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TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: Key role of the excited-state hydrogen-bond strengthening
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 49–53
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: Key role of the excited-state hydrogen-bond strengthening Sheng-Cheng Lan a,b, Yu-Hui Liu a,⇑ a b
Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
DFT/TDDFT method has been
performed to study the ESIPT. The nature of excited-state hydrogen
bond dynamic has been investigated. ESIPT of 8-hydroxyquinoline is
facilitated by strengthening of the electronic excited-state hydrogen bond. The radiationless deactivation via internal conversion is demonstrated.
a r t i c l e
i n f o
Article history: Received 9 October 2014 Received in revised form 1 December 2014 Accepted 10 December 2014 Available online 19 December 2014 Keywords: Hydrogen bond Time-dependent density functional theory Excited-state intramolecular proton transfer Internal conversion
a b s t r a c t Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (–OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O–HN) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excitedstate hydrogen bond (O–HN). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Photo-induced proton-transfer reactions are of importance in photochemical and photobiological processes. Numerous studies have been conducted by various experimental [1–4] and theoretical [5–8] methods since the first experimental observation of the
⇑ Corresponding author. Tel.: +86 416 3400145; fax: +86 416 3400149. E-mail address: [email protected] (Y.-H. Liu). http://dx.doi.org/10.1016/j.saa.2014.12.015 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
phenomenon by Weller in the middle of the last century [9]. ESIPT occurs in molecules containing both acidic and basic groups in close proximity that may rearrange in the electronic excited state through a proton or hydrogen atom transfer. The fast reorganization of the charge distribution resulting from the tautomerization renders these molecules of interest for the design and operation of fluorescent sensors [10,11], organic light-emitting diodes (OLEDs) [12,13], and molecular switches [14]. Compounds in which excited-state proton transfer could occur three categories [15]. The first group consists of compounds in
50
S.-C. Lan, Y.-H. Liu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 49–53
which donor acidic and acceptor basic groups are in close proximity, linked by hydrogen bonds. ESIPT occurs after the excited enol form converts into the keto form. Later, the molecule returns to the electronic ground state by fluorescence or internal conversion, and the proton is finally transferred back to the initial position [16–20]. The emission spectrum of the keto form may be strongly red-shifted compared to the absorption spectrum of the enol form. Such compounds were reported by Weller in 1956 [9]. The second series of compounds are those in which the functional groups are in suitable positions to allow photo-induced tautomerization as a result of concerted double-proton transfer from a donor group to an acceptor group in a hydrogen-bonded system. This is exemplified by 7-azaindole [21]. Proton transfer can also occur through bridging solvent molecules. This occurs, for instance, in 7-hydroxyquinoline (7HQ) in ammonia clusters [22,23] and in 8HQ in aqueous solutions [24–26]. The third group of compounds are those in which the donor acidic and acceptor basic groups are far from each other; unconcerted intermolecular proton transfer is facilitated by the surrounding solvent molecules, as can occur for 6-hydroxyquinoline (6HQ) in aqueous solution [27]. In summary, hydrogen bonding plays an important role in the excited-state intramolecular proton-transfer reaction. Recently, Zhao and Han [6,7,16,28– 32], using DFT and TDDFT, demonstrated the feasibility of an excited-state hydrogen-bond strengthening mechanism, as opposed to the hydrogen-bond cleavage mechanism for coumarin 102 proposed by Chudoba and co-workers in 1998 [33]. This seminal work can successfully explain many photochemical phenomena, such as internal conversion [34], intermolecular charge transfer [35], and excited-state proton transfer [36]. Due to development in experimental techniques and subsequent studies of excited-state hydrogen-bond dynamics, many photophysical and photochemical phenomena involving photo-induced excited-state reactions need to be reconsidered. Hydroxyquinoline is a good model molecular system for exploring proton-transfer reactions. We have previously investigated the proton-transfer reaction dynamics of 6HQ and 7HQ clusters by means of quantum chemical calculations [37–39]. However, 8HQ is different from other hydroxyquinolines because its donor acidic and acceptor basic groups are in close proximity and linked by a hydrogen bond. Thus, intramolecular and intermolecular protontransfer reactions could occur in both the ground and excited states. In 1970, Goldman and Wehry pointed out that different solvents and temperature can directly influence the quantum yield of 8HQ [40]. Later, Bardez confirmed that the intermolecular protontransfer process can occur in 8HQ in aqueous solution, but intrinsic intramolecular proton transfer between the hydroxyl group (–OH) and the heterocyclic ring nitrogen atom (-N-) could not be ruled out [27]. In 2007, Amati and co-workers demonstrated low fluorescence yields in aqueous and some organic solutions. They also reported on the 1:1 water–8HQ adduct and the 2:1 water–8HQ adduct on the basis of experimental measurements and theoretical methods [24]. To date, few experimental studies and quantum chemical calculations have been performed on 8HQ [24–27,40– 43]. In addition, little is known about the role of the excited-state hydrogen-bond dynamics. As mentioned above, the proton-transfer reaction occurs through hydrogen bonding, hence the role of the excited-state hydrogen-bond dynamics is very important for understanding the process. In this work, the ESIPT reaction of 8HQ has been investigated by a time-dependent density functional theory (TDDFT) method. The geometrical structures have been globally optimized in both the ground and the excited electronic states. Moreover, the steady-state absorption and fluorescence spectra of 8HQ have been simulated. The potential energy curves for the ground and excited states have also been calculated.
