Anastasia B. S. Elliott

‘Click’ to functionalise: synthesis, characterisation and enhancement of the physical properties of a series of exo- and endo-functionalised Pd2L4 nanocages

The synthesis of self-assembled metallosupramolecular architectures has been of steadily growing interest in recent years due to their diverse applications. Appending additional functionality to the ligands of these architectures has been limited as this often involves incorporation of coordinating groups that can potentially disrupt formation of the desired structure. Herein we report the use of the facile, functional group tolerant and high yielding CuAAC ‘click’ reaction to attach a variety of functional moieties to a tripyridyl ligand system. Despite the presence of the potentially coordinating 1,2,3-triazole rings, self-assembly of quadruply-stranded dipalladium(II) cage architectures in the presence of Pd(II) ions was almost universally observed for the functionalised “click” ligands. The only system which did not assemble into the expected cage featured a 2-(1,2,3-triazol-4-yl)pyridine binding pocket which sequestered the Pd(II) ions. Blocking this chelating pocket with an inert [Re(CO)3Cl] moiety restored the ability of the ligand to self-assemble into the desired quadruply-stranded dipalladium(II) cage, generating a heterometallic cage architecture. All ligands and cage architectures have been characterised using 1H, 13C and DOSY NMR, IR, UV-Vis and emission spectroscopies, mass spectrometry and in some cases by X-ray crystallography. Whilst the parent cage system is devoid of useful physical properties and displays a limited range of solubility, the CuAAC methodology provides a facile method to enhance the cages properties. A variety of fluorescent, redox active and biologically relevant species have been appended to the external surface of these cages. These groups enabled the generation of a series of aqueous soluble, fluorescent and electrochemically active Pd2L4 cages in a modular fashion.

Enhanced performance of dye-sensitized solar cells using carbazole-substituted di-chromophoric porphyrin dyes

The purpose of this work is to investigate the origin of improved photovoltaic performance of a series of di-chromophoric carbazole-substituted porphyrin dyes employed as sensitizers in dye-sensitized solar cells. Five di-chromophoric zinc porphyrin dyes with the same porphyrin core, a carbazole unit attached in the meso-position through a phenylethenyl linkage, and substituents spanning a range of electron affinities, in an attempt to tune the electronic level of the carbazole unit, have been synthesized (CZPs). Density functional theory (DFT) calculations predicted the nature of the electronic transitions observed in the CZP systems, showing a large degree of orbital mixing. In contrast, UV-vis absorption, resonance Raman spectroscopy and differential pulse voltammetry investigations suggested negligible interaction between the porphyrin and carbazole chromophores. Carbazole substitution led to a moderate increase in photon absorption intensity within the ∼300 nm to 400 nm wavelength region, a smaller but broader Soret band absorption and slightly increased photon absorption intensity in the 550 nm to 650 nm Q band region. Despite the rather small changes in light harvesting and negligible changes in the HOMO/LUMO electronic levels, the photovoltaic performance of the new dyes is increased by as much as 30% compared to the single chromophore Zn porphyrin dye 5-(4-(2-cyano-2-carboxyethenyl)phenyl-15-phenyl-10,20-bis(2,4,6-trimethylphenyl)porphyrinato zinc(II) (ZP1), leading to over 6% power conversion efficiencies (PCEs). Both open circuit voltage (VOC) and short circuit current (JSC) have increased. The increased VOC is attributed to increased electron lifetimes due to a steric blocking effect. Analysis of the increased short circuit current (ΔJSC) showed that only less than 10% of ΔJSC originates from increased light absorption under simulated air mass 1.5 illumination, while the rest of the improvements are attributed to a steric effect enhancing the electron injection efficiency. These results suggest that developing non-conjugated multichromophoric dyes can lead to simultaneous increases in both the photocurrent and the photovoltage of dye-sensitized solar cells.

Luminescent Cages: Pendant Emissive Units on [Pd2L4]4+ “Click” Cages

The photophysics of a family of exo-functionalized [Pd2L4](4+) metallo-supramolecular cage architectures constructed from a tripyridyl 1,2,3-triazole backbone are reported. Several spectroscopic techniques are employed including both electronic (steady-state and transient absorption and emission) and vibrational (resonant and nonresonant Raman) methods. These experimental results are interpreted alongside simulated results from density functional theory calculations of the systems vibrational and electronic properties. The ligands and cages are shown to be essentially insulated from the exo-functionalization. They exhibit electronic transitions in the UV region and excited-state properties that are little affected by formation of the cage. Upon functionalization, characteristic Raman bands, electronic transitions, and emission bands associated with, and confined to, the substituent are observed.

Long-Lived Charge Transfer Excited States in HBC-Polypyridyl Complex Hybrids

The synthesis of two bipyridine-hexa-peri-hexabenzocoronene (bpy-HBC) ligands functionalized with either (t)Bu or C12H25 and their Re(I) tricarbonyl chloride complexes are reported and their electronic properties investigated using spectroscopic and computational methods. The metal complexes show unusual properties, and we observed the formation of a long-lived excited state using time-resolved infrared spectroscopy. Depending on the solvent, this appears to be of the form Rebpy(•-)HBC(•+) or a bpy-centered π,π* state. TD-DFT calculations support the donor-acceptor charge transfer character of these systems, in which HBC is the donor and bpy is the acceptor. The ground state optical properties are dominated by the HBC chromophore with additional distinct transitions of the complexes, one associated with MLCT 450 nm (ε > 17 000 L mol(-1) cm(-1)) and another with a HBC/metal to bpy charge transfer, termed the MLLCT band (373 nm, ε = 66 000 L mol(-1) cm(-1)). These assignments are also supported by resonance Raman spectroscopy.

Application of terpyridyl ligands to tune the optical and electrochemical properties of a conducting polymer

We present a simple and effective way of using metal and metal–ligand modifications to tune the electrochemical and optical properties of conducting polymers. To that end, a polyterthiophene functionalized with terpyridine moieties was synthesized and then the resulting films surface or bulk was modified with different metal ions, namely Fe2+, Zn2+ and Cu2+ and terpyridine. The modification of the terpyridine functionalized polyterthiophene film by Fe2+ increased the absorptivity and electrochemical capacitance of the conducting polymer, and improved its conjugation. Further modification by Zn2+ and Cu2+ resulted in dramatically different spectroelectrochemical properties of the film. Moreover, the influence of the solvents (ACN and 1 : 1 ACN : H2O) in conjunction with the metal ion applied for the modification was found crucial for the electrochemical and optical properties of the films.

Chapter 7:Vibrational spectroscopy of N-donor ligand metal complexes: probing excited states

This report reviews work on vibrational spectroscopy of N-donor ligand metal complexes from 2008–2013. The particular focus is on the examination of excited states using resonance Raman spectroscopy, transient resonance Raman spectroscopy and time-resolved infrared spectroscopy. The report describes some of the basic concepts around each technique and then highlights work from the review period that exemplifies the utility of each method. In addition tables are included that summarise the work using these methods over the review period.