Picosecond observation of cation-stepwise delayed and cation-triggered photoinduced intramolecular charge transfer in fluorescent cation probes

Time-resolved transient absorption and gain spectra with subpicosecond laser excitation are reported for donor-donor and acceptor-donor stilbene-crowns (in each compound the electron donor group is the 15-aza-5-crown macrocycle, D-I). The effect of the calcium cation on the photoinduced intramolecular charge transfer (ICT) rate constants are measured. When the cation is on the donor side of the molecular system (class 1: DICS-Crown), the photoinduced ICT process is slowed down and goes through several intermediates characterized by a distancing of the cation, while a solvent molecule enters its coordination sphere, When the cation is on the acceptor side (class 2: DIDS-Crown,DIDB-Crown and DIDCS-Crown), the photoinduced ICT process becomes too fast to be measured, even on the picosecond scale.