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Theoretical Investigation of the Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of Cresol Derivatives Yunfan Yang, Yanzhen Ma, Yu Zhao, Yanliang Zhao, and Yong-Qing Li J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11571 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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The Journal of Physical Chemistry

Theoretical Investigation of the Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of Cresol Derivatives Yunfan Yang,a, b Yanzhen Ma,a Yu Zhao,a Yanliang Zhao,c and Yongqing Lia, b⃰ a

Department of Physics, Liaoning University, Shenyang 110036, P. R. China.

b

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China. c

School of Physics and Optoelectronics Engineering, Ludong University, Yantai

264025, P. R. China.

ABSTRACT: The novel photodeamination process of cresol derivatives 1 and 3 has been reported experimentally [Škalamera, D. et al. J. Org. Chem. 2015, 80, 10817]. However, a full theoretical interpretation of the mechanism is still lacking. In the present study, we aim to provide insight into the factors that promote the deamination reaction through density functional theory (DFT) and time-dependent DFT methods. Calculated absorption and emission spectra are in good agreement with the experimental results. Hydrogen-bond strengthening in the excited state has been verified by analyzing relevant bond parameters and vibrational frequencies as well as frontier molecular orbitals (FMOs), implying that hydrogen-bond interaction acts as the important parameter for the excited-state intramolecular proton-transfer (ESIPT) reaction. The proton-transfer and deamination reactions have been qualitatively analyzed through Gibbs free-energy reaction profiles in different electronic states. It can be concluded that the ESIPT and photodeamination reactions occur in the excited state. To further illustrate the photodeamination mechanism, the constructed 2D potential-energy surface indicates that the photodeamination reaction is infeasible without the ESIPT reaction. This work provides the first theoretical rationale for 1

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ESIPT-induced photodeamination occurring spontaneously because of protonation of a basic nitrogen atom.

*Corresponding author: email: [email protected]

1. INTRODUCTION Hydrogen bonds occur extensively in biomolecules, and play a crucial role in chemistry, biology, and biochemistry.1 A hydrogen bond (X–H···Y) comprises a proton-donor group, X, and a proton-acceptor group, Y. The proton donor and acceptor groups typically contain strongly electronegative atoms (O, N, S).2 Their interaction is analogous to that in a covalent bond, and can stabilize the structures of biomolecules and influence their physical and chemical properties.3 For example, N–H···O and N–H···N hydrogen bonds are significant interactions in establishing the double-helical and helical forms of DNA and proteins, respectively.4,5 Generally, proton-transfer reactions occur through hydrogen-bonding interactions.6–10 Hydrogen bonds can be divided into two categories, intermolecular and intramolecular hydrogen bonds, as proton transfer may occur either between molecules or within a molecule.11,12 Excited state proton transfer (ESIPT) processes have been extensively researched through theoretical and experimental means since the phenomenon was first experimentally observed by Weller et al. in 1955.13 Subsequently, both intramolecular and intermolecular photo-induced proton-transfer reactions have been widely investigated. ESIPT plays an important role in chemistry, physics, biochemistry, and other relevant disciplines, since it affects the photophysical and photochemical properties of substances.14–18 An ESIPT-generated dual-fluorescence phenomenon is an ultrafast process occurring on the femto- to picosecond time scale, as reported by Kasha et al. in 1979.19 Generally, the fluorophore induced by the ESIPT process exhibits a significant Stokes shift compared with the absorption spectrum, because the generated tautomer (T*) has an extremely different structure compared with the normal (N*) structure in the excited state.20–23 2


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Quinone methides (QMs) as reactive intermediates show high reactivity towards some biological organic molecules, such as amino acids, nucleic acids, base pairs, DNA, and proteins.24–26 Both thermodynamic27,28 and photochemical methods29,30 are available for generating QMs. The photochemical methods involve more facile and controllable reaction conditions, as discussed by Kralj et al.31 Previous experimental studies have revealed many instances of photodeamination reactions. For example, Hlavka et al. studied photodeamination in the tetracycline series in 1966, and found that this photochemical reaction proceeded spontaneously upon illumination.32 Modica et al. reported the photodeamination of Mannich salts in 2001.33 Doria et al. studied the effects of conjugation, substituents, and solvent on the photodeamination reaction in 2016.34 Husak et al. experimentally investigated the photodeamination mechanism generating active QMs in 2016.35 However, to the best of our knowledge, related theoretical research reports have been lacking. Škalamera et al.36 confirmed that the ESIPT process can promote the photodeamination of cresol derivatives, thereby accelerating the generation of QMs. As shown in Scheme 1, the tautomers 1-T and 3-T derived from compounds 1 and 3 undergo photo-induced ESIPT reactions. Protonation of the NMe2 groups then further enhances the deamination reactions. In the present work, we have sought to provide insight into the factors that promote the deamination reactions, and the processes whereby QMs are generated have been dissected. The time-dependent density functional theory (TDDFT) methods have been applied throughout, since they are accurate and time-saving for investigating these photochemical reactions, and have been extensively applied in the past decades.16,37–44 Therefore, we can obtain a reasonable interpretation of the reaction mechanisms leading to QMs.

