1 Electronic Supplementary Information for Photophysical Properties and Intracellular Imaging of Water-Soluble Porphyrin Dimers for Two-Photon Excited Photodynamic Therapy Marina K. Kuimova, a Hazel A. Collins, b Milan Balaz, b Emma Dahlstedt, b James A. Levitt, c Nicolas Sergent, c Klaus Suhling, c Mikhail Drobizhev, d Aleksander Rebane, d Harry L. Anderson* ,b and David Phillips* ,a a Chemistry Department, Imperial College London, Exhibition Road, London SW7 2AZ, UK; b Department of Chemistry, Oxford University, Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA, UK; c Department of Physics, King’s College London, Strand, London WC2R 2LS, UK; d Department of Physics, Montana State University, Bozeman, Montana 5917-384, USA. Email: Harry.Anderson@chem.ox.ac.uk and D.Phillips@imperial.ac.uk Supplementary Material (ESI) for Organic & Biomolecular Chemistry This journal is (c) The Royal Society of Chemistry 2009