- Email: [email protected]
amine charge-transfer complexes studied by optical spectroscopy
3 September 1999
Chemical Physics Letters 310 Ž1999. 373–378 www.elsevier.nlrlocatercplett
Optical transition and ionicity of C 60ramine charge-transfer complexes studied by optical spectroscopy Masao Ichida
a,b,)
, Takamitsu Sohda b, Arao Nakamura
a,b
a
b
Center for Integrated Research in Science and Engineering, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-8603, Japan Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-8603, Japan Received 13 January 1999; in final form 17 March 1999
Abstract We have investigated absorption spectra of charge-transfer ŽCT. complexes of C 60 with various aromatic amines with different ionization potentials. An absorption band due to CT transition is observed in the visible region; with increasing ionization potential of amines, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. We extract degrees of charge transfer by analyzing the transition energy of the CT band as a function of ionization potential and Raman spectra. The experimental results show that the CT complexes studied here have a neutral character in their ground state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Since the discovery and establishment of the bulk preparation of fullerenes, in particular C 60 w1x, a great deal of experimental and theoretical work has been done. The photophysics of C 60 molecules in solution is quite well understood. Recently, the donor–acceptor ŽD–A. behavior of fullerenes has received considerable attention from the viewpoint of both basic fullerene chemistry and fullerene-based applications. A C 60 molecule can act as an electron acceptor with a variety of electron donors, and optical and electronic properties for C 60 with various donors have been reported by several groups w2–7x. Treatment of C 60 with amines and their derives results in formation of both ionic and neutral fullerene ) Corresponding author. Fax: q81 52 789 3724; e-mail: [email protected]
amino derivatives. The treatment of C 60 with the tertiary amine tetrakis Ždimethylamino .ethylene ŽTDAE. leads to the salt TDAEq Cy 60 which reveals an unusual molecular ferromagnetism w2x. Sibley et al. have reported CT absorption spectra for C 60 CT complexes with aniline, o-toluidine, etc. w6x. They observed that the CT absorption peak shifts to the higher energy side for different donors. However, an oscillator strength of the CT transition and a degree of charge transfer of the CT complex have not yet been studied. The ionicity which is determined by the degree of charge transfer is one of the important parameters which determines electronic and optical properties in the derives. In this Letter, we have systematically investigated absorption spectra of CT complexes of C 60 with various aromatic amines, and extracted degrees of charge transfer and oscillator strengths of the CT transition as a function of the ionization potential.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 7 7 - 6
374
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
From analysis of the transition energy, depending on the ionization potential, we have obtained degrees of CT r in various C 60 amine CT complexes. The value of r obtained is 0.04–0.17, which indicates that the CT complexes studied here have a neutral character in their ground state.
2. Experimental Pure C 60 Ž) 99.9%. powders and commercially purchased amines which were purified by distillation were used for all measurements. We prepared C 60 CT complexes with different amines: aniline ŽANI., o-toluidine Ž o-TOL., N-methylaniline Ž N-MAN., N, N-dimethylaniline ŽDMA., and N, N-diethylaniline ŽDEA.. The ionization potentials of isolated ANI, o-TOL, N-MAN, DMA, and DEA are 7.72, 7.44, 7.32, 7.12 and 6.95 eV, respectively w8x. These amines consist of a one-amino group. We also prepared CT complexes with other types of donors such as pyrene and N, N, N X , N X-tetramethyl-1,4-phenylenediamine ŽTMPD.. Pyrene has no amino group and TMPD consists of two amino groups. The ionization potentials of pyrene and TMPD are 7.37 and 6.20 eV, respectively. Here, we use the adiabatic ionization potentials instead of vertical ones, because the values of the adiabatic ionization potential are known for all the donors used in this study. Spectroscopicgrade toluene was used as solvent for C 60 ramines and C 60 rpyrene. We used 1-chloronaphthalene as solvent for C 60 rTMPD. The concentrations of C 60 in solution are 2 = 10y4 M for ANI, 5 = 10y4 M for o-TOL, 1 = 10y3 M for N-MAN, DMA, and DEA, 5 = 10y3 M for TMPD. Absorption spectra were taken using a Hitachi U-3500 spectrometer. The equilibrium constants of solutions were determined by analyzing the absorption spectra as a function of amine concentration. All spectra were run in matched quartz cuvettes against the corresponding amine blank in order to cancel out amine absorbance and to correct for variations in dielectric constant of the solutions. Raman spectra were measured using Jovin-Yvon U1000 monochromator with liquid N2cooled CCD. The spectral resolution of Raman spectrum is ; 0.8 cmy1 and the precision of Raman shift is ; 0.2 cmy1 by using line shape analysis.
