Photoinduced electron transfer across ortho-oligo-phenylenes and novel luminophores based on earth-abundant metals

Long-range electron-transfer is of high interest for many fields, such as artificial photosynthesis and molecular electronics. Donor – bridge – Acceptor compounds, wherein photoinduced electron transfer takes place were thoroughly investigated in this regard. Especially para-phenylene systems were chosen due to their rigid, rod-like wire behaviour. Depending on the electronic coupling of the bridge, para-phenylenes reveal �-values ranging from 0.2 to 0.8 °A−1. Their ortho-connected relatives are completely unexplored until now. In chapter I of this thesis the motivation for this work and the theoretical background for electron transfer will be given. This will be followed by a few examples of electron transfers across para-phenylenes, to put the herein presented work into perspective. In chapter II photoinduced electron-transfer across an ortho-phenylene wire consisting of 2 to 6 phenyl units will be presented. A Ru(II)-photosensitiser and a triarylamine electron donor were chosen to investigate the kinetics of the charge-shift reaction. The photoinduced forward, as well as the thermal back-reaction, were explored with time resolved measurements. Due to the flexibility of the bridge and slowly interconverting conformers in solution, analysis of charge-separation remains turbid, but a coherent analysis can be made for charge-recombination. The main discovery is that ortho-phenylenes possess very low �-values for charge-transfer, with a �-value in acetonitrile of 0.04 °A−1. The mechanism for the hole transfer is coherent tunnelling and a relevant aspect seems to be the �-pathway, which is shorter for ortho-phenylenes than for para-phenylenes. Ortho-phenylenes can therefore be considered as a new class of molecular wires. Chapter III will then present the results of photoinduced long-range electron transfer through ortho-naphthalenes, which can form different atropisomers and electron transfer is studied in these systems for the first time. Photoinduced electron transfer would not be possible without photosensitisers, which allow for an enough long living excited state with enough reducing or oxidising power, that electron transfer reactions can take place. Most photosensitisers today which reach the photophysical goals for being applied, for example in photoredox catalysis or in long-range electron transfer, rely on noble metals, such as Ru(II) or Ir(III). Therefore it would be desirable, to shift from noble metals as centres to more earth abundant metals. Nickel(0) allows for an MLCT transition when the ligand orbitals are of the right energy. In chapter IV first preliminary results of a nickel(0) bis(diphenylphosphino)naphthalene complex will be presented, which were accompanied by DFT calculations. Following this approach of using more earth-abundant metals, in chapter V the possibility of titanium(IV) complexes is considered, which could possibly undergo LMCT emission transitions. Different possible approaches towards a titanium-based luminophore will be given

Luminescent Ni(0) complexes

With its 3d10 valence electron configuration Ni(0) is isoelectronic with Cu(I). While many Cu(I) complexes emitting from metal-to-ligand charge transfer (MLCT) excited states have been explored, the number of luminescent Ni(0) complexes known to date is very limited. Ni(0) is typically stabilized by carbonyls, phosphines or isocyanides due to the π-acceptor properties of these ligands, and photoluminescence has been reported in a few selected cases that are reviewed herein. Recent studies indicate that chelating isocyanide ligands are promising for obtaining Ni(0) complexes with long-lived 3MLCT states, and this could be interesting for a similar range of applications as with photoactive Cu(I) complexes, including for example luminescent devices, solar cells, and organic photoredox reactions

Electron Transfer across o-Phenylene Wires

Photoinduced electron transfer across rigid rod-like oligo-p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)32+ (bpy =2,2′-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120–220 ns in deaerated CH3CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor–acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion

Directing Energy Transfer in Panchromatic Platinum Complexes for Dual Vis–Near-IR or Dual Visible Emission from σ‑Bonded BODIPY Dyes

We report on the platinum complexes <i>trans</i>-Pt­(BODIPY)­(8-ethynyl-BODIPY)­(PEt₃)₂ (<b>EtBPtB</b>) and <i>trans</i>-Pt­(BODIPY)­(4-ethynyl-1,8-naphthalimide)­(PR₃)₂ (R = Et, <b>EtNIPtB-1</b>; R = Ph, <b>EtNIPtB-2</b>), which all contain two different dye ligands that are connected to the platinum atom by a direct σ bond. The molecular structures of all complexes were established by X-ray crystallography and show that the different dye ligands are in either a coplanar or an orthogonal arrangement. π-stacking and several CH···F and short CH···π interactions involving protons at the phosphine substituents lead to interesting packing motifs in the crystal. The complexes feature several strong absorptions (ε = 3.2 × 10⁵–5.5 × 10⁵ M^–1 cm^–1) that cover the regime from 350 to 480 nm (<b>EtNIPtB-1</b> and <b>EtNIPtB-2</b>) or from 350 to 580 nm (<b>EtBPtB</b>). Besides the typical absorption bands of both kinds of attached dyes, they also feature an intense band near 400–420 nm, which is assigned by time-dependent density functional theory calculations to a higher-energy transition within the ethynyl-BODIPY (EtB) ligand or to charge transfer between the BODIPY (B) and naphthalimide (NI) chromophores. All complexes show dual fluorescence and phosphorescence emission from either the B (<b>EtNIPtB-1</b> and <b>EtNIPtB-2</b>) or EtB (<b>EtBPtB</b>) ligand with a maximum phosphorescence quantum yield of 41% for <b>EtNIPtB-1</b>. The latter seems to be the highest reported value for room temperature phosphorescence from a BODIPY dye. The complete quenching of the emission from the chromophore absorbing at the higher energy and the appearance of the corresponding absorption bands in the fluorescence and phosphorescence excitation spectra indicate complete and rapid energy transfer to the chromophore with the lower-energy excited state, i.e., EtNI → B in <b>EtNIPtB-1</b> and <b>EtNIPtB-2</b> and B → EtB in <b>EtBPtB</b>. The latter process was further investigated by transient absorption spectroscopy, indicating that energy transfer is complete within 0.6 ns. <b>EtNIPtB-1</b> catalyzes the photooxidation of 1,5-dihydroxynaphthalene with photogenerated ¹O₂ to Juglone at a much faster rate than methylene blue but with only modest quantum yields of 37% and with the onset of photodegradation after 60 min