A Molybdenum(0) Isocyanide Analogue of [Ru(2,2'-Bipyridine)3]2+: Strong Reductant for Photoredox Catalysis

We report the first homoleptic Mo0 complex with bidentate isocyanide ligands, which exhibits metal-to-ligand charge transfer (3MLCT) luminescence with quantum yields and lifetimes similar to Ru(bpy)32+ (bpy=2,2′-bipyridine). This Mo0 complex is a very strong photoreductant, which manifests in its capability to reduce acetophenone with essentially diffusion-limited kinetics as shown by time-resolved laser spectroscopy. The application potential of this complex for photoredox catalysis was demonstrated by the rearrangement of an acyl cyclopropane to a 2,3-dihydrofuran, which is a reaction that requires a reduction potential so negative that even the well-known and strongly reducing Ir(2-phenylpyridine)3 photosensitizer cannot catalyze it. Our study thus provides the proof-of-concept for the use of chelating isocyanides to obtain Mo0 complexes with long-lived 3MLCT excited states that are applicable to unusually challenging photoredox chemistry

Homoleptic and heteroleptic complexes of chromium(III) containing 4'-diphenylamino-2,2':6',2''-terpyridine ligands

Two heteroleptic bis(2,2′:6′,2″-terpyridine)chromium(III) complexes [Cr(1)(4′-(4-tolyl)tpy)][CF3SO3]3 and [Cr(2)(4′-(4-tolyl)tpy)][CF3SO3]3 (1 = 4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-diphenylaniline, 2 = 4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-bis(4-methoxyphenyl)aniline, 4′-(4-tolyl)tpy = 4′-(4-tolyl)-2,2′:6′,2″-terpyridine) have been prepared and their spectroscopic and electrochemical properties compared with those of [Cr(4′-(4-tolyl)tpy)2][CF3SO3]3 and [Cr(1)2][CF3SO3]3. The single crystal structure of [Cr(4′-(4-tolyl)tpy)2][CF3SO3]3·2MeCN is presented, and the effects of accommodating three triflate anions and two MeCN molecules per cation are discussed in terms of related structures. The coordination of 1 or 2 to chromium(III) red-shifts the intra-ligand charge transfer (ILCT) band and this band exhibits a negative solvatochromic effect in some solvents. However, in H2O, MeOH, DMSO and DMF, the tpy ligands are labile; changes in the absorption spectra of solutions of [Cr(2)(4′-(4-tolyl)tpy)][CF3SO3]3 are consistent with the formation of [Cr(4′-Xtpy)(Solv)3]3+ (Solv = solvent) rather than complete ligand displacement or a ligand redistribution

Catechol[4]arene: The Missing Chiral Member of the Calix[4]arene Family

A missing, inherently chiral member of the calix[4]arene family denoted "catechol[4]arene" was synthesized. Its properties were studied and compared to the ones of its close relatives resorcin[4]arene and pyrogallol[4]arene. This novel supramolecular host exhibits binding capabilities that are superior to its sister molecules in polar media. The enantiomerically pure forms of the macrocycle display modest recognition of chiral ammonium salts

Shine bright or live long: substituent effects in [Cu(N^N)(P^P)]+-based light-emitting electrochemical cells where N^N is a 6-substituted 2,2'-bipyridine

We report [Cu(P^P)(N^N)][PF6] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 6-methyl-2,2′-bipyridine (Mebpy), 6-ethyl-2,2′-bipyridine (Etbpy), 6,6′-dimethyl-2,2′-bipyridine (Me2bpy) or 6-phenyl-2,2′-bipyridine (Phbpy). The crystal structures of [Cu(POP)(Phbpy)][PF6]·Et2O, [Cu(POP)(Etbpy)][PF6]·Et2O, [Cu(xantphos)(Me2bpy)][PF6], [Cu(xantphos)(Mebpy)][PF6]·CH2Cl2·0.4Et2O, [Cu(xantphos)(Etbpy)][PF6]·CH2Cl2·1.5H2O and [Cu(xantphos)(Phbpy)][PF6] are described; each copper(I) centre is distorted tetrahedral. In the crystallographically determined structures, the N^N domain in [Cu(xantphos)(Phbpy)]+ and [Cu(POP)(Phbpy)]+ is rotated ∼180° with respect to its orientation in [Cu(xantphos)(Mebpy)]+, [Cu(POP)(Etbpy)]+ and [Cu(xantphos)(Etbpy)]+; in each complex containing xantphos, the xanthene ‘bowl’ retains the same conformation in the solid-state structures. The two conformers resulting from the 180° rotation of the N^N ligand were optimized at the B3LYP-D3/(6-31G**+LANL2DZ) level and are close in energy for each complex. Variable temperature NMR spectroscopy evidences the presence of two conformers of [Cu(xantphos)(Phbpy)]+ in solution which are related by inversion of the xanthene unit. The complexes exhibit MLCT absorption bands in the range 378 to 388 nm, and excitation into each MLCT band leads to yellow emissions. Photoluminescence quantum yields (PLQYs) increase from solution to thin-film and powder; the highest PLQYs are observed for powdered [Cu(xantphos)(Mebpy)][PF6] (34%), [Cu(xantphos)(Etbpy)][PF6] (37%) and [Cu(xantphos)(Me2bpy)][PF6] (37%) with lifetimes of 9.6–11 μs. Density functional theory calculations predict that the emitting triplet (T1) involves an electron transfer from the Cu–P^P environment to the N^N ligand and therefore shows a 3MLCT character. T1 is calculated to be ∼0.20 eV lower in energy than the first singlet excited state (S1). The [Cu(P^P)(N^N)][PF6] ionic transition-metal (iTMC) complexes were tested in light-emitting electrochemical cells (LECs). Turn-on times are fast, and the LEC with [Cu(xantphos)(Me2bpy)][PF6] achieves a maximum efficacy of 3.0 cd A−1 (luminance = 145 cd m−2) with a lifetime of 1 h; on going to the [Cu(xantphos)(Mebpy)][PF6]-based LEC, the lifetime exceeds 15 h but at the expense of the efficacy (1.9 cd A−1). The lifetimes of LECs containing [Cu(xantphos)(Etbpy)][PF6] and [Cu(POP)(Etbpy)][PF6] exceed 40 and 80 h respectively

