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Deuteration effects on the phosphorescence of aromatic hydrocarbons
_ ;
VoIume39;number3,
CHEkAi
:
_
DEUiERATION
EFFE.tiS
1 May 1976
__ &‘YSIC~ LETTERS __
ON THE PtiOSPHORESCEKE
-.’
:
OF ~AROMATIC HYDRO@ARBO&S
-.
J.B. BIR?XS, T.ti.S.‘&WLTON 77teSchusterLa&oratory.Universityof Manchester. Manchestef, UK and J. NAJBAR Instituteof Ckemistry.
Jagiellonian hiversity.
Cracow, Poland
Received 12 January 1976
bbservations of the phosphorescence decay of isotopic mixtures of naphthalene-ha and d p enanthrene-h 10 and -dlo, &hD and chrysene-h Iz and d12, in ethanol solutions at 77 K are anaIyscd to determine !he ratio .k~/k~ of the tripIet radiative rate parameters of the perprotonated and perdeuterated compounds. The ratio is 1.20 (eO.07) for naphthalene, 1.39 00.06) for phenanthrene, and 0.98 (kO.04) for chrysene.
The triplet (T,) lifetime 7T (= ilkT) of an aromatic hydrocarbon in a low temperature rigid solution is sensitive to deutzration. In general < > 77, where superscripts H and D refer to perprotonated and perdeuterated molecules, respectively. Un’Gl recently this effect was attributed solely to the T 1 -+ So intersystem crossin.~ rate k,,, with k + > kg*- Siebrand [ l] evaluated bT md k,,D from 7T and 7; for many aromatic hydrocarbons, by assuming that the T, + So phosphorescence rate km is independent of perdeuteration and of the nature of the compound, i.e. ,H +r
=kD = Fr
0.03 s-1,
(I)
The k& and k& values are practically unchanged and the quantitative model [l] developed to explain them is unaffected, even if relation (1) is only approximately valid. Lii and co-workers [2-41 reported a deuteration effect on km with kHm> k!& for aromatic hydrocarbons in EPA and other solvents at 77 K. The deuteration effect on li, is much less lihan that on kGT, i.e. k$/kh 4 kH /kD . In contrast, Johnson and Struder [5] observed G% km GTD = km, within the experimental error, foi toluene and naphthalene in an aigon matrix at 20 K. Both grqups measured the triplet lifetime TT) the fluorescence quantum efficiency qFS, and the
phosphorescence quantum yield QpT with S, excitation, and they evaluated kpy = $-&(I
-&’
(2)
assuming that the S, + T, inters stem crossing quan4; is low for some tum efficiency &m = 1 - qFs_ fpm aromatic hydrocarbons and this may introduce errors. The present paper describes a method of determining kh/k&. which does not involve direct observations of &T* If a dilute solution of unexcited aromatic molecules is excited into S, by a short duration light pulse at time t = 0, the phosphorescence response function [6] is ‘hrt>
kTSkFT = (kS _ AT) {exp(-kTtj
- exp(-kgt)),
(3)
where k,, is the SI * TI intersystem crossing rate and kS (= 11~~) is the total S, decay rate. At ;Z+ 7s the phosphorescence decay function is iL(l)
= qmkpr
exp(-kTt).
(4)
If the sb&tion is in photostationary ec@Iibrium under steady S, excitation, and the excitation is terminated at t = 0, the phosphorescence response function is 445
. __ )
“ck, exp(+.r)
-kT &p(7ksr)}, (5)
where qh (5 km/k& is the phosphorescence tum efficiency: This Simplifies, a.t t > 7s, to phorescence decay function &.(f) = qkqpyexp(---kTf;
the
quanphos-
= Qur e& (--k+f).
(6)
Consider g mixed dike solution of perprotonated (33) and perdeuteratkd (D) aromatic molecules of the
same tiompovnd. IffH andfD (= 1 -fli) are the fractions of the absorbed excitation photons absorbed by H and D respectively, then the total phosphorescence decay function of the mixed solution is $#)
= fH i”,tG + f,$!#),
(7)
where ih(t) and i&.(f) are the phosphorescence functions of H and D, respectively. In general
~,Cd
+Jo)
= A exp(-k;t)
decay
i- (1 -A)exp(-kyt),
(8)
where 1/A = 1 + B(&,‘f;)_
(9)
The parameter B depends on the mode of initiation
of the phosphorescence decay. For pulsed excitation of an unexcited system [4] BE = qsk;/qskk.
