Electronically excited states of protonated phenol and para-substituted phenol

Chemical Physics Letters 555 (2013) 19–25

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Electronically excited states of protonated phenol and para-substituted phenol Shirin Azizkarimi a, Reza Omidyan a,b,⇑, Gholamhasan Azimi a a b

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran Centre Laser de l’Université Paris Sud (LUMAT, FR, 2764), Bât. 106, Univ. Paris-Sud 11, 91405 Orsay Cedex, France

a r t i c l e

i n f o

Article history: Received 1 September 2012 In final form 18 October 2012 Available online 26 October 2012

a b s t r a c t The low-lying electronic excited states of protonated phenol and para-Fluorophenol have been investigated extensively by RI-MP2/RI-CC2 methods. Although, protonation of phenol leads to a small redshift-effect on the S1–S0 (pp⁄) electronic transition in respect to its neutral homologue, a large redshift-effect, on the same electronic transitions of para-substituted phenol has been predicted. The pp⁄ excited state of protonated phenol stays in the UV range (4.34 eV), while its pr⁄ state lies in the VUV region (8.3 eV). The S1 excited-state geometry optimization of protonated phenol predicted unstable S1 state owing to the strong out-of-plane deformation in the benzene ring. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Recent advances in the laser spectroscopy and ab initio computational methods provided a good situation to study the isolated protonated aromatic molecules in the gas phase [1–6]. During the last decades, several of these compounds have been studied either by IR, or UV–Vis laser spectroscopy, associated with theoretical methods [7–12]. Rode et al. showed that protonated benzene undergoes a strong deformation due to S1–S0 transition; this leads to a short life excited state which can predict a broad and structureless spectrum [13]. In contrast to protonated benzene and benzene dimer, the longer lifetime of the S1 excited state, was produced in the cases of protonated benzaldehyde and salicylaldehyde which we could record the well-resolved vibrational spectra of them [7–9]. The ‘ab initio’ investigations showed that the optimized geometry of S1 excited state of these molecules doesn’t show a significant deformation in comparison with their optimized ground state structure [7–9]. Aromatic molecules are important in the wide range of science. Particularly, their biological role for constructing the molecular building blocks, such as proteins and DNA, are crucial. Phenol is one of the simplest aromatic molecules, which has been extremely attractive in photophysics. In addition, phenol is one of the simplest aryl alcohols with several applications in the organic chemistry, biology, and bio-chemistry. Hence, heavy experimental and theoretical studies were carried out on the electronic transitions and electronic structures of this molecule [14–18].

⇑ Corresponding author at: Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran. E-mail addresses: [email protected], [email protected] (R. Omidyan). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.053

Excited state hydrogen detachment (ESHD) is a well understood process which plays an essential role in photochemistry of aromatic molecules involving the acidic group such as pyrole, indole and phenol [19–22]. This process was experimentally confirmed by detection the H atom after photo-excitation of bare molecules, such as phenol (PhOH), substituted PhOHs pyrrole, and indole [23–26]. This is well known that the ESHD acts as the fast nonradiative relaxation channel of electronically excited states of these molecules [21]. The fast internal conversion (IC) between the S2 (pr⁄) to S1(pp⁄) and the S0 (ground state) is the main reason for ESHD [21,25,26]. The repulsive pr⁄ potential energy surface along the O–H stretching forms a fast IC with pp⁄ state and later with the S0 (pp) state [26,27]. Several mechanisms for the ESHD were presented in the literatures [23–29]. Replacement of the para hydrogen atom of phenol either by a fluorine atom (which is an electron attractive) or a –CH3 group (which is an electron donating) can influence properties such as molecular geometry, ionization energy, electronic transitions, reactivity, and dynamics of excited states [30–32]. So far, a variety of ab initio methods were employed for calculating the electronic transition energies of organic compounds. Among them, the CASPT2 and CC2 methods were reported to be more powerful and accurate [33]. Particularly, the comparison between the ‘CC2 calculation results with the cc-pVDZ or aug-cc-pVDZ basis sets’ and ‘experimental data’ for a set of neutral and protonated benzene derivatives was performed by Alata et al. [34]. The CC2 results were quite in well agreement with experiments. In this Letter, we present results of ab initio calculations on the geometry and electronic excited state properties of protonated phenol as well as protonated para substituted phenols. Also, it will be interesting to follow the sign of dissociative nature of pr⁄ state in protonated phenol.

