Donor-π-acceptor type porphyrins with large two-photon absorption cross section

Article abstract of DOI:10.1016/j.dyepig.2015.03.033

The third-order nonlinear optical properties of 5-(4-pyrenylalkynylphenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin (1) and 5-(4-iodophenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin (2) have been comparatively investigated with symmetrical 5,10,15,20-tetra(4-octyloxyphenyl)porphyrin (3) and pyrene (4) as reference using femtosecond-pulsed Z-scan technique. Due to the D-π-A structure, both A₃B type asymmetric porphyrins 1 and 2 were revealed to exhibit larger two-photon absorption (2PA) cross sections over the symmetrical analogue 3 with the largest value of 6.1 × 10⁵ GM revealed for 1 because of the intramolecular energy transfer from the excited pyrene moiety to the porphyrin moiety as indicated by the fluorescence spectroscopic result. The good 2PA properties of these A₃B type porphyrins with ultrafast response indicate their good application potential in nonlinear photonic devices.

Full text of DOI:10.1016/j.dyepig.2015.03.033

Accepted Manuscript
Donor-π-acceptor type porphyrins with large two-photon absorption cross section
Ning Sheng, Dahui Liu, Jialu Wu, Bing Gu, Zhuyuan Wang, Yiping Cui
PII:
S0143-7208(15)00108-4
10.1016/j.dyepig.2015.03.033
DYPI 4714
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 16 February 2015
Revised Date: 23 March 2015
Accepted Date: 26 March 2015
Please cite this article as: Sheng N, Liu D, Wu J, Gu B, Wang Z, Cui Y, Donor-π-acceptor type
porphyrins with large two-photon absorption cross section, Dyes and Pigments (2015), doi: 10.1016/
j.dyepig.2015.03.033.
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ACCEPTED MANUSCRIPT
Donor-π-acceptor type porphyrins with large two-photon absorption
cross section
Ning Sheng,a,b Dahui Liu,a Jialu Wu,a Bing Gu,a Zhuyuan Wang,a Yiping Cui*a
a Advanced Photonics Center, Southeast University, Nanjing 210096, Jiangsu, China.
b Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of
Chemistry and Chemical Engineering, Jining University, Qufu, 273155, Shandong,
China
*Corresponding author: cyp@seu.edu.cn
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Abstract:
5-(4-pyrenylalkynylphenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin
5-(4-iodophenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin (2)
comparatively investigated with
The
third-order
nonlinear
optical
properties
(1)
have
of
and
been
symmetrical
5,10,15,20-tetra(4-octyloxyphenyl)porphyrin (3) and pyrene (4) as reference using
femtosecond-pulsed Z-scan technique. Due to the D-π-A structure, both A3B type
asymmetric porphyrins 1 and 2 were revealed to exhibit larger two-photon absorption
(2PA) cross sections over the symmetrical analogue 3 with the largest value of
6.1×105 GM revealed for 1 because of the intramolecular energy transfer from the
excited pyrene moiety to the porphyrin moiety as indicated by the fluorescence
spectroscopic result. The good 2PA properties of these A3B type porphyrins with
ultrafast response indicate their good application potential in nonlinear photonic
devices.
Keywords: Porphyrin; Pyrene; Two-photon absorption; Donor-π-acceptor;
Fluorescence resonance energy transfer; Femtosecond-pulsed Z-scan technique
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1. Introduction
Porphyrins are versatile functional pigments with a wide range of applications not
only because of their two-dimensional conjugated electronic molecular structure but
also associated with their versatile functionalization at the peripheral positions and
the central metal ions. Recently, porphyrins have attracted considerable attention due
to their potential applications in 3-D microfabrication, optical power limiting,
two-photon photodynamic therapy, and fluorescence microscopy [1-5]. However,
most of porphyrins have relatively low two-photon absorption (2PA) cross sections
(<100 GM) [6-8], therefore limiting their usefulness in 2PA applications. In order to
improve the 2PA cross section, considerable efforts have been continuously devoted
to the exploration of novel porphyrin derivates [9,10] including porphyrin arrays [11],
dimers [12,13], self-assembled porphyrins [14,15], and expanded porphyrins [16,17].
