J. Phys. Chem. 1995, 99, 2143-2150
2143
Molecular Orientation at Dielectric Surfaces by Angle-Resolved Photoacoustic Spectroscopy Susan K. Doughty and Kathy L. Rowlen” Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 Received: September 13, 1994; In Final Form: November 21, 1994@
Angle-resolved photoacoustic spectroscopy (ARPAS) has been developed as a sensitive (sub-monolayer detection) and nearly universal method to study molecular orientation on dielectric surfaces or in thin films. ARPAS is based on the photoacoustic signal generated as the angle between incident pulsed, plane-polarized light and the sample is vaned. The theory required to describe the angular dependence of the acoustic signal, experimental details, and two comparative molecular orientation studies are presented herein. Specifically, the orientation of 1,4-bis(2-methylstyryl)benzene partitioned into a chromatographic alkyl phase was investigated as a function of solvent overlayer. ARPAS yielded approximately the same average angles of orientation as previously determined by fluorescence anisotropy. The orientation of dimethylphenylsilane covalently bound to quartz was also investigated. A value of 71’ f 3’ with respect to the surface normal was obtained for the transition centered near 260 nm. This value provides supporting evidence that the phenyl ring is tilted -70’ with respect to the surface normal.
Introduction
“tilted plate” method in which the sample plane is tilted with respect to the electric field vector. The main advantage of Oriented molecular systems, particularly thin organic films, absorbance LD is its broad applicability; the molecule need only are of great interest in a variety of research For absorb light and contain reasonably well-defined transition example, oriented azo dyes exhibit nonlinear optical properties4 dip01es.l~ Although there are a few exception^,'^ the general and have potential applications in the field of optoelectr~nics.~,~ limitation to absorbance LD is sensitivity, which is often The average angle of molecular orientation with respect to a insufficient for monolayer detection. For example, a new LD given axis and the degree of molecular order in thin films are technique, polarization-modulation microscopy, has recently key factors in developing thin film technology. While there been demonstrated for the study of molecular order in LB films are several spectroscopic techniques currently employed for the of phthalocyanine dyes.16 The technique is limited to investigastudy of molecular orientation in thin films, most are limited tion of two or more LB layers of a strongly absorbing dye. by either inadequate sensitivity or by stringent substrate Fluorescence LD offers adequate sensitivity but is limited to requirements. For the study of alkyl self-assembled monolayers fluorescent molecules. In addition, energy transfer can be and Langmuir-Blodgett (LB) films, Fourier transform infrared problematic in fluorescence studies of molecular orientation. spectroscopy (FTIR) is the most widely applied spectroscopic SHG is particularly useful for direct measurement of molecular technique?-’ FTIR provides vibrational information useful for orientation and nonlinear optical properties of interfacial investigation of both orientation and intermolecular interactions. systems.’*-** The largest drawback to SHG is the requirement FTIR of thin organic films is typically conducted in either of a tensorial description of the molecule’s polarizability;2 the attenuated total reflection (ATR) or grazing angle mode.6 The lack of such information often leads to only approximate primary limitation of ATR is that the sample must be placed in determinations of molecular orientation. contact with, or derivatized onto, an ATR crystal (Si, Ge, or The new technique presented here, angle-resolved photoZnSe). Grazing angle FTIR is particularly useful for probing acoustic spectroscopy (ARPAS), is based on the absorbance of vibrations with transition dipoles normal to a reflective (usually plane-polarized light as the angle between the incident electric metal) surface. The main limitation of grazing angle FTIR is field vector is varied with respect to molecular transition dipoles the necessity of a reflective surface. Surface-enhanced Raman oriented on a flat surface. Generation of excited states with spectroscopy (SERS) also provides detailed vibrational informapulsed excitation and subsequent nonradiative decay results in tion and can be used as a qualitative probe of orientation, but an acoustic wave. The amplitude of a photoinduced acoustic is limited by the requirement of a roughened metal surface.12 wave is In systems which contain chromophores with transitions in the ultraviolet (UV) and visible (vis) regions of the spectrum, S=- KPvJoa (1) such as.oriented dyes, it is important to determine the orientation QCP of electronic transition dipoles with respect to a laboratory axis (usually normal to the surface). Arguably, the three predominant where S is the acoustic signal amplitude, K is the instrument techniques employed for these systems are linear dichroism function, /3 is the volumetric expansion coefficient, va is the (LD), polarized luminescence, and second-harmonic generation acoustic velocity in the medium of interest, l o is the excitation (SHG). In absorbance LD, the ratio of absorbance for parallel intensity, a is the absorptivity (cm-I), 8 is the density, and C, and perpendicular polarized light is measured. Mathematical is the heat capacity of the medium.23 Since the magnitude of treatment of this ratio yields information about molecular the photoacoustic wave is directly proportional to the intensity orientation in the plane of the film.13 In order to probe transition of excitation and sample absorbance, photoacoustic spectroscopy dipoles normal to the film plane, Norden et al.14 developed a offers the combined advantages of sensitivity equivalent to that of fluorescence and the near universal utility and simplicity of Abstract published in Advance ACS Abstracts, February 1, 1995. absorbance detection. In addition, since the photoacoustic sig-
