Linear dichroism and trans → cis photo-isomerization studies of azobenzene molecules in oriented polyethylene matrix

Fur, Polym. J. Vol. 27, No. 1, pp. 41-43, 1991 Printed in Great Britain. All rights reserved

0014-3057/91 $3.00+ 0.00 Copyright © 1991 PergamonPress pie

LINEAR DICHROISM A N D T R A N S ---, C I S PHOTO-ISOMERIZATION STUDIES OF AZOBENZENE MOLECULES IN ORIENTED POLYETHYLENE MATRIX P. UZNANSKI, 1 M. KRYSZEWSKI 1. and E. W. THULSTRUP2 ~Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 E'6d~, Poland 2The World Bank, 1818H St. N.W., Washington, DC 20433, U.S.A. (Received 29 March 1990)

Abstract--U.v.-vis linear dichroism spectra of trans- and cis-azobenzene in stretched poly(ethylene) (PE) films are reported. Trans --,cis photo-isomerization kinetics were measured at 337 nm using polarized light with the electric vector either parallel or perpendicular to the stretching direction of the matrix. Three types of samples were examined, viz. unoriented, kept at constant elongation stretched to (800%) and relaxed (stretched to 600%). Analysis of the kinetic data reveals that the photo-isomerization is purely exponential in time up to 83% conversion, with nearly the same rate constants in both directions. It is faster in the stretched than in the unstretched sample. This suggest that trans---,cis isomerization of azobenzene in nonpolar solid solution occurs according to an inversion mechanism. Cis-azobenzene molecules obtained from phototransformation of the trans species in stretched PE attain globular geometry with the phenyl rings twisted perpendicular to the azo plane.

INTRODUCTION

n n * transition can occur by rotation [6]. From exper-

imental results on the molecular orientation effect of coiour fading in spiropyrans, different photo-isomerization kinetics for molecules with different positions in the oriented polymer matrix (parallel or perpendicular alignment) would be expected. In this system, photochromism is indeed related to the reduction of segmental mobility because the rotational pathway requires substantially more local free volume.

The photochromic probe technique is one of few which makes it possible to evaluate average free volume and its distribution in amorphous polymers [1-3]. A m o n g m a n y photochromic molecules employed in these studies, azobenzene and its derivatives have considerable interest because they have only two ground-state configurations (trans and cis) while other photochromic probes (such as spiropyran derivatives) have several iso-energetic trans-conformers of the opened merocyanine form [4]. In addition, u.v. illumination to photo-equilibrium of the latter can lead to local polymer re-organization forced by energy dissipation. Recently we used linear dichroism absorption spectroscopy to determine the molecular geometry as well as the number, positions and polarizations of electronic transitions in trans-azobenzene, aligned in uniaxially stretched polyethylene (PE) films [5]. We also demonstrated that trans-azobenzene (t-ab) has a non-planar geometry and that the polarization direction of the nn* transition is different from that of the first n n * absorption. Here we report further results using the stretched sheet method to elucidate some spectral and dynamical properties of both transand cis-ab species, especially the photo-isomerization. Two basic mechanisms for isomerization are considered viz. rotation about the N - - N bond following bond rupture, or alternatively inversion following a (sp-hybridized) linear transition state (Fig. 1). Although there are theoretical and experimental indications that for m a n y azobenzenes phototransformation occurs by the inversion process, some authors suggest that photo-isomerization at the first

EXPERIMENTAL PROCEDURES

For high degree of orientation of solute molecules, we used low-density PE. Preparation of samples, i.e. stretching and doping in the case of azobenzene, is relatively simple. We introduced azobenzene molecules into the polymer from the vapour phase by storing the commercial polymer sheet over small amount of substrate. Stretching was done on an Instron tensile machine at a drawing rate of 5 cm/min, or on a stretcher, a mechanical device which could be mounted in a spectrophotometer. The final drawing ratio of the PE sheets was 800% (strained samples at constant elongation) or 600% (released or stretched specimens). We measured separate dichroic spectra Ez(~) and Ey(~) of the dopant, where as usual Ez(~) is recorded with the electric vector of the analysing light along the sample axis Z (the stretching direction) and F~@) is recorded with the electric vector perpendicular to Z. We used the TEM [7-9] stepwise reduction procedure in order to decompose u.v. spectra into components of different transition moment directions. First we determined the set of functions Ez(~)- d'Ey(~) which allows separation of contributions to the spectrum by different polarized overlapping transitions. A dichroic ratio d = dr is determined for each spectral feature f (i.e. maximum or shoulder); the feature f disappears from the combination Ez - d. Ey ; df determines the orientation factor Kf = (cos2(Z, Mf))--tbe average deviation of the transition moment from the stretching direction Z: Kf = df/(df + 2). For trans- to cis- phototransformation of azobenzene, we used nonpolarized nitrogen laser light (337 rim). The measurements were carried out at ambient temperature.

