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Naked diazaborepin dyes: Synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process
Journal Pre-proof Naked diazaborepin dyes: Synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process Nuonuo Zhang, Genjiang Liu, Jiaying Yan, Tingting Zhang, Xiang Liu PII:
S0143-7208(19)32700-7
DOI:
https://doi.org/10.1016/j.dyepig.2019.108128
Reference:
DYPI 108128
To appear in:
Dyes and Pigments
Received Date: 20 November 2019 Revised Date:
8 December 2019
Accepted Date: 10 December 2019
Please cite this article as: Zhang N, Liu G, Yan J, Zhang T, Liu X, Naked diazaborepin dyes: Synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108128. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Naked diazaborepin dyes: synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process Nuonuo Zhanga, Genjiang Liua, Jiaying Yana,b*, Tingting Zhanga, Xiang Liuac* a
College of Materials and Chemical Engineering, Key Laboratory of Inorganic Nonmetallic
Crystalline and Energy Conversion Materials, China Three Gorges University, Hubei, Yichang 443002, PR China b
State Key Laboratory of Coordination Chemistry, Nanjing University, Jiangsu, Nanjing 210093, PR China
c
Material Analysis and Testing Center, China Three Gorges University, Hubei, Yichang 443002, PR China
ABSTRACT
A series of new 3H-Pyrrolizine fused ‘naked’ diazaborepin (NDABs) have been strategically designed and synthesized by replacing one of the hydrogens on pyrrole and/or indole units with electron-donating/withdrawing groups. NDABs contain a seven-membered N-H··· N (H-bond) between proton donor (pyrrole) and proton acceptor (indole) moieties. The structures are
1
characterized by
1
H-NMR,
13
C-NMR, and high-resolution mass spectrum (HR-MS).
Photophysical properties are discussed by the absorption and emission spectra, DFT calculation. NDABs show characteristic strong absorption bands with high molar absorption coefficients, and emission bands with high quantum yields (0.14-0.47) and large Stokes shifts (7270-8025 cm-1). Based on this benchmark, ESIPT process was studied by electronic spectra, geometry, dynamics and thermodynamics analysis. The results demonstrate that the different substitutions at various positions of parent molecule, altered the strength of intramolecular H-bond, tuned the speed of ESIPT process which is consistent with the experimentally observed. On this -NH- type system, ESIPT process has been dramatically affected by the substitution on proton donor moiety and slightly on the proton acceptor moiety.
Keywords: Naked diazaborepin, Excited-State Intramolecular Proton Transfer, Seven-membered Ring, TDDFT, Potential Energy Surface
Introduction Excited-state intramolecular proton transfer (ESIPT) process is one of important photophysical phenomenon, plays a vital role in biological systems and has drawn much attention in recent years. The proton-transfer rate is connected with the strength of hydrogen bond, which is intimately associated with the application of ESIPT system[1]. Those system with different transfer speeds and large Stokes shift revealing normal and/or tautomer emission would be jointed in heterocyclic compounds[2-6] and applied in sensing[7-17], optoelectronics device [1012, 18, 19], biological applications[20-24] and organic light-emitting diodes (OLED)[25-27]. Unlike the most well-studied ESIPT systems containing the hydroxyl group (-OH), the ESIPT process rarely takes place in secondary amino-type (-NH-) system due to its weaker acidity and
2
intramolecular hydrogen bond. Because of that, the weaker and highly tunable H-bond leads to moderate exergonic process with a small energy barrier in excited state. This finite transfer rate of ESIPT suitable for experimental exploration also makes the fundamental study and provides more useful information on kinetics and/or thermodynamics. Although ESIPT reaction was observed and proposed in 1956[28], the -NH- based ESIPT systems were first discovered in 1991 (1-(acylamino)anthraquinone, Scheme 1)[29], in which the acidity was obviously enhanced. Since then, -NH- type ESIPT systems obtained much attention. It has been usually incorporated into heterocyclic compounds, such as pyrroles[30, 31], imidazoles[32, 33] and fluorophores[34-39] (Scheme 1).
Scheme 1. The representative -NH- type ESIPT systems. In most cases, the -NH- type ESIPT process is suppressed because H-bond is too weak to drive this reaction. Recently, it has focused on the molecule modification to enhance the acidity leading to a greater drive force. The most popular method is replacing one of the amino hydrogens with electron-donating/withdrawing groups, such as 2-(2ʹ-aminophenyl)benzothiazole (ABT)[36], ABDI)[33],
5-(2-aminobenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one 2-(imidazo-1,2-apyridin-2-yl)
aniline
(IPA)[40],
(o-
10-aminobenzohquinoline
(ABQ)[34]. The effects of electron-withdrawing or donating group substituents on the proton
3
donor part were also studied in these works. The results shown that the introduction of electronwithdrawing substituents increased the acidity of the molecular leading to faster speed of ESIPT process. An investigation of ESIPT process based on ‘naked’ diazaborepins with a seven-membered ring, -NH- type hydrogen bond of pyrrole-indole moiety (NDABs) was reported in our previous work[41-43]. The result showed that the ESIPT rate was tuned by the substitution on the proton acceptor moiety to some extent. However, the research of substitution effects both on proton acceptor part is limited. In this report, a series of new NDABs (NDAB-6, NDAB-8, NDAB-9) has been strategically designed and synthesized by the reaction of substituted 2,3,3trimethylindole and 4-acetyl-2-formylpyrrole with organocatalyst in 4 hours (Scheme 2). Electron-withdrawing or donating groups were introduced both on the proton donor and acceptor part. All the compounds are theoretically calculated by DFT calculation method for further discussing the ESIPT process.
