"Push-no-pull" porphyrins for second harmonic generation imaging by Lopez-Duarte, I, Reeve, J, Reeve, J, Perez-Moreno, J, Perez-Moreno, J, Boczarow, I, Boczarow, I, Depotter, G, Depotter, G, Fleischhauer, J, Fleischhauer, J, Clays, K, Clays, K, Anderson, H, Anderson, H