Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group.

A series of photosensitizers comprised of both an inorganic and an organic chromophore are investigated in a joint synthetic, spectroscopic, and theoretical study. This bichromophoric design strategy provides a means by which to significantly increase the excited state lifetime by isolating the excited state away from the metal center following intersystem crossing. A variable bridging group is incorporated between the donor and acceptor units of the organic chromophore, and its influence on the excited state properties is explored. The Franck-Condon (FC) photophysics and subsequent excited state relaxation pathways are investigated with a suite of steady-state and time-resolved spectroscopic techniques in combination with scalar-relativistic quantum chemical calculations. It is demonstrated that the presence of an electronically conducting bridge that facilitates donor-acceptor communication is vital to generate long-lived (32 to 45 µs), charge-separated states with organic character. In contrast, when an insulating 1,2,3-triazole bridge is used, the excited state properties are dominated by the inorganic chromophore, with a notably shorter lifetime of 60 ns. This method of extending the lifetime of a molecular photosensitizer is, therefore, of interest for a range of molecular electronic devices and photophysical applications.

Exploiting Supramolecular Interactions to Control Isomer Distributions in Reduced-Symmetry [Pd2L4]4+ Cages.

High-symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis, and drug delivery. Recently, there have been increasing efforts to enhance those applications by generating reduced-symmetry MSAs. Here we report our attempts to use supramolecular (dispersion and hydrogen-bonding) forces and solvophobic effects to generate isomerically pure [Pd2(L)4]4+ cage architectures from a family of new reduced-symmetry ditopic tripyridyl ligands. The reduced-symmetry tripyridyl ligands featured either solvophilic polyether chains, solvophobic alkyl chains, or amino substituents. We show using NMR spectroscopy, high-performance liquid chromatography, X-ray diffraction data, and density functional theory calculations that the combination of dispersion forces and solvophobic effects does not provide any control of the [Pd2(L)4]4+ isomer distribution with mixtures of all four cage isomers (HHHH, HHHT, cis-HHTT, or trans-HTHT, where H = head and T = tail) obtained in each case. More control was obtained by exploiting hydrogen-bonding interactions between amino units. While the cage assembly with a 3-amino-substituted tripyridyl ligand leads to a mixture of all four possible isomers, the related 2-amino-substituted tripyridyl ligand generated a cis-HHTT cage architecture. Formation of the cis-HHTT [Pd2(L)4]4+ cage was confirmed using NMR studies and X-ray crystallography.

A Lantern-Shaped Pd(II) Cage Constructed from Four Different Low-Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach.

One of the key challenges of metallo-supramolecular chemistry is to maintain the ease of self-assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern-shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi-ligand) assemblies, or 2) homoleptic assemblies from low-symmetry ligands. Heteroleptic cages formed from low-symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen-bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low-symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low-symmetry ligands.

Post-Synthetic Modification of a Porous Hydrocarbon Cage to Give a Discrete Co24 Organometallic Complex.

A new alkyne-based hydrocarbon cage was synthesized in high overall yield using alkyne-alkyne coupling in the cage forming step. The cage is porous and displays a moderately high BET surface area (546 m2 g-1 ). The cage loses crystallinity on activation and thus is porous in its amorphous form, while very similar cages have been either non-porous, or retained crystallinity on activation. Reaction of the cage with Co2 (CO)8 results in exhaustive metalation of its 12 alkyne groups to give the Co24 (CO)72 adduct of the cage in good yield.

A Switchable Palladium(II) Trefoil Entangled Tetrahedron with Temperature Dependence and Concentration Independence.

Self-assembly makes metallo-interlocked architectures attractive targets, but being in equilibrium with smaller species means that they can suffer from dilution effects. We show that a junctioned system gives rise to a [Pd4 (L)2 ]8+ trefoil entangled tetrahedron irrespective of concentration. Heating the sample reversibly shifts the equilibrium from the knot to an isomeric non-interlocked dual metallo-cycle, demonstrating that thermodynamic equilibria can still be exploited for switching even in the absence of concentration effects.

Heterotrimetallic Double Cavity Cages: Syntheses and Selective Guest Binding.

A strategy for the generation of heterotrimetallic double cavity (DC) cages [Pdn Ptm L4 ]6+ (DC1: n=1, m=2; and DC2: n=2, m=1) is reported. The DC cages were generated by combining an inert platinum(II) tetrapyridylaldehyde complex with a suitably substituted pyridylamine and PdII ions. 1 H and DOSY nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization mass spectrometry (ESIMS) data were consistent with the formation of the DC architectures. DC1 and DC2 were shown to interact with several different guest molecules. The structure of DC1, which features two identical cavities, binding two 2,6-diaminoanthraquinone (DAQ) guest molecules was determined by single-crystal X-ray crystallography. In addition, DC1 was shown to bind two molecules of 5-fluorouracil (5-FU) in a statistical (non-cooperative) manner. In contrast, DC2, which features two different cage cavities, was found to interact with two different guests, 5-FU and cisplatin, selectively.

