Subscriber access provided by ECU Libraries
Article
Regulatory Mechanism and Kinetic Assessment of Energy Transfer Catalysis Mediated by Visible Light Lishuang Ma, Weihai Fang, Lin Shen, and Xuebo Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00146 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Regulatory Mechanism and Kinetic Assessment of Energy Transfer Catalysis Mediated by Visible Light Lishuang Ma, Wei-Hai Fang, Lin Shen*, Xuebo Chen* Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China ABSTRACT: The visible-light-mediated energy transfer catalysis plays a pivotal role in the photochemical synthesis. Although many significant advances in this field have been achieved within the last decade, the knowledge of the photo-chemically tunable metal-ligand interaction for photocatalysts, the manipulation principle of excited-state properties and the available electronic excitation for the free and bound substrates, which makes it possible to design some photo- and auxiliary catalysts based on the proposed mechanism, is still sparse. In the present work, we investigated the paradigm example of intermolecular [2+2] photocycloaddition reactions for 2-hydroxychalcones coordinated by the chiral Lewis acids, using tris(bipyridyl) ruthenium(II) as a photosensitizer. The electronic structure calculations at the CASPT2//CASSCF/PCM level of theory, as well as the kinetic assessment of energy transfer process using the Fermi’s golden rule and the Dexter model, were performed to provide useful benchmarks for the elucidation of energy transfer photocatalysis. The excitation properties for the enone substrate are photo-chemically tunable in the presence of various metal ion based chiral Lewis acids, which rules out the background reaction of excited state intramolecular proton transfer (ESIPT). The preferable photosensitized pathway with dual catalysts can be also regulated cooperatively as a priority by the introduction of high valance d0 ions that notably decreases the triplet energy for the photocatalysis reaction but without an efficient improvement on the intersystem crossing rate of metal-chelated substrates. Our kinetic evaluation method, which has been applied to different catalysis systems, reveals various factors that determine the energy transfer efficiency, including the rigidity of substrate-chiral Lewis acid complexes, the reasonable triplet energy gap between donor and acceptor, the molecular orientation of complexes and the electronic characters of triplet excited states. KEYWORDS: energy transfer, kinetic assessment, energy regulation, [2+2] photocycloaddition, CASPT2//CASSCF calculations
INTRODUCTION The past several decades have witnessed the boom in the development of photochemical synthesis1-12 to create a myriad of unique bond constructions,6 a broad variety of innovative functionalization methods7 and the diverse chemical transformations8 based on the versatile platform of photo-catalysis. The utilization of photochemical steps notably simplifies the synthesis procedure and considerably shortens the overall synthesis route, thereby producing highly functionalized structures starting from simple substrates through the high levels of control over the stereo- and regio-selectivity of complexity-building reactions.4-12 Recent breakthroughs have shown the impactful advances of photochemical synthesis in various fields ranging from the pharmaceutical industry,9 materials science,10 commodity chemicals11 to the natural product total synthesis.12 However, these photochemical transformations frequently suffer from one drawback that the substrate of most organic molecules tend to be photoactivated only by the high-energy photons of ultraviolet (UV) region, which leads to the unintended UV photo-degradation and inevitably limits the development and evolution of the photochemical reactions.13-15
Moreover, the operation of enantioselectivities in a given photochemical reaction is a considerable challenge that modest differences of optimal catalyst and substrate structures can have profound influence on the yield and enantiomeric excess (ee) of the products, which is largely attributed to the highly reactivity, short lifetime, and weak inter- or intramolecular interaction of the excited-state species.16-18 As a promising strategy, the chiral Lewis acid catalysts have been adopted successfully to channel the photochemical transformation into a highly productive and enantioselective pathway, in which the energy levels of different excited states are subtly altered for the reaction species while the asymmetric induction can be achieved in the presence of chiral ligands.17-29 As early as the 1980s, the Lewis acid catalysts (i.e., BF3 and EtAlCl2) were first observed to regulate the spectroscopic properties and photochemical behaviors of the catalyst-substrate complexes.19 Based on this landmark discovery, a series of oxazaborolidine/AlBr3-based catalysts were examined to enable the enantioselective photoreactions of coumarins in Bach’s group.20 In our group this interesting photochemical transformation has been theoretically rationalized by a
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 15