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S. Azizkarimi et al. / Chemical Physics Letters 555 (2013) 19–25

2. Computational details

3.1. Ground state structures of protonated phenol

The ‘ab initio’ calculations have been performed with the TURBOMOLE (V. 6.2) program suit [32,35], making use of the resolution-of-the-identity (RI) approximation for the evaluation of the electron-repulsion integrals. The equilibrium geometry of the protonated phenol and para Fluro-phenol at the ground electronic states (S0) has been determined at the MP2 level (Möller-Plesset second order perturbation theory). Excitation energies and equilibrium geometry of the lowest excited singlet states have been determined at the RI-CC2 level. The calculations were performed with a few basis sets as following: The correlation-consistent polarized valence double-zeta (cc-pVDZ) and the aug-cc-pVDZ [36] were used for most of the calculations. The ‘charge distribution calculations’ were performed based on the Natural Population Analysis algorithm (NPA) [37] implemented on TURBOMOLE program. In most of the calculations, the starting geometry of the ground-state optimization was constructed with the Cs molecular symmetry. The benzene ring lies in the symmetry plane. Within the Cs point group, the wave function of the ‘1pp⁄ and 1pr⁄ state’ transform according to the A0 and A00 representations respectively, thus the excited states can be well distinguished. The geometry optimization of excited states was carried out, mostly, with symmetry restriction. Also in some complementary calculations, the full S1 optimization has been done (i.e. no symmetry was considered). In order to optimize the excited state geometries, the optimized geometry of the ground state has been selected as the starting point for two lowest excited states of A0 and A00 states.

The relative stability of protonated isomers of phenol is presented in Table 1. Corresponding to the different protonation sites (Figure 1), the isomers could be divided into four classes:

3. Results The geometric properties of the bare phenol and para Fluorophenol in the S0 ground state were reported experimentally and theoretically [38–40]. In order to evaluate our computational level and to predict the influence of proton on the geometric properties, the bare phenol and para Fluorophenol are reinvestigated here. The equilibrium structures have been optimized using the MP2/ccpVDZ method and the results are presented in the ESI file (see Table SM1). Our MP2/cc-pVDZ results show the C–C bonds are roughly 0.005 Å larger than the experimental values, nevertheless, the C–C–C bond angles are more comparable to the experimental values.

(I) Para-Isomer (C4); which is the most stable isomer of protonated phenol (0.0 eV). (II) Ortho Isomers (C2 and C6); the energetic levels of these isomers at the ground state are 0.13 and 0.18 eV respectively (at the CC2/aug-cc-pVDZ level of theory). (III) Meta Isomers (C3 and C5); the energetic levels of these isomers at the ground state are 0.69 and 0.66 eV respectively (at the same level) (IV) Oxygen location (Ph–OH2)+; this isomer is 0.69 eV higher than the most stable isomer (Para, C4) in energy level. One may think about the C1 protonated isomer of phenol, whereas the proton locates on the C1 (see Figure 1). We considered this isomer too, but in the MP2 ground state geometry optimization, this isomer converted to the C2 (see the Cartesian Coordinates, Table SM2, in the ESI file). In order to explain the protonation effect on the geometry of phenol, only the optimized geometric parameters of the C4 most stable isomer of protonated phenol (PhH+) have been summarized in Table 2. The equilibrium structure of C4 isomer in its ground electronic state has the Cs symmetry. The structure is planar with the exception of the CH2 group (–C4H2H3), (Figure 1a). Comparing the optimized geometric parameters of protonated and neutral phenol (Table SM1 in the ESI file), substantial alterations over the bond lengths and bond angles has been predicted. Exception the C6–C1, all the other C–C bonds is elongated around 0.1 Å in the protonated phenol respect to the neutral molecule. Protonation at the C4 position elongates the C3–C4 and C4–C5 distances to 1.493 and 1.502 Å respectively. The corresponding aromatic C–C bonds in the isolated neutral phenol lie between 1.402 and 1.407 Å. The C1–O1 bond in the protonated species is slightly shorter (1.349 Å) than that of the neutral phenol (1.370 Å), while, the O–H bond in the protonated phenol is longer than the corresponding bond in the neutral phenol (0.977 Å compared to 0.968 Å). The strongest alteration in the C–C–C bond angles is related to the C3–C4–C5 bond which decreases from 120° (in neutral phenol) to 111.4° in protonated one. Also, the C6–C1–C2 angle decreases from 120° to 117°. The other C–C–C angle more or less increased from 120°.

