Theoretical Study on the Size Dependence of Excited State Proton

Article pubs.acs.org/JPCB

Theoretical Study on the Size Dependence of Excited State Proton Transfer in 1‑Naphthol−Ammonia Clusters Toshihiko Shimizu,† Shunpei Yoshikawa,† Kenro Hashimoto,‡ Mitsuhiko Miyazaki,† and Masaaki Fujii*,† †

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, 192-0397, Japan

‡

S Supporting Information *

ABSTRACT: The geometries and energetics of the ground and lower-lying singlet excited states S0, La, and Lb of 1-naphthol (NpOH)−(NH3)n (n = 0−5) clusters have been computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Cluster size dependence of the excited state proton transfer (ESPT) reaction was investigated by the vertical transitions from the geometries that can be populated in the molecular beam experiments. For the n = 3 and 4 clusters, the proton-transferred geometries cannot be accessible without significant geometrical rearrangement from the initially populated isomers. For the n = 5 clusters, the proton-transferred structure is found in the La excited state of the isomer that can be populated in the beam. Thus, ESPT is possible by the optically prepared Lb state via internal conversion to La. We concluded that the threshold cluster size of ESPT is n = 5 under the experimental condition with low excess energy.

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INTRODUCTION

Most of all, the 1-NpOH−(NH3)n clusters are important because they require only a few solvent molecules to promote ESPT. Thus, they have been considered as one of the prototype systems for ESPT. The first evidence of ESPT in 1-NpOH− (NH3)n clusters was found in fluorescence spectra.9 The n = 4 cluster shows a broad, strongly red-shifted fluorescence spectrum, which is close to the fluorescence spectrum of NpO− in basic solution.9 On the other hand, smaller clusters show only structured spectra; thus, Cheshnovsky and Leutwyler concluded that the minimum size to promote ESPT is n = 4 in 1-NpOH−(NH3)n. Fisher and co-workers also concluded that the minimum size is n = 4 from a sudden change of the time evolution by picosecond pump−probe experiments. On the other hand, Zewail and co-workers interpreted that three ammonias are sufficient to trigger ESPT from the cluster size dependence on lifetime.4 The same conclusion was reported by Bernstein and co-workers from nanosecond and picosecond laser spectroscopy. Later, Dedonder-Lardeux et al. studied evaporation of the clusters and reassigned the cluster size in the mass-selected REMPI spectra. From the reassignments, they concluded that the threshold size of ESPT should be 5.12,13 So far, despite decades of studies, the threshold size of ESPT in 1NpOH−(NH3)n has not been established yet.

Proton transfer is a fundamentally significant process that plays a key role in a large variety of chemical and biological processes, e.g., acid−base reactions, electrophilic additions, enzymatic catalysis, and the primary photosynthetic steps.1−5 Particularly, the excited state proton transfer (ESPT) reaction attracts many researchers1−41 because the reaction can be controllable by photoexcitation. Originally, ESPT was found in an aqueous solution of naphthol (NpOH) by visible emission after UV excitation.42 The origin of the visible emission was interpreted as the formation of an anion such as NpO−; therefore, this emission was thought to release a proton in the excited state. Such proton release corresponds to the increase of acidity, and thus, pKa values of phenol and naphthols were assumed to decrease by S1 ← S0 photoexcitation such as 9.1 (S0) to 0.5 (S1) in 1-NpOH. From the drastic decrease of pKa, phenol and naphthols and related molecules are called photoacids. Later, various laser spectroscopies have been applied to the solvated clusters to elucidate the mechanism of ESPT at the molecular level. For example, ESPT and related phenomena have been examined for phenol−(H2O)n/(NH3)n, 1-NpOH− (H2O)n/(NH3)n, 2-NpOH−(H2O)n/(NH3)n, hydroxyquinoline−(NH3)n, 7-azaindole−(NH3)n, and other systems by laser-induced fluorescence (LIF), dispersed fluorescence, resonant enhanced multiphoton ionization (REMPI), ion dip IR spectroscopy, time-resolved pump−probe experiments, picosecond time-resolved IR dip spectroscopy, and so forth.3−6,8,17,21−23,25,27−29,32−34,36,37 It has been reported that ESPT takes place in the small 1-NpOH clusters with strong bases (ammonia, piperidine, and triethylamine).9,43,44 © XXXX American Chemical Society

Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: July 19, 2014 Revised: September 9, 2014

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dx.doi.org/10.1021/jp507222n | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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Figure 1. Calculated structures (left) of 1-naphthol−(NH3)n (n = 0−2) in the ground state by DFT optimized at the M06-2X/cc-pVTZ level and those (center and right) in the Lb and La states by TD-DFT optimized at the M06-2X/cc-pVDZ level. Each length of OH distance is given in angstroms. Relative binding energies are also presented in kcal/mol.

