ARTICLE pubs.acs.org/JPCB
Ultrafast Intramolecular Energy Relaxation Dynamics of Benzoporphyrins: Influence of Fused Benzo Rings on Singlet Excited States Pyosang Kim,† Jooyoung Sung,† Hiroki Uoyama,‡ Tetsuo Okujima,‡ Hidemitsu Uno,*,‡ and Dongho Kim*,† † ‡
Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
bS Supporting Information ABSTRACT: We have investigated the role of fused benzo rings on the electronic structures and intramolecular energy relaxation dynamics in a series of benzoporphyrins (Bp1, synBp2, anti-Bp2, Bp3, and Bp4) by using time-resolved fluorescence measurements and theoretical calculations. Interestingly, even though anti- and syn-Bp2 have the same number of fused benzo rings, in the respective absorption spectra, anti-Bp2 shows an obvious splitting of Bx (Q x) and By (Q y) states, whereas syn-Bp2 exhibits degenerate B and Q bands. These features provide two dynamical models for the effect of the position of substituted benzo rings on the intramolecular energy relaxation dynamics. syn-Bp2 gives rise to similar intramolecular dynamics from the B state to the Q state in the case of ZnTPP having D4h molecular symmetry. On the other hand, anti-Bp2 shows split B and Q bands in the order By > Bx > Q x > Q y, which leads a superimposition of the Q x (0,0) and Q y (1,0) bands. This overlap generates a strong coupling between these two states, which results in a direct internal conversion from Bx (0,0) to Q y (0,0). This observation suggests that the anti-type fused position of benzo rings leads to a new mechanism in internal conversion from the B to the Q state. On the basis of this work, further insight was obtained into the effect of fused benzo rings on the photophysical properties of benzoporphyrins, providing a detailed understanding of the structureproperty relationship in a series of benzoporphyrins.
I. INTRODUCTION Porphyrins are a class of molecules playing key roles in many important biological processes. This fact has inspired numerous research activities ranging from fundamental studies of their chemical versatility and photophysical properties to practical applications such as molecular photonic devices, artificial photosynthesis systems, and dye-sensitized solar cells.113 Strenuous research efforts have focused on developing new types of porphyrin chromophores that display certain characteristics for applications.14 In this context, a detailed understanding of the relationship between structure and photophysical properties is crucial for the fabrication of novel porphyrins targeted for specific applications. Among the various porphyrin systems, zinc(II) tetraphenylporphyrin (ZnTPP) and free-base tetraphenylporphyrin (H2TPP) have been studied most extensively, both experimentally and theoretically.1519 ZnTPP shows two characteristic absorption bands, B and Q, which are doubly degenerate states, Bx (Q x) and By (Q y), with D4h symmetry, as predicted according to Gouterman’s four-orbital model.15 From a dynamic point of view, ZnTPP displays a dual fluorescence from the two singlet excited states: S2 (B) f S0 and S1 (Q) f S0.2022 In particular, the observation of emission from B state is considered to be a well-known violation r 2011 American Chemical Society
of Kasha’s rule.23 Since the S2 fluorescence of ZnTPP was first observed in 1975,20 many groups have reported S2 lifetimes ranging from ∼1 to ∼3 ps in various solvents as determined using femtosecond time-resolved spectroscopic tools.2428 On the other hand, the photophysical properties of free-base tetraphenylporphyrin (H2TPP) have also been thoroughly characterized by femtosecond up-conversion and transient absorption measurements. In contrast with the electronic structures of ZnTPP, the Q bands of H2TPP split into Qx and Qy states mainly by reducing the molecular symmetry from D4h to D2h, which leads to ultrafast internal conversion from the B state to the Qx state within ∼50 fs, followed by complicated intramolecular vibrational energy redistributions.29 These previous reports revealed that the molecular symmetry largely affects the intramolecular energy relaxation dynamics in porphyrin derivatives. Recently, our group reported the synthesis of various benzoporphyins, namely, monobenzo- (Bp1), dibenzo- (syn-Bp2 and antiBp2), tribenzo- (Bp3), and tetrabenzoporphyrin (Bp4), without peripheral substituents, which allows these porphyrins to have nearly Received: January 17, 2011 Revised: March 8, 2011 Published: March 22, 2011 3784