Scheme 2. Synthetic routes for NDAB-6, 8, 9.
Experimental Section 1 Synthesis and Characterizations The parent compound NDAB-H derivatives, ethyl 6-(3,3-dimethyl-3H-indol-2-yl)-5-(4(ethoxycarbonyl)-1H-pyrrol-2-yl)-3H-pyrrolizine-2-carboxylate
(NDAB-6),
ethyl
5-(4-
(ethoxycarbonyl)-1H-pyrrol-2-yl)-6-(5-methoxy-3,3-dimethyl-3H-indol-2-yl)-3H-pyrrolizine-2-
4
carboxylate (NDAB-8), ethyl 2-(2-(ethoxycarbonyl)-5-(4-(ethoxycarbonyl)-1H-pyrrol-2-yl)-3Hpyrrolizin-6-yl)-3,3-dimethyl-3H-indole-5-carboxylate (NDAB-9) were synthesized[44, 45]. The synthetic routes for these compounds are showed in Scheme 2. Detailed synthesis and characterizations are provided in the Supporting Information (SI). 2 Spectroscopic Measurements UV/Vis spectra in various solvents were detected on Shimadzu UV-2600 spectrometer in 10 mm quartz cell. Fluorescence spectra were obtained using a Shimadzu FL-4500 spectrometer. Their wavelengths λ are reported in nm. 3 Computational details In this work, the geometries have been optimized by the density functional theory (DFT) with a CAM-B3LYP functional, and the 6-311*G(d) basis set is used for all atoms [46, 47]. In order to simulate the solvent effect, toluene has been selected as the solvent in these calculations based on the polarizable continuum model (PCM)[48, 49]. To further understand the specific ESIPT process, the potential energy curves in both S0 and S1 states are scanned by constrained optimizations with fixing the length of N and H (-NH-) bond. Considering the calculation time, the 6-31G(d) basis set is used for all atoms in the potential energy curves [41, 50]. All calculations of the present work are performed using Gaussian 09 program package[51].
Results and Discussion 1 Synthesis
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(a)
(b)
(c)
(d)
16.4
16.3
16.2
16.1
16.0
15.9
15.8
15.7
15.6
15.58.3
8.2
8.1
8.0
7.9
7.8 7.7 f1 (ppm)
7.6
7.5
7.4
7.3
7.2
7.1
7.0
6.9
6.8
6.7
6.6
6.5
6.4
6.
Figure 1. Partial 1H-NMR spectra of NDAB-H (a), NDAB-6 (b), NDAB-8 (c), NDAB-9 (d). * is the signal of residual solvent CDCl3. NDAB-6, 8, 9 are synthesized under organo-catalyst piperidinium acetate via Mannich-type condensation in yield ranging from 26 to 37%. The methoxyl and ethyl ester groups on the fifth position of 2,3,3-trimethylindolenine are introduced for NDAB-8, 9 compared to NDAB-6 shown in Scheme 1. 4-acetyl-2-formylpyrrole is used to obtain NDABs in this work. 1H-NMR spectroscopic data for NDABs are shown in Figure 1, consisting with their structure information. The proton of hydrogen bond of NDABs appears at 15.7-16.3 ppm. The protons of pyrrolizine and pyrrole ring appear as a singlet at 6.4-7.1 ppm for NDABs. The aromatic protons of indole appear at a singlet and doublet at 6.4-8.2 ppm. The remaining protons of methyl and methylene unit appear at 1.0-5.2 ppm.
