidine as halogen bonding acceptors were studied. Structural analysis of the mono-N-oxides allows comparison of the competitive occurrence of N···I vs O···I interactions and the relative strength and directionality of these two types of interactions. Of the aromatic heterocyclic diazine mono-N-oxide organoiodine cocrystals examined, six exhibited 1:1 stoichiometry, forming chains that utilized both N···I and O···I interactions. Two cocrystals presented 1:1 stoichiometry with exclusive O···I interactions. Two cocrystals displayed a 2:1 stoichiometry-one characterized solely by O···I interactions and the other solely by N···I interactions. We have also compared these interactions to those present in the corresponding diazines, some of which we report here and some which have been previously reported. In addition, a computational analysis using density functional theory (M062X/def2-SVPD) was performed on these two systems and has been compared to the experimental results. The calculated complex formation energies were, on average, 4.7 kJ/mol lower for the I···O halogen bonding interaction as compared to the corresponding N···I interaction. The average I···O interaction distances were calculated to be 0.15 Å shorter than the corresponding I···N interactions.

Hydrogen-bonding interactions in 5-fluorocytosine-urea (2/1), 5-fluorocytosine-5-fluorocytosinium 3,5-dinitrosalicylate-water (2/1/1) and 2-amino-4-chloro-6-methylpyrimidine-6-chloronicotinic acid (1/1).

Three new compounds, namely, 5-fluorocytosine-urea (2/1), 2C4H4FN3O·CH4N2O, (I), 5-fluorocytosine-5-fluorocytosinium 3,5-dinitrosalicylate-water (2/1/1), 2C4H4FN3O·C4H5FN3O+·C7H2N2O7-·H2O, (II), and 2-amino-4-chloro-6-methylpyrimidine-6-chloronicotinic acid (1/1), C6H4ClNO2·C5H6ClN3, (III), have been synthesized and characterized by single-crystal X-ray diffraction. In compound (I), 5-fluorocytosine (5FC) molecules A and B form two different homosynthons [R22(8) ring motif], one formed via N-H...O hydrogen bonds and the second via N-H...N hydrogen bonds. In addition to this interaction, a sequence of fused-ring motifs [R21(6), R33(8), R22(8), R43(10) and R22(8)] are formed, generating a supramolecular ladder-like hydrogen-bonded pattern. In compound (II), 5FC and 5-fluorocytosinium are linked by triple hydrogen bonds, generating two fused-ring motifs [R22(8)]. The neutral 5FC and protonated 5-fluorocytosinum cation form a dimeric synthon [R22(8) ring motif] via N-H...O and N-H...N hydrogen bonds. On either side of the dimeric synthon, the neutral 5FC, 5-fluorocytosinium cation, 3,5-dinitrosalicylate anion and water molecule are hydrogen bonded through N-H...O, N-H...N, N-H...OW and OW-HW...O hydrogen bonds, forming a large ring motif [R1010(56)], leading to a three-dimensional supramolecular network. In compound (III), 2-amino-4-chloro-6-methylpyrimidine (ACP) interacts with the carboxylic acid group of 6-chloronicotinic acid via N-H...O and O-H...O hydrogen bonds, generating an R22(8) primary ring motif. Furthermore, the ACP molecules form a base pair via N-H...N hydrogen bonds. The primary motif and base pair combine to form tetrameric units, which are further connected by Cl...Cl interactions. In addition to this hydrogen-bonding interaction, compounds (I) and (III) are further enriched by π-π stacking interactions.

9,10-Bis(iodo-ethyn-yl)anthracene.

The title compound, C18H8I2, is an ethynyl-substituted anthracene. The C-C-I bond angles deviate from 180°, being affected by inter-molecular I⋯π inter-actions. These inter-actions form a two-dimensional supra-molecular structure further supported by offset π-π stacking of neighboring anthracene moieties.

