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Triplet-triplet spectra and triplet quantum yields of some aromatic hydrocarbons in liquid solution
TRIPLET-TRIPLET OF
15 September 1969
CHEMICAL PHYSICS LETTERS
Volume -4, number 1
SOME
SPECTRA
AROMATIC
AND
TRIPLET
HYDROCARBONS
IN
QUANTUM LIQUID
W. HEINZELMANN and H. LABHART Physikalisch-chemisches Insfituf der Universitat Ziirich, Switzerland
YIELDS
SOLUTION
Ziirich,
Received 23 July 1969
The extinction coefficient of triplet-triplet absorption has been measured for aeven aromatic hydrocarbon molecules in a wavelength interval extending into the region of singlet-singIet absorption. The. experimental method also permits the determination of the triplet quantum yield 4~ and of the triplet decay constant kT. With two of the compounds the dependence of these two properties from concentra-tion has been inv&tigatcd and discussed.
The present knowledge on spectral properties and de-activation mechanisms of electronically excited organic molecules in solution being still scarce, we measured triplet-triplet absorption spectra
and triplet
quantum
yields
of seven
aro-
matic hydrocarbons. Special attention was paid to the question of the importance of Sl .+ So internal conversion, concentration quenching and to the T-T absorption in the spectral region of singlet absxption which is missing from flash experiments. The method applied has been described more extensively in an earlier paper [l] and in the Ph. D.-thesis of one of us [2]. It consists in measuring the small changes in optical density of a solution illuminated by chopped light of relatively low intensity. The lifetime of molecules in the triplet state being much larger than in the excited singlet state, the change in optical density AD upon illumination is AD =
at low chopper
frequencies
it was possible
to
CT(ET-ES)~
where ET and ES are the extinction coefficients in the Tl- and So-state respectively; d is the depth of the illuminated area of the cell and CT the concentration of molecules in the triplet state. Procedures for determining CT and cT from the wavelergth dependence of AD with an accuracy of 5% in favourable cases and of 10 to 20% in less favourable cases as well as for corrections for nonuniform illumination have been given [l, 21. If the inverse chopper frequency becomes smaller than the lifetime of the triplet state, the concentration CT will not reach its 20
steady-state value during the illumination period, nor will it decay completely during the dark ‘period. The dependence of the measured effect from the chopper speed thils yields a means for determining triplet lifetime 7~ on the same sample. This together with a measurement of the spectral energy distribution of the exciting light and calculation of the number of molecules excited per second and cm3 permits the determination of the triplet quantum yield @T. The layout of the apparatus is given in fig. 1. By varying the intensity of the exciting light
Fig. 1. Experimental set-up. -. -. - probing light beam: ----_ excixing light beam; XI xenon high pressure lamp (CSRAM XBO 450 W) ; X2 xenon-mercury high pressure lamp (HANOVIA 901-Bl) ; LI - L6 quartz lenses; MI, M2 grating monochromators; PM photomultiplier (PHILIPS 150 WP); P photocell (reference signal); TI , T2 tungsten lamps; FI, F2 filters; S sample cell; C light chopper; A lock-in amplifier; R recorder.
VohIme
C!HEMICA.L. PHYSICS
4, number 1
L5 September
LETTERS
/ 43
E-
’
250
-41
I
250
300
I
300
450
350
1 350
I
100
I
450
.
I
tS69
C,
500
I silo
I
A lnml
Fig. 2. Pyrene. Upper Curves: (1) extinction Cof?rficicnt of S-S-absorption; (2) extinction coefficient of T-T-absorption (n-hexane, 300°K, thus work); (3) relative T-T-absorption (liquid paraffin. 300%, ref. pi); (4) relative T-Tabsorption (EPA, ?@‘K, ref. [4]); Lower curve: relative change of transmitted light intensity upon illiunination.
