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Photophysical Heavy-Atom Effect in Iodinated Metallocorroles: Spin–Orbit Coupling and Density of States
journal contribution
posted on 2018-08-24, 00:00 authored by Enrico Pomarico, Petr Pospíšil, Marine E. F. Bouduban, Jenya Vestfrid, Zeev Gross, Stanislav Záliš, Majed Chergui, Antonín VlčekExcited-state
dynamics and electronic structures of Al and Ga corrole
complexes were studied as a function of the number of β-pyrrole
iodine substituents. Using spectrally broad-band femtosecond-resolved
fluorescence upconversion, we determined the kinetics of the Soret
fluorescence decay, the concomitant rise and subsequent decay of the
Q-band fluorescence, as well as of the accompanying vibrational relaxation.
Iodination was found to accelerate all involved processes. The time
constant of the internal conversion from the Soret to the Q states
decreases from 320–540 to 70–185 fs upon iodination.
Vibrational relaxation then occurs with about 15 and 0.36–1.4
ps lifetime for iodine-free and iodinated complexes, respectively.
Intersystem crossing to the lowest triplet is accelerated up to 200
times from nanoseconds to 15–24 ps; its rate correlates with
the iodine p(π) participation in the corrole π-system
and the spin–orbit coupling (SOC) strength. TDDFT calculations
with explicit SOC show that iodination introduces a manifold of low-lying
singlet and triplet iodine → corrole charge-transfer (CT) states.
These states affect the photophysics by (i) providing a relaxation
cascade for the Soret → Q internal conversion and cooling and
(ii) opening new SOC pathways whereby CT triplet character is admixed
into both Q singlet excited states. In addition, SOC between the higher
Q singlet and the Soret triplet is enhanced as the iodine participation
in frontier corrole π-orbitals increases. Our observations that
iodination of the chromophore periphery affects the whole photocycle
by changing the electronic structure, spin–orbit coupling,
and the density of states rationalize the “heavy-atom effect”
and have implications for controlling excited-state dynamics in a
range of triplet photosensitizers.
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Photophysical Heavy-Atom EffectCT triplet characteriodinationparticipationSoret fluorescence decayQ singletrelaxationcorrole π- systemβ- pyrrole iodine substituentsbroad-band femtosecond-resolved fluorescence upconversionGa corrole complexesfrontier corrole π- orbitals increasesSOCTDDFTStates Excited-state dynamicsQ states decreases
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