Two-photon fluorescence and femtosecond two-photon absorption studies of MeLPPP, a ladder-type poly(phenylene) with low intra-chain disorder

MeLPPP is a ladder-type conjugated poly(phenylene) with particularly low in tra-chain disorder and inhomogeneous broadening of only 20 meV. The two-pho ton excited state (mA(g)), investigated by two-photon fluorescence excitati on spectroscopy, is much broader [E(mA(g)) approximate to 3.4 +/- 0.2 eV] a nd without well-resolved vibronic structure. This agrees with a broad under lying feature in the electroabsorption spectra of MeLPPP. Femtosecond pump- probe measurements reveal transient absorption into the mA(g) state within the duration of the pump pulse (140 fs) and rapid subsequent decay (tau app roximate to 150 fs) to the 1B(u) exciton, with photoluminescence clearly vi sible 600 fs after excitation. A value of the nonlinear absorption coeffici ent beta approximate to 11 cm(2)/GW is calculated for the two-photon absorp tion, mA(g) <-- 1A(g). (C) 1999 Elsevier Science B.V. All rights reserved.