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Study of ground state EDA complex formation between [70]fullerene and a series of polynuclear aromatic hydrocarbons
Spectrochimica Acta Part A 58 (2002) 289– 298 www.elsevier.com/locate/saa
Study of ground state EDA complex formation between [70]fullerene and a series of polynuclear aromatic hydrocarbons Sumanta Bhattacharya a, Sandip K. Nayak b, Subrata Chattopadhyay b, Manas Banerjee a, Asok K. Mukherjee a,* a b
Department of Chemistry, The Uni6ersity of Burdwan, Golapbag, Burdwan 713104, India Bio-Organic Di6ision, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 2 April 2001; accepted 14 May 2001
Abstract [70]fullerene has been shown to form 1:1 EDA complex with anthracene, naphthalene, phenanthrene, pyrene and acenaphthene in CCl4 medium. Charge transfer (CT) bands have been detected in all the cases. Isosbestic points have been observed in the cases of phenanthrene and acenaphthene complexes. Ionisation potentials of the donors and CT transition energies have been found to correlate in accordance with Mulliken equation and from this correlation the electron affinity of C70 has been found to be 2.59 eV. Enthalpies and entropies of formation of the complexes have been estimated from the formation constants of the complexes determined spectrophotometrically at three different temperatures. © 2002 Elsevier Science B.V. All rights reserved. Keywords: [70]fullerene; EDA complex; Electron affinity; Complexation enthalpy and entropy
1. Introduction The novel material [70]fullerene is finding [1,2] interesting applications in the fields of organic chemistry [3–7] and in photophysics [8 – 10]. Theoretical [11 –13] and electrochemical studies [14 – 16] reveal that C70 can accept a maximum of six electrons and reduction potentials corresponding to [C70]n − , n=3–6, have been determined. Thus * Corresponding author. Tel.: + 91-342-558-545; fax: + 91342-64-452. E-mail address: [email protected] (A.K. Mukherjee).
C70 is expected to behave as an efficient electron acceptor in forming charge transfer (CT) complexes. However, the number of such complexes studied in solution with C70 is much fewer [17,18] than those with C60 as acceptor [19 –24]. Konarev et al. [17] have determined and utilised the CT transition energies of the complexes of [60] and [70]fullerenes with a series of tetrathiafulvalene and related donors to estimate the electron affinities of C60 and C70. Recently [25] EDA interaction of C70 with a series of methylbenzenes have been studied. However, no study has yet been carried out to quantify the interactions of C70 with non-
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S. Bhattacharya et al. / Spectrochimica Acta Part A 58 (2002) 289–298 Table 1 CT absorption maxima and transition energies of C70–PAH complexes and ionisation potentials of the donors Donor
uCT (nm)a
hwCT (eV)
ID (eV)
Naphthalene Anthracene Acenaphthene Pyrene phenanthrene
477 486 566 521 619
2.60 2.55 2.19 2.38 2.00
8.12 7.40 7.66 7.55 7.85
a
Fig. 1. Absorption spectra of C70 and three of its EDA complexes in CCl4: (1) C70 (5.4977× 10 − 5 mol dm − 3)+ pyrene (0.761 mol dm − 3); (2) C70 (5.4977× 10 − 5 mol dm − 3)+ acenaphthene (0.947 mol dm − 3); (3) C70 (5.4977× 10 − 5 mol dm − 3)+ naphthalene (1.627 mol dm − 3) against the pristine C70 solution as reference.
The longest wavelength in case of multiple CT peaks.
substituted multi-ring aromatic hydrocarbons, and no thermodynamic data have so far been presented for C70-aromatic p-donor complexes. The present paper reports a series of results for complexation of C70 with anthracene, naphthalene, phenanthrene, pyrene and acenaphthene in CCl4 medium by electronic absorption spectroscopy. C70 is shown to form 1:1 EDA complexes with the above donors. CT bands have been detected in all the cases. Isosbestic points have been observed in the cases of phenanthrene and acenaphthene complexes. Dependence of the CT transition energies (hwCT) of the C70 complexes on donor ionisation potentials have been analysed
Fig. 2. Plot of 2I vD –hwCT against I vD (I vD −hwCT) for C70 – PAH system.
