Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 814–820

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Photoinduced excited state intramolecular proton transfer and spectral behaviors of Aloesaponarin 1 Yonggang Yang a, Yufang Liu a,⇑, Dapeng Yang b, Hui Li a, Kai Jiang c, Jinfeng Sun a a

College of Physics and Information Engineering, Henan Normal University, Xinxiang 453007, China Physics Laboratory, North China University of Water Resources and Electric Power, Zhengzhou 450045, China c College of Chemistry and Environmental Science, Henan Normal University, Xinxiang 453007, China b

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

 We confirm that the ESIPT process of

AS1 is along HB1 while not HB2.  The novel infrared spectral of AS1

demonstrate the ESIPT process indirectly.  The potential energy curves of AS1 describe the gradual process and dynamical behaviors of ESIPT.  The large Stokes shifts of AS1 explain part of its UV protection property.

a r t i c l e

i n f o

Article history: Received 2 April 2015 Received in revised form 14 June 2015 Accepted 7 July 2015 Available online 8 July 2015 Keywords: Aloesaponarin 1 ESIPT Hydrogen bond Spectral behaviors UV protection

a b s t r a c t The novel spectral behaviors of Aloesaponarin 1 (AS1) are investigated by studying the dynamics process of excited state intramolecular proton transfer (ESIPT). Two intramolecular hydrogen bonds (HB1 and HB2) are formed between hydroxyl and carbonyl groups of AS1. The calculated potential energy curves of AS1 demonstrate that the ESIPT process along HB1 is energy favorable while not along HB2. The analysis of potential energy curves describes clearly the dynamic behaviors of the proton transfer process from hydroxyl group to carbonyl group along HB1. The infrared spectra of AS1 confirm that the stretching absorption peak of hydroxyl group in HB1 disappears and that a new peak corresponding to hydroxyl group appears in the first excited state, which depicts the ESIPT process indirectly. The fluorescence peaks of AS1 (636 nm), AS2 (Aloesaponarin 1 3-O-methyl ether, 629 nm) and AS3 (Aloesaponarin 1 8-O-methyl ether, 522 nm) demonstrate that the fluorescence behavior of AS1 is primarily effected by HB1 rather than HB2. The large Stokes shifts of AS1 (206 nm) indicate that the absorbed energy is partly transferred to non-harmful long fluorescence through ESIPT process, which plays important role in the explanation for the UV protection property of AS1. The inducement and influence factors of ESIPT process of AS1 are illustrated by analyzing electrostatic potential, molecular orbital and natural bond orbital. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Aloe is an active and frequently used ingredient in hundreds of skin lotions, sun blocks for UV protection, and different cosmetics ⇑ Corresponding author. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.saa.2015.07.046 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

for its cosmetic and healing properties [1–11]. Numerous Aloe species around the world are used in conditions ranging from dermatitis to cancer [1–3]. Aloesaponarin 1 (AS1) is a characteristic constituent of the roots of Aloe species with the following features: anti-bacterial activity against mycobacterium tuberculosis in the Microplate Alamar Blue Assay (MABA) and Low Oxygen Recovery Assay (LORA) [4–6]. It has been reported that AS1 prevents

