Intramolecular triplet energy transfer in flexible molecules: Electronic, dynamic, and structural aspects

Article abstract of DOI:10.1021/ja990224l

Exothermic intramolecular triplet energy transfer (TET) rate constants in various flexible bichromophoric systems D-(CH₂)_n-O-A (D = benzoyl, 4-methylbenzoyl; A = 2-naphthyl, 4-, 3-, 2-biphenyl; n = 3-14) have been determined from steady-state quenching and quantum yield measurements. The magnitude of the rate constants in molecules where n = 3 is comparable to those in molecules with a rigid spacer between chromophores, so that a through-bond mechanism is presumed to remain important. A very gradual drop in TET rate constants as the connecting polymethylene chain becomes longer indicates that through-space interactions compete and apparently provide the only mechanism responsible for transfer when n ≥ 5. Rate constants in long molecules (n = 11-14) remain remarkably high (～10⁸ s^-1) - lower than in those with four-atom tethers by only 1 order of magnitude. This effect is explained on the basis of rapid conformational equilibria always keeping a sufficient fraction of the molecules coiled so that the two chromophores are close enough to interact within 10 ns, the time required for the competing γ-hydrogen abstraction used to monitor triplet lifetimes. Energy transfer accounts for 40-75% of triplet decay for the longer molecules. This high efficiency indicates that only a small fraction involves static quenching in ground-state conformers with the two ends within 4 A. The majority must represent a combination of rate-determining bond rotations to such geometries and equilibrated conformations with their ends farther apart but still able to undergo energy transfer within 10 ns. Thus, the measured rate constants are, in fact, a weighted average of three different conformational mechanisms. The decrease in rate constant with tether length is not monotonic: a relative increase in rate for medium-chain-length molecules is explained by a larger number of favorable conformers and further, in biphenyl derivatives, by a rotation along the terminal O-C bond between the tether and the aromatic ring. As was expected, replacement of the polymethylene tether with poly(ethylene oxide) promotes better flexibility and thus higher transfer rates. Rate constants were found to be lower by a factor of ～2 when biphenyl rather than naphthyl is the acceptor, in agreement with earlier bimolecular measurements. With the 4-methylbenzoyl group (π,π* lowest triplet) as donor instead of benzoyl (n,π* lowest triplet), a small (～1.5x) but consistent rate increase occurred for all tether lengths.

Full text of DOI:10.1021/ja990224l

9626
J. Am. Chem. Soc. 1999, 121, 9626-9635
Intramolecular Triplet Energy Transfer in Flexible Molecules:
Electronic, Dynamic, and Structural Aspects
Peter J. Wagner* and Petr Kla´n†
Contribution from the Chemistry Department, Michigan State UniVersity, East Lansing, Michigan 48824
ReceiVed January 22, 1999. ReVised Manuscript ReceiVed August 20, 1999
Abstract: Exothermic intramolecular triplet energy transfer (TET) rate constants in various flexible bichro-
mophoric systems D-(CH2)n-O-A (D ) benzoyl, 4-methylbenzoyl; A ) 2-naphthyl, 4-, 3-, 2-biphenyl; n )
3-14) have been determined from steady-state quenching and quantum yield measurements. The magnitude
of the rate constants in molecules where n ) 3 is comparable to those in molecules with a rigid spacer between
chromophores, so that a through-bond mechanism is presumed to remain important. A very gradual drop in
TET rate constants as the connecting polymethylene chain becomes longer indicates that through-space
interactions compete and apparently provide the only mechanism responsible for transfer when n g 5. Rate
constants in long molecules (n ) 11-14) remain remarkably high (∼108 s-1)slower than in those with four-
atom tethers by only 1 order of magnitude. This effect is explained on the basis of rapid conformational
equilibria always keeping a sufficient fraction of the molecules coiled so that the two chromophores are close
enough to interact within 10 ns, the time required for the competing γ-hydrogen abstraction used to monitor
triplet lifetimes. Energy transfer accounts for 40-75% of triplet decay for the longer molecules. This high
efficiency indicates that only a small fraction involves static quenching in ground-state conformers with the
two ends within 4 Å. The majority must represent a combination of rate-determining bond rotations to such
geometries and equilibrated conformations with their ends farther apart but still able to undergo energy transfer
within 10 ns. Thus, the measured rate constants are, in fact, a weighted average of three different conformational
mechanisms. The decrease in rate constant with tether length is not monotonic: a relative increase in rate for
medium-chain-length molecules is explained by a larger number of favorable conformers and further, in biphenyl
derivatives, by a rotation along the terminal O-C bond between the tether and the aromatic ring. As was
expected, replacement of the polymethylene tether with poly(ethylene oxide) promotes better flexibility and
thus higher transfer rates. Rate constants were found to be lower by a factor of ∼2 when biphenyl rather than
naphthyl is the acceptor, in agreement with earlier bimolecular measurements. With the 4-methylbenzoyl group
(π,π* lowest triplet) as donor instead of benzoyl (n,π* lowest triplet), a small (∼1.5×) but consistent rate
increase occurred for all tether lengths.
Introduction
the donor and the acceptor are in a single molecule, the same
factors presumably hold, with the added complexity that the
intervening molecular skeleton affects the distance between the
two chromophores.
Photosensitization induced by electronic energy transfer is
an important process in nature and thus has been one of the
most studied aspects of photochemistry. It is generally accepted
that energy transfer from an excited triplet to the ground state
of another chromophore proceeds via an exchange mechanism,
in which the rate constant shows an inverse exponential
dependence on the distance between the two chromophores.1,2
It is also important that the chromophores have the proper
electronic orientation.3,4 For a solution encounter between
excited donor and acceptor molecules, the actual energy transfer
rate usually exceeds 1010 s-1 when energy transfer is exother-
mic,5 such that bimolecular energy transfer is partially or fully
diffusion controlled, depending on solvent viscosity.6,7 When
There has been a long-standing interest in determining the
extent to which intramolecular electronic interactions between
two functional groups occur by a through-bond or a through-
space mechanism. Following studies with Miller that revealed
a strong through-bond component in intramolecular electron
transfer, Closs and co-workers extended this interest to triplet
energy transfer (TET).8 They investigated compounds in which
a 2-naphthyl group was connected via a rigid spacer to a
4-benzoylphenyl chromophore, with the spacer either a cyclo-
hexane or a decalin ring.9 Rate constants for such transfer
dropped ∼1 order of magnitude for each additional bond
between chromophores. When the chromophores were attached
†Katedra organicke` chemie, Masarykova univerzita, Kotl‚jsk‚ 2, 611 37
Brno, Czech Republic.
(1) (a) Dexter, D. L. J. Chem. Phys. 1953, 21, 836. (b) Yardley, J. T.
Introduction to Molecular Energy Transfer; Academic Press: New York,
1980.
(2) Lamola, A. A. Electronic Energy Transfer in Solution: Theory and
Applications. In Techniques of Organic Chemistry; Weisberger, A., Ed.;
Wiley: New York, 1969; Vol. 14.
(5) Anderson, R. W.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. J. Chem.
Phys. 1974, 61, 2500.
(6) Wagner, P. J.; Kochevar, I. J. Am. Chem. Soc. 1968, 90, 2232.
(7) Saltiel, J.; Shannon, P. T.; Zafiriou, O. C.; Uriarte, A. K. J. Am.
Chem. Soc. 1980, 102, 6799. Makemura, T.; Baba, H.; Fujita, M. Bull.
Chem. Soc. Jpn. 1973, 46, 2625.
(3) Letchen, P.; Turro, N. J. Angew. Chem., Int. Ed. Engl. 1973, 12,
314.
(4) Scaiano, J. C.; Leigh, W. J.; Meador, M. A.; Wagner, P. J. J. Am.
Chem. Soc. 1985, 107, 5806.
(8) (a) Calcaterra, L. T.; Closs, G. L.; Miller, J. R. J. Am. Chem. Soc.
1983, 105, 670. (b) Closs, G. L.; Miller, J. R. Science 1988, 240, 440. (c)
Johnson, M. D.; Miller, J. R.; Green, N. S.; Closs, G. L. J. Phys. Chem.
1989, 93, 1173.
10.1021/ja990224l CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/06/1999

Triplet Energy Transfer in Flexible Molecules
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9627
to the spacer in all-equatorial positions, TET rate constants were
about 1 order of magnitude higher than those for equatorial-
axial attachments, consistent with a predominant through-bond
mechanism. It is interesting that these studies extrapolate to the
low rate constant measured in a very early study with a steroid
spacer and the same chromophores.10
Scheme 1
Studies of chemical reactions or physical processes occurring
between groups attached at the ends of a flexible tether are
widespread in the literature.11 However, intramolecular triplet
energy transfer in flexible molecules has been studied only for
systems with relatively short tethers. Some early phosphores-
cence measurements12 established only that TET is very rapid.
