English

• 1. INTRODUCTION

  Hydrogen bond, as one of most fundamental weak interactions, has been an important research subjects for a long time due to its pervasiveness in biology, physics and chemistry fields^[1-9]. It plays a critical role in the stabilization of polypeptides and proteins as well as the accumulation of crystals. Since the dynamics of excited hydrogen bonds were proposed, excited state hydrogen bond was widely found in many photophysical and photochemical processes, such as intramolecular charge transfer (ICT)^{[10, 11]}, photoinduced electron transfer (PET)^{[12, 13]}, fluorescence resonance energy transfer (FRET)^[14], fluorescence quenching (FQ)^{[15, 16]}, and excited state intramolecular proton transfer (ESIPT)^[17]. ESIPT reaction with hydrogen bonding should be one of the most important dynamic processes. Commonly, the molecules with ESIPT properties involve a heterocyclic ring which possesses proton donor (-OH or -NH₂) and acceptor (-C=O or -N=) bridged by intramolecular hydrogen bond. In excited states, the proton would transfer from donor to acceptor rapidly. Previous work has shown that the ESIPT reaction was a process of four-level enol-keto-phototautomerism cycle^[18-22], in which, dual fluorescence and a large stokes shift between absorption and tautomer emission could be observed^{[23, 24]}. These remarkable natures make ESIPT molecules widely developed in a variety of applications, fluorescence sensors, laser dyes, UV-absorbers, molecular switches, fluorescent probes, bioimaging and OLEDs^[25-31].

  ESIPT compounds include the derivatives of salicylate and aniline salicylate, flavonoids, benzoazoles and chalcolones^[32]. Their photophysical properties could be modulated via introducing electron withdrawing and donating substituents, replacing heteroatom or changing the conjugation of structures^[33]. Up to now, the effects of substituents have received intense attention both experimentally and theoretically. By introducing different substituents into the benzothiazolyl ring, Li et al. synthesized a series of novel 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives^[34]. The emission intensity of these derivatives decreases with the enhancement of electron-withdrawing ability of the substituents and further demonstrated that electron-withdrawing substituents are not beneficial to the ESIPT reactions. In the experiment, Araki's team observed the blue- and red-shift of fluorescence emission peaks of 1-(2΄-hydroxyphenyl)-1H-imidazo[4, 5-c]pyridine (HPIP) derivatives would be tuned by introducing electron withdrawing and electron donating groups into the part of proton donor, respectively^[35]. However, the introduction of such groups into the part of proton accepter led to opposite results. Tong and co-workers investigated the asymmetric substitution effect on the optical properties of keto-salicylaldehyde azine (KSA), which is valuable to design and develop novel KSA fluorescent probes^[36]. Very recently, Nachtsheim and co-workers have developed an efficient method to synthesize oxazoline-directed ortho C(sp²)–H hydroxylation using molecular oxygen or air as green oxidants. The emission properties of selected phenols were investigated in dichloromethane, showing almost exclusive ESIPT keto-emission and exhibiting large Stokes shifts up to 12, 000 cm^-1. Depending on the substitution pattern and the π-extension of luminophore, the emission wavelengths range from blue to green and red^[37]. However, the deeper investigations into luminescent properties and the detailed ESIPT dynamical behaviors of ortho-hydroxylated oxazolines are deficient. In this study, the ESIPT reactions and spectral properties of three typical ortho-hydroxylated oxazolines, 2-(4, 4-dimethyl-4, 5-dihydrooxazol-2-yl)-phenol (DDOP), 4-(4, 4-dimethyl-4, 5-dihydrooxazol-2-yl)-[1, 1΄-biphenyl]-3-ol (DDOP-C₆H₅) and 4-(4, 4-dimethyl-4, 5-dihydrooxazol-2-yl)-3-hydroxy-ben-zoni-trile (DDOP-CN) (Fig. 1) were systemically investigated in dichloromethane solvent by using DFT and TD-DFT methods. The geometries for different electronic states were optimized and their bond lengths, bond angles and infrared vibrational spectra associated with the hydrogen bonds were analyzed. The frontier molecular orbitals (FMOs) and reduced density gradient (RDG) function were also discussed. To further reveal the ESIPT reactions, we constructed the S₀ and S₁ state potential energy curves.