Y. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 814–820

photosensitization by absorbing harmful UV radiation and dissipating energy as heat through excited-state intramolecular proton transfer (ESIPT) process which is performed by intramolecular hydrogen bond interaction [10,11]. Being a hot topic in biology and chemistry, intramolecular and intermolecular hydrogen bond interactions of some molecular systems have been extensively studied by both experimental and theoretical methods [12–25]. The ESIPT process through hydrogen bond interaction has been used to interpret the spectral behaviors of molecular system due to its important role in practical application [26–28]. The molecules undergoing ESIPT process can be candidates for molecular systems, such as optical memory, photolabeling, proton-transfer laser and an information storage device at the molecular level [29–40]. The ESIPT processes of some molecular systems often occur through intermolecular or intramolecular hydrogen bond, which are central to understanding their spectral behaviors and microscopic structure [12–25]. Previous AS1 studies mainly focus on the relationship between ESIPT process and its UV protection property or singlet-oxygen quenching activity [39,40]. Nevertheless, the mechanism of novel spectral behaviors of AS1 has not been comprehensively studied yet. In this work, we will pay our main attention on the dynamics behaviors of the ESIPT process of AS1 before and after photoexcitation in order to study its spectral behaviors. The derivatives of AS1, Aloesaponarin 1 3-O-methyl ether (AS2) and Aloesaponarin 1 8-O-methyl ether (AS3), are provided to analyze the relevant factors of the ESIPT process. 2. Computational details The geometric structure, electronic and infrared spectra, and potential energy curves of AS1 are all calculated using Gaussian 09 program suite [41]. Becke’s three-parameter hybrid exchange function with Lee–Yang–Parr gradient-corrected correlation functional (B3-LYP functional), in combination with 6-311++G(d, p) basis set, is used in both the Density functional theoretical (DFT) and TDDFT methods [42,43]. Natural bonding orbital (NBO) method has been demonstrated as a reliable tool in measuring the intermolecular delocalization and hyperconjugation of molecular. The NBO calculations are therefore performed using NBO program in Gaussian 09W package at B3LYP/6-311++G(d, p) level in order to understand various second order interactions between the filled orbital of one subsystem and vacant of the other subsystem [42,43]. Ethanol is used as solvent in the CPCM calculations to evaluate the solvent effect. The entire local minimums are confirmed by the absence of any imaginary frequency in vibrational analysis calculations. The potential energy curves of AS1 and AS2 have been scanned by constrained optimizations on the bond lengths of hydroxyl groups (O1–H1 and O4–H2) or intramolecular hydrogen bond (H1–O2) to obtain the thermodynamic corrections in their excited S1 states. Such as, Fig. 4A, the scanned bond lengths of hydroxyl group O1–H1 are set to a constant value with re-optimization of other coordinates, keeping the O1–H1 bond length from 0.900 to 1.600 Å in steps of 0.05 Å. 3. Results and discussion 3.1. Optimized geometric structures in the ground state and the S1 state The optimized geometric structures of AS1, AS2 and AS3 in the ground state (S0) and the first excited (S1) state are plotted in Fig. 1. AS2 and AS3 are provided in order to explain the contribution of their single intramolecular hydrogen bond to the ESIPT process and their spectral behaviors. The bond lengths of the functional

815

Fig. 1. The calculated bond lengths of AS1, AS2 and AS3 in ground and the excited S1 states (the numbers in parentheses).

groups involved in formation of HB1 and HB2 are provided in the right side of Fig. 1 with the numbers in parentheses showing bond lengths in the S1 state. For AS1, the bond lengths of O1–H1 and HB1 in the ground state are calculated as 0.991 and 1.648 Å, which change to 1.494 and 1.208 Å respectively in the S1 state accompanied with the cleavage of O1–H1 and the formation of new bond O2–H1. The bond lengths of C1–O1 and carbonyl group C2–O2 in the ground state are 1.343 and 1.244 Å, which change to 1.275 and 1.314 Å in the S1 state. These changes indicate that the ESIPT process of AS1 occurs due to the proton transfer from O1–H1 to carbonyl group C2–O2. For HB2, the bond lengths of O4–H2 and C3–O3 are lengthened by 0.004 and 0.002 Å respectively after photoexcitation, and the bond length of HB2 decreases to 1.662 Å in the S1 state from 1.690 Å in the ground state. This result shows that HB2 is strengthened in the S1 state without undergoing the ESIPT process. The above analysis confirms that the ESIPT process of AS1 occurs along HB1 rather than along HB2, which demonstrates the theoretical model provided by Nagaoka et al. [11]. For AS2, the only intramolecular hydrogen bond, HB1, is formed as the substitute of methyl on atom O4. The HB1 of AS2 undergoes the similar ESIPT process to that of AS1 for the reason of photoexcitation. For AS3, the intramolecular hydrogen bond HB2 is strengthened without undergoing ESIPT process after being photo-excited to the S1 state. 3.2. Frontier molecular orbital analysis The most important frontier molecular orbitals, such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), play a crucial role in the chemical stability of molecules [44,45]. The frontier molecular orbitals (MOs) and their transition energies of AS1, AS2 and AS3 are provided in Fig. 2. The S1 states of AS1, AS2 and AS3 are all of HOMO ? LUMO transition. For AS1, the electron density of HOMO is mainly delocalized at the phenyl ring and the hydroxyl groups, and the electron density of LUMO is mainly located at the phenyl ring, middle six-membered ring and the carbonyl group. The electron density of hydroxyl group O1–H1 in HOMO decreases when being transited to LUMO, and the electron density of carbonyl group C2–O2 increases after orbital transition. The electron transfer from hydroxyl group O1–H1 to carbonyl group C2–O2 is along HB1, implying that the intramolecular hydrogen bond interaction between hydroxyl group O1–H1 and atom O2 is strengthened. The electron density of hydroxyl group O4–H2 in HOMO decreases while that of C3–O3 increases after orbital transition, indicating the strengthening of the interaction of HB2 after

