Imidazophenothiazine-Based Thermally Activated Delayed Fluorescence Materials with Ultra-Long-Lived Excited States for Energy Transfer Photocatalysis

Abstract

Triplet-triplet energy transfer (EnT) is a powerful activation pathway in photocatalysis that unlocks new organic transformations and improves the sustainability of organic synthesis.Many current examples, however, still rely on platinum-group metal complexes as photosensitizers, with associated high costs and environmental impacts. Photosensitizers that exhibit thermally activated delayed fluorescence (TADF) are attractive fully organic alternatives in EnT photocatalysis. However, TADF photocatalysts incorporating heavy atoms remain rare, despite their utility in inducing efficient spin-orbit-coupling, intersystem-crossing, and consequently a high triplet population. Here, we describe the synthesis of imidazophenothiazine (IPTZ), a sulfur-containing heterocycle with a locked planar structure and a shallow LUMO level. This acceptor is used to prepare seven TADF-active photocatalysts with triplet energies up to 63.9 kcal mol −1 . We show that sulfur incorporation improves spin-orbit coupling and increases triplet lifetimes up to 3.64 ms, while also allowing for tuning of photophysical properties via oxidation at the sulfur atom. These IPTZ materials are applied as photocatalysts in five seminal EnT reactions: [2 + 2] cycloaddition, the disulfide-ene reaction, and Ni-mediated C−O and C−N cross-coupling to afford etherification, esterification, and amination products, outcompeting the industry-standard TADF photocatalyst 2CzPN in four of the five studied scenarios. Detailed photophysical and theoretical studies are used to understand structure−activity relationships and to demonstrate the key role of the heavy atom effect in the design of TADF materials with superior photocatalytic performance.

A Paradigm Shift in Catalysis: Electro- and Photomediated Nickel-Catalyzed Cross-Coupling Reactions

“…That collaboration led to the design of SACR-IPTZ, an imidazophenothiazine (IPTZ)-based, fully organic material with a high triplet energy and exceptionally long-lived excited triplet state (τ = 367 μs). 45 SACR-IPTZ was designed to use structural constraint and the heavy atom effect (sulfur atom) to improve excited-state lifetimes and spin−orbit coupling for utility in EnT photocatalysis. SACR-IPTZ efficiently promoted Ni-catalyzed C−O (etherification and esterification), C−N cross-couplings (Figure 8D), and other chemical reactions.…”

“…We recognized that photosensitizers exhibiting thermally activated delayed fluorescence (TADF) could be an attractive fully organic alternative in EnT photocatalysis and collaborated with Professor Zachary Hudson to explore TADF materials in Ni-catalyzed C–heteroatom cross-couplings. That collaboration led to the design of SACR-IPTZ , an imidazophenothiazine (IPTZ)-based, fully organic material with a high triplet energy and exceptionally long-lived excited triplet state (τ = 367 μs) …”

Role of Bridging Groups in Regulating the Luminescence and Charge Transfer Properties of Thermally Activated Delayed Fluorescence Molecules: A Theoretical Perspective

“…Thermally activated delayed fluorescence (TADF) materials have recently garnered extensive attention as promising luminophores for applications in organic light-emitting diodes (OLEDs), biomedical fields, and photocatalysis. − This is attributed to their remarkable features, including bright fluorescence, being noble-metal-free, and exhibiting low cytotoxicity. Typically, TADF emitters consist of donor (D) and acceptor (A) units arranged in a twisted configuration.…”

Organic Photothermal Materials Obtained Using Thermally Activated Delayed Fluorescence Design Principles

“…This property has made TADF materials useful for efficiently harvesting excitons in organic light-emitting diodes, [18,19] for use in time-resolved fluorescence imaging, [20,21] and as materials for photoredox and energy transfer photocatalysis. [22][23][24] Due to their fully organic structures, TADF materials have been widely explored as biocompatible imaging and phototherapeutic agents. [25] In TADF materials design, there is a well-known trade-off between increasing the donor-acceptor dihedral angle (to decrease ΔE ST ) and decreasing this dihedral angle (to improve the oscillator strength of the HOMO-LUMO transition, and thus the photoluminescent quantum yield, or PLQY).…”