3150
J. Phys. Chem. 1996, 100, 3150-3156
Langmuir-Blodgett Films and Photophysical Properties of a C60-Sarcosine Methyl Ester Derivative, C60(C5H9NO2) Dejian Zhou,† Liangbing Gan,† Chuping Luo,† Haisong Tan,† Chunhui Huang,*,† Guangqing Yao,† Xinsheng Zhao,‡ Zhongfan Liu,‡ Xiaohua Xia,‡ and Bei Zhang§ State Key Laboratory of Rare Earth Material Chemistry and Applications, Department of Chemistry, and Department of Physics, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: July 12, 1995; In Final Form: October 20, 1995X
A new C60-sarcosine methyl ester derivative, C60(C5H9NO2) (1), was prepared, and its Langmuir films with and without dioctadecyldimethylammonium bromide (DODMAB) were formed at the air/water interface. Pure 1 forms monolayer or multilayer films depending on the experimental conditions. The 1:1 mixed Langmuir film of 1 and DODMAB was transferred onto hydrophilic quartz substrates both in Z and Y type. Macroscopic second-harmonic generation from the mixed LB film was observed, and the second-order molecular susceptibility χ(2) and hyperpolarizability β were evaluated to be (2.1 ( 0.8) × 10-7 and (1.2 ( 0.4) × 10-28 esu. The first observations of room-temperature photoluminescence of 1 from both the mixed LB film and its CHCl3 solution and its fluorescence quenching by concentration and N,N-dimethylaniline (DMA) were reported. Its singlet energy is estimated to be 40.2 kcal/mol. Its fluorescence lifetime in CHCl3 solution is determined to be 1.0 ( 0.3 ns using the frequency-domain method.
Introduction Fullerenes and their derivatives are of great interest due to their unique molecular structure, fascinating physical and chemical properties, and important application aspects.1 It is very interesting that they can form stable rigid Langmuir and Langmuir-Blodgett films at the air/water interface,2-19 but in most cases only the multilayer films were obtained.9-19 The monolayers were found to be very sensitive to vibration, easy to aggregate, and very difficult to transfer onto solid substrates.2-5 This can be explained from their molecular structure. Fullerenes are very rigid ball-shape molecules with high hydrophobicity. They are not typical amphiphilic molecules for conventional Langmuir-Blodgett studies. Heiney et al.4 observed that the fullerene epoxide C60O forms monolayers much easier than pure C60. Recently, we discovered that the introduction of polar atoms, such as nitrogen or oxygen, into the close-caged molecule can not only enhance their Langmuir film stability but also improve their monolayer transformation ability. Their monolayers could be readily transferred onto hydrophilically pretreated quartz substrates to form high-quality LB films.6,7 The hydrophilic-substituted oxygen or nitrogen atoms play an important role in enhancing their monolayer stability and transformation ability. Besides, fullerenes and their derivatives can also form stable mixed Langmuir films with long-chain fatty acid or alcohols.5,10-12 These mixed films could be easily transferred onto solid substrates with a transfer ratio around unity. However, their 1:1 mixed Langmuir films were not ideally mixed films at the air/water interface, because the limiting area found for C60 (typically less than 0.10 nm2/ molecule) was much smaller than that it should be (around 0.95 nm2/molecule). The C60 molecule seemed more likely to be squeezed into the hydrophobic long chains instead of standing at the air/water interface due to its high hydrophobicity.10,12 The introduction of a hydrophilic group into the close-caged structure † State Key Laboratory of Rare Earth Material Chemistry and Applications. ‡ Department of Chemistry. § Department of Physics. X Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3150$12.00/0
should not only enhance the monolayer stability but also improve this ideal mixed Langmuir film formation ability, in which the fullerene molecules really stand at the air/water interface. The fluorescence properties of fullerenes and their derivatives are of considerable interests to chemists and spectroscopists.20-31 Knowledge of the optical spectral properties of these compounds is valuable to a fundamental understanding of the electronoptical and photochemical properties of them and would allow the most straightforward correlation with theoretical calculations of their electronic and vibrational structures. In spite of extensive investigations, earlier studies have failed to detect their fluorescence at room temperature presumably because of the very low fluorescence quantum yield.20 Only recently have the room-temperature fluorescence from the solutions of C60 and C70 been observed.22-24 However, the reported results about the fluorescence properties of fullerenes are rather quite different. For example, Wang24 observed that the fluorescence quantum yield of C60 varied with the excitation wavelength, but Sun et al.27 reported that its quantum yield is excitation wavelength independent. The fluorescence lifetime of