- Email: [email protected]
Fluorescence yields of azulene derivatives
:.
Volume 13; nun&r
2
‘,
..’ ”
.
CHEMtCAL ki$Y&ZS l&T&S ‘, ,:” ‘, ., ,. .. ,:_,..: .: ‘. ‘. ‘. ,‘,, ,. :
..
. . .
,, :
.,
,_‘,
.-
.’
~L~O~ESCE~CE
..
:
15 February l-972 (.
.’
(.
..
‘,
“‘.
”
s. ~&RATA,
YIELDS OF kX.ikNE c. ~W~AGA,
Depawment
D&~A’@E~ ~..’ T. TODA. and ti. K~KUBW.
of ehemist&,
ToimZU University,
: Sendai, Japan
.,
K&+.%ved 30
:.
: 1 ,I,
November 1971 .I’ -:
:
;’
._
‘.
.’
”
:
..
.. ‘. .’
:
.. ‘. ;
,:
:
‘.
‘.
‘. Fluorescence pie& of a number of azuiene derivatives were d~ter~i~~~~ c&ohe~&~e at ro+m temperature. it was found that the yield decreases with the decrease of the energy gap, A!?, between tie first and the- second ex- .‘. cited states and that the rate constant of radiationfess transition increases exponentiaffy with decreasing hE: :
-.
O-0 bands itt the absorption Sp+.rum, : Xt is well kn&n t&t azulene emits intense fluor& Table 1,shows ihat @f has no relation with the cence from the ‘second excited state [I J . This fluoresrmmber of chlorine atoms in a molecule. Thus the cence is called “anoma.Zous”, since it is an exception heavy atom efrect is not responsibie foi the variation to Kasha’s rule. -The fluorescence of some azulene dein $?,:A larger AE results in a larger 0, irrespective of rivatives is also ,=ornalous [2-S] _ the number and.the kind of substituentsas shown in Beer and Longuet-Higgins have suggested that the fig. 2. It is apparent that Iaige AE is one of the impor; large energy gap, AE, between the first and the second tant factors for the fluorescence anomaly of azuient: excited’states is responsible for the fluqrcscetice anomand its derivatives. The deviation of comp&nds sly of azulene, i.e., large AI!?makes the internal con32~34 may be attributed to the enhanced intersys* version’from S2 to S1 slow. Although the th?ory of tern crossing dtie io the heavy atom effect ‘and to radiationltiss-transitions suppbrts this point of view, there seems to be.na systematic experimental study some photochemical processes. It is of interest to dete?ine the qtiantum yields concerning this point. Furthermore, the effect of inof internal’ conversion and intersystem crossing from tersystem crossing on the fluorescence yield of azu-’ S, _However, in the present state it is quite difficult tene remains compjetely tinknown. to discriminate both processes. ’ In this work the compounds given in fig; i were inThe, close correlation between @f and AE suggests vestigated. Fluorescence from the second excited the importance of internal ctinversion S2--+G, in the state ivas observed for all the compounds’except 15, Fadiationless decay,. fn tirder.to study this in more de32, 33, and 34.This was confirmed by iheexcitation tail we have evaluated *&erate constant, kd, of radiaspectrum or by a mirror-image ~lations~p between tionless tiansition absorption and, fluorescence spectra. The normal fluoi rescence was not obsetied for all the compounds stu: *r=k&Ck;) ; kd =.k&Dfl -y I) , (f) died. I&&Its are summar~~d in table 1. As a.sta.ndard bf the fluoresce’nce’yield, @f, azulene’s value 0.20, in where kf is &e rate donsttit of flubiescence emission cyclohexane ob,tained by’,Ber&m (0.24) [6] and carmd. kd is the sum of the rate’ constant of internal con* r&ted by-.Birh [7] was adopted:, The energy.ievels version to S, (kiC_) ;an$ th+t of irit&ys~em~~~oss~ng 1” Esx and:E, were_estimated from the positions of the. a’ (disc ):, I+m$ converSion to S&is safely,neglected.:the’~~Flified:Sttickfer~Beig ‘.. -_: kf,ti& e$uated’by _:. . *, B&i& i71 ‘pihts oht’that tlPe’[email protected] ref. [q shou$ ‘, :‘. ‘. ail be dimhiished b$,Z7$%.Sfzea& ef.‘!8] .,: ._ ., .., ‘. : _’ .,_..”
e@a+i: ‘: ,_ _
f :, :’ .I
_,, -, ;.
_,-.: :_;
::
”
.. _I. ‘: ‘_
_ ‘,L.