For relaxation from a photostationary
161 BR= &$$&,q,I3 D =g&&.
(10)
excited system
NINA charge quantizer and a Northern S&e&tic 600 muitichannkl calyser and teletype.’ Be decay fun& tions were processed using an Odra 1204 computer to evaluate the mean lifetimes. The samples were excited with steady illumination for at least 5 triplet lifetimes to achieve photostationary conditions, and the phosphorescence decay was obsei-ved after the iliurknation was terminated by a mechanical shutter. i’he fluorescence lifetimes of phenanthreneYh10 and *IO in ethanol at 77 K were measured using a single-photon fluorometer. Fig. 1 plots the parameter l/A, evaluated from (8) and the observed phbsphorescence decay functions, against the isotopic molar concentration ratio [HI/ [D] for naphthalene, phenanthrene and chrysene in ethanol at 77 K. [H] / [D] -is equated to f&&, since the mean molar extinction coefficients of H and D over the excitation spectral region are equal within the experimental error. The linear plots of fig. 1 are consistent with (9)_ Least squares computer fits to their gradients yielded the mean BR (= H /c&$,) vr4ues &d probable errors listed in table 1. PT and 7T were calculated both from the zeroth and first order moments of the phosphorescence decay functions, and their mean values and probable errors are listed in table 1. Table 2 lists data on the S, propertips of the compounds in solution at 77 K. Certain features may be noted. The direct qTs values [7] agree satisfactorily with those obtained assuming qm = 1 -q s. For I-F = q&, phenanthrene and chrysene ks = kg, qTs within tie experimental error, but for naphthalene
(11)
In order to Faximize the phosphorescence intensity, the relaxation method [ 1 l] has been used in the present experiments, but the pulsed method [lo] provides an alternative mzans of determining k&/k&.
2.0 -
Observations have been made of the phosphores-
cence decay functions of isotopic mixtures of naphthd+e-hg and dcig (total concentration lo-%), phenanthrene-12 to and ;Lilo (total concentration 10d3M), and chrysene-h12 and -d12 (total concentration 5 X 1OyM) in ethanol bolution at 77 K. The samp&s in Pyrex tubes (2 mm i.d.) were immersed directly in liquid nitrogen in a transparent Pyrex dewar. The phosphorescence decay functions were measurid using a Fanand spectrofluorimeter; EM1 photomultiplier, ..
Fig. 1. Phosphorescence decay function parameter (l/A) against isotopic molar concentration ratio [H]/[DI far naphthalene (N), phenanthrene Cp) and chxysene (C) in ethanol solutions at 77 K. I _.
EPA EtOH CA BPA EtOH CA
EPA
phcnanthrene-frto
chrysene-lr12 clnyscnc?llz
a)CA = celhllose ncctatc,
p~cn~nthrene~lo
t:; [61 161
141
L!&*nsl i"il iLxi*QLQl [41 _ . I
0.77 f 0.01 I.1 0.77 f 0.01b)
La-
14.0t 0.3 13*tl* 0.3
.
