Comparative photophysical properties of free-base, bis-Zn(II), bis-Cu(II), and bis-Co(II) doubly N-confused hexaphyrins(1.1.1.1.1.1)

Article abstract of DOI:10.1021/jp056083t

We have comparatively investigated the photophysics of a series of bis-metal doubly N-confused hexaphyrins(1.1.1.1.1.1) using time-resolved fluorescence, femtosecond transient absorption, two-photon absorption measurements, and geometry-optimized ab initio calculations. Bis-Zn(II) and free-base doubly N-confused hexaphyrins exhibit well-resolved and red-shifted B- and Q-like absorption bands compared with porphyrins. Their allowed transitions are (π,π*) transitions of the hexaphyrin ring, as confirmed by the HOMO and LUMO frontier orbitals based on ab initio calculations at the B3LYP/6-31G level. On the other hand, the absorption spectra of bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins are relatively broad, presumably due to large couplings between the metal d-orbitals and π-electrons of the hexaphyrin ring. Owing to these couplings, bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins have much shorter excited-state lifetimes of 9.4 ± 0.3 ps and 670 fs, respectively, than those (267 ± 16 and 62.4 ± 1.2 ps, respectively) of bis-Zn(II) and free-base doubly N-confused hexaphyrins. The two-photon absorption cross section (σ⁽²⁾) values, which are believed to depend strongly on the ring planarity (π-conjugation), are in line with the excited-state lifetime trends.

Full text of DOI:10.1021/jp056083t

J. Phys. Chem. B 2006, 110, 11683-11690
11683
Comparative Photophysical Properties of Free-Base, Bis-Zn(II), Bis-Cu(II), and Bis-Co(II)
Doubly N-Confused Hexaphyrins(1.1.1.1.1.1)
Jung Ho Kwon, Tae Kyu Ahn, Min-Chul Yoon, Deok Yun Kim, Mi Kyoung Koh, and
Dongho Kim*
Center for Ultrafast Optical Characteristics Control and Department of Chemistry, Yonsei UniVersity,
Seoul 120-749, Korea
Hiroyuki Furuta*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity,
Fukuoka 812-8581, Japan
Masaaki Suzuki and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science CREST of JST, Kyoto UniVersity,
Kyoto 606-8502, Japan
ReceiVed: October 24, 2005; In Final Form: April 13, 2006
We have comparatively investigated the photophysics of a series of bis-metal doubly N-confused hexaphyrins-
(1.1.1.1.1.1) using time-resolved fluorescence, femtosecond transient absorption, two-photon absorption
measurements, and geometry-optimized ab initio calculations. Bis-Zn(II) and free-base doubly N-confused
hexaphyrins exhibit well-resolved and red-shifted B- and Q-like absorption bands compared with porphyrins.
Their allowed transitions are (π,π*) transitions of the hexaphyrin ring, as confirmed by the HOMO and
LUMO frontier orbitals based on ab initio calculations at the B3LYP/6-31G level. On the other hand, the
absorption spectra of bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins are relatively broad, presumably
due to large couplings between the metal d-orbitals and π-electrons of the hexaphyrin ring. Owing to these
couplings, bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins have much shorter excited-state lifetimes
of 9.4 ( 0.3 ps and 670 fs, respectively, than those (267 ( 16 and 62.4 ( 1.2 ps, respectively) of bis-Zn(II)
and free-base doubly N-confused hexaphyrins. The two-photon absorption cross section (σ(2)) values, which
are believed to depend strongly on the ring planarity (π-conjugation), are in line with the excited-state lifetime
trends.
Introduction
(Scheme 1). Thus if one or more carbon atoms in the cavity is
replaced by nitrogens, the metal coordination seems to be
plausible. This idea has led to the synthesis of doubly N-
confused hexaphyrins(1.1.1.1.1.1) where two confused pyrroles
are introduced into the core of normal hexaphyrin. Doubly
N-confused hexaphyrins were initially prepared via a stepwise
route16 and were synthesized via an unexpected double pyrrolic
rearrangement route in improved yields.17 It is notable that these
doubly N-confused hexaphyrins accommodate two metal ions
within their flat rectangular coordination site quite smoothly,
in sharp contrast to normal hexaphyrins(1.1.1.1.1.1).
