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Triplet-singlet radiationless energy transfer between benzophenone and perylene in vitreous solution
‘. Volume 29, num&r 3
:.
CHEhffCAL PHYSICk L~l+EI~S .:
‘., ..
. . _’ :
-
‘:
-,,. . .
(. ,’ ‘:’ _ ,,
:
‘.
;
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.
‘: l.Deceinb~ 1974
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‘. .- ;
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.,
.( :. . (.
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TRIPL~~-SfFaCLET'R~~~ATIoNLESS'ENERGy'TRANSFER: ,' ": .,.,:' .,BETWEENBENZOPHENONEANDPERYLENE:~VITREO~SSOLUT~ON' ,: .' ., ‘. .. " J.L LAPORTE, Y. ROUSS& ‘.
‘,
.’
:_ .
. ..
Laborat&& d’Optique.n~ooleculuire, Universire de f:eimS, Reims, France .. ,:
,and
.:
.-
..
:
,‘. P. I?ERETTIand
,‘,’
.‘.‘
,_
:-. .
,” I.
.‘, .‘,_
-
P. RANSON
~&memenz des hecherches Physiques, Untvers#
.
.,’
,de Paris Vi, fanis, fiance
.. Received 16 Jilly 1974
:
1 ._
,., \Ye&ve m&u& at 77 K the “criticf iadius” F& of the resonant intesmolecu& energy transfer from the tiiplet : I state oi’benzophendne to the shgfet state of perylene, these two cornporents being in vitrek solution.
spin f&bidden); however, the effici&cy of this tiansfer may be importarit, because the large donor lifetime. ErmolaeL and Sveshnikova [6] first demonstrated triplet-shglet transfers by leashing the decay time of the‘ acceptor fluorescence which appears within the donor lifetime. The investigation of the shortening of the donor fluorescenze lifetime’for various concentrations of the acceptor gives the acceptor “critical concentration’! Co. The “critic& sadi&” Ro(Aj = 7.3 5 Cii3 is the
.‘.- Intermolecular radiationless transfer of electromc energy involves the sim~taneous d&excitation of the .’ donor D*.,and excitationof the acceptor A. men the ” interaction energyis smaller than the vibronic, band..widt.hs 4Q of the moiecules (very weak interaction) .casc), the electroi$c.energy transfer is slow compared to the vibrational relaxation in the donor’and acceptor.. The interaction of continua model is here apyropriate &d the rate HDP-+A for transfer is given by timedependerttperturbation
separation
theory:
between
the~donor
the rate of intermolecular ..
,wh&-e p is the interaction
and
ncceptoi
at which
energy transfer is equal to
the sum of’ the rates-for all other donor d+excitation processes. ‘ihe agreement between the experimental and theqre tical values of R. (according to the Forster transfer rate theory) shows,ffiat.this triplet-singlet, transfer is.:1resonant energy transfer. When tf:e solution is excited by .a light fl&h whose
matrix &ment bet&en the initial’and final states and the Cx’Ix) are Franckdondon factors, the sum being’over all possible final resonant starei and a Boltzmann distribution of initi;il time is smaller than the decay time of the excited state states. .Iti the case of ‘adipoleidipol& ikeractibn, p2 is ‘, .of the donor molecule, Bennett et’al. [7) have calculated the decay process df the excited energy donor: $oportionaI to &~~~&(t?)/K6 where &+ and &A .are &n&on dipole moments, R the distance apart, aid,‘@(B) a term r&&e to the r3rientation of the -_molecules fly53: :_ ., ‘. ., Irrthe case of.triplet-singlet. i~dia&mIess energy *hereRi ‘is the distance bet&en D* ‘znd A &diV~ ‘. tr3nsfTr,,i&;A,is very w&k (the donor transition is :
-,
. . .,
,. . .
.,,
:
,_:
.:’
,,
CHEMICAL PHYSICS LETPERS
Volume 29, number 3
f December L974
is the number of surround~g A molecules. One can put iVk i=,‘the;refore the average of the probabilityy P(t) that D is in the excited state is @en by [2] :
where I(t) is the intensity of the luminescence decay, and CA the acceptor concentration. By plotting the
507
h(nm)
t
r
436
405
curves, Bennett gave the ‘Lcritical” values of the transfer in good agreement with the values c~culated from the Fijrster theory.
Fig. 1. Phosphorescence spectra at 77 K OFsolutions OFbentophenone (lo-* M) with concentration CA of peryIene. (1) CA = 0, C-0CA = 1.35 x io--!j M, (3) CA= 4.05 x x!-5 hi, (41 CA = 1.22 x 1o-4 M, (5) CA = 3.66 x lo-= hf, (6) CA = 1.11 X 10Y3 M.
