Electronically Excited States of Protonated Aromatic Hydrocarbons

Article pubs.acs.org/JPCA

Electronically Excited States of Protonated Aromatic Hydrocarbons: Phenanthrene and Pyrene Behnaz Saed† and Reza Omidyan†,‡,* †

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

‡

S Supporting Information *

ABSTRACT: The first and second electronic excited states (S1 and S2) of protonated phenanthrene and protonated pyrene, having the ππ* nature, are strongly red-shifted compared to corresponding electronic transitions in neutral homologues. The CC2 calculations identify an out-of-plane deformation as the most important photochemical reaction coordinate in protonated phenanthrene as well as protonated benzene. It was shown that the excited S1 states of protonated phenanthrene and protonated benzene are unstable via a torsional motion, which provides a fast access to a S1−S0 conical intersection. From the conical intersection, a barrier-less reaction path directs the system back to the minimum of the S0 potentialenergy surface. In contrast to the most stable isomer of protonated phenanthrene, the most stable structure of protonated pyrene shows planar structure in both the S1 and S2 excited states, without considerable geometry deformations.

1. INTRODUCTION Protonated aromatic hydrocarbon molecules are important constituents of the fundamental class of organic compounds. During the past decade, new developments in high resolution time-of-flight mass spectrometers, improved light sources, sensitive IR spectroscopic detection and ion trapping techniques associated with the high-level quantum mechanics methods such as multiconfigurational second-order perturbation method (CASPT2)1 and the second-order approximate coupled cluster singles and doubles model (CC2)2 have provided significant progress in the study of these important molecules.3−9 In regards to experimental methods, highresolution spectra were recorded with the use of sensitive infrared photodissociation schemes for gas-phase AH+ ions, which provided valuable spectroscopic information on the chemical and structural properties of these reactive intermediates and the preferred protonation site(s) in their ground electronic states.3,5,6,8,10−21 Spectroscopic characterization of the most stable structures of protonated aromatic amino acids, their fragmentations after electronic excitation and their excited-state lifetimes were obtained by IR and UV nanosecond and femtosecond lasers.22−26 The excitation spectra of the protonated benzene dimer and protonated linear-polycyclic aromatic-hydrocarbons (PAHs,) up to tetracene, have been recorded by the jet-cooled gas phase setup of Orsay,27−31 supported by ab initio calculations. The most important conclusion of these studies was reported as a red shift effect on the S1−S0 electronic transition of protonated species from the UV in neutral compounds to the visible in protonated homologues. In contrast to the broad spectrum recorded for © 2013 American Chemical Society

the protonated benzene dimer, well-resolved electronic spectra for the S1−S0 electronic transition of several protonated species such as naphthalene, anthracene, tetracene, and fluorene have been recorded up to now.27−31 Obviously, interpretation of spectra for such molecules could be performed only by the help of ab initio calculations. Consequently, theoretical investigations suggested that a charge transfer character should be the main reason for the read shift effect of electronic transitions in protonated PAHs.28−30 So far, several computational methods have been applied for studying both neutral and protonated aromatic hydrocarbons. Among them, the CASPT2 and CC2 methods were reported to be more powerful and accurate for calculating the electronic transition energies of these organic compounds.32 Comprehensive theoretical33−35 and spectroscopic studies have been performed on phenanthrene, pyrene, and other small PAHs.36−55 Recently, the observed fluorescence excitation spectra of the S1 ← S0 transition of jet-cooled phenanthrene has been recorded by Yasuyuki Kowaka et al.56 They analyzed the rotational structure of the 000 band of phenanthrene by ultrahigh-resolution spectroscopy using a continuous-wave single-mode laser. It has been demonstrated that the stable geometrical structure of phenanthrene (planar structure, C2v symmetry) is markedly changed upon the S1 ← S0 electronic excitation. It was suggested that structural changes can be considered as the main cause of fast internal conversion (IC) in the S1 state of neutral phenanthrene.56 In addition, protonated Received: January 17, 2013 Published: February 26, 2013 2499

dx.doi.org/10.1021/jp400554h | J. Phys. Chem. A 2013, 117, 2499−2507

The Journal of Physical Chemistry A

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Figure 1. Optimized geometries and numbering pattern: (a) neutral phenanthrene; (b) neutral pyrene; (c) the most stable isomer of protonated phenanthrene PhC9H+; (d) the most stable isomer of protonated pyrene PyC1H+; (e) the optimized geometry and numbering pattern of protonated benzene; (f) geometry of the S1−S0 conical intersection of PhC9H+.