Despite great efforts paid in this direction, most of the newly developed porpphyrin
derivatives still exhibit the 2PA cross sections in the range of 103~106 GM as
revealed by the Z-scan measurements under the excitation of femtosecond laser
pulses. However, a number of factors have been found to influence the 2PA
magnitudes, among which π-electronic delocalization plays important role [18,19].
As a consequence, special attention nowadays is given to the molecule with donor
and acceptor sets intervened by a π-conjugation system. Such systems carry an
electron donating group on one side of the porphyrin macrocycle and an electron
withdrawing group on the other side. The effect of varying the electron-donating or
electron-accepting intensity of the end groups as well as introducing additional
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groups to perturb the charge redistribution has been considered in designing new
molecular structures [20], resulting in a series of the donor-π-acceptor (D-π-A) type
systems such as A2B2 and A2BC porphyrins showing excellent nonlinear optical
(NLO) properties with large 2PA cross section. For example, Osuka reported
5,15-A2BC-bis(azulenylethynyl) zinc(II) porphyrins with the maximum 2PA cross
section value of 8030 GM at 1400 nm [20]. Porphyrins with
2,4-bis(dimethylamino)phenylethynyl donor and various acceptor groups were
calculated using density functional theory (B3LYP) to evaluate the 2PA cross
sections between 400 and 3600 GM at 1300-1500 nm [21]. A similar study on
5,15-A2B2 porphyrins with various donor or acceptor groups gave the values of 2PA
cross sections between 300 and 3300 GM [22]. However, little research is focused on
the 2PA properties of A3B type asymmetric porphyrins with donor and acceptor
groups.
In the present paper, the third-order nonlinear optical properties of
5-(4-pyrenylalkynylphenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin
5-(4-iodophenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin (2)
comparatively investigated with
(1)
have
and
been
symmetrical
5,10,15,20-tetra(4-octyloxyphenyl)porphyrin (3) and pyrene (4) as reference using
femtosecond-pulsed Z-scan technique, Fig. 1. Due to the D-π-A structure, both A3B
type asymmetric porphyrins 1 and 2 were revealed to exhibit larger 2PA cross
sections over the symmetrical analogue 3 with the largest value of 6.1×105 GM
revealed for 1 because of the intramolecular energy transfer from the excited pyrene
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moiety to the porphyrin moiety as indicated by the fluorescence spectroscopic result.
The good 2PA properties of these A3B type porphyrins with ultrafast response
indicate their good application potential in nonlinear photonic devices.
OR
OR
N
N
N
N
H
N
H
N
RO
I
RO
H
N
H
N
OR
OR
1
2
OR
N
N
H
N
RO
OR
H
N
OR
R = C8H17
4
3
Fig. 1. Schematic molecular structures of A3B type 5-(1-pyrenylalkynylphenyl)-10,15,20-tris
(4-octyloxyphenyl)porphyrin (1) and 5-(4-iodophenyl)-10,15,20-tris (4-octyloxyphenyl)porphyrin
(2) with 5,10,15,20-tetra (4-octyloxyphenyl)porphyrin (3) and pyrene (4) as references.
2. Results and discussion
2.1 Synthesis
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In order to obtain suitable D-π-A structure with expanded π-electronic
delocalization and good solubility in most organic solvent, a A3B type
5-(4-pyrenylalkynylphenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin
designed and synthesized. The
(1)
was
asymmetric
5-(4-iodophenyl)-10,15,20-tris(4-octyloxyphenyl)porphyrin (2) substituted with three
4-octyloxyphenyl groups and one 4-iodophenyl group in the meso-positions was
synthesized according to the method reported previously [25]. Subsequently, as
shown in Fig. 2, porphyrin 1 was prepared from the Pd(0)-catalyzed cross-coupling
reaction of porphyrin 2 with 1-ethynylpyrene [23] under mild basic condition without
using any copper for the purpose of avoiding the oxidative dimerization of the alkyne
component and metalation of the porphyrin unit [24]. In addition, the symmetric
porphyrin 3 substituted with four 4-octyloxyphenyl groups in the meso-positions was
also synthesized according to the established Adler-Longo method as the reference
compound [26,27]. All these porphyrin compounds were purified and isolated by
silica-gel column chromatography. The newly prepared porphyrins were
unambiguously confirmed by various spectroscopic methods in addition to elemental
analysis.
