Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 282–287
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Effects of hydrogen bond on 2-aminopyridine and its derivatives complexes in methanol solvent Jinfeng Zhao a,b, Peng Song a,⇑, Yanling Cui a, Xuemei Liu a, Shaowu Sun a, Siyao Hou a, Fengcai Ma a,⇑ a b
Department of Physics and Chemistry, Liaoning University, Shenyang 110036, PR China State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR 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
Intermolecular hydrogen bond is
strengthened in excited state. Multiple hydrogen bond enhances the
hydrogen bond binding energy. Three kinds of binding sites and seven
kinds of binding modes are reported.
a r t i c l e
i n f o
Article history: Received 2 March 2014 Received in revised form 14 April 2014 Accepted 22 April 2014 Available online 30 April 2014 Keywords: Frontier molecular orbitals Electronically excited state Infrared spectra Fluorescence spectrum Intermolecular hydrogen bond
a b s t r a c t In the present work, the time-dependent density functional theory (TD-DFT) method was adopted to investigate the excited state hydrogen-bond dynamics of 2-aminopyridine monomer (2AP) and its derivatives in hydrogen donating methanol solvent. The calculated steady-state absorption and fluorescence spectra agree well with the experimental results. Theoretical results state that the bond lengths of both O–H and N–H bands are lengthened, while the intermolecular hydrogen bond lengths are shortened in the excited state. Further, the intermolecular hydrogen bonds are proved to be strengthened according to the calculated binding energy. As a reasonable explanation, the hydrogen bonds binding energy increases with multiple hydrogen-bonding interactions in the electronically excited state. In addition, the hydrogen bonding dynamics in the excited state were visualized by the spectral shifts of vibrational modes. The calculated infrared spectra of both O–H and N–H stretching vibrational regions revealed that the O–H and N–H stretching bands red-shift. Ó 2014 Elsevier B.V. All rights reserved.
Introduction The excited state relaxation has not only been recognized for its importance in physics, chemistry, and biology, but also it is of paramount importance in the photochemical process [1–7]. Relatively important part of it can be induced through intermolecular ⇑ Corresponding authors. Tel.: +86 24 62202306; fax: +86 24 62202304. E-mail addresses: [email protected] (P. Song), [email protected] (F. Ma). http://dx.doi.org/10.1016/j.saa.2014.04.116 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
hydrogen bond. The excited-state dynamics of hydrogen-bonded complexes occurs on ultrafast time scales mainly set by vibrational motions of the hydrogen donor and acceptor groups. A number of studies have been studied about the vibrational motions of hydrogen-bonded excited-state dynamics by Han and co-workers [8– 10]. Indeed, the solute–solvent interactions play a significant role for molecular nonequilibrium processes in liquids, and many experimental researches of it have been tested. Subsequently,
J. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 282–287
283
Fig. 1. The calculated absorption and fluorescence spectrum of the 2AP and its derivatives in methanol.
many a theoretical methods have been adopted to explain the phenomenon obtained from experiments [11–17]. 2-Aminopyridine (2AP) is a heterocyclic molecule containing two nitrogen atoms, and it can be used as a model molecule for studying the photo-physics of biomolecules. Further, its dimer has been deliberated from the viewpoint of a nucleobase pair [18]. Several studies on clusters of 2AP with methanol, H2O, ethanol, and threshold photoionization spectroscopy have been reported by Hager and Wallace [19,20]. The resonant two-photon ionization spectrum of the 2AP dimer near by the electronic origin of the S0 to S1 transition and the red shift rationalized by a strengthening of the dimer bonds in electronic excited state have been reported by them. However, multiple hydrogen-bonding interactions and the influence bringing from multiple intermolecular hydrogen bonds in methanol are known fewer. In this study, we have investigated the 2AP and its derivatives complexes adopting the density functional theory (DFT) method for optimizing ground state and TD-DFT method for the calculations of the excited state minimum geometries, IR spectrum, and steady-state absorption and fluorescence spectra in methanol. Three kinds of binding sites and seven kinds of binding modes have been considered from the optimized geometries, and further we focused our attention on the changes of intermolecular hydrogen bonds among these 2AP derivatives complexes. Computational details In the present work, the ground state and electronically excited state geometric optimizations were performed using the density
Fig. 2. Frontier molecular orbitals (MOs) of the hydrogen-bonded 2AP7 complex.
functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, respectively [21–24]. Becke’s threeparameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) and 6-311+G (d) basis set were used throughout. There were no constraints to all the atoms, bonds, angles or dihedral angles during the geometric optimization. To evaluate the solvent effect, methanol was used as solvent in the calculations depended on the model of Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM). All the local minima were confirmed by the absence of an imaginary mode in vibrational analysis calculations. Fine quadrature grids of size 4 were also employed. Harmonic vibrational frequencies in ground state and the excited state 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. The infrared intensities were determined from the gradients of the dipole moment. All the electronic calculations were carried out depending on the Gaussian 09 program suite [25].
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J. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 282–287
2AP
2AP1
2AP4
2AP2
2AP5
2AP3
2AP6
2AP7
Fig. 3. Optimized geometric structures of the isolated 2AP and its derivatives.
Table 1 The calculated lengths of the hydrogen-bonded groups LO–H and LH–N (Å) in ground state and electronic excited state.
