Femtosecond to subnanosecond multistep calcium photoejection from a crown ether-linked merocyanine

Photoinduced calcium release from the crown ether-linked merocyanine DCM-crown is reexamined by femtosecond transient absorption spectroscopy with sub-100 fs time resolution. Photodisruption of the bond linking the cation to the nitrogen atom shared by the crown and the chromophore is found to take place in 130 fs. Confirming our previous reports, the photoinduced intraligand charge transfer is observed in the picosecond regime but kinetics involving three-components (1 ps, 8 ps and 77 ps), together with a 56 ps stimulated-emission time-resolved red shift, are found in the present study. Both delayed intraligand charge transfer and cation release are assumed to occur in this time range due to repulsion effects between the positively charged nitrogen of the crown ether moiety and the cation. In the subnanosecond regime, a 670 ps time-resolved red shift of the stimulated-emission spectrum of the charge-transfer state, similar to the shift previously observed with Sr(2+), demonstrates the motion of the cation away from the crown to the bulk. A thorough examination of the present data allows us to conclude that calcium ion is photoejected to the bulk in a multistep process.