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Theoretical studies of conjugation and substituent effect on intramolecular proton transfer in the ground and excited states
Chemical Physics 322 (2006) 382–386 www.elsevier.com/locate/chemphys
Theoretical studies of conjugation and substituent effect on intramolecular proton transfer in the ground and excited states Ping G. Yi, Yong H. Liang
*
School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China Received 16 June 2005; accepted 2 September 2005 Available online 21 October 2005
Abstract The ground- and excited-state intramolecular proton transfer (GSIPT and ESIPT) for 8-hydroxy-4H-naphthalen-1-one (HNA), 5hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1-indenone (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene-3,10-quinone (DHP) are studied at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level. The calculated results show that the PES of GSIPT for HNA, HNQ and HAQ exhibit a single minimum in the enol zone, while for 7-HIN, DHNQ and DHP exhibit a double minimum and a high barrier between the two minima. The barrierless ESIPT for HNA is predicted, however, the PES of ESIPT for HNQ, HAQ, 7HIN, DHNQ and DHP exhibit a high barrier in the S1 tautomerism. 2005 Elsevier B.V. All rights reserved. Keywords: Excited-state intramolecular proton transfer; B3LYP; TD; PES
1. Introduction In recent years, excited state intramolecular proton transfer (ESIPT) has been a topic of much interest because of its importance in many chemical and biological processes [1,2] and a wide range of application, such as polymer stabilizers [3], information storage [4] and antitumor activity [5]. ESIPT is a very simple chemical process and is readily accessible to both accurate measurements and quantitative theoretical analyses, however, still poses challenges for the complexity of its physical and chemical nature. Since the Albert WellerÕs pioneering work on the ESIPT of methyl salicylate (MS) [6], many systems have been developed to study the ESIPT, such as o-hydrobenzoyl [7–9], flavonoids [10–12], etc. In the early stage of investigations, a double minimum potential associating with the enol-keto tautomerism had been proposed for the reaction coordinate in S0 and S1 [13]. Recent studies in experimental
*
Corresponding author. Tel.: +86 732 829 0317; fax: +86 732 829 0001. E-mail address: [email protected] (Y.H. Liang).
0301-0104/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.09.019
and theoretical points of view [14–17] have abandoned such a double minimum potential, but proposed a single minimum potential for the S0 and S1 state and the ESIPT process with no barrier in the S1 state. However, Petrich et al. [5,18,19] recently reported the fluorescence up-conversion measurements of hypericin and hypocrellin A in a variety of solvents. Hypericin executes ESIPT in 10 ps and is independent of solvent, on the other hand, the H-atom transfer time for hypocrellin A in ethanol and octanol is range from 50 to 100 ps. From these results, they believed that an energy barrier exist in the intramolecular proton transfer of hypocrellin A (rate processes observed for the barrierless ESIPT is 10 11–10 14 s). And the theoretical investigations [20–23] on the GSIPT and ESIPT in the perylenequinonoid derivatives showed that intramolecular proton transfer exhibit a double minimum and a barrier in the S0 and S1 state using the HF and CIS methods, respectively. On the other hand, Dogra et al. [24] reported the photophysics of 1-hydroxy-9-fluorenone by theory and experiment, and stated the absence of excited state intramolecular proton transfer reaction recently. These results show that GSIPT and ESIPT are different in the variety of molecule systems.
P.G. Yi, Y.H. Liang / Chemical Physics 322 (2006) 382–386
383
(B3LYP)/6-31G(d,p) theoretical calculations for the B3LYP/6-31G(d,p) ground state optimized structures. All the calculations presented here have been performed with the Gaussian 03 series of program [29]. 3. Results and discussions
Fig. 1. Structures of 8-hydroxy-4H-naphthalen-1-one (HNA), 5-hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1H-inden-1-one (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene-3,10-quinone (DHP), with enumeration of oxygen atoms indicated.
