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Phosphorescence of aromatic hydrocarbons in the vapour phase
Volume 13. number 6
CHEMICAL PHYSICS LETTERS
PHOSPHORESCENCE
OF AROMATIC J3YDROCARBONS
W.H. VAN LEEUWEN, J. LANGELAAR for Physicd
Chemistry.
Univem’ty
15 April 1972
IN THE VAPOUR PHASE
and J.D.W. VAN VOORST
of Amsterdam,
Amsterdam,
The Netherlands
Received 14 January 1972
Phosphorescence from pyrene and naphthalene in the vapour phase has been obsrved. The lifetime and quantum between 0.04 and 10 torr.
yields are studied
1. introduction Investigations of the phosphorescence of aromatic hydrocarbons are generally performed at low temperatures in rigid matrices, because non-radiative processes dominate the decay of triplet states under all other conditions. In the last few years phosphorescence of some aromatic hydrocarbons in pure molecular crystals [I ,2] and in fluid solutions [3,43 has been observed. To our knowledge phosphorescence of these molecules in the vapour phase has not been published as yet, although delayed fluorescence [S] and T-T absorption experiments [63 in the gas phase indicated that the triplet lifetime is about 1 msec for most aromatic hydrocarbons. The reason for this lack can be due to an extremely low phosphorescence quantum yield in the gas phase so that a gas phase phosphorescence study may only lead to results if suitable equipment is available.
3I. ‘ExperirnentaJ Gas phase phosphorescence experiments were carried out with the spectrophosphorirneter described elsewhere [7] , which has proven to be suitable for the measurement of low qu,antum yield emissions [4,8-101. For the extreme weak signals the phasesensitive detection system was extended with a pulsequantizer (PAR). Pyrene was excited at 31400 em-l; naphthalene at 36 300 cm-l. However, for some experiments with naphthalene we used the frequency 622
doubled green line (2573 A = 38 900 cm-L) of an argon-ion laser instead of a xenon arc. With this equipment we were able to observe the gas phase phosphorescence of pyrene down tc? 150°C (4 X 10e2 torr) and of naphthalene down to 20°C (4 X 10e2 torr). For the experiments we used vacuum isolated rectangular quartz cuvettes with an excess of aromatic molecules heated in a two-level oven similar to the one used by other investigators [ 11, 121. The phosphorescence quantum efficiencies were measured relative to the fluorescence yields taken from the literature.
3.
Results and discussion
The gas phase phosphorescence spectra of pyrene 10m2 torr) and naphthalene (8 X IO-1 torr) are given in figs, 1 and 2 respectively. The spectra are corrected for spectrometer sensitivity. The correction of the spectra in the red part (dotted lines) is not very reliable due to the low signal-to-noise ratio as a result of the low detector sensitivity (see insert fig. 1). For comparison the peaks of the emission obtained in a rigid matrix 14, 131 are given in the same figure. The additional bands at 17 350 cm-r and 17 750 cm-l in the phosphorescence spectrum of pyrene are due to hot band emission. Due to the low phosphorescence intensity, the resolution in the gas phase experiments is restricted by the bandwidth of the detection monochromator. The values of the triplet state lifetimes observed (4 X
Volume
13, number
CHEMICAL
6
PHYSKS
LETTERS
1.5 April 1973
Fig. 2. Corrected phosphorescence spectrum of naphthalene at 70°C and at a concentration of 3.8 X 10s5 mole/l (= 0.8 torr). The vertical lines indicate the band positions in a rigid matrix [ 13 1.
Fig. 1. Phosphorescence spectrum of pyrene vapor at 170°C and concentration of 1.4 X 10m6 mole/l (= 0.04 ton). Above: recorded spectrum, below: .ume spectrum after correction. The phosphorescence band position: in a rigid matrix are indicated by vertical tines 141.
from the phosphorescence emission are in close agreement with the resclts obtained by T-T absorption experiments under similar conditions, but with a low pressure of a foreign gas [6]. Using a value of 0.9 for the fluorescence efficiency obtained in liquid solutions
[4],
the phosphorescence
quantum
yield
for pyrene can be calculated to be 2 X 10m6 at 17O’C and a concentration of 1.4 X 10B6 moie/l. The lifetime is measured to be 3 msec and is, within the experimental accuracy, not or only slightly dependent on temperature (ISO-240°C) and concentration (10-G-3 x 10-a mole/I; i.e., pressure: 0.04-10 torr). For naphthalene similar results are obtained. The phosphbrescence quantum yield turned out to be 1P6 when excited at 36 300 cm-l and IO-5 at 38 900 cm-l. Here we have used for the calculation a fluorescence efficiency of 0.45 and 0.30 respectively [6]. The lifetime of the triplet state, which was measured to be 0.5 msec at both excitation frequencies and also the phosphorescence yields are within experimental error independent of temperature and concentration. (Ranges: 70°--150°C and 4.5 X IO-S-
5 X 10v4 mole/l for excitation at 36 300 cm-l; 20°-70°C and 2 X 10-6-4.5 X 10v5 mole/l at 38 900 cm-l .) Only at higher concentration @=-3 X IO-4 mole/l) can a considerable decrease be observed due to an increase of the T-T annihilation. From the obtained quantum yields and the lifetimes we calculated for both pyrene and naphthalene a triplet radiative lifetime of the order of 100 set, in agreement with previous results in liquid solutions [4]. We have to realise that, at the pressures used, the molecules in the triplet state are far from isolated (at 4 X 10-Z torr the collision time is about 2 X 10c6 set, i.e., IO3 coilisions during a triplet lifetime). This means that, although the phosphorescence quantum yield and the iifetime of the triplet state are almost independent of concentration, one cannot conclude that the triplet decay is determined by an intramolecular radiationless process 161.
