Versatile photophysical properties of meso-aryl-substituted subporphyrins: Dipolar and octupolar charge-transfer interactions

Article abstract of DOI:10.1002/chem.200901671

Donor-acceptor systems based on subporphyrins with nitro and amino substituents at meta and para positions of the meso-phenyl groups were synthesized and their photophysical properties have been systematically investigated. These molecules show two types

Full text of DOI:10.1002/chem.200901671

FULL PAPER
DOI: 10.1002/chem.200901671
Versatile Photophysical Properties of meso-Aryl-Substituted Subporphyrins:
Dipolar and Octupolar Charge-Transfer Interactions
Shanmugam Easwaramoorthi,[a] Jae-Yoon Shin,[a] Sung Cho,[a] Pyosang Kim,[a]
Yasuhide Inokuma,[b] Eiji Tsurumaki,[b] Atsuhiro Osuka,*[b] and Dongho Kim*[a]
Abstract: Donor–acceptor systems
based on subporphyrins with nitro and
amino substituents at meta and para
positions of the meso-phenyl groups
were synthesized and their photophysi-
cal properties have been systematically
investigated. These molecules show
two types of charge-transfer interac-
tions, that is, from center to periphery
and periphery to center depending on
the peripheral substitution, in which
the subporphyrin moiety plays a dual
role as both donor and acceptor. Based
on the solvent-polarity-dependent pho-
tophysical properties, we have shown
that the fluorescence emission of para
isomers originates from the solvato-
chromic, dipolar, symmetry-broken,
and relaxed excited states, whereas the
non-solvatochromic fluorescence of
meta isomers is of the octupolar type
with false symmetry breaking. The re-
stricted meso-(4-aminophenyl) rotation
at low temperature prevents the intra-
molecular charge-transfer (ICT)-form-
ing process. The two-photon absorption
(TPA) cross-section values were deter-
mined by photoexcitation at 800 nm in
nonpolar toluene and polar acetonitrile
solvents to see the effect of ICT on the
TPA processes. The large enhancement
in the TPA cross-section value of ap-
10ꢀ50 cm4 sphotonꢀ1) with donor–ac-
ceptor substitution has been attributed
to the octupolar effect and ICT interac-
tions. A correlation was found between
the
electron-donating/-withdrawing
abilities of the peripheral groups and
the TPA cross-section values, that is, p-
aminophenyl>m-aminophenyl>nitro-
phenyl. The increased stability of octu-
polar ICT interactions in highly polar
solvents enhances the TPA cross-sec-
tion value by a factor of approximately
2 and 4, respectively, for p-amino- and
m-nitrophenyl-substituted subporphyr-
ins. On the other hand, the stabilization
of the symmetry-broken, dipolar ICT
state gives rise to a negligible impact
on the TPA processes.
proximately
3200 GM
(1 GM=
Keywords: absorption
·
charge
transfer · donor–acceptor systems ·
photophysics · porphyrinoids
Introduction
tical storage,[2–6] micro- to nanofabrication,[7–9] and three-di-
mensional imaging of biological systems.[10–13] As part of the
mission of increasing TPA efficiency, molecules showing in-
tramolecular charge-transfer (ICT) interactions between
donor (D) and acceptor (A) groups through intervening p-
conjugation bridges have been identified to be quite promis-
ing.[14–17] For several decades along this line, much attention
has been focused on the dipolar molecules, in which the
charge asymmetry created by the permanent dipole moment
is redistributed under the influence of an intense electro-
magnetic field through a highly polarizable p-conjugated
system, which contributes to the higher nonlinear optical
properties of molecules. Recently, attention has turned from
these dipolar molecules to branched multipolar molecules
such as quadrupolar (D–p-A–p-D or A–p-D–p-A)[18,19] and
octupolar systems (AD3 or DA3), which show non-monoton-
ically increased TPA properties as compared with the sum
of their individual dipolar branches.[20–25] Important parame-
ters that influence the TPA properties in octupolar systems
Two-photon absorption (TPA) processes have received in-
creasing attention over the past twenty years. The multitude
of applications include spectroscopy,[1] three-dimensional op-
[a] Dr. S. Easwaramoorthi, J.-Y. Shin, Dr. S. Cho, P. Kim,
Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems
and Department of Chemistry
Yonsei University, Seoul 120-749 (Korea)
Fax : (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
[b] Y. Inokuma, E. Tsurumaki, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901671.
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are the following: 1) a first hyperpolarizability that qualita-
tively exhibits a linear relationship with the TPA processes,
2) multidimensionality of core-to-periphery or periphery-to-
core charge transfer, 3) donor and/or acceptor strengths of
constitutional segments, and 4) additional coherent coupling
between the branches.[26–30]
Typical examples of octupolar molecular systems are
three-dimensional p-conjugated systems such as triphenyl-
benzene[21,26] and triphenylamine derivatives,[17,20] metal–
ligand complexes,[31,32] subporphyrins,[33,34] and subphthalo-
cyanines.[35,36] Among these, subporphyrins and subphthalo-
cyanines deserve particular attention because of their rich
p-electron networks that communicate through the triangu-
lar cone-shaped structure. Since our inception in 2006, we
have been continuously exploring the chemistry of subpor-
phyrins,[37] which includes the large substituent effects of
meso-aryl substituents and peripheral b substituents on the
electronic properties,[38] remarkable ICT interactions, and
cation-sensing by fluorescence spectroscopy.[39] Importantly,
we have unveiled the progressive increase of TPA cross-sec-
tion values (s(2)) from approximately 80 GM (1 GM=
10ꢀ50 cm4 sphotonꢀ1) for triphenyl subporphyrin (1) to
1340 GM for tris-oligo(1,4-phenyleneethynylene) subpor-
phyrins upon 800 nm photoexcitation, which is essentially
originating from the octupolar effect.[33] Further enhance-
ment of the s(2) value to 2800 GM has been attained by at-
taching six phenylethynyl substituents at b positions.[34] In
contrast, porphyrins do not show such enhancement with
either donor/acceptor perturbation or elongated p conjuga-
tion at the meso-phenyl group on account of its negligible
electronic communication with the porphyrin core caused
by the mutually orthogonal orientation. Nevertheless, larger
TPA efficiencies of porphyrins have been achieved by the
extended p conjugation through multiporphyrins using dif-
ferent linkages.[40,41]
reduced by SnCl2·2H2O in a mixture of aqueous EtOH/HCl
to afford p-NH2 and m-NH2, respectively.
