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Study of triplet quantum yields using a tunable dye laser
Volutie 13, ntimber. 3
CHEhfICALPHYSICS LETTERS., :. :.
l’bfarW5+
‘.
: ;
‘.
.-
_ STUDY
6FTRIPLErQiJANTUM
YIELDS
iJSING A TUNABLE
DYE
.’ :
LASER
B. SCIEP, A. KELLMANN, M. MARTiN zind .i. LINDQVIST Lubbratoire de Photophysique Mole’culoire *. UniversitG de ParfsSud, 91_Orsay, fiance Received 3 January 1972
A method of determining triplet quantum yields using a dye laser isdescrl%cd. Triplet yiekkwere measured for eosin, fluorescein, acridine orange, proflavine, and pentacene in fluid solutions. The’results reveal the occurrence of an efficient internal conversion process for the three latter timpounds.
1. Ikroduction
2. Experimental
Several techniques have been developed to determine, by direct [l-4] ‘or indirect [5-81 methods, the quantum yieids of population of the triplet state following photoexcitation of a molecular system. In spite of this effort, only a very limited number of triplet yields have been reported, illustrating the experimental difficulties associated with such measurements and.the need for further investigation_ The present paper reports on the possibilities of the tunable dye laser in the study of triplet yields. The system to be studied is exposed to a pulse of laser light and the transient absorption changes in the systern - due to triplet population - are recorded. Knowing the energy of the laser.pulse and the extinction coefficient of the tripiet, an absolute value of the triplet yield is obtained. This method is similar to a technique previously used by Bowers and Porter [3] except for the us+ of a laser instead of a flash lamp as exciting light source._The principal advantages of-the dye Iaser in this connection are the high spectral purity, the ideal collimating properties, the short duratioq, and the high poker.of the laser pulse. The main limitation is the,restricti& in the spectral range ofe.mission. The method was usea.to determine triplet yields for eosin, fluoresce&
The experimental setup,is shqwn in fig. 1. The sample, contained in a 1 cm square silica cell**‘with polished sides, is exposed to a laser pulse of circular cross section with a diameter slightly smaller than the cell width. A mirror placed behind the cell increases the uniformity of excitation, The part of light absorbed in the cell is calculated from the OD of the solution, taking window reflections &to account. (A correction, < 2%, was made in the present runs for ** In the fluorescein study a 0.5 by 1 cm cell was used, with the laser light incident on the wider side.
x&ii LAMP BEAM SPLITTER
acridine orange; pr6flavine;and .
..
* Groupe _derecherche dti CNRS. .‘.. ;
--
‘_ >
x
CELL
,
pentacene ifi fluid,solutionk
.,
SAMPLE
:
Fig. 1; Experimkntal setup for detkmination of triplet quz+ .. tum.yields by mea&of a dye laser. ‘.
,;. _... .y
.. ‘. ,.;,.:-, ‘..
:
.’
.,I’
;’ .,
,.. :
.., __
._.
‘.
