Tuning Light Absorption in Platinum(II) Terpyridyl π-Conjugated Complexes: A First-Principle Study

Platinum(II) terpyridyl complexes with a donor-acceptor (D-A) framework have long been considered as a promising component of dye-sensitized solar cells (DSSCs). To revealing the structure-property relationship of these highly modular systems, we have conducted a first-principle study at the time-dependent density functional theory (TDDFT) level on the [Pt(^tBu₃tpy)(-C≡C-Ph)_n]⁺ (^tBu₃tpy is 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine) complexes. It was found that their visible absorbance could be improved by elongating the donor chain with n (-C≡C-Ph) units, reaching a maximum at n = 16. It is noteworthy that such a simple concatenating protocol enables a remarkable charge transfer distance as long as 5 nm, implying a promising solution for the bottleneck problem of low charge separation rate in DSSCs. Furthermore, using a A-D-A system (two Pt(^tBu₃tpy) acceptors bridged by one donor) effectively doubles the visible-harvesting ability, and twisting an benzene ring in the chain of donors to break π-conjugations can tune down light absorption in a quantitatively angular dependent manner. Finally, replacing the C≡C bond linker with C═C double bond in donor leads to comparable light absorption ability while bestowing structural flexibility. These structure-property relationships thus provide efficient knobs for molecular rational design toward high performance dye-sensitized solar cells.