Steroids as molecular photonic wires. Z → E olefin photoisomerization by intramolecular triplet-triplet energy transfer with and without intervening olefinic gates

Article abstract of DOI:10.1021/ja9808595

The steroids 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5α- androstane (1), 3β((dimethyl-phenylsilyl)oxy)-17-(Z)-ethylidene-5-androstene (2), 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5α- androstane (3a), and 3α((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6- methylene-5α-androstane (3b) have been prepared. The triplet-triplet excited-state energy transfer (TTET) that occurs from the C3 aryl 'donor' group to the C17 ethylidene 'acceptor', has been studied in detail at 10 mM steroid concentration. Irradiation with 266 nm light results in Z → E olefin isomerization of the C17 ethylidene group, a consequence of both intra- and interTTET. Φ(Z→E) = 0.037, 0.018, 0.028, and 0.004 for 1, 2, 3a, and 3b, respectively. Detailed kinetic analyses of these compounds and appropriate models, with and without added olefin quenchers, provide a complete set of rate constants which are determined relative to an assumed energy transfer rate constant to piperylene of 7.0 x 109 M^-1 s^-1. In particular, k(intra)TTET for 1 = [1.7 (±0.7)] x 10⁶ s^-1. Isomerization at C17 in 2, due to intraTTET, is reduced (83% vs 1) but not completely eliminated by the endocyclic alkene in ring B, which functions as a 'triplet gate'. The exocyclic methylene group in 3b is more efficient in gating the intraTTET than it is in 3a (Φ(TTET) = 0.08 vs 0.71, respectively). This higher level of gating that occurs in 3b is attributed tO a much shorter lifetime of the axial DPSO triplet, caused by an efficient through-space intraTTET from the axial DPSO group to the C6 exocyclic olefin.

Full text of DOI:10.1021/ja9808595

3618
J. Am. Chem. Soc. 1999, 121, 3618-3632
Steroids as Molecular Photonic Wires. Z f E Olefin
Photoisomerization by Intramolecular Triplet-Triplet Energy Transfer
with and without Intervening Olefinic Gates1
Larry D. Timberlake and Harry Morrison*
Contribution from the Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
ReceiVed March 16, 1998
Abstract: The steroids 3â-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5R-androstane (1), 3â-((dimethyl-
phenylsilyl)oxy)-17-(Z)-ethylidene-5-androstene (2), 3â-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-meth-
ylene-5R-androstane (3a), and 3R-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5R-androstane (3b)
have been prepared. The triplet-triplet excited-state energy transfer (TTET) that occurs from the C3 aryl
“donor” group to the C17 ethylidene “acceptor” has been studied in detail at 10 mM steroid concentration.
Irradiation with 266 nm light results in Z f E olefin isomerization of the C17 ethylidene group, a consequence
of both intra- and interTTET. ΦZfE ) 0.037, 0.018, 0.028, and 0.004 for 1, 2, 3a, and 3b, respectively. Detailed
kinetic analyses of these compounds and appropriate models, with and without added olefin quenchers, provide
a complete set of rate constants which are determined relative to an assumed energy transfer rate constant to
piperylene of 7.0 × 109 M-1 s-1. In particular, kintraTTET for 1 ) [1.7 ((0.7)] × 106 s-1. Isomerization at C17
in 2, due to intraTTET, is reduced (83% vs 1) but not completely eliminated by the endocyclic alkene in ring
B, which functions as a “triplet gate”. The exocyclic methylene group in 3b is more efficient in gating the
intraTTET than it is in 3a (ΦTTET ) 0.08 vs 0.71, respectively). This higher level of gating that occurs in 3b
is attributed to a much shorter lifetime of the axial DPSO triplet, caused by an efficient through-space intraTTET
from the axial DPSO group to the C6 exocyclic olefin.
Introduction
an antenna to absorb incident radiation, multiplicity switches
that convert singlet energy to triplet energy, gates and relays
that impede or facilitate energy migration, and an acceptor
which is the ultimate site of chemical reaction. The energy
transmission in these systems occurs, at least in part, by a
through-bond interaction (TBI) and, hence, the steroid frame-
work serves as a “photonic wire”. Others have used steroids in
a similar way. Recent examples include a series of studies of
long-distance energy transfer to a norbornadiene acceptor via a
TBI exchange mechanism in several steroids.6 With androst-
5-ene as the spacer and the benzophenone and norbornadiene
moieties attached to C17 and C3, respectively, the authors
observed photoisomerization of the norbornadiene group upon
excitation of the remote benzophenone chromophore with a rate
constant for triplet-triplet energy transfer of 1.5 × 105 s-1 and
a quantum efficiency of 22%.6b,7
The study of molecular dimension “photonic wires” has
continued to receive significant attention.2 As part of our
ongoing efforts in exploring the photoinitiated activation of
functional groups distal from “antenna” chormophores, we have
been elaborating the capability of the steroid framework to act
as a photonic wire through which excitation energy can migrate.3
We have used the 5R-androstane4 and 5â-androstane5 skeletons
to mount various functionalities that fulfill specific electronic
functions, with the intent of developing an understanding of
how different stereoelectronic and functional group modifica-
tions affect the energy transfer process. These groups include
(1) Organic Photochemistry. Part 117. Part 116: Waugh, T.; Morrison,
H. J. Am. Chem. Soc. 1999, 121, 3083.
(2) Examples with leading references: Wagner, R. E.; Lindsey, J. S. J.
Am. Chem. Soc. 1994, 116, 9759. Wagner, R. W.; Lindsey, J. S.; Seth, J.;
Palaniappan, V.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 3996. Wagner,
R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1996, 118, 11166.
Hsiao, J.-S.; Kruger, B. P.; Wagner, R. W.; Johnson, T. E.; Delancy, J. K.;
Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.; Donohoe,
R. J. J. Am. Chem. Soc. 1996, 118, 11181. Seth, J.; Palaniappan, V.; Wagner,
R. W.; Johnson, T. E.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc.
1996, 118, 11194. Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117,
12593. Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707. Grosshenny,
V.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2705.
Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem. Soc.
1998, 120, 5118.
In our laboratories, we have used the (dimethylphenylsilyl)-
oxy (DPSO) group as the antenna, due to its favorable properties
which, as outlined earlier,8a include a relatively short (ca. 1 ns)
excited-state singlet lifetime to minimize intermolecular
(6) (a) Tung, C.-H.; Zhang, L.-P.; Yi, L.; Cao, H.; Tanimoto, Y. J. Am.
Chem. Soc. 1997, 119, 5348. (b) Tung, C.-H.; Zhang, L.-P.; Yi, L.; Cao,
H.; Tanimoto, Y. J. Phys. Chem. 1996, 100, 4480. (c) Tung, C.-H.; Zhang,
L.-P.; Yi, L. Chin. J. Chem. 1996, 14, 377. (d) Cao, H.; Akinoto, Y.;
Fujiwara, Y.; Tanimoto, Y.; Zhang, L.-P.; Tung, C.-H. Bull. Chem. Soc.
Jpn. 1995, 68, 3411.
(3) (a) Agyin, J. K.; Timberlake, L. D.; Morrison, H. J. Am. Chem. Soc.
1997, 119, 7945. (b) Agyin, J. K.; Morrison, H.; Siemiarczuk, A. J. Am.
Chem. Soc. 1995, 117, 3875.
(4) (a) Wu, Z.-Z.; Morrison, H. J. Am. Chem. Soc. 1989, 111, 9267. (b)
Wu, Z.-Z.; Morrison, H. Tetrahedron Lett. 1990, 31 (41), 5865. (c) Wu,
Z.-Z.; Morrison, H. J. Am. Chem. Soc. 1992, 114, 4119. (d) Wu, Z.-Z.;
Nash, J.; Morrison, H. J. Am. Chem. Soc. 1992, 114, 6640.
(7) Other frameworks have also been shown to be capable of energy
transfer via TBI. Some of these include bicyclo[2.2.1]heptanes, bicyclo-
[2.2.2]octanes, oligo[1.1.1]propellanes, polyenes, polyynes, polyphenylenes,
oligothiophenes, polypeptides, polyethers, and block polymers. See citation
5 in ref 3a.
(8) (a) Morrison, H.; Agyin, K.; Jiang, A.; Xiao, C. Pure Appl. Chem.
1995, 67, 111. (b) Wu, Z.-Z.; Morrison, H. Photochem. Photobiol. 1989,
50(4), 525.
(5) Jiang, S. A.; Xiao, C.; Morrison, H. J. Org. Chem. 1996, 61, 7045.
10.1021/ja9808595 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/02/1999

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3619
processes.8b Ketones have served as the recipients of intra-
molecular singlet-singlet and triplet-triplet energy transfer
(intraSSET and intraTTET, respectively), whereas olefins have
been used to probe intraTTET exclusively.3,4 These studies
demonstrated the use of intervening ketone groups as “singlet-
triplet switches” in the steroidal photonic wires.3,4 Thus, we have
reported that in the molecule 3R-((dimethylphenylsilyl)oxy)-
17-(Z)-ethylidene-5R-androstane (3RDPSO/17Z), excitation of
the DPSO antenna leads to triplet photochemistry at the distal
C17 ethylidene (Z f E photoisomerization) with a quantum
yield of ΦZfE ) 0.043.3a Insertion of a ketone at C6 in 3R-
((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5R-androstan-6-
one (3RDPSO/6/17Z) improved the quantum yield of isomer-
ization almost 10-fold to ΦZfE ) 0.36 upon DPSO excitation.
It was shown that the quantum efficiency of intraSSET from
3RDPSO to the C6 ketone is ca. 90%, so that the greater
efficiency of olefin isomerization requires that a significant
portion (91%) of the singlet energy transferred to C6 is switched
to triplet energy before ultimate transfer to C17 (82%; k ) 8.3
× 108 s-1).
Chart 1
Most of our previous steroid studies have involved the use
of the DPSO antenna as a singlet energy donor. However, as
noted above, relatively inefficient olefin isomerization was
indeed observed in the nonketonic compound 3RDPSO/17Z.
Though this chemistry must be a consequence of TTET from
the DPSO group to the remote olefin, the details of the process
were not examined in depth. In particular, the antenna would
now be operating as a donor through its relatively long-lived
triplet state, thus creating the potential for a duality of intra-
and intermolecular TTET processes. We thus report a detailed
study of the DPSO triplet-sensitized olefin isomerization of the
17-(Z)-ethylidene group in 3â-DPSO-17-(Z)-ethylidene-5R-
androstane (1; Chart 1).
This paper also explores the efficiency through which
intervening olefins may function as triplet energy gates to
modify TBI intraTTET. To this end, we have placed both
endocyclic and exocyclic olefins on the B-ring of the steroid
nucleus through the synthesis of 3â-((dimethylphenylsilyl)oxy)-
17-(Z)-ethylidene-5-androstene (2), 3â-((dimethylphenylsilyl)-
oxy)-17-(Z)-ethylidene-6-methylene-5R-androstane (3a), and
3R-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-
5R-androstane (3b). The model compounds 17â-((dimethyl-
phenylsilyl)oxy)-5R-androstane (4), 3â-((tert-butyldimethyl-
silyl)oxy)-17-(Z)-ethylidene-5R-androstane (5), 3â-((tert-butyl-
dimethylsilyl)oxy)-17-(ethylenedioxy)-5-androstene (6a), 3â-
((dimethylphenylsilyl)oxy)-17-(ethylenedioxy)-5-androstene (6b),
3â-((tert-butyldimethylsilyl)oxy)-6-methylene-5R-androstan-
17â-ol (7a), 3â-((dimethylphenylsilyl)oxy)-6-methylene-5R-
androstane-17â-ol (7b), and 3â-((dimethylphenylsilyl)oxy)-17-
ethylenedioxy-5R-androstane (8) have been prepared and will
also be discussed.
in Scheme 1. Testosterone acetate was brominated with NBS
followed by hydrolysis in HCl/methanol to produce 17â-
hydroxy-5R-androstan-3,6-dione, as described earlier.11 This
material was treated with 2-ethyl-2-methyl-1,3-dioxolane at
reflux for 5 min to selectively form the 3-ketal in 42% isolated
yield.12 The 3-(ethylenedioxy)-17â-hydroxy-5R-androstan-6-one
was treated with the ylide of methyltriphenylphosphonium
bromide in THF to form the 6-methylene compound.13 The
Results
Synthesis of Target Compounds 1, 2, 3a, and 3b. The C-17
monoolefin 1 was prepared via a Wittig reaction of epiandros-
terone followed by silylation with chlorodimethylphenylsilane.
The material was formed predominantly as the Z isomer, based
on literature precedent.9 Compound 2 was prepared in like
manner from dehydroisoandrosterone, giving predominantly the
Z isomer.10 Compound 3a was prepared in eight steps as outlined
(11) (a) Reference 4d. (b) Djerassi, C.; Rosenkranz, G.; Romo, J.;
Kaufmann, S.; Pataki, J. J. Am. Chem. Soc. 1950, 72, 4534. (c) Fried, J.
H.; Nutile, A. N.; Arth, G. E. J. Am. Chem. Soc. 1960, 82, 5704.
(12) Rosenkranz, G.; Valasco, M.; Sondheimer, F. J. Am. Chem. Soc.
1954, 76, 5024.
(9) (a) Drefahl, G.; Ponsold, K.; Schick, H. Chem. Ber. 1965, 98, 604.
(b) Baggiolini, E. G.; Iacobelli, J. A.; Hennessy, B. M.; Uskokovic, M. R.
J. Am. Chem. Soc. 1982, 104, 2945.
(10) Hazra, B. G.; Pore, V. S., Joshi, P. L. J. Chem. Soc., Perkin. Trans.
1 1993, 1819.
(13) Barnikol-Oettler, K.; Zepter, R.; Heller, K. J. Prakt. Chem. 1965,
27, 18.

