Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 76–80
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Study of fluorescence probe transfer mechanism based on a new type of excited-state intramolecular proton transfer Yumei Dai a,1, Jinfeng Zhao b,1, Yanling Cui b, Qianyu Wang b, Peng Song b,c,⇑, Fengcai Ma b,c,⇑, Yangyang Zhao a a b c
Normal College, Shenyang University, Shenyang 110044, China College of Physics, Liaoning University, Shenyang 110036, China Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials, Liaoning University, Shenyang 110036, 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
The proton transfer mechanism of 3-
HF and its derivatives have been studied theoretically. Analysis of the absorption and fluorescence spectra explains its physical mechanism. The ESIPT processes have been justified to occur in the first excited state.
a r t i c l e
i n f o
Article history: Received 4 November 2014 Received in revised form 3 January 2015 Accepted 19 February 2015 Available online 28 February 2015 Keywords: Fluorescence probe Hydrogen bond Density functional theory Ultraviolet–visible Excited-state intramolecular proton transfer
a b s t r a c t 3-Hydroxyflavone (3-HF) is a typical representative of a new type of fluorescent molecular probe. The intramolecular proton transfer mechanisms of 3-HF and its derivatives have been studied theoretically based on detailed density functional theory. An optical physical cycle diagram of intramolecular proton transfer of 3-HF and its derivatives has been found based on the optimal configuration before and after proton transfer. An analysis of the absorption and fluorescence spectra of these probes explains their optical physical mechanism, which agrees well with experimental results. This correlation indicates that the adopted theory is reasonable and effective. The primary bond lengths, angles and infrared vibrational spectra indicate that the intramolecular hydrogen bonds were strengthened, which is an indication of the excited-state intramolecular proton transfer (ESIPT) processes. The constructed potential energy curves of the ground and first excited state based on these three chromophores provide the ESIPT mechanism, which demonstrates that potential barriers lower than the 6 kcal/mol and justifies the ESIPT processes occur in the first excited state. The fluorescence quenching phenomenon has been explained based on the ESIPT mechanism. Ó 2015 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors at: Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials, Liaoning University, Shenyang 110036, China. Tel.: +86 24 62202306; fax: +86 24 62202304. E-mail addresses: [email protected] (P. Song), [email protected] (F. Ma). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.saa.2015.02.098 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
Proton transfer (PT) is a fundamental photochemistry that occurs everywhere as an elementary process [1,2]. Photo-induced PT processes can be considered to consist of a series of elementary steps, which have been described by the Eigen–Weller mechanism [3,4]. The steps include electronic redistribution upon photo-excitation,
Y. Dai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 76–80
hydrogen bond rearrangement, proton transfer, ion pair formation and diffusion. The intermolecular hydrogen bond, as a site-specific interaction between solute and solvent, plays an important role in photophysics, photochemistry and photobiology. Much attention has been given to investigations of hydrogen bond dynamics since Han and co-workers determined that these bonds were strengthened theoretically in excited states [5–11]. A new mechanism for photoinduced hydrogen bond strengthening or weakening in the excited state was proposed based on the electronic spectra [1–7], which rejected the cleavage mechanism of the hydrogen bond in the excited state reported by Elsaesser and co-workers [12]. Especially, the recent theoretical results predicted two excited state proton transfer mechanisms of 3-hydroxyisoquinoline in cyclohexane and acetic acid, suggesting the different double-proton transfer mechanism from that proposed previously [11]. The effective rule, excited state hydrogen bond strengthening or weakening, was used to prove that the electronic spectral red- or blue-shift was induced by intra- and inter-molecular hydrogen bonding interactions. Many important photochemical phenomena, such as tuning effects, fluorescence quenching and PT, have been explained based on the new mechanism. Because of these recent advances in the excited state, many phenomena involving excited-state hydrogen bonding need to be revisited.
O3
H2 O1
O
77
3-Hydroxyflavone (3-HF) is a natural flavonoid probe that exists in the plant kingdom. Its extensive physiological activity has raised chemists’ interest [13–15]. 3-HF derivatives are well-known fluorophores that possess many favorable optical properties such as good photostabilities, reasonable fluorescent quantum yields and easy synthesis [16–19]. Their widespread use has led to the study of molecular interactions in solution and in biological systems [20– 24]. Bai et al. reported on a probe for detecting palladium species based on 3-HF derivatives [25] and Wang et al. reported on a thiol sensor based on 3-HF derivatives [26]. 3-HF is one of the most extensively studied systems for understanding the mechanism and dynamics of intramolecular PT reactions [27–31]. Woolfe et al. studied spectral quality based on time-resolved fluorescence techniques. The dependence of ESIPT dynamics on solvent and temperature were reported by Itoh and co-workers [27,28]. Barbara and co-workers revealed bi-exponential PT kinetics based on steady-state and time-resolved dynamics, which showed that the 3-HF chromophore exists as an enol with an intramolecular hydrogen bond in the S0 state, and has a keto-tautomer form through intramolecular PT reaction [29]. The effect of solvent on the PT reaction of 3-HF has been investigated by Kasha et al. [30] and subsequently by Strandjord and Barbara in detail [31]. Even though the intramolecular PT of 3-HF has been found to be a typical excited state reaction in previous experiments, limited knowledge is available on the transfer mechanism of 3-HF, and theoretical research on 3-HF and its derivatives is limited. Spectroscopic techniques can only provide indirect information about geometries involved in sensing. Therefore, in this work, we report on a theoretical study concerning 3-HF and its derivatives (enol forms are shown in Fig. 1) to explain the ESIPT phenomenon obtained from experiments. We also focus on the mechanism relating to hydrogen-bonding interactions in the S0 and S1 states. Computational details
(a) O3
In this work, the S0 and S1 state geometric optimizations of 3-HF and its derivatives were performed using density functional theory (DFT) and time-dependent density functional theory (TDDFT), respectively [32–36], without constraint of bonds, angles and dihedral angles. Becke’s three-parameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) and the triple-f valence quality with one set of polarization functions (TZVP) basis set were used. All local minima were confirmed by the absence of an imaginary mode in the vibrational analysis calculations. The S0 and S1 potential energy curves (PESs) of 3-HF and its derivatives were scanned by constrained optimizations and frequency analyses to obtain the thermodynamic corrections in the corresponding electronic state, and by keeping the O–H distance fixed at a series of values. Fine quadrature size 4 grids were used. Harmonic vibrational frequencies in the ground and excited states were determined by diagonalization of the Hessian. The excited-state Hessian was obtained by numerical differentiation of analytical gradients using central differences and default displacements of 0.02 Bohr. Infrared intensities were determined from the gradients of the dipole moment. All electronic calculations were carried out using the Gaussian 09 program suite [37].
