Yunjeong
Lee‡
a,
Byung Hak
Jhun‡
b,
Sihyun
Woo‡
c,
Seoyeon
Kim
a,
Jaehan
Bae
a,
Youngmin
You
*b and
Eun Jin
Cho
*a
aDepartment of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea. E-mail: ejcho@cau.ac.kr
bDepartment of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea. E-mail: odds2@yonsei.ac.kr
cDivision of Chemical Engineering and Materials Science, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea
First published on 28th June 2024
Synthetic photochemistry has undergone significant development, largely owing to the development of visible-light-absorbing photocatalysts (PCs). PCs have significantly improved the efficiency and precision of cycloaddition reactions, primarily through energy or electron transfer pathways. Recent research has identified photocatalysis that does not follow energy- or electron-transfer formalisms, indicating the existence of other, undiscovered photoactivation pathways. This study unveils an alternative route: a charge-neutral photocatalytic process called charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry. Our investigations revealed CRTS occurrence in DeMayo-type [2 + 2] cycloaddition reactions catalyzed by indole-fused organoPCs. Our mechanistic investigations, including steady-state and transient spectroscopic analyses, electrochemical investigations, and quantum chemical calculations, suggest a mechanism involving substrate activation through photoinduced electron transfer, followed by charge recombination, leading to substrate triplet state formation. Our findings provide valuable insights into the underlying photocatalytic reaction mechanisms and pave the way for the systematic design and realization of innovative photochemical processes.
In this study, we investigated an alternative route: charge-neutral photocatalytic process via charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry (Fig. 1d).32,33 To investigate this uncommon CRTS mechanism, we utilized an indole-based polycyclic organoPC series,34,35 developed in our laboratory, for a DeMayo-type [2 + 2] cycloaddition36–44 between methyl-2-(quinolin-2-yl)acetate (1) and various styrene derivatives. Detailed studies involving photophysical and electrochemical measurements and computational analyses provide insights into this unique CRTS pathway.
In addition to our indole-fused organoPCs, we also evaluated widely used PCs such as Eosin Y, 4-CzIPN, and [Ir{dF(CF3)ppy}2(dtbbpy)]PF6.41 However, none of these PCs exhibited superior reactivity compared to A and H. Control experiments showed that in the absence of an organoPC or visible light, the reaction did not proceed at all, confirming its photocatalytic nature.
The quantum yield for the reaction of 1 and 2a, which was determined using the standard ferrioxalate actinometry, is 49% with A and 48% with H. The other photocatalysts show quantum yields for the reaction in the range 4–23% (Table S1†). The values lower than 100% suggest the absence of intermediacy of any chain reaction.
First, the photoluminescence (fluorescence) spectra were recorded for Ar-saturated 1,4-dioxane containing 1.0 mM A and increasing concentrations of 1 (0–500 mM) under 345 nm photoexcitation. As shown in Fig. 2a, the photoluminescence intensity of A decreased with increasing 1 concentration, suggesting a nonradiative interaction between 1 and excited state A (A*). The quenching interaction hardly required a ground-state association between the two species, as seen from the decreasing photoluminescence lifetime (τobs) of A* without a multiphasic transition in the decay trace (Fig. 2b; see Fig. S2† for results for the other organoPCs, B–H). The photoluminescence quenching rate constant (kobs) is estimated from the relationship: kobs = 1/τobs(1) − 1/τobs(0), where τobs(1) and τobs(0) are τobs in the presence and absence, respectively, of 1; the kobs increases with the molar concentration of 1 ([1]), consistent with bimolecular quenching.
Pseudo-first-order kinetic analyses of kobs yielded the rate constant for hetero-bimolecular quenching (kQ) of A–H in the range of 1.2–2.7 × 108 M−1 s−1, which approaches the diffusion-limited regime in 1,4-dioxane at 298 K (Fig. 2c and Table 2). Notably, kQ does not correlate with the yield of the DeMayo-type [2 + 2] cycloaddition reactions of 1 (Fig. 2d), which suggests that the quenching of excited-state organoPCs by the substrate is not a rate-determining step in the overall photocatalytic cycle.