Excited-State Switching in Rhenium(I) Bipyridyl Complexes with Donor-Donor and Donor-Acceptor Substituents.

The optical properties of two Re(CO)3(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA2) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA2 the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH2Cl2 to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin-orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states.

Discrete Self-Assembled Metallo-Foldamers with Heteroleptic Sequence Specificity.

Discrete and structurally diverse foldamer sequences are constructed in both natural and abiotic systems primarily using inert connectivity with irreversible organic covalent bonds, serving to preserve the identity of the sequence. The formation of sequences under thermodynamic control using labile coordination bonds would be attractive for synthetic ease and modular capability, but this presents issues regarding sequence preservation. Here is presented an approach integrating palladium(II) metal ions into the sequence itself, with fidelity maintained through use of complementary pairings of ligand arrangements at the metal centre. This is accomplished using sites of different denticity and/or hydrogen bonding capability. In this fashion, discrete and ordered metallo-sequences are formed as thermodynamic products in a single step, and these then fold into defined conformations due to π-π interactions between electron-rich and -poor aromatic regions of the combined componentry.

Quadruply Stranded Metallo-Supramolecular Helicate [Pd2(hextrz)4]4+ Acts as a Molecular Mimic of Cytolytic Peptides.

[Pd2(hextrz)4]4+ is a quadruply stranded helicate, a novel bioinorganic complex designed to mimic the structure and function of proteins due to its high stability and supramolecular size. We have previously reported that [Pd2(hextrz)4]4+ exhibited cytotoxicity toward a range of cell lines, with IC50 values ranging from 3 to 10 µM. Here we demonstrate that [Pd2(hextrz)4]4+ kills cells by forming pores within the cell membrane, a mechanism of cell death analogous to the naturally occurring cytolytic peptides. [Pd2(hextrz)4]4+ induced cell death is characterized by an initial influx of Ca2+, followed by nuclear condensation and mitochondrial swelling. This is accompanied by progressive cell membrane damage that results in the formation of large blebs at the cell surface. This allows the efflux of molecules from the cell leading to loss of cell viability. These data suggest that it may be possible to design metallo-supramolecular complexes to mimic the cytotoxic action of pore forming proteins and peptides and so provide a new class of drug to treat cancer, autoimmune disorders, and microbial infection.

Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes.

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 µs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing 3MLCT and 3ILCT contributions across the series.

Reversible Transformation between a [PdL2 ]2+ "Figure-of-Eight" Complex and a [Pd2 L2 ]4+ Dimer: Switching On and Off Self-Recognition.

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with PdII metal ions, a spiro-type [PdL2 ]2+ "Figure-of-eight" complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of PdII metal ion results in the conversion to a dimeric [Pd2 L2 ]4+ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of PdII metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL2 ]2+ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:PdII stoichiometry.

Dramatic Alteration of 3ILCT Lifetimes Using Ancillary Ligands in [Re(L)(CO)3(phen-TPA)] n+ Complexes: An Integrated Spectroscopic and Theoretical Study.

The ground and excited state photophysical properties of a series of fac-[Re(L)(CO)3(α-diimine)] n+ complexes, where L = Br-, Cl-, 4-dimethylaminopyridine (dmap) and pyridine (py) have been extensively studied utilizing numerous electronic and vibrational spectroscopic techniques in conjunction with a suite of quantum chemical methods. The α-diimine ligand consists of 1,10-phenanthroline with the highly electron donating triphenylamine (TPA) appended in the 5 position. This gives rise to intraligand charge transfer (ILCT) states lying lower in energy than the conventional metal-to-ligand charge transfer (MLCT) state, the energies of which are red and blue-shifted, respectively, as the ancillary ligand, L becomes more electron withdrawing. The emitting state is 3ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR2), time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Systematic modulation of the ancillary ligand causes unanticipated variation in the 3ILCT lifetime by 2 orders of magnitude, ranging from 6.0 µs for L = Br- to 27 ns for L = py, without altering the nature of the excited state formed or the relative order of the other CT states present. Temperature dependent lifetime measurements and quantum chemical calculations provide no clear indication of close lying deactivating states, MO switching, contributions from a halide-to-ligand charge transfer (XLCT) state or dramatic changes in spin-orbit coupling. It appears that the influence of the ancillary ligand on the excited state lifetime could be explained in terms of energy gap law, in which there is a correlation between ln( knr) and Eem with a slope of -21.4 eV-1 for the 3ILCT emission.

Strategies for Reversible Guest Uptake and Release from Metallosupramolecular Architectures.

