Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 20, Issue 18, Pages 12483-12492Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c8cp00657a
Keywords
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Funding
- Deutsche Forschungsgemeinschaft [RTG 1986]
- project Equip@Meso of the program Investissements d'Avenir'' [ANR-10-EQPX-29-01]
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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 similar to 20 fs, while the subsequent N -> 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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