Photoreaction of anthracenyl phosphine oxides: Usual reversible photo- and heat-induced emission switching, and unusual oxidative P&C bond cleavage

Article abstract of DOI:10.1016/j.tetlet.2019.06.063

Anthracenyl(diphenyl)phosphine oxide and dianthracenylphenylphosphine oxide as photoactive compounds have been synthesized. Anthracenyl group of these compounds indicate the multi-functional roles such as an emission component, a photodimerization component, and a leaving group. When the light irradiation was performed under an oxygen atmosphere, photo-oxidative P&C bond cleavage to leave the antharacenyl group was observed. Moreover, phosphonyl radical was produced and then P&P bond formation to form diphosphane was observed.

Full text of DOI:10.1016/j.tetlet.2019.06.063

Tetrahedron Letters 60 (2019) 2026–2029
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Tetrahedron Letters
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Photoreaction of anthracenyl phosphine oxides: Usual reversible
photo- and heat-induced emission switching, and unusual
oxidative PAC bond cleavage
a
a
a
b
Kosuke Katagiri a, , Yukina Yamamoto , Yuui Takahata , Ryoga Kishibe , Naoki Fujimoto
⇑
a Department of Chemistry, Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto, Higashinada-ku, Kobe, Hyogo 658-8501, Japan
b Graduate School of Natural Science, Konan University, 8-9-1 Okamoto, Higashinada-ku, Kobe, Hyogo 658-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Anthracenyl(diphenyl)phosphine oxide and dianthracenylphenylphosphine oxide as photoactive com-
pounds have been synthesized. Anthracenyl group of these compounds indicate the multi-functional
roles such as an emission component, a photodimerization component, and a leaving group. When the
light irradiation was performed under an oxygen atmosphere, photo-oxidative PAC bond cleavage to
leave the antharacenyl group was observed. Moreover, phosphonyl radical was produced and then
PAP bond formation to form diphosphane was observed.
Received 15 May 2019
Revised 24 June 2019
Accepted 27 June 2019
Available online 28 June 2019
Keywords:
Ó 2019 Elsevier Ltd. All rights reserved.
Phosphine oxide
Photodimerization
Anthracenyl group
Oxidative cleavage
Introduction
and the photoreactivity of anthraphane crystals by [4 p + 4 p] pho-
todimerization has been demonstrated [22]. Moreover, the lumi-
nescence behavior of a photoactive anthracene-based ligand and
its Ir(III) complexes can be switched by photo-oxidation [23].
Recently, non-covalent interactions such as charge transfer (CT)
in have been used to achieve various tailor-made molecular
organic co-crystals with tunable fluorescence, with wide applica-
tions in optoelectronics [24–26]. Binary-component CT complexes
which use anthracene derivatives as donors and 1,2,4,5-tetra-
cyanobenzene (TCNB) as an acceptor have been reported [27]. In
these materials, emission color changes are due to the regulation
Photoactive organic molecules such as dithienylethene, azoben-
zene and anthracene have exhibited reversible emission and struc-
tural changes induced by light irradiation and heating.
Considerable attention has been focused on these molecules,
mainly concerning the intriguing photocyclization of the dithieny-
lethene unit [1], E/Z isomerization of the azobenzene unit [2], and
[4p + 4 p] photodimerization [3–6] and photo-oxidation reactions
of the anthracene unit [7]. Combining these photoactive units with
polymers [8–11], complexes [12–16], supramolecular polymers
[17] and supramolecules [18–20], is very interesting, as the resul-
tant materials have the potential to show important stimuli-
responsive behaviors. In particular, functionalized anthracenes
and materials containing anthracene have been extensively
reported as anthracene is easily functionalized. Furthermore, the
of the intermolecular interactions, such as
p/p stacking between
the anthracene moieties by grinding and heating. However, it is
unclear whether it is optimal to use a grinding/heating process to
regulate the intermolecular interactions in materials containing
anthracene moieties and to control the luminescence behavior.
Therefore, more anthracene derivatives must be designed and syn-
thesized to understand the relationship between the molecular
stacking mode and the grinding-/heating-responsive emission.
Meanwhile, phosphorus compounds are well known and widely
used as ligands for catalysts [28–30], ligands for coordination poly-
mers [31,32], and Lewis base catalysts [33–35] because of the fea-
tures that govern P(III)/P(V) chemistry, the potential to be
connected to three-, four-, or five-connected building blocks and
Lewis basicity. We have previously been interested in the develop-
ment of novel luminescent phosphine materials [36]. Hence, it is a
natural extension to create the phosphorus compounds with an
molecular stacking modes such as
p / p and CH/ p interactions,
which play a key role in obtaining the desired luminescent proper-
ties, can be regulated. For example, an anthracene derivative with
an imidazole displays three different packing structures, which
show significantly different solid-state luminescence behaviors.
Supramolecular interactions in these crystals can be reversibly
changed by grinding and heating [21]. Anthracene-based nano-
cages such as anthraphane contain reactive packings in the crystal,
⇑
Corresponding author.
E-mail address: katagiri@konan-u.ac.jp (K. Katagiri).
https://doi.org/10.1016/j.tetlet.2019.06.063
0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.