21
-Volume 4. number 1 -. check
that the decay
CHEMICAL of the triplet
state
PHYSJCS LETTERS
was first
order within the limits of error. Thus triplettriplet annihilation is shown to influence the steady-state triplet concentration by at most 10%; -CT ranged in the order of lo-4 t2 IO-3 times the concedtration of molecules in the So state. At1 compounds except 9, lo-clihydrophenauthrene have been purified by zone-refiiing; 9, lodihydrophenanthrene has been recrystallized and distilled under high vacuum. In ail experiments the solvent was n-h-e (spectral quality UVASOL-Merck) which was used without further purification. The solutions have been o?.tgassed by repeated freeze and melting cycles until the pressure of remaining gases was below 10-4 Torr; they were sealed under vacuum in quartz ceils. Some of the compounds showed photoreactions. The degree of decomposition was repeatedly checked by taking absorption spectra, and the triplet quenching by photoproducts as found with 1, 2-benzanthracene was eliminated by renewing the solution frequently. Fig. 2 shows the measured values of the reiative change of light transmitted upon illumination as well as the singlet-singlet spectra and the triplet-triplet spectra of pyrene. Figs. 3 to 5 display the S-S spectra and the T-T spectra of 1,2-benzanthxcene, chrysene, fluorene, 9, lod!hydrophenanthrene, diphenyi, and p-terphenyi determined in the same way. As demonstrated by these figures, our method allows the determination of therrbsolute extinction coefficient of triplet-triplet spectra with an accuracy of about 10% even in the spectral region of singlet absorption. As far as data are available in the iiterature [3-141, the positions of tne band’maxima at longer wavelengths coincide, within the limits due to solvent- and temperature-shifts, with our measurements. Only for chrysene and 1,2-benzanthracex z%&te values of extinction coeff icier&s from the literature can be compared with our measure,nents. The large deviations are partly due to differences in solvents and temperature, partly they may be attributed to uncertainties of the methods used so far. Table 1 contains the triplet decay constants kT and triplet quantum yields f&T determined by this method as well as fluorescence quantum yields OF taken from the references given there. For 1,2_benzanthracene and for pyrene, KT increases quite markedly with increasing concentration. This suggests that one can put kT ‘&!$j
+k2[soj
where k$ comprises the radiative as well as nonradiative decay mechanisms in the solvent at the
22
15 September
1969
*t E-10,J 3-
2-
l-
c.
-
*\. 300
400
500
r\lnmJ
300
400
500
l(nm)
Blo-’ 2-
Alnml 9,10-g;drophenanthrere. diphenyl and p-terphenyl. (1) extinction coefficient of S-S-absorption; (2) extinction coefficient of T-T-absorption (n-hexane; 300%. this work);
Fig. 3. Absorptfo: spectra of figtene,
(3) relattive T T-absprption (liquid paraffin, 300%. ref. [3j;; (4) relative T-T-absorption (isopentane/me*-hylcyciohexane.
77°K. -ref. [4]).