Crystal structure and NMR spectroscopy: A single crystal of
p-NH2 suitable for X-ray diffraction analysis was obtained
by recrystallization from THF/methanol. The crystallograph-
ic data of p-NH2 are summarized in the Experimental Sec-
tion, and the structure is shown in Figure 1. The crystal lat-
tice includes two independent p-NH2 molecules, in which
the dihedral angles of the meso-aryl rings with respect to
the mean plane of the subporphyrin core are 41.2, 45.4, and
57.98 for one subporphyrin, and 44.1, 48.2, and 58.68 for the
other. As seen in meso-(N,N-dibenzylamino)phenyl subpor-
phyrin 3,[39] p-NH2 also exhibits considerable bond-length al-
ternation for its all-meso 4-aminophenyl rings. The bond
ꢀ
ꢀ
ꢀ
Inspired by the versatile electronic properties of octupolar
subporphyrin molecules, herein we report the synthesis and
photophysical and TPA properties of meta and para isomers
of meso-aminophenyl- (m-NH2 and p-NH2, respectively) and
meso-nitrophenyl-substituted subporphyrins (m-NO2 and p-
NO2, respectively). Aminophenyl-substituted subporphyrins
can be regarded as a three-donor–one-acceptor system
(D3A) and nitrophenyl-substituted subporphyrins can be re-
garded as a one-donor–three-acceptor system (DA3). Sys-
tematic investigations on the photophysical properties of
these subporphyrins reveal the nature and degree of ICT in-
teractions that can be correlated with the TPA efficiencies.
lengths between C(1) C(2), C(3) C(4), C(4) C(5), and
ꢀ
C(6) C(1) are in the range from 1.386–1.401, whereas those
ꢀ
ꢀ
of C(2) C(3) and C(5) C(6) are in the range from 1.376–
1.383 ꢁ, thereby indicating a clear quinonoid-type bond al-
ternation. On the other hand, 4-nitrophenyl analogue p-
NO2, as we reported in our previous paper,[38] does not show
such distinctive bond-length alternation for its meso-aryl
ꢀ
rings, although each C C bond length is slightly different
from that of ideal benzene rings. Besides, structural parame-
ters for the subporphyrin cores of p-NH2 and p-NO2 are es-
sentially the same. These results might be indicative of
rather stronger interactions between the meso-(4-amino-
phenyl) substituent and the subporphyrin core than those
between the 4-nitrophenyl and the core in the ground state.
Although a lot of intermolecular hydrogen bonds, which
might change the stable dihedral angles in the solid state,
were observed in the structure, such low dihedral angles sug-
gest a low rotational barrier of meso-aryl rings and thus
strong conjugative interactions between the meso-aryl sub-
stituents and subporphyrin core. More evidently, the
1H NMR spectrum of p-NH2 shows a couple of doublets at
d=7.86 and 7.00 ppm due to the meso-aryl protons, thereby
Results
Synthesis: We have synthesized meta and para isomers of
amino- (p-NH2 and m-NH2, respectively) and nitrophenyl-
(p-NO2 and m-NO2, respectively) substituted subporphyrins.
meso-Nitrophenyl-substituted subporphyrins p-NO2 and m-
NO2 were synthesized according to our modified Adler pro-
tocol, in which peripheral nitro groups were quantitatively
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Aryl-Substituted Subporphyrins
FULL PAPER
tal energy levels of both
HOMO and LUMO are desta-
bilized due to the electron-do-
nating character of the amino
groups.
Importantly,
the
largest
impact of the amino groups is
seen in the HOMO(a1) of p-
NH2, in which the orbital coef-
ficients are fully delocalized on
both the subporphyrin core
and meso-aryl substituents; the
destabilization energy from the
HOMO of 1 was calculated to
be 0.60 eV. On the other hand,
the destabilization energy of
m-NH2 has been estimated to
be only 0.22 eV. These results
delineate that the substituent
effect on para positions is sig-
nificantly larger than that of
meta positions. The energy dif-
ference in the HOMOꢀ1(a2)
between p-NH2 and m-NH2 is
quite small, which is attributed
to ineffective conjugation with
meso substituents through
node points at meso positions.
In the cases of p-NO2 and
m-NO2, the frontier orbitals
are largely stabilized due to
the electron-withdrawing char-
acter of the nitro group. Inter-
estingly, the LUMOs of p-NO2
and m-NO2 are spread over
the meso-attached nitrophenyl
moieties. Subporphyrin p-NO2
exhibits the highest stabiliza-
tion in LUMO energy (ca.
1.1 eV). As a consequence, the
Figure 1. The crystal structure of p-NH2.
HOMO–LUMO gaps were cal-
culated to be 3.18, 2.94, 3.13,
indicating its rapid rotation in solution on the NMR spectro-
scopic timescale. The solid-state structure of p-NO2 is simi-
lar to that of p-NH2.[38] In addition, the meta-substituted sub-
porphyrins m-NH2 and m-NO2 exhibit one singlet each due
to the b protons and a single signal set due to the meta-sub-
stituted phenyl groups at room temperature in CDCl3.
These results show that the free rotation of meso-aryl
groups is also allowed for m-NH2 and m-NO2, and thus p-
conjugative interactions between the meso-aryl group and
core are significant both for m-NH2 and m-NO2.
2.94, and 3.16 eV for 1, p-NH2, m-NH2, p-NO2, and m-NO2,
respectively. It can be concluded that the smallest HOMO–
LUMO gaps of 2.94 eV in p-NH2 and p-NO2 are caused by
a large electronic interaction at para positions, namely, the
significant destabilization of the HOMO for p-NH2 or stabi-
lization of the LUMO for p-NO2 by the electron-donating
and -withdrawing characters of the amino and nitro groups,
respectively.
Steady-state spectral properties: Figure 3 shows the steady-
state absorption and fluorescence spectra of 1, p-NH2, m-
NH2, p-NO2, and m-NO2 in toluene. In general, the para iso-
mers exhibit redshifted B- and Q-like bands compared with
the corresponding meta isomers. In fact, both the absorption
and fluorescence spectra of the latter ones are almost identi-
MO calculations: Figure 2 shows the MO diagrams of the
subporphyrins 1, p-NH2, m-NH2, p-NO2, and m-NO2 calcu-
lated by using the Gaussian 03 program at the B3LYP/6-
31G* level.[42] In the MOs of subporphyrins p-NH2, the orbi-
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A. Osuka, D. Kim et al.
(m-NO2) indicate significant
structural changes between the
ground and excited states. In
contrast, the electronic proper-
ties of meso-aryl-substituted
porphyrins remain relatively
unperturbed and insensitive to
meta or para substituents due
to their nearly orthogonal ori-
entations.
Solvatochromism: Encouraged
by the results of our previous
investigations on the ICT inter-
actions of meso-tris(4-N,N-ben-
zylaminophenyl)subporphyr-
in[39] and meso-tris(4-nitrophe-
nyl)subporphyrin,[38]
we
became interested in the struc-
ture–property relationship be-
tween meta and para positions
and ICT processes. Subpor-
phyrins p-NH2 and p-NO2 ex-
hibit redshifted fluorescence
with increased solvent polarity
(Figure S1 in the Supporting
Information). The quantitative
analysis of solvatochromism by
the Lippert–Mataga plot[43,44]
provided
the
slopes
of
2235 cmꢀ1 for p-NH2 and
1061 cmꢀ1 for p-NO2. These
large slopes suggest more polar
excited states for both subpor-
phyrins but in opposite direc-
tions. On the contrary, the ab-
sence of such strong solvato-
chromism for 1, m-NH2, and
m-NO2 suggests that a change
in the excited-state dipole
moment induced by the solvent
polarity is negligible. The ob-
served small Stokes shifts
Figure 2. Frontier molecular orbitals (B3LYP/6-31G(d)) of optimized structures of 1, p-NH2, m-NH2, p-NO2,
and m-NO2.
