2934
J. Phys. Chem. 1981, 85,2934-2937
Infrared Linear Dichroism of Solutes in Stretched Polyethylene Using Fourier-Transform Spectroscopy. Pyrene Jullusr G. Radrlsrewskl and Josef Michi" Department of Chemistry, UnlversW of Utah, SaR Lake CW, Utah 84 1 12 (Received:January 26, 198 1; In Final Form: April 14, 198 1)
A Fourier transform-infrared spectrometer has been used to measure the linear dichroism of pyrene contained in stretched low-density polyethylene. Symmetries of -60 pyrene vibrations have been assigned, and the orientation factors were determined unambiguously. They agree very well with those determined previously from UV measurements assuming pure polarizations of the origins of the UV transitions and thus permit a resolution of a current controversy concerning the interpretation of linear dichroism in the UV region. The orientation distribution appears to be uniaxial but is far from rodlike.
Introduction The uniaxial partial alignment imparted to solutes included in stretched polymers causes their absorption to be dichroic. While many papers have been devoted to investigations of such linear dichroism in the UV-visible region and analyzed it to obtain information about polarization directions of electronic transitions in the solutes,l only a few studies described similar measurements on solutes in the infrared region. These were mostly limited to a few bands in solutes containing groups such as carbonyl or cyano which exhibit particularly intense bands in the IR,233and the method has not been generally considered very useful for vibrational assignments. We now wish to report that the use of thicker polymer sheets and of modern Fourier-transform instrumentation changes the picture dramatically, and the present paper reports a measurement of IR linear dichroism of -60 bands of the aromatic hydrocarbon pyrene (1)in stretched polyethylene.
1
This permits a symmetry assignment of these vibrations and an unambiguous determination of the orientation factors. These are then used to resolve a current controversy concerning the interpretation of the UV dichroic data of pyrene.
Experimental Section Infrared spectra were recorded on a Nicolet 7000 Fourier-transform spectrometer (4000-200 cm-l) with a resolution of 1 cm-', using aluminum grid polarizers (Cambridge Physical Sciences, Ltd.). Pyrene was purified by gradient sublimation. Linear low-density polyethylene pellets (research grade, obtained from Du Pont) were melted at 165 "C for 15 min, pressed at 2000 psi, and (1)For lists of references, see: (a) B. Norden, Appl. Spectrosc. Reu., 14,157(1978);(b) E.W.Thulstrup and J. Michl, J. Phys. Chem., 84,82 (1980);.(c) E.W.Thulstrup, "Aspects of the Linear and Magnetic Circular Dichroism of Planar Organic Molecules", Springer-Verlag, New York, 1980. (2) J. Kern, Z. Naturforsch. A, 17,271 (1962);H.Jakobi, A. Novak, end H. Kuhn, Z . Electrochem., 66,8631962;N. S. Gangakhedkar, A. V. Namjoski, P. S.Tamtrane, and N. K. Chaudhwi, J.Chem. Phys., 60,2584 (1974);I. Jonii and B. Norden, Spectrochim. Acta, Part A , 32,427(1976). (3)R. T. Ingwall, C. Gilon, and M. Goodman, J. Am. Chem. SOC.,97, 4356 (1975).
quenched in water. Unstretched sheets were soaked in a saturated chloroform solution of pyrene for 3 days at 40 OC or 6 h at 55 "C, then dried for 2 days in the air or 0.5 h on a vacuum line and stretched 600%. Complete removal of residual chloroform is only necessary if reproducible values for the orientation factors K are desired. The assignment of vibrational symmetries alone can be completed in less than 1day on several samples simultaneously. The samples were (10 X 5) mm2 in area and 0.4 mm thick and had very slightly nonparallel faces in order to reduce interference fringes. The reference (base-line) spectra were obtained from another sample of the same shape and thickness. A series of measurements was made on samples of different thickness and pyrene concentration in order to bring all bands to the optimum range of 0.1-0.3 absorbance units. For measurement at 77 K, the sheet was mounted at the head of a closed-cycle helium cryostat (Air Products Co.) between two CsI plates.
