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Excited state intramolecular proton transfer and substituent effect of 10-hydroxybenzo[h]quinoline: A time-dependent density functional theory study
Computational and Theoretical Chemistry 1034 (2014) 80–84
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Excited state intramolecular proton transfer and substituent effect of 10-hydroxybenzo[h]quinoline: A time-dependent density functional theory study Shuo Chai a,b,⇑, Shu-Lin Cong a a b
School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 17 December 2013 Received in revised form 3 February 2014 Accepted 18 February 2014 Available online 24 February 2014 Keywords: Excited state intramolecular proton transfer Hydrogen bond Substituent effect Time-dependent density functional theory
a b s t r a c t The excited state intramolecular proton transfer (ESIPT) and the substituent effect of 10-hydroxybenzo[h]quinoline (HBQ) compounds are investigated using the time-dependent density functional theory (TDDFT) method. With the spectra and potential energy curve calculations we have demonstrated the occurrence of an ultrafast excited state intramolecular proton transfer reaction in HBQ compounds. The HBQ-a and HBQ-b in the enol conformations can convert into the keto tautomers in the excited state S1. The significant Stokes shift of HBQ-a is observed, as large as 300 nm, which is much larger than that of HBQ-b. The calculated molecular orbital gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for HBQ-a is 3.33 eV, smaller than the 3.79 eV for HBQ-b. The calculations demonstrate that it is much easier to take place the ESIPT reaction for HBQ-a than HBQ-b. The reaction mechanism of ESIPT is analyzed with theoretical potential energy curves and the ESIPT reaction energy barriers of 3.73 kcal/mol for HBQ-a and 17.75 kcal/mol for HBQ-b are obtained. These results clearly indicate that the substituent with electron-withdrawing groups in the hydrogenaccepting moiety in HBQ-a facilitate the proton transfer in the excited state. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Excited state intramolecular proton transfer (ESIPT) as a significant photoinduced isomerization process widely exists in chemical and biological systems [1–11]. The ESIPT reaction involves the proton transfer from the hydrogen-donating group to the hydrogen-accepting group in an organic molecule in the excited state. Generally, the donating moiety possesses the structure with a hydroxyl or amino moiety. Therefore the organic dyes may take place a conversion either between enol and keto forms, or between amino and imino forms [12–14]. The enol-keto conversion is a widespread ESIPT process and attracts considerable attentions of experimental and theoretical researchers in time-resolved ultrafast spectroscopy experiments and first-principle quantum chemistry calculations [15–19]. In the ground state, the ESIPT molecules exist as the enol form which is stabilized by the intramolecular hydrogen bonding interactions. Upon photoexcitation, the molecule in the excited-state enol form experiences an ultrafast intramolecular ⇑ Corresponding author at: School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China. E-mail address: [email protected] (S. Chai). http://dx.doi.org/10.1016/j.comptc.2014.02.019 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
proton transfer which gives rise to the excited-state keto tautomeric form. After decaying to the ground state, the keto form returns to the enol form by a reverse proton transfer reaction [3,20–22]. In this photoinduced process, we can observe the large Stokes shift, which contributes to the obvious difference in photophysical properties between enol and keto conformations. At present, the ESIPT compounds have been used as molecular switches, fluorescence probe, biology sensor, and so on [23–30]. In the photoisomerization process, the intramolecular hydrogen bond plays a significant part in the conformation and stability of the molecule. Most ESIPT molecules in the ground state possess a stable structure with a five- or six-membered ring, connecting by the intramolecular hydrogen bonds between the OAH (or NAH) and C@O (or pyridinic nitrogen). Suitable structures with intramolecular hydrogen bond are in favor of the occurrence of ESIPT reaction. The hydrogen bond strengthening or weakening in the ground and excited states influences the photophysical processes of organic chromophores, which has been demonstrated by Zhao et al. theoretically [31–37]. 10-Hydroxybenzo[h]quinoline (HBQ) is an organic heterocyclic molecule and exhibits a photoisomerization character [38–41]. The enol and keto conformation conversion of HBQ takes place in
