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A theoretical study on the effect of cyano group on the proton transfer process of 10-hydroxybenzo[h]quinoline
Author’s Accepted Manuscript A theoretical study on the effect of cyano group on the proton transfer process of 10hydroxybenzo[h]quinoline Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang LIU, Yufang Liu, Yonggang Yang www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(18)31523-0 https://doi.org/10.1016/j.jlumin.2019.01.057 LUMIN16255
To appear in: Journal of Luminescence Received date: 18 August 2018 Revised date: 25 December 2018 Accepted date: 29 January 2019 Cite this article as: Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang LIU, Yufang Liu and Yonggang Yang, A theoretical study on the effect of cyano group on the proton transfer process of 10-hydroxybenzo[h]quinoline, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.01.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A theoretical study on the effect of cyano group on the proton transfer process of 10-hydroxybenzo[h]quinoline Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang LIU, Yufang Liu* Yonggang Yang*,
Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang Liu, Yufang Liu (), Yonggang Yang () College of Physics and Materials Science, Henan Normal University Xinxiang 453007, China Email: [email protected]
[email protected] Fax: +86 373 3329297
ABSTRACT: The proton transfer processes of 10-hydroxybenzo[h]quinoline (HBQ) and its cyano derivatives (4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) in ethyl acetate. The optimized geometric structures and infrared spectra show that the intramolecular hydrogen bond of enol form structure is weakened when the cyano group substitutes on the pyridine ring and that is strengthened when the cyano group substitutes on the phenol ring in the S0 state. Electron spectra show that the substitution of cyano group also has a significant effect on electron spectra. The reason for the change in the electron spectra can be obtained from the analysis of frontier molecular orbitals (MOs). The calculated potential barriers for proton transfer of these four molecules are 6.83, 7.83, 4.87 and 2.79 kcal/mol respectively in the S0 state, illustrating that the presence, position and number of cyano groups affect the proton transfer processes to some extent. After vertical excitation to the S1 state, the proton transfer processes of these four molecules become barrierless because of the promotion of charge transfer. Through the analysis of Mulliken charge, it can be found
that the cyano group, an electron-withdrawing group, can change the strength of hydrogen bond by withdrawing the electronic charge from hydroxyl oxygen or neighboring nitrogen atom of its ring and thus affects the barrier in the S0 state. Keywords: Density functional theory; Infrared spectra; Hydrogen bond; Proton transfer; Potential barrier.
1. Introduction The proton transfer is an important chemical reaction in the essence of acid-base neutralization reaction is proton transfer.[1-3]. In 1965, the excited-state intramolecular proton transfer (ESIPT) was first discovered by Weller and co-workers in the experiment of methyl salicylate[4]. At the end of 20th century, people began to realize the fundamental role of weak interactions in proton transfer (PT) processes[5-7]. Proton transfer molecules usually consist of a proton transfer donor (e.g. hydroxyl and amine) and a proton transfer acceptor (e.g. carbonyl oxygen or heterocyclic nitrogen)[8]. In general, the proton on molecular hydroxyl (or amine) group may not transfer to oxygen (or nitrogen) acceptor atom in the S0 state. After photo-excitation, the intramolecular hydrogen bond is strengthened, providing a driving force for the ESIPT process[9]. Fluorescence spectroscopy can be used to determine the occurrence of ESIPT because the molecular emission spectrum after proton transfer will have a large red shift with respect to the absorption spectrum[10]. Due to their unique spectroscopic properties and powerful charge recombination capability, these proton transfer molecules can be used in fluorescent probes[11], laser dyes[12], light emitting diodes (OLEDs)[13-15], molecular switches[16] and so on. Though ESIPT reaction has been widely studied both experimentally and theoretically, there still exist some unsolved problems because the infrared spectroscopic techniques only present indirect information about the PT process[17]. To better explore the mechanism of the PT process in depth, DFT and TD-DFT quantum mechanical calculations can be used[9, 18-20].
In recent years, the influence of various substituents (e.g. -NO2，-COOH or -CN) on the proton transfer process has attracted the attention of many researchers[21-25]. For example, the effect of amino group on the ESIPT mechanism of 2-(2'-hydroxy phenyl)benzoxazole (HBO) was studied by Li and co-workers[21-25]. As a typical proton transfer type molecule, the ESIPT mechanism of 10-hydroxybenzo[h]quinoline (HBQ) has been widely studied by different techniques[26-29]. At the same time, the effect of different substituents on proton transfer of HBQ has also been widely studied.[25, 30, 31]. Via a rational derivatization of HBQ, Chen and co-workers tuned the proton-transfer emission from 550 nm to 675 nm, generating a new family of proton transfer laser dyes[21]. Chai compared the difference in ESIPT process of 4CN-HBQ and 7NO2-HBQ[31]. In this study, HBQ and its cyano derivatives are selected from the previous two papers to study the effect of cyano group on proton transfer process of HBQ by DFT and TD-DFT quantum mechanical calculations. The optimized geometric structures and infrared spectra were monitored to observe the change of hydrogen bond strength. The electronic spectra, frontier molecular orbitals and potential energy curves were presented to demonstrate the proton transfer process in detail. The calculated results show that the proton transfers of these four molecules are different because of the addition of cyano group. The energy gap between HOMO and LOMO and the intramolecular charge analysis explain why cyano group has an effect on the intramolecular proton transfer. 2. Computational details All the results in this study were obtained by Gaussian 09 program suite[32]. The geometries of HBQ and its cyano derivatives in S0 and S1 states were optimized by DFT and TD-DFT quantum mechanical calculations, respectively. This method has been regarded as a reliable way to study the hydrogen interaction in hydrogen-bonded systems[33]. Becke's three-parameter hybrid exchange functional with Lee-Yang-Parr gradient-corrected correlation functional (B3LYP) and the TZVP basis set were applied in the whole calculation[34-37]. In order to simulate the experimental environment, ethyl acetate was selected as the solvent based on the Polarizable