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0022-365419512099-2143$09.00/0
0 1995 American Chemical Society
Doughty and Rowlen
2144 J. Phys. Chem., Vol. 99, No. 7, 1995
A
rotating
Glan prism Nd:YAG Laser
r?-j
~. .........., F ........!:::
.'.?? .......... I
B
A ot-
\
cell support
preamplifier
oscilloscope
Figure 1. (a) Side view of the experimental apparatus. (b) Side view of the sample cell.
nal depends only on the amount of light absorbed, it is insensitive to scattered light,23 which can be troublesome in many spectroscopic techniques. Here, the theory required to describe the angular dependence of the acoustic signal is developed, details of the experiment are provided, and molecular orientation angles are examined for two systems. Specifically, the two molecular systems investigated are (1) the orientation of a single layer of phenylsilane covalently bound to quartz15 and (2) the effect of solvent on the orientation of an alkyl chromatographic stationary Theory The probability of photon absorbance by a molecule is proportional to the square of the molecule's transition dipole projected onto the electric vector of the electromagnetic field. For a pure transition, the probability (p) in terms of angle between the two vectors is
where p is the transition moment and 4 is the angle between the transition dipole vector and the electric field vector.13 For a molecule fixed in a coordinate system, the orientation of a given transition dipole within that coordinate system can be accessed by varying the angle of linearly polarized light with respect to the transition dipole. For a molecule oriented on a flat, transparent surface in a sample cell (see Figure lB), changes in the incident electric field as it traverses the sample cell can be accounted for by Fresnel's laws of reflection. For parallel polarized light incident on a transparent dielectric (e.g., quartz),
the intensity of light refracted into the cell as a function of incident angle is given by tan2(Oi - e,) 1-
tan2(Oi
+ e,)
(3)
where t9i and 8, are the angles of incidence and refraction, respectively.26 This term, which has a value of approximately 1 for incident angles 160, also takes into account the change in beam area as a function of incident angle. As the angle of incidence at the top surface is varied, the angle of the electric field vector with respect to the surface, or surface normal, can be obtained via Snell's law and straightforward geometrical analysis (the assumption is made that the refractive index of a monolayer covered by solvent is the same as that of the bulk solvent). The angle of a pure molecular transition with respect to the surface plane is defined as Om. The angle of the electric field vector with respect to the surface plane (x,y-plane) is defined as Be. Thus, for a single molecule oriented 30" from the surface plane, for example, the absorbance would progress from an initial nonzero value through a maximum at 8, = 30" (i.e., 8, = e,,.,). For the molecular systems presented in this work, however, alignment of the transition dipoles with respect to the z-axis occurs with equal probability in the x,y-plane (i.e., uniaxial). The angle y is defined in Figure 2 as the angle of the transition dipole with respect to the y-axis (in the x,y-plane). The effect of orientational distribution of the transition dipole on the angular absorbance signal is derived here by projecting the molecular transition dipole vector onto the electric field vector. Utilizing the angles shown in Figure 2, the magnitude
J. Phys. Chem., Vol. 99, No. 7,1995 2145
Molecular Orientation at Dielectric Surfaces
molecule in which the principal axes of the molecule (x, y , z) are known, eq 4 can be simplified using "orientation factor" n0tati0n.l~ From literature based on the tilted plate method of linear dichroism, the absorbance (A,) of a uniaxial sample as a function of the electric field angle is given by the sum of two independent components:
A, = A, sin2
e, + A, cos2 e,
(5)
or
-Y
L
/ X
Figure 2. Molecular orientation in a nonrigid system. The molecule is assumed to be bound to the surface at only one point, with free rotation about that point. The angles of Oe and 8, are defined in the text. The angle y is defined with respect to the y-axis. 0.8
0.7
0.5
8
0.4
K
- ~ ~cos2 ( e, 3 - 2)1
(7)
Note that Ki = 1/3 is the magic angle condition such that the amplitude is independent of excitation angle and A, = '/31p1*. Also, at cos2 8, = 213 (Le., 8, = 35.3"), A, = '/31pI2. For a pure transition moment that lies along a principal molecular axis, eqs 4 and 7 yield identical results.
