Revealing the ultrafast photophysical dynamics of selected platinum(II) and ruthenium(II) complexes by time-resolved spectroscopy

Abstract

Femtosecond (fs) and nanosecond (ns) time-resolved transient absorption spectroscopies (fs-TA, ns-TA) and other spectroscopic methods are helpful in studying the photophysical processes of metal complexes by direct observations of short-lived species. In this thesis, metal complexes like platinum(II) complexes that contain tetraphenylethylene (TPE) ligands and ruthenium(II) complexes that contain fused-ring ligands were prepared and characterized. The intriguing photophysical processes of these complexes were examined by utilizing resonance Raman spectroscopy, ns-TA, fs-TA, time-resolved fluorescence spectroscopy and supporting time-dependent density functional theory (TD-DFT) computations. TPE has evoked great interest due to its application in the rapid developing aggregation-induced emission (AIE) field. Observation of the TPE related triplet excited state has barely been reported to date. Thus, two different Pt(II) acetylide complexes were introduced to link with TPE aiming to obtain the TPE related triplet excited states by use of the heavy atom effect. TA spectroscopy measurements demonstrate that two kinds of metal-to-ligand charge transfer (MLCT) excited states are involved and they are the 3MLCT (Pt → tpy/bpy) and the 3MLCT (Pt → TPE) excited states, in which the former 3MLCT excited states are much shorter-lived than the latter one. However, the lifetime of the 3MLCT (Pt → TPE) excited state is determined to be around 50 ns, which is also relatively short compared to other triplet excited states with microsecond lifetimes. This may be due to the non-radiative decay of the 3MLCT (Pt → TPE) excited state being accelerated by the intramolecular rotations of the TPE molecule. In Chapter 4, two state-of-art push-pull fused-ring ligands (L1, L2) were synthesized and Ru1/Ru2 were obtained by incorporating L1/L2 with the Ru(bpy)2Cl2 complexes. Bithiophene (bth) is chosen as the electron donor in such fused-ring ligands and the electron acceptors are phenazine (phz) for L1 and quinoxalino[2,3-b]quinoxaline (qxq) for L2. The photoinduced intramolecular charge transfer (ICT) and intersystem crossing (ISC) were directly seen in the TA measurements. The qxq ligand that has more nitrogen atoms put into the fused-ring ligand was found to be a stronger electron acceptor and process promoted ISC efficiency. On the other hand, the Ru(II) complexes bearing L1 and L2 as ligands, showed similar photophysical dynamics that were mainly associated to the interplay of the 3MLCT (Ru → phen), 3π-π* and the triplet intraligand charge-transfer [3ILCT (bth → phz/qxq)] excited states. The 3ILCT excited state is found in such [Ru(bpy)2 (dppz)]2+-like complexes for the first time. The lifetimes of the triplet excited states of both the pure ligands and the Ru(II) complexes were associated to the 3CT (bth → phz/qxq) excited states, but the origins of the final state from them were different, which made the triplet-excited-state lifetimes of the fused-ring ligands and the corresponding Ru(II) complexes different.