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 intra-chain disorder and inhomogeneous broadening of only 20 meV. The two-photon excited state (mA(g)), investigated by two-photon fluorescence excitation spectroscopy, is much broader [E(mA(g)) approximate to 3.4 +/- 0.2 eV] and without well-resolved vibronic structure. This agrees with a broad underlying 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 approximate to 150 fs) to the 1B(u) exciton, with photoluminescence clearly visible 600 fs after excitation. A value of the nonlinear absorption coefficient beta approximate to 11 cm(2)/GW is calculated for the two-photon absorption, mA(g) <-- 1A(g). (C) 1999 Elsevier Science B.V. All rights reserved.