Experimental and Theoretical Investigation for the Level of Conjugation in Carbazole-Based Precursors and Their Mono-, Di-, and Polynuclear Pt(II) Complexes

A series of trimethylsilyl-protected monoalkynes (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 monoalkynes (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 diynes, 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 polynuclear Pt(II) σ-acetylide complexes have been characterized spectroscopically, with a subset analyzed 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 modeling 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.