Fluorescence and phosphorescence polarization of molecules oriented in stretched polymers. General description

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19,

number 4

IS April

CHEMICAL PHYSICS LE’ITERS

FLUORESCENCE OF MOLECULES

AND PHOSPHORESCENCE ORIENTED IN STRETCHED GENERAL DESCRIPTION

1973

POLARIZATION POLYMERS.

J.J. DEKKERS, G.Ph. HOORNWEG, C. MACLEAN and N.H. VELTHORST Chemical Laboratory

of the Free Utzivern@, Amsterdam.

Received 9 February

T%e Netherlands

1973

A description is given of an experimental technique used !o measure fluorescence and phosphorescence polarization spectra of molecules oriented in a stretched polymer sheet. T.his method of orienting moIccuIes is applied to diphenylhexatriene, a molecule with a large long-axis-short-axis ratio and to coronene, a molscule with a three-fold Lois of symmetry. The spectra observed indicate that both compounds are oriented considerably. The fluorescence of the first molecule is long-axis polarized: the phosphorescence of coronene is predomirunt!y polarized out-of-plane. The merits of the stretched film technique are discussed.

1. Introduction In recent years several studies have been published on the absorption and emission’of light by oriented molecules [ 1, 21. These optical experiments provide useful data about the transition moment direction of the electronic bands, which is an important quantity for band assignments. Moreover, polarization measurements are used for the analysis of vibrational structure of electronic transitions. In 1935 Jablonski [3] adsorbed dye molecules on a cellophane sheet. From the emission spectra he concluded that the molecules were at least partially oriented. Since that time this method has hardly been applied to determine the polarization of emission; on the contrary several studies have been published about optical absorption in stretched polymers like polyethylene [4-61 and polyvinylalcohol [7-91. We have investigated fluorescence and phosphorescence polarization spectra of aromatic hydrocarbons dissolved in polyethylene sheets. It is supposed that stretching of the sheets causes a preferential orientationsuch that the long axis of the molecules are mainly aligned along the direction of stretch [5]. In this paper we describe our procedure to measure emission polarizations and we introduce a correction factor f for the self-polarization of the equip

ment. On the basis of the emission data of diphenylhexatriene and coronene we will discuss the merits of this technique.

2. Methods for orienting molecrrles A number of methods to orient moIecules have been described in the literature. We mention briefly the principal ones. (a) Orientation in single and mixed crystals [IO-121. This method is ideal in that the mole&es are oriented perfectly. Nevertheless the interpretation of the spectra may not be straiglztforward, e.g., there may be several molecules in the unit cell. In pure crystals Davidov splittings may be Fresent. Unfortunately it is not always possible to find a suitable crystalline host to dissolve a particular hydrocarbon. (b) Orientation of molecules in Iiquid crystals [ 13-15). The alignment transferred on dissolved molecules can be ConsiderabIe, but often the liquid crystat displays optical absorption and emission in the wavelength region of the sample invtstigatcd. More-. over the liquid crystals may quench the emission of

the guest molecules. Temperature dependent studies are in general not possible. (c) Ojetitation.of PO!? molecules in strong elec-

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CHEMICAL

tric fields 116, 171. The force of this method lies in the simpli&of the orienting mechanism. The effects attainable are rather small and this technique can only be applied to polar molecules. (d) Photoselection [ 1S] . From the molecules in an isotropic rigid solution an assembly is selected with a preferential orientation along the electric vector of the linearly polarized exciting light. Pen-in [ 191 calculated the limits of the degree of polarization, defied as p=

u,,-Q&

+Q

Iii and 1, are, respectively, the intensi?ies of the

emitted light polarized parallel and perpendicular to the exciting light. For an isotropic distribution of molecules the values of P are between -l/3 and +1/Z. We emphasize that rhis method gives information about the relative orientation of the several transition moments of a molecule. We will discuss the stretched fiim technique in comparison with the photoselection method. (e) Orientation in stretched polymers. With this method we will be concerned in the following sec!ions.