6
In those -NH- type systems, stronger intramolecular hydrogen bond induces more downfield chemical shift of NH signal in the 1H NMR spectra[36, 52, 53]. The N-H chemical shift (ppm) of NDABs in 1H NMR spectra is concluded in the Table S1. The chemical shift of the N-H proton is obviously downfield shifted from δ = 15.78 ppm to 16.30 ppm (NDAB-H vs NDAB-6, CDCl3), suggesting that the strength of hydrogen bond is enhanced by the introduction of the strong withdrawing group on proton donor unit. Meanwhile, N-H chemical shift (ppm) of NDAB-6, NDAB-8, NDAB-9 are similar. When the strong withdrawing group is decorated on the proton donor unit of NDABs, the substitution on the proton acceptor unit plays a relatively minor role on those ESIPT process, due to the longer geometric distance between the substitution position and the -NH- group. 2 Photophysical Properties
Figure 2. UV-Vis (solid line) and Fluorescence (dash line) spectra of NDAB-6, 8, 9 in toluene (Left) and NDAB-9 in various solvents (Right). The absorption and emission spectra have been carried out in toluene, dichloromethane (DCM), methanol (MeOH), dimethylsulfoxide (DMSO), respectively, (Figure 2, 3 and S1-2). The data, namely their maximum absorption and emission wavelengths, molar extinction coefficients, Stokes shifts, and fluorescence quantum yields, have been summarized in Table S2. 7
Solvents with different polarity have no dramatically effect on the photophysical properties. Figure 2 shows absorption and the normalized emission spectra of selected NDABs. NDABs shows characteristic strong absorption bands around 400 nm, and their emission bands were observed at 570-600 nm as a sole peak with high molar absorption coefficients (ε = 2.14-4.43× 104 M-1 cm-1). All NDABs show higher quantum yields (0.14-0.47) and larger Stokes shifts (7270-8025 cm-1). The strong absorption bands are due to the S0–S1 ( π - π *) transition. Obviously, the large Stokes shifts have been detected in NDAB-H, NDAB-6, NDAB-8, NDAB9, considered as fast speed ESIPT system compared with NDAB-COOEt shown obviously dual peak[41] and their BF2 complex DABs[45, 54], BOPYINs[55] without ESIPT process.
3 Calculation For detail discussion of the ESIPT process of NDABs, frontier molecular orbitals (MOs), the geometry configurations of ground state, excited state and potential energy surfaces have been calculated. NDABs with substituents both on pyrrole as proton donor and indole as proton acceptor unit are discussed for the first time in a qualitative manner. 3.1 Geometries of ground and excited states Ground and excited states optimized structures of NDABs are calculated by DFT methods. Our work is divided into two sections to discuss. On one hand, NDAB-H, NDAB-6, NDAB-7 (with H, -COOEt, -NEt2 groups on proton donor part, without substitution on proton acceptor part) are chosen for discussing the relationship between substitutions on proton donor part and the ESIPT process. On the other hand, the ESIPT process of NDAB-6, NDAB-8, NDAB-9 (with ethyl ester on proton donor part, different substitution on proton acceptor part) are discussed by experiment and calculation.
8
In our opinion, the strength of intramolecular H-bond can be speculated by the change of Hbonding length form ground to excited state. Take NDAB-6 for example (Figure 3), when it is excited from the normal ground (N-S0) state to the normal excited (N-S1) state, the distance of intramolecular hydrogen bond is decreased. Meanwhile, the bond angel of ∠ N-H⋯N is enhanced. The similar trend in bond length of hydrogen bonds and bond angles is also exhibited in other NDABs (Table S3, Figure S3-4) suggesting that the ESIPT reaction occurred smoothly in this system. From the kinetics point of view, shorter distance of hydrogen bond in S1 states leads to stronger hydrogen bond, which is connected with the rate of ESIPT reaction [56]. On the one hand, the length of hydrogen bond of NDAB-6 is much shorter than NDAB-H in N-S1 states, otherwise the shortened length of NDAB-6 from N-S1 to N-S0 is larger than NDAB-H. The intramolecular hydrogen bond become stronger in NDAB-6 which accelerates the rate of ESIPT process. NDAB-7 with strong donating group on pyrrole unit could not detect the excited state of tautomeric structure by calculation, which means ESIPT process is suppressed. On the other hand, comparing NDAB-6, NDAB-8, NDAB-9 with different substitution on proton acceptor part. The bond lengths of hydrogen bond in S1 states are in the order of NDAB-9 < NDAB-8 ~ NDAB-6. This implies that different substitution on proton acceptor unit has weak effect on the ESIPT process in the present of ester group on proton donor unit. The ester substitution on proton donor moiety plays a major role in the process of ESIPT reaction.
9
Figure 3. Optimized S0 and S1 state structures of NDAB-6. 3.2 Charge distribution and transfer In order to further investigate the detail process of electronic transition upon excitation in those system, the four frontier molecular orbitals (MOs) of the NDABs are calculated and exhibited in Figure 4 and Table S4-S5. With the ethyl ester substitution on pyrrole unit, the energy of four frontier molecular orbitals decreased obviously and the electronic transferred from one pyrrole to another, finally moved to indole unit. This speculates that the electron distribution in N atom of pyrrole unit transfer to N atom of indole unit serving as the driving force for ESIPT process. The energy gaps of the NDABs shown in Figure 4 are following the order of NDAB-H (5.82 eV) > NDAB-6 (5.72 eV) > NDAB-8 (5.65 eV) ~ NDAB-9 (5.63 eV), which is in good correlation with the speed of NDAB-6, NDAB-8, NDAB-9 with ethyl ester substitution on pyrrole unit (Figure 1). Their speed is faster than NDAB-H. Accordingly, compared with NDAB-H, energy change of LUMO for NDAB-6 with electronwithdrawing group on proton donor part is more than that of HOMO (Table S4). NDAB-6, NDAB-8 and NDAB-9 with different substitution on indole unit have no dramatically effect on the energy of HOMOs and LUMOs. Four frontier orbitals are slightly enhanced by the donating
10
group on indole ring (NDAB-8), and meanwhile slightly decreased by the withdrawing group (NDAB-9). However, the energies of four frontier orbitals are obviously decreased to NDAB-H (Table S5).
