Partially fluorinated dimyristoylphosphatidylcholines (DMPCs) involving double alkyl chains are employed to control the phonon generation in thin films, which is examined by infrared (IR) spectroscopy coupled with multiple-angle incidence resolution spectrometry (MAIRS). technique. Compounds having perfluoroalkyl (Rf) chains are known to exhibit phonon bands in IR spectra because of the strong dipole–dipole interactions. Since the phonon bands of an organic matter have a similar shape to the normal absorption bands, however, recognition of the phonon modes is difficult and confusing for IR spectroscopists. Here, we show that MAIRS works out for finding phonon modes in monolayers: the Berreman shift is readily captured by the MAIRS in-plane and out-of-plane (OP) spectra. By measuring the longitudinal-optic (LO) energy-loss function spectrum of a bulk sample, the degree of molecular aggregation in the monolayer is also revealed by comparing the OP spectrum of the monolayer to the LO one. In addition, partially fluorinated DMPC compounds having both hydrocarbon and Rf chains are prepared, and they are used to obstruct the self-aggregation of the Rf groups in the film. As a result, the phonon characteristics are mostly lost in the MAIRS spectra as expected.

Infrared (IR) spectroscopy1 of condensed matters can be divided into two classes of inorganic and organic matters. A typical example of the inorganic ones is given by ionic crystals, such as sodium chloride.2,3 In sodium chloride, sodium cations and chloride anions are alternately located, which are strongly interacted with each other by the Coulomb interaction, and the structural unit is repeated three dimensionally to make the crystal. In this case, the concept of “a single molecule” has eventually no meaning especially in terms of vibrational spectroscopy since the molecular vibration of a single molecule cannot be isolated and vibrations of neighboring molecules are strongly coupled with each other via the Coulomb interaction to generate a coupled oscillator over the crystal. In fact, the term of “normal mode” of inorganic ionic crystals is used for the whole crystal, but not for a single molecule.

On the other hand, organic condensed matters have a largely different schematic in terms of vibrational spectroscopy no matter how the crystallinity is. In an organic matter, organic molecules are packed with relatively weak interactive forces represented by van der Waals (vdW) forces4 and hydrogen bonding. Each organic molecule is, on the other hand, of strongly connected atoms by the very strong covalent bonds. In this manner, in an organic condensed matter, two largely different covalent and vdW forces coexist, which is symbolically different from inorganic crystals.5 In most cases, the weak intermolecular interaction makes intermolecular vibrational couplings ignorable in organic matters, and therefore, the normal mode can approximately be on a single molecule in an isolated manner. In fact, the vibrational spectra of organic matters can be readily calculated by numerical simulations on quantum mechanics that mostly considers a single molecule only.

On closer inspection, nevertheless, single-molecule-based calculations do not work well for some chemical groups having a large permanent dipole moment represented by the C–O,6,7 C=O,8 and C–F8,9 bonds even in organic matters. These strong dipoles make group vibrations of the neighboring molecules be a coupled oscillator, i.e., “phonons,”2,10,11 which results in band shifts via the change of electric permittivity at the relevant wavenumber positions as obviously found in “thin films.” In addition, the phonon-specific modes or the so-called “virtual modes”5,8,10 appear when oblique-angle incidence measurements are performed using the p-polarization for the thin film having strong IR absorbing groups, which makes the IR spectra more complicated. In a thin film, the electric field of the incident IR light is distributed in an anisotropic manner, and the IR spectra of the in-plane (IP) and out-of-plane (OP) components of the transition moments in the film are ruled by different functions of the transverse-optic (TO) and longitudinal optic (LO) energy-loss functions (ELFs), respectively,1,12
ELFTO=Imεr,x,
(1)
ELFLO=Im1εr,z.
(2)
Here, ɛr is the complex electric relative permittivity and the indices of x and z indicate the surface-parallel and surface-perpendicular directions in the film, respectively, indicating the surface selection rules. ɛr is given by the following equation as a function of angular frequency, ω:
εrω=εr,+ωp2jfjωj2ω2iγjω.
(3)

