Photoinduced Electron Transfer between Acenaphthylene and Tetracyanoethylene: Effect of Irradiation Mode on Reactivity of the Charge-Transfer Complex and the Resulted Radical Ion Pair in Solution and Crystalline State

Article abstract of DOI:10.1021/jo9801824

The mechanism of photodimerization of acenaphthylene (ACN) and of reactions with tetracyanoethylene (TCNE) by electron transfer (ET) has been investigated in solution and solid state to elucidate the role of the radical cation of ACN (ACN^+?) in formation of the cisoid-dimer (cisoid-1) and the transoid-dimer (transoid-1) of ACN and addition products to TCNE. Selective excitation of the 1:1 charge-transfer (CT) complex between ACN and TCNE with light of >500 nm did not result in any reaction in acetonitrile (AN) or 1,2-dichloroethane (DCE). On the other hand, direct irradiation of ACN with light of >400 nm in solution in the presence of TCNE gave cisoid-1 and transoid-1 as the major products together with a [2 + 2]-adduct (2) and two isomeric [2 + 2 + 2]-adducts (3 and 4) of ACN and TCNE as minor products. Distinction of photochemical reactivity between selective CT excitation and direct excitation of ACN can be attributed to faster backward electron transfer (BET) from the contact radical ion pair (CIP) on CT excitation than from the solvent-separated radical ion pair (SSIP) on direct excitation of ACN due to very low energy for BET, as low as 1.34 V. Effect of [TCNE] on quantum yield for the dimerization of ACN and on the cisoid / transoid ratio of the resulted 1 rationalizes the mechanism involving the singlet and triplet SSIP; the former tends to undergo BET, but the latter undergoes dissociation to ACN^+?, followed by formation of dimeric radical cation of ACN, ACN₂^+?, finally leading to 1. A possible mechanism for formation of 3 and 4 is discussed on the basis of concentration dependence of ACN. Contrary to photochemical inertness of the CT complex in solutions, CT excitation of the 1:1 crystal of ACN and TCNE (ACN·TCNE) gave 2 as the sole product. The selective formation of 2 indicates that fixation of the two alkenic C=C double bonds in ACN·TCNE separated by 3-4 A in both the excited CT state and the resulted CIP retards the deactivation and BET but enables them to undergo cycloaddition.

Full text of DOI:10.1021/jo9801824

5372
J . Org. Chem. 1998, 63, 5372-5384
P h otoin d u ced Electr on Tr a n sfer betw een Acen a p h th ylen e a n d
Tetr a cya n oeth ylen e: Effect of Ir r a d ia tion Mod e on Rea ctivity of
th e Ch a r ge-Tr a n sfer Com p lex a n d th e Resu lted Ra d ica l Ion P a ir
in Solu tion a n d Cr ysta llin e Sta te
Naoki Haga,*,† Hiroyuki Nakajima,† Hiroaki Takayanagi,† and Katsumi Tokumaru‡
School of Pharmaceutical Sciences, Kitasato University, Minato-ku, Tokyo 108, J apan, and University of
Tsukuba, Tsukuba, Ibaraki 305, J apan
Received February 3, 1998
The mechanism of photodimerization of acenaphthylene (ACN) and of reactions with tetracyano-
ethylene (TCNE) by electron transfer (ET) has been investigated in solution and solid state to
elucidate the role of the radical cation of ACN (ACN+•) in formation of the cisoid-dimer (cisoid-1)
and the transoid-dimer (transoid-1) of ACN and addition products to TCNE. Selective excitation
of the 1:1 charge-transfer (CT) complex between ACN and TCNE with light of >500 nm did not
result in any reaction in acetonitrile (AN) or 1,2-dichloroethane (DCE). On the other hand, direct
irradiation of ACN with light of >400 nm in solution in the presence of TCNE gave cisoid-1 and
transoid-1 as the major products together with a [2 + 2]-adduct (2) and two isomeric [2 + 2 +
2]-adducts (3 and 4) of ACN and TCNE as minor products. Distinction of photochemical reactivity
between selective CT excitation and direct excitation of ACN can be attributed to faster backward
electron transfer (BET) from the contact radical ion pair (CIP) on CT excitation than from the
solvent-separated radical ion pair (SSIP) on direct excitation of ACN due to very low energy for
BET, as low as 1.34 V. Effect of [TCNE] on quantum yield for the dimerization of ACN and on the
cisoid/ transoid ratio of the resulted 1 rationalizes the mechanism involving the singlet and triplet
SSIP; the former tends to undergo BET, but the latter undergoes dissociation to ACN+•, followed
by formation of dimeric radical cation of ACN, ACN2+•, finally leading to 1. A possible mechanism
for formation of 3 and 4 is discussed on the basis of concentration dependence of ACN. Contrary
to photochemical inertness of the CT complex in solutions, CT excitation of the 1:1 crystal of ACN
and TCNE (ACN‚TCNE) gave 2 as the sole product. The selective formation of 2 indicates that
fixation of the two alkenic CdC double bonds in ACN‚TCNE separated by 3-4 Å in both the excited
CT state and the resulted CIP retards the deactivation and BET but enables them to undergo
cycloaddition.
In tr od u ction
been proposed for this reaction. However, due to the
generally low yield of the cycloadducts and formation of
undesirable polymeric products, it seems that until today
less effort has been devoted to revealing the role of the
singlet and triplet of alkene and the wavelength effect
on reactivity and product distribution.
Charge-transfer (CT) interaction between an electron
donor and an acceptor in the ground state1 and photo-
induced electron transfer (ET)2 have been accepted to be
general phenomena in organic chemistry. The photo-
chemical [2 + 2]-cycloaddition of alkenes in the presence
of various electron acceptors has been studied as one of
the representative models of cycloaddition of an alkene
which proceeds via ET.3-10 The mechanism which in-
volves formation of a radical cation of alkenes followed
by nucleophilic attack by another alkene molecule has
Acenaphthylene (ACN), one of the most simple nonal-
ternant aromatic hydrocarbons, has been known to
(6) (a) Farid, S.; Shealer, S. E. J . Chem. Soc., Chem. Commun. 1973,
677-678. (b) Mattes, S. L.; Farid, S. J . Am. Chem. Soc. 1982, 104,
1454-1456. (c) Farid, S.; Shealer, S. E. J . Chem. Soc., Chem. Commun.
1973, 296-297. (d) Mizuno, K.; Kaji, R.; Otsuji, Y. Chem. Lett. 1977,
1027-1030. (e) Pac, C.; Mizuno, K.; Sugioka, T.; Sakurai, H. Chem.
Lett. 1973, 187-188. (f) Calhoun, G. C.; Schuster, G. B. J . Am. Chem.
Soc. 1986, 108, 8021-8027.
(7) (a) Asanuma, T.; Gotoh, T.; Tsuchida, A.; Yamamoto, M.;
Nishijima, Y. J . Chem. Soc., Chem. Commun. 1977, 485-486. (b)
Yamamoto, M.; Asanuma, T.; Nishijima, Y. J . Chem. Soc., Chem.
Commun. 1975, 53-54. (c) Asanuma, T.; Yamamoto, M.; Nishijima,
Y. J . Chem. Soc., Chem. Commun. 1975, 56-57, 608-609. (d) Kojima,
M.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1981, 22, 2889-
2892.
(8) (a) Mattes, S. L.; Farid, S. J . Am. Chem. Soc. 1986, 108, 7356-
7361. (b) Mattes, S. L.; Farid, S. J . Am. Chem. Soc. 1983, 105, 1386-
1387. (c) Arnold, D. R.; Neunteufel, R. A. J . Am. Chem. Soc. 1973, 95,
4080-4081.
(9) Kuwata, S.; Shigemitsu, Y.; Odaira, Y. J . Chem. Soc., Chem.
Commun. 1972, 2.
† Kitasato University.
‡ University of Tsukuba.
(1) Foster, R. F. Charge-Transfer Complexes in Organic Chemistry;
Academic Press: New York, 1963.
(2) (a) Mattes, R. F.; Farid, S. Org. Photochem. 1983, 6, 233-326.
(b) Mattes, R. F.; Farid, S. Acc. Chem. Res. 1982, 15, 80-86. (c) Mattay,
J . Angew. Chem., Int. Ed. Engl. 1987, 26, 825-845. (d) Mattay, J .
Synthesis 1989, 233-251. (e) Osa, T., Ed. Kikan Kagaku Sosetsu, No.
2; Gakkai Shuppan Center: Tokyo, 1988. (f) Oh-hashi, M., Ed. Kagaku
Sosetsu, No. 33.; Gakkai Shuppan Center: Tokyo, 1982. (g) Kavarnos,
G. J . Fundamentals of Photoinduced Electron Transfer; VCH Publish-
ers: New York, 1993. (h) Leffler, J . E. An Introduction to Free Radicals;
J ohn Wiley & Sons: New York, 1993; pp 230-270.
(3) (a) Pages 75-96 in ref 2e. (b) Pages 38-54, 103-118 in ref 2g.
(4) Kricka, L. J .; Ledwith, A. Synthesis 1974, 539-548.
(5) (a) Bell, F. A.; Crellin, R. A.; Fujii, H.; Ledwith, A. J . Chem. Soc.
Chem. Commun. 1969, 251-252. (b) Carruthers, R. A.; Crellin, R. A.;
Ledwith, A. J . Chem. Soc., Chem. Commun. 1969, 252-253.
(10) (a) Ohashi, M.; Tanaka, Y.; Yamada, S. Tetrahedron Lett. 1977,
41, 3629-3632. (b) Ohashi, M.; Tanaka, Y.; Yamada, S. J . Chem. Soc.,
Chem. Commun. 1976, 800.
S0022-3263(98)00182-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/08/1998

