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.