0.6
8
+ (1 - K,)F,} cos2 8, (6)
where Fj = Zlp12cos2 8,j, which is the sum over all transitions which contribute to absorbance at the excitation wavelength, Ki = (cos2 &,), 8i, being the angle between the molecular axis (i) and the laboratory z-axis (normal to the surface). Assuming that only one transition contributes to absorbance at the excitation wavelength and that the transition lies along a molecular orientation axis, eq 6 simplifies toI3
1 A, = p 2 [ c 0 s 2e,
0.9
8
(1 - K,)F,
0.3 0.2
Experimental Section
0.1
Instrument Design. A schematic of the instrument is shown in Figure 1A. A Q-switched Nd:YAG laser (Spectra Physics GCR- 11) with frequency doubling and mixing crystals provided three UV-vis excitation lines (266, 355, 532 nm). Average power at 532 nm (-15 mW) was measured with a calibrated Scientech 380101 power meter. Average power at 266 and 355 nm (-15 mW) was measured with a calibrated Scientech 360001 power meter. The repetition rate was 10 Hz with a pulse width of 8 ns. Although the polarization ratio is quoted as 100:1 from the manufacturer, the excitation beam was passed through a Glan laser prism (Melles Griot 03PGL301) to ensure a high degree of polarization. For the same reason, the excitation beam was not focused. For all of the results presented herein, parallel polarized light was used for excitation. As shown in Figure lB, the sample cell consisted of two quartz flats (A1 or SlUV, 1/16 in. thick, 1 in. diameter, ESCO Products, Oak Ridge, NJ). A fused quartz microscope slide (ESCO Products) was used to physically support the cell as well as to couple the cell to the detector. A piezoelectric transducer ( E T , Panametrics V103, 0.5 or 1 in. diameter, 1 MHz resonance) served as the acoustic detector. The PZT was rigidly affixed to the bottom of the quartz microscope slide with a cyanoacrylic adhesive (Superglue). The cell was held in place on top of the quartz support with a precise volume of coupling liquid (typically, 10 p L of water). The molecule of interest was derivatized onto the top surface of the bottom flat, as indicated in Figure 1B. The two flats were held together by surface tension from a 10 pL volume of coupling fluid. The fluid is essential to ensure adequate acoustic energy transfer. With water as the coupling fluid the path length through the cell was -20 pm. The PZT was connected directly to a preamplifier (Panametrics 5660B, 0.02-2 MHz, 40 or 60 dB) with a 90" BNC union.
0 0
5
10152025m3S4045 Angle of ElectricVector
Figure 3. Simulated photoacoustic angle response. While the angle of incidence can be varied from 0" to 90".due to refractive index effects, the angle of the electric vector (ee,as defined with respect to the surface plane) does not exceed -47" (Snell's Law). In this simulation, 8, is vaned from 1" to 89" (lo, lo", 20°, 30°, 35.3", 40°, 50°, 60",70". 80", and 89"). The dashed line is for 8, = 35.3" with respect to the surface plane (unoriented system). of the transition dipole along the electric field vector is
where ppmj is the magnitude of the projected moment. The angular response is obtained for a given molecular transition dipole orientation, Om, by integrating y from 0 to 2n at each value of 8,. The magnitude of ppmj is plotted in Figure 3 as a function of both excitation angle and dipole orientation (taking into account eq 3; see eq 8). Note that the signals all converge at 35.3" from the surface plane, or 54.7" (the magic angle) from the z-axis. As expected, for a molecular system oriented at 54.7" with respect to a laboratory-defined z-axis (surface normal) the signal has no angular dependence; the same is true for an isotropic solution. If the angle (8,i) between the transition dipole (m) and a molecular axis (i = x, y , or z) is known, determination of 8, leads directly to molecular orientation. For a symmetrical
2146 J. Phys. Chem., Vol. 99, No. 7, 1995
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Doughty and Rowlen
0.02
i
E0, e 0.015
c
s"8
0.01
0
f
0.005
1 t
0 0
1
0
2
0
3
0
4
0
5
0
Time bsec)
Figure 4. Photoacoustic wave form from 0 to 50 ys. The darker trace is the photoacoustic signal for a water blank. The lighter trace (large amplitude) is the signal for a solution of Rose Bengal (5 pL of 2 x M). Note that the signal from light scattered directly onto the
PZT appears at times less than 5 ys.