LOO* 0‘02
,
13.6 $0.9 /41
1to1 i 0.0s
&I&
f!L&Ql
’
0.99 f 0.02
2.28 $0.08
2.80 f 0.10
kTS
0.98 f 0.04 1.35 1.43 1:0.17
13.8 t 0,9
-p
0,194 f 0.004 0.205 0.175 * 0.008
141
0.57t 0.03 b)
0.61 f 0.02 b)
‘ITS
!Ul.Q*:JwlQfi 0.28 0.25 1 0,028
1.39 f 0.06 1.44 1.29 1 0.16
1.20 f 0,07 1.69 1*4510‘17
QJ.2*U!!L
I.w.*!UL% 141 -
Q.a~QJa
ws
‘I kSSUmillg qTS = 1 L &JFS, cl Presentwork,
EPA
%! wa 2.&Q
EPA OtOH EPA &OH CA”)
naphtlmlene-~~o
naphthalene-d~
TS(n,s)
Sol&t
Molecule
Table 2 S1 propcrticsof aronmtichydrocarbonsin solutionnt 77 K (experimentaldata underlined)
QaJ~swL1
Qi
EtOII EPA [ 31 EPA 141
chrysone
0.345 0.312 x 0,037
0,232 1 0.004 0,243 0.245 t 0.008
f!aLa*t
EtOH EPA [ 3] EPA 14)
phcnanthrcne 1123 U*Q.L!J
0,118 f 0.003 0,125 0.124 1:0,003
Solvent
nupl~tIl~lcne EtOH 5 EPh[3] ,e EPA [4]
MoiecuIe
Table 1 Tl propertiesof aromatic hydrocarbonsin solutionat 77 K (experiments data ul~dcrlined)
Vo!uril& 39, &mber
..CHEMICAL
3.
PHIiSICS
1 May 1976
LETTERS
_..:
k!s > kg, & > 4%. The clqse agreement betwken the T_~Value-sin ethanol and EPA solutions
L$/&
= $&$@&i-$~~
-
(12)
The experimental values of $, r:, $& and &. for EPA solutions [3,4] and the corresponding values of &!&“,, q/$ and kH /kh are also listed in rable 1. The values of +f~= rsrfor the Three compounds in ethanol
and EPA solutions
are in reasonable
agreement.
topic mixtures the phosphbrescence intensities of H and D are excited and observed under identical spectroscopic and environmental conditions, and the comparison is made for a-series of mixtures of different isotopic fractions. This procedure is expected to yield more reliable values of q$$/& than separate observations of GH pi and & for individual solutions of H and D, particularly when r& Is low as for naphthalene and chrysene. The two sets of EPA solution data [3,4] in table 1 show the different values of $3 and & that can be obtained from such observations, even in the same laboratory.
For phenan+&ene the values of @$i@& and k&/kk in the ttio solvents also agree within the experimental
The experimental work was undertaken at the University of Manchester during J. Najbar’s tenure of a British Council Scholarship, and leave of absence
error, confkming
from Jagiellonian
For
that k&
> kg
for this compound.
naphthalece and chrysene the ethanol solution and k&/I& are significantly lower values of $!+& thvl the EPA solution values. The value of k&/i?& = 1.20 f 0.07 for naphrhalene in ethanol solution is intermediate between the values of 1.45 + 0.17 obtained in EPA solution [4J and 0.96 f 0.20 obtained in solid argon solution at 20 K [5], dthough it lies within the range of their probable errors. The ethanol solution data confirm that kh/kk > 1 for naphthalene, but they indicate that its magnitude is less than either the original 131 or subsequent [4] EPA solution value.‘For chrysene in ethanol solution i?& = kh within the experimental error, a result in clear disagreement with the EPA solution data [3,4]. Comparison of the experimental parameters for the ethanol and EPA solutions (table 1) shows that the major discrepancies are in the values of $g/&&. In the phosphorescence decay measurements on the iso-
448
University.
The subsequent
tation and data analysis were undertaken
compu-
at both insti-
tutions.
References [l] W. Siebrand. ia: The triplet state, ed. A.B. Zahlan (Cambridge Univ. Press, London, 1976) p_ 31. [2] SF. Fischer and EC. Lim, Chem. Phys. Letters
14 (1972)
[3] FLi and EC. Lim, J. Chem. Phys. 57 (1972) 605. [4] N. Kanamaru, H.R. Bhattacharjee and E.C. Lim, Chem. Phys. Letters 26 (1974) 174. [5] P.hI. Johnson and h1.C. Struder, Chem. Phys. Letters 18 (1973) 341. [6] P.F. Jones and A.R. Galloway, J. Chim. Phys. Suppl. Transitions non radiatives dans les moldcules (1970) p_ 110. [7] R.E. Kellogg ad R-G. Bennett, J. Chem. Phys. 41 (1964) 3042.
