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A pOH Jump Driven by N=N Out-of-Plane Motion in the Photo-Isomerization of Water-Solvated Triazabutadiene Hongmei Xiao, Lishuang Ma, Wei-Hai Fang, and Xuebo Chen J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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The Journal of Physical Chemistry

A pOH Jump Driven by N=N Out-of-Plane Motion in the Photo-isomerization of Water-Solvated Triazabutadiene Hongmei Xiao, Lishuang Ma, Weihai Fang and Xuebo Chen*

Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Department of Chemistry, Beijing Normal University, Xin-wai-da-jie No. 19, Beijing, 100875, People´s Republic of China

* Email: [email protected] (XC)

ACS Paragon Plus Environment

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ABSTRACT

Utilization of photo-initiated isomerization reaction has recently emerged as a very promising platform to modulate the basicity of compounds, however theoretical insight to its regulatory mechanism remains largely unknown and needs to be addressed. For the first time, an unexpected trans-cis photoisomerization via the N=N out of plane (NOOP) motion triggered by an in-plane inversion of N-N=N moiety was computationally demonstrated to regulate the pOH jump of water-solvated triazabutadiene by using the multi-configurational perturbation theory together with the calculation of rate constants of protonation-deprotonation reactions. Kinetic analyses show that the dramatic pOH change can be attributed to the reinforced intramolecular hydrogen bonding resulting from water cluster reorientation and the enhanced coupling between rotated π orbital and N lone pair of triazabutadiene in the remarkable trans-cis photoisomerization.


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1. Introduction The utility of acid-base chemistry of electronically excited states is naturally appealing to chemists, the study of which is essential for the understanding of a multitude of chemical1-2 and biochemical3-6 processes, including enzymatic catalysis, biochemical energy storage, and photosynthesis.7-8 The idea was inspired by using the change in acidity or basicity of a molecule upon irradiation to spatially and temporally regulate the excited-state proton transfer (ESPT) processes and proton concentration. It has long been understood that Brønsted acids and bases that lose or accept more rapidly a proton upon electronic excitation, thus exhibiting an enhanced excited-state acidity and basicity compared with those in the ground state.9-10 The proton-transfer dynamics of photoacids can be examined by using the time-resolved fluorescence or optical pump-probe spectroscopy.11-12 These events are generally classified to several fundamental steps, which include the breaking/formation of hydrogen bonds in hundreds of femtoseconds, solvent reorientation and relaxation in picoseconds, proton dissociation/transfer and relaxation in tens to hundreds ps, and finally diffusion and geminate recombination of the dissociated proton with the conjugated photobase followed by fluorescence emission in nanoseconds.9, 13 As the prototypical examples of photoacid, the aromatic compounds substituted with a hydroxy group, like 1naphthol,14-16 2-naphthol,17-18 pyranine,19-20 hydroxyquinoline,21−25 and quinine cyanine26−28 are shown to be capable of an acidity enhancement upon photoexcitation with a significant pKa increase (up to 12 units).29 In contrast to the wealth of investigations on photoacids, few examples of photobases have been reported up to date. The pOH jump was first observed by Weller in photoexcitation of acridine or 6-hydroxyquinoline as early as the mid-20th century.30 The similar photobasic phenomenon was recorded in the analogues of 6-methoxyquinoline31and 6-aminoquinoline32 by