Through the various experimental studies, the ESPT mechanism in 1-NpOH clusters often has been discussed by the two-step−three-state model. The naphthalene ring has the La and Lb electronic states, and Lb is the lowest excited state in the monomer.45 The polar La state becomes more stabilized by solvation, and then, internal conversion to La (first step) triggers the proton transfer (second step). For this issue, theoretical study should be able to give a good clue to reveal the mechanism and the definite conclusion on the size dependence of ESPT. However, in our best knowledge, only

one theoretical study has been reported for 1-NpOH−(NH3)n clusters.44 The structures of the clusters were calculated up to n = 4 in the excited state (CIS/6-31G* level), and the threshold size of n = 4 was concluded. However, the character of the excited state (La and Lb) was not specified, and the used methods were too low in the present level of computational chemistry. Recent progress of computer technology and time-dependent density functional theory (TD-DFT) enables us to study structures, vibrations, and chemical reactions in the excited B



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Figure 2. Calculated structures (left) of 1-naphthol−(NH3)3 in the ground state by DFT optimized at the M06-2X/cc-pVTZ level and those (center and right) in the Lb and La states by TD-DFT optimized at the M06-2X/cc-pVDZ level. Each length of OH distance is given in angstroms. Relative binding energies are also presented in kcal/mol.

the TD-DFT method, in which the interaction between La and Lb is included. The geometry optimizations of Lb and La were carried out at the M06-2X/cc-pVDZ level. Single point energy calculations were performed on the optimized geometries with CISD/aug-cc-pVDZ. The equilibrium configurations which are stable in S0 were used as initial configurations to optimize the structures in Lb and La. Here, Lb and La were distinguished by the direction of the dipole moment.45 The convergence to the energy minimum was checked by calculating the vibrational frequencies. All structures have been confirmed to have all real vibrational frequencies. The relative solvation enthalpies at 0 K were computed with the zero-point vibrational correction by scaling factors of 0.943 and 0.948 for the ground and excited states, respectively. These values were determined from the νOH ratio of the experimental and computational frequencies of the 1-NpOH monomer. The program used was Gaussian 09.46

molecules with sufficient level. In this paper, we report the quantum chemical calculations of 1-naphthol ammonia clusters (n ≤ 5) in the ground state (S0) and the first and second excited states (Lb and La). Relative electronic energies are compared between non-ESPT and ESPT species in the ground and excited states. The excited state geometry is optimized starting from the ground state geometry, and the size dependence of ESPT was discussed by the vertical transitions from the geometries that can be populated in the molecular beam experiments.

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METHODS

Molecular structures of 1-NpOH−(NH3)n (n = 0−5) in S0 were optimized by using the DFT method. The M06-2X/ccpVTZ level of approximation was used in the geometry optimization in S0. The initial geometries were generated from the smaller cluster by adding an ammonia molecule to the possible hydrogen-bonding sites. Since these clusters are relatively small, we can pick up all the possible hydrogenbonded structures. In the present work, we extended our study to 1-NpOH−(NH3)n (n = 0−5) in the excited singlet states by

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RESULTS AND DISCUSSION Structures of 1-NpOH−(NH3)n (n = 0−5) in the Ground and Excited States. The optimized geometries and their relative energies of 1-NpOH−(NH3)n (n = 0−2) in S0, Lb, and C



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La are shown in Figure 1. The optimized geometry Oa of trans1-NpOH has Cs symmetry, and the OH bond length is 0.961 Å. Meanwhile, the optimized geometry Ob of cis-1-NpOH has C1 symmetry and the OH bond length is 0.959 Å. The cis-1NpOH is less stable by 1.2 kcal/mol relative to trans-1-NpOH in S0. On the other hand, the cis-1-NpOH is more stable than trans-1-NpOH in both the Lb and La ππ* states. This inversion of the relative energies is consistent with the experimental estimations.47 For the n = 1 clusters, we have found two isomers in the ground state: Ia and Ib. The structure of Ia is Cs symmetry in which 1-NpOH acts as a proton donor. The OH bond length is 0.982 Å in S0, while it is elongated to 0.996 and 1.002 Å in Lb and La, respectively. The longer bond length is consistent with the higher acidity in the excited state. Another isomer Ib, cis-1-

