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Issue 18, 2018
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Ultrafast photochemistry of free-base porphyrin: a theoretical investigation of B → Q internal conversion mediated by dark states

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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 → 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 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 → 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 Qx and Qy states is obtained.

Graphical abstract: Ultrafast photochemistry of free-base porphyrin: a theoretical investigation of B → Q internal conversion mediated by dark states

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Publication details

The article was received on 30 Jan 2018, accepted on 19 Apr 2018 and first published on 19 Apr 2018


Article type: Paper
DOI: 10.1039/C8CP00657A
Citation: Phys. Chem. Chem. Phys., 2018,20, 12483-12492
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    Ultrafast photochemistry of free-base porphyrin: a theoretical investigation of B → Q internal conversion mediated by dark states

    K. Falahati, C. Hamerla, M. Huix-Rotllant and I. Burghardt, Phys. Chem. Chem. Phys., 2018, 20, 12483
    DOI: 10.1039/C8CP00657A

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