Regio- and stereo-selective [4+4] photodimerization of angular-shaped dialkyltetracenedithiophene

Article abstract of DOI:10.1039/C8CC09485C

Angular-shaped dialkyltetracenedithiophenes (aTDTs) undergo [4+4] photodimerization in solution to form a butterfly-shaped skeleton. This reaction proceeds in a regio- and stereo-selective manner, forming only a single planosymmetric syn-[2,2]-daTDT out of six possible products. The photocycloaddition of aTDTs can take place topochemically in the thin-film state while maintaining regio- and stereo-selectivity. Stronger aliphatic dispersion forces and π-π interactions play important roles in forming the eclipsed dimeric complex that leads to the syn-[2,2]-daTDT isomer.

Full text of DOI:10.1039/C8CC09485C

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Regio- and stereo-selective [4+4]
photodimerization of angular-shaped
dialkyltetracenedithiophene†
Cite this: Chem. Commun., 2019,
55, 381
Received 29th November 2018,
Accepted 30th November 2018
Tze-Gang Hsu,a Hsiao-Chieh Chou,a Ming-Ju Liang,a Yu-Ying Lai *b and
a
Yen-Ju Cheng
*
DOI: 10.1039/c8cc09485c
rsc.li/chemcomm
Angular-shaped dialkyltetracenedithiophenes (aTDTs) undergo (tetracene, pentacene or larger acenes) are able to participate in
[4+4] photodimerization in solution to form a butterfly-shaped cycloaddition. Some photodimers from a single acene cannot
skeleton. This reaction proceeds in a regio- and stereo-selective be easily purified and characterized. In addition, it is difficult
manner, forming only a single planosymmetric syn-[2,2]-daTDT out to obtain single crystals of the dimeric molecules suitable for
of six possible products. The photocycloaddition of aTDTs can take decisive X-ray diffraction analysis.4d,5
place topochemically in the thin-film state while maintaining regio-
The unpredictable or unidentified structures of the photo-
and stereo-selectivity. Stronger aliphatic dispersion forces and p–p dimers greatly hinder the photochromic applications of acene
interactions play important roles in forming the eclipsed dimeric derivatives.6 For example, photoinduced-dimerization of
complex that leads to the syn-[2,2]-daTDT isomer.
unsubstituted tetracene forms centrosymmetric (head-to-tail)
and planosymmetric (head-to-head) dimers (Scheme 1). The
Acenes, polycyclic aromatic hydrocarbons (PAHs) comprising centrosymmetric photodimer is found to be the major product4f,7
linearly fused benzene rings, have exhibited great potential as due to the smaller steric hindrance and repulsion of the p-electron
organic semiconductors.1 Tetracene and pentacene derivatives system of the bimolecular sandwich-like alignment during the
are especially of current interest due to their superior hole transition state.4b,c,f When tetracene is further substituted with
mobility due to the enhancement of p–p stacking in the solid solubilizing groups, more geometric arrangements of two inter-
state.2 Increasing the number of benzene rings in acenes acting monomers are possible in the transition state, leading to
simultaneously results in a narrower optical bandgap and thus the formation of more complicated isomeric products.4d,7c For
higher photo-reactivity.1b,3 Highly planar acene derivatives instance, photodimerization of 2,3-didecyloxytetracene leads to
undergo visible-light induced [4+4] cycloaddition to furnish the formation of four different regio- and stereo-isomers in a
3-dimensional dimers with unique butterfly-shaped skeletons. similar ratio.5 Therefore, controlling the selectivities of acene-
The light-triggered dimeric formation with concomitant disrup- based [4+4] photo-cycloadditions remains challenging and
tion of the p-conjugation significantly alters the optical absorption unsolved.
and emission wavelengths. The dramatic change in the steric and
So far only scarce tetracene derivatives have been prepared
electronic properties between acene and the photodimer has on account of their poor solubility, instability, and lack of a
attracted much research interest to experimentally or theoretically
investigate the reaction mechanism.2f,4 The most problematic
drawback of this chemistry is that photodimerization of an acene
inevitably yields a mixture of geometric isomers due to the fact
that multiple reactive benzene moieties embedded in an acene
a Department of Applied Chemistry, National Chiao Tung University,
1001 University Road, Hsinchu, Taiwan 300, Republic of China.
E-mail: yjcheng@mail.nctu.edu.tw
b Institute of Polymer Science and Engineering, National Taiwan University,
No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, Republic of China.