Photoinduced Electron Transfer between ACN and TCNE
J . Org. Chem., Vol. 63, No. 16, 1998 5373
Sch em e 1
Photoinduced ET between electron donors and accep-
tors can be classified into two categories in terms of the
mode of production of the radical ions, i.e., ET at
encounter between an excited state and a ground state
where no interaction occurs in their ground states and
ET occurring on excitation of the ground-state CT
complex.23 Mataga and co-workers,24,25 have observed
that, in some donor-acceptor systems, radical ion pairs
resulting from excitation of the CT complex undergo
backward electron transfer (BET) with larger rate con-
stants, kBET, than those from the encounter mode in the
same solvents. It follows that, if the CT complex could
be irradiated independently, distinctive features in pho-
tochemical reactivity due to the difference of kBET should
be observed between the two modes of excitation.
Tetracyanoethylene (TCNE),26 which is one of the most
powerful electron acceptors with a reduction potential
(E1/2Red) of 0.24 V (vs SCE in AN),27 forms a brown-colored
CT complex with ACN in the ground state. Because
selective excitation of the CT band is readily accom-
plished, we have chosen the ACN-TCNE system as a
suitable one to examine distinction between the two
modes of irradiation.
Apart from reactivity on irradiation in solution, pho-
tochemical reactions in solid states have received much
attention and have been developed due to its specific
features such as stereoselectivity, regioselectivity (speci-
ficity), and generation of chirality.28 Many CT complexes
can be isolated as crystals whose crystal structures have
been elucidated by X-ray crystallographic technique.29
Though the physical properties of crystalline CT com-
plexes have been investigated due to their intriguing
properties such as ferromagnetism,30 few studies have
so far been done on their photochemical behavior in the
crystal.31 Irradiation of crystalline CT complexes must
provide a unique opportunity to study the reactions via
ET included in a highly organized crystalline lattice.
To elucidate the mechanisms of the photochemical
reactions of ACN involving ET, we focused herein on
three points as follows: (1) examine the wavelength effect
of irradiation to compare reactivity of CIP and the SSIP,
(2) determine product distribution and quantum yields
on irradiation of ACN-TCNE system over a wide range
of concentrations of ACN and TCNE to elucidate the
participation of ET from the S1 and T1 states of ACN,
undergo dimerization on irradiation into two isomeric
dimers (cisoid-1 and transoid-1)11-16 in quantitative yield
(Scheme 1). The mechanism of this reaction has been
extensively studied, because of different stereoselectivity
for dimerization from the singlet and triplet states13-16
together with unique photophysical properties such as
short lifetime of its singlet (S1) state,17,18 very weak
fluorescence,18 and extremely low quantum yield for
intersystem crossing, ΦST.19,20 It has been concluded that
on direct irradiation of ACN, transoid-1 is formed from
the triplet (T1) state and cisoid-1 is derived from both
the S1 and T1 states, though mainly from the former.13,14
Our recent study has clearly quantified the role of the
pathways from the S1 and T1 states, based on quantum
yields for the reaction in solutions over a wide range of
concentrations of ACN and the effect of a triplet sensi-
tizer, a triplet quencher, and heavy atom-containing
solvents.21
On the other hand, photochemical reactions of ACN
in the presence of electron acceptors have not been
studied except by Shirota and co-workers.22 Because on
irradiation with 435.8 nm monochromatic light ACN is
converted into the isomeric dimers without producing
byproducts such as polymers,14b,21 we considered that
ACN is an appropriate model to study the photodimer-
ization via ET, focusing on the effect of reduction
potential of the acceptor and concentration of the donor
and the acceptor on quantum yields and product distri-
bution.
(11) (a) Dziewonski, K.; Rapalski, G. Chem. Ber. 1912, 45, 2491-
2495. (b) Dziewonski, K.; Paschalski, C. Chem. Ber. 1913, 46, 1986-
1992. (c) Dziewonski, K.; Leyko, Z. Chem. Ber. 1914, 47, 1679-1990.
(12) (a) Bowen, E. J .; Marsh, J . D. F. J . Chem. Soc. 1947, 109-110.
(b) Griffin, G. W.; Veber, D. F. J . Am. Chem. Soc. 1960, 82, 6417. (c)
Hartmann, I.; Hartmann, W.; Schenck, G. O. Chem. Ber. 1967, 100,
3146-3155. (d) Devidson, R. S. J . Chem. Soc., Chem. Commun. 1969,
1450-1451. (e) Plummer, B. F.; Hall, R. A. J . Chem. Soc., Chem.
Commun. 1970, 44-45.
(13) Livingston, R.; Wei, K. S. J . Phys. Chem. 1967, 71, 541-547.
(14) (a) Cowan, D. O.; Drisco, R. L. Tetrahedron Lett. 1967, 1255-
1258. (b) Cowan, D. O.; Drisco, R. L. J . Am. Chem. Soc. 1970, 92, 6286-
6291. (c) Cowan, D. O.; Drisco, R. L. J . Am. Chem. Soc. 1970, 92, 6281-
6285. (d) Cowan, D. O.; Kozier, J . C. J . Am. Chem. Soc. 1974, 96, 1229-
1230. (e) Cowan, D. O.; Kozier, J . C. J . Am. Chem. Soc. 1975, 97, 249-
254.
(23) 23. Mataga, N. Photochemical Energy Conversion; Elsevier:
Amsterdam, 1989; pp 32-46.
(24) (a) Miyasaka, H.; Ojima, S.; Mataga, N. J . Phys. Chem. 1989,
93, 3380-3382. (b) Ojima, S.; Miyasaka, H.; Mataga, N. J . Phys. Chem.
1990, 94, 5834-5839.
(25) (a) Asahi, T.; Ohkohchi, M.; Mataga, N. J . Phys. Chem. 1993,
97, 13132-13137. (b) Mataga, N.; Shioyama, H.; Kanda, Y. J . Phys.
Chem. 1987, 91, 314-317. (c) Ojima, S.; Miyasaka, H.; Mataga, N. J .
Phys. Chem. 1990, 94, 7534-7539. (d) Miyasaka, H.; Nagata, T.; Kiri,
M.; Mataga, N. J . Phys. Chem. 1992, 96, 8060-8065. (e) Asahi, T.;
Mataga, N. J . Phys. Chem. 1989, 93, 6575-6578. (f) Asahi, T.;
Ohkohchi, M.; Matsusaka, R.; Mataga, N.; Zhang, R. P.; Osuka, A.;
Maruyama, K. J . Am. Chem. Soc. 1993, 115, 5665-5674. (g) Asahi,
T.; Mataga, N. J . Phys. Chem. 1991, 95, 1956-1963.
(15) Koser, G. F.; Liu, V.-S. J . Org. Chem. 1978, 43, 478-481.
(16) White, E. H.; Wildes, P. D.; Wiecko, J .; Doshan, H.; Wei, C. C.
J . Am. Chem. Soc. 1973, 95, 7050-7058.
(17) Samanta, A.; Devadoss, C.; Fessenden, R. W. J . Phys. Chem.
1990, 94, 7106-7110.
(18) (a) Brown, E. J . Advances in Photochemistry; J ohn Wiley and
Sons: New York, 1963; Vol. 1, pp 23-42. (b) Plummer, B. F.;
Hopkinson, M. J .; Zoeller, J . K. J . Am. Chem. Soc. 1979, 101, 6779-
6781. (c) Castellan, G.; Dumartin, G.; Bouas-Laurent, H. Tetrahedron
1980, 36, 97-103.
(26) (a) Fatiadi, A. J . Synthesis 1986, 249-284; (b) 1987, 749-789.
(27) (a) Mulvaney, J . E.; Cramer, R. J .; Hall, H. K. J . Polym. Sci.,
Polym. Chem. Ed. 1983, 21, 309-314. (b) Olbrich-Deussner, B.; Kaim,
W.; Gross-Lannert, R. Inorg. Chem. 1989, 28, 3113-3120.
(28) Ramamurthy, V., Ed. Photochemistry in Organized and Con-
strained Media; VCH Publishers: New York, 1991.
(19) Dunsbach, R.; Schmidt, R. J . Photochem. Photobiol. A: Chem.
1994, 83, 7-13.
(29) Pages 216-251, 276-302 in ref 1.
(20) Samanta, A.; Fessenden, R. W. J . Phys. Chem. 1989, 93, 5823-
5827.
(21) Haga, N.; Takayanagi, H.; Tokumaru, K. J . Org. Chem. 1997,
62, 3734-3743.
(22) Shirota, Y.; Nagata, J .; Mikawa. Chem. Lett. 1972, 49-50.
(30) Kollmac, C.; Kahn, O. Acc. Chem. Res. 1993, 26, 259-265.
(31) (a) Suzuki, T.; Fukushima, T.; Yamashita, Y.; Miyashi, T. J .
Am. Chem. Soc. 1994, 116, 2793-2803. (b) Koshima, H.; Ding, K.;
Chisaka, Y.; Matsuura, T.; Ohashi, Y.; Mukasa, M. J . Org. Chem. 1996,
61, 2352.