OR
OR
N
N
H
N
N
H
Pd(PPh3)4
RO
I
+
RO
H
N
H
N
N
N
triethylamine, N2
R = C8H17
OR
OR
2
1
Fig. 2. Synthesis of 5-(1-pyrenylalkynylphenyl)-10,15,20-tris (4-octyloxyphenyl) porphyrin (1).
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2.2. Electronic absorption spectra
The electronic absorption spectra of A3B type asymmetric porphyrin 1 and 2 in
CHCl3 are shown in Fig. 3. For comparative purpose, the electronic absorption
spectra of the reference compounds 3 and 4 are also shown in the same figure. It is
worth noting that because of the poor solubility in CHCl3, the spectrum of 4 was
recorded in methanol. Compared with the symmetrical porphyrin 3, the absorption
spectra of the asymmetric porphyrins 1 and 2 show similar Soret band at 423-425 nm
and Q bands between 500 and 660 nm, in line with the results reported previously
[28,29]. In addition, the common bands at 280 and 370 nm for porphyrin 1 can be
ascribed to the absorption of pyrenyl group. The absorption spectrum of 1 is identical
with the sum of the spectra of porphyrin 3 and pyrene 4, indicating that there is no
appreciable interaction between porphyrin moiety and pyrenly group in compound 1
in the ground-state [30]. In comparison with porphyrin 3, little red-shift takes place
on the Soret band of porphyrin 1, by 2 nm, which demonstrates that the pyrene
substituent is a stronger electron donor than that of OC8H17 substituent.
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0.5
0.4
0.3
0.2
0.1
0.0
1
2
3
4
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. The electronic absorption spectra of A3B type porphyrin 1 and 2 with reference porphyrin 3
and pyrene 4 recorded at room temperature. The concentrations were held at 10-6 mol/L.
2.3 Fluorescence spectra
Figs. 4 and 5 show the fluorescence emission spectra of A3B type asymmetric
porphyrin 1 and 2 as well as 3 and 4 excited at 410 and 270 nm, respectively, with
their concentrations being kept constant at 10-6 mol/L. As shown in Fig. 4, the
fluorescence spectra of asymmetric porphyrin 1 and 2 showed similar emission bands
at about 660 nm compared with the symmetrical porphyrin 3 when porphyrin was
selectively excited at 410 nm, which suggest the porphyrin origin of these emission
bands [28,31-33].
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350
300
250
200
150
100
50
1
2
3
0
600
650
700
750
800
Wavelength (nm)
Fig. 4. Fluorescence emission spectra of A3B type porphyrin 1 and 2 with reference porphyrin 3
recorded at room temperature. The concentrations were held constant to 10-6 mol/L for all of the
species. λex = 410 nm.
As shown in Fig. 5(a), excitation of 4 at 270 nm results in the expected emission
from the pyrene with intense peaks at 380 and 392 nm [31]. However, when the
pyrene unit in porphyrin 1 was selectively excited at 270 nm, the intense fluorescence
peak of pyrene was significantly quenched. Simultaneously, the characteristic
emission peak of porphyrin at 660 nm is observed in porphyrin 1, Fig. 5(b). This is,
however, not true for porphyrins 2 and 3 since no fluorescence signals were detected
in both cases. These results indicate the efficient fluorescence resonance energy
transfer (FRET) from the pyrene donor to the porphyrin acceptor due to the rigid
molecular structure as well as the good overlap between emission of the pyrene group
and absorption of the porphyrin unit in 1. The fluorescence excitation spectroscopic
result gives further confirmation for the presence of FRET in porphyrin 1. As shown
in Fig. 6, an obvious band at 280 nm attributed to pyrenyl group and a series of bands
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between 400-600 nm attributed to porphyrin are observed for 1. However, only
characteristic porphyrin bands are observed for porphyrins 2 and 3.
1200
1000
1
2
a
800
600
400
200
0
3
4
300
350
400
450
500
Wavelength (nm)
50
40
30
20
10
0
1
2
3
b
600
650
700
750
Wavelength (nm)
Fig. 5. Fluorescence emission spectra of A3B type porphyrin 1 and 2 with reference porphyrin 3
and pyrene 4 recorded at room temperature. The concentrations were held constant to 10-6 mol/L
for all of the species. λex = 270 nm.