A
cal to their parent compound 1. This feature is ascribed to
the inherently weaker influences of meta substituents on the
electronic property of the subporphyrin core. In addition,
the broad single Q-like band of p-NH2 changes into two
are attributed to the refractive index changes of the solvent
rather than solvent-polarity-induced dipole moment change.
This feature suggests a different operating ICT mechanism
(vide infra) for meta and para isomers.
well-resolved peaks corresponding to QACTHNUGTRENNUG(1,0) and QAHCTUNGTRNE(NUGN 0,0)
transitions in m-NH2, and the lowest energy Q-band of p-
NO2 becomes smeared in m-NO2. The steady-state fluores-
cence spectra were observed at 518, 571, 522, 533, and
517 nm for 1, p-NH2, m-NH2, p-NO2, and m-NO2, respec-
tively, with the mirror image of their respective Q-bands.
The redshifted and intensified fluorescence spectrum of p-
NH2 is impressive. In addition, the larger Stokes shifts of
2095 (p-NH2), 1251 (m-NH2), 1481 (p-NO2), and 2256 cmꢀ1
Fluorescence quantum yields and lifetimes: The fluores-
cence quantum yields (Ff) for subporphyrins in the solvents
with different polarities have been determined with refer-
ence to Nile red (0.72) in dioxane (Table 1). All the substi-
tuted subporphyrin derivatives are more strongly fluorescent
in toluene than 1. For instance, the Ff of p-NH2 in toluene is
about 57%, which is approximately four times larger than
that of 1 (Ff =13%). The fluorescence quantum yields of all
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Aryl-Substituted Subporphyrins
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The solvent-dependent time-resolved fluorescence decay
profiles of m-NH2 and m-NO2 are shown in Figure 4 (for p-
NH2, see Figure S3 in the Supporting Information). The flu-
orescent decays can be fitted with a single-exponential func-
tion and the lifetime (tf) values are given in Table 1. Owing
to the limitation in the time resolution of our time-correlat-
ed single-photon counting (TCSPC) setup (ꢁ100 ps), the
fluorescence lifetimes of p-NO2 and m-NO2 in acetonitrile
were obtained by using a fluorescence up-conversion tech-
nique, and the decay profiles are presented in Figure 4c and
4d, respectively. The fluorescence lifetime of p-NH2, unlike
its Ff values, remains insensitive to all the solvents studied
here, which has not been observed for other derivatives.
Nonetheless, in our recent report we have unveiled solvent-
insensitive Ff values and fluorescence lifetimes when the
amino group is N-substituted with an aryl group.[39] These
properties stress the importance of the electron-donor
strength or hydrogen-bonding ability for the fluorescence
property of p-NH2. In contrast, the fluorescence lifetime of
the corresponding meta derivative m-NH2 was found to be
reduced with increasing solvent polarity (Table 1). On the
other hand, the fluorescence lifetime of m-NO2 (3.9 ns) is
longer than p-NO2 (2.8 ns) in toluene, but in strongly polar
acetonitrile that of m-NO2 becomes considerably shorter
(0.03 ns) by a factor of at least four compared to p-NO2
(0.14 ns) under similar experimental conditions. To obtain a
quantitative relationship, the nonradiative (nr) rate con-
stants calculated from the fluorescence quantum yields and
lifetimes (knr =(1ꢀFf)/tf) are plotted against the solvent-po-
larity parameters ET(30)[45] (Figure S4 in the Supporting In-
formation); p-NO2 and m-NO2 are found to show, respec-
tively, 35 and 150 times enhanced nonradiative decay rates
in acetonitrile as compared with the nonpolar solvent, tolu-
ene, whereas the nonradiative decay rates of p-NH2 and m-
NH2 are rather insensitive to ET(30). In the absence of a
specific correlation between the nonradiative deactivation
rate and the emission energy, the role of internal conversion
or intersystem crossing can be neglected in accounting for
the faster excited-state deactivation. Notably, despite the ab-
sence of solvatochromism in the fluorescence spectra for
meta isomers, the interesting solvent-dependent fluorescence
lifetimes and quantum yields imply the possibility of ICT in-
teractions.
Figure 3. UV/Vis absorption (c) and fluorescence (b) spectra of
subporphyrins in toluene.
Table 1. Photophysical properties of subporphyrin p-NH2, m-NH2, p-NO2
and m-NO2 in various solvents.
[c]
Solvent
Ff[a]
tf [ns][b]
kr [108 sꢀ1
]
p-NH2
m-NH2
p-NO2
m-NO2
toluene
chloroform
CH2Cl2
acetone
CH3CN
toluene
chloroform
CH2Cl2
acetone
CH3CN
toluene
chloroform
CH2Cl2
acetone
CH3CN
toluene
chloroform
CH2Cl2
0.57
0.46
0.41
0.29
0.26
0.20
0.09
0.09
0.06
0.02
0.29
0.24
0.22
0.20
0.02
0.27
0.05
0.02
0.03
0.005
4.63
4.59
4.88
4.71
4.70
2.93
2.13
2.04
2.14
1.12
2.80
2.20
1.92
2.31
0.18
3.90
0.85
0.39
0.57
0.03
1.24
1.00
0.84
0.62
0.55
0.68
0.42
0.39
0.28
0.16
1.04
1.09
1.15
0.87
1.00
0.69
0.57
0.46
0.52
1.41
Spectroscopic measurements at low temperatures: Absorp-
tion and fluorescence spectra were measured at low temper-
atures in both
a nonpolar toluene/methylcyclohexane
(MCH; 1:4) mixture and polar butyronitrile solvents. The
absorption spectra were recorded in the temperature range
of 173–298 K to explore the influence of free rotation of
meso-aryl groups on the spectral properties of subporphyr-
in.[38] Upon lowering the temperature, the absorption spectra
of subporphyrin derivatives in the toluene/MCH mixture ex-
hibit substantial changes, which is characteristic of the elec-
tron-donating/-withdrawing nature of the substituent and its
para/meta position (Figure S5 in the Supporting Informa-
tion). For instance, as the temperature was lowered, the B-
acetone
CH3CN
[a] Fluorescence quantum yield. [b] Fluorescence lifetime obtained by
the TCSPC method. [c] Radiative rate constant calculated from the fluo-
rescence quantum yield and lifetime (see main text).
donor/acceptor-substituted subporphyrins, however, were
found to decrease gradually with an increase in solvent po-
larity.
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A. Osuka, D. Kim et al.
