Understanding the Structural Modulations in Twisted Donor–Acceptor–Donor (D-A-D) Systems for Boosting Type I Photosensitizing Photocatalytic Activity

Supramolecular assemblies based on the twisted donor–acceptor–donor
(D-A-D) building block Qx-Ind have been developed, which
interestingly, due to the balanced angle of twist (38.28°), high
intermolecular charge transfer, and crystallization induced emission
(CIE) characteristics, exhibit high molar absorptivity and a long-lived
“lighted” excited state at the supramolecular level.
The validity of the design concept was examined by preparing CIE active
D-A-D system Qx-Indaz (weak donor, low angle of twist:
35.45°), in which, due to the insertion of an additional binding
site for noncovalent interactions, a drastic change in the photophysical
behavior is observed. The combined spectroscopic studies of all the
compounds unveil the strong impact of modulation of the angle of twist
and intermolecular charge transfer upon photophysical behavior in
the aggregated state. Due to the favorable photophysical behavior,
the supramolecular assemblies of Qx-Ind exhibit high
type I photosensitizing activity in comparison to Qx-Indaz. The superior type I photosensitizing activity of Qx-Ind assemblies is manifested in their ability to efficiently catalyze
the aerobic oxidative synthesis of quinazolin-4(3H)-ones (via type I ROS) from 2-aminobenzamide and aromatic aldehydes
in the absence of additional additives (base/oxidant). Unlike photocatalytic
nanoassemblies reported in the literature, due to the CIE characteristics, Qx-Ind does not require preliminary preparation and could
be directly introduced in the solid state to reaction media. Thus,
the present work demonstrates a simple strategy of upgrading type
I photosensitizing activity by improving the ground/excited state
behavior of a twisted D-A-D system through modulation of the angle
of twist and charge transfer characteristics.