Excited‐State Symmetry Breaking in an Aza‐Nanographene Dye

Abstract

The photophysics of a structurally unique aza‐analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture was studied in detail. The combined experimental and computational investigation reveals that the lowest excited state has charge‐transfer character, in spite of the absence of any peripheral electron‐withdrawing groups. The exceptionally electron‐rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, which is qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states that are close in energy but have different polarization directions.

The Energy Gap Law at Work: Emission Yield and Rate Fluctuations of Single NIR Emitters

“…Moreover, DFT calculations did predict a different molecular geometry in S 1 , which, however, was not further specified . Tentatively, we assume that geometry changes in S 1 give rise to an excited state symmetry breaking and an asymmetric charge distribution . Since such behavior was not predicted from the calculations in vacuum, the solvent may play a crucial role.…”

Bowl‐Shaped Pentagon‐ and Heptagon‐Embedded Nanographene Containing a Central Pyrrolo[3,2‐b]pyrrole Core

“…Positive solvatochromic shift in the emission spectra of certain TAPP derivatives,a ttributed to the excited-state symmetry breaking, has been lately observed and closely studied. [53,[66][67][68] This prompted us to examine the spectral features dependence on solvent polarity for the synthesized pyrrolo[3,2b]pyrroles 4-7.I na ddition to measurements performed in dichloromethane,r espective emission spectra were collected for toluene and methyl-tetrahydrofuran.C omparison of fluorescence maxima indicate that, indeed, dyes 5-7 display solvatofluorochromism with concomitant decrease of emission intensity from less polar to more polar solvents (see ESI, Figures S5-S7). To further elucidate the nature of the electronic transitions,a bsorption spectra of 7-H were obtained with the aid of two different theoretical methods.T he spectra are qualitatively similar to each other and both are composed of several overlapping absorption bands of increasing intensity.The lowest absorption band, assigned to the S 0 !S 1 transition involves an intramolecular charge-transfer from the central pyrrolo [3,2-b]pyrrole unit to the side benzene rings as it is illustrated in Figure S13 and Table S3 of the ESI.…”

Synthesis and Characterization of Peralkylated Pyrrole-Fused Azacoronene

“…Polycyclic heteroaromatic molecules (PHAs), including pyrrole units, have attracted considerable attention for research in nanographene chemistry because of their near-infrared (NIR) absorption, intense fluorescence emission, molecular recognition ability, and redox properties . Hexapyrrolohexaazacoronene (HPHAC) is an archetypal molecule of pyrrole-fused azacoronene, which is comprised of six pyrrole units fused in a ring and bound to a central benzene through each pyrrole nitrogen (Figure , 2a ) .…”