Dark states of symmetric polymethine dyes revealed by pump–dump–probe spectroscopy

We have applied three-pulse transient absorption spectroscopy to investigate the ultrafast photoisomerization dynamics in two symmetric polymethine dyes. Pump-deplete-probe experiments have revealed that the excited state manifold of these molecules contains two closely lying excited states in the dynamic equilibrium. One of these states is emissive, while the other is largely dark. It is the dark state that ultimately results in the photoisomer formation and is the main channel of excited state decay in these dyes. We have shown that excited state populations do flow between these two states and therefore it can be inferred that the photoisomerization pathway is not predetermined by ground state distortions of the molecular structure. The decision whether the molecule will isomerize or not seems to be taken in the excited state. Global analysis of three-pulse transient data allowed us to determine the spectra of the excited and ground intermediate states and build a comprehensive picture of photoinduced dynamics in symmetric polymethine dyes.