Scheme 1. Schematic presentation of enantioselective intermolecular [2+2] photocycloaddition reaction for 1 coordinated by the chiral Lewis acids (MLn) with a diene substrate (2) through the catalyzed energy transfer from an electronically excited photosensitizer (4). Numbering scheme is shown in blue for substrates 1 and 2. noticeable energy inversion of the nπ*/ππ* states of the substrate in the catalyzed reaction, and an enhanced spinorbit coupling caused by relativistic effects resulting from heavy atoms in the chiral Lewis acid catalyst.21 Up until now, many other metal-based Lewis acids including main group,22,23 transition- or lanthanide-metal-based,24-27 and the chiral-at-metal auxiliary catalysts28,29 were employed successfully to provide the effective enantio-differentiating environments for a wide range of mechanistically distinct organic reactions. In the practical photochemical transformations, the catalyst-substrate complex can be photoexcited preferentially over the unbound achiral substrate, in which the participation of racemic background reactions is minimized considerably to obtain the chiral product with high ee under relatively mild conditions.17,18,20,22-29 Although a significant advance has been achieved, the contribution of uncatalyzed background reaction cannot be eliminated completely in the presence of a unique catalyst of chiral Lewis acid while the catalyzed reactions require careful irradiation with a monochromatic light source and high catalyst loadings (typically ~50 mol%).20,24 To overcome these shortcomings, the dual-catalysis strategy was developed recently by the introduction of an extra photocatalyst, such as Ir or Ru transition metal complexes, to regulate cooperatively the catalysis reaction with the auxiliary catalyst of chiral Lewis acids.30-32 As a paradigm example shown in Scheme 1, tris(2,2-bipyridine) ruthenium(II) complex was applied in Yoon’s group to act as a photosensitizer enabling the enantioselective intermolecular [2+2] photocycloaddition (PCA) reaction for the substrate 2-hydroxychalcone (1) coordinated by the chiral Lewis acid with another substrate diene (2).25 Catalyzed [2+2] PCA reaction takes place under the mild condition, in which the energy level of precursor state is considerably lowered for the coordinated substrate 1 in the presence of the chiral Lewis acid. As an important consequence, the coordinated substrate 1 can be photochemically activated by the Ru photocatalyst (4) upon the visible-light irradiation through an intermolecular triplet-triplet energy transfer (EnT). In this case, the unbound substrate 1 is totally inactivated, which eliminates largely the racemic background reaction, thus making it possible to reduce catalyst loadings by using simple household light sources or
even promisingly the sunlight. Dual-catalysis has been considered up to now as an attractive strategy to furnish enantioselective products by using the combination of chiral and achiral catalysts, which is generally believed to proceed via the single electron transfer (SET) or EnT processes between the photo-sensitizer and various substrates, thereby enabling the visible-light-induced photochemical transformations.30-40 The merger of photosensitizer and auxiliary catalysts offers a robust, flexible strategy to control the reactions among dual catalyst systems and the different substrates. The introduction of extra-reductant or oxidant has been usually considered as the judging criteria to determine the reaction channel of photochemical transformation.25a,33,34,37 However, the knowledge of the photo-chemically tunable metal-ligand interaction for photocatalysts, the manipulation principle of excited-state properties and the available electronic excitation for the free and bound substrates, is generally required but is still sparse at the present stage.41-44 The competitive coexistence SET and EnT pathways should be also identified precisely. In this paper, we first report the intermolecular EnT path of the paradigm example of asymmetric [2+2] PCAs of 2-hydroxychalcones (1), using tris(bipyridyl) ruthenium(II) as a photosensitizer (see Scheme 1). The EnT process can be compared with the SET and energy regulatory pathways mediated by the single catalyst of Lewis acid or transition (actinide) metal complex in our previous works.45-47 This study aims to provide the evaluation strategy of photocatalytic efficiency for dual catalyst system based on the accurate electronic structure calculations and kinetic estimation of EnT rate and to facilitate the mechanism-based design for single or dual catalyst applications. METHODS The ab initio electronic structure calculations of the free 2-1 substrate and its complexes coordinated by three metal-based Lewis acids (2-1-ML3, M=Sc3+, Al3+ and Y3+; L= Tridentate chiral ligand (S,S)-t-BuPyBox) as well as the photocatalysts of [Ru(bpy)3]2+ (4) were primarily performed at the complete active space self-consistent field (CASSCF) level of theory. To perform multi-configurational CASSCF calculations, numerous test calculations are
ACS Paragon Plus Environment
Page 3 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
