Photodimerization of anthracene derivatives in their neat solid state and in solid molecular compounds

Article abstract of DOI:10.1039/c0ce00739k

The Cambridge Crystallographic Data Center provides valuable data for analysis and detection of compounds with potential to undergo photodimerization in the solid-state. Such analysis is presented for anthracene derivatives, and showed that there are many

Full text of DOI:10.1039/c0ce00739k

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PAPER
Photodimerization of anthracene derivatives in their neat solid state and in
solid molecular compounds†
a
b
a
a
a
a
I. Zouev, Den-Ke Cao, T. V. Sreevidya, M. Telzhensky, M. Botoshansky and M. Kaftory*
Received 19th October 2010, Accepted 6th January 2011
DOI: 10.1039/c0ce00739k
The Cambridge Crystallographic Data Center provides valuable data for analysis and detection of
compounds with potential to undergo photodimerization in the solid-state. Such analysis is presented
for anthracene derivatives, and showed that there are many compounds that their packing in the crystal
meets the requirement needed for [4 + 4] photodimerization in the solid-state. The crystal structures of
various neat acyl anthracenes before and after exposure to UV light are described and the molecular
structures of the dimers are compared with those obtained by exposing molecular compounds
possessing a light-stable host (1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol) and light-sensitive acyl
anthracenes. While exposing neat compounds to UV light causes disintegration of the single crystal, the
reaction in the molecular compounds proceeds in a single-crystal to single-crystal mode until a certain
rate of conversion, and then disintegrates.
Introduction
guest molecule and its surrounding by changing the host mole-
3
cule, the other advantage is that in many cases the photo-
The product of a photochemical reaction in the solid state is
dependent on the constraint imposed by the neighbor molecules.
The consequence is that if the reaction takes place in the neat
compound a single product is expected, unless the reactive
compound crystallizes in different polymorphic forms. Also in
most cases the reaction was followed by destruction of the
integrity of the single crystal therefore it is not possible to follow
changes in the molecular structure. Although this draw back is
somewhat changed since the discovery that irradiating a single
crystal at the wavelength of the tail of the absorption spectrum
chemical reaction is carried out in a single-crystal to single-
crystal mode with a simple broadband irradiation.
Photodimerization in the solid state is highly dependent on the
relative position and orientation of the monomers. The
geometric criteria have been established by Schmidt in the sixties
4
of last century. Others added geometry criteria for different
5,6
types of photochemical reactions. The vast amount of crystal
structures collected in the Cambridge Crystallographic Data
7
Center (CCDC) provide the opportunity to examine an enor-
mous amount of compounds for their potential to undergo
photochemical reactions in the solid state. To what degree of
confidence we can predict that a solid compound will undergo
photochemical reaction is too early to say.
1
leads to a single-crystal to single-crystal transformation.
Carrying out a photochemical reaction on reactive molecules
embedded within molecular compounds started by the work of
2
Tanaka and Toda leads to a new and productive method for an
We wanted to use the CCDC data to examine compounds
possessing an anthracene skeleton for their photodimerization
abilities in the solid state and to compare the outcome with
respect to the molecular structure in different environments. The
search was limited to anthracene derivatives with intermolecular
distances between the potentially reactive atoms (see the struc-
alternative type of solid state photochemistry. One of the
advantages of carrying out the reaction in molecular compounds
consisting of light-sensitive guest and light-stable host molecules
is that it enables modifying the structure of the light-sensitive
˚
ture below) in the range of 3.2–4.5 A. The search yielded 128 hits.
The crystal structures of only 11 dimers of the listed monomers
are known. Definition of the parameters used for the analysis of
the mutual geometry with respect to the potential of these
compounds to undergo photochemical [4 + 4] dimerization is
given in Fig. 1. Comparison of the crystal and molecular struc-
tures of a series of closely related neat anthracene derivatives and
their molecular compounds with a light-stable host, before and
after exposure to UV light, is also described.
