Photoinduced charge separation in platinum acetylide oligomers

The series of three donor-spacer-acceptor complexes, DPAF-Ptn-NDI, has been synthesized and characterized using time-resolved absorption spectroscopy. In these complexes, the donor is a (diphenylamino)-2,7-fluorenylene (DPAF) unit, the acceptor is a naphthalene diimide (NDI), and the spacers are a series of platinum acetylides of varying lengths, [-Pt(PBu(3))(2)-C≡C-Ph-C≡C-](n) (where Bu = n-butyl, Ph = 1,4-phenylene and n = 1, 2, and 3). Electrochemistry indicates that the DPAF-Ptn-NDI system has a charge transfer state at ca. 1.5 eV above the ground state that is based on one electron transfer from the DPAF donor to the NDI acceptor. Transient absorption spectroscopy on time scales ranging from 0.2 ps to 1 μs reveals that excitation of all of the complexes leads to production of the charge transfer state with nearly unit quantum efficiency. The rates for charge separation and charge recombination are not strongly dependent upon the length of the platinum acetylide spacer, suggesting that the spacer is actively involved in the electron (hole) transport processes. Analysis of the experimental results leads to a model in which charge separation and charge recombination occur by hole-hopping via states localized on the [-Pt(PBu(3))(2)-C≡C-Ph-C≡C-](n) bridge.