C60 from as long as 1.45 ns to as short as 0.033 ns has been reported. Typically, the fluorescence lifetime is around 1.2 ns for C60 and 0.70 ns for C70, with fluorescence quantum yields around 2 × 10-4 for C60 and 8 × 10-4 for C70. However, studies on isomerically pure fullerene derivatives are still rare.30,31 Only very recently have Foote et al.30 reported the photophysical property of 1,9-(4-hydroxycyclohexano)buckminsterfullerene(60), but they did not measure its fluorescence lifetime and fluorescence quenching by electron donors. The effect of substituents on the photoluminescence property of C60 remains to be well-defined. In this paper, a new C60-sarcosine methyl ester derivative, C60(C5H9NO2) (1), was synthesized, and its Langmuir films with and without dioctadecyldimethylammonium bromide (DODMAB) at the air/water interface were studied. We also report the first observations of roomtemperature photoluminescence (PL) of 1, as well as its fluorescence lifetime and fluorescence quenching by concentration and electron donor, N,N-dimethylaniline (DMA). © 1996 American Chemical Society
LB Films and Photophysical Properties of C60(C5H9NO2)
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Experimental Section Materials and Synthesis. C60 was prepared by contact ac arc1a and purified through an adsorption method based on active carbon.32 Its purity is more than 99%, as confirmed by HPLC analysis. Sarcosine methyl ester hydrochloride was purchased from Fluka Chemika Co., and its purity is more than 95%. Dioctadecyldimethylammonium bromide (DODMAB) was also purchased from Fluka Chemika Co. with purity higher than 99%. Compound 1 was prepared by the photochemical reaction between C60 and sarcosine methyl ester in mixed solvents of toluene/methanol, then chromatographed on a silica gel column, and finally recrystallized from CH2Cl2/petroleum ether (1:1) as irregular black crystals.33 Elemental analysis found: C 92.43, H 1.12, N 1.52. Calcd for C60(C5H9NO2)‚0.5H2O: C 92.41, H 1.19, N 1.66. Spectrosopic data34 indicate its molecular structure as shown in eq 1:
Langmuir Film Formation. The spreading solutions were prepared by dissolving a small amount of 1 crystals in chloroform (AR, Beijing Chemical Factory) to get a concentration of 0.54 g/L (solution A); further solutions were diluted with chloroform or toluene to get a concentration of 0.120 (solution B), 0.054 g/L in CHCl3 (solution C), and 0.054 g/L in 9:1 toluene/CHCl3 (solution D). The mixed spreading solution (solution E) was prepared by dissolving compound 1 and DODMAB (1:1, molar ratio) in CHCl3 to get a concentration of 0.312 mM for 1. The spreading solution of DODMAB was dissolved in CHCl3 at a concentration of 0.312 mM (solution F). The substrates were all hydrophilically pretreated according to the procedure described in ref 6. A FACE single Langmuir trough (made in Kyowa Kaimenkagaku Co. Ltd., Japan) was employed for the LangmuirBlodgett study.7 The air/water interface was thoroughly cleaned, so for complete barrier movement (surface area change from 920 to 60 cm2) the surface pressure changed by less than 0.1 mN/m. To compare the surface pressure-area isotherms at different experimental condition, a NIMA 622 computercontrolled Langmuir trough was also employed. An accurate amount of the above solutions was carefully deposited on the pure water subphase (pH ) 5.60, 15 ( 1 °C) in approximately 5 µL droplets. After standing for 15 min (solution A, C, E, or F) or 30 min (solution D), allowing the solvents to evaporate, the floating films were compressed at a rate of 15 cm2/min (6.5 × 10-4 nm2 molecule-1 s-1), and the surface pressure-area (π-A) isotherm was recorded. Spectroscopy Measurement. The absorption spectra of the LB films and 1 in CH2Cl2 solution were measured on a Shimadazu UV-3100 spectrophotometer. A low-angle X-ray diffractogram of the LB film was obtained on a Rigaku D/max3B diffractometer, using the Cu KR radiation (λ ) 0.154 nm). Second-harmonic generation (SHG) from the monolayer was measured in the transmission, using a Y-cut quartz plate as reference and with a Nd:YAG laser beam (λ ) 1064 nm) at an angle of 45° to the film surface.35,36 The room-temperature photoluminescence (PL) was measured by excitation of the sample with an unfocused Ar ion laser beam (λ ) 488.0 nm) at an incident power density of 1.5 W/cm2. The PL was performed on a HRD-1 monochrometer system, and then it was acquired from a 5101 lock-in amplifier connected with a S-1 photomultiplier detector cooled at -40
Figure 1. Surface pressure-area isotherms of compound 1 at the air/ water interface: (I) multilayer isotherms on the FACE Langmuir trough; (a) 100 µL solution A, (b) 1000 µL of solution C, (c) 1000 µL of solution D; (II) multilayer isotherms on the FACE Langmuir trough; (d) the first compression (1000 µL of solution C), (e) recompression of curve (d) released from 20 to less than 0.1 mN/m; (III) monolayer isotherms on the NIMA Langmuir trough; 200 µL of solution B.
