Solvent spectral shifts in perfluoroalkane solvents

Volume 163, number 1

CHEMICAL PHYSICS LETTERS

3 November 1989

SOLVENT SPECTRAL SHIFTS IN PERFLUOROALKANE SOLVENTS Andrzej MACIEJEWSIU Photochemistry Laboratorv. Faculty of Chemistry, A. Mickiewicz University,60- 780 PoznaL Poland Received 9 March 1989: in final form 18 August 1989

Absorption and emission spectra (‘Lb and ‘L, bands) of benzene, naphthalene, anthracene and tetracene were measured in six pertluoroalkanes (PF), six alkaues and a few of their derivatives. The solvent spectral shift (SSS) resulting from dispersion interactions of aromatic hydrocarbons with PF may be described by expressions derived for macroscopic dielectric cavity models. Reasons for the inconsistency between those expressions and the SSS for aromatic hydrocarbon-alkane systems are discussed. The role of repulsion, specific, and higher-order solute-solvent interactions, as well as solute and solvent structures, in SSS are discussed. A similarity between PF and argon with respect to their interactions with aromatic hydrocarbons is pointed out.

1. Introduction In experimental studies, much attention has been devoted to the influence of solvent (s) and solute (S) properties on the spectral shift observed in electronic absorption and emission spectra [l-lo]. In the simplest case when both S and s have no permanent dipole moment (p= O), the attractive interactions between them are due to dispersion forces. They are responsible for a bathochromic (red) spectral shift Arr(v-s) of the order of 1O-2000 cm- ‘, with respect to the gas (v) phase. There are some expressions, usually based on the Onsager reaction field model [ 11,12,], and on the quantum statistical mechanical theory [ 131, which enable the value of the solvent spectral shift (SSS) due to solute-solvent (Ss) dispersion interactions [l-3,5,14-16] to be determined. In many experimental works, see e.g. refs. [ 1- 10 1, the linear relationship between A8(v-s) (solute) and (n2- 1)/(2n2+ 1) (solvent), where n is the refractive index, has been found to be relatively well fultilled in the case of dispersion interactions. However, some inconsistencies have been found between the experimental data and results of theoretical calculations (e.g. from eq. ( 1) below) as to the SSS results. The most important among them are: (i ) different slopes for particular groups of solvents, although the same relation has been predicted

to hold for all solvents with fi =O [2,7,9], (ii) theinterceptfor (n2-1)/(2n’+l)=Oisdifferent from zero and for various solutes A9(v-s) = -3OOf 100 cm-’ [ l-101, (iii) the deviation from the linear dependence of AI(v-s) on (pt’-1)/(2n2+1) (knownasaBayliss factor) [ 1,2,10,13]. So far, n-alkanes (AL) have commonly been used in spectral studies as inert and weakly interacting solvents (see e.g. refs. [ l-10, 17-20 ] ), whereas per(PF) have rarely been used fluoroalkanes [ 1, 10,17,19 1. However, there is experimental evidence that PF are distinctly more inert and weakly interacting than AL and in this respect resemble rare gases such as argon and krypton [ 19,21-241; this is particularly evident from studies of spectral, photophysical and photochemical properties of ketones and thioketones in PF and AL [ 25-271. It might be expected that the assumptions based on the Onsager cavity model and applied in the expressions describing SSS (e.g. eq. ( 1) below) would be better fulfilled for inert and weakly interacting PF than for AL and other groups of solvents. The aim of our work is to study SSS as a result of S-s dispersion interaction in the absorption spectra of aromatic hydrocarbons: benzene, naphthalene, anthracene and tetracene in six different PF solvents and to compare the results with those obtained in six

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AL as well as in some PF and AL derivatives.

