Langmuir 1995,11, 860-863
860
Effects of Chloromethanes on the Photoionization of Methylphenothiazine in Silica Gels at Room Temperature Bosong Xiang and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received August 1, 1994. I n Final Form: January 6,1995@ Chloromethanes were added to silica gel of 14 nm pore size with methylphenothiazine (PC1) trapped in them. The methylphenothiazine was photoionized by 320-nm irradiation by a one-photon process to form stable methylphenothiazine cation radical (PC1+)which was detected by electron spin resonance. The addition of CC14 or CHCb increases the net PC1+photoyield when the samples are irradiated at room temperature while CHzClz and CH3C1have no effect. Thus, CHCl3 and cc14 are suggested to act as better electron acceptors in competition with the silica gel framework while CHzClz and CH3Cl do not seem t o be able to compete with the silica gel framework. A chloromethane with more C1 atoms has a greater electron affinity which supports this interpretation. No effects of these chloromethanes on the net photoionization yield of PC1 in silica gels at 77 K were observed, probably due to the immobilization of the chloromethanes at 77 K. Introduction
The design of efficient artificial photoredox systems for storinglight energy must be able to minimize back electron t r a n ~ f e r . l - Heterogeneous ~ materials, such as micelles, vesicles, molecular sieves, and silica gel, have been found to be good host materials for such photoredox systems. The photochemistry of molecules on oxide surfaces has been in~estigated.~,' We have reported the photoionization of tetramethylbenzidine and phenothiazine derivatives and the photoreduction of methylviologen in silica gels at room temperature.s-10 It was shown that stable charge separation could be achieved and that the silica gel pore size had strong effects on both the photoionization and photoreduction efficiencies. In a previous study of the photoionization of phenothiazine derivatives, includingmethylphenothiazine, in silica gels, no molecular electron acceptors were added to the system and thus the silica gel framework was suggested to be the electron acceptor.1° Here we add a series of chloromethanes with different electron acceptor efficiencies to demonstrate competition with the silica gel framework for photoinduced electron transfer with silica gel of 14 nm pore size. Halogenated compounds, such as c c 4 , CHCls, CHzClz, and CH3C1, have been used as electron acceptors or excited state quenchers in flash photolysis studies of the photoionization of molecules such as pyrene, NJVJVJV-tetramethylbenzidine (TMB) and N,NJVJV-tetramethyl-pphenylenediamine (TMPD).11-16 The photosensitive electron donor molecules, pyrene, TMB, or TMPD were ~
~~
Abstract published in Advance ACS Abstracts, February 15, 1995. (1)Borgarello, E.; Kiwi, J.;Pelizzetti, E.;Visca,M.; Gratzel, M.Nature 1981,289,158. (2) Connolly, J. S. Photochemical Conversion and Storage of Solar Energy: Academic, New York, 1981. (3) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J . A m . Chem. SOC. 1988,110,8232. (4) Vermeulen, L. A.; Thompson, M. E. Nature 1992,358,656. (5) Slama-Schwok,A,;Avnir, D.;Ottolenghi, M.Photochem. Photobiol. 1991,54, 525. (6) Oelkrug, D.; Flemming, W.; Fulleman, R.; Gunther, R.; Honne, W.; Krabichler, G.; Schafer, M.; Uhl, S. Pure Appl. Chem. 1986,58, @
"An ILUI.
(7) Thomas, J. K. Chem. Rev. 1993,93,301. ( 8 ) Xiang, B.; Kevan, L. Colloids Surf. A 1993,97,2027. (9)Xiang, B.; Kevan, L. J . Phys. Chem. 1994,98,5120. (10)Xiang, B.; Kevan, L. Langmuir 1994,10, 2688. (11)Saperstein, D.; Levine, E. J . Chem. Phys. 1976,62,3560. (12) Piciulo, P. L.; Thomas, J. K. J . Chem. Phys. 1978,68,3260.