generally required to obtain an appropriate active space that can be used to represent a variety of electronic states and photochemical reactions, including but not limited to different types of charge transfer (CT) transitions, the excited state intra-molecular proton transfer (ESIPT) reaction, and the [2+2] PCA reaction with or without the Lewis acid catalysts, in order to achieve a good balance between computational cost and accuracy. In brief, a total of 14e/12o, 12e/11o, 12e/9o and 4e/4o active space was employed for the free 2-1, 2-1-ML3 complexes, the photosensitizer 4 and the 4-1-ML3 complexes, respectively. More details on all active orbitals for the 2-1, 2-1-ML3, 4 and 4-1-ML3 are described in supporting information (SI) and schematically shown in Figures S1-S4. The pseudopotential basis sets, which were developed by Dolg’s group48 and applied to simulate photocatalysis with metal ions successfully in our previous works,45-47 were used for metal atoms. To reduce the unaffordable computational cost on the complexes involving metal-based Lewis acids at the CASPT2//CASSCF level, the 6-31G(d) basis set was employed for all other atoms in 4 and free 2-1 and the nonmetal heavy atoms at the reaction center in 4-1-ML3 and 21-ML3, while a smaller STO-3G basis set was applied to the remaining atoms in 4-1-ML3 and 2-1-ML3. Note that the mixed basis sets should be validated using the dividing strategy by setting boundary at the appropriate σ-bonding region without losing the conjugative effect. More details on the choices of basis sets for different calculations are shown in SI. Solvent effect was considered using the polarizable continuum model (PCM)49 for the acetonitrile matrix for all calculations. The preliminary geometry optimizations and frequency analyses for all stationary points (minima and transition states) were performed at the density functional theory (DFT) level using the B3LYP functional. The results of frequency analyses on minima were also necessary for our calculations on energy transfer rates as discussed below. The structures of stationary points were further optimized at the CASSCF level. Several IRC computations50 were followed to obtain the minimum energy profiles (MEPs) that connect critical points in multiple electronic states. Other critical points such as conical intersections and singlet-triplet crossings (STCs) were obtained using CASSCF optimizations. To consider dynamic electron correlation effects, the refined single-point energy calculations on critical points as well as the intermediate structures along the MEPs were performed at the multiconfiguration second-order perturbation (CASPT2) level of theory based on the zeroth-order multiple roots stateaveraged CASSCF wave functions. The vertical excitation energies and oscillator strengths (f) in the Franck-Condon region were obtained from 13 (4), 6 (2-1-ML3 and 1-ML3), 5 (2-1) roots state-averaged CASSCF state interaction (CASSI) computations. The spin-orbit coupling (SOC) between the singlet and triplet states was also computed using CASSI with effective spin-orbit terms for the metal atoms.51 All DFT and CASSCF calculations were implemented using the Gaussian program package,52 while the CASPT2 computations were carried out with the Molcas 8.0 program package.53
The triplet-triplet energy transfer rate was calculated using the approach within the general formalism of nonradiative transition models, which was first developed by Lin et al.,54-57 and successfully applied to study the mechanism of tunable emission for a real WOLED of FPt in our group.58 Based on the Born–Oppenheimer approximation and the Fermi’s golden rule, the energy transfer rate constant is given by 2 2 2 (1) i Hˆ f Piu iu fv E fv Eiu Wi f = h u ,v where Φ and Θ are electronic and vibrational wavefunctions, respectively, i and f denote the initial and final adiabatic electronic states, respectively, Ĥ is the transition operator that represents the nonadiabatic effect and perturbs the system from state i to state f, u and v denote nuclear vibrational states corresponding to electronic states i and f, respectively, Eiu and Efv are energies of vibronic states, and Piu is the Boltzmann factor. The nuclear part in Eq (1), denoted as the Franck-Condon (FC) term, can be obtained based on the multidimensional harmonic oscillator model as56 2 1 it (2) Piu iu fv E fv Eiu dte i f G j t 2 h u ,v j where
G j (t )
P
iu j
u j ,v j
iu fv j
2 j
e
1 it v j j 2
e
1 it u j j 2
(3)
j is the vibrational frequency of the jth normal mode,
represents the nuclear wavefunction of harmonic oscillators, and if is the adiabatic energy difference between electronic states i and f.54-57 Only displacements of normal modes between two electronic states are considered in our calculations, but it is reasonable for the present systems with high rigidity on the geometry structures during the whole photochemical process. The electronic part in Eq (1) under the Dexter model is the square of electronic coupling between two electronic states of the donor-acceptor complex as54,59 i Hˆ f
2
D* 1 A 2
1 A* 1 D 2 r12
2
(4)
where φD*, φD, φA* and φA are singly occupied orbitals of the donor (denoted as D) and the acceptor (denoted as A), respectively, and 1 and 2 denote the two exchanged electrons during the Dexter EnT. The wavefunctions of the donor-acceptor complex can be used to calculate the electronic coupling term as 1 D* 1 A 2 A* 1 D 2 (5) r12
c c c c ij kl i
D*
j
A*
k
A
l
D
i , j , k ,l
where (ij|kl) is a two-electron integral, (i, j) and (k, l) are the basis sets associated with electron 1 and 2, respectively, and c is the coefficient of the singly occupied orbitals. More details on our computational method can be seen from SI and Ref 58.
