1002
NOELS. BAYLISSAND EIONG. MCRAE
Vol. 58
SOLVENT EFFECTS IN ORGANIC SPECTRA: DIPOLE FORCES AND THE FRANCK-CONDON PRINCIPLE BY NOELS. BAYLISS AND EION G. MCRAE Contribution from the Department of Chmistrg of the University of Western Australia, Nedlands, Western Australia Reeaived March 16, 1964
All organic electronic spectra in solution are subject to a generalized polarization red shift which is due to solvent polarieation by the transition dipole and which depends on the solvent refractive index. This can be obscured by the effect of dipole-dipole and dipole-polarization forces if the solute is polar, when the application of the Franck-Condon shows that the solvent cage around the excited solute molecule is momentarily strained. Orientation strain an8$!$; strain are defined, of which the former is more important. The absor tion frequencies of polar solutes are shifted to the red in solution if the dipole moment increases during the transition; tffey may be shifted to the blue (relative t o the gas) if the dipole moment decreases. Four cases are discussed according to whether solute and solvent are polar or non-polar. The place of T* +- n transitions ie diecussed.
There have been several papers recently in which solvent effects on organic spectra have been correlated with various properties of the solute and the solvent. For example Kasha' and McConnel12 have proposed the use of solvent effects as an aid to the distinction between T * + u and T * n transitions, Nagakura and Baba3 have explained solvent shifts in aromatic compounds by assuming that hydrogen bonding causes an electron migration in the solute molecule, and Brooker4 and also Ungnade6 have suggested that some solvent effects are due to the stabilization of preferred resonance structures in the solute by certain solvents as the result of their dielectric constants or of their electron donor-acceptor properties. Coggeshall and Lang6 in dealing with solvent effects in the spectra of phenols have drawn attention to the importance of dipole-dipole interaction, of hydrogen bonding, and of the change in dipole moment during the solute transition. Now the interpretation of solvent effects is made difficult because they are often small and not easy to measure precisely, and also because they are often the resultants of several individual effects which sometimes reinforce one another and sometimes cancel out. There is also some difficulty in the fact that the most easily measured and most often recorded solvent effect is the displacement or shift of the absorption maximum, whereas theoretical considerations of electronic, energy states should be related to the position of the (0, 0) band, which is not necessarily affected in the same way as the maximum. Since it is practically impossible to locate the (0, 0) band in diffuse or structureless solution spectra, the spectral shifts discussed here and in the following paper are referred to absorption maxima, which provide the only possible experimental reference points. It is the purpose of this paper to present a scheme by which many solvent effects in organic spectra can be interpreted at least qualitatively in terms of dipole, polarization and hydrogen bonding forces between solute and solvent, bearing in mind three factors that have not always been given due recognition, namely, the (a) momentary transition dipole during the optical M. Kasha, Disca. Faraday Soc., No. 9, 14 (1950). H. McConnell, J. Chem. Phys., 'do, 700 (1952). 8. Nagakura and H. Baba, J . A m . Chem. SOC.,72, 5693 (1952). L. G. S. Brooker, et al., ibid., 78, 5332, 5350 (1951). H. E. Ungnade, ibid., 76, 432 (1953). (8) N. D. Coggeshall and E.M. Lang, ibid,, 70, 3283 (1948).
(1) (2) (3) (4) (5)
absorption process, (b) the difference in permanent dipole moment between the ground and excited states of the solute and (c) the effect of the FranckCondon principle. It will be seen that the effect of hydrogen bonding fits naturally into the proposed scheme, which can provide a consistent explanation of the experimental results referred to in the preceding paragraph. A following paper' will give additional experimental examples. Two recent papers8.9 from this Laboratory have dealt with the solvent effect that results from the momentary polarization that is induced in the solvent by the transition dipole of the solute, the predicted result being a red shift that is a function of the solvent refractive index, the transition intensity and the size of the solute molecule. The theory of this polarization shift, as we may call it, is quite general even though it is approximate, and therefore should be applicable to all solution spectra. However although the polarization shift seems experimentally to be dominant when the solute is nonpolar, we shall see below that it can be obscured by other effects when the solute is polar,l0 including those cases when there is hydrogen bonding between solute and solvent. It is convenient to relate the frequency shift of an absorption spectrum in solution to what we shall for the present call the solvation energies of the solute in its ground and excited states. If these are s" and s', respectively, it is obvious from Fig. 1 that the frequency displacement Av = ~(soln)v(gas) is given by Av = s" - s f , s being taken as positive if the energy of the solute is decreased in solution. The important point is that while SI' is the normal solvation energy of the solute in its ground state and in equilibrium with the solvent, the application of the Franck-Condon principle will show that the appropriate s' for the excited solute molecule is not necessarilyzthe equilibrium value. Solvation energies depend on various types of intermolecular interaction such as dipole-dipole, dipole-polarization, dispersion and hydrogen bonding forces. Dispersion forces are usually smaller than the others mentioned, but in any case they are (7) N. 8. Bayliss and E. G . McRae. THEJOURNAL,68, 1006 (1954). (8) N. S. Bayliss, J. Chcm. Phys.. 18, 292 (1950). (9) N. S. Bayliss and L. Hulme, Australian J . Chem., 6 , 257 (1953). (10) We use the term potor t o describe 8 molecule with a permanent dipole moment, or with strongly localized groups that are permanent
dipoles.