3. Results and discussion Changing donor concentrations, we measured absorption spectra and determined equilibrium constants. The inset of Fig. 1 shows the absorption spectra of C 60 in toluene with addition of o-TOL. The weak absorption band due to the HOMO–LUMO ŽS 0 –S 1 . forbidden transition is observed around 2 eV and the strong absorption bands due to allowed S 0 –S 2 and S 0 –S 3 transitions are observed above 3.5 eV Žnot shown.. A new absorption band due to the formation of CT complexes in solution appears at ; 2.7 eV and grows with increasing o-TOL concentration. The equilibrium process of complex formation is written as FqD|C ,
Ž 1.
where F, D, and C represent C 60 , amine, and complex, respectively. The equilibrium constant K, defined as wCxrwFxwDx, can be determined by monitoring the CT absorption intensity as a function of amine concentration wDx. By plotting Ž1 y OD 0r OD.rwDx versus OD 0rOD ŽBenesi–Hildebrand plot.,
Fig. 1. Absorption spectra for CT complexes of C 60 with various amines at room temperature: Ža. ANI, Žb. o-TOL, Žc. N-MAN, Žd. DMA, Že. DEA, and Žf. C 60 toluene solution. Broken curves: fitted Gauss functions. Inset: Absorption spectra of C 60 Ž5=10y4 M. in toluene with addition of o-TOL. The o-TOL concentrations are 0, 0.93, 2.80, 4.67, 6.53, and 9.33 M Žfrom bottom to top..
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
where OD is the absorbance of C 60 rtoluene-amine solution and OD 0 is the absorbance in the absence of amines, we fit a straight line to the data. The intercept of the straight line yields yK. The obtained value of K for C 60 ro-TOL complex is 0.04. The equilibrium constants of C 60 CT complex with various amines are listed in Table 1. The K value of C 60 rDEA is comparable to the result reported by Wang et al. w4x. The K values for C 60 rANI and C 60 rDMA are much smaller than the results reported by Sibley et al. for C 60 rANI w6x and by Seshadri et al. for C 60 rDMA w5x. The discrepancy may be due to the donor concentration range in the Benesi–Hildebrand plot. We measured the absorption spectra in the wide concentration range Ž0–11 M. compared to their measurements Ž0–1.5 M.. The Benesi–Hildebrand plot in the wide concentration range yields a more accurate value of the equilibrium constant. The value of K increases with decreasing ionization potential of amine. This result indicates that a donor with a lower ionization potential more easily forms a CT complex in solution. Fig. 1 shows the absorption spectra of C 60 CT complexes with various amines. These spectra were obtained by subtracting the absorption component of C 60 molecules from the spectra measured for the solvent of C 60 ramine. We observed the absorption band due to the CT complex which shifts to the higher energy side depending on the donor species. The molar absorption coefficient a is also dependent on the donor species and increases with increasing ionization potential. The absorption spectrum was analyzed by fitting to the Gauss function, and the results are illustrated by broken curves in Fig. 1.
Table 1 Ionization potential Ž I P ., equilibrium constant Ž K ., peak energy of CT band Ž " v CT ., CT band width Ž D ., oscillator strength Ž f . with substituted amines Amine
IP K ŽeV.
" v CT ŽeV.
D ŽeV.
f
ANI o-TOL N-MAN DMA DEA TMPD
7.72 7.44 7.32 7.12 6.95 6.20
2.82"0.01 2.70"0.01 2.48"0.01 2.33"0.01 2.19"0.01 1.67"0.01
0.54"0.02 0.50"0.02 0.48"0.02 0.46"0.02 0.42"0.01 0.45"0.02
0.75"0.23 0.46"0.12 0.31"0.04 0.24"0.03 0.18"0.01 0.12"0.01
0.02"0.01 0.04"0.01 0.06"0.01 0.09"0.01 0.11"0.01 0.17"0.01
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Fig. 2. Ža. Peak energy and degree of charge transfer of the CT band for C 60 ramine CT complexes as a function of ionization potential. Open circles: peak energies of C 60 rANI, o-TOL, NMAN, DMA, DEA. Open triangle: peak energies of C 60 rpyrene. Square: peak energies of C 60 rTMPD. Broken curve: fitted result based on Eq. Ž1.. Closed circles, closed square, and closed triangle illustrate the degree of charge transfer obtained by Eq. Ž6.. Žb. Oscillator strength of the CT band as a function of ionization potential. Open circles: C 60 rANI, o-TOL, N-MAN, DMA and DEA. Open square: C 60 rTMPD.