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400
300
200
100
0
1
2
3
300
400
500
600
Wavelength (nm)
Fig. 6. Excitation spectra of A3B type asymmetric porphyrin 1 and 2 with reference symmetric
porphyrin 3 recorded at room temperature. λem = 650 nm.
2.4 Molecular structure of A3B type porphyrin 1
To obtain the information about the molecular structure for A3B type porphyrin 1,
PM3 method [35,36] in Gaussian 98W [37] was carried out on the basis of simplified
molecular models in which the octyloxy groups are replaced with methyloxy groups
on the porphyrin unit. The optimized structure for 1 obtained is shown in Fig. 7. It
appears that the meso-phenyl moiety and the pyrenyl moiety are apparently coplanar
through the alkynyl bridge. The porphyrin ring is approximately vertical to the meso-
phenyl moiety and the pyrenyl group. Obviously the D-π-A system gets expanded in
porphyrin 1, resulting in the elongation of the π-conjugation length by introducing a
pyrene group to porphyrin linked by ethynyl bond.
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Top view
Side view
Fig. 7. PM3 method optimized structure of A3B type porphyrin 1 in the top and side view. The
three OC8H17 groups attached on the meso-phenyls of porphyrin are replaced by OCH3 groups for
clarity.
2.5 2PA properties
Open-aperture Z-scan measurements were carried out for the solution of
asymmetric porphyrins 1 and 2 as well as symmetrical porphyrin 3 in CHCl3 and
pyrene (4) in methanol with the concentrations of 10-3 mol/L contained in 1 mm thick
quartz cell. As shown in Fig. 8, all Z-scan traces display a symmetrical valley with
respect to the focus, suggesting that all these compounds in respective solution exhibit
the positive nonlinear absorption in nature. Interestingly, the experimental data are in
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good agreement with the theoretical fitting using Z-scan theory as described in
literature [38], Fig. 8. At the end of this section, it is noteworthy that for pure
chloroform or methanol in quartz cell, the nonlinear absorption effect can be safely
ignored because no peak or valley was observed according to the open-aperture
Z-scan experiments.
1.00
0.95
1
2
3
0.90
4
0.85
0.80
0.75
-7.5
-5.0
-2.5
0.0
2.5
5.0
7.5
Z/mm
Fig. 8. Z-scan data for A3B type porphyrin 1 and 2 with reference porphyrin 3 and pyrene 4
obtained under an open-aperture configuration. The concentration of them is 10-3 mol/L. The dots
are the experimental data while the solid curves are the theoretical fit.
As shown in Fig. 3, all porphyrins exhibit strong absorption band centered at
423-425 nm, resulting in the highly transparent nature for their CHCl3 solution in the
near infrared range. As a consequence, the nonlinear absorption effect measured at the
wavelength of 800 nm is only able to originate from 2PA. By best fittings between the
measured Z-scan traces and the Z-scan theory for characterizing two-photon absorber
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[39], the 2PA coefficients were extracted with β = 0.148 and 0.076 for the asymmetric
A3B type porphyrin 1 and 2 and 0.062 cm/GW for the symmetrical porphyrin 3,
respectively. Furthermore, according to the formula σ2PA = ħωβ/N, where ħω is the
incident photon energy, and N is the number density of sample in solution the 2PA
coefficient was converted into the 2PA cross section with the value of σ2PA at 800 nm,
6.1×10-45 cm4•s•photon-1 and 3.1×10-45 cm4•s•photon-1 for porphyrin 1 and 2 and
2.5×10-45 cm4•s•photon-1 for 3, respectively. According to 1 GM = 10-50
cm4•s•photon-1, the values of σ2PA at 800 nm are estimated to be 6.1×105 GM for 1,
3.1×105 GM for 2, and 2.5×105 GM for 3. In spite of the obvious NLO absorptive
effect of the porphyrin, no obvious NLO absorption was observed for pyrene (4).
For comparative reason, some of corresponding results for porphyrin and porphyrin
derivatives including monomer porphyrins [40], porphyrin arrays, self-assembled
porphyrins and expanded porphyrins reported previously have been listed in Table 1.
