Platinum(II) Diimine Complexes with Catecholate Ligands Bearing Imide Electron-Acceptor Groups: Synthesis, Crystal Structures, (Spectro)Electrochemical and EPR studies, and Electronic Structure

Multicenter interactions and ligand field effects in platinum(II) tripyrrindione radicals

The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum(II) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert-butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt(II) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand (i.e., aqua vs. isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt(II)-Pt(II) contact in the case of the aqua complex. Overall, the properties of Pt(II) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.

Dinuclear ligand-to-ligand charge transfer complexes

The synthesis and characterization of dinuclear ligand-to-ligand charge transfer complexes are described. Each complex is comprised of square-planar platinum(II) coordinated to a 3-tert-butyl-orthocatecholate donor and a 4,4'-di-tert-butyl-2,2'-bipyridine acceptor. Both complexes exhibit donor → acceptor ligand-to-ligand charge transfer (LL'CT) bands in the visible spectrum. The platinum complexes are covalently attached at the catecholate 5-position to either a meta- or para-phenylene bridge fragment. Both cyclic voltammetry and electronic absorption spectroscopy exhibit features characteristic of intramolecular interaction between the platinum centres. The LL'CT excited state lifetimes are ∼twofold longer than the mononuclear parent complex. The properties of these complexes are discussed and compared to similar complexes in the literature.

Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes

A series of new charge transfer (CT) chromophores of "α-diimine-M^II-catecholate" type (where M is 3d-row transition metals-Cu, Ni, Co) were derived from 4,4'-di-tert-butyl-2,2'-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) in accordance with three modified synthetic approaches, which provide high yields of products. A square-planar molecular structure is inherent for monomeric [Cu^II(3,6-Cat)(bipy^tBu)]∙THF (1) and Ni^II(3,6-Cat)(bipy^tBu) (2) chromophores, while dimeric complex [Co^II(3,6-Cat)(bipy^tBu)]₂∙toluene (3) units two substantially distorted heteroleptic D-M^II-A (where D, M, A are donor, metal and acceptor, respectively) parts through a donation of oxygen atoms from catecholate dianions. Chromophores 1-3 undergo an effective photoinduced intramolecular charge transfer (λ = 500-715 nm, extinction coefficient up to 10⁴ M^-1·cm^-1) with a concomitant generation of a less polar excited species, the energy of which is a finely sensitive towards solvent polarity, ensuring a pronounced negative solvatochromic effect. Special attention was paid to energetic characteristics for CT and interacting HOMO/LUMO orbitals that were explored by a synergy of UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT study. The current work sheds light on the dependence of CT peculiarities on the nature of metal centers from various groups of the periodic law. Moreover, the "α-diimine-M^II-catecholate" CT chromophores on the base of "late" transition elements with differences in d-level's electronic structure were compared for the first time.

Trinuclear ReFePt clusters with a μ3-phenylvinylidene ligand: synthetic approaches, rearrangement of vinylidene, and redox-induced transformations

A series of trinuclear μ₃-vinylidene ReFePt clusters were synthesized by the application of two approaches: (i) reactions of the binuclear RePt μ-vinylidene complexes with Fe₂(CO)₉; (ii) ligand substitution or exchange reactions at the Pt atom in the synthesized ReFePt clusters. The molecular structures of CpReFePt(μ₃-CCHPh)(CO)₅[P(OEt)₃]L [L = CO; P(OEt)₃] were determined by an X-ray diffraction study. The obtained compounds were studied by IR and ¹H, ¹³C and ³¹P NMR spectroscopy. The spectroscopic study revealed that the clusters CpReFePt(μ₃-CCHPh)(CO)₅[P(OEt)₃]L [L = CO; P(OEt)₃] and CpReFePt(μ₃-CCHPh)(CO)₆[P(OPrⁱ)₃] undergo isomerization upon dissolution, resulting in three isomers with different positions of the μ₃-vinylidene ligand over the ReFePt core. The redox properties of the clusters were studied by electrochemical methods. The relatively stable cation-radicals obtained by chemical oxidation of CpReFePt(μ₃-CCHPh)(CO)₆[P(OPrⁱ)₃] and CpReFePt(μ₃-CCHPh)(CO)₅[P(OEt)₃]₂ with ferrocenium tetrafluoroborate were characterized by EPR spectroscopy.

