Experimental and theoretical investigation for level of conjugation in carbazole-based precursors and their mono-,di- and polynuclear Pt(II) complexes

A series of trimethylsilyl-protected mono-alkynes (Me3SiC≡C−R) and bis-alkynes (Me3 SiC≡C−R−C≡CSiMe3) incorporating carbazole spacer groups (R = carbazole-2-yl, carbazole-3-yl, carbazole-2,7-diyl, N-(2-ethylhexyl)carbazole-2,7-diyl, carbazole-3,6-diyl, N-(2-ethylhexyl)carbazole-3,6-diyl), together with the corresponding terminal mono-alkynes (H−C≡C−R) and bis-alkynes (H−C≡C−R−C≡C−H), have been synthesized and characterized. The CuI-catalyzed dehydrohalogenation reaction between trans-[(Ph)(Et3P)2PtCl], trans-[(Et3P)2PtCl2] and trans-[(PnBu3)2PtCl2] and the terminal alkynes in iPr2NH/CH2Cl2 affords a series of Pt(II) mono- and di-ynes, while the dehydrohalogenation polycondensation reactions with trans-[(PnBu3)2PtCl2] under similar reaction conditions yields four Pt(II) poly-ynes of the form trans-[(PnBu3)2Pt−C≡C−R−C≡C−]n. The acetylide-functionalized carbazole ligands and the mono-, di- and poly-nuclear Pt(II) σ-acetylide complexes have been characterized spectroscopically, with a subset analysed using single-crystal X-ray diffraction. The Pt(II) mono-, di- and poly-ynes incorporating the carbazole spacers are soluble in common organic solvents, and solution absorption spectra show a consistent red-shift between the 2- and 2,7- as well as 3- and 3,6-carbazole complexes. Computational modelling is used to explain the observed spectral shifts, which are related to the enhanced electronic delocalization in the latter systems. These results also indicate that the inclusion of carbazole-2,7-diyl units into rigid-rod organometallic polymers should enhance electronic transport along the chains.