  Figure 1

  

  Figure 1.  Optimized structures of the enol forms in the S₀ state and keto forms in the S₁ state for DDOP, DDOP-C₆H₅, and DDOP-CN

  DownLoad: Full-Size Img PowerPoint

  2. COMPUTATIONAL DETAILS

  In this work, all the quantum-chemical computations about the electronic structures of the ground states (S₀) and the first excited states (S₁) were obtained on the basis of DFT and TDDFT methods with B3LYP functional^[38] in combination with 6-31+G(d) basis set by Gaussian 09 programs^[39]. All the compounds were optimized with no constraints, and the most stable structures are given without imaginary frequencies. The absorption and emission properties were calculated based on the optimized S₀ and S₁ structures. Simultaneously, the experimental environment was simulated in view of the polarizable continuum model (PCM) incur-porating dichloromethane as the solvent^[40-42]. In order to precisely clarify the ESIPT mechanism, the potential energy curves (PECs) in the S₀ and S₁ states were constructed by scanning the O(1)–H(2) bond length for a fixed step size from 0.8 to 2.2 Å at a step of 0.1 Å. In addition, the reduced density gradient (RDG) analysis was obtained by Multiwfn^[43] and VMD^[44] softwares.

  3. RESULTS AND DISCUSSION

  3.1 Geometric structures

  All geometry structures of the S₀ and S₁ states of three ortho-hydroxylated oxazolines (DDOP, DDOP-C₆H₅, and DDOP-CN) are optimized in dichloromethane solvent based on B3LYP/6-31+G(d) and TD-B3LYP/6-31+G(d) levels. The optimized enol forms in the S₀ states and keto forms in the S₁ states of DDOP, DDOP-C₆H₅ and DDOP-CN are shown in Fig. 1, and the major bond lengths and bond angles of each structure are provided in Table 1. The calculated results show that there are two stable forms (enol and keto) for all compounds in the S₀ state, but no stable DDOP-enol and DDOP-CN-enol forms in the S₁ states can be optimized. In other words, when we optimized the DDOP-enol(S₁) and DDOP-CN-enol(S₁) forms, the stable states turn out to be DDOP-keto (S₁) and DDOP-CN-keto (S₁) forms. It shows that the ESIPT reactions of DDOP and DDOP-CN molecules may be a non-barrier process, which will be discussed in detail in the section of potential energy curves. However, when introducing an electron donating group (-C₆H₅) at the para position of the phenyl ring, the corresponding molecule is DDOP-C₆H₅. There are two stable forms (enol and keto) in the S₁ states, which would lead to an energy barrier in the ESIPT reaction. As a result, the ESIPT reaction of DDOP-C₆H₅ is more difficult than that of DDOP and DDOP-CN to some extent.

  Table 1

  

  Table 1.  Calculated Geometric Parameters (Bond Lengths in Å and Bond Angles in °) for DDOP, DDOP-C₆H₅, and DDOP-CN in S₀ and S₁ States Based on the DFT and TD-DFT Methods, respectively

  DownLoad: CSV

  	DDOP-enol		DDOP-keto		DDOP-C6H5-enol		DDOP-C6H5-keto		DDOP-CN-enol		DDOP-CN-keto
  State	S0	S1	S0	S1	S0	S1	S0	S1	S0	S1	S0	S1
  O(1)−H(2)	1.000	-	1.820	2.018	1.000	1.010	1.819	2.007	1.001	-	1.826	2.011
  H(2)−N(3)	1.750	-	1.032	1.025	1.750	1.710	1.032	1.023	1.742	-	1.032	1.022
  O(1)−H(2)···N(3)	146.7	-	131.5	127.0	146.8	148.8	131.5	126.6	146.5	-	131.1	125.2

  In view of the DDOP-C₆H₅-enol form, it can be seen that DDOP-C₆H₅ changed from S₀(enol) to S₁(enol) upon photoexcitation, O(1)−H(2) bond length increased from 1.000 to 1.010 Å, and the H(2)···N(3) bond length reduced significantly from 1.750 to 1.710 Å. In addition, the O(1)–H(2)···N(3) bond angle in the S₀ state (146.8°) increased significantly in the S₁ state (148.8°). The lengthening of the O(1)–H(2) bond length, the shortening of the H(2)···N(3) bond length and the increase of O(1)–H(2)···N(3) bond angle suggest that the hydrogen-bond O(1)–H(2)···N(3) is reinforced in the S₁ state, which can facilitate the ESIPT reaction. As to the keto forms, in the S₁ state, the O(1)···H(2) bond lengths of DDOP, DDOP-C₆H₅, and DDOP-CN are 2.018, 2.007 and 2.011 Å, and the H(2)–N(3) bond lengths are 1.025, 1.025 and 1.022 Å, respectively, which confirm that the H(2) atoms have migrated from O(1) atoms to N(3) atoms and form new covalent bonds with N(3) atoms. In addition, the O(1)···H(2) bond lengths of DDOP, DDOP-C₆H₅, and DDOP-CN are decreased to 1.820, 1.819 and 1.826 Å in the S₀ states. And the H(2)–N(3) bond lengths are increased to 1.320 Å in the S₀ states. Meanwhile, the bond angles O(1)···H(2)–N(3) are enlarged from 127.0°, 126.6° and 125.2° in the S₁ states to 131.5°, 131.5° and 131.1° in the S₁ states. It can be concluded that the hydrogen bonds O(1)···H(2)–N(3) are more stable in the S₀ states. That is, the keto forms of the S₁ states are likely to undergo radiative transition to the S₀ states, forming stable intramolecular hydrogen bond O(1)···H(2)–N(3) after the ESIPT process.