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using ultrafast spectroscopy. In aqueous solution, these compounds are the weak bases in the ground state, which transform into the strong bases in their excited states through the rapid proton abstraction from water, producing the ion pair of PhN*H+ and OH-.33 Since its inception, the photobasic phenomenon was reproduced in a series of compounds containing the heterocyclic nitrogen, particularly in a naphthalene framework such as curcumin,34 xanthone,35 3-styrylpyridine36 and 9-Hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine.37 In addition, a family of bifunctional photoacids with a strong basic side group was explored experimentally to control their excited-state reactivity, leading to an simultaneous enhancement in acidity-basicity through the functional OH and COO- groups.38-39 Unlike the traditional case by employing ESPT reaction, the utilization of photoinduced trans-cis isomerization reactions followed by the chemical bond fission has been recently developed to alter considerably the Lewis basicity of triazabutadiene scaffold 1 upon the irradiation of hand-held UV lamp.40-41 As shown in Scheme 1, the photobasic functional group of N1=N2-N3 (see scheme 1 for numbering) is initially activated by photoexcitation to trigger instantaneously a fast isomerization reaction, producing a more unstable Z-form (1-Z). This leads to a quick pH jump of solution to 9.8 from 9.0 via the N3 or N1 protonation reactions in the early phase after UV irradiation. The pH jump is suppressed to some extent since the weak basic 2 is significantly formed through a thermal N2-N3 bond fission of protonated Z [1-Z(N3H+)] and E [1-E(N3H+)] isomers, thus liberating a protected aryl diazonium species. In most cases, 2 is significantly formed through a thermal N2-N3 bond fission of protonated Z [1-Z(N3H+)] and E [1-E(N3H+)] isomers, thus liberating a protected aryl diazonium species. In most cases, however, the reaction can proceed along the predominant N1 protonation pathway but without the


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Scheme 1. A tunable photobase is shown through the photo-isomerization of triazabutadiene 1 followed by the protonation and N-N bond fission reactions. Numbering scheme is given in red. occurrence of N2-N3 bond cleavage (see left panel of Scheme 1), which ensures a basicity enhancement of the aqueous solution containing the water-soluble 1. The pioneering experiments have launched a promising platform to design various photobases with long-term stability and operational ease.40-41 This inspires researchers to contribute a mechanistic elucidation of all possible influence factors regarding the controversial photo-isomerization mechanism via rotation or inversion fashion around N=N bond,42-49 and to assess the kinetic control of the bond-cleaving reactivity in a pH-dependent manner. In this work, we therefore employed a multi-configurational quantum chemical approach to map the reaction pathways of photoisomerization of 1 and the subsequent protonation and bond fission, and to perform the rate calculation of protonation reactions (for computational details see Supporting Information, SI). The aim is to disclose the control factors for the pH modulation of photobase


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by the utility of photo-isomerization and the associated mechanism-based principle for the ideal design of photobases containing with labile group. 2. Computational methods 2.1 Complete active space self-consistent field calculations The complete active space self-consistent field (CASSCF) method 50-51was used to perform the ab initio calculations of 1. After a series of computational tests, it turned out that a balanced active space with a total 12 electrons in 11 orbitals, referred to as CAS(12e/11o) hereafter, was chosen for the present CASSCF computations of 1. To characterize the precursor nπ* state for the photo-isomerization reaction, the active space includes one lone pair that delocalizes three nitrogen atoms of N1=N2-N3 moiety, respectively, and the lowest unoccupied π* orbital. To account for all possible ππ* transitions and the conjugated effects, the rest 10e/9o active orbitals originate from high-lying occupied π and low-lying π* orbitals that distribute in the benzene ring (4e/4o), imidazole (4e/4o), and the one more π orbital of N1=N2-N3 moiety, respectively. It should be noticed that the nπ* state optimization is often interfered by the correlated ππ* transition of N1=N2-N3 group. Therefore, the π orbital of N1=N2-N3 moiety was excluded out of active space when the photo-isomerization path computations were performed. As a result, the CAS(10e/10o) active space was employed to map the minimum energy path (MEP) of photo-isomerization in the nπ* state. All these orbitals in active space were schematically shown in the Table 1. 2.2 Complete active space perturbation theory and minimum energy profiles mapping The state-averaged CASSCF method52 was taken to determine the geometry of the stationary points on the excited states. A two-root (S0, SNP) state-averaged procedure with equal


