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Interactions between exciplexes in solution
Volume
7, number
4
CHEMICAL
INTERACTIONS
PIiYSICS
BETWEEN
LETTERS
EXCIPLEXES
IN SOLUTION
The lifetime shortening of heteroexclmer as \vell as escimer has been ot)ser\ed I))- usm!: a Q-s\~itchctl laser for exciting the solution. This pnrticulur behnvior c:m he ollscrvcd onI> when the conccntr:>tcd solutions are irradintcd by :I iaser pulse of high intenslt)‘. It h:w heen shwrn that the lifetime shortcnlng is due to the bimolcculnr process, i. c. the intern&ion tJt%trvccnthe csciplcscs. ruby
Since the irradiation of solutions with some kind of laser can produce excited states quite densely, it may be expected that some interactions arise between those excited states. Actually, an interaction between pyrene escimcrs has been
demonstrated
by using
a pulsed N2 laser
[I]. It
may be expected also that an interaction between exciplexes can be observed not only in the case oE excimer but also in the case of heteroescimer. In this paper the results on anthracene-N. N-dimethylaniline (DMA) heteroexcimer and pyrene escimer will be reported. Anthracene is an appropriate solute for studying the interaction between the heteroexcimers, since it forms no excimer. Pyrene forms cxcimcr readily and is also an appropriate solute, although its molecular e.xtinction coefficient at 347 nm is rather small. The high intensity irradiation of these concentrated solutions leads to the lifetime shortenings and non-exponential decay of exciples fluorescence. The analysis of fluorescence decay curves shows that the bimolecular interaction between esciplexes contributes to the disappearance of exciplexes. The Q-switched ruby laser and associated experimental apparatus are the same as those repcrted before [2]. The output power of the laser pulse is ca. 1.5 joule and a few % of the energy of 694 nm beam is converted to that of 347 nm beam by the ADP frequency doubler. The pulse has a duration of 15-20 nsec. All solutions were degassed by the freeze-pump-thaw method. The solutions of anthracene-DMA system and those of pyrene were excited with !‘le 347 nm pulse. For the excitation with relatively low intensity light, the laser pulse was adjusted bq’ using neutral filters composed of wire gauzes. The filters reduced the light intensity to 0.3%, 8%
and 31.6%, respectively. For the high intensity excitation, the pulsed beam without filters was focussed on the front face of the solution. In the case of rather dilute solutions, such as ethylether solution of 5 y 10W4M nnthracene and LOwlM DhIA and cyclohesane solution of lo-2M pyrene, the decay curves have nothing to do with the cscitation light intensity and are esponential. In the case of concentrated solutions, such as benzene solution of 0.05M anthracene and 2M DMA and toluene solution of 0.25M pyrene, the decay curves are exponential only when the excitation intensity is reduced to 0.35 of 34’7 nm output. With higher excitation intensity, however, the fluorescence decay of these concentrated solutions are shortened and .-,re not exponential. iu’ci-c?rtheless, in the latter case. the curves are esponential at the later stage of the decay and the lifetimes estimated from these esponential part arc identical with those obtained in the case of the excitation intensity reduced to 0.3%. The halfvalue-width of the fluorescence decay function at 500 nm, at which no monomer emission is observed, are listed in table 1. The shape and band position of the fluorescence spectra of escipies have nothing to do with the excitation light intensity. Usually the exciples fluorescence disappears unimolecularly and the decay CLII-ve is given as follows. (11 I;=Foe-ai * (l/P)
= (l./Fo)
.af
,
(la)
where I: is the fluorescence intensity of esciplt~x, FO is the value of F al t = 0 and (1 is the rate constant of unimolecular decnY. When the esciplcs is produced densly with laser escitation, It is es-
CHEXICAL
PHYSICS
-_
that the intermolecular interaction of exziples can occur and by this interaction the exciplex decays bimolecularly. In this case, the rate equation can be given as
petted
dF’dt
=-crF-bF2
,
b is the rate constant of bimolecular The integrated solution of eq. (2) is where
1 ‘F : (1 FO + 7)/n) ent - b a .