Theoretical methods By means of density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, geometry optimizations of the ground state and the excited state of the 8HQ molecule were accomplished without symmetry constraints. Frequency calculations at the same levels of theory were then performed to confirm that each optimized structure was a real minimum. In addition, potential energy curves in both the ground state and electronic excited state were calculated through optimizing the excited state by increasing the hydroxyl group (–OH) bond length in increments of 0.1 Å. Becke’s three-parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation (B3LYP) was used in both the DFT and TDDFT methods [44]. The triple-n valence quality with one set of polarization functions (TZVP) was chosen as the basis set [45]. In view of the solvent effect, dichloromethane was used as solvent in the SCRF calculations using IEFPCM throughout. All electronic structure calculations were carried out using the TURBOMOLE program suite [46,47]. Results and discussion Optimized geometric structure in the S0 state 8HQ has two isomeric forms in the ground state, cis-8HQ and trans-8HQ, with different geometrical configurations. The donor acidic and acceptor basic groups of trans-8HQ are far from each other, essentially precluding ESIPT [24]. Herein, we focus exclusively on cis-8HQ, since it can form an intramolecular hydrogen bond between the donor acidic and acceptor basic groups. The geometric structure was fully optimized by the DFT method. Fig. 1a shows the optimized structure of 8HQ in the ground state, which can be seen to be planar. One can note that an intramolecular hydrogen bond (O–HN) can be formed between the protondonating hydroxyl group (–OH) and the accepting heterocyclic ring nitrogen (-N-) atom. Table 1 lists some salient geometrical parameters, that is, bonds and angles. The calculated absorption spectra The electronic excitation energies and corresponding oscillator strengths of 8HQ are presented in Table 2. The result of this simulation is shown in Fig. 2, where the vertical lines denote the corresponding peaks in the experimental spectrum. A very strong absorption is observed at higher electronic energy levels. In the long-wavelength region, the calculated absorption maximum for 8HQ in dichloromethane solution is located at 329 nm, which is in good agreement with the experimental value (309 nm) [15]. From the calculated electronic excitation energies, we can expect that 8HQ may be electronically excited to the S1 state upon photo-excitation by a laser pulse of 305 nm [15]. Thus, we consider only the S1 state in this work. Molecular orbital analysis Before discussing the ESIPT dynamics, it is important to understand the nature of the excited states of the reactants and products. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 8HQ–enol and 8HQ–keto have been calculated. As shown in Fig. 3, for both complexes the p character of the HOMO and the p⁄ character of the LUMO reveal that the first excited state, S1, has pp⁄ character. The HOMO and LUMO of 8HQ–keto are almost identical to those of 8HQ–enol. In addition, the electron density at the hydroxyl oxygen atom in the
S.-C. Lan, Y.-H. Liu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 49–53
51
Fig. 1. Optimized geometric structures of 8HQ in the ground state and its tautomer in the electronic excited state. Gray: C, azury: H, red: O, blue: N (the dash line indicates intramolecular hydrogen bond O–HN). The dash line indicates intramolecular hydrogen bond. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The calculated bond lengths (angstrom) and angles (degree) for 8HQ in the ground state (GS) and in the excited state (ES). The LH–N bond denotes hydrogen bond length.