°C.37 The fluorescence lifetime determination was performed on a SLM 48000 multifrequency phase fluorometer, using the frequency-domain method with glycogen as reference. The lifetime determination method is the same as that described in refs 37-39. Results and Discussion Langmuir Films of 1 at the Air/Water Interface. The surface pressure Vs area (π-A) isotherms of the Langmuir films of 1 on the FACE Langmuir trough are different when using different spreading solutions, and they are shown in Figure 1. A 100 µL sample of solution A gives reproducible multilayer isotherms (Figure 1 Ia). The limiting area per molecule, obtained by extrapolation of the rising portion of the isotherm to π ) 0, was 0.26 nm2/molecule, typical for fullerene multilayer films. When the diluted solutions were used, for example, a 1000 µL of solution C or D, another kind of multilayer π-A isotherms were obtained (Figure 1, Ib and Ic). The later isotherms are more compressible than the former. The slopes of the solid phase are smaller and the limiting areas are bigger (curve Ib, 0.37 nm2/molecule, and curve Ic, 0.39 nm2/molecule). All these multilayers collapse below 30 mN/m, lower than those
3152 J. Phys. Chem., Vol. 100, No. 8, 1996 reported for C60, but comparable with our earlier results for C60glycine ester derivatives.7 We did not obtain a real monolayer of 1 at the air/water interface using the above FACE Langmuir trough, subphase, and spreading solutions. The π-A isotherms were almost the same when using different diluted spreading solutions (C or D, the diluted solutions would be better to form fullerene monolayers2,5) and different volumes of the spreading solutions (because the smallest area limit of the Langmuir trough is 60 cm2, the smallest volume used was 150 µL). Reexpansion of the multilayer film of 1 at a surface pressure of about 20 mN/m (solution C or D was used) resulted in a decrease in the surface pressure to a value close to 0. Upon subsequent recompression, the surface pressure remains close to 0 until the area per molecule is decreased to well below that of the initial compression, at which point the surface pressure increased rapidly to afford a different pressure-area isotherm (Figure 1, II). This behavior is just like that of C60 at the air/water interface.5 This hysteresis can be rationalized by a model in which the multilayer film fragments into rafts of aggregations which do not relax to fill the increasing surface area when the surface pressure is released. These rafts would stack up upon subsequent recompression.5 However, the monolayer of 1 was obtained by using a NIMA 622 computer-controlled Langmuir trough. A 200 µL sample of solution B was very carefully deposited on the above pure water subphase. After standing for 30 min, allowing the CHCl3 solvent to evaporate, the floating film was compressed at a rate of 5 cm2/min. (4.9 × 10-4 nm2 molecule-1 s-1) and the surface pressure-area isotherm was recorded and is shown in Figure 1, III. The limiting area obtained by extrapolation of the rising portion of the isotherm to π ) 0 was 0.95 nm2 per molecule of 1. By using the method in refs 4 and 6, the nearest neighbor distance of 1.09 ( 0.05 nm is obtained. These values are in fairly good agreement with those of the monolayers of C60 and its other derivatives,2,4,6 indicating that the substituted atoms in 1 do not affect the distances between the molecules at the air/ water interface. This is reasonable, considering the molecular orientation of 1 at the air/water interface: the hydrophilicsubstituted group is attracted under the water surface, and only the hydrophobic fullerene ball stands on the surface.4 The monolayer can sustain at least to 32 mN/m, and it is rather reversible. However, the monolayer is more sensitive to vibration and more difficult to obtain than those of our previous studies;6,7 this may be due to less hydrophilicity in 1 compared with the other derivatives studied. This verifies that introducing hydrophilic atoms can enhance the Langmuir film formation ability and stability of fullerene derivatives. Mixed Langmuir Film At the Air/Water Interface. A 200 µL sample of solution E spread very well on the pure water subphase and gave an excellent reproducible surface pressure Vs area isotherm (Figure 2). It shows a condensed region at the surface pressure from 1 to 47 mN/m with a plateau at even higher pressure (which can stand at least to 52 mN/m). This Langmuir film is very stable. Compressing the film to a surface pressure of 30 mN/m for 2 h, it shows only negligible area loss. Expansion and recompression of a previously compressed film reveal very little hysteresis up to the point at which the expansion was initiated (for a surface pressure less than 45 mN/ m). Even if the Langmuir film is compressed into the plateau region, the film will not collapse. Expansion and recompression reveals only small hysteresis (Figure 2, b). The effective limiting area per molecule 1 calculated from the π-A isotherm (equals the area occupied by 1 and DODMAB minus the area of blank DODMAB from the isotherm) increases with the surface pressure: for example, 0.14 nm2 at 0 mN/m (at the rising
Zhou et al.