’ :
.-.;:241 ‘,’ _.,
CHEMICAL PHYSIC3 LETTERS. 1 March 1972,: :.y’V&me: 13, numb& 3 _ .: :“;; ,’ ‘I : ~,_ ;.: ,.‘. ” :‘. _,.. ,,,:/ ; :. 2. Results :- the’b decrease.during the.pulse due to depopulation .’ of,the solute, giound state.) The ia.ser energy was l-12 In the absence of photochemical reactions or spe~ .mJ’in the present study, at a pulse halfwidth of 500 cific interactions with other molecules, a lightexcited ., n&c and a spectral width of less than 5 nm. Tuning molecuIe wiil return directly to the ground state by .ti+i obtained:by means of a birefringent plate inside fluorescence or internal conversion, or it may undergo ,:the l&i-cavitqr [9] . -intersyste’m crossing to the triplet state. The relative “,‘;-A photodiode.(CSF, CPA 1143).receiving a small probabilities of these processes are expressed by the : .&action of the laser light measu’res the laser energy. corresponding quantum yields (OF, QIc, and Q+, reThe integrated photocurrent is recorded on a dualspectively) the sum of which is unity. beam oscilloscope and converted to pulse energy, apThe compounds’studied and the solgents used are plyiitg aconversion factor determined by calibrating given in table 1. Maximum solute concentration was the photodiode alternatively against one of two fac3 X low5 M. The table summarizes the results of the tory-hlibrated detectors (a laser calorimeter - CSF, triplet yield study, made at room temperature; it also C 23 - and a photodiode - ITT; 4OOOS2OUVG) inindicates the triplet.monitoring wavelength (AT) and terceptmgthe l&r beam. The two calibrations agreed the triplet extinction coefficient (cT) at wavelength -, within 10%. 1,. Values of eT were. taken from the literature or ‘,The ana!yzing light beam, obtained from a xenon determined by a light-saturation technique [lo] using dc arc lamp, is diaphragmed to cover the width of the a flash photolysis apparatus described in ref. [ 1 l] _ cell ana sligbt!y n&i Sn*the height of the laserFluorescence yields given in the table are literature irradiated part of the soluticn. Variations in light values or relative values determined at this labcratory. trarlsmission-of the, irradiated aoluticn are measured Detailed flash-photolytic stud& made at this lab.._photoelectritilly at selected wavelengths. The triplet oratory have shown that the triplet is the only primaconcentration is obtained from the difference between ry species formed in the stage immediately following the OD at the.end of the laser pulse and that before photoexcitation of the compounds listed, under the irradiation, using known values of the triplet extincpresent experimental conditions. One may then contion coefficient. Since the triplet concentration is not clude that no primary photochemical reactions are uniform over.the volume viewed by the analysis light, occurring and obtains from the relation @rc = only very weak OD changes were studied (< 0.05): 1 - &F - %, values of &Ic given in table 1. the, transient OD is then directly proportional to the A check of the validity of the present method of measured intensity variation to a close approximadetermining @-I.is obtained from the results for eosin. tion, and a mean value of the triplet concentration is This compound was previously studied by Bowers obtained. The number of molecules brought to the and Porter [13-l who found QI- = 0.7 1, in good agreetriplet state is obtained is the product of the mean ment with our result. A value of 0.64 was obtained by triplet .concentration and the volume defined by the an indirect method [13] _ Literature vahies of % for cell dirrrensions and the height of the analyzing beam. fluorescein in alkaline aqueous solution are 0.05 [ 121 This number divided by the number of absorbed phoa.nd 0.02 [ 131 , and in methanol 0.03 [ 141. tons gives the triplet yield, provided that no noticeIt is seen from table 1 that for,eosin and fluores-able decay occurs during the Iaser pulse, a condition cein the internal conversion is ne&ible. Comparing largely fulfilled in the present study. fluorescein in aqueous and alcoholic solution one .’ .The dyes studied ivere purified by chromatography; fllds indication, however, of a slight amount of interpe_ntacene.yas sublime&Bidistilled water, twice fracnal conversion hi aqueous solution. Seybold, Gouter,. :tion&y. vacuum-distilled l-chloro-ntiphthalene ‘md man, and Callis [15] observed a slightly lower FF in : spectro$ade ethanol - were used as solvents. Oxygen aqueous solution; in connection with the present q .‘..ivas removed by the,free&-pump-thaw technique, values one may conclude that this difference is due ?C~I the..‘$e .of.p&itacene’b&ore dissolution.
.,
..,:,:.,_, .. .’ .._,_.., f’ : I
‘.
