1 Introduction

Proton transfer (PT) plays a significant part in all sorts of chemical and biological systems [1,2,3,4,5,6]. Proton transfer can happen in the ground or excited state. From the viewpoint of photochemistry, the excited state proton transfer (ESPT) and excited state intramolecular proton transfer (ESIPT) begin with photoexcitation, in which the driving force based on acidity or basicity is strengthened, and then, PT occurs. In recent years, researchers have developed the application of ESIPT molecules and found them to be extremely powerful application value in light stabilizers [7], laser fuels [8], photoexcited materials [9], light-emitting materials in luminescent devices [10], fluorescent probes [11] and biological systems [12]. ESIPT molecule also can be applied to be a preferred material for photonic devices such as optical switches, optical limiting, optical waveguides and real-time optical storage. Studying the ESIPT reaction is of great significance for photochemical and photobiological processes and thus has become one of the hotspots of experimental and theoretical researches in the chemical field [13,14,15].

ESPT takes place from the proton donor to the proton acceptor among different molecules by forming dimers or complexes via intermolecular hydrogen-bonding (H-bonding). ESIPT is a single molecular reaction. Since the proton donor and acceptor are present in the same molecule and have a suitable distance, the proton transfer process can be accomplished with the aid of intramolecular H-bonding. Hence, the formation of intramolecular H-bond is a necessary step for proton transfer in the excited state. Most of the ESIPT systems involve hydroxyl or amino groups serving as the proton donors and carbonyl oxygen or azo nitrogen serving as the proton acceptors. Along the intramolecular H-bonding, a five-, six-, seven- or even eight-membered ring structure can be formed between proton donor and proton acceptor. In other words, at least a five-membered ring can undergo ESIPT [16,17,18,19,20,21,22,23,24].

ESIPT reaction is a four-level photoinduced tautomerism process. Generally, ESIPT molecule in its enol (E) form is the most stable in the ground state (S0), whereas its keto (K) form is more stable in the first singlet excited state (S1). Upon photoexcitation, the enol (E) structure should be excited to the excited state form (E*), and then, a proton can migrate from donor to acceptor with the structure transformation from E* to K*. After intense fluorescence emission from K* to K with a large Stokes shift (up to 10,000 cm−1), K structure returns back to the E structure via ground state intramolecular proton transfer (GSIPT) to accomplish the four-level process [25,26,27,28,29].

Most ESIPT processes have much less barrier height due to the strong intramolecular H-bond in ESIPT compound. The close distance between proton donor and proton acceptor in the ESIPT molecule is important to the ESIPT process [30, 31]. Namely, the strength of intramolecular H-bond has crucial effect on the photophysical properties of the ESIPT rate. Proton transfer reactions usually happen via forming a six-membered ring with a strong intramolecular H-bond between the proton donor and acceptor [32,33,34,35,36,37,38,39,40,41,42,43]. ESIPT occurring in five-membered [44,45,46] and seven-membered [17, 47] systems is not common. The H-bond distance in the five-, six- and seven-membered ring varies on sub-angstrom scale with their ESIPT ability [48]. On account of the previous studies, the subtle changes of H-bond distance may result in distinct effect on the fluorescence properties [8]. However, no systematical theoretical studies on five-, six- and seven-membered ring H-bonding systems have been reported. In this respect, it is very significant to research the ESIPT process of the five-, six- and seven-membered ring H-bonding systems, which is meaningful to interpret biochemical phenomena and design new ESIPT molecules.

A representative five-membered ring intramolecular H-bond system undergoing ESIPT process is 3-hydroxyflavone (3HF) derivatives. The emitting fluorescence properties of 3HF are very sensitive to H-bonding perturbations [49], and the intramolecular H-bond strength of 3HF can be tuned by adjusting the pi electron system conjugation through the two rings of the molecule. Sholokh et al. [50] synthesized a fluorescent l-amino acid containing 4′-methoxy-3-hydroxyflavone fluorescent group, which has a methoxy group in the para-position on the 2-phenyl moiety and exhibits double emission due to ESIPT. Another classical ESIPT molecule is 2-(2-hydroxyphenyl) benzothiazole (HBT), which has a six-membered intramolecular H-bond ring between proton donor (–OH) and proton acceptor (–N=). HBT and its derivatives are organic light-emitting and fluorescent probing materials and can occur ESIPT process in hundreds femtoseconds [51, 52]. Recently, a new compound of 2-(5-(4-carboxyphenyl)-2-hydroxyphenyl) benzothiazole was designed and synthesized by Li et al. based on the HBT form [53]. In contrast to a large number of studies on six-membered H-bonding ESIPT molecule [32,33,34,35,36,37,38,39,40,41,42,43], studies on the seven-membered ring intramolecular H-bonding system are very rare because of the weaker H-bond strength [17,18,19, 54,55,56,57]. For the seven-membered ring H-bonding ESIPT molecule, a typical example is 4-(2-hydroxy-benzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), which has the similar structure to the core chromophore of green fluorescent protein [17]. Upon electronic excitation, o-HBDI takes place ESIPT and leads to a large Stokes-shifted tautomer emission at 605 nm. Recently, based on the rule that the quantum yield of fluorescence emission would be increased with the structural rigidity, a structurally locked o-HBDI core chromophore, o-LHBDI has been reported by Hsu et al. [56].