GS ES a b
2AP1
2AP2
2AP3
2AP4
LO–H 0.984 0.987
LH–Na 1.019 1.028
LH–Nb 1.017 1.027
LH–Na 1.011 1.022
2AP5 LH–Nb 1.016 1.026
LO–H 0.983 0.985
2AP6 LH–Na 1.017 1.027
LO–H 0.986 0.988
2AP7 LH–Nb 1.017 1.027
LO–H 0.993 0.996
LH–Na 1.017 1.030
LH–Nb 1.015 1.026
LO H 1.954 1.837
LH N 1.807 1.782
The bond length between nitrogen and the hydrogen in the NH2 group which is close to the aromatic nitrogen. The hydrogen pointing away from the aromatic nitrogen.
Table 2 The calculated lengths of the hydrogen bond LO H and LH N (Å) in ground state and electronic excited state.
GS ES a
2AP1
2AP2
2AP3
2AP4
LH N 1.867 1.864
LO H 1.988 1.835
LO H 1.946 1.812
LO Ha 2.433 2.004
2AP5 LO H 1.940 1.819
LO H 1.996 1.852
2AP6 LH N 1.884 1.858
LO H 1.944 1.817
2AP7 LH N 1.852 1.843
LO Ha 1.995 1.843
The hydrogen bond O H–N which is close to the aromatic nitrogen.
Table 3 Calculated hydrogen bond binding energies (kJ/mol) of the hydrogen-bonded 2AP derivatives in ground state and electronic excited state.
GS ES
2AP1
2AP2
2AP3
2AP4
2AP5
2AP6
2AP7
24.43 31.74
9.86 21.30
13.83 24.03
38.98 55.14
34.96 45.54
39.28 55.31
71.78 92.53
Results and discussion Steady-state electronic spectra Although the steady-state spectral characters of 2AP have been deliberated in the previous works [18,26,27], three kinds of binding sites and seven kinds of hydrogen bond combination for 2AP in methanol have been given here for the first time. The calculated steady-state absorption and fluorescence spectra of 2AP and its derivatives in methanol have been shown in Fig. 1. To distinctly observe the shapes of the absorption and fluorescence spectra, the calculated spectrum from 200 to 350 nm and 200 to 450 nm have been shown in Fig. 1, respectively. It should be noted that the absorption spectra shown in Fig. 1(A) has two peaks at the spectral range of about 220–240 nm and 260–290 nm. It is obvious that the absorption peaks are red-shifted due to the formation of
intermolecular hydrogen bonds for both the S1 and S2 states [28]. Furthermore, the red-shift of different levels also manifests that the intermolecular hydrogen bonding interaction among 2AP derivatives plays a dominant role on the steady-state and the dynamic spectral properties of the 2-aminopyridine chromophore in methanol solvent. The calculated S1 absorption peak for the monomer is located at 270 nm, which is in consistent with the experimental value 287 nm [28]. It is confirmed that 2AP and its derivatives complexes in methanol presented here can be regarded as a good model to simulate the solute-solvent interaction. Also, the calculated value of fluorescence spectra for the isolated 2AP and its derivatives in methanol are shown in Fig. 1(B). The excitation band of higher intensity is located at 239 nm for the isolated 2AP monomer, which is consistent with the experimental result 230 nm [28]. However, they are 241, 243, 245, 246, 243, 246 and 249 nm for derivatives from 2AP1 to 2AP7. The intermolecular hydrogen bond between derivatives of 2AP and methanol can induce fluorescence peak shifts to red. The excitation band of lower intensity is situated at about 310.6 nm, which is also in accordance with the value 322 nm of experiment [29]. At the same time, it is still demonstrated that the calculated excited states of isolated 2AP and the hydrogen-bonded derivatives–methanol complexes using the TD-DFT method can delineate the excited states of the non-hydrogen-bonded and hydrogen-bonded forms in solution well further.
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285
Fig. 4. Calculated IR spectra of hydrogen-bonded 2AP and its derivatives at the spectral region of O–H stretching band, vertical line denotes the corresponding peak in experiment.
For each 2AP hydrogen-bonded derivatives–methanol complexes, the first excited singlet state has pp*-type character from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Here, we only take 2AP7 for example. As mentioned above, the TD-DFT calculations revealed that the S1 state is dominated by the HOMO–LUMO transition in 2AP7 molecule. So only the HOMO and LUMO orbitals are exhibited here. The shapes of the Frontier molecular orbitals of it are shown in Fig. 2. Note that the HOMO and LUMO are localized on different parts. From the HOMO orbital, the whole occupied amino moiety can be seen, whereas the only nitrogen atom occupied could be found in the LUMO orbital. In other words, the electron density of the amino group decreases after the transition from HOMO to LUMO. Therefore, the S1 state involving intramolecular charge transfer should be concluded. The change of electron density in the NH2 group can directly influence the intermolecular hydrogen bonding between amino moiety and H2O molecular.
2AP1
2AP5
Fig. 6. Calculated IR spectra of hydrogen-bonded 2AP derivatives at the spectral region of N–H stretching band.
2AP6
2AP7
Fig. 5. Assignment of the vibrational modes (both displacement vector and dipole derivative unit vector) of hydrogen-bonded 2AP and its derivatives at the spectral region of O–H stretching band.
