- Email: [email protected]
Theoretical study on the electronic structures and intramolecular charge transfer of naamine A, naamidine A and naamidine G
Chemical Physics Letters 501 (2011) 534–539
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Theoretical study on the electronic structures and intramolecular charge transfer of naamine A, naamidine A and naamidine G Kai Jiang a,⇑, Yonggang Yang a,b, Yufang Liu b, Deheng Shi b, Jinfeng Sun b a b
Department of Chemistry, Henan Normal University, Xinxiang 453007, China Department of Physics, Henan Normal University, Xinxiang 453007, China
a r t i c l e
i n f o
Article history: Received 5 August 2010 In final form 23 November 2010 Available online 26 November 2010
a b s t r a c t The ground and excited states of naamine A, naamidine A and naamidine G have been investigated using the density functional theory and the time-dependent density functional theory methods. The geometric structures of naamine A, naamidine A and naamidine G in their ground and excited states are considered in detail, and significant changes in angle and bond length are discovered. Furthermore, through the analysis of the frontier molecular orbitals and changes in atomic charge, the excited S1 states of naamine A, naamidine A and naamidine G demonstrate the intramolecular charge transfer. The infrared spectra are also provided for in-depth study. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Marine sponges have proven to be a valuable source of medicinally relevant 2-aminoimidazole natural products, many of which display interesting biological properties [1–4]. These compounds can be divided into 1-unsubstituted and 1-substituted 2-aminoimidazoles. As typical and large parts of 1-substituted 2-aminoimidazoles, naamine and isonaamine have been reported to possess biological activity including antitumor and antifungal properties [5–12]. Naamidine A has attracted enormous attention for its fascinating biological activity, making it the prominent one of 2-aminoimidazole alkaloids. The initial report by Ireland’s group demonstrated that naamidine A has modest antitumor activity, via the inhibition of epidermal growth factor-dependent DNA synthesis and cell proliferation in an A431 tumor model. A subsequent report from the same group revealed that treatment of A431 cells with naamidine A result in cell cycle arrest in the G1 phase, which attracted many researchers to study its mechanism and methods of synthesis [9– 12]. These previous studies have indicated that the p-hydroxybenzyl unit of naamidine A contributes to the activity of naamidine A, while the p-methoxybenzyl unit and the methyl unit do not [12]. This is also confirmed by naamidine G, which does not have p-hydroxybenzyl unit and has been reported as inactive [13,14]. Moreover, as the presumed biosynthetic precursor of naamidine A, naamine A lacks the dehydrohydantoin moiety and has been demonstrated to be essentially inactive. This indicate the contribution of dehydrohydantoin unit to the biological activity of naamidine A [12]. ⇑ Corresponding author. Fax: +86 373 3328507. E-mail address: [email protected] (K. Jiang). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.077
It should be noted that all these studies focused their attention on the synthesis of naamidine A and its analogues, plus the mechanisms of their biological activity [9–16]. However, little has been reported on its ground and excited sate electronic structures, as well as its spectral properties, which provide a clear opportunity for the study of the structure–activity relationship [12]. In the Letter, time-dependent density functional theory (TDDFT) method has been employed to study the excited states of naamine A, naamidine A and naamidine G. The geometric structures of them are given an analysis in detail. Moreover, it is found that the S1 states of naamine A, naamidine A and naamidine G have intramolecular charge transfer (ICT). ICT is considered as a fundamental and crucial feature of the photophysical and photochemical properties of compound and has been studied by many researchers [17–28]. Therefore, in this work, we have motivated to calculate the ground and excited states of naamine A, naamidine A and naamidine G and carry out a detailed analysis of their ICT states. 2. Theoretical methods The electronic structure, molecular orbital and atomic charges are calculated using TURBOMOLE program [29–38]. Generalized gradient approximation (GGA) for exchange correlation potential [B-P86] is employed for both the ground states and excited states calculation, using the density functional theory (DFT) method and the time-dependent functional theory (TDDFT) method respectively [29,35–38]. Zhao et al. have used the DFT/TDDFT method to investigate the excited-state hydrogen bonding dynamics in their benchmark works [30–34]. To improve the efficiency of the method, the resolution-of-the-identity (RI) approximation is applied without sacrificing the accuracy of the results [35,36]. The triple-f valence quality with one set of polarization functions
535
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
(TZVP) is chosen as the basis set throughout both the ground states and the excited states and the corresponding auxiliary basis sets for the RI approximation [37]. The Mulliken charges of all the atoms are calculated in order to study their change between ground and excited states. The changes of atomic charges can provide sufficient proof for the charge transfer (CT) study. In this Letter, the functional B-P86 has been used and given reliable results. Therefore, it is demonstrated well for the ICT study. Nevertheless, it has been reported that TDDFT method underestimated the position of the charge transfer (CT) state and the CAMB3-LYP functional has been taken as the successful attempt [39–41].
