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Radiationless decay processes in some alkyl aromatics
CHEMICAL
PHYSICS
LETTERS
RADIATIONLESS
1 (1968)
DECAY
623 -624.
NORTH-HOLLAND
PROCESSES
PUBLISHXNG
COMPANY.
IN SOME ALKYL
AMSTERDAM
AROMATICS
l
T. E. iiL4RTIN and k H. KALANTAB Department
of Cl’+emistry,’ University Edmontin. Received
Albefia,
of Alberta,
Chmda
19 February
1968
Phosphorescence lifetime measurements Indicate that alkyl CH modes do not contribute to the radiationless deactivation of the triplet state. CH modes associated with equivalent aryi hydrogens contribute to different extents.
The observed phosphorescence decay time, 7, for aromatic hydrocarbons is increased upon partial [I] or complete CZ] deuteratioll; this is known as the deuterium effect. Both theoreticai considerations, based upon Franck-Condon factors, and experimental evidence suggest that CH stretching modes are principally responsible for the larger radiationless rate constant, P, in depopulating the first excited triplet state of C6H6 and ClOH3, e.g. The high CH stretching frequency allows a large Franck-Condon overlap between the triplet state and the quasi-continuum of the ground singlet state to which tunnelling occurs
Observed
Toluenes
A recent study of some partially deuterated benzenes rl] indicates that only a few CH stretching normal modes are important to B. In particular, some non-totally symmetric CH stretches appear to contribute little to the decay processes. The sensitivity of /3to the normal mode raises a question about the effect of CH stretches associated with inequivalent hydrogen& These could be substituent group hydrogens of hydrogen atoms in inequivalent positions on aromatics less symmetric than benzene. Consequently, lifetimes of some selected toluenes and naphthalenes have been measured in 3-methylpentane glasses ** at 77OK [3].
Mole SEa) of isomer
i (sec1 6.02
C6H5CH3 C6H5CD3 b,
98
8.03
C6D5CH3 b)
96
10.24
C6D5CD3 c)
94
LO.54 are not
C6H6CH3 and C6HgCD3 and for C6DgCH3 and C6D5CD3 indicate that the a&y1 vibratiod modes are unimportant to the deactivation of the triplet state. These equalities are in contrast to the changes in 7 found in the deuterobenzenes [l]. Fof_ these, r lengthens by - 6% on going from CgHg to C&D and by - 66% from c(jIfDS to C6D6. In view of this, the virtually identical T for C6D3CH3 and C6D3CD3 are especidLy significant. Replacement of three alkyl CH stretching modes by CD modes increases r a mere 3% in this case. Therefore changes in unconjugated aJky1 CH modes do not affect T and make no significant contribution to the nonradiative decay process. Thus the deuterium effect arises from changes in only the aryl CH modes of the hydrocarbons. This suggests that the alkyl CA modes do not contribute in any way to the quasicontimnim of the ground state. The alkyd CR frequencies are 100 - 200 cm-l lower than those of the aryL CH
Toluenes
The effect of the methyl CH modes was studied by determining the lifetimes of selected deuterotoluenes. The equalities of the T (table 1) for
* Supported in part by the National Research Council of Canada. ** Glass relaxation phenomena 13) have been allowed for. Thus quoted 7 are reproducible to f 1%. 1968
times of some tolu-
a) Low eV mass spectral data. The lifetimes adjusted to 100 mole ‘% of isomer. 5) Merck, Sharp and Dohme. Montreal. c) Stohler Isotope Chemicals, Montreal.
PI*
April
‘Fable 1 phcaphorescence decay ene isomers
623
T.E.MARTIN
624
and A.H.RALANTAR
modes. Thus one might consider them slightly less important contributors to the quasicontinuum for these large singlet-tripiet energy gaps. However, the lifetime data show that any contributions to the quasicontinuum by the a&y1 modes are, in fact, negligible. Siebrand has suggested [2] that CH stretching anharmonicities are important to radiationless decay. In this context, the alkyl modes are expected to be either fairly harmonic (unlikely) or to have no anharmonic coupling with aryl CH modes. Naphthalenes The CYand /3positions on naphthalene are electronically and vibrationally inequivalent. Replacement of a hydrogen by deuterium in these positions leads only to changes in the pattern of vibrational modes. These inequivalent hydrogens give rise to different sets of CH stretching normal modes, which may contribute to fl differently, as is the case for bexcne [I]. This can be determined by measuring the lifetimes of compounds whose CH stretching modes, associated with either the cy or B positions, have been completely removed (as in (Y or &da naphthalene). Recently, the lifetimes of this key pair of tetradzutero naphthalenes have been reported (table 2) and can now be interpreted. The different T for these compounds indicate that the CH modes arising from the c! hydrogens are more efficient in the radiationless depopulation of the triplet
state than the 13CH modes. In more accurate work on the monodeuteronaphthalenes, Watts and Strickler [5] have recently confirmed this same effect for a case involving replacement of only a single hydrogen (table 2). lIeplacement of the (Y and I3 hydrogen9 can also be accomplished by methyl substitution. A single alkyl group is electronically the most innocuous ” and the toluene results show that its CH modes do not affect T. The lifetimes of the ethyl- and methyf. naphthalenes (table 2) are in the same order as those for the deuteronaphthalenes. If such a pattern exists for other monomethylhydrocarbons, then this is a simple method for determining the relative contribution to p of CH modes associated with inequivalent hydrogens. The different efficiencies of the CH modes associated with the cy and /3hydrogens may be related to their anharmonicities. Then Siebrand’s findings require that the CH stretching modes associated with the (Y hydrogens be more anharmanic than /3CH modes. This means that the (Y CH modes would be relatively more important to the quasicontinuum of the ground singlet state of naphthalene **. We thank S. Strickler for talks and for sharing his results prior to publication.