The peak energies Ž " v CT . obtained from the fitting analysis are listed in Table 1 and plotted in Fig. 2a as a function of ionization potential. The peak energy dependence on the ionization potential is approximately linear in this ionization potential range. When we use the vertical ionization potential, a similar dependence is obtained. For a series of related donors with a single acceptor, the peak energy of a CT band is approximately given by " v CT s I P y EA q C1 q
C2 I P y EA q C1
,
Ž 2.
where C1 and C2 are constants for a given acceptor, and I P and EA are the ionization potential of the donor and the electron affinity of acceptor, respectively w9x. For C 60 molecules, EA is estimated as 2.67 eV w10x. We analyzed the experimental data using Eq. Ž2. and the ionization potential dependence of the peak energy was well reproduced taking C1 of y2.38 eV and C2 of 0.543 eV 2 .
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
376
From the absorption spectra, we can extract an oscillator strength. The oscillator strength f is estimated using the formula fs
10 3 = 9m 0 c ´ b
(
2 p 2 e 2 NA Ž ´ b q 2 .
2
Ha Ž v . d v ,
Ž 3.
where a is the molar absorption coefficient, and ´ b and NA are the dielectric constant of solution and the Avogadro number, respectively. We used ´ b s 1.5 which is the refractive index of toluene. The obtained oscillator strengths of the CT band are summarized in Table 1 and shown in Fig. 2b as a function of ionization potential. The oscillator strength increases with increasing ionization potential from f s 0.18 for C 60 rDEA to 0.73 for C 60 rANI. These values are one order of magnitude larger than that of C 60 Ž; 0.014 for the S 0 –S1 transition.. Now, we discuss degrees of charge transfer of C 60 ramine CT complexes. In a Mulliken two-state model w9x, the ground Ž cg . and excited Ž ce . state wave functions of the CT complex are described by a linear combination of no-bond c ŽD 0 ,A0 . and ionic c ŽDq,Ay . states,
(
cg s '1 y r c Ž D 0 ,A0 . q 'r c Ž Dq,Ay .
Ž 4.
ce s '1 y r c Ž Dq,Ay . y 'r c Ž D 0 ,A0 . ,
Ž 5.
where r is the degree of charge transfer. c ŽDq,Ay . differs from c ŽD 0 ,A0 . by the promotion of an electron from the donor to acceptor. In this model, the peak energy of the CT band Ž " v CT . is expressed by Eq. Ž2. and r is given by
rs
C2r2 2 Ž IP y EA q C1 . q C2r2
.
have calculated the charge distribution by the INDO method using the CT-complex configuration that an ANI molecule arranges perpendicular to the spherical surface of C 60 and the amino group is located at the nearest position from C 60 w11x. The calculated value is much larger than the experimental one, which suggests the CT complex configuration used in the calculation is inconsistent with our experiment. We have performed Raman scattering experiments to confirm our observation by using another technique. Fig. 3 shows Raman spectra of C 60 molecules in toluene solution and C 60 rDMA CT complexes. The Raman peak originated from AgŽ2. mode of C 60 molecule is observed. By fitting the spectrum to a Gauss function, the peak frequencies are obtained as 1470.9 cmy1 for C 60 in toluene and 1470.1 cmy1 for C 60 rDMA. C 60rDMA CT complex shows the red shift of 0.8 cmy1 . It is well known that the AgŽ2. mode shifts to the lower wavenumber side with electron doping into a C 60 molecule and the coefficient is y6.1 cmy1 per one electron w12,13x. Thus, r of C 60 rDMA is estimated as ; 0.13 from the measured shift. Although this value is larger than the value determined from the peak energy data Ž0.06., this result supports that r is of the order of 0.1, and indicates that the CT complex configuration used in the calculation of Ref. w11x is not suitable for the C 60 ramine CT complex. We further investigated the ionicity of CT complex with other types of donors to get information on the configuration of C 60 and donor molecules. The distance d between the C 60 and donor molecules affects the CT transition energy " v CT because the
Ž 6.
