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The photochemical and electrochemical properties of chiral porphyrin dimer and self-aggregate nanorods of cobalt(II) porphyrin dimer
Inorganica Chimica Acta 363 (2010) 317–323
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The photochemical and electrochemical properties of chiral porphyrin dimer and self-aggregate nanorods of cobalt(II) porphyrin dimer Ximing Guo a,b,c,*, Bin Guo a,*, Tongshun Shi c a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China The Academy of Science and Technology, Harbin Institute of Technology, Heilongjiang 150001, People’s Republic of China c College of Chemistry of Jilin University, Chang Chun 130023, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 15 September 2009 Accepted 22 October 2009 Available online 28 October 2009 Keywords: Chiral porphyrin dimer Nanorods Luminescence Electrochemistry
a b s t r a c t In this paper, the novel chiral porphyrin dimer ligand and its cobalt(II) porphyrin dimer were synthesized by using a glutamate bridging group. The FT-IR and Raman spectra of the chiral porphyrin dimer were investigated. Furthermore, the photochemical and electrochemical properties of dimer were studied. In addition, we prepared the nanorods of the cobalt(II) porphyrin dimer using liquid–solid-solution (LSS) technologies. The shape and dimension of the spontaneous aggregates of cobalt(II) porphyrin dimer were characterized by the transmission electron microscopy (TEM). The results show the diameter and shape of the aggregates can be controlled by refining the stocked solution temperature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Chemistry as a significant bridge between the bioscience and material sciences, its many disciplines allow explanation of how biomolecules self-assemble to form functional nanostructures and provide the tools to synthesize analogous structures in nonbiological environments. The fabrication of well-defined nanoscale objects is essential for the understanding and development of potential applied materials [1–3]. In this respect, porphyrins constitute an important class of synthetic and natural compounds because they take vital role in bioscience and advanced materials. Recently, the self-aggregate porphyrins with well-defined shapes and dimensions are of great applications in magnetic properties, photo-catalytic, electronics, photonics and light-energy conversion. For example, in artificial light-harvesting systems [4–6], it is expected that the porphyrin nanorods will have unique photonic properties not obtainable by other analogous inorganic and organic nanoparticles, larger-scaled materials containing the macrocycle, or by the molecules themselves. However, the aggregate morphologies of porphyrin completely depend on the exterior conditions (pH, ionic strength, solvent, temperature and so on), which is advantageous for aggregate porphyrin to prepare the different functional materials. A challenging aspect in these systems is the
* Corresponding authors. Address: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. Tel./fax: +86 451 8640 1179. E-mail addresses: [email protected], [email protected] (X.m. Guo), [email protected] (B. Guo). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.10.015
possibility of modulating the structure of the final aggregates by tuning the properties of porphyrin monomer or exterior conditions to obtain new materials. The amphiphilic porphyrin aggregates in aqueous solution [7,8] have demonstrated the form of fibers, ribbons and tubules. In communication [9], we have also reported the changes of UV–Vis and CD spectral with the various aggregate size and formation of the chiral cobalt(II) porphyrin dimer [10–12]. But the vibrational spectrum, photochemical and electrochemical properties of the chiral porphyrin dimers have not been investigated. In this study, we qualitatively assign the FT-IR and Raman bands of the novel chiral porphyrin dimers. The photochemical and electrochemical properties of the novel chiral porphyrin dimers are investigated. In addition, the nanorods of cobalt(II) porphyrin dimer are prepared by the liquid–solid-solution (LSS) technologies. The shape and dimensions of the cobalt(II) porphyrin dimer aggregates are characterized by the Transmission electron microscopy (TEM). The results show that the diameter and shape of the aggregates were dependent on the temperature of the solution. 2. Experimental 2.1. Materials and methods 5-Hydroxyl-10,15,20-triphenylporphyrin(MHTPP) was synthesized as in literature [13]. Methylene dichloride and chloroform (being further purified by the traditional methods before using) were purchased from Tiantai Company. Pyrrole was from Aldrich (A.R grade), anhydrous ethanol, aluminum oxide (200–300 mesh)
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silica gel G were from the Chemical Reagent Company of Shanghai Instrument. Element analyses were carried out with a Perkin–Elmer 240C auto element analyzer; electronic absorption spectra were measured on Shimadzu UV-2400 spectrophotometer; FAB mass spectrum was recorded with a JEOL AX-505 spectrometer: FT-IR spectrum was measured on an impact Nicolet 5PC FT-IR spectrophotometer with KBr disks. Raman spectra were recorded with a Jobin Yvon Raman spectrophotometer equipped with an integral microscopy. Radiation of 457.9 nm was obtained from an Ar+ laser. The fluorescence measurements were performed in 1 cm quartz cell using Shimadzu RF-5301 spectrofluorimeter at excitation wavelength of 400 nm and scan range from 500 nm to 800 nm. The nanocomposites were characterized by transmission electron microscope (JEOL 2010 microscope operated at 120 kV); the cyclic voltammetry was carried out in dried DMF (0.1 mol/L TBAP as a supporting electrolyte) by CHI660A electrochemical analyzer. A three-electrode system was used: Platinum wires as working and counter electrodes; Ag/Ag+ as reference electrode (0.01 mol/L AgNO3 in acetonitrile solution). All solvent/supporting electrolyte solutions were deaerated using high-purity Ar. The reversibility of the electrochemical processes was evaluated by standard procedures. 2.2. Synthesis of porphyrin dimer ligand To a 250-mL three-necked round-bottomed flask containing thionyl chloride (15 mL, 0.21 mmol) were added glutamic acid (amidogen was protected) (14.70 mg, 0.1 mmol), then the solution was stirring for 2 h at the atmosphere bath. Followed by adding the 5-hydroxyl-10,15,20-triphenylporphyrin (631 mg, 1 mmol) dissolved in anhydrous benzene (120 mL) to the reaction solution, the mixture was stirred for 4 h in the boiled benzene, after cooling to room temperature. The mixture was washed with 100 mL 0.5 M sodium carbonate solution to remove of excess thionyl chloride and glutamic acid. Then the solution was washed three times with the double distilled water and dried with anhydrous sodium sulfate overnight. After removing of the solvent, the solid was dissolved in the chloroform and loaded on aluminum oxide column, the products were eluted by the different ratios of acetone and chloroform and collected main band. According to literature’s report [13,14] amidogen was deprotected, the crude products was purified with silica gel column and concentrated. The product was further recrystallized from chloroform using absolute methanol and dried on vacuum, the structure of porphyrin dimer was displayed in Scheme 1, Gained 88.3 mg, Yield, 12.8%, Mass [FAB], m/z: calc. 1372.4, found 1374.5 [M+1]; Elemental Anal. Calc. for C93H65N9O4: calc. C, 81.38; H, 4.77; N, 9.18. Found: C, 81.41; H, 4.76; N, 9.24%. 1H NMR (DMSO-d6, 333 k): d:-2.93 (d, 4H), 0.85–
0.83 (t, 2H), 1.06 (m, 1H), 1.17–1.14 (t, 2H), 6.50 (s, 2H, NH), 7.42–7.45 (m, 4H, m-C6H4OH), 7.53–7.59 (m, 2H, m-C6H4OH), 7.60–7.67 (2H, m-C6H4OH), 7.74–7.69 (t, 2H, m-C6H5), 7.75–7.93 (m, 18H, p, m, o-C6H5), 7.95–8.12 (d, 2H, m-C6H5), 8.14 (s, 6H, mC6H5), 8.24–8.37 (m, 2H, p-C6H5), 8.38–8.49 (d, 2H pyrrole b-H), 8.57–8.51 (d, 2H pyrrole, b-H), 8.57–8.91 (t, 12H pyrrole b-H); UV–Vis [DMF, kmax/nm (e 105)]: 420(8.5) 515(0.13) 552(0.08) 589(0.04) 649(0.05). 2.3. Synthesis of cobalt(II) porphyrin dimer A mixture of porphyrin dimer ligand (160 mg, 0.12 mmol) and hydrous cobalt chloride (0.485 g) in DMF (30 mL) was heated at 80 °C for 1.5 h with stirring under nitrogen atmosphere. After cooling to room temperature, the mixture was added to 100 mL chloroform. The solution was washed three times with the double distilled water, after removing excess cobalt chloride and DMF, the solution was dried with anhydrous sodium sulfate overnight, concentrated and loaded on aluminum oxide column chromatography with the mixture of acetone and chloroform as elute, The purple–red crude product was gave and further purified on Al2O3 column. The product was recrystallized from chloroform using absolute methanol and dried on vacuum, the structure of cobalt(II) porphyrin dimer was displayed in Scheme 1, Gained 121.3 mg, Elemental Anal. Calc. for C93H61N9O4Co2: C, 75.15; H, 4.14; N, 8.48. Found: C, 75.41; H, 4.72; N, 8.24%. 1H NMR (300 MHz, DMSO), d: 12.48 (br, 16H, pyrrole-H), 10.3 (s, 4H, m-C6H4O), 8.57 (m, 18H, m, p-C6H5), 8.19 (s, 4H, o-C6H4O), 7.95–7.88 (t, 12H, o-C6H5),2.89 (s, 2H, CH2), 2.73 (s, 2H, CH2) 1.22–1.14 (d, 2H NH2), 0.85 (br, s, 1H, CH); UV–Vis [DMF, kmax/nm (e 105)]: 409 (6.4), 536 (0.04). 2.4. Preparation of nanorods The procedure to prepare nanorods of cobalt(II) porphyrin dimer was initiated by dissolving 14.86 mg porphyrin dimer in 10 mL DMF, which was then equally divided into five. Each was vigorously stirred under different designated temperatures (a273 K, b-298 K, c-308 K d-318 K and e-328 K). As followed, the sodium chloride aqueous solution with 0.01 mol/L was added slowly to each of the five, and then the system was treated for 12 h. After adding 6 mL CHCl3 to each of the five stocked solution, which was divided into two phases, respectively: a and b retain purple–red in the below solution and colorless in the above solution; c, d and e also retain purple–red in the below solution and light red in the above solution. For c, d and e, the suspension appeared in the above solution, which suggested that the metalloporphyrin was dispersed into the mixture of DMF and water. This was explained by chloroform volatilization or the solubility of aggregate decline in chloroform with the solution temperature increase. A spontaneous phase-separation process then occurred because of the aggregate metalloporphyrin and the incompatibility between the hydrophobic surfaces and their hydrophilic surroundings, such the metalloporphyrin nanocrystals can be easily collected in the chloroform at the bottom of container. 3. Result and discussion 3.1. The spectral properties of chiral porphyrin dimer
Scheme 1. The structure of porphyrin dimer.