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the S1 state, the electrophilic attack ability of atom O1 and the nucleophilic attack ability of H1 are obviously strengthened, which induces the strengthening of intramolecular hydrogen bond interaction. In contrast to HB1, the electron density of atom O4 and H2 in HB2 is hardly changed before and after photoexcitation to the S1 state. The red region of AS2 is also localized on the oxygen atoms of hydroxyl group O1–H1 and carbonyl group C2–O2. The substitution of methyl at atom O4 induces the cleavage of electron density transfer along intramolecular hydrogen bond HB2. The electrophilic attack of atom O2 and the nucleophilic reaction H1 of HB2 is strengthened. For AS3, the change of HB2 is similar to that of AS1. These ESP results are consistent with the analysis of molecular orbital. 3.4. Natural bond orbital (NBO) analysis

Fig. 2. The frontier molecular orbitals of AS1, AS2 and AS3 corresponding to excited S1 state and their transition energy.

molecular orbital transition. The electron distribution of HB1 of AS2 in HOMO and LUMO is similar to that of AS1, and the electron distribution of the moieties involved in the formation of HB2 of AS3 is similar to that of AS1. The energy gap between HOMO and LUMO reflects the biological activity, optical polarizability and chemical hardness–softness of the molecule. A small frontier orbital gap is generally associated with a high-chemical reactivity and low kinetic stability [38]. The orbital transition energy gaps between HOMO and LUMO of AS1 and AS2 are calculated to be 3.436 and 3.471 eV, which are both much smaller than that of AS3 (3.546 eV). The comparative analysis implies that AS1 and AS2 have higher chemical reactivity and lower kinetic stability compared to AS3. That is, AS1 and AS2 are more likely to undergo the ESIPT process than AS3. Moreover, the transition energy comparison implies that the transition energy of HOMO ? LUMO of AS1 relies more on HB1 than on HB2. 3.3. Electrostatic potential analysis As a visual method to understand the relative polarity of the molecule, molecular electrostatic potential (ESP) is related to the electron density of molecular and depicts the size, shape, charge density and site of chemical reactivity of the molecules [46–48]. ESP is widely used in understanding the sites for electrophilic attack and nucleophilic reactions, such as intermolecular or intramolecular hydrogen bonding interactions [46–48]. An electron density isosurface is mapped with electrostatic potential surface. The different colors of electrostatic potential represent different values, with red representing regions of most electron negative electrostatic potential, blue representing regions of the most positive electrostatic potential, and green representing regions of zero potential. The ESPs of AS1, AS2 and AS3 in the ground state and the excited S1 state are provided in Fig. 3. It can be seen that the red regions are located at the electronegative atoms, and that the blue regions are located at the hydrogen atoms. For AS1, the red region is localized on the oxygen atoms of hydroxyl group O1–H1 and O4–H2, carbonyl group C2–O2 and C3–O3, and the blue region is localized on the hydrogen atoms of O1–H1 and O4–H2. This distribution indicates that the oxygen atom of C2–O2 and the hydrogen atom of hydroxyl group O1–H1 are the possible sites for electrophilic attack and nucleophilic attack respectively, which induces the formation of hydrogen bond interaction between O1–H1 and C2–O2. The red region of atom O3 and the blue region of atom H2 are lighter than those of H1 and O2. When excited to