We can estimate a r value using Eq. Ž6. and the values of C1 and C2 which were determined from the fitting of the peak energy data to Eq. Ž2.. We show r as a function of ionization potential in Fig. 2a by closed circles. The value of r is 0.04–0.07, and decreases with increasing ionization potential. This result indicates that the CT complexes studied here have an almost neutral character in the ground state. In the theoretical calculation, Li et al. have reported r s 0.571 for C 60 rANI CT complex. They
Fig. 3. Raman spectra of C 60 toluene solution and C 60 rDMA CT complex measured at 77 K. The broken curves indicate the fitted Gauss functions.
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
parameter C1 in Eq. Ž2. is related to d by the following equation: C1 ;
e2
´bd
.
Ž 7.
We prepared C 60 CT complexes with two amino groups and without an amino group, and measured absorption spectra. Fig. 4a,b are the absorption spectra of C 60 CT complexes with TMPD Žtwo amino groups. and pyrene Žno amino group., respectively. The absorption bands due to the CT transition are observed in near-infrared and visible regions. The absorption peak energies are plotted for TMPD Žopen square. and pyrene Žopen triangle. in Fig. 2a. These peak energies are well fitted to the same dependence obtained for the CT complexes with the one amino group. We also estimated the degrees of charge transfer for C 60 rTMPD and C 60rpyrene as 0.17 and 0.05, respectively, by using Eq. Ž6. and the values of C1 and C2 which were determined for the CT complex with the one amino group donor. The results are shown in Fig. 2a Žclosed square and closed triangle.. The oscillator strength of C 60 rTMPD CT complex was obtained as 0.12 from the absorption spectrum, and shown in Fig. 2b Žopen square.. This value agrees well with the dependence on the ionization potential obtained for other C 60 ramine CT com-
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plexes. These results indicate that the distance between the C 60 and donor molecules is almost independent of the molecular structure of the amino group. Thus, we propose that the donor molecule in CT complexes studied here arranges parallel to the C 60 spherical surface. In such a configuration, the distance and the degree of charge transfer are not sensitive to the amino group structure. Finally, it is worth mentioning that we need a proper calculation of oscillator strengths of C 60 ramine CT complexes. As the oscillator strength is very sensitive to the molecular configuration and the electron charge distribution in the CT complex, we call for a calculation based on the configuration proposed here. In the C 60 CT complexes, we cannot use a simple model assuming a charge localized at a certain site of the C 60 sphere, since the C 60 molecule has p bonds which are directed radially with a node on the molecular cage. A more realistic model taking into account the molecular orbitals characteristic of C 60 is necessary to explain the observed dependence of the oscillator strength on the ionization potential.
4. Conclusions We have investigated optical transition and ionicity of CT complexes of C 60 with various aromatic amines. We have determined equilibrium constants for complexation: K s 0.02–0.17 depending on the ionization potential of amine. An absorption band due to CT transition is observed in the visible region. With increasing ionization potential, the CT band shifts to the lower energy side and the oscillator strength of CT transition increases. We have obtained the degree of CT r by analyzing the transition energy of the CT band and Raman spectra. The value of r is 0.04–0.17 and decreases with increasing ionization potential. This result indicates that the CT complexes studied here have a neutral character in their ground state.
Acknowledgements Fig. 4. Absorption spectra for CT complexes of C 60 with TMPD Ža. and pyrene Žb.. The broken curves indicate the fitted Gauss functions.
We greatly thank Dr. K. Ishiguro for useful discussions and sample preparation. This work was supported by the Grant-in-Aid for General Scientific
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M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
Research from the Ministry of Education, Science, Sports and Culture of Japan, and the Tatematsu Foundation.
References w1x W. Kratschmer, L.D. Lamb, K. Forstiropoulos, D.R. Huffman, Nature 347 Ž1990. 354. w2x P.A. Allemand, K.C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Gruner, J.D. Thompson, Science 253 Ž1991. 1154. w3x Y. Wang, J. Phys. Chem. 96 Ž1992. 764. w4x Y. Wang, L.-T. Cheng, J. Phys. Chem. 96 Ž1992. 1530.