λ abs (nm, ε/104 M−1 cm−1) | E S1 (eV) | λ em (nm) | τ obs (ns) | Φ PL | k r (107 s−1) | k nr (107 s−1) | k ISC (107 s−1) | J (1012 M−1 cm−1 nm4) | k Q (108 M−1 s−1) | k PeT (108 M−1 s−1) | k CR (108 M−1 s−1) | |
---|---|---|---|---|---|---|---|---|---|---|---|---|
a Absorption peak wavelength (molar absorbance). b The first singlet state energy was determined from the UV-Vis absorption spectra onset wavelengths. c Emission peak wavelength. d Photoluminescence lifetime determined through nonlinear least-squares fitting to a mono-exponential decay model of decay traces after picosecond pulsed 345 nm excitation. e Photoluminescence quantum yield was determined using 9,10-diphenylanthracene as a standard (toluene, ΦPL = 1.00). f Radiative rate constant, kr = ΦPL/τobs. g Nonradiative rate constant, knr = (1 − ΦPL)/τobs. h Intersystem crossing rate. i Spectral overlap is integral between the absorption spectrum of the substrate and the photocatalyst emission spectrum. j The bimolecular quenching rate constant was determined from pseudo-first-order kinetic analyses of the photocatalyst fluorescence quenching rates with the substrate. k The rate constant for the bimolecular photoinduced electron transfer rate was determined through the relationship kPeT = −(kQ × kdiff)/(kQ − kdiff), where kdiff is the 1,4-dioxane diffusion rate constant at 298 K, estimated from the Stokes–Einstein–Smoluchowski equation. l The rate constant for charge recombination within the radical ion pair [PC˙−⋯1˙+] to form 31*. | ||||||||||||
A | 294 (4.40) | 2.74 | 489 | 11 | 0.21 | 2.0 | 7.5 | 1.7 | 1.5 | 1.3 | 1.3 | 6.9 |
B | 303 (1.29) | 2.86 | 484 | 8.8 | 0.20 | 2.3 | 9.1 | — | 6.2 | 1.8 | 1.8 | 0.66 |
C | 290 (1.88) | 2.78 | 491 | 10 | 0.25 | 2.6 | 7.7 | 0.63 | 1.2 | 1.4 | 1.4 | 1.8 |
D | 302 (2.00) | 2.93 | 539 | 1.2 | 0.0045 | 0.37 | 83 | — | — | — | — | 0.66 |
E | 290 (1.39) | 2.83 | 479 | 12 | 0.47 | 4.0 | 4.6 | — | 2.5 | 1.5 | 1.6 | 1.9 |
F | 290 (1.43) | 2.85 | 478 | 8.9 | 0.21 | 2.3 | 8.9 | 10 | 2.0 | 1.2 | 1.3 | 3.8 |
G | 378 (1.38) | 2.81 | 486 | 8.5 | 0.23 | 2.7 | 9.1 | — | — | — | — | 4.1 |
H | 412 (1.57) | 2.73 | 462 | 3.4 | 0.39 | 11 | 18 | 1.0 | 2.7 | 2.7 | 2.9 | 9.3 |
1 | 316 (0.42) | 2.69 | 365 | — | — | — | — | — | — | — | — | — |
The quenching of A* by 1 does not result from EnT. We found that kQ exhibited a poor proportional relationship with the spectral overlap integral (J) between the UV-Vis absorption spectrum of 1 and the photoluminescence spectrum of A (Fig. S3 and S4†). The absence of proportionality indicates that the interaction between A* and 1 does not originate from singlet–singlet EnT that follows the Förster formalism. Moreover, the photoluminescence quenching behaviors are inconsistent with Förster EnT theories (see Fig. S5† for further discussion). In addition, even though the singlet sensitization of 1 to its excited state (11*) by the organoPCs would take place, 11* is unlikely to be converted to its triplet state (31*); our quantum calculations, based on the method of Ma et al.,46 indicate negligibly low rates (kISCs) for intersystem crossing (ISC): 1.4 × 102 s−1 for the benzenoid form and 2.7 × 103 s−1 for the quinoid form of 1 (Table S2†). These results support the hypothesis that singlet–singlet EnT is not a productive pathway. Furthermore, the formation of an EDA complex and an exciplex can be excluded based on the absence of new bands in the UV-Vis absorption and photoluminescence spectra of a mixture of A and 1 or 2a (Fig. S6 and S7†).