5374 J . Org. Chem., Vol. 63, No. 16, 1998
Haga et al.
at 520 nm to avoid overlap with the absorption of ACN
with varying concentration of ACN and TCNE to get Keq
for the 1:1 CT complex (Scheme 2) by eq 1.
[TCNE]
ACT
1
ꢀCT
1
)
+
(1)
ꢀCTKeq[ACN]
Here, ꢀCT represents the molar extinction coefficient of
the CT complex at 520 nm (instead of ꢀ at maximum
wavelength, λmax). By employing eq 1, Keq is determined
as 1.54 ( 0.23 and 2.20 ( 0.38 M-1 with ꢀCT of 1320 and
500 M-1 cm-1 in DCE and AN, respectively. Relatively
small values for Keq in both solvents are similar to those
of other CT complexes of TCNE38 and indicate that the
intermolecular interaction between ACN and TCNE is
moderate.
F igu r e 1. Absorption spectra in DCE: (a) CT complex of ACN
and TCNE ([ACN] ) 2.5 × 10-2 M and [TCNE] ) 1.0 × 10-2
M); (b) ACN (2.5 × 10-2 M); (c) ACN (8.0 × 10-4 M). TCNE is
transparent in this wavelength region.
Ir r a d ia tion of ACN in th e P r esen ce of TCNE in
Solu tion . When a solution containing 1.0 × 10-2 M ACN
and 1.0 × 10-2 M TCNE in AN or DCE was irradiated
with >500-nm light to excite the CT band of the complex
selectively, the color from the complex did not bleach at
all after 24 h of exposure of the light. Thin-layer
Sch em e 2
1
chromatography (TLC) or H NMR analysis of the solu-
tion showed that no reaction occurred on the CT excita-
tion.
However, on irradiation of this solution with light of
wavelength longer than 400 nm, which excites uncom-
plexed ACN, ACN was converted into a [2 + 2]-cycload-
duct (2) between ACN and TCNE and [2 + 2 + 2]-cy-
cloadducts (3 and 4) together with cisoid-1 and transoid-1
as characterized by chromatographic isolation (Scheme
3).
and (3) irradiate the crystalline CT complex of ACN and
TCNE to examine characteristic features to the photo-
induced ET in the organic solid state. Furthermore, as
a reference, we also made irradiation of the ACN in the
presence of fumaronitrile (FN), which is a weaker accep-
tor than TCNE and does not form a CT complex with
ACN in the ground state.
The structure of 222,36 was identified by comparison of
the spectral properties with that of an authentic sample
which was prepared by an alternative method. Composi-
tions of 3 and 4 were deduced by mass spectroscopy and
elemental analysis. The structure of 3 was established
successfully by X-ray crystallography of a single crystal.
The crystallographic data are given in Table 1. The
ORTEP diagram in Figure 2 shows that this compound
has a structure with the apparent stereoselective reaction
of two molecules of ACN with one molecule of TCNE,
where the four methine protons originating from alkenic
ones of ACN are arranged asymmetrically. In the 1H
NMR spectrum, the pair of double-doublet resonance at
δ 4.68 and 5.02 is readily assigned to H2 and H3,
respectively. Similarly, the pair of doublet at δ 4.28 and
4.68 is assigned to H1 and H4, respectively, with coupling
constants (J ) of 10.8, 5.3, and 6.2 Hz for J 1-2, J 2-3, and
J 3-4, respectively. Though the conformation of the cy-
clohexane ring of 3 is not a typical chair form, the J
values of the protons in this ring are similar to those of
axial-axial and axial-equatorial protons in a cyclohex-
ane ring with a chair form. The configurations of the
aliphatic tertiary protons in 4 were determined on the
Resu lts
CT Com p lex. When a solution of TCNE was added
to a solution of ACN in DCE or AN, immediately tan-
brown coloration developed. Figure 1 shows the absorp-
tion spectra accompanying this spectral change together
with that of ACN in DCE. Since neither ACN nor TCNE
absorbs in the 460-700-nm visible region, the broad
absorption band is readily assigned to a CT complex
produced between ACN and TCNE (Scheme 2), as
reported for CT complexes of various aromatic hydrocar-
bons with TCNE.26,29,32-35 The CT band in Figure 1 is
not well-separated from the absorption band of ACN in
the region of 400-500 nm. Therefore, the λmax of the CT
complex seems concealed among the intensive absorption
of ACN. However, according to the empirical equation
relating the charge-transfer energies with ionization
potential of donors,36 the λmax can be estimated as 480
nm, which agrees with the present outcome.
The equilibrium constant, Keq, of the CT complex
between ACN and TCNE was determined spectrophoto-
metrically by the Benesi-Hildebrand procedure.37 Thus
the absorbance due to the CT band, ACT, was determined
1
basis of comparison of its H NMR spectrum with that
of 3. Thus, 12 nonequivalent aromatic protons and 4
nonequivalent aliphatic tertiary protons agree with
asymmetric structure similar to that in 3. Chemical
shifts of H1, H2, H3, and H4 are 4.56, 4.36, 4.71, and 4.64,
respectively, with J 1-2, J 2-3, and J 3-4 of 10.5, 10.0, and
6.7 Hz, respectively.
(32) (a) Cram, D. J .; Bauer, R. H. J . Am. Chem. Soc. 1959, 81, 5971-
5977. (b) Sheehan, M.; Cram, D. J . J . Am. Chem. Soc. 1969, 91, 3553-
3558.
(33) Kuroda, H.; Ikemoto, I.; Akamatsu, H. Bull. Chem. Soc. J pn.
1966, 39, 1842-1849.
(34) Vincent, V. M.; Wright, J . D. J . Chem. Soc. Faraday 2 1974,
70, 58-71.
(35) Eckhardt, C. J .; Hood, R. J . J . Am. Chem. Soc. 1979, 101, 6170-
6174.
(37) (a) Pages 125-178 in ref 1. (b) Benesi, H. A.; Hildebrand, J . H.
J . Am. Chem. Soc. 1949, 71, 2703-2707.
(38) Pages 276-302 in ref 1.
(36) Shirota, Y.; Nagata, J .; Nakano, Y.; Nogami, T.; Mikawa, H. J .
Chem. Soc., Perkin Trans. 2 1977, 14-18.

Photoinduced Electron Transfer between ACN and TCNE
Sch em e 3
J . Org. Chem., Vol. 63, No. 16, 1998 5375
Ta ble 1. Cr ysta l Da ta for 3, 7, a n d ACN‚TCNE
3
7
ACN‚TCNE
C18H8N4
empirical
formula
C
30H16N4
C16H10N2
color, habit
dimensions
(mm)
colorless, prism
0.30 × 0.30 × 0.20
colorless, prism
0.20 × 0.20 × 0.20
tan-brown, prism
0.40 × 0.60 × 0.30
crystal system
lattice
parameters
orthorhombic
a ) 21.258(3) Å
orthorhombic
a ) 9.841(2) Å
monoclinic
a ) 7.240(1) Å
b ) 23.556(3) Å
c ) 9.451(2) Å
V ) 4733(1) Å3
b ) 21.813(2) Å
c ) 5.405(1) Å
V ) 1160.3(5) Å3
b ) 7.680(1) Å
c ) 26.548(9) Å
V ) 1468(5) Å3
â ) 96.02(8)°
P21/n (No. 14)
4
space group
Pbca (No. 61)
P212121 (No. 19)
4
1.318
1244
Z value
8
D
calc (g cm-3
)
1.259 (1.223)a
4716
1.268
2813
no. reflections
measured
no. reflections
for structure
solution and
refinement
R
1468
1034
1157
0.085
0.074
0.060
0.068
0.094
0.098
Rw
a
Determined value in aqueous potassium iodide.
Ta ble 2. P r od u ct Distr ibu tion on Dir ect Ir r a d ia tion of
ACN w ith Ligh t of Lon ger Wa velen gth th a n 400 n m in
th e P r esen ce of TCNE in DCE a n d AN
yield (%)a
[ACN] [TCNE]
cisoid/
(M)
(M)
solvent cisoid-1 transoid-1 transoid
2
3
4
10-3
10-3
10-3
10-3
10-1
10-1
10-1
10-1
10-3
10-3
10-3
10-3
10-1
10-1
10-1
10-1
0
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
AN
AN
AN
AN
AN
37
78
80
80
43
48
67
74
63
82
83
81
68
77
80
78
63
16
15
15
57
47
19
11
37
15
12
12
32
18
11
8
0.59
4.75
5.33
5.33
0.74
1.02
3.29
6.80
1.67
5.60
6.80
6.75
2.15
4.25
7.28
9.20
10-3
10-2
10-1
0
6
5
5
0
0
0
0
0
0
10-3
10-2
10-1
0
5
3
2
0
5
7
0
5
6
10-3
10-2
10-1
0
3
4
6
0
0
0
0
0
0
F igu r e 2. ORTEP drawing of compound 3.
10-3
10-2
10-1
AN
AN
AN
5
3
3
0
3
5
0
2
5
When each of the dimers (cisoid-1 and transoid-1) and
2 was irradiated (λ > 400 nm) independently in the
presence of TCNE, the starting compound was completely
recovered without producing 3 and 4 at all. Furthermore,
all of those five products (cisoid-1, transoid-1, 2-4) were
stable against irradiation of this wavelength of light
without producing any photochemical secondary prod-
ucts.
Effects of concentration of ACN and TCNE on product
distribution were examined on irradiation (λ > 400 nm)
with the conversion of ACN in the range of 35-70%. The
results are collected in Table 2. In each case, yields of
cisoid-1 and transoid-1 are higher than those of 2-4 even
under conditions with excess of TCNE. It is noteworthy
that cisoid-1 prevails over transoid-1 in the presence of
a
1
Yields based on consumed ACN determined by H NMR with
conversion of ACN of 35-60%.
TCNE in each solvent. Among 2-4, yields of 3 and 4
are higher than that of 2 at high concentrations of ACN
(1.0 × 10-1 M) in the presence of TCNE g 1.0 × 10-2 M,
whereas the opposite was the case in irradiation at low
concentration of ACN. No priority was observed between
3 and 4.
Qu a n tu m Yield s a n d cisoid -1/tr a n soid -1 Ra tio
(Cisoid /Tr a n soid Ra tio) in th e P h otod im er iza tion
of ACN in th e P r esen ce of TCNE. To quantify the
influence of concentration of TCNE on the efficiency and
the stereoselectivity of the dimerization of ACN, quantum

5376 J . Org. Chem., Vol. 63, No. 16, 1998
Haga et al.
Sch em e 4
higher for higher [ACN]. Especially at low [ACN] this
trend is similar to the increase of ΦR with [TCNE] (Figure
3a). Thus, the cisoid/ transoid ratio rapidly increased on
addition of TCNE to attain a plateau of =4.6 and =6.5
in DCE and AN, respectively.
P h otoch em ica l Rea ction of ACN w ith F N. To
examine the effect of electron-accepting capability of
Red
TCNE, FN, a weaker electron acceptor with E1/2
of
-1.36 V (vs SCE in AN),39 has been used in place of
TCNE.
The free energy change, ∆GET
singlet state of ACN to these acceptors can be calculated
,
40 for ET from the excited
Ox
according to eq 2 with values of oxidation potential (E1/2
as 1.58 V41 (vs SCE in AN) and excitation energy (∆Eexcit
as 257 kJ mol-1 41b,c,42 for ACN.
)
)
∆GET ) F[E1/2Ox(D) - E1/2Red(A)] - ∆Eexcit
+
∆ECoulomb (2)
Here, ∆ECoulomb represents Coulomb energy between the
resulted radical ions. The calculated ∆GET on direct
excitation of ACN is -118.3 and -132.0 kJ mol-1 for
TCNE and 28.4 and 14.6 kJ mol-1 for FN in DCE and
AN, respectively. Therefore, the ∆GET value from ACN
to FN is slightly positive, but highly negative from ACN
to TCNE.
When ACN was mixed with FN in solution, no char-
acteristic coloration due to development of the CT
complex was observed even in a polar solvent such as
AN. Absorption spectrum of this solution corresponds
to the sum of the spectrum of ACN and FN. Therefore,
in contrast to the ACN-TCNE system, ACN and FN do
not form a CT complex in the solvents used.
F igu r e 3. Plots of quantum yield (Q.Y.) for photodimerization
of ACN, ΦR, and the cisoid/ transoid ratio versus concentration
of TCNE, [TCNE], in DCE and AN: (a) [ACN] ) 1.0 × 10-3
M; (b) [ACN] ) 2.0 × 10-2 and 2.0 × 10-1 M, together with
calculated values employing eq 7, ΦRcalc, at [ACN] ) 2.0 × 10-1
M.
yields for the dimerization, ΦR, and the cisoid/ transoid
ratio of 1 were determined at varying [TCNE]. Figure 3
plots ΦR and the cisoid/ transoid ratio of 1, determined
by GC on 435.8 nm irradiation of ACN against [TCNE]
in DCE and AN. At low concentration of ACN ([ACN] )
1.0 × 10-3 M), the ΦR values steeply increased as [TCNE]
increased from 0 to 1.0 × 10-3 M (Figure 3a). For
example, upon addition of 1.0 × 10-3 M TCNE to ACN
in DCE, ΦR jumped nearly 3-fold from 1.4 × 10-3 to 3.6
On irradiation of a solution containing ACN and FN
in DCE with light of >400 nm, three products were
isolated in addition to the two dimers (cisoid-1 and
transoid-1). On the basis of EI-MS, elemental analysis,
1
and H NMR spectrometry, these products are identified
as three isomeric [2 + 2]-cycloadducts (5-7) between
ACN and FN (Scheme 4). Other products, such as the
[2 + 2 + 2]-cycloadducts, were not found. The configu-
ration at positions 1-4 of 7 was unambiguously deter-
mined by X-ray crystallography of a single crystal. The
crystallographic data for this compound are summarized
× 10-3
. On further increase of [TCNE], however, ΦR
approached a plateau of =4.1 × 10-3. On the other hand,
at high [ACN] ([ACN] ) 2.0 × 10-2 and 2.0 × 10-1 M),
the ΦR values decreased as [TCNE] increased from 0 to
5.0 × 10-2 M (Figure 3b). However, higher [ACN] gives
higher ΦR than lower [ACN], and this trend is particu-
larly remarkable when [TCNE] < 1.0 × 10-2 M. Similar
trend was observed in AN. Thus, with increase of
[TCNE], the ΦR increased to a plateau (=3.4 × 10-3) at
low [ACN] condition but decreased to approach a constant
value at high [ACN].
(39) (a) Iwai, K.; Takemura, F.; Furue, M.; Nozakura, S. Bull. Chem.
Soc. J pn. 1984, 57, 763-767. (b) Kemp, T. J .; Parker, A. W.; Wardman,
P. J . Chem. Soc., Perkin Trans. 2 1987, 397-403.
(40) (a) Rehm, D.; Weller, A. Israel J . Chem. 1970, 8, 259-271. (b)
Leonhardt, H.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1963, 67, 791-
795.
(41) (a) Tanimoto, I.; Kushioka, K.; Maruyama, K. Bull. Chem. Soc.
J pn. 1979, 52, 3586-3591. (b) Santa Cruz, T. D.; Akins, D. L.; Birke,
R. L. J . Am. Chem. Soc. 1976, 98, 1677-1682. (c) Siegel, T. M.; Mark,
H. B., J r. J . Am. Chem. Soc. 1972, 94, 9020-9027.
(42) Levanon, H.; Neta, P.; Trozzolo, A. M. Chem. Phys. Lett. 1978,
54, 181-185.
The cisoid/ transoid ratio increased rapidly with in-
crease of [TCNE] to give a nearly plateau value at both
low (Figure 3a) and high (Figure 3b) [ACN], which is