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weights was used in SNP(1nπ*) state calculation while single root optimization was employed in ground state. The optimized critical points such as Franck-Condon (FC) geometries, minima and conical intersections were successively connected by intrinsic reaction coordinate (IRC) computations53-54 to map the MEPs of the photoisomerization reaction for 1. To consider the dynamical electron correlation effects for these points, the refined single-point energy was recalculated at the multiconﬁguration second-order perturbation theory level (CASPT2)55-56 using a six roots state averaged CASSCF (12e/11o) with standard zeroth order hamiltonian (H0) wave function. To account for the external part of the zero order Hamiltonian, a shift value is set to 0.2. The charge distributions were obtained using a full Mulliken population analysis at the CASPT2 level of theory. The 6-31G* basis set was applied for all atoms. Therefore, the MEPs for photoisomerization reaction of 1 were eventually computed at the CASPT2//IRC/CASSCF/631G* level of theory along the unbiased reaction coordinates to understand how the isomerization reactions take place in the different electronic states. 2.3 The MEPs mapping for deprotonation-protonation reactions by using DFT calculations It is well established that two coexisted structures of the hydrated excess proton, the Zundel (H5+O2) and the Eigen cation [H3O+·(H2O)3], have been identified by spectroscopy of water clusters.57-58 Recently, H5+O2 was demonstrated to be at least as important as the Eigen cation or its variant H3+O in Nibbering’ group, which are the smallest clusters of water to capture the fluxional nature of the hydrated proton.59 Computational tests were conducted to inspect the structural character of water cluster in the solvent surrounding of triazabutadiene. However, the N1 and N3 protonation states of 1 can not be obtained by the minimum optimizations of 1(Z, E)H5+O2 water complexes. In contrast, protonation reactions proceed smoothly by using the large water cluster of the Eigen cation [H3O+·(H2O)3]. This indicates that a large water cluster is


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required to stabilize the N1 and N3 protonation states for triazabutadiene with the large-scale spatial structure. Eigen’s protonated water structure has been verified to be remarkable stability compared to other typical water clusters, which can also be demonstrated directly by mass spectrometry.60 A "four-cluster" structure (H9O4+) has been hypothesized as the most probable hydrating unit for the proton in acid-base equilibria, charge transfer, and other chemical systems.61 Therefore, a solvated hydronium complex of H3O+·(H2O)3 (Eigen cation) was used to calculate the MEPs for the N1 and N3 protonation reactions in the ground state. Test computations were performed using density functional theory (DFT) at the B3lYP/6-31G* level to determine the structural configuration of the 1(Z, E)-H3O+.(H2O)3 water complexes that will be employed as the simulation model for the protonation reactions. The frequency calculations were numerically performed to confirm minima or transition states (maxima) for selective stationary points of different compounds at B3LYP level of theory. Solvent effect was included using the polarizable continuum model (PCM) for the water matrix (ε=78.3) at the temperature of 298.15 K for all the optimization and IRC computation of protonation reactions. In this work, all of the DFT and CASSCF calculations, together with the IRC pathway calculations, were performed using the Gaussian program package,62 whereas the CASPT2 computations were carried out with the Molcas program package.63 2.4 Calculation of pKb for various isomers The conventional acid–base equilibria of a Brønsted base in water can be written as:

(Eq. 1) With an equilibrium constant, Kb, given by, 32


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(Eq. 2) The equilibria of reactant 1 with H9O4+ in the case of this work can be written as:

(Eq. 3) Finally, the total rate constant can be expressed as:

(Eq. 4)

(Eq. 5) The forward (k+) and backward (k-) rate constants can be obtained on the basis of transition-state theory (TST) by using the Arrhenius law (Equation 5). 64-66

k   Ae

E  / RT

(Eq. 6)

where A is a pre-exponential factor with the dimensions of s-1 for unimolecular reaction, activation energy (E) is the classical potential barrier along the minimum energy path with zero-point energy (ZPE) correction. A is a frequency factor (units s-1) given by Eq.7.

s   i i 1  A  

s 1  2   i  i1 

(Eq. 7)

The unit conversion from cm-1 for normal mode vibration frequencies and frequency factor A to rate unit s-1 is taken by using the following formula


