LIU Gi-Mei MA Wei-Ji WANG Yn,b②YANG Yn SONG Xin-Jin

a (School of Chemistry and Environmental Engineering, Hubei Minzu University, Enshi 445000, China)

b (Key Laboratory of Magnetic Resonance in Biological Systems, National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China)

ABSTRACT Excited-state intramolecular proton transfer (ESIPT) reactions of three ortho-hydroxylated oxazolines, 2-(4,4-dimethyl-4,5-dihydro-oxazol-2-yl)-phenol (DDOP), 4-(4,4-dimethyl-4,5-dihydro-oxazol-2-yl)-[1,1΄-biphenyl]-3-ol (DDOP-C6H5) and 4-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxy-benzonitrile(DDOP-CN), have been systematically explored by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Two stable configurations (enol and keto forms) are found in the ground states(S0) for all the compounds while the enol form only exists in the first excited states (S1) for the compound modified with electron donating group (-C6H5). In addition, the calculated absorption and emission spectra of the compounds are in good agreements with the experiments. Infrared vibrational spectra at the hydrogen bond groups demonstrate that the intramolecular hydrogen bond O(1)−H(2)···N(3) in DDOP-C6H5 is strengthened in the S1 states, while the frontier molecular orbitals further reveal that the ESIPT reactions are more likely to occur in the S1 states for all the compounds. Besides, the proton transfer potential energy curves show that the enol forms can barely convert into keto forms in the S0 states because of the high energy barriers. Meanwhile, intramolecular proton transfer of all the compounds could occur in S1 states. The ESIPT reactions of the ortho-hydroxylated oxazolines are barrierless processes for unsubstituted DDOP and electron withdrawing substituted DDOP-CN, while the electron donating substituted DDOP-C6H5 has a small barrier, so the electron donating is unfavorable to the ESIPT reactions of ortho-hydroxylated oxazolines.

Keywords: excited-state intramolecular proton transfer, electron donating group, ortho-hydroxylated oxazolines, potential energy curves; DOI: 10.14102/j.cnki.0254–5861.2011–2990

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)-1Himidazo[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 oxazolinedirected 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 theπ-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-dihy-

drooxazol-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-zonitrile (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 theS0andS1state potential energy curves.

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 optimizedS0andS1structures.Simultaneously, the experimental environment was simulated in view of the polarizable continuum model (PCM) incurporating dichloromethane as the solvent[40-42]. In order to precisely clarify the ESIPT mechanism, the potential energy curves (PECs) in theS0andS1states 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 theS0andS1states 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 theS0states and keto forms in theS1states of DDOP, DDOP-C6H5and 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 theS0state, but no stable DDOP-enol and DDOP-CN-enol forms in theS1states 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 theS1states, which would lead to an energy barrier in the ESIPT reaction. As a result, the ESIPT reaction of DDOPC6H5is more difficult than that of DDOP and DDOP-CN to some extent.

Fig. 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

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

In view of the DDOP-C6H5-enol form, it can be seen that DDOP-C6H5changed fromS0(enol) toS1(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 theS0state (146.8°)increased significantly in theS1state (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 theS1state, which can facilitate the ESIPT reaction. As to the keto forms, in theS1state, 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 theS0states. And the H(2)–N(3) bond lengths are increased to 1.320 Å in theS0states. Meanwhile, the bond angles O(1)···H(2)–N(3) are enlarged from 127.0°, 126.6° and 125.2° in theS1states to 131.5°, 131.5° and 131.1° in theS1states. It can be concluded that the hydrogen bonds O(1)···H(2)–N(3) are more stable in theS0states. That is, the keto forms of theS1states are likely to undergo radiative transition to theS0states, 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 bothS0andS1states, and the infrared vibrational spectra linking the hydrogen bond in dichloromethane solvent are shown in Fig. 2. As can be seen, theS1states vibration frequencies of H(2)–N(3) group in DDOP-keto, DDOPC6H5-keto, and DDOP-CN-keto are 3421, 3454, and 3480 cm-1, decreased to 3297, 3295, and 3130 cm-1in theS0states,respectively. The 124, 159, and 350 cm-1blue-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 theS0states. For DDOP-C6H5-enol, the computed O(1)–H(2) stretching vibrational frequency is located at 3160 cm-1in theS0state, whereas 2934 cm-1in theS1state. A significant red-shift of 226 cm-1for the O(1)–H(2)stretching band indicates that the O(1)–H(2)···N(3) hydrogen bond is strengthened in theS1state. Therefore, the ESIPT reaction of DDOP-C6H5might be promoted by the strengthened hydrogen bond, which is in agreement with the result based on analyzing bond lengths and bond angles.

Fig. 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

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 optimizedS1states 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 theS1states. 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. 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

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. TheS0→S1transition 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 characteristicπ→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.

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

3. 3 Potential energy curves

To further elucidate the mechanism of ESIPT reactions in the ortho-hydroxylated oxazolines, theS0andS1states potential energy curves of DDOP, DDOP-C6H5and DDOPCN 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 theS0state, and the potential barriers are 6.26, 6.24 and 5.58 kcal·mol-1for DDOP, DDOP-C6H5and 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-1for DDOP, DDOP-C6H5and 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.

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

For theS1states, 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 theS1state. In contrast to DDOP and DDOP-CN, there are two minimum energy points and a small barrier of 1.26 kcal·mol-1along 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 S1state. 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-C6H5is 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 theS0state.As reported by previous work[49], the positive values of sign(λ2)ρ, the values of sign(λ2)ρclose to zero and the negative sign(λ2)ρrepresent 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 theS0state. This phenomenon once again illustrates the intramolecular hydrogen bond O(1)–H(2)···N(3)for DDOP and its derivatives.

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

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 theS0andS1states 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 theS1state.All three compounds show large Stokes shifts of ～10000 cm-1due 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 characteristicπ→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 bothS0andS1states further confirm that the proton transfer reactions can take place in theS1state easier than in theS0state. More importantly, theS1proton 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.