Thiolate-Thione Redox-Active Ligand with a Six-Membered Chelate Ring via Template Condensation and Its Pt(II) Complexes

A new template condensation reaction has been discovered in a mixture of Pt(II), thiobenzamide, and base. Four complexes of the general form [Pt(ctaPh^R)₂], R = CH₃ (1a), H (1b), F (1c), Cl (1d), cta = condensed thioamide, have been prepared under similar conditions and thoroughly characterized by ¹H NMR and UV-vis-NIR spectroscopy, (spectro)electrochemistry, elemental analysis, and single-crystal X-ray diffraction. The ligand is redox active and can be reduced from the initial monoanion to a dianionic and then trianionic state. Chemical reduction of 1a with [Cp₂Co] yielded [Cp₂Co]₂[Pt(ctaPh^CH3)₂], [Cp₂Co]₂[1a], which has been similarly characterized with the addition of EPR spectroscopy and SQUID magnetization. The singly reduced form containing [1a]^1-, (nBu₄N)[Pt(ctaPh^CH3)₂], has been generated in situ and characterized by UV-vis and EPR spectroscopies. DFT studies of 1b, [1b]^1-, and [1b]^2- confirm the location of additional electrons in exclusively ligand-based orbitals. A detailed analysis of this redox-active ligand, with emphasis on the characteristics that favor noninnocent behavior in six-membered chelate rings, is included.

Square-Planar Heteroleptic Complexes of α-Diimine-NiII-Catecholate Type: Intramolecular Ligand-to-Ligand Charge Transfer

Two heteroleptic NiII complexes combined the redox-active catecholate and 2,2'- bipyridine ligand platforms were synthesized to observe a photoinduced intramolecular ligand-to-ligand charge transfer (LL'CT, HOMOcatecholate → LUMOα-diimine). A molecular design of compound [NiII(3,6-Cat)(bipy)]∙CH3CN (1) on the base of bulky 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) was an annelation of the ligand with an electron donor glycol fragment, producing derivative [NiII(3,6-Catgly)(bipy)]∙CH2Cl2 (2), in order to influence the energy of LL'CT transition. A substantial longwave shift of the absorption peak was observed in the UV-Vis-NIR spectra of 2 compared with those in 1. In addition, the studied NiII derivatives demonstrated a pronounced negative solvatochromism, which was established using a broad set of solvents. The molecular geometry of both compounds can be ascribed as an insignificantly distorted square-planar type, and the π-π intermolecular stacking of the neighboring α-diimines is realized in a crystal packing. There is a lamellar crystal structure for complex 1, whereas the perpendicular T-motifs with the inter-stacks attractive π-π interactions form the packing of complex 2. The redox-active nature of ligand systems was clearly shown through the electrochemical study: a quasi-reversible one-electron reduction of 2,2'-bipyridine and two reversible successive one-electron oxidative conversations ("catecholate dianion-o-benzosemiquinonato radical anion-neutral o-benzoquinone") were detected.

Redox-triggered reversible modulation of intense near-infrared and visible absorption using a paddlewheel-type diruthenium(III) complex

A new paddlewheel-type diruthenium complex with 2-amino-3-(trifluoromethyl)pyridine (amtfmp) [Ru2(amtfmp)4Cl2] ([1]), which shows intense and characteristic near-infrared (NIR) and visible absorption, has been developed and structurally characterized by single crystal X-ray diffraction (SCXRD) analyses. This complex exhibits reversible and dramatic NIR and visible electrochromic behavior from deep-blue ([1]) to pink ([1]-) due to the ON-OFF switching of its characteristic ligand-to-metal charge transfer (LMCT) and d-d absorption bands in response to an external voltage or chemical reagent such as decamethylcobaltocene (CoCp*2). The one-electron reduced species of [1], i.e., [CoCp*2][1], was successfully isolated and fully characterized via SCXRD, the temperature dependence of the magnetic susceptibility, and mass spectroscopy, proving that this electrochromic behavior occurs without significant structural reorganization of [1].

