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BY 4.0 license Open Access Published by De Gruyter Open Access August 24, 2022

Theoretical investigations on the excited-state intramolecular proton transfer in the solvated 2-hydroxy-1-naphthaldehyde carbohydrazone

  • Jing Huang EMAIL logo , Lei Yang , Minglian Fu , Zhangxu Chen and Xiaojin Huang
From the journal Open Chemistry

Abstract

The vast applications of 2-hydroxy-1-naphthaldehyde-derived systems in the sensors originate from their unusual excited-state intramolecular proton transfer (ESIPT) fluorescence in the molecules. The mechanism of ESIPT fluorescence in the solvated 2-hydroxy-1-naphthaldehyde carbohydrazone (HNLSC) system was investigated by ab initio time-dependent density functional theory (TDDFT) calculation. The solvation stabilized both ground state and excited state in the enol form, and the medium intramolecular interaction ensured the bond break, bond-forming and proton transfer in the conversion from excited enol form to keto form. ESIPT reaction from the enol form to the keto form had a low barrier of 2.54 kcal mol−1 in the cyclohexane solvation, and all the calculated emission was consistent with the experimental findings. Moreover, the disaggregation of excited enol form was favoured instead of the IPT conversion to keto form, vanishing the specific ESIPT pathway in the protic surroundings. Our research can give a meaningful insight into the two kinds of fluorescence spectroscopy in the HNISC system found by experimental measurement and be potential guidance to the application of 2-hydroxy-1-naphthaldehyde-derived systems in the development of new-type sensors, nonlinear optical materials and biochemical probes.

1 Introduction

Excited-state intramolecular proton transfer (ESIPT) was defined as tautomerism from the original excited form to another electronic structure via intramolecular hydrogen bond in the condition of heat, ray and electricity [1,2,3]. The ESIPT effects can be found in many organic compounds and biological systems. Since the first research on ESIPT by Waller in the 1950s [4], more than 4,000 ESIPT phenomena and applications [3] were investigated in the area of energy fuels, [5,6] nonlinear optical materials [7,8], laser dye suppliers [9,10], chemosensors [11,12] and biological probes [13,14]. The general scheme of ESIPT fluorescence was shown in Scheme 1a. In some systems, the excited keto isomer was more stable than the enol isomer. The electron in the enol ground state E1(S0) absorbed the energy of photons and transited to the first excited state E 1 ( S 1 ) in the principle of the frank-condon rule. There were two fates of enol excited state: one was the internal conversion and geometric relaxation to relative stable excited state E 2 ( S 1 ) followed by frank-condon emission and the geometric relaxation to the ground state E1(S0) marked in a blue dash line, and the other was the tautomerism from E 1 ( S 1 ) enol form to K*(S1) keto form, the keto emission from K*(S1) to K(S0), the tautomerism from keto ground state K(S0) back to enol ground state E1(S0). The fluorescence translation happened in the way of second emissions. The ESIPT process meant the enol E*(S1) → keto K*(S1) conversion via IPT shown in Scheme 1b.

Scheme 1 
               The general emission and keto-enol tautomerism: (a) the general scheme of ESIPT fluorescence; (b) the enol-keto tautomerism with the proton transfer between donor and acceptor.
Scheme 1

The general emission and keto-enol tautomerism: (a) the general scheme of ESIPT fluorescence; (b) the enol-keto tautomerism with the proton transfer between donor and acceptor.