Figure 2. Surface pressure-area isotherms of the 1:1 mixed Langmuir film of 1 and DODMAB at the air/water interface on the FACE Langmuir trough: (a) 200 µL of solution E, (b) recompression of curve (a) released from 52 to less than 0.1 mN/m, (c) 200 µL of solution F.
Figure 3. Proposed ideal molecular arrangements of the 1:1 mixed Langmuir film of 1 and DODMAB at the air/water interface (R ) N(Me)CH2COOMe): (I) at low surface pressure, the long chains of DODMAB are very tilted and the effective area occupied by molecule 1 is not obvious; (II) at high surface pressure, the long chains of DODMAB are not so tilted; thus the effective area occupied by molecule 1 becomes predominant, but is still smaller than the area it really occupied.
point of the surface pressure), 0.38 nm2 at 10 mN/m, 0.52 nm2 at 20 mN/m, 0.61 nm2 at 30 mN/m, and 0.62 nm2 at 40 mN/m. This behavior is significantly different from those of mixed Langmuir films studied, in which the area occupied by the fullerene molecule decreases as the pressure increases.5,10-12 This indicates that the 1 molecules really stand at the air/water interface instead of being squeezed into the long chains, and the molecules of 1 and DODMAB are mixed at the air/water interface. This is the first observation of this kind of mixed Langmuir film for fullerene and their derivatives. The above phenomena can only be explained as that both the molecules of 1 and DODMAB stand at the interface. The proposed ideal molecular arrangement in the mixed Langmuir film is shown in Figure 3. Of course, maybe the real molecular organization at the air/water interface is not so well separated, and two or even more molecules of 1 separated by two or more DODMAB molecules may also exist. Because the two hydrophobic alkyl long chains of DODMAB are connected through a sp3 hybridized N atom and they are tilted at the surface, the effective area occupied by molecule 1 is not obvious at low pressure. When the surface pressure increases, the long chains become more and more perpendicular, so the effective area occupied by molecule 1 becomes more and more predominant. However, even if the film is compressed up to 40 mN/m, the long chains are still tilted at the water surface, so the effective
LB Films and Photophysical Properties of C60(C5H9NO2) area of 1 obtained from the isotherm (0.62 nm2/molecule) is still smaller than what is expected (around 0.95 nm2/molecule), but it is much bigger than that of C60 in the mixed Langmuir films. The plateau can be explained as follows: the 1 molecules can squeeze into the hydrophobic long chains when the surface pressure becomes very high. However, most of the squeezed molecules can reverse to the interface when the surface pressure is released, because the hydrophilic atoms in 1 make it more stable to stand at the air/water interface than to be squeezed into the long chains. A small amount of irreversibly squeezed atoms (may be due to the aggregation of the molecules in the hydrophobic long chains; this is typical for fullerene LB films) lead to the small hysteresis in the second compression stage. Formation of Langmuir-Blodgett Films. The multilayers of 1 could be transferred onto hydrophilic quartz plates by the vertical dipping method. However, we observed that during the compression of these multilayers, visible yellow patchy domains that coexisted with a fluid isotropic phase on the surface usually formed at a surface pressure around 8 mN/m. These solid domains appeared close to the moving barrier in the form of elongated, bright needles. These domains remained at the air/water interface when the surface pressure was released, and they still existed when they were transferred onto hydrophilic quartz substrates with the film. This behavior is just like that of C60, indicating the large aggregation of the molecules in the multilayers.5 The monolayer film of 1 could be