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Fig. 3. Absorption spectra of (1) C70 (3.0247× 10 − 5 mol dm − 3) +phenanthrene (0.032 mol dm − 3); (2) C70 (3.0247 ×10 − 5 mol dm − 3)+ phenanthrene (0.089 mol dm − 3); (3) C70 (3.0247 ×10 − 5 mol dm − 3) +phenanthrene (0.106 mol dm − 3); (4) C70 (3.0247× 10 − 5 mol dm − 3) in CCl4 against the solvent as reference.
in the light of Mulliken’s theory [26] and thence the electron affinity of C70 has been found to be 2.59 eV. Enthalpies and entropies of formation of the
complexes have been estimated from the formation constants of the complexes determined spectrophotometrically at three different temperatures.
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Fig. 4. Benesi – Hildebrand plot for C70 – acenaphthene system at 299 K.
Fig. 5. Plot for determination of enthalpy of formation of the C70 – phenanthrene complex.
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Table 2 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–naphthalene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
298
0.242 0.385 0.315 0.224 0.216 0.242 0.385 0.224 0.291 0.471 0.367 0.242 0.315 0.224 0.367 0.291
3.553
0.385 0.373 0.380 0.396 0.382 0.387 0.390 0.394 0.403 0.370 0.401 0.385 0.372 0.381 0.361 0.387
10.6
4496.2
306
316
a
3.553
3.553
8.9
6.5
Measured against the solvent as reference.
Table 3 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–pyrene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
298
0.022 0.029 0.059 0.066 0.071 0.086 0.099 0.022 0.029 0.044 0.059 0.071 0.022 0.029 0.059 0.066 0.071 0.086 0.099
1.589
0.345 0.489 0.398 0.448 0.446 0.476 0.442 0.353 0.376 0.356 0.395 0.415 0.378 0.369 0.389 0.435 0.386 0.402 0.522
64.6
14 705.8
307
317
a
Measured against the solvent as reference.
1.589
1.589
51.1
32.6
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294
2. Materials and methods [70]fullerene, was collected from SES Research, Houston, TX. Naphthalene and phenanthrene were purified by sublimation. Anthracene from Sigma was used without further purification. Acenaphthene was purified by recrystallisation from purified ethanol. Pyrene was purified by recrystallisation form dry ethanol in dark. The solvent, CCl4 was of HPLC grade. All spectral measurements were carried out in a Shimadzu UV-2101 PC model spectrophotometer fitted with TB-85 thermo bath.
taining C70 + anthracene and C70 + phenanthrene. Except for anthracene, multiple CT peaks were observed for each of the donors studied. The wavelengths at these new absorption maxima and the corresponding CT transition energies (hwCT) are summarised in Table 1. While calculating hwCT the longest wavelength peaks were considered for the complexes having multiple peaks. According to Mulliken’s theory [26] the CT transition energies are related to the vertical ionisation potentials (I vD) of the donors by the relation, hwCT = I vD –C1 + C2/(I vD –C1)
(1)
Here, C1 = E vA + G1 + G0
3. Results and discussion
(2)
v A
3.1. CT spectra Fig. 1 shows the electronic absorption spectra of three mixtures containing respectively acenaphthene +C70, pyrene +C70 and naphthalene+ C70 in CCl4 medium against the respective pristine C70 solutions as reference. It is observed that new absorption peaks appear in the visible range where the PAH donors do not absorb. Similar spectral features were obtained with mixtures con-
where E is the vertical electron affinity of the acceptor, G0 is the sum of several energy terms (like dipole–dipole, van der Waals interaction, etc) in the ‘no-bond’ state and G1 is the sum of a number of energy terms in the ‘dative’ state. In most cases, G0 is small and can be neglected while G1 is largely the electrostatic energy of attraction between D+ and A−. The term C2 in Eq. (1) is related to the resonance energy of interaction between the ‘no-bond’ and ‘dative’ forms in the ground and excited states and for a given acceptor
Table 4 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–phenanthrene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
299
0.107 0.122 0.200 0.299 0.032 0.107 0.122 0.299 0.052 0.032 0.122 0.089 0.107 0.153 0.217
3.025
0.522 0.583 0.445 0.544 0.485 0.483 0.495 0.512 0.515 0.473 0.527 0.436 0.434 0.443 0.494
59.2
3013.3
309
317
a
Measured against the solvent as reference.
