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J. Phys. Chem. B 2009, 113, 5098–5103
Chiral Sum Frequency Spectroscopy of Thin Films of Porphyrin J-Aggregates Tetsuhiko Nagahara,†,| Kenji Kisoda,‡ Hiroshi Harima,§ Misako Aida,† and Taka-aki Ishibashi*,† Center for Quantum Life Sciences and Graduate School of Science, Hiroshima UniVersity, Higashi-Hiroshima 739-8530, Japan, Physics Department, Wakayama UniVersity, Sakaedani, Wakayama 640-8510, Japan, and Graduate School of Electronics, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan ReceiVed: NoVember 30, 2008; ReVised Manuscript ReceiVed: February 4, 2009
Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering. Introduction J-Aggregates of porphyrin have been widely studied, since porphyrin aggregates are known to play central roles in biological events such as photosynthetic light energy conversion. For water-soluble tetrakis(4-sulfonatophenyl)porphyrin (TSPP), it is known that, upon addition of aqueous acid to free-base TSPP in water, the absorption spectrum changes successively from free-base to N-protonated monomer (diacid), and then to J-aggregate (Scheme 1), resulting in blue- and red-shifted sharp absorption bands at ∼420 nm (H- band) and ∼490 nm (J-band) in the Soret region, respectively.1 Nagahara et al. investigated morphological and photophysical properties of thin films of TSPP J-aggregates, which were prepared by casting the acidified solution on a glass plate, by scanning near-field optical microscopy (SNOM) and found that the thin film consisted of quasi-one-dimensional microcrystals (height, 5-10 nm; width, ∼100 nm; length, ∼1 µm) whose optical transition dipole of J-band is parallel to the long axis of the microcrystal.2,3 The mesoscopic structure of the aggregate in water may be very similar to that on the glass substrate, since photophysical properties of the J-aggregates as revealed by absorption and fluorescence spectroscopy in thin films are very similar to those in acidic water, and the quasi-one-dimensional structure was also suggested by flow-induced linear dichroism measurements.1–3 The aggregates of achiral porphyrin derivatives are known to show induced circular dichiroism (CD) by addition of chiral ingredients, such as DNA.4–6 Ohno et al. reported that achiral TSPP formed stable and CD-active J-aggregates in acidic water by addition of L- and D-tartaric acid.1 Similar chiral aggregates were also reported by addition of tryptophan.7,8 These results suggest that noncovalent intermolecular interactions introduced into TSPP molecules result in the controlled formation of supramolecular structures. The molecular thin films of the chiral * To whom correspondence should be addressed. Fax: +81-82-424-0727. E-mail:
[email protected]. † Hiroshima University. ‡ Wakayama University. § Kyoto Institute of Technology. | Present address: Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan.
SCHEME 1: Molecular Structure of TSPP in the Diacid Form Used in This Experiment and a Schematic Illustration of a Proposed Model of TSPP J-Aggregate1
TSPP J-aggregates may be prepared by casting the suspensions on a glass plate in a similar manner to that without additives reported previously,2,3 if the CD-active supramolecular structures formed in solutions are retained. The chiroptical properties of such molecular thin films provide useful information not only of the molecular structures but also of their biological activities. In this study, we have utilized chiral-specific sum frequency generation (SFG) spectroscopy to probe the chirality of the thin films of TSPP J-aggregates, since chiral SFG may have much higher sensitivity than conventional methods such as CD or Raman optical activity (see below). IR-visible SFG has found a wide range of applications to obtain vibrational spectra of molecules on surfaces and at interfaces. In this process, two input pulsed laser beams at visible (ωVIS) and infrared (ωIR) overlapped spatially and temporally on a sample are used to generate the sum frequency beam at ωSFG ) ωVIS + ωIR. In IR-visible SFG, a doubly resonant (DR) enhancement effect is expected if vibrational and electronic transitions probed by ωIR and ωSFG, respectively, are coupled.9 Chiral DR SFG, which can selectively probe optical activity under certain polarization combinations, is very useful to detect