During the last decade expanded porphyrins, possessing more
than four pyrrole rings to form a π-conjugated macrocyclic ring,
have attracted much attention due to their rich chemistry such
as versatile ways of connecting pyrrole rings to produce a variety
of expanded porphyrins, anion or neutral receptors, organic
nonlinear optical materials, and new IR dyes and the interplay
between aromaticity and antiaromaticity.1-5 Another important
aspect as aromatic ligands in porphyrins is the chelation of
various transition metals in central cores formed by the
framework of four pyrrole rings.6 While only one metal site is
available for the relatively small size cavity of porphyrins, bis-
metal coordination can be envisaged for expanded porphyrins
due to their expanded inner cavities. In this regard, representative
expanded porphyrins, hexaphyrins, can be promising candidates
for bis-metal complexation due to their planar conformation as
well as rectangular cavity framework. Sessler et al. reported
that [24]hexaphyrin(1.0.0.1.0.0), amethyrin, could accommodate
two metal ions with six inward pointing pyrrole nitrogens.7,8 In
contrast, normal [26]hexaphyrin(1.1.1.1.1.1)5,9-12 does not form
a bis-metal complex13,14 except a bis-Au(III) complex,15 despite
its planar conformation, because it requires the involvement of
inner CH of the N2C2 core in the coordination with the metal
In metalloporphyrins, it has been well established that the
central metals play an important role in photophysical, electro-
chemical, catalytic, and electronic properties. In this context, it
is relevant to investigate the bis-metal doubly N-confused
hexaphyrins such as bis-Zn(II), bis-Cu(II), and bis-Co(II)
complexes to explore the electronic interactions between the
metal and hexaphyrin ring and to compare their photophysical
properties with free-base doubly N-confused hexaphyrins (Scheme
1). The choice of filled d-orbital Zn(II) and unfilled d-orbital
Cu(II) and Co(II) as central metals in bis-metal doubly N-
confused hexaphyrins is expected to provide a rationale to
distinguish (d,d), LMCT (π,d), MLCT (d, π*), and ring (π,π*)
states in the deactivation processes of photoexcited bis-metal
doubly N-confused hexaphyrins.
* To whom correspondence should be addressed. E-mail: dongho@
yonsei.ac.kr, hfuruta@cstf.kyushu-u.ac.jp, osuka@kuchem.kyoto-u.ac.jp.
10.1021/jp056083t CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/26/2006

11684 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Kwon et al.
SCHEME 1: Molecular Structures of Free-Base, Bis-
Zn(II), Bis-Cu(II), and Bis-Co(II) Doubly N-Confused
Hexaphyrins(1.1.1.1.1.1) (top) and
meso-Hexakis(pentafluorophenyl)
[26]Hexaphyrin(1.1.1.1.1.1) (bottom)
was demetalated to give its free base 2 quantitatively upon
treatment with trifluoroacetic acid.
General Comments. Absorption spectra were acquired with
a UV-vis spectrometer (Shimadzu, UV-3101PC). Solvents of
HPLC grade were purchased from Aldrich and used without
further purification. All measurements were carried out at room
temperature (23 ( 2 °C).
Near-IR Fluorescence Spectrum and Lifetime Measure-
ments. The fluorescence emission was detected using a near-
IR photomultiplier (Hamamatsu, H9170-75) and a lock-in
amplifier (EG&G, 5210) combined with a chopper after laser
excitation at 442 nm from a CW He-Cd laser (Melles Griot,
Omnichrome 74). Time-resolved fluorescence was detected
using a time-correlated single-photon-counting (TCSPC) tech-
nique.18 As an excitation light source, we used a homemade
cavity-dumped Ti:Sapphire oscillator, which provides a high
repetition rate (200-400 kHz) of ultrashort pulses (100 fs at
full width at half-maximum (fwhm)) pumped by a CW Nd-
YVO4 laser (Spectra-Physics, Millennia V). The output pulses
of the oscillator were frequency-doubled with a second-harmonic
crystal. The TCSPC detection system consisted of a near-IR
photomultiplier (Hamamatsu, H9170-75), a TAC (EG&G Ortec,
457), two discriminators (EG&G Ortec, 584 (signal) and
Canberra, 2126 (trigger)), and two wideband amplifiers (Philip
Scientific (signal) and Mini Circuit (trigger)). A personal
computer with a multichannel analyzer (Canberra, PCA3) was
used for data storage and signal processing. The overall
instrumental response function by an IR dye (Aldrich, IR1100)
was about 350 ps (fwhm). For the deconvolution procedure,
the IRF function was obtained by detecting emission from a IR
dye molecule (Aldrich, IR1100) with a known lifetime of ∼6
ps.
In this work, we have explored the photophysical properties
of 1, 2, 3, and 4 including excited singlet- and triplet-state
dynamics by various time-resolved spectroscopic techniques and
quantum mechanical calculations on HOMO-LUMO transitions
based on the B3LYP/6-31G level. Much different photophysical
properties of 3 and 4 compared with 1 and 2 demonstrate the
involvement of metal d-electrons in the formation of low-lying
quenching states below the normally emissive ring (π,π*) states
of 1 and 2. Since our work is the first attempt to explore the
fundamental photophysical properties of bis-metal complexes
of expanded porphyrins, we believe that our results will provide
a firm basis for the characterization of multimetal-chelated
expanded porphyrins.