Cowan and Baum observed a continuous drop in rate constants
of energy transfer as the number of methylenes connecting
benzoyl and styryl chromophores increased from two to four.13
In R-benzoyl-ω-azidoalkanes, quenching of the benzoyl chro-
mophore’s triplet chemistry by the azido group decreased 3
orders of magnitude as the tether length increased from three
to five methylenes.14 Since these effects were comparable to
those observed in Closs’s more rigid systems, it was impossible
to distinguish between through-space and through-bond effects.
To distinguish these, we investigated energy transfer in the
cinnamyl esters of ω-benzoylcarboxylic acids.15 For these
flexible 4-7-atom tethers, energy transfer rate constants de-
creased much more slowly than in Closs’s rigid systems, and
we concluded that through-space transfer became the predomi-
nant mechanism in molecules with longer chains. We made a
similar conclusion in a study of energy transfer in unsymmetric
diketones.16
Strategy
The Norrish type II reaction19 has proven to be a uniquely
useful tool for monitoring competing triplet reactions, both inter-
and intramolecular. This reaction proceeds exclusively via
intramolecular γ-hydrogen abstraction by the excited carbonyl
group, which produces a 1,4-biradical as the primary photo-
product.20 In simple phenyl and substituted phenyl ketones, the
intersystem crossing quantum yield is unity,21 and hydrogen
abstraction proceeds exclusively from the triplet excited state.20
The biradical undergoes three competing reactions: cleavage
to an alkene and the enol of acetophenone, which rapidly
tautomerizes to acetophenone; ∼10% cyclization to a 1-aryl-
cyclobutanol; and disproportionation back to ground-state
ketone. The latter reaction can be suppressed by use of Lewis
base additives.20,22 When the molecule contains another func-
tional group with which the triplet carbonyl can react, both the
quantum yield for type II product formation and the triplet
lifetime are reduced proportionately, providing two separate
measures of the relative rates for hydrogen abstraction and the
competing reaction. Since energy transfer is one such reaction
that can compete with hydrogen abstraction, we utilized this
approach in our earlier energy transfer studies14-16 and have
applied it in this work as well, the kinetics of γ-hydrogen
abstraction being adopted as the “system clock” to calculate
energy transfer rate constants. Scheme 1 exemplifies the
competition between energy transfer and the Norrish type II
reaction, Ar being any of the aromatic groups that we studied.
Since hydrogen abstraction is unavoidable with a polymethylene
spacer between donor and acceptor, quantum yield studies are
absolutely required in order to assess the ratio of energy transfer
and hydrogen abstraction. Picosecond flash kinetics would be
required in order to measure triplet lifetimes directly, but
quenching studies have been shown to be just as accurate. These
have shown that the rate constants for γ-hydrogen abstraction
With these two examples in hand, we decided to extend our
studies to even longer R,ω-substituted polymethylenes with
chromophores as similar as possible to those used by Closs. In
a preliminary communication, we reported our study of in-
tramolecular TET in ω-naphthyl- and ω-4-biphenylylalkanophe-
nones, in which the donor is the benzoyl group.17 Although
phenyl alkyl ketones have slightly higher triplet energies than
benzophenone, their triplets are very similar electronically, and
Closs has shown that acetophenone and benzophenone donors
provide nearly identical TET rate constants to naphthalene
acceptor.18 We found that transfer rate constants decrease only
1 order of magnitude as the tether length increases from 5 to
14 methylenes and concluded, as we had in our 1992 papers,
that energy transfer occurs primarily by through-space interac-
tions in these long, flexible molecules. We have expanded this
work to include additional donors and acceptors, and we now
wish to report our complete findings on how electronic,
dynamic, and structural factors can change the course of
intramolecular TET.
(9) (a) Closs, G. L.; Piotrowiak, P.; MacInnis, J. M.; Fleming, G. R. J.
Am. Chem. Soc. 1988, 110, 2652. (b) Closs, G. L.; Johnson, M. D.; Miller,
J. R.; Piotrowiak, P. J. Am. Chem. Soc. 1989, 111, 3751.
(10) Keller, R. A.; Dolby, L. J. J. Am. Chem. Soc. 1969, 91, 1293. Breen,
D. E.; Keller, R. A. J. Am. Chem. Soc. 1968, 90, 1935.
(11) Winnik, M. A. Chem. ReV. 1981, 81, 491.
(12) Lamola, A. A.; Leemakers, P. A.; Byers, G. W.; Hammond, G. S.
J. Am. Chem. Soc. 1965, 87, 2322.
(13) Cowan, D. O.; Baum, A. A. J. Am. Chem. Soc. 1971, 93, 1153.
(14) Wagner, P. J.; Scheve, B. J. J. Am. Chem. Soc. 1979, 101, 378.
(15) Wagner, P. J.; El-Taliawi, G. M. J. Am. Chem. Soc. 1992, 114,
8325.
(16) Wagner, P. J.; Giri, B. P.; Frerking, H. W., Jr.; DeFrancesco, J. J.
Am. Chem. Soc. 1992, 114, 8326.
(19) Wagner, P. J. Acc. Chem. Res. 1971, 4, 168. Wagner, P. J.; Park,
B.-S. Photoinduced Hydrogen Atom Abstraction by Carbonyl Compounds.
In Organic Photochemistry; Padwa, A., Ed.; 1991; Vol. 11, p 111.
(20) Wagner, P. J. J. Am. Chem. Soc. 1967, 89, 5898. Wagner, P. J.;
Kelso, P. A.; Zepp, R. G. J. Am. Chem. Soc. 1972, 94, 7480.
(21) Lamola, A. A.; Hammond, G. S. J. Chem. Phys. 1965, 43, 2129.
(22) Wagner, P. J.; Kochevar, I. E.; Kemppainen, A. E. J. Am. Chem.
Soc. 1972, 94, 7489.
(17) Klan, P.; Wagner, P. J. J. Am. Chem. Soc. 1998, 120, 2198.
(18) Sigman, M. E.; Closs, G. L. J. Phys. Chem. 1991, 95, 5012.

9628 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Wagner and Kla´n
Table 1. Photokinetics of ω-Phenoxy Ketones in Cyclohexanea
Scheme 2
D-n-OA
ΦII
kqτ, M-1
1/τ (kH), 108 s-1
Bz-3-OPh
Bz-4-OPh
Bz-5-OPh
Bz-7-OPh
Bz-10-OPh
Bz-11-OPh
MeBz-3-OPh
MeBz-4-OPh
MeBz-5-OPh
Bz-3-O-2-OPh
0.45 ( 0.00
0.44 ( 0.01
0.35 ( 0.01
0.29 ( 0.00
0.26 ( 0.01
0.26 ( 0.01
0.61 ( 0.02
0.56 ( 0.02
0.43 ( 0.02
0.44 ( 0.01
31 ( 0.4
232 ( 16
90 ( 0.3
62 ( 2.3
57 ( 1.2
58 ( 0.4
153 ( 2.7
1090 ( 87
412 ( 1.5
25 ( 1.6
2.6
0.34
0.89
1.3
1.4
1.4
0.50
0.07
0.19
3.2
a Measured at room temperature; values represent average of two
measurements.
do not vary significantly as the alkyl group gets longer.23,24 Such
rate constants depend on the equilibrium population of gauche
conformers around the R-â C-C bond, in which a γ-hydrogen
is within bonding distance of the carbonyl. Scheme 1 also shows
this equilibrium. We have performed some simple molecular
mechanics and semiempirical calculations that indicate no
significant difference in the equilibrium as R gets larger.
Based on the following known triplet excitation energies,
energy transfer in all systems except the ω-phenoxyketones must
be exothermic and irreversible (all values in kcal/mol): benzoyl,
∼73;25 4-methylbenzoyl, ∼72;25,26 phenoxy, ∼81;27 2-naphthyl-
oxy, ∼62;28 2-, 3-, and 4-biphenyloxy, 68-69.29
Table 2. Photokinetics of Naphthyloxy Bichromophores in
Cyclohexanea
kH, 108 s-1
from Φ
D-n-OA
ΦII
kqτ, M-1 1/τ, 108 s-1
Bz-3-ONp
Bz-4-ONp
Bz-5-ONp
Bz-6-ONp
Bz-7-ONp
Bz-9-ONp
Bz-10-ONp
Bz-11-ONp
Bz-14-ONp
MeBz-3-ONp
MeBz-4-ONp
MeBz-5-ONp
MeBz-6-ONp
MeBz-7-ONp
0.13 ( 0.00
4.2 ( 0.1
19.0
4.9
4.1
3.6
3.4
5.3
5.9
3.2
2.6
23.5
5.1
3.9
3.4
3.1
2.8
5.5
0.4
1.9
1.8
2.0
2.1
2.9
2.2
1.5
1.2
0.1
0.3
0.35
0.4
0.5
0.04 ( 0.00 16.3 ( 0.5
0.16 ( 0.00 19.5 ( 0.1
0.16 ( 0.00 22.2 ( 0.7
0.17 ( 0.00 23.2 ( 0.0
0.11 ( 0.02 15.0 ( 1.4
0.13 ( 0.01 13.6 ( 1.1
0.18 ( 0.00 24.7 ( 0.2
0.15 ( 0.00 30.8 ( 3.0
Results
0.03 ( 0.00
3.4 ( 0.0
Scheme 2 lists the bichromophoric ketones we have studied.