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The Journal of Physical Chemistry B Scheme 1. Molecular Structures of Zinc(II) Porphine (ZnP) and Mono-, Di-, Tri-, and Tetrabenzoporphyrin (Bp1, synBp2, anti-Bp2, Bp3, and Bp4)
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without peripheral substituents were prepared according to the reported methods.30 High-performance-liquid-chromatography(HPLC-) grade pyridine and toluene, as well as ZnTPP, were purchased from Sigma-Aldrich and used without further purification. All measurements were carried out at ambient temperature (23 ( 2 °C). Steady-State Absorption and Fluorescence Spectra. Absorption spectra were obtained using a UVvisNIR spectrometer (Varian, Cary5000), and steady-state fluorescence spectra were measured on a Hitachi model F-2500 fluorescence spectrophotometer. Steady-state excitation anisotropy spectra were obtained by aligning the polarization of fluorescence detection either parallel or perpendicular to the polarization of the excitation light. The fluorescence excitation anisotropy values were then calculated as r ¼
planar geometry (Scheme 1).30 Interestingly, the symmetries induced by the positions and numbers of benzene substituents give rise to two completely different absorption spectra, in which anti-Bp2 and Bp3 show split B and Q bands in contrast with single B and Q bands in Bp1, syn-Bp2, and Bp4. On the basis of magnetic circular dichroism studies and quantum mechanical calculations, Kobayashi et al. reported that low symmetry significantly affects electronic structures, resulting in the splitting of degenerate Bx (Q x) and By (Q y) states in various benzo-fused azaporphyrins31 and porphyrins.32 Nevertheless, little is known about the split B- and Q-state properties, especially in relation to the intramolecular energy relaxation dynamics in benzoporphyrins. As a consequence, these molecules are good candidates for systematic studies of the effect of fused benzo rings on the intramolecular energy relaxation dynamics in porphyrin derivatives as compared to zinc(II) porphyrins having nearly D4h symmetry such as ZnTPP. Therefore, in this work, we investigated the relationship between the molecular symmetry and intramolecular energy relaxation processes of Bp1Bp4 in pyridine as compared to those of ZnTPP in toluene using steady-state spectroscopic measurements, femtosecond fluorescence up-conversion techniques, and quantum mechanical calculations. In particular, even though anti- and syn-Bp2 have the same number of fused benzo rings, anti-Bp2 shows an obvious splitting of Bx (Q x) and By (Q y) states in contrast with the degenerate B and Q states of syn-Bp2. From this contrasting feature in electronic transitions between syn-Bp2 and anti-Bp2, we focused our attention on revealing the effect of the substituted positions of benzo rings on electronic transitions and the intramolecular energy relaxation dynamics. In contrast to syn-Bp2, anti-Bp2 exhibits an additional ultrafast depolarization process during internal conversion from the B state to the lowest Q state, which is caused by the overlap of Q y (1,0) and Q x (0,0). This feature breaks the energy gap law and suggests a new mechanism in internal conversion from the B to the Q state that is modified by the anti-type fused position of benzo rings.
II. EXPERIMENTAL METHODS Sample Preparation. Monobenzo- (Bp1), dibenzo- (syn-Bp2 and anti-Bp2), tribenzo- (Bp3), and tetrabenzoporphyrin (Bp4)
IVV GI VH IVV þ 2GIVH
