The dynamics of the ring-closure reaction of poly-1,2-bis(2-methylthien-3-yl)perfluorocyclopentene in chloroform solution is investigated. Ultrafast transient absorption spectra are measured using 260 nm pump pulses. It is shown that the optical switching occurs on a sub-200 fs timescale. The presence of a transient band around 450 nm, which disappears in similar to200 ps. is ascribed to excited-state absorption of the dithiophene moiety. (C) 2002 Elsevier Science B.V. All rights reserved.