J. Phys. Chem. 1995,99, 5817-5825
5817
Solvent Effects on the Electronic Spectrum of C ~ O Sean H. Gallagher: Robert S. Armstrong,*>+ Peter A. Lay,*$+and Christopher A. Reed’ Division of Inorganic Chemistry, University of Sydney, Sydney, New South Wales, 2006 Australia, and Department of Chemistry, University of Southem Califomia, Los Angeles, California 90089-0744 Received: August 15, 1994; In Final Form: October 24, 1994@
UV-vis absorption spectra of CWhave been measured in 15 different organic solvents. There are substantial shifts in A,, values between gas phase spectra, n-hexane, aromatic solvents, and CS2. The solvatochromic shifts of two of the bands due to the symmetry-allowed transitions, the HOMO-LUMO A0 transition (1’Tl”liAg) around 405 nm and the C transition (3ITl,-liAg) around 330 nm, have been analyzed statistically in terms of a number of solvent parameters. These include the index of refraction, n, polarizability parameters, (n2 - 1)/(2n2 1 ) and (n2 - l ) / ( n 2 2), polarity parameter, (8- 1)/(2c2 l), dielectric parameter, ( 6 l ) / ( e 2), molecular volume, V, Hildebrand solubility parameter, &, n* dipolarity/polarizability parameter and its polarizability correction term, &*, and solubility. The general theory that the energy shift should be mainly dependent on the polarizability of the solvent is not obeyed. Although polarizability is a major contributor, parameters related to the polarity of the solvent, such as the polarity parameter, dielectric parameter, and n*,are also statistically significant in determining the energy shift. By modifying the classical theory to take account of changes in quadrupole moments during the electron transition, good agreement between theoretical and observed solvatochromism has been established. This indicates that c 6 0 has a marked change in electron distribution upon excitation into the LUMO (and states of like symmetry) and suggests the formation of an axial quadrupole in the excited electronic state. This excited state is preferentially stabilized by polar solvents and those with a tendency to interact through n-stacking.
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Introduction The solubility of c 6 0 in a variety of solvents has been investigated extensively by a number of authors. The results, in general, support the age-old principle “similia similibus solvuntur”, i.e., like dissolves like. C a has a large refractive index (about 1.96), is nonpolar (dielectric constant 3.61), has a Hildebrand solubility parameter around 10 Cali’* ~ m - ~ / and *, prefers solvents with parameters of similar The electronic absorption spectra of C a have been analyzed in detail by Leach et aL3 They reveal a plethora of symmetry-allowed and symmetry-forbidden electronic transitions across the UV and visible regions that contain extensive vibronic fine structure. Apart from a recent paper on the solvent dependence in alkane solvents? the solvatochromic effects of different solvents on the electronic spectra of c60, however, are unexplored. Ruoff et al.’ made a passing comment on these solvent effects that “the wavelength of the maximum of each Cm absorbance peak is significantly solvent dependent”. Using the electronic spectrum of C a in n-hexane as a benchmark (part of the electronic spectrum of C a in the gas phase has been recorded but is not yet c ~ m p l e t e ) , ~solvents .~ with appreciable C a solubility result in varying bathochromic (red) shifts across the entire spectrum. Solvatochromism is a change in position and intensity of an electronic transition band induced by a change in the medium. The effect is particularly pronounced in molecules that have a significantly different charge distribution of ~t electrons in the excited state compared to that in the ground state.6 According to Bayliss and McRae,’ the solvatochromism of a nonpolar solute in a polar solvent is purely dependent on dispersion forces which produce a bathochromic (red) shift. McRae8 illustrated that the frequency shift is linearly dependent upon the polarizability parameter, (n2 - 1)l(2n2 1). For a polar solute in a
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’ University of Sydney. @
University of Southern California. Abstract published in Advance ACS Abstracrs, February 15, 1995.