where IVV (or IVH) is the fluorescence excitation spectrum when the excitation light is vertically polarized and only the vertically (or horizontally) polarized portion of the fluorescence is detected; that is, the first and second subscripts represent excitation and detection polarization, respectively. The factor G is defined as IHV/IHH, which is equal to the ratio of the sensitivities of the detection system for vertically and horizontally polarized fluorescence. Picosecond Time-Resolved Fluorescence Decay and Anisotropy. A time-correlated single-photon-counting (TCSPC) system was used for measurements of spontaneous fluorescence decay and fluorescence anisotropy decay. As an excitation light source, we used a mode-locked Ti:sapphire laser (Spectra Physics, MaiTai BB) that provides ultrashort pulses [80 fs, full width at halfmaximum (fwhm)] with a high repetition rate (80 MHz). This high repetition rate was slowed to 1000800 kHz by using a homemade pulse picker. The pulse-picked output pulse was frequency-doubled with a 1-mm thickness of a β-barium borate (BBO) crystal (EKSMA). The fluorescence was collected by a microchannel plate photomultiplier (MCP-PMT, Hamamatsu, R3809U-51) with a thermoelectric cooler (Hamamatsu, C4878) connected to a TCSPC board (Becker & Hickel SPC-130). The overall instrumental response function was about 25 ps (fwhm). A vertically polarized pump pulse from a Glan-laser polarizer was irradiated onto samples, and a sheet polarizer, set at an angle complementary to the magic angle (54.7°), was placed in the fluorescence collection path to obtain polarization-independent fluorescence decays. Time-resolved fluorescence anisotropy decays were obtained by changing the detection polarization on the fluorescence path to be parallel or perpendicular to the polarization of the excitation pulses. The anisotropy decay was then calculated as rðtÞ ¼
IVV ðtÞ GIVH ðtÞ IVV ðtÞ þ 2GIVH ðtÞ
where IVV(t) [or IVH(t)] is the fluorescence decay when the excitation light is vertically polarized and only the vertical (or horizontally) polarized portion of fluorescence is detected; that is, first and second subscripts represent excitation and detection polarization, respectively. The factor G is defined as IHV(t)/IHH(t), which is equal to the ratio of the sensitivities of the detection system for vertical and horizontal polarizations. 3785
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Figure 1. Steady-state absorption (black lines) and fluorescence (gray lines) spectra of benzoporphyrins in pyridine. Asterisks (*) indicate the Raman peak of pyridine.
Femtosecond Time-Resolved Fluorescence Decay and Anisotropy. A femtosecond fluorescence up-conversion apparatus
was used for the time-resolved spontaneous fluorescence measurements. The beam sources for B- and Q-state fluorescence were provided by a mode-locked Ti:sapphire laser also used in TCSPC system. The second harmonic of the fundamental generated by a 200-μm-thick BBO crystal served as the pump pulse. Residual fundamental pulse was used as a gate pulse. The pump beam was focused onto a 500-μm-thick quartz cuvette containing sample solution using a 5-cm-focal-length plano-convex lens with a magic angle (54.7°) in order to prevent polarization-dependent signals. The cuvette was mounted on a motor-driven stage and moved constantly back and forth to minimize photodegradation. Collection of the fluorescence and focusing into a 1-mm-thick BBO crystal for frequency conversion were achieved with a reflecting microscope objective lens (Coherent). The fwhm of the cross-correlation function between the scattered pump pulse and the gate pulse was measured to be ∼310 fs. The average excitation power was kept at a level below 2 mW in order to minimize thermal lens effects. In this excitation intensity regime, the fluorescence dynamics was found to be independent of the excitation intensity for all samples. For the anisotropy measurements, IVV and IVH were recorded to calculate the anisotropy used in TCSPC, where IVV(t) and IVH(t) represent upconverted signals with excitation and fluorescence vertically and horizontally polarized, respectively. The correction factor G was obtained by tail matching of the fluorescence (IVV and IVH) for Coumarin 1 at long times. Computational Methods. Quantum mechanical calculations were carried out with the Gaussian 03 program suite.33 Geometry optimizations were performed by density functional theory (DFT) with Becke’s three-parameter hybrid exchange functionals and the LeeYangParr correlation functional (B3LYP), employing a 6-31G(d) basis set. The oscillator strength was calculated by performing time-dependent (TD) DFT calculations.
