Excited state characterization of a polymeric indigo

A comprehensive investigation of the solution photophysics of a 5,5'-methylene-bridged polymeric indigo, a statistical copolymer consisting of indigo and N-acetylindigo units, was performed in organic solvents at room temperature and further compared with indigo. A complete spectral and photophysical characterization based on photoacoustic calorimetry, steady-state and time-resolved fluorescence data was undertaken. A fluorescence quantum yield of 0.00037 and an intersystem crossing singlet-to-triplet quantum yield of 0.006 (close to the value for indigo) were obtained, leading to a value of 0.9936 for the S(1) → S(0) internal conversion (IC) quantum yield. Spectral and photophysical characteristics similar to indigo were obtained with, however, a special signature: it (mainly) decays single exponentially (in contrast with indigo, found to decay bi-exponentially), with a decay time value of 40-50 ps and an even more efficient S(1) → S(0) IC deactivation channel, related to an efficient energy migration within an energetic ladder of the polymer chromophoric segments. The photochemistry of this polymer, namely the degradation under light excitation, was also investigated and the obtained photoreaction quantum yield (ϕ(R)) in DMF was found to be 0.003, which is lower than the previously determined value for indigo in the same solvent (ϕ(R) = 0.0078). The overall data indicate that although the polymer and indigo have a close finger-print, the former is more stable which is suggested to be due to the additional intramolecular energy transfer process (within different chromophoric units) found with the polymer.