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Triplet excimer emission from pyrene single crystals
CHEMICAL PHYSICSLETTERS
Volume 5. number 5
TRIPLET
EXCIMER
EMISSION
0. L. J. GIJZEMAN. Labouatoq
FROM
1.5 April I970
PYRENE
SINGLE
CRYSTALS
.I. LANGELAAR and J. D. W. VAN VOORST
for Physical
Chemislvy.
Anzst.crdam,
7%~
Unircvsify
of Anlsteudunl.
ZVcthcrionds
Received 6 March 1970
Two nelv. but distinct delayed emissions nre found from pyrene single crystals in the temperature gion 77O- 300°K. which arc nttributed to the emission of two types of triplet escimer.
The following results were obtained: 1. At room temperature a broad and structureless delayed emission band is present around
1. INTRODUCTION Aromatic grouped
crystals
in pairs,
in which molecules
parallel
to each
other
are a@
re-
with
21 800 cm-l_
This
band.
which
is identical
to
relatively small interplanar distances (3.5A), are known to give rise to singiet excimer fluo-
the ‘direct’ excimer fluorescence reported by Birks [4] and Fer&uson [I], cari be assigned
rescence
to the delayed
[l,
21. In principle
offer possibilities emission. Pyrene
these
of perceiving
crystals
triplet
also
excimer
[3] was chosen to study this
effect. 2: EXPERIMENTAL The crystals used in our experiments were small flakes, grown by sublimation from highly purified pyrene. For powdered pyrene, preliminary measurements at room temperature on the lifetime of the prompt excimer fluorescence, using a frequency doubled ruby laser pulse for excitation,
yielded
values
of 109 * 10 nanosec-
onds. The observed lifetime indicated a high purity as has been established by Birks [4]. The emission spectra were recorded using the spectrophosphorimeter described elsewhere [5]. Lifetime measurements in the millisecond range were done with the aid of a Varian C-1024 time averaged computer. 3. RESULTS The delayed emission and measured
spectra,
shown in fig. 1
with the two choppers
out of phase
[5], are representative sitivity
mator.
fluorescence
of pyrene.
2. Under the same tureless
studied.
excimer
caused by the annihilation of two triplets. This delayed emission has also been found in fIuid solutions [6 J.
for the crystal flakes The spectra are corrected for the sen-
of the detector
and analysing
monochro-
conditions a we& and strucband is observed at 13 800 cm-l_
3. Upon lowering the temperature a new, but structured emission is gradually seen to arise with pronounced maxima at 15 200, 14 800 and 14400 cm-l. At the same time some structure is developed in the 13 000 - 14 000 cm-l legton. Together with the appearance and increase of the emission, the intensity of the delayed fluorescence is seen to decrease with respect to the red emission.
4. Lifetime measurements
were performed at
different temperatures on the bands at 21800, 15 200 and 13 800 cm-l. The results for a representative crystal are shown in fig. 2. Within experimental error the lifetime of the 15 200 cm-1 band, when present, was always equal to the lifetime of the 13 800 cm-l band. 5. There appears to be a distinct reLation between the lifetime of the delayed excimer fluorescence and the emission at 13 800 cm-1 (and also at 15 200 cm-Ii. This is illustrated in fig. 3, where the correIation between the lifetime of the delayed excimer fluorescence (horizontal axis) and the lifetime of the 13800 cn.~-~ band (vertical axis) is shown.
The line is drawn with slope 2. The insert
shows the correlation
between the lifetimes
of 269
Volume 5. number 5
CHEMICAL PHYSICS
LETTERS
1,213
15 April
1970
‘K B
A
I 13
I
I
I
14
15
16
,/I ’
I
16
’
I
I 20
I,,
fyL;\
,
22
24-
13
14
15
16
T:173*K
18
20
,
,
22
24
D
I
Xl
,:;.:,;;;,
,
,
,,_‘_
CM-‘x10-’
,
,
13
14
Fig.
\ 18
1. Delnyeci emission
spectr:l
20
(corrected)
22
_ --,-
24
from
pyrcnc
13
single
14
crystal
15
flakes.
16
The
increase
‘red region with respect to the blue is indicated in each spectrum. 800 and 15
-----____
16
20
in sensitivity
22
24
for the
cm-1
6.
,>I_ fig.