Results and Discussion Figure 1 shows a polarized IR spectrum of pyrene in stretched polyethylene along with the base line obtained on a blank polyethylene sheet. Many peaks due to pyrene are distinctly seen in the regions of low polyethylene absorption. The 715-735-,1455-1485, and 2800-3000-~m-~ regions are blocked by the absorption of the polymer. Figure 2 shows an example of pyrene spectra polarized parallel (E,) and perpendicular (Ey) to the stretching direction after computer subtraction of the base line. The regions blocked by polymer absorption are left blank. Those peaks which do not overlap others clearly fall into three categories, characterized by dichroic ratios d = EZ/Ey = 2.17 f 0.17, 1.03 f 0.05, and 0.27 f 0.03 and labeled by circle, dot, and arrow in Figures 2 and 3, respectively. A particularly nice illustration is provided by peaks in the 470--570-~m-~ region shown expanded in Figure 3. Because of the D% symmetry of pyrene, each transition is polarized along one of its three symmetry axes. The average degrees of alignment of the three axes are obtained from the relationlbl4 (cos2u ) = d,/(d, + 2)) where cos u is the angle between the uth axis and the polymer stretching direction, 2, d, is the observed dichroic ratio for transitions polarized along the uth axis, and pointed brackets indicate ensemble averaging. Labeling the axes z , y, and x in the order of decreasing average alignment, we obtain the results shown in Table I. The three orientation factors add up to unity within experimental error. (4)E.W.Thulstrup, J. Michl, and J. H. Eggers, J. Phys. Chem., 74, 3868 (1970).
0022-365418112085-2934$01.25/0 0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 20, 7987
Infrared Llnear Dichroism of Solutes
2935
TABLE I : Orientation Factors f o r Pyrene' room t e m p
-
77 K
uv
IR
uv
IR
0.56 i 0.02 0.58 i 0.03 0.33 i 0.01 0.34 i 0.02 0.10 i 0.01 0.08 i 0.05 a T h e measurement of UV dichroism was performed in this laboratory by Dr.M. Gisin, using standard procedures (E. W. Thulstrup. P.L. Case. a n d J. Michl. Chem. Phys., 6,410-8 (1974)). T h e pyrene concentration was considerably lower than 0.50 i 0.03 0.34 * 0.02 0.16 t 0.05
0.52 i 0.02 0.34 f 0.01 0.12 * 0.01
Kz = (COS' u') K , = (cos' y ) K, = (COS' X )
those used i n the IR measurements. WRVENUIIBERS
0
?
40 o
3 ~ 6 - 3 0 0 zobo
v
36b0
32b0
zsbo
zrbo
robo
WWENUMBERS
i6bo
izbo
sbo
iabo
i ~ b o ~ r b o Izbn
lobo
ROO
~ h o
1
C
rbo
Flgure 1. Infrared absorptlon of pyrene in stretched polyethylene (solid line) and base line (dashed Ilne). Polarization: perpendicular to the stretching direction.