NpOH−NH3, is 1.4 kcal/mol more unstable than the trans-1NpOH−NH3 (Ia). Experimentally, the cis-1-NpOH−NH3 has not been observed; thus, the calculated energetics is consistent. The study on the fully resolved S1−S0 fluorescence excitation spectrum showed that the heavy atom separation (O−H−N) of trans-1-NpOH−NH3 is 2.86 Å in S0 and 2.72 Å in S1,48 while our calculated one is 2.82 Å in S0 and 2.76 Å in S1, respectively. Therefore, we can say that the calculations in the present work are reliable because the theoretical analysis has good agreement with experiments. For n = 2, the optimized structures in S0 are shown in IIa− IIc. The complexes IIa and IIb are chain structures in which an ammonia dimer is bound to 1-NpOH by one of the nitrogen atoms through a single hydrogen bond. The 1-NpOH molecule is the trans form in IIa, while it is the cis form in IIb. In the D



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The left column in Figure 2 displays six equilibrium structures IIIa−IIIf for the n = 3 clusters in S0. The complexes IIIa and IIIe are chain structures where a three-membered ammonia chain is bound to 1-NpOH at the end of the chain through a single hydrogen bond. The structure IIIa where 1NpOH is the trans form is more stable than IIIe where 1NpOH is the cis form by 4.1 kcal/mol. On the other hand, the complexes IIIb, IIIc, and IIId are bifurcated structures where a three-membered ammonia chain is bound to 1-NpOH through a single hydrogen bond of the central ammonia molecule. The energies for IIIb and IIId where 1-NpOH is the cis form are 1.6 and 2.7 kcal/mol relative to IIIa, respectively, and that for IIIc where 1-NpOH is the trans form is 1.7 kcal/mol. For the structure IIIf, 1-NpOH acts not only as a proton donor for the two-membered ammonia chain but also as a proton acceptor for the single ammonia molecule. The energy for IIIf is higher than that for IIIa by 5.7 kcal/mol and is the least stable in all

structure IIc, 1-NpOH acts not only as a proton donor but also as a proton acceptor, where no interaction is found between two ammonia molecules. The trans-1-NpOH−(NH3)2 (IIa) is the most stable, and the relative energies to IIb and IIc are 0.5 and 5.2 kcal/mol, respectively. The structure of IIa is very close to the observed one determined by the rotational coherence spectroscopy.49 Thus, the population will be concentrated to this structure in a molecular beam. In the Lb excited state, cis-1NpOH−(NH3)2 (IIbLb) is more stable than trans-1-NpOH− (NH3)2 (IIaLb) by 0.9 kcal/mol. Though the equilibrium structure IIc which is stable in S0 was used as the initial configuration to optimize the structure in Lb, IIcLb converged to the same structure as IIaLb. Likewise, in the Lb state, cis-1NpOH−(NH3)2 (IIbLa) is more stable than trans-1-NpOH− (NH3)2 (IIaLa) by 3.6 kcal/mol. IIcLa also converged to the same structure as IIaLa. E



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Figure 5. (a) Energetics diagram for the 1-naphthol−(NH3)3 isomers relative to the energy of the ground state of the most stable S0-optimized structure IIIa, (b and c) corresponding to 1-naphthol−(NH3)4 and 1-naphthol−(NH3)5, respectively. The relative energies are plotted from the most stable species IVa and Va, respectively. Calculated relative energies are presented in kcal/mol.

isomers. The optimized structures for the n = 3 clusters in Lb and La are also depicted in the center and right columns of Figure 2. The complexes IIIaLb and IIIeLb are also chain structures as well as the case in S0. The structure IIIaLb is more stable than IIIeLb by 4.2 kcal/mol in the same manner. The complexes IIIbLb, IIIcLb, and IIIdLb are also bifurcated structures as well, and their relative energies to the structure IIIaLb are 1.0, 2.7, and 1.3 kcal/mol, respectively, while structure IIIfLb is less stable than IIIaLb by 7.9 kcal/mol. In the Lb state, the proton in naphtholic OH is still close to the NpOH molecule and thus no ESPT is found. On the other

hand, the naphtholic proton in the most stable structure in the La state (IIIbLa) is located 2.017 Å from the oxygen atom and forms NH4+. Such ESPT is also found in the structures IIIdLa and IIIeLa. Thus, n = 3 can be the smallest cluster for the ESPT reaction. The non-ESPT structures, IIIaLa, IIIcLa, and IIIfLa, are more unstable than the ESPT ones; thus, the proton transfer significantly contributes to the stabilization of the clusters in La. For the n = 4 clusters, we have found eight structural isomers IVa−IVh, as shown in Figure 3. The complexes IVa, IVb, IVc, and IVe are chain structures, where a four-membered ammonia chain is bound to the 1-NpOH at one end through a single F



