Effective expansion of the subporphyrin chromophore through conjugation with meso-oligo(1,4-phenyleneethynylene) substituents: Octupolar effect on two-photon absorption

Article abstract of DOI:10.1002/anie.200801192

(Chemical Equation Presented) Go with the glow: A series of meso-oligo(1,4-phenyleneethynylene) substituted subporphyrins shows a significant amplification in absorption coefficient and fluorescence quantum yield as the chain length of the substituents increases (see picture). Particularly, enhancement of the two-photon absorption properties along the series can be characterized by an octupolar effect rather than a conjugation effect.

Full text of DOI:10.1002/anie.200801192

Communications
DOI: 10.1002/anie.200801192
Porphyrinoids
Effective Expansion of the Subporphyrin Chromophore Through
Conjugation with meso-Oligo(1,4-phenyleneethynylene) Substituents:
Octupolar Effect on Two-Photon Absorption**
Yasuhide Inokuma, Shanmugam Easwaramoorthi, So Young Jang, Kil Suk Kim, Dongho Kim,*
and Atsuhiro Osuka*
Subporphyrin, a ring-contracted porphyrin with a bowl-
shaped structure, has emerged as a promising functional
pigment because of its intense absorption and bright green
fluorescence which arise from its 14p-conjugated aromatic
circuit.[1–3] The chemistry of subporphyrins 1 and 2 has
remained in its infancy, compared to the chemistry of
subphthalocyanines 3 that has been extensively studied[4]
since the first report by Meller and Ossko in 1972.[5] In
porphyrin chemistry, the meso-aryl substituents in 4 have
served as key synthetic handles for the covalent linkage of
functional units, the anchoring of chelating units for control-
ling the reactivity of metalloporphyrins, for providing steric
hindrance to the central metal center thus protecting metal-
loporphyrins from intermolecular interference, and so forth.[6]
As such, the roles of meso-aryl substituents are in most cases
limited to being structural through providing sites for func-
tional fabrications with well-defined geometries with respect
to the porphyrin plane. In other words, the electronic impact
of meso-aryl substituents on the porphyrin p-system is only
marginal, this is due mainly to their restricted perpendicular
arrangements to the porphyrin plane. This marginal influence
is consistent with small 1 values for Hammett plots of the
redox potentials of porphyrins (1 = 0.073 and 0.065 V in the
plot of E1/2,red or E1/2,ox vs. 4 s).[7] In contrast, we have recently
revealed that the 1 values are large (0.124 and 0.105 V) for
subporphyrins,[1b] which indicates a large electronic influence
of the meso-aryl substituents. This interesting property is
aided by the free rotation of the meso-aryl substituents and
the large orbital coefficients at the meso-position in the
HOMO and LUMO. Herein, we report the synthesis and
nonlinear optical (NLO) properties of 5,10,15-tris-oligo(1,4-
phenyleneethynylene) substituted subporphyrins 10–13 (see
Scheme 1), in which the p-electron system of the subporphyr-
ins is effectively delocalized over the whole molecule includ-
ing the oligo(1,4-phenyleneethynylene)[8] arms.
Scheme 1 outlines the synthetic routes to 10–13. Sonoga-
shira coupling reactions of 4-bromophenyl-substituted sub-
porphyrin 5 with trimethylsilylacetylene (6), phenylacetylene
(7), and 4-(phenylethynyl)phenylacetylene (8) proceeded
quantitatively to give 10–12 in 96, 96, and 95%, yields
respectively. It is worth noting that subporphyrins 10–12 are
readily soluble in CH2Cl2, CHCl3, and toluene despite the
presence of three oligo(1,4-phenyleneethynylene) units with-
out any bulky substituents. The yield of 13 was only 58%,
probably because of the poor solubility of the acetylene
precursor 9. Subporphyrins 10–13 exhibit similar 1H NMR
spectral patterns, featuring a singlet in the range of d = 8.12–
8.17 ppm for the peripheral b-protons and a single set of
signals without discrimination of the two phenylene protons
at the 2,6- or 3,5-positions for the 1,4-phenyleneethynylene
groups. These 1H NMR data indicate the C3-symmetry for the
subporphyrins 10–13 and free rotation of the meso-aryl
substituents.
[*] Y. Inokuma, 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: dongho@yonsei.ac.kr
Dr. S. Easwaramoorthi, S. Y. Jang, K. S. Kim, Prof. Dr. D. Kim
Centre for Ultrafast Optical Characteristics Control and Department
of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: osuka@kuchem.kyoto-u.ac.jp
[**] This work was partly supported by a Grants-in-Aid (A) (No.
19205006) for Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology (Japan). Y.I. thanks the
JSPS Research Fellowships for Young Scientists. The work at Yonsei
University was supported by the Star Faculty program from the
Ministry of Education and Human Resources Development, Korea.
S.E., S.Y.J., and K.S.K. acknowledge BK21 fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 1. Synthesis of subporphyrins 10–13; TBAF=tetrabutyl ammonium fluoride.