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c

 

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(Eq. 8)

Where ν is the unimolecular vibration frequencies,  is the wave number and c is the velocity of light. Therefore, the expression of frequency factor can be written as follows: s     (31010 )  i1 i  A s 1  2  i  i  1  

(Eq. 9)

So finally the unimolecular rate constant can be expressed as:

s    (31010 )  i1 i  k  e s 1 2     i1 i 

E  / RT

(Eq. 10)

3. Results and discussion 3.1 Photoisomerization reaction of triazabutadiene in 1nπ* state Table1 summarizes the vertical excitation energies (∆E, kcal/mol), oscillator strengths ( f ), and dipole moments (DM, Debye) of the different transitions for the 1 and the assignment of the excited-state character. The lowest-lying excited state of the S0SNP(1nπ*) transition originates from the promotion of the one lone pair that distributes respectively three nitrogen atoms of N1=N2-N3 moiety to a π* orbital that is delocalized along the N1=N2-N3 and phenyl ring moieties. The S0SNP(1nπ*) transition of 1 shows a small change in the dipole moment (9.910.8 D) and the small oscillator strength (0.0047). This suggests a “dark” spectroscopic


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Table 1. Vertical excitation energies (∆E, kcal/mol), oscillator strengths (ƒ), dipole moments (D.M., Debye), and the character of singly occupied orbitals for different transitions of 1 calculated at the CASPT2//CASSCF(12e/11o)/6-31G* level of theory. Transitions S0 S0→SNP(1nπ*) S0→SCT(1ππ*) S0→SPP1(1ππ*) S0→SPP2(1ππ*) S0→SPP3(1ππ*)

D.M. 9.9 10.8 16.3 9.6 9.5 8.7

ƒ 0.0047 1.0 0.033 0.011 0.026

∆E 0.0 67.3 77.1 96.8 116.1 127.1

Singly occupied orbitals n πN=N-N π3 π1 πN=N-N

πN=N-N* πN=N-N* πN=N-N* πN=N-N* π2*

state and a lower charge transfer (CT) character for the nπ* transition, which is a common feature for compounds containing the azobenzene chromophore.46 Unlike the nπ* transition, the lowest-lying 1ππ* excited state was found for 1 that has a larger CT character and is therefore referred as SCT(1ππ*). According to the population analysis and charge translocation calculation, the photoinitiated charge translocation for S0SCT(1ππ*) transition takes place from the imidazole ring to the N3-N1=N2 moiety and phenyl ring. Accordingly, the dipole moment


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significantly increases from 9.9 D in the S0 state to 16.3 D in the SCT(1ππ*) state. The S0SCT(1ππ*) state transition has a large oscillator strength (∼1.0) and a vertical excitation energy (77.1 kcal/mol) corresponding to a ∼371 nm UV excitation that excellently coincides with the experimental absorption spectra peaked at 370 nm.40 As shown in Table 1, the other ππ* transitions investigated here are associated with electron promotion from the π orbital of phenyl or imidazole ring, which exhibit the small oscillator strengths and the relatively high excitation energies. These findings give strong support to the view that the charge transfer ππ* transition is responsible for the initial population of the photoexcited triazabutadiene under the experimental condition. As shown in Figure 1, 371 nm photoexcitation of triazabutadiene takes the system to the Franck-Condon (FC) region of the SCT(1ππ*) state. Following the initial FC excitation, transtriazabutadiene (1-E) undergoes a rapidly relaxation to the minimum of SCT(1ππ*) state, SCT-Min that is energetically 3.6 kcal/mol more stable than that of FC. The photo-initiated CT along the desired direction leads to the weakened N1=N2 bond (1.241.26 Å) with a concomitant decrease in the N1-C4 bond length (1.421.37 Å). The reinforced N1-C4 bond imposes the considerable restrictions on the rotation of phenyl ring, which indicates that the initially populated SCT(1ππ*) unlikely functions as an effective precursor state for the occurrence of photoisomerization reaction. Careful CASPT2 energy analysis revealed that SCT-Min is energetically degenerate with the SNP(1nπ*) state, which corresponds to the peaked conical intersection between SCT(1ππ*) and SNP(1nπ*) states and is referred as the CI(SCT/SNP).


