J . Phys. Chem. 1989, 93, 3483-3487 a I
I I / - / -
I / I
b
\
I \
I
I
\ \
I
\ 1 1 I
- \ - / - I
\
I
\
I
I
\
\
I
1 \
\
\
Figure 11. Profiles of the vibrational displacements of H-bonded a , w dihydroxy-n-alkanes: (a) LAM-1 of dimer; (b) LAM-3 of tetramer. The
longitudinal displacements are represented as transverse. very small: the present procedure preserves the simplicity of fitting most of the results with a single pair of force constants, rather than treating f and F as adjustable parameters. As suggested by Hsu and Krimm,8 the longitudinal vibrations of the a,w-disubstituted n-alkanes can also be well modelled by the vibrations of uniform rods with appropriately chosen modulus and density but with end masses and end forces varied according to end group. A study'j of the effect on LAM-1 of bromination of polyethylene single crystals is related to our work. It was observed that the LAM-1 frequency was lowered by bromination of chains in the single-crystal surface layers. However, in contrast to the present work, quantitative comparison with theory was not (13) Runt, J.; Harrison, I. R. J . Macromol. Sci., Phys. 1980, B18, 83.
3483
straightforward, because of uncertainty regarding the site and extent of bromination as well as the crystalline-stem length itself. Effects of H Bonding. The high values found for LAM-1 for the a,w-dihydroxy-n-alkanes can be roughly reproduced within the model by use of a higher value of the interchain force constant, Le., f = 19 N m-l, as is appropriate for hydrogen-bonded chains9 (see Figure 9). The low-frequency spectra of the a,w-dihydroxy-n-alkanes (see Figures 1 and 7) contain intense bands in the range 37 em-l (n = 8) to 22 cm-l ( n = 16), Le., at frequencies lower than those of the bending vibrations (IPB-1, OPB-1). These bands can be assigned to non-zero-frequency translational modes, first described by Minoni and ZerbiS for coupled 1-alkanoic acids and later applied to octade~anol.~ Their frequencies can be satisfactorily modelled (see Figure 10) by the one-node longitudinal vibration of a coupled dimer of 2n methylene groups with a central weak hydrogen bond and with free ends (see Figure 1la). The band is rendered Raman active by the large change in polarizability of the H bond. A band of the same frequency of vibration would arise from the three-node vibration of an H-bonded tetramer, the five-node vibration of an H-bonded hexamer, etc, Le., generally, the r - 1 mode of an H-bonded r-mer (r even); see Figure 1 1b. For the reasons given above, it is not thought worthwhile to discuss the small deviations of experimental results from the calculated curves apparent in Figures 9 and 10. Acknowledgment. We thank Peter Kobryn for help with the experimental work. The Royal Society and the Science and Engineering Research Council provided financial support. Registry No. CI(CH2)loC1,2162-98-3; Br(CH&Br, 629-03-8; Br(CH,)8Br, 4549-32-0; Br(CH2)loBr,4 101-68-2; Br(CH2)12Br,3344-70-5; I(CH2)& 24772-63-2; I(CH2)loI,16355-92-3;I(CH2)121,24772-65-4; HO(CH2)80H, 629-41-4; HO(CH,),oOH, 1 12-47-0; HO(CH2)120H, 5675-51-4; HO(CH2)16OH, 7735-42-4; H$O2C(CH2)6CO*CH3, 173209-8; H s C O ~ C ( C H ~ ) ~ C O ~106-79-6; C H ~ , H3C02C(CH2)loC02CH,, 1731-79-9.
Pressure-Induced Solvatochromism of the Charge-Transfer Transitions in Pyridinium Betaines William S. Hammack,',* David N. Hendrickson,**' and Harry G. Drickamer*>'** School of Chemical Sciences, Department of Physics, and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received: August 17, 1988)
Pressure is used to induce the solvatochromism of the charge-transfer (CT) absorption bands of several pyridinium N-phenoxide betaine dyes in methanol, ethanol, 2-propanol, poly(methy1 methacrylate), and poly(styrene). The major conclusion of this study is that the peak maxima of the CT bands correlate with the solvent polarity function (e - l ) / ( t + 2). The spectral changes observed are very similar to the solvatochromic behavior found at ambient pressure with solvents of varying polarity. Pressure-inducedsolvatochromism,though, has several advantages over ambient pressure methods: (1) the dielectric properties of the solvent change in a smooth and continuous fashion and (2) any specific solute-solvent interactions that occur are, to first order, constant with pressure. The experiments reported in this paper are an implementation of a general principle of high-pressure research: by varying the bulk properties of a material, either continuously or discontinuously,the relationship between the bulk and local properties can be determined.