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the ground and excited states, accompanied with the photoexcitation and fluorescence emission. Piechowska et al. synthesized a series of HBQ compounds and performed the steady-state and time-resolved spectrum measurements to investigate the optical properties [40]. In their experiments, the fluorescence emission from the keto tautomer was identified and the occurrence of ESIPT was demonstrated. However, there are still many questions about the reaction route and the mechanism of the excited state photoisomerization process. The calculations may explain the reaction mechanism and explore the substituent effect on the photophysical properties. In the present work we investigate the photophysical properties of HBQ compounds to understand the dynamic process of ESIPT and clarify the substituent and solvent effects on the photophysical properties. The electron-withdrawing groups CN and NO2 are attached to the heterocyclic structure in different positions. The orbital energy levels and ESIPT reactions of HBQ compounds are studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The bulk effect of the solvation shells is considered in our calculations by using the polarizable continuum model (PCM) package. The theoretical results of steady-state spectra are compared to the experimental measurements. The potential energy curves of the ESIPT are given to understand the reaction mechanism. 2. Computational details The structural and electronic properties and the relevant photophysical processes of 10-hydroxybenzoquinoline (HBQ) compounds are investigated by using DFT and TDDFT methods. The DFT calculation is performed to obtain the optimized geometries in the ground state. Both the steady-state spectrum calculation and geometric optimization in the excited state are performed by using the TDDFT method. The Becke’s three parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation (B3LYP functional) and 6-311++G(d, p) basis set are employed in all the DFT and TDDFT calculations [42–45]. In the present work, the quantum chemistry calculations are performed using the Gaussian 09 program suite [46]. The PCM package is employed to study the solvent effect, where acetonitrile and toluene are chosen as the solvents. The potential energy curves are scanned as a function of O2–H1 distance in the ground and excited states. 3. Results and discussions In our calculation, the electron-withdrawing substituents CN and NO2 are attached to the HBQ heterocycle in different positions and the two HBQ compounds are named as HBQ-a and HBQ-b. Fig. 1 shows the structures of HBQ-a and HBQ-b, where the relevant atoms are numbered. In HBQ-a, the electron-withdrawing group CN is substituted at the hydrogen-accepting moiety. In
HBQ-b, however, the group NO2 is connected with the hydrogendonating moiety. The intramolecular hydrogen bond exists between the hydroxyl and nitrogen atom in conjugated ring of the two HBQ compounds. The six-membered ring is formed via the connection of the intramolecular hydrogen bond. The HBQ-a and HBQ-b in the ground state are optimized by using the DFT method at the B3LYP/6-311++G(d, p) level. The two compounds in the optimized geometries are photoexcited. The electronic excitation energies (EE) and corresponding oscillator strengths (OS) are listed in Table 1. Both HBQ-a and HBQ-b are excited to the S1 state with the strongest oscillator strengths. The excitation energy to the S1 state is 2.90 eV for HBQ-a, which is less than the 3.38 eV for HBQ-b. In the excited states S1, we optimize the geometries of HBQ-a and HBQ-b and find that the enol-keto conversion in the two compounds occurs very rapidly after excitation. We can only get the optimized keto form geometries rather than the enol forms in the excited state. Piechowska et al. detected the emission from the excited keto tautomer rather than enol form because of the occurrence of excited state intramolecular proton transfer [40]. The optimized geometries of HBQ-a and HBQ-b in the ground state and keto tautomers in the excited state S1 are shown in Fig. 2. The intramolecular hydrogen bond lengths are also labeled in Fig. 2. The relevant geometric parameters, including bond lengths, bond angles, and dihedral angles, are listed in Table 2. In the ground state, the difference in the intramolecular hydrogen bond length between HBQ-a and HBQ-b is obvious. The intramolecular hydrogen bond lengths are 1.72 Å for HBQ-a and 1.65 Å for HBQ-b. The hydrogen bond in HBQ-b in the ground state is much stronger, and the six-membered hydrogen-bond structure is more stable than that in HBQ-a. In the excited state S1, the molecules exist as the keto forms and the intramolecular hydrogen bond structure of keto tautomer in HBQ-a is similar to that in HBQ-b. However, the hydrogen bond length of HBQ-a is shorter than that of HBQ-b in the excited state S1. HBQ-a easily generate the keto tautomer in the excited state via an intramolecular proton transfer reaction. Because of the substituent effects with the electron-withdrawing groups at the hydrogen-donating and hydrogen-accepting moieties, the bond lengths and angles relating to the intramolecular hydrogen bond formation are different in HBQ-a and HBQ-b. For example, the bond angle AH1–N6–C5 is 101.2° in HBQ-a and 112.0° in HBQ-b. The investigated dihedral angles in HBQ-a and HBQ-b in the ground and excited states are almost zero. In the HBQ conformation, the six-membered intramolecular hydrogen bond structure contributes to the planar molecular conformation. By using the TDDFT method the steady-state spectra of HBQ-a and HBQ-b in gas phase, acetonitrile solution and toluene solution are calculated and shown in Fig. 3, where the corresponding experimental results are labeled as comparison [40]. The solvent effect is considered by using the PCM method in the calculations. For HBQa, the calculated absorption maxima in the gas phase, acetonitrile solution and toluene solution are 427, 422 and 430 nm, respectively, which agree with the experimental measurements of 402 nm in acetonitrile and 411 nm in toluene. The fluorescence
Table 1 The electronic excitation energies (EE, in eV) and corresponding oscillator strengths (OS) of HBQ-a and HBQ-b compounds. HBQ-a
Fig. 1. Structures and atom numbers of HBQ compounds.