Single crystals of 10 and 11 suitable for X-ray
diffraction analysis were grown in a mixture of
CH2Cl2/methanol.[9] These two structures show
bowl-shaped triangular frameworks, in which the
sides are approximately 17 Š for 10 and 24 Š for 11
(Figure 1). The bowl-depth, defined by the distance
between center boron atom and the mean plane of
peripheral six b-carbon atoms, has been calculated
to be 1.33 Š both for 10 and 11. The dihedral angles
of the meso-aryl substituents with respect to the
mean plane are 50.2, 51.5, and 57.38 for 10, and
45.8, 48.7, and 52.08 for 11.
Upon elongation of the meso-oligo(1,4-phenyl-
eneethynylene) chain lengths, the Soret- and Q-
bands are red-shifted and progressively intensify
(Figure 2 and Table 1). While the Q(1,0) band is
Figure 2. UV/Vis absorption (solid lines) and fluorescence (dashed lines) spectra
of subporphyrins 2 and 10–13 in CH2Cl2.
more intense than the Q(0,0) band in the case of 2,
the relative intensity of Q(0,0) band progressively
increases with the increasing
length of the substituted 1,4-
phenyleneethynylene chains.
These spectral changes reveal
the electronic interactions
between subporphyrin core
and meso-oligo(phenyleneethy-
nylene) chains, however it
reaches a saturation point at
12, beyond that no significant
spectral change occurs. None-
theless the absorption bands
characteristic of oligo(1,4-phe-
nyleneethynylene)
moieties
observed at 309, 333, and,
347 nm for 11, 12, and 13,
respectively, are in line with
the elongation of p-conjugation
length. The steady-state fluores-
cence spectra are observed at
518, 536, 554, 554, and 555 nm
for 2, 10, 11, 12, and 13, respec-
tively, and are a mirror image of
Figure 1. Crystal structures of A) 10 and B) 11.
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Communications
Table 1: Summary of optical measurements.
Compd
Absorption
Fluorescence
FF
TPA
lmax [nm] (e [105 mÀ1 cmÀ1][a])
lmax [nm][b]
tf [ns][d]
kr [sÀ1 [e]
]
s(2) [GM][f]
[c]
2
10
11
12
13
373 (1.66)
380 (1.75)
388 (2.11)
392 (3.17)
394 (3.42)
461 (0.13)
465 (0.15)
466 (0.14)
470 (0.24)
470 (0.23)
484 (0.09)
493 (0.15)
499 (0.24)
500 (0.38)
500 (0.36)
518
536
554
554
555
0.13
0.15
0.29
0.34
0.35
2.37
2.01
1.93
1.77
1.62
5.5”107
7.5”107
1.5”108
1.9”108
2.2”108
88
270
660
960
1340
[a] Absorption coefficient. [b] Excited at each absorption maximum (373–394 nm). [c] Absolute fluorescence quantum yield. [d] Fluorescence life time.
[e] Natural radiative rate constant. [f] TPA cross-section values obtained by excitation at 800 nm (1 GM=10À50 cm4 s photonÀ1
)
the Q-band spectral changes. Thus, these spectral features
indicate that the effective conjugative interactions with the
subporphyrin moiety are limited up to the 1,4-bis(phenyle-
thynyl)benzene group, that is, 12. Since the natural radiative
lifetime (t0) is expected to have a correlation with the actual
radiative size of the chromophore and fluorescence lifetime,
the natural radiative rate constants calculated from the
fluorescence quantum yield and fluorescence lifetime accord-
ing to the relationship of (kr = 1/t0, t0 = tf/Ff) are 5.5 ” 107 sÀ1,
7.5 ” 107 sÀ1, 1.5 ” 108 sÀ1, 1.9 ” 108 sÀ1, and 2.2 ” 108 sÀ1 for 2,
10, 11, 12, and 13, respectively, in agreement with the
increasing size of the effective radiative chromophore with
the elongation of meso-oligo(1,4-phenyleneethynylene) sub-
stituents.[10]
Further information on the electronic conjugative effect
of meso-oligo(1,4-phenyleneethynylene) substituents on the
subporphyrin ring is gleaned from two-photon absorption
(TPA) cross-section (s(2)) values, which are largely propor-
tional to the electron delocalization strength of the mole-
cule.[11] The TPA cross-section values were measured by the
open aperture z-scan method by exciting the molecule at
800 nm using femtosecond Ti:sapphire regenerative amplifier
system with 130 fs pulse width.[11] The TPA cross-section
values of subporphyrins increase gradually from 88 to
1340 GM as the length of the substituent group increases
(Table 1). The TPA cross-section value of10 is approximately
triple that of 2 which can not be solely explained by the
elongation of the p-conjugation length by an additional
acetylene group. Thus, the observed behavior is attributed to
the octupolar structure of these cone shaped subporphyrins
because these molecules exhibit larger first-order hyper-
polarizabilities (b) than dipolar molecules.[12] The hyper-
polarizability (b) and TPA cross-section (s(2)) values exhibit a
qualitatively linear relationship and these values increase
with the increasing p-conjugation length.[13]
whereas the HOMOÀ1 has nodal points at meso-positions.
