Average Orientation of a Molecular Rotor Embedded in a Langmuir

ARTICLE pubs.acs.org/Langmuir

Average Orientation of a Molecular Rotor Embedded in a Langmuir
Blodgett Monolayer Deborah L. Casher,† Luk
as Kobr,† and Josef Michl*,†,‡ † ‡

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo n
am. 2, Prague 16610, Czech Republic

bS Supporting Information ABSTRACT: A molecular rotor in which a naphthalene rotator is attached through a silicon atom to three fatty acid chains has been synthesized, and Langmuir
Blodgett techniques were used to deposit on silica surfaces monolayers of its calcium salt, both neat and diluted with stearic acid salts. The monolayer films have been characterized by ellipsometry and Fourier transform infrared (FT-IR) grazing-incidence attenuated total internal reflection (GATR) spectroscopy on Si-SiO2 and by UV
vis absorption spectroscopy on SiO2. The measurements were combined with calculations of the electronic (INDO/S) and vibrational (DFT) transition moment directions to deduce the average orientation of the rotor molecules, including the naphthalene ring, relative to the surface. In both neat and mixed films, the naphthalene ring is found to preferentially tilt toward the surface, enough that its rotation is most likely hindered. A comparable picture was obtained from molecular mechanics calculations on a mixed film of the naphthalene rotor and stearic acid.

’ INTRODUCTION Surface-Mounted Artificial Azimuthal Molecular Rotors. Many of the potential applications of artificial molecular rotors1
5 would benefit from their firm and rigid mounting on a surface. Various modes of surface attachment have been examined experimentally, ranging from mere physical contact 6,7 to covalent bonding.8
11 Some possibilities have also been investigated computationally.12
16 At present, we are interested in azimuthal molecular rotors, which consist of (i) an anchor or groups that attach to a surface, (ii) an axle mounted perpendicular8,9 to the surface, plus (iii) a rotator, free to turn about the axle and carrying a permanent dipole moment, and preferably also a UV
visible transition moment,17 both oriented perpendicular to the axle. One possible mode of surface attachment of azimuthal molecular rotors that has not received much attention so far would be anchoring within pure or mixed monolayer films. In mixed rotor films, a transparent diluent would serve the purpose of inhibiting energy transfer between rotors with UV
visible chromophores. This would be important in studies of rotor motion by fluorescence depolarization. Presently, we examine how such rotors and diluents could be incorporated and how they would be likely to orient within and at the outside surface of the film. We have prepared 1-naphthyltris(15-carboxypentadec-1-yloxy)silane (1), a molecular rotor with a rotator consisting of a simple naphthalene derivative, connected through a C
Si single bond axle to an anchor group consisting of three fatty acids (Chart 1). The fatty acid chains are designed to incorporate in a Langmuir
Blodgett (LB) monolayer, and we have prepared monolayers of 1 and monolayers of 1 diluted with octadecanoic (stearic) acid (2), both pure and mixed with their respective salts (1a and 2a), as r 2011 American Chemical Society

shown in Chart 1. The films were characterized by ellipsometry and Fourier transform infrared (FT-IR) grazing-incidence attenuated total internal reflection (GATR) on Si-SiO2, and by UV
vis absorption spectroscopy on SiO2. Fluorescence along with steady-state excitation and fluorescence polarization measurements on commercially available 1-naphthyltriethoxysilane (3), a model compound for 1 that remains dissolved in a variety of glassy solvents at 77 K, were also performed to explore electronic transition moment polarization in the naphthalene rotator. Orientation Determination by FT-IR GATR and UV
Vis Spectroscopy. LB films made from 2 or similar compounds in the presence of divalent cations, such as Ca2+, Cd2+, and Pb2+, are generally highly organized. Grazing incidence X-ray diffraction18,19 and polarized FT-IR ATR20,21 and reflection absorption22,23 measurements of LB films on a range of substrates have shown a mostly perpendicular orientation of the alkyl chains with respect to the substrate, with average tilt angles ranging from ∼0 to 40
. UV
vis chromophores incorporated into the films allowed further characterization of chromophore orientation23,24 and were found to form dimers and excimers.25 In LB films of alkyl chains containing a chromophore, the chromophore is constrained by the zigzag conformation of the chain inside the film and tilts accordingly. The terminal attachment of our naphthalene chromophore to three fatty acid chains can be expected to give it the freedom to Received: September 27, 2011 Revised: November 9, 2011 Published: December 01, 2011 1625

dx.doi.org/10.1021/la2037789 | Langmuir 2012, 28, 1625–1637

Langmuir Chart 1. 1-Naphthyltris(15-carboxypentadec-1-yloxy)silane (the naphthalene rotor, 1), Mixtures of 1 and Its Calcium Salt (1a), Stearic Acid (2), Mixtures of 2 and Calcium Stearate (2a), and Model Compounds 3 and 4a

ARTICLE

A quantitative orientation analysis of the naphthalene chromophore and alkyl chains can be obtained from FT-IR GATR spectra and the knowledge of vibrational transition moment directions. The theory behind GATR measurements on thin films on Si covered with a native SiO2 layer has been previously described.11 In the present study, we used Harrick’s GATR, a single reflection grazing-incidence ATR instrument32 in which films are pressed against a Ge internal reflection element. With an incident angle of 65
, the light is totally internally reflected and an evanescent wave is established in the film and in the underlying Si
SiO2 substrate. According to Maxwell’s equations, the electric field perpendicular to the Ge-film interface is enhanced as much as ∼10-fold compared to the incident beam, while the field parallel to the surface is hardly enhanced at all.33 Using the Fresnel equations for a range of plausible refractive indices,34,35 we have calculated that only 2
3% of the total electric field interacting with the sample on the GATR is s-polarized.36 Thus, only the component of the fth transition moment Mf that is directed along the surface normal Z contributes significantly to the spectrum and the probability of absorption by a molecule is proportional to |Z 3 Mf|2, where Z is a unit vector in direction Z. The relative weight with which the absorbance |Z 3 Mf|2 of the fth transition enters into the spectrum obtained in the thin-layer GATR experiment can be written as37 AGATR ¼ cjMf j2

∑u Ku cos2 θuf

ð1Þ

and for an ordinary isotropic sample, where Ku = 1/3 for any choice of u, it is AISO ¼ c0 ð1=3ÞjMf j2

ð2Þ

where Ku = Æcos uZæ (u = x, y, z, where uZ is the angle that the molecular axis u makes with Z) are the three orientation factors that describe the orientation of the principal framework of axes, cos θ fu gives the orientation of the transition moment Mf in the molecular frame, and c and c0 are proportional to the number of molecules observed in each spectrum (cf. Supporting Information). A comparison of relative peak intensities in the GATR and ordinary isotropic spectra therefore gives information on molecular orientation in the thin film. 2

a

DFT/B3LYP/6-31G(d,p) calculated transition moment directions of selected bands in 4 are shown assigned to observed bands in spectra of 1 and 1a.

orient in a variety of ways. If the three acid chains prefer to be equidistant from the substrate surface, the rotator axle could even be oriented upright, as shown in the idealized structure of 1a in Chart 1. Orientation determination by polarization spectroscopy requires a thorough understanding of transition moment directions. Much is known about the naphthalene chromophore from prior work. The origin of its lowest energy electronic transition moment (into the 1Lb state, using Platt’s nomenclature26) is polarized along its long axis.27 Polarized UV
vis absorption and fluorescence measurements of naphthalene crystals28 and linear dichroism (LD) of naphthalene in stretched polyethylene29 show that the lowest energy absorption band and the fluorescence of naphthalene are of significantly mixed polarization. The mixed polarization has been attributed to vibronic coupling of the relatively weak 1Lb transition with the short axis polarized 1La transition.28 Substitution in position 1, as in compounds 1 and 3, has little effect on the nodal properties of molecular orbitals30,31 and is expected to preserve the electronic transition moment directions of naphthalene. In our UV
vis absorption and fluorescence experiments, only the 1La transition is well-defined and accessible, and this results in an only partial picture of the orientation of the naphthalene ring.