GS ES
LC–O
LO–H
LH–N
LC–C
LC–N
\COH
\OHN
\CNH
1.355 1.318
0.975 0.999
2.096 1.893
1.425 1.433
1.358 1.357
105 103
117 122
81 83
Table 2 Calculated electronic excitation energies (EEE) (eV) and corresponds oscillator strengths (OS) for 8HQ, the orbital transitions (OT) contributions are also listed. H, HOMO; L, LUMO. States
EEE (eV)
OS
OT
S1
Abs: 3.7629 eV (329.49 nm) Flu: 3.0136 eV (411.42 nm) 4.3686 4.7249 5.3008 5.6571 5.7417
0.0556 0.0706 0.0058 0.0026 0.8574 0.0000 0.0081
H ? L 96% H ? L 98% H-1 ? L 37%; H ? L + 1 61% H-3 ? L H-3 ? L + 1; H-1 ? L H-2 ? L + 1 H-3 ? L; H-1 ? L + 1
S2 S3 S4 S5 S6
Fig. 3. Calculated frontier molecular orbitals of the enol and keto forms of 8HQ.
donor and acceptor. As a result, it is favorable to induce ESIPT in 8HQ. Optimized geometric structure in the S1 state
Fig. 2. Simulated absorption and emission spectra of 8HQ in dichloromethane solution. The vertical lines denote the corresponding peaks in the experimental spectra.
The geometric structures were also globally optimized for the electronic excited state and its tautomeric form (shown in Fig. 1b) by the TDDFT method. As can be seen in Table 1, the intramolecular hydrogen-bond distance was significantly decreased by about 0.21 Å from 2.096 Å in the ground state to 1.893 Å in the excited state. However, the length of the hydroxyl group (–OH) was slightly increased by 0.024 Å from 0.975 Å in the ground state to 0.999 Å in the excited state. The neighboring C–O distance was concomitantly shortened by 0.037 Å. The length of the C–N bond was increased by 0.01 Å in the excited state as compared to the ground state. The other bond lengths and angles were correspondingly changed in the excited state compared to those in the ground state. The calculated emission spectra
HOMO is significantly decreased while that at the nitrogen (-N-) atom is increased after the transition to the LUMO for 8HQ–enol. The intramolecular charge redistribution in the S1 state is somewhat influenced by the acid–base properties between the proton
Similarly, the emission spectrum was simulated, as shown in Fig. 2. According to Ballard and Edwards [48], after excited-state proton transfer, the resulting ketonic form of 8HQ is barely emissive. Thus, the calculated normal emission peak for a solution in
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Fig. 4. Calculated vibrational absorption spectra of 8HQ in the S0 and S1 states. Fig. 6. Schematic view of the excitation and relaxation processes for 8HQ.
The calculated potential energy curves in both S0 and S1 states
Fig. 5. Calculated potential energy curves along the proton-transfer reaction coordinate for both the S0 and S1 states.
dichloromethane is 411 nm, which is also in agreement with the experimental value (391 nm) [15]. This further verified the reliability of the TDDFT calculation for 8HQ in the S1 state. From the geometrical configuration and molecular orbitals (MOs) in the ground and excited states obtained by DFT and TDDFT methods, we can conclude that the intramolecular hydrogen bond is significantly strengthened in the S1 state. As the ESIPT of 8HQ takes place through intramolecular hydrogen bonding, the proton-transfer reaction should be facilitated by the excited-state hydrogen-bond strengthening in the S1 state. The calculated vibrational absorption spectra Ultrafast hydrogen-bonding dynamics can be investigated by monitoring the vibrational absorption spectra of some hydrogenbonded groups in different electronic states [6,7,49]. The infrared spectra of 8HQ in the ground and excited states were calculated using the DFT and TDDFT methods. As shown in Fig. 4, the hydroxyl group (–OH) stretching band was remarkably red-shifted by 402 cm 1 from 3610 cm 1 in the ground state to 3208 cm 1 in the excited state. It is well known that an excited-state hydrogen bond is strengthened if the infrared bands of groups show a redshift on going from the ground to the excited state, whereas a blue-shift of the infrared bands means that the hydrogen bond is weakened. Hence, our results further demonstrate that the intramolecular hydrogen bond becomes distinctly strengthened in 8HQ. At the same time, the infrared spectrum is consistent with the optimized structure.
