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High-field anisotropies in the effect of magnetic field on d.c. photoconductivity of anthracene and tetracene
Volume 30A, number 6
PHYSICS L E T T E R S
HIGH-FIELD ANISOTROPIES IN THE EFFECT OF O N D. C . P H O T O C O N D U C T I V I T Y OF ANTHRACENE
17 November 1969
MAGNETIC FIELD AND TETRACENE*
P. DELANNOY and M. SCHOTT Groupe de P h y s i q u e des Solides de I'E. N. S. ** T o u r 23, 9,quai S a i n t - B e r n a r d - P a r i s Ve, F r a n c e
Received 11 August 1969
The maximum magnetic-field effect on photoconductivity occurs in the same directions as the effect on delayed (in anthracene) or normal (in tetracene) fluorescence, indicating that the magnetic field sensitive state is the same in all cases, namely the triplet exciton.
F r a u k e v i c h showed that a static m a g n e t i c field d e c r e a s e s the d.c. photoconductivtty [1] and r e c o m b i n a t i o n r a d i a t i o n [2] in a n t h r a c e n e c r y s t a l s . He proposed that r e c o m b i n a t i o n p r o c e e d s via longlived m a g n e t i c - f i e l d s e n s i t i v e W a n n i e r excttons, which he also c a l l s " l a r g e r a d i u s excttous" or "excftons with Charge t r a n s f e r " although they app e a r to be different f r o m the m o r e tightly bound "charge t r a n s f e r excttons" studied by o t h e r s [3]. Such W a n n i e r eXcttons have also been postulated by P e s t e f l [4] tO explain " b y d r o g e n - l t k e " s y s t e m s of a b s o r p t i o n bands on the long wavelength side of the f i r s t i n t e n s e a b s o r p t i o n of n u m e r o u s a r o m a t i c c r y s t a l s and m o r e r e c e n t l y by Donntnt and Abetino [5] to eXplain the e n h a n c e m e n t of the t e t r a ceue c r y s t a l f l u o r e s c e n c e by a m a g n e t i c field. A study of the v a r i a t i o n of these effects with the d i r e c t i o n of the m a g n e t i c field would be very i n f o r m a t i v e . A n t s o t r o p i e s w e r e found by J o h n s o n et al. [6] in the m a g n e t i c field effect on delayed f l u o r e s c e n c e o f a n t h r a c e n e c r y s t a l s and they showed that the s e n s i t i v e step is the t r i p l e t t r i p l e t annihilation. An a n i s o t r o p y in the m a g n e t ic field e n h a n c e m e n t of the i n s t a n t a n e o u s f l u o r e s cence of t e t r a c e n e was r e c e n t l y found [7,8] and it was proposed that the s e n s i t i v e step is the d i s sociation of the s i n g l e t exciton into two t r i p l e t s . M e r r t f l e l d [9] explained the a n t h r a c e n e r e s u l t s by a s s u m i n g that the i m p o r t a n t p a r a m e t e r is the n u m b e r of d o u b l e - t r i p l e t s t a t e s having s i n g l e t c h a r a c t e r . At "high field", where the Z e e m a n e n e r g y is much l a r g e r than the z e r o - f i e l d s p l i t ting, the spin wavefuncttons a r e quanttzed along the field and a r e independent of its s t r e n g t h , * Work supported by the DGRST under contract 66.0299.01 ** Laboratoire associ~ au CNRS.