The 2-hydroxy-1-naphthaldehyde was considered to be the “binding site-signalling subunit” for related sensors [15], as its unusual ESIPT fluorescence in the molecules. The ESIPT fluorescence in the 2-hydroxy-1-naphthaldehyde-derived systems was widely investigated [16,17,18,19,20,21]. Experimentally, Xu et al. found the regular fluorescence at 415 nm and the unusual ESIPT fluorescence at 435 nm in the cyclohexane solvated 2-hydroxy-1-naphthaldehyde carbohydrazone (HNLSC) [17,18]. They also stressed that the ESIPT fluorescence would disappear after deprotonation in the polar protic solvent [17,18]. Konoshima et al. analysed the crystal structures as well as fluorescence spectra of the 2-(2′-hydroxyphenyl)benzimidazole and found the ESIPT phenomenon in the 2-(2′-hydroxyphenyl)benzimidazole crystals [19]. Stasyuk et al. reported the ESIPT fluorescence in 2′-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines with the high fluorescence quantum yields [20]. Theoretically, Jiang and Peng qualified the ESIPT reaction of 3-hydroxyflavonein methylcyclohexane solvent via quantum chemical calculation [21]. Li et al. investigated the ESIPT reaction of 6-amino-2-(2′-hydroxyphenyl)benzoxazole in dichloromethane and methanol solvents using time-dependent density functional theory (TDDFT) calculations [22]. The HNLSC had the –NH-spacer structure and was potential to be applied as an anion sensor [22]. Additionally, Bose et al. used the photochemical methods and theoretical methods to explore the ESIPT of 2-hydroxy-1-naphthaldehyde semicarbazone and suggested that the IPT happened in the first exited state [23]. So here we deeply investigated the HNLSC by the computational modelling to inspect the triggering mechanism of the ESIPT as the detailed ESIPT process was not well suggested for HNLSC in the previous experimental findings [17,18]. Moreover, the various polarized and acidic surroundings were also employed to reveal their influences on the ESIPT reaction of the 2-hydroxy-1-naphthaldehyde-derived system.

2 Methods

All calculations were performed in the Gaussian 16 software [24] in this work. The equilibrium geometries both of the ground state and the excited state were optimized at B3LYP/6-31G(d,p) and TDB3LYP/6-31G(d,p) levels for their forms (enol form, keto form and ionic form, shown in Scheme 2) of HNLSC, as the B3LYP function was demonstrated to provide reliable results in the 2-hydroxy-1-naphthaldehyde-derived systems [20,21] and the ESIPT calculation [25,26]. Three solvents cyclohexane (nonpolar solvent), triethylamine (weak polarized solvent) and water (polar solvent) were selected for the different polarized surroundings during the optimization along with the calculation in the gas phase. Combined implicit-explicit solvation model was used to describe the solvation in this modelling. The first shell solvation was considered as the implicit molecules with the ONIOM model [27] and the bulk solvation used the IEFPCM model [28,29]. The transition states of ESIPT were located by integrating the intrinsic reaction coordinate in each solvation [30,31].

Scheme 2 
               (a) The enol form, (b) the keto form and (c) the ionic form of HNLSC.
Scheme 2

(a) The enol form, (b) the keto form and (c) the ionic form of HNLSC.

In the principle of frank-condon rule, the single points of enol form/keto form/ionic form were determined by B3LYP/6-31++G(d,p) for ground state as well as TDB3LYP/6-31++G(d,p) for the first excited state at all the optimized geometries in their respective environments. The fluorescence quantum yield showed the percentage of the number of photons emitting secondary radiation fluorescence to the number of primary radiation photons absorbing excitation light in unit time [32]. It seemed very complex to accurately calculate the quantum yield in this study, as there may were two kinds of fluorescence (one was normal fluorescence and the other was ESIPT fluorescence) and one photochemical reaction (ESIPT reaction) happened after frank-condon excitation. So here we did the very roughly evaluation of the quantum yields for both the normal fluorescence and ESIPT fluorescence qualitatively with the following formula [32]:

(1) Φ = I I 0 × 100 % ,

where Φ is the qualitatively calculated fluorescence quantum yield, I 0 is the intensity of E1(S0) → E 1 ( S 1 ) frank-condon excitation and I is the intensity of E 2 ( S 1 ) → E2(S0) for normal fluorescence and K*(S1) → K(S0) for ESIPT fluorescence. Here, the photons absorption during the E 2 ( S 1 ) → K*(S1) photochemical reaction and the difference in molar absorption coefficient ε of the enol form and keto form were ignored in the calculation of quantum yield.