3. Results and discussion The optimized geometric structures of naamine A, naamidine A and naamidine G in the ground state are shown in Figure 1, and some important functional groups of naamidine A are labeled. It can be seen that naamidine A consists of a 2-aminoimidazole central core and its four substituents: N1 methyl, an unusual C4 dehydrohydantoin, C1 p-methoxybenzyl unit, and the C2 p-hydroxybenzyl unit [12]. For the isomer naamidine A, we mainly discuss the center core and the C4, C1, C2 substituents. Firstly, we pay attention to the center core, 2-aminoimidazole, which is a five-membered heterocyclic with C@C and C@N double bonds. Both the C@C and C@N bonds can serve as hydrogen bond acceptors. The center core 2-aminoimidazole displays a broad range of interesting biological properties and serves as an important precursor in drug design and natural products synthesis [42– 44,1,45–49]. Secondly, the C4 dehydrohydantoin unit is also a five-membered heterocyclic with two C@O bonds and one C@N bond, the three of these being hydrogen bond acceptors which can hydrogen bonded with other polar solvents containing hydrogen atoms. In addition, this typical five-membered heterocyclic is almost in the plane of the 2-aminodazole and has interesting photophysical and photochemical properties. Thirdly, the C1 p-methoxybenzyl unit has benzene and oxygen methyl groups, with oxygen atom serving as hydrogen bond acceptors. Fourthly,
the C2 p-hydroxybenzyl unit has benzene and a hydroxyl group. The benzene is typical hydrogen bond acceptor as discussed widely previously, and the hydroxyl group can act as both hydrogen bond donor and acceptor in many molecular systems. Moreover, naamine A and naamidine G are also shown in Figure 1. Compared with naamidine A, naamine A is similar to naamidine A except the five-numbered cyclic substituent of N3 atom and naamidine G has two p-methoxybenzyl units but without p-hydroxybenzyl unit . The bond length and angle of the major functional groups of naamine A, naamidine A and naamidine G in their ground states and excited S1 states are shown in Table 1. For naamidine A, the result demonstrates that the C1–C2 and C4–N2 groups undergo different changes although they are in the same five-membered heterocyclic ring. The C1@C2 bond length is shortened from 1.389 to 1.380 Å after excitation to S1 sate, while C4–N2 lengthens from 1.331 to 1.321 Å. The bond of functional groups of C4 dehydrohydantoin unit, such as C5@N4, C6@O1 and C8@O2, lengthen in the excited S1 state. In contrast, C2 p-hydroxybenzyl unit appears to undergo no obvious change, especially for the functional group O3–H2 having the same bond length 0.973 Å in the ground and excited S1 states. For the C1 p-methoxybenzyl unit, the C14–O4 and O4–C15 bonds undergo contrary changes, which shortened and lengthened, respectively. Moreover, the calculated result shows that the angle of C4N3C5 is about 129.2°. The angle reduced to 113.0° in the S1 excited state, which indicates the distinct structural changes. Both the C4–N3 and N3–C5 bonds lengthened when excited to the S1 state compared to the ground state. The angle of C1C12C13 is calculated to be 113.9°, which is almost perpendicular to the center core. This angle increases to 117.7° in the S1 state. In addition, the angle C2C9C10 is about 116.0° in the ground state and 116.5° in the excited S1 state. It is noted that the bond length and angle of naamidine G in ground and excited S1 state are very close to naamidine A, which indicated that the replacement of H2 atom by methoxyl cause little change between them. All these data changes in Table 1 indicate the structural changes of naamidine A in the ground and excited states are consistent with the electron density changes shown in Table 3. By comparison with naamidine A, naamine A does not have the five-numbered cyclic substituent of N3 atom. The bond length of C1–C2 and C4–N3 lengthened about 0.058 and 0.057 Å, respectively. This is contrary to the change of naamidine A. The N1–C3 and C4–N2 bonds of naamine A and naamidine A display similar change between ground and excited S1 states. The angle C2C9C10 shortened
Table 1 Calculated bond lengths L (Å) and angles () of naamine A, naamidine A and naamidine G in the ground state (GS) and the electronically excited state (ES) states. Naamine A GS
Figure 1. The optimized geometric structures of naamine A, naamidine A and naamidine G. Some important functional groups are labeled.
C1–C2 N1–C3 C4–N2 C4–N3 C4N3C5 N3–C5 N3–H1 C5–N4 C6–O1 N5–C7 C8–O2 C2C9C10 O3–H2 O3–C11 C1C12C13 C14–O4 O4–C15
1.381 1.453 1.321 1.400 – – – – – – – 115.9 0.973 1.376 114.7 1.375 1.427
EX 1.439 1.473 1.341 1.343 – – – – – – 119.0 0.973 1.393 113.5 1.368 1.429
Naamidine A
Naamidine G
GS
EX
GS
EX
1.389 1.462 1.321 1.401 129.2 1.344 1.023 1.310 1.213 1.451 1.224 116.0 0.973 1.374 113.9 1.373 1.428
1.380 1.461 1.331 1.405 113.0 1.440 1.023 1.332 1.239 1.443 1.255 116.5 0.973 1.376 117.7 1.331 1.455
1.389 1.463 1.321 1.401 129.2 1.344 1.023 1.310 1.213 1.451 1.224 116.3 – 1.371 113.9 1.373 1.428
1.380 1.461 1.331 1.406 113.1 1.439 1.023 1.332 1.239 1.442 1.255 116.3 – 1.372 117.7 1.332 1.455
536
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
Table 2 Calculated electronic excited energies E (eV) and corresponding oscillator strengths (in the parenthesis) of isolated naamine A, naamidine A and naamidine G.
S1 T1 S2 S3 S4 S5 S6
Naamine A
Naamidine A
Naamidine G
3.291 3.266 3.684 3.372 3.757 4.143 4.161
1.908 1.835 2.276 2.497 2.877 3.082 3.241
1.910(0.0093) 1.829(0.2194) 2.180(0.0140) 2.478(0.2169) 2.878(0.0002) 3.082(0.0032) 3.180(0.0002)
(0.0027) (0.0326) (0.0021) (0.0001) (0.0038) (0.0210) (0.0020)
(0.0089) (0.2029) (0.0307) (0.1980) (0.0002) (0.0033) (0.0000)
about 4° comparing with ground state. However, the hydroxyl group of naamine A does not undergo any change between ground and excited S1 state. The bond length changes of naamine A, naamidine A and naamidine G are well consistent with their electron density changes. The calculated electronic excitation energies and corresponding oscillator strengths of naamine A, naamidine A and naamidine G are listed in the Table 2. It can be seen that the excitation energies of naamidine A are lower than naamine A, which once more indicate the influence of C4 dehydrohydantoin unit on naamidine A. In
addition, we find that the S1 states of them do not correspond to maximum absorption but with lowest excitation energy. The maximum absorption peaks shift to longer wavelength, corresponding to S5 state for naamine A and S3 state for naamidine A, respectively. The electronic excitation energies and corresponding oscillator strengths of naamidine A is very close to naamidine A. In this Letter, we pay our mainly attention to the S1 states of naamine A, naamidine A and naamidine G, which are also their fluorescence states. The triplet T1 states of naamine A, naamidine A and naamidine G are also listed in Table 2. According to the results, for naamine A, the electronic excitation energy of T1 state is calculate to be 3.266 eV, which is very close to excitation energy 3.291 eV for singlet S1 state. The similar phenomenon occurs to naamidine A and naamidine G, whose excitation energy of T1 states are calculated to be 1.835 and 1.829 eV, a little lower than their excitation energy of excited S1 states. It is well-known that analysis of molecular orbitals (MOs) can provide insight into the nature of the excited states [50]. Compared with naamine A, the C4 dehydrohydantoin substituent reduce the levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Figure 2 shows the relative HOMO and LUMO energy levels as well as energy
Table 3 Calculated frontier molecular orbitals and corresponding orbital energies (in eV) for naamine A, naamidine A and naamidine G. Naamine A
Naamidine A
Naamidine G
S1
H
4.607
L
H
4.607
L+1
1.324
H
5.273
L
3.399
L
3.399
L
3.399
H
5.254
L
3.381
5.609
L
3.381
S2
0.944
H
2
5.725
H
2
H
1
5.443
L
3.381
H
3
6.258
L
3.381
H
6.348
L
3.381
4
H
5
6.559
L
3.381
S3
H H
1
5.051
L
1
5.477
1.324
S4
H
4.607
L+2
H
4.607
L+3
0.869
H
3
6.276 L
3.399
3.399
S5
0.510
H
4
6.366
L
H
5
6.638
L
S6
H
1
5.051
L+2
0.869
3.399
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
Figure 2. The HOMO and LUMO energy level of naamine A, naamidine A and naamidine G.