They were prepared by standard synthetic methods and purified
to 99+% in all cases, except for compounds with the 3-biphenyl
group as acceptor, which contained 2-3% of their 4-biphenyl
isomers. The ω-phenoxy compounds were prepared as model
compounds in which there could be no internal energy transfer,
since the triplet energy of anisole is much higher than that of
phenyl ketones.27 The 7-phenoxy and -(4-biphenyl)oxy versions
of 1-phenyl-5-oxa-1-heptanone (Bz-3-O-2-OA) were also pre-
pared and studied.
Cyclohexane or benzene solutions 0.001 M in bichromophore
and containing different concentrations of 2,5-dimethyl-2,4-
hexadiene as quencher were irradiated at 366 nm, where only
the benzoyl group absorbs. Either valerophenone or 4-meth-
ylvalerophenone solutions, also 0.001 M, were irradiated
simultaneously as actinometers for quantum yield measure-
ments.22,26 Triplet lifetimes were determined by Stern-Volmer
steady-state quenching techniques.32 All Stern-Volmer plots
were linear, with correlation coefficients 0.97-0.99, their slopes
equaling kqτ. The type II product yields (acetophenone or
4-methylacetophenone) were determined by HPLC; conversions
0.01 ( 0.00 15.7 ( 0.2
0.03 ( 0.00 20.3 ( 0.0
0.04 ( 0.00 23.7 ( 0.6
0.05 ( 0.00 25.8 ( 1.1
MeBz-11-ONp 0.06 ( 0.00 28.8 ( 1.6
a Measured at room temperature; values represent average of two
measurements.
were kept under 15%. Minor HPLC peaks were observed in
some ketones and assumed to be the expected cyclobutanol
coproducts.
Tables 1-3 list Norrish type II quantum yield, kqτ, and 1/τ
values for all systems. Quantum yields were corrected for optical
density at 366 nm in cyclohexane or benzene. Most bichro-
mophores used in this work were not very soluble in cyclo-
hexane, and therefore it was difficult to obtain precise optical
density and molar absorbtivity values in this solvent. For many
of the longer molecules, 0.001 M concentration was nearly a
saturated solution. Typical values of molar absorbtivities ꢀ366
were as low as 3-5 L mol-1 cm-1. Reciprocal lifetimes 1/τ
were calculated from kqτ and the known bimolecular rate
constants (kq), 8 × 109 M-1 s-1 for cyclohexane6 and 6 × 109
M-1 s-1 for benzene.4 It is possible that increasing the length
of the bichromophore could lower the probability of energy
transfer during a solution encounter and thus cause kq to drop
slightly, but the effect would be masked by the near diffusion
control. Therefore, it was assumed throughout that the near-
diffusion-controlled values of kq measured for various smaller
ketones4 remain constant as n increases.
Energy Transfer Rate Constants. Energy transfer rate
constants (kET) were calculated from the measured triplet
lifetimes by two methods: (1) subtracting 1/τ of the model
ω-phenoxy compound from that of the corresponding bichro-
mophore and (2) partitioning triplet lifetimes between hydrogen
abstraction and energy transfer by comparing quantum yields
to those of the model compounds. The kET values are listed in
Tables 4 and 5, along with quantum yields for energy transfer
as gleaned from the measured decreases in triplet lifetimes and
acetophenone quantum yields. Determination of γ-hydrogen
(23) Wagner, P. J.; Kemppainen, A. E. J. Am. Chem. Soc. 1972, 94,
7495.
(24) Pitts, J. N., Jr.; Burley, D. R.; Mani, J. C.; Broadbent, A. D. J. Am.
Chem. Soc. 1968, 90, 5900.
(25) Kearns, D. R.; Case, W. A. J. Am. Chem. Soc. 1966, 88, 5087.
Yang, N. C.; McClure, D. S.; Murov, S. L.; Houser, J. J.; Dusenbery, R. J.
Am. Chem. Soc. 1967, 89, 5466.
(26) Wagner, P. J.; Thomas M. J.; Harris, E. J. Am. Chem. Soc. 1976,
98, 7675.
(27) Handbook of Photochemistry; Murov, S. L., Carmichael, I., Hug,
G. L., Eds.; Marcel Dekker: New York, 1993.
(28) Marchetti, A. P.; Kearns, D. R. J. Am. Chem. Soc. 1967, 89, 768.
(29) Estimated from ET ) 64 kcal/mol for 4-hydroxybiphenyl30and the
large Stokes shift for biphenyls.31
(30) Taylor, H. V.; Allred, A. L.; Hoffman, B. M. J. Am. Chem. Soc.
1973, 95, 3215.
(31) Wagner, P. J. J. Am. Chem. Soc. 1967, 89, 2820. Hutchison, C. A.,
Jr.; Kemple, M. D. J. Chem. Phys. 1979, 71, 866. Takei, Y.; Yamaguchi,
T.; Osamura, Y.; Fuke, K.; Kaya, K. J. Phys. Chem. 1988, 92, 577.
(32) Stern, O.; Volmer, M. Phys. Z. 1919, 20, 183. Wagner, P. J.
Handbook of Organic Photochemistry; CRC Press: Boca Raton, FL, 1989;
Vol. 2, p 38.

Triplet Energy Transfer in Flexible Molecules
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9629
Table 3. Photokinetics of Biphenyloxy Bichromophores in
Table 5. Intramolecular Energy Transfer Rate Constants in
Biphenyloxy Ketones
Cyclohexanea
kH, 108 s-1
from Φ
kET, 108 s-1 kH, 108 s-1 kET, 108 s-1
D-n-OA
ΦII
kqτ, M-1 1/τ, 108 s-1
D-n-OA
from 1/τ
from Φ
from Φ
ΦET (%)
Bz-3-O4Bp
Bz-4-O4Bp
Bz-5-O4Bp
Bz-6-O4Bp
Bz-7-O4Bp
Bz-9-O4Bp
Bz-10-O4Bp
Bz-11-O4Bp
Bz-14-O4Bpb
Bz-3-O3Bp
Bz-6-O3Bp
Bz-7-O3Bp
Bz-9-O3Bpc
Bz-10-O3Bp
Bz-14-O3Bp
Bz-3-O2Bp
Bz-4-O2Bp
Bz-5-O2Bp
Bz-6-O2Bp
Bz-7-O2Bp
Bz-9-O2Bp
Bz-10-O2Bp
Bz-11-O2Bp
Bz-14-O2Bp
0.17 ( 0.02 7.7 ( 0.6
0.05 ( 0.00 20.7 ( 0.3
0.13 ( 0.00 27.3 ( 0.9
0.14 ( 0.00 30.4 ( 0.2
0.17 ( 0.01 32.5 ( 0.1
0.13 ( 0.00 20.5 ( 0.2
0.17 ( 0.01 23.3 ( 0.4
0.21 ( 0.01 33.4 ( 0.7
0.24 ( 0.00 28.4 ( 0.2
0.20 ( 0.00 10.1 ( 0.1
0.17 ( 0.01 32.8 ( 0.2
0.18 ( 0.01 27.5 ( 0.0
10.4
3.9
2.9
2.6
2.5
3.9
3.4
2.4
2.1
7.9
2.4
2.9
2.2
2.8
2.2
6.7
2.2
2.5
3.1
3.7
2.7
3.3
2.2
2.0
9.0
7.6
3.9
0.45
1.1
1.1
1.5
1.8
2.1
1.9
1.9
3.5
1.3
1.8
1.2
1.9
1.4
3.4
0.2
1.2
1.3
1.7
1.3
1.65
1.6
1.5
0.6
2.9
Bz-3-O4Bp
Bz-4-O4Bp
Bz-5-O4Bp
Bz-6-O4Bp
Bz-7-O4Bp
Bz-9-O4Bp
Bz-10-O4Bp
Bz-11-O4Bp
Bz-14-O4Bp
Bz-3-O3Bp
Bz-6-O3Bp
Bz-7-O3Bp
Bz-9-O3Bp
Bz-10-O3Bp
Bz-14-O3Bp
Bz-3-O2Bp
Bz-4-O2Bp
Bz-5-O2Bp
Bz-6-O2Bp
Bz-7-O2Bp
Bz-9-O2Bp
Bz-10-O2Bp
Bz-11-O2Bp
Bz-14-O2Bp
MeBz-3-O4Bp
Bz-3-O-2-O4Bp
7.8
3.5
2.0
1.6
1.2
2.5
2.0
1.0
0.7a
5.3
1.4
1.6
0.8
1.4
0.8
4.1
1.8
1.5
2.1
2.4
1.3
1.9
0.8
0.6
8.5
4.4
3.9
0.45
1.1
1.1
1.5
1.8
0.9
1.9
1.9
3.5
1.3
1.8
1.2
1.9
1.4
3.4
0.2
1.2
1.3
1.7
1.3
1.65
1.6
1.5
0.6
2.9
6.5
3.5
1.8
1.5
1.0
2.1
2.5
0.5
0.2a
4.4
1.2
1.1
1.0
0.9
0.8
3.3
2.0
1.3
1.9
2.0
1.4
1.7
0.6
0.5
8.4
4.7
62-75
90
62-69
60
40-48
49-64
59-73
21-42
10-33
56-67
50-58
38-55
36-45
32-50
36
49-61
82-91
52-60
61-68
54-65
50
0.15
36
0.18 ( 0.01 28.6 ( 0.1
0.17 ( 0.00 35.7 ( 0.5
0.23 ( 0.01 11.9 ( 0.1
0.05 ( 0.00 36.7 ( 0.0
0.17 ( 0.00 32.5 ( 0.3
0.13 ( 0.00 25.7 ( 0.6
0.13 ( 0.00 21.6 ( 0.4
0.13 ( 0.01 29.8 ( 0.6
0.13 ( 0.00 23.9 ( 0.9
0.19 ( 0.01 35.7 ( 2.0
0.20 ( 0.01 39.2 ( 0.6
52-58
27-36
28
94
58-62
MeBz-3-O4Bp 0.04 ( 0.00 8.9 ( 0.1
Bz-3-O-2-O4Bp 0.17 ( 0.00 10.5 ( 0.9
a Measured at room temperature; values represent average of two
measurements. b In benzene. c Single measurement.