S1 S2 S3 S4 S5
HBQ-b
EE
OS
EE
OS
2.90 3.61 3.84 4.23 4.34
0.147 0.001 0.059 0.001 0.004
3.38 3.62 3.78 4.09 4.24
0.24 0.021 0.033 0.028 0.104
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Fig. 2. The optimized enol form geometries in the ground state and keto form geometries in the excited state of HBQ-a and HBQ-b, with hydrogen bond length (in Å).
Table 2 Geometric parameters of HBQ-a and HBQ-b in the ground state and keto tautomers in the excited state S1 (the bond lengths in Å, bond angles in ° and dihedral angles in °). GS
LH1–O2 LO2–C3 LC5–N6 LN6–H1 AH1–O2–C3 AO2–H1–N6 AH1–N6–C5 DH1–O2–C3–C4 DH1–N6–C5–C4
ES
HBQ-a
HBQ-b
HBQ-a Taut
HBQ-b Taut
0.99 1.34 1.36 1.72 107.5 147.5 101.2 0 0
1.00 1.33 1.36 1.65 107.4 148.7 101.6 0.4 0.4
1.65 1.27 1.36 1.06 103.3 143.5 112.0 0 0
1.75 1.26 1.37 1.03 104.5 138.3 114.2 2.9 1.2
emission from the keto tautomer of HBQ-a is distributed in the region of 500800 nm, and the fluorescence maxima are 660, 710 and 684 nm in three different cases. The experimental fluorescence peaks are 712 and 717 nm in acetonitrile and toluene solutions, respectively. The large Stokes shifts of 300 nm are observed for HBQ-a. In Fig. 3(b), the calculated absorption maxima of HBQ-b are 367, 388 and 379 nm in the gas phase and acetonitrile, toluene solutions. The calculated absorption spectra are also consistent with the experimental data of 368 and 370 nm in acetonitrile and toluene solutions. The calculated fluorescence emission of HBQ-b from the keto form is around 560 nm and the Stokes shift is about 200 nm. The different substituents obviously influence the spectrum distribution of HBQ compounds. The Stokes Shift of HBQ-a is much larger than that of HBQ-b. The large Stokes shifts originate from the significant geometric change in the ground and excited states. The Stokes shift character of HBQ compounds promises the considerable applications in photochemistry and photobiology. Generally, the solution environments will lead to a small shift of the steady-state spectra, but are unable to change the photophysical properties of organic dyes. Therefore, in the following discussion we use the gas phase calculation results to analyze the ESIPT reaction for simplicity. The molecular orbital energy levels and energy gap of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are calculated based on the optimized structures in the ground state, as shown in Fig. 4. HBQ-a and HBQ-b have similar geometries but different substituted groups. Different substituents will lead to an obvious difference in the orbital level. The HOMO energy level for HBQ-a is 6.21 eV, which is higher than the 6.55 eV for HBQ-b, and the LUMO energy level for HBQ-a is lower than that for HBQ-b by 0.12 eV. As a result,
Fig. 3. Absorption and fluorescence spectra of (a) HBQ-a and (b) HBQ-b in the gas phase, acetonitrile solution and toluene solution (the corresponding experimental results are labeled in the spectra [40]).
