Intramolecular Cycloadditions of Photogenerated Azaxylylenes: An Experimental and Theoretical Study

The
mechanism of intramolecular cycloadditions of azaxylylenes
photogenerated via excited-state intramolecular proton transfer (ESIPT)
in aromatic o-amido ketones and aldehydes bearing
unsaturated functionalities was studied experimentally and computationally.
In time-correlated single-photon counting experiments, no relation
was found between lifetimes of singlet species and the nature of the
amide pendant, either unsaturated furanpropanamide, capable of photocyclization,
or the acetamide control. Steady-state emission for amido-tetralone
derivatives showed comparable dual emission bands, but bromo substitution
decreased the intensity of the ESIPT band. The most reactive derivatives
of amidobenzaldehydes were virtually lacking the ESIPT band. The quantum
yield of cycloaddition is decreased in the presence of triplet quenchers,
O2 or trans-piperylene, and improved with
heavy atom substitution in the aromatic ring, providing further evidence
for the initial mechanistic hypothesis in which the fast singlet-state
ESIPT is accompanied by the ISC in the tautomer (azaxylylene), which
undergoes stepwise addition to the tethered unsaturated pendants.