3620 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
Scheme 1
3-ketal was cleaved and the 17-alcohol was oxidized to the
ketone in a single step by the use of the Jones reagent.14 This
material (6-methylene-5R-androstane-3,17-dione) had properties
identical with those described in the literature.15 We found that
reduction of the C17 ketone can be suppressed at low temper-
atures so that treatment with lithium tri-tert-butoxyalumino-
hydride at -78 °C allowed stereoselective reduction of the
3-ketone16 to give compound 15. A Wittig reaction of 15,
followed by reaction with chlorodimethylphenylsilane, gave the
target compound 3a.
forming 3R-hydroxy-6-methylene-5R-androstan-17-one. This
material was reacted with the ylide of ethyltriphenylphosphon-
ium bromide as described above, and the product was silylated
with chlorodimethylphenylsilane to give 3b.
Synthesis of Model Compounds for Kinetic Studies.
Compound 4 was prepared from the parent alcohol and
chlorodimethylphenylsilane. 5 was prepared from 3â-hydroxy-
17-(Z)-ethylidene-5R-androstane (prepared as described above)
and tert-butyldimethylsilyl chloride (TBDMS chloride). To
prepare compound 6a, dehydroisoandrosterone was treated with
2-ethyl-2-methyl-1,3-dioxolane to form the 17-ketal,17 followed
by treatment with TBDMS chloride. Treatment of the same ketal
with chlorodimethylphenylsilane gave 6b. Compound 7a was
prepared from 15 (Scheme 1) by silylation with TBDMS
chloride, followed by reduction of the C17 ketone with lithium
tri-tert-butoxyaluminohydride at -20 °C. Compound 7b was
prepared from 15 (Scheme 1) by reaction with chlorodimethyl-
Compound 3b was synthesized in three steps from 3R-
hydroxy-5R-androstan-6,17-dione4d by first conducting a Wittig
reaction with methyltriphenylphosphonium bromide at -40 °C,
(14) (a) Bowers, A.; Halsall, T. G.; Jones, E. H. R.; Lemin, A. J. J.
Chem. Soc. 1953, 2549. (b) Org. Synth. 1965, 45, 28.
(15) Davies, M. T.; Ellis, B.; Kirk, D. N.; Petrow, V. Tetrahedron 1965,
21, 3185.
(16) Wheeler, O. H.; Mateos, J. L. Can. J. Chem. 1958, 36, 1431. Fajkosˇ,
J. Collect. Czech. Chem. Commun. 1959, 24, 2285. Brown, H. C.; McFarlin,
R. F. J. Am. Chem. Soc. 1956, 78, 252.
(17) (a) Fieser, L. F. J. Am. Chem. Soc. 1954, 76, 1945. (b) Caballero,
G. M.; Gros, E. G. Synth. Commun. 1995, 25(3), 395.

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3621
Table 1. Fluorescence Quantum Yields and Phosphorescence Area
Percentages for 1-4a
b
b
φf (×10-2
)
% phosc
φf (×10-2
)
% phosc
1
2
3a
1.2
1.2
1.2
23
0
0
3b
4
1.1
1.3
0
66
a Using 254 nm excitation in cyclohexane. b Measured using toluene
as the reference (φf ) 0.14) in cyclohexane at room temperature.20
Accuracy is estimated at 10%. c Measured in a methylcyclohexane glass
at 77 K. Values are the integrated area of the phosphorescence emission
relative to the area of the total emission; all data are corrected for
photomultiplier response.
phenylsilane, followed by reduction of the C17 ketone with tri-
tert-butoxyaluminohydride at -10 °C. 8 was prepared by
ketalization of epiandrosterone18 followed by reaction with
chlorodimethylphenylsilane. The stereochemistry of all C3 and
C17 alcohols described above is easily assigned on the basis of
1H NMR analysis (see Experimental Section).19
Figure 1. Total emission spectra in a methylcyclohexane glass at 77
Spectroscopy. Absorption Spectra. UV absorption spectra
of 1-4 and 8 show the typical π-π* aryl transition with λmax
258 nm. The presence of the unconjugated ∆5 or 6-methylene
olefins had no significant affect on the intensity or wavelength
of the aryl absorption, indicating that the aryl π-π* transition
is not perturbed by the olefins and that no notable ground-state
interactions exist between these chromophores.
Fluorescence Quantum Yields and Singlet Lifetimes. The
room-temperature fluorescence emission spectra of 1-4 were
obtained in cyclohexane with 254-nm excitation. Each consists
of a featureless band from 265 to 340 nm with λmax at 280 nm.
The fluorescence quantum yields are shown in Table 1 and are
identical within experimental error, indicating that the olefinic
groups have no affect on the DPSO singlet state. Additionally,
the singlet lifetimes for 4 and a C3-DPSO model (3R-DPSO-
17-(Z)-ethylidene-5R-androstane)3 were determined to be 2.5
and 2.6 ns, respectively.
K.
106 kcal/mol. The DPSO triplet energy, as estimated from the
onset of the phosphorescence emission at 348 nm, is ET ) 82
kcal/mol.
Total Emission of 4 with Added 5 Quencher. The total
emission spectrum of a 1.4 × 10-3 M solution of 4 in a
methylcyclohexane glass was obtained in the presence and
absence of added equimolar amounts of 5. The phosphorescence
emission constituted 63% and 65% of the total emission area
with and without the added steroid olefin, respectively. This
supports our conclusion that energy transfer in 1, 2, 3a, and 3b
is completely intramolecular in the methylcyclohexane glass at
77 K.
Photochemistry. Irradiation of 1, 2, 3a, and 3b. Separate
irradiations of 2.0 mL of argon-degassed 1.0 × 10-2
M
cyclohexane solutions of 1, 2, and 3a using the 266 nm line of
a Nd:YAG laser (30 mW power) produced major photoproducts
with GC retention times identical with the small amounts of E
isomers formed during the synthesis of these substrates (eq 1).
Fluorescence Emission of 1 with Added cis-2-Heptene
Quencher. The fluorescence emission of a 5.7 × 10-4
M
cyclohexane solution of 1 was obtained using 254 nm excitation
with and without 75 mM cis-2-heptene. The integrated fluo-
rescence areas were 215 and 214 area units, respectively,
indicating that there is no interaction of the 2-heptene at this
concentration with the DPSO singlet state.
Fluorescence Emission of 1 with Added cis-Piperylene. The
fluorescence emission of 1 was determined with and without 1
mM piperylene. The integrated emission areas were virtually
identical, indicating that piperylene at this concentration does
not interact with the DPSO singlet state.
Total Emission at 77 K in a Methylcyclohexane Glass. The
total emission spectra elicited by 254 nm excitation for
compounds 1-4 are shown in Figure 1 (see also Table 1). The
DPSO phosphorescence emission is evident for compounds 1
and 4 at 345-485 nm, with that for 1 being diminished relative
to the model 4. No phosphorescence emission is visible in any
of the B-ring olefin compounds. These findings are consistent
with intraTTET occurring in 1, 2, 3a, and 3b. The total emission
spectrum of a 3â-DPSO steroid, 3â-((dimethylphenylsilyl)oxy)-
5R-androstane, was found to be identical with that for 4.
The 0-0 transition energy for the DPSO S0 f S1 transition
is estimated from the fluorescence onset at 270 nm to be ES )
(1)
For 3b similar results were obtained using 2.0 mL of argon-
degassed 6.1 × 10-3 cyclohexane solutions. In one case, that
of compound 1, the E isomer was independently synthesized
by irradiation of 3â-hydroxy-17-(Z)-ethylidene-5R-androstane
at 254 nm in cyclohexane with toluene (ET ) 83 kcal/mol)20 as
the sensitizer. The photolyzate was isolated and then treated
with chlorodimethylphenylsilane to give a product as two
separate peaks in the GC with retention times identical with
those described for the irradiation of 1 directly. The identifica-
(18) Marquet, A.; Dvolaitzky, M.; Kagan, H. B.; Mamlok, L.; Ouannes,
C.; Jacques, J. Bull. Soc. Chim. Fr. 1961, 1827.
(19) Bridgeman, J. E.; Cherry, P. C.; Clegg, A. S.; Evans, J. M.; Jones,
E. R. H.; Kasal, A.; Kumar, V.; Meakins, G. D.; Morisawa, Y.; Richards,
E. E.; Woodgate, P. D. J. Chem. Soc. C 1970, 250.
(20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993; p 50.

3622 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
Table 2. Irradiation of Target Molecules with Various
Concentrations of cis-2-Heptene
Table 3. Quenching of cis-2-Heptene Isomerization Using Various
Model Compounds as Sensitizers and Quenchers
sensitizer
quencher
kqτDPSO-H (M-1
)
R2
1/ΦhZefptE vs 1/[heptene]
Stern-Volmer plot of C17
isomerizn
reciprocal plot data
4
4
8
6b
7b
6a
7a
5
5
5
42 ( 4
47 ( 2
221 ( 11
50 ( 3a
123 ( 4b
0.97
0.99
0.98
0.99
0.99
steroida
slope (M)
intercept R2
kqτ (M-1
)
R2
4
1
2
3a
0.051 ( 0.005 23.4 ( 1.3 0.97
0.564 ( 0.004 25.4 ( 1.2 0.99
45 ( 2
23 ( 1
36 ( 3
0.99
0.99
0.99
a kqτDPSO-5-H b kqτDPSO-6-H; see text.
.
a Steroid concentration 10 mM, ca. 3.0 mL. Irradiation times were
as follows: 4, 25 min; 1, 15 min; 2, 30 min; 3a, 20 min.
cis-2-heptene (2.0 × 10-3 to 3.0 × 10-2 M) were irradiated in
the Rayonet reactor using 4 × 254 nm lamps for 25 min. The
flux was determined by using (E)-1-phenyl-2-butene actinom-
etry.21,22 GC analysis was used to determine the amount of trans-
hept
2-heptene formed, and a plot of 1/Φ
vs 1/[heptene] was
ZfE
linear (Figure 2, Table 2).
Irradiation of 4 and 8 with cis-2-Heptene and Various
Olefin Quenchers. Several quartz tubes containing 3.0 mL of
a 1.0 × 10-2 M solution of 8 (argon degassed, cyclohexane),
5.0 × 10-3 M cis-2-heptene, and various concentrations of 5
as the quencher (0-15 mM) were irradiated in the Rayonet
reactor using 4 × 254 nm lamps for 15 min. GC analysis was
used to determine the amount of trans-2-heptene formed, and
the data were analyzed by using the Stern-Volmer treatment.
A plot of Φ0/Φ vs [5] gave a straight line (Table 3) with kqτ )
221 ( 11 M-1, where kq is actually the intermolecular rate
constant for TTET from 3â-DPSO to the C17 olefin, kinter17
,
and τDPSO-H is the lifetime of the 3â-DPSO group in the
presence of 5 mM cis-2-heptene.
By similar treatment, quenching of the 4-sensitized cis-2-
heptene isomerization with 6a and 7a as quenchers gave
kqτDPSO-H values of 42 ( 4 and 47 ( 2 M-1, respectively (Table
3). Here, τDPSO-H represents the lifetime of the 17â-DPSO group
in the presence of 5 mM cis-2-heptene.
Figure 2. Reciprocal plots of isomerization of cis-2-heptene using 1
and 4 as sensitizers.
tion of the photoproducts of the other compounds as the E
isomers is by analogy with the above data and with the
knowledge that the characteristic reaction of the olefins with
triplet donors is geometrical isomerization.3a For the irradiation
of 1, 2, 3a, and 3b, the quantum yields of isomerization are
ΦZfE ) 0.037 ( 0.001, 0.018 ( 0.002, 0.028 ( 0.002, and
0.0035 ( 0.0006, respectively.
Irradiation of 1, 2, 3a and 4 with cis-2-Heptene. Irradiation
of 3.0 mL of a 1.0 × 10-2 M solution of 1 (argon degassed) in
separate quartz tubes containing various amounts of cis-2-
heptene was conducted in the Rayonet reactor using 4 × 254
nm lamps for 15 min. The flux was determined by using (E)-
1-phenyl-2-butene actinometry.21,22 The amount of E isomer of
1 formed was determined, and the data were analyzed by using
the Stern-Volmer treatment. A plot of Φ0/Φ vs [heptene] was
linear (Table 2) with kqτ ) 45 ( 2 M-1. Irradiations of 2 and
3a were conducted in the same manner to give linear Stern-
Volmer plots with the data summarized in Table 2. All results
are reported with standard deviations obtained from linear
regression analysis.
Irradiation of 6b and 7b with cis-2-Heptene Using 5 as
the Quencher. Using the same procedure as described above,
6b and 7b were used as sensitizers of cis-2-heptene isomeriza-
tion using 5 as the quencher. The slopes of the Stern-Volmer
plots for 6b and 7b gave kinter17τDPSO-5-H ) 50 ( 3 M-1 and
kinter17τDPSO-6-H ) 123 ( 4 M-1, respectively. In these plots,
τDPSO-n-H is the lifetime of the given 3â-DPSO group in the
presence of either the C5 or C6 olefin, respectively (Table 3).
Irradiation of 1 with cis-Piperylene Quencher. Several
tubes containing 3.0 mL of a 1.0 × 10-2 M solution of 1 (argon
degassed, cyclohexane) with cis-piperylene as the quencher (1.0
× 10-4 to 2.0 × 10-3 M) were irradiated in the Rayonet reactor
using 4 × 254 nm lamps for 20 min. GC analysis was used to
determine the amount of steroid C17 E isomer formed, and the
data were analyzed by using the Stern-Volmer treatment. A
plot of Φ0/Φ vs [piperylene] was linear (Figure 3) with kqτ )
744 ( 13 M-1, where kq here is the intermolecular rate constant
for TTET from DPSO to piperylene, kinterP, which is the fastest
rate constant determined in this study.
Irradiation of 1 at Various Concentrations. Cyclohexane
solutions of 1 ranging in concentration from 9.8 × 10-2 to 1.0
× 10-4 M were irradiated in the Rayonet reactor using 4 ×
254 nm lamps for 30 min. The amount of isomerized olefin
was measured by GC and was corrected for the fraction of light
absorbed. The results are shown in Figure 4 and show a
concentration dependence until ca. 1 mM steroid is reached.
Quantum Yields for Photosensitization of cis-2-Heptene
Isomerization Using Different DPSO Models. The quantum
yields for the sensitized isomerization of 12.5 M cis-2-heptene
by 6.3 mM 4 and of 6.6 mM cis-2-heptene by 4.9 mM 3â-
DPSO-5R-androstane, in cyclohexane, were determined at 266
The amount of trans-2-heptene formed during the irradiation
hept
ZfE
of 1 was also determined. A plot of 1/Φ
Vs. 1/[heptene]
was linear, as shown in Figure 2. All reciprocal plot data are
summarized in Table 2.
Five quartz tubes containing 2.7 mL of a 1.0 × 10-2
M
solution of 4 (argon degassed) with various concentrations of
(21) For (E)-1-phenyl-2-butene, ΦEfZ ) 0.20: Morrison, H.; Peiffer,
R. J. Am. Chem. Soc. 1968, 90, 3428.
(22) All olefin quantum yields are corrected for back-reaction, which
takes into account the small amount of product isomer initially present.
See: Lamola, A. A.; Hammond, G. S. J. Chem. Phys. 1965, 43, 2129.
Palensky, F. J. Ph.D. Thesis, Purdue University, May 1977. See also: Saltiel,
J.; Marinari, A.; Chang, D. W.-L.; Mitchener, J. C.; Megarity, E. D. J. Am.
Chem. Soc. 1979, 101, 2982.