put either in the pure liquid of the halogenated compound or in other organic solvents with a specific concentration of the halogenated compound. Time-resolved optical spectroscopy was used to study the photoionization of pyrene and other aromatic compounds.llJ2 A timeresolved microwave dielectric technique was also used to study the photoionization of TMB and TMPD.14-16 The photoionization of TMPD was also studied by using timeresolved electron spin resonance (ESR)spectroscopy.17 Electron transfer from the photosensitive electron donor molecules to halogenated compounds was confirmed with these techniques. The formation of contact ion pairs was also indicated.17 In the current study, the chloromethanes, CC4,CHC13, CHzC12, and CHsCI, were added to silica gels containing trapped methylphenothiazine. By comparison of the net photoyields of the methylphenothiazine cation radicals of these samples with those of samples without the chloromethanes, it was found that a t room temperature only the addition of CC14 and CHCl3 increased the photoionization yield of methylphenothiazine in silica gel, while CHzClz and CH3C1 did not enhance the photoionization. This indicates that CC14 and CHC13 act as more efficient electron acceptors in comparison with the silica gel framework, while CHzClz and CH3C1are less efficient than the silica gel framework. Experimental Section Commercial methylphenothiazine from Eastman Kodak Co. (1207-72-3) is not pure since it shows a dark color. Thus, methylphenothiazinewas synthesized from phenothiazine."JJ8J9 Synthesis of Methylphenothiazine(PCd. Twelve grams of phenothiazine (Aldrich, 98%)was dissolved in 250 m L of tetrahydrofuran (THF) which had been dried with sodium metal and degassed by nitrogen bubbling. The reaction system was kept under nitrogen. Then 1.6 g of sodium hydride (Aldrich) was added into the solution slowly with stirring. After 1 h of stirring at room temperature, 8.55 g of CHJ (Aldrich)was added (13) Alford, P. C.; Cureton, C. G.; Lampert, R. A.; Phillips, D. Chem. Phys. 1983,76,103. (14) Shimamori, H.; Uegaito, H. J . Phys. Chem. 1991,95,6218. (15) Shimamori, H.; Tatsumi, Y. J . Phys. Chem. 1993,97,9408. (16) Shimamori, H.; Okuda, T. J . Phys. Chem. 1994,98,2576. (17) Honma, H.: Murai. H.: Kuwata, K. J . Phys. Chem. 1994,98,
2571.
(18)Gilman, H.; Ingham, R. K.; Champaigne, J. G.; Diehl, J. W.; Rank, R. 0. J . O g . Chem. 1964,19,560. (19) Kang, Y . S.; Baglioni, P.; McManus, H. J. D.; Kevan, L. J . Phys. Chem. 1991,95, 7944.
0743-7463/95/2411-0860$09.00/00 1995 American Chemical Society
Langmuir, Vol. 11, No. 3, 1995 861
Photoionization of Methylphenothiazine Pump
0
I I
g = 2.0055
Figure 2. Room temperature ESR spectrum of sample PC1/ Fluran Tubing and Clamp
IP14 with CHC13 (0.35 ymol of PC1 and 0.169 pmol of CHC13) recorded after 4 min of room temperature irradiation at 320 nm.
0.0121
Q,
Figure 1. Vacuum system used to evacuate silica gel samples and introduce chloromethanes. Pis a pressure gauge. K1, Kz, &,,&,,, &,, and &,,are stopcocks. & and & a r e Hoffman clamps.
l
+-
2
0.008-
0.0060.0040.00,
and the solution was stirred overnight. The mixture was first extracted with water to remove inorganic impurities. The remaining THF solution, which contained the product, was concentrated and purified further by liquid chromatography (Aldrich silica gel, 60-200 mesh, hexanes as eluant). The 'HNMR spectrum of the product in CDC13 showed absorption peaks at 3.45 ppm (CH3N) and 6.8-7.5 ppm (ring protons).l0J8Jg Silica gel powder, Aldrich (No. 24,398-11, with a 14.0-nm average pore diameter, as given by the supplier, was used. No information is available on the pore size distribution. Methylphenothiazine (PC1) was introduced into the pores of a silica gel by impregnation. Silica gel powder (-6.0 mL or 2.00 g) was M PCl/CHC13 solution overnight. immersed in 10 mL of the The residual solution was removed and the silica gel powder was evacuated below 1 Torr for 2 h at room temperature to remove the solvent (CHC13). The amount of PC1 trapped in the silica gel was measured from the change in the amount of PC1 in the impregnation liquid. The impregnated samples are designated Pc1/P14 where 14 is the average pore diameter of the silica gel in nanometers. The impregnated powder was filled into 2 mm i.d. by 3 mm 0.d. Suprasil quartz tubes. I t was further evacuated and then a known amount of chloromethane gas was introduced via a vacuum line using the design shown in Figure 1. CCld, CHC13, and CHZC12 were degassed by repeating a freeze-pump-thaw cycle four times. The sample was connected to the vacuum line by fluran tubing (Fisher, AGN00002) which was inert to chloromethanes. The sample is evacuated to 3 mTorr with &, and &,,closed but all other stopcocks (KI, Kz, &,,and &,,) and the two clamps (& and &) open. After evacuation, & and K1 are closed and I(4,and I(4" were opened to introduce a specific amount of the chloromethane gas into the quartz tube between & and & by controlling the pressure. Then & was closed and &was opened. The sealed end ofthe sample tube was immersed in liquid nitrogen so that all the chloromethane was adsorbed onto the silica gel powder and then the top of the sample tube was flame sealed. A Cermax 150-W xenon lamp (ILC-LX150F) was used for irradiation. The light was passed through a 10-cm water filter and a Corning 7-54 glass filter to give 320-nm irradiation of 1.3 x lo6 ergs cm-2 s-l in the range from 240 to 400 nm. This was sufficient intensity to carry out monophotonicZ0photoionization of methylphenothiazine. Monophotonic photoionization for our system was verified by observing t h a t the photoyield varied linearly with light intensity. For irradiation a t 77 K the sample (20) Alkaitis, S. A.; Beck, G.; Gratzel, M. J.Am. Chem. SOC.1975, 97, 5723.
