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 (σ ) 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, σ =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S →S 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.

Synthesis of Bis(arylethynyl)pyrrolo[3,2‐b]pyrroles and Effect of Intramolecular Charge Transfer on Their Photophysical Behavior

“…The combination of π‐expansion and presence of an EWG, such as in dye 15 , was found to be the best strategy to shift absorption and emission. The strong symmetry breaking in the excited state of TAPPs has been observed numerous times . In particular, for 2,5‐bis‐(4‐nitrophenyl)pyrrolopyrrole a significant redshift of the emission and even a stronger decrease of fluorescent quantum yields have been noticed with increasing polarity of solvent .…”

“…The strong symmetry breaking in the excited state of TAPPs has been observed numerous times . In particular, for 2,5‐bis‐(4‐nitrophenyl)pyrrolopyrrole a significant redshift of the emission and even a stronger decrease of fluorescent quantum yields have been noticed with increasing polarity of solvent . Indeed, positive solvatofluorochromism was observed for all dyes 13 – 18 (Table ) although for compound 16 it was rather weak.…”

Excited‐State Symmetry Breaking in an Aza‐Nanographene Dye

“…The pyrrolo[3,2‐ b ]pyrrole (PP), which is an electron rich heterocyclic unit, behaves as an energy donor moiety in A‐π‐D‐π‐A‐type chromophores, when coupled to strong electron withdrawing moieties like cyano, nitro and carbonyl groups . PP‐derivative 1 (Figure ) has a qualitatively different structure from previously investigated PP‐based quadrupolar dyes.…”

FCI Literature Prize: T. Schirmeister, C. Schmuck, P. R. Wich / Foundation for Polish Science Award: D. T. Gryko / Heinz Award: J. M. DeSimone

“…Gryko's research interests include the synthesis and spectroscopic properties of functional dyes such as porphyrins, corroles, chlorins, diketopyrrolopyrroles, and pyrrolopyrroles. He has reported in Chemistry—A European Journal on the hydrogen‐bond‐driven oligomerization of amido‐corroles, and in the Asian Journal of Organic Chemistry on fluorescent probes based on a rhodal chromophore . Gryko is on the Editorial Boards of ChemistrySelect and ChemPhotoChem .…”