Insight in the conjugation and substituent effect on GSIPT and ESIPT, we investigate the ground- and excited-state intramolecular proton transfer (GSIPT and ESIPT) for 8-hydroxy-4H-naphthalen-1-one (HNA), 5hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1-indenone (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene3,10-quinone (DHP) (Fig. 1) at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level in this paper. The purpose of this work is to clarify the following aspects: (a) whether six aromatic molecules have one or more stationary points at ground- and excited-state; (b) whether the first singlet excited state of six aromatic molecules is 1(p,p*) or (n,p*); (c) can ESIPT happen in all six aromatic molecules. 2. Computational section The molecular systems studied in this work are illustrated in Fig. 1. The B3LYP hybrid density functional of Becke [25,26] in combination with the split valence 631G(d,p) basis set, referred to as B3LYP/6-31G(d,p), is employed for geometry optimizations. The ground state intramolecular proton transfer curves are constructed at ˚ range. a fixed O2–H distances over the 0.95–1.65 A There exist a number of different theoretical methods, such as CASSCF, CASPT2, CCSD and TDDFT, which are capable in principle of treating a proton transfer in a given excited state. However, the large size of the systems is still precluded an advanced theoretical characterization (CASSCF, CCSD and CASPT2) of the potential energy surfaces of ESIPT. Fortunately, the recent calculations [14,27] showed that TDDFT can reliably predict the potential energy surface of ESIPT. In this work, we constructed the potential energy surface (PES) of ESIPT by TD [28]
In this section we will discuss ESIPT and GSIPT in six aromatic molecules. Previous studied results [15] show that the lowest two singlet excited state order has two cases, case A is 1(p,p*) < (n,p*) and case B is (n,p*) < 1(p,p*). When the first singlet excited state is 1(p,p*) and enol 1(p,p*) < keto 1(p,p*), ESIPT is likely to happen, on the other hand, when the first singlet excited state is (n,p*), ESIPT cannot do. In order to examine the lowest two singlet excited state order, we calculate the lowest three singlet excited state for six aromatic molecules. As shown in Fig. 2, the first excited states in the enol zone are 1(p,p*) in our studied six aromatic molecules except HNQ and HAQ and we will discuss GSIPT and the first excited state ESIPT in this section. 3.1. HNA, HNQ, and HAQ Fig. 2(a)–(c) shows the proton transfer potential energy curves either in the ground or excited state for the molecules HNA, HNQ and HAQ, respectively. As can be seen, the calculations at the level B3LYP/6-31G(d,p) provide the ground state intramolecular proton transfer curves with a single minimum at the equilibrium O–H distance of the ˚ ), this markedly stabilized enol form (O2–H = 0.99 A GSIPT curve for the enol form also found for the molecules 2-hydroxybenzoy [15]. And the keto form (O2– ˚ ) is found at about 10 kcal/mol above the enol H = 1.60 A form. In the previous studies, Catala´n and his coworkers [16] suggested that the transition state is the structure with the shortest O1–O2 length, so we construct the Fig. 3 by RO1–O2 vs. RO2–H. As depicted in Fig. 3, when the O2–H ˚ , the RO1–O2 for HNA, bond length is amount to 1.15 A HNQ and HAQ has the shortest length, so the barrier of GSIPT for HNA, HNQ and HAQ is amount to 6.15, 6.25 and 4.75 kcal/mol as shown in Table 1, respectively. Based on Fig. 2(a), the PES for HNA at the first excited state exhibits a single minimum in the transferred zone and has no barrier for proton transfer. On the other hand, the first excited state for HNQ and HAQ at TDB3LYP/631G(d,p) level is 1(n,p*) in the enol zone (Fig. 2(b, c)). And the PES of 1(p,p*) exhibit a single minimum in the enol ˚ and zone, where the equilibrium O–H distance is 1.04 A somewhat longer than that in the ground state. Therefore, ESIPT cannot happen in the molecules HNQ and HAQ. The absorption spectrum of HAQ in [30,31] is consistent with our results. However, No ESIPT in HAQ is in contradiction with previous theoretical at HF/4-31G(d) and CIS/ 4-31(d) level calculations and experimental results [30–33] that a large Stoke-shift exists in fluorescence spectrum and ESIPT with no barrier can occur in HAQ.
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Fig. 2. GSIPT curves (–·–) obtained from B3LYP/6-31G(d,p) optimized structures and (n,p*)1 (–h–), 1(p,p*)1 (–s–), and 2(p,p*)1 (–n–) Franck– Condon ESIPT curves constructed by using TD B3LYP/6-31G(d,p) calculations.