3 Results

3.1 Optimized geometry

The key optimized parameters of the enol form and keto form in the ground state (S0) and excited state (S1) are shown in Table 1. The intramolecular hydrogen bond which was essential to the ESIPT reaction can be determined both in the ground state (S0) and in the excited state (S1). For example, in the cyclohexane solvation, The N1⋯H lengths were 1.764 and 1.653 Å in the S0 state and S1 state, respectively, for the enol form of the HNLSC. HNLSC in the polar solvation had similar medium intramolecular hydrogen bond to that in the nonpolar solvation. The N1⋯H/O1⋯H distances did not change much but the whole system were stabilized in the polar surroundings with the total energy about 5.00 eV (Table 2) lower than that in the gas phase, when the solvation varied from gas, nonpolar to polar solvents in both the S0 state and S1 state.

Table 1

The DFT/TDDFT optimized geometries in the various solvations

enol (gas) keto (gas) enol (cyclohexane) keto (cyclohexane)
Electronic state S0 S1 a S0 S1 S0 S1 a S0 S1
r(O1–H) Å 0.982 1.759 0.983 1.010 1.773 1.555
r(N1–H) Å 1.763 1.032 1.764 1.653 1.029 1.072
∠(O1–H–N1) deg 145.6 131.5 145.5 148.2 131.1 142.4
enol (triethylamine) keto (triethylamine) enol (water) keto (water)
r(O1–H) Å 0.982 1.011 1.780 1.564 0.984 1.010 1.800 1.630
r(N1–H) Å 1.761 1.647 1.031 1.073 1.756 1.652 1.030 1.073
∠(O1–H–N1) deg 145.7 148.9 130.9 142.0 145.4 148.6 130.2 140.1

aThe S1 state of HNLSC was not stable in the gas phase.

Table 2

The relative energy of enol form and keto form at the optimized geometry in the various solvationsa

Solvation enol form keto form
S0 S1 S0 S1
Gas 0.00 0.28
Cyclohexane −0.36 3.00 0.07 2.94
Triethylamine −0.21 3.02 0.03 2.90
Water −0.47 2.61 -0.26 2.59

aThe unit was in eV.

3.2 Energy profile of the photochemical process.

In Table 2, the relative energy of each state was shown against to the S0 state of enol form in the gas phase. Generally, the solvation stabilized and the polar solvation stabilized more in the both S0 state and S1 state. The S0 state of enol form has lower energies than that of keto form and the S1 state of enol form has higher energies than that of keto form. For example, the relative energies of enol S0 state, keto S0 state, enol S1 state and keto S1 state were −0.21, 0.03, 3.02 and 2.90 eV, respectively, in the triethylamine solvation. Thus, the intramolecular tautomerism E 1 ( S 1 ) → K*(S1) and K(S0) → E1(S0) caused the unusual ESIPT fluorescence in the HNLSC. On the other hand, E 2 ( S 1 ) had less than 0.08 eV relative energy above K*(S1) state. The normal fluorescence transition E1(S0) → E 1 ( S 1 ) → E2(S0) → E1(S0) was also favourable in the excited HNLSC system. These results were consistent with the experimental observation of normal fluorescence and unusual ESIPT fluorescence in the HNLSC [17,18,23].

The energy profile of the fluorescence is shown in Figure 1 in the cyclohexane solvation. The energy of E1(S0) state in the cyclohexane solvation was −0.36 eV relative to the ground state in the gas phase. The transition of E1(S0) → E 1 ( S 1 ) E 2 ( S 1 ) → E2(S0) → E2(S0) caused the normal fluorescence and the emission of E1(S0) → E 1 ( S 1 ) E 2 ( S 1 ) → K*(S1) → K(S0) → E1(S0) led to the ESIPT fluorescence. The calculated absorption wavelength of frank-condon excitation E1(S0) → E 1 ( S 1 ) was 360 nm, consistent well with the experimental result (369 nm). Then, as the energy of E 1 ( S 1 ) was not the minimum in the S1 potential surface, the internal conversion and geometric relaxation of E 1 ( S 1 ) led to the S1 minimum state E 2 ( S 1 ) . The frank-condon emission E 2 ( S 1 ) → E2(S0) generated the normal fluorescence. The calculated fluorescence of 418 nm also agreed well with the experimental spectral result of 412 nm. In the HNLSC system, ESIPT fluorescence would happen along with the normal fluorescence, which was found in the previous experimental investigation [15]. ESIPT process was an chemical reaction in the excited state from enol form to keto form with an transition state, unlike E 1 ( S 1 ) E 2 ( S 1 ) with the barrierless relaxation of geometry along the potential surface. The barrier of ESIPT reaction was 0.11 eV (2.54 kcal mol−1) but K*(S1) has 0.06 eV (2.30 kcal mol−1) relative energy lower than E 2 ( S 1 ) . The completed choice of E 2 ( S 1 ) between the E 2 ( S 1 ) → E2(S0) and E 2 ( S 1 ) → K*(S1) → K(S0) emitted normal and ESIPT fluorescence at the same time. The calculated ESIPT fluorescence was 436 nm, in accordance with 435 nm in the experimental findings [17]. Moreover, the calculated normal fluorescence of 446 nm in the water solvation agreed well with the experimental measurement of 453 nm [17,18,23], as shown in Table 3.