difference. In contrast with naamine A, the HOMO and LUMO energy levels of naamidine A are decreased, and the energy difference changes to 1.874 eV. The energy difference of naamidine G is 1.873 eV, which is very close to naamidine A. Therefore, all these changes should correspond to the C4 dehydrohydantoin substituent. Table 3 shows the calculated molecular orbitals of naamine A, naamidine A and naamidine G in their six excited states. It can be seen that the S1 states of naamine A, naamidine A and naamidine G correspond to both HOMO and LUMO transition. For the isolated naamine A, the electron density of HOMO is mainly localized on the 2-aminoimidazole central core and the N3 atom, whereas
537
the electron density of LUMO is mainly localized on C2 p-hydroxybenzyl unit. The electron density transferred from central core and the N3 atom (donor) to C2 p-hydroxybenzyl unit (acceptor) upon excitation. While for naamidine A, the electron density transfer occurs from the 2-aminoimidazole central core (conjugated unit) and C1 p-methoxybenzyl (donor) to C4 dehydrohydantoin (acceptor) upon molecular orbital transition. In contrast to naamidine A, the HOMO ? LUMO transition of naamidine G is very close to naamidine A. The intramolecular charge transfer (ICT) of their excited S1 states can be observed obviously. The other five excited states of naamine A, naamidine A and naamidine G with their main molecular orbital transition are also shown in Table 3. The excited S2, S4, S5 states of naamine A have similar characteristics with excited S1 state, while the S3 state has obvious electron density transfer from C1 p-methoxybenzyl unit to C2 p-hydroxybenzyl unit. Otherwise, besides the excited S1 state, the other five excited states of naamidine A and naamidine G corresponding to the transition from HOMO to n (n = 1–5) to LUMO. The other four states have the similar characteristics with S1 state except the S5 state. The localize excitation (LE) properties of S5 state can be obviously found. The atomic charge distributions of naamine A, naamidine A and naamidine G are shown in Figure 3. The green arrowhead denoted the charge change trend. In order to describe the change of atomic charges obviously, we divided molecular structures into several parts and summed the atomic charge of each unit. Naamine A was divided into four parts, C2 p-hydroxybenzyl unit, C1 p-methoxybenzyl unit, 2-aminoimidazole central core and N1 methyl unit. The sum of atomic charges of C2 p-hydroxybenzyl unit in the ground state is calculated to be 0.055, which changes to 0.091 in the excited S1 state. The similar phenomenon happened
Figure 3. The atomic charge distribution of naamine A, naamidine A and naamidine G. The sum of atom charges of divided units display in both ground state and excited S1 state (in parentheses).
538
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
to 2-aminoimidazole central core, the sum of which changed from 0.050 to 0.347. Both of them undergo significant change, from negative to positive number. However, the sum of C1 p-methoxybenzyl unit displays an opposite tendency of change compared with above-mentioned. All of this demonstrated the charge transfer from C2 p-hydroxybenzyl unit and 2-aminoimidazole central core to C1 p-methoxybenzyl unit. Therefore, we can conclude that the S1 state of naamine A is an intramolecular charge transfer (ICT) state. For naamidine A, the sum of atomic charge of C2 p-hydroxybenzyl unit in ground state is calculated to be 0.031, which changed to 0.051 after excitation, while C1 p-methoxybenzyl unit displays little change. The 2-aminoimidazole central core and C4 dehydrohydantoin unit undergo a different tendency of change, which can be seen obviously from Figure 3. Therefore, the charge transfer (CT) from C4 dehydrohydantoin unit and C1 p-methoxybenzyl unit to 2-aminoimidazole central core is confirmed. For naamidine G, the sum of atomic charge of C1 p-methoxybenzyl unit is calculated to be 0.032 in the ground state, which changes to 0.813 after excitation to S1 state. It changes from negative to positive. The sums of atomic charges of 2-aminoimidazole central core and C4 dehydrohydantoin unit display similar tendency of change. Both of them become more negative. Therefore, it is demonstrated that the atomic charge transfer (CT) mainly from C1 p-methoxybenzyl to 2-aminoimidazole central core and C4 dehydrohydantoin unit, which is consistent with electron density change of HOMO ? LUMO transition. All of above demonstrates the intramolecular charge transfer (ICT) for the S1 state of naamidine A and naamidine G. The infrared spectra of naamine A, naamidine A and naamidine G for both the ground states and excited states are shown in Figure 4. It can be seen obviously that the infrared spectral of naamine A lack the vibration absorption peak around 1700 cm 1, which correspond to C@O group. For naamine A, the stretching vibration wavenumber 3685 cm 1 should corresponds to O–H group, which redshift to 3639 cm 1 when excited to S1 state. In addition, the wavenumber 3526 cm 1 should correspond to N–H bond, which hardly change between the ground and excited states. For isomer naamidine A, the stretching vibration absorption 3690 cm 1 obviously corresponds to the O–H bond, which changes to 3685 cm 1 when excited to the S1 state. In addition, the wavenumber 3436 cm 1 should correspond to the N–H bond, which redshifts to 3428 cm 1 when excited to the S1 state. It can be seen that both
Figure 4. The infrared spectra of naamine A (NA), naamidine A (ND) and naamidine G (NG) in both ground and excited states.