Fig. 3. Variation of the O1–O2 length with RO2–H in the ground state proton transfer calculated at the B3LYP/6-31G(d,p) level. Bond length ˚. in A
3.2. 7HIN PES of the ground and excited state for 7HIN are calculated with the B3LYP and TDB3LYP methods as functions of RO2–H is shown in Fig. 2(d). The GSIPT curve for this compound contains a minimum that is located in
˚ ), and this curve also apthe enol zone (O2–H = 0.979 A pears to have an incipient minimum in the keto zone ˚ ), with a low barrier to the enol form (O2–H = 1.860 A (about 1.1 kcal/mol). However, it is contrast with the previous report for 7-hydroxy-3H, 4H-indon-1-one [14,15] which is absent the double bond in the five-member ring, that the GSIPT curve exhibits a single minimum in the enol zone. These results prove the Nagoaka and his coworkersÕ prediction [34] that a double minimum exists on the GSIPTÕs PES. As shown in Fig. 3, when the O2-H bond ˚ , the RO1–O2 for 7HIN has the length is amount to 1.15 A shortest length, and RO–2H is longer than these of above three molecules. The corresponding ESIPT curves for this compound perform some interesting difference with the 7hydroxy-3H, 4H-indon-1-one [14,15] and 1-hydroxy-9-fluorenone [24]. This curve in the S1 state exhibits two minima corresponding to the enol and keto forms, respectively, and the proton transfer reaction is exothermic by 0.3 kcal/mol. The height of the barrier between two minima is 4.2 kcal/ mol. However, the ESIPT curve of the 1(p,p*)1 state for 7-hydroxy-3H, 4H-indon-1-one [14,15] exhibits a single minimum in the keto zone and its first singlet excited state is (n,p*), on the other hand, PES exhibits a single minimum in the enol zone for 1-hydroxy-9-fluorenone [24].
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Table 1 Calculated property for six aromatic compounds in the ground- and excited-state at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level Compound HNA HNQ HAQ 7HIN DHNQ DHP
E (Hartree) 536.348996 610.359532 764.026086 497.013420 685.593288 1069.127571
DE(enol S0) (kcal)
DE(keto S0) (kcal)
DEa(S0) (kcal)
DE(enol S1) (kcal)
DE(keto S1) (kcal)
DEa(S1) (kcal)
0.00 0.00 0.00 0.00 0.00 0.00
11.64 10.02 9.61 14.60 4.54 1.10
6.15 6.25 4.75 15.74 5.42 2.37
80.16 63.88 68.94 70.75 56.62 53.90
73.27 – – 70.65 59.75 53.70
– – – 4.15 3.63 1.70
3.3. DHNQ and DHP The GSIPT curves have been constructed from the energies obtained at the B3LYP/6-31G(d,p) level for the molecular structures of DHNQ (Fig. 2(e)) and DHP (Fig. 2(f)) which have a double H-bonding O2–H O1 and O3– H O4. Based on the GSIPT curves, which are quite consistent with previous experimental and theoretical report that proton transfer reaction for the perylenequinonoid derivatives (hypocrellin, calphostin, hypericin and hypomycin A) is a stepwise mechanism [20–23], DHNQ and DHP exhibit a double minimum in the enol and keto positon. The keto forms are found to be more stable than their enol form by 4.54 and 1.10 kcal/mol, respectively. And our recent calculated results [35] show that the GSIPT PES for hypocrellin A also have a double minimum at B3LYP/431G level.