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Geometric structures of hydrogen-bonded complexes In Fig. 3, the geometric structures of the hydrogen-bonded fully formations of the 2AP and its derivatives in methanol in ground state are shown. According to the calculations, the angle of the monomer between the plane through the amino group and the ring plane is 32.5°, which is in agreement with the 32° measured by Kydd and Mills [30]. Furthermore, the distance H N of the two equivalent hydrogen bonds is 2.04 Å, and the distance of N– H N is 3.06 Å (not shown), which is consistent with the experimental value (3.07 Å), as measured for the dimer in a crystal [31]. While, we just want to discuss the three kinds of binding sites and seven kinds of combinations for hydrogen bond, which is between 2AP monomer and methanol. From Table 1, the calculated lengths of the H-bond, O–H and H– N for 2AP derivatives are shown in ground state and electronically excited state, respectively. For 2AP monomer, the length of hydrogen-bonded group H–N is 1.009 Å that is close to the aromatic nitrogen, and another length of H–N is 1.008 Å in ground state. One can be noted that when the hydrogen bond is formed between O element of methanol and H–N of amino-group of 2AP derivatives in ground state, the length of H–N can be lengthened than 2AP monomer shown in Table 1. The variable-length of H–N is obviously due to the formation of intermolecular hydrogen bonds. From optimized excited state geometric structure of isolated 2AP (not showed), it can be found that the H–N bond length is increased from 1.008 to 1.011 Å. However, in electronically excited state, the H-N bond continues to be lengthened for derivatives of 2AP, which may be also due to the formation of intermolecular hydrogen bond O H–N. In addition, it is to be noted that the hydrogen bond length of O H–N is shortened corresponding to ground state. On the other hand, we have also deliberated hydrogen-bonded group O–H depending on DFT for ground state and TDDFT for excited state. Similarly, we find O–H bond is lengthened and H-bond length of O–H N is shorted in excited state shown in Table 2, all of which demonstrate that the binding energy of hydrogen bond can be increased in excited state. In order to account for it further, we calculated hydrogen bond binding energies of 2AP derivatives in ground state and electronic excited state presented in Table 3. One should be noted that along with the increase in the number of hydrogen bond formations, the hydrogen bond binding energy is increased. And the gap between ground and excited state is increased in the wake of the increase
in the number of the formation of hydrogen bonds. Therefore, from the discussed above, we can conclude that not only the energy of H-bond could be strengthened by the formation of intermolecular hydrogen bond in excited state, but also the more hydrogen bonds form, the gap of hydrogen bond binding energy become greater between ground state and excited state possibly. Infrared spectra of ground and excited states The geometric optimizations of excited state for the isolated monomer and its derivatives have also been performed depending on the TDDFT method. Because of the vibrational frequencies of the stretching vibrations of N–H and O-H groups involved in hydrogen bonds can provide a clear dynamic of the hydrogen-bonding [32], the infrared spectra both in ground state and the excited state are calculated here. In excited states the calculation of the IR spectra is difficult and time-consuming [2,33]. The calculated IR spectra of O–H stretching vibration in different derivatives at the spectral range from 3000 to 4000 cm 1 are shown in Fig. 4. For the 2AP1 in ground state, the calculated stretching vibrational mode of the O– H group is 3384 cm 1, while the mode in electronically excited state is 3309 cm 1, which is in good accordance with the experimental value 3305 cm 1 [34]. Moreover, one can be noted that the stretching vibrational mode of the O–H group is red-shifted by 75 cm 1 in the excited state of the 2AP1 compared with that in ground state owing to the formation of the intermolecular hydrogen bond. In order to see the conditions of the vibration much clearly, the vibrational modes (both displacement vector and dipole derivative unit vector) of hydrogen-bonded 2AP derivatives at the spectral region of O–H stretching band have been shown by us in Fig. 5. The analogous red-shift of the O–H stretching vibrational mode appear on 2AP5, 2AP6 and 2AP7 from 3379, 3325 and 3187 cm 1 in ground state to 3346, 3277 and 3139 cm 1 in excited state because of the intermolecular hydrogen bond N H–O, respectively. Furthermore, the red-shift of the O–H stretching vibrational mode means that the O–H bond is lengthened upon electronic excitation. The changes of the intermolecular hydrogen bond formed by N– H N can be monitored by the spectral shift of the stretching vibrational mode of the N–H group. We also calculated the stretching vibrational mode of the N–H in the case of N–H N shown in Fig. 6. Through the calculation of isolated 2AP monomer, the free N–H stretching mode is 3571 cm 1, which agrees with experimen-
2AP2
2AP3
2AP4
2AP5
2AP6
2AP7
Fig. 7. Assignment of the vibrational modes (both displacement vector and dipole derivative unit vector) of hydrogen-bonded 2AP derivatives at the spectral region of N–H stretching band.
J. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 282–287
tal data 3542 cm 1 [34], and the mode is 3568 cm 1 in excited state (not shown). However, taking the 2AP2 for example, the stretching mode is 3455 cm 1 and 3295 cm 1, respectively. It should be paid attention to that the formation of intermolecular hydrogen bond for 2AP2 induces a large red-shift of the N–H stretching frequency in the electronic excited state 273 cm 1 than that in ground state 112 cm 1. Similarly, from the 2AP derivatives shown in Fig. 6, the N–H stretching frequency of 2AP5 and 2AP6 in excited state relative to ground state can induce 148 cm 1 and 153 cm 1 of red-shift, respectively. Analogously, the stretching frequency for 2AP4 and 2AP7 are 153 cm 1 and 201 cm 1. Furthermore, Fig. 7 reveals the vibrational modes of hydrogen-bonded 2AP derivatives at the spectral region of N–H stretching band. Since the stretching mode can undergo a shift to lower frequency for stronger hydrogen-bonding interaction, the large spectral red-shift discussed above suggests that the intermolecular hydrogen bond N–H N is strengthened in the electronic excited state of 2AP derivatives. Conclusion In summary, the excited state of hydrogen-bonded 2AP and its derivatives complexes in methanol depending on TDDFT method have been studied carefully. The steady state absorption and fluorescence spectrum have been calculated, and they are all in agreement with experiments. Further, we can conclude that the TDDFT method can be used to decipher the experiment reasonably. From the calculated bond lengths, it has been founded that both the O–H and N–H bonds are lengthened in the excited state of 2AP derivatives complexes, while the length of hydrogen bond is shortened. Through calculating hydrogen bond binding energy, we could find that the intermolecular hydrogen bonds are strengthened in the electronically excited state. Furthermore, we notice that maybe the more hydrogen bonds form, the gap of hydrogen bond binding energy become greater between ground and excited state. On the other hand, depending on spectral shifts of some vibrational modes involving in the formation of hydrogen bond, we can monitor hydrogen-bonding dynamics. The infrared spectrums were calculated at both O–H and N–H stretching vibrational regions in the ground state and excited state. It is very important to note that the large red-shifts of both the O–H and the N–H stretching bands were founded in the excited state due to the formation of intermolecular hydrogen bond. Therefore, it is clearly manifested that the intermolecular hydrogen bond is strengthened in excited state. Acknowledgements This work was financially supported by the Program of Shenyang Key Laboratory of Optoelectronic Materials and Technology
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(Grant No. F12-254-1-00), the National Natural Science Foundation of China (Grant Nos. 11274149, 11374353 and11304135). Xuemei Liu acknowledges financial support from the Undergraduate Training Programs for Innovation and Entrepreneurship of Liaoning University. References [1] D. Laage, I. Burghardt, T. Sommerfeld, J.T. Hynes, Chem. Phys. Chem. 4 (2003) 61–66. [2] M.T. Sun, J. Chem. Phys. 124 (2006) 054903. [3] D.P. Yang, Y.G. Yang, Y.F. Liu, Commun. Comput. Chem. 1 (2013) 205. [4] S.G. Ramesh, S. Re, J.T. Hynes, J. Phys. Chem. A 112 (2008) 3391. [5] V. Ramamurthy, N.J. Turro, Chem. Rev. 93 (1993) 1. [6] G.R. Fleming, M.H. Cho, Annu, Rev. Phys. Chem. 47 (1996) 109. [7] J. Waluk, Acc. Chem. Res. 36 (2003) 832. [8] G.J. Zhao, K.L. Han, ChemPhysChem 9 (2008) 1842. [9] D.P. Yang, Y.G. Yang, Y.F. Liu, J. Chin. Chem. 60 (2013) 618. [10] G.J. Zhao, K.L. Han, J. Phys. Chem. A 113 (2009) 14329. [11] M.T. Sun, Z.L. Zhang, P.J. Wang, Q. Li, F.C. Ma, H.X. Xu, Light: Sci. Appl. 68 (2013) 10. [12] F. Tschirschwitz, E.T. Nibbering, J. Chem. Phys. Lett. 312 (1999) 169. [13] E.T.J. Nibbering, F. Tschirschwitz, C. Chudoba, T. Elsaesser, J. Phys. Chem. A 104 (2000) 4236. [14] E.T.J. Nibbering, T. Elsaesser, Appl. Phys. B 71 (2000) 439. [15] M. Rini, A. Kummrow, J. Dreyer, E.T.J. Nibbering, T. Elsaesser, Faraday Discuss. 122 (2002) 27. [16] E.T.J. Nibbering, H. Fidder, E. Pines, Annu. Rev. Phys. Chem. 56 (2005) 337. [17] P. Hamm, M. Lim, R.M. Hochstrasser, Phys. Rev. Lett. 81 (1998) 5326. [18] R. Wu, B. Brutschy, J. Phys. Chem. A 108 (2004) 9715–9720. [19] J.W. Hager, S.C. Wallace, J. Phys. Chem. 89 (1985) 3833. [20] J.W. Hager, G.W.L. Leach, D.R. Demmer, S.C. Wallace, J. Phys. Chem. 91 (1987) 3750. [21] F. Furche, R. Ahlrichs, J. Chem. Phys. 117 (2002) 7433. [22] J.L. Whitten, J. Chem. Phys. 58 (1973) 4496. [23] A. Schafer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. [24] G.J. Zhao, K.L. Han, Acc. Chem. Res. 45 (2011) 404–413. [25] 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. [26] R. Wu, P. Nachtigall, B. Brutschy, Phys. Chem. Chem. Phys. 6 (2004) 515. [27] R. Wu, S. Vaupel, P. Nachtigall, B. Brutschy, J. Phys. Chem. A 108 (2004) 3338. [28] D.L. Wilson, D.R. Wirz, G.H. Schenk, Anal. Chem. 8 (1973) 45. [29] G.J. Zhao, K.L. Han, Phys. Chem. Chem. Phys. 11 (2009) 4385–4390. [30] R.A. Kydd, I.M. Mills, J. Mol. Spectrosc. 42 (1972) 320. [31] M. Chao, E. Schempp, R. Rosenstein, D. Acta Crystallogr. Sect. B 31 (1975) 2922. [32] A.L. Sobolewski, W. Domcke, J. Phys. Chem. A 108 (2004) 10917. [33] C.A. Southern, D.H. Levy, G.M. Florio, A.T. Lougarte, S. Zwier, J. Phys. Chem. A 107 (2003) 4032. [34] Y.J. Yamada, H. Ohba, Y. Noboru, S. Daicho, Y. Nibu⁄, J. Phys. Chem. A 116 (2012) 9271–9278.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Effects of hydrogen bond on 2-aminopyridine and its derivatives complexes in methanol solvent Jinfeng Zhao a,b, Peng Song a,⇑, Yanling Cui a, Xuemei Liu a, Shaowu Sun a, Siyao Hou a, Fengcai Ma a,⇑ a b
Department of Physics and Chemistry, Liaoning University, Shenyang 110036, PR China State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR 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
Intermolecular hydrogen bond is
strengthened in excited state. Multiple hydrogen bond enhances the
hydrogen bond binding energy. Three kinds of binding sites and seven
kinds of binding modes are reported.
