Internal Heavy-Atom Effect on Visible-Light-Induced Cyclization Reaction in Diarylethene–Perylenebisimide Dyads

All-visible-light
switchable diarylethene–perylenebisimide
(DAE-PBI) dyads having bromine heavy atoms in the molecule were designed
and synthesized. Very recently, we found a unique visible-light-induced
cyclization reaction in a DAE-PBI dyad. The dyad exhibited reversible
cyclization and cycloreversion reactions upon alternate irradiation
with green (500–550 nm) and red (>600 nm) light. From the
experimental
results, it was suggested that the triplet state of DAE unit was generated
via multiplicity conversion based on intramolecular energy transfer
from the singlet excited state of PBI unit and that the cyclization
reaction of DAE unit proceeded from the triplet state. In addition,
it was revealed that the reactivity remarkably increased in a solvent
containing heavy atoms such as carbon tetrachloride and iodoethane
(i.e., external heavy-atom effect). Based on such results, in this
study, we attempted to design and synthesize novel DAE-PBI dyads introducing
bromine heavy atoms at different positions in the molecule. The synthesized
dyads exhibited higher quantum yields of photocyclization reaction
under visible-light irradiation even in a heavy-atom-free solvent
compared to the previous dyad having no heavy atoms. The magnitude
of enhancement well correlated to the contribution ratio of atomic
orbital of bromine to the molecular orbital in LUMOs. These results
indicated that the internal heavy atom effectively contributed to
the visible-light-induced cyclization reaction in DAE-PBI dyads. Such
an internal heavy-atom effect will pave the way for new molecular
design to develop all-visible-light-activatable molecular switches.