a
Schulich Faculty of Chemistry, Technion-Israel Institute of Technology,
Haifa, 32000, Israel. E-mail: kaftory@tx.technion.ac.il
b
State Key Laboratory of Coordination Chemistry, Coordination
Chemistry Institute, Nanjing University, Nanjing, 210093, People’s
Republic of China
†
CCDC reference numbers 812116–812121, 806062–806064. For
crystallographic data in CIF or other electronic format see DOI:
0.1039/c0ce00739k
1
4
376 | CrystEngComm, 2011, 13, 4376–4381
This journal is ª The Royal Society of Chemistry 2011

organic phase of the mixture was separated, and the aqueous
phase was further extracted with 2 ꢁ 100 mL of ether–CS (v/v ¼
2
4
3
/1). Finally, the combined organic phase was washed with
ꢁ 100 mL of 2 M HCl, 3 ꢁ 100 mL of water, and 3 ꢁ 100 mL
4
of NaCl solution, and dried with MgSO . After removal of the
solvent, the pale yellow residue was refluxed in hexane (40 mL) to
yield powder. After recrystallization in CH Cl –hexane, the
colorless needle-like crystals were collected with 89% yield.
-(9-Anthryl)ethanone (9-acetylanthracene) (1) (Scheme 1) is
2
2
8
1
commercially available and it was purchased from Sigma-
Aldrich.
Methyl 9-anthracenecarboxylate (2) (Scheme 1), n-Pr
9
-anthracenecarboxylate (3), and i-Pr 9-anthracenecarboxylate
9–11
(
4) were synthesized according to known procedures.
Compounds 1, 3, and 4 were crystallized from ethanol,
yielding yellow prism crystals. Crystallization of i-Pr 9-anthra-
cenecarboxylate (4) yielded two additional unknown polymorphs
Fig. 1 Relevant geometric parameters.
(
4b and 4c). The dimers of n-Pr 9-anthracenecarboxylate (3) and
i-Pr 9-anthracenecarboxylate (4) (labeled as 3d and 4d respec-
tively) were crystallized from ethyl acetate, yielding colorless
prism crystals. The molecular compound composed of 1,1,6,6-
tetraphenyl-2,4-hexadiyne-1,6-diol (I) and n-Pr 9-anthracene-
carboxylate (3) (labeled as II-3) was crystallized from a mixture
of ethyl acetate and heptane yielding yellow block crystals.
Irradiation
Irradiation of single crystals was carried out with a high power
UV LED (Rapp OptoElectronic) system with a wavelength of
3
65 nm. Powder illumination was carried out in sunlight for 8 h.
Crystal structure determination
X-Ray diffraction intensities were measured by a KappaCCD
˚
Nonius Diffractometer (Mo Ka, l ¼ 0.71073 A). The crystal
Experimental section
12
structures were solved by direct methods (SHELX97). All non-
hydrogen atoms were refined anisotropically. The positions of
the hydrogen atoms were located in difference Fourier maps, but
refined in calculated positions by riding on their parent atoms.
The experimental data for crystal structure determination are
given in Table 1.
Synthesis
,1,6,6-Tetraphenyl-2,4-hexadiyne-1,6-diol (I) (Scheme 1)—
1
Cu(OAc) (5 mmol, 0.9 g) was dissolved in 60 mL methanol–
2
pyridine (1 : 1), and then the solution of 1,1-diphenylpropyn-1-ol
(
purchased from Sigma-Aldrich) (10 mmol, 2.08 g) in 60 mL of
methanol–pyridine (purchased from Sigma-Aldrich) (1 : 1) was
ꢀ
Results and discussion
dropped with stirring in an oil bath (63 C). After the mixture
was stirred for 10 h, it was cooled to room temperature. The
solvent was evaporated, and the resulting residue was mixed with
The geometric parameters served in the analysis are shown in
Fig. 1. They consist of the following: the distance between the
potentially reacting atoms (d), the distance between the mean
planes (p), the lateral distance between the p-orbitals of the
reacting atoms (l), and (t) the angle between (d) and (p). The
histograms of the four parameters are given in Fig. 2, omitting
compounds where the two molecules are not symmetry related
and therefore their mean planes are not absolutely parallel. There
are 103 hits of such systems in the CCDC. Two main prerequisite
geometric conditions are needed to ensure a successful photo-
chemical [4 + 4] dimerization of the anthracene derivatives; the
distances between the potentially reacting atoms and the degree
of overlap between the p-orbital of these atoms. There are 91 hits
of compounds in which the distances between the potentially
1
00 mL of ether–CS
2
(v/v ¼ 4/1) and 250 mL of 2 M HCl. The
˚
reactive atoms are meeting the limit of 4.2 A set up by Schmidt.