10.1021/jp8105138 CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
SFG of Thin Films of Porphyrin J-Aggregates chiral molecular monolayers and thin films, since it is a dipoleallowed process under electric-dipole approximation, while conventional methods such as CD and ROA, which rely on higher order interactions, are dipole-forbidden.10–13 The sensitivity of chiral SFG is thus superior to that of conventional methods. Belkin et al. reported that the antisymmetric part of the Raman tensor, which comes from Albrecht’s B-term14 and can become comparable to its symmetric counterpart near electronic resonance, plays an important role in vibrational chiral SFG, while the symmetric part contributes to achiral SFG.15 It is known that the antisymmetric part necessarily vanishes under the Born-Oppenheimer adiabatic approximation. However, Simpson and co-workers suggested that significant chiroptical effects would be observed in vibrational SFG of achiral chromophores in chiral oriented films, and in this case a breakdown of the Born-Oppenheimer approximation to show chirality in the molecular tensor is not required.16–18 It would thus be interesting to study the contribution of the antisymmetric B-term in chiral SFG. In principle, contributions of the B-term in resonance Raman can be studied by Raman excitation profile, since the profiles of A- and B-terms differ from one another. Such an excitation profile can also be obtained by DR SFG. In addition, measurement of the depolarization ratio of resonance Raman (F) is effectual because F > 0.75 for randomly oriented molecular systems is a sufficient condition for the existence of the antisymmetric B-term. Based on these viewpoints, we have measured electronic resonance profiles of chiral and achiral SFG vibrational bands of the thin films of TSPP aggregates, and the depolarization ratio of resonance Raman signals of solutions of TSPP aggregates. Experimental Section TSPP (Dojin) and hydrochloric acid (Wako) were used without further purification. Water was purified to the resistivity of ∼18 MΩ cm by using a water purification system (Arium 611UV, Sartorius). TSPP J-aggregates in acidic water with/ without addition of L-, D-, and DL-tartaric acids (Wako) were prepared as described in the literature.1 Electronic absorption spectra of the aggregate suspensions with tartaric acids were identical to those without tartaric acid. CD spectra of the suspensions were recorded with a commercial spectropolarimeter (J-500CH, JASCO). By casting the suspensions onto glass substrates, solid thin films of aggregates were prepared. The morphologies of the thin films were measured by contact-mode atomic force microscopy (AFM; SPA-400 and SPI3800N, SII). Quasi-one-dimensional microcrystals (5-10 nm height, ∼100 nm width, ∼1 µm length), similar to those reported previously for the films without chiral additives, were observed in this study regardless of whether tartaric acids had been added as the chiral additive.2,3 The chiral molecules may be located inside the microcrystals or attached to the surface of the microcrystals. Since observed shapes of microcrystals with and without tartaric acids are very similar, we could not determine the specific locations of the tartaric acids. However, it is likely that the chiral molecules are intercalated inside microcrystals, since tetraphenyl porphyrin molecules are known to form clathrate lattices as revealed by X-ray crystallography.19 In our study, the thickness of the thin film sample without addition of tartaric acid was determined to be ∼40 nm by AFM observations. We attempted to estimate the surface coverage of TSPP of the thin films described as follows: The samples on the glass substrate were resolubilized in basic water, and the absorbances of the monomer solutions were measured to
J. Phys. Chem. B, Vol. 113, No. 15, 2009 5099
Figure 1. Schematic illustration of polarization combinations of incident visible (VIS)/IR lasers and generated sum frequency (SF) signal. Optically active signal without background is observed using PSP polarization combination (SF, VIS, and IR being P, S, and P polarized, respectively), while achiral SFG is observed using PPP. Mixed polarizations (M+) and (M-) refer to linear polarizations at (45° from its plane of incidence.