2. Experimental results and discussion
phenone concentration
parameter y = CA/C~ with the experimental decay
various samples: each of them having the same benzo. (c = 10e2 M) and a variable concentration (1.6 X 10-3 hf to 2 x lo-4 M). The excitation source is the second hamonic of a Qswitched ruby Iaser (frequency ft = 28800 cm-f)_ The przCiiCa8y gaussian light pulses have a halfwidth durapeIykne
We have studied the benzophe~one-pe~~ene pair in a vitreous medium (a mixture of chloroform-ethylether, four to one in volume, at 77 K). The spectro-
tion of about 10 ns. The emission intensities were with a 55 UVP (RTC) photomultiplier. The experimental results are shown in fig. 2. If Wf2put I’(t) = ~~&~~~(~~~, (1) can be rewritten as:
scopic study of the observed luminescence is realised by means of a mechanical phosphoroscope (variable rotating disc-type) IS]. We feund two emissions: the benzophenone phosphorescence and also a delayed fluorescence of perylene with the same decay time. From the relative variations Lntensity
a transfer
in regard
lOgf(t)
of the two emissions’
to the concentration.
mxsured
= -k
+G
-
2
we can deduce
.
mechanism between the’tripletstate of ben-
zophenoneand the singletsfate of pexylene(fig. 1). ‘Ike decrease of the decay time of .Shc two emissions in regard to the perylene concentration increasing in the media shows clearly that we are observing a non: radiative resonant transfer T’benzophenone + S$@ne. The presence of radiative transfer (by means of a trivial reabsorption of the emission) prevents a quanti-
33 .20 c ,o. ; g 5
tative study of the radiationless transfer from this relative variation in intensity; bmthe analysis of the luminescence decay time is not affected by this mechanism. This radiative transfer becomes predominant at low concentrations of perylene and explains in this
4 Fig. 2.
3
T:MEcm,;
: ‘.’
7
8
solutions
peryfene, (I) CA = 0,
C2)CA = 2 x t0-4 M. (3) CA = 4 x lo_4 Bf, (4) C;, = 8 X 1O-4 M, (5) CA = 1.6 x 1O-3 M.
,.. .
6
PhosRhorescence decay of benzopheaone
NJ,y2 MI with a concentrstion CA of
case the ~orn~o~ls important e~ssio~,of.~e acceptor. We have measured the luminescence decay.time of
‘.
2
,. :
.:. 445
., .,
:,
‘,
..
Volume 29,.rjlimb& >
;
’
CHEhlICAL qHY&CS LETTERS
,,
-‘, ...-.: .>
.
.. .
-. : :,
‘.:
;.
1 December 1974
..: .‘.. : The fik tech of .jhis k&lity is ehsil~~cal~ulated : ‘from the experk&l results. We obtain by a comparison b‘:tween the v(f) functions (fig. .3) and the .: calculated curves for different values of the “dritical co~centr:~ti~n” Co,-acting as a $ar&net& (we follow : a Seth&. :ecentIy_%sed by ,&$ataga191) C’Q3 1.2 X. .. 113-.~M and therefore the “kitical radius” IQ-, =!68 A. ’ Tliis distilrrce is in agreement &th results obtained for other pairs i&olving a resonant Geigy transfer. .: References
-,‘.
:.
0.5
. . .
.
‘I “’
-‘t/Y,
a.
1;s :’
Fig. 3. &&rison”~f experiment& and &xla~d v&es of the parameter-v(t) for different concentiations
Co=1x~0-3M,(2)Co=1.2~!0-3M,(3)Co=1.4x’ 1073 M. ,-, or.
:
-CA
.,.. ..
[lf .A.A. Lamola,and NJ. Turro, Energy transfer and orgakc photc chemistry (Wiley, New York, 1969). [2] k. M&g+ and T. Kubota, Molecular interactions and ele-ctronicspectra (Deklter, New.York, 1970). [3] T. FSrster, Discussions FYadiy Sac. 27 (1959) 7,. [4] D.L. IDexter, I:Chem. Phys. 21 (1953) 836. [S] G,W.
Robinson and R.P. Frosch, J. C&em. Phys. 38, (1969) 1187, ” ‘. [6] V.L. Ermblaev and E.B. Sveshnikova, Akad..Nauk USSR ?73. [7] R.G. Ber&ert, R.P. Schwenker
HogI’
.:
,’ ‘,
Bull. Phys. 26 (1962) 29; Soviet Phys. Doklady 8 (1963)
...
tlS&bo17~
‘.
‘.
and R.E. kellog& J. Chem. Fhys 41 (1964) 3&O. [8] Y. RouTset, F._Dupuy and e. Lo&et, J. Phys. Appl. 3 (196:3) 250. {9] N. Mntaga, Chbm. Fhys. L&&20 (1973) 376. 1
:.