;
‘..;
Volume 13. number
I March 1972
CHEMICAL PHYSICS LETTERS
3
:
Table 1’ Results of the t&let y&M study System eosin in water, 0.01 hl NaOH fluorescein
OF
@IC
...hT G-m)
0.22 [22]
0.02
630
4800 (231
0.032 f 0.005
0.92 [24]
0.05
570
8400 [lo]
dianion
0.035 + 0.005
0.97 I1S.J
0.00
570
6800
neutiai
0.37
f 0.05
0.20
0.43
410
42000
cation
0.10
f 0.03
0.46 [22]
0.44
490
13500
ution
0.22
r 0.04
0.40 [22j
0.38
4.55
6000
neutral
0.16
f 0.02
0.08
0.76
505
9900 [ 251
Species
@T
dianion
0.76
dianion
f 0.08
$-I
cm-‘)
in
water, 0.01 M NaOH fluorescein in ethanol, 0.01 M KOH n&dine or-e in ethanol, 0.01 M KOH acridine orange in ethanol, 0.01 hi HCI proflavinc in ethanol, 0.01 hf HCI pentxene in I-Cl-naphthalene
to an internal conversion process in aqueous solution. A previous study [ 161 of the temperature variations
of the excited’singlet state of pentacene of 3 X TO-* set (calculated from the absorption spectrum), one
in @F and QT also indicated the presence of internal
obtains from the present results ti internal conversion rate constant of 3 X lo* see-l. The expression
conversion
for fluorescein
in aqueous
solution
at
room temperature, with a minimum value aIC = 0.03. The study of the acridine dyes makes evident the occurrence of an efficient internal conversion process. The effect of protonaiion
of acridine
orange on this
process and on the intersystem crossing was shown by studying the neutral molecule and the corresponding cation. It is seen from the table that the protonation decreases @T significantly; however, (I+ is increased $0 approximately the same extent, giving as result almost no change in the extent of internal conversion. Substituent. effect on the quantum yields were studied by comparing the cationic forms of proflavine .and acridine orange. The.substitution of the hydrogens in the amino groups of proflavine by methyl groups is seen to decrease % and increase G+ ; Gpc is almost the same for the two compounds. The internal conversion process is particularly important for pentacene. This result may be compared to data for the lower aceries. Naphthalene and anthracene do not seem to be subject to internal conversion [Z, 12;17,18] ; for tetracene a value of qc = 0.2 has been found .[7,19] _Siebrand and-Williams [20] have derived a.semiempirical expression relating the probability of internal conversiott to the Franck-Condon factor of the transition. Assuming a radiative lifetime. ./ ,._ : : : ‘,..,. : :. :
deduced by Siebrand and Williams gives a value of 2 X IO* set-l, in excellent agreement with the experimental result.
A previous study [2 I] demonstrated
the efficiency
of internal conversion for a xanthene dye: 6.hydzoxy9-phenyl-fluoron. The present results give further ex-
amples of dyes of high internal conversion efftciency, and also a particularly clearcut example of internal conversion in an aromatic hydrocarbon.
References [ 11 H. Labhart, Helv. Chim. Acta 47 (1964) 2279. [2] T. Medinger and p. Wilkinson, Trans Farsday Sot. 61 (1965) 620. [3] P.G. Bowersand G. Porter, Proc. Roy. Sot. 296A (1967) 435. [4] ‘J.B. Callis, M. Gouterr& and J.D.S. Danielson, Rev. Sci. Instr. 40 (1969) 1399,. [S] AA. +mola and G.S. Hammond, J. Chem; Phys. 43 (,l965) 2129. [6 J C.A. Parker and T.&Joyce, Chem. Commun. (1966) 108. [7] B. Stevens and BX.. A&r,
Chem; Phys. Letters
1 (1967)
‘. 58. .‘: [8] bi. Nemoto, HI Kokubuu and M..Koizumi, EuB’Cheui~ .: “. Sot. Japrur 42.(1969) 1’22?. : ‘. ‘. ._I ;. ;.: .., ,,,.
.‘/. .:
., .,: 24;.