transferred onto a hydrophilic quartz plate by the vertical dipping method. The monolayer was transferred in Y type at a surface pressure of 30.0 ( 0.1 mN/m at a dipping speed of 5 mm/min. A transfer ratio of 0.85 ( 0.05 in the up stroke and 0-0.4 in the down stroke was usually obtained. This smaller transfer ratio of the monolayer may be due to its weaker hydrophilicity compared with other derivatives studied.6,7 There are no visible domains in the LB film, quite different from the LB film of C60, indicating that the LB film is much more uniform than that of pure C60.11 The 1:1 mixed Langmuir film of 1 and DODMAB exhibits an excellent film transformation ability. The mixed Langmuir film was transferred in Z type or Y type onto hydrophilic quartz plates for different purposes. The transfer was performed by vertical dipping, with a dipping speed of 5 mm/min (Z type, up stroke only) or 7 mm/min (Y type) at the surface pressure of 30.0 ( 0.1 mN/m. The transfer ratio was found to be always maintained within 1.00 ( 0.05 for the up stroke and 0.95 ( 0.05 for the down stroke. The transfer ratio of the down stroke is much bigger than that of the 1:1 mixed Langmuir film of C60 and DODMAP11 (only 0-0.4, DODMAP ) dioctadecyldimethylammonium perchlorate). The hydrophilic oxygen and nitrogen atoms in 1 play an important role in enhancing the mixed film transformation ability. The LB films on the quartz plate are apparently uniform; no visibly domains could be observed. Low-Angle X-ray Diffraction. The low-angle X-ray diffractogram of a 53-layer mixed Y type LB film is shown in
J. Phys. Chem., Vol. 100, No. 8, 1996 3153
Figure 4. Low-angle X-ray diffraction of a 53-layer mixed LB film on a quartz plate.
Figure 4. It shows a predominant peak at 2θ of 2.40° and reveals a well-defined film structure. The peak is assigned to the (001) Bragg diffraction. Using the well-known Bragg equation, we calculated the average layer spacing to be 3.68 nm. Thus, we obtained the single-layer film thickness of 1.84 nm based on the Y type film structure (one averaged layer spacing contains two single layers). This X-ray diffraction pattern indicates that the LB film has an ordered layer structure. The single-layer film thickness is smaller than the calculated value of 2.51 nm based on the fact that the alkyl long chains in DODMAB are vertically arranged. This can be explained as follows: the long chains have a tilt angle from the vertical arrangement and the long chains may have some insertion within each double layer. Second-Harmonic Generation (SHG). C60 should have a large nonlinear optical response because of the existence of abundant π electrons in the close-caged molecular structure. It exhibits strong third-order nonlinearity, and its third-order molecular susceptibility χ(3) and hyperpolarizability γ have been evaluated to be 3.3 × 10-9 and 1.6 × 10-31 esu, respectively.40 However, it does not show second-harmonic generation (SHG) due to its central symmetry structure. We note that the monofunctionized derivative 1 has no central symmetry, and the Z type LB film generates a statistical noncentral symmetrical environment, so the derivative should have macroscopic SHG response in highly ordered Z type mixed films. We assume that the refractive index is 1.70 (smaller than that of C60,41 but comparable with that of many organic dyes). Using the methods in ref 42, a film thickness obtained from low-angle X-ray diffraction (1.84 nm), and a limiting area of 1.39 nm2/molecule (the SHG response of DODMAB is relatively small and can be neglected), the second molecular hyperpolarizability β and susceptibility χ(2) can be evaluated. They are given in Table 1. From Table 1 it can be seen that the χ(2) value of the mixed LB film of 1 is comparable with those of the C60 derivatives studied, but the β value is about 2-4 times bigger than those of the former derivatives studied. This can be explained as follows: the chromophore (molecule 1) was separated in an ordered manner by the large DODMAB molecule, and this ordered separation of the chromophore in the LB film can