Quantum yields of fluorescence Substance
.-
---
Table 1 and intersystem-crossing
(moloc 1-Q 0.9
1,2-benzanthracene
I5 September 2969
CHEMICAL PHYSICS LETTERS
Volume 4, number 1
x 10-5
triplet-decay
constants
(S2’t
(7.5 *o-2) x 102
0.79 4: 0.04
0.18 L 0.03a)-
1.25 x 10-4
(1.3 iO.05) x 103
0.73 t 0.07
0.18 L 0.03af
1.0
(1.9 +o.l)
0.54 -L0.16
x 10-3
x 103
0.8
x10-5
(5.65 ~0.05) v: 102
0.38 * 0.02
0.65b)
1.0
x 10-4
(6.4 iO.1) X 102
0.26 & 0.03
0.60 b1
1.0
x 10-3
(9.0 *0.05) x 102
0.12 * 0.04
o.ssbl
1.0
x 10-2
(3.4 *0.2) x 103
0.0s f 0.03
5.0
x 10-2
(l.5
0.08 -I 0.04
o.ssdt
chrysene
1.0
x10-5
(1.4 *o.oq
0.81 + 0.08
0.1&4
fluorene
1.3
x 10-3
(6.5 iO.1) X 103
0.10 i: 0.05
0.52 L O.IOa)
9, lo-~ydro~h~nanthreno
1.0
x10-3
(8.4 “0.6) x 103
0.13 * 0.05
0.48 = 0.08 3
diphenyl
1.0
x10-3
(7.7 *0.3) x 103
0.51 4 0.20
0.12 * 0.02 G
p-terphenyl
2.2
X 10-4
(2.2 *0.2) x 103
0.11 4. 0.04
0.9;; 4
pyrene
a) ref. [lS] (zz-hexane) b) ref. [l@J (sum of monomer and excimer fiuorescence c) ref. [17] (cyclohesane) a) ref. {15] (ethanol)
I-0.1) x 104 x 103
in ethanol)
Fig. 4. Absorption spectra of chrysene. (l) extinction coefficient of S-S-absorption; (2) extinction coefficient OET-Tabsorption (n-hexane, 3009(. this work ; (3) refative T-T-absorption (EPA, 77qc. ref. f4J); (4) extinction coefffcient of T-T-absorption (n-bexane, 300 a , ref. [3])_ A T-T extinction coefficient is given in ref. [13J (EPA, 77% for 585 nm: ET = 48000;
23
Volume 4. number 1
CHRMICAL PHYSICS LETTERS lay the
/ od? H/. \
tic
-I b
”
1
.5
~
limit of zero concentration; k2 describes the quenching by solute molecules in the ground state or by an impurity of the solute. With k!$ = 555 set-l and k2 = 2.8 x 105 l/mole set the experimental data for pyrene were very well reproduced; k2 compares reasonably with a value found by Wilkinson [15] when investigating carf.fully purified pyrene in ethanol. With 1, 2-benzanthracene we get k” - ‘710 set-1, k2 = 7 x lo5 l/mole set and k, = T-.5 x 108 l/mole set, where k, describes the very pronounced triplet quenching by a photoproduct already mentioned The values of kg are quite small compared with what would be expected for a diffusion controlled reaction. The close agreement between Wilkinson’s and our value of k2 in the case of pyrene suggests that quenching is due rather to pyrene molecules in the ground state than to an impurity. This indicates either that only a small fr--ction of encounters leade to radiationless deactivation or that triplet excimers are formed which have rather high transition rates to the ground state.. The latter possibility is supported
24
of Langelaar
etaL
[16]who
found a probabIe excimer phosphorescence with phenanthrene and naphthalin at low temperature. The data in table 1 show that for pyrene and benzanthracene ~#ITseems to decrease with increasing concentration. For pyrene this has already been stated by Wilkinson [15]. At higher concentrations, $F and #T no longer sum up to unity. We have found the same behaviour for diphenyl, fluorine and 9, lo-dihydrophenanthrene. However, in Wilkinson’s and our measurements it is assumed that all triplet-triplet absorption is due to triplet monomers. Thus, if a considerable amount of triplets would form triplet extimers, the extinction coefficient of which diifers from that of the monomer at the wavelength of measurement, the values of qT ought to be modified We are planning to compare the T-T absorption spectra at low and at higher concentrations over a large wavelength interval. Only if there appears no triplet excimer absorption we may conclude that the decrease of c$T at higher concentrations is due to rapid and competitive
Fig. 5. Absorption spectra of 1.2-benzanthracene. (1) extinction coefficient of S-S-absorption; (2) extinction coefficient of T-T-absorption (n-hexene. 300°K. this work); (3) extinction coefficient of T-T-absorption (EPA, 770K, ref. 1141); (4) extinction coefficient of T-T-absorption (liquid paraffin, 300°K, ref. [s]); (5) relative T-T-absorption (EPA, 7?K. ref. [4]).
experiments
15 September 1969
internal
conversion
of the singlet.
