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Fluorescence of saturated hydrocarbons. II. The effect of alkyl substituents
Volume 5. number 5
CHEMICAL
FLUORESCENCE THE
EFFECT
OF
SATURATED OF
15 Aprii 1970
PHYSICS LETTERS
ALKYL
HYDROCARSONS. SUBSTXTUENTS
II. *
Received 23 February 1970 Fiuorcscencc spectra and quantum yields have been determinud for n vnriety of normal and op9ic alIn the cast of normal alkancs. the quantum yield inknnes mcl for some of their alk~l dcrkativos. crexes almost linenrly with number of carbon :ltoms from pcntane to heptndecane. whereas the emission spectrum rcmnins unchanged. The addition of n methyl group strongi?; reduces the qunnlum yield and shifts the spectrum to the red. For c.vclon1knne.s.a relativei!: intense emission is observed only for
c~cl&e_itno znci its nlkyl dcrivntives, one e.tiibit no fluarsccnce.
whereas cyciopentzmc.
Fluorescence spectra and quantum yields have previous!y been reported for a variety of saturated hydrocarbons [l]. These measurehave now been extended to include a larger number of normal alkanes and cycloatkanes and to examine systematically the effects of manoalkyl substi~tion. In tables 1 and 2 are presented results for alkanes and cycloalkanes respectively. The sample was the neat, deoxygenated liquid. AL1 fluorescence quantum yields f&f) reported*in this communication are for excitation at 1470A. For longer wavelength excitation, &f increases and most prominently for the lower members of the alkane series. However, the fluorescence spectrum is independent of exciting wavelength. For this reason we were able to obtain more reliable spectra for the smaller branches alkanes by exciti?g at longer wavelengths, most usually at 1660A $. All normai alkanes that we have examined emit with essentially the same spectral distribution of intensity with maximum at 207nm. This spectrum has been presenied previously [I]. Additionally, we have now confirmed that the fluorescence quantum yield increases monotonically from pentane to heptadecane. The dependence on number of carbon atoms is approximately linear with some indication of a slower increase in #f for the higher members of the series. * This work was supported by the U, S, Atomic Energy Commission, DocumczX No. COO-313-35. ** E. I, DupontDe Nemours 8 Co. Summer Research Fellow, 1969. $ Detailed studies of the xmvclength dependence of #f will be reported elsewhere.
c>-cloheptnnc. cgclooctnne.
and cyelodec-
The substitution of a methyl group to make a br~ched alkane invariably shifts the fluorescence to the red. Increasing the number of carbon ato_ms of the parent alkane consistently decreases the red shift (e.g., cf. Z-methyl alkane series). Additionally, it should be noted that the magnitude of the red shift is always largest for substitution at the 3 position. The fluorescence is structureless, as is the case for normal alkanes [I] and, indeed, with appropriate shift of o&in
is otherwise
very
similar
to the n-alkane
spectrum. The iluorescence quantum yields of the branched alkanes are strongly reduced from the parent normal al!tane. For all 2-methyl derivatives, c)f is reduced by about a factor of 10. As the branch point moves further from the end of the chain there is a tendency for r#~fto aiways
incresse. The ratio of c$f to the fluorescence lifetime (as estimated from oxygen quenching measurements) has been previously reported for the normal alkanes to be independent of number of carbon atoms [l]. This ratio is now found not to change markedly even on introduction of an alkyl substituent, regardless of its position in the chain or the length of the chain. The impIication is that variations in #f from one compund to another are due predominantly to variation in some non-radiative
rate constant.
To errplain these results,
it is suggested that the emitting state of a branched alkane is char-
acterized by relatively Sigh localization of the electronic excitation in the vicinity of the branch point. Equivalently, the carbon configuration _
Volume
5. number
3
CHEMICAL
Table 1 Fluorescence masimn fimns)a) and fluoresccncc qunnturn yields (&) for l-&?OA excitation of normal and branched alknnes at 25’ C Compound
&mas (nm)
6f(l47O;i)
Pentnnc Z-methyl3-methyl-
207 230 231
0.06 0.01 0.03
Hexnne 2-mcthyl3-methyl-
207 223 225
0.3 0.03 0.07
Hcptane 2-mefhyl3-methyl4-mcthyl-
207 220 223 218
0-i O.Oi 0.11 0.12
Octane 2-methyl3-methyl&methyl-
207 217 220 217
o.lW
Konanc ch-rdhyl3-methyl-I-methylJ-methyl-
20i 95 220 216 216
1.6 0.1,
Decane 2-mcthyl3-methgl4-mcthyl-
207 212 215 212
2.0 0.25 0.29 0.39
Undccane 2-mcthyl-
207 212
2.s 0.36
Dodccane
207
3.2
Tridecanc
207
3.5
Tctr.?dec.ule
207
3.9
Pentndccanc
2Oi
4.oc)
Hexadecanc
207
4.4
Heptndcc.anc
207
4.6
0.11 0.10 0.22
0.22 0.28 0.35
the analyzing system.