w5x R. Seshadri, C.N.R. Rao, H. Pal, T. Mukherjee, J.P. Mittal, Chem. Phys. Lett. 205 Ž1993. 395. w6x S.P. Sibley, R.L. Campbell, H.B. Silber, J. Phys. Chem. 99 Ž1995. 5274. w7x A.S. Lobach, N.F. Goldshleger, M.G. Kaplunov, A.V. Kulikov, Chem. Phys. Lett. 243 Ž1995. 22. w8x K. Seki, Mol. Cryst. Liq. Cryst. 171 Ž1989. 255. w9x R.S. Mulliken, W.B. Person, Molecular Complexes Ža lecture and reprint., vol. 419, 1969. w10x C. Brink, L.H. Andersen, P. Hvelplund, D. Mathur, J.D. Voldstad, Chem. Phys. Lett. 233 Ž1995. 52. w11x J. Li, J. Feng, C. Sun, J. Phys. Chem. 98 Ž1994. 8636. w12x P.C. Eklund, A.M. Rao, Y. Wang, P. Zhou, K.-A. Wang, J.M. Holden, M.S. Dresselhaus, G. Dresselhaus, Thin Solid Films 257 Ž1995. 211. w13x Y. Iwasa, T. Kaneyasu, Phys. Rev. B 51 Ž1995. 3678.
Chemical Physics Letters 310 Ž1999. 373–378 www.elsevier.nlrlocatercplett
Optical transition and ionicity of C 60ramine charge-transfer complexes studied by optical spectroscopy Masao Ichida
a,b,)
, Takamitsu Sohda b, Arao Nakamura
a,b
a
b
Center for Integrated Research in Science and Engineering, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-8603, Japan Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya UniÕersity, Chikusa-ku, Nagoya 464-8603, Japan Received 13 January 1999; in final form 17 March 1999
Abstract We have investigated absorption spectra of charge-transfer ŽCT. complexes of C 60 with various aromatic amines with different ionization potentials. An absorption band due to CT transition is observed in the visible region; with increasing ionization potential of amines, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. We extract degrees of charge transfer by analyzing the transition energy of the CT band as a function of ionization potential and Raman spectra. The experimental results show that the CT complexes studied here have a neutral character in their ground state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Since the discovery and establishment of the bulk preparation of fullerenes, in particular C 60 w1x, a great deal of experimental and theoretical work has been done. The photophysics of C 60 molecules in solution is quite well understood. Recently, the donor–acceptor ŽD–A. behavior of fullerenes has received considerable attention from the viewpoint of both basic fullerene chemistry and fullerene-based applications. A C 60 molecule can act as an electron acceptor with a variety of electron donors, and optical and electronic properties for C 60 with various donors have been reported by several groups w2–7x. Treatment of C 60 with amines and their derives results in formation of both ionic and neutral fullerene ) Corresponding author. Fax: q81 52 789 3724; e-mail: [email protected]
amino derivatives. The treatment of C 60 with the tertiary amine tetrakis Ždimethylamino .ethylene ŽTDAE. leads to the salt TDAEq Cy 60 which reveals an unusual molecular ferromagnetism w2x. Sibley et al. have reported CT absorption spectra for C 60 CT complexes with aniline, o-toluidine, etc. w6x. They observed that the CT absorption peak shifts to the higher energy side for different donors. However, an oscillator strength of the CT transition and a degree of charge transfer of the CT complex have not yet been studied. The ionicity which is determined by the degree of charge transfer is one of the important parameters which determines electronic and optical properties in the derives. In this Letter, we have systematically investigated absorption spectra of CT complexes of C 60 with various aromatic amines, and extracted degrees of charge transfer and oscillator strengths of the CT transition as a function of the ionization potential.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 7 7 - 6
374
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
From analysis of the transition energy, depending on the ionization potential, we have obtained degrees of CT r in various C 60 amine CT complexes. The value of r obtained is 0.04–0.17, which indicates that the CT complexes studied here have a neutral character in their ground state.
