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Excited state properties of 5H-dibenzo(a, d) cycloheptene
Journal of Luminescence 21(1980) 239 © North-Holland Publishing Company
246
EXCITED STATE PROPERTIES OF SH-DIBENZO(a, d) CYCLOHEPTENE A.R. WATKINS
*
Max Planck Institut für Biophysikalische Chemie, 34 Göttingen-Nikolausberg, West Germany and
Fuat BAYRAK~EKEN Department of Theoretical Chemistry, Middle East Technical University, Ankara, Turkey Received 10 December 1979
A number of photophysical properties of 5H-dibenzo(a,d) cycloheptene have been measured including the emission and triplet-triplet absorption spectra. The quantum yields and decay parameters for the triplet state of this molecule as a function of temperature have also been measured. The results are discussed and compared to those of molecules.
I. Introduction Relatively little is known concerning the photophysical properties of the molecule 5H-dibenzo(a,d) cycloheptene (hereafterDBCH),
which corresponds, at least in its structure, to the cis-isomer of stilbene. The only previous study of this molecule was carried out by Herkstroeter and McClure [1], who were mainly concerned with the triplet-triplet absorption spectrum of DBCH. In particular, there is generally a paucity of information regarding the temperature dependence of such parameters as intersystem crossing yields. The work reported here was undertaken with the ajm of determining the values and the temperaturedependent behaviour of a number of photophysical parameters which are of significance to the photochemistry of the excited singlet and triplet states.
*
Present Address: CSIRO, Division of Chemical Physics, P.O. Box 160, Clayton, 3168, Victoria, Australia. 239
240
AR. Watkins and F. Bayrakceken
/ Properties of 5H-Dibenzo(a,d)cycloheptene
2. Experimental DBCH was purified by repeated recrystallization; all other chemicals used were analytical reagent grade. Fluorescence and phosphorescence spectra and intensities were measured on a Hitachi Perkin-Elmer MPF2A spectrofluorimeter, and the fluorescence lifetime of DBCH was determined with a pulsed nitrogen laser. Delayed fluorescence and triplet-triplet absorption experiments were carried out using a flash photolysis apparatus described previously [21.Concentration-dependent flash photolysis experiments were carried out in an all-glass cell without stopcocks [3].
3. Results and discussion The fluorescence lifetime of DBCH was determined to be 5.8 + 0.3 ns in a solvent consisting of 20% isopentane- 80% methylcyclohexane, and the fluorescence spectrum of DBCH in methylcyclohexane is shown in fig. 1. The peak corresponds to a first
I’
Ii
I
II I!
H
\\
\\
I1/ 1/
Cr) z
LU H
I II
z
\
I,
I,
UJ C) Z
ji
1/
H
1/ I
LU UJ
\ \ \
0
320
360
400
440
480
EM~SSI0N WAVELENGTH (nm)
Fig. 1. Prompt and delayed fluorescence spectra of SH-dibenzo [a,d] cycloheptene — Prompt fluorescence spectrum of a 2 X M solution of DBCH in methylcyclohexane at room temperature; excitation wavelength 320 nm. Delayed fluorescence spectrum of DBCH in 20%-isopentane-80%-methylcyclohexane.
/ Properties of 5H-Dibenzo(a,d)cycloheptene
A .R. Watkinsand F. Bayrakceken
241
excited singlet state having an energy of 3.33 eV. These two observations can be contrasted with the situation in cis-stilbene (an analogue of DBCH in which rotation about the double bond is unhindered), where radiationless processes chiefly the transition to the trans isomer are so fast that no fluorescence emission can be observed at room temperature [4]. Apparently the absence of rotation about the double bond has brought about the stability of the cis-stilbene analogue DBCH. A similar fluorescence spectrum, consisting of a broad emission with a peak centred on 400 nm, has been observed for the cis-stilbene analogue 1 ,2-diphenylcyclopentene [4]. These authors also detected a weak emission from cis-stilbene itself in a solvent consisting of 5 parts methylcyclohexane to 1 part isopentane at 77 K: the maximum was at 400 nm which, in view of their results for 1 ,2-diphenylcyclopentene and ouls for DBCH, is consistent with the emission fiom cis-stilbene being fluorescence. Quenching of the DBCH singlet by inorganic salts and heavy atom quenchers also occurs, and some figures are given in the table. It is interesting to observe that the quenching rate constant for KI in acetonitrile (14 X i0~M l~ 1) is about what would be expected for an aromatic molecule of this singlet energy [5], although the correlation is uncertain in the absence of a value for the reduction potential of DBCH. The triplet state of DBCH is similarly stable. Fig. 2 shows the phosphorescence spectrum of DBCH in methylcyclohexane at liquid nitrogen temperature; the spectrum is structured, and the origin is easily identified as the peak at 435 rim (2.85 eV~.This lies at a rather higher energy than the origin of the singlet-triplet spectra of cis-stilbene observed in heavy-atom solvents (2.5 eV) [6,7].