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Figure 1. The minimum energy profiles for the photo-isomerization of triazabutadiene in the S0, SNP(1nπ*), and SCT(1ππ*) electronic states, obtained at the CASPT2//IRC//CASSCF level of theory. Selected stationary structures are given with their key bond lengths in Å and the N3N2N1 and N2N1C4 angles as well as N3N2N1C4 (D1) dihedral angles in . Passing through the non-adiabatic rely of CI(SCT/SNP), the decay in the SNP(1nπ*) state is characterized by the notably increased N3N2N1 (110.1124.4) and N2N1C4 (114.6126.6) angles along a sharp downhill path and ultimately reaches its quasi-minimum of SNP-Min that is obtained by full system optimizations in SNP(1nπ*) state. As illustrated in Figure 1 and Scheme 2, the imidazole and phenyl rings remain intact in the early phase of SNP(1nπ*) state relaxation,


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which are mainly attributed to the presence of attached large groups and the locked N1-C4 bond for the restricted rotation of imidazole and phenyl rings, respectively. This provides evidence that the structural deformation significantly takes place in the manner of inversion in the decay of CI(SCT/SNP)SNP-Min, which has been repeatedly proposed for nπ* state decay in many azobenzene derivatives.42-49

Scheme 2. Schematic diagram of the inversion, NOOP motion and rotation pathways of the transcis photoisomerization of triazabutadiene. Skeletonal structures for the key stationary points are schematically given in top (left) and side (right) views, respectively. Substituents of the imidazole ring are omitted for clarification. N3N2N1C4 dihedral angle (D1) are given in . As shown in structural parameter of SNP-Min (Figure 1), the N1-C4 bond is still locked at 1.37 Å with quasi-double bond character together with the sterically blocked imidazole ring in


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the presence of large substituents. These two aspects of structural hindrance cause a significant increase in intramolecular tension in the decay path of SNP(1nπ*) state. To our surprise, the N1=N2 out-of-plane (NOOP) motion was found to occur from SNP-Min, which can be seen in the top view of structural evolution in Scheme 2. This is reflected in the NOOP dihedral angle of 105.7 (N3N2N1C4) at the peaked conical intersection between SNP(1nπ*) and ground states that is denoted as CI(SNP/S0) compared with -177.8 in SNP-Min. The analogous isomerization model through hydrogen-out-of-plane (HOOP) motion was observed in presence of rigid steric constraints from the chromophore67-68 or protein environment.69 As far as we know, the NOOP motion is computationally rationalized for the first time to be driven by the increased tensile forces. In this process, N1=N2 out-of-plane (NOOP) motion plays a decisive role to lower the energy level of SNP(1nπ*) state, releasing intramolecular tension. Meanwhile, the strong NOOP distortion (twist >70, see Figure 1) results in a sharp increase in the energy of ground state and gives rise to the CI(SNP/S0) that is energetically 8.0 kcal/mol lower than that of SNP-Min. It is necessitated by the rigid steric constraints imposed by in the presence of large substituents for the rotor of imidazole ring and the formation of tight linker through the conjugated quasi-double N1-C4 bond between the rotor of phenyl ring and the rotational axis N1-N2 bond, which prohibit the one-bond flip photoisomerization and instead favor a hula-twist like mechanism characterized by the NOOP motion in the central N1=N2 bond moiety. Following the aforementioned NOOP motion, N1-C4 bond is slightly elongated to 1.39 Å in CI(SNP/S0) from 1.37 Å in SNP-Min. This loosened rotation linker allows the occurrence of single bond rotational isomerization in the ground state funneling through the non-adiabatic rely of CI(SNP/S0). As illustrated in Figure 1, the rotation pathway occurs by an out of plane torsion of the N3N2N1C4 dihedral angle labeled by D1. Like case of azobenzene derivatives, 42-48 the