Acknowledgement The authors arc indebted to Messrs. D. Bebelaar and G. Jansen for their assistance during the experiments. The investigations were supported in part by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). 623
V&.rme
13, number6
CHEMICAL
PHYSICS
Refennces (11 D.F.Williams and W.G.Schncider, J. Chem. Phys. 45 (1966) 4756. 121. E.B.Priestley and A.Haug, J. Chem. Phys 49 (1968) 622. [ 31 C.A.Parker, Photoluminescence of solutions (Elsevier, Amsterdam, 1968). [4] J,Langelaar, R.P.H.Rettschnick and G.J.Hoytink, J. Chem. Phys 54 (1971) 1. [S] G.Finger and A.B.Zahlan, J. Chem. Phys. 50 (1969) 25. [6] S.J.Formosinho,Thesi; University of London (1971). [7 J J.Langehar, G.A.de Vries and D.Babelaar, J. Sci Instr. 46 (1969) 149.
LETTERS
15 April 1972
jsy P&MM&f, R.p.>J.Xett &XI C.J.IfO~ti& C&iT. Phys Letters 4 (1969) 59. [9] C.J.hf.Brugman, R.P.H.Rettsch&k and G.J.Hoytink Chem. Phys Letters 8 (1971) 574. [ 10) O.L.J.Gijzeman, J. Langelaar and J.D.W.van Voorst, Chem. Phys. Letters 5 (1970) 269. E.Bowen and S.Veljkovic, Prac. Roy. Sot. A236 (1956) [ill 1. J. Chem. Phys 43 1121 W.R.Ware and P.T.Cunningham, (1965) 3826. Z. [131 J.CzekaUa, G.Briegleb, W.Herre and H.J.Vahlensieck, Electrochem. 63 (1959) 71.5.
CHEMICAL PHYSICS LETTERS
PHOSPHORESCENCE
OF AROMATIC J3YDROCARBONS
W.H. VAN LEEUWEN, J. LANGELAAR for Physicd
Chemistry.
Univem’ty
15 April 1972
IN THE VAPOUR PHASE
and J.D.W. VAN VOORST
of Amsterdam,
Amsterdam,
The Netherlands
Received 14 January 1972
Phosphorescence from pyrene and naphthalene in the vapour phase has been obsrved. The lifetime and quantum between 0.04 and 10 torr.
yields are studied
1. introduction Investigations of the phosphorescence of aromatic hydrocarbons are generally performed at low temperatures in rigid matrices, because non-radiative processes dominate the decay of triplet states under all other conditions. In the last few years phosphorescence of some aromatic hydrocarbons in pure molecular crystals [I ,2] and in fluid solutions [3,43 has been observed. To our knowledge phosphorescence of these molecules in the vapour phase has not been published as yet, although delayed fluorescence [S] and T-T absorption experiments [63 in the gas phase indicated that the triplet lifetime is about 1 msec for most aromatic hydrocarbons. The reason for this lack can be due to an extremely low phosphorescence quantum yield in the gas phase so that a gas phase phosphorescence study may only lead to results if suitable equipment is available.
3I. ‘ExperirnentaJ Gas phase phosphorescence experiments were carried out with the spectrophosphorirneter described elsewhere [7] , which has proven to be suitable for the measurement of low qu,antum yield emissions [4,8-101. For the extreme weak signals the phasesensitive detection system was extended with a pulsequantizer (PAR). Pyrene was excited at 31400 em-l; naphthalene at 36 300 cm-l. However, for some experiments with naphthalene we used the frequency 622
doubled green line (2573 A = 38 900 cm-L) of an argon-ion laser instead of a xenon arc. With this equipment we were able to observe the gas phase phosphorescence of pyrene down tc? 150°C (4 X 10e2 torr) and of naphthalene down to 20°C (4 X 10e2 torr). For the experiments we used vacuum isolated rectangular quartz cuvettes with an excess of aromatic molecules heated in a two-level oven similar to the one used by other investigators [ 11, 121. The phosphorescence quantum efficiencies were measured relative to the fluorescence yields taken from the literature.
3.