3. The stretched film method

of emission

transitions

of compounds

dissolved in polymer sheets, will be described. The correction factor, necessary to eliminate instrumental errors will be evaluated. Finally the preparation of the samples will be reported. 3.1. A ppararus The optical arrangement is shown schematically in fig. 1. Light emitted by a 100 W mercury arc (1) passes a Bausch and Lomb grating monochromator (2) to select the desired wavelength. With suitable Schqtt filters (3) the second order light is screened. The exciting beam is polarized by a Clan prism (4); in .our experiments we only use exciting light with the electric vector parallel to the stretching direction of the sheet. The polarized light is focussed on the film which has been stretched in a film-holder (5); this at-

1973

tachment can be placed in a cooling vessel (6) with a suprasil window to allow measurements from room temperature to liquid nitrogen temperature. The emitted light is analyzed by means of a polaroid sheet (7) in two mutually perpendicular directions (III : parallel to the stretching direction;I1: perpendicular to the stretching direction ). Then the beam is focussed on the entrance slit of a Carl Leiss monochromator (8) and is dispersed by a flint-glass prism or a quartz prism (9). Before entering the photomultiplier tube (11) the beam is chopped (10) at 1200 Hz. The signal of the photomuitipher and a 1200 Hz reference signal (I2j are supplied to a lock-in amplifier (13). whereafter the signal is recorded (14); the photo-

multiplier signal can also be visualized on an oscilloscope (I 5j. A mercury arc is used for wavelength calibration. A check has been made by measuring the sharp fluorescence lines of coronene in crystalline n-heptane, which are compared with data given by Bowen and Brocklehurst [20, 211. We estimate that the wavelength can be detemlined to + 0.5 8, in the blue and to * 2.5 A in the yel!ow wavelength regions. 3.2. Corrections The degree of polarization P should be corrected for the self-polarization of the apparatus [22]. The correct expression from which P can be determined is

This section deals with the experimental procedure. The home-made equipment, used to investigate the polarization

15 April

PHYSICS Xl-TERS

in which

f is a correction

factor.

The main contribu-

tion to f is made by the unequal reflection of the components of the light beam perpendicular and parallel to the reflecting plane of the prism. If the angle of incidence and the indices of refraction are known f can be calculated from Fresnel’s reflection formulae. In fig. 2,fdc is given as a function of the wavelength for the flint-glass and quartz prism; isotropy of the prisms has been assumed. The factorf has also been determined expedmentally. When isotropic light passes through the prism the ratio between I,, and I, gives the experimental value of the correction factor. We have used light emitted by isotropic solutions of naphthalene, phenanthrene, coronene, tetracene or benzanthrone in diethylether at zoom temperature to measure I,, and 1, at different wavelengths. The ratio I,,/I, is represented in fig. 2 for

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7OOW mercury w-c Bausch 8 Lomb grating ~nochmmat~r

Schott filters Gian prism Film-holdar Cooling vessel

CHEMICAL

PH:SICS

LElTERS

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Polaroid sheet moncxhromator El Carl t&s Flint SF5 9 ( c&art 2 50 Optical chopper ‘I200Ht RCA 7265 11 F~oto~~t~pl~rtu~e 12 fi~b-~i-02 signal

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1973

PAR preci?&n Lock-in ampfifier WR-8 Kipp recorder micrbgmpt-i f3D5 686 E Tektronix asciftoscape 561 A

Ten- turn potentiom&Qt” XL0R Synchronous motor Q.OM -Ifir.p.m. 76 2V horb3ntaf cteffection osciltoscope

Fig. I. Diagram of the optical ~rr~n~ernc~t.

both prisms. We have ernp~oyed~~~~ in the determination ofP, as this factor includes ali effects of the ex~riment~ arrangement on P.

shape of the spectra but do not influence the degree of pofarization.