Immobilizing a π-Conjugated Catecholato Framework on Surfaces of SiO2 Insulator Films via a One-Atom Anchor of a Platinum Metal Center to Modulate Organic Transistor Performance

Chemical modification of insulating material surfaces is an important methodology to improve the performance of organic field-effect transistors (OFETs). However, few redox-active self-assembled monolayers (SAMs) have been constructed on gate insulator film surfaces, in contrast to the numerous SAMs formed on many types of conducting electrodes. In this study, we report a new approach to introduce a π-conjugated organic fragment in close proximity to an insulating material surface via a transition metal center acting as a one-atom anchor. On the basis of the reported coordination chemistry of a catecholato complex of Pt(II) in solution, we demonstrate that ligand exchange can occur on an insulating material surface, affording SAMs on the SiO₂ surface derived from a newly synthesized Pt(II) complex containing a benzothienobenzothiophene (BTBT) framework in the catecholato ligand. The resultant SAMs were characterized in detail by water contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and cyclic voltammetry. The SAMs served as good scaffolds of π-conjugated pillars for forming thin films of a well-known organic semiconductor C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), accompanied by the engagements of the C8-BTBT molecules with the SAMs containing the common BTBT framework at the first layer on SiO₂. OFETs containing the SAMs displayed improved performance in terms of hole mobility and onset voltage, presumably because of the unique interfacial structure between the organic semiconducting and inorganic insulating layers. These findings provide important insight into creating new elaborate interfaces through installing coordination chemistry in solution to solid surfaces, as well as OFET design by considering the compatibility between SAMs and organic semiconductors.

Tuning PtII -Based Donor-Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

Impact of Group 10 Metals on the Solvent-Induced Disproportionation of o-Semiquinonato Complexes

The oxidation of [M^II (3,5-DTBCat)(DTBbpy)] (M=Ni ([Ni]), Pd ([Pd]), and Pt ([Pt]); 3,5-DTBCat=3,5-di-tert-butylcatecholato; DTBbpy=4,4'-di-tert-butyl-2,2'-bipyridine) afforded the dimeric {[Ni^II (3,5-DTBSQ)(DTBbpy)](PF₆ )}₂ ({[Ni](PF₆ )}₂ ; 3,5-DTBSQ=3,5-di-tert-butylsemiquinonato) and monomeric semiquinonato (SQ) complexes [M^II (3,5-DTBSQ)(DTBbpy)](PF₆ ) (M=Pd ([Pd](PF₆ )) and Pt ([Pt](PF₆ ))). The negative solvatochromic properties of the SQ complexes allowed us to estimate the relative order of their dipole moments: [Pd](PF₆ )>[Pt](PF₆ )>{[Ni](PF₆ )}₂ . The complexes [Pd](PF₆ ) and [Pt](PF₆ ) adopt monomeric structures and are stable in CH₂ Cl₂ and toluene, whereas they gradually disproportionate at room temperature to [M] and 3,5-di-tert-butylbenzoquinone (3,5-DTBBQ) in polar solvents such as THF, MeOH, EtOH, DMF, or DMSO. The results of spectroscopic studies suggested that the oxidized nickel complex adopts a monomeric structure ([Ni](PF₆ )) in CH₂ Cl₂ , but a dimeric structure ({[Ni](PF₆ )}₂ ) in the other investigated solvents. In polar solvents, {[Ni](PF₆ )}₂ may disproportionate to [Ni] and 3,5-DTBBQ at 323 K, thereby demonstrating a significant solvent- and metal-dependence in temperature. The relative activities of {[Ni](PF₆ )}₂ and [M](PF₆ ) toward disproportionation are related to the electrochemically estimated K_dis values in CH₂ Cl₂ and DMF. The present work demonstrates that solvent polarity and the dipole moments of the SQ complexes promote disproportionation, which can be controlled by a judicious choice of the metal ion, solvent, and temperature.

New π-extended catecholato complexes of Pt(ii) and Pd(ii) containing a benzothienobenzothiophene (BTBT) moiety: synthesis, electrochemical behavior and charge transfer properties

