This
work demonstrates how push–pull substitution can induce spectral tuning toward the
visible range and improve the photoisomerization efficiency of azobenzene-based
photoswitches, making them good candidates for technological and biological
applications. The red-shifted bright ππ* state (S2) behaves like the lower and more productive dark nπ*
(S1) state because less potential energy along the planar
bending mode is available to reach higher energy unproductive nπ*/S0 crossing regions, which are responsible for the lower quantum
yield of the parent compound. The stabilization of the bright ππ*
state and the consequent increase in isomerization efficiency may
be regulated via the strength of push–pull substituents. Finally, the torsional
mechanism is recognized here as the unique productive route because
structures with bending values attributable to the inversion mechanism
were never detected, out of the 280 ππ* time-dependent
density functional theory (RASPT2-validated) dynamics simulations.
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