Structural Modulation of the Absorption Spectrum in Cyclometalated Iridium Complexes for Artificial Photosynthesis

ANA FOI; JUAN RODRIGUEZ SAMIA; DAMIAN E. BIKIEL; FABIO DOCTOROVICH; FLORENCIA DI SALVO

Chascomús

Encuentro; IV Latin American Meeting on Biological Inorganic Chemistry - V WOQUIBIO; 2014

FCEN, UBA

Introduction Photosynthesis is the most important photoreactive process known.1 Using light from the Sun, plants and other organisms are capable to produce high-energy compounds. Mimicking Nature using synthetic compounds for capturing solar energy is one of the approaches most interesting to overcome the future global energy needs. The most popular sensitizers are the ones derived from [Ru(bpy)3]2+ (bpy = 2,2'-bipyridyl) which have been studied during the last 30 years.2 After absorbing a photon, the ground state of these compounds reaches an excited state that is comparatively a more powerful oxidant. Our goal is the develop of new sensitizers using [Ir(ppy)2Cl]2 (ppy = 2-Phenylpyridine) as starting material, which demonstrated to be an excellent template for the synthesis of coordination compounds with interesting photoluminiscent and structural properties. Result & Discusion Synthesis and Characterization. In order to study the photoluminiscent and structural properties in Ir(III) complexes, we synthesized three complexes following the general procedure described by Bernhard.3 Since the previous synthesis were achieved by combinatorial chemistry methods, the characterization of the reported complexes was incomplete. We present the synthesis of three complexes (Scheme 1), and the full characterization by means of UV-Vis and FT-IR spectroscopy, NMR, MS Spectrometry, and Elemental Analysis. UV-Vis spectra at room temperature for both the Pyridine (2), and the dimethylaminopyridine (3) complexes are shown in Figure 3. In order to estimate the pKa for the carboxylic acid groups present in complex 4, a titration procedure adding acid to the solution of the complex was followed by UV-Vis. The presence of 2 isosbestic points allows us to estimate the pH range in which the carboxylic acids is protonated (Figure 4). Crystal Structure. Single crystal X-ray diffraction structures for complexes 2, 3, 4 were successfully obtained (Figure 5 and Table 1). The most relevant intermolecular interactions observed in the crystal structure of complex 2 are the C-H...Cl ones. Due to these non-covalent contacts, infinite linear chains are described along crystallographic axis c (Figure 6). As a consequence of these interactions, a columnar arrangement is observed as well (Figure 7). Conclusions & Perspectives - Three new compounds of the family derived from the dimer [Ir(ppy)2Cl]2 were synthesized and fully characterized including XRD structural determinations. Supramolecular structure of complex 2 shows a different behavior in comparison with the other complexes due to the presence of the chloride ligand which give place to non-conventional C-H...Cl H-bond interactions. In contrast, complexes 3 and 4 share dimeric columnar arrangement. - Complex 4 presents an easy way to modulate the total charge of the complex. Evidence of this is observed in the modification of the UV spectrum upon changing the pH. - The photoproperties of these complexes are clearly modulated by exchanging the auxiliary ligand, which shows a whole new variety of complexes that can be synthesized. One of the ligands that is under investigation is the py-CH2-py one, which will provide light in the effect of the ring rotation over the electronic structure of the Ir(ppy)2L complexes. REFERENCES (1). Meyer T. J., Acc. Chem. Res. (1989), 22:163-170. (2). Meyer T. J., Progress in Inorganic Chemistry (1983), 30:389-440. (3). A) Lowry M. S., Hudson W. R., Pascal R. A., Bernhard S., J. Am. Chem. Soc. (2004), 126: 14129-14135. b) Cline E. D., Bernhard S., CHIMIA Int. J. Chem. (2009), 63:709-713