second order perturbation theory) level.64,65 Excitation energies and equilibrium geometry of the lowest excited singlet states (S0) have been determined at the RI-CC2 (the second-order approximate coupled-cluster method).2,66,67 The CC2 method was chosen because it gives reasonable results for medium-size closed shell molecules for a moderate computational time.32 The calculations were performed with a few basis sets as following: the correlation-consistent polarized valence double-ζ (cc-pVDZ) and the aug-cc-pVDZ68 were used for most of the calculations, and the TZVP basis set was used for a few complementary calculations. The isomers of protonated phenanthrene and pyrene are noted PhCnH+, PyCnH+, depending on the carbon atom where the proton attached, while the neutral phenanthrene and pyrene are denoted Ph and Py, respectively. The numbering of the carbon atoms is shown in Figure 1. The charge distribution calculations were performed based on the Natural Population Analysis algorithm (NPA)69 implemented on the TURBOMOLE program. To optimize the excited state geometries, the optimized geometry of the ground state was selected as the starting point for two lowest excited states of S1, S2 states. All of calculations, on the ground and excited states, were performed without symmetry restriction. Regarding the electronic states and PES interpretations, we preferred the familiar nomenclature in the literature mostly used by A. L. Sobolewski and W. Domcke.70−73

phenanthrene was studied in strong acidic solutions and its spectrum exhibits two broad absorptions with maxima at 410 and 520 nm.57,58 These electronic systems were tentatively assigned to two isomers, corresponding to different positions of carbon sites for locating the proton (1H-Ph+ and 9H-Ph+). Electronic spectra of protonated phenanthrene and pyrene in the neon matrix have been recorded by Garkush et al.59,60 They discussed the astrophysical relevance of phenanthrene and pyrene as possible carriers of UV−vis bands of interstellar space. Recently, fluorescence excitation spectra for the S1(1B3u) state of pyrene were observed, and the lifetime for single vibronic level excitation was determined by Y. Kowaka et al.54 They accurately ascertained the vibrational energies and fluorescence lifetimes for the observed vibronic bands. The lifetime at the zero-vibrational level of pyrene was reported to 1480 ns, and it becomes gradually shorter as the vibrational excess energy increases. This indicates that the radiation-less transition is very slow in the S1(1B3u) state of pyrene.54 In the present work, we have investigated the protonation effect on the electronic properties of protonated phenanthrene and pyrene. It was shown that the nonadiabatic deactivation pathway plays the main role on photophysical behavior of protonated aromatic molecules. Following the suggestion of Rode et al.,61 we examined the PE profiles of protonated benzene for finding a conical intersection between the S1 and S0 potential energy curves of BH+ too. The CC2 method was employed to explore the photophysical dynamics of protonated phenanthrene and benzene after the photoexcitation to the S1 (ππ*) electronic state. The reaction pathways in which the excited molecules decay to the ground state via a low-lying conical intersection between the S1 and S0 is being analyzed.

3. RESULTS AND DISCUSSION A. Geometry Properties. The first step of this research is to look for the most stable isomer of each molecule. Thus, the MP2 geometry optimization for all of protonated isomers was carried out. The optimized geometries of neutral Ph and Py are planar, and they belong to the C2V and D2h molecular symmetries, respectively. Figure 1 shows the optimized geometries and numbering scheme of neutral phenanthrene which is applicable to pyrene as well. Therefore, one may predict seven protonated isomers for protonated phenanthrene and four different isomers for protonated pyrene, corresponding to different carbon sites on two molecules.