4
Scheme 1
We may evaluate the limit of the lateral shift (l) between the
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 4376–4381 | 4377

p-orbitals by examining these values in anthracene derivatives
that are known to undergo [4 + 4] dimerization. The largest
˚
lateral shift of 1.904 A was observed in dimethyl 9-anthrylme-
13
thylphosphonate (CCDC refcode MAHNEO). In 71 hits the
lateral shifts of the p-orbitals were found to be less than this limit.
Can we expect that all 71 compounds will undergo [4 + 4]
photodimerization in the solid upon exposure to UV light?
Certainly not, because there are other factors that have to be
considered such as the bulkiness and the electronic properties of
the substituents at atoms C1 and C8 (Scheme 1). We do not
intend to examine the assumption that most of the compounds
will undergo photodimerization if they will be subjected to
irradiation. We have selected a series of known and new
anthracene derivatives (see 1–4 in Scheme 1) crystallized their
neat phases as well their molecular compounds with 1,1,6,6-tet-
raphenyl-hexa-2,4-diyne-1,6-diol (I in Scheme 1) as the host
molecule and compared their behavior under exposure to UV
light. The crystal structures of some of the compounds have been
published before, however, we have repeated the crystal structure
determination of few of them.
Neat compounds
The crystal structures of 1, 2 and 2d (refcodes DEZCAM,
MOCANT, and QQQFEJ respectively in the CCDC) have been
14–16
published earlier
but we have repeated the crystal structure
determination and refinement of 1 and 2d. Albeit the similarity
between the molecules, it is interesting to note that the mutual
geometry of the monomers is different. The difference is
expressed by the relative orientation of the substituent at position
C1 (see Scheme 1) of the anthracene skeleton. In all three
structures the two monomers are related by an inversion center,
however, while in 1 the carbonyl oxygen atoms of the two
monomers are pointing inward with regard to the space between
the molecules, they are pointing outward with regards to the
space between the molecules in compounds 2 and 3 (Fig. 3). The
prerequisite geometric parameters needed to allow solid state
photodimerization meet in compounds 1, 2, and 3. The distances
˚
between the reacting atoms are 3.786(3), 3.679, and 3.725 A
respectively. The lateral shifts of the p-orbitals are 1.611, 1.736,
˚
and 1.233 A in compounds 1, 2, and 3 respectively. The crystal
17
structure of 4 was published (refcode NUKMAH) (4a) and the
relative geometry (Fig. 4, top) suggests that exposure to irradi-
ation will not produce a dimer. However, irradiation of a sample
of newly prepared 4 shows to yield a dimer. Eventually it was
shown that the dimer was obtained by irradiation of two new
polymorphs of 4 (4b and 4c) (Fig. 4, bottom). The distances
˚
between the reacting atoms are 3.769(4) and 3.802(4) A in 4b and
4
c respectively. The lateral shifts of the p-orbitals of these atoms
˚
are 1.605 in 4b and 0.986 A in 4c. The difference between the two
polymorphs is the relative orientation of the substituent on
C1 (Scheme 1) of the anthracene skeleton; the carbonyl oxygen
atoms are pointing outward with respect to the space between the
molecular planes in 4b, and pointing inward in 4c. This obser-
vation might suggest that we should not be surprised to find
other polymorphs of 1–3.
We had hoped that irradiation of 1–4 will be carried out in
a single-crystal to single-crystal mode so that the crystal structure
could be studied as the reaction will proceed. Monitoring such
4
378 | CrystEngComm, 2011, 13, 4376–4381
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Fig. 2 Histograms of geometric parameters: (top left)—distances between potential reactive atoms (d), (top right)—distances between mean planes (p),
bottom left)—lateral shifts between p-orbitals (l), and (bottom right)—angles between d and p.
(
Fig. 3 Relative geometry of two monomers in compounds: 1 (top), 2
bottom left) and 3 (bottom right). (Hydrogen atoms were removed for
Fig. 4 Relative geometry of two monomers in three polymorphs of 4: 4a
(
(
(
refcode NUKMAH) (top), 4b (bottom left) and 4c (bottom right).
clarity reasons.)