Figure 4. Computed HOMO and LUMO orbitals for NDABs in their normal form involved in the first singlet excitation in toluene. 3.3 Potential energy curves The strength of intramolecular H-bond also can be approximated by the energy change in excited state. Potential energy curves are investigated to study the mechanism of proton transfer reaction in detail. Considering the calculation time, the potential energy curves are constructed by increasing the N-H bond length in enhancement of 0.06 Å for 12 steps using DFT/B3LYP method in the S0 state and using TDDFT/B3LYP method in the S1 state. From the view of thermodynamics, the calculated energies △E1 (between the excited state of tautomer and normal structures) and △E2 ( energy barriers between the excited state of highest energy structures and normal structures) are concluded in Table S6, which related to the speed of ESIPT process[57]. In the excited states, energy barriers (△E2) are 2.32, 0.87, 2.09, 1.49
11
Kcal/mol. All the compounds have smaller energy barriers with the order of NDAB-6 < NDAB9 < NDAB-8 < NDAB-H. Meanwhile, the energy gaps between N-S1 and T-S1 (△E1) states are 0.01, -3.85, -2.41, -1.84 Kcal/mol for NDAB-H, NDAB-6, NDAB-8, NDAB-9, respectively. △ E1 of NDAB-H is slightly positive and NDAB-6, NDAB-8, NDAB-9 are obvious negative. The energies of T-S1 state are much higher than that of N-S1 state. The proton transfer from N-S1 to T-S1 state is exergonic with the fast or even ultrafast speed ESIPT process. However, the substitution of withdrawing group or donating group on proton acceptor unit has the similar trend on the energy change, energy barrier and the energy between N-S1 and T-S1 state. Thus, substitution on the proton acceptor unit of NDAB-6 has a minor effect on the ESIPT process.
Figure 5. Calculated potential energy curves of the S0 (black line) and S1 (red line) states along the proton tautomerization reactions of NDABs. Conclusion 12
Three new -NH- type ESIPT molecules NDABs based on seven-membered pyrrole-indole systems have been strategically synthesized in 26-37% yield. The all NDABs show characteristic strong absorption bands around 400 nm with high molar absorption coefficients (ε = 2.144.43×104 M-1 cm-1), and emission bands around 570-600 nm with high quantum yields (0.140.47). The obvious sole emission peak and large Stokes shift (7270-8025 cm-1) have been detected by experiment. This is the first time to study the effect of substitutions both on proton donor and proton acceptor part in the system. Conjugated with geometry, dynamics and thermodynamics analysis, ESIPT process has been demonstrated. ESIPT of NDABs has been dramatically affected by the substitution on proton donor part and slightly on the proton acceptor moiety. The acidity of NDABs is more closely related to the substitution on donor sites. Withdrawing group on donor part leads to an increase of hydrogen-bonding strength, which is associated with ESIPT rate in those seven-membered -NH- type system. Corresponding Author *Corresponding authors. E-mail addresses: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 21606145, 21805166), The 111 Project of Hubei Province (Grant No. 2018-19-1), State Key Laboratory of Coordination Chemistry Foundation of Nanjing University (No. SKLCC1811). REFERENCES [1] Zhou P, Han K. Unraveling the Detailed Mechanism of Excited-State Proton Transfer.
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[49] Cammi R, Tomasi J. Remarks on the use of the apparent surface charges (ASC) methods in solvation problems: Iterative versus matrix-inversion procedures and the renormalization of the apparent charges. Journal of Computational Chemistry. 1995;16(12):1449-58. [50] Hehre WJ, Ditchfield R, Pople JA. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. The Journal of Chemical Physics. 1972;56(5):2257-61. [51] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 09, revision A.02. Wallingford CT: Gaussian Inc; 2009 [52] Lin T-Y, Tang K-C, Yang S-H, Shen J-Y, Cheng Y-M, Pan H-A, et al. The Empirical Correlation between Hydrogen Bonding Strength and Excited-State Intramolecular Proton Transfer in 2-Pyridyl Pyrazoles. The Journal of Physical Chemistry A. 2012;116(18):4438-44. [53] Yamauchi K, Kuroki S, Fujii K, Ando I. The amide proton NMR chemical shift and hydrogen-bonded structure of peptides and polypeptides in the solid state as studied by highfrequency solid-state 1H NMR. Chemical Physics Letters. 2000;324(5):435-9. [54] Zhang T, Li Y, Yan J, Liang Y, Xu B, Zheng K, et al. Syntheses and spectroscopic properties of substituted diazaborepins with large stokes shift. Tetrahedron. 2018;74(22):2807-11. [55] Zhang T, Yan J, Hu Y, Liu X, Wen L, Zheng K, et al. A Simple Central Seven-Membered BOPYIN: Synthesis, Structural, Spectroscopic Properties, and Cellular Imaging Application. Chemistry – A European Journal. 2019;25(39):9266-71. [56] Li Y, Wang L, Guo X, Zhang J. A CASSCF/CASPT2 insight into excited-state intramolecular proton transfer of four imidazole derivatives. Journal of Computational Chemistry. 2015;36(32):2374-80. [57] Tseng H-W, Liu J-Q, Chen Y-A, Chao C-M, Liu K-M, Chen C-L, et al. Harnessing ExcitedState Intramolecular Proton-Transfer Reaction via a Series of Amino-Type Hydrogen-Bonding Molecules. The Journal of Physical Chemistry Letters. 2015;6(8):1477-86.
