Enhancement of intramolecular photocycloaddition of bichromophoric compounds via inclusion in low-density polyethylene films

Article abstract of DOI:10.1021/jo990246q

The photocycloaddition of tetra- and penta(ethylene glycol) labeled at the chain terminals with 2-naphthoyl groups (N-P(n)-N; n = 4 or 5) and tetra(ethylene glycol) terminated by 7-(4-methylcoumarinyl) groups (C-P₄-C) has been investigated in h

Full text of DOI:10.1021/jo990246q

5156
J . Org. Chem. 1999, 64, 5156-5161
En h a n cem en t of In tr a m olecu la r P h otocycloa d d ition of
Bich r om op h or ic Com p ou n d s via In clu sion in Low -Den sity
P olyeth ylen e F ilm s
Chen-Ho Tung,* Zhen-Yu Yuan, and Li-Zhu Wu
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China
Richard G. Weiss
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227
Received February 9, 1999
The photocycloaddition of tetra- and penta(ethylene glycol) labeled at the chain terminals with
2-naphthoyl groups (N-Pn-N; n ) 4 or 5) and tetra(ethylene glycol) terminated by 7-(4-methylcou-
marinyl) groups (C-P4-C) has been investigated in hexane solutions and in low-density polyethylene
(LDPE) films. At loading levels less than 1 × 10-2 mol/g-film, irradiation of the compounds in LDPE
leads to formation of intramolecular photocyclomers to the exclusion of intermolecular products.
Irradiations in hexane containing 1 × 10-3 M of a bichromophoric compound produce large amounts
of oligomeric material. The difference between the results in solution and the film is explained in
terms of the compartmentalization of the guest molecules in the LDPE amorphous regions. Cold-
stretching the LDPE films significantly increases the relative efficiency of the intramolecular
photocycloadditions due to reduction of the free volume at the reaction sites. This work reports a
new approach to synthesize large-ring compounds in high chemical yields at relatively high substrate
concentrations.
In tr od u ction
rotational diffusion and conformational changes (which
is a prerequisite for intramolecular reactions), it is
possible to synthesize large-ring compounds in high
yields under high substrate concentrations. We find that
the sizes and shapes of reaction cavities of low-density
polyethylene (LDPE) films can accommodate our bichro-
mophoric substrates and allow relatively free rotational/
conformational changes while retarding translational
diffusion. Thus, intramolecular reaction can occur with-
out competition even at high loading levels since indi-
vidual reaction cavities prefer to include only one sub-
strate molecule and diffusion between cavities is slow on
the molecular reaction time scale.
The construction of macrocyclic compounds continues
to be an important topic in synthetic organic chemistry.1,2
A molecule with two reactive groups separated by a
flexible link may undergo either intramolecular or in-
termolecular reactions. Intramolecular reaction gives
macrocyclic ring-closure products, while intermolecular
reaction results in dimers, oligomers, and polymers.
Thus, cyclization suffers from the competition from
intermolecular reactions. The rates of the latter are
dependent on substrate concentration, while those of the
former are not, since the effective concentration for
intramolecular reaction is constant by virtue of the tether
between the two functional groups. Hence, in normal
solutions where diffusion is rapid, high substrate con-
centrations favor polymerization while cyclization occurs
in good chemical yields only at low concentrations.