Continuum Model (PCM) using the integral equation formalism of the Polarizable Continuum Model (IEFPCM)[38-41]. The symmetry, bonds, angles and dihedral angles were not limited in the geometric optimizations. All the local minima were confirmed by the absence of an imaginary mode in the vibrational analysis calculations. The potential energy curves of the molecules have been scanned by increasing the O-H bond length at a fixed step size. Harmonic vibrational frequencies in both the ground state and excited state were got by diagonalization of the Hessian[42]. The excited state Hessian was obtained by numerical differentiation of the analytical gradients using central differences and default displacements of 0.02 Bohr[43]. The infrared intensities were determined using the gradients of the dipole moment. 3. Results and discussion 3.1 Optimized geometric structures and infrared spectra. The structures were optimized by DFT and TD-DFT methods with B3LYP function at TZVP basis set (shown in Fig 1). The local minima in both S0 and S1 states have real frequencies. The structure parameters are listed in Table 1. It can be seen that the calculated bond lengths of H···N for HBQ-enol, 4CN-HBQ-enol, 7CN-HBQ-enol and 7,9CN-HBQ-enol are 1.682, 1.697, 1.651 and 1.612 Å respectively in the S0 state. The order of intramolecular hydrogen bond strength is 4CN-HBQ-enol < HBQ-enol < 7CN-HBQ-enol < 7,9CN-HBQ-enol in the S0 state. The result reflects that the position and number of substituents can affect the strength of intramolecular hydrogen bond. It is easy to see that, compared with HBQ-enol, the hydrogen bond is weakened when the cyano group appears on the proton acceptor ring while the hydrogen bond is strengthened when the cyano group appears on the proton donor ring. Through the comparison of the hydrogen bond lengths of 7CN-HBQ-enol and 7,9CN-HBQ-enol, it can be noted that the hydrogen bond strength is stronger with increasing number of cyano groups on the proton donor ring in the S0 state. The enol form of four molecules in S1 state and the keto form of 4CN-HBQ in the S0 state have no stable structure and the reason will be discussed in part 3.3. For the keto forms in the S1 state, the bond lengths of O···H for 4CN-HBQ,
HBQ, 7CN-HBQ and 7,9CN-HBQ are 1.729, 1.798, 1.823 and 1.826 Å, respectively, indicating that the order of intramolecular hydrogen bond strength is 4CN-HBQ-keto > HBQ- keto > 7CN-HBQ- keto >7,9CN-HBQ- keto, which is opposite to that of enol forms in the S0 state. This phenomenon also indicates that the addition of cyano groups can affect the strength of intramolecular hydrogen bond, which can influence the PT process. In addition, the bond lengths of O···H for HBQ, 7CN-HBQ and 7,9CN-HBQ shorten to 1.509, 1.572, 1.625 Å in the S0 state. The intramolecular hydrogen bonds of keto forms significantly become strengthened from S1 state to S0 state, which can facilitate the reverse proton transfer reaction in the S0 state. The infrared (IR) vibrational spectra of functional groups can reflect the change of the hydrogen bond strength by the red-shift and blue-shift. The calculated infrared spectra of O-H bonds and N-H bonds are shown in Fig 2. It can be seen that the O-H stretching vibrational frequencies of HBQ, 4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ normal structures are located at 3064.90, 3133.43, 2945.46 and 2758.41 cm-1 in S0 state. The order of intramolecular hydrogen bond strength is 4CN-HBQ-enol < HBQ-enol < 7CN-HBQ-enol < 7,9CN-HBQ-enol in the S0 state. The difference in hydrogen bond strength of the four molecules is due to the presence, location and number of cyano group in HBQ framework. In the S1 state , the N-H infrared vibrational frequencies of keto forms are located at 3315.51, 3132.01, 3393.81 and 3445.13 cm-1, indicating that the order of intramolecular hydrogen bond strength is 4CN-HBQ-keto > HBQ-keto > 7CN-HBQ-keto > 7,9CN-HBQ-keto in this time. The N-H infrared vibrational frequencies of keto forms are 2447.23 cm-1 for HBO, 2684.22 cm-1 for 7CN-HBQ and 2878.13 cm-1 for 7,9CN-HBQ in the S0 state and the all of them have red-shifts compared with that in the S1 state. It illustrates that the intramolecular hydrogen bond N-H···O is significantly strengthened from the S1 state to the S0 state.
Fig.1. The optimized enol and keto-tautomer structures of HBQ and its cyano derivatives at the B3LYP/TZVP theoretical level. Table 1 The calculated bond lengths (Å) and angles (︒) of HBQ and its cyanide derivatives in the S0 and S1 states at the B3LYP/TZVP theoretical level. state
H-N
O-H
(O-H-N)
HBQ-enol
S0
1.682
1.00
149.0
HBQ-keto
S0
1.088
1.509
147.0
S1
1.026
1.798
137.6
4CN-HBQ-enol
S0
1.697
0.996
148.0
4CN-HBQ-keto
S1
1.038
1.729
140.3
7CN-HBQ-enol
S0
1.651
1.007
149.2
7CN-HBQ-keto
S0
1.068
1.572
144.4
S1
1.021
1.823
136.0
7,9CN-HBQ-enol
S0
1.612
1.017
149.6
7,9CN-HBQ-keto
S0
1.054
1.625
142.1
S1
1.018
1.846
134.3
Fig.2. Calculated IR spectra at the B3LYP/TZVP theoretical level: (1) the O-H vibration of enol forms in S0 state; (2) the N-H vibration of keto forms in S1 state; (3) the N-H vibration of keto forms in S0 state.