  As is well known, an effective signature for the evidence of excited state hydrogen bond strengthening or weakening can be estimated by the peak red-shift or blue-shift of the stretching vibrations of O–H moiety involved in hydrogen bond^[45-48]. In this work, the infrared spectra were performed in both S₀ and S₁ states, and the infrared vibrational spectra linking the hydrogen bond in dichloromethane solvent are shown in Fig. 2. As can be seen, the S₁ states vibration frequencies of H(2)–N(3) group in DDOP-keto, DDOP-C₆H₅-keto, and DDOP-CN-keto are 3421, 3454, and 3480 cm^-1, decreased to 3297, 3295, and 3130 cm^-1 in the S₀ states, respectively. The 124, 159, and 350 cm^-1 blue-shifts of H(2)–N(3) stretching frequency obviously reveal the O(1)–H(2)···N(3) hydrogen bonds of these three compounds are strengthened in the S₀ states. For DDOP-C₆H₅-enol, the computed O(1)–H(2) stretching vibrational frequency is located at 3160 cm^-1 in the S₀ state, whereas 2934 cm^-1 in the S₁ state. A significant red-shift of 226 cm^-1 for the O(1)–H(2) stretching band indicates that the O(1)–H(2)···N(3) hydrogen bond is strengthened in the S₁ state. Therefore, the ESIPT reaction of DDOP-C₆H₅ might be promoted by the strengthened hydrogen bond, which is in agreement with the result based on analyzing bond lengths and bond angles.

  Figure 2

  

  Figure 2.  Calculated IR vibrational spectra for H(2)−N(3) stretching bands of (A) DDOP-keto, (B) DDOP-C₆H₅-keto, (C) DDOP-CN-keto and for O(1)−H(2) vibrational mode of (D) DDOP-C₆H₅-enol

  DownLoad: Full-Size Img PowerPoint

  3.2 Frontier molecular orbitals and electronic spectra

  The calculated Stokes shifts, absorption and emission peaks, oscillator strengths (f), and the corresponding compositions based TD-B3LYP/6-31+G(d) level are reported in Table 2, together with the available experimental values^[37]. It should be noted that the calculated absorption peak of DDOP, DDOP-C₆H₅, and DDOP-CN are 287, 306 and 311 nm, respectively, which are in good agreements with the corresponding experimental data (303, 316 and 326 nm)^[37]. The calculated fluorescence emission peaks based on the optimized S₁ states of DDOP-keto, DDOP-C₆H₅-keto, and DDOP-CN-keto are located at 450, 447 and 446 nm, which also match well with the experimental ESIPT-based emission values (470, 472 and 471 nm). In addition, all three compounds show large Stokes shifts, the calculated Stokes shifts of DDOP, DDOP-C₆H₅, and DDOP-CN are 12621 cm^-1, 10308, and 9733, which are well consistent with the experimental values (11730, 10460, and 9440 cm^-1). That is to say, the absence of the emission of the enol form would confirm that the ESIPT reactions are instantaneously in the S₁ states. In a word, the experimental absorption and emission spectra are well reproduced based on our calculated results, which demonstrate that the theoretical level is reasonable and effective.

  Table 2

  

  Table 2.  Calculated Absorption and Emission Peaks (nm), Oscillator Strengths (f), and the Corresponding Compositions (CI) for DDOP, DDOP-C₆H₅, and DDOP-CN in Dichloromethane Solvent, along with the Experimental Data

  DownLoad: CSV

  Compound	Absorption					Emission				Stokes shift (cm-1)	Stokes shift exp(cm-1)
  	Composition (CI%)	λ(nm)	f	λexp(nm)		Composition (CI%)	λ(nm)	f	λexp(nm)
  DDOP	HOMO→LUMO(92%)	287	0.1567	303		HOMO→LUMO(97%)	450	0.1328	470	12621	11730
  DDOP-C6H5	HOMO→LUMO(90%)	306	0.4672	316		HOMO→LUMO(98%)	447	0.1345	472	10308	10460
  DDOP-CN	HOMO→LUMO(93%)	311	0.2072	326		HOMO→LUMO(97%)	446	0.1598	471	9733	9440

  In addition, it is well known that the nature of the excited state can be directly exhibited by analyzing the frontier molecular orbitals (FMOs). The highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) are displayed in Fig. 3. The S₀→S₁ transition is mainly relative to the HOMO→LUMO for DDOP, DDOP-C₆H₅, and DDOP-CN molecules, in which their orbital transition contribution rates are 92%, 90% and 93%, as shown in Table 2. It can be distinctly found that the excitation processes have significant characteristic π→π* transitions from the HOMO to the LUMO for these compounds. As shown in Fig. 3, the electron density distributions of HOMO and LUMO are different. Herein, we mainly focus on the differences about the moiety involved in intramolecular H-bond O(1)–H(2)···N(3). The HOMO→LUMO transition make the electron density of O(1) atoms decrease and the electron densities of N(3) atom increase. As a result, the intramolecular hydrogen bond O(1)–H(2)···N(3) is strengthened, which may further trigger the proton transfer.