Fig. 4. Frontier molecular orbital energy levels (in eV) and plots of HOMOs and LUMOs of HBQ-a and HBQ-b.
the energy gap between the LUMO and HOMO for HBQ-a is 3.33 eV, which is smaller than the 3.79 eV for HBQ-b. The molecular orbital energy levels are influenced by the molecular substituents. The molecular modification with electron-withdrawing substituent in the hydrogen-accepting moiety can lower the LUMO molecular orbital and decrease the energy gap between the LUMO and HOMO. From the orbital plots, we notice that the charges are distributed throughout the entire structure. It is found that there is a charge redistribution between HOMO and LUMO in HBQ-a, from hydrogen-donating moiety to hydrogen-accepting moiety,
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which is also attributed to the substituent effect with the electronwithdrawing CN in the hydrogen-accepting moiety. The potential energy curves as a function of O2–H1 distance in HBQ-a and HBQ-b are shown in Fig. 5. The intramolecular proton transfer in the ground and excited states are calculated using DFT and TDDFT methods to describe the reaction route. The potential energies of optimized enol and keto forms in the ground and excited states are used, and the other potential energies with O2– H1 distance changing every 0.1 Å are obtained from single-point energy calculations. From the potential energy curves we can analyze the proton transfer reaction mechanism in the ground and exicted states. The enol form in the ground state S0 is photoexcited to the excited state S1 with absorption wavelengths of 427 and 367 nm for HBQ-a and HBQ-b, as discussed in the above section. For HBQ-a, the proton in the enol form in the excited state transfers along the hydrogen bond coordinate, and then comes across the energy barrier of 3.73 kcal/mol. It finally gets to the nitrogen part in the hydrogen-accepting moiety. The keto geometry is generated in the excited state, and then relaxes to the ground state via the fluorescence emission. The reverse intramolecular proton transfer will take place in the ground state from the keto form to enol form in a non-barrier process. The intramolecular pronton transfer process of HBQ-b is very similar to that of HBQ-a, in which the energy barrier in the excited state is 17.75 kcal/mol. Therefore, the intramolecular proton transfer reaction for HBQ-a is much easier than HBQ-b, which is consistent with the above discussion. The ESIPT reaction is enhanced by the molecular modification with electron-withdrawing substituent CN in the hydrogen-accepting
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moiety. We also notice that the energy barriers for the forward and backward intramolecular proton transfer reactions in the excited state are on the same order of magnitude, so the two processes coexist in the excited state. With the help of potential energy curve calculation, the relaxation process from the ground state to the excited state and back to the ground state is clarified and the ESIPT reaction mechanism is understood. 4. Conclusions The ESIPT reactions and the photophysical properties of HBQ-a and HBQ-b compounds have been investigated using the TDDFT method at the level of B3LYP with 6-311++G(d, p) basis set. It is demonstrated that the proton transfer motion is facilitated by the functionalization with the electron-withdrawing group in the hydrogen-accepting moiety. When the HBQ molecule is photoexcited, the proton moves rapidly along the intramolecular hydrogen bond, which causes the hydrogen bond O2–H1 N6 to break and the hydrogen bond O2 H1–N6 to form. The HBQ molecule is initially excited to the S1 state, and the corresponding electronic excitation energy of HBQ-a is less than that of HBQ-b. The steady-state absorption and fluorescence spectra are calculated in the gas phase, acetonitrile solution and toluene solution. We find that the solvent environment is of little importance to the photophysical properties of HBQ compounds. The remarkable Stokes shifts of HBQ compounds appear in the steady-state spectra, which are attributed to the photoinduced isomerization. All the calculated spectral features are in good agreement with the spectral results recorded in experiments. With the frontier molecular orbital analysis, we find that the molecular orbital energy gap of HBQ-a is smaller than that of HBQ-b. Moreover, the charges in HBQ-a have a redistribution which originates from the substituent effect with the electron-withdrawing group in the hydrogen-accepting moiety. Furthermore, the potential energy curves of HBQ-a and HBQ-b are calculated as a function of O2–H1 distance. The energy barriers of ESIPT reactions are 3.73 kcal/mol for HBQ-a and 17.75 kcal/mol for HBQ-b. It is indicated that the occurrence of ESIPT reaction in HBQ-a is much easier than in HBQ-b, because of the substituent with the electron-withdrawing group in the hydrogen-accepting moiety. The reaction mechanism is clarified with the help of the calculations. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 11304029) and Fundamental Research Funds for Central Universities of China (Grant No. DUT12RC(3)55). References
Fig. 5. Calculated potential energy curves of (a) HBQ-a and (b) HBQ-b as a function of O2–H1 distance in the ground and excited states.