Fig. 4. Variation of the O3–H length with RO2–H for DHNQ (A) and DHP (B) in the ground state at the B3LYP/6-31G(d,p) level. The inset figures ˚. show the O3–O4 length with RO2–H. Bond length in A
We also find that the H-bonding O3–H O4 play an important role in stabilizing the keto form. As shown the Fig. 4, the length of the hydrogen bond for DNHQ and DHP is substantially shorter in the keto form ˚ , respectively) than in the enol (RO3–O4 = 2.55 and 2.49 A ˚ , respectively), whereas the form (RO3–O4 = 2.58 and 2.51 A RO3–H in the keto form is also longer than that in the enol ˚ for DNHQ and 1.023 vs. 1.014 A ˚ form (1.018 vs. 0.996 A for DHP, respectively). When rotate the O3–H group of the keto form of DHNQ and DHP, giving birth to a keto form without the H-bonding O3–H O4 (Scheme 1), and optimize this initial structure at B3LYP/6-31G(d,p) level. The optimized structure for DHNQ is not the initial structure, and an enol structure without H-bonding O3–H O4 is obtained. In other words, the hydrogen atom on the atom O1 transfers back to the atom O2 when optimize this molecule (Scheme 1). These results show that the keto form without the H-bonding O3–H O4 is not a stable rotamer for DHNQ and the H-bonding O3–H O4 play a vital role in stabilize the keto form. And the optimized structure for DHP is a keto structure without H-bonding O3–H O4, and is more unstable than the keto form with H-bonding by 19.4 kcal/mol. The potential energy curves of the existed-state for DHNQ and DHP are shown in Fig. 2(e) and (f). PES of DHNQ in the S1 state exhibits a single minimum in the enol keto zone. This curve also appears to have an incipient minimum in the keto zone, with a very low barrier to the enol form (about 0.5 kcal/mol). Thereof, ESIPT cannot happen in this molecule. However, the corresponding ESIPT curves for DHP are consistent with the experimental evidence. The PES in the S1 state exhibits a double minimum in the enol and keto zones, and the keto form is calculated to be more stable than the enol form by 0.2 kcal/mol. And the height of the barrier between the
Scheme 1.
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two minima is 1.70 kcal/mol. These results are consistent with the prediction of Petrich et al. [18,19] that a barrier exists for the intramolecular proton transfer in hypocrellin A which has seven-member ring. 4. Conclusion In this work, PES for ground- and existed-state intramolecular proton transfer of HNA, HNQ, HAQ, 7HIN, DHNQ and DHP have been calculated by using B3LYP and TDB3LYP methods. The main results are as follows: (a) PES for ground-state intramolecular proton transfer of HNA, HNQ, HAQ and 7HIN exhibit a single minimum, however, PES for DHNQ and DHP exhibit a double minimum and the H-bonding O3–H O4 plays a vital role in stabilizing the keto form. (b) PES for HNA show that ESIPT is barrierless, on the other hand, PES for 7HIN and DHP show that ESIPT in these molecules exist a high barrier, but ESIPT for HNQ, HAQ and DHNQ cannot occur because PES of the S1 state exhibit a double minimum but the enol form is stabilized. Acknowledgements This research was supported by the National Natural Science Foundation of China (20173050), Natural Science Foundation of Hunan Province (04JJY40010) and Key Technologies R&D Programme of China (2004BA308A22-1). References [1] Special Issue on Proton Transfer, Chem. Phys. 136 (1989). [2] Special Issue on Proton Transfer, J. Phys. Chem. 95 (1991). [3] J. Keck, M. Roessler, S. Schroeder, G.J. Stueber, F. Waiblinger, M. Stein, D. Legourrie´recm H, E.A. Kramer, H. Hoier, S. Henkel, P. Fischer, H. Port, T. Hirsch, G. Tytz, P. Hayoz, J. Phys. Chem. B 102 (1998) 6975. [4] R.W. Munn, Chem. Br. (1989) 517. [5] G.A. Kraus, W. Zhang, M.J. Fehr, J.W. Petrich, Y. Wannemuenler, S. Carpenter, Chem. Rev. 96 (1996) 523. [6] A. Weller, Z. Elektrochem. 60 (1956) 1144. [7] S. Nagaoka, N. Hirota, M. Sumitani, K. Yoshihara, E. LipczynskaKochany, H. Iwamura, J. Am. Chem. Soc. 106 (1984) 6913. [8] C. Lu, R.M. Hsieh, L.R. Lee, P.Y. Cheng, Chem. Phys. Lett. 310 (1999) 103. [9] J.A. Organero, M. Moreno, L. Santos, J.M. Lluch, A. Douhal, Chem. Phys. Lett. 328 (2000) 83. [10] T.C. Swinney, D.F. Kelley, J. Chem. Phys. 99 (1993) 211.
