Energy Cascades, Excited State Dynamics, and Photochemistry in Cob(III)alamins and Ferric Porphyrins

• Published in: Accounts of chemical research 2015-03, Vol.48 (3), p.860-867
• Main Authors: Rury, Aaron S, Wiley, Theodore E, Sension, Roseanne J
• Format: Article
• Language: English
• Subjects: Bonding; Cascades; Cobalt - chemistry; Dynamics; Electronics; Excitation; Ferric Compounds - chemistry; Ligands; Metalloporphyrins - chemistry; Molecular Structure; Organometallic Compounds - chemistry; Photochemical Processes; Photochemistry; Porphyrins; Quantum Theory
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/ar5004016

CONSPECTUS Porphyrins and the related chlorins and corrins contain a cyclic tetrapyrrole with the ability to coordinate an active metal center and to perform a variety of functions exploiting the oxidation state, reactivity, and axial ligation of the metal center. These compounds are used in optically activated applications ranging from light harvesting and energy conversion to medical therapeutics and photodynamic therapy to molecular electronics, spintronics, optoelectronic thin films, and optomagnetics. Cobalt containing corrin rings extend the range of applications through photolytic cleavage of a unique axial carbon–cobalt bond, permitting spatiotemporal control of drug delivery. The photochemistry and photophysics of cyclic tetrapyrroles are controlled by electronic relaxation dynamics including internal conversion and intersystem crossing. Typically the electronic excitation cascades through ring centered ππ* states, ligand to metal charge transfer (LMCT) states, metal to ligand charge transfer (MLCT) states, and metal centered states. Ultrafast transient absorption spectroscopy provides a powerful tool for the investigation of the electronic state dynamics in metal containing tetrapyrroles. The UV–visible spectrum is sensitive to the oxidation state, electronic configuration, spin state, and axial ligation of the central metal atom. Ultrashort broadband white light probes spanning the range from 270 to 800 nm, combined with tunable excitation pulses, permit the detailed unravelling of the time scales involved in the electronic energy cascade. State-of-the-art theoretical calculations provide additional insight required for precise assignment of the states. In this Account, we focus on recent ultrafast transient absorption studies of ferric porphyrins and corrin containing cob­(III)­alamins elucidating the electronic states responsible for ultrafast energy cascades, excited state dynamics, and the resulting photoreactivity or photostability of these compounds. Iron tetraphenyl porphyrin chloride (Fe(III)TPPCl) exhibits picosecond decay to a metal centered d → d* 4T state. This state decays on a ca. 16 ps time scale in room temperature solution but persists for much longer in a cryogenic glass. The photoreactivity of the 4T state may lead to novel future applications for these compounds. In contrast, the nonplanar cob­(III)­alamins contain two axial ligands to the central cobalt atom. The upper axial ligand can be an alkyl group as in the two biologi