Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 529–533
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
TDDFT study on intramolecular hydrogen bond of photoexcited methyl salicylate Peng Qu, Dongxu Tian ⇑ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, 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
Intramolecular hydrogen bond of MS is
strengthened upon photoexcitation. Experimental peak at 310 nm in UV
absorption spectrum corresponds to the S1. The S3 state is the main state of the low-lying excited states. The S1 and S3 are predicted to be the p ? p⁄ transition mechanism.
a r t i c l e
i n f o
Article history: Received 6 May 2013 Received in revised form 1 November 2013 Accepted 26 November 2013 Available online 9 December 2013 Keywords: Hydrogen bonded complex Orbital transition Absorption spectra Vibrational redshift TDDFT
a b s t r a c t The equilibrium geometries, IR-spectra and transition mechanism of intramolecular hydrogen-bonded methyl salicylate in excited state were studied using DFT and TDDFT with 6-31++G (d, p) basis set. The length of hydrogen bond OAH O@C is decreased from 1.73 Å in the ground state to 1.41 and 1.69 Å in the excited S1 and S3 states. The increase of bond length for HAO and C@O group also indicates that in excited state the hydrogen bond OAH O@C is strengthened. IR spectra show HAO and C@O stretching bands are strongly redshifted by 1387 and 67 cm 1 in the excited S1 and S3 states comparing to the ground state. The excitation energy and the absorption spectrum show the S3 state is the main excited state of the low-lying excited states. By analyzing the frontier molecular orbitals, the transition from the ground state to the excited S1 and S3 states was predicted to be the p ? p⁄ mode. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The hydrogen bond plays an important role in physics, chemistry and biology fields. Hydrogen bond is a very important type of solute–solvent interaction that has been investigated extensively by diverse experimental and theoretical methods [1–20]. Additionally, hydrogen bonding is of fundamental importance in molecular and supramolecular photochemistry [4–11]. Hydrogen bond and its properties have been extensively studied ⇑ Corresponding author. Tel.: +86 411 84986074. E-mail address: [email protected] (D. Tian). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.112
in previous works [12–20]. Mataga et al. have reported that the hydrogen bond is important in reactions and interactions between drugs and DNA, especially the reaction rate and path in photochemistry and photobiology [11–14]. Hydrogen bond interactions have been studied using electronic and infrared spectroscopic techniques, which offer the effective means for studying hydrogen bonds [15–17]. Sobolewski and Domcke have reported theoretical results about intramolecular hydrogen bonds in excited states, which role is important in the phenomenon of excited-state intramolecular proton transfer [18–20]. Upon photoexcitation, on account of significant difference in charge distribution for the different electronic states, the solute
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and solvent molecules in the formation of hydrogen bonds need to reorganize themselves [21], which affects the ground and excitedstate properties of hydrogen-bonded complexes. The changes in site-specific hydrogen-bonding interactions can induce spectral shifts of some characteristic vibrational modes involved in the formation of hydrogen bonds [21]. Early time hydrogen-bonding is mainly monitored by characteristic vibrational motions of the hydrogen bond, and it is potential for experiments using the femtosecond time-resolved vibrational spectroscopy to monitor hydrogen-bonding dynamics. Zhao et al. have reported the hydrogen-bonding dynamics of photoexcited coumarin 102 and investigated the effects of hydrogen-bond strengthening and weakening [22–26]. Methyl salicylate (MS) is getting more and more attention because of its special fluorescence property [27]. The two forms (shown in Fig. 1) of MS with an intramolecular hydrogen bond (IMHB) are responsible for the three fluorescence emissions [27]. UV–vis spectroscopy, fluorescence spectra and fluorescence excitation spectra have been used to explain the dual-fluorescence phenomenon [28–31]. However, to our knowledge, little is known about the excited state property of MS. Theoretical study on the geometries, IR-spectra and transition mechanism of intramolecular hydrogen-bonded methyl salicylate in the excited states can provide insight into the fluorescence property on photoexcitation. The present work investigated the equilibrium geometries, absorption and infrared spectra of MS with intramolecular hydrogen bond for respective ground and excited states using DFT and TDDFT method, and the transient mechanism was studied. It is illustrated that the intramolecular hydrogen bond between the C@O group and the HAO group is strengthened upon photoexcitation. In comparison with the calculated HAO stretching mode in the ground state (3425 cm 1), it is strongly redshifted to 2038 cm 1 in the excited S1 state. The calculated C@O stretching mode in the ground state (1716 cm 1), is redshifted to 1649 cm 1 in the excited S1 state. In addition, the calculated electronic excitation energies and corresponding oscillator strengths show the S3 state is the main excited state of the low-lying excited states. By analyzing the frontier molecular orbitals of methyl salicylate, the S1 state of the locally excited state, and the transition from the ground state to the excited state was predicted to be the p ? p⁄ mode.
