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Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as microchemical reactor
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
289
Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as microchemical reactor Su Young Ryu and Minjoong Yoon* Department of Chemistry, Chungnam National University, Daejon, 305-764, Korea We have encapsulated porphyrin derivatives into MCM-41, TiMCM-41, and Cu°-A1MCM41, and its photoinduced electron transfer were investigated by steady state and ultrafast timeresolved spectroscopy. All spectroscopic results measured for tetraphenyl Porphine Manganese (III) Chloride (Mn(III)TPPCl) encapsulated in MCM-41 and TiMCM-41 reveal that framework modification by incorporating the Ti02 into the MCM-41 enhances the electron accepting ability of the MCM-41 framework. And also, Raman and UV-Vis spectroscopic investigations for Zn" tetraphenylporphyrin (Zn^TPP) encapsulated into MCM41 and CU"A1MCM-41 allow us to conclude that Zn"TPP in MCM-41 is oxidized to a considerable extent. Furthermore, central metal ion exchange of Zn"TPP encapsulated into CU"A1MCM-4 1 gives Cu" tetraphenylporphyrin (CU"TPP) with almost unit transformation efficiency. In conclusion, metal ion exchanged MCM-41, Cu°AlMCM-41, might be used for microchemical reactor metal-ion exchange reactions of the porphyrin derivatives. 1. INTRODUCTION Among the heterogeneous hosts, a mesoporous silica, MCM-41, whose pore size is between 2 and 10 nm, has been known to have promising usage for catalysis due to its regular hexagonal array of uniform silica tube with a narrowly distributed diameters.' These unique properties of MCM-41 might allow one to selectively ionize porphyrin macrocycles and reduce the back electron transfer rate by separating the donor and acceptor, which will eventually increase the ionization efficiency."^ If the tetrahedral Si^"* of the mesoporous silica is replaced with transition metal, it is known to be more acidic as well as more reactive with adsorbates than that of MCM-41.^ Due to the catalytic potential of transition metal ions adsorbed, the synthesis and characterization of MCM-41 modified by exchanging the metal ions is one of active research fields."* In this work, we have attempted to investigate the fast electron or charge-transfer processes between metallo-porphyrin and mesoporous silica by using the ultrafast time-resolved diffuse reflectance spectroscopy. And Zn^TPP encapsulated into CU^'A1MCM-41, was also investigated with respect to the utility of mesoporous silica as a michrochemical reactor to control the product selectivity of the central metal exchange reaction in porphyrin macrocycles. 2. EXPERIMENTAL 1.
The synthesis procedure and characterization of the mesoporous silica used in this work are already reported elsewhere.^ X-ray diffractograms were recorded by M03X-HF diffractometer. The diffuse reflectance UV-VlS spectra were recorded by using a Shimadzu
To whom correspondence should be addressed. This work was financially supported by the Korea Research Foundation and the Korea science and Engineering Foundation.