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Theoretical studies of conjugation and substituent effect on intramolecular proton transfer in the ground and excited states Ping G. Yi, Yong H. Liang
*
School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China Received 16 June 2005; accepted 2 September 2005 Available online 21 October 2005
Abstract The ground- and excited-state intramolecular proton transfer (GSIPT and ESIPT) for 8-hydroxy-4H-naphthalen-1-one (HNA), 5hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1-indenone (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene-3,10-quinone (DHP) are studied at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level. The calculated results show that the PES of GSIPT for HNA, HNQ and HAQ exhibit a single minimum in the enol zone, while for 7-HIN, DHNQ and DHP exhibit a double minimum and a high barrier between the two minima. The barrierless ESIPT for HNA is predicted, however, the PES of ESIPT for HNQ, HAQ, 7HIN, DHNQ and DHP exhibit a high barrier in the S1 tautomerism. 2005 Elsevier B.V. All rights reserved. Keywords: Excited-state intramolecular proton transfer; B3LYP; TD; PES
1. Introduction In recent years, excited state intramolecular proton transfer (ESIPT) has been a topic of much interest because of its importance in many chemical and biological processes [1,2] and a wide range of application, such as polymer stabilizers [3], information storage [4] and antitumor activity [5]. ESIPT is a very simple chemical process and is readily accessible to both accurate measurements and quantitative theoretical analyses, however, still poses challenges for the complexity of its physical and chemical nature. Since the Albert WellerÕs pioneering work on the ESIPT of methyl salicylate (MS) [6], many systems have been developed to study the ESIPT, such as o-hydrobenzoyl [7–9], flavonoids [10–12], etc. In the early stage of investigations, a double minimum potential associating with the enol-keto tautomerism had been proposed for the reaction coordinate in S0 and S1 [13]. Recent studies in experimental
*
Corresponding author. Tel.: +86 732 829 0317; fax: +86 732 829 0001. E-mail address: [email protected] (Y.H. Liang).
0301-0104/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.09.019
and theoretical points of view [14–17] have abandoned such a double minimum potential, but proposed a single minimum potential for the S0 and S1 state and the ESIPT process with no barrier in the S1 state. However, Petrich et al. [5,18,19] recently reported the fluorescence up-conversion measurements of hypericin and hypocrellin A in a variety of solvents. Hypericin executes ESIPT in 10 ps and is independent of solvent, on the other hand, the H-atom transfer time for hypocrellin A in ethanol and octanol is range from 50 to 100 ps. From these results, they believed that an energy barrier exist in the intramolecular proton transfer of hypocrellin A (rate processes observed for the barrierless ESIPT is 10 11–10 14 s). And the theoretical investigations [20–23] on the GSIPT and ESIPT in the perylenequinonoid derivatives showed that intramolecular proton transfer exhibit a double minimum and a barrier in the S0 and S1 state using the HF and CIS methods, respectively. On the other hand, Dogra et al. [24] reported the photophysics of 1-hydroxy-9-fluorenone by theory and experiment, and stated the absence of excited state intramolecular proton transfer reaction recently. These results show that GSIPT and ESIPT are different in the variety of molecule systems.
P.G. Yi, Y.H. Liang / Chemical Physics 322 (2006) 382–386
383
(B3LYP)/6-31G(d,p) theoretical calculations for the B3LYP/6-31G(d,p) ground state optimized structures. All the calculations presented here have been performed with the Gaussian 03 series of program [29]. 3. Results and discussions
Fig. 1. Structures of 8-hydroxy-4H-naphthalen-1-one (HNA), 5-hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1H-inden-1-one (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene-3,10-quinone (DHP), with enumeration of oxygen atoms indicated.