Femtosecond Transient Absorption Measurements The
dual-beam femtosecond time-resolved transient absorption
spectrometer consisted of two independently tunable homemade
noncollinear optical parametric amplifiers (NOPA) pumped by
a Ti:sapphire regenerative amplifier system (Spectra-Physics,
Hurricane X) operating at 5 kHz repetition rate and an optical
detection system.19 The NOPA systems were based on noncol-
linearly phase-matching geometry and easily color-tuned by
controlling a delay between white light continnum and pump
pulses. The generated visible NOPA pulses had a pulse width
of ∼35 fs and an average power of 10 mW at 5 kHz repetition
rate in the range 500-700 nm. The probe beam was split into
two parts. One part of the probe beam was overlapped with the
pump beam at the sample to monitor the transient (signal), while
the other part of the probe beam was passed through the sample
without overlapping the pump beam to compensate the fluctua-
tion of the probe beam (reference). The time delay between
pump and probe beams was carefully controlled by making the
pump beam travel along a variable optical delay (Newport,
ILS250). To obtain the time-resolved transient absorption
difference signal at a specific wavelength, the monitoring
wavelength was selected by using an interference filter. By
chopping the pump pulses at 47 Hz, the modulated probe pulses
as well as the reference pulses were detected by two separate
photodiodes. The modulated signals of the probe pulses were
measured by a gated-integrator (SRS, SR250) and a lock-in
amplifier (EG&G, DSP7265) and stored in a personal computer
for further signal processing. The polarization angle between
pump and probe beam was set at the magic angle (54.7°) in
order to prevent polarization-dependent signals.
Experimental Section
Sample Preparation. The details of the synthesis of 1-4
are described elsewhere17 and in the Supporting Information.
Under aerobic conditions, to a solution of meso-hexakis-
(pentafluorophenyl) [26]hexaphyrin (5) (400 mg) in pyridine
(100 mL) was added CuCl (2 g), and the resulting mixture was
stirred for 3 h. TLC analysis of the reaction mixture revealed
the appearance of a pale blue band faster than 5. After usual
workup followed by chromatographic separation through a silica
gel column, reddish solids (104 mg) were isolated as the sole
major product 3. Positive-mode high-resolution electrospray
ionization mass spectroscopy (HR-ESI-TOF-MS) exhibited the
parent ion peak at m/z ) 1616.9052 (M + H) (cald for
C66H11F30N6O2Cu2, 1616.9050), thus demonstrating that two
oxygen atoms had been incorporated in addition to two copper
ions. The structure of this complex has been revealed by single-
crystal X-ray diffraction analysis to be bis-Cu(II) doubly
N-confused hexaphyrin 3, where the two Cu atoms are each
bound to three pyrrolic nitrogen atoms and a carbonyl oxygen
atom with a Cu-Cu distance of 4.896(1) Å in a planar
macrocycle with the mean plane deviation of only 0.079 Å and
small displacements of the Cu ions (0.059 Å). Bond distances
are Cu-NA (2.043(4) Å), Cu-NB (1.961(4) Å), Cu-NC (2.052-
(4) Å), and Cu-O (1,932(3) Å), respectively. Remarkably, both
the inverted pyrrole rings of 5 underwent simultaneous rear-
rangements to form N-confused pyrroles with concomitant
oxygenation at the internal pyrrolic R-position in 3. Complex 3
Nanosecond Flash Photolysis Measurements. The nano-
second transient absorption spectra were obtained by nanosecond

Properties of Doubly N-Confused Hexaphyrins(1.1.1.1.1.1)
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11685
flash photolysis techniques.20 An excitation pulse of 532 nm
was generated from the second-harmonic output of a Q-switched
Nd:YAG laser (Continuum, Surelite). The time duration of the
excitation pulse was ca. 6 ns, and the pulse energy was ca. 2
mJ/pulse. A CW Xe lamp (150 W) was used as a probe light
source for transient absorption measurement. The probe light
was collimated on the sample cell and then spectrally resolved
by using a 15 cm monochromator (Acton Research, SP150)
equipped with a 600 grooves/mm grating after passing the
sample. The spectral resolution was about 3 nm for the transient
absorption experiment. The light signal was detected by using
photomultiplier tubes (Hamamatsu, R928 and R5108). The
output signal from the PMT was recorded with a 500 MHz
digital storage oscilloscope (Tektronix, TDS3052) for the
temporal profile measurement. Since the triplet-state dynamics
of molecules in solution is strongly dependent on the concentra-
tion of oxygen molecules dissolved in solution, we tried to
remove oxygen rigorously by repeated freeze-pump-thaw
cycles. To ensure our data, we first examined the triplet-state
dynamics of Zn(II)TPP in toluene at room temperature, which
gives the lifetime of 1 ms by our flash photolysis setup. Since
the intrinsic triplet-state lifetime of Zn(II)TPP is 25 ms at 77
K, the triplet-state lifetime of 1 ms at room temperature is
limited by collisional quenching.21 Since the concentration of
molecules also affects significantly the excited triplet-state
lifetime due to triplet-triplet annihilation processes, we kept
the concentration at 10-5 M with relatively low photoexcitation
density at 532 nm produced by the second-harmonic output of
a Q-switched Nd:YAG laser.