2. Experimental Pure C 60 Ž) 99.9%. powders and commercially purchased amines which were purified by distillation were used for all measurements. We prepared C 60 CT complexes with different amines: aniline ŽANI., o-toluidine Ž o-TOL., N-methylaniline Ž N-MAN., N, N-dimethylaniline ŽDMA., and N, N-diethylaniline ŽDEA.. The ionization potentials of isolated ANI, o-TOL, N-MAN, DMA, and DEA are 7.72, 7.44, 7.32, 7.12 and 6.95 eV, respectively w8x. These amines consist of a one-amino group. We also prepared CT complexes with other types of donors such as pyrene and N, N, N X , N X-tetramethyl-1,4-phenylenediamine ŽTMPD.. Pyrene has no amino group and TMPD consists of two amino groups. The ionization potentials of pyrene and TMPD are 7.37 and 6.20 eV, respectively. Here, we use the adiabatic ionization potentials instead of vertical ones, because the values of the adiabatic ionization potential are known for all the donors used in this study. Spectroscopicgrade toluene was used as solvent for C 60 ramines and C 60 rpyrene. We used 1-chloronaphthalene as solvent for C 60 rTMPD. The concentrations of C 60 in solution are 2 = 10y4 M for ANI, 5 = 10y4 M for o-TOL, 1 = 10y3 M for N-MAN, DMA, and DEA, 5 = 10y3 M for TMPD. Absorption spectra were taken using a Hitachi U-3500 spectrometer. The equilibrium constants of solutions were determined by analyzing the absorption spectra as a function of amine concentration. All spectra were run in matched quartz cuvettes against the corresponding amine blank in order to cancel out amine absorbance and to correct for variations in dielectric constant of the solutions. Raman spectra were measured using Jovin-Yvon U1000 monochromator with liquid N2cooled CCD. The spectral resolution of Raman spectrum is ; 0.8 cmy1 and the precision of Raman shift is ; 0.2 cmy1 by using line shape analysis.
3. Results and discussion Changing donor concentrations, we measured absorption spectra and determined equilibrium constants. The inset of Fig. 1 shows the absorption spectra of C 60 in toluene with addition of o-TOL. The weak absorption band due to the HOMO–LUMO ŽS 0 –S 1 . forbidden transition is observed around 2 eV and the strong absorption bands due to allowed S 0 –S 2 and S 0 –S 3 transitions are observed above 3.5 eV Žnot shown.. A new absorption band due to the formation of CT complexes in solution appears at ; 2.7 eV and grows with increasing o-TOL concentration. The equilibrium process of complex formation is written as FqD|C ,
Ž 1.
where F, D, and C represent C 60 , amine, and complex, respectively. The equilibrium constant K, defined as wCxrwFxwDx, can be determined by monitoring the CT absorption intensity as a function of amine concentration wDx. By plotting Ž1 y OD 0r OD.rwDx versus OD 0rOD ŽBenesi–Hildebrand plot.,
Fig. 1. Absorption spectra for CT complexes of C 60 with various amines at room temperature: Ža. ANI, Žb. o-TOL, Žc. N-MAN, Žd. DMA, Že. DEA, and Žf. C 60 toluene solution. Broken curves: fitted Gauss functions. Inset: Absorption spectra of C 60 Ž5=10y4 M. in toluene with addition of o-TOL. The o-TOL concentrations are 0, 0.93, 2.80, 4.67, 6.53, and 9.33 M Žfrom bottom to top..
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
where OD is the absorbance of C 60 rtoluene-amine solution and OD 0 is the absorbance in the absence of amines, we fit a straight line to the data. The intercept of the straight line yields yK. The obtained value of K for C 60 ro-TOL complex is 0.04. The equilibrium constants of C 60 CT complex with various amines are listed in Table 1. The K value of C 60 rDEA is comparable to the result reported by Wang et al. w4x. The K values for C 60 rANI and C 60 rDMA are much smaller than the results reported by Sibley et al. for C 60 rANI w6x and by Seshadri et al. for C 60 rDMA w5x. The discrepancy may be due to the donor concentration range in the Benesi–Hildebrand plot. We measured the absorption spectra in the wide concentration range Ž0–11 M. compared to their measurements Ž0–1.5 M.. The Benesi–Hildebrand plot in the wide concentration range yields a more accurate value of the equilibrium constant. The value of K increases with decreasing ionization potential of amine. This result indicates that a donor with a lower ionization potential more easily forms a CT complex in solution. Fig. 1 shows the absorption spectra of C 60 CT complexes with various amines. These spectra were obtained by subtracting the absorption component of C 60 molecules from the spectra measured for the solvent of C 60 ramine. We observed the absorption band due to the CT complex which shifts to the higher energy side depending on the donor species. The molar absorption coefficient a is also dependent on the donor species and increases with increasing ionization potential. The absorption spectrum was analyzed by fitting to the Gauss function, and the results are illustrated by broken curves in Fig. 1.
Table 1 Ionization potential Ž I P ., equilibrium constant Ž K ., peak energy of CT band Ž " v CT ., CT band width Ž D ., oscillator strength Ž f . with substituted amines Amine
IP K ŽeV.
" v CT ŽeV.