The cavities of metallosupramolecular cages can be used to mimic the central spaces of naturally occurring proteins and bind a wide variety of molecular guests. A range of potential applications have arisen from this capacity for host-guest chemistry. However, to truly harness the opportunities thus afforded, methodologies to controllably allow the release and reuptake of guests from the cavities of metallosupramolecular cages are required. Methods to accomplish this have centered upon reversibly altering the character of either the guest or host. This minireview outlines the current approaches used to carry out the binding and release of guests from metallosupramolecular hosts using important examples from the field.

Metallosupramolecular Architectures Formed with Ferrocene-Linked Bis-Bidentate Ligands: Synthesis, Structures, and Electrochemical Studies.

The self-assembly of ligands of different geometries with metal ions gives rise to metallosupramolecular architectures of differing structural types. The rotational flexibility of ferrocene allows for conformational diversity, and, as such, self-assembly processes with 1,1'-disubstituted ferrocene ligands could lead to a variety of interesting architectures. Herein, we report a small family of three bis-bidentate 1,1'-disubstituted ferrocene ligands, functionalized with either 2,2'-bipyridine or 2-pyridyl-1,2,3-triazole chelating units. The self-assembly of these ligands with the (usually) four-coordinate, diamagnetic metal ions Cu(I), Ag(I), and Pd(II) was examined using a range of techniques including 1H and DOSY NMR spectroscopies, high-resolution electrospray ionization mass spectrometry, X-ray crystallography, and density functional theory calculations. Additionally, the electrochemical properties of these redox-active metallosupramolecular assemblies were examined using cyclic voltammetry and differential pulse voltammetry. The copper(I) complexes of the 1,1'-disubstituted ferrocene ligands were found to be coordination polymers, while the silver(I) and palladium(II) complexes formed discrete [1 + 1] or [2 + 2] metallomacrocyclic architectures.

A Nona-nuclear Heterometallic Pd3 Pt6 "Donut"-Shaped Cage: Molecular Recognition and Photocatalysis.

We report a simple, low-symmetry 2-(1-(pyridine-4-methyl)-1H-1,2,3-triazol-4-yl)pyridine ligand that has both monodentate and bidentate binding sites. With platinum(II) and/or palladium(II) ions, two examples of a new nona-nuclear metallo-assembly have been accessed. These complexes were characterized by NMR spectroscopy, electrospray mass spectrometry (ESI-MS), and in key cases, X-ray crystallography. The cages possess three clefts comprised of planar cationic panels. This structural feature enables the binding of planar aromatic guests such as anthracene. More interestingly, the heterometallic assembly was able to catalyze the light-induced [4+2] cycloaddition of anthracene with singlet oxygen.

Multicavity [PdnL4]2n+ Cages with Controlled Segregated Binding of Different Guests.

Multicavity [Pdn(L)4]2n+ metallosupramolecular cages based on long backboned ligands are an attractive approach to increasing molecular size without loss of the binding specificity conferred by small cavity [Pd2(L)4]4+ assemblies. We herein report the synthesis of two double cavity polypyridyl [Pd3(L)4]6+ cages that bind cisplatin [Pt(NH3)2Cl2] within their internal cavities and interact with triflate (TfO-) on their exohedral faces. We also report the first example of a triple cavity [Pd4(L)4]8+ cage. This cage differs in that the central cavity is phenyl-linked rather than having the pyridyl core as in the peripheral cavities. The difference in cavity character results in selective guest binding of cisplatin in the peripheral cavities, with triflate binding within the central cavity and on the exohedral faces of the peripheral palladium(II) ions. All the cavities could be simultaneously filled by introducing both cisplatin and triflate concurrently, providing the first example of a discrete metallosupramolecular architecture with segregated guest binding in different designed internal cavities. The ligands and cages were characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and, in one case, X-ray crystallography.

Solid-State Gas Adsorption Studies with Discrete Palladium(II) [Pd2 (L)4 ]4+ Cages.

The need for effective CO2 capture systems remains high, and due to their tunability, metallosupramolecular architectures are an attractive option for gas sorption. While the use of extended metal organic frameworks for gas adsorption has been extensively explored, the exploitation of discrete metallocage architectures to bind gases remains in its infancy. Herein the solid state gas adsorption properties of a series of [Pd2 (L)4 ]4+ lantern shaped coordination cages (L = variants of 2,6-bis(pyridin-3-ylethynyl)pyridine), which had solvent accessible internal cavities suitable for gas binding, have been investigated. The cages showed little interaction with dinitrogen gas but were able to take up CO2 . The best performing cage reversibly sorbed 1.4 mol CO2 per mol cage at 298 K, and 2.3 mol CO2 per mol cage at 258 K (1 bar). The enthalpy of binding was calculated to be 25-35 kJ mol-1 , across the number of equivalents bound, while DFT calculations on the CO2 binding in the cage gave ΔE for the cage-CO2 interaction of 23-28 kJ mol-1 , across the same range. DFT modelling suggested that the binding mode is a hydrogen bond between the carbonyl oxygen of CO2 and the internally directed hydrogen atoms of the cage.