The cell, PZT, and preamplifier were mounted on an optical rail. The laser beam was directed via a mirror/rotation stage arrangement above the cell. The angle of incidence on the cell was varied by sliding the optical rail mount and adjusting the rotatable mirror accordingly. The entire instrument was constructed on an air-suspended laser table (Newport type XL-A). Data Processing. Once amplified, the signal was collected and digitized with a Hewlett-Packard 54501A digitizing oscilloscope. The Q-switch sync-out from the laser was used to trigger the oscilloscope. Signal was acquired over a specified time period after each pulse, typically 50 ,us. Signal averaging of 100 pulses was employed to remove random noise. Lab Windows (National Instruments) software was used to configure the scope and to transfer ASCII data files to a computer (386, Advanced Logic Research PowerFlex 205x, Irvine, CA) over an IEEE interface. A representative photoacoustic wave form is shown in Figure 4. Typically the amplitude of the first positive excursion is reported. However, on the basis of a detailed study of both the time and frequency domain, integration of the entire photoacoustic wave form is necessary to prevent loss of i n f ~ r m a t i o n .In ~ ~this study, data were processed using a custom-designed program with a user-defined integration range. After normalization of the dc offset, the wave form was typically integrated over the time period of 5-50 ,us. Due to the relatively long travel time of the acoustic wave, contributions from light scattered directly onto the PZT were easily avoided by not integrating the signal at times less than 5 ,us. The integrated wave form yielded the corresponding amplitude at a given excitation angle of incidence. The amplitude as a function of incident angle was fit, via nonlinear regression (Levenberg-Marquardt algorithm)28to the following form:
where a is a weighting factor which includes the normalized photoacoustic signal described in eq 1. In order to test the accuracy of the fit, multiple initial parameters (a and 6,) were used for each experimental data set. From x2 minimization,
the average relative error in the value of Q for the phenylsilane data set was 1%, and the average relative error in 8, was 4%. Slide Preparation. Silanization of quartz slides with chlorodimethylphenylsilane was carried out in a glovebag under a nitrogen (USP grade) atmosphere. The quartz slides were precleaned by soaking in three consecutive concentrated sulfuric acid baths for 3 min each. Next, the slides were immersed in a 4:l (v/v) NH40WH202 solution for 3 min. The slides were then rinsed in deionized water and dried with NZ (USP). Silanization was performed by exposing the slide to either neat chlorodimethylphenylsilane (99%, Aldrich Chemical Co., Milwaukee, WI) or a toluene mixture of the silane for approximately 10 min. The slides were then rinsed sequentially with dichloromethane, toluene, and acetone. Derivatization of quartz slides with dimethyloctadecylchlorosilane (C18, Huls America, Inc., Piscataway, NJ) was performed using standard procedure^.