Solvent tuning of photochemistry upon excited-state symmetry breaking

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 S 2 ← S 1 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.

Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor‐Acceptor Dyes

“…Until now this process was never observed in non-polar environments, even with molecules characterised by torsional disorder in the electronic ground state, [38] unless a photochemical transformation with substantial structural changes takes place. [39] This suggests that the instantaneous asymmetric arrangement of the solvent molecules around the dye, i. e. the asymmetry of the solvent field, and the ensuing solvent relaxation play crucial roles in ES-SB, as confirmed by recent theoretical modelling of this phenomenon. [40][41][42] Depending on the dye, non-dipolar but quadrupolar solvents were found to generate sufficient asymmetry in the local field to induce ES-SB.…”

“…A similar broad background signal measured earlier with other multipolar dyes was attributed to the S 2 S 1 transition of the delocalised excitonic state. [39,64] This is thus a characteristic electronic signature of the symmetric quadrupolar excited state. The energy of this transition coincides with the energy difference of the OPA and TPA band maxima reported in Ref.…”

Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor–acceptor branch length

“…Structural disorder or photochemical transformation were shown to be insufficient for ES-SB to take place in non-polar environments. 39,40 .…”

Porous shape-persistent rylene imine cages with tunable optoelectronic properties and delayed fluorescence

“…Both the experiment and theory show that J Coul does not vary correctly with the solvent polarity ( Table 2 ) and, therefore, it is inconsistent with the excited state behavior that we observe for samples of 1c in different solvents. In addition, rapid excited state symmetry-breaking is induced by solvent field (polar medium) or structural (non-polar medium) fluctuations when the interchromophore coupling is very small 27 and, therefore, the exciton must quickly localize on a single PDI unit in 1c . Nevertheless, the presence of this small coupling is sufficient to reduce the k rad and Φ em in 1c (toluene, Table 2 ) when compared to 3c in the three studied solvents (Tables S7 and S8 † ).…”