Raman spectral shifts relevant to electron delocalization in polydiacetylenes

Abstract

The extent of π electron delocalization in solid‐state polymerized diacetylenes is derived from observed Raman vibrational frequencies. Raman intense C=C and C≡C vibrations indicate that the polymer backbone is best represented by (=C–C≡C–C=)_n. However, v_C=C is linearly correlated with v_C≡C over a wide frequency range and both frequencies increase with decreasing phase perfection. This suggests a significant fractional contribution δ from the mesomeric structure (–C=C=C=C–)_n, which decreases the π bond order of C=C to 1‐δ and C≡C to 2‐δ. Both v_C=C and v_C≡C are calculated as a function of δ and compared with observed frequencies for variously substituted polydiacetylenes in different physical states. Resulting Δv_C=C/Δv_C≡C values obtained using two different bond‐length to bond‐order approximations are in good agreement with the observed value (1.19±0.12).