Computational details Molecular geometries of the hydrogen-bonded MS complexes were optimized using the Becke’s three-parameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) [32] for ground state and timedependent density functional theory (TD-DFT) for excited state [33–34]. The standard polarized and diffuse 6-31++G (d, p) basis set was used. Harmonic vibration frequencies in the ground state and the excited state were determined by diagonalization of the Hessian. The IR intensities were determined from the gradients of the dipole moment. NBO was used to analyze the electronic properties and the constitution of molecular orbitals. All calculations in this work were performed using the Guassian 09 program package [35] and completed purely in gas phase.
Results and discussion Geometry of MS in the ground and excited states The MS complex in ground and excited states (S1 and S3) with IMHB between the C@O and HAO groups have been studied using DFT and TDDFT with 6-31++G (d, p) basis set. The optimized geometric structures of MS in the ground state and excited S1 and S3 states are shown in Fig. 1. On photoexcitation, the bond length of HAO group is increased from 0.99 Å in the ground state to 1.08 and 1.01 Å in the excited S1 and S3 states, and the bond length of C@O group is increased from 1.25 Å of the ground state to 1.28 and 1.29 Å in the excited S1 and S3 states, respectively. The hydrogen bond length of OAH O@C between the H and O atoms is 1.73 Å in the ground state, while it is respective 1.41 and 1.69 Å in the excited S1 and S3 states. Comparing with the ground state, the decrease of the hydrogen bond length indicates that the hydrogen bond OAH O@C in the excited state is strengthened. It is in agreement with the research results that the hydrogen bond is strengthened in the excited state of photoexcited coumarin 102 by Zhao and Han [22]. Upon photoexcitation, the benzene ring skeleton atoms and hydrogen bond OAH O@C remain in the plane. Sobolewski and Domcke have reported the hydrogen bond properties of salicylic acid (SA) [19]. The structure difference between MS and SA is only a methyl group, which will not affect
Fig. 1. Geometric structures of intramolecular hydrogen-bonded complexes in different electronic states, S0 on the top left, S1 on the top right and S3.
P. Qu, D. Tian / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 529–533
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the intramolecular hydrogen bond properties significantly. It was also found that the hydrogen bond of SA is strengthened in the excited state. The bond length of HAO group of SA is increased from 0.992 Å in the ground state to 1.098 Å in the S1 state, and the bond length of C@O group is increased from 1.233 Å in the ground state to 1.281 Å in the S1 state. The hydrogen bond length of OAH O@C between the H and O atoms is 1.708 Å in the ground state, while it is 1.386 Å in the S1 state. Absorption spectroscopy and the low-lying excited states The low-lying electronically excited states of MS complexes with IMHB were investigated. The electronic excitation energies and the corresponding oscillator strengths of the low-lying excited states are presented in Table 1. In comparison with the oscillator strengths, the excited S3, S6, S8 and S18 states of MS with IMHB are identified as the main excited states. Thus, the MS complexes with IMHB may be initially photoexcited to the S3, S6, S8 and S18 states. The S3 state is the main excited state of the low-lying excited states, and this result was supported by the experimental UV spectrum reported by Seskar et al. [36]. However, other excited states need to be tested by further spectroscopic experiments. The absorption spectrum of MS complex with IMHB was calculated and shown in Fig. 2. The absorption band corresponding to the relatively significant excited states, S1, S3, S4, S6, S8 and S18, were all plotted in Fig. 2. The calculated and experimental absorption peak values were marked. In Fig. 2, the calculated spectrum exhibits an absorption peak value at 296 nm, which corresponds to the excited S1 state. It is in good agreement with the experimental value of 310 nm in UV–visible absorption spectrum [27] reported by Catalán et al. The excited energy corresponding to the S1 state is 4.20 eV as shown in Table 1, which is closed to the excited energy of 4.19 eV for SA calculated by Sobolewski and Domcke [19]. The calculated spectrum also exhibits a strong absorption peak at the value of 236 nm for the low-lying excited states, which corresponds to the excited S3 state. This result is in very good agreement with the experimental value 237.9 nm by Seskar et al. [36]. The calculated result show the absorption peaks at 310 and 237.9 nm in experimental UV spectrum corresponds to the S1 and S3 excited states, respectively. From the calculated absorption spectrum, absorption bands corresponding to S4, S6, S8 and S18 excited states may be identified in further UV–visible absorption spectra. The transition mechanism and frontier molecular orbitals Upon photoexcitation, the MS with IMHB complex reorganize themselves as a consequence of significant difference in charge distribution for the different electronic states. Frontier molecular orbitals (MOs) analysis was performed to provide insight into the transition mechanism of the excited states [37]. The calculated
Table 1 Calculated the electronic excitation energies (eV), corresponding oscillator strengths (in the parenthesis) and absorption wavelength (nm) for MS. Excited states
Energies (oscillator strengths)
Wavelength
S1 S2 S3 S4 S5 S6 S8 S18
4.19 5.12 5.26 5.53 5.82 5.98 6.31 6.97
296 242 236 224 213 207 196 178
(0.0951) (0.0000) (0.1558) (0.0029) (0.0008) (0.3116) (0.1819) (0.1720)
Fig. 2. Calculated absorption spectroscopy of intramolecular hydrogen-bonded MS.