’ EXPERIMENTAL PART AND CALCULATIONS Materials. Stearic acid (2), 1-naphthyltriethoxysilane (3), and starting materials for the synthesis of 1-naphthyltris(15-carboxypentadec-1-yloxy)silane (1) were purchased from Sigma-Aldrich and used as received. To avoid unnecessary contact with air and moisture, 1 was stored under Ar in sealed glass vials. All solvents used for sample preparation and measurement were of spectroscopic quality. EPA is a 5:5:2 by volume mixture of 2-methylbutane:diethyl ether/EtOH. Synthesis. The synthesis and characterization of 1-naphthyltris(15-carboxypentadec-1-yloxy)silane (1) are described in the Supporting Information. Sample Preparation. In an Ar-purged glovebag, ∼1 mg/mL solution samples of 1 and mixed samples of 1 and 2 in chloroform were made and used immediately for measurement or refrigerated at
3
C and used within 24 h for experiments using a Langmuir trough (KSV 5000, KSV Instruments, Ltd., Helsinki, Finland) with an aqueous (∼10
4
10
3 M Ca2+ from CaCl2) subphase at room temperature and at a pH of 6.2
6.8. Depositions were performed on Si(111) wafers (Silicon Quest International, 1
5 Ω 3 cm resistivity) with a native oxide layer (Si-SiO2) 1626

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir

ARTICLE

Scheme 1

or SiO2 wafers (NSG Precision Cells, Inc., P/N 10040). The substrate cleaning and handling procedures are described in the Supporting Information. To prepare films, chloroform solutions of 1, 2, or 1 with 2 at various molar ratios were added dropwise to the trough and left to sit for ∼15 min while the chloroform evaporated. Monolayer films were assembled by first compressing the barriers at 10 mm/min to a surface pressure of 30 mN/m, increasing the barrier distance at 5 mm/min to ∼0 mN/m, and recompressing at 2 mm/min to give a final surface pressure of 30 mN/m. Compression isotherms were recorded for both the first compression (“preannealed film”) and for the recompressed (annealed) film, using software that was provided with the KSV 5000 instrument. Following the recompression, films were held at a constant surface pressure for several minutes and appeared to be quite stable. After ∼5 min, the substrate (previously buried in the trough) was pulled up through the film at 0.5 mm/min while the surface pressure was held constant at 30 mN/m. Transfer ratios, F, were recorded using software for the KSV 5000 and ranged between 0.8 and 1.5 for films of 1a and 1
1.25 for films of 2a and mixed films. Transfer ratios in excess of unity have been previously reported for 1a in this pH range and are likely due to the solution of ionized material into the trough.38 Ellipsometry. Film thickness on Si
SiO2 was measured with a helium
neon laser (632.8 nm) beam at an incidence angle of 70
using a Rudolph model 437 ellipsometer (Rudolph Research, Flanders, NJ) equipped with an automatic rotating analyzer. An average of three or more measurements was used to determine the thickness of each film (Supporting Information). UV
Vis Absorption and Fluorescence. The room temperature (RT) UV
vis absorption spectrum of 1 was measured in 3-methylpentane (3MP), MeOH, and in EPA in a 1 cm path length Suprasil quartz cell using a Hewlett-Packard 8452A diode array spectrophotometer. The spectrum consists of a single scan integrated for 0.1 s and was corrected against a solvent blank. UV
vis absorption spectra of 1a and mixed films of 2a-1a (6.5:1 and 14:1) on SiO2 were measured with the same spectrophotometer, with a home-built mount that positions the substrate with its face perpendicular to the source beam. Monolayer spectra were corrected against the spectrum of 2a on SiO2. RT fluorescence spectra of 1 in EPA and 3 in MeOH and in hexanes (excitation at 280 and 300 nm), and 77 K fluorescence (excitation at 282 nm) of 1 in EPA and fluorescence (excitation at 282 nm) and excitation (emission monitored at 318 nm) of 3 in 3MP were measured in a 1 cm path length Suprasil quartz cell with a SPEX spectrofluorimeter with a 0.5 m double monochromator for excitation and a 0.75 m monochromator for emission. Polarized Emission and Excitation. Steady state polarized fluorescence (excitation at 282 nm) and excitation (emission observed at 318 nm) spectra of 3 in 3MP in a 1 cm path length Suprasil quartz cell that was immersed in a quartz-windowed Dewar filled with liquid N2 were measured with the SPEX spectrofluorimeter described above. A Glan-Taylor polarizer was positioned between the excitation source and the sample, a Polacoat39 polarizer was placed between the sample and the detector, and a 14
angle was maintained between the excitation light and the collection optics (Supporting Information). In the absence of spectral overlap, the fluorescence anisotropy of a vitrified dilute solution

Figure 1. Representative isotherms of 1a before (thin dashed line) and after (thick black line) annealing, 2a (thin dotted line), and mixed films of 2a
1a in ratios of 18.2:1 (full green line), 11.5:1 (purple dash-dot line), and 6.1:1 (red dashed line). is related to the angle β between the absorption and emission transition dipoles by r0 ¼ ð2=5Þð3cos2 β
1Þ=2:

ð3Þ

FT-IR Spectroscopy. FT-IR measurements were made in a dry airpurged sample chamber of a Nicolet Nexus 670 spectrometer equipped with a DTGS detector. Pressed KBr disks containing 1 and 2 were measured in transmittance mode for 100
200 scans at 2 cm
1 resolution. Spectra were corrected against a blank pressed KBr disk. Monolayer films of 1a, 2a, and mixtures of the two on Si
SiO2 were measured with the above instrument fitted with a Ge total internal reflection accessory (GATR, Harrick Scientific Products), which uses a single reflection at an incident angle of 65
. Contact between the sample and the Ge reflection element was maintained at a torque of 49.6 in. 3 oz (0.35 N 3 m). Monolayer spectra were measured at a resolution of 2 cm
1 and are the accumulated averages of up to 20 000 scans. Spectra were corrected against a spectrum of freshly cleaned Si-SiO2. Further baseline corrections were applied to all of the monolayer spectra. Computational Methods. The geometry of 1-naphthyltris(carboxymethoxy)silane (4), a model compound for 1, was built in ChemDraw 7.0 Ultra (CambridgeSoft Corporation, Cambridge, MA) and optimized with imposed Cs symmetry in Gaussian 9840 at the B3LYP/6-31G(d,p) level of density functional theory (DFT). The electronic spectrum, including transition moment directions, was calculated using the INDO/S Hamiltonian, the Zerner atomic parameter set, and the Weiss approximation for one-center two-electron integrals as detailed by Ridley and Zerner.41 The vibrational spectrum, including transition energies, oscillator strengths, and transition moment directions (dipole derivatives), was calculated with the B3LYP/6-31G(d,p) method. To explore the preferred conformational structure of our LB films, 1 and 2 were built using Materials Studio (Version 2.1, Accelrys San Diego, CA) and assembled into a 1:5 film. Both a single molecule of 1 and the 1:5 film were optimized using TINK,12 an empirical force field simulation package. A Powell minimization was followed by a truncated Newton minimization, using the Universal Force Field (UFF) of Rapp
e et al.42 The molecular visualization program VMD43 (Version 1.8.3) was used to visualize the output from TINK calculations.