3.025
3.553
25.5
12.3
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Table 5 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–acenaphthene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
299
0.087 0.157 0.269 0.250 0.380 0.417 0.535 0.586 0.849 0.087 0.157 0.269 0.250 0.380 0.417 0.535 0.586 0.849 0.087 0.157 0.269 0.250 0.380 0.417 0.535 0.586 0.849
1.589
0.387 0.406 0.407 0.417 0.426 0.424 0.446 0.414 0.436 0.366 0.385 0.408 0.417 0.427 0.414 0.423 0.381 0.439 0.357 0.388 0.391 0.422 0.432 0.411 0.440 0.471 0.450
28.9
14 615.8
309
317
a
1.589
1.589
22.0
11.1
Measured against the solvent as reference.
it may be supposed constant [26]. A rearrangement of Eq. (1) yields 2I vD –hwCT =(1/C1)I vD(I vD – hwCT) + (C1 +C2/C1) (3) The ionisation potentials of anthracene, naphthalene, phenanthrene, pyrene and acenaphthene were collected from Ref. [27]. With the observed transition energies we have obtained the correlation, 2I vD –hwCT =(0.148890.0052)I vD(I vD – hwCT) +(6.90879 0.2197)
(4)
with a correlation coefficient of 0.99 (Fig. 2). This confirms the CT nature of the transition observed and yields C1 = 6.72 eV. Neglecting G0 and taking
the typical D–A distance in p-type EDA complexes to be 3.5 A, the major part of G1 is estimated as e 2/4ym0r= 4.13 eV. Now using Eq. (2) the electron affinity (EA) of C70 in solution is found to be 2.59 eV, which is in fair agreement with the value 2.73 eV obtained in gas phase by Boltalina et al. [28]. The difference (0.14 eV) between the electron affinity values in gas phase and in solution is due to solvation effect which has significant contribution to G1. Owing to lack of suitable data this contribution can not be estimated correctly. Isosbestic points were observed in the cases of C70 –acenaphthene (Fig. 3) and C70 –phenanthrene system at 407 and 574 nm, respectively. With the other three PAHs no such points were observed.
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Table 6 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–anthracene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
301
0.0037 0.0075 0.0110 0.0112 0.0116 0.0185 0.0056 0.0055 0.0074 0.0116 0.0075 0.0110 0.0112 0.0185 0.0037 0.0056 0.0055 0.0074 0.0116 0.0075 0.0110 0.0112
1.589
0.369 0.343 0.343 0.362 0.335 0.343 0.348 0.325 0.313 0.316 0.329 0.320 0.297 0.322 0.302 0.332 0.293 0.312 0.277 0.290 0.283 0.304
489.7
3939.8
309
317
a
1.589
1.589
387.3
239.8
Measured against the solvent as reference.
3.2. Determination of formation constant (K) Owing to low intensities of the CT peaks, variation of their intensity with change in donor concentration could not be employed for determination of stoichiometry and formation constants (K) of the complexes. Instead, it was observed that the intensity of the broad absorption band of C70 (measured against the solvent CCl4 as reference), which is centered more or less at 472 nm, decreased systematically with gradual addition of the donors and this fact was utilised to determine K by using Benesi– Hildebrand [29] equation for cells with 1 cm optical path length: [A]0[D]0/d / =[D]0/m / +1/Km /
(5)
absorbance of the donor–acceptor mixture at 472 nm measured against the solvent as reference, d 0A and d 0D are the absorbances at 472 nm of the acceptor and donor solutions with same molar concentrations as in the mixture. The quantity m / = mc − mA − mD means the molar absorptivity of the complex and mA and mD are those of the acceptor and the donor respectively at 472 nm. K is the formation constant of the complex. Eq. (5) is valid [29] under the condition [D]0 [A] for 1:1 donor–acceptor complex. Experimental data are shown in Tables 2–6. In all the cases very good linear plots according to Eq. (5) were obtained, one typical case being shown in Fig. 4. Values of K and m / of the complexes obtained from such plots are also shown in Tables 2–6.
with d / = d −d 0A −d 0d
(6)
Here [A]0 and [D]0 are the initial concentrations of the acceptor and donor respectively, d is the
3.3. Enthalpies (ZH 0f ) and entropies of formation (ZS 0f ) of the complexes The enthalpies (DH 0f ) and entropies of form-
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297
Table 7 Enthalpies and entropies of formation of the C70–PAH complexes Donor
Temperature (K)
Formation constant (K) (mol dm−3)
DH 0f (kJ mol−1)
Naphthalene
298 306 316 299 309 317 298 307 317 301 309 317 301 309 317
10.6 8.9 6.5 59.2 25.5 12.3 64.6 51.1 32.6 489.7 387.3 239.9 28.9 22.0 11.1
−21.49 2.5
−52.098.2
−65.79 0.5
−185.891.5
Phenanthrene
Pyrene
Anthracene
Acenaphthene
ation (DS 0f ) of the complexes have been determined by using the Eq. (7). As measured, these terms will represent the net change in enthalpy and entropy for the solvated species. ln K = − DH 0f /RT +constant
(7)
Values of K at three different temperatures are summarised in Table 7. The following linear regression equations have been obtained with the present data: Naphthalene–C70 complex: ln K =2574/T − 6.25; correlation coefficient =0.95.