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Highlights A series of naked diazaborepin dyes (NDABs) have been designed and synthesized. NDABs exhibit high quantum yields (0.14-0.47) and large Stokes shifts (167-205 nm). ESIPT process of NDABs was investigated by experiment and theoretical calculation. ESIPT process has been dramatically affected by the substitutions on proton donor unit.
Author statement
Nuonuo Zhang: Data curation, Supervision, Writing Original draft; Genjiang Liu: Investigation, Resources; Jiaying Yan: Conceptualization, Methodology, Software, Project administration; Tingting Zhang: Formal analysis; Xiang Liu: Writing - Review & Editing, Funding acquisition
The authors declare no competing financial interest.
S0143-7208(19)32700-7
DOI:
https://doi.org/10.1016/j.dyepig.2019.108128
Reference:
DYPI 108128
To appear in:
Dyes and Pigments
Received Date: 20 November 2019 Revised Date:
8 December 2019
Accepted Date: 10 December 2019
Please cite this article as: Zhang N, Liu G, Yan J, Zhang T, Liu X, Naked diazaborepin dyes: Synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108128. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Naked diazaborepin dyes: synthesis, photophysical properties, substituent effects and theoretical calculations on ESIPT process Nuonuo Zhanga, Genjiang Liua, Jiaying Yana,b*, Tingting Zhanga, Xiang Liuac* a
College of Materials and Chemical Engineering, Key Laboratory of Inorganic Nonmetallic
Crystalline and Energy Conversion Materials, China Three Gorges University, Hubei, Yichang 443002, PR China b
State Key Laboratory of Coordination Chemistry, Nanjing University, Jiangsu, Nanjing 210093, PR China
c
Material Analysis and Testing Center, China Three Gorges University, Hubei, Yichang 443002, PR China
ABSTRACT
A series of new 3H-Pyrrolizine fused ‘naked’ diazaborepin (NDABs) have been strategically designed and synthesized by replacing one of the hydrogens on pyrrole and/or indole units with electron-donating/withdrawing groups. NDABs contain a seven-membered N-H··· N (H-bond) between proton donor (pyrrole) and proton acceptor (indole) moieties. The structures are
1
characterized by
1
H-NMR,
13
C-NMR, and high-resolution mass spectrum (HR-MS).
Photophysical properties are discussed by the absorption and emission spectra, DFT calculation. NDABs show characteristic strong absorption bands with high molar absorption coefficients, and emission bands with high quantum yields (0.14-0.47) and large Stokes shifts (7270-8025 cm-1). Based on this benchmark, ESIPT process was studied by electronic spectra, geometry, dynamics and thermodynamics analysis. The results demonstrate that the different substitutions at various positions of parent molecule, altered the strength of intramolecular H-bond, tuned the speed of ESIPT process which is consistent with the experimentally observed. On this -NH- type system, ESIPT process has been dramatically affected by the substitution on proton donor moiety and slightly on the proton acceptor moiety.
Keywords: Naked diazaborepin, Excited-State Intramolecular Proton Transfer, Seven-membered Ring, TDDFT, Potential Energy Surface
Introduction Excited-state intramolecular proton transfer (ESIPT) process is one of important photophysical phenomenon, plays a vital role in biological systems and has drawn much attention in recent years. The proton-transfer rate is connected with the strength of hydrogen bond, which is intimately associated with the application of ESIPT system[1]. Those system with different transfer speeds and large Stokes shift revealing normal and/or tautomer emission would be jointed in heterocyclic compounds[2-6] and applied in sensing[7-17], optoelectronics device [1012, 18, 19], biological applications[20-24] and organic light-emitting diodes (OLED)[25-27]. Unlike the most well-studied ESIPT systems containing the hydroxyl group (-OH), the ESIPT process rarely takes place in secondary amino-type (-NH-) system due to its weaker acidity and
2
intramolecular hydrogen bond. Because of that, the weaker and highly tunable H-bond leads to moderate exergonic process with a small energy barrier in excited state. This finite transfer rate of ESIPT suitable for experimental exploration also makes the fundamental study and provides more useful information on kinetics and/or thermodynamics. Although ESIPT reaction was observed and proposed in 1956[28], the -NH- based ESIPT systems were first discovered in 1991 (1-(acylamino)anthraquinone, Scheme 1)[29], in which the acidity was obviously enhanced. Since then, -NH- type ESIPT systems obtained much attention. It has been usually incorporated into heterocyclic compounds, such as pyrroles[30, 31], imidazoles[32, 33] and fluorophores[34-39] (Scheme 1).