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Excited state intramolecular proton transfer and substituent effect of 10-hydroxybenzo[h]quinoline: A time-dependent density functional theory study Shuo Chai a,b,⇑, Shu-Lin Cong a a b
School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 17 December 2013 Received in revised form 3 February 2014 Accepted 18 February 2014 Available online 24 February 2014 Keywords: Excited state intramolecular proton transfer Hydrogen bond Substituent effect Time-dependent density functional theory
a b s t r a c t The excited state intramolecular proton transfer (ESIPT) and the substituent effect of 10-hydroxybenzo[h]quinoline (HBQ) compounds are investigated using the time-dependent density functional theory (TDDFT) method. With the spectra and potential energy curve calculations we have demonstrated the occurrence of an ultrafast excited state intramolecular proton transfer reaction in HBQ compounds. The HBQ-a and HBQ-b in the enol conformations can convert into the keto tautomers in the excited state S1. The significant Stokes shift of HBQ-a is observed, as large as 300 nm, which is much larger than that of HBQ-b. The calculated molecular orbital gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for HBQ-a is 3.33 eV, smaller than the 3.79 eV for HBQ-b. The calculations demonstrate that it is much easier to take place the ESIPT reaction for HBQ-a than HBQ-b. The reaction mechanism of ESIPT is analyzed with theoretical potential energy curves and the ESIPT reaction energy barriers of 3.73 kcal/mol for HBQ-a and 17.75 kcal/mol for HBQ-b are obtained. These results clearly indicate that the substituent with electron-withdrawing groups in the hydrogenaccepting moiety in HBQ-a facilitate the proton transfer in the excited state. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Excited state intramolecular proton transfer (ESIPT) as a significant photoinduced isomerization process widely exists in chemical and biological systems [1–11]. The ESIPT reaction involves the proton transfer from the hydrogen-donating group to the hydrogen-accepting group in an organic molecule in the excited state. Generally, the donating moiety possesses the structure with a hydroxyl or amino moiety. Therefore the organic dyes may take place a conversion either between enol and keto forms, or between amino and imino forms [12–14]. The enol-keto conversion is a widespread ESIPT process and attracts considerable attentions of experimental and theoretical researchers in time-resolved ultrafast spectroscopy experiments and first-principle quantum chemistry calculations [15–19]. In the ground state, the ESIPT molecules exist as the enol form which is stabilized by the intramolecular hydrogen bonding interactions. Upon photoexcitation, the molecule in the excited-state enol form experiences an ultrafast intramolecular ⇑ Corresponding author at: School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China. E-mail address: [email protected] (S. Chai). http://dx.doi.org/10.1016/j.comptc.2014.02.019 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
proton transfer which gives rise to the excited-state keto tautomeric form. After decaying to the ground state, the keto form returns to the enol form by a reverse proton transfer reaction [3,20–22]. In this photoinduced process, we can observe the large Stokes shift, which contributes to the obvious difference in photophysical properties between enol and keto conformations. At present, the ESIPT compounds have been used as molecular switches, fluorescence probe, biology sensor, and so on [23–30]. In the photoisomerization process, the intramolecular hydrogen bond plays a significant part in the conformation and stability of the molecule. Most ESIPT molecules in the ground state possess a stable structure with a five- or six-membered ring, connecting by the intramolecular hydrogen bonds between the OAH (or NAH) and C@O (or pyridinic nitrogen). Suitable structures with intramolecular hydrogen bond are in favor of the occurrence of ESIPT reaction. The hydrogen bond strengthening or weakening in the ground and excited states influences the photophysical processes of organic chromophores, which has been demonstrated by Zhao et al. theoretically [31–37]. 10-Hydroxybenzo[h]quinoline (HBQ) is an organic heterocyclic molecule and exhibits a photoisomerization character [38–41]. The enol and keto conformation conversion of HBQ takes place in
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the ground and excited states, accompanied with the photoexcitation and fluorescence emission. Piechowska et al. synthesized a series of HBQ compounds and performed the steady-state and time-resolved spectrum measurements to investigate the optical properties [40]. In their experiments, the fluorescence emission from the keto tautomer was identified and the occurrence of ESIPT was demonstrated. However, there are still many questions about the reaction route and the mechanism of the excited state photoisomerization process. The calculations may explain the reaction mechanism and explore the substituent effect on the photophysical properties. In the present work we investigate the photophysical properties of HBQ compounds to understand the dynamic process of ESIPT and clarify the substituent and solvent effects on the photophysical properties. The electron-withdrawing groups CN and NO2 are attached to the heterocyclic structure in different positions. The orbital energy levels and ESIPT reactions of HBQ compounds are studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The bulk effect of the solvation shells is considered in our calculations by using the polarizable continuum model (PCM) package. The theoretical results of steady-state spectra are compared to the experimental measurements. The potential energy curves of the ESIPT are given to understand the reaction mechanism. 