of them undergo little change. The 1724 and 1777 cm 1 signals should correspond to the C@O groups existing in C4 dehydrohydantoin unit, both of which redshifted when excited to the S1 state. The infrared spectra of naamidine G is very similar with naamidine A in ground state and excited S1 state except lack the stretching absorption of O–H group. It is observed that the changes in the spectra between the ground and excited states are consistent with the bond length changes. 4. Summary Naamidine A has attracted much attention for its biological activity. Many studies have been conducted to study the mechanism of its biological activity and the methods of synthesis. In this Letter, we focused on naamine A, naamidine A and naamidine G of their properties in both the ground and excited states. The electronic structures of naamine A, naamidine A and naamidine A in the ground states and excited states were calculated by the DFT and TDDFT methods respectively, whereby we provided electronic structure information of them for the study of the structure–activity relationship. From the calculated results, the changes of bond length and angle are discussed in detail. The electronic structure change of naamidine G in ground and excited S1 state are very close to naamidine A. In addition, by the analysis of molecular orbitals and atomic charges, the intramolecular charge transfers (ICT) of the S1 states for naamine A, naamidine A and naamidine G have been theoretically demonstrated. The infrared spectra of naamine A, naamidine A and naamidine G are also provided in order to indepth study their structure–property relationship. Our theoretical study will be useful in further understanding of the photophysics and photochemistry of naamidine A as well as its biological activity. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 20571025 and 60977063), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province, China (Grant No. 084100510011) and the Innovation Talents of Institution of Higher Education of Henan Province, China (Grant No. 2006KYCX002). References [1] R.L. Giles, J.D. Sullivan, A.M. Steiner, R.E. Looper, Angew. Chem. Int. Edit. 48 (2009) 3116. [2] H. Gross, S. Kehraus, G.M. König, G. Woerheide, A.D. Wright, J. Nat. Prod. 65 (2002) 1190. [3] R.A. Edrada, C.C. Stessman, P. Crews, J. Nat. Prod. 66 (2003) 939. [4] D.C. Dunbar, J.M. Rimoldi, A.M. Clark, M. Kelly, M.T. Hamann, Tetrahedron 56 (2000) 8795. [5] S. Tsukamoto et al., J. Nat. Prod. 70 (2007) 1658. [6] W. Hassan et al., J. Nat. Prod. 67 (2004) 817. [7] P. Ralifo, P. Crews, J. Org. Chem. 69 (2004) 9025. [8] S. Nakamura, I. Kawasaki, M. Kunimura, M. Matsui, Y. Noma, M. Yamashita, S. Ohta, J. Chem. Soc., Perkin Trans. 1 (2002) 1061. [9] B.R. Copp, C.R. Fairchild, L. Cornell, A.M. Casazza, S. Robinson, C.M. Ireland, J. Med. Chem. 41 (1998) 3909. [10] R.D. James, D.A. Jones, W. Aalbersberg, C.M. Ireland, Mol. Cancer Ther. 2 (2003) 747. [11] D.V. LaBarbera et al., Anticancer Drugs 20 (2009) 425. [12] N. Aberle, J. Catimel, E.C. Nice, K.G. Watson, Bioorg. Med. Chem. Lett. 17 (2007) 3741. [13] P.B. Koswatta, C.J. Lovely, Tetrahedron Lett. 51 (2010) 164. [14] I. Mancini, G. Guella, C. Debitus, F. Pietra, Helv. Chim. Acta 78 (1995) 1178. [15] N.S. Aberle, G. Lessene, K.G. Watson, Org. Lett. 8 (2006) 419. [16] S. Ohta, N. Tsuno, S. Nakamura, N. Taguchi, M. Yamashita, I. Kawasaki, M. Fujieda, Heterocycles 53 (2000) 1939. [17] G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 113 (2009) 14329. [18] G.-J. Zhao, K.-L. Han, J. Chem. Phys. 127 (2007) 024306. [19] B.K. Paul, A. Samanta, S. Kar, N. Guchhait, J. Lumin. 130 (2010) 1258. [20] G.-J. Zhao et al., Chem. Eur. J. 14 (2008) 6935.
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539 [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
G.-J. Zhao, K.-L. Han, J. Comput. Chem. 29 (2008) 2010. G.-J. Zhao, K.-L. Han, P.J. Stang, J. Chem. Theory Comput. 5 (2009) 1955. G.-J. Zhao, K.-L. Han, Phys. Chem. Chem. Phys. 12 (2010) 8914. G.-J. Zhao, B.H. Northrop, K.-L. Han, P.J. Stang, J. Phys. Chem. A 114 (2010) 9007. A.K. Satpati, M. Kumbhakar, S. Nath, H. Pal, Photochem. Photobiol. 85 (2009) 119. K. Rotkiewicz, K. Rechthaler, A. Puchała, D. Rasała, S. Styrcz, G. Köhler, J. Photochem. Photobiol. A 98 (1996) 15. Z. Xu, G. Bai, C. Dong, Spectrochim. Acta, Part A 62 (2005) 987. S. Song et al., Talanta 77 (2009) 1707. J.L. Whitten, J. Chem. Phys. 58 (1973) 4496. G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 2469. G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 9218. G.-J. Zhao, J.-Y. Liu, L.-C. Zhou, K.-L. Han, J. Phys. Chem. B 111 (2007) 8940. G.-J. Zhao, K.-L. Han, ChemPhysChem 9 (2008) 1842. G.-J. Zhao, K.-L. Han, Biophys. J. 94 (2008) 38. B.I. Dunlap, J.W.D. Connolly, J.R. Sabin, J. Chem. Phys. 71 (1979) 3396. O. Vahtras, J. Almlöf, M.W. Feyereisen, Chem. Phys. Lett. 213 (1993) 514. A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829.
