Asymmetric dinuclear bis(dipyrrinato)zinc(ii) complexes: Broad absorption and unidirectional quantitative exciton transmission

Article abstract of DOI:10.1039/c4cc01573h

Asymmetric, heteroleptic dinuclear bis(dipyrrinato)zinc(ii) complexes were synthesized and demonstrated to collect a wide range of light across the UV and visible regions (340-655 nm). The complexes also conveyed excitons across their structure from one end to the other, with spectroscopic studies demonstrating quantitative and fast energy transfer. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01573h

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Asymmetric dinuclear bis(dipyrrinato)zinc(II)
complexes: broad absorption and unidirectional
quantitative exciton transmission†
Cite this: Chem. Commun., 2014,
50, 5881
Received 2nd March 2014,
Accepted 9th April 2014
Mizuho Tsuchiya,a Ryota Sakamoto,*a Shinpei Kusaka,a Yasutaka Kitagawa,b
Mitsutaka Okumurab and Hiroshi Nishihara*a
DOI: 10.1039/c4cc01573h
www.rsc.org/chemcomm
Asymmetric, heteroleptic dinuclear bis(dipyrrinato)zinc(II) com- dipyrrin metal complexes receiving little attention as photo-
plexes were synthesized and demonstrated to collect a wide range active materials. We note that multinuclear dipyrrinato-metal
of light across the UV and visible regions (340–655 nm). The complexes, hybrids of boron and zinc complexes, have been
complexes also conveyed excitons across their structure from reported to date, though none is described to be fluorescent.9d–f
one end to the other, with spectroscopic studies demonstrating
quantitative and fast energy transfer.
We recently developed heteroleptic bis(dipyrrinato)zinc(II)
complexes with high fluorescent quantum yields.9b In the present
work, we report the synthesis of asymmetric dinuclear bis-
Natural photosynthetic systems employ light-harvesting antenna (dipyrrinato)zinc(II) complexes 6–8 (Scheme 1) that collect a wide
pigments that funnel excitons to a single point, the reaction centre. range of light across the UV and visible regions (340–655 nm) and
Inspired by such systems, various artificial light-harvesting molec- quantitatively convey excitons from one end of the molecular
ular systems have been proposed.1,2 Their development might system to the other.
allow the fabrication of ‘‘molecular photonic wires’’, a class of
The complexes 6–8 each contain three types of dipyrrinato
nano-sized optic fibres for molecular photonic systems that are ligands (Scheme 1, see Scheme S1 in the ESI† for their syn-
similar to the natural photosynthetic antennae.3–5 They would thesis). Dipyrrin ligand 2-H10 comprises anthracene and
enable the transport of excitons over long distances from one end a,a0-dimethyldipyrrin. The former acts as a UV absorbent
of a molecular system to the other.
(340–400 nm); visible light (400–520 nm) is acquired by the latter.
Dipyrrin, or dipyrromethene, is composed of two pyrrole
rings connected to one another at their alpha-positions by a
methine bridge.6 It may serve as a bidentate ligand to cations.
Its boron difluoride complexes (BODIPYs) show strong absorp-
tion in the visible region (e B 105 Mꢀ1 cmꢀ1), high fluorescence
quantum yields, and chemical and photochemical stability,
thereby finding many applications as fluorescent dyes.7 Bis-
and tris(dipyrrinato) metal complexes can be designed to form
oligomeric and polymeric structures through the coordination
of two and three dipyrrinato ligands, respectively, to one metal
centre.8,9 This provides an advantage over BODIPYs for supra-
molecular designs. However, their fluorescence quantum yields
are generally inferior to those of BODIPYs, which has led to
a Department of Chemistry, Graduate School of Science, The University of Tokyo,
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: sakamoto@chem.s.u-tokyo.ac.jp, nisihara@chem.s.u-tokyo.ac.jp;
Fax: (+81) 3-5841-4489; Tel: (+81) 3-5841-4348
b Department of Chemistry, Graduate School of Science, Osaka University, 1-1,
Machikaneyama, Toyonaka, Osaka 056-0043, Japan
† Electronic supplementary information (ESI) available: Detailed synthetic and
measurement procedures, DFT calculation, fluorescence lifetime decay, and
detailed procedures of estimation of rate constants. See DOI: 10.1039/c4cc01573h
Scheme 1 Synthesis of dinuclear complexes 6–8 and ligand molecules
used therein.
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Ligand 1-H is equipped with two a,a0-dimethyldipyrrin ligands
tethered by a durene linker, which allows it to act as a bridging
ligand to form a dinuclear configuration. The variation among the
complexes 6–8 arises through the third ligand, with 3-H,11 4-H,12
or 5-H12 being used as p-extended (b-arylethynyl and a-styryl)
dipyrrins to absorb wavelengths longer than those absorbed by
a,a0-dimethyldipyrrin (520–655 nm). Complexes 6–8 were charac-
Table 1 Spectroscopic properties of 6–8
emax (104 Mꢀ1 cmꢀ1
)
lem (nm)
jfd
th (ns)
a
b
c
labs (nm)
6
7
8
366
491
556
347
491
611
363
491
623
1.98
21.1
9.51
6.49
20.1
13.5
6.76
20.6
12.6
578
578
578
619
619
619
631
631
631
0.65
0.68
0.68
0.74e
0.78
0.77f
0.71e
0.74
0.78g
2.6i
2.6 j
2.6k
3.5i
3.6 j
3.7k
3.4i
3.4 j
3.5k
1
terized using H and 13C NMR, and high resolution electrospray
ionization mass spectrometry (HR-ESI-MS) (ESI†). Additionally,
the molecular structures of 6 and 7 were estimated by means of
DFT calculation (Fig. S1 and Tables S1 and S2, ESI†). This series of