3.2. Electronic spectra The calculated absorption and fluorescence spectra of HBQ and its cyano derivatives are shown in Fig.3. The absorption spectra of HBQ, 4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ are located at 356.36, 423.79, 347.15 and 350.40 nm, respectively. The fluorescence spectra of these keto forms are located at 588.32, 696.58, 565.12 and 538.95 nm, respectively. Lack of enol-form emission is caused by the barrierless process of ESIPT, which is shown in part 3.3 of this paper. The experimental absorption spectra of HBQ, 7CN-HBQ and 7,9CN-HBQ are 374, 375 and 373 nm, which are closed to our calculated values (356.36, 347.15 and 350.40 nm)[21]. This confirms the reliability of our calculation. Compared with the absorption spectrum of HBQ, the absorption spectrum of 4CN-HBQ has an obvious red shift of 69.43 nm (4465 cm-1) and the absorption spectra of 7CN-HBQ and 7,9CN-HBQ have no significant shifts. This phenomenon indicates an interesting phenomenon that the absorption spectra can have red shifts when -CN substitutes on the proton acceptor ring and that have no significant shift when -CN substitutes on the proton donor ring. Compared with the fluorescence spectrum of HBQ, the fluorescence spectrum of 4CN-HBQ have an obvious red shift of 108.26 nm (2642 cm-1) while the fluorescence spectra of 7CN-HBQ and 7,9CN-HBQ have blue shifts of 23.20 nm (698 cm-1) and 49.37 nm (1557 cm-1), respectively. This phenomenon indicates that it causes red (blue) shifts of the fluorescence spectra when -CN substitutes on the proton (donor) acceptor ring and the number of substituents can also affect the fluorescence spectra. From the discussion above, it can be concluded that the location and number of cyano group in HBQ framework can influence the distribution of the spectra bands. The cause of electronic spectra changes is analyzed in part 3.4 of this paper in detail. The calculated electronic excited state energies and oscillator strengths of the first six states for HBQ and its derivatives are listed in the Table 2. The oscillator strengths of transitions for HBQ and its derivatives are 0.1884, 0.2058, 0.2109 and 0.2731, respectively, showing that there is no transition forbidden from the S0 state to the S1 state. Because oscillator strengths of S0 to S2-S5 are much lower than 0.1 and
the excitation energy from S0 to S6 is too high to reach, we only discuss the transition of S0→S1.
Fig.3. The calculated absorption and fluorescence spectra of HBQ and its derivatives at the B3LYP/TZVP theoretical level. Table 2 The calculated electronic excitation energy (nm) and corresponding oscillator strengths (OS) of HBQ and its derivatives. HBQ
4CN-HBQ
7CN-HBQ
4,7CN-HBQ
E (nm)
OS
E (nm)
OS
E (nm)
OS
E (nm)
OS
S1
356
0.188
424
0.206
347
0.211
350
0.273
S2
320
0.028
339
0.001
320
0.097
318
0.085
S3
283
0.091
319
0.087
278
0.158
279
0.237
S4
268
0.045
293
0.001
268
0.069
271
0.027
S5
266
0.001
284
0.011
267
0.001
271
0.001
S6
253
0.156
271
0.059
258
0.487
261
0.483
3.3. Potential energy curves In order to reveal the proton transfer mechanism of HBQ molecule and its cyanide derivatives, the potential energy curves of the S0 state and S1 state have been scanned based on the optimized geometric structure in the corresponding electronic states with O-H bond length fixed at a series of values. The scanning range of O-H bond is from 1.0 Å to 1.8 Å in step of 0.05 Å. The potential energy curves are shown in Fig5, which were obtained by TDDFT/B3LYP/TZVP calculation method. The previous studies indicated that this method is reliable enough to explore the shape of the hydrogen-transfer potential energy curves[44, 45].
Fig.4. The potential energy curves of the S0 and S1 states for HBQ and its derivatives. The corresponding optimized geometries are inset.
As shown in Fig.4, the potential barrier of HBQ is 6.83 kcal/mol in the S0 state. HBQ undergoes a proton transfer process to form a stable keto form in the S0 state. The proton transfer process of the S1 state is much easier than that of the S0 state since the S1 potential energy curve is barrierless with the increase of O-H bond length. After photo-excitation, HBQ undergoes a barrierless process to the stable keto form in S1 state. For 4CN-HBQ, the barrier is 7.83 kcal/mol in the S0 state, which is 1.00
kcal/mol higher than that of HBQ, illustrating that the cyano group on the pyridine ring may lead to the rise of the barrier in the S0 state. With the increase of the O-H bond length, the energy of 4CN-HBQ is increased in the S0 state and there is no stable energy point on the potential energy curve. Therefore 4CN-HBQ will not undergo the intramolecular proton transfer in the S0 state. However, 4CN-HBQ can take a proton transfer process in the S1 state as HBQ, and then go back to its original enol form structure along the potential curve of the S0 state. The barriers of 7CN-HBQ and 7,9CN-HBQ are 4.87 and 2.79kcal/mol in the S0 states, which are smaller than HBQ and 4CN-HBQ. It can be concluded that the cyano group on the phenol ring leads to a decrease of the barrier. The more cyano groups substitute on the phenol ring, the smaller the barrier is. In addition, 7CN-HBQ and 7,9CN-HBQ have stable energy points at 1.572 Å and 1.625 Å, respectively. Therefore, the proton transfer processes of 7CN-HBQ and 7, 9CN-HBQ can occur in the S0 state, and then return to the original enol structure through the reverse proton transfer process. The potential energy curve of the S1 state has no potential barrier, indicating that the proton transfer processes of 7CN-HBQ and 7,9CN-HBQ are barrierless, which are similar to HBQ and 4CN-HBQ. In summary, the appearance of cyano group in HBQ framework can affect the proton transfer process. The proton transfer process of 4CN-HBQ can only occur in the S1 state while the proton transfer processes of HBQ, 7CN-HBQ and 7,9CN-HBQ can occur in the S0 and S1 states. 3.4. Frontier molecular orbitals and Mulliken charge analysis. The PT processes of HBQ and its cyanide derivatives were shown clearly in the part of 3.3. While the reason why the proton transfer processes are different and the proton transfer in the S1 state is much easier than that in the S0 state are not explicit. The frontier MOs can throw light on the nature of electronic state and electron transfer in the excited-state. The frontier MOs of HBQ and its derivatives were calculated in ethyl acetate. Since the transition of S0→S1 is associated with the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), this study only discussed these two orbitals. They are shown in Fig.4. It can be found that the HOMO orbitals are π character while the LUMO orbitals are π*