References Observed
Table 2 phosphorescence decay times, naphthalenes
7. of selected
o!
rs
methyl
2.45
2.33
ethyl
2.50.
2.35
-d, a)
2.9
_d; b)
a) See ref. [5]. b) The reported [4] lifetimes
and A.H.
Kalantar,
J.Chem.
Phys.
. in
erences therein. [3] T.E.AIartin and A. H.Kalantar. J. Phys.Chem., press. [4] N.Hirotaand C.A.Hutchinson Jr., J.Chem.Phys. 47 (1967) 1561. [5] R. Watts and S. J.Striclcler, to be published.
in
2.6
-6
d8
T. E.Martin press.
[2] W. Siebrand, J. Chem. Phys. 47 (1967) 2411 and ref-
(set) Substitution position 7
Naphthalene compound
[l]
-4 21.0
are 5.4 and 4.8 set for o! and @d4, respectively, in durene -d14 at 77’K. These values have been adjusted because the compounds used were impure (the C?!-d4had too little deuterium and the p-b.4 too much). *Account was also taken of the different efficiencies of the Q! and B hydrogens .
* Different substituents contribute varying electronic effects, thereby changing T and complicating effects due to vibrations. Consequently the 7 for naphthalenes with different substituents are not directly compared. ** Strickler and Watts have independently come to the same conclusion (private communication).
PHYSICS
LETTERS
RADIATIONLESS
1 (1968)
DECAY
623 -624.
NORTH-HOLLAND
PROCESSES
PUBLISHXNG
COMPANY.
IN SOME ALKYL
AMSTERDAM
AROMATICS
l
T. E. iiL4RTIN and k H. KALANTAB Department
of Cl’+emistry,’ University Edmontin. Received
Albefia,
of Alberta,
Chmda
19 February
1968
Phosphorescence lifetime measurements Indicate that alkyl CH modes do not contribute to the radiationless deactivation of the triplet state. CH modes associated with equivalent aryi hydrogens contribute to different extents.
The observed phosphorescence decay time, 7, for aromatic hydrocarbons is increased upon partial [I] or complete CZ] deuteratioll; this is known as the deuterium effect. Both theoreticai considerations, based upon Franck-Condon factors, and experimental evidence suggest that CH stretching modes are principally responsible for the larger radiationless rate constant, P, in depopulating the first excited triplet state of C6H6 and ClOH3, e.g. The high CH stretching frequency allows a large Franck-Condon overlap between the triplet state and the quasi-continuum of the ground singlet state to which tunnelling occurs
Observed
Toluenes
A recent study of some partially deuterated benzenes rl] indicates that only a few CH stretching normal modes are important to B. In particular, some non-totally symmetric CH stretches appear to contribute little to the decay processes. The sensitivity of /3to the normal mode raises a question about the effect of CH stretches associated with inequivalent hydrogen& These could be substituent group hydrogens of hydrogen atoms in inequivalent positions on aromatics less symmetric than benzene. Consequently, lifetimes of some selected toluenes and naphthalenes have been measured in 3-methylpentane glasses ** at 77OK [3].