Fig. 3. Frontier molecular orbitals (MOs) of MS complex.
frontier MOs of MS complex were shown in Fig. 3. The excited S1 state corresponds to the orbital transition from HOMO to LUMO (95.13%) and from HOMO 1 to LUMO+1(4.87%). The excited S3 state originates from the orbital transition from HOMO 1 to LUMO (82.55%) and from HOMO to LUMO+1(17.45%). From the p and p⁄ character of HOMO, HOMO 1 and LUMO, LUMO+1, respectively, the S1 or S3 state was predicted to be the p ? p⁄ mode upon photoexcitation. In comparison with the calculations about SA reported by Sobolewski and Domcke [19], the transition from S0 to S1 was also found to be p ? p⁄ mode and it is consistent with the present results. When MS complex with IMHB is photoexcited from the ground state S0 to the excited S1 and S3 states, the electron density distribution of frontier molecular orbitals over the C@O and the HAO group changes as shown in Fig. 3. The electron density of the O atom in the C@O group increases with orbital transition from HOMO to LUMO, while the electron density of the O atom of the OAH group decreases. When MS is photoexcited from the S0 to S3 state corresponding to the transition from HOMO 1 to LUMO, the electron density of the O atom in the C@O group increases. The electron density of the hydrogen bond OAH O@C increases in the excited S1 and S3 states. The increase of the electron density indicates the IMHB is strengthened in the excited states.
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The intramolecular hydrogen bond of SA with a very significant strengthening in the S1 state was also observed by Sobolewski and Domcke [19]. Natural bond orbitals (NBO) analysis was performed to investigate the transition process. The orbital coefficients of frontier MOs of some nature atomic orbitals (NAO) were presented as shown in Table 2. The atoms listed in Table 2 were marked as shown in Fig. 1. By analyzing the NAOs [38], the constitution of frontier MOs can be calculated. The results of all frontier MOs were presented in the parenthesis following the orbital coefficients in Table 2. The HOMO 1 is constituted by 17.20% 2Pz (C1) + 25.51% 2Pz (C3) + 21.66% 2Pz (C4) + 26.92% 2Pz (C6) + 4.27% 2Pz (O12) + 2.40% 2Pz (O13). According to the results of calculations, all the atomic orbitals which have significant contributions to the frontier MOs are 2Pz. As shown in Fig. 1, upon photoexcitation, the benzene ring skeleton atoms and hydrogen bond OAH O@C remain in the plane of MS. Hydrogen bond OAH O@C and benzene ring skeleton atoms constitute a delocalized p bond. This result is in agreement with transition mechanism that the transition from the ground state S0 to the S1 or S3 state was predicted to be the p ? p⁄. The transition from HOMO to LUMO is the major photoexcited channel for S0 ? S1. By analyzing the HOMO, the contribution of benzene ring (C1AC6) atoms and C11, O12 and O13 atoms involved in the hydrogen bond to the HOMO is 76.11% and 1.71%, respectively. LUMO is constituted by benzene ring atoms (58.66%), C11, O12 and O13 atoms (39.19%) and O18 atom (2.14%). The orbital transition from HOMO to LUMO leads to the p electron of benzene ring transfer to the C11, O12 and O13 atoms involved in the hydrogen bond. The contribution of O18 atom to HOMO and LUMO changes significantly from 22.10% to 2.15%. O18 atom may be the channel of p electron transfer from the benzene ring to hydrogen bond. The transition from HOMO 1 to LUMO is the major mode for S0 ? S3. The contribution of C1, C3, C4 and C6 atom to the HOMO 1 (the total is 93.19%) is higher than that to the LUMO (the total is 51.12%). While the change of C5, C11, O12 and O18 atom shows the opposite tendency (from 4.36% to 44.33%). The contribution of O13 atom has no significant change (from 2.45% to 4.55%). The charge distribution for the different electronic states in the hydrogen bond O18AH O12@C11, especially the C11 atom with very significant increase (from 0% to 21.82% for S0 ? S1 and S0 ? S3), leads to the strengthened intramolecular hydrogen bond upon photoexcitation. Infrared spectra The calculated IR spectra in the ground and excited S1 and S3 states of MS with IMHB at the range from 1000 to 3500 cm 1 were shown in Fig. 4. Due to the changes in site-specific hydrogen-bonding interactions, spectral shifts of some characteristic vibrational modes involved in hydrogen bonds occur. A clear-cut signature
Fig. 4. Calculated HAO and C@O stretching bands of MS in the S0, S1 and S3 states shown in blue, red and green lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of the hydrogen-bonding dynamics can be got from the vibrational frequencies of the stretching vibrations of HAO and C@O group involved in hydrogen bonds. The stretching bands of MS in the ground and excited S1 and S3 states are shown in the blue and red lines, respectively. The calculated HAO stretching band in the ground state is 3425 cm 1, while it is strongly redshifted by 1387 cm 1 to 2038 cm 1 in the excited S1 state upon electronic excitation. It was also found a 1395 cm 1 redshift of the OAH stretch frequency for SA excited from S0 to S1 by Sobolewski and Domcke [19]. When S0 is excited to S3 state, 421 cm 1 redshift occur for HAO stretching band. In comparison with the calculated C@O stretching band of 1716 cm 1 in the ground state, it is