Characterization of polarons and excitons in monodisperse platinum acetylide materials.

摘要

The overall goal of this work was to design, synthesize and characterize various series of platinum-acetylide oligomers in order to better understand the structure-property relationships in these types of materials. Specifically, this research sought to further define the photophysical properties of both triplet excitons and negative polarons in these molecules by means of both steady-state and time-resolved methods. The results of this work not only provide insight into the synthesis of monodisperse organometallic oligomers, but they also provide a more in-depth understanding of both the structure and dynamics of excited and charged states within conjugated systems that feature organometallic or metal-organic moieties.;First, we report the synthesis and characterization of a series of monodisperse platinum-acetylide oligomers PtnNDI2, where NDI is an easily reducible end group. The oligomers were synthesized via an iterative-convergent approach utilizing organometallic synthons that feature orthogonally protected terminal acetylene units. The oligomers were characterized by electrochemistry, UV-visible absorption and photoluminescence spectroscopy. The emission spectra reveal that the triplet exciton is efficiently quenched in the NDI end-capped oligomers, and the quenching is thought to arise due to exciton migration followed by photoinduced charge separation.;Variable temperature steady-state emission studies, pulse radiolysis measurements and ultrafast transient absorption measurements were applied to examine the transport dynamics of negative polaron and triplet exciton states in the PtnNDI2 series. Ultrafast single shot experiments, compared to simulated random-walk data, reveal trapping of the anion radical for Pt10NDI2 in short times with the lifetime for polaron diffusion estimated to be ∼27 ps. Low temperature emission studies reveal that the rate-limiting step for exciton quenching is exciton diffusion. Femtosecond transient absorption data was coupled with simulation data to extract a triplet exciton hopping rate of ∼27.2 ps.;Another series of platinum-acetylide oligomers PhnPt 2 were also synthesized, consisting of a phenylene ethynylene core of varying length capped by platinum end groups. The study was meant to examine the effects on excited state properties between two platinum moieties of increasing organic spacer distances. Optical data conclude that the singlet exciton remains highly delocalized throughout the series, but approaching its limit. The extent of delocalization in triplet exciton seems to have been reached by Ph 4Pt2, and phosphorescence yields at ambient temperatures decreased with increasing spacer length until emission was almost elusive in the n=9 oligomer.