290
UV-3101PC spectrometer. The details of femtosecond diffuse reflectance spectroscopic system have been reported elsewhere.^' ^ Briefly, a light source of self-mode-rocked Ti:sapphire laser pumped by an Ar^ laser and a Tiisapphire regenerative amplifier system with Q-switched Nd:YAG laser. Resonance Raman signal upon photoexcitation at 442 and 458 nm from HeCd and Ar ion laser, respectively, was dispersed by a Raman UIOOO double monochromator and detected with cooled photomultiplier in a photon counting configuration. To determine which metals were altered by the encapsulation of Zn°TPP into Cu AIMCM41, the metal concentration in the toluene solution extracted from the used mesoporous silica was measured by inductively coupled plasma mass spectroscopy(ICP-MS). 3. RESULTS AND DISCUSSION 3.1. The photoinduced electron transfer of Mn (III) chloride tetraphenylporphine incorporated TiMCM-41 and MCM-41 The efficiency of photoinduced electron transfer is generally limited by the occurrence of deactivating back electron transfer, which completes with other reactive pathway of the generated radical ion pairs. The large pore molecular sieves like MCM-41 is used to provide an appropriate microenvironment for retarding dramatically the back electron transfer and increasing enormously the lifetime of the photogenerated ion pairs. The ground-state absorption spectra of Mn°^TPP(Cl) encapsulated into MCM-41 and TiMCM-41 show a dramatically change compare to that in benzene, i.e. i) the Q-band was blue-shifted, ii) the ratio of Soret bands to Q-bands was reduced, and iii) the Soret bands became broader. These absorption spectral changes indicate that Mn"' TPP(Cl) molecules are adsorbed well onto MCM-41 and TiMCM-41, and that the porphyrin Ji-electrons interact with the surface hydroxyl group of MCM-41 and TiMCM-41.^'' ^^^ To understand the microenvironmental effects on the photophysical dynamics of Mn TPP(Cl), we have performed the femtosecond diffuse reflectance photolysis. Figure la shows the transient absorption spectra of Mn"' TPP(Cl) in benzene at delay times 1 ps with an excitation of the soret band at 390 nm. Since Mn'" TPP(Cl) has a d"^ ground-state electron configuration, the (ji, ji*) transition states of complex are consist of a quintet singlet state ( S i ) and a "tripmultiplet" manifold (^Ti, '^Ti, ^Ti). The transient species at 450-500 nm with a decay time of approximately 6 ps in benzene is assigned that the tripquintet, Ti (;i, JT ), which is apparently formed rapidly via intersystem crossing from the lowest singquintet, Si(jr, JT ) decaying very rapidly (2-3 ps).'^''^ The transient spectra in MCM-41 and TiMCM-41 were observed greatly different with that in benzene as shown fig. 1(b) and 1(c) : In MCM-41 and TiMCM-41, its spectral feature shows the broad transient absorption not only around 450-500 nm but also in the low energy region around 550-800 nm. According to suggestion by Holtern et al.,^^' ^^ we ascribe the transient absorption around 550-800 nm to a
400
500
600
700
wavetengji/nm
450
500
550
600
650
Wavetength/nm
700
750
450
500
550
600
650
Wav^engih/nm
Fig 1. Transient absorption spectra Mn"'TPP(Cl) in benzene (a), in MCM-41(b), and in TiMCM-41(c).
^CO 750
291
quintet CT state. The temporal decay profile of the transient absorption indicates that two different transient species (c.a. 10 ps and c.a. 80 ps) were observed in MCM-41 and TiMCM41. The short-lived component should be originated from relaxation of a "tripmultiplet" state and the longer-lived component should be attributed to the spin-forbidden relaxation via a quintet CT state. And also, we found that the longer-lived component in TiMCM-41 is even more enhanced than in MCM-41, indicating that the framework of TiMCM-41 could be more easily interacted with Mn^TPP(Cl) compared with one of MCM-41, and the spin-forbidden relaxation via a quintet CT state were favorable in the order TiMCM-41> MCM-41. After irradiation, MnTPPCl^* radicals are detected in MCM-41 or TiMCM-41, indicating that the mesoporous silicate framework plays good electron acceptor. Furthermore, it has been found that the formation MnTPPCr* is easier in TiMCM-41 than in MCM-41, indicating that framework modification by incorporating the Ti"^^ into the MCM-41 enhances the electronaccepting ability of the MCM-41 framework. Therefore, those photogenerated electrons and Mn TPP(Cl) cation radical in this system could be applied to the photocatalytic reaction. 3.2. Resonance raman studies on Zn" Tetraphenylporphyrin encapsulated into MCM-41 and Cu^AlMCM-41: catalytic ionization of Zn^TPPand its central metal ion exchange We have encapsulated Zn°TPP into MCM-41 and Cu°AlMCM-41. Fig. 2(a) shows the resonance Raman spectrum of Zn"TPP encapsulated into MCM-41.The resonance Raman spectra of Zn"TPP in crystal and toluene were also displayed in Fig. 2(b) and 2(c) for comparison. The frequency and half-width of the Raman band in crystal are identical to those in toluene solution. The most evident change due to encapsulation of Zn"TPP into MCM-41 is the enhancement of u 4 mode intensity as well as the broadening of u 2 mode to the lower frequency region. It is interesting to compare Raman spectrum of Zn"TPP-MCM-41 with that of Zn"TPP radical cation electrochemically generated in solution because mesoporous silica is well known to have an oxidative catalytic properties, and the radical cation is rather stabilized to be long-lived.^' ^ The metallo-porphyrin radical cation like Zn^TPP^ and Cu"TPP^exhibit a rather strong enhancement in the intensity of the Raman modes related to phenyl substituent such as u 4 and u 1 modes as well as appreciable down Shift of u 2 modes.''' These observations were consistent well with the results of the observed Zn"TPP-MCM-41. All the
1300 1400 1500 R a m a n S h ift ( c m ' ' !