Insight in the conjugation and substituent effect on GSIPT and ESIPT, we investigate the ground- and excited-state intramolecular proton transfer (GSIPT and ESIPT) for 8-hydroxy-4H-naphthalen-1-one (HNA), 5hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1-indenone (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene3,10-quinone (DHP) (Fig. 1) at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level in this paper. The purpose of this work is to clarify the following aspects: (a) whether six aromatic molecules have one or more stationary points at ground- and excited-state; (b) whether the first singlet excited state of six aromatic molecules is 1(p,p*) or (n,p*); (c) can ESIPT happen in all six aromatic molecules. 2. Computational section The molecular systems studied in this work are illustrated in Fig. 1. The B3LYP hybrid density functional of Becke [25,26] in combination with the split valence 631G(d,p) basis set, referred to as B3LYP/6-31G(d,p), is employed for geometry optimizations. The ground state intramolecular proton transfer curves are constructed at ˚ range. a fixed O2–H distances over the 0.95–1.65 A There exist a number of different theoretical methods, such as CASSCF, CASPT2, CCSD and TDDFT, which are capable in principle of treating a proton transfer in a given excited state. However, the large size of the systems is still precluded an advanced theoretical characterization (CASSCF, CCSD and CASPT2) of the potential energy surfaces of ESIPT. Fortunately, the recent calculations [14,27] showed that TDDFT can reliably predict the potential energy surface of ESIPT. In this work, we constructed the potential energy surface (PES) of ESIPT by TD [28]
In this section we will discuss ESIPT and GSIPT in six aromatic molecules. Previous studied results [15] show that the lowest two singlet excited state order has two cases, case A is 1(p,p*) < (n,p*) and case B is (n,p*) < 1(p,p*). When the first singlet excited state is 1(p,p*) and enol 1(p,p*) < keto 1(p,p*), ESIPT is likely to happen, on the other hand, when the first singlet excited state is (n,p*), ESIPT cannot do. In order to examine the lowest two singlet excited state order, we calculate the lowest three singlet excited state for six aromatic molecules. As shown in Fig. 2, the first excited states in the enol zone are 1(p,p*) in our studied six aromatic molecules except HNQ and HAQ and we will discuss GSIPT and the first excited state ESIPT in this section. 3.1. HNA, HNQ, and HAQ Fig. 2(a)–(c) shows the proton transfer potential energy curves either in the ground or excited state for the molecules HNA, HNQ and HAQ, respectively. As can be seen, the calculations at the level B3LYP/6-31G(d,p) provide the ground state intramolecular proton transfer curves with a single minimum at the equilibrium O–H distance of the ˚ ), this markedly stabilized enol form (O2–H = 0.99 A GSIPT curve for the enol form also found for the molecules 2-hydroxybenzoy [15]. And the keto form (O2– ˚ ) is found at about 10 kcal/mol above the enol H = 1.60 A form. In the previous studies, Catala´n and his coworkers [16] suggested that the transition state is the structure with the shortest O1–O2 length, so we construct the Fig. 3 by RO1–O2 vs. RO2–H. As depicted in Fig. 3, when the O2–H ˚ , the RO1–O2 for HNA, bond length is amount to 1.15 A HNQ and HAQ has the shortest length, so the barrier of GSIPT for HNA, HNQ and HAQ is amount to 6.15, 6.25 and 4.75 kcal/mol as shown in Table 1, respectively. Based on Fig. 2(a), the PES for HNA at the first excited state exhibits a single minimum in the transferred zone and has no barrier for proton transfer. On the other hand, the first excited state for HNQ and HAQ at TDB3LYP/631G(d,p) level is 1(n,p*) in the enol zone (Fig. 2(b, c)). And the PES of 1(p,p*) exhibit a single minimum in the enol ˚ and zone, where the equilibrium O–H distance is 1.04 A somewhat longer than that in the ground state. Therefore, ESIPT cannot happen in the molecules HNQ and HAQ. The absorption spectrum of HAQ in [30,31] is consistent with our results. However, No ESIPT in HAQ is in contradiction with previous theoretical at HF/4-31G(d) and CIS/ 4-31(d) level calculations and experimental results [30–33] that a large Stoke-shift exists in fluorescence spectrum and ESIPT with no barrier can occur in HAQ.
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Fig. 2. GSIPT curves (–·–) obtained from B3LYP/6-31G(d,p) optimized structures and (n,p*)1 (–h–), 1(p,p*)1 (–s–), and 2(p,p*)1 (–n–) Franck– Condon ESIPT curves constructed by using TD B3LYP/6-31G(d,p) calculations.
