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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Reaction Mechanism of Photodeamination Induced by ExcitedState Intramolecular Proton Transfer of Anthrols Molecule Yunfan Yang, Yong Ding, Yu Zhao, Feng-Cai Ma, and Yong-Qing Li J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of Anthrols Molecule Yunfan Yang†, Yong Ding†, Yu Zhao，Fengcai Ma* and Yongqing Li* Department of Physics, Liaoning University, Shenyang 110036, P. R. China.

Abstract Photodeamination reaction of Anthrols molecule generating the high-activity quinone methides intermediate had been investigated (J. Org. Chem. 2017, 82, 6006-6021), while lacking careful explanation for its reaction mechanism. In our research, the mechanism of Anthrols molecule photodeamination induced by excited state intramolecular proton transfer was reported for the first time. Absorption and emission spectra calculated for the work presented here agreed well with experimental results. To propose a molecular-level explanation of photodeamination reaction, we calculated bond parameters, corresponding infrared vibrational frequencies, Mayer bond orders and visualized frontier molecular orbitals of different molecules, and the hydrogen bond strengthening behavior in excited states was confirmed, enhancing the excited state intramolecular proton transfer of Anthrols molecule. Moreover, we concluded that photoisomerization weakened the bond strength between nitrogen and carbon atoms, which promoted photodeamination reaction. Finally, for visually and quantificationally revealing the photodeamination mechanism, we have established the three-dimensional potential energy surfaces for deamination reaction in different electronic states and calculated the corresponding reaction Gibbs free energies, it can be confirmed that the photodeamination reaction of Anthrols molecule must be induced by proton transfer reaction in excited state.

† These authors contributed equally to this work. *E-mail: [email protected] and [email protected].

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Introduction Excited state intramolecular proton transfer (ESIPT) acts as the most significant photochemical switching processes of excited-state hydrogen bonding dynamics.1-8 Hydrogen bond originating from electrostatic interaction offers driving force for proton transfer reactions, which depend on the changes of hydrogen-bond properties in different electronic states.9 Usually, the proton transfer reaction is an endothermic process in ground (S0) state, the keto form of molecule cannot be obtained from enol form, since keto form is thermodynamically unstable in S0 state. However, upon the photoexcitation process, the electron population will present distinct changes, its redistribution results in the stronger acidity in proton donor group and the stronger basicity in proton acceptor group, strengthening hydrogen bond interaction facilitates hydrogen proton to transfer to acceptor group.10 The exothermic photoisomerization process generates stable keto form in the first excited (S1) state. On the basis of Kasha’s rule, if a molecule emits fluorescence, in general it will be from the lowest excited state to ground state,11 while comparing with excitation energy, the emission of keto form is of a large Stokes’ shift with 8000-10000 cm-1,12 therefore the distinct dual fluorescent phenomenon originating enol and keto in S1 state will be observed, which is an important reason that the ESIPT reactions play a key role in lots of fields. For example, white organic light-emitting diodes (OLED)13,14, newer arenas of biotechnology15,16, fluorescence imaging17,18, fluorescence probing19-21, optoelectronic devices22, laser dyes23,24 and so on. Since the ESIPT reaction in salicylate derivatives was reported firstly in experiment by Weller,25 during the past five decades, the ESIPT reactions have been widely studied in theory and experiment by researchers.26-36 Recently, for example, Zhou et al. investigated the effect of different conditions and intermolecular hydrogen bond sites on ESIPT reaction for methyl salicylate in 2015.37 Zhao et al. proposed theoretically the new mechanism of the simultaneous excited state intramolecular double proton transfer in 2015.38 Li et al. summarized an advanced review of the fluorescence-based sensing mechanisms involving in ESIPT reactions in 2017.39 ACS Paragon Plus Environment