350 400 450 500 550 600 650 700 750 800 W a v e l e n g t h (nm )
Fig. 2. Resonance Raman Spectra of Zn'^P-MCM-
Fig. 3. UV-Vis absorption spectra of Zn"TPP-MCM-41
41(a), Zn"TPP crystal (b), Zn°TPP-Cu"AlMCM-
(a), ZnVPP-Cu°AlMCM^l (b), sample in toluene
41(c), Zn"TPP (d), and Cu°TPP in solution (e).
extracted &om Zn°TPP-MCM41 (c), and Zn'^PCU"A1MCM-41 (d), neat Cu°TPP in toluene (e).
292
results led us safely to propose that MCM-41 can efficiently oxidize Zn"TPP encapsulated. Fig. 3a shows absorption spectra of Zn"TPP-MCM-41, the Soret absorption band maximum of Zn"TPP was observed blue shifts accompanied with an appreciable broadening compare to that in toluene solution. In addition to this, the red shifts of Q-band with a new band at 650 nm could be also observed. The characteristic features in the electronic spectra of porphyrin cation radical compared to those of neutral porphyrins are a diminished intensity, the broadening of the Soret band and the appearances of new visible bands at 600-700nm.^^ Therefore, The broadening and blue shifts observed in the Soret band and the additional band at 650 run in Q-band can be explained in terms of the catalytic oxidation of Zn°TPP into MCM-41. Raman and UV-Vis spectroscopic investigations allows us to conclude that Zn TPP in MCM-41 is oxidized to a considerable extent. Tabel 1 Resonance Raman Frequencies (cm"^) for Zn"TPP(crystal), Zn"TPP in solution, Zn"TPPMCM-41, Zn"TPP-Cu'^AlMCM-41, and CU"TPP in solution. Zn"TPP-
Zn"TPP in solution
Cu"TPP in solution
CU"AIMCM-41
(a)
Zn"TPP (crystal) (b)
(c)
(d)
(e)
1234
1234
1236
1234
1238
1292
-
-
-
-
1351
1352
1368
1348
1366
1414 1467
-
-
1492
1490
-
1490
-
Vll
1545
1545
1561
1548
1563
V2
1593
1592
1601
1597
1601
4
Zn"TPPMClVI-41
assignment
Vi
" V4
V2«
The encapsulation of Zn"TPP into CU"A1MCM-41 exhibits a broad Soret band at 410 nm (Figure 3 (b)), which is lower wavelength compared to that of Zn"TPP-MCM-41. The Q-band of Zn"TPP-Cu"AlMCM-41 also exhibits the spectral features quite different from that of Zn"TPP-MCM-41. The resonance Raman spectrum in CU"A1MCM-41 is shown in Figure 2 (c). Of very interest it is to note that o 2 and u 4 modes is up-shifted more than 10 cm ' compared to those for Zn"TPP in crystal and toluene. It should be also noticed that the spectrum is quite different from that of Zn"TPP-MCM-41 (See Table 1). Surprisingly, both UV-Vis absorption and Raman spectral feature of Zn"TPP-Cu"AlMCM-41 were almost identical to the previous reported spectra of Cu" TPP in solution.