f
a axis
Fig. 1. Decrease in d.c. photoeurrent in anthracene
plotted versus the angle 8 between the a axis and magnetic field. Circles and crosses correspond to two different experiments. H = 7kG in bothocases. Positive electrode illuminated with 3650 A light. hence the constancy of the effect. T h e r e is s t i l l however an a n i s o t r o p y in the e n e r g i e s , r e m i n i s cent of the z e r o - f i e l d splittlng and t h e r e may be c r o s s i n g for c e r t a i n d i r e c t i o n s of the field, hence the auisotropy. We r e p o r t h e r e our m e a s u r e m e n t s of highfield a n i s o t r o p i e s which appear in the d.c. photoconductivity of a n t h r a c e n e and t e t r a c e n e . The c r y s t a l s were s u b l i m e d f r o m p u r i f i e d m a t e r i a l , with well developed (a,b) p l a n e s , 10 - 50 /1 thick; they w e r e mounted In ~plexiglas" h o l d e r s , s a n d wiched between aqueous e l e c t r o d e s and i l l u m i nated with different l i n e s of a P h i l i p s SP 500 m e r c u r y l a m p , isolated by f i l t e r s . The m a g n e t i c
357
Volume 30A, number 6
PHYSICS L E T T E R S
field was in the (a,b) plane. All e x p e r i m e n t s were p e r f o r m e d at r o o m t e m p e r a t u r e . In a n t h r a c e n e , the photoconductivity is d e p r e s sed by a few p e r cent in "high" magnetic fields. Above 3 kG, the d e c r e a s e is independent of the magnitude of the field, but depends s t r o n g l y on its d i r e c t i o n in the (a,b) plane. The magnitude of the effect v a r i e s somewhat f r o m c r y s t a l to c r y s t a l , but not the a n i s t r o p y (fig. 1). The d i r e c t i o n s of m a x i m u m d e c r e a s e a r e exactly the s a m e a s those calculated, u s i n g the known zero field splitting of a n t h r a c e n e t r i p l e t excitlon [10]. T h i s i n d i c a t e s that the s p e c i e s s e n s i t i v e to the magnetic field in the photoconductivity is the t r i p l e t , e i t h e r through t r i p l e t - t r i p l e t or through t r l p l e t - d o u b l e t i n t e r action, a s both will give the s a m e a n i s o t r o p y (see [11] ). The t r i p l e t s play a role in the g e n e r ation (by s u r f a c e ionization) but also in detrapping (by t r a n s f e r of their e n e r g y to t r a p p e d c a r r i e r ) , e t c . . . T h e r e f o r e the d e t a i l s of the effects of the m a g n e t i c field on the o v e r a l l photoconduction p r o c e s s a r e not yet known and s t i l l u n d e r study. In t e t r a c e n e , the conductivity is enhanced in a h i g h - f i e l d (>~3kG). The s a m e a n i s o t r o p y is then found whatever the p o l a r i t y of the i l l u m i n a t e d e l e c t r o d e , with two s h a r p m a x i m a o c c u r i n g when the d i r e c t i o n of the m a g n e t i c field is a p p r o x i m a t e l y at 34 ° and 18 ° of the b axis. Its position c o i n c i d e s exactly with the a n i s o t r o p y a l r e a d y found for f l u o r e s c e n c e [7,8] although the e n h a n c e ment is l e s s in conductivity (< 10% as c o m p a r e d to ~27%). T h i s o b s e r v a t i o n , together with the p r e c e d i n g one on a n t h r a c e n e , s u p p o r t s the a s s u m p t i o n that the " m a g n e t i c a l l y active s p e c i e s " in t e t r a c e n e is also the t r i p l e t . No detail c a l c u l a t i o n is p o s s i b l e u n t i l the zero field splitting p a r a m e t e r s of the c r y s t a l l i n e t r i p l e t exciton a r e known. In both c a s e s , the behaviour of the photocon-
358
17 November 1969
ductivity is p a r a l l e l to that of the singlet density governing the f l u o r e s c e n c e . T h i s has been noticed for a long t i m e for the e x t r i n s i c photoconductivity in a r o m a t i c c r y s t a l s [e.g. 12]. Additional i n f o r m a t i o n wiU be provided by low field s t u d i e s c u r r e n t l y in p r o g r e s s . The p r e s e n t r e s u l t s indicate that one should t r y to avoid the i n t r o d u c t i o n of " l a r g e r a d i u s e x c i t o n s " , but t r y to explain the magnetic field effects d i s c u s s e d h e r e only through t r i p l e t s . We thank P. Quedec for purifying and p r e p a r i n g the t e t r a c e n e c r y s t a l s .
References 1. E.L. Frankevieh and E. L Balabanov, Soviet Physics Solid State 8(1966) 855. E. L. Frankevich, Soviet Physics JETP 23 (1966)814 and following papers. 2. E.L. Frankevieh and V. M. 11umyantsev, Soviet Physics JETP 26 (1968) 1102. 3. S.I. Choi, J.Jortner, S.A. Rice and 11. Silbey, J. Chem. Phys. 41 (1964) 3294. 4. P. Pesteil, J. Chim. Phys. 58(1961) 661. M. Benarroche, Ann, Phys. 2 (1967) 27. 5. J. M. Donnini and F. Abetino, Compt. Rend. 266 B (1968) 1618. 6. 1t. C. Johnson, 1t. E. Merrifield, P. Avakian and 11. B. Flippen, Phys. Rev. Letters 19 (1967) 285. 7. N. Geacintov, M. Pope and F. Vogel, Phys. Rev. Letters 22 (1969) 593. 8. 11. E. Merrffield, P. Avakian and 1t. P. Groff, Chem. Phys. Letters 3 (1969) 155. 9. 1t. E. Merrifield, J. Chem. Phys. 48 (1968) 4318. 10. D. Haarer, D. Schmid and H. C. Wolf, Phys. Stat. Solidi 23 (1967) 633. 11. V. Ern and 11. E. Merrffield, Phys. Rev. Letters 21 (1968) 609. 12. O. H. Leblanc Jr. in Physics and chemistry of the organic solid state, III, eds. F. Fox, M. Labes and A. Weissberger (Wiley, 1967), p. 133.