3.1.1. UV–Vis spectra Typical porphyrin absorption bands include an intense near-UV band (Soret) and two visible bands (Q): Q (0,0) represents excitation from the lowest vibrational level of the ground singlet electronic state to the lowest vibrational level of the first excited singlet electronic state, and Q (1,0) bands have one quantum of
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vibration in the first excited singlet electronic state. In metalloporphyrins with square symmetry, these bands are due to degenerate excited state with x and y polarization. In the free base porphyrin the Qx (0,0) and Qy (0,0) bands are no longer degenerate due to the proton axis. The absorption bands of free base porphyrin dimer ligands appear at 420,515, 552, 589 and 649 nm; the absorption bands of cobalt(II) porphyrin dimer appear at 409 nm and 536 nm (Fig. 1). Fig. 1 shows that the Soret-band and Q bands of the dimer red-shifted compared with those of MHTPP, the Soretband red-shifted from 418 to 420 nm. The four Q bands of MHTPP appeared at 515, 550, 590 and 650 nm; the four Q absorbance bands of the dimer ligand appeared at 515, 552, 589, and 649 nm, respectively (Fig. 1), these changes can be attributed to the interaction between amidogen of amino acid and N–H porphyrin ring. The interaction causes either a linear action with respect to the direction of Qx-, Bx-transition dipole moments or, in the opposite case, to the direction of the Qy-, By-transition dipole moments. Both the types of interaction within the dimer ligand enable the generation of parallel and perpendicular interaction simultaneously because the transition dipole moments of Qx-, Bx- and Qy-, By-type transitions have parallel and perpendicular orientations. The transition dipole moments of the Qx- and Bx-bands lay along the axis connecting the two inner hydrogen atoms while the Qy- and By-transitions belong to the transition dipole moments, which are perpendicular to the former axis [13].
3.1.2. FT-IR spectra FT-IR spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer are shown in Fig. 2. The FT-IR bands at 3500 cm 1 and 3429 cm 1 of porphyrin dimer ligand are assigned to N–H stretching vibration of radical amino acid and the band of cobalt(II) porphyrin dimer at 3440 cm 1 was observed. The C–H stretching vibration of porphyrin dimers appears at 2850 cm 1 and 2920 cm 1. The bands at 3313, 964 cm 1 are attributed to the N–H stretching and bending vibration of the porphyrin ligand core. In cobalt(II) porphyrin dimer, these bands disappear and a new band appears at about 1002 cm 1, which is the characteristic of metalloporphyrin. The C@O stretching vibration band of the porphyrin dimer ligand is weak due to the intramolecular hydrogenbonding, however, The vibrational bands of cobalt(II) porphyrin dimer appears at 1741 and 1704 cm 1.The band of the porphyrin dimer appears 1654 cm 1 which was ascribed to the N–H bending
Fig. 1. UV–Vis spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
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Fig. 2. FT-IR spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
vibration of the amino acid. The bands in the range of 1519– 1535 cm 1 are assigned to N–H in-plane bending (amidogen II) [13]. The bands at 1265 and 1216 cm 1 was ascribed to the C–O stretching vibration in the porphyrin dimer ligand, however, the bands of cobalt porphyrin complexes was displayed 1259 and 1205 cm 1 which can be attributed to the enhancement of both the dihedral angel of the two porphyrin chromophores planes by metalation and reduction of the distance of porphyrin ring intradimer. The bands at 1471 cm 1, 1170 cm 1, 1072 cm 1,798 cm 1 were ascribed to the skeleton vibration of porphyrin dimer ligand, the skeleton vibration model of cobalt porphyrin complexes appeared at the 1349 cm 1, 1170 cm 1, 1072 cm 1, 796 cm 1 [11]. The bands at 717–721 cm 1 are assigned to the methylene inplane rocking vibration of straight alkyl chain. 3.1.3. Raman spectra The Raman spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer using 457.9 nm excitation are displayed in Fig. 3, those Raman bands exhibited a sizable frequency difference between porphyrin dimer ligand and cobalt(II) porphyrin dimer. In low-frequency region, the Raman spectra of porphyrin dimer ligand are significantly different from those of porphyrin dimer ligand. For porphyrin dimer ligand, the only weak Raman
Fig. 3. Raman spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
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band in low-frequency region was observed at 330 cm 1. This band was assigned to the m8 mode. The m8 mode consists of an in-plane translational motion of the pyrrole rings, which can be described as a uniform breathing of the whole porphine ring accompanied by an in-plane CaCmCa deformation in the pyrrole rings [15]. While a very strong band and two weak bands were observed for cobalt(II) porphyrin dimer (at 396 cm 1vs and 253 cm 1w, 334 cm 1w, respectively). The differences of Raman spectra in low-frequency region suggest that the structures or vibrational dynamics especially around the CaCmCa bond-angles are altered by the metal ions. In the 900–1650 cm 1 high-frequency region of the Raman spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer, bands generally arise from the totally symmetric vibrational modes such as CaCm, CaCb, CbCb, pyrrole quarter-ring, and pyrrole half-ring stretching. The wavenumber positions of Raman bands in the high-frequency region are sensitive to the core size, axial ligation and electron density of the central metal ion [16–18]. In this region, the band at 1550 cm 1 of porphyrin dimer ligand is assigned to CbCb stretch m2 mode, which is up-shifted to 1565 cm 1 in cobalt(II) porphyrin dimer. The m2 mode is observed with enhanced intensity in cobalt(II) porphyrin dimer. In fact, it is one of the most intense bands in the high-frequency region. The band at 1498 cm 1 of porphyrin dimer ligand is assigned to the vibration of phenyl ring; the band at about 1455 and 1453 cm 1 of porphyrin dimer ligand is assigned to CaCm stretch m3 mode and these bands do not appear in cobalt(II) porphyrin dimer. The m2 mode arises mainly from the symmetric CaCm stretching, which is mixed with the CbCb stretching of the same symmetry. The m3 mode has a noticeable component of the symmetric stretching motion of CaCm bonds. On the other hand, the m2 mode has a major contribution from the in-phase CbCb stretching along with a smaller contribution from the CaCm stretching. Thus, mixing of these stretching motions causes nearly complete cancellation of the intensity for the m3 mode, while the intensity of the m2 mode is increased in metallo-tetraphenylporphines under D4h point group symmetry [19]. Raman bands in 1300–1450 cm 1 are due to the out-of-phase coupled CaCb/CaN stretching modes. The 1358 and 1325 cm 1 bands of porphyrin ligand are assigned to the m4 and m12 modes, respectively. The m4 mode of cobalt(II) porphyrin dimer appears at 1369 cm 1, while its m12 band shifts to 1311 cm 1. The 1239 cm 1 band of porphyrin dimer ligand and the 1237 cm 1 band of cobalt(II) porphyrin dimer are attributed to Cm-ph stretching m1 mode. The band at 1080 cm 1 of porphyrin ligand is assigned to the vibration of pyrrole Cb–H stretching m9 mode, which does not shift in cobalt(II) porphyrin dimer. The band at 999 cm 1 of porphyrin ligand is assigned to the vibration of pyrrole breathing and phenyl stretching m15 mode, which is up-shifted to 1009 cm 1 in cobalt(II) porphyrin dimer. The band at about 960 cm 1 of porphyrin ligand is assigned to pyrrole breathing m6 mode, but it disappeared in cobalt(II) porphyrin dimer because the hydrogen atom in the N–H bonding is replaced by metal ion. 3.1.4. Luminescence The optical properties of porphyrin can be related to its molecular skeleton. The nucleus of porphyrin consists of four pyrrole rings linked by methine-bridge: the corresponding macrocycle is fully conjugated plane, containing an 18-electron aromatic p-system. The extended aromatic system is responsible for the high molar absorbance. Porphyrins hold the S2 (B band) emission band and the S1 (Q band) emission band. The S2 fluorescence band is attributed to the transition from the second excited singlet state S2 to the ground state S0, which is much weaker than that of the S1 ? S0 transition of the Q band emission. Its quantum yield is so low that the fluorescence becomes unobservable in this work. Fluorescence of S1 consists of two bands Q(0 1) and Q(0 2). When the hydrogen atoms in the center of porphyrin ring are replaced
Fig. 4. The fluorescence spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
