Early Events
in the Nonadiabatic Relaxation Dynamics
of 4‑(<i>N</i>,<i>N</i>‑Dimethylamino)benzonitrile
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Abstract
4-(<i>N</i>,<i>N</i>-Dimethylamino)benzonitrile
(DMABN) is the archetypal system for dual fluorescence. Several past
studies, both experimental and theoretical, have examined the mechanism
of its relaxation in the gas phase following photoexcitation to the
S<sub>2</sub> state, without converging to a single description. In
this contribution, we report first-principles simulations of the early
events involved in this process performed using the nonadiabatic trajectory
surface hopping (TSH) approach in combination with the ADC(2) electronic
structure method. ADC(2) is verified to reproduce the ground- and
excited-state structures of DMABN in reasonably close agreement with
previous theoretical benchmarks. The TSH simulations predict that
internal conversion from the S<sub>2</sub> state to the S<sub>1</sub> takes place as early as 8.5 fs, on average, after the initial photoexcitation,
and with no significant torsion of the dimethylamino group relative
to the aromatic ring. As evidenced by supporting EOM-CCSD calculations,
the population transfer from S<sub>2</sub> to S<sub>1</sub> can be
attributed to the skeletal deformation modes of the aromatic ring
and the stretching of the ring-dimethylamino nitrogen bond. The non-
or slightly twisted locally excited structure is the predominant product
of the internal conversion, and the twisted intramolecular charge
transfer structure is formed through equilibration with the locally
excited structure with no change of adiabatic state. These findings
point toward a new interpretation of data from previous time-resolved
experiments