These orbital characteristics of meso-aryl substituted subpor-
phyrins allow large orbital interactions with the meso-aryl
ring to be predicted. This feature has been confirmed by the
calculated molecular orbitals of 10–13. Pairs of strongly
degenerated LUMOs are stabilized through the increasing
conjugative interactions with the meso-1,4-phenyleneethyny-
lene groups progressively from 10 to 13, although this trend
becomes almost saturated at 13. In contrast, the energy levels
of the HOMO orbitals are accidentally rather constant for 2,
10, 11, 12, and 13, hence giving rise to a continuous decrease in
the HOMO–LUMO gap. To check the validity of these
molecular orbitals, the electrochemical properties of the
subporphyrins have been examined by cyclic voltammetry
(CV) in CH2Cl2 containing 0.10m Bu4NPF6 as a supporting
electrolyte. The first one-electron oxidation potentials were
observed at 0.76, 0.72, and 0.71 V for 11, 12, and 13,
respectively, versus the ferrocene/ferrocenium ion couple,
while the first one-electron reduction potentials of 10–13 were
observed at À1.83 to À1.81 V, which are distinctly less
negative than that of 2 (À1.97 V). Experimentally determined
electrochemical HOMO–LUMO gaps are roughly consistent
with the molecular orbital calculation and optical HOMO–
LUMO gaps (Table 2 and SI).
In summary, meso-oligo(1,4-phenyleneethynylene)-sub-
stituted subporphyrins 10–13 were synthesized by Sonoga-
shira-coupling of meso-(4-bromophenyl)subporphyrin 5 with
ethynes 6–9. The electronic p-networks of subporphyrins are
effectively expanded through the conjugation of meso-
oligo(1,4-phenyleneethynylene) substituents, as has been
clearly indicated by the optical and electrochemical proper-
ties. These results illustrate the highly tunable electronic
properties of subporphyrins by meso-aryl substituents, prop-
erties which are impossible for porphyrins, hence underscor-
ing versatile and promising potentials of subporphyrinic
It is noteworthy that unlike one-photon absorption, the
TPA cross-section values do not show any saturation behavior
with respect to meso-substituent chain length, hence the
enhancement in TPA properties in meso substituted subpor-
phyrins would be determined by the octupolar effect rather
than the conjugation effect, which shows a saturation
behavior as inferred from their one-photon absorption
spectra.
Calculations were performed at the B3LYP/6-31G* level
with Gaussian03 package.[14] The subporphyrin 2 has degen-
erate HOMOs and LUMOs. Its HOMO, LUMO and
LUMO + 1 have large coefficients at the meso-positions,
Table 2: Electrochemical, optical, and theoretical HOMO–LUMO gaps.
Compd
Redox potential [V][a]
HOMO–LUMO gap [eV]
Eox,1
Ered,1
CV
UV/Vis[b]
Calculation[c]
2
10
11
12
13
0.71
0.86
0.76
0.72
0.71
À1.97
À1.81
À1.83
À1.82
À1.81
2.68
2.67
2.59
2.54
2.52
2.56
2.51
2.48
2.48
2.48
3.18
3.05
2.91
2.83
2.80
[a] vs. Ferrocene/ferrocenium ion pair. [b] Calculated from Q(0,0) bands.
[c] Performed at the B3LYP/6-31G* level.
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subporphyrins show higher two-photon absorption efficiency
by virtue of an octupolar effect, which can be favorably tuned
by increasing the arm length. Along this line, our current
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[7] K. M. Kadish, M. Morrison, J. Am. Chem. Soc. 1976, 98, 3326.
[8] U. H. F. Bunz, Chem. Rev. 2000, 100, 1605.
[9] Crystallographic data for 10: C40H24B1N3O1·CH3OH, Mr =
605.47,
15.304(9) Š, b = 96.020(18)8, V= 3305(3) Š3, T= 123(2) K,
space group P21/a, Z = 4, 1 = 1.217 gcmÀ3
m(MoKa) =
monoclinic,
a = 11.032(6),
b = 19.682(8),
c =
,
0.075 mmÀ1, 24698 reflections measured, 5778 unique (Rint
=
Received: March 12, 2008
Published online: May 21, 2008
0.1496) which were used in all calculations, R(F2) = 0.1679 (all
data), R1 = 0.0979 (I > 2s(I)), GOF = 1.039. Crystallographic
data for 11: C58H36BN3O, Mr = 801.71, orthorhombic, a =
24.337(4), b = 17.749(3), c = 22.934(3) Š, V= 9906(2) Š3, T=
Keywords: conjugation · fluorescence · porphyrinoids ·
.
subporphyrins · two-photon absorption
123(2) K, space group Pbcn (no. 60), Z = 8, 1 = 1.075 gcmÀ3
,
m(MoKa) = 0.064 mmÀ1, 73730 reflections measured, 8703 unique
(Rint = 0.1495) which were used in all calculations, R(F2) = 0.1545
(all data), R1 = 0.0974 (I > 2s(I)), GOF = 1.073. CCDC-680553
(10) and CCDC-680554 (11) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The SQUEEZE program
in PLATON was used for analysis to remove the solvent
densities: a) P. van der Sluis, A. L. Spek, Acta Crystallogr. Sect. A
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