Table 1 Ground state relative stabilities and electronic transition energies (vertical and adiabatic) – for different isomers of protonated phenol. The calculations performed at the CC2 level, using two different basis sets. All the values are in eV and the values in parenthesis are corresponding to the oscillator strength.

C2 (1A0 , pp⁄) C2 (1A00 , rp⁄) C3 (1A0 , pp⁄) C3 (1A00 , rp ⁄) C4 (1A0 , pp⁄) C4 (1A00 , rp⁄) C5 (1A10 , pp⁄) C5 (1A00 , rp⁄) C6 (1A0 , pp⁄) C6 (1A00 , rp⁄) OH2+ (1A0 , pp⁄) OH2+ (1A00 , pr⁄) Neutral (1A0 , pp⁄) Neutral (1A00 , pr⁄) *

S0 (eV)

Vertical transitions cc-pVDZ

aug-cc-pVDZ

cc-pVDZ

aug-cc-pVDZ

0.13

3.76 5.92 3.27 5.38 4.77 6.01 3.26 5.41 3.75 5.85 5.37 6.47 5.03 7.00

3.68 5.84 3.23 5.35 4.69 5.94 3.24 5.39 3.65 5.76 5.28 5.87 4.88 5.35

3.49 5.26 2.91 4.62 4.43 5.30 2.88 4.67 3.49 5.15 5.17 Not converged* 4.83 Not converged

3.39 5.17 2.86 4.54 4.34 5.22 2.85 4.59 3.37 5.04 5.08 Not converged 4.67 Not converged

0.69 0 0.66 0.18 0.69 8.55

Adiabatic transitions

(0.178) (0.004) (0.110) (0.002) (0.123) (0.000) (0.104) (0.003) (0.171) (0.001) (0.003) (0.000) (0.048) (0.000)

In these cases, the strong geometry deformation in the S1 excited state leads to the falling of the energy gap between the ground and excited states. This suggests a conical intersection as the main interpretation for the ‘Not converged’ CC2-optimization jobs.

S. Azizkarimi et al. / Chemical Physics Letters 555 (2013) 19–25

(b)

(a)

H2 H3

O1

C2

O1

C2

H1

C1

H1

C3

C6

C4

C3

C1

H2

C4

C6

C5

C5

H3

F1

Figure 1. (a) MP2 optimized geometry of the most stable protonated isomer of phenol (C4 isomer) and the numbering pattern. (b) MP2 optimized geometry of the most stable protonated isomer of para Fluorophenol (C2).

Table 2 CC2/cc-pVDZ optimized geometric parameters of the local minima of the first excited state of protonated Phenol (PhH+) and protonated para Fluorophenol (F-PhH+) with and without the Cs symmetry consideration compared to the MP2/cc-pVDZ optimized S0 equilibrium structures. Ground state

S1 excited state

PhH+

Cs symmetry

C1 symmetry

PhH+

F-PhH+

PhH+

F-PhH+

F-PhH+

Distances (Å) C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C6–C1 C1–O1 O1–H1 F1–C4 C2–H2 C4–H2

1.493 1.419 1.493 1.502 1.415 1.401 1.349 0.977 – 1.093 1.112

1.483 1.485 1.367 1.433 1.388 1.421 1.308 0.977 1.327 1.112 –

1.493 1.419 1.493 1.509 1.415 1.481 1.349 0.978 – 1.097 1.115

1.503 1.488 1.403 1.470 1.401 1.431 1.433 0.988 1.311 1.114 –

1.421 1.410 1.509 1.502 1.481 1.427 1.332 0.978 – 1.097 1.102

1.509 1.496 1.418 1.445 1.405 1.441 1.341 0.979 1.305 1.107 –

Angles (deg) C6–C1–C2 C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C5–C6–C1 C1–O1–H1 H2–C2–H3 H2–C4–H3

