Luminescent Platinum(II) Complexes with Stimuli-Responsive Flexible Lewis Pair Ligands: Spectroscopic and Computational Studies.

A series of new luminescent bimetallic platinum(II) complexes with stimuli-responsive flexible Lewis pair (FlexLP) ligands are described. The FlexLP ligands consist of a dimesitylboron Lewis acid and diphenylphosphine oxide Lewis base which are in equilibrium between the unbound open form and the Lewis adduct, controlled by the hydrogen bond donating strength of the solvent. Spectroscopic techniques and density functional theory (DFT) calculations were used to interpret the photophysics of the platinum(II) complexes. All complexes exhibit tunable absorption in the region of 300-500 nm and green to orange photoluminescence, depending on the ratio of weak (THF) to strong (MeOH) hydrogen bond donating solvent employed. Spectroscopic and computational data shows that phosphine and peripheral acetylide ligands on the platinum(II) centers have limited influence on the emission energy, indicating the emission originates from the FlexLP-dominated fluorescence. Using time-resolved transient absorption spectroscopy it is shown that the complexes undergo intersystem crossing (ISC) to the triplet excited state upon photoexcitation, and the ISC efficiency is affected by the peripheral acetylide ligands. The triplet excited state lifetime can also be manipulated by the state of the FlexLP ligand, with the closed form complexes having longer lifetimes than the open form complexes.

Rhenium(I) Complexes with Sulfur-Bridged Dipyridyl Ligands: Structural, Photophysical, and Computational Studies.

A series of six new rhenium(I) tricarbonyl complexes [Re(CO)3(N-N)Br] bearing sulfur-bridged dipyridyl (N-N) ligands with three different oxidation states (sulfide (S), sulfoxide (SO), and sulfone (SO2)) are described. Spectroscopic studies show that changing the oxidation state of the ligands influences the photophysical properties of the complexes, with complexes 3 and 6 containing the sulfone ligand exhibiting a lower energy MLCT absorption band tailing into the visible region. Solution-state emission measurements show that these complexes exhibit readily tunable emission energies from 480 to 610 nm, depending on the oxidation state of the sulfur bridge and the presence of substituents on the pyridyl rings. Solid-state emission measurements show that the emission is significantly red-shifted upon oxidation of the sulfur bridge to sulfone with enhanced photoluminescence quantum yield.

Restricted rotation and tunable fluorescence in atropisomeric naphthyl pyridine chromophores.

Enhanced fluorescence quantum yields are enabled by simple reactions at the heterocyclic nitrogen in naphthyl-pyridine chromophores in which the electronic properties can be tuned through protonation, oxidation, and alkylation at the nitrogen center. Fluorescence quantum yield is increased by reacting the pyridine lone pair with either a proton or an alkyl group. Restricted intramolecular rotation (RIR) was observed upon alkylation, as evidenced by the presence of atropisomers. These simple structural changes allow application-driven tuning of electronic properties.

Effect of Oxidation on the Chiroptical Properties of Sulfur-Bridged Binaphthyl Dimers.

A series of axially chiral sulfur-bridged dimers were prepared from 1,1'-binaphthyl-2,2'-diol and subsequently oxidized to the respective sulfones. The chiroptical properties of the chiral chromophores were studied as a function of the oxidation state. Upon oxidation, an increase in quantum yields was observed for directly linked sulfur bridged binaphthyls (0.04 to 0.32), and a modest increase in dissymmetry factor was observed for diphenylsulfide-bridged binaphthyls (-8.9 × 10-4 to -1.4 × 10-3). Computational calculations were used to elucidate the changes in photophysical properties.

Lewis Pair-Functionalized Pt(II) Complexes with Tunable Emission Color and Triplet-State Properties.

Two emissive Pt(II) complexes containing dynamic "flexible" Lewis pair (FlexLP) ligands are reported. The FlexLP ligand encompasses a diphenylphosphine oxide Lewis base and a dimesitylborane Lewis acid attached to a bithiophene scaffold, which can switch between an open unbound Lewis pair and a bound P-O-B Lewis adduct depending on the hydrogen bond-donating (HBD) strength of the solvent. [Pt(FlexLP)2] contains two FlexLP ligands, and [Pt(FlexLP)(Py)] contains one FlexLP ligand and one pyrene ligand. UV-vis absorption and fluorescence studies demonstrate that the FlexLP ligands switch between the open Lewis pair and the closed Lewis adduct in MeOH, a strong HBD solvent, and acetone, a weak HBD solvent, respectively, and exhibit tunable emission color depending on the acetone/MeOH solvent ratio. Transient absorption spectroscopy reveals a large difference in the triplet-state lifetime depending on the conformation of the FlexLP ligands for both complexes. In the closed form, the triplet-state lifetimes of the two complexes are over an order of magnitude longer compared to that of the complexes in the open conformation. Calculations of optimized geometries suggest that this difference in triplet-state lifetime is due to a difference in the thiophene-thiophene torsion angle between the two conformations.