a In benzene.
physical decay processes are known to be too slow to compete
at all with hydrogen abstraction. The fact that quantum yields
for acetophenone formation are at least as large as those for
comparably long alkanophenones without the ω-phenoxy group,24
whose triplets decay only by hydrogen abstraction, verifies this
view.
If kH in a given bichromophore is assumed equal to that in
the corresponding ω-phenoxy compound, the intramolecular
energy transfer rate constants kET can be calculated according
to eq 1, where τ is the triplet lifetime of the ω-aryloxyketone,
Table 4. Intramolecular Energy Transfer Rate Constants in
Naphthyloxy Ketones
kET, 108 s-1 kH, 108 s-1 kET, 108 s-1
D-n-OA
from 1/τ
from Φ
from Φ
ΦET (%)
Bz-3-ONp
Bz-4-ONp
Bz-5-ONp
Bz-6-ONp
Bz-7-ONp
Bz-9-ONp
Bz-10-ONp
Bz-11-ONp
Bz-14-ONp
MeBz-3-ONp
MeBz-4-ONp
MeBz-5-ONp
MeBz-6-ONp
MeBz-7-ONp
MeBz-11-ONp
16.3
4.5
3.1
2.5
2.0
3.8
4.4
1.7
1.2
23
5.0
3.6
3.1
2.8
2.5
5.5
0.4
1.9
1.8
2.0
2.1
2.9
2.2
1.5
1.2
0.1
0.3
0.35
0.4
0.5
13.5
4.5
2.2
1.8
1.4
3.2
3.0
1.0
1.1
22
5.0
3.6
3.1
2.7
2.3
71-86
92
54-76
50-69
41-59
60-72
51-75
31-53
46-58
95-97
98
92
90
89
86
kET ) 1/τ - kH - k2[K]
(1)
kH is the rate constant for hydrogen abstraction (1/τ of the
corresponding model ketone in Table 135), k2 is the rate constant
for bimolecular quenching by a ground-state bichromophore’s
aryloxy group (8 × 109 M-1 s-1 for naphthalene, 3 × 109 M-1
s-1 for biphenyl),4 and [K] is the bichromophore concentration.
Because aryloxy ketone concentrations were only 0.001 M,
bimolecular quenching rates were 1-2 orders of magnitude
slower than the intramolecular rates. Thus, bimolecular quench-
ing could be neglected in most of our calculations, except for
the longest lived triplets, where it was taken into account.
Equation 1 does ignore the possibility that some intramolecular
charge-transfer quenching occurs in the triplets of the shorter
bichromophores, since naphthoxy and biphenyloxy groups
should be better electron donors than phenoxy. In such cases,
the calculated kET value would be somewhat too high.
abstraction rate constants (kH) in bichromophores from these
data is crucial for kET calculations because, provided that there
are no other reactions, kH competes only with energy transfer
(Scheme 1). Comparison of the two methods establishes the
internal consistency of the data. As Tables 4 and 5 show, the
two methods provide slightly different kET values for a given
compound but nearly identical variations with structural change.
The two methods are described more fully below.
Rate Constants kH Obtained Only from 1/τ Values of
Model Phenoxy Ketones. The model phenoxy ketones provide
values for the quantum efficiencies of product formation and
the rate constants for γ-hydrogen abstraction in the absence of
triplet energy transfer. Rapid γ-hydrogen abstraction is presumed
to be their only triplet reaction, since energy transfer to the
phenoxy group is highly endothermic, charge-transfer quenching
of triplet phenyl ketones by anisole is slow,33,34 and all other
Rate Constants kH Calculated from Quantum Yields and
Triplet Lifetimes. The quantum yield of the Norrish type II
reaction is expressed in eq 2,36 where ΦISC is the quantum yield
of intersystem crossing and PII is the probability that the
(34) Wolf, M. W.; Brown, R. E.; Singer, L. A. J. Am. Chem. Soc. 1977,
99, 526.
(35) Since the ω-phenoxy ketones studied did not include all of the n
values, a few quantum yield and kH values were interpolated from the
measured values of Table 1.
(33) Kochevar, I.; Wagner, P. J. J. Am. Chem. Soc. 1972, 94, 3859.

9630 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
ΦII ) ΦISCkHτPII
Wagner and Kla´n
(2)
biradical will form products rather than revert to starting ketone.
Inasmuch as the model ω-phenoxy ketone triplets undergo only
γ-hydrogen abstraction, their product quantum yields equal PII.
Since all the compounds have the same absorbing chromophore,
with ΦISC ) 1,20,21 this factor cancels out in all calculations.
However, should there be any additional contribution to 1/τ
besides kH + kET, such as the charge transfer suggested above,
it would lower the product quantum yield. The quantum yields
thus provide direct information about how kH values may vary
between a bichromophoric ketone and its phenoxy model. With
ΦïII being the quantum yield for the phenoxyketone, Tables 2
and 3 include kH values derived from eq 3, which must be
compared to the 1/τ values in Table 1. The values of kH depend
kH ) (1/τ)(ΦII/ΦïII)
(3)
on the nature of the aryloxy group in D-n-OA, especially in
those with three and four methylene tethers. It is known that
intramolecular abstraction of a γ-hydrogen atom is strongly
affected by inductive effects of γ and δ substituents.23 Thus,
the rate constant kH may differ slightly in a bichromophore and
its model compound. However, in both cases, kH is largest when
n ) 3 and much smaller when n ) 4, the former because the
aryloxy group weakens the γ C-H bond, and the latter because
the aryloxy group slows hydrogen abstraction by its electron-
withdrawing character.23
Figure 1. Rate constants for triplet energy transfer as a function of
the number x of connecting atoms between the donor and acceptor:
9, PhCOPh-“rigid spacer”-2Np (refs 9 and 10); 0, Bz-n-ONp; b, Bz-
(CH2)n-styryl (ref 13); O, Bz-(CH2)nCO2CH2-styryl (ref 15).
The rate constants for energy transfer can be calculated from
eq 1 with the eq 3 value of kH. The variations in these kET values
match the trends of the values derived from only lifetime
measurements. Their statistical accuracy may not be as high,
since each Stern-Volmer experiment consists of 20 independent
HPLC measurements while quantum yield experiments involve
only four. For that reason, the lifetime-based kH values are
plotted in the figures discussed below. Equation 3, like eq 1,
still provides too large a value of kET should any other triplet
decay process exist.
