Mandal Amit Kumar, Taniguchi Masahiko, Diers James R, Niedzwiedzki Dariusz M, Kirmaier Christine, Lindsey Jonathan S, Bocian David F, Holten Dewey
Department of Chemistry, Washington University , St. Louis, Missouri 63130-4889, United States.
Department of Chemistry, North Carolina State University , Raleigh, North Carolina 27695-8204, United States.
J Phys Chem A. 2016 Dec 15;120(49):9719-9731. doi: 10.1021/acs.jpca.6b09483. Epub 2016 Dec 6.
Six free base porphyrins bearing 0-4 meso substituents have been examined by steady-state and time-resolved absorption and fluorescence spectroscopy in both toluene and N,N-dimethylformamide (DMF). The lifetime of the lowest singlet excited state (S) decreases with an increase in the number of meso-phenyl groups; the values in toluene are HP-0 (15.5 ns) > HP-1 (14.9 ns) > HP-2c (14.4 ns) > HP-2t (13.8 ns) ∼ HP-3 (13.8 ns) > HP-4 (12.8 ns), where "HP" refers to the core free base porphyrin, the numerical suffix indicates the number of meso-phenyl groups, and "c" and "t" refer to cis and trans, respectively. The opposite trend is found for the fluorescence quantum yield; the values in toluene are HP-0 (0.049) < HP-1 (0.063) ∼ HP-2c (0.063) < HP-2t (0.071) < HP-3 (0.073) < HP-4 (0.090). Similar trends occur in DMF. All radiative and nonradiative (internal conversion and intersystem crossing) rate constants for S decay increase with the increasing number of meso-phenyl groups. The increase in the rate constant for fluorescence parallels an increase in oscillator strength of the S → S absorption manifold. The trend is reproduced by time-dependent density functional theory calculations. The calculations within the context of the four-orbital model reveal that the enhanced S ↔ S radiative probabilities derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital, which underpins a parallel bathochromic shift in the S → S absorption wavelength. Polarizations of the S and S excited states with respect to molecular structural features (e.g., the central proton axis) are analyzed in the context of historical conventions for porphyrins versus chlorins and bacteriochlorins, where some ambiguity exists, including for porphine, one of the simplest tetrapyrroles. Collectively, the study affords fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins that should aid their continued widespread use as benchmarks for tetrapyrrole-based architectures in chemical, solar-energy, and life-sciences research.
通过稳态和时间分辨吸收光谱以及荧光光谱，在甲苯和N,N-二甲基甲酰胺（DMF）中对六种带有0 - 4个中位取代基的游离碱卟啉进行了研究。最低单重激发态（S）的寿命随着中位苯基数量的增加而缩短；在甲苯中的数值为HP - 0（15.5纳秒）> HP - 1（14.9纳秒）> HP - 2c（14.4纳秒）> HP - 2t（13.8纳秒） ∼ HP - 3（13.8纳秒）> HP - 4（12.8纳秒），其中“HP”指核心游离碱卟啉，数字后缀表示中位苯基的数量，“c”和“t”分别指顺式和反式。荧光量子产率呈现相反的趋势；在甲苯中的数值为HP - 0（0.049）< HP - 1（0.063） ∼ HP - 2c（0.063）< HP - 2t（0.071）< HP - 3（0.073）< HP - 4（0.090）。在DMF中也出现类似趋势。S态衰变的所有辐射和非辐射（内转换和系间窜越）速率常数都随着中位苯基数量的增加而增大。荧光速率常数的增加与S → S吸收多重态的振子强度增加平行。这种趋势通过含时密度泛函理论计算得以重现。在四轨道模型的背景下进行的计算表明，增强的S ↔ S辐射概率源于中位苯基优先提高最高占据分子轨道能量的效应，这导致S → S吸收波长出现平行的红移。在卟啉与二氢卟吩和细菌二氢卟吩的历史惯例背景下，分析了S态和S*态相对于分子结构特征（例如中心质子轴）的极化情况，其中存在一些模糊性，包括对于最简单的四吡咯之一的卟吩。总体而言，该研究为中位苯基卟啉的光物理性质和电子结构提供了基本见解，这将有助于它们在化学、太阳能和生命科学研究中作为基于四吡咯结构的基准继续广泛应用。
