Supplementary Information Effects of Conjugation Length and Resonance Enhancement on Two-Photon Absorption in Phenylene-Vinylene Oligomers Mette Johnsen, ‡ Martin J. Paterson, ‡§ Jacob Arnbjerg, ‡ Ove Christiansen, ‡ Christian B. Nielsen, ‡† Mikkel Jørgensen † and Peter R. Ogilby ‡ * ‡ Center for Oxygen Microscopy and Imaging, Department of Chemistry, University of Aarhus, DK-8000, Århus, Denmark, † Polymer Department, Risø National Laboratory, DK-4000, Roskilde, Denmark, and § School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, Scotland * To whom correspondence should be addressed: progilby@chem.au.dk Contents: 1. Discussion on the origin of background signals observed in the two-photon experiments. 2. Preparation and characterization of the phenylene-vinylene (PV) oligomers. 3. Excitation-wavelength-dependence of the PV3 and PV4 emission spectra. 1