Triplet-Forming Thionated Donor-Acceptor Chromophores for Electrochemically Amphoteric Photosensitization

A series of piperidine-naphthalene monoimide, donor (cid:0) acceptor (D (cid:0) A) chromophores and their thionated derivatives were prepared. The physical properties of the chromophores were studied by spectroscopic and electrochemical measurements and DFT calculations. Relative to their non-thionated counter-parts, the thionated D (cid:0) A chromophores display enhanced visible-light absorption and 1 O 2 -sensitization quantum yields and negligible fluorescence, suggesting facile triplet formation of this class of molecules. This behavior originates from the significant n-π * character in the singlet excited state, resulting in large spin-orbit coupling between the singlet/triplet manifolds regardless of the degree of D/A interactions. With the electrochemically active D/A moieties and high triplet energy, the thionated chromophores can be used as amphoteric photosensitizers to catalyze reductive and oxidative photo-reactions with efficiency comparable to Ru(bpy) 3 2 + as the sensitizer. Our results demonstrate that thiocarbonylation can be utilized in amide/imide-containing D (cid:0) A chromophores to provide a wide range of heavy-atom-free redox-active photo-sensitizers.

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A series of piperidine-naphthalene monoimide, donorÀacceptor (DÀA) chromophores and their thionated derivatives were prepared. The physical properties of the chromophores were studied by spectroscopic and electrochemical measurements and DFT calculations. Relative to their non-thionated counterparts, the thionated DÀA chromophores display enhanced visible-light absorption and 1 O 2 -sensitization quantum yields and negligible fluorescence, suggesting facile triplet formation of this class of molecules. This behavior originates from the significant n-π* character in the singlet excited state, resulting in large spin-orbit coupling between the singlet/triplet manifolds regardless of the degree of D/A interactions. With the electrochemically active D/A moieties and high triplet energy, the thionated chromophores can be used as amphoteric photosensitizers to catalyze reductive and oxidative photoreactions with efficiency comparable to Ru(bpy) 3 2 + as the sensitizer. Our results demonstrate that thiocarbonylation can be utilized in amide/imide-containing DÀA chromophores to provide a wide range of heavy-atom-free redox-active photosensitizers.
With the rapid advances in photocatalysis, [1] photon up conversion, [2] and photodynamic therapy, [3] organic triplet photosensitizers have received reinvigorated attention in recent years for their atomic abundance and versatility in comparison to their inorganic counterparts. [1a,2b,4] Long-lived triplet species, necessary for the intermolecular energy/electron transfer processes in these applications, are generated after electronic excitation followed by intersystem crossing (ISC). Conventionally, organic chromophores, especially those of polycyclic aromatic hydrocarbons, suffer from fast, spin-allowed S 1 -S 0 decay that outcompetes ISC. [5] Strategies based on heavy atoms, [6] radical pair recombination, [7] twisted π-conjugation, [3a,8] and radical-enhanced ISC [9] have been identified to increase the rate of triplet formation. However, these methods do not always produce triplet-forming molecules with high predictability, and a careful structural design is often required to optimize the coupling and driving force factors for ISC. Incorporation of heavy atoms is a more universal approach; however, the resultant sensitizers inevitably comprise weak carbon-heavy atom bonds that are prone to degradation. [10] Therefore, renovated molecular designs of organic triplet chromophores are continuously being explored to unlock the full potential of heavy-atom-free photosensitizers, [3a,11] with the introduction of supramolecular interactions to enhance the bimolecular reaction kinetics and the substrate specificity being some of the latest developments. [12] Inspired by the prosperous use of organometallic photosensitizers, we propose a new class of organic photosensitizers based on thiocarbonyl donorÀacceptor (DÀA) chromophores. The n-π* excited-state character introduced by the thiocarbonyl (C=S) functionality should enable fast ISC (see below), [3b,13] whereas intramolecular DÀA interactions can permit strong light absorption that is otherwise weak for n-π* chromophores. These two features mimic the key characteristics of Ru-or Irbased polypyridyl complexes, [14] where the presence of heavy metals provides a reliable ISC mechanism, and the metal-toligand/ligand-to-ligand charge transfer (MLCT/LLCT) gives optical absorption stronger than the metal-centered transitions. Mild decoupling of the mechanisms of ISC and optical absorption allows the modification of the latter property without jeopardizing triplet formation. Incorporation of the electron D and A moieties further allows the as-formed triplet species to participate in oxidative or reductive electron transfer reactions.
Organic electron acceptors featuring amide/imide functionalities are ubiquitous in organic optoelectronic applications. [15] Established synthetic methodologies exist for structural modification, and their carbonyl units can be thionated by Lawesson's reagent to give the corresponding thioamide/-imides, [16] which are known to display superior stability compared to thioketo/-ester molecules. [17] Taking these into account, we chose naphthalene monoimide (NMI) as the model acceptor in this study. Facile reduction of the parent NMI molecule takes place at � À1.35 V vs. saturated calomel electrode (SCE) [18] and its small π-system ensures the energetic triplet excited state, favorable for photosensitization, to be generated from NMI derivatives.
