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Electronically Excited States of Neutral, Protonated #Naphthol and Their Water Clusters: a Theoretical Study Reza Omidyan, Gholamhasan Azimi, Zahra Heidari, and Mohammad Salehi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b02249 • Publication Date (Web): 29 May 2015 Downloaded from http://pubs.acs.org on June 3, 2015

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The Journal of Physical Chemistry

Electronically Excited States of Neutral, Protonated α-Naphthol and their Water Clusters: a Theoretical Study Reza Omidyan*, Zahra Heidari, Mohammad Salehi and Gholamhassan Azimi Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran Abstract The RI-MP2 and RI-CC2 methods have been employed to determine the potential energy profiles of neutral and protonated α-Naphthol, in their individual forms and microhydrated with 1 and 3 water molecules, at different electronic states. According to calculated results, it has been predicted that dynamics of nonradiative processes in protonated α-Naphthol is essentially different from that of its neutral homologue. In protonated α-Naphthol, the calculations reveal that 1σπ* state, is the most important photophysical state, having a bound nature with a broad potential curve along the OH coordinate of isolated system, while it is dissociative in monohydrated homologue. In neutral system, similar to phenol, the 1πσ* state, plays the fundamental relaxation role along the O-H stretching coordinate. Moreover, microhydration strongly affects the photophysical properties of α-Naphthol, mostly by alteration 1ππ* PE profile, from bound state in isolated analogue to a dissociative state in hydrated systems. Furthermore, it has been found that three water molecules are necessary for ground state proton transfer between protonated α-Naphthol and water; with a small barrier; (∆E< 0.1 eV).

Keywords: α-Naphthol, Excited States, RI-MP2 and RI-CC2 methods, red shift effect, Conical Intersections.

1-Introduction During the last two decades a great attention has been paid to the study of protonated aromatic molecules (AH+), since of their wide range of applications in science and technology1-7. As it has been frequently remarked in recent reports, protonation of aromatic systems is accompanied with a significant red shift effect on the S1-S0 electronic transition1-3,8-10.

*

Corresponding author, E-mail: [email protected], [email protected], Fax: (+98) 311 6689732

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The dynamics of photoinduced bond cleavages in aromatic molecules, including heteroatms, like azoles11, phenols12-23, pyrrole24-29 and so forth, have been the subjective of several studies. According to recent investigations, it has been emphasized that dissociative trend of 1πσ* has the most important role in the photo-resistive properties of chromophores of aromatic amino acids and DNA bases12,24,30-33. α-Naphthol has been considerably studied34-50 due to its essential alteration in acidity, following photo-excitation to S1 state39. This remarkable change in acidity has encouraged the use of αNaphthol as a photo-initiated acid39. It has been also established that following photo-excitation of Naphthol with a short pulse laser, it is possible to follow fast proton transfer and solvent reorientation phenomena39,48. In addition, the mass resolved excitation spectra of α-Naphthol and its clusters with ammonia and water have been investigated by several groups42,45,51. The electronic band origin of the S1S0 transition for jet-cooled α-Naphthol has been reported by J. M. Hollas42 and later by C. Lakshminarayan48, locating at 31458 cm-1 (3.90 eV). The fluorescence excitation spectrum of αand β-Naphthol in the origin region have been reported by Johnson et al51. In the jet experiments, they observed that each molecule exhibits two electronic origins; 31118, 31455 cm-1 for αNaphthol and 30586, 30903 cm-1 for β-Naphthol. Also, Pratt43 measured rotationally resolved fluorescence excitation spectra of the electronic origin of α-Naphthol-NH3 and determined the ground- and excited state rotational constants, and H-bond geometries. Moreover, the spectral hole-burning study and fluorescence measurements for the α-Naphthol-(NH3) and α-Naphthol(ND3) complexes in their low frequency vibronic band region have been reported by Hensler et al.40 Proton transfer is an important process in biochemistry34-36. Nevertheless, the mechanism and kinetics of acid–base reactions strongly depend on the solvent structure; hence, great attention has been paid to the study of proton transfer in molecular clusters of small size systems for a solvent environment35. The α-Naphthol-(NH3)n cluster series has been the subject of numerous experimental studies, because ammonia is strong enough to allow excited-state proton transfer (ESPT) in clusters with n>439,52,53. In this work, our calculated results on the electronic structures and photophysical properties of α-Naphthol in their neutral and protonated forms are presented. In addition to 2 ACS Paragon Plus Environment

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ground- and excited-state optimized structures, the vertical and adiabatic transition energies of neutral and protonated isomers of α-Naphthol will be accurately addressed. We will also discuss and explain the deactivation pathways of excited species of neutral/protonated α-Naphthol as well as their water clusters. The RI-CC2 method has been selected because it gives reliable results for small sized organic systems, for a reasonable computational time2,10,53-55.

2- Computational Details: The “ab initio” calculations have been performed with the TURBOMOLE program suit (V 6.3)56,57, making use of the resolution-of-identity58, (RI) approximation for the evaluation of electron repulsion integrals. The equilibrium geometry of all systems at the ground state has been determined at the RI-MP2 (Moller-Plesset second order perturbation theory) level59,60. Excitation energies and equilibrium geometry of the lowest excited singlet states have been determined at the RI-CC2 (the second-order approximate coupled-cluster method)61-63. The Dunning’s correlation consistent split-valence double-ζ basis set (cc-pVDZ)64,65 and the augmented ccpVDZ by diffuse functions on all atoms (aug-cc-pVDZ)66 have been used for determination of transition energies and oscillator strengths. The potential energy profiles have been calculated using aug-cc-pVDZ basis function. The calculations relevant to the transition energies and PE profiles have been determined either by considering the C1 or Cs symmetry point groups; (imposing the Cs symmetry will be addressed as well). Within the Cs point group the wave function of the ground state and S1 (1ππ*) state transform according to the A′ representation and thus they are more distinguishable either with the 1πσ* in neutral molecules, or with the 1σπ* excited state in protonated homologues; (both belong to the A″ symmetry representation). The numbering pattern of the carbon atoms is shown in Figure 1. The abbreviations of Naph, NaphH+, and Naph-Wn will be employed instead of neutral, protonated and water clusters of α-Naphthol respectively.