redshifted by 67 cm 1 to 1649 cm 1 in the excited S1 state with a weak IR intensity. Sobolewski and Domcke [19] reported that the redshift of the C@O stretch frequency for SA from S0 to S1 is 60 cm 1. Additionally, when S0 is excited to S3 state, 133 cm 1 redshift occur for C@O stretching band. These results show both HAO and C@O bonds are strongly weakened upon photoexcitation. The weakened HAO and C@O bonds are induced by the strengthened hydrogen bond of OAH O@C on photoexcitation, in good agreement with previous calculations of SA [19]. The spectral shift of the HAO stretching band in the excited state can be driven by changes of electron density redistribution analyzed by orbital constitution. Additionally, the IR intensity of the HAO stretching band in the excited state is stronger than that in
Table 2 Calculated the orbital coefficients of natural bond orbitals (NBO) for HOMO 1, HOMO, LUMO, and LUMO+1. Calculated the weight of atomic orbitals in the parenthesis to HOMO 1, HOMO, LUMO, and LUMO+1. Atom
Orbital
Orbital coefficients HOMO 1
C1 C2 C3 C4 C5 C6 C11 O12 O13 O18
2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz
0.4210(17.56%) – 0.5051(26.04%) 0.4654(22.11%) – 0.5188(27.48%) – 0.2066(4.36%) 0.1550(2.45%) –
HOMO 0.2769 (7.87%) 0.5109(26.76%) – 0.4013(16.53%) 0.3778(14.65%) 0.3180(10.3%) – 0.1276(1.71%) – 0.4640(22.10%)
LUMO 0.3900(15.81%) – 0.4397(20.08%) 0.2477(6.38%) 0.2693(7.54%) 0.2917(8.85%) 0.4582(21.82%) 0.3512(12.82%) 0.2092(4.55%) 0.1427(2.15%)
LUMO+1 0.4449(22.04%) 0.4721(24.84%) – 0.4019(17.99%) 0.5079(28.75%) – 0.1010(1.14%) – – 0.2169(5.24%)
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the ground state. When the MS with the hydrogen bond between the C@O and the HAO is photoexcited the transition dipole moment increases, leading to the increase of IR intensity. Massaro et al. [31] have reported the calculated OAH and C@O stretching frequency of ground state MS under BPW91/6-311++G, and the respective 3262 and 1647 cm 1 were obtained by a scaling factor of 0.977. The calculated OAH and C@O stretching of ground state for SA under B3LYP/cc-pVDZ reported by Sobolewski and Domcke [19] is 3332 and 1746 cm 1, respectively. The experimental value of 3200 cm 1 for OAH [39] and 1698 cm 1 for C@O [40] were reported, respectively. For the other rotamer of MS (ketoA) with a hydrogen bond between OAH and OACH3, a calculated 3633 cm 1 for OAH stretching band, and 1775 cm 1 for C@O stretching band were reported under HF/6-31G (d) by a scaling factor value of 0.8929 [41]. Under the BPW91/6-311++G, a 3387 cm 1 band for OAH and a 1655 cm 1 band for C@O [31] were observed. The experimental value of C@O for ketoA reported by Varghese et al. [41] is 1682 cm 1. There exists a big deviation between the experimental and calculated values. More vibration analysis for MS, especially the OAH mode, needs to be tested in further IR spectra experiments and the calculations in the future work. Conclusion In summary, we theoretically studied the ground and excited states of intramolecular hydrogen-bonded MS complex, especially focusing on the transition mechanism. It is distinctly illustrated that the intramolecular hydrogen bond OAH O@C between the C@O and HAO groups is strengthened upon photoexcitation. The length of hydrogen bond OAH O@C is decreased from 1.73 Å in the ground state to 1.41 and 1.69 Å in the excited S1 and S3 states. In comparison with the calculated HAO stretching band in the ground state (3425 cm 1), it is strongly redshifted to 2038 cm 1 in the excited S1 state upon electronic excitation. The calculated C@O stretching band in the ground state (1716 cm 1), is redshifted to 1649 cm 1 in the excited S1 state. The intramolecular hydrogen bond strengthening and the redshift of OAH and C@O stretching bands on photoexication was also found for SA [19]. The calculated spectrum at 296 nm of the S1 excited state is in good agreement with the experimental value of 310 nm in UV absorption spectrum. Additionally, the calculated electronic excitation energies and corresponding oscillator strengths show the S3 is the main excited state of the low-lying states. This result is in very good agreement with the experimental value 237.9 nm reported by Seskar et al. The electron densities of all the frontier molecular orbitals have the p or the p⁄ bonding character and the transition from the ground state to the excited state is the p ? p⁄ mode. NBO analysis shows the p electrons of benzene ring transfer to the C11, O12 and O13 atoms involved in the hydrogen bond leads to the strengthened hydrogen bond. Acknowledgements This work was supported by the National Science Foundation of China (21001019) and the Fundamental Research Funds for the Central Universities (DUT12LK26). The results were obtained on
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
TDDFT study on intramolecular hydrogen bond of photoexcited methyl salicylate Peng Qu, Dongxu Tian ⇑ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, 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
Intramolecular hydrogen bond of MS is
strengthened upon photoexcitation. Experimental peak at 310 nm in UV
absorption spectrum corresponds to the S1. The S3 state is the main state of the low-lying excited states. The S1 and S3 are predicted to be the p ? p⁄ transition mechanism.