The jth absorption peak appears at ωj with a bandwidth associated with the damping factor, γj. The peak intensity is directly correlated with the oscillator strength, fj, and ωp is the plasma frequency. If the permittivity function is put in Eqs. (1) and (2), the peak positions of TO and LO ELFs are different from each other only when the absorption is as strong as C–O, C=O, and C–F bonds, whereas the alkyl chains consisted of the C–H groups show ignorable shifts. The TO absorption peak appears at nearly the same position as that of a “bulk” sample, and the LO peak can directly be measured by using reflection–absorption (RA) spectrometry only in “thin films,” which is known as Berreman’s effect.13 

This thin-film specific effect is recognized in IR spectroscopy especially when the RA and normal-incidence transmission (Tr) spectra are directly compared with each other. For example, the IR RA and Tr spectra of Langmuir–Blodgett (LB) films14,15 of metal stearate show differences not only for relative band intensity reflecting the molecular orientation but also for the band positions related to the strong IR absorption bands, i.e., the antisymmetric and symmetric COO stretching vibration bands.16,17 In particular when the transition moment has a large component in the surface perpendicular direction, in theory, the Berreman shift becomes large, as schematically shown in Fig. 1(a). In fact, in the LB film of cadmium stearate, the symmetric COO stretching vibration band exhibits a large shift of 10 cm−1 between the RA and Tr spectra, while the antisymmetric band shows as small as 4 cm−116,17 since cadmium stearate has a nearly perpendicular orientation in the film. (For the details of the assignment, the reader is referred to Ref. 17.) The rest bands related to hydrocarbons show no (or ignorable) Berreman’s shift. In this manner, the C–O bond is truly found to give phonon modes in thin films because of its large permanent dipole moment via the dipole–dipole (D–D) coupling.

In short, strong IR absorbing chemical groups having the transition moments in the OP direction exhibit a higher wavenumber shift in IR spectra because of the Berreman effect. The concept of phonons is thus commonly used for both inorganic and organic materials. In the case of inorganic ionic crystals, in particular, the phonon modes are sometimes coupled very strongly with IR light to make “polaritons”2,3,5–9 that are easily recognized by the unique band shape of a plateau-shape peak top. This is known as the Reststrahlen bands that are found for both phonons and polaritons, indicating that ionic crystals are of a collection of large dipoles. On the other hand, in the case of organic matters, the phonon bands largely lose the characteristic shape because of the relatively small oscillator strength due to weak dipole moments, which makes it difficult to discriminate the normal IR peaks from the phonon bands.5–8 

Here, the character that phonon modes exhibit an anisotropic band shift in a thin film (Berreman’s effect) can be used for finding them out in the IR spectra. Fortunately, a spectroscopic technique of multiple-angle incidence resolution spectrometry (MAIRS)18–21 is suitable for this purpose since this technique enables us to measure both IP and OP spectra simultaneously on an identical thin-film sample with a common ordinate scale.22,23 In fact, the Berreman shift of the COO stretching vibration bands is clearly revealed by MAIRS on a single sample.18 

At this situation, we are interested in a problem: what would happen if the D–D interaction is obstructed by introducing hydrocarbons, as schematically shown in Fig. 1(b)? In theory, the obstruction of the D–D couplings would break the phonon, and the phonon-specific IR bands would be changed to the single-molecular normal modes, which should make both anisotropic band intensity and wavenumber shift lost in the IR MAIRS spectra.

Here, we have to note that perfluoroalkyl (Rf) chains are strongly interacted with each other especially when the carbon numbers, n, is eight (C8) or longer.4,24 Once the Rf groups are segregated by the spontaneous aggregation, it would be quite difficult to make the homogeneously blended molecular arrangement, as shown in Fig. 1(b). This experimental difficulty can be overcome by introducing a molecular skeleton involving double alkyl chains as dimyristoylphosphatidylcholine (DMPC). For our experimental purposes, two series of compounds as shown in Fig. 2 are prepared.