Benzothienobenzothiophene (BTBT) and derivatives have received increasing attention as organic field-effect transistor materials and molecular conductors. This report presents the first synthesis of metal complexes involving a BTBT moiety, which was achieved by complexation of 2,2'-bipyridyl complexes of Pt(ii) and Pd(ii) with dihydroxy-substituted BTBT (1) as a new π-extended catecholato ligand (^tBu₂Bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). The resulting complexes M(^tBu₂Bpy)(O₂BTBT) (M = Pt (3Pt) and Pd (3Pd)) were characterized by UV-vis spectroscopy, density functional theory (DFT) calculations, and cyclic voltammetry. The electron donating ability of BTBT was substantially enhanced upon including two oxygen substituents followed by metal coordination. This enabled chemical oxidation of 3Pt and 3Pd with a mild chemical oxidant (ferrocenium hexafluorophosphate) and formation of the one-electron-oxidized state. While 3Pt and 3Pd exhibited an absorption band originating from a catecholate → Bpy ligand-to-ligand charge transfer transition typical of this class of catecholato complexes, the radical cations exhibited a unique π-π* intramolecular charge transfer (ICT) transition absorption in which the π and π* orbitals were the newly incorporated benzothienothiophene-based donor and semiquinonato-based acceptor, respectively. The BTBT⁺ skeleton was electronically divided into two sites by the present chemical modification. The ICT properties of the complexes were found to be modulated by varying the metal ions. These findings offer a new approach to molecular design for (semi)conducting materials using optical properties.

Stabilising the lowest energy charge-separated state in a {metal chromophore - fullerene} assembly: a tuneable panchromatic absorbing donor-acceptor triad

Photoreduction of fullerene and the consequent stabilisation of a charge-separated state in a donor-acceptor assembly have been achieved, overcoming the common problem of a fullerene-based triplet state being an energy sink that prevents charge-separation. A route to incorporate a C60-fullerene electron acceptor moiety into a catecholate-Pt(ii)-diimine photoactive dyad, which contains an unusually strong electron donor, 3,5-di-tert-butylcatecholate, has been developed. The synthetic methodology is based on the formation of the aldehyde functionalised bipyridine-Pt(ii)-3,5-di-tert-butylcatechol dyad which is then added to the fullerene cage via a Prato cycloaddition reaction. The resultant product is the first example of a fullerene-diimine-Pt-catecholate donor-acceptor triad, C60bpy-Pt-cat. The triad exhibits an intense solvatochromic absorption band in the visible region due to catechol-to-diimine charge-transfer, which, together with fullerene-based transitions, provides efficient and tuneable light harvesting of the majority of the UV/visible spectral range. Cyclic voltammetry, EPR and UV/vis/IR spectroelectrochemistry reveal redox behaviour with a wealth of reversible reduction and oxidation processes forming multiply charged species and storing multiple redox equivalents. Ultrafast transient absorption and time resolved infrared spectroscopy, supported by molecular modelling, reveal the formation of a charge-separated state [C60˙-bpy-Pt-cat˙+] with a lifetime of ∼890 ps. The formation of cat˙+ in the excited state is evidenced directly by characteristic absorption bands in the 400-500 nm region, while the formation of C60˙- was confirmed directly by time-resolved infrared spectroscopy, TRIR. An IR-spectroelectrochemical study of the mono-reduced building block (C60-bpy)PtCl2, revealed a characteristic C60˙- vibrational feature at 1530 cm-1, which was also detected in the TRIR spectra. This combination of experiments offers the first direct IR-identification of C60˙- species in solution, and paves the way towards the application of transient infrared spectroscopy to the study of light-induced charge-separation in C60-containing assemblies, as well as fullerene films and fullerene/polymer blends in various OPV devices. Identification of the unique vibrational signature of a C60-anion provides a new way to follow photoinduced processes in fullerene-containing assemblies by means of time-resolved vibrational spectroscopy, as demonstrated for the fullerene-transition metal chromophore assembly with the lowest energy charge-separated excited state.

BODIPY-Appended 2-(2-Pyridyl)benzimidazole Platinum(II) Catecholates for Mitochondria-Targeted Photocytotoxicity

Platinum(II) complexes of the type [Pt(L)(cat)] (1 and 2), in which H2 cat is catechol and L represents two 2-(2-pyridyl)benzimidazole ligands with 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) pendants, were synthesized to achieve mitochondria-targeted photocytotoxicity. The complexes showed strong absorptions in the range λ=510-540 nm. Complex 1 exhibited intense emission at λ=525 nm in 1 % DMSO/water solution (fluorescence quantum yield of 0.06). Nanosecond transient absorption spectral features indicated an enhanced population of the triplet excited state in di-iodinated complex 2. The generation of singlet oxygen by complex 2 upon exposure to visible light, as evidenced from experiments with 1,3-diphenylisobenzofuran, is suitable for photodynamic therapy because of the remarkable photosensitizing ability. The complexes resulted in excellent photocytotoxicity in HaCaT cells (half maximal inhibitory concentration IC50 ≈3 μm, λ=400-700 nm, light dose=10 J cm(-2) ), but they remained non-toxic in the dark (IC50 >100 μm). Confocal microscopy images of 1 and Pt estimation from isolated mitochondria showed colocalization of the complexes in the mitochondria. Complex 2 displayed generation of reactive oxygen species induced by visible light, disruption of the mitochondrial membrane potential, and apoptosis.