character, indicating that excitation processes are π→π* transitions from HOMO to LUMO for HBQ and its derivatives. In addition, the frontier MOs shows that the electron distribution in the two orbitals is different. The electron density in the oxygen atoms of the hydroxyl group decreased and the electron density in the nitrogen atoms of the pyridine ring increased through the transition from HOMOs to LOMOs. This phenomenon indicates that the negative charge transfer from the oxygen atom of the hydroxyl group to the nitrogen of the pyridine ring after light excitation. The charge transfer enhances the strength of hydrogen bond, which can facilitate intramolecular proton transfer in the S1 state. The LUMO-HOMO energy gap can explain the distribution change of the electronic spectra bands in part 3.2. Compared with the MOs of HBQ, the level of HOMO and LUIMO energy for cyano derivatives all decrease but the change of LUMO-HOMO energy gap is different, which causes the shift of the electronic spectra. Adding a cyano group on the pyridine ring decreases the LUMO-HOMO energy gap of enol form from 4.01 eV to 3.40 eV and the LUMO-HOMO energy gap of keto form from 2.50eV to 2.08eV. Therefore, the excitation and emission energy are reduced, resulting in red shifts of absorption and emission spectra of 4CN-HBQ relative to HBQ. The LUMO-HOMO energy gaps of 7CN-HBQ and 7,9CN-HBQ enol form are 4.09eV and 4.04 eV. The LUMO-HOMO energy gaps of 7CN-HBQ and 7,9CN-HBQ keto form are 2.61eV and 2.74 eV. It may be concluded that adding cyano groups on the phenol ring has little change on the LUMO-HOMO energy gap of enol form but can increase the LUMO-HOMO energy gap of keto form. In addition, the LUMO-HOMO energy gap of keto form increases with the increasing number of cyano groups on the phenol ring. Therefore, the absorption spectra of 7CN-HBQ and 7,9CN-HBQ change little relative to HBQ. The fluorescence spectra of 7CN-HBQ and 7,9CN-HBQ keto form blue-shift to 565.12nm and 538.95nm, respectively.
Fig.5. The MOs of enol and keto forms of HBQ and its derivatives at the calculated level of TDDFT/B3LYP/TZVP.
The effect of cyano group on the electron density distribution of molecules cannot be observed from the frontier MOs. The Mulliken charge distributions of the hydroxyl moiety O, the neighboring N and -CN for enol form structure of these four molecules were calculated, which are listed at Table 3. It can be seen that the Mulliken charges of the hydroxyl O for HBQ and its cyano derivatives are -0.300, -0.292, -0.278 and -0.253 in the S0 state, respectively. The Mulliken charge of the N of the pyridine ring for these four molecules are -0.210, -0.196, -0.211 and -0.209 respectively in the S0 state. As a strong electron-withdrawing group, the cyano group that substitutes on the pyridine ring can withdraw the electrons from N atom, which results in the decrease of the Mulliken charge of N atom from -0.210 to -0.196. This
result leads to the weakening of the electrostatic attraction of N atom to hydrogen proton. This is the reason why the hydrogen bond of 4CN-HBQ is weakened and the PT potential barrier becomes high in the S0 state contrast to HBQ. Compared with HBQ, the Mulliken charge of hydroxyl O atoms of 7CN-HBQ and 7,9CN-HBQ decrease to -0.278 and -0.253, respectively. This decrease shows that the cyano group substituted on the phenol ring withdraws the electrons from O atom of hydroxyl. The binding force of O atom to hydrogen proton is weakened. Therefore the hydrogen bond becomes strong and the PT potential barrier becomes low in the S0 state relative to HBQ. The data comparison of 7CN-HBQ and 7,9CN-HBQ shows that the change of the the hydroxyl O atom’s Mulliken charge is more obvious with the more cyano groups are introduced. In addition, through the transition from S0 state to S1 state, the charge of cyano increases from -0.178 to -0.259 for 4CN-HBQ, while it decreases from -0.226 to -0.118 for 7-CN-HBQ, -0.416 to -0.320 for 7,9CN-HBQ, respectively. This shows that the location of the cyano group has an effect on its own electron-withdrawing ability. The electron-withdrawing ability of cyano group becomes strong (weak) on the pyridine (phenol) ring from S0 state to S1 state. This also indicates that the location of the cyano group has an effect on the entire charge density distribution of the entire molecule. Table 3 The Mulliken charge (in a.u.) of the hydrogen moiety O, the neighboring N and -CN for HBQ and its cyanide derivatives were calculated by B3LYP/TZVP. HBQ- enol O
N
4CN-HBQ- enol O
N
CN
7CN-HBQ- enol O
N
CN
7,9CN-HBQ- enol O
N
CN
S0 -0.300 -0.210
-0.292 -0.196 -0.178
-0.278 -0.211 -0.226
-0.253 -0.209 -0.416
S1 -0.206 -0.264
-0.201 -0.260 -0.259
-0.195 -0.253 -0.118
-0.194 -0.238 -0.320
4. Conclusion In this work, the proton transfer processes of HBQ and its cyano derivatives were studied by using DFT and TD-DFT methods. It is found that the addition of cyano group affect the intramolecular hydrogen bond and the electronic spectra in some extent. The potential energy curves show that the proton transfer processes of these four molecules are barrierless in the S1 state. The calculated potential barriers of
these four molecules are 6.83, 7.83, 4.87 and 2.79 kcal/mol respectively in the S0 state, illustrating that the presence, position and number of cyano group can affect the proton transfer process. Through the analysis of the frontier MOs and Mulliken charge, it can be found that the charge transfer promotes the ESIPT reaction. Adding the electron-withdrawing cyano group in different position can cause blue shift or red shift for the electronic spectra by changing the LUMO-HOMO energy gap. The cyano group in the HBQ framework can change the strength of the hydrogen bond by withdrawing the electronic charge from hydroxyl oxygen or nitrogen atom of its ring. This is also the reason why HBQ and its cyano derivatives have different PT potential barriers in the S0 state. In summary, this work explored the effect of cyano groups on the proton transfer processes of HBQ and explained the reason for this effect. Our findings might be enlightening in the design of proton transfer materials in the future.
Acknowledgments This work is supported by National Natural science Foundation of China (Grant No. 11274096), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 124200510013) and Innovative Research Team in Science and Technology in University of Henan Province (Grant No. 13IRTSTHN016).
