Photophysics and non-linear absorption of Au(i) and Pt(ii) acetylide complexes of a thienyl-carbazole chromophore

Abstract

In order to understand the photophysics and non-linear optical properties of carbazole containing π-conjugated oligomers of the type ET-Cbz-TE (E = ethynylene, T = 2,5-thienylene, Cbz = 3,6-carbazole), a detailed investigation was carried out on a series of oligomers that feature Au(I) or Pt(II) acetylide “end groups”, as well as a Pt(II)-acetylide linked polymer (CBZ-Au-1 and CBZ-Pt-1, CBZ-Poly-Pt). These organometallic chromophores were characterized by UV-visible absorption and variable temperature photoluminescence spectroscopy, nanosecond transient absorption spectroscopy, open aperture nanosecond z-scan and two photon absorption (2PA) spectroscopy. The Au(I) and Pt(II) oligomers and polymer exhibit weak fluorescence in fluid solution at room temperature. Efficient phosphorescence is observed from the Pt(II) systems below 150 K in a solvent glass; however, the Au(I) oligomer exhibits only weak phosphorescence at 77 K. Taken together, the emission results indicate that the intersystem crossing efficiency for the Pt(II) chromophores is greater than for the Au(I) oligomer. Nonetheless, nanosecond transient absorption indicates that direct excitation affords moderately long-lived triplet states for all of the chromophores. Open aperture z-scan measurement shows effective optical attenuation can be achieved by using these materials. The 2PA cross section in the degenerate S₀→S₁ transition region was in the range 10–100 GM, and increased monotonically toward shorter wavelengths, reaching 800–1000 GM at 550 nm.