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Absorption spectrum of pyrene in the excited singlet state measured by the flash method
ABSUK~“‘I‘IUN
IN
THE
MEASURED
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I
EXCITED BY
LLU~~
va
a
SINGLET THE
FLASH
a____..
STATE METHOD
Y. NAKATO, N. YAMAMOTO and H. TSUBOMTYRA Department of Ckemistry , Faculty of Engineering Science, Osaka University. Toyonaka. Osaka, Japan Received 1 April 1968
An absorption spectrum attributable to the transition from the first excited singlet state to higher singlet states (S’ Q--S transition) of pgrene in soIution has been observed by the flash method.
Very recently Novak and Windsor reported that they developed a laser photolysis and spectroscopy apparatus in the nanosecond time range and observed an absorption spectrum from the first excited singlet state (S’ - S absorption) in coronene [l]. We report here that by the ordinary flash-spectrographic technique we have similarly observed an absorption spectrum which is probably caused by the transition from the lowest excited singlet state of pyr?:. The flash apparatus used in this experiment was the same as described in a previous paper [2]. The transient absorption spectrum was measured photographically with Fuji Neopan SSS film (ASA 200) and was recorded by using a microphotometer. The duration time of the photolysis-flash was 4 psec and that of the spectroflash was 3 ~.~sec or less. It is found from actinometric measurements that the photolysis-flash emits about 1017 - 1018 photons/cm2 at the surface of the sample cell in the region between 300 rnp and 400 m/-L.The fluorescence lifetime of pyrene was measured by Mataga, Okada and Yamamoto to be 380 nanosec in n-hexane [3]. The quartz cell used was 20 cm long and 1.5 cm in diameter. From these data, the number of the pyrene molecules in the lowest excited singlet state at the peak of the photolysis-flash can be estimated to be around 1016 - 1017 per l-cm cell length since it is reasonably assumed that all photons were absorbed by pyrene. Under the supposition that the excited species is formed uniformly in the cell, its concentration becomes 10-5 - IO-4 mol/l. With this concentration, the optical density pf the S’ - S spectrum may reach 2.0 or more if its molecular extinction coefficient
is around IO4 1 mol’l cm-l. Thus it may be understood that all the conditions are sufficient to obtain the S’+ S absorption spectrum of pyrene. A result obtained for a 10-4 mol/l deoxygenated liquid paraffin solution of pyrene is shown in fig. 1. Viscous liquid paraffin was chosen as
fi .:
~
~;~~.__
350
iI
450
550
/L-_-A
350
(1
450 WAVELENGTH
550 ( rry 1
Fig. 1. (A) shows transient absorption spectra obtained by the flash illumination of pyrene: (a) is for the cnse of no delay between the photolpsis- and the spectroflashes, and (b) for the case where the delay between both flashes was 10 pet. (B) shows :he new bands which are probably caused by the S’ - S absorption. 57
.
...”
“_“I
. .
. .
..a.#
_.A..
L--e..,
.s”-..---*
II--.
a--1-.-1
was 10 psec or longer. Curve (b) may be regarded to represent essentially the T' T absorption of pyrene and it agrees with the T’ +-. T spectrum obtained by Porter and Windsor and by Craig and Ross [4], while curve (a) contains some additional bands in the regions around 470 rnp and 370 rnp. These new bands are shown more clearly in fig. 1 (B), obtained by expanding curve (a) so that it has the same height at 520 rnp as that of curve (b) and by taking the difference between it and curve (b). The absorption maximum at 370 rnp is made a Iittle ambiguous by the ground state absorption starting at 375 rnp, because the concentration of the ground state pyrene is reduced while pyrene is excited to the metastable states. Since the intensity of the new absorption bands runs parallel with the intensity profile of the photolysis flash light, it can be concluded that the decay time of the intermediate is less than 1 ,usec. This finding is in harmony with our conl-
clusion
58
that these
new bands
are due to the
.,__ =-----
- -,
=----D_
---_
__~_‘
__________
_-
-___
ca gel and alumina, foilowed by vacuum sublimation and further purified by zone melting. The flash light in the wavelength region shorter than 310 rn~ was cut with a glass filter. The authors are greatly indebted to Prof. N. Mataga, Mr. Kobashi and Mr. Oohari for gifts of very purified pyrene.
Refel*ences [ll J. R. Novak and M. W. Windsor. J. Chem.Phys. 47 (1967) 3075. 121 N. Yamamoto. Y. Nakato and H. Tsubomura. Bull. Chem.Soc. Japan 39 (1966) 2603. 131 N. Mataga. T. Okada and N. Yamamoto. Chem. Phys Letters 1 (1967) 119. [4] G. Porter and M. W. Windsor. Proc. Roy.Soc. (London) A245 (1958) 238: D. P. Craig and I. G. Ross. J. Chem.Soc. (1954) 1589.