III. RESULTS Steady-State Spectroscopic Measurements. In Figure 1 are shown the ground-state absorption and fluorescence spectra of a
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series of benzoporphyrins, Bp1Bp4; their peak positions are reported in Table 1. To eleiminate aggregation phenomena, these benzoporphyrins are dissolved in strongly coordinating pyridine solvent. ZnTPP dissolved in toluene displays the absorption spectrum consisting of two well-resolved bands, the B or Soret band and the Q band, traditionally identified as the S0 f S2 (B) and S0 f S1 (Q) transitions (not shown).15 These peaks were observed in the absorption spectra at 415 and 550 nm and are assigned as B(0,0) and Q(1,0), respectively. Additional features at ∼380 and 590 nm are assigned to B(1,0) and Q(0,0), respectively.15 As in the case of ZnTPP, Bp4 also exhibits B (433 nm) and Q (628 nm) bands in its absorption spectrum. The degenerate B and Q bands in ZnTPP and Bp4 are generally observed in D4h-symmetry porphyrin systems. Based on the absorption features of ZnTPP, the B band of Bp1 can be assigned as B(0,0) at 416 nm with a shoulder of B(1,0) at ∼380 nm. The Q bands are assigned as Q(1,0) at 547 nm and Q(0,0) at 586 nm with a shoulder at ∼530 nm. Similarly to ZnTPP and Bp1, synBp2 also exhibits an intense B(0,0) band at 422 nm and relatively weak Q(0,0), Q(1,0), and B(1,0) bands at 592, 554, and 400 nm, respectively. Interestingly, however, the B and Q bands of antiBp2 become split into two components with unequal intensities, Bx, By and Q x, Q y, respectively. Whereas the B band shows wellresolved peaks at 380, 413, and 428 nm, the complicated spectral feature observed in the Q-band region shows peaks at 561, 568, and 612 nm, along with structureless bands between 520 and 550 nm. As for anti-Bp2, Bp3 also exhibits split B bands at 423 and 433 nm with a smeared band at ∼400 nm, as well as Q bands at 597 and 618 nm with structureless bands between 550 and 570 nm. The fluorescence spectra in Figure 1 obtained by photoexcitation in the 400410-nm region clearly show both S1 f S0 and S2 f S0 fluorescence emission. Whereas the S1 fluorescence appears around 590620 nm, the S2 fluorescence is observed in the range 415435 nm. As for the lowest B and Q bands in the absorption spectra, the S2 and S1 fluorescence spectra are redshifted with increasing number of benzene substituents. The sharp emission at around 450 nm corresponds to the unresolved ∼3060 cm1 CH stretching Raman peak of the pyridine solvent. To obtain information on the relative orientation between the absorption and emission dipoles of benzoporphyrins, the steadystate fluorescence excitation anisotropy spectra were comparatively measured as shown in Figure 2. The anisotropy spectra show that the relative orientation between the transition dipoles of the B state and the lowest Q states of anti-Bp2 and Bp3 are completely different from those of Bp1 and syn-Bp2. It is noteworthy that the fluorescence excitation anisotropy spectra of Bp1 and syn-Bp2 show spectral features close to those of ZnTPP, suggesting that the B bands of these molecules arise from the transition to the nearly degenerate states. However, the B-band anisotropy spectra of anti-Bp2 and Bp3 exhibit a differential form, indicating that the blue side of the B band has a parallel transition dipole with respect to that of the lowest Q band, in contrast to the orthogonal one on the red side of the B band, as well as a broken degeneracy between the two B states. The steady-state spectroscopic data clearly show that Bp1, synBp2, and Bp4 have electronic transitions that are completely different from those of anti-Bp2 and Bp3. In this context, we measured the time-resolved fluorescence decay from the B and Q states to investigate the electronic transitions of syn-Bp2 and anti-Bp2. 3786
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Table 1. Photophysical Parameters for ZnTPP in Toluene and Benzoporphyrins in Pyridine fluorescence (nm)
absorption (nm) molecule
a
B band
Q.Y.a
Q band
B band
Q band
B band
Q band
S1 state lifetime (ns)
ZnTPP
415
550, 585
420
590
0.0014
0.03
2.20
Bp1
416
547, 586
418
590
0.0016
0.079
2.36
syn-Bp2
422
554, 592
426
595
0.0010
0.10
1.72
anti-Bp2
413, 428
568, 612
432
616
0.0010
0.059
1.30
Bp3
423, 433
597, 618
436
621
0.0012
0.059
1.47
Bp4
433
580, 628
442
634
0.003
0.12
1.56
Fluorescence quantum yields of B and Q bands were measured relative to those of ZnTPP in ethanol (0.0018 in the B band and 0.03 in the Q band).21
Figure 3. Fluorescence transients of syn-Bp2 and anti-Bp2 in pyridine.
Figure 2. Steady-state fluorescence excitation anisotropy spectra (black lines) and absorption spectra (gray lines) of benzoporphyrins in pyridine recorded at the maximum of the emission band. The shaded areas represent the region without absorption, where the anisotropy value should be discarded.