REFERENCES [l] H. Labhart,
Helv. Chim. Acta 47 (1964) 2279. [2] W. Heinzeimann, Ph. D. thesis, Zirrich (1969). [3] G. Porter and M. W. Windsor, Proc. Roy. Sot. (London) A245 0958) 238. [4] D. P. Craig and LG. Ross, J. Chem. Sot. (1954) 1589. [5] 0. S. McClure, J. Chem. Phys. 19 (1951) 670. [S] LA. Zhmyreva, V. P.Kolobkov and S. V. Volkov, Opt.Spectry. 20 (1966) 162. [7] R. A.Keller and S. G. Hadley. J. Chem. Phys. 42 (1965) 2382. [8] G. N-Lewis. D. Lipkin and T. T. Magel, J.Am. Chem.Soc. 63 (1941) 3005. 191 L.Lindcvist, ArkivKemi 16 (1960) 79. [iOj lK W.Windsbr and J. R.Nova.k. in:.The triplet state. ed. A.B. Zahlan (Cambridge. London. 1967). [ll] R. Astier and Y. Meyer,‘ J. Chim:Phys. 64 (1967) 919. [12] B. R-Henry and M.Kasha, J. Chem. Phys. 47 (1967) 3319. (131 M.W. Windsor and W. R. Dawson, Mol. Cryst. 4 0965) 253. [14] W. R. Dawson, J.Opt. Soc.Am. 58 (1968) 222. [15] T. Medinger and F. Wilkinson, Trans. Faraday Sec. 62 (1966) 1785. 1161J. Langelaar, R. P. Rettschnick. A. M. F. LambAoy and G. J. Hoyti~k. Chem. Phys. Letters 1 (1968) 609. [17j I. B. Berhnan, Handbook of fluorescence spectra (New York, 1965). 1181 C. A. Parker and C. C. Hatchard, Trans. Faraday Sot. 59 (1963) 284. [lS] E. R. Pantke, unpublished results.
15 September 1969
CHEMICAL PHYSICS LETTERS
Volume -4, number 1
SOME
SPECTRA
AROMATIC
AND
TRIPLET
HYDROCARBONS
IN
QUANTUM LIQUID
W. HEINZELMANN and H. LABHART Physikalisch-chemisches Insfituf der Universitat Ziirich, Switzerland
YIELDS
SOLUTION
Ziirich,
Received 23 July 1969
The extinction coefficient of triplet-triplet absorption has been measured for aeven aromatic hydrocarbon molecules in a wavelength interval extending into the region of singlet-singIet absorption. The. experimental method also permits the determination of the triplet quantum yield 4~ and of the triplet decay constant kT. With two of the compounds the dependence of these two properties from concentra-tion has been inv&tigatcd and discussed.
