Synthesis and properties of a through-space-conjugated dimer

Article abstract of DOI:10.1246/cl.131059

We describe the synthesis and energy-transfer characteristics of a [2.2]paracyclophane-based through-space-conjugated dimer consisting of two stacked π-electron systems. Photoexcited energy transfer by the Forster mechanism from the donor to acceptor π-electron systems occurred efficiently (energy-transter efficiency: ET > 0.999) with a rate constant (kET) of 1012 s11.

Full text of DOI:10.1246/cl.131059

CL-131059
Received: November 11, 2013 | Accepted: December 6, 2013 | Web Released: December 12, 2013
Synthesis and Properties of a Through-space-conjugated Dimer
Yasuhiro Morisaki,* Naoya Kawakami, Tatsuya Nakano, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510
(E-mail: ymo@chujo.synchem.kyoto-u.ac.jp, chujo@chujo.synchem.kyoto-u.ac.jp)
We describe the synthesis and energy-transfer characteristics
and M29 that behave as energy donor and acceptor, respectively.
Figure 2 shows their UV-vis absorption and photoluminescence
(PL) spectra obtained at an excitation wavelength of 290 nm
using dilute solutions in CHCl3 (1.0 © 10¹5 M for UV and
of a [2.2]paracyclophane-based through-space-conjugated dimer
consisting of two stacked π-electron systems. Photoexcited
energy transfer by the Förster mechanism from the donor to
acceptor π-electron systems occurred efficiently (energy-transter
(A)
¹1
efficiency: ΦET > 0.999) with a rate constant (kET) of 1012
s .
[2.2]Paracyclophane is an attractive molecule that consists
of face-to-face benzene rings with two ethylene chains.1 Since
the first report on [2.2]paracyclophane in 19492 and the
development of its practical synthesis in 1951,3 intensive studies
on the reactivities, optoelectronic properties, and theoretical
studies have been carried out in the field of organic chemistry.
We have focused on the unique structure of [2.2]paracyclophane
and applied it in polymer chemistry.4,5 Specifically, we used a
[2.2]paracyclophane skeleton as a scaffold to stack π-electron
systems one-dimensionally. The obtained polymers exhibited
through-space conjugation,6a in which photoexcited charge6a
and energy6b were transferred through the stacked π-electron
systems. Our next target is to construct an energy-transfer system
in which the energy-transfer process involves unidirectional
transfer of photoexcitation energy from one terminal π-electron
system to another; such a structure effectively functions as a
through-space-conjugated single-molecular wire. Taking a step
further toward the syntheses of trimers, tetramers, and so on, in
the present study, we synthesized a through-space-conjugated
dimer consisting of an energy donor and an acceptor. The optical
properties and energy-transfer abilities are discussed in detail.
We designed and synthesized a through-space-conjugated
dimer D1 (Figure 1)7 composed of two π-electron systems M18
(B)
(C)
¹5
Figure 2. UV-vis absorption spectra (in CHCl3, 1.0 © 10
M) and photoluminescence spectra (in CHCl3, 1.0 © 10¹6 M,
excited at 290 nm) of (A) M1, (B) M2, and (C) D1.
Figure 1. Structures of target through-space-conjugated dimer
D1, model compounds M1 and M2 synthesized in this study.
426 | Chem. Lett. 2014, 43, 426–428 | doi:10.1246/cl.131059
© 2014 The Chemical Society of Japan

