- Email: [email protected]
Temperature dependence of the deactivation of electronically excited indazoles in solution
CHEMICAL PHYSICS LIXTERS
‘Volume 11, number 3
TEMPERATURE DEPENDENCE OF THE DEACTIVATION C?F ELECTRONICALLY EXCITED INDAZOLES IN SOLUTION P. BIRCHER, E.R. PANTKE and H. LABHART P~lysikclliscir-Cllenrisches Institut der UntierskYt Ziirich. CH-8001 Zwikh. Switzerland Received 2 September 1971
The T-T spectra, the triplei quantum yield $IJ- and the fluorescence quantum yield QF of 2-tert butyI+methylindazole and 1,3dimethylindazole have been measuredin the temperature range of +25”to -L96”C in solucnts of different viscosities. It could be shown that in all c;ises where photochemical reactions from the fust excited singlet state are absent, the sum #JF + @ equals unity within the limits of error.
Contrary to the quite numerous experimental results on the deactivation mechanisms of electronic excited
states
of aromatic
hydrocarbon
molecules
[l--9],
only little is known about heterocyclic compotinds. IMeasurements of the T-T absorption spectra of several compounds of this type have been published [lo-171 , but ociy one paper [ 181 deals with the question of whether at low temperatures internal conversion from the first excited singlet state to the ground state is as unimportant as it has been found to be in hydrocarbons. In order to get additional information about these problems we measured the T-T absorption spectra, the triplet quantum yield QT, and the fluorescence quantum yield + of two indazole derivatives in solvents of different \%cosities and in a temperature range between +2S°C and -196’C. The photochemical reactions of these cornpounds are studied at our institute. The method used to measure the T-T absorption spectra and the triplet quantum yields has been described earlier 119,201 . The change in optical density 0f.a probe illuminated by a chopped light beam of medium intensity is
where CT is the triplet concentration, eG and cT are&Lhe extinction coefficients of the singlet and the triplet staies, end d is ihe depth of the measurihg 4. Pro-, ., ‘. cedures h&e, been-deicribed to qbtairi E-+) when ., I’ .-
:
.;,. : ..-_ .... ,’‘._ -,:.., .;.-.’ .‘_‘.
.‘, ..,
:..
..
1.
and Q(X) are known [l9,2d]. In order to obtain the quantum efficiency *, one has to know the tid the number of molecules excited pet unit tie rate constant kT for the unimolecular decay of the triplet state. A few modifications to the earlier experimental setup [20] w&e empioyed: (i) the probe was placed in a cryostat capable of AD@)
maintaining -196’C
_.
,,. _. .,,.
.I .;
i-25’C
and
decay constants kr smaUer than 100 set-L could not be determined from the frequency dependence of the amplitude of AD, but could be obtained from the decay curves of the phosphorescence. A phosphore&ope of the Lewis and Kasha type [2 I] together with a Boxcar integrator (PAR CW-1) allowed the exact determination of kT down to a value of 1.O set-l . Decay curves with kT-vdues smaller than 1 :Oset-1 could be displayed on a servorecorder. In this case iz could be shown that the lifetime of the phosphorescence was the same as the lifetime of the tranient absorbing species which thus is identified as the triplet state. The compounds chosen were 2-tert. butyl4-methylindazole (2,4 BMI) and I ,3dimethyLndazoie (I,3 DME), “0th showing practicalIy no irreversible photochemistj at IoW temperatures. Purification was achieved by chromatcjgraphy and subsequent.recrysiallizatiibn frdm.n-penian& The solvents used werðanbl UVksOL (Mer’ck) 96% and a mixture .
. .’ :
,‘.,
.‘._ 347.
.,
,:i -, .- ., ..,,, “. ., ;.,:: ,- -.: _‘,...’ : : ..,.,*._;-:_., -.. .. ..,,. :. _:__. .’,.(,.,,, ” - ‘, .r. -.:y ,I__;,. .:’ ( : _ .,;, ‘.- /,..’ ,, . ....::;...-.,. ,.I I-
., _,..:
betweerc
O.l°C;
(ii) triplet
: .:
an;’ temperature
within
:
:
.,
j.