  Figure 3

  

  Figure 3.  HOMO and LUMO of DDOP, DDOP-C₆H₅, and DDOP-CN based on the calculated level of TD-DFT/B3LYP/6-31+g(d)

  DownLoad: Full-Size Img PowerPoint

  3.3 Potential energy curves

  To further elucidate the mechanism of ESIPT reactions in the ortho-hydroxylated oxazolines, the S₀ and S₁ states potential energy curves of DDOP, DDOP-C₆H₅ and DDOP-CN have been scanned along the proton transfer pathways based on constrained optimizations at the fixed O(1)–H(2) distance from 0.8 to 2.2 Å in a step of 0.1 Å, and the potential energy curves are shown in Fig. 4. The potential energy curves show two minimum energy points in the S₀ state, and the potential barriers are 6.26, 6.24 and 5.58 kcal·mol^-1 for DDOP, DDOP-C₆H₅ and DDOP-CN, respectively. It indicates that the stable enol forms can barely convert into the keto ones because of their high energy barriers for these three compounds. In comparison, the energy barriers for the RGSIPT processes are 3.46, 3.51 and 3.91 kcal·mol^-1 for DDOP, DDOP-C₆H₅ and DDOP-CN, respectively, which are significantly lower than that of the GSIPT processes. That is to say, the RGSIPT processes are easier to occur than that of the GSIPT processes to some extent.

  Figure 4

  

  Figure 4.  Potential energy curves of the S₀ and S₁ states for DDOP, DDOP-C₆H₅, and DDOP-CN along with the proton transfer coordinate

  DownLoad: Full-Size Img PowerPoint

  For the S₁ states, there is only minimum energy point for DDOP and DDOP-CN, and the ESIPT reactions are assumed to be a barrierless process, which are in accordance with the optimized geometries and explain why the optimized geometries of DDOP-enol (S₁) and DDOP-CN-enol (S₁) are not obtained. It means that the proton transfer processes are spontaneously in the S₁ state. In contrast to DDOP and DDOP-CN, there are two minimum energy points and a small barrier of 1.26 kcal·mol^-1 along the ESIPT process for DDOP-C₆H₅, indicating that the DDOP-C₆H₅-enol can be easily isomerized into DDOP-C₆H₅-keto (minimum energy point) by crossing a low barrier in the S₁ state. Hence, it could conceivably be concluded that the donating group (-C₆H₅) at the para position of the phenyl ring hinders the ESIPT reactions. Furthermore, the reverse proton transfer barrier of DDOP-C₆H₅ is relatively higher (14.09 kcal·mol^-1), and the DDOP-C₆H₅-keto can barely convert into DDOP-C₆H₅-enol. As a result, there is no enol form emission due to the high reverse proton transfer barrier and the ultrafast ESIPT process following the excitation, which is in accordance with the experimental emission spectra^[37].

  3.4 Discriminate of weak interaction types by RDG

  To visualize the hydrogen bonding interactions in the real space, we calculated the reduced density gradient (RDG) scatter plots and the corresponding isosurfaces in the S₀ state. As reported by previous work^[49], the positive values of sign(λ2)ρ, the values of sign(λ2)ρ close to zero and the negative sign(λ2)ρ represent the steric effect, Van der Waals (VDW) interactions and hydrogen bonding interactions, respectively. As shown in Fig. 5, the spikes locates around –0.01 a.u. in the S₀ state. This phenomenon once again illustrates the intramolecular hydrogen bond O(1)–H(2)···N(3) for DDOP and its derivatives.

