close

Вход

Забыли?

вход по аккаунту

?

Deactivation of Excited States.

код для вставкиСкачать
Deactivation of Excited Statesi**l
By L. M. Stephenson and G. S. Hammondr*J
Nature abhors an electronically excited state and strives to convert this energy to other
forms. This article is concerned with the variouspathways involved in the degradation of
electronic excitation to ordinary “thermal” forms, but will primarily discuss the quenching
of excited states by other molecules. The authors include as examples only those phototransformations and interactions encountered in solution.
1. Introduction
During phototransformations and interactions occurring in solution, the high frequency of collision
with solvent in general establishes a Boltzmann distribution among vibrational and rotational levels in
the lowest lying, electronically excited states within a
very short time span (< 10-11 sec) following excitation.
Reference to a ,Jablonski energy diagram (Figure 1)
c
Fig. 1 . Jablonski diagram. Solid arrows show absorption and emission
of light, wavy arrows indicate radiationless processes.
will serve to clarify this point. The energy gap between the ground state and the lowest lying excited
states is generally sufficiently large so that the vibrational overlap between these states is slight. As a consequence, modes of deactivation other than nonradiative decay are frequently sought from lowest excited
singlet and triplet states. Vibrational overlap between
higher excited states, which are often separated by only
small energy gaps, is much larger and leads to extremely rapid rates of nonradiative decay to the lowest excited states in any multiplicity manifold.
[*] Dr. L. M. Stephenson and Prof. G. S. Hammond
California Institute of Technology
Pasadena, California, 91 109 (USA)
[**I Contribution No. 3625 from the Gates and Crellin Laboratories of Chemistry.
[I] The importance of vibrational overlap in determining rates
of nonradiative decay has been delineated by G. W. Robinson and
R. P. Frosch, J. chem. Physics 37, 1962; 38, 1187 (1963).
Angew. Chem. internal. Edit.1 VoI. 8 (1969) / NL..4
Most organic molecules contain even numbers of
electrons which are spin paired and occupy orbitals
that may be classified as localized, delocalized (as in
conjugated unsaturated systems), or nonbonding
(such as the pairs on oxygen and nitrogen). In addition, organic molecules, which are composed primarily
of light atoms, have rigidly defined spin selection
rules for electronic transitions. These molecules usually
have ground singlet configuration, and display only
spin allowed, singlet-singlet transitions in absorption
spectra in the absence of strong external perturbation.
Following the rapid deactivation of the initially produced excited-singlet state to the lowest excited-singlet
state, what pathways of deactivation do exist for this
momentarily “trapped” molecule? Physical processes
which degrade singlet states are familiar; fluorescence
and nonradiative decay, both to the ground state and
crossing t o the excited triplet manifold. This latter
process, known as intersystem crossing has proved to
be of immense importance to photochemistry. The
triplet state populated by the intersystem crossing
process may also emit light (phosphorescence) or may
return to the ground state by nonradiative paths.
Events of more interest to photochemists include bond
formation or fragmentation in the excited state (chemical quenching), energy transfer to suitable substrates,
and formation of excited complexes (exiplexes) having
decay characteristics different from those of the excited
monomers.
2. The Chemical Quenching of Excited States
Because of the considerable energy possessed by electronically excited states, activation barriers for chemical reactions are frequently easily overcome. Conceptually, the quenching of excited states by bond
formation or fragmentation is very simple; in practice
the details are often difficult to explore. Two quenching mechanisms of this type have received considerable attention in our laboratories, i.e. the photoreduction and photoreactions of ketones and the photodimerization of conjugated dienes.
The photochemical reaction of ketones with alcohols
is a familiar method for the production of pinacols [21.
[2] A . Schonberg and A . Mustafa, Chem. Reviews 40, 181 (1947).
26 1
Until the late 195O's, however, little effort had been
directed toward elucidation of the reaction mechanism,
although the involvement of free radicals was generally
accepted. In 1960, Backstrom and Sandros 131 were able
to show compelling evidence for the identification of
the reactive species as the triplet state of the ketone.
Their results were focused on a study of the energy
transfer from benzophenone ( I ) to biacetyl, monitored
by the observation of induced phosphorescence from
the latter. Hammond, Moore, and Foss I43 were able t o
obtain results in substantial agreement with those of
Backstrom and Sandros. By using benzhydrol (2) as
the hydrogen donor the reaction was simplified by
virtue of the fact that only a single kind of free radical
was produced.
(C6Hs)2CO*
+
(C6Hs)zCHOH
(1) *
2 (C6fI&kOH
kr
---f
(2)
-+ (C~H~)~COH-COH(C~HS)~
(1)
The dependence of the quantum yield of benzophenone
disappearance ( @ - - ( I ) ) on benzhydrol concentration
was explained in terms of the competition of unimolecular decay [reaction (2)] with pinacol formation
[reaction (l)]
Treatment of (C&&CO
led to eq. (3).
* as an unstable intermediate
The constant a is the yield of benzophenone ( I ) .
Experimental results showed that a = 1.0 and that
kd/kr = 0.05 moles/liter. Since kr cannot be larger
than the diffusion-controlled limit, the data demanded
the conclusion that the excited state (C6H5)2CO* be
longer lived than the excited singlet state. The hypothesis that the excited state produced by absorption
of light by benzophenone undergoes quantitative
intersystem crossing to yield a long lived triplet excited state, which is the chemically reactive species,
has thus proved durable.
In a subsequent study, Hammond and Leermakers I51 showed
that carbonyl compounds such as naphthaldehyde and acetonaphthone, which possess lowest triplet states of x -+ x*
rather than n -f x* configuration, are relatively unreactive
in photoreduction. The effect is striking because the lifetimes
of x -f x * triplets are longer than those of n + x* triplets.
The strong dependence of chemical reactivity on electronic
configuration of the excited states dispels the vague notion
that energy rich, excited molecules are nondiscriminating in
their chemical reactivity.
