Comparative photophysics of [26]- and [28]hexaphyrins(1.1.1.1.1.1): Large two-photon absorption cross section of aromatic [26]hexaphyrins(1.1.1.1.1.1)

Article abstract of DOI:10.1021/ja050895l

We have explored the electronic natures of representative expanded porphyrins, [26]- and [28]-hexaphyrins, to investigate the interplay between the aromaticity and antiaromaticity that is brought by two electron oxidation/reduction processes. The excited

Full text of DOI:10.1021/ja050895l

Published on Web 08/27/2005
Comparative Photophysics of [26]- and
[28]Hexaphyrins(1.1.1.1.1.1): Large Two-Photon Absorption
Cross Section of Aromatic [26]Hexaphyrins(1.1.1.1.1.1)
Tae Kyu Ahn,†,| Jung Ho Kwon,† Deok Yun Kim,† Dae Won Cho,‡
Dae Hong Jeong,§ Seong Keun Kim,| Masaaki Suzuki, Soji Shimizu,
Atsuhiro Osuka,*, and Dongho Kim*,†
Contribution from the Center for Ultrafast Optical Characteristics Control and Department of
Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, Department of Chemistry, Seonam
UniVersity, Namwon 590-711, Korea, Department of Chemistry Education and Nano-Systems
Institute (NSI-NCRC), and School of Chemistry, Seoul National UniVersity,
Seoul 151-747, Korea, and Department of Chemistry, Graduate School of Science CREST of
JST, Kyoto UniVersity, Kyoto 606-8502, Japan
Received February 11, 2005; E-mail: osuka@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr
Abstract: We have explored the electronic natures of representative expanded porphyrins, [26]- and [28]-
hexaphyrins, to investigate the interplay between the aromaticity and antiaromaticity that is brought by two
electron oxidation/reduction processes. The excited singlet and triplet states of [26]hexaphyrin in solution
exhibit lifetimes of 125 ps and 1.8 µs, respectively, as revealed by various time-resolved spectroscopic
measurements. On the other hand, [28]hexaphyrin shows faster singlet and triplet lifetimes than those of
[26]hexaphyrin, which is largely in accordance with the perturbation of aromaticity due to the π electron
formulation of [4n] in [28]hexaphyrins. The two-photon absorption cross-section values at 1200 nm for
[26]hexaphyrins show ca. 9890 GM which is >102 larger than those of porphyrins. The reduced TPA values
of 2600 and 810 GM of [28]hexaphyrin and perfluorinated [28]hexaphyrin, respectively, match well with
their relatively short excited-state lifetimes. Overall, the enhanced excited-state lifetimes for various
hexaphyrins go in line with the increased TPA cross-section values and the ring planarity.
Introduction
of expanded porphyrins give rise to considerably red-shifted
HOMO-LUMO transitions relative to those of porphyrins.
Recently, expanded porphyrins with more than four pyrrole
subunits in the porphyrin macrocycle have emerged as a
promising class of molecules in light of versatilities in their
optical, electrochemical, and coordination properties.1-5 Because
of an increased π conjugation pathway, the electronic structures
Numerous kinds of expanded porphyrins have so far been
prepared by taking advantage of the flexibilities in connecting
pyrrole rings, overall structures, and heteroatom substitution on
pyrrole nitrogens. Despite the synthetic diversity of expanded
porphyrins, their characterizations are mainly limited to the
structural analyses such as 1H NMR and X-ray, along with some
absorption spectra.6 In this regard, it is relevant to investigate
the photophysical properties of representative expanded por-
phyrins [26] and [28]hexaphyrins(1.1.1.1.1.1) to provide a
platform for further investigations on the electronic structures
of a series of expanded porphyrins.
† Yonsei University.
‡ Seonam University.
§ Department of Chemistry Education and Nano-Systems Institute, Seoul
National University.
| School of Chemistry, Seoul National University.
Kyoto University.
(1) (a) Strickler, J. H.; Webb, W. W. Opt. Commun. 1991, 16, 1780. (b) Kawata,
S.; Sun, H.-B.; Tanaka, T.; Takada, K. Nature 1999, 398, 697.
(2) (a) Anderson, H. L.; Martin, S. J.; Bradleu, D. D. C. Angew. Chem., Int.