Figure 4. Fluorescence decay profiles of a) m-NH2 and b) m-NO2 in various solvents obtained by the TCSPC method; c) p-NO2 and d) m-NO2 in acetoni-
trile obtained by the femtosecond fluorescence up-conversion method (the decays are monitored at the respective fluorescence maximum; IRF=instru-
mental response function).
like bands of 1, p-NO2, and m-NO2 were slightly intensified
with small redshifts (ca. 2 nm), whereas those of p-NH2 and
m-NH2 were broadened with larger redshifts. The tempera-
ture dependence of p-NH2 and m-NH2 is similar to that ob-
served for highly conjugated meso-alkynyl-substituted multi-
porphyrins.[46–48] In addition, the notable redshift of the Q-
like band for p-NH2 (21 nm) compared with m-NH2 (2 nm)
is attributable to the stronger electronic interactions of the
para-aminophenyl group. Another important parameter is
the full width at half-maximum (FWHM) of the absorption
bands, which are obviously influenced by the meso-aryl
group dihedral angle distribution. Accordingly, the differen-
ces in FWHM values for B-like bands between 173 and
298 K are ꢀ308, 1470, 891, 586, and ꢀ235 cmꢀ1, respectively,
for 1, p-NH2, m-NH2, p-NO2, and m-NO2. The larger
FWHM values for p-NH2 and m-NH2 are due mainly to the
broad absorption bands at low temperatures, whereas the
observed negative FWHM differences for 1 and m-NO2 are
caused by the sharper absorption band at low temperatures.
The fluorescence spectra at different temperatures were
measured by excitation at three different wavelengths, in
which the temperature-change-induced absorption changes
were found to be minimal so that the observed trend was in-
dependent of the excitation wavelength. As a general trend,
a slightly redshifted narrow spectrum was observed upon
lowering the temperature, but the changes in the Stokes
shift remain fairly small and nearly temperature independ-
ent. In addition, the FWHM values of fluorescence spectra
were found to be smaller at low temperatures and the differ-
ences between the highest (298 K) and lowest (178 K) tem-
perature in a mixture of toluene/MCH were calculated to be
656, 368, 73, 452, and 166 cmꢀ1, respectively, for 1, p-NH2,
m-NH2, p-NO2, and m-NO2, and a similar trend was also ob-
served for butyronitrile solvent. Figure 5 shows the fluores-
cence spectra of p-NH2 in a toluene/MCH mixture and bu-
tyronitrile at temperatures ranging from 163 to 298 K. Evi-
dently, p-NH2 shows strongly suppressed fluorescence in a
toluene/MCH mixture at low temperature, but in contrast, a
small increase is noted in butyronitrile. The reference mole-
cule 1, meso-phenyl-substituted analogues m-NH2 (toluene/
MCH and butyronitrile), p-NO2 (toluene/MCH), and m-
NO2 (butyronitrile) show enhanced fluorescence as the tem-
perature decreases, whereas that of p-NO2 in butyronitrile is
nearly intact (Figure S6 in the Supporting Information). A
quantitative relationship with temperature can be obtained
from the relative fluorescence quantum-yield measurements
(corrected for the absorbance changes) in comparison with
that at room temperature as a reference.
Consequently, the fluorescence quantum yield of p-NH2
in a toluene/MCH mixture decreases gradually from 57% at
298 K to 14% at 193 K and becomes saturated beyond
193 K, but in butyronitrile it remains fairly independent of
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Aryl-Substituted Subporphyrins
FULL PAPER
Table 2. Summary of two-photon absorption cross-section values ob-
tained by 800 nm, 120 fs photoexcitation in different solvents.
Molecule Solvent s(2)
[GM]
Fs(2)
s
(2)/Mr
Fs(2)/Mr
[GM] [GMgꢀ1 molꢀ1
]
[GMgꢀ1 molꢀ1
]
1
CH2Cl2
80Æ30
9
0.16
2.57
5.87
1.56
1.69
0.57
0.71
0.63
0.02
1.46
1.53
0.31
0.03
0.16
0.01
0.17
0.01
p-NH2
toluene 1400Æ200 798
CH3CN 3200Æ300 832
m-NH2
p-NO2
m-NO2
toluene
CH3CN
toluene
CH3CN
CH2Cl2
850Æ100 170
920Æ100
360Æ50
450Æ50
400Æ50
18
104
9
108
CH2Cl2/ 1600Æ200
8[a] 2.52
CH3CN
(1:2)
3
toluene 1900Æ200 1116
3.02
2.86
1.77
1.57
CH3CN 1800Æ200 988
[a] The Ff value in acetonitrile is used.
m-NO2 were carried out in dichloromethane and a dichloro-
methane/acetonitrile (1:2) mixture due to the poor solubility
in toluene. The determined s(2) values roughly follow the
order of p-NH2 >m-NH2 >p-NO2ffim-NO2. Notably, p-NH2
and m-NH2 exhibit larger TPA values than p-NO2 and m-
NO2. As the TPA properties of molecules are influenced by
the magnitude of ICT interactions, we have also measured
the s(2) values in acetonitrile, in which the ICT state is ex-
pected to be considerably more stabilized than that in tolu-
ene. Evidently, the s(2) value of p-NH2 is enhanced signifi-
cantly from 1400 to 3200 GM in going from toluene to ace-
tonitrile solvent. Similar enhancement of the s(2) value of m-
NO2, from 400 to 1600 GM, is also observed in going from
dichloromethane to dichloromethane/acetonitrile (1:2) mix-
ture. However, those of m-NH2 and p-NO2 remain rather
constant within the experimental uncertainty.
Figure 5. Temperature-dependent fluorescence spectra of p-NH2 in a) a
MCH/toluene mixture and b) butyronitrile in the temperature range
from 163 to 298 K at 15 K intervals.
temperature (Figure 6). In contrast, 1, m-NH2, p-NO2, and
m-NO2 exhibit nearly temperature-independent fluorescence
quantum yields within the experimental error.
Discussion
ICT in octupolar molecules: The charge resonance for the
linear, dipolar molecule with a donor–acceptor pair can be
described by two electronic states,[50] DA$D+Aꢀ, which
have been accompanied by a larger dipole moment change
between the ground and excited states, apparently identified
by strong fluorescence solvatochromism. On the other hand,
multibranched, multipolar systems comprising more than
one donor–acceptor group exhibit at least two types of
charge-resonance structures that depend upon the cumula-
tive or subchromophoric behaviors of donor–acceptor pairs.
Specifically, one of the plausible charge-resonance structures
for quadrupolar (ADA or DAD) and octupolar systems
(AD3 or DA3), respectively, can be denoted as DAD$Dd+
Figure 6. A plot of relative fluorescence quantum yields of p-NH2 and 1
versus temperature (the maximum possible error limit in fluorescence
*
quantum yields is 10%): p-NH2 in MCH/toluene; & p-NH2 in butyroni-
~
trile; 1 in MCH/toluene.