^^ The slides were first cleaned in 70:30 v/v H2S04/H202 until bubbling of the solution ceased. The slides were then immersed in hot concentrated nitric acid for 5 min. The slides were rinsed with distilled, deionized water after removal from each cleaning solution. The reaction was conducted in a glovebag under a nitrogen atmosphere with hexadecane (Aldrich, 99%) as the solvent and pyridine (Mallinckrodt, analytical reagent) as the catalyst. Dimethyloctadecylchlorosilane was added to the solvent, and the reaction mixture, including the slides, was heated for 3 h. After removal and sequential rinsing with toluene, hexane, and methanol, the surface of each slide was hydrophobic, as determined by a large contact angle. The slides were end-capped with chlorotrimethylsilane (Aldrich, 98%) using a hexadecane reflux and pyridine catalyst for 30 min. The slides were then rinsed as described above. Angle-Resolved Measurements. Angle-resolved measurements were performed using 266 nm excitation for dimethylphenylsilane and 355 nm for the C18 surfaces. The control surface for the chlorodimethylphenylsilane-derivatized slide was the same slide prior to derivatization. Water was employed as the coupling fluid. The control surface for the bis-MSBK18 experiments was a C18-derivatized slide immersed in methanol for 15 min and in the solvent of interest for 15 min. Saturated aqueous solutions of 1-heptanol (98%, Aldrich), 1-octanol (99+%, Aldrich), and 1-decanol (99%, Aldrich) were equilibrated for at least 30 min before use. ,In order to incorporate the probe molecule, the C18-derivatized slide was then immersed in a M solution of 1,4-bis(2-methylstyryl)benzene (bis-MSB, Aldrich, 99%) in methanol (Mallinckrodt HPLC grade) for 15 min. Excess bis-MSB was removed with successive methanol rinsing. The slide was then immersed in the solvent of interest for 15 min before the angle-resolved data were taken. The solvent of interest served as the coupling liquid.
Results and Discussion Calibration. Instrument response was fust characterized with an isotropic dye solution. Rose Bengal was chosen as the dye because it has significant extinction at all three excitation wavelengths; the absorbance spectrum of Rose Bengal in water is shown in Figure 5. As mentioned in the Introduction, the photoacoustic amplitude should be directly proportional to both laser intensity and molecular absorptivity. The signals as a function of laser power at the three excitation wavelengths are also shown in Figure 5. The photoacoustic signal is linear over the entire concentration range investigated in this study ( to lo-* M). For this range, linear regression to the concentration calibration data yields a slope of 4.6 (f0.14) x lo3, R = 0.996. The detection limit for this experimental system is determined by background photoacoustic signal from the quartz cell and
Molecular Orientation at Dielectric Surfaces
J. Phys. Chem., Vol. 99, No. 7, 1995 2147
0.22
0.17
0.12
190
290
590
490
390 Wavelength (nm)
Figure 5. Absorbance spectrum of 4.32 x M Rose Bengal in a 1 cm2cuvette. Inset shows Rose Bengal (10 p L of a 4.32 x M solution) photoacoustic signal for 532, 355, and 266 nm excitation. From linear regression of the data for each wavelength, the following results were obtained: R = 0.99, y = 0.028 0.039 (f0.0033) mW; R = 0.98, y = -0.011 0.012 (f0.0019) mW; and R = 0.97, y = -0.15 0.026 (f0.0032) mW for 532,355, and 266 nm, respectively. Note that the slope of each linear plot is proportional to Rose Bengal's extinction at the excitation wavelength. The reported laser power is measured prior to sample incidence. Since 266 nm light is absorbed most strongly by the sample cell, the nonzero intercept of the regression for 266 nm excitation is attributed to noise-dominated response below 20 mW.