Ed. Engl. 1994, 33, 655. (b) Screen, T. E. O.; Throne, J. R. G.; Denning,
R. G.; Bucknall, D. G.; Anderson, H. L. J. Am. Chem. Soc. 2002, 124,
9712. (c) LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang, C. H.;
Therien, M. J. J. Am. Chem. Soc. 1996, 118, 1497.
[26]Hexaphyrins(1.1.1.1.1.1) and [28]hexaphyrins(1.1.1.1.1.1)
are attractive molecules in view of aromaticity/antiaromaticity
with a 26/28π-electronic circuit, intense UV-absorption, a small
(3) Large TPA cross-section values (σ(2) > 103 GM) have been reported at
around 800 nm in various porphyrin derivatives, using the femtosecond
apertureless Z-scan method: (a) Ogawa, K.; Ohashi, A.; Kobuke, Y.;
Kamada, K.; Ohta, K. J. Am. Chem. Soc. 2003, 125, 13356. (b) Drobizhev,
M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane, A.; Taylor, P. N.;
Anderson, H. L. J. Am. Chem. Soc. 2004, 126, 15352. (c) Drobizhev, M.;
Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane, A.; Taylor, P. N.;
Anderson, H. L. J. Phys. Chem. B 2005, 109, 7223.
(5) (a) 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. (b) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.;
Osuka, A. J. Am. Chem. Soc. 2001, 123, 7190. (c) Taniguchi, R.; Shimizu,
S.; Suzuki, M.; Shin, J.-Y.; Furuta, H.; Osuka, A. Tetrahedron Lett. 2003,
44, 2505. (d) Suzuki, M.; Osuka, A. Org. Lett. 2003, 5, 3943.
(6) Studies on photphysical properties of expanded porphyrins are rather limited.
(a) Levanon, H.; Regev, A.; Michaeli, S.; Galili, T.; Cyr, M.; Sessler, J. L.
Chem. Phys. Lett. 1990, 174, 235. (b) Maiya, B. G.; Cyr, M.; Harriman,
A.; Sessler, J. L. J. Phys. Chem. 1990, 94, 3597. (c) Guldi, D. M.; Mody,
T. D.; Gerashimchuk, N. N.; Magda, D.; Sessler, J. L. J. Am. Chem. Soc.
2000, 122, 8289. (d) Shionoya, M.; Furuta, H.; Lynch, V.; Harriman, A.;
Sessler, J. L. J. Am. Chem. Soc. 1992, 114, 5714.
(4) The synthetic design of centrosymmetric molecules with large TPA cross-
section values was investigated: Albota, M.; Beljonne, D.; Bredas, J.-L.;
Ehrlich, J. E.; Fu, J.-Y.; Heikel, A. A.; Hess, S. E.; Kogej, T.; Levin, M.
D.; Marder, S. R.; McCord-Maughon, D.; Perry, J. W.; Rockel, H.; Rumi,
M.; Subramaniam, G.; Webb, W. W.; Wu, X.-L.; Xu, C. Science 1998,
281, 1653 and references are therein.
9
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J. AM. CHEM. SOC. 2005, 127, 12856-12861
10.1021/ja050895l CCC: $30.25 © 2005 American Chemical Society

Photophysics of [26]- and [28]Hexaphyrins(1.1.1.1.1.1)
A R T I C L E S
Chart 1. Structures of Hexaphyrins(1.1.1.1.1.1)
Figure 1. Absorption and fluorescence spectra of 1a in toluene. Inset shows
a time-resolved fluorescence decay with τ ) ∼125 ps.
HOMO-LUMO gap, and rectangular molecular shape, hence
possessing an important position bridging porphyrin and larger
expanded porphyrins.5a Another useful character for the ex-
panded π conjugated molecular system is optical nonlinear
properties such as two-photon absorption (TPA).
In this paper, we report comparative photophysical properties
including excited singlet and triplet state dynamics and TPA
cross-section values of meso-aryl substituted [26]- and [28]-
hexaphyrins(1.1.1.1.1.1) using fluorescence lifetime, femtosec-
ond transient absorption, nanosecond flash photolysis, and
apertureless Z-scan measurements.7 Overall, aromatic [26]-
hexaphyrin gives a longer excited-state lifetime and larger TPA
value than [28]hexaphyrin with antiaromaticity.