NBO analysis is proven to be an effective tool for studying intra or intermolecular hydrogen bonding and hyper conjugative interaction among different bonds [49]. Moreover, it provides a convenient basis for investigating charge transfer from filled lone electron pairs of n(D) of the ‘‘Lewis base’’ D into the unfilled antibond r⁄(A–H) of the ‘‘Lewis acid’’ A–H in A–H  D hydrogen bonding systems. The calculated results of NBO analysis are estimated by the second order energy lowering equation and provided in Table 1. The strength of stabilization energy (E(2)) between different types of interactions is associated with the electron delocalization between NBO(i) and acceptor NBO(j) delocalization. The larger the stabilization energy is, the more donation tendency from electron donors to electron acceptor there will be. For this reason, there will be greater extent of conjugation of the whole system [45]. The NBO results related to the formation of intramolecular hydrogen bond HB1 and HB2 in the ground and the excited S1 state are provided in Table 1. The intra molecular interactions in molecule AS1 are formed by the orbital overlap between (r, p)(C–C, C–O, O–H) and their anti bonding orbital, which results in intra molecular charge transfer (ICT) from donor to acceptor and causes stabilization of the molecular system [40]. The stabilization energy of intra molecular hyper conjugative interaction of r(C1–C6) distributed to r⁄(C1–O1) is 0.51 kcal/mol, which is much smaller than that of r⁄(C1–C7) nearly 3.34 kcal/mol. The stabilization energy of r(C1–C7) bond to r⁄(O1–H1) involved in the formation of HB1 is 1.66 kcal/mol. The stabilization energies of r(C1–O1) distributed to r⁄(C1–C6) and r⁄(C1–C7) are 1.00 and 0.90 kcal/mol respectively, which are much smaller than that of r⁄(O4–H2), nearly 16.99 kcal/mol for hyperconjugative. The charge transfer from r(C2–O2) to r⁄(O1–H1) shows the stabilization energy of about 5.21 kcal/mol. Moreover, the intramolecular interaction due to the lone pair electron donation from LP(1)O2 and LP(2)O2 to the anti bonding orbital r⁄(O1–H1) stabilizes the molecular system by 4.11 and 23.45 kcal/mol, which strengthens intramolecular hydrogen bond O1–H1  O2–C2. When excited to the S1 state, the stabilization energies of r(C1–O1) distributed to r⁄(C1–C6) and r⁄(C1–C7) are 1.00 and 0.90 kcal/mol respectively. 3.5. Potential energy curves analysis The potential energy curves of AS1 and AS2 are calculated and plotted in Fig. 4 in order to study the dynamics behavior of the ESIPT process in detail. The bond lengths of O1–H1, O2–H1, and O4–H2 of AS1 and the bond length of H1–O2 of AS2 are scanned by step-length (0.05 Å) for their corresponding energy in the S1 states. Fig. 4A–C correspond to the potential curve of AS1 located at the bond lengths of O1–H1 O2–H1, and O4–H2 involved in the formation of HB1 and HB2 respectively. The arrows plotted in Fig. 4 denote the optimized structures of AS1 and AS2 in the excited S1 states. In Fig. 4A, the energy difference between original structure

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Fig. 3. The molecular electrostatic potentials AS1, AS2 and AS3 in ground and the excited S1 states.