Photoinduced Electron Transfer between ACN and TCNE
J . Org. Chem., Vol. 63, No. 16, 1998 5377
Ta ble 3. P r od u ct Distr ibu tion on Dir ect Ir r a d ia tion of ACN w ith Ligh t of Lon ger Wa velen gth th a n 400 n m in th e
P r esen ce of F N in DCE a n d AN
yield (%)a
[ACN] (M)
[FN] (M)
solvent
cisoid-1
transoid-1
cisoid/transoid
5
6
7
1.0 × 10-3
1.0 × 10-2
5.0 × 10-2
2.0 × 10-1
1.0 × 10-3
1.0 × 10-2
5.0 × 10-2
2.0 × 10-1
1.0 × 10-3
1.0 × 10-2
5.0 × 10-2
2.0 × 10-1
1.0 × 10-3
1.0 × 10-2
5.0 × 10-2
2.0 × 10-1
DCE
DCE
DCE
DCE
AN
AN
AN
AN
25
39
0.64
0.88
1.41
1.50
2.00
1.99
3.94
4.60
30.2
35.8
17.2
44
48.3
45.0
14.3
34.3
26.0
11.5
22
24.2
11.9
3.1
8.8
6.9
1.1
7.8
4.0
5.9
4.8
1.0
tb
4.8
3.3
6.5
3.1
1.3
5.4
3.0
0.9
tb
Yields determined by means of 1H NMR based on consumed ACN. Conversion of ACN is 45-85%. Trace.
a
b
F igu r e 4. ORTEP drawing of compound 7.
in Table 1. The ORTEP diagram in Figure 4 shows the
structure of 7, where both pairs of protons (H1 and H4,
H2 and H3) are cis and two protons (H2 and H3) are
aligned as anti with the two cyano groups. The struc-
tures of 5 and 6 are established on the basis of compari-
1
son of their H NMR spectra with that of 7. Thus, three
equivalent pairs of aromatic protons, two equivalent
aliphatic tertiary protons, and a smaller J 1-2 value than
that of 7 support the structure for 6. Similarly, the non-
equivalent six aromatic protons and the nonequivalent
aliphatic tertiary protons support the structure for 5.
Yields of the above products obtained in various
concentrations of ACN and FN in DCE and AN are shown
in Table 3. In each case, formation of cisoid-1 and
transoid-1 prevails over that of 5-7. Addition of FN
slightly increased the cisoid/ transoid ratio, but the effect
was not so remarkable in addition of TCNE. No selectiv-
ity among 5-7 was noted.
F igu r e 5. Plots of quantum yield (Q.Y.) for photodimerization
of ACN, ΦR, and the cisoid/ transoid ratio versus [FN] in DCE
and AN: (a) [ACN] ) 1.0 × 10-3 M; (b) [ACN] ) 2.0 × 10-2
and 2.0 × 10-1 M.
Quantum yields (ΦR) of the dimerization of ACN were
determined in varying [FN] on irradiation of 435.8 nm
light. Figure 5 plots ΦR and the cisoid/ transoid ratio
against [FN] in DCE and AN. Both ΦR and the cisoid/
transoid ratio are almost insensitive to [FN] at low
(Figure 5a) and high (Figure 5b) [ACN]. This means that
the addition of FN does not affect the dimerization of
ACN, which is quite in contrast to the ACN-TCNE
system (Figure 3).
P r ep a r a tion of a Cr ysta llin e CT Com p lex of ACN
w ith TCNE a n d Its Ir r a d ia tion in th e Solid Sta te.43
An equimolar solution of ACN and TCNE were mixed in
ethyl acetate, and the mixture was slowly evaporated at
ambient temperature to give a crystalline 1:1 CT complex
between ACN and TCNE (ACN‚TCNE), as tan-brown
cubes, mp 103.0-105.0 °C. The 1:1 composition and
purity of the crystalline CT complex were satisfactorily
verified by elemental analysis. From the X-ray crystal-
lographic analysis, as listed in Table 1, the crystalline
CT complex is monoclinic and belongs to the space group
of P21/n. Figure 6a shows the molecular packing in ACN‚
TCNE along the a axis. One-dimensional columns of
ACN‚TCNE are constituted along this axis, where ACN
and TCNE are stacked alternately. Molecular arrange-
ment in the crystal viewed along the b axis is shown in

5378 J . Org. Chem., Vol. 63, No. 16, 1998
Haga et al.
Ta ble 4. P r od u ct Distr ibu tion of P h otoch em ica l Rea ction s of ACN in th e Solid Sta te
yields (%)a
substrate
ACN
ACN‚TCNE
ACN‚TCNE
ACN‚TCNE
ACN‚TCNE
ACN‚TCNE
state
crystal
crystal
crystal
crystal
mixed powder
mixed powder
hν (nm)
temp (°C)
conversion (%)
cisoid-1b
transoid-1b
cisoid/transoid
2c
>400
>500
>500
>400
>500
>400
25
25
-10
25
25
25
14.5
25.4
26.9
38.5
30.6
52.5
15.8
0
0
0
5.2
13.1
83.2
0
0
0
7.3
65.6
0.19
80.7
78.2
73.3
75.8
12.2
0.71
0.20
a
b
Yields based on 1 recovered. Determined by GC. c Isolated by flash column chromatography.
into 2 in 80.7% yield (based on consumed ACN) deter-
mined by chromatographic isolation (Table 4). This is
in contrast to CT excitation in solutions which did not
result in any net photochemical reaction. The quantum
yield for formation of 2 was ca. 4 ((1) × 10-3 under
irradiation of 546.1 nm monochromatic light. This solid-
state reaction was not affected by reaction temperature
(-10-25 °C). Wavelength of the irradiation light es-
sentially did not affect the reaction except that we
obtained higher yields of 2. When the crystalline CT
complex was irradiated with light > 400 nm, 2 was
formed in a 73.2% yield with a conversion of 38.5%, which
is in contrast to the irradiation in solutions giving five
products in appropriate yields (Table 2). For comparison,
a single crystal of ACN was irradiated (λ > 400 nm) at
25 °C to give cisoid-1 and transoid-1 with the cisoid/
transoid ratio of 0.19.
To compare the behavior of the crystalline CT complex
in a topochemically ordered crystal lattice with that in
the solid state where molecules randomly arrange, ir-
radiation of a ground powder of a mixture of ACN and
TCNE was examined. Thus, an equimolar mixture of
ACN and TCNE was pulverized in a mortar, and the
resulted tan-brown-colored fine powder was irradiated
under the same conditions as in the case of ACN‚TCNE.
As included in Table 4, irradiation of the mixed powder
with >500 nm light afforded 2 accompanied by lesser
amounts of cisoid-1 and transoid-1 with the cisoid/
transoid ratio of 0.71. The above cisoid/ transoid ratio
of the dimer is much larger than that of the irradiation
of ACN crystal without TCNE and those reported by
Cohen et al.44 and Livingston and Wei.,45 who described
transoid-1 as the sole product at 20 °C. It is noteworthy
that direct excitation of ACN in the mixed powder with
>400-nm light resulted in decrease of 2 and instead
increase of 1 with the cisoid/ transoid ratio of 0.20, which
is close to that for the direct irradiation of ACN crystal
without TCNE (Table 4). Formation of 3 and 4 was not
observed in the solid-state irradiation.
F igu r e 6. Molecular arrangement in the crystalline ACN‚
TCNE along the a axis (a) and the b axis (b).
F igu r e 7. Molecular pair in the crystalline ACN‚TCNE.
Figure 6b. One unit cell consists of four columns, in
which two ACN‚TCNE molecules are included. Two
molecules of ACN in the adjacent columns in a same
lattice are aligned in parallel and face the opposite
direction. A pair of ACN‚TCNE is given in Figure 7. The
dihedral angle between the plane of ACN and TCNE in
the crystal is 0.8° with a torsion angle between two
alkene parts of ACN and TCNE of 62.2°.
ACN‚TCNE was irradiated with >500-nm light under
argon atmosphere at 25 °C. Surprisingly enough, under
this CT excitation condition, photochemical reaction
between ACN and TCNE took place to give 2 as a sole
product without giving any other products such as cisoid-
1, transoid-1, 3, and 4.43 For example, on irradiation of
ACN‚TCNE for 7.0 h, 25.0% of ACN‚TCNE was converted
Discu ssion
(a ) SSIP Mech a n ism for th e Dim er iza tion of ACN.
Direct excitation of ACN in solution in the presence of
TCNE gave not only cisoid-1 and transoid-1 but also 2-4.
As we discuss below, TCNE works as an electron acceptor
to either the singlet or the triplet excited state of ACN
to give a singlet or triplet solvent-separated radical ion
pair (SSIP) particularly at high or low [ACN], respec-
tively; among them the singlet radical ion pair (singlet
(43) Haga, N.; Nakajima, H.; Takayanagi, H.; Tokumaru, K. Chem.
Commun. 1997, 1171-1172.
(44) Cohen, M. D.; Ron, I.; Schmidt, G. M. J .; Thomas, J . M. Nature
1969, 224, 167-168.
(45) Wei, K. S.; Livingston, R. J . Phys. Chem. 1967, 71, 548-549.