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

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts. Cp2Ti(C2Ph)2 (where C2Ph = phenylethynyl) was reported to be weakly emissive in room-temperature (RT) fluid solution from its phenylethynyl-to-Ti 3LMCT state but readily photodecomposes. Coordination of CuX between the alkyne ligands to give Cp2Ti(C2Ph)2CuX (X = Cl, Br) has been shown to significantly increase the photostability, but such complexes are not emissive in RT solution. Herein, we investigate whether inhibition of alkyne-Ti-alkyne bond compression might be responsible for the increased photostability of the CuX complexes by investigating the decomposition of a structurally constrained analogue, Cp2Ti(OBET) (OBET = o-bis(ethynyl)tolane). To investigate the mechanism of nonradiative decay from the 3LMCT states in Cp2Ti(C2Ph)2CuX, the photophysical properties were investigated both upon deuteration and upon rigidifying in a poly(methyl methacrylate) film. These investigations suggested that inhibition of structural rearrangement may play a dominant role in increasing emission lifetimes and quantum yields. The bulkier Cp*2Ti(C2Ph)2CuBr was prepared and is emissive at 693 nm in RT THF solution with a photoluminescent quantum yield of 1.3 × 10-3 (τ = 0.18 µs). Time-dependent density functional theory (TDDFT) calculations suggest that emission occurs from a 3LMCT state dominated by Cp*-to-Ti charge transfer.

New Fluorine-Containing Diamine Monomers for Potentially Improved Polyimides.

Two new fluorine-containing diamine monomers were designed with the goal of reducing charge transfer complex (CTC) interactions between neighboring chains in polyimides (i.e., high transparency/low color) while hopefully maintaining the well-known thermal stability and flexibility generally associated with polyimides. The proposed diamines have been prepared through (1) the functionalization of 1,3-bis[(pentafluorobenzyl)oxy]benzene with 4-aminophenol and (2) the addition of 2-chloro-5-nitrobenzotrifluoride to 4,4'-bicyclohexanol followed by reduction of the resulting dinitro compound. The new compounds have been characterized by multinuclear NMR and IR spectroscopy and high-resolution liquid chromatography-mass spectrometry as well as single-crystal X-ray diffraction on the new diamine prepared from 4,4'-bicyclohexanol. Not only was the structure of the proposed new diamine confirmed, but another interesting example of hydrogen bonding between an N-H proton and the π-system of an aromatic ring was observed and documented. Initial polymerizations have been carried out via the two-step imidization process.

Supramolecular interactions in some organic hydrated 2,4,6-triaminopyrimidinium carboxylate and sulfate salts.

Four salts, namely, 2,4,6-triaminopyrimidinium 6-chloronicotinate dihydrate, C4H8N5+·C6H3ClNO2-·2H2O, (I), 2,4,6-triaminopyrimidinediium pyridine-2,6-dicarboxylate dihydrate, C4H9N52+·C7H3NO42-·2H2O, (II), 2,4,6-triaminopyrimidinediium sulfate monohydrate, C4H9N52+·SO42-·H2O, (III), and 2,4,6-triaminopyrimidinium 3,5-dinitrobenzoate dihydrate, C4H8N5+·C7H3N2O6-·2H2O, (IV), were synthesized and characterized by X-ray diffraction techniques. Proton transfer from the corresponding acid to the pyrimidine base has occurred in all four crystal structures. Of the four salts, two [(I) and (IV)] exist as monoprotonated bases and two [(II) and (III)] exist as diprotonated bases. In all four crystal structures, the acid interacts with the pyrimidine base through N-H...O hydrogen bonds, generating an R22(8) ring motif. The sulfate group mimics the role of the carboxylate anions. The water molecules present in compounds (I)-(IV) form water-mediated large ring motifs. The formation of water-mediated interactions in these crystal structures can be used as a model in the study of the hydration of nucleobases. Water molecules play an important role in building supramolecular structures. In addition to these strong hydrogen-bonding interactions, some of the crystal structures are further enriched by aromatic π-π stacking interactions [(I) and (II)].

