Nonplanar Butterfly‐Shaped π‐Expanded Pyrrolopyrroles

Abstract

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9'-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.

Excited‐State Symmetry Breaking in an Aza‐Nanographene Dye

“…The oscillator strength associated to the lowest‐energy absorption band amounts to approximately 0.3, corresponding to a transition dipole moment of about 6 D. The emission band is largely solvatochromic, and redshifts from 451 nm in cyclohexane to 505 nm in dimethyl sulfoxide (DMSO). The fluorescence quantum yield increases from 17 to 32 % from toluene to DMSO, with lifetimes ranging from 16.8 to 18.9 ns . From the Weisskopf–Wigner equation for the radiative decay rate, the transition dipole moment associated with the emission is estimated to amount to approximately 3 D, quite independently of the solvent.…”

“…Fluorescence excitation spectra of 1 in Figure nicely match absorption spectra, showing two groups of closely spaced signals, one in the 300–350 nm region, with a sharp peak at approximately 350 nm, and another beyond 400 nm, with a maximum around 430 nm. Vibronically resolved emission spectra are obtained in frozen solvents with maximum intensity at 450 nm.…”

“…To reduce the computational cost, tert ‐butyl groups were replaced with hydrogen atoms, with marginal effects on calculated spectral properties. The folded form was found to be less stable than the twisted form, the energy difference between the conformers amounting to 9.9 kJ mol −1 in DMSO, in agreement with previously published data . Accordingly, at equilibrium, the twisted form is largely favored over the folded form and dominates the spectroscopic behavior of 1 .…”

“…Two conformational isomers of 1 were found in the crystalline phase: a folded meso diastereoisomer (belonging to the C i point group) and a helical twisted form of C 2 symmetry, existing in the ( P , P ) and ( M , M ) enantiomers (Figure , right). The isomers could not be distinguished in solution because of low interconversion energy barrier (56.0 kJ mol −1 ) and fast interconversion rate …”

A Nitrogen‐Doped Hexapole [7]Helicene versus Its All‐Carbon Analogue

“…Herein, we present ap air of propeller-shaped NGs: [8][9][10] the nitrogen-doped hexapole [7]helicene (H7H) N-H7H [11,12] and its all-carbon analogue C-H7H. N-H7H and C-H7H share acommon hexabenzocoronene (HBC) platform, to which six dibenzoullazine (DBU) [13] or dibenzopyrene (DBP) blades are attached at the periphery in ahelical structure ( Figure 1).…”