Vibrational spectroscopic investigation of rigid-rod platinum σ-acetylide polymers containing variable acetylenic microstructures

A detailed study of the vibrational spectra of the rigid-rod Pt(II)-σ-acetylide polymers, [(PⁿBU₃)₂Pt-C≡CC≡CC≡C-] _n, [(PⁿBu₃)₂Pt-C≡CC≡C-]_n, [(PⁿBu₃)₂Pt-C≡CC₆H ₄C≡C-]_n, [(PⁿBU₃)₂Pt-C≡CC≡CC ₆H₄C≡CC≡C-]_n, and [(AsⁿBu₃)₂Pt-C≡CC₆H ₄C≡C-]_n is reported. The backbone chain vibrations in these highly conjugated organometallic polymers can be analyzed by relating them to their centrosymmetric monomeric structures with an approximate C_2h or D_2h local symmetry. The extent of π-conjugation in the backbone can be monitored from the associated changes in the vibrational frequencies, in particular, the v_C≡C modes. In general, the vibrational spectra of the polymers suggest that the backbone is essentially alternating -C≡C- in nature, without any contribution from allene-type structures (=C=C=). The interruption of the acetylenic linkages with phenyl groups reduces the π-conjugation. The near-infrared FT-Raman spectra of the polymers show resonance enhancement of selected vibrations which occur along the direction of the polymer chain, presumably due to coupling with electronic transitions of the backbone. In addition, these polymers exhibit strong and substituent-sensitive fluorescence in their FT-Raman spectra.