The first one as represented in Fig. 2(a) is a compound whose tail ends of the two alkyl chains are replaced by Rf groups with the same length of n, which is named “FnFn-DMPC.” If a monolayer of this compound is prepared by LB technique, the schematic in Fig. 1(a) can be generated. On the other hand, if only a single alkyl chain is replaced by an Rf group as shown in Fig. 2(b), the monolayer of this compound should have another schematic of Fig. 1(b). This single-substituted compound having both Rf and normal alkyl groups is named “FnF0-DMPC.” With the use of the four synthesized compounds (n = 4 and 8), IR spectroscopic properties of phonons will be discussed.

FnFn-DMPCs were synthesized by the synthetic method established by one of the authors (T.T.): dehydration–condensation of sn-glycero-3-phosphocholine with myristic acids containing a terminal Rf group (Fn-MA) with n = 4 and 8.25 For the synthesis of FnF0-DMPCs, (S)-1-(benzyloxy)-3-(trityloxy)propan-2-ol was synthesized as the glycerol skeleton, of which protecting groups are introduced at 1- and 3-positions and the final products were obtained by stepwise deprotection and dehydration–condensation reactions for three positions. The primary chemical structure of the compounds was determined by 1H and 19F NMR spectrometry.

LB films of the DMPC-involved compounds were prepared by transferring a Langmuir monolayer on pure water onto Si substrates using the LB (vertical dipping) method at 25.0 mN m−1 with the constant withdrawing rate of 5 mm min−1. A double-side polished HT Si wafer having a thickness of 1.0 mm was purchased from Silicon Technology Corporation (Tokyo, Japan) and cut to the size of 4 × 2 cm2 for the LB transfer. The substrates were cleaned by sonication with pure water, ethanol, acetone, and dichloroethane for about 1 min each and further cleaned using a BioForce Nanosciences (Ames, IA) UV/ozone Procleaner Plus ozone cleaner for 10 min.

IR MAIRS2 measurements were carried out using a Thermo Fischer Scientific (Madison, WI) Nicolet iS50 FT-IR spectrometer equipped with a Thermo Fischer Scientific (Yokohama, Japan) automatic MAIRS accessory (TN10-3001). The angle of incidence was fixed at 45° during the measurements, which is the optimal angle on MAIRS2 measurements using a Si substrate.20 The polarization angle was automatically changed from 0° to 90° by 15° steps, where the angles of 0° and 90° correspond to the s- and p-polarizations, respectively, by using an angle-controllable wire-grid linear polarizer made of Ge incorporated in the spectrometer. A mercury cadmium telluride (MCT) detector was employed for detecting the FT-modulated IR ray. The accumulation number of the single-beam spectra with a wavenumber resolution of 4 cm−1 was 512 for each polarization angle. All of the measurements were performed under a dry air condition. The absorbance of the obtained OP spectra was corrected by multiplying by a factor of n4H, where n and H are the refractive index of the film and the substrate-specific constant, respectively (for details, see Refs. 20 and 23). In this study, n = 1.35 and H = 0.33 were used for all the film samples.

As a reference, IR spectra of powder samples were obtained by the IR attenuated total reflection (ATR) technique. IR ATR measurements were performed using a Thermo Fischer Scientific Nicolet 6700 FT-IR spectrometer equipped with a Spectra-Tech (Oak Ridge, TN, USA) Foundation Thunder Dome ATR accessory. The accessory has a prism made of Ge, and the angle of incidence was fixed at 45°. Un-polarized light was used for the measurements and detected by using a MCT detector. The accumulation number of the single-beam spectra with a wavenumber resolution of 4 cm−1 was 300.

To begin with, the results of n = 4 are shown in Fig. 3. Figures 3(a) and 3(b) show IR MAIRS spectra of monolayer LB films of F4F4-DMPC and F4F0-DMPC, respectively, in the C–F stretching vibration region. Since F4F4-DMPC and F4F0-DMPC have double and single Rf chains, respectively, the scales of the two figure panels [Figs. 3(a) and 3(b), respectively] are twice as different in size.

The symmetric CF2 stretching vibration (νsCF2) band appears at different positions of 1137 and 1137 cm−1 for the IP and OP spectra for F4F4-DMPC, respectively, whereas it appears at the same position at 1136 cm−1 for both IP and OP spectra for F4F0-DMPC. The band positions and the shift are summarized in Table I.

