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Excited‐State Symmetry Breaking in an Aza‐Nanographene Dye
Author(s) -
Bardi Brunella,
Krzeszewski Maciej,
Gryko Daniel T.,
Painelli Anna,
Terenziani Francesca
Publication year - 2019
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201902554
Subject(s) - excited state , dipole , symmetry breaking , conjugated system , molecule , molecular physics , anisotropy , fluorescence anisotropy , pyrrole , chemistry , atomic physics , fluorescence , materials science , chemical physics , photochemistry , physics , quantum mechanics , organic chemistry , polymer
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.