’ RESULTS Synthesis. Compound 1 was prepared by the reaction of 1-naphthyltrichlorosilane44 and 16-hydroxyhexadecanoic acid in the presence of triethylamine (40% yield) as shown in Scheme 1. 1627

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir

ARTICLE

Langmuir Film Isotherms. Representative isotherms for films of 1a (pre- and postannealing), 2a, and 2a
1a at different ratios are shown in Figure 1. An average of three or more isotherms each of 1a and 2a gave mean molecular areas of 63 ( 2 and 20.4 ( 0.8 Å2, respectively. Average areas per molecule for isotherms of mixed films were generally larger by ∼1.5
3 Å2 than expected from the ratios of 2 to 1 in the original solution and the mean molecular areas determined for pure films of 1a and 2a. Ellipsometry. Within ∼20 min of deposition, films of 1a and 2a showed ellipsometric thicknesses of 30 ( 3 and 26.9 ( 0.6 Å, respectively. Mixed films with ratios of 2a
1a from 5.3:1 to 11.5:1 all showed similar ellipsometric thicknesses of 28.0 ( 0.2 Å. Over time, films occasionally showed small (1
3 Å) increases or decreases in thickness. Assuming a metal ion layer of 2
3 Å thickness,18,45 the expected thickness for fully extended, upright films of 1a and 2a is ∼29
32 and ∼25
28 Å, respectively. UV
Vis Absorption and Steady State Polarization. Figure 2 shows the UV
vis absorption spectrum of 1 in EPA, along with monolayer spectra of 1a and mixed films of 2a
1a (6.5:1, 14:1) on SiO2, and the calculated spectrum of 4 (described later). Table 1 lists the observed and calculated band frequencies and relative intensities with the calculated transition moment directions for 4. Two main absorption bands are observed, a stronger one at 220
228 nm (∼44 700 cm
1) shown normalized, and a weaker one centered at 280
282 nm (∼35 600 cm
1). A third band at 318 nm (31 400 cm
1) is barely apparent above the noise in the spectra of 1 and 1a (shown in the inset of Figure 2). Compared with the solution spectrum maximum at 224 nm (∼44 600 cm
1), the corresponding band in the spectrum of 1a

is somewhat red-shifted and significantly broadened, and those in the spectra of mixed films are slightly blue-shifted, broadened (in the case of the 6.5:1 film) or of similar line width (in the 14:1 film). The band at 280
282 nm is about half as intense in the monolayer films relative to the solution spectrum. In 3MP and MeOH, 1 was not sufficiently soluble to measure accurate extinction coefficients. However, the relative intensities of the different absorption bands in EPA, 3MP, and MeOH (the latter two not shown) were identical, allaying concern that reaction of the Si
O bond of 1 with alcohol in the solvent, to form the corresponding silanetriol (C10H7Si(OH)3), would noticeably change the spectrum. The 77 K fluorescence and excitation spectra and the steady state fluorescence and excitation anisotropy of 3 in 3MP are also shown in Figure 2. At 77 K, 1 in EPA glass (not shown) had very weak fluorescence and a steady state emission anisotropy around zero, possibly due to insufficient solubility and aggregation or precipitation. We observed no difference in the RT fluorescence spectra of 1 and 3 and expect little to no difference in their respective electronic transition moment directions. With excitation at 282 nm (∼35 500 cm
1), the fluorescence anisotropy of 3 varies significantly across the band between
0.09 at ∼31 400 cm
1 to a maximum of 0.16 at ∼31 000 cm
1. Naphthalene also shows variation in the anisotropy of its fluorescence, which ranges from ∼0.08 to ∼0.16 with excitation at 313 nm (∼31 900 cm
1).46 Polarized excitation spectra, observed at 318 nm (the fluorescence maximum at 77 K), revealed a steep drop in anisotropy between ∼32 300 and 33 300 cm
1 from 0 to

Figure 2. UV
vis absorption (normalized) of 1 in EPA solution (gray line) and LB films on SiO2 of 1a (black line) and 2a
1a in ratios of 6.5:1 (red dashed line) and 14:1 (purple dash-dot line), and INDO/S calculated transitions and transition dipole moment directions from the naphthalene y0 axis of 4 (Chart 1). Normalized fluorescence (gray line) and excitation (gray line) and corresponding anisotropies (orange line) of 4 in 3MP at 77 K. Inset: ∼100 magnification from 30 000 to 33 000 cm
1.

Figure 3. Uncorrected FT-IR spectrum of 1 in KBr (gray line), GATR spectra of 1a (black line), 2a (dotted line), and 2a
1a in ratios of 5.3:1 (red dashed line) and 11.5:1 (purple dash-dot line), and DFT/B3LYP/ 6-31G(d,p) calculated transitions (scaled as described in the text) in 4.

Table 1. Observed UV
Vis Absorption Spectra of 1a, 1ab, and 2a
1a (6.5:1),b and the Calculated Spectrum of 4,c Compared with Literature Valuesd,e λ/nm 1a

1ab

2a
1ab

224

228

282

282

318 a

318 b

c

|ϕ|/(deg)f

rel. intensity/(arb. units) 4c

litd

220

218

224

280

272

282

not obs.

307

312

1a

1ab

2a:1ab

1g

1

1

1

1

0.12h

0.076

0.056

0.09

0.1

0.001

0.006

∼0.01

∼0.009

d

4c

not obs. e

litd

4c

lite

0

0

88

90

15

0i

f

In EPA. On SiO2. By INDO/S (see text for details). 1-Methylnaphthalene in EtOH (ref 27.). Naphthalene (ref 27.). Absolute value of the transition moment direction with respect to the y0 axis (see Chart 1). g ε = 61 000 M
1 cm
1. h ε = 7100 M
1 cm
1. i In 1-methylnaphthalene, this transition moment lies ∼15
from the y0 -axis (ref 31). 1628

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir
0.1 and a gradually increasing anisotropy from ∼33 300 to 37 000 cm
1 of
0.1 to ∼0. FT-IR Spectroscopy. Uncorrected FT-IR spectra of 1 in KBr and monolayer films on Si
SiO2 of 1a, 2a, and mixtures (5.3:1 and 11.5:1) of 2a
1a are shown in Figure 3. Selected regions of the spectra of 1, 1a, 2a, and of the mixed films are shown in Figure 4 to elucidate the chain organization and naphthalene ring orientation in the monolayer samples. Experimental band frequencies and intensities of 1, 1a, and the 5.3:1 mixed film of 2a
1a are given in Table 2 along with the calculated frequencies, intensities, and transition moment polarization directions for vibrations associated with the naphthalene ring of 3, and band assignments from the literature. A detailed description of the observed bands can be found in the Supporting Information. Calculated Electronic and Vibrational Spectra. The INDO/S calculated electronic spectrum of 4 is included in Table 1 and plotted in Figure 2 with the observed spectra of 1, 1a, and mixed films. The spectra correspond to expectations for the naphthalene chromophore, with an intense long-axis polarized Bb transition at high energy followed by a weaker short-axis polarized La transition, and finally a very weak Lb transition at lower energies. The calculated energies are all slightly higher than observed for 1 and 1a, but the agreement between the calculated and experimental spectra is otherwise excellent. The relative intensities of the calculated bands qualitatively match the experimental intensities in the solution spectrum of 1 and also the literature values for 1-methylnaphthalene in EtOH27 (Table 1). Calculated electronic transition moment directions for 4 are in excellent agreement with those reported for naphthalene27 and several 1-substituted naphthalenes.31,47 The DFT/B3LYP/6-31G(d,p) calculated vibrational spectrum of 4 is presented in Table 2 and plotted with the observed spectra in Figures 3 and 4. The B3LYP force field generally overestimates vibrational frequencies58 with the overestimation decreasing with decreasing wavenumber. Although the average error between experimental and unscaled calculated frequencies can be relatively high for the B3LYP method, scaling the force field or resulting frequencies by a factor or several factors for different frequency regions has been shown to result in lower average errors when compared with several other methods.58,59 In order to obtain reasonable agreement with the experimental spectra, and with the observed ring band frequencies in particular, calculated frequencies were scaled by three different factors: 0.956 above 3000 cm
1, 0.967 between 2999 and 1300 cm
1, and 0.98 below 1300 cm
1. The latter two frequency scaling factors are identical to those used by Langhoff60 in his DFT/B3LYP calculation of the spectrum of naphthalene with the 6-31G(d) basis set. Band assignments given in Table 2 are based on the calculations and have been confirmed using the references provided in the table. Bands associated with the alkyl chain and acid groups have been included in the table for completeness; however, differences between the calculated model compound 4 and measured compounds 1 and 1a are expected given that we have not treated the full alkyl chain nor the carboxylate group in the calculation. It is clear that some of the calculated frequencies associated with the chain do not match the experimental spectra well (alkyl C
H and CdO stretching frequencies, for example), and that different scaling factors would model these vibrations better.58 Fortunately, the order in which most bands appear in the calculated spectrum is consistent with their order in the experimental spectra and bands are easily assigned by the calculated atom displacements for each mode. A detailed description of the