(2) decay. (2a)
Plotting 1 F against en/, straight lines are espetted in both cases of eq. (la) and eq. (Pa). The difference bctlveen eq. (la) and eq. (2a) is in the value of the intercepts of these straight lines. Our esperimental results were analyzed according to the above method. as indicated in fig.1 and fig. 2. In the case of low intensity irradiation of concentrated solutions and in the case of dilute solutions. the obtained lines pass the origin. ~lowever, in the case of high intensity irradiation of concentrated solutions, the value of the intercept is negative. Then, it can be concluded that the intermolecular interaction of heteroexcimer
Fig. 1. Kinetic :mnlysis on nnthr;tccne-DNA hC>teroeXc,mcr. eztcitcd :i> Anthraccnc 0.05X anti DJIA 231 in benzene, with fr~cusscd loser p”lsc. 1,) Xnthwcene U.O.S>i and DX4 2X in benzcnc. escitcd v:ith 0.37~ intensity of the pulse. cj Anthrnccne ,5 x 10-451 and DJIA lU-l~l in ethylether, escited xv~th normal intensity of laser.
418
15 November 1970
LETTERS
as well as excimer occurs in the latter case. An estimation of the effective concentration of exciplex in the range of thickness Y IO-3 cm at the front face gives ca. 5 x 10-31’vI. Under this concenrration the bimolecular collisional interaction of exciplexes controlled by diffusion, can occur. In the above discussion the bimolecular collision of exciplexes is assumed to lead to the radiatior.less degradation of both exciplexes. But it may also be expected that the excited state of esciples is formed under the bimolecular collision in the way similar to the case of triplet-triplet annihilation. Although the produced excited state of esciplex has a much larger energy compared to the binding energy of the exciplex, the rapid radiationless transition to the fluorescent state of csciplex might occur. Such circumstances are similar to those observed when the ground state EDA complex is excited. The rate equation of these processes may be written as follows. dE/dl = -crE -i,E2 dlC*,‘dt = crbE2 -cE*
+ CE’
,
)
(3) (4!
Fig. 2. Kinetic nnnlysis on pyrene excimer. a) Pyrcne 0.25M in toluene, exited with focusscd lnser pUlSe. b) I’yrcnc 0.2551 in tolucne, excited with 0.35, intcnsit> of the pulse. c) Pyrcne 1O-251 in cyclohexrtne. excited with normal intensity of laser.
Volume 7, n u m b e r 4
CItEMICAL PIIYSICS LE'I"I'I-~RS
Ilalf-value-width
Table I of the fluorescence decay functmn.
15 N o v e m b e r 197o
All ~ a l u e s i s n s e c Excztation light mten~zty I (/() ;
Foellast.d with. [en:s
45
35
:t0-37
,',5
7C~
~(}
65
t0
.15
2s
25
65
60
~;~)
Solutions ............................................
0 . 3 "~
Anthracene
0.05M and DMA 25I m benzene
65
Anthracene
5 × 10-'tM
and DMA 10-IM
Pyrene
0 . 2 5 5 I in t o l u e n e
Pyrene
I0
in e t h y l e t h e r
"M m cyelohexane t5-~;5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
whereE and E* represent, respectively, the fluorescent and excited states of exciplcx, c r e p r e s e n t s t h e r a t e c o n s t a n t of t h e r a d i a t i o n l e s s t r a n s i t i o n f r o m t h e e x c i t e d s t a t e to t h e g r o u n d s t a t e o[ e x c i p l e x , a a n d b a r e t h e s a m e q u a n t i t i e s a s g i v e n in e q . ( 1 } a n d e q . ( 2 ) , a i s t h e q u a n t u m y i e l d of t h e f o r m a t i o n o f t h e e x c i t e d s t a t e of e x c i p l e x due to the d i f f u s i o n - c o n t r o l l e d c o l l i s i o n of t h e e x c i p l e x e s . S i n c e eq.{2) c a n r e p r o d u c e t h e experimental results quite satisfactorily, it m a y b e a r g u e d t h a t t h e e f f e c t of t h i s E - E a n n i h i l a t i o n p r o c e s s on t h e f l u o r e s c e a c e d e c a y is v e r y s m a l l . t I o w e v e r , it m a y b e p o s s i b l e a l s o t i m t t h e r a t e c o n s t a n t s in e q . (2) c o n t a i n t h e E - E a n n i h i l a t i o n e f f e c t i m p l i c i t l y . F o r a c l o s e r e x a m i n a t i o n of t h i s p r o b l e m , it w o u l d b e n e c e s s a r y t o s o l v e t h e c o u p l e d e q s . (3) a n d (4) n u m e r i c a l l y , t h o u g h w e do n o t a t t e m p t it h e r e . In t h e c a s e o f a n t h r a c e n e - D M A h e t e r o e x c i m e r the h i g h e r e x c i t e d s t a t e of e x e i p l e x can be p r o d u c e d a l s o b y t h e i r r a d i a t i o n o f b o t h 347 n m a n d