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rotational isomerization proceeds smoothly around N1-N2 bond in the ground state which is characterized by a sharp downhill pathway. With the decreased D1 dihedral angle, triazabutadiene progressively exhibits cis arrangement in which N1=N2 restores its double bond character. As an important consequence, the cis-isomer (Z) is ultimately produced and is 5.7 kcal/mol less stable than the starting reactant of trans one (E). 3.2 pOH jump driven by photo-isomerization followed by the N protonation reaction The unstable cis-isomer (Z) exhibits a considerable advantage in the electronic feature and the geometric arrangement for the subsequent N1 protonation reaction (Z-N1) compared with the N3 protonation of cis-isomer (Z-N3) and these two reaction channels for trans-isomer that are labelled by E-N1 and E-N3. The MEPs for these protonation reactions in the ground state and the related kinetic parameters have been summarized in Figure 2 (a) and (b) as well as Table 2. A H3O+·(H2O)3 water cluster undergoes solvent orientation and is finally placed in the top of Z-N1R isomer through the formation of strong intramolecular hydrogen bond (1.63 Å, see left panel of Figure 2a). In contrast, the relatively weak intramolecular hydrogen bonds (1.72-1.83 Å) of ZN3-R, E-N1-R and E-N3-R were found to be non-covalently bound with water clusters in the bottom of various isomers. The test computations indicate that the H3O+·(H2O)3 cluster in E-N1R can orientate from N1 to N3 in transcis photoisomerization process, which inevitably leads to an unstable isomer of Z-N3-R. This indicates that the steric hindrance resulting from the rotary of phenyl ring drives the solvent orientation from the bottom to top of triazabutadiene. This generates the spatially allowed isomer of Z-N1-R with the strong hydrogen bonding that facilitates the subsequent N1 pronation reaction.


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Figure 2. The MEPs of proton transfer in ground state for cis (Z, a) and trans (E, b) isomers of 1H3O+·(H2O)3 complex through N1 and N3 protonation reaction channels that are labelled by Z-


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N1, Z-N3, E-N1 and E-N3, respectively, in water solution, obtained at B3LYP/6-31G*/PCM level of theory. The relative energies are shown along MEPs without the consideration of zeropoint correction. The key stationary points of reactant (R), transition state (TS) and product (P) are given with the selective bond lengths in Å. Substituents of the imidazole ring are omitted for clarification. As shown in Table 2, the rate of forward N1 protonation reaction (K+) for cis-isomer (1-ZN1) is estimated to be 11.6 folds larger than that of trans one (1-E-N1). To our surprise, the former barrier (0.29 kcal/mol) of N1 protonation is even slightly higher than latter one (0.22 kcal/mol), which unlikely accounts for the rate increase of cis-isomer N1 protonation compared with the case of trans one. Careful data analyses reveal the rate difference mainly arises from the frequency changes in the special modes associated with the O-H stretching vibration in H3O+·(H2O)3 cluster of reactants and the corresponding transition states (see section 1 in SI). As a result, the pre-exponential frequency factor (A) for the K+ of cis-isomer is 13.3 folds improved with respect to that of trans one, which is caused by the reinforced hydrogen bond resulting from water reorientation in the transcis photoisomerization. The orbital composition analyses show that the contribution of phenyl ring π orbital increase ca. 10% for the N1 lone pair of 1-Z-N1 compared with that of 1-E-N1 in the process of transcis conversion with the concomitant water reorientation (see Table S3 in the section 1 of SI). This is largely due to the enhanced coupling between rotated π orbital and lone pair of N1 in the molecular plane for 1-Z-N1 while these two orbitals adopt a perpendicular orientation in 1-E-N1. The enhanced π coupling facilitates the dispersion of positive charge when proton attacks the lone pair of N1, thereby stabilizing the protonation product and reducing the rate of backward N1 protonation reaction (K-). Consistently, the K- of 1-Z-N1 is calculated to be one third of that of 1-E-N1 (see Table 2).