Table 1 Second order perturbation theory analysis of fock matrix in NBO basis for AS1 in the ground state and the excited S1 state. Donor

ED/e

Acceptor

ED/e

E(2)a

Ground state BD(1)C1–C6

1.97438

BD(1)C1–C7

1.97751

BD⁄(1)C1–C7 BD⁄(1)C1–O1 BD⁄(1)C1–C6 BD⁄(1)C1–O1 BD⁄(1)O1–H1 BD⁄(1)C1–C6 BD⁄(1)C1–C7 BD⁄(1)O4–H2 BD⁄(1)C1–C6 BD⁄(1)C1–C7 BD⁄(1)C2–O2 BD⁄(1)C2–C5 BD⁄(2)C2–O2 BD⁄(1)O1–H1 BD⁄(1)C3–O3 BD⁄(2)C2–O2 BD⁄(1)O1–H1 BD⁄(2)C2–O2 BD⁄(1)O4–H2 BD⁄(1)O4–H2 BD⁄(1)C1–C7 BD⁄(2)C2–O2 BD⁄(1)O1–H1 BD⁄(1)O1–H1 BD⁄(1)O1–H1

0.02377 0.01878 0.03431 0.01878 0.06123 0.03431 0.02377 0.05119 0.03431 0.02377 0.01097 0.05354 1.96296 0.06123 1.99493 1.96296 0.06123 1.96296 0.05119 0.05119 0.02377 1.96296 0.06123 0.06123 0.06123

3.34 0.51 3.60 0.54 1.66 1.00 0.90 16.99 1.90 2.19 1.15 1.45 32.99 5.21 16.47 25.62 0.82 37.41 2.88 19.71 0.53 2.63 0.88 4.11 23.45

BD(1)C1–O1

BD(1)C2–C6

a

1.99396

1.97631

BD(1)C2–O2 BD(2)C2–O2

1.99125 1.96296

BD(1)C3–C4

1.97289

BD(1)C3–O3

1.99493

BD(1)C8–O4 LP(1)O2 LP(2)O2

1.99356 1.96997 1.87244

Donor

ED/e

Acceptor

ED/e

E(2)a

Excited S1 state BD(1)C1–C6

1.97445

BD (1)C1–C7

1.97874

BD(1)C1–O1

1.99379

BD(1)C2–C6

1.97505

BD(1)C2–O2 BD(1)C3–O3

1.99195 1.99485

LP(1)O1

1.97156

LP(2)O1 LP(2)O2

1.76426 1.96665

LP(1)O3

1.96819

BD⁄(1)C1–C7 BD⁄(1)C1–O1 BD⁄(1)C2–C6 BD⁄(1)C1–C6 BD⁄(1)C1–O1 BD⁄(1)C2–C6 BD⁄(1)C1–C6 BD⁄(1)C1–O1 BD⁄(1)C2–C6 BD⁄(1)C2–O2 BD⁄(1)C1–C7 BD⁄(1)C2–C5 BD⁄(1)C1–O1 BD⁄(1)C2–C6 BD⁄(2)C2–O2 BD⁄(1)C1–C6 BD⁄(1)C1–C7 BD⁄(2)C2–O2 BD⁄(1)C1–O1 BD⁄(1)C2–C5 BD⁄(2)C2–O2 BD⁄(1)C1–O1 BD⁄(1)C2–C6 BD⁄(2)C2–O2 BD⁄(1)C3–C4

0.02532 0.01633 0.04888 0.03814 0.01633 0.04888 0.03814 0.01633 0.04888 0.01273 0.02532 0.04361 0.01633 0.04888 0.33476 0.03814 0.02532 0.33476 0.01633 0.04361 0.33476 0.01633 0.04888 0.33476 0.05168

2.68 0.63 43.05 2.87 1.49 43.91 0.74 1.19 2.31 0.63 1.98 1.92 69.03 326.03 33.95 6.88 0.51 0.87 1.68 13.89 1.46 47.05 787.70 27.24 6.65

E(2) means energy of hyper conjugative interaction stabilization energy (kcal/mol).