Platinum(ii) complexes of mixed-valent radicals derived from cyclotricatechylene, a macrocyclic tris-dioxolene

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4'-bis(tert-butyl)-2,2'-bipyridyl { t Bu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180 mV, corresponding to stepwise oxidation of the [ctc]6- catecholato rings to the semiquinonate level. The redox series [1]0/1+/2+/3+ and [3]0/1+/2+/3+ have been characterised by UV/vis/NIR spectroelectrochemistry. The mono- and di-cations have class II mixed valent character, with reduced radical delocalisation compared to an analogous bis-dioxolene system. The SOMO composition of [1˙]+ and [3˙]+ has been delineated by cw EPR, ENDOR and HYSCORE spectroscopies, with the aid of two monometallic model compounds [PtL(DBsq˙)]+ (DBsqH = 3,5-bis(tert-butyl)-1,2-benzosemiquinone; L = dppe or t Bu2bipy). DF and time-dependent DF calculations confirm these interpretations, and demonstrate changes to spin-delocalisation in the ctc macrocycle as it is sequentially oxidised.

Tuning of intramolecular charge transfer properties and charge distributions in ferrocene-appended catechol derivatives by chemical substitution

In this study, we report intramolecular charge transfer (ICT) properties and charge distributions in a series of FcC derivatives (FcC = 4-ferrocenylcatecholate where Fc = ferrocene and C = catecholate). This series consists of a previously reported complex FcV (4-ferrocenylveratrole) and newly synthesized complexes FcA (4-ferrocenylcatechol bis(acetate) and Pt((t)Bu2bpy)(FcC) ((t)Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). An electrochemical analysis of Pt((t)Bu2bpy)(FcC) using cyclic voltammetry revealed two well-defined, reversible waves which were assigned to the sequential oxidation of the Pt((t)Bu2bpy)(C) and Fc moieties. The potential splitting between the waves (524 mV) indicated that there was an electronic interaction between both moieties. ICT property and charge distribution of [Pt((t)Bu2bpy)(FcC)]˙(+) were rationalized by comparison with the [FcV]˙(+) and [FcA]˙(+) (4-ferrocenylcatechol bis(acetate)). DFT calculations and UV-vis-NIR spectroscopy revealed that [Pt((t)Bu2bpy)(FcC)]˙(+), [FcV]˙(+), and [FcA]˙(+) were ferrocenium (Fc(+))-centered rather than semiquinone ligand-centered and that these complexes exhibited ICT transition bands from the catechol-derivatized framework to the Fc(+) moiety in the near infrared (NIR) region. Both the electronic coupling parameter (HAB) and delocalization parameter (α) increased in value as the electron-donating strength of the substituent groups in the catechol-derivatized framework increased (OCOCH3 ([FcA]˙(+)) < OCH3 ([FcV]˙(+)) < O(-) ([Pt((t)Bu2bpy)(FcC)]˙(+))). The electronic interactions between the organometallic center and the non-innocent framework were tuned by changing the substituents. The potential energy surfaces of the Fc(+) derivatives, obtained using two-state Marcus-Hush theory, can be modulated by changing the energy level of the molecular orbitals of the appended catechol-derivatized moieties.

Exploring excited states of Pt(II) diimine catecholates for photoinduced charge separation

