Photophysics of meso-β doubly linked Ni(II) porphyrin arrays:: Large two-photon absorption cross-section and fast energy relaxation dynamics

Two-photon absorption (TPA) properties and energy relaxation dynamics of meso-beta doubly linked Ni(II) porphyrin arrays DLNin, n = 2-5) in CHCl3 have been investigated. DLNin exhibits near-IR-extended absorption bands up to 1000 nm and enlarged TPA cross-section values without any one-photon contribution as the number of subunits increases mainly due to an elongation of effective pi-conjugation length. Especially, in the case of pentamer, DLNi5 had a TPA cross-section value of 41 000 GM, which is the largest value per subunit (sigma((2))/n similar to 8000 GM) among the conjugated porphyrins ever studied. Fast energy relaxation processes within a few picoseconds were observed by femtosecond transient absorption measurements, indicating that various charge transfer and (d,d) states of the central Ni(II) ion are involved in deactivation pathways as the ladder states in DLNin.