The present knowledge on spectral properties and de-activation mechanisms of electronically excited organic molecules in solution being still scarce, we measured triplet-triplet absorption spectra
and triplet
quantum
yields
of seven
aro-
matic hydrocarbons. Special attention was paid to the question of the importance of Sl .+ So internal conversion, concentration quenching and to the T-T absorption in the spectral region of singlet absxption which is missing from flash experiments. The method applied has been described more extensively in an earlier paper [l] and in the Ph. D.-thesis of one of us [2]. It consists in measuring the small changes in optical density of a solution illuminated by chopped light of relatively low intensity. The lifetime of molecules in the triplet state being much larger than in the excited singlet state, the change in optical density AD upon illumination is AD =
at low chopper
frequencies
it was possible
to
CT(ET-ES)~
where ET and ES are the extinction coefficients in the Tl- and So-state respectively; d is the depth of the illuminated area of the cell and CT the concentration of molecules in the triplet state. Procedures for determining CT and cT from the wavelergth dependence of AD with an accuracy of 5% in favourable cases and of 10 to 20% in less favourable cases as well as for corrections for nonuniform illumination have been given [l, 21. If the inverse chopper frequency becomes smaller than the lifetime of the triplet state, the concentration CT will not reach its 20
steady-state value during the illumination period, nor will it decay completely during the dark ‘period. The dependence of the measured effect from the chopper speed thils yields a means for determining triplet lifetime 7~ on the same sample. This together with a measurement of the spectral energy distribution of the exciting light and calculation of the number of molecules excited per second and cm3 permits the determination of the triplet quantum yield @T. The layout of the apparatus is given in fig. 1. By varying the intensity of the exciting light
Fig. 1. Experimental set-up. -. -. - probing light beam: ----_ excixing light beam; XI xenon high pressure lamp (CSRAM XBO 450 W) ; X2 xenon-mercury high pressure lamp (HANOVIA 901-Bl) ; LI - L6 quartz lenses; MI, M2 grating monochromators; PM photomultiplier (PHILIPS 150 WP); P photocell (reference signal); TI , T2 tungsten lamps; FI, F2 filters; S sample cell; C light chopper; A lock-in amplifier; R recorder.
VohIme
C!HEMICA.L. PHYSICS
4, number 1
L5 September
LETTERS
/ 43
E-
’
250
-41
I
250
300
I
300
450
350
1 350
I
100
I
450
.
I
tS69
C,
500
I silo
I
A lnml
Fig. 2. Pyrene. Upper Curves: (1) extinction Cof?rficicnt of S-S-absorption; (2) extinction coefficient of T-T-absorption (n-hexane, 300°K, thus work); (3) relative T-T-absorption (liquid paraffin. 300%, ref. pi); (4) relative T-Tabsorption (EPA, ?@‘K, ref. [4]); Lower curve: relative change of transmitted light intensity upon illiunination.
21
-Volume 4. number 1 -. check
that the decay
CHEMICAL of the triplet
state
PHYSJCS LETTERS
was first
order within the limits of error. Thus triplettriplet annihilation is shown to influence the steady-state triplet concentration by at most 10%; -CT ranged in the order of lo-4 t2 IO-3 times the concedtration of molecules in the So state. At1 compounds except 9, lo-clihydrophenauthrene have been purified by zone-refiiing; 9, lodihydrophenanthrene has been recrystallized and distilled under high vacuum. In ail experiments the solvent was n-h-e (spectral quality UVASOL-Merck) which was used without further purification. The solutions have been o?.tgassed by repeated freeze and melting cycles until the pressure of remaining gases was below 10-4 Torr; they were sealed under vacuum in quartz ceils. Some of the compounds showed photoreactions. The degree of decomposition was repeatedly checked by taking absorption spectra, and the triplet quenching by photoproducts as found with 1, 2-benzanthracene was eliminated by renewing the solution frequently. Fig. 2 shows the measured values of the reiative change of light transmitted upon illumination as well as the singlet-singlet spectra and the triplet-triplet spectra of pyrene. Figs. 3 to 5 display the S-S spectra and the T-T spectra of 1,2-benzanthxcene, chrysene, fluorene, 9, lod!hydrophenanthrene, diphenyi, and p-terphenyi determined in the same way. As demonstrated by these figures, our method allows the determination of therrbsolute extinction coefficient of triplet-triplet spectra with an accuracy of about 10% even in the spectral region of singlet absorption. As far as data are available in the iiterature [3-141, the positions of tne band’maxima at longer wavelengths coincide, within the limits due to solvent- and temperature-shifts, with our measurements. Only for chrysene and 1,2-benzanthracex z%&te values of extinction coeff icier&s from the literature can be compared with our measure,nents. The large deviations are partly due to differences in solvents and temperature, partly they may be attributed to uncertainties of the methods used so far. Table 1 contains the triplet decay constants kT and triplet quantum yields f&T determined by this method as well as fluorescence quantum yields OF taken from the references given there. For 1,2_benzanthracene and for pyrene, KT increases quite markedly with increasing concentration. This suggests that one can put kT ‘&!$j
+k2[soj
where k$ comprises the radiative as well as nonradiative decay mechanisms in the solvent at the
22
15 September
1969
*t E-10,J 3-
2-
l-
c.