PHYSICS L E T T E R S
HIGH-FIELD ANISOTROPIES IN THE EFFECT OF O N D. C . P H O T O C O N D U C T I V I T Y OF ANTHRACENE
17 November 1969
MAGNETIC FIELD AND TETRACENE*
P. DELANNOY and M. SCHOTT Groupe de P h y s i q u e des Solides de I'E. N. S. ** T o u r 23, 9,quai S a i n t - B e r n a r d - P a r i s Ve, F r a n c e
Received 11 August 1969
The maximum magnetic-field effect on photoconductivity occurs in the same directions as the effect on delayed (in anthracene) or normal (in tetracene) fluorescence, indicating that the magnetic field sensitive state is the same in all cases, namely the triplet exciton.
F r a u k e v i c h showed that a static m a g n e t i c field d e c r e a s e s the d.c. photoconductivtty [1] and r e c o m b i n a t i o n r a d i a t i o n [2] in a n t h r a c e n e c r y s t a l s . He proposed that r e c o m b i n a t i o n p r o c e e d s via longlived m a g n e t i c - f i e l d s e n s i t i v e W a n n i e r excttons, which he also c a l l s " l a r g e r a d i u s excttous" or "excftons with Charge t r a n s f e r " although they app e a r to be different f r o m the m o r e tightly bound "charge t r a n s f e r excttons" studied by o t h e r s [3]. Such W a n n i e r eXcttons have also been postulated by P e s t e f l [4] tO explain " b y d r o g e n - l t k e " s y s t e m s of a b s o r p t i o n bands on the long wavelength side of the f i r s t i n t e n s e a b s o r p t i o n of n u m e r o u s a r o m a t i c c r y s t a l s and m o r e r e c e n t l y by Donntnt and Abetino [5] to eXplain the e n h a n c e m e n t of the t e t r a ceue c r y s t a l f l u o r e s c e n c e by a m a g n e t i c field. A study of the v a r i a t i o n of these effects with the d i r e c t i o n of the m a g n e t i c field would be very i n f o r m a t i v e . A n t s o t r o p i e s w e r e found by J o h n s o n et al. [6] in the m a g n e t i c field effect on delayed f l u o r e s c e n c e o f a n t h r a c e n e c r y s t a l s and they showed that the s e n s i t i v e step is the t r i p l e t t r i p l e t annihilation. An a n i s o t r o p y in the m a g n e t ic field e n h a n c e m e n t of the i n s t a n t a n e o u s f l u o r e s cence of t e t r a c e n e was r e c e n t l y found [7,8] and it was proposed that the s e n s i t i v e step is the d i s sociation of the s i n g l e t exciton into two t r i p l e t s . M e r r t f l e l d [9] explained the a n t h r a c e n e r e s u l t s by a s s u m i n g that the i m p o r t a n t p a r a m e t e r is the n u m b e r of d o u b l e - t r i p l e t s t a t e s having s i n g l e t c h a r a c t e r . At "high field", where the Z e e m a n e n e r g y is much l a r g e r than the z e r o - f i e l d s p l i t ting, the spin wavefuncttons a r e quanttzed along the field and a r e independent of its s t r e n g t h , * Work supported by the DGRST under contract 66.0299.01 ** Laboratoire associ~ au CNRS.
f
a axis
Fig. 1. Decrease in d.c. photoeurrent in anthracene
plotted versus the angle 8 between the a axis and magnetic field. Circles and crosses correspond to two different experiments. H = 7kG in bothocases. Positive electrode illuminated with 3650 A light. hence the constancy of the effect. T h e r e is s t i l l however an a n i s o t r o p y in the e n e r g i e s , r e m i n i s cent of the z e r o - f i e l d splittlng and t h e r e may be c r o s s i n g for c e r t a i n d i r e c t i o n s of the field, hence the auisotropy. We r e p o r t h e r e our m e a s u r e m e n t s of highfield a n i s o t r o p i e s which appear in the d.c. photoconductivity of a n t h r a c e n e and t e t r a c e n e . The c r y s t a l s were s u b l i m e d f r o m p u r i f i e d m a t e r i a l , with well developed (a,b) p l a n e s , 10 - 50 /1 thick; they w e r e mounted In ~plexiglas" h o l d e r s , s a n d wiched between aqueous e l e c t r o d e s and i l l u m i nated with different l i n e s of a P h i l i p s SP 500 m e r c u r y l a m p , isolated by f i l t e r s . The m a g n e t i c
357
Volume 30A, number 6
PHYSICS L E T T E R S
