- Email: [email protected]
Intersystem crossing in 9-carbonyl derivatives of anthracene
Volume
i3; nuriiber1
,.
‘i F&y
_::
,!
,_ ,‘.
.’ ..
-.
..‘.;.
.’
: :. :. : : ,I,
INTER~Y~TEMCR~~~IN~ZN~-(~~RB~NYLDERIVATIVESOFANTHRAC~E' :'
:
.’
Faculty
.. “y.:.
T.MATSWMO~O;M.S~TO and S. HlkAYAMk'. : of Textile Schnce, Kydto T&hnicd University. Matmgasakf, Sakyo-ku, Kyoro, Received 5 November 1971
_,
“. . . .
;
Japn
‘.
An unusual temperature ,effect on ihe intensity of fluorescence of 9-carbonyl derivhtives of anthracene is observed:ihis is interpreted in terms of an intersystem crossing processfrom the lowest excited singlet state, !+r to the higher excited triplet state T,&_ ,.
Investigations of temperature
effects on fluorescence
spectra and on the quantum yields of anthracenk and. its derivatives made by Lim et al. [I] and’others [2,3] have revealed the important role played by the higher excited triplet states in the radiationless intersystem crossing prdcess. It is said that the interSystem crossing process occurs from the lowest excited singlet
.state S, not to the lowest t;_ipiet state T, but rsther to the s&and triplet state Tz, which is believed to have an energy nearly equal to that of S1. In mesoderivatives of anthracene, S, lies lower than T, on account of the substituent effect, so at low temperatures the radiationkss process, S, UIT, ,which needs some activation eneigy, becomes greatly prohibited and as a result the quantum yield of fluorescence approaches. unity. The carbonyl mesodkrivatives of kthracene, 9CH3CO-A, 9CH3CH2CO-A, 9-PhCO-A and 9,10diPhCO-A, behave in a different way: At room
Si -So
‘can be neglected.for these compounds just ,.’ as it is neglected ,for other antbacene derivatives; it is supposed that the non-fluorescent property at iooti temperature and the large temperature’effect on the fluorescence intensity of these carbonyl compqunds ... ., orginate from a special intersystem crossing process, other thanSl*T2. Emission spectra in EPA nt 77’K, are shown in figi 1 for 9CH3CO-A and 9CH#XZCO-A. Except for
other carbonyl compounds were VJOdiPhCO-A, finally purified by sublimation. In dilute solutions the intensities of emission were fcund to be proportional to concentration. me absorption spectra at 77% which had more pronounced vibrational structures, only shifted to lower wavklengths by2 w’cpmpared witl~ those at room temperature and no spectral changes
due to conforkatiotial changes were found; The +-ror image relationship holds approximately between the emission and absorpti& spectra at 77°K. From the&z temperature they are all non-fluorescent in polar facts the emission spectra observed at low temperatures media as well as in nqn-polar media, i.e., they show no .can be assigned to the fluorescence spectra of the car; “fluorescence activation”. We found, however, that ‘the ,’ responding compourids id we are assured that the _ fist two.showed .quite a large increak of fluorescence ‘appearance of the emission is not associated with-some ., : intensity at temperatures near-77°K. The absorption : specific solutti-solute interadtioris. spectra of,thesk’compounds, except for 9,10-diPhCO_A, ihe fluorescence qu&um,yields at 77”K,%&e tie similar to that of tithracenk so_the rate of the : 0.30 (EPA) and 0.1..7 (EMMA, polymer of methyl.. radiative process will not diffei from that of anthracene methacjlate) forPCH&O-A. and 0.63.(EPA) and, for:these compounds. In +e dega&ed media t& T-T”. 0.37 (PMMA) for 9CH3CH2CO-A. RIiod~in&‘B Was’. abstirption spectra of these &rbonyl~&npounds could used as’s &u&xri dciunter and the quantum yi+ of :’ easily be observed &-fiash irradiation .[4], &towing g,lO-dichloroan.~racene_~~ 77v,K.wa~_asstimed t&b+ : the existence of an’inter@stem crossing procek’to ,. unity:9-PhC&A $nd ?,IO&PhCO-A.were @ill.pon- :‘....I. Geld a triplet state.‘As the radiatibnless,~~ocess of. fiuorescent ~t~77‘YThese~re~ults are ndt cornpat@ ;,2 1 :
‘..