117.0 121.8 123.1 111.4 125.5 121.4 109.0 – 102.6

120.9 117.1 119.7 120.8 122.9 118.5 111.5 103.1 –

117.0 121.8 123.1 111.1 125.5 121.4 109.7 – 103.1

125.4 110.5 122.5 124.8 114.8 121.9 110.8 104.0 –

113.9 118.2 120.8 89.8 115.1 115.8 110.4 – 112.2

118.7 98.5 117.4 121.4 113.2 119.0 110.0 110.6 –

0.0 1.0

1.0 1.0

0.0 0.0

8.2 121.1

133.3 164.1

Dihedral angle (deg) C2–C1–C6–C3 1.00 C4–C3–C5–C6 1.00

3.2. Ground state structures of protonated substituted phenol To estimate the substitution effect, we considered Fluorine, Chlorine, and Methyl substitutions on the para position with respect to the hydroxyl group of phenol. Obviously, by moving the substitution at the ortho, meta and para positions, several isomers are produced. For simplicity, we ignore other isomers and concentrate on those which the substitutes have a fixed position on the C4 (or para position in respect to the OH group). We abbreviate the protonated substituted phenols as X-PhH+ (X will be F, Cl or –CH3). Similar to phenol, there are several positions for protonation of para substituted phenols. The relative stability of protonated isomers of para Fluorophenol is presented in Table 3. The isomers are divided into four groups: (I) Ortho Isomers (C2 and C6); the C2 isomer is the most stable (0.0 eV at the MP2/cc-pVDZ level) and the energetic level of C6 is only 0.04 eV higher than C2. The calculations predict a

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potential energy barrier for converting the C6 isomer to the C2 one, by a torsional motion of O–H moiety. Figure 2 shows the potential energy profile for converting the C6 isomer to C2 one. As shown, in respect to the local minimum of C6 isomer, the barrier height for such transformation is 0.56 eV. The energetic values were determined by the single point calculations at the CC2/cc-pVDZ level on the MP2 optimized ground state structures. The reaction coordinate is also variation of the O1–H1–C1–C2 dihedral angle from 0° to 180°. (II) Meta Isomers (C3 and C5); the energetic levels of these two isomers are 0.33 eV and 0.27 eV higher than the most stable protonated isomer of C2 respectively (at the RI-CC2/cc-pVDZ level). (III) C1 and C4; among the protonated isomers of para substituted phenol, the C1 isomer is in the highest energetic-level at the ground state (+0.76 eV higher than the local minimum of the most stable C2 isomer). The C4 ground state energetic level is also 0.23 eV higher than the C2 isomer. (IV) Oxygen position (which was abbreviated by X-Ph–OH2+): The energetic level of this isomer is +0.44 eV higher than the C2. Excluding the C1 and C4, other isomers belong to the Cs molecular symmetry (see Table SM3, the Cartesian Coordinates of protonated p-Fluorophenol in the ESI file). In order to investigate the protonation effect on the geometry of para Fluorophenol, only the optimized geometric parameters of C2 -the most stable isomer of protonated para Fluorophenol- have been considered (see Table 2). For comparing, the geometric parameters of neutral para Fluorophenol were recalculated at the same level and presented in the ESI file (see Table SM1). The equilibrium structure of C2 most stable isomer of protonated para Fluorophenol (F-PhH+) in its ground electronic state is planar with the exception of the CH2 group (–C2H2H3), (Figure 1b) and belongs to the Cs molecular symmetry. Protonation at the C2 position elongates almost of the C–C bonds. The most important effect is related to the C1–C2 and C2–C3 distances which they are increased around 0.08 Å. The corresponding aromatic C–C bonds in the isolated neutral para Fluorophenol, lie in the range of 1.396 to 1.407 Å. The C1–O bond in the protonated molecule is slightly shorter than the one of neutral molecule (1.308 Å compared to 1.370 Å) and the O–H bond lengths in the protonated phenol is longer than the corresponding bond length in the neutral phenol (0.977 Å compared to 0.968 Å). The strongest alteration in the C–C–C bond angles is related to the C1–C2–C3 bond which decreases from 120° in a neutral molecule to 117.1° in protonated species. Other C–C–C angles are less affected. The H2–C2–H3 bond angle amounts to 103.1° for the minimum. 3.3. Geometry optimization of the excited states In order to determine the lowest adiabatic transition energies, two lowest excited states with the A0 and A00 symmetry-representations were optimized at the RI-CC2/cc-pVDZ and aug-cc-pVDZ levels. The calculations were performed for all of protonated isomers as well as the neutral molecules. The ground state structure of every protonated isomer of phenol is planar. In the Ph–OH2+ isomer, the geometry optimization of the 1A00 excited state leads to the bond cleavage between the C1 and OH2 group (C1–OH2+ bond), thus; the OH2 abstraction is the main result. In other cases, no important geometry deformation, either in the 1A0 or in 1A00 excited states has been predicted by calculations. The S1 optimized geometric parameters under the Cs symmetry constraint, are more or less the same as corresponding

