title: Understanding strong two-photon absorption in pi-conjugated porphyrin dimers via double-resonance enhancement in a three-level model.
abstract: We present the two-photon absorption (2PA) spectra of a series of conjugated porphyrin dimers and show that they possess extremely large intrinsic (femtosecond) peak 2PA cross sections, up to sigma2 = 1 x 104 GM in the near-IR region; these are among the highest values measured for any organic molecule. Moreover, we demonstrate that the second-order perturbation theory applied to a simple three-level model gives a perfect quantitative description of the observed 2PA cross section. By comparing all the factors of the three-level model for dimers with those of corresponding monomer (for which sigma2 = 20 GM), we explain an approximately 500-fold cooperative enhancement in sigma2 and find that the most important factor is the strength of excited-state transition. The matrix element of dipole moment of this transition amounts gigantic values of 30-40 D for conjugated porphyrin dimers, which can be accounted for a large delocalization radius (large electron-hole separation) in this state. We also demonstrate efficient generation of singlet oxygen upon one- and two-photon excitation of these porphyrin dimers, which can be useful for two-photon initiated photodynamic therapy of cancer.
authors.affiliation: Department of Physics, Montana State University, Bozeman, MT 59717, USA.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Drobizhev
authors.forename: Yuriy
authors.initials: Y
authors.lastname: Stepanenko
authors.forename: Yuliya
authors.initials: Y
authors.lastname: Dzenis
authors.forename: Aliaksandr
authors.initials: A
authors.lastname: Karotki
authors.forename: Aleksander
authors.initials: A
authors.lastname: Rebane
authors.forename: Peter N
authors.initials: PN
authors.lastname: Taylor
authors.forename: Harry L
authors.initials: HL
authors.lastname: Anderson