In contrast, our femtosecond and nanosecond transient absorption spectroscopy experiments revealed a relatively fast ISC for the organoPCs, occurring at a rate of kISC = 0.63–10 × 107 s−1 (Fig. S8† and Table 2). The ISC of A occurs at a rate (kISC = 1.7 × 107 s−1) twice as fast as its kobs in the presence of 0.050 M 1 (i.e., 6.5 × 106 s−1 = 1.3 × 108 M−1 s−1 × 0.050 M). Based on the ISC behavior, one might consider that 1 undergoes triplet sensitization through consecutive processes involving the ISC of an organoPC, followed by triplet–triplet EnT from the organoPC triplet state to 1. However, quantum chemical calculations at the B3LYP-D3(BJ) level of theory using the TZP basis set, followed by time-dependent w-B97X-D calculations considering solvation effects, contradict this scenario. As shown in Fig. 3, the triplet (T1) states of the B–H ground-state geometries (1.78–2.01 eV) were located below the T1 state of the quinoid form of the 1 ground state (2.05 eV). In particular, the significant T1 state energy difference between H and 1 (0.27 eV) strongly indicates that substrate activation via triplet–triplet EnT is unfavorable, except for A. Notably, H catalyzes the DeMayo reaction of 1, with yields as high as 93%. In the case of A, its T1 state is isoenergetic with the T1 state of 1 (2.05 eV). Our quantum chemical calculations for 1 reveal localization of the spin density within the heterocycle and methyne units (Fig. S9†), which suggests an occurrence of the [2 + 2] cycloaddition reaction with 2a, but refutes the possibility for the [4 + 2] cycloaddition.47 In addition, the singlet excited (S1) states of the A–H ground-state geometries (3.02–3.22 eV) are predicted to be lower in energy than the S1 state of the quinoid form of 1 (3.24 eV). These energetic alignments indicate that EnTs from the excited-state organoPCs to 1 are endoergic, suggesting that triplet sensitization of 1via EnT is not feasible.
The Dexter theory of electron exchange predicts that the process rate decreases exponentially with increasing distance between the catalyst and the substrate. Good linearity was observed between the logarithm of the photoluminescence quenching rate constant, kobs, and [1]−1/3 (Fig. S5b†). This adherence to Dexter formalism implies that an excited-state organoPC is quenched by unidirectional or bidirectional ET—that is, the formation of a radical ion pair or EnT, respectively. Because EnT has been refuted, it is tempting to assert that unidirectional, photoinduced hetero-bimolecular ET forms radical ion pairs of the organoPC and substrate. To examine this possibility, we determined the oxidation (Eox) and reduction (Ered) potentials of A–H and 1 using cyclic and differential pulse voltammetry (Fig. S10 and S11†). The excited-state redox potentials of organoPCs were subsequently calculated from the relationships = Eox − ES1 and = Ered + ES1, where is the excited-state oxidation potential, ES1 is the S1 state energy determined from the onset wavelength of the UV-Vis absorption spectrum, and is the excited-state reduction potential. The ground- and excited-state redox potentials are compiled in Table 3. The (−1.52–1.37 V vs. standard calomel electrode (SCE)) of A–H are found to be more cathodic than the Ered (−1.13 V vs. SCE) of the 1 quinoid form. This electrochemical disposition allows photoinduced ET from organoPCs to 1, forming a geminate radical ion pair consisting of a one-electron oxidized species of organoPC (denoted as PC˙+) and a one-electron reduced form of 1 (denoted as 1˙−). The driving