539
[38] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 162 (1989) 165. [39] D.J. Tozer, R.D. Amos, N.C. Handy, B.O. Roos, L. Serrano-Andres, Mol. Phys. 97 (1999) 859. [40] T. Yanai, D.P. Tew, N.C. Handy, Chem. Phys. Lett. 393 (2004) 51. [41] M.J.G. Peach, P. Benfield, T. Helgaker, D.J. Tozer, J. Chem. Phys. 128 (2008) 044118. [42] J.C. Kaila, A.B. Baraiya, A.N. Pandya, H.B. Jalani, K.K. Vasu, V. Sudarsanam, Tetrahedron Lett. 50 (2009) 3955. [43] M.del.R.S. Salvatori, R. Abou-Jneid, S. Ghoulami, M.-T. Martin, A. Zaparucha, A. AI-Mourabit, J. Org. Chem. 70 (2005) 8208. [44] J.D. Sullivan, R.L. Giles, R.E. Looper, Curr. Bioact. Compd. 5 (2009) 39. [45] T.L. Little, S.E. Webber, J. Org. Chem. 59 (1994) 7299. [46] D.S. Ermolat’ev, E.V. Van der Eycken, J. Org. Chem. 73 (2008) 6691. [47] D.S. Ermolat’ev, E.V. Babaev, E.V. Van der Eycken, Org. Lett. 8 (2006) 5781. [48] C.H. Soh, W.K. Chui, Y. Lam, J. Comb. Chem. 10 (2008) 118. [49] J.C. Kaila, A.B. Baraiya, K.K. Vasu, V. Sudarsanam, Tetrahedron Lett. 49 (2008) 7220. [50] V. Lemaur, M. Steel, D. Beljonne, J.-L. Brédas, J. Comil, J. Am. Chem. Soc. 127 (2005) 6077.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Theoretical study on the electronic structures and intramolecular charge transfer of naamine A, naamidine A and naamidine G Kai Jiang a,⇑, Yonggang Yang a,b, Yufang Liu b, Deheng Shi b, Jinfeng Sun b a b
Department of Chemistry, Henan Normal University, Xinxiang 453007, China Department of Physics, Henan Normal University, Xinxiang 453007, China
a r t i c l e
i n f o
Article history: Received 5 August 2010 In final form 23 November 2010 Available online 26 November 2010
a b s t r a c t The ground and excited states of naamine A, naamidine A and naamidine G have been investigated using the density functional theory and the time-dependent density functional theory methods. The geometric structures of naamine A, naamidine A and naamidine G in their ground and excited states are considered in detail, and significant changes in angle and bond length are discovered. Furthermore, through the analysis of the frontier molecular orbitals and changes in atomic charge, the excited S1 states of naamine A, naamidine A and naamidine G demonstrate the intramolecular charge transfer. The infrared spectra are also provided for in-depth study. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Marine sponges have proven to be a valuable source of medicinally relevant 2-aminoimidazole natural products, many of which display interesting biological properties [1–4]. These compounds can be divided into 1-unsubstituted and 1-substituted 2-aminoimidazoles. As typical and large parts of 1-substituted 2-aminoimidazoles, naamine and isonaamine have been reported to possess biological activity including antitumor and antifungal properties [5–12]. Naamidine A has attracted enormous attention for its fascinating biological activity, making it the prominent one of 2-aminoimidazole alkaloids. The initial report by Ireland’s group demonstrated that naamidine A has modest antitumor activity, via the inhibition of epidermal growth factor-dependent DNA synthesis and cell proliferation in an A431 tumor model. A subsequent report from the same group revealed that treatment of A431 cells with naamidine A result in cell cycle arrest in the G1 phase, which attracted many researchers to study its mechanism and methods of synthesis [9– 12]. These previous studies have indicated that the p-hydroxybenzyl unit of naamidine A contributes to the activity of naamidine A, while the p-methoxybenzyl unit and the methyl unit do not [12]. This is also confirmed by naamidine G, which does not have p-hydroxybenzyl unit and has been reported as inactive [13,14]. Moreover, as the presumed biosynthetic precursor of naamidine A, naamine A lacks the dehydrohydantoin moiety and has been demonstrated to be essentially inactive. This indicate the contribution of dehydrohydantoin unit to the biological activity of naamidine A [12]. ⇑ Corresponding author. Fax: +86 373 3328507. E-mail address: [email protected] (K. Jiang). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.077
It should be noted that all these studies focused their attention on the synthesis of naamidine A and its analogues, plus the mechanisms of their biological activity [9–16]. However, little has been reported on its ground and excited sate electronic structures, as well as its spectral properties, which provide a clear opportunity for the study of the structure–activity relationship [12]. In the Letter, time-dependent density functional theory (TDDFT) method has been employed to study the excited states of naamine A, naamidine A and naamidine G. The geometric structures of them are given an analysis in detail. Moreover, it is found that the S1 states of naamine A, naamidine A and naamidine G have intramolecular charge transfer (ICT). ICT is considered as a fundamental and crucial feature of the photophysical and photochemical properties of compound and has been studied by many researchers [17–28]. Therefore, in this work, we have motivated to calculate the ground and excited states of naamine A, naamidine A and naamidine G and carry out a detailed analysis of their ICT states. 2. Theoretical methods The electronic structure, molecular orbital and atomic charges are calculated using TURBOMOLE program [29–38]. Generalized gradient approximation (GGA) for exchange correlation potential [B-P86] is employed for both the ground states and excited states calculation, using the density functional theory (DFT) method and the time-dependent functional theory (TDDFT) method respectively [29,35–38]. Zhao et al. have used the DFT/TDDFT method to investigate the excited-state hydrogen bonding dynamics in their benchmark works [30–34]. To improve the efficiency of the method, the resolution-of-the-identity (RI) approximation is applied without sacrificing the accuracy of the results [35,36]. The triple-f valence quality with one set of polarization functions
535
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
(TZVP) is chosen as the basis set throughout both the ground states and the excited states and the corresponding auxiliary basis sets for the RI approximation [37]. The Mulliken charges of all the atoms are calculated in order to study their change between ground and excited states. The changes of atomic charges can provide sufficient proof for the charge transfer (CT) study. In this Letter, the functional B-P86 has been used and given reliable results. Therefore, it is demonstrated well for the ICT study. Nevertheless, it has been reported that TDDFT method underestimated the position of the charge transfer (CT) state and the CAMB3-LYP functional has been taken as the successful attempt [39–41].
