title: Alternative selection rules for one- and two-photon transitions in tribenzotetraazachlorin: quasi-centrosymmetrical π-conjugation pathway of formally non-centrosymmetrical molecule.
abstract: We compare the two-photon absorption (2PA) spectra of non-centrosymmetrical metal-free tribenzo-tetraazachlorin (H2TBTAC) and analogous symmetrical tetra-tert-butyl-phthalocyanine (H2TtBuPc). Surprisingly, despite formal lack of center of inversion, the 2PA spectrum of H2TBTAC displays a two-photon allowed transition at 935 nm, similar to gerade-gerade (g-g) transitions observed in H2TtBuPc and in other symmetrical phthalocyanines. This transition is even better resolved in the singlet-singlet excited-state absorption spectrum. We tentatively explain the survival of the g-g transition in H2TBTAC by assuming that the main π-electron conjugation pathway in the tetraaza-substituted tetrapyrrole macrocycle bypasses the outer parts of the two oppositely located isoindole rings and thus renders the optically responsive core of the chromophore quasi-centrosymmetrical. By using the independently measured ground- and excited-state absorption extinction coefficients, we also show that the two-photon absorptivity can be quantitatively explained by a simple three-level model with the lowest energy Q1 state serving as an intermediate level.
authors.affiliation: Department of Physics, Montana State University, Bozeman, Montana 59717, USA.
authors.forename: Nikolay S
authors.initials: NS
authors.lastname: Makarov
authors.forename: Mikhail
authors.initials: M
authors.lastname: Drobizhev
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Wicks
authors.forename: Elena A
authors.initials: EA
authors.lastname: Makarova
authors.forename: Evgeny A
authors.initials: EA
authors.lastname: Lukyanets
authors.forename: Aleksander
authors.initials: A
authors.lastname: Rebane
mesh_terms: D008968:Molecular Conformation; D017785:Photons; D011166:Porphyrins / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D011166:Porphyrins