-::‘Voltimk13;humber:3 ,.-,. .‘-:,. l,..
:
”
‘. _. ..,
,.:
CHEMICALPHYSICS ‘.
‘.[12! -P.G. Bcwer$and_G. ,34& : : . . : .
Porter, Prqc. Roy. Sot; 299A (1967;.
:-i [ii]. M; Nemoto; H. i(okubun and hi. Koiaumi, Bull. Chem. ,: +C. J&a&(196!& i464. ..-..[i&l. K. ~c+nick kd G.O. Schen&, Purc.AppL Chem. 9 -. :’ -,(19$4)507. [ 151 P.G. Seybold;.M. Gouterman and J:CalJis, Photpchem. G : PJtotobioL9 (1969) 229. ‘Tii%] -M; Martin and L. Lindqvi$ J. Chin-r. Phys., Trans.Non : RadJative Mols 2@me Reunion Sot. Chhn. Phys. (1969) 144. ‘. . [ !‘!I .A.R. ,Ho&ocks‘and F. Wilkinson, Proc Roy. Sot. 306A (1968) 257.
LETTERS .[18]
:
,’ 1 March 1972.
CA. Parker and T.A. Joyce, Chem. Commuo. .
(1986)
.’ ‘I191 ztearwell and F. Wilkinson Chem. Phys. titters’11 ‘. ‘, (1971j 472. .1201 W. Siebrand and,D_F. W.iUiarns, J. Chem. Phys 49 (1968) 1860. [21] L. Lindqvist and G.W. Lundecn, J. Chem. Ph&. 44 (1966) 1711. .. [22] C.A. Parker, Photoluminesc&ce of scIutions (Elscvier, Amsterdam, 1968). [23] V. Kasche and L. Lindqvist, Photochem. PhotobioL 4 (1965) 923. [24] G. Weber and F.W.J. Teale, Trans. Faraday Sot. 54 (1958) 640. [25] C. HelIner, P. Roberge and L. Linclqvist, to be publish+
‘-
CHEhfICALPHYSICS LETTERS., :. :.
l’bfarW5+
‘.
: ;
‘.
.-
_ STUDY
6FTRIPLErQiJANTUM
YIELDS
iJSING A TUNABLE
DYE
.’ :
LASER
B. SCIEP, A. KELLMANN, M. MARTiN zind .i. LINDQVIST Lubbratoire de Photophysique Mole’culoire *. UniversitG de ParfsSud, 91_Orsay, fiance Received 3 January 1972
A method of determining triplet quantum yields using a dye laser isdescrl%cd. Triplet yiekkwere measured for eosin, fluorescein, acridine orange, proflavine, and pentacene in fluid solutions. The’results reveal the occurrence of an efficient internal conversion process for the three latter timpounds.
1. Ikroduction
2. Experimental
Several techniques have been developed to determine, by direct [l-4] ‘or indirect [5-81 methods, the quantum yieids of population of the triplet state following photoexcitation of a molecular system. In spite of this effort, only a very limited number of triplet yields have been reported, illustrating the experimental difficulties associated with such measurements and.the need for further investigation_ The present paper reports on the possibilities of the tunable dye laser in the study of triplet yields. The system to be studied is exposed to a pulse of laser light and the transient absorption changes in the systern - due to triplet population - are recorded. Knowing the energy of the laser.pulse and the extinction coefficient of the tripiet, an absolute value of the triplet yield is obtained. This method is similar to a technique previously used by Bowers and Porter [3] except for the us+ of a laser instead of a flash lamp as exciting light source._The principal advantages of-the dye Iaser in this connection are the high spectral purity, the ideal collimating properties, the short duratioq, and the high poker.of the laser pulse. The main limitation is the,restricti& in the spectral range ofe.mission. The method was usea.to determine triplet yields for eosin, fluoresce&
The experimental setup,is shqwn in fig. 1. The sample, contained in a 1 cm square silica cell**‘with polished sides, is exposed to a laser pulse of circular cross section with a diameter slightly smaller than the cell width. A mirror placed behind the cell increases the uniformity of excitation, The part of light absorbed in the cell is calculated from the OD of the solution, taking window reflections &to account. (A correction, < 2%, was made in the present runs for ** In the fluorescein study a 0.5 by 1 cm cell was used, with the laser light incident on the wider side.
x&ii LAMP BEAM SPLITTER
acridine orange; pr6flavine;and .