+
+
+
TABLE 1: Amplitudes and Orientation Angles for Isotropic Solutione Rose Bengal (mM) 0.0
0.024 0.073 0.24 0.52
amplitude 0.0025 f 0.000 0.0035 i0.0001 0.0058 f 0.0002 0.0167 f 0.002 0.034 i0.005
em
53" f 0.4" 58' f 1" 56" f 2' 56" f 7" 58" f 9"
Angles are defmed with respect to the surface normal. The values reported are the mean and standard deviation from three measurements for 0.024,0.24, and OS2 mM concentrations. Two measurements were used for 0.0 and 0.073 mM concentrations. The amplitude parameter R = 0.999. is linear with concentration;y = 0.002 0.0614(f0.0012)~,
+
support slide. Although there is still room for improvement, the signal-to-background ratio is 1.4 for 10 ,uL of a M solution of Rose Bengal excited at 532 nm. Assuming the molecular radius of Rose Bengal is -5 A, the area of one molecule would be 78.5 A2. Using a 1 in. diameter flat, one monolayer of Rose Bengal on the surface would be approximately 1 nmol. On the basis of these estimates and the experimentally determined limit of detection, the effective coverage detection limit for the ARPAS instrument described here is -0.1 monolayer of Rose Bengal. Angle-Resolved Measurements. The angular response for both the background (quartz/water/quartz) and the anisotropic solution (quartzRose Bengalaq/quartz) should be flat, i.e., the magic angle response. Indeed, the angular response of the photoacoustic amplitude for millimolar solutions of Rose Bengal in water yields 8, values close to the magic angle (see Table 1). The slightly elevated orientation angles for aqueous Rose Bengal solutions (+2") are attributed to Rose Bengal's high affinity for glass, which could result in a partially oriented system. In order to extract meaningful results from the angular response for molecules covalently bound and oriented at a surface, a reasonably flat surface must be employed.29 The
quartz slides utilized in this study were investigated by atomic force microscopy (AFM,Nanoscope E, Digital Instruments). Both nanometer and micrometer scale images of S 1 quartz are displayed in Figure 6. Quantitative analysis of nanometer scale micrographs indicates that the surface features are on the order of 1.7 (f0.8) 8, in height (68 measurements) and 3.5 (i1.6) A in length (47 measurements), with a "valley" width of 3.1 ( f l ) 8, (37 measurements); thus, on a molecular scale the surface is reasonably flat. The geometry of the valleys is such that a C18 molecule is most likely to bind to a surface that is parallel to the macroscopic surface plane. As observed in Figure 6, there are a few larger scale features ("dips and stacks") that may influence angular measurements; however, these are thought to be negligible.29 The first oriented molecular system investigated was dimethylphenylsilane covalently bound to a quartz slide. This molecular system has been previously studied by both linear dichroism15 and nuclear magnetic resonance.30 Utilizing the conclusions of Slotfeldt-Ellingsen and Resing with respect to bond geometry,30and assuming a tetrahedral configuration of the Si atom, Cropek and Bohn predicted the angle of orientation between the long axis of the phenyl ring and the surface normal to be 70.5", as shown in Figure 7. They assumed that the dipole moment of the n to n* transition near 260 nm, shown in Figure 7, lies primarily along the Si-C,m bond axis.15 Cropek and Bohn, utilizing "indirect" waveguide linear dichroism, with excitation at 35 1 nm, experimentally determined the molecular orientation angle to be 61" with respect to the surface normal. However, it is not clear that the 260 nm transition in dimethylphenylsilane is along the long axis of the molecule; in benzene the transition at 260 nm has been attributed to the forbidden 'A[, 1B2u.31 For a monosubstituted benzene with CzV symmetry, the 0,O transition excited state would be B1, with a transition moment along the short axis.32 It is quite possible that the band centered near 260 nm is not pure and represents a mixture of transitions polarized perpendicular and parallel to the long axis. In such a case, the angular dependence represents a combination of transition moments, and the molecular orientation can only be extracted if the relative amplitudes of the transitions, and their orientation with respect to the molecular axes, are known. The photoacoustic amplitude for a single layer of phenylsilane (based on a 3 pmol/m2 concentration of reactive sites, a single layer of phenylsilane would be -0.02 of a close-packed monolayer) as a function of excitation angle is presented in Figure 8. The control (quartz/water/quartz) angular response is also shown. Fitting the angular response of phenylsilane to eq 8 yields a value of 19" i 3" for 8, (from 20 measurements over a period of 5 months), or 71" with respect to the surface normal. This value compares quite well with the predicted value (70.5") and reasonably well with the previously measured angle (61"). However, as is likely, if 266 nm excitation probes a short axis transition, interpretation of 8, is not straightforward. Using NMR, Slotfeldt-Ellingsen and Resing observed the plane of the phenyl ring in phenylsilane bound to Si02 to be fixed near 65" with respect to the plane of the 0-Si-C bond.30 Such orientation places the short axis transition (B1) at 65" with respect to the laboratory z-axis. This is a special case, since both short axis and long axis (vibronically coupled) transitions would yield an orientation vector near 70" with respect to the z-axis. A detailed investigation of the polarization of the 260 nm transition in phenylsilane is required for absolute determination of molecular orientation. Note in Figure 8, that the water blank yields an angular value of 56" f 3" from surface nor-
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2148 J. Phys. Chem., Vol. 99, No. 7, 1995
Doughty and Rowlen
Y
0-
w
nu
t
a 0
0
9 T
UU
Figure 6. Atomic force micrographs of S 1UV quartz flats. The z-axis is to scale in both micrographs. The top micrograph is over a 10 nm scan size, and the bottom micrograph shows a 5 p m scan size.