LDPE is a very complex family of materials that consist
of ca. 50% of crystalline regions (wherein chains are
tightly and regularly packed) and ca. 50% of amorphous
or interfacial regions (wherein chains are in somewhat
random conformations).3 The glass transition of the
amorphous part occurs near -30 °C, and the melting
transition is >100 °C.4 A wide variety of organic mol-
ecules can be incorporated into LDPE by soaking its films
in a swelling solvent containing the guest. It has been
established that guest molecules are excluded from the
crystalline portions of LDPE at temperatures below the
melting transition. Their principal locations are the
amorphous parts and the interfacial regions between
crystalline and amorphous domains.3b,d Due to its aniso-
The working definitions of “high” and “low” concentra-
tion ranges are also dependent upon solvent viscosity (i.e.,
solute diffusion rates). In a certain reaction medium, if
the rates of translational diffusion (which is a prerequi-
site for intermolecular reactions) of the substrate mol-
ecules can be attenuated much more than those for
(1) (a) Evans, P. A.; Holmers, A. B. Tetrahedron 1991, 47, 9131. (b)
Illuminati, G.; Mandolini, L. Acc. Chem. Res. 1981, 14, 95. (c)
Griesbeck, A. G.; Henz, A.; Hirt, J . Synthesis 1996, 11, 1261. (d)
Mandolini, L. In Advanced Physical Organic Chemistry; Gold, V.,
Bethell, D., Eds.; Academic Press: London, 1986; Vol. 22, pp 1-112
and references therein. (e) Lehn, J . M. Supramolecular Chemistry;
VCH: Weinheim, 1995. (f) Vo¨gtle, F., Ed. Top. Curr. Chem. 1983, 113.
(g) Weber, E., Vogtle, F., Eds. Top. Curr. Chem. 1991, 161. (h) Proter,
N. W. J . Am. Chem. Soc. 1986, 108, 2787.
(3) (a) Peterlin, A. Macromolecules 1980, 13, 777. (b) Phillips, P. J .
Chem. Rev. 1990, 90, 425 and references therein. (c) Axelson, D. E.;
Levy, G. C.; Mandelkern, L. Macromolecules 1979, 12, 41. (d) J ang, Y.
T.; Phillips, P. J .; Thulstrup, E. W. Chem. Phys. Lett. 1982, 93, 66.
(4) Quirk, R. P.; Alsamarraie, M. A. A. In Polymer Handbook;
Brandup, J ., Immergut, E. H., Eds.; Wiley: New York, 1989; p V/15.
(2) J iang, X. K.; Hui, Y. Z.; Fei, Z. X. J . Chem. Soc., Chem. Commun.
1988, 689.
10.1021/jo990246q CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/22/1999

Photocycloaddition of Bichromophoric Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5157
Sch em e 1
tropic nature, LDPE has been used as a reaction medium
to control the reaction pathways of a variety of guest
molecules.5,6 Although several interesting examples in
this topic have appeared,5-7 we are aware of only one
example in which it is used to enhance the formation of
large-ring compounds.7a The sites where guest molecules
reside need not resemble those in the native polymer.
The swelling process by which species are introduced
“opens” the network of polymethylene chains in the
amorphous and interfacial regions; when the swelling
liquid is removed, the nearby chains move into van der
Waals contact with the reactive guest molecules that
have been left behind. In this way, the free volume of
the reaction sites can be made sufficiently large8 to accept
a molecule with two reactive groups linked by a flexible
chain,9 yet small enough to force the two terminal groups
in proximity. Furthermore, according to a Poisson dis-
tribution, there is less than a 5% probability to find two
guests in one cavity if the loading level is less than one
guest molecule per 10 reaction cavities. As long as this
condition is met and the rate of site exchange by guests
is slower than that of conformational changes of the guest
molecule, intermolecular reactions should be inhibited,
while intramolecular reactions still can occur.10 We report
that this is indeed the case for the photocyclizations of
tetra- and penta(ethylene glycol) terminated by 2-naph-
thoyl groups (N-Pn-N) or tetra(ethylene glycol) termi-
nated by 7-(4-methylcoumarinyl) groups (C-P4-C).11
Np-COO-(CH2CH2O)n-OC-Np
N-Pn-N, (n ) 4, 5; Np ) 2-naphthyl)
Cu-O(CH2CH2O)n-Cu
C-P4-C, (Cu ) 7-(4-methylcoumarinyl))
Resu lts a n d Discu ssion
Gen er a l. LDPE films, from Du Pont of Canada, have
been characterized in detail.12 Incorporation of a sub-
strate into a film was achieved by soaking in a chloroform
solution. After several hours, the solvent was removed,
and the film was washed with methanol (a nonswelling
liquid) to remove surface-occluded substrate. Loading
levels of ca. 1 × 10-3 mol/g-film for N-Pn-N and 1 × 10-2
mol/g-film for C-P4-C were attained. The films were then
transferred to a Pyrex vessel and irradiated under a
nitrogen atmosphere. Generally, to complete the conver-
sion required 5 h of irradiation for N-Pn-N and 10 h for
C-P4-C; in hexane, the conversions of analogous molar
concentrations and absolute numbers of moles were
accomplished after about 3 h irradiation for N-Pn-N and
5 h for C-P4-C. After irradiation in the films, the products
were extracted with chloroform and analyzed by HPLC.