[3] H . L . J. Brickstrom and K. Sandros, Acta chem. scand. I4,48
(1960).
[4] W. M. Moore, G. S. Hammond, and R . P. Foss. J. Amer.
chem. SOC.83, 2189 (1961).
[5] G. S. Hammond and P. A. Leermakers, J. h e r . chem. SOC.
84, 207 (1962).
262
Other typical photoreactions have also been shown to occur
from n -+ x* lowest excited triplet states, including the widely
studied oxetane-forming reaction [6,7J.
Arnold and GIick[a],and Hammond et al.191 have obtained,
for example, the oxetane ( 3 a ) and the two dioxaspiroheptanes (Sa) and (Sb) from the allene (4) and benzophenone.
Further data obtained by Leermakers and Hammondclol confirmed the supposition that triplet states
are involved in the photoreductions and photoreactions of ketones. A number of compounds were found
to quench the photoreduction of benzophenone by
benzhydrol. Interestingly, many quenchers, such as 2acetonaphthone, naphthalene, cis- and trans-l,3pentadiene, azulene, I-naphthaldehyde, display remarkably similar quenching efficiencies. Since the
compounds above all possess triplet energies lower
than that of benzophenone it was inferred that the
quenching mechanism must involve energy transfer
from benzophenone at close to the same rate for all
quenchers, presumably at or near that of a diffusion
controlled process. Other quenchers, with triplet
energies higher than that of benzophenone, such as
cyclohexene, are considerably less efficient.
The use of specific quenchers for triplet states is now
a commonplace technique for determining the spin
multiplicity of a photoreactive state. An often overlooked criterion for energy transfer, however, is the
production of an excited state of the quencher. This
is perhaps most easily demonstrated when employing
the conjugated dienes as quenchers [10-121.
Irradiation of solutions of conjugated diene and benzophenone (at wavelengths only absorbed by benzophenone) leads to cis-trans isomerization in the case
of 1,3-pentadiene, and dimerization occurs with many
[6] N . C. Yang: Organic Photochemistry. Butterworths, London 1965; Pure appl. Chem. 9, 591 (1964).
[7] Oxetane formation which proceeds through an excited
singlet n, x* state has recently been reported. See N. J . Turro,
P. Wriede, J . C. Dalton, D . A . Arnold, and A. H . Glick, J. Amer.
chem. SOC.89, 3950 (1967).
[8] R. Arnold and A . Glick, Chem. Commun. 1966, 813.
[9] H. Gotthardt, R . Steinmetz, and G. S . Hammond Chem.
Commun. 1967, 480; H. Gotthardt, R. Steinmetz, and G. S.
Hammond, J. org. Chemistry, 33, 2114 (1968).
[lo] P. A. Leermakers and G. S. Hammond, J. physic. Chem. 66,
1148 (1962).
[ll] G . S. Hammond, J. Turro, and P. A . Leermakers, J. physic.
Chem. 66,1142 (1962).
1121 This fact coupled with the existence of an unusually large
singlet-triplet splitting has made 1,3-pentadiene the "quencher
of choice" in many recent photochemical studies.
Angew. Chem. internat. W i t . 1 Vol. 8 (I969) 1 No. 4
dienes at high concentrations. Neither reaction
proceeds with high efficiency from the excited singlet
states of the diene, and the product distribution for
dimers in no way resembles that formed from ground
state molecules in thermal reactions. The triplet state
was thus specifically implicated in the photoreactions
of the ketones quenched by the dienes.
Triplet quenchers, such as 1,3-pentadiene, were employed more recently by Hammond and Wagner C13.141
to prove the reactive character of both singlet and
triplet states in Norrish Type I1 photoelimination.
The cyclobutane derivative is usually only a minor
product.
-Ketone
4[a1 DS[bl
2-Pentanone 0.44
2-Hexanone 0.50
0.05
0.21
Qt = @ o - @ ~
4Ial
I
+Acetone
@t = @o-QS
QS [bl
I
I :2 1 8:;: 1 82
0.39
0.29
having the same electronic configuration (e.g., n --f x*
or x
x*) are likely to show similar chemical reactivities. The importance of the triplet state in photochemistry lies in its lifetime, which is often sufficiently
long to allow photochemistry to compete with modes
--f
- --0
I
,
020
4-
I
0.10
030
CtH2=tH-C"=C"-CH1
010
lmolelll
8.0
&+%
H H
Fig. 2. Ratio (@,/@) of quantum yield for disappearance of ketone with
no quencher to quantum yield at given quencher concentrations vs concentration of quencher (1,3-~ntadiene). Do = quantum yield for disappearance of 2-pentanone ( 0 )and 2-hexanone ( 0 )without quencher;
0 = quantum yield with 1.3-pentadiene as quencher.
Quenching data in Figure 2 show that, even at high
pentadiene concentration (8 M), 2-hexanone still gives
a significant amount of cleavage [see reaction (4)], and
YH
1
+
CH3
that a lesser, but still significant portion of the 2pentanone reaction is unquenched. The most obvious
interpretation is that the phototransformations have
sizable singlet-btate contribution (see Table 1). These
results underline the fact that singlet and triplet states
[13] P. J. Wagner and G . S. Hammond, J. Amer. chem. SOC. 87,
4009 (1966).
1141 P. J. Wagner and G . S.Hammond, J. Amer. chem. SOC. 88,
1245 (1966).
Angew. Chem. internat. Edit.
VoI. 8 (1969) J No. 4
CH3
Table 2. Quantum yield (a)for formation of isoprene
dimers (in 10 M solution).
benzophenone
8-acetonaphthone
Ruorenone
I i:
0.92
:1:x
0.29
263
of physical deactivation, whereas corresponding excited singlets decay much more rapidly.