(8a)
ln K = 7902/T − 22.3; correlation coefficient (8b)
Pyrene–C70 complex: ln K =3200/T − 6.5; correlation coefficient =0.97. Anthracene–C70 complex:
(8c)
ln K = 4060/T −7.2; correlation coefficient = 0.98. Acenaphthene–C70 complex:
−26.69 6.6
−54.6918.1
−33.79 5.95
−60.5919.7
−45.39 10.1
−122.4933.6
ln K= 5446/T − 14.7; correlation coefficient = 0.97.
(8e)
DH 0f and DS 0f determined from the above correlations are shown in Table 7. One typical plot of ln K against 1/T is shown in Fig. 5. To calculate DS 0f the following equation was employed. DS 0f = (RT ln K+ DH 0f )/T
(9)
4. Conclusion
Phenanthrene–C70 complex:
=0.97.
DS 0f (J K−1 mol−1)
(8d)
The current study demonstrates significant interaction of C70 with polynuclear aromatic hydrocarbons. The high formation constant of the anthracene complex compared to those of the other PAHs studied may be attributed to the fact that the linear arrangement of the hexagonal rings of anthracene molecule with long p-conjugation is similar to that in the belt region of the C70 molecule. The CT nature of the transitions in the complexes has been established and a reasonable value of electron affinity of C70 has been determined. The vertical EA of C70 estimated from hwCT energies in solution has been found to be 0.14 eV lower than those in the gas phase presumably due to solvation effects.
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Acknowledgements S. Bhattacharya thanks the Council of Scientific and Industrial Research (C.S.I.R.), India for a Junior Research Fellowship (J.R.F.). Financial assistance by the U.G.C., India extended through the DSA project in Chemistry, is also gratefully acknowledged. References [1] E.A. Rohlfing, D.M. Cox, A. Kaldor, J. Chem. Phys. 81 (1984) 3322. [2] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162. [3] A.G. Avent, P.R. Birkett, A.D. Darwish, W. Kroto, R. Taylor, D.R.M. Walton, J. Chem. Soc., Perkin Trans. 2 (2001) 68. [4] R. Taylor, A.K. Abdul-Sada, O.V. Boltalina, J.M. Street, J. Chem. Soc., Perkin Trans. 2 (2000) 1013. [5] K.M. Rogers, P.W. Fowler, Chem. Comm., (1999) 2357. [6] P.W. Fowler, J.P.B. Sandall, R. Taylor, J. Chem. Soc., Perkin Trans. 2 (1997) 419. [7] R.G. Bergosh, M.S. Meier, J.A. Laske-Cooke, H.P. Spielmann, B.R. Weedon, J. Org. Chem. 62 (1997) 7667. [8] A. Graja, R. Lipiec, S. Waplak, S. Krol, I. Turowskatyrk, N.V. Drichko, Chem. Phys. Lett. 313 (1999) 725. [9] D.M. Guldi, D. Liu, P.V. Kamat, J. Phys. Chem. A 101 (1997) 6195. [10] O. Ito, Y. Sasaki, A. Watanabe, R. Hoffmann, C. Siedschlag, J. Mattay, J. Chem. Soc., Perkin Trans. 2 (1997) 1007. [11] Q. Xie, E. Perez-Cordero, L. Echegoyen, J. Am. Chem. Soc. 114 (1992) 11004. [12] K.M. Rogers, P.W. Fowler, J. Chem. Soc., Perkin Trans.