a r t i c l e
i n f o
Article history: Received 6 May 2013 Received in revised form 1 November 2013 Accepted 26 November 2013 Available online 9 December 2013 Keywords: Hydrogen bonded complex Orbital transition Absorption spectra Vibrational redshift TDDFT
a b s t r a c t The equilibrium geometries, IR-spectra and transition mechanism of intramolecular hydrogen-bonded methyl salicylate in excited state were studied using DFT and TDDFT with 6-31++G (d, p) basis set. The length of hydrogen bond OAH O@C is decreased from 1.73 Å in the ground state to 1.41 and 1.69 Å in the excited S1 and S3 states. The increase of bond length for HAO and C@O group also indicates that in excited state the hydrogen bond OAH O@C is strengthened. IR spectra show HAO and C@O stretching bands are strongly redshifted by 1387 and 67 cm 1 in the excited S1 and S3 states comparing to the ground state. The excitation energy and the absorption spectrum show the S3 state is the main excited state of the low-lying excited states. By analyzing the frontier molecular orbitals, the transition from the ground state to the excited S1 and S3 states was predicted to be the p ? p⁄ mode. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The hydrogen bond plays an important role in physics, chemistry and biology fields. Hydrogen bond is a very important type of solute–solvent interaction that has been investigated extensively by diverse experimental and theoretical methods [1–20]. Additionally, hydrogen bonding is of fundamental importance in molecular and supramolecular photochemistry [4–11]. Hydrogen bond and its properties have been extensively studied ⇑ Corresponding author. Tel.: +86 411 84986074. E-mail address: [email protected] (D. Tian). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.112
in previous works [12–20]. Mataga et al. have reported that the hydrogen bond is important in reactions and interactions between drugs and DNA, especially the reaction rate and path in photochemistry and photobiology [11–14]. Hydrogen bond interactions have been studied using electronic and infrared spectroscopic techniques, which offer the effective means for studying hydrogen bonds [15–17]. Sobolewski and Domcke have reported theoretical results about intramolecular hydrogen bonds in excited states, which role is important in the phenomenon of excited-state intramolecular proton transfer [18–20]. Upon photoexcitation, on account of significant difference in charge distribution for the different electronic states, the solute
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and solvent molecules in the formation of hydrogen bonds need to reorganize themselves [21], which affects the ground and excitedstate properties of hydrogen-bonded complexes. The changes in site-specific hydrogen-bonding interactions can induce spectral shifts of some characteristic vibrational modes involved in the formation of hydrogen bonds [21]. Early time hydrogen-bonding is mainly monitored by characteristic vibrational motions of the hydrogen bond, and it is potential for experiments using the femtosecond time-resolved vibrational spectroscopy to monitor hydrogen-bonding dynamics. Zhao et al. have reported the hydrogen-bonding dynamics of photoexcited coumarin 102 and investigated the effects of hydrogen-bond strengthening and weakening [22–26]. Methyl salicylate (MS) is getting more and more attention because of its special fluorescence property [27]. The two forms (shown in Fig. 1) of MS with an intramolecular hydrogen bond (IMHB) are responsible for the three fluorescence emissions [27]. UV–vis spectroscopy, fluorescence spectra and fluorescence excitation spectra have been used to explain the dual-fluorescence phenomenon [28–31]. However, to our knowledge, little is known about the excited state property of MS. Theoretical study on the geometries, IR-spectra and transition mechanism of intramolecular hydrogen-bonded methyl salicylate in the excited states can provide insight into the fluorescence property on photoexcitation. The present work investigated the equilibrium geometries, absorption and infrared spectra of MS with intramolecular hydrogen bond for respective ground and excited states using DFT and TDDFT method, and the transient mechanism was studied. It is illustrated that the intramolecular hydrogen bond between the C@O group and the HAO group is strengthened upon photoexcitation. In comparison with the calculated HAO stretching mode in the ground state (3425 cm 1), it is strongly redshifted to 2038 cm 1 in the excited S1 state. The calculated C@O stretching mode in the ground state (1716 cm 1), is redshifted to 1649 cm 1 in the excited S1 state. In addition, the calculated electronic excitation energies and corresponding oscillator strengths show the S3 state is the main excited state of the low-lying excited states. By analyzing the frontier molecular orbitals of methyl salicylate, the S1 state of the locally excited state, and the transition from the ground state to the excited state was predicted to be the p ? p⁄ mode.
