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

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.

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.

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.

Palladium(ii) and platinum(ii) complexes of glyoxalbis(N-aryl)osazone: molecular and electronic structures, anti-microbial activities and DNA-binding study

A new family of palladium(ii) and platinum(ii) complexes of redox non-innocent osazone ligands that exhibit moderate antileishmanial activity were isolated.

Directly Coupled Versus Spectator Linkers on Diimine PtII Acetylides-Change the Structure, Keep the Function?

Modification of light-harvesting units with anchoring groups for surface attachment often compromises light-harnessing properties. Herein, a series of [donor-acceptor-anchor] platinum(II) diimine (bis-)acetylides was developed in order to systematically compare the effect of conjugated versus electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of light-harvesting units. The first examples of "decoupled" phosphonate diimine Pt^II complexes are reported, and their properties are compared and contrasted to those of carboxylate analogues studied by a diversity of methods. Ultrafast time-resolved IR and transient absorption spectroscopy revealed that all complexes have a charge-transfer (CT) lowest excited state with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of CT states were identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy redistribution accompanied by structural changes was directly captured in the CT states. A significant difference between the structures of the electronic ground and CT excited states, as well as differences in the structural reorganisation in the complexes bearing directly attached or electronically decoupled anchoring groups, was discovered. This work demonstrates that decoupling of the anchoring group from the light-harvesting core by a saturated spacer is an easy approach to combine surface attachment with high reduction potential and ten times longer lifetime of the CT excited state of the light-absorbing unit, and retain electron-transfer photoreactivity essential for light-harvesting applications.

Anionic cyclometallated Pt(ii) square-planar complexes: new sets of highly luminescent compounds

Novel anionic Pt(ii) complexes were synthesized, displaying an outstanding enhancement of the emission efficiency in the solid state.

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.

Near-IR absorbing donor-acceptor ligand-to-ligand charge-transfer complexes of nickel(ii)

A new series of square-planar nickel(ii) donor-acceptor complexes exhibiting ligand-to-ligand charge-transfer (LL'CT) transitions have been prepared. Whereas the use of a catecholate donor ligand in conjunction with a bipyridyl acceptor ligand affords a complex that absorbs throughout the visible region, the use of a azanidophenolate donor ligands in conjunction with a bipyridyl acceptor ligand affords complexes that absorbs well into the near-IR region of the solar spectrum. Three new complexes, (cat)Ni(bpy t Bu2) (1; (cat)2- = 3,5-di-tert-butyl-1,2-catecholate; bpy t Bu2 = 4,4'-di-tert-butyl-2,2'-bipyridine), (ap)Ni(bpy t Bu2) (2; (ap)2- = 4,6-di-tert-butyl-2-(2,6-diisopropylphenylazanido)phenolate), and (apPh)Ni(bpy t Bu2) (3; (apPh)2- = 10-(2,6-diisopropylphenylazanido)-9-phenanthrolate), have been prepared and characterized by structural, electrochemical, and spectroscopic methods. Whereas all three square-planar complexes show multiple reversible one-electron redox-processes and strong LL'CT absorption bands, in azanidophenolate complexes 2 and 3, the LL'CT absorption covers the near-IR region from 700-1200 nm. The electronic absorption spectra and ground state electrochemical data for 2 and 3 provide an estimate of their excited-state reduction potentials, E+/*, of -1.3 V vs. SCE, making them as potent as the singlet excited state of [Ru(bpy)3]2+.

Highly polar solvent-induced disproportionation of a cationic Pt(ii)–diimine complex containing an o-semiquinonato

The unprecedented disproportionation of [Pt^II(3,5-DTBSQ)(DTBbpy)]⁺PF₆⁻ in highly polar solvents, DMSO and DMF, was elucidated for the first time.