VoIume39;number3,
CHEkAi
:
_
DEUiERATION
EFFE.tiS
1 May 1976
__ &‘YSIC~ LETTERS __
ON THE PtiOSPHORESCEKE
-.’
:
OF ~AROMATIC HYDRO@ARBO&S
-.
J.B. BIR?XS, T.ti.S.‘&WLTON 77teSchusterLa&oratory.Universityof Manchester. Manchestef, UK and J. NAJBAR Instituteof Ckemistry.
Jagiellonian hiversity.
Cracow, Poland
Received 12 January 1976
bbservations of the phosphorescence decay of isotopic mixtures of naphthalene-ha and d p enanthrene-h 10 and -dlo, &hD and chrysene-h Iz and d12, in ethanol solutions at 77 K are anaIyscd to determine !he ratio .k~/k~ of the tripIet radiative rate parameters of the perprotonated and perdeuterated compounds. The ratio is 1.20 (eO.07) for naphthalene, 1.39 00.06) for phenanthrene, and 0.98 (kO.04) for chrysene.
The triplet (T,) lifetime 7T (= ilkT) of an aromatic hydrocarbon in a low temperature rigid solution is sensitive to deutzration. In general < > 77, where superscripts H and D refer to perprotonated and perdeuterated molecules, respectively. Un’Gl recently this effect was attributed solely to the T 1 -+ So intersystem crossin.~ rate k,,, with k + > kg*- Siebrand [ l] evaluated bT md k,,D from 7T and 7; for many aromatic hydrocarbons, by assuming that the T, + So phosphorescence rate km is independent of perdeuteration and of the nature of the compound, i.e. ,H +r
=kD = Fr
0.03 s-1,
(I)
The k& and k& values are practically unchanged and the quantitative model [l] developed to explain them is unaffected, even if relation (1) is only approximately valid. Lii and co-workers [2-41 reported a deuteration effect on km with kHm> k!& for aromatic hydrocarbons in EPA and other solvents at 77 K. The deuteration effect on li, is much less lihan that on kGT, i.e. k$/kh 4 kH /kD . In contrast, Johnson and Struder [5] observed G% km GTD = km, within the experimental error, foi toluene and naphthalene in an aigon matrix at 20 K. Both grqups measured the triplet lifetime TT) the fluorescence quantum efficiency qFS, and the
phosphorescence quantum yield QpT with S, excitation, and they evaluated kpy = $-&(I
-&’
(2)
assuming that the S, + T, inters stem crossing quan4; is low for some tum efficiency &m = 1 - qFs_ fpm aromatic hydrocarbons and this may introduce errors. The present paper describes a method of determining kh/k&. which does not involve direct observations of &T* If a dilute solution of unexcited aromatic molecules is excited into S, by a short duration light pulse at time t = 0, the phosphorescence response function [6] is ‘hrt>
kTSkFT = (kS _ AT) {exp(-kTtj
- exp(-kgt)),
(3)
where k,, is the SI * TI intersystem crossing rate and kS (= 11~~) is the total S, decay rate. At ;Z+ 7s the phosphorescence decay function is iL(l)
= qmkpr
exp(-kTt).
(4)
If the sb&tion is in photostationary ec@Iibrium under steady S, excitation, and the excitation is terminated at t = 0, the phosphorescence response function is 445
. __ )
“ck, exp(+.r)
-kT &p(7ksr)}, (5)
where qh (5 km/k& is the phosphorescence tum efficiency: This Simplifies, a.t t > 7s, to phorescence decay function &.(f) = qkqpyexp(---kTf;
the
quanphos-
= Qur e& (--k+f).
(6)
Consider g mixed dike solution of perprotonated (33) and perdeuteratkd (D) aromatic molecules of the
same tiompovnd. IffH andfD (= 1 -fli) are the fractions of the absorbed excitation photons absorbed by H and D respectively, then the total phosphorescence decay function of the mixed solution is $#)
= fH i”,tG + f,$!#),
(7)
where ih(t) and i&.(f) are the phosphorescence functions of H and D, respectively. In general
~,Cd
+Jo)
= A exp(-k;t)
decay
i- (1 -A)exp(-kyt),
(8)
where 1/A = 1 + B(&,‘f;)_
(9)
The parameter B depends on the mode of initiation
of the phosphorescence decay. For pulsed excitation of an unexcited system [4] BE = qsk;/qskk.