ARTICLE

Figure 4. Expanded view of corrected FT-IR spectrum of 1 in KBr (gray line), GATR spectra of 1a (black line), 2a (dotted line), 5.3:1 (red dashed line) and 11.5:1 (purple dash-dot line) mixed films of 2a
1a, and DFT/B3LYP/6-31G(d,p) calculated transitions. (A) Normalized absorbance in the C
H stretching region. Inset: Relative absorbance of the monolayer films. (B) CdO and COO
stretching and C
H deformation bands with film intensities shown relative to one another and the CdO band of 1 in KBr normalized to that in the 11.5:1 film. (C) Aromatic C
H stretching region approximately normalized to the band near 3045 cm
1. (D) Naphthalene ring bands (corrected for the COO
stretch above 1475 cm
1) with film intensities shown relative to one another and the outof-plane polarized band at 775 cm
1 in 1 normalized to that in 1a, and the spectrum of 5 in KBr (0) for comparison with calculated ring bands (9). Inset: Spectra normalized to the oop polarized band at 775 cm
1.

calculated vibrational spectrum of 4 can be found in the Supporting Information. 1629

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

N/A

N/A

N/A

1562

N/A

1467
1459

obsc.

1428

N/A

N/A

N/A

N/A

N/A

1472, 1463

1431

obsc.

2878

2871

1700

2916

2917

1704

2965

2954

not obs.

N/A

N/A

∼2670

N/A

N/A

2848

N/A

N/A

2849

not obs.

2ab

∼3320, ∼3050

2a

1630

obsc.

1436

1471, 1464

1505

N/A

1569

1589

1619

1705

∼2670

2849

N/A

2916

N/A

3041

3054

not obs.

∼3340, ∼3100

1a

~v/cm
1

1430

obsc.

1468

1506

1544, 1536

1570

1590

not obs.

1738, 1721

not obs.

2850

N/A

2917

N/A

3042

3060

not obs.

not obs.

1ab

1429

obsc.

1467
1457

1506

1562

1569

1591

not obs.

1698

not obs.

2853

2877

2926

2961

not obs.

3061

not obs.

not obs.

2a-1ab


5


4


4


4

0.00203

0.0138

0.0249




0.00269

0.00779

0.00275

0.0967 0.469

0.533





4




0.274

0.633

0.248

7.40  10
4

0.00402







COO
sym str.47

COOH CO str. + OH def.47

Ar CdC str.49


y (15)

CH2 CH def.48

Ar CH def.49 z (26)


CH2 CH def.51

Ar CdC str.48,49

COO
asym str.47

Ar CdC str.48,49

Ar CdC str.48,49

Ar CdC str.48,50

COOH HO 3 3 3 H47 CdO str.47




y (30)




z (12)

z (22)

y (19)














CH2 CH sym str.48





CH3 CH sym str.48





0.0565

0.00104

obsc.

0.00359

4.49  10

0.00246

0.00244

0.00195

CH2 CH asym str.48





CH3 CH asym str.48

Ar CH str.48

Ar CH str.48

Ar CH str.49

COOH HO 3 3 3 H str.48

band assignment

axis (
)




0.0502

0.0580

1429

obsc.

3.49  10


4

7.14  10


4

6.42  10


4

3.40  10


4

not obs.

0.00192

not obs.

0.00413

0.00162

0.0135

obsc.

0.398, 0.386


4

2.99  10

not obs.

4.91  10
4

5.54  10

not obs.

0.00113

N/A

1442

obsc.

0.601

0.181

N/A

0.113

0.0952

0.0135

0.672

∼0.3

0.926

N/A

0.0213

2.93  10

obsc.

0.529, 0.439

N/A

0.506

N/A

N/A

N/A

0.303

not obs.

0.602

0.309

0.00745

1443

1454

1460

N/A

N/A


1505

N/A

N/A

N/A

0.687

∼0.3

0.916

1565

1592

1619

1799 1781

1812




2922

2942

2943

0.489

0.00158

0.00209

0.0145


1

N/A

0.00741

0.976

N/A

2951


1

0.394

2988

2995

0.588




z (19)

y (31) y (38)

z (10)




0.0339

0.0464 0.0318

0.0106

y (26)




z (18)

not obs.

1.46  10

0.0102

z (39)

5.68  10
5

8.71  10

not obs.

2.86  10
4

0.460

0.458

not obs.







Me

0.00865

N/A

N/A

not obs.

0.142

4c,d

3030

N/A

N/A

N/A

not obs.

2a-1ab

3033

3040

3058 3054

3067

3111

N/A

not obs.

1ab

0.0877

∼0.1, ∼0.4

1a,d

0.00706

not obs.

2ab,d

3629

∼0.08, ∼0.3

2a,d

3632

3632

4c

intensity/au

Table 2. Observed and DFT/B3LYP/6-31G(d,p) Vibrational Spectra of 1a, 1ab, 2a, 2ab and a 5.3:1 Mixed Filmb of 2a
1a with Calculated Transition Moment Directions of the Naphthalene Ring Bands in 4h

Langmuir ARTICLE

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

not obs. 1278

1262

obsc.

1298 1279

1260

1241

1631

1103 1050
1013

obsc. not obs.

1103 not obs.

N/A 989

obsc. ∼1152

not obs. ∼1310

1331 1313

1186 N/A

1381 1370 ∼1349

obsc. 1372 1355

1227 obsc. N/A obsc.

obsc.

not obs.

N/A 1223 N/A 1202

obsc.

2ab

1412

2a

Table 2. Continued

f

1025 990

obsc. 1085

1187 1152

N/A 1222 obsc. 1205

1246

1263

1297 1285

1330 1312

obsc. obsc. 1351

∼1389

1411

1a

~v/cm
1

1025 992

obsc. 1072

obsc. 1146

1230 obsc. obsc. obsc.

obsc.

obsc.

1301 obsc.

1025 991

1100 obsc.

obsc. 1151

1228 obsc. obsc. obsc.

obsc.

obsc.

not obs. obsc.

obsc. 1320

∼1336 1315 f

1382 1370 1357

obsc.

obsc.

2a-1ab

obsc. 1368 1355

1391

1408, 1402

1ab

y(12)



0.0387

z (23)


981

0.00281 0.00142




0.00228 0.00169

0.00108

0.248 0.174 1008

N/A not obs.

1.26  10
4

Si
SiO2 SiO253 z (28) z (11)



Ar CCC def.49

Ar CH def.52 Ar CH def.49,52 CH2C
CH2 rock + CC str.51

SiOCH247

COOH CO str.47 Si
SiO2 SiO2 asym str.,53

Ar CH def.49,52 z (29)

1012

N/A 0.0762

Ar CH def.49 CH2 CH def.47 SiOCH2
SiOC def.47




z (3)



y (17) z (42)


0.00586 obsc.

Si
SiO2 SiO254 CH2 CH def.47 Ar ip CH bend55 CH2 CH def.47





z (10)





CH2 CH def.47 CH2 CH def.47 COOH OH def.47 CH2 CH def.47 CH2 CH def.47 Ar CdC str.49,53 CH2 CH def.47

CH2 CH def.47 CH2 CH def.47 Ar CC str.49

CC str.+ CH2 CH wag48,52 CH3 CH sym def.48 CC str.+ CH2 CH wag48,51 Ar CdC str.49




z(27)
y(23)

Ar CH def.49


CH2COOH CH def.47

band assignment

axis (
)


z(19)




Me

0.317 0.00519 0.0197 0.00181

obsc. 3.34  10
4

1120 1071 1035 1017

obsc. 0.422

0.0971 0.0458

0.160 0.415

0.00487 0.0245 0.840 1.00

obsc. obsc.