Scheme 1. The representative -NH- type ESIPT systems. In most cases, the -NH- type ESIPT process is suppressed because H-bond is too weak to drive this reaction. Recently, it has focused on the molecule modification to enhance the acidity leading to a greater drive force. The most popular method is replacing one of the amino hydrogens with electron-donating/withdrawing groups, such as 2-(2ʹ-aminophenyl)benzothiazole (ABT)[36], ABDI)[33],
5-(2-aminobenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one 2-(imidazo-1,2-apyridin-2-yl)
aniline
(IPA)[40],
(o-
10-aminobenzohquinoline
(ABQ)[34]. The effects of electron-withdrawing or donating group substituents on the proton
3
donor part were also studied in these works. The results shown that the introduction of electronwithdrawing substituents increased the acidity of the molecular leading to faster speed of ESIPT process. An investigation of ESIPT process based on ‘naked’ diazaborepins with a seven-membered ring, -NH- type hydrogen bond of pyrrole-indole moiety (NDABs) was reported in our previous work[41-43]. The result showed that the ESIPT rate was tuned by the substitution on the proton acceptor moiety to some extent. However, the research of substitution effects both on proton acceptor part is limited. In this report, a series of new NDABs (NDAB-6, NDAB-8, NDAB-9) has been strategically designed and synthesized by the reaction of substituted 2,3,3trimethylindole and 4-acetyl-2-formylpyrrole with organocatalyst in 4 hours (Scheme 2). Electron-withdrawing or donating groups were introduced both on the proton donor and acceptor part. All the compounds are theoretically calculated by DFT calculation method for further discussing the ESIPT process.
Scheme 2. Synthetic routes for NDAB-6, 8, 9.
Experimental Section 1 Synthesis and Characterizations The parent compound NDAB-H derivatives, ethyl 6-(3,3-dimethyl-3H-indol-2-yl)-5-(4(ethoxycarbonyl)-1H-pyrrol-2-yl)-3H-pyrrolizine-2-carboxylate
(NDAB-6),
ethyl
5-(4-
(ethoxycarbonyl)-1H-pyrrol-2-yl)-6-(5-methoxy-3,3-dimethyl-3H-indol-2-yl)-3H-pyrrolizine-2-
4
carboxylate (NDAB-8), ethyl 2-(2-(ethoxycarbonyl)-5-(4-(ethoxycarbonyl)-1H-pyrrol-2-yl)-3Hpyrrolizin-6-yl)-3,3-dimethyl-3H-indole-5-carboxylate (NDAB-9) were synthesized[44, 45]. The synthetic routes for these compounds are showed in Scheme 2. Detailed synthesis and characterizations are provided in the Supporting Information (SI). 2 Spectroscopic Measurements UV/Vis spectra in various solvents were detected on Shimadzu UV-2600 spectrometer in 10 mm quartz cell. Fluorescence spectra were obtained using a Shimadzu FL-4500 spectrometer. Their wavelengths λ are reported in nm. 3 Computational details In this work, the geometries have been optimized by the density functional theory (DFT) with a CAM-B3LYP functional, and the 6-311*G(d) basis set is used for all atoms [46, 47]. In order to simulate the solvent effect, toluene has been selected as the solvent in these calculations based on the polarizable continuum model (PCM)[48, 49]. To further understand the specific ESIPT process, the potential energy curves in both S0 and S1 states are scanned by constrained optimizations with fixing the length of N and H (-NH-) bond. Considering the calculation time, the 6-31G(d) basis set is used for all atoms in the potential energy curves [41, 50]. All calculations of the present work are performed using Gaussian 09 program package[51].
Results and Discussion 1 Synthesis
5
(a)
(b)
(c)
(d)
16.4
16.3
16.2
16.1
16.0
15.9
15.8
15.7
15.6
15.58.3
8.2
8.1
8.0
7.9
7.8 7.7 f1 (ppm)
7.6
7.5
7.4
7.3
7.2
7.1
7.0
6.9
6.8
6.7
6.6
6.5
6.4
6.
Figure 1. Partial 1H-NMR spectra of NDAB-H (a), NDAB-6 (b), NDAB-8 (c), NDAB-9 (d). * is the signal of residual solvent CDCl3. NDAB-6, 8, 9 are synthesized under organo-catalyst piperidinium acetate via Mannich-type condensation in yield ranging from 26 to 37%. The methoxyl and ethyl ester groups on the fifth position of 2,3,3-trimethylindolenine are introduced for NDAB-8, 9 compared to NDAB-6 shown in Scheme 1. 4-acetyl-2-formylpyrrole is used to obtain NDABs in this work. 1H-NMR spectroscopic data for NDABs are shown in Figure 1, consisting with their structure information. The proton of hydrogen bond of NDABs appears at 15.7-16.3 ppm. The protons of pyrrolizine and pyrrole ring appear as a singlet at 6.4-7.1 ppm for NDABs. The aromatic protons of indole appear at a singlet and doublet at 6.4-8.2 ppm. The remaining protons of methyl and methylene unit appear at 1.0-5.2 ppm.