H2 O1
O
N
(b) O3
H2 O1
O O
N
Results and discussion Structure optimization
(c) Fig. 1. The enol configurations of 3-HF and its derivatives: (a) 3-HF, (b) FE, (c) FA.
The optimized keto structures of the 3-HF and its derivatives, 40 -dimethylamino-3-hydroxyflavone (FE) and 2-(6-dimethylaminobenzene and [6] furan base)-3-hydroxyflavone (FA) were
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primary structure parameters involved in the intramolecular hydrogen bonds are shown in Table 1. The calculated lengths of O1–H2 and H2 O3 for three enol form chromophores are 0.98 0
0
0
and 1.97 Å A, 0.98 and 1.98 Å A, and 0.98 and 2.04 Å A in the S0 state, respectively. However, after photoexcitation, the O1–H2 bonds 0
were lengthened to 1.01, 1.00 and 0.99 Å A, whereas the H2 O3 0
bonds were shortened to 1.76, 1.80 and 1.92 Å A, respectively. The variable-length O1–H2 bands and variable-short H2 O3 bonds demonstrate that the intramolecular hydrogen bonds were strengthened in the S1 state. The vibrational frequencies of the O–H stretching vibration involved in the hydrogen bonds can provide a clear-cut signature of the hydrogen bonding dynamics [5–11]. The calculated vibrational frequencies of the O–H stretching vibrations are shown in Fig. 3. For the 3-HF-enol in the ground state, the calculated stretching vibrational mode of the O–H group is located at 3539 cm1, whereas the mode in the first excited state is 3049 cm1. The red-shift of 490 cm1 for the O–H group stretching vibrational mode results from the S1 state, from which it can be concluded that the intramolecular hydrogen bond was strengthened. Homoplastically, upon electronic excitation, the O–H stretching band is red-shifted from 3544 cm1 in the ground state to 3194 cm1 in the S1 state for the FE and from 3516 cm1 to 3348 cm1 for the FA chromophore. This indicates that the intramolecular hydrogen bonds were strengthened in the first excited state, which may be an indication of the ESIPT reaction. Ultraviolet–visible spectra and molecular orbital analysis
Fig. 2. The optimized keto configurations of 3-HF and its derivatives: (a) 3-HF, (b) FE, (c) FA.
obtained at the B3LYP function with TZVP basis set level of theory (as shown in Fig. 2), with a subsequent vibrational frequency analysis to ensure the validity of the stationary points. The most
We have calculated the electronic transition energies and the corresponding oscillator strengths of 3-HF, FE and FA based on TDDTF/B3LYP/TZVP according to ground-optimized geometries. The absorbing transitions were predicted in our calculations shown in Fig. 4. Their fluorescence spectra were calculated based on the optimized geometries of the first singlet excited states. The absorption peak of 3-HF was located at 345.7 nm and the emission peak was calculated at 392.8 nm. The bathochromic-shift of 47 nm that corresponds to an absorption peak can be ascribed to
Table 1 0 The calculated primary bond lengths (Å A) and dihedral angles (°) in the S0 and S1 state of 3-HF, FE and FA based on the DFT and TDDFT methods, respectively. Electronic state
3-HF-enol
3-HF-keto
FE-enol
FE-keto
FA-enol
FA-keto
S0
S1
S0
S1
S0
S1
S0
S1
S0
S1
S0
S1
O1–H2
0.98
1.01
2.02
1.91
0.98
1.00
1.99
2.04
0.98
0.99
2.01
2.00
H2–O3 d(O1–H2–O3)
1.97 120°
1.76 127°
0.99 115°
0.99 120°
1.98 120°
1.80 127°
0.99 117°
0.98 116°
2.04 118°
1.92 123°
0.99 117°
0.99 118°
Fig. 3. The calculated IR spectra of 3-HF, FE and FA structures at the spectral region of O–H stretching band based on TDDFT/B3LYP/TZVP theoretical level .