It should be noted that the kH values are fairly constant at (2
( 0.5) × 108 s-1 when n > 5. If kq values decreased as n
increased, then the apparent values of kH would also decrease,
since triplet lifetimes would then be longer than the tables
indicate. Since the value of kH is determined by the local
conformations of the CCdO-CR, CR-Câ, and Câ-Cγ bonds and
since these conformations are little influenced by the length of
the remaining chain, the observed near constancy of kH strongly
suggests that the triplet lifetimes measured by Stern-Volmer
quenching are accurate within a factor of 1.5, and thus that the
trends observed in the kET values are also accurate.
Figure 2. Rate constants for TET as a function of the number of
connecting atoms between the donor and acceptor: 2, Bz-n-ONp; O,
MeBz-n-ONp; 9, Bz-n-O4Bp.
assume that the major conformers have their aryloxy chro-
mophore anti to the polymethylene chain. Benzoyl groups often
eclipse CR-Câ bonds,37 so there should be a significant
population of conformers with the benzoyl nearly gauche (see
Scheme 1). Figure 1 also displays rate constants for two
relatively short, flexible benzoyl/styryl bichromophore sys-
tems.13,15
Figure 2 compares intramolecular TET rate constants for three
of our bichromophoric systems, Bz-n-ONp, Bz-n-O4Bp, and
MeBz-n-ONp. There are several aspects of the plots in Figure
2 that require separate discussion: the curvature of each plot,
with a sharp decrease in the slope as n increases; a bump in the
plots for n ) 9-11; and small differences in kET as either the
donor or the acceptor changes. Since the general shape of the
plots is the same for both acceptor chromophores, we shall
discuss first how kET varies with n and then how it varies with
chromophore.
Discussion
Correlation of kET Values. Figure 1 compares intramolecular
TET rate constants for our Bz-n-ONp bichromophores with
those for rigid molecules in which the benzophenone donor and
the naphthalene acceptor are attached in all-equatorial positions
to a cyclohexane or decalin ring9 or 10 carbons apart on a
steroid.10 Such isomers, with each chromophore anti to the
skeleton, display through-bond energy transfer rate constants
about 10 times higher than those of isomers with substituents
in equatorial-axial positions (i.e., anti-gauche). These differ-
ences presumably reflect the geometric requirements for elec-
tronic coupling though σ bonds. In our acyclic compounds, we
Length of the Tether. The shape of the plots in Figure 2 is
very different from those associated with covalent bonding38
(37) Wagner, P. J.; Zhou, B.; Hasegawa, T.; Ward, D. L. J. Am. Chem.
Soc. 1991, 113, 9640.
(36) Wagner, P. J.; Kemppainen, A. E. J. Am. Chem. Soc. 1968, 90,
5896.
(38) Illuminati, G.; Mandolini, L. Acc. Chem. Res. 1981, 14, 95.

Triplet Energy Transfer in Flexible Molecules
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9631
or excimer and exciplex formation between39 two functional
groups at opposite ends of a molecular chain. This is not
surprising, since energy transfer occurs over longer distances
than does either covalent bonding or excimer formation. Rate
constants for exothermic bimolecular energy transfer between
the chromophores used in this work are nearly diffusion
controlled.6,27 The efficiency of correspondingly fast intramo-
lecular processes can be controlled by bond rotation rates and/
or by ground-state conformational distribution.11,40 Under such
conditions, ratios of intramolecular/bimolecular rate constants
are not measures of “effective molarity” such as one observes
with slow ground-state reactions under conditions of rapid
conformational equilibrium.38 In particular, the kET values in
Tables 4 and 5 are not simple weighted averages over all
conformations;14 thus, we must carefully dissect the various
conformational contributions to energy transfer.
produce the same overall rate constant. On the other hand, direct
connection of the carbonyl to the linking skeleton in our and
Cowan’s compounds could enhance through-bond coupling
relative to that in Closs’s systems, in which the connection was
para on the benzoyl ring. If there is, indeed, such an enhance-
ment, it does not produce larger kET values in the shortest
flexible bichromophores than those in Closs’s analogous
compounds, a fact that must reflect how the percentage of
gauche conformations increases with x.
In our flexible bichromophores with five-atom links, kET is
lower than that for the corresponding molecules with a four-
atom link by a factor of 2-3 but higher by about a factor of 5
than that in the rigid model. The 90% efficiency of energy
transfer is a combination of very slow hydrogen abstraction (the
inductive effect of δ-alkoxy substitution23) and a still relatively
high kET. Molecular flexibility apparently produces sufficient
conformations in which the two chromophores are close enough
that through-space contributions become competitive with
through-bond interaction. Such coiled conformations must
contain several gauche C-C bond arrangements, which presum-
ably weaken through-bond interaction but allow through-space
interaction.
Triplet excitation of the carbonyl chromophore is unlikely
to significantly alter either the conformational equilibrium or
the rotational kinetics of the bichromophoric molecules from
their ground-state behavior. Each conformation has its own
probability of undergoing energy transfer that depends mainly
on the distance between chromophores and their orientations.
All conformers whose chromophores are 6 Å or less apart should
contribute to the total transfer. Those in which the chromophores
are within 3-4 Å of each other should undergo energy transfer
in 100 ps or less,5-7 as observed for in-cage bimolecular energy
transfer, where chromophores presumably approach van der
Waals separation. Whatever small fraction of our aryloxy
ketones that preexist in such geometries before excitation would
undergo instantaneous energy transfer, since bond rotation would
barely compete. Such ground-state control41 is the intramolecular
equivalent of static quenching.6,11 Likewise, any such geometries
formed by bond rotations competing with γ-hydrogen abstrac-
tion would also undergo efficient energy transfer so rapidly as
to make the bond rotation largely irreversible. Such rotational
control is the intramolecular equivalent of diffusion-controlled
quenching.11,40 Conformers with chain ends 5-6 Å apart, where
For flexible bichromophores with n g 5, the rate constants
remain remarkably high, no longer falling an order of magnitude
per additional bond as they do for molecules with rigid spacers.
In fact, the overall drop on going from n ) 5 to 14 is a factor
of only 0.20-0.25, or 0.86 per bond. (We have too few data
for our flexible benzoyl/styrene system15 to provide a meaningful
comparison with the current results.) The rate constants for our
longest bichromophores are only 1 order of magnitude lower
than those for molecules with four-atom tethers. The rate
constant for TET through a rigid steroid molecule in which a
benzophenone donor is separated by 11 bonds from a naphtha-
lene acceptor was found to be 25 s-1 10
, 7 orders of magnitude
smaller than that for our corresponding flexible molecule Bz-
9-ONp (3.8 × 108 s-1). Given these huge differences between
rigid and flexible spacers, Figures 1 and 2 suggest that >99%
of the total energy transfer occurs through space when n g 5.
From the slow decrease in kET values as n increases, we
conclude that, for molecules with six or more connecting bonds
between chromophores, conformational factors produce a nearly
constant percentage of coiled geometries in which the two
chromophores are close enough for through-space energy
transfer to compete with triplet decay, in this case γ-hydrogen
abstraction. Similar plateaus have been observed in various
studies on electron transfer,43 spin-orbit exchange interaction
in biradicals,44 and many cyclization reactions.11,38 Various
Monte Carlo calculations45 as well as exact enumeration
calculations of Sisido and Shimada46 predict that, even for very
long molecules with 10-25-atom chains, there exists a certain
small fraction of conformations in which the two ends are close
enough to interact. What we can conclude is that this small
fraction could not produce the observed high energy transfer
efficiencies and rate constants.
kET ) 108-109 s-1 42
should also contribute directly to the
,
observed value of kET, but now with efficiencies determined
largely by conformational equilibria. Thus, all three kinetics
boundary conditions for intramolecular reactions are likely to
coexist in our bichromophores, with individual kET values
comprising different combinations of the three.
We include the data for benzoyl-to-styrene TET in Figure 1
to illustrate that, for very short, flexible tethers (3-4 bonds),
rate constants are nearly the same as those in Closs’s rigid
systems, dropping 1 order of magnitude with each additional
connecting bond. Likewise, kET for Bz-3-ONp is nearly identical
in value to that for Closs’s rigid benzophenone-naphthalene
system with five connecting bonds. We might conclude that
through-bond interactions predominate in all four-atom (and
shorter) tethered molecules, were it not for the strong likelihood
of significant population of conformers with gauche attachment
of the terminal chromophores in the flexible systems. These
presumably would have weaker through-bond interactions,9 in
which case there must be enough through-space interaction to
(43) (a) Shimada, K.; Shimozato, Y.; Szwarc, M. J. Am. Chem. Soc.