  Figure 5

  

  Figure 5.  RDG scatter plots and isosurfaces for DDOP, DDOP-C₆H₅, and DDOP-CN in S₀ state

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In this present work, DFT and TD-DFT calculations with B3LYP functional in combination with the 6-31+G(d) basis set are performed to explore the ESIPT mechanisms of three ortho-hydroxylated oxazolines (DDOP, DDOP-C₆H₅, and DDOP-CN), and their absorption and fluorescence spectra are simulated and their potential energy curves involving the S₀ and S₁ states are constructed. Via analyzing the bond lengths, bond angles, and infrared vibrational spectra of these three stable structures, we confirm that the intramolecular hydrogen bond O(1)−H(2)···N(3) should be strengthened in the S₁ state. All three compounds show large Stokes shifts of ~10000 cm^-1 due to the absence of the emission of enol form. Our calculated results reproduce well the experimental absorption and emission spectra and the first electronic transitions of all three compounds have significant characteristic π→π* nature. The HOMO→LUMO transition results in the redistribution of charge and would facilitate the ESIPT reaction. In addition, the constructed potential energy curves of both S₀ and S₁ states further confirm that the proton transfer reactions can take place in the S₁ state easier than in the S₀ state. More importantly, the S₁ proton transfer potential energy curves of DDOP and DDOP-CN have been determined to be barrierless, indicating a fast dynamics mechanism, but there is a small barrier for DDOP-C₆H₅, which further verifies that the electron donating is unfavorable to the ESIPT reactions of ortho-hydroxylated oxazolines.

  
  
• 1. [1]

     Chen, M. J.; Runge, T.; Wang, L. L.; Li, R.; Feng, J.; Shu, X. L.; Shi, Q. S. Hydrogen bonding impact on chitosan plasticization. Carbohyd. Polym. 2018, 200, 115−121. doi: 10.1016/j.carbpol.2018.07.062

  2. [2]

     Zuo, H. Y.; Christopher, J. R.; Wong, T. H. F.; Tong, K. Y.; Chan, J.; Au-Yeung, H. Y. Activity-based sensing of ascorbate by using copper-mediated oxidative bond cleavage. Chem. Eur. J. 2020, 26, 1−8. doi: 10.1002/chem.201905547

  3. [3]

     Dalchand, N.; Cui, Q.; Geiger, F. M. Electrostatics, hydrogen bonding, and molecular structure at polycation and peptide: lipid membrane interfaces. ACS Appl. Mater. Inter. 2020, 12, 2149−2158.

  4. [4]

     Sessler, C. D.; Rahm, M.; Becker, S.; Goldberg, J. M.; Wang, F.; Lippard, S. J. CF₂H, a hydrogen bond donor. J. Am. Chem. Soc. 2017, 139, 9325−9332. doi: 10.1021/jacs.7b04457

  5. [5]

     Yang, Z. G.; Cao, J. F.; He, Y. X.; Yang, J. H.; Kim, T.; Peng, X. J.; Kim, J. S. Macro-/micro-environment-sensitive chemosensing and biological imaging. Chem. Soc. Rev. 2014, 43, 4563−4601. doi: 10.1039/C4CS00051J

  6. [6]

     Brovarets', O. O.; Yurenko, Y. P.; Hovorun, D. M. Intermolecular СН···О/N Н-bonds in the biologically important pairs of natural nucleobases: a thorough quantum-chemical study. J. Biomol. Struct. Dyn. 2014, 32, 993−1022. doi: 10.1080/07391102.2013.799439

  7. [7]

     Liang, S. Z.; Hammond, G. B.; Xu, B. Hydrogen bonding: regulator for nucleophilic fluorination. Chem. Eur. J. 2017, 23, 17850−17861. doi: 10.1002/chem.201702664

  8. [8]

     Crabtree, R. H. Hypervalency, secondary bonding and hydrogen bonding: siblings under the skin. Chem. Soc. Rev. 2017, 46, 1720−1729. doi: 10.1039/C6CS00688D

  9. [9]

     Robertson, C. C.; Wright, J. S.; Carrington, E. J.; Perutz, R. N.; Hunter, C. A.; Brammer, L. Hydrogen bonding vs. halogen bonding: the solvent decides. Chem. Sci. 2017, 8, 5392−5398. doi: 10.1039/C7SC01801K

  10. [10]

      Wen, Z. C.; Jiang, Y. B. Ratiometric dual fluorescent receptor for anions under intramolecular charge transfer mechanism. J. Cheminform. 2005, 36, 11109−11115.

  11. [11]

      Kumari, N.; Jha, S.; Bhattacharya, S. Colorimetric probes on anthraimidazolediones for selective sensing of fluoride and cyanide ion via intramolecular charge transfer. J. Organomet. Chem. 2011, 76, 8215−8222.

