Theoretical Investigation of an Excited-State ... - ACS Publications

Subscriber access provided by READING UNIV

Article

Theoretical Investigation of Excited-State Intramolecular Proton Transfer Mechanism for an Asymmetric Structure of 3,7-Dihydroxy-4oxo-2-phenyl-4H-chromene-8-carbaldehyde: Single or Double? Zhe Tang, Yi Wang, DongShuai Bao, Meiheng Lv, Yi Yang, Jing Tian, and Liang Dong J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08266 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Theoretical Investigation of Excited-State Intramolecular Proton Transfer Mechanism for an Asymmetric Structure of 3,7-Dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde: Single or Double? Zhe Tanga,b

Yi Wanga* Dongshuai Baob

Meiheng Lvb

Yi Yangc

Jing Tiana

Liang Donga

a School of Biological Engineering, Dalian Polytechnic University, Dalian 116034, China b State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China c School of Light Industry & Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China

E-mail: [email protected] Fax: +86 0411-86323646

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

Abstract 3,7-Dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde

in

methylcyclohexane solvent was chosen to investigate excited-state intramolecular proton transfer mechanisms by using a time-dependent density-functional theory method. The results show that the single- and double-proton-transfer mechanisms are related and exist simultaneously in the excited states, which differs from those reported in previous experiments (Serdiuk et al., RSC. Adv. 2015, 5, 102191−102203). The analyses of bond distance, bond angle, the molecular electrostatic potential surface and the infrared vibrational spectra show that two intramolecular hydrogen bonds were formed in the S0 state, and upon excitation, the two intramolecular hydrogen bonds were strengthened in the S1 state, which can facilitate the proton-transfer process. The calculated absorption and fluorescence spectra agree well with the experimental results. The constructed potential-energy surfaces on the S1 and S0 states can explain the proton-transfer process. In the S1 state, three types of proton-transfer processes exist as type 1 (single-proton transfer: H2 from O1 to O3), type 2 (single-proton transfer: H5 from O4 to O6) and type 3 (double-proton transfer). The relationship of the potential barrier is: type 1 (1.02 kcal/mol) < type 2 (1.57 kcal/mol) < type 3 (2.29 kcal/mol), which indicates that type 1 is most susceptible to proton transfer.

2


Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The excited-state proton transfer reaction has been studied extensively experimentally and theoretically because of its important role in a variety of biological and physicochemical processes.1-9 In 1956, Weller and co-workers observed the excited-state intramolecular proton transfer (ESIPT) reaction experimentally.10 Research into the ESIPT reaction has attracted increased attention from scholars because of its wide application to systems such as laser dyes and light-emitting diodes,11,12 ultraviolet-filters,13,14 molecular switches,15,16 fluorescence probes,17 and so on.18–21 In general, the ESIPT process includes single-proton transfer (PT) via the existence of an intramolecular hydrogen bond between a protic acid group (–OH, –NH2) and a basic site (–C=O, –C=N) in close proximity. Upon photo-excitation, the intramolecular hydrogen bond is strengthened, which promotes the occurrence of an ESIPT reaction.22–24 Such a single-PT process may not be valid to simulate the double-proton reaction in materials and biological systems. Therefore, researchers

are

increasingly

concerned

with

excited-state

intramolecular

double-proton transfer (ESIDPT) processes because they simulate the inherent nature of proton relays in important materials and biological systems.25–38 The ESIDPT process is a special ESIPT reaction that involves two PT sites in a single molecule. In recent years, a variety of biological functional double-proton organic compounds have been designed for synthesis, including derivatives of salicylic acid,39 chromones 40–46 and oxazoles,28,47,48 and the ESIDPT process has been investigated based on steady-state and time-dependent absorption and fluorescent spectroscopies, and a series of theoretical calculation techniques.28,39–48 Research focused on axisymmetric compounds and the same two PT sites, and studied the stepwise or concerted ESIDPT mechanism. Recently, Serdiuk et al. synthesized two novel asymmetric flavonoid derivatives and used spectroscopic techniques to study their ESIPT mechanisms.49 Among them, they believe that the newly synthesized 3,7-Dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde

(2a)

asymmetric

flavonoid derivatives behave in the electron-excited state similar to the 3-hydroxyflavone (3-HF) derivatives. In 1979, Sengupta and Kasha first reported that 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

this dual fluorescence results from ESIPT and that “blue” fluorescence occurs from N*, whereas “green” fluorescence originates from T* (see Scheme 1).50 Many investigations have also demonstrated the mechanism and dynamics of the system.50–58 However, the conclusion that the 2a compound is similar to the electronically excited state behavior of the 3-HF derivatives by spectral investigation is not comprehensive because of the two PT sites. Firstly, in the previous experimental reports, there are no direct information about the photophysical and photochemical properties for the 2a compound. Secondly, there is a lack of detailed discussion on the ESIPT process. Finally, it is necessary to further demonstrate whether the 2a compound can undergo an ESIDPT process. Therefore, we will conduct a detailed theoretical investigation of the 2a compound. In this work, the theoretical investigation of the ESIPT mechanism of 2a compound in methylcyclohexane (MCH) solvent focuses on the discussion of singleor double-proton reactions in the excited state. We have optimized all conformations in the ground and excited states using the density-functional theory (DFT) and time-dependent density-functional theory (TDDFT) methods as shown in Figure 1. The calculated primary structural parameters and infrared (IR) vibrational spectra have been monitored to predict changes in intramolecular hydrogen bonds. The frontier molecular orbitals (MOs), molecular electrostatic potential surface (MEPS), Mulliken's charge analyses and natural population analysis (NPA) methods understand the charge distribution, which provides the tendency for the ESIPT process. To provide a more detailed and clear ESIPT mechanism, we also construct the S0 and S1 states of the potential-energy surfaces (PESs).