2. Computational details The structural and electronic properties and the relevant photophysical processes of 10-hydroxybenzoquinoline (HBQ) compounds are investigated by using DFT and TDDFT methods. The DFT calculation is performed to obtain the optimized geometries in the ground state. Both the steady-state spectrum calculation and geometric optimization in the excited state are performed by using the TDDFT method. The Becke’s three parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation (B3LYP functional) and 6-311++G(d, p) basis set are employed in all the DFT and TDDFT calculations [42–45]. In the present work, the quantum chemistry calculations are performed using the Gaussian 09 program suite [46]. The PCM package is employed to study the solvent effect, where acetonitrile and toluene are chosen as the solvents. The potential energy curves are scanned as a function of O2–H1 distance in the ground and excited states. 3. Results and discussions In our calculation, the electron-withdrawing substituents CN and NO2 are attached to the HBQ heterocycle in different positions and the two HBQ compounds are named as HBQ-a and HBQ-b. Fig. 1 shows the structures of HBQ-a and HBQ-b, where the relevant atoms are numbered. In HBQ-a, the electron-withdrawing group CN is substituted at the hydrogen-accepting moiety. In
HBQ-b, however, the group NO2 is connected with the hydrogendonating moiety. The intramolecular hydrogen bond exists between the hydroxyl and nitrogen atom in conjugated ring of the two HBQ compounds. The six-membered ring is formed via the connection of the intramolecular hydrogen bond. The HBQ-a and HBQ-b in the ground state are optimized by using the DFT method at the B3LYP/6-311++G(d, p) level. The two compounds in the optimized geometries are photoexcited. The electronic excitation energies (EE) and corresponding oscillator strengths (OS) are listed in Table 1. Both HBQ-a and HBQ-b are excited to the S1 state with the strongest oscillator strengths. The excitation energy to the S1 state is 2.90 eV for HBQ-a, which is less than the 3.38 eV for HBQ-b. In the excited states S1, we optimize the geometries of HBQ-a and HBQ-b and find that the enol-keto conversion in the two compounds occurs very rapidly after excitation. We can only get the optimized keto form geometries rather than the enol forms in the excited state. Piechowska et al. detected the emission from the excited keto tautomer rather than enol form because of the occurrence of excited state intramolecular proton transfer [40]. The optimized geometries of HBQ-a and HBQ-b in the ground state and keto tautomers in the excited state S1 are shown in Fig. 2. The intramolecular hydrogen bond lengths are also labeled in Fig. 2. The relevant geometric parameters, including bond lengths, bond angles, and dihedral angles, are listed in Table 2. In the ground state, the difference in the intramolecular hydrogen bond length between HBQ-a and HBQ-b is obvious. The intramolecular hydrogen bond lengths are 1.72 Å for HBQ-a and 1.65 Å for HBQ-b. The hydrogen bond in HBQ-b in the ground state is much stronger, and the six-membered hydrogen-bond structure is more stable than that in HBQ-a. In the excited state S1, the molecules exist as the keto forms and the intramolecular hydrogen bond structure of keto tautomer in HBQ-a is similar to that in HBQ-b. However, the hydrogen bond length of HBQ-a is shorter than that of HBQ-b in the excited state S1. HBQ-a easily generate the keto tautomer in the excited state via an intramolecular proton transfer reaction. Because of the substituent effects with the electron-withdrawing groups at the hydrogen-donating and hydrogen-accepting moieties, the bond lengths and angles relating to the intramolecular hydrogen bond formation are different in HBQ-a and HBQ-b. For example, the bond angle AH1–N6–C5 is 101.2° in HBQ-a and 112.0° in HBQ-b. The investigated dihedral angles in HBQ-a and HBQ-b in the ground and excited states are almost zero. In the HBQ conformation, the six-membered intramolecular hydrogen bond structure contributes to the planar molecular conformation. By using the TDDFT method the steady-state spectra of HBQ-a and HBQ-b in gas phase, acetonitrile solution and toluene solution are calculated and shown in Fig. 3, where the corresponding experimental results are labeled as comparison [40]. The solvent effect is considered by using the PCM method in the calculations. For HBQa, the calculated absorption maxima in the gas phase, acetonitrile solution and toluene solution are 427, 422 and 430 nm, respectively, which agree with the experimental measurements of 402 nm in acetonitrile and 411 nm in toluene. The fluorescence
Table 1 The electronic excitation energies (EE, in eV) and corresponding oscillator strengths (OS) of HBQ-a and HBQ-b compounds. HBQ-a
Fig. 1. Structures and atom numbers of HBQ compounds.