Two-photon absorption processes: The TPA cross-section
(s(2)) values were obtained by using a Z-scan technique[49]
by excitation at 800 nm with a femtosecond (ꢁ120 fs
FWHM) Ti:sapphire oscillator system. The typical Z-scan
traces of p-NH2 and m-NO2 are given in the Supporting in-
formation (Figures S7–S8) and the s(2) values in toluene and
acetonitrile solvents are presented in Table 2. As revealed in
Table 2, the s(2) values were found to increase significantly
from approximately 80 (1) to 360 (p-NO2), 850 (m-NH2),
and 1400 GM (p-NH2) in toluene. The s(2) measurements of
AꢀDd+ and DA(D)D$Dd+Aꢀ
(Dd+)Dd+. Because of their
ACHTUNGTRENNUNG
highly symmetric structures, the CT has not been accompa-
nied by a significant dipole moment change, and hence, is
expected to show non-solvatochromic fluorescence.[51,52]
Nonetheless, several examples have been reported in the lit-
erature of quadrupolar[53] and octupolar molecules[22,54,55]
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A. Osuka, D. Kim et al.
that exhibit solvatochromic fluorescence similar to dipolar
molecules, which is related to the symmetry breaking in the
excited state by the formation of a relaxed, dipolar excited
state localized on one of the donor–acceptor pairs. The CT
states for quadrupolar and octupolar molecules can be de-
scribed by three (DAD$D+AꢀD$DAꢀD+) and four
charge-resonance structures, respectively (Scheme S1 in the
Supporting Information).
Despite the octupolar symmetry of subporphyrins, a large
Stokes shift of para isomer with increasing solvent polarity,
reminiscent of that of dipolar chromophores, can be attrib-
uted to the excited-state symmetry-breaking through the for-
mation of solvatochromic, dipolar, and a relaxed excited
state.[54–58] Thus, the formed CT state can be explained by
three charge-resonance structures in which only one meso-
aryl group takes part in the excited charge-separated state
simultaneously (Scheme S1).[59,60] In particular, the longer
fluorescence lifetime and higher fluorescence quantum yield
of p-NH2 indicate the formation of an intramolecular-conju-
gated CT state with a quinonoid-type structure localized on
one of the meso-aminophenyl groups. The CT state of p-
NH2 is depicted in Scheme 1, which can be regarded as a ho-
mologue of boron dipyrromethane (BODIPY) dye.[61]
This consideration has been strongly supported by the
Figure 7. Potential-energy diagram of relative energy levels of charge-res-
onance structures in nonpolar solvent at a) RT and b) low temperature
(Abs=absorption, Fl=fluorescence, Ea =activation energy).
Scheme 1. The symmetry-broken CT state of p-NH2.
facts that the observed solvatochromic behaviors such as
fluorescence quantum yield and lifetime and the slope of
the Lippert–Mataga plot of p-NH2 are nearly identical to
those of mono-N,N-dibenzylaminophenyl-substituted sub-
porphyrin 2.[39] The radiative rate constant calculated from
the fluorescence quantum yield and lifetime using the rela-
tionship of kr =1/t0 and t0 =tf/Ff decreases gradually from
1.24ꢂ108 to 0.55ꢂ108 sꢀ1 in going from nonpolar toluene to
polar acetonitrile (Table 1). In contrast, the radiative rate re-
mains virtually solvent independent for tris-N,N-dibenzyla-
minophenyl-substituted subporphyrin 3.[39] As p-NH2 and 3
show similar ICT interactions, the difference in the radiative
rate could arise from the hydrophobic nature and different
the fluorescence of p-NH2 shows high dependence upon the
solvent polarity and temperature.
In 3, the ICT state is thought to be truly localized with
the involvement of one N,N-dibenzylaminophenyl group,
which may explain the observed solvent-insensitive intense
fluorescence. Meanwhile, the facile rotation of the unsubsti-
tuted aniline groups in p-NH2 leads to effective charge de-
localization between three equivalent, degenerate ICT
states as a result of fast interconversion (tunneling) between
the resonance structures, thus decreasing the radiative decay
rate.[60] Indeed, a similar behavior has been noted previously
for multibranched molecules[62] in which those with three
side arms show an increased radiative decay rate relative to
monomers and dimers due to the delocalization of excitonic
states between the branches. On the other hand, p-NO2 un-
dergoes a photoinduced ICT processes from the subporphy-
hydrogen-bonding
characters
of
N,N-dibenzylamino
groups.[38,39] The symmetry-broken localized ICT state is
probably not accessible through relaxation of the Franck–
Condon excited state but through an activated rotation pro-
cess of the meso-aryl group, as shown in Figure 7.[51,60] Thus,
AHCTUNGTRENNUNG
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Aryl-Substituted Subporphyrins
FULL PAPER
meso-nitrophenyl groups and the subporphyrin core, and
thus it shows a solvent-polarity-induced Stokes shift. Indeed,
a substantial slope in the Lippert–Mataga plot for p-NO2
(1061 cmꢀ1) was observed, which is, however, smaller than
that of p-NH2 (2235 cmꢀ1).
phenyl and the subporphyrin core. But the fluorescence at
low temperature arises from the relaxed excited state, as can
be seen from the relatively smaller FWHM values of the
fluorescence band. Particular attention in this section is paid
to p-NH2, because it shows dramatically decreased fluores-
cence intensity in a mixture of methylcyclohexane/toluene
upon lowering the temperature. Such changes are crucial to
gaining information on the charge-transfer mechanism be-
cause structural changes are accompanied by intramolecular
charge transfer.[65] In this context, the temperature-depen-
dent Ff values were examined to elucidate the role of con-
formational relaxation in the formation of the ICT state as
discussed below.
Interestingly, the absence of a solvatochromic redshift for
m-NH2 and m-NO2 suggests that the ICT interaction is of
octupolar nature and therefore, unlike their para congeners,
is not accompanied by the solvent-induced dipole moment
changes. Furthermore, the accelerated nonradiative decay of
meta isomers in polar solvents ascertains stronger charge-
transfer interactions than para isomers. In fact, different
types of ICT interactions of ortho-, meta-, and para-phenyl-
substituent positions[63,64] have been well established for the
dipolar molecules with a single donor–acceptor pair. Never-
theless, this is the first report, to the best of our knowledge,
that the octupolar molecule exhibits two different types of
ICT mechanisms depending on the meso-phenyl-substitution
position. This result may be interpreted as follows: A re-
stricted meso-phenyl rotational motion by meta-phenyl sub-
stituents (relative to para-phenyl substituents) due to the
steric hindrance between the substituent and subporphyrin
core, which prevents a symmetry-broken, dipolar, ICT-form-
ing process. However, considering the distance between the
meta substituent and subporphyrin core, the free rotation of
the meso-phenyl ring is seemingly not affected by the meta
substituents. In addition, NMR spectroscopic data also con-
firms the free meso-phenyl rotation of meta isomers, thus
the influence of steric factors on the different ICT interac-
tions can be ruled out. The other possibility would be the
appreciable difference in electronic coupling between the
meta and para isomeric forms. As the ICT interactions are
driven by through-bond interactions, the direct linkage
across meta positions is less effective due to their inherently
weaker electronic coupling than the linkage across para po-
sitions. Thus the poor electron delocalization through meta-
phenyl positions accounts for the unfavorable symmetry-
broken, dipolar, ICT-forming process, thereby resulting in
non-solvatochromic octupolar ICT interactions.