2. Experimental Of the compounds used in these studies, benzeneh6 for fluorescence (Merck), benzene-$ (Merck), naphthalene, anthracene and tetracene (Aldrich ) , the latter three were crystallized. Perfluoroalkanes (PCR, Aldrich, Fluka, and Ventron) were purified by fractional distillation and column chromatography. Other solvents, mostly special for fluorescence (Merck ) or for spectroscopy (Merck, Baker, Aldrich), were used as received. Absorption spectra were measured using Specord M-40 (C. Zeiss-Jena) and Car-y 118 C (Varian) spectrophotometers with spectral resolution = 5 cm- l. Emission spectra were recorded on MPF-3 (modified) and MPF-44 spectrofluorimeters (Perkin-Elmer). All instruments were carefully calibrated with respect to wavelength. Refractive indices of some PF solvents were measured with an PR-2 (C. Zeiss-Jena) refractometer. The measurement of position (r) of each absorption band for which SSS was to be determined was repeated at least ten times to make the error of measurement ~3 cm-’ for benzene and G5 cm-r for the other solutes. All experiments were carried out at room temperature.

3 November 1989

compounds (e.g. refs. [ 1,2,4,6,9,10,13] ). The solvents used were chosen so that their refractive indices would cover a range as broad as possible, which should produce the greatest spectral shift from S-s dispersion interactions. The results of SSS for the *Lb band in C6H6 and C6D, in PF and AL solvents are given in fig. 1 and in table 1. For the sake of comparison, fig. 1 also includes results obtained for C6D6, CH4 and CzH6 at cryogenic temperatures ( T< 150 K) [ 28 1. The results of analogous measurements for anthracene ( ‘L, band) in PF, AL and their derivatives are given in weak anfig. 2 and in tables 1 and 2. Exceptionally thracene-PF interactions are illustrated by low values of the Stokes shift (table 2). The significantly different slope and intercept obtained from SSS measurements for benzene and authracene in PF and AL (figs. 1 and 2, and tables 1 and 2 ) were also confirmed for the So+!& (lb) transition in naphthalene and So&, (IL,) in tetracene in 5 PF and 5 AL (table 1) . The SSS values were determined for all S not only for the (O-O) or 6: bands (tables 1 and 2, and figs. 1 and 2) but additionally for a few other

Benzene -‘Lb

‘v

‘86

1

3. Results SSS resulting from S-s dispersion interactions were measured for the ‘Lt, and ‘L. bands of aromatic hydrocarbons: benzene&, benzene-de, naphthalene, anthracene and tetracene. The reasons for choosing those compounds as S besides their zero dipole moment and rigidity are the following: (i) the shape of their absorption spectra does not change for the solvents of one group, (ii} their spectra show a well resolved and a relatively narrow O-O band and a few other vibronic bands, (iii ) apart from absorption they also show fluorescence which enables us to determine their Stokes shift, A8’O-O’(abs-em), (iv) the literature provides spectral data on these 82

Fig. 1. Solvent spectral shift (relative to the vapour) of the ‘L,, absorption (66) band ofbenzene versus (n2-1)/(2n’+l) at room temperature. For the values of intercept and slope, see table 2; (a) represents perlluoro-1,3-diiethylcyclohexane, (b) perfluorodecalin. Points 23 and 24 are fmm ref. [28].

CHEMICALPHYSICSLETTERS

Volume 163, number 1 Table I Spectral characteristic of aromatic hydkarbons Solute

Transition

benzene-h, benzene-d, naphtha&z anthracene tetracene c)

‘La %

3 November 1989

in perfluoroakanes (PF) and n-alkanes (AL) at room temperature

AC(v-s) (cm-‘)

Intercept b, (cm-‘)

Slope ‘rb) (cm-‘)

Slope PF/AL

C&4

C&M

PF

AL

PF

AL

5 14 60 490 525

210 252 244 949 975

-26 -19 -30 +15 -10

-316 -353 -310 -241 -400

235 241 650 3423 3850

2848 3264 3050 6292 7400

0.083 0.074 0.21 0.54 0.52

I) Linear correlation coefficient is always > 0.99. b, See figs. 1 and 2 and text. ‘) From fluorescence excitation spectra, as the absorption spectra were too weak due to too low solubility in PF.

