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ODMR as a method to detect mobility of triplet excitation
CHEMICAL PHYSICS LETTERS
Volume 32, number 1
ODMR
AS.A
METHOD
R. SCHMIDBERGER P?~ysikaJiscJ~er Inrtitut,
TO DETECT MOBILITY
OF’TRIPLET
I April 1975
EXCITATION
and H-C. WOLF Teilinstitut’3,
Univern’th’t Stuttgurt,
Stuttgart.
Germany
Received 15 September 1974 Some special aspects of ODMR spectroscopy on linear triplet escitons are discussed for the use of 1.4 DBN sinzle crystals. It is sflown that from the ODhlR lineshape it can be decided whether an ESR line is homogeneously broadened or nit. F&tl,crmorc.thc ODMR lineshape and intensity can be used as a sensitive detector for Slow motion Of ex~t~tion, such as out of chain hopping of linear escitons.
1. Intro duction
mental set-up for our experiments viously [2,4].
Opticai detection of magnetic resonance (ODMR) in the phosphorescent triplet state of organic molecules and excitons in molecular crystals i,s a,favourable method.for ESR detection of triplet states [I] at low concentrations. Recently we reported a method of ODMR [2], where the Zeeman levels of the triplet state are optically resolved in a magnetic field of 12 kG.
2. Homogeneous
pre-
Iines
Fig. 1 shows optically .detected ESR hnes of exci,tons and X-traps (isolated host molecules) in an orientation of the magnetic field parallel to the 6 crystal axis. We used thz phosphorescence of the lowest Zee-
The ESR is detected by the change in intensity of the individual Zeeman components during microwave saturation. This technique requiresspecial ratios of the radiative and radiationless’constants as in the case of unresolved.Zeeman components [1,3]. In our experiments on linear triplet excitons in I,4 dibromonaphthalene we used ODMR in addition to conventional ESR techniques [2,4]. ...In addition to the dominant exciton migration in c-axis direction [S] in DBN there exists a transition probability.for the excitons between adjacent molecu!es in-the ob plane which are magnetically inequivalent.of about lCl!S;l (A-B hopping probability). III ihis paper .we present some aspects of FDMR. of tripIet excitoris which mi&t be of more general interest and applicable to other systems: It’is shown~~ that with the help of ODOR one can easil? decide whether tin ESR line is homogeneo,usly-broadened or -.not: Fur$&ore, the .6DMR linefhape’a.nd +nsity in some’r,as.es is.inore. sensitive against excitpq m&io?’ than the line&p& of conventional ESR- l-he ex$ri-, . .,- ‘;.. ,: ; ‘, :’ ‘.,‘, : ., ;. ‘.: ;,: .,‘. .:_ ,:. ., : :’ -’ ;, ,; ..I I’, .,_.:‘., :. ‘,. ;” ;_... ..,.. -...
and inhomageneous
was described
man components of either exciton or X-trap O-Cl EXCITON
-*“A*,
c,
N ;
X-TRL?
/
. . ,++L - .. ;’
15:nsY: H-WC<
V fe_,h’
*%.__ ‘T
\
-
i-y ‘i
I/’
I
:\,
/
f-zd
Icoirtir
v
- 10t
5aG
Fig. 1. Comparison of the OD@ lineshape of evcitons and .X-traps in 14 DBN bith and tithoucmagn:tic-field modub‘kn’at 1.4K. The orien’ation of the magnetic field is parallel to:the b axis. The phosphorezccncz inteniiry of the Iowcst
‘; .'
‘,
:ze WI%In~Cor&oncntis
tr& 0~0 ttisition
rrionitoredfkihecsciton
(left)Or x-
&htL
The microwhve laturation occurs : ‘bctwen the lowestand kddle Zeeman ieve[. The mayhum
,,. in phosph.orescence intensjty is about 30%. chanti ,,. _ ,,;: : .,: .. : :_ -,: ...I .. ‘. ;. ..: .’ .,-. .,; : .’ _,
21
_-.
..
Volume
35- f-lb.er
:
1
1 April 1975
CUEMICA~~PLiYS1CS~l-T~~
..
_. .. .
:.
with the ma@etk field paraUeI.to the symmetry gxcs. (integral over A-and B
-.
The total ODMR i&k&ty
outside-the sytimetj axes therefore is twice the intensity for &e magnetic field + symmetric di-
:
’ The conditkm for this intensity doubIkg effect is the A-B t&&ion of the excitor& is faster than SL.R
and t&l decay: PAB > Z/FL, I?. Otherwise one
can conclude that if a’doubling effedt is (not) observed in ODM_R,the A-B hopping pro@biIity is (less) larger than the .5LR piobability:
z
If one-knows
..