determine number densities of TSPP. The obtained number densities were ∼3 × 1019 molecules/m2. From the results, the thicknesses of the thin film samples on glass surface were estimated to be ∼120 TSPP monolayers, regardless of whether tartaric acids had been added, by assuming the porphyrin plane lies parallel to the substrate surface. The details of our SFG spectrometer using wavelength-tunable optical parametric amplifiers (OPAs) and a multiplex detection scheme can be found elsewhere.20–22 Briefly, our light sources were based on an amplified Ti:sapphire laser (2.5 mJ, 1 kHz, TITAN-II, Quantronix). Femtosecond broadband IR probe pulses (∼1 µJ @ ∼1100 cm-1, ∼200 cm-1 fwhm) and picosecond narrow-band visible pulses (∼0.1 µJ, ∼8 cm-1 fwhm) were generated by using femtosecond and picosecond OPAs (TOPAS, Light Conversion), respectively. It should be noted that an SFG spectrometer that is both IR and visible wavelength tunable is indispensable to satisfy the doubly resonant condition of a sample. The two input probe pulses were overlapped spatially and temporally on the sample, and the sum frequency (SF) signal generated was detected by a spectrograph [asymmetric double spectrograph consisting of a prism premonochromator stage (CT-25UV, JASCO) and a grating polychromator stage (TRIAX550, Horiba Jobin Yvon)]/LN-cooled CCD (Roper Scientific) combination. A GaAs(110) wafer was used as a reference of the SFG signal. Polarization combinations of SF, visible, and IR beams (see below) were controlled by polarizers and half-wave plates. In our experiment, chiral-specific SFG, which is an optically active signal without background, was observed by using PSP polarization combination (SF, visible, and IR being P, S, and P polarized, respectively), while achiral SFG was observed by PPP (see Figure 1).10,11 As reported by Belkin et al., the intensities of chiral (IPSP) and achiral SFG signals (IPPP) are described as follows:
{
| | | |
IPSP ∝ χPSP 2
( )
IPPP ∝ χPPP
(2)
2
(1) 2
where χPSP(2) and χPPP(2) are effective nonlinear susceptibilities accessed by polarization combinations PSP and PPP, respectively. The effective nonlinear susceptibilities are related to the second-order molecular hyperpolarizability through molecular orientational averages. The hyperpolarizability, R(2)ijk, of the molecular frame i, j, and k, is described as follows:
5100 J. Phys. Chem. B, Vol. 113, No. 15, 2009
Rijk(2) ∝ Aijµk
Nagahara et al.
(2)
where Aij and µk denote the anti-Stokes Raman tensor and infrared transition moment, respectively. From eq 2, it is seen that SFG can be considered as an infrared excitation of a vibrational mode followed by an anti-Stokes Raman transition. It should be noted here that the electronic resonance effect of vibrational SFG arises from the Raman tensor, Aij. Although χPSP(2) shown in eq 1 is known to have opposite signs for the two enantiomers, we cannot distinguish between them from the PSP signal (IPSP). However, Belkin et al. proposed that the signs of χPSP(2) can be obtained by taking the difference of IP(M+)P and IP(M-)P, where mixed-polarizations (M+) and (M-) refer to linear polarizations at (45° from the plane of incidence, respectively (Figure 1).10
IP(M()P ∝
|
1 χ (2) ( χPPP(2) 2 PSP
|
2
IP(M+)P - IP(M-)P ∝ χPSP(2)χPPP(2)* + c.c.
(3)
(4)
The difference spectra thus obtained must be inverted for the two enantiomers. In this study, a P(M()P difference spectrum measurement was used to distinguish the chiralities of the thin films of TSPP J-aggregates. Resonance Raman spectra of sample solutions in a square cuvette made of fused silica were obtained by using a Raman spectrometer comprising an Ar+ ion laser (488 nm, Stabilite 2017, SpectraPhysics), a double monochromator (f ) 85 cm, SPEX 1403), and a LN-cooled CCD detector (PI-1340/100B, Roper Scientific) with a 90° scattering geometry. Since the aggregate solutions are known to show flow induced linear dichroism, we used neither rotating cells nor flow cells. The frequencies of Raman bands were calibrated with radiation lines of a Ne lamp. Depolarization ratios (F ) I⊥/I|) were calculated from parallel (I|) and perpendicularly polarized Raman spectra (I⊥) obtained by using a film polarizer and a quartz depolarizer.