:11.6 ~'~
694 n m p u l s e s s i n c e t h e e x c i t e d S - S a b s o r p t i o n o f h e t e r o e x c i m e r i s o b s e r v e d in t h e r e g i o n o f 600 ~ 1000 n , n [3 l. T h u s , w e c a n s e e w h e t h e r t h e e x c i t a t i o n of t h i s e x c i p l e x a f f e c t s i t s b e h a v i o r s i g n i f i c a n t l y o r not. I r r a d i a t i n g t h e s o l u t i o n s w i t h 347 n m a n d 694 n m p u l s e d , t h e o b s e r v e d d e c a y c u r v e s of t h e e x c i p l e x w e r e t h e s a m e a s t h o s e o b t a i n e d w i t h 347 n m i r r a d i a t i o n o n l y . r h e r e f o r e t h e e x c i t e d s t a t e of t h e e x c i p t e x p r o d u c e d b y E - E a n n i h i l a t i o n o r s u c c e s s i v e tcvo q u a n t u m a b sorption, seems to make a rapid internal convers i o n to i t s f l u o r e s c e n t s t a t e a n d no s u b s t a n t i a l e f f e c t s p e c i f i c t o it s e e m s t o a r i s e . REFERENCES [I] C . R . Gohl.-,chmz,lt, Y . T . T o m k l e w i c z and l . B . B e r l - - ~ man, C h e m . Phys. L e t t e r s 2 (19ti~.~ 520. [2I H. Masuh:t t'a :rod N. Mata,:;a, C h e m . Ph.'. s . I x,tte r - ~1 (1 !)70) ~JOS. [3] C'. R. (;oId.-.chmidt and M. t)ttoh'nghl, t'ht.-m. I'll.'..-:. l.ettL'rs -t (lb70) 570.
41. °
7, number
4
CHEMICAL
INTERACTIONS
PIiYSICS
BETWEEN
LETTERS
EXCIPLEXES
IN SOLUTION
The lifetime shortening of heteroexclmer as \vell as escimer has been ot)ser\ed I))- usm!: a Q-s\~itchctl laser for exciting the solution. This pnrticulur behnvior c:m he ollscrvcd onI> when the conccntr:>tcd solutions are irradintcd by :I iaser pulse of high intenslt)‘. It h:w heen shwrn that the lifetime shortcnlng is due to the bimolcculnr process, i. c. the intern&ion tJt%trvccnthe csciplcscs. ruby
Since the irradiation of solutions with some kind of laser can produce excited states quite densely, it may be expected that some interactions arise between those excited states. Actually, an interaction between pyrene escimcrs has been
demonstrated
by using
a pulsed N2 laser
[I]. It
may be expected also that an interaction between exciplexes can be observed not only in the case oE excimer but also in the case of heteroescimer. In this paper the results on anthracene-N. N-dimethylaniline (DMA) heteroexcimer and pyrene escimer will be reported. Anthracene is an appropriate solute for studying the interaction between the heteroexcimers, since it forms no excimer. Pyrene forms cxcimcr readily and is also an appropriate solute, although its molecular e.xtinction coefficient at 347 nm is rather small. The high intensity irradiation of these concentrated solutions leads to the lifetime shortenings and non-exponential decay of exciples fluorescence. The analysis of fluorescence decay curves shows that the bimolecular interaction between esciplexes contributes to the disappearance of exciplexes. The Q-switched ruby laser and associated experimental apparatus are the same as those repcrted before [2]. The output power of the laser pulse is ca. 1.5 joule and a few % of the energy of 694 nm beam is converted to that of 347 nm beam by the ADP frequency doubler. The pulse has a duration of 15-20 nsec. All solutions were degassed by the freeze-pump-thaw method. The solutions of anthracene-DMA system and those of pyrene were excited with !‘le 347 nm pulse. For the excitation with relatively low intensity light, the laser pulse was adjusted bq’ using neutral filters composed of wire gauzes. The filters reduced the light intensity to 0.3%, 8%