Nov., 1954
SOLVENT EFFECTS IN ORGANIC SPECTRA
always operative whether solute and solvent are polar or non-polar. I n this paper we shall regard them as contributing equally to s" and s', and we shall therefore confine our attention to the dipoledipole, dipole-polarization and hydrogen bonding forces. We first consider the nature sf the optical transition and the effect of the Franck-Condon principle. The Optical Transition.-In classical theory the absorption of light is associated with the forced oscillation of an electric dipole. In quantum theory the absorbing molecule undergoes a change in charge distribution which may or may not cause a change in the permanent dipole moment. In the simplest possible example of the H atom it is obvious from the wave functions that the 2p + Is transition causes a change in charge distribution, yet neither state has a dipole moment. I n the more complex example of a non-polar molecule such as benzene, it is clear from symmetry considerations that the excited states as well as the ground state have zero dipole moment, and it is the instantaneous change in charge distribution without the creation of a permanent dipole that leads to the polarization shift that is mentioned above. On the other hand it is to be expected that a molecule that is polar in its ground state will have in each of its excited states an altered dipole moment which may be greater or less than the ground state moment. The Franck-Condon Principle.-Consider a solute molecule in its ground state which is in equilibrium (modified by thermal motion) with the surrounding solvent molecules that form its cage. The solvation energy of this equilibrium state involves (a) a packing factor depending on the geometry of the solute and solvent molecules, and (b) a factor which depends on the degree of mutual orientation interaction if solute and solvent are polar or if there is hydrogen bonding between them. Bohon and Claussen11 for example have discussed the contributions of packing and orientstion entropy to the entropy of solution of aromatic compounds in water. To the extent that the size, charge distribution and dipole moment of the solute molecule are different in the excited state, the configuration of the solvent cage in equilibrium with the excited state is differentfrom that of the ground stage cage. Now it is the essence of the FranckCondon principle that an optical transition occurs within a time that is short compared with the period of nuclear motions. At the instant of its formation, Le., when it is in what might be called the Franck-Condon state, the excited solute molecule is momentarily surrounded by a solvent cage whose size and orientation are those that are appropriate to the ground state, The equilibrium excited configuration is only reached subsequently by a process of relaxation, which requires a t least several molecular vibration periods (-lO-l* sec.) as far as readjustment for size is concerned, and st time of the order of lo-" sec.12if a readjustment of the solvent Orientation is required. The lifetime of the excited state is known to be of the order of 10-8 see., which is ample for equilibrium with the solvent to be es(11) R. L. Bohon and W.F. Clauaaen, J . A m . Chcm. SOC.,1 8 , 1571 (1951). (12) D.H.Whiffen, Quart. Rara., 4, 131 (1950).
1003
\
Fig. 1.-Formal diagram of the effect of the solvation energies s' and 8" on the relation between the absorption frequencies in the gas state and in solution.
tablished before deactivation eventually occurs. (This time factor may be of importance in comparing solvent effects in absorption and in fluorescence.) The Franck-Condon excited molecule and its solvent cage are thus in a state of strain whose energy is necessarily greater than that of the equilibrium state, and the excited state solvation energy S' to be used in finding A v is less than the equilibrium value (and in certain cases as in case IVa below it may even be negative). In discussing this strain it is convenient to use the general term Franclc-Condon strain, while its two components can be called packing strain and orientation strain. Orientation strain may be expected when solute and solvent are polar, and when the solute dipole moment changes during the transition. It is similarly to be expected if there is hydrogen bonding between solute and solvent that is changed in degree as a result of the transition. Paulingla showed that what we have here called orientation strain is a major factor in causing the large positive displacement (to the blue) of the absorption spectra of halide ions in solution. Packing strain is to be expected when the solute molecule is substantially bigger in the excited than in the ground state. This concept has been used by one of the authors14 and by to explain the positive shift in the "brown" solutions of iodine and bromine, and although an alternative explanation of this phenomenon in terms of complex formation is currently accepted,16packing strain seems to be the only way of accounting for Prikhotko's observation'' of a positive shift of 1300 cm.-' in the spectrum of bromine in pentane a t low temperatures. 14b However, packing strain is probably unimportant in many organic spectra where the relative changes in size on excitation are not marked. Blurring of Vibrational Structure.-There is an important consequence of the concept of FranckCondon strain and of the relaxation time of the solvent cage. The Franck-Condon excited state as such is of very short life owing to the relaxation of the solvent during a time of the order of molecular vibrations. In terms of the principle of indeterminacy this will prevent the establishment of (13) L. Pauline, Phy8. R ~ v .$4, , 954 (1929). (14) (a) N. S. Bayliss and A. L. G. Rees, J. Chsm. Phya., 8, 377 (1940): (b) N . 8. Bayliss, A. R. H. Cole and B. G. Green, Australian
".giR. S.