..
* Groupe _derecherche dti CNRS. .‘.. ;
--
‘_ >
x
CELL
,
pentacene ifi fluid,solutionk
.,
SAMPLE
:
Fig. 1; Experimkntal setup for detkmination of triplet quz+ .. tum.yields by mea&of a dye laser. ‘.
,;. _... .y
.. ‘. ,.;,.:-, ‘..
:
.’
.,I’
;’ .,
,.. :
.., __
._.
‘.
’ :
.-.;:241 ‘,’ _.,
CHEMICAL PHYSIC3 LETTERS. 1 March 1972,: :.y’V&me: 13, numb& 3 _ .: :“;; ,’ ‘I : ~,_ ;.: ,.‘. ” :‘. _,.. ,,,:/ ; :. 2. Results :- the’b decrease.during the.pulse due to depopulation .’ of,the solute, giound state.) The ia.ser energy was l-12 In the absence of photochemical reactions or spe~ .mJ’in the present study, at a pulse halfwidth of 500 cific interactions with other molecules, a lightexcited ., n&c and a spectral width of less than 5 nm. Tuning molecuIe wiil return directly to the ground state by .ti+i obtained:by means of a birefringent plate inside fluorescence or internal conversion, or it may undergo ,:the l&i-cavitqr [9] . -intersyste’m crossing to the triplet state. The relative “,‘;-A photodiode.(CSF, CPA 1143).receiving a small probabilities of these processes are expressed by the : .&action of the laser light measu’res the laser energy. corresponding quantum yields (OF, QIc, and Q+, reThe integrated photocurrent is recorded on a dualspectively) the sum of which is unity. beam oscilloscope and converted to pulse energy, apThe compounds’studied and the solgents used are plyiitg aconversion factor determined by calibrating given in table 1. Maximum solute concentration was the photodiode alternatively against one of two fac3 X low5 M. The table summarizes the results of the tory-hlibrated detectors (a laser calorimeter - CSF, triplet yield study, made at room temperature; it also C 23 - and a photodiode - ITT; 4OOOS2OUVG) inindicates the triplet.monitoring wavelength (AT) and terceptmgthe l&r beam. The two calibrations agreed the triplet extinction coefficient (cT) at wavelength -, within 10%. 1,. Values of eT were. taken from the literature or ‘,The ana!yzing light beam, obtained from a xenon determined by a light-saturation technique [lo] using dc arc lamp, is diaphragmed to cover the width of the a flash photolysis apparatus described in ref. [ 1 l] _ cell ana sligbt!y n&i Sn*the height of the laserFluorescence yields given in the table are literature irradiated part of the soluticn. Variations in light values or relative values determined at this labcratory. trarlsmission-of the, irradiated aoluticn are measured Detailed flash-photolytic stud& made at this lab.._photoelectritilly at selected wavelengths. The triplet oratory have shown that the triplet is the only primaconcentration is obtained from the difference between ry species formed in the stage immediately following the OD at the.end of the laser pulse and that before photoexcitation of the compounds listed, under the irradiation, using known values of the triplet extincpresent experimental conditions. One may then contion coefficient. Since the triplet concentration is not clude that no primary photochemical reactions are uniform over.the volume viewed by the analysis light, occurring and obtains from the relation @rc = only very weak OD changes were studied (< 0.05): 1 - &F - %, values of &Ic given in table 1. the, transient OD is then directly proportional to the A check of the validity of the present method of measured intensity variation to a close approximadetermining @-I.is obtained from the results for eosin. tion, and a mean value of the triplet concentration is This compound was previously studied by Bowers obtained. The number of molecules brought to the and Porter [13-l who found QI- = 0.7 1, in good agreetriplet state is obtained is the product of the mean ment with our result. A value of 0.64 was obtained by triplet .concentration