Figure 2. (a) Electronic absorption spectra of free-base (pH 10, dotted curve), diacid monomer (pH 7, dashed curve), and aggregate (pH 1, solid curve) of TSPP observed in water. H-band (∼420 nm) and J-band (∼490 nm) of TSPP aggregates are marked as H and J, respectively. Identical spectra were obtained for solutions with chiral additives of D-, L-, and DL-tartaric acids. (b) Electronic absorption CD spectra obtained from aggregate solutions (pH 1) with D-tartaric acid (D, dotted curve), with L-tartaric acid (L, solid curve,), and without tartaric acid (WO, dashed curve). CD signals were observed at both the H-band and J-band. We could not detect CD signals from the cast thin films used for SFG experiments.
Results and Discussion The absorption spectra of TSPP in water at pH 10, 7, and 1 shown in Figure 2a are ascribed to free-base monomer, diacid monomer, and J-aggregate, respectively.1,3 At pH 1, blue- and red-shifted absorption bands at 420 nm (H-band) and 490 nm (J-band) in the Soret region were observed.1 It is known that the J-aggregate of TSPP shows the blue-shifted H-band whose transition dipole is perpendicular to the long axis of the aggregate, in addition to the red-shifted J-band whose dipole is parallel to the long axis.1 The absorption spectra of solutions with D-, L-, and DL-tartaric acid and without tartaric acid (hereafter referred to as D, L, DL, and WO solution) were identical. This finding indicates that interactions between porphyrin macrocycles, which is the origin of the exciton-split aggregate spectrum, are not influenced significantly by the addition of tartaric acids. D and L solutions showed significant CD signals on both Hand J-bands, while that of DL and WO provided a baseline spectrum (Figure 2b). Nevertheless, no CD signals from the thin film samples were observed. This may have been due to the following reasons: (1) low sensitivity of CD measurement, since it relies on the dipole-forbidden process as mentioned before and (2) chiral supramolecular structures formed in solutions were not retained on the glass surface.
Figure 3. SFG spectra of D (upper panel) and L (lower panel) thin film samples obtained with PPP (dashed curves) and PSP (solid curves) polarization combinations. Visible probe at 518.186 nm was used. Intense SFG peaks at 990, 1090, 1130, 1190, and 1230-1250 cm-1 are seen in chiral specific PSP spectra compared to those of PPP.
In contrast to the CD measurements, we succeeded in detecting chiral SFG signals at ∼490 nm in reflection from the D and L films by using PSP polarization combinations (Figure 1: SF, visible, and IR being P, S, and P polarized, respectively), whose intensities are comparable to that of achiral ones by PPP (Figure 3). The PSP signals from DL and WO films were below the detection limit, while plots of the PPP signals from them showed very similar spectra to those of PPP from D and L films. Chiral SFG vibrational spectra of D and L films are very similar, since the signal intensities are proportional to |χPSP(2)|2, irrespec-
SFG of Thin Films of Porphyrin J-Aggregates
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TABLE 1: Vibrational Bands (in cm-1) Observed by SFG and by Resonance Raman resonance Raman (this study)
SFG (this study)
Raman shift
depolarization ratio of WO/ average ratio of D and L
1230
1231
0.48/1.43
1230
1190 1130
1194 1125 1116 1083 1016
0.80/1.90 0.44/1.53 0.46/1.33 0.41/1.05 0.39/1.05
1195 1124 1082 1014
986
0.43/1.38
985
1090 990 a
resonance Ramana assignmenta ν(Cm-Ph) and ν(CR-N) ν(CR-Cβ) ν(CR-N) δ(Cβ-H) ν(CR-Cβ)/ ν(CR-N) ν(CR-Cβ)/ ν(CR-N)
From ref 24. Figure 5. Electronic resonance profiles of SFG vibrational bands for thin film samples of TSPP aggregates. Electronic absorption spectrum of TSPP aggregates in water, which is identical to that shown in Figure 1, is also shown as smooth curves. (a) Resonance profiles of 990 cm-1 obtained with PSP polarization combination (open circles, solid lines), 990 cm-1 with PPP (open squares, dashed lines), 1190 cm-1 with PSP (closed circles, solid lines), and 1190 cm-1 with PPP (closed squares, dashed lines) are shown. (b) Resonance profiles of 1090 cm-1 obtained with PSP (open circles, solid lines), 1090 cm-1 with PPP (open squares, dashed lines), 1130 cm-1 with PSP (closed circles, solid lines), and 1130 cm-1 with PPP (closed squares, dashed lines) are shown. PSP profiles show maxima at ∼480 nm, while those of PPP show maxima at ∼500 nm. These wavelengths do not agree with the wavelength of the J-band (490 nm).