Mole SEa) of isomer
i (sec1 6.02
C6H5CH3 C6H5CD3 b,
98
8.03
C6D5CH3 b)
96
10.24
C6D5CD3 c)
94
LO.54 are not
C6H6CH3 and C6HgCD3 and for C6DgCH3 and C6D5CD3 indicate that the a&y1 vibratiod modes are unimportant to the deactivation of the triplet state. These equalities are in contrast to the changes in 7 found in the deuterobenzenes [l]. Fof_ these, r lengthens by - 6% on going from CgHg to C&D and by - 66% from c(jIfDS to C6D6. In view of this, the virtually identical T for C6D3CH3 and C6D3CD3 are especidLy significant. Replacement of three alkyl CH stretching modes by CD modes increases r a mere 3% in this case. Therefore changes in unconjugated aJky1 CH modes do not affect T and make no significant contribution to the nonradiative decay process. Thus the deuterium effect arises from changes in only the aryl CH modes of the hydrocarbons. This suggests that the alkyl CA modes do not contribute in any way to the quasicontimnim of the ground state. The alkyd CR frequencies are 100 - 200 cm-l lower than those of the aryL CH
Toluenes
The effect of the methyl CH modes was studied by determining the lifetimes of selected deuterotoluenes. The equalities of the T (table 1) for
* Supported in part by the National Research Council of Canada. ** Glass relaxation phenomena 13) have been allowed for. Thus quoted 7 are reproducible to f 1%. 1968
times of some tolu-
a) Low eV mass spectral data. The lifetimes adjusted to 100 mole ‘% of isomer. 5) Merck, Sharp and Dohme. Montreal. c) Stohler Isotope Chemicals, Montreal.
PI*
April
‘Fable 1 phcaphorescence decay ene isomers
623
T.E.MARTIN
624
and A.H.RALANTAR
modes. Thus one might consider them slightly less important contributors to the quasicontinuum for these large singlet-tripiet energy gaps. However, the lifetime data show that any contributions to the quasicontinuum by the a&y1 modes are, in fact, negligible. Siebrand has suggested [2] that CH stretching anharmonicities are important to radiationless decay. In this context, the alkyl modes are expected to be either fairly harmonic (unlikely) or to have no anharmonic coupling with aryl CH modes. Naphthalenes The CYand /3positions on naphthalene are electronically and vibrationally inequivalent. Replacement of a hydrogen by deuterium in these positions leads only to changes in the pattern of vibrational modes. These inequivalent hydrogens give rise to different sets of CH stretching normal modes, which may contribute to fl differently, as is the case for bexcne [I]. This can be determined by measuring the lifetimes of compounds whose CH stretching modes, associated with either the cy or B positions, have been completely removed (as in (Y or &da naphthalene). Recently, the lifetimes of this key pair of tetradzutero naphthalenes have been reported (table 2) and can now be interpreted. The different T for these compounds indicate that the CH modes arising from the c! hydrogens are more efficient in the radiationless depopulation of the triplet
state than the 13CH modes. In more accurate work on the monodeuteronaphthalenes, Watts and Strickler [5] have recently confirmed this same effect for a case involving replacement of only a single hydrogen (table 2). lIeplacement of the (Y and I3 hydrogen9 can also be accomplished by methyl substitution. A single alkyl group is electronically the most innocuous ” and the toluene results show that its CH modes do not affect T. The lifetimes of the ethyl- and methyf. naphthalenes (table 2) are in the same order as those for the deuteronaphthalenes. If such a pattern exists for other monomethylhydrocarbons, then this is a simple method for determining the relative contribution to p of CH modes associated with inequivalent hydrogens. The different efficiencies of the CH modes associated with the cy and /3hydrogens may be related to their anharmonicities. Then Siebrand’s findings require that the CH stretching modes associated with the (Y hydrogens be more anharmanic than /3CH modes. This means that the (Y CH modes would be relatively more important to the quasicontinuum of the ground singlet state of naphthalene **. We thank S. Strickler for talks and for sharing his results prior to publication.
References Observed
Table 2 phosphorescence decay times, naphthalenes
7. of selected
o!
rs
methyl
2.45
2.33
ethyl
2.50.
2.35
-d, a)
2.9
_d; b)
a) See ref. [5]. b) The reported [4] lifetimes
and A.H.
Kalantar,
J.Chem.
Phys.
. in
erences therein. [3] T.E.AIartin and A. H.Kalantar. J. Phys.Chem., press. [4] N.Hirotaand C.A.Hutchinson Jr., J.Chem.Phys. 47 (1967) 1561. [5] R. Watts and S. J.Striclcler, to be published.
in
2.6
-6
d8
T. E.Martin press.
[2] W. Siebrand, J. Chem. Phys. 47 (1967) 2411 and ref-
(set) Substitution position 7
Naphthalene compound
[l]
-4 21.0
are 5.4 and 4.8 set for o! and @d4, respectively, in durene -d14 at 77’K. These values have been adjusted because the compounds used were impure (the C?!-d4had too little deuterium and the p-b.4 too much). *Account was also taken of the different efficiencies of the Q! and B hydrogens .
* Different substituents contribute varying electronic effects, thereby changing T and complicating effects due to vibrations. Consequently the 7 for naphthalenes with different substituents are not directly compared. ** Strickler and Watts have independently come to the same conclusion (private communication).