The extent of subsequent thionation can be controlled to some degree by the amount of reagents and reaction time to provide the mono-thionated chromophores as the major product. The fully thionated chromophores are not considered here as their triplet energies and hence the excited-state redox power are significantly lower than the mono-thionated ones. [16a,21] Due to the molecular symmetry, only one monothionated product (N-PipNMI-S and N-PhPipNMI-S) is expected for N-donor-substituted chromophores. The piperidinyl donor in asymmetric 4-PipNMI does not provide sufficient electronic bias to distinguish the reactivity of the two carbonyl groups towards Lawesson's reagent; thionation resulted in an inseparable mixtures 4-PipNMI-S of nearly equal amounts according to 1 H NMR. Such a regioisomeric issue was resolved for 2-PipNMI-S, where the additional steric hindrance caused by the 2piperidinyl substituent guarantees that thionation only takes place once even in the presence of excess Lawesson's reagent. The distal-site selectivity was unambiguously confirmed by the single crystal X-ray structure of 2-PipNMI-S (Figure 1). Despite the change in the electron-accepting power of the naphthalene imide moiety, the difference in the dihedral angle between the D and A moieties is negligible upon thionation. Since all structural parameters from the single-crystal analysis are similarly reproduced by the DFT-optimized structures at the level of ωB97X-D/6-31G(d,p), these observations indicate that the structural twisting in the present series of DÀA chromophores are primarily governed by intramolecular steric factors.
The absorption spectra for each chromophore were measured in CH 2 Cl 2 ( Figure 2 and Table 1). For 4-PipNMI and 2-PipNMI, the low-energy, broad absorption > 400 nm slightly blue shifts in toluene ( Figure S1), whereas the high energy bands remain unchanged. The solvatochromism in the visible absorption reflects the mixed intramolecular charge-transfer   and π-π* characters of these DÀA chromophores, supported by natural transition orbital (NTO) analysis (see Figure 3 and SI Section 7). As the piperidinyl donor is at the imide position through which the NMI-localized LUMO nodal planes pass in N-PipNMI and N-PhPipNMI, the charge-transfer absorption is strongly forbidden, [22] and thus only the NMI-originated absorption (< 370 nm) was observed. Thionation resulted in pronounced light absorption of all four chromophores in the visible region. The red shift has been similarly reported for other thionated rylene imides, and is often attributed to the reduction in the LUMO energy. [13a,17] The improved visible-light absorption, especially significant for N-PipNMI-S and N-PhPipNMI-S, together with the substantial extinction coefficients of these thionated DÀA chromophores (e. g., ɛ = 1.8 × 10 4 m À1 cm À1 for 2-PipNMI-S, cf. ɛ = 1.4 × 10 4 m À1 cm À1 for Ru(bpy) 3 Cl 2 ), [23] suggest potential solar-energy utilization of these chromophores.
While 4-PipNMI gives bright green fluorescence (Φ FL = 0.43), weak to no emission was observed for all other DÀA chromophores (Φ FL < 0.05). The absence of fluorescence is indicative of fast non-radiative relaxation to the ground state or efficient formation of the triplet excited state. Using 1,3diphenylisobenzofuran as a chemical probe, it was found that all thionated compounds sensitized triplet molecular oxygen to give singlet oxygen ( 1 O 2 , quantum yield Φ Δ 16-52 %); thus, triplet formation is likely to take place efficiently. For the nonthionated molecules (except for N-PhPipNMI, see below), the ISC process is, however, outcompeted by radiative and nonradiative internal conversion in the singlet manifold.
Facile triplet formation of the thionated DÀA chromophores can be attributed to the high spin-orbit coupling matrix elements hS n j Ĥ SO j T m i, calculated at the level of TD-ωB97XÀD/6-31G(d,p) using Gaussian 09 interfaced with PySOC ( Figure 3, and SI Section 7). [24] hS 1 j Ĥ SO j T 1 i above 100 cm À1 were found,  which is 2-3 orders of magnitude larger than that of nonthionated precursors and notably larger than that of Ru(bpy) 3 2 + (hS 1 j Ĥ SO j T 1 i = 39 cm À1 ). [25] By NTO analysis, it is clear that the S 1 and T 1 states of all thionated DÀA chromophores feature n-π* and π-π* transitions, respectively; the change in orbital spatial orientation facilitates ISC, in accordance with the El-Sayed rule (Figure 3b and Figure 3d). In the absence of the C=S unit, both the S 1 and T n states are dominated by the π-π* character and thus small spin-orbit coupling (< 1 cm À1 ). ISC is comparatively less efficient given such a low value and the sizable energy difference between the singlet and triplet states (ΔE ST ) in these molecules. However, it is worth noting that due to the orientation of the N lone-pair in N-PipNMI or the aniline ring in N-PhPipNMI, these energetically high-lying orbitals can mix well with the in-plane p orbitals of C=O oxygen, giving the n-π* character to the S 1 state of these two non-thionated molecules (Figure 3c). This unusual scenario provides hS 1 j Ĥ SO j T n i � 3 cm À1 for N-PipNMI and N-PhPipNMI that are noticeably higher than that of common π-π* chromophores. Efficient triplet formation (Φ Δ = 72 %) of organic chromophores with hS 1 j Ĥ SO j T 1 i � 2 cm À1 has been reported. [26] Additionally, the orthogonal orientation of the D/A π systems in N-PhPipNMI may facilitate ISC through the spin-orbit charge transfer (SOCT) mechanism, where charge recombination is accompanied with a molecular orbital angular moment change due to the large twist in structure. [7a,27] These factors seem contribute to the triplet formation and 1 O 2 sensitization of N-PhPipNMI, and may explain the unusual, delayed fluorescence observed from the crystals of analogous N- [4-(dimethylamino) phenyl]-NMI. [22] However, as this molecule only absorbs in the UV region, we will focus on the triplet-forming, visible-light absorbing thionated chromophores.
