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Abstract
The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment. Solvent-driven localization breaks the symmetry and traps the exciton in one branch. Using a combination of ultrafast spectroscopies, we investigate how such excited-state symmetry breaking affects the photochemical reactivity of quadrupolar and octupolar A–(π-D)2,3 molecules with photoisomerizable A–π–D branches. Excited-state symmetry breaking is identified by monitoring several spectroscopic signatures of the multipolar delocalized exciton, including the S2 ← S1 electronic transition, whose energy reflects interbranch coupling. It occurs in all but nonpolar solvents. In polar media, it is rapidly followed by an alkyne–allene isomerization of the excited branch. In nonpolar solvents, slow and reversible isomerization corresponding to chemically-driven symmetry breaking, is observed. These findings reveal that the photoreactivity of large conjugated molecules can be tuned by controlling the localization of the excitation.
Highlights
The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment
Excitedstate symmetry breaking (ES-SB) can be viewed as the decoherence of a multipolar exciton evenly delocalized over all branches of the molecule that eventually results in its confinement on a single branch
We report on our investigation of the excited-state dynamics and photochemistry of octupolar (O) and quadrupolar (Q) dyes consisting of a triazine core decorated with dialkylanilines connected through alkyne π-bridges (Fig. 1a)
Summary
The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment. Excited-state symmetry breaking is identified by monitoring several spectroscopic signatures of the multipolar delocalized exciton, including the S2 ← S1 electronic transition, whose energy reflects interbranch coupling It occurs in all but nonpolar solvents. Exhibiting advantageous two-photon absorption (TPA) properties[14,15], they attract attention in various fields, including 3D nanofabrication[16,17,18], in vivo fluorescence microscopy[19,20,21,22], power limiting[23,24] and optical uncaging[25] among others[26,27,28,29] Photoexcitation of these molecules leads to a delocalized and symmetric multipolar exciton that can undergo symmetry breaking due to surrounding solvent fluctuations[30,31,32]. The results demonstrate that the delocalization of the excitation leads to local changes of electronic distribution that are too weak to favor efficient photochemistry, and does not result in qualitatively different photochemical pathways that would involve its inherent coherent nature and lead to functional symmetry[50]
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