Theoretical insights on spectral and electrochemical properties of BODIPY-bipyridine and Re(BODIPY-bipyridine) (CO)3Cl dyes for dye-sensitized solar cells

Density functional theory calculations provided structural, spectral, and electrochemical properties of BODIPY-bipyridine conjugates as potential dyes for dye-sensitized solar cells. Our BODIPY, B, carries styryl and pyrrole arms at positions C3 and C5, respectively, but lacks substituents at positions C1 and C7 and its spectrum has peaks at 627 nm (band-1) and 413 nm (band-2). One and two B units are attached via their C8 or C7 atoms to the xx’ atoms of 2,2′-bipyridine, b, (Bbx/Bbx_C7 and BBbxx’/BBbxx’_C7 conjugates). These attachments do not alter the HOMO but allow the LUMO and HOMO-1 orbitals to spread over the b fragment, affecting their orbital energies and absorption wavelengths. The C8 connection increases the intensity and red-shifts band-2, whereas the C7 one benefits band-1. The greater the number of B units, the greater the intensity of the bands. In Re_BBb55′ and Re_BBb55'_C7 complexes the Re orbitals do not participate in the electronic transitions. BBb55′ and BBb55’_C7 are the best candidates since they exhibit high light harvesting efficiencies, reasonable driving forces for electron injection and dye regeneration, and the lowest reorganization energy. They adsorb on the surface of TiO₂ nanotubes, rendering photoexcitations from the B units to b, the two B, and TiO₂.