J . Phys. Chem. 1995, 99, 5274-5276
5274
Solution and Solid State Interactions of
c 6 0
with Substituted Anilines
Scott P. Sibley,*J Robert L. Campbell, and Herbert B. Silber Department of Chemistry, San Jost State University, San Jost, California 95192 Received: October 17, 1994; In Final Form: January 3, 1995@
The equilibrium constant for complexation of c 6 0 and aniline in toluene has been determined by UV/vis spectroscopy, with a value of K = 0.28 f 0.04 obtained. The first extracted and well-defined charge-transfer (CT) bands have been observed for complexes of C a with a variety of aromatic amines, with C T band maxima moving to higher frequency with increasing amine ionization potential. FTIR spectra of solid c 6 0 with substituted anilines show slight blue shifts in C ~ bands, O indicative of a small amount of donor-acceptor interaction.
Introduction Since their discovery,'%* there has been a considerable amount of work done on the spectroscopic characterization of c 6 0 fullerene molecules in both ground and excited states3x4 However, it is only recently that the donor-acceptor (D-A) behavior of fullerenes has received increased attention. Though it is now known that c 6 0 can act as an electron acceptor with a variety of electron donor^,^-^ accurate determination of equilibrium constants in solution has been difficult due to the small magnitude of the K values obtained. Wang6 reported a spectroscopic study of the interaction of c 6 0 with N,Ndiethylaniline (DEA) in the ground state, suggesting an equilibrium constant K = 0.18 f 0.04 M-' in benzene. In a similar study, Seshadri et al.' reported values of K = 0.1-0.3 M-', depending upon the nature of the solvent. Sension8 has obtained a smaller value of 0.047 M-' for the CdN,N-dimethylaniline (DMA) system. However, an additional study9 of C6@MA complexation in toluene by Uvlvis absorption spectra determined K to be 0.20 M-l, a substantially larger result. Though a relationship has been postulated between oxidation potential of the donor and complexation e n t h a l ~ y , ~ the variation in results by different workers and in different solvents has led to a need for further studies to discern trends in the donor-acceptor behavior of C ~ with O aromatic amines. In order to determine the dependence of complex stability upon basicity of the aromatic amine donor, we have extended equilibrium constant measurements to study of the C6~aniline system by UVIvis electronic spectroscopy in toluene. Aside from the measurement of the stability constant, we have used difference spectra to observe well-defined charge-transfer (CT) bands for C6daromatic amine complexation. While it has been reported that no distinct CT bands are present for c 6 0 with DEA or DMA,'O we have found that subtraction of the acceptor spectrum leads to Gaussian-like CT bands in the visible region for a series of donor amines. In addition, we have carried out high-resolution FTIR measurements of shifts in infrared bands of solid c 6 0 with various donor amines. This paper presents the results and preliminary analysis of the data for these systems.
Experimental Section Pure, sublimed c 6 0 (Term Ltd., >99.98%) was used for all measurements. The purity of the samples was further verified by UV-vis spectroscopy. Fullerene samples were stored in a
' Camille and Henry Dreyfus Postdoctoral Fellow. @
Abstract published in Advance ACS Abstracts, March 15, 1995.
0022-365419.512099-5274$09.00/0
vacuum desiccator between uses to minimize exposure to atmospheric oxygen. Commercially purchased amines were purified by passing though neutral aluminum oxide columns minutes prior to use.6 Spectroscopic grade toluene (EM Science, >99.9%) and HPLC grade dichloromethane (EM Science, '99.8%) were used for solution measurements. Data for equilibrium constant determination were recorded using a Hewlett-Packard 8542A UV-vis diode-may spectrophotometer, with temperature control. Spectral measurements of CT bands were performed using a Cary 13 double-beam spectrophotometer. All Cdamine solution spectra were run in matched quartz cuvettes against the corresponding amine blank in order to cancel out amine absorbances and to correct for variations in dielectric constant of the solutions. Infrared spectra were run on a Perkin-Elmer Model 1800 FTIR spectrometer. Samples were prepared by grinding 5-10 mg of c 6 0 with a slight excess of liquid amine, followed by mounting between KJ3r plates.