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Goswami et al. reported the photophysical phenomenon of near infra-red emission induced by ESIPT reaction for acetate in 2013.40 Wang et al. discussed the mechanism of ESIPT-induced photocyclization reactions in 2011.41 Škalamera et al.42 (in experiment) and our group43 (in theory) reported the mechanism of photodeamination induced by ESIPT reaction for p-cresol derivatives in 2015 and 2017, respectively. In 2017, Škalamera et al. have synthesized series of Anthrols derivative, and reported that the high-activity quinone methides (QMs) were observed by laser flash photolysis method in experiment. They referred to the deamination of Anthrols in CH3OH solvent might be induced by ESIPT reaction,44 while this conclusion lacks a carefully theoretical interpretation. To quantitatively and qualitatively illustrate the mechanism of photodeamination induced by ESIPT reaction, the theoretical calculation methods are carried out in our research work. The molecular-level explanations are presented by analyzing bond parameters and corresponding infrared (IR) vibrational frequencies, visualizing frontier molecular orbital (FMOs), calculating Mayer bond orders, constructing three-dimensional potential energy surfaces (3DPES) and calculating reactive Gibbs free energies.

Computational Method In order to obtain correct molecular constructions, the implicit solvation model based on density (SMD)45 is considered with density functional theory (DFT) and its Time-Dependent (TD) method. The TD-DFT functional method has been confirmed by contemporary researchers that it is efficient and accurate measure to investigate photochemical reactions.46-57 Dielectric constant (ε) has been set as 38.8 to simulate the acetonitrile solution environment in experiment. All geometric constructions are fully optimized by Gaussian 09 software58 carried out B3LYP-D3/6-31G(d,p) calculation level, the transition-state constructions are calculated by Berny arithmetic method.59 Based on the same calculation level as optimization, frequency analyses are conducted to ensure that each of the stable geometries is a real minimum, free from imaginary, while the transition-state constructions are confirmed by sole imaginary



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frequency of specific vibrational mode. The veracity of transition-state constructions can be further confirmed by calculating intrinsic reaction coordinate (IRC).60 The geometries of Anthrols, isomer Anthrols molecule (Anthrols-T) and final product QM molecules are visualized with VMD program.61 Frontier molecular orbitals (FMOs) isosurfaces are visualized with Chemcraft program.62 Mayer bond orders63 of molecule are calculated and analyzed by Multiwfn program.64 The Mayer orders between N atom and C atom are defined as α β I NC = I NC + I NC = 2 ∑∑ [( P α S )ba ( P α S ) ab + ( P β S )ba ( P β S ) ab ]

(1)

a∈ N b∈C

Where P α and P β represents alpha and beta density matrix, respectively, S is overlap matrix, total spin density matrix is P = P α + P β . For restricted close-shell circumstance, total spin density matrix P = 0 , the equation (1) can be written as I NC =

∑∑ ( PS )

ba

( PS ) ab

(2)

a∈ N b∈C

The total valence of N atom in Multiwfn64 is defined as

VN = 2 ∑ ( PS ) aa − ∑ ∑ ( PS )ba ( PS ) ab a∈ N

(3)

a∈ N b∈ N

The free valence is defined as

FN = VN −

∑I

NC

C≠N

= ∑ ∑ ( P S S )ba ( P S S ) ab

(4)

a∈ N b∈ N

For our restrained closed shell molecular system, spin density matrix P S = 0 , the free valence FN of N atom is zero, its total valence is sum of the related Mayer bond orders:

VN =

∑I

NC

(5)

C≠N

Therefore, the bonding capacity of atoms can denote as the Mayer bond order. The three-dimensional potential energy surfaces (3DPES) in different electronic states are built by constraining optimizations method as function of bond O1-H2 and N3-C4, their step size is fixed at 0.1 Å to better clarify the photodeamination mechanism induced by ESIPT reaction. The Gibbs free energy correction values (∆Gcorr) of all stable and transition-state structures are calculated by B3LYP-D3/6-31G(d,p) calculation level upon 1 atm and 298.15 K. In addition, the larger atomic basis set


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def2-tzvp65 is used for eliminating the impact of ampliative basis set and obtaining the more accurate single-point energy (SPE), the formula calculating to Gibbs free energies (G) as followes: G=SPE+∆Gcorr All theoretical calculation DFT/TDDFT methods consider the dispersion correction with ultrafine integration grid (99, 590 points).