Quantum Mechanical Calculation. Quantum mechanical
calculations were performed with the Gaussian 98 program
suite.22 All calculations were carried out by the density
functional theory (DFT) method employing the B3LYP func-
tional as 6-31G basis set. The X-ray crystal structures were used
as initial geometry for geometry optimization. The excited-state
calculations were performed with the time-dependent density
functional theory (TD-DFT) method using the same functional
and basis set as those used for geometry optimizations.
Measurement of Two-Photon Absorption Cross-section
(σ(2)). The TPA spectra were measured at 1200 nm by using
the open-aperture Z-scan method23 with ∼130 fs pulses from
an optical parametric amplifier (Light Conversion, TOPAS)
operating at 5 kHz repetition rate generated from a Ti:sapphire
regenerative amplifier system (Spectra-Physics, Hurricane). The
laser beam was divided into two parts. One was monitored by
a Ge-PIN photodiode (New Focus) as intensity reference, and
the other was used for transmittance measurement. After passing
through an f ) 10 cm lens, the laser beam was focused and
passed through a quartz cell. The position of the sample cell
could be varied along the laser beam direction (z-axis), so the
local power density within the sample cell could be changed
under a constant laser power level. The thickness of the cell
was 1 mm. The transmitted laser beam from the sample cell
was then detected by the same photodiode as used for reference
monitoring. The on-axis peak intensity of the incident pulses
at the focal point, I0, ranged from 40 to 60 GW cm-2. Assuming
a Gaussian beam profile, the nonlinear absorption coefficient â
can be obtained by curve fitting to the observed open-aperture
traces with the following equation:
Figure 1. Absorption (solid line) and fluorescence (dashed line) spectra
of 1 (a) and 2 (b) in toluene. Inset shows the fluorescence decay profile
of 1 at 1045 nm with photoexcitation at 400 nm.
After obtaining the nonlinear absorption coefficient â, the
TPA cross-section σ(2) of one solute molecule (in units of 1
GM ) 10-50 cm4‚s/photon‚molecule) can be determined by
using the following relationship:
σ(2)NAd × 10-3
â )
(2)
hν
where NA is the Avogadro constant, d is the concentration of
the TPA compound in solution, h is the Planck constant, and ν
is the frequency of the incident laser beam.
We obtained the TPA cross-section σ(2) values of hexaphyrins
at 1200 nm, where linear absorption is negligible, to satisfy the
condition of R0l , 1 in retrieving the pure TPA σ(2) values in
the simulation procedure. We also measured the TPA cross-
section value of AF-50 as a reference compound, which is 50
GM at 800 nm.
Results and Discussion
Photophysical Properties of 1 and 2. The crystal structures
of doubly N-confused hexaphyrins(1.1.1.1.1.1) 1-4 were
reported to have a relatively flat rectangular shape with two
inverted pyrrole rings, where the two metal atoms are each
bound to three pyrrolic nitrogen atoms and a carbonyl oxygen
atom with a metal-metal distance of 4.9 Å in a planar
macrocycle (Supporting Information, Figure S1). The doubly
N-confused hexaphyrins exhibit aromaticity due to 26 π-elec-
trons around the hexaphyrin ring, as judged from their strong
diatropic ring currents.16,17 The absorption spectrum of 1 exhibits
a strong B-like absorption band at 608 nm and rather weak
âI0(1 - e-R l
)
0
T(z) ) 1 -
(1)
2R0(1 + (z/z0)2)
where R0 is the linear absorption coefficient, l the sample length,
and z0 the diffraction length of the incident beam.

11686 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Kwon et al.
Figure 2. Frontier orbitals of 1 (a) and 2 (b) by ab initio calculations
at the B3LYP/6-31G level.
Figure 3. Transient absorption decay profiles of 1-4 (a) and 1 on a
longer time scale (b) in toluene pumped and probed at the corresponding
absorption maxima.
Q-like ones at 779, 883, 912, and 1042 nm (Figure 1a). Overall,
the absorption bands are red-shifted compared with those of
porphyrins mainly due to the enhanced π-conjugation pathway.
The absorption spectrum of 2 has a B-like band at 571 nm and
several weak Q-like ones at 727, 798, 910, and 1051 nm (Figure
1b). Interestingly, the bathochromic B-like and the hypsochromic
Q-like (0,0) bands of 1 relative to 2 are similar to those of
metalloporphyrins relative to free-base porphyrins.6 The fluo-
rescence spectra of 1 and 2 show peaks at 1044 and 1058 nm
and weak vibronic bands at around 1250 nm, respectively.