Results and Discussion
UV/vis Spectroscopic Measurements. Addition of aniline to C6dtoluene solution leads to increased absorbance, characteristic of donor-acceptor interaction. Results for a series of solutions of varying donor concentration at 20 "C are shown in Figure 1. A general increase in the intensity of c 6 0 absorbance is noted with increasing amine concentration. Infrared absorption spectra had no new features and gave no evidence of reaction of c 6 0 upon amine addition. No isosbestic point is observed in the spectra. Unlike the results reported for other systems,l0 the change in the c 6 0 absorption spectrum is noticeably larger in the region around 450 nm. This point will be discussed later. Equilibrium constants were obtained from the concentration dependence of the spectral data by using the Benesi-Hildebrand equation and assuming 1:1 complexation.11s12Data were taken at 440 nm, and results of the analysis are shown in Figure 2. Linear regression of the plot leads to values of K = 0.28 f 0.04, a result slightly larger than the value of 0.20 M-' reported by Seshadri et aL9 for C6d dimethylaniline in the same solvent. It is also higher than the result reported by Wang6 of K = 0.18 for C6@EA in the aromatic solvent benzene. Since aniline should be a poorer donor based on its higher ionization potential, l 3 the larger magnitude of the observed stability constant suggests the importance of steric effects on complexation in solution. As can be seen from Figure 1, the relative increase in absorbance upon addition of amine is largest in the shorter wavelength region. For a series of related donors with a single
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 15, 1995 5275
Interactions of C60 with Substituted Anilines
h
m
c)
.e
c a
+-i 9
0.05
0 0
B 2 s
e
(0
0
460
410
400
450
550
500
600
650
700
wavelength (nm)
510
560
610
660
wavelength (nm) Figure 3. Charge-transfer bands for Cw with various amines: (a) aniline, (b) o-toluidine, (c) 2,6-dimethylaniline, (d) N-methylaniline, (e) NJ-dimethylaniline, and (f)NJ-diethylaniline.
Figure 1. Representative absorbance spectra of Cm (2 x M) in toluene with addition of aniline. Aniline concentrations of 0,0.5,0.85, 1.0, 1.15, and 1.5 M are shown, with increasing concentrations from bottom to top. 16
I
I
7
4
0 0.5
I .5
I
2
1 /[aniline] Figure 2. Benesi-Hildebrand plot for complexation of Cdaniline in toluene. acceptor, the peak maximum of a CT band can be approximately given by14
hvCT = I D - c1
+ Cz/(ID - c,)
(1)
where C1 and Cz are constants for a given acceptor (Cl >> C2) and ID is the ionization potential of the donor. Over small ranges, then, a plot of h v a versus ZD should be roughly linear, with
hvcT >> I,, - constant
7.5
8
Donor I.P. (eV) Figure 4. Plot of energy of maximum absorbance of charge-tranfer bands of Cdamine complexes versus ionization potentials13J6of the respective amines.
(2)
This behavior has been empirically observed for iodine and other acceptor m01ecules.l~ In order to investigate the behavior of Cm, UV/vis spectra were recorded for c 6 0 with various amines added. The 25 "C spectra, which were referenced against the appropriate amine solutions, were then subtracted from the spectrum of C6ddiChlOrOmethane in the absence of amine. Results are shown in Figure 3. There are some slight oscillations in the spectra due to the base line correction process and the low intensity of the c 6 0 CT bands. As can be seen from the plot, maxima in the CT bands vary from 420 nm for aniline to nearly 560 nm for DEA. The maxima of the CT bands do not coincide with the maxima of the respective amine absorbances, and intensities of the CT bands are sometimes larger than those of the amine solutions. Results are summarized in
TABLE 1: Ionization Potentials and Charge-Transfer Band Maxima for Ca with Substituted Anilines compound IP (eV). CT peak max (eV) aniline 8.1 2.95 o-toluidine 7.83b 2.82 2,6-dimethylaniline 7.75 2.71 N-methylaniline 7.65 2.54 NJ-dimeth y laniline 7.45 2.46 N,N-diethy hiline 7.2 2.21 a Ionization values from ref 16 unless otherwise noted. Value from ref 13.