Computational details Molecular geometries of the hydrogen-bonded MS complexes were optimized using the Becke’s three-parameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) [32] for ground state and timedependent density functional theory (TD-DFT) for excited state [33–34]. The standard polarized and diffuse 6-31++G (d, p) basis set was used. Harmonic vibration frequencies in the ground state and the excited state were determined by diagonalization of the Hessian. The IR intensities were determined from the gradients of the dipole moment. NBO was used to analyze the electronic properties and the constitution of molecular orbitals. All calculations in this work were performed using the Guassian 09 program package [35] and completed purely in gas phase.
Results and discussion Geometry of MS in the ground and excited states The MS complex in ground and excited states (S1 and S3) with IMHB between the C@O and HAO groups have been studied using DFT and TDDFT with 6-31++G (d, p) basis set. The optimized geometric structures of MS in the ground state and excited S1 and S3 states are shown in Fig. 1. On photoexcitation, the bond length of HAO group is increased from 0.99 Å in the ground state to 1.08 and 1.01 Å in the excited S1 and S3 states, and the bond length of C@O group is increased from 1.25 Å of the ground state to 1.28 and 1.29 Å in the excited S1 and S3 states, respectively. The hydrogen bond length of OAH O@C between the H and O atoms is 1.73 Å in the ground state, while it is respective 1.41 and 1.69 Å in the excited S1 and S3 states. Comparing with the ground state, the decrease of the hydrogen bond length indicates that the hydrogen bond OAH O@C in the excited state is strengthened. It is in agreement with the research results that the hydrogen bond is strengthened in the excited state of photoexcited coumarin 102 by Zhao and Han [22]. Upon photoexcitation, the benzene ring skeleton atoms and hydrogen bond OAH O@C remain in the plane. Sobolewski and Domcke have reported the hydrogen bond properties of salicylic acid (SA) [19]. The structure difference between MS and SA is only a methyl group, which will not affect
Fig. 1. Geometric structures of intramolecular hydrogen-bonded complexes in different electronic states, S0 on the top left, S1 on the top right and S3.
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the intramolecular hydrogen bond properties significantly. It was also found that the hydrogen bond of SA is strengthened in the excited state. The bond length of HAO group of SA is increased from 0.992 Å in the ground state to 1.098 Å in the S1 state, and the bond length of C@O group is increased from 1.233 Å in the ground state to 1.281 Å in the S1 state. The hydrogen bond length of OAH O@C between the H and O atoms is 1.708 Å in the ground state, while it is 1.386 Å in the S1 state. Absorption spectroscopy and the low-lying excited states The low-lying electronically excited states of MS complexes with IMHB were investigated. The electronic excitation energies and the corresponding oscillator strengths of the low-lying excited states are presented in Table 1. In comparison with the oscillator strengths, the excited S3, S6, S8 and S18 states of MS with IMHB are identified as the main excited states. Thus, the MS complexes with IMHB may be initially photoexcited to the S3, S6, S8 and S18 states. The S3 state is the main excited state of the low-lying excited states, and this result was supported by the experimental UV spectrum reported by Seskar et al. [36]. However, other excited states need to be tested by further spectroscopic experiments. The absorption spectrum of MS complex with IMHB was calculated and shown in Fig. 2. The absorption band corresponding to the relatively significant excited states, S1, S3, S4, S6, S8 and S18, were all plotted in Fig. 2. The calculated and experimental absorption peak values were marked. In Fig. 2, the calculated spectrum exhibits an absorption peak value at 296 nm, which corresponds to the excited S1 state. It is in good agreement with the experimental value of 310 nm in UV–visible absorption spectrum [27] reported by Catalán et al. The excited energy corresponding to the S1 state is 4.20 eV as shown in Table 1, which is closed to the excited energy of 4.19 eV for SA calculated by Sobolewski and Domcke [19]. The calculated spectrum also exhibits a strong absorption peak at the value of 236 nm for the low-lying excited states, which corresponds to the excited S3 state. This result is in very good agreement with the experimental value 237.9 nm by Seskar et al. [36]. The calculated result show the absorption peaks at 310 and 237.9 nm in experimental UV spectrum corresponds to the S1 and S3 excited states, respectively. From the calculated absorption spectrum, absorption bands corresponding to S4, S6, S8 and S18 excited states may be identified in further UV–visible absorption spectra. The transition mechanism and frontier molecular orbitals Upon photoexcitation, the MS with IMHB complex reorganize themselves as a consequence of significant difference in charge distribution for the different electronic states. Frontier molecular orbitals (MOs) analysis was performed to provide insight into the transition mechanism of the excited states [37]. The calculated
Table 1 Calculated the electronic excitation energies (eV), corresponding oscillator strengths (in the parenthesis) and absorption wavelength (nm) for MS. Excited states
Energies (oscillator strengths)
Wavelength
S1 S2 S3 S4 S5 S6 S8 S18
4.19 5.12 5.26 5.53 5.82 5.98 6.31 6.97
296 242 236 224 213 207 196 178
(0.0951) (0.0000) (0.1558) (0.0029) (0.0008) (0.3116) (0.1819) (0.1720)
Fig. 2. Calculated absorption spectroscopy of intramolecular hydrogen-bonded MS.
