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Article

Effects of Residual Water on Proton Transfer-Switching Molecular Device

by
Hiroto Tachikawa
Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Photochem 2024, 4(4), 462-473; https://doi.org/10.3390/photochem4040028
Submission received: 20 September 2024 / Revised: 28 October 2024 / Accepted: 28 October 2024 / Published: 31 October 2024
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Abstract

:
The excited state proton transfer (ESPT) reaction plays a crucial role in DNA defense and ON-OFF proton-switching molecular devices. o-Hydroxybenzaldehyde (OHBA) is the simplest model-molecule for the ESPT reactions where a proton is transferred from OH to C=O carbonyl groups by photo-excitation. In the present study, the reaction mechanism of ESPT in OHBA was investigated by means of the direct ab initio molecular dynamics (AIMD) method. The triplet (T1) state of OHBA, OHBA(T1), was considered as the excited state of OHBA. The dynamic calculations showed that fast PT occurred from OH to C=O carbonyl groups at the T1 state. The time of PT was calculated to be 34–57 fs in OHBA(T1). The spin density was mainly distributed on the benzene ring (Bz) at time zero. The density was gradually transferred from Bz to C=O as a function of time on the T1 surface. When the spin density on C=O was larger than that on Bz (at time = 35–43 fs), the proton of OH was rapidly transferred to C=O. The localization of spin density on C=O dominated strongly the PT rate. Next, the effects of residual water (H2O) on the PT rate were investigated using OHBA-H2O 1:1-complexes to elucidate the effects of H2O on the PT rate in the ON-OFF proton-switching molecular devices. The PT rates were strongly dependent on the position of H2O around OHBA. The reaction mechanism is discussed based on theoretical results.

Graphical Abstract

1. Introduction

Proton transfer (PT) is one of the simplest chemical reactions that play an important role in many areas. [1,2,3,4,5,6] For example, PT acts as a DNA defense reaction (Figure 1) [7,8,9] and a photo-induced ON-OFF switching device [10].
o-Hydroxybenzaldehyde (OHBA) is the simplest molecule that causes intramolecular PT at the first excited S1 state [11,12,13]. It is also a model molecule for ON-OFF optical switching devices. Hence, OHBA and related molecules have been the subject of much research [14,15,16]. Nagaoka et al. observed high-resolution fluorescence spectra of OHBA by means of emission spectroscopy [17,18,19]. They showed that the first and second excited states are expressed as S1(π, π*) and S2(π, π*) states, respectively. At the S1 state, the excited state (ES) PT occurs, as shown in Figure 2. With the help of quantum chemical calculations, they revealed that the energy minimum point of the S1(π, π*) state in OHBA is largely distorted from that of the S0 state (ground state), and the minimum point of the S2(π, π*) state is close to that of the S0 state: normal emission of OHBA occurs from S2 state and the red-shifted PT band appears from the S1 state.
A number of theoretical calculations have been carried out by several research groups to understand the PT process in OHBA [20,21,22]. De et al. calculated potential energy curves (PECs) for the PT processes in OHBA using B3LYP/6-31G(d) and time-dependent (TD)-B3LYP/6-31G(d) levels of theory [23]. Their calculations showed that the energy minimum point at the S1 state was far away from that of the ground S0 state, suggesting that the large Stokes shifts are expected in the fluorescence spectrum of OHBA. In contrast, the energy minimum point at the S2 state was close to that of the S0 state. These results support Nagaoka’s experiments [20,21,22].
Thus, PT in OHBA at the first excited S1 state has been the subject of a great deal of research. Almost all works were carried out for the S1 state. In contrast, the excited triplet (T1) state has not been studied. The information on the reaction of the T1 state is scarcely known. In the present study, we focus on PT at the T1 state of OHBA because the T1 state has a long lifetime and is important in molecular switching devices. The effect of H2O on the PT rate was also studied using the OHBA-H2O 1:1 complex.
Residual H2O in the air has a significant impact on the operation of semiconductors and photo-induced ON-OFF switching devices [24,25,26,27]. For example, it is known that the performance of a thin-film transistor (TFT) is significantly degraded by H2O in the air. Recently, it was shown that defending against H2O adsorption can increase the transistor’s operating speed by a factor of ten in In2O3-based TFT [27]. Ghediya et al. suggested that coating of the In2O3 surface prevents the adsorption of H2O and increases its ability as a semiconductor [27]. Therefore, the study of the effect of H2O on the PT speed of optical switching devices is an extremely important topic. In the present calculation, the effects of H2O on the PT rate in OHBA(T1) were investigated in order to shed light on the mechanism of PT in the ON-OFF switching devices.