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Table 3 Ground state relative stabilities and electronic transition energies (vertical and adiabatic) of different isomers of protonated para Fluorophenol. The calculations performed at the CC2 level with two different basis sets. All the values are in eV and the values in parenthesis correspond to the oscillator strength. S0 (eV)

C1 (S1) C1 (S2) C2 (1A0 , pp⁄) C2 (1A00 , rp⁄) C3 (1A0 , pp⁄) C3 (1A00 , rp⁄) C4 S1 C4 S2 C5 (1A0 , pp⁄) C5 (1A00 , rp⁄) C6 (1A0 , pp⁄) C6 (1A00 , rp⁄) OH2+ (1A0 , pp⁄) OH2+ (1A00 , pr⁄) Neutral (1A0 , pp⁄) Neutral (1A00 , pr⁄)

Vertical Transitions

0.76 0.00 0.33 0.23 0.27 0.04 0.44 8.16

Adiabatic Transitions

cc-pVDZ

aug-cc-pVDZ

cc-pVDZ

aug-cc-pVDZ

3.88 4.52 3.43 5.77 3.01 5.46 4.33 5.15 3.04 5.51 3.42 5.76 5.28 6.24 4.84 6.86

3.89 4.49 3.36 5.73 2.96 5.42 4.24 5.09 3.01 5.48 3.34 5.70 5.18 5.65 4.68 5.27

2.93 Not converged 3.15 5.22 2.64 4.92 2.93 Not converged 2.69 4.99 3.15 5.16 5.08 Not converged 4.62 Not converged

2.86 Not converged 3.06 5.14 2.56 4.88 Not converged Not converged 2.62 4.95 3.04 5.06 4.97 Not converged 4.45 Not converged

(0.040) (0.103) (0.151) (0.004) (0.107) (0.003) (0.071) (0.239) (0.099) (0.003) (0.142) (0.002) (0.006) (0.002) (0.048) (0.000)

0.7

0.6

Energy (eV)

0.5

0.4

0.3

C6 Protonated isomer of F-PhH+

Figure 3. Optimized geometry of C4 protonated isomer of phenol at the S1 (pp⁄) excited state: (a) Under the Cs symmetry constraint. (b) No symmetry was taken to account.

0.2

0.1

C2 Protonated isomer of F-PhH+

0.0 0

20

40

60

80

100

120

140

160

180

200

H-O-C1-C2/deg Figure 2. The potential energy profile along the C6, C2 isomer transformation coordinate of para Fluorophenol. The energetic values were determined at the CC2/ cc-pVDZ single point calculations on the MP2 ground state minimum structures. The reaction coordinate is the dihedral angle of O1–H1–C1–C2 from 0° (correspond to C6 isomer) to 180° (correspond to C2 isomer).