The shift of 3 cm−1 is typically due to the Berreman effect, which implies that F4F4-DMPC molecules make a weak phonon. F4F0-DMPC ones have no phonon character that the IP and OP bands have comparable intensities as well as the same band positions, i.e., shift is 0. This simply implies that the Berreman effect is lost, meaning that the D–D interaction is also lost as expected.

Here, the change of the intensity ratio of the νsCF2 band between the IP and OP spectra may make us consider an orientation change of the Rf group. This possibility can be, however, denied by looking at the band at about 1357 cm−1. This band is assigned to the symmetric CF3 stretching vibration (νsCF3) band,24,26 whose transition moment is mostly parallel to the molecular axis of the Rf group.4,26 This band is active in the OP spectrum only for both F4F4-DMPC and F4F0-DMPC, while it is absent in the IP spectrum. This very clear MAIRS dichroic ratio indicates that the Rf groups are perpendicularly oriented in the monolayers.27 Therefore, the band intensity ratio of the νsCF2 band is simply influenced by the degree of phonon formation.

A similar thing is expected for the antisymmetric CF2 stretching vibration (νaCF2) band, but this band is heavily overlapped with the neighboring virtual-mode (VM) band5 at about 1240 cm−1. This band has already been assigned to a phonon mode that is discussed on the Berreman effect.5,8 In a polymer of PTFE, this band is so strong that it is beyond phonons to have polaritons that give negative electric permittivity, in fact.8 For F4F4-DMPC, the band is dominated by the VM band at 1245 cm−1 in the OP spectrum and the same thing happens at 1234 cm−1 in the IP one, which is a typical Berreman’s shift as much as 11 cm−1. Here, we have to note that such a short Rf chain as n = 4 is known to be not aggregated spontaneously through both experiences and theory. In fact, when the stratified dipole-array (SDA) theory4,24 is referred, and the “spontaneous” molecular aggregation is not expected for the Rf chain with n = 7 or shorter. Simply judging from this criterion, n = 4 is thus too short to make the molecules aggregated and phonons are not expected. We have to note, however, that the molecular aggregates in a monolayer for the MAIRS measurements are not spontaneously generated, but they are “artificially” prepared by sliding the compression bar on the Langmuir trough keeping at the surface pressure of 25.0 mN m−1. In other words, even if a short Rf compound is employed, the Langmuir technique makes the molecules highly condensed having a random orientation for the short Rf compound, which generates phonons to some extent.

Then, if we had another spectrum of “spontaneously” aggregated molecules of the same compound as a reference, we would be able to discuss the MAIRS spectra more comprehensively comparing with the reference spectrum. Since powder samples are of spontaneously aggregated compounds (mostly crystalline), the reference spectrum is obtained by the ATR technique1,12 with powder samples. Here, we have to note that an ATR spectrum is of a linear combination of both LO and TO spectra, which cannot be directly compared to the MAIRS spectra.1 Therefore, in the present study, the ATR spectrum was converted to an LO spectrum by using the Kramers–Kronig (K–K) relationship.1,28,29

FT-IR gives us information of reflectance, Rω, along with the ATR measurement, but the phase information, ϕω, is annihilated. Fortunately, however, ϕω and Rω are of the complex reflection coefficient, rω, and they are mutually correlated with each other by the K–K relationship [Eq. (4)].29 Thus, ϕω can be calculated from Rω by using Eq. (5) where θ is the angle of incidence and ɛr, is the high-frequency limit of the permittivity (1.77 and 2.19 for Rf-involved compounds and DMPC, respectively24). Once ϕω is calculated, the complex refractive index of ñ=n+ik is readily obtained by Eq. (6),
lnrω=12lnRω+iϕω,
(4)
ϕω=2ωπ0lnRϖϖ2ω2dϖ+2tan1εr,sin2θcosθ,
(5)
nω=1Rω12Rωcosϕω+Rω,andkω=2Rωsinϕω12Rωcosϕω+Rω.
(6)
Since ñω is simply correlated to εrω=ñω2, we readily obtain εrω that further produces LO ELF through Eq. (2). In this fashion, the LO function is calculated from the ATR spectrum (see Fig. S1 of the supplementary material), which is presented in the black curve in Fig. 3(a).