2. Computational details Theoretical calculations were accomplished based on the Gaussian 09 programs suite.59 Geometry optimizations for the ground and excited states were finished based on the DFT and TDDFT methods with Becke One Parameter Hybrid Functionals (B1B95) and the triple-ζ valence quality with one set of polarization functions (TZVP).60,61 To be consistent with previous experiments,49 a polarizable continuum 4


Page 4 of 27

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


model (PCM) that uses the integral equation formalism variant (IEFPCM) was applied with MCH as the solvent. The geometries of the ground and excited states were optimized without constraints of bonds and angles. All optimized structures of the ground and excited states were proven to be local minima without imaginary modes in the vibrational analysis calculations. We calculated the Mayer bond order, Wiberg bond order, Mulliken's charge analysis and NPA. All calculations were performed by using the Multiwfn program.62 To determine the choice of B1B95, we tested a number of functions, among which the B1B95 function provides the most satisfactory agreement with the experimental results in Table S1. The computational details are presented in the Supporting Information (SI).

3. Results and discussion 3.1 Optimized geometric structures and infrared vibrational spectra The structures of the ground and excited states of 2a-N, 2a-T1 (single-PT form: H2 from O1 to O3), 2a-T2 (single-PT form: H5 from O4 to O6) and 2a-T3 (double-PT form) have been optimized by DFT and TDDFT methods at the B1B95/TZVP level in MCH solvent based on the IEFPCM solvation model (shown in Figure 1). We have used serial numbers to mark the most significant atoms that were involved in the intramolecular hydrogen bonds. The primary bond lengths and angles of the 2a-N, 2a-T1, 2a-T2 and 2a-T3 forms have been listed in Table 1. For the 2a-N form, the hydrogen bond lengths of O1–H2 and O4–H5 are increased from 0.976 Å (S0) to 0.995 Å (S1) and 0.989 Å (S0) to 0.990 Å (S1), respectively. The hydrogen bond lengths of H2···O3 and H5···O6 are decreased from 1.972 Å (S0) to 1.838 Å (S1) and 1.678 Å (S0) to 1.587 Å (S1), respectively, which indicates that the intramolecular hydrogen bond in the S1 state is strengthened. The increase in bond angles of O1-H2···O3 and O4-H5···O6 confirms the enhancement of hydrogen bonds in the S1 state. For the 2a-T1 form, because of the ESIPT, atoms H2 and O3 form a bond, and form a hydrogen bond with O1. The hydrogen bonds of H5···O6 are still enhanced in the S1 state, and the length of the bond reduction according to hydrogen is 0.087 Å. In summary, the primary bond length and angle of the 2a compound indicate that the intramolecular 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

hydrogen bonds are enhanced in the S1 state, which means the ESIPT reaction is more likely to occur in the S1 state. MEPS has been used extensively to understand sites for nucleophilic reactions and electrophilic attack, such as intramolecular or intermolecular hydrogen bonding interactions.63,64 The MEPS of the 2a-N form is shown in Figure 2. At the negative and positive electrostatic potential, a significant polarization distribution exists on the surface. The blue and red of the electrostatic potential represent the region of the most positive and negative electrostatic potential, respectively. The positive electrostatic potential on the H2 and H5 atom proves its function as a hydrogen-bond donor site. The negative potential on the O3 and O6 atom indicates that it is an obvious hydrogen-bond acceptor site, which illustrates that intramolecular PT occurs by attraction between an electropositive H atom (H2 and H5) and an electronegative O atom (O3 and O6). We have calculated the IR vibrational spectra of the 2a-N form in the S0 and S1 states as provided in Figure 3. Upon excitation, the calculated O1-H2 bond stretching vibrational frequency at 3601 cm−1 in the S0 state changes to 3317 cm−1 in the S1 state. The 284 cm−1 red-shift of the O1-H2 stretching frequency shows that the intramolecular hydrogen bond (O1-H2···O3) is enhanced in the S1 state. To prove that the hydrogen bond is strengthened in the S1 state and the trend of double-PT, we calculated the vibration frequency of the O4-H5 bond. The stretching vibrational model of the O4-H5 bond is located at 3301 cm−1 in the S0 state, whereas it is located at 2920 cm−1 in the S1 state. The 171 cm−1 red-shift of the O4-H5 bond stretching frequency shows that the intramolecular hydrogen bond (O4-H5···O6) of the 2a-N form is stronger in the S1 state. Therefore, the change in stretching vibration frequency between the proton donor and the proton acceptor is affected by the formation of hydrogen bonds, which provides the basis for the ESIPT process. 3.2 Electronic spectra, frontier molecular orbitals, and charge distribution The

corresponding

absorption

and

fluorescence

spectra

by

the

TD-DFT/B1B95/TZVP level are shown in Figure 4. The absorption peak of 2a-N exists at 361 nm, which is consistent with experimental data at 337 nm.49 The calculated normal fluorescence peak of 2a-N is at 463 nm, which shows a large stokes 6


Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shift of approximately 103 nm. However, the experiment only reported a fluorescence peak of 2a-T1 at 551 nm, which agrees well with our calculated result (584nm). To explore other single-PT and double-PT forms, we calculated the fluorescence peaks of 2a-T2 and 2a-T3. By calculating the results, we find that 2a-T2 and 2a-T3 form fluorescence peaks at 502 nm and 619 nm, respectively. As shown in Figure 4, we can see that the oscillator strength of 2a-N, 2a-T2 and 2a-T3 is relatively weak compared with 2a-T1, which is one of the reasons that only the fluorescence peak is reported. To make the data more intuitive, we have calculated the 2a-N, 2a-T1, 2a-T2 and 2a-T3 forms of the oscillator strength, as listed in Table S1. Two reasons exist for reporting only one fluorescence peak in the experiment. One reason is that the potential barriers in the excited states 2a-N to 2a-T1 and 2a-N to 2a-T3 are too high to cross the available energy. Another reason is that the energy barrier between the 2a-N, 2a-T1, 2a-T2 and 2a-T3 forms on the excited state is not very high and stabilizes at the lowest point of energy 2a-T1 form and occurs with fluorescent radiation. We provide a reasonable explanation for this phenomenon in the following sections. To investigate the charge distribution and charge transfer of the excited state, we analyzed the frontier MOs of the 2a-N in MCH solvent and explored the nature of the excited state (shown in Figure 5). We also calculated electronic transition energies, and the corresponding oscillator strengths are listed in Table 2. The calculated results show that the oscillator strength of the first excited state is 0.1584, which is significantly higher than that of the second excited state. Hence, we only calculated the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbital (shown in Figure 5). The HOMO exhibits a π character and the LUMO exhibits a π* character, which leads to the distinct ππ* feature of the S0→ S1 transition. The ππ* transition is an allowed transition. In general, the system prefers to deactivate from S1 to S0 by radiation (emitting fluorescence) rather than by nonradiative deactivation. It is well known that the study of biological activity, chemical hardness–softness and molecular optical polarizability can be reflected on the basis of energy gap (Egap). A low kinetic stability and a high chemical reactivity are associated with a small frontier orbital Egap.65 The calculated Egap between HOMO 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and LUMO is 4.204 eV, which indicates that 2a-N has a low kinetic stability and a high chemical reactivity. Figure 5 shows that the HOMO and LUMO are localized on different parts of the 2a-N, and the first excited state involves intramolecular charge transfer. The electron density of the O2 and O6 atom increases and that of the benzene ring moiety decreases, which improves the attraction to H and promotes the ESIPT process that occurs upon photo-excitation. To make the calculation results more reliable, we also calculate the frontier molecular orbital based on the TDDFT/CAM-B3LYP/TZVP level, and the results are consistent (shown in Figure S1 and Table S2). The phenomenon of ESIPT is a substantial modulation of the electron charge density distribution on heavy atoms caused by photo-excitation. A detailed study of the charge distribution over the atoms in the intramolecular hydrogen bonds by Mulliken's charge analysis and NPA charge methods may provide evidence for exploring the ESIPT mechanism. Based on Mulliken's charge analysis for 2a-N, a decrease in the negative charge of the O1 atom from –0.274 in the S0 state to –0.263 in the S1 state occurs with an increase in the negative charge of the O3 atom from –0.390 in the S0 state to –0.403 in the S1 state. The negative charge of the O4 atom decreases from –0.239 in the S0 state to –0.236 in the S1 state and the negative charge of the O6 atom does not change in the S1 and S0 states. To analyze our calculation results better, the NPA charge method has been compared with the Mulliken's charge analysis to provide more reliable evidence. The negative charges of the O1 and O4 atom based on the NPA charge decrease from –0.636 to –0.631 and –0.610 to –0.609, respectively. The negative charges of the O3 and O6 atom based on the NPA charge decrease from –0.599 to –0.608 and –0.560 to –0.567, respectively (the data are listed in Table S3). Our calculations indicate that the results obtained by the two types of charge analysis methods are consistent. The calculated Mayer and Wiberg bond orders are listed in Table S4. The O1-H2 and O4-H5 bonds are strengthened and the H2-O3 and H5-O6 bonds are weakened, which agrees with our conclusion above. Therefore, intramolecular charge transfer and intramolecular hydrogen-bond enhancement will facilitate the ESIPT process. 8