Hydrogen atoms were removed for clarity reasons.)
reaction provides interesting information about the structural
3,18a–e
we could not succeed in preparing I-4. The crystal structures of
course of the reaction.
For example 9-methyl-anthracene
17
I-1 and I-2 have been described before. In the three molecular
compounds the host and the guest molecules are hydrogen
bonded through the hydroxyl group of the host molecule and the
carbonyl group of the guest molecule (Fig. 6). However, while in
I-1 and in I-3 the isolated two guest molecules are related by an
inversion center, in I-2 the guest molecules are arranged in stacks
and the inversion center is between each pair of guest molecules
undergoes single-crystal to single-crystal transformation during
19
the photodimerization. However, irradiation of single crystals
of compounds 1–4 did not show single-crystal to single-crystal
transformation. Therefore the irradiation was carried out on
powder samples and the products were recrystallized. In all the
dimers shown in Fig. 5, the orientation of the substituent on C1
of the anthracene skeleton is such that the carbonyl oxygen
atoms are pointing inward with respect to the space where the
new bonds were formed (the torsion angle O1–C15–C1–C8* is
(
Fig. 6). Also, it should be noted that there are two crystallo-
graphic independent molecules in I-1. While isolated cavities are
formed by the host molecules in I-1 and I-3, channel-like cavities
are formed in I-2 (Fig. 7–9. The relevant distances are 3.805(4)
ꢀ
ca. 0 ).
˚
and 3.714(5) A in the two sites (A and B) and the lateral shifts of
˚
the p-orbitals are 1.476 and 1.422 A in the two sites, of I-1. The
Molecular compounds
˚
geometric values in compound I-3 are 3.771(3) and 1.139 A. The
Compounds 1, 2, and 3 were crystallized with the host mole-
cule1,1,6,6-tetraphenyl-hexa-2,4-diyne-1,6-diol (I in Scheme 1)
to form the molecular compounds I-1, I-2, and I-3, unfortunately
pair of monomers in I-2 is not related by the inversion center.
Therefore the mean planes of the molecules are not parallel
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Fig. 5 Molecular structure of the dimers: 1d (top left), 2d (top right), 3d
bottom left), and 4d (bottom right). (Hydrogen atoms were removed for
(
clarity reasons.)
Fig. 8 Isolated cavities formed by the host molecules in I-3.
Fig. 6 Relation between the guest molecules in I-1 (top left), I-2
(bottom), and I-3 (top right). (Hydrogen atoms were removed for clarity
reasons.)
Fig. 9 Channel cavities formed by the host molecules in I-2.
The relative geometry with respect to the orientation of the
substituent on C1 of the anthracene skeleton is different in the
three molecular compounds. The carbonyl oxygen atoms are
pointing inward with regard to the space between the two
monomers in compound I-1, they are pointing outward from the
space between the two monomers in I-3, and they are arranged in
an alternate fashion in the stack of guest molecules in I-2. As
a result it is expected that each will lead to a different dimer with
respect to the orientation of the substituent on C1 of the
anthracene skeleton as long as the reaction takes place in the
solid. The orientation may be changed upon recrystallization of
the product. Irradiation of a single crystal of each of the
molecular compounds causes [4 + 4] photodimerization that
Fig. 7 Isolated cavities formed by the host molecules in I-1. A and B
marked the different cavity types.
ꢀ
making an angle of 3.8 . As a result the distances between the
˚
reactive atoms are not equivalent (3.946(4) and 4.401(4) A) and
the lateral shifts between the p-orbitals of these atoms are 1.874
17
˚
and 2.455 A. Nevertheless, photodimerization takes place.
4
380 | CrystEngComm, 2011, 13, 4376–4381
This journal is ª The Royal Society of Chemistry 2011

proceeds by a single-crystal to single-crystal transformation to
a limited percentage of conversion, before the crystal is
disintegrated (11%, 37%, and 18% in I-1, I-2, and I-3 respec-
tively). The photodimerization takes place at these cavities. As
a result of the presence of two crystallographic independent
molecules in I-1, there are two different cavities as shown in
Fig. 7 (marked by A and B). Interestingly, the reaction takes
place in only one of the two different sites (B) before the crystal
References
1
(a) V. Enkelmann, G. Wegner, K. Novak and K. B. Vagener, J. Am.