Considering the absorption edge, the Stokes shift of 1 was
estimated to be ∼30 cm-1, which implies negligible structural
changes between ground and excited states. The larger Stokes
shift of ∼80 cm-1 of 2 than that of 1 indicates that the molecular
structure of 1 is more planar and rigid than that of 2 due to the
anchoring of a flexible hexaphyrin ring by bis-Zn(II) coordina-
tion.
Given the crystal structures as initial input geometries, ab
initio calculations on the B3LYP/6-31G level were performed
to calculate the molecular orbitals of 1 and 2. The HOMO and
LUMO frontier orbitals of 1 and 2 show large electron densities
around their pyrrole rings, which matches well with the eg and
a2u orbitals of porphyrins (Figure 2), and the S1 states can be
assigned to (π,π*) transitions. While the frontier orbitals of 1
have a slight electron density on the central Zn(II) metals, the
frontier orbitals of 1 and 2 exhibit high aromaticity and ring
planarity.
nm (inset of Figure 1a). It is noteworthy that this is much shorter
than that of Zn(II)TPP (∼2.54 ns). Since the IRF function of
our TCSPC setup in the IR region was estimated to be ca. 350
ps, we could not resolve the fluorescence decay of 2. According
to the energy gap law,24 the reduced HOMO-LUMO gap
mainly due to the enhanced π-conjugation of the 26 π-electron
system seems to accelerate the internal conversion rate, yielding
a much shorter lifetime than that of porphyrins with 18
π-electrons.6,25
To explore the excited-state dynamics of 1 and 2, we have
measured the transient absorption decay profiles (Figure 3a).
The bleaching recovery time of 267 ( 16 ps for 1 matches well
with its fluorescence lifetime of ∼255 ps. In addition, the long-
lived component (>10 ns) of 1 observed as a tail in the transient
absorption decay can be assigned as the decay of the triplet
state (Figure 3b). Interestingly, the shorter excited singlet-state
lifetime26 of 62.4 ( 1.2 ps of 2 than that of 1 was observed in
the transient absorption decay profile. Since Zn(II) metal
enhances the intersystem crossing rate due to spin-orbit
coupling as observed in the fluorescence lifetimes of 2.54 ns
for Zn(II)TPP and 13.6 ns for H2TPP, the longer singlet excited-
state lifetime and smaller Stokes shift of 1 compared with 2
are ascribed to the higher planarity of 1 than 2, leading to the
deceleration of nonradiative decay channels despite its enhanced
intersystem crossing from the S1 to T1 state.
To examine the excited triplet-state dynamics, we have carried
out nanosecond flash photolysis for 1 and 2 in argon-saturated
toluene solutions with nanosecond pulse excitation at 532 nm.
The weak and broad T-T absorption spectra of 1 and 2 with
The fluorescence lifetime of 1 was measured by the time-
correlated single-photon-counting (TCSPC) technique, which
gave a lifetime of ∼255 ps at 1045 nm with excitation at 400

Properties of Doubly N-Confused Hexaphyrins(1.1.1.1.1.1)
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11687
Figure 5. Absorption spectra of 3 (a) and 4 (b) in toluene.
Figure 4. Nanosecond flash photolysis spectra of 1 (a) and 2 (b) in
argon-saturated toluene. The pump source at 532 nm is a frequency-
doubled Nd:YAG laser.
apparent bleaching signals at the indicated delays are shown in
Figure 4. The triplet-state lifetime and quantum yield of 1 were
found to be 2.3 ( 0.1 µs and 0.2, respectively. Although a longer
triplet-state lifetime of 9.5 ( 1.2 µs was observed for 2, the
triplet-state quantum yield was estimated to be less than 0.01.
The relatively short excited triplet-state lifetime and high triplet
quantum yield of 1 compared with 2 are attributable to the
enhanced spin-orbit coupling by the two central Zn(II) metals.
Photophysical Properties of 3 and 4. The absorption spectra
of 3 and 4 are similar to those of 1 and 2 but exhibit relatively
broad features (Figure 5). The B-like bands of 3 and 4 are
located at 619 and 616 nm, and their lowest Q-like bands are
observed at 1048 and 1043 nm, respectively. Importantly, the
smeared absorption spectra of 3 and 4 presumably arise from
strong interactions between unoccupied d-orbitals of central bis-
metals and hexaphyrin ring π-electrons. To examine the metal
contribution to the excited-state formation, we have investigated
the frontier orbitals of 3 and 4 by using ab initio calculations
on the B3LYP/6-31G level. Contrary to the frontier orbitals of
1 and 2, those of 3 and 4 exhibit large contributions by the
central metal ions, indicating that the metal d-orbitals are heavily
involved in the formation of excited states of 3 and 4 (Figure
6). It is noteworthy that the HOMO-1 and LUMO+1 of 3 and
4 have an energy gap similar to the HOMO and LUMO of 1
and 2. It should also be mentioned that the contributions by
metal d-orbitals to the HOMO and LUMO of 3 and 4 are
significantly enhanced compared with those of 1 and 2.