Results and discussion Analyses of geometric constructions

To propose the molecular-level explanation about photodeamination mechanism induced by ESIPT reaction, the molecular configurations of Anthrols, isomer Anthrols-T and final product QM have been fully optimized as shown in Figure 1. To conveniently illustrate the changes of bond parameters during photochemical reactions, key atoms are numbered 1-4. The violet imaginary lines represent hydrogen bonds, proton transfer reactions can occur along them. Deamination reactions occur in bond N3-C4 moiety in different electronic states. The corresponding bond parameters are presented in Table 1. For Anthrols molecule, the bond length of O1-H2 is 1.009 Å in S0 state, which increases to 1.029 Å in S1 state, while the bond length of N3-H2 reduces from 1.716 Å in S0 state to 1.644 Å in S1 state. The hydrogen bond’s angle of δ (O1-H2-N3) is also exhibited, which changes from 152.1° to 152.9° from S0 to S1 state. It is concluded that the hydrogen bond O1-H2···N3 is strengthened upon photo-induced process. ESIPT reaction can be facilitated, since the hydrogen bond interaction provides driving force for it. Note that bond length of N3-C4 is 1.481 Å in S0 state, while the length is 1.487 Å in S1 state. The little change of bond N3-C4 is ∆r=0.00551 Å, which indicates that the intensity of bond N3-C4 hardly changes under the photoexcitation process, so photo-excitation

will not affect the deamination

reaction of Anthrols molecule. For isomer Anthrols-T, its instability of structure in S0 state has been verified via theoretical calculation method. Upon photo-induced process the isomer Anthrols-T can be obtained by ESIPT reaction, bond parameters of



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isomer form occur notable change compared with that of normal Anthrols form in Table 1. Table 1 Bond Lengths (Å) and Angles (°) for the Anthrols, isomer Anthrols-T and final product QM molecules in S0 and S1 state. Anthrols

Anthrols-T

QM

S0

S1

S0

S1

S0

S1

O1-H2

1.00861

1.02883

-

1.85902

-

3.48437

N3-H2

1.71643

1.64426

-

1.04170

-

1.01731

δ(O1-H2-N3)

152.121

152.855

-

139.687

-

105.213

N3-C4

1.48109

1.48660

-

1.52971

-

2.30835

Note that the bond length of N3-C4 on Anthrols-T molecule changes to 1.530 Å from 1.487 Å on Anthrols molecule in S1 state. We can conclude that the intensity of bond N3-C4

became

weaker

on

Anthrols-T

molecule,

which

indicates

that

photoisomerization will promote the photodeamination reaction. For final product QM molecule, we only obtain its construction in S1 state, thereinto, the bond length of O1-H2 and N3-C4 are 3.484 Å and 2.308 Å separately, which indicates that hydrogen bond O1···H2-N3 and covalent bond N3-C4 are interrupted.

Fig. 1 Optimized geometrical configurations of Anthrols, isomer Anthrols-T and final product QM molecules. Key atoms are numbered 1−4. Atom color coding: O, red; N, blue; C, cyan; H, gray.