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(a)

(b)

H12

O11 9 8 7

6

10 5

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(c)

1 2 4

3

Figure 1: Optimized geometries of: (a) cis α-Naphthol (b) trans α-Naphthol and numbering pattern, (c) the most stable isomer of protonated α-Naphthol (C4 isomer). 3. Results and Discussions: 3-A: Geometry and Electronic Structures: Neutral and Protonated Isomers of α-Naphthol:

The optimized structural parameters of α-Naphthol at ground-state have been reported by several authors and do not need to be discussed here37,42,48,51. It has been established that two origins in the S1-S0 spectrum of α-Naphthol exist since of the existence of rotational cis/trans conformations; differing in the orientation of the O-H bond with respect to the naphthalene frame. Based on literature reports48, we utilize the "cis" and "trans" terms indicating to different orientations of -OH group with respect to single bonds (see Figure 1). Oikawa et al50 assigned the lower energy origin (31182 cm-1) as being due to excitation of the molecule in the cis configuration, and the higher energy origin (31457 cm-1) to the trans configuration, based on differences in the energy shifts of the two bands that occur in the spectra of hydrogen-bonded complexes of Naphthol. We also, determined the minimum potential energy (MPE) profile of trans/cis conversion at the RI-MP2/cc-pVDZ level of theory; (See Figure S1, ESI file). It has been found that a barrier of 0.18 eV (17 kJ mol-1) in the middle of reaction coordinate, hinders the cis/trans conversions.


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1.5 1.38 C5 1.19 1.2

1.13 C1

Energy (eV)

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C10

0.9

0.79

0.77

C3

C7

0.6

0.51

0.62 C8 0.55 C9

C6 0.3

0.2 C2 0.0

0.0 C4

Protonated Isomers of α-Naphthole Figure 2: MP2 calculated results indicating relative stability of different isomers associated with protonation of α-Naphthol. According to our MP2/cc-pVDZ calculated results, it has been revealed that trans conformer is 0.07 eV (~560 cm-1) more stable than the cis one, which is qualitatively in agreement with results of Johnson et al51. However, in the present work, we only considered the most stable conformer of trans-αNaphthol for performing more calculations and investigations relevant to protonation effect. Figure 1 shows the equilibrium structure and numbering pattern of α-Naphthol. Concerning the main idea of present work, we neglect to present further details from geometry properties of αNaphthol, and briefly discuss its protonated isomers. In this regard, we have performed an exploration for finding most attractive carbon site on the Naphthalene frame of α-Naphthol for protonation. Therefore, corresponding to 10 carbon sites of α-Naphthol, 10 protonated isomers have been considered. The MP2 method has been employed for geometry optimization of all protonated isomers. In Figure 2, the ground state relative stabilities of isomers associated with protonation of α-Naphthol have been presented. The ground state energetic levels for different isomers have been determined comparing the internal energy of each isomer with that of C4 (the 5 ACS Paragon Plus Environment


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most stable one). As shown, the C4 isomer is the most stable protonated isomer of α-Naphthol, and other isomers comprise 0.2-1.3 eV higher than C4 protonated isomer. With exception of three isomers, (C1, C5 and C10), other isomers are planar (i.e. two aromatic rings stay planar following RI-MP2 optimization). In addition, by inspecting Figure 2, it is seen that the ground-state energetic differences between protonated isomers of the first ring; (phenolic part, C1-C4), are more pronounced than those of second ring (C5-C10). Particularly, there are significant diversities between the stability of first ring’s protonated isomers, indicating an efficient-stabilization role of OH group on special carbon sites of the first ring (C2 and C4). Moreover, locating the excess proton on each position of C1, C5 or C10 obtains a non-planar protonated isomer. Thus, low possibility for populating of these type isomers can be expected because of the large internal energy of these isomers (1.19 eV, 1.38 eV and 1.13 eV respectively for C1, C5 and C10 isomers, see Fig. 2). Hence, no further details on the excited state of these type isomers have been provided. Table 1 represents the selected bond-lengths and bond-angels of the most stable protonated isomer of C4, at the ground and S1 excited states. Comparison the equilibrium geometric parameters of protonated α-Naphthol with those of neutral homologue (Table SM1 in the ESI file), it is remarkable that protonation is accompanied with essential alterations over geometric properties of α-Naphthol. These alterations on the C-C bond lengths and bond angles in protonated ring of α-Naphthol are more prominent than those of neutral ring (i.e. unprotonated). In neutral α-Naphthol, the aromatic C-C bonds, lie between 1.389-1.427 Å. The C2-C3 bond in protonated species is slightly shorter (1.376 Å) than that of neutral α-Naphthol (1.412 Å). Also, the C1-O11 bond length increases from 1.312 Å in neutral to 1.370 Å in protonated C4 isomer and the O11-H12 bond length roughly decreases from 0.976 Å to 0.969 Å in protonated system. Moreover, the largest adjustment in the C-C-C bond angles, upon to protonation is related to the C3-C4-C5 which decreases from 121.1º (in neutral) to 115.9º in protonated species. Further geometric parameters of α-Naphthol along with the xyz coordinates of neutral and protonated isomers are available in supplementary file (ESI). .