0022-365419512099-5817$09.00/0
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polar solvent, quadratic terms involving both the refractive index and the static dielectric constant also contribute, but these effects are normally small and can be neglected.8 Recently, the solvent dependence work by Cata16n4 on c 6 0 in nonpolar aliphatic solvents has shown very good agreement on the basis of this model. If, however, the redistribution of charge in the excited state produces dipolar or quadrupolar structure in the solute, polarity parameters might be expected to be important as well. Such a case is not relevant to the simple aromatics considered by Bayliss and McRae79*but needs to be considered in light of the following information. The lowest-lying excited states of c 6 0 are related to the C ~ O ion, since both involve putting an electron into the tl, LUMO. Information on the ground state charge distribution in c60- may therefore be suggestive of that in the TI, excited states of c60. Recent calculations by Morokuma and coworkers9 have predicted that population of the LUMO in c60- results in a significant change in the electron distribution compared to c 6 0 . Experimental confirmation of charge asymmetry in c60- comes from the low-temperature EPR studies of Reed et al.,I0 which reveal anisotropic spectra. A solvent dependence was also observed. In this paper, the solvent-dependent shifts in the electronic spectra of C a are investigated in terms of the general theory of the Bayliss-McRae model’s8 and the redistribution of charge upon population of the LUMO by excitation. The results are discussed in terms of relative stabilization of the ground and excited states by solvent dipole interactions and It-stacking. The prediction of gas phase values for electronic transitions of c 6 0 will also be considered in light of recent result^.^,^ Experimental Section Cm was synthesized and purified according to methods reported in the literature.i’ The purity was verified by I3C NMR spectroscopy recorded on a 400-MHz Bruker AMX-400 spec0 1995 American Chemical Society
5818 J. Phys. Chem., Vol. 99, No. 16, 1995 trometer. A single peak in a benzene-d6 solution at -143 ppm was revealed, which corresponded well with the literature value. The purities of the solvents used were generally 99% or better. Solvents of purity better than 99.8% included carbon tetrachloride (Merck), N,N-dimethylacetamide (Aldrich), and pyridine (Aldrich). Solvents with a purity greater than 99.5% were n-hexane (May & Baker Aust.), cyclohexane (Univar), 1,4dioxane (Merck), chlorobenzene (Fluka), benzene (Merck), and toluene (Fluka). Those of less than 99.5% purity were fluorobenzene (Merck, 99%), anisole (Merck, 99%), 1,Zdichlorobenzene (Merck, 99%), benzonitrile (Merck, 99%), carbon disulfide (Merck, 99%), and mesitylene (Merck, 98%). Carbon tetrachloride was stored over molecular sieves, chlorobenzene was stored over P2O5, and benzene was stored over sodium. The remaining solvents were used as received. The electronic spectra were recorded on a Cary 05E spectrophotometer at a resolution of 0.5 nm. Enough C a to produce a saturated solution was added to 5-mL serum bottles containing 1-2 mL, of solvent and sealed with a septum cap. Each solution was agitated for 5 min, and then the remaining solid was allowed to settle before the solution was transferred by glass pipe to a 1-cm-pathlength Infrasil cuvette. A spectrum of the concentrated solution was taken immediately. Whilst this concentration was necessary to gain sufficient definition of the less intense transitions, it proved to be too concentrated for the strongly absorbing allowed transitions, and a further dilution to about one-fourth the original concentration was required. In all cases, except for CS2, which is intensely absorbing from 300 to 350 nm, it was possible to subtract the solvent to reveal the electronic spectrum of c60 from -300 nm through the visible region. The solvatochromic shifts were gauged from the energy of the 31T1,-1’A, (C) and llTlu-liAg (Ao) transitions relative to the energy of the respective transitions in n-hexane. The values of the energies of the & transition were found using the curvefitting program, J-Plot,I3 whilst the energies of the C transition were determined directly from the value of A,, in the spectra. The solvent shifts were statistically analyzed using the MINITAB package.I4 This package performs least-squares analyses of all experimental data to determine significant correlations. Multiple linear regressions were obtained using the “stepwise” and “regress” programs. The stepwise program considers all the solvent parameters to find which correlates most strongly with the appropriate physical property. It then finds the next most important parameter that correlates with the difference between calculated and observed values from the firs: correlation. After least-squares analysis of both solvent parameters it retains the second solvent parameter if it is statistically significant, (Le., with a t statistic of 2 or greater, where t = F’’2).14 The iteration continues until the addition of further solvent parameters are no longer significant. The regress program allows the number and type of solvent parameters to be chosen and obtains the best fit of all of the chosen parameters in a multiple linear regression. The values of f, R 2 , and R2‘ ( R corrected for the number of degrees of freedom) obtained are then used to test the significance of the correlation. These statistical analyses are presented where appropriate in the tables. Results The difference in spectral resolution of the bands due to the electronic transitions of c 6 0 in aliphatic and aromatic solvents is quite dramatic (Figure 1). Most of the peaks are well-defined in aliphatic solvents, whereas in aromatic solvents there is extensive broadening (see Figure 1). There is considerable blumng and obliteration of the less intense bands due to the
Gallagher et al.