Chem. Soc., 1993, 115, 10390; (b) V. Enkelmann, Adv. Polym. Sci., 63,
91.
2 K. Tanaka and F. Toda, in Organic Solid-State Reactions, ed. F.
Toda, Kluwer Academic Publishers, Dodrecht, 2002, and references
therein.
3
T. Lavy, Y. Sheynin, H. A. Sparkes, J. A. K. Howard and
M. Kaftory, CrystEngComm, 2008, 10, 734.
17
4 (a) G. M. J. Schmidt, J. Chem. Soc., 1964, 2014; (b) G. M. J. Schmidt,
Pure Appl. Chem., 1971, 27, 647.
is disintegrated.
5
S. K. Kearsley, Organic Solid State Chemistry, ed. G. R. Desiraju,
Elsevier Science Publishers, Amsterdam, 1987, pp. 69–115.
M. Leibovitch, G. Olovsson, J. R. Scheffer and J. Trotter, J. Am.
Chem. Soc., 1998, 120, 12755.
Conclusions
6
The dimers obtained by exposure of the neat solid compounds to
UV light were recrystallized. In all cases they adopt the same
conformation with respect to rotation of the substituent on C1.
In these cases the carbonyl oxygen atoms are pointing inward
with respect to the newly formed bonds between the two parent
monomers. This is believed to be the thermodynamically more
stable conformation. However, when the monomers are
embedded in the host environment the monomers are bound to
adopt the conformation set by the thermodynamically stable
crystalline molecular compound. Therefore the dimers obtained
by irradiation of the crystals are dependent on these conforma-
tions. This is why there are differences between the molecular
conformations of the dimers upon irradiation of neat monomers
and molecular compounds of these monomers.
7
8
F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58, 380.
Y. Kuwatani, G. Yamamoto, M. Oda and M. Iyoda, Bull. Chem. Soc.
Jpn., 2005, 78, 2188.
9
R. C. Parish and L. M. Stock, Tetrahedron Lett., 1964, 5, 1285.
10 R. C. Parish and L. M. Stock, J. Org. Chem., 1965, 30, 927.
1
1 L. M. Sweeting, A. L. Rheingold, J. M. Gingerich, A. W. Rutter,
R. A. Spence, C. D. Cox and T. J. Kim, Chem. Mater., 1997, 9, 1103.
2 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2007,
64, 112.
1
13 W. Bhanthumnavin, S. Ganapathy, A. M. Arif and W. G. Bentrude,
Heteroat. Chem., 1998, 9, 155.
1
4 K. Anderson, H. Becker, L. M. Engelhardt, L. Hansen and
A. H. White, Aust. J. Chem., 1984, 37, 1337.
15 J. C. J. Bart and G. M. J. Schmidt, Isr. J. Chem., 1971, 9, 429.
1
1
1
6 E. Heller and G. M. J. Schmidt, Isr. J. Chem., 1971, 9, 449.
7 I. Zouev, T. Lavy and M. Kaftory, Eur. J. Org. Chem., 2006, 4164.
8 (a) D. B. Varshney, G. S. Papaefstathiou and L. R. MacGillivray,
Chem. Commun., 2002, 1964; (b) S. Ohba, H. Hosomi and Y. Ito, J.
Am. Chem. Soc., 2001, 123, 6349; (c) S.-L. Zheng,
M. Messerschmidt and P. Coppens, Acta Crystallogr., Sect. B:
Struct. Sci., 2007, 63, 644; (d) S.-L. Zheng, M. Messerschmidt and
P. Coppens, Chem. Commun., 2007, 2735; (e) T. Fri ꢁs ^c i ^c and
L. R. MacGillivray, Z. Kristallogr., 2005, 220, 351.
Acknowledgements
The work was supported by the Israel Science Foundation, #
4
99/08. We would like to thank Prof. Israel Goldberg from Tel-
Aviv University for his help in X-ray diffraction data collection
when needed.
1
9 I. Turowska-Tyrk and E. Trzop, Acta Crystallogr., Sect. B: Struct.
Sci., 2003, 59, 779.
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