and the volume defined by the an indirect method [13] _ Literature vahies of % for cell dirrrensions and the height of the analyzing beam. fluorescein in alkaline aqueous solution are 0.05 [ 121 This number divided by the number of absorbed phoa.nd 0.02 [ 131 , and in methanol 0.03 [ 141. tons gives the triplet yield, provided that no noticeIt is seen from table 1 that for,eosin and fluores-able decay occurs during the Iaser pulse, a condition cein the internal conversion is ne&ible. Comparing largely fulfilled in the present study. fluorescein in aqueous and alcoholic solution one .’ .The dyes studied ivere purified by chromatography; fllds indication, however, of a slight amount of interpe_ntacene.yas sublime&Bidistilled water, twice fracnal conversion hi aqueous solution. Seybold, Gouter,. :tion&y. vacuum-distilled l-chloro-ntiphthalene ‘md man, and Callis [15] observed a slightly lower FF in : spectro$ade ethanol - were used as solvents. Oxygen aqueous solution; in connection with the present q .‘..ivas removed by the,free&-pump-thaw technique, values one may conclude that this difference is due ?C~I the..‘$e .of.p&itacene’b&ore dissolution.
.,
..,:,:.,_, .. .’ .._,_.., f’ : I
‘.
;
‘..;
Volume 13. number
I March 1972
CHEMICAL PHYSICS LETTERS
3
:
Table 1’ Results of the t&let y&M study System eosin in water, 0.01 hl NaOH fluorescein
OF
@IC
...hT G-m)
0.22 [22]
0.02
630
4800 (231
0.032 f 0.005
0.92 [24]
0.05
570
8400 [lo]
dianion
0.035 + 0.005
0.97 I1S.J
0.00
570
6800
neutiai
0.37
f 0.05
0.20
0.43
410
42000
cation
0.10
f 0.03
0.46 [22]
0.44
490
13500
ution
0.22
r 0.04
0.40 [22j
0.38
4.55
6000
neutral
0.16
f 0.02
0.08
0.76
505
9900 [ 251
Species
@T
dianion
0.76
dianion
f 0.08
$-I
cm-‘)
in
water, 0.01 M NaOH fluorescein in ethanol, 0.01 M KOH n&dine or-e in ethanol, 0.01 M KOH acridine orange in ethanol, 0.01 hi HCI proflavinc in ethanol, 0.01 hf HCI pentxene in I-Cl-naphthalene
to an internal conversion process in aqueous solution. A previous study [ 161 of the temperature variations
of the excited’singlet state of pentacene of 3 X TO-* set (calculated from the absorption spectrum), one
in @F and QT also indicated the presence of internal
obtains from the present results ti internal conversion rate constant of 3 X lo* see-l. The expression
conversion
for fluorescein
in aqueous
solution
at
room temperature, with a minimum value aIC = 0.03. The study of the acridine dyes makes evident the occurrence of an efficient internal conversion process. The effect of protonaiion
of acridine
orange on this
process and on the intersystem crossing was shown by studying the neutral molecule and the corresponding cation. It is seen from the table that the protonation decreases @T significantly; however, (I+ is increased $0 approximately the same extent, giving as result almost no change in the extent of internal conversion. Substituent. effect on the quantum yields were studied by comparing the cationic forms of proflavine .and acridine orange. The.substitution of the hydrogens in the amino groups of proflavine by methyl groups is seen to decrease % and increase G+ ; Gpc is almost the same for the two compounds. The internal conversion process is particularly important for pentacene. This result may be compared to data for the lower aceries. Naphthalene and anthracene do not seem to be subject to internal conversion [Z, 12;17,18] ; for tetracene a value of qc = 0.2 has been found .[7,19] _Siebrand and-Williams [20] have derived a.semiempirical expression relating the probability of internal conversiott to the Franck-Condon factor of the transition. Assuming a radiative lifetime. ./ ,._ : : : ‘,..,. : :. :
deduced by Siebrand and Williams gives a value of 2 X IO* set-l, in excellent agreement with the experimental result.