force for the oxidative quenching of excited-state organoPC (−ΔGoxPeT) can be estimated from the relationship −ΔGoxPeT = e[(PC) − Ered(1)], where e is the elementary charge; (PC) is the of an organoPC; and Ered(1) is the Ered of 1. The Coulomb term was ignored in this estimation owing to the use of polar 1,4-dioxane. The −ΔGoxPeT spans the range 0.24–0.39 eV (Table 3), which indicates that the oxidative quenching of A–H by 1 is exergonic. In addition to oxidative quenching, the reductive quenching of A–H to form a geminate radical ion pair of the organoPC radical anion (PC˙−) and the 1 radical cation (1˙+) is thermodynamically allowed. The driving force for the reductive quenching (−ΔGredPeT), estimated from the relationship −ΔGredPeT = e[Eox(1) − (PC)] where Eox(1) and (PC) are Eox of 1 and the excited-state reduction potential of an organoPC, respectively, is in the range 0.31–0.55 eV (Table 3). Taken together, our electrochemical analyses suggest the formation of radical ion pairs of [PC˙+⋯1˙−] or [PC˙−⋯1˙+]. To monitor the radical ion pairs directly, we performed nanosecond laser flash photolysis (LFP) experiments on Ar-saturated 1,4-dioxane containing 1.0 mM A and 100 mM 1. The mixture was photo-irradiated under nanosecond pulsed laser excitation at 355 nm. As shown in Fig. 4a, a weak photoinduced absorption (PIA) signal emerged at wavelengths greater than 800 nm, together with significant photoinduced bleaching (PIB) in the shorter wavelength region due to the stimulated emission of A*. The PIA spectrum consisted of bands with approximately 840 and 1400 nm peak wavelengths.
E T1 (eV) | E ox (V vs. SCE) | E red (V vs. SCE) | (V vs. SCE) | (V vs. SCE) | −ΔGoxPeTd (eV) | −ΔGredPeTe (eV) | −ΔGCRf (eV) | (eV) | |
---|---|---|---|---|---|---|---|---|---|
a The T1 state energies of the photocatalysts were calculated at the w-B97X-D/TZP//B3LYP-D3(BJ)/TZP level with COSMO parameterized for 1,4-dioxane. b Excited-state oxidation potential, = Eox − ES1. c Excited-state reduction potential, = Ered + ES1. Table 2 presents the ES1 values of the samples. d Driving force for oxidative electron transfer from the excited-state catalyst to 1, −ΔGoxPeT = e[(PC) − Ered(1)]. e Driving force for reductive electron transfer from 1 to the excited-state organoPC, −ΔGredPeT = e[Eox(1) − (PC)]. f Driving force for the charge recombination reaction, PC˙− + 1˙+ → PC + 1, −ΔGCR = e[Eox(1) − Ered(PC)]. g Driving force for the charge recombination reaction, PC˙− + 1˙+ → PC + 31*, = −ΔGCR − ET1(1). | |||||||||
A | 2.05 | 1.37 | −1.50 | −1.37 | 1.24 | 0.24 | 0.35 | 2.39 | 0.34 |
B | 2.01 | 1.47 | −1.42 | −1.39 | 1.44 | 0.26 | 0.55 | 2.31 | 0.26 |
C | 1.98 | 1.38 | −1.45 | −1.40 | 1.33 | 0.27 | 0.44 | 2.34 | 0.29 |
D | 1.93 | — | — | — | — | — | — | — | — |
E | 2.01 | — | — | — | — | — | — | — | — |
F | 1.93 | 1.44 | −1.46 | −1.41 | 1.39 | 0.28 | 0.50 | 2.35 | 0.30 |
G | 1.98 | — | — | — | — | — | — | — | — |
H | 1.78 | 1.21 | −1.53 | −1.52 | 1.20 | 0.39 | 0.31 | 2.42 | 0.37 |
1 | 2.05 | 0.89 | −1.13 | −1.80 | 1.56 | — | — | — | — |
On the other hand, the spectrum of the control solution devoid of 1 exhibited negligible PIA bands in the range 1200–1600 nm. The PIA band at 1400 nm coincides with the visible-NIR absorption spectrum of A˙− electrochemically generated at −2.0 V vs. Ag+/0 (ε at 1400 nm is 285 M−1 cm−1). The simulated UV-Vis-NIR absorption spectrum of A˙− further supports these observations (the fourth panel in Fig. 4a). These results provide compelling evidence for the photoinduced formation of [A˙−⋯1˙+].