3. Results and discussion The optimized geometric structures of naamine A, naamidine A and naamidine G in the ground state are shown in Figure 1, and some important functional groups of naamidine A are labeled. It can be seen that naamidine A consists of a 2-aminoimidazole central core and its four substituents: N1 methyl, an unusual C4 dehydrohydantoin, C1 p-methoxybenzyl unit, and the C2 p-hydroxybenzyl unit [12]. For the isomer naamidine A, we mainly discuss the center core and the C4, C1, C2 substituents. Firstly, we pay attention to the center core, 2-aminoimidazole, which is a five-membered heterocyclic with C@C and C@N double bonds. Both the C@C and C@N bonds can serve as hydrogen bond acceptors. The center core 2-aminoimidazole displays a broad range of interesting biological properties and serves as an important precursor in drug design and natural products synthesis [42– 44,1,45–49]. Secondly, the C4 dehydrohydantoin unit is also a five-membered heterocyclic with two C@O bonds and one C@N bond, the three of these being hydrogen bond acceptors which can hydrogen bonded with other polar solvents containing hydrogen atoms. In addition, this typical five-membered heterocyclic is almost in the plane of the 2-aminodazole and has interesting photophysical and photochemical properties. Thirdly, the C1 p-methoxybenzyl unit has benzene and oxygen methyl groups, with oxygen atom serving as hydrogen bond acceptors. Fourthly,
the C2 p-hydroxybenzyl unit has benzene and a hydroxyl group. The benzene is typical hydrogen bond acceptor as discussed widely previously, and the hydroxyl group can act as both hydrogen bond donor and acceptor in many molecular systems. Moreover, naamine A and naamidine G are also shown in Figure 1. Compared with naamidine A, naamine A is similar to naamidine A except the five-numbered cyclic substituent of N3 atom and naamidine G has two p-methoxybenzyl units but without p-hydroxybenzyl unit . The bond length and angle of the major functional groups of naamine A, naamidine A and naamidine G in their ground states and excited S1 states are shown in Table 1. For naamidine A, the result demonstrates that the C1–C2 and C4–N2 groups undergo different changes although they are in the same five-membered heterocyclic ring. The C1@C2 bond length is shortened from 1.389 to 1.380 Å after excitation to S1 sate, while C4–N2 lengthens from 1.331 to 1.321 Å. The bond of functional groups of C4 dehydrohydantoin unit, such as C5@N4, C6@O1 and C8@O2, lengthen in the excited S1 state. In contrast, C2 p-hydroxybenzyl unit appears to undergo no obvious change, especially for the functional group O3–H2 having the same bond length 0.973 Å in the ground and excited S1 states. For the C1 p-methoxybenzyl unit, the C14–O4 and O4–C15 bonds undergo contrary changes, which shortened and lengthened, respectively. Moreover, the calculated result shows that the angle of C4N3C5 is about 129.2°. The angle reduced to 113.0° in the S1 excited state, which indicates the distinct structural changes. Both the C4–N3 and N3–C5 bonds lengthened when excited to the S1 state compared to the ground state. The angle of C1C12C13 is calculated to be 113.9°, which is almost perpendicular to the center core. This angle increases to 117.7° in the S1 state. In addition, the angle C2C9C10 is about 116.0° in the ground state and 116.5° in the excited S1 state. It is noted that the bond length and angle of naamidine G in ground and excited S1 state are very close to naamidine A, which indicated that the replacement of H2 atom by methoxyl cause little change between them. All these data changes in Table 1 indicate the structural changes of naamidine A in the ground and excited states are consistent with the electron density changes shown in Table 3. By comparison with naamidine A, naamine A does not have the five-numbered cyclic substituent of N3 atom. The bond length of C1–C2 and C4–N3 lengthened about 0.058 and 0.057 Å, respectively. This is contrary to the change of naamidine A. The N1–C3 and C4–N2 bonds of naamine A and naamidine A display similar change between ground and excited S1 states. The angle C2C9C10 shortened
Table 1 Calculated bond lengths L (Å) and angles () of naamine A, naamidine A and naamidine G in the ground state (GS) and the electronically excited state (ES) states. Naamine A GS
Figure 1. The optimized geometric structures of naamine A, naamidine A and naamidine G. Some important functional groups are labeled.
C1–C2 N1–C3 C4–N2 C4–N3 C4N3C5 N3–C5 N3–H1 C5–N4 C6–O1 N5–C7 C8–O2 C2C9C10 O3–H2 O3–C11 C1C12C13 C14–O4 O4–C15
1.381 1.453 1.321 1.400 – – – – – – – 115.9 0.973 1.376 114.7 1.375 1.427
EX 1.439 1.473 1.341 1.343 – – – – – – 119.0 0.973 1.393 113.5 1.368 1.429
Naamidine A
Naamidine G
GS
EX
GS
EX
1.389 1.462 1.321 1.401 129.2 1.344 1.023 1.310 1.213 1.451 1.224 116.0 0.973 1.374 113.9 1.373 1.428
1.380 1.461 1.331 1.405 113.0 1.440 1.023 1.332 1.239 1.443 1.255 116.5 0.973 1.376 117.7 1.331 1.455
1.389 1.463 1.321 1.401 129.2 1.344 1.023 1.310 1.213 1.451 1.224 116.3 – 1.371 113.9 1.373 1.428
1.380 1.461 1.331 1.406 113.1 1.439 1.023 1.332 1.239 1.442 1.255 116.3 – 1.372 117.7 1.332 1.455
536
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
Table 2 Calculated electronic excited energies E (eV) and corresponding oscillator strengths (in the parenthesis) of isolated naamine A, naamidine A and naamidine G.
S1 T1 S2 S3 S4 S5 S6
Naamine A
Naamidine A
Naamidine G
3.291 3.266 3.684 3.372 3.757 4.143 4.161
1.908 1.835 2.276 2.497 2.877 3.082 3.241
1.910(0.0093) 1.829(0.2194) 2.180(0.0140) 2.478(0.2169) 2.878(0.0002) 3.082(0.0032) 3.180(0.0002)
(0.0027) (0.0326) (0.0021) (0.0001) (0.0038) (0.0210) (0.0020)
(0.0089) (0.2029) (0.0307) (0.1980) (0.0002) (0.0033) (0.0000)
about 4° comparing with ground state. However, the hydroxyl group of naamine A does not undergo any change between ground and excited S1 state. The bond length changes of naamine A, naamidine A and naamidine G are well consistent with their electron density changes. The calculated electronic excitation energies and corresponding oscillator strengths of naamine A, naamidine A and naamidine G are listed in the Table 2. It can be seen that the excitation energies of naamidine A are lower than naamine A, which once more indicate the influence of C4 dehydrohydantoin unit on naamidine A. In
addition, we find that the S1 states of them do not correspond to maximum absorption but with lowest excitation energy. The maximum absorption peaks shift to longer wavelength, corresponding to S5 state for naamine A and S3 state for naamidine A, respectively. The electronic excitation energies and corresponding oscillator strengths of naamidine A is very close to naamidine A. In this Letter, we pay our mainly attention to the S1 states of naamine A, naamidine A and naamidine G, which are also their fluorescence states. The triplet T1 states of naamine A, naamidine A and naamidine G are also listed in Table 2. According to the results, for naamine A, the electronic excitation energy of T1 state is calculate to be 3.266 eV, which is very close to excitation energy 3.291 eV for singlet S1 state. The similar phenomenon occurs to naamidine A and naamidine G, whose excitation energy of T1 states are calculated to be 1.835 and 1.829 eV, a little lower than their excitation energy of excited S1 states. It is well-known that analysis of molecular orbitals (MOs) can provide insight into the nature of the excited states [50]. Compared with naamine A, the C4 dehydrohydantoin substituent reduce the levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Figure 2 shows the relative HOMO and LUMO energy levels as well as energy
Table 3 Calculated frontier molecular orbitals and corresponding orbital energies (in eV) for naamine A, naamidine A and naamidine G. Naamine A
Naamidine A
Naamidine G
S1
H
4.607
L
H
4.607
L+1
1.324
H
5.273
L
3.399
L
3.399
L
3.399
H
5.254
L
3.381
5.609
L
3.381
S2
0.944
H
2
5.725
H
2
H
1
5.443
L
3.381
H
3
6.258
L
3.381
H
6.348
L
3.381
4
H
5
6.559
L
3.381
S3
H H
1
5.051
L
1
5.477
1.324
S4
H
4.607
L+2
H
4.607
L+3
0.869
H
3
6.276 L
3.399
3.399
S5
0.510
H
4
6.366
L
H
5
6.638
L
S6
H
1
5.051
L+2
0.869
3.399
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
Figure 2. The HOMO and LUMO energy level of naamine A, naamidine A and naamidine G.