..,,
..,
;_
.,
;,
-
:..
__
._
..
.:
:_ ‘,
,I
V&itie 11, number 3
,.-j+
”
.’:,
_ .:
,_
j .’ : ‘. : ,.
CHEMICALPHYSICS LEI-I-ERS
:
: :’
‘.
‘._ ‘, ,_. ‘,
15 Oct&er 197:
Fig. 1. Sk absorption (7) and,T-T abkptioi spectra (-: - -1of.&text. buty~methylindizole and [email protected] ethanol it low. tgiq&ittue. ._: : -’ :. . . . . ‘I ;,. ‘: ,, ” :- ‘.” /’ ...,:.._ ,,’ -., _..;:
:
of the latter with ethyleneglycol (Schuchardt) 1;:1 by volume..Both solvents were spectrograde quality and w&e used without further purification. Viscosities of &thesoIutions as a function of t~rn~ra~re were measured with the rotating cylinder method [2’L]T IXtsrminations of the fluorescence quantum yields at various temperatures were carried out using anthracene in ethanol as a standard [+ = 0.28 (25’C)I. TiG T-T absorption spectra of the two compounds in ethanol are shown in fig. 1. The coKe~pond~g spectra
in
15 October
CHEMICAL PHYSICS LWRS
Volume 11, number 3
ethanol/ethyleneg.Iycol
a+e
identical
within
,$e limits of error with those measured in etha11o1at
temperatures where &e solvent viscosities are equal. The quantum yields for fluorescence #F and intersystern ciossing h for several temperatures are compifed in table 1. We conclude from our experiments that in all but one of the cases stu’died these two processes are the only important pathways for deactivation of the first excited singlet state. In the case of 2,4 BMI in e~~ol/e~ylene~ycol at --50X, where q+ + 4)r < 1, an additional deactivation process of the S, state seems to be involved. From investigations of the, photochemical behaviour of this compound, the existence of a short lived phototropic state called 2’ is inferred which can deactivate by a direct radiationless transition to the ground state of ind&ole [23,24]. From this we concliude that, in the mo!ecules studied, : Internal conversion cannot compete withthe processes of fluorescence,‘intersystem crossing and photoreaction from the first excited sin&et state.. The first order triplet decay constants have beeti mea&red by the methods described above and are presented in table 2. The strong rjse of the observed decay constant /CTwith increasing temperiture cc&Id be explained quantitatively by diffusibn-controlled ‘impurity quenching., Similar results have been obt+ned by other workers :[25-291. :
:
Table 2 First order’triplet decay constants &‘(sec-;) in ethanol ,. -’ (WASOL)
1971
We thank the Swiss National ScienceFoundation for-support of the work described in this paper.
Refer&es E.C.L&, J.D.Laposa and J.M.H.Yu, J. Mot Spekry. 19 (1966) 412. R.G.Bennett and P.J.McCartin, I. Chem. E’hys44 (1969) 1969. BStevens and M.F.‘Fhontzz, C’hem, Phys. Letters 1
(1968) 549. B.Stwens and M.F,ihomaz, (1968) 535.
Chem. Phys. Lerfers
1
T.F.Hunter end R.F.Wyatt, Chem. Phys Letters 6 (1970) 221. P.F.Jones and S.Sie@, Chem. Phys.’ Letters 2 (1968) ~~.D~~~n and J.L.K.ropp. 3. pfifs. Chem. 73 (1969) 693. B.Stewns, M.F.Thomaz and P.FITones, J. Chem. Phys. 46 (1967) 405. J.L.Kropp. W.RDaiwson and M.W.Windsor, I. Phys.
