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.

中文翻译：

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碘化金属茂中的光物理重原子效应：自旋轨道耦合和态密度。

研究了Al和Ga络合物的激发态动力学和电子结构与β-吡咯碘取代基数量的关系。使用光谱飞秒解析的宽带荧光上转换，我们确定了Soret荧光衰减，Q波段荧光随之上升和随后衰减以及伴随的振动弛豫的动力学。发现碘化可以加速所有涉及的过程。碘化后，从Soret到Q状态的内部转换的时间常数从320–540 fs减小到70–185 fs。对于无碘和碘化的络合物，振动松弛的寿命分别约为15和0.36-1.4 ps。跨系统到达最低三重态的速度从纳秒加速到15-24 ps，最多可加速200倍；其速率与碘在π-体系中的参与度和自旋-轨道耦合（SOC）强度有关。用显式SOC进行的TDDFT计算表明，碘引入了一系列低价单线态和三线态碘→腐蚀电荷转移（CT）状态。这些状态通过（i）为Soret→Q内部转换和冷却提供弛豫级联，以及（ii）开辟新的SOC途径，从而将CT三重态特征混合到两个Q单重态激发态中，从而影响光物理。此外，随着碘对前沿腐蚀π轨道的增加，高Q单重态与Soret三重态之间的SOC增强。我们的观察发现，发色团外围的碘化会通过改变电子结构，自旋-轨道耦合来影响整个光循环，