Abstract
A number of donor-acceptor compounds have been investigated using a range of spectroscopic and computational techniques. Donor-acceptor compounds are widely used in molecular electronics applications, and it is of interest to investigate how their properties can be manipulated and probed.
Ground state properties are characterised with Fourier Transform (FT) - Raman spectroscopy, which is also used to verify density functional theory calculations. The initially formed excited state (Franck-Condon state) is characterised with electronic absorption and resonance Raman spectroscopy. Transient absorption spectroscopy tends to probe the longest-lived excited state, while time-resolved infrared spectroscopy can probe intermediate states and kinetic processes as the time between pump and probe is varied. Experimental techniques are complemented by the use of density functional theory (DFT) calculations.
A series of complexes containing dimethyl or diphenyl amine- substituted dipyrido[3,2-a:2',3'-c]phenazine (dppz) in which the ancillary ligand at the metal centre was varied were studied. The properties of these ligands and complexes were found to be dominated by a strong intra-ligand charge transfer (ILCT) transition from the amine to dppz, with little contribution from typical low-energy metal-to-ligand charge transfer (MLCT) transitions for complexes. Protonation shifted this ILCT transition to the near-IR; this was characterised with resonance Raman spectroscopy and time-dependent DFT (TD-DFT) calculations. Based on this study, amine-substituted dppz systems were altered in various ways in order to manipulate this ILCT transition.
The effect of changing the distance between the amine donor and the dppz acceptor was investigated; this was found to influence the energy of the ILCT transition, the relative intensity of vibrational modes associated with different parts of the molecule, and the excited state lifetime, but the ILCT character appeared to be retained. The effect of the angle between the donor and acceptor units for dppz was altered in various ways. Increasing the donor-acceptor angle was found to increase the energy and decrease the intensity of the ILCT transition, and increase the intensity of an MLCT transition for Re(I) complexes. Experimental and calculated non-resonant Raman cross sections also decreased as donor-acceptor angle was increased. The effect of changing the bridge type between donor and acceptor from conducting thiophene to more insulating triazole was investigated. This increased the energy of the lowest electronic transition, and reduced the degree of ILCT in this transition, with behaviour tending towards more typical dppz as the linker became more insulating.
The properties of the ligand 5,6,11,12,17,18 - hexaazatrinaphthalene (HATN) substituted with electron-donating sulfur groups were investigated. The mono-, bi- and tri-Re(CO)3Cl complexes of this ligand showed broad and intense visible absorption, which TD-DFT calculations suggested involved mixed MLCT/ILCT transitions. Resonance Raman spectroscopy was consistent with this, and time-resolved infrared (TRIR) spectroscopy provided evidence for a mixed MLCT/ILCT excited state.
Finally, a series of donor-acceptor compounds that are used in dye-sensitised solar cells are discussed, in order to try and understand what makes some more efficient than others. The first series were zinc porphyrin -based with carbazole -thiophene chains, which increased their visible absorption. The second series were organic dyes, also using a carbazole donor and thiophene chain, but for these compounds the reason for differences in solar cell performance could not be established with the techniques used.