by metal ion, the fluorescence quenching is a well-known phenomenon [13]. Fig. 4 shows the fluorescence emission spectra of the porphyrin dimer ligand and cobalt complex in DMF. The absorbance of two compounds is almost coincident at vicinity of excitation wavelength, so we can directly compare their fluorescence intensities in experimental conditions. It can be seen from Fig. 4 that Q(0 1) and Q(0 2) emission bands of them are in the regions of 654 and 719 nm, respectively. However, the fluorescent intensity of the cobalt(II) porphyrin dimer (654 nm) is only 5.3% of that of porphyrin dimer ligand, which indicates that the metal ions inner metalloporphyrin efficiently quenches the fluorescence intensities of porphyrin moiety. This result has been reasonably explained by the classic Marcus theory, for such a derivative, the photoinduced intramolecular electron transfer from the singlet excited state inner free base porphyrin to one low-spin cobalt(II) [14]. The electron transfer is the primary de-excitation path for the excited state and the electron transfer rates depend on the angle and distance of donor–acceptor in the classic Marcus theory. For nonadiabatic electron transfer, fluorescence can become the dominant decay pathway for the excited free base porphyrin. The S1 ? S0 (Q band) quantum yield depends on the relative rates of the radiative process S1 ? S0 and two radiationless processes S1 ? S0 and S1 ? Tn. Thus the excited state S1 is primarily deactivated by radiationless decay. That the spin forbidden process S1 ? Tn is predominant for radiationless deactivation of the excited state S1. The fluorescence intensity of cobalt complex largely declined which was attributed to enhancing the S1 ? Tn radiationless process due to cobalt(II) [20]. 3.1.5. Electrochemistry The cyclic voltammetry of porphyrin dimer ligand and cobalt(II) porphyrin dimer is performed to evaluate their redox potentials. In the tetraphenylporphyrin complex, the cobalt has a stable two valence oxidation state in air. So two metal redox reactions can take place, specifically an oxidation: Co(II) ? Co(III) and a reduction: Co(II) ? Co(I). In addition, two ring reductions and one ring oxidation is observed (Fig. 5). The first electroreduction occur 0.448 V corresponds to the Co(III)/Co(II) electrode reaction. The half wave potentials at E1/2 = 0.22 V was negative-shift relative to that of TPPCo. The change of E1/2 for Co(III) ? Co(II) of cobalt complex was attributed to the Co(III) axial interaction with the amido of amino acid. A second reduction occurs at 1.29 V, which was assigned to Co(II)/Co(I) [21–26]. The oxidation of Co(II)/Co(I) is faint which can be attributed to forming intramolecular or intermolecular polymer due to the Co(I) bearing strong nucleophility [27]. For
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Fig. 5. Cyclic voltammograms of porphyrin dimer ligand and cobalt(II) porphyrin dimer in DMF + 0.1 mol/L TBAP.
the ring redox of porphyin dimer ligand, the first ring reduction of the ligand occurs at 1.58 V and the second reduction has not been observed; in the cobalt complex, the first and second reduction of porphyrin ring occurs at 1.56 V and 1.93 V, respectively. This was corresponds to the formation of p anion radical and the second reduction leads to the formation of porphyrin dianion. However, the oxidation peak is broad which can be attributed to intramolecular charge-transfer between porphyrin and amino acid or porphyrin and porphyrin [28]. The reduction peak twice higher than that of oxidation peak which can be ascribed to two-electron reduction and one-electron oxidation in cobalt porphyrin dimer. An irreversible ring oxidation peak occurs at 0.56 V and 0.60 V corresponds to the electronoxydation of ligand and cobalt complex, respectively. 3.2. Spectral properties of self-assembly nanorods of cobalt(II) porphyrin dimer under different conditions General speaking, The primary reaction in the preparation of metalloporphyrin nanocrystals through LSS involved the p–p stacking, hydrophilic and hydrophobic grounds interaction at the interfaces of metalloporphyrin (solid), chloroform–liquid phase (liquid) and water–DMF (N,N-dimethylformamide) solutions (solu-
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tion) and other interaction (metal–metal, intermolecular electrostatic et al). For cobalt(II) porphyrin dimer, the aggregate can be adopted a head-to-tail (J-aggregate), face-to-face (H-aggregate) or nonspecific aggregate arrangement in different experimental conditions. When the solution was placed at lower temperature (sample a, 273 K and sample b, 298 K), The H-aggregate or nonspecific aggregate is dominant in aggregate morphologies of cobalt(II) porphyrin dimer. However, as temperature increase, the J-aggregate take vital role in the aggregate morphologies of cobalt(II) porphyrin dimer because the J-aggregate is stable in thermodynamics while H-aggregate is stable in dynamics. The aggregate morphologies transform from H- to J-aggregate with the stocked solution temperature increase. Thus, the aggregate species grow single which leads to forming homogeneous and ordered chiral nanorods of cobalt(II) porphyrin dimer [29]. Fig. 6 shows the transmission electron microscope (TEM) images of self-aggregate porphyrin dimer at different temperature. From Fig. 6 we can see that the aggregate shape of porphyrin dimer changes from disordered stacking to ordered aggregates; the aggregate configuration of dimer transform from fractal, big brick up to rod-like particles; the diameter of aggregates changes from micron to nanometer, and the diameter size distribution of the nanorods changes from inhomogeneity to homogeneity (Fig. 5e) with solution temperature increase.
3.2.1. The UV–Vis properties of self-assembly nanorods The UV–Vis of cobalt(II) porphyrin dimer demonstrated great differences with the change of the stocked solution temperature. The exciton theory developed by Kasha predicts the occurrence of hypsochromic or bathochromic shifts for the relevant absorption bands, in the case of H-aggregate or J-aggregate interactions respectively. The UV–Vis of the cobalt(II) porphyrin dimer with the changes of the temperature was displayed in Fig. 7. We find that the shoulder peak appears at 435 nm, the variational tendency to absorbance at 415 nm and 435 nm of dimer have been inserted into Fig. 7 with the changes of the temperature, the absorbance intensities of 415 nm decline and that of 435 nm rise with the stocked solution temperatures increase, which can be attributed to forming aggregates of cobalt(II) porphyrin dimer. However, the absorbance values of 415 nm and 435 nm have an abrupt change when the stocked solution is between 338 K and 343 K, which
Fig. 6. TEM images of porphyrin nanorods at the different temperature, (a) 273 K, (b) 298 K, (c) 308 K, (d) 318 K and (e) 328 K.
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Fig. 7. The changes of UV–Vis spectra of cobalt(II) porphyrin dimer in water–DMF with temperature varying from 303 K to 383 K, the interval of temperature is 5°. The variational tendency to absorbance at 415 nm and 435 nm of dimer have been inserted into figure with the changes of the temperature.
can be isosbestic temperature of the transformation from H- to Jaggregate. In communication [9], the UV–Vis of the cobalt(II) porphyrin dimer deposited in water–DMF for 12 h under different temperatures has been analyzed as Refs. [30–33]. We speculated this resulted from the following reasons: face–face assembly of cobalt(II) porphyrin dimer results in the splitting of the excited state because of the exciton-coupling between the two porphyrin molecules. When the two porphyrins are stacked with little or no offset (i.e., an ‘‘H” aggregate), electronic transition is allowed to the upper state but not to the lower state, which results in the observed shift of the absorption spectrum to a shorter wavelength. When two dimer molecules are assembled adjacent to one another in the minor groove, a secondary coupling arises because of the end–end interaction between dimer molecules. This causes an additional splitting of the excited state, which is normally manifested in the UV–Vis spectrum as a broadening or splitting of the absorption band, finally, the absorption of the cobalt(II) porphyrin dimer was red-shifted which was that the porphyrin dimer transforms from H-aggregate to J-aggregate as the stocked solution temperature increases. The results can help us to well-understand the electronic coupling of the chromophors [34,35]. 3.2.2. Circular dichroism (CD) specropolarimetry of self-aggregate nanorods The CD spectra of the cobalt(II) porphyrin dimer have been measured and discussed under different temperature. The results show a weak single CD band under lower temperature (273 K and 298 K), a small bisignate curve under medium temperature (308 K) and a stronger bisignate curve under higher temperature (318 K and 328 K) [9]. The spectral shifts and splitting observed in the CD spectra of the aggregate cobalt(II) porphyrin dimer, which was rationalized by a simple exciton-coupling model, based on work by Kasha et al. [36] and Davydov [37]. When the solution was under lower or medium temperature, the H-aggregate or nonaggregate of cobalt(II) porphyrin dimer is dominant in the stocked solution. There was weak exciton-coupling or no exciton-coupling in H-aggregate or non-aggregate of cobalt(II) porphyrin; (Fig. 8) while the solution was under higher temperature, the J-aggregate of cobalt(II) porphyrin dimer is dominant in the stocked solution, the strong and splitting CD signal observed in the CD spectra [38,39]. It is known that such an interaction depends on inter chromophoric distance and twist, as well as the conformational rigidity around the porphyrin L-glutamic acid linkage [40,41]. The intramolecular exciton-coupling was reinforced due to both the enhancement of the dihedral angel and reduction of the distance of
Fig. 8. The comparison of face–face and end–end couplings of H-and J-aggregated chromophores.