1800

22*

Anthrocene - 'La

1

well resolved vibronic bands for which very similar Av(v-s) values were obtained. The results of spectral measurements in the vapour at room temperature for benzene-&, ~(6:)=38610 cm-‘, for benzene-d,, V(@,)~3877 1 cm-’ and for naphthalene, IT(O-O) = 32018 cm - ‘, obtained to calculate AP( vs ), are consistent with literature data [ 29-3 11. For anthracene we got ~(0-0) =27630 cm-’ (see table 2), and for tetracene the value F( O-O) = 22300 cm-’ was taken from ref. [ 32 ] #I. To determine the role of S-s specific interactions and solvent properties (local dipole, structure, anisotropy, steric hindrance), as well as higher-order S-s interactions, we measured SSS for anthracene in PF and AL derivatives including several functional groups (e.g. C=C, C-H, O-H, C-Cl, C-S), fig. 2.

600

4. Discussion 500 400 300'

0.10 I

0.12 I OXI

0.16 I

018 I

0.20 I 0.22 ! 4 In*-11/!Zn2+11

Fig. 2. Solvent spectral shift (relative to the vapour) of the ‘L,, absorption (O-O) bandofanthraceneversus (n2-1)/(2n2t 1) at room temperature. For the values of intercept and slope, see table 2; (a) represents perfluoro-I ,3dimethylcyclohexane, (b) perfluorodecalin, (c) perfluoro-2-methylpentene-2, (d) pdioxane, (e) 2,2,4,6,6qentamethylheptane, (f) 2,2,4,4,6,8,8_heptamethylnonane. Points 20 and 21 are from ref. [ 91. The difference of solvent spectral shift between point 9 ( 0 ) and 9 ( l ) as well as between I9 (0 ) and 19 ( l ) is a consequence of the solvent dipole-solute-induced-dipole interactions disregarded in the casesof9(o)and19(o).

The equation, uum models [ 11,121, which sulting from [8,14,15,19]

based on dielectric cavity contin(Onsager, BGttcher, Wertheim) is frequently used for the SSS reS-s dispersion interactions, is

2 n2-1 E, hcAf(v-s) = - ___ a3 2n2+ 1 El +I& X[K,12-~2(~3

--~,)I,

(1)

where hco is the transition energy, v and s indicate values measured in the vapour and solvent, a is an *’ The difference between i(O-0) in a supersonic jet and in the gas phase at room temperature was assumed to be 60 cm-’ by analogy to experimental results obtained for anthracene.

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Table 2 Spectral properties of the transition Sc-&r (O-O band) in anthracene (c= lfl-’ M) in several solvents at room temperature Solvent

I (cm-‘)

Aqv-s) (cm-‘)

vapour PF-n-pentane PF-n-heptane n-hexane cyclohexane benzene

27630 a’ 27160 27126 26681 26596 26392

0 470 504 949 1034 1238

b,

nz-1 2n2+ I

M(a-Q)(abs-em) (cm-r)

0.000 0.131 0.141 0.186 0.205 0.228

7 15 118 140 200

‘r Obtained with an error d 30 cm-’ by extrapolation from 328 to 393 K. b, v, vapour; s, solvent.

effective Onsager cavity radius, n the solvent refractive index, E, and E2 the mean excitation energies usually replaced by the ionization potential of the solvent and solute, respectively, Pegis the electric dipole transition moment and is related to the oscillator strength of the electronic transition between the ground (g) and excited (e) states of the solute, and c~is the solute polarizability. In eq. (l), the values a 3, E,, 1Pa I* and cy,-c~, describing the S properties as well as E, value characterizing s are assumed constant for a given electronic transition in S. That is why, by choosing solvents of p=O and n varying within a sufficiently wide range, we could study the dependenceofAu(v-s) on (n2-1)/(2n2+1). In eq. ( 1 ), as well as in analogous equations [ l5,13-l 61, the solvent properties are described only by its refractive index, and a spherical symmetry of S is assumed. Thus, such important properties of S and s as their molecular geometry, anisotropy of their polarizability, and additionally repulsive, specific and higher-order interactions between them have not been taken into account. No wonder therefore, that numerous experimental SSS results [l-10,17,18] differ from the predictions of eq. ( 1). On the other hand, the SSS results obtained in this work for five S in six PF were found to obey eq. ( 1) unexpectedly well. All experimental points for each S were found to lie on a single straight line with high accuracy (figs. 1 and 2), and the intercept value was found to be close to zero (table 2). The results obtained have shown that because of exceptionally weak S-PF interactions in solution, PF are practically the only group of solvents that obey the Onsager reaction field mode1 assumptions. It means that PF can be treated as a continuum described by the refractive index 84