^^ To obtain further spectroscopic data on this peculiar system, UV-Vis spectrum of the extracts with toluene from the solid Zn"TPP-Cu"AlMCM-41 is shown in Figure 3 (d). For comparison. Figure 3 (c) shows the electronic absorption spectrum observed from the toluene extracts from Zn"TPPMCM-41. Of quite interest is to note that the two spectra are different from each other even though the same porphyrin, Zn"TPP is initially encapsulated. To gain better understandings of the interesting central metal ion exchange occurred in mesoporous silica, we have measured UV-Vis as well as Raman spectra on the extracted solutions from both Zn"TPP-Cu"AlMCM41 and Zn"TPP-MCM-41. Zn"TPP is recovered without any noticeable changes in both UV-
293
Vis and Raman spectrum after encapsulation into MCM-41. However, the extracted solutions from Zn"TPP-Cu ' A I M C M - 4 1 shows that the spectroscopic observations are identical to those from C U " T P P dissolved in toluene. From the ICP-MS studies the relative ratio between Cu" to Zn" quantity in mole fraction is found to be (32.5±1.0) (not shown). Central metal ion exchange of Zn^TPP encapsulated into Cu°AlMCM-41 gives Cu ° tetraphenylporphyrin with almost unit transformation efficiency. All the experimental results in addition to the above considerations led us to safely suppose that the mobile Cu" ions in CU"A1MCM-41 might replace Zn" ions from the porphyrin macrocycles, if the resultant C U " T P P is stable in the mesoporous silica (see Scheme 1).
ClPTPP Scheme 1. Microreactor controlled metal-ion exchanged reactions of porphyrin derivatives 4. CONCLUSION Mesoporous MCM-41 and TiMCM-41 molecular sieves are found to be promising hosts for photoinduced charge separation of adsorbed Mn"'TPP(Cl). In MCM-41 or TiMCM-41, Mn'" TPPCl *' radicals are generated by irradiation, indicating that the mesoporous silicate framework plays good electron acceptor. The Mn'" TPPCl *' generation increases in the order MCM-41 < TiMCM-41, indicating that framework modification by incorporating the Ti^^ into the MCM-41 enhances the electron-accepting ability of the MCM-41 frame work. Also the metal ion exchanged MCM-41 and Cu°AlMCM-41 might be used for microchemical reactor controlled metal-ion exchange reactions of the porphyrin derivatives.
REFERENCES P Selvam, S.K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237. H. M. Sung-Suh, Z. Luan, L. Kevan, J. Phys. Chem. B 101 (1997) 10455 J. Xu, J.-S. Yu, S.J. Lee, B.Y. Kim, L. Kevan, J. Phys. Chem. B 104 (2000) 1307. S. Sinlapadech, R.M. Krishna, Z. Luan, L. Kevna, J. Phys. Chem. B 105 (2001) 4350. Y. Kim, J.R. Choi, M. Yoon, A. Furube, T. Asahi, H. Masuhara, J. Phys. Chem. B 105 (2001)8513. 6. A. Furube, T. Asahi, H. Masuhara, H. Yamashita, M. Anpo, J. Phys. Chem B 103 (1999) 3120-3127. T. Asahi, A. Furube, H. Fukimura, M. Ichikawa, H. Masuhara, Rev. Sci. Instrum. 69 (1998)361. H. M. Sung-Suh, Z. Luan, L. Kevan, J. Phys. Cheln. B 101 (1997) 10455-10463. R A. Leennker, H. T. Thomas, L. D. Weis, R C. James, J. Am. Chem Soc. 88 (1966) 5075.
1. 2. 3. 4. 5.