6
In those -NH- type systems, stronger intramolecular hydrogen bond induces more downfield chemical shift of NH signal in the 1H NMR spectra[36, 52, 53]. The N-H chemical shift (ppm) of NDABs in 1H NMR spectra is concluded in the Table S1. The chemical shift of the N-H proton is obviously downfield shifted from δ = 15.78 ppm to 16.30 ppm (NDAB-H vs NDAB-6, CDCl3), suggesting that the strength of hydrogen bond is enhanced by the introduction of the strong withdrawing group on proton donor unit. Meanwhile, N-H chemical shift (ppm) of NDAB-6, NDAB-8, NDAB-9 are similar. When the strong withdrawing group is decorated on the proton donor unit of NDABs, the substitution on the proton acceptor unit plays a relatively minor role on those ESIPT process, due to the longer geometric distance between the substitution position and the -NH- group. 2 Photophysical Properties
Figure 2. UV-Vis (solid line) and Fluorescence (dash line) spectra of NDAB-6, 8, 9 in toluene (Left) and NDAB-9 in various solvents (Right). The absorption and emission spectra have been carried out in toluene, dichloromethane (DCM), methanol (MeOH), dimethylsulfoxide (DMSO), respectively, (Figure 2, 3 and S1-2). The data, namely their maximum absorption and emission wavelengths, molar extinction coefficients, Stokes shifts, and fluorescence quantum yields, have been summarized in Table S2. 7
Solvents with different polarity have no dramatically effect on the photophysical properties. Figure 2 shows absorption and the normalized emission spectra of selected NDABs. NDABs shows characteristic strong absorption bands around 400 nm, and their emission bands were observed at 570-600 nm as a sole peak with high molar absorption coefficients (ε = 2.14-4.43× 104 M-1 cm-1). All NDABs show higher quantum yields (0.14-0.47) and larger Stokes shifts (7270-8025 cm-1). The strong absorption bands are due to the S0–S1 ( π - π *) transition. Obviously, the large Stokes shifts have been detected in NDAB-H, NDAB-6, NDAB-8, NDAB9, considered as fast speed ESIPT system compared with NDAB-COOEt shown obviously dual peak[41] and their BF2 complex DABs[45, 54], BOPYINs[55] without ESIPT process.
3 Calculation For detail discussion of the ESIPT process of NDABs, frontier molecular orbitals (MOs), the geometry configurations of ground state, excited state and potential energy surfaces have been calculated. NDABs with substituents both on pyrrole as proton donor and indole as proton acceptor unit are discussed for the first time in a qualitative manner. 3.1 Geometries of ground and excited states Ground and excited states optimized structures of NDABs are calculated by DFT methods. Our work is divided into two sections to discuss. On one hand, NDAB-H, NDAB-6, NDAB-7 (with H, -COOEt, -NEt2 groups on proton donor part, without substitution on proton acceptor part) are chosen for discussing the relationship between substitutions on proton donor part and the ESIPT process. On the other hand, the ESIPT process of NDAB-6, NDAB-8, NDAB-9 (with ethyl ester on proton donor part, different substitution on proton acceptor part) are discussed by experiment and calculation.
8
In our opinion, the strength of intramolecular H-bond can be speculated by the change of Hbonding length form ground to excited state. Take NDAB-6 for example (Figure 3), when it is excited from the normal ground (N-S0) state to the normal excited (N-S1) state, the distance of intramolecular hydrogen bond is decreased. Meanwhile, the bond angel of ∠ N-H⋯N is enhanced. The similar trend in bond length of hydrogen bonds and bond angles is also exhibited in other NDABs (Table S3, Figure S3-4) suggesting that the ESIPT reaction occurred smoothly in this system. From the kinetics point of view, shorter distance of hydrogen bond in S1 states leads to stronger hydrogen bond, which is connected with the rate of ESIPT reaction [56]. On the one hand, the length of hydrogen bond of NDAB-6 is much shorter than NDAB-H in N-S1 states, otherwise the shortened length of NDAB-6 from N-S1 to N-S0 is larger than NDAB-H. The intramolecular hydrogen bond become stronger in NDAB-6 which accelerates the rate of ESIPT process. NDAB-7 with strong donating group on pyrrole unit could not detect the excited state of tautomeric structure by calculation, which means ESIPT process is suppressed. On the other hand, comparing NDAB-6, NDAB-8, NDAB-9 with different substitution on proton acceptor part. The bond lengths of hydrogen bond in S1 states are in the order of NDAB-9 < NDAB-8 ~ NDAB-6. This implies that different substitution on proton acceptor unit has weak effect on the ESIPT process in the present of ester group on proton donor unit. The ester substitution on proton donor moiety plays a major role in the process of ESIPT reaction.