Chem. 73 (1969) 1747.. A.Keltmann and L,Cim.iquist, i.x The tripbt state, ed. A.B.ZahIan (Cambridge Univ. Press, London, 1967) p. 439. B.R.Henryand hI.Kasha, 3. Chem. Phys. 47 (1967) 3319. R.&tier and Y.H.Meyer, I. Ck_im. Phys. 64 (1967) 9?9.
iXP.Craigand I.G.Ross. 1. Cheti Sot. (i954) L.589. S.G.Hadfey, J. Phys. Ghem. 74_(L97? 3551. J.S.Brinen, in: MoIecuIzr luminescence, ed. ECLim (Benjamin, New York, i969) p. 333. V.Zanker and B.Schneider, Z. mysik. Chem. 68 (1969)
19. V.&ker and GSrelI, Ber. Bunsenges. Physik. @em 73 (1969) 791. J.R.MerkiU and RG.Bermett, J. Cbem. Fhys. 43 (1965) 1410. H.&bbart, HeIv. Chim. Actz 47 (IP&) 2279. W.Heixizelmannand H.L+bhacc,Chem. Phys Letters 4
(1969) 19. G.N,Lewisa;ld Rf.Ka&a,3. Am. Chem. Sot. 66 (1944) 2100. G.A.von S&s and H.labbrt, ,’ 752.
1. Phys. C&em_72 (1968)
W.Heinzeham, tpivate comnwnicaltian. H&b&& W.He&eImann and J.P.Dubois, Pure A?$ Ckem; 24 (1970) 495. J.W.Hilpkm, G.Portetand L.I.Stief, Rot. Roy. Sot. A277 ($964)437: ‘. G.&&son and RLivingston.:J: C&m, Phys. 35 (1% I) 2182; : ‘. mange*, Thesis, vniy. of&sterda7n (1969). R.Lrringstoa ar;d.W.R.W~;J. Cl@k Phys. 39 (1963) ‘.
2y)rj*
.; I ‘.
.,. ..
I
B;Sic,&ns andk$.Wa&&+oc
_
Roy. ,&:
A281 (!Yt&
‘Volume 11, number 3
TEMPERATURE DEPENDENCE OF THE DEACTIVATION C?F ELECTRONICALLY EXCITED INDAZOLES IN SOLUTION P. BIRCHER, E.R. PANTKE and H. LABHART P~lysikclliscir-Cllenrisches Institut der UntierskYt Ziirich. CH-8001 Zwikh. Switzerland Received 2 September 1971
The T-T spectra, the triplei quantum yield $IJ- and the fluorescence quantum yield QF of 2-tert butyI+methylindazole and 1,3dimethylindazole have been measuredin the temperature range of +25”to -L96”C in solucnts of different viscosities. It could be shown that in all c;ises where photochemical reactions from the fust excited singlet state are absent, the sum #JF + @ equals unity within the limits of error.
Contrary to the quite numerous experimental results on the deactivation mechanisms of electronic excited
states
of aromatic
hydrocarbon
molecules
[l--9],
only little is known about heterocyclic compotinds. IMeasurements of the T-T absorption spectra of several compounds of this type have been published [lo-171 , but ociy one paper [ 181 deals with the question of whether at low temperatures internal conversion from the first excited singlet state to the ground state is as unimportant as it has been found to be in hydrocarbons. In order to get additional information about these problems we measured the T-T absorption spectra, the triplet quantum yield QT, and the fluorescence quantum yield + of two indazole derivatives in solvents of different \%cosities and in a temperature range between +2S°C and -196’C. The photochemical reactions of these cornpounds are studied at our institute. The method used to measure the T-T absorption spectra and the triplet quantum yields has been described earlier 119,201 . The change in optical density 0f.a probe illuminated by a chopped light beam of medium intensity is
where CT is the triplet concentration, eG and cT are&Lhe extinction coefficients of the singlet and the triplet staies, end d is ihe depth of the measurihg 4. Pro-, ., ‘. cedures h&e, been-deicribed to qbtairi E-+) when ., I’ .-
:
.;,. : ..-_ .... ,’‘._ -,:.., .;.-.’ .‘_‘.