[ 11,121, and a random PF distribution around the S molecule can be assumed. This would also imply that the assumption of spherical symmetry for S (even for tetracene) and the neglect of the influence of changes in other properties of S and s, apart from PF refractive index, on the SSS value seems to be justified. The SSS’results in PF reported in this paper are in agreement with those obtained for the same aromatic hydrocarbons as a result of dispersion interactions and van der Waals complex formation with rare gases (RG) [ 32-371, studied by the supersonic free-jet technique, table 3. The linear relation between microscopic red shift and the polarizability of five rare gases was well fulfilled and the straight line passed through the origin [33,34,37-391. The SSS values obtained by us in PFH were unexpectedly lower than in Ar both in solution and in the supersonic free jet under a high enough pressure (pAr= 11 atm ) (table 3). The difference in SSS is particularly pronounced for the ‘Lb transition in C6H6which may be a consequence of a significantly greater contribution of S-s repulsive interaction in PF solution [ 18,23,32,33]. A more detailed comparison of the spectral results in PF and AL will be the subject of a separate publication [ 401. Here we note that for all S studied in both types of solvent, a perfectly linear dependence Afl(v-s) on Bayliss factor yet considerably different SSS, slope and intercept in PF and AL were obtained (figs. 1 and 2, and table 1). Solvent spectral shiji differences between PF and AL are greater than expected from the difference in refractive indices of these compounds. It has commonly been assumed that the slope is the same for

Volume 163, number 1

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3 November 1989

Table 3

A comparison of the spectral shift for benzene, anthracene and tetracene in perfluoro-n-hexane, n-hexane and argon Solute

benzene-h,

Solvent

Ar

Ar n-C&, n-C& anthracene

tetracene

ti(v-s) (cm-‘)

&L/2

21

4

60 5 210

100 95 345

Ar

45

Ar

760

200

n-CsFId

490

220

n-&HI4 949

390

Ar fG%,

86 265 320

n-C6H14

709 525 975

Transition

Conditions of measurement

Ref.

‘Lb

vdW complex 1: 1,in supersonic jet

[331

(6f,b;lIld)

solution, T= 145 K solution, T= 295 K solution, T= 295 K

[421 a) a)

‘L.

vdW complex 1: 1, in supersonic jet vdW complex 1:n(Ar), n t 10, in supersonic jet solution, T= 295 K solution, T= 295 K

1341

(O-O band)

a) a)

‘La (O-O band)

vdW complex 1: n(Ar), n L 10, in supersonic jet, pAr= 11atm solution, T= 295 K solution, T= 295 K

[351 ~231 ~231

(cm-‘)

2.5

[361

a’This work.

all solvents with p=O [l-6,14-16] and that the increase in repulsive interactions resulting from excitation is much less than the increase in attraction [ 1,3-5,14- 161 (see below). It follows from our results (tables 1-3) for aromatic hydrocarbons in PF and AL that these assumptions are not fulfilled. Considerable differences in SSS values were obtained for the IL,, transition in C6H6 and C6D6 in PF and AL solvents (tables 2 and 3, and fig. 1). We note that our results in PF are in good agreement with the seemingly strange results obtained for C6H6 (IL,, transition) in PFH under high pressure [ 171. The intercept is a measure of increase in repulsion resulting from electronic excitation [ 4,17,18,33,4 11. A near-zero value of the intercept in PF for all transitions in all S (figs. 1 and 2, and table 1 ), is in agreement with a low value of the Stokes shift obtained for anthracene in PF (table 2). This is a consequence of the shape of the intermolecular energy curve describing aromatic hydrocarbon-PF interactions (fig. 3a). The curve is flat and shallow and resembles those obtained for aromatic hydrocarbons Ar [ 33,341. This resemblance may follow from similar physical properties of PF and Ar [ 2 I ,23 ] as well as similar values of SSS (table 3) and Stokes shift [ 35 1. On the other hand, the stronger interactions of S-AL (fig. 3b) [ 421, explain the intercept -300 + 100 cm- ’ (figs. 1 and 2 and table 1) and larger Stokes shift than in PF (table 2 )_ The slope ratio PF/AL= 0.5 for the ‘L, transition