At the νsCF2 and phonon bands, the calculated LO peak positions of 1135 and 1240 cm−1 are both lower than those of the measured OP bands of 1137 and 1245 cm−1, respectively. This proves that the bulk sample giving the ATR spectrum is composed of F4F4-DMPC with relatively weak “spontaneous” aggregations, which makes incomplete phonons. In this manner, the phonon band responds to the molecular aggregation sensitively, and it can be used as an index of phonon formation. In addition, the clear band intensity ratio of the OP band to the IP one also supports the phonon formation on considering the Berreman effect.

When F4F0-DMPC is employed for the same experiment, largely different results are obtained, as shown in Fig. 3(b). In this case, the hydrocarbon chain is introduced so that the direct aggregation of Rf chains is obstructed in the monolayer, which means that formation of phonons should be inhibited. In fact, both intensity and locations of the VM bands in IP and OP become very close to each other at about 1244 cm−1. With the decrease of the phonon character, the νaCF2 band appears as a shoulder at 1227 cm−1 that is figured out by the second-derivative calculation. It is of interest, on the other hand, that the reference LO spectrum of powder shown by the black curve seems to be an averaged spectrum of IP and OP spectra. The “shape of the LO spectrum” makes an index of phonon formation: an OP-like spectrum appears if a phonon is strongly generated, while the IP-like spectrum appears if no phonon is generated.5 Therefore, the average-like shape indicates a middle state of phonon formation in the powder reference. The peak position of the LO one is located at 1258 cm−1 that is significantly higher than the OP position. This suggests that the spontaneous aggregation in powder would be like clustering of the Rf chains that is not found in the fresh (not aged) LB film.

In this manner, we have found that the phonon formation in a monolayer can be checked out by using IR MAIRS with a help of the reference spectrum of a bulk sample. In the next paragraph, a similar study is performed by using F8-compounds having long Rf chains with the length of n = 8, which should spontaneously aggregate with each other judging from the SDA theory.

Figures 4(a) and 4(b) show IR MAIRS spectra of monolayer LB films of F8F8-DMPC and F8F0-DMPC, respectively, in the C–F stretching vibration region. Since the long-chain Rf group, F8, has a structurally ordered arrangement of the –(CF2)8 groups,4,24 the IR active E1 modes (mostly assigned to the symmetric and antisymmetric CF2 stretching vibration modes)26 are more enhanced than the F4 compounds. In particular, the antisymmetric mode appears clearly mainly in the IP spectrum at about 1205 cm−1. The orientation of F8 compounds is actually perpendicular to the film surface judging from the νsCF3 band at 1370 or 1333 cm−1 as discussed for F4 compounds, although the intensity is not large enough since the terminal CF3 group has less presence relative to the many CF2 groups of F8.

In the case of F8F8-DMPC, the νsCF2 band exhibits a clear Berreman’s effect appearing split at 1150 and 1153 cm−1 in the IP and OP spectra, respectively (Table I), which means that the Rf groups are adequately interacted with each other to readily generate phonon. Of interest is that the reference LO spectrum has a very similar shape to the OP spectrum, which clearly indicates that a phonon is firmly generated, which is clearly different from F4F4-DMPC. Here, a difference from the F4F4-DMPC case is found that the OP position (1153 cm−1) is slightly less than the reference LO position (1154 cm−1) calculated from the ATR spectrum of a powder sample. This indicates that the molecules in powder are spontaneously aggregated a little bit stronger than the monolayer, which indicates that the aggregation (crystallization) of the long F8 Rf chain occurs spontaneously.

A very similar thing happens for the VM band at about 1250 cm−1. This mode is clearly split into two bands at 1241 and 1250 cm−1 for the IP and OP spectra, respectively. The large splitting width of 9 cm−1 is comparable to the case of F4F4-DMPC. From another point of view, a large intensity ratio of the VM band in OP to IP is found at 1250 cm−1. In addition, this mode also has a similar difference to the νsCF2 band that the OP position is lower than the LO position at 1252 cm−1. This supports again that the spontaneously aggregated powder sample (crystalline) has a still better order than the artificially compressed monolayer. Through these results, the spontaneously aggregated molecules with a chain length of n = 8 have proved to be intrinsically stronger than the shorter chain compounds as expected by the SDA theory.