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PII: DOI: Reference:
S0022-2313(18)31523-0 https://doi.org/10.1016/j.jlumin.2019.01.057 LUMIN16255
To appear in: Journal of Luminescence Received date: 18 August 2018 Revised date: 25 December 2018 Accepted date: 29 January 2019 Cite this article as: Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang LIU, Yufang Liu and Yonggang Yang, A theoretical study on the effect of cyano group on the proton transfer process of 10-hydroxybenzo[h]quinoline, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.01.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A theoretical study on the effect of cyano group on the proton transfer process of 10-hydroxybenzo[h]quinoline Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang LIU, Yufang Liu* Yonggang Yang*,
Yuanyuan He, Chaozheng Li, Xueli Jia, Qianfei Ma, Yang Liu, Yufang Liu (), Yonggang Yang () College of Physics and Materials Science, Henan Normal University Xinxiang 453007, China Email: [email protected]
[email protected] Fax: +86 373 3329297
ABSTRACT: The proton transfer processes of 10-hydroxybenzo[h]quinoline (HBQ) and its cyano derivatives (4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) in ethyl acetate. The optimized geometric structures and infrared spectra show that the intramolecular hydrogen bond of enol form structure is weakened when the cyano group substitutes on the pyridine ring and that is strengthened when the cyano group substitutes on the phenol ring in the S0 state. Electron spectra show that the substitution of cyano group also has a significant effect on electron spectra. The reason for the change in the electron spectra can be obtained from the analysis of frontier molecular orbitals (MOs). The calculated potential barriers for proton transfer of these four molecules are 6.83, 7.83, 4.87 and 2.79 kcal/mol respectively in the S0 state, illustrating that the presence, position and number of cyano groups affect the proton transfer processes to some extent. After vertical excitation to the S1 state, the proton transfer processes of these four molecules become barrierless because of the promotion of charge transfer. Through the analysis of Mulliken charge, it can be found
that the cyano group, an electron-withdrawing group, can change the strength of hydrogen bond by withdrawing the electronic charge from hydroxyl oxygen or neighboring nitrogen atom of its ring and thus affects the barrier in the S0 state. Keywords: Density functional theory; Infrared spectra; Hydrogen bond; Proton transfer; Potential barrier.
1. Introduction The proton transfer is an important chemical reaction in the essence of acid-base neutralization reaction is proton transfer.[1-3]. In 1965, the excited-state intramolecular proton transfer (ESIPT) was first discovered by Weller and co-workers in the experiment of methyl salicylate[4]. At the end of 20th century, people began to realize the fundamental role of weak interactions in proton transfer (PT) processes[5-7]. Proton transfer molecules usually consist of a proton transfer donor (e.g. hydroxyl and amine) and a proton transfer acceptor (e.g. carbonyl oxygen or heterocyclic nitrogen)[8]. In general, the proton on molecular hydroxyl (or amine) group may not transfer to oxygen (or nitrogen) acceptor atom in the S0 state. After photo-excitation, the intramolecular hydrogen bond is strengthened, providing a driving force for the ESIPT process[9]. Fluorescence spectroscopy can be used to determine the occurrence of ESIPT because the molecular emission spectrum after proton transfer will have a large red shift with respect to the absorption spectrum[10]. Due to their unique spectroscopic properties and powerful charge recombination capability, these proton transfer molecules can be used in fluorescent probes[11], laser dyes[12], light emitting diodes (OLEDs)[13-15], molecular switches[16] and so on. Though ESIPT reaction has been widely studied both experimentally and theoretically, there still exist some unsolved problems because the infrared spectroscopic techniques only present indirect information about the PT process[17]. To better explore the mechanism of the PT process in depth, DFT and TD-DFT quantum mechanical calculations can be used[9, 18-20].
In recent years, the influence of various substituents (e.g. -NO2，-COOH or -CN) on the proton transfer process has attracted the attention of many researchers[21-25]. For example, the effect of amino group on the ESIPT mechanism of 2-(2'-hydroxy phenyl)benzoxazole (HBO) was studied by Li and co-workers[21-25]. As a typical proton transfer type molecule, the ESIPT mechanism of 10-hydroxybenzo[h]quinoline (HBQ) has been widely studied by different techniques[26-29]. At the same time, the effect of different substituents on proton transfer of HBQ has also been widely studied.[25, 30, 31]. Via a rational derivatization of HBQ, Chen and co-workers tuned the proton-transfer emission from 550 nm to 675 nm, generating a new family of proton transfer laser dyes[21]. Chai compared the difference in ESIPT process of 4CN-HBQ and 7NO2-HBQ[31]. In this study, HBQ and its cyano derivatives are selected from the previous two papers to study the effect of cyano group on proton transfer process of HBQ by DFT and TD-DFT quantum mechanical calculations. The optimized geometric structures and infrared spectra were monitored to observe the change of hydrogen bond strength. The electronic spectra, frontier molecular orbitals and potential energy curves were presented to demonstrate the proton transfer process in detail. The calculated results show that the proton transfers of these four molecules are different because of the addition of cyano group. The energy gap between HOMO and LOMO and the intramolecular charge analysis explain why cyano group has an effect on the intramolecular proton transfer. 2. Computational details All the results in this study were obtained by Gaussian 09 program suite[32]. The geometries of HBQ and its cyano derivatives in S0 and S1 states were optimized by DFT and TD-DFT quantum mechanical calculations, respectively. This method has been regarded as a reliable way to study the hydrogen interaction in hydrogen-bonded systems[33]. Becke's three-parameter hybrid exchange functional with Lee-Yang-Parr gradient-corrected correlation functional (B3LYP) and the TZVP basis set were applied in the whole calculation[34-37]. In order to simulate the experimental environment, ethyl acetate was selected as the solvent based on the Polarizable