values in the ground state (see Table 2). When the Cs symmetry is not taken to account, the strong out-of-plane deformation is the main result of the S1 optimization. As shown in Table 2, the C4– C3–C5–C6 dihedral angle is 121.1° in the S1 state while that is 1° in the ground state. On the other hand, all of the C–C–C angles are strongly changed in the S1 state. e.g., the C3–C4–C5 decreases from 111.1° to 89.8° from S0 to S1 state, respectively. When the optimization leads to a strong change in geometry, the excited state optimization does not converge. It often corresponds to cases where the energy gap between the ground and excited state becomes very small, suggesting a conical intersection that cannot be calculated here. Such cases are denoted ‘Not converged’ in Tables 1 and 3. Figure 3a and b shows the optimized geometry of C4, most stable isomer of protonated phenol, with or without symmetry consideration at the S1 excited state. The unconstrained S1 geometry-optimization leads to a chair like structure. The S1 (1A0 ) state, being of pp⁄ orbital character, is unstable with respect to such an out-of-plane deformation of the benzene ring [13,41–43].

Obviously, the strong geometry deformation of the S1 excited state for protonated phenol might lead to the weak Franck–Condon factors consequently (Figure 3b).

3.4. Vertical and adiabatic electronic transitions 3.4.1. Neutral molecules In order to evaluate our computational level, we have determined the adiabatic transition energies for neutral phenol and para substituted phenols for comparing with the values of literatures. In this regard, the adiabatic transition energy of 1A0 (pp⁄) state was calculated accurately. For neutral phenol, our calculations obtained the value of 4.67 eV for adiabatic transition energy of 1A0 (pp⁄) at the CC2/ aug-cc-pVDZ level. This value is a little higher than the value of 4.24 eV which was reported by Sobolewski and Domcke at the CASPT2 level of theory [18]. Also, Pino et al. reported the experimental value of 4.50 eV for the 0–0 band of 1A0 (pp⁄) state of phenol [31]. When the difference between the zero point vibrational energy of S1–S0 transition is taken to account (DZPE = 0.19 eV), our adiabatic value (4.48 eV) is in the best agreement with the experiment. At the CC2/aug-cc-pVDZ level of theory, our calculations obtained the adiabatic-transition-energies of 4.45 and 4.52 eV for the 1A0 (pp⁄) state of neutral para F-Ph–OH and para CH3–Ph–OH respectively. These results are well comparable with the experimental results of Pino et al. [31]. For para F-Ph–OH and para CH3–Ph–OH, they reported the experimental values of 4.35 and 4.38 eV respectively intended for the 0–0 band of the S1 (pp⁄) state.

S. Azizkarimi et al. / Chemical Physics Letters 555 (2013) 19–25

3.4.2. Protonated phenol (C4 the most stable isomer) The vertical transition energies of the 1A0 and 1A00 have been determined at the CC2 method using the aug-cc-pVDZ basis set. The first A0 transition is corresponding to HOMO–LUMO electronic transition and the first A00 (S3) transition is also corresponding to the (HOMO-2)–LUMO electronic transition. The first A0 transition of protonated phenol is a pp⁄ and the first A00 transition is almost always p⁄ r with a small contribution of p⁄ n (see Table SM4 in the ESI file). In protonated phenol, the pr⁄ state, which is a well known state in the neutral phenol, has a notably higher-energy than the pp⁄ and rp⁄ states. The vertical transition energy of the first pr⁄ state amounts to 8.29 eV in protonated phenol at the CC2/aug-cc-pVDZ level of calculation, which is higher than its corresponding pr⁄ state in the neutral phenol. 3.4.3. Other isomers of protonated phenol The vertical and adiabatic 1A0 and 1A00 excited states of protonated isomers of phenol have been determined at the CC2/aug-ccpVDZ level of theory. The MP2 optimized geometry of each protonated isomer has been selected for the starting point of the CC2 calculations. The results are presented in Table 1. According to the Cs symmetry of protonated isomers, the excited 1A0 state, mostly, has the pp⁄ nature and its adiabatic transition energies lie between 2.85 and 5.08 eV. The 1A00 transition, mostly, has the rp⁄ character and its adiabatic transition energy lies between 4.54 eV in the C3 isomer and 5.22 eV in the C4. Although, under the Cs symmetry, the geometry optimization of 1A0 and 1A00 excited state undergo an equilibrium minimum in almost of the protonated isomers, the CC2 optimization of 1A00 state for the Ph–OH2+ isomer leads to the O–C1 bond cleavage and it releases the H2O and C6H5 radical cation. The optimized geometries of protonated isomers of phenol at the 1A0 , 1A00 excited states are presented in Table SM5 in the ESI file. 3.4.4. Protonated para Fluorophenol Excluding the C1 and C4 isomers of protonated para Fluorophenol (F-PhH+), other isomers belong to the Cs symmetry. Thus, the excited states of these isomers have either A0 or A00 representations. The adiabatic electronic transitions for protonated isomers of para Fluorophenol have been calculated, and the results are presented in Table 3. The adiabatic transition energies for the Cs isomers lie in the range of 2.56–3.06 eV. Only, when proton is located on the oxygen atom (F-Ph–OH2+ isomer), the optimization at the 1A00 state was failed and the OH2 moiety was separated (Table SM6 in the ESI file). Considering the orbitals, the first two A0 transitions have the pp⁄ and the first two A00 states have the rp⁄ characters (Table SM4 in the ESI file). For the C1 and C4 isomers of para Fluorophenol, the CC2 optimization of the excited states has been done without symmetry constraint. Both of the S1 and S2 excited states showed a strong deformation in comparison with the ground state structure. The unconstrained S1 geometry-optimization leads to the chair like structure as well as protonated phenol. The CC2 calculations predict that the S1(1A0 ) state, in protonated Fluorophenol should be unstable as a consequence of the strong out-of-plan deformation in the benzene ring [13].