TABLE 1: Second-Harmonic-Generation Comparison of the Monolayers of Fullerene Derivatives and Typical SHG Materials compound
limiting area (nm2/molecule)
refractive index
film thickness (nm)
χ(2) (10-7 esu)
β (10-29 esu)
ref
C60(C4H8N2) C60(C6H9NO4) C60(C8H13NO4) (1:1) 1/DODMAB Aa B C
0.89 0.67 0.71 1.39 0.75 0.86 0.95
1.90 1.90 1.90 1.70 1.60 1.60 1.60
1.5 1.50 1.50 1.84 3.4 2.55 2.83
1.8 ( 0.8 1.5 ( 0.5 1.1 ( 0.5 2.1 ( 0.9 24 ( 12 12 ( 2 21 ( 8
3.6 ( 1.2 2.6 ( 1.0 1.8 ( 0.8 12 ( 4 90 ( 40 75 ( 15 200 ( 80
[6] [7] [7] this work [42a] [35] [36]
A: Me2NC6H4CHdCHC5H4NC18H37‚C18H37OSO3. b B: Me2NC6H4CHdCHC5H4NC16H33‚Eu(NTA)4, NTA ) 2-naphthyltrifluoroacetone. c C: Et2NC6H4NdNC5H4NC18H37‚Dy(PMBP)4, PMBP ) 1-phenyl-3-methyl-4-benzoylpyrazolone-5. a
3154 J. Phys. Chem., Vol. 100, No. 8, 1996
Zhou et al.
Figure 6. Plot of the absorbance at 258 nm of the 1:1 mixed LB film against the layer number.
Figure 5. Absorption spectra of compound 1: (I) CH2Cl2 solution; (II) seven-layer 1:1 mixed LB film (solid line), two-layer LB film of 1 (broken line) on a quartz plate.
effectively restrain the J-aggregations which affect the SH intensity and thus lead to a much bigger β value.42 However, both the χ(2) and β values are relatively small compared with typical SHG materials,35,36,42 maybe due to the lack of strong electron donor and acceptor groups in the molecule. Absorption Spectra. The absorption spectra of 1 in CH2Cl2 and in the LB films are shown in Figure 5. The absorption spectrum of the CH2Cl2 solution shows three major bands at 255, 308, and 429 nm; another band around 220 nm could not be observed due to the absorption limit of the CH2Cl2 solvent. The onset absorption lies at about 720 nm, with the first small evident peak at 698 nm. The seven-layer mixed LB film shows two major bands characteristic of 1 at 222 and 258 nm, with a weak broad shoulder around 330 nm. The sharp absorption peak at 429 nm and the well-defined weak bands in the 700-400 region cannot be distinguished; instead a flat curve is observed. The 255 nm band of 1 red shifts only 3 nm in the mixed LB film compared with its corresponding CH2Cl2 solution. The two-layer Y type LB film of 1 also exhibit two major bands of compound 1 at 230 and 267 nm, with a broad shoulder around 340 nm. The three bands all red shift about 10 nm compared with those of the 1:1 mixed LB film, maybe due to the fact that the aggregation in the LB film is much larger than that in the mixed LB film.6,7,11 This is more evidence that the molecular aggregation in the mixed LB film is effectively restrained. In a plot of the absorbance of the 1:1 mixed LB films at 258 nm against the layer number, a linear line is obtained (Figure 6), indicating that the mixed Langmuir film is effectively transferred onto the quartz substrates, in good agreement with the transfer ratio always maintained around unity. Room-Temperature Photoluminescence (PL). The PL spectra of 1 in a CHCl3 solution and a 53-layer mixed LB film excited by an Ar ion laser beam (λ ) 488.0 nm) are shown in Figure 7. The PL of 1 in CHCl3 exhibits two major bands monitored at 715 and 788 nm, with a broad weak shoulder around 880 nm. The first band is the strongest of the three, and the three bands are separated by about 1300 cm-1. Because of the width of the bands, the uncertainty is greater than (100 cm-1. This progression is assigned to the totally symmetric pentagonal pinch mode, which has been observed in the Raman spectrum as a strong peak at 1461 cm-1. Both the peak position
Figure 7. PL spectra of compound 1 in CHCl3 (1.23 × 10-3 mol/L, broken line) and a 53-layer 1:1 mixed LB film (solid line) excited by an unfocused Ar laser beam (λ ) 488.0 nm).