294
10. A. Ron, M. Folman, 0. Schnepp, J. Phys. Chem 36 (1962) 2449. 11. L. Mochida, K. Tsuji, H. Fujitsu, K. Takeshida, J. Am Chem.Soc. 84 (1980) 3159. 12. M. Goutennan, In The Polphyrins, Dolphin. D., Ed., Academic: New York. Vol.3 (1978) 1-165. 13. M. P. Irvine, R. J. Harrison, M. A. Stahand, G. S. Beddard, Bunsen-Ges. Ber, Phys. Chem 89 (1985) 226-232. 14. X. Yan, C. Kinnaier, D. Holten, Inorg. Chem. 25 (1986) 4774-4777. 15. D. Holten, M. Gouterman, In Optical Properties and Strucfure of Tetrapyrroles, Blauer, G. Sund, H., Eds., de Gruyter: Berlin. (1985) 64. 16. L. Pekkarinen, H. Linschitz, J. Am. Chem Soc. 82 (1960) 2407-241150 17. R.S. Czernuszewicz, K.A. Macor, X.-Y. Li, J.R. Kincaid, T.G. Spiro, J. Am. Chem. Soc. I l l (1989) 3860. 18. R.H. Felton, D. Dolphin, The Porphyrins, Academic Press, New York, 5 (1978) Chapter 3.
289
Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as microchemical reactor Su Young Ryu and Minjoong Yoon* Department of Chemistry, Chungnam National University, Daejon, 305-764, Korea We have encapsulated porphyrin derivatives into MCM-41, TiMCM-41, and Cu°-A1MCM41, and its photoinduced electron transfer were investigated by steady state and ultrafast timeresolved spectroscopy. All spectroscopic results measured for tetraphenyl Porphine Manganese (III) Chloride (Mn(III)TPPCl) encapsulated in MCM-41 and TiMCM-41 reveal that framework modification by incorporating the Ti02 into the MCM-41 enhances the electron accepting ability of the MCM-41 framework. And also, Raman and UV-Vis spectroscopic investigations for Zn" tetraphenylporphyrin (Zn^TPP) encapsulated into MCM41 and CU"A1MCM-41 allow us to conclude that Zn"TPP in MCM-41 is oxidized to a considerable extent. Furthermore, central metal ion exchange of Zn"TPP encapsulated into CU"A1MCM-4 1 gives Cu" tetraphenylporphyrin (CU"TPP) with almost unit transformation efficiency. In conclusion, metal ion exchanged MCM-41, Cu°AlMCM-41, might be used for microchemical reactor metal-ion exchange reactions of the porphyrin derivatives. 1. INTRODUCTION Among the heterogeneous hosts, a mesoporous silica, MCM-41, whose pore size is between 2 and 10 nm, has been known to have promising usage for catalysis due to its regular hexagonal array of uniform silica tube with a narrowly distributed diameters.' These unique properties of MCM-41 might allow one to selectively ionize porphyrin macrocycles and reduce the back electron transfer rate by separating the donor and acceptor, which will eventually increase the ionization efficiency."^ If the tetrahedral Si^"* of the mesoporous silica is replaced with transition metal, it is known to be more acidic as well as more reactive with adsorbates than that of MCM-41.^ Due to the catalytic potential of transition metal ions adsorbed, the synthesis and characterization of MCM-41 modified by exchanging the metal ions is one of active research fields."* In this work, we have attempted to investigate the fast electron or charge-transfer processes between metallo-porphyrin and mesoporous silica by using the ultrafast time-resolved diffuse reflectance spectroscopy. And Zn^TPP encapsulated into CU^'A1MCM-41, was also investigated with respect to the utility of mesoporous silica as a michrochemical reactor to control the product selectivity of the central metal exchange reaction in porphyrin macrocycles. 2. EXPERIMENTAL 1.