In the case of F8F0-DMPC, first of all, the VM bands appear with a similar intensity in both IP and OP spectra, which implies that the phonon-property is annihilated as found in the case of F4F0-DMPC [Fig. 3(b)]. On the other hand, however, the peak position of the VM bands exhibits a difference of 4 cm−1. In theory, the intensity change and the peak shift should have a good correlation, but it does not hold for this case. As a matter of fact, in the case of F0, both F4F0 and F8F0 show a difference of the peak-top positions of the VM bands (∼3 cm−1), whereas the intensities are fairly the same as each other. This may happen because the broad νaCF2 band that overlapped with the phosphate band (∼1230 cm−1) influences the VM band. In the case of F4F4 and F8F8, the shift is fairly significant (∼10 cm−1), which is recognized to be out of the interference. In other words, the phonon analysis on the peak shift alone would be a little bit dangerous especially for a minor shift. Since the benefit of using MAIRS is that the peak intensities can be compared quantitatively, the VM intensity is thus found to be the first choice for discussing the obstruction of phonons.

Of another interest here is that the reference LO spectrum still has a similar shape to that of the OP spectrum, indicating that F8F0-DMPC crystalline (powder) keeps the phonon character that is lost in the monolayer. This suggests that the Rf groups form clusters in the powder sample even with the hydrocarbons after aging. The reason why the peak position of the LO spectrum of F8F0-DMPC [1254 cm−1 in Fig. 4(b)] is even higher than that of F8F8-DMPC [1252 cm−1 in Fig. 4(a)] should also be attributed to the small clusters.

Double-chains involved phospholipids, DMPC, were employed as a molecular skeleton to have nearly symmetric and asymmetric compounds having normal alkyl and Rf groups. The symmetric compounds exhibited strong phonon bands irrespective of the chain length because the molecules were artificially compressed on water in the Langmuir trough. In the case of the powder crystalline samples, the phonon formation relied on the intrinsic molecular aggregation property that is driven by the length of the Rf group. When the length is short (n = 4), the phonon formation was very weak, making the LO ELF have an intermediate shape between the MAIRS-IP and MAIRS-OP spectra of the compressed monolayer. In this manner, IR MAIRS has been found to be a powerful tool to find phonon bands that are readily discriminated from the normal IR bands. In addition, the long Rf-chain compounds showed a largely different result from the short ones, and the LO spectrum of powder crystalline was very similar to the OP spectrum of the monolayer in shape, which can readily be understood in the SDA theory manner.

See the supplementary material for Fig. S1 that represents IR ATR spectra of powder samples of the four compounds of F4F4-, F4F0-, F8F8-, and F8F0-DMPC before any mathematical treatment; these “raw” spectra were subjected to the Kramers–Kronig relationship as shown by Eqs. (5) and (6) to have the LO spectra in Figs. 3 and 4.

This work was supported by a Grant-in-Aid Challenges in Research (Exploratory) [No. 21K18979 (T.H.)], a Grant-in-Aid for Scientific Research (C) [No. 23K04805 (T.S.)], and Grant-in-Aid for Early-Career Scientists [No. 19K15602 (N.S.)] from the Japan Society for the Promotion of Science (JSPS), a grant from Iketani Science and Technology Foundation (ISTF) [No. 0351056-A (M.S.)], the Sasakawa Scientific Research Grant from the Japan Science Society [No. 2023-3025 (A.N.)], the Asahi Glass Foundation (T.H.), and the Collaborative Research Program of Institute for Chemical Research (ICR), Kyoto University (No. 2023-72), to which the authors’ thanks are due.

The authors have no conflicts to disclose.

The manuscript was written through contributions of all authors.