-
*\. 300
400
500
r\lnmJ
300
400
500
l(nm)
Blo-’ 2-
Alnml 9,10-g;drophenanthrere. diphenyl and p-terphenyl. (1) extinction coefficient of S-S-absorption; (2) extinction coefficient of T-T-absorption (n-hexane; 300%. this work);
Fig. 3. Absorptfo: spectra of figtene,
(3) relattive T T-absprption (liquid paraffin, 300%. ref. [3j;; (4) relative T-T-absorption (isopentane/me*-hylcyciohexane.
77°K. -ref. [4]).
Quantum yields of fluorescence Substance
.-
---
Table 1 and intersystem-crossing
(moloc 1-Q 0.9
1,2-benzanthracene
I5 September 2969
CHEMICAL PHYSICS LETTERS
Volume 4, number 1
x 10-5
triplet-decay
constants
(S2’t
(7.5 *o-2) x 102
0.79 4: 0.04
0.18 L 0.03a)-
1.25 x 10-4
(1.3 iO.05) x 103
0.73 t 0.07
0.18 L 0.03af
1.0
(1.9 +o.l)
0.54 -L0.16
x 10-3
x 103
0.8
x10-5
(5.65 ~0.05) v: 102
0.38 * 0.02
0.65b)
1.0
x 10-4
(6.4 iO.1) X 102
0.26 & 0.03
0.60 b1
1.0
x 10-3
(9.0 *0.05) x 102
0.12 * 0.04
o.ssbl
1.0
x 10-2
(3.4 *0.2) x 103
0.0s f 0.03
5.0
x 10-2
(l.5
0.08 -I 0.04
o.ssdt
chrysene
1.0
x10-5
(1.4 *o.oq
0.81 + 0.08
0.1&4
fluorene
1.3
x 10-3
(6.5 iO.1) X 103
0.10 i: 0.05
0.52 L O.IOa)
9, lo-~ydro~h~nanthreno
1.0
x10-3
(8.4 “0.6) x 103
0.13 * 0.05
0.48 = 0.08 3
diphenyl
1.0
x10-3
(7.7 *0.3) x 103
0.51 4 0.20
0.12 * 0.02 G
p-terphenyl
2.2
X 10-4
(2.2 *0.2) x 103
0.11 4. 0.04
0.9;; 4
pyrene
a) ref. [lS] (zz-hexane) b) ref. [l@J (sum of monomer and excimer fiuorescence c) ref. [17] (cyclohesane) a) ref. {15] (ethanol)
I-0.1) x 104 x 103
in ethanol)
Fig. 4. Absorption spectra of chrysene. (l) extinction coefficient of S-S-absorption; (2) extinction coefficient OET-Tabsorption (n-hexane, 3009(. this work ; (3) refative T-T-absorption (EPA, 77qc. ref. f4J); (4) extinction coefffcient of T-T-absorption (n-bexane, 300 a , ref. [3])_ A T-T extinction coefficient is given in ref. [13J (EPA, 77% for 585 nm: ET = 48000;
23
Volume 4. number 1
CHRMICAL PHYSICS LETTERS lay the
/ od? H/. \
tic
-I b
”
1
.5
~
limit of zero concentration; k2 describes the quenching by solute molecules in the ground state or by an impurity of the solute. With k!$ = 555 set-l and k2 = 2.8 x 105 l/mole set the experimental data for pyrene were very well reproduced; k2 compares reasonably with a value found by Wilkinson [15] when investigating carf.fully purified pyrene in ethanol. With 1, 2-benzanthracene we get k” - ‘710 set-1, k2 = 7 x lo5 l/mole set and k, = T-.5 x 108 l/mole set, where k, describes the very pronounced triplet quenching by a photoproduct already mentioned The values of kg are quite small compared with what would be expected for a diffusion controlled reaction. The close agreement between Wilkinson’s and our value of k2 in the case of pyrene suggests that quenching is due rather to pyrene molecules in the ground state than to an impurity. This indicates either that only a small fr--ction of encounters leade to radiationless deactivation or that triplet excimers are formed which have rather high transition rates to the ground state.. The latter possibility is supported