I
_, :.,_: : _’
._..’
.
_.
.Y:.,._ .,
,...
.,
.‘. ,,.’ .. : : _’ .-. L .. .. ., ” ~ :..
.‘. _ .,, :,_:;:_:~::_ ,, :, .. .., ,’ .-. 13 .‘..I..
-1 .‘Vol?e!3;n%.ber ,., :I _; .: : .I .: :.. .‘. .. ‘L .. .:_;
1 ‘: ;:
CtiEMICAL
PHYSIdS
LETTERS
.,
1 February
1972
;:
... . ‘Fig. 1. &O~~SE~CZ spectra at 77°K in EPA. Solid Iine: :: 9;CH3CH;COLA’(c.a. IO-’ moie/l). Broken line: 9-CHsCO-A (ca. 10m5 mole/l). ,, : .;-
.. with the conclusiork drawn by Lim et al. [I] on the inteisystem .cr&ing Process of the mesoderivatives
of anthracene..
‘If
The temperature dependences of the fluorescence intensity observed at 410 rnp in several degassed media are shown in fig. 2 for oCH$H$O-A. Corrections were made to take into account the temperature dependences of the densities of the media. In EPA and ,.PMMA siririlarresults were obtained for 9-CH$O-A.
Fig. 3. Plot of log [(l/Qf)-l] YWXI~ l/T for 9CH$H2CO-A, the upper curve in EPA and the lower one in PMhm.. As is shown
in fig. 2. the media differ from each other in their effectiveness for promoting fluorescence, so it is presumed that not only the temperature effect (as in PMMA) but also the rigidity of the medium play dominant roles in the appearance of fluorescence?‘. Furthermore, it can be inferred from the shape of the curves in fig. 2 and from the small values of the quantum yield that there still.occurs a comparatively efficient intersystem crossing process even at 77%. Therefore much lower temperatures will be necessary to attain a value ofthe quantum yield close to unity. It has been shown [4] that the intensities of T-T’ absorptions observed with 9CH3CO-A and 9CHO-A in PPMA at 77’K by flash irradiation were almost the same as those observed at room temperature,-i.e., the
,:Fig. 2:Fluorescence inter&y change of 3dH$I!H$O-A : ‘with temperature inseveral media. The intensities at 77°K are all arb,i&rily
put ‘&ual to unity. 1
isopentane,
,_,., : 1 ? ..EP)...’ ,.. PW+. -.-., methyicycloh&ne. .‘ -.,. _.a- ‘_..: :.i, ;_; : ..:: ::., .,.:’ ,.‘_ :l~..(:,::~..:..,:.‘ ...T ._.. :.,;: ,.. _‘,,I ?
- - -
‘,‘.. ,;-
x 103
:,
t As shown in fE:2,‘ar’inflection point~&is& in the temperature-fluorescence intensit? curve 6btained in EPA (5:5:2 by volume of ether, isopentane, and alcohol). This is prob.. ably due to a sudden change in rigidity of the medium at that temperature.
..‘,
-:
~Volunie 13, number 1
CHEMICAL PHYSICS LETTERS
intersystem dressing process at 77°K was as efficient as that at room temperattire: To interpret the observed change in the fluorescence Werisities with temperature, it is assumed that the rate cc&ant of fhe mdiative process, k,, is independent of temperature and that the only temperature dependent process is the intersystem crossing process specified by a rate cotitant, kb exp(-MIRT). The?, the quantum yield of fluorescence will be given by
kf
cp, =
ki + k, exp(-M/RT)
*
In fig. 3 a plot of log (l/Qf-I) versus I/T is given for 9CH3CH2CO-A. The calculated values of AE are 310 .cal/mole (low temperature portion) and 2690 Cal/mole (high temperature portion) in EPA and 430 cal/mole in PMMA. The error in af at a smaller value of Qf will cause a greater error in AE as is clear from the equation -$-In f
[(l/af)-
l] =-
“1 Febru&
.’
