A Direct S₀→T_n Transition in the Photoreaction of Heavy‐Atom‐Containing Molecules

Abstract

According to the Grotthuss–Draper law, light must be absorbed by a substrate to initiate a photoreaction. There have been several reports, however, on the promotion of photoreactions using hypervalent iodine during irradiation with light from a non‐absorbing region. This contradiction gave rise to a mystery regarding photoreactions involving hypervalent iodine. We demonstrated that the photoactivation of hypervalent iodine with light from the apparently non‐absorbing region proceeds via a direct S0→Tn transition, which has been considered a forbidden process. Spectroscopic, computational, and synthetic experimental results support this conclusion. Moreover, the photoactivation mode could be extended to monovalent iodine and bromine, as well as bismuth(III)‐containing molecules, providing new possibilities for studying photoreactions that involve heavy‐atom‐containing molecules.

Direct Photoexcitation of Ethynylbenziodoxolones: An Alternative to Photocatalysis for Alkynylation Reactions**

“…Herein, we report the serendipitous discovery of a different alkynylation approach via the visible light photoexcitation of aryl‐EBX reagents, alleviating the need for a fine‐tuned photocatalyst (Scheme 1 C). Visible light irradiation can promote the excitation of a variety of hypervalent iodine reagents through spin forbidden transitions [8] . Nevertheless, to the best of our knowledge, this activation mode has never been reported for EBXs.…”

Development of a Platform for Near-Infrared Photoredox Catalysis

“…S 0 → T 1 excitation has been reported for Ru(II), 7 Ir(III), 8 and Fe(II), 9 but with inconsequential extinction coefficients for photoredox catalysis. While this concept has been applied to excitation of reagents, 10 , 11 identifying new photocatalysts with strong SOC will allow direct access to long-lived redox-active T 1 states via S 0 → T 1 excitation and minimize ISC-specific energy loss. Fundamentally, this creates a two-state system paradigm for transition metal-based photoredox catalysis by direct S 0 → T 1 excitation, obviating the limitations associated with accessing the T 1 state by initial S 0 → S 1 excitation.…”

Visible‐Light‐Driven Fluoroalkylation of Tryptophan Residues in Peptides

“…A fraction of the radicals can trigger the fluoroalkylation reaction. Nakajima and co‐workers have recently discussed a possibility of direct S 0 →T n visible light excitation of hypervalent iodine‐containing reagents that induces formation of fluorinated radicals [41] . However, the calculated energy of the lowest excited triplet state of 1 a is significantly lower than that of the used light and therefore such process is unlikely in our system.…”