*To whom all correspondence should be addressed. 41

42

P. UZNANSKIet al.

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Fig. 1. Formula and scheme of photochromism for azobenzene. 0.6

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Fig. 2. Dichroic spectra of t-ab before (E') and after illumination at photo-equilibrium (E'+c) in oriented PE. Polarization of the analysing light was parallel ( ) and perpendicular (. . . . ) to the stretching direction Z. RESULTS AND DISCUSSION

The polarized u.v.-vis spectra Ez(~) and Ev(~) of t-ab before (pure trans-) and after (mixture of cisand trans- content) illumination at photo-equilibrium in stretched PE sheet are shown in Fig. 2. the spectra of t-ab demonstrate that the first 7rn* band is polarized close to the long molecular axis while the second nn* transition is more perpendicular to it. In c-ab (Fig. 3), the two spectra are very similar indicating that the alignment of the globular cis form is poor. The bottom part of Fig. 3 shows linear combinations of the polarized absorption spectra E z ( ¢ ) - d. Ev(¢). For each spectral feature, the orientation factors Kr at wavenumber ~ can be determined. In the case of t-ab, it was possible to determine the angles between "long molecular axis" and electronic transition moment Mr from Kr values [5], using orientation factors K, (u - - x , y, z) for the principal orientation axes x, y, z (connected with molecular framework) obtained from i.r. dichroism measurement. For c-ab, the information obtained from u.v. and i.r. spectra did not allow such a determination because of the small differences between the two dichroic spectra. The peaks in the i.r. spectrum of the illuminated sample are due to a mixture of vibrations of the trans and cis forms. We consider only the Kr

i t l i t , l l l l l i , t ,

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Fig. 3. Dichroic spectra of c-ab in oriented PE obtained as the linear combination E' + c - 0.05.E' for both analysing directions. The light polarization was parallel ( ) and perpendicular (. . . . ) to the stretch direction Z. Bottom: linear combination of the polarized u.v. absorption spectra Ez(¢) - d ' Ey(¢) (0.8 < d < 1.4). The orientation factor for the transitions at 22.5 kK and 35 kK is Kf = 0.33; for the transition at 40.5 kK, it is Kf--0.37. values of new peaks, which have appeared in the spectrum after the trans-~cis photo-isomerization, i.e. at 495, 701,759cm -~, with K f = 0.29 and at 593, 860, 1587, 1591 cm -~ with K f = 0.33. If we combine the Kf values of these with those obtained from the u.v. measurements, we note that they fall into three groups. The sum of the three values is quite accurately unity. c-ab can belong to either of the TEM categories of low symmetry or high symmetry molecules. The point group symmetry is C2, when the phenyl rings lie in the plane of the azo bridge or are perpendicular to it and

Photo-isomerization studies of azobenzene t0

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Fig. 4. Plots for trans-*c& photo-isomerization of azobenzene in unoriented (©) and in stretched (up to 600%) PE. The electric vector of the analysing light was parallel (F1) or perpendicular ( I ) to the stretching direction. C i (C2) when they are twisted in the same (opposite)