Fig. 3. Variation of the O1–O2 length with RO2–H in the ground state proton transfer calculated at the B3LYP/6-31G(d,p) level. Bond length ˚. in A
3.2. 7HIN PES of the ground and excited state for 7HIN are calculated with the B3LYP and TDB3LYP methods as functions of RO2–H is shown in Fig. 2(d). The GSIPT curve for this compound contains a minimum that is located in
˚ ), and this curve also apthe enol zone (O2–H = 0.979 A pears to have an incipient minimum in the keto zone ˚ ), with a low barrier to the enol form (O2–H = 1.860 A (about 1.1 kcal/mol). However, it is contrast with the previous report for 7-hydroxy-3H, 4H-indon-1-one [14,15] which is absent the double bond in the five-member ring, that the GSIPT curve exhibits a single minimum in the enol zone. These results prove the Nagoaka and his coworkersÕ prediction [34] that a double minimum exists on the GSIPTÕs PES. As shown in Fig. 3, when the O2-H bond ˚ , the RO1–O2 for 7HIN has the length is amount to 1.15 A shortest length, and RO–2H is longer than these of above three molecules. The corresponding ESIPT curves for this compound perform some interesting difference with the 7hydroxy-3H, 4H-indon-1-one [14,15] and 1-hydroxy-9-fluorenone [24]. This curve in the S1 state exhibits two minima corresponding to the enol and keto forms, respectively, and the proton transfer reaction is exothermic by 0.3 kcal/mol. The height of the barrier between two minima is 4.2 kcal/ mol. However, the ESIPT curve of the 1(p,p*)1 state for 7-hydroxy-3H, 4H-indon-1-one [14,15] exhibits a single minimum in the keto zone and its first singlet excited state is (n,p*), on the other hand, PES exhibits a single minimum in the enol zone for 1-hydroxy-9-fluorenone [24].
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Table 1 Calculated property for six aromatic compounds in the ground- and excited-state at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level Compound HNA HNQ HAQ 7HIN DHNQ DHP
E (Hartree) 536.348996 610.359532 764.026086 497.013420 685.593288 1069.127571
DE(enol S0) (kcal)
DE(keto S0) (kcal)
DEa(S0) (kcal)
DE(enol S1) (kcal)
DE(keto S1) (kcal)
DEa(S1) (kcal)
0.00 0.00 0.00 0.00 0.00 0.00
11.64 10.02 9.61 14.60 4.54 1.10
6.15 6.25 4.75 15.74 5.42 2.37
80.16 63.88 68.94 70.75 56.62 53.90
73.27 – – 70.65 59.75 53.70
– – – 4.15 3.63 1.70
3.3. DHNQ and DHP The GSIPT curves have been constructed from the energies obtained at the B3LYP/6-31G(d,p) level for the molecular structures of DHNQ (Fig. 2(e)) and DHP (Fig. 2(f)) which have a double H-bonding O2–H O1 and O3– H O4. Based on the GSIPT curves, which are quite consistent with previous experimental and theoretical report that proton transfer reaction for the perylenequinonoid derivatives (hypocrellin, calphostin, hypericin and hypomycin A) is a stepwise mechanism [20–23], DHNQ and DHP exhibit a double minimum in the enol and keto positon. The keto forms are found to be more stable than their enol form by 4.54 and 1.10 kcal/mol, respectively. And our recent calculated results [35] show that the GSIPT PES for hypocrellin A also have a double minimum at B3LYP/431G level.