Exploring simple ancillary ligands in copper-based dye-sensitized solar cells: effects of a heteroatom switch and of co-sensitization

The copper( I ) complexes [Cu(1) 2 ][PF 6 ], [Cu(2) 2 ][PF 6 ], [Cu(3) 2 ][PF 6 ] and [Cu(4) 2 ][PF 6 ] (1 . 2-(1H-imidazol-2-yl)- 6-methylpyridine, 2 . 2-(6-methylpyridin-2-yl)oxazole, 3 . 2-(6-methylpyridin-2-yl)thiazole and 4 . 2- methyl-6-(1-methyl-1H-imidazol-2-yl)pyridine) are reported. The crystal structures of [Cu(2) 2 ][PF 6 ]$ 0.5CH 2 Cl 2 and [Cu(3) 2 ][PF 6 ] confirm N,N 0 -chelation modes for 2 and 3, and tetrahedral copper( I ). In the solution absorption spectra, the MLCT band shifts to lower energy with a change in heteroatom (O, 424 nm; NH, 435 nm; NMe, 446 nm; S, 465 nm). [Cu(1) 2 ][PF 6 ] and [Cu(4) 2 ][PF 6 ] undergo copper-centred oxidative processes at lower potential than the complexes with O or S heteroatoms. Heteroleptic complexes [Cu(5)(L)] + (5 . ((6,6 0 -dimethyl-[2,2 0 -bipyridine]-4,4 0 -diyl)bis(4,1-phenylene)bis(phosphonic acid)), L . 1–4) were assembled on FTO/TiO 2 electrodes. The shift in the MLCT band (O NH < NMe < S) in the solid-state absorption spectra of the dye-functionalized electrodes parallels that of solution. The photoconversion efficiencies (h) of masked, dye-sensitized solar cells (DSCs) containing [Cu(5)(L)] + (L . 1– 4) dyes and an I/I 3 redox shuttle follow the order [Cu(5)(1)] + (3.03%) > [Cu(5)(3)] + (2.88%) > [Cu(5)(4)] + (2.71%) > [Cu(5)(2)] + (2.62%) relative to 7.55% for N719. Ancillary ligand 1 (with NH) leads to the highest open-circuit voltage (V OC . 608 mV) whilst 3 (S-heteroatom) gives the highest short-circuit current density (J SC . 7.76 mA cm 2 ). The performances of [Cu(5)(1)] + and [Cu(5)(3)] + are understood with the aid of electrochemical impedance spectroscopy (EIS). The DSC with [Cu(5)(1)] + exhibits a high chemical capacitance (C m ) and a low recombination resistance (R rec ); since the latter is offset by a low transport resistance (R tr ), a high J SC and V OC are observed for [Cu(5)(1)] + . DSCs with [Cu(5)(3)] + have the lowest R tr of all four devices. The performance of DSCs sensitized by a combination of [Cu(5)(1)] + and [Cu(5)(3)] + were assessed in order to capitalize upon the high V OC of [Cu(5)(1)] + and the high J SC of [Cu(5)(3)] + . After FTO/ TiO 2 functionalization with anchor 5, the electrodes were treated with a 1 : 1 mix of [Cu(5)(1)] + and [Cu(5)(3)] + or sequentially with [Cu(5)(3)] + then [Cu(5)(1)] + , or [Cu(5)(1)] + then [Cu(5)(3)] + . The DSC performances and the EIS parameters are consistent with competition between 1 and 3 for surface binding-sites; 1 dominates over 3, both in binding and in contribution to the overall photoresponse