.E
.,:.
‘(
200G
;
:...,
.-.
-_. Fig. 3; &v-field cart _qf the X&p OD’kR spectrum in’1,i DEW for Mln and some fur$kr orie+tions.in the ah plane :_ : at ‘2K’The total intensity (integrated ar&) of.th~eIin& is .. .. constant fo; all’&ie.$ations_.. : :. :’
. .. &II&S the toti &kritensity
:
of one kind df m&cules (A.qr B) for an arbitrary orientation bf the. I i &aetic. fieid is onI+ a fractign of tk intensity ,me&-
u&d ina symmetric dricktation (in an id&aIiz&d Fe; where,the. radiative decay ‘constants.~fe._tfiesanie’for .. : .the thrkk triplet kvels, the ratjo, df th,e ikten$ties.is 1 :2). ...I .;. .._. ; :y..I’. : ~. .,’ .:--In tI@:case of .&near excitobs, u$ikh mdve,.slbwJy,
shape. The effect can be tised to determine energy transfer processes *whichare appreciabIy stower than -those measured by the conventional ESR detection. .. AcIpowledgement
Fe crystals fd; our exceriments were grown in the’_Stuttgartef &istallabtir. This work was done with the f&an&l su&oti of the. Deutsche Forschungsge.. :. meinschaft..
’ . ., l
d
._ -
-.
.
-_ References ; ;__ ; ,y .. -[l] ,M. Shtinoff,Symp. Faraday Sot. 3 (c9691 137. [2] R:Sch&&@er atid KC. Wo!f, Chem. Phye Letters 16
betweeri non-equival@~ch&ns (A aqd.B chains) we. behav$kr. If:&k saturates, t&e ESR :.. (1972)402, transit&n bf .one-chain (A & .B) k, .ati orienttitiork ..- .: ‘_ .- [3] H:,Sixl and
find, a different
&e can $ve a
lower (upper) Emi! for the A--B transition probability .. _. .. : s_ .:, of linear excitons if a doubling’effect in ODMR intensity is (not) 0bseye.d. This may be of interest in cases 1 where the ESR linewidth is determined primarily not by the A-B hoppiiig and the hopping probability _,;o. therefore cannot be determined from the ESR link..
: :
. ..
the SFR-probability
.,
M; Sihwotier, Chem. Phys. Letters 6 (f9701 whek t@ 4 and-8 chains gre.non-eqyi;valent,.the’‘. ; 21. : saturation is.car+ed to th&:other chain by Pr_B.jtimps : j [!I R. Schmidbeiger and l!&W4f, Chekihys. Letters 25 ;,a&ice ve& ift& ho&& p&ability,@ greater. 1 : .--. (1974) 185, :: ::. thiht~k SL~-pro~abili~..Therres~~t is tb&q$de’ ’ : -f’: [S] R._hf:-Hochstrasskr-akdJ.D. @h&em&, 3. Che&. Phys. ‘.:56 (1972) 5935. .-. ‘, :t& syr&n$ry q% the’:ODMg,&ensity of one @d ._ .: 1. ::- _ [cl ‘R. Schnzidberger and H.C. W&f, Chem. Phqg. Letters 32 1 ?of Ii&r-&kit& (Aeof 5) is~as.great:g~t,hB.intehsily .I: -, ‘. irs,sj l8. : I :_ :: _ : ,~ .. ; .- ;, ,.. ,, ‘, . .,. y: .y-: :_._-.~ !‘_. ,‘. --~I .--. r-‘:.’::T.-:‘. z:.~.I. ._:.,_‘:, I ...‘ ;~. .-.,. . .__::_ ;,: .:,. -_ .. ,_ .:.., :_.I..f;,:. _‘:; ,_,._:.__ . . . .‘. .: . .L-. -. :y .‘-.I .L.. :=y~ : _i..;y: -; .i_ .?.I.- ... _y- :: ._--,: __,I-(, -_ .... _~_ _..-..._.__ ;_. .__ _;. :____ .__ :. : ~. __..;.. : ‘ , . . ‘ ,.f._ ;, .:::. I ‘ -;,,.. ::,. .‘ L, ,,, “ y ‘ ... ._ ‘.....’ .‘._-:.-‘:‘ : ... ‘ ._>.,_ ._ -_ .’ ~, .; __ _ ., :-:, .:.. (T. ” .(( ,:;-5 ,,._. .; ,(_.,_ I :,. .. : ., : .. I. ,_,,;. ,, .--. _:.; ;..,., ...;, .:-,..;~_r.. ‘..: --_ : -,-, -...I . I_: ..,,(-1.. it-;:.,.; :. -.Y_.. )_. :r: :,. _._ _yl ..,, ,... _. -. -. -_..~,; ;_ ._. , ,’.‘.‘5’ ,. ~.,...,’ _” .” ,...(‘. ._ ._-y_ ._ .;. _. ,:. :. _:_.: :_“ 1 .:. 1-z . . -:_..:. . ..” .‘ I:, _J:,_; _, ‘ ;, ,.I .