Photoinduced Electron Transfer between ACN and TCNE
J . Org. Chem., Vol. 63, No. 16, 1998 5379
Sch em e 5
TCNE will proceed from the T1 state. Thus, the T1 state
will either add to the ground-state ACN with a rate
constant of krT collapsing to cisoid-1 and transoid-1 with
an efficiency of â or undergo ET to TCNE with a rate
constant of kETT to give a triplet radical ion pair of ACN+•
and TCNE-• leading to cisoid-1 and transoid-1 with an
efficiency of xT competing with deactivation with a
lifetime, τT. Then the ΦR is given by eq 3.
kr [ACN]â + kETT[TCNE]xT
T
ΦR ) ΦST
(3)
-1
T
τT + kr [ACN] + kETT[TCNE]
When [TCNE] is sufficiently high, ET from the T1 state
to TCNE will proceed much faster than deactivation and
addition to the ground-state ACN (kr of 2.1 × 106 and
T
2.6 × 106 M-1 s-1 and τT of 2.9 × 10-6 and 2.2 × 10-6 s in
DCE and AN,14e,21 respectively). Accordingly, eq 3 can
be approximated as eq 4.
ΦR ) ΦSTxT
(4)
Equation 4 means that, under the above conditions,
where the dimerization occurs from ACN+•, the ΦR
attains a plateau value, ΦST‚xT. This satisfactorily agrees
with the results in Figure 3a. Based on the plateau ΦR
value at 2.0 × 10-3 M TCNE, 0.0040 in DCE and 0.0035
in AN, xT can be obtained as 0.023 and 0.11 in DCE and
AN, respectively (Table 5). More than 5 times lower xT
value in DCE than that in AN may reflect more facile
reversion of the triplet SSIP to the singlet one in DCE
than in AN probably due to the heavy atom effect of
chlorine atoms of DCE. The cisoid/ transoid ratio of 1
observed as a plateau value at high [TCNE], 4.5 and 6.2
in DCE and AN, respectively, corresponds to the cisoid/
transoid ratio of the dimer formed from ACN+•
.
At low [TCNE] (<1.0 × 10-3 M), where the T1 state
predominantly undergoes unimolecular deactivation,
T
kr [ACN] and kETT[TCNE] in the denominator of eq 3 are
much smaller than τT and can be neglected. Also
T
kr [ACN]â in the numerator can be omitted. Therefore,
the ΦR increases linearly with [TCNE] at low [TCNE] as
in eq 5. From the linear relation between ΦR and [TCNE]
ΦR ) ΦSTτTkETT[TCNE]xT
(5)
SSIP) mostly undergoes BET preceding diffusion prob-
ably through the triplet SSIP, but the triplet SSIP under-
goes diffusion to give the radical ion pair ACN+• and
TCNE-•. Then ACN+• adds to ACN to give ACN2+•, which
will react with TCNE-• to afford 1 or reacts with TCNE-•
to give the 1:1 cycloadduct 2. The mechanism for the
reaction is depicted in Scheme 5 and briefly in Scheme
6. The present mechanism well fits the various obser-
vation, particularly the effect of [ACN] and [TCNE] or
the ΦR and the cisoid/ transoid ratio of 1 as described
below.
at the low [TCNE] in Figure 3a, one can obtain τTΦSTkETTxT
as a slope. On the basis of the slope (2.09 and 2.39 M in
T
DCE and AN, respectively) and the data in Table 5, kET
was determined as 2.3 × 108 and 3.4 × 108 M-1 s-1 in
T
DCE and AN. The obtained kET values are 100 times
smaller than the diffusion-controlled rate constant. This
may be due to activation energy for ET from the T1 state
of ACN to TCNE, though the triplet state of ACN (179-
46
181 kJ mol-1
)
lies higher in energy than that of the
First, we discuss the effect of [TCNE] on the ΦR and
the cisoid/ transoid ratio when [ACN] is low such as 1.0
× 10-3 M. When [ACN] ) 1.0 × 10-3 M, both ΦR and
the cisoid/ transoid ratio steeply increase with increase
of [TCNE] to attain a plateau value (Figure 3a). Thus,
addition of TCNE enhances production of 1, accompany-
ing the increase of its cisoid/ transoid ratio. In the
absence of TCNE, as we have previously reported, at
[ACN] ) 1.0 × 10-3 M, ΦST (0.18 in DCE and 0.029 in
AN) is much larger than the quantum yield for the
SSIP (121 kJ mol-1).
Second, we discuss the effect of [TCNE] at high [ACN].
On the contrary to the acceleration of the dimerization
by TCNE at low [ACN], at higher [ACN] (2.0 × 10-1 and
2.0 × 10-2 M), increase of [TCNE] reduces the ΦR to an
almost constant value (Figure 3b) but increases the
cisoid/ transoid ratio to get a near plateau similar to at
low [ACN] when [ACN] is as high as 1.0 × 10-1 M. This
suppression of dimerization by added TCNE is not due
dimerization from the S1 state, ΦR (5.7 × 10-4 in DCE
S
(46) Kobashi, H.; Ikawa, H.; Kondo, R.; Morita, T. Bull. Chem. Soc.
J pn. 1982, 55, 3013-3018.
and 2.8 × 10-4 in AN).21 Therefore, at low [ACN] ET to

5380 J . Org. Chem., Vol. 63, No. 16, 1998
Haga et al.
Sch em e 6
Ta ble 5. Va r iou s Kin etic Da ta a n d Qu a n tu m Yield s for
In ter system Cr ossin g Rela tin g to th e Dim er iza tion of
ACN in th e P r esen ce of TCNE
DCE
AN
Ra
0.257
0.286
S
a
kr (M-1 s-1
)
6.3 × 109
2.6 × 109
âa,b
0.85
0.18
1.05
0.029
a
ΦST
T
a,c
kr (M-1 s-1
)
2.13 × 106
2.93 × 10-6
0.337
1.78 × 106
2.24 × 10-6
0.618
τT (s)a,c
γa,c
T
kET (M-1 s-1
)
)
2.3 × 108
3.3 × 1011
0.023
3.4 × 108
5.8 × 1011
0.11
S
kET (M-1 s-1
xT
xS
0.017
0.015
a
b
From ref 21. Determined by Eosin-Y-sensitized reaction on
irradiation at 546.1 nm. c Determined by quenching with ferrocene.
F igu r e 8. Stern-Volmer plots for quenching of the photo-
dimerization of ACN by TCNE at [ACN] of 2.0 × 10-1 M.
to increase of proportion of the CT complex. For example,
at [ACN] ) 2.0 × 10-1 M in AN, increase of [TCNE] from
0 to 1.0 × 10-3, 1.0 × 10-2, and 5.0 × 10-2 M reduced the
ΦR from 0.048 to 0.027, 0.016, and 0.015, whereas nearly
20% out of total ACN formed the CT complex in each
case. Therefore, decrease of free ACN by forming the CT
complex, which is inert for photochemical reactions,
cannot be ascribed to this suppression of dimerization.
At low [ACN], the active excited state is the T1 state
of ACN as described above. On the other hand, at high
[ACN], as reported previously,21 in the absence of TCNE,
the singlet state of ACN can add to ACN competing with
deactivation to the ground state and to the triplet state.
For simplicity, let us assume the suppression of ΦR with
TCNE as quenching of the dimerization from the S1 state
by TCNE; TCNE quenches the S1 state and will give a
singlet SSIP, which will subsequently undergo facile BET
without giving net products. Then, the ratio of the
quantum yields in the absence of TCNE, ΦR0, to that in
the presence of TCNE, ΦRTCNE, is represented by eq 6,
than those commonly accepted for ET probably due to
very simplified treatment of the data.
Generally, at high [ACN] four pathways can give 1,
that is, dimerization of ACN from the S1 and T1 states
without ET, and via ET from the S1 and T1 states to
TCNE (Scheme 5). The S1 state undergoes ET to TCNE
with a rate constant kETS, and the resulting SSIP col-
lapses to the products with an efficiency of xS. The
quantum yield for the dimerization (ΦR) at high [ACN]
is given in eq 7. At high [TCNE], the S1 state is assumed
ΦR
)
krT[ACN]â + kETT[TCNE]xT
krS[ACN]R + kETS[TCNE]xS + ΦST
-1
T
τT + krT[ACN] + kET [TCNE]
(
)
τS + krS[ACN] + kETS[TCNE]
-1
(7)
S
S
where τs, kr , and kET stand for the singlet lifetime, rate
constant for addition of the S1 state to the ground state
of ACN, and rate constant for quenching of the S1 state
by TCNE by ET, respectively.
to undergo ET to TCNE with a rate constant much larger
than that for the addition to the ground-state ACN.
Furthermore, ET from the S1 state to TCNE must be
sufficiently faster than deactivation of the S1 state when
[TCNE] > 1.0 × 10-2 M. Then, at high [TCNE], eq 7
can be simplified as eq 8. At high [TCNE], the first term
0
ΦR
kETS[TCNE]
) 1 +
(6)
TCNE
-1
S
ΦR
τS + kr [ACN]
S
kr [ACN]R + ΦSTxT
ΦR )
+ xS
(8)
kETS[TCNE]
Figure 8 presents typical Stern-Volmer plots for
[ACN] ) 2.0 × 10-1 M in DCE and AN, which afford a
linear relationship with an intercept close to unity. The
observed slope is 81.3 in DCE and 174.7 M-1 in AN.
S
in eq 8 can be neglected (kETS[TCNE] . kr [ACN]R +
Φ
STxT). Hence, ΦR is to be reduced to a nearly constant
Applying τs-1 of 2.8 × 108 s-1 17,18 and kr of 6.3 × 109
value with increase of [TCNE], which agrees with the
results in Figure 3b. On the basis of ΦR values, xS can
be determined from eq 8 as 0.017 and 0.015 in DCE and
AN, respectively. These data are collected in Table 5.
S
S
and 2.6 × 109 M-1 s-1 in DCE and AN,21 kET value can
be given as 3.3 × 1011 and 5.8 × 1011 M-1 s-1 in DCE
and AN, respectively, though these values are higher