IN
THE
MEASURED
3~5~
I
EXCITED BY
LLU~~
va
a
SINGLET THE
FLASH
a____..
STATE METHOD
Y. NAKATO, N. YAMAMOTO and H. TSUBOMTYRA Department of Ckemistry , Faculty of Engineering Science, Osaka University. Toyonaka. Osaka, Japan Received 1 April 1968
An absorption spectrum attributable to the transition from the first excited singlet state to higher singlet states (S’ Q--S transition) of pgrene in soIution has been observed by the flash method.
Very recently Novak and Windsor reported that they developed a laser photolysis and spectroscopy apparatus in the nanosecond time range and observed an absorption spectrum from the first excited singlet state (S’ - S absorption) in coronene [l]. We report here that by the ordinary flash-spectrographic technique we have similarly observed an absorption spectrum which is probably caused by the transition from the lowest excited singlet state of pyr?:. The flash apparatus used in this experiment was the same as described in a previous paper [2]. The transient absorption spectrum was measured photographically with Fuji Neopan SSS film (ASA 200) and was recorded by using a microphotometer. The duration time of the photolysis-flash was 4 psec and that of the spectroflash was 3 ~.~sec or less. It is found from actinometric measurements that the photolysis-flash emits about 1017 - 1018 photons/cm2 at the surface of the sample cell in the region between 300 rnp and 400 m/-L.The fluorescence lifetime of pyrene was measured by Mataga, Okada and Yamamoto to be 380 nanosec in n-hexane [3]. The quartz cell used was 20 cm long and 1.5 cm in diameter. From these data, the number of the pyrene molecules in the lowest excited singlet state at the peak of the photolysis-flash can be estimated to be around 1016 - 1017 per l-cm cell length since it is reasonably assumed that all photons were absorbed by pyrene. Under the supposition that the excited species is formed uniformly in the cell, its concentration becomes 10-5 - IO-4 mol/l. With this concentration, the optical density pf the S’ - S spectrum may reach 2.0 or more if its molecular extinction coefficient
is around IO4 1 mol’l cm-l. Thus it may be understood that all the conditions are sufficient to obtain the S’+ S absorption spectrum of pyrene. A result obtained for a 10-4 mol/l deoxygenated liquid paraffin solution of pyrene is shown in fig. 1. Viscous liquid paraffin was chosen as
fi .:
~
~;~~.__
350
iI
450
550
/L-_-A
350
(1
450 WAVELENGTH
550 ( rry 1
Fig. 1. (A) shows transient absorption spectra obtained by the flash illumination of pyrene: (a) is for the cnse of no delay between the photolpsis- and the spectroflashes, and (b) for the case where the delay between both flashes was 10 pet. (B) shows :he new bands which are probably caused by the S’ - S absorption. 57
.
...”
“_“I
. .
. .
..a.#
_.A..
L--e..,
.s”-..---*
II--.
a--1-.-1
was 10 psec or longer. Curve (b) may be regarded to represent essentially the T' T absorption of pyrene and it agrees with the T’ +-. T spectrum obtained by Porter and Windsor and by Craig and Ross [4], while curve (a) contains some additional bands in the regions around 470 rnp and 370 rnp. These new bands are shown more clearly in fig. 1 (B), obtained by expanding curve (a) so that it has the same height at 520 rnp as that of curve (b) and by taking the difference between it and curve (b). The absorption maximum at 370 rnp is made a Iittle ambiguous by the ground state absorption starting at 375 rnp, because the concentration of the ground state pyrene is reduced while pyrene is excited to the metastable states. Since the intensity of the new absorption bands runs parallel with the intensity profile of the photolysis flash light, it can be concluded that the decay time of the intermediate is less than 1 ,usec. This finding is in harmony with our conl-
clusion
58
that these
new bands
are due to the
.,__ =-----
- -,
=----D_
---_
__~_‘
__________
_-
-___
ca gel and alumina, foilowed by vacuum sublimation and further purified by zone melting. The flash light in the wavelength region shorter than 310 rn~ was cut with a glass filter. The authors are greatly indebted to Prof. N. Mataga, Mr. Kobashi and Mr. Oohari for gifts of very purified pyrene.
Refel*ences [ll J. R. Novak and M. W. Windsor. J. Chem.Phys. 47 (1967) 3075. 121 N. Yamamoto. Y. Nakato and H. Tsubomura. Bull. Chem.Soc. Japan 39 (1966) 2603. 131 N. Mataga. T. Okada and N. Yamamoto. Chem. Phys Letters 1 (1967) 119. [4] G. Porter and M. W. Windsor. Proc. Roy.Soc. (London) A245 (1958) 238: D. P. Craig and I. G. Ross. J. Chem.Soc. (1954) 1589.