title: Extremely strong near-IR two-photon absorption in conjugated porphyrin dimers: quantitative description with three-essential-states model.
abstract: Two-photon absorption spectra (2PA) of a series of conjugated dimers and the corresponding monomer were studied in the near-IR region. All of the dimers show very large peak cross section values, sigma(2) = (3-10) x 10(3) GM (1 GM = 1 x 10(-50) cm(4) s photon(-1)), which is several hundred times larger than that obtained for the corresponding monomer in the same region. We explain such dramatic cooperative enhancement by a combination of several factors, such as strong enhancement of the lowest one-photon Q-transition, better resonance conditions in the three-level system, dramatic enhancement of the excited-state singlet-singlet transition, and parallel arrangement of consecutive transitions in dimers, as compared to perpendicular arrangement in the monomer. We show that the absolute values of the 2PA cross section in these molecules are quantitatively described by a quantum-mechanical expression, derived for the three-level model. We also demonstrate the possibility of singlet oxygen generation upon one- and two-photon excitation of these dimers, which makes them particularly attractive for photodynamic therapy.
authors.affiliation: Department of Physics, Montana State University, Bozeman, Montana 59717, P. N. Lebedev Physics Institute, Leninsky Pr., 53, 119991 Moscow, Russia.
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
mesh_terms: D000042:Absorption; D002627:Chemistry, Physical / Q000379:methods*; D019281:Dimerization; D007834:Lasers; D008956:Models, Chemical; D016011:Normal Distribution; D017785:Photons; D011166:Porphyrins / Q000737:chemistry*; D026082:Singlet Oxygen / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013055:Spectrophotometry, Infrared / Q000379:methods*; D019265:Spectroscopy, Near-Infrared; D013816:Thermodynamics
chemical_list: D011166:Porphyrins; D026082:Singlet Oxygen