Material balance in general was greater than 90%.
P h otocycloa d d ition of N-P n -N. Irradiation of alkyl
2-naphthoates is known to yield a “cubane”-like photo-
cyclomer as the unique product (Scheme 1), although six
isomeric cyclomers are formally possible.11,13 This selec-
tivity originates from two restrictions. First, the photo-
cycloaddition occurs only between the substituted rings.
Second, in the cyclomer, the substituents are in a head-
to-tail orientation. Irradiation of N-Pn-N in organic
solvents, such as hexane, can lead either to intra- or
(5) Weiss, R. G. Spectrum 1994, 7(4), 1.
(6) (a) Weiss, R. G.; Ramamurthy, V.; Hammond, G. S. Acc. Chem.
Res. 1993, 26, 530. (b) Ramamurthy, V.; Weiss. R. G.; Hammond, G.
S. In Advances in Photochemistry; Volman, D. H., Hammond, G. S.,
Neckers, D. C., Eds.; Wiley: New York, 1993; Vol. 18, p 67.
(7) (a) Ramesh, V.; Weiss, R. G., Macromolecules 1986, 19, 1489.
(b) Cui, C.; Weiss, R. G. J . Am. Chem. Soc. 1993, 115, 9820. (c) Aviv,
G.; Sagiv, J .; Yogev, A. Mol. Cryst. Liq. Cryst. 1976, 36, 349. (d) Li, S.
K. L.; Guillet, J . E. Polym. Photochem. 1984, 4, 21.
(8) (a) J enkins, R. M.; Hammond, G. S.; Weiss, R. G. J . Phys. Chem.
1992, 96, 496. (b) He, Z.; Hammond, G. S.; Weiss, R. G. Macromolecules
1992, 25, 1568.
(9) Zimerman, O. E.; Weiss, R. G. J . Phys. Chem. 1998, 102A, 5364.
(10) Assuming that the substrate molecules are randomly distrib-
uted over the reaction cavities in an LDPE film (but not the total
volume), their distribution can be expressed by the Poisson equation.
(11) (a) Tung, C. H.; Li, Y.; Yang, Z. Q. J . Chem. Soc., Faraday
Trans. 1994, 90(7), 947. (b) Tung, C. H.; Wang, Y. M. J . Am. Chem.
Soc. 1990, 112, 6322. (c) Tung, C. H.; Xu, C. B. In Photochemistry and
Photophysics; Rabek, J . F., Ed.; CRC Press: Boca Raton, 1991; Vol. 4,
Chapter 3, p 177. (d) Tung, C. H.; Wu, L. Z.; Yuan, Z. Y.; Su, N. J . Am.
Chem. Soc. 1998, 120, 11594.
(12) Zimerman, O. E.; Cui, C.; Wang, X.; Atvars, T. D. Z.; Weiss, R.
G. Polymer 1998, 39, 1177.
(13) (a) Collin, P. J .; Roberts, D. B.; Sugowdz, G.; Wells, D.; Sasse,
W. H. F. Tetrahedron Lett. 1972, 321. (b) Kowala, C.; Sugowdz, G.;
Sasse, W. H. F.; Wunderlich, J . A. Tetrahedron Lett. 1972, 4721. (c)
Teitei, T.; Wells, D.; Sasse, W. H. F. Aust. J . Chem. 1976, 29, 1783.