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 15
Table 1. Vertical excitation energies (E ⊥, kcal/mol) compared with the experimental values in parentheses if available,25a,60 oscillator strengths (f), dipole moments (D.M., Debye), the character of singly occupied molecular orbitals (SOMOs) for various electronic transitions of 1 and 1-ML3 (M=Sc3+, Al3+ and Y3+), as well as the schematic orbitals of 1-ScL3 as an example.
1
Transitions
E⊥
f
D.M.
SOMO
S0SCT-D (1ππ*)
96.4 (90.7)
0.82
4.210.8
π1 π*
S0SCT-P (1ππ*)
79.7 (81.7)
0.40
4.28.6
π2 π*
87.9
4.610-6
4.22.1
n π*
S0SNP 1-ScL3
1-AlL3
(1nπ*)
S0SCT-D
(1ππ*)
66.2 (67.9)
1.32
17.410.0
π1 π*
S0SCT-P
(1ππ*)
99.4
0.35
17.421.5
π2 π*
S0SNP (1nπ*)
107.6
0.02
17.420.9
n π*
S0SCT-D (1ππ*)
68.7
1.20
18.911.5
π1 π*
S0SCT-P
(1ππ*)
104.1
0.32
18.920.6
π2 π*
(1nπ*)
108.0
0.04
18.915.3
n π*
(1ππ*)
70.2
1.12
18.310.7
π1 π*
S0SCT-P (1ππ*)
104.9
0.32
18.321.7
π2 π*
S0SNP (1nπ*)
98.1
0.05
18.321.5
n π*
S0SNP
S0SCT-D 1-YL3
π1
π2
n
π*
Scheme 2. The diagrams of PICT for 1, metal-enhanced PICT and metal-reduced PICT for 1-ScL3, and their intrinsic dipole moment vectors. RESULTS AND DISCUSSION Tunable Excited State Properties for the Enone Substrate via the Chiral Lewis Acid―2-hydroxychalcone Coordination. Table 1 summarizes vertical excitation energies (E ⊥ , kcal/mol), oscillator strengths (f), and the changes of dipole moment (D.M., Debye) of different transitions for the 1 and its complexes (1-ML3, M=Sc3+, Al3+ and Y3+) as well as the assignment of the excited-state character. A pair of bright spectroscopic states were found for free 1 with the same magnitude of oscillator strengths (0.82 and 0.40) that are much larger than that of the dark nπ* transition. Population analyses reveal more details on their excitation characters, both of which exhibit a significant charge transfer (CT) feature. One originates from the C3=C4 double bond to the C2=O1 carbonyl moiety
conjugated with the phenylethylene group, denoted as SCT-D (1ππ*). According to CASPT2 computations, about 0.28 e is immigrated from phenyl-ethylene to carbonyl group upon the FC excitation. The other originates from the phenolyl ring to the same conjugated moiety as SCT-D(1ππ*), denoted as SCT-P(1ππ*). The photo-initiated charge transfer (PICT) was observed in this case that 0.48 e is bifurcated towards two chromo-phores of C2=O1 (0.31 e) and phenylethylene (0.17 e) moieties, respectively (see scheme 2). As shown in Table 1, the PICT directions for these two excitations agree well with the changes on dipole moments. In the presence of the substrate-chiral Lewis acids with different metals (Sc3+, Al3+ and Y3+), a dramatic energy level inversion was obtained computationally between two bright spectroscopic states of SCT-D(1ππ*) and SCT-P(1ππ*). As
ACS Paragon Plus Environment
Page 5 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
shown in Table 1, the S0SCT-D(1ππ*) transition demonstrates a considerable bathochromic shift from ultraviolet (1: 96.4 kcal/mol, 297 nm) to visible spectral region (1-ML3: 66.2-70.2 kcal/mol, 432-407 nm). Chiral Lewis acid coordination also leads to a noticeable increase of oscillator strength (1: 0.821-ML3: 1.12-1.32). It is consistent with experimental observations that the maximum absorption of the isolated trans-1 was measured at 315 nm60 and redshifted to 421 nm25a for 1-ScL3 complex. Similar redshifts (> 100 nm) of the maximum absorption with one-fold intensity increase were repeatedly detected in a series of Lewis acid-1 complexes chelated by the electron deficiency boron.61 Unlike the decreased energy level of SCT-D(1ππ*) state, the vertical excitation energies of SCT-P(1ππ*) state are shifted up to 99.4-104.9 kcal/mol for 1-ML3 complexes from 79.7 kcal/mol of free 1. The shifted-up SCT-P(1ππ*) state for 1-ML3 complexes coincidentally overlaps with the maximum absorption of S0SCT-D(1ππ*) for the free 1 (96.4 