Figure 2. Energy levels of frontier orbitals of [26]hexaphyrin from the
geometry optimization process based on the ab initio calculation on B3LYP/
6-31G level.
Results and Discussion
Ab initio calculations on the B3LYP/6-31G level were
performed to examine the frontier orbitals as well as the
optimized geometry (Figure 2). It is noteworthy that the overall
features of the HOMO and LUMO of 1a are, respectively,
similar to the a1u and a2u HOMOs and two degenerate eg*
LUMOs of porphyrins, which explains the spectral resemblance
between 1a and porphyrin.9
The femtosecond transient absorption spectra of 1a were
recorded in the region of 400-1100 nm following photoexci-
tation at 400 nm in toluene. Aside from a rather featureless and
broad induced absorption, strong ground-state bleaching was
obtained (Figure 3). The decay profiles of bleaching and induced
absorption at 570 and 640 nm, respectively, yielded a time
constant of ∼98 ps (inset, Figure 3), which is quite comparable
to the fluorescence lifetime for the S1 state lifetime.
To examine the excited triplet state dynamics, we carried out
nanosecond flash photolysis for 1a at 532 nm in toluene under
an anaerobic condition (Figure 4). The T-T absorption spectrum
recorded after photoexcitation is characterized by the T1-Tn
absorption centered at 630 nm to the red side of the main
bleaching band at 568 nm. The triplet state lifetime was
measured at 530 and 640 nm, both of which yielded a lifetime
of 1.8 µs at room temperature. Just as in fluorescence lifetime,
the triplet state lifetime is significantly reduced for 1a as
compared to porphyrins. The quantum yield of triplet state
formation was estimated to be ΦISC ) 0.01 for 1a using the
Photophysics of [26]Hexaphyrin(1.1.1.1.1.1). The crystal
structure of meso-hexakis(pentafluorophenyl) [26]hexaphyrin-
(1.1.1.1.1.1) 1a (Chart 1)8 has a flat rectangular conformation
with two inverted pyrrole rings, leading to the expected
aromaticity of [4n+2] π electrons. The absorption spectrum of
1a shows a strong band at 568 nm, which presumably corre-
sponds to the formally allowed Soret (B) band of porphyrins.
Several low-energy bands at 712, 898, and 1026 nm may
correspond to the weaker Q-bands of porphyrins (Figure 1). An
increased conjugation of hexaphyrins is responsible for the large
bathochromic shift and decreased HOMO-LUMO gap relative
to porphyrins. The fluorescence of 1a was measured with
photoexcitation at 442 nm, whose spectrum shows the fluores-
cence maximum at 1036 nm with a weak vibronic band at 1205
nm (Figure 1). As compared to the absorption edge of 1026
nm, the Stokes shift is rather small (94 cm-1), indicating a
minimal structural change in the S1 state relative to the ground
state. The fluorescence quantum yield was roughly estimated
to be less than 10-4. The fluorescence lifetime was measured
by the time-correlated single-photon counting (TCSPC) tech-
nique, which yielded a lifetime of ∼125 ps at 1030 nm upon
photoexcitation at 405 nm (inset, Figure 1). It should be noted
that this is much shorter than those of analogous molecules such
as free-base porphyrin (∼11.3 ns) and sapphyrin (∼4.7 ns).6d
(7) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. G.; van Stryland, E.
W. IEEE J. Quantum Electron. 1990, 26, 760.
(8) Preparation and compound data are given in refs 3b, 3d, and the Supporting
Information.
(9) Chirvony, V. S.; Hoek, A.; Galievsky, V. A.; Sazanovich, I. V.; Schaafsma,
T. J.; Holten, D. J. Phys. Chem. B 2000, 104, 9909.
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A R T I C L E S
Ahn et al.
Figure 3. Femtosecond transient absorption spectra of 1a at zero time delay.
Inset shows temporal decay curves at 570 and 640 nm for ground-state
bleaching and induced absorption signals.
Figure 5. Absorption and fluorescence spectra of 2 in toluene at room
temperature. The inset shows time-resolved fluorescence decay profile at
1030 nm with photoexcitation at 405 nm. The decay profile was fitted by
a deconvolution procedure with an IRF function, which gives ∼205 ps
lifetime.