The synthesis procedure and characterization of the mesoporous silica used in this work are already reported elsewhere.^ X-ray diffractograms were recorded by M03X-HF diffractometer. The diffuse reflectance UV-VlS spectra were recorded by using a Shimadzu
To whom correspondence should be addressed. This work was financially supported by the Korea Research Foundation and the Korea science and Engineering Foundation.
290
UV-3101PC spectrometer. The details of femtosecond diffuse reflectance spectroscopic system have been reported elsewhere.^' ^ Briefly, a light source of self-mode-rocked Ti:sapphire laser pumped by an Ar^ laser and a Tiisapphire regenerative amplifier system with Q-switched Nd:YAG laser. Resonance Raman signal upon photoexcitation at 442 and 458 nm from HeCd and Ar ion laser, respectively, was dispersed by a Raman UIOOO double monochromator and detected with cooled photomultiplier in a photon counting configuration. To determine which metals were altered by the encapsulation of Zn°TPP into Cu AIMCM41, the metal concentration in the toluene solution extracted from the used mesoporous silica was measured by inductively coupled plasma mass spectroscopy(ICP-MS). 3. RESULTS AND DISCUSSION 3.1. The photoinduced electron transfer of Mn (III) chloride tetraphenylporphine incorporated TiMCM-41 and MCM-41 The efficiency of photoinduced electron transfer is generally limited by the occurrence of deactivating back electron transfer, which completes with other reactive pathway of the generated radical ion pairs. The large pore molecular sieves like MCM-41 is used to provide an appropriate microenvironment for retarding dramatically the back electron transfer and increasing enormously the lifetime of the photogenerated ion pairs. The ground-state absorption spectra of Mn°^TPP(Cl) encapsulated into MCM-41 and TiMCM-41 show a dramatically change compare to that in benzene, i.e. i) the Q-band was blue-shifted, ii) the ratio of Soret bands to Q-bands was reduced, and iii) the Soret bands became broader. These absorption spectral changes indicate that Mn"' TPP(Cl) molecules are adsorbed well onto MCM-41 and TiMCM-41, and that the porphyrin Ji-electrons interact with the surface hydroxyl group of MCM-41 and TiMCM-41.^'' ^^^ To understand the microenvironmental effects on the photophysical dynamics of Mn TPP(Cl), we have performed the femtosecond diffuse reflectance photolysis. Figure la shows the transient absorption spectra of Mn"' TPP(Cl) in benzene at delay times 1 ps with an excitation of the soret band at 390 nm. Since Mn'" TPP(Cl) has a d"^ ground-state electron configuration, the (ji, ji*) transition states of complex are consist of a quintet singlet state ( S i ) and a "tripmultiplet" manifold (^Ti, '^Ti, ^Ti). The transient species at 450-500 nm with a decay time of approximately 6 ps in benzene is assigned that the tripquintet, Ti (;i, JT ), which is apparently formed rapidly via intersystem crossing from the lowest singquintet, Si(jr, JT ) decaying very rapidly (2-3 ps).'^''^ The transient spectra in MCM-41 and TiMCM-41 were observed greatly different with that in benzene as shown fig. 1(b) and 1(c) : In MCM-41 and TiMCM-41, its spectral feature shows the broad transient absorption not only around 450-500 nm but also in the low energy region around 550-800 nm. According to suggestion by Holtern et al.,^^' ^^ we ascribe the transient absorption around 550-800 nm to a
400
500
600
700
wavetengji/nm
450
500
550
600
650
Wavetength/nm
700
750
450
500
550
600
650
Wav^engih/nm
Fig 1. Transient absorption spectra Mn"'TPP(Cl) in benzene (a), in MCM-41(b), and in TiMCM-41(c).