0.0699
0.245

0.0419
0.167 0.0866


0.0217 0.0464 1.00  10
4 0.161 2.37  10
4 1.52  10
4

1157 1146 1143 1128

obsc. obsc.

0.01523 obsc. obsc. obsc.

obsc.

obsc.

not obs. obsc.


0.00326

0.146

0.0690 0.229

obsc. obsc. obsc. obsc.

obsc.

obsc.

∼1  10 obsc.


4

obsc. 0.0011

6.87  10
5 6.59  10
5

0.0665
0.00426 0.00174

0.00777

0.0189

4c,d

0.00171

obsc. 0.169

N/A 0.116 obsc. 0.125

0.147

0.110

0.172 0.163

0.0524 0.162

0.00167 0.00150 0.00133

obsc.

obsc.

2a-1ab

obsc. 1.67  10
4 1.41  10
4

∼0.142 obsc. obsc. 0.0204

5.22  10
4 5.66  10
4


4

5.42  10

1ab

0.240

1a,d

1167

0.178 N/A

1218
1212

0.659 obsc. N/A obsc.

obsc.

0.0949

not obs. 0.0328

not obs. ∼0.08

0.155 0.0702 0.0718

obsc.

obsc.

2ab,d

1204

N/A 0.219 N/A 0.197

0.215

0.248

0.392 0.267

1258
1241 1234


1299 1285 1282 1270 1267 1261

0.173 0.305

obsc. 0.102 0.144

1383
1379 1350
1317

not obs.

0.205

2a,d

1386

1397

4c

intensity/au

Langmuir ARTICLE

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

1632

914

obsc.

not obs.

N/A

not obs.

N/A

not obs.

724

obsc.

910f

not obs.

not obs.

N/A

not obs.

N/A

not obs.

729, 719

obsc.

702

∼699

776 obsc.

f

obsc.

797

obsc.

833

not obs.

917

obsc.

1ab

727, 720

776

obsc.

798

obsc.

833

obsc.

913

949

1a

~v/cm
1

702

obsc.

777

obsc.

799

obsc.

832

not obs.

∼924

obsc.

2a-1ab

715

734

776

786

799 789

815

829

860

obsc.

0.229

N/A

N/A

obsc.

N/A

obsc.

not obs.

obsc.

0.549

N/A

N/A

not obs.

N/A

not obs.

obsc.
4

0.00404

obsc.

∼0.24 0.299

0.00249

obsc.

0.00196

obsc.

9.81  10

not obs.

0.369

obsc.

0.299

obsc.

0.208

obsc.

0.00286

obsc.

0.00174

obsc.

0.00108

obsc.

0.0011

not obs.


4

0.0870

0.00178

0.0290

0.113

0.0455 0.00577

0.165

0.0381

1.68  10

0.0374

0.00256

0.0508

0.00288

846

0.116

846

1

1.56  10
4

4c,d

0.0460 0.00273

0.133

obsc.

2a-1ab

854 913

obsc.

1ab

0.00107

0.0791

1a,d 0.00322

obsc.

2ab,d

940

0.240

2a,d

966

972

4c

intensity/au

Ar CH oop55 Ar CC str.49 SiOCH2
g Ar CH oop55

x y (4)
x

z (9)

Ar CCC ip def.49

CH2 CH rock47,50,54,57

Ar CH oop55

SiOCH2
g




x

Ar CC str.49

Ar CH oop56

SiOH SiO str.c

oop def.47

COOH HO ...H

z (19)

x

x

SiOCH2
sym str.47 Ar CH oop49

COOH HO 3 3 3 H oop def.47 x

x

band assignment Ar CH oop49

x

Me

axis (
)

In KBr, measured by transmittance FTIR. b Monolayer on Si
SiO2, measured by GATR FTIR. c Calculated by DFT/B3LYP/6-31G(d, p), scaled by 0.956 above 3000 cm
1, 0.967 between 2999 and 1300 cm
1, and 0.98 below 1300 cm
1 (see text for more details). d Normalized to the most intense band in the spectrum. e Ring bands only. See Chart 1 for molecular framework axes. f Shoulder. g Observed in spectra of phenyltriethoxysilane. (See text for details.) h Not obs. = not observed; N/A = not applicable, obsc.= obscured by a more intense band or by noise; def. = deformation;
= not included in calculation.

a

obsc.

2ab

943

2a

Table 2. Continued

Langmuir ARTICLE

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir

Figure 5. Molecular mechanics models of 1 (left) and a 1:5 mixture of 1 and 2 (right).

Of all the ring bands, only three or four were sufficiently intense, separable, and secure enough in their assignment in each of the spectra to be used for quantitative analysis of the orientation of the naphthalene ring. The calculated absolute transition moment directions for these bands are shown in Chart 1. Based on previous estimates61 and on computations performed for this study (Supporting Information), we assume an uncertainty in the calculated transition moment directions of ((5
10)
. Calculated Film Structure. Figure 5 shows the results of molecular mechanics calculations of the structure of 1 and a 1:5 mixed film of 1 and 2. The optimized geometry of 1 shows a slight kink in one of the chains, which lifts one of the acid mounting groups upward and results in a tilt of the short axis of naphthalene with respect to the chain axis. The optimized 1:5 film shows a variety of preferred orientations of the naphthalene chromophore, most of which are tilted as above and some which show the ring plane bent toward the film surface. In other work not shown here, we have explored the effect of a plane of Ca2+ cations both constrained and unconstrained on COOH and COO
head groups (which were free to move). When the Ca2+ ions were constrained to lie in a single plane, the head groups remained close to the surface. When unconstrained, the ions no longer occupied a distinct plane and some of the alkyl chains shifted vertically from one another as seen in the film modeled in this work.

’ DISCUSSION The goal of this work was to characterize the order and orientation in monolayer LB films of 1a, 2a, and mixed films of the two. We look first at the ordering of the carboxy CdO and COO
head groups and the differences between 1a and the other monolayer films based on their FT-IR GATR spectra. Second, we use results of the compression isotherms, ellipsometry, and FT-IR spectroscopy to determine qualitatively, and quantitatively where possible, the tilt angles of the alkyl chains in these films. Third, we use the FT-IR, UV
vis, and fluorescence polarization spectra in conjunction with calculations to examine the orientation of the naphthalene ring in pure and mixed films of 1a. Carboxy CdO and COO
Head Group Organization and Orientation. FT-IR GATR spectra of monolayers of 1a, 2a, and

mixtures of the two in Figure 4B give a complex and somewhat inconclusive picture of the nature of the carboxy CdO and COO
head groups in these films. Having both COOH and COO
groups present is likely to introduce some amount of disorder into the headgroup packing at the surface that would be lessened by working at pH values above 9 or below 5, where either only the carboxylate COO
or only acid COOH is present. However, the sensitivity of the Si
O bond in 1 to acidic and

ARTICLE

Figure 6. (Left) Cartoon of a calcium stearate chain tilted by angle α with respect to surface normal, Z. (Right) Transition moment directions with respect to the molecular framework axes approximated from the normal mode descriptions for (upper) the C
H stretching bands of an alkyl chain and (lower) the COO
stretching bands.