Figure 4. Nanosecond flash photolysis spectra of 1a in toluene under
unaerobic conditions with photoexcitation at 532 nm. The triplet state
lifetimes were measured at 530 and 640 nm for ground-state bleaching and
induced absorption, respectively, which all give the lifetime of 1.8 µs at
room temperature.
Figure 6. Femtosecond transient absorption spectra of 2 in toluene at zero
time delay with photoexcitation at 400 nm at 296 K. The inset shows
temporal decay curves at 580 and 660 nm for induced absorption and
ground-state bleaching, respectively, after photoexcitation at 400 nm. The
dual decay times were estimated as 115 ( 5 ps and 1.25 ( 0.1 ns.
known value of ΦISC ) 0.89 for free-base porphyrin. Overall,
the reduced HOMO-LUMO transition energy in 1a dramati-
cally enhances the internal conversion from the S1 to the S0
state with a very low quantum yield for triplet state formation.10
Excited-State Dynamics of [28]Hexaphyrin(1.1.1.1.1.1). It
is known that 1a undergoes reduction with NaBH4 to produce
meso-hexakis(pentafluorophenyl) [28]hexaphyrin(1.1.1.1.1.1) 2
with a vivid color change from violet to blue. The latter is easily
converted to the former by oxidation with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ).5 Although the solid-state
crystal structure of 2 is unavailable, its conformation is
conjectured to be a largely planar rectangular shape similar to
1a as judged from the crystal structure of a related [28]-
distorted nonplanar geometry, in contrast to the relatively flat
structure for 1 (Supporting Information, Figure S1). The
fluorescence lifetime of 2 revealed a lifetime of ∼205 ps at
1030 nm with excitation at 405 nm that is considerably longer
than that of 1a (inset, Figure 5).
The transient absorption decay profile of 2 at 570 nm
exhibited a double exponential decay with time constants of
∼115 ps (85%) and 1.25 ns (15%) at 296 K (Figure 6). Because
no signal was observed in nanosecond flash photolysis, the
long decay component with the time constant of 1.2 ns may
be assigned as an intersystem crossing. Considering these
spectroscopic features of 2, we suggest that the conforma-
tional dynamics may play an important role in the energy
relaxation pathways as illustrated in the previous work by
Chirvony et al. on the photophysical properties of free-base
diacid porphyrins.9 To check the possibility of conformational
change in the excited state, we examined transient absorption
decay kinetics of 2 by lowering the temperature to 273 K, which
yielded the decreased time constants of ∼26 ps (72%) and 1.25
ns (28%).
1
hexaphyrin molecule.9 The H NMR spectra of 2 revealed no
signal reflecting the anticipated antiaromatic ring current effect,
which is presumably due to the slightly nonplanar geometry of
2 as compared to 1a. The absorption spectrum for 2 exhibits a
relatively broad Soret-like band with smeared Q-band-like
transitions (Figure 5). Dual fluorescence bands with vibronic
structures were observed at 1046 and 1210 nm with a relatively
large Stokes shift of 290 cm-1 (Figure 5). The ab initio
calculation for 2 on the B3LYP/3-21G* level yielded a slightly
(10) Cho, H. S.; Ahn, T. K.; Yang, S. I.; Jin, S. M.; Kim, D.; Kim, S. K.; Kim,
H. D. Chem. Phys. Lett. 2003, 375, 292.
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Photophysics of [26]- and [28]Hexaphyrins(1.1.1.1.1.1)
A R T I C L E S
Figure 7. The transient absorption decay profiles of 1a and 2 at 570 nm
at 296 and 273 K. Those of [28]hexaphyrin show exponential decays with
time constants of ∼115 ps (85%) and 1.25 ns (15%) at 296 K and ∼26 ps
(72%) and 1.25 ns (28%) at 273 K, which supports the conformational
change in the excited state of 2.