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All the calculated results for n = 3 are represented in Figure 5a. The vertical transition energies Lb and La were shown in the left column with the color of purple and red horizontal bars. For the most stable species IIIa, this set of energies is indicated as S0-opt. The energies La and Lb at the optimized structure for the Lb state were plotted in the center column (Lb-opt) after the energy correction to S0. Similarly, the energies of La and Lb at the La-optimized structure were shown in the right column (La-opt). The same set of energies (S0-, Lb-, and La-opt) is also shown for each isomer. In the S0 state, the population is thought to concentrate to IIIa, because the second stable species IIIb is 1.6 kcal/mol unstable from IIIa. The single species in n = 3 is consistent with the experimental results.9 The vertical transition from IIIa will prepare the Lb state. The optimized structure of Lb is more stable than that of La; thus, the excited cluster will stay in Lb, and no PT will be expected in this geometry. In the IIIb, IIId, and IIIe isomers, the proton-transferred structures (indicated by PT in the figure) were found in the La states. Thus, if geometrical conversion to other isomers is possible, ESPT takes place in n = 3. However, such an isomerization would have a high barrier and significant excess energy should be required to promote ESPT in n = 3. Let us discuss the relation to the experimental results. From the above discussion, ESPT will not occur when the n = 3 cluster is excited to the origin or the low vibrational excited states. This corresponds to the experimental conditions by Leutwyler’s group9 and Fisher’s group.26,49 On the other hand, ESPT will be promoted if the cluster has sufficient internal energy to cross over the barrier. Such high internal energy can be prepared by the excitation to the high vibronic states. The experimental results by Zewail’s group and Bernstein’s group, i.e., ESPT from n = 3, would correspond to this condition. Figure 5b shows the energies of the S0, Lb, and La states in the n = 4 clusters. Unlike n = 3, the clusters populate not only in IVa but also in IVb, since the second stable species IVb is only 0.4 kcal/mol unstable from IVa in S0. The population in IVc would be negligibly small because IVc is 0.7 kcal/mol unstable from IVa. The vertical transition from IVa and IVb will prepare the Lb state which is more stable than La in each isomer. Here the Lb states in both isomers are not the protontransferred structures; thus, no ESPT will be expected within these isomers. In other words, ESPT in n = 4 has to be associated with the isomerization to other isomers in which the proton-transferred structures are the most stable. The isomers IVc, IVf, IVg, and IVh have the proton-transferred structures in the excited states; thus, the isomerization to these isomers over potential barriers should be included if ESPT takes place in n = 4. Similarly to n = 3, ESPT in n = 4 depends on the internal excess energy in the prepared clusters. No ESPT is expected if the n = 4 cluster is prepared to the origin or the low vibrational excited states, while ESPT can be promoted if the internal energy is sufficient to overcome barriers for isomerization. In the experiments, originally Leutwyler’s and Fisher’s groups reported that ESPT was observed in the n = 4 cluster. However, the assignments of the cluster size were revised24 and the originally reported “n = 4” cluster would correspond to the clusters with five ammonia molecules. Similarly, the original “n = 3” cluster is reassigned to the n = 4 cluster. If we will take the revised assignments, the experimental results by Leutwyler’s, Fisher’s, and Jouvet’s groups mean no ESPT in n = 4. These were measured under the low excess energy; thus, it is