Exploring the Chemoselectivity towards Cysteine Arylation by Cyclometallated AuIII Compounds: New Mechanistic Insights.

To gain more insight into the factors controlling efficient cysteine arylation by cyclometallated AuIII complexes, the reaction between selected gold compounds and different peptides was investigated by high-resolution liquid chromatography electrospray ionization mass spectrometry (HR-LC-ESI-MS). The deduced mechanisms of C-S cross-coupling, also supported by density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) calculations, evidenced the key role of secondary peptidic gold binding sites in favouring the process of reductive elimination.

Fluorescence Switching of Intramolecular Lewis Acid-Base Pairs on a Flexible Backbone.

A class of intramolecular Lewis acid-base pairs containing Lewis acidic dimesitylboranes paired with phosphine oxide, sulfoxide, and sulfone Lewis basic groups are explored. The absorption and emission properties of the compounds are investigated in different solvent environments, and switching of the photophysical behavior between the Lewis acid-base adducts and free acid-base pairs is examined. We find that phosphine oxide Lewis base groups are effective partners in flexible Lewis pairs; however, the analogous sulfoxide and sulfone groups are not. Additionally, the absorption and emission wavelengths can be systematically tuned by varying the conjugation length and electron-donating and -accepting substituents on the backbone.

Solvent- and Temperature-Responsive Platinum(II)-Functionalized Flexible Lewis Pairs.

Metal-functionalized flexible Lewis pair (flexLP) complexes are described. Platinum(II) terpyridine (Pt-tpy) units added to a flexLP backbone through copper(I)-catalyzed coupling reactions yield monometallic (1) and bimetallic (2) Pt-tpy flexLP complexes. NMR and UV-vis absorption studies demonstrate that the complexes switch between the open unbound Lewis pair and closed Lewis adduct with solvents of varying hydrogen-bond-donating (HBD) strength as well as varying temperature. The charge-transfer (CT) energies of the complexes can be tuned by controlling the equilibrium between open and closed forms, demonstrating the ability to alter the photophysical behavior of the metal complexes by stimuli-responsive flexLP ligands.

Influence of Sulfur Oxidation State and Substituents on Sulfur-Bridged Luminescent Copper(I) Complexes Showing Thermally Activated Delayed Fluorescence.

Copper(I) complexes are seen as more sustainable alternatives to those containing metal ions such as iridium and platinum for emitting devices. Copper(I) complexes have the ability to radiatively decay via a thermally activated delayed fluorescence (TADF) pathway, leading to higher photoluminescent quantum yields. In this work we discuss six new heteroleptic Cu(I) complexes of the diphosphine-diimine motif. The diphosphine ligands employed are (oxidi-2,1-phenylene)bis(diphenylphosphine) (DPEPhos), and the diimine fragments are sulfur-bridged dipyridyl ligands (DPS) which are functionalized at the 6,6'-positions of the pyridyl rings (R = H, Me, Ph) and have varying oxidation states at the bridging sulfur atom (S, SO2). The proton (Cu-DPS, Cu-DPSO2) and phenyl (Cu-Ph-DPS, Cu-Ph-DPSO2) substituted species are found to form monometallic complexes, while those with methyl substitution (Cu-Me-DPS, Cu-Me-DPSO2) are found to have a "Goldilocks" degree of steric bulk leading to bimetallic species. All six Cu(I) complexes show emission in the solid state, with the photophysical properties characterized by low temperature steady-state and time-resolved spectroscopies and variable temperature time-correlated single photon counting. Cu-DPS, Cu-DPSO2, Cu-Me-DPS, Cu-Me-DPSO2, and Cu-Ph-DPSO2 were shown to emit via a TADF mechanism, while Cu-Ph-DPS showed photoluminescence properties consistent with triplet ligand-centered (3LC) emission.

Photoswitching of Copper(I) Chromophores with Dithienylethene-Based Ligands.