Free-base diacid porphyrins (H4TPP2+ and H4OEP2+) are
known to exhibit nonplanar structural distortions relative to their
corresponding neutral porphyrins (H2TPP and H2OEP), which
was confirmed by semiempirical PM3 calculations and X-ray
crystallography.9 As compared to neutral porphyrins, diacid
porphyrins exhibit (1) spectral broadening in absorption and
fluorescence spectra, (2) large Stokes shift, and (3) reduced
excited-state lifetimes mainly due to their enhanced internal
conversion (S1fS0) rates. In addition, H4TPP2+ was reported
to experience a fluorescence lifetime change according to a
change in temperature. They interpreted this temperature-
dependent excited-state dynamics in terms of conformational
change occurring in the excited state, which involves the
activation process over the energy barrier leading to different
nonradiative decay rates.9 These features are quite similar to
our spectroscopic observations on the excited-state dynamics
of 2. We also observed a slightly smaller Stokes shift in
fluorescence emission at 273 K than at 296 K, supporting our
interpretation of the conformational dynamics in the excited state
of 2 (Supporting Information, Figure S2). On the contrary, the
transient absorption decay of 1a remains unchanged (∼100 ps)
with a change in temperature from 296 to 273 K, which
precludes the possibility of conformational dynamics in the
excited state of 1a (Figure 7). Figure 8 presents a comparative
schematic energy diagram based on our interpretations of the
excited-state dynamics of 1a and 2.
Two-Photon Absorption Behaviors of [26]- and [28]-
Hexaphyrins(1.1.1.1.1.1). 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 photody-
namic therapy, three-dimensional micro-fabrication and optical
data storage, and optical limiting.1 Among these, porphyrins
with 18π-electronic system are quite promising, but high TPA
performance has been achieved only when π-electronic networks
are considerably enlarged either by conjugation with peripheral
substituents and/or covalent and noncovalent assembling, be-
cause normal porphyrin monomers exhibit only small σ(2) values
of less than 102 GM.2,3 We examined the TPA properties of 1
and 2 by an open-aperture Z-scan method (Supporting Informa-
tion, Figure S3) with ∼130 fs pulses at 1200 nm, and the results
are summarized in Table 1. The TPA cross-section (σ(2)) value
Figure 8. Schematic diagrams of the excited states of 1a and 2 to explain
the temperature-dependent transient profiles. At 273 K, the excited-state
conformer blocks the conformational change because of its energy barrier.
Table 1. Absorption Band, TPA Cross Section, and S1-State and
T1-State Lifetimes of Hexaphyrins(1.1.1.1.1.1)
hexaphyrin
λmax/nm
σ
(2)/GM
τ
S/psf
τ
T/nsg
1a
1b
1c
1d
2
567,a 1018b
574,a 1038b
574,a 1033b
568,a 1038b
594,a 1009b,c
498
9890 ( 230d
9490 ( 150d
7790 ( 260d
7640 ( 190d
2600 ( 260d
810 ( 50e
98
103
100
101
26
1800
665
h
-
52
h
-
-
h
3
9.2
a Soret-like absorption band. b Lowest energy Q-band-like absorption
band. c Broad band. d Two-photon absorption cross-section (σ(2)) value
measured at 1200 nm. e σ(2) value measured at 800 nm. f S1-state lifetime.
g T1-state lifetime. h No transient was observed by nanosecond flash
photolysis.
of 1a was measured to be 9890 GM at 1200 nm where the linear
absorption is negligible (Supporting Information, Figure S4).3,4,10
This value is much larger than those of porphyrin monomers
by a factor of >102, being the largest among porphyrin-related
monomeric pigments. Hexaphyrins 1b, 1c, and 1d all exhibit
large σ(2) values, 9490, 7790, and 7640 GM, respectively. The
1H NMR spectral data of 1b-1d all indicate a strong diatropic
ring current similar to 1a. Therefore, it is concluded that the
hexaphyrins 1a-1d take the similar molecular shape and
aromatic 26π-electronic system (Supporting Information, Figures
S5-S7). Despite large and polarizable aromatic substituents
such as 9-anthryl groups at the meso-positions (1c), the σ(2)
values were not much influenced probably due to their tilting
conformations against the hexaphyrin plane. Interestingly, the
S1-state lifetimes of 1a-1d are nearly the same, ca. 100 ps,
while their T1-state lifetimes exhibit large variations depending
on the meso-aryl substituents (Table 1).