Synthesis and crystal structure of a bench-stable pyridinium ketene hemiaminal: 1-(1-eth-oxy-ethen-yl)-2-[meth-yl(phen-yl)amino]-pyridin-1-ium tri-fluoro-methane-sulfonate.

The novel bench-stable N-quaternized ketene N,O-acetal, C16H19N2O+·CF3O3S-, was synthesized and its structure determined. The title compound is a rare example of a pyridinium ketene hemiaminal for which a crystal structure has been determined, joining the 2-chloro-1-(1-ethyoxyethen-yl)pyridin-1-ium tri-fluoro-methane-sulfonate salt from which it was synthesized. The cationic species of the title compound can be defined by three individually planar fragments assembling into a non-coplanar cation. The phenyl substituent extending from the amino nitro-gen atom and the ethyoxyvinyl substituent extending from the pyridine N atom are oriented on the same side of the mol-ecule and maintain the closest coplanar relationship of the three fragments. Supra-molecular inter-actions are dominated by C-H⋯O inter-actions from the cation to the SO3 side of the tri-fluoro-methane-sulfonate anion, forming a two-dimensional substructure.

A New Motif in Halogen Bonding: Cooperative Intermolecular S-Br⋠⋠⋠O, O⋠⋠⋠F, and F⋠⋠⋠F Associations in the Crystal Packing of α,ω-Di(sulfonyl bromide) Perfluoroalkanes.

We recently reported the first examples of S-Cl⋅⋅⋅O halogen bonding complemented by short F⋅⋅⋅F contacts between neighboring chains that resulted in stabilized crystals of ClSO2 (CF2 )4 SO2 Cl and ClSO2 (CF2 )6 SO2 Cl. More recently, other researchers studied our crystallographic data further using an Independent Gradient Model (IGM), and they suggested if one goes beyond IUPAC's proposed 'less than the sum of the van der Waals radii' criterion that even more noncovalent interactions between fluorine atoms on neighboring chains as well as Cl⋅⋅⋅Cl, Cl⋅⋅⋅S, O⋅⋅⋅F, and O⋅⋅⋅S attractive interactions can be found. With that said, we have prepared samples of the related BrSO2 (CF2 )n SO2 Br derivatives (where n=4, 6, 8, and others), which give rise to even stronger S-Br⋅⋅⋅O halogen bonding interactions complemented minimally by O⋅⋅⋅F and F⋅⋅⋅F intermolecular interactions as shown by X-ray crystallography and computational chemistry using IGM isosurface plots. Additional spectroscopic characterization (multinuclear NMR, FT-IR, and MS) of the disulfonyl bromide derivatives BrSO2 (CF2 )4 SO2 Br, BrSO2 (CF2 )6 SO2 Br, and BrSO2 (CF2 )8 SO2 Br has also been obtained as well as some preliminary spectroscopic evidence for BrSO2 (CF2 )2 SO2 Br and BrSO2 CF2 O(CF2 )2 OCF2 SO2 Br. The implication of these results toward the preparation of the corresponding disulfonyl iodides is discussed.

Supramolecular interactions in salts/cocrystals involving pyrimidine derivatives of sulfonate/carboxylic acid.