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Scheme 1. Proposed mechanism for the formation of QM1 and QM3 by deamination of compounds 1 and 3, respectively.

2. COMPUTATIONAL DETAILS The Gaussian 09 suite of program was applied for the calculations in this study.45 Ground-state and excited-state structures were thoroughly optimized by means of density functional theory (DFT) and TDDFT methods, respectively. The long-range corrected hybrid functional ωB97XD46 was used in structure optimization and corresponding frequency calculations. The 6-31+G (d) atomic basis set was used to perform the above calculations, since it is accurate and not time-consuming. Frequency analyses were conducted to ensure that each of the stable geometries was a real minimum on the 2D potential-energy surface (PES), free from imaginary vibrational frequencies. Vertical transition and 0-0 transition energies were calculated based on Handy’s OPTX modification of Becke’s three-parameter hybrid exchange functional and the Lee–Yang–Parr gradient-corrected correlation function (O3LYP)47 with the 6-311+G (2d, p) basis set, because this basis set can be used to accurately perform DFT calculations. Many researchers have demonstrated that the calculation of energies based on fully optimized constructions can be performed with different level of computation from the optimization.48–50 The 0-0 transition energies were calculated as follows. The adiabatic energy was obtained as the difference between the minima in the ground and excited states, respectively, that is: ES GS E adia = Emin − Emin

(1)

To obtain the 0-0 transient energy, we need to deduct the difference in the zero-point 4


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vibrational energies (∆ZPE) between the excited and ground states.51 The adiabatic energy is then corrected by the non-negligible ∆ZPE, that is:

E 0 − 0 = E adia − ∆ZPE

(2)

To accurately simulate the experimental environment, the implicit solvation model IEFPCM is considered.52 The solvent dielectric constant (ε) of acetonitrile is 38.8. In addition, the structures of the cresol derivatives and isomers were represented using the VMD program53. The transition-state energies and structures were calculated by the Berny arithmetic method54. The transition-state structures in the S0 state were verified by the intrinsic reaction coordinate55 (IRC), whereas the transition-state structures in the lowest excited (SL) state were verified by IRC55 with the Gaussian 16 suite56.

3. RESULTS AND DISCUSSION 3.1 Optimization of configurations To carefully dissect the dynamic behavior of the generated QM molecules, as shown in Figure 1, the normal and isomeric forms of the free amines, compounds 1 and 3, were fully optimized. Their frequencies were also calculated, and no imaginary frequencies were found. Thus, these forms were confirmed as the real, most stable structures. The bond lengths and angles of the hydrogen bonds in 1, 1-T, 3, and 3-T are listed in full in Tables 1 and 2, respectively.

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Figure 1. The optimized geometrical configurations of compounds 1 and 3 along with isomers 1-T and 3-T in the SL state. Key atoms are numbered 1–8. Atom color coding: O - red; N - blue; C - cyan; H - gray.

For compound 1, the O1-H2 and H2-N3 bond lengths are 0.997 and 1.787 Å in the S0 state, which change to 1.053 and 1.549 Å, respectively, in the SL state. (Table 1) In addition, the bond angle of the hydrogen bond changes from 149.0° to 155.6°. Therefore, hydrogen bond O1–H2···N3 is obviously strengthened in the photo-induced process. In 1-T, the bond lengths O1–H2 and H2–N3 of hydrogen bond O1···H2–N3 are 1.588 and 1.079 Å in the S0 state, which change to 1.842 and 1.039 Å, respectively, in the SL state. The corresponding bond angle O1–H2–N3 changes from 150.0° to 141.0°, which indicates that hydrogen bond O1···H2-N3 is weaker in the SL state compared with that in the S0 state. For compound 3 and tautomer 3-T, the salient parameters of the hydrogen bonds are listed in Table 2.