The identification of the three axes with actual directions in the molecular framework can be performed by comparison with previous analyses based on single crystal IR data: 5-8 z is the long in-plane axis, y is the short in-plane axis, and x is the out-of-plane axis. This is as expected considering what is known about relations between molecular shape and alignment in stretched polyethylenegand is also in agreement with the analysis of linear dichroism in the UV region4 The results are summarized in Table 11, which also contains information on the polarization of partially overlapping transitions, which were resolved by using the stepwise reduction procedure of ref 4. The agreement of the presently determined polarizations with those accepted previously' is generally very good. There are four cases in which our assignment differs substantially from those proposed previously: (i) The 963-cm-' vibration assigned as B2, in ref 7 and as B3, in ref 5 is now confirmed to be of B3, symmetry. We propose that the B2, fundamental calculated' to lie in this vicinity corresponds to the y-polarized peak at 955 cm-'. The peak at 891 cm-' reported in ref 6 is not present in spectra of samples which do not contain crystalline pyrene. (ii) The strong band at 1600 cm-l is definitely of B1, symmetry and not Bauas assigned before.6 We assign it as a fundamental instead of the 5-times weaker band reported at 1584 cm-l and assigned as a fundamental in ref 7 (we observe it at 1578 cm-' and assign it as a combina(5) S. Califano and G. Abbondanza, J.Chem. Phys., 39, 1016 (1963). (6) A. Bree, R. A. Kydd, T. N. Misra, and V. V. B. Vilkos, Spectrochim. Acta, Part A , 27, 2315 (1971). (7) S. J. Cyvin, B. N. Cyvin, J. Brunvoll, J. C. Whitmer, P. Klaeboe, and J. E. Gustavsen, 2.Nuturforsch. A , 34, 876 (1979). (8) N. Net0 and C. di Lauro, Spectrochim. Acta, Part A , 26, 1175 (1970). (9) 3.Michl and E. W. Thulstrup, Spectrosc. Lett., 10,401 (1977);P. E. W. Thulstrup and J. Michl, unpublished results.
in
Figure 2. Polarized infrared spectra of pyrene in stretched polyethylene: (top) E;, (bottom) E Assigned polarizations: B,, (circles), BPU(dots), B,, (arrows). Crosses indicate peaks due to chloroform, to overlapping pyrene peaks of different polarizations which could not be analyzed, or to polyethylene. The intensity of the peak at 1433 cm-' is distorted in the bottom spectrum due to baseline subtraction.
.,
tion). We propose that the B,, fundamental calculated to lie in this vicinity corresponds to the y-polarized peak at 1611. cm-'. (iii) The 1206-cm-' band assigned as a B2, fundamental in ref 7 only appears in spectra of crystal-containing samples. We propose that the B2"fundamental expected in this region corresponds to the 1191-cm-' y-polarized peak observed in our spectra. (iv) The sharp band at 1002 cm-' rather than the broad peak at 1061 cm-' may be the B1, fundamental expected in this region; the latter then would be a combination band. This assignment is in better agreement with calculations." Parenthetically, we note that the solvent effects on the frequencies of fundamental vibrations are very small, usually less than 1 cm-l and only in one or two cases as large as 5-7 cm-'. On the other hand, almost all combination bands exhibit quite large solvent shifts, as much as 12 cm-l (Table 11). The peak positions shift to higher
2936
The Journal of Physical Chemistty, Vol. 85, No. 20, 1981
Radziszewski and Michl
TABLE 11: Observed Vibrations of Pyrene and Their Symmetries" wavenumber species B,,
K
CH,Cl,
496 674 819
498 667b 821 997 1002 1063 1085 1095 (1143) 1243 1289 1409 1449 1467 1524 1572 1587 1597 1600 1673 1703 1722
498 677' 821 (989) 1002 1061 1085 1095 (1143) 1242 1284 1409 1448 1514 (1562) 1578 1593 1600 1663 1690 1709
0.54 0.55 0.50 0.49 0.49 0.49 0.50 0.51
1769 1804 1873 1927 1941 3043 3046 3082b 3103b 353 54 2 812
1756 1789 1861 1915 1928 3040 3046 3080 3102 3 52 54 2 (800)
0.52 0.53 0.47 0.50 0.51 0.47 0.48 0.50 0.48 0.33 0.33
fundamental6 fundamental6 fundamental' fundamental? g fundamental or 542(B,,) + 408(A,) = 950 or 498(B,,) 594(A,) + 542(B,,) = 1136 fundamental' fundamental
1062 1095 1241 1446 1462 1584
3039
B,,
Polyethylene
lit.