Photoinduced Electron Transfer between ACN and TCNE
J . Org. Chem., Vol. 63, No. 16, 1998 5381
As to xS and xT, the fractions of the S1 and T1 states of
ACN quenched with TCNE leading to dimerization,
respectively, in AN xT is nearly 10 times larger than xS.
This will reflect the difference of the nature of the
overlap conformation of the two aryl chromophores.47 The
same goes for dimeric radical cations in poly(vinylpyrenes)
generated by pulse radiolysis.48 This attractive force,
which is defined as charge-resonance energy,49 is esti-
mated as several kilocalories per mole in magnitude from
their charge-resonance band. The present results indi-
cate that addition of ACN+• to ACN gives (cisoid-1)+•
several times more efficiently than (transoid-1)+•, since
in the former the two naphthyl chromophores can adopt
a somewhat parallel sandwich-like conformation and can
exert an attractive interaction to delocalize the positive
charge to stabilize this species.
resulting triplet and singlet SSIP of ACN+• and TCNE-•
.
On the other hand, in DCE xT is no more than twice of
xS. This may reflect the heavy atom effect of chlorine
atoms in DCE as mentioned before.
Using eq 7, ΦR values at various [TCNE] can be
calculated, and the calculated values, ΦRcalc, are included
calc
in Figure 3b. As Figure 3b indicates, plots for ΦR
satisfactorily agree well with those for the observed ΦR.
This substantiates the validity of the mechanism shown
in Scheme 5. The opposite effect of TCNE on ΦR at
between low and high [ACN], that is, to increase ΦR at
low [ACN] but to reduce ΦR at high [ACN], is reasonably
attributed to difference of efficiency to give product from
the singlet SSIP, xS, and from the triplet SSIP, xT. Thus,
at low [ACN] ET to TCNE exclusively proceeds from the
T1 state, which gives the products in relatively high
efficiency (xT ) 0.023 and 0.11 in DCE and AN, respec-
tively), due to relatively slow BET from the triplet SSIP.
On the other hand, at high [ACN] ET mainly proceeds
from the S1 state which gets to the products with low
efficiency (xS ) 0.017 and 0.015 in DCE and AN,
respectively) because of faster BET from the singlet SSIP.
Therefore, at high [ACN], TCNE functions as a quencher
of the S1 state rather than an effective reactant.
The plateau ΦR value for the dimerization in the
presence of TCNE, which is much smaller than unity,
may rule out a chain reaction mechanism which proceeds
+•
by the way of ET from the ground-state ACN to ACN2
giving ACN+• and 1 (cisoid or transoid), though E1/2Ox for
cisoid-1 (1.98 V50 vs SCE in AN) is higher than that for
ACN (1.58 V41 vs SCE in AN).
(b) F or m a tion of 2-4 fr om ACN+•. Excitation of
ACN in the presence of TCNE afforded 2-4 besides the
dimers (cisoid-1 and transoid-1). Formation of 2 from
irradiation of ACN in the presence of TCNE in DCE with
light of >300 nm was previously reported by Shirota et
al.22
In the present reaction, 3 and 4 were produced only
when [ACN] was as high as 1.0 × 10-1 M and [TCNE]
was higher than 1.0 × 10-2 M. If 3 and 4 were produced
by reaction between ACN2 and TCNE-•, this process
+•
Variation of the cisoid/ transoid ratio of 1 with [TCNE]
in DCE is similar at high [ACN] and at low [ACN]
affording a plateau value of 5.0 (Figure 3). This indicates
that both the singlet and the triplet SSIP give 1 in the
same cisoid/ transoid ratio in DCE. This again suggests
that in DCE the singlet SSIP will easily convert to the
triplet SSIP, subsequently undergoing diffusion to give
free ACN+•, which then reacts with ACN finally affording
cisoid-1 and transoid-1 in a ratio of 5.0. On the other
hand, in AN the cisoid/ transoid ratio attained at high
[TCNE] is higher at high [ACN] (11) than at low [ACN]
(6.2). The reason for this difference is not clear. How-
ever, one could suppose that an ACN+• site in a singlet
SSIP might interact with an ACN molecule out of the
pair with cisoid-type configuration leading to a cage-wall
cycloaddition as suggested for photocycloaddition be-
tween two molecules of 1,1-diphenylethylene sensitized
with 9,10-dicyanoanthracene.8a
should take place even at low [ACN] competing with
production of cisoid- and transoid-1 by charge recombi-
nation between these species. Therefore, the present
results suggest a possible mechanism in which an ACN
molecule can interact with a singlet SSIP to give 3 and
4 particularly at high [ACN]; the stationary concentration
of the SSIP will be sufficiently high at high [TCNE] for
the above reaction to occur. High [ACN] will be effective
to undergo addition to ACN in the SSIP. Furthermore,
production of 3 and 4 at a slight expense of production
of 2 with increase of [TCNE] suggests that 2 also can
result from geminate reaction in the SSIP as well as from
coupling of free ACN+• and TCNE-•
.
(c) P h otoch em ica l In er tn ess of CT Com p lex of
ACN-TCNE in Solu tion s (CIP Mech a n ism ). The
tan-brown coloration observed immediately after mixing
solutions of ACN and TCNE shows formation of a CT
complex between ACN and TCNE in the ground state.
The intensity of the color increases with increase of the
initial concentration of ACN and TCNE as shown in
Figure 1, obeying eq 1 for complex formation with the
1:1 stoichiometry in Scheme 2.
Selective excitation of the CT complex with light of
>500 nm did not result in any reaction in contrast to
excitation of ACN with >400-nm light in the presence of
TCNE in solution. Excitation of a CT complex generally
The dissociated ACN+• from the singlet or triplet SSIP
reacts with the ground-state ACN to give a dimeric
radical cation, ACN2+• (Scheme 5). The ACN2+• can take
cisoid- and transoid-type configuration corresponding to
(cisoid-1)+• and (transoid-1)+•. Therefore, the stereo-
chemistry of the resulting 1 will be determined when
ACN+• adds to ACN. Charge recombination between
+•
ACN2 and TCNE-• gives cisoid-1 and transoid-1 in a
cisoid/ transoid ratio depending on the solvents used.
Why does formation of cisoid-1 notably prevail over
formation of transoid-1 in the radical cation mechanism?
Generally speaking, in the case of photodimerization of
alkenes proceeding via ET mechanism, little consistency
has been observed for stereoselectivity for the dimeric
products.5-10 It is worth noting that photochemically
generated radical cations of some 1,3-diarylpropanes, i.e.,
1,3-dinaphthylpropane, 1,3-dipyrenylpropane, and 1,3-
dicarbazolylpropane, are stabilized in a sandwich-like
(47) (a) Washio, M.; Tagaya, S.; Tabata, Y. Polym. J . 1981, 13, 935-
938. (b) Tsuchida, A.; Tsujii, Y.; Ito, Y.; Yamamoto, S. J . Phys. Chem.
1989, 93, 1244-1248. (c) Tsujii, Y.; Tsuchida, A.; Onogi, Y.; Yamamoto,
M. Macromolecules 1990, 23, 4019-4023. (d) Tsuchida, A.; Tsujii, Y.;
Ohoka, M.; Yamamoto, M. J . Phys. Chem. 1991, 95, 5797-5802. (e)
Tsuchida, A.; Yamamoto, M. J . Photochem. Photobiol. A: Chem. 1992,
65, 53-59.
(48) Tsuchida, A.; Ikawa, T.; Yamamoto, M.; Ishida, A.; Takamuku,
S. J . Phys. Chem. 1995, 99, 14793-14797.
(49) Yamamoto, M.; Tsujii, Y.; Tsuchida, A. Chem. Phys. Lett. 1989,
154, 559-562.
(50) Haga, N. Unpublished results.

