1
Investigation on TPPS4 nanoporphyrin aggregation
Using UV–Vis and Atomic Force Microscopy
A. Shokuhi Rad
Department of Chemical Engineering, Islamic Azad University, Science and
Research Branch of Tehran, Tehran, Iran
Abstract
The aggregates of - tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) in acidic solution were investigated. The
spatial structure of TPPS4 deposited on the surface was imaged using scanning atomic force microscopy (AFM).
The aggregation of TPPS4 was studied using UV–Vis spectroscopy. The TPPS4 nanotubes could be used in future
as electronic devices.
Keywords: aggregation, nanotube, porphyrin, AFM, TPPS4.
1.Introduction
Porphyrins and porphyrin derivatives present in all living systems of biochemistry. They form the main structure
of pigments like chlorophyll and heme, which involved in important processes of life [1]. Self-assembled
nanostructures are of great current interest [2-4]. Porphyrins are attractive building blocks for these
nanostructures because of their electronic, optical, catalytic properties and medical applications of porphyrins like
photodynamic therapy.
Substantial amount of researches has been done on the medical applications of porphyrins [5-7]. In this present
article, We worked on metalloporphyrins as anti-cyanide agents. It was founded that nickel (II) and copper(II)
can be removed from their N-methyl-tetra(4-sulfonato) porphyrins using cyanide and hydrogen cyanide [8].
Enthalpically interaction between the π-systems of porphyrins caused to - self-association and formation of
molecular’s stacks in aqueous solutions [9].
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One type of molecular assemblies is called J-aggregates [10]. The aggregates are characterized by one optical
absorption band (J-band), which has a red shift with respect to the absorption band of the monomers at about 400
nm (known as the Soret band) and several weaker absorptions (Q-bands) at higher wavelengths [11–17]. The
structure of-TPPS4 porphyrin molecule in acid aqueous solution was illustrated in Fig. 1. In the presence of chiral
groups, the structure has the optical activity of J-aggregates [18, 19]. The supramolecular J-aggregate structures
are not completely understood yet and hence, the spectroscopic determination of aggregation numbers doesn’t
correspond to the geometrical size of - aggregates [20, 21]. This conclusions was supported by the results of
picoseconds spectroscopy, suggesting large aggregates composed of thousands of dye monomers [22].
2. Experimental
Tetraphenylporphine was synthesized by the method of Adler [23], sulfonated [24] and metalated [25]. The Jaggregate solutions were prepared by dissolving TPPS4 in acidic aqueous medium (HCl was used to adjusting the
pH value at 1) in the range of concentration between 1×10 −4 to 2×10−6 M. To stabilization of the aggregates
formation, the solution was kept at room temperature (25 ċ) for about 10 days. The nanotubes deposited on the
glass surface using setting it in TPPS solution for 5 minutes. The glass dried in oven at 70
ċ for 30 minutes.
Atomic Force Microscopy confirmed the results of J-aggregates of TPPS4. UV-Vis spectroscopy used for study
of absorption changes at different acidic solutions and different time of aggregation.
3. Results and discussion
3.1. Acid aggregation in TPPSx derivatives
Fleischer [26] found that H2TPPS4 in acidic media at pH=4, forms a di-acid porphyrin , the related bands were
observed at 435, 593 and 645 nm, and at pH values less than 2, the bands appeared at 435, 491, 645 and 707 nm
as shown in Fig. 2. Pasternack [27] reported about aggregation of H2TPPS3 in different ionic strength solutions
and he studied the extent depending on the identification of the cation used to production of the ionic media. In
this present work, other TPPSx derivatives in 0.1 M HCl was studied and it founded that the aggregation is in the
order of trans-TPPS2 (fully aggregated)>> cis-TPPS2 >TPPS3 ~ TPPS4 [28]. The order of porphyrin solubility
was H2TPPS4 (very soluble) > H2TPPS3 (soluble) > cis-H2TPPS2 (slightly soluble) > trans-H2TPPS2 (very slightly
soluble) >> H2 TPPS1 (fairly insoluble). The comparison of these two series shows that the order of porphyrin
solubility was in contrast to aggregation. By filtering the aggregated TPPS4 through a 0.45 μm Millipore filter,
the porphyrin di-acid passed through and the aggregation form was left in the filter as shown in Fig.2. The
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H4TPPSx compounds could overlap in an edge-to-edge aggregation larger than 0.45 μm and cause a red shift in
the soret band of absorption spectra.