The large Ff value of p-NH2 in a mixture of methylcyclo-
hexane/toluene that arises from the localized ICT state has
largely been suppressed at low temperatures because of a
certain activation barrier. Quite intriguingly, the Ff value of
p-NH2 at 173 K is equivalent to that of 1. This feature can
be directly correlated to the formation of a dipolar, local-
ized, emissive ICT state that is accessible through meso-4-
aminophenyl group rotation and not through the Frank–
Condon relaxation process. At low temperatures, the mole-
cule in the S1 state cannot undergo such a process to form a
localized ICT state (Figure 7b), since the rotational motion
of the meso-aminophenyl group is significantly restricted.
Thus the fluorescence at low temperatures comes from the
Frank–Condon state with low Ff. Similar phenomena have
commonly been observed for the molecules that show twist-
ed intramolecular charge-transfer (TICT) processes in which
the low-energy TICT emission is considerably reduced or
vanished at low temperatures due to the frozen conforma-
tional relaxation that prevents the formation of the electron-
ically decoupled TICT state.[65] Furthermore, our interpreta-
tion is also in a good agreement with the previous reports
on the multibranched quadrupolar and octupolar structures
for which the symmetry breaking in nonpolar solvent de-
pends on the height of the energy barrier that separates the
identical, degenerate charge-resonance structures.[60,66] In
this case, the symmetry breaking occurs through the exciton-
ic interaction between the branches; however, in the present
subporphyrin case, it occurs through the p-conjugation path-
way of the subporphyrin moiety accompanied by meso-aryl
group rotation. On the contrary, despite enhanced mesomer-
ic interactions upon lowering the temperature, for p-NH2 in
polar solvent, butyronitrile, the Ff remains nearly constant
within the experimental uncertainty. This observation is con-
sistent with our interpretation (vide supra) that the ICT in-
teraction in polar solvent at room temperature is of an octu-
polar nature due to the facile charge resonance between all
the meso-p-aminophenyl groups (three-dimensional charge
transfer) and hence has not been significantly affected by
the restricted meso-aryl group rotation at low temperatures,
which is responsible for the unchanged Ffl irrespective of
temperature, similar to its meta congener.
Temperature-dependent fluorescence: As a unique feature
of octupolar donor–acceptor systems, subporphyrins exhibit
highly variable optical properties that arise from large elec-
tronic interactions between the polar meso-aryl substituents
and the subporphyrin core. In this context, the rotational
freedom of the meso-aryl group gains much significance.
Thus we have examined the fluorescence spectra at different
temperatures to reveal the role of such rotational motions
of the meso-aryl substituents in the electronic interactions
systematically. Explicitly, the larger electronic interactions
due to the enhanced coplanarity can be seen from the tem-
perature-dependent absorption spectral changes such as the
redshifted B-like absorption bands (Figure S5 in the Sup-
porting Information). Besides, the broad absorption peaks
with large FWHM values at low temperatures, except for 1
and m-NO2, indicate the existence of various conformers
with a distribution of dihedral angles between the meso-
Two-photon absorption processes: The third-order nonlinear
optical properties of donor–acceptor-substituted subporphy-
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12013

A. Osuka, D. Kim et al.
A
very small or negligible increase in the s(2) value for p-NO2
and m-NH2 is probably due to the comparatively smaller
ICT interactions. For p-NO2, another probable reason would
be the stabilization of the symmetry-broken, stabilized, di-
polar state in acetonitrile, which can be negated by reduced
octupolar contribution, thus leading to the constant s(2)
value. On the contrary, the much larger s(2) value of p-NH2
in acetonitrile could arise from the enhanced charge delocal-
ization in acetonitrile over toluene. To confirm this hypothe-
sis, we have also measured the s(2) value of 3 under similar
conditions, which turned out to be (1850Æ50) GM in both
toluene and acetonitrile. This feature has led to our inter-
pretation that increased octupolar ICT interaction and its
subsequent stabilization by polar acetonitrile is responsible
for the enhanced s(2) value for p-NH2, thereby further un-
derscoring the superiority of the octupolar state over the
symmetry-broken dipolar ICT state for the enhanced TPA
property of octupolar molecules. This result is in good
agreement with the previous reports on triphenylamine de-
rivatives in which three-dimensional charge-transfer interac-
tions are responsible for higher s(2) values.[62] In addition,
hyper-Rayleigh scattering (HRS) and electric-field-induced
second-harmonic generation (EFISH) studies of nitro-substi-
tuted subphthalocyanines indicate that the second-order
nonlinear optical properties of subphthalocyanines are also
sensitive to the dipolar and octupolar contribution, which in
fact can further be tailored by the substitution pattern,[75]
similar to subporphyrins. However, in contrast with subpor-
phyrins, higher nonlinear optical (NLO) efficiencies of sub-
phthalocyanines originate from the acceptor substituents
rather than the donor substituents.[76] Thus, we can conclude
that simple fabrications of subporphyrins by the introduc-
tion of meso-aryl substituents allows for the tailoring of di-
polar and octupolar contributions, and subsequently their
TPA properties.
drastic enhancement of s(2), from approximately 4 to the
maximum of about 32 times, depending on the substituents,
can be ascribed to the ICT interactions between donor and
acceptor moieties as well as the octupolar effect. The higher
TPA efficiencies of subporphyrins originate from the larger
first hyperpolarizability due to the octupolar structure that
qualitatively possesses a linear relationship with the s(2)
values, and both of them increase monotonically with an in-
crease in the donor and acceptor strength of the substitu-
ents.[30] A comparison between the p-conjugative fabrication
and donor–acceptor substituents at the meso-aryl group of
the subporphyrin provides further insight into the structure–
property relationship, though both of them are found to en-
hance the TPA efficiency. The meso-oligo(1,4-phenylenee-
thynylene)-substituted subporphyrins[33] exhibit the s(2) value
of 1340 GM, but here it has been shown that even higher
TPA efficiency is achieved by p-NH2, a far simpler subpor-
phyrin. This result is promising when one considers the
smaller molecular weight of p-NH2, since such views of
atom economy or atom efficiency will be more important in
actual applications. These results illustrate the advantages of
the charge-transfer interaction over the chain-length elonga-
tion in octupolar systems on TPA properties.
In contrast to the investigations on the structure–property
relationship of TPA processes at the molecular level, the
study of solvent effect is still in a rudimentary stage. Impor-
tantly, the magnitude of charge-transfer interactions,[15,67]
one of the desired requirements for the TPA property, can
be largely perturbed by solvent polarity, and such solvent ef-
fects are expected to change the s(2) values. However, there
have been few theoretical and computational studies to ad-
dress the solvent effect, and unfortunately the published re-
sults indicate a lack of consensus.[66,68–72] A very recent inde-
pendent experimental study using vinylbenzene[73] and vinyl-
fluorene[74] derivatives reveals that the two-photon transi-
tions do not correlate with the polarity and polarizability of
the solvent unlike one-photon transition. Furthermore,
Bazan et al.[53] reported that the solvent-dependent s(2) value
of distyrylbenzene chromophores is nonmonotonic; the
maximum s(2) value was observed in the solvent with inter-
mediate polarity. On the other hand, the TPA cross-section
values of octupolar molecules are expected to show little
solvent dependence due to a lack of permanent dipole
moment. However, theoretical studies indicate the influence
of dipolar, quadrupolar, octupolar, and higher-order interac-
tions with solvent.[71]
The solvent-dependence study in the present investigation
is aimed at elucidating the ICT effect on the TPA processes.