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Analyses of infrared (IR) vibrational spectral The IR vibrational frequencies of Anthrols, Anthrols-T and final product QM molecules were also calculated at the same theory level as the optimization, and no imaginary frequencies were found. Thus, these forms were confirmed as the real, most stable structures. The shift in the IR vibrational frequency of a chemical bond can usually be taken as an indicator of bond strength.66 As shown in Figure 2 (a), the IR vibrational frequencies of hydrogen-bond O1-H2 moiety are exhibited in S0 and S1 state for normal Anthrols form, which are 2945 cm-1 and 2602 cm-1 in S0 and S1 state, respectively. After the photoexcitation process, the IR vibrational frequency of O1-H2 is of a large red shift with 343 cm-1. It indicates that the intensity of bond O1-H2 weaken in S1 state, leading to the strengthening in hydrogen bond O1-H2···N3. The IR vibrational frequencies of bond N3-C4 of Anthrols and Anthrols-T forms in S1 state are shown in Figure 2 (a), corresponding values of Anthrols and Anthrols-T are 1055.3 cm-1 and 945.4 cm-1, respectively. The IR vibrational frequency of bond N3-C4 has a red shift with 109.9 cm-1, which confirms that the ESIPT reaction causes the weakening of bond N3-C4, and then it is helpful to conduct the deamination.

Fig. 2 Infrared (IR) vibrational frequencies of (a) bond O1−H1 in the S0 and S1 states for Anthrols; (b) bond N3-C4 for Anthrols and isomer Anthrols-T in the S1 state.

Calculated frontier molecular orbitals and Mayer orders

The changes of bond length or IR vibrational frequency in most case serve as the



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indicators of bond strength. To essentially explain the changes of bond strength for hydrogen bond and N3-C4, the frontier molecular orbitals (FMOs) and Mayer orders of molecule have been calculated. Upon photo-induced process, electron population will redistribute in molecule, further influencing the hydrogen-bond strength originated from electrostatic interaction. In Table 2, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) occupy predominant transition component with 98 % from S0 to S1 state. The relevant oscillator strength (f) is 0.0658, the excitation energy is 411 nm.

Table 2 Transition types with different excitation energies λ (nm) of Anthrols and their oscillator strength (f) and their transition compositions. process

excitation

species

Anthrols

transition

λ (nm)

f

compositions

S0→S1

411

0.0658

H→L (98%)

S0→S2

336

0.0299

H-1→L (60.7%) H→L+1 (36.7%) S0→S3

310

0.0033

H-2→L (97%)

The isosurfaces of FMOs are present in Figure 3, the electron population redistribution is characterized by ππ* in electronic transition process. Note that the electron density of proton donor group decreases locating in region in green ellipse from HOMO to LUMO process, which facilitates hydrogen proton to move to proton acceptor (dimethylamino nitrogen atom) because of the stronger acidity of proton donor group (carbonyl oxygen atom), therefore hydrogen bond O1-H2···N3 is strengthened in photoexcitation process. To investigating effect of ESIPT reaction on intensity of bond N3-C4, we calculate the Mayer orders of Anthrols and Anthrols-T molecules. The value of Mayer order of bond N3-C4 is 0.903 on normal Anthrols molecule, while it decreases to 0.829 on isomer Anthrols-T molecule. Obvious reduction of Mayer orders indicates the intensity of bond N3-C4 weakening through ESIPT reaction, which further facilitates the occurrence of deamination reaction.


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Fig. 3 Frontier molecular orbitals HOMO and LUMO of Anthrols molecule (isovalue = 0.03 a. u.). The green elliptical regions are labeled with hydrogen bond moiety.

Analyses of electron spectra

The electronic transition energies can be calculated by using B3LYP-D3 method and def2-tzvp65 atomic basis set to reproduce experimental photophysical phenomena.44 According to Frank-Condon principle67, the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment. The target state is named as Frank-Condon state, the transition belongs to vertical transition, which means that the calculation of absorption and emission energy based on ground-state geometry and excited-state geometry, respectively.68 On the basis of Kasha’s rule, the processes of fluorescent radiation are only presented from the lowest excited state to the ground state.11 To accurately calculate absorption and fluorescence between S0 and S1 state, we solve three excited states (nstates=3). In Figure 4, the absorption and fluorescence spectra are depicted and broadened by GaussView program based on its default Gaussian and Lorentz function.69 Herein, X and Y axis represent wave length and molar absorption coeffcient, respectively. The value of absorption peak locates in 411 nm, coinciding with value 420 nm of absorption in experiment. Note that emission spectra exhibit a distinct dual-fluorescent phenomenon in Figure 4, the large Stocks’ shift is about 147