P(n) ) λn exp(-λ)/n!
λ is the ratio of the number of substrate molecules to the number of
occupiable cavities, and P(n) is the probability of n substrate molecules
being in a cavity. In the case of one substrate molecule per 10 cavities
(λ ) 0.1), P(1) and P(2) are 0.09 and 4.5 × 10-3, respectively. Thus,
the probability to find one substrate molecule in one cavity is 20 times
that to find two substrate molecules in one cavity. It follows that, on
a purely statistical basis, the amount of intermolecular photodimer
will be less than 5% of the intramolecular cyclomer.

5158 J . Org. Chem., Vol. 64, No. 14, 1999
Tung et al.
Ta ble 1. 1H NMR Da ta a n d Str u ctu r a l Assign m en ts of
N-P 5-N
δ (ppm)
peak multiplicity
integration
assignment
6.95-7.00
4.52
4.39
4.25
3.86
m
d
dd
m
d
8H
2H
2H
4H
2H
Ar
Ha
Hc
Hd
Hb
He
F igu r e 1. Absorption spectra of N-P4-N in LDPE as a function
of irradiation time. Loading level: 1 × 10-3 mol/g-film: s, 0
h; - - -, 1 h; ‚‚‚, 2 h, -‚-, 5 h.
3.6-3.8
m
16H
1
able to the naphthyl group were detected in the H NMR
spectra of the products. The structure proposed for the
1
cubane-like photocyclomer rests mainly on its H NMR
spectrum, which is similar to that reported.11,13 The
spectral details and assignments for the intramolecular
photocyclomer of N-P5-N are given in Table 1, and those
for N-P4-N are given in the Experimental Section.
Due to the immiscibility of the polyether chains of the
N-Pn-N with polyethylene, the maximal loading of N-Pn-N
in LDPE film we could reach was ca. 1.2 × 10-3 mol/g-
film. Since the LDPE films employed are 42% crystal-
line,12 the actual volume in which the guest molecules
reside is only ca. 60% of the total. As a result, the true
concentrations in the amorphous regions are at least 1.7
× 10-3 mol/g-film. Despite this, the exclusive formation
of intramolecular photocyclomers suggests that each
occupied site contains one molecule of N-Pn-N.
To obtain further information on the cavities experi-
enced by the N-Pn-N, their fluorescence spectra were
examined. At the loading level of 1 × 10-3 mol/g-film,
the N-Pn-N show monomeric and weak excimer emis-
sions. A model compound, ethyl 2-naphthoate, gives only
monomer emission under analogous conditions. The
excimer/monomer fluorescence intensity ratios for the
N-Pn-N are independent of concentration, indicating that
the excimer is intramolecular. Since the rate of transla-
tional diffusion of the naphthoate groups in the LDPE
films is slow on the time scale of the decay of the excited
singlet states,5,8,14 the excimers probably form from
chromophores that are at or near excimer-like conforma-
tions prior to excitation. On the basis of the relative
weakness of the intramolecular excimer emissions, we
propose that only a small fraction of N-Pn-N molecules
have their naphthoyl groups in an excimer-like confor-
mation in the ground state at any given time; the
chromophores of most N-Pn-N molecules do not overlap
and provide monomer fluorescence upon excitation. How-
ever, two naphthoyl groups may diffuse into proximity
on longer time scales. When one of them is excited,
intramolecular photocycloaddition occurs. In this way,
long-term irradiation of N-Pn-N in a film can eventually
lead to complete conversion.
F igu r e 2. HPLC chromatogram of N-P4-N after 2 h irradia-
tion in LDPE.
intermolecular cycloadditions. At concentrations higher
than 1 × 10-3 M, the main product is intermolecular
photocyclomers.