2*oli
!
f 1.0
400
300
500 Wavelength (nm)
600
700
’
1 -
I
\
1.0
I
. 0.5
400
300
500 Wavelength (nm)
600
700
Figure 1. Comparison of absorption spectra of Cm in the region 300700 nm in (a) n-hexane (6.0 x M for expanded spectrum, 1.5 x M for main spectrum) and (b) benzene (2.4 x M for expanded spectrum, 6.0 x M for main spectrum) at room temperature.
symmetry-forbidden electronic transitions and the symmetryallowed vibronic transitions. This broadening is not concentration dependent, as shown in Figure 1 for the C transition. The present studies are confined to the & and C transitions occurring at about 405 and 330 nm, respectively, since they are the only bands with well-defined energies over a range of solvents. The shifts in energies of the C and & transitions from those observed in n-hexane, AVCand A v A ~respectively, , are given in Table 1, along with various solvent and solubility parameters. The A0 Transition. The energy of the & electronic transition was solvent dependent and varied by 360 cm-’ (4.3 kJ mol-’) on going from n-hexane to carbon disulfide. At 0.5nm resolution, the error is f 3 0 cm-I. By comparison, the difference between the energies of the bands in carbon disulfide and the gas phase4 is 695 cm-’ or 8.3 kJ mol-’. When correlations were made between AVA and various solvent parameters (Table 2), the strongest correlation was observed with the index of refraction, n, and the polarizability parameter, (n2 - 1)/(2n2 1). The polarizability-based parameters were the only statistically significant predictors with the bathochromic shift using the stepwise program. However, in multiple linear regressions, the index of refraction, n, the polarizability parameter, (n2 - 1)/(2n2 l), the dielectric parameters, (E 1 ) / ( ~ 2) and (E* - 1)/(2c2 1) (where E is the dc dielectric constant), and n* (which is a measure of the molecular solvent dipolarity/polari~ability)~~-~* were all statistically significant. Although the d,* t e ~ m ’ ~ was - ’ ~ not statistically significant, its inclusion strengthened the significance of the contributions of the other parameters and increased the adjusted R 2 value.
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Solvent Effects on the Electronic Spectrum of Cm
J. Phys. Chem., Vol. 99, No. 16, 1995 5819
TABLE 1: Bathochromic Shifts in Energy of the Electronic Transitions of C a as a Function of the Solvent and Values for the Solvent Parameters Used in Correlations solvent n-hexane cyclohexane carbon tetrachloride 1.4-dioxane NNdimethylacetamide fluorobenzene anisole benzonitrile 1,2-dichlorobenzene chlorobenzene pyridine benzene toluene mesitylene carbon disulfide
AVC"
Avbb
(n2 - 1)/ nc,d,e (2n2 l y
cc.d,g
0 -56 -148 -193 -148 -376 -421 -421 -421 -466 -556 -600 -689 -690 -940p
0 -42 -96 -92 -105 -113 -150 -160 -168 -180 -180 -183 -178 -248 -364
1.38 1.43 1.46 1.42 1.44 1.47 1.52 1.53 1.55 1.53 1.51 1.50 1.50 1.50 1.63
1.89 2.02 2.24 2.21 37.9 5.42 4.33 25.6 9.93 5.71 12.30 2.28 2.44 2.28 2.64
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0.188 0.205 0.215 0.202 0.209 0.218 0.232 0.236 0.242 0.233 0.230 0.227 0.227 0.227 0.262
(€
(E
- 1)/
+ 2)h
( € 2 - 1)/ (2c2 2)'