As mentioned above, the photo-induced excited-state protontransfer reaction along an intramolecular hydrogen bond is very significant in photochemistry. However, the details of the protontransfer mechanism still need to be investigated. Potential energy curves are important for investigating the proton-transfer reaction mechanism [50]. Fig. 5 shows the calculated potential energy curves along the proton-transfer coordinates. Potential energy curves in both the ground state and electronic excited state were calculated through optimizing the excited state by increasing the hydroxyl group (–OH) bond length in steps of 0.1 Å. The results indicated that the enol form is stable in the S0 state and the keto form is most stable in the S1 state. In addition, upon photo-excitation to the S1 state of 8HQ, it can clearly be seen that the proton-transfer reaction is an almost barrierless process (2.24 kcal/mol) in the excited state. Thus, the proton-transfer reaction proceeds easily in the excited state. On the contrary, a large barrier (16.44 kcal/mol) prevents the proton-transfer reaction in the ground state. From our discussions above, it is evident that excited-state intramolecular hydrogen bonding plays a very important role in the ESIPT reaction, and we have confirmed that the intramolecular hydrogen bond becomes significantly strengthened in the S1 state upon photo-excitation. Calculated potential energy curves have further demonstrated the key role of hydrogen-bond strengthening in the ESIPT reaction. Excited-state hydrogen-bond strengthening also plays an important role in increasing the IC process from the fluorescent state to the ground state [6]. As shown in Fig. 6, optical excitation of the S0 state to the S1 Franck–Condon (FC) state is followed by a relaxation process to the minimum S1 state because of the charge distribution all over the molecule. At the same time, some of the 8HQ molecules revert to the ground state through fluorescence. On the other hand, the ESIPT reaction occurs and the enol form is largely converted to the keto form. The IC rate constant is closely related to the relevant energy gap. The calculated energy gaps between the S1 and S0 states are about 3.01 eV for the enol form and 1.52 eV for the keto form. The smaller energy gap for the keto form results in the excited-state molecule reverting to the ground state through the IC process rather than by emitting fluorescence. Thus, the photophysical and photochemical processes of 8HQ can be fully rationalized. Conclusion DFT and TDDFT calculations have been performed to investigate the proton-transfer reaction in 8HQ. The intramolecular hydrogen
S.-C. Lan, Y.-H. Liu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 49–53
bonding (O–HN) has been investigated by the TDDFT method. The hydroxyl group (–OH) stretching frequency in the excited state of 8HQ is clearly red-shifted in comparison to that in the S0 state, which suggests that intramolecular hydrogen bonding is significantly strengthened upon photo-excitation. ESIPT potential energy curves in both the S0 and S1 states have been calculated. We can conclude that intramolecular proton transfer occurs easily in the excited state due to an almost barrierless process, whereas ground-state intramolecular proton transfer does not occur because of a large barrier. Thus, it is demonstrated that the proton-transfer reaction is facilitated by hydrogen-bond strengthening in the excited state, which reveals the important role of hydrogen bonding in the ESIPT reaction. In addition, we have confirmed that the IC process becomes the primary deactivation way for 8HQ. Acknowledgment This work was supported by NSFC (No. 21203011). References [1] J. Herbich, C.Y. Hung, R.P. Thummel, J. Am. Chem. Soc. 118 (1996) 3508–3518. [2] L. Sandra, A. Katrin, T.J. Nibbering, S.B. Victor, J. Phys. Chem. A 117 (2013) 5269–5279. [3] J. Lee, T. Joo, Bull. Korean Chem. Soc. 35 (2014) 881–885. [4] P.D. Alexander, K.-C. Tang, P.-T. Chou, Chem. Soc. Rev. 42 (2013) 1379–1408. [5] M.A. Rios, M.C. Rios, J. Phys. Chem. A 102 (1998) 1560–1567. [6] G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 9218–9223. [7] G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 2469–2474. [8] P. Jaramillo, K. Coutinho, S. Canuto, J. Phys. Chem. A 113 (2009) 12485–12495. [9] A. Weller, Z. Elektrochem. Phys. Chem. 60 (1956) 1144–1147. [10] F.B. Yu, P. Li, B.S. Wang, K.L. Han, J. Am. Chem. Soc. 135 (2013) 7674–7680. [11] F.B. Yu, P. Li, G.J. Zhao, T.S. Chu, K.L. Han, J. Am. Chem. Soc. 133 (2011) 11030– 11033. [12] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913–915. [13] M. Amati, F. Lelj, J. Phys. Chem. A 107 (2003) 2560–2569. [14] T. Naoto, M. Hiroshi, Chem. Rev. 100 (2000) 1875–1890. [15] B. Elisabeth, D. Isabelle, J. Phys. Chem. B 101 (1997) 7786–7793.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: Key role of the excited-state hydrogen-bond strengthening Sheng-Cheng Lan a,b, Yu-Hui Liu a,⇑ a b
Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
DFT/TDDFT method has been
performed to study the ESIPT. The nature of excited-state hydrogen
bond dynamic has been investigated. ESIPT of 8-hydroxyquinoline is
facilitated by strengthening of the electronic excited-state hydrogen bond. The radiationless deactivation via internal conversion is demonstrated.