”
., _ :.
: 1. .’
.
..’
.i
IfOQfy, .’
...
.
15 Februark~1972’
: ,,
1,; .;.
‘.
8’., -;5 7 1, ,9. 4. 8 .‘2'
:
12.7
‘. ; '.
13 10
13.7 .13.4 14.3 13.8 13.7 14.9. -14.5 15.4 lS.5 15.8 16.3 l6.4 116.7‘ 17.3 16.0
12 11 18 17 15
.33
,:
.~
27.0 27.7 27.9 :28.3 27.2 26.7 27.8 27.9 27.3
.. ”
g-37.: '-14.3, i4.3 .. 0.23 . 14.2 1.’ 0.20.. .’ 14.9, .0.20 13.4 ., : 0.092 13.0 ,0:076 12.9 0.052 12.5 12.8 '. .’ -p.o40 0.043 "
27.0 27.2 27.2 26.9 26.5 26.9 24.8
11.5 11.4 ,-‘10.9, 10.5
9.8 9.6 8.8
14 31 ., 32
16.7 15.0 13.3
23.9 27.i 26.4
7.2 12.1 13.1
34
13.8 14.2
26.6 26.1
12.8 11.9
kf 7 2.88 X. 10-gn2c;:fi,) :
-'
I e(T) dz ,
(2)
where n, ca and ;;f are, respectively, the refractive index of the solvent, the mean wavenumbers of absorption tid fluorescence. Eq. (2) yields kf = 3.4 X lo7 smrl for azulene, which is a factor of 4.1 smaller than the value dalculated from the observed values of T (I.4 nsec) [6] and Qf (0.20)*. The origin of this large discrepancy is ‘not clear and it was assumed that
:
Fluorescence~viekiand the ener&ga~ -~ - _ in &clbhekie. ‘. .., ,’ _,c: .; ,, : ,, ,:_.
,’
. .
4.9.: ,’ 4.9 .::,I5.2' 5.7 ,’ .’ i7.,- “., ,23 -” 24 ,' 41 3?
: ,.
.’ .. ,’
,:
-0.001 --0.0005
i.6
.a1606
=-0.0005
1.7 ..
.&3300
510" =10-s
_.;
‘.
.’
=1o-s 0.024
2.6
-
-i1g
; :
10"
‘.” 10
.
lo”: .-
Q g IO’Om
3!
-13
._
.'
,,-
=O.O@L'
0.2-
Fie. m-m2.
..,
:- 2.3 :. 15. 15 1.4. : 1:s .'gI 1.3 ‘. 1.6 1.7
.’ ., -sac-‘:for’tiukhe: ,.:, I------. ,. _’ ,‘_ -’ ...- .
.:;
..
14
‘.””
j03 .:,
.,
2.. CHEMICALPHYSICS LETTERS : .. 15 February 1973; .., ;. : ,:. .’ :. ,._;,.‘. ,, .’ --kf~j$&es~@~~ other &m&k& are ‘equal to the ones exists. !n addition, no external he& atoti;effect by ” ob&ned by ,eq: (2) multiplied by a factor 6f,4.1_’ The bromobenzene (up tb 2 m&/j)_& the fluorescerice &its thus ,evaluated ire skk-kaiized in table 1 and intensity of azulene has be@ detected., This experi: d&$&e&in fig. 3, Q,varies exponentially with AE rnkkl etidence.seems to support the ‘con&ion that .m _:8nd the c&elation is very good. It is reasonable to the quantum-yield of intersystem cidssing is small. .: This seems reasonable since the flud&cence lifetime c’otisidtr that th+ was brought about by the predomiof zulene is very short. If we assunk that kisc is the : nant internal conversion in the radiationless decay. : same order bf magnitude with that of naphthalene Adcording to Englman a$ Jortner [9]‘, the rate ., (8.4 X ,106 .~ec-l), then kisc 7 % 0.01. constant, W, of radiationless transitions between two To calcuiate the kd values, we h’aie made the aselectronic states of a large molecule is expressed as. sumption that the kf values are equal to the ones ., follows: .. ‘:-, Noluine, 13, A&er
evaluated by eq. (2) multiplied by a factor of 4.1. If this factor vaties largely from compound to compound, the close correlation between kd and AE will
not exist. Hence, the above assumption may be cor, (3)
matrix.element, w the frequency of the normal molecular vibration, LI the reduced displacement of the equilibrium position, A.E &e energy gap, and M refers to the vibrational mode(s) with the highest freque’ncy’ in the molecule. Eq. (3) may be applicable to the internal conversion from Sa. to S 1 .of azulenc derivatives. Since we are dealing with azzulerie derivatives without any large change. in 7relectron systems, Cand A,, may be regarded to tie practically constant_ Thus eq. (3) states that the dependence of ki, on &? is exponential (or rather superexponential). This is in qualitative agreement with the, result given in fig. 3. AS def’med above, kd = ki, + kisc_ It is kd, and not kit that has been plotted in fig. 3. If kisc is compara-* the close correlation bebie with or greater than Aic, tween kd and AE as well as the expdnential dependence of kd on OE will not exist, since ki,c should not be correlated with AE. The result in fig. 3 suggests that disc is small. Compounds 2-7 are chlorine-substituted aulenesj, .yet no evidence of internal heavy atom effect on k, where Cis the electronic
* Si&la.r equations have b&en derived by Fischer [ 101 and dy .F&kd and iorttier [ll].
