Charge transfer induced enhancement of near-IR two-photon absorption of 5,15-bis(azulenylethynyl) zinc(II) porphyrins

Article abstract of DOI:10.1039/b704986b

Intramolecular charge transfer in 5,15-bis(azulenylethynyl) substituted zinc(ii) porphyrin leads to a significant enhancement of two-photon absorption at near-IR region, which has been investigated by femtosecond Z-scan method. The Royal Society of Chemis

Full text of DOI:10.1039/b704986b

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Charge transfer induced enhancement of near-IR two-photon absorption
of 5,15-bis(azulenylethynyl) zinc(II) porphyrins{
Kil Suk Kim,a Su Bum Noh,a Takayuki Katsuda,b Shuji Ito,c Atsuhiro Osuka*b and Dongho Kim*a
Received (in Cambridge, UK) 2nd April 2007, Accepted 10th May 2007
First published as an Advance Article on the web 23rd May 2007
DOI: 10.1039/b704986b
bisphenylethynyl-linked zinc(II) porphyrin as a reference molecule
(1, 2, 3 and 4 in Fig. 1). Bisethynylated porphyrin was employed as
a highly polarizable potent pigment on the basis of the previous
extensive studies on this class of molecules. Therien and co-
workers reported the large first-order hyperpolarizability for push–
pull 5,15-bis(arylethynyl) zinc(II) porphyrins.10 Anderson et al.
reported the very large third-order nonlinear optical properties for
meso-to-meso butadiyne-linked porphyrin oligomers.11 Azulene is
also a polarizable group and can serve as either an electron donor
or an electron acceptor depending on the nature and connectivity.
The electron-rich nature of the five-membered ring and electron-
deficient nature of the seven-membered rings are well known12
and have been confirmed for directly azulene-linked zinc(II)
porphyrins.13
Intramolecular charge transfer in 5,15-bis(azulenylethynyl)
substituted zinc(II) porphyrin leads to a significant enhance-
ment of two-photon absorption at near-IR region, which has
been investigated by femtosecond Z-scan method.
Recently, p-conjugated organic molecules with large two-photon
absorption (TPA) cross section have attracted much attention due
to numerous applications in material science1 for optical limiters,
optical data storage, 3-D microfabrication, optical switching, and
in biological fields2 for imaging with two-photon excited
fluorescence. Among a variety of molecules studied, porphyrins
and their derivatives have emerged as promising candidates for
nonlinear optical materials because of their rigid two-dimensional
and highly p-conjugated structures.3,4 One of the main issues in
two-photon absorption is to find new molecular systems with large
TPA cross sections. In parallel with this, a considerable progress
has been made in elucidating structure–property relationships for
large TPA cross section.5 A number of factors influence the TPA
magnitudes, among which the two major considerations are
p-electronic delocalization6,7 and charge separation efficiency.8 As
a consequence, to increase TPA cross section values in molecular
systems, molecular designs involving donor/acceptor sets inter-
vened with a p-conjugation system in a symmetrical (D–p–D or
A–p–A) or asymmetrical (D–p–A) arrangement have been
proposed.9 The effect of varying the electron-donating or electron-
accepting strength of the end groups, introducing additional
groups to perturb the charge redistribution, and elongating the
effective p-conjugation length has been considered to design new
molecular structures with enhanced two-photon absorption
behaviors. Asymmetric D–p–A perturbation is effective for
enhancement of second-order polarizability of p-conjugated
systems but has never been tested for enhancement of third order
polarizability.
Reference porphyrin 1 was prepared by the method by Therien
and co-workers.10a Azulene-linked molecules 2, 3 and 4 were
prepared by the route developed by Therien and Anderson
with some modifications. Sonogashira coupling reactions of
In this work, we examined the TPA properties of azulenyl-
ethynyl-linked zinc(II) porphyrins with a particular attention
to the degree of charge separation in comparison with
aDepartment of Chemistry, Yonsei University, Seoul, 120-749, Korea.
E-mail: dongho@yonsei.ac.kr; Fax: 82-2-2123-2434;
Tel: 82-2-2123-2652
bDepartment of Chemistry, Graduate School of Science, Kyoto
University, Sakyo-ku, Kyoto, 606-8502, Japan.
E-mail: osuka@kuchem.kyoto-u.ac.jp; Fax: 81-75-753-3970;
Tel: 81-75-753-4008
cDepartment of Material Science and Technology, Faculty of Science
and Technology, Hirosaki University, Hirosaki, 036-8561, Japan.
Fax: 81-172-39-3541; Tel: 81-172-39-3568
{ Electronic supplementary information (ESI) available: Two-photon
absorption cross-section s(2) and schematic diagram of femtosecond open-
aperture Z-scan set-up. See DOI: 10.1039/b704986b
Fig. 1 Schematic molecular structures of 1, 2, 3 and 4.