a r t i c l e
i n f o
Article history: Received 9 October 2014 Received in revised form 1 December 2014 Accepted 10 December 2014 Available online 19 December 2014 Keywords: Hydrogen bond Time-dependent density functional theory Excited-state intramolecular proton transfer Internal conversion
a b s t r a c t Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (–OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O–HN) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excitedstate hydrogen bond (O–HN). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Photo-induced proton-transfer reactions are of importance in photochemical and photobiological processes. Numerous studies have been conducted by various experimental [1–4] and theoretical [5–8] methods since the first experimental observation of the
⇑ Corresponding author. Tel.: +86 416 3400145; fax: +86 416 3400149. E-mail address: [email protected] (Y.-H. Liu). http://dx.doi.org/10.1016/j.saa.2014.12.015 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
phenomenon by Weller in the middle of the last century [9]. ESIPT occurs in molecules containing both acidic and basic groups in close proximity that may rearrange in the electronic excited state through a proton or hydrogen atom transfer. The fast reorganization of the charge distribution resulting from the tautomerization renders these molecules of interest for the design and operation of fluorescent sensors [10,11], organic light-emitting diodes (OLEDs) [12,13], and molecular switches [14]. Compounds in which excited-state proton transfer could occur three categories [15]. The first group consists of compounds in
50
S.-C. Lan, Y.-H. Liu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 49–53
which donor acidic and acceptor basic groups are in close proximity, linked by hydrogen bonds. ESIPT occurs after the excited enol form converts into the keto form. Later, the molecule returns to the electronic ground state by fluorescence or internal conversion, and the proton is finally transferred back to the initial position [16–20]. The emission spectrum of the keto form may be strongly red-shifted compared to the absorption spectrum of the enol form. Such compounds were reported by Weller in 1956 [9]. The second series of compounds are those in which the functional groups are in suitable positions to allow photo-induced tautomerization as a result of concerted double-proton transfer from a donor group to an acceptor group in a hydrogen-bonded system. This is exemplified by 7-azaindole [21]. Proton transfer can also occur through bridging solvent molecules. This occurs, for instance, in 7-hydroxyquinoline (7HQ) in ammonia clusters [22,23] and in 8HQ in aqueous solutions [24–26]. The third group of compounds are those in which the donor acidic and acceptor basic groups are far from each other; unconcerted intermolecular proton transfer is facilitated by the surrounding solvent molecules, as can occur for 6-hydroxyquinoline (6HQ) in aqueous solution [27]. In summary, hydrogen bonding plays an important role in the excited-state intramolecular proton-transfer reaction. Recently, Zhao and Han [6,7,16,28– 32], using DFT and TDDFT, demonstrated the feasibility of an excited-state hydrogen-bond strengthening mechanism, as opposed to the hydrogen-bond cleavage mechanism for coumarin 102 proposed by Chudoba and co-workers in 1998 [33]. This seminal work can successfully explain many photochemical phenomena, such as internal conversion [34], intermolecular charge transfer [35], and excited-state proton transfer [36]. Due to development in experimental techniques and subsequent studies of excited-state hydrogen-bond dynamics, many photophysical and photochemical phenomena involving photo-induced excited-state reactions need to be reconsidered. Hydroxyquinoline is a good model molecular system for exploring proton-transfer reactions. We have previously investigated the proton-transfer reaction dynamics of 6HQ and 7HQ clusters by means of quantum chemical calculations [37–39]. However, 8HQ is different from other hydroxyquinolines because its donor acidic and acceptor basic groups are in close proximity and linked by a hydrogen bond. Thus, intramolecular and intermolecular protontransfer reactions could occur in both the ground and excited states. In 1970, Goldman and Wehry pointed out that different solvents and temperature can directly influence the quantum yield of 8HQ [40]. Later, Bardez confirmed that the intermolecular protontransfer process can occur in 8HQ in aqueous solution, but intrinsic intramolecular proton transfer between the hydroxyl group (–OH) and the heterocyclic ring nitrogen atom (-N-) could not be ruled out [27]. In 2007, Amati and co-workers demonstrated low fluorescence yields in aqueous and some organic solutions. They also reported on the 1:1 water–8HQ adduct and the 2:1 water–8HQ adduct on the basis of experimental measurements and theoretical methods [24]. To date, few experimental studies and quantum chemical calculations have been performed on 8HQ [24–27,40– 43]. In addition, little is known about the role of the excited-state hydrogen-bond dynamics. As mentioned above, the proton-transfer reaction occurs through hydrogen bonding, hence the role of the excited-state hydrogen-bond dynamics is very important for understanding the process. In this work, the ESIPT reaction of 8HQ has been investigated by a time-dependent density functional theory (TDDFT) method. The geometrical structures have been globally optimized in both the ground and the excited electronic states. Moreover, the steady-state absorption and fluorescence spectra of 8HQ have been simulated. The potential energy curves for the ground and excited states have also been calculated.