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Ground state cc-pVDZ

1

aug-ccpVDZ

Excited State

cc-pVDZ

aug-cc-pVDZ

Distances/Å C1-C2

1.437

1.436

1.403

1.403

C2-C3

1.366

1.369

1.392

1.395

C3-C4

1.483

1.438

1.504

1.505

C4-C5

1.500

1.500

1.501

1.502

C5-C6

1.407

1.408

1.460

1.460

C6-C7

1.400

1.402

1.415

1.417

C7-C8

1.415

1.417

1.393

1.393

C8-C9

1.393

1.394

1.451

1.453

C9-C10

1.423

1.424

1.412

1.414

C10-C1

1.437

1.435

1.474

1.472

C1-O11

1.312

1.321

1.366

1.376

O11-H12

0.976

0.975

0.975

0.974

C4-H15

1.110

1.109 Angles/deg

1.113

1.113

C1-C2-C3

119.7

119.6

121.7

121.5

C2-C3-C4

122.7

122.6

122.6

122.5

C3-C4-C5

115.9

116.0

114.1

114.1

C4-C5-C6

120.7

120.7

119.0

118.9

C5-C6-C7

120.5

120.5

122.6

122.7

C6-C7-C8

120.7

120.8

117.5

117.5

C7-C8-C9

120.1

120.2

120.5

120.5

C8-C9-C10

119.1

119.0

122.1

122.1

C9-C10-C1

120.5

120.7

121.6

121.8

C10-C1-O11

116.7

116.5

113.4

113.4

C1-O11-H12

111.0

111.7

108.6

109.6

Table 1: The MP2/CC2 equilibrium geometry parameters of protonated Naphthol (C4 isomer), at the ground and S1 excited states.



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3-B: Vertical and Adiabatic Transition Energies: Neutral and Protonated α-Naphthol

In order to obtain information from protonation effect on the electronic transition energies of αNaphthol, we have determined the adiabatic and vertical excitation energies for two lowestsinglet electronic transitions (S1 and S2) of neutral and protonated species. For each isomer of protonated α-Naphthol, the ground and excited state geometry optimization were performed at the RI-MP2/RI-CC2 (cc-pVDZ and aug-cc-pVDZ) levels; (see Table 2). The vertical transitions were calculated on the optimized geometry of ground states, while the adiabatic S1-S0 and S2-S0 transitions were calculated at the corresponding S1 and S2 excited state optimized geometries. With exception of C3 protonated isomer, the S1 excited-state geometry optimization of other protonated isomers didn’t show important alterations with respect to their ground state optimized structures (See Figure S2, ESI file). Nevertheless, for C3 protonated isomer, an out-of-plan deformation of protonated ring, from -CH2 position, at S1 excited-state geometry optimization has been predicted. For such cases, the RI-CC2 cycle cease to converge, thus we were unable to provide the corresponding adiabatic transition energies. It is noteworthy that seminal work of Jouvet’s group on protonated benzene dimer, demonstrated that strong deformation of protonated ring at S1 state, leads to a broad and structure-less S1-S0 electronic transition3. This deformation has been predicted to occur in protonated benzene67 and phenanthrene54 too. Nevertheless, the well resolved S1-S0 electronic spectra of protonated Naphthalene2 and Benzaldehyde10, have been interpreted in terms of planar structures at S1 state, without important geometry deformation with respect to corresponding ground state structures. Regarding the electronic transitions, in both of neutral and protonated α-Naphthol (C4, the most stable protonated isomer) the first electronic transition (S1−S0), corresponds to Lumo←Homo single electron transition (94%), while the second electronic transition (S2−S0) corresponds to the Lumo+1←Homo in neutral and Lumo←Homo-1 transition in protonated species (~80%, see ESI file). The frontier molecular orbitals (MOs) of neutral and protonated αNaphthol (most stable isomer) are depicted in Table 3. As shown, the first two electronic transitions in neutral α-Naphthol have 1ππ* nature. In addition, in protonated isomers, the C2, C3, C4, C6, C8, and C9 isomers belong to the Cs molecular symmetry and their S1−S0 , S2-S0 transitions could be identified by 1ππ* character.


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Vertical Transition/eV Protonated isomer and relative energy/eV

Adiabatic Transition/eV

aug-cc-pVDZ a

cc-pVDZ

cc-pVDZb

aug-cc-pVDZ

S1 (1ππ*)

S2 (1ππ*)

S1 (1ππ*)

S2 (1ππ*)

S1 (1ππ*)

S1 (1ππ*)

C2 0.20

2.99

4.22

2.89 (0.074)

4.13 (0.194)

2.68

2.59

C3 0.79

2.43

3.70

2.42 (0.118)

3.68 (0.002)

*

*

C4 0.00

3.46

3.93

3.39 (0.027)

3.86 (0.281)

3.08 [-0.162 eV]

2.98

C6 0.51

2.47

3.65

2.41 (0.088)

3.58 (0.262)

2.08

2.01

C7 0.77

2.22

4.04

2.21 (0.087)

4.00 (0.071)

1.89

1.87

C8 0.62

2.45

4.33

2.40 (0.121)

4.25 (0.009)

2.15

2.08

C9 0.55

2.26

3.48

2.26 (0.035)

3.44 (0.282)

1.88

1.88

4.36

4.67

4.23 (0.019)

4.46 (0.098)

4.20 [-0.180 eV] # 3.90

4.08

Neutral αNaphthol

Table 2: Vertical and adiabatic transition energies of neutral and protonated α-Naphthol, computed at the RI-MP2/RI-CC2 levels with two different basis sets. a The values in parenthesis and in bracket b indicate to oscillator strength and ∆ZPE respectively. # The experimental values for the 0-0 band of S1-S0 transition of neutral α-Naphthol, has been taken from Ref. 48. *The S1 geometry optimization for the C3 isomer leads to ring deformation and probably causes a fast internal conversion to the ground state via a conical intersection67, thus the CC2 cycle cease to converge. In order to evaluate our method and basis set, we recalculated the electronic transition energies of neutral α-Naphthol, for which the jet cooled experimental data is available. As shown in Table 2, there is a good agreement between experimental42,48,51 band origin of S1-S0 electronic 9 ACS Paragon Plus Environment


