Molecular engineering and solvent dependence of excitation energy hopping in self-assembled porphyrin boxes

Article abstract of DOI:10.1039/c2cc30834g

It has been demonstrated that the direction and magnitude of transition dipole moments, and hence rates in the excitation energy hopping in the self-assembled porphyrin boxes can be tuned by insertion of ethynyl groups as well as the dielectric constant of solvent.

Full text of DOI:10.1039/c2cc30834g

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COMMUNICATION
Molecular engineering and solvent dependence of excitation energy
hopping in self-assembled porphyrin boxeswz
Hee Won Bahng,a Pyosang Kim,a Young Mo Sung,a Chihiro Maeda,*b Atsuhiro Osuka*c
and Dongho Kim*a
Received 6th February 2012, Accepted 6th March 2012
DOI: 10.1039/c2cc30834g
It has been demonstrated that the direction and magnitude of
transition dipole moments, and hence rates in the excitation
energy hopping in the self-assembled porphyrin boxes can be
tuned by insertion of ethynyl groups as well as the dielectric
constant of solvent.
Recently, supramolecular chemistry utilizing a strategy of
non-covalent self-assembly has become a desirable choice to
prepare multi-porphyrin arrays as mimicries of light harvesting
(LH) antennae complexes.1 Interesting examples of porphyrin
systems so far reported include conjugated porphyrin ladders,2 a
tetrameric cage that encapsulates a guest porphyrin,3 and linear
giant porphyrin arrays assembled by imidazole–zinc(II) porphyrin
coordination.4 As such, molecular self-assembly can translate the
covalent connectivity and molecular shape of the components into
Chart 1 Molecular structures of D1–D3 and B1–B3.
rates, since the magnitude of the transition dipole moment has
a significant role in the Forster-type energy transfer process.9
¨
Encouraged by these results, we attempted to engineer the
tertiary structure.
In this context, the molecular component of a meso–meso-linked
diporphyrin5 is quite attractive because of its perpendicular
conformation, which may lead to unique architectures. In addition,
two porphyrins in the component are strongly coupled mainly by
intra-box EEH in B1 by inserting ethynyl units either along or
perpendicular to the long molecular axis to form B2 or B3 with
intention to increase the transition dipole moments along such
excitonic interactions but not by p-conjugation.6 This feature
directions (Chart 1). As described later, porphyrin boxes B2
and B3 were spontaneously self-assembled in non-polar solvents,
hence allowing the assessment of the effects of magnitude and
direction of transition dipole moment on excitonic interactions
encouraged us to explore self-assembled porphyrin boxes including
B1 as models for LH antennae.1 Recently, tetrameric squares
formed from 4-pyridine appended zinc(II) chlorophyll or 4-pyridyl-
and EEH processes.
Full synthetic details of D2 and D3 are given in ESI.z The
ethynyl-substituted zinc(II) porphyrin were reported to exhibit
excitation energy hopping (EEH),7 whose rates were accelerated
as compared to that of a porphyrin square formed from 4-pyridine-
appended zinc(II) porphyrins.8 In both cases, larger transition
dipole moments are considered to give rise to enhanced EEH
1H NMR spectra of Dn in CDCl3 exhibit single sets of signals
with distinct up-field shifts for the pyridyl protons (ESIz),
indicating the quantitative formation of Bn. In contrast,
such up-field shifts disappeared in coordinating pyridine-d5,
indicating the dissociation as Dn, which have been taken as a
a Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: dongho@yonsei.ac.kr; Fax: +82 2-2123-2434;
Tel: +82 2-2123-2652
non-assembled reference. The formations of Bn were also
indicated by their shorter retention times in analytical gel
permeation chromatography (Fig. S3, ESIz).
b Department of Applied Chemistry, Faculty of Science and
Technology, Keio University, Kohoku-ku, Yokohama 223-8522,
Japan. E-mail: cmaeda@applc.keio.ac.jp; Fax: +81 45-566-1551;
Tel: +81 45-566-1551
The absorption spectra of Dn are characteristic of free
meso–meso-linked zinc(II) diporphyrin, exhibiting split Soret
bands due to the well-established exciton coupling (Fig. 1).6,10,11
The insertion of triple bonds gives rise to bathochromic shifts in
the Soret bands of D2 and D3 as a whole but causes different
influences on the spectral shape; an intensified low-energy split
band in D2 and an intensified high-energy band in D3, in line
with the exciton coupling scheme. In addition, the inserted
triple bonds reduce the molecular symmetry, hence increasing
c Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
E-mail: osuka@kuchem.kyoto-u.ac.jp; Fax: +81 75-753-3970;
Tel: +81 75-753-4008
w This article is part of the ChemComm ‘Porphyrins and phthalocya-
nines’ web themed issue.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc30834g
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4181–4183 4181

