5604
J. Phys. Chem. 1996, 100, 5604
Reply to Comment on “Solvent Effects on the Electronic Spectrum of C60” Robert S. Armstrong,* Sean H. Gallagher, Peter A. Lay,* and Jeffrey R. Reimers School of Chemistry, UniVersity of Sydney, NSW, 2006, Australia
Christopher A. Reed Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089-0744 ReceiVed: NoVember 22, 1995 In the preceding comment1 a number of criticisms were raised concerning our paper2 on the solvatochromism of C60. These criticisms were based, in part, on a recently published theory of the effects of solute quadrupoles on the solvatochromism.3 The purpose of this comment is to correct some misconceptions in the previous comment and to address the remaining points raised. Suppan1 has asserted incorrectly that in our paper1 the quadrupole in the excited state had a moment in “some undefined direction”. The proposed axial quadrupole was illustrated clearly in Figure 5 of the original article,2,4 and the equation defining the quadrupole was given in eq 15;2 therefore, this assertion is without basis. It was also stated,1 on the basis of a previous theoretical paper,3 that a change in quadrupole moment from zero in the ground state to a nonzero value in the excited state cannot result in solvatochromism. Such a postulate is unlikely to be correct, since in the analogous prediction for systems incorporating solvent dipoles, contributions involving products of the polarizability with both µg∆µ and (∆µ)2 are important.5 This is the basis of our suggestion2,6 that terms involving ∆Q (i.e., f[Qg∆Q] and f[(∆Q)2]) are important, which will be addressed in detail elsewhere.7 Suppan is correct in stating1 that there cannot be a term involving solvent polarity for C60 when using a point charge (quadrupole) theory, since this term is a function of Qg∆Q and, hence, would be zero assuming that C60 has no ground state quadrupole. His theory3 also predicts that only the first allowed transition should show significant solvatochromism due to this term. While this has been verified experimentally with molecules, such as benzene,3 similar behavior is not observed in the solvatochromism of C60.2 The model proposed by Suppan neglects several features of the C60 molecule that we have addressed elsewhere.2,8,9 The first is that the C60 molecule is poorly described as a theoretical point quadrupole. It is more akin to an activated surface interacting quite strongly with the surrounding solvent. These localized effects are likely to invalidate a point charge model with fullerenes, and this is one reason why we have not defined an equation involving ∆Q in the original paper2 (another criticism in Suppan’s comment1). In addition, the solvent/solute interactions involving the C60
0022-3654/96/20100-5604$12.00/0
molecule are likely to reduce the symmetry of the molecule in the ground state from the spherical symmetry of the molecule in the gas phase, producing a multipole in the solvated ground state. This solvent-induced distortion of the symmetry is evident in the resonance Raman spectrum of C608,9 and is supported by studies on the rotational reorientation dynamics.10 These results have been interpreted10 as having the C60 molecule spinning on an axis in solution, which implies the formation of an axial quadrupole in the ground state due to specific solvent/solute interactions. Consistent with these results is the observation that the molecule spins more slowly in less viscous solvents, where the solvent/solute interactions are strongest, compared to more viscous solvents where these interactions are weak.10 It should be noted that these solvent effects are not the solventinduced instantaneous changes in the multipoles on C60 that would give rise to the Stark effect mentioned in the comment.1 The three factors that have been outlined above are the likely reasons for the observed solvatochromism of C60 and serve to highlight the deficiencies of the model proposed by Suppan,3 in both a general sense and specifically for fullerene molecules. The observation that C70, which has a ground state multipole and a finite quadrupole moment,11 exhibits a similar solvatochromism to C6012 suggests that the Ih symmetry of the ground state of C60 in the gas phase is not particularly important in understanding its solvatochromism. Acknowledgment. The authors are grateful for support from the Australian Postgraduate Award scheme (S.H.G.), the Australian Research Council (R.S.A., P.A.L., and J.R.R.), and the National Institutes of Health (GM 23851, C.A.R.). References and Notes (1) Suppan, P. J. Phys. Chem. 1996, 100, 5603. (2) Gallagher, S. H.; Armstrong, R. S.; Lay, P. A.; Reed, C. A. J. Phys. Chem. 1995, 99, 5817. (3) Ghoneim, N.; Suppan, P. Spectrochim. Acta 1995, 51A, 1043. (4) Figure 5 was incorrectly referenced to ref 26 in the original article. This should have been ref 28, as given correctly in the text. (5) Liptay, W. In Modern Quantum Chemistry; Sinanoglu, O., Ed.; Academic: New York, 1965; Vol. 3, p 45. Rettig, W. J. Mol. Struct. 1982, 84, 303. (6) There are typographical errors in eqs 22 and 23 of ref 2. There should be a minus sign instead of a plus sign separating the polarity and polarizability terms in each of the last two terms in these equations. (7) Armstrong, R. S.; Bolskar, R. D.; Gallagher, S. H.; Lay, P. A.; Reed, C. A.; Reimers, J. R. Manuscript in preparation. (8) Gallagher, S. H.; Armstrong, R. S.; Lay, P. A.; Reed, C. A. J. Am. Chem. Soc. 1994, 116, 12091. (9) Gallagher, S. H.; Armstrong, R. S.; Lay, P. A.; Reed, C. A. Chem. Phys. Lett. 1996, 248, 353. (10) Rubstov, I. V.; Khudiakov, D. V.; Nadtochenko, V. A.; Lobach, A. S.; Moravskii, A. P. Chem. Phys. Lett. 1994, 229, 517. (11) Roduner, E.; Reid, I. D. Chem. Phys. Lett. 1994, 223, 149. (12) Gallagher, S. H.; Armstrong, R. S.; Bolskar, R. D.; Lay, P. A.; Reed, C. A. In Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; Electrochemical Society: Pennington, NJ; Vol. 2, in press.
JP953435V
© 1996 American Chemical Society