b) The previously
reported vniue for octane (0.0 x10e3) [l] is slightly altered now due to the effects of improved purification.
C) The absorption spectrum of this sample indicated the presence of an impurity which we have so far
been unable to remove.
this point n?ay be approximately
LETTERS
regarded
as an emitting chromophore. Our previous explanation for the decrease in Qf with decreasing number of carbon atoms for normal alkanes [l] can now again be invoked to explain the decrease
of pf on introduction of a branch: Localized electronic excitation is accompanied by relatively
large nuclear distortion and this, in turn, causes increase in the rate of some non-radiative process. A delocalization of excitation away
15 April
1970
Table 2 Fluorescence maxima (&m.zu)a) nnd fiuorescence quantum yields (&j for 147&I excitation of cyclic and substituted cyclic alknnes at 25O C
x103
a) These are maxima of the spectral distribution (quanta/cm-l) corrected for spectra: rcsponsc of
about
PHYSICS
Compound
*max
Wn)
d?f(L47&
Cyclopcntanc
%103
< 0.01
methyl-
c
Cyclohcsane methylCthYlhexyl1 .I dimcthgl-b)
201 214 214
3.5 5.5
'7.0 12
214 214
Cyclohcptabc methyl-
0.01
16 < 0.01 0.04
2%
Cyclooctnnc mcthyl-
.,< 0.01 < 0.01
Cgclodecanc
< 0.01
:I) These are maxima GEtbc spectral distribution (quanta/cm-‘) corrected for spectrni response
of
analvzing system. b, The 1.4’dimethvlcvclohesane used was a mixture oc . . cis and tvans isomers. (A difference in emission characteristics of cis-fun% isomers of saturated hydrocarbons is possible when the two isomers hare different conformations. Indeed. in the case of decalin, we have now observed an emission s,epararel! the
from
both cis
(A,,,=
233nm.
@f-2.1
x10--) and isomers which and boat-ho:& conforma-
tram (Am,= 220nm. 6f= 1-S xIO-~) exist
in the chair-chair
tions rcspcctively [G].)
from the branch point appears to occur on increasing the number of carbon atoms in the parent alkane. This is best illustrated in the Zmethyl aLkane series which exhibits both an increase in Qf and an approach of the emission spectrum to that of the normal aikane as the number of carbon atoms increases. The larger red shift observed for aLI S-meth-
yl compounds does not appear to be accompanied by any irregularity in Qf. It is interesting to note that a similar peculiarity of the S-methyl compounds is manifested in their boiling points, all of which are invariably higher than those of the 2,4, or 5 methyl substituted isomers [Z]. The spectral anomaly may, therefore, not be intrinsic to the molecule but rather caused by some curious intermolecular interaction in the condensed phase. Unfortunately, however, due to the very low fluorscence quantum yields of branched alkanes, we have so far been unsuccessful in sufficiently resolving a vapor spectrum
to adequately
confirm
this point.
Emission from cyclohexane has been previously reported [l] and is reZativeIy intense (+f = 3.5 x 10e3) whereas no fluorscence (Le., $f < 10e5) has been observed from cyclopentane, 297
Volume 5. number 3
CHEMICAL
eycloheptane, cyclooctane, or cyclodecane. However, microcrystalline cyclododecane flueresees weakly (Amax z 210nm). This peculiar variation in fluorescence quantum yield correlates surprisingly well with reported variations in ring-strain energies of cycloalkanes as estimated from heats of combustion *. Relative to
cyclohexane.
A& per CH2 group is 1.3 kcal
(cyclopentane).
0.0 kcal
(cycfohexane),
(cycloheptanef, (cyclodecane).
1.2 kcal (cyclooctane),
0.9
kcal
1.2 kcal
and 0.3 kcal (cyclododecane) 13). A possible explanation for this correlation may be developed on the assumption that these differences
in ground
state
strain
energies
are
re-
duced in the equiiibrium configurations of the excited states.