basic hydrolysis restricted our working range to close-to-neutral pH values where COOH and COO
coexist in the presence of Ca2+. In addition to having both head groups present, frequencies and line widths of the carboxy CdO and COO
bands show, respectively, that a variety of H-bonded configurations of the carboxy CdO group is present and that COO
adopts a wide range of H-bonded geometries and possibly more than one type of coordination with Ca2+ in all of these films. Since the films were formed under similar conditions and the relative intensities of the carboxy CdO and COO
bands are comparable in each, it is notable that this region of the spectrum is so different for 1a and in the other films. The mostly nonhydrogen-bonded and singly hydrogen-bonded carbonyl oxygen atoms in monolayers of 1a suggest that a number of the COOH groups are in a position where they cannot interact with carbonyl groups on neighboring chains. The situation may be similar in mixed films, with the broad carboxy CdO band masking a relatively small contribution of potentially noninteracting chains from the fraction of 1a in these samples. The frequency of the asymmetric COO
band and the splitting of the asymmetric and symmetric COO
stretching frequencies are also quite different in 1a compared to the other films, likely a result of different types of coordination.62
64 While the organization and orientation of the COO
headgroup is too complex to explore here in a quantitative fashion, FT-IR spectra do give a qualitative picture that supports a preferred orientation of the COO
groups at the surface. IR spectra of isotropic fatty acid salts (not shown) have a relatively intense asymmetric COO
stretch and a significantly weaker symmetric COO
stretch.65 In contrast, our monolayer spectra of 2a and mixed films show the opposite trend: a symmetric COO
stretch at ∼1430 cm
1 that is enhanced with respect to the asymmetric COO
stretch at 1562 cm
1. Based on the transition moment directions shown in Figure 6 and the surface normal direction of the electric field in our FT-IR experiments, and considering that the probability of absorption goes as the square of the scalar product of the two, the relative band intensities in the monolayer samples suggest a preferred orientation in which the symmetry axis of the COO
group is oriented upright, with the COO
plane more perpendicular than parallel to the surface, and with the two oxygen atoms roughly equidistant from the surface as illustrated in Figure 6. The smaller intensity difference between the asymmetric and symmetric COO
bands in 1a supports a somewhat less upright orientation of this group in this film. 1633

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir Despite the overall complexity of the environment of the COOH and COO
groups, there is still an apparent preferred average orientation of the COO
head groups in all of these films. Alkyl Chain Organization and Orientation. Simultaneous consideration of mean molecular areas, ellipsometric thickness, and FT-IR spectra provides much insight into the alkyl chain organization in our LB films. Molecular areas around 20.4 Å2 for 2a are in excellent agreement with the literature.66 Considering the area required for a single chain, average molecular areas of 63 Å2 in films of 1a are consistent with what we would expect for all three chains of the molecule to be fixed in the bulk of the film. The slightly larger than expected areas per molecule in mixed films of 1a and 2a suggest that packing may be different and probably contains a fair amount of defects compared to that in single component films, a conclusion further supported by spectroscopic evidence discussed below. Ellipsometric thickness gives a first-order estimate of orientation in these films. The measured thickness of films of 1a, 2a, and mixed films (30, 27, and 28 Å, respectively) all fall within the expected ranges for fully extended, upright films. Previous measurements of stearate LB films by ellipsometry (using a refractive index of 1.51 for the film),67 AFM,68 and X-ray diffraction18 have all shown a thickness of 25.0
25.4 Å. Using a refractive index of 1.51 in our experiments on 2a gives a thickness of 26.0 ( 0.8 Å, identical to these previous determinations. Given the 26
27 Å length of fully extended calcium stearate, a measured thickness of 26 ( 0.8 Å would correspond to a film in which the alkyl chains were tilted on average between 0 and 21
with respect to the surface normal. FT-IR GATR spectra of 1a, 2a, and mixtures of the two on Si
SiO2 give a more detailed picture of alkyl chain organization and orientation. The C
H stretching frequencies of the asymmetric and symmetric CH2 bands in monolayers of 1a and 2a show highly ordered chains with a predominantly trans conformation.69 In the spectra of mixed films, the increased frequencies, line widths, and intensities of the CH2 bands all support an increase in the number of gauche interactions in the chains. The higher intensities could additionally signify a greater tilt angle of the alkyl chains, but the relative values of ellipsometric thickness do not support significantly different tilt angles. Assuming the increase in CH2 band frequencies, widths, and intensities in mixed films is due to an increased number of gauche interactions might suggest that 1a and 2a are truly mixed rather than phase-segregated into macroscopic domains. UV
vis absorption, which shows a reduction in aggregate formation with increasing amounts of 2a, supports this picture. We conclude that gauche defects introduced by mixing 1a and 2a account for some if not all of the greater disorder apparent in these films and that the mixed films are still highly oriented, based on the ellipsometric thickness. The GATR spectra yield a more quantitative picture of the alkyl chain orientation in some of these films. Their intensities provide a measure of the orientation of transition moments with respect to surface normal, and can be used to determine the orientation of the molecule with respect to the surface. Figure 6 shows an octadecyl chain tilted away from surface normal by an angle α, the coordinate systems chosen to define the laboratory and the molecular frameworks, and the directions of the C
H stretching transition moments in the molecular framework approximated from normal mode descriptions. As shown, the long axis of the chain is the z0 axis and the symmetric and asymmetric CH2 stretching band transition moments lie

ARTICLE

perpendicular to z0 , with the former along y0 , in the CCC plane, bisecting the HCH angle, and the latter along x0 , perpendicular to the CCC plane. For an all trans-chain, the CCC bond angles are approximately 112
. The symmetric CH3 stretching band transition moment lies along the C
CH3 bond, hence at ∼34
from the z0 axis. The asymmetric CH3 stretching band transition moment has two components, both perpendicular to the symmetric band moment, one at ∼56
from the z0 axis (∼34
from the y0 axis) and the other along x0 . The assumed transition moment directions may differ from the actual directions by several degrees and we account for this in an analysis of the uncertainty of the orientation angles between the molecular x0 y0 z0 axes and the Z axis. To solve for the orientation of the molecular framework in the laboratory framework, we assume a uniaxial distribution of orientations of the chains, an assumption we can test by the orientation angles determined for x0 and y0 . For each of the C
H stretching bands, eqs 1 and 2 give the following expressions: ISO 0 AGATR CH2as =ACH2as ¼ ðc=c ÞKx ISO 0 AGATR CH2s =ACH2s ¼ ðc=c ÞKy ISO 0 2 2 AGATR CH3s =ACH3s ¼ ðc=c Þ½Ky sin 34
þ Kz cos 34
þ 2Kyz cos 34
sin 34
 ISO 0 2 2 AGATR CH3as =ACH3as ¼ ðc=c Þ½Kx þ Ky cos 34
þKz sin 34
þ2Kyz cos 34
sin 34


ð4Þ where “as” refers to the asymmetric stretch and “s” to the symmetric stretch. Note that symmetry induces Kxy = Æcos x0 Z cos y0 Zæ = 0 and Kxz = Æcos x0 Z cos z0 Zæ = 0. The condition Kx + Ky + Kz = 1 permits an algebraic solution for the orientation factors K using a linear least-squares regression. The orientation analysis assumes the presence of a single conformer only and we can therefore only evaluate films in which the chains appear to be all trans, as in 1a and 2a. In the GATR spectrum of 1a, no well-defined bands with a component of the transition moment polarized along the z0 axis are present, and this excludes this film from analysis, too. Therefore, we restrict the quantitative analysis to the orientation of 2a and invoke the relative ellipsometric thickness to draw conclusions about the orientation of alkyl chains in films of 1a and in the mixed films. In the spectrum of 2a the C
H stretching bands overlap. In order to get reasonable absorption intensities, we fit the spectrum of 2a and also that of a polycrystalline isotropic sample of C20H42 in pressed KBr70 with eight Lorentzians, each based on literature assignments.70,71 The spectra with fitted curves are shown in the Supporting Information (Figure 1S). Intensities of the bands used in the orientation analysis were calculated as the band amplitude (or sum of the maximum amplitudes of all bands contributing to a particular band). The two bands due to a Fermi resonance, which have different polarization directions from the asymmetric CH2 band with which they overlap,71 have been excluded from the analysis. The regression gave: 0.00307 e Kx e 0.159, 0.00193 e Ky e 0.139, and 0.702 e Kz e 0.995. The average chain tilt angle, α, with respect to the Z axis, is therefore α ¼ cos
1 Kz 1=2 ¼ 19 ð( 15Þ