Table 1. Bond lengths and angles for the studied compound 1 in acetonitrile. state

species

N3–C4 (Å)

O1–H2 (Å)

H2–N3 (Å)

O1–H2–N3 (°)

S0

1

1.469

0.997

1.787

149.0

SL

1

1.479

1.053

1.549

155.6

S0

1-T

1.503

1.588

1.079

150.0

SL

1-T

1.519

1.842

1.039

141.0

Table 2. Bond lengths and angles for the studied compound 3 in acetonitrile. state

species

N7–C8 (Å)

O5–H6 (Å)

H6–N7 (Å)

O5–H6–N7 (°)

S0

3

1.469

0.997

1.776

149.4

SL

3

1.479

1.050

1.553

155.8

S0

3-T

1.503

1.588

1.078

149.2

SL

3-T

1.521

1.838

1.040

140.2

6


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It can be seen that the bond lengths O5-H6 and H6-N7 change from 0.997 and 1.776 Å in the S0 state to 1.050 and 1.553 Å, respectively, in the SL state. The angle of the hydrogen bond increases from 149.4° to 155.8°. Hence, the hydrogen bond O5–H6···N7 is also strengthened in the SL state for compound 3. Besides, the hydrogen bond O5···H6–N7 is weaker in the SL state compared to that in the S0 state, since the bond lengths O5–H6 and H6–N7 change from 1.588 and 1.078 Å to 1.838 and 1.040 Å, respectively. The angle of the hydrogen bond decreases from 149.2° to 140.2°. It can be concluded that the strengthening of the hydrogen bonds O1–H2···N3 and O5–H6···N7 in compounds 1 and 3 reveals that the driving force of the proton-transfer reaction is stronger in the SL state. However, the weaker hydrogen bonds O1···H2–N3 and O5···H6–N7 reflect stabilization of the tautomer in the SL state. Moreover, it can also be seen in Table 1 that the bond length N3–C4 in compound 1 is 1.469 and 1.479 Å in the S0 and SL states, respectively, but in the tautomer these values increase to 1.503 and 1.519 Å in the S0 and SL states, respectively. Similarly, in Table 2, it can be seen that the length of bond N7–C8 also obviously increases after the proton-transfer process, irrespective of the electronic state. Therefore, protonation of the basic nitrogen increases the length of the N–C bond, thereby facilitating breaking of this bond.

3.2 Infrared (IR) vibrational spectral analysis The shift in the stretching frequency of an X–H bond (as a component of a hydrogen bond) can usually be taken as an indicator of hydrogen-bond strength.57–62 In the present study, vibrational frequency analyses were performed at the same level of theory as the optimization of configurations. In this way, the hydrogen-bond strengthening in the photo-induced process was further verified. The IR vibrational spectra of bonds O1–H2 and O5–H6 in the S0 and SL states are depicted in Figure 2. The stretching vibrational frequency of bond O1–H2 is seen to decrease from 3205 cm−1 in the S0 state to 2203 cm−1 in the SL state (Figure 2[a]). Thus, there is an enormous red-shift of 1002 cm−1 for compound 1 in the photo-induced process. Similarly, the stretching vibrational frequency of bond O5-H6 for compound 3 7



exhibits an apparent red-shift of 964 cm−1 from 3207 cm−1 in the S0 state to 2243 cm−1 in the SL state (Figure 2[b]). This implies that the hydrogen bonds O1–H2···N3 and O5–H6···N7 are strengthened in the SL state, which would promote the ESIPT reactions of compounds 1 and 3.

Figure 2. Calculated stretching vibrational frequencies of the O–H bonds in the S0 and SL states for compounds 1 (a) and 3 (b). 3.3 Analysis of frontier molecular orbitals The changes of bond length and stretching vibrational frequency of bond O1–H2 only act as the important parameter for ESIPT reaction. In the photo-excitation process, the redistributing electronic density of molecule leads to the changes of hydrogen-bond strength, since the hydrogen bonding force originates from electrostatic interaction. The frontier molecular orbitals (FMOs) can clearly present the electron population in the photo-excitation process as shown in Figure 3. It can be found that the FMOs of compounds 1 and 3 exhibit the π-π* character from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). In this redistribution the electron density of proton accepter group hydroxyl oxygen nearly reduces to zero, so the proton accepter groups present the stronger acidity, which enhances the hydrogen-bond interaction, and then promotes the ESIPT reaction.