___-~ .& :._; ‘ , z: .. -,,I ._. I. ;.:: ,-:‘- : : .. : :_,_;,, ; _‘_‘ _--.; .:: : : .._, ‘, __ . . _,:_ : .,:y.. ... _I’ .\- ,._:’ _,._ ._:: .....1, ._ _ -; ;’ ,_.I ~_:_ i’j,. ; ;. ._. ._ .-.‘::.. _.; y _.-. -1 .-I ; _ .<:,::_: :_.~_; __;;;, _: ,._ : .- , .:.._.::i~‘:;..,.,.. .: .. _._.,_: ., .:I.‘ ,.- f,-.. I,... ‘..: ‘y: .,-.-.., _;_,--..’ . .‘,,, _c I...,‘ ~ : ..b’ _.l._.. .23 _ :: ..% .i. ,. : ,: ....;,::. :Y .:. : ...-I.,.,l:+._.y;_:‘ :_ t- ..,, .‘-.).+:.” (,..-i.-. ,: _. .~~_y--,yy-- ,-:.-; _ ~_ -: -..y.. ->~~_~,_:;,:_.‘T:‘ .:.: ..... ..._: ~.~I.;-:;.~~_-:.::._ *_. :’ : ::~~~ &-: ..:: _f _ ,:-:_::._._’ .:.’-:~~,-,‘:_~‘~~.;j..~~~~:_~I’ :T ,‘;:‘-_-_-; ~: .‘_ ., .~_. : :,. . : .,.- (_~ .-.- ; .,.
Volume 32, number 1
ODMR
AS.A
METHOD
R. SCHMIDBERGER P?~ysikaJiscJ~er Inrtitut,
TO DETECT MOBILITY
OF’TRIPLET
I April 1975
EXCITATION
and H-C. WOLF Teilinstitut’3,
Univern’th’t Stuttgurt,
Stuttgart.
Germany
Received 15 September 1974 Some special aspects of ODMR spectroscopy on linear triplet escitons are discussed for the use of 1.4 DBN sinzle crystals. It is sflown that from the ODhlR lineshape it can be decided whether an ESR line is homogeneously broadened or nit. F&tl,crmorc.thc ODMR lineshape and intensity can be used as a sensitive detector for Slow motion Of ex~t~tion, such as out of chain hopping of linear escitons.
1. Intro duction
mental set-up for our experiments viously [2,4].
Opticai detection of magnetic resonance (ODMR) in the phosphorescent triplet state of organic molecules and excitons in molecular crystals i,s a,favourable method.for ESR detection of triplet states [I] at low concentrations. Recently we reported a method of ODMR [2], where the Zeeman levels of the triplet state are optically resolved in a magnetic field of 12 kG.
2. Homogeneous
pre-
Iines
Fig. 1 shows optically .detected ESR hnes of exci,tons and X-traps (isolated host molecules) in an orientation of the magnetic field parallel to the 6 crystal axis. We used thz phosphorescence of the lowest Zee-
The ESR is detected by the change in intensity of the individual Zeeman components during microwave saturation. This technique requiresspecial ratios of the radiative and radiationless’constants as in the case of unresolved.Zeeman components [1,3]. In our experiments on linear triplet excitons in I,4 dibromonaphthalene we used ODMR in addition to conventional ESR techniques [2,4]. ...In addition to the dominant exciton migration in c-axis direction [S] in DBN there exists a transition probability.for the excitons between adjacent molecu!es in-the ob plane which are magnetically inequivalent.of about lCl!S;l (A-B hopping probability). III ihis paper .we present some aspects of FDMR. of tripIet excitoris which mi&t be of more general interest and applicable to other systems: It’is shown~~ that with the help of ODOR one can easil? decide whether tin ESR line is homogeneo,usly-broadened or -.not: Fur$&ore, the .6DMR linefhape’a.nd +nsity in some’r,as.es is.inore. sensitive against excitpq m&io?’ than the line&p& of conventional ESR- l-he ex$ri-, . .,- ‘;.. ,: ; ‘, :’ ‘.,‘, : ., ;. ‘.: ;,: .,‘. .:_ ,:. ., : :’ -’ ;, ,; ..I I’, .,_.:‘., :. ‘,. ;” ;_... ..,.. -...