ð5Þ

The values of Kx and Ky correspond to average angles of 77 ((10)
and 78 ((10)
between the x0 and y0 axes with Z, respectively. We conclude that the assumption of a uniaxial distribution of transition moments about the z0 axis is reasonable. Given the predominantly trans conformation of the alkyl chains 1634

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir

ARTICLE

Table 3. Best Fit Orientation Factors, Ku, and Corresponding Average Angles, θu0 , of the Molecular u0 Axes (u0 = x0 , y0 , z0 , Chart 1) of Naphthalene with Respect to Surface Normal (Z axis) θx0

Kx

Ky

θy0

Kz

θz0

1a

0.42
0.58

45 ( 5

0.037
0.19

72 ( 7

0.38
0.39

52 ( 1

2a
1a (5.3:1)

0.36
0.52

48 ( 5

0.0069
0.19

75 ( 11

0.45
0.47

47 ( 1

in films of 2a, it is almost certain that the sample is actually composed of numerous small biaxial domains that are randomly oriented with respect to rotation about the surface normal to produce an overall uniaxial distribution when averaged over the whole sample. The average tilt angle of 2a is consistent with the ellipsometric thickness found by assuming a refractive index of 1.51 (see ref 67) and an average length per molecule of 27 Å, since 27 Å (cos 19
) = 26 Å. Using a refractive index of 1.51 to model 1a and mixed films of 1a and 2a, gives thicknesses of 29 ((3) Å and 27 ((1) Å, respectively. With the range of molecular lengths for 1a given in the Results section, the average tilt angle lies somewhere between 0 and 25 ( 15
. The mixed films, which all displayed a thickness between the values found for 1a and 2a, appear to be oriented similarly. Compared to a single chain, the three chain anchor we use appears to be similarly tilted, on average, with respect to surface normal. Naphthalene Ring Orientation. GATR spectra of 1a and mixed films, in conjunction with the isotropic spectrum of 1a in KBr and the DFT/B3LYP/6-31G(d,p) calculated transition moment directions for bands associated with the naphthalene ring, allow for a quantitative determination of the orientation of the naphthalene chromophore in the monolayer films. We use a similar analysis to that described above for 2a with the most reliable ring bands around 1590, 1505, and 775 cm
1 where the absorbances, Ai, were determined from the band intensities of the spectra shown in Figure 4D and listed in Table 2, and the angles were determined by projection of the calculated transition moments on the x0 , y0 , and z0 axes as shown in Chart 1. For the reliable ring bands, eqs 1 and 2 give the following expressions: ISO 0 AGATR 775 =A775 ¼ ðc=c ÞKx ISO 0 2 2 AGATR 1505 =A1505 ¼ ðc=c Þ½Kz cos 60
þ Ky sin 60
 ISO AGATR 1590 =A1590

0

ð6Þ

¼ ðc=c Þ½Kz cos 22
þ Ky sin 22
 2

2

where we have assumed negligible coupling of the vibrations and electronic excitation in the naphthalene rotator to the rest of the molecule. A 3  3 matrix results, allowing us to solve for Kx, Ky, and Kz using a linear least-squares regression. The best fit values for 1a and the 5.3:1 mixed film of 2a
1a are shown in Table 3. The uncertainty in the calculated transition moment directions is likely to be as high as (()10
61 and is probably the largest source of error in our determination of Ku and θu0 . A range of possible Ku values was estimated and used to determine the uncertainty in θu0 . If the terminal CC bond that attaches the naphthalene rotator to the anchor had the same (average) direction as the alkyl chains, we would expect values of θx0 close to 90
, θz0 close to the chain tilt angle of 0
25 ( 15
, and θy0 around 65
90
. Only θy has the anticipated value. The value of θx0 = 45 ( 5
observed in the film of 1a (or 48 ( 5
in the 5.3:1 mixed film) and the values obtained for θz0 (52 ( 1
, or 47 ( 1
in the 5.3:1 mixed film) show that the ring plane of naphthalene tilts about half way toward the surface. The molecular mechanics geometry of a

1:5 ratio of a 1:2 film shown in Figure 5 illustrates this tilting quite clearly. In comparison to the IR results, the data obtained in the UV
vis are much less revealing, but we can get a qualitative picture of the orientation from the calculated electronic transition moment directions in 4, measurement of the excitation anisotropy of 3, and absorption of monolayer films containing the rotor. Calculations suggest that the two relatively intense transitions in the absorption spectrum at 224 and 282 nm are polarized along the y0 and z0 axes, respectively (Chart 1), in agreement with the literature for naphthalene.27 The excitation anisotropy of the second of these bands varies between ∼-0.1 and ∼0, giving a 55
66
angle (eq 3) with the emission transition dipole at 318 nm. It is safe to assume that the transition moment for the band at 282 nm is polarized much closer to the z0 than the y0 axis of naphthalene. With our experimental setup for UV
vis absorption measurements of rotor films, in which the exciting light is perpendicular to the substrate, we selectively excite transition moments parallel to the plane of the substrate. Normalized to the 224 nm band, the monolayer film spectra clearly show a decrease in intensity of the 282 nm band relative to the solution spectrum, indicating that the ring is tilted, and that the molecular z0 axis lies closer to surface normal than the y0 axis, consistent with the IR results. Molecular mechanics calculations on a mixed film of 1 and 2 show a behavior similar to that observed in the IR and UV
vis spectra of mixed films of 1a and 2a. All of the rotor molecules in Figure 5 show significant tilting of the z0 axis of naphthalene from the chain axis and many show the ring plane tilted toward the film surface. We did not account for the presence of Ca2+ or COO
groups in these calculations, but unpublished results show that unless Ca2+ groups are constrained at the surface (which is possible due to attraction to the underlying water layers expected on SiO2, but not necessarily the case), the COO
and/or COOH mounting groups of the anchor may move upward to accommodate other geometrical needs of the system. Molecular mechanics results also show that rotation of the naphthalene group in a film with this degree of tilting would be seriously hindered. The UV
vis absorption spectrum gives additional insight into the nature of organization in films containing 1a. Red-shifting and broadening of the 224 nm absorption band in the spectrum of 1a with respect to the solution spectrum of 1 are consistent with an end-on type interaction72 between adjacent naphthalene rings in 1a. At ∼6 Å long, the naphthalene unit is effectively larger than the average diameter of the chains (5
5.3 Å), so end-on physical dimer formation is plausible. Other, more stable dimer conformations, in which the ring planes of adjacent molecules lie parallel to one another, are not likely given the small (∼3.6 Å) intermolecular distances required to form them.73 As films become increasingly dilute with 1a, the 224 nm band is no longer red-shifted with respect to the solution spectrum and its bandwidth decreases to that seen in the solution spectrum. This supports the formation of homogeneous, mixed films as suggested by the IR and ellipsometry results. 1635