K. Katagiri et al. / Tetrahedron Letters 60 (2019) 2026–2029
2027
anthracenyl group as a stimuli-responsive luminescent material. In
this paper, we have designed and synthesized an anthracenyl
(diphenyl)phosphine oxide (1) and a dianthracenylphenylphos-
phine oxide (5) as photoactive organic compounds. The anthra-
cenyl group in 1 and 5 plays the role of the luminescent unit, the
quenching unit via [4
p + 4 p] and [4 p + 2 p] photodimerization,
and the elimination group to demonstrate unusual PAC bond
cleavage and PAP bond formation.
Results and discussion
Fig. 1. Crystal structure of 1 (a) and 5 (b) in the thermal ellipsoid model. The
ellipsoids of all non-hydrogen atoms have been drawn at 50% probability.
The synthesis of 1 and 5 are shown in Scheme 1. After the halo-
gen/lithium exchange reaction of 9-bromoanthracene, chloro
(diphenyl)phosphine or dichlorophenylphosphine was added and
the resulting product was oxidized with aq. H2O2. The product 1
and 5 were afforded as yellow solid and were characterized by
1H NMR and 31P NMR. Recrystallization of 1 or 5 from CH3CN
yielded yellow crystals suitable for single crystal X-ray diffraction
(SCXRD) analysis. The material 1 crystallized as a monoclinic sys-
tem with space group P21/n, and each unit cell included 16 mole-
cules of 1. As shown in Fig. 1, the anthracenyl group was almost
vertical against the P@O bond (the torsion angle of OAPACAC is
ca. 26.9°). The material 5 crystallized as a monoclinic system with
space group P21/n, and each unit cell included 4 molecules of 5
(Fig. 1b). In the packing diagram of crystals 1 and 5, the anthra-
cenyl group was not aligned in face-to-face type stacking
(Figs. S34 and S40).
Scheme 2. Photodimerization reaction of 1 and heating under N2 atmosphere.
Although several organocrystals containing the anthracenyl
group revealed that single-crystal-to-single-crystal (SCSC) dimer-
ization reaction between face-to-face stacked anthracenes
occurred, phosphine oxide 1 and 5 did not react to construct a pho-
todimer in the solid state. No color or shape change was observed
upon light irradiation of the yellow solid of 1 and 5. In contrast,
after light irradiation (365 nm) with N2 bubbling through a chloro-
form or acetonitrile solution of 1, the solution color changed from
yellow to light yellow. 1H NMR was performed in CD3CN to observe
any structural changes (Fig. S22). Although the proton signals of
the phenyl groups were not shifted, remarkable upfield shifts of
the anthracenyl proton signals were observed. The spectra were
reversibly returned to the original by heating at 80 °C. This evi-
dence suggests that a structural change between monomer 1 and
Fig. 2. Crystal structure of photodimer 2 in the thermal ellipsoid model. The
ellipsoids of all non-hydrogen atoms have been drawn at 50% probability. (a) Top
view. (b) Side view.
photodimer 2 by [4
p + 4 p] photodimerization of the anthracene
unit of 1 had occurred (Scheme 2). Recrystallization of an irradi-
ated solution of 1 from CH3CN yielded colorless crystals suitable
for SCXRD analysis. The material crystallized as a triclinic system
with space group P-1, and each unit cell included two molecules
of photodimer 2. The crystal structure of centrosymmetric pho-
todimer 2 is shown in Fig. 2. One of the 9- and 10-positions of
one anthracene unit is linked to the 9- and 10-positions of the
other anthracene. After the heating the light yellow solution of 2,
the solution color returned to yellow. 1H NMR and single crystal
X-ray diffraction analysis revealed that the monomer 1 was
obtained under these conditions. Moreover, to study the emission
behavior of the solution, the UV/vis absorption and fluorescence
spectral studies in CH3CN solutions (ca. 1 Â 10À5 M) were mea-
sured. The absorption and emission spectra of compounds 1 and
2 are shown in Fig. 3. Characteristic absorption bands and emission
bands of the anthracene moieties (350–400 nm and 450–500 nm,
respectively) were observed for 1, and photodimerization of the
anthracenyl group led to the disappearance of these bands.
When the CHCl3 or CH3CN solution of dianthra-
cenylphenylphosphine oxide (5) was irradiated at 365 nm with
N2 bubbling, the solution color changed from yellow to colorless.
However, usual intermolecular [4
p + 4 p] photodimerization was
not observed, and the proton signals of anthracenyl groups were
changed to complex. Although the pure products were not isolated,
colorless crystals suitable for SCXRD were obtained by recrystal-
lization of an irradiated solution of 5 from CH3CN. As shown in
Fig. 4, one of the 9- and 10-positions of an anthracene unit is linked
to the 1- and 2-positions of the other anthracene unit in the same
molecule. This fact suggested that the intramolecular [4
photoreaction was occurred instead of [4 + 4 ] photodimeriza-
tion reaction (Scheme 3). Generally, [4 + 2 ] cyclization is recog-
p + 2 p]
p
p
Scheme 1. Synthesis of anthracenyl(diphenyl)phosphine oxide
cenylphenylphosphine oxide 5.
1 or dianthra-
p
p