For relaxation from a photostationary
161 BR= &$$&,q,I3 D =g&&.
(10)
excited system
NINA charge quantizer and a Northern S&e&tic 600 muitichannkl calyser and teletype.’ Be decay fun& tions were processed using an Odra 1204 computer to evaluate the mean lifetimes. The samples were excited with steady illumination for at least 5 triplet lifetimes to achieve photostationary conditions, and the phosphorescence decay was obsei-ved after the iliurknation was terminated by a mechanical shutter. i’he fluorescence lifetimes of phenanthreneYh10 and *IO in ethanol at 77 K were measured using a single-photon fluorometer. Fig. 1 plots the parameter l/A, evaluated from (8) and the observed phbsphorescence decay functions, against the isotopic molar concentration ratio [HI/ [D] for naphthalene, phenanthrene and chrysene in ethanol at 77 K. [H] / [D] -is equated to f&&, since the mean molar extinction coefficients of H and D over the excitation spectral region are equal within the experimental error. The linear plots of fig. 1 are consistent with (9)_ Least squares computer fits to their gradients yielded the mean BR (= H /c&$,) vr4ues &d probable errors listed in table 1. PT and 7T were calculated both from the zeroth and first order moments of the phosphorescence decay functions, and their mean values and probable errors are listed in table 1. Table 2 lists data on the S, propertips of the compounds in solution at 77 K. Certain features may be noted. The direct qTs values [7] agree satisfactorily with those obtained assuming qm = 1 -q s. For I-F = q&, phenanthrene and chrysene ks = kg, qTs within tie experimental error, but for naphthalene
(11)
In order to Faximize the phosphorescence intensity, the relaxation method [ 1 l] has been used in the present experiments, but the pulsed method [lo] provides an alternative mzans of determining k&/k&.
2.0 -
Observations have been made of the phosphores-
cence decay functions of isotopic mixtures of naphthd+e-hg and dcig (total concentration lo-%), phenanthrene-12 to and ;Lilo (total concentration 10d3M), and chrysene-h12 and -d12 (total concentration 5 X 1OyM) in ethanol bolution at 77 K. The samp&s in Pyrex tubes (2 mm i.d.) were immersed directly in liquid nitrogen in a transparent Pyrex dewar. The phosphorescence decay functions were measurid using a Fanand spectrofluorimeter; EM1 photomultiplier, ..
Fig. 1. Phosphorescence decay function parameter (l/A) against isotopic molar concentration ratio [H]/[DI far naphthalene (N), phenanthrene Cp) and chxysene (C) in ethanol solutions at 77 K. I _.
EPA EtOH CA BPA EtOH CA
EPA
phcnanthrene-frto
chrysene-lr12 clnyscnc?llz
a)CA = celhllose ncctatc,
p~cn~nthrene~lo
t:; [61 161
141
L!&*nsl i"il iLxi*QLQl [41 _ . I
0.77 f 0.01 I.1 0.77 f 0.01b)
La-
14.0t 0.3 13*tl* 0.3
.
