Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts

Abstract

A key feature of prominent transition-metal-containing photoredox catalysts (PCs) is high quantum yield access to long-lived excited states characterized by a change in spin multiplicity. For organic PCs, challenges emerge for promoting excited-state intersystem crossing (ISC), particularly when potent excited-state reductants are desired. Herein, we report a design exploiting orthogonal π-systems and an intermediate-energy charge-transfer excited state to maximize ISC yields (Φ) in a highly reducing ( E* = -1.7 V vs SCE), visible-light-absorbing phenoxazine-based PC. Simple substitution of N-phenyl for N-naphthyl is shown to dramatically increase Φ from 0.11 to 0.91 without altering catalytically important properties, such as E*.

Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers

“…Evaluation of CT from 1 PC* can estimate the CT character of 3 PC*, as CT singlet and triplet excited states are expected to be energetically degenerate, with low Δ E ST . A high fluorescence quantum yield ( Φ f ) can indicate a lack of CT, as CT states have been shown to minimize fluorescence and increase triplet yields . Consistent with previous observations in N , N ‐diaryl dihydrophenazine and N ‐aryl phenoxazine studies, for dimethyl dihydroacridines subtle N‐aryl substitution differences were significant in influencing the nature of the experimentally observed CT character.…”

“…In these systems, electron density is transferred from the electron‐rich core to either the N‐aryl or core substituent, generating a shift in charge density within the molecule in its excited state, which is dictated by the electron‐accepting ability of the aryl “acceptor” as well as the environment surrounding the “donor” tricyclic core. The connection between PC CT and superior O‐ATRP performance was investigated in two separate studies, but different conclusions were drawn: For N‐aryl phenoxazines, increasing CT character has been shown to augment triplet yields, positing that higher concentrations of 3 PC* promotes fast activation . Conversely, studies with N , N ‐diaryl dihydrophenazine PCs suggest that CT lowers *PC reduction potentials, slowing down activation, reducing radical concentrations, and minimizing termination …”

Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers

“…[24] Ah igh fluorescence quantum yield (F f )can indicate alack of CT,asCT states have been shown to minimize fluorescence and increase triplet yields. [27] Consistent with previous observations in N,N-diaryl dihydrophenazine and N-aryl phenoxazine studies,f or dimethyl dihydroacridines subtle N-aryl substitution differences were significant in influencing the nature of the experimentally observed CT character.Ofthese candidates, 1 (Figure 2d), 2, and 7 displayed the largest degree of CT through the largest measured Stokes shifts (ranging from 126 to 180 nm) paired with low F f (0.1 %t o8 .7 %), and the most dramatic solvatochromism spanning blue to yellow wavelengths of emission ( Figure 2d;s ee Figure S1). By the same analysis, 4 and 5 (Figure 2e and f) displayed am oderate degree of CT, while 3 and 6 displayed the least amount of CT character (see Figure S1).…”

“…[25, 15a] In these systems,e lectron density is transferred from the electron-rich core to either the N-aryl or core substituent, generating as hift in charge density within the molecule in its excited state,w hich is dictated by the electron-accepting ability of the aryl "acceptor" as well as the environment surrounding the "donor" tricyclic core.T he connection between PC CT and superior O-ATRP performance was investigated in two separate studies,but different conclusions were drawn:F or N-aryl phenoxazines,increasing CT character has been shown to augment triplet yields, positing that higher concentrations of 3 PC* promotes fast activation. [27] Conversely,s tudies with N,N-diaryl dihydrophenazine PCs suggest that CT lowers *PC reduction potentials,slowing down activation, reducing radical concentrations,and minimizing termination. [15c] Computationally,C Tc haracteristics can be predicted through the presence of charge-separated singly occupied molecular orbitals (SOMOs) for 3 PC*.…”

Visible‐Light‐Mediated Liberation and In Situ Conversion of Fluorophosgene

“…[14] However,d uring measurements at catalyst concentrationso f3 0mm anda bove (compared to 2mm under the reaction conditions), we observed ac harge-transfer band originating from the radical cation part in the catalyst CT-complex dimer. 15 The lifetimeo ft his band decreases significantly by addition of DIPEA (Figure 2a). This indicates, that the charger ecombination might be slowed down by interaction of the radi-cal cation part of the CT-complex with DIPEA.…”