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transition (31457 cm-1, 3.90 eV), and our calculated values for α-Naphthol at the RI-CC2/aug-ccpVDZ level of theory (4.08 eV). The value obtained by smaller basis set (4.20 eV at RI-CC2/ccpVDZ), is also comparable with experiment. When the difference between zero point vibrational energy of the ground and excited state (∆ZPE=-0.18 eV) is taken to account, the S1-S0 transition energy of the RI-CC2/cc-pVDZ level (4.02 eV, ∆ZPE corrected), is also consistent with experimental band origin. Moreover, the calculated adiabatic S1-S0 transitions of protonated isomers lie in a narrow range, from visible to the near IR (NIR), (2.98-1.88 eV at the RICC2/aug-cc-pVDZ level). The lowest value is related to the C7 isomer (1.87 eV) with an oscillator strength of 0.087, and the highest transition energy is related to the C4 (2.98 eV) with the oscillator strength of 0.027. Additionally, the CC2 geometry optimization at the S2 electronic state for almost of protonated isomers, leads to strong prefulvenic deformation of protonated ring. Thus, we couldn’t determine the S2 adiabatic transition energies.

Homo (π1)

Lumo ( π1*)

Lumo+1 (π2*)

Lumo+2 (σ1*)

Homo-3 (σ1)

Homo-1 (π1)

Homo (π2)

Lumo (π1*)

Naph

NaphH+

Table 3: Schemes of valence orbitals of neutral and protonated α-Naphthol involved in the first 1 ππ* , 1πσ* and 1σπ* electronic states. Additionally, the vertical electronic transition energies of the neutral and the most stable isomer of protonated α-Naphthol (C4 isomer) under the Cs symmetry have been determined; (Table SM3 and SM4, ESI file). In the Cs symmetry point group, it has been determined that S1 to S4 electronic states of neutral and protonated α-Naphthol transform according to A′ representation,


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while the first 1A″ electronic state corresponds to the S5-S0 electronic transition, having 1πσ* and 1

σπ* natures respectively in neutral and protonated α-Naphthol. The first two electronic

transitions in neutral and protonated species (C4 isomer), with 1ππ* nature, belong to the 1A′ representation. Comparison the adiabatic transition energies of S1-S0 transition of C4 protonated isomer (2.98 eV at CC2/aug-cc-pVDZ) and that of neutral α-Naphthol (4.08 eV at the same level of theory and experimental value of 3.90 eV for 0-0 band of α-Naphthol reported by Lakshminarayan and Knee48), this is notable that a strong red sift effect on the S1-S0 transition energy of α-Naphthol, ~1.0 eV, is the main consequence of protonation.

3-c Neutral and Protonated α-Naphthol-(H2O)n Clusters (n=1, 3): Geometry Properties and Electronic States The intracluster proton transfer processes in small size water clusters of α-Naphthol has been extensively investigated35,36,44-46. According to results of Kim et al.45,46, the proton transfer reaction does not occur in α-Naphthol-Wn systems, because the proton affinity of water is low and because the Naphtholate anion is not well stabilized or solvated by water. Concerning the cluster-size effects on the electronic structures and photophysics of neutral and protonated α-Naphthol, we have investigated the small clusters of these species with one and three water molecules ([Naph-W1,3] and [Naph-W1,3]H+). The n=1 and 3 for water molecules, have been selected, since they are critical to obtain small sized cluster structures, having Cs molecular symmetry. As mentioned, the Cs symmetry is necessary to distinguish the 1ππ* and 1

πσ*, or 1σπ* states as well. In this point group, the ground state and 1ππ* transform as 1A′ while

the 1πσ*, or 1σπ* transform as A″ representations. The hydrogen bond between OH group and water molecule has been considered as a critical point for stabilization of monohydrated clusters. For three hydrated systems, there are several possibilities for interaction of additional water molecules either with aromatic rings or with the first water molecule. Our previous study on protonated phenol reveals that the latter case, for which three molecules of water are connected to each other by two hydrogen bond network, is more favored than other structures68. The equilibrium structure of α-Naphthol-H2O (Naph-W1) complex optimized at the electronic ground and excited states are shown in Figure 3. In the Naph-W complex, a strong O-H(Naph) 11 ACS Paragon Plus Environment


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⋯O(W) hydrogen bond (of 1.810 Å, at MP2/cc-pVDZ level), connects two molecules. Our calculated value of hydrogen bond length (1.810 Å), is slightly longer than the value of 1.783 Å reported by R. Yoshino et al.69 (at the RI-MP2/6-31G level of calculation). By increasing the number of water molecules, significantly shortening of OH(Naph)⋯O(w) hydrogen-bond has been predicted in [Naph-W3] complex (from 1.810 Å in Naph-W1 to 1.636 Å in [Naph-W3]). In addition, the optimized structure of protonated α-Naphthol, water complexes ([Naph-W1,3]H+) optimized at the electronic ground and excited states are shown in Figure 3. Similar to the neutral analogue, the [Naph-W]H+ complex is bound by a strong hydrogen bond, while there is a network of hydrogen bonds in [Naph-W3]H+ complex, between water molecules. The RIMP2/cc-pVDZ level of calculation employed for determination of optimized structures leads to an O-H⋯O(w) bond length of 1.579 Å in monohydrated NaphH+, while in three-hydrated complex, [Naph-W3]H+, the S0 geometry optimization has been accompanied with hydrogen transfer to adjacent water molecule. Although in the ground state equilibrium minimum of [Naph-W3]H+, the hydrogen (or proton) is attached to water molecule; (RO-H(W)= 1.088 Å), the distance between oxygen of α-Naphthoxide and transferred proton (H+) is very short (RO…..H= 1.356 Å). Thus, a large attractive-force between negative O and proton (H+), can be expected to facilitate the back proton transfer from water to Naphthoxide system, either in the ground or excited states. The xyz coordinates of selected water clusters and monomer of NaphH+ are presented in ESI file. The vertical and adiabatic excitation energies of the neutral and protonated water clusters of αNaphthol, calculated under the Cs symmetry point group, are given in Table 4. According to RICC2 calculations, in both of neutral and protonated forms of mono- and thri-hydrated systems, the first electronic transition of 1A′ (S1−S0) corresponds to Lumo←Homo single electron transition (~ 94%) which has the 1ππ* nature. The first 1A″ electronic transition has 90% 1πσ* character in neutral clusters, mainly corresponding to the Lumo+2←Homo electronic transitions (See Table 4, Table 5 and ESI file).