0D C
= >, 0
>.-
H Cf
z
U)
H
z
z 0
Cl) Cf
I 360
400
I
I
I
I
440
480
520
560
WAVELENGTH (nm)
Fig. 2. Phosphorescence spectrum of 5H-dlbenzo[a,d] cycloheptene in methylcyclohexane at 77K.
242
A.R. Watkins and F. Bayrakceken /Properties of 5H.Dibenzo(a,d)cycloheptene
0012
-
:: 350
400
450
500
WAVELENGTH (nm) Fig. 3. Triplet-triplet absorption spectrum of 5H-dibenzo[a,d] cycloheptene in cyclohexane at room temperature. The path length of the cell is 10 cm.
The triplet-triplet absorption spectrum of DBCH is shown in fig. 3, and the delayed emission spectrum of DBCH appears as the dashed curve in fig. 1. The spectrum of Fig. 3 agrees partially with that determined by Herkstroeter and McClure [1] for DBCH in an isopentane-3-methylpentane glass at liquid nitrogen temperature; in both spectra a peak at about 425 nm is the main feature. The fact that our experiments were carried out at room temperature instead of in solid solution at 77 K as in [1] probably accounts for the lack of structure in fig. 3, particularly at shorter wavelengths. The spectrum of fig. 3 shows that the DBCH triplet is stable at room temperature; this conclusion is reinforced by the appearance of delayed fluorescence. Delayed fluorescence is produced by the P-type mechanism (the wide singlet-triplet energy gap, amounting to at least 0.8 eV, would appear to preclude an E-type mechanism [8]); the identity of the delayed fluorescence and prompt fluorescence spectra excludes the possibility of the triplet state spectra observed being due to an impurity with a low-lying triplet state. Decay curves for the DBCH triplet, recorded at 420 nm, were found to follow an exponential decay law: [3DBCH] = [3DBCH]0 e kit (1) where k1 is a constant. The variation of ln k1 with 1/Tis shown in fig. 4 for DBCH in acetonitrile and in a 20%-isopentane-80%-methylcyclohexane solvent mixture. Although the points for acetonitrile cover a limited temperature range (acetonitrile becomes solid at C), both sets of points appear to follow an Arrhenius law with an activation energy ~G in both cases of 1.0 kcal mole If the data avail4570
‘.
A.R. Watkins and F. Bayrakceken /Properties of 5H-Dibenzo(a,d)cycloheptene
243
8->o
In K
1
l/T
(K
x l0~)
Fig. 4. Temperature dependence of the triplet decay rate constant k1 for 5H-dibenzo[a,dj cycloheptene in o acetonitrile and • 20%-isopentane-8 0%-methylcyclohexane.
able for the temperature dependence of the viscosity of these two solvents [9] are fitted to an expression similar to eq. (1), “activation energies” of viscositydependent diffusion equal to 2.4 kcal mole~for methylcyclohexane (no data were available for the solvent mixture used here) and 1.8 kcal mole 1 for acetonitrile are obtained. These values are higher than those obtained here, which suggests that the deactivation mechanism may not be bimolecular, but rather an intramolecular process which does not depend on the solvent. This idea receives support from the fact that our values of z.~Gare the same in both solvent systems. Quenching of excited singlet states by inorganic anions is known to produce triplet states with unit efficiency in many cases [10]. This can be used to determine the efficiency of the intersystem crossing process which produces the triplet in the unquenched molecule [11]. In the presence of a quencher Q which quenches fluorescence with a Stern-Volmer constant Ksv, the ratio of the triplet-triplet absorption in the presence of quencher E to the triplet-triplet absorption in the absence of any quencher E0 will be given by E/E0(1 +Ksv[Q])
1
+Ksv[Q]/40
(2)
where /~is the intersystem crossing quantum yield [12]. In fig. 5 equation (2) is plotted for DBCH quenched by KI in acetonitrile, giving a value of ~ = 0.32 ±0.06 for the intersystem crossing quantum yield of DBCH. The added KI can also quench the DBCH triplet state; the rate constant for this process is given in table 1. The way
244
A.R.