A previous study [2 I] demonstrated
the efficiency
of internal conversion for a xanthene dye: 6.hydzoxy9-phenyl-fluoron. The present results give further ex-
amples of dyes of high internal conversion efftciency, and also a particularly clearcut example of internal conversion in an aromatic hydrocarbon.
References [ 11 H. Labhart, Helv. Chim. Acta 47 (1964) 2279. [2] T. Medinger and p. Wilkinson, Trans Farsday Sot. 61 (1965) 620. [3] P.G. Bowersand G. Porter, Proc. Roy. Sot. 296A (1967) 435. [4] ‘J.B. Callis, M. Gouterr& and J.D.S. Danielson, Rev. Sci. Instr. 40 (1969) 1399,. [S] AA. +mola and G.S. Hammond, J. Chem; Phys. 43 (,l965) 2129. [6 J C.A. Parker and T.&Joyce, Chem. Commun. (1966) 108. [7] B. Stevens and BX.. A&r,
Chem; Phys. Letters
1 (1967)
‘. 58. .‘: [8] bi. Nemoto, HI Kokubuu and M..Koizumi, EuB’Cheui~ .: “. Sot. Japrur 42.(1969) 1’22?. : ‘. ‘. ._I ;. ;.: .., ,,,.
.‘/. .:
., .,: 24;.
-::‘Voltimk13;humber:3 ,.-,. .‘-:,. l,..
:
”
‘. _. ..,
,.:
CHEMICALPHYSICS ‘.
‘.[12! -P.G. Bcwer$and_G. ,34& : : . . : .
Porter, Prqc. Roy. Sot; 299A (1967;.
:-i [ii]. M; Nemoto; H. i(okubun and hi. Koiaumi, Bull. Chem. ,: +C. J&a&(196!& i464. ..-..[i&l. K. ~c+nick kd G.O. Schen&, Purc.AppL Chem. 9 -. :’ -,(19$4)507. [ 151 P.G. Seybold;.M. Gouterman and J:CalJis, Photpchem. G : PJtotobioL9 (1969) 229. ‘Tii%] -M; Martin and L. Lindqvi$ J. Chin-r. Phys., Trans.Non : RadJative Mols 2@me Reunion Sot. Chhn. Phys. (1969) 144. ‘. . [ !‘!I .A.R. ,Ho&ocks‘and F. Wilkinson, Proc Roy. Sot. 306A (1968) 257.
LETTERS .[18]
:
,’ 1 March 1972.
CA. Parker and T.A. Joyce, Chem. Commuo. .
(1986)
.’ ‘I191 ztearwell and F. Wilkinson Chem. Phys. titters’11 ‘. ‘, (1971j 472. .1201 W. Siebrand and,D_F. W.iUiarns, J. Chem. Phys 49 (1968) 1860. [21] L. Lindqvist and G.W. Lundecn, J. Chem. Ph&. 44 (1966) 1711. .. [22] C.A. Parker, Photoluminesc&ce of scIutions (Elscvier, Amsterdam, 1968). [23] V. Kasche and L. Lindqvist, Photochem. PhotobioL 4 (1965) 923. [24] G. Weber and F.W.J. Teale, Trans. Faraday Sot. 54 (1958) 640. [25] C. HelIner, P. Roberge and L. Linclqvist, to be publish+
‘-