Single photon triggered dianion formation in TCNQ and F4TCNQ crystals

Excited state dynamics in two strong organic electron acceptor systems, TCNQ and F(4)TCNQ single crystals, was studied. After absorption of a single photon, dianions are formed in both crystals on ultrashort timescale: TCNQ tau < 50 fs, F(4)TCNQ tau = 4 ps. By use of transient absorption spectroscopy, we demonstrate that the dianion formation in F(4)TCNQ is mediated by the radical anion precursor which is described by a two-step model. Our measurements show the phenomenon that in this quinoid acceptor crystals in the absence of additional donor molecule, it is possible to resolve the two step formation of a doubly charged anion upon absorption of a single low energy photon (2.6 eV).