Figure 1 
                  The energy profile of the fluorescence in the HNLSC with the cyclohexane solvation.
Figure 1

The energy profile of the fluorescence in the HNLSC with the cyclohexane solvation.

Table 3

The excitation energy, normal fluorescence and ESIPT fluorescence in the various surroundings

Solvation Frank-condon excitation (nm) Normal fluorescence (nm) ESIPT fluorescence (nm)
Gas 351
Cyclohexane 360 (369a) 418 (412a) 436 (435a)
Triethylamine 358 419 439
Water 355 (359a) 446 (453a)

aThe experimental results from ref. [17].

However, in the polar solvation, the ionic state (Scheme 2c) would be stabilized by the protic surroundings in the excited state. The mechanism of disaggregation reaction of E 2 ( S 1 ) is shown in Figure 2 in the water solvation. Each state was stabilized more in the protic solvation like water than in the nonpolar solvation. For example, the relative energy of E1(S0) was −0.47 eV in the water solvation while E1(S0) had the relative energy of −0.36 eV in the cyclohexane solvation. The normal fluorescence was generated by the transition E 2 ( S 1 ) → E2(S0) after the frank-condon excitation E1(S0) → E 1 ( S 1 ) and geometric relaxation E 1 ( S 1 ) E 2 ( S 1 ) . Two competed reactions would be started from E 2 ( S 1 ) . One was the ESIPT reaction from enol form E 2 ( S 1 ) to keto form K*(S1), and the other was the disaggregation from the enol form E 2 ( S 1 ) to the ionic form I*(S1). The reaction barrier of E 2 ( S 1 ) → I*(S1) was only 0.07 eV (1.61 kcal mol−1), and the ESIPT barrier was 0.26 eV (6.00 kcal mol−1) in the water solvation. So the disaggregation was favourable in the water solvation rather than the ESIPT reaction, leading to the vanish of ESIPT fluorescence. These results were in accordance with the experimental findings [16,17].

Figure 2 
                  The fluorescence in the water solvation.
Figure 2

The fluorescence in the water solvation.

The fluorescence quantum yields in the different solvents are listed in Table 4. The HNLSC has the comparative ability to emit the normal fluorescence and ESIPT fluorescence in the nonpolar solvation, as the quantum yields accounted 35.9% and for 43.2% for normal and ESIPT fluorescence, respectively, in the solvation of cyclohexane. The intensity of normal fluorescence increased and the magnitude of ESIPT fluorescence dropped, when the polarity raised. Moreover, as the ESIPT fluorescence quenching, the total fluorescence quantum yield in the water solvation (38.7%) was approximately half of those in the cyclohexane (79.1%) and triethylamine (78.0%) solvation. Similar spectral phenomena were also examined by Bose et al. that Fluorescence intensity in n-heptane (nonpolar solvent) was nearly double the intensity in methanol (polar solvent) [23].

