Photophysical Heavy-Atom Effect in Iodinated Metallocorroles: Spin–Orbit Coupling and Density of States

Abstract

Excited-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.

Phosphorus corrole complexes: from property tuning to applications in photocatalysis and triplet–triplet annihilation upconversion

“…This observation can be attributed to the enhanced ISC in the iodinated corroles, i.e. the heavy atom effect 51–53. The photophysical parameters for the phosphorus corroles in toluene are compiled in Table 1.…”

Photophysical properties and singlet oxygen generation ofmeso-iodinated free-base corroles

“…Such a CT state accelerates ISC from nanosecond to picosecond. 22 From our study, the lack of red-shi of the emission maximum of Q band may indicate that the change of the energy of electronic states should be negligible. Therefore, this CT state may not exist in the present meso-iodination corroles, because of the very tiny conjugation between the corrole and the meso-phenyl p systems.…”

“…Similarly, Pomarico et al studied the impact of b-iodination on the excitedstate dynamics and electronic structures of Al and Ga corrole complexes. 22 They regarded s  as 1/k ISC , and found k ISC drastically increased from 2.3 Â 10 8 s À1 for 0-Ga (no iodine) to 6.7 Â 10 10 s À1 for 4-Ga (4 iodines). Both of these two studies show a two-fold order of magnitude increase of k ISC by b-halogenation.…”

“…On the other hand, the timescale of the Soret / Q conversion accelerates upon b-iodination from 320 fs (0-Ga) to 180 fs (3-Ga) and 70 fs (4-Ga). 22 It should be attributed to the manifold of CT excited states between Soret and Q states present in the iodinated complexes. In the present system, the meso-iodine atoms may exhibit neither enough withdrawal effect nor CT state to accelerate the IC timescale.…”

“…Iodination could accelerate all of the involved processes, including the internal conversion from the Soret to the Q states (IC), vibrational cooling (VC) and ISC. 22 Although much work has focused on corroles substituted with halogens at the beta-pyrrole positions, the study of heavy halide atoms effect at meso-phenyl rings is limited. Our group synthesized a series of halogenated free-base corroles, including two C 6 F 5 and one 2-hydroxy-5-halogen-phenyl meso substituents (F 10 -OH-F, F 10 -OH-Cl, F 10 -OH-I).…”