rect
to some
extent.
De&Is and further where..
results will be published
else-
H.K. and T.T. wish to thank the Ito Science Foundation for fimancial support. ’
References [l] hl. Beer and H.C. Longuet-Higgins, J. Chem. Phys. 23 (1955) 1390. [2] G. VisvJanath and hi. Kasha, J. Chem. Phys. 24 (1956) 574. 131 G. Binsch, E. Heibronner, R. Jankow and D. Schmidt,
Chem. Phys. Letters 1 (1968) 135. [4] DE. F&on, T.R. Evans and P.A. Leermakers,
Photochem. l(1969) 347. [5 ] iA. Poole, R.C. Dhingra and B. Gebmt,
Mol.
J. Chem. Phys. 52 (1970) 464. [6] I.B. Berlman, Handbook of fluorescence spectra of ‘aromatic molecules (.4cademic Press, New York, 1965). [7j J.B. Birks. Photophysics of aromatic molecules (Wiley, New York, 1970) p. 103. [8] J.N. Demas and C.A. Crosby, J. Fhys. Chem. 75 (1971) 991; (91 R. Englman and.J. jortner, Mol. Phys. 18 (1970) 145. [lo] S. Fischer, (3hem. Phys. Letters 4 (1969) 333. i11] KP. Freed and J. Jortner, J. Chem. Phys. 52 (1970) 6272.
Volume 13; nun&r
2
‘,
..’ ”
.
CHEMtCAL ki$Y&ZS l&T&S ‘, ,:” ‘, ., ,. .. ,:_,..: .: ‘. ‘. ‘. ,‘,, ,. :
..
. . .
,, :
.,
,_‘,
.-
.’
~L~O~ESCE~CE
..
:
15 February l-972 (.
.’
(.
..
‘,
“‘.
”
s. ~&RATA,
YIELDS OF kX.ikNE c. ~W~AGA,
Depawment
D&~A’@E~ ~..’ T. TODA. and ti. K~KUBW.
of ehemist&,
ToimZU University,
: Sendai, Japan
.,
K&+.%ved 30
:.
: 1 ,I,
November 1971 .I’ -:
:
;’
._
‘.
.’
”
:
..
.. ‘. .’
:
.. ‘. ;
,:
:
‘.
‘.
‘. Fluorescence pie& of a number of azuiene derivatives were d~ter~i~~~~ c&ohe~&~e at ro+m temperature. it was found that the yield decreases with the decrease of the energy gap, A!?, between tie first and the- second ex- .‘. cited states and that the rate constant of radiationfess transition increases exponentiaffy with decreasing hE: :
-.