Y. Dai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 76–80
79
Fig. 4. The calculated electronic spectra of 3-HF (a), FE (b) and FA (c) at TDDFT/B3LYP/TZVP level.
the Stokes shift. Similarly, the bathochromic shift of 47 nm for FE and 165 nm for FA are ascribed to the Stokes shift. Calculations of these chromophores have a second emission peak, which is ascribed to the 3-HF, FE and FA keto forms (not shown). The phenomenon of dual fluorescence spectra demonstrates the occurrence of excited-state PT. To investigate the nature of the excited state, the charge distribution in the electronic excited state should be investigated. The frontier molecular orbitals (MOs) of the 3-HF, FE and FA based on TDDTF/B3LYP/TZVP are shown in Fig. 5. Since the S1 states of these molecules correspond to the orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) according to our TDDFT results, only the HOMO and LUMO orbitals are exhibited here. The p character for the HOMO and p⁄ character for the LUMO can be seen clearly, which indicates that the first excited state results from p–p⁄ tran-
sition. It should be noted that the HOMO and LUMO are localized on different parts of the molecules. From the LUMO orbital, we can see the complete occupied the whole molecule, while the outer unit of Hydroxyflavone moiety unoccupied could be found in the HOMO orbital. In other words, the electron density of the Hydroxyflavone group increases after the transition from HOMO to LUMO. In addition, for the part involved in the intramolecular hydrogen bond, O–H O, the electron density of the hydroxide radical moiety increases after the transition from HOMO to LUMO. Therefore, the first excited state involving intramolecular charge transfer should be concluded, and the change in electron density in the hydroxide radical moiety can influence the intramolecular hydrogen bonding O–H O directly. The H O bond length could therefore be shorted upon excitation to the first excited state. The ESIPT process could therefore occur because of intramolecular charge transfer.
Fig. 5. The calculated frontier molecular orbitals HOMO and LUMO for 3-HF (a), FE (b) and FA (c) at TDDFT/B3LYP/TZVP theoretical level.
Fig. 6. Potential energy curves of the ground and the first excited states for 3-HF (a), FE (b) and FA (c) chromophores along with O1–H2 bond length. The inset shows the detailed configurations.
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Potential energy curves To understand the ESIPT mechanism clearly, the ground state and first excited state potential energy curves of the 3-HF, FE and FA chromophores were scanned based on constrained optimizations in their corresponding electronic states at fixed O–H distance for a series of values. Even though the TDDFT/B3LYP calculated level may not be expected to be sufficiently accurate to surmount the correct ordering of the closely spaced excited states, previous research indicates that this method may be reliable regarding the shape of the hydrogen-transfer potential energy curves [38–46]. The potential energy curves with only the variable parameter of 0
O1–H2 bond length from approximately 0.9 to 2.4 Å A among the enol 0
and keto form geometries in steps of 0.05 Å A are shown in Fig. 6. These curves can provide qualitative energetic pathways description for the ESIPT process. The energy of the ground state increases with O–H bond relaxation from an optimized length of 0
approximately 1.0 Å A and the potential barriers are above 10 kcal/ mol. PT processes therefore occur with difficulty in the ground state. The first excited state potential energy curves exhibit a barrier of 1.99, 4.39 and 5.57 kcal/mol between the reagent (from the vertical transition at the geometry of the minimum in the ground state) and the product. This indicates that the PT can easily take place for 3-HF by undergoing a relatively low barrier of 1.99 kcal/ mol. While for FE and FA chromophores, proton transfers along with O1–H2 bond should overcome the larger barriers of 4.39 and 5.57 kcal/mol, respectively. Moreover, the ESIPT processes are therefore more likely to proceed in the first excited state than in the ground state. Keto⁄ forms were found with corresponding O–H bond lengths (Fig. 6). The keto⁄ form decays to the ground state keto form following with radiating fluorescence. Up to now, the ESIPT processes for these chromophores can be explained as: after photo-excitation, the intramolecular hydrogen bonds can form in the first excited-state, following which the hydrogen atoms of the hydroxyl group are removed based on the neighboring O atom forming keto⁄ forms. Radiating fluorescence then occurs with the keto⁄ form decaying to the ground state, which provides a possible explanation for the fluorescence quenching. Conclusion The ESIPT mechanisms of 3-HF, FE and FA chromophores were investigated theoretically using TDDFT. The intramolecular hydrogen bonds of these three chromophores were strengthened based on the primary bond lengths, angles and infrared vibrational spectra involved in the hydrogen bonds, which indicates an ESIPT reaction. The phenomenon of dual fluorescence spectra demonstrates the occurrence of excited-state PT. Corresponding frontier molecular orbitals were analyzed and indicate that the ESIPT process could occur because of intramolecular charge transfer. Based on an analysis of the potential energy curves for 3-HF, FE and FA chromophores, almost no PT occurs in the ground state. A corresponding low barrier (less than 6 kcal/mol) between enol⁄ and keto⁄ forms in the first excited state vitrified ESIPT processes occurs for these three chromophores. The phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Acknowledgements This work was supported by the National Science Foundation of China (Grant Nos. 11304135, 11374353 and 11274149), the Liaoning Natural Science Foundation of China (2013004003), the Program for Liaoning Excellent Talents in University, China