is the same for anthracene and tetracene, as for naphthalene and benzene [ 401, To explain the difference in the slope, which has not been predicted by the theories describing S-s dispersion interaction, one should take into account that for S the values of a,-ok, increase with increasing S-s interaction [ 43,441, and the value of a3 for S in PF may bc higher than in AL because of greater free volume in PF [ 2 1,231. Moreover, the S-AL interactions are strong enough so that anisotropic polarizability of S (especially of tetracene) and AL [ 2,13,23,43,44], their effective dielectric constant [ 11,12 1, and probably higher-order interactions between S and AL [l3,13,23], should be taken into account. Brady and Carr [45] obtained a similar slope ratio PF/ALx 0.5 for the dependence of A*on Bayliss factor, which for AL and PF is a measure of S-s dispersion interaction. The slope ratio PF/AL for the IL,, transition for all S studied is not only distinctly lower than for the IL, transition but variable as well (table 1) . It follows from the fact that in PF the S-s attractive interactions are not significantly greater than the repulsive ones and additionally their relative contribution changes when we come from benzene to naphthalene and phenanthrene [ 40 1. This is the first evidence of an essential influence of the S-s repulsive interactions not only on SSS but also on the slope. These results disprove the widespread opinion that the slope is the same for CsH6 in all solvents including PF [ 1,4,10,28].

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a

b

Fig. 3. Qualitative potential energy curves for aromatic hydrocarbonperfluoroalkane (PF) interactions (a), and for aromatic hydrocarbon-alkane (AL) interactions (b).

The linear dependence of AJ(v-s) on the Bayliss factor obtained for all S in PF and AL made it possible to assume the deviation from the straight line to be a measure of specific local and higher-order Ss interactions (fig. 2), These interactions were measured for the ‘L, band of anthracene in the solvents of zero dipole moment but of different local structure and other characteristics such as local dipole moment (e.g. p-dioxane, CS2, CCL,), steric hindrance (t-BuOH), quadrupole and higher-order interactions (CsH6, CS2, p-dioxane), the lack of or smaller anisotropy of polarizability than in linear AL (neopentane, pentamethylheptane, heptamethylnonane) . We also studied those interactions for the IL, band of anthracene in PF derivative solvents with an active group, e.g. the C=C bond in perfluoro-2methyl-Zpentene, the C-H bond in lH-perfluoroheptane, the O-H bond in hexafluoroisopropanoi. As a consequence of these interactions, the SSS value was found to vary from 39 to 350 cm-’ (fig. 2); a more detailed discussion will be given in ref. [ 40 ] .

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5. Conclusions

We have found that only the SSS results obtained for aromatic hydrocarbons in PF solution are well described by the expressions (e.g. eq. ( 1) ) [ l-4,1416 ] derived on the basis of the Onsager reaction field model [ 11,12 1. They provide a good explanation of data concerning S-PF interactions reported earlier [ 17,45,46]. Moreover, in the light of the results obtained in this work, the interpretation of SSS data, e.g. refs. [47-491, demands some changes. Our spectral results are a good illustration of essential differences in AL and PF properties and, at the same time, point to similarities between the latter and rare gases, in particular Ar.

Acknowledgement

The author thanks Dr. J. Makarewicz, Dr. J. Dr. M. Szyma6ski and Dr. B. Ferchminowa for valuable discussions. Thanks also due to Ms. M. Fidecka and Mr. Z. Szeluga for assistance in the experimental work. The author acknowledges the fi-

Kaput,

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nancial support of Polish Research Project CPBP 01.19.

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