9
Figure 3. Optimized S0 and S1 state structures of NDAB-6. 3.2 Charge distribution and transfer In order to further investigate the detail process of electronic transition upon excitation in those system, the four frontier molecular orbitals (MOs) of the NDABs are calculated and exhibited in Figure 4 and Table S4-S5. With the ethyl ester substitution on pyrrole unit, the energy of four frontier molecular orbitals decreased obviously and the electronic transferred from one pyrrole to another, finally moved to indole unit. This speculates that the electron distribution in N atom of pyrrole unit transfer to N atom of indole unit serving as the driving force for ESIPT process. The energy gaps of the NDABs shown in Figure 4 are following the order of NDAB-H (5.82 eV) > NDAB-6 (5.72 eV) > NDAB-8 (5.65 eV) ~ NDAB-9 (5.63 eV), which is in good correlation with the speed of NDAB-6, NDAB-8, NDAB-9 with ethyl ester substitution on pyrrole unit (Figure 1). Their speed is faster than NDAB-H. Accordingly, compared with NDAB-H, energy change of LUMO for NDAB-6 with electronwithdrawing group on proton donor part is more than that of HOMO (Table S4). NDAB-6, NDAB-8 and NDAB-9 with different substitution on indole unit have no dramatically effect on the energy of HOMOs and LUMOs. Four frontier orbitals are slightly enhanced by the donating
10
group on indole ring (NDAB-8), and meanwhile slightly decreased by the withdrawing group (NDAB-9). However, the energies of four frontier orbitals are obviously decreased to NDAB-H (Table S5).
Figure 4. Computed HOMO and LUMO orbitals for NDABs in their normal form involved in the first singlet excitation in toluene. 3.3 Potential energy curves The strength of intramolecular H-bond also can be approximated by the energy change in excited state. Potential energy curves are investigated to study the mechanism of proton transfer reaction in detail. Considering the calculation time, the potential energy curves are constructed by increasing the N-H bond length in enhancement of 0.06 Å for 12 steps using DFT/B3LYP method in the S0 state and using TDDFT/B3LYP method in the S1 state. From the view of thermodynamics, the calculated energies △E1 (between the excited state of tautomer and normal structures) and △E2 ( energy barriers between the excited state of highest energy structures and normal structures) are concluded in Table S6, which related to the speed of ESIPT process[57]. In the excited states, energy barriers (△E2) are 2.32, 0.87, 2.09, 1.49
11
Kcal/mol. All the compounds have smaller energy barriers with the order of NDAB-6 < NDAB9 < NDAB-8 < NDAB-H. Meanwhile, the energy gaps between N-S1 and T-S1 (△E1) states are 0.01, -3.85, -2.41, -1.84 Kcal/mol for NDAB-H, NDAB-6, NDAB-8, NDAB-9, respectively. △ E1 of NDAB-H is slightly positive and NDAB-6, NDAB-8, NDAB-9 are obvious negative. The energies of T-S1 state are much higher than that of N-S1 state. The proton transfer from N-S1 to T-S1 state is exergonic with the fast or even ultrafast speed ESIPT process. However, the substitution of withdrawing group or donating group on proton acceptor unit has the similar trend on the energy change, energy barrier and the energy between N-S1 and T-S1 state. Thus, substitution on the proton acceptor unit of NDAB-6 has a minor effect on the ESIPT process.
Figure 5. Calculated potential energy curves of the S0 (black line) and S1 (red line) states along the proton tautomerization reactions of NDABs. Conclusion 12
Three new -NH- type ESIPT molecules NDABs based on seven-membered pyrrole-indole systems have been strategically synthesized in 26-37% yield. The all NDABs show characteristic strong absorption bands around 400 nm with high molar absorption coefficients (ε = 2.144.43×104 M-1 cm-1), and emission bands around 570-600 nm with high quantum yields (0.140.47). The obvious sole emission peak and large Stokes shift (7270-8025 cm-1) have been detected by experiment. This is the first time to study the effect of substitutions both on proton donor and proton acceptor part in the system. Conjugated with geometry, dynamics and thermodynamics analysis, ESIPT process has been demonstrated. ESIPT of NDABs has been dramatically affected by the substitution on proton donor part and slightly on the proton acceptor moiety. The acidity of NDABs is more closely related to the substitution on donor sites. Withdrawing group on donor part leads to an increase of hydrogen-bonding strength, which is associated with ESIPT rate in those seven-membered -NH- type system. Corresponding Author *Corresponding authors. E-mail addresses: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 21606145, 21805166), The 111 Project of Hubei Province (Grant No. 2018-19-1), State Key Laboratory of Coordination Chemistry Foundation of Nanjing University (No. SKLCC1811). REFERENCES [1] Zhou P, Han K. Unraveling the Detailed Mechanism of Excited-State Proton Transfer.
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Highlights A series of naked diazaborepin dyes (NDABs) have been designed and synthesized. NDABs exhibit high quantum yields (0.14-0.47) and large Stokes shifts (167-205 nm). ESIPT process of NDABs was investigated by experiment and theoretical calculation. ESIPT process has been dramatically affected by the substitutions on proton donor unit.
Author statement
Nuonuo Zhang: Data curation, Supervision, Writing Original draft; Genjiang Liu: Investigation, Resources; Jiaying Yan: Conceptualization, Methodology, Software, Project administration; Tingting Zhang: Formal analysis; Xiang Liu: Writing - Review & Editing, Funding acquisition
The authors declare no competing financial interest.