and 31.6%, respectively. For the high intensity excitation, the pulsed beam without filters was focussed on the front face of the solution. In the case of rather dilute solutions, such as ethylether solution of 5 y 10W4M nnthracene and LOwlM DhIA and cyclohesane solution of lo-2M pyrene, the decay curves have nothing to do with the cscitation light intensity and are esponential. In the case of concentrated solutions, such as benzene solution of 0.05M anthracene and 2M DMA and toluene solution of 0.25M pyrene, the decay curves are exponential only when the excitation intensity is reduced to 0.35 of 34’7 nm output. With higher excitation intensity, however, the fluorescence decay of these concentrated solutions are shortened and .-,re not exponential. iu’ci-c?rtheless, in the latter case. the curves are esponential at the later stage of the decay and the lifetimes estimated from these esponential part arc identical with those obtained in the case of the excitation intensity reduced to 0.3%. The halfvalue-width of the fluorescence decay function at 500 nm, at which no monomer emission is observed, are listed in table 1. The shape and band position of the fluorescence spectra of escipies have nothing to do with the excitation light intensity. Usually the exciples fluorescence disappears unimolecularly and the decay CLII-ve is given as follows. (11 I;=Foe-ai * (l/P)
= (l./Fo)
.af
,
(la)
where I: is the fluorescence intensity of esciplt~x, FO is the value of F al t = 0 and (1 is the rate constant of unimolecular decnY. When the esciplcs is produced densly with laser escitation, It is es-
CHEXICAL
PHYSICS
-_
that the intermolecular interaction of exziples can occur and by this interaction the exciplex decays bimolecularly. In this case, the rate equation can be given as
petted
dF’dt
=-crF-bF2
,
b is the rate constant of bimolecular The integrated solution of eq. (2) is where
1 ‘F : (1 FO + 7)/n) ent - b a .
(2) decay. (2a)
Plotting 1 F against en/, straight lines are espetted in both cases of eq. (la) and eq. (Pa). The difference bctlveen eq. (la) and eq. (2a) is in the value of the intercepts of these straight lines. Our esperimental results were analyzed according to the above method. as indicated in fig.1 and fig. 2. In the case of low intensity irradiation of concentrated solutions and in the case of dilute solutions. the obtained lines pass the origin. ~lowever, in the case of high intensity irradiation of concentrated solutions, the value of the intercept is negative. Then, it can be concluded that the intermolecular interaction of heteroexcimer
Fig. 1. Kinetic :mnlysis on nnthr;tccne-DNA hC>teroeXc,mcr. eztcitcd :i> Anthraccnc 0.05X anti DJIA 231 in benzene, with fr~cusscd loser p”lsc. 1,) Xnthwcene U.O.S>i and DX4 2X in benzcnc. escitcd v:ith 0.37~ intensity of the pulse. cj Anthrnccne ,5 x 10-451 and DJIA lU-l~l in ethylether, escited xv~th normal intensity of laser.
418
15 November 1970
LETTERS
as well as excimer occurs in the latter case. An estimation of the effective concentration of exciplex in the range of thickness Y IO-3 cm at the front face gives ca. 5 x 10-31’vI. Under this concenrration the bimolecular collisional interaction of exciplexes controlled by diffusion, can occur. In the above discussion the bimolecular collision of exciplexes is assumed to lead to the radiatior.less degradation of both exciplexes. But it may also be expected that the excited state of esciples is formed under the bimolecular collision in the way similar to the case of triplet-triplet annihilation. Although the produced excited state of esciplex has a much larger energy compared to the binding energy of the exciplex, the rapid radiationless transition to the fluorescent state of csciplex might occur. Such circumstances are similar to those observed when the ground state EDA complex is excited. The rate equation of these processes may be written as follows. dE/dl = -crE -i,E2 dlC*,‘dt = crbE2 -cE*
+ CE’
,
)
(3) (4!
Fig. 2. Kinetic nnnlysis on pyrene excimer. a) Pyrcne 0.25M in toluene, exited with focusscd lnser pUlSe. b) I’yrcnc 0.2551 in tolucne, excited with 0.35, intcnsit> of the pulse. c) Pyrcne 1O-251 in cyclohexrtne. excited with normal intensity of laser.