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CHEMICAL PHYSICS LEIITERS

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measurements, as it is transparent for light with wavelengths longer than 2100 A. The hydrocarbon molecules are introduced into the unstretched sheet by dipping it into a solution of the compound in diethylether or chioroform. The molecules diffuse into the amorphous parts of the polymer sheet. After a few hours the fdm is washed with the solvent to remove small crystallites from the surface. Then the film is clamped into a film-holder and stretched six- or sevenfold.

It is also possible to introduce the solute into the sheet after stretching. As expected, the solubility of the compound in a stretched film is much lower than that in an unstretched one [23].

I-.

I

400

500 -h(nm)

7co

600

4. Experimental Fig. 2. The correction factorffor the self-polarization of the apparatus as a function of the wavelength. experimental, --- theoreiicul.

results and discussion

To investigate method

the suitability of the stretched film for fluorescence and phosphorescence we

A

I

400

I

450

L

500

Fig. 3. Polarization flubrescence spectra of diphenylhexatriene at.room temperature (A) and at -190°C (B). stretching direction, --- perpendicular to the stretching direct+ -520.

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y

..’

:

parark to the

Volume 19, number 4

CHEhlICAL PHYSICS LETTERS

have used all-trans diphenylhexatriene as an example of a molecule with a large long-axis-short-axis ratio (D,, symmetry) and coronene, as an example of a molecule with a three-fold axis of symmetry (D,, symmetry). 4.1. Dipheuylhexatn’ene The experimental results at room temperature and at -190°C (fig. 3) show that 111is much more intense than IL. Evidently, the fluorescence of djphenylhexatriene is polarized

along the long molecular

axis. At

room temperature the corrected value of P is about + 0.89 for all vibrational bands. In the evaluation of P, overlap of the vibrational bands has been neglected. For comparison we give the value observed by Sackman and Rehm [ 131 in a liquid crystalline mixture of cholesterylchloride and cholesteryllaurate, oriented by an electric field at +30°C:P= +0.30. We have also measured P at room temperature as a function of the degree of stretching (s> of the sheet (fig. 4). At first Pincreases rapidly with S, but for S > 4 a nearly constant value of P is reached. From the fluorescence spectra recorded at -190°C and at room temperature it is clear that P which has the values +0.58 and eO.89, respectively, is temperature dependent, a phenomenon to be studied further. 4.2. Coronene In fig. 5 the uncorrected fluorescence and phosphorescence spectra of coronene, measured at -19O”C, are given. The details of the spectra are in agreement with those published in the literature 124, 2.51. The corrected values of P for all vibrational bands are schcmatically presented in a histogram (fig. 5). From this behaviour we conclude that coronene is partially oriented in a stretched fim. We assume that the molecules are aligned such that their molecular planes are approximately parallel to and randomly distributed about fhe stretching direction. In this case the maximum theoretical degree of polarization of the fluorescence (Pujamounts to i-O.33 and that of the phosphorescence (P,,), if polarized perpendicular to the molecular plane, to - 1. It appears from our experiments that the value of Pn is about +0.2>, whereas P,& is about -0.54. The opposite signs of Pfl and Pph indicate that the phosphorescence of coronene is predominantly polarized out-of-plane. Stretching of the sheet results in a six- or seven-

Fig. 4. Corrected polarization

degree of diphcnylhe~triene ;1 function of S, the degree of stretching.

as

fold increase of its length and a three-fold decrease of its thickness. The differences between the esperimental and theoretical values of P can probably be ascribed to deviations from the distributians described above.

