title: Strong cooperative enhancement of two-photon absorption in double-strand conjugated porphyrin ladder arrays.
abstract: We present the two-photon absorption (2PA) spectra of a series of conjugated porphyrin oligomers containing N = 2, 4, 8, and ca. 13 monomer units, meso-meso connected with butadiyne linkers. We demonstrate that, in the coplanar double-strand arrays, self-assembled upon addition of 4,4'-bipyridyl, the conjugation length increases dramatically, leading to very strong cooperative enhancement of 2PA. We analyze the scaling of 2PA in both the double-strand and rotationally free single-strand arrays and show how the effective conjugation length in both cases is linked to the observed 2PA properties. By introducing a "conjugation signature" for the 2PA strength, we show that, in double-strand arrangement, the conjugation embraces the whole molecule up to the tetramer level, whereas in single-strand arrangement, it is always less than N, except for N = 2, but keeps increasing until N = 8. Our finding of extremely strong 2PA cross section, sigma2 approximately 105 GM, in double-strand oligomers peaking at 1.3 mum can find use for signal processing in fiber-optic devices.
authors.affiliation: Department of Physics, Montana State University, Bozeman, Montana 59717-384, USA.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Drobizhev
authors.forename: Yuriy
authors.initials: Y
authors.lastname: Stepanenko
authors.forename: Aleksander
authors.initials: A
authors.lastname: Rebane
authors.forename: Craig J
authors.initials: CJ
authors.lastname: Wilson
authors.forename: Thomas E O
authors.initials: TE
authors.lastname: Screen
authors.forename: Harry L
authors.initials: HL
authors.lastname: Anderson