Time-Resolved Fluorescence Decay Profiles. To track down the intramolecular energy relaxation processes taking place in the excited states of benzoporphyrins, time-resolved isotropic (magic-angle) spontaneous fluorescence transients were recorded with the femtosecond fluorescence up-conversion technique. Figure 3 shows the temporal decay profiles of S2 and S1 fluorescence arising from the B band and the lowest Q bands, respectively, with photoexcitation of the B band at 410 and 435 nm in syn-Bp2 and anti-Bp2, respectively (Tables 1 and 2). The S2 fluorescence decay of ZnTPP in toluene with photoexcitation at 410 nm can be fit well as a single-exponential decay with a time constant of 1.5 ps, and the signal at 590 nm, corresponding to the peak of the S1 emission spectrum, shows a rise with the same time constant of 1.5 ps. This result suggests that the internal conversion between the B and Q states occurs in about 1.5 ps. syn-Bp2 and anti-Bp2 also show S2 fluorescence decays with time constants of 1.28 and 1.11 ps, respectively, as well as rise components in the S1 fluorescence with time
constants of 1.28 and 1.11 ps, respectively. Because the S2 state lifetimes of these molecules are nearly the same, it is noteworthy that the split the B and Q bands of anti-Bp2 do not largely affect the internal conversion processes from the B state to the lowest Q state. Furthermore, in the case of anti-Bp2, the same rate of internal conversion processes between photoexcitations at 410 and 435 nm suggests that there are not two distinct parallel channels (Bx f Qx and By f Qy) that could allow the detection of separate and distinguishable S2 decay rates. The lifetimes of S1 states were also measured using the TCSPC technique with photoexcitation at 410 nm in syn-Bp2 and at 435 nm in anti-Bp2, revealing time constants of 1.72 ns for syn-Bp2 and 1.30 ns for anti-Bp2 (Supporting Information). These results also show no considerable effect of split B and Q bands on the S1 state lifetime in anti-Bp2. Time-Resolved Fluorescence Anisotropy Decay Profiles. Time-resolved fluorescence anisotropy decay measurements can provide detailed information on intra- and intermolecular energy relaxation dynamics. When an initial excitation relaxes to a state whose transition dipole moment points in a different direction than that of the initial state, the anisotropy decay represents this energy relaxation rate. As shown in Figures 4 and 5, the fluorescence anisotropy decay profiles of syn-Bp2 and anti-Bp2 were measured up to 2 ps by the femtosecond fluorescence upconversion technique. As reported in previous works, ultrafast anisotropy decay for the S1 state of a magnesium tetraphenylporphyrin (MgTPP) is composed of three anisotropy time components with time constants of 210 fs, 1.6 ps, and 3787
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Table 2. Fitted B- and Q-State Fluorescence Lifetime and Anisotropy Decay Parameters of ZnTPP, syn-Bp2, and anti-Bp2 B state r0a
anisotropy decay (fs)
ZnTPP
0.33
120
syn-Bp2
0.26
120
anti-Bp2 (λex= 410 nm)
0.047
244
anti-Bp2 (λex = 435 nm)
0.35
molecule
a
Q state fluorescence decay (ps)
r0a
0.10
1.50
0.1
0.10
1.50
0.10
1.28
0.1
0.10
1.28
0.05
1.12
0.16
Bx > Q x > Q y. The transition dipole moments of the Bx (Qx) and By (Qy) bands are orthogonal to each other and lie along the x and y axes, respectively. Figure 9 illustrates the relationship between the MOs and the electronic structures of syn-Bp2 and anti-Bp2 by different configuration interactions. In the case of syn-Bp2, the degenerate B and Q
Figure 9. Schematic layout for (top) MO and (bottom) electronic states generated by configuration interaction in syn-Bp2 and anti-Bp2.