In this work, we employed the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to systemically research the ESIPT processes of the typical five-, six- and seven-membered ring intramolecular H-bond molecules 4′-methoxy-3-hydroxyflavone (5-HB), 2-(5-(4-carboxyphenyl)-2-hydroxyphenyl) benzothiazole (6-HB) and o-LHBDI (7-HB) (see Fig. 1). The potential energy surfaces along the PT reactions both in the ground and excited states were described, and the structures, the barrier height of PT and spectral properties with vertical electronic absorption and emission were investigated. We hope that these detailed theoretical researches can throw a light on the correlation with the size of intramolecular H-bond and ESIPT mechanism.

Fig. 1
figure 1

Enol structures of 5-HB, 6-HB and 7-HB

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2 Computational details

The structures of 5-HB, 6-HB and 7-HB were optimized in the gas phase by using DFT and TDDFT methods for the S0 and S1 states, respectively. The hybrid functional of Truhlar and Zhao [58] (M06-2X) and 6-31 + G(d,p) basis set in the Gaussian 09 program [59] were employed. The frequency calculation has been carried out at the same computational level after the geometry optimization in order to testify the minima (reactant and product) and transition state (TS). Only one imaginary frequency and no imaginary frequency for the TS and minima along the potential surface of ESPT were found, respectively. The absorption and fluorescence spectra were performed at TD-M06-2X/6-31 + G(d,p) level with the S0 and S1 optimized structures. In order to deeply understand the ESIPT process, the S0 and S1 constrained potential energy curves (PECs) were scanned by point-to-point optimizations at M06-2X/6-31 + G(d,p) and TD-M06-2X/6-31 + G(d,p) levels, respectively. For each stationary point optimization, the reaction coordinate O1–H2 distance was fixed at a given range, while the other parameters are fully optimized without any constraint. Along the PECs, the increment of O1–H2 distance is 0.05 Å.

3 Results and discussion

3.1 Geometric structures

In this part, we studied the enol form structures of 5-HB, 6-HB and 7-HB first obtained by using M06-2X and TD-M06-2X methods in both S0 and S1 states, respectively. The optimized structural parameters related to the H-bonds of 5-HB, 6-HB and 7-HB in the ground (S0) and excited states (S1) are listed in Table 1. Based on our calculated results, the bond lengths of O1–H2, H2–O3 and O1–O3 of 5-HB are 0.976 Å, 2.032 Å and 2.632 Å in the S0 state, respectively. After excitation to S1 state, the corresponding bond distances are 0.998 Å, 1.838 Å and 2.535 Å, respectively. At the same time, the O1–H2–O3 bond angle changes from 117.8° in the S0 state to 124.0° in the S1 state. With comparison to the corresponding values in the S0 state, the bond length of O1–H2 in 5-HB increases by 0.022 Å in the S1 state, but H2–O3 and O1–O3 distances shorten by 0.194 Å and 0.097 Å, respectively. In addition, the O1–H2–O3 bond angle in the S1 state increases by 6.2° than that value in the S0 state. These results indicate that intramolecular H-bond H2–O3 is enhanced in the S1 state. Similarly, the O1–H2 bond length of 6-HB elongates from 0.987 Å in the S0 state to 1.063 Å in the S1 state. On the contrary, H2–N3 and O1–N3 distances shorten 0.267 Å and 0.150 Å in the excited state, respectively. The O1–H2–N3 bond angle of 6-HB in the S1 state increases by 6.5°. In the 7-HB compound, the O1–H2, H2–N3 and O1–N3 bond distances in the S1 state increase by 0.047 Å and decrease by 0.155 Å and 0.105 Å, respectively, compared to those values in the S0 state. It is obvious that the H2–N3 intramolecular H-bonds of 6-HB and 7-HB in the S1 state are stronger than those in the S0 state. Moreover, H2–N3 H-bond distances of 7-HB and 6-HB in the S1 state are with small difference, but both are much shorter than the corresponding H-bond of 5-HB, and the O1–H2–N3 bond angle of 7-HB is much larger than those of 6-HB and 5-HB. All these results indicate that the intramolecular H-bond strengths of 7-HB and 6-HB in the S1 state are stronger than that of 5-HB and is anticipated to promote the ESIPT process in 7-HB and 6-HB. Furthermore, simulating the infrared (IR) vibrational spectra of 5-HB, 6-HB and 7-HB may provide an effective way to further explain the changes of H-bond in the S0 and S1 states. As shown in Fig. 2 and Table 1, the calculated frequencies of O1–H2 stretching vibration of 5-HB, 6-HB and 7-HB are situated at 3702 cm−1, 3358 cm−1 and 3057 cm−1 in the S0 state, while 3300 cm−1, 2138 cm−1 and 2140 cm−1 in the S1 state. It is worth noting that the 402 cm−1, 1220 cm−1 and 917 cm−1 redshift of the O–H stretching frequency demonstrates that intramolecular H-bonding (O1–H2···O3/N3) is strengthened in the S1 state.