5382 J . Org. Chem., Vol. 63, No. 16, 1998
Haga et al.
Sch em e 7
CIP is essentially fast, the retention of proximity and
rigid mobility of ACN and TCNE in the crystal should
retard the BET but enable the two alkenic parts to
undergo cycloaddition. Moreover, the absence of solvent
molecules does not stabilize the CIP, which will also
retard the BET. On the other hand, in solution the
excited CT complex and the CIP are solvated, tend to
undergo rapid BET due to small -∆GBET, will not
maintain favorable configuration to undergo addition
because of their mobility, and therefore result in facile
deactivation or BET to the ground state.
In the crystal lattice, two alkenic parts of ACN and
TCNE are aligned parallel with a torsion angle of 62.2°
with the interatomic distances of C1-C13 and C2-C14
of 3.43 and 3.65 Å, respectively (Figure 7). This feature
of molecular arrangement satisfies the requirements for
the [2 + 2]-cycloaddition of ACN to TCNE in the solid
state under the topochemical control in the crystal lattice
(Schmidt’s rule).52 Another combination of the alkenic
carbon atoms, C1-C14 and C2-C13 with distances of
3.97 and 3.33 Å, respectively, is also possible for the
cycloaddition, which satisfies Schmidt’s rule. Consider-
ing the more remote distance of C1-C14, the former
combination is more favorable than the latter. Topologi-
cal transformation is necessarily accompanied from the
torsion angle of 62.2° between the two alkenic CdC
double bonds to the parallel C-C single bond of the
cyclobutane moiety in 2. Though this movement requires
unoccupied large space in the crystal lattice, significantly
low density (1.268 g cm-3) of the crystalline CT complex
can satisfy this requirement.
Much higher reactivity of ACN on direct excitation
than that by CT excitation may be the reflection of more
effective ET in the crystalline CT complex due to increase
of photons absorbed. In this condition, however, not only
CT excitation of ACN‚TCNE but also local excitation of
ACN is accomplished.
In contrast to proximity of the alkenic carbons of ACN
and TCNE, those of two molecules of ACN in the
crystalline CT complex are aligned unfavorably for the
dimerization reaction to give the dimers (cisoid-1 and
transoid-1) with interatomic distances longer than 6 Å.
Apparently, these distances are out of order of the limit
value (4.2 Å) to undergo reaction in solid state.52 The
crystal structure of ACN‚TCNE is necessarily advanta-
geous for formation of 2 and disadvantageous for cisoid-1
and transoid-1. Moreover, formation of 3 and 4 in the
crystalline CT complex is definitely unfavorable, because
it is necessary to generate ACN2+•, which is the common
intermediate of cisoid-1 and transoid-1.
gives a contact RIP (CIP).23-25 In some donor-acceptor
systems, BET proceeds much faster from CIP than SSIP
as observed by transient absorption spectroscopy using
femtosecond laser photolysis.25 The present result indi-
cates that the resulting CIP very rapidly undergoes BET
preceding dissociation to free radical ions through SSIP.
This is attributed to a very low net energy for BET, as
low as 1.34 V, since CIP is known to undergo rapid BET
particularly when the CIP lies very close in energy to
the ground state.25 According to Miyasaka et al., CIP
with a -∆GBET of around 1.34 V quickly deactivates with
a rate constant of 1011 s-1 24
The mechanism involving
.
CIP formed by excitation of the CT complex rationalizes
this phenomenon (Scheme 5).
Distinction of reactivity and product distribution be-
tween selective CT excitation and bimolecular quenching
has been observed for the photoreduction of chloranil by
acenaphthene.51 J ones et al. rationalized this distinction
in terms of spin multiplicity of the resulted radical ion
pair.
(d ) Rea ction of ACN w ith F N. As shown in Figure
5, on irradiation of ACN with FN both ΦR and the cisoid/
transoid ratio for the dimerization do not increase with
increase of [FN]. Dimerization of ACN seems to proceed
irrespective of the presence of FN. This is quite in
contrast to those in the presence of TCNE. The above
results indicate that the radical ions, ACN+• and FN-•
do not participate in the dimerization. This reflects that
ET from the S1 state of ACN to FN is endergonic as
described before.
Furthermore, formation of the [2 + 2 + 2]-cycloadducts
of ACN and FN was not observed, whereas the corre-
sponding products (3 and 4) were isolated on direct
irradiation of ACN in the presence of TCNE. This again
shows no participation of ACN+• and ACN2+•. Alterna-
tively, a mechanism shown in Scheme 7 involving a
biradical (8) as an intermediate is in accord with the
present outcome. Because if the excited ACN underwent
addition to FN via a concerted process, 6 and 7 would
never be produced.
,
(e) P h otoch em ica l Rea ction of th e Cr ysta llin e CT
Com p lex u n d er Top och em ica l Con tr ol. It is quite
remarkable that CT excitation of the crystalline CT
complex (ACN‚TCNE) gave 2 as a sole product with a
ΦR of 4 × 10-3 in contrast to the inertness of the CT
complex in solution. This is attributed to retention of
proximity and rigid mobility of the ACN and TCNE
molecules in the crystalline lattice (Figures 6 and 7).
Thus, molecules of ACN and TCNE in the crystalline CT
complex are continually held in a favorable configuration
and distance for ET to take place; that is, the distance
between the ACN and TCNE plane is 3.5 Å with a
dihedral angle of 0.8°. Though the rate for BET from
It is quite suggestive to compare the photochemical
behavior in the topochemically ordered crystalline CT
complex with that in the randomly disordered mixed
powder. Irradiation (λ > 500 nm) of the mixed powder
of an equimolar mixture of ACN and TCNE afforded not
only 2 but also a small amount of cisoid-1 and transoid-1
with a cisoid/ transoid ratio of 0.71 (Table 4). A higher
yield of 2 than cisoid-1 and transoid-1 in this condition
gives evidence for formation of ACN‚TCNE in high
proportion by pulverization. Furthermore, the formation
of cisoid-1 and transoid-1 indicates that a part of ACN+•
produced by CT excitation of the CT complex can add to
a ground-state ACN to give ACN2+•. A looser and more
random arrangement of molecules of ACN and TCNE in
(51) (a) J ones, G., II; Haney, W. A.; Phan, X. T. J . Am. Chem. Soc.
1988, 110, 1922-1929. (b) J ones, G., II; Mouli, N.; Haney, W. A.;
Bergmark, W. R. J . Am. Chem. Soc. 1997, 119, 8788-8794.
(52) Pages 133-184 in ref 32.

Photoinduced Electron Transfer between ACN and TCNE
J . Org. Chem., Vol. 63, No. 16, 1998 5383
the mixed powder, where two molecules of ACN can be
located proximately, should allow to undergo the dimer-
ization. A much higher cisoid/ transoid ratio on CT
excitation than those on direct excitation of ACN in the
mixed powder and on irradiation of the ACN crystal
without TCNE (Table 4)44,45 supports preferable forma-
tion of ACN2+• which is stabilized by charge resonance47-49
under irradiation of >500-nm light to excite the CT
complex. Absence of 3 and 4 in this condition is in accord
with the requirement of enormous movement (topological
transformation) of the resulting CIP to interact with the
ground-state ACN in the crystalline lattice.
On the other hand, use of FN in place of TCNE did
not affect at all ΦR and the cisoid/ transoid ratio of 1.
Therefore, ACN+• and FN-• do not participate in the
dimerization because of the slightly positive ∆GET value
for ET in ACN-FN.
In contrast to direct excitation of ACN, selective
excitation of the CT complex between ACN and TCNE
in DCE and AN did not result in any reaction. Excitation
of the CT complex results in formation of CIP which
undergoes very rapid BET preceding dissociation to free
radical ions through SSIP since the CIP lies only 1.34 V
above the ground state.
It is noteworthy that CT excitation of the 1:1 crystalline
CT complex between ACN and TCNE afforded 2 as the
sole product in contrast to ineffective CT excitation in
solution. This distinctive reactivity and selectivity for
product distribution in the crystalline CT complex is
attributed to proximity and rigid mobility of the alkenic
CdC double bonds of ACN and TCNE in ACN‚TCNE,
which retards the deactivation and BET but enables the
two alkenic parts to undergo cycloaddition. Formation
of ACN+• as an intermediate was also confirmed by
formation of cisoid-1 and transoid-1 on CT excitation of
the mixed powder of ACN and TCNE.
Formation of a lower yield of 2 and higher yields of
cisoid-1 and transoid-1 on direct excitation of ACN in the
mixed powder than on CT excitation shows contribution
of addition of the S1 or T1 state to the ground state of
ACN to production of 1 which proceeds competitively with
the radical ion pathway in the mixed powder due to the
loose and random structure of the mixed powder. A lower
cisoid/ transoid ratio of 0.20 than that in the CT excita-
tion, which is close to that for the dimerization in the
topochemically controlled crystal of ACN in the absence
of TCNE (Table 4),44,45 clearly indicates that the excited
state of ACN adds to ACN in these conditions.
Exp er im en ta l Section
Con clu sion s
Gen er a l. Common analytical instruments (melting point,
1H NMR, 13C NMR, UV, EI-MS, GC) are the same as in our
previous papers.21 Flash column chromatography and thin-
layer chromatography (TLC) were performed on silica gel as
described elsewhere.53 Microanalyses were carried out in the
microanalytical laboratory of our school.
Ma ter ia ls a n d Solven ts. Purification of ACN has been
described elsewhere.21 TCNE (Tokyo Kasei) was purified by
recrystallization from 1,2-dichlorobenzene followed by subli-
mation to give colorless prisms with mp 200.5-202.0 °C in a
sealed capillary (lit.26 mp 200-202 °C), which was analytically
pure. No trace of impurity was detected on 13C NMR spec-
trum. FN (Tokyo Kasei) was recrystallized from methanol.
Potassium ferrioxalate was synthesized according to the
literature.54 Spectral-grade DCE and AN (Dojin) as solvents
for photolysis were used as supplied.
Ap p a r a tu s for P h otolysis. Apparatus kit for preparative-
scale irradiation in solution has been described elsewhere.21,53,55
As the light source, a high-pressure mercury lamp was
employed with color-glass filters. For isolation of light of
wavelength longer than 300 nm, a jacket made of Pyrex glass
was employed in stead of a quartz one. For reaction in solid
state, a Pyrex vessel, in which disk color-glass filters with
samples and argon gas are sealed, was used in a thermostat.
Merry-go-round apparatus for analytical-scale irradiation
is described elsewhere.55 For irradiation with light of wave-
length longer than 400 and 500 nm, additional color-glass
filters, HOYA L-42 (corresponds to Corning 3389) and HOYA
Y-50 (corresponds to Corning 3486), were used, respectively.
Temperature of sample solutions was maintained at 20 ( 2
°C in a thermostat.
Ir r a d ia tion of ACN in th e P r esen ce of TCNE or F N.
As a typical run, irradiation of ACN in the presence of TCNE
in DCE will be described. A solution of 259 mg (1.70 mmol)
of ACN and a solution of 218 mg (1.70 mmol) of TCNE in 170
mL of DCE was mixed and purged with nitrogen for 30 min
prior to the irradiation. This solution was irradiated with
>300-nm light at ambient temperature for 12 h. Bubbling of
nitrogen gas was continued during the irradiation. After
This work has revealed unique features of photoin-
duced ET and the behavior of the resulting radical ions
(ACN+• and TCNE-•) in the ACN-TCNE system in
solution and solid state. Participation of two distinct
radical ion pairs was confirmed depending on the mode
of excitation: the CIP from the CT excitation of the CT
complex and the SSIP in the direct excitation of ACN.
Direct irradiation of ACN in solution in the presence
of TCNE produces the SSIP, which gives cisoid-1 and
transoid-1 as the major products together with 2-4 as
minor products.
At low [ACN], the ΦR and the cisoid/ transoid ratio for
the dimerization of ACN steeply increased and leveled
off with increase of [TCNE]. Under this condition, the
T1 state of ACN undergoes ET to TCNE to generate the
triplet SSIP competing with addition to the ground state
of ACN. The dimerization exclusively proceeds via the
triplet SSIP at high [TCNE] corresponding to the plateau
region of ΦR and the cisoid/ transoid ratio. The triplet
SSIP dissociates to free radical ions, ACN+• and TCNE-•
and the former reacts with the ground-state ACN to give
,
a dimeric radical cation, cisoid-1+• and transoid-1+•
,
followed by charge recombination with TCNE-• to give
cisoid-1 and transoid-1, respectively. The total efficiency
(xT) from the T1 state to afford 1 can be obtained as 0.023
and 0.11 in DCE and AN, respectively.
At high [ACN], increase of [TCNE] reduced the ΦR to
an almost constant value but increased the cisoid/
transoid ratio up to a nearly plateau value. In this
condition the S1 state of ACN undergoes ET to TCNE to
afford the singlet SSIP competing with deactivation to
the ground state. The fraction of survival (xS) from the
singlet SSIP to 1 is estimated to be 0.017 and 0.015 in
DCE and AN, respectively.
A cycloadduct (2) results from geminate reaction in the
SSIP as well as from coupling of free ACN+• and TCNE-•
Formation of 3 and 4 only at high [ACN] is attributed to
a preferred interaction of an ACN molecule with a singlet
SSIP.
.
(53) Haga, N.; Takayanagi, H. J . Org. Chem. 1996, 61, 735-745.
(54) Kuhn, H. J .; Braslavsky, S. E.; Schmidt, R. Pure. Appl. Chem.
1989, 61, 188-210.
(55) Haga, N.; Kuriyama, Y.; Takayanagi, H.; Ogura, H.; Tokumaru,
K. Photochem. Photobiol. 1995, 61, 557-562.