and the optimized structure denoted as arrow is 9.34 kcal/mol, indicating that the proton transfer from O1–H1 to atom O2 is energy favorable and is a barrierless process. With the lengthening of O1–H1, the corresponding energy of AS1 decreases to the minimum point, which corresponds to the optimized structure in the S1 state, implying that the ESIPT process of AS1 occurs along the intramolecular hydrogen bond HB1 naturally. When the hydrogen atom of O1–H1 moves gradually close to the oxygen atom of O2–C2, the hydroxyl group O1–H1 cleaves and a new hydroxyl group O2–H1 forms. In Fig. 4B, with the shortening of O2–H1, the energy of AS1 drops to the minimum point of the curve with energy decreasing of 9.34 kcal/mol, which is consistent with the results of Fig. 4A. In Fig. 4C, with the lengthening of O4–H2, the energy decreases to the lowest point of the potential curve which corresponds to the optimized structure of AS1 in the S1 state. When passing through the minimum, the energy of AS1 increases with the increasing of the bond length of O4–H2. The energy barrier between the minimum and the largest point is 11.85 kcal/mol,

which is so large that the ESIPT process is not energy favorable. It should be noted that, unlike HB1, HB2 does not undergo ESIPT process. The analysis of potential energy curves of Fig. 4A and B indicates that the ESIPT process of AS1 occurs along HB1 rather than HB2 and gives a clear picture of the dynamics process of AS1 before and after photoexcitation. In Fig. 4D, the bond length of H1–O2 denotes the intramolecular hydrogen bond HB1. With the decrease of the bond length of H1–O2, the energy of AS2 decreases to the minimum point corresponding to the optimized structure in the S1 state. The energy difference is calculated to be 4.79 kcal/mol, implying that the ESIPT process of AS2 along HB1 is energy favorable. 3.6. Infrared spectra analysis To delineate the changes of HB1 and HB2 before and after photoexcitation, the infrared spectra of AS1 and AS2 in ground and the excited S1 states are calculated and provided in Fig. 5. In Fig. 5A, the

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Fig. 4. The potential energy curves scanned on the bond length of O1–H1, O2–H1, and O4–H2 of AS1 and that of H1–O2 of AS2 with step-length (0.05 Å) and their corresponding energy in the S1 states.

Fig. 5. The infrared spectra of AS1 and AS2 in ground and the excited S1 states with the groups involved in the formation of intramolecular hydrogen bond.

peaks at 1652 and 3267 cm 1 correspond to C2–O2 and O1–H1 of AS1 respectively and disappear when excited to the S1 state. It is interesting to find that two new and strong stretching vibration peaks located at 2588 and 1364 cm 1 appear in the excited S1 state and correspond to functional groups O2–H1 and C1–O1 respectively. It should be noticed that the HB1 formed in the ground state is cleaved after photoexcitation to the S1 state, accompanying the cleavage of bond O1–H1 and C2–O2 and the formation of carbonyl group C1–O1 and hydroxyl group O2–H1. This change demonstrates that the ESIPT process of AS1 transfers from atom H1 of O1–H1 to atom O2 along HB1. In Fig. 5A, the stretching absorption peaks located at 3382 and 1681 cm 1 correspond to the O4–H2 and C3–O3 groups of AS1 respectively, red-shifting to 3306 and 1670 cm 1 when excited to the S1 state. This red-shifting of functional groups O4–H2 and C3–O3 indicates that the intramolecular hydrogen bond HB2 is strengthened rather than cleaves like HB1

after photoexcitation to the S1 state. This situation is consistent with the results of bond length analysis, charge analysis and NBO analysis of AS1 before and after photoexcitation. In Fig. 5B, the stretching absorption peaks at 1653 and 3261 cm 1 correspond to functional groups C2–O2 and O1–H1 respectively, both of which disappear when excited to the S1 state. It is noted that two new peaks are formed and located at 1435 and 2610 cm 1 respectively, corresponding to new bonds C1–O1 and O2–H1. The infrared spectra of AS1 and AS2 demonstrate the ESIPT process of AS1 and AS2 along HB1 indirectly. 3.7. Absorption and fluorescent spectra analysis The calculated absorption and fluorescent spectra of AS1, AS2 and AS3 are provided in Fig. 6. The absorption peak located at 430 nm corresponds to the transition to the S1 state of AS1, which