1975, 97, 5834 and references therein. (b) Yonemoto, E. H.; Saupe, G. B.;
Schmehl, R. H.; Hubig, S. H.; Riley, R. L.; Iverson, B. L.; Mallouk, T. E.
J. Am. Chem. Soc. 1994, 116, 4786. (c) Park, J. W.; Lee, B. A.; Lee, S. Y.
J. Phys. Chem. B 1998, 102, 8209. (d) Staerk, H.; Busmann, H. G.; Ku¨hnte,
W.; Treichel, R. J. Phys. Chem. 1991, 95, 1907.
(39) (a) Zachariasse, K.; Ku¨hnle, W. Z. Phys. Chem., N. F. 1976, 101,
267. (b) Halpern, A. A.; Legenze, M. W.; Ramachandran, J. J. Am. Chem.
Soc. 1979, 101, 5736.
(40) Wagner, P. J. Acc. Chem. Res. 1983, 16, 461.
(44) Forbes, M. D. E.; Closs, G. L.; Calle, P.; Gautam, P. J. Phys. Chem.
1993, 97, 3384. Forbes, M. D. E.; Schulz, G. R. J. Am. Chem. Soc. 1994,
116, 10174.
(45) Closs, G. L.; Forbes, M. D. E.; Piotrowiak, P. J. Am. Chem. Soc.
1992, 114, 3285. Avdievich, N. I.; Forbes, M. D. E. J. Phys. Chem. 1995,
99, 9660. Werner, U.; Staerk, H. J. Phys. Chem. 1993, 97, 9274.
(46) Sisido, M.; Shimada, K. J. Am. Chem. Soc. 1977, 99, 7785.
(41) Baldwin, J. E.; Kroeger, N. S. J. Am. Chem. Soc. 1969, 91, 6444.
Dauben, W. G.; Williams, R. G.; McKelvey, R. D. J. Am. Chem. Soc. 1973,
95, 3932. Lewis, F. D.; Johnson, R. W.; Kory, D. R. J. Am. Chem. Soc.
1974, 96, 6090.
(42) Terenin, A.; Ermolaev, V. Trans. Faraday Soc. 1956, 52, 1042.
Ermolaev, V. L. SoV. Phys., Dokl. 1967, 6, 600.

9632 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Scheme 3. D-9-OA
Wagner and Kla´n
case 5 shows a typical low-energy “W” conformation,38,47 in
which donor and acceptor are not in close proximity and/or their
orientations are not favorable. However, a through-space
(solvent) energy hop from the chromophore to a chain atom
(eventually from chain to chain) might contribute to the total
energy transfer. The transfer rate constant is assumed to be high
because the total transfer path is about 4.5 Å. Calculations of
electronic coupling in rigid systems using ab initio MO theory
have revealed that paths involving hops which skip over bonds
can make the largest contribution to the total through-bond
coupling.48,49
Equation 4 describes how the quantum yield of energy
transfer depends on the three independent kinetic pathways, with
ΦET ) ΦISC[ø3 + ø5(k5ET + k5rot)/(kE5T + kr5ot + kH) +
øuk3rot/(k3rot + kH)] (4)
The quantum efficiency of energy transfer in our longer
bichromophores (30-60%) is much higher than the expected44-46
few percent of ground-state conformations with the two ends
close enough (3-4 Å) to interact in e100 ps. Therefore, only
a few percent of the total energy transfer can be attributed to
static quenching in this small fraction of the ground-state
conformational distribution. The major portion of the energy
transfer must be provided by a combination of other conformers
irreversibly rotating into such static quenching geometries within
the lifetime of the triplet ketone and by conformationally
equilibrated conformers with their ends 5-6 Å apart undergoing
direct energy transfer with rate constants of 108-109 s-1. Both
of these processes are controlled by rapid conformational
interconversion, with bond rotations being rate determining in
the former case and energy transfer in the latter. The overall
efficiency of energy transfer thus is determined by a combination
of ground-state control, rotational control, and conformational
equilibrium, as suggested above. The kET values in the tables
are only apparent rate constants that must be apportioned into
three components on the basis of what fraction of the overall
quantum yield of energy transfer each contributes.
ø3 equaling the percent population of ground-state conformers
with the two end chromophores 3-4 Å apart, ø5 equaling the
equilibrium population of conformers with the two end chro-
mophores 5-6 Å apart, k5ET equaling the average rate constant
for energy transfer by that group of conformers, k5rot equaling
the average rate constant for rotation by that group of conformers
to one with the two ends 3-4 Å apart, øu equaling the major
fraction of the conformers with their ends too far apart to
interact, and k3ET equaling an average rate constant for these
unreactive conformers to rotate irreversibly into ones with their
ends 3-4 Å apart. Although kH certainly depends on ketone
conformation, its value does not vary much since the geometry
near the carbonyl is relatively fixed in alkanophenones of all
lengths (Scheme 1).
We have already noted that ground-state-controlled static
quenching (the first term in brackets) contributes, at most, a
few percent to the quantum yield, so that the kET values are
comprised mainly of ø5k5 + øuk3 , each of which represents a
ET
rot
weighted average over all appropriate conformations. Given the
large number of conformations possible in these molecules, we
cannot assess the contributions of the two kinetic processes
dependent on conformational interconversion with any accuracy.
If it were 50:50, we would have to conclude an average rate
constant of ∼108 s-1 for irreversible rotation by something like
70% of the molecules into ones with the ends 3-4 Å apart and
about a 30% equilibrium population of conformers 5-6 Å apart
that can undergo energy transfer in 1-10 ns.
The behavior of Bz-3-O-2-O4Bp emphasizes the importance
of conformational factors, its kET value being 3 times higher
than that of Bz-6-O4Bp, which also has a seven-atom tether.
As has often been demonstrated,43d,50 replacing a methylene
group with an oxygen atom reduces gauche torsional strain and
thus increases chain flexibility and coiling.
As useful as Monte Carlo calculations are in estimating
conformational distributions, it is the dynamics of conforma-
tional change that must be understood. For the moment, we can
merely offer Scheme 3 to portray, with five representative
conformations of a bichromophore connected by a nine-carbon
chain, various examples of the above conditions. In what is
probably the lowest energy but most sparsely populated
conformation, the totally stretched 4, the donor and the acceptor
are about 14 Å apart and cannot undergo internal energy transfer.
Likewise, there are scores of conformers with one or two gauche
twists that do not bring the two chromophores close enough to
interact in the 10-ns lifetime of the triplet carbonyl. In both 2
and 3, energy transfer can compete with hydrogen abstraction.
3 depicts one conformer with three gauche twists that produce
a 6-Å D-A distance, which corresponds to an energy transfer
rate constant of ∼108 s-1. Four gauche twists can produce a
conformation such as 1 that, with a 3-Å D-A distance,
presumably undergoes energy transfer in e100 ps, or one such
as 2, in which irreversible rotation along one C-C bond can
bring the acceptor as close to the donor as in 1. As discussed
above, the key necessity for efficient energy transfer is that rates
for rotation of unreactive geometries such as 4 and 2 into ones
such as 1, and for establishing equilibrium populations of ones
such as 3, be appreciably faster than the rate of hydrogen
abstraction by the triplet benzoyl, i.e., g109 s-1. The special
The 2-3-fold jump in energy transfer rates for both Bz-x-
O-2Np and Bz-x-O-4Bp with 10- and 11-atom links in Figure
1 represents a major difference between this and some intramo-
lecular electron transfer reactions. This phenomenon will be
discussed more fully below, since the behavior of the isomeric
biphenyloxy acceptors may offer an explanation.
(47) Goodman, J. M.; Hoffmann, H. M. R.; Vinter, J. G. Tetrahedron
Lett. 1995, 36, 7757. Vinter, J. G.; Hoffmann, H. M. R. J. Am. Chem. Soc.
1974, 96, 5466.
(48) Chattoraj, M.; Bal, B.; Closs, G. L.; Levy, D. H. J. Phys. Chem.
1991, 95, 9666. Chattoraj, M.; Paulson, B.; Shi, Y.; Closs, G. L.; Levy, D.
H. J. Phys. Chem. 1993, 97, 13046.
(49) Mutz, M. W.; McLendon, G. L.; Wishart, J. F.; Gaillard, E. R.;
Corin, A. F. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 9521.
(50) Mark, J. E.; Flory, P. J. J. Am. Chem. Soc. 1966, 88, 3702.

Triplet Energy Transfer in Flexible Molecules
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9633
Scheme 4
Figure 3. Rate constants for triplet energy transfer, kET, as a function
of the number of atoms connecting the donor and the acceptor: 9,
Bz-x-O4Bp; 3, Bz-x-O3Bp; 0, Bz-x-O2Bp.
