Rotational Diffusion of the Mode-locicing Dye Dodci and Its Photoisomer

Pïcosecond time dependent fluorescente depolarization has been used to measure the rotational diff=sion time of 3,3’iliethyloxadicarjocyanine iodide (DODCI) in solution at 293 K: the stable form of DODCI was shown to have a “coiled” form, the cis,&-1 ,%conformation, whereas the photoisomer formed bu tiradiating DODCI in solution was shown to be tbe elongated all-trnrzs-isomer. The stable form has a rotational correlation time of 160 k 30 ps in ethanol and 320 k40 ps in isopropanol; the fluorescente lifetimes were 1.2 2 0.1 ns (ethanol) and 1.6 5 0.1 ns (isopropanol). The all-fuans-photoisomer bas a rotational correlation time of ,700 ps in isopropanol and a fluorescente lifetime of 0.42 kO.05 ns. Ftirthermore, the rotational diffusion times agree closely with those predicted on the basis of the Stokes-Einstein hydrodynamic model, indicating a lack of any solvent attachment to DODCI in solution.