states are reasonably interpreted by the fact that the configuration interaction is similar between the By, Q y (b1 f a2 and a2 f b1) and Bx, Q x (b1 f b1 and a2 f a2) pairs because of the same energy difference. However, anti-Bp2 shows more complicated configuration interaction than that of syn-Bp2. The y-polarized transition (b3u f b1g and au f b2g) leads to smaller configuration interaction than that in the x-polarized transition (b3u f b2g and au f b1g) because of larger energy difference in the former. This smaller configuration interaction conclusively generates new electronic states in the order By > Bx > Q x > Q y. From this calculation result, we can assign 413, 428, 568, and 612 nm in the absorption spectrum of anti-Bp2 as By, Bx, Q x and Q y, respectively. The successive relaxation steps of excited syn-Bp2 and antiBp2 are clearly resolved in the measurements of fluorescence from B and Q states (Figure 3). The decay and rise kinetic curves observed in fluorescence up-conversion experiment gave an identical time constant for these two processes. This result enables us to cast one question. According to the energy gap law, the rate decreases exponentially with increasing the energy gap. This relationship has been invoked in previous reports about the changes in B state lifetimes depending on environments.36 H2TPP shows that the energy gap between B (0,0) and Q y (0,0) is ∼5650 cm1 in benzene solvent, which exhibits a lifetime of less than 50 fs for B band, while in ZnTPP the corresponding lifetime is ∼1.5 ps notwithstanding the B(0,0) - Q(0,0) gap of ∼6670 cm1.29 Moreover, the previous reports also suggest that symmetry reduction, which increases the density of vibronic states in the accessible bath, may also play a role in reducing the lifetime of H2TPP relative to ZnTPP.29 Considering the energy gaps in syn-Bp2 and anti-Bp2, the energy gap between B (0,0) and Q (0,0) of syn-Bp2 is ∼6800 cm1, while in anti-Bp2 the Bx (0,0) - Q x (0,0) gap is ∼5760 cm1. If the correlation between the energy gap and internal conversion rate is considered in these molecules based on previous reports, a femtosecond rate would be expected for an energy gap of 5760 cm1. However, fluorescence up-conversion experiment reveals that the internal conversion rate is similar between syn-Bp2 (∼1.3 ps) and anti-Bp2 (∼1.1 ps) in pyridine, which are also not largely different from 3790
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The Journal of Physical Chemistry B Scheme 2. Schematic Diagram of the Energy Relaxation Dynamics of ZnP, syn-Bp2, and anti-Bp2
the rate of ZnTPP (1.5 ps) in toluene with the energy gap of ∼7000 cm1. This unexpected result suggests the general energy gap law and low-symmetry are not suitable for explanation of internal conversion process in anti-Bp2. The fluorescence anisotropy changes measured by the fluorescence up-conversion technique directly reveal the mechanism of intramolecular energy relaxation dynamics in anti-Bp2 as compared to syn-Bp2, as illustrated in Scheme 2. As mentioned above, because syn-Bp2 does not exhibit split B and Q bands, the fluorescence anisotropy decay of syn-Bp2 can be interpreted as a dephasing and energy equilibration process between the degenerate Bx (Q x) and By (Q y) states, just as the case of ZnTPP. On the other hand, upon photoexcitation of the Bx state in anti-Bp2, the fluorescence of Qy(0,0) exhibits ultrafast anisotropy decay ( Bx > Q x > Q y, which induces a superimposition of Q x (0,0) and Q y (1,0) bands. This overlap generates a strong coupling between these two states, which induces a direct internal conversion from Bx (0,0) to Q y (0,0). Based on these observations, we have provided a dynamical model for the intramolecular relaxation processes strongly influenced by the position of fused benzo rings. Collectively, we believe that the molecular symmetry tuned by fused benzo rings can modify internal conversion processes from B to Q state in various porphyrinoid systems. ’ ASSOCIATED CONTENT
bS
Supporting Information. S1 fluorescence decay profiles obtained using TCSPC; anisotropy decay profiles of B- and Q-state fluorescence of ZnTPP in toluene obtained using the fluorescence up-conversion technique; four frontier MOs of ZnP and benzoporphyrins; and calculated transition energies, oscillator strengths, and configuration interactions of B and Q bands in ZnP and benzoporphyrins. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (D.K.), uno.hidemitsu.mm@ ehime-u.ac.jp (H.U.).
’ ACKNOWLEDGMENT The work at Yonsei University was supported by the Midcareer Researcher Program (2010-0029668) and World Class University (R32-2010-000-10217) Programs of the Ministry of Education, Science, and Technology (MEST) of Korea and an AFSOR/AOARD grant (FA2386-10-1-4080). The work at Ehime University was supported by Grant-in-Aids for Scientific Research [20550047 and 21108517 (π-space)] from the Japanese Ministry of Education, Culture, Sports, Science and Technology. The quantum calculations were performed using the supercomputing resources of the Korea Institute of Science and Technology Information (KISTI). P.K. acknowledges a 3791
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The Journal of Physical Chemistry B Seoul Science Fellowship. H.U. thanks JSPS for a Research Fellowship for Young Scientists.
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