Figure 4. SFG difference spectra between P(M+)P and P(M-)P polarization combinations of thin film samples of TSPP aggregates. The observed PMP spectra before subtractions are shown in S1 in the Supporting Information. The difference spectra of D and L are reversed, while that of DL (and also WO) results in a baseline. Thus, we have distinguished the chirality of the thin film samples. The fine structure seen in the spectrum of DL at ∼980 cm-1 is due to defect pixels of the CCD.
tive of the chirality. These findings are due to the chiral specificity of χPSP(2), which is nonzero only for the chiral samples and changes its sign with the enantiomers. Bands at 1230, 1190, 1130, 1090, and 990 cm-1, which were reported by resonance Raman studies of acidic WO solution, are seen in these spectra, although a band at 1014 cm-1 was not observed in our SFG experiment (Table 1).1,23–25 These bands are ascribed principally to skeletal vibrational modes of the porphyrin macrocycle. Band shapes in the 1230-1250 cm-1 region are complex, suggesting that these features originate from more than two bands. By subtracting observed spectra of P(M+)P and P(M-)P (see S1, Supporting Information), as described before, we have distinguished the chirality of the thin films (Figure 4). These difference spectra are reversed when switched from D to L, while that of DL and WO result in a baseline. From these results, it has been revealed that the supramolecular chiral structures of J-aggregates formed in water are retained on the glass surface. We have thus succeeded in detecting chirality of the thin film samples, which was not detected by electronic absorption CD spectroscopy. We believe that detecting vibrational CD (VCD) from the chiral thin films of TSPP is practically impossible because VCD is less sensitive than the ordinary electronic CD in the UV/vis regions. As mentioned before, the antisymmetric part of the Raman tensor plays an important role in vibrational chiral SFG, while the symmetric part contributes to achiral SFG. This suggests that electronic resonance profiles of chiral and achiral SFG may
be different from each other since they rely on different parts of the Raman tensor. To elucidate contributions of antisymmetric and symmetric parts to the SFG signals, we measured electronic resonance profiles of chiral (PSP) and achiral (PPP) SFG of TSPP J-aggregates. The electronic resonance profiles of D and L films were measured at various VIS wavelengths. In this experiment, each vibrational SFG spectrum obtained at a visible wavelength was fitted by the general SFG model function shown in eq 5.
