A comprehensive photophysical study has been carried out on a series of platinum(II)-containing phenylethynyl oligomers. The compounds are composed of a platinum center attached to two tributylphosphine ligands and two ligands that vary the number of repeat phenyl-ethynyl units (-PhsCtC-). The objective of this work is to understand the effects of increased conjugation and the influence of the platinum on the overall electronic structure of the molecule. This was done by utilizing steady-state absorption, steady-state emission, picosecond pump-probe, and nanosecond laser flash photolysis techniques. The effect of increased conjugation is a red shift of S 0 -S 1 and T 1 -T n and an increase in both the S 0 -S 1 and T 1 -T n molar extinction coefficients. The spin-orbit coupling effect of platinum on the ground and excited-state properties is reduced with increased conjugation length because the S 0 -S 1 transition is more localized on the ligand. As the ligand becomes larger, it takes on more π-π* character and therefore is spatially further away from the platinum center.

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.