2. COMPUTATIONAL DETAILS The ab initio calculations have been performed with the TURBOMOLE program suit,62,63 making use of the resolutionof-identity (RI) approximation for the evaluation of the electron-repulsion integrals. The equilibrium geometry of protonated phenanthrene and pyrene at the ground electronic states (S0) has been determined at the MP2 (Möller-Plesset 2500



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Table 1. Optimized Geometry Parameters of PhC1H+ Protonated Isomer of Phenanthrene. Calculated Values Were Obtained by the MP2/CC2 Methods for the Ground and S1 Excited States, Respectively

Figure 2 represents the results of ab initio calculations, in particular the ground state relative stabilities of isomers

ground state cc-pVDZ C1−C2 C2−C3 C3−C4 C1−C10 C9−C8 C8−C7 C7−C6 C1−H1 C1−H15 C14−H14 C2−H2 C10−C1−C2 C1−C2−C3 C2−C3−C4 C10−C9−C8 C9−C8−C7 C8−C7−C6 C7−C6−C5 C7−C6−C11 C6−C11−C12

Figure 2. Relative stability for different isomers of (a) protonated phenanthrene and (b) protonated pyrene, calculated at the MP2/ccpVDZ.

associated with the protonation of Ph and Py. The ground state energies of the different isomers of Ph and Py are given relative to the most stable PhC9H+ and PyC1H+ isomers, respectively. In fact, our MP2/CC2 results are roughly different with the reported values of Garkusha et al.,59 at the DFT/B3LYP level, where the PhC1H+ was reported as the most stable isomer. Considering the phenanthrene, there are two classes of protonated isomers: (I) Planar isomers: four planar isomers corresponding to protonation of C1, C2, C3, C4, and C9 atoms are expected (i.e., benzene rings stay planar following MP2 optimization). Relative ground state energies of these isomers are very close to each other. Also, two isomers (PhC1H+ and PhC4H+) have the same ground state energies, very close to the PhC9H+ isomer, 0.02 eV (1.9 kJ.mol−1) and other isomers have slightly higher ground-state energies than PhC9H+ and PhC1H+ (see Figure 2a). (II) Nonplanar isomers: obviously, the protonated isomers which are produced by locating the proton on a connection point of two benzene rings, (i.e., C5 and C10 atoms), are not planar. These isomers (PhC5H+ and PhC10H+) have very large internal energy compared to the class I isomers, (0.57 and 0.70 eV for PhC5H+ and PhC10H+ isomers, respectively), they are less stable than the others, therefore, no more calculations have been done for excited state of these two isomers. Also, for protonated pyrene, one may predict five different isomers, among them, PyC1H+, PyC2H+, and PyC9H+ are planar, and PyC4H+, PyC5H+ are nonplanar. The latter two isomers have higher ground state energy than the others, the PyC1H+ is the most stable isomer, and PyC2H+ and PyC9H+ have significantly higher energy than PyC1H+ (0.69 and 0.47 eV, respectively). Typically, to study the protonation effect on geometry parameters, selected bond lengths and bond angles for one of protonated isomers of phenanthrene have been presented in Table1. The structure is planar with the exception of the CH2 group (−CHH). In a comparison of the optimized geometric

1.478 1.376 1.423 1.495 1.392 1.427 1.438 1.112 1.113 1.095 1.096 117.4 120.8 119.7 120.3 122.1 119.5 117.8 118.3 120.6

aug-cc-pVDZ Distances (Å) 1.478 1.379 1.423 1.495 1.393 1.429 1.440 1.112 1.113 1.094 1.095 Angles (deg) 117.5 120.8 119.7 120.2 122.0 119.7 117.7 118.5 120.5