Thus with increasing
A& there
contrary
of sub-
is expected to be increasing disparity in equilibrium configuration of ground and excited states and this relative nuclear displacement we presume again leads ultimatefy to an increase in the radiationless transition probability. A&y1 substitution of cycfohexane appears to produce still another type of chromophore with characteristic emission at 214nm (see table 2). However.
quite
to the effect
stitution in the n-alkane series,
it will be noted
that now &f incrsases on ~trodu~tion of the aIkyl substituent. Since, once again, in the neighborhood of the chromophore there is likely to be relatively large nuclear distortion. it appears necessary to require that, for the cycloalkane, a more than compensating decrease in the rate of non-radiative transition be effected by ‘Yemouing electronic excitation irom the ring. The * WC are indebted to Professor G.Van-Catledge for bringing this correlation to our attention.
298
PHYSICS
LETTERS
1.5 April
1370
of a fluorescence upon methyl substitution of cyclopheptane which is otherwise non-fluorescent is most reasonably explained in this way. The continued absence of an emission on methyl substi~tion of cyclopentane and cyclooctane is consistent with their relatively higher ring-strain energies.
appearance
The fluorescence characteristics
of more
highfy substituted alkanes are currently being determined. In the case of disubstituted alkanes, preliminary results indicate a profound influence of the relative positions of the two branches. For example, no fluorescence has been detected f&i
least as strongiy as the parent alkanes (with hmax =: 240nm), whereas for the 2,4-, 2,5- and 2,6-dimethyl alkanes, bf is reduced to approximately the same order of magnitude as observed for the monosubstituded compounds. The systematics of these variations will be presented elsewhere.
REFERENCES F.Hirayama *and S.Lipsky, J.Chem.Phys. 51 (1969) 3616. [2] F. D. Rossini et al., Selected mlues of physical and thermodynamic properties of hydrocarbons and reIated compounds, A. P, I. Res.Proj. 44 (Carnegie Press, Pittsburgh. 1953). [3] E.L.Eliel, N.L.Allinger, S.J.Angyal andG.A. Morrison, Conformation4 Analysis (Interscience, New York, 1963) p. 193. [4j %I. S. Newman. ed., in : Steric effects in organic [l]
chemistry (Wiley, New York, 1965) p-23.
CHEMICAL
FLUORESCENCE THE
EFFECT
OF
SATURATED OF
15 Aprii 1970
PHYSICS LETTERS
ALKYL
HYDROCARSONS. SUBSTXTUENTS
II. *
Received 23 February 1970 Fiuorcscencc spectra and quantum yields have been determinud for n vnriety of normal and op9ic alIn the cast of normal alkancs. the quantum yield inknnes mcl for some of their alk~l dcrkativos. crexes almost linenrly with number of carbon :ltoms from pcntane to heptndecane. whereas the emission spectrum rcmnins unchanged. The addition of n methyl group strongi?; reduces the qunnlum yield and shifts the spectrum to the red. For c.vclon1knne.s.a relativei!: intense emission is observed only for
c~cl&e_itno znci its nlkyl dcrivntives, one e.tiibit no fluarsccnce.
whereas cyciopentzmc.
Fluorescence spectra and quantum yields have previous!y been reported for a variety of saturated hydrocarbons [l]. These measurehave now been extended to include a larger number of normal alkanes and cycloatkanes and to examine systematically the effects of manoalkyl substi~tion. In tables 1 and 2 are presented results for alkanes and cycloalkanes respectively. The sample was the neat, deoxygenated liquid. AL1 fluorescence quantum yields f&f) reported*in this communication are for excitation at 1470A. For longer wavelength excitation, &f increases and most prominently for the lower members of the alkane series. However, the fluorescence spectrum is independent of exciting wavelength. For this reason we were able to obtain more reliable spectra for the smaller branches alkanes by exciti?g at longer wavelengths, most usually at 1660A $. All normai alkanes that we have examined emit with essentially the same spectral distribution of intensity with maximum at 207nm. This spectrum has been presenied previously [I]. Additionally, we have now confirmed that the fluorescence quantum yield increases monotonically from pentane to heptadecane. The dependence on number of carbon atoms is approximately linear with some indication of a slower increase in #f for the higher members of the series. * This work was supported by the U, S, Atomic Energy Commission, DocumczX No. COO-313-35. ** E. I, DupontDe Nemours 8 Co. Summer Research Fellow, 1969. $ Detailed studies of the xmvclength dependence of #f will be reported elsewhere.