2. Computational Details

2.1. Ab Initio Calculations

The geometries of the neutral states of OHBA and OHBA-H2O 1:1 complexes were fully optimized using the CAM-B3LYP/6-311++G(d,p) method [28]. Thereafter, the ground and first excited triplet states were expressed as OHBA(S0) and OHBA(T1), respectively. The second-order perturbation Møller–Plesset (MP2) method with a 6-311++G(d,p) basis set was also used for comparison [29,30]. Atomic and molecular charges were calculated using the natural population analysis (NPA) approach. Standard Gaussian 09 and 16 program packages were used for all static ab initio calculations [31,32].

2.2. Direct Ab Initio Molecular Dynamics (AIMD) Calculations

In the methodology of direct AIMD calculation, dynamical trajectories are generated by using forces computed “on the fly” from ab initio calculations. The equations of motion for N atoms in the reaction system are given by the following:
d Q j d t = ∂ H ∂ P j d P j d t = − ∂ H ∂ Q j = − ∂ U ∂ Q j
where j = 1–3N, H is the classical Hamiltonian, Qj is the Cartesian coordinate of the j-th mode, and Pj is the conjugated momentum. These equations were numerically solved. The velocity Verlet algorithm, with a time step of 0.10–0.25 fs, was used to solve the equations of motion for the system. The maximum simulation time was 2.0 ps. The total energy drift in all trajectory calculations was <0.01 kcal/mol.
Two methods were used in the direct AIMD calculations for OHBA(T1): (a) the optimized structure of OHBA(S0) was used as the starting structure at time zero without the zero-point vibrational energy (ZPE), and (b) OHBA(T1) including ZPE. In the direct AIMD calculation of OHBA(T1), OHBA(S0) was first optimized at the CAM-B3LYP/6-311++G(d,p) level. Thereafter, the trajectory of OHBA(T1) was started from the vertical excitation point from the S0 structure. The rotational temperature, momentum vector, and excess energy of OHBA(T1) were set as zero (time = 0 fs). No symmetry restriction was applied to calculate the energy gradient. The vertical excitation from OHBA(S0) to OHBA(T1) was assumed. For micro-hydrated systems, OHBA-H2O, similar calculations were applied using OHBA-H2O 1:1 complexes. The bulk solvent was not considered in the present calculations.
The effects of ZPE on the reaction mechanism were investigated using the classical vibrational sampling method (microcanonical ensemble) [33,34,35]. The calculations including ZPE were performed at the CAM-B3LYP/6-31G(d) level. Direct AIMD calculations were carried out using our code [36,37,38]. The drifts of the total energies were less than 1.0 × 10−2 kcal/mol in all trajectory calculations. These levels of theory gave reasonable features for the reaction dynamics of several molecular systems [36,37,38].