and inhomageneous
was described
man components of either exciton or X-trap O-Cl EXCITON
-*“A*,
c,
N ;
X-TRL?
/
. . ,++L - .. ;’
15:nsY: H-WC<
V fe_,h’
*%.__ ‘T
\
-
i-y ‘i
I/’
I
:\,
/
f-zd
Icoirtir
v
- 10t
5aG
Fig. 1. Comparison of the OD@ lineshape of evcitons and .X-traps in 14 DBN bith and tithoucmagn:tic-field modub‘kn’at 1.4K. The orien’ation of the magnetic field is parallel to:the b axis. The phosphorezccncz inteniiry of the Iowcst
‘; .'
‘,
:ze WI%In~Cor&oncntis
tr& 0~0 ttisition
rrionitoredfkihecsciton
(left)Or x-
&htL
The microwhve laturation occurs : ‘bctwen the lowestand kddle Zeeman ieve[. The mayhum
,,. in phosph.orescence intensjty is about 30%. chanti ,,. _ ,,;: : .,: .. : :_ -,: ...I .. ‘. ;. ..: .’ .,-. .,; : .’ _,
21
_-.
..
Volume
35- f-lb.er
:
1
1 April 1975
CUEMICA~~PLiYS1CS~l-T~~
..
_. .. .
:.
with the ma@etk field paraUeI.to the symmetry gxcs. (integral over A-and B
-.
The total ODMR i&k&ty
outside-the sytimetj axes therefore is twice the intensity for &e magnetic field + symmetric di-
:
’ The conditkm for this intensity doubIkg effect is the A-B t&&ion of the excitor& is faster than SL.R
and t&l decay: PAB > Z/FL, I?. Otherwise one
can conclude that if a’doubling effedt is (not) observed in ODM_R,the A-B hopping pro@biIity is (less) larger than the .5LR piobability:
z
If one-knows
..
.E
.,:.
‘(
200G
;
:...,
.-.
-_. Fig. 3; &v-field cart _qf the X&p OD’kR spectrum in’1,i DEW for Mln and some fur$kr orie+tions.in the ah plane :_ : at ‘2K’The total intensity (integrated ar&) of.th~eIin& is .. .. constant fo; all’&ie.$ations_.. : :. :’
. .. &II&S the toti &kritensity
:
of one kind df m&cules (A.qr B) for an arbitrary orientation bf the. I i &aetic. fieid is onI+ a fractign of tk intensity ,me&-
u&d ina symmetric dricktation (in an id&aIiz&d Fe; where,the. radiative decay ‘constants.~fe._tfiesanie’for .. : .the thrkk triplet kvels, the ratjo, df th,e ikten$ties.is 1 :2). ...I .;. .._. ; :y..I’. : ~. .,’ .:--In tI@:case of .&near excitobs, u$ikh mdve,.slbwJy,
shape. The effect can be tised to determine energy transfer processes *whichare appreciabIy stower than -those measured by the conventional ESR detection. .. AcIpowledgement
Fe crystals fd; our exceriments were grown in the’_Stuttgartef &istallabtir. This work was done with the f&an&l su&oti of the. Deutsche Forschungsge.. :. meinschaft..
’ . ., l
d
._ -
-.
.
-_ References ; ;__ ; ,y .. -[l] ,M. Shtinoff,Symp. Faraday Sot. 3 (c9691 137. [2] R:Sch&&@er atid KC. Wo!f, Chem. Phye Letters 16
betweeri non-equival@~ch&ns (A aqd.B chains) we. behav$kr. If:&k saturates, t&e ESR :.. (1972)402, transit&n bf .one-chain (A & .B) k, .ati orienttitiork ..- .: ‘_ .- [3] H:,Sixl and
find, a different
&e can $ve a
lower (upper) Emi! for the A--B transition probability .. _. .. : s_ .:, of linear excitons if a doubling’effect in ODMR intensity is (not) 0bseye.d. This may be of interest in cases 1 where the ESR linewidth is determined primarily not by the A-B hoppiiig and the hopping probability _,;o. therefore cannot be determined from the ESR link..