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nm that is generated by the ESIPT reaction, some contemporary researchers have attributed this photophysical phenomenon to photo-isomerization processes.70-76 The fluorescence signals of 478 nm and 558 nm are attributed to Anthrols and Anthrols-T molecules, respectively, which are agreement with the corresponding values in experiment. In Table 3, the obtained values of absorption and emission with theoretical computation approaches are compared with corresponding values in experiment. All results within the margin of error of 0.2 eV. It concludes that the theoretical methods that we have carried out are reliable to investigate relevant photochemical reactive mechanisms of Anthrols molecule.

Fig. 4 Absorption and fluorescence spectra of Anthrols and isomer Anthrols-T molecules employing the B3LYP-D3/def2-tzvp calculation level. The large Stokes’ shift is labeled as the difference of spectra between enol absorption and keto emission.

Table 3 Absorption and emission spectra peak values (nm) based on B3LYP-D3/def2-tzvp calculation level and corresponding values (nm) in experiment, the difference between them is represented as ∆λ (nm), E represents electron volt (eV) in parenthesis. B3LYP-D3/def2-tzvp

Theor.

Exp.

∆λ(E)

Anthrols(abs.)

411

420

18(0.13)

Anthrols(flu.)

478

450

28(0.16)

Anthrols-T(flu.)

558

575

17(0.07)


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Analysis of the reaction pathways and mechanisms

To better explain the mechanisms of deamination, the reactive three-dimensional potential energy surfaces (3DPES) as function of bond O1-H2 and N3-C4 are built by restricted optimization method in Figure 5 and Figure 6. Herein, the X and Y axis denote the linear changes of bond O1-H2 and N3-C4, respectively. The Z axis represents single-point energies with the changing of configuration.

Fig. 5 (a) 3D potential-energy surface as functions of bond lengths O1−H2 (from 1.00 to 2.20 Å) and N3−C4 (from 1.47 to 2.67 Å) of Anthrols in the S0 state. The white imaginary arrow represents the infeasible reactions, (b) the corresponding contour map.

Fig. 6 3D potential-energy surface as functions of bond lengths O1−H2 (from 1.00 to 2.20 Å) and N3−C4 (from 1.47 to 2.22 Å) of Anthrols in the S1 state. The white imaginary arrow represents the infeasible reaction; the white solid arrows represent workable reactions, (b) the corresponding contour map.

In Figure 5 (a) the 3DPES indicates that both direct and induced by proton transfer ACS Paragon Plus Environment


deamination reaction are infeasible in S0 state. The corresponding contour map has been exhibited on Figure 5 (b), note that Anthrols molecule (a) is incapable of generating thermodynamically stable Anthrols-T (b) and final QM (c) in S0 state. Figure 6 (a) shows the 3DPES of photodeamination reaction in S1 state, which indicates that the direct photodeamination reaction of Anthrols molecule (A) that plotted by imaginary line still is infeasible, while it is workable induced by ESIPT reaction. In Figure 6 (b) the isomer Anthrols-T (B) and final product QM (C) are locally most stable points in contour map, the reaction pathway of photodeamination is A→B→C. To quantificationally illustrate occurrence of photodeamination reaction, the Gibbs free energies have been calculated to approximate reactive activation energies. Herein, the transition-state constructions are sought with Berny arithmetic method59, their accuracy is confirmed via analyses of IR vibrational frequencies, that is, there is only one imaginary frequency and its vibrational mode is consistent with the reactive orientation. In addition, we calculate intrinsic reaction coordinate (IRC) to verify the transition-state constructions as shown in Figure 7. In Figure 7 (a) the summit TS1 represents transition-state construction of ESIPT reaction; the IRC connects correct reactant (Anthrols) and product (Anthrols-T). In Figure 7 (b) the summit TS2 represents transition-state construction of photodeamination reaction; the IRC connects correct reactant (Anthrols-T) and product (QM). Gibbs free energy corrected values (∆Gcorr) and single-point energies (SPE) of these constructions are calculated, the Gibbs free energies (G) are calculated with equation G=SPE+∆Gcorr as shown in Table 4. To obviously present the activation energies of ESIPT and photodeamination reactions, relevant schematic diagram is shown in Figure 8.


