1-Hydroxy-2-acetonaphthone
(HAN) has been extensively studied both
experimentally and computationally to ascertain the existence of the
excited-state proton transfer process. However, the process of full
photocycle including the nonradiative relaxation pathways is yet to
be proposed. Therefore, in the present study, we aim at providing
a comprehensive picture of the excited-state processes in HAN including
the proton transfer and relaxation processes through electronic structure
calculations at second-order algebraic diagrammatic construction (ADC(2))
and complete active space second-order perturbation theory (CASPT2)//complete
active space self-consistent field (CASSCF) and dynamics simulations
at ADC(2) levels. Our studies show that the proton transfer process
in the S1 state is barrierless and produces a stable keto
form, which is in accordance with previous experimental and computational
studies. Adiabatic dynamics simulations at the ADC(2) level confirmed
the ultrafast process with an average proton transfer time of 43 fs.
The resultant keto conformer then undergoes torsional rotation, leading
to a conical intersection that mediates the internal conversion process
to the ground state. Our dynamics simulation predicted that this deactivation
process occurs at a time scale beyond 600 fs of simulation time. We
also explored nonradiative relaxation from the enol Franck–Condon
region, and this process was found to be improbable from the static
point of view at both the ADC(2) and CASPT2 levels of theory due to
a high energy barrier along the torsional coordinate.
Molecular-level
understanding of photochemistry in simple
vinylene-linked
systems such as ethylene and stilbene has been a major area of research.
However, the effect of replacing the two benzene rings by five-membered
heterocyclic rings, thiophene and pyrrole, is yet to be reported.
In the present theoretical study, our aim is to illustrate photoinduced
processes in a vinylene-linked thiophene–pyrrole system. Computational
studies are carried out at the RI-MP2/RI-ADC(2)/cc-pVTZ level to explore
different isomerization pathways. Minimum-energy conical intersection
(MECI) structures are categorized into two types: closed-ring and
twisted-pyramidalized structures. Relaxation through the former MECIs
is accessible only from the cis isomers. However,
the latter MECIs are inaccessible due to high energy barriers along
the linear interpolation in internal coordinate paths.
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