: :
. ..
the SFR-probability
.,
M; Sihwotier, Chem. Phys. Letters 6 (f9701 whek t@ 4 and-8 chains gre.non-eqyi;valent,.the’‘. ; 21. : saturation is.car+ed to th&:other chain by Pr_B.jtimps : j [!I R. Schmidbeiger and l!&W4f, Chekihys. Letters 25 ;,a&ice ve& ift& ho&& p&ability,@ greater. 1 : .--. (1974) 185, :: ::. thiht~k SL~-pro~abili~..Therres~~t is tb&q$de’ ’ : -f’: [S] R._hf:-Hochstrasskr-akdJ.D. @h&em&, 3. Che&. Phys. ‘.:56 (1972) 5935. .-. ‘, :t& syr&n$ry q% the’:ODMg,&ensity of one @d ._ .: 1. ::- _ [cl ‘R. Schnzidberger and H.C. W&f, Chem. Phqg. Letters 32 1 ?of Ii&r-&kit& (Aeof 5) is~as.great:g~t,hB.intehsily .I: -, ‘. irs,sj l8. : I :_ :: _ : ,~ .. ; .- ;, ,.. ,, ‘, . .,. y: .y-: :_._-.~ !‘_. ,‘. --~I .--. r-‘:.’::T.-:‘. z:.~.I. ._:.,_‘:, I ...‘ ;~. .-.,. . .__::_ ;,: .:,. -_ .. ,_ .:.., :_.I..f;,:. _‘:; ,_,._:.__ . . . .‘. .: . .L-. -. :y .‘-.I .L.. :=y~ : _i..;y: -; .i_ .?.I.- ... _y- :: ._--,: __,I-(, -_ .... _~_ _..-..._.__ ;_. .__ _;. :____ .__ :. : ~. __..;.. : ‘ , . . ‘ ,.f._ ;, .:::. I ‘ -;,,.. ::,. .‘ L, ,,, “ y ‘ ... ._ ‘.....’ .‘._-:.-‘:‘ : ... ‘ ._>.,_ ._ -_ .’ ~, .; __ _ ., :-:, .:.. (T. ” .(( ,:;-5 ,,._. .; ,(_.,_ I :,. .. : ., : .. I. ,_,,;. ,, .--. _:.; ;..,., ...;, .:-,..;~_r.. ‘..: --_ : -,-, -...I . I_: ..,,(-1.. it-;:.,.; :. -.Y_.. )_. :r: :,. _._ _yl ..,, ,... _. -. -. -_..~,; ;_ ._. , ,’.‘.‘5’ ,. ~.,...,’ _” .” ,...(‘. ._ ._-y_ ._ .;. _. ,:. :. _:_.: :_“ 1 .:. 1-z . . -:_..:. . ..” .‘ I:, _J:,_; _, ‘ ;, ,.I .___-~ .& :._; ‘ , z: .. -,,I ._. I. ;.:: ,-:‘- : : .. : :_,_;,, ; _‘_‘ _--.; .:: : : .._, ‘, __ . . _,:_ : .,:y.. ... _I’ .\- ,._:’ _,._ ._:: .....1, ._ _ -; ;’ ,_.I ~_:_ i’j,. ; ;. ._. ._ .-.‘::.. _.; y _.-. -1 .-I ; _ .<:,::_: :_.~_; __;;;, _: ,._ : .- , .:.._.::i~‘:;..,.,.. .: .. _._.,_: ., .:I.‘ ,.- f,-.. I,... ‘..: ‘y: .,-.-.., _;_,--..’ . .‘,,, _c I...,‘ ~ : ..b’ _.l._.. .23 _ :: ..% .i. ,. : ,: ....;,::. :Y .:. : ...-I.,.,l:+._.y;_:‘ :_ t- ..,, .‘-.).+:.” (,..-i.-. ,: _. .~~_y--,yy-- ,-:.-; _ ~_ -: -..y.. ->~~_~,_:;,:_.‘T:‘ .:.: ..... ..._: ~.~I.;-:;.~~_-:.::._ *_. :’ : ::~~~ &-: ..:: _f _ ,:-:_::._._’ .:.’-:~~,-,‘:_~‘~~.;j..~~~~:_~I’ :T ,‘;:‘-_-_-; ~: .‘_ ., .~_. : :,. . : .,.- (_~ .-.- ; .,.