Time-resolved resonance Raman studies of ruthenium (II) complexes in solution and zeolite
Abstract
The properties of Ruthenium (Ru) polypyridine complexes make them promising choice as photosensitizers for artificial solar energy conversion devices. My work focuses on the study of the properties of the excited states of these complexes in solution and in zeolites by using transient Resonance Raman (TR 2 ) or Time-resolved Resonance Raman (TR 3 ) spectroscopy. To investigate the effects of spectator ligands on excited state properties of 3 MLCT states, Resonance Raman (RR) and Transient Resonance Raman (TR 2 ) spectra are acquired for a series of heteroleptic complexes of ruthenium; i.e., RuL 2 (bpz) 2+ , where bpz is 2,2'-bipyrazine and L is 2,2'-bipyridine (bpy) or an alkylated 2,2'-bipyridine (i.e., diazafluorene, daf or 4,4'-dimethyl-5,5'-diethyl-2,2'-bipyridine, dmdeb). Resonance Raman spectra acquired at different excitation lines for the ground state complexes reveal selective enhancement of modes associated with the coordinated bpz or the spectator ligands, as expected. The TR 2 spectra, acquired with excitation at 355 nm, confirm selective population of a bpz-localized 3 MLCT excited state for each complex. Both the ground state RR spectra and the excited state TR 2 data indicate that only slight shifts are observed in a few modes as the donor strength of the spectator ligands are varied. Such data for a systematically manipulated set of complexes, acquired here for the first time, imply that both the RR and TR 2 spectral parameters are reliably characteristic for a given ligand, varying only slightly as the nature of other ligands in the complex are changed. The second project deals with the orientation effects on the three bpy ligands of a Ru (bpy) 3 2+ complex entrapped within the supercage of Y-type zeolite. Recently it has been proposed that, owing to the particular geometrical arrangement of the supercage, the last (i.e., 3 rd ) ligand added assumes a special orientation, directly facing the adjacent supercage, while the other two face the negatively charged zeolite framework. Resonance Raman and its temporal variants (TR 2 and TR 3 ) provide uniquely powerful methods for selectively characterizing the structure of the individual ligands of the entrapped complex; i.e., the study of selectively deuterated analogues, Z-Ru(bpy) 2 (d 8 -bpy) and Z-Ru(bpy) 2 (d 8 -bpy), provides distinct spectroscopic signals for the bpy and d 8 -bpy fragments of each complex. Results of the TR 2 spectra and TR 3 spectra provide no definitive evidence for selective localization of excited state electron density on the third ligand. Among the bridging ligands used to construct various multinuclear Ru complexes, dipyridylpyrazine (dpp) has attracted much attention. In this work, both the ground state and the excited state properties of monomeric and dimeric complexes based on the (2,3-bis (2-pyridyl) pyrazine) dpp ligand are studied by RR and TR 2 techniques. Specifically, the complexes studied here include the monomers Ru(bpy) 2 (2,3-dpp) 2+ , Ru(bpz) 2 (2,3-dpp) 2+ , Ru(2,3-dpp) 3 2+ , (bPY) 2 Ru(dpp)Ru(bpy) 2 4+ , and (bpy) 2 Ru(dpp)Ru(bpz) 2 4+ . Comparisons of the ground state RR spectra of symmetric and asymmetric dimers demonstrate that some of the dpp peripheral modes shift to higher frequency upon asymmetric changes, indicating a shift in ground state electronic structure. The spectrum of the excited state of the monomeric complexes, Ru(bpy) 2 (2,3-dpp) 2+ and Ru(bpz) 2 (2,3-dpp) 2+ , have been obtained by TR 2 , using 368 nm excitation and provide documentation for specific localization on either the dpp or bpz ligands. Unfortunately, the excited state modes of the dimer have not been observed, even at low temperature (77K), although the dimer complex excited state has a life time ∼1μs, which is certainly long enough to be probed by TR 2 . The reason why they are not detected may be because the laser line we used (368 nm) is not in resonance with the dpp ·- π*-π* absorption for the dimer complex; unfortunately, some confusion still exists regarding the position of this electronic absorption band. To resolve this, further studies of the transient absorption spectrum of the dpp dimer, or a trial and error approach to the TR 2 method, would be necessary.
This paper has been withdrawn.