S1 excited state cc-pVDZ 1.485 1.399 1.404 1.504 1.394 1.420 1.440 1.115 1.115 1.097 1.098 114.7 122.1 121.5 122.0 118.6 120.3 120.2 117.4 121.9

aug-cc-pVDZ 1.486 1.402 1.404 1.505 1.396 1.421 1.440 1.115 1.115 1.096 1.097 114.6 122.2 121.5 122.0 118.5 120.4 120.4 117.5 121.9

parameters of this isomer with the corresponding values in the neutral molecule (see Table SM1 in the ESI file), substantial alterations over the bond lengths and bond angles have been predicted. Both of the C−H covalent bonds in the aliphatic CH2 moiety are slightly elongated (1.112 and 1.113 Å) as compared to the corresponding aromatic C−H bonds in the isolated phenanthrene molecule (1.093, 1.095, and 1.096 Å). Protonation at the C1 position elongates the C1−C2, and C3− C4 distances to 1.478 and 1.423 Å, respectively. The corresponding aromatic C−C bonds in the isolated neutral phenanthrene lie between 1.392 to 1.394 Å. The C2−C3 bond in the protonated species is slightly shorter (1.376 Å) than one of the neutral phenanthrene (1.412 Å). The strongest alteration in the C−C−C bond angles is related to the C10−C1−C2 bond which decreases from 121.0° (in neutral) to 117.4° in the protonated species. Furthermore, the proton affinity (PA) of phenanthrene and pyrene was calculated. The MP2/cc-pVDZ calculation, obtained 8.61 eV (830.69 kJ.mol−1) for proton affinity of phenanthrene and 9.14 eV (881.82 kJ.mol−1) for pyrene. The calculated results are in good agreement with the experimental values of Hunter and Lies (825.7 kJ.mol−1 for phenanthrene and 869.2 kJ.mol−1 for pyrene74). The internal energy of neutral phenanthrene and pyrene are +8.61 eV (830.69 kJ.mol−1) and 9.14 eV (881.82 kJ.mol−1) higher than energy of the most stable protonated isomer of phenanthrene (PhC9H+) and pyrene (PyC1H+) at the MP2/cc-pVDZ level respectively. B. Vertical and Adiabatic Transition Energies. 1. Neutral and Protonated Phenanthrene (Ph and PhCnH+). For each isomer produced by protonation of phenanthrene, the ground and excited state geometry optimization were performed at the MP2/CC2 (cc-pVDZ and aug-cc-pVDZ) 2501



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Table 2. Relative Ground State Energies along with Excited Transition Energies (Vertical and Adiabatic) for Different Isomers of Protonated Phenanthrene (PhCnH+), Computed at the MP2/CC2 Levels with Two Different Basis Sets. The Values in Parentheses Correspond to the Oscillator Strength adiabatic transitions vertical transitions ground state relative energy/eV [kJ·mol−1] PhC1H+ 0.02 [1.9] PhC2H+ 0.12 [11.5] PhC3H+ 0.07 [6.75] PhC4H+ 0.02 [1.9] PhC9H+ 0.00 neutral-Ph

S1← S2← S1← S2← S1← S2← S1← S2← S1← S1← S1← S2←

S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0

cc-pVDZ

cc-pVDZ

aug-cc-pVDZ

energy (Ev)

ΔZPE (eV)

2.43 (0.216) 2.82 (0.019) 2.34 (0.047) 3.22 (0.290) 2.63 (0.242) 2.81 (0.023) 2.19 (0.045) 2.90 (0.305) 2.41 (0.161) 2.81 (0.036) 4.06 (0.001) 4.77 (0.076)

2.37 2.77 2.31 3.19 2.58 2.75 a a 2.37 2.77 3.97 4.61

2.21 2.51 2.12 3.06 2.45 a 1.90 2.68 a a 3.90 4.54

−0.13

2.08

−0.12

2.00

−0.15

2.30

−0.10

1.80

corrected for ZPE

a −0.10

3.80 (3.63b)

aug-cc-pVDZ 2.14 2.45 2.08 a 2.39 a a a 2.09 a 3.80 4.39

a CC2 cycles cease to converge due to strong geometry deformations. bThe experimental value for the 0−0 band of S1−S0 transition of neutral phenanthrene was taken from ref 75.