The s(2) values of subporphyrins remain solvent insensitive
except for p-NH2 and m-NO2, which, respectively, show ap-
proximately two- and fourfold enhancement in going from
toluene to acetonitrile and from dichloromethane to a 1:2
mixture of dichloromethane/acetonitrile, respectively. These
behaviors substantiate the significant role played by solvents
in influencing the degree of ICT interactions. On the other
hand, despite the enhanced degree of ICT interactions, the
We have also calculated the action cross section[17] defined
as the product of the fluorescence quantum yield (Ff) multi-
plied by the s(2) values (Table 2). As a consequence of the
enhancement of both s(2) and Ff values, the amino-substitut-
ed subporphyrins p-NH2 and m-NH2 exhibit an approxi-
mately one-hundredfold increase in the action cross-section
values with respect to 1. Indeed, the enhancement effect is
modulated by the strength of the donor group as can be
seen from the order of s(2)Ff values NBn2 >p-NH2 >m-NH2.
To compare the different types of octupolar molecules, the
s(2)/molecular weight ratio and s(2)Ff/molecular weight ratio
are shown to be the relevant figures of merit (Table 2).[17] In
this aspect, p-NH2 and its N-substituted analogue are one of
the most efficient TPA chromophores of similar molecular
size; their s(2)/Mr and s(2)Ff/Mr values are 5.87 and 1.53, re-
spectively.[17] Furthermore, in most of the branched systems
previously reported, the increased s(2) values with ICT inter-
actions have been accompanied by a decrease in the fluores-
cence quantum yield and hence these molecules are not
useful for applications that utilize two-photon excited fluo-
rescence. These problems are overruled for our molecules,
particularly 3, which shows a higher fluorescence quantum
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Aryl-Substituted Subporphyrins
FULL PAPER
benzaldehyde (3.10 g, 20.8 mmol) in 1% yield (41 mg) by following the
same procedure. Aminophenyl-substituted subporphyrins p-NH2 and m-
NH2 were prepared by reduction of corresponding nitrophenyl-substitut-
ed subporphyrins p-NO2 and m-NO2, respectively.
yield irrespective of the surrounding environment. That
makes this molecule an ideal candidate for two-photon fluo-
rescence imaging in a heterogeneous biological environ-
ment. Given the smaller size of the molecule with its well-
connected p network, the easy functionalization at the b
and meso positions, and higher sensitivity to meso-aryl sub-
stituents, the TPA properties of subporphyrins are easily
tunable by a judicious choice of substituents.
Subporphyrin p-NO2 or m-NO2 (ca. 10–50 mg) was dissolved in a mini-
mal amount of chloroform, and the solution was diluted with ethanol
(15 mL). Next, 1m aq HCl (15 mL) and SnCl2·2H2O (20 equiv) were
added to the solution. The resulting solution was vigorously stirred at
708C for 7 h. After cooling, 1m aq NaOH was added to make the solu-
tion basic and the products were repeatedly extracted with CH2Cl2 until
the aqueous layer became colorless. The combined organic layer was
washed with brine, dried over anhydrous Na2SO4, and the solvent was
evaporated. The residue was dissolved in methanol (ca. 20 mL) and the
solution was heated at reflux for 30 min. After evaporating the solvent,
axially methoxo-coordinated product was recrystallized from CH2Cl2/
hexane to give meso-aminophenyl-substituted subporphyrin (p-NH2 as a
red solid or m-NH2 as an orange solid) quantitatively.
Conclusion
Herein, we present a systematic study on the photophysical
and TPA properties of donor–acceptor-substituted octupolar
subporphyrins. Solvatochromic photophysical properties
demonstrate the ICT interactions between the donor and ac-
ceptor groups either from the center to the periphery or
vice versa, depending on the electron-withdrawing/-donating
nature of the peripheral substituent. Furthermore, the oppo-
site solvatochromic behavior between para and meta isomers
allows us to quantify the emissive excited state of para iso-
mers as the localized, dipolar state due to symmetry break-
ing, whereas that of meta congeners is of an octupolar
nature that arises from false symmetry breaking. We have
also shown that restricted rotation of a para-aminophenyl
subporphyrin in nonpolar solvents at low temperatures pre-
vents the formation of a localized excited state, thereby
leading to the fluorescence from the Frank–Condon state.
Finally, the TPA cross-section values of subporphyrins are
drastically enhanced with donor/acceptor substituents by
means of octupolar as well as ICT interactions. Given the
high fluorescence quantum yield and TPA cross-section
value, p-aminophenyl-substituted subporphyrins like p-NH2
and 3 would be an ideal candidate for two-photon fluores-
cence imaging. In fact, the stabilization of octupolar ICT
states by polar solvents has proved to be effective for TPA
enhancement, whereas that of symmetry-broken dipolar ex-
cited states has a negligible or very small influence. We be-
lieve that the present study will provide a firm basis for un-
derstanding the octupolar and dipolar ICT contributions to
the third-order nonlinear optical properties and will guide
the design of novel molecules with high versatility.
Methoxo[5,10,15-tri(3-nitrophenyl)subporphyrinato]boronACTHUNRGTNEUNG(III) (m-NO2):
1H NMR (600 MHz, CDCl3): d=8.85 (s, 3H; meso-Ar-o-H), 8.50 (d, J=
7.4 Hz, 3H; meso-Ar-o or p-H), 8.43 (d, J=7.4 Hz, 3H; meso-Ar-o or p-
H), 8.14 (s, 6H; b-H), 7.92 (t, J=7.4 Hz, 3H; meso-Ar-m-H), 0.82 ppm
(s, 3H; axial-OMe); 11B NMR (193 MHz, CDCl3): d=ꢀ15.4 ppm (s, 1B);
13C NMR (150 MHz, CDCl3): d=148.6, 141.4, 138.8, 138.5, 129.9, 127.4,
123.1, 122.7, 118.6, 46.8 ppm (axial-OMe); UV/Vis (in CH2Cl2): l (e)=
373 (141000), 460 nm (14000 mꢀ1 cmꢀ1); fluorescence (in CH2Cl2, lex
=
373 nm); lmax =512 nm, FF =8ꢂ10ꢀ3; HR-ESI TOF-MS (positive mode):
m/z calcd for C33H18N6B1O6: 605.1381 [MꢀOMe]+; found: 605.1396.
Methoxo[5,10,15-tri(4-aminophenyl)subporphyrinato]boronACTHUNRGTNEUNG(III) (p-NH2):
1H NMR (600 MHz, CDCl3): d=8.08 (s, 6H; b-H), 7.86 (d, J=8.7 Hz,
6H; meso-Ar-o-H), 7.00 (d, J=8.7 Hz, 6H; meso-Ar-m-H), 3.94 (brs,
6H; NH2), 0.81 ppm (s, 3H; axial-OMe); 11B NMR (193 MHz, CDCl3):
d=ꢀ15.1 ppm (s, 1B); 13C NMR (150 MHz, CDCl3): d=146.4, 140.7,
134.3, 127.8, 122.0, 120.5, 115.4, 46.9 ppm (axial-OMe); UV/Vis (in
CH2Cl2): l (e)=388 (165000), 510 nm (23000 mꢀ1 cmꢀ1); fluorescence (in
CH2Cl2, lex =388 nm); lmax =580 nm, FF =0.41; HR-ESI TOF-MS (posi-
tive mode): m/z calcd for C33H24N6B1: 515.2156 [MꢀOMe]+; found:
515.2154.