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Figure 3. Calculated frontier molecular orbitals HOMO and LUMO of the compounds 1 and 3. Atom color coding: O - red; N - blue; C - black; H - green. 3.4 Calculated absorption and emission spectra The vertical absorption energies of 1 and 3 as well as the vertical emission energies of their normal and tautomeric structures were calculated based on TDDFT methods and the hybrid exchange functional O3LYP. To ensure computational accuracy, the 6-311+G (2d, p) basis set was applied to calculate vertical transition and 0-0 transition energies. State-specific calculations were also performed at the same level in the available error range. The corresponding major components of the orbital transitions, transition wavelengths ( λ ) and oscillator strengths ( f ) of compounds 1 and 3 as well as their isomers are given in Table 3 and Table 4, respectively. On the basis of the Kasha’s principle63, the processes of fluorescent radiation are only presented from the lowest excited state to the ground state. We neglect the mode of transition when the corresponding f value is less than 0.01. In Table 3, for compound 1 the f value on excitation from the S0 to the first excited (S1) state is 0.0660, whereas the f value on excitation from the S0 to the second excited (S2) state is 0.0013. Therefore, the electronic excitation of compound 1 originates from the S0 to the S1 state, its major components of the orbital transition is from highest occupied 9



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molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). The f value of its normal structure and isomer 1-T on emission processes are 0.0842 and 0.0734, respectively. In addition, their major components of the orbital transitions are the same as that on excitation process from HOMO to LUMO (H→L). So we conclude that the lowest excited (SL) state is the S1 state. The electronic transitions of compound 1 occur between S0 and S1 state. Table 3. Transition wavelengths λ (nm) for the low-lying singlet excited states and corresponding oscillator strengths f and major components of the orbital transitions for compound 1. processes

species

transition

λ (nm)

f

Components

Excitation

1

S0→S1

268

0.0660

H→L

S0→S2

251

0.0013

H→L+1&L+2

1

S0←S1

305

0.0842

H→L

1-T

S0←S1

358

0.0734

H→L

Emission

In Table 4, for compound 3 the f value on excitation from the S0 to the S1 state was 0.0007, whereas the f value on excitation from the S0 to the S2 state was 0.0660. Therefore, the electronic excitation of compound 3 leads to a transition from the S0 to the S2 state, its major components of the orbital transition is from HOMO to LUMO (H→L). If the emissive processes originate from S1 to S0 state, its f value will be very low. However, the f values of its normal structure and isomer 3-T on emission processes are 0.0965 and 0.0878, respectively. So the lowest excited (SL) state is not the S1 state. In Table 4, we can further find that the major components of the orbital transition is H→L from SL to S0 state, which is the same as that of excitation process from S0 to S2 state. It concludes that the SL state is S2 state for compound 3. The absorption and emission spectra of 1 and 3 are shown in Figure 4 (a, b), respectively.

Table 4. Transition wavelengths λ (nm) for the low-lying singlet excited states and corresponding oscillator strengths f and major components of the orbital transitions for compound 3. 10


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processes

species

transition

λ (nm)

f

Components

Excitation

3

S0→S1

279

0.0007

H-1→L

S0→S2

271

0.0877

H→L

3

S0←S2

307

0.0965

H→L

3-T

S0←S2

361

0.0878

H→L

Emission

It is noteworthy that the calculated absorption maxima are at 268 and 271 nm for compounds 1 and 3, respectively, close to the 260 nm observed experimentally.36 The 0-0 transitions of compounds 1 and 3 are at 275 and 277 nm, respectively, consistent with that of 283 nm observed experimentally.36 Moreover, the emission maxima of 1 and 3 are seen at 305 and 307 nm, respectively, very close to the 308 nm observed experimentally.36 The emission maxima of isomers 1-T and 3-T derived from ESIPT were calculated as 358 and 361 nm, respectively, consistent with the experimental value of 370 nm.36 As mentioned earlier, our chosen theoretical methods are evidently sufficiently reliable to reproduce the experimental results.

Figure 4. Calculated absorption and fluorescence spectra of (a) 1 and 1-T, (b) 3 and 3-T employing the O3LYP/6-311+G (2d, p) basis level. The corresponding experimental values are shown in parentheses.

3.5 Analysis of the reaction pathways and mechanisms The reaction pathways of deamination were carefully investigated in the S0 and SL states, the corresponding transition-state energies and structures were calculated by the Berny arithmetic method.54 In Figure 5, the structures of the intermediate products 11



IM1 and IM3 and final products QM1 and QM3 are presented. The reaction pathways from compounds 1 and 3 in the S0 and SL states are shown in Figure 6.

Figure 5. The optimized intermediate products IM1 and IM3 and final products QM1 and QM3 in the deamination reaction process. Atom color coding: O - red, N blue, C - cyan, H - gray.