^CO 750
291
quintet CT state. The temporal decay profile of the transient absorption indicates that two different transient species (c.a. 10 ps and c.a. 80 ps) were observed in MCM-41 and TiMCM41. The short-lived component should be originated from relaxation of a "tripmultiplet" state and the longer-lived component should be attributed to the spin-forbidden relaxation via a quintet CT state. And also, we found that the longer-lived component in TiMCM-41 is even more enhanced than in MCM-41, indicating that the framework of TiMCM-41 could be more easily interacted with Mn^TPP(Cl) compared with one of MCM-41, and the spin-forbidden relaxation via a quintet CT state were favorable in the order TiMCM-41> MCM-41. After irradiation, MnTPPCl^* radicals are detected in MCM-41 or TiMCM-41, indicating that the mesoporous silicate framework plays good electron acceptor. Furthermore, it has been found that the formation MnTPPCr* is easier in TiMCM-41 than in MCM-41, indicating that framework modification by incorporating the Ti"^^ into the MCM-41 enhances the electronaccepting ability of the MCM-41 framework. Therefore, those photogenerated electrons and Mn TPP(Cl) cation radical in this system could be applied to the photocatalytic reaction. 3.2. Resonance raman studies on Zn" Tetraphenylporphyrin encapsulated into MCM-41 and Cu^AlMCM-41: catalytic ionization of Zn^TPPand its central metal ion exchange We have encapsulated Zn°TPP into MCM-41 and Cu°AlMCM-41. Fig. 2(a) shows the resonance Raman spectrum of Zn"TPP encapsulated into MCM-41.The resonance Raman spectra of Zn"TPP in crystal and toluene were also displayed in Fig. 2(b) and 2(c) for comparison. The frequency and half-width of the Raman band in crystal are identical to those in toluene solution. The most evident change due to encapsulation of Zn"TPP into MCM-41 is the enhancement of u 4 mode intensity as well as the broadening of u 2 mode to the lower frequency region. It is interesting to compare Raman spectrum of Zn"TPP-MCM-41 with that of Zn"TPP radical cation electrochemically generated in solution because mesoporous silica is well known to have an oxidative catalytic properties, and the radical cation is rather stabilized to be long-lived.^' ^ The metallo-porphyrin radical cation like Zn^TPP^ and Cu"TPP^exhibit a rather strong enhancement in the intensity of the Raman modes related to phenyl substituent such as u 4 and u 1 modes as well as appreciable down Shift of u 2 modes.''' These observations were consistent well with the results of the observed Zn"TPP-MCM-41. All the
1300 1400 1500 R a m a n S h ift ( c m ' ' !
350 400 450 500 550 600 650 700 750 800 W a v e l e n g t h (nm )
Fig. 2. Resonance Raman Spectra of Zn'^P-MCM-
Fig. 3. UV-Vis absorption spectra of Zn"TPP-MCM-41
41(a), Zn"TPP crystal (b), Zn°TPP-Cu"AlMCM-
(a), ZnVPP-Cu°AlMCM^l (b), sample in toluene
41(c), Zn"TPP (d), and Cu°TPP in solution (e).
extracted &om Zn°TPP-MCM41 (c), and Zn'^PCU"A1MCM-41 (d), neat Cu°TPP in toluene (e).
