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Ultrafast photochemistry of free-base porphyrin: a theoretical investigation of B -\textgreater Q internal conversion mediated by dark states

Abstract : We examine the mechanism of ultrafast internal conversion between the B band (Soret band) and the Q band in porphine (H2P), the prototypical free-base porphyrin, using electronic structure studies and on-the-fly surface-hopping nonadiabatic dynamics. Our study highlights the crucial role of dark states within the N band which are found to mediate B/Q state transfer, necessitating a treatment beyond Gouterman's classic four-orbital model. The sequential B -\textgreater N -\textgreater Q pathway dominates largely over the direct B -\textgreater Q pathway which is found to be energetically unfavorable. Potential energy surface cuts and conical intersections between excited states are determined by TDDFT and validated by CASSCF/CASPT2 and XMCQDPT2 calculations. Both the static analysis and on-the-fly surface-hopping calculations suggest a pathway which involves minor structural deformations via in-plane vibrations. The B -\textgreater N conversion is a bar rierless adiabatic process occurring within similar to 20 fs, while the subsequent N -\textgreater Q conversion occurs via a conical intersection within similar to 100 fs, in agreement with time-resolved experiments for porphine and related free base porphyrins. Furthermore, evidence for both sequential and direct transfer to the Q(x) and Q(y) states is obtained.
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https://hal-amu.archives-ouvertes.fr/hal-02091908
Contributor : Didier Gigmes <>
Submitted on : Saturday, April 6, 2019 - 5:17:52 PM
Last modification on : Thursday, December 5, 2019 - 10:40:03 AM

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Konstantin Falahati, Carsten Hamerla, Miquel Huix-Rotllant, Irene Burghardt. Ultrafast photochemistry of free-base porphyrin: a theoretical investigation of B -\textgreater Q internal conversion mediated by dark states. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2018, 20 (18), pp.12483--12492. ⟨10.1039/c8cp00657a⟩. ⟨hal-02091908⟩

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