c>-cloheptnnc. cgclooctnne.
and cyelodec-
The substitution of a methyl group to make a br~ched alkane invariably shifts the fluorescence to the red. Increasing the number of carbon ato_ms of the parent alkane consistently decreases the red shift (e.g., cf. Z-methyl alkane series). Additionally, it should be noted that the magnitude of the red shift is always largest for substitution at the 3 position. The fluorescence is structureless, as is the case for normal alkanes [I] and, indeed, with appropriate shift of o&in
is otherwise
very
similar
to the n-alkane
spectrum. The iluorescence quantum yields of the branched alkanes are strongly reduced from the parent normal al!tane. For all 2-methyl derivatives, c)f is reduced by about a factor of 10. As the branch point moves further from the end of the chain there is a tendency for r#~fto aiways
incresse. The ratio of c$f to the fluorescence lifetime (as estimated from oxygen quenching measurements) has been previously reported for the normal alkanes to be independent of number of carbon atoms [l]. This ratio is now found not to change markedly even on introduction of an alkyl substituent, regardless of its position in the chain or the length of the chain. The impIication is that variations in #f from one compund to another are due predominantly to variation in some non-radiative
rate constant.
To errplain these results,
it is suggested that the emitting state of a branched alkane is char-
acterized by relatively Sigh localization of the electronic excitation in the vicinity of the branch point. Equivalently, the carbon configuration _
Volume
5. number
3
CHEMICAL
Table 1 Fluorescence masimn fimns)a) and fluoresccncc qunnturn yields (&) for l-&?OA excitation of normal and branched alknnes at 25’ C Compound
&mas (nm)
6f(l47O;i)
Pentnnc Z-methyl3-methyl-
207 230 231
0.06 0.01 0.03
Hexnne 2-mcthyl3-methyl-
207 223 225
0.3 0.03 0.07
Hcptane 2-mefhyl3-methyl4-mcthyl-
207 220 223 218
0-i O.Oi 0.11 0.12
Octane 2-methyl3-methyl&methyl-
207 217 220 217
o.lW
Konanc ch-rdhyl3-methyl-I-methylJ-methyl-
20i 95 220 216 216
1.6 0.1,
Decane 2-mcthyl3-methgl4-mcthyl-
207 212 215 212
2.0 0.25 0.29 0.39
Undccane 2-mcthyl-
207 212
2.s 0.36
Dodccane
207
3.2
Tridecanc
207
3.5
Tctr.?dec.ule
207
3.9
Pentndccanc
2Oi
4.oc)
Hexadecanc
207
4.4
Heptndcc.anc
207
4.6
0.11 0.10 0.22
0.22 0.28 0.35
the analyzing system.
b) The previously
reported vniue for octane (0.0 x10e3) [l] is slightly altered now due to the effects of improved purification.
C) The absorption spectrum of this sample indicated the presence of an impurity which we have so far
been unable to remove.
this point n?ay be approximately
LETTERS
regarded
as an emitting chromophore. Our previous explanation for the decrease in Qf with decreasing number of carbon atoms for normal alkanes [l] can now again be invoked to explain the decrease
of pf on introduction of a branch: Localized electronic excitation is accompanied by relatively
large nuclear distortion and this, in turn, causes increase in the rate of some non-radiative process. A delocalization of excitation away
15 April
1970
Table 2 Fluorescence maxima (&m.zu)a) nnd fiuorescence quantum yields (&j for 147&I excitation of cyclic and substituted cyclic alknnes at 25O C
x103
a) These are maxima of the spectral distribution (quanta/cm-l) corrected for spectra: rcsponsc of
about
PHYSICS
Compound
*max
Wn)
d?f(L47&
Cyclopcntanc
%103
< 0.01
methyl-
c
Cyclohcsane methylCthYlhexyl1 .I dimcthgl-b)
201 214 214
3.5 5.5
'7.0 12
214 214
Cyclohcptabc methyl-
0.01
16 < 0.01 0.04
2%
Cyclooctnnc mcthyl-
.,< 0.01 < 0.01
Cgclodecanc
< 0.01
:I) These are maxima GEtbc spectral distribution (quanta/cm-‘) corrected for spectrni response
of
analvzing system. b, The 1.4’dimethvlcvclohesane used was a mixture oc . . cis and tvans isomers. (A difference in emission characteristics of cis-fun% isomers of saturated hydrocarbons is possible when the two isomers hare different conformations. Indeed. in the case of decalin, we have now observed an emission s,epararel! the
from
both cis
(A,,,=
233nm.