3. Results

3.1. Structures of OHBA Systems

The optimized structure of OHBA(S0) calculated at the CAM-B3LYP/6-311++G(d,p) level is given in Figure 3. The O-H bond length in the O-H group and the distance of the proton (H) from the carbonyl oxygen were R1 = 0.981 and R2 = 1.761 Å, respectively. The bond length of the C=O carbonyl was RCO = 1.221 Å. The atomic charges were O1(−0.320), O2(−0.252), and H(+0.320) and at the neutral state of OHBA(S0). At the vertical excited triplet state (T1), the atomic charges were changed to O1(−0.370), O2(−0.181), and H(+0.324), indicating that the positive charge on the carbonyl oxygen atom (O1) increased at the excited T1 states, while the positive charge on the OH group increased to +0.068 (S0) and +0.143 (T1). This means that the vertical excitation to the T1 state causes a slight charge transfer from the n-orbital of the OH group to the π* orbital of the C=O carbonyl.
The optimized structures of micro-hydrated OHBA (OHBA-H2O 1:1 complex) are given in Figure 3. Several geometries of OHBA(S0)-H2O were examined as the initial structures in the geometry optimization, and three stable structures were obtained from the calculations and expressed as types 1, 2, and 3. In type 1, H2O interacted with the carbonyl group of OHBA, expressed as C=O-H2O. Two hydrogen bonds were connected with H2O, and the distances were r1 = 1.927 and r2 = 2.593 Å. In type 2, H2O interacted with the O-H group of OHBA, expressed as OH-H2O and the distances were r1 = 1.953 and r2 = 2.411 Å. In type 3, H2O interacted with a bridge site composed of both carbonyl and OH groups. H2O made a bridge between OH and C=O groups, and the distances were r1 = 1.965 and r2 = 2.036 Å.
The binding energies between H2O and OHBA(S0) for types 1, 2, and 3 were calculated to be 6.4, 5.9, and 4.6 kcal/mol, respectively and the relative energies were 0.0, 0.5, and 1.8 kcal/mol, respectively, at the CAM-B3LYP/6-311++G(d,p) level. Three structures have similar binding energies.
The same calculations were carried out at the MP2/6-311++G(d,p) level. Similar structures and energetics were obtained: the binding energies of H2O to OHBA(S0) for types 1, 2, and 3 were calculated as 5.9, 6.4, and 4.8 kcal/mol, respectively. The accordance suggests that the CAM-B3LYP/6-311++G(d,p) level of theory gives reasonable structures and energetics for the OHBA systems.

3.2. Proton Transfer Dynamics on T1 Potential Energy Surface of OHBA

Snapshots and the time evolution of the potential energy (PE) of OHBA(T1) are given in Figure 4. Zero level of PE corresponds to the total energy of OHBA(T1) at the vertical excitation point from OHBA(S0). At time zero, the O-H bond length was R1 = 0.990 Å and the proton was located at R2 = 1.390 Å from the oxygen atom of C=O carbonyl. After the vertical excitation to the T1 state, PE decreased suddenly to −5.5 kcal/mol at time= 0.0–6.0 fs. This energy decrease was caused by the structural change in the molecular skeleton in OHBA(T1). After the energy lowered, PE vibrated periodically in the ranges PE= (−6.0)–(−3.0) kcal/mol during time = 6.0–35.0 fs. At 38.3 fs, the proton of OH was located at R1 = 1.070 and R2 = 1.444 Å, suggesting that the proton was still bonded to the OH group. At 43.4 fs, the O-H bond was elongated and the proton was located at R1 = 1.159 and R2 = 1.343 Å.
In the final stage of the PT reaction (50.2 fs), the proton was fully transferred to C=O (R1 = 1.397 and R2 = 1.006 Å). The time of PT was calculated as 50.2 fs for this trajectory. PE was −13.0 kcal/mol at 50.2 fs, indicating that the PT reaction was exothermic in OHBA(T1). After PT, the structural relaxation of OHBA(T1) occurred and was completed in 80 fs (PE was −18.0 kcal/mol).
The results of MP2/6-311++G(d,p) are given in Figure S1 in the Supporting Information (SI). Similar reaction dynamics were obtained by the MP2 level, where the time of PT was calculated as 42.9 fs.
Figure 5 shows the time evolution of the atomic spin densities of OHBA(T1) after the excitation to the T1 state. The spin densities of carbon and oxygen atoms in the C=O carbonyl were C(C=O)= 0.12 (C) and O(C=O)= 0.21 (O) at time zero, respectively. In contrast, the carbon atoms in the benzene ring of OHBA(T1), expressed as Bz(C), had larger spin densities of 0.40 (Bz(C), indicating that Bz(C) spin densities were significantly larger than those of C=O. After excitation to the T1 state, the structure of OHBA gradually deformed on the T1 surface against to the most stable structure. The spin density distributions gradually varied with the structural change. The value of the spin densities on C(C=O) and O(C=O) coincided with those of Bz(C) at 38.2 fs (the values were about 0.25). At 43.4 fs, the densities were completely reversed: 0.32 for C=O and 0.14 for Bz(C). PT occurred rapidly at this reverse region. Namely, PT takes place after the spin density transfer from Bz(C) to C=O. At 50.2 fs, the densities of C=O and Bz(C) were 0.47 and 0.02, respectively. The time of PT is strongly dependent on the rate of transfer of the spin density from Bz to C=O caused by the structural deformation.