a r t i c l e
i n f o
Article history: Received 2 March 2014 Received in revised form 14 April 2014 Accepted 22 April 2014 Available online 30 April 2014 Keywords: Frontier molecular orbitals Electronically excited state Infrared spectra Fluorescence spectrum Intermolecular hydrogen bond
a b s t r a c t In the present work, the time-dependent density functional theory (TD-DFT) method was adopted to investigate the excited state hydrogen-bond dynamics of 2-aminopyridine monomer (2AP) and its derivatives in hydrogen donating methanol solvent. The calculated steady-state absorption and fluorescence spectra agree well with the experimental results. Theoretical results state that the bond lengths of both O–H and N–H bands are lengthened, while the intermolecular hydrogen bond lengths are shortened in the excited state. Further, the intermolecular hydrogen bonds are proved to be strengthened according to the calculated binding energy. As a reasonable explanation, the hydrogen bonds binding energy increases with multiple hydrogen-bonding interactions in the electronically excited state. In addition, the hydrogen bonding dynamics in the excited state were visualized by the spectral shifts of vibrational modes. The calculated infrared spectra of both O–H and N–H stretching vibrational regions revealed that the O–H and N–H stretching bands red-shift. Ó 2014 Elsevier B.V. All rights reserved.
Introduction The excited state relaxation has not only been recognized for its importance in physics, chemistry, and biology, but also it is of paramount importance in the photochemical process [1–7]. Relatively important part of it can be induced through intermolecular ⇑ Corresponding authors. Tel.: +86 24 62202306; fax: +86 24 62202304. E-mail addresses: [email protected] (P. Song), [email protected] (F. Ma). http://dx.doi.org/10.1016/j.saa.2014.04.116 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
hydrogen bond. The excited-state dynamics of hydrogen-bonded complexes occurs on ultrafast time scales mainly set by vibrational motions of the hydrogen donor and acceptor groups. A number of studies have been studied about the vibrational motions of hydrogen-bonded excited-state dynamics by Han and co-workers [8– 10]. Indeed, the solute–solvent interactions play a significant role for molecular nonequilibrium processes in liquids, and many experimental researches of it have been tested. Subsequently,
J. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 282–287
283
Fig. 1. The calculated absorption and fluorescence spectrum of the 2AP and its derivatives in methanol.
many a theoretical methods have been adopted to explain the phenomenon obtained from experiments [11–17]. 2-Aminopyridine (2AP) is a heterocyclic molecule containing two nitrogen atoms, and it can be used as a model molecule for studying the photo-physics of biomolecules. Further, its dimer has been deliberated from the viewpoint of a nucleobase pair [18]. Several studies on clusters of 2AP with methanol, H2O, ethanol, and threshold photoionization spectroscopy have been reported by Hager and Wallace [19,20]. The resonant two-photon ionization spectrum of the 2AP dimer near by the electronic origin of the S0 to S1 transition and the red shift rationalized by a strengthening of the dimer bonds in electronic excited state have been reported by them. However, multiple hydrogen-bonding interactions and the influence bringing from multiple intermolecular hydrogen bonds in methanol are known fewer. In this study, we have investigated the 2AP and its derivatives complexes adopting the density functional theory (DFT) method for optimizing ground state and TD-DFT method for the calculations of the excited state minimum geometries, IR spectrum, and steady-state absorption and fluorescence spectra in methanol. Three kinds of binding sites and seven kinds of binding modes have been considered from the optimized geometries, and further we focused our attention on the changes of intermolecular hydrogen bonds among these 2AP derivatives complexes. Computational details In the present work, the ground state and electronically excited state geometric optimizations were performed using the density
Fig. 2. Frontier molecular orbitals (MOs) of the hydrogen-bonded 2AP7 complex.
functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, respectively [21–24]. Becke’s threeparameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) and 6-311+G (d) basis set were used throughout. There were no constraints to all the atoms, bonds, angles or dihedral angles during the geometric optimization. To evaluate the solvent effect, methanol was used as solvent in the calculations depended on the model of Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM). All the local minima were confirmed by the absence of an imaginary mode in vibrational analysis calculations. Fine quadrature grids of size 4 were also employed. Harmonic vibrational frequencies in ground state and the excited state 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. The infrared intensities were determined from the gradients of the dipole moment. All the electronic calculations were carried out depending on the Gaussian 09 program suite [25].
284
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2AP
2AP1
2AP4
2AP2
2AP5
2AP3
2AP6
2AP7
Fig. 3. Optimized geometric structures of the isolated 2AP and its derivatives.
Table 1 The calculated lengths of the hydrogen-bonded groups LO–H and LH–N (Å) in ground state and electronic excited state.