Table 4

The calculated quantum yields of normal fluorescence and ESIPT fluorescence in the various surroundings

Solvation Normal fluorescence (%) ESIPT fluorescence (%)
Cyclohexane 35.9 43.2
Triethylamine 37.9 40.1
Water 38.7

3.3 Frontier molecular orbitals

The frontier molecular orbitals, the oscillator strengths and the orbital contributions of the relative states are shown in Figure 3 and Table 5. The oscillator strength of 0.4266 was considerable for the typical π → π* excitation transition in the E1(S0) → E 1 ( S 1 ) and the HOMO[E1(S0)] → LUMO[E1(S0)] was dominant counting to 69.86% orbital transition contribution for the frank-condon E1(S0) → E 1 ( S 1 ) transition. In the normal fluorescence, the oscillator strength was 0.4450, and the H → L was also the main contribution for the E 2 ( S 1 ) → E2(S0). Moreover, the ESIPT fluorescence K*(S1) → K(S0) has a strong oscillator strength of 0.4213 with the main contribution (70.30%) of LUMO[K*(S1)] → HOMO[K*(S1)].

Figure 3 
                  The molecular orbital with the cyclohexane solvation of (a) HOMO at the optimized enol S0 state HOMO[E
                     
                        1
                     
                     (S
                     
                        0
                     )], (b) LUMO at the optimized enol S0 state LUMO[E
                     
                        1
                     
                     (S
                     
                        0
                     )], (c) LUMO at the optimized enol S1 state LUMO[
                     
                        
                           
                           
                              
                                 
                                    E
                                 
                                 
                                    1
                                 
                                 
                                    ⁎
                                 
                              
                              (
                              
                                 
                                    S
                                 
                                 
                                    1
                                 
                              
                              )
                           
                           {{\bf{E}}}_{{\bf{1}}}^{{\boldsymbol{\ast }}}{\boldsymbol{(}}{{\bf{S}}}_{{\bf{1}}}{\boldsymbol{)}}
                        
                     ], (d) HOMO at the optimized keto S1 state HOMO[K*(S
                     
                        1
                     )] and (e) LUMO at the optimized keto S1 state LUMO[K*(S
                     
                        1
                     )].
Figure 3

The molecular orbital with the cyclohexane solvation of (a) HOMO at the optimized enol S0 state HOMO[E 1 (S 0 )], (b) LUMO at the optimized enol S0 state LUMO[E 1 (S 0 )], (c) LUMO at the optimized enol S1 state LUMO[ E 1 ( S 1 ) ], (d) HOMO at the optimized keto S1 state HOMO[K*(S 1 )] and (e) LUMO at the optimized keto S1 state LUMO[K*(S 1 )].

Table 5

The oscillator strengths and the orbital transition contributions of electronic transitions in the cyclohexane solvation

Electronic transition Transition type Oscillator strength Orbital contribution
E1(S0) → E 1 ( S 1 ) Absorption 0.4266 H → L(69.86%)
E 2 ( S 1 ) → E2(S0) Normal fluorescence 0.4450 H → L(70.27%)
K*(S1) → K(S0) ESIPT fluorescence 0.4213 H → L(70.30%)

4 Discussion

4.1 Hydrogen bonding interaction

In Table 1, The O1⋯H distances were 1.773 and 1.555 Å in the S0 state and S1 state, respectively, for the keto form. The distance of 1.6 ∼ 1.7 Å and the ∠(O1–H–N1) angle 131° ∼ 148° were proper for N1 or O1 atoms to form a medium-strength hydrogen bond with nearby H atom. The strength of the so-formed intramolecular hydrogen bond in the excited state was neither too strong nor too weak, ensuring the O1–H cleavage and N1–H formation in the IPT from enol form to keto form and causing the ESIPT fluorescence. On the other hand, in terms of the hydrogen bonding interaction between HNLSC and the solvent, the formation of keto form was terminated and the ESIPT would be disappeared. The hydrogen transferred more difficult from O1 atom to N1 atom in the case of the solute–solvent hydrogen bond, causing the enlargement of E 2 ( S 1 ) → K*(S1) barrier. At the same time, the disaggregation was favourable due to the stabilization of ionic form by these solute–solvent hydrogen bonding interactions.

4.2 Mechanism of ESIPT fluorescence

The mechanism of the fluorescence in the HNLSC with the nonpolar solvation is shown in Figure 4. The total mechanism contained four sub-procedures: frank-condon excitation, internal relaxation, ESIPT and fluorescence. Frank-condon excitation was that the S0 → S1 transition at the geometry of S0 as the electron transfer was much faster than the nuclear relaxation. The internal relaxation was the geometric relaxation to the energetic minimum of certain state. The ESIPT was the reaction of the IPT in the excited state. The fluorescence happened when the electron transferred from excited state back to the ground state in the geometry of excited state. So besides the normal fluorescence, the mechanism of ESIPT fluorescence was (i) the frank-condon excitation from the enol form ground state to the first excited state, (ii) the ESIPT reaction from enol form excited state to more stable keto form excited state, (iii) the fluorescence from keto form excited state to keto form ground state, and (iv) the internal geometric relaxation of keto form to enol form ground state.