Table 1. Ionization potentials are those reported by Maier et al.16 unless otherwise noted. A plot of the maximum of the charge transfer peak versus ionization potential is shown in Figure 4. The data are linear for all of the substituted anilines shown. Diphenylamine is not included since it did not fit the pattern, possibly due to different resonance interactions as a result of the additional phenyl ring. The full width at halfmaximum (fwhm) of the bands increases slightly as the bands shift to lower energy, suggesting increased interactions with the donors of lowest ionization potential. It is clear from the results of this study that the observed changes in the Cdamine spectra are indeed due to the presence of CT absorptions. Spectra changes due to a generalized perturbation would result in an increased absorption over the entire envelope, which is not the case. The position of the C&aniline CT band (420 nm) is found to be at higher energy than predicted by recent theoretical calculation^,^^ which placed it between 650 and 800 nm. Overall, the results suggest that in spite of its large size and
5276 J. Phys. Chem., Vol. 99, No. 15, 1995
Sibley et al.
on donors overcomes the expected decrease in frequency which
590
580
570
Wavenumbers (cm-') Figure 5. Overlay plot of FTIR scans of C d a m i n e samples. From top to bottom: Csdaniline, pure Cw, CdNfl-dimethylaniline, and C d N,N-diethylaniline. Intensities are not normalized.
unique symmetry, c60 acts in a manner similar to that of other
x* acceptors with regard to its donor-acceptor behavior in solution.
FTIR Studies. In order to gather further evidence about the strength of C a interaction with substituted anilines, FTIR spectra were run of c60 with excess amine. Though the spectrum of Cm appears to be on the whole unchanged, high-resolution scans (0.1 cm-' resolution) reveal slight shifts in the bands. Scans of the 576 cm-' peak with various amines added are shown in Figure 5. Other c60 infrared vibrational bands display qualitatively similar behavior, though exact analysis of these is complicated by lower peak intensity andor overlap of amine absorbances. While the addition of aniline results in peak broadening as well as a slight red shift, addition of other amines results in bands that are blue-shifted relative to that of pure c60. The observed blue shifts were found not to correlate with the refractive index of the amine, and addition of Nujol caused no observable shifts in spectra. All spectral features are reproducible to f O . l cm-'. The blue shifts are unexpected, since electron transfer to the antibonding18 LUMO of c60 would be predicted to cause a red shift in some normal modes due to lowering of bond force constants. While rare, similar behavior has been found in Raman spectra of a variety of aromatic nitro-acceptor molecules in complexes of substituted a n i l i n e ~ . 'Some ~ complexes of DMA and DEA showed blue shifts in some NO2 stretching vibrations of up to 20 cm-I. Any blue shifts in the vibrations of the acceptor C ~ O should be smaller due to its degree of delocalization and small extent of interaction with anilines. This is consistent with our results. It is interesting to note that electron-transfer-induced blue shifts of C a vibrational frequencies have already been observed. Raman spectra have been obtained of intercalation compounds of C ~ with O the strong R-type donor alkali metals K and Rb.*O The results show that Raman bands in the 500 cm-I region (axial modes) of the spectra are blue-shifted up to 10 wavenumbers relative to the modes of pure c60. In addition, this shift is seen to increase as the coordination number of positive charges increases from three to six and as the size of the unit cell decreases. Though no explanation has been given for these shifts, the results suggest that the proximity of positive charges
should occur upon electron transfer. It has been notedz1 that if D-A interaction does not affect the force constant of the molecule, vibrational frequencies should increase as a result of the interaction, neglecting small shifts due to electrostatic effects. Also, the shift should increase as the strength of the interaction increases. Because of the large size and delocalized nature of the fullerene orbitals, changes in force constant due to partial charge transfer in these weak complexes should be minimal. The observed shifts thus suggest an interaction order for solid c60 of DEA > DMA > aniline, consistent with the results expected solely on the basis of ionization potentials. This does not appear to agree with the results obtained thus far from UV/vis spectra in solution, where solvent interactions may play a greater role. This interaction order is also suggested by the shapes of the IR bands, since reproducible features are seen on the high-frequency side of the peaks in the most blue-shifted spectra, resulting in the asymmetric appearance of the bands. Though more work needs to be done on characterization of the infrared bands, the FTIR results suggest another method for probing the interactions between c60 and various amine donors.