Fig. 9. Exciton-coupling model explain how spectral shifts relate to the aggregate structure.
porphyrin ring intradimer when the aggregate morphologies of cobalt(II) porphyrin dimer transform from H-aggregate to J-aggregate [42–45]. As shown in Fig. 9, the transformation from H-aggregate to J-aggregate decrease the face–face coupling, while enhance the end–end coupling. The much larger intramolecular coupling is reflective of the more extensive orbital overlap between two porphyrin rings in dimer compared with the end-to-end interactions between adjacent dimers. So the shape and intensity of the CD spectrum should be highly dependent on the aggregate formation and the conformation of the chiral porphyrin dimer.
4. Conclusions In this study, we successfully synthesized novel chiral porphyrin dimer ligand and its cobalt(II) porphyrin dimer by using a glutamate bridging group. We assigned the FT-IR and Raman bands of the chiral porphyrin dimers. Furthermore, the photochemical and electrochemical properties of dimers were detailedly studied. In addition, the nanorods of cobalt(II) porphyrin dimer were prepared by adjusting the stocked solution temperature, according to our results, the different sized and functional nanoparticles of porphyrin can be attained by refining the exterior conditions. The UV–Vis spectra were always acting as the indicator of porphyrin aggregates. However, for the chiral porphyrin, the changes of CD spectra can help us to well-understand inter- and intramolecular electronic coupling and the aggregate formation of the chiral porphyrin.
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Acknowledgements The project was supported by natural scientific research innovation Foundation in Harbin Institute of Technology (HIT NSRIF.2008.09). References [1] G.M. Whitesides, J.P. Mathias, C.T. Seto, Science 254 (1991) 1312. [2] N.B. Bowden, M. Weck, I.S. Choi, G.M. Whitesides, Acc. Chem. Res. 34 (2001) 231. [3] Special section on ‘‘Supramolecular Chemistry and Self-Assembly” Science 295 (2002) 2395–2399. [4] M.E. Michel-Beyerle, J. Deisenhofer, H. Michel, M. Bixon, L. Jorter, Biochem. Biophys. Acta 932 (1988) 52. [5] A. Lamrabte, J. Janot, M.E. Bienvenue, M. Momenteau, P. Seta, Photochem. Photobiol. 54 (1991) 123. [6] P.G. Schouten, J.M. Warman, M.P. Haas, M.A. Fox, H.L. Pan, Nature 353 (1991) 736. [7] W.B.H. West, in: T.H. James (Ed.), The Theory of Photographic Processes, third ed., The McMillan Company, New York, 1966. [8] T. Kobayashi, J-Aggregates, World Scientific, Singapore, 1996. [9] X.M. Guo, T.S. Shi, Inorg. Chem. 46 (2007) 4766. [10] X.M. Guo, Y.H. Shi, Y.H. Zhang, L.X. Yu, T.S. Shi, Chin. J. Organ. Chem. 26 (2006) 247. [11] X.M. Guo, L.J. Su, L.X. Yu, T.S. Shi, Chem. J. Chin. Univer. 27 (2006) 410. [12] X.M. Guo, T.S. Shi, Acta Chim. Sin. 64 (2006) 1218. [13] X.M. Guo, T.S. Shi, J. Mol. Struct. 789 (2006) 8. [14] Y.Q. Weng, Y.L. Teng, F. Yue, Y.R. Zhong, B.H. Ye, Inorg. Chem. Commun. 10 (2007) 443. [15] P.M. Kozlowski, A.A. Jarzecki, P. Pulay, X.Y. Li, M.Z. Zgierski, J. Phys. Chem. 100 (1996) 13885. [16] G.S.S. Saini, Spectrochim. Acta Part A 64 (2006) 981. [17] X.Y. Li, R.S. Czernuszewicz, J.R. Kincaid, Y.O. Su, T.G. Spiro, J. Phys. Chem. 94 (1990) 31. [18] F. Paulat, V.K.K. Praneeth, C. Nather, N. Lehnert, Inorg. Chem. 45 (2006) 2835. [19] X.Y. Li, M.Z. Zgierski, J. Phys. Chem. 95 (1991) 4268.
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
[37] [38] [39] [40] [41]
[42] [43] [44] [45]
323
M.H. Qi, G.F. Liu, J. Phys. Chem. B 107 (2003) 7640. F.A. Walker, D. Beroiz, K.M. Kadish, J. Am. Chem. Soc. 98 (1976) 3484. L.A. Truxillo, D.G. Davis, Anal. Chem. 47 (1975) 2260. A. Wolberg, Isr. J. Chem. 12 (1974) 1031. A. Wolberg, J. Manassen, J. Am. Chem. Soc. 92 (1970) 2982. R.H. Felton, M. Linschitz, J. Am. Chem. Soc. 88 (1966) 1113. X.Q. Lin, B. Boisselier-Cocolios, K.M. Kadish, Inorg. Chem. 25 (1986) 3242. G. Gritzner, Pure Appl. Chem. 62 (1990) 1839. H.Y. Liu, J.W. Huang, X. Tian, X.D. Jiao, G.T. Luo, L.N. Ji, Chem. Commun. 16 (1997) 1575. W. Xu, H. Guo, D.L. Akins, J. Phys. Chem. B 105 (2001) 1543. N.C. Maiti, N. Periasamy, J. Phys. Chem. B 102 (1998) 1528. C.A. Hunter, J.K.M. Sanders, J. Am. Chem. Soc. 112 (1990) 5525. K. Kano, H. Minamizono, T. Kitae, S. Negi, J. Phys. Chem. A 101 (1997) 6118. C.M. Drain, F. Nifiatis, A. Vasenko, J.D. Batteas, Angew. Chem., Int. Ed. 37 (1998) 2344. C.M. Drain, J.M. Lehn, J. Chem. Soc., Chem. Commun. (1994) 2313. M. Kasha, Physical Processes in Radiation Biology, Academic Press, New York, 1964. M. Kasha, H. Rawls, R.M. Ashraf El-Bayoumi, The exciton model in molecular spectroscopy, in: Molecular Spectroscopy Proceedings of the VIII European Congress of Molecular Spectroscopy, Butterworths, London, 1965. A.S. Davydov, Theory of Molecular Excitons, Plenum Press, New York, 1971. T.N. Milic, N. Chi, D.G. Yablon, G.W. Flynn, J.D. Batteas, C.M. Drain, J.D. Batteas, Angew. Chem., Int. Ed. 42 (2002) 2117. M. Balaz, A.E. Holmes, M. Benedetti, P.C. Rodriguez, N. Berova, K. Nakanishi, G. Proni, J. Am. Chem. Soc. 127 (2005) 4172. F.D. Lewis, Y. Wu, L. Zhang, X. Zuo, R.T. Hayes, M.R. Wasielewski, J. Am. Chem. Soc. 126 (2004) 8206. N. Berova, K. Nakanishi, Exciton chirality method: principles and applications, in: N. Berova, K. Nakanishi, R.W. Woody (Eds.), Circular Dichroism, Principles and Applications, Wiley-VCH, New York, 2000. T. Arai, K. Araki, N. Maruo, Y. Sumid, C. Korosue, K. Fukuma, T. Katob, N. Nishino, New J. Chem. 28 (2004) 1151. C.A. Hunter, P. Leighton, J.K.M. Sanders, J. Chem. Soc., Perkin Trans. 1 (1989) 547. M. Kasha, H.R. Rawls, A. Shraf El-Bayoumi, Pure Appl. Chem. 11 (1965) 371. E. Peeters, M.P.T. Christiaans, R.A.J. Janssen, H.F.M. Schoo, H.P.J.M. Dekkers, E.W. Meijer, J. Am. Chem. Soc. 119 (1997) 9909.