E-mail: yuyinglai@ntu.edu.tw
Scheme 1 (a) Planosymmetric (left) and centrosymmetric (right) photo-
dimers of tetracene (b) photodimerization of 2,3-didecyloxytetracene and
(c) photodimerization of angular tetracenedithiophene (aTDT) in this work.
† Electronic supplementary information (ESI) available. CCDC 1836546. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8cc09485c
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versatile synthetic methodology, which limits further investiga-
tion. It is of substantial interest to develop a new tetracene-based
system which favors selectively a butterfly-shaped photodimer and
allows for fundamental understanding of structure–selectivity
relationships. To this end, terminally fusing a tetracene with
two thiophenes to afford tetracenedithiophene (TDT) is a rational
design to enhance p–p interactions and improve molecular
stability.8 It is documented that angular-shaped acenedithio-
phenes possess lower-lying HOMO levels than the corresponding
linear-shaped one,9 resulting in a better ambient stability, which
is advantageous to handle and examine the [4+4] photodimeriza-
tion product.10 Herein, we have successfully developed a new
methodology to synthesize an angular tetracenedithiophene
(aTDT) derivative with two lateral aliphatic side chains at the
4 and 11 positions to impart solubility. The aTDT is thermally
stable but undergoes dimerization upon illumination with
visible light (420–460 nm). Six possible photodimers (daTDT)
including two enantiomers (syn-[2,2]-daTDT, syn-[2,3]-daTDT,
anti-[2,2]-daTDT and its enantiomer anti-[2,2]0-daTDT, and anti-
[2,3]-daTDT and its enantiomer anti-[2,3]0-daTDT) are theoreti-
cally anticipated (Chart 1). daTDT is the abbreviation of dimeric
aTDT. syn stands for the two up and down thiophenes aligning
syn to each other and anti represents an anti-configuration. 2
and 3 denote that the bridge carbon lies on the second and
Scheme 2 Synthetic route of aTDT1 and aTDT2.
Fig. 1 Time-dependent UV-Vis spectra of aTDT1 under light illumination
(420–460 nm); inset: the green emission is bleached after irradiation.
third benzene ring of the central tetracene core numbered from photodimer whose UV-Vis spectra and 1H NMR spectra are
the left, respectively. Surprisingly, this reaction exclusively affords presented in Fig. 1 and 2, respectively. The absorption intensity
the planosymmetric syn-[2,2]-daTDT. To the best of our knowl- of aTDT1 from 350 to 500 nm gradually diminished as a
edge, this is the first time that photodimerization of an acene function of irradiation time, implying the formation of the
derivative forms a planosymmetric dimer with such a high regio- butterfly-skeleton dimer via photo-induced [4+4] cycloaddition
and stereo-selectivity.
with disconnection of the original conjugation (Fig. 1). In
Synthesis of aTDT1 with decanyl side chains is described in addition, the dimerization is reversible when syn-[2,2]-daTDT1
Scheme 2. Stille coupling of 1 with tributyl(2-thienyl)stannane is stored in the dark without interruption. Although not fully
furnished 2. Subsequently, Sonogashira cross-coupling reaction reverted, the reversibility of syn-[2,2]-daTDT1 is clearly observed
of 2 with 1-dodecyne afforded 3a. PtCl2-catalyzed 6-electrocyclization after months (Fig. S1, ESI†). This newly-formed compound is
of 3a led to the formation of the central tetracene core of aTDT1 highly viscous and the attempt at recrystallization of this
with two side chains at the 4 and 11-positions. The structure of compound was not successful owing to its high solubility even
1
aTDT1 was unambiguously confirmed by single-crystal X-ray crystallo- in apolar organic solvents. Judging from the H NMR spectra
graphy. To investigate the side-chain steric effect, aTDT2 with monitored as a function of irradiation time (Fig. 2), it can be
branched 2-octyldodecyl groups was also prepared for comparison.
concluded that only a single photodimer out of the six possible
When a CDCl3 solution of aTDT1 was illuminated by visible isomers was gradually formed during the reaction. The chemical
light (420–460 nm), aTDT1 gradually converted into a new
Chart 1 Six possible stereoisomers and corresponding symmetry point
groups for daTDT, in which anti-[2,2]-daTDT/anti-[2,2]0-daTDT and anti-
[2,3]-daTDT/anti-[2,3]0-daTDT are enantiomers.