Theoretical methods By means of density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, geometry optimizations of the ground state and the excited state of the 8HQ molecule were accomplished without symmetry constraints. Frequency calculations at the same levels of theory were then performed to confirm that each optimized structure was a real minimum. In addition, potential energy curves in both the ground state and electronic excited state were calculated through optimizing the excited state by increasing the hydroxyl group (–OH) bond length in increments of 0.1 Å. Becke’s three-parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation (B3LYP) was used in both the DFT and TDDFT methods [44]. The triple-n valence quality with one set of polarization functions (TZVP) was chosen as the basis set [45]. In view of the solvent effect, dichloromethane was used as solvent in the SCRF calculations using IEFPCM throughout. All electronic structure calculations were carried out using the TURBOMOLE program suite [46,47]. Results and discussion Optimized geometric structure in the S0 state 8HQ has two isomeric forms in the ground state, cis-8HQ and trans-8HQ, with different geometrical configurations. The donor acidic and acceptor basic groups of trans-8HQ are far from each other, essentially precluding ESIPT [24]. Herein, we focus exclusively on cis-8HQ, since it can form an intramolecular hydrogen bond between the donor acidic and acceptor basic groups. The geometric structure was fully optimized by the DFT method. Fig. 1a shows the optimized structure of 8HQ in the ground state, which can be seen to be planar. One can note that an intramolecular hydrogen bond (O–HN) can be formed between the protondonating hydroxyl group (–OH) and the accepting heterocyclic ring nitrogen (-N-) atom. Table 1 lists some salient geometrical parameters, that is, bonds and angles. The calculated absorption spectra The electronic excitation energies and corresponding oscillator strengths of 8HQ are presented in Table 2. The result of this simulation is shown in Fig. 2, where the vertical lines denote the corresponding peaks in the experimental spectrum. A very strong absorption is observed at higher electronic energy levels. In the long-wavelength region, the calculated absorption maximum for 8HQ in dichloromethane solution is located at 329 nm, which is in good agreement with the experimental value (309 nm) [15]. From the calculated electronic excitation energies, we can expect that 8HQ may be electronically excited to the S1 state upon photo-excitation by a laser pulse of 305 nm [15]. Thus, we consider only the S1 state in this work. Molecular orbital analysis Before discussing the ESIPT dynamics, it is important to understand the nature of the excited states of the reactants and products. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 8HQ–enol and 8HQ–keto have been calculated. As shown in Fig. 3, for both complexes the p character of the HOMO and the p⁄ character of the LUMO reveal that the first excited state, S1, has pp⁄ character. The HOMO and LUMO of 8HQ–keto are almost identical to those of 8HQ–enol. In addition, the electron density at the hydroxyl oxygen atom in the
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Fig. 1. Optimized geometric structures of 8HQ in the ground state and its tautomer in the electronic excited state. Gray: C, azury: H, red: O, blue: N (the dash line indicates intramolecular hydrogen bond O–HN). The dash line indicates intramolecular hydrogen bond. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The calculated bond lengths (angstrom) and angles (degree) for 8HQ in the ground state (GS) and in the excited state (ES). The LH–N bond denotes hydrogen bond length.