(LR2012001). F.C. Ma is indebted to the support from the Program of Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials. Peng Song acknowledges financial support from the Undergraduate Training Programs for Innovation and Entrepreneurship of Liaoning Province. References [1] B.J. Siwick, H.J. Bakker, J. Am. Chem. Soc. 129 (2007) 13412. [2] M.T. Sun, Z.L. Zhang, P.J. Wang, Q. Li, F.C. Ma, H.X. Xu, Light: Sci. Appl. 2 (2013) e112. [3] A. Weller, Prog. React. Kinet. 1 (1961) 187. [4] M. Eigen, W. Kruse, G. Maass, L. De Maeyer, Prog. React. Kinet. 2 (1964) 285. [5] G.J. Zhao, K.L. Han, J. Phys. Chem. A 111 (2007) 2469. [6] M.T. Sun, J. Chem. Phys. 124 (2007) 054903. [7] G.J. Zhao, B.H. Northrop, P.J. Stang, K.L. Han, J. Phys. Chem. A 114 (2010) 3418. [8] M.T. Sun, Z.L. Zhang, H.R. Zheng, H.X. Xu, Sci. Rep. 2 (2012) 647. [9] G.J. Zhao, J.Y. Liu, L.C. Zhou, K.L. Han, J. Phys. Chem. B 111 (2007) 8940. [10] Y.Z. Huang, Y.R. Fang, Z.L. Zhang, L. Zhu, M.T. Sun, Light: Sci. Appl. 3 (2014) e199. [11] J.F. Zhao, J.S. Chen, Y.L. Cui, J. Wang, L.X. Xia, Y.M. Dai, P. Song, F.C. Ma, Phys. Chem. Chem. Phys. 17 (2015) 1142. [12] C. Chudoba, E.T.J. Nibbering, T. Elsaesser, Phys. Rev. Lett. 81 (1998) 3010. [13] M.T. Sun, Z.L. Zhang, Z.H. kim, H.R. Zheng, H.X. Xu, Chem. Eur. J. 19 (2013) 14958. [14] S.I. Tanaka, J. Miyazaki, D.K. Tiwari, T. Jin, Y. Inouye, Angew. Chem. Int. Ed. 50 (2011) 431. [15] V.I. Tomin, R. Jaworski, Opt. Spectrosc. 109 (2010) 279. [16] V.I. Tomin, R. Javorski, Opt. Spectrosc. 103 (2007) 952. [17] R. Casadesus, O. Vendrell, M. Moreno, J.M. Lluch, K. Morokuma, Chem. Phys. 325 (2006) 243. [18] A.J.G. Strandjord, S.H. Courtney, D.M. Friedrich, P.F. Barbara, J. Phys. Chem. 87 (1983) 1125. [19] M. Buffa, S. Carturan, A. Quaranta, G. Maggioni, G.D. Mea, Opt. Mater. 34 (2012) 1219. [20] K. Chevalier, M.M.N. Wolf, A. Funk, M. Andres, M. Gerhards, R. Diller, Phys. Chem. Chem. Phys. 14 (2012) 15007. [21] J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, Chem. Soc. Rev. 40 (2011) 3483. [22] V.F. Valuk, G. Duportail, V.G. Pivovarenko, J. Photochem. Photobiol., A 175 (2005) 226. [23] V.I. Tomin, R. Javorski, Opt. Spectrosc. 109 (2010) 279. [24] R. Villamil-Ramos, V. Barba, A.K. Yatsimirsky, Analyst. 137 (2012) 5229. [25] B. Liu, H. Wang, T. Wang, Y. Bao, F. Du, J. Tian, Q. Li, R. Bai, Chem. Commun. 48 (2012) 2867. [26] M. Lan, J. Wu, W. Liu, H. Zhang, W. Zhang, X. Zhuang, P. Wang, Sens. Actuators, B 156 (2011) 332. [27] G.J. Woolfe, P.J. Thistlethwaite, J. Am. Chem. Soc. 103 (1981) 6916. [28] M. Itoh, K. Tokumura, Y. Tanimoto, Y. Okada, H. Takeuchi, K. Obi, I. Tanaka, J. Am. Chem. Soc. 104 (1982) 4146. [29] A.J.G. Strandjord, S.H. Courtney, D.M. Friedrich, P.F. Barbara, J. Phys. Chem. 87 (1983) 1125. [30] D. Mcmorrow, M. Kasha, J. Am. Chem. Soc. 105 (1983) 5133. [31] A.J.G. Strandjord, D.E. Smith, P.F. Barbara, J. Phys. Chem. 89 (1985) 2362. [32] F. Furche, R. Ahlrichs, J. Chem. Phys. 117 (2002) 7433. [33] J.L. Whitten, J. Chem. Phys. 58 (1973) 4496. [34] A. Schafer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. [35] G.J. Zhao, K.L. Han, Acc. Chem. Res. 45 (2011) 404. [36] R. Wu, P. Nachtigall, B. Brutschy, Phys. Chem. Chem. Phys. 6 (2004) 515. [37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Scalmani, G. Cheeseman, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford, CT, 2009. [38] D. Yang, Y. Yang, Y. Liu, Commun. Comput. Chem. 1 (2013) 205–215. [39] J.F. Zhao, P. Song, Y.L. Cui, X.M. Liu, S.W. Sun, S.Y. Hou, F.C. Ma, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 131 (2014) 282–287. [40] G.Y. Liu, Y.H. Li, H. Zhang, G.H. Cui, Commun. Comput. Chem. 1 (2013) 88–98. [41] D.D. Wang, G.J. Zhao, Commun. Comput. Chem. 1 (2013) 181–190. [42] M.Z. Zhang, B.P. Ren, Y. Wang, C.X. Zhao, Commun. Comput. Chem. 1 (2013) 216–224. [43] Y. Liu, S.C. Lan, Commun. Comput. Chem. 1 (2013) 235–243. [44] L. Serrano-Andres, M. Merchan, J. Photochem. Photobiol. C. Photochem. Rev. 10 (2009) 21–32. [45] Y. Saga, Y. Shibata, H. Tamiaki, J. Photochem. Photobiol. C. Photochem. Rev. 11 (2010) 15–24. [46] A.L. Sobolewski, W. Domcke, Phys. Chem. Chem. Phys. 1 (1999) 3065–3072.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Study of fluorescence probe transfer mechanism based on a new type of excited-state intramolecular proton transfer Yumei Dai a,1, Jinfeng Zhao b,1, Yanling Cui b, Qianyu Wang b, Peng Song b,c,⇑, Fengcai Ma b,c,⇑, Yangyang Zhao a a b c
Normal College, Shenyang University, Shenyang 110044, China College of Physics, Liaoning University, Shenyang 110036, China Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials, Liaoning University, Shenyang 110036, 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
The proton transfer mechanism of 3-
HF and its derivatives have been studied theoretically. Analysis of the absorption and fluorescence spectra explains its physical mechanism. The ESIPT processes have been justified to occur in the first excited state.