hydrogen bond and they are the most stable structures. Meanwhile, the complexes IVd, IVf, and IVg are bifurcated structures where four-membered branched ammonia moieties are bound to 1-NpOH through a single hydrogen bond. The relative energies of IVb−IVg to the structure IVa are 0.4, 0.7, 1.6, 1.7, 3.5, and 4.2 kcal/mol, respectively. The structure IVh which is the proton-transferred form is much less stable than IVa by 10.7 kcal/mol. As for the Lb state, the optimized structures are similar to those in S0 except that IVfLb is the proton-transferred form. However, the proton-transferred structure IVhLb is the most stable. The relative energies of IVaLb−IVgLb to the structure IVhLb are 1.0, 1.4, 1.9, 3.0, 2.8, 5.0, and 6.3 kcal/mol, respectively. Similar to the case in Lb, the proton-transferred structure IVhLa is the most stable in La. The structures IVcLa and IVfLa−IVhLa which are the protontransferred species are much more stable than the nonproton-transferred species by more than 6 kcal/mol. The relative energies of IVaLa−IVgLa are 6.9, 8.0, 0.5, 10.0, 8.0, 0.12, and 0.13 kcal/mol, respectively. The optimized equilibrium structures obtained for n = 5 are Va−Vg, as shown in Figure 4. We found the double cyclic form Va is the most stable. The complexes Vc, Vd, Ve, and Vf are also double cyclic forms aside from Va. On the other hand, Vb is another type of cyclic structure where the two cyclic rings of ammonia molecules are formed separately across the aromatic ring. The complex Vg is a proton-transferred form which is the least stable of all. The relative energies of Vb−Vg to the structure Va are 0.8, 1.9, 2.5, 3.8, 4.2, and 10.2 kcal/mol, respectively. Thus, Va is expected to be more abundant than the others. As with the excited state Lb, the optimized structures are similar to those in S0 other than VdLb and VeLb which are also proton-transferred forms. However, the proton-transferred structure VgLb is the most stable just like the case in n = 4. The relative energies of VaLb−VfLb to the structure VgLb are 0.1, 1.8, 3.5, 2.7, 0.7, and 5.5 kcal/mol, respectively. Interestingly, in the excited state La, all the isomers are proton-transferred forms and the most stable one is VgLa as well as the case in Lb. The relative energies of VaLa−VfLa to the structure VgLa are 2.3, 3.4, 4.5, 4.7, 4.0, and 5.8 kcal/mol, respectively. Calculated Lb−S0 transition energies are compared to the observed ones in Table S1 in the Supporting Information. All of the calculated geometries are included in Table S2 in the Supporting Information. Mechanism of ESPT. We have elucidated the energy relationship in each of the S0-, Lb-, and La-optimized geometries for 1-NpOH−(NH3)n (n = 0−5). In this section, we would like to discuss the ESPT mechanism and the relation to the observed photochemical reactivity. Considering the correspondence to experiments, it is necessary to calculate the vertical transition energies to the electronic excited states. Here, we calculated the vertical transition energies to the Lb and La states at the optimized structures in the S0 state. Independently, the optimized structure in the Lb state was calculated. To compare the energies between the vertical and optimized geometries in Lb, it is necessary to calculate the energies of S0 and La at the Lboptimized (Lb-opt) structure. Similarly, the energies of S0 and Lb were calculated at the La-optimized structure. If we add the energies of S0 to the energies of La and Lb, it is possible to compare the Lb and La energies among isomers. In other words, all the calculated energies can be compared from the energy of S0 in the most stable isomers such as IIIa. G



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consistent with the present theoretical interpretations. The ESPT in “n = 3” reported by Zewail’s and Bernstein’s groups can be reassigned to the ESPT at n = 4, and it would correspond to the high excess energy condition. So far, the reaction mechanism of ESPT is essentially the same in the n = 3 and n = 4 clusters, and the isomerization from the initially prepared isomer is required to promote ESPT. The different ESPT mechanism can be expected in the n = 5 cluster (see Figure 5c). The ground state population is limited to the most stable isomer Va because the second stable species is already 0.8 kcal/mol unstable. According to the calculated vertical transition energies, the initially prepared state by the photoexcitation is the Lb state. The different points from n = 3 and 4 are the relative energies between La and Lb. For n = 5, the La state is significantly more stable than Lb in the same isomer Va; thus, the internal conversion to La is expected after the excitation to Lb. The La state has the proton-transferred structure; thus, the internal conversion corresponds to ESPT. As a result, in the n = 5 cluster, ESPT is possible without geometrical conversion, i.e., isomerization. The barrier in the Lb−La internal conversion would not be high because the La state is more than 10 kcal/mol more stable than the Lb state. Therefore, we concluded that ESPT occurs in the n = 5 cluster even with low excess energy. According to the revised assignments on the cluster size, the originally reported ESPT threshold at “n = 4” corresponds to the n = 5 cluster. The assignment of the cluster size is fully studied by the high resolution mass spectrum by Jouvet and co-workers. It is consistent with our theoretical interpretation that the ESPT mechanism is changed at n = 5.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated Lb−S0 transition energies are compared to the observed ones in Table S1. All of the calculated geometries are included in Table S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by KAKENHI on innovative area (2503) and the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, the Core-to-Core Program 22003 from the Japan Society for the Promotion of Science (JSPS). The computations were performed using Research Center for Computational Science, Okazaki, Japan.