By contrast, irradiation of N-Pn-N in LDPE films yields
only intramolecular photocyclomers. The progress of the
photocycloadditions was followed by UV absorption spec-
troscopy. Figure 1 shows changes in the absorption
spectra of N-P4-N as a function of irradiation time;
similar results were obtained when N-P5-N was the guest
molecule. Evidently, after 5 h of irradiation, the conver-
sion approached 100%. The products were extracted from
the LDPE film with chloroform and analyzed by HPLC.
Figure 2 is the HPLC profile of the products during
irradiation of N-P4-N. Only the intramolecular ring-
closure photocyclomers were evident; no intermolecular
products were detected. The isolated yields of intramo-
lecular products were >90% on the basis of consumption
of the starting materials.
The conclusion that the photoreactions are almost
exclusively intramolecular is based on the observation
that m/z values of the molecular ions in the mass spectra
of photoproducts are identical to those of the correspond-
ing starting materials. Furthermore, no protons assign-
(14) (a) Lu, L.; Weiss, R. G. Macromolecules 1994, 27, 219. (b) He,
Z.; Hammond, G. S.; Weiss, R. G. Macromolecules 1992, 25, 501. (c)
Naciri, J .; Weiss, R. G. Macromolecules 1989, 22¸ 3928.

Photocycloaddition of Bichromophoric Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5159
Ta ble 2. Con ver sion of N-P 4-N in Str etch ed a n d
Un str etch ed LDP E F ilm s a s a F u n ction of Ir r a d ia tion
Tim e (Loa d in g Level: 1 × 10-3 m ol/g-film )a
tramolecular cyclomers. Steric factors introduced in the
corresponding molecule by methyl groups at the 4-posi-
tions of the coumarins led predominantly the syn head-
to-tail cyclomer. At higher concentrations, the amounts
of intermolecular photoproducts increase as expected.
conversion (%)
1 h
1.5 h
2 h
C-P4-C can undergo intra- and intermolecular photo-
reactions. Irradiation of <5 × 10-4 M C-P4-C in an
organic solvent such as benzene results primarily in
intramolecular syn head-to-tail cyclomer (Scheme 2), as
demonstrated by mass spectral and NMR analyses. The
m/z value of the molecular ion of the product is identical
to that of C-P4-C, and there are no protons assignable to
stretched film
unstretched film
efficiency ratio for stretched
to unstretched samples
24.7
17.6
1.40
37.9
27.4
1.38
48.3
33.5
1.44
a
Averaged on five samples; error limits were ca. 5%.
The effect of stretching LDPE films doped with N-P4-N
on its photochemical reactivity was also examined. It has
been found previously that even when one type of LDPE
is employed reproducible results are limited to films that
share the same history.5,7 Thus, we cut a doped film into
several pieces, half of which were stretched manually to
4.5 times their original length. The relative reactivities
for intramolecular photcycloadditions of the N-P4-N in
stretched and unstretched samples are indicated in Table
2. The results indicate that dimerization efficiency in the
stretched films is ca. 1.4 times greater than in the
unstretched ones. This observation has ample precedent.7
The product ratios from competitive intramolecular Pa-
terno-Buchi photocyclization, and intermolecular pho-
toreduction of 10-undecenyl benzophenone-4-carboxylate
have been employed to probe the free volume changes
that occur at guest sites when LDPE films are stretched.7a
A remarkable increase in the relative yield of the pho-
tocyclization product was observed when the films were
stretched to ca. 500% their original length. Aviv and co-
workers studied the photodimerization of tetraphenylbu-
tatriene in LDPE films and compared the reactivity in
stretched and unstretched samples.7c A 3-fold increase
in quantum efficiency for photodimerization of the sub-
strate upon stretching the LDPE films was reported. The
interpretation advanced to explain these results is that
film stretching decreases the average free volume of guest
sites by aligning partially the polymethylene chains that
constitute the cavity walls. Therefore, the two photore-
active groups of our substrates will reside, on average,
closer to each other in stretched films than in unstretched
ones; intramolecular photocycloaddition will be enhanced.