a r t i c l e
i n f o
Article history: Received 4 November 2014 Received in revised form 3 January 2015 Accepted 19 February 2015 Available online 28 February 2015 Keywords: Fluorescence probe Hydrogen bond Density functional theory Ultraviolet–visible Excited-state intramolecular proton transfer
a b s t r a c t 3-Hydroxyflavone (3-HF) is a typical representative of a new type of fluorescent molecular probe. The intramolecular proton transfer mechanisms of 3-HF and its derivatives have been studied theoretically based on detailed density functional theory. An optical physical cycle diagram of intramolecular proton transfer of 3-HF and its derivatives has been found based on the optimal configuration before and after proton transfer. An analysis of the absorption and fluorescence spectra of these probes explains their optical physical mechanism, which agrees well with experimental results. This correlation indicates that the adopted theory is reasonable and effective. The primary bond lengths, angles and infrared vibrational spectra indicate that the intramolecular hydrogen bonds were strengthened, which is an indication of the excited-state intramolecular proton transfer (ESIPT) processes. The constructed potential energy curves of the ground and first excited state based on these three chromophores provide the ESIPT mechanism, which demonstrates that potential barriers lower than the 6 kcal/mol and justifies the ESIPT processes occur in the first excited state. The fluorescence quenching phenomenon has been explained based on the ESIPT mechanism. Ó 2015 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors at: Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials, Liaoning University, Shenyang 110036, China. Tel.: +86 24 62202306; fax: +86 24 62202304. E-mail addresses: [email protected] (P. Song), [email protected] (F. Ma). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.saa.2015.02.098 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
Proton transfer (PT) is a fundamental photochemistry that occurs everywhere as an elementary process [1,2]. Photo-induced PT processes can be considered to consist of a series of elementary steps, which have been described by the Eigen–Weller mechanism [3,4]. The steps include electronic redistribution upon photo-excitation,
Y. Dai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 76–80
hydrogen bond rearrangement, proton transfer, ion pair formation and diffusion. The intermolecular hydrogen bond, as a site-specific interaction between solute and solvent, plays an important role in photophysics, photochemistry and photobiology. Much attention has been given to investigations of hydrogen bond dynamics since Han and co-workers determined that these bonds were strengthened theoretically in excited states [5–11]. A new mechanism for photoinduced hydrogen bond strengthening or weakening in the excited state was proposed based on the electronic spectra [1–7], which rejected the cleavage mechanism of the hydrogen bond in the excited state reported by Elsaesser and co-workers [12]. Especially, the recent theoretical results predicted two excited state proton transfer mechanisms of 3-hydroxyisoquinoline in cyclohexane and acetic acid, suggesting the different double-proton transfer mechanism from that proposed previously [11]. The effective rule, excited state hydrogen bond strengthening or weakening, was used to prove that the electronic spectral red- or blue-shift was induced by intra- and inter-molecular hydrogen bonding interactions. Many important photochemical phenomena, such as tuning effects, fluorescence quenching and PT, have been explained based on the new mechanism. Because of these recent advances in the excited state, many phenomena involving excited-state hydrogen bonding need to be revisited.
O3
H2 O1
O
77
3-Hydroxyflavone (3-HF) is a natural flavonoid probe that exists in the plant kingdom. Its extensive physiological activity has raised chemists’ interest [13–15]. 3-HF derivatives are well-known fluorophores that possess many favorable optical properties such as good photostabilities, reasonable fluorescent quantum yields and easy synthesis [16–19]. Their widespread use has led to the study of molecular interactions in solution and in biological systems [20– 24]. Bai et al. reported on a probe for detecting palladium species based on 3-HF derivatives [25] and Wang et al. reported on a thiol sensor based on 3-HF derivatives [26]. 3-HF is one of the most extensively studied systems for understanding the mechanism and dynamics of intramolecular PT reactions [27–31]. Woolfe et al. studied spectral quality based on time-resolved fluorescence techniques. The dependence of ESIPT dynamics on solvent and temperature were reported by Itoh and co-workers [27,28]. Barbara and co-workers revealed bi-exponential PT kinetics based on steady-state and time-resolved dynamics, which showed that the 3-HF chromophore exists as an enol with an intramolecular hydrogen bond in the S0 state, and has a keto-tautomer form through intramolecular PT reaction [29]. The effect of solvent on the PT reaction of 3-HF has been investigated by Kasha et al. [30] and subsequently by Strandjord and Barbara in detail [31]. Even though the intramolecular PT of 3-HF has been found to be a typical excited state reaction in previous experiments, limited knowledge is available on the transfer mechanism of 3-HF, and theoretical research on 3-HF and its derivatives is limited. Spectroscopic techniques can only provide indirect information about geometries involved in sensing. Therefore, in this work, we report on a theoretical study concerning 3-HF and its derivatives (enol forms are shown in Fig. 1) to explain the ESIPT phenomenon obtained from experiments. We also focus on the mechanism relating to hydrogen-bonding interactions in the S0 and S1 states. Computational details
(a) O3
In this work, the S0 and S1 state geometric optimizations of 3-HF and its derivatives were performed using density functional theory (DFT) and time-dependent density functional theory (TDDFT), respectively [32–36], without constraint of bonds, angles and dihedral angles. Becke’s three-parameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) and the triple-f valence quality with one set of polarization functions (TZVP) basis set were used. All local minima were confirmed by the absence of an imaginary mode in the vibrational analysis calculations. The S0 and S1 potential energy curves (PESs) of 3-HF and its derivatives were scanned by constrained optimizations and frequency analyses to obtain the thermodynamic corrections in the corresponding electronic state, and by keeping the O–H distance fixed at a series of values. Fine quadrature size 4 grids were used. Harmonic vibrational frequencies in the ground and excited states were determined by diagonalization of the Hessian. The excited-state Hessian was obtained by numerical differentiation of analytical gradients using central differences and default displacements of 0.02 Bohr. Infrared intensities were determined from the gradients of the dipole moment. All electronic calculations were carried out using the Gaussian 09 program suite [37].