24
of Langelaar
etaL
[16]who
found a probabIe excimer phosphorescence with phenanthrene and naphthalin at low temperature. The data in table 1 show that for pyrene and benzanthracene ~#ITseems to decrease with increasing concentration. For pyrene this has already been stated by Wilkinson [15]. At higher concentrations, $F and #T no longer sum up to unity. We have found the same behaviour for diphenyl, fluorine and 9, lo-dihydrophenanthrene. However, in Wilkinson’s and our measurements it is assumed that all triplet-triplet absorption is due to triplet monomers. Thus, if a considerable amount of triplets would form triplet extimers, the extinction coefficient of which diifers from that of the monomer at the wavelength of measurement, the values of qT ought to be modified We are planning to compare the T-T absorption spectra at low and at higher concentrations over a large wavelength interval. Only if there appears no triplet excimer absorption we may conclude that the decrease of c$T at higher concentrations is due to rapid and competitive
Fig. 5. Absorption spectra of 1.2-benzanthracene. (1) extinction coefficient of S-S-absorption; (2) extinction coefficient of T-T-absorption (n-hexene. 300°K. this work); (3) extinction coefficient of T-T-absorption (EPA, 770K, ref. 1141); (4) extinction coefficient of T-T-absorption (liquid paraffin, 300°K, ref. [s]); (5) relative T-T-absorption (EPA, 7?K. ref. [4]).
experiments
15 September 1969
internal
conversion
of the singlet.
REFERENCES [l] H. Labhart,
Helv. Chim. Acta 47 (1964) 2279. [2] W. Heinzeimann, Ph. D. thesis, Zirrich (1969). [3] G. Porter and M. W. Windsor, Proc. Roy. Sot. (London) A245 0958) 238. [4] D. P. Craig and LG. Ross, J. Chem. Sot. (1954) 1589. [5] 0. S. McClure, J. Chem. Phys. 19 (1951) 670. [S] LA. Zhmyreva, V. P.Kolobkov and S. V. Volkov, Opt.Spectry. 20 (1966) 162. [7] R. A.Keller and S. G. Hadley. J. Chem. Phys. 42 (1965) 2382. [8] G. N-Lewis. D. Lipkin and T. T. Magel, J.Am. Chem.Soc. 63 (1941) 3005. 191 L.Lindcvist, ArkivKemi 16 (1960) 79. [iOj lK W.Windsbr and J. R.Nova.k. in:.The triplet state. ed. A.B. Zahlan (Cambridge. London. 1967). [ll] R. Astier and Y. Meyer,‘ J. Chim:Phys. 64 (1967) 919. [12] B. R-Henry and M.Kasha, J. Chem. Phys. 47 (1967) 3319. (131 M.W. Windsor and W. R. Dawson, Mol. Cryst. 4 0965) 253. [14] W. R. Dawson, J.Opt. Soc.Am. 58 (1968) 222. [15] T. Medinger and F. Wilkinson, Trans. Faraday Sec. 62 (1966) 1785. 1161J. Langelaar, R. P. Rettschnick. A. M. F. LambAoy and G. J. Hoyti~k. Chem. Phys. Letters 1 (1968) 609. [17j I. B. Berhnan, Handbook of fluorescence spectra (New York, 1965). 1181 C. A. Parker and C. C. Hatchard, Trans. Faraday Sot. 59 (1963) 284. [lS] E. R. Pantke, unpublished results.