292
results led us safely to propose that MCM-41 can efficiently oxidize Zn"TPP encapsulated. Fig. 3a shows absorption spectra of Zn"TPP-MCM-41, the Soret absorption band maximum of Zn"TPP was observed blue shifts accompanied with an appreciable broadening compare to that in toluene solution. In addition to this, the red shifts of Q-band with a new band at 650 nm could be also observed. The characteristic features in the electronic spectra of porphyrin cation radical compared to those of neutral porphyrins are a diminished intensity, the broadening of the Soret band and the appearances of new visible bands at 600-700nm.^^ Therefore, The broadening and blue shifts observed in the Soret band and the additional band at 650 run in Q-band can be explained in terms of the catalytic oxidation of Zn°TPP into MCM-41. Raman and UV-Vis spectroscopic investigations allows us to conclude that Zn TPP in MCM-41 is oxidized to a considerable extent. Tabel 1 Resonance Raman Frequencies (cm"^) for Zn"TPP(crystal), Zn"TPP in solution, Zn"TPPMCM-41, Zn"TPP-Cu'^AlMCM-41, and CU"TPP in solution. Zn"TPP-
Zn"TPP in solution
Cu"TPP in solution
CU"AIMCM-41
(a)
Zn"TPP (crystal) (b)
(c)
(d)
(e)
1234
1234
1236
1234
1238
1292
-
-
-
-
1351
1352
1368
1348
1366
1414 1467
-
-
1492
1490
-
1490
-
Vll
1545
1545
1561
1548
1563
V2
1593
1592
1601
1597
1601
Zn"TPPMClVI-41
assignment
Vi
" V4
V2«
The encapsulation of Zn"TPP into CU"A1MCM-41 exhibits a broad Soret band at 410 nm (Figure 3 (b)), which is lower wavelength compared to that of Zn"TPP-MCM-41. The Q-band of Zn"TPP-Cu"AlMCM-41 also exhibits the spectral features quite different from that of Zn"TPP-MCM-41. The resonance Raman spectrum in CU"A1MCM-41 is shown in Figure 2 (c). Of very interest it is to note that o 2 and u 4 modes is up-shifted more than 10 cm ' compared to those for Zn"TPP in crystal and toluene. It should be also noticed that the spectrum is quite different from that of Zn"TPP-MCM-41 (See Table 1). Surprisingly, both UV-Vis absorption and Raman spectral feature of Zn"TPP-Cu"AlMCM-41 were almost identical to the previous reported spectra of Cu" TPP in solution.^^ To obtain further spectroscopic data on this peculiar system, UV-Vis spectrum of the extracts with toluene from the solid Zn"TPP-Cu"AlMCM-41 is shown in Figure 3 (d). For comparison. Figure 3 (c) shows the electronic absorption spectrum observed from the toluene extracts from Zn"TPPMCM-41. Of quite interest is to note that the two spectra are different from each other even though the same porphyrin, Zn"TPP is initially encapsulated. To gain better understandings of the interesting central metal ion exchange occurred in mesoporous silica, we have measured UV-Vis as well as Raman spectra on the extracted solutions from both Zn"TPP-Cu"AlMCM41 and Zn"TPP-MCM-41. Zn"TPP is recovered without any noticeable changes in both UV-
293
Vis and Raman spectrum after encapsulation into MCM-41. However, the extracted solutions from Zn"TPP-Cu ' A I M C M - 4 1 shows that the spectroscopic observations are identical to those from C U " T P P dissolved in toluene. From the ICP-MS studies the relative ratio between Cu" to Zn" quantity in mole fraction is found to be (32.5±1.0) (not shown). Central metal ion exchange of Zn^TPP encapsulated into Cu°AlMCM-41 gives Cu ° tetraphenylporphyrin with almost unit transformation efficiency. All the experimental results in addition to the above considerations led us to safely suppose that the mobile Cu" ions in CU"A1MCM-41 might replace Zn" ions from the porphyrin macrocycles, if the resultant C U " T P P is stable in the mesoporous silica (see Scheme 1).
ClPTPP Scheme 1. Microreactor controlled metal-ion exchanged reactions of porphyrin derivatives 4. CONCLUSION Mesoporous MCM-41 and TiMCM-41 molecular sieves are found to be promising hosts for photoinduced charge separation of adsorbed Mn"'TPP(Cl). In MCM-41 or TiMCM-41, Mn'" TPPCl *' radicals are generated by irradiation, indicating that the mesoporous silicate framework plays good electron acceptor. The Mn'" TPPCl *' generation increases in the order MCM-41 < TiMCM-41, indicating that framework modification by incorporating the Ti^^ into the MCM-41 enhances the electron-accepting ability of the MCM-41 frame work. Also the metal ion exchanged MCM-41 and Cu°AlMCM-41 might be used for microchemical reactor controlled metal-ion exchange reactions of the porphyrin derivatives.
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