Metal-containing dithienylethenes offer new opportunities for variations in photochromic behavior. This work reports a series of copper(I) complexes containing dithienylethene-based bidentate phosphine ligands displaying varying photochromic properties. A copper dimer is used as a common precursor, allowing diverse photochromic functionality to be achieved. Calculations show that ring closing of the peripheral dithienyl moiety leads to a slight expansion of the five-membered metallacycle. The photochromic properties and photoreaction quantum yields of these complexes are tunable by variation of the ancillary ligands. Photoswitchable catalysis of a hydroboration reaction with one of the copper complexes is demonstrated.

The Influence of para Substituents in Bis(N-Heterocyclic Carbene) Palladium Pincer Complexes for Electrocatalytic CO2 Reduction.

The effect of modifying the pyridyl para position of lutidine-linked bis(N-heterocyclic carbene) Pd pincer complexes is studied both experimentally (R = OMe, H, Br, and COOR) and computationally, showing a strong effect on the first reduction potential of the complex and allowing the reduction potential to be tuned over a wide range in relation to the Hammett σp constant of the para substituent. The effect of the pyridyl para substituent on electron density of the metal center, frontier orbital energies, and dissociation energy of the trans ligand are also investigated in the context of reactivity with CO2 through electrochemical characterization of the complexes under N2 and CO2 and controlled potential electrolysis experiments where CO2 is reduced to CO.

Tunable Emission of Iridium(III) Complexes Bearing Sulfur-Bridged Dipyridyl Ligands.

A series of six new cyclometalated iridium(III) complexes [Ir(ppy)2(N∧N)][PF6] (ppy = 2-phenylpyridine) is reported herein. Proligands bis(pyridin-2-yl)sulfane (1a), 2,2'-sulfinyldipyridine (1b), 2,2'-sulfonyldipyridine (1c), bis(4-methylpyridin-2-yl)sulfane (2a), 2,2'-sulfinylbis(4-methylpyridine) (2b), and 2,2'-sulfonylbis(4-methylpyridine) (2c) were synthesized, characterized, and employed as the N∧N ancillary ligand. Changing the oxidation state of the sulfur atom serves as a switch to alter the emissive state from that of mainly 3LC character (blue-green emission) to one of 3MLCT/3LLCT character (yellow emission). Sulfide and sulfoxide complexes Ir(1a), Ir(1b), Ir(2a), and Ir(2b) show identical, vibrationally structured emission profiles with maxima at 478, 510, 548 nm in CH2Cl2 solutions resulting from a 3LC state. In contrast, sulfone complexes Ir(1c) and Ir(2c) show broad, red-shifted 3CT emission (552 and 537 nm, respectively).

Biological Imaging with Medium-Sensitive Bichromatic Flexible Fluorescent Dyes.

A family of environment-sensitive shape-shifting molecules have been developed as flexible fluorescent (FlexFluor) dyes for biological imaging applications. These compounds feature a flexible bithiophene-based fluorophore that gives rise to different emission colors in lipophilic or hydrophilic environments, as well as side groups that can be synthetically modified with ease. FlexFluor dyes are the first fluorescent dyes in which emission color can be used to indicate lipid/water environments. The behavior of these dyes in different solvents was studied, and used to simultaneously highlight lipid and water contents in adipose and brain tissues using optical fluorescence microscopy.

Dual-Emissive Platinum(II) Metallacycles with Thiophene-Containing Bisacetylide Ligands.

Three cis-diphosphino Pt(II) metallacycles with thiophene-containing bisacetylide ligands were synthesized and their absorption and emission properties examined. These properties are explained by DFT and TD-DFT analysis of ground-state as well as singlet- and triplet-state energies and geometries. Two of the metallacycles show room-temperature dual emission (fluorescence and phosphorescence) with different intensity ratios. Replacing the phenylene or ethynylene groups with thiophene rings in the bisacetylide ligand of the metallacycles results in modulation of the S1 and T1 states to show stronger fluorescence and weaker phosphorescence. More rigid and planar metallacycles suffer less from energy loss due to smaller degrees of structural reorganization and thermal deactivation. These are important factors to consider when designing single-component dual-emissive materials for applications such as white-emitting OLEDs, self-referencing oxygen sensors, and hypoxia contrast agents.

Tunable luminescence of bithiophene-based flexible Lewis pairs.

Bithiophene-based flexible Lewis pairs with P(O)R2 (R = phenyl, isopropyl) and BMes2 (Mes = 2,4,6-trimethylphenyl) functionalities are able to toggle between closed, Lewis adduct and open, unbound Lewis pair structures. The open structure is favored in strong hydrogen bond donating solvents or at higher temperatures giving rise to an intense charge-transfer (CT) luminescence, while the closed structure without this emission dominates in non-hydrogen bond donating solvents or at lower temperatures. Intermediate solvents result in an equilibrium mixture of both structures, which shows unusual mixed emission that is dependent on excitation wavelength.