As shown in Table 1, both A3B type asymmetric porphyrins 1 and 2 exhibit large
2PA cross sections. Their σ2PA values are even on the same order of magnitude with
porphyrin arrays [11,15], self-assembled porphyrins [14,16], expanded porphyrins
[17], and trisporphyrins [41] reported previously, indicating that they are excellent
candidates for 2PA materials.
Table 1 Optical parameters of selected 2PA compounds.
σ2PA
(GM)
Compounds
Technique λ (nm) Solvent Reference
H2TPP
16
Z-scan fs
810
CHCl3
42
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CuTPP
101
Z-scan fs
Z-scan fs
Z-scan fs
Z-scan fs
Z-scan fs
810
873
780
800
887
800
800
800
800
CHCl3
CHCl3
CH2Cl2
toluene
CHCl3
THF
42
16
Conjugated
bis-(imidazolylporphyrin)
4.4×105
0.8×105
0.9×104
Expanded porphyrins
Porphyrin arrays
17
11
Self-assembled porphyrins 7.6×103
14
tris-ZnPor
1.18×105 Z-scan fs
41
A3B type porphyrin 1
A3B type oorphyrin 2
Symmetric porphyrin 3
6.1×105
3.1×105
2.5×105
Z-scan fs
Z-scan fs
Z-scan fs
CHCl3
CHCl3
CHCl3
This work
This work
This work
Previous theoretical and experimental results have demonstrated that organic
molecules with strong electron donor and electron acceptor groups that are connected
by a large conjugated π-electron system should have high nonlinear effects [41,42].
As shown in Fig. 1, the pyrene unit serves as donor and the three octyloxy groups of
porphyrin serve as accepter to form a D-π-A system in porphyrin 1. Similarly, iodine
substituent serves as acceptor and the three octyloxy groups of porphyrin serve as
donors in porphyrin 2. These two A3B type porphyrins provide the structure with
highly delocalized π-electron system, which induces the enhancement of 2PA
response. It is worth noting that both A3B type asymmetric porphyrins 1 and 2 exhibit
larger 2PA cross sections over the symmetrical analogue 3.
Pyrenyl group-substituted A3B type porphyrin 1 shows more excellent 2PA
property than porphyrin 2. Several factors are considered to be responsible for the
enhancement of 2PA cross section. The expansion of the D-π-A system by
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introducing a pyrenyl group to porphyrin linked by ethynyl group is believed to play
an important role in contributing to increasing the σ2PA value of porphyrin 1. In
addition, due to the good overlap between the emission of pyrenyl group and the
absorption of porphyrin, the pyrenyl group transfers energy with high efficiency to the
porphyrin unit in porphyrin 1, which is another factor for leading to the large σ2PA
value of porphyrin 1. In addition, previous research has proved that elongating the
effective π-conjugation length is effective to improve 2PA behavior [20,41,42]. Since
the alkynyl bond increases the conjugated path of porphyrin 1, the σ2PA value of
porphyrin 1 is larger than that of porphyrin 2.
3. Experimental Section
3.1 Materials and measurements
Column chromatography was carried out on silica gel (Merck, Kieselgel 60,
200-300 mesh) with the indicated eluents. CH2Cl2 and triethylamine were purified by
distillation over CaH2. All other reagents and solvents were used as received. The
compound 1-ethynylpyrene was prepared according to the published procedures [23].
5-(4-iodophenyl)-10,15,20-tris(4-octyloxyphenyl) porphyrin (2) was prepared
according to the method we have reported previously [25].
1H NMR spectra were recorded on a Bruker DPX 300 spectrometer (300 MHz) in
CDCl3. Spectra were referenced internally by using the residual solvent resonance (δ
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= 7.26 for CDCl3) relative to SiMe4. Electronic absorption spectra were recorded with
a Shimadzu UV-3600 spectrophotometer. Fluorescence experiments were performed
with a RF-5301 PC spectrofluorophotometer. MALDI-TOF mass spectra were taken
on a Bruker BIFLEX III ultra-high resolution mass spectrometer with
α-cyano-4-hydroxycinnamic acid as matrix. Elemental analyses for C, H and N were
performed with a Vario EL III elemental analyzer.