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REFERENCES

(1) David, O.; Dedonder-Lardeux, C.; Jouvet, C. Is There an Excited State Proton Transfer in Phenol (or 1-Naphthol)- Ammonia Clusters? Hydrogen Detachment and Transfer to Solvent: A Key for Nonradiative Processes in Clusters. Int. Rev. Phys. Chem. 2002, 21, 499− 523. (2) Ebata, T.; Mizuochi, N.; Watanabe, T.; Mikami, N. OH Stretching Vibrations of Phenol-(H2O)1 and Phenol-(H2O)3 in the S1 State. J. Phys. Chem. 1996, 100, 546−550. (3) Harris, C. M.; Sellnge, B. K. Acid-Base Properties of l-Naphthol. Proton-Induced Fluorescence Quenching. J. Phys. Chem. 1980, 84, 1366−1371. (4) Kim, S. K.; Wang, J.-K.; Zewail, A. H. Femtosecond pH Jump: Dynamics of Acid-Base Reactions in Solvent Cages. Chem. Phys. Lett. 1994, 228, 369−378. (5) Lakshminarayan, C.; Knee, J. L. Spectroscopy and Dynamics of the S, State of Jet-Cooled 1-Naphthol. J. Phys. Chem. 1990, 94, 2637− 2643. (6) Andrei, H.-S.; Solca, N.; Dopfer, O. Ionization-induced Switch in Aromatic Molecule−nonpolar Ligand Recognition: Acidity of 1naphthol (1-Np+) rotamers probed by IR spectra of 1-Np+−Ln complexes (L = Ar/N2, n ≤ 5). Phys. Chem. Chem. Phys. 2004, 6, 3801−3810. (7) Ashfold, M. N. R.; Cronin, B.; Devine, A. L.; Dixon, R. N.; Nix, M. G. D. The Role of πσ* Excited States in the Photodissociation of Heteroaromatic Molecules. Science 2006, 312, 1637−1640. (8) Bernstein, E. R. Chemical Reactions in Clusters. J. Phys. Chem. 1992, 96, 10105−10115. (9) Cheshnovsky, O.; Leutwyler, S. Proton Transfer in Neutral Gasphase Clusters: α-Naphthol-(NH3)n. J. Chem. Phys. 1988, 88, 4127− 4138. (10) Daigoku, K.; Ishiuchi, S.; Sakai, M.; Fujii, M.; Hashimoto, K. Photochemistry of Phenol−(NH3)n Clusters: Solvent Effect on a Radical Cleavage of an OH Bond in an Electronically Excited State and Intracluster Reactions in the Product NH4(NH3)n‑1 (n ≤ 5). J. Chem. Phys. 2003, 119, 5149−5158. (11) David, O.; Dedonder-Lardeux, C.; Jouvet, C.; Sobolewsk, A. L. Role of the Intermolecular Vibrations in the Hydrogen Transfer Rate: The 3-Methylindole-NH3 Complex. J. Phys. Chem. A 2006, 110, 9383−9387. (12) Dedonder-Lardeux, C.; Grosswasser, D.; Jouvet, C.; Martrenchard, S. Dissociative Hydrogen Transfer in Indole-(NH3)n Clusters. PhysChemComm 2001, 4, 1−3.

CONCLUSIONS

We have discussed the size dependence of ESPT in 1naphthol−ammonia clusters. We have found 6, 8, and 7 stable isomers for the clusters of n = 3, 4, and 5. From the calculated relative energies in S0, the initial population was estimated. For n = 3 and 5, the population is concentrated to the most stable isomers IIIa and Va, respectively. For n = 4, we considered the initial population in IVa and IVb because of the small relative energy. Optical transitions from these initial isomers always produce the Lb states. For the n = 3 clusters, the optimized structure of Lb is more stable than that of La in the most stable isomer IIIa. Thus, the excited cluster will stay in Lb, and no ESPT will be expected in the geometry that can be accessible by the vertical transitions. The PT structures exist but in other isomers. This means that the isomerization is required prior to ESPT, and therefore, ESPT would not occur at low excess energy. This interpretation is essentially the same for the n = 4 clusters, although we have to consider two initial geometries IVa and IVb. In the case of 1-NpOH−(NH3)5, the PT structure is found in the La state of the initially populated isomer Va. Thus, the internal conversion from Lb to La corresponds to ESPT. Since the internal conversion does not require a significant geometrical change, ESPT in n = 5 will take place even without high excess energy. These theoretical conclusions are consistent with the experimental results. Therefore, we concluded that the threshold size of ESPT is n = 5 in the low excess energy conditions. H