The crystal structures of three compounds involving aminopyrimidine derivatives are reported, namely, 5-fluorocytosinium sulfanilate-5-fluorocytosine-4-azaniumylbenzene-1-sulfonate (1/1/1), C4H5FN3O+·C6H6NO3S-·C4H4FN3O·C6H7NO3S, I, 5-fluorocytosine-indole-3-propionic acid (1/1), C4H4FN3O·C11H11NO2, II, and 2,4,6-triaminopyrimidinium 3-nitrobenzoate, C4H8N5+·C7H4NO4-, III, which have been synthesized and characterized by single-crystal X-ray diffraction. In I, there are two 5-fluorocytosine (5FC) molecules (5FC-A and 5FC-B) in the asymmetric unit, with one of the protons disordered between them. 5FC-A and 5FC-B are linked by triple hydrogen bonds, generating two fused rings [two R22(8) ring motifs]. The 5FC-A molecules form a self-complementary base pair [R22(8) ring motif] via a pair of N-H...O hydrogen bonds and the 5FC-B molecules form a similar complementary base pair [R22(8) ring motif]. The combination of these two types of pairing generates a supramolecular ribbon. The 5FC molecules are further hydrogen bonded to the sulfanilate anions and sulfanilic acid molecules via N-H...O hydrogen bonds, generating R44(22) and R66(36) ring motifs. In cocrystal II, two types of base pairs (homosynthons) are observed via a pair of N-H...O/N-H...N hydrogen bonds, generating R22(8) ring motifs. The first type of base pair is formed by the interaction of an N-H group and the carbonyl O atom of 5FC molecules through a couple of N-H...O hydrogen bonds. Another type of base pair is formed via the amino group and a pyrimidine ring N atom of the 5FC molecules through a pair of N-H...N hydrogen bonds. The base pairs (via N-H...N hydrogen bonds) are further bridged by the carboxyl OH group of indole-3-propionic acid and the O atom of 5FC through O-H...O hydrogen bonds on either side of the R22(8) motif. This leads to a DDAA array. In salt III, one of the N atoms of the pyrimidine ring is protonated and interacts with the carboxylate group of the anion through N-H...O hydrogen bonds, leading to the primary ring motif R22(8). Furthermore, the 2,4,6-triaminopyrimidinium (TAP) cations form base pairs [R22(8) homosynthon] via N-H...N hydrogen bonds. A carboxylate O atom of the 3-nitrobenzoate anion bridges two of the amino groups on either side of the paired TAP cations to form another ring [R32(8)]. This leads to the generation of a quadruple DADA array. The crystal structures are further stabilized by π-π stacking (I and III), C-H...π (I and II), C-F...π (I) and C-O...π (II) interactions.

Halogen, chalcogen, and hydrogen bonding in organoiodine cocrystals of heterocyclic thiones: imidazolidine-2-thione, 2-mercaptobenzimidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzoxazole, and 2-mercaptobenzothiazole.

Through the combination of heterocyclic thiones with variation in the identity of the heterocyclic elements, namely, imidazolidine-2-thione, 2-mercaptobenzimidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzoxazole, and 2-mercaptobenzothiazole with the common halogen-bond donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene, 1,3,5-trifluorotriiodobenzene, and tetraiodoethylene, a series of 18 new crystalline structures were characterized. In most cases, N-H...S hydrogen bonding was observed, with these interactions in imidazole-containing structures typically resulting in two-dimensional motifs (i.e. ribbons). Lacking the second N-H group, the thiazole and oxazole hydrogen bonding resulted in only dimeric pairs. C-I...S and C-I...I halogen bonding, as well as C=S...I chalcogen bonding, served to consolidate the packing by linking the hydrogen-bonding ribbons or dimeric pairs.

Bis[3,5-di-fluoro-2-(pyridin-2-yl)phen-yl](4,4'-di-meth-oxy-2,2'-bi-pyridine)-iridium(III) hexa-fluorido-phosphate.

The title cyclo-metalated distorted octa-hedral iridium complex, [Ir(C11H6F2N)2(C12H12N2O2)]PF6, exhibits elongated Ir-N bonds to the dimeth-oxy bi-pyridine ligand [2.128 (3) and 2.136 (3) Å] where these nitro-gen atoms are trans to the Ir-C bonds of the two cyclo-metalating di-fluoro-phenyl-pyridine ligands. The angles between the mean planes of the phenyl and pyridyl fragments within the individual ligands range from 3.5 (2) to 11.4 (2)° to deviate slightly from coplanarity.

N-(2,3,5,6-Tetrafluoropyridin-4-yl)formamide.

