Estudo teórico sobre a transferência de carga fotoinduzida de ftalocianinas de Ru (II) para um fragmento de anatase

Abstract

The present study investigated qualitative and quantitative aspects of electronic properties related to different derivatives of ruthenium (II) phthalocyanine and it respective free base (Complexo I - dipyridine(2-carboxyl- 16-dietylamine-phthalocyaninate-ruthenium(II)); Complexo II - dipyridine(2- carboxyl-9,16-(bis)dietylamine-phthalocyaninate-ruthenium(II)); Complexo III - dipyridine(2,9-dicarboxyl-16,26-(bis)dietylamine-phthalocyaninate-uthenium(II)); Complexo IV - dipyridine(2-carboxil-9,16,26-(tris) dietylamine- phthalocyaninate-ruthenium(II)); Complexo IV-BL - dipyridine(2-carboxyl- 9,16,26-(tris)dietylamine-phthalocyanine), to evaluate its applicability in energy conversion processes. Such properties have been theoretically evaluated with the employ of density functional theory (DFT) methods and its time-dependent approach (TDDFT). All the studied complexes must absorb light in the visible region. Data from the electronic structure analysis, such as population analysis of molecular orbitals, density of states and natural transition orbitals suggest that there is no charge transfer for the conduction band of anatase in the electronic transitions of the first absorption band. However, a study performed for Complex IV and Complex IV-BL, involving the use of atomic bases with relativistic correction (DZPDKH), and a long range corrected functional (CAM- B3LYP), revealed the importance of central metal (Ru (II)) in photoinduced charge transfer (PICT) process. While for the complex formed by the association of the free base and anatase cluster, even after excitation the electronic density remains restricted to the macrocycle, for the Ru(II)-based complex, it was found a PICT involving the S0®S1 transition, with the majority participation of the LUMO and LUMO+1 orbitals. Moreover, the occurrence of spin-orbit coupling and consequent intersystem crossing due to the presence of Ru (II), suggests that the charge transfer should mainly occur from the triplet state (T1), which is also shifted to the semiconductor region. Thermodynamic parameters related to the operation of a dye sensitized solar cell show that both the oxidation potential, the ground state (-335.13 kJ mol-1) and excited (-488.17 kJ mol-1) are consistent with expectations for conduction band charge injection. The magnitude of the Gibbs free energy of electron injection suggests that of recombination between the injected electrons and the oxidized dye should be unlikely, and the values of Gibbs free energy of the dye regeneration, considering the electrolyte chosen in this work, should ensure a good operation of a hypothetic dye sensitized solar cell based on the system with metal.