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3623
expectedly longer lifetimes of triplet states relative to their
singlet counterparts. One might also anticipate a possible
reduction in rates for triplet vs singlet intramolecular energy
transfer. Both factors would result in a greater opportunity for
the donor and acceptor chromophores on different molecules
to interact.
Kinetics. Intrinsic DPSO Triplet Lifetime. Having noted
that both intra- and intermolecular TTET play a role in the
sensitized isomerization reaction, we set about to differentiate
these two processes and to elaborate their relative efficiencies
and rate constants. Our approach involved using a Stern-
Volmer kinetic analysis of various triplet quenching reactions
to calculate the inter- and intramolecular quantum efficiency
components involved in the isomerization of the steroids. These
quantum efficiencies were then used to predict the overall
isomerization quantum yields (ΦZfE), which could be compared
to the experimentally measured values as a check in the
consistency of the method. We chose this approach over the
obvious alternative of “isolating” the intramolecular process by
reducing the steroid concentration to the point where intermo-
lecular processes do not compete (see Figure 4) for two reasons.
First, we found that the low concentrations involved in such
high-dilution studies required very large corrections of the data
to take into account the marginal light absorption of the
solutions. Second, reproducibility in the irradiation of such dilute
reaction solutions was problematic and gave less accurate results.
The quantum yields of isomerization (ΦZfE) were therefore
determined at 6-10 mM steroid concentration, a range of
concentrations in which the DPSO group absorbs at least 95%
of the light. The data indicate that the overall ΦZfE is most
efficient for 1 (0.037) and is diminished in each of the additional
steroids bearing an alkene group in ring B. The relative rate
constants for interTTET and intraTTET at 10 mM steroid were
then obtained by using cis-2-heptene in various competitive
olefin quenching and sensitization experiments. Since such
experiments only provide relative rates, our experiments were
normalized to the quenching efficiency observed for 1 with cis-
piperylene. The triplet energy of this quencher (ET ) 57 kcal/
mol),24 relative to that of the DPSO group (ET ) 82 kcal/mol),
justifies the assumption that the rate of quenching of 1 by the
diene would be diffusion-controlled.25 The reported diffusion-
controlled rate constants in cyclohexane are 7.0 × 109 and 1.3
Figure 3. Stern-Volmer plot for quenching of C17 ethylidene
isomerization in 1 using cis-piperylene as quencher.
Figure 4. Irradiation of 1 at various concentrations (y axis normalized
to photons absorbed).
nm by using the Nd:YAG laser at 30 mW power. The results
were ΦZfE ) 3.34 × 10-2 and 3.45 × 10-2 for the two
sensitizers, respectively. Additionally, 5 mM cis-2-heptene was
irradiated in cyclohexane (254 nm) using 10 mM 4 or 10 mM
8 as sensitizers. The amount of trans-2-heptene formed was
12.5% and 11.7% for 4 and 8, respectively.
Discussion
× 1010 M-1 s-1 26,27
.
We have chosen to use the lower value
Photochemistry. As with our earlier observations for 3R-
DPSO-17-(Z)-ethylidene-5R-androstane,3 excitation of com-
pounds 1, 2, 3a, and 3b using light absorbed by the antenna
DPSO group leads to olefin Z f E isomerization at the remote
C17 position (cf. eq 1). Significantly, the photoisomerization
in 1 was found to be quenchable by the addition of equimolar
amounts of cis-2-heptene. The fact that the aryl fluorescence is
unquenched by 75 mM cis-2-heptene confirms that the interac-
tion involves the DPSO triplet state. Thus, at the 10 mM
concentrations of 1 used in these experiments, TTET from C3
to C17 is not exclusively an intramolecular process.23 That this
is the case is also evidenced by the observed dependence of
the photoisomerization reaction on the concentration of the
substrate (cf. Figure 4). These observations contrast with our
previous observations involving DPSO singlet-singlet energy
transfer in steroid substrates, where no intermolecular component
to the sensitization was observed.3,4,5,8 As noted in the Introduc-
tion, this difference in behavior could be anticipated from the
because it has been shown that TTET is typically less than
diffusion-controlled, especially in nonviscous solvents.28,29
Quenching of the C17 ethylidene isomerization in 1 by
piperylene gave a slope in the Stern-Volmer plot of kinterPτDPSO-17
) 744 ( 13 M-1, where kinterP corresponds to the rate constant
for TTET from 3â-DPSO to the piperylene (see Figure 3) and
τDPSO-17 is the triplet lifetime of the 3â-DPSO group in the
presence of the C17 olefin. Using this slope and assuming that
kinterP is diffusion-controlled, the 3â-DPSO triplet lifetime in 1
(τDPSO-17) is calculated to be 106 ( 2 ns at 10 mM concentra-
tion.
(24) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993; p 71.
(25) The fluorescence emission of 1 was measured with and without 1
mM piperylene and showed no singlet quenching.
(26) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Mill Valley, CA, 1991; p 314.
(27) Saltiel, J.; Atwater, B. W. In AdVances in Photochemistry; Vollman,
D. H., Hammond, G. S., Gollnick, K. K., Eds.; Interscience: New York,
1988; Vol. 14, p 1.
(28) Wagner, P. J.; Kochevar, I. J. Am. Chem. Soc. 1968, 90, 2232.
(29) All of the rate constants would be correspondingly altered were the
actual intermolecular rate constant to be significantly different, but the
quantum efficiencies we derive from these rates would remain unchanged.
(23) The reported3a lack of quenching of 3RDPSO/17Z was in error.
Reanalysis of this compound showed ca. 30% quenching in the presence
of equimolar amounts of cis-2-heptene, similar to 1 in the present study.
The reported3a lack of quenching of 3RDPSO/6-ketone/17Z with equimolar
cis-2-heptene has, however, been confirmed.

3624 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
The analogous experiment was run using cis-2-heptene as
the quencher, with the resulting slope of the Stern-Volmer plot
being kinterHτDPSO-17 ) 45 ( 2 M-1 (Table 2). Here kinterH is
the bimolecular rate constant for energy transfer from the 3â-
DPSO group to the cis-2-heptene quencher. Since τDPSO-17 is
the same for these two reactions, we calculate kinterH ) [4.2
is the lifetime of the DPSO chromophore in the presence of 5
mM cis-2-heptene.32 This lifetime is defined as shown in eq 7.
1
τDPSO-H
)
(7)
kinterH[hept] + kTS
((0.2)] × 108 M-1 s-1
.
Using the values of kinterH and kTS determined above, we
calculate the DPSO triplet lifetime in these runs to be τDPSO-H
) 330 ( 16 ns. From this value, and the slopes of the Stern-
Volmer plots (Table 3), we determined the intermolecular rate
We can now use this information to determine the intrinsic
triplet decay rate constant for a DPSO group (kTS) by irradiating
the model compound 4 with varying amounts of cis-2-heptene.30
constants to be kinter17 ) [6.7 ((0.1)] × 108 M-1 s-1, kinter5
)
The amount of trans-2-heptene formed was determined, and
hept
ZfE
[1.3 ((0.1)] × 108 M-1 s-1, and kinter6 ) [1.4 ((0.1)] × 108
M-1 s-1. We note that the rate constant for intermolecular energy
transfer to the C17 ethylidene group (kinter17) is ca. 5-fold greater
than the rate constants for transfer to either of the B-ring
olefins.33
the data were plotted as 1/Φ
vs. 1/[hept]. If one assumes
that the only decay paths available to the DPSO triplet in this
model compound are the intrinsic triplet decay and energy
transfer to the heptene, the quantum efficiency of heptene
isomerization and the energy transfer quantum efficiency are
given by eqs 2 and 3, respectively. Here, FDfE is the fraction
Rate Constants for IntraTTET. With a complete set of
values of the rate constants for interTTET in hand, and the
knowledge of the DPSO intrinsic triplet decay rate (kTS), we
can now calculate the intraTTET rate constants. First, for energy
transfer from the C3â-DPSO group to the C17 ethylidene in
compound 1 (k(in1t)ra17), the Stern-Volmer analysis of the quench-
ing of the C17 isomerization with cis-2-heptene is represented
by eq 8. As already noted, the slope of this line is 45 ( 2 M-1
(Table 2). In this equation, τDPSO-17 is the triplet lifetime of the
3â-DPSO group in 1 (106 ( 2 ns) at 10 mM steroid
concentration (eq 9) and Φ0 is the quantum yield of the C17
ethylidene isomerization in 1 in the absence of the cis-2-heptene
quencher.
hept
ZfE
Φ
) ΦiscΦTTETFDfE
(2)
(3)
k
interH[hept]
ΦTTET
)
kinterH[hept] + kTS
of heptene diradical triplets that decay to the E isomer.
Substitution of eq 3 into eq 2 gives eq 4 as the overall expression
for heptene isomerization. After inverting, this provides the final
ΦiscFDfEkinterH[hept]
hept
ZfE
Φ
)
(4)
kinterH[hept] + kTS
Φ
0 ) 1 + kinterH
τ
DPSO-17[hept]
(8)
(9)
Φ
expression (eq 5) in a form useful for plotting. A plot of the
1
+ kinter17[DPSO] + kTS
τDPSO-17
)
(1)
kTS
1
1
)
+
(5)
k
intra17
hept
ΦiscFDfE
ΦiscFDfEkinterH[hept]
Φ
ZfE
The 3â-DPSO triplet lifetime in 1 (τDPSO-17) is concentration-
data using eq 5 is shown in Figure 2. From the slope and
intercept (Table 2) we can calculate the ratio of rate constants
for 4 (eq 6). Using eq 6 and the value for kinterH determined
dependent, since it depends on both intraTTET and interTTET
(1)
intra17
to the C17 olefin (eq 9).36 In this equation, k
and kinter17
correspond to the intramolecular and bimolecular rate constants
for energy transfer from the 3â-DPSO donor to the C17
ethylidene, and kTS is the intrinsic triplet decay rate constant
for DPSO intersystem crossing to the ground state. Inserting
kinterH
) 460 ( 50 M-1
(6)
kTS
the slope of 45 ( 2 M-1 as the value for kinterH
τDPSO-17 in eq 8,
and solving for k(in1t)ra17 in eq 9, we obtain ki(n1t)ra17 ) [1.7 ((0.6)]
above we obtain kTS ) [9.2 ((1.1)] × 105 s-1. The intrinsic
DPSO triplet lifetime (τDPSO) is therefore 1.1 ( 0.1 µs.
× 106 s-1
.
(1)
intra17
Alternatively, the value for k
can be derived from the
Rate Constants for InterTTET. The value for interTTET
from the 3â-DPSO group to C17 was determined by using
compound 8 sensitization of cis-2-heptene with 5 as quencher.
The values for the rate constants corresponding to interTTET
from a DPSO group to the B-ring olefins present in 2 and 3a
were determined by using the steroid-olefin model compounds,
6a and 7a as quenchers of the compound 4 (10 mM) DPSO-
sensitized isomerization of cis-2-heptene (5 mM).31 The results
are summarized in Table 3, where the slope of the line is
kqτDPSO-H, kq is the intermolecular rate constant for energy
transfer to the given olefin (kinter17, kinter5, or kinter6), and τDPSO-H
same experiment, but by analyzing the amount of isomerized
heptene using a reciprocal plot (Figure 2). The quantum yield
for heptene isomerization as defined in eq 2 still applies.
However, with the additional modes of triplet decay now
(32) Since the 3â-DPSO and 17â-DPSO chromophores display the same
fluorescence quantum yields, singlet lifetimes, and phosphorescence/
fluorescence ratios, we assume their intrinsic triplet lifetimes are the same.
(33) These relative rates presumably reflect a combination of sterics and
relative triplet energies. Triplet energies for representative olefins are as
follows (kcal/mol): CH2dC(CH3)2, 80.5; CH2dC(CH3)C2H5, 78.3;
CH3CHdC(CH3)2, 77.2; (CH3)2dC(CH3)2, 75.8.34 Sterics have been shown
to have a modest effect on TTET by inhibiting favorable orbital overlap
between a donor and an acceptor.35
(30) We have demonstrated that the 17â-DPSO model (4), the 3â-DPSO
model (8), and 3â-DPSO-5R-androstane are equally effective in sensitizing
cis-2-heptene isomerizations (see Results).
(34) Ni, T.; Caldwell, R. A.; Melton, L. A. J. Am. Chem. Soc. 1989,
111, 457.
(31) Compound 4 was used for these studies because 8 only became
available in the latter part of this work. We believe that the cis-2-heptene
results30 justify our assumption that, for the relatively unencumbered C5
and C6 steroid olefins, the calculated intermolecular rate constants would
not significantly differ for 4 vs 8.
(35) Scaiano, J. C.; Leigh, W. J.; Meador, M. A.; Wagner, P. J. J. Am.
Chem. Soc. 1985, 107, 5806.
(36) The [DPSO] term in this and subsequent equations represents the
concentration of the appropriate DPSO-steroid for the system under
consideration.