0
0
/;
f
-x--_x-
-xz
I
lx
0
\
-150
-100
’
t +.; -2 I
-50
1,
where
only
intensities
within
one
figure
should be compared_) 7. Besides the emission bands as shown in fig. 1 no other delayed luminescence could be observed. 8. The quantum yield of the phosphorescence emission at 93’K (fig. 1 d) was estimated to be in the order of 10-l.
0
D
“x\ ’
‘\\
\+\ --
lC
_I
4. DISCUSSION
0
The structured emission, emerging at lower temperature may be attributed to crystal phosphorescence in view of the following facts:
270
Volume
5, number
5
CHEMICAL
PHYSICS
LETTERS
15 April 1970
PY RENE
lK
T-93
=
. .
t
tJ .
/
-
2-
._
:
-
*
=2
:
2r
I I
.*
0
I 2
0
______
x x x x
x
‘/ ii .
.
Y
x
%
4?L ____ 4
I_______1 2
/
z oEF(msec) -
I 4
I 6
13
Fig. 3. Correlation between the lifetimes measured at 13 800 cm-l (71) and 21800 cm-l (7~ E F ). The line
dr;rxr-n with slorxe 2.
Insert:
correlnti&vbe’trveen
Iine drawn
hns slope
14
5x lo-%
Iife-
PYRCNE IN ETliANOL
times mensurdd at 13800 cm-l (71) and 15200 cm-l (1-2). The
? I’
CRYSTAL
SINGLE
T.90.K
1.
1. The delayed excimer fluorescence
intensity decreases upon lowering the temperature, whereas the intensity of the structured emission increases.
This
suggests,
tured emission originates as less triplets annihilate
that the struc-
in the triplet state,
to form delayed extimer fluorescence. 2. Since delayed excimer fluorescence intensity depends upon the square of the triplet exciton
concentration and phosphorescence intensity is linearly related to the number of triplet excitons present, a 1 : 2 ratio in lifetimes is expected [7-g]. This has actually been observed (see fig. 3). 3. The structured spectrum emitted by the crystal looks very similar to the pyrene monomer phosphorescence as measured in fluid and glassy solutions [?I*. This is illustrated in fig. 4. However, one cannot attribute this emission to the triplet exciton for the following reasons. * An excerpt of this J. Chem.
Phys.
thesis
is
to be published
in the
~~ /
I
ch4-’ xia-3 1
15
\ I
16
I
17
\
,
18
4. The phosphorescence of pyrene in ethanoL and the delayed crystal emission spectra. Note the red shift of the crystal spectrum. Fig.
Although similar in appearance, the emission from the crystal is shifted over 1700 cm-l to the red as compared to the phosphorescence spectra in fluid and glassy solutions. This is more 271
Volume 5. number 5
CHEIWCAL
PHYSICS
ever found in triplet exciton phosphorescence [9, lo]. Secondly Avakian and Abramson [ll] have shown that the S - T absorption in the crystal has the same location as that found by the heavy atom effect in fluid solutions [12]. On the other hand the decrease in the ilitensity of the delayed fluorescence, due to triplet exciton annihilation, upon increase in intensity of the than
structured
emission
suggests
that the mobjlity
the triplet excitons is decreased lowering become
the temperature important
and that upon
relatively
in trapping
shallow
the triplet
of
traps
excitons.
Such a trap might be the sandwich-like
wavenumbers. which is consistent with the fact that this structured emission can be observed only at Lower temperatures. The major part of the shift can then be attributed to the ground state repulsion of the two unexcited molecules. From direct singlet excimer fluorescence experiments [13] it has been shown that this ground state repulsion may be as large as 2 8’70 cm-l. In anthracene or naphthalene [9, lo], with different crystal structures, this trapping situation cannot occur, hence no appreciable shift is observed; At higher temperatures (> ZOOoK) we observe a broad emission band around 23 800 cm-l_ It is doubtless present at lower temperatures too, but overlaps the structured spectrum. (Compare figs. 1 a-d.) We assign this broad band to a different kind of triplet excimer than the one that causes the structured emission. The broadness of the band can be explained on the basis of lattice defects. Two neighbouring molecules may have virtually any conformation in such a defect, thus leading to differences in excimer stabilisation energy and ground state repulsion for suchlike pairs. The broadness of the band is then caused by the overlap of many lines,
pair.