Introduction The energy associated with an optical excitation frequently differs from solvent to solvent. This may be because there are different degrees of molecular or ionic association or aggregation in different solvents. In some cases the spin states of transition-metal ions in organometallic compounds may differ in different media. More often this is because the excited state of the School of Chemical Sciences. (2) Department of Physics and Materials Research Laboratory.
(1)
0022-3654/89/2093-3483$01.50/0
molecular interacts differently with the surroundings then the ground state. For many bonding-antibonding excitations (e.g., the r-n* excitation of organic molecules) the excited state is more polarizable than the ground state. In solvents where van der Waals interactions are dominant the excited state is then stabilized with respect to the ground state as one moves to more polarizable solvents. A plot of peak position as a function of pressure versus (n2 - l)/(n2 2 ) ( n is the refractive index) for the a-n* fluorescence emission of diphenyl polyenes shows an excellent correlation and illustrates the effect of polarizability on bonding-antibonding excitations and emissions.3 Since pressure in-
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0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
creases n, one can increase the solvent polarizability in a continuous fashion and thus establish in a more quantitative way the relation between excitation energy and solvent properties. Although strictly speaking solvatochromism should refer to changes in excitation energy in different solvents whatever the cause, it is frequently applied in a restricted sense to molecules with different dipole moments in the two states. It is in this restricted sense that we use it here. There are many theories which describe the interaction of a solute with the surrounding solvent. These theories relate the absorption or emission band maximum of a solute to the solvent’s dielectric and optical properties. The majority of these theories are based on a model due to Onsager for the reaction field of a point dipole in a spherical ~ a v i t y . Extensions ~ of this model are due to M C R ~Ooshika,6 ~ , ~ Mataga,’ Amos,8 L i ~ p e r tLiptay,lo ,~ and Bakhshiev.”s’2 Sutin and co-workers have shown that a nonequilibrium thermodynamic approach developed by Marcus can yield the same expressions as the reaction field methods above.13 In addition there have been some attempts to develop microstructural models of solvatochromism which try to take into account the actual molecular shape and charge distribution of the solute rather than the spherical approximations described above.’4J5 One feature common to all these approaches is a dependence of the solute band maximum on some function,f(e), of the solvent’s static dielectric constant, t. In many of these models the functionality is of the form f(t)
=
e-1
t+2
Rettig has denoted eq 1 as the solvent polarity function.15 The ambient pressure solvatochromism of the betaine dyes has been studied extensively. In order to determine the dependence of the C T band maxima on the solvent polarity function, (e - l)/(t + 2), the absorption spectra of the betaine dyes are measured in a variety of solvents with different dielectric constants. Using the data from ref 16, 17,and 18,we note that in general a plot of the C T peak maximum against (e - l ) / ( t 2) shows that there is a rough correlation between these two quantities. There are two general trends from these plots that are germane to the results discussed below. The trends as ( t - l ) / ( t 2) increases are (1) the C T band maxima shifts toward higher energy and (2) the intensity of the bands decrease. Additionally, these plots show that hydrogen-bonding solvents correlate differently from the rest of the solvents. Specifically, the slope of the line through the hydrogen-bonding solvents is steeper than that of the other solvents by roughly a factor of 5 or 6. (Others have previously noted that hydrogen-bonding solvents behave differently from aprotic solvents for compounds 1 and 2.’9920) The correlation between peak
+
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(3) Brey, L.; Schuster, G. B.; Drickamer, H. G. J . Chem. Phys. 1979, 71, 2765. (4) (5) (6) (7)
Onsager, L. J . Am. Chem. SOC.1936, 58, 1486. McRae, E. G. J . Phys. Chem. 1957, 61, 562. Ooshika, Y. J . Phys. Chem. SOC.Jpn. 1954, 9, 594. Mataga, N.; Kubota, T. Molecular Interaction and Electronic Spectra; Marcel Dekker: New York, 1970; pp 371-410. (8) Amos, A. T.; Burrows, B. L. Adv. Quantum Chem. 1973, 7, 289. (9) Lippert, E. Z . Naturforsch. 1955, IOA, 541. (10) Liptay, W. Excited States 1974, 1 , 129. (11) Bakhshiev, N. G. Opt. Spectrosc. 1962, 13, 104; Opt. Spektrosk.
1962, 13, 192. (12) Bakhshiev, N. G.; Knyazhanskii, M. I.; Minkin, V. I.; Osipov, 0. A,; Saidov, G. V. Russ. Chem. Reu. (Engl. Transl.) 1969, 38, 740; Usp. Khim. 1969, 38, 1644. (13) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J . Am. Chem. SOC.1987, 91, 4714. (14) Nolte, K. D.; Dahne, S . Adu. Mol. Relax. Interact. Processes 1977, 10, 299. (15) Rettig, W. J . Mol. Struct. 1982, 84, 303. (16) Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Liebigs Ann. Chem. 1963, 661, 1. (17) Fowler, F. W.; Katritzky, A. R.; Rutherford, R. J. J . Chem. SOC.B 1971, 460. (18) Reichardt, C.; Harbush-Bornet, E. Liebigs Ann. Chem. 1983, 721, 11.