complexes can be regarded as a model for molecular photonic
wires: photoexcitation of the anthracene moiety corresponds to an
a
c
b
Absorption maximum wavelength. Maximum molar absorptivity.
d
Fluorescence maximum wavelength. Fluorescence quantum yield
e
f
excited at the absorption maximum. Excited at 385 nm. Excited at
g
600 nm due to a small Stokes shift. Excited at 605 nm due to a small
h
i
j
optical input, the a,a0-dimethyldipyrrin array serves as an exciton Stokes shift. Fluorescence lifetime. Excited at 365 nm. Excited at
k
470 nm. Excited at 590 nm.
transmitter, and the p-extended dipyrrins receive excitons, thereby
emitting luminescence as an output.
UV-vis absorption spectra of the dinuclear zinc(II) complexes
6–8 are depicted in Fig. 1 and Table 1. Each complex possesses
three chief absorption bands; for example, complex 6 shows
two strong absorptions with maxima at 490 and 556 nm and a
moderate one with a developed vibronic structure spanning
340–380 nm. The two strong absorptions are assignable to the 1p–p*
transitions of the a,a0-dimethyl9b and b-arylethynyldipyrrins,9b
respectively; the other absorption to the anthracene moiety.13
The absorption spectrum of 6 corresponds to the sum of its
components’ spectra, indicating that there is minimal ground
state interaction among the pigments. A similar discussion is
applicable to 7 and 8, except that another absorption of
a-styryldipyrrins 4-H and 5-H at around 350 nm overlaps with
that of anthracene.
Steady-state fluorospectrometry acquired fluorescence spectra
and fluorescence quantum yields (ff) are shown in Fig. 1 and
Table 1. Each complex was irradiated at three different wave-
lengths corresponding to its three absorption bands. The fluores-
cence spectrum exhibits only one emission band, irrespective
of the choice of the excited absorption band. For example, 6
fluoresces exclusively with a maximum at 578 nm, which corre-
sponds to luminescence from b-arylethynyldipyrrin 3-H,9b thus
indicating the lack of contribution by emissions from the
anthracene and a,a0-dimethyldipyrrin moieties, which would be
expected at around 400 nm13 and 510 nm,9b respectively. Simi-
larly, the luminescence of 7 and 8 (with maxima at 619 nm and
631 nm) is derived from a-styryldipyrrins 4-H and 5-H, respec-
tively. High ff values (0.65–0.78) are noted (photographs of
solutions of 6–8 are shown in Fig. 2). What is also important is
that ff is constant regardless of the photoexcitation wavelength
(Table 1). This series of results indicates the quantitative migration
of excitons from either the anthracene or the a,a0-dimethyldipyrrin
moiety to the p-extended dipyrrin moiety. Fluorescence lifetime
studies for the complexes (Table 1 and Fig. S2, ESI†) show that
each complex exhibits a first-order decay with a lifetime (t) of a few
nanoseconds, which is typical for bis(dipyrrinato)zinc(II) com-
plexes.9 The lifetime appears independent of the excitation wave-
length, suggesting that exciton transfers from the anthracene to
the a,a0-dimethyldipyrrin and from the a,a0-dimethyldipyrrin to
the p-extended dipyrrin are both very fast processes compared
with the radiative processes from the anthracene and the
a,a0-dimethyldipyrrin moieties. For anthracene, ff = 0.36
and t = 5.25 ns (in cyclohexane);13 the respective values for
Fig. 2 Photographs of toluene solutions of 6–8 under ambient light (left)
and under illumination of 365 nm light (right). The fluorescence of 6–8 is
prominent even under ambient light, so that the solutions look cloudy (left).
Fig. 1 UV-vis absorption and fluorescence spectra for 6–8: (A) complex 6;
(B) complex 7; (C) complex 8.
5882 | Chem. Commun., 2014, 50, 5881--5883
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a,a0-dimethyldipyrrin are 0.18 and 3.9 ns (in toluene). These
values along with the ff 4 0.001 detection limit of the fluoro-
spectrometer employed here suggest that the rate constant
for the exciton transfer from the anthracene moiety to the
a,a0-dimethyldipyrrin is greater than 7 ꢁ 1010 sꢀ1 and that
from the a,a0-dimethyldipyrrin to the p-extended dipyrrin is
greater than 5 ꢁ 1010 sꢀ1 in 6 (see the ESI† for derivation).
In conclusion, asymmetric and dinuclear bis(dipyrrinato)zinc(II)
complexes 6–8 were demonstrated to be potential artificial light-
harvesting systems and molecular photonic wires. They collected a
broad range of UV and visible light (340–655 nm) and underwent
quantitative and unidirectional exciton transfer from one end
(the anthracene part) of the molecular system to the other
(the p-extended dipyrrin unit), resulting in bright emission
(jf = 0.65–0.78). Elongation to multinuclear complexes is
feasible,14 with such systems also expected to show efficient
unidirectional exciton transfer.
The authors acknowledge Grants-in-Aid from MEXT of Japan
(No. 21108002, 24750054, 25107510, 26708005, 26107510,
26620039, areas 2107 [Coordination Programming], 2406 [All
Nippon Artificial Photosynthesis Project for Living Earth], 2506
[Science of Atomic Layers]), and a JSPS fellowship for young
scientists. R.S. is grateful to the Tokuyama Science Foundation,
the Ogasawara Foundation for the Promotion of Science &
Engineering, the Kao Foundation for Arts and Sciences, the
Asahi Glass Foundation, the Noguchi Institute, and Japan
Association for Chemical Innovation for financial supports.
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