Figure 6. Frontier orbitals of 3 (a) and 4 (b) by ab initio calculations
at the B3LYP/6-31G level.
Furthermore, we have calculated the electronic transitions of
1-4 using the time-dependent density functional theory (TD-
DFT) method. In the cases of 1 and 2, we observed the strong
B- and weak Q-like absorption bands, which is largely in
accordance with their absorption spectra. For 3 and 4, however,
the calculated absorption spectra exhibit several weak transitions

11688 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Kwon et al.
TABLE 1: Excited-State Lifetimes, Fluorescence Quantum
Yields, and Triplet-State Quantum Yields of a Series of
Doubly N-Confused Hexaphyrins
a
sample
τ1/ps
τ2/ps
Φfl τT/µs
ΦT
1
2
3
4
267 ( 16 (85%) >10 000 (15%)
62.4 ( 1.2
9.4 ( 0.3 (59%) 29.2 ( 0.9 (41%)
0.67 ( 0.02
5.46 2.3
0.2
0.61 9.5 <0.01
b
b
c
c
c
c
a The relative fluorescence quantum yields were estimated by that
of meso-hexakis(pentafluorophenyl) [26]hexaphyrin(1.1.1.1.1.1) as
unity. b No fluorescence was observed. c No signal was observed in
nanosecond flash photolysis.
around B- and Q-like bands in addition to strong B- and weak
Q-like bands. Moreover, several low-lying states with weak
oscillator strength were clearly observed (Supporting Informa-
tion, Table S1). Possible explanations are that the HOMO and
LUMO produce low-lying excited states between the allowed
ring (π,π*) transitions, which are responsible for the quenching
process of the singlet excited (π,π*) states.
No fluorescence emission was detected near the absorption
edge of 3 and 4, implying that the quenching states are involved
in their relaxation processes from the normally emissive ring
1(π,π*) states. From the femtosecond transient absorption decay
profiles, the excited states of 3 and 4 were observed to have
lifetimes of 9.4 ( 0.3 (59%) with 29.2 ( 0.9 ps (41%) and
0.67 ( 0.02 ps, respectively (Figure 3 and Table 1). The
population of the excited triplet state seems to be inefficient,
since no signal of 3 and 4 in the nanosecond flash photolysis
measurement was observed. It was reported for Cu(II)TPP that
the unpaired metal electron in the d9 complex split the 3(π,π*)
Figure 7. Transient absorption decay profiles of 3 and 4 in toluene
and benzonitrile pumped and probed at the absorption maxima.
2
4
into a triple doublet T(π,π*) and a quartet T(π,π*), which
2
are responsible for fast S1(π,π*) state dynamics. The spin-
multiplet states seem to be produced in hexaphyrin rings through
electronic mixing between the odd electron in the dz orbital of
(93%) and 18.5 ( 0.4 ps (7%), which are apparently longer
than 0.67 ( 0.02 ps in toluene (Figure 7).
2
the Cu(II) d9 system and the normal singlet and triplet
hexaphyrin ring (π,π*) states, which can be inferred from the
photophysics of Cu(II) porphyrin. It should be noted that the
two Cu(II) ions in the hexaphyrin ring should be involved
simultaneously in the electronic mixing with the hexaphyrin ring
(π,π*) states in bis-Cu(II) doubly N-confused hexaphyrin.
Accordingly, in a manner similar to the case of Cu(II)TPP, the
double-exponential decays of 3 with the time constants 9.4 (
0.3 (59%) and 29.2 ( 0.9 ps (41%) can be assigned as the
formation of triple multiplet (3T/5T(π,π*)) excited states from
the initially populated single triplet 3S1(π,π*) state and a
subsequent relaxation to the ground state, respectively.6,27-29
Since the initially excited 3S1(π,π*) state of 3 is followed by
3
a rapid equilibrium between triple triplet T(π,π*) and triple
quintet 5T(π,π*) states and a subsequent relaxation to the ground
state, the overall energy relaxation dynamics should be insensi-
tive to the polarity of the solvent. On the other hand, the slower
relaxation processes of 4 observed in benzonitrile are regarded
to be associated with the MLCT (d,π*) state, which is consistent
with the theoretical calculations of the HOMO and LUMO of
4. But there still exists a possibility for the involvement of the
LMCT (π,d) state in the relaxation process of 4, because the
stabilization of this state is also sensitive to the solvent polarity.
Thus, on the basis of our theoretical and spectroscopic inves-
tigations we can suggest that the central metals in bis-metal
doubly N-confused hexaphyrins play an important role in the
energy relaxation processes, as observed in metalloporphyrins.
On the basis of our theoretical and spectroscopic investigations,
we can propose the schematic diagrams for energy relaxation
processes of a series of bis-metal and free-base doubly N-
confused hexaphyrins(1.1.1.1.1.1) as shown in Figure 8.
