Anion responsive dyad system of porphyrin and N-confused porphyrin

Article abstract of DOI:10.1039/b907775h

A dyad of porphyrin and N-confused porphyrin is synthesized for the first time, in which an efficient excitation energy transfer as well as enhancement of emission quantum yield induced by anion binding is observed.

Full text of DOI:10.1039/b907775h

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Anion responsive dyad system of porphyrin and N-confused porphyrin†
Motoki Toganoh, Hironao Miyachi, Hisanori Akimaru, Fuyuki Ito, Toshihiko Nagamura and Hiroyuki Furuta*
Received 17th April 2009, Accepted 12th June 2009
First published as an Advance Article on the web 17th June 2009
DOI: 10.1039/b907775h
A dyad of porphyrin and N-confused porphyrin is synthe-
sized for the first time, in which an efficient excitation energy
transfer as well as enhancement of emission quantum yield
induced by anion binding is observed.
BF3·OEt2 (9%). The structural assignment of 1 is derived from
the mass and 1H NMR spectra. In the mass spectrum (MALDI,
positive), the parent ion peak is observed at m/z = 1756.40 (calcd:
1756.68). The 1H NMR spectrum of 1 in CDCl3 is consistent with
the assigned structure. The signals attributable to the outer (Ha)
and inner (Hb) protons of the confused pyrrole moiety are observed
at d 8.99 and -5.07 ppm, respectively. The signal due to the inner
NH protons for the porphyrin moiety is found at d -2.54 ppm,
while the respective signals for the NCP moiety are at d -2.16 and
The study of porphyrin photophysics is of great importance,
because such fundamental information is a prerequisite for the
development of artificial light harvesting complexes,1 photosyn-
thetic reaction centers,2 and molecular devices.3 As a simple
model, a porphyrin dyad system has been often constructed and
examined closely.4,5 Besides, porphyrin dyads bearing switching
ability are often investigated to obtain further information on
photophysics among porphyrin chromophores.6 Despite many
examples, the switching moiety is usually installed in the bridge
part and is hardly ever introduced in the chromophore itself.
Herein, a dyad of porphyrin and N-confused porphyrin (NCP)7
was synthesized for the first time, where the NCP can be used as a
switchable chromophore by anion binding.8 In this dyad system,
enhancement of emission quantum yield as well as acceleration of
excitation energy transfer was induced by anion binding (Fig. 1).
-2.14 ppm. The signals for the p-phenylene spacer resonate at d
3
8.57 and 8.69 ppm, and are coupled to each other with JHH
=
7.9 Hz. The rest of the signals such as the pyrrole b-protons (d
8.6–9.3 ppm), the meso-aryl protons (d 7.8–8.2 ppm), and the
protons of the tert-butyl groups (d 1.55 and 1.58 ppm) can be
assigned. 15-Phenyl-5,10,20-tris(pentafluorophenyl) NCP (4) was
also synthesized in 20% yield by a similar [2 + 2] condensation
method as a model compound.
The absorption spectrum of 1 in CH2Cl2 is shown in Fig. 2
together with the model compounds with the NCP moiety (4)
and the porphyrin moiety (5). The spectrum profile of 1 can
be simulated simply by the sum of those of 4 and 5, suggesting
negligible interaction between the two chromophores in the
ground state. Hence, the Q4-band (649 nm) originates from the
porphyrin moiety and the Q5-band (716 nm) originates from the
NCP moiety. This is further supported by theoretical study as
discussed below.
The structure of 1 was fully optimized at the B3LYP/6-31G**
level and the Kohn–Sham orbitals were analyzed at the same level.
The orbital energy levels of the HOMO (0~-3) and the LUMO
(0~+3) are summarized in Fig. 3 along with selected Kohn–
Sham orbitals. Undoubtedly, orbitals of the porphyrin moiety and
the NCP moiety are observed independently. The HOMO and
HOMO - 1 are exclusively composed of the porphyrin moiety,
while HOMO - 2 and HOMO - 3 are of the NCP moiety. Further,
the LUMO and LUMO + 1 are composed of the NCP moiety,
while LUMO + 2 and LUMO + 3 are of the porphyrin moiety.
As a result, the Q5-band should be derived from the transition
from HOMO - 2 to LUMO, while the Q4-band from HOMO to
LUMO + 2. The absorption corresponding to the HOMO–LUMO
transition is not observed in 1. Such a clear differentiation
of molecular orbitals between the donor and the acceptor
would be beneficial for the photophysical study of the dyad
system.11
Interestingly, addition of Bu4N+F- to 1 in CH2Cl2 changed
the absorption spectrum significantly (Fig. 4a), suggesting the
occurrence of anion binding at the peripheral NH group of the
NCP as reported previously.8 Isosbestic points were observed at
446 and 702 nm, indicating a 1:1 binding mode between 1 and
F-. Bu4N+F- interacts only with the NCP moiety and negligible
interaction with the porphyrin moiety is expected, since almost no
Fig. 1 Anion responsive dyad system.
Synthesis of the porphyrin–NCP dyad (1) was achieved by an
acid-catalyzed [2 + 2] condensation reaction of porphyrin-attached
normal-type dipyrromethane (2) and N-confused dipyrromethane
dicarbinol (3) as shown in Scheme 1.9 The pentafluorophenyl
groups of the NCP moiety in 1 contribute better yields in dyad
synthesis as well as stability against photo-irradiation.10 The
reaction with CF3CO2H (16% yield) as an acid-catalyst, so far
afforded better yield than the reaction with CH3SO3H (8%) or
Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.
E-mail: hfuruta@cstf.kyushu-u.ac.jp; Fax: (+81)92-802-2865; Tel: (+81)
92-802-2865
† Electronic supplementary information (ESI) available: Synthesis and
characterization of 1 and 4, additional spectroscopic data, and details
of DFT study. See DOI: 10.1039/b907775h
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Scheme 1 Preparation of porphyrin-NCP dyad (1).