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Ground state optimized geometry

1

A′ Excited state optimized geometry

a

a

Naph-W1

b θ

1

A″ Excited state optimized geometry

0.979 1.810 θ 107.8º

0.986 1.772 108.0

2.072 1.003 117.3

1.006 1.636 108.0

1.018 1.590 108.4

1.801 0.990 138.5

0.969 107.1

0.972 106.8

1.980 98.9

0.971 108.6

0.975 108.6

0.976 109.6

1.013 1.579 112.4

0.998 1.670 108.6

1.005 1.629 109.9

1.356 1.088 118.8

1.038 1.490 108.5

1.700 0.991 160.7

b

Naph-W3

Naph

NaphH+

[Naph-W1]H+

[Naph-W3]H+

Figure 3: Optimized geometry structures of neutral and protonated water clusters of αNaphthol’s complexes at the S0, 1A′ , and the 1A″ electronic states. In each panel, three numbers have been presented; (from top to bottom: a, b and θ refer to the O-H and OH⋯O(w) hydrogen bond lengths (in Å) and C-O-H bond angle (in degree) respectively.



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In protonated water clusters, (mono and three hydrated NaphH+), the first 1A″ electronic transition has almost 1σπ* character (~80%), corresponding to the Lumo←Homo-3 electronic transitions (See ESI file). Also, the adiabatic transition energies of neutral and protonated α-Naphthol with one and three water molecules have been determined; the results are tabulated in Table 4. As shown the S1−S0 electronic transition energy of mono- and thri-hydrated α-Naphthol have been determined respectively to 4.02 and 3.98 eV. The experimental origin of the S1 (0−0) band of neutral αNaph-W1 in the gas phase molecular beam was reported by Kim et al.45 amount to 3.88 eV (31310 cm-1). Thus, our calculated values at the RI-CC2/aug-cc-pVDZ (4.02 eV) and also, at the cc-pVDZ level; (3.97 eV, ∆ZPE corrected), for S1-S0 electronic transition are in good agreement with experiment. Comparing the adiabatic S1-S0 electronic transition energy of neutral mono- and thrihydrated α-Naphthol (4.02 and 3.98 eV respectively) with those of protonated species (3.17 eV and 3.29 eV for [Naph-W]H+ and [Naph-W3]H+ at the same level of calculation), significant red shift effects have been predicted upon to protonation of microhydrated α-Naphthol. In addition, one may compare the S1-S0 electronic transition energy of individual NaphH+ with those of micro hydrated. Comparing the adiabatic S1-S0 transition energy of NaphH+, 2.98 eV with corresponding values of mono- and thri-hydrated NaphH+ (3.17 eV and 3.29 eV respectively), a remarkable blue-shift effect on the S1-S0 transition has been predicted upon to micro-hydration of protonated α-Naphthol. Nevertheless, there is no significant discrepancy between the S1-S0 transition energies of individual α-Naphthol (neutral) and its hydrated species (4.08 eV, 4.02 eV and 3.98 eV respectively for the bare, mono and tri-hydrated α-Naphthol, see Table 2 and Table 4). Additionally, the excited state equilibrium geometry of the first 1A′ (1ππ*) and 1A″ (1πσ* or 1

σπ*) for individual and water clusters of neutral and protonated α-Naphthol; (with 1 and 3 water

molecules), have been determined (See Figure 3). For neutral clusters, with the exception of significant shortening of OH(Naph)⋯HO(w) hydrogen bond and lengthening of O12-H, the S1 (1A′,1ππ*) state geometry optimization did not show significant geometry variation. In protonated [Naph-W1]H+ system, the RI-CC2 geometry optimization of S1 (1ππ*) state leads to elongation of OH(NaphH+)⋯OH(w) hydrogen-bond from 1.579 Å (in ground state) to 1.670 Å in


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Naph-Wa Basis Set

1

Adiabatic Transitions /eV

1

1

1

1

[Naph-W3]H+ 1

1

A′ ( ππ*)

A″ ( πσ*)

A′ ( ππ*)

A″ ( πσ*)

A′ ( ππ*)

A″ ( σπ*)

A′ ( ππ*)

A″ ( σπ*)

cc-pVDZ

4.33

6.86

4.33

7.13

3.68

5.62

4.24

5.28

aug-ccpVDZ

4.22 (0.024)

6.67 (0.000)

4.23 (0.017)

6.95 (0.002)

3.58 (0.027)

5.51 (0.000)

4.09 (0.031)

5.21 (0.000)

cc-pVDZ [∆ZPE/eV]

4.180 [-0.183]

4.18

3.310 [-0.165]