Electronic Factors. The measured kET values for 4-biphenyl
acceptor are consistently lower by a factor of ∼2 than those
for the naphthyl acceptor, although both systems exhibit the
same profile for the dependence of kET on tether length. Since
the length of a 2-naphthyl group resembles that of 4-biphenyl,
as will be discussed below, electronic effects seem to be the
only factors causing differences in behavior of these two
bichromophores. It is known that the lowest energy conforma-
tion of the biphenyl ground state in solution is twisted, while
in the T1 state the two rings are coplanar.51 This conformational
change in biphenyl slows energy transfer because of unfavorable
Franck-Condon factors, as was shown in both bimolecular4,51a
and intramolecular energy transfer52 studies. Combining our
current results with the earlier ones indicates that this effect is
independent of whether through-bond or through-space interac-
tion predominates. Discussion below of the 2- and 3-biphenyl-
oxy acceptors elaborates on this phenomenon.
Energy transfer rate constants are higher by a factor of roughly
1.5 for all tether lengths when a 4-methylbenzoyl (p-toluyl)
group rather than a benzoyl group is the donor. The difference
is small but consistent, and a shift from through-bond to through-
space mechanism obviously does not play any role in this case.
We studied the p-toluyl donor to determine the extent to which
different electronic configurations of the donor influence energy
transfer rates, since the lowest triplet state of phenyl alkyl
ketones is n,π*, while that of p-tolyl alkyl ketones is π,π*,25
with only some 10% population of their upper n,π* triplet.26
Whereas the two singly occupied molecular orbitals (SOMOs)
of n,π* ketone triplets are heavily localized on the carbonyl
group,53 π,π* excitation is centered primarily on the benzene
ring.54 That the different electronic distributions of the two states
greatly affects rates of hydrogen transfer is well known25b,26,55
and is reflected in the kH values in the tables. Scheme 4 depicts
our expectation for the likely difference in molecular geometries
for efficient energy transfer from π,π* and n,π* donors, based
on the concept of two one-electron transfers between highest
occupied molecular orbitals and lowest unoccupied molecular
orbitals. Despite the slightly greater exothermicity for energy
transfer from an n,π* triplet, two major factors would suggest
slower energy transfer from the n,π* state. First, whereas the
acceptor π and π* orbitals are parallel, the donor n and π*
orbitals are orthogonal. Energy transfer would appear to require
a change of orbital orientation during the process, experimental
evidence for which occurs in the negative ∆S for excitation of
benzophenone.56 Second, efficient overlap of the half-occupied
oxygen n orbital with the acceptor’s π system requires a different
geometry for both the tether and the ketone function than that
required for simple parallel overlap of donor and acceptor π
systems.57 (These same orbital geometry expectations often have
been used to analyze charge-transfer reactions of triplet ketones
with aromatic donors.34,58) Nevertheless, the difference between
benzoyl and p-toluyl donors was found to be very small. This
finding might suggest that there is little energy difference
between the connecting chain geometries required for efficient
n or π overlap of ketone with acceptor and/or that efficient
electron exchange can occur with less than ideal orbital
orientations, as Ghiggino59 and Levy60 have suggested. How-
ever, the effect of orientational factors on those portions of the
intramolecular energy transfer that are static or rotationally
controlled would be diminished by the rapidity of short-range
energy transfer compared to bond rotations; only the slower
TET in conformationally equilibrated molecules could show any
dramatic effect.
Isomeric Differences. Figure 3 compares the behavior of the
three bichromophoric systems with 4-, 3-, and 2-biphenyloxy
acceptors, the three resulting plots of kET vs n + 1 each having
rate jumps at longer n values, as observed for the 2-naphthoxy
compounds, but with maxima at different n values such that
the relative rate constants for the three isomers vary considerably
with n.
The compounds with 4-6-atom tethers display normal
descending reactivity as n increases, with the para isomer
reacting twice as fast as the ortho, and the meta in between.
The barrier to rotation of an ortho substituent on biphenyl is
significantly higher than those for meta and para substituents
due to steric interference with the other ring.61 Likewise, the
change from twisted ground state into coplanar triplet in the
2-biphenyloxy group is more energetically difficult than in the
other isomers,62 and Franck-Condon factors slow energy
(56) Gessner, F.; Scaiano, J. C. J. Am. Chem. Soc. 1985, 107, 7206.
(57) Mirbach, M. F.; Ramamurthy, V.; Mirbach, M. J.; Turro, N. J.;
Wagner, P. J. NouV. J. Chim. 1980, 4, 471.
(58) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.;. Wake, R. J. Am.
Chem. Soc. 1986, 108, 7727.
(59) Harcourt, R. D.; Ghiggino, K. P.; Scholes, G. D.; Speiser, S. J.
Chem. Phys. 1996, 105, 1897.
(60) Yip, Y. T.; Levy, D. H.; Kobetic, R.; Piotrowiak, P. J. Phys. Chem.
A 1999, 103, 10.
(51) (a) Wagner, P. J. J. Am. Chem. Soc. 1967, 89, 2820. (b) Hutchison,
C. A., Jr.; Kemple, M. D. J. Chem. Phys. 1979, 71, 866. (c) Takei, Y.;
Yamaguchi, T.; Osamura, Y.; Fuke, K.; Kaya, K. J. Phys. Chem. 1988, 92,
577.
(52) Sigman, M. E.; Closs, G. L. J. Phys. Chem. 1991, 95, 5012. Miller,
J. R.; Paulson, B. P.; Bal, R.; Closs, G. L. J. Phys. Chem. 1995, 99, 6923.
(53) Hochstrasser, R. M.; Scott, G. W.; Zewail, A. H. Mol. Phys. 1978,
36, 475.
(61) Katritzky, A. R.; Topsom, R. D. Chem. ReV. 1977, 77, 639. Topsom,
R. D. Acc. Chem. Res. 1983, 16, 292. Wolf, C.; Ko¨nig, W. A.; Roussel, C.
Liebigs Ann. Chem. 1995, 781.
(54) Wagner, P. J.; Siebert, E. J. J. Am. Chem. Soc. 1981, 103, 7335.
(55) Wagner, P. J.; Kemppainen, A. E.; Schott, H. N. J. Am. Chem. Soc.
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9634 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Wagner and Kla´n
Scheme 5
Figure 4. Rate constants as a function of the number of bonds
connecting the donor and acceptor: b, triplet energy transfer in Bz-
n-ONp; electron transfer in 0, pyrene-(CH2)n-dimethylaniline (ref 43d),
O, anthracene-CO2-(CH2)n-NMe2 (ref 64), and 4, phthalimide-(CH2)n-
phthalimide•- (right Y axis, ref 43a).
transfer even more, as we showed years ago.63 On the other
hand, ortho and especially para alkoxy groups stabilize a planar
biphenyl conformation by increasing the electron density at the
carbon in the pivot bond, thus decreasing the rotational energy
barrier by resonance.61 We conclude that the 2-biphenyloxy
acceptor shows lower energy transfer rates because of steric
crowding and that the 4-biphenyloxy acceptor produces the
highest rate constants because of a resonance effect that is absent
in 3-substituted biphenyls. The behavior of the longest bichro-
mophores (12- and 15-atom tethers) is comparable in that kET
values are nearly the same for all three isomers. However, the
situation for molecules with tethers of medium length is much
more complex. In the region of 7-11-atom tethers, Bz-n-O4Bp
shows a single rate constant jump for n ) 8-10, whereas Bz-
n-O3Bp and Bz-n-O2Bp show two such humps, one maximized
at n ) 7 and one at n ) 10. These anomalies result in an ortho
> meta > para rate constant ratio at n ) 7 and a para ≈ ortho
> meta ratio at n ) 10.
We have not explained the rate jumps at n ) 9-11, which
obviously have nothing to do with rotations around the terminal
acceptor-chain bond. Two possible reasons come to mind.
There may be a larger percentage of chain conformations that
allow close approach of the chain ends when n ) 9 or 10 than
when n > 11. Alternatively, we could consider the minimum
at n ) 7 for the naphthoxy and 4-biphenyloxy acceptors to be
the normal effect of forming medium-sized rings, which is offset
by favorable rotamers of the other biphenyloxy isomers, as just
explained. In that case, the rise in rates as n approaches 10 would
also be normal, but the drop in rates for larger n would require
a new explanation for how increasing chain length affects the
dynamics and thermodynamics for bringing the two ends close
enough to interact.