0.229 0.254 0.292 0.287 0.925 0.596 0.672 0.891 0.749 0.611 0.790 0.299 0.324 0.299 0.353
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0.316 0.336 0.364 0.361 0.499 0.475 0.461 0.499 0.492 0.477 0.495 0.368 0.384 0.368 0.399
B#
dHCsdm
VOlC~d"
-0.08 0.00 0.28 0.55 0.88 0.62 0.73 0.90 0.80 0.71 0.87 0.59 0.54
0.00 0.00 0.50 0.00 0.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
131 108
0.62
0.00
14.85 16.77 17.59 20.50 22.14 18.45 9.50 17.10 20.40 9.20 10.70 18.82 18.20 8.80 20.50
n*j,k
SOlC
"
0.43 0.036 0.32 0.041
80 85 93 94 109 97 113 102 80 89 106 139 54
0.59 5.6 0.41 27.0 7.0 0.89 1.7 2.80 1.50 7.9
a Bathochromic shift for the C transition with respect to the n-hexane C transition, cm-l. Bathochromic shift for the A,, transition with respect to the n-hexane AOtransition, cm-'. See ref 1. CRC Handbook of Chemistry and Physic,s 61st ed.; 1980-1981. e Index of refraction. f Polarizability parameter. Dielectric constant. Dielectric parameter. Polarity parameter. x* parameter. See ref 15. Polarizability correction term. Hildebrand solubility parameter, cm-312." Molar volume, cm3 mol-!. Solubility, mg/mL. p Calculated from eqs 9-1 1.
TABLE 2: Significant Correlations of AVA,with Solvent Parameters at 25 "Ce constant"
+ 1)
(n2 - 1)/(2n2
n
value
P
a
P
1817 830 1907 870 1897 820 1557 711 1527 604 1592 741 1562 645
7.09 6.24 7.88 7.21 7.76 6.81 7.40 6.10 6.82 4.30 7.79 6.79 6.61 4.41
-1319
7.68
-1403 -1449 -1147 -1247 -1174 -1265
b
P
-4378
7.37
-4743
8.48
-4944
8.15
-3889
6.97
-4252
7.99
-4058
7.73
-4379
8.02
(E
- I)/(€
+ 2) P
C
8.48
71.8 83.3
d
272 327 149 153
&*
n*
P
e
P
f
R2
r'
R2
R2'
1.33 1.65 0.66 1.01
81.9 80.7 85.8 85.8 85.4 85.5 93.8 92.3 93.3 92.1 94.8 94.1 93.6 92.9
80.6 79.2 83.4 83.4 82.9 83.1 91.9 90.1 91.3 89.7 92.5 91.5 90.8 89.8
1.68 1.99
2.98 2.76
8.79 8.27
~~
1.80 2.06
8.11 7.87
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(e2 - 1)/(2c2 1)
61.3 64 1 148 150
2.76 2.66
3.07 2.93
8.51
579 583
2.46 2.36
-117 -110 -124 -118 -132 -126 -125 -118
2.40 2.00 2.29 1.99 2.73 2.43 2.25 2.00
21.6 28.8 12.1 19.8
Constant of proportionality. Mesitylene has been omitted from equations including the n* parameter. R *' represents the R adjusted value. The t-function is a measure of the significance of each added value to the fit. (The higher the t-value, the better the fit.) e a-fare the corresponding coefficients.
Further, it makes sense chemically to include the 6,* parameter, as it is a polarizability correction term that is used to distinguish between the n* values of certain groups of organic solvents, i.e., the aliphatics and aromatic^.'^ The difference between correlations including the (E - 1 ) / ( ~ 2) and (e2 - 1)/(2e2 1) parameters is subtle. Either of these dielectric-based parameters is statistically significant. Inclusion of the ( E - 1)/ (E 2 ) parameter yielded a slightly better fit in the multiple linear regression, with the calculated values of the energy of the transition, using eq 1-4, being within experimental error. The data best fit equations of the form
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AvA, = 1592 - 1174n
+ 148 (2T -
3
132n*
AvA, = 741 - 4058 (;;
