Oligo(phenylene-vinylene) terpyridine ligands and their ruthenium(II), platinum(II) and zinc(II) complex systems: Synthesis, photophysical and photochemical properties
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Description
Polypyridine d6 and d8 transition metal complexes have received a great deal of attention because of their wide applications in various areas such as photo-induced energy- and electron-transfer process, artificial photosynthetic centers, electroluminescent displays, dye-sensitized solar cell, and molecular-level machines, etc Compared to 2, 2'-bipyridine-type complexes, 2, 2':6', 2″-terpyridine(tpy)-type complexes have been much less thoroughly examined, because the weak ligand field of tpy-type ligands results in the weak emissive and short-lived excited states of their complexes. However, tpy-type complexes have geometrical advantages, which are achiral and could lead to linear structures upon introduction of substituents at 4'-position of tpy The goal of this work is to overcome the short excited state lifetime drawback of the tpy-type complexes by the development of novel tpy-type ligands, so as to develop long-lived excited state sensitizers potentially for light-harvesting applications Oligo(phenylene-vinylene) (OPV) and derivatives, which are strong luminescent chromophores with semiconducting and electroluminescent properties, were employed to modify the tpy ligand at the 4'-position. Two series of OPV-tpy ligands (mono-tpy and bridging tpy-OPV-tpy ligands) with varying substituents like electron-donating groups (n-C8H17O- and Me2N-) and electron-withdrawing group(-CN) at different positions, were successfully synthesized by means of Wittig-type reactions and the Knoevenagel reaction OPV-tpy and bridging tpy-OPV-tpy ligands show strong luminescence. Their Zn(II) complexes also display strong luminescence with relatively higher quantum yields than the corresponding free ligands. The absorption and luminescent spectra of the free ligands were found to be red-shifted by increasing the length of the phenylene-vinylene or introducing electron donating groups. The Me2N- containing ligand, Me2Npvpt shows strong emission solvatochromism. By varying the solvent polarity, its emission colors were finely tuned to cover the entire visible range from blue (426 nm, Ф = 0.25) in the non polar solvent hexane, to red (626 nm, Ф = 0.52) in the highly polar solvent DMSO. The Zn(II)-induced solvatochromism wase observed in OPV-tpy ligands A series of monometallic Ru(OPV-tpy)22+ complexes and bimetallic [(mpt)Ru(tpy-OPV-tpy)Ru(tpm)]4+ complexes were prepared. All the complexes show very close energy singlet metal-ligand charge transfer (MLCT) transitions and similar electrochemical behaviors. However, progressive changes in the emission, transient absorption and excited state lifetime (tau) were observed. The tau shows a 6000 fold magnitude difference from one vinylene Ru1vRu complex (tau = 14ns, lambdaem max,r,t = 684nm), two vinylene Ru2vRu (tau = 280ns, lambda em max,r,t = 697nm), to four vinylene Ru4vRu complex (tau = 60,000 ns, lambdaem max,r,t = 773nm). Combined spectral data suggest that the energy gap (DeltaE) between triplet MLCT and triplet intraligand (3IL) transitions determines the excited state lifetimes. When DeltaE is large, if 3MLCT is the lowest lying state, tau is very short; if 3IL is the lowest lying state, tau is significantly long. When DeltaE is very small, a long-lived 3MLCT transition might occur, due to reversible energy transfer from 3IL to 3MLCT A series of [Pt(OPV-typ)Cl]+ complexes were synthesized. The long-lived (1 mus - 10mus) phosphorescence with predominant 3pi-pi* in nature was observed in both non-coordinating and coordinating solvents. The transient absorption spectra show ground state bleaching and excited state absorption from visible to far red