Synthesis and spectroscopic study of phenylene-(poly)ethynylenes substituted by amino or amino/cyano groups at terminal(s): electronic effect of cyano group on charge-transfer excitation of acetylenic π-systems

To gain insight into substituent electronic effect on charge-transfer excitation of acetylenic π-systems, phenylene-(poly)ethynylenes substituted by Ph2N or Ph2N/cyano groups were synthesized by combination of Sonogashira coupling and double elimination protocol of β-substituted sulfones. These substituted phenyleneethynylenes showed large molar absorption coefficients ɛ, and emitted strong fluorescence upon UV light irradiation. Phenylene-(poly)ethynylenes, which involve butadiyne or hexatriyne motifs, emitted fluorescence in remarkably lower fluorescence quantum yields ΦF as their polyethynylene motifs -(CC)n- expanded. The drastic decrease of fluorescence quantum yields ΦF were explained in terms of increasing nonradiative reaction rate constants knr, which had been determined by the corresponding fluorescence quantum yields ΦF and lifetime values τ. The emission underwent a large bathochromic shift in polar solvents because the charge-separated excited state is more stabilized than the ground state. Comparison of slope values ρ in Lippert/Mataga plot for the Ph2N and Ph2N/cyano-substituted phenylene-(poly)ethynylenes revealed that the latter underwent large change of dipole moments upon photo-excitation although highly expanded acetylenic π-systems with cyano group did little.