GS ES
LC–O
LO–H
LH–N
LC–C
LC–N
\COH
\OHN
\CNH
1.355 1.318
0.975 0.999
2.096 1.893
1.425 1.433
1.358 1.357
105 103
117 122
81 83
Table 2 Calculated electronic excitation energies (EEE) (eV) and corresponds oscillator strengths (OS) for 8HQ, the orbital transitions (OT) contributions are also listed. H, HOMO; L, LUMO. States
EEE (eV)
OS
OT
S1
Abs: 3.7629 eV (329.49 nm) Flu: 3.0136 eV (411.42 nm) 4.3686 4.7249 5.3008 5.6571 5.7417
0.0556 0.0706 0.0058 0.0026 0.8574 0.0000 0.0081
H ? L 96% H ? L 98% H-1 ? L 37%; H ? L + 1 61% H-3 ? L H-3 ? L + 1; H-1 ? L H-2 ? L + 1 H-3 ? L; H-1 ? L + 1
S2 S3 S4 S5 S6
Fig. 3. Calculated frontier molecular orbitals of the enol and keto forms of 8HQ.
donor and acceptor. As a result, it is favorable to induce ESIPT in 8HQ. Optimized geometric structure in the S1 state
Fig. 2. Simulated absorption and emission spectra of 8HQ in dichloromethane solution. The vertical lines denote the corresponding peaks in the experimental spectra.
The geometric structures were also globally optimized for the electronic excited state and its tautomeric form (shown in Fig. 1b) by the TDDFT method. As can be seen in Table 1, the intramolecular hydrogen-bond distance was significantly decreased by about 0.21 Å from 2.096 Å in the ground state to 1.893 Å in the excited state. However, the length of the hydroxyl group (–OH) was slightly increased by 0.024 Å from 0.975 Å in the ground state to 0.999 Å in the excited state. The neighboring C–O distance was concomitantly shortened by 0.037 Å. The length of the C–N bond was increased by 0.01 Å in the excited state as compared to the ground state. The other bond lengths and angles were correspondingly changed in the excited state compared to those in the ground state. The calculated emission spectra
HOMO is significantly decreased while that at the nitrogen (-N-) atom is increased after the transition to the LUMO for 8HQ–enol. The intramolecular charge redistribution in the S1 state is somewhat influenced by the acid–base properties between the proton
Similarly, the emission spectrum was simulated, as shown in Fig. 2. According to Ballard and Edwards [48], after excited-state proton transfer, the resulting ketonic form of 8HQ is barely emissive. Thus, the calculated normal emission peak for a solution in
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Fig. 4. Calculated vibrational absorption spectra of 8HQ in the S0 and S1 states. Fig. 6. Schematic view of the excitation and relaxation processes for 8HQ.
The calculated potential energy curves in both S0 and S1 states
Fig. 5. Calculated potential energy curves along the proton-transfer reaction coordinate for both the S0 and S1 states.
dichloromethane is 411 nm, which is also in agreement with the experimental value (391 nm) [15]. This further verified the reliability of the TDDFT calculation for 8HQ in the S1 state. From the geometrical configuration and molecular orbitals (MOs) in the ground and excited states obtained by DFT and TDDFT methods, we can conclude that the intramolecular hydrogen bond is significantly strengthened in the S1 state. As the ESIPT of 8HQ takes place through intramolecular hydrogen bonding, the proton-transfer reaction should be facilitated by the excited-state hydrogen-bond strengthening in the S1 state. The calculated vibrational absorption spectra Ultrafast hydrogen-bonding dynamics can be investigated by monitoring the vibrational absorption spectra of some hydrogenbonded groups in different electronic states [6,7,49]. The infrared spectra of 8HQ in the ground and excited states were calculated using the DFT and TDDFT methods. As shown in Fig. 4, the hydroxyl group (–OH) stretching band was remarkably red-shifted by 402 cm 1 from 3610 cm 1 in the ground state to 3208 cm 1 in the excited state. It is well known that an excited-state hydrogen bond is strengthened if the infrared bands of groups show a redshift on going from the ground to the excited state, whereas a blue-shift of the infrared bands means that the hydrogen bond is weakened. Hence, our results further demonstrate that the intramolecular hydrogen bond becomes distinctly strengthened in 8HQ. At the same time, the infrared spectrum is consistent with the optimized structure.