Page 8 of 27

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3 Potential-energy surfaces To illustrate the ESIPT mechanism further and to explore whether single- or double-PT occurs, the geometrical structures in the ground- and excited-state were optimized with the O1-H2 and O4-H5 bond length fixed at a series of values at the TD-DFT/B1B95/TZVP level with the IEFPCM solvation model for MCH. The PESs have been scanned with bond lengths of O1-H2 and O4-H5 ranging from 0.87 to 2.27 with a step size of 0.2 Å in the S0 state and from 0.89 to 2.29 with a step size of 0.2 Å in the S1 state, respectively. Figure 6a shows the PESs of the S1 state. The coordinates of the four minimum points are N (1.09, 1.10), T1 (1.89, 1.10), T2 (1.09, 1.50) and T3 (1.89, 1.50), respectively, with the energy relationship ET2 > EN > ET3 > ET1. For the PESs of the S0 state, the coordinates of the two minimum points are N (1.07, 1.08) and T1 (1.67,1.08) as shown in Figure 6b. The precise values of these four minima are based on the B1B95/TZVP calculation level as listed in Table 3. 2a-T1 is the most stable form in the S1 state. To determine the specific values of the protons that are transferred on the PESs, the potential-energy barriers among the four minimum values are listed in Table S5. The potential-energy barriers are 1.02 kcal/mol (N→T1), 1.57 kcal/mol (N→T2), 2.29 kcal/mol (N→T2), 2.34 kcal/mol (T1→T3) and 1.38 kcal/mol (T2→T3). Through these potential barrier values, it can be shown that single- and double-PT processes may occur in the S1 state. The reverse PT barriers are 7.58 kcal/mol (T1→N), 0.32 kcal/mol (T2→N), 6.91 kcal/mol (T3→N), 0.137 kcal/mol (T3 →T1) and 6.71 kcal/mol (T3→T2), respectively. The potential barrier between the reverse PT is not high, which indicates that four minimum points can coexist. In addition, we also calculate the barrier between the lowest points in the S0 state. To provide a more reasonable and intuitive explanation, we consider the entire molecule to summarize the PT process in the S1 and S0 states as shown in Figure 7. 2a is excited from the S0 state to the S1 state with a form identified as point 2a-N. The PT process can follow three paths. Type 1 is the transfer of a H2 atom from the O1 atom to the O3 atom to form 2a-T1. Type 2 is the transfer of a H5 atom from the O4 atom to the O6 atom to form 2a-T2. Type 3 is when the H2 and H5 atoms undergo PT simultaneously to form 2a-T3. The relationship between the potential barrier is type 1 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

< type 2 < type 3, which shows that type 1 is most prone to PT. The 2a-T2 and 2a-T3 forms can also be transferred to the 2a-T1 form because they are not the most stable form. In other words, upon photo-excitation, the 2a compound reaches the S1 state and undergoes proton transfer along the three types to form the 2a-T1, 2a-T2 and 2a-T3 forms, respectively, and then returned to the S0 state by fluorescent radiation transition. In particular, since the energy of 2a-T1 is lower than that of 2a-T2 and 2a-T3, the potential barrier between them is not high, so that the fluorescence is mainly emitted in 2a-T1. Thus, this can be another reason for the experiment to provide only one fluorescence spectrum.49 Subsequently, the 2a-T1, 2a-T2 and 2a-T3 forms in the S0 state are unstable and collapse back to the 2a-N form. Therefore, the construction of the PESs of the 2a compound contributes to the investigation of the ESIPT and ESIDPT processes. In addition, we further demonstrate the existence of the ESIDPT process and explain the reason that the ESIDPT process is not trapped in the experiment.

4. Conclusion The ESIPT mechanism of the 2a molecules was investigated based on the geometric structures, electronic spectra, photo-physical properties and PESs using the DFT and TDDFT methods. By calculating the bond length, bond angle and IR vibrational spectra of the geometric structure, it was shown that the intramolecular hydrogen bond in the S1 state was enhanced significantly. The absorption and fluorescence spectra are consistent with the experimental results, which indicates that the calculated methods are valid. We also discussed the charge distribution and charge transfer. The frontier MOs, MEPS, Mulliken's charge analysis and NPA charge methods show that the intramolecular charge-transfer phenomenon arises upon photo-excitation and predicts the occurrence of a PT reaction. The PESs of the S1 and S0 states were constructed based on the O1-H2 and O4-H5 bond lengths that were fixed at a series of values. The PESs in the S1 state proved the rationality of the single- and double-PT mechanisms. Single-PT occurs via the transfer of a hydroxyl proton to the O3 or O6 atom along the hydrogen bonds. The double-proton mechanism transfers the 10


Page 10 of 27

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


other proton after a single-PT or transfers both protons along two hydrogen bonds simultaneously. In particular, we further demonstrate the existence of the 2a-T2 and 2a-T3 forms by calculating the oscillator strength and energy barrier. Moreover, we have explained the only one fluorescence is measured in the experiment reasonably. The first reason is that the oscillator strengths of 2a-T2 and 2a-T3 are lower than that of 2a-T1, and the second reason is that the energy of 2a-T1 is lower than that of 2a-T2 and 2a-T3 in the S1 state. Therefore, we provide a detailed mechanism of single-PT and double-PT, which is theoretical significance for the future study of 2a compound. Acknowledgements This work was supported by the Open Project of SKLMRD-K2017_5 (Open Project of State Key Laboratory of Molecular Reaction Dynamics). The results of quantum chemical calculations described in this paper were obtained on the homemade Linux cluster of group 1101, Dalian Institute of Chemical Physics.