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character, while, the first A00 state is a rp⁄ state. In contrast to the neutral phenol, the first two A00 transition have mainly rp⁄ character. The vertical transition energy for the first pr⁄ (7A00 ) state of protonated phenol amounts to 8.29 eV (at the CC2/augcc-pVDZ level of theory). Although the energetic level of pr⁄ (7A00 ) in protonated phenol is higher than that of neutral phenol, the CC2 geometry optimization of this state doesn’t show the stable minimum as the same as its neutral analogue. The first pr⁄ (7A00 ) state in the protonated phenol is dissociative, in fact since the protective energy barrier is likely below the zero-energy vibrational motion. In Figure 4a we present the r⁄ orbital for the pr⁄ (7A00 ) state at PhO–H = 1.0 Å (a geometry close to the minimum of the ground state). In Figure 4b the r⁄ orbital calculated at PhO–H = 2.0 Å is shown for comparison. Figure 4a shows that the r⁄ orbital is diffuse and is largely localized near the proton of the hydroxyl group. Its antibonding character with respect to the OH bond is clearly visible. Upon detachment of the proton, the r⁄ orbital contracts and evolves to the 1s orbital of hydrogen (Figure 4b). Comparing the first adiabatic transition energy (S1–S0) of protonated phenol (4.34 eV) and the same transition in the neutral phenol (4.50 eV reported by Pino et al. [31]), only a weak red-shift effect was predicted upon to protonation. The calculated charge distributions at the ground and 1A0 excited states of neutral and protonated phenol don’t show an important charge transfer character along with the S1–S0 excitation. The total charge transferred from the hydroxyl group (OH) toward the benzene ring is +0.04q in the neutral phenol and 0.06q in the protonated C4 isomer of phenol (data are not shown).

4.2. Comparing the electronic excited states of protonated para substituted phenol and their neutral homologues The adiabatic transition energies of the 1A0 and 1A00 excited state of protonated phenol and para substituted phenol (F, Cl and CH3 substituted) were calculated at the CC2/aug-cc-pVDZ level of theory. The results are presented in Table 4. The calculations predict a large red shift effect in the S1–S0 electronic transition of protonated para substituted phenols, in respect to their neutral homologues. The adiabatic S1–S0 transition energy of 2.93 eV has been calculated for protonated para Chlorophenol. This is significantly lower than that of the neutral para Chlorophenol (4.44 eV). In the protonated para CH3–Ph–OH, similar to the halo phenols, the S1 transition energy showed a red-shift amount of 1.35 eV comparing the corresponding transition energy in the neutral analogue. In spite of the efforts made in our previous works, the reason for this red-shift effect has not been known precisely. In protonated benzene dimer (BBH+)[7], similar to protonated naphthalene and tetracene [34,44], the first excited state corresponds to the charge transfer (CT) state where an electron of a p orbital (HOMO orbital) localized on the neutral benzene molecule is promoted to the