kcal/mol), which may cause some confusions in differentiating signal origin, especially for the case of low Lewis acid loading.20e,25a The energy level inversion for 1ML3 complexes is due to the significant electron-withdrawing effect of the positively charged metal ions on the carbonyl group. This effect promotes the PICT of S0SCT-D (1ππ*) transition along a favorable direction from C3=C4 to C2=O1 moiety (0.28 e for free 1 and 0.38 e for 1-ScL3), named as metal-enhanced PICT. Conversely, for the S0SCT1 P( ππ*) excitation the transferred charge from the phenolyl ring is decreased by 20% and the bifurcated PICTs proceed mostly along the reverse direction compared with the metal-enhanced PICT, named as metal-reduced PICT. More details can be seen from the results on dipole moment calculations in Table 1 and Scheme 2. Besides two tunable CT ππ* state, the nπ* state is also energetically regulated to the medium ultra-violet region (~100.0 kcal/mol, 286 nm) upon complexation with Lewis acid, which has been discussed in our previous works.21 As an important consequence, the SCT-D(1ππ*) state escapes successfully from the interference from the phenol group involved CT and nπ* states via an effective metal-induced energy separation (> 27 kcal/mol). Other possible unexpected deactivation channels are excluded. On one hand, the optimal performance of metal-tuned excited state properties for 2-hydroxychalcone based substrates can be ascribed to the introduction of an additional orthocoordinate site (i.e., phenolic hydroxyl) for the metal center of Lewis acid, which considerably stabilizes the metalcarbonyl complexation in enone moiety, facilitates the metal-enhanced PICT and reduces the energy level of the PCA reaction precursor of double bond involved CT state. On the other hand, since the regulatory function of phenol group involved CT state can be ruled out by the metalreduced PICT for 1-ML3 complexes, the utilized phenol chromophore unlikely gets involved in the relaxation of PCA reaction precursor state. These structural advantages can be further optimized by the incorporation of electrondonating group into the phenylethylene moiety rather than the substitution in the phenol group. Indeed, in Yoon’s
laboratory the electron-donating group substitutions were frequently adopted in phenylethylene moiety based on the chelating phenolyl molecule skeleton in Lewis acid catalyzed sensitization of chalcones, providing the cyclobutane products with high yields and excellent enantioselectivities.25 Racemic Background Reaction of 2-hydroxychalcone with diene. The racemic background reaction of the 2-1 complex without chiral-center ligands was illustrated in Figure 1. Upon UV light photoexcitation at 297 nm, the complex is instantaneously promoted in the FC region of the double bond involved SCT-D(1ππ*) state. The initial decay of enone form 1(E) in SCT-D(1ππ*) state is characterized structurally by the elongated C2=O1 (1.221.29 Å) and C3=C4 bonds (1.351.43 Å) accompanied by geometric adjustments of C-C bonds in the phenyl ring, which exactly reflects the character of the singly occupied orbitals for the S0SCT-D (1ππ*) transition. These structural changes produce a 13.3 kcal/mol energy decrease in SCT-D (1ππ*) state and then lead to the conical intersection (CI) of SCT-D/SCT-P. A skeleton rearrangement associated with the reinforced C6=O7 bond (1.341.27 Å) is further imposed by the electron deficiency of phenol ring due to the PICT of SCT-P(1ππ*) state relaxation, leading to an approximate equal length of C6=O7 and C2=O1 (~1.28 Å) bonds at the minimum of SCT-P(1ππ*) state, SCT-P-Min. Besides the structural changes during the aforementioned process, an abundant amount of negative charge was found to be gathered considerably around the proton acceptor of carbonyl O1, i.e, S0(E) (-0.59) CI(SCT-D/SCT-P) (-0.75) SCTP-Min (-0.80), which imposes a strong attraction