Acknowledgment. We thank the Camille and Henry Dreyfus Foundation Scholar/Fellow Program and the National Institutes of Health Minority Biomedical Research Support (MBRS) Program for Grant GM08192 for the financial support for this research. References and Notes (1) Kroto, H. W; Heath, J. R.; O'Brien, S . C.: Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Curl, R. F.; Smalley. R. E. Science 1988, 242, 1017. (3) Ajie, H.; Alvarez, M. M.; Anz, S . J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kratschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J. Phys. Chem. 1990, 94, 8630. (4) Bethune, D. S . ; Meijer, G.; Tang, W. C.; Rosen, H. J.; Golden, W. G.; Seki, H.; Brown, C. A.; DeVries, M. S . Chem. Phys. Lett. 1991, 179, 2, 181-185. (5) Pradeep, T.; Singh, K. K.; Sinha, A. P. B.; Morris, D. E. J . Chem. Soc., Chem. Commun. 1992, 33, 2069. (6) Wang, Y. J . Phys. Chem. 1992, 96, 764-7. (7) Seshadri, R.; Rao, C. N. R.; Pal, H.; Mukherjee, T.: Mittal, J. P. Chem. Phys. Lett. 1993, 205, 4-5, 396. (8) Sension, R. J.; Szarka, A. Z.; Smith, G. R.; Hochstrasser, R. M. Chem. Phys. Lett. 1991, 195, 2073. (9) Seshadri, R.; D'Souza, F.; Krishnan, V.; Rao, C. N. R. Chem. Lett. 1993, 21 7-220. (IO) Rao C. N. R.; Seshadri, R.; Govindaraj, A,; Mittal, J. P.; Pal. H.; Mukherjee, T. J. Mol. Strucr. 1993, 300, 290. (11) Benesi, H. A.; Hildebrand, J. H. J.Am. Chem. SOC.1949, 71,2705. (12) Connors, K. A. Binding Constants: The Measurement of Molecular Complex Stabilio; Wiley: New York, 1987. (13) Kobayashi, T.; Nagakura, S. Bull. Chem. Soc. Jpn. 1974,47,2565. (14) Mulliken, R. S.; Person, W. B. Molecular Complexes; WileyInterscience: New York, 1969; pp 419-427. (15) McConnell, H.; Ham, J. S . ; Platt, J. R. J. Chem. Phys. 1953, 2 I , 66. (16) Maier, J. P.; Turner, D. W. J . Chem. SOC., Faraday Trans. 2 1973, 69, 521-524. (17) Li, J; Feng, J.; Sun, C. J. Phys. Chem. 1994, 98, 8636. (18) Fowler, P. W.; Woolrich, J. Chem. Phys. Letr. 1986, 127, 78-83. (19) Hindawey, A. M.; Nassar, A. M. G.; Issa, R. M.; Issa, Y. M. Indian J. Chem. 1980, 19A, 615-617. (20) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. J. Mater. Res. 1993, 8, 2068. (21) Mulliken, R. S . ; Person. W. B. Molecular Complexes; WileyInterscience: New York, 1969. JP942786R