Supporting Information The comparisons of different functionals for 2a in MCH solvent (Table S1); Electronic excitation energy (nm), corresponding oscillator strengths and the corresponding compositions for 2a based on the TDDFT/CAM-B3LYP/TZVP level (Table S2); calculated Mulliken's charge and natural population analysis of O1, H2, O3, O4, H5 and O6 atom (Table S3); calculated Mayer and Wiberg bond order of O1-H2, H2-O3, O4-H5 and O5-H6 (Table S4); the potential energy barriers among configurations 2a-N,2a-T1,2a-T2 and 2a-T3 in S0 and S1 states; and frontier molecular orbitals (HOMO and LUMO) of 2a-N based on TDDFT/CAM-B3LYP/TZVP level.

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

References: (1) Sytnik, A.; Kasha, M. Excited-state intramolecular proton transfer as a fluorescence probe for protein binding-site static polarity. Proc. Natl. Acad. Sci. U. S. A. 1994, 91, 8627−8630. (2) Kukura, P.; McCamant, D. W.; Mathies, R. A. Femtosecond Stimulated Raman Spectroscopy. Rev. Phys. Chem. 2007, 58, 461−488. (3) Meech, S. R. Excited State Reactions in Fluorescent Proteins. Chem. Soc. Rev. 2009, 38, 2922−2934. (4) Martinez, T. J. Insights for Light-Driven Molecular Devices from ab initio Multiple Spawning Excited-State Dynamics of Organic and Biological Chromophores. Acc. Chem. Res. 2006, 39, 119−126. (5) Han, K. L.; He, G. Z. Photochemistry of aryl halides: Photodissociation dynamics. J. Photochem. Photobiol. C: Photochem. Rev. 2007, 8, 55−66. (6) Chai, S.; Zhao, G. J.; Song, P.; Yang, S. Q.; Liu, J. Y.; Han, K. L. Reconsideration of the Excited-state Double Proton Transfer (ESDPT) in 2-aminopyridine/acid Systems: Role of the Intermolecular Hydrogen Bonding in Excited States. Phys. Chem. Chem. Phys. 2009, 11, 4385−4390 (7) Zhou, Q.; Du, C.; Yang, L.; Zhao, M.Y.; Dai, Y. M.; Song, P. Mechanism for the Excited-State Multiple Proton Transfer Process of Dihydroxyanthraquinone Chromophores. J. Phys. Chem. A. 2017, 121, 4645−4651. (8) Zhang, M. X.; Zhou, Q.; Du, C.; Ding, Y.; Song, P. Detailed theoretical investigation on ESIPT process of pigment yellow 101. RSC Adv. 2016, 6, 59389−59394. (9) Zhao, J. F.; Chen, J. S.; Cui, Y. L.; Wang, J.; Xia, L. X.; Dai, Y. M.; Song, P.; Ma, F.C.

A

questionable

excited-state

double-proton

transfer

mechanism

3-hydroxyisoquinoline. Phys. Chem. Chem. Phys. 2015, 17, 1142−1150. (10) Weller, A. Innermolekularer protonenubergang im angeregten zustand, Z. Elektrochem. 1956, 60, 1144–1147.

12


for

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Ma, D. G.; Liang, F. S.; Wang, L. X.; Lee, S. T.; Hung, L. S. Blue organic light-emitting devices with an oxadiazole-containing emitting layer exhibiting excited state intramolecular proton transfer. Chem. Phys. Lett. 2002, 358, 24–28. (12) Chou, P. T.; Studer, S. L.; Martinez, M. L. Practical and Convenient 355-nm and 337-nm Sharp-Cut Filters for Multichannel Raman Spectroscopy. Appl. Spectrosc. 1991, 45, 513-515. (13) Catalán, J.; Valle, J. C. D.; Fabero, F.; Garcia, N. A. The influence of molecular conformation on the stability of ultraviolet stabilizers toward direct and dye-sensitized photoirradiation: the case of 2-(2′- hydroxy-5′-methyphenyl) benzotriazole (TIN P). Photochem. Photobiol. 1995, 61, 118−123 (14) Chou, P. T.; Martinez, M. L.; Studer, S. L. The Design of an Effective “Fluorescence Filter” for Raman Spectroscopy. Appl. Spectrosc. 1991, 45, 918-921. (15) Chou, P. T.; Martinez, M. L.; Cooper, W. C.; Chang, C. P. Photophysics of 2-(4′-Dialkylaminophenyl)benzothialzoles: Their Application for Near-UV Laser Dyes. Appl. Spectrosc. 1994, 48,604−606. (16) Cui, Y.; Li, P.; Wang, J.; Song, P.; Xia, L. An Investigation of Excited-State Intramolecular Proton Transfer Mechanism of New Chromophore. J. At. Mol. Sci. 2015, 6, 23−33. (17) Lou, Z. R.; Li, P.; Han, K. L. Redox-Responsive Fluorescent Probes with Different Design Strategies. Acc. Chem. Res. 2015, 48, 1358−1368. (18) Lim, S. J.; Seo, J.; Park, S. Y. Photochromic Switching of Excited-State Intramolecular Proton-Transfer (ESIPT) Fluorescence: A Unique Route to High-Contrast Memory Switching and Nondestructive Readout. J. Am. Chem. Soc. 2006, 128, 14542−14547. (19) Tang, K. C.; Chang, M. J.; Lin, T. Y.; Pan, H. A.; Fang, T. C.; Chen, K. Y.; Hung, W. Y.; Hsu, Y. H.; Chou, P. T. Fine Tuning the Energetics of Excited-State Intramolecular Proton Transfer (ESIPT): White Light Generation in A Single ESIPT System. J. Am. Chem. Soc. 2011, 133, 17738−17745. (20) Liu, Y. H.; Lan, S. C.; Zhu, C. Y.; Lin, S. H. Intersystem Crossing Pathway in Quinoline Cpyrazole Isomerism: A Timedependent Density Functional Theory Study 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