|∑
Aj eiθj ωj - ω - iΓj j)1 n
ISFG(ω) )
|
2
(5)
where Aj, θj, ωj, and Γj are amplitude, phase, frequency, and bandwidth of a vibrational band (j) at a specified visible wavelength, respectively. The vibrationally nonresonant SFG term without an energy denominator was not included in eq 5 because of its negligibly small contribution. The amplitudes of each vibrational band obtained from the thin films were normalized by that obtained for a vibrational band (1160 cm-1) of a y-cut quartz crystal with a PPP polarization combination, which is known to be a Raman-active vibrational band in quartz.26 We have averaged the amplitudes of D and L for each set of polarization combinations, SFG wavelengths, and vibrational modes, since the amplitudes are the same irrespective of D or L. Though we measured the SFG spectra in the 950-1250 cm-1 region, reliable analysis was difficult for the 1230-1250 cm-1 region, since more than two bands exist and overlap each other. In the following part, we will discuss the bands in the 950-1230 cm-1 region. Amplitudes thus obtained are plotted against the SFG wavelength in Figure 5. Almost all the electronic resonance profiles of the SFG amplitude increase at longer wavelengths
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(>520 nm). This might be due to the electronic postresonance of the Q-band at longer wavelengths (600-750 nm). Resonance profiles of both chiral (PSP) and achiral (PPP) responses at 990 cm-1 (Figure 5a) show maxima at 490 nm, which is equal to the wavelength of the J-band maximum. The amplitude of the 1190 cm-1 band by achiral (PPP) SFG, which is very large compared to that of the chiral one (Figure 3), also shows a maximum at 490 nm, while that from chiral (PSP) SFG was very small. The electronic resonance profiles of chiral and achiral SFG at 1190 cm-1 seem somewhat different. The most striking difference between the profiles of chiral and achiral SFG can be seen in the 1090 and 1130 cm-1 bands in Figure 5b. In addition, these features do not agree with the electronic absorption of the J-band. We will discuss below these profiles at 1090 and 1130 cm-1. The amplitude of chiral response shows a maximum at 480 nm, while that of the achiral response shows a maximum at 500 nm. The difference of these amplitude peaks is about 830 cm-1, which is close to the vibrational frequency of the mode (1090 and 1130 cm-1) and is beyond the experimental uncertainty. The peculiar profiles for the 1090 and 1130 cm-1 bands are likely due to the interference effect of terms in the Raman tensor (see below), analogous to the antiresonance effect in resonance Raman.27,28 The resonance Raman tensor for a vibrational mode is a sum of the distinct terms, each of which is related to a distinct vibronic level as the intermediate state in the Raman process.29 The terms may interfere with each other, since the Raman intensity and also the SFG intensity are proportional to the square of the tensor. The interference effect is destructive or constructive depending on the relative phase of the interfering terms. Destructive interference deepens the excitation profile and pushes the adjacent maxima away from one another, while constructive interference fills in the valleys and pulls the adjacent peaks toward one another. The peak position in an excitation profile of resonance Raman can thus be different from that of electronic resonance due to the interference effect, and this is called the antiresonance effect. It was reported that the interference effect of terms from 0-0 and 0-1 may be constructive or destructive in Albrecht’s B-term14 but always constructive in the A-term.30,31 Stein et al. reported that interference between A- and B-terms of different electronic states results in antiresonance.28 If antisymmetric Raman tensor elements coming from the B-term contribute significantly to chiral SFG signals and symmetric counterparts contribute to achiral SFG as mentioned before,15 destructive and constructive interference effects might also be observed in our resonance profiles of chiral and achiral SFG, respectively. By using the simplified model shown below, we have simulated the observed resonance profiles of chiral and achiral SFG for the 1090 and 1130 cm-1 bands. The antisymmetric (AAS) and the symmetric (AS) parts of the B-term components can be described as follows:
( (
AS
) )
1 1 + Ω0 - ω - iΓ Ω1 - ω - iΓ c exp(iθAS) 1 1 ) (a + b) + + Ω0 - ω - iΓ Ω1 - ω - iΓ c exp(iθS) (6)
AAS ) (a - b)
where a, b, and c are constant values; Ω0 and Ω1 are 0-0 and 0-1 transition energies, respectively; Γ is the damping constant
Figure 6. Simulated electronic resonance profiles of antisymmetric (solid curve) and symmetric components (dashed curve); absolute squares of AAS and AS in eq 6 are plotted against wavelength. Contributions from the antisymmetric part maximizes at 480 nm, while that from the symmetric part maximizes at 500 nm. Both wavelengths do not agree with that of the J-band (490 nm). These features are qualitatively similar to the observed profiles of SFG shown in Figure 5b.