The intense absorption in the red part of the visible range, and the presence of a lowest charge-transfer excited state, render Platinum(II) diimine catecholates potentially promising candidates for light-driven applications. Here, we test their potential as sensitisers in dye-sensitised solar cells and apply, for the first time, the sensitive method of photoacoustic calorimetry (PAC) to determine the efficiency of electron injection in the semiconductor from a photoexcited Pt(II) complex. Pt(II) catecholates containing 2,2′-bipyridine-4,4′-di-carboxylic acid (dcbpy) have been prepared from their parent iso-propyl ester derivatives, complexes of 2,2′-bipyridine-4,4′-di-C(O)OiPr, (COOiPr)2bpy, and their photophysical and electrochemical properties studied. Modifying diimine Pt(II) catecholates with carboxylic acid functionality has allowed for the anchoring of these complexes to thin film TiO2, where steric bulk of the complexes (3,5-di(t)Bu-catechol vs. catechol) has been found to significantly influence the extent of monolayer surface coverage. Dye-sensitised solar cells using Pt(dcbpy)((t)Bu2Cat), 1a, and Pt(dcbpy)(pCat), 2a, as sensitisers, have been assembled, and photovoltaic measurements performed. The observed low, 0.02–0.07%, device efficiency of such DSSCs is attributed at least in part to the short excited state lifetime of the sensitisers, inherent to this class of complexes. The lifetime of the charge-transfer ML/LLCT excited state in Pt((COO(I)Pr)2bpy)(3,5-di-(t)Bu-catechol) was determined as 250 ps by picosecond time-resolved infrared spectroscopy, TRIR. The measured increase in device efficiency for 2a over 1a is consistent with a similar increase in the quantum yield of charge separation (where the complex acts as a donor and the semiconductor as an acceptor) determined by PAC, and is also proportional to the increased surface loading achieved with 2a. It is concluded that the relative efficiency of devices sensitised with these particular Pt(II) species is governed by the degree of surface coverage. Overall, this work demonstrates the use of Pt(diimine)(catecholate) complexes as potential photosensitizers in solar cells, and the first application of photoacoustic calorimetry to Pt(II) complexes in general.

Ultrafast charge transfer dynamics in supramolecular Pt(ii) donor–bridge–acceptor assemblies: the effect of vibronic coupling

Thanks to major advances in laser technologies, recent investigations of the ultrafast coupling of nuclear and electronic degrees of freedom (vibronic coupling) have revealed that such coupling plays a crucial role in a wide range of photoinduced reactions in condensed phase supramolecular systems. This paper investigates several new donor–bridge–acceptor charge-transfer molecular assemblies built on a trans-Pt(ii) acetylide core. We also investigate how targeted vibrational excitation with low-energy IR light post electronic excitation can perturb vibronic coupling and affect the efficiency of electron transfer (ET) in solution phase. We compare and contrast properties of a range of donor–bridge–acceptor Pt(ii) trans-acetylide assemblies, where IR excitation of bridge vibrations during UV-initiated charge separation in some cases alters the yields of light-induced product states. We show that branching to multiple product states from a transition state with appropriate energetics is the most rigid condition for the type of vibronic control we demonstrate in our study.

Electrochemistry, chemical reactivity, and time-resolved infrared spectroscopy of donor-acceptor systems [(Q(x))Pt(pap(y))] (Q = substituted o-quinone or o-iminoquinone; pap = phenylazopyridine)

The donor-acceptor complex [((O,N)Q(2-))Pt(pap(0))] (1; pap = phenylazopyridine, (O,N)Q(0) = 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [((O,N)Q(•-))Pt(pap(0))](BF4) ([1]BF4) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of (O,N)Q in [1](+). Simulation of the cyclic voltammograms of 1 recorded in the presence of PPh3 elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh3. The thermodynamically stable product of this reaction, complex [((O,N)Q(•-)) Pt(PPh3)2](PF6) ([2]PF6), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex 1b (one of the positional isomers of 1) and its analogue [((O,O)Q(2-))Pt(pap(0))] (3; (O,O)Q = 3,5-di-tert-butyl-o-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex 1b is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of 3. Density functional theory calculations on [1](n+) provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of 1 with electron donors and propose a mechanism for this process.

Isomeric separation in donor-acceptor systems of Pd(II) and Pt(II) and a combined structural, electrochemical and spectroelectrochemical study

Compounds of the form [(pap)M(Q(2-))] (pap = phenylazopyridine; Q = 3,5-di-tert-butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl-benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one-pot reaction. The geometrical isomers, which are possible because of the built in asymmetry of these ligands, have been separated by using different temperatures and variable solubility. Structural characterization of 1b shows that the metal centers are in a square planar environment, the pap ligand is in the unreduced neutral state and the quinones are in the doubly reduced, Q(2-) catecholate form. Cyclic voltammetric measurements on the complexes display two one-electron oxidations and two one-electron reductions. EPR and vis-NIR spectra of the one-electron oxidized forms of the complexes indicate that the first oxidation takes place on the Q(2-) ligands to produce a metal bound semiquinone (Q˙(-)) radical. Reduction takes place on the pap ligand, generating metal bound pap˙(-) as seen from the (14)N (I = 1) coupling in their EPR spectrum. All the complexes in their [(pap)M(Q(2-))] neutral forms show strong absorptions in the NIR region which are largely LLCT (ligand to ligand charge transfer) in origin. These NIR bands can be tuned over a wide energy range by varying the metal center as well as the Q ligand. In addition, the intensity of NIR bands can be switched on and off by a simple electron transfer at relatively low potentials. DFT studies were used to corroborate these findings.