Watkins and F. Bayrakçeken /Properties of 5H-Dibenzo(~a,d,)cyc1oheptene
0
~sV [Ku
Fig. 5. Plot of equation (2) (see text) for 5H-dibenzo[a,d] cycloheptene quenched by KI in acetonitrile.
in which DBCH triplet yields vary with temperature is shown in fig. 6 for the two solvent systems acetonitrile and 20%-isopentane-80%-methylcyclohexane; the temperature dependence of the triplet yield differs markedly on going from one solvent to the other. In reality the temperature dependence of the quantity k15~ris being measured, where k~5~ is the rate constant for intersystem crossing and r is the lifetime of the excited singlet state. The two linear portions observed with DBCH in 20%-isopentane-80%-methylcyclohexane suggest that, at least in this solvent, both ~ and r are temperature dependent. If we assume that the linear portion of the curve at high values of 1/Tis mainly determined by the temperature variation of ~ (the temperature variation of r would be expected to have the opposite trend), we
Table 1 Quencher
KI p-lodotoluene KI
State quenched
Solvent
Singlet Singlet
CH3CN 20%-isopentane80%-methylcyclohexane CH3CN
Triplet
Ksv1 (M 81.6
)
Kq (M 14
x io~ 9
46.3
8 X 10 5.8 X 10
Singlet and triplet state quenching of 511-dibenzo[a,d] cycloheptene. Ksy is the Stern-Volmer quenching constant; the singlet quenching rate constants K~have been calculated using the measured value of fluorescence lifetime r— 5.8 ns.
AR. Watkins and F. Bayrakçeken I Properties of 5H-Dibenzo(a,d)cycloheptene
In E
~
245
-
4•0
3
I
I
I
I
4
5
6
7
Fig.
6. remperature dependence of the relative triplet yield of 5H-dibenzo[a,d] cycloheptene in o acetonitrile and. 20%-isopentane-80%-methylcyclohexane.
obtain an activation energy of 1.1 kcal mole 1 This is again lower than the value to be expected for a diffusion-controlled bimolecular reaction. While it is clear that care must be exercised in applying the results presented here to cis-stilbene, the idea of using structurally similar molecules to obtain information about molecules which are inaccessible to direct measurement appears to hold promise. It has been suggested [13], on theoretical grounds, that the success of this method with cis-stilbene is due not only to its formal similarity to DBCH, but also to the similarity in conformation of the two molecules. Similar studies using dibenzocyclopentene as an analogue of cis-stilbene [4] and indeno [2,1-a] indene as an analogue of trans-stilbene [14] indicate that this approach may have wide application as a method of analysing photochemical reactions. -
Acknowledgements We are grateful to Dr. K.H. Grellmann for the use of the nitrogen laser, and to NATO for a supporting grant (No. 1175) for one of us (FB.).
References [1] W.G. Herkstroeter and D.S. McClure, J. Amer. Chem. Soc. 90(1968)4522. L21 K-H Grellmann, A.R. Watkins and A. Weller, J. Luminescence. 1, 2(1970) 678. [3) A.R. Watkins. Mol. Photochem. 6 (1974) 325.
246
A.R. Watkins and F. Bayrakceken
/ Properties of5H.Dibenzo(a,d)cycloheptene
[4] A.A. Lamola, G.S. Hammond and lB. Mallory, Photochem. Photobiol. 4 (1965) 259. [51AR. Watkins, J. Phys. Chem. 78 (1974) 2555. [6] F. Lippert, Z. Physik. Chem. 42 (1964) 125. [7] H. Stegemeyer, Z. Physik. Chem. 51(1966) 95. [8] C.A. Parker, Photoluminescence of Solutions (Elsevier, London, 1968). [9] Landolt-BOrnstein, Zahlenwerte und Funktionen, Vol. II Part 5, pp. 159, 246. [10] AR. Watkins, J. Phys. Chem. 78 (1974) 1885. [11] T. Medinger and l~.Wilkinson, Trans. l~arad.Soc. 61(1965) 620. [12] J.B. Birks, Photophysics of Aromatic Molecules (Wiley-Interscience, London, 1970) p. 197. [13] T. Toth and L. Kiasinic, Z. Naturforsch. 29a (1974) 1371. [14] J. Saltiel, D.W.C. Chang, ED. Margarity, AD. Rousseau, PT. Shannon, B. Thomas and AK. Uriarte, Pure Appl. Chem. 41(1975) 559.
