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 (mAg), investigated by two-photon fluorescence excitation spectroscopy, is much broader [E(mAg)≈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 mAg state within the duration of the pump pulse (140 fs) and rapid subsequent decay (τ≈150 fs) to the 1Bu exciton, with photoluminescence clearly visible 600 fs after excitation. A value of the nonlinear absorption coefficient β≈11 cm2/GW is calculated for the two-photon absorption, mAg←1Ag.