on the opposite proton H8 of phenol ring. Consequently, intramolecular hydrogen bonding (O1H8) is remarkably reinforced in SCT-P(1ππ*) state to trigger immediately an ESIPT. A tiny barrier (1.3 kcal/mol) for ESIPT was found computationally, which is in accordance with the experimentally observed ultrafast tautomer-ization within a picosecond timescale.62 The excited-state ketone form intermediate (SCT-P-K) is then generated and undergoes an excited state isomerization of phenol ring followed by the ground state cyclization reaction and enol-keto tautomeric reaction assisted by aqueous solution, eventually yielding a flavanone product. More details are shown and discussed in the section 2 of SI and the previous experimental works.63 Another nonradiative channel for the SCT-P-Min was revealed through a singlet-triplet crossing (STC) of SCT-P/ TCT-D. After reaching the minimum in TCT-D(3ππ*) state (TCT-D -Min) with a growing diradical character to attract the approaching of diene 2, the 2-1 complex evolves into STC(TCT-D/S0) and then converts into the final cycloaddtion products (3 and its enantiomer ent-3) in the ground state. Although the barrierless reaction paths in the triplet and ground states have been observed computationally, the whole transformation efficiency is predominated by the intersystem crossing (ISC) from SCT-P(1ππ*) to TCT-D(3ππ*) at the starting point in the triplet state. The ISC rate was calculated as 7.1108 s-1 due to the small spin-orbital coupling (0.5 cm-1) at the STC(SCT-P/TCT-D), which is much
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 15
Figure 1. MEPs for the competitive coexistence of [2+2] PCA and ESIPT reactions for 2-1 complex at the CASPT2//IRC/ CASSCF(14e/12o)/PCM level of theory. The highlighted critical points of the 2-1 complex with their key bond distances are schematically shown in SI. slower than the competing ESIPT within picosecond timescale.62 It is consistent with experiments that a considerable yield of flavanone (up to 91%) was observed in photochemical cyclization of the isolated 163c while the cycloadditions of 2-hydroxychalcone with dienes can exhibit modest levels of background cycloaddition with near UV irradiation.25 Catalyzed PCA Reaction Mediated by the Chiral Lewis Acid Catalysts. As shown in Table S3 of SI, the difference on the vertical excitation energies between 1-ScL3 and 2-1ScL3 can be neglected. Upon visible light irradiation (430440 nm), 2-1-ScL3 complex is initially populated in the double bond involved SCT-D(1ππ*) state and rapidly decays to SCT-D-Min with 4.0 kcal/mol energy decrease (see Figure 2). However, the following nonradiative channels for 1-ML3 and 2-1-ML3 in the SCT-D state are different from that for free 1. First, the intermediate relay of the phenol group involved CT state (SCT-P) is unlikely to participate in the photochemical processes of 1-ML3 and 2-1-ML3 complexes because of the metal-reduced PICT as discussed above. Second, the rates of a direct ISC from SCT-D(1ππ*) to TCT3 7 -1 3+ D( ππ*) were calculated to be ~10 s for 1-ML3 (M=Sc , Al3+ and Y3+) complexes resulting from the small SOCs (0.20.4 cm-1) and the large singlet-triplet energy gaps (> 12.0 kcal/mol, see Table S1 in SI). The nonradiative transitions to the triplet state are 3-fold slower than fluorescencedecay process with experi-mentally measured lifetimes within sub-nanosecond-to-nanosecond range for the boron coordinated 2-hydroxy-chalcone,61a decreasing the PCA yield through the triplet state transformation. It is advantageous in practice because the unfavorable PCA reaction with a unique catalyst of chiral Lewis acid, which is inferior to the dual catalyst reaction as discussed above, can be precluded due to the small ISC rate. Experimentally,
metal chelated chalcones were usually developed as the promising fluorophores for applications.61,64 Experimentally, a tiny quantum yield (