Hammack et al.
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position and (t - l)/(t 2) is somewhat rough since specific solute-solvent interactions may differ from solvent to solvent. In this paper pressure is used to induce the solvatochromism of compounds 1, 2, and 3 in a series of alcohols and polymeric media. The basic idea is that with pressure we change the quantity (t - I)/(€ + 2) and observe any changes in the betaine’s absorption spectra. Essentially, with pressure, we are able to perform, in a single solvent, the same experiment as using solvents with different values of (e - l)/(t + 2) at ambient pressure. Pressure-induced solvatochromism, though, has several clear-cut advantages over ambient pressure methods: (1) the dielectric properties of the solvent change in a smooth and continuous fashion and (2) any specific solute-solvent interactions that occur are, to first order, constant with pressure. The experiments reported in this paper are an implementation of a general principle of high-pressure research, viz., by varying the bulk properties of a material, either continuously or discontinuously, the relationship between bulk and molecular properties can be delineated. Experimental Section
High-pressure Measurements. Solution absorption spectra were taken by placing the alcohol solutions in a stainless-steel inner cell with pistons containing sapphire windows.2’ This cell was placed in a larger bomb which was filled with the pressurizing fluid, isobutyl alcohol. The polymers and dyes were dissolved in CH2C12and spin dried or vacuum evaporated. Dye concentrations were 1% (monomer basis). The films appeared uniform under the microscope, and peak locations and shapes were clearly different from those we observed for the crystalline solids. A 100-W Oriel quartz tungsten halogen lamp with a Kratos quartermeter monochromator using either a 400 or a 1000 blaze grating was used. Transmitted light was detected by an EM1 #9558 PM tube. All pressure-dependent phenomena were reversible and reproducible. Dielectric Constants. The change of the static dielectric constant, e, with pressure was calculated by using a modified form of the Clausius-Mossotti equation
-
-t --1 1 t+2p
= constant
The modification, due to Debye, makes the expression valid for molecules with permanent dipole moments.22 It is evident from eq 2 that the quantity (c - I ) / ( € + 2) will increase with compression since it is inversely proportional to the solvent density. Danforth has noted that if the reciprocal of the Clausius-Mossotti expression is plotted against density that a straight line is obtained.23 The slope of this line depends on the nature of the solvent studied. For highly polar solvents such as water, acetone, and methanol the slopes were almost identical ranging from 0.0964 to 0.988.24 A less polar solvent has a smaller slope, e.g., for n-pentane the slope is 0.526.24In order to calculate the change with pressure of c we used a slope of 0.988 for the alcohols and 0.526 for poly(styrene) and pdy(methy1 methacrylate). The change of density of the solvents used is from Bridgman.2s-27 For the polymers over the 10 kbar range the change in polarity constant was not sensitive to the exact value of the coefficient. For example, if one uses 0.65 the polarity constant for 10 kbar is moved by less than 0.01. Materials. Compound 1, pyridinium, 1 -(2’-hydroxy[ 1,1’:3,1”-terphenyl]-5’-yl)-2,4,6-triphenyl-,hydroxide, inner salt; compound 2, pyridinium, 1-(3,5-di-tert-butyl-4-hydroxy(19) Jouanne, J.; Palmer, D. A,; Kelm, H. Bull. Chem. SOC.Jpn. 1978, 51, 463. (20) Kjaer, A. M.; Ulstrup, J. J . Am. Chem. SOC.1987, 109, 1934. (21) Okamoto, B. Y. Ph.D. Thesis, University of Illinois, Urbana, IL, 1974. (22) Debye, P. Polar Molecules; Dover: New York, 1945. (23) Danforth, W. E. Phys. Rev. 1931, 38, 1224. (24) Salman, 0. A,; Drickamer, H. G. J . Chem. Phys. 1982, 77, 3329. (25) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1913, 49, 3. (26) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1931, 66, 185. (27) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1948, 76, 72.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3485
C T Transition in Pyridinium Betaines
(07
R2 = -H
Compound 2: R1 = -C(CH&
R2 = -H
Compound 1: R1 =
C(CH3)3
R2 = --C(CH3)3
Figure 1. General structure of a pyridinium N-phenoxide betaine dye.
5
Figure 3. Peak position of the charge-transfer band of compound 1 versus (e - I ) / ( € + 2). 2-propanol (A),ethanol (A), methanol (A). 19000
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