2 (2001) 18. [13] W.-L. Jhu, C.M. Puah, K.C. Ng, H.-L. Jiang, X.-J. Tan, K.-X. Chen, J. Chem. Soc., Perkin Trans. 2 (2001) 233. [14] D. Dubosis, K.M. Kadish, S. Flanagan, L.J. Wilson, J. Am. Chem. Soc 113 (1991) 7773. [15] A.G. Avent, P.R. Birkett, M. Carano, A.D. Darwish, H.W. Kroto, J.O. Lopez, F. Paolucci, S. Roffia, R. Taylor, N. Wachter, D.R.M. Walton, F. Zerbetto, J. Chem. Soc., Perkin Trans. 2 (2001) 140. [16] Q. Xie, F. Arias, L. Echegoyen, J. Am. Chem. Soc 115 (1993) 9818. [17] D.V. Konarev, R.N. Lyubovskaya, N.V. Drichko, V.N. Semkin, A. Graja, Chem. Phys. Lett. 314 (1999) 570. [18] V.V. Gritsenko, O.A. D/yachenko, D.V. Konarev, R.N. Lyubovskaya, Russ. Chem. Bull. 49 (2000) 102. [19] S.P. Sibley, Y.T. Nguyen, R.L. Campbell, H.B. Silber, Spectrochim. Acta Part A 53 (1997) 679. [20] S. Nath, D.K. Palit, A.V. Sapre, Chem. Phys. Lett. 330 (2000) 255. [21] D.V. Konarev, A. Yu. Kovalevsky, P. Coppens, R.N. Lyubovskaya, Chem. Comm. (2000) 2357. [22] D.V. Konarev, R.N. Lyubovskaya, N.V. Drichko, E.I. Yudanova, Yu. M. Shul’ga, A.L. Litvinov, V.N. Semkin, B.P. Tarasov, J. Mater. Chem., (2000) 803. [23] K. Datta, M. Banerjee, B.K. Seal, A.K. Mukherjee, J. Chem. Soc., Perkin Trans. 2 (2000) 531. [24] S. Bhattacharya, M. Banerjee, A.K. Mukherjee, Spectrochim. Acta Part A 57 (2001) 1463. [25] S. Bhattacharya, S.K. Nayak, S.K. Chattopadhyay, M. Banerjee, A.K. Mukherjee, Spectrochim. Acta Part A 57 (2001) 309. [26] R.S. Mulliken, J. Am. Chem. Soc. 74 (1952) 811. [27] J.B. Birks, Photophysics of Aromatic Molecules, Wiley Interscience, New York, 1969, pp. 459 – 460. [28] O.V. Boltalina, E.V. Dashkova, L.N. Sidorov, Chem. Phys. Lett. 256 (1996) 253. [29] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.
Study of ground state EDA complex formation between [70]fullerene and a series of polynuclear aromatic hydrocarbons Sumanta Bhattacharya a, Sandip K. Nayak b, Subrata Chattopadhyay b, Manas Banerjee a, Asok K. Mukherjee a,* a b
Department of Chemistry, The Uni6ersity of Burdwan, Golapbag, Burdwan 713104, India Bio-Organic Di6ision, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 2 April 2001; accepted 14 May 2001
Abstract [70]fullerene has been shown to form 1:1 EDA complex with anthracene, naphthalene, phenanthrene, pyrene and acenaphthene in CCl4 medium. Charge transfer (CT) bands have been detected in all the cases. Isosbestic points have been observed in the cases of phenanthrene and acenaphthene complexes. Ionisation potentials of the donors and CT transition energies have been found to correlate in accordance with Mulliken equation and from this correlation the electron affinity of C70 has been found to be 2.59 eV. Enthalpies and entropies of formation of the complexes have been estimated from the formation constants of the complexes determined spectrophotometrically at three different temperatures. © 2002 Elsevier Science B.V. All rights reserved. Keywords: [70]fullerene; EDA complex; Electron affinity; Complexation enthalpy and entropy
1. Introduction The novel material [70]fullerene is finding [1,2] interesting applications in the fields of organic chemistry [3–7] and in photophysics [8 – 10]. Theoretical [11 –13] and electrochemical studies [14 – 16] reveal that C70 can accept a maximum of six electrons and reduction potentials corresponding to [C70]n − , n=3–6, have been determined. Thus * Corresponding author. Tel.: + 91-342-558-545; fax: + 91342-64-452. E-mail address: [email protected] (A.K. Mukherjee).