on Excited-state Intramolecular Proton Transfer. J. Phys. Chem. A. 2015, 119, 6269−6274. (21) Yu, F. B.; Li, P.; Wang, B. S.; Han, K. L. Reversible Near-Infrared Fluorescent Probe Introducing Tellurium to Mimetic Glutathione Peroxidase for Monitoring the Redox Cycles between Peroxynitrite and Glutathione in Vivo. J. Am. Chem. Soc. 2013, 135, 7674−7680. (22) Zhao, G. J.; Han, K. L. Early Time Hydrogen-Bonding Dynamics of Photoexcited Coumarin 102 in Hydrogen-Donating Solvents: Theoretical Study. J. Phys. Chem. A. 2007, 111, 2469−2474. (23) Zhao, G. J.; Han, K. L. Time-dependent density functional theory study on hydrogen-bonded

intramolecular

charge-transfer

excited

state

of

4-dimethylamino-benzonitrile in methanol. J. Comput. Chem. 2008, 29, 2010−2017. (24) Zhao, G. J.; Han, K. L. Hydrogen Bonding in the Electronic Excited State. Acc. Chem. Res. 2012, 45, 404−413. (25) Sekiya H,; Sakota K. Excited-state double-proton transfer in a model DNA base pair: resolution for stepwise and concerted mechanism controversy in the 7-azaindole dimer revealed by frequency-and time-resolved spectroscopy. J. Photochem. Photobiol. C: Photochem. Rev. 2008, 9, 81–91. (26) Plasser, F.; Barbatti, M.; Aquino, A. J. A.; Lischka, H. Excited-State Diproton Transfer in [2,2′-Bipyridyl]-3,3′-diol: the Mechanism Is Sequential, Not Concerted. J. Phys. Chem. A. 2009, 113, 8490−8499. (27) Gil M.; Waluk, J. Vibrational Gating of Double Hydrogen Tunneling in Porphycene. J. Am. Chem. Soc. 2007, 129, 1335−1341. (28) Mordzinski, A.; Grabowska, A.; Kuhnle, W.; Krowczynski, A. Intramolecular single and double proton transfer in benzoxazole derivatives. Chem. Phys. Lett. 1983, 101, 291−296. (29) Chu, Q.; Medvetz, D. A.; Pang, Y. A Polymeric Colorimetric Sensor with Excited-State Intramolecular Proton Transfer for Anionic Species. Chem. Mater. 2007, 19, 6421−6429.

14


Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Weiß J.; May, V.; Ernsting, N. P.; Farztdinov, V.; Mühlpfordt, A. Frequency and time-domain analysis of excited-state intramolecular proton transfer. Double-proton transfer in 2,5-bis(2-benzoxazolyl)-hydroquinone? Chem. Phys. Lett. 2001, 346, 503−511. (31) Wnuk, P.; Burdzinski, G.; Sliwa, M.; Kijak, M.; Grabowska, A.; Sepiot, J. Kubicki, J. of

From ultrafast events to equilibrium-uncovering the unusual dynamics

ESIPT

reaction:

the

case

of

dually

fluorescent

diethyl-2,5-(dibenzoxazolyl)-hydroquinone. Phys. Chem. Chem. Phys. 2014, 16, 2542−2552. (32) Lim, S. J.; Seo J.; Park, S. Y. Photochromic Switching of Excited-State Intramolecular Proton-Transfer (ESIPT) Fluorescence: A Unique Route to High-Contrast Memory Switching and Nondestructive Readout. J. Am. Chem. Soc. 2006, 128, 14542−14547. (33) Nagaoka, S.; Uno H.; Huppert, D. Ultrafast Excited-State Intramolecular Proton Transfer of Aloesaponarin I. J. Phys. Chem. B. 2013, 117, 4347−4353. (34) Wang, J.; Chu, Q.; Liu, X.; Wesdemiotis C.; Pang, Y. Large Fluorescence Response by Alcohol from a Bis(benzoxazole)-Zinc(II) Complex: The Role of Excited State Intramolecular Proton Transfer. J. Phys. Chem. B. 2013, 117, 4127−4133. (35) Ma, C; Yang, Y. G.; Li, C. Z.; Liu, Y. F. TD-DFT Study of the Double Excited-State

Intramolecular

Proton

Transfer

Mechanism

of

1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole. J. Phys. Chem. A. 2015, 119, 12686−12692. (36) Serdiuk, I. E.; Roshal, A. D. Exploring double proton transfer: A review on photochemical features of compounds with two proton-transfer sites. Dyes and Pigments. 2017, 138, 223−244. (37) Piechowska, J.; Virkki, K.; Sadowski, B.; Lemmetyinen, H.; Tkachenko, N. V.; Gryko, D. T. Excited State Intramolecular Proton Transfer in Pi-Expanded Phenazine-Derived Phenols. J. Phys. Chem. A. 2014, 118, 144−151.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

(38) Hao, Y. C.; Chen, Y. Excited-state intramolecular single and double proton transfer emission of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol. Dyes and Pigments. 2016, 129, 186−190. (39) Randino, C.; Ziółek, M.; Gelabert, R.; Organero, J. A.; Gil, M.; Moreno, M.; Lluch J. M.; Douhal, A. Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies.