Self-Assembly with 2,6-Bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine: Silver(I) and Iron(II) Complexes.

A new "click" ligand, 2,6-bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine (L) featuring a tridentate 2,6-bis(1,2,3-triazol-4-yl)pyridine (tripy) pocket and two pyridyl (py) units was synthesized in modest yield (42%) using the copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The coordination chemistry of the ligand with silver(I) and iron(II) ions was examined using a battery of solution (¹H and DOSY (diffusion ordered spectroscopy) nuclear magnetic resonance (NMR), infrared and absorption spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS)), and solid state (X-ray crystallography, elemental analysis) techniques. When treated with silver(I) ions, the ligand forms discrete [Ag(L)]⁺ (X-, where X- = BF4-, NO3- or SbF6-) complexes in dimethyl sulfoxide (DMSO) solution but these complexes crystallize as coordination polymers. The addition of [Fe(H2O)6](BF4)2 to an acetonitrile solution of the ligand forms the expected monomeric octahedral [Fe(L)2]2+ complex and treatment of the iron(II) complex with AgBF4 generates a heterometallic linear coordination polymer.

Controlled Formation of Heteroleptic [Pd2(La)2(Lb)2](4+) Cages.

Metallosupramolecular architectures are beginning to be exploited for a range of applications including drug delivery, catalysis, molecular recognition, and sensing. For the most part these achievements have been made with high-symmetry metallosupramolecular architectures composed of just one type of ligand and metal ion. Recently, considerable efforts have been made to generate metallosupramolecular architectures that are made up of multiple different ligands and/or metals ions in order to obtain more complex systems with new properties. Herein we show that the addition of an electron-rich 2-amino-substituted tripyridyl ligand, 2,6-bis(pyridin-3-ylethynyl)pyridine (2A-tripy), to a solution of the [Pd2(tripy)4](4+) cage resulted in the clean generation of a heteroleptic [Pd2(tripy)2(2A-tripy)2](4+) architecture. The formation of the mixed-ligand cage [Pd2(tripy)2(2A-tripy)2](4+) was confirmed using (1)H NMR spectroscopy, diffusion-ordered spectroscopy, and rotating-frame nuclear Overhauser effect spectroscopy and high-resolution electrospray ionization mass spectrometry. Density functional theory calculations suggested the cis isomer was more stable that the trans isomer. Additionally, the calculations indicated that the heteroleptic palladium(II) cages are kinetically metastable intermediates rather than the thermodynamic product of the reaction. Competition experiments supported that finding and showed the cages are long-lived in solution at room temperature. Finally, it was shown that the addition of 2A-tripy to a range of preformed [Pd2(Ltripy)4](4+) cages cleanly generated the mixed-ligand systems. Three other systems displaying different exo and endo functionalities within the cage assembly were generated, suggesting that this method could be applied to synthesize a range of highly functionalized heteroleptic cis-[Pd2(La)2(Lb)2](4+) cages.

Heterometallic [MnPtn(L)2n](x+) Macrocycles from Dichloromethane-Derived Bis-2-pyridyl-1,2,3-triazole Ligands.

There is continued interest in the assembly of heterometallic metallosupramolecular assemblies due to the potential for new structural types and/or interesting chemical and physical properties. Two novel methylene-linked bis-2-pyridyl-1,2,3-triazole ligands (1a, 1b) were synthesized through CuAAC "click" chemistry between dichloromethane (DCM), sodium azide, and the appropriate ethynylpyridine, giving the shortest possible "regular" bis-pyridyl-1,2,3-triazole. The first example of bis-coordination of two 2-pyridyl-1,2,3-triazoles (Rpytri) around one Pt(II) center is reported, giving exclusive formation of head-to-tail [Pt(Rpytri)2](2+) complexes with vacant binding sites suitable for complexation with other metals and the formation of heterometallic assemblies. The concentration-dependent formation of an equilibrium mixture of a heterometallic [Pd2Pt2(L)4](8+) [4 + 4] square and [Pd3Pt3(L)6](12+) [6 + 6] hexagon was observed, and at lower concentrations ([reactants] = 1.5 mM) hexamer formation was negligible. The [Pt(L)2](2+) building block could also be utilized in the synthesis of a concentration-independent [Cu2Pt2(L)4](6+) metallomacrocyle. These compounds were characterized with (1)H, (13)C, and (1)H DOSY NMR spectroscopies, elemental analysis, mass spectrometry, and in some cases X-ray crystallography.