The reaction of conjugated dienes in their ground
states with excited triplet states of dienes to produce
dimers furnishes our second example of chemical
quenching. Dimers are produced smoothly when the
excited triplet states are produced via energy transfer
from benzophenone or other appropriate triplet
energy donors.
The dimerization of 1,3-butadiene
1,3-~entadiene
andisoprene [161,1,3-cyclopentadiene1171,andl,3-cyclohexadiene [I*] has been studied in detail. Product
mixtures, although complex, could be accounted for
by assuming that the addition reaction occurs in two
discrete steps, first formation of an intermediate biradical, and then cyclization of the biradical to form
the final products. The process is illustrated below for
the dimerization of 1,3-cyclohexadiene (7) (Table 2 ) 1191
and isoprene (8).
A plot of l/(yield of dimer) vs l/(diene concentration)
in a set of matched samples subjected to equivalent
irradiation is linear. Information concerning the lifetime of the diene triplet was obtained by use of azulene
quenching. In the presence of a high concentration of
diene, small amounts of azulene are not expected to
influence the transfer of energy from sensitizer to diene.
However, addition of slightly more than 1 0 - 2 ~
azulene is capable of quenching the dimerization by
more than a factor of 1/2 (see Figure 3). Clearly the
Fig. 3. Quantum yield for dimerization of isoprene in the presence of
varying concentrations of azulene.
azuIene is capable of quenching the diene triplet state.
From the slope of the plot the ratio of the quenching
constant to the rate constant for the attack of diene
triplet on ground state molecules is found to be
1.2 x 103. This indicates that the rate constant far
reaction must be about 2 x 106 liter mole-1 sec-1 or
less; analysis of the dependence of quantum yield on
[15] G. S. Hammond, N . J . Turro, and A . Fischer, J. Amer. chem.
SOC.83, 4674 (1961).
1161 G . S. Harnmond and R . S . H . Liu, J. Amer. chem. SOC.85,
477 (1963).
[I71 J. Turro and G . S.Hammond, J . Amer. chem. SOC.84, 2841
(1962).
[18] D. Valentine, N. J . Turro, and G . S . Hammond, J. Amer.
chem. SOC.86, 5202 (1964).
[I91 R. S. H . Liu, N . J . Turro, and G. S. Hammond, J. Amer.
chem. SOC.87, 3406 (1965).
264
diene concentration gives the ratio of the rate constant
for the dimerization reaction to that for radiationless
decay of triplets and places a lower limit of 2 x 10-8
sec on the triplet lifetime 1201.
Attempts to extend these methods to cross additions of dienes
have led, not unexpectedly, to complex mixtures of products.
In addition, attempts to add dienes to olefins photochemically
have met with little success[211. It is important to note, however, that bond formation between the excited state and other
molecules is a n important quenching pathway, even for rather
short-lived states, and that high yields of products are obtained from such interactions. This fact will be of importance
later in discussions of other quenching phenomena [221 (see
Section 4.1).
3. Triplet Energy Transfer in Solution
The transfer of triplet excitation energy from donors
to acceptors is an efficient pathway for the quenching
of excited states. The photochemical significance of the
production of chemically reactive states of the acceptor
was illustrated in the previous section. The energy
transfer process will now be outlined for the photochemical cis-trans isornerization of the stilbenes 124-261.
The following generalized mechanism will prove
adequate for discussion. Here S is sensitizer in a singlet,
S*(l), or triplet, S*(3), excited state, with c = cis olefin,
t = trans olefin, and T = olefin triplet states1271.
Three types of measurements have been made: quantum yields of sensitized cis + trans isomerization,
measurements of the stationary state composition of
the system following prolonged irradiation, and flash
spectrophotometric measurement of the rates of decay
of sensitizer triplets in the presence of the isomerizable
olefins.
-___ .
[20] This derives from the hypothesis that azulene will quench at
(or probably less than) a diffusion controlled rate, estimated at
6 x 109 liters mole-] sec-1. The deactivation of a diene triplet
state probably involves a process in which a twisted triplet
reverts to a planar ground-state form. Energy transfer requiring
a change in geometry of one of the reactions will, in all probability, be slower than the diffusion controlled rate (see Section 3).
1,3-Cyclohexadiene, which has recently been studied by G . F.
Vesley in these laboratories, appears to be incapable of undergoing very extensive distortion in excited states. Consequently, the
high susceptibility of cyclohexadiene to quenching by azulene
confirms this viewpoint.
[21] R. S. H. Liu and G . S . Hammond, J. Amer. chem. SOC.86,
1892 (1964).
[22] This also seems to be at the heart of the most satisfying
argument against a mechanism for energy transfer involving
bonding of the sensitizer and substrate, rather than physical excitation transfer. See references [19] and [231.
[23] G. 0.Schenck and R . Steinmetz, Bull. SOC.chim. Belgique
71, 781 (1962).
1241 G. S. Hammond et. al, J. Amer. chem. SOC.86, 3197 (1964),
and references cited therein.
[25] K. A. Muszkat, D . Cegion, and E. Fischer, J. Amer. chem.
SOC.89, 4814 (1967).
[26] G. S.Hammond, Chem. and Ind. chem. (Kagaku to Kogyo)
18, 1464 (1965).
[27] The mechanism implies that cisoid, transoid, and nonplanar
triplet states of the olefin are indistinguishable or are rapidly
interconverting species. Although data indicate that decay to
trans stilbene becomes slightly more favorable at higher temperature, thus indicating a small barrier to interconversion between
transoid and nonplanar (or cisoid) triplet states, no evidence forcing the abandonment of the common triplet hypothesis has been
found with a-methyl- or a,a’-dimethyl-stilbenes.