and the relative intensity of the two major bands are quite different from that of C60 in room-temperature solution,23,29 but rather close to that of C60 film deposited on a CaF2 plate at 20 K.21 Using the intersection of the absorption and emission maxima,30 the singlet energy is estimated to be 40.2 kcal/mol, about 6 kcal/mol lower than that of C60 and almost the same as the value of another C60 derivative, 1,9-(4-hydroxycyclohexano)buckminsterfullerene(60) (40.2 kcal/mol). This similarity between the two fullerene derivatives is not surprising, considering that they are both C60 derivatives at the 6,6-junction.30 The PL spectrum of the 53-layer mixed LB film shows only a broad band from 680 to 980 nm. The two major bands which appear in the CHCl3 solution cannot be distinguished; instead, a wide band monitored around 744 nm appears. Comparing the PL spectrum of the LB film with that of the CHCl3 solution, we can see that the PL spectrum in the mixed LB film is evidently broadened. The onset of the PL appears at about 680 nm in the solution; however, it blue shifts to about 620 nm in the LB film. We did not observe the white light emission from the mixed LB films, although the PL in the LB film is evidently broadened. This broadening may be the results of the interaction of 1 and DODMAB, and maybe the interaction in the LB film is not strong enough to perturb the molecular orbitals, which leads to the transition between the ground and lowest excited states becoming dipole allowed and leads to the strong visible white light emission as observed from C60 confined in zeolite cages.43 However, this is the first observation of roomtemperature photoluminescence from LB films of fullerenes and their derivatives. Fluorescence Lifetime. Because of the time lag between absorption and emission, the emission is delayed to the
LB Films and Photophysical Properties of C60(C5H9NO2)
J. Phys. Chem., Vol. 100, No. 8, 1996 3155
Figure 9. (I) Fluorescence quenching ratio I0/I of 1 at different DMA concentration. (II) Treated fluorescence quenching ratio at different DMA concentrations based on a model in which both static and dynamic quenching are considered. Figure 8. Dependence of the PL intensity of 1 in CHCl3 on concentration (excited at 570 nm): (I) relative PL intensity of 1 at different concentrations; (II) relative PL intensity per unit concentration of 1 at different concentrations.
modulated excitation. The fluorescence lifetime could be evaluated by measuring the phase shift and the relative modulation of the sample and the glycogen reference. By using a least-squares analysis of the measured phase shifts and relative modulations at different frequencies, we evaluated the fluorescence lifetime of 1.0 ( 0.3 ns for compound 1 in CHCl3 solution. The deviation ((0.3 ns) is relatively big due to the very low quantum yield (e.g. very weak fluorescence intensity) of 1. The lifetime value of 1 is almost the same compared with the previously reported value for C60,23 indicating that the substituted sarcosine ester group in 1 does not affect the lifetime of C60 significantly. This phenomena is also observed by Lin et al.31 Fluorescence Quenching by Concentration. The relative PL intensity of 1 is different at different concentrations and is shown in Figure 8. It can be seen that the strongest PL intensity occurs around the concentration of 1.5 mM. At the low concentration region, the PL intensity increases with the concentration; however, it decreases with the concentration at the high concentration region (Figure 8, I). When the relative PL intensity of per unit concentration of 1 (equals the PL intensity divided by its concentration) is plotted against the concentration, a decreased relative PL intensity with the concentration is obtained (Figure 8, II). This indicates that the fluorescence intensity of 1 is effectively quenched by concentration. This is the first observation of concentration quenching for fullerenes and their derivatives. Fluorescence Quenching by DMA. Fullerenes are good electron acceptors, they could interact with electron donors through charge-transfer interaction, and their fluorescence could be quenched by electron donors. The fluorescence quenching of C60 and C70 by electron donors has been reported.24,25 However, no such report for isomerically pure fullerene derivatives has been investigated. In CHCl3, the PL intensity of 1 is effectively quenched by N,N-dimethylaniline (DMA), and the pattern of the spectrum shows hardly any change. Ratios of the fluorescence intensity without DMA (I0) and with DMA (I) are plotted as a function of DMA quencher concentration and
are shown in Figure 9. According to the Stern-Volmer equation, a linear relationship between the quenching ratio (I0/ I) and quencher concentration is expected:39
I0/I ) 1 + KSV[DMA]
(1)
where KSV is the Stern-Volmer constant and [DMA] is the molar concentration of DMA. An estimate of the Stern-Volmer constant can be made by considering the data at low DMA concentrations (