3.3. Proton Transfer Dynamics in OHBA(T1)-H2O

The static ab initio and DFT calculations gave three types of structures of OHBA(S0)-H2O (types 1, 2, and 3). In this section, direct AIMD calculations were carried out for the three structures. The dynamics calculations for types 1 and 2 showed that the time of PT was 61.5 fs (type 1) and 43.7 fs (type 2). These time scales were close to that of OHBA without H2O (50.2 fs). The snapshots and energy profiles for types 1 and 2 were similar to OHBA, as shown in Figures S2 and S3 in SI.
In contrast, the time of PT in type 3 was significantly longer than that of types 1 and 2. Snapshots and the time evolution of PE of OHBA(T1)-H2O (type 3) are given in Figure 6. At time zero, H2O was located at r1 = 1.965 and r2 = 2.036 Å. H2O was bound to the bridge site composed of OH and C=O groups of OHBA, namely, the oxygen and proton of H2O bind to OH and C=O, respectively. The distances of H2O were r1 = 2.466 and r2 = 2.086 Å at 144.5 fs, and those were r1 = 2.792 and r2 = 1.962 Å at 219.2 fs, indicating that the movement of H2O occurred from the bridge to C=O sites during time = 0–220 fs in addition to the structural deformation of the molecular skeleton of OHBA(T1). After the vertical excitation to the T1 state, PE decreased to −5.5 kcal/mol within 5.0 fs and vibrated periodically in the ranges PE= (−6.0)–(−2.0) kcal/mol. At 219.2 fs, the proton was located at R1 = 1.035 and R2 = 1.497 Å, suggesting that the proton was still bound to the OH group. At 251.2 fs, the proton was suddenly transferred to C=O (R1 = 1.376 and R2 = 1.019 Å). The time of PT was 251.2 fs for this trajectory. PE was down to −14.0 kcal/mol, indicating that the PT reaction was exothermic in OHBA(T1)-H2O. After PT, the structural relaxation of OHBA(T1) occurred.
Figure 7 shows the time evolution of atomic spin densities of OHBA(T1)-H2O (type 3) after the excitation to the T1 state. The spin densities of carbon and oxygen atoms in the C=O carbonyl were C(C=O)= 0.12 (C) and O(C=O)= 0.27 (O) at time zero, respectively. In contrast, the carbon atoms in the benzene ring of OHBA(T1)-H2O, expressed as Bz(C), had larger spin densities of 0.40 (Bz(C)), indicating that the spin density (unpaired electron) was mainly distributed on Bz(C) at time zero. After the excitation to the T1 state, the movement of H2O around OHBA(T1) gradually occurred. The spin density distributions were changed from Bz(C) to C=O. The value of spin densities on C(C=O) and O(C=O) coincided with those of Bz(C) at 220.0 fs (the values were about 0.25–0.28). At 251.2 fs, the densities were completely reversed, 0.68 for C=O and 0.07 for Bz(C). At this point, PT occurs rapidly from OH to C=O. Namely, PT takes place after spin density transfer from Bz(C) to C=O as well as OHBA(T1) without H2O. The time of PT is strongly dependent on the rate of transfer of spin density (unpaired electron) from Bz to C=O caused by the structural deformation of OHBA(T1) and movement of H2O around OHBA(T1).