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(13) Dedonder-Lardeux, C.; Grosswasser, D.; Jouvet, C.; Martrenchard, S.; Teahu, A. Evaporation after Ionization in Molecular Clusters: Application to 1-Naphthol−(NH3)n. Phys. Chem. Chem. Phys. 2001, 3, 4316−4324. (14) Domcke, W.; Sobolewski, A. L. Chemistry. Unraveling the Molecular Mechanisms of Photoacidity. Science 2003, 302, 1693− 1694. (15) Esboui, M.; Jouvet, C.; Dedonder, C.; Ebata, T. Excited-State Dynamics of the 2-Hydroxypyridine-Ammonia Complex. J. Phys. Chem. A 2010, 114, 3060−3066. (16) Grégoire, G.; Jouvet, C.; Dedonder, C.; Sobolewski, A. L. On the Role of Dissociative πσ* States in the Photochemistry of Protonated Tryptamine and Tryptophan: An ab initio Study. Chem. Phys. 2006, 324, 398−404. (17) Hineman, M. F.; Brucker, G. A.; Kelley, D. F.; Bernstein, E. R. Excited-state Proton Transfer in 1-naphthol-Ammonia Clusters. J. Chem. Phys. 1992, 97, 3341−3347. (18) Ishiuchi, S.; Daigoku, K.; Saeki, M.; Sakai, M.; Hashimoto, K.; Fujii, M. Hydrogen Transfer in Photoexcited Phenol/Ammonia Clusters by UV−IR−UV Ion Dip Spectroscopy and ab initio Molecular Orbital Calculations. I. Electronic Transitions. J. Chem. Phys. 2002, 117, 7077−7082. (19) Ishiuchi, S.; Daigoku, K.; Saeki, M.; Sakai, M.; Hashimoto, K.; Fujii, M. Hydrogen Transfer in Photo-excited Phenol/ammonia Clusters by UV−IR−UV Ion Dip Spectroscopy and ab initio Molecular Orbital Calculations. II. Vibrational Transitions. J. Chem. Phys. 2002, 117, 7083−7093. (20) Ishiuchi, S.; Sakai, M.; Daigoku, K.; Hashimoto, K.; Fujii, M. Hydrogen Transfer Dynamics in a Photoexcited Phenol/Ammonia (1:3) Cluster Studied by Picosecond Time-resolved UV-IR-UV Ion Dip Spectroscopy. J. Chem. Phys. 2007, 127, 1−8. (21) Kelley, D. F.; Bernstein, E. R. Comment on “Excited-state Proton Transfer in 1-Naphthol(NH3)n Complexes: The Threshold Size is n = 4” [Chem. Phys. Lett. 293 (1998) 191]. Chem. Phys. Lett. 1999, 305, 230−232. (22) Knochenmuss, R. Excited-state proton transfer in 1-naphthol(NH3)n Complexes: The Threshold Size is n = 4. Chem. Phys. Lett. 1998, 293, 191−196. (23) Knochenmuss, R. Reply to comment by Kelley and Bernstein on Chem. Phys. Lett. 293 (1998) 191, ‘‘Excited-state Proton Transfer in 1-Naphthol(NH3)n Complexes: The Threshold Size is n = 4’’. Chem. Phys. Lett. 1999, 305, 233−237. (24) Knochenmuss, R.; Leutwyler, S. Proton Transfer from 1Naphthol to Water: Small Clusters to the Bulk. J. Chem. Phys. 1989, 91, 1268−1278. (25) Leutwyler, S.; Burgi, T.; Schiitz, M.; Taylor, A. Structures,Dynamics and Vibrations of Cyclic (H2O)3 and Its Phenyl and Naphthyl Derivatives. Faraday Discuss. 1994, 97, 285−297. (26) Lührs, D. C.; Knochenmuss, R.; Fischer, I. Excited-state Proton Transfer in 1-Naphthol(NH3)n Clusters: Wavelength-dependence of the Picosecond Pump−probe Spectra. Phys. Chem. Chem. Phys. 2000, 2, 4335−4340. (27) Matsumoto, Y.; Ebata, T.; Mikami, N. Characterizations of the Hydrogen-bond Structures of 2-Naphthol-(H2O)n (n = 0−3 and 5) Clusters by Infrared-ultraviolet Double-resonance Spectroscopy. J. Chem. Phys. 1998, 109, 6303−6311. (28) Matsumoto, Y.; Ebata, T.; Mikami, N. Structures and Vibrations of 2-Naphthol-(NH3)n (n = 1−3) Hydrogen-bonded Clusters Investigated by IR-UV Double-resonance Spectroscopy. J. Mol. Struct. 2000, 552, 257−271. (29) Matsumoto, Y.; Ebata, T.; Mikami, N. Photofragment-Detected IR Spectroscopy (PFDIRS) for the OH Stretching Vibration of the Hydrogen-Bonded Clusters in the S1 States-Application to 2Naphthol-B (B = H2O and CH3OH) Clusters. J. Phys. Chem. A 2001, 105, 5727−5730. (30) Pino, G.; Grégoire, G.; Dedonder-Lardeux, C.; Jouvet, C.; Martrenchard, S.; Solgadi, D. A Forgotten Channel in the Excited State Dynamics of Phenol-(Ammonia)n Clusters: Hydrogen Transfer. Phys. Chem. Chem. Phys. 2000, 2, 893−900.

