Diers James R, Kirmaier Christine, Taniguchi Masahiko, Lindsey Jonathan S, Bocian David F, Holten Dewey
Department of Chemistry, University of California, Riverside, CA 92521-0403, USA.
Department of Chemistry, Washington University, St. Louis, MO 63130-4889, USA.
Phys Chem Chem Phys. 2021 Sep 15;23(35):19130-19140. doi: 10.1039/d1cp01943k.
Tetrapyrrole macrocycles serve a multitude of roles in biological systems, including oxygen transport by heme and light harvesting and charge separation by chlorophylls and bacteriochlorophylls. Synthetic tetrapyrroles are utilized in diverse applications ranging from solar-energy conversion to photomedicine. Nevertheless, students beginning tetrapyrrole research, as well as established practitioners, are often puzzled when comparing properties of related tetrapyrroles. Questions arise as to why optical spectra of two tetrapyrroles often shift in wavelength/energy in a direction opposite to that predicted by common chemical intuition based on the size of a π-electron system. Gouterman's four-orbital model provides a framework for understanding these optical properties. Similarly, it can be puzzling as to why the oxidation potentials differ significantly when comparing two related tetrapyrroles, yet the reduction potentials change very little or shift in the opposite direction. In order to understand these redox properties, it must be recognized that structural/electronic alterations affect the four frontier molecular orbitals (HOMO, LUMO, HOMO-1 and LUMO+1) unequally and in many cases the LUMO+1, and not the LUMO, may track the HOMO in energy. This perspective presents a fundamental framework concerning tetrapyrrole electronic properties that should provide a foundation for rational molecular design in tetrapyrrole science.
四吡咯大环在生物系统中发挥着多种作用，包括血红素的氧运输以及叶绿素和细菌叶绿素的光捕获与电荷分离。合成四吡咯被用于从太阳能转换到光医学等各种应用中。然而，刚开始进行四吡咯研究的学生以及经验丰富的从业者在比较相关四吡咯的性质时，常常感到困惑。问题在于，为何两种四吡咯的光谱在波长/能量上的移动方向，往往与基于π电子系统大小的常见化学直觉所预测的方向相反。古特曼的四轨道模型为理解这些光学性质提供了一个框架。同样，在比较两种相关四吡咯时，氧化电位为何有显著差异，而还原电位变化很小或朝相反方向移动，这也可能令人困惑。为了理解这些氧化还原性质，必须认识到结构/电子变化对四个前沿分子轨道（最高占据分子轨道、最低未占分子轨道、最高占据分子轨道-1和最低未占分子轨道+1）的影响是不平等的，而且在许多情况下，最低未占分子轨道+1而非最低未占分子轨道，可能在能量上与最高占据分子轨道相关。这一观点提出了一个关于四吡咯电子性质的基本框架，应为四吡咯科学中的合理分子设计奠定基础。