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3625
Table 4. Summary of Kinetic Parametersa
rate const
available to the DPSO group in 1 (i.e. intraTTET and inter-
TTET), the efficiency of energy transfer from the antenna to
the heptene is defined as in eq 10. Substitution of this expression
lifetime (ns)b
kTS
[9.2 ((1.1)] × 105 s-1
τDPSO ) 1090 ( 130
τDPSO-H ) 330 ( 16
τDPSO-17 ) 106 ( 2
τDPSO-5-17 ) 54 ( 4
τDPSO-6-17 ) 85 ( 7
τDPSO-5-H ) 75 ( 5
τDPSO-6-H ) 184 ( 7
kinterH
kinter5
kinter6
kinter17
kintra5
[4.2 ((0.2)] × 108 M-1 s-1
[1.3 ((0.1)] × 108 M-1 s-1
[1.4 ((0.1)] × 108 M-1 s-1
[6.7 ((0.1)] × 108 M-1 s-1
[9.1 ((0.8)] × 106 s-1
k
interH[hept]
ΦTTET
)
(10)
(1)
intra17
kinterH[hept] + kTS + k
+ kinter17[DPSO]
kintra6
[1.0 ((0.3)] × 106 s-1
into eq 2 and inverting provides a form useful for plotting (eq
11). The results of this analysis for the sensitization of cis-2-
[1.7 ((0.7)] × 106 s-1
(1)
intra17
(2)
intra17
(3a)
intra17
k
k
k
[6.2 ((8)] × 105 s-1
[1.6 ((1.2)] × 106 s-1
(1)
kTS + k + kinter17[DPSO]
intra17
1
1
a All rates are relative to kinterP ) kdiff ) 7.0 × 109 M-1 s-1
.
b τDPSO-X
)
+
(11)
hept
φiscFDfE
φiscFDfEkinterH[hept]
represents the lifetime of the DPSO group in the presence of the given
olefin, where H is heptene (5 mM) and 5, 6, and 17 are the olefinic
positions on the steroid.
Φ
ZfE
heptene isomerization by compound 1 are given in Table 2.
From the slope and intercept we calculate k
(1)
intra17
) [1.7
The Stern-Volmer analysis of the quenching of the isomer-
ization of the C17 ethylidene in 2 by cis-2-heptene gave
kinterHτDPSO-5-17 ) 23 ( 1 M-1 (Table 2). Using the value for
((0.7)] × 106 s-1, which is the same as that obtained from the
quenching data independently derived above. This rate constant
is similar to that found by Tung et al. (k ) 1.5 × 105 s-1) for
intraTTET from C17 to C3 in a benzophenone donor/norbor-
nadiene acceptor system.6b,37
k
interH determined above, we find τDPSO-5-17 ) 54 ( 4 ns. Using
this value in eq 14, and the values of the other rate constants
determined previously, gives k
The high value for the error is a result of the propagation of
errors, combined with the low value of the final result.
For 3a, Stern-Volmer analysis of the quenching of the
isomerization of the C17 ethylidene by cis-2-heptene gave
kinterHτDPSO-6-17 ) 36 ( 3 M-1 (Table 2). Using the value for
(2)
intra17
) [6.2 ((8)] × 105 s-1
.
To determine the rate constants for intraTTET from the 3â-
DPSO group to the C5 and C6 olefins in 2 and 3a (kintra5 and
k
intra6, respectively), quenching of the 6b- or 7b-sensitized cis-
2-heptene photoisomerization by 5 was conducted. In this
Stern-Volmer analysis, Φ0 is the quantum yield of cis-2-
heptene isomerization in the absence of the compound 5
quencher. Beginning with 6b, the lifetime of the DPSO group
in the presence of the C5 olefin depends on both inter- and
intramolecular energy transfer to that olefin, as shown in eq
12. The lifetime also depends on kinterH and kTS, which were
previously determined. Using the slope of the Stern-Volmer
k
interH determined above gives τDPSO-6-17 ) 85 ( 7 ns. Insertion
of this value, and the values of the other rate constants
(3a)
intra17
determined previously, into eq 15 gives k
) [1.6 ((1.2)]
× 106 s-1. All rate constants and lifetimes are summarized in
Table 4.
We diverge briefly to discuss the intersystem crossing
efficiency of a DPSO group, and the possibility of other
quenching mechanisms, before proceeding to calculate the
quantum efficiencies for intramolecular energy migration in these
compounds.
plot (kinter17
appropriate rate constants, we calculate kintra5 ) [9.1 ((0.8)] ×
τ
DPSO-5-H) of 50 ( 3 M-1 and the values for the
106 s-1. The analogous analysis using 7b (eq 13) gives kintra6
)
[1.0 ((0.3)] × 106 s-1
.
Intersystem Crossing. The efficiency of intersystem crossing
can be determined from the intercept of the reciprocal plots
discussed above (Figure 2, Table 2) using eq 5 or 11. The
fraction of heptene diradical triplets that decay to the E isomer
(FDfE) has been previously determined.38 The calculated Φisc
values, obtained using compounds 1 and 4 as sensitizers of cis-
2-heptene isomerization, are 0.079 ( 0.004 and 0.086 ( 0.005,
respectively. The average value is Φisc ) 0.083 ( 0.005.
A previous determination of Φisc for the DPSO group using
photoacoustic calorimetry (PAC) gave Φisc ) 0.19,4c a value
considerably larger than the number obtained from our present
triplet counting technique. If, during the interaction of the DPSO
triplet with heptene, there were other modes of DPSO triplet
decay that did not result in olefin isomerization (charge transfer,
exciplex formation, etc.), then our value for Φisc would indeed
be low. The existence of such processes would have the effect
of adding a fractional term to eq 2 to account for incomplete
energy transfer to the heptene by DPSO triplets. The fraction
necessary to reduce the PAC Φisc value from 0.19 to our value
of 0.083 is 0.44. Thus, instead of multiplying ΦTTETFDfE by a
1
τDPSO-5-H
)
)
kinterH[hept] + kinter5[DPSO] + kintra5 + kTS
(12)
1
τDPSO-6-H
kinterH[hept] + kinter6[DPSO] + kintra6 + kTS
(13)
These results can now be used to determine the values of the
intramolecular rate constant for energy transfer to the C17
olefins in 2 and 3a, k(in2t)ra17, and k(in3tar)a17, respectively. Using the
cis-2-heptene quenching of the C17 ethylidene isomerization
reaction for 2 and 3a, (Table 2) the lifetimes of the 3â-DPSO
triplet can be represented as shown in eqs 14 and 15, where all
DPSO triplet-decay modes to the respective B-ring olefins are
now included.
τDPSO-5-17
)
1
(14)
(15)
(37) It is interesting that a rate constant for electron transfer from a C16
biphenyl anion to a C3 naphthyl group has been determined to be 1.5 ×
106 s-1. See: Johnson, M. D.; Miller, J. R.; Green, N. S.; Closs, G. L. J.
Phys. Chem. 1989, 93, 1173.
(38) We have used a value of 0.50 previously determined for FDfE for
2-heptene; see: Golub, M. A.; Stevens, C. L.; Brash, J. L. J. Chem. Phys.
1966, 45, 1503. Morrison, H.; Pajak, J.; Peiffer, R. J. Am. Chem. Soc. 1971,
93, 3978.
(2)
intra17
k
+ kinter17[DPSO] + kintra5 + kinter5[DPSO] + kTS
τDPSO-6-17
)
1
(3a)
intra17
k
+ kinter17[DPSO] + kintra6 + kinter6[DPSO] + kTS

3626 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
Table 5. Comparison of Measureda and Predicted ΦZfE Values for 1-3a Using ΦTTET Calculated from either the Gate or the Relay
b
Assumptionc
gate
relay
Φ
ZfE calcdd from
measd ΦZfE
Φ
ZfE calcdd from
ΦTTET
ΦintraTTET
Φ
TTET (×10-2
)
(×10-2
)
ΦTTET
ΦintraTTET
Φ
TTET (×10-2
)
1e
2
3a
0.90 ( 0.10
0.39 ( 0.04
0.71 ( 0.13
0.18 ( 0.07
0.03 ( 0.04
0.14 ( 0.11
3.9 ( 0.4
1.7 ( 0.2
3.1 ( 0.6
3.7 ( 0.1
1.8 ( 0.2
2.8 ( 0.2
0.90 ( 0.10
0.95 ( 0.08
0.92 ( 0.14
0.18 ( 0.07
0.52 ( 0.07
0.22 ( 0.11
3.9 ( 0.4
4.1 ( 0.4
4.0 ( 0.6
b
a Measured with a Nd:YAG laser at 266 nm, 30 mW power, 10 Hz pulse. ΦTTET and ΦintraTTET refer to C3 f C17 energy transfer. c “Gate”
assumes that all energy reaching the B-ring olefins is decayed and not transferred to the C17 olefin. “Relay” assumes that all energy reaching the
d
B-ring olefins is passed on to the C17 olefin. ΦZfE ) ΦiscΦTTETFDfE ) (0.083)ΦTTET(0.52), where FDfE was obtained from photostationary data
for the analogous 3R-DPSO-17-(Z)-ethylidene-5R-androstan-6-one.3a e Since no olefins are present in the B-ring, the “gate” and “relay” cases are
equivalent.
Φisc value of 0.083, we would multiply by (0.19)(0.44);
obviously, the net effect is the same. In other words, the
involvement of additional decay paths during the interaction of
DPSO triplets with heptene would not affect the calculated
isomerization quantum yields. We are currently seeking an
additional, independent, measurement of Φisc to resolve the
discrepancy between the PAC value and the chemical triplet-
counting value.39,40
and is not passed on to the C17 olefin. In this case, the B-ring
olefins act as triplet energy gates. The TTET quantum efficiency
is represented by eqs 17 and 18 for 2 and 3a, respectively.
ΦTTET
)
(2)
intra17
k
+ kinter17[DPSO]
(17)
(2)
intra17
k
+ kinter17[DPSO] + kintra5 + kinter5[DPSO] + kTS
Quantum Efficiencies of Energy Migration in 1. Having
determined the rate constants for the various process that affect
the triplet state in 1, we can now calculate the quantum
efficiency of energy transfer and dissect it into its inter- and
intramolecular components. In 1, the overall quantum efficiency
of energy transfer from the 3â-DPSO group to the C17
ethylidene is given by eq 16. Substitution of the appropriate
ΦTTET
)
(3a)
intra17
k
+ kinter17[DPSO]
(18)
(3a)
intra17
k
+ kinter17[DPSO] + kintra6 + kinter6[DPSO] + kTS
Alternatively, we may assume that all of the energy that
reaches the B-ring olefins is passed on to the C17 olefin; i.e.,
the B-ring olefins are triplet energy relays. Such a situation will
result in ΦTTET expressions for 2 and 3a as shown in eqs 19
and 20, respectively.
(1)
intra17
k
+ kinter17[DPSO]
ΦTTET
)
(16)
(1)
intra17
k
+ kinter17[DPSO] + kTS
rate constants into eq 16 provides the calculated value for the
overall TTET quantum efficiency as ΦTTET ) 0.90 ( 0.10. The
intramolecular component, obtained by using only k(in1t)ra17 in the
numerator, is ΦintraTTET ) 0.18 ( 0.07. The overall quantum
efficiency for isomerization at C17 (ΦZfE) in 1 can be calculated
an a manner analogous to that presented in eq 2 using the ΦTTET
value. This calculated value and the corresponding experimental
result are presented in Table 5. We believe that the match
between the calculated and experimentally measured ΦZfE
values is quite reasonable when one considers the multiple
measurements involved in determining the calculated value.
Quantum Efficiencies of Energy Migration in 2 and 3a:
The “Gate” Effect. There are two limiting possibilities for the
TTET quantum efficiencies from the 3â-DPSO group to the
C17 ethylidene in 2 and 3a. In the first, we assume that all of
the energy reaching the B-ring olefins decays unproductively
ΦTTET
)
(2)
intra17
k
+ kinter17[DPSO] + kintra5 + kinter5[DPSO]
(19)
(20)
(2)
intra17
k
+ kinter17[DPSO] + kintra5 + kinter5[DPSO] + kTS
ΦTTET
)
(3a)
intra17
k
+ kinter17[DPSO] + kintra6 + kinter6[DPSO]
(3a)
intra17
k
+ kinter17[DPSO] + kintra6 + kinter6[DPSO] + kTS
The results of both computations from these limiting cases
for 2 and 3a are presented in Table 5. It is clear from this table
that the ΦTTET values obtained using the gate assumption lead
to calculated ΦZfE values which best parallel the measured
results, i.e., the C5 and C6 olefins function as triplet energy
gates and are not relays. The relative efficiencies of the
endocyclic and exocyclic gates can be illustrated by comparing
just the intramolecular “wire” portion of the TTET. As shown
in the gate section of Table 5, ΦintraTTET is reduced in 2 by 83%
relative to that calculated for 1, i.e., the C5 gate allows only a
small amount of energy to reach C17. The energy at C17 could
be due to a small fraction of energy being relayed from C5. It
is interesting, however, that the through-bond mode of energy
transfer in the steroids allows for the possibility of multiple
energy transfer pathways between C3 and C17, so that a
“northern” route is available even if the “southern” route is
completely blocked.42 In either event, that we observe any
C17 isomerization at all in 2 is due primarily to the interTTET
that is present at the concentrations used.
(39) We do have some evidence in hand, however, which discounts the
existence of additional decay paths during the interaction of the DPSO triplet
with heptene. The fact that we obtained the same intermolecular rate
constants by a Stern-Volmer analysis of the quenching of the C17
ethylidene isomerization and by measuring the amount of isomerization
that occurred in the quencher via a reciprocal plot lends support to there
being no additional quenching processes.
(40) The sum of the intersystem crossing and fluorescence efficiencies
for the DPSO group is Φisc + Φf ) 0.083 + 0.012 ) 0.095. This leaves
the 91% of the DPSO singlets unaccounted for as nonradiative decay (Φnr).
Clearly, this extensive nonradiative decay must be the source of the short
singlet lifetime (ca. 1 ns) of the DPSO group. This is reminiscent of the
“R-substitution effect” which we documented two decades ago41 wherein a
monosubstituted benzene having methyl, ethyl, isopropyl, or tert-butyl
groups displayed fluorescence lifetimes of 35.2, 35.1, 24.5, and 10.0 ns,
respectively. The Φisc values decreased accordingly (i.e. 0.52, 0.44, 0.34,
and 0.086, respectively). The phenomenon was attributed to an increase in
Φnr with increasing R methylation. The tert-butyl like substitution of the
DPSO group may be causing a similar acceleration of radiationless decay.
(41) Froehlich, P. M.; Morrison, H. J. Phys. Chem. 1972, 76, 3506.
Schloman, W. W., Jr.; Morrison, H. J. Am. Chem. Soc. 1977, 99, 3342.