3.61

4.02 3.88

3.98

3.17

3.29

1

Vertical Transitions /eV

1

[Naph-W]H+

Naph-W3

aug-ccPVDZ

#

1

1

1

1

1

1

1

Table 4: Vertical and adiabatic transition energies of neutral and protonated α-Naphthol clusters with one and three water molecules. The adiabatic values for 1A″ states have not been reported, because of the large geometry deformations upon to CC2 optimization. a The values in parenthesis correspond to the oscillator strength. # The experimental value for the 0-0 band of S1-S0 transition of α-Naphthol-H2O complex has been adopted from Ref. 45.

the 1ππ* state, which is a sign of hydrogen-bond weakening in the S1 excited state of monohydrated NaphH+ system, (See Figure 3). For ground-state optimized structure of trihydrated NaphH+ system, although, the excess proton is attached to water molecule, corresponding to a Naph-[H3O(H2O)2]+ system, that is still very close to the oxygen atom of Naphthol system (RO11-H12=1.356 Å). The structure is similar to a Zundel70,71 structure in which the excess proton is roughly shared between two neighboring molecules. The S1 (1ππ*) geometry optimization on this system is accompanied with the back proton transfer from water to Naphthol , while the 1A"(1σπ*) geometry optimization leads to strong deformation on orientation and also distance of H3O+(H2O)2 network with respect to Naphthol moiety; (See Figure 3). As shown in Figure 3, the ∠C1O11H12 shows a large alteration from 118.8°(in ground state) to 160.8°, in the 1

A",1σπ* excited state).



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Homo (π1)

Lumo+1 (π2*)

Lumo+2 (σ1*)

Homo-3 (σ1)

Homo (π1)

Lumo (π1*)

Homo (π1)

Lumo+1 (π2*)

Lumo+2 (σ1*)

Homo-3 (σ1)

Homo (π1)

Lumo (π1*)

Naph-W1

[Naph-W1]H+

Naph-W3

[Naph-W3]H+

Table 5: Schemes of valence orbitals of neutral and protonated α-Naphthol with 1 and 3 water clusters. Only the MOs with significant contribution on the valance excitations have been depicted.

3-d: Photophysical Behavior: Potential Energy Profiles and Internal Conversions: 3-dI. Neutral and Protonated α-Naphthol: In contrast to the first 1A′ , the geometry optimizations at the CC2 level for the first 1A″ excited state, which is of 1πσ* character in neutral α-Naphthol, leads to hydrogen detachment along the O-H stretching bond (See Figure 3). This geometry alteration motivated us to investigate the photophysical behavior of neutral and protonated α-Naphthol along the O-H 16 ACS Paragon Plus Environment

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coordinate. The potential energy profiles for both cases (neutral and protonated), have been determined at the RI-CC2/RI-MP2 levels of theory and the results have been presented in Figure 4 and Figure 5. The minimum-energy paths for H-atom detachment of neutral α-Naphthol has been presented in Figure 4-a. The Cs symmetry constraint has been imposed on the calculations of the ground and excited states. For simplicity, only the lowest 1ππ*, and 1πσ* states and the electronic ground state have been shown. For the case of individual α-Naphthol, the O-H bond distance has been selected as reaction coordinate and the minimum structures of excited states have been determined along this coordinate, whereas the 1πσ* optimized structures have been considered for determination of ground-state energy profile (dash lines). From minimum potential energy curves of Figure 4-a, it is seen that the ground state PE profiles and also the lowest valence 1ππ* excited state show increasing trend with elongation of O-H bond, whereas the PE profile of the 1

πσ* state, having repulsive trend decreases along this reaction coordinate. Thus, the 1πσ* PE

profile intersects 1ππ* state at the beginning of reaction coordinate. The 1ππ*-1πσ* curve crossings in Figure 4-a, develops into the conical intersections (CIs) in a multidimensional picture. Particularly, the PECs of the different electronic states (1ππ* and 1πσ*), have been determined separately at different geometry optimizations, thus, the energetic position of these CIs cannot be accurately determined from Fig. 4. However, previous reports, verify the credibility of CC2 method for determination of potential energy profiles11,30,72, and our results relevant to the CIs are good signs for the region of conical intersections qualitatively. Moreover, the adiabatic PE sheet of the coupled 1ππ*-1πσ* states, exhibited as a red curve on Fig. 4-a, reveals a large barrier in the vicinity of the conical intersection. This barrier has been estimated by breaking the Cs symmetry of the system (to allow vibronic coupling between 1ππ* and 1πσ* states). The barrier has been approximately determined to 1.0 eV. Thus only the excited system involving sufficient excess energy above the band origin (~1 eV) of S1-S0 transition, will be able to pass the barrier and approach the dissociative region of S1 PE profile. In addition, from inspection of Figure 4-a, it is seen that repulsive 1πσ* PE profile, at an O-H distance of ~2.1 Å, crosses with PE function of the ground state, making another conical intersection. In the bare α-Naphthol, this conical intersection is expected to lead the excited system to the ground state via ultrafast internal conversions.



7

(c)

(b)

(a) 1

πσ*

6

1

1

πσ*

5

πσ*

1

ππ* S1

1

ππ*

4

Energy (eV)

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1

ππ*

3

2

S0 (πσ*) S0 (πσ*)

1

S0 (S0)

S0 (πσ*)

S0 (S0)

S0 (S0)

0 1.2

1.5

1.8

2.1

O-H/ [A]

-0.8 -0.4

0.0

0.4

0.8 -0.6

OH(Naph)-OH(w) / [A]

-0.3

0.0

0.3

0.6

OH(Naph)-OH(w) / [A]

Figure 4: The CC2 potential energy profiles of the electronic ground state (circles), the lowest 1 A′ excited state (squares) and the lowest 1A″ state (triangles), calculated along the hydrogen detachment or hydrogen transfer reaction coordinates of (a) neutral α-Naphthol, (b) monohydrated and (c) thri-hydrated α-Naphthol. The red-color curves in panels (a) and (b) represents the PE profile of S1 state without Cs symmetry constraint.