Continuum Model (PCM) using the integral equation formalism of the Polarizable Continuum Model (IEFPCM)[38-41]. The symmetry, bonds, angles and dihedral angles were not limited in the geometric optimizations. All the local minima were confirmed by the absence of an imaginary mode in the vibrational analysis calculations. The potential energy curves of the molecules have been scanned by increasing the O-H bond length at a fixed step size. Harmonic vibrational frequencies in both the ground state and excited state were got by diagonalization of the Hessian[42]. The excited state Hessian was obtained by numerical differentiation of the analytical gradients using central differences and default displacements of 0.02 Bohr[43]. The infrared intensities were determined using the gradients of the dipole moment. 3. Results and discussion 3.1 Optimized geometric structures and infrared spectra. The structures were optimized by DFT and TD-DFT methods with B3LYP function at TZVP basis set (shown in Fig 1). The local minima in both S0 and S1 states have real frequencies. The structure parameters are listed in Table 1. It can be seen that the calculated bond lengths of H···N for HBQ-enol, 4CN-HBQ-enol, 7CN-HBQ-enol and 7,9CN-HBQ-enol are 1.682, 1.697, 1.651 and 1.612 Å respectively in the S0 state. The order of intramolecular hydrogen bond strength is 4CN-HBQ-enol < HBQ-enol < 7CN-HBQ-enol < 7,9CN-HBQ-enol in the S0 state. The result reflects that the position and number of substituents can affect the strength of intramolecular hydrogen bond. It is easy to see that, compared with HBQ-enol, the hydrogen bond is weakened when the cyano group appears on the proton acceptor ring while the hydrogen bond is strengthened when the cyano group appears on the proton donor ring. Through the comparison of the hydrogen bond lengths of 7CN-HBQ-enol and 7,9CN-HBQ-enol, it can be noted that the hydrogen bond strength is stronger with increasing number of cyano groups on the proton donor ring in the S0 state. The enol form of four molecules in S1 state and the keto form of 4CN-HBQ in the S0 state have no stable structure and the reason will be discussed in part 3.3. For the keto forms in the S1 state, the bond lengths of O···H for 4CN-HBQ,
HBQ, 7CN-HBQ and 7,9CN-HBQ are 1.729, 1.798, 1.823 and 1.826 Å, respectively, indicating that the order of intramolecular hydrogen bond strength is 4CN-HBQ-keto > HBQ- keto > 7CN-HBQ- keto >7,9CN-HBQ- keto, which is opposite to that of enol forms in the S0 state. This phenomenon also indicates that the addition of cyano groups can affect the strength of intramolecular hydrogen bond, which can influence the PT process. In addition, the bond lengths of O···H for HBQ, 7CN-HBQ and 7,9CN-HBQ shorten to 1.509, 1.572, 1.625 Å in the S0 state. The intramolecular hydrogen bonds of keto forms significantly become strengthened from S1 state to S0 state, which can facilitate the reverse proton transfer reaction in the S0 state. The infrared (IR) vibrational spectra of functional groups can reflect the change of the hydrogen bond strength by the red-shift and blue-shift. The calculated infrared spectra of O-H bonds and N-H bonds are shown in Fig 2. It can be seen that the O-H stretching vibrational frequencies of HBQ, 4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ normal structures are located at 3064.90, 3133.43, 2945.46 and 2758.41 cm-1 in S0 state. The order of intramolecular hydrogen bond strength is 4CN-HBQ-enol < HBQ-enol < 7CN-HBQ-enol < 7,9CN-HBQ-enol in the S0 state. The difference in hydrogen bond strength of the four molecules is due to the presence, location and number of cyano group in HBQ framework. In the S1 state , the N-H infrared vibrational frequencies of keto forms are located at 3315.51, 3132.01, 3393.81 and 3445.13 cm-1, indicating that the order of intramolecular hydrogen bond strength is 4CN-HBQ-keto > HBQ-keto > 7CN-HBQ-keto > 7,9CN-HBQ-keto in this time. The N-H infrared vibrational frequencies of keto forms are 2447.23 cm-1 for HBO, 2684.22 cm-1 for 7CN-HBQ and 2878.13 cm-1 for 7,9CN-HBQ in the S0 state and the all of them have red-shifts compared with that in the S1 state. It illustrates that the intramolecular hydrogen bond N-H···O is significantly strengthened from the S1 state to the S0 state.
Fig.1. The optimized enol and keto-tautomer structures of HBQ and its cyano derivatives at the B3LYP/TZVP theoretical level. Table 1 The calculated bond lengths (Å) and angles (︒) of HBQ and its cyanide derivatives in the S0 and S1 states at the B3LYP/TZVP theoretical level. state
H-N
O-H
(O-H-N)
HBQ-enol
S0
1.682
1.00
149.0
HBQ-keto
S0
1.088
1.509
147.0
S1
1.026
1.798
137.6
4CN-HBQ-enol
S0
1.697
0.996
148.0
4CN-HBQ-keto
S1
1.038
1.729
140.3
7CN-HBQ-enol
S0
1.651
1.007
149.2
7CN-HBQ-keto
S0
1.068
1.572
144.4
S1
1.021
1.823
136.0
7,9CN-HBQ-enol
S0
1.612
1.017
149.6
7,9CN-HBQ-keto
S0
1.054
1.625
142.1
S1
1.018
1.846
134.3
Fig.2. Calculated IR spectra at the B3LYP/TZVP theoretical level: (1) the O-H vibration of enol forms in S0 state; (2) the N-H vibration of keto forms in S1 state; (3) the N-H vibration of keto forms in S0 state.