H2 O1
O
N
(b) O3
H2 O1
O O
N
Results and discussion Structure optimization
(c) Fig. 1. The enol configurations of 3-HF and its derivatives: (a) 3-HF, (b) FE, (c) FA.
The optimized keto structures of the 3-HF and its derivatives, 40 -dimethylamino-3-hydroxyflavone (FE) and 2-(6-dimethylaminobenzene and [6] furan base)-3-hydroxyflavone (FA) were
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primary structure parameters involved in the intramolecular hydrogen bonds are shown in Table 1. The calculated lengths of O1–H2 and H2 O3 for three enol form chromophores are 0.98 0
0
0
and 1.97 Å A, 0.98 and 1.98 Å A, and 0.98 and 2.04 Å A in the S0 state, respectively. However, after photoexcitation, the O1–H2 bonds 0
were lengthened to 1.01, 1.00 and 0.99 Å A, whereas the H2 O3 0
bonds were shortened to 1.76, 1.80 and 1.92 Å A, respectively. The variable-length O1–H2 bands and variable-short H2 O3 bonds demonstrate that the intramolecular hydrogen bonds were strengthened in the S1 state. The vibrational frequencies of the O–H stretching vibration involved in the hydrogen bonds can provide a clear-cut signature of the hydrogen bonding dynamics [5–11]. The calculated vibrational frequencies of the O–H stretching vibrations are shown in Fig. 3. For the 3-HF-enol in the ground state, the calculated stretching vibrational mode of the O–H group is located at 3539 cm1, whereas the mode in the first excited state is 3049 cm1. The red-shift of 490 cm1 for the O–H group stretching vibrational mode results from the S1 state, from which it can be concluded that the intramolecular hydrogen bond was strengthened. Homoplastically, upon electronic excitation, the O–H stretching band is red-shifted from 3544 cm1 in the ground state to 3194 cm1 in the S1 state for the FE and from 3516 cm1 to 3348 cm1 for the FA chromophore. This indicates that the intramolecular hydrogen bonds were strengthened in the first excited state, which may be an indication of the ESIPT reaction. Ultraviolet–visible spectra and molecular orbital analysis
Fig. 2. The optimized keto configurations of 3-HF and its derivatives: (a) 3-HF, (b) FE, (c) FA.
obtained at the B3LYP function with TZVP basis set level of theory (as shown in Fig. 2), with a subsequent vibrational frequency analysis to ensure the validity of the stationary points. The most
We have calculated the electronic transition energies and the corresponding oscillator strengths of 3-HF, FE and FA based on TDDTF/B3LYP/TZVP according to ground-optimized geometries. The absorbing transitions were predicted in our calculations shown in Fig. 4. Their fluorescence spectra were calculated based on the optimized geometries of the first singlet excited states. The absorption peak of 3-HF was located at 345.7 nm and the emission peak was calculated at 392.8 nm. The bathochromic-shift of 47 nm that corresponds to an absorption peak can be ascribed to
Table 1 0 The calculated primary bond lengths (Å A) and dihedral angles (°) in the S0 and S1 state of 3-HF, FE and FA based on the DFT and TDDFT methods, respectively. Electronic state
3-HF-enol
3-HF-keto
FE-enol
FE-keto
FA-enol
FA-keto
S0
S1
S0
S1
S0
S1
S0
S1
S0
S1
S0
S1
O1–H2
0.98
1.01
2.02
1.91
0.98
1.00
1.99
2.04
0.98
0.99
2.01
2.00
H2–O3 d(O1–H2–O3)
1.97 120°
1.76 127°
0.99 115°
0.99 120°
1.98 120°
1.80 127°
0.99 117°
0.98 116°
2.04 118°
1.92 123°
0.99 117°
0.99 118°
Fig. 3. The calculated IR spectra of 3-HF, FE and FA structures at the spectral region of O–H stretching band based on TDDFT/B3LYP/TZVP theoretical level .