Fig. 3. Frontier molecular orbitals (MOs) of MS complex.
frontier MOs of MS complex were shown in Fig. 3. The excited S1 state corresponds to the orbital transition from HOMO to LUMO (95.13%) and from HOMO 1 to LUMO+1(4.87%). The excited S3 state originates from the orbital transition from HOMO 1 to LUMO (82.55%) and from HOMO to LUMO+1(17.45%). From the p and p⁄ character of HOMO, HOMO 1 and LUMO, LUMO+1, respectively, the S1 or S3 state was predicted to be the p ? p⁄ mode upon photoexcitation. In comparison with the calculations about SA reported by Sobolewski and Domcke [19], the transition from S0 to S1 was also found to be p ? p⁄ mode and it is consistent with the present results. When MS complex with IMHB is photoexcited from the ground state S0 to the excited S1 and S3 states, the electron density distribution of frontier molecular orbitals over the C@O and the HAO group changes as shown in Fig. 3. The electron density of the O atom in the C@O group increases with orbital transition from HOMO to LUMO, while the electron density of the O atom of the OAH group decreases. When MS is photoexcited from the S0 to S3 state corresponding to the transition from HOMO 1 to LUMO, the electron density of the O atom in the C@O group increases. The electron density of the hydrogen bond OAH O@C increases in the excited S1 and S3 states. The increase of the electron density indicates the IMHB is strengthened in the excited states.
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The intramolecular hydrogen bond of SA with a very significant strengthening in the S1 state was also observed by Sobolewski and Domcke [19]. Natural bond orbitals (NBO) analysis was performed to investigate the transition process. The orbital coefficients of frontier MOs of some nature atomic orbitals (NAO) were presented as shown in Table 2. The atoms listed in Table 2 were marked as shown in Fig. 1. By analyzing the NAOs [38], the constitution of frontier MOs can be calculated. The results of all frontier MOs were presented in the parenthesis following the orbital coefficients in Table 2. The HOMO 1 is constituted by 17.20% 2Pz (C1) + 25.51% 2Pz (C3) + 21.66% 2Pz (C4) + 26.92% 2Pz (C6) + 4.27% 2Pz (O12) + 2.40% 2Pz (O13). According to the results of calculations, all the atomic orbitals which have significant contributions to the frontier MOs are 2Pz. As shown in Fig. 1, upon photoexcitation, the benzene ring skeleton atoms and hydrogen bond OAH O@C remain in the plane of MS. Hydrogen bond OAH O@C and benzene ring skeleton atoms constitute a delocalized p bond. This result is in agreement with transition mechanism that the transition from the ground state S0 to the S1 or S3 state was predicted to be the p ? p⁄. The transition from HOMO to LUMO is the major photoexcited channel for S0 ? S1. By analyzing the HOMO, the contribution of benzene ring (C1AC6) atoms and C11, O12 and O13 atoms involved in the hydrogen bond to the HOMO is 76.11% and 1.71%, respectively. LUMO is constituted by benzene ring atoms (58.66%), C11, O12 and O13 atoms (39.19%) and O18 atom (2.14%). The orbital transition from HOMO to LUMO leads to the p electron of benzene ring transfer to the C11, O12 and O13 atoms involved in the hydrogen bond. The contribution of O18 atom to HOMO and LUMO changes significantly from 22.10% to 2.15%. O18 atom may be the channel of p electron transfer from the benzene ring to hydrogen bond. The transition from HOMO 1 to LUMO is the major mode for S0 ? S3. The contribution of C1, C3, C4 and C6 atom to the HOMO 1 (the total is 93.19%) is higher than that to the LUMO (the total is 51.12%). While the change of C5, C11, O12 and O18 atom shows the opposite tendency (from 4.36% to 44.33%). The contribution of O13 atom has no significant change (from 2.45% to 4.55%). The charge distribution for the different electronic states in the hydrogen bond O18AH O12@C11, especially the C11 atom with very significant increase (from 0% to 21.82% for S0 ? S1 and S0 ? S3), leads to the strengthened intramolecular hydrogen bond upon photoexcitation. Infrared spectra The calculated IR spectra in the ground and excited S1 and S3 states of MS with IMHB at the range from 1000 to 3500 cm 1 were shown in Fig. 4. Due to the changes in site-specific hydrogen-bonding interactions, spectral shifts of some characteristic vibrational modes involved in hydrogen bonds occur. A clear-cut signature