3.2 2PA measurements
The 2PA properties of asymmetric porphyrin 1 and 2 with symmetrical porphyrin 3
and pyrene (4) were measured by using an open-aperture Z-scan method [38]. The
laser source for the experiments was a Ti: sappire regenerative amplifier (Coherent
Inc.), operating at 800 nm wavelength with a pulse duration of 170 fs and a repetition
rate of 1 kHz. The laser pulses had near-Gaussian spatial and temporal profiles. In the
Z-scan measurements, the laser beam was focused by a lens with a 150 mm focal
length, producing the beam waist at the focus w0 = 15 ꢀm ± 2 and the corresponding
Rayleigh length is 0.88 ꢀm. To carry out Z-scan measurements, we perform the
Z-scan measurements at the intensity of I0 = 132.96 GW/cm2, the sample was scanned
across the focus along the optical axis using a computer-controlled stage, while the
transmitted pulse energies were probed by a detector, producing the open-aperture
Z-scan trace. The Z-scan system was calibrated with a piece of cadmium sulfide bulk
crystal and the experimental uncertainty should be within ±20%.
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3.3 Preparation of 5-(1-pyrenylalkynylphenyl)-10,15,20-tris (4-octyloxyphenyl)
porphyrin (1)
A deaerated solution of pophyrin 2 (0.05 mmol, 56 mg) in 5 mL of triethylamine
and CH2Cl2 (1:1) was stirred with 5 mg (0.005 mmol) of Pd(PPh3)4 at room
temperature for 15 min under nitrogen atmosphere, then 1-ethynylpyrene (0.05 mmol,
11 mg) was added. The resulting mixture was heated to reflux with stirring under
nitrogen atmosphere for 2 h. After being cooled to room temperature, the volatiles
were removed under the reduced pressure and the residue was purified by
chromatography on a silica-gel column with CHCl3 as the eluent to give a fraction
containing the desired compound. Repeated chromatography followed by
recrystallisation from CHCl3 and CH3OH gave pure 1 as purple powder (23 mg, 38%).
1H NMR (300 Hz, 25°C, TMS): δ -2.79 (s, 2H, NH), 0.96 (t, 9H, CH3), 1.40 (m, 24H,
CH2CH2CH2CH2), 1.69 (m, 6H, CH2), 2.01 (m, 6H, CH2), 4.28 (t, 6H, OCH2), 7.29
(AA'BB', 6H, ArH), 8.15 (AA'BB', 6H, ArH), 8.09 (AA'BB', 2H, ArH), 8.89 (AA'BB',
2H, ArH), 8.18-8.40 (9H, pyrene), 8.94 (m, 8H, β-pyrrole); MS (MALDI-TOF): m/z =
1223.7 [calcd. for C86H86N4O3 (M+) 1223.6]. Anal. calcd. (%) for C86H86N4O3: C
84.41, H 7.08, N 4.58. Found: C 84.76, H 7.27, N 4.70.
3.4 Preparation of 5,10,15,20-tetra(4-octyloxyphenyl)porphyrin (3)
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5,10,15,20-tetra(4-octyloxyphenyl)porphyrin (3) was synthesized by the
Adler-Longo method [26,27]. A mixture of 4-octyloxybenzaldehyde (4.68 g, 20
mmol), and pyrrole (1.34 g, 20 mmol) in propyl acid (150 mL) was refluxed for 2 h,
and CH3OH (400 mL) was added into the reaction mixture after being cooled to room
temperature. The precipitate was filtered and washed with CH3OH, then subjected to
chromatography on a silica gel column with CHCl3 and petroleum ether (7:3) as
eluent. After removing the solvent in vacuo, the residue was re-chromatographed
under similar condition followed by recrystallisation from CHCl3/CH3OH to give the
target compound 3 (1.65g, 30%). 1H NMR (300 Hz, 25°C, TMS): δ -2.73 (s, 2H, NH),
0.95 (t, 12H, CH3), 1.46 (m, 32H, CH2CH2CH2CH2), 1.67 (m, 8H, CH2), 2.01 (m, 8H,
CH2), 4.27 (t, 8H, OCH2), 7.30 (AA'BB', 8H, ArH), 8.15 (AA'BB', 8H, ArH), 8.89 (m,
8H, β-pyrrole); MS (MALDI-TOF): m/z = 1127 [calcd. for C76H94N4O4 (M+) 1127.6]
Anal. calcd. (%) for C76H94N4O4: C 80.95, H 8.40, N 4.97. Found: C 80.67, H 8.38, N
4.60.