S1 S2 S3 S4 S5
HBQ-b
EE
OS
EE
OS
2.90 3.61 3.84 4.23 4.34
0.147 0.001 0.059 0.001 0.004
3.38 3.62 3.78 4.09 4.24
0.24 0.021 0.033 0.028 0.104
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Fig. 2. The optimized enol form geometries in the ground state and keto form geometries in the excited state of HBQ-a and HBQ-b, with hydrogen bond length (in Å).
Table 2 Geometric parameters of HBQ-a and HBQ-b in the ground state and keto tautomers in the excited state S1 (the bond lengths in Å, bond angles in ° and dihedral angles in °). GS
LH1–O2 LO2–C3 LC5–N6 LN6–H1 AH1–O2–C3 AO2–H1–N6 AH1–N6–C5 DH1–O2–C3–C4 DH1–N6–C5–C4
ES
HBQ-a
HBQ-b
HBQ-a Taut
HBQ-b Taut
0.99 1.34 1.36 1.72 107.5 147.5 101.2 0 0
1.00 1.33 1.36 1.65 107.4 148.7 101.6 0.4 0.4
1.65 1.27 1.36 1.06 103.3 143.5 112.0 0 0
1.75 1.26 1.37 1.03 104.5 138.3 114.2 2.9 1.2
emission from the keto tautomer of HBQ-a is distributed in the region of 500800 nm, and the fluorescence maxima are 660, 710 and 684 nm in three different cases. The experimental fluorescence peaks are 712 and 717 nm in acetonitrile and toluene solutions, respectively. The large Stokes shifts of 300 nm are observed for HBQ-a. In Fig. 3(b), the calculated absorption maxima of HBQ-b are 367, 388 and 379 nm in the gas phase and acetonitrile, toluene solutions. The calculated absorption spectra are also consistent with the experimental data of 368 and 370 nm in acetonitrile and toluene solutions. The calculated fluorescence emission of HBQ-b from the keto form is around 560 nm and the Stokes shift is about 200 nm. The different substituents obviously influence the spectrum distribution of HBQ compounds. The Stokes Shift of HBQ-a is much larger than that of HBQ-b. The large Stokes shifts originate from the significant geometric change in the ground and excited states. The Stokes shift character of HBQ compounds promises the considerable applications in photochemistry and photobiology. Generally, the solution environments will lead to a small shift of the steady-state spectra, but are unable to change the photophysical properties of organic dyes. Therefore, in the following discussion we use the gas phase calculation results to analyze the ESIPT reaction for simplicity. The molecular orbital energy levels and energy gap of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are calculated based on the optimized structures in the ground state, as shown in Fig. 4. HBQ-a and HBQ-b have similar geometries but different substituted groups. Different substituents will lead to an obvious difference in the orbital level. The HOMO energy level for HBQ-a is 6.21 eV, which is higher than the 6.55 eV for HBQ-b, and the LUMO energy level for HBQ-a is lower than that for HBQ-b by 0.12 eV. As a result,
Fig. 3. Absorption and fluorescence spectra of (a) HBQ-a and (b) HBQ-b in the gas phase, acetonitrile solution and toluene solution (the corresponding experimental results are labeled in the spectra [40]).