difference. In contrast with naamine A, the HOMO and LUMO energy levels of naamidine A are decreased, and the energy difference changes to 1.874 eV. The energy difference of naamidine G is 1.873 eV, which is very close to naamidine A. Therefore, all these changes should correspond to the C4 dehydrohydantoin substituent. Table 3 shows the calculated molecular orbitals of naamine A, naamidine A and naamidine G in their six excited states. It can be seen that the S1 states of naamine A, naamidine A and naamidine G correspond to both HOMO and LUMO transition. For the isolated naamine A, the electron density of HOMO is mainly localized on the 2-aminoimidazole central core and the N3 atom, whereas
537
the electron density of LUMO is mainly localized on C2 p-hydroxybenzyl unit. The electron density transferred from central core and the N3 atom (donor) to C2 p-hydroxybenzyl unit (acceptor) upon excitation. While for naamidine A, the electron density transfer occurs from the 2-aminoimidazole central core (conjugated unit) and C1 p-methoxybenzyl (donor) to C4 dehydrohydantoin (acceptor) upon molecular orbital transition. In contrast to naamidine A, the HOMO ? LUMO transition of naamidine G is very close to naamidine A. The intramolecular charge transfer (ICT) of their excited S1 states can be observed obviously. The other five excited states of naamine A, naamidine A and naamidine G with their main molecular orbital transition are also shown in Table 3. The excited S2, S4, S5 states of naamine A have similar characteristics with excited S1 state, while the S3 state has obvious electron density transfer from C1 p-methoxybenzyl unit to C2 p-hydroxybenzyl unit. Otherwise, besides the excited S1 state, the other five excited states of naamidine A and naamidine G corresponding to the transition from HOMO to n (n = 1–5) to LUMO. The other four states have the similar characteristics with S1 state except the S5 state. The localize excitation (LE) properties of S5 state can be obviously found. The atomic charge distributions of naamine A, naamidine A and naamidine G are shown in Figure 3. The green arrowhead denoted the charge change trend. In order to describe the change of atomic charges obviously, we divided molecular structures into several parts and summed the atomic charge of each unit. Naamine A was divided into four parts, C2 p-hydroxybenzyl unit, C1 p-methoxybenzyl unit, 2-aminoimidazole central core and N1 methyl unit. The sum of atomic charges of C2 p-hydroxybenzyl unit in the ground state is calculated to be 0.055, which changes to 0.091 in the excited S1 state. The similar phenomenon happened
Figure 3. The atomic charge distribution of naamine A, naamidine A and naamidine G. The sum of atom charges of divided units display in both ground state and excited S1 state (in parentheses).
538
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539
to 2-aminoimidazole central core, the sum of which changed from 0.050 to 0.347. Both of them undergo significant change, from negative to positive number. However, the sum of C1 p-methoxybenzyl unit displays an opposite tendency of change compared with above-mentioned. All of this demonstrated the charge transfer from C2 p-hydroxybenzyl unit and 2-aminoimidazole central core to C1 p-methoxybenzyl unit. Therefore, we can conclude that the S1 state of naamine A is an intramolecular charge transfer (ICT) state. For naamidine A, the sum of atomic charge of C2 p-hydroxybenzyl unit in ground state is calculated to be 0.031, which changed to 0.051 after excitation, while C1 p-methoxybenzyl unit displays little change. The 2-aminoimidazole central core and C4 dehydrohydantoin unit undergo a different tendency of change, which can be seen obviously from Figure 3. Therefore, the charge transfer (CT) from C4 dehydrohydantoin unit and C1 p-methoxybenzyl unit to 2-aminoimidazole central core is confirmed. For naamidine G, the sum of atomic charge of C1 p-methoxybenzyl unit is calculated to be 0.032 in the ground state, which changes to 0.813 after excitation to S1 state. It changes from negative to positive. The sums of atomic charges of 2-aminoimidazole central core and C4 dehydrohydantoin unit display similar tendency of change. Both of them become more negative. Therefore, it is demonstrated that the atomic charge transfer (CT) mainly from C1 p-methoxybenzyl to 2-aminoimidazole central core and C4 dehydrohydantoin unit, which is consistent with electron density change of HOMO ? LUMO transition. All of above demonstrates the intramolecular charge transfer (ICT) for the S1 state of naamidine A and naamidine G. The infrared spectra of naamine A, naamidine A and naamidine G for both the ground states and excited states are shown in Figure 4. It can be seen obviously that the infrared spectral of naamine A lack the vibration absorption peak around 1700 cm 1, which correspond to C@O group. For naamine A, the stretching vibration wavenumber 3685 cm 1 should corresponds to O–H group, which redshift to 3639 cm 1 when excited to S1 state. In addition, the wavenumber 3526 cm 1 should correspond to N–H bond, which hardly change between the ground and excited states. For isomer naamidine A, the stretching vibration absorption 3690 cm 1 obviously corresponds to the O–H bond, which changes to 3685 cm 1 when excited to the S1 state. In addition, the wavenumber 3436 cm 1 should correspond to the N–H bond, which redshifts to 3428 cm 1 when excited to the S1 state. It can be seen that both
Figure 4. The infrared spectra of naamine A (NA), naamidine A (ND) and naamidine G (NG) in both ground and excited states.