P h otocycloa d d iton s of C-P 4-C. The photochemistry
of coumarin and its derivatives has been the subject of
numerous investigations, mainly as a consequence of its
importance in biological systems.15 Direct irradiation of
coumarin itself in solution results in highly inefficient
formation of the syn head-to-head and syn head-to-tail
cyclomers as the major and minor products, respectively;
triplet sensitization is more efficient and yields the anti
head-to-head cyclomer as the major product (Scheme 2).
De Schryver and co-workers16 found that irradiation of
7,7′-polymethylenedi(oxycoumarin)s at low concentra-
tions yields only syn head-to-head and head-to-tail in-
1
coumarinyl in the H NMR spectrum. The syn head-to-
tail nature of the cyclomer is based on comparison of its
1H NMR spectrum (Figure 3 and Table 3) with those of
related molecules.16 Upon irradiation of more concen-
trated solutions, a large amount of oligomeric material
was formed. For example, at a concentration of 1 × 10-3
M, ca. 20% of oligomers were present in the product
mixture.
As observed in the case of the N-Pn-N, irradiation of 1
× 10-2 mol/g-film C-P4-C in LDPE resulted in conversion
to intramolecular cyclomer. Generally, the conversion
was close to 100% after 10 h of irradiation. The products
were extracted from the LDPE films and identified by
their spectral properties and by comparison with authen-
tic samples. As in the case of irradiations in homogeneous
solutions, only the syn head-to-tail cyclomer was detected.
Since the material balance was >90%, any unidentified
products must be minor. Thus, at loading levels <1 ×
10-2 mol/g-film, each reaction cavity in LDPE must
contain no more than one C-P4-C molecule: intermolecu-
lar reaction is inhibited and intramolecular reaction is
enhanced.
We also examined the effect of stretching the LDPE
film on the photochemistry of C-P4-C. Films doped with
C-P4-C were stretched manually to 4.5 times their
original length and then irradiated. Again, at 1 × 10-2
mol/g-film C-P4-C, only the intramolecular syn head-to-
tail cyclomer was obtained. However, compared with the
unstretched sample, the efficiency of the photocycload-
dition is increased 1.2 times. This observation is consis-
tent with the hypothesis that film stretching decreases
the free volume of the reaction cavities6,7 and, thereby,
enhances intramolecular reactions by forcing the reactive
groups to reside nearer each other.
Con clu sion s
Low-density polyethylene can incorporate bichro-
mophoric molecules composed of oligo(ethylene glycol)
chains terminated by naphthyl or coumarinyl groups. At
loading levels <1 × 10-2 mol/g-film, host reaction cavities
are singly occupied, allowing the substrate molecules to
be isolated from each other. As a result, irradiation of
our guest molecules produces intramolecular photocy-
clomers exclusively; no intermolecular products could be
detected. Consistent with the hypothesis that stretching
LDPE films reduces the free volume of their reaction
sites, the time required to achieve full conversion of the
guest molecules in stretched films was shorter than the
period in unstretched ones.
(15) (a) Melo, J . S. S.; Becker, R. S.; Macanita, A. L. J . Phys. Chem.
1994, 98, 6054. (b) Hallberg, A.; Isaksson, R.; Martin, A. R.; Sandstro¨m,
J . J . Am. Chem. Soc. 1989, 111, 4387. (c) Sakellariou-Fargues R.;
Maurette, M. T.; Oliveros E.; Riviere M.; Lattes, A. Tetrahedron 1984,
40, 2381. (d) Muthuramu, K.; Ramnath, N.; Ramamurthy, V. J . Org.
Chem. 1983, 48, 1872. (e) Morrison, H.; Curtis, H.; McDowell T. J .
Am. Chem. Soc. 1966, 5415. (f) Moorthy, J . N.; Venkatesan, K.; Weiss,
R. G. J . Org. Chem. 1992, 57, 3292. (g) Lewis, F. D.; Barancyk, S. V.
J . Am. Chem. Soc. 1989, 111, 8653. (h) Lewis, F. D.; Howard, D. K.;
Oxman, J . D. J . Am. Chem. Soc. 1983, 105, 3344. (i) Muthuramu, K.;
Ramamurthy, V. J . Org. Chem. 1982, 47, 3976.