Phys. Chem. Chem. Phys. 2011, 13,

14960−14972. (40) Falkovskaia, E.; Pivovarenko V. G.; Valle, J. C. Interplay between Intra- and Intermolecular Excited-State Single- and Double-Proton-Transfer Processes in the Biaxially

Symmetric

Molecule

3,7-Dihydroxy-4H,6H-pyrano[3,2-g]-chromene-4,6-dione. J. Phys.Chem. A, 2003, 107, 3316−-3325. (41) Roshal, A. D.; Moroz, V. I.; Pivovarenko, V. G.; Wróblewska A.; Błażejowski, J. Spectral

and

Acid−Base

Features

of

3,7-Dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-dione (Diflavonol)A Potential Probe for Monitoring the Properties of Liquid Phases. J. Org. Chem. 2003, 68, 5860−5869. (42) Moroz, V. V.; Chalyi, A. G.; Serdiuk, I. E.; Roshal, A. D.; Zadykowicz, B.; Pivovarenko, V. G.; Wróblewska A.; Błażejowski, J. Tautomerism and Behavior of 3-Hydroxy-2-phenyl-4H-chromen-4-ones

(Flavonols)

and

3,7-Dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-diones (Diflavonols) in Basic Media: Spectroscopic and Computational Investigations. J. Phys. Chem. A. 2013, 117, 9156−9167. (43)

Svechkarev,

D.

A.;

Doroshenko

A.

1,4-bis-(3-hydroxy-4-oxo-4H-chromen-2-yl)-benzene

O.;

Kolodezny,

(bis-flavonol):

D.

Y.

synthesis,

spectral properties and principle possibility of the excited state double proton transfer reaction Cent. Eur. J. Chem. 2012, 10, 205−215.

16


Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Cui, Y. L.; Li, P. Y.; Wang, J.; Song P.; Xi, L. X. An investigation of excited-state intramolecular proton transfer mechanism of new chromophore. J. At. Mol. Sci. 2015, 6, 23−33. (45) Huang, J. D.; Wu, J. Y.; Dong, H.; Song, P.; Zhao, J. F. Straightforward Stepwise Excited State Dual Proton Transfer Mechanism for 9-10-HBQ System. Commun. Comput. Chem. 2017, 5, 27−36. (46) Zhao, J. F.; Yao, H. B.; Liu, J. Y.; Hoffmann, M. R. New Excited-State Proton Transfer Mechanisms for 1,8-Dihydroxydibenzo[a,h]phenazine. J. Phys. Chem. A. 2015, 119, 681−688. (47) Wortmann, R.; Lebus, S.; Reis, H.; Grabowska, A.; Kownacki K.; Jarosz, S. Spectral and electrooptical absorption and emission studies on internally hydrogen bonded benzoxazole `double' derivatives: 2,5-bis(benzoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Single versus double proton transfer in the excited BBPC revisited. Chem. Phys. 1999, 243, 295−304. (48) Enchev, V.; Markova, N.; Stoyanova, M.; Petrov, P.; Rogozherov, M.; Kuchukova, N.; Timtcheva, I.; Monev, V.; Angelova S.; Spassova, M. Excited state proton transfer in 3,6-bis(4,5-dihydroxyoxazo-2-yl)benzene-1,2-diol. Chem. Phys. Lett. 2013, 563, 43−49. (49) Serdiuk, I. E.; Roshalb, A. D. Single and double intramolecular proton transfers in the electronically excited state of flavone derivatives. RSC. Adv. 2015, 5, 102191−102203. (50) Sengupta, P. K.; Kasha, M. Excited state proton-transfer spectroscopy of 3-hydroxyflavone and quercetin. Chem. Phys. Lett. 1979, 68, 382−385 (51) Woolfe, G. J.; Thistlethwaite, P. J. Direct Observation of Excited State Intramolecular Proton Transfer Kinetics in 3-Hydroxyflavone. J. Am. Chem. Soc. 1981, 103, 6916−6923. (52) Itoh, M.; Tokumura, K.; Tanimoto, Y.; Okada, Y.; Takeuchi, H.; Obi, K.; Tanaka, I. Time-Resolved and Steady-State Fluorescence Studies of the Excited-State Proton Transfer in 3-Hydroxyflavone and 3-Hydroxychromone. J. Am. Chem. Soc. 1982, 104, 4146−4150. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(53) McMorrow, D.; Kasha, M. Proton-transfer spectroscopy of 3-hydroxychromones. Extreme sensitivity to hydrogen-bonding perturbations. J. Am. Chem. Soc. 1983, 105, 5133−5134. (54) Strandjord, A. J. G.; Barbara, P. F. Hydrogen/deuterium isotope effects on the excited-state proton transfer kinetics of 3-hydroxyflavone. Chem. Phys. Lett. 1983, 98, 21−26. (55) Strandjord, A. J. G.; Barbara, P. F. The proton-transfer kinetics of 3-hydroxyflavone: solvent effects. J. Phys. Chem. 1985, 89, 2355−2361. (56) Barbara, P. F.; Walsh, P. K.; Brus, L. E. Picosecond kinetic and vibrationally resolved spectroscopic studies of intramolecular excited-state hydrogen atom transfer. J. Phys. Chem. 1989, 93, 29−34. (57) Brewer, W. E.; Studer, S. L.; Chou, P. T. Temperature-dependent study of the ground-state reverse proton transfer of 3-hydroxyflavone. Chem. Phys. Lett. 1989, 158, 345−350. (58) Schwarz, B. J.; Peteanu, L. A.; Harris, C. B. Direct observation of fast proton transfer: femtosecond photophysics of 3-hydroxyflavone. J. Phys. Chem. 1992, 96, 3591−3598. (59) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Scalmani, J. R.; Cheeseman, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (60) Schafer A,; Horn H,; Ahlrichs R. Fully optimized contracted gaussian-basis sets for atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571−2577. (61) Feller, D. The Role of Databases in Support of Computational Chemistry Calculations. J. Comput. Chem. 1996, 17, 1571−1586. (62) Lu, T.; Chen, F. W. Multiwfn: A multifunctional wavefunction analyzer, J. Comp. Chem. 2012, 33, 580−592. (63) Galabov, B.; Bobadova-Parvanova, P. Molecular Electrostatic Potential as Reactivity Index in Hydrogen Bonding: Ab Initio Molecular Orbital Study of Complexes of Nitrile and Carbonyl Compounds with Hydrogen Fluoride. J. Phys. Chem. A. 1999, 103, 6793−6799. 18