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
The photochemical and electrochemical properties of chiral porphyrin dimer and self-aggregate nanorods of cobalt(II) porphyrin dimer Ximing Guo a,b,c,*, Bin Guo a,*, Tongshun Shi c a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China The Academy of Science and Technology, Harbin Institute of Technology, Heilongjiang 150001, People’s Republic of China c College of Chemistry of Jilin University, Chang Chun 130023, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 15 September 2009 Accepted 22 October 2009 Available online 28 October 2009 Keywords: Chiral porphyrin dimer Nanorods Luminescence Electrochemistry
a b s t r a c t In this paper, the novel chiral porphyrin dimer ligand and its cobalt(II) porphyrin dimer were synthesized by using a glutamate bridging group. The FT-IR and Raman spectra of the chiral porphyrin dimer were investigated. Furthermore, the photochemical and electrochemical properties of dimer were studied. In addition, we prepared the nanorods of the cobalt(II) porphyrin dimer using liquid–solid-solution (LSS) technologies. The shape and dimension of the spontaneous aggregates of cobalt(II) porphyrin dimer were characterized by the transmission electron microscopy (TEM). The results show the diameter and shape of the aggregates can be controlled by refining the stocked solution temperature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Chemistry as a significant bridge between the bioscience and material sciences, its many disciplines allow explanation of how biomolecules self-assemble to form functional nanostructures and provide the tools to synthesize analogous structures in nonbiological environments. The fabrication of well-defined nanoscale objects is essential for the understanding and development of potential applied materials [1–3]. In this respect, porphyrins constitute an important class of synthetic and natural compounds because they take vital role in bioscience and advanced materials. Recently, the self-aggregate porphyrins with well-defined shapes and dimensions are of great applications in magnetic properties, photo-catalytic, electronics, photonics and light-energy conversion. For example, in artificial light-harvesting systems [4–6], it is expected that the porphyrin nanorods will have unique photonic properties not obtainable by other analogous inorganic and organic nanoparticles, larger-scaled materials containing the macrocycle, or by the molecules themselves. However, the aggregate morphologies of porphyrin completely depend on the exterior conditions (pH, ionic strength, solvent, temperature and so on), which is advantageous for aggregate porphyrin to prepare the different functional materials. A challenging aspect in these systems is the
* Corresponding authors. Address: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. Tel./fax: +86 451 8640 1179. E-mail addresses: [email protected], [email protected] (X.m. Guo), [email protected] (B. Guo). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.10.015
possibility of modulating the structure of the final aggregates by tuning the properties of porphyrin monomer or exterior conditions to obtain new materials. The amphiphilic porphyrin aggregates in aqueous solution [7,8] have demonstrated the form of fibers, ribbons and tubules. In communication [9], we have also reported the changes of UV–Vis and CD spectral with the various aggregate size and formation of the chiral cobalt(II) porphyrin dimer [10–12]. But the vibrational spectrum, photochemical and electrochemical properties of the chiral porphyrin dimers have not been investigated. In this study, we qualitatively assign the FT-IR and Raman bands of the novel chiral porphyrin dimers. The photochemical and electrochemical properties of the novel chiral porphyrin dimers are investigated. In addition, the nanorods of cobalt(II) porphyrin dimer are prepared by the liquid–solid-solution (LSS) technologies. The shape and dimensions of the cobalt(II) porphyrin dimer aggregates are characterized by the Transmission electron microscopy (TEM). The results show that the diameter and shape of the aggregates were dependent on the temperature of the solution. 2. Experimental 2.1. Materials and methods 5-Hydroxyl-10,15,20-triphenylporphyrin(MHTPP) was synthesized as in literature [13]. Methylene dichloride and chloroform (being further purified by the traditional methods before using) were purchased from Tiantai Company. Pyrrole was from Aldrich (A.R grade), anhydrous ethanol, aluminum oxide (200–300 mesh)
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silica gel G were from the Chemical Reagent Company of Shanghai Instrument. Element analyses were carried out with a Perkin–Elmer 240C auto element analyzer; electronic absorption spectra were measured on Shimadzu UV-2400 spectrophotometer; FAB mass spectrum was recorded with a JEOL AX-505 spectrometer: FT-IR spectrum was measured on an impact Nicolet 5PC FT-IR spectrophotometer with KBr disks. Raman spectra were recorded with a Jobin Yvon Raman spectrophotometer equipped with an integral microscopy. Radiation of 457.9 nm was obtained from an Ar+ laser. The fluorescence measurements were performed in 1 cm quartz cell using Shimadzu RF-5301 spectrofluorimeter at excitation wavelength of 400 nm and scan range from 500 nm to 800 nm. The nanocomposites were characterized by transmission electron microscope (JEOL 2010 microscope operated at 120 kV); the cyclic voltammetry was carried out in dried DMF (0.1 mol/L TBAP as a supporting electrolyte) by CHI660A electrochemical analyzer. A three-electrode system was used: Platinum wires as working and counter electrodes; Ag/Ag+ as reference electrode (0.01 mol/L AgNO3 in acetonitrile solution). All solvent/supporting electrolyte solutions were deaerated using high-purity Ar. The reversibility of the electrochemical processes was evaluated by standard procedures. 2.2. Synthesis of porphyrin dimer ligand To a 250-mL three-necked round-bottomed flask containing thionyl chloride (15 mL, 0.21 mmol) were added glutamic acid (amidogen was protected) (14.70 mg, 0.1 mmol), then the solution was stirring for 2 h at the atmosphere bath. Followed by adding the 5-hydroxyl-10,15,20-triphenylporphyrin (631 mg, 1 mmol) dissolved in anhydrous benzene (120 mL) to the reaction solution, the mixture was stirred for 4 h in the boiled benzene, after cooling to room temperature. The mixture was washed with 100 mL 0.5 M sodium carbonate solution to remove of excess thionyl chloride and glutamic acid. Then the solution was washed three times with the double distilled water and dried with anhydrous sodium sulfate overnight. After removing of the solvent, the solid was dissolved in the chloroform and loaded on aluminum oxide column, the products were eluted by the different ratios of acetone and chloroform and collected main band. According to literature’s report [13,14] amidogen was deprotected, the crude products was purified with silica gel column and concentrated. The product was further recrystallized from chloroform using absolute methanol and dried on vacuum, the structure of porphyrin dimer was displayed in Scheme 1, Gained 88.3 mg, Yield, 12.8%, Mass [FAB], m/z: calc. 1372.4, found 1374.5 [M+1]; Elemental Anal. Calc. for C93H65N9O4: calc. C, 81.38; H, 4.77; N, 9.18. Found: C, 81.41; H, 4.76; N, 9.24%. 1H NMR (DMSO-d6, 333 k): d:-2.93 (d, 4H), 0.85–
0.83 (t, 2H), 1.06 (m, 1H), 1.17–1.14 (t, 2H), 6.50 (s, 2H, NH), 7.42–7.45 (m, 4H, m-C6H4OH), 7.53–7.59 (m, 2H, m-C6H4OH), 7.60–7.67 (2H, m-C6H4OH), 7.74–7.69 (t, 2H, m-C6H5), 7.75–7.93 (m, 18H, p, m, o-C6H5), 7.95–8.12 (d, 2H, m-C6H5), 8.14 (s, 6H, mC6H5), 8.24–8.37 (m, 2H, p-C6H5), 8.38–8.49 (d, 2H pyrrole b-H), 8.57–8.51 (d, 2H pyrrole, b-H), 8.57–8.91 (t, 12H pyrrole b-H); UV–Vis [DMF, kmax/nm (e 105)]: 420(8.5) 515(0.13) 552(0.08) 589(0.04) 649(0.05). 2.3. Synthesis of cobalt(II) porphyrin dimer A mixture of porphyrin dimer ligand (160 mg, 0.12 mmol) and hydrous cobalt chloride (0.485 g) in DMF (30 mL) was heated at 80 °C for 1.5 h with stirring under nitrogen atmosphere. After cooling to room temperature, the mixture was added to 100 mL chloroform. The solution was washed three times with the double distilled water, after removing excess cobalt chloride and DMF, the solution was dried with anhydrous sodium sulfate overnight, concentrated and loaded on aluminum oxide column chromatography with the mixture of acetone and chloroform as elute, The purple–red crude product was gave and further purified on Al2O3 column. The product was recrystallized from chloroform using absolute methanol and dried on vacuum, the structure of cobalt(II) porphyrin dimer was displayed in Scheme 1, Gained 121.3 mg, Elemental Anal. Calc. for C93H61N9O4Co2: C, 75.15; H, 4.14; N, 8.48. Found: C, 75.41; H, 4.72; N, 8.24%. 1H NMR (300 MHz, DMSO), d: 12.48 (br, 16H, pyrrole-H), 10.3 (s, 4H, m-C6H4O), 8.57 (m, 18H, m, p-C6H5), 8.19 (s, 4H, o-C6H4O), 7.95–7.88 (t, 12H, o-C6H5),2.89 (s, 2H, CH2), 2.73 (s, 2H, CH2) 1.22–1.14 (d, 2H NH2), 0.85 (br, s, 1H, CH); UV–Vis [DMF, kmax/nm (e 105)]: 409 (6.4), 536 (0.04). 2.4. Preparation of nanorods The procedure to prepare nanorods of cobalt(II) porphyrin dimer was initiated by dissolving 14.86 mg porphyrin dimer in 10 mL DMF, which was then equally divided into five. Each was vigorously stirred under different designated temperatures (a273 K, b-298 K, c-308 K d-318 K and e-328 K). As followed, the sodium chloride aqueous solution with 0.01 mol/L was added slowly to each of the five, and then the system was treated for 12 h. After adding 6 mL CHCl3 to each of the five stocked solution, which was divided into two phases, respectively: a and b retain purple–red in the below solution and colorless in the above solution; c, d and e also retain purple–red in the below solution and light red in the above solution. For c, d and e, the suspension appeared in the above solution, which suggested that the metalloporphyrin was dispersed into the mixture of DMF and water. This was explained by chloroform volatilization or the solubility of aggregate decline in chloroform with the solution temperature increase. A spontaneous phase-separation process then occurred because of the aggregate metalloporphyrin and the incompatibility between the hydrophobic surfaces and their hydrophilic surroundings, such the metalloporphyrin nanocrystals can be easily collected in the chloroform at the bottom of container. 3. Result and discussion 3.1. The spectral properties of chiral porphyrin dimer
Scheme 1. The structure of porphyrin dimer.
