Two-step field-induced singlet dissociation in a fluorene trimer

Abstract

We time resolve the event of singlet separation into free carriers in an oligofluorene using femtosecond field assisted pump-probe technique. Steady state photoinduced absorption (PA) measurements have also been employed to identify triplet absorption $({T}_{1}\ensuremath{-}{T}_{n})$ in the spectral region of $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. As a result of an applied electric field of $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{MV}∕\mathrm{cm}$ singlets are observed to undergo a dissociation process consisting of first singlet splitting into bound polaron pairs and second further dissociation of polaron pairs into free carriers. Subsequent free carrier recombination yields a new population of singlets and triplets. Our results suggest that nongeminate recombination leads to the formation of 35% singlets and 65% triplets.