Further comments on the nature of the reaction
cavities are appropriate.6 The intrinsic size of guest sites
in LDPE should preclude any of the bichromophoric
(16) Leenders, L. H.; Schouteden, E.; De Schryver, F. C. J . Org.
Chem. 1973, 38, 957.

5160 J . Org. Chem., Vol. 64, No. 14, 1999
Tung et al.
1
F igu r e 3. 400 M Hz H NMR spectrum of the cyclomer from C-P4-C.
Sch em e 2
Ta ble 3. 1H NMR Da ta a n d Str u ctu r a l Assign m en ts of
th e Cyclom er fr om C-P 4-C
molecules in this study from entering them.17 However,
their mode of inclusion, swelling of the amorphous parts
of the polymer matrix, opens spaces inside the films large
enough for the guests to enter. When the swelling solvent
is removed selectively (by its more rapid diffusion to the
air), the guest molecules are trapped. Coincidentally, the
“walls” of the reaction cavities collapse around the guest
molecules in orientations that maximize van der Waals
contacts.
We have suggested that in the LDPE inclusion sample
the groups tethered to the ends of chains experience a
more mobile environment than in the bulk of the native
LDPE due to the disordering effect caused by guest
molecule; large guest molecules can plasticize the chains
of polyethylene that constitute the walls of the reaction
δ (ppm)
peak multiplicity
integration
assignment
cages (making them “softer”6).9 In this way, slow confor-
mational changes bring the two reactive groups at the
chain ends into sufficient proximity for them to react
within an excited-state lifetime. At any moment, the
fraction of molecules in reactive conformations may be
7.05
d
d
s
m
s
s
2H
2H
2H
16H
2H
6H
Ha
Hb
Hc
Hf
Hd
He
6.63
6.04
3.70-3.95
3.40
1.66
small, but eventually all can adopt the necessary shapes.
The key is that the rates of these conformational changes
(17) Serna, J .; Abbe, J . Ch.; Duplatre, G. Phys. Status Solidi AI
1989, 115, 389.

Photocycloaddition of Bichromophoric Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5161
(0.02 mol) of 7-hydroxy-4-methylcoumarin were added to a
solution of 5 g (0.01 mol) of the di(p-toluenesulfonyl) tetraeth-
ylene glycol in 80 mL of acetonitrile. The mixture was stirred
under reflux for 12 h. After being cooled, the solvent was
removed under vacuum to yield a solid residue. The product
(3 g, 60%) was isolated by column chromatography on silica
(ethyl acetate/petroleum ether (4:1, v/v)). 1H NMR (CDCl3)
(ppm): δ7.5 (d, 2H), 6.9 (d, 2H), 6.8 (s, 2H), 6.15 (s, 2H), 4.2
(t, 4H), 3.9 (t, 4H), 3.75 (t, 8H), 2.4 (s, 6H).
1H NMR spectra were recorded at 400 MHz with a Varian
XL-400 spectrometer. MS spectra were run on a VG ZAB
spectrometer. UV absorption spectra were measured with a
Shimadzu UV 1601PC spectrophotometer. Photocycloaddition
products were separated and analyzed using a Shimadzu LC-
10AS liquid chromatograph with a 25 cm Hyper ODS2 C18
column (eluted with methanol/water (85/15); UV detection at
254 nm).
are significantly faster than the rates at which two
molecules meet in one reaction cavity. We suspect that
polyethylene films will be useful to synthesize a large
variety of macrocycles under concentration conditions
that would lead to significant intermolecular (i.e., po-
lymerization) reactions. Since each kind of polyethylene
differs from the others in some morphological details,
exact reproducibility from laboratory to laboratory should
not be expected. However, the gross observationsthat
bichromophoric molecules can be (reactively) isolated
from each other at relatively high bulk concentrationss
should be common to all polyethylenes!