Table 1. Data of optical properties
¹1
c
¹1 f
¹1 g
Comp
-
abs/nm,a ¾/M¹1 cm
-
PL,max/nmb
ΦPL
¸/nsd (»2)
kr/©109 s
knr/©109 s
M1
288, 25200
313, 21500
320, 36400
372, 40000
313, 44600
376, 39000
320
0.25d
0.44 (1.03)
0.57
1.7
M2
D1
401
406
0.40d
1.27 (1.01)
0.31
0.32
0.47
0.44
0.42d
0.002e
1.32 (1.08)
n.d.
c
aIn CHCl3 (1.0 © 10¹5 M). bIn CHCl3 (1.0 © 10¹6 M), excited at 290 nm. Photoluminescence quantum efficiency. dCalculated at
PL,max; all decay curves could be fitted by single-exponential decay. eCalculated at around 320 nm. fRadiative rate. gNonradiative rate.
-
1.0 © 10¹6 M for PL). The concentration used for PL measure-
ments (1.0 © 10¹6 M) was confirmed to be sufficiently dilute
(Figure S12 in the Supporting Information (SI)); therefore,
intermolecular π-π interactions could be ignored. The results are
summarized in Table 1, which also includes the results for the
PL decay studies (Figures S8, S10, and S13 in the SI). In the
absorption spectrum of M1 (Figure 2A), the π-π* transition
band of the tolane moiety was observed between 250 and
320 nm. Upon excitation at 290 nm, a PL peak with a vibrational
structure was observed at around 320 nm. As shown in
Figure 2B, M2 exhibited a typical absorption band of the
π-π* transition of the p-arylene-ethynylene moiety, and the peak
wavelengths -max of M2 were at 310 and 376 nm. By photo-
exciting M2 at 290 nm, a PL peak with a vibrational structure
was observed at around 400 nm.
The absorption spectrum of D1 was similar to that of M2
(Figure 2C). The molar extinction coefficient (¾) for the peak at
maximum absorption (313 nm) increased because of the overlap
between the absorption bands of the M1 and M2 moieties. On
the other hand, an overlap between the PL spectra of the M1 and
M2 units was not observed in the PL spectrum of D1. The
emission from the M1 moiety was nearly undetectable (PL
quantum efficiency: ΦPL = 0.002), and the emission peak from
the M2 moiety appeared (ΦPL = 0.42) with a vibrational
structure. As indicated by the PL lifetime of M2 (¸ = 1.27 ns,
»2 = 1.01) and D1 (¸ = 1.32 ns, »2 = 1.08) in Table 1, the
emission from the M1 moiety in D1 was not detected.10 The PL
intensity of D1 increased in comparison with that of M2 because
of the energy transfer from the M1 to the M2 unit in D1; in other
words, the antenna effect occurred. Fluorescence decay calcu-
lations suggest that the energy-transter efficiency (ΦET) of this
system was >99%. Identical PL behaviors were observed in
other good solvents such as THF.
Figure 3. (A) Molecular orbitals of the D1 model including
the oscillator strengths ( f ) of HOMO/LUMO and HOMO¹1/
LUMO+1.
To estimate theoretically the value of ΦET, density func-
tional theory (DFT) and time-dependent DFT calculations were
carried out on M1, M2, and D1 models. The -OC12H25 group
was replaced with -OCH3 for simplicity. The optimized
structure of the D1 model is shown in Figure S16 and Chart S1
in the SI; both stacked π-electron systems were planar in the
ground state, and the center-to-center distance (r) was calculated
to be 11.25 ¡. As shown in Figure 3, molecular orbitals of the
M1 and M2 moieties in D1 were found to lie mainly on the
HOMO¹1/LUMO+1 and HOMO/LUMO levels, respectively.
In addition, the oscillator strengths ( f ) of their π-electron
systems were estimated to be 1.79 and 0.66, respectively.11
These results indicate that each π-electron system in D1 existed
almost independently. The arrangement of the dipole moments
of the M1 and M2 moieties in D1 was favorable for the
fluorescence resonance energy transfer (FRET);12 in particular,
they were aligned along the long axis of the stacked π-electron
systems (Chart S1). Consequently, a high value of the orienta-
tion factor (¬2 = 3.59) was obtained from the combination of the
M1 and M2 moieties based on the [2.2]paracyclophane skeleton.
Chart S1 in the SI summarizes the parameters of the energy-
transfer system determined on the basis of the Förster mecha-
nism.12-14 The spectral overlap integral (J) was calculated from
the PL spectrum of M1 and the absorption band of M2 had a
value of 3.10 © 1014 M¹1 cm3. The values of J, refractive index
of CHCl3 (n = 1.446), ΦPL of M1, and ¬2 provided a relatively
large Förster radius (R0 = 42.1 ¡). As shown in Figure 4 and
Chart S1, the ΦET and energy-transfer rate kET were calculated to
Chem. Lett. 2014, 43, 426–428 | doi:10.1246/cl.131059
© 2014 The Chemical Society of Japan | 427