4. Discussions 4.1. Comparing the electronic excited states of protonated phenol and neutral phenol The nature and energetic level of electronic transitions associated with the neutral and protonated phenol are totally different. The first A0 electronic transition of protonated phenol has the pp⁄

Figure 4. r⁄ orbital obtained for the pr⁄ state of the C4 isomer of protonated phenol: (a) at the ground-state and (b) at the (7A00 ) pr⁄-state minimum geometries.

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Table 4 Vertical and adiabatic electronic transition energies of neutral/protonated phenol and para substituted phenols. The calculations performed at the MP2/CC2, aug-cc-pVDZ levels of theory. Electronic excited states Phenol (PhOH)

Protonated (C4, 0.0 eV) Neutral

para-Chloro-PhOH

Protonated (C2, 0.0 eV) Neutral

para-Fluoro-PhOH

Protonated (C2, 0.0 eV) Neutral

para-Methyl-PhOH

Protonated (C2, 0.0 eV) Neutral

1A0 1A0 1A0 1A0 1A0 1A0 1A0 1A0 1A’ 1A0 1A0 1A0 1A0 1A0 1A0 1A0

(pp⁄) (rp⁄) (pp⁄) (pr⁄) (pp⁄) (rp⁄, np⁄) (pp⁄) (pr⁄) (pp⁄) (rp⁄) (pp⁄) (pr⁄) (pp⁄) (rp⁄) (pp⁄) (pr⁄)

LUMO p⁄ orbital localized on the protonated benzene moiety. We also examined the CT theorem; perhaps, the CT is responsible for red shift effect of protonation in substituted phenol. The calculated charge distributions at the ground and 1A0 excited states of neutral and protonated para Fluorophenol don’t show a significant charge transfer along with the S1-S0 excitation (The total charge transferred from the OH-hydroxyl group to the benzene ring is +0.04q in the neutral para Fluorophenol and +0.01q in the protonated C2 isomer of para Fluorophenol). Nevertheless, the basis of the red shift effect on the protonated aromatic molecules, especially those contain a functional group, (e.g., benzaldehyde [8]), is unclear. Therefore, to be able to describe such an important effect, more study on the protonated aromatic molecules is required!.

5. Conclusion A theoretical study of the low lying electronic excited states of protonated phenol and para substituted phenols have been carried out by means of ab initio (MP2 and CC2) methods. In summery: At the CC2 level, the first adiabatic pp⁄ electronic transition of protonated phenol, protonated para Fluorophenol, Chlorophenol, and para Cresol have been obtained to 4.34, 3.06, 2.93 and 3.17 eV respectively. In comparison to their neutral homologues, the main effect of protonation is a red shift of the first pp⁄ electronic transition of protonated phenol and para-substituted phenols. This effect in the para substituted phenols is much larger (at least 1.35 eV) than the bare phenol (0.33 eV). The unconstrained geometry optimization of the first pp⁄ electronically excited state of protonated phenol and para substituted phenol, showed a drastic geometry deformation, which leads to a chair like structure in the benzene ring. This variation may lead to the short lived excited state of protonated phenol and their para substituted phenol, which can be accompanied with a broad and structureless electronic spectrum.

Acknowledgments This research was supported by the research project No. 900401 from the Research Council of University of Isfahan. The authors are grateful to Professor Christophe Jouvet from his valuable comments. Also, the calculations have been done at the GMPCS center of the University Paris Sud.

Vertical transition energy (eV)

Adiabatic transition energy (eV)

4.69 5.94 4.88 5.35 3.22 4.84 4.66 5.28 3.36 5.73 4.68 5.27 3.46 5.72 4.73 5.14

4.34 5.22 4.67 – 2.93 4.47 4.44 – 3.06 5.14 4.45 – 3.17 4.80 4.52 –

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