Angew. Chem. internat. Edit. 1 Vol. 8 (1969)
/ No. 4
excitation
(5)
decay to ground state
(6)
intersystem crossing
(7)
energy transfer to cis or
trans olefin
(8)
In a closely related study, Herkstroeter, Jones, and
Hammond 1301 have shown evidence for considerable
steric interference in energy transfer from benzophenone derivatives with bulky ortho substituents, with
transfer rates to cis-stilbene being more strongly
affected than those to trans-stilbene. Figure 4 shows
(9)
decay of substrate triplet
states to cis or trans
ground states
(10)
(11)
t
-
90 -
I
U
80-
i
s
-70-
Applying reactions (5)-(11) the stationary state
measurements are expected to be determined by the
product of two functions, an excitation ratio and a
decay ratio [eq. (12)]. From this several predictions
can be made regarding the behavior of the photo-
stationary state with respect to the sensitizer triplet
energy. Barring steric effects, or other unanticipated
difficulties, sensitizers with triplet energies greatly in
excess of both cis- (57 kcallmole) and trans-stilbene
(50 kcal/mole) [28J should transfer energy at equal
rates to both isomers, and, if the decay ratio is independent of sensitizer, as the mechanism implies, a
high energy region of similar photostationary states
would be obtained. As the sensitizer energy is varied
between
57 and
50 kcal/mole a falloff in transfer
rates to cis stilbene is expected, thus pumping the
system to cis rich mixtures. As the sensitizer energy
drops below 50 kcal/mole, it is expected that the
stationary state will again level off, and with the deficiency made up as activation energy the excitation
ratio would assume the value
-
kzlkl
=
-
4 5 7 000-50000)/RT
e
and lead to an extremely cis rich mixture.
Values obtained for the cisltrans photostationary state
ratios of stilbene with varying sensitizer triplet energies
agree with these predictions to only a limited extent
(Fig. 4). In the high energy region a near "ideal" behavior is noted, but, although rate constants near that of
a diffusion controlled reaction are noted, a slightly
sloping line cannot be ignored.
Valentine has considered various factors as sources of possible inefficiency in exothermic energy transfer [291. H e concludes that energy transfer t o the cis isomer is not diffusion
controlled and becomes more rapid as the sensitizer energy
increases. Steric factors may well be important and it should
be noted that the structure of the sensitizer becomes less
complex with higher energy.
[28] D. F. Evans, J. chem. SOC.(London) 1957, 1351; from oxygen perturbed singlet-triplet spectrum
[291 D. Valentine, Ph.D. Thesis, California Institute of Technology, Pasadena, California, 1966.
Angew. Chem. internaf. Edit. / Vo1. 8 (1969)
/ No. 4
.Y
0
60 -
196
5
-
Fig. 4. Photostationary states for the stilbenes, measured (0)
and predicted from kq measurements (m). ET = triplet energy of sensitizer.
Sensitizers: 1, cyclopropyl phenyl ketone. 2, acetophenone. 3, benzophenone. 4, thioxanthone. 5, anthraquinone. 6, flavone. 7, Michler's
ketone. 8, 2-naphthyl phenyl ketone. 9, 2-naphthaldehyde. 10, 2-acetonaphthone. 11, I-naphthyl phenyl ketone. 12. chrysene. 13, l-naphthaldehyde. 14, biacetyl. 15, 2,3-pentadione. 16, fluorenone. 17,
fluoranthrene. 18, dibenz[a,hlanthracene. 19, duroquinone. 20. benzil.
21, dibenz[a,clanthracene. 22. pyrene. 23, benz[a]anthracene. 24, benzanthrone. 25, 3-acetylpyrene. 26, acridine. 27, 9,10-dimethylbenz[alanthracene. 28, anthracene. 19, henz[cdlpyrene.
-
that as the sensitizer triplet energy drops from
60
kcal/mole to
50 kcal/moIe [311 the photostationary
states indeed move to cis-rich mixtures. At low triplet
energies, however, instead of leveling off, the curve
again returns to trans-rich mixtures, clearly unexpected
behavior. This result may be rationalized in two ways;
either transfer of energy to cis-stilbene is decreasing
less rapidly than predicted, or rates of transfer to
trans-stilbene decrease more rapidly than predicted
(or both).
-
In preliminary communications, Hammond and
SaZtieZ1321 adopted the former approach. In particular,
they suggested that cis-stilbene is converted directly to
a transoid or nonplanar (shown above) excited triplet
during energy transfer. The planar trans triplet is
known to possess a lower energy than the planar cis
form and nonplanar triplets are predicted, on the basis
of calculations 133,341, to have lower energy. Consequently, transitions from cis-stilbene to either the
[30] W. G. Herkstroeter, L. B. Jones, and G. S. Hammond, J.
Amer. chem. SOC.88, 4777 (1966).
[31] The 3-5 kcal/mole excess energy needed to obtain efficient
transfer is not well understood but is found in many systems,
including ones more rapid than stilbene.
[32] G. S. Hammond and J. SaltieI, J. Amer. chem. Soc. 85, 2516
(1963).
[33] D. S. McClure, Symposium on Photochemistry, California
Institute of Technology, Pasadena (1966).
1341 P. Borrell and H . H. Greenwood, Proc. Roy. SOC.(London)
1967. 453.
265
trans, planar, or twisted triplet states should require
less energy than excitation to the planar cis triplet.
This process is not observed spectroscopically (it
would violate the Franck-Condon principle) and hence
is termed a non-vertical transition or a non-vertical
energy transfer. The key postulate is that energy
transfer with non-vertical excitation of the acceptor
may occur when vertical transitions are forbidden for
energetic reasons. The phenomenon is expected to be
a n important factor only if one or both of the partners
is a flexible molecule, i.e., a molecule having significantly different equilibrium geometry in ground and
excited states.
Confirmation of this point of view was later obtained
by Hammond and Herkstroeter 1351 in a study of these
energy transfer rates by kinetic flash spectrophotometryI361. A solution containing a sensitizer and one
of the isornerizable substrates is flashed with a highintensity light pulse. A significant fraction of the sensitizer molecules are promoted to excited triplet states
by light absorption followed by intersystem crossing.