O-0 bands itt the absorption Sp+.rum, : Xt is well kn&n t&t azulene emits intense fluor& Table 1,shows ihat @f has no relation with the cence from the ‘second excited state [I J . This fluoresrmmber of chlorine atoms in a molecule. Thus the cence is called “anoma.Zous”, since it is an exception heavy atom efrect is not responsibie foi the variation to Kasha’s rule. -The fluorescence of some azulene dein $?,:A larger AE results in a larger 0, irrespective of rivatives is also ,=ornalous [2-S] _ the number and.the kind of substituentsas shown in Beer and Longuet-Higgins have suggested that the fig. 2. It is apparent that Iaige AE is one of the impor; large energy gap, AE, between the first and the second tant factors for the fluorescence anomaly of azuient: excited’states is responsible for the fluqrcscetice anomand its derivatives. The deviation of comp&nds sly of azulene, i.e., large AI!?makes the internal con32~34 may be attributed to the enhanced intersys* version’from S2 to S1 slow. Although the th?ory of tern crossing dtie io the heavy atom effect ‘and to radiationltiss-transitions suppbrts this point of view, there seems to be.na systematic experimental study some photochemical processes. It is of interest to dete?ine the qtiantum yields concerning this point. Furthermore, the effect of inof internal’ conversion and intersystem crossing from tersystem crossing on the fluorescence yield of azu-’ S, _However, in the present state it is quite difficult tene remains compjetely tinknown. to discriminate both processes. ’ In this work the compounds given in fig; i were inThe, close correlation between @f and AE suggests vestigated. Fluorescence from the second excited the importance of internal ctinversion S2--+G, in the state ivas observed for all the compounds’except 15, Fadiationless decay,. fn tirder.to study this in more de32, 33, and 34.This was confirmed by iheexcitation tail we have evaluated *&erate constant, kd, of radiaspectrum or by a mirror-image ~lations~p between tionless tiansition absorption and, fluorescence spectra. The normal fluoi rescence was not obsetied for all the compounds stu: *r=k&Ck;) ; kd =.k&Dfl -y I) , (f) died. I&&Its are summar~~d in table 1. As a.sta.ndard bf the fluoresce’nce’yield, @f, azulene’s value 0.20, in where kf is &e rate donsttit of flubiescence emission cyclohexane ob,tained by’,Ber&m (0.24) [6] and carmd. kd is the sum of the rate’ constant of internal con* r&ted by-.Birh [7] was adopted:, The energy.ievels version to S, (kiC_) ;an$ th+t of irit&ys~em~~~oss~ng 1” Esx and:E, were_estimated from the positions of the. a’ (disc ):, I+m$ converSion to S&is safely,neglected.:the’~~Flified:Sttickfer~Beig ‘.. -_: kf,ti& e$uated’by _:. . *, B&i& i71 ‘pihts oht’that tlPe’[email protected] ref. [q shou$ ‘, :‘. ‘. ail be dimhiished b$,Z7$%.Sfzea& ef.‘!8] .,: ._ ., .., ‘. : _’ .,_..”
e@a+i: ‘: ,_ _
f :, :’ .I
_,, -, ;.
_,-.: :_;
::
”
.. _I. ‘: ‘_
_ ‘,L.
”
., _ :.
: 1. .’
.
..’
.i
IfOQfy, .’
...
.
15 Februark~1972’
: ,,
1,; .;.
‘.
8’., -;5 7 1, ,9. 4. 8 .‘2'
:
12.7
‘. ; '.
13 10
13.7 .13.4 14.3 13.8 13.7 14.9. -14.5 15.4 lS.5 15.8 16.3 l6.4 116.7‘ 17.3 16.0
12 11 18 17 15
.33
,:
.~
27.0 27.7 27.9 :28.3 27.2 26.7 27.8 27.9 27.3
.. ”
g-37.: '-14.3, i4.3 .. 0.23 . 14.2 1.’ 0.20.. .’ 14.9, .0.20 13.4 ., : 0.092 13.0 ,0:076 12.9 0.052 12.5 12.8 '. .’ -p.o40 0.043 "
27.0 27.2 27.2 26.9 26.5 26.9 24.8
11.5 11.4 ,-‘10.9, 10.5
9.8 9.6 8.8
14 31 ., 32
16.7 15.0 13.3
23.9 27.i 26.4
7.2 12.1 13.1
34
13.8 14.2
26.6 26.1
12.8 11.9
kf 7 2.88 X. 10-gn2c;:fi,) :
-'
I e(T) dz ,
(2)
where n, ca and ;;f are, respectively, the refractive index of the solvent, the mean wavenumbers of absorption tid fluorescence. Eq. (2) yields kf = 3.4 X lo7 smrl for azulene, which is a factor of 4.1 smaller than the value dalculated from the observed values of T (I.4 nsec) [6] and Qf (0.20)*. The origin of this large discrepancy is ‘not clear and it was assumed that
:
Fluorescence~viekiand the ener&ga~ -~ - _ in &clbhekie. ‘. .., ,’ _,c: .; ,, : ,, ,:_.
,’
. .
4.9.: ,’ 4.9 .::,I5.2' 5.7 ,’ .’ i7.,- “., ,23 -” 24 ,' 41 3?
: ,.
.’ .. ,’
,:
-0.001 --0.0005
i.6
.a1606
=-0.0005
1.7 ..
.&3300
510" =10-s
_.;
‘.
.’
=1o-s 0.024
2.6
-
-i1g
; :
10"
‘.” 10
.
lo”: .-
Q g IO’Om
3!
