Platinum Acetylide Two-Photon Chromophores

• Published in: Inorganic chemistry 2007-08, Vol.46 (16), p.6483-6494
• Main Authors: Rogers, Joy E, Slagle, Jonathan E, Krein, Douglas M, Burke, Aaron R, Hall, Benjamin C, Fratini, Albert, McLean, Daniel G, Fleitz, Paul A, Cooper, Thomas M, Drobizhev, Mikhail, Makarov, Nikolay S, Rebane, Aleksander, Kim, Kye-Young, Farley, Richard, Schanze, Kirk S
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic700549n

To explore the photophysics of platinum acetylide chromophores with strong two-photon absorption cross-sections, we have investigated the synthesis and spectroscopic characterization of a series of platinum acetylide complexes that feature highly π-conjugated ligands substituted with π-donor or -acceptor moieties. The molecules (numbered 1−4) considered in the present work are analogs of bis(phenylethynyl)bis(tributylphosphine)platinum(II) complexes. Molecule 1 carries two alkynyl-benzothiazolylfluorene ligands, and molecule 2 has two alkynyl-diphenylaminofluorene ligands bound to the central platinum atom. Compounds 3 and 4 possess two dihexylaminophenyl substituents at their ends and differ by the number of platinum atoms in the oligomer “core” (one vs two in 3 and 4, respectively). The ligands have strong effective two-photon absorption cross-sections, while the heavy metal platinum centers give rise to efficient intersystem crossing to long-lived triplet states. Ultrafast transient absorption and emission spectra demonstrate that one-photon excitation of the chromophores produces an S1 state delocalized across the two conjugated ligands, with weak (excitonic) coupling through the platinum centers. Intersystem crossing occurs rapidly (k isc ≈ 1011 s-1) to produce the T1 state, which is possibly localized on a single conjugated fluorenyl ligand. The triplet state is strongly absorbing (εTT > 5 × 104 M-1 cm-1), and it is very long-lived (τ > 100 μs). Femtosecond pulses were used to characterize the two-photon absorption properties of the complexes, and all of the chromophores are relatively efficient two-photon absorbers in the visible and near-infrared region of the spectrum (600−800 nm). The complexes exhibit maximum two-photon absorption at a shorter wavelength than 2λ for the one-photon band, consistent with the dominant two-photon transition arising from a two-photon-allowed gerade−gerade transition. Nanosecond transient absorption experiments carried out on several of the complexes with excitation at 803 nm confirm that the long-lived triplet state can be produced efficiently via a sequence involving two-photon excitation to produce S1, followed by intersystem crossing to produce T1.