272
each originating
in a particular
The fact that the emission
sandwich
is relatively
in-
LETTERS
15 April 1970
sensitive to temperature effects supports this interpretation. At any temperature all traps (lattice defects) are completely filled. This requires a stabiiisation energy of only some 300 cm-l. Alternatively, one could try to attribute this band to an emission from a state in which the ex-
citation exchange between is fast,
compared
to the intramolecular
molecules
vibration
frequency. This might explain the lack of structure_ It implies, however, a Large stabilisation energy of the triplet excimer (more than. say 1000 cm-l) which we think is unlikely. We conclude our discussion by remarking that for powdered pyrene at lower temperatures (s: 200oK) the same spectra can be observed. Even at liquid helium temperature the delayed red emission remains relatively unchanged. Above 200’K the delayed emission is almost indetectable owing to the very short triplet lifetime. A further study is being carried out in our laboratory. The investigations the Netherlands
were supported in part by
Foundation
for
Chemical
Re-
search (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). REFERENCES [l] J. Feryson, J. Chum. Phys. 28 (1958) iG5. [21 B. Stevens. Spcctrochim. dcta 15 (1962) 439. (31 J. 11. Robertson and J. G.White. J. Chem. Sot. (19+7) 358. [A] J. 3. Birlis. A. A. Liazznz and T. A. King. Proc. Sot. AZ91 (1966)556. [5JJ.Langelaar, G. A.de Yries and D.Bebelnar, Sci.Instr. 46 (1969) 149.
Roy.
J.
16) G. F. Moore and I. H. Munro, Spcctrochim. Acta
23.1 (1966) 1291. 171 J. Langelanr. Thesis, University of Amsterdam (1969). [8] B. Stevens and hl. S. IVnlker. Proc. Chem. Sot. (19G4)26: Proc. Roy. Sot. A281 (1964) 420. 191 G. C.Smith, Phys.Rcv. 166 (1968) 839. [lo] E.B. Priestley and A. Haug. J. Chem. Phys. 49 (1968) 622. 1111P. Avakinn nnd E. Abramson, J. Chem. Phys. 48 (196Sj 821. [12] C. Dijkgmaf, Thesis, University of Amsterdam (1962). I131 J. B. Birks and A. A. Kazzaz. Chem. Phys_ Letters 1 (136s) 307.
Volume 5. number 5
TRIPLET
EXCIMER
EMISSION
0. L. J. GIJZEMAN. Labouatoq
FROM
1.5 April I970
PYRENE
SINGLE
CRYSTALS
.I. LANGELAAR and J. D. W. VAN VOORST
for Physical
Chemislvy.
Anzst.crdam,
7%~
Unircvsify
of Anlsteudunl.
ZVcthcrionds
Received 6 March 1970
Two nelv. but distinct delayed emissions nre found from pyrene single crystals in the temperature gion 77O- 300°K. which arc nttributed to the emission of two types of triplet escimer.
The following results were obtained: 1. At room temperature a broad and structureless delayed emission band is present around
1. INTRODUCTION Aromatic grouped
crystals
in pairs,
in which molecules
parallel
to each
other
are a@
re-
with
21 800 cm-l_
This
band.
which
is identical
to
relatively small interplanar distances (3.5A), are known to give rise to singiet excimer fluo-
the ‘direct’ excimer fluorescence reported by Birks [4] and Fer&uson [I], cari be assigned
rescence
to the delayed
[l,
21. In principle
offer possibilities emission. Pyrene
these
of perceiving
crystals
triplet
also
excimer
[3] was chosen to study this
effect. 2: EXPERIMENTAL The crystals used in our experiments were small flakes, grown by sublimation from highly purified pyrene. For powdered pyrene, preliminary measurements at room temperature on the lifetime of the prompt excimer fluorescence, using a frequency doubled ruby laser pulse for excitation,
yielded
values
of 109 * 10 nanosec-
onds. The observed lifetime indicated a high purity as has been established by Birks [4]. The emission spectra were recorded using the spectrophosphorimeter described elsewhere [5]. Lifetime measurements in the millisecond range were done with the aid of a Varian C-1024 time averaged computer. 3. RESULTS The delayed emission and measured
spectra,
shown in fig. 1
with the two choppers
out of phase
[5], are representative sitivity
mator.
fluorescence
of pyrene.