field was in the (a,b) plane. All e x p e r i m e n t s were p e r f o r m e d at r o o m t e m p e r a t u r e . In a n t h r a c e n e , the photoconductivity is d e p r e s sed by a few p e r cent in "high" magnetic fields. Above 3 kG, the d e c r e a s e is independent of the magnitude of the field, but depends s t r o n g l y on its d i r e c t i o n in the (a,b) plane. The magnitude of the effect v a r i e s somewhat f r o m c r y s t a l to c r y s t a l , but not the a n i s t r o p y (fig. 1). The d i r e c t i o n s of m a x i m u m d e c r e a s e a r e exactly the s a m e a s those calculated, u s i n g the known zero field splitting of a n t h r a c e n e t r i p l e t excitlon [10]. T h i s i n d i c a t e s that the s p e c i e s s e n s i t i v e to the magnetic field in the photoconductivity is the t r i p l e t , e i t h e r through t r i p l e t - t r i p l e t or through t r l p l e t - d o u b l e t i n t e r action, a s both will give the s a m e a n i s o t r o p y (see [11] ). The t r i p l e t s play a role in the g e n e r ation (by s u r f a c e ionization) but also in detrapping (by t r a n s f e r of their e n e r g y to t r a p p e d c a r r i e r ) , e t c . . . T h e r e f o r e the d e t a i l s of the effects of the m a g n e t i c field on the o v e r a l l photoconduction p r o c e s s a r e not yet known and s t i l l u n d e r study. In t e t r a c e n e , the conductivity is enhanced in a h i g h - f i e l d (>~3kG). The s a m e a n i s o t r o p y is then found whatever the p o l a r i t y of the i l l u m i n a t e d e l e c t r o d e , with two s h a r p m a x i m a o c c u r i n g when the d i r e c t i o n of the m a g n e t i c field is a p p r o x i m a t e l y at 34 ° and 18 ° of the b axis. Its position c o i n c i d e s exactly with the a n i s o t r o p y a l r e a d y found for f l u o r e s c e n c e [7,8] although the e n h a n c e ment is l e s s in conductivity (< 10% as c o m p a r e d to ~27%). T h i s o b s e r v a t i o n , together with the p r e c e d i n g one on a n t h r a c e n e , s u p p o r t s the a s s u m p t i o n that the " m a g n e t i c a l l y active s p e c i e s " in t e t r a c e n e is also the t r i p l e t . No detail c a l c u l a t i o n is p o s s i b l e u n t i l the zero field splitting p a r a m e t e r s of the c r y s t a l l i n e t r i p l e t exciton a r e known. In both c a s e s , the behaviour of the photocon-
358
17 November 1969
ductivity is p a r a l l e l to that of the singlet density governing the f l u o r e s c e n c e . T h i s has been noticed for a long t i m e for the e x t r i n s i c photoconductivity in a r o m a t i c c r y s t a l s [e.g. 12]. Additional i n f o r m a t i o n wiU be provided by low field s t u d i e s c u r r e n t l y in p r o g r e s s . The p r e s e n t r e s u l t s indicate that one should t r y to avoid the i n t r o d u c t i o n of " l a r g e r a d i u s e x c i t o n s " , but t r y to explain the magnetic field effects d i s c u s s e d h e r e only through t r i p l e t s . We thank P. Quedec for purifying and p r e p a r i n g the t e t r a c e n e c r y s t a l s .
References 1. E.L. Frankevieh and E. L Balabanov, Soviet Physics Solid State 8(1966) 855. E. L. Frankevich, Soviet Physics JETP 23 (1966)814 and following papers. 2. E.L. Frankevieh and V. M. 11umyantsev, Soviet Physics JETP 26 (1968) 1102. 3. S.I. Choi, J.Jortner, S.A. Rice and 11. Silbey, J. Chem. Phys. 41 (1964) 3294. 4. P. Pesteil, J. Chim. Phys. 58(1961) 661. M. Benarroche, Ann, Phys. 2 (1967) 27. 5. J. M. Donnini and F. Abetino, Compt. Rend. 266 B (1968) 1618. 6. 1t. C. Johnson, 1t. E. Merrifield, P. Avakian and 11. B. Flippen, Phys. Rev. Letters 19 (1967) 285. 7. N. Geacintov, M. Pope and F. Vogel, Phys. Rev. Letters 22 (1969) 593. 8. 11. E. Merrffield, P. Avakian and 1t. P. Groff, Chem. Phys. Letters 3 (1969) 155. 9. 1t. E. Merrifield, J. Chem. Phys. 48 (1968) 4318. 10. D. Haarer, D. Schmid and H. C. Wolf, Phys. Stat. Solidi 23 (1967) 633. 11. V. Ern and 11. E. Merrffield, Phys. Rev. Letters 21 (1968) 609. 12. O. H. Leblanc Jr. in Physics and chemistry of the organic solid state, III, eds. F. Fox, M. Labes and A. Weissberger (Wiley, 1967), p. 133.