Fig. 1 Steady-state absorption spectra (left) and fluorescence (right)
spectra of Dn (blue) and Bn (orange). The fluorescence spectra were
obtained using an excitation wavelength of 430 nm.
the intensity of the Q(0,0) band.12,13 The fluorescence spectra
of D2 and D3 are red-shifted from that of D1. As in the case of
D1, the relatively broad fluorescence spectra of D2 and D3
became sharpened in B2 and B3, probably due to the structural
rigidification particularly with regard to the dihedral angle of the
meso–meso linked diporphyrin upon box formation.14 In line
with this interpretation, the Stokes shifts in B2 and B3 are much
less than those in D2 and D3. These characteristic fluorescence
spectra of B2 and B3 are rather concentration independent up to
1.0 ꢀ 10ꢁ8 M, from which the association constants have been
Fig. 2 Steady-state fluorescence excitation anisotropy spectra (black)
and absorption spectra (blue) of Dn and Bn in CH2Cl2 (A) and toluene
(B). The spectra were obtained using parallel and perpendicular
orientations between excitation and emission polarizations at the fixed
emission wavelengths of 630 nm (D1/B1) and 650 nm (D2/B2, D3/B3).
smaller in going from Dn to Bn, indicating the presence of
additional depolarization channels in Bn, which are contributed
by the EEH processes within Bn.
estimated to be at least larger than 1024 Mꢁ3
.
To investigate the fast EEH processes within Bn, pump-power
dependent transient absorption (TA) decays were measured in
CH2Cl2 and toluene (Fig. 3), where the Q band excitations,
i.e., lpump = 570 nm (B1), lpump = 590 nm (B2/B3) were
employed to avoid the involvement of S2–S1 relaxation.16 The
TA decays of Dn were also measured as references. Dn revealed
pump-power independent slow TA decays in agreement with the
S1-state lifetimes (B1.2–1.6 ns) observed in the fluorescence
lifetime measurements. On the other hand, Bn showed relatively
fast pump-power dependent TA decays in the time region of
The steady-state fluorescence excitation anisotropy spectra were
comparatively measured. While D1(B1) and D2(B2) displayed the
common anisotropy profiles exhibiting negative anisotropy in the
high-energy Soret band and positive anisotropies in the low-energy
Soret and Q bands,6,10 D3(B3) showed a distinctly inverted feature
(Fig. 2). From these results, we concluded that the direction of
transition dipole moment of the emissive state is along the long axis
of diporphyrin (green) in D1(B1) and D2(B2), but perpendicular in
D3(B3) (blue). These features indicate that the insertion of the triple
bonds dictates the directions of the transition dipole moments that
participate in EEH. This trend is independent of solvent for B2 and
B3, while B1 displays solvent-dependent fluorescence anisotropy,
indicating that the predominant direction of the transition dipole
moments of emissive states in B1 changes from the long axis in
CH2Cl2 to the short axis in toluene.
The time-resolved fluorescence and its anisotropy decays of
Dn and Bn were measured in CH2Cl2 (Fig. S4, ESIz). Time-
resolved polarization anisotropy measurement is informative
in revealing the EEH process, because the energy migration
between the same molecular units with different orientations
provides a depolarization channel.15 But in the present case,
the fluorescence anisotropy decay time constants trot, however, do
not reflect the energy migration process within Bn, because its time
scale (B2.5–4.9 ns) is too slow for the energy transfer time. The
fluorescence anisotropy decay time constants trot increase in Bn
compared with those of Dn, reflecting an increased molecular
volume. Therefore, the trot values have been interpreted to
originate from the rotational diffusion motions in solution. The
initial r0 value of the fluorescence anisotropy decay became
Fig. 3 Transient absorption decay profiles of Bn that include pump-
power dependences. The transient absorption decay profiles of Dn are
shown as insets in the upper right corner of each panel. In the experiments,
the pump wavelengths were 570 nm (D1/B1), 590 nm (D2/B2, D3/B3) and
the probe wavelengths were 490 nm (D1/B1), 510 nm (D2/B2, D3/B3)
which correspond to the induced absorption bands.
c
4182 Chem. Commun., 2012, 48, 4181–4183
This journal is The Royal Society of Chemistry 2012