As mentioned above, the photo-induced excited-state protontransfer reaction along an intramolecular hydrogen bond is very significant in photochemistry. However, the details of the protontransfer mechanism still need to be investigated. Potential energy curves are important for investigating the proton-transfer reaction mechanism [50]. Fig. 5 shows the calculated potential energy curves along the proton-transfer coordinates. Potential energy curves in both the ground state and electronic excited state were calculated through optimizing the excited state by increasing the hydroxyl group (–OH) bond length in steps of 0.1 Å. The results indicated that the enol form is stable in the S0 state and the keto form is most stable in the S1 state. In addition, upon photo-excitation to the S1 state of 8HQ, it can clearly be seen that the proton-transfer reaction is an almost barrierless process (2.24 kcal/mol) in the excited state. Thus, the proton-transfer reaction proceeds easily in the excited state. On the contrary, a large barrier (16.44 kcal/mol) prevents the proton-transfer reaction in the ground state. From our discussions above, it is evident that excited-state intramolecular hydrogen bonding plays a very important role in the ESIPT reaction, and we have confirmed that the intramolecular hydrogen bond becomes significantly strengthened in the S1 state upon photo-excitation. Calculated potential energy curves have further demonstrated the key role of hydrogen-bond strengthening in the ESIPT reaction. Excited-state hydrogen-bond strengthening also plays an important role in increasing the IC process from the fluorescent state to the ground state [6]. As shown in Fig. 6, optical excitation of the S0 state to the S1 Franck–Condon (FC) state is followed by a relaxation process to the minimum S1 state because of the charge distribution all over the molecule. At the same time, some of the 8HQ molecules revert to the ground state through fluorescence. On the other hand, the ESIPT reaction occurs and the enol form is largely converted to the keto form. The IC rate constant is closely related to the relevant energy gap. The calculated energy gaps between the S1 and S0 states are about 3.01 eV for the enol form and 1.52 eV for the keto form. The smaller energy gap for the keto form results in the excited-state molecule reverting to the ground state through the IC process rather than by emitting fluorescence. Thus, the photophysical and photochemical processes of 8HQ can be fully rationalized. Conclusion DFT and TDDFT calculations have been performed to investigate the proton-transfer reaction in 8HQ. The intramolecular hydrogen
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bonding (O–HN) has been investigated by the TDDFT method. The hydroxyl group (–OH) stretching frequency in the excited state of 8HQ is clearly red-shifted in comparison to that in the S0 state, which suggests that intramolecular hydrogen bonding is significantly strengthened upon photo-excitation. ESIPT potential energy curves in both the S0 and S1 states have been calculated. We can conclude that intramolecular proton transfer occurs easily in the excited state due to an almost barrierless process, whereas ground-state intramolecular proton transfer does not occur because of a large barrier. Thus, it is demonstrated that the proton-transfer reaction is facilitated by hydrogen-bond strengthening in the excited state, which reveals the important role of hydrogen bonding in the ESIPT reaction. In addition, we have confirmed that the IC process becomes the primary deactivation way for 8HQ. Acknowledgment This work was supported by NSFC (No. 21203011). References [1] J. Herbich, C.Y. Hung, R.P. Thummel, J. Am. Chem. Soc. 118 (1996) 3508–3518. [2] L. Sandra, A. Katrin, T.J. Nibbering, S.B. Victor, J. Phys. Chem. A 117 (2013) 5269–5279. [3] J. Lee, T. Joo, Bull. Korean Chem. Soc. 35 (2014) 881–885. [4] P.D. Alexander, K.-C. Tang, P.-T. Chou, Chem. Soc. Rev. 42 (2013) 1379–1408. [5] M.A. Rios, M.C. Rios, J. Phys. Chem. A 102 (1998) 1560–1567. [6] G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 9218–9223. [7] G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 2469–2474. [8] P. Jaramillo, K. Coutinho, S. Canuto, J. Phys. Chem. A 113 (2009) 12485–12495. [9] A. Weller, Z. Elektrochem. Phys. Chem. 60 (1956) 1144–1147. [10] F.B. Yu, P. Li, B.S. Wang, K.L. Han, J. Am. Chem. Soc. 135 (2013) 7674–7680. [11] F.B. Yu, P. Li, G.J. Zhao, T.S. Chu, K.L. Han, J. Am. Chem. Soc. 133 (2011) 11030– 11033. [12] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913–915. [13] M. Amati, F. Lelj, J. Phys. Chem. A 107 (2003) 2560–2569. [14] T. Naoto, M. Hiroshi, Chem. Rev. 100 (2000) 1875–1890. [15] B. Elisabeth, D. Isabelle, J. Phys. Chem. B 101 (1997) 7786–7793.
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