Schiff base ancillary ligands in bis(diimine)copper(I) dye-sensitized solar cells

Five 6,6'-dimethyl-2,2'-bipyridine ligands bearing N -arylmethaniminyl substituents in the 4- and 4'-positions were prepared by Schiff base condensation in which the aryl group is Ph ( 1 ), 4-tolyl ( 2 ), 4- t BuC 6 H 4 ( 3 ), 4-MeOC 6 H 4 ( 4 ), and 4-Me 2 NC 6 H 4 ( 5 ). The homoleptic copper(I) complexes [CuL 2 ][PF 6 ] (L = 1 - 5 ) were synthesized and characterized, and the single crystal structure of [Cu( 1 ) 2 ][PF 6 ] . Et 2 O was determined. By using the "surfaces-as-ligands, surfaces-as-complexes" (SALSAC) approach, the heteroleptic complexes [Cu( 6 )(L ancillary )] + in which 6 is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid)) and L ancillary = 1 - 5 were assembled on FTO-TiO 2 electrodes and incorporated as dyes into n-type dye-sensitized solar cells (DSCs). Data from triplicate, fully-masked DSCs for each dye revealed that the best-performing sensitizer is [Cu( 6 )( 1 )] + , which exhibits photoconversion efficiencies ( η ) of up to 1.51% compared to 5.74% for the standard reference dye N719. The introduction of the electron-donating MeO and Me 2 N groups (L ancillary = 4 and 5 ) is detrimental, leading to a decrease in the short-circuit current densities and external quantum efficiencies of the solar cells. In addition, a significant loss in open-circuit voltage is observed for DSCs sensitized with [Cu( 6 )( 5 )] + , which contributes to low values of η for this dye. Comparisons between performances of DSCs containing [Cu( 6 )( 1 )] + and [Cu( 6 )( 4 )] + with those sensitized by analogous dyes lacking the imine bond indicate that the latter prevents efficient electron transfer across the dy

Unravelling the conductance path through single-porphyrin junctions

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance

A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of [Fe(2,2'-Bipyridine)3]2+

A meta-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand (CNtBuAr3NC) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr(CNtBuAr3NC)3 complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 °C) is nearly 2 orders of magnitude longer than the current record lifetime for isoelectronic Fe(II) complexes, which are of significant interest as earth-abundant sensitizers in dye-sensitized solar cells. Due to its chelating ligands, Cr(CNtBuAr3NC)3 is more robust than Cr(0) complexes with carbonyl or monodentate isocyanides, manifesting in comparatively slow photodegradation. In the presence of excess anthracene in solution, efficient energy transfer and subsequent triplet–triplet annihilation upconversion is observed. With an excited-state oxidation potential of −2.43 V vs Fc+/Fc, the Cr(0) complex is a very strong photoreductant. The findings presented herein are relevant for replacement of precious metals in dye-sensitized solar cells and in luminescent devices by earth-abundant elements

Highly Stable Red-Light-Emitting Electrochemical Cells

The synthesis and characterization of a series of new cyclometalated iridium(III) complexes [Ir(ppy) 2 (N ∧ N)][PF 6 ] in which Hppy = 2-phenylpyridine and N ∧ N is (pyridin-2-yl)benzo[ d ]thiazole ( L1 ), 2-(4-( tert -butyl)pyridin-2-yl)benzo[ d ]thiazole ( L2 ), 2-(6-phenylpyridin-2-yl)benzo[ d ]thiazole ( L3 ), 2-(4-( tert -butyl)-6-phenylpyridin-2-yl)benzo[ d ]thiazole ( L4 ), 2,6-bis(benzo[ d ]thiazol-2-yl)pyridine ( L5 ), 2-(pyridin-2-yl)benzo[ d ]oxazole ( L6 ), or 2,2′-dibenzo[ d ]thiazole ( L7 ) are reported. The single crystal structures of [Ir(ppy) 2 ( L1 )][PF 6 ]·1.5CH 2 Cl 2 , [Ir(ppy) 2 ( L6 )][PF 6 ]·CH 2 Cl 2 , and [Ir(ppy) 2 ( L7 )][PF 6 ] have been determined. The new complexes are efficient red emitters and have been used in the active layers in light-emitting electrochemical cells (LECs). The effects of modifications of the 2-(pyridin-2-yl)benzo[ d ]thiazole ligand on the photoluminescence and LEC performance have been examined. Extremely stable red-emitting LECs are obtained, and when [Ir(ppy) 2 ( L1 )][PF 6 ], [Ir(ppy) 2 ( L2 )][PF 6 ], or [Ir(ppy) 2 ( L3 )][PF 6 ] are used in the active layer, device lifetimes greater than 1000, 6000, and 4000 h, respectively, are observe