3.1.1. UV–Vis spectra Typical porphyrin absorption bands include an intense near-UV band (Soret) and two visible bands (Q): Q (0,0) represents excitation from the lowest vibrational level of the ground singlet electronic state to the lowest vibrational level of the first excited singlet electronic state, and Q (1,0) bands have one quantum of
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vibration in the first excited singlet electronic state. In metalloporphyrins with square symmetry, these bands are due to degenerate excited state with x and y polarization. In the free base porphyrin the Qx (0,0) and Qy (0,0) bands are no longer degenerate due to the proton axis. The absorption bands of free base porphyrin dimer ligands appear at 420,515, 552, 589 and 649 nm; the absorption bands of cobalt(II) porphyrin dimer appear at 409 nm and 536 nm (Fig. 1). Fig. 1 shows that the Soret-band and Q bands of the dimer red-shifted compared with those of MHTPP, the Soretband red-shifted from 418 to 420 nm. The four Q bands of MHTPP appeared at 515, 550, 590 and 650 nm; the four Q absorbance bands of the dimer ligand appeared at 515, 552, 589, and 649 nm, respectively (Fig. 1), these changes can be attributed to the interaction between amidogen of amino acid and N–H porphyrin ring. The interaction causes either a linear action with respect to the direction of Qx-, Bx-transition dipole moments or, in the opposite case, to the direction of the Qy-, By-transition dipole moments. Both the types of interaction within the dimer ligand enable the generation of parallel and perpendicular interaction simultaneously because the transition dipole moments of Qx-, Bx- and Qy-, By-type transitions have parallel and perpendicular orientations. The transition dipole moments of the Qx- and Bx-bands lay along the axis connecting the two inner hydrogen atoms while the Qy- and By-transitions belong to the transition dipole moments, which are perpendicular to the former axis [13].
3.1.2. FT-IR spectra FT-IR spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer are shown in Fig. 2. The FT-IR bands at 3500 cm 1 and 3429 cm 1 of porphyrin dimer ligand are assigned to N–H stretching vibration of radical amino acid and the band of cobalt(II) porphyrin dimer at 3440 cm 1 was observed. The C–H stretching vibration of porphyrin dimers appears at 2850 cm 1 and 2920 cm 1. The bands at 3313, 964 cm 1 are attributed to the N–H stretching and bending vibration of the porphyrin ligand core. In cobalt(II) porphyrin dimer, these bands disappear and a new band appears at about 1002 cm 1, which is the characteristic of metalloporphyrin. The C@O stretching vibration band of the porphyrin dimer ligand is weak due to the intramolecular hydrogenbonding, however, The vibrational bands of cobalt(II) porphyrin dimer appears at 1741 and 1704 cm 1.The band of the porphyrin dimer appears 1654 cm 1 which was ascribed to the N–H bending
Fig. 1. UV–Vis spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
319
Fig. 2. FT-IR spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
vibration of the amino acid. The bands in the range of 1519– 1535 cm 1 are assigned to N–H in-plane bending (amidogen II) [13]. The bands at 1265 and 1216 cm 1 was ascribed to the C–O stretching vibration in the porphyrin dimer ligand, however, the bands of cobalt porphyrin complexes was displayed 1259 and 1205 cm 1 which can be attributed to the enhancement of both the dihedral angel of the two porphyrin chromophores planes by metalation and reduction of the distance of porphyrin ring intradimer. The bands at 1471 cm 1, 1170 cm 1, 1072 cm 1,798 cm 1 were ascribed to the skeleton vibration of porphyrin dimer ligand, the skeleton vibration model of cobalt porphyrin complexes appeared at the 1349 cm 1, 1170 cm 1, 1072 cm 1, 796 cm 1 [11]. The bands at 717–721 cm 1 are assigned to the methylene inplane rocking vibration of straight alkyl chain. 3.1.3. Raman spectra The Raman spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer using 457.9 nm excitation are displayed in Fig. 3, those Raman bands exhibited a sizable frequency difference between porphyrin dimer ligand and cobalt(II) porphyrin dimer. In low-frequency region, the Raman spectra of porphyrin dimer ligand are significantly different from those of porphyrin dimer ligand. For porphyrin dimer ligand, the only weak Raman
Fig. 3. Raman spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
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band in low-frequency region was observed at 330 cm 1. This band was assigned to the m8 mode. The m8 mode consists of an in-plane translational motion of the pyrrole rings, which can be described as a uniform breathing of the whole porphine ring accompanied by an in-plane CaCmCa deformation in the pyrrole rings [15]. While a very strong band and two weak bands were observed for cobalt(II) porphyrin dimer (at 396 cm 1vs and 253 cm 1w, 334 cm 1w, respectively). The differences of Raman spectra in low-frequency region suggest that the structures or vibrational dynamics especially around the CaCmCa bond-angles are altered by the metal ions. In the 900–1650 cm 1 high-frequency region of the Raman spectra of porphyrin dimer ligand and cobalt(II) porphyrin dimer, bands generally arise from the totally symmetric vibrational modes such as CaCm, CaCb, CbCb, pyrrole quarter-ring, and pyrrole half-ring stretching. The wavenumber positions of Raman bands in the high-frequency region are sensitive to the core size, axial ligation and electron density of the central metal ion [16–18]. In this region, the band at 1550 cm 1 of porphyrin dimer ligand is assigned to CbCb stretch m2 mode, which is up-shifted to 1565 cm 1 in cobalt(II) porphyrin dimer. The m2 mode is observed with enhanced intensity in cobalt(II) porphyrin dimer. In fact, it is one of the most intense bands in the high-frequency region. The band at 1498 cm 1 of porphyrin dimer ligand is assigned to the vibration of phenyl ring; the band at about 1455 and 1453 cm 1 of porphyrin dimer ligand is assigned to CaCm stretch m3 mode and these bands do not appear in cobalt(II) porphyrin dimer. The m2 mode arises mainly from the symmetric CaCm stretching, which is mixed with the CbCb stretching of the same symmetry. The m3 mode has a noticeable component of the symmetric stretching motion of CaCm bonds. On the other hand, the m2 mode has a major contribution from the in-phase CbCb stretching along with a smaller contribution from the CaCm stretching. Thus, mixing of these stretching motions causes nearly complete cancellation of the intensity for the m3 mode, while the intensity of the m2 mode is increased in metallo-tetraphenylporphines under D4h point group symmetry [19]. Raman bands in 1300–1450 cm 1 are due to the out-of-phase coupled CaCb/CaN stretching modes. The 1358 and 1325 cm 1 bands of porphyrin ligand are assigned to the m4 and m12 modes, respectively. The m4 mode of cobalt(II) porphyrin dimer appears at 1369 cm 1, while its m12 band shifts to 1311 cm 1. The 1239 cm 1 band of porphyrin dimer ligand and the 1237 cm 1 band of cobalt(II) porphyrin dimer are attributed to Cm-ph stretching m1 mode. The band at 1080 cm 1 of porphyrin ligand is assigned to the vibration of pyrrole Cb–H stretching m9 mode, which does not shift in cobalt(II) porphyrin dimer. The band at 999 cm 1 of porphyrin ligand is assigned to the vibration of pyrrole breathing and phenyl stretching m15 mode, which is up-shifted to 1009 cm 1 in cobalt(II) porphyrin dimer. The band at about 960 cm 1 of porphyrin ligand is assigned to pyrrole breathing m6 mode, but it disappeared in cobalt(II) porphyrin dimer because the hydrogen atom in the N–H bonding is replaced by metal ion. 3.1.4. Luminescence The optical properties of porphyrin can be related to its molecular skeleton. The nucleus of porphyrin consists of four pyrrole rings linked by methine-bridge: the corresponding macrocycle is fully conjugated plane, containing an 18-electron aromatic p-system. The extended aromatic system is responsible for the high molar absorbance. Porphyrins hold the S2 (B band) emission band and the S1 (Q band) emission band. The S2 fluorescence band is attributed to the transition from the second excited singlet state S2 to the ground state S0, which is much weaker than that of the S1 ? S0 transition of the Q band emission. Its quantum yield is so low that the fluorescence becomes unobservable in this work. Fluorescence of S1 consists of two bands Q(0 1) and Q(0 2). When the hydrogen atoms in the center of porphyrin ring are replaced
Fig. 4. The fluorescence spectra of the porphyrin dimer ligand and cobalt(II) porphyrin dimer.