Y. Dai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 76–80
79
Fig. 4. The calculated electronic spectra of 3-HF (a), FE (b) and FA (c) at TDDFT/B3LYP/TZVP level.
the Stokes shift. Similarly, the bathochromic shift of 47 nm for FE and 165 nm for FA are ascribed to the Stokes shift. Calculations of these chromophores have a second emission peak, which is ascribed to the 3-HF, FE and FA keto forms (not shown). The phenomenon of dual fluorescence spectra demonstrates the occurrence of excited-state PT. To investigate the nature of the excited state, the charge distribution in the electronic excited state should be investigated. The frontier molecular orbitals (MOs) of the 3-HF, FE and FA based on TDDTF/B3LYP/TZVP are shown in Fig. 5. Since the S1 states of these molecules correspond to the orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) according to our TDDFT results, only the HOMO and LUMO orbitals are exhibited here. The p character for the HOMO and p⁄ character for the LUMO can be seen clearly, which indicates that the first excited state results from p–p⁄ tran-
sition. It should be noted that the HOMO and LUMO are localized on different parts of the molecules. From the LUMO orbital, we can see the complete occupied the whole molecule, while the outer unit of Hydroxyflavone moiety unoccupied could be found in the HOMO orbital. In other words, the electron density of the Hydroxyflavone group increases after the transition from HOMO to LUMO. In addition, for the part involved in the intramolecular hydrogen bond, O–H O, the electron density of the hydroxide radical moiety increases after the transition from HOMO to LUMO. Therefore, the first excited state involving intramolecular charge transfer should be concluded, and the change in electron density in the hydroxide radical moiety can influence the intramolecular hydrogen bonding O–H O directly. The H O bond length could therefore be shorted upon excitation to the first excited state. The ESIPT process could therefore occur because of intramolecular charge transfer.
Fig. 5. The calculated frontier molecular orbitals HOMO and LUMO for 3-HF (a), FE (b) and FA (c) at TDDFT/B3LYP/TZVP theoretical level.
Fig. 6. Potential energy curves of the ground and the first excited states for 3-HF (a), FE (b) and FA (c) chromophores along with O1–H2 bond length. The inset shows the detailed configurations.
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Potential energy curves To understand the ESIPT mechanism clearly, the ground state and first excited state potential energy curves of the 3-HF, FE and FA chromophores were scanned based on constrained optimizations in their corresponding electronic states at fixed O–H distance for a series of values. Even though the TDDFT/B3LYP calculated level may not be expected to be sufficiently accurate to surmount the correct ordering of the closely spaced excited states, previous research indicates that this method may be reliable regarding the shape of the hydrogen-transfer potential energy curves [38–46]. The potential energy curves with only the variable parameter of 0
O1–H2 bond length from approximately 0.9 to 2.4 Å A among the enol 0
and keto form geometries in steps of 0.05 Å A are shown in Fig. 6. These curves can provide qualitative energetic pathways description for the ESIPT process. The energy of the ground state increases with O–H bond relaxation from an optimized length of 0
approximately 1.0 Å A and the potential barriers are above 10 kcal/ mol. PT processes therefore occur with difficulty in the ground state. The first excited state potential energy curves exhibit a barrier of 1.99, 4.39 and 5.57 kcal/mol between the reagent (from the vertical transition at the geometry of the minimum in the ground state) and the product. This indicates that the PT can easily take place for 3-HF by undergoing a relatively low barrier of 1.99 kcal/ mol. While for FE and FA chromophores, proton transfers along with O1–H2 bond should overcome the larger barriers of 4.39 and 5.57 kcal/mol, respectively. Moreover, the ESIPT processes are therefore more likely to proceed in the first excited state than in the ground state. Keto⁄ forms were found with corresponding O–H bond lengths (Fig. 6). The keto⁄ form decays to the ground state keto form following with radiating fluorescence. Up to now, the ESIPT processes for these chromophores can be explained as: after photo-excitation, the intramolecular hydrogen bonds can form in the first excited-state, following which the hydrogen atoms of the hydroxyl group are removed based on the neighboring O atom forming keto⁄ forms. Radiating fluorescence then occurs with the keto⁄ form decaying to the ground state, which provides a possible explanation for the fluorescence quenching. Conclusion The ESIPT mechanisms of 3-HF, FE and FA chromophores were investigated theoretically using TDDFT. The intramolecular hydrogen bonds of these three chromophores were strengthened based on the primary bond lengths, angles and infrared vibrational spectra involved in the