Comparison with Electron Transfer. Given the similar
exchange interactions responsible for both energy and electron
transfer, one might expect a resemblance between our plots in
Figure 2 and comparable plots of electron transfer. Figure 4
compares our results with three such investigations of electron
transfer between donors and acceptors connected by a poly-
methylene chain: electron exchange between two phthalimide
groups43a and electron transfer from a dimethylamino group to
an excited singlet anthracene64 and from dimethylaniline to
excited singlet pyrene.43d It is interesting that none of the three
have remotely similar profiles, the thermoneutral one showing
the same bump at 11-12 bonds as ours. The variability in the
shapes of such plots for rates of intramolecular covalent bonding
is ascribed to varying orientational requirements for the actual
transitions state for bonding;38 both electron and energy transfer
may be subject to variable orientational requirements. Electron-
transfer rates in the pyrene-(CH2)n-dimethylaniline system43d
provide a plot similar to ours but without any significant hump.
Since that study was performed in acetonitrile, we suspect that
the essentially constant kET value of (5-6) × 108 s-1 for n )
8-16 represents extensive solvent-induced chain coiling, which
could mask normal conformational effects as well as enhance
through-space interactions beyond those derived from solvent-
independent conformations.
All four acceptors show a rate jump maximized at n ) 9 or
10, while only the two with 2- and 3-biphenyloxy acceptors
show an additional jump at n ) 7, which amounts to a 4-fold
jump in the ortho/para rate ratio and a 2-fold jump in the meta/
para ratio. Scheme 5 presents a possible explanation for the n
) 7 jump based on the outcome of a single bond rotation in a
single chain conformer. The four acceptors are compared in
terms of a “reactive volume” originating from the rotation. The
scheme exemplifies how rotation about the C-O bond of a
single D-6-OA conformation affects the distance between the
far ring of the acceptor and the carbonyl oxygen. The position
of the phenyl ring attached to oxygen is not influenced by the
rotation and is identical for all four acceptors. The more distant
phenyl in the 4-biphenyl and 2-naphthyl chromophores obvi-
ously gets little closer to the carbonyl donor during the rotation
and so cannot enhance energy transfer. Rotation of the 3- and
especially 2-biphenyl groups does, however, cause the more
distant ring to approach the donor carbonyl close enough to
promote more efficient energy transfer. For both the 2- and
3-biphenyloxy acceptors, in one favorable chain conformation
half of the C-O rotamers position the distal phenyl ring closer
to the carbonyl than can occur with the 4-biphenyloxy acceptor.
Of course, the overall rate effect is too large to be explained by
a single conformer. We conclude that, in the torsionally
challenged medium-ring chains, there are significant populations
of such geometries in which the orientation of the second phenyl
ring in the biphenyls has a large effect on energy transfer,
whereas no such effect that can be explained by a C-O rotation
manifests itself for either the short molecules or the very long
ones.
Summary
This study presents a very consistent picture that shows that
intramolecular triplet energy transfer in flexible molecules is
dominated by through-space/-solvent interactions when the
(64) Vanderauwera, P.; DeSchryver, F. C.; Weller, A.; Winnik, M. A.;
Zachariasse, K. A. J. Phys. Chem. 1984, 88, 2964.
(63) Wagner P. J.; Scheve, B. J. J. Am. Chem. Soc. 1977, 99, 2888.

Triplet Energy Transfer in Flexible Molecules
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9635
Quencher. 2,5-Dimethyl-2,4-hexadiene (Aldrich) was allowed to
sublime in a refrigerator.
Actinometers and Standards. Acetophenone, 4-methylacetophe-
none, methyl benzoate, valerophenone, and 4-methylvalerophenone
were all commercially obtained materials (Aldrich) and were purified
by fractional distillation before use.
ω-Aryloxy Ketones. These were all synthesized, characterized, and
purified by following standard procedures, as described in the Sup-
porting Information.
Analytical Procedures. 1H and 13C NMR spectra were obtained on
either a 300-MHz Varian Gemini or a 300-MHz Varian VXR-300
instrument. IR spectra were recorded on a Nicolet IR/42 Fourier
transform IR spectrometer, and samples were prepared using a pressed
KBr disk technique. UV spectra were recorded on a Shimadzu UV-
160 spectrometer with matched 1.0-cm quartz cells. High-resolution
mass spectra were obtained on a Joel JMS-HX110 double-focusing mass
spectrometer in the MSU Mass Spectroscopic Facility: the electron
impact and direct probe methods usually were used. Melting points
(uncorrected) were measured on a Thomas-Hoover capillary melting
point apparatus. HPLC analyses were performed on either a Rainin
HPXL apparatus equipped with a Dynamax UV-D absorbance detector
or a Dynamax SD-200 system equipped with a PDA-1 Dynamax diode
array detector, in both cases with a normal-phase Rainin Microsorb
Si80-125-CS silica gel column and a 100-µL loop.
connecting tether contains five or more bonds, since rate
constants for energy transfer are decreased by a factor of only
0.86 for every additional intervening C-C bond, in contrast to
the factor of 0.1 observed for systems with rigid cycloalkane
spacers between donor and acceptor. Thus, the compound Bz-
(CH2)14ONp, with a 15-atom tether, undergoes internal triplet
energy transfer 10% as rapidly as does Bz(CH2)3ONp, which
presumably operates primarily by through-bond energy transfer.
Exchanging one CH2 group with an oxygen atom increases kET
by a factor of 3, a result that stresses the importance of
conformational dynamics of the connecting chain in maintaining
a significant fraction of molecules in coiled geometries with
their ends close enough for electron exchange to compete with
the γ-hydrogen abstraction that occurs in each of the molecules.
The bumps in the kET vs n plots for the 2- and 3-biphenyloxy
acceptors at n ) 7 accentuate the importance of short end-to-
end polymethylene chain distances, since it is only in such
geometries that rotation around the O-biphenyl bond can produce
carbonyl-biphenyl distances shorter than possible for the para
isomer. The bump at n ) 10 or 11 for all acceptors cannot have
similar close end-to-end approaches as an explanation and so
may, in fact, merely reflect the normal drop in rate constants
for formation of medium-sized 8-10-carbon rings, offset by
the differences in the isomeric biphenyl acceptors, plus a drop-
off in the availability of sufficiently close end-to-end approach
as n increases beyond 11.
Identification of Photoproducts. The only photoproducts common
for all reactants in this work were acetophenone and 4-methylacetophe-
none. Their identification was based on 1H NMR, 13C NMR, and HPLC
comparisons with authentic samples (Aldrich).
Photokinetic Measurements. Irradiation procedures were as in
earlier work.14 Samples of known concentrations were prepared: 2.8-
mL aliquots were placed in 13- × 100-mm Pyrex tubes that were then
degassed in three freeze-pump-thaw cycles before being sealed.
Samples were irradiated in a “merry-go-round” apparatus65 immersed
in a water bath. The 366-nm band from a medium-pressure 450-W
Hanovia mercury arc lamp was isolated by filtration with Corning 7-83
filters. Samples were analyzed by HPLC with response factors for
products and methyl benzoate internal standard calibrated with authentic
samples. Parallel irradiation of 10-3 M valerophenone solutions, where
the quantum yield of acetophenone production is 0.30,22 provided
actinometry.
Energy transfer to 4-biphenyl acceptors is slower than that
to naphthyl, as previously shown for both bimolecular and
intramolecular quenching of triplet ketones. This study shows
that the 2-3-fold rate decrease caused by the change in
coplanarity of the two rings of biphenyl is of the same size for
both through-bond and through-space energy transfer, as
expected for a purely electronic effect.
Comparison of rate constants for p-toluyl and benzoyl as
triplet donors indicates that the π,π* triplet of the former
transfers energy only 1.5 times faster than does the n,π* triplet
of the latter. This result is most interesting in view of the
orthogonality of the relevant SOMOs in the n,π* triplet; there
do not appear to have been any other such measurements.
These results furnish only a semiquantitative picture of how
connecting chain length affects energy transfer rates. Higher
level calculations are necessary for a more complete understand-
ing of both conformational dynamics and thermodynamics in
such systems.
Acknowledgment. This work was supported by NSF Grants
CHE91-20931 and CHE95-08308. We thank Dr. Jun Qiao for
the synthesis of Bz-n-OA compounds with n ) 3-5 and A )
Ph and 4Bp. We also thank a reviewer for informing us of
Staerk’s work.
Supporting Information Available: Synthesis and charac-
terization of all the ω-aryloxyalkyl phenyl ketones (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Experimental Section
Solvents. Reagent grade benzene (J. T. Baker) was washed with
sulfuric acid until further portions remained colorless and then washed,
dried, and distilled over P2O5, bp 80 °C. Reagent grade cyclohexane
(CCI) was washed with sulfuric acid until further portions remained
colorless and then washed, dried, and distilled over CaH2, bp 81 °C.
JA990224L
(65) Moses, F. G.; Liu, R. S. H.; Monroe, B. M. Mol. Photochem. 1969,
1, 245.