Abstract
The photophysical properties of a range of transition metal 1,10-phenanthroline (phen) complexes have been investigated. A suite of electronic and vibrational spectroscopic techniques including steady state and transient absorption, resonance Raman and transient resonance Raman (TR2) spectroscopy have been used in the study of the compounds. The experimental techniques are complemented by density functional theory (DFT) and time-dependent (TD) DFT calculations.
The compounds investigated in this thesis were designed to possess close lying excited states of different electronic character. The studied transition metal complexes possess metal-to-ligand charge transfer (MLCT) states which absorb in the visible region. Additional low energy states were introduced by modification of the phen ligand with substituents with electron donating capabilities. Tuning of the excited state energies was performed by systematic structural modifications, generating different, controlled series of compounds such that the origin of the photophysical changes could be attributed to an isolated molecular component.
The photophysical properties of a series of Ru(II) complexes of the type, [Ru(phen)2(phen-R)]2+ were studied in order to examine the role of the R substituent. The R groups were designed to function as pseudo electron donors, in that they raise the energy of phen π* orbitals and contribute to greater delocalisation of electron density, but do not form formal charge-separated excited states. It was found that when naphthalene groups were appended to the phen ligand, a 40 μs dark state is formed. The dark state was shown to be of triplet, ligand-centred (3LC) character and to exist alongside two interacting, emissive 3MLCT states.
Low energy intra-ligand charge transfer (ILCT) states were introduced by incorporation of a triphenylamine (TPA) group to the phen ligand. The electron donating abilities of TPA are well established as it readily oxidises to TPA•+. The phen-TPA ligand was coordinated to a {Re(L)(CO)3} core, allowing population of both MLCT and ILCT states. Variation of the ancillary ligand, L, successfully modified the energies of both charge transfer states, however the photophysical properties were dominated by ILCT states which remained the lowest in energy. The thermally equilibrated excited (THEXI) state was characterised as 3ILCT in nature. Although the ancillary ligand was not directly involved in this state, it had an indirect effect of varying the excited state lifetime by two orders of magnitude, from 6 μs to 27 ns. The relationship between the excited state lifetime and emission energy could be expressed in terms of the energy gap law.
The energy of the phen-TPA based ILCT state was further tuned by appending electron donating methoxy (OMe) and electron withdrawing cyano (CN) groups to the TPA donor, giving phen-TPA-R ligands where R = OMe, H and CN. The success of the electronic tuning was observed in the emission spectra of the ligands, where a blue-shift in the emission wavelength, λem, is seen upon decreasing the donating ability of the TPA group. The phen-TPA-R ligands were coordinated to a {ReCl(CO)3} core and variation to the ILCT state energy was observed through changes in the electronic absorption and resonance Raman spectra of the complexes. The energy gap between the Franck-Condon MLCT and ILCT states decreases with the decreasing donating ability of the TPA group, with the MLCT and ILCT states becoming approximately isoenergetic in [ReCl(CO)3(phen-TPA-CN)]. At 298 K [ReCl(CO)3(phen-TPA-CN)] and [ReCl(CO)3(phen-TPA)] display isoenergetic emission at ~ 610 nm while [ReCl(CO)3(phen-TPA-OMe)] is non-emissive. However at 77 K, the λem varies systematically with the electron donating ability of the TPA group, occurring at 530, 555 and 597 nm, respectively. This implies that considerable solvent reorganisation occurs in the excited state triplet manifold. Evidence of 3ILCT contribution to the THEXI state was gained from transient absorption and TR2 spectra of the complexes.
The role of the transition metal centre was explored by incorporation of the same phen-TPA-R ligands to a Pt(II) bis-acetylide core, giving complexes of the type [Pt(C≡C)2(phen-TPA-R)]. The acetylide ligands function to raise the dπ orbitals such that the metal based transitions occurred at lower energies than in the Re(I) complexes. This resulted in a closer interaction of metal-ligand-to-ligand charge transfer (MLLCT) and ILCT states in the Pt(II) complexes than in their Re(I) counterparts. The nature of the lowest lying absorbing state varies across the series, changing from ILCT for [Pt(C≡C)2(phen-TPA-OMe)], to isoenergetic MLLCT/ILCT states in [Pt(C≡C)2(phen-TPA)] and finally to a MLLCT state in [Pt(C≡C)2(phen-TPA-CN)]. However, as with the [ReCl(CO)3(phen-TPA-R)] complexes the excited state properties were unanticipated, as the emission energy is largely unaffected by variation of the TPA donor or the transition metal centre, with all complexes displaying near isoenergetic emission at 298 K.
Lastly, the effect of incorporation of a bridging group between the phen acceptor and TPA donor was investigated for both Re(I) and Pt(II) complexes. The ground state properties of the bridged ligands and complexes behaved in a predictable manner consistent with previous studies. The electronically conducting acetylide and thiophene bridges promote donor-acceptor communication, red-shifting the ILCT transition while the electronically insulating triazole bridge has the opposite effect. The thiophene complexes exhibit prompt, singlet state emission but also have long-lived dark states which decay biexponentially. The transient absorption spectra and excited state lifetimes of the thiophene complexes were unaffected by variation of the metal centre, highlighting the ligand localised nature of the state. Time-resolved resonance Raman (TR3) spectroscopy was used to aid in assignment of the THEXI state as a mixed 3LC/3ILCT state. No such dark states were observed in the complexes with insulating bridges. The effect of donor-acceptor distance on electron transfer rates was examined using the semiclassical Marcus model for the phenylene bridged and parent complexes.
These series of complexes with close lying charge transfer states highlight the differences between the ground and excited properties. The ground state properties of the series are chemically intuitive. Predictable changes to the energies of the initially populated Franck-Condon states occur as a result of controlled structural modification. However, in the majority of molecular electronics applications, it is the properties of the excited states which control useful or desirable behaviours such as the redox capabilities, lifetime or emission energy. The studied compounds show an intricate interaction and reorganisation of excited states following initial excitation. The excited states of these systems exhibit some unanticipated, unpredictable properties, some of which have not been seen previously in transition metal polypyridyl complexes. This work has demonstrated that, for this class of compounds, the rules that govern the ground state behaviour have far less relevance when it comes to predictability of the excited state properties.