’ a+-(1 -@fj
Hitherto we have emphasized the importance of the intersystem crossing process in relation to the observed temperature effect on the intensity of fluorescence. Let us next ask what kind of intersystem crossing is involved. It is evident from our results that this process would be closely related to the carbonyl substituents. El-Sayed and Lower [5] have shown that the intersystem crossing process between the nn* state and the nn* state is more efficient than the one between electronic states of.the same kind by an order of two or three. All 9-carbonyl derivatives of anthracene studied here have mr* excited states’in addition to 5t7~*excited states.. The lowest excited singlet states of these compounds are evidentlynn” states, so following the conclusions f El-Sayed [5] , an efficient intersystem crossing pro-
‘197i ,_
: ,.
tb T,,,+,would be conceivable for our cess from S, compounds_ Furthermore, the small energy gap ‘. bettieen San+ and T,+ , which can be estimated’from a typical value for each electronic state assuming that the electronic energy of the.nn* sfa?e.does not change much with variation ‘of conjugated.systems, favoursthe radiationless process, S,,.$---‘T,,t .‘Therefore it is l
thought
that th,e acti%tion
9-CH3CH$O-A system crossing
are mainly process.
energies
observed
associated
for
with that inter-
In conclusion, the prominent temperature effect on fluorescence intensity, the small fluorescence qur+_mi yields at 77”K.observed for 9-CH3CO’-A and 9$H,Cl$CO-A, and the non-fluorescent property of 9-PhCO-A and 9,10-diPhCO-A even at 77°K &I be well undcrstocd if we consider an intersystem crossing fiiocess, &+--YT,.,+, which is’more efficient than the process, S-T,. The latter process has b&en suggested for the ordinary m,eso-derivatives ofanthracene. From dur results alone, however, possibilities for the occurrence of other efficient arid temperature dependent radiationless transitions from S 1 cannot be excluded comple’tely [6] .
References [l] [2] i3] (41 [S] [6]
E.C. Lim, J.D. Laposa and J.M.H. Yu, J. Mol. Spectry,’ 19 (1966) 412. T.F. Hunter and R.F. Wyatt, Chem. Phys. Letters 6 (1950) 221. E’..Bowen and J. Sahu, J. r’hys. Chem. 63 (1959) 4. S. Hirayama and J. Osugi,.Nippon Kagaku Zasshi 92 (1971) 825. SK Lower and M.A.EISayed, Cbem. Rev. 66 (1966) 199. E.C. iim, Molecular luminescence (Benjam& New York, 1969) p. 469.
i3; nuriiber1
,.
‘i F&y
_::
,!
,_ ,‘.
.’ ..
-.
..‘.;.
.’
: :. :. : : ,I,
INTER~Y~TEMCR~~~IN~ZN~-(~~RB~NYLDERIVATIVESOFANTHRAC~E' :'
:
.’
Faculty
.. “y.:.
T.MATSWMO~O;M.S~TO and S. HlkAYAMk'. : of Textile Schnce, Kydto T&hnicd University. Matmgasakf, Sakyo-ku, Kyoro, Received 5 November 1971
_,
“. . . .
;
Japn
‘.
An unusual temperature ,effect on ihe intensity of fluorescence of 9-carbonyl derivhtives of anthracene is observed:ihis is interpreted in terms of an intersystem crossing processfrom the lowest excited singlet state, !+r to the higher excited triplet state T,&_ ,.