-13
._
.'
,,-
=O.O@L'
0.2-
Fie. m-m2.
..,
:- 2.3 :. 15. 15 1.4. : 1:s .'gI 1.3 ‘. 1.6 1.7
.’ ., -sac-‘:for’tiukhe: ,.:, I------. ,. _’ ,‘_ -’ ...- .
.:;
..
14
‘.””
j03 .:,
.,
2.. CHEMICALPHYSICS LETTERS : .. 15 February 1973; .., ;. : ,:. .’ :. ,._;,.‘. ,, .’ --kf~j$&es~@~~ other &m&k& are ‘equal to the ones exists. !n addition, no external he& atoti;effect by ” ob&ned by ,eq: (2) multiplied by a factor 6f,4.1_’ The bromobenzene (up tb 2 m&/j)_& the fluorescerice &its thus ,evaluated ire skk-kaiized in table 1 and intensity of azulene has be@ detected., This experi: d&$&e&in fig. 3, Q,varies exponentially with AE rnkkl etidence.seems to support the ‘con&ion that .m _:8nd the c&elation is very good. It is reasonable to the quantum-yield of intersystem cidssing is small. .: This seems reasonable since the flud&cence lifetime c’otisidtr that th+ was brought about by the predomiof zulene is very short. If we assunk that kisc is the : nant internal conversion in the radiationless decay. : same order bf magnitude with that of naphthalene Adcording to Englman a$ Jortner [9]‘, the rate ., (8.4 X ,106 .~ec-l), then kisc 7 % 0.01. constant, W, of radiationless transitions between two To calcuiate the kd values, we h’aie made the aselectronic states of a large molecule is expressed as. sumption that the kf values are equal to the ones ., follows: .. ‘:-, Noluine, 13, A&er
evaluated by eq. (2) multiplied by a factor of 4.1. If this factor vaties largely from compound to compound, the close correlation between kd and AE will
not exist. Hence, the above assumption may be cor, (3)
matrix.element, w the frequency of the normal molecular vibration, LI the reduced displacement of the equilibrium position, A.E &e energy gap, and M refers to the vibrational mode(s) with the highest freque’ncy’ in the molecule. Eq. (3) may be applicable to the internal conversion from Sa. to S 1 .of azulenc derivatives. Since we are dealing with azzulerie derivatives without any large change. in 7relectron systems, Cand A,, may be regarded to tie practically constant_ Thus eq. (3) states that the dependence of ki, on &? is exponential (or rather superexponential). This is in qualitative agreement with the, result given in fig. 3. AS def’med above, kd = ki, + kisc_ It is kd, and not kit that has been plotted in fig. 3. If kisc is compara-* the close correlation bebie with or greater than Aic, tween kd and AE as well as the expdnential dependence of kd on OE will not exist, since ki,c should not be correlated with AE. The result in fig. 3 suggests that disc is small. Compounds 2-7 are chlorine-substituted aulenesj, .yet no evidence of internal heavy atom effect on k, where Cis the electronic
* Si&la.r equations have b&en derived by Fischer [ 101 and dy .F&kd and iorttier [ll].
rect
to some
extent.
De&Is and further where..
results will be published
else-
H.K. and T.T. wish to thank the Ito Science Foundation for fimancial support. ’
References [l] hl. Beer and H.C. Longuet-Higgins, J. Chem. Phys. 23 (1955) 1390. [2] G. VisvJanath and hi. Kasha, J. Chem. Phys. 24 (1956) 574. 131 G. Binsch, E. Heibronner, R. Jankow and D. Schmidt,
Chem. Phys. Letters 1 (1968) 135. [4] DE. F&on, T.R. Evans and P.A. Leermakers,
Photochem. l(1969) 347. [5 ] iA. Poole, R.C. Dhingra and B. Gebmt,
Mol.
J. Chem. Phys. 52 (1970) 464. [6] I.B. Berlman, Handbook of fluorescence spectra of ‘aromatic molecules (.4cademic Press, New York, 1965). [7j J.B. Birks. Photophysics of aromatic molecules (Wiley, New York, 1970) p. 103. [8] J.N. Demas and C.A. Crosby, J. Fhys. Chem. 75 (1971) 991; (91 R. Englman and.J. jortner, Mol. Phys. 18 (1970) 145. [lo] S. Fischer, (3hem. Phys. Letters 4 (1969) 333. i11] KP. Freed and J. Jortner, J. Chem. Phys. 52 (1970) 6272.