Decay of sensitizer triplets is then monitored by absorption spectroscopy. Decay is accelerated in the
presence of substances to which the sensitizers can
transfer their excitation.
Measurement of the dependence of decay rates on the
concentration of the quencher allows evaluation of
bimolecular rate constants for the quenching reaction.
A number of sensitizers were studied using the isomeric stilbenes and 1,2-diphenylpropenes (a-methyl-
9
a
t 7
P
s
-
m
0
6
5
I
$16
L2
,
46
I
-
50 5L 58
E, Ikcallmolel
62
,
66
Fig. 6. Rate constants for quenching of sensitizers by cis- (0)
and
rrans-1,2-diphenylpro~ene
( 0 ) .kq = quenching constant. ET = triplet
energy of sensitizers. Sensitizers: 1, triphenylene. 2, thioxanthone. 3, 2acetonaphthone. 4, I-naphthyl phenyl ketone. 5, fluorenone. 4 dibenz[u,h]anthracene. 7, duroquinone. 8, benzil. 9, dibenz[a,c]anthracene.
10, pyrene. 11, benz[ulanthracene. 12, benzanthrone. 13, 3-acetylpyrene.
14, acridine. 15,9,1O-dimethylbenz[a]anthracene.16, anthracene.
stilbenes) as quenchers. Plots of the logarithms of the
quenching constants (log k,) against the triplet
energies of the sensitizers (ET) are shown in Figures 5
and 6. If the energy deficiency for the low energy sensitizers were made up as activation energy, the straight
line portion of these figures should follow equation(13).
Alog kq/AET
=
-1/(2.303 R T )
(13)
The slope predicted by this equation is observed for
trans-stilbene. For cis-stilbene the rates of energy
transfer fall off much more slowly than a classical
mechanism would imply. The postulate of nonvertical
energy transfer is fully consistent with these results.
Nonvertical energy transfer is inferred to be operative
with both isomers of a-methylstilbene, since slopes
smaller than the classical value are observed with both
isomers.
Several other systems have also been discussed in terms of
nonvertical energy transfer, among these the photosensitized
interconversion of norbornadiene (16) and quadricyclene
(17) 1371 and the cis + trans isomerization of 1.2-diphenylcyclopropane (18) [381.
I
42
46
I
I
I
50 54 58
ET I kcallmolel
I
I
62
66
hv; S e n s .
hv;Sens.
Fig. 5. Rate constants for quenching of sensitizers by cis- (0)
and
trans-stilbene ( 0 ) .kq = quenching constant. ET = triplet energy of
sensitizer. Sensitizers: 1, triphenylene. 2, thioxanthone. 3, phenanthrene.
4, 2-acetonaphthone. 5, I-naphthyl phenyl ketone. 6, chrysene. 7.
fluorene. 8, dibenz[a,hlanthracene. 9, benzil. 10, dibenz[u.clanthracene.
11, pyrene. 12, benz[ulanthracene. 13, benzanthrone. 14, 3-acetylpyrene.
15. acridine. 16, 9,10-dimethylbenz[ulanthracene. 17, anthracene. 18,
benz[cdlp yrene.
[35] W . G. Herkstroeter and G . S . Hammond, J. Amer. chem.
SOC. 88,4169 (1966).
1361 Described by G . Porter in S. L. Fress, E. S. Lewis, and A .
Weissburger: Technique of Organic Chemistry. Interscience,
New York 1965, Vol. VIII, Part 11, p. 1055.
266
[37] G. S. Hammond, N . J. Turro, and A . Fischer, J. Amer. chem.
SOC.83,4674 (1961).
[381 G. S. Hammond, P. W y a f f ,C. D. DeBoer, and N . J. Turro,
J . Amer. chem. SOC.86,2532 (1964).
Angew. Chem. internat. Edit. ] Vol. 8 (1969) I No. 4
4.1. Chemical Spectroscopy
The triplet excitation energy of quadricyclene has been
measured as 92 kcal mole-' [391 and the vertical excitation
energy of diphenylcyclopropane is probably nearly as high as
that of benzene. Consequently, it was felt that energy transfer, coupled with bond breakage, could be involved - another
example of nonvertical energy transfer. However, recent
investigations (401 have shown that the quadricyclene - norbornadiene interconversion involves strong interaction with
excited singlet states of the sensitizers in many cases and that
triplet energy transfer may not be involved at all.
In similar findings, Cole and H a m m o n d ~ 4 1 1were able to
induce asymmetry into trans-diphenylcyclopropane using
optically active naphthalene derivatives as sensitizers; once
again, singlet state interactions are responsible for the
sensitized reaction[@]. Application of the concept of the
nonvertical mechanism for triplet energy transfer of these
systems was obviously a n overenthusiastic application of the
theory. However, re-examination of the original cases confirms the utility of the hypothesis in those examples. The
scope of the phenomenon is essentially undefined at this time.
Table 3.
-
An examination of the revised Saltiel plot (Figure 4)
shows several interesting features 1421.
The chemical response of the system (e.g., the [cis]/
[trans] ratio) changes abruptly as the triplet energy of
the donor passes through a point of the energy scale
2-3 kcal/mole higher than that of the acceptor. Note
that with the stilbenes the curve begins a steep rise in
the region of 60 kcal/mole and a descent in the region
of 50 kcal/mole. If the notions of the previous sections
concerning energy transfer rates are correct these
points should be near the spectroscopic excitation
energies of the triplet states of the stilbenes.
The photochemical dimerization of isoprene leads to
the compounds ( 9 ) - ( 1 5 ) , which may be classed as
Composition of products from photosensitized dimerization of isoprene (8).
