Unusually short excited state lifetimes of indenofluorene and fluorenofluorene derivatives result from a conical intersection

The ultrafast excited state decay rates for indenofluorene and fluorenofluorene derivatives as measured by transient absorption spectroscopy are reported. The excited state lifetimes of the molecules were extremely short (ca. 9-12 ps) and are a result of the p-xylylene motif in indenofluorenes and the corresponding expanded 2,6-naphthoquinonedimethide in fluorenofluorene. Quantum chemical calculations indicate that the fast relaxation to the ground state results from a potential energy surface crossing between the S0 and S1 states. This process in turn provides an efficient route for excited state deactivation and explains why this class of quinoidal molecules is non-emissive.