In contrast, we observed the TPA value of 2600 GM for 2 at
1200 nm, which is consistent with the reduced aromaticity as
compared to 1a. Besides aromaticity determined by the number
of π electrons, the planarity of the hexaphyrin ring is likely to
play an important role in maintaining the π conjugation
(Supporting Information, Figure S4). Furthermore, we measured
the σ(2) value of perfluorinated meso-hexakis(pentafluorophenyl)
[28]hexaphyrin(1.1.1.1.1.1) 3 that has a totally nonplanar figure
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A R T I C L E S
Ahn et al.
dumped Ti:Sapphire oscillator, which provided a high repetition rate
(200-400 kHz) of ultrashort pulses (100 fs at full width half-maximum-
(fwhm)) pumped by a CW Nd-YVO4 laser (Spectra-Physics, Millen-
nia). The output pulses of the oscillator were frequency-doubled with
a second harmonic crystal. The TCSPC detection system consisted of
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 processing.
The overall instrumental response function by an IR dye (Aldrich,
IR1100) was about 410 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.
eight structure due to a lack of ring electron density, and its
absorption spectrum exhibits a broad absorption band centered
at 498 nm without any Q-band like low-energy absorption bands,
and no fluorescence was detected,13 hence featuring typical
nonaromatic nature. The excited-state lifetime has been revealed
quite short (9.2 ps) on the basis of the femtosecond transient
absorption spectroscopy (Supporting Information, Figure S8).
Interestingly, the σ(2) value of 3 was determined to be 810 GM
at 800 nm, again reinforcing the dependence of the TPA
absorption cross section on the aromaticity. Yet these results
indicate that the extension of cyclic electronic network from
porphyrin to hexaphyrin(1.1.1.1.1.1) leads to enhancement of
σ(2) value even for the nonaromatic one. Thus, as the overall
ring structure becomes planar in [26]hexaphyrins, a large TPA
cross section is observed. It is noteworthy that the changes in
the excited-state lifetimes for various [26]hexaphyrins are
parallel with the increase of TPA cross-section values and the
enhancement of ring planarity revealed by X-ray crystal-
lography.
Femtosecond Transient Absorption Measurements. The dual-
beam femtosecond time-resolved transient absorption spectrometer
consisted of a self-mode-locked femtosecond Ti:sapphire laser (Coher-
ent, MIRA), a Ti:sapphire regenerative amplifier (Clark MXR, CPA-
1000) pumped by a Q-switched Nd:YAG laser (ORC-1000), a pulse
stretcher/compressor, OPG-OPA system, and an optical detection
system.15 The pump beam was focused to a 1 mm diameter spot, and
laser fluence was adjusted less than ∼1.0 mJ cm-2 by using a variable
neutral-density filter. The fundamental beam remaining in the OPG-
OPA system was focused onto a flowing water cell to generate white
light continuum, which was again split into two parts. One part of the
white light continuum was overlapped with the pump beam at the
sample to probe the transient, while the other part of the beam was
passed through the sample without overlapping the pump beam. The
time delay between pump and probe beams was controlled by making
the pump beam travel along a variable optical delay. The white
continuum beams after sample were sent to a 15 cm focal length
spectrograph (Acton Research) through each optical fiber and then
detected by the dual 512 channel photodiode arrays (Princeton
Instruments). The intensity of the white light of each 512 channel
photodiode array was processed to calculate the absorption difference
spectrum at the desired time delay between pump and probe pulses.
Nanosecond Flash Photolysis Measurements. The nanosecond
transient absorption spectra were obtained by nanosecond flash pho-
tolysis technique.16 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 transient absorption experiment.
The light signal was detected via a photomultiplier tube (Hamamatsu,
R928). The output signal from the PMT was recorded with a 500 MHz
digital storage oscilloscope (Tektronix, TDS3052) for the temporal
profile measurement. Because the triplet state dynamics of molecules
in solution is strongly dependent on the concentration of oxygen
molecules dissolved in solution, we have 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
under unaerobic conditions, which gives about 1 ms lifetime at room
temperature. Because the concentration of molecules also affects
significantly the excited triplet state lifetime due to triplet-triplet
annihilation processes, we have kept the concentration down to 10-5
M with relatively low photoexcitation density at 532 nm produced by
the second harmonic output of a Q-switched Nd:YAG laser.
Conclusion
In this work, we have revealed the overall dynamics of the
S1 and T1-excited states of hexaphyrins 1, 2, and 3, which
constitute a useful platform for further investigations of the
excited states of expanded porphyrins. Especially, we proved
the conformational dynamics involved in the excited-state
relaxation of 2 by temperature-dependent transient absorption
decay measurements. In addition, large TPA cross sections (ca.