Wiley-VCH, Weinheim, 2004. doi:10.1002/3527603964.
C. J. Brown, A. C. Farthing, Nature 1949, 164, 915.
D. J. Cram, H. Steinberg, J. Am. Chem. Soc. 1951, 73, 5691.
a) Y. Morisaki, Y. Chujo, Macromolecules 2002, 35, 587. b)
Y. Morisaki, Y. Chujo, Angew. Chem., Int. Ed. 2006, 45,
6430. c) Y. Morisaki, Y. Chujo, Prog. Polym. Sci. 2008, 33,
346. d) Y. Morisaki, Y. Chujo, Polym. Chem. 2011, 2, 1249.
e) Y. Morisaki, Y. Chujo, Chem. Lett. 2012, 41, 840.
a) L. Guyard, P. Audebert, Electrochem. Commun. 2001, 3,
164. b) L. Guyard, P. Audebert, W. R. Dolbier, Jr., J.-X.
Duan, J. Electroanal. Chem. 2002, 537, 189. c) F. Salhi, B.
Lee, C. Metz, L. A. Bottomley, D. M. Collard, Org. Lett.
2002, 4, 3195. d) F. Salhi, D. M. Collard, Adv. Mater. 2003,
15, 81. e) S. P. Jagtap, D. M. Collard, J. Am. Chem. Soc.
2010, 132, 12208. f) S. Clément, T. Goudreault, D. Bellows,
D. Fortin, L. Guyard, M. Knorr, P. D. Harvey, Chem.
Commun. 2012, 48, 8640.
a) Y. Morisaki, S. Ueno, A. Saeki, A. Asano, S. Seki, Y.
Chujo, Chem.®Eur. J. 2012, 18, 4216. b) Y. Morisaki, S.
Ueno, Y. Chujo, J. Polym. Sci., Part A: Polym. Chem. 2013,
51, 334.
Synthetic route is shown in Scheme S1 in the Supporting
Information (SI). The SI is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
2
3
4
5
Figure 4. Energy transfer from the M1 to M2 moieties in D1.
be >0.999 and 6.28 © 1012 ¹1, respectively. The ΦET value
s
6
7
agrees with the experimental value of >0.99 (vide supra).
Recently, triple energy transfer (the Dexter-type energy trans-
fer15) along [2.2]paracyclophane-based polymers has been
reported.5f The kET value has been estimated to be approximately
on the order of 105 s¹1, which is much smaller than the value
in the present study (kET of 6.28 © 1012 ¹1).16 Therefore, the
s
photoexcited energy in D1 was dominantly transferred by FRET.
In summary, a [2.2]paracyclophane-based through-space-
conjugated dimer, in which the donor and acceptor π-electron
systems are stacked in close proximity, was synthesized. The
stacked π-electron systems existed independently, and the
8
9
K. Park, G. Bae, J. Moon, J. Choe, K. H. Song, S. Lee,
J. Org. Chem. 2010, 75, 6244.
Y. Morisaki, N. Wada, M. Arita, Y. Chujo, Polym. Bull.
2009, 62, 305.
photoexcitation energy was transferred efficiently (ΦET
>
10 The PL spectrum of the 1:1 mixture of M1 and M2 in CHCl3
(1.0 © 10¹6 M) is shown in Figure S14 in the SI. The both
emission peaks from M1 and M2 were observed.
11 Molecular orbitals of M1, M2, and D1 models as well as the
oscillator strengths are summarized in Figure S15 in the SI.
12 T. Förster, Naturwissenschaften 1946, 33, 166.
13 Principles of Fluorescence Spectroscopy, 3rd ed., ed. by
J. R. Lakowicz, Springer, New York, 2006, pp. 443-475.
doi:10.1007/978-0-387-46312-4_13.
0.999 and kET = 6.28 © 1012 ¹1) from the donor to the acceptor
s
π-electron system via the Förster mechanism. Thus, the
[2.2]paracyclophane skeleton provides ideal conditions for
FRET, such as the close proximity between π-electron systems
(small r value) and proper orientation (large ¬2 value). Further
investigations on the syntheses of [2.2]paracyclophane-based
through-space trimers and tetramers toward single molecular
wires, in which energy flows unidirectionally and efficiently
over a long distance, are currently under way.
14 The parameter is also summarized with equations in detail in
the SI.
This work was partially supported by Grant-in-Aid for
Young Scientists (A) (No. 24685018) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of
Japan.
15 D. L. Dexter, J. Chem. Phys. 1953, 21, 836.
16 [2.2]Paracyclophane-based electron-transfer rate constant
was reported (order of 109 s¹1). a) A. Molina-Ontoria, M.
Wielopolski, J. Gebhardt, A. Goulumis, T. Clark, D. M.
Guldi, N. Martín, J. Am. Chem. Soc. 2011, 133, 2370. b) M.
Wielopolski, A. Molina-Ontoria, C. Schubert, J. T. Margraf,
E. Krokos, J. Kirschner, A. Gouloumis, T. Clark, D. M.
Guldi, N. Martín, J. Am. Chem. Soc. 2013, 135, 10372.
References and Notes
1
a) F. Vögtle, Cyclophane Chemistry: Synthesis, Structures
and Reactions, John Wiley & Sons, Chichester, 1993. b)
Modern Cyclophane Chemistry, ed. by R. Gleiter, H. Hopf,
428 | Chem. Lett. 2014, 43, 426–428 | doi:10.1246/cl.131059
© 2014 The Chemical Society of Japan