The title compound, C6H2F4N2O, displays amide bond lengths of 1.218 (3) Šand 1.366 (3) Šfor the C=O and C-N bonds, respectively. The Cp-N-C-O (p = pyridine) torsion angle of 179.0 (2)° indicates an anti-conformation for the grouping. Inter-molecular hydrogen bonding is observed between the amine N-H group and the carbonyl O atom, which generates chains of mol-ecules propagating along the b-axis direction.

Ln2 (SeO3 )2 (SO4 )(H2 O)2 (Ln=Sm, Dy, Yb): A Mixed-Ligand Pathway to New Lanthanide(III) Multifunctional Materials Featuring Nonlinear Optical and Magnetic Anisotropy Properties.

Bottom-up assembly of optically nonlinear and magnetically anisotropic lanthanide materials involving precisely placed spin carriers and optimized metal-ligand coordination offers a potential route to developing electronic architectures for coherent radiation generation and spin-based technologies, but the chemical design historically has been extremely hard to achieve. To address this, we developed a worthwhile avenue for creating new noncentrosymmetric chiral Ln3+ materials Ln2 (SeO3 )2 (SO4 )(H2 O)2 (Ln=Sm, Dy, Yb) by mixed-ligand design. The materials exhibit phase-matching nonlinear optical responses, elucidating the feasibility of the heteroanionic strategy. Ln2 (SeO3 )2 (SO4 )(H2 O)2 displays paramagnetic property with strong magnetic anisotropy facilitated by large spin-orbit coupling. This study demonstrates a new chemical pathway for creating previously unknown polar chiral magnets with multiple functionalities.

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 PtII 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)PtC2CF3 (phbpy = 6-phenyl-2,2'-dipyridyl), has also been characterized by single-crystal X-ray diffraction. All complexes reported are emissive in both RT CH2Cl2 solution (ΦPL = 0.007 to 0.027) and PMMA film (ΦPL = 0.25 to 0.42). The trifluoropropynyl ligand elevates the energy of the MLCT and LL'CT states above that of the IL π-π* state, resulting in IL emission in all cases. The emission energies of the trifluoropropynyl compounds are also blue-shifted relative to the analogous pentafluorophenylethynyl compounds, suggesting that the trifluoropropynyl ligand is one of the most electron-withdrawing alkynyl ligands. Rate constants for radiative and nonradiative deactivation were determined from experimentally determined values of ΦPL and excited-state lifetimes in both solution and PMMA films. The increase in ΦPL upon incorporation into PMMA film (rigidoluminescence) results from a decrease in the rate constant for non-radiative relaxation. Experimental activation energies for excited-state decay in combination with TDDFT are consistent with the rigidoluminescence resulting from an increase in the energy of the non-emissive triplet metal-centered state. Two of the complexes investigated, (Ph2bpy)Pt(C2CF3)2 and (t-Bu2bpy)Pt(C2CF3)2, where t-Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl and Ph2bpy = 4,4'-diphenyl-2,2'-dipyridyl, exhibit concentration-dependent excimer emission (orange) along with monomer emission (blue), enabling fine-tuning of the emission color. However, excimer emission was absent in cured PMMA films up to the solubility limit for solution processing of (Ph2bpy)Pt(C2CF3)2 in CH2Cl2, demonstrating the diffusional nature of excimer formation.

Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes.

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 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 Cp2Ti(C2Ar)2·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. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp2Ti(C2Ph)2. TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium 3LMCT in all cases except Cp2Ti(C2Ph)2·CuBr, which is dominated by CuBr-to-Ti charge transfer. In room-temperature fluid solution, only Cp2Ti(C2Ph)2 and Cp2Ti(C2Ph)2·AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10-4. The parent complexes photodecompose in room-temperature solution with quantum yields, Φrxn, between 0.25 and 0.99. The coordination of MX decreases Φrxn by two to three orders of magnitude. There is a clear trend that Φrxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the 3LMCT state and the singlet transition state for decomposition.