Table 3. The Highest Occupied and the Lowest Unoccupied MOs of Neutral and Protonated Phenanthrene and Pyrene

S1(0−0) band of neutral phenanthrene in the gas phase jetcooled molecular beam was reported by Hager and Wallace as 3.63 eV (341.0 nm).75 Thus, the calculated value of the present study at the CC2/cc-pVDZ level of theory (3.80 eV, ΔZPE corrected) is in good agreement with experiment. For protonated isomers, the calculated adiabatic S1−S0 transitions lie in a narrow range, from of visible to the near IR (NIR), (2.03−1.80 eV at the CC2/cc-pVDZ level and ΔZPE corrected). The lowest value is related to the PhC4H+ isomer (1.80 eV) with an oscillator strength of 0.045, and the highest transition energy is related to the PhC3H+ (2.30 eV) with the value of 0.242 oscillator strength. Unfortunately, there is no experimental report for the gas phase of protonated phenanthrene in a jet-cooled molecular beam to compare with our results, the only experimental data which we can refer to is the low lying electronic spectrum of protonated phenanthrene in the 9 K neon matrix reported by Garkusha et al.59 Only a few values calculated in this work are in agreement with the experimental origin (0−0) bands of Garkusha et al. For instance, the adiabatic value of S1−S0 and

levels. Vertical and adiabatic transition energies of S1 and S2 excited states have been calculated for neutral and protonated isomers of PhCnH+ (see Table 2). The vertical transitions were calculated on the optimized geometry of ground states, while the adiabatic S1 and S2 transitions were calculated at the corresponding (S1 and S2) excited state optimized geometries. According to RI-CC2 calculations, in both of the neutral and protonated phenanthrene (PhC9H+, the most stable protonated isomer), the first electronic transition (S1−S0) corresponds to LUMO ← HOMO and the second electronic transition (S2−S0) corresponds to the LUMO ← HOMO-1 transition (see Table 3). Therefore, the first electronic transition in neutral phenanthrene has a ππ* nature. In the protonated isomers, the PhC1H+, PhC2H+, PhC3H+, PhC4H+, and PhC9H+ isomers belong to the Cs symmetry and their S1−S0 could be known as the ππ* transition. The calculated S1−S0 adiabatic transition energy of neutral phenanthrene is 3.80 eV, (the difference between the ground and excited state zero-point vibrational energy is taken into account). The experimental origin of the 2502



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Table 4. Relative Ground State Energies along with Excited Transition Energies (Vertical and Adiabatic) for Different Isomers of Protonated Pyrene (PyCnH+), which Were Computed at the CC2 Level with Two Different Basis Sets. All the Values are in eV; the Values in Parentheses Correspond to the Oscillator Strength adiabatic transitions vertical transitions ground state relative energy/eV [kJ·mol−1] PyC1H+ 0.00 PyC2H+ 0.69 [66.6] PyC9H+ 0.47[45.3] Neutral Py a

electronic transition S1← S2← S1← S2← S1← S2← S1← S2←

S0 S0 S0 S0 S0 S0 S0 S0

cc-pVDZ

cc- pVDZ

aug- ccpVDZ

energy/ eV

ΔZPE (eV)

corrected for ΔZPE (eV)

2.80 (0.182) 3.02 (0.193) 2.02 (0.067) 3.18 (0.037) 1.86 (0.049) 2.53 (0.110) 3.70 (0.0004) 4.05 (0.381)

2.76 2.99 1.98 3.15 1.83 2.49 3.63 3.93

2.67 2.91 1.86 3.03 1.57 2.30 3.58 3.89

-0.15

2.52

−0.08

1.78

−0.10

1.47

−0.14

3.44 (3.38a)

aug-ccpVDZ 2.63 2.86 1.83 2.98 1.53 2.25 3.51 3.76

The experimental value for the 0−0 band of S1−S0 transition of neutral pyrene was taken from ref 75.