C70 is expected to behave as an efficient electron acceptor in forming charge transfer (CT) complexes. However, the number of such complexes studied in solution with C70 is much fewer [17,18] than those with C60 as acceptor [19 –24]. Konarev et al. [17] have determined and utilised the CT transition energies of the complexes of [60] and [70]fullerenes with a series of tetrathiafulvalene and related donors to estimate the electron affinities of C60 and C70. Recently [25] EDA interaction of C70 with a series of methylbenzenes have been studied. However, no study has yet been carried out to quantify the interactions of C70 with non-
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 5 4 3 - 1
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S. Bhattacharya et al. / Spectrochimica Acta Part A 58 (2002) 289–298 Table 1 CT absorption maxima and transition energies of C70–PAH complexes and ionisation potentials of the donors Donor
uCT (nm)a
hwCT (eV)
ID (eV)
Naphthalene Anthracene Acenaphthene Pyrene phenanthrene
477 486 566 521 619
2.60 2.55 2.19 2.38 2.00
8.12 7.40 7.66 7.55 7.85
a
Fig. 1. Absorption spectra of C70 and three of its EDA complexes in CCl4: (1) C70 (5.4977× 10 − 5 mol dm − 3)+ pyrene (0.761 mol dm − 3); (2) C70 (5.4977× 10 − 5 mol dm − 3)+ acenaphthene (0.947 mol dm − 3); (3) C70 (5.4977× 10 − 5 mol dm − 3)+ naphthalene (1.627 mol dm − 3) against the pristine C70 solution as reference.
The longest wavelength in case of multiple CT peaks.
substituted multi-ring aromatic hydrocarbons, and no thermodynamic data have so far been presented for C70-aromatic p-donor complexes. The present paper reports a series of results for complexation of C70 with anthracene, naphthalene, phenanthrene, pyrene and acenaphthene in CCl4 medium by electronic absorption spectroscopy. C70 is shown to form 1:1 EDA complexes with the above donors. CT bands have been detected in all the cases. Isosbestic points have been observed in the cases of phenanthrene and acenaphthene complexes. Dependence of the CT transition energies (hwCT) of the C70 complexes on donor ionisation potentials have been analysed
Fig. 2. Plot of 2I vD –hwCT against I vD (I vD −hwCT) for C70 – PAH system.
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291
Fig. 3. Absorption spectra of (1) C70 (3.0247× 10 − 5 mol dm − 3) +phenanthrene (0.032 mol dm − 3); (2) C70 (3.0247 ×10 − 5 mol dm − 3)+ phenanthrene (0.089 mol dm − 3); (3) C70 (3.0247 ×10 − 5 mol dm − 3) +phenanthrene (0.106 mol dm − 3); (4) C70 (3.0247× 10 − 5 mol dm − 3) in CCl4 against the solvent as reference.
in the light of Mulliken’s theory [26] and thence the electron affinity of C70 has been found to be 2.59 eV. Enthalpies and entropies of formation of the
complexes have been estimated from the formation constants of the complexes determined spectrophotometrically at three different temperatures.
292
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Fig. 4. Benesi – Hildebrand plot for C70 – acenaphthene system at 299 K.
Fig. 5. Plot for determination of enthalpy of formation of the C70 – phenanthrene complex.
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Table 2 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–naphthalene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
298
0.242 0.385 0.315 0.224 0.216 0.242 0.385 0.224 0.291 0.471 0.367 0.242 0.315 0.224 0.367 0.291
3.553
0.385 0.373 0.380 0.396 0.382 0.387 0.390 0.394 0.403 0.370 0.401 0.385 0.372 0.381 0.361 0.387
10.6
4496.2
306
316
a
3.553
3.553
8.9
6.5
Measured against the solvent as reference.
Table 3 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–pyrene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
298
0.022 0.029 0.059 0.066 0.071 0.086 0.099 0.022 0.029 0.044 0.059 0.071 0.022 0.029 0.059 0.066 0.071 0.086 0.099
1.589
0.345 0.489 0.398 0.448 0.446 0.476 0.442 0.353 0.376 0.356 0.395 0.415 0.378 0.369 0.389 0.435 0.386 0.402 0.522
64.6
14 705.8
307
317
a
Measured against the solvent as reference.