246
EXCITED STATE PROPERTIES OF SH-DIBENZO(a, d) CYCLOHEPTENE A.R. WATKINS
*
Max Planck Institut für Biophysikalische Chemie, 34 Göttingen-Nikolausberg, West Germany and
Fuat BAYRAK~EKEN Department of Theoretical Chemistry, Middle East Technical University, Ankara, Turkey Received 10 December 1979
A number of photophysical properties of 5H-dibenzo(a,d) cycloheptene have been measured including the emission and triplet-triplet absorption spectra. The quantum yields and decay parameters for the triplet state of this molecule as a function of temperature have also been measured. The results are discussed and compared to those of molecules.
I. Introduction Relatively little is known concerning the photophysical properties of the molecule 5H-dibenzo(a,d) cycloheptene (hereafterDBCH),
which corresponds, at least in its structure, to the cis-isomer of stilbene. The only previous study of this molecule was carried out by Herkstroeter and McClure [1], who were mainly concerned with the triplet-triplet absorption spectrum of DBCH. In particular, there is generally a paucity of information regarding the temperature dependence of such parameters as intersystem crossing yields. The work reported here was undertaken with the ajm of determining the values and the temperaturedependent behaviour of a number of photophysical parameters which are of significance to the photochemistry of the excited singlet and triplet states.
*
Present Address: CSIRO, Division of Chemical Physics, P.O. Box 160, Clayton, 3168, Victoria, Australia. 239
240
AR. Watkins and F. Bayrakceken
/ Properties of 5H-Dibenzo(a,d)cycloheptene
2. Experimental DBCH was purified by repeated recrystallization; all other chemicals used were analytical reagent grade. Fluorescence and phosphorescence spectra and intensities were measured on a Hitachi Perkin-Elmer MPF2A spectrofluorimeter, and the fluorescence lifetime of DBCH was determined with a pulsed nitrogen laser. Delayed fluorescence and triplet-triplet absorption experiments were carried out using a flash photolysis apparatus described previously [21.Concentration-dependent flash photolysis experiments were carried out in an all-glass cell without stopcocks [3].
3. Results and discussion The fluorescence lifetime of DBCH was determined to be 5.8 + 0.3 ns in a solvent consisting of 20% isopentane- 80% methylcyclohexane, and the fluorescence spectrum of DBCH in methylcyclohexane is shown in fig. 1. The peak corresponds to a first
I’
Ii
I
II I!
H
\\
\\
I1/ 1/
Cr) z
LU H
I II
z
\
I,
I,
UJ C) Z
ji
1/
H
1/ I
LU UJ
\ \ \
0
320
360
400
440
480
EM~SSI0N WAVELENGTH (nm)
Fig. 1. Prompt and delayed fluorescence spectra of SH-dibenzo [a,d] cycloheptene — Prompt fluorescence spectrum of a 2 X M solution of DBCH in methylcyclohexane at room temperature; excitation wavelength 320 nm. Delayed fluorescence spectrum of DBCH in 20%-isopentane-80%-methylcyclohexane.
/ Properties of 5H-Dibenzo(a,d)cycloheptene
A .R. Watkinsand F. Bayrakceken
241
excited singlet state having an energy of 3.33 eV. These two observations can be contrasted with the situation in cis-stilbene (an analogue of DBCH in which rotation about the double bond is unhindered), where radiationless processes chiefly the transition to the trans isomer are so fast that no fluorescence emission can be observed at room temperature [4]. Apparently the absence of rotation about the double bond has brought about the stability of the cis-stilbene analogue DBCH. A similar fluorescence spectrum, consisting of a broad emission with a peak centred on 400 nm, has been observed for the cis-stilbene analogue 1 ,2-diphenylcyclopentene [4]. These authors also detected a weak emission from cis-stilbene itself in a solvent consisting of 5 parts methylcyclohexane to 1 part isopentane at 77 K: the maximum was at 400 nm which, in view of their results for 1 ,2-diphenylcyclopentene and ouls for DBCH, is consistent with the emission fiom cis-stilbene being fluorescence. Quenching of the DBCH singlet by inorganic salts and heavy atom quenchers also occurs, and some figures are given in the table. It is interesting to observe that the quenching rate constant for KI in acetonitrile (14 X i0~M l~ 1) is about what would be expected for an aromatic molecule of this singlet energy [5], although the correlation is uncertain in the absence of a value for the reduction potential of DBCH. The triplet state of DBCH is similarly stable. Fig. 2 shows the phosphorescence spectrum of DBCH in methylcyclohexane at liquid nitrogen temperature; the spectrum is structured, and the origin is easily identified as the peak at 435 rim (2.85 eV~.This lies at a rather higher energy than the origin of the singlet-triplet spectra of cis-stilbene observed in heavy-atom solvents (2.5 eV) [6,7].