Figure 4 
                  The mechanism of ESIPT fluorescence in the nonpolar solvation.
Figure 4

The mechanism of ESIPT fluorescence in the nonpolar solvation.

4.3 Influence of solvation

From the results and discussion above, it was obvious to understand the fluorescence in different surroundings. No fluorescence is emitted in the gas phase, and fluorescence can be checked in both the nonpolar solvation and polar surroundings. The kinds of fluorescence spectroscopy were different in the nonpolar and polar solvation. Normal fluorescence and ESIPT fluorescence occurred in the nonpolar solvation, whereas the unusual ESIPT fluorescence was not favourable in the polar solvation. These results were also addressed experimentally [17,18,23]. Besides the influence of solvents, the additions of base, acid and other ionic compounds were examined to change the fluorescence character of HNLSC experimentally [18]. CH3COO and CO3 2− can enhance the intensity of ESIPT fluorescence in the acetonitrile solvent by regulating the O1⋯H⋯N1 intramolecular hydrogen bond strength studied by Xu et al. experimentally [18]. So the HNLSC system was the star molecule of fluorescence properties. The fluorescence type and the fluorescence intensity can be controlled and regulated by exchanging different solvents and surroundings for various applications.

5 Conclusion

The mechanism of ESIPT fluorescence in the HNLSC was addressed via theoretical investigations. The following conclusions were obtained in the TDDFT calculation of the solvated HNLSC. The structure of the excited state was stabilized by the solvated surroundings. The structure with proper length N1⋯H/O1⋯H and angle O1⋯H⋯N1 suggested the medium strength hydrogen bonds in the excited enol form and keto form. These medium hydrogen bonding interactions ensured the O1–H cleavage and N1–H formation in the IPT from enol form to keto form, followed by the emission of excited keto form, causing the ESIPT fluorescence. The ESIPT fluorescence was generated in the way of E1(S0) → E 1 ( S 1 ) E 2 ( S 1 ) → K*(S1) → K(S0) → E1(S0) in the nonpolar solvation. Molecular orbital analysis showed that the contributions of HUMO → LUMO (or LUMO → HUMO) were dominant in all the absorption E1(S0) → E 1 ( S 1 ) , normal fluorescence E 2 ( S 1 ) → E2(S0) and the special ESIPT emission K*(S1) → K(S0) spectrum. Moreover, the ESIPT fluorescence was found to be forbidden in the protic surroundings as the disaggregation of E 2 ( S 1 ) had a lower barrier than the ESIPT reaction in our theoretical prediction. All the calculation results were consistent with the previous experimental findings. The theoretical results can provide useful guidance in the following research and application of HNLSC and the 2-hydroxy-1-naphthaldehyde-derived systems.

Acknowledgment

J. H. thanks the scientific research team of Putian University authorized by Lijun Fu and Jianhui Huang.

  1. Funding information: This work was supported by the fund of NSFP of Fujian Province (No. 2020J05210, No. 2021J011105, No. 2022J01132911, and No. 2022J01132905).

  2. Author contributions: Conceptualization: Jing Huang; data curation: Jing Huang, Minglian Fu, Xiaojin Huang; funding acquisition, Lei Yang, Zhangxu Chen; writing – original draft: Jing Huang; writing-review and editing: Jing Huang, Lei Yang. All authors have approved the final version of the manuscript.

  3. Conflict of interest: The authors declared that they have no conflict of interest.

  4. Ethical approval: The conducted research is not related to either human or animal use.

  5. Data availability statement: All data generated or analysed during this study are included in this published article.

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Received: 2022-07-08
Revised: 2022-08-01
Accepted: 2022-08-03
Published Online: 2022-08-24

© 2022 Jing Huang et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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