Negative Polaron and Triplet Exciton Diffusion in Organometallic “Molecular Wires”

Abstract

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.

Effect of Oligomer Length on Photophysical Properties of Platinum Acetylide Donor–Acceptor–Donor Oligomers

“…In order for annihilation to occur, the localized triplet exciton may diffuse along the oligomer chain via Dexter exchange hopping between adjacent [TBT-PtL 2 ] chromophore units. 59,60 As suggested here, triplet-triplet annihilation results in net production of a single triplet exciton from two excitons. We point out that an important consequence of the intrachain annihilation is that it ultimately limits the triplet population that can be achieved in polymers when they are exposed to saturating laser pulse energies.…”

Triplet Transfer Mediates Triplet Pair Separation during Singlet Fission in 6,13‐Bis(triisopropylsilylethynyl)‐Pentacene

“…Despite the limitations, both triplet transfer and charge transfer depend on local orbital overlap interactions . Therefore, we suggest these tractable charge transfer integral calculations may provide a valuable tool for predicting whether potential singlet fission sensitizers will undergo rapid CTP separation to form multiplied triplet excitons.…”

Effect of Conjugation Length on Photoinduced Charge Transfer in π-Conjugated Oligomer-Acceptor Dyads

“…The naphthalene diimide acceptor was used due to its relatively low reduction potential, combined with relatively high singlet and triplet energies, which preclude OPE to NDI energy transfer from competing with electron transfer. [26][27] In addition, the NDI anion radical (NDI -. ) has distinct absorption features which are easily identified by visible transient absorption spectroscopy.…”