by metal ion, the fluorescence quenching is a well-known phenomenon [13]. Fig. 4 shows the fluorescence emission spectra of the porphyrin dimer ligand and cobalt complex in DMF. The absorbance of two compounds is almost coincident at vicinity of excitation wavelength, so we can directly compare their fluorescence intensities in experimental conditions. It can be seen from Fig. 4 that Q(0 1) and Q(0 2) emission bands of them are in the regions of 654 and 719 nm, respectively. However, the fluorescent intensity of the cobalt(II) porphyrin dimer (654 nm) is only 5.3% of that of porphyrin dimer ligand, which indicates that the metal ions inner metalloporphyrin efficiently quenches the fluorescence intensities of porphyrin moiety. This result has been reasonably explained by the classic Marcus theory, for such a derivative, the photoinduced intramolecular electron transfer from the singlet excited state inner free base porphyrin to one low-spin cobalt(II) [14]. The electron transfer is the primary de-excitation path for the excited state and the electron transfer rates depend on the angle and distance of donor–acceptor in the classic Marcus theory. For nonadiabatic electron transfer, fluorescence can become the dominant decay pathway for the excited free base porphyrin. The S1 ? S0 (Q band) quantum yield depends on the relative rates of the radiative process S1 ? S0 and two radiationless processes S1 ? S0 and S1 ? Tn. Thus the excited state S1 is primarily deactivated by radiationless decay. That the spin forbidden process S1 ? Tn is predominant for radiationless deactivation of the excited state S1. The fluorescence intensity of cobalt complex largely declined which was attributed to enhancing the S1 ? Tn radiationless process due to cobalt(II) [20]. 3.1.5. Electrochemistry The cyclic voltammetry of porphyrin dimer ligand and cobalt(II) porphyrin dimer is performed to evaluate their redox potentials. In the tetraphenylporphyrin complex, the cobalt has a stable two valence oxidation state in air. So two metal redox reactions can take place, specifically an oxidation: Co(II) ? Co(III) and a reduction: Co(II) ? Co(I). In addition, two ring reductions and one ring oxidation is observed (Fig. 5). The first electroreduction occur 0.448 V corresponds to the Co(III)/Co(II) electrode reaction. The half wave potentials at E1/2 = 0.22 V was negative-shift relative to that of TPPCo. The change of E1/2 for Co(III) ? Co(II) of cobalt complex was attributed to the Co(III) axial interaction with the amido of amino acid. A second reduction occurs at 1.29 V, which was assigned to Co(II)/Co(I) [21–26]. The oxidation of Co(II)/Co(I) is faint which can be attributed to forming intramolecular or intermolecular polymer due to the Co(I) bearing strong nucleophility [27]. For
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Fig. 5. Cyclic voltammograms of porphyrin dimer ligand and cobalt(II) porphyrin dimer in DMF + 0.1 mol/L TBAP.
the ring redox of porphyin dimer ligand, the first ring reduction of the ligand occurs at 1.58 V and the second reduction has not been observed; in the cobalt complex, the first and second reduction of porphyrin ring occurs at 1.56 V and 1.93 V, respectively. This was corresponds to the formation of p anion radical and the second reduction leads to the formation of porphyrin dianion. However, the oxidation peak is broad which can be attributed to intramolecular charge-transfer between porphyrin and amino acid or porphyrin and porphyrin [28]. The reduction peak twice higher than that of oxidation peak which can be ascribed to two-electron reduction and one-electron oxidation in cobalt porphyrin dimer. An irreversible ring oxidation peak occurs at 0.56 V and 0.60 V corresponds to the electronoxydation of ligand and cobalt complex, respectively. 3.2. Spectral properties of self-assembly nanorods of cobalt(II) porphyrin dimer under different conditions General speaking, The primary reaction in the preparation of metalloporphyrin nanocrystals through LSS involved the p–p stacking, hydrophilic and hydrophobic grounds interaction at the interfaces of metalloporphyrin (solid), chloroform–liquid phase (liquid) and water–DMF (N,N-dimethylformamide) solutions (solu-
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tion) and other interaction (metal–metal, intermolecular electrostatic et al). For cobalt(II) porphyrin dimer, the aggregate can be adopted a head-to-tail (J-aggregate), face-to-face (H-aggregate) or nonspecific aggregate arrangement in different experimental conditions. When the solution was placed at lower temperature (sample a, 273 K and sample b, 298 K), The H-aggregate or nonspecific aggregate is dominant in aggregate morphologies of cobalt(II) porphyrin dimer. However, as temperature increase, the J-aggregate take vital role in the aggregate morphologies of cobalt(II) porphyrin dimer because the J-aggregate is stable in thermodynamics while H-aggregate is stable in dynamics. The aggregate morphologies transform from H- to J-aggregate with the stocked solution temperature increase. Thus, the aggregate species grow single which leads to forming homogeneous and ordered chiral nanorods of cobalt(II) porphyrin dimer [29]. Fig. 6 shows the transmission electron microscope (TEM) images of self-aggregate porphyrin dimer at different temperature. From Fig. 6 we can see that the aggregate shape of porphyrin dimer changes from disordered stacking to ordered aggregates; the aggregate configuration of dimer transform from fractal, big brick up to rod-like particles; the diameter of aggregates changes from micron to nanometer, and the diameter size distribution of the nanorods changes from inhomogeneity to homogeneity (Fig. 5e) with solution temperature increase.
3.2.1. The UV–Vis properties of self-assembly nanorods The UV–Vis of cobalt(II) porphyrin dimer demonstrated great differences with the change of the stocked solution temperature. The exciton theory developed by Kasha predicts the occurrence of hypsochromic or bathochromic shifts for the relevant absorption bands, in the case of H-aggregate or J-aggregate interactions respectively. The UV–Vis of the cobalt(II) porphyrin dimer with the changes of the temperature was displayed in Fig. 7. We find that the shoulder peak appears at 435 nm, the variational tendency to absorbance at 415 nm and 435 nm of dimer have been inserted into Fig. 7 with the changes of the temperature, the absorbance intensities of 415 nm decline and that of 435 nm rise with the stocked solution temperatures increase, which can be attributed to forming aggregates of cobalt(II) porphyrin dimer. However, the absorbance values of 415 nm and 435 nm have an abrupt change when the stocked solution is between 338 K and 343 K, which
Fig. 6. TEM images of porphyrin nanorods at the different temperature, (a) 273 K, (b) 298 K, (c) 308 K, (d) 318 K and (e) 328 K.
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Fig. 7. The changes of UV–Vis spectra of cobalt(II) porphyrin dimer in water–DMF with temperature varying from 303 K to 383 K, the interval of temperature is 5°. The variational tendency to absorbance at 415 nm and 435 nm of dimer have been inserted into figure with the changes of the temperature.
can be isosbestic temperature of the transformation from H- to Jaggregate. In communication [9], the UV–Vis of the cobalt(II) porphyrin dimer deposited in water–DMF for 12 h under different temperatures has been analyzed as Refs. [30–33]. We speculated this resulted from the following reasons: face–face assembly of cobalt(II) porphyrin dimer results in the splitting of the excited state because of the exciton-coupling between the two porphyrin molecules. When the two porphyrins are stacked with little or no offset (i.e., an ‘‘H” aggregate), electronic transition is allowed to the upper state but not to the lower state, which results in the observed shift of the absorption spectrum to a shorter wavelength. When two dimer molecules are assembled adjacent to one another in the minor groove, a secondary coupling arises because of the end–end interaction between dimer molecules. This causes an additional splitting of the excited state, which is normally manifested in the UV–Vis spectrum as a broadening or splitting of the absorption band, finally, the absorption of the cobalt(II) porphyrin dimer was red-shifted which was that the porphyrin dimer transforms from H-aggregate to J-aggregate as the stocked solution temperature increases. The results can help us to well-understand the electronic coupling of the chromophors [34,35]. 3.2.2. Circular dichroism (CD) specropolarimetry of self-aggregate nanorods The CD spectra of the cobalt(II) porphyrin dimer have been measured and discussed under different temperature. The results show a weak single CD band under lower temperature (273 K and 298 K), a small bisignate curve under medium temperature (308 K) and a stronger bisignate curve under higher temperature (318 K and 328 K) [9]. The spectral shifts and splitting observed in the CD spectra of the aggregate cobalt(II) porphyrin dimer, which was rationalized by a simple exciton-coupling model, based on work by Kasha et al. [36] and Davydov [37]. When the solution was under lower or medium temperature, the H-aggregate or nonaggregate of cobalt(II) porphyrin dimer is dominant in the stocked solution. There was weak exciton-coupling or no exciton-coupling in H-aggregate or non-aggregate of cobalt(II) porphyrin; (Fig. 8) while the solution was under higher temperature, the J-aggregate of cobalt(II) porphyrin dimer is dominant in the stocked solution, the strong and splitting CD signal observed in the CD spectra [38,39]. It is known that such an interaction depends on inter chromophoric distance and twist, as well as the conformational rigidity around the porphyrin L-glutamic acid linkage [40,41]. The intramolecular exciton-coupling was reinforced due to both the enhancement of the dihedral angel and reduction of the distance of
Fig. 8. The comparison of face–face and end–end couplings of H-and J-aggregated chromophores.