hydrogen bonds, which indicates an ESIPT reaction. The phenomenon of dual fluorescence spectra demonstrates the occurrence of excited-state PT. Corresponding frontier molecular orbitals were analyzed and indicate that the ESIPT process could occur because of intramolecular charge transfer. Based on an analysis of the potential energy curves for 3-HF, FE and FA chromophores, almost no PT occurs in the ground state. A corresponding low barrier (less than 6 kcal/mol) between enol⁄ and keto⁄ forms in the first excited state vitrified ESIPT processes occurs for these three chromophores. The phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Acknowledgements This work was supported by the National Science Foundation of China (Grant Nos. 11304135, 11374353 and 11274149), the Liaoning Natural Science Foundation of China (2013004003), the Program for Liaoning Excellent Talents in University, China
(LR2012001). F.C. Ma is indebted to the support from the Program of Liaoning Key Laboratory of Semiconductor Light Emitting and Photocatalytic Materials. Peng Song acknowledges financial support from the Undergraduate Training Programs for Innovation and Entrepreneurship of Liaoning Province. References [1] B.J. Siwick, H.J. Bakker, J. Am. Chem. Soc. 129 (2007) 13412. [2] M.T. Sun, Z.L. Zhang, P.J. Wang, Q. Li, F.C. Ma, H.X. Xu, Light: Sci. Appl. 2 (2013) e112. [3] A. Weller, Prog. React. Kinet. 1 (1961) 187. [4] M. Eigen, W. Kruse, G. Maass, L. De Maeyer, Prog. React. Kinet. 2 (1964) 285. [5] G.J. Zhao, K.L. Han, J. Phys. Chem. A 111 (2007) 2469. [6] M.T. Sun, J. Chem. Phys. 124 (2007) 054903. [7] G.J. Zhao, B.H. Northrop, P.J. Stang, K.L. Han, J. Phys. Chem. A 114 (2010) 3418. [8] M.T. Sun, Z.L. Zhang, H.R. Zheng, H.X. Xu, Sci. Rep. 2 (2012) 647. [9] G.J. Zhao, J.Y. Liu, L.C. Zhou, K.L. Han, J. Phys. Chem. B 111 (2007) 8940. [10] Y.Z. Huang, Y.R. Fang, Z.L. Zhang, L. Zhu, M.T. Sun, Light: Sci. Appl. 3 (2014) e199. [11] J.F. Zhao, J.S. Chen, Y.L. Cui, J. Wang, L.X. Xia, Y.M. Dai, P. Song, F.C. Ma, Phys. Chem. Chem. Phys. 17 (2015) 1142. [12] C. Chudoba, E.T.J. Nibbering, T. Elsaesser, Phys. Rev. Lett. 81 (1998) 3010. [13] M.T. Sun, Z.L. Zhang, Z.H. kim, H.R. Zheng, H.X. Xu, Chem. Eur. J. 19 (2013) 14958. [14] S.I. Tanaka, J. Miyazaki, D.K. Tiwari, T. Jin, Y. Inouye, Angew. Chem. Int. Ed. 50 (2011) 431. [15] V.I. Tomin, R. Jaworski, Opt. Spectrosc. 109 (2010) 279. [16] V.I. Tomin, R. Javorski, Opt. Spectrosc. 103 (2007) 952. [17] R. Casadesus, O. Vendrell, M. Moreno, J.M. Lluch, K. Morokuma, Chem. Phys. 325 (2006) 243. [18] A.J.G. Strandjord, S.H. Courtney, D.M. Friedrich, P.F. Barbara, J. Phys. Chem. 87 (1983) 1125. [19] M. Buffa, S. Carturan, A. Quaranta, G. Maggioni, G.D. Mea, Opt. Mater. 34 (2012) 1219. [20] K. Chevalier, M.M.N. Wolf, A. Funk, M. Andres, M. Gerhards, R. Diller, Phys. Chem. Chem. Phys. 14 (2012) 15007. [21] J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, Chem. Soc. Rev. 40 (2011) 3483. [22] V.F. Valuk, G. Duportail, V.G. Pivovarenko, J. Photochem. Photobiol., A 175 (2005) 226. [23] V.I. Tomin, R. Javorski, Opt. Spectrosc. 109 (2010) 279. [24] R. Villamil-Ramos, V. Barba, A.K. Yatsimirsky, Analyst. 137 (2012) 5229. [25] B. Liu, H. Wang, T. Wang, Y. Bao, F. Du, J. Tian, Q. Li, R. Bai, Chem. Commun. 48 (2012) 2867. [26] M. Lan, J. Wu, W. Liu, H. Zhang, W. Zhang, X. Zhuang, P. Wang, Sens. Actuators, B 156 (2011) 332. [27] G.J. Woolfe, P.J. Thistlethwaite, J. Am. Chem. Soc. 103 (1981) 6916. [28] M. Itoh, K. Tokumura, Y. Tanimoto, Y. Okada, H. Takeuchi, K. Obi, I. Tanaka, J. Am. Chem. Soc. 104 (1982) 4146. [29] A.J.G. Strandjord, S.H. Courtney, D.M. Friedrich, P.F. Barbara, J. Phys. Chem. 87 (1983) 1125. [30] D. Mcmorrow, M. Kasha, J. Am. Chem. Soc. 105 (1983) 5133. [31] A.J.G. Strandjord, D.E. Smith, P.F. Barbara, J. Phys. Chem. 89 (1985) 2362. [32] F. Furche, R. Ahlrichs, J. Chem. Phys. 117 (2002) 7433. [33] J.L. Whitten, J. Chem. Phys. 58 (1973) 4496. [34] A. Schafer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. [35] G.J. Zhao, K.L. Han, Acc. Chem. Res. 45 (2011) 404. [36] R. Wu, P. Nachtigall, B. Brutschy, Phys. Chem. Chem. Phys. 6 (2004) 515. [37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Scalmani, G. Cheeseman, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford, CT, 2009. [38] D. Yang, Y. Yang, Y. Liu, Commun. Comput. Chem. 1 (2013) 205–215. [39] J.F. Zhao, P. Song, Y.L. Cui, X.M. Liu, S.W. Sun, S.Y. Hou, F.C. Ma, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 131 (2014) 282–287. [40] G.Y. Liu, Y.H. Li, H. Zhang, G.H. Cui, Commun. Comput. Chem. 1 (2013) 88–98. [41] D.D. Wang, G.J. Zhao, Commun. Comput. Chem. 1 (2013) 181–190. [42] M.Z. Zhang, B.P. Ren, Y. Wang, C.X. Zhao, Commun. Comput. Chem. 1 (2013) 216–224. [43] Y. Liu, S.C. Lan, Commun. Comput. Chem. 1 (2013) 235–243. [44] L. Serrano-Andres, M. Merchan, J. Photochem. Photobiol. C. Photochem. Rev. 10 (2009) 21–32. [45] Y. Saga, Y. Shibata, H. Tamiaki, J. Photochem. Photobiol. C. Photochem. Rev. 11 (2010) 15–24. [46] A.L. Sobolewski, W. Domcke, Phys. Chem. Chem. Phys. 1 (1999) 3065–3072.