Y. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 814–820

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(206 nm) and AS2 (202 nm) are much larger than that of AS3 (98 nm). The analysis of Stokes shift demonstrates that the absorbed energies of AS1 and AS2 are partly transferred to non-harmful long fluorescence under ESIPT process, which plays important role in the explanation for the UV protection property of AS1 and AS2. In this work, the dynamics behaviors of the ESIPT process and novel spectra of AS1 are comprehensively interpreted, which might provide adequate dynamics information for further study of the UV protection property of AS1. Acknowledgments This work is supported by National Natural Science Foundation of China (Grant No. 11274096), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 124200510013) and Supported by Program for Innovative Research Team (in Science and Technology) in University of Henan Province (Grant No. 13IRTSTHN016). Fig. 6. The absorption spectra and fluorescence spectra of AS1, AS2 and AS3 and their Stokes shift.

is slightly larger than that of AS2 (427 nm). The fluorescent peak of AS1 is located at 636 nm, larger than that of AS2 (629 nm). It is found that the non-formation of HB2 of AS2 hardly induce any change of absorption and fluorescence spectra compared to that of AS1. The Stokes shift of AS1 is calculated to be 206 nm, slightly larger than that of AS2 (202 nm). The absorption peak of AS3 is located at 424 nm, close to that of AS1 and AS2. But, the fluorescent peak of AS3 (522 nm) is much smaller than that of AS1 and AS2. The Stokes shift of AS1 and AS2 is significantly larger than that of AS3 (98 nm). Moreover, the fluorescent spectra of AS1 are considerably influenced by HB1 rather than HB2. This can explain why AS1 and AS2 undergo ESIPT process along intramolecular hydrogen bond HB1 while AS3 does not undergo this process. The large Stokes shifts of AS1 (206 nm) and AS2 (202 nm) indicate that the absorbed energy is partly transferred to non-harmful long fluorescence under ESIPT process, which offers part of the explanation for the UV protection property of AS1 and AS2. 4. Conclusions The TDDFT method is performed in this work for the study of dynamics behavior of the ESIPT process and the spectral behaviors of AS1 in excited state. Firstly, the analysis of geometric structure provides direct evidence that AS1 undergoes the ESIPT process along intramolecular hydrogen bond O1–H1  O2–C2 rather than along O4–H2  O3–C3. The frontier orbital and electrostatic potential interpret the reason for the occurrence of ESIPT process of AS1 and AS2. Secondly, the infrared peak corresponding to the O1–H1 of AS1 disappears while a new peak corresponding to a new bond O2–H1 (located at 2588 cm 1) appears after photoexcitation to the first excited state. Moreover, the functional groups involved in the formation of O4–H2  O3–C3 are both red-shifted after photoexcitation to the S1 state. The analysis of infrared demonstrates that the ESIPT process of AS1 occurs along intramolecular hydrogen bond O1–H1  O2–C2. Thirdly, the potential energy curves of AS1 interpret clearly the dynamics process of proton transfer from hydroxyl group O1–H1 to carbonyl group C2–O2 accompanied with the cleavage of O1–H1 and formation of new bond O2–H1. The analysis of potential energy curves demonstrates that the ESIPT process of AS1 along the intramolecular hydrogen bond O1–H1  O2–C2 is energy favorable while that along the O4–H2  O3–C3 is energy prohibited. Finally, we demonstrate that AS1, AS2 and AS3 have similar absorption peaks (430, 427 and 424 nm respectively), and that the Stokes shifts of AS1

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Photoinduced excited state intramolecular proton transfer and spectral behaviors of Aloesaponarin 1.

The novel spectral behaviors of Aloesaponarin 1 (AS1) are investigated by studying the dynamics process of excited state intramolecular proton transfe...
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