Abuhadba Sara, Fuqua Charlotte, Maltese Anthony, Schwinn Caroline, Lin Neo, Chen Angela, Martzloff Rilee, Esipova Tatiana V, Mani Tomoyasu
Department of Chemistry and Biochemistry, Loyola University Chicago, Chicago, Illinois 60660, United States.
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, United States.
JACS Au. 2024 Nov 18;4(12):4892-4898. doi: 10.1021/jacsau.4c00877. eCollection 2024 Dec 23.
Red-light absorbing photoredox catalysts offer potential advantages for large-scale reactions, expanding the range of usable substrates and facilitating bio-orthogonal applications. While many red-light absorbing/emitting fluorophores have been developed recently, functional red-light absorbing photoredox catalysts are scarce. Many photoredox catalysts rely on long-lived triplet excited states (triplets), which can efficiently engage in single electron transfer (SET) reactions with substrates. However, triplets of π-conjugated molecules are often significantly lower in energy than photogenerated singlet excited states (singlets). Combined with the inherent low energy of red light, this could limit the reductive/oxidative powers. Here, we introduce a series of sustainable heavy atom-free photoredox catalysts based on red-light absorbing dibenzo-fused BODIPY. The catalysts consist of two covalently linked units: a dibenzo-fused BODIPY fluorophore and an electron donor, arranged orthogonally. Excitation of the dibenzoBODIPY unit induces charge separation (CS) from the donor to the dibenzoBODIPY unit, forming a radical pair (RP) state. Unlike the regular BODIPY counterparts, these catalysts do not form triplets. Instead, SET occurs from the high-energy singlet-born RP states, preventing energy loss and effectively utilizing the low-energy red light. The proximity of donor molecules allows efficient charge separation despite the CS being uphill in energy. The molecules demonstrate efficient catalysis of Atom Transfer Radical Addition (ATRA) reaction, yielding products with high yields ranging from 70 to 90%, while the molecule without a donor group does not exhibit catalytic activity. The mechanistic studies by transient absorption and electron paramagnetic resonance (EPR) spectroscopy methods support the proposed mechanism. The study presents a new molecular design strategy for converting noncatalytic fluorophores to efficient photoredox catalysts operating in the red spectral region.
吸收红光的光氧化还原催化剂在大规模反应中具有潜在优势，可扩大可用底物的范围并促进生物正交应用。尽管最近已开发出许多吸收/发射红光的荧光团，但功能性吸收红光的光氧化还原催化剂却很稀少。许多光氧化还原催化剂依赖于长寿命的三重激发态（三线态），其可与底物有效地进行单电子转移（SET）反应。然而，π共轭分子的三线态能量通常比光生单重激发态（单线态）低得多。结合红光固有的低能量，这可能会限制还原/氧化能力。在此，我们介绍了一系列基于吸收红光的二苯并稠合BODIPY的可持续无重原子光氧化还原催化剂。这些催化剂由两个共价连接的单元组成：一个二苯并稠合BODIPY荧光团和一个电子供体，它们正交排列。二苯并BODIPY单元的激发诱导电荷从供体分离到二苯并BODIPY单元，形成自由基对（RP）状态。与常规的BODIPY对应物不同，这些催化剂不会形成三线态。相反，SET发生在高能单线态产生的RP状态，从而防止能量损失并有效利用低能量红光。尽管电荷分离在能量上是上坡的，但供体分子的接近允许有效的电荷分离。这些分子证明了对原子转移自由基加成（ATRA）反应的有效催化，产率高达70%至90%，而没有供体基团的分子则没有催化活性。通过瞬态吸收和电子顺磁共振（EPR）光谱方法进行的机理研究支持了所提出的机理。该研究提出了一种新的分子设计策略，可将非催化荧光团转化为在红色光谱区域运行的高效光氧化还原催化剂。