Page 18 of 27

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(64) Kenny, P. W. Hydrogen Bonding, Electrostatic Potential, and Molecular Design, J. Chem. Inf. Model. 2009, 49, 1234−1244. (65) Luber, S.; Adamczyk, K.; Nibbering, E. T. J.; Batista, V. S. Photoinduced Proton Coupled Electron Transfer in 2-(2′-Hydroxyphenyl)-Benzothiazole. J. Phys. Chem. A. 2013, 117, 5269−5279.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Table 1 Calculated primary bond lengths (Å) and angles (°) of 2a-N, 2a-T1, 2a-T2 and 2a-T3 forms in the S0 and S1 states.

2a-N

2a-T1

2a-T2

2a-T3

S0

S1

S0

S1

S0

S1

S0

S1

O1-H2

0.976

0.995

1.740

1.937

-

0.995

-

1.906

H2-O3

1.972

1.838

1.007

0.982

-

1.825

-

0.985

O4-H5

0.989

1.007

0.990

1.002

-

1.473

-

1.396

H5-O6

1.678

1.587

1.699

1.612

-

1.041

-

1.076

δ(O1-H2-O3)

119.7

122.2

126.7

118.9

-

122.8

-

119.8

δ(O4-H5-O6)

147.6

153.7

147.5

152.2

-

155.2

-

156.8

Table 2 Electronic excitation energy (nm), corresponding oscillator strengths and the corresponding compositions for the 2a-N form based on the TDDFT/B1B95/TZVP level.

transition

λ(nm)

f

composition

CI(%)

S0→S1

361.50

0.1584

H→L

97.83

H-5→L

54.47

H-5→L+1

17.16

H-4→L

25.29

H-4→L+1

3.07

2a-N S0→S2

315.37

0.0070

20


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 Potential energies (Hartree) of these stable structures on the S0 and S1 states based on the DFT and TD-DFT methods, respectively.

2a-N

2a-T1

2a-N

2a-T1

S0 Energy -991.7972

2a-T2

2a-T3

S1 -991.7824

-991.6881 -991.6985 -991.6861 -991.6946

Scheme 1 The 3-hydroxyflavone of a general ESIPT process

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Structures of 2a-N, 2a-T1, 2a-T2 and 2a-T3 from.

Figure 2. The total electron density isosurface mapped with molecular electrostatic potential surface (MEPS) for 2a-N form. From negative (red) to positive (bluish): −0.063 a.u.–0.063 a.u.

22


Page 22 of 27

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Calculated IR spectra of 2a-N form in the spectral region of O1-H2 and O4-H5 stretching bands in the S0 and S1

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. The calculated electronic spectra of 2a-N, 2a-T1, 2a-T2 and 2a-T3 at the TDDFT/B1B95/TZVP/IEFPCM(MCH) theoretical level.

24


Page 24 of 27

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Frontier molecular orbitals (HOMO and LUMO) of 2a-N form corresponding to S1 state and its transition energy

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. PESs of the S0 and S1 states of 2a molecule as functions of the O1-H2 and O4-H5 lengths both from 0.9 to 2.3 Å. (a) S1 state PES; (b) S0 state PES.

26


Page 26 of 27

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. The proton transfer processes of the 2a molecule in S0 and S1 states.

27


Theoretical Investigation of an Excited-State ... - ACS Publications

Recommend Documents