Methoxo[5,10,15-tri(3-aminophenyl)subporphyrinato]boron
G
(m-
NH2): 1H NMR (600 MHz, CDCl3): d=8.13 (s, 6H; b-H), 7.45 (t, J=
7.6 Hz, 3H; meso-Ar-m-H), 7.41 (s, 3H; meso-Ar-o-H), 7.40 (d, J=
6.4 Hz, 3H; meso-Ar-o-H), 6.92 (d, J=8.7 Hz, 3H; meso-Ar-p-H), 3.92
(brs, 6H; NH2), 0.81 ppm (s, 3H; axial-OMe); 11B NMR (193 MHz,
CDCl3): d=ꢀ15.3 ppm (s, 1B); 13C NMR (150 MHz, CDCl3): d=146.6,
140.9, 138.4, 129.5, 123.9, 122.2, 120.7, 120.0, 114.6, 46.8 ppm (axial-
OMe); UV/Vis (in CH2Cl2):
l
(e)=377 (170000), 489 nm
(12000 mꢀ1 cmꢀ1); fluorescence (in CH2Cl2, lex =377 nm); lmax =522 nm,
FF =0.09; HR-ESI TOF-MS (positive mode): m/z calcd for C33H24N6B1:
515.2156 [MꢀOMe]+; found: 515.2159.
Sample preparation: All the solvents used were of spectroscopic grade
(Aldrich) and used without further purification. The fluorescence quan-
tum yields were determined with reference to Nile red in dioxane (0.72)
at ambient temperature ((22Æ1)8C).
Steady-state absorption and fluorescence spectra: Absorption spectra
were obtained using a Varian Cary 5000 UV-Vis-NIR spectrophotometer,
and steady-state fluorescence spectra were measured using a Hitachi
model F-2500/F-4500 fluorometer at room temperature. Temperature-de-
pendent absorption and fluorescence studies were carried out using an
Oxford instruments, Optistat DN-V, liquid nitrogen optical spectroscopy
cryostat.
Experimental Section
Synthesis and characterization: All reagents and solvents were of com-
mercial reagent grade and were used without further purification. H, 11B,
1
and 13C NMR spectra were recorded using a JEOL delta-600 spectrome-
ter, and chemical shifts were reported as the delta scale in ppm relative
to internal standards, CHCl3 (d=7.26 ppm for 1H, 77.16 ppm for 13C),
and an external standard, BF3·OEt2 in CDCl3 (d=0.00 ppm for 11B). ESI-
TOF-MS spectra were recorded using a BRUKER DALTONICS micro
TOF LC using positive-ion mode.
Time-correlated single-photon counting: Time-resolved fluorescence was
detected using a time-correlated single-photon counting (TCSPC) tech-
nique. A homemade cavity-dumped Ti:sapphire oscillator pumped by a
CW Nd-YVO4 laser (Coherent, Verdi) was used as the excitation light
source; this provided ultrashort pulses (100 fs at FWHM) and allowed
for a high repetition rate (ca. 200–400 kHz). The output pulse of the os-
Methoxo(5,10,15-triphenylsubporphyrinato)boron
thoxo(5,10,15-tri(4-nitrophenyl)subporphyrinato)boron
were prepared according to the reported procedure.[38] Methoxo(5,10,15-
tri(3-nitrophenyl)subporphyrinato)boron(III) (m-NO2) was also synthe-
sized from pyridine·tri-N-pyrrolylborane (2.00 g, 6.94 mmol) and 3-nitro-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
cillator was frequency-doubled with
a second-harmonic crystal. The
G
TCSPC detection system consisted of a multichannel plate photomultipli-
er (Hamamatsu, R3809U-51) with a cooler (Hamamatsu, C4878), a time-
Chem. Eur. J. 2009, 15, 12005 – 12017
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12015

A. Osuka, D. Kim et al.
to-amplitude converter (TAC; EG&G Ortec, 457), two discriminators
(EG&G Ortec, 584 as signal, and Canberra, 2126 as trigger), and two
wide-band amplifiers (Philip Scientific as signal, and Mini Circuit as trig-
ger). A personal computer with a multichannel analyzer (Canberra,
PCA3) was used for data storage and processing. The overall instrumen-
tal response function (IRF) was about 60 ps (FWHM). A sheet polarizer,
set at an angle complementary to the magic angle (54.78), was placed in
the fluorescence collection system. The decay fittings were made by
using a least-squares deconvolution fitting process.
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
Acknowledgements
Femtosecond time-resolved fluorescence measurements: Femtosecond
fluorescence up-conversion apparatus was used for the time-resolved
spontaneous fluorescence. The beam source for the S1 fluorescence was a
femtosecond regenerative amplifier consisting of a self-mode-locked fem-
tosecond Ti:sapphire laser (Coherent, MIRA), a Ti:sapphire regenerative
amplifier (Clark MXR, CPA-1000) pumped by a Q-switched Nd:YAG
laser (ORC-1000), a pulse stretcher/compressor, and an optical detection
system. The third harmonic (267 nm) of the fundamental pulses generat-
ed by two 100 mm-thick BBO crystals served as the pump pulse. Residual
fundamental pulses were used as gate pulses. The pump beam was fo-
cused at a 500 mm-thick cuvette containing sample solution by a 5 cm
focal length plano-convex lens. The cuvette was mounted on a motor-
driven state and moved constantly back and forth across the beam to
minimize photodegradation. Collecting the fluorescence and focusing it
into a BBO crystal for the frequency conversion was achieved by a re-
flective microscope objective lens (Coherent). We used two kinds of
mixing BBO crystals to improve the signal-to-noise ratio. We measured
the S2 fluorescence by using a 0.5 mm crystal to prevent pulse broadening
in the mixing crystal. The FWHM of the cross-correlation functions be-
tween the scattered pump pulses and the gate pulses was 300 fs for S1
fluorescence.
The work at Yonsei was financially supported by the Star Faculty and
World Class University (no. 2008-8-1955) programs of the Ministry of
Education, Science and Technology of Korea and the AFSOR/AOARD
grant (no. FA4869-08-1-4097). S.E., J.-Y.S, S.C., and P.K. thank the BK21
fellowships. The quantum chemical calculations were performed using
the supercomputing resources of the Korea Institute of Science and Tech-
nology Information (KISTI). The work at Kyoto was supported by the
Grants-in-Aid for Scientific Research Program (A) of MEXT (nos.
19205006 and 20108001 “pi-Space“). Y.I. thanks the JSPS Research Fel-
lowship for Young Scientists.
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Published online: September 22, 2009
Chem. Eur. J. 2009, 15, 12005 – 12017
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www.chemeurj.org
12017