0.04
0.4
0.35 0.3
0.03
0.25
2
8 t
ga
0.2
0.15
0.02
2 a
0.1 0.05
0.01
0
[
t 5
10
15
20
25
30 35
40 45
Angb of Electric Vector
0 200
250
300
350
Wavelength (nm) Figure 7. Absorbance spectrum of a quartz flat derivatized with chlorodimethylphenylsilane. The spectrum was obtained on a HewlettPackard 8452A spectrophotometer. Inset shows the predicted orientation of dimethylphenylsilane on the surface. mal, or 34" with respect to the surface, close to the value predicted for an unoriented sample.
Figure 8. Photoacoustic angular response (266 nm excitation) for a single layer of dimethylphenylsilane (solid circles) on a quartz flat. The open circles represent the signal from the same slide prior to derivatization. For both cases, the symbols represent the average of three consecutive measurements; the error bars are the standard deviation of those points, and the solid line represents the nonlinear regression of the average data to eq 8. The possibility of photoinduced degradation was addressed by varying the order in which the angular response was obtained. The experiment was carried out by rotating from low-to-high and high-to-low angles of incidence. No systematic differences
Molecular Orientation at Dielectric Surfaces
0.2' 0
"
5
"
"
'
J. Phys. Chem., Vol. 99, No. 7, 1995 2149
'
I
1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5
Angle of Electric Vector
Figure 9. Photoacoustic angular response for water/C18/MSB (circles) and octanol(aq)/ClB/MSB (diamonds). The solid line through the points is the nonlinear regression.
TABLE 2: Comparison of ARPAS and Fluorescence Anisotropy Measurement of Average Angle of Orientation for Bis-MSB in C18 solvent fluorescence ARPAS runs 79.6" f 0.4" 78' f 4" 11 water 72.0" f 0.2" 64" f 3" 17 MeOH (aq) 69" f 3" 16 1-propanol(aq) 69.0" f 0.2" 54.8" f 0.2" 64" f 4" 12 1-heptanol (aq) 53.9" f 0.2" 56" f 2" 8 1-octanol (aq) 56" f 2" 8 1-decanol (aq) 52.9" f 0.2" a Angles are reported with respect to the surface normal. Aqueous solutions of methanol and propanol are 20 and 5% by volume, respectively. The remaining aqueous alcohol solutions are saturated. The values listed under the fluorescence column are data reported by Montgomery and Wi~th.2~ The errors reported for the angles determined by ARPAS are standard error of the mean; the number of experiments is listed in the final column. 0.8
I
were observed in the angular response, indicating no contribution from photobleaching. The amount of heat deposited in the system by the laser can be calculated using a heat capacity of 4.184 J/g "C for water, 10 Hz laser repetition rate, 10 p L of water, and an average laser power of 15 mW. In the worst case, with every photon absorbed and a nonradiative quantum yield of 1, the temperature rise would be only 0.04 "C per laser pulse. At 10 Hz repetition rate, the time between pulses (100 ms) is sufficiently long to allow the system to relax back to its initial state. An additional molecular system was investigated in order to further test the angular resolution of ARPAS. Wirth and coworkers probed the orientation of bis-MSB in a C18 monolayer on silica using frequency-domain fluorescence a n i s o t r ~ p y . ~ ~ ~ ~ ~ Bis-MSB was found to partition into the C18 layer and align Figure 10. Simulated absorbance spectrum as a function of excitation with the alkyl chains, providing indirect orientational informaangle. Each transition is modeled as a Gaussian, with the angular tion about the alkyl phase. The influence of solvent on response at each wavelength modeled by eq 8. Approximating two orthogonal transitions, the values of em were 1" for the 250-300 nm chromatographic alkyl surfaces is a long-standing question. transition and 89" for the 300-400 nm transition. Wirth and co-workers found that as the solvent overlayer was varied from water to aqueous solutions of alcohols, the two techniques yield approximately the same average angle for chromatographic alkyl chains were "fluffed up", Le., tilted bis-MSB in C18. The general trend as a function of solvent, farther away from the surface plane. The solvent effect was i.e., a tilt up toward surface normal with increasing alcohol chain greater for longer alkyl chain alcohols. length, is also in agreement with that observed by Wirth and Since the photoacoustic signal depends on nonradiative co-workers. transitions from electronically excited states, any molecule The relatively large error