3-dII: α-Naphthol Clusters with One and Three Water Molecules: In Figure 4-b, the minimum-energy path profiles calculated along hydrogen transfer between αNaphthol and one water molecule are presented. The reaction coordinate is defined as the difference between the phenolic O-H and water molecule, describing the position of the hydrogen relative to the oxygen atom (from phenolic OH group) and water. As shown in Figure 4-b, hydration of α-Naphthol molecule, alters the slope of 1ππ*, 1πσ* PE profiles, moving the 1

ππ*-1πσ* intersection to the longer region of O-H distance compared to that of individual α-

Naphthol. In addition, monohydrating of α-Naphthol has a removal effect on the conical 18 ACS Paragon Plus Environment

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intersection of the 1πσ* state with the S0 state, which is in well agreement with the result of Sobolewski and Domcke12 on the phenol-water clusters. In comparison with Figure 4-a, the 1πσ* energy is moved essentially downward, nevertheless, the S0 energy increases significantly less than in bare molecules for large OH distances. As a result, the 1πσ* state shows a minimum at RHT~0.8 Å and the intersection with the ground state is removed. At the minimum, the hydrogen of OH group is transferred to the water molecule. Thus, the large barrier of the S1 PE profile strongly hinders the PT process when the system is excited at the band origin of S1-S0 transition. Nevertheless, the excited state proton transfer from α-Naphthol to water requires roughly 1.0 eV (~96.5 kJ mol-1) excess energy above the band origin of the S1 (1ππ*) state, for passing the excited system through the barrier of reaction coordinate. Comparison between the energetic level of S1 (1ππ*) cluster at the end of reaction coordinate (4.82 eV) with that of band origin of local S1 minimum (4.12 eV), it is seen that PT process is approximately 0.7 eV endoenergetic in the S1 state.

In addition, the ultrafast internal-conversion channel, existing in the bare α-

Naphthol systems is also eliminated in the cluster systems. In Figure 4-c, the RI-CC2 PE profiles calculated along the minimum-energy path for hydrogen transfer between α-Naphthol and three water molecules are presented. The reaction coordinate is defined as the difference between the Naphtholic O-H and adjacent water molecule and describes the position of the hydrogen relative to the oxygen atom (from OH group of Naphthol) and its neighborhood water. Inspection of Figure 4-c reveals three important points: 1- The S0 PE profile in Naph-W3, increases with significantly lower slope than that of monohydrated analogue. It reveals that PT process in the ground state of Naph-W3 is significantly less endoenergetic than Naph-W1. 2- The 1ππ* MEP profile shows significantly different trend, not only with that of isolated αNaphthol but also with monohydrated. As seen, the 1ππ* MEP at the beginning of reaction coordinate is roughly bound state while it shows a flat trend after middle of reaction coordinate. Thus, in the larger size of α-Naphthol-(H2O)n clusters (n≥3), slightly excess energy above the band origin of 1ππ* state, may trigger the PT process, from Naphthol to adjacent water molecule. 3- In contrary to isolated and monohydrated α-Naphthol, the 1πσ* state in tri-hydrated system is flat around the Franck-Condon region while that is dissociative at the end of reaction coordinate. 19 ACS Paragon Plus Environment


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The above mentioned points reveal essential effects of hydration on photophysics of α-Naphthol and most probably its similar systems. Figure 5-a shows the CC2 potential energy curves of protonated α-Naphthol (C4 isomer), determined along the minimum-energy path (MEP) for elongation of O-H group. For simplicity, only the lowest 1A′ (1ππ*) and 1A″(1σπ*) states and the electronic ground state of protonated Naphthol are shown. The MEPs (full lines) were obtained by calculation of minimum-geometry structures for fixed values of reaction coordinate. From inspecting of the PE curves presented in Figure 5-a, it is seen that both of the PE profiles of 1(1A′) and 1(1A″) excited states (i.e. the lowest 1ππ* and 1σπ* states) of protonated Naphthol, increase along the reaction coordinate of O-H distance, owing to their bounding characters. These results are in good agreement with those of CC2 geometry-optimizations at the 1

ππ* and 1σπ* excited states for individual α-Naphthol, where no sign of excited state proton

detachment has been predicted. In contrary, the CC2 geometry optimization for cluster of protonated α-Naphthol with one water molecule, [Naph-W1]H+, shows a proton transfer, taking place from OH of Naphthol moiety to the water molecule (See Figure 3). With rearrangements, it can be suggested that proton transfer, lead the excited system to produce Naphthol and hydronium ion (H3O+). However, this geometry alteration motivated us to investigate the potential energy profiles of ground state as well as 1ππ* and 1σπ* excited states of [Naph-W1]H+ system along the PT reaction coordinate. Figure 5-b, reveals the MPE profiles for proton transfer between NaphH+ and water. The reaction coordinate is defined as the difference of the OH(NaphH+) and OH(w) bond lengths and explains the position of proton relative to the oxygen atom of protonated Naphthol and water, respectively. One may compare the ground state internal energy of individual and monohydrated NaphH+ at the end of proton detachment and PT reaction coordinate (Figure 5, panel a and b). The minimum energy determined for the proton detached system (Naphthoxid and H+) has been determined to 4.0 eV more than the internal energy of the NaphH+ global minimum, while the Naphthol+H3O systems produced at the end of panel b (Fig. 5), include only 0.8 eV, more than that of global minimum. Thus, it can be concluded that complexation of NaphH+ with water molecules facilitate the PT process. Concerning the excited state potential energy profiles, it is seen from Figure 5-b, that 1(1ππ*) excited state is slightly rising with increasing reaction coordinate, and the 1σπ* (1A″) profile smoothly decreases. Following a flat pattern at the beginning, the 1σπ* state shows a dissociative trend, crosses with 20 ACS Paragon Plus Environment

Page 21 of 28

1

ππ* at the end of PT reaction path. However, after a small barrier (~0.10 eV, 9.6 kJ mol-1) on

the 1σπ* PE sheet, the dissociative trend of 1σπ* profile, leads the excited system to a proton transfer process from NaphH+ system to the neighboring water molecule.