~~~
Distribution of dimers, %
ET
No
Sensitizer [a]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Propiophenone
Cyclopropyl phenyl ketone
Acetophenone
1,3,5-Triacetylbenzcne
Benzaldehyde
o-Dibenzoylbenzene
Benzophenone
Thioxanthone [b]
2-Acetylfluorene [b]
Anthraquinone [b]
Flavone [bl
Michlers' ketone [b]
Acetylbiphenyl
8-Naphthyl phenyl ketone
P-Acetonaphthone
P-Naphthaldehyde
a-Naphthyl phenyl ketone
a-Acetonaphthone
a-Naphthaldehyde
Biacetyl
2,J-Pentanedione
Benzil
Fluorenone
Dibenz[u,hlanthracene [b]
Pyrene [bl
Benzanthrone [h]
3-Acetylpyrene
9,10-Dimethyldibenz[a]anthracene[b]
Anthracene [b]
9,IO-Dibromoanthracene [b]
(kcall
mole)
[cl
74.6
74.4
73.6
73.3
71.9
68.7
68.5
65.5
67.5
62.4
62.0
61.0
60.6
59.6
59.3
59.5
57.5
56.4
56.3
54.9
54.7
53.7
53.3
52.3
38.7
47
45
44.4
42.5
40.2
93
92
92
92
90
92
93
92
91
91
90
91
90
81
81
76
75
66
60
53
49
45
43
38
36
35
51
58
87
94
29.7
28.0
29.0
29.5
28.4
29.6
29.0
28.6
29.2
28.6
26.2
27.8
28.6
24.4
26.3
22.1
23.2
19.6
18.3
18.8
14.5
13.5
12.7
11.9
12.1
11.9
13.5
17.5
27.3
30.8
(12)
(13)
__
--
(14) and
(15)
9.7
8.0
8.2
6.7
7.5
7.7
9.0
8.0
8.0
7.5
5.8
7.7
8.3
8.7
9.3
6.3
5.4
8.7
5.2
4.1
4.9
3.5
4.6
2.6
2.1
2.2
6.4
5.0
8.2
7.9
4.4
5.4
4.8
4.6
6.3
4.9
4.7
5.2
6.1
6.2
7.8
5.8
6.8
13.7
14.5
15.9
18.7
27.0
29.2
34.6
38.6
38.5
42.5
13.3
15.4
46.6
36.6
30.3
8.5
3.6
2.2
2.9
3.0
3.7
3.4
2.8
2.1
3.2
3.0
3.2
3.9
3.3
3.1
5.9
4.5
7.5
6.9
7.7
11.0
12.7
12.4
16.9
14.6
18.6
18.4
18.7
12.3
12.1
4.9
2.1
23.3
25.2
25.4
25.8
28.2
23.9
25.2
28.9
23.8
25.1
28.1
25.8
24.2
21.1
18.2
26.5
20.9
17.4
16.8
10.0
15.4
15.3
14.1
13.6
9.5
8.7
15.7
16.1
21.4
21.8
(11)
(9)
30.7
30.5
29.7
29.6
26.1
31.2
30.1
26.0
30.0
29.5
28.3
29.5
29.1
26.9
27.2
21.7
25.0
19.5
19.3
19.8
14.6
12.4
11.7
10.0
12.4
11.8
15.5
19.0
29.8
33.8
[a] Sensitizer was 0.1 M in neat isoprene unless otherwise stated.
[b] Saturated solution of sensitizer in isoprene.
[cl ET = triplet energy of sensitizer.
[dl Sum of percentages of (9). (lo), ( l l ) , (14). and (15).
4. Applications of Quenching Studies
Apart from yielding some understanding of triplet
energy transfer and the behavior of excited states of
various substrates, the studies of cis-trans isomerization and other modes of chemical deactivation of excited states have pointed to several useful applications.
[39] P. S. Wei, Ph.D. Dissertation, California Institute of Technology, Pasadena, California, 1968.
[40] S. Murov, R. S. Cole, and G. S. Hammond, J. Amer. chem.
SOC.PO, 2959 (1968); S. Murov and G. S. Hammond, J. physic.
Chem. 72, 3797 (1968).
[41] C. S. Hammond and R. S. Cole, J. Amer. chem. SOC. 87,
3256 (1965).
Angew. Chem. internat. Edit.
Vol. 8 (1969) 1 No. 4
cyclobutanes, cyclohexenes, and cyclooctadienes.
Table 3 gives the product distribution with various
sensitizers. Five of the compounds, (9), (IO), ( I ] ) ,
(14), and (15), behave as a unit and their formation
is competitive with the production of the two cyclohexenes (12) and (13). These trends are shown in
[42] Reference [25] gives a Saltiel plot displaying much more
fine structure than can be found in Figure 4. Many of the minima
responsible for this structure have now been removed. It is found
that the isomerization induced by several sensitizers, namely the
quinones and the brominated sensitizers such as eosin, proceeds
by mechanisms other than energy transfer. It is significant that
many of these stationary states are closer to the composition at
thermal equilibrium than that mixture induced by normal sensitization. A thorough analysis of this problem is incomplete.
267
Figure 7 where the sum of the yields of the cyclobutanes (9)-(11) and cyclooctadienes (14) and (IS)
is plotted against the triplet excitation energies of the
sensitizers. The reasonable postulate is made that the
value of 50 kcalimole [431. Using high pressures of 0 2 ,
Evans [441 has observed these highly forbidden So -+ T
absorptions and places the energy for isoprene triplet
(a model transoid diene) at 60 kcal/mole and for cyclohexadiene (a model cisoid diene) an energy of 53 kcal/
mole. Obviously, the method of chemical spectroscopy provides a useful adjunct t o optical spectroscopy
in this case.