Takeshi Hasegawa: Conceptualization (lead); Formal analysis (lead); Funding acquisition (equal); Investigation (lead); Project administration (lead); Supervision (lead); Writing – original draft (lead). Ai Nakagawara: Data curation (lead); Investigation (equal); Resources (equal); Writing – review & editing (supporting). Toshiyuki Takagi: Resources (equal). Takafumi Shimoaka: Supervision (supporting); Writing – review & editing (supporting). Nobutaka Shioya: Data curation (supporting); Writing – review & editing (supporting). Masashi Sonoyama: Project administration (supporting); Resources (supporting); Supervision (supporting); Writing – review & editing (supporting).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
T.
Hasegawa
,
Quantitative Infrared Spectroscopy for Understanding of a Condensed Matter
(
Springer
,
Tokyo
,
2017
).
2.
C.
Kittel
,
Introduction to Solid State Physics
(
Phonons
,
2020
).
3.
R.
Fuchs
and
K. L.
Kliewer
, “
Optical modes of vibration in an ionic crystal slab
,”
Phys. Rev.
140
(
6A
),
2076
2088
(
1965
).
4.
T.
Hasegawa
, “
Physicochemical nature of perfluoroalkyl compounds induced by fluorine
,”
Chem. Rec.
17
(
10
),
903
917
(
2017
).
5.
A.
Fukumi
,
T.
Shimoaka
,
N.
Shioya
,
N.
Nagai
, and
T.
Hasegawa
, “
Infrared active surface modes found in thin films of perfluoroalkanes reveal the dipole–dipole interaction and surface morphology
,”
J. Chem. Phys.
153
,
044703
(
2020
).
6.
N.
Nagai
,
M.
Okawara
, and
Y.
Kijima
, “
Infrared response of sub-micron-scale structures of polyoxymethylene: Surface polaritons in polymers
,”
Appl. Spectrosc.
70
(
8
),
1278
1291
(
2016
).
7.
N.
Nagai
,
H.
Okada
,
Y.
Amaki
,
M.
Okamura
,
T.
Fujii
,
T.
Suzuki
,
A.
Takayanagi
, and
S.
Nakagawa
, “
Anomalous high-infrared reflectance of extruded polyoxymethylene
,”
AIP Adv.
10
(
9
),
095201
(
2020
).
8.
N.
Nagai
,
H.
Okada
, and
T.
Hasegawa
, “
Morphology-sensitive infrared absorption bands of polymers derived from surface polaritons
,”
AIP Adv.
9
(
10
),
105203
(
2019
).
9.
T.
Hasegawa
, “
Understanding of the intrinsic difference between normal- and perfluoro-alkyl compounds toward total understanding of material properties
,”
Chem. Phys. Lett.
627
,
64
66
(
2015
).
10.
V. M.
Agranovich
and
D. L.
Mills
,
Surface Polaritons
(
North-Holland Publishing Company
,
1982
).
11.
R.
Ruppin
and
R.
Englman
, “
Optical phonons of small crystals
,”
Rep. Prog. Phys.
33
(
1
),
149
196
(
1970
).
12.
V. P.
Tolstoy
,
I. V.
Chernyshova
, and
V. A.
Skryshevsky
,
Handbook of Infrared Spectroscopy of Ultrathin Films
(
Wiley
,
Hoboken, NJ
,
2003
).
13.
D. W.
Berreman
, “
Infrared absorption at longitudinal optic frequency in cubic crystal films
,”
Phys. Rev.
130
(
6
),
2193
2198
(
1963
).
14.
J.
Umemura
,
T.
Kamata
,
T.
Kawai
, and
T.
Takenaka
, “
Quantitative evaluation of molecular orientation in thin Langmuir-Blodgett films by FT-IR transmission and reflection-absorption spectroscopy
,”
J. Phys. Chem.
94
(
1
),
62
67
(
1990
).
15.
C.
Naselli
,
J.
Rabolt
, and
J.
Swalen
, “
Order–disorder transitions in Langmuir–Blodgett monolayers. I. Studies of two-dimensional melting by infrared spectroscopy
,”
J. Chem. Phys.
82
(
4
),
2136
2140
(
1985
).
16.
T.
Hasegawa
,
S.
Takeda
,
A.
Kawaguchi
, and
J.
Umemura
, “