GS ES a b
2AP1
2AP2
2AP3
2AP4
LO–H 0.984 0.987
LH–Na 1.019 1.028
LH–Nb 1.017 1.027
LH–Na 1.011 1.022
2AP5 LH–Nb 1.016 1.026
LO–H 0.983 0.985
2AP6 LH–Na 1.017 1.027
LO–H 0.986 0.988
2AP7 LH–Nb 1.017 1.027
LO–H 0.993 0.996
LH–Na 1.017 1.030
LH–Nb 1.015 1.026
LO H 1.954 1.837
LH N 1.807 1.782
The bond length between nitrogen and the hydrogen in the NH2 group which is close to the aromatic nitrogen. The hydrogen pointing away from the aromatic nitrogen.
Table 2 The calculated lengths of the hydrogen bond LO H and LH N (Å) in ground state and electronic excited state.
GS ES a
2AP1
2AP2
2AP3
2AP4
LH N 1.867 1.864
LO H 1.988 1.835
LO H 1.946 1.812
LO Ha 2.433 2.004
2AP5 LO H 1.940 1.819
LO H 1.996 1.852
2AP6 LH N 1.884 1.858
LO H 1.944 1.817
2AP7 LH N 1.852 1.843
LO Ha 1.995 1.843
The hydrogen bond O H–N which is close to the aromatic nitrogen.
Table 3 Calculated hydrogen bond binding energies (kJ/mol) of the hydrogen-bonded 2AP derivatives in ground state and electronic excited state.
GS ES
2AP1
2AP2
2AP3
2AP4
2AP5
2AP6
2AP7
24.43 31.74
9.86 21.30
13.83 24.03
38.98 55.14
34.96 45.54
39.28 55.31
71.78 92.53
Results and discussion Steady-state electronic spectra Although the steady-state spectral characters of 2AP have been deliberated in the previous works [18,26,27], three kinds of binding sites and seven kinds of hydrogen bond combination for 2AP in methanol have been given here for the first time. The calculated steady-state absorption and fluorescence spectra of 2AP and its derivatives in methanol have been shown in Fig. 1. To distinctly observe the shapes of the absorption and fluorescence spectra, the calculated spectrum from 200 to 350 nm and 200 to 450 nm have been shown in Fig. 1, respectively. It should be noted that the absorption spectra shown in Fig. 1(A) has two peaks at the spectral range of about 220–240 nm and 260–290 nm. It is obvious that the absorption peaks are red-shifted due to the formation of
intermolecular hydrogen bonds for both the S1 and S2 states [28]. Furthermore, the red-shift of different levels also manifests that the intermolecular hydrogen bonding interaction among 2AP derivatives plays a dominant role on the steady-state and the dynamic spectral properties of the 2-aminopyridine chromophore in methanol solvent. The calculated S1 absorption peak for the monomer is located at 270 nm, which is in consistent with the experimental value 287 nm [28]. It is confirmed that 2AP and its derivatives complexes in methanol presented here can be regarded as a good model to simulate the solute-solvent interaction. Also, the calculated value of fluorescence spectra for the isolated 2AP and its derivatives in methanol are shown in Fig. 1(B). The excitation band of higher intensity is located at 239 nm for the isolated 2AP monomer, which is consistent with the experimental result 230 nm [28]. However, they are 241, 243, 245, 246, 243, 246 and 249 nm for derivatives from 2AP1 to 2AP7. The intermolecular hydrogen bond between derivatives of 2AP and methanol can induce fluorescence peak shifts to red. The excitation band of lower intensity is situated at about 310.6 nm, which is also in accordance with the value 322 nm of experiment [29]. At the same time, it is still demonstrated that the calculated excited states of isolated 2AP and the hydrogen-bonded derivatives–methanol complexes using the TD-DFT method can delineate the excited states of the non-hydrogen-bonded and hydrogen-bonded forms in solution well further.
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Fig. 4. Calculated IR spectra of hydrogen-bonded 2AP and its derivatives at the spectral region of O–H stretching band, vertical line denotes the corresponding peak in experiment.
For each 2AP hydrogen-bonded derivatives–methanol complexes, the first excited singlet state has pp*-type character from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Here, we only take 2AP7 for example. As mentioned above, the TD-DFT calculations revealed that the S1 state is dominated by the HOMO–LUMO transition in 2AP7 molecule. So only the HOMO and LUMO orbitals are exhibited here. The shapes of the Frontier molecular orbitals of it are shown in Fig. 2. Note that the HOMO and LUMO are localized on different parts. From the HOMO orbital, the whole occupied amino moiety can be seen, whereas the only nitrogen atom occupied could be found in the LUMO orbital. In other words, the electron density of the amino group decreases after the transition from HOMO to LUMO. Therefore, the S1 state involving intramolecular charge transfer should be concluded. The change of electron density in the NH2 group can directly influence the intermolecular hydrogen bonding between amino moiety and H2O molecular.
2AP1
2AP5
Fig. 6. Calculated IR spectra of hydrogen-bonded 2AP derivatives at the spectral region of N–H stretching band.
2AP6
2AP7
Fig. 5. Assignment of the vibrational modes (both displacement vector and dipole derivative unit vector) of hydrogen-bonded 2AP and its derivatives at the spectral region of O–H stretching band.