CHEhffCAL PHYSICk L~l+EI~S .:
‘., ..
. . _’ :
-
‘:
-,,. . .
(. ,’ ‘:’ _ ,,
:
‘.
;
’ ..
.
‘: l.Deceinb~ 1974
..(.
‘. .- ;
. :
.,
.( :. . (.
1’ :
:
‘,‘. ” ,,‘.,’ ..I .,
..
..,_:. :,,
.
:
,.
.:.
TRIPL~~-SfFaCLET'R~~~ATIoNLESS'ENERGy'TRANSFER: ,' ": .,.,:' .,BETWEENBENZOPHENONEANDPERYLENE:~VITREO~SSOLUT~ON' ,: .' ., ‘. .. " J.L LAPORTE, Y. ROUSS& ‘.
‘,
.’
:_ .
. ..
Laborat&& d’Optique.n~ooleculuire, Universire de f:eimS, Reims, France .. ,:
,and
.:
.-
..
:
,‘. P. I?ERETTIand
,‘,’
.‘.‘
,_
:-. .
,” I.
.‘, .‘,_
-
P. RANSON
~&memenz des hecherches Physiques, Untvers#
.
.,’
,de Paris Vi, fanis, fiance
.. Received 16 Jilly 1974
:
1 ._
,., \Ye&ve m&u& at 77 K the “criticf iadius” F& of the resonant intesmolecu& energy transfer from the tiiplet : I state oi’benzophendne to the shgfet state of perylene, these two cornporents being in vitrek solution.
spin f&bidden); however, the effici&cy of this tiansfer may be importarit, because the large donor lifetime. ErmolaeL and Sveshnikova [6] first demonstrated triplet-shglet transfers by leashing the decay time of the‘ acceptor fluorescence which appears within the donor lifetime. The investigation of the shortening of the donor fluorescenze lifetime’for various concentrations of the acceptor gives the acceptor “critical concentration’! Co. The “critic& sadi&” Ro(Aj = 7.3 5 Cii3 is the
.‘.- Intermolecular radiationless transfer of electromc energy involves the sim~taneous d&excitation of the .’ donor D*.,and excitationof the acceptor A. men the ” interaction energyis smaller than the vibronic, band..widt.hs 4Q of the moiecules (very weak interaction) .casc), the electroi$c.energy transfer is slow compared to the vibrational relaxation in the donor’and acceptor.. The interaction of continua model is here apyropriate &d the rate HDP-+A for transfer is given by timedependerttperturbation
separation
theory:
between
the~donor
the rate of intermolecular ..
,wh&-e p is the interaction
and
ncceptoi
at which
energy transfer is equal to
the sum of’ the rates-for all other donor d+excitation processes. ‘ihe agreement between the experimental and theqre tical values of R. (according to the Forster transfer rate theory) shows,ffiat.this triplet-singlet, transfer is.:1resonant energy transfer. When tf:e solution is excited by .a light fl&h whose
matrix &ment bet&en the initial’and final states and the Cx’Ix) are Franckdondon factors, the sum being’over all possible final resonant starei and a Boltzmann distribution of initi;il time is smaller than the decay time of the excited state states. .Iti the case of ‘adipoleidipol& ikeractibn, p2 is ‘, .of the donor molecule, Bennett et’al. [7) have calculated the decay process df the excited energy donor: $oportionaI to &~~~&(t?)/K6 where &+ and &A .are &n&on dipole moments, R the distance apart, aid,‘@(B) a term r&&e to the r3rientation of the -_molecules fly53: :_ ., ‘. ., Irrthe case of.triplet-singlet. i~dia&mIess energy *hereRi ‘is the distance bet&en D* ‘znd A &diV~ ‘. tr3nsfTr,,i&;A,is very w&k (the donor transition is :
-,
. . .,
,. . .
.,,
:
,_:
.:’
,,
CHEMICAL PHYSICS LETPERS
Volume 29, number 3
f December L974
is the number of surround~g A molecules. One can put iVk i=,‘the;refore the average of the probabilityy P(t) that D is in the excited state is @en by [2] :
where I(t) is the intensity of the luminescence decay, and CA the acceptor concentration. By plotting the
507
h(nm)
t
r
436
405
curves, Bennett gave the ‘Lcritical” values of the transfer in good agreement with the values c~culated from the Fijrster theory.
Fig. 1. Phosphorescence spectra at 77 K OFsolutions OFbentophenone (lo-* M) with concentration CA of peryIene. (1) CA = 0, C-0CA = 1.35 x io--!j M, (3) CA= 4.05 x x!-5 hi, (41 CA = 1.22 x 1o-4 M, (5) CA = 3.66 x lo-= hf, (6) CA = 1.11 X 10Y3 M.