2. Under the same tureless
studied.
excimer
caused by the annihilation of two triplets. This delayed emission has also been found in fIuid solutions [6 J.
for the crystal flakes The spectra are corrected for the sen-
of the detector
and analysing
monochro-
conditions a we& and strucband is observed at 13 800 cm-l_
3. Upon lowering the temperature a new, but structured emission is gradually seen to arise with pronounced maxima at 15 200, 14 800 and 14400 cm-l. At the same time some structure is developed in the 13 000 - 14 000 cm-l legton. Together with the appearance and increase of the emission, the intensity of the delayed fluorescence is seen to decrease with respect to the red emission.
4. Lifetime measurements
were performed at
different temperatures on the bands at 21800, 15 200 and 13 800 cm-l. The results for a representative crystal are shown in fig. 2. Within experimental error the lifetime of the 15 200 cm-1 band, when present, was always equal to the lifetime of the 13 800 cm-l band. 5. There appears to be a distinct reLation between the lifetime of the delayed excimer fluorescence and the emission at 13 800 cm-1 (and also at 15 200 cm-Ii. This is illustrated in fig. 3, where the correIation between the lifetime of the delayed excimer fluorescence (horizontal axis) and the lifetime of the 13800 cn.~-~ band (vertical axis) is shown.
The line is drawn with slope 2. The insert
shows the correlation
between the lifetimes
of 269
Volume 5. number 5
CHEMICAL PHYSICS
LETTERS
1,213
15 April
1970
‘K B
A
I 13
I
I
I
14
15
16
,/I ’
I
16
’
I
I 20
I,,
fyL;\
,
22
24-
13
14
15
16
T:173*K
18
20
,
,
22
24
D
I
Xl
,:;.:,;;;,
,
,
,,_‘_
CM-‘x10-’
,
,
13
14
Fig.
\ 18
1. Delnyeci emission
spectr:l
20
(corrected)
22
_ --,-
24
from
pyrcnc
13
single
14
crystal
15
flakes.
16
The
increase
‘red region with respect to the blue is indicated in each spectrum. 800 and 15
-----____
16
20
in sensitivity
22
24
for the
cm-1
6.
,>I_ fig.
0
0
/;
f
-x--_x-
-xz
I
lx
0
\
-150
-100
’
t +.; -2 I
-50
1,
where
only
intensities
within
one
figure
should be compared_) 7. Besides the emission bands as shown in fig. 1 no other delayed luminescence could be observed. 8. The quantum yield of the phosphorescence emission at 93’K (fig. 1 d) was estimated to be in the order of 10-l.
0
D
“x\ ’
‘\\
\+\ --
lC
_I
4. DISCUSSION
0
The structured emission, emerging at lower temperature may be attributed to crystal phosphorescence in view of the following facts:
270
Volume
5, number
5
CHEMICAL
PHYSICS
LETTERS
15 April 1970
PY RENE
lK
T-93
=
. .
t
tJ .
/
-
2-
._
:
-
*
=2
:
2r
I I
.*
0
I 2
0
______
x x x x
x
‘/ ii .
.
Y
x
%
4?L ____ 4
I_______1 2
/
z oEF(msec) -
I 4
I 6
13
Fig. 3. Correlation between the lifetimes measured at 13 800 cm-l (71) and 21800 cm-l (7~ E F ). The line
dr;rxr-n with slorxe 2.
Insert:
correlnti&vbe’trveen
Iine drawn
hns slope
14
5x lo-%
Iife-
PYRCNE IN ETliANOL
times mensurdd at 13800 cm-l (71) and 15200 cm-l (1-2). The
? I’
CRYSTAL
SINGLE
T.90.K
1.
1. The delayed excimer fluorescence
intensity decreases upon lowering the temperature, whereas the intensity of the structured emission increases.
This
suggests,
tured emission originates as less triplets annihilate
that the struc-
in the triplet state,
to form delayed extimer fluorescence. 2. Since delayed excimer fluorescence intensity depends upon the square of the triplet exciton
concentration and phosphorescence intensity is linearly related to the number of triplet excitons present, a 1 : 2 ratio in lifetimes is expected [7-g]. This has actually been observed (see fig. 3). 3. The structured spectrum emitted by the crystal looks very similar to the pyrene monomer phosphorescence as measured in fluid and glassy solutions [?I*. This is illustrated in fig. 4. However, one cannot attribute this emission to the triplet exciton for the following reasons. * An excerpt of this J. Chem.