direction. The fact that only three different K-values are observed indicate that only three different transition moment directions occur. This suggests that we can assume a C2v symmetry for c-ab, probably with nonplanar geometry. Now these three Kf values can be referred to orientation factors K, (u = x, y, z) with Kx = 0.29, Ky = 0.33, Kz = 0.37. Possible orientations of transition moments are limited to the x, y, z axes within the molecular framework. The dichroic spectra of the mixture of isomers give also information on the extent of photo-isomerization of azobenzene in'stretched PE matrix. It is 95% both for molecules oriented parallel and perpendicular to the stretching. Figure 4 shows kinetic data for trans---,cis photo-isomerization as a function of time after irradiation using polarized light with electric vector either parallel or perpendicular to the stretching direction of the matrix at 337 nm. The trans --,cis isomerization was studied in three samples, viz. unstretched, stretched to 600%, and strained at constant elongation (800%). Analysis of the kinetic data reveals that the photo-isomerization is purely exponential in time up to 83% conversion and then deviates from first order kinetics. Table 1 collects all kinetic constants and the trans-fraction at photo-equilibrium measured with light polarization parallel or perpendicular to the stretching direction Z as well as the dichroic ratios. The data show that the magnitudes of kz and kv are not strongly affected by stretching while d is affected significantly. It may therefore be inferred that the transformation of azobenzene molecules is not influenced much by stretching while their orientation is affected significantly. When the polymer is stretched Table 1. The dichroicratios(d) for t-ab at 337 nm, firstorder kinetic constants (k) and trans fractioncontent(t) at photo-equilibriumin PE measured parallel (Z) and perpendicular (Y) to the stretch direction Z Specimen d kz[1/sec] kv[I/sec] tz tv Unoriented 1.0 0.00193 0.00193 0.175 0.175 Stretched 600% 2.5 0.00249 0.00231 0.105 0.146 Strained 800% 3.45 0.00218 0.00197 0.105 0.146

43

600%, the process becomes a little faster and the extent of photo-isomerization decreases. Further stretching cause a small opposite effect. The most striking observation is the small change in the trans-cis transformation process as a function of stretching and the lack of significant differences in k for molecules oriented parallel (kz) or perpendicular (kv) to Z. The first observation is probably a reflection of different solubilization sites for molecules before and after stretching of the matrix. The second provides strong evidence that oriented solubilization sites do not prevent the isomerization. It therefore appears that trans-cis isomerization of azobenzene in nonpolar solid solution occurs according to an inversion mechanism, because this does not require extra free volume. In summary, u.v. and i.r. linear dichroism studies of c-ab, obtained from phototransformation of the trans species in stretched PE, show that it attains a globular geometry with the phenyl rings twisted perpendicular to the plane determined by C - - N - - N - - - C . The nn* and the first nn* transition at 35 kK are polarized perpendicular to N ~ N whereas the second nn* transition at 40.5 kK is polarized parallel to azo group. The trans-eis photo-isomerization is scarcely affected by the stretching. The data indicate that, although trapping sites are anisotropic in shape for orientation, they do not provide anisotropy for the relaxation process of azobenzene molecules. We suggest that the latter phenomenon can be explained by acceptance of an inversion mechanism for phototransformation. It seems that the problems studied are closely connected with the location and mechanism of alignment of organic molecules in drawn PE sheets which is still a matter of controversy and remains a subject for further studies [10, 11]. Acknowledgement--This research is partially supported by Danish Natural Science Research Council and CPBP 01.19 Project.

REFERENCES

I. J. G. Victor and J. M. Torkelson. Macromolecules 20, 2241 (1987). 2. W.-Ch. Yu, Ch. S. P. Sung and R. E. Robertson. Macromolecules 21, 355 (1988). 3. I. Mira, K. Horie and K. Hirao. Macromolecules 22, 558 (1989). 4. K. Horie, K. Hirao, N. Kenmochi and I. Mita. Macromolec. Chem.; Rapid Commun. 9, 267 (1988). 5. P. Uznanski, M. Kryszewski and E. W. Thulstrup. Spectrochim. Acta in press (1990). 6. H. Rau and E. J. Luddecke. J. Am. chem. Soc. 104, 1616 (1982). 7. J. Michl and E. W. Thulstrup. Spectroscopy with polarized light. Solute Alignment by Photoselection in Liquid Crystals, Polymers and Membranes. VCH, Deerfield Beach, Fla. (1986). 8. E. W. Thulstrup and J. Michl. Spectrochim. Acta 44A, 767 (1988). 9. E. W. Thulstrup and J. Michl. Elementary Polarization Spectroscopy. VCH, Deerfield Beach, Fla (1989). 10. Y. T. Jang, P. J. Phillips and E. W. Thulstrup. Chem. Phys. Lett. 93, 66 (1982). 11. A. Kaito, K. Nakayama and H. Kanetsuna. J. Macromolec. Sci.-Phys. B26(3), 281 (1987).