5. Concluding

remarks

The stretched fim method is attractive in view of the attainable limits of P: -1 f P< f 1. Moreover it is an absolute method in contrast with photoselection, which yields relative polarizations. For molecules with a large Iongaxis-short-axis ratio the film method does not distinguish a polarization of the phosphorescence along the short axis from a polarization out of the molecular plane. However, for molecules with a three-fold axis of symmetry, the distinction between a polarization in or out the molecular plane can be made, as has been pointed out For coronene. Photoselection provides information about this direction for both types of moIecules [25]. Polarization of transitions to higher excited states - 521

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CHEMICAL PHYSICS LETTERS

15 April 1973 8x-

L 450

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550

600

650

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X(nm)

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5. Polakation fIuorcscence and phosphorescence spectra of coroncnc at -190°C . parallel to the stretching --- perpendicular to the stretching direction. In the histogram the cqrrected \-alues of P are given.

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Volume 19, number 4

CHEMICAL PHYSICS J,ETTERS

can be determined with the photoselection method by varying the wavelength of the exciting light. The stretched film method can give the absolute polarization of these transitions by measuring the absorption spectra directly. At low temperatures a well-resolved vibrational fine structure can often be found, a feature useful for vibrational analysis. This will discussed in a subse-, quent paper, by investigating a series of related hydrocarbon compounds.

References hf. Held and F. D’drr, Angew. Chem. 72 (1960) 287. F. Dijrr, Angew. Chem. 78 (1966) 457. A. Jablonski, Acta Phys. Polon. 3 (1934) 421. J.H. Eggers and E.W. Thulstrup, Chcm. Phys. Letters 1 (1968) 690. J.H. Eggers, E.W. Thuistrup and .I. hlichl, J. Phys. Chem. 74 (1970) 3868. (61 J. Michl, E.W. Thulstrup and J.H. Eggcrs, J. Phys. Chem. 74 (1970) 3878. I71 H. Inouc, T. Hoshi, T. hiasamoto, J. Shiraishi and Y. Tanizaki, Bcr. Bunsengcs. Physik. Chem. 75 (1971) 441. 181T. Hoshi, H. Inouc, J. Yoshino’, T. Mnsamoto and Y. Tanizaki, Z. Physik. Chem. N. F. 81 (1972) 23.

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191T. Hoshi, J. Yoshino, H. Funami, H. Kadoi and H.Lnoue, Ber. Bunsenges. Physik. Chcm. 76 (1972) 888.

ilO1 G. Scheibe, St. Hartwig and R. Miilier. 2. EIcctrochcm. 49 (1943) 372.

(111 D.P. Craig, PC. Hobbins and J.R. WaIsh, J. Chem. Phys. 22 (1954) 1616.

1121J.W. Sidman and D.S. hlcCIure, I. Chem. Phys. 24 (1956) 757.

1131 E. Sackmann and D. Rehm,Chem.

Phys. Letters4 (1970) 537. I141 E. Sacltmann, J. Am. Chem. Sot. 90 (1968) 3569. 1151 G.P. Ceasx and H.B. Gray, J. Am. Chem. Sac. 91 (1969) 191. 1161H. Labhart, Tetrahedron 19. suppl. 2 (1963) 223 [171 J. Czckalla, Bcr. Bunscnges. Physik. Chem. 64 (1960) 1221. [iSI AC. Albrecht, J. hlol. Spcctry. 6 (1961) S4. I191 F. Perrin, Acta Phys. Polon. 5 (1936) 335. 1201 E.J. Bowen and B. Brocklehurst, I. Chcm. Sot. (19.54) 3875. 1211 E.J. Bowen and B. Brocklehurst, 1. Chcm. Sot. (1955) 4320. 1221 T. Azumi and S.P. McClynn, I. Chem. Phys. 37 (L962) 2413. 1231 A. Petcrlin and H.G. Olf, J. Polymer Sci. Ser. Za, 4 (1966) 587. v41 H. Zimmerman and N. Joop, Ber. Bunsenges. Physik. Chem. 65 (1961) 138. 1251 F. D&r and H. Gropper, Ber. Bunsenges. Physik. Chem. 67 (1963) 193.

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