5384 J . Org. Chem., Vol. 63, No. 16, 1998
Haga et al.
evaporation of the solvent under reduced pressure, the residual
solid (428 mg) was flash-chromatographed (silica gel, eluent:
hexane), which gave 48.1 mg (18.6%) of ACN, 25.2 mg (9.7%)
of cisoid-1, and 39.3 mg (15.2%) of transoid-1. Further elution
(hexane/dichloromethane, 2:1 then 1:1) gave 32.2 mg (8.8%)
of the 3, 19.5 mg (5.3%) of 4, and 14.2 mg (3.0%) of 2.
Irradiation in AN gave a similar result.
The five products decompose in this irradiation condition,
which caused low yield for them. Light of wavelength longer
than 400 nm was used in an analytical scale, and the product
distribution was determined by 1H NMR without isolation.
The results are summarized in Table 2.
8a ,t-6b,c-6c,c-8a ,6b,6c,12b-Tetr a h yd r o[c,e]d ia cen a p h -
th ylen e-13,13,14,14-tetr a ca r bon itr ile (3): recrystallized
from dichloromethane-hexane, colorless prisms, mp 263.0-
269.0 °C (dec.) EI-MS: m/e 432 (M+). 1H NMR (CDCl3): δ
7.82-7.95 (m, 5H), 7.77 (d, 1H, J ) 8.2 Hz), 7.60-7.73 (m,
4H), 7.40 (dd, 1H, J ) 8.2, 7.0 Hz), 7.28 (dd, 1H, J ) 7.0, 1.3
Hz), 5.28 (dd, 1H, J ) 6.2, 5.3 Hz), 4.69 (d, 1H, J ) 6.2 Hz),
4.69 (dd, J ) 10.8, 5.3 Hz), 4.28 (d, 1H, J ) 10.8 Hz). 13C
NMR (CDCl3): δ 138.56, 137.54, 137.17, 137.01, 136.70,
134.74, 131.73, 131.67, 128.62, 128.42, 128.32 × 2, 127.62,
126.36, 125.44, 122.73, 122.08, 120.42, 119.46, 111.87, 110.87,
109.31, 109.22, 52.14, 47.93, 47.34, 47.20, 44.46, 43.29. Anal.
Calcd for C30H16N4: C, 83.32; H, 3.73; N, 12.95. Found: C,
83.52; H, 3.78; N, 12.97.
P r ep a r a tion of th e Cr ysta llin e CT Com p lex of ACN
a n d TCNE. An equimolar solution of ACN and TCNE in ethyl
acetate, which was prepared independently, was mixed. The
solution was allowed to stand in the dark at ambient temper-
ature for several days to evaporate the solvent slowly. Tan-
brown prisms obtained were collected by filtration and dried
in vacuo. Other solvents except ethyl acetate gave worse
results. ACN‚TCNE: mp 103.0-105.0 °C. Anal. Calcd for
C18H8N4: C, 77.13; H, 2.88; N, 19.99. Found: C, 77.10; H,
3.05; N, 20.19.
Ir r a d ia tion of th e Cr ysta llin e CT Com p lex. The crys-
talline CT complex (40.0 mg) was placed between two disk
glass filters (Y-50) with a diameter of 120 mm and was
irradiated for 7.0 h under argon atmosphere. The temperature
was maintained at 20.0 ( 1.0 °C in a thermostat by circulating
purified water. The resulting mixture was isolated by flash-
chromatography (eluent: hexane then hexane-dichloro-
methane, 1:1), which gave 15.2 mg (74.6%) of ACN and 8.2
mg (80.7%) of 2 based on ACN consumed. When ACN was
excited directly in the crystalline CT complex, L-42 filter was
used instead of Y-50 filter.
In the case of irradiation of the mixed powder, an equimolar
mixture of ACN and TCNE was pulverized in a mortar made
of agate to develop tan-brown colorization. This powder was
placed on the color-glass filter and irradiated in a similar
manner as described above. The quantum yield for the solid-
state reaction was determined according to Ito et al.56
X-r a y Cr ysta llogr a p h ic An a lysis. A single crystal with
dimensions of 0.3 × 0.3 × 0.2 mm (3), 0.2 × 0.2 × 0.2 mm (7),
and 0.4 × 0.6 × 0.3 mm (ACN‚TCNE) was used for the
analysis. Crystals of 3 undergo gradual crumbling on stand-
ing, so the single crystal was coated with adhesion during the
data collection. The cell dimension and diffraction intensities
were measured on a Rigaku AFC5R diffractometer using
graphite monochromatic Cu Ka radiation (λ ) 1.541 78 Å) and
12 kW rotating anode generator at 23 °C. The data were
collected using the ω - 2θ scan technique in the range of 2θ
< 140.1°, 140.1°, and 126.5, for 3, 7, and ACN‚TCNE,
respectively. Scans of (1.10 + 0.30 tan θ)° at a speed of 32.0°
min-1, (1.78 + 0.30 tan θ)° at a speed 8.0° min-1, and (1.78 +
0.30 tan θ)° at a speed of 32.0° min-1 were made for 3, 7, and
ACN‚TCNE, respectively. Total reflections of 4716, 1244, and
2813 were collected for 3, 7, and ACN‚TCNE for Lorentz and
polarization factors but not for absorption. The structures
were elucidated by a direct method using TEXSAN.57 At the
final stage, the non-hydrogen atoms were refined anisotropi-
cally by the full-matrix least-squares refinement. A difference
Fourier synthesis was calculated, and the positions of all
hydrogen atoms were found and refined isotropically. Details
of the crystal data are summarized in Table 1.
8a ,t-6b,t-6c,c-8a ,6b,6c,12b-Tetr a h yd r o[c,e]d ia cen a p h -
th ylen e-13,13,14,14-tetr a ca r bon itr ile (4): recrystallized
from dichloromethane, pale-brown prisms, mp 284.0-292.0 °C
(dec.) EI-MS: m/e 432 (M+). 1H NMR (CDCl3): δ 8.00 (d, 1H,
J ) 7.0 Hz), 7.91-7.96 (m, 2H), 7.80-7.90 (m, 4H), 7.63-7.79
(m, 5H), 4.71 (dd, J ) 10.0, 6.6 Hz), 4.64 (d, J ) 6.7 Hz), 4.50
(d, J ) 10.5 Hz), 4.36 (dd, J ) 10.5, 10.0 Hz). Anal. Calcd for
C
30H16N4: C, 83.32; H, 3.73; N, 12.95. Found: C, 83.56; H,
3.75; N, 12.94.
6b,8a -Dih yd r ocyclobu t[a ]a cen a p h th ylen e-7,7,8,8-tet-
r a ca r bon itr ile (2): recrystallized from dichloromethane,
colorless prisms, slightly colored at 194 °C and decomposed
at 293.0 °C in a sealed tube (lit.36 dec. at 186 °C). EI-MS: m/e
280 (M+). 1H NMR (CDCl3): δ 7.96 (d, 2H, J ) 8.4 Hz), 7.73
(t, 2H, J ) 8.4 Hz), 7.65 (d, 2H, J ) 8.4 Hz), 5.13 (s, 2H). Anal.
Calcd for C18H8N4: C, 77.13; H, 2.88; N, 19.99. Found: C,
77.03; H, 3.12; N, 19.89.
Irradiation of ACN in the presence of FN was done in a
similar manner as described above.
6b,t-7,t-8,c-6b,8a -Dih yd r ocyclobu t[a ]a cen a p h th ylen e-
7,8-d ica r bon itr ile (5): recrystallized from dichloromethane-
hexane, colorless needles, mp 186-187 °C. EI-MS: m/e 230
(M+). 1H NMR (CDCl3): δ 7.79 (d, 2H, J ) 8.0 Hz), 7.58 (dd,
2H, J ) 8.0, 7.1 Hz), 7.46 (d, 2H, J ) 7.1 Hz), 4.70 (d, 2H, J
) 4.0 Hz), 3.3.41 (dd, 2H, J ) 4.0, 1.4 Hz). Anal. Calcd for
C
16H10N2: C, 83.46; H, 4.38; N, 12.17. Found: C, 83.55; H,
Ack n ow led gm en t. This work was partly supported
by a Grant-in Aid for Scientific Research (No. 08640696,
N.H.) from the Ministry of Education, Science, and
Culture, J apan, and Kitasato University Research
Grant for Young Researchers.
4.52; N, 12.04.
6b,t-7,c-8,t-6b,8a -Dih yd r ocyclobu t[a ]a cen a p h th ylen e-
7,8-d ica r bon itr ile (6): recrystallized from ethanol, colorless
fine fibers, mp 314-319 °C (dec. in a sealed tube). EI-MS:
m/e 230 (M+). 1H NMR (CDCl3): δ 7.83 (d, 1H, J ) 8.2 Hz),
7.78 (d, 1H, J ) 8.2 Hz), 7.65 (dd, J ) 8.0, 7.0 Hz), 7.52-7.57
(m, 2H), 7.40 (d, 1H, J ) 7.0 Hz). Anal. Calcd for C16H10N2:
C, 83.46; H, 4.38; N, 12.17. Found: C, 83.70; H, 4.47; N, 12.06.
6b,c-7,c-8,c-6b,8a -Dih yd r ocyclobu t[a ]a cen a p h th ylen e-
7,8-d ica r bon itr ile (7): recrystallized from acetone, colorless
prisms, mp 228-230 °C. EI-MS: m/e 230 (M+). 1H NMR
(CDCl3): δ 7.83 (d, 2H, J ) 8.2 Hz), 7.63 (dd, 2H, J ) 8.2, 7.0
Hz), 7.46 (d, 2H, J ) 7.0 Hz), 4.60 (d, 2H, J ) 10.0 Hz), 4.23
(d, 2H, J ) 10.0 Hz). Anal. Calcd for C16H10N2: C, 83.46; H,
4.38; N, 12.17. Found: C, 83.72; H, 4.54; N, 12.26.
Qu a n tu m Yield s for P h otod im er iza tion of ACN in th e
P r esen ce of TCNE or F N on Dir ect Excita tion of ACN.
Monochromatic light for measurement of quantum yields of
435.8 and 546.1 nm light has been described elsewhere.21
Detailed procedure for the quantum yields measurements is
similar as in the case our previous report employing ferriox-
alate actinometry.21 Irradiation time was controlled to keep
the conversion low.
Registry Numbers: ACN, 208-96-8; TCNE, 670-54-2;
cisoid-1, 15065-28-8; transoid-1, 14620-98-5; 2, 35427-
85-1; FN, 764-42-1.
Su p p or tin g In for m a tion Ava ila ble: Positional param-
eters, B(eq), bond distances, bond angles, and torsion or
conformation angles for 3, 7, and ACN‚TCNE (27 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
J O9801824
(56) Ito, Y.; Matsuura, T.; Fukuyama, K. Tetrahedron Lett. 1988,
29, 3087-3090.
(57) TEXRAY Structure Analysis Package; Molecular Structure
Corp., 1985.