30806 309V 351 540 89 1 963 1185 1206 1272 1312 1433 1484
0.53 0.48 0.50 0.53 0.55 0.55 0.55 0.48 0.52 0.53 0.54 0.50
e
1246 21g6 4846
955 (1136) 1184 1192
955 (1135) 1182 1191
0.36 0.33 0.33 0.34
1276b
1272
0.34
1312 141gb 1434 1487 1499 1551
1311 1418 1433 1485 1490 1542
0.34 0.33 0.34 0.33 0.33 0.32
1610 1652 1671 1755 1856
1611 1642 1659 1742 1844 i 219 487 (703) 711 744 84 2 962
0.36 0.33 0.35 0.32 0.33
7085 7455 8405
9635
i
220 489 706b 713b 744b 847 969
fundamental? 821(BlU)t 594(A,) = 1415 fundamental' fundamental? 773(B,,) t 744(B,,) = 1517 1066(Ag)t 498(B,,) = 1564 d fundamen tal 1095(B,,) t 594(Ag) = 1689 or 1182(B,,) t 505(B,,)= 1687 1359(B,,) t 352(B,,) = 1711 or 1174(B,,) t 542(B,,) = 1716 or 970(B, ) t 744(B,,) = 1714 1311(B,,~t 450(B,,) = 1761 1276(B,,) t 505(B,,)= 1781 1371(B,,) t 542(B,,)= 1913 1433(B,,) t 505(B3,) = 1938 fundamental
f
1593
B,,
fundamental fundamental? fundamental? 542(B,,) t 450(B,,) = 992 or 773(B,,) t 219(B,,) = 992 fundamental 842(B,,) t 221(B, ) = 1063d 594(Ag) t 498(B1,7 = 1092 or 970(B,,) t 124(B,,) = 1094 fundamental7
0.13 0.12 0.11 0.12 0.13 0.13 0.12
+
450(B,,)= 948
f
fundamental7or 821(B1,) + 450(B,,) = 1271 or 775(B,,) t 498(B1,) = 1273 fundamental" 1066(Ag)+ 352(B,,)= 1418 fundamental7 fundamental 1 1 4 2 ( 4 ) t 352(B,,) = 1494 1095(B1,) -t 450(B,,)= 1545 h fundamental 1242(B,,) t 505(B,,)= 1746 1174(B,,) t 677(B,,) = 1851 or 1107(B,,) t 821(B1,) = 1928 fundamen tal6 fundamental6 fundamental6 fundamental5 fundamental5 fundamental5 fundamental5
a Values in parentheses refer to weak bands overlapping with stronger bands and are less accurate. The Raman frequencies were taken from ref 6 (Bzg) and ref 7 (A B ). The literature IR frequencies were taken from ref 7 unless indicated otherwise. b CHC1, solution. ' Observed at %w'&emperature only (20 K). Assigned as fundamental in ref 7 (see text). e Could not be observed because of strong polyethylene absorption. f No peak was observed at this position. g This This vibration is presently assigned as B,, (see text). Lies outside of the previbration is presently as B,, (see text). sently investigated frequency region.
The Journal of Physical Chemistry, Vol. 85, No. 20, 1981 2937
Infrared Linear Dichroism of Solutes
e
. 530
550
550
5’10
WAVENUMBERS
4bO
4’lO
Ftgure 3. Polarized infrared spectra of pyrene in stretched polyethylene in the 470-570-cm-’ region: (dashed line) E=; (solid line) Ey. Assigned polarizations: B,, (circle), B2, (dot), B,, (arrow).
