Investigations of organic and inorganic chromophores towards greater purposes

The vibrational and electronic properties of six systematically altered donor-acceptor dyes were investigated with density functional theory (DFT), spectroscopy and electrochemical techniques. The dyes incorporated a carbazole donor connected to a dithieno[3,2-b:2’,3’-d]thiophene linker at either the C₂ (m) or C₃ (p) positions. Indane-based acceptors contained either dimalononitrile (IndCN), ketone and malononitrile (InOCN) or diketone (IndO) electron accepting groups. Molecular geometries modelled by DFT using the BLYP functional and def2-TZVP basis set showed planar geometries containing large, extended π-systems and produced Raman spectra consistent with the experimental data. Electronic absorption spectra had transitions with π–π^∗ character at wavelengths below 325 nm, and a charge transfer (CT) transition region from 500−700 nm. The peak wavelength was dependent on the donor and acceptor architecture, with each modulating the HOMO and LUMO levels respectively, supported by TD-DFT estimates using the LC-ωPBE* functional and 6-31g(d) basis set. The compounds showed emission in solution with quantum yields ranging from 0.004−0.6 and lifetimes less than 2 ns. These were assigned to either π–π^∗ or CT emissive states. Signals attributed to CT states exhibited positive solvatochromism and thermochromism. The spectral emission behaviour of each compound trended with the acceptor unit moieties, where malononitrile units lead to greater π–π^∗ character, while ketones exhibited greater CT character.

Two novel donor-acceptor-donor ligands and their respective ruthenium bis-bipyridine complexes have been synthesised and analysed using spectroscopic and computational techniques. The ligands were synthesised in suitable yields using two sequential Sonogashira cross coupling reactions, with geometries altered by substitutions of the two TPA donor and alkyne bridge units onto the 4, and 4′(bpy-pEDPA), or 5 and 5′(bpy-mEDPA) positions of the bpy acceptor group. Both ligands were complexed to ruthenium bis-bipyridine complexes, [Ru(bpy)₂(bpy-pEDPA)]²⁺ and [Ru(bpy)₂(bpy-mEDPA)]²⁺. Ground state properties were analysed with Raman spectroscopy, X-ray crystallography, and compared with DFT frequency calculations. The B3LYP35 functional and def2SVP basis set gave simulations which matched best with experimental measurements. TD-DFT calculations matched well with UV-vis absorption spectroscopy, where differences in ligand spectra resulted from their respective geometries and orbital overlap. Resonance Raman spectroscopy of the complexes elucidated the nature of peaks in the electronic absorption spectrum. [Ru(bpy)₂(bpy-pEDPA)]²⁺ exhibited three potential enhancement patterns, assigned to π–π^∗ transitions in the violet region (λ_ex = 355−375 nm), MLCT in the blue (λ_ex = 406−457 nm) and ILCT to the red of these (λ_ex = 448−515 nm). The [Ru(bpy)₂(bpy-mEDPA)]²⁺ complex exhibited two enhancement patterns, corresponding to π–π^∗ and ILCT in two regions (λ_ex = 355−375 and > 491 nm), or MLCT in the blue (λ_ex = 406−457). Both complexes exhibited similar excited state lifetimes at room temperature. These were assigned as ³ILCT for [Ru(bpy)₂(bpy-pEDPA)]²⁺ and ³MLCT for [Ru(bpy)₂(bpy-mEDPA)]²⁺ evidenced by Marcus theory, transient resonance Raman and triplet state TD-DFT calculations.

[Re(CO)₃(L)(X)] complexes, where L corresponds to bpy-pEDPA or bpy-mEDPA and X corresponds to the ancillary ligand Br, Cl, DMAP or py, were studied. Synthesis of the bpy-pEDPA and py complex was unsuccessful. The ground state characteristics have been analysed using Raman spectroscopy, and DFT calculations. Calculations using the B3LYP35 functional and def2SVP basis set matched well with experimental spectra. Electronic UV-vis spectroscopy was measured in DCM, where all bpy-mEDPA complexes ([Re(M)(CO)₃(X)]) were red shifted from their analogous bpy-pEDPA complexes ([Re(P)(CO)₃(X)]), and both saw a red shifting of peaks with increasing electron withdrawing capability of the ancillary ligand. All complexes exhibited ILCT and MLCT transitions in the CT region, the low energy ILCT’s red shifted with the electron withdrawing capability of the ancillary ligand, while MLCT’s were blue shifted. [Re(P)(CO)₃(B)], and [Re(P)(CO)₃(C)] exhibited close lying ILCT’s and MLCT’s leading to mixing of states, while all other complexes observed less mixing. Several enhancement patterns in resonance Raman were observed for [Re(P)(CO)₃(X)] complexes, assigned to π–π^∗ and ILCT^∗ transitions from λ_ex = 355−375 nm, and a mixture of MLCT and ILCT at longer wavelengths with tuning according to the ancillary. [Re(M)(CO)₃(X)] complexes saw π–π^∗ and high energy ILCT enhancement from λ_ex = 355−457 nm tuned by the ancillary. Longer λ_ex showed an alternative enhancement pattern assigned to lower energy ILCT transitions. Emission spectroscopy observed triplet state phosphorescence in each complex, which red shifted with the electron withdrawing capability of the ancillary. Excited state lifetimes of the [Re(P)(CO)₃(X)] complexes were ∼ 300 ns, while [Re(M)(CO)₃(X)] was extended to ∼3000 ns at room temperature. Triplet state calculations, transient resonance Raman spectroscopy and transient absorption all suggested the difference in lifetime was a result of the ³ILCT nature of the [Re(P)(CO)₃(X)] complexes, compared to the ³LC excited state of the [Re(M)(CO)₃(X)] series.

[Re(NPDC)(CO)₃(X)] complexes, utilising the polyaromatic bidentate ligand, naphtho[2,3-f][1,10]phenanthroline-9,14-dicarbonitrile (NPDC), where X corresponds to Br or Cl, were synthesised, characterised and analysed using spectroscopy and computational modelling. Ground state analysis was performed with Raman spectroscopy, X-ray crystallography and modelled with DFT calculations using the B3LYP35 functional and def2SVP basis set. The ground state structure of these systems was strained by ∼8◦ from the additional cyano-groups attached to the naphthalene unit. Aromatic stacking was observed in dimeric patterns in the X-ray crystal structures. Electronic spectroscopy displayed a plethora of π–π^∗ states. MLCT transitions, from Re to the phen or naph orbitals were modelled by TD-DFT, but were not observed in experimental spectra, owing to their weak absorbance. Resonance Raman spectroscopy predominantly displayed enhancement from π–π^∗ transitions, with one phen breathing mode at 1583 cm⁻¹ postulated to be enhanced by the lower energy MLCT at longer excitation wavelengths. Emission spectroscopy observed high energy emissions with vibronic structure and small Stokes shifts resulting from π–π^∗ states in both the naked ligand and [Re(N)(CO)3(B)] complex. [Re(N)(CO)₃(C)] observed alternative emission signals postulated to result from a photo-degradation product. Further examination of these compounds was discontinued or unable to be performed due to photo-degradation, or solubility issues.