Quantitative analysis of uniaxial molecular orientation in Langmuir-Blodgett films by infrared reflection spectroscopy
,”
Langmuir
11
(
4
),
1236
1243
(
1995
).
17.
T.
Hasegawa
,
J.
Nishijo
,
J.
Umemura
, and
W.
Theiss
, “
Simultaneous evaluation of molecular-orientation and optical parameters in ultrathin films by oscillators-model simulation and infrared external-reflection spectrometry
,”
J. Phys. Chem. B
105
(
45
),
11178
11185
(
2001
).
18.
T.
Hasegawa
, “
A novel measurement technique of pure out-of-plane vibrational modes in thin films on a nonmetallic material with no polarizer
,”
J. Phys. Chem. B
106
(
16
),
4112
4115
(
2002
).
19.
T.
Hasegawa
, “
Advanced multiple-angle incidence resolution spectrometry for thin-layer analysis on a low-refractive-index substrate
,”
Anal. Chem.
79
(
12
),
4385
4389
(
2007
).
20.
N.
Shioya
,
K.
Tomita
,
T.
Shimoaka
, and
T.
Hasegawa
, “
Second generation of multiple-angle incidence resolution spectrometry
,”
J. Phys. Chem. A
123
(
32
),
7177
7183
(
2019
).
21.
T.
Hasegawa
and
N.
Shioya
, “
MAIRS: Innovation of molecular orientation analysis in a thin film
,”
Bull. Chem. Soc. Jpn.
93
(
9
),
1127
1138
(
2020
).
22.
N.
Shioya
,
S.
Norimoto
,
N.
Izumi
,
M.
Hada
,
T.
Shimoaka
, and
T.
Hasegawa
, “
Optimal experimental condition of IR pMAIRS calibrated by using an optically isotropic thin film exhibiting the Berreman effect
,”
Appl. Spectrosc.
71
(
5
),
901
910
(
2016
).
23.
N.
Shioya
,
T.
Shimoaka
,
R.
Murdey
, and
T.
Hasegawa
, “
Accurate molecular orientation analysis using infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) considering the refractive index of the thin film sample
,”
Appl. Spectrosc.
71
(
6
),
1242
1248
(
2016
).
24.
T.
Hasegawa
,
T.
Shimoaka
,
N.
Shioya
,
K.
Morita
,
M.
Sonoyama
,
T.
Takagi
, and
T.
Kanamori
, “
Stratified dipole-arrays model accounting for bulk properties specific to perfluoroalkyl compounds
,”
ChemPlusChem
79
(
10
),
1421
1425
(
2014
).
25.
M.
Yoshino
,
H.
Takahashi
,
T.
Takagi
,
T.
Baba
,
K.
Morita
,
H.
Amii
,
T.
Kanamori
, and
M.
Sonoyama
, “
Effect of partial fluorination in the myristoyl groups on thermal and interfacial properties of dimyristoylphosphatidylcholine
,”
Chem. Lett.
41
(
11
),
1495
1497
(
2012
).
26.
Y.
Nojima
,
T.
Shimoaka
,
T.
Hasegawa
, and
T.
Ishibashi
, “
Molecular orientations of myristic acid derivatives with different perfluoroalkyl chain lengths at the air/water interface evaluated by heterodyne-detected sum frequency generation spectroscopy
,”
J. Phys. Chem. C
127
(
25
),
12349
12356
(
2023
).
27.
K.
Honda
,
M.
Morita
,
H.
Otsuka
, and
A.
Takahara
, “
Molecular aggregation structure and surface properties of poly(fluoroalkyl acrylate) thin films
,”
Macromolecules
38
(
13
),
5699
5705
(
2005
).
28.
J. S.
Plaskett
and
P. N.
Schatz
, “
On the robinson and price (Kramers-Kronig) method of interpreting reflection data taken through a transparent window
,”
J. Chem. Phys.
38
(
3
),
612
617
(
1963
).
29.
A.
Ikehata
,
N.
Higashi
, and
Y.
Ozaki
, “
Direct observation of the absorption bands of the first electronic transition in liquid H2O and D2O by attenuated total reflectance far-UV spectroscopy
,”
J. Chem. Phys.
129
(
23
),
234510
(
2008
).

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