1.589
1.589
51.1
32.6
S. Bhattacharya et al. / Spectrochimica Acta Part A 58 (2002) 289–298
294
2. Materials and methods [70]fullerene, was collected from SES Research, Houston, TX. Naphthalene and phenanthrene were purified by sublimation. Anthracene from Sigma was used without further purification. Acenaphthene was purified by recrystallisation from purified ethanol. Pyrene was purified by recrystallisation form dry ethanol in dark. The solvent, CCl4 was of HPLC grade. All spectral measurements were carried out in a Shimadzu UV-2101 PC model spectrophotometer fitted with TB-85 thermo bath.
taining C70 + anthracene and C70 + phenanthrene. Except for anthracene, multiple CT peaks were observed for each of the donors studied. The wavelengths at these new absorption maxima and the corresponding CT transition energies (hwCT) are summarised in Table 1. While calculating hwCT the longest wavelength peaks were considered for the complexes having multiple peaks. According to Mulliken’s theory [26] the CT transition energies are related to the vertical ionisation potentials (I vD) of the donors by the relation, hwCT = I vD –C1 + C2/(I vD –C1)
(1)
Here, C1 = E vA + G1 + G0
3. Results and discussion
(2)
v A
3.1. CT spectra Fig. 1 shows the electronic absorption spectra of three mixtures containing respectively acenaphthene +C70, pyrene +C70 and naphthalene+ C70 in CCl4 medium against the respective pristine C70 solutions as reference. It is observed that new absorption peaks appear in the visible range where the PAH donors do not absorb. Similar spectral features were obtained with mixtures con-
where E is the vertical electron affinity of the acceptor, G0 is the sum of several energy terms (like dipole–dipole, van der Waals interaction, etc) in the ‘no-bond’ state and G1 is the sum of a number of energy terms in the ‘dative’ state. In most cases, G0 is small and can be neglected while G1 is largely the electrostatic energy of attraction between D+ and A−. The term C2 in Eq. (1) is related to the resonance energy of interaction between the ‘no-bond’ and ‘dative’ forms in the ground and excited states and for a given acceptor
Table 4 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–phenanthrene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
299
0.107 0.122 0.200 0.299 0.032 0.107 0.122 0.299 0.052 0.032 0.122 0.089 0.107 0.153 0.217
3.025
0.522 0.583 0.445 0.544 0.485 0.483 0.495 0.512 0.515 0.473 0.527 0.436 0.434 0.443 0.494
59.2
3013.3
309
317
a
Measured against the solvent as reference.
3.025
3.553
25.5
12.3
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295
Table 5 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–acenaphthene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
299
0.087 0.157 0.269 0.250 0.380 0.417 0.535 0.586 0.849 0.087 0.157 0.269 0.250 0.380 0.417 0.535 0.586 0.849 0.087 0.157 0.269 0.250 0.380 0.417 0.535 0.586 0.849
1.589
0.387 0.406 0.407 0.417 0.426 0.424 0.446 0.414 0.436 0.366 0.385 0.408 0.417 0.427 0.414 0.423 0.381 0.439 0.357 0.388 0.391 0.422 0.432 0.411 0.440 0.471 0.450
28.9
14 615.8
309
317
a
1.589
1.589
22.0
11.1
Measured against the solvent as reference.
it may be supposed constant [26]. A rearrangement of Eq. (1) yields 2I vD –hwCT =(1/C1)I vD(I vD – hwCT) + (C1 +C2/C1) (3) The ionisation potentials of anthracene, naphthalene, phenanthrene, pyrene and acenaphthene were collected from Ref. [27]. With the observed transition energies we have obtained the correlation, 2I vD –hwCT =(0.148890.0052)I vD(I vD – hwCT) +(6.90879 0.2197)
(4)
with a correlation coefficient of 0.99 (Fig. 2). This confirms the CT nature of the transition observed and yields C1 = 6.72 eV. Neglecting G0 and taking
the typical D–A distance in p-type EDA complexes to be 3.5 A, the major part of G1 is estimated as e 2/4ym0r= 4.13 eV. Now using Eq. (2) the electron affinity (EA) of C70 in solution is found to be 2.59 eV, which is in fair agreement with the value 2.73 eV obtained in gas phase by Boltalina et al. [28]. The difference (0.14 eV) between the electron affinity values in gas phase and in solution is due to solvation effect which has significant contribution to G1. Owing to lack of suitable data this contribution can not be estimated correctly. Isosbestic points were observed in the cases of C70 –acenaphthene (Fig. 3) and C70 –phenanthrene system at 407 and 574 nm, respectively. With the other three PAHs no such points were observed.
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Table 6 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C70–anthracene complex at three different temperatures Temperature (K)
Donor concentration (mol dm−3)
105[A]0 (mol dm−3)
Absorbancea (d) at 472 nm
K (dm3 mol−1)
m / (dm3 mol−1 cm−1)
301
0.0037 0.0075 0.0110 0.0112 0.0116 0.0185 0.0056 0.0055 0.0074 0.0116 0.0075 0.0110 0.0112 0.0185 0.0037 0.0056 0.0055 0.0074 0.0116 0.0075 0.0110 0.0112
1.589
0.369 0.343 0.343 0.362 0.335 0.343 0.348 0.325 0.313 0.316 0.329 0.320 0.297 0.322 0.302 0.332 0.293 0.312 0.277 0.290 0.283 0.304
489.7
3939.8
309
317
a
1.589
1.589
387.3
239.8
Measured against the solvent as reference.