High triplet energy (E T1 = 1.78-1.95 eV) estimated computationally for the thionated DÀA chromophores supports the energetic driving force required for triplet energy transfer to sensitize 1 O 2 as mentioned above. These values further suggest ample excited-state redox power in the triplet chromophores. Due to the presence of both the electron donor and acceptor in the same molecule, these electrochemically amphoteric chromophores are reduced at E RED = À1.20 to À1.02 V and oxidized at E OX = + 0.94 to + 1.24 V vs. SCE, offering excited-state redox power of E T,RED � + 0.74 to + 0.86 V and E T,OX � À0.84 to À0.66 V.
To assess the amphoteric photoredox capacity of the thionated DÀA chromophores, photooxidative aza-Henry [23b] and photoreductive dehalogenation [28] reactions were chosen ( Table 2). The reactions were conducted in the presence of 1-5 mol% thionated DÀA chromophores as the photocatalyst under anaerobic conditions. The photo aza-Henry reaction involves the generation of the amino radical cation of Nphenyltetrahydroisoquinoline (A) at E OX = + 0.78 V, [23b] close to the minimal E T,RED available from the thionated DÀA chromophores. On the other hand, reduction of α-bromoacetophenone at E RED � À0.5 V, [29] more positive than E T,OX of the present chromophores, is required to provide the key radical intermediate (B) in the debromination reaction. Additionally, the thionated DÀA chromophores were also tested as energy transfer (EnT) photocatalysts using a [2 + 2] cycloaddition reaction ( Table 2). [30] The yields of the photo products catalyzed by the thionated DÀA chromophores were found comparable to those obtained using Ru(bpy) 3 Cl 2 as the sensitizer. It is also notable that each thionated DÀA chromophore could be recovered after the reaction by chromatography. Although the yields display no apparent correlation with the 1 O 2 -generation quantum yield or the photoredox driving force, these results provide clear evidence that the photoexcited thionated DÀA chromophores can engage in reductive/oxidative single electron transfer (SET) and EnT photocatalytic processes. Among these chromophores, the strong visible-light absorption and high triplet energy and excited-state redox power of 2-PipNMI-S make it a highly promising SET and EnT photocatalyst. Rateconstant characterization of the excited-state physical and chemical processes will provide more quantitative insights into the photocatalytic behaviors of these sensitizers in future study.
In summary, we synthesized, characterized, and tested the photo-reactivity of a series of thionated DÀA chromophores. In comparison with the non-thionated precursors, these chromophores display strong visible-light absorption, negligible fluorescence, redox amphoteric activity, and moderate 1 O 2 sensitization. The combination of the latter two enables the application of these thionated chromophores in both reductive and oxidative photocatalytic reactions. This proof-of-concept study demonstrates the generality of thionating DÀA chromophores to provide heavy-atom-free organic triplet photosensi- [a] The yield was calculated by quantitative 1 H NMR using 2,5-dimethylfuran as the internal standard (See SI Section 6); [b] 2-phenyl-1,2,3,4tetrahydroisoquinoline (60 mg), photocatalyst (0.01 eq) and CH 3 NO 2 (3 mL); the reaction was irradiated for 4 h; [c] α-bromoacetophenone (80 mg), photocatalyst (0.05 eq), i-PrNEt 2 (8 eq) and MeCN (4 mL tizers. The effective design can be attributed to the decoupling of molecular optical/electrochemical properties from their triplet formation mechanism. The involvement of the n-π* excited state promotes ISC regardless of the detailed DÀA interactions. Since many commercial pigments or high-performance organic LED and photovoltaics materials contain amide/ imide functionalities, thionation of these chromophores can provide a wide range of photosensitizers with bespoke photoredox activity. As hinted by the comparison between N-PhPipNMI and N-PhPipNMI-S, additional ISC channels may exist due to charge separation and/or the twisting between D/A moieties. Our future study will unravel the interplay between different ISC mechanisms for triplet formation and evaluate this complex scenario for sensitizer design, in addition to the quantitative rate-constant characterization.

Single Crystal X-Ray Data
Deposition