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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


Table 2. The barriers (kcal/mol) with the consideration of zero-point correction for the forward (∆E+)/backward (∆E-) protonation reactions in ground state, the forward (k+)/backward (k-) rates (s-1) and their ratio (k+/k-) as well as the pkb for 1-Z-N1, 1-Z-N3, 1-E-N1 and 1-E-N3 isomers of 1-H3O+·(H2O)3 and 2-H3O+·(H2O)3 complexes in water solution, obtained at B3LYP/631G*/PCM level based on the transition state theory.

compounds

∆E+

∆E-

k+

k-

k+/k-

pKb

1-Z-N1

0.29

10.82

2.41×1014

1.67×1005

1.44×1009

4.8

1-Z-N3

3.93

10.38

8.77×1009

3.90×1002

2.25×1007

6.6

1-E-N1

0.22

9.05

2.07×1013

5.94×1005

3.49×1007

6.5

1-E-N3

2.11

6.11

3.54×1010

5.24×1003

6.75×1006

7.2

2

0.23

12.37

3.42×1011

5.80×1003

5.91×1007

6.2

As a result, the dynamic equilibrium is established between Z-N1-R and Z-N1-P tautomers through the forward/backward protonation with different rates (2.41×1014/1.67×1005), producing the enhanced basicity (pKb =4.8). Thus, the pOH jump is estimated from 6.5 for trans-1 to 4.8 for cis isomer, which quantitatively agrees with the experimental measurements (7.05.1) upon the transcis photoisomerization.40 As illustrated Figure 2 and Table 2, the N3 protonation reaction proceed more difficult than those for N1 in different isomers, resulting in an ignorable pOH jump, which is in good agreement with experimental observations.40 The N2-N3 bond fissions for trans and cis isomers were also inspected to require 14.0-20.0 kcal/mol energies, producing the diazonium species and 2 that undergoes further N3 protonation reaction with the low efficiency and makes a minor contribution for the pOH jump (see Table 2, section 1 and 2 of SI).


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4. Conclusions In summary, accurate quantum chemical calculations at the CASPT2//CASSCF level of theory have been performed to provide the first regulation theory for the pOH jump driven by the photo-isomerization

of

water-solvated

triazabutadiene.

An

unexpected

trans-cis

photoisomerization was first found to proceed through the N=N out of plane (NOOP) motion in 1

nπ* excited state, which has benefited from the initially structural distortion via inversion

fashion in the early phase of 1nπ* relaxation. Funneling through the non-adiabatic relay between 1

nπ* and ground states, the thermal rotary motion of phenyl ring is instantaneously triggered by

the loosened rotation linker, thereby producing an unstable cis-isomer with the concomitant water cluster reorientation. As a result, the quick pOH jump is ultimately achieved through the establishment of dynamic equilibrium between the fast forward protonation and the slow backward de protonation reactions. This dramatic pOH change is computationally rationalized by the reinforced intramolecular hydrogen bonding and the enhanced coupling between rotated π orbital and N lone pair of triazabutadiene. These computational insights not only contribute a detailed understanding of the NOOP photoisomerization mechanism and its specific features in the restricted environment, but also disclose the principal driving force for the pOH jump initiated by the remarkable trans-cis photoisomerization, which facilitates mechanism-based principle for the ideal design of photobases.

ASSOCIATED CONTENT Supporting Information. Analyses of pre-exponential factor, MEPs of decomposition reaction for 2, MEPs of protonation reaction for 2, Tables of absolute/relative energies and Cartesian coordinates of the optimized structures. (PDF)


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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] ORCID Xuebo Chen: 0000-0002-9814-9908 Author Contributions H. M. Xiao and L. S. Ma contributed equally to this work Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Financial support for this work was provided by the NSFC 21373029 and NSFC21421003. References (1)

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A pOH Jump Driven by N N Out-of-Plane Motion in ... - ACS Publications

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