@f-2.1
x10--) and isomers which and boat-ho:& conforma-
tram (Am,= 220nm. 6f= 1-S xIO-~) exist
in the chair-chair
tions rcspcctively [G].)
from the branch point appears to occur on increasing the number of carbon atoms in the parent alkane. This is best illustrated in the Zmethyl aLkane series which exhibits both an increase in Qf and an approach of the emission spectrum to that of the normal aikane as the number of carbon atoms increases. The larger red shift observed for aLI S-meth-
yl compounds does not appear to be accompanied by any irregularity in Qf. It is interesting to note that a similar peculiarity of the S-methyl compounds is manifested in their boiling points, all of which are invariably higher than those of the 2,4, or 5 methyl substituted isomers [Z]. The spectral anomaly may, therefore, not be intrinsic to the molecule but rather caused by some curious intermolecular interaction in the condensed phase. Unfortunately, however, due to the very low fluorscence quantum yields of branched alkanes, we have so far been unsuccessful in sufficiently resolving a vapor spectrum
to adequately
confirm
this point.
Emission from cyclohexane has been previously reported [l] and is reZativeIy intense (+f = 3.5 x 10e3) whereas no fluorscence (Le., $f < 10e5) has been observed from cyclopentane, 297
Volume 5. number 3
CHEMICAL
eycloheptane, cyclooctane, or cyclodecane. However, microcrystalline cyclododecane flueresees weakly (Amax z 210nm). This peculiar variation in fluorescence quantum yield correlates surprisingly well with reported variations in ring-strain energies of cycloalkanes as estimated from heats of combustion *. Relative to
cyclohexane.
A& per CH2 group is 1.3 kcal
(cyclopentane).
0.0 kcal
(cycfohexane),
(cycloheptanef, (cyclodecane).
1.2 kcal (cyclooctane),
0.9
kcal
1.2 kcal
and 0.3 kcal (cyclododecane) 13). A possible explanation for this correlation may be developed on the assumption that these differences
in ground
state
strain
energies
are
re-
duced in the equiiibrium configurations of the excited states.
Thus with increasing
A& there
contrary
of sub-
is expected to be increasing disparity in equilibrium configuration of ground and excited states and this relative nuclear displacement we presume again leads ultimatefy to an increase in the radiationless transition probability. A&y1 substitution of cycfohexane appears to produce still another type of chromophore with characteristic emission at 214nm (see table 2). However.
quite
to the effect
stitution in the n-alkane series,
it will be noted
that now &f incrsases on ~trodu~tion of the aIkyl substituent. Since, once again, in the neighborhood of the chromophore there is likely to be relatively large nuclear distortion. it appears necessary to require that, for the cycloalkane, a more than compensating decrease in the rate of non-radiative transition be effected by ‘Yemouing electronic excitation irom the ring. The * WC are indebted to Professor G.Van-Catledge for bringing this correlation to our attention.
298
PHYSICS
LETTERS
1.5 April
1370
of a fluorescence upon methyl substitution of cyclopheptane which is otherwise non-fluorescent is most reasonably explained in this way. The continued absence of an emission on methyl substi~tion of cyclopentane and cyclooctane is consistent with their relatively higher ring-strain energies.
appearance
The fluorescence characteristics
of more
highfy substituted alkanes are currently being determined. In the case of disubstituted alkanes, preliminary results indicate a profound influence of the relative positions of the two branches. For example, no fluorescence has been detected f&i
least as strongiy as the parent alkanes (with hmax =: 240nm), whereas for the 2,4-, 2,5- and 2,6-dimethyl alkanes, bf is reduced to approximately the same order of magnitude as observed for the monosubstituded compounds. The systematics of these variations will be presented elsewhere.
REFERENCES F.Hirayama *and S.Lipsky, J.Chem.Phys. 51 (1969) 3616. [2] F. D. Rossini et al., Selected mlues of physical and thermodynamic properties of hydrocarbons and reIated compounds, A. P, I. Res.Proj. 44 (Carnegie Press, Pittsburgh. 1953). [3] E.L.Eliel, N.L.Allinger, S.J.Angyal andG.A. Morrison, Conformation4 Analysis (Interscience, New York, 1963) p. 193. [4j %I. S. Newman. ed., in : Steric effects in organic [l]
chemistry (Wiley, New York, 1965) p-23.