title: Platinum acetylide two-photon chromophores.
abstract: 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 pi-conjugated ligands substituted with pi-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 (Kisc approximately 1011 s-1) to produce the T1 state, which is possibly localized on a single conjugated fluorenyl ligand. The triplet state is strongly absorbing (epsilonTT > 5 x 104 M-1 cm-1), and it is very long-lived (tau > 100 micro s). 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 2lambda 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.
authors.affiliation: Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA.
authors.forename: Joy E
authors.initials: JE
authors.lastname: Rogers
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Slagle
authors.forename: Douglas M
authors.initials: DM
authors.lastname: Krein
authors.forename: Aaron R
authors.initials: AR
authors.lastname: Burke
authors.forename: Benjamin C
authors.initials: BC
authors.lastname: Hall
authors.forename: Albert
authors.initials: A
authors.lastname: Fratini
authors.forename: Daniel G
authors.initials: DG
authors.lastname: McLean
authors.forename: Paul A
authors.initials: PA
authors.lastname: Fleitz
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Cooper
authors.forename: Mikhail
authors.initials: M
authors.lastname: Drobizhev
authors.forename: Nikolay S
authors.initials: NS
authors.lastname: Makarov
authors.forename: Aleksander
authors.initials: A
authors.lastname: Rebane
authors.forename: Kye-Young
authors.initials: KY
authors.lastname: Kim
authors.forename: Richard
authors.initials: R
authors.lastname: Farley
authors.forename: Kirk S
authors.initials: KS
authors.lastname: Schanze