D ŽeV.
f
ANI o-TOL N-MAN DMA DEA TMPD
7.72 7.44 7.32 7.12 6.95 6.20
2.82"0.01 2.70"0.01 2.48"0.01 2.33"0.01 2.19"0.01 1.67"0.01
0.54"0.02 0.50"0.02 0.48"0.02 0.46"0.02 0.42"0.01 0.45"0.02
0.75"0.23 0.46"0.12 0.31"0.04 0.24"0.03 0.18"0.01 0.12"0.01
0.02"0.01 0.04"0.01 0.06"0.01 0.09"0.01 0.11"0.01 0.17"0.01
375
Fig. 2. Ža. Peak energy and degree of charge transfer of the CT band for C 60 ramine CT complexes as a function of ionization potential. Open circles: peak energies of C 60 rANI, o-TOL, NMAN, DMA, DEA. Open triangle: peak energies of C 60 rpyrene. Square: peak energies of C 60 rTMPD. Broken curve: fitted result based on Eq. Ž1.. Closed circles, closed square, and closed triangle illustrate the degree of charge transfer obtained by Eq. Ž6.. Žb. Oscillator strength of the CT band as a function of ionization potential. Open circles: C 60 rANI, o-TOL, N-MAN, DMA and DEA. Open square: C 60 rTMPD.
The peak energies Ž " v CT . obtained from the fitting analysis are listed in Table 1 and plotted in Fig. 2a as a function of ionization potential. The peak energy dependence on the ionization potential is approximately linear in this ionization potential range. When we use the vertical ionization potential, a similar dependence is obtained. For a series of related donors with a single acceptor, the peak energy of a CT band is approximately given by " v CT s I P y EA q C1 q
C2 I P y EA q C1
,
Ž 2.
where C1 and C2 are constants for a given acceptor, and I P and EA are the ionization potential of the donor and the electron affinity of acceptor, respectively w9x. For C 60 molecules, EA is estimated as 2.67 eV w10x. We analyzed the experimental data using Eq. Ž2. and the ionization potential dependence of the peak energy was well reproduced taking C1 of y2.38 eV and C2 of 0.543 eV 2 .
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From the absorption spectra, we can extract an oscillator strength. The oscillator strength f is estimated using the formula fs
10 3 = 9m 0 c ´ b
(
2 p 2 e 2 NA Ž ´ b q 2 .
2
Ha Ž v . d v ,
Ž 3.
where a is the molar absorption coefficient, and ´ b and NA are the dielectric constant of solution and the Avogadro number, respectively. We used ´ b s 1.5 which is the refractive index of toluene. The obtained oscillator strengths of the CT band are summarized in Table 1 and shown in Fig. 2b as a function of ionization potential. The oscillator strength increases with increasing ionization potential from f s 0.18 for C 60 rDEA to 0.73 for C 60 rANI. These values are one order of magnitude larger than that of C 60 Ž; 0.014 for the S 0 –S1 transition.. Now, we discuss degrees of charge transfer of C 60 ramine CT complexes. In a Mulliken two-state model w9x, the ground Ž cg . and excited Ž ce . state wave functions of the CT complex are described by a linear combination of no-bond c ŽD 0 ,A0 . and ionic c ŽDq,Ay . states,
(
cg s '1 y r c Ž D 0 ,A0 . q 'r c Ž Dq,Ay .
Ž 4.
ce s '1 y r c Ž Dq,Ay . y 'r c Ž D 0 ,A0 . ,
Ž 5.
where r is the degree of charge transfer. c ŽDq,Ay . differs from c ŽD 0 ,A0 . by the promotion of an electron from the donor to acceptor. In this model, the peak energy of the CT band Ž " v CT . is expressed by Eq. Ž2. and r is given by
rs
C2r2 2 Ž IP y EA q C1 . q C2r2
.
have calculated the charge distribution by the INDO method using the CT-complex configuration that an ANI molecule arranges perpendicular to the spherical surface of C 60 and the amino group is located at the nearest position from C 60 w11x. The calculated value is much larger than the experimental one, which suggests the CT complex configuration used in the calculation is inconsistent with our experiment. We have performed Raman scattering experiments to confirm our observation by using another technique. Fig. 3 shows Raman spectra of C 60 molecules in toluene solution and C 60 rDMA CT complexes. The Raman peak originated from AgŽ2. mode of C 60 molecule is observed. By fitting the spectrum to a Gauss function, the peak frequencies are obtained as 1470.9 cmy1 for C 60 in toluene and 1470.1 cmy1 for C 60 rDMA. C 60rDMA CT complex shows the red shift of 0.8 cmy1 . It is well known that the AgŽ2. mode shifts to the lower wavenumber side with electron doping into a C 60 molecule and the coefficient is y6.1 cmy1 per one electron w12,13x. Thus, r of C 60 rDMA is estimated as ; 0.13 from the measured shift. Although this value is larger than the value determined from the peak energy data Ž0.06., this result supports that r is of the order of 0.1, and indicates that the CT complex configuration used in the calculation of Ref. w11x is not suitable for the C 60 ramine CT complex. We further investigated the ionicity of CT complex with other types of donors to get information on the configuration of C 60 and donor molecules. The distance d between the C 60 and donor molecules affects the CT transition energy " v CT because the
Ž 6.