4.2. Quantum Yields of Isomerization.
A Method of TripIet Counting
I
I
LO
50
E, Ikcallnolei
I
I
I
60
70
75
Fig. 7. Dependence of isoprene dimer composition on triplet energy of
sensitizer. C = sum of (9). (IO), (11). ( 1 4 ) , and (15). ET = triplet
energy of sensitizer. For sensitizers 1-30, see Table 3.
cyclohexenes are derived from interaction between
fransoid ground states and cisoid excited triplet states
and that the cyclobutanes are derivable from transoid
ground and excited states. The cyclooctadienes, while
in principle derivable from an interaction between
cisoid ground- and cisoid excited-states, may actually
be formed by thermal rearrangement of the cis-dirnethylcyclobutanes [see reaction (14)].
In addition to placing nonvertical energy transfer on a
firm experimental basis, the flash spectroscopic results
of Herkstroeter also demonstrated the insensitivity of
the olefin triplet decay ratio to the structure of the sensitizer. Using the triplet decay ratio and the ratio of
rate constants for quenching by cis and trans species,
values of expected photostationary state compositions
were calculated and found to be in good agreement
with experimental values.
Recognizing this fact, Lamola and Hammond [461 were
able to show that the quantum yield for cis + trans
isomerization of an olefin is a function of only three
variables; the yield of sensitizer triplets produced by
intersystem crossing, the efficiency of energy transfer
to the olefin, and the triplet decay ratio of the olefin
triplet. In addition, the work of Herkstroeter had made
it clear that energy transfer rates could compete very
favorably with triplet decay rates with all but the very
lowest energy sensitizers. Thus, nearly 100% efficiency
in triplet energy transfer could be effected in most
cases even at relatively low acceptor concentrations.
Preliminary studies with benzophenone confirmed
these ideas, since this sensitizer was already known to
intersystem cross with unit efficiency (see Table 4).
Table 4. Quantum yields for cis-trans olefin isomerization by benzophenone (c = cis, f = irans).
The quantum yield for sensitized, cis + trans isomerization in the absence of the trans isomer is given by
Equation (15).
0
If the interpretation of the origin of the products is
correct, the first break in the curve should correspond
to a decrease in the rate of energy transfer to the
transoid diene, predominantly present in solution
(- 95 % in isoprene at room temperature). Thus the
lowest triplet state of the transoid olefin was assigned
a n energy of approximately 60 kcal/mole. The next
break in chemical response occurs at
50 kcal/mole
and represents a decrease in the rate of energy transfer
to the cisoid form, whose triplet energy was assigned a
-
268
[43] Figure 7 is reminiscent of the Saltiel stilbene curve (Figure 4),
and again would seem to demand the postulation of a nonvertical
energy transfer. Flash kinetic studies which confirm this feeling
even for the relatively rigid 1,3-cyclohexadiene, have been performed by G. F. Vesley. However, recent work by Liu and Edman [45] shows that sensitization with low-energy sensitizers
sometimes involves energy transfer from more highly excited
triplet states. This phenomenon may have made some contribution to data shown in Figure 7.
I441 D.F. Evans, J. chem. SOC.(London), 1960, 1735.
[45] R. S. H. Liu and J . R. Edman, J. Amer. chem. SOC.90, 213
(1968).
[46] A . A . Lamola and G. S . Hammond, J. chem. Physics 43,2129
(1965).
Angew. Chem. internaf. Edit. 1 Vol. 8 (1969) 1 No. 4
kic = rate constant for intersystem crossing by the sensitizer.
k f = rate constant for fluorescence by the sensitizer.
k l = rate constant for triplet energy transfer to the cis substrate.
k , , kc = rate constants for decay of substrate triplets to trans
and cis ground states.
kd = rate constant for unimolecular decay of sensitizer triplets
to the ground state.
If, as with benzophenone, energy transfer is 100%
efficient 1471, the intersystem crossing yield (@iC) of the
sensitizer is obtained by merely summing the quantum
yields of isomerization.
The intersystem crossing yield of a sensitizer is a matter
of some practical interest to a photochemist. This
measurement, coupled with measurements of fluorescence and phosphorescence yields, allows a complete
detailing of the extent of nonradiative decay [1,48,491
from both the singlet and triplet manifolds.
Hammond and Lamola, exploiting the method outlined
above, were able to develop a simple and accurate
method for the measurement of intersystem crossing
efficiencies in solution at room temperature. Unfortunately, the highly variable rates of this reaction
could not be rationalized in terms of molecular structure, or other obvious parameters. The data seemed
to demonstrate the occurrence of some nonradiative
decay from the excited singlet state in several systems
(notably naphthalene and derivatives, see Section
4.3), in contradiction to the implication of theory [481.
counter because of its high singlet energy and low
triplet energy (> 100 and < 60 kcal/mole, respectively), formed the focus of this study. The finding that
conjugated dienes are capable of quenching the fluorescence of aromatic hydrocarbons revealed one important source of error in triplet counting experiments 1521.
It is further observed that products characteristic of
the diene triplet photoreactions do not arise from the
singlet quenching.
Figures 8,9, and 10 show the quenching of naphthalene
fluorescence by cis-l,3-pentadiene, a Stern-Volmer
treatment of the quenching data, and the decreasing
cis + trans quantum yield as the concentration of cis1,3-pentadiene is increased. Data of this type have
allowed corrections to be applied to the earlier results
obtained by Lamola for several aromatic hydrocarbons, in particular naphthalene and derivatives.
These data agree closely with those obtained by Wilkinson and co-workers [53,541 who employed a different
Fig. 8. Naphthalene fluorescence in the presence of a) 0.0, b) 0.I , c) 0.4,
d) 0.8 mole/] of cir-l,3-pentadiene (relative).
4.3. Complications in the Triplet Counting Scheme;
Complex Formation in the Excited State
The necessity for the postulation of nonradiative
decay from the first excited-singlet state of aromatic
hydrocarbons, as suggested by the data obtained by
Lamola, has found strong disagreement in the literature. Deuteration has no effect on singlet lifetimes,
indicating a lack of nonradiative decay from that
state 1471. Furthermore, Lamolu's unpublished value
for benzene intersystem crossing, 0.24, disagreed
sharply with values estimated by Lim (0.80 1501) and
Noyes and Ishikawa (0.78 1511)
The inherent simplicity and usefulness of the method
made an investigation of these anomalies desirable.