Once formed, the radical ion pair [A˙−⋯1˙+] rapidly disappears through charge recombination via back-ET from A˙− to 1˙+ within the solvent cage. Note that 1˙+ or its resonance benzylic radical derivative might react with the alkene; however, we excluded this pathway because it cannot lead to DeMayo-type [2 + 2] cycloaddition, which should occur through biradical intermediates (see Fig. S9 and Table S17†). This charge recombination typically produces charge-neutral ground-state species (that is, A and 1). When the driving force for charge recombination (−ΔGCR) is greater than the constituent species T1 state energy, charge recombination sensitizes the triplet state. We have previously reported CRTS in organic light-emitting devices48,49 and photoredox catalysis.50 The −ΔGCR for [A˙−⋯1˙+] amounts to 2.39 eV, as estimated by −ΔGCR = e[Eox(1) − Ered(A)]. This −ΔGCR value is greater than the T1 state energy (2.05 eV) of the quinoid form of 1 (Fig. 3 and Table 3). The corresponding driving force for charge-recombinative sensitization of 31* is estimated to be 0.34 eV for A from the relationship = −ΔGCR − ET1(1). Although we could not directly observe the spectroscopic signatures of 31*, presumably because of its weak signals, the positive indicates the spontaneity of the CRTS of 31*. The values for A–H are in the range 0.26–0.37 eV (Table 3).
The question remains as to why A–H produce different yields in DeMayo-type [2 + 2] cycloaddition reactions (Table 1). These differences may originate from their different rates of charge-recombinative substrate triplet sensitization. We determined the charge recombination rate (kCR) through second-order kinetic analyses of the A˙− PIA decay traces, which were recorded at a wavelength of 1400 nm using the molar absorbance determined spectrophotometrically (ε = 285 M−1 cm−1) (Fig. 4b). kCR was determined to be as large as 6.9 × 108 M−1 s−1 for A˙− and 1˙+. The kCR values for the other organoPCs, B–H, were also determined (Table 2). As shown in Fig. 4c, kCR increases with . Based on the Jortner ET formalism,49 our analyses suggest that charge recombination occurs in the Marcus normal region of ET with a reorganization energy of 1.2 eV. More importantly, a mild linear relationship was observed between kCR and the yield for the DeMayo-type [2 + 2] cycloaddition reaction (Fig. 4d). For instance, A and H, which had the largest kCR values, produced the best catalytic performance, whereas D, which had the smallest kCR, elicited the lowest reaction yield. Although inconclusive, the linearity strongly suggests that charge recombination, which generally causes quenching of the reaction, is actually a key step in our results. Therefore, it can be concluded that the charge-recombinative triplet-state generation step governs the overall catalytic performance.
Based on these results, we propose a plausible mechanism involving 1 and 2a (Fig. 4e). Because the DeMayo-type [2 + 2] cycloaddition proceeds from the T1 state, substrate triplet activation is essential. Our proposed mechanism involves a photoinduced cycle of consecutive steps, including (i) initial photon absorption by an organoPC to form an excited-state organoPC (1PC*) that is intrinsically deactivated at a rate of 2.0 × 107 s−1 in the case of A; (ii) diffusion-controlled formation of an encounter complex between 1PC* and a substrate [1PC*⋯substrate];51 (iii) ET from the substrate to 1PC* to form a radical ion pair [PC˙−⋯substrate˙+]; (iv) charge recombination within the radical ion pair to form the substrate T1 state; (v) dissociation of the triplet substrate and the PC. The triplet substrate subsequently reacts with styrene 2a to generate a biradical adduct, or the biradical intermediate may cyclize readily to form a cyclobutane intermediate. Finally, ring-opening52 followed by aromatization produces 3a.
Footnotes |
† Electronic supplementary information (ESI) available. CCDC 2225142. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc02601b |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2024 |