Three Ru(II) dipyrrinate complexes, [Ru(bpy)2(3-TDP)](PF 6) (1), [Ru(H2dcbpy)(Hdcbpy)(3-TDP)] (2), and [Ru(H 2dcbpy)(Hdcbpy)(TPADP)] (3) (bpy = 2,2′-bipyridine; dcbpy = 4,4′-dicarboxylato-2,2′-bipyridine; 3-TDP = 3-thienyl-dipyrrinate; TPADP = triphenylamino-dipyrrinate), have been synthesized and characterized by electrochemical and photophysical methods as well as by theoretical electronic structure calculations on the DFT level. The complexes exhibit panchromatic light harvesting due to complementary ligand-based absorption around 450 nm and metal-to-ligand charge transfer (MLCT) absorption around 530 nm. Complexes 2 and 3 have been investigated as potential sensitizers for the dye-sensitized solar cell (DSSC). Time-dependent DFT calculations reveal the preferential localization of an excited-state electron on the H2dcbpy ligands, leading to a favorable scenario for electron injection from these anchoring ligands into TiO2. Solution-phase transient absorption spectroscopy was used to follow the excited state dynamics of methyl-ester derivatives of 2 and 3. Excitation with a 400-nm laser pulse resulted in two bleaches centered at 460 and 540 nm and corresponding to the ligand-based and MLCT transitions, respectively. A rapid 2 ps loss of the ligand based bleach corresponded with a growth of the MLCT bleach that were interpreted as the result of vibrational relaxation between metal-centered and ligand-centered frontier orbitals. The electron-injection kinetics were studied on dyes 2 and 3 anchored on the TiO2 surface. The excited state electron-injection yield and incident photon-to-current efficiency were dramatically suppressed by addition of tert-butylpyridine (TBP), indicating that the lowest excited state was positioned close in energy to the TiO2 acceptor states. Nevertheless, the injection yield measured with 416-nm excitation was less sensitive to the TBP concentration than that measured with 532-nm excitation, thus suggesting the possibility of hot-electron injection from the upper excited states generated by the ligand-based excitations. Based on the findings of this study, a pathway is proposed for improving the electron-injection yield of Ru-dipyrrinate dyes and increasing the power-conversion efficiency of the DSSC incorporating these dyes.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films