Fig. 4. Calculated HAO and C@O stretching bands of MS in the S0, S1 and S3 states shown in blue, red and green lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of the hydrogen-bonding dynamics can be got from the vibrational frequencies of the stretching vibrations of HAO and C@O group involved in hydrogen bonds. The stretching bands of MS in the ground and excited S1 and S3 states are shown in the blue and red lines, respectively. The calculated HAO stretching band in the ground state is 3425 cm 1, while it is strongly redshifted by 1387 cm 1 to 2038 cm 1 in the excited S1 state upon electronic excitation. It was also found a 1395 cm 1 redshift of the OAH stretch frequency for SA excited from S0 to S1 by Sobolewski and Domcke [19]. When S0 is excited to S3 state, 421 cm 1 redshift occur for HAO stretching band. In comparison with the calculated C@O stretching band of 1716 cm 1 in the ground state, it is redshifted by 67 cm 1 to 1649 cm 1 in the excited S1 state with a weak IR intensity. Sobolewski and Domcke [19] reported that the redshift of the C@O stretch frequency for SA from S0 to S1 is 60 cm 1. Additionally, when S0 is excited to S3 state, 133 cm 1 redshift occur for C@O stretching band. These results show both HAO and C@O bonds are strongly weakened upon photoexcitation. The weakened HAO and C@O bonds are induced by the strengthened hydrogen bond of OAH O@C on photoexcitation, in good agreement with previous calculations of SA [19]. The spectral shift of the HAO stretching band in the excited state can be driven by changes of electron density redistribution analyzed by orbital constitution. Additionally, the IR intensity of the HAO stretching band in the excited state is stronger than that in
Table 2 Calculated the orbital coefficients of natural bond orbitals (NBO) for HOMO 1, HOMO, LUMO, and LUMO+1. Calculated the weight of atomic orbitals in the parenthesis to HOMO 1, HOMO, LUMO, and LUMO+1. Atom
Orbital
Orbital coefficients HOMO 1
C1 C2 C3 C4 C5 C6 C11 O12 O13 O18
2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz 2Pz
0.4210(17.56%) – 0.5051(26.04%) 0.4654(22.11%) – 0.5188(27.48%) – 0.2066(4.36%) 0.1550(2.45%) –
HOMO 0.2769 (7.87%) 0.5109(26.76%) – 0.4013(16.53%) 0.3778(14.65%) 0.3180(10.3%) – 0.1276(1.71%) – 0.4640(22.10%)
LUMO 0.3900(15.81%) – 0.4397(20.08%) 0.2477(6.38%) 0.2693(7.54%) 0.2917(8.85%) 0.4582(21.82%) 0.3512(12.82%) 0.2092(4.55%) 0.1427(2.15%)
LUMO+1 0.4449(22.04%) 0.4721(24.84%) – 0.4019(17.99%) 0.5079(28.75%) – 0.1010(1.14%) – – 0.2169(5.24%)
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the ground state. When the MS with the hydrogen bond between the C@O and the HAO is photoexcited the transition dipole moment increases, leading to the increase of IR intensity. Massaro et al. [31] have reported the calculated OAH and C@O stretching frequency of ground state MS under BPW91/6-311++G, and the respective 3262 and 1647 cm 1 were obtained by a scaling factor of 0.977. The calculated OAH and C@O stretching of ground state for SA under B3LYP/cc-pVDZ reported by Sobolewski and Domcke [19] is 3332 and 1746 cm 1, respectively. The experimental value of 3200 cm 1 for OAH [39] and 1698 cm 1 for C@O [40] were reported, respectively. For the other rotamer of MS (ketoA) with a hydrogen bond between OAH and OACH3, a calculated 3633 cm 1 for OAH stretching band, and 1775 cm 1 for C@O stretching band were reported under HF/6-31G (d) by a scaling factor value of 0.8929 [41]. Under the BPW91/6-311++G, a 3387 cm 1 band for OAH and a 1655 cm 1 band for C@O [31] were observed. The experimental value of C@O for ketoA reported by Varghese et al. [41] is 1682 cm 1. There exists a big deviation between the experimental and calculated values. More vibration analysis for MS, especially the OAH mode, needs to be tested in further IR spectra experiments and the calculations in the future work. Conclusion In summary, we theoretically studied the ground and excited states of intramolecular hydrogen-bonded MS complex, especially focusing on the transition mechanism. It is distinctly illustrated that the intramolecular hydrogen bond OAH O@C between the C@O and HAO groups is strengthened upon photoexcitation. The length of hydrogen bond OAH O@C is decreased from 1.73 Å in the ground state to 1.41 and 1.69 Å in the excited S1 and S3 states. In comparison with the calculated HAO stretching band in the ground state (3425 cm 1), it is strongly redshifted to 2038 cm 1 in the excited S1 state upon electronic excitation. The calculated C@O stretching band in the ground state (1716 cm 1), is redshifted to 1649 cm 1 in the excited S1 state. The intramolecular hydrogen bond strengthening and the redshift of OAH and C@O stretching bands on photoexication was also found for SA [19]. The calculated spectrum at 296 nm of the S1 excited state is in good agreement with the experimental value of 310 nm in UV absorption spectrum. Additionally, the calculated electronic excitation energies and corresponding oscillator strengths show the S3 is the main excited state of the low-lying states. This result is in very good agreement with the experimental value 237.9 nm reported by Seskar et al. The electron densities of all the frontier molecular orbitals have the p or the p⁄ bonding character and the transition from the ground state to the excited state is the p ? p⁄ mode. NBO analysis shows the p electrons of benzene ring transfer to the C11, O12 and O13 atoms involved in the hydrogen bond leads to the strengthened hydrogen bond. Acknowledgements This work was supported by the National Science Foundation of China (21001019) and the Fundamental Research Funds for the Central Universities (DUT12LK26). The results were obtained on
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