  12. [12]

      Khrenova, M. G.; Nemukhin, A. V.; Domratcheva, T. Photoinduced electron transfer facilitates tautomerization of the conserved signaling glutamine side chain in BLUF protein light sensors. J. Phys. Chem. B 2013, 117, 2369−2377. doi: 10.1021/jp312775x

  13. [13]

      Hankache, J.; Hanss, D.; Wenger, O. S. Hydrogen-bond strengthening upon photoinduced electron transfer in ruthenium-anthraquinone dyads interacting with hexafluoroisopropanol or water. J. Phys. Chem. A 2012, 116, 3347−3358. doi: 10.1021/jp300090n

  14. [14]

      Xu, W. J.; Liu, S. J.; Sun, H. B.; Zhao, X. Y.; Zhao, Q.; Sun, S.; Cheng, S.; Ma, T. C.; Zhou, L. X.; Huang, W. RET-based probe for fluoride based on a phosphorescent iridium(III) complex containing triarylboron groups. J. Mater. Chem. 2011, 21, 7572−7581. doi: 10.1039/c1jm00071c

  15. [15]

      Barman, N.; Singha, D.; Sahu, K. Fluorescence quenching of hydrogen-bonded coumarin 102-phenol complex: effect of excited-state hydrogen bonding strength. J. Phys. Chem. A 2013, 117, 3945−3953. doi: 10.1021/jp4019298

  16. [16]

      Chang, D. H.; Ou, C. L.; Hsu, H. Y.; Huang, G. J.; Kao, C. Y.; Liu, Y. H.; Peng, S. M.; Diau, E. W. G.; Yang, J. S. Cooperativity and site-selectivity of intramolecular hydrogen bonds on the fluorescence quenching of modified GFP chromophores. J. Org. Chem. 2015, 80, 12431−12443. doi: 10.1021/acs.joc.5b02303

  17. [17]

      Cao, H.; Liu, G. M.; Cai, J.; Wang, Y. Excited-state intramolecular proton transfer mechanisms of thiazole-based chemosensor: a TD-DFT study. Chin. J. Struct. Chem. 2020, 11, 1993−1940.

  18. [18]

      Yang, D. P.; Yang, G.; Jia, M.; Song, X. Y.; Zhang, Q. L. Comparing the substituent effects about ESIPT process for HBO derivatives. Comput. Theor. Chem. 2018, 1131, 51−56. doi: 10.1016/j.comptc.2018.03.016

  19. [19]

      Li, Y. Q.; Ma, Y. Z.; Yang, Y. F.; Shi, W.; Lan, R. F.; Guo, Q. Effects of different substituents of methyl 5-R-salicylates on the excited state intramolecular proton transfer process. Phys. Chem. Chem. Phys. 2018, 20, 4208−4215. doi: 10.1039/C7CP06987A

  20. [20]

      Berbigier, J. F.; Duarte, L. G. T. A.; Zawacki, M. F.; Araújo, B. B.; Santos, C. M.; Atvars, T. D. Z.; Gonçalves, P. F. B.; Petzhold, C. L.; Rodembusch, F. S. ATRP initiators based on proton transfer benzazole dyes: solid state photoactive polymer with very large stokes shift. ACS Appl. Polym. Mater. 2020, 2, 1406−1416. doi: 10.1021/acsapm.0c00102

  21. [21]

      Padalkar, V. S.; Seki, S. Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters. Chem. Soc. Rev. 2015, 45, 169−202.

  22. [22]

      Kwon, J. E.; Park, S. Y. Advanced organic optoelectronic materials: harnessing excited-state intramolecular proton transfer (ESIPT) process. Adv. Mater. 2011, 23, 3615−3642. doi: 10.1002/adma.201102046

  23. [23]

      Sedgwick, A. C.; Wu, L. L.; Han, H. H.; Bull, S. D.; He, X. P.; James, T. D.; Sessler, J. L.; Tang, B. Z.; Tian, H.; Yoon, J. Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents. Chem. Soc. Rev. 2018, 47, 8842−8880. doi: 10.1039/C8CS00185E

  24. [24]

      Zhang, Q. L.; Zhao, Z. J.; Cheng, S. B.; Yang, G.; Zhang, T. J.; Jia, M.; Song, X. Y. A theoretical investigation on the excited state intramolecular single or double proton transfer mechanism of a salicyladazine system. J. Chin. Chem. Soc. 2019, 66, 1416−1421. doi: 10.1002/jccs.201800490

  25. [25]

      Zhang, Y. W.; Sun, Q. K.; Li, Z. P.; Zhi, Y. F.; Li, H.; Li, Z. P.; Xia, H.; Liu, X. M. Light-emitting conjugated microporous polymers based on an excited-state intramolecular proton transfer strategy and selective switch-off sensing of anions. Mater. Chem. Front. 2020, 4, 3040−3046. doi: 10.1039/D0QM00384K

  26. [26]

      Sugiyama, K.; Tsuchiya, T.; Kikuchi, A.; Yagi, M. Optical and electron paramagnetic resonance studies of the excited triplet states of UV-B absorbers: 2-ethylhexyl salicylate and homomenthyl salicylate. Photochem. Photobiol. Sci. 2015, 14, 1651−1659. doi: 10.1039/c5pp00138b

  27. [27]