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3627
Comparison of the gating ability of the exocyclic olefin in
3a with the endocyclic olefin in 2 reveals that the exocyclic
alkene is an ineffective gate, leading to a 22% reduction in
Φ
intraTTET in 3a vs 1. This is due to the greatly diminished rate
constant for intramolecular triplet energy transfer to the exo-
cyclic alkene (kintra6) relative to the endocyclic alkene (kintra5
)
(Table 4) that makes the former much less effective. This was
frankly unexpected, since we anticipated that the exocyclic
methylene triplet would exhibit facile bond rotation and hence
more rapidly and effectively dissipate triplet energy via the “free
rotor effect”.43
It is interesting that, in the benzophenone-steroid-norbor-
nadiene system studied by Tung et al., the steroid contains an
endocyclic B-ring olefin.6b Although these workers did not
compare their results with a saturated B-ring analogue, there is
no indication of impedance by the olefin. This is not surprising,
since the olefin triplet energy is much higher than the triplet
energies of the benzophenone and norbornadiene groups and,
thus, should not act as a triplet gate in this system.
Figure 5. Molecular mechanics optimized structure for one rotamer
of 3b.
Comparison of 3â- vs 3r-DPSO as a Triplet Donor.
Virtually all of our effort was devoted to the 3â-series of
substrates represented by 1, 2, and 3a. An insufficient quantity
of the 3R-steroid 3b prevented us from conducting a complete
kinetic analysis of this compound. However, the remarkably
low ΦZfE found for 3b (0.0035) vs that for 3a (0.028) suggests
that any intraTTET to C17 in this compound must be minimal.
Using the measured ΦZfE for 3b, we calculate ΦTTET ) 0.08.44
This compares with the value ΦTTET ) 0.71 for compound 3a
computed from the rate constants. Clearly, the lifetime of the
axial 3R-DPSO triplet is much shorter than that for the 3â-
isomer, due to the more efficient quenching by the C6 methylene
group. We have seen a similar enhancement of singlet energy
transfer for an axial vs equatorial DPSO donor with a ketone
acceptor at C63a,4d and noted that this result was inconsistent
with the general observation that TBI in polycyclics is favored
when donors and acceptors are equatorial.45 We believe the more
efficient energy transfer to C6 from the axial C3 donors now
can be explained in terms of an added through-space mecha-
nism. A molecular mechanics optimized structure for 3b shows
the distance from the aryl group to the methylene group to be
about 4 Å in one conformation (Figure 5). In fact, an X-ray
structure of a compound similar to 3b, but with a carbonyl group
at C6 in place of the exocyclic olefin, shows the DPSO group
to indeed be tucked under the steroid. The distance from the
aryl group to the C6 carbonyl is 6.45 Å.46-48
Energy Transfer in a Glass at Low Temperature. As one
would expect, the total emission spectra for compounds 2, 3a
and 3b show no evidence of DPSO phosphorescence (Figure
1). This is consistent with complete intraTTET from the DPSO
triplet to the alkenes in these compounds.49 However, though
compound 1 shows a diminution in the relative amount of
phosphorescence emission compared with the nonolefinic model
4, phosphorescence is not completely eliminated.50 The observa-
tion of such emission is surprising, since the relatively slow
triplet emission rate constants should not be competitive with
the rate of intraTTET calculated for 1 above. We presume that
the reduced rate of intraTTET in the glass may be a consequence
of the restricted rotational movement of the DPSO group at 77
K. Thus, we speculate that conformations which are particularly
inefficient in coupling to the steroid are extensively populated
and frozen into place in the glass.
Conclusions
Triplet-triplet energy transfer in these steroid systems at
millimolar concentrations consists of intra- and interTTET
components. The use of external quenchers has allowed for the
determination of a complete set of both the rate constants and
the quantum efficiencies for these two modes of energy transfer.
Partial gating of the through-bond intraTTET from C3 to C17
can be achieved by inserting endocylic and exocyclic alkenes
in ring B, with the effect significantly more pronounced for the
endocyclic alkene. The more efficient intraTTET from an axial
C3 DPSO group to an exocyclic alkene at C6 is attributed to
an added through-space intraTTET pathway between these
groups.
(42) For discussions of multiple through-bond pathways, see: Paddon-
Row, M. N.; Shephard, M. J. J. Am. Chem. Soc. 1997, 119, 5355. Paulson,
B. P.; Curtiss, L. A.; Bal, B.; Closs, G. L.; Miller, J. R. J. Am. Chem. Soc.
1996, 118, 378. Ratner, M. A. J. Phys. Chem. 1990, 94, 4877. Onuchic, J.
N.; de Andrade, P. C. P.; Beratan, D. N. J. Chem. Phys. 1991, 95(2), 1131.
Onuchic, J. N.; Beratan, D. N. J. Am. Chem. Soc. 1987, 109, 6771.
(43) Zimmerman, H. E.; Epling, G. A. J. Am. Chem. Soc. 1972, 94, 8749.
Zimmerman, H. E.; Albrecht, F. X.; Haire, M. J. J. Am. Chem. Soc. 1975,
97, 3726.
The rate constant for intraTTET from the 3â-DPSO antenna
to the C17 ethylidene acceptor (k(in1t)ra17) in 1 is 1.7 × 106 s-1
.
(44) Calculated by using ΦZfE ) ΦiscΦTTETFDfE: 0.0035 ) (0.083)ΦTTET
-
The approximate distance between the DPSO and the ethylidene
groups of 11 Å cannot support this magnitude of rate constant
via a simple through-space exchange mechanism; for a donor
and an acceptor at this distance the rate would be on the order
(0.52).
(45) Closs, G. L.; Piotrowiak, P.; MacInnis, J. M.; Fleming, G. R. J.
Am. Chem. Soc. 1988, 110, 2652. Closs, G. L.; Johnson, M. D.; Miller, J.
R.; Piotrowiak, P. J. Am. Chem. Soc. 1989, 111, 3751.
(46) Agyin, J. K, Ph.D. Thesis, Purdue University, Aug 1996.
of 103 s-1 51
We therefore invoke a TBI-mediated electron
.
(47) For some examples of the involvement of through-space intramo-
lecular energy transfer involving flexible tethers connecting donor and
acceptor groups, see: Haggquist, G. W.; Katayama, H.; Tsuchida, A.; Ito,
S.; Yamamoto, M. J. Phys. Chem. 1993, 97, 9270. Wagner, P. J.; El-Taliawi,
G. M. J. Am. Chem. Soc. 1992, 114, 8325. Katayama, H.; Ito, S.; Yamamoto,
M. J. Phys. Chem. 1992, 96, 10115. Katayama, H.; Maruyama, S.; Ito, S.;
Tsujii, Y.; Tsuchida, A.; Yamamoto, M. J. Phys. Chem. 1991, 95, 3480.
(48) There is evidence that for conformers having the donor and acceptor
within 3-4 Å of each other, energy transfer occurs in 100 ps or less; cf.:
Kla´n, P.; Wagner, P. J. J. Am. Chem. Soc. 1998, 120, 2198.
(49) The total emission spectrum of 4 was measured at 77 K in a
methylcylohexane glass containing an equimolar amount of 5 added as a
competitive olefin quencher. The spectrum was unchanged, evidence that
there is no interTTET in the steroidal olefins under these conditions.
(50) We have consistently observed phosphorescence emission from the
R and â C3-DPSO/C17 and C17-DPSO/C3 olefins.
(51) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Mill Valley, CA, 1991; p 320.

3628 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
use. Fluorescence quantum efficiencies were obtained by using toluene
as the reference20 and correcting for differences in detector gain,
absorbance, and refractive index. Total emission spectra were obtained
in a methylcyclohexane glass using a 2 mm × 5 cm cylindrical quartz
cell in an optical Dewar at 77 K after at least five freeze-pump-thaw
degassing cycles (2 × 10-5 Torr).
exchange mechanism for intraTTET in the steroid molecule;
i.e., the steroid acts as a photonic wire, allowing excitation
energy to be passed from donor to acceptor through the
intervening σ-bond framework.52 This study therefore represents
our first unambiguous example of TBI within a steroid, since
in the SSET cases we studied previously,3-6 the possibility of
resonance energy transfer was also present.
Photochemical Apparatus. All irradiations were conducted at room
temperature in solutions that were argon-degassed at least 25 min prior
to use. The kinetic determinations were conducted in a Model RPR-
100 Rayonet reactor available from Southern New England Ultraviolet
Co. Cylindrical quartz tubes were used (1 cm × 9 cm). The reactor
was equipped with 4 × 254 nm lamps and a merry-go-round turntable
apparatus that was positioned approximately 2 cm from the lamps. Up
to eight tubes could be irradiated simultaneously. Laser irradiations
were conducted at 266 nm and 30 mW power with a Continuum NY-
61 Nd:YAG laser equipped with a frequency quadrupler (10 Hz, ca.
3.0 mJ/pulse). A 2× beam enlarger was used in front of the sample
cell to avoid cell damage. Square Vycor sample cells (1 cm) were used.
Chromatography. Samples were analyzed by analytical GLC using
various capillary columns on a Varian Model Star 3400 CX gas
chromatograph. Detection was with a flame-ionization detector utilizing
a Hewlett-Packard 3390A integrator. Steroids were analyzed by using
a DB-1 capillary column (15 m × 0.25 mm × 0.25 µm film thickness)
with the following temperature parameters: injector and detector 300
°C, column 240 °C for 1 min, 5 °C/min to a final temperature of 280
°C. Retention times were 6-12 min for silylated steroids and 4-6 min
for the steroid alcohols, depending on the structure. cis-2-Heptene and
(E)-1-phenyl-2-butene were analyzed by using a DB-5 capillary column
(30 m × 0.25 mm × 0.25 µm film thickness) with injector and detector
set at 250 °C and the column set at either 40 °C (cis-2-heptene, retention
time 4 min) or 100 °C ((E)-1-phenyl-2-butene, retention time 6 min).
Flash chromatography was performed by using hexane/ethyl acetate
(v/v) solutions with a 50 mm × 36 in. column packed with 6 in. of
230 × 400 mesh silica gel (EM-9385). Analytical TLC was performed
on silica gel 60 Å, 250 µm, coated on a glass support (Whatman) and
visualized using a cerium sulfate/ammonium molybdate/sulfuric acid
solution or a UV lamp as appropriate. Semipreparative HPLC was
conducted by using a Waters 4000 HPLC system with a Waters 486
tunable absorbance detector set at 254 nm. The column used was a
Beckman Model 235328 Ultrasphere 5 µm C-18 column of dimensions
10 mm × 25 cm with a mobile phase of 100% acetonitrile (isocratic)
at a flow rate of 9 mL/min.
Experimental Section
Chemicals. The following chemicals were obtained from Aldrich
Chemical Co., stored at room temperature, and used without further
purification unless otherwise stated: epiandrosterone; chlorodimethyl-
phenylsilane (stored in a desiccator); ethyltriphenylphosphonium
bromide; methyltriphenylphosphonium bromide; triethylamine (distilled
from CaH2); N,N-dimethylformamide (anhydrous); potassium tert-
butoxide (1.0 M in THF); 2-methyl-2-ethyl-1,3-dioxolane; tert-butyl-
dimethylsilyl chloride; imidazole; cis-2-heptene and cis-piperylene
(stored in the freezer and distilled prior to use); lithium tri-tert-
butoxyaluminohydride (1.0 M in THF); p-toluenesulfonic acid mono-
hydrate (p-TSA, stored in a desiccator). All silylated steroids synthesized
in this study were stored at room temperature in amber vials in a
desiccator.
The following chemicals were obtained from other suppliers and
stored at room temperature unless otherwise indicated: 5R-androstane-
17â-ol, dehydroepiandrosterone, and testosterone (all from Sigma);
pyridine, chromium trioxide, and sodium bicarbonate (all from Mallinck-
rodt); chlororform-d (Cambridge Isotope Laboratories, stored over
sodium carbonate); anhydrous magnesium sulfate (EM); hydrochloric
and sulfuric acids (Fisher).
The following solvents were purchased from various suppliers and
used as received: acetonitrile, 2-propanol, and methylene chloride
(Fisher); acetone, ethyl acetate, hexanes, and toluene (Mallinckrodt).
Tetrahydrofuran (THF, Fisher) was distilled under nitrogen from
sodium-benzophenone ketyl before use. Spectrophotometric grade
solvents were used in the photochemical and spectroscopic studies
without further purification: cyclohexane (Fisher) and methylcylohex-
ane (Aldrich). The photochemical solvents were stored under argon
and purged with argon after removing a portion for use.
Instrumentation. Melting points were determined with a Fisher-
Johns melting point apparatus and are uncorrected. H NMR spectra
1
were obtained in CDCl3 with either a GE spectrometer operating at
300 MHz or a Varian Gemini spectrometer operating at 200 MHz, with
chemical shifts reported relative to residual chloroform at 7.27 ppm.
13C NMR were obtained in CDCl3 with either a GE spectrometer
operating at 75.6 MHz or a Varian Gemini spectrometer operating at
50 MHz, with chemical shifts reported relative to residual chloroform
at 77.0 ppm unless otherwise noted. Mass spectra were recorded on
Finnigan 4000 mass spectrometers operating with a source temperature
of 250 °C with direct probe sample introduction. Electron impact (EI)
and chemical ionization (CI) mass spectra were recorded at 70 eV, the
latter with a 2-methylpropane pressure of 0.30 Torr. High-resolution
mass spectra were recorded on a Kratos Model MS-50 instrument that
was calibrated to a resolution of 10 000, with a 10% valley between
peaks using perfluorokerosene. Absorption spectra were recorded in 1
cm cells against a solvent blank on a Cary Model 100 UV-vis
spectrophotometer (dual beam) interfaced to a computer (Pentium P5-
100) using Cary software. Steady-state fluorescence spectra were
obtained on a SLM Aminco SPF-500 spectrophotometer using a 250
W xenon arc lamp operating in the A/B mode. Detection was normal
to the excitation light. All fluorescence samples used 1 cm square
fluorescence cells and were purged with argon at least 25 min prior to
Syntheses. All reactions were conducted in oven-dried glassware,
sealed with rubber septa, and run under nitrogen unless otherwise
indicated.
3â-((Dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5r-androstane
(1). Wittig Reaction: Illustration of General Procedure. Epiandros-
terone (3.00 g, 10.3 mmol) was dissolved in 15 mL of THF. In a
separate vessel, potassium tert-butoxide (53 mL, 1.0 M solution in THF)
was added to a slurry of ethyltriphenylphosphonium bromide (19.2 g,
51.6 mmol) in 50 mL of THF. After 5 min of stirring, the steroid
solution was added to the Wittig reagent via cannula over 5 min at
ambient temperature. The mixture was stirred for 4.5 h and, after
removal of a large portion of the Wittig byproducts by recrystallization
from 7/3 hexane/ethyl acetate, the filtrate containing the crude product
was concentrated and purified by flash chromatography (7/3 hexane/
ethyl acetate). The 17-(Z)-ethylidene-3â-hydroxy-5R-androstane product
was obtained in 92.1% yield (2.88 g; mp 138-144 °C from cyclohex-
ane, lit.9a mp 152-154 °C) and used for the next reaction without further
purification. 1H NMR: δ 5.12 (q, 1 H), 3.59 (m, 1 H), 2.40-0.82 (m,
26 H), 0.86 (s, 3 H), 0.81 (s, 3 H). 13C NMR: δ 150.3, 113.1, 71.2,
56.2, 54.3, 44.8, 44.3, 38.1, 37.1, 36.9, 35.5, 35.0, 31.8, 31.4, 31.3,
28.6, 24.3, 21.4, 16.8, 13.0, 12.3. MS (CI): m/z 302 (67, M+), 287
(70). HRMS (CI): m/z calculated for C21H35O (M + H) 303.2688, found
303.2672.
(52) In the absence of a comprehensive rate vs distance study, one can
use the relationship kintraTTET ) k0(exp) - [â(no. of bonds) - 1], with k0 as
the commonly accepted value53 of 1013 s-1 and an 11-bond separation, to
estimate the value â ) 1.6. This is close to the â ) 1.4 value obtained
from ab initio calculations for TBI mediated intraTTET in a naphthyl-
polynorbornyl-naphthyl system.54
(53) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
(54) Clayton, A. H. A.; Scholes, G. D.; Ghiggino, K. P.; Paddon-Row,
M. N. J. Phys. Chem. 1996, 100, 10912.
DPSO Reaction: Illustration of General Procedure. 17-(Z)-
Ethylidene-3â-hydroxy-5R-androstane (2.62 g, 8.66 mmol) was dis-
solved in anhydrous DMF (30 mL) containing triethylamine (6.1 mL,
43.3 mmol). The solution was cooled in an ice bath, chlorodimethyl-
phenylsilane (1.45 mL, 8.66 mmol) was added via syringe over 5 min,
and the resulting slurry was mixed for 2.5 h. TLC (9/1 hexane/ethyl