Two-Photon Absorption Behaviors of 1, 2, 3, and 4. There
have been numerous research activities to search for new optical
nonlinear materials with large two-photon absorption (TPA)
cross sections (σ(2)) due to potential applications in a variety of
fields including photodynamic therapy, three-dimensional mi-
crofabrication and optical data storage, and optical limiting.
Recently, expanded porphyrins containing more than four
pyrrole rings have received much attention due to their high
TPA values contributed by a large number of π-electrons. For
enhanced TPA values of expanded porphyrins, it is relevant to
While the HOMOs (HOMO, HOMO-1, and HOMO-2) of
4 reveal large metal contributions, the LUMOs (LUMO,
LUMO+1, and LUMO+2) of 4 are characterized by negligible
metal contributions, leading to the possibility of metal-to-ligand
(MLCT) (d,π*) charge transfer (CT) transitions between the
allowed (π,π*) transitions. Thus the low-lying MLCT (d,π*)
state is suggested to be responsible for the quenching of
fluorescence emission from the ring (π, π*) state in 4.
To confirm our interpretations of the quenching states of 3
and 4, we have investigated the transient absorption decay
profiles of 3 and 4 in benzonitrile (Figure 7). The best fitted
curves of 3 in benzonitrile exhibit double-exponential decay with
time constants of 6.1 ( 0.1 (55%) and 25.4 ( 0.5 ps (45%),
yielding the weight-averaged time constant of 14.6 ( 0.3 ps,
which is comparable to that (17.6 ( 0.4 ps) in toluene. However,
the transient absorption decay profile of 4 in benzonitrile gives
double-exponential decay with time constants of 0.91 ( 0.02

Properties of Doubly N-Confused Hexaphyrins(1.1.1.1.1.1)
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11689
absorption cross-section (σ(2)) value of 3450 GM. On the other
hand, bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins
exhibit much shorter excited-state lifetimes of 9.4 ( 0.3 with
29.2 ( 0.9 ps and 670 ( 20 fs, respectively. The ultrafast
relaxation processes for photoexcited 3 and 4 are ascribed to
the formation of triple multiplet states (3T(π,π*) and 5T(π,π*))
and the LMCT(d,π*) states, respectively, both of which are
located energetically below their band-gaps.
Overall, our comprehensive comparative spectroscopic in-
vestigations of various bis-metal doubly N-confused hexaphy-
rins(1.1.1.1.1.1) will serve as platforms for the further under-
standing of electronic structures of expanded porphyrins, which
also will be useful in future applications in photodynamic
therapy, anion sensing, drug delivery, optical nonlinear materials,
and so on.
Acknowledgment. The work at Yonsei University was
financially supported by the Star Faculty Program of the
Ministry of Education and Human Resources. The work at
Kyoto University was supported by the CREST of JST. Special
thanks are extended to Prof. Osamu Ito at Tohoku University
for his allowance of J.H.K.’s stay at his laboratory for
nanosecond flash photolysis experiments.
Supporting Information Available: X-ray crystallographic
structures, Z-scan trace, and TD-DFT calculations of electronic
transitions are available free of charge via the Internet at http://
pubs.acs.org.
Figure 8. Schematic energy relaxation diagrams of 1 and 2 (a), 3 (b),
and 4 (c).
References and Notes
(1) Jasat, A.; Dolphin, D. Chem. ReV. 1997, 97, 2267.
(2) Sessler, J. L.; Seidel, D. Angew. Chem., Int. Ed. 2003, 42, 5134.
(3) Lash, T. D. Angew. Chem., Int. Ed. 2000, 39, 1763.
(4) Furuta, H.; Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795.
(5) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A. J. Am.
Chem. Soc. 2001, 123, 7190.
(6) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.;
Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. In The Porphyrin
Handbook; Kadish, K., Smith, K. M., Guilard, R., Eds.; Academic Press:
San Diego, 1999; Vol. 3, p 1.
(7) Sessler. J. L.; Weghorn, S. J.; Hosaeda, Y.; Lynch, V. Chem. Eur.
J. 1995, 1, 56.
(8) (a) Weghorn, S. J.; Sessler, J. J.; Lynch, V.; Baumann, T. F.; Sibert,
J. W. Inorg. Chem. 1996, 35, 1089. (b) Hannah, S.; Seidel, D.; Sessler, J.
L.; Lynch, V. Inorg. Chim. Acta 2001, 317, 211.
(9) Ahn, T. K.; Kwon, J. H.; Kim, D. Y.; Cho, D. W.; Jeong, D. H.;
Kim, S. K.; Suzuki, M.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2005, 127,
12856.
(10) Neves, M. G. P. M. S.; Martins, R. M.; Tome´, A. C.; Silvestre, A.