0D C
= >, 0
>.-
H Cf
z
U)
H
z
z 0
Cl) Cf
I 360
400
I
I
I
I
440
480
520
560
WAVELENGTH (nm)
Fig. 2. Phosphorescence spectrum of 5H-dlbenzo[a,d] cycloheptene in methylcyclohexane at 77K.
242
A.R. Watkins and F. Bayrakceken /Properties of 5H.Dibenzo(a,d)cycloheptene
0012
-
:: 350
400
450
500
WAVELENGTH (nm) Fig. 3. Triplet-triplet absorption spectrum of 5H-dibenzo[a,d] cycloheptene in cyclohexane at room temperature. The path length of the cell is 10 cm.
The triplet-triplet absorption spectrum of DBCH is shown in fig. 3, and the delayed emission spectrum of DBCH appears as the dashed curve in fig. 1. The spectrum of Fig. 3 agrees partially with that determined by Herkstroeter and McClure [1] for DBCH in an isopentane-3-methylpentane glass at liquid nitrogen temperature; in both spectra a peak at about 425 nm is the main feature. The fact that our experiments were carried out at room temperature instead of in solid solution at 77 K as in [1] probably accounts for the lack of structure in fig. 3, particularly at shorter wavelengths. The spectrum of fig. 3 shows that the DBCH triplet is stable at room temperature; this conclusion is reinforced by the appearance of delayed fluorescence. Delayed fluorescence is produced by the P-type mechanism (the wide singlet-triplet energy gap, amounting to at least 0.8 eV, would appear to preclude an E-type mechanism [8]); the identity of the delayed fluorescence and prompt fluorescence spectra excludes the possibility of the triplet state spectra observed being due to an impurity with a low-lying triplet state. Decay curves for the DBCH triplet, recorded at 420 nm, were found to follow an exponential decay law: [3DBCH] = [3DBCH]0 e kit (1) where k1 is a constant. The variation of ln k1 with 1/Tis shown in fig. 4 for DBCH in acetonitrile and in a 20%-isopentane-80%-methylcyclohexane solvent mixture. Although the points for acetonitrile cover a limited temperature range (acetonitrile becomes solid at C), both sets of points appear to follow an Arrhenius law with an activation energy ~G in both cases of 1.0 kcal mole If the data avail4570
‘.
A.R. Watkins and F. Bayrakceken /Properties of 5H-Dibenzo(a,d)cycloheptene
243
8->o
In K
1
l/T
(K
x l0~)
Fig. 4. Temperature dependence of the triplet decay rate constant k1 for 5H-dibenzo[a,dj cycloheptene in o acetonitrile and • 20%-isopentane-8 0%-methylcyclohexane.
able for the temperature dependence of the viscosity of these two solvents [9] are fitted to an expression similar to eq. (1), “activation energies” of viscositydependent diffusion equal to 2.4 kcal mole~for methylcyclohexane (no data were available for the solvent mixture used here) and 1.8 kcal mole 1 for acetonitrile are obtained. These values are higher than those obtained here, which suggests that the deactivation mechanism may not be bimolecular, but rather an intramolecular process which does not depend on the solvent. This idea receives support from the fact that our values of z.~Gare the same in both solvent systems. Quenching of excited singlet states by inorganic anions is known to produce triplet states with unit efficiency in many cases [10]. This can be used to determine the efficiency of the intersystem crossing process which produces the triplet in the unquenched molecule [11]. In the presence of a quencher Q which quenches fluorescence with a Stern-Volmer constant Ksv, the ratio of the triplet-triplet absorption in the presence of quencher E to the triplet-triplet absorption in the absence of any quencher E0 will be given by E/E0(1 +Ksv[Q])
1
+Ksv[Q]/40
(2)
where /~is the intersystem crossing quantum yield [12]. In fig. 5 equation (2) is plotted for DBCH quenched by KI in acetonitrile, giving a value of ~ = 0.32 ±0.06 for the intersystem crossing quantum yield of DBCH. The added KI can also quench the DBCH triplet state; the rate constant for this process is given in table 1. The way
244
A.R.