      Kanamori, D.; Okamura, T. A.; Yamamoto, H.; Ueyama, N. Linear-to-turn conformational switching induced by deprotonation of unsymmetrically linked phenolic oligoamides. Angew. Chem. Int. Ed. 2005, 44, 969−972. doi: 10.1002/anie.200461842

  28. [28]

      Yan, C. C.; Wang, X. D.; Liao, L. S. Organic lasers harnessing excited state intramolecular proton transfer process. ACS Photonics. 2020, 7, 1355−1366. doi: 10.1021/acsphotonics.0c00407

  29. [29]

      Zhao, Y.; Ding, Y.; Yang, Y.; Shi, W.; Li, Y. Fluorescence deactivation mechanism for a new probe detecting phosgene based on ESIPT and TICT. Org. Chem. Front. 2019, 6, 597−602. doi: 10.1039/C8QO01320A

  30. [30]

      Kaur, I.; Sharma, V.; Mobin, S. M.; Khajuria, A.; Ohri, P.; Kaur, P.; Singh, K. Aggregation tailored emission of a benzothiazole based derivative: photostable turn on bioimaging. RSC Adv. 2019, 9, 39970−39975. doi: 10.1039/C9RA08149F

  31. [31]

      Duarte, L. T.; Germino, J. C.; Berbigier, J. F.; Barboza, C. A.; Faleiros, M. M.; Simoni, D. A.; Galante, M. T.; Holanda, M. S.; Rodembusch, F. S.; Atvars, T. D. Z. White-light generation from all-solution-processed OLEDs using a benzothiazole-salophen derivative reactive to the ESIPT process. Phys. Chem. Chem. Phys. 2019, 21, 1172−1182. doi: 10.1039/C8CP06485G

  32. [32]

      Watwiangkham, A.; Roongcharoen, T.; Kungwan, N. Effect of nitrogen substitution and conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives. J. Photoch. Photobio. A Chem. 2019, 389, 112267−11.

  33. [33]

      Berbigier, J. F.; Duarte, L. T.; Zawacki, M.; Araújo, B.; Santos, C.; Atvars, T. Z.; Goncalves, P. F. B.; Petzhold, C. L.; Rodembusch, F. S. ATRP initiators based on proton transfer benzazole dyes: solid state photoactive polymer with very large stokes shift. ACS Appl. Polym. Mater. 2020, 2, 1406−1416. doi: 10.1021/acsapm.0c00102

  34. [34]

      Wang, R. J.; Liu, D.; Xu, K.; Li, J. Y. Substituent and solvent effects on excited state intramolecular proton transfer in novel 2-(2-hydroxyphenyl) benzothiazole derivatives. J. Photochem. Photobio. A Chem. 2009, 205, 61−69. doi: 10.1016/j.jphotochem.2009.03.025

  35. [35]

      Mutai, T.; Sawatani, H.; Shida, T.; Shono, H.; Araki, K. Tuning of excited-state intramolecular proton transfer (ESIPT) fluorescence of imidazo[1, 2 a]pyridine in rigid matrices by substitution effect. J. Org. Chem. 2013, 78, 2482−2489. doi: 10.1021/jo302711t

  36. [36]

      Tong, J. L.; Zhang, K. X.; Wang, J.; Li, H.; Zhou, F.; Wang, Z. M.; Zhang, X. J.; Tang, B. Z. Keto-salicylaldehyde azine: asymmetric substituent effect on their optical properties via electron-donating group insertion. J. Mater. Chem. C 2020, 8, 996−1001. doi: 10.1039/C9TC05822B

  37. [37]

      Göbel, D.; Clamor, N.; Lork, E.; Nachtsheim, B. J. Aerobic C(sp²)-H hydroxylations of 2-aryloxazolines: fast access to excited-state intramolecular proton transfer (ESIPT)-based luminophores. Org. Lett. 2019, 21, 5373−5377. doi: 10.1021/acs.orglett.9b01350

  38. [38]

      Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. doi: 10.1063/1.464913

  39. [39]

      Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B. 01, Gaussian, Inc., Wallingford CT 2009.

  40. [40]

      Mennucci, B.; Cancès, E.; Tomasi, J. Evaluation of solvent effects in isotropic and anisotropic dielectrics and in ionic solutions with a unified integral equation method: theoretical bases, computational implementation, and numerical applications. J. Phys. Chem. B 1997, 101, 10506−10517. doi: 10.1021/jp971959k

  41. [41]

      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. J. Comput. Chem. 1995, 16, 1449−1458. doi: 10.1002/jcc.540161202

  42. [42]

      Miertuš, S.; Scrocco, E.; Tomasi, J. Electrostatic interaction of a solute with a continuum. A direct utilizaion of ab initio molecular potentials for the prevision of solvent effects. Chem. Phys. 1981, 55, 117−129. doi: 10.1016/0301-0104(81)85090-2

  43. [43]

