It Takes More Than an Imine: The Role of the Central Atom on the Electron-Accepting Ability of Benzotriazole and Benzothiadiazole Oligomers

We report on the comparison of the electronic and photophysical
properties of a series of related donor–acceptor–donor
oligomers incorporating the previously known 2H-benzo­[d]­[1,2,3]­triazole (BTz) moiety as the acceptor
and the recently reported BTzTD acceptor, a hybrid of BTz and 2,1,3-benzothiadiazole (BTD). Although
often implied in the polymer literature that BTz has
good acceptor character, we show that this moiety is best described
as a weak acceptor. We present electrochemical, computational, and
photophysical evidence supporting our assertion that BTzTD is a strong electron acceptor while maintaining the alkylation ability
of the BTz moiety. Our results show that the identity
of the central atom (N or S) in the benzo-fused heterocyclic ring
plays an important role in both the electron-accepting and the electron-donating
ability of acceptor moieties with sulfur imparting a greater electron-accepting
ability and nitrogen affording greater electron-donating character.
We report on the X-ray crystal structure of a BTzTD trimer,
which exhibits greater local aromatic character in the region of the
triazole ring and contains an electron-deficient sulfur that imparts
strong electron-accepting ability. Additionally, we examine the transient
absorption spectra of BTzTD and BTz oligomers
and report that the BTz core promotes efficient intersystem
crossing to the triplet state, while the presence of the thiadiazole
moiety in BTzTD leads to a negligible triplet yield.
Additionally, while BTz does not function as a good acceptor,
oligomers containing this moiety do function as excellent sensitizers
for the generation of singlet oxygen.