.‘, ..,
:..
..
1.
and Q(X) are known [l9,2d]. In order to obtain the quantum efficiency *, one has to know the tid the number of molecules excited pet unit tie rate constant kT for the unimolecular decay of the triplet state. A few modifications to the earlier experimental setup [20] w&e empioyed: (i) the probe was placed in a cryostat capable of AD@)
maintaining -196’C
_.
,,. _. .,,.
.I .;
i-25’C
and
decay constants kr smaUer than 100 set-L could not be determined from the frequency dependence of the amplitude of AD, but could be obtained from the decay curves of the phosphorescence. A phosphore&ope of the Lewis and Kasha type [2 I] together with a Boxcar integrator (PAR CW-1) allowed the exact determination of kT down to a value of 1.O set-l . Decay curves with kT-vdues smaller than 1 :Oset-1 could be displayed on a servorecorder. In this case iz could be shown that the lifetime of the phosphorescence was the same as the lifetime of the tranient absorbing species which thus is identified as the triplet state. The compounds chosen were 2-tert. butyl4-methylindazole (2,4 BMI) and I ,3dimethyLndazoie (I,3 DME), “0th showing practicalIy no irreversible photochemistj at IoW temperatures. Purification was achieved by chromatcjgraphy and subsequent.recrysiallizatiibn frdm.n-penian& The solvents used werðanbl UVksOL (Mer’ck) 96% and a mixture .
. .’ :
,‘.,
.‘._ 347.
.,
,:i -, .- ., ..,,, “. ., ;.,:: ,- -.: _‘,...’ : : ..,.,*._;-:_., -.. .. ..,,. :. _:__. .’,.(,.,,, ” - ‘, .r. -.:y ,I__;,. .:’ ( : _ .,;, ‘.- /,..’ ,, . ....::;...-.,. ,.I I-
., _,..:
betweerc
O.l°C;
(ii) triplet
: .:
an;’ temperature
within
:
:
.,
j.
..,,
..,
;_
.,
;,
-
:..
__
._
..
.:
:_ ‘,
,I
V&itie 11, number 3
,.-j+
”
.’:,
_ .:
,_
j .’ : ‘. : ,.
CHEMICALPHYSICS LEI-I-ERS
:
: :’
‘.
‘._ ‘, ,_. ‘,
15 Oct&er 197:
Fig. 1. Sk absorption (7) and,T-T abkptioi spectra (-: - -1of.&text. buty~methylindizole and [email protected] ethanol it low. tgiq&ittue. ._: : -’ :. . . . . ‘I ;,. ‘: ,, ” :- ‘.” /’ ...,:.._ ,,’ -., _..;:
:
of the latter with ethyleneglycol (Schuchardt) 1;:1 by volume..Both solvents were spectrograde quality and w&e used without further purification. Viscosities of &thesoIutions as a function of t~rn~ra~re were measured with the rotating cylinder method [2’L]T IXtsrminations of the fluorescence quantum yields at various temperatures were carried out using anthracene in ethanol as a standard [+ = 0.28 (25’C)I. TiG T-T absorption spectra of the two compounds in ethanol are shown in fig. 1. The coKe~pond~g spectra
in
15 October
CHEMICAL PHYSICS LWRS
Volume 11, number 3
ethanol/ethyleneg.Iycol
a+e
identical
within
,$e limits of error with those measured in etha11o1at
temperatures where &e solvent viscosities are equal. The quantum yields for fluorescence #F and intersystern ciossing h for several temperatures are compifed in table 1. We conclude from our experiments that in all but one of the cases stu’died these two processes are the only important pathways for deactivation of the first excited singlet state. In the case of 2,4 BMI in e~~ol/e~ylene~ycol at --50X, where q+ + 4)r < 1, an additional deactivation process of the S, state seems to be involved. From investigations of the, photochemical behaviour of this compound, the existence of a short lived phototropic state called 2’ is inferred which can deactivate by a direct radiationless transition to the ground state of ind&ole [23,24]. From this we concliude that, in the mo!ecules studied, : Internal conversion cannot compete withthe processes of fluorescence,‘intersystem crossing and photoreaction from the first excited sin&et state.. The first order triplet decay constants have beeti mea&red by the methods described above and are presented in table 2. The strong rjse of the observed decay constant /CTwith increasing temperiture cc&Id be explained quantitatively by diffusibn-controlled ‘impurity quenching., Similar results have been obt+ned by other workers :[25-291. :
:
Table 2 First order’triplet decay constants &‘(sec-;) in ethanol ,. -’ (WASOL)
1971
We thank the Swiss National ScienceFoundation for-support of the work described in this paper.