7

(b)

(a)

6 1

1

σπ*

σπ*

5

Energy (eV)

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


4

1

1

ππ*

ππ*

3

2

S0 (σπ*)

S0 (σπ*)

1

S0 (S0)

S0 (S0)

0 1.2

1.5

1.8

O-H/ [A]

-0.4

-0.2

0.0

0.2

0.4

O-H(NaphH+)-OH(W)/ [A]

Figure 5: Potential energy curves of the S0 state (circles), the 1A'(1ππ*, squares) and 1A" (1σπ*, triangles) state, of (a) individual form of NaphH+ and (b) Water cluster of NaphH+, for proton detachment and proton transfer reactions respectively.

In contrast to 1σπ* state, no possibility for excited state proton transfer process at 1ππ* state is expected, since of its bounding nature along the PT reaction coordinate. From Figure 3, it is seen that ground state geometry optimization of tri-hydrated NaphH+, is accompanied with a proton transfer from hydroxyl group of Naphthol moiety to the neighboring water molecule. In order to investigate the ground state proton transfer (GSPT) process in [NaphW3]H+, we have determined the minimum potential-energy (MPE) profile. The results are presented in Figure 6. The difference between the Naphtholic O-H and adjacent water molecule has been selected as the reaction coordinate, describing the position of the hydrogen relative to 21 ACS Paragon Plus Environment


the oxygen atom of Naphthol and its neighborhood water. The minimum potential energy curve shows a small barrier of 0.08 eV (7.8 kJ mol-1), roughly at the middle of reaction coordinate (R≈0, where proton locates in the same distance from both sides). The barrier is not so large that hinders the [Naph-W3]H+ from GSPT, especially from the large possibility of hydrogen tunneling through the barrier, owing to its small mass. However, it can be suggested that in heavier clusters of [Naph-Wn]H+, (n>3), the excess proton prefers to locate on water instead of Naphthol chromophore. However, it will be interesting if one investigate the GSPT process by the Laser Infrared spectroscopy in [Naph-Wn]H+ complex with n≥3 in a jet cooled experimental setup.

0.09

0.06

0.03

Energy (eV)

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

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0.00

-0.03

-0.06

-0.09 -0.4

-0.2

0.0

0.2

0.4

0.6

O-H(NaphH+)-OH(W)/ [A]

Figure 6: Minimum potential energy curve of the [Naph-W3]H+ along the PT reaction path from NaphH+ to adjacent water molecule, determined at the RI-MP2/cc-pVDZ level of theory. 4- Conclusion

The hydrogen and proton transfer processes in the neutral/protonated α-Naphthol and their water clusters have been investigated at MP2/CC2 methods. It has been predicted that three water molecules promote the ground state proton transfer (GSPT) reaction from protonated αNaphthol to adjacent water moiety. Regarding the excited states properties, the lowest 1σπ* state 22 ACS Paragon Plus Environment

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has been found to play as the most important role in photochemistry of protonated α-Naphthol with less than three water molecules. In the bare NaphH+, the 1σπ* state is a bound state with a deep minimum, while it is dissociative in monohydrated systems. In the clusters of [NaphW1]H+, the 1σπ* state is dissociative along the PT reaction with a small barrier. This dissociative trend of 1σπ* state leads the excited system of [Naph-W1]H+ to the prompted PT process after photoexcitation. Regarding the photophysical feature of neutral α-Naphthol, it has been predicted that the lowest 1πσ* is the most important electronic state, as the same as Phenol. The 1πσ* state is essentially dissociative and predissociates the bound S1 (1ππ*) state, and then connects to a conical intersection with the S0 state in the bare α-Naphthol. Furthermore, according to the RI-CC2 calculated results, it has been predicted that protonation significantly red shifts the S1-S0 electronic transition energy of the bare and microhydrated clusters of α-Naphthol within 1.0-0.7 eV. *Supporting Information The xyz coordinates of neutral, protonated and water clusters of α-Naphthol and additional information about transition energies are available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment The research council of Isfahan University is acknowledged for financial support. Also, the use of computing facility cluster GMPCS of the LUMAT federation (FR LUMAT2764) for partially performance of our calculations is kindly appreciated. References:

(1) Alata, I.; Dedonder, C.; Broquier, M.; Marceca, E.; Jouvet, C. Role of The Charge-Transfer State in the Electronic Absorption of Protonated Hydrocarbon Molecules. J. Am. Chem. Soc. 2010, 132, 17483-17489. (2) Alata, I.; Omidyan, R.; Broquier, M.; Dedonder, C.; Dopfer, O.; Jouvet, C. Effect of Protonation on The Electronic Structure of Aromatic Molecules: Naphthaleneh(+). Phys. Chem. Chem. Phys. 2010, 12, 14456-14458. (3) Chakraborty, S.; Omidyan, R.; Alata, I.; Nielsen, I. B.; Dedonder, C.; Broquier, M.; Jouvet, C. Protonated Benzene Dimer: an Experimental and ab Initio Study. J. Am. Chem. Soc. 2009, 131, 1109111097.



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Graphical Abstract:

Ground state MPE profile for the [Naph [Naph-W3]H+ along proton transfer reaction coordinated from

NaphH+ to adjacent water molecule, computed at the RI RI-MP2/cc-pVDZ pVDZ level of theory.

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Electronically Excited States of Neutral, Protonated ... - ACS Publications

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