Spectroscopy And Excited State Dynamics Of Nearly Infinite Polyenes

Steady-state and transient absorption spectra with <50 fs time resolution were obtained for two conjugated polymers, both with approximate to 200 conjugated double bonds (N), constrained in planar, stable, polyene frameworks. Solutions of the polymers exhibit the same S-2 -> S-1 -> S* -> S(0)decay pathway observed for theN= 11-19 polyene oligomers and for zeaxanthin homologues withN= 11-23. Comparisons with the excited state dynamics of polydiactylene and a much longer, more disordered polyene polymer (poly(DEDPM)) show that the S-2, S-1, and S* lifetimes of the four polymers are almost identical. The S* signals in the polymers are assigned to absorption from vibrationally excited ground states. In spite of significant heterogeneities and variations in conjugation lengths in these long polyenes, their S-0 -> S(2)absorptions are vibronically-resolved in room temperature solutions with electronic origins at approximate to 600 nm. The limiting wavelength for the S-0 -> S(2)transitions is consistent with the persistence of bond length alternation in the electronic ground states and a HOMO-LUMO band gap in polyenes withN approximate to 200. The coincidence of the well-resolved S-0 -> S(2)electronic origins and the convergence of the excited state lifetimes in the four polymers point to a common, "nearly infinite" polyene limit.