Structure and ultrafast dynamics of the charge-transfer excited state and redox activity of the ground state of mono- and binuclear platinum(II) diimine catecholate and bis-catecholate complexes: a transient absorption, TRIR, DFT, and electrochemical study

A series of mononuclear complexes of the type [Pt(Bu(2)cat)(4,4'-R(2)-bipy)] [where Bu(2)cat is the dianion of 3,5-(t)Bu(2)-catechol and R = H, (t)Bu, or C(O)NEt(2)] and analogous dinuclear complexes based on the "back-to-back" bis-catechol ligand 3,3',4,4'-tetrahydroxybiphenyl have been studied in detail in both their ground and excited states by a range of physical methods including electrochemistry, UV/vis/near-IR, IR, and electron paramagnetic resonance spectroelectrochemistry, and time-resolved IR (TRIR) and transient absorption (TA) spectroscopy. Density functional theory calculations have been performed to support these studies, which provide a detailed picture of the ground- and excited-state electronic structures, and excited-state dynamics, of these complexes. Notable observations include the following: (i) for the first time, the lowest-energy catecholate → bipyridine (bpy) ligand-to-ligand charge-transfer (LL'CT) excited states of these chromophores have been studied by TRIR spectroscopy, showing a range of transient bands associated with the bpy radical anion and semiquinone species, and back-electron-transfer occurring in hundreds of picoseconds; (ii) strong electronic coupling between the two catecholate units in the bridging ligand of the dinuclear complexes results in a delocalized, planar (class 3) "mixed-valence" catecholate(2-)/semiquinone(•-) state formed by one-electron oxidation of the bridging ligand; (iii) in the LL'CT excited state of the dinuclear complexes, the bridging ligand is symmetrical and delocalized, whereas the bpy radical anion is localized at one terminus of the complex. This study is the first example of an investigation of excited-state behavior in platinum(II) catecholate complexes, performed with the use of picosecond TRIR and femtosecond TA spectroscopy.

Rhenium(I) and platinum(II) complexes with diimine ligands bearing acidic phenol substituents: hydrogen-bonding, acid-base chemistry and optical properties

Tricarbonylchloro-rhenium(i) (1-4) and catecholato-platinum(ii) complexes (6, 7) of diimine ligands bearing phenol and O-protected phenol substituents have been prepared and fully characterised including single crystal structure analyses of 1, 4 and 7. The redox behaviour of the catecholato platinum(ii) complexes 6 and 7 has been probed by cyclic voltammetry, preparative oxidation and EPR spectroscopy (6˙(+), 7˙(+)). Reversible deprotonation of the hydroxy substituted complexes 1, 3 and 6 to 1(-), 3(-) and 6(-) resulted in significant changes in their electronic spectra. The luminescence properties of the diamagnetic complexes have been investigated using emission spectroscopy. DFT and TD-DFT calculations were invoked to shed some light onto the geometric and electronic structures as well as the experimental spectroscopic properties of the neutral complexes 1-7, the radical cations 6˙(+) and 7˙(+) and the conjugate bases 1(-), 3(-) and 6(-).

Synthesis, Characterisation and Properties of Diazoketiminato-Chelate Complexes of Os(II)

Reactions of 2-(arylazo)aniline, L-NH2 [L-NH2 = p-RC6H4N=NC6H4NH2; where R=H for L1-NH2, CH3 for L2-NH2 and Cl for L3-NH2] with [Os(H)Br(CO)(PPh3)3] gave the complexes [OsBr(L-NH)(CO)(PPh3)2] for which cyclic volatammograms exhibited one quasi-reversible oxidative response within the range 0.54–0.6 V vs SCE. The X-ray crystal structure of [OsBr(L1-NH)(CO)(PPh3)2] has been determined and its electronic structure was evaluated on the basis of single point DFT calculations using its crystallographic coordinates.