Fig. 4. Frontier molecular orbital energy levels (in eV) and plots of HOMOs and LUMOs of HBQ-a and HBQ-b.
the energy gap between the LUMO and HOMO for HBQ-a is 3.33 eV, which is smaller than the 3.79 eV for HBQ-b. The molecular orbital energy levels are influenced by the molecular substituents. The molecular modification with electron-withdrawing substituent in the hydrogen-accepting moiety can lower the LUMO molecular orbital and decrease the energy gap between the LUMO and HOMO. From the orbital plots, we notice that the charges are distributed throughout the entire structure. It is found that there is a charge redistribution between HOMO and LUMO in HBQ-a, from hydrogen-donating moiety to hydrogen-accepting moiety,
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which is also attributed to the substituent effect with the electronwithdrawing CN in the hydrogen-accepting moiety. The potential energy curves as a function of O2–H1 distance in HBQ-a and HBQ-b are shown in Fig. 5. The intramolecular proton transfer in the ground and excited states are calculated using DFT and TDDFT methods to describe the reaction route. The potential energies of optimized enol and keto forms in the ground and excited states are used, and the other potential energies with O2– H1 distance changing every 0.1 Å are obtained from single-point energy calculations. From the potential energy curves we can analyze the proton transfer reaction mechanism in the ground and exicted states. The enol form in the ground state S0 is photoexcited to the excited state S1 with absorption wavelengths of 427 and 367 nm for HBQ-a and HBQ-b, as discussed in the above section. For HBQ-a, the proton in the enol form in the excited state transfers along the hydrogen bond coordinate, and then comes across the energy barrier of 3.73 kcal/mol. It finally gets to the nitrogen part in the hydrogen-accepting moiety. The keto geometry is generated in the excited state, and then relaxes to the ground state via the fluorescence emission. The reverse intramolecular proton transfer will take place in the ground state from the keto form to enol form in a non-barrier process. The intramolecular pronton transfer process of HBQ-b is very similar to that of HBQ-a, in which the energy barrier in the excited state is 17.75 kcal/mol. Therefore, the intramolecular proton transfer reaction for HBQ-a is much easier than HBQ-b, which is consistent with the above discussion. The ESIPT reaction is enhanced by the molecular modification with electron-withdrawing substituent CN in the hydrogen-accepting
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moiety. We also notice that the energy barriers for the forward and backward intramolecular proton transfer reactions in the excited state are on the same order of magnitude, so the two processes coexist in the excited state. With the help of potential energy curve calculation, the relaxation process from the ground state to the excited state and back to the ground state is clarified and the ESIPT reaction mechanism is understood. 4. Conclusions The ESIPT reactions and the photophysical properties of HBQ-a and HBQ-b compounds have been investigated using the TDDFT method at the level of B3LYP with 6-311++G(d, p) basis set. It is demonstrated that the proton transfer motion is facilitated by the functionalization with the electron-withdrawing group in the hydrogen-accepting moiety. When the HBQ molecule is photoexcited, the proton moves rapidly along the intramolecular hydrogen bond, which causes the hydrogen bond O2–H1 N6 to break and the hydrogen bond O2 H1–N6 to form. The HBQ molecule is initially excited to the S1 state, and the corresponding electronic excitation energy of HBQ-a is less than that of HBQ-b. The steady-state absorption and fluorescence spectra are calculated in the gas phase, acetonitrile solution and toluene solution. We find that the solvent environment is of little importance to the photophysical properties of HBQ compounds. The remarkable Stokes shifts of HBQ compounds appear in the steady-state spectra, which are attributed to the photoinduced isomerization. All the calculated spectral features are in good agreement with the spectral results recorded in experiments. With the frontier molecular orbital analysis, we find that the molecular orbital energy gap of HBQ-a is smaller than that of HBQ-b. Moreover, the charges in HBQ-a have a redistribution which originates from the substituent effect with the electron-withdrawing group in the hydrogen-accepting moiety. Furthermore, the potential energy curves of HBQ-a and HBQ-b are calculated as a function of O2–H1 distance. The energy barriers of ESIPT reactions are 3.73 kcal/mol for HBQ-a and 17.75 kcal/mol for HBQ-b. It is indicated that the occurrence of ESIPT reaction in HBQ-a is much easier than in HBQ-b, because of the substituent with the electron-withdrawing group in the hydrogen-accepting moiety. The reaction mechanism is clarified with the help of the calculations. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 11304029) and Fundamental Research Funds for Central Universities of China (Grant No. DUT12RC(3)55). References
Fig. 5. Calculated potential energy curves of (a) HBQ-a and (b) HBQ-b as a function of O2–H1 distance in the ground and excited states.
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