Table 1 Bond lengths (in Å), bond angles (in °) and IR vibrational frequencies of O1–H2 of the S0 and S1 states for the studied compounds 5-HB, 6-HB and 7-HB in gas obtained at the M06-2X/6-31 + G(d,p) and TD-M06-2X/6-31 + G(d,p) levels
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Fig. 2
figure 2

IR spectra for 5-HB, 6-HB and 7-HB in the region of the O1–H2 stretching vibration frequencies in the S0 and S1 states at the M06-2X/6-31 + G(d,p) and TD-M06-2X/6-31 + G(d,p) levels

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3.2 Electronic spectra and frontier molecular orbitals

The first excited state structures of 5-HB, 6-HB and 7-HB compounds are completely optimized at TD-M06-2X/6-31 + G(d,p) level. The calculated absorption and fluorescence spectra of 5-HB, 6-HB and 7-HB compounds in the gas phase obtained at the TD-M06-2X/6-31 + G(d,p) level are displayed in Fig. 3. The optimized geometries in the S0 and S1 states are served as the initial structure to obtain the S0–S1 vertical excitation energy and electronic spectra. It can be found that the calculated absorption peak for 5-HB, 6-HB and 7-HB lies on 313.7 nm, 310.8 nm and 341.4 nm, which are consistent with experimental values (5-HB: 350–355 nm; 6-HB: 282–390 nm; 7-HB: ~ 380 nm) [50, 53, 56].

Fig. 3
figure 3

Calculated absorption and fluorescence spectra of 5-HB, 6-HB and 7-HB in the gas phase at the TD-M06-2X/6-31 + G(d,p) level

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Moreover, the fluorescence properties of 5-HB, 6-HB and 7-HB are also simulated at the enol and keto forms. The calculated fluorescence emission peaks of 5-HB-enol and 5-HB-keto are situated at 360.1 nm and 491.6 nm, respectively, which are in agreement with the experimental data (enol form: 410–420 nm; keto form: 530–541 nm) [60]. Evidently, the emission peak of 5-HB-enol redshifts 46.3 nm compared to the absorption peak, which would be attributed to the Stokes shift, whereas the 5-HB-keto emission peak has a large redshift of 177.9 nm compared to the absorption peak. Similarly, for 6-HB and 7-HB, dual fluorescent emission peaks are obtained at 366.5 nm, 397.4 nm for enol form and 471.7 nm, 472.1 nm for the keto form, respectively. Our theoretical fluorescence emission peaks of 6-HB and 7-HB are consistent with the experimental values [6-HB: 398 nm (enol form), 540 nm (keto form); 7-HB: 480 nm (enol form), 585 nm (keto form)]. The fluorescence emission peaks exhibit the Stokes shift of 55.7 nm and 56.0 nm for the enol form of 6-HB and 7-HB molecules, respectively. In addition, the keto forms of 6-HB and 7-HB molecules have a large redshift of 160.9 nm and 130.8 nm, respectively, between the fluorescent emission peak and the absorption peak. The double emission peaks mean that 5-HB, 6-HB and 7-HB molecules have two isomers (enol and keto) in the S1 state, and the keto form isomer of 5-HB, 6-HB and 7-HB generated due to ESIPT process.

