First‐Row d ⁶ Metal Complex Enables Photon Upconversion and Initiates Blue Light‐Dependent Polymerization with Red Light

Abstract

Photosensitizers for sensitized triplet‐triplet annihilation upconversion (sTTA‐UC) often rely on precious heavy metals, whereas coordination complexes based on abundant first‐row transition metals are less common. This is mainly because long‐lived triplet excited states are more difficult to obtain for 3d metals, particularly when the d‐subshell is only partially filled. Here, we report the first example of sTTA‐UC based on a 3d ⁶ metal photosensitizer yielding an upconversion performance competitive with precious metal‐based analogues. Using a newly developed Cr ⁰ photosensitizer featuring equally good photophysical properties as an Os ^II benchmark complex in combination with an acetylene‐decorated anthracene annihilator, red‐to‐blue upconversion is achievable. The upconversion efficiency under optimized conditions is 1.8 %, and the excitation power density threshold to reach the strong annihilation limit is 5.9 W/cm ². These performance factors, along with high photostability, permit the initiation of acrylamide polymerization by red light, based on radiative energy transfer between delayed annihilator fluorescence and a blue light absorbing photo‐initiator. Our study provides the proof‐of‐concept for photon upconversion with elusive first‐row analogues of widely employed precious d ⁶ metal photosensitizers, and for their application in photochemical reactions triggered by excitation wavelengths close to near‐infrared.