Thiocyanate‐free ruthenium(II) tetrabenzoporphyrin sensitizers for photoelectrochemical cell: A DFT/TD‐DFT probe for stability of axial donor ligands

Although previously studied [(HOOC)₄(TBPor)Ru(NCS)₂]^2– ( A ; TBPor = tetrabenzoporphrin) avoided the intrinsic π‐stacking aggregation of planar metallophorphryins via incorporating two axial ligands, these isothiocyanato groups are believed to be the weakest part of the sensitizer while operating in dye‐sensitized solar cells (DSSCs). In this work, a series of thiocyanate‐free ruthenium porphyrin complexes featuring with phenyl/substituted‐phenyl axial groups, [(HOOC)₄(TBPor)Ru(L′)₂]^2– (L′ = Ph ( 1 ), PhF₂ ( 2 ), PhCl₂ ( 3 ), PhBr₂ ( 4 ), and PhI₂ ( 5 )), have been examined using density functional theory (DFT) and time‐dependent DFT (TD‐DFT). Both analyses of electronic structures and calculations of interaction energies demonstrate that the Ru‐L′ interaction in 1 – 5 is significantly enhanced relative to the Ru‐NCS in A , which will raise chemical stability of the former in DSSCs. Single‐electron oxidation mechanism has been proposed. Oxidation potentials (E⁰) are increased by 0.2–0.6 V when changing axial groups from NCS to Ph/PhX₂. The spin‐orbit coupling (SOC) relativistic effects can be negligible in computing E⁰ values. TD‐DFT calculations show that 1 – 5 have more intense Q band in the visible region than A does. Taken together, high chemical stability, suitable oxidation potential and expanding absorption spectra would allow for potential applications of the thiocyanate‐free sensitizers in DSSCs. © 2016 Wiley Periodicals, Inc.