3.4. Effects of Zero-Point Energy (ZPE) on the Reaction Mechanism

Direct AIMD calculations of OHBA(T1), including ZPE calculations, were performed at the CAM-B3LYP/6-31G(d) level. Thirty trajectories were performed. The reaction time for PT was distributed in the range of 10.0–58.8 fs, with an average of 34.3 fs. In contrast, the reaction time in the absence of ZPE was 52.5 fs. These results indicate that the ZPE accelerated the reaction time for PT in OHBA(T1).
In the case of OHBA(T1)-H2O (type 3), the time of PT including ZPE was calculated as 153.7 fs, which was about a third of the time of PT without ZPE (502.4 fs). These results indicate that the ZPE also accelerated the reaction time in OHBA(T1)-H2O.

3.5. Summary of Trajectory Calculations

Table 1 shows the summary of the time of PT calculated at several levels of theory. The calculations were carried out at the CAM-B3LYP/6-311++G(d,p), CAM-B3LYP/6-311G(d,p), CAM-B3LYP/6-31G(d), and wB97XD/6-311G(d,p) level. All levels of theory gave a similar tendency at the time of PT. The time of PT was calculated as 50.2 fs (OHBA), 61.5 fs (type 1), 43.7 fs (type 2), and 251.2 fs (type 3) at the CAM-B3LYP/6-311++G(d,p) level. The time of PT for types 1 and 2 was close to that of OHBA without H2O. In contrast, the time of PT for type 3 was significantly longer than those of OHBA and types 1 and 2. The water molecule in type 3 binds to the bridge site composed of both OH and C=O groups. After the excitation, H2O in type 3 moved a long distance to localize the spin density on the C=O, involving the transfer of H2O from OH to C=O groups being required. This is why it took so long for type 3. After the excitation to the T1 state, H2O in types 1 and 2 hardly moved and were still bonded to OHBA(T1). The effects of the water molecule were significantly small in types 1 and 2.

4. Discussion

4.1. Reaction Model

On the basis of the calculations, the reaction model for PT on the T1 surface of OHBA is proposed. Figure 8 shows a schematic illustration of the mechanism of PT in OHBA(T1). The initial state is OHBA(S0) at the ground state (no figure). The electronic state is changed from S0 to the T1 state by UV-light irradiation via the S1 state. At time zero of OHBA(T1) (Figure 8a), the spin density is mainly distributed on carbon atoms of the benzene ring, expressed as Bz(C). On the T1 potential energy surface, the structure of OHBA(T1) is gradually deformed against the stable structure. The spin density is also gradually transferred from Bz(C) to C=O with this deformation (time 34–43 fs) (Figure 8b). When the density on C=O is larger than that of Bz(C) and is localized (Figure 8c), PT rapidly takes place from OH to C=O (Figure 8d). Thus, the localization of spin density of C=O plays an important role in the PT reaction.

4.2. Effects of H2O on Proton Transfer-Switching Devices

The addition of H2O to OHBA affected the time of PT. The time of PT in OHBA-H2O was dependent on the position of H2O around OHBA. The solvation of H2O to OH or C=O groups (types 1 or 2) had no significant effect on the reaction rate because H2O hardly moved around OHBA after the excitation to the T1 state in types 1 and 2. In contrast, the solvation in the bridge site (OH-H2O-C=O site: type 3) decelerated the time significantly of PT (50.2 vs. 251.2 fs). This is due to the fact that the movement of H2O is required from the bridge to C=O sites before PT. Therefore, a long time is needed in type 3.
OHBA is one of the simplest molecules for ON-OFF proton transfer-switching devices. Previously, it was known that PT occurred at the excited S1 state of OHBA. The present study indicated that PT takes place on the T1 state surface. The time of PT is very fast, less than 50 fs. Hence, OHBA can be effective for a PT switching device. It was also found that the localization of spin density on C=O is important for PT to proceed.
It is known that the residual water affects strongly the properties of semiconductors. Recently, it was experimentally shown that defending against H2O adsorption can increase the transistor operating speed by a factor of ten in In2O3-based TFT [27]. Ghediya et al. suggested that the coating of the In2O3 surface prevents the adsorption of H2O and increases the ability as semiconductor [27]. Therefore, studying the effect of H2O on PT is important in the development of an effective semiconductor. In the present study, the effects of H2O on the PT rate were investigated by means of the direct AIMD method. The PT rates were strongly dependent on the position of H2O around OHBA. In types 1 and 2, PT rates were close to that of OHBA without H2O. In contrast, H2O in the bridge site (type 3) affected strongly to the PT rate. This is due to the fact that H2O in the bridge site prevented the localization of spin density on the C=O carbonyl of OHBA(T1).