Investigations of temperature
effects on fluorescence
spectra and on the quantum yields of anthracenk and. its derivatives made by Lim et al. [I] and’others [2,3] have revealed the important role played by the higher excited triplet states in the radiationless intersystem crossing prdcess. It is said that the interSystem crossing process occurs from the lowest excited singlet
.state S, not to the lowest t;_ipiet state T, but rsther to the s&and triplet state Tz, which is believed to have an energy nearly equal to that of S1. In mesoderivatives of anthracene, S, lies lower than T, on account of the substituent effect, so at low temperatures the radiationkss process, S, UIT, ,which needs some activation eneigy, becomes greatly prohibited and as a result the quantum yield of fluorescence approaches. unity. The carbonyl mesodkrivatives of kthracene, 9CH3CO-A, 9CH3CH2CO-A, 9-PhCO-A and 9,10diPhCO-A, behave in a different way: At room
Si -So
‘can be neglected.for these compounds just ,.’ as it is neglected ,for other antbacene derivatives; it is supposed that the non-fluorescent property at iooti temperature and the large temperature’effect on the fluorescence intensity of these carbonyl compqunds ... ., orginate from a special intersystem crossing process, other thanSl*T2. Emission spectra in EPA nt 77’K, are shown in figi 1 for 9CH3CO-A and 9CH#XZCO-A. Except for
other carbonyl compounds were VJOdiPhCO-A, finally purified by sublimation. In dilute solutions the intensities of emission were fcund to be proportional to concentration. me absorption spectra at 77% which had more pronounced vibrational structures, only shifted to lower wavklengths by2 w’cpmpared witl~ those at room temperature and no spectral changes
due to conforkatiotial changes were found; The +-ror image relationship holds approximately between the emission and absorpti& spectra at 77°K. From the&z temperature they are all non-fluorescent in polar facts the emission spectra observed at low temperatures media as well as in nqn-polar media, i.e., they show no .can be assigned to the fluorescence spectra of the car; “fluorescence activation”. We found, however, that ‘the ,’ responding compourids id we are assured that the _ fist two.showed .quite a large increak of fluorescence ‘appearance of the emission is not associated with-some ., : intensity at temperatures near-77°K. The absorption : specific solutti-solute interadtioris. spectra of,thesk’compounds, except for 9,10-diPhCO_A, ihe fluorescence qu&um,yields at 77”K,%&e tie similar to that of tithracenk so_the rate of the : 0.30 (EPA) and 0.1..7 (EMMA, polymer of methyl.. radiative process will not diffei from that of anthracene methacjlate) forPCH&O-A. and 0.63.(EPA) and, for:these compounds. In +e dega&ed media t& T-T”. 0.37 (PMMA) for 9CH3CH2CO-A. RIiod~in&‘B Was’. abstirption spectra of these &rbonyl~&npounds could used as’s &u&xri dciunter and the quantum yi+ of :’ easily be observed &-fiash irradiation .[4], &towing g,lO-dichloroan.~racene_~~ 77v,K.wa~_asstimed t&b+ : the existence of an’inter@stem crossing procek’to ,. unity:9-PhC&A $nd ?,IO&PhCO-A.were @ill.pon- :‘....I. Geld a triplet state.‘As the radiatibnless,~~ocess of. fiuorescent ~t~77‘YThese~re~ults are ndt cornpat@ ;,2 1 :
‘..
I
_, :.,_: : _’
._..’
.
_.
.Y:.,._ .,
,...
.,
.‘. ,,.’ .. : : _’ .-. L .. .. ., ” ~ :..
.‘. _ .,, :,_:;:_:~::_ ,, :, .. .., ,’ .-. 13 .‘..I..
-1 .‘Vol?e!3;n%.ber ,., :I _; .: : .I .: :.. .‘. .. ‘L .. .:_;
1 ‘: ;:
CtiEMICAL
PHYSIdS
LETTERS
.,
1 February
1972
;:
... . ‘Fig. 1. &O~~SE~CZ spectra at 77°K in EPA. Solid Iine: :: 9;CH3CH;COLA’(c.a. IO-’ moie/l). Broken line: 9-CHsCO-A (ca. 10m5 mole/l). ,, : .;-
.. with the conclusiork drawn by Lim et al. [I] on the inteisystem .cr&ing Process of the mesoderivatives
of anthracene..
‘If
The temperature dependences of the fluorescence intensity observed at 410 rnp in several degassed media are shown in fig. 2 for oCH$H$O-A. Corrections were made to take into account the temperature dependences of the densities of the media. In EPA and ,.PMMA siririlarresults were obtained for 9-CH$O-A.
Fig. 3. Plot of log [(l/Qf)-l] YWXI~ l/T for 9CH$H2CO-A, the upper curve in EPA and the lower one in PMhm.. As is shown
in fig. 2. the media differ from each other in their effectiveness for promoting fluorescence, so it is presumed that not only the temperature effect (as in PMMA) but also the rigidity of the medium play dominant roles in the appearance of fluorescence?‘. Furthermore, it can be inferred from the shape of the curves in fig. 2 and from the small values of the quantum yield that there still.occurs a comparatively efficient intersystem crossing process even at 77%. Therefore much lower temperatures will be necessary to attain a value ofthe quantum yield close to unity. It has been shown [4] that the intensities of T-T’ absorptions observed with 9CH3CO-A and 9CHO-A in PPMA at 77’K by flash irradiation were almost the same as those observed at room temperature,-i.e., the
,:Fig. 2:Fluorescence inter&y change of 3dH$I!H$O-A : ‘with temperature inseveral media. The intensities at 77°K are all arb,i&rily
put ‘&ual to unity. 1
isopentane,
,_,., : 1 ? ..EP)...’ ,.. PW+. -.-., methyicycloh&ne. .‘ -.,. _.a- ‘_..: :.i, ;_; : ..:: ::., .,.:’ ,.‘_ :l~..(:,::~..:..,:.‘ ...T ._.. :.,;: ,.. _‘,,I ?