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3629
acetate) showed the reaction to be complete. The slurry was diluted
with 100 mL of toluene and washed successively with cold 5% aqueous
sodium bicarbonate, cold 5% HCl, and cold 5% aqueous sodium
bicarbonate. The organic phase was dried (MgSO4) and concentrated
under vacuum to give a colorless oil. The material was purified by
using flash chromatography (9/1 hexane/ethyl acetate) to provide 3â-
((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5R-androstane as a color-
less oil, which was recrystallized (2×) from acetonitrile to give a white
solid (2.39 g, 63.3% yield, mp 57-58 °C). GC analysis showed the
64.6, 54.9, 51.5, 48.8, 43.6, 41.9, 38.2, 38.0, 37.1, 36.1, 34.0, 31.4,
31.0, 23.8, 21.6, 12.2, 11.6.
3-(Ethylenedioxy)-6-methylene-5R-androstan-17â-ol (4.9 g, 14.1
mmol) was dissolved in 300 mL of acetone in an open flask equipped
with a mechanical stirrer. Jones reagent (18 mL) was added by pipet
while the ca. 20 °C reaction temperature was maintained. (The Jones
reagent was prepared by dissolving 67 g of chromium trioxide in 125
mL of water and slowly adding 58 mL of sulfuric acid with cooling.)
After the reaction slurry was stirred for 1 h, a small amount of
2-propanol was added to destroy any excess oxidant. The slurry was
worked up by removing most of the acetone under reduced pressure
and pouring the residue into 400 mL of water. The product was
extracted with ethyl acetate (3×), and the organic phase was washed
with 5% sodium bicarbonate and water. The product solution was dried
(MgSO4), and the solvent was removed under reduced pressure. The
crude 6-methylene-5R-androstane-3,17-dione was used in the next
reaction without further purification (GC assay 92.9%). An analytical
specimen was purified by recrystallization from cyclohexane (mp 186-
1
product to contain ca. 2.5% of the E isomer. H NMR: δ 7.36-7.61
(m, 5 H), 5.10 (q, 1 H), 3.59 (m, 1 H), 2.04-0.86 (m, 25 H), 0.85 (s,
3 H), 0.79 (s, 3 H), 0.38 (s, 6 H). 13C NMR: δ 150.5, 138.6, 133.5,
129.4, 127.7, 113.2, 72.3, 56.2, 54.4, 44.9, 44.4, 38.4, 37.2, 37.1, 35.5,
35.0, 31.9, 31.7, 31.4, 28.7, 24.4, 21.4, 16.9, 13.1, 12.3, -0.9, -1.0.
MS (CI): m/z 437 (13, M + H), 285 (100). HRMS (CI): m/z calculated
for C29H45OSi (M + H) 437.3240, found 437.3241.
3â-((Dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5-androstene (2).
17-(Z)-Ethylidene-3â-hydroxy-5-androstene was obtained from dehy-
droisoandrosterone (2.89 g, 10.0 mmol) in 92.7% yield (2.79 g) using
the general Wittig procedure and used for the next reaction without
further purification (mp 133-135 °C from cyclohexane, lit.10 mp 136-
1
187 °C, lit.15 mp 187-188 °C). H NMR: δ 4.82 (d, 1 H), 4.47 (d, 1
H), 2.55-0.95 (m, 20 H), 0.91 (s, 3 H), 0.87 (s, 3 H). 13C NMR: δ
220.4, 211.8, 147.2, 107.3, 54.1, 51.1, 50.8, 47.7, 40.5, 40.4, 38.0, 37.9,
37.7, 36.7, 35.7, 31.3, 21.7, 20.9, 13.8, 11.6.
1
137 °C). H NMR: δ 5.36 (m, 1 H), 5.13 (q, 1 H), 3.53 (m, 1 H),
2.42-1.04 (m, 23 H), 1.02 (s, 3 H), 0.90 (s, 3 H). 13C NMR: δ 150.3,
140.8, 121.6, 113.5, 71.7, 56.5, 50.1, 44.0, 42.3, 37.2, 36.9, 36.5, 31.7,
31.6, 31.42, 31.37, 24.5, 21.2, 19.4, 16.6, 13.1. MS (EI): m/z 300 (100,
M+), 285 (38), 267 (70);. HRMS (CI): m/z calculated for C21H33O (M
+ H) 301.2531, found 301.2530.
6-Methylene-5R-androstane-3,17-dione (3.4 g, 11.4 mmol) was
dissolved in 115 mL of THF. The solution was cooled to -78 °C, and
a 1.0 M solution of lithium tri-tert-butoxyaluminohydride (18.6 mL)
was added over 2.5 h with reaction monitoring by GC. The solution
was poured into 500 mL of 5% HCl, and the resulting slurry was
extracted with methylene chloride (3×). The combined methylene
chloride layers were washed with water and 5% sodium bicarbonate.
The solution was dried (MgSO4), concentrated under reduced pressure,
and purified with flash chromatography (1/1 hexane/ethyl acetate) to
give 2.4 g of 3â-hydroxy-6-methylene-5R-androstan-17-one (15; mp
135-136 °C, toluene) and 0.5 g of recovered starting material (83%
yield based on recovered starting material). 1H NMR: δ 4.76 (d, 1 H),
4.51 (d, 1 H), 3.64 (m, 1 H), 2.49-0.88 (m, 21 H), 0.85 (s, 3 H), 0.71
(s, 3 H). 13C NMR: δ 220.9, 148.5, 106.6, 71.2, 54.7, 51.3, 49.3, 47.9,
40.7, 37.8, 36.9, 36.5, 35.8, 33.6, 31.4, 31.2, 21.7, 20.7, 13.8, 12.4.
MS (EI): m/z 302 (100, M+), 284 (41), 269 (42), 91 (72). HRMS
(CI): m/z calculated for C20H31O2 (M + H) 303.2324, found 303.2323.
17-(Z)-Ethylidene-3â-hydroxy-5-androstene (1.08 g, 3.61 mmol) was
treated with chlorodimethylphenylsilane (0.61 mL, 3.61 mmol) ac-
cording to the general DPSO procedure. The material was purified by
using flash chromatography (20/1 hexane/ethyl acetate) to provide the
product as a white solid, which was recrystallized (2×) from acetonitrile
to give white crystals (0.953 g, 60.7% yield). GC analysis showed the
product to contain ca. 2.0% of the E isomer (mp 73-74 °C). 1H
NMR: δ 7.36-7.60 (m, 5 H), 5.26 (m, 1 H), 5.14 (q, 1 H), 3.56 (m,
1 H), 2.42-1.02 (m, 22 H), 1.00 (s, 3 H), 0.88 (s, 3 H) 0.390 (s, 3 H),
0.385 (s, 3 H). 13C NMR: δ 150.3, 141.3, 138.5, 133.4, 129.5, 127.7,
121.2, 113.4, 72.7, 56.5, 50.1, 44.0, 42.5, 37.2, 37.0, 36.6, 31.8, 31.7,
31.42, 31.37, 24.5, 21.2, 19.3, 16.6, 13.1, -1.0, -1.1. MS (EI): m/z
434 (16, M+), 356 (48), 135 (100). HRMS (CI): m/z calculated for
C29H43OSi (M + H) 435.3083, found 435.3083.
3â-Hydroxy-6-methylene-5R-androstan-17-one (1.0 g, 3.3 mmol)
was treated with potassium tert-butoxide (16.6 mL, 1.0 M solution in
THF) and ethyltriphenylphosphonium bromide (6.1 g, 16.6 mmol)
according to the general Wittig procedure. The 17-(Z)-ethylidene-3â-
hydroxy-6-methylene-5R-androstane product was obtained in 75% yield
(0.78 g, mp 155-157 °C, cyclohexane) and used for the next reaction
3â-((Dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5r-
androstane (3a). 17â-Hydroxy-5R-androstane-3,6-dione was prepared
by following the reported literature procedure in 65% overall yield from
testosterone acetate.11 1H NMR: δ 3.69 (t, 1 H), 2.61-1.18 (m, 21 H),
0.97 (s, 3 H), 0.78 (s, 3 H).
1
without further purification. H NMR: δ 5.12 (q, 1 H), 4.73 (d, J )
17â-Hydroxy-5R-androstane-3,6-dione was treated on the basis of
the procedure of Rosenkranz et al.12 This steroid (11.53 g, 37.9 mmol)
was placed in a dry flask with 138 mL of 2-methyl-2-ethyl-1,3-
dioxolane and 0.46 g of p-TSA. The mixture was quickly heated to
reflux and held at reflux for 5 min. The resulting reaction solution was
cooled in an ice bath, during which time crystals appeared. Isolation
of the filter cake gave 5.59 g of 3-(ethylenedioxy)-17â-hydroxy-5R-
androstan-6-one (42.4% yield, GC assay 97.6%; mp 189-190 °C from
1.46, 1 H), 4.47 (d, J ) 1.46, 1 H), 3.65 (m, 1 H), 2.36-0.89 (m, 24
H), 0.86 (s, 3 H), 0.70 (s, 3 H). 13C NMR: δ 150.1, 149.4, 113.4,
106.0, 71.5, 56.1, 54.7, 49.3, 44.4, 41.8, 37.8, 37.1, 37.0, 36.5, 33.7,
31.4, 31.3, 24.3, 21.7, 16.9, 13.1, 12.4. MS (EI): m/z 314 (100, M+),
299 (68), 281 (24). HRMS (CI): m/z calculated for C22H35O (M + H)
315.2688, found 315.2688.
17-(Z)-Ethylidene-3â-hydroxy-6-methylene-5R-androstane (0.76 g,
2.42 mmol) was treated with chlorodimethylphenylsilane (0.41 mL,
2.4 mmol) according to the general DPSO procedure. The material was
purified by using flash chromatography (9/1 hexane/ethyl acetate) to
provide 3a as a colorless oil. After it was held under vacuum overnight,
the material formed a fused solid, which was crystallized from
acetonitrile to give white needles (0.70 g, 80% yield; mp 82-83 °C).
GC analysis showed the product to contain ca. 1.0% of the E isomer.
The material was recrystallized again prior to use (0.4% E isomer). 1H
NMR: δ 7.63-7.35 (m, 5 H), 5.12 (q, 1 H), 4.70 (m, 1 H), 4.44 (m,
1 H), 3.62 (m, 1 H), 2.33-1.16 (m, 23 H), 0.85 (s, 3 H), 0.68 (s, 3 H)
0.40 (s, 6 H). 13C NMR: δ 150.2, 149.6, 138.5, 133.5, 129.5, 127.8,
113.3, 105.9, 72.5, 56.1, 54.7, 49.4, 44.4, 41.8, 37.8, 37.1, 37.0, 36.6,
34.0, 31.5, 31.4, 24.3, 21.6, 16.9, 13.1, 12.4, -0.9, -1.0. MS (EI):
m/z 448 (97, M+), 433 (39), 281 (44), 137 (100). HRMS (CI): m/z
calculated for C30H45OSi (M + H) 449.3240, found 449.3240.
1
cyclohexane/ethyl acetate). H NMR: δ 3.87-3.99 (m, 4 H), 3.69 (t,
1 H, C17RH), 2.55-1.13 (m, 21 H), 0.78 (s, 3 H, C19-CH3), 0.75 (s,
3 H, C18-CH3). 13C NMR: δ 211.1. 109.1, 81.6, 64.4, 64.3, 56.2,
53.8, 51.5, 46.2, 43.5, 41.0, 38.0, 36.4, 35.8, 30.9, 30.5, 29.9, 23.3,
21.2, 12.6, 11.2. MS (EI): m/z 348 (91, M+), 319 (47), 99 (100). HRMS
(CI) m/z calculated for C21H33O4 (M + H) 349.2379, found 349.2378.
3-(Ethylenedioxy)-17â-hydroxy-5R-androstan-6-one (5.59 g, 16.1
mmol) was treated with potassium tert-butoxide (48.2 mL, 1.0 M
solution in THF) and methyltriphenylphosphonium bromide (17.2 g,
48.15 mmol) according to the general Wittig procedure. The mixture
was stirred overnight, and the isolated product was purified by flash
chromatography (1/1 hexane/ethyl acetate). The 3-(ethylenedioxy)-6-
methylene-5R-androstan-17â-ol product13 was obtained in 93% yield
1
(5.2 g) and used for the next reaction without further purification. H
NMR: δ 4.7 (d, J ) 1.47 Hz, 1 H), 4.4 (d, J ) 1.47 Hz, 1 H), 3.96-
3.93 (m, 4 H), 3.54 (t, 1 H), 2.24-1.00 (m, 21 H), 0.72 (s, 3 H, C19-
CH3), 0.70 (s, 3 H, C18-CH3). 13C NMR: δ 149.9, 110.1, 105.2, 82.3,
3r-((Dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5r-
androstane (3b). 3R-Hydroxy-5R-androstane-6,17-dione was prepared