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Geometric structures of hydrogen-bonded complexes In Fig. 3, the geometric structures of the hydrogen-bonded fully formations of the 2AP and its derivatives in methanol in ground state are shown. According to the calculations, the angle of the monomer between the plane through the amino group and the ring plane is 32.5°, which is in agreement with the 32° measured by Kydd and Mills [30]. Furthermore, the distance H N of the two equivalent hydrogen bonds is 2.04 Å, and the distance of N– H N is 3.06 Å (not shown), which is consistent with the experimental value (3.07 Å), as measured for the dimer in a crystal [31]. While, we just want to discuss the three kinds of binding sites and seven kinds of combinations for hydrogen bond, which is between 2AP monomer and methanol. From Table 1, the calculated lengths of the H-bond, O–H and H– N for 2AP derivatives are shown in ground state and electronically excited state, respectively. For 2AP monomer, the length of hydrogen-bonded group H–N is 1.009 Å that is close to the aromatic nitrogen, and another length of H–N is 1.008 Å in ground state. One can be noted that when the hydrogen bond is formed between O element of methanol and H–N of amino-group of 2AP derivatives in ground state, the length of H–N can be lengthened than 2AP monomer shown in Table 1. The variable-length of H–N is obviously due to the formation of intermolecular hydrogen bonds. From optimized excited state geometric structure of isolated 2AP (not showed), it can be found that the H–N bond length is increased from 1.008 to 1.011 Å. However, in electronically excited state, the H-N bond continues to be lengthened for derivatives of 2AP, which may be also due to the formation of intermolecular hydrogen bond O H–N. In addition, it is to be noted that the hydrogen bond length of O H–N is shortened corresponding to ground state. On the other hand, we have also deliberated hydrogen-bonded group O–H depending on DFT for ground state and TDDFT for excited state. Similarly, we find O–H bond is lengthened and H-bond length of O–H N is shorted in excited state shown in Table 2, all of which demonstrate that the binding energy of hydrogen bond can be increased in excited state. In order to account for it further, we calculated hydrogen bond binding energies of 2AP derivatives in ground state and electronic excited state presented in Table 3. One should be noted that along with the increase in the number of hydrogen bond formations, the hydrogen bond binding energy is increased. And the gap between ground and excited state is increased in the wake of the increase
in the number of the formation of hydrogen bonds. Therefore, from the discussed above, we can conclude that not only the energy of H-bond could be strengthened by the formation of intermolecular hydrogen bond in excited state, but also the more hydrogen bonds form, the gap of hydrogen bond binding energy become greater between ground state and excited state possibly. Infrared spectra of ground and excited states The geometric optimizations of excited state for the isolated monomer and its derivatives have also been performed depending on the TDDFT method. Because of the vibrational frequencies of the stretching vibrations of N–H and O-H groups involved in hydrogen bonds can provide a clear dynamic of the hydrogen-bonding [32], the infrared spectra both in ground state and the excited state are calculated here. In excited states the calculation of the IR spectra is difficult and time-consuming [2,33]. The calculated IR spectra of O–H stretching vibration in different derivatives at the spectral range from 3000 to 4000 cm 1 are shown in Fig. 4. For the 2AP1 in ground state, the calculated stretching vibrational mode of the O– H group is 3384 cm 1, while the mode in electronically excited state is 3309 cm 1, which is in good accordance with the experimental value 3305 cm 1 [34]. Moreover, one can be noted that the stretching vibrational mode of the O–H group is red-shifted by 75 cm 1 in the excited state of the 2AP1 compared with that in ground state owing to the formation of the intermolecular hydrogen bond. In order to see the conditions of the vibration much clearly, the vibrational modes (both displacement vector and dipole derivative unit vector) of hydrogen-bonded 2AP derivatives at the spectral region of O–H stretching band have been shown by us in Fig. 5. The analogous red-shift of the O–H stretching vibrational mode appear on 2AP5, 2AP6 and 2AP7 from 3379, 3325 and 3187 cm 1 in ground state to 3346, 3277 and 3139 cm 1 in excited state because of the intermolecular hydrogen bond N H–O, respectively. Furthermore, the red-shift of the O–H stretching vibrational mode means that the O–H bond is lengthened upon electronic excitation. The changes of the intermolecular hydrogen bond formed by N– H N can be monitored by the spectral shift of the stretching vibrational mode of the N–H group. We also calculated the stretching vibrational mode of the N–H in the case of N–H N shown in Fig. 6. Through the calculation of isolated 2AP monomer, the free N–H stretching mode is 3571 cm 1, which agrees with experimen-
2AP2
2AP3
2AP4
2AP5
2AP6
2AP7
Fig. 7. Assignment of the vibrational modes (both displacement vector and dipole derivative unit vector) of hydrogen-bonded 2AP derivatives at the spectral region of N–H stretching band.