3.2. Electronic spectra The calculated absorption and fluorescence spectra of HBQ and its cyano derivatives are shown in Fig.3. The absorption spectra of HBQ, 4CN-HBQ, 7CN-HBQ and 7,9CN-HBQ are located at 356.36, 423.79, 347.15 and 350.40 nm, respectively. The fluorescence spectra of these keto forms are located at 588.32, 696.58, 565.12 and 538.95 nm, respectively. Lack of enol-form emission is caused by the barrierless process of ESIPT, which is shown in part 3.3 of this paper. The experimental absorption spectra of HBQ, 7CN-HBQ and 7,9CN-HBQ are 374, 375 and 373 nm, which are closed to our calculated values (356.36, 347.15 and 350.40 nm)[21]. This confirms the reliability of our calculation. Compared with the absorption spectrum of HBQ, the absorption spectrum of 4CN-HBQ has an obvious red shift of 69.43 nm (4465 cm-1) and the absorption spectra of 7CN-HBQ and 7,9CN-HBQ have no significant shifts. This phenomenon indicates an interesting phenomenon that the absorption spectra can have red shifts when -CN substitutes on the proton acceptor ring and that have no significant shift when -CN substitutes on the proton donor ring. Compared with the fluorescence spectrum of HBQ, the fluorescence spectrum of 4CN-HBQ have an obvious red shift of 108.26 nm (2642 cm-1) while the fluorescence spectra of 7CN-HBQ and 7,9CN-HBQ have blue shifts of 23.20 nm (698 cm-1) and 49.37 nm (1557 cm-1), respectively. This phenomenon indicates that it causes red (blue) shifts of the fluorescence spectra when -CN substitutes on the proton (donor) acceptor ring and the number of substituents can also affect the fluorescence spectra. From the discussion above, it can be concluded that the location and number of cyano group in HBQ framework can influence the distribution of the spectra bands. The cause of electronic spectra changes is analyzed in part 3.4 of this paper in detail. The calculated electronic excited state energies and oscillator strengths of the first six states for HBQ and its derivatives are listed in the Table 2. The oscillator strengths of transitions for HBQ and its derivatives are 0.1884, 0.2058, 0.2109 and 0.2731, respectively, showing that there is no transition forbidden from the S0 state to the S1 state. Because oscillator strengths of S0 to S2-S5 are much lower than 0.1 and
the excitation energy from S0 to S6 is too high to reach, we only discuss the transition of S0→S1.
Fig.3. The calculated absorption and fluorescence spectra of HBQ and its derivatives at the B3LYP/TZVP theoretical level. Table 2 The calculated electronic excitation energy (nm) and corresponding oscillator strengths (OS) of HBQ and its derivatives. HBQ
4CN-HBQ
7CN-HBQ
4,7CN-HBQ
E (nm)
OS
E (nm)
OS
E (nm)
OS
E (nm)
OS
S1
356
0.188
424
0.206
347
0.211
350
0.273
S2
320
0.028
339
0.001
320
0.097
318
0.085
S3
283
0.091
319
0.087
278
0.158
279
0.237
S4
268
0.045
293
0.001
268
0.069
271
0.027
S5
266
0.001
284
0.011
267
0.001
271
0.001
S6
253
0.156
271
0.059
258
0.487
261
0.483
3.3. Potential energy curves In order to reveal the proton transfer mechanism of HBQ molecule and its cyanide derivatives, the potential energy curves of the S0 state and S1 state have been scanned based on the optimized geometric structure in the corresponding electronic states with O-H bond length fixed at a series of values. The scanning range of O-H bond is from 1.0 Å to 1.8 Å in step of 0.05 Å. The potential energy curves are shown in Fig5, which were obtained by TDDFT/B3LYP/TZVP calculation method. The previous studies indicated that this method is reliable enough to explore the shape of the hydrogen-transfer potential energy curves[44, 45].
Fig.4. The potential energy curves of the S0 and S1 states for HBQ and its derivatives. The corresponding optimized geometries are inset.
As shown in Fig.4, the potential barrier of HBQ is 6.83 kcal/mol in the S0 state. HBQ undergoes a proton transfer process to form a stable keto form in the S0 state. The proton transfer process of the S1 state is much easier than that of the S0 state since the S1 potential energy curve is barrierless with the increase of O-H bond length. After photo-excitation, HBQ undergoes a barrierless process to the stable keto form in S1 state. For 4CN-HBQ, the barrier is 7.83 kcal/mol in the S0 state, which is 1.00
kcal/mol higher than that of HBQ, illustrating that the cyano group on the pyridine ring may lead to the rise of the barrier in the S0 state. With the increase of the O-H bond length, the energy of 4CN-HBQ is increased in the S0 state and there is no stable energy point on the potential energy curve. Therefore 4CN-HBQ will not undergo the intramolecular proton transfer in the S0 state. However, 4CN-HBQ can take a proton transfer process in the S1 state as HBQ, and then go back to its original enol form structure along the potential curve of the S0 state. The barriers of 7CN-HBQ and 7,9CN-HBQ are 4.87 and 2.79kcal/mol in the S0 states, which are smaller than HBQ and 4CN-HBQ. It can be concluded that the cyano group on the phenol ring leads to a decrease of the barrier. The more cyano groups substitute on the phenol ring, the smaller the barrier is. In addition, 7CN-HBQ and 7,9CN-HBQ have stable energy points at 1.572 Å and 1.625 Å, respectively. Therefore, the proton transfer processes of 7CN-HBQ and 7, 9CN-HBQ can occur in the S0 state, and then return to the original enol structure through the reverse proton transfer process. The potential energy curve of the S1 state has no potential barrier, indicating that the proton transfer processes of 7CN-HBQ and 7,9CN-HBQ are barrierless, which are similar to HBQ and 4CN-HBQ. In summary, the appearance of cyano group in HBQ framework can affect the proton transfer process. The proton transfer process of 4CN-HBQ can only occur in the S1 state while the proton transfer processes of HBQ, 7CN-HBQ and 7,9CN-HBQ can occur in the S0 and S1 states. 3.4. Frontier molecular orbitals and Mulliken charge analysis. The PT processes of HBQ and its cyanide derivatives were shown clearly in the part of 3.3. While the reason why the proton transfer processes are different and the proton transfer in the S1 state is much easier than that in the S0 state are not explicit. The frontier MOs can throw light on the nature of electronic state and electron transfer in the excited-state. The frontier MOs of HBQ and its derivatives were calculated in ethyl acetate. Since the transition of S0→S1 is associated with the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), this study only discussed these two orbitals. They are shown in Fig.4. It can be found that the HOMO orbitals are π character while the LUMO orbitals are π*
character, indicating that excitation processes are π→π* transitions from HOMO to LUMO for HBQ and its derivatives. In addition, the frontier MOs shows that the electron distribution in the two orbitals is different. The electron density in the oxygen atoms of the hydroxyl group decreased and the electron density in the nitrogen atoms of the pyridine ring increased through the transition from HOMOs to LOMOs. This phenomenon indicates that the negative charge transfer from the oxygen atom of the hydroxyl group to the nitrogen of the pyridine ring after light excitation. The charge transfer enhances the strength of hydrogen bond, which can facilitate intramolecular proton transfer in the S1 state. The LUMO-HOMO energy gap can explain the distribution change of the electronic spectra bands in part 3.2. Compared with the MOs of HBQ, the level of HOMO and LUIMO energy for cyano derivatives all decrease but the change of LUMO-HOMO energy gap is different, which causes the shift of the electronic spectra. Adding a cyano group on the pyridine ring decreases the LUMO-HOMO energy gap of enol form from 4.01 eV to 3.40 eV and the LUMO-HOMO energy gap of keto form from 2.50eV to 2.08eV. Therefore, the excitation and emission energy are reduced, resulting in red shifts of absorption and emission spectra of 4CN-HBQ relative to HBQ. The LUMO-HOMO energy gaps of 7CN-HBQ and 7,9CN-HBQ enol form are 4.09eV and 4.04 eV. The LUMO-HOMO energy gaps of 7CN-HBQ and 7,9CN-HBQ keto form are 2.61eV and 2.74 eV. It may be concluded that adding cyano groups on the phenol ring has little change on the LUMO-HOMO energy gap of enol form but can increase the LUMO-HOMO energy gap of keto form. In addition, the LUMO-HOMO energy gap of keto form increases with the increasing number of cyano groups on the phenol ring. Therefore, the absorption spectra of 7CN-HBQ and 7,9CN-HBQ change little relative to HBQ. The fluorescence spectra of 7CN-HBQ and 7,9CN-HBQ keto form blue-shift to 565.12nm and 538.95nm, respectively.