3630 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
as described earlier.4d 1H NMR: δ 4.18 (s, 1 H, 3â H), 0.88 (s, CH3),
0.77 (s, CH3).
slurry was held at 40 °C for 2.5 h, and TLC analysis (7/3 hexane/ethyl
acetate) indicated the reaction was complete. The reaction mixture was
diluted with 50 mL of toluene and washed successively with water,
5% bicarbonate, and water. The organic phase was dried (MgSO4),
and the solvent was removed under reduced pressure to give a white
solid. The crude solid was purified by using flash chromatography with
20/1 hexane/ethyl acetate as the eluant. The isolated material was
recrystallized from acetonitrile containing a small amount of ethyl
acetate to give 1.2 g of white needles. A second crop of crystals
provided an additional 0.12 g (overall yield 82%). The main crop was
recrystallized again before use (GC 99.74% Z, 0.26% E; mp 143-144
3R-Hydroxy-5R-androstane-6,17-dione (1.0 g, 3.3 mmol, contami-
nated with a small amount of the 3â-hydroxy isomer) was dissolved
in 12 mL of THF. In a separate vessel, potassium tert-butoxide (9.9
mL, 1.0 M solution in THF) was added to a slurry of methyltriphenyl-
phosphonium bromide (3.5 g, 9.9 mmol) in 12 mL of THF. After 5
min of stirring, the Wittig reagent was cooled to -60 °C and the steroid
solution was added via cannula over 8 min. The mixture was stirred
for 3 h, and the temperature was slowly increased to -15 °C. The
reaction mixture was poured into ice-water and extracted with
methylene chloride. The crude product was purified by flash chroma-
tography (1/1 hexane/ethyl acetate). 3R-Hydroxy-6-methylene-5R-
androstan-17-one was isolated in 39% yield (0.38 g, mp 151-171 °C)
and contained a trace of the 3â-alcohol impurity. 1H NMR: δ 4.74 (d,
J ) 1.52 Hz, 1 H), 4.45 (d, J ) 1.52 Hz, 1 H), 4.17 (m, 1 H), 2.41-
1.24 (m, 21 H), 0.85 (s, 3 H), 0.68 (s, 3 H). 13C NMR: δ 221.4, 149.5,
106.1, 66.1, 55.0, 51.5, 48.0, 43.6, 41.0, 38.5, 37.1, 35.9, 31.7, 31.5,
31.3, 28.6, 21.8, 20.4, 13.96, 11.6. MS (EI): m/z 302 (21 M+), 284
(75), 269 (75), 55 (100). HRMS (CI): m/z calculated for C20H31O2 (M
+ H): 303.2324, found 303.2320.
3R-Hydroxy-6-methylene-5R-androstan-17-one (0.38 g, 1.3 mmol)
was dissolved in 6 mL of THF and treated with potassium tert-butoxide
(6.3 mL, 1.0 M solution in THF) and ethyltriphenylphosphonium
bromide (2.3 g, 6.3 mmol) according to the general Wittig procudure.
The solid crude product was purified by flash chromatography (1/1
hexane/ethyl acetate). The 17-(Z)-ethylidene-3R-hydroxy-6-methylene-
5R-androstane product was obtained in 59% yield (0.23 g) and used
for the next reaction without further purification. 1H NMR: δ 5.12 (q,
1 H), 4.70 (d, J ) 1.5, 1 H), 4.41 (d, J ) 1.5, 1 H), 4.17 (m, 1 H),
2.34-1.21 (m, 24 H), 0.86 (s, 3 H), 0.67 (s, 3 H). 13C NMR: δ 150.3,
113.5, 105.5, 66.4, 56.3, 54.7, 44.6, 43.6, 42.1, 38.5, 37.2, 31.7, 31.5,
31.4, 28.6, 24.4, 21.3, 17.0, 13.2, 11.5. MS (EI): m/z 314 (48 M+),
299 (23), 281 (59), 91 (100). HRMS (FAB, CH2Cl2-PEG): m/z
calculated for C22H35O (M + H) 315.2688, found 315.2687.
1
°C). H NMR: δ 5.12 (q, 1 H), 3.57 (m, 1 H), 2.45-0.95 (m, 25 H),
0.89 (s, 9 H), 0.86 (s, 3 H), 0.81 (s, 3 H), 0.05 (s, 6 H). 13C NMR: δ
150.5, 113.2, 72.2, 56.3, 54.5, 45.0, 44.4, 38.7, 37.3, 37.1, 35.6, 35.1,
32.0, 31.4, 28.7, 26.0, 24.4, 21.4, 18.3, 16.9, 13.1, 12.3, -4.6. MS
(CI): m/z 417 (7, M + H), 285 (100). HRMS (CI): m/z calculated for
C27H49OSi (M + H) 417.3553, found 417.3552.
3â-((tert-Butyldimethylsilyl)oxy)-17-ethylenedioxy-5-andros-
tene (6a). Dehydroisoandrosterone (2.00 g, 6.94 mmol), 2-ethyl-2-
methyl-1,3-dioxolane (10 mL), and p-TSA (0.066 g, 0.35 mmol) were
mixed, and the reaction slurry was heated to reflux, producing a solution.
The reaction solution was held at reflux for 4.5 h and cooled to room
temperature. The solution was diluted with ether and washed succes-
sively with 5% sodium bicarbonate and water. The organic layer was
dried (MgSO4) and the ether was removed under reduced pressure to
give a light yellow solid. The crude solid was purified by using flash
chromatography (7/3 hexane/ethyl acetate). Recrystallization from
cyclohexane containing a trace of ethyl acetate gave 1.37 g of 17-
(ethylenedioxy)-3â-hydroxy-5-androstene (59.2% yield; mp 163-165
°C, lit.17a mp 161-165 °C).
17-(Ethylenedioxy)-3â-hydroxy-5-androstene (1.365 g, 4.106 mmol)
was treated with tert-butyldimethylsilyl chloride (0.74 g, 4.93 mmol)
according to the general TBDMS procedure. The solid product was
purified by using flash chromatography (9/1 hexane/ethyl acetate)
followed by crystallization from acetonitrile to give 1.5 g of 6a (81%
17-(Z)-Ethylidene-3R-hydroxy-6-methylene-5R-androstane (0.22 g,
0.68 mmol) was treated with chlorodimethylphenylsilane (0.11 mL,
0.68 mmol) according to the general DPSO procedure. The material
was purified by using flash chromatography (9/1 hexane/ethyl acetate)
to provide 3b as a colorless oil. The product was then purified by
semipreparative HPLC. The product could not be recrystallized and
was isolated as an oil (0.14 g, 47% yield). GC analysis showed the
product to contain ca. 1.8% of the E isomer. After NMR analysis, the
material was purified again by twice repeating the HPLC purification
1
yield) as white flakes (mp 121-122 °C). H NMR: δ 5.31 (m, 1 H),
3.88 (m, 4 H), 3.45 (m, 1 H), 2.35-1.05 (m, 19 H), 1.01 (s, 3 H), 0.89
(s, 9 H), 0.86 (s, 3 H), 0.06 (s, 6 H). 13C NMR: δ 141.4, 120.9, 119.5,
72.5, 65.1, 64.5, 50.6, 50.0, 45.7, 42.8, 37.3, 36.6, 34.2, 32.2, 32.0,
31.3, 30.6, 25.9, 22.8, 20.4, 19.4, 18.2, 14.2, -4.6. MS (CI): m/z 447
(100, M + H), 315 (66). HRMS (CI): m/z calculated for C27H47O3Si
(M + H) 447.3295, found 447.3294.
3â-((Dimethylphenylsilyl)oxy)-17-(ethylenedioxy)-5-androstene (6b).
17-(Ethylenedioxy)-3â-hydroxy-5-androstene (1.50 g, 4.51 mmol) was
treated with chlorodimethylphenylsilane (0.76 mL, 4.51 mmol) ac-
cording to the general DPSO procedure. The material was purified by
using flash chromatography (9/1 hexane/ethyl acetate) to provide 6b
as a white solid. The product was crystallized from acetonitrile (2×)
to give 1.5 g of white needles (74% yield, mp 115-116 °C). 1H
NMR: δ 7.61-7.36 (m, 5 H), 5.25 (m, 1 H), 3.83-3.92 (m, 4 H),
3.52 (m, 1 H), 2.40-1.34 (m, 19 H), 0.99 (s, 3 H), 0.85 (s, 3 H), 0.39
and 0.38 (s, 6 H). 13C NMR: δ 141.3, 138.6, 133.6, 129.6, 127.9, 121.2,
119.6, 72.7, 65.3, 64.7, 50.7, 50.1, 45.8, 42.6, 37.4, 36.7, 34.3, 32.3,
31.9, 31.4, 30.7, 22.9, 20.6, 19.5, 14.3, -0.86, -0.95. MS (CI): m/z
467 (60, M + H), 315 (100). HRMS (CI): m/z calculated for C29H43O3-
Si (M + H) 467.2982, found 467.2982.
1
and drying on high vacuum for several days before use (1.7% E). H
NMR: δ 7.61-7.37 (m, 5 H), 5.13 (q, 1 H), 4.68 (d, J ) 1.5 Hz, 1 H),
4.35 (d, J ) 1.5 Hz, 1 H), 4.12 (s, 1 H), 2.35-1.22 (m, 23 H), 0.87 (s,
3 H), 0.65 (s, 3 H), 0.35 (s, 6H). 13C NMR: δ 151.0, 150.4, 139.0,
133.6, 129.5, 127.9, 113.5, 105.2, 67.3, 56.3, 54.7, 44.6, 43.7, 42.1,
38.4, 37.25, 37.23, 32.0, 31.98, 31.6, 29.3, 24.5, 21.4, 17.1, 13.3, 11.8,
-0.8, -0.9. MS (EI): m/z 448 (21, M+), 370 (35), 296 (38), 281 (100).
HRMS (FAB, PEG): m/z calculated for C30H45OSi (M + H) 449.3240,
found 449.3250.
17â-((Dimethylphenylsilyl)oxy)-5r-androstane (4). 5R-Androstan-
17â-ol (1.98 g, 7.18 mmol) was treated with chlorodimethylphenylsilane
(1.20 mL, 7.18 mmol) according to the general DPSO procedure. Flash
chromatography (9/1 hexane/ethyl acetate) provided the product as a
colorless oil, which was crystallized from acetone to give a white solid
3â-((tert-Butyldimethylsilyl)oxy)-6-methylene-5r-androstan-17â-
ol (7a). 3â-Hydroxy-6-methylene-5R-androstan-17-one (1.4 g, 4.6
mmol) was treated with tert-butyldimethylsilyl chloride (0.84 g, 5.6
mmol) according to the general TBDMS procedure. The crude solid
product was purified by using flash chromatography (8/2 hexane/ethyl
acetate), giving 1.7 g (87% yield) of 3â-((tert-butyldimethylsilyl)oxy)-
6-methylene-5R-androstan-17-one. The material was used in the next
1
(1.787 g, 60.6% yield, mp 59-60 °C). H NMR: δ 7.35-7.59 (m, 5
H), 3.56 (t, 1 H), 1.82-0.80 (m, 24 H), 0.77 (s, 3 H), 0.74 (s, 3 H),
0.333 (s, 3 H), 0.328 (s, 3 H). 13C NMR: δ 138.9, 133.5, 129.3, 127.6,
82.1, 55.0, 50.7, 47.1, 43.2, 38.7, 37.1, 36.3, 35.6, 31.8, 30.8, 29.05,
28.97, 26.8, 23.5, 22.2, 20.4, 12.3, 11.5, -0.9, -1.0. MS (EI): m/z
410 (15, M+), 332 (26), 258 (32), 135 (100). HRMS (CI): m/z
calculated for C27H43OSi (M + H) 411.3083, found 411.3080.
3â-((tert-Butyldimethylsilyl)oxy)-17-(Z)-ethylidene-5r-andros-
tane (5). TBDMS Reaction: Illustration of General Procedure. 17-
(Z)-Ethylidene-3â-hydroxy-5R-androstane (1.20 g, 3.97 mmol) and
imidazole (0.68 g, 9.92 mmol) were dissolved in anhydrous DMF (10
mL) at 40 °C under nitrogen. tert-Butyldimethylsilyl chloride (0.72 g,
4.76 mmol) was added all at once to produce a thick slurry. The reaction
1
reaction without further purification. GC: 96.6%. H NMR: δ 4.76
(s, 1 H), 4.52 (s, 1 H), 3.58 (m, 1 H), 2.41-1.24 (m, 20 H), 0.90 (s, 9
H), 0.85 (s, 3 H), 0.71 (s, 3 H), 0.06 (s, 6 H). 13C NMR: δ 220.9,
148.8, 106.5, 72.2, 54.9, 51.4, 49.5, 47.9, 40.8, 37.9, 37.0, 36.7, 35.8,
34.1, 31.7, 31.5, 26.0, 21.7, 20.8, 18.3, 13.8, 12.5, -4.6. MS (EI): m/z
416 (2, M+), 401 (1), 359 (100). HRMS (CI): m/z calculated for
C26H45O2Si (M + H) 417.3189, found 417.3184.