In order to explore the nature of the conformations of charge distribution in the S1 state and ESIPT dynamics, the frontier MO of 5-HB, 6-HB and 7-HB in gas was analyzed and is shown in Fig. 4. Herein, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are depicted in the S1 state. From Fig. 4, it can be clearly seen that the HOMO and LUMO in the S1 state have π and π* character localized on different parts of 5-HB, 6-HB and 7-HB molecules, respectively, which demonstrates that the transition from the HOMO and LUMO is a predominant ππ*-type transition with a charge transfer character. It is worth noting that the charge densities of hydroxyl moiety (O1–H2) and proton acceptor (O3/N3) decrease and increase through the transition from HOMO to LUMO, respectively. According to valence bond theory, the added electron density of acceptor O3/N3 atoms would be of great importance in strengthening the intramolecular H-bond and then facilitates ESIPT [61,62,63,64,65,66,67,68]. The chemical activity of the molecule can be reflected by the energy gap between HOMO and LUMO. The high chemical activity and low kinetic stability mean a small energy gap [69, 70]. The energy gaps of 5-HB, 6-HB and 7-HB compounds are 5.20 eV, 5.36 eV and 4.99 eV, respectively. The energy gap of 7-HB is smaller than those of 5-HB and 6-HB, meaning that 7-HB has a high chemical activity and low kinetic stability. Namely, it is much easier to occur ESIPT process for 7-HB compound.

Fig. 4
figure 4

Frontier molecular orbitals (HOMO and LUMO) of 5-HB, 6-HB and 7-HB in the S1 state

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3.3 Potential energy curves

In order to better understand the ESIPT processes of 5-HB, 6-HB and 7-HB molecules, we constructed the potential energy curves (PECs) in the S0 and S1 states by M06-2X/6-31 + G(d,p) and TD-M06-2X/6-31 + G(d,p) methods, respectively. Potential energy curves are scanned by using the constrained optimizations in the S0 and S1 states with fixed O1–H2 distance in a given range and in increments of 0.05 Å. The range of O1–H2 distance in PEC was selected because all the enol, transition state and keto structures can be contained in it. The information of qualitative energetic pathways for the ESIPT process can be obtained by PECs.

As shown in Fig. 5, the relative energy of 5-HB, 6-HB and 7-HB in the S0 state is lower than that in the S1 state, which means that the enol form of 5-HB, 6-HB and 7-HB compounds in the S1 state is unstable and ESIPT process is apt to happen by crossing a small barrier (1.87 kcal/mol for 5-HB, 0.23 kcal/mol for 6-HB and 0.34 kcal/mol for 7-HB). In addition, the reverse ESIPT barriers are 10.9 kcal/mol, 4.73 kcal/mol and 1.42 kcal/mol for 5-HB, 6-HB and 7-HB, respectively, and all the barriers are bigger than ESIPT barriers, demonstrating that 5-HB, 6-HB and 7-HB in the S1 state exist in the keto forms. It can also be seen that no stationary points for 6-HB and 7-HB-keto forms in the ground state can be obtained, which indicates that proton transfer processes cannot occur. The proton transfer is also hard to proceed for the enol forms of 5-HB molecule in the S0 state to turn in the keto forms because of the relatively high barrier (5-HB: 16.0 kcal/mol) and endothermic reaction. On the contrary, the reverse barrier of 5-HB is very small (0.88 kcal/mol), so 5-HB is inclined to exist in the enol structure in the S0 state. Based on the previous discussions, it can be concluded that the ability of ESIPT varies along this order: 6-HB~7-HB > 5-HB.

Fig. 5
figure 5

Potential energy curves of 5-HB, 6-HB and 7-HB in the S0 and S1 states

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4 Concluding remarks

In summary, the photophysical properties and ESIPT process of 4′- methoxy-3-hydroxyflavone (5-HB), 2-(5-carboxyphenyl)-2-hydroxyphenyl) benzothiazole (6-HB) and o-LHBDI (7-HB) compounds in the gas phase are theoretically studied by using M06-2X and TD-M06-2X methods. The ground and excited state structural parameters and IR vibrational spectra, electronic spectra, frontier MOs and the potential energy curves are investigated to analyze the ESIPT process. The results indicate that the intramolecular H-bonds are obviously enhanced in the S1 state based on the shortened H-bond distance and the redshift of IR vibrational frequency of O1–H2, which can promote the ESIPT reactions. The intramolecular H-bonds of 6-HB and 7-HB in the S1 state are not much different, both are much stronger than that of 5-HB. The energy gap between HOMO and LUMO indicates that 7-HB has a higher chemical activity and lower kinetic stability than 5-HB and 6-HB. The calculated potential energy curves and potential barriers in the S0 and S1 states reveal that PT process is apt to happen in the S1 state, and it is hard to occur in the S0 state. The H-bonds in the six- and seven-membered ring intramolecular H-bond of 6-HB and 7-HB are stronger than that in the five-membered ring intramolecular H-bond of 5-HB, causing that the ESIPT processes of the former are nearly barrierless, but the latter needs to overcome some barriers. The ESIPT processes of 6-HB and 7-HB are much easier and faster than that of 5-HB. It is obvious that the size of intramolecular H-bonding ring may affect the ESIPT process.