Fig. 2 Time-dependent 1H NMR spectra of photo-dimerization of aTDT1
(420–460 nm) and labels for syn-[2,2]-daTDT1 (see Chart 1).
382 | Chem. Commun., 2019, 55, 381--384
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shifts at 6.29 and 6.43 ppm in Fig. 2 are assigned to the typical second (HbR) carbons. In the same way, B is the coupling
hydrogens on the sp3-bridgehead carbon of the butterfly-dimer.4d,5 between HaL and HbL. Searching along the horizontal dotted
The neighboring two bridgehead protons in syn-[2,3]-daTDT1 and line of B, signals C (HaL–Hinternal0) and D (HaL–Hinternal0) are
anti-[2,2]-daTDT1 should couple with each other to exhibit two perceived. These signals, based on the HSQC and HMBC
doublets, because they are chemically nonequivalent. On the other interpretation, are characterized to be the correlations of
hand, the bridge head hydrogens on syn-[2,2]-daTDT1 and anti- HaL–Hinternal or HaL–Hinternal0. However, the analogous HaR–Hinternal
[2,3]-daTDT1 are chemically equivalent and expected to exhibit two or HaR–Hinternal0 coupling on the naphthothiophene side of daTDT1
singlets. The observation of two singlet peaks instead of doublet is not yet detected. It should be emphasized that neither
peaks explicitly rules out the possibility of syn-[2,3]-daTDT1 and HaL–Hinternal nor HaR–Hinternal coupling is observed in the
anti-[2,2]-daTDT1. In other words, only syn-[2,2]-daTDT1 or ROESY NMR spectrum of the aTDT1 monomer (Fig. S8, ESI†).
0
anti-[2,3]-daTDT1 could possibly be the product.
The exclusive detection of HaL–Hinternal couplings in daTDT1
With the help of heteronuclear single quantum coherence strongly indicates that the top and bottom decanyl moieties
spectroscopy (HSQC) and heteronuclear multiple bond correlation attached to the benzothiophene segments are in close proximity,
spectroscopy (HMBC), the individual carbons and hydrogens of which is only possible for syn-[2,2]-daTDT1 with the two decanyl
daTDT1 can be assigned accordingly (Fig. S2–S7, ESI†). The groups situated on the same side.11 Another probable photo-
numbering of the side chains for the hindsight structure is shown dimer, anti-[2,3]-daTDT1, with the upper and lower side chains
in Fig. 3. We define a, b, and ‘‘internal’’ for the first, second, and pointing in different directions is reasonably ruled out. syn-
internal carbons on the decanyl group respectively. ‘‘L’’ denotes [2,2]-daTDT1 can be concluded as the product of the photo-
the left-sided decanyl group attached to the benzothiophene induced dimerization. The chemical shift of each proton in
moiety while ‘‘R’’ signifies the right-sided decanyl group linking syn-[2,2]-daTDT1 is assigned in Fig. 2 after 45 min irradiation.
0
to the naphthothiophene segment and prime is used to distin- To further discuss the ROESY HaL–Hinternal signals in Fig. 4,
guish the upper and lower moieties. Nevertheless, the combi- the encounter of the aliphatic side chains induces inter-chain
nation of HSQC and HMBC spectroscopy is not sufficient to nuclear Overhauser effect (NOE) enhancement. On the other
elucidate the exact stereochemistry of daTDT1. ROESY (rotating- hand, the absence of a long-range NOE signal for the naphthothio-
frame nuclear Overhauser effect correlation spectroscopy) was phene parts suggests that the upper and lower decanyl groups in
then employed to further characterize daTDT1 (Fig. 4).
the naphthothiophene sides are too far away in comparison to the
In the resultant spectrum, signal A corresponds to the benzothiophene side.