Fig.5. The MOs of enol and keto forms of HBQ and its derivatives at the calculated level of TDDFT/B3LYP/TZVP.
The effect of cyano group on the electron density distribution of molecules cannot be observed from the frontier MOs. The Mulliken charge distributions of the hydroxyl moiety O, the neighboring N and -CN for enol form structure of these four molecules were calculated, which are listed at Table 3. It can be seen that the Mulliken charges of the hydroxyl O for HBQ and its cyano derivatives are -0.300, -0.292, -0.278 and -0.253 in the S0 state, respectively. The Mulliken charge of the N of the pyridine ring for these four molecules are -0.210, -0.196, -0.211 and -0.209 respectively in the S0 state. As a strong electron-withdrawing group, the cyano group that substitutes on the pyridine ring can withdraw the electrons from N atom, which results in the decrease of the Mulliken charge of N atom from -0.210 to -0.196. This
result leads to the weakening of the electrostatic attraction of N atom to hydrogen proton. This is the reason why the hydrogen bond of 4CN-HBQ is weakened and the PT potential barrier becomes high in the S0 state contrast to HBQ. Compared with HBQ, the Mulliken charge of hydroxyl O atoms of 7CN-HBQ and 7,9CN-HBQ decrease to -0.278 and -0.253, respectively. This decrease shows that the cyano group substituted on the phenol ring withdraws the electrons from O atom of hydroxyl. The binding force of O atom to hydrogen proton is weakened. Therefore the hydrogen bond becomes strong and the PT potential barrier becomes low in the S0 state relative to HBQ. The data comparison of 7CN-HBQ and 7,9CN-HBQ shows that the change of the the hydroxyl O atom’s Mulliken charge is more obvious with the more cyano groups are introduced. In addition, through the transition from S0 state to S1 state, the charge of cyano increases from -0.178 to -0.259 for 4CN-HBQ, while it decreases from -0.226 to -0.118 for 7-CN-HBQ, -0.416 to -0.320 for 7,9CN-HBQ, respectively. This shows that the location of the cyano group has an effect on its own electron-withdrawing ability. The electron-withdrawing ability of cyano group becomes strong (weak) on the pyridine (phenol) ring from S0 state to S1 state. This also indicates that the location of the cyano group has an effect on the entire charge density distribution of the entire molecule. Table 3 The Mulliken charge (in a.u.) of the hydrogen moiety O, the neighboring N and -CN for HBQ and its cyanide derivatives were calculated by B3LYP/TZVP. HBQ- enol O
N
4CN-HBQ- enol O
N
CN
7CN-HBQ- enol O
N
CN
7,9CN-HBQ- enol O
N
CN
S0 -0.300 -0.210
-0.292 -0.196 -0.178
-0.278 -0.211 -0.226
-0.253 -0.209 -0.416
S1 -0.206 -0.264
-0.201 -0.260 -0.259
-0.195 -0.253 -0.118
-0.194 -0.238 -0.320
4. Conclusion In this work, the proton transfer processes of HBQ and its cyano derivatives were studied by using DFT and TD-DFT methods. It is found that the addition of cyano group affect the intramolecular hydrogen bond and the electronic spectra in some extent. The potential energy curves show that the proton transfer processes of these four molecules are barrierless in the S1 state. The calculated potential barriers of
these four molecules are 6.83, 7.83, 4.87 and 2.79 kcal/mol respectively in the S0 state, illustrating that the presence, position and number of cyano group can affect the proton transfer process. Through the analysis of the frontier MOs and Mulliken charge, it can be found that the charge transfer promotes the ESIPT reaction. Adding the electron-withdrawing cyano group in different position can cause blue shift or red shift for the electronic spectra by changing the LUMO-HOMO energy gap. The cyano group in the HBQ framework can change the strength of the hydrogen bond by withdrawing the electronic charge from hydroxyl oxygen or nitrogen atom of its ring. This is also the reason why HBQ and its cyano derivatives have different PT potential barriers in the S0 state. In summary, this work explored the effect of cyano groups on the proton transfer processes of HBQ and explained the reason for this effect. Our findings might be enlightening in the design of proton transfer materials in the future.
Acknowledgments This work is supported by National Natural science Foundation of China (Grant No. 11274096), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 124200510013) and Innovative Research Team in Science and Technology in University of Henan Province (Grant No. 13IRTSTHN016).
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