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Fig. 7 Calculated Intrinsic Reaction Coordinate (IRC) of transition states, (a) ESIPT reaction, (b) photodeamination reaction.

Table 4 Single-point energies (SPE), Gibbs free energy corrected values (∆Gcorr) and Gibbs free energies (G) of minimum points and transition-state constructions. The all numerical values are of atomic unit (a. u.) Anthrols

TS1

(A)

Anthrols-T

TS2

(B)

QM (C)

SPE

-866.94240

-866.94130

-866.95290

-866.94330

-866.94690

∆Gcorr

0.307643

0.303453

0.309645

0.306546

0.302740

G=SPE+∆Gcorr

-866.63476

-866.63785

-866.64326

-866.63676

-866.64420

The free energy barrier ∆G1 of ESIPT reaction (A→B) is -1.95 Kcal/mol, it is a common phenomenon that obtained negative value is due to use the Gibbs free energy correction.77-80 The potential barrier of ESIPT reaction with 0.69 Kcal/mol is obtained by calculating single-point energies. The free energy barrier ∆G1r of reversed ESIPT



reaction (B→A) is 3.40 Kcal/mol. For photodeamination reaction (B→C), free energy barrier ∆G2 is 4.13 Kcal/mol, its reversed (C→B) free energy barrier ∆G2r is 4.69 Kcal/mol. Therefore, it concludes that the photodeamination reaction induced by ESIPT reaction occurs only in S1 state.

Fig. 8 Schematic diagram of ESIPT and photodeamination reaction, relevant reactive free energy barriers are calculated.

Conclusion In this work, the mechanism of ESIPT-induced photodeamination reaction is given a molecular-level explanation based on theoretical research. Absorption and emission spectra coincide with observed results in experiment. The analyses of bond parameters and IR vibrational frequencies of different constructions confirm that the intramolecular hydrogen bond O1-H2···N3 is strengthened upon photoexcitation, facilitating the occurrence of ESIPT reaction, this conclusion also confirms that the photoisomerization further weaken the strength of bond N3-C4, facilitating its


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breakage. To essentially explain changes of strength of hydrogen bond O1-H2···N3 and bond N3-C4, the FMOs and Mayer bond orders of Anthrols molecule are calculated. Because of electron density redistribution upon photoexcitation process, enhancing acidity of donor group facilitate hydrogen proton to transfer to acceptor group, which represents as hydrogen bond O1-H2···N3 strengthening. The weaker bonding capacity of N3 and C4 atoms is exhibited in Anthrols-T molecule, since Mayer bond orders of bond N3-C4 is decreasing compared with that in Anthrols molecule, which indicates that the strength of bond N3-C4 is weaken by ESIPT reaction. According to the constructed 3DPES and calculated reactive Gibbs free energies barriers, we can clearly indicate that deamination or photodeamination reactions cannot direct occur wherever in S0 or S1 state, while the photodeamination reaction can be induced only by ESIPT reaction.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11474141 and 11274149), the Natural Science Foundation of Liaoning Province (Grant No. 20170540408), and the Science and Technology Plan Project of Shenyang City (Grant No.17-231-1-06).



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