frequencies as one goes from hydrocarbon solvents to polyethylene, carbon tetrachloride, chloroform, methylene chloride, and acetone. The generality of this observation is now being examined. In our opinion, the primary significance of the present results for IR spectroscopy of large molecules lies not as much in their contribution to the problem of IR assignments in pyrene, but rather in that they illustrate how IR polarizations can be obtained in a fraction of the time which would be required for single crystal measurements. The knowledge of IR polarizations is likely to be of use in at least two ways: first, it will aid in spectroscopic assignments and normal mode analysis; and second, it will aid in structural and stereochemical assignments on those large molecules which contain groups with characteristic frequencies and polarizations. The main limitation of the method is the presence of strong absorption of the polymer in certain spectral regions, and limited solubility of some substrates. The use of several polymers for the same solute, e.g., polyethylene and perdeuterated polyethylene, should remove most of the first restriction. The small differencesbetween the K values of vibrations of like polarization shown in Table I1 are partly reproducible, but more accurate measurements are needed before they can be assigned with certainty to symmetry-lowering site effects. It is interesting to compare the IR results for pyrene with those obtained previously4 in the UV region. The orientation factors obtained by using the reduction procedure of ref 4 on the La, Bb, and B, electronic m*transitions of pyrene contained in stretched polyethylene are identical
with those obtained from the IR spectra within experimental error. For instance, using the same sample of polyethylene as that used in the IR work, UV measurements yield the values listed in Table I for K, (from La and B,), Ky (from Bb), and K, (by difference to unity). These values compare very well with those obtained from IR measurements. The values reported in ref 4 from UV measurements on pyrene in a different sample of polyethylene at room temperature were also quite similar to the presently obtained room-temperature IR values, K, = 0.50, K y = 0.32, K, = 0.18 (the value of K, is less accurate since it contains both the error in the determination of K, and that in the determination of Ky).In our experience, the relative independence of the exact nature of the polyethylene sample is quite general. The agreement between the orientation factors obtained from UV and from IR measurements implies that the fundamental assumption in the stepwise reduction of the UV spectra is correct, i.e., that the zero-zero components of the three strong electronic transitions, La,Bb, and B,, are purely polarized. This assumption has been recently challenged by Yoshinaga et al.,l0 who interpreted their stretched-sheet data as implying that the long-axis polarized La and B, transitions in pyrene contain short-axis polarized perfectly overlapping contributions and labeled them E2 and E4, respectively (the short-axis polarized Bb transition was assumed to be purely polarized). The present results demonstrate unambiguously that the original assumption4 was correct. The Ezand E4bands of ref 10 are artifacts due to invalid assumptions inherent in the Tanizaki reduction procedure (for a detailed analysis of the procedure see ref lb). The long-axis polarized spectrum shown in ref 10 agrees with that of ref 4 and is compatible with the present IR results. The long-axis polarized band E3 is probably of vibrionic origin4 We have now measured also the extremely weak Lb region of the UV spectrum and confirm the mixed polarization first reported in ref 10. It was assigned to vibronic interactions there, but we suspect that it is more likely due to the symmetry-loweringeffects due to the environment since it appears even in the 0-0 component of the Lb band. Finally, it should be noted that the inequality of K, and K y proves that there is a considerable difference between the alignment of the in-plane (y) and out-of-plane ( x ) short axes of pyrene. This provides a further warning against the indiscriminate use of the Fraser-Beer model for solute orientation, in which K, and K y are assumed to be equal and all angles of rotation around the solute orientation axis z are assumed to be equally probable (rodlike orientation distribution). While most likely quite correct for the polymer itself, this assumption is unphysical as far as the solute is ~oncerned.~ At one time, the application of this model to solutes contained in stretched polymers unfortunately enjoyed considerable popularity since it requires only one orientation parameter and was even defended as exact; l1 the model is occasionally still being u ~ e d . ~AJ ~ more detailed discussion of its problems can be found in ref lb.
Acknowledgment. This work was supported by a U S . Public Health Service grant (GM-19450). (IO) T. Yoshinaga, H. Hiratsuka, and Y. Tanizaki, Bull. Chem. SOC. Jpn., 50,3096 (1977). (11)A.Yogev, L. Margulies, and Y. Mazur, Chem. Phys. Lett., 8,157 (1971). (12)J. Sagiv, Tetrahedron, 33, 2303 (1977).