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Table 1 Excitation wavelength (nm), sample concentrations (mM)
and TPA cross section values (GM) of 1, 2, 3 and 4 in toluene
Sample
Wavelength/nm
Concentration/mM
s(2)/GM
1 (P–p–P)
2 (D–p–D)
3 (D–p–A)
4 (A–p–A)
1260
1360
1400
1430
0.12
0.14
0.13
0.13
1890
3520
8030
5490
amplifier system (Spectra-Physics, Hurricane) (ESI{). Sufficiently
high power pulses easily afford an adequate power density for
TPA process without tight focusing, which eliminates unwanted
nonlinear effects such as self-focusing and white-light continuum
generation. The maximum TPA cross section s(2) values are
1890 GM at 1260 nm for 1, 3520 GM at 1360 nm for 2, 8030 GM
at 1400 nm for 3, and 5490 GM at 1430 nm for 4, respectively
(Table 1).
Scheme 1 Reagents and conditions: (i) LiHMDS, TIPSCl, pyridine,
THF; (ii) 7, Pd(PPh3)4, CuI, Et3N, THF; (iii) TBAF, THF; (iv) 10,
Pd(PPh3)4, CuI, Et3N, THF.
Also, the TPA spectra of these porphyrin derivatives were
recorded in the 600–850 nm region, indicating that two-photon
allowed states should exist nearby 630–720 nm, slightly higher and
wider than one-photon allowed Q bands (Fig. 3). Since the
electron densities of the meso-carbon atoms of the Zn(II) porphyrin
ring are very high, the attachment of acceptor moieties at meso-
carbons is more effective in elongating the p-conjugation pathway
throughout the molecular framework along the triple bond
linkages. Thus, it is expected that A–p–A type 4 is more effective
in elongating p-conjugation pathway than D–p–D type 2, which is
in accord with the red-shifted absorption spectra and larger s(2)
value of 4 as compared with 2. Importantly, D–p–A type
porphyrin 3 exhibits the TPA cross-section value of 8030 GM at
1400 nm, which is distinctly larger than that of 4. The
enhancement in the TPA value of 3 may be ascribed to CT
character. It is noteworthy that the TPA cross section s(2) values of
2, 3 and 4 are larger than that of 1, indicating that electron
donating/accepting substituents play important roles in the
enhancement of TPA values in addition to the elongation of the
p-conjugation pathway through p-bond linkages. To gain further
insight into the enhancement of TPA values, the molecular orbital
structures of 1, 2, 3 and 4 were calculated on B3LYP/6-31G* level
(Fig. 4). The HOMO and LUMO of 4 are characteristic of an
elongated p-electron conjugation pathway throughout the mole-
cular framework, which naturally results in an enhancement in the
TPA value as compared with 2. On the other hand, the CT
5.15-diethynyl zinc(II) porphyrin 5 with 1-iodo-3-hexyloxycarbonyl
azulene (7) and 6-bromo-1,3-dihexyloxycarbonylazulene (10)
provided 2 and 4 in 43 and 87% yields, respectively. Porphyrin 3
was prepared via partially silylated intermediate 6, which was
prepared from 5. Sequential Sonogashira coupling reaction
starting from 6 afforded 3 via intermediates 8 and 9 with a 87%
isolated yield for the final reaction of 8 and 10 (Scheme 1).
In the steady-state absorption spectra, the Soret and Q-bands
are observed at 449 and 641 nm for 1, 471 and 685 nm for 2, 473
and 713 nm for 3, and 489 and 723 nm for 4, respectively (Fig. 2).
Since the Q- band (HOMO–LUMO transition) is sensitive to an
enlarged p-conjugation pathway of porphyrin ring,14 the p-con-
jugation is estimated to increase in the order of 1 , 2 , 3 , 4,
which is also in line with the relative intensity of the Q-band. This
may be caused by charge transfer property induced by electron-
donating and electron-accepting azulenylethynyl substituents. The
oscillator strengths calculated by TD-DFT method are shown in
Fig. 2. Various transitions around the Soret band of 3 suggest the
involvement of CT transitions in the absorption spectra, which is
consistent with the broader spectral features of 3 in comparison
with the relatively narrow absorption bands of 1.
The TPA cross-section values were measured by an open-
aperture Z-scan method15 with wavelength tunable 130 fs pulses at
5 kHz repetition rate generated from a Ti:sapphire regenerative
Fig. 2 Steady-state absorption spectra (solid line) of 1, 2, 3 and 4 in
toluene and calculated oscillator strength (vertical line) by TD-DFT
method.
Fig. 3 One-photon absorption spectra (solid line) and two-photon
absorption spectra (closed circle) of 1–4 in toluene.
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The work at Yonsei University was financially supported by the
Star Faculty Program of the Ministry of Education and Human
Resources Development. The work at Kyoto University was
supported by the CREST of JST. K. S. K. acknowledges the
fellowship of the BK 21 program from the Ministry of Education
and Human Resources Development.
Notes and references
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This journal is ß The Royal Society of Chemistry 2007
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