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir

’ CONCLUSIONS We have made highly oriented monolayer LB films of stearate and a naphthalene-carrying molecular rotor, and homogeneous mixed films of the two, in the presence of Ca2+. In both pure and mixed films containing the naphthalene rotor, the anchor which comprises three fatty acid chains that attach the rotator to the surface was found to have average tilt angles between 0 and 25 ((15)
with respect to the surface normal, similar to the tilt angles observed in monolayers of single alkyl chains. The naphthalene ring plane preferentially orients at 45
48
on average from the surface. While rotation of the naphthalene group is anticipated to be strongly hindered at this angle, it would probably still be possible at room temperature. The results suggest that a preparation of freely rotating surface-mounted molecular rotors could be achieved by insertion of a rigid axle that elevates the naphthalene by a few angstroms. ’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed descriptions of the synthesis and the experimental and calculated vibrational transitions. More information regarding the orientation determination and various experimental and computational methods. Complete ref 43. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT We are grateful for DOE support of this project through Grant DE-FG02-08ER15959. ’ REFERENCES (1) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. Rev. 2005, 105, 1281. (2) Michl, J.; Sykes, E. C. H. ACS Nano 2009, 3, 1042. (3) Browne, W. R.; Feringa, B. L. Annu. Rev. Phys. Chem. 2009, 60, 407. (4) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72. (5) Garcia-Garibay, M. A. Nat. Mater. 2008, 7, 431. (6) Gimzewski, J. K.; Joachim, C.; Schlitter, R. R.; Langlais, V.; Tang, H.; Johannsen, I. Science 1998, 281, 531. (7) Hersam, M. C.; Guisinger, N. P.; Lyding, J. W. Nanotechnology 2000, 11, 70. (8) Clarke, L. I.; Horinek, D.; Kottas, G. S.; Varaksa, N.; Magnera, T. F.; Hinderer, T. P.; Horansky, R. D.; Michl, J.; Price, J. C. Nanotechnology 2002, 13, 533. (9) Jian, H.; Tour, J. M. J. Org. Chem. 2003, 68, 5091. (10) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540. (11) Mulcahy, M. E.; Magnera, T. F.; Michl, J. J. Phys. Chem. C 2009, 113, 20698. (12) Vacek, J.; Michl, J. New J. Chem. 1997, 21, 1259. (13) Vacek, J.; Michl, J. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 5481. (14) Horinek, D.; Michl, J. J. Am. Chem. Soc. 2003, 125, 11900. (15) Hou, S.; Sagara, T.; Xu, D.; Kelly, T. R.; Ganz, E. Nanotechnology 2003, 14, 566. (16) Fendrich, M.; Wagner, Th.; Stohr, M.; Moller, R. Phys. Rev. B 2006, 73, 115433. (17) Casher, D. L.; Kobr, L.; Michl, J. J. Phys. Chem. A 2011, 11167–11178. (18) Malik, A.; Durbin, M. K.; Richter, A. G.; Huang, K. G.; Dutta, P. Thin Solid Films 1996, 284/285, 144. (19) Malik, A.; Durbin, M. K.; Richter, A. G.; Huang, K. G.; Dutta, P. Phys. Rev. B 1995, 52, R11654.

ARTICLE

(20) Ahn, D. J.; Franses, E. I. J. Phys. Chem. 1992, 96, 9952. (21) Jang, W.-H.; Miller, J. D. J. Phys. Chem. 1995, 99, 10272. (22) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (23) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (24) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. (25) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1987, 91, 3572. (26) Platt, J. R. J. Chem. Phys. 1949, 17, 484. (27) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (28) McClure, D. S. J. Chem. Phys. 1954, 22, 1668. (29) Davidsson, Å.; Nord
en, B. Chem. Phys. Lett. 1974, 28, 221. (30) Kouteck
y, J. J. Chem. Phys. 1967, 47, 1501. (31) Berden, G.; Meerts, W. L.; Plusquellic, D. F.; Fujita, I.; Pratt, D. W. J. Chem. Phys. 1996, 104, 3935. (32) Mulcahy, M. E.; Berets, S. L.; Milosevic, M.; Michl, J. J. Phys. Chem. B 2004, 108, 1519. (33) Milosevic, M.; Milosevic, V.; Berets, S. L. Appl. Spectrosc. 2007, 61, 530. (34) Tsuji, K.; Yamada, H. J. Phys. Chem. 1972, 76, 260. (35) Pinkley, L. W.; Sethna, P. P.; Williams, D. J. Phys. Chem. 1978, 82, 1532. (36) S- and p-polarized contributions to the IR absorption of a 26.2 Å thick film were calculated for a range of refractive indices, n = 1.4
1.52, and extinction coefficients, k = 0.01
0.15, based on a 45:1 mol ratio of CH2:naphthalene groups as in 1 at 2940, 1000, and 775 cm
1, assuming a 65
incident angle of the exciting light with nSi = 3.85 and nGe = 4. The equations used may be found in Milosevic, M.; Berets, S. L. Appl. Spectrosc. 1993, 47, 566 and in Appendix B of Mulcahy, M. E. The Deposition and Characterization of Molecular-Sized Rotors on Gold and Glass Surfaces. Ph.D. Dissertation; University of Colorado at Boulder, 2006. (37) Michl, J.; Thulstrup, E. W. Spectroscopy With Polarized Light; VCH Publishers, Inc.: New York, 1995. (38) Handy, R. M.; Scala, L. C. J. Electrochem. Soc. 1966, 2, 109. (39) McDermott, M. N.; Novick, R. J. Opt. Soc. 1961, 51, 1008. (40) Frisch, M. J., et al. Gaussian 98, revision A.6; Gaussian, Inc.: Pittsburgh, PA, 1998. (41) Ridley, J.; Zerner, M. Theor. Chim. Acta 1973, 32, 111. (42) Rapp
e, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (43) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33. (44) Corriu, R. J. P.; Poirier, M.; Royo, G. C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Sci. Terre Univers 1990, 310, 1337. (45) Harding, M. M. Acta Crystallogr. 1999, D55, 1432. (46) Czekalla, J.; Liptay, W.; D€ollefeld, E. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 80. (47) Hollas, J. M.; Thakur, S. N. Mol. Phys. 1974, 27, 1001. (48) Socrates, G. Infrared Characteristic Group Frequencies, 2nd ed.; John Wiley & Sons: New York, 1994. (49) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; John Wiley & Sons: New York, 1975; Vol. 1. (50) Michaelian, K. H.; Ziegler, S. M. Appl. Spectrosc. 1973, 27, 13. (51) Stein, R. S.; Sutherland, G. B. B. M. J. Chem. Phys. 1953, 21, 370. (52) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85. (53) Friedman, H. M. Appl. Spectrosc. 1970, 24, 44. (54) Olsen, J. E.; Shimura, F. J. Appl. Phys. 1989, 66, 1353. (55) Tao, Y.-T.; Lee, M.-T.; Chang, S.-C. J. Am. Chem. Soc. 1993, 115, 9547. (56) Wilberly, S. E.; Gonzalez, R. D. Appl. Spectrosc. 1961, 15, 174. (57) Song, Y. P.; Petty, M. C.; Taywood, J. Langmuir 1992, 8, 257. (58) El-Azhary, A. A.; Suter, H. U. J. Phys. Chem. 1996, 100, 15056. (59) Rauhut, G.; Pulay, P. J. Phys. Chem. 1995, 99, 3093. (60) Langhoff, S. R. J. Phys. Chem. 1996, 100, 2819. (61) Radziszewski, J. G.; Downing, J. W.; Gudipati, M. S.; Balaji, V.; Thulstrup, E. W.; Michl, J. J. Am. Chem. Soc. 1996, 118, 10275. 1636

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637

Langmuir

ARTICLE

(62) Gericke, A.; Huhnerfuss, H. Thin Solid Films 1994, 245, 74. (63) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. (64) Tackett, J. E. Appl. Spectrosc. 1989, 43, 483. (65) Rabolt, J. F.; Burns, F. C.; Sclotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (66) Gaines, G. L. Insoluble Monolayers at Liquid Gas Interfaces; Interscience: New York, 1966. (67) Lee, S.; Virtanen, J. A.; Virtanen, S. A.; Penner, R. M. Langmuir 1992, 8, 1243. (68) Costa, N.; Aindow, M.; Marquis, P. M. Mater. Res. Soc. Symp. Proc. 1994, 351, 97. (69) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (70) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (71) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (72) Scholes, G. D.; Turner, G. O.; Ghiggino, K. P.; Paddon-Row, M. N.; Piet, J. J.; Schuddeboom, W.; Warman, J. M. Chem. Phys. Lett. 1998, 292, 601. (73) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikamia, M. J. Chem. Phys. 2004, 120, 647.

1637

dx.doi.org/10.1021/la2037789 |Langmuir 2012, 28, 1625–1637