Fig. 4. Variation of the O3–H length with RO2–H for DHNQ (A) and DHP (B) in the ground state at the B3LYP/6-31G(d,p) level. The inset figures ˚. show the O3–O4 length with RO2–H. Bond length in A
We also find that the H-bonding O3–H O4 play an important role in stabilizing the keto form. As shown the Fig. 4, the length of the hydrogen bond for DNHQ and DHP is substantially shorter in the keto form ˚ , respectively) than in the enol (RO3–O4 = 2.55 and 2.49 A ˚ , respectively), whereas the form (RO3–O4 = 2.58 and 2.51 A RO3–H in the keto form is also longer than that in the enol ˚ for DNHQ and 1.023 vs. 1.014 A ˚ form (1.018 vs. 0.996 A for DHP, respectively). When rotate the O3–H group of the keto form of DHNQ and DHP, giving birth to a keto form without the H-bonding O3–H O4 (Scheme 1), and optimize this initial structure at B3LYP/6-31G(d,p) level. The optimized structure for DHNQ is not the initial structure, and an enol structure without H-bonding O3–H O4 is obtained. In other words, the hydrogen atom on the atom O1 transfers back to the atom O2 when optimize this molecule (Scheme 1). These results show that the keto form without the H-bonding O3–H O4 is not a stable rotamer for DHNQ and the H-bonding O3–H O4 play a vital role in stabilize the keto form. And the optimized structure for DHP is a keto structure without H-bonding O3–H O4, and is more unstable than the keto form with H-bonding by 19.4 kcal/mol. The potential energy curves of the existed-state for DHNQ and DHP are shown in Fig. 2(e) and (f). PES of DHNQ in the S1 state exhibits a single minimum in the enol keto zone. This curve also appears to have an incipient minimum in the keto zone, with a very low barrier to the enol form (about 0.5 kcal/mol). Thereof, ESIPT cannot happen in this molecule. However, the corresponding ESIPT curves for DHP are consistent with the experimental evidence. The PES in the S1 state exhibits a double minimum in the enol and keto zones, and the keto form is calculated to be more stable than the enol form by 0.2 kcal/mol. And the height of the barrier between the
Scheme 1.
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two minima is 1.70 kcal/mol. These results are consistent with the prediction of Petrich et al. [18,19] that a barrier exists for the intramolecular proton transfer in hypocrellin A which has seven-member ring. 4. Conclusion In this work, PES for ground- and existed-state intramolecular proton transfer of HNA, HNQ, HAQ, 7HIN, DHNQ and DHP have been calculated by using B3LYP and TDB3LYP methods. The main results are as follows: (a) PES for ground-state intramolecular proton transfer of HNA, HNQ, HAQ and 7HIN exhibit a single minimum, however, PES for DHNQ and DHP exhibit a double minimum and the H-bonding O3–H O4 plays a vital role in stabilizing the keto form. (b) PES for HNA show that ESIPT is barrierless, on the other hand, PES for 7HIN and DHP show that ESIPT in these molecules exist a high barrier, but ESIPT for HNQ, HAQ and DHNQ cannot occur because PES of the S1 state exhibit a double minimum but the enol form is stabilized. Acknowledgements This research was supported by the National Natural Science Foundation of China (20173050), Natural Science Foundation of Hunan Province (04JJY40010) and Key Technologies R&D Programme of China (2004BA308A22-1). References [1] Special Issue on Proton Transfer, Chem. Phys. 136 (1989). [2] Special Issue on Proton Transfer, J. Phys. Chem. 95 (1991). [3] J. Keck, M. Roessler, S. Schroeder, G.J. Stueber, F. Waiblinger, M. Stein, D. Legourrie´recm H, E.A. Kramer, H. Hoier, S. Henkel, P. Fischer, H. Port, T. Hirsch, G. Tytz, P. Hayoz, J. Phys. Chem. B 102 (1998) 6975. [4] R.W. Munn, Chem. Br. (1989) 517. [5] G.A. Kraus, W. Zhang, M.J. Fehr, J.W. Petrich, Y. Wannemuenler, S. Carpenter, Chem. Rev. 96 (1996) 523. [6] A. Weller, Z. Elektrochem. 60 (1956) 1144. [7] S. Nagaoka, N. Hirota, M. Sumitani, K. Yoshihara, E. LipczynskaKochany, H. Iwamura, J. Am. Chem. Soc. 106 (1984) 6913. [8] C. Lu, R.M. Hsieh, L.R. Lee, P.Y. Cheng, Chem. Phys. Lett. 310 (1999) 103. [9] J.A. Organero, M. Moreno, L. Santos, J.M. Lluch, A. Douhal, Chem. Phys. Lett. 328 (2000) 83. [10] T.C. Swinney, D.F. Kelley, J. Chem. Phys. 99 (1993) 211.
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