Absorption and emission studies on pure and mixed J-aggregates of pseudoisocyanine

Fluorescence excitation experiments are used to extract the absorption spectra of the two different aggregate species observed for pseudoisocyanine bromide at 77 K. Energy transfer between the species indicates that both can be present on the same aggregate fibre. The blue species' radiative lifetime is determined to be ∼1.5-1.7 times longer than that of the red species, and at 77 K the blue species has a 3 times lower fluorescence quantum yield. An aggregate unit cell is proposed that reproduces many features of the absorption spectra, and predicts a dihedral angle of ∼60°, in reasonable agreement with crystallographic data. Experiments and theory on emission from the exciton-band to ground-state-vibrations are presented. The influence of the mixing ratio on absorption spectra of self-assembled mixed aggregates of pseudoisocyanine iodide and its aza-analogue is investigated, and compared with numerical simulations.