of the transitions; θΑS and θS are the phases; and ω is the SFG photon energy. Only the vibrational ground and first excited states are taken into account in this model. This approximation is justified when Franck-Condon activity is absent for the electronic transition under consideration. This is generally true for the nontotally symmetric vibrational modes, and it is also true for totally symmetric ones because the J-band of the TSPP aggregates has no vibrational progression. We also assumed equal damping constants for both 0-0 and 0-1 transitions. The first terms in both equations with energy denominators are exactly the same as those used in the simulation by Mortensen.30 Additional contributions from another electronic state (i.e., Q-band) are approximated by the constant c without energy denominators. By taking absolute squares of eq 6, the simulated profiles for a ) 1, b ) 0.05, c ) 0.005, Ω0 ) 20408 cm-1 (corresponding to 490 nm), Ω1 ) 20408 + 1100 ) 21 508 cm-1, Γ ) 400 cm-1, θAS ) π, and θS ) 0 are shown in Figure 6 as a function of wavelength. Contribution from the antisymmetric part maximizes at 500 nm, while that from the symmetric part maximizes at 480 nm. Both of the wavelengths do not agree with the wavelength of the J-band (490 nm). The observed peculiar resonance profiles of SFG of the 1090 and 1130 cm-1 bands, shown in Figure 5b, are qualitatively reproduced by the simple model: The profiles of chiral (PSP) and achiral (PPP) SFG correspond to those of antisymmetric (AAS) and symmetric contributions (AS), respectively.10,11 Although the choices of parameters are rather arbitrary at this stage, the simulation shows that the observed antiresonance effect in the chiral/achiral vibrational SFG signal can be explained by the interference effect of B-term components of different levels (0-0 and 0-1) and different electronic states. The symmetry of the terms of the Raman tensor contributing to the SFG signal was also studied by the depolarization ratio of resonance Raman. Parallel and perpendicularly polarized resonance Raman spectra of WO, D, L, and DL solutions were measured by exciting at 488 nm (see the Supporting Information S2). Obtained Raman shifts and depolarization ratios of WO and averaged ratios of D and L are summarized in Table 1, together with reported Raman shifts and their assignments. Startling increases in depolarization ratios (F > 0.75, anomalously polarized) by addition of D- and L-tartaric acid are evident, and they clearly show that the antisymmetric component of the B-term contributes significantly to the depolarization ratio. Thus,
SFG of Thin Films of Porphyrin J-Aggregates it seems reasonable to expect that this antisymmetric component also contributes significantly to the chiral SFG signal we observed. In conclusion, we have detected chiral vibrational SFG signals from thin films of chiral J-aggregates of TSPP and distinguished the chirality of the thin films. These facts indicate that CDactive supramolecular chiral structures of TSPP aggregates formed in water are retained in the cast thin films on glass plates. We have also studied electronic resonance profiles of vibrational band amplitudes for chiral and achiral SFG of TSPP aggregates, and found that the profiles of the 1090 and 1130 cm-1 vibrational bands are different from each other and that both of them do not maximize at the wavelength of absorption maximum. To explain the difference, the interference effect of terms in the Raman tensor that is known in resonance Raman scattering has been proposed as a mechanism, in which the antisymmetric part of Albrecht’s B-term Raman tensor and that of the symmetric counterpart contribute to chiral and achiral SFG, respectively. Acknowledgment. The authors thank N-BARD of Hiroshima University for CD measurements, and Y. Fujiwara and Y. Tanimoto for AFM measurements. This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government [No. 19056007, Priority Areas (477)] and NaBiT program. Supporting Information Available: Spectra obtained by P(M+)P and P(M-)P polarization combinations (S1). Resonance Raman spectra observed at parallel and perpendicular polarizations (S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1993, 99, 4128– 4139. (2) Nagahara, T.; Imura, K.; Okamoto, H. Chem. Phys. Lett. 2003, 381, 368–375. (3) Nagahara, T.; Imura, K.; Okamoto, H. Scanning 2004, 26, I-10– 15.
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