We can estimate a r value using Eq. Ž6. and the values of C1 and C2 which were determined from the fitting of the peak energy data to Eq. Ž2.. We show r as a function of ionization potential in Fig. 2a by closed circles. The value of r is 0.04–0.07, and decreases with increasing ionization potential. This result indicates that the CT complexes studied here have an almost neutral character in the ground state. In the theoretical calculation, Li et al. have reported r s 0.571 for C 60 rANI CT complex. They
Fig. 3. Raman spectra of C 60 toluene solution and C 60 rDMA CT complex measured at 77 K. The broken curves indicate the fitted Gauss functions.
M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
parameter C1 in Eq. Ž2. is related to d by the following equation: C1 ;
e2
´bd
.
Ž 7.
We prepared C 60 CT complexes with two amino groups and without an amino group, and measured absorption spectra. Fig. 4a,b are the absorption spectra of C 60 CT complexes with TMPD Žtwo amino groups. and pyrene Žno amino group., respectively. The absorption bands due to the CT transition are observed in near-infrared and visible regions. The absorption peak energies are plotted for TMPD Žopen square. and pyrene Žopen triangle. in Fig. 2a. These peak energies are well fitted to the same dependence obtained for the CT complexes with the one amino group. We also estimated the degrees of charge transfer for C 60 rTMPD and C 60rpyrene as 0.17 and 0.05, respectively, by using Eq. Ž6. and the values of C1 and C2 which were determined for the CT complex with the one amino group donor. The results are shown in Fig. 2a Žclosed square and closed triangle.. The oscillator strength of C 60 rTMPD CT complex was obtained as 0.12 from the absorption spectrum, and shown in Fig. 2b Žopen square.. This value agrees well with the dependence on the ionization potential obtained for other C 60 ramine CT com-
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plexes. These results indicate that the distance between the C 60 and donor molecules is almost independent of the molecular structure of the amino group. Thus, we propose that the donor molecule in CT complexes studied here arranges parallel to the C 60 spherical surface. In such a configuration, the distance and the degree of charge transfer are not sensitive to the amino group structure. Finally, it is worth mentioning that we need a proper calculation of oscillator strengths of C 60 ramine CT complexes. As the oscillator strength is very sensitive to the molecular configuration and the electron charge distribution in the CT complex, we call for a calculation based on the configuration proposed here. In the C 60 CT complexes, we cannot use a simple model assuming a charge localized at a certain site of the C 60 sphere, since the C 60 molecule has p bonds which are directed radially with a node on the molecular cage. A more realistic model taking into account the molecular orbitals characteristic of C 60 is necessary to explain the observed dependence of the oscillator strength on the ionization potential.
4. Conclusions We have investigated optical transition and ionicity of CT complexes of C 60 with various aromatic amines. We have determined equilibrium constants for complexation: K s 0.02–0.17 depending on the ionization potential of amine. An absorption band due to CT transition is observed in the visible region. With increasing ionization potential, the CT band shifts to the lower energy side and the oscillator strength of CT transition increases. We have obtained the degree of CT r by analyzing the transition energy of the CT band and Raman spectra. The value of r is 0.04–0.17 and decreases with increasing ionization potential. This result indicates that the CT complexes studied here have a neutral character in their ground state.
Acknowledgements Fig. 4. Absorption spectra for CT complexes of C 60 with TMPD Ža. and pyrene Žb.. The broken curves indicate the fitted Gauss functions.
We greatly thank Dr. K. Ishiguro for useful discussions and sample preparation. This work was supported by the Grant-in-Aid for General Scientific
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M. Ichida et al.r Chemical Physics Letters 310 (1999) 373–378
Research from the Ministry of Education, Science, Sports and Culture of Japan, and the Tatematsu Foundation.
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