2. Experimental results and discussion
phenone concentration
parameter y = CA/C~ with the experimental decay
various samples: each of them having the same benzo. (c = 10e2 M) and a variable concentration (1.6 X 10-3 hf to 2 x lo-4 M). The excitation source is the second hamonic of a Qswitched ruby Iaser (frequency ft = 28800 cm-f)_ The przCiiCa8y gaussian light pulses have a halfwidth durapeIykne
We have studied the benzophe~one-pe~~ene pair in a vitreous medium (a mixture of chloroform-ethylether, four to one in volume, at 77 K). The spectro-
tion of about 10 ns. The emission intensities were with a 55 UVP (RTC) photomultiplier. The experimental results are shown in fig. 2. If Wf2put I’(t) = ~~&~~~(~~~, (1) can be rewritten as:
scopic study of the observed luminescence is realised by means of a mechanical phosphoroscope (variable rotating disc-type) IS]. We feund two emissions: the benzophenone phosphorescence and also a delayed fluorescence of perylene with the same decay time. From the relative variations Lntensity
a transfer
in regard
lOgf(t)
of the two emissions’
to the concentration.
mxsured
= -k
+G
-
2
we can deduce
.
mechanism between the’tripletstate of ben-
zophenoneand the singletsfate of pexylene(fig. 1). ‘Ike decrease of the decay time of .Shc two emissions in regard to the perylene concentration increasing in the media shows clearly that we are observing a non: radiative resonant transfer T’benzophenone + S$@ne. The presence of radiative transfer (by means of a trivial reabsorption of the emission) prevents a quanti-
33 .20 c ,o. ; g 5
tative study of the radiationless transfer from this relative variation in intensity; bmthe analysis of the luminescence decay time is not affected by this mechanism. This radiative transfer becomes predominant at low concentrations of perylene and explains in this
4 Fig. 2.
3
T:MEcm,;
: ‘.’
7
8
solutions
peryfene, (I) CA = 0,
C2)CA = 2 x t0-4 M. (3) CA = 4 x lo_4 Bf, (4) C;, = 8 X 1O-4 M, (5) CA = 1.6 x 1O-3 M.
,.. .
6
PhosRhorescence decay of benzopheaone
NJ,y2 MI with a concentrstion CA of
case the ~orn~o~ls important e~ssio~,of.~e acceptor. We have measured the luminescence decay.time of
‘.
2
,. :
.:. 445
., .,
:,
‘,
..
Volume 29,.rjlimb& >
;
’
CHEhlICAL qHY&CS LETTERS
,,
-‘, ...-.: .>
.
.. .
-. : :,
‘.:
;.
1 December 1974
..: .‘.. : The fik tech of .jhis k&lity is ehsil~~cal~ulated : ‘from the experk&l results. We obtain by a comparison b‘:tween the v(f) functions (fig. .3) and the .: calculated curves for different values of the “dritical co~centr:~ti~n” Co,-acting as a $ar&net& (we follow : a Seth&. :ecentIy_%sed by ,&$ataga191) C’Q3 1.2 X. .. 113-.~M and therefore the “kitical radius” IQ-, =!68 A. ’ Tliis distilrrce is in agreement &th results obtained for other pairs i&olving a resonant Geigy transfer. .: References
-,‘.
:.
0.5
. . .
.
‘I “’
-‘t/Y,
a.
1;s :’
Fig. 3. &&rison”~f experiment& and &xla~d v&es of the parameter-v(t) for different concentiations
Co=1x~0-3M,(2)Co=1.2~!0-3M,(3)Co=1.4x’ 1073 M. ,-, or.
:
-CA
.,.. ..
[lf .A.A. Lamola,and NJ. Turro, Energy transfer and orgakc photc chemistry (Wiley, New York, 1969). [2] k. M&g+ and T. Kubota, Molecular interactions and ele-ctronicspectra (Deklter, New.York, 1970). [3] T. FSrster, Discussions FYadiy Sac. 27 (1959) 7,. [4] D.L. IDexter, I:Chem. Phys. 21 (1953) 836. [S] G,W.
Robinson and R.P. Frosch, J. C&em. Phys. 38, (1969) 1187, ” ‘. [6] V.L. Ermblaev and E.B. Sveshnikova, Akad..Nauk USSR ?73. [7] R.G. Ber&ert, R.P. Schwenker
HogI’
.:
,’ ‘,
Bull. Phys. 26 (1962) 29; Soviet Phys. Doklady 8 (1963)
...
tlS&bo17~
‘.
‘.
and R.E. kellog& J. Chem. Fhys 41 (1964) 3&O. [8] Y. RouTset, F._Dupuy and e. Lo&et, J. Phys. Appl. 3 (196:3) 250. {9] N. Mntaga, Chbm. Fhys. L&&20 (1973) 376. 1