tens of picoseconds in addition to long-lived components
corresponding to the S1-state decays. When the pump power
was increased, the relative contributions of the fast t1 and t2
components were enhanced as compared to the slowest t3 one
(Table S4, ESIz). The pump-power dependence on the TA
decay is indicative of singlet–singlet annihilation processes,
because the intense excitation or high density of photons
generates two or more excitons in one assembly unit, and then
the recombination between the excitons gives rise to fast
deactivation processes. In this regard, while the fastest t1
components are likely to arise from multiple exciton generation
processes, the second fastest t2 components from the migration
limited exciton–exciton recombination processes between
meso–meso-linked zinc(II) diporphyrins rather than zinc(II)
porphyrin monomers, because this process does not occur in Dn.
The following formula relates the annihilation time (ta) to the
excitation energy hopping (EEH) time (th) for multichromophoric
systems where N is the number of hopping sites:17
Thus, the t2 components occurring in a few tens of picoseconds
seem to be related with the EEH processes in which the exact
hopping time can be derived by using a theoretical modeling of
the multichromophoric system as a planar 2-dimensional
polygon structure (ESIz). In this sense, it is difficult to estimate
the EEH times accurately from the TAA decays of Bn because
of their 3-dimensional structures. Nevertheless, the trends
observed in the t2 components are in good accordance with
the TA decays.
In summary, our attempt to engineer the direction and
magnitude of transition dipole moments of Bn by inserting ethynyl
units along or perpendicular to the long axis of meso–meso linked
diporphyrin has been verified by our combined photophysical
measurements. In addition, we have found that the EEH rates
in Bn depend on the dielectric constant of solvent.
Notes and references
1 (a) J.-M. Lehn, Science, 2002, 295, 2400; (b) M. D. Hollingsworth,
Science, 2002, 295, 2410.
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ta = (N2 ꢁ 1)/24th
(1)
Though Bn have eight mutually perpendicular porphyrin
monomer units, the number of hopping sites is conceived as
N = 4, because there are four major transition dipole
moments in Bn as shown in Fig. 2, contributing to the lowest
S1 states from which the EET processes occur. Thus, by using
eqn (1), the EEH times th of B1, B2 and B3 were evaluated to
be 28.8, 27.2, and 22.4 ps in CH2Cl2 and 17.6, 5.4, and 3.4 ps in
toluene, respectively. Therefore, the insertion of triple bonds
causes certain enhancement in EEH rate, which is, however,
more dependent upon solvent, only modest in CH2Cl2 but
distinct in toluene. The observed solvent effect may be explained
in terms of different dielectric constant; 8.93 for CH2Cl2 and
2.38 for toluene. The dielectric constants stand for the storage
capacity of electric energy, implying that CH2Cl2 can store
more electric charge than toluene. As a result, the magnitude of
effective transition dipole moments participating in the EEH
process should be larger in toluene than in CH2Cl2. It should be
noted that the decay profiles of B1 in TA measurements do not
seem to be affected by the solvent environment. This may be
accounted for in terms of the change in the transition dipole
moments participating in the EEH from the long to short
molecular axes upon the solvent change.
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Dn to Bn indicate the presence of fast depolarization channels
contributed by the EEH processes because the change in the
orientation of transition dipole moments is accompanied as
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4181–4183 4183