S2−S0 transitions of PhC2H+ isomer obtained were 2.00 and 3.06 eV at the CC2/cc-pVDZ level of calculation (ΔZPE correction was considered for the S1−S0 transition) in our work, while the origin (0−0) band of S1−S0 and S2−S0 transitions were reported to 2.13 eV (583.33 nm) and 2.97 eV (417.40 nm), respectively. Nevertheless, there are several discrepancies: the calculations showed a strong geometry deformation for the S1 state of PhC9H+ and for the S2 state of PhC3H+. The CC2/cc-pVDZ geometry deformation of the S1 state of PhC9H+ (similar to Figure 1f), was verified by using the CC2/TZVP level of calculation too. These deformations on the excited states of these cases lead to the weak Franck− Condon factors and cause a broad and structure-free spectrum for such isomers in the gas phase. 2. Neutral and Protonated Pyrene (Py and PyCnH+). The calculated vertical and adiabatic transition energy for the neutral and protonated isomers of pyrene at the MP2/CC2 (cc-pVDZ and aug-cc-pVDZ) level are presented in Table 4. The optimized geometry of neutral pyrene at the ground state belongs to the D2h symmetry point group, and its S1 and S2 transition energies are assigned as the ππ* transition. In protonated isomers, the PyC1H+, PyC2H+, and PyC9H+ isomers belong to the Cs symmetry point group and their S1−S0 transition could be known as the ππ* also. The frontier MOs which participated in the S1 and S2 transitions of neutral and protonated pyrene are shown in Table 3. According to RICC2 calculations, one electron excitation that especially contributes to the S1 state is the LUMO←HOMO transition. The LUMO←HOMO-1 transition gives rise to the second electronic transition of S2−S0. The experimental origin band of the S1(ππ*) transition of the neutral pyrene was reported to be 3.38 eV75 (367.45 nm). Our calculations gave a value of 3.58 eV (at the CC2/cc-pVDZ level). When the ΔZPE (−0.14 eV) is taken into account, the calculated value (3.44 eV) is in good agreement with experiment. In contrast to some cases in protonated phenanthrene for which the S1 or S2 geometry optimizations at the CC2 level lead to the strong out-of-plane deformation, the same level of calculations for all of protonated isomers of pyrene showed a planar structure without significant geometry alterations (see Supporting Information, Table SM2). The S1 transition energies in protonated isomers of pyrene are closed to one another. The lowest S1 transition energy (1.47 eV) is related to the PyC9H+ isomer (with the oscillator strength of 0.049) and the highest S1 transition energy (2.52

eV) is related to the PyC1H+ protonated isomer which is the most stable protonated isomer of pyrene. In addition, the S2 vertical and adiabatic transition energies of protonated and neutral pyrene have been calculated at the CC2/cc-pVDZ and aug-cc-pVDZ level of theory (see Table 4). The geometry optimization has been done at the S2 excited state for calculating the S2 adiabatic transition energies for each protonated isomer and neutral case. The values for the adiabatic transition of S2−S0 for the protonated isomers of pyrene, as is the same as S1−S0 transition energies, lie in the visible region (from 2.30 to 2.91 eV at the CC2/cc-pVDZ level of calculation). Considering the values of oscillator strength, in PyC2H+ the S2−S0 transition is weaker than the S1−S0 transition, but in the PyC1H+, PyC9H+, and neutral pyrene, the S2−S0 transitions are stronger than the S1−S0 transitions. C. Potential Energy Profiles and Internal Conversion: Protonated Benzene and Phenanthrene. Protonated benzene was the first aromatic system for which the protonation effect was investigated by Rode et al.61 They suggested that excited states of protonated benzene probably undergo a very fast internal conversion and should be very short-lived.61 Indeed, in the first excited states, the system has to lose its planar symmetry, and a conical intersection can be expected with the ground state which arises along out-of-plane bending coordinates. As shown in Figure 1f, such an out-ofplane deformation happened in protonated phenanthrene (PhC9H+, most stable isomer) too. This ring deformation should be a sign for a CI in the potential energy curves (PECs) of protonated phenanthrene as well as protonated benzene. The potential energy profiles along the minimum energy paths (MEP) for torsion of the −CH2 (C9−C8−C10−C5 dihedral angle, see Figure 1a for numbering) in protonated phenanthrene in the S0 and S1 states have been calculated. The results are shown in Figure 3a (full curves). The coordinatedriven minimum-energy paths for out-of-plane deformation have been obtained by fixing the θ(C9−C8−C10−C5) and optimizing the lowest 1ππ* state with respect to all other coordinates. The geometry optimizations have been performed with the CC2 method. The energies at the optimized geometries have been calculated at the CC2/cc-pVDZ level. Figure 3a shows the resulting PE profile of the 1ππ* state of protonated phenanthrene as a function of θ(C9−C8−C10−C5), (solid lines with the filled circles). The PE profiles of the S0 state, calculated at the 1ππ* optimized geometries (dashed lines with hollow squares), as well as at the S0 optimized geometries, 2503