(31) Pino, G. A.; Dedonder-Lardeux, C.; Grégoire, G.; Jouvet, C.; Martrenchard, S.; Solgadi, D. Intracluster Hydrogen Transfer Followed by Dissociation in the Phenol−(NH3)3 Excited State: PhOH(S1)− (NH3)3→PhO·+(NH4)(NH3)2. J. Chem. Phys. 1999, 111, 10747− 10749. (32) Saeki, M.; Ishiuchi, S.; Sakai, M.; Fujii, M. Structure of 1Naphthol/Alcohol Clusters Studied by IR Dip Spectroscopy and ab Initio Molecular Orbital Calculations. J. Phys. Chem. A 2001, 105, 10045−10053. (33) Saeki, M.; Ishiuchi, S.; Sakai, M.; Hashimoto, K.; Fujii, M. Structural Evolution of (1-NpOH)n Clusters Studied by R2PI and IR Dip Spectroscopies. J. Phys. Chem. A 2010, 114, 11210−11215. (34) Schmitt, M.; Jacoby, C.; Gerhards, M.; Unterberg, C.; Roth, W.; Kleinermanns, K. Structures and Vibrations of Phenol(NH3)2−4 Clusters. J. Chem. Phys. 2000, 113, 2995. (35) Sobolewski, A. L.; Domcke, W.; Dedonder-Lardeux, C.; Jouvet, C. Excited-state Hydrogen Detachment and Hydrogen Transfer Driven by Repulsive 1πσ* States: A New Paradigm for Nonradiative Decay in Aromatic Biomolecules. Phys. Chem. Chem. Phys. 2002, 4, 1093−1100. (36) Stanley, R. J.; Castleman, A. W., Jr. Cluster Ion Dip Spectroscopy of Hydrogen Bonded Phenol(H2O)n Clusters, n = 0− 4. J. Chem. Phys. 1991, 94, 7744−7756. (37) Steen, H. B.; Bowman, M. K.; Kevan, L. Temperature Dependence of a Process Competing with S2-S1 Internal Conversion in Indole and Phenol in Aqueous Solutions. J. Phys. Chem. 1976, 80, 482−486. (38) Tanner, C.; Manca, C.; Leutwyler, S. Probing the Threshold to H Atom Transfer Along a Hydrogen-bonded Ammonia Wire. Science 2003, 302, 1736−1739. (39) Tseng, C. M.; Lee, Y. T.; Ni, C. K. H Atom Elimination from the πσ* State in the Photodissociation of Phenol. J. Chem. Phys. 2004, 121, 2459−2461. (40) Miyazaki, M.; Kawanishi, A.; Nielsen, I.; Alata, I.; Ishiuchi, S.; Dedonder, C.; Jouvet, C.; Fujii, M. Ground state Proton Transfer in Phenol-(NH3)n (n ≤ 11) Clusters Studied by Mid-IR Spectroscopy in 3−10 μm Range. J. Phys. Chem. A 2013, 117, 1522−1530. (41) Shimizu, T.; Yoshino, R.; Ishiuchi, S.-i.; Hashimoto, K.; Miyazaki, M.; Fujii, M. Structure of 1-Naphthol−Water Clusters in the S1 State Studied by UV−IR Fluorescence Dip Spectroscopy and ab initio Molecular Orbital Calculations. Chem. Phys. Lett. 2013, 557, 19− 25. (42) Hercules, D. M.; Rogers, L. B. Absorption and Fluorescence Spectra of Some Mono- and Di-hydroxy Naphthalenes. Spectrochim. Acta 1959, 15, 393−408. (43) Knochenmuss, R.; Cheshnovsky, O.; Leutwyler, S. Proton Transfer Reactions in Neutral Gas-Phase Clusters: l-Naphthol with H2O, D2O, CH3OH, NH3 and Piperidine. Chem. Phys. Lett. 1988, 144, 317−323. (44) Mataga, N.; Kaifu, Y. Charge Transfer and Proton Transfer Reactions in the Excited Hydrogen Bonded Complex in Non-polar Solvents. Mol. Phys. 1964, 7, 137−147. (45) Platt, J. R. Classification of Spectra of Cata-Condensed Hydrocarbons. J. Chem. Phys. 1949, 17, 484−495. (46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; I



Article

Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (47) Lakshminarayan, C.; Smith, J. M.; Knee, J. L. Laser photoelectron spectroscopy of l- and 2-Naphthol: Relative Stability of the cis and trans Cation Rotamers. Chem. Phys. Lett. 1991, 182, 656−662. (48) Humphrey, S. J.; Pratt, D. W. Acid−base Chemistry in the Gas Phase: The trans-1-Naphthol-NH3 Complex in Its S0 and S1 Electronic States. J. Chem. Phys. 1996, 104, 8332−8340. (49) Knochenmuss, R.; Fischer, I. Excited-state Proton Transfer in Naphthol/Solvent Clusters: The Current State of Affairs. Int. J. Mass Spectrom. 2002, 220, 343−357.

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Theoretical Study on the Size Dependence of Excited State Proton

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