Results and discussion
The gas phase phosphorescence spectra of pyrene 10m2 torr) and naphthalene (8 X IO-1 torr) are given in figs, 1 and 2 respectively. The spectra are corrected for spectrometer sensitivity. The correction of the spectra in the red part (dotted lines) is not very reliable due to the low signal-to-noise ratio as a result of the low detector sensitivity (see insert fig. 1). For comparison the peaks of the emission obtained in a rigid matrix 14, 131 are given in the same figure. The additional bands at 17 350 cm-r and 17 750 cm-l in the phosphorescence spectrum of pyrene are due to hot band emission. Due to the low phosphorescence intensity, the resolution in the gas phase experiments is restricted by the bandwidth of the detection monochromator. The values of the triplet state lifetimes observed (4 X
Volume
13, number
CHEMICAL
6
PHYSKS
LETTERS
1.5 April 1973
Fig. 2. Corrected phosphorescence spectrum of naphthalene at 70°C and at a concentration of 3.8 X 10s5 mole/l (= 0.8 torr). The vertical lines indicate the band positions in a rigid matrix [ 13 1.
Fig. 1. Phosphorescence spectrum of pyrene vapor at 170°C and concentration of 1.4 X 10m6 mole/l (= 0.04 ton). Above: recorded spectrum, below: .ume spectrum after correction. The phosphorescence band position: in a rigid matrix are indicated by vertical tines 141.
from the phosphorescence emission are in close agreement with the resclts obtained by T-T absorption experiments under similar conditions, but with a low pressure of a foreign gas [6]. Using a value of 0.9 for the fluorescence efficiency obtained in liquid solutions
[4],
the phosphorescence
quantum
yield
for pyrene can be calculated to be 2 X 10m6 at 17O’C and a concentration of 1.4 X 10B6 moie/l. The lifetime is measured to be 3 msec and is, within the experimental accuracy, not or only slightly dependent on temperature (ISO-240°C) and concentration (10-G-3 x 10-a mole/I; i.e., pressure: 0.04-10 torr). For naphthalene similar results are obtained. The phosphbrescence quantum yield turned out to be 1P6 when excited at 36 300 cm-l and IO-5 at 38 900 cm-l. Here we have used for the calculation a fluorescence efficiency of 0.45 and 0.30 respectively [6]. The lifetime of the triplet state, which was measured to be 0.5 msec at both excitation frequencies and also the phosphorescence yields are within experimental error independent of temperature and concentration. (Ranges: 70°--150°C and 4.5 X IO-S-
5 X 10v4 mole/l for excitation at 36 300 cm-l; 20°-70°C and 2 X 10-6-4.5 X 10v5 mole/l at 38 900 cm-l .) Only at higher concentration @=-3 X IO-4 mole/l) can a considerable decrease be observed due to an increase of the T-T annihilation. From the obtained quantum yields and the lifetimes we calculated for both pyrene and naphthalene a triplet radiative lifetime of the order of 100 set, in agreement with previous results in liquid solutions [4]. We have to realise that, at the pressures used, the molecules in the triplet state are far from isolated (at 4 X 10-Z torr the collision time is about 2 X 10c6 set, i.e., IO3 coilisions during a triplet lifetime). This means that, although the phosphorescence quantum yield and the iifetime of the triplet state are almost independent of concentration, one cannot conclude that the triplet decay is determined by an intramolecular radiationless process 161.
Acknowledgement The authors arc indebted to Messrs. D. Bebelaar and G. Jansen for their assistance during the experiments. The investigations were supported in part by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). 623
V&.rme
13, number6
CHEMICAL
PHYSICS
Refennces (11 D.F.Williams and W.G.Schncider, J. Chem. Phys. 45 (1966) 4756. 121. E.B.Priestley and A.Haug, J. Chem. Phys 49 (1968) 622. [ 31 C.A.Parker, Photoluminescence of solutions (Elsevier, Amsterdam, 1968). [4] J,Langelaar, R.P.H.Rettschnick and G.J.Hoytink, J. Chem. Phys 54 (1971) 1. [S] G.Finger and A.B.Zahlan, J. Chem. Phys. 50 (1969) 25. [6] S.J.Formosinho,Thesi; University of London (1971). [7 J J.Langehar, G.A.de Vries and D.Babelaar, J. Sci Instr. 46 (1969) 149.
LETTERS
15 April 1972
jsy P&MM&f, R.p.>J.Xett &XI C.J.IfO~ti& C&iT. Phys Letters 4 (1969) 59. [9] C.J.hf.Brugman, R.P.H.Rettsch&k and G.J.Hoytink Chem. Phys Letters 8 (1971) 574. [ 10) O.L.J.Gijzeman, J. Langelaar and J.D.W.van Voorst, Chem. Phys. Letters 5 (1970) 269. E.Bowen and S.Veljkovic, Prac. Roy. Sot. A236 (1956) [ill 1. J. Chem. Phys 43 1121 W.R.Ware and P.T.Cunningham, (1965) 3826. Z. [131 J.CzekaUa, G.Briegleb, W.Herre and H.J.Vahlensieck, Electrochem. 63 (1959) 71.5.