~ ~ , $ ; ~ ~ s P~h y ~ s . , 4 429 ~ (1940). ~ G m , (16) Mulliken, J . A m . Chsm. Soc.. T 2 , 600 (1950); 14, 811 (1952). (17) A. Prikhotko, Acta Phyeicochim. U.R.S.S., 16, 126 (1942).
NOELS. BAYLISS AND EIONG. MCRAE
1004 Franck-Condon
Equilibrium state
Vol. 58
solvent refractive index. The effect of slight packing strain will probably be more important than in case I, since dipolar and more particularly hydrogen bonding forces between the solvent molecules themselves will tend to increase the solvent cage relaxation time after the transition.14* Thus there may be increased blurring though not necessarily obliteration of vibrational structure. A_--Case 1IIa.-Since the solvent is non-polar, there “\ will be no orientation strain. The forces contributing to the solvation energy are dispersion forces and dipole-polarization forces (polarization of solvent molecules by the solute dipole), and the latter are probably the greater. When the solute dipole moment decreases as a result of the transition, the contribution of the dipole-polarization forces to the solvation energy is decreased, and s’ is less than s” (see Fig, 2a). This will cause the solution spectrum to shift to the blue by an amount that depends on the solvent refractive index and on the Fig. 2.-The effect of solvation energies on the eolution absorption frequency when dipole-polarization forces are change in the solute dipole moment. On this shift dominant and when there is no orientation strain, Le., will be superimposed the polarization red shift, and when the Franck-Condon and the equilibrium excited state the resultant shift may be either to the red or to the have the same solvation energy: (a) solute dipole moment blue, depending on the relative magnitudes of decreases, and (b) it increases, during the transition. two effects. Vibrational structure will tend to be vibrational quantization in the excited state, with preserved. Case 1IIb.-The argument is the same as in case the result that the vibrational structure of the spec- IIIa, that {he increased solute dipole trum is blurred. This we believe is a frequent momentexcept in the excited state makes s‘ greater than cause of the blurring or obliteration of structure in s” (see Fig. Thus the solution spectrum is the spectra of polar molecules in polar solvents. shifted to the2b). red by an amount depending on the In cases such as non-polar benzene, where strain is solvent refractive index. The polarization red shift absent, vibrational structure is preserved in polar is also operative, Bo that the resultant is always to solvents.4 the red. Vibrational structure will tend to be preWe now discuss typical cases as follows served. Case I, non-polar solute in non-polar solvent Case 1Va.-To make the argument clear we take Case 11, non- olar solute in polar solvent the extreme case where the dipole moment in the Case IIIa, pogr solute in non-polar solvent; solute dipoIe excited state is zero, The ground state solvation moment decreases during the transition Case IIIb, polar solute in non-polar solvent; solute dipole energy is largely due to dipole-dipole forces, and moment increases during the transition the solvent cage is oriented. In the equilibrium exCase IVa, polar solute in polar solvent; solute dipole cited state the solvation energy is much smaller moment decreases during the transition Case IVb, polar solute in polar solvent; solute dipole since there is no dipole-dipole contribution (see Fig. 3a). In the Franck-Condon state, the nonmoment increases during the transition polar excited solute is in a cage of oriented dipoles Cases IVa and IVb include hydrogen bonding (orientation strain), and, as PaulingIs showed, this that is, respectively, decreased or increased by the contributes a negative term to the Franck-Condon transition. solvation energy which is equal to the energy input Case 1.-The solvation energy in both ground required to orient solvent dipoles around a non-polar and excited states is due to dispersion forces and molecule. As shown in Fig. 3a the effect on the is assumed to be about the same in the two states. solution spectrum will be to give it a positive disSince there is no dipole moment in either state, placement to the blue as compared with the gas. there are no solute-solvent orientation forces, and hence there is no orientation strain in the Franck- The effect will be less, but in the same direction, if Condon state. I n the absence of packing strain, the dipole moment in the excited state is not zero, which as mentioned above seems usually to be provided it is less than the ground state moment. The magnitude of the blue shift will depend on small in the case of relatively large organic moleseveral factors, including the magnjtude of the cules, the solution spectrum is shifted to the red owing to the polarization effect; the shift depends on change in dipole moment during the transition, the the solvent refractive index.