5. Conclusions

The excited state of the PT reaction plays a crucial role in DNA defense and ON-OFF proton-switching molecular devices. OHBA is the simplest model molecule for the PT reactions caused by photo-excitation. In the present study, the reaction mechanism of PT in OHBA was investigated by means of the direct AIMD method. The triplet (T1) state of OHBA, OHBA(T1), was considered as the excited state because there is no information on the reaction of OHBA(T1).
The present dynamic calculations for OHBA(T1) showed that fast PT occurred from the OH to C=O carbonyl groups at the T1 state as well as the S1 state. The time of PT was calculated to be 34–57 fs, indicating that PT in OHBA(T1) was a very fast process. The localization of spin density on C=O dominates strongly the PT rate.
Next, the effects of residual water (H2O) on the PT rate were investigated using the OHBA-H2O 1:1 complexes. The PT rates were affected by the position of H2O around OHBA, where the localization of spin density on C=O was dependent on the position of H2O. The present calculations provided important information on the effects of residual water on PT rate in molecular devices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/photochem4040028/s1, Figure S1. Time evolution of (A) snapshots and (B) potential energy of OHBA(T1). Figure S2. Time evolution of snapshots and potential energy of OHBA(T1)-H2O(type 1). Figure S3. Time evolution of (A) snapshots and (B) potential energy of OHBA(T1)-H2O(type 2).