1,3-Pentadiene, the most widely employed triplet
[47] This is most easily demonstrated by showing that
QC
,,
is
invariant with increasing olefin concentration.
[48]See, for example, J. D . Laposa, E. C. Lim, and R . E. KeIIogg,
J. chem. Physics 42, 3025 (1965).
I491 M . Gouterman, J. chem. Physics 36, 2846 (1962).
[50] E. C. Lim,J. chem. Physics 36, 3497 (1962).
[511 H . Ishikawa and W. A . Noyes, j r . , J. chem. Physics 37, 583
(1962).
Angew. Chem. internat. Edit. i Val. 8 (1969)
1 No. 4
Fig.9. Stern-Volmer treatment of naphthalene quenching by cis-1,3pentadiene
[52] L . M. Stephenson, D . G. Whitten, G. F. Vesley, and G. S .
Hammond, J . Amer. chem. SOC.88, 3665 (1966).
[531 T. Medinger and F. Wilkinson, Trans. Faraday SOC.61, 620
(1965); A . R . Harroeks, T . Medinger, and F. Wilkinson, Chem.
Commun. 1965,452
[54] F. WiIkinson, private communication.
269
and the diene but the quantum yields of such products
are far too small to account for more than a small part
of the quenching. "Chemical quenching" and transfer
of singlet energy are thus eliminated as mechanistic
possibilities.
0.321i
0.28
For the most part, this quenching must be described
as a cataIysis of the nonradiative decay of the quenchers. The rates observed for naphthalene quenching
are shown in Table 5 and do not exceed those expected
in diffusion controlled reactions. Results also show [55J
that addition of alkyl substituents to naphthalene, as
in (19) and (20), reduces sensitivity to quenching by
1,3-pentadiene [(19), k, = 13 xi 106 1 mole-1 sec-1;
(ZO), kq = 8.7 x 106 1 mole-1 sec-1; naphthalene,
140 x 1 0 6 1 mole-1 sec-11.
Fig. 10. The cis-trans isomerization quantum yield for 1,3-pentadiene
sensitized by naphthalene as a function of initial cis-1,3-pentadiene
concentration.
method of triplet counting. Substantial support is
found for the conclusion that, in the absence of specific
quenchers, nonradiative decay from the first excitedsinglet state of aromatic hydrocarbons is unimportant.
The mechanism of this singlet quenching process is a
subject of continuing study. Quenching by conjugated
dienes is ineffective in inducing photochemical transformations typical of the diene triplet state. In addition, careful searches have revealed no evidence for
products which characteristically arise from excited
singlet states of the dienes. In particular, n o trace of
cyclobutene is produced from 1,3-pentadiene, hexatriene is not formed from cyclohexadiene, nor are any
of the several rearrangement products of myrcene
found. All processes are known to proceed from the
excited singIet following direct irradiation. Adducts
are sometimes formed from the aromatic compound
Table 5. Quenchingof fluorescenceof aromatic hydrocarbons by dienes.
Aromatic
Diene
kq x 10-9[bI
(Imole-1 sec-2)
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
Naphthalene
I-Methylnaphthalene
Anthracene
2.63
2,5-Dimethyl-2,4-hexadiene
3.95
1,3-Cyclohexadiene
4-Methyl-l,3-pentadiene
8.76
frans-2-fruns-4-Hexadiene
16.8
20.5
fruns-Z-cis-4-Hexadiene
45.3
cis-2-cb-4-Hexadiene
61.2
3-Methylenecyclohexene
rrans-Z-Methyl-I,3-~entadienc
: 94.2
95.0
rrons-l,3-Pentadiene
cis-l,3-Pentadiene
113
2,3-DimethyI-l,3-butadiene
680
12
1,3-CycIohexadiene
3.96
2.50
1.20
0.62
0.51
0.23
0.17
0.11
0.11
0.092
0.015
1.1
1,3-Cyclohexadiene
67
2.96
[a] c % is the concentration required to quench half the fluorescence.
[bI Calculated assuming T~ for naphthalene = 77 nanoseconds, as
estimated by oxygen quenching.
270
We tentatively conclude that quenching not only
requires that the two molecules be in contact but that
the contact be sufficiently intimate to be subject to
steric hindrance.
A model designed to account for the reactivity relationships has been formulated and assumes that,
within a complex formed between excited singlet state
quenchee (A) and ground state quencher diene (Q), a
small amount of the electronic excitation is delocalized
to the quencher.
A*Q
t-f
A.Q*
Since the singlet excitation energies of the dienes are
considerably greater than those of the quenchers, the
contribution from the second configuration is expected
to be small. However, the importance of the A . . Q*
structure arises primarily from its effect on the rate of
internal conversion, not from its contribution to the
binding energy of the complex. This suggests that the
weak coupling may allow the excited complex to
borrow some of the very rapid, nonradiative decay
characteristics of the diene.
This work was supported by the National Science
Foundation and the Directorate of Chemical Sciences,
Air Force Office of Scientific Research, under Contract
NO. AF 49 (638)-1479.
[A 688 IEI
Received: March 26, 1968
German version: Angew. Chem. 81. 279 (1969)
[55] L. M. Stephenson, D . G. Whitten, and G. S. Hammond,
Proc. Conf. Radiation Chem. and Photochem., Newcastle upon
Tyne (England) 1967, p. 35. Taylor and Francis, London.
Angew. Chem. internat. Edit. 1 Vol. 8 (1969) 1 No. 4
Документ
Категория
Без категории
Просмотров
1
Размер файла
951 Кб
Теги
state, excited, deactivation
1/--страниц
Пожаловаться на содержимое документа