Watkins and F. Bayrakçeken /Properties of 5H-Dibenzo(~a,d,)cyc1oheptene
0
~sV [Ku
Fig. 5. Plot of equation (2) (see text) for 5H-dibenzo[a,d] cycloheptene quenched by KI in acetonitrile.
in which DBCH triplet yields vary with temperature is shown in fig. 6 for the two solvent systems acetonitrile and 20%-isopentane-80%-methylcyclohexane; the temperature dependence of the triplet yield differs markedly on going from one solvent to the other. In reality the temperature dependence of the quantity k15~ris being measured, where k~5~ is the rate constant for intersystem crossing and r is the lifetime of the excited singlet state. The two linear portions observed with DBCH in 20%-isopentane-80%-methylcyclohexane suggest that, at least in this solvent, both ~ and r are temperature dependent. If we assume that the linear portion of the curve at high values of 1/Tis mainly determined by the temperature variation of ~ (the temperature variation of r would be expected to have the opposite trend), we
Table 1 Quencher
KI p-lodotoluene KI
State quenched
Solvent
Singlet Singlet
CH3CN 20%-isopentane80%-methylcyclohexane CH3CN
Triplet
Ksv1 (M 81.6
)
Kq (M 14
x io~ 9
46.3
8 X 10 5.8 X 10
Singlet and triplet state quenching of 511-dibenzo[a,d] cycloheptene. Ksy is the Stern-Volmer quenching constant; the singlet quenching rate constants K~have been calculated using the measured value of fluorescence lifetime r— 5.8 ns.
AR. Watkins and F. Bayrakçeken I Properties of 5H-Dibenzo(a,d)cycloheptene
In E
~
245
-
4•0
3
I
I
I
I
4
5
6
7
Fig.
6. remperature dependence of the relative triplet yield of 5H-dibenzo[a,d] cycloheptene in o acetonitrile and. 20%-isopentane-80%-methylcyclohexane.
obtain an activation energy of 1.1 kcal mole 1 This is again lower than the value to be expected for a diffusion-controlled bimolecular reaction. While it is clear that care must be exercised in applying the results presented here to cis-stilbene, the idea of using structurally similar molecules to obtain information about molecules which are inaccessible to direct measurement appears to hold promise. It has been suggested [13], on theoretical grounds, that the success of this method with cis-stilbene is due not only to its formal similarity to DBCH, but also to the similarity in conformation of the two molecules. Similar studies using dibenzocyclopentene as an analogue of cis-stilbene [4] and indeno [2,1-a] indene as an analogue of trans-stilbene [14] indicate that this approach may have wide application as a method of analysing photochemical reactions. -
Acknowledgements We are grateful to Dr. K.H. Grellmann for the use of the nitrogen laser, and to NATO for a supporting grant (No. 1175) for one of us (FB.).
References [1] W.G. Herkstroeter and D.S. McClure, J. Amer. Chem. Soc. 90(1968)4522. L21 K-H Grellmann, A.R. Watkins and A. Weller, J. Luminescence. 1, 2(1970) 678. [3) A.R. Watkins. Mol. Photochem. 6 (1974) 325.
246
A.R. Watkins and F. Bayrakceken
/ Properties of5H.Dibenzo(a,d)cycloheptene
[4] A.A. Lamola, G.S. Hammond and lB. Mallory, Photochem. Photobiol. 4 (1965) 259. [51AR. Watkins, J. Phys. Chem. 78 (1974) 2555. [6] F. Lippert, Z. Physik. Chem. 42 (1964) 125. [7] H. Stegemeyer, Z. Physik. Chem. 51(1966) 95. [8] C.A. Parker, Photoluminescence of Solutions (Elsevier, London, 1968). [9] Landolt-BOrnstein, Zahlenwerte und Funktionen, Vol. II Part 5, pp. 159, 246. [10] AR. Watkins, J. Phys. Chem. 78 (1974) 1885. [11] T. Medinger and l~.Wilkinson, Trans. l~arad.Soc. 61(1965) 620. [12] J.B. Birks, Photophysics of Aromatic Molecules (Wiley-Interscience, London, 1970) p. 197. [13] T. Toth and L. Kiasinic, Z. Naturforsch. 29a (1974) 1371. [14] J. Saltiel, D.W.C. Chang, ED. Margarity, AD. Rousseau, PT. Shannon, B. Thomas and AK. Uriarte, Pure Appl. Chem. 41(1975) 559.