      Tian, L.; Chen, F. W. Multiwfn: a multifunctional wavefunction analyzer. J. Comput. Chem. 2012, 33, 580−592. doi: 10.1002/jcc.22885

  44. [44]

      Humphery, W.; Dalke, A.; Shulten, K. VMD: visual molecular dynamics. J. Mol. Graph. 1996, 14, 33−38. doi: 10.1016/0263-7855(96)00018-5

  45. [45]

      Zhao, G. J.; Han, K. L. Hydrogen bonding in the electronic excited state. Acc. Chem. Res. 2012, 45, 404−413. doi: 10.1021/ar200135h

  46. [46]

      Zhao, G. J.; Han, K. L. pH-Controlled twisted intramolecular charge transfer (TICT) excited state via changing the charge transfer direction. Phys. Chem. Chem. Phys. 2010, 12, 8914−8918. doi: 10.1039/b924549a

  47. [47]

      Zhao, G. J.; Han, K. L. Early time hydrogen-bonding dynamics of photoexcited coumarin 102 in hydrogen-donating solvents: theoretical study. J. Phys. Chem. A 2007, 111, 2469−2474. doi: 10.1021/jp068420j

  48. [48]

      Zhao, G. J.; Han, K. L. Time-dependent density functional theory study on hydrogen-bonded intramolecular charge-transfer excited state of 4-dimethylamino-benzonitrile in methanol. J. Comput. Chem. 2008, 29, 2010−2017. doi: 10.1002/jcc.20957

  49. [49]

      Johnson, E. R.; Keinan, S.; Mori-Sanchez, P.; Contreras-Carcis, J.; Cohen, A. J.; Yang, W. Revealing noncovalent interactions. J. Am. Chem. Soc. 2010, 132, 6498−6506. doi: 10.1021/ja100936w

• 

  Figure 1  Optimized structures of the enol forms in the S₀ state and keto forms in the S₁ state for DDOP, DDOP-C₆H₅, and DDOP-CN

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Calculated IR vibrational spectra for H(2)−N(3) stretching bands of (A) DDOP-keto, (B) DDOP-C₆H₅-keto, (C) DDOP-CN-keto and for O(1)−H(2) vibrational mode of (D) DDOP-C₆H₅-enol

  下载: 全尺寸图片 幻灯片
  

  Figure 3  HOMO and LUMO of DDOP, DDOP-C₆H₅, and DDOP-CN based on the calculated level of TD-DFT/B3LYP/6-31+g(d)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Potential energy curves of the S₀ and S₁ states for DDOP, DDOP-C₆H₅, and DDOP-CN along with the proton transfer coordinate

  下载: 全尺寸图片 幻灯片
  

  Figure 5  RDG scatter plots and isosurfaces for DDOP, DDOP-C₆H₅, and DDOP-CN in S₀ state

  下载: 全尺寸图片 幻灯片

  Table 1.  Calculated Geometric Parameters (Bond Lengths in Å and Bond Angles in °) for DDOP, DDOP-C₆H₅, and DDOP-CN in S₀ and S₁ States Based on the DFT and TD-DFT Methods, respectively

  	DDOP-enol		DDOP-keto		DDOP-C6H5-enol		DDOP-C6H5-keto		DDOP-CN-enol		DDOP-CN-keto
  State	S0	S1	S0	S1	S0	S1	S0	S1	S0	S1	S0	S1
  O(1)−H(2)	1.000	-	1.820	2.018	1.000	1.010	1.819	2.007	1.001	-	1.826	2.011
  H(2)−N(3)	1.750	-	1.032	1.025	1.750	1.710	1.032	1.023	1.742	-	1.032	1.022
  O(1)−H(2)···N(3)	146.7	-	131.5	127.0	146.8	148.8	131.5	126.6	146.5	-	131.1	125.2

  下载: 导出CSV

  Table 2.  Calculated Absorption and Emission Peaks (nm), Oscillator Strengths (f), and the Corresponding Compositions (CI) for DDOP, DDOP-C₆H₅, and DDOP-CN in Dichloromethane Solvent, along with the Experimental Data

  Compound	Absorption					Emission				Stokes shift (cm-1)	Stokes shift exp(cm-1)
  	Composition (CI%)	λ(nm)	f	λexp(nm)		Composition (CI%)	λ(nm)	f	λexp(nm)
  DDOP	HOMO→LUMO(92%)	287	0.1567	303		HOMO→LUMO(97%)	450	0.1328	470	12621	11730
  DDOP-C6H5	HOMO→LUMO(90%)	306	0.4672	316		HOMO→LUMO(98%)	447	0.1345	472	10308	10460
  DDOP-CN	HOMO→LUMO(93%)	311	0.2072	326		HOMO→LUMO(97%)	446	0.1598	471	9733	9440

  下载: 导出CSV
•