title: Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism and Two-photon Absorption.
abstract: Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ2PA) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ2PA=770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S0→S1transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.
authors.affiliation: Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44-52, 01-224, Warsaw, Poland.
authors.forename: Łukasz G
authors.initials: ŁG
authors.lastname: Łukasiewicz
authors.affiliation: Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44-52, 01-224, Warsaw, Poland.|Department of Chemistry, POSTECH, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyungbuk, 37673, Korea.
authors.forename: Hye Gun
authors.initials: HG
authors.lastname: Ryu
authors.affiliation: Department of Physics, Montana State University, Bozeman, MT, 59717, USA.
authors.forename: Alexander
authors.initials: A
authors.lastname: Mikhaylov
authors.affiliation: CEISAM laboratory-UMR 6230, University of Nantes, 2, rue de la Houssinière, 44322, Nantes, France.
authors.forename: Cloé
authors.initials: C
authors.lastname: Azarias
authors.affiliation: Institute of Physics PAS, Al. Lotników 32/46, 02-668, Warsaw, Poland.
authors.forename: Marzena
authors.initials: M
authors.lastname: Banasiewicz
authors.affiliation: Institute of Physics PAS, Al. Lotników 32/46, 02-668, Warsaw, Poland.
authors.forename: Bolesław
authors.initials: B
authors.lastname: Kozankiewicz
authors.affiliation: Department of Chemistry, POSTECH, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyungbuk, 37673, Korea.
authors.forename: Kyo Han
authors.initials: KH
authors.lastname: Ahn
authors.affiliation: CEISAM laboratory-UMR 6230, University of Nantes, 2, rue de la Houssinière, 44322, Nantes, France.|Institut Universitaire de France, 1, rue Descartes, 75231, Paris Cedex 05, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Jacquemin
authors.affiliation: Department of Physics, Montana State University, Bozeman, MT, 59717, USA.|National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618, Tallinn, Estonia.
authors.forename: Aleksander
authors.initials: A
authors.lastname: Rebane
authors.affiliation: Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44-52, 01-224, Warsaw, Poland.
authors.forename: Daniel T
authors.identifier: http://orcid.org/0000-0002-2146-1282
authors.initials: DT
authors.lastname: Gryko
keywords: donor-acceptor systems; dyes/pigments; fluorescence; fused-ring systems; structure-activity relationship