Phys.
thesis
is
to be published
in the
~~ /
I
ch4-’ xia-3 1
15
\ I
16
I
17
\
,
18
4. The phosphorescence of pyrene in ethanoL and the delayed crystal emission spectra. Note the red shift of the crystal spectrum. Fig.
Although similar in appearance, the emission from the crystal is shifted over 1700 cm-l to the red as compared to the phosphorescence spectra in fluid and glassy solutions. This is more 271
Volume 5. number 5
CHEIWCAL
PHYSICS
ever found in triplet exciton phosphorescence [9, lo]. Secondly Avakian and Abramson [ll] have shown that the S - T absorption in the crystal has the same location as that found by the heavy atom effect in fluid solutions [12]. On the other hand the decrease in the ilitensity of the delayed fluorescence, due to triplet exciton annihilation, upon increase in intensity of the than
structured
emission
suggests
that the mobjlity
the triplet excitons is decreased lowering become
the temperature important
and that upon
relatively
in trapping
shallow
the triplet
of
traps
excitons.
Such a trap might be the sandwich-like
wavenumbers. which is consistent with the fact that this structured emission can be observed only at Lower temperatures. The major part of the shift can then be attributed to the ground state repulsion of the two unexcited molecules. From direct singlet excimer fluorescence experiments [13] it has been shown that this ground state repulsion may be as large as 2 8’70 cm-l. In anthracene or naphthalene [9, lo], with different crystal structures, this trapping situation cannot occur, hence no appreciable shift is observed; At higher temperatures (> ZOOoK) we observe a broad emission band around 23 800 cm-l_ It is doubtless present at lower temperatures too, but overlaps the structured spectrum. (Compare figs. 1 a-d.) We assign this broad band to a different kind of triplet excimer than the one that causes the structured emission. The broadness of the band can be explained on the basis of lattice defects. Two neighbouring molecules may have virtually any conformation in such a defect, thus leading to differences in excimer stabilisation energy and ground state repulsion for suchlike pairs. The broadness of the band is then caused by the overlap of many lines,
pair.
272
each originating
in a particular
The fact that the emission
sandwich
is relatively
in-
LETTERS
15 April 1970
sensitive to temperature effects supports this interpretation. At any temperature all traps (lattice defects) are completely filled. This requires a stabiiisation energy of only some 300 cm-l. Alternatively, one could try to attribute this band to an emission from a state in which the ex-
citation exchange between is fast,
compared
to the intramolecular
molecules
vibration
frequency. This might explain the lack of structure_ It implies, however, a Large stabilisation energy of the triplet excimer (more than. say 1000 cm-l) which we think is unlikely. We conclude our discussion by remarking that for powdered pyrene at lower temperatures (s: 200oK) the same spectra can be observed. Even at liquid helium temperature the delayed red emission remains relatively unchanged. Above 200’K the delayed emission is almost indetectable owing to the very short triplet lifetime. A further study is being carried out in our laboratory. The investigations the Netherlands
were supported in part by
Foundation
for
Chemical
Re-
search (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). REFERENCES [l] J. Feryson, J. Chum. Phys. 28 (1958) iG5. [21 B. Stevens. Spcctrochim. dcta 15 (1962) 439. (31 J. 11. Robertson and J. G.White. J. Chem. Sot. (19+7) 358. [A] J. 3. Birlis. A. A. Liazznz and T. A. King. Proc. Sot. AZ91 (1966)556. [5JJ.Langelaar, G. A.de Yries and D.Bebelnar, Sci.Instr. 46 (1969) 149.
Roy.
J.
16) G. F. Moore and I. H. Munro, Spcctrochim. Acta
23.1 (1966) 1291. 171 J. Langelanr. Thesis, University of Amsterdam (1969). [8] B. Stevens and hl. S. IVnlker. Proc. Chem. Sot. (19G4)26: Proc. Roy. Sot. A281 (1964) 420. 191 G. C.Smith, Phys.Rcv. 166 (1968) 839. [lo] E.B. Priestley and A. Haug. J. Chem. Phys. 49 (1968) 622. 1111P. Avakinn nnd E. Abramson, J. Chem. Phys. 48 (196Sj 821. [12] C. Dijkgmaf, Thesis, University of Amsterdam (1962). I131 J. B. Birks and A. A. Kazzaz. Chem. Phys_ Letters 1 (136s) 307.