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tal data 3542 cm 1 [34], and the mode is 3568 cm 1 in excited state (not shown). However, taking the 2AP2 for example, the stretching mode is 3455 cm 1 and 3295 cm 1, respectively. It should be paid attention to that the formation of intermolecular hydrogen bond for 2AP2 induces a large red-shift of the N–H stretching frequency in the electronic excited state 273 cm 1 than that in ground state 112 cm 1. Similarly, from the 2AP derivatives shown in Fig. 6, the N–H stretching frequency of 2AP5 and 2AP6 in excited state relative to ground state can induce 148 cm 1 and 153 cm 1 of red-shift, respectively. Analogously, the stretching frequency for 2AP4 and 2AP7 are 153 cm 1 and 201 cm 1. Furthermore, Fig. 7 reveals the vibrational modes of hydrogen-bonded 2AP derivatives at the spectral region of N–H stretching band. Since the stretching mode can undergo a shift to lower frequency for stronger hydrogen-bonding interaction, the large spectral red-shift discussed above suggests that the intermolecular hydrogen bond N–H N is strengthened in the electronic excited state of 2AP derivatives. Conclusion In summary, the excited state of hydrogen-bonded 2AP and its derivatives complexes in methanol depending on TDDFT method have been studied carefully. The steady state absorption and fluorescence spectrum have been calculated, and they are all in agreement with experiments. Further, we can conclude that the TDDFT method can be used to decipher the experiment reasonably. From the calculated bond lengths, it has been founded that both the O–H and N–H bonds are lengthened in the excited state of 2AP derivatives complexes, while the length of hydrogen bond is shortened. Through calculating hydrogen bond binding energy, we could find that the intermolecular hydrogen bonds are strengthened in the electronically excited state. Furthermore, we notice that maybe the more hydrogen bonds form, the gap of hydrogen bond binding energy become greater between ground and excited state. On the other hand, depending on spectral shifts of some vibrational modes involving in the formation of hydrogen bond, we can monitor hydrogen-bonding dynamics. The infrared spectrums were calculated at both O–H and N–H stretching vibrational regions in the ground state and excited state. It is very important to note that the large red-shifts of both the O–H and the N–H stretching bands were founded in the excited state due to the formation of intermolecular hydrogen bond. Therefore, it is clearly manifested that the intermolecular hydrogen bond is strengthened in excited state. Acknowledgements This work was financially supported by the Program of Shenyang Key Laboratory of Optoelectronic Materials and Technology
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(Grant No. F12-254-1-00), the National Natural Science Foundation of China (Grant Nos. 11274149, 11374353 and11304135). Xuemei Liu acknowledges financial support from the Undergraduate Training Programs for Innovation and Entrepreneurship of Liaoning University. References [1] D. Laage, I. Burghardt, T. Sommerfeld, J.T. Hynes, Chem. Phys. Chem. 4 (2003) 61–66. [2] M.T. Sun, J. Chem. Phys. 124 (2006) 054903. [3] D.P. Yang, Y.G. Yang, Y.F. Liu, Commun. Comput. Chem. 1 (2013) 205. [4] S.G. Ramesh, S. Re, J.T. Hynes, J. Phys. Chem. A 112 (2008) 3391. [5] V. Ramamurthy, N.J. Turro, Chem. Rev. 93 (1993) 1. [6] G.R. Fleming, M.H. Cho, Annu, Rev. Phys. Chem. 47 (1996) 109. [7] J. Waluk, Acc. Chem. Res. 36 (2003) 832. [8] G.J. Zhao, K.L. Han, ChemPhysChem 9 (2008) 1842. [9] D.P. Yang, Y.G. Yang, Y.F. Liu, J. Chin. Chem. 60 (2013) 618. [10] G.J. Zhao, K.L. Han, J. Phys. Chem. A 113 (2009) 14329. [11] M.T. Sun, Z.L. Zhang, P.J. Wang, Q. Li, F.C. Ma, H.X. Xu, Light: Sci. Appl. 68 (2013) 10. [12] F. Tschirschwitz, E.T. Nibbering, J. Chem. Phys. Lett. 312 (1999) 169. [13] E.T.J. Nibbering, F. Tschirschwitz, C. Chudoba, T. Elsaesser, J. Phys. Chem. A 104 (2000) 4236. [14] E.T.J. Nibbering, T. Elsaesser, Appl. Phys. B 71 (2000) 439. [15] M. Rini, A. Kummrow, J. Dreyer, E.T.J. Nibbering, T. Elsaesser, Faraday Discuss. 122 (2002) 27. [16] E.T.J. Nibbering, H. Fidder, E. Pines, Annu. Rev. Phys. Chem. 56 (2005) 337. [17] P. Hamm, M. Lim, R.M. Hochstrasser, Phys. Rev. Lett. 81 (1998) 5326. [18] R. Wu, B. Brutschy, J. Phys. Chem. A 108 (2004) 9715–9720. [19] J.W. Hager, S.C. Wallace, J. Phys. Chem. 89 (1985) 3833. [20] J.W. Hager, G.W.L. Leach, D.R. Demmer, S.C. Wallace, J. Phys. Chem. 91 (1987) 3750. [21] F. Furche, R. Ahlrichs, J. Chem. Phys. 117 (2002) 7433. [22] J.L. Whitten, J. Chem. Phys. 58 (1973) 4496. [23] A. Schafer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. [24] G.J. Zhao, K.L. Han, Acc. Chem. Res. 45 (2011) 404–413. [25] 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. [26] R. Wu, P. Nachtigall, B. Brutschy, Phys. Chem. Chem. Phys. 6 (2004) 515. [27] R. Wu, S. Vaupel, P. Nachtigall, B. Brutschy, J. Phys. Chem. A 108 (2004) 3338. [28] D.L. Wilson, D.R. Wirz, G.H. Schenk, Anal. Chem. 8 (1973) 45. [29] G.J. Zhao, K.L. Han, Phys. Chem. Chem. Phys. 11 (2009) 4385–4390. [30] R.A. Kydd, I.M. Mills, J. Mol. Spectrosc. 42 (1972) 320. [31] M. Chao, E. Schempp, R. Rosenstein, D. Acta Crystallogr. Sect. B 31 (1975) 2922. [32] A.L. Sobolewski, W. Domcke, J. Phys. Chem. A 108 (2004) 10917. [33] C.A. Southern, D.H. Levy, G.M. Florio, A.T. Lougarte, S. Zwier, J. Phys. Chem. A 107 (2003) 4032. [34] Y.J. Yamada, H. Ohba, Y. Noboru, S. Daicho, Y. Nibu⁄, J. Phys. Chem. A 116 (2012) 9271–9278.