Steroids as Molecular Photonic Wires
J. Am. Chem. Soc., Vol. 121, No. 15, 1999 3631
3â-((tert-Butyldimethylsilyl)oxy)-6-methylene-5R-androstan-17-
one (1.6 g, 3.8 mmol) was dissolved in 25 mL of THF, and the solution
was cooled to -20 °C. Lithium tri-tert-butoxyaluminohydride (5 mL,
1.0 M in THF) was added via syringe, and the reaction solution was
held at -20 °C for 2 h. The reaction solution was worked up by dilution
with 80 mL of toluene, followed by pouring into 20 mL of 5% HCl.
The mixture was immediately washed with 5% sodium bicarbonate
and water. The organic phase was dried with MgSO4, and the solvent
was removed under reduced pressure. The crude product was purified
by flash chromatography using 7/3 hexane/ethyl acetate to give 1.4 g
of 7a (87% yield). The product was crystallized (2×) from acetonitrile
with the same procedure. The fluorescence emission for the sample
was acquired at 254 nm excitation, and the instrumental conditions
were optimized to give ca. 90% of full intensity at the maximum
emission wavelength. The toluene standard was run using identical
instrumental conditions.
Total Emission Spectra. A steroid sample in methylcylohexane
giving an optical density of ca. 0.3 in a 1 cm cuvette was transferred
to a 2 mm phosphorescence cell equipped with a vacuum stopcock.
After it was degassed with at least five freeze-pump-thaw cycles,
the sample was inserted into a liquid nitrogen optical Dewar and the
spectrum was acquired with 254 nm excitation.
Steroid Quantum Yields ΦZfE. Several laser cells were filled with
2 or 3 mL of steroid solution at 10 mM concentration in cyclohexane
(optical density ca. 2.5). The solutions were argon-degassed for 25 min
and sealed with rubber septa. The power was monitored with a OPHIR
Model AN/2 power meter. The solutions were placed in the 266 nm
beam of the laser and irradiated for ca. 5 min (30 mW power, 10 Hz
rep rate). Typical product conversions were 5-7%. This was repeated
with the other sample cells, and the results were averaged. All results
were corrected for the small amount of back-reaction. A dark control
sample was analyzed simultaneously. The product formed was deter-
mined by GC analysis using an internal standard of 3R-((dimethylphen-
ylsilyl)oxy)-17-methylene-5R-androstane or 6a.
The quantum yields obtained using this laser technique were
compared with those obtained using the 254-nm Rayonet reactor for
the compound 3R-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5R-
androstane. The results were ΦZfE ) 0.043 and 0.041, respectively.
Thus, two-photon processes occurring due to laser irradiation can be
ruled out.
Stern-Volmer Quenching of C17 Isomerization in 1 with cis-
2-Heptene. Illustration of Typical Procedure. Stock solutions of 1
and cis-2-heptene in cyclohexane were prepared. Six quartz tubes were
filled with 1 to give a final concentration of 10 mM, and four of the
tubes were filled with various amounts of cis-2-heptene to produce
final concentrations of 0.2 × 10-2, 1 × 10-2, 2 × 10-2, and 5 × 10-2
M. The final volume was 3.0 mL. The tubes were argon-degassed for
25 min with a slow stream of argon and were sealed with rubber septa.
The tubes were irradiated at 254 nm in the Rayonet reactor simulta-
neously for 15 min. After irradiation, a cyclohexane solution of the
internal standard (3R-((dimethylphenylsilyl)oxy)-17-methylene-5R-an-
drostane) was added and the tubes were analyzed by GC to determine
the amount of product formed. The results were corrected for the small
amount of back-reaction.
Quenching of 8-Sensitized cis-2-Heptene Isomerization with
Steroid-Olefin Quencher. Illustration of Typical Procedure. Stock
solutions of 8, 5, and cis-2-heptene in cyclohexane were prepared. Seven
photolysis tubes were filled with 10 mM 8 and 5 mM cis-2-heptene.
Five of the tubes were filled with various concentrations of 5 at 1 ×
10-2, 5 × 10-2, 10 × 10-2, and 15 × 10-2 M (includes one replicate).
The final volume was 3.0 mL. The tubes were argon-degassed for 25
min with a slow stream of argon and were sealed with rubber septa.
The tubes were irradiated at 254 nm in the Rayonet reactor simulta-
neously for 15.0 min, and the amount of trans-2-heptene was determined
by GC analysis. The amount of trans-2-heptene formed ranged between
3.5% and 11.2%. The initial amount of trans-2-heptene present in the
cis-2-heptene solution was 1.2%. After correction for back-reaction,
the data were plotted according to the Stern-Volmer equation.
cis-2-Heptene Isomerization with Steroid Sensitizer. Illustration
of Typical Reciprocal Plot Procedure. Stock solutions of 4, cis-2-
heptene, and (E)-1-phenyl-2-butene (actinometer) in cyclohexane were
prepared. Five quartz tubes were filled with 10 mM 4 and various
1
prior to use (mp 140-141 °C). H NMR: δ 4.71 (d, J ) 1.20 Hz, 1
H), 4.47 (s, J ) 1.20 Hz, 1 H), 3.64 (m, 2 H), 2.31-0.91 (m, 21 H),
0.90 (s, 9 H), 0.72 (s, 3 H), 0.69 (s, 3 H), 0.06 (s, 6 H). 13C NMR: δ
149.3, 106.0, 81.8, 72.3, 54.9, 51.0, 49.5, 43.1, 41.5, 38.0, 37.5, 36.7,
36.6, 34.1, 31.7, 30.5, 25.9, 23.3, 21.1, 18.2, 12.5, 11.1, -4.6. MS
(CI): m/z 419 (64, M + H), 401 (25), 287 (100). HRMS (CI): m/z
calculated for C26H47O2Si (M + H) 419.3345, found 419.3345.
3â-((Dimethylphenylsilyl)oxy)-6-methylene-5r-androstan-17â-
ol (7b). 3â-Hydroxy-6-methylene-5R-androstan-17-one (1.4 g, 4.6
mmol) was treated with chlorodimethylphenylsilane (0.78 mL, 4.6
mmol) according to the general DPSO procedure. The material was
purified by using flash chromatography (8/2 hexane/ethyl acetate) to
provide 3â-((dimethylphenylsilyl)oxy)-6-methylene-5R-androstan-17-
1
one as a colorless oil (1.3 g). H NMR: δ 7.64-7.37 (m, 5 H), 4.75
(d, 1 H), 4.49 (d, 1H), 3.62 (m, 1 H), 2.39-1.27 (m, 20 H), 0.85 (s, 3
H), 0.70 (s, 3 H), 0.41 (s, 6 H). 13C NMR: δ 220.9, 148.6, 138.4,
133.4, 129.5, 127.8, 106.5, 72.3, 54.7, 51.3, 49.4, 47.8, 40.7, 37.8, 36.9,
36.6, 35.8, 33.9, 31.4, 21.7, 20.7, 13.8, 12.4, -0.96, -1.05. MS (EI):
m/z 436 (9, M+), 421 (100). HRMS (CI): m/z calculated for C28H41O3-
Si (M + H) 437.2876, found 437.2875.
3â-((Dimethylphenylsilyl)oxy)-6-methylene-5R-androstan-17-one (1.2
g, 2.7 mmol) was dissolved in 10 mL of THF, and the solution was
cooled to -10 °C. Lithium tri-tert-butoxyaluminohydride (4.1 mL, 1.0
M in THF) was added via syringe, and the reaction solution was held
at -10 °C for 30 min. The reaction solution was worked up by dilution
with 80 mL of cold toluene, followed by pouring into 20 mL of cold
5% HCl and quickly washing the mixture with 5% sodium bicarbonate
and water. The organic phase was dried with MgSO4, and the solvent
was removed under reduced pressure. The crude product was purified
by flash chromatography using 7/3 hexane/ethyl acetate to give 0.9 g
of 7b as a white solid. The product was crystallized (2×) from hexane
prior to use in photolysis experiments (mp 102-104 °C). 1H NMR: δ
7.62-7.37 (m, 5 H), 4.70 (d, 1 H), 4.49 (d, 1 H), 3.61 (m, 2 H), 2.39-
1.00 (m, 20 H), 0.72 (s, 3 H), 0.69 (s, 3 H), 0.40 (s, 6 H). 13C NMR
δ 149.2, 138.5, 133.4, 129.4, 127.7, 106.0, 81.8, 72.4, 54.8, 51.0, 49.4,
43.1, 41.5, 37.7, 37.5, 36.6, 33.9, 31.5, 30.5, 23.3, 21.1, 12.5, 11.1,
-0.9, -1.0. MS (CI): m/z 439 (52, M + H), 421 (34), 287 (100).
HRMS (CI): m/z calculated for C28H43O2Si (M + H) 439.3032, found
439.3031.
3â-((Dimethylphenylsilyl)oxy)-17-(ethylenedioxy)-5r-andros-
tane (8). Epiandrosterone (2.0 g, 6.9 mmol) was refluxed in 20 mL of
2-ethyl-2-methyl-1,3-dioxolane with p-TSA (0.07 g) overnight to give
17-(ethylenedioxy)-5R-androstan-3â-ol (mp 141-148 °C, lit.18 mp
152-154 °C). This compound (1.76 g, 5.27 mmol) was treated with
chlorodimethylphenylsilane (0.88 mL, 5.27 mmol) according to the
general DPSO procedure. The product was isolated by flash chroma-
tography (9/1 hexane/ethyl acetate) to give 1.92 g of 8 (78% yield).
The material was crystallized (2×) from methanol before use (mp 106-
1
108 °C). H NMR: δ 7.62-7.36 (m, 5 H), 3.91-3.83 (m, 4 H), 3.48
(m, 1 H), 1.97-0.88 (m, 22 H), 0.83 (s, 3 H), 0.79 (s, 3 H), 0.38 (s, 6
H). 13C NMR: δ 138.6, 133.4, 129.4, 127.7, 119.5, 72.2, 65.1, 64.5,
54.1, 50.3, 45.9, 44.9, 38.4, 37.1, 35.7, 35.5, 34.2, 31.7, 31.3, 30.7,
28.5, 22.6, 20.6, 14.4, 12.3, -0.9, -1.0. MS (EI): m/z 468 (29, M+),
453 (4), 99 (100). HRMS (CI): m/z calculated for C29H45O3Si (M +
H) 469.3138, found 469.3140.
amounts of cis-2-heptene to produce final concentrations of 2 × 10-3
,
5 × 10-3, 10 × 10-3, 15 × 10-3, and 30 × 10-3 M. Three quartz tubes
were filled with 21 mM of (E)-1-phenyl-2-butene solution. All tubes
contained 2.7 mL of solution. The tubes were argon-degassed for 30
min with a slow stream of argon and were sealed with rubber septa.
The tubes were irradiated at 254 nm in the Rayonet reactor. The steroid
samples were irradiated for 25 min, and the actinometer samples were
irradiated for 7-8 min simultaneously. The amounts of trans-2-heptene
and (E)-1-phenyl-2-butene were determined by GC analysis. The results
were corrected for the small amount of back-reaction.
Fluorescence Quantum Yields. A 1 cm square quartz fluorescence
cell was filled with a given steroid solution at an optical density of ca.
0.1 at the excitation wavelength. The solution was argon-degassed for
30 min and sealed with a rubber septum wrapped with Parafilm. A
solution of toluene was also placed in a fluorescence cell and treated

3632 J. Am. Chem. Soc., Vol. 121, No. 15, 1999
Timberlake and Morrison
Acknowledgment. We are grateful for financial support
of this research from the National Science Foundation (Grant
No. CHE 9311828). A research grant from Great Lakes
Chemical Corp. is also gratefully acknowledged. We thank Ms.
Fatemeh Olang for the quenching study on 3RDPSO/6-ketone/
17Z.
Supporting Information Available: Stern-Volmer plots
for all quenching reactions not shown in the text (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA9808595