3.2. Determination of formation constant (K) Owing to low intensities of the CT peaks, variation of their intensity with change in donor concentration could not be employed for determination of stoichiometry and formation constants (K) of the complexes. Instead, it was observed that the intensity of the broad absorption band of C70 (measured against the solvent CCl4 as reference), which is centered more or less at 472 nm, decreased systematically with gradual addition of the donors and this fact was utilised to determine K by using Benesi– Hildebrand [29] equation for cells with 1 cm optical path length: [A]0[D]0/d / =[D]0/m / +1/Km /
(5)
absorbance of the donor–acceptor mixture at 472 nm measured against the solvent as reference, d 0A and d 0D are the absorbances at 472 nm of the acceptor and donor solutions with same molar concentrations as in the mixture. The quantity m / = mc − mA − mD means the molar absorptivity of the complex and mA and mD are those of the acceptor and the donor respectively at 472 nm. K is the formation constant of the complex. Eq. (5) is valid [29] under the condition [D]0 [A] for 1:1 donor–acceptor complex. Experimental data are shown in Tables 2–6. In all the cases very good linear plots according to Eq. (5) were obtained, one typical case being shown in Fig. 4. Values of K and m / of the complexes obtained from such plots are also shown in Tables 2–6.
with d / = d −d 0A −d 0d
(6)
Here [A]0 and [D]0 are the initial concentrations of the acceptor and donor respectively, d is the
3.3. Enthalpies (ZH 0f ) and entropies of formation (ZS 0f ) of the complexes The enthalpies (DH 0f ) and entropies of form-
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297
Table 7 Enthalpies and entropies of formation of the C70–PAH complexes Donor
Temperature (K)
Formation constant (K) (mol dm−3)
DH 0f (kJ mol−1)
Naphthalene
298 306 316 299 309 317 298 307 317 301 309 317 301 309 317
10.6 8.9 6.5 59.2 25.5 12.3 64.6 51.1 32.6 489.7 387.3 239.9 28.9 22.0 11.1
−21.49 2.5
−52.098.2
−65.79 0.5
−185.891.5
Phenanthrene
Pyrene
Anthracene
Acenaphthene
ation (DS 0f ) of the complexes have been determined by using the Eq. (7). As measured, these terms will represent the net change in enthalpy and entropy for the solvated species. ln K = − DH 0f /RT +constant
(7)
Values of K at three different temperatures are summarised in Table 7. The following linear regression equations have been obtained with the present data: Naphthalene–C70 complex: ln K =2574/T − 6.25; correlation coefficient =0.95.
(8a)
ln K = 7902/T − 22.3; correlation coefficient (8b)
Pyrene–C70 complex: ln K =3200/T − 6.5; correlation coefficient =0.97. Anthracene–C70 complex:
(8c)
ln K = 4060/T −7.2; correlation coefficient = 0.98. Acenaphthene–C70 complex:
−26.69 6.6
−54.6918.1
−33.79 5.95
−60.5919.7
−45.39 10.1
−122.4933.6
ln K= 5446/T − 14.7; correlation coefficient = 0.97.
(8e)
DH 0f and DS 0f determined from the above correlations are shown in Table 7. One typical plot of ln K against 1/T is shown in Fig. 5. To calculate DS 0f the following equation was employed. DS 0f = (RT ln K+ DH 0f )/T
(9)
4. Conclusion
Phenanthrene–C70 complex:
=0.97.
DS 0f (J K−1 mol−1)
(8d)
The current study demonstrates significant interaction of C70 with polynuclear aromatic hydrocarbons. The high formation constant of the anthracene complex compared to those of the other PAHs studied may be attributed to the fact that the linear arrangement of the hexagonal rings of anthracene molecule with long p-conjugation is similar to that in the belt region of the C70 molecule. The CT nature of the transitions in the complexes has been established and a reasonable value of electron affinity of C70 has been determined. The vertical EA of C70 estimated from hwCT energies in solution has been found to be 0.14 eV lower than those in the gas phase presumably due to solvation effects.
298
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