J. D.; Silva, A. M. S.; Fe´lix, V.; Drew, M. G. B.; Cavaleiro, J. A. S. Chem.
Commun. 1999, 385.
(11) Suzuki, M.; Osuka, A. Org. Lett. 2003, 5, 3943.
(12) Shimizu, S.; Shin, J.-Y.; Furuta, H.; Ismael, R.; Osuka, A. Angew.
Chem., Int. Ed. 2003, 42, 78.
consider various factors to affect the TPA values such as the
π-conjugation pathway, the number of π-electrons (Hu¨ckel’s
[4n+2] rule vs aromaticty/antiaromaticity), ring planarity, and
the central metals. In this regard, we have examined the TPA
properties of 1, 2, 3, and 4 by an open-aperture Z-scan method
with ∼130 fs pulses. Because of the structural rigidity due to
the coordination with central Zn(II) metals in 1, the relatively
high TPA cross-section value of 3450 GM was obtained at 1200
1
nm. The faster S(π,π*) state decay with the time constant of
62 ps and smaller TPA value of 2250 GM at 1150 nm of 2
were measured as compared with 1. This feature is probably
attributable to the flexible hexaphyrin ring without coordination
with bis-Zn(II) metals in 2. In the cases of 3 and 4, however,
relatively low TPA values of <100 GM were observed
presumably due to short-lived excited states contributed by
d-orbitals in the formation of their excited electronic states,
although the absorption spectra of 3 and 4 are quite similar to
those of 1.
(13) Shimizu, S.; Anand, V. G.; Taniguchi, R.; Furukawa, K.; Kato, T.;
Yokoyama, T.; Osuka, A. J. Am. Chem. Soc. 2004, 126, 12280.
(14) Mori, S.; Shimizu, S.; Taniguchi, R.; Osuka, A. Inorg. Chem. 2005,
44, 4127.
(15) Mori, S.; Osuka, A. J. Am. Chem. Soc. 2005, 127, 8030.
(16) Srinivasan, A.; Ishiduka, T.; Osuka, A.; Furuta, H. J. Am. Chem.
Soc. 2003, 125, 878.
(17) Suzuki, M.; Yoon, M.-C.; Kim, D. Y.; Kwon, J. H.; Furuta, H.;
Kim, D.; Osuka, A. Chem. Eur. J. 2006, 12, 1754.
(18) Hwang, I.-W.; Cho, H, S.; Jeong, D. H.; Kim, D.; Tsuda, A.;
Nakamura, T.; Osuka, A. J. Phys. Chem. B 2003, 107, 9977.
(19) Cho, H. S.; Song, J. K.; Ha, J.-H.; Cho, S.; Kim, D.; Yoshida, N.;
Osuka, A. J. Phys. Chem. A 2003, 107, 1897.
Conclusion
We have comparatively explored the photophysical properties
of a series of bis-metal doubly N-confused hexaphyrins-
(1.1.1.1.1.1). As for bis-Zn(II) and free-base doubly N-confused
hexaphyrins, the allowed transitions are π-π* transitions around
a hexaphyrin ring, which is confirmed by ab initio calculations
at the B3LYP/6-31G level. The two Zn(II) metals of 1 enhance
the intersystem crossing rate to give the high triplet quantum
yield of 0.2. Furthermore, the coordination by the two Zn(II)
metal ions leads to the rigid planarity of the hexaphyrin ring,
resulting in a longer singlet excited-state lifetime (267 ( 16
ps) than that (62.4 ( 1.2 ps) of 2 and a high two-photon
(20) Song, N. W.; Cho, H. S.; Yoon, M. C.; Aratani, N.; Osuka, A.;
Kim, D. Bull. Korean Soc. 2002, 23, 271.
(21) Photochemistry of Polypyridine and Porphyrin Complexes; Kalya-
nasundaram, K., Ed.; Academic Press: New York, 1991; p 405.

11690 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Kwon et al.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A. 11; Gaussian, Inc.: Pittsburgh,
PA, 2001.
(23) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. G.; van
Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760.
(24) Lumpkin, R. S.; Meyer T. J. J. Phys. Chem. B 1986, 90, 5307.
(25) Chirvony, V. S.; Hoek, A.; Galievsky, V. A.; Sazanovich, I. V.;
Schaafsma, T. J.; Holten, D. J. Phys. Chem. B 2000, 104, 9909.
(26) The slightly decreased lifetime of 62.4 ( 1.2 ps for 2 compared
with that (98 ps) of meso-hexakis(pentafluorophenyl) [26]hexaphyrin-
(1.1.1.1.1.1) may be ascribed to structural distortion originated from internal
double hydrogen bonds between oxygen and pyrrolic NH.
(27) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989,
111, 6500.
(28) Tait, C. D.; Holten, D.; Gouterman, M. Chem. Phys. Lett. 1983,
100, 268.
(29) Tait, C. D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984,
106, 6653.