Fig. 9. Exciton-coupling model explain how spectral shifts relate to the aggregate structure.
porphyrin ring intradimer when the aggregate morphologies of cobalt(II) porphyrin dimer transform from H-aggregate to J-aggregate [42–45]. As shown in Fig. 9, the transformation from H-aggregate to J-aggregate decrease the face–face coupling, while enhance the end–end coupling. The much larger intramolecular coupling is reflective of the more extensive orbital overlap between two porphyrin rings in dimer compared with the end-to-end interactions between adjacent dimers. So the shape and intensity of the CD spectrum should be highly dependent on the aggregate formation and the conformation of the chiral porphyrin dimer.
4. Conclusions In this study, we successfully synthesized novel chiral porphyrin dimer ligand and its cobalt(II) porphyrin dimer by using a glutamate bridging group. We assigned the FT-IR and Raman bands of the chiral porphyrin dimers. Furthermore, the photochemical and electrochemical properties of dimers were detailedly studied. In addition, the nanorods of cobalt(II) porphyrin dimer were prepared by adjusting the stocked solution temperature, according to our results, the different sized and functional nanoparticles of porphyrin can be attained by refining the exterior conditions. The UV–Vis spectra were always acting as the indicator of porphyrin aggregates. However, for the chiral porphyrin, the changes of CD spectra can help us to well-understand inter- and intramolecular electronic coupling and the aggregate formation of the chiral porphyrin.
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Acknowledgements The project was supported by natural scientific research innovation Foundation in Harbin Institute of Technology (HIT NSRIF.2008.09). References [1] G.M. Whitesides, J.P. Mathias, C.T. Seto, Science 254 (1991) 1312. [2] N.B. Bowden, M. Weck, I.S. Choi, G.M. Whitesides, Acc. Chem. Res. 34 (2001) 231. [3] Special section on ‘‘Supramolecular Chemistry and Self-Assembly” Science 295 (2002) 2395–2399. [4] M.E. Michel-Beyerle, J. Deisenhofer, H. Michel, M. Bixon, L. Jorter, Biochem. Biophys. Acta 932 (1988) 52. [5] A. Lamrabte, J. Janot, M.E. Bienvenue, M. Momenteau, P. Seta, Photochem. Photobiol. 54 (1991) 123. [6] P.G. Schouten, J.M. Warman, M.P. Haas, M.A. Fox, H.L. Pan, Nature 353 (1991) 736. [7] W.B.H. West, in: T.H. James (Ed.), The Theory of Photographic Processes, third ed., The McMillan Company, New York, 1966. [8] T. Kobayashi, J-Aggregates, World Scientific, Singapore, 1996. [9] X.M. Guo, T.S. Shi, Inorg. Chem. 46 (2007) 4766. [10] X.M. Guo, Y.H. Shi, Y.H. Zhang, L.X. Yu, T.S. Shi, Chin. J. Organ. Chem. 26 (2006) 247. [11] X.M. Guo, L.J. Su, L.X. Yu, T.S. Shi, Chem. J. Chin. Univer. 27 (2006) 410. [12] X.M. Guo, T.S. Shi, Acta Chim. Sin. 64 (2006) 1218. [13] X.M. Guo, T.S. Shi, J. Mol. Struct. 789 (2006) 8. [14] Y.Q. Weng, Y.L. Teng, F. Yue, Y.R. Zhong, B.H. Ye, Inorg. Chem. Commun. 10 (2007) 443. [15] P.M. Kozlowski, A.A. Jarzecki, P. Pulay, X.Y. Li, M.Z. Zgierski, J. Phys. Chem. 100 (1996) 13885. [16] G.S.S. Saini, Spectrochim. Acta Part A 64 (2006) 981. [17] X.Y. Li, R.S. Czernuszewicz, J.R. Kincaid, Y.O. Su, T.G. Spiro, J. Phys. Chem. 94 (1990) 31. [18] F. Paulat, V.K.K. Praneeth, C. Nather, N. Lehnert, Inorg. Chem. 45 (2006) 2835. [19] X.Y. Li, M.Z. Zgierski, J. Phys. Chem. 95 (1991) 4268.
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
[37] [38] [39] [40] [41]
[42] [43] [44] [45]
323
M.H. Qi, G.F. Liu, J. Phys. Chem. B 107 (2003) 7640. F.A. Walker, D. Beroiz, K.M. Kadish, J. Am. Chem. Soc. 98 (1976) 3484. L.A. Truxillo, D.G. Davis, Anal. Chem. 47 (1975) 2260. A. Wolberg, Isr. J. Chem. 12 (1974) 1031. A. Wolberg, J. Manassen, J. Am. Chem. Soc. 92 (1970) 2982. R.H. Felton, M. Linschitz, J. Am. Chem. Soc. 88 (1966) 1113. X.Q. Lin, B. Boisselier-Cocolios, K.M. Kadish, Inorg. Chem. 25 (1986) 3242. G. Gritzner, Pure Appl. Chem. 62 (1990) 1839. H.Y. Liu, J.W. Huang, X. Tian, X.D. Jiao, G.T. Luo, L.N. Ji, Chem. Commun. 16 (1997) 1575. W. Xu, H. Guo, D.L. Akins, J. Phys. Chem. B 105 (2001) 1543. N.C. Maiti, N. Periasamy, J. Phys. Chem. B 102 (1998) 1528. C.A. Hunter, J.K.M. Sanders, J. Am. Chem. Soc. 112 (1990) 5525. K. Kano, H. Minamizono, T. Kitae, S. Negi, J. Phys. Chem. A 101 (1997) 6118. C.M. Drain, F. Nifiatis, A. Vasenko, J.D. Batteas, Angew. Chem., Int. Ed. 37 (1998) 2344. C.M. Drain, J.M. Lehn, J. Chem. Soc., Chem. Commun. (1994) 2313. M. Kasha, Physical Processes in Radiation Biology, Academic Press, New York, 1964. M. Kasha, H. Rawls, R.M. Ashraf El-Bayoumi, The exciton model in molecular spectroscopy, in: Molecular Spectroscopy Proceedings of the VIII European Congress of Molecular Spectroscopy, Butterworths, London, 1965. A.S. Davydov, Theory of Molecular Excitons, Plenum Press, New York, 1971. T.N. Milic, N. Chi, D.G. Yablon, G.W. Flynn, J.D. Batteas, C.M. Drain, J.D. Batteas, Angew. Chem., Int. Ed. 42 (2002) 2117. M. Balaz, A.E. Holmes, M. Benedetti, P.C. Rodriguez, N. Berova, K. Nakanishi, G. Proni, J. Am. Chem. Soc. 127 (2005) 4172. F.D. Lewis, Y. Wu, L. Zhang, X. Zuo, R.T. Hayes, M.R. Wasielewski, J. Am. Chem. Soc. 126 (2004) 8206. N. Berova, K. Nakanishi, Exciton chirality method: principles and applications, in: N. Berova, K. Nakanishi, R.W. Woody (Eds.), Circular Dichroism, Principles and Applications, Wiley-VCH, New York, 2000. T. Arai, K. Araki, N. Maruo, Y. Sumid, C. Korosue, K. Fukuma, T. Katob, N. Nishino, New J. Chem. 28 (2004) 1151. C.A. Hunter, P. Leighton, J.K.M. Sanders, J. Chem. Soc., Perkin Trans. 1 (1989) 547. M. Kasha, H.R. Rawls, A. Shraf El-Bayoumi, Pure Appl. Chem. 11 (1965) 371. E. Peeters, M.P.T. Christiaans, R.A.J. Janssen, H.F.M. Schoo, H.P.J.M. Dekkers, E.W. Meijer, J. Am. Chem. Soc. 119 (1997) 9909.