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Ultrafast photochemistry of free-base porphyrin: A theoretical investigation of B → 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 (H 2 P), 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 → N → Q pathway dominates largely over the direct B → 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 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 → N conversion is a barrierless adiabatic process occurring within ∼20 fs, while the subsequent N → Q conversion occurs via a conical intersection within ∼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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Konstantin Falahati, Carsten Hamerla, Miquel Huix-Rotllant, Irene Burghardt. Ultrafast photochemistry of free-base porphyrin: A theoretical investigation of B → 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-02079633⟩

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