UltrafastExcited State Dynamics in 9,9′-Bifluorenylidene.

9,9′-Bifluorenylidene hasbeen proposed as an alternativeand flexible electron acceptor in organic photovoltaic cells. Herewe characterize its excited state properties and photokinetics, combiningultrafast fluorescence and transient IR measurements with quantumchemical calculations. The fluorescence decay is ultrafast (sub-100fs) and remarkably independent of viscosity. This suggests that largescale structure change is not the primary relaxation mode. The ultrafastdecay populates a dark state characterized by distinct vibrationaland electronic spectra. This state decays with a 6 ps time constantto a hot ground state that ultimately populates the initial statewith a 20 ps time constant; these times are also insensitive to solventviscosity. No metastable intermediate structures are resolved in thephotocycle after population of the dark state. The implications ofthese results for the operation of 9,9′-bifluorenylidene asan electron acceptor and as a potential molecular switch are discussed. [ABSTRACT FROM AUTHOR]