Refer&es E.C.L&, J.D.Laposa and J.M.H.Yu, J. Mot Spekry. 19 (1966) 412. R.G.Bennett and P.J.McCartin, I. Chem. E’hys44 (1969) 1969. BStevens and M.F.‘Fhontzz, C’hem, Phys. Letters 1
(1968) 549. B.Stwens and M.F,ihomaz, (1968) 535.
Chem. Phys. Lerfers
1
T.F.Hunter end R.F.Wyatt, Chem. Phys Letters 6 (1970) 221. P.F.Jones and S.Sie@, Chem. Phys.’ Letters 2 (1968) ~~.D~~~n and J.L.K.ropp. 3. pfifs. Chem. 73 (1969) 693. B.Stewns, M.F.Thomaz and P.FITones, J. Chem. Phys. 46 (1967) 405. J.L.Kropp. W.RDaiwson and M.W.Windsor, I. Phys.
Chem. 73 (1969) 1747.. A.Keltmann and L,Cim.iquist, i.x The tripbt state, ed. A.B.ZahIan (Cambridge Univ. Press, London, 1967) p. 439. B.R.Henryand hI.Kasha, 3. Chem. Phys. 47 (1967) 3319. R.&tier and Y.H.Meyer, I. Ck_im. Phys. 64 (1967) 9?9.
iXP.Craigand I.G.Ross. 1. Cheti Sot. (i954) L.589. S.G.Hadfey, J. Phys. Ghem. 74_(L97? 3551. J.S.Brinen, in: MoIecuIzr luminescence, ed. ECLim (Benjamin, New York, i969) p. 333. V.Zanker and B.Schneider, Z. mysik. Chem. 68 (1969)
19. V.&ker and GSrelI, Ber. Bunsenges. Physik. @em 73 (1969) 791. J.R.MerkiU and RG.Bermett, J. Cbem. Fhys. 43 (1965) 1410. H.&bbart, HeIv. Chim. Actz 47 (IP&) 2279. W.Heixizelmannand H.L+bhacc,Chem. Phys Letters 4
(1969) 19. G.N,Lewisa;ld Rf.Ka&a,3. Am. Chem. Sot. 66 (1944) 2100. G.A.von S&s and H.labbrt, ,’ 752.
1. Phys. C&em_72 (1968)
W.Heinzeham, tpivate comnwnicaltian. H&b&& W.He&eImann and J.P.Dubois, Pure A?$ Ckem; 24 (1970) 495. J.W.Hilpkm, G.Portetand L.I.Stief, Rot. Roy. Sot. A277 ($964)437: ‘. G.&&son and RLivingston.:J: C&m, Phys. 35 (1% I) 2182; : ‘. mange*, Thesis, vniy. of&sterda7n (1969). R.Lrringstoa ar;d.W.R.W~;J. Cl@k Phys. 39 (1963) ‘.
2y)rj*
.; I ‘.
.,. ..
I
B;Sic,&ns andk$.Wa&&+oc
_
Roy. ,&:
A281 (!Yt&