Propriedades espectroscópicas dos complexos bipiridí nicos de rênio (I)

Abstract

Since 1984, the photophysical properties of fac --[Re(bpy)( 3 Cl] complex have been studied in order to optimize its catalytic application on CO 2 reduction, this step is crucial to artificial photosynthesis. Therefore, the electronic spectroscopy of emission and absorption in ultra violet and visible region of spectrum (UV Vis) have been widely used. In this work density functional theory DFT and time dependent functional theory ( TD DFT calculation s were used to a systemic evaluation of the effects in the complex's spectrum front of ligand changes in bipyridine ring. This changes leads to 21 complexes which were investigated. The main results indicate that adding each one of the ligand F, NH 2 and COOH in the bpy ring leads to intensity decreases in the highest intensity band in the region between 200 and 300 nm, except to the Re bpy 2C OOH complex. In the NH 2 complexes instance, which is a good donor of electronic density by induction, in the bands over 300 nm blue shifts were observed, varying from 5.5 to 99.7 nm, moreover intensity decreases were also computed. Comparing the results of substituted complexes with electronic withdrawn effect F and COOH, small intensity changes and shifts varying from 3.2 to 24.9 nm can be observed in bands over 300 nm caused by fluoro ligand presence. To carboxyl ligands, interesting results have been fo und to this substituted complex, the spectra showed an increase in the bands number and its intensities, in particular, those located in the visible region of electromagnetic spectra. Furthermore, bathochromic shifts varying from 5.0 to 165.1 nm were obser ved in almost all bands over 300 nm. These results, caused by the presence of carboxyl group, should be due its higher efficiency of the electronic withdrawn effects promoted by the electronic resonance between this group and the aromatic rings of the inve stigated complexes. Overall these results are very significant, once it is well known that the bipyridine complexes of Re(I) tends to present low intensity band over 300 nm, these bands are attributed to a process called metal to ligand charge transfer (ML CT), this process is essential to the photocatalytic reduction of CO 2 . The aromatic increase, cause by the substitution of the bpy ring to 1,10 phenantrolinic (phen) and benzo f 1,10 phenantrolinic (benz phen) were also evaluated, showing high intensity decreases which reaches 76.7%. The results obtained to the triplet transitions are favorable to the non radioactive photochemical processes of internal conversion and intersystem crossing to the COOH substituted complexes, mainly due the smaller diff erences between its singlets excited states, as well between the singlet and triplet excited states. Consequently, the radioactive photophysical processes of fluorescence and phosphorescence should be very active in these complexes. The results obtained by this work indicate that the COOH substituted complexes as promising catalysts in the CO 2 reduction process.