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Figure 4. Comparison the S1−S0 adiabatic transition energy of neutral and protonated small PAHs. The blue and red colors represent the S1−S0 adiabatic transition energies of neutral and protonated PAHs, respectively. The Δ denotes to difference between the S1−S0 adiabatic transition energy of neutral and protonated homologues in eV.

Figure 3. Potential energy curves of the S0 state (squares) and the S1(ππ*) state (circles), determined at the CC2/cc-pVDZ level for torsion of the −CH2 (a) protonated phenanthrene (PhC9H+ isomer) and (b) protonated benzene, The energy origin is the energy of minimum of each compound at the ground state. The full lines represent the energy profiles of reaction paths determined in the same electronic state and the dashed lines show the energy profiles of reaction paths determined in the complementary electronic state.

et al.27,28 For other small PAHs, (i.e., phenanthrene-H+, pyreneH+, and pentacene-H+), there are no experimental values in the literature. To compare the S1−S0 electronic transition energies between neutral and protonated aromatic compounds, we considered the values which have been obtained in the same theoretical methods. With the exception of protonated phenanthrene, the CC2/cc-pVDZ results are related to the most stable isomers of protonated PAHs (corresponding to experiment). For comparison, the adiabatic values were calculated for naphthalene-H+, anthracene-H+, fluorene-H+, and tetracene-H+; the values are 2.56, 2.66, 2.87, and 1.86 eV, respectively. The average difference between the experimental and theoretical values is −0.10 eV. Therefore the CC2/ccpVDZ values are in good agreement with experiment. (The ΔZPE correction between the S1 and S0 states was not considered). The adiabatic S1−S0 transition energies for the neutral and protonated PAHs (from naphthalene to pentacen), are collected in Figure 4. At least, four important points can be presented by considering the data from Figure 4: (1) In contrast to neutral molecules, the electronic transition energy of protonated PAHs is not a linear function of number of benzene rings. (2) Unlike the neutral PAHs, among them, a linear PAH has smaller S1−S0 transition energy than its nonlinear homologue; there is no regular trend in the protonated species. (3) The protonation process leads to a significant red shift effect on the S1−S0 electronic transition in small PAHs. This transition-energy-movement is more effective in nonlinear PAHs than their linear analogues (i.e., the red shift in phenanthrene is larger (1.69 eV) than in anthracene (0.81 eV), and also that in pyrene (0.91 eV) is larger than in tetracene (0.88 eV)). Nevertheless, this suggestion needs more study on PAHs to conclude whether that is true in larger PAHs or not. (4) It appears that protonation causes a smaller red-shift effect on the S1−S0 electronic transition of large PAHs rather than on that of small PAHs. Looking for the accurate reason for this suggestion will provide interesting future studies. Despite efforts made in our previous works, the reason for the red-shift result following protonation of PAHs, is not precisely known. In protonated anthracene, tetracene,28 and naphthalene,29 the first excited state corresponds to the charge

