Platinum Acetylide Two-Photon Chromophores

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 (kisc ≈ 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.