Figure 6. Gibbs free-energy reaction profiles for deamination reactions in the S0 state (a) and the SL state (b). The ∆G values were calculated under conditions of 1 atmosphere and 298.15 K. The normal structures of compounds 1 and 3 were designated as the zero free-energy points, respectively. The details are shown in the corresponding legends.

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In Figure 6 (a), the deamination reactions of compounds 1 and 3 are seen to involve three stages in the S0 state. (i) Proton transfer: the proton is transferred from the oxygen atom of the keto donor group to the N atom of the Me2N acceptor group. The energy barriers associated with transition states TS1 and TS4 generating tautomers 1-T and 3-T are 5.99 and 5.93 kcal/mol, respectively. (ii) Hydrogen-bond cleavage: the O···H–N hydrogen bond is mainly cleaved through twisting of the protonated dimethylamino group. The energy barriers associated with transition states TS2 and TS5 generating IM1 and IM3 are 11.39 and 11.28 kcal/mol, respectively. (iii) Deamination: the protonated dimethylamino group is lost through breaking of the N–C bond. The energy barriers associated with transition states TS3 and TS6 generating QM1 and QM3 are 20.36 and 19.56 kcal/mol, respectively. The deamination reactions of compounds 1 and 3 do not occur easily because of the high energy barriers in the S0 state. The photodeamination reactions of compounds 1 and 3 involve two stages in the SL state (Figure 6[b]). (i) ESIPT: in the photoinduced process, proton transfer is facilitated by strengthening of the hydrogen-bond interaction. The energy barriers associated with transition states TS7 and TS9 generating isomers 3-T and 1-T are 1.07 and 1.05 kcal/mol, respectively. (ii) Photodeamination: dissociation of the protonated dimethylamino group is spontaneous in this photochemical process, because the energy barriers associated with transition states TS8 and TS10 generating QM3 and QM1 are negligible at 0.88 and 0.83 kcal/mol, respectively. It can be concluded that ESIPT is the first step in route to photodeamination in compounds 1 and 3. To verify the accuracy of the transition states, the corresponding IRCs are depicted in the Supporting Information. To further analyze the effect of ESIPT on the photodeamination reaction, we selected compound 1 to continue our research on the S1 state. The 2D potential-energy surface, as functions of bond lengths O1–H2 and N3-C4, is shown in Figure 7. It can clearly be seen that the photodeamination reaction is infeasible without the ESIPT process due to the enormous energy barrier. On the contrary, photodeamination induced by ESIPT can occur spontaneously.

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Figure 7. The 2D potential-energy surface as functions of bond lengths O1–H2 (from 0.9 to 2.4 Å) and N3-C4 (from 0.9 to 2.4 Å) of compound 1 in the S1 state. The white imaginary arrow represents the infeasible photodeamination reaction involving direct cleavage of the N3–C4 bond without ESIPT; the white solid arrows represent the initial ESIPT and subsequent spontaneous photodeamination reactions.

4. CONCLUSIONS We have investigated the photodeamination reactions of isomers 1-T and 3-T based on theoretical wB97XD/6-31+G (d) and O3LYP/6-311+G (2d, p) methods for the first time, and have calculated the pertinent absorption and emission spectra. The calculated spectra are consistent with the corresponding experimental measurements, validating the reliability of our chosen theoretical methods. Comparing the bond parameters and stretching vibrational frequencies as well as the FMOs for the cresol derivatives 1 and 3, we have demonstrated that hydrogen-bond interactions are strengthened in the SL state. Thus, we have confirmed that the isomers 1-T and 3-T can be obtained by ESIPT reactions of compounds 1 and 3, respectively. Potential-energy curves for cresol derivatives 1 and 3 have also been calculated, and demonstrated that the ESIPT and photodeamination reactions occur spontaneously in 14


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the SL state, since the energy barriers associated with these reactions are negligible. It is worth noting that the constructed 2D PES implies that the photodeamination reaction can occur smoothly following ESIPT. Consequently, we can reliably conclude that the enhanced hydrogen-bond strength facilitates the ESIPT processes of compounds 1 and 3, which further promotes the occurrence of photodeamination to generate the final QM products.

ASSOCIATED CONTENT SUPPORTING INFORMATION The IRC graphs of proton transfer and deamination processes have been described to verify validities of transition states in S0 and SL state.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 11474141 and 11604333), the Natural Science Foundation of Liaoning Province (Grant No. 20170540408), the Science and Technology Plan Project of Shenyang City (Grant No.17-231-1-06), and the Program for Liaoning Excellent Talents in University (Grant No. LJQ2015040).

ORCID Yongqing Li: 0000-0001-7673-1844

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Theoretical Investigation of the Reaction Mechanism of

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