in the average angle determined having a finite nonradiative quantum yield, which includes most by ARPAS is attributed to the rather simplistic cell design fluorescent molecules, is amenable to photoacoustic detection. utilized for this study. The cell is dismantled and reassembled Therefore, the system investigated by Wirth and co-workers for each experiment and the angle of incidence is adjusted represents an excellent, unambigous, test of the utility of manually. It is anticipated that a rigid, flow-through cell and ARPAS. Excitation at 355 nm accesses the pure long axis stepper motor control would result in greater accuracy and transition of bis-MSB, as determined experimentally (stretched precision. Further, differences in sample preparation may thin film) and theoretically (AM1) by Wirth and c o - ~ o r k e r s ~ ~ account for the lack of perfect agreement in 8,. Despite the and c o n f i i e d in our laboratory with Pariser-Parr-Pople (PPP) crude cell design and a static rather than flow-through system, molecular orbital calculations. The orientation of bis-MSB in on the basis of comparison with the two molecular systems C18 covalently bound to quartz was investigated for the solvents investigated it is clear that ARPAS is a sensitive and straightutilized by Wirth and co-workers: water, 20% methanol in forward technique for extracting orientational information. water, 5% 1-propanol in water, and saturated aqueous solutions The Next Generation. The final test of ARPAS is to of heptanol, octanol, and d e c a n 0 1 . ~Due ~ ~ ~to~the high affinity examine a molecular system in which orthogonal transitions of bis-MSB for the alkyl phase, in the presence of aqueous within the same molecule can be probed. Although the current solvents there is little or no bis-MSB in solution, Le., negligible instrumental limitation is lack of excitation tunability, investigasolution-phase contributions to the photoacoustic signal, tion of phenylazo dye layers? which have long and short axis transitions at Nd:YAG wavelengths, is underway. Ultimately, Figure 9 is a plot of photoacoustic amplitude as a function the goal is to conduct angle-resolved spectroscopy, Le., probe of angle for bis-MSB/C18 with water and octanol, overlayers. the entire absorbance spectrum of an oriented system as a Table 2 is a comparison of the molecular orientation angles function of excitation angle, Such an experiment would yield determined by ARPAS with those obtained by fluorescence anisotropy (expressed with respect to the surface normal). The the polarized spectrum and could be extremely useful in 24325
2150 J. Phys. Chem., Vol. 99, No. 7, 1995 assigning transition moment vectors within molecules of interest. For example, consider a molecule that contains two purely polarized orthogonal transitions and is immobilized on a flat surface. Figure 10 illustrates the three-dimensional data set that could be generated by varying both wavelength and angle of incidence. By fitting each band with eq 8 (or eq 7), the angle of each transition moment can be extracted. Straightforward geometrical analysis should then yield a complete description of the electronic transition moments. For molecules that can be reproducibly immobilized on a surface, ARPAS could be a powerful tool for exploring both orientation and electronic structure.
Acknowledgment. This work was supported by a Young Investigator (K.L.R.) grant from the Beckman Foundation. The authors gratefully acknowledge Prof. Josef Michl for helpful discussions and Brian Moore for programming assistance. References and Notes (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312-1319. (2) Rasing, T.; Berkovic, G.; Shen, Y. R.; Grubb, S. G.; Kim, M. W. Chem. Phys. Lett. 1986, 130, 1-5. (3) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461-3470. (4) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485-1487. ( 5 ) Dagani, R. Chem. Eng. News. 1994, 72 (6), 26-27. (6) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: San Diego, CA, 1991. (7) Greenler, R. G. J . Chem. Phys. 1966, 44, 310-315. (8) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J . Am. Chem. Soc. 1988, 110, 6136-6144. (9) Kessel, C. R.; Granick, S. Langmuir 1991, 7 , 532-538. (10) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (11) Nishikawa, Y.; Fujiwara, K.; Ataka, K-i.; Osawa, M. Anal. Chem. 1993, 65, 556-562.
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