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K. Katagiri et al. / Tetrahedron Letters 60 (2019) 2026–2029
ment suggested the formation of the oxidative species by observa-
tion of the ion peaks corresponding to the oxygen linked [1 + 2OH]+
at m/z of 412.9 (Fig. S26). This inspired us to attempt the reaction
under oxidative conditions. Under an oxygen atmosphere, the
anthracenyl group of 1 act as a leaving group to form the anthra-
quinone via PAC bond cleavage, and the phosphorus unit was
transformed to diphenyl phosphinic acid (4) (Scheme 4). Similarly,
photooxidation reaction of 5 under oxygen atmosphere also sug-
gested PAC bond cleavage to afford the phosphinic acid and
anthraquinone (Scheme 5) and some phosphine oxide compounds.
Recrystallization of an irradiated solution of 5 from CH3CN yielded
colorless crystals suitable for SCXRD analysis. Interestingly, unu-
sual PAP bond formation by the coupling reaction of phosphonyl
radical to form the anthracenylphenyldiphosphane 7 was observed
in this reaction (Fig. 5).
Scheme 6 shows a postulated mechanism for this reaction.
When light irradiation under N2 atmosphere was performed, pho-
tochemical allowed [4
p + 4 p] photodimerization reaction was
occurred. In contrast, when light irradiation under O2 atmosphere
was performed, photo-oxidation such as oxygen addition via radi-
cal path was proceeded. After the homolysis of oxygen adducts,
Fig. 3. (a) Photograph of irradiated crystal 1 under lamp with 365 nm wavelength
light. (b) Photograph of crystal 1. (c) Absorption and emission spectra (k = 382 nm)
of 1 and 2 in CH3CN solutions (ca. 1 Â 10À5 M).
Scheme 4. Photooxidation reaction of 1 under an O2 atmosphere.
Fig. 4. Crystal structure of 6 in the thermal ellipsoid model. The ellipsoids of all
non-hydrogen atoms have been drawn at 50% probability.
Scheme 5. Photooxidation reaction of 5 under an O2 atmosphere.
Scheme 3. Photoreaction of 5 and heating under N2 atmosphere.
nized as photochemical forbidden [37–41]. As shown in Fig. 1, 9-
and 10-positions of an anthracene and 1- and 2-positions of the
other anthracene of compound 5 is located near each other. Usual
[4
steric hinderence of anthracenyl groups. Therefore, photochemical
forbidden [4 + 2 ] cyclization reaction was allowed. Sakurai et al
reported this type [4 + 2 ] cyclization reaction of anthracenyl
p + 4 p] photodimerization reaction was interrupted because of
p
p
p
p
silane and anthracenyl german compounds [42].
When light irradiation without N2 bubbling was performed,
photodimerization was observed in a low yield, and anthraquinone
(3) was obtained as a by-product. Positive mode ESI-MS measure-
Fig. 5. Crystal structure of 7 in the thermal ellipsoid model. The ellipsoids of all
non-hydrogen atoms have been drawn at 50% probability.

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2029
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We have synthesized phosphine oxide compounds with anthra-
cenyl group such as anthracenyl(diphenyl)phosphine oxide (1) and
dianthracenylphenylphosphine oxide (5), and have demonstrated
the roles of the anthracenyl group. The emission behavior of phos-
phine oxide 1 and 5 can be reversibly switched by photoirradiation
under a N2 atmosphere with heating. Characteristic absorption and
emission spectra of an anthracenyl group were observed in com-
pound 1. In contrast, [4
p + 4 p] photodimerization reaction of
the anthracenyl groups lead to formation of photodimer 2 with
no fluorescence. In compound 1, the anthracenyl moiety plays
the role of the photoresponsive group. Upon irradiation under an
O2 atmosphere, PAC bond cleavage was observed to form an
anthraquinone. In this reaction, the anthracene fragment plays
the specific role of a leaving group to form the diphosphane struc-
ture by PAP bond formation between the phosphonyl radicals. The
presence of the P–anthracenyl bond is an important factor for the
formation of the diphosphane. Our findings are expected to pave
the way for the creation of new and more efficient photoresponsive
materials based on phosphorus compounds containing an anthra-
cenyl group.
Acknowledgments
We thank Alison McGonagle, PhD, from Edanz Group (www.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2019.06.063.