4. Conclusion
In summary, the 2PA properties of two A3B type porphyrins 1 and 2 have been
comparatively investigated with symmetrical porphyrin 3 as reference using
femtosecond-pulsed Z-scan technique. Due to the D-π-A structure, both A3B type
asymmetric porphyrins 1 and 2 were revealed to exhibit larger 2PA cross sections
over the symmetrical analogue 3 with the largest value of 6.1×105 GM revealed for 1
because of the intramolecular energy transfer from the excited pyrene moiety to the
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porphyrin moiety as indicated by the fluorescence spectroscopic result. The
experimental results confirm that construction of high polarizable π-electronic
systems is one of the useful strategies to improve the 2PA properties of porphyrins
and FRET from suitable substituent to porphyrin is considered to be another effective
method to enhance the 2PA cross section of porphyrins.
Acknowledgements
Financial support from the Natural Science Foundation of China (21401075,
11174160 and 11474052), Shandong Provincial Natural Science Foundation
(ZR2012BL01), A Project of Shandong Province Higher Educational Science and
Technology Program (J12LD54) and the Jiangsu Planned Projects for Postdoctoral
Research Funds (No. 1401034B).
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Table 1 Optical parameters of selected 2PA compounds.
σ2PA
(GM)
Compounds
Technique λ (nm) Solvent Reference
H2TPP
CuTPP
16
Z-scan fs
Z-scan fs
Z-scan fs
Z-scan fs
Z-scan fs
Z-scan fs
810
810
873
780
800
887
800
800
800
800
CHCl3
CHCl3
CHCl3
CH2Cl2
toluene
CHCl3
THF
42
101
42
16
Conjugated
bis-(imidazolylporphyrin)
4.4×105
0.8×105
0.9×104
Expanded porphyrins
Porphyrin arrays
17
11
Self-assembled porphyrins 7.6×103
14
tris-ZnPor
1.18×105 Z-scan fs
41
A3B type porphyrin 1
A3B type oorphyrin 2
Symmetric porphyrin 3
6.1×105
3.1×105
2.5×105
Z-scan fs
Z-scan fs
Z-scan fs
CHCl3
CHCl3
CHCl3
This work
This work
This work
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Figure Captions
Fig.
1.
Schematic
molecular
structures
of
A3B
type
5-(1-pyrenylalkynylphenyl)-10,15,20-tris (4-octyloxyphenyl)porphyrin (1) and
5-(4-iodophenyl)-10,15,20-tris (4-octyloxyphenyl)porphyrin (2) with 5,10,15,20-tetra
(4-octyloxyphenyl)porphyrin (3) and pyrene (4) as references.
Fig. 2. Synthesis of 5-(1-pyrenylalkynylphenyl)-10,15,20-tris (4-octyloxyphenyl)
porphyrin (1).
Fig. 3. The electronic absorption spectra of A3B type porphyrin 1 and 2 with reference
porphyrin 3 and pyrene 4 recorded at room temperature. The concentrations were held
at 10-6 mol/L.
Fig. 4. Fluorescence emission spectra of A3B type porphyrin 1 and 2 with reference
porphyrin 3 recorded at room temperature. The concentrations were held constant to
10-6 mol/L for all of the species. λex = 410 nm.
Fig. 5. Fluorescence emission spectra of A3B type porphyrin 1 and 2 with reference
porphyrin 3 and pyrene 4 recorded at room temperature. The concentrations were held
constant to 10-6 mol/L for all of the species. λex = 270 nm.
Fig. 6. Excitation spectra of A3B type asymmetric porphyrin 1 and 2 with reference
symmetric porphyrin 3 recorded at room temperature. λem = 650 nm.
Fig. 7. PM3 method optimized structure of A3B type porphyrin 1 in the top and side
view. The three OC8H17 groups attached on the meso-phenyls of porphyrin are
replaced by OCH3 groups for clarity.
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Fig. 8. Z-scan data for A3B type porphyrin 1 and 2 with reference porphyrin 3 and
pyrene 4 obtained under an open-aperture configuration. The concentration of them is
10-3 mol/L. The dots are the experimental data while the solid curves are the
theoretical fit.
29