Thus, LDPE complements zeolites, which we demon-
strated earlier to serve a similar role in isolating mol-
ecules for intramolecular cyclizations.11d By contrast, the
reaction cavities of zeolites have fixed volumes and
shapes and very “hard” walls.6 Each medium has its
relative merits; now, it is possible to select the one that
is more compatible with a particular reactant and cy-
clization reaction.
In clu sion of Su bstr a tes w ith in LDP E a n d P h otoir r a -
d ia t ion . A LDPE film was immersed in a 0.2 M substrate
solution in chloroform. After equilibration was reached, the
solvent was removed, and the film surface was washed rapidly
with methanol and dried with a nitrogen stream. The substrate
content was calculated from UV absorption spectra of the
doped films, assuming Beers law behavior and molar extinction
coefficients of the guest molecules in nonpolar solvents. The
sample was placed in a Pyrex glass cell, which was purged
with nitrogen and irradiated with a 450-W Hanovia medium-
pressure mercury lamp. After the irradiation period, the
products were extracted with chloroform and analyzed by
HPLC. Structures of all products were identified by MS and
1H NMR spectra.
Intramolecular cyclomer of N-P4-N: 1H NMR (CDCl3)
(ppm): δ 6.95-6.97 (8H, m, ArH), 4.55 (2H, d, Ha), 4.42 (2H,
dd, Hc), 4.35 (4H, m, Hd), 3.92 (2H, d, Hb), 3.65-3.85 (12H, m,
He). MS: m/e 502 (M+), 199 (NpCOOC2H4+), 172 (NpCO2H+),
155 (NpCO+), 127 (Np+). Intramolecular cyclomer of C-P4-C:
1H NMR (CDCl3) (ppm): δ 7.05 (d, 2H), 6.63 (d, 2H), 6.04 (s,
2H), 3.70-3.95 (m, 16H), 3.4 (s, 2H), 1.66 (s, 6H).
Exp er im en ta l Section
Ma ter ia ls a n d In str u m en ts. Spectral-grade chloroform,
hexane, and benzene were used without further purification
as the swelling solvent for LDPE and the solvent for photo-
chemical reactions, respectively. Low-density polyethylene
films (42% crystallinity, mp 116 °C, 0.918 g/cm3, 70 µm thick12
)
were a gift from DuPont of Canada. Before use, films were
immersed overnight in chloroform to remove antioxidants and
plasticizers, washed with methanol, and dried in a stream of
nitrogen. Naphthalene end-labeled poly(ethylene glycol) (N-
Pn-N) was prepared as reported previously.11b
7-Coumarinyl end-labeled tetra(ethylene glycol) (C-P4-C)
was synthesized by the following procedure. To a mixture of 7
mL (0.04 mol) of tetraethylene glycol and 11.7 mL (0.08 mol)
of triethylamine in 100 mL of 1,2-dichloroethane was slowly
added 15.6 g (0.08 mol) of p-toluenesulfonyl chloride over 1 h.
After being stirred for 12 h at room temperature, the reaction
mixture was filtered to remove the resulting salt. The solvent
was evaporated, and the products were separated by column
chromatography on silica (ethyl acetate/petroleum ether (2:1,
v/v)); 12 g (60%) of R,ω-di(p-toluenesulfonyl)tetraethylene
glycol was obtained. 1H NMR (CDCl3) (ppm): δ7.82 (d, 4H),
7.38 (d, 2H), 4.2 (m, 4H), 3.75 (m, 4H), 3.6 (s, 8H), 2.5 (s, 6H).
Anhydrous potassium carbonate (2.8 g (0.02 mol)) and 3.5 g
Ack n ow led gm en t. The authors in the Institute of
Photographic Chemistry thank the National Science
Foundation of China and the Bureau for Basic Research
of the Chinese Academy of Sciences for financial sup-
port. R.G.W. thanks the U.S. National Science Founda-
tion for partial support of this work.
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