LOO* 0‘02
,
13.6 $0.9 /41
1to1 i 0.0s
&I&
f!L&Ql
’
0.99 f 0.02
2.28 $0.08
2.80 f 0.10
kTS
0.98 f 0.04 1.35 1.43 1:0.17
13.8 t 0,9
-p
0,194 f 0.004 0.205 0.175 * 0.008
141
0.57t 0.03 b)
0.61 f 0.02 b)
‘ITS
!Ul.Q*:JwlQfi 0.28 0.25 1 0,028
1.39 f 0.06 1.44 1.29 1 0.16
1.20 f 0,07 1.69 1*4510‘17
QJ.2*U!!L
I.w.*!UL% 141 -
Q.a~QJa
ws
‘I kSSUmillg qTS = 1 L &JFS, cl Presentwork,
EPA
%! wa 2.&Q
EPA OtOH EPA &OH CA”)
naphtlmlene-~~o
naphthalene-d~
TS(n,s)
Sol&t
Molecule
Table 2 S1 propcrticsof aronmtichydrocarbonsin solutionnt 77 K (experimentaldata underlined)
QaJ~swL1
Qi
EtOII EPA [ 31 EPA 141
chrysone
0.345 0.312 x 0,037
0,232 1 0.004 0,243 0.245 t 0.008
f!aLa*t
EtOH EPA [ 3] EPA 14)
phcnanthrcne 1123 U*Q.L!J
0,118 f 0.003 0,125 0.124 1:0,003
Solvent
nupl~tIl~lcne EtOH 5 EPh[3] ,e EPA [4]
MoiecuIe
Table 1 Tl propertiesof aromatic hydrocarbonsin solutionat 77 K (experiments data ul~dcrlined)
Vo!uril& 39, &mber
..CHEMICAL
3.
PHIiSICS
1 May 1976
LETTERS
_..:
k!s > kg, & > 4%. The clqse agreement betwken the T_~Value-sin ethanol and EPA solutions
L$/&
= $&$@&i-$~~
-
(12)
The experimental values of $, r:, $& and &. for EPA solutions [3,4] and the corresponding values of &!&“,, q/$ and kH /kh are also listed in rable 1. The values of +f~= rsrfor the Three compounds in ethanol
and EPA solutions
are in reasonable
agreement.
topic mixtures the phosphbrescence intensities of H and D are excited and observed under identical spectroscopic and environmental conditions, and the comparison is made for a-series of mixtures of different isotopic fractions. This procedure is expected to yield more reliable values of q$$/& than separate observations of GH pi and & for individual solutions of H and D, particularly when r& Is low as for naphthalene and chrysene. The two sets of EPA solution data [3,4] in table 1 show the different values of $3 and & that can be obtained from such observations, even in the same laboratory.
For phenan+&ene the values of @$i@& and k&/kk in the ttio solvents also agree within the experimental
The experimental work was undertaken at the University of Manchester during J. Najbar’s tenure of a British Council Scholarship, and leave of absence
error, confkming
from Jagiellonian
For
that k&
> kg
for this compound.
naphthalece and chrysene the ethanol solution and k&/I& are significantly lower values of $!+& thvl the EPA solution values. The value of k&/i?& = 1.20 f 0.07 for naphrhalene in ethanol solution is intermediate between the values of 1.45 + 0.17 obtained in EPA solution [4J and 0.96 f 0.20 obtained in solid argon solution at 20 K [5], dthough it lies within the range of their probable errors. The ethanol solution data confirm that kh/kk > 1 for naphthalene, but they indicate that its magnitude is less than either the original 131 or subsequent [4] EPA solution value.‘For chrysene in ethanol solution i?& = kh within the experimental error, a result in clear disagreement with the EPA solution data [3,4]. Comparison of the experimental parameters for the ethanol and EPA solutions (table 1) shows that the major discrepancies are in the values of $g/&&. In the phosphorescence decay measurements on the iso-
448
University.
The subsequent
tation and data analysis were undertaken
compu-
at both insti-
tutions.
References [l] W. Siebrand. ia: The triplet state, ed. A.B. Zahlan (Cambridge Univ. Press, London, 1976) p_ 31. [2] SF. Fischer and EC. Lim, Chem. Phys. Letters
14 (1972)
[3] FLi and EC. Lim, J. Chem. Phys. 57 (1972) 605. [4] N. Kanamaru, H.R. Bhattacharjee and E.C. Lim, Chem. Phys. Letters 26 (1974) 174. [5] P.hI. Johnson and h1.C. Struder, Chem. Phys. Letters 18 (1973) 341. [6] P.F. Jones and A.R. Galloway, J. Chim. Phys. Suppl. Transitions non radiatives dans les moldcules (1970) p_ 110. [7] R.E. Kellogg ad R-G. Bennett, J. Chem. Phys. 41 (1964) 3042.