of them undergo little change. The 1724 and 1777 cm 1 signals should correspond to the C@O groups existing in C4 dehydrohydantoin unit, both of which redshifted when excited to the S1 state. The infrared spectra of naamidine G is very similar with naamidine A in ground state and excited S1 state except lack the stretching absorption of O–H group. It is observed that the changes in the spectra between the ground and excited states are consistent with the bond length changes. 4. Summary Naamidine A has attracted much attention for its biological activity. Many studies have been conducted to study the mechanism of its biological activity and the methods of synthesis. In this Letter, we focused on naamine A, naamidine A and naamidine G of their properties in both the ground and excited states. The electronic structures of naamine A, naamidine A and naamidine A in the ground states and excited states were calculated by the DFT and TDDFT methods respectively, whereby we provided electronic structure information of them for the study of the structure–activity relationship. From the calculated results, the changes of bond length and angle are discussed in detail. The electronic structure change of naamidine G in ground and excited S1 state are very close to naamidine A. In addition, by the analysis of molecular orbitals and atomic charges, the intramolecular charge transfers (ICT) of the S1 states for naamine A, naamidine A and naamidine G have been theoretically demonstrated. The infrared spectra of naamine A, naamidine A and naamidine G are also provided in order to indepth study their structure–property relationship. Our theoretical study will be useful in further understanding of the photophysics and photochemistry of naamidine A as well as its biological activity. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 20571025 and 60977063), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province, China (Grant No. 084100510011) and the Innovation Talents of Institution of Higher Education of Henan Province, China (Grant No. 2006KYCX002). References [1] R.L. Giles, J.D. Sullivan, A.M. Steiner, R.E. Looper, Angew. Chem. Int. Edit. 48 (2009) 3116. [2] H. Gross, S. Kehraus, G.M. König, G. Woerheide, A.D. Wright, J. Nat. Prod. 65 (2002) 1190. [3] R.A. Edrada, C.C. Stessman, P. Crews, J. Nat. Prod. 66 (2003) 939. [4] D.C. Dunbar, J.M. Rimoldi, A.M. Clark, M. Kelly, M.T. Hamann, Tetrahedron 56 (2000) 8795. [5] S. Tsukamoto et al., J. Nat. Prod. 70 (2007) 1658. [6] W. Hassan et al., J. Nat. Prod. 67 (2004) 817. [7] P. Ralifo, P. Crews, J. Org. Chem. 69 (2004) 9025. [8] S. Nakamura, I. Kawasaki, M. Kunimura, M. Matsui, Y. Noma, M. Yamashita, S. Ohta, J. Chem. Soc., Perkin Trans. 1 (2002) 1061. [9] B.R. Copp, C.R. Fairchild, L. Cornell, A.M. Casazza, S. Robinson, C.M. Ireland, J. Med. Chem. 41 (1998) 3909. [10] R.D. James, D.A. Jones, W. Aalbersberg, C.M. Ireland, Mol. Cancer Ther. 2 (2003) 747. [11] D.V. LaBarbera et al., Anticancer Drugs 20 (2009) 425. [12] N. Aberle, J. Catimel, E.C. Nice, K.G. Watson, Bioorg. Med. Chem. Lett. 17 (2007) 3741. [13] P.B. Koswatta, C.J. Lovely, Tetrahedron Lett. 51 (2010) 164. [14] I. Mancini, G. Guella, C. Debitus, F. Pietra, Helv. Chim. Acta 78 (1995) 1178. [15] N.S. Aberle, G. Lessene, K.G. Watson, Org. Lett. 8 (2006) 419. [16] S. Ohta, N. Tsuno, S. Nakamura, N. Taguchi, M. Yamashita, I. Kawasaki, M. Fujieda, Heterocycles 53 (2000) 1939. [17] G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 113 (2009) 14329. [18] G.-J. Zhao, K.-L. Han, J. Chem. Phys. 127 (2007) 024306. [19] B.K. Paul, A. Samanta, S. Kar, N. Guchhait, J. Lumin. 130 (2010) 1258. [20] G.-J. Zhao et al., Chem. Eur. J. 14 (2008) 6935.
K. Jiang et al. / Chemical Physics Letters 501 (2011) 534–539 [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
G.-J. Zhao, K.-L. Han, J. Comput. Chem. 29 (2008) 2010. G.-J. Zhao, K.-L. Han, P.J. Stang, J. Chem. Theory Comput. 5 (2009) 1955. G.-J. Zhao, K.-L. Han, Phys. Chem. Chem. Phys. 12 (2010) 8914. G.-J. Zhao, B.H. Northrop, K.-L. Han, P.J. Stang, J. Phys. Chem. A 114 (2010) 9007. A.K. Satpati, M. Kumbhakar, S. Nath, H. Pal, Photochem. Photobiol. 85 (2009) 119. K. Rotkiewicz, K. Rechthaler, A. Puchała, D. Rasała, S. Styrcz, G. Köhler, J. Photochem. Photobiol. A 98 (1996) 15. Z. Xu, G. Bai, C. Dong, Spectrochim. Acta, Part A 62 (2005) 987. S. Song et al., Talanta 77 (2009) 1707. J.L. Whitten, J. Chem. Phys. 58 (1973) 4496. G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 2469. G.-J. Zhao, K.-L. Han, J. Phys. Chem. A 111 (2007) 9218. G.-J. Zhao, J.-Y. Liu, L.-C. Zhou, K.-L. Han, J. Phys. Chem. B 111 (2007) 8940. G.-J. Zhao, K.-L. Han, ChemPhysChem 9 (2008) 1842. G.-J. Zhao, K.-L. Han, Biophys. J. 94 (2008) 38. B.I. Dunlap, J.W.D. Connolly, J.R. Sabin, J. Chem. Phys. 71 (1979) 3396. O. Vahtras, J. Almlöf, M.W. Feyereisen, Chem. Phys. Lett. 213 (1993) 514. A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829.
539
[38] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 162 (1989) 165. [39] D.J. Tozer, R.D. Amos, N.C. Handy, B.O. Roos, L. Serrano-Andres, Mol. Phys. 97 (1999) 859. [40] T. Yanai, D.P. Tew, N.C. Handy, Chem. Phys. Lett. 393 (2004) 51. [41] M.J.G. Peach, P. Benfield, T. Helgaker, D.J. Tozer, J. Chem. Phys. 128 (2008) 044118. [42] J.C. Kaila, A.B. Baraiya, A.N. Pandya, H.B. Jalani, K.K. Vasu, V. Sudarsanam, Tetrahedron Lett. 50 (2009) 3955. [43] M.del.R.S. Salvatori, R. Abou-Jneid, S. Ghoulami, M.-T. Martin, A. Zaparucha, A. AI-Mourabit, J. Org. Chem. 70 (2005) 8208. [44] J.D. Sullivan, R.L. Giles, R.E. Looper, Curr. Bioact. Compd. 5 (2009) 39. [45] T.L. Little, S.E. Webber, J. Org. Chem. 59 (1994) 7299. [46] D.S. Ermolat’ev, E.V. Van der Eycken, J. Org. Chem. 73 (2008) 6691. [47] D.S. Ermolat’ev, E.V. Babaev, E.V. Van der Eycken, Org. Lett. 8 (2006) 5781. [48] C.H. Soh, W.K. Chui, Y. Lam, J. Comb. Chem. 10 (2008) 118. [49] J.C. Kaila, A.B. Baraiya, K.K. Vasu, V. Sudarsanam, Tetrahedron Lett. 49 (2008) 7220. [50] V. Lemaur, M. Steel, D. Beljonne, J.-L. Brédas, J. Comil, J. Am. Chem. Soc. 127 (2005) 6077.