Sulfur-Bridged Terthiophene Dimers: How Sulfur Oxidation State Controls Interchromophore Electronic Coupling.

Symmetric dimers have the potential to optimize energy transfer and charge separation in optoelectronic devices. In this paper, a combination of optical spectroscopy (steady-state and time-resolved) and electronic structure theory is used to analyze the photophysics of sulfur-bridged terthiophene dimers. This class of dimers has the unique feature that the interchromophore (intradimer) electronic coupling can be modified by varying the oxidation state of the bridging sulfur from sulfide (S), to sulfoxide (SO), to sulfone (SO2). Photoexcitation leads to the formation of a delocalized charge resonance state (S1) that relaxes quickly (<10 ps) to a charge-transfer state (S1*). The amount of charge-transfer character in S1* can be enhanced by increasing the oxidation state of the bridging sulfur group as well as the solvent polarity. The S1* state has a decreased intersystem crossing rate when compared to monomeric terthiophene, leading to an enhanced photoluminescence quantum yield. Computational results indicate that electrostatic screening by the bridging sulfur electrons is the key parameter that controls the amount of charge-transfer character. Control of the sulfur bridge oxidation state provides the ability to tune interchromophore interactions in covalent assemblies without altering the molecular geometry or solvent polarity. This capability provides a new strategy for the design of functional supermolecules with applications in organic electronics.

Polyannulated Bis(N-heterocyclic carbene)palladium Pincer Complexes for Electrocatalytic CO2 Reduction.

Phenanthro- and pyreno-annulated N-heterocyclic carbenes (NHCs) have been incorporated into lutidine-linked bis-NHC Pd pincer complexes to investigate the effect of these polyannulated NHCs on the ability of the complexes to electrochemically reduce CO2 to CO in the presence of 2,2,2-trifluoroacetic acid and 2,2,2-trifluoroethanol as proton sources. These complexes are screened for their ability to reduce CO2 and modeled using density functional theory calculations, where the annulated phenanthrene and pyrene moieties are shown to be additional sites for redox activity in the pincer ligand, enabling increased electron donation. Electrochemical and computational studies are used to gain an understanding of the chemical significance of redox events for complexes of this type, highlighting the importance of anion binding and dissociation.

Direct C-F bond formation using photoredox catalysis.

We have developed the first example of a photoredox catalytic method for the formation of carbon-fluorine (C-F) bonds. The mechanism has been studied using transient absorption spectroscopy and involves a key single-electron transfer from the (3)MLCT (triplet metal-to-ligand charge transfer) state of Ru(bpy)3(2+) to Selectfluor. Not only does this represent a new reaction for photoredox catalysis, but the mild reaction conditions and use of visible light also make it a practical improvement over previously developed UV-mediated decarboxylative fluorinations.

Electrocatalytic reduction of CO2 with palladium bis-N-heterocyclic carbene pincer complexes.

A series of pyridine- and lutidine-linked bis-N-heterocyclic carbene (NHC) palladium pincer complexes were electrochemically characterized and screened for CO2 reduction capability with 2,2,2-trifluoroethanol, acetic acid, or 2,2,2-trifluoroacetic acid (TFA) as proton sources. The lutidine-linked pincer complexes electrocatalytically reduce CO2 to CO at potentials as low as -1.6 V versus Ag/AgNO3 in the presence of TFA. The one-electron reduction of these complexes is shown to be chemically reversible, yielding a monometallic species, with density functional theory studies indicating charge storage on the redox-active ligand, thus addressing a major source of deactivation in earlier triphosphine electrocatalysts.

Tuning the extended structure and electronic properties of gold(I) thienyl pyrazolates.

A series of thienyl pyrazole proligands and gold(I) thienyl pyrazolate cyclic trinuclear complexes (CTCs) have been synthesized. The relationship between the structure and emission properties of bridging thienyl pyrazolates within gold(I) cyclic trinuclear complexes suggests that the nature of dual emission is sensitive to ligand conjugation length. Density functional theory has been used to support the assignment of metal-sensitized, ligand-localized phosphorescence from monothienyl complexes, while low-lying, ligand-localized LUMOs present in bithienyl systems prohibit metal-sensitized phosphorescence. Soluble n-hexyl derivatives have been synthesized to explore the electrochemical properties of gold(I) thienyl pyrazolates CTCs, and conductive electropolymerized thin films were realized.