Symmetry Breaking in Pyrrolo[3,2‐b]pyrroles: Synthesis, Solvatofluorochromism and Two‐photon Absorption

• Published in: Chemistry, an Asian journal an Asian journal, 2017-07, Vol.12 (14), p.1736-1748
• Main Authors: Łukasiewicz, Łukasz G., Ryu, Hye Gun, Mikhaylov, Alexander, Azarias, Cloé, Banasiewicz, Marzena, Kozankiewicz, Bolesław, Ahn, Kyo Han, Jacquemin, Denis, Rebane, Aleksander, Gryko, Daniel T.
• Format: Article
• Language: English
• Subjects: Broken symmetry; Chemistry; Conjugation; Cross-sections; Dihedral angle; Dipole moments; donor–acceptor systems; dyes/pigments; Electric dipoles; Emission analysis; Fluorescence; fused-ring systems; Ground state; Photon absorption; Polarity; Pyrroles; Solvents; structure–activity relationship; Symmetry
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.201700159

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→S1 transition 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. Breaking news: The strong solvatofluorochromism for centrosymmetric pyrrolo[3,2‐b]pyrroles possessing two nitro groups is a general phenomenon which disappears completely while moving to analogous dyes possessing dipolar architecture. The two‐photon absorption spectra provide evidence that symmetry breaking may already be present in the ground state, that is, prior to solute–solvent excited‐state relaxation.