coupling between hydrogens on the right-sided first (HaR) and
The formation of syn-[2,2]-daTDT1 from two aTDT1 mole-
cules is not only regioselective but also stereoselective. The
X-ray crystallography of aTDT1 is shown in Fig. 5 to illustrate
the dimeric packing extracted from the extended stacks in the
crystal structure. It is found that the two aTDT1 molecules align
in a syn manner rather than an anti one, which may be linked to
the presence of two lateral decanyl groups. The dispersion forces
induced by the aliphatic side chains might play a significant role
in aligning the eclipsed face-to-face dimeric complex leading to
the preferable syn-[2,2]-daTDT isomer. Moreover, the emission
spectra for aTDT1 are listed in Fig. S10 (ESI†). The increase in
concentration from 1.04 ꢀ 10ꢁ3 M to 5.24 ꢀ 10ꢁ3 M gives rise to
the emergence of a new emission band at lower energies, suggest-
ing the formation of aTDT excimer.
Fig. 3 Labels of the side chain hydrogens for syn-[2,2]-daTDT1.
To investigate the general photo-selectivity of the aTDT
system, we synthesized aTDT2 with branched 2-octyldodecyl
Fig. 5 Extracted dimeric packing of aTDT1 in the single crystal structure
Fig. 4 ROESY spectrum of syn-[2,2]-daTDT1 in a selected range.
of X-ray crystallography: (a) top view and (b) side view.
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side chains that impose larger steric hindrance during dimeriza- system provides guidelines to design smart acene-based materials
tion. The NMR analyses again demonstrate that syn-[2,2]-daTDT2 which are promising for photochromic and photo-responsive
is still the single photodimer with identical regioselectivity and applications.
stereoselectivity in solution. It is of great interest to further
This work is supported by Ministry of Science and Technology,
investigate the photodimerization in the solid state. The X-ray Taiwan (grant No. MOST 107-3017-F009-003 and 107-2628-M-009-
diffraction (XRD) patterns of aTDT1 powder and single crystal are 003-MY3) and Ministry of Education, Taiwan.
compared in Fig. S11 (ESI†). The close resemblance between the
two patterns suggests that the powder sample possesses a similar
solid-state stacking structure to the single crystal shown in Fig. 5.
Upon direct exposure to irradiation, the aTDT1 powder remain
unreacted. Solvent vapour annealing of the aTDT1 powder was
Conflicts of interest
There are no conflicts to declare.
therefore attempted. When aTDT1 is saturated with d-chloroform
vapour in a closed jar under 420–460 nm irradiation, photo-
Notes and references
dimerization proceeds gradually to form a single isomer which
is also determined to be syn-[2,2]-daTDT1. d-Chloroform vapour
would penetrate into the powder to slightly dissolve the aTDT1
molecules, rendering them with sufficient freedom for structural
transformation. It is envisaged that the two aTDT1 molecules in
the syn geometry undergo molecular slippage from the offset
face-to-face stacking to the eclipsed face-to-face stacking before
dimerization, leading to the syn-[2,2]-daTDT1 product. In addition
to the aliphatic dispersion interactions, it is also envisaged that
the eclipsed fact-to-face dimeric stacking would have stronger p–p
interactions than the offset fact-to-face stacking. The thermo-
dynamically more stable eclipsed complex might account for the
selectivity of syn-[2,2]-daTDT over syn-[2,3]-daTDT.
Furthermore, an aTDT2 thin film was prepared by spin-
coating of its solution on a glass slide. After 15 min light
irradiation, the original yellow thin film became totally trans-
parent, demonstrating that photodimerization can successfully
take place in the thin-film state (Fig. S12, ESI†). More importantly,
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as the single product again. Compared with the XRD of aTDT2
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be polymorphic. Considering that the photodimerization of the
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polymorphic packing in the aTDT2 thin film might be responsible
for the distinct photo-reactivity. To our knowledge, this study is
the first demonstration that a tetracene-based material can carry
out a light-induced topochemical reaction with high regio- and
stereo-selectivity.
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In summary, we have developed dialkyl angular-shaped
TDTs (aTDT1 and aTDT2) which undergo [4+4] photodimerization
reaction to form a butterfly-shaped syn-[2,2]-daTDT. Instead of the
sterically-favored centrosymmetric anti-[2,3]-daTDT, the plano-
symmetric syn-[2,2]-daTDT surprisingly turns out to be the single
isomer out of six possible products. The structure of syn-[2,2]-
daTDT is carefully determined by a series of NMR techniques
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