104 GM) of the aromatic hexaphyrins 1 have been demonstrated,
and their dependence on the aromaticity has been established.
These large TPA cross sections of 1 are quite attractive and
will be further enhanced by the extension of their π-electronic
systems through suitable peripheral modification with conjugated
substituents or covalent and/or noncovalent assembling.
Experimental Section
Sample Preparation. The details of the synthesis of 1a-1d, 2, and
3 are described elsewhere5,11 and in the Supporting Information. For
example, in case of 1a, a solution of methanesulfonic acid in CH2Cl2
(2.5 M, 12.5 mL) was added to a solution of pentafluorobenzaldehyde
(0.5 mmol) and meso-pentafluorophenyl dipyrromethane (0.5 mmol)
in dry CH2Cl2 (15 mL). The resulting solution was stirred for 2 h at 0
°C under nitrogen. After addition of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ, 500 mg), the solution was stirred for 3 h and
passed through a short alumina column for neutralization and removal
of tar with 10% methanol in CH2Cl2. After removal of the solvent by
a rotary evaporator, the reaction mixture was separated by silica gel
column chromatography with a mixture of CH2Cl2 and hexane (30:70)
as an eluent.
Near-IR Fluorescence Spectrum and Lifetime Measurements.
The fluorescence emission was detected using a near-IR photomultiplier
(Hamamatsu, H9170-75), 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)
technique.14 As an excitation light source, we used a homemade cavity
Measurement of Two-Photon Absorption Cross Section (σ (2)).
The TPA spectra were measured at 1200 nm by using the open-aperture
(11) Suzuki, M.; Shimizu, S.; Shin, J.-Y.; Osuka, A. Tetrahedron Lett. 2003,
44, 4597.
(12) Kim, D. Y.; Ahn, T. K.; Kwon, J. H.; Kim, D.; Ikeue, T.; Aratani, N.;
Osuka, A.; Shigeiwa, M.; Maeda, S. J. Phys. Chem. A 2005, 109, 2996.
(13) Shimizu, S.; Shin, J.-Y.; Furuta, H.; Ismael, R.; Osuka, A. Angew. Chem.,
Int. Ed. 2003, 42, 78.
(15) Cho, H. S.; Song, N. W.; Kim, Y. H.; Jeoung, S. C.; Hahn, S.; Kim, D.;
Kim, S. K.; Yoshida, N.; Osuka, A. J. Phys. Chem. A 2000, 104, 3287.
(16) Song, N. W.; Cho, H. S.; Yoon, M. C.; Aratani, N.; Osuka, A.; Kim, D.
Bull. Korean Chem. Soc. 2002, 23, 271.
(14) Hwang, I.-W.; Cho, H. S.; Jeong, D. H.; Kim, D.; Tsuda, A.; Nakamura,
T.; Osuka, A. J. Phys. Chem. B 2003, 107, 9977.
9
12860 J. AM. CHEM. SOC. VOL. 127, NO. 37, 2005

Photophysics of [26]- and [28]Hexaphyrins(1.1.1.1.1.1)
A R T I C L E S
Z-scan method7 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) (Figure S5). The laser beam was divided
into two parts. One was monitored by a Ge/PN 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 is 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:7
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 exhibits 50 GM at 800 nm.17
Acknowledgment. The work at Yonsei University was
supported by the National Creative Research Initiatives Program
of KOSEF. The work at Kyoto University was supported by
the CREST of JST.
âI0(1 - e-R l
)
0
Supporting Information Available: Synthetic details (PDF);
X-ray crystallographic data (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
T(z) ) 1 -
(1)
2R0(1 + (z/z0)2)
where R0 is the linear absorption coefficient, l is the sample length,
and z0 is the diffraction length of the incident beam.
JA050895L
After obtaining the nonlinear absorption coefficient â, the TPA cross-
(17) Kim, O. K.; Lee, K. S.; Woo, H. Y.; Kim, K. S.; He, G. S.; Swiatkiewicz,
section σ(2) of one solute molecule (in units of 1 GM ) cm4‚s/photon‚
J.; Prasad, P. N. Chem. Mater. 2000, 12, 284.
9
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