Volume 7, n u m b e r 4
CItEMICAL PIIYSICS LE'I"I'I-~RS
Ilalf-value-width
Table I of the fluorescence decay functmn.
15 N o v e m b e r 197o
All ~ a l u e s i s n s e c Excztation light mten~zty I (/() ;
Foellast.d with. [en:s
45
35
:t0-37
,',5
7C~
~(}
65
t0
.15
2s
25
65
60
~;~)
Solutions ............................................
0 . 3 "~
Anthracene
0.05M and DMA 25I m benzene
65
Anthracene
5 × 10-'tM
and DMA 10-IM
Pyrene
0 . 2 5 5 I in t o l u e n e
Pyrene
I0
in e t h y l e t h e r
"M m cyelohexane t5-~;5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
whereE and E* represent, respectively, the fluorescent and excited states of exciplcx, c r e p r e s e n t s t h e r a t e c o n s t a n t of t h e r a d i a t i o n l e s s t r a n s i t i o n f r o m t h e e x c i t e d s t a t e to t h e g r o u n d s t a t e o[ e x c i p l e x , a a n d b a r e t h e s a m e q u a n t i t i e s a s g i v e n in e q . ( 1 } a n d e q . ( 2 ) , a i s t h e q u a n t u m y i e l d of t h e f o r m a t i o n o f t h e e x c i t e d s t a t e of e x c i p l e x due to the d i f f u s i o n - c o n t r o l l e d c o l l i s i o n of t h e e x c i p l e x e s . S i n c e eq.{2) c a n r e p r o d u c e t h e experimental results quite satisfactorily, it m a y b e a r g u e d t h a t t h e e f f e c t of t h i s E - E a n n i h i l a t i o n p r o c e s s on t h e f l u o r e s c e a c e d e c a y is v e r y s m a l l . t I o w e v e r , it m a y b e p o s s i b l e a l s o t i m t t h e r a t e c o n s t a n t s in e q . (2) c o n t a i n t h e E - E a n n i h i l a t i o n e f f e c t i m p l i c i t l y . F o r a c l o s e r e x a m i n a t i o n of t h i s p r o b l e m , it w o u l d b e n e c e s s a r y t o s o l v e t h e c o u p l e d e q s . (3) a n d (4) n u m e r i c a l l y , t h o u g h w e do n o t a t t e m p t it h e r e . In t h e c a s e o f a n t h r a c e n e - D M A h e t e r o e x c i m e r the h i g h e r e x c i t e d s t a t e of e x e i p l e x can be p r o d u c e d a l s o b y t h e i r r a d i a t i o n o f b o t h 347 n m a n d
:11.6 ~'~
694 n m p u l s e s s i n c e t h e e x c i t e d S - S a b s o r p t i o n o f h e t e r o e x c i m e r i s o b s e r v e d in t h e r e g i o n o f 600 ~ 1000 n , n [3 l. T h u s , w e c a n s e e w h e t h e r t h e e x c i t a t i o n of t h i s e x c i p l e x a f f e c t s i t s b e h a v i o r s i g n i f i c a n t l y o r not. I r r a d i a t i n g t h e s o l u t i o n s w i t h 347 n m a n d 694 n m p u l s e d , t h e o b s e r v e d d e c a y c u r v e s of t h e e x c i p l e x w e r e t h e s a m e a s t h o s e o b t a i n e d w i t h 347 n m i r r a d i a t i o n o n l y . r h e r e f o r e t h e e x c i t e d s t a t e of t h e e x c i p t e x p r o d u c e d b y E - E a n n i h i l a t i o n o r s u c c e s s i v e tcvo q u a n t u m a b sorption, seems to make a rapid internal convers i o n to i t s f l u o r e s c e n t s t a t e a n d no s u b s t a n t i a l e f f e c t s p e c i f i c t o it s e e m s t o a r i s e . REFERENCES [I] C . R . Gohl.-,chmz,lt, Y . T . T o m k l e w i c z and l . B . B e r l - - ~ man, C h e m . Phys. L e t t e r s 2 (19ti~.~ 520. [2I H. Masuh:t t'a :rod N. Mata,:;a, C h e m . Ph.'. s . I x,tte r - ~1 (1 !)70) ~JOS. [3] C'. R. (;oId.-.chmidt and M. t)ttoh'nghl, t'ht.-m. I'll.'..-:. l.ettL'rs -t (lb70) 570.
41. °