Fig. 2 Absorption spectra of 1, 4, and 5 in CH2Cl2. The maximum absorption wavelengths are shown in the parentheses.
change was observed in intensity and wavelength of the Soret-band
corresponding to the porphyrin moiety at 421 nm. Meanwhile the
similar band in the NCP moiety shifted from 434 to 450 nm.
Besides, the original Q5-band at 716 nm, which was derived from
the NCP moiety, completely disappeared and a new Q-band
appeared at 683 nm. This spectral change is consistent with the
NH tautomerism induced by anion binding in NCP.8,12 In contrast,
almost no change was observed by the addition of Bu4N+F- to 5
in CH2Cl2. Thus, the fluoride ion is placed near the peripheral NH
moiety and the counter ammonium cation might be placed close
to the NCP. This means that the photophysical properties of NCP
chromophore can be switched by anion binding even in the dyad
system, where a similar trend is not possible by using the porphyrin
chromophore. The addition of Bu4N+BPh4- brought no change in
the absorption spectrum, while Bu4N+Cl- afforded a similar result
as in the case of Bu4N+F-.
To examine the relationship between the excitation energy
transfer and the anion binding, the fluorescence spectral change
of 1 with increasing amounts of Bu4N+F- was measured (Fig. 4b).
A diluted solution of 1 in CH2Cl2 was excited at 421 nm, where
the light was absorbed mainly by the porphyrin moiety. By the
addition of Bu4N+F-, the emission bands at 730 and 803 nm, which
were from the NCP chromophore, shifted to 703 and 765 nm,
respectively. The emission intensity directly from the porphyrin
chromophore (around 650 nm) was always negligible regardless of
the amounts of Bu4N+F- (less than 2% in area ratio) used. This
suggested efficient excitation energy transfer from the porphyrin to
the NCP chromophore in both cases, before and after the addition
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Fig. 5 Absolute quantum yields of 1, 4, and 6 in CH2Cl2.
0.006) was observed with Bu4N+Cl- or Bu4N+F-. Unexpectedly,
the addition of Bu4N+BPh4 also gave slightly better emission
-
efficiency although no change was observed in the absorption
spectrum. The enhancing effect on emission quantum yields by
the added halide anion was remarkable in 4 presumably due to
favorable induced dipole moment through anion binding. Similar
enhancement of the absolute quantum yields from Uem = 0.0053 to
0.0153 was observed for 1 in the presence of Bu4N+F- or Bu4N+Cl-.
This means that NCP would be a fascinating chromophore in
the dyad system, because its switching ability by anion binding
can coexist with excitation energy transfer. Note that essentially
the same quantum yields were observed in 1 when the NCP
chromophore was directly excited.14
Fluorescence lifetime measurements on 1 gave further informa-
tion on excitation energy transfer from porphyrin to NCP.15 The
porphyrin chromophore was predominantly excited at 420 nm
in the lifetime measurements. In the absence of ammonium salt,
lifetime of excitation energy transfer was relatively long (40 ps) and
emission lifetime was relatively short (370 ps). Expectedly, longer
emission lifetimes were observed after the addition of Bu4N+Cl-
(660 ps) or Bu4N+F- (770 ps) with shorter excitation energy
Fig. 3 Selected orbital energy levels and Kohn–Sham orbitals of 1
calculated at the B3LYP/6-31G** method.
of Bu4N+F-. The blue shift in the emission spectra (from 730
to 703 nm) was consistent with the blue shift in the absorption
spectrum (from 716 to 683 nm) of 1.
Absolute emission quantum yields (Uem) of 1, 4, and
tetrakis(pentafluorophenyl) NCP (6) in CH2Cl2 were measured
with a calibrated integrating sphere13 in the presence or absence
of excess amounts (~10000 equiv) of ammonium salts (Fig. 5).
4 and 6 were excited at their Soret-bands and 1 was excited at
the Soret band of the porphyrin chromophore (421 nm). In 6, a
small enhancement of emission efficiency (from Uem = 0.004 up to
Fig. 4 Anion binding behavior of 1 in CH2Cl2; (a) absorption spectrum change through addition of Bu4N+F- (6.6 mM of 1; 0–18 equivalent of Bu4N+F-),
(b) fluorescence spectrum change through addition of Bu4N+F- (0.69 mM of 1; 0–32 equivalent of Bu4N+F-; excited at 421 nm).
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transfer lifetime (both <10 ps), which was consistent with the
better emission quantum yields as described above.16 Acceleration
of energy transfer by anion binding could be explained by increase
of a normalized overlap integration factor between absorption
of NCP and emission of porphyrin in the Fo¨rster equation (see
Fig. S5 in ESI).17
In conclusion, we have demonstrated the construction of an
anion responsive dyad system by connecting a porphyrin and an
NCP. When the halogen anion was recognized by the NCP part,
the emission quantum yield was significantly improved and the
excitation energy transfer became faster. In addition to the anion
binding studies, one of the interesting properties of NCP is to
generate a wide variety of metal complexes with variable oxidation
states and leads to peculiar structures.18 Some of such complexes
possess unique emission properties.19 Combining a dyad system
and the coordination chemistry of NCP would afford interesting
molecular devices in the future.
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and gradually decomposed by photo-irradiation.
11 An insulating effect imposed by the perpendicular 1,4-phenylene bridge
moiety in 1 should be important also.
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The present work was supported by the Grant-in-Aid for
Scientific Research (19750036) and the Global COE Program
“Science for Future Molecular Systems” from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
14 5 (Uem = 0.0833) showed slightly less emission quantum yield in the
Notes and references
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©