determined with the MP2 method (solid lines with filled squares), are also shown. The S(S1) curve in Figure 3a is rising 0 by increasing θ(C9−C8−C10−C5), while, the energy profile of the S1 state calculated along the S1 reaction path (S(S1) 1 ) in Figure 3a indicates a barrier-free reaction coordinate which leads to the conical intersection. In the S1 state, the MEP for the CH2 torsion could be determined with the exception of geometries which are very close to the S1−S0 conical intersection, where the CC2 iteration cycle fails to converge. It is seen that the energy profile corresponding to pure twisting of the CH2 group exhibits roughly a flat reaction path before the CI region. Around 35° in PhC9H+, the S0/S1 potential energy profiles cross with each other and obtain a conical intersection (see Figure 1f). As shown in Figure 3b, the PE profiles of protonated benzene (BH + ), show the same trend as protonated phenanthrene. The reaction coordinate in BH+ is the torsion of −CH2 (dihedral angle of ϕ(C1−C6−C5−C3), see Figure 1e for numbering), and the CI region falls to ϕ(C1−C6−C5−C3) around 14°. While the CC2 method is not accurate for the location of the precise geometry of the conical intersection, the conical intersection structure is estimated by the intersection of the CC2 energies roughly before and after the CI point (Figure 1f). Actually, in the regions in which the energetic levels of S1 approach the one of the S0 state, the CC2 optimization is faced with problems and ceases to converge. After the intersection point, the reaction coordinates (θ and ϕ) increasing to more values, the CI leads the flat trend of S(S1) and S(S1) curves (data 0 1 were not shown). D. Comparison the S1−S0 Transition Energy in Protonated and Neutral Molecules of Small PAHs. The adiabatic S1−S0 electronic transitions for seven polycyclic aromatic hydrocarbons are collected in Figure 4. The experimental values from the S1(0−0) band of naphthaleneH+, anthracene-H+, fluorene-H+, and tetracene-H+ have been reported to 2.46, 2.51, 2.77, and 1.82 eV, respectively, by Alata 2504



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transfer (CT) state where an electron of a π orbital (highest occupied molecular orbital (HOMO)) localized on the neutral benzene ring is promoted to the lowest unoccupied molecular orbital (LUMO) (π*) localized on the protonated benzene moiety. Because of a positive charge on the protonated ring, the ionization potential of the protonated benzene is higher than that of the neutral one. Thus, the HOMO (and also HOMO − 1) is more stabilized at the protonated moiety than the neutral part. So, a CT character may be an explanation for such redshift trend.28 According to calculations that were performed in the present study, in PhC1H+, the S1−S0 transition corresponds to the HOMO−LUMO excitation. The charge distribution analysis performed for the ground state, and for the first excited states of the PhC1H+ isomer, verifies that the CT hypothesis is valid for protonated phenthrene as well as for naphthalene and anthracene. The excitation of PhC1H+ to the ππ* singlet state involves a −0.20 q charge transfer from the neutral part to the protonated ring (for more details, see Supporting Information, Table SM4). Similar to protonated phenanthrene, a CT character may be an alternative explanation for the red-shift trend in PyC1H+ too. The NPA charge distribution calculations in PyC9H+ verify that −0.10 q charge transfers from the neutral part to the protonated ring (see Supporting Information, Table SM5).

ACKNOWLEDGMENTS The research council of University of Isfahan is acknowledged for financial support. We would express our gratitude to Prof. C. Jouvet (University of Aix-Marseille) for his helpful comments. We also acknowledge the use of the computing facility cluster GMPCS of the LUMAT federation (FR LUMAT 2764).

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSION A conical intersection between the S1 and S0 potential energy curves has been found to be responsible for the fast deactivation of the S1 excited state in protonated phenanthrene (PhC9H+) and protonated benzene (BH+). At the S1−S0 conical intersection, the system switches from the S1 surface to the S0 surface via a nonadiabatic transition. It is thus possible that the global minimum structure of BH+ and PhC9H+ is restored with a probability very close to unity, which is a requirement for the function of BH+ and PhC9H+ as effective photostabilizers. Red-shifting of the S1−S0 electronic transition is an essential consequence of protonation of small PAHs. The CC2 calculations verify that the first electronic transition energy of the smaller PAHs (i.e., naphthalene, anthracene, and phenanthrene) becomes more affected by protonation in comparison to this effect in larger PAHs (such as pentacene). Considering the PAHs with the same molecular formula and different structures, linear and nonlinear PAHs such as anthracene and phenanthrene, it seems that the red-shift effect associated with protonation is stronger in nonlinear PAHs than their linear analogues. More theoretical and experimental studies on larger PAHs are required to confirm this suggestion concerning the larger PAHs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Fax: (+98) 311 6689732. Notes

The authors declare no competing financial interest. 2505



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