* The solution spec- value of the solvent dipole moment, the extent to trum will tend to retain vibrational structure if it is which the solute and solvent dipoles are “exposed,” and the sizes of solute and solvent molecules. If present in the gas. Case 11.-Although the solvent is polar, the the solvent molecules are small, for example, more absence of a solute dipole moment means that there of them can get close to the solute dipole with the are no solute-solvent orientation forces and orienta- result of greater interaction. The superimposed tion strain is therefore absent. The case is thus polarization red shift will usually be dominated by identical with case I, and the dominant solvent ef- this dipole blue shift. fect is the red polarization shift depending on the The same arguments apply if the solute-solvent
---
IS‘)
I
---
Nov., 1954
SOLVENT EFFECTS IN ORGANIC SPECTRA
forces (including orientation) are largely due to hydrogen bonding provided it is less in the excited state. Owing to the existence of orientation strain, vibrational structure will be obliterated. Case 1Vb.-The dipole-dipole forces between solute and solvent are greater in the excited state than in the ground state. The Franck-Condon excited state is formed in an already partly oriented solvent cage, so that even though there is orientation strain,’s s’ will be greater than s“, and the solution spectrum is shifted to the red. The energy relations are shown in Fig. 3b. The superimposed polarization shift is also to the red. The magnitude of the shift depends on the same factors as in case IVa. Vibrational structure is probably blurred or obliterated; but it might be preserved if there is orientation saturation in the ground state.Is The same red shift will occur in the case of hydrogen bonding that is increased in the excite4 state. Discussion.-The 2600 and 2000 A. transitions of benzene are examples of the non-polar solute of cases I and 11. The frequency displacements are to the red by amounts depending on the solvent refractive index with slighto anomalies in water (2600 A.) and ethanol (2000 A.).9 Condensed aromatic hydrocarbons also show the polarization shift of cases I and I119s20 although exceptions have been reported.21 The solvent effects in the spectra of toluenoe (2600 A.) and chlorobenzene (2600 and 2000 A.) are also dominated by the red polarization shift of cases I and 1119 even though the solutes are polar. The reason is doubtless that these transitions are R* c R and closely resemble the corresponding benzene transitions, and that the electron displacement in the R shell has little effect on the dipole moment of the -CH3 and -C1 substituent groups. However when the spectrum of a polar solute is definitely associated with the polar group, either one of cases I11 and IV should apply. Thus Hammond and Modic22&ave recently shown that the characteristic 2600 A. band of aromatic nitro compounds has an increasing red shift in the solvents ethanol, water and sulfuric acid in that order. This is a case IVb effect, consistent with the increase in dipole moment if the transition involves obvious resonance forms such as
The observed red shift is also consistent with increased H-bonding in the excited state as suggested by Hammond and Modic.22 The red shift in nitroo l e h spectra in ethanol as compared with n-hexane as a solvent, described by Braude, Jones and (18) If solute-solvent orientation has reached ”saturation” in the ground state, there will be no orientation strain in the Franck-Condon state since no further orientation is possible. (19) N. D. Coggeshall and A. Pozefsky, J . Chem. Phye., 19, 980 (1951). (20) G. M. Badger and R. 8. Pearce, Speclrochim. Ada, 4, 280 (1951). (21) R. Schnurrnann and W. F. Maddame, J . Chem. Phys., 19,1430 (1951). (22) G.8. Hammond and F. J. Modic, J . Am. Chem. Soc., 76, 1386 (1953). (23) E.A. Braude, E. R. H. Jones and G. G. Rose, J . Chem. Soc., 1104 (1947).
Franck-Condon state
1005 Equilibrium state
/”T
ti--‘
I -
I
‘\.
Jv14‘” ::=
Idhdn) P‘ .l - --
(b)
Fig. 3.--Solution spectrum when dipole-dipole forces are dominant between solute and solvent, and when there is orientation strain in the Franck-Condon excited state: (a) dipole moment of solute decreases, and (b) it increases, during the transition.
is also case IVb, suggesting a transition such as )c=C-&y
0 \O-
+ Y-C=N