Funding

This research was funded by JSPS KAKENHI (Grant Numbers: 21K04973).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of the deactivation process of excess energy in DNA base pair (aminopyridine dimer). The excess energy of photo-irradiation to DNA base pair is released by proton transfer.
Figure 1. Schematic illustration of the deactivation process of excess energy in DNA base pair (aminopyridine dimer). The excess energy of photo-irradiation to DNA base pair is released by proton transfer.
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Figure 2. Schematic illustration of the energy diagram for the proton transfer (PT) process in OHBA caused by UV-irradiation. The presented calculations were carried out on PT at the triplet state (T1) potential energy surface.
Figure 2. Schematic illustration of the energy diagram for the proton transfer (PT) process in OHBA caused by UV-irradiation. The presented calculations were carried out on PT at the triplet state (T1) potential energy surface.
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Figure 3. Optimized structure of OHBA and micro-hydrated OHBA(OHBA-H2O 1:1 complexes) calculated at the CAM-B3LYP/6-311++G(d,p) level.
Figure 3. Optimized structure of OHBA and micro-hydrated OHBA(OHBA-H2O 1:1 complexes) calculated at the CAM-B3LYP/6-311++G(d,p) level.
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Figure 4. Time evolution of (A) snapshots and (B) potential energy of OHBA(T1) reaction system. Direct AIMD calculation were performed at the CAM-B3LYP/6-311++G(d,p) level. The CAM-B3LYP/6-311++G(d,p)-optimized geometry of OHBA(S0) was used as the initial geometries of OHBA(T1) at time zero.
Figure 4. Time evolution of (A) snapshots and (B) potential energy of OHBA(T1) reaction system. Direct AIMD calculation were performed at the CAM-B3LYP/6-311++G(d,p) level. The CAM-B3LYP/6-311++G(d,p)-optimized geometry of OHBA(S0) was used as the initial geometries of OHBA(T1) at time zero.
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Figure 5. Time evolution of atomic spin densities of OHBA(T1). Bz(C) means spin densities of carbon atoms in benzene ring of OHBA(T1). O(C=O) and C(C=O) mean spin densities of oxygen and carbon atoms in C=O carbonyl group of OHBA(T1). Direct AIMD calculation were performed at the CAM-B3LYP/6-311++G(d,p) level.
Figure 5. Time evolution of atomic spin densities of OHBA(T1). Bz(C) means spin densities of carbon atoms in benzene ring of OHBA(T1). O(C=O) and C(C=O) mean spin densities of oxygen and carbon atoms in C=O carbonyl group of OHBA(T1). Direct AIMD calculation were performed at the CAM-B3LYP/6-311++G(d,p) level.
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Figure 6. Time evolution of (A) snapshots and (B) potential energy of OHBA(T1)-H2O (type 3) reaction system. Direct AIMD calculations were performed at the CAM-B3LYP/6-311++G(d,p) level. The CAM-B3LYP/6-311++G(d,p)-optimized geometry of OHBA(S0)-H2O (type 3) was used as the initial geometries of OHBA(T1) at time zero.
Figure 6. Time evolution of (A) snapshots and (B) potential energy of OHBA(T1)-H2O (type 3) reaction system. Direct AIMD calculations were performed at the CAM-B3LYP/6-311++G(d,p) level. The CAM-B3LYP/6-311++G(d,p)-optimized geometry of OHBA(S0)-H2O (type 3) was used as the initial geometries of OHBA(T1) at time zero.
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Figure 7. Time evolution of atomic spin densities of OHBA(T1). Bz(C) means the spin densities of carbon atoms in the benzene ring of OHBA(T1). O(C=O) and C(C=O) mean the spin densities of oxygen and carbon atoms in the C=O carbonyl group of OHBA(T1), respectively. Direct AIMD calculations were performed at the CAM-B3LYP/6-311++G(d,p) level.
Figure 7. Time evolution of atomic spin densities of OHBA(T1). Bz(C) means the spin densities of carbon atoms in the benzene ring of OHBA(T1). O(C=O) and C(C=O) mean the spin densities of oxygen and carbon atoms in the C=O carbonyl group of OHBA(T1), respectively. Direct AIMD calculations were performed at the CAM-B3LYP/6-311++G(d,p) level.
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Figure 8. Schematic illustration of reaction model for proton transfer (PT) process in OHBA(T1). (a) Structure at time zero, (b) a slight deformed structure on T1 state, (c) relaxed structure, and (d) proton transferred structure.
Figure 8. Schematic illustration of reaction model for proton transfer (PT) process in OHBA(T1). (a) Structure at time zero, (b) a slight deformed structure on T1 state, (c) relaxed structure, and (d) proton transferred structure.
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Table 1. Summary for the time of proton transfer (PT) in OHBA(T1) calculated at several levels of theory (in fs). ZPE means the results of trajectory calculations including zero-point vibrational energy at the CAM-B3LYP/6-31G(d) level. Results of MP2/6-311++G(d,p) calculation was given in parenthesis.
Table 1. Summary for the time of proton transfer (PT) in OHBA(T1) calculated at several levels of theory (in fs). ZPE means the results of trajectory calculations including zero-point vibrational energy at the CAM-B3LYP/6-31G(d) level. Results of MP2/6-311++G(d,p) calculation was given in parenthesis.
Functional CAM-B3LYPCAM-B3LYPCAM-B3LYPwB97XDZPE
basis set /6-311++G(d,p)/6-311G(d,p)/6-31G(d)/6-311G(d,p)34.3
OHBAOHBA50.2 (42.9)48.252.557.3
OHBA-H2Otype 161.559.064.466.9
type 243.741.743.448.3
type 3251.2320.5502.4442.6153.7
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Tachikawa, H. Effects of Residual Water on Proton Transfer-Switching Molecular Device. Photochem 2024, 4, 462-473. https://doi.org/10.3390/photochem4040028

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Tachikawa H. Effects of Residual Water on Proton Transfer-Switching Molecular Device. Photochem. 2024; 4(4):462-473. https://doi.org/10.3390/photochem4040028

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Tachikawa, Hiroto. 2024. "Effects of Residual Water on Proton Transfer-Switching Molecular Device" Photochem 4, no. 4: 462-473. https://doi.org/10.3390/photochem4040028

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Tachikawa, H. (2024). Effects of Residual Water on Proton Transfer-Switching Molecular Device. Photochem, 4(4), 462-473. https://doi.org/10.3390/photochem4040028

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