H2P porphyrin molecules protonate at the central nitrogen atoms to formation of H 3P+ or H4P2+ in the acidic
medium with pH values range of 0 to 3. The equilibrium constants for protonation reactions are:
To decrease the presence of dimmers, the total porphyry’s concentrations were near 10 -7 M. The absorption
spectra were plotted as a function of pH. The relationship between the observed absorbance A x, the total
porphyrin concentration,[P]T, and [H+] is:
Where є2 , є3 and є4 are the molar absorptivities of the free base, mono-and di-cations, respectively. Fig. 2 shows
a plot of Ax /[P]T versus pH for H2T(2,6-F)PPS .
3.2. UV-Vis and AFM studies of TPPS4 aqueous acid solutions
The measured UV–Vis spectrum of the acid solutions (Fig. 3) has maximum absorption at 433, 490 and 705 nm,
which has to be attributed to the formation of J-aggregates (490 nm).
AFM image of TPPS4 samples deposited on silica, prepared from TPPS 4 aqueous acid solutions at pH=1. The
dispersion of nanotubes on glass substrate is given in Figs. 4 and 5. The size of nanotubes from section analysis
calculated by section analysis (fig. 6), was about (20×100×200) nm.
Comparison between spectroscopic data and AFM images implies that the absorption band at 423 nm and the Jband at 490 nm both together reflect the formation of the ordered structure of TPPS tube - like aggregates [29].
UV–Vis absorption spectra of TPPS4 solution recorded in the middle of aggregation as shown in Fig.7. The
absorption decreased by the time ahead and the final spectrum was the nanotube absorption bands.
The possible reasons for differences in spectral observed between J-aggregates present in aqueous solution and
those formed in the presence of acid, could be related to the assembling and subsequent superficial localization of
size-limited aggregated structures. The necessary conditions for the co-precipitation seem to require the presence
of TPPS4 monomers as well as highly aggregated species serving as a substrate finalizing the cohesion.
4. Conclusions
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Aggregated porphyrin materials studied showed nanoporous surfaces, and their applications are promising. Due
to the high surface area, nanoporphyrins usually act high efficiency for sensing devices and using different metals
makes them suitable for sensors. The J-aggregate composition of - tube walls indicates strong electronic coupling
of multiple porphyrin subunits, which might be expected to facilitate the electron transport that is necessary to
growing the nanowire. Wide ranges of such properties have been explored, including various non-linear optical
behavior and low-dimensional electric conduction.
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Fig. 1. The structure of the TPPS4 porphyrin diacid (H6TPPS42+) in pH =1 solution.
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Fig. 2. pH profile of the protonation reactions of H 2T(2,6-F)PPS (0.1M NaNO3). Insert: the four-band spectrum
of aggregated TPPS4 after passage through a 0.45 μm millipore filter.
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3
2,5
Absorbance
2
1,5
1
0,5
0
350
400
450
500
550
600
650
700
750
800
wavelength (nm)
Fig. 3. UV–Vis absorption spectra of TPPS4 solution by changing the pH from 4 to 1.
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Fig. 4. AFM image of TPPS4 sample on silica prepared from TPPS4 aqueous acid solutions (pH=1).
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Fig. 5. AFM image (from above) of TPPS4 sample on silica.
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Fig. 6. AFM section analysis of TPPS4 samples on silica.
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3
82 min
90 min
2,5
98 min
115 min
Absorbance
2
127 min
1,5
140 min
160 min
1
180 min
195 min
0,5
210 min
14 hour
0
350
400
450
500
550
600
650
700
750
wavelength (nm)
Fig. 7. UV–Vis absorption spectra of TPPS4 solution by the different time of aggregation.