- - -
‘,‘.. ,;-
x 103
:,
t As shown in fE:2,‘ar’inflection point~&is& in the temperature-fluorescence intensit? curve 6btained in EPA (5:5:2 by volume of ether, isopentane, and alcohol). This is prob.. ably due to a sudden change in rigidity of the medium at that temperature.
..‘,
-:
~Volunie 13, number 1
CHEMICAL PHYSICS LETTERS
intersystem dressing process at 77°K was as efficient as that at room temperattire: To interpret the observed change in the fluorescence Werisities with temperature, it is assumed that the rate cc&ant of fhe mdiative process, k,, is independent of temperature and that the only temperature dependent process is the intersystem crossing process specified by a rate cotitant, kb exp(-MIRT). The?, the quantum yield of fluorescence will be given by
kf
cp, =
ki + k, exp(-M/RT)
*
In fig. 3 a plot of log (l/Qf-I) versus I/T is given for 9CH3CH2CO-A. The calculated values of AE are 310 .cal/mole (low temperature portion) and 2690 Cal/mole (high temperature portion) in EPA and 430 cal/mole in PMMA. The error in af at a smaller value of Qf will cause a greater error in AE as is clear from the equation -$-In f
[(l/af)-
l] =-
“1 Febru&
.’
’ a+-(1 -@fj
Hitherto we have emphasized the importance of the intersystem crossing process in relation to the observed temperature effect on the intensity of fluorescence. Let us next ask what kind of intersystem crossing is involved. It is evident from our results that this process would be closely related to the carbonyl substituents. El-Sayed and Lower [5] have shown that the intersystem crossing process between the nn* state and the nn* state is more efficient than the one between electronic states of.the same kind by an order of two or three. All 9-carbonyl derivatives of anthracene studied here have mr* excited states’in addition to 5t7~*excited states.. The lowest excited singlet states of these compounds are evidentlynn” states, so following the conclusions f El-Sayed [5] , an efficient intersystem crossing pro-
‘197i ,_
: ,.
tb T,,,+,would be conceivable for our cess from S, compounds_ Furthermore, the small energy gap ‘. bettieen San+ and T,+ , which can be estimated’from a typical value for each electronic state assuming that the electronic energy of the.nn* sfa?e.does not change much with variation ‘of conjugated.systems, favoursthe radiationless process, S,,.$---‘T,,t .‘Therefore it is l
thought
that th,e acti%tion
9-CH3CH$O-A system crossing
are mainly process.
energies
observed
associated
for
with that inter-
In conclusion, the prominent temperature effect on fluorescence intensity, the small fluorescence qur+_mi yields at 77”K.observed for 9-CH3CO’-A and 9$H,Cl$CO-A, and the non-fluorescent property of 9-PhCO-A and 9,10-diPhCO-A even at 77°K &I be well undcrstocd if we consider an intersystem crossing fiiocess, &+--YT,.,+, which is’more efficient than the process, S-T,. The latter process has b&en suggested for the ordinary m,eso-derivatives ofanthracene. From dur results alone, however, possibilities for the occurrence of other efficient arid temperature dependent radiationless transitions from S 1 cannot be excluded comple’tely [6] .
References [l] [2] i3] (41 [S] [6]
E.C. Lim, J.D. Laposa and J.M.H. Yu, J. Mol. Spectry,’ 19 (1966) 412. T.F. Hunter and R.F. Wyatt, Chem. Phys. Letters 6 (1950) 221. E’..Bowen and J. Sahu, J. r’hys. Chem. 63 (1959) 4. S. Hirayama and J. Osugi,.Nippon Kagaku Zasshi 92 (1971) 825. SK Lower and M.A.EISayed, Cbem. Rev. 66 (1966) 199. E.C. iim, Molecular luminescence (Benjam& New York, 1969) p. 469.