y
0.000~ ' 0 2
1 i 4
'
8
8
'
8
6
8 10 12 14 16 Irradiation Time, min
Figure 3. Increase of t h e amount of t h e photoproduced PCl+ radical at room temperature of sample PC1/IP14 with CHC13 (0.35 ymol of PCI and 0.35 pmol of CHC13) versus 320 n m irradiation time. tube was immersed in liquid nitrogen. For such a n opaque powder the total photon absorption is unknown so no estimate of the quantum yield can be made. Electron spin resonance was used to detect the photoproduced methylphenothiazine cation radical (PCl+). ESR spectra were recorded on a Bruker ESP 300 ESR spectrometer with 3 G magnetic field modulation and a microwave power of 10 mW. The doubly integrated intensities of the ESR spectra were calculated using the spectrometer's software. The absolute amount ofthe photoproduced PC1+radical was determined using 4-hydroxy-TEMPO nitroxide radical (Aldrich) trapped in silica gel (Aldrich, No. 24,398-1) as a standard sample. The standard sample was prepared by impregnating the nitroxide radical ( M in CzH50H) into the silica gel, removing the solvent by evacuation, and then sealing it in 2 mm i.d. by 3 mm 0.d. Suprasil quartz tubes under 3 mTorr vacuum.
Results The PC1 impregnated silica gel sample PCJIP14 with adsorbed CHCl3 showed no ESR signal before being photoirradiated. It showed a strong ESR signal (Figure 2) typical of the PC1+ cation radical at room temperature after being irradiated by 320-nm light at room temperature for 4 min.1° This confirms the photoionization of PC1 into PC1+in the silica gel. No ESR signals of other radicals, such as CHCl3- or CHC12, were observed in the room temperature ESR spectra.21 Figure 3 shows that PC1increases with irradiation time and reaches a plateau in about 4 min. An irradiation time of 4 min was selected for comparative yield and stability studies. Figure 4 shows the increase of the photoproduced PCl+ radical intensity with an increasing amount of CHC13 in a PCJIP14 sample. PC1+increases with increasing CHCl3 (21) Bonazzola, L.; Michaut, J. P.; Roncin, J. Chem.Phys. Lett. 1988, 153, 52.
862 Langmuir, Vol. 11, No. 3, 1995
Xiang and Kevan PCi /IP14, PC 1 = 0.35 prole CC14 orCHCl3 -0.169 wmola
a -
0.01 0
0.008
0.004v 0.0024 0.0
3
0.1
0.2 0.3 CHCl3, p mole
0.4
0
Figure 4. Increase ofthephotoproducedPC1+radical in sample PC1/IP14 versus the amount of CHC13. The ESR spectra were recorded at room temperature after 4 min of room temperature irradiation at 320 nm.
0. 0 i 4 1 6 0.01 2
5
0.010
PC 1 /1P14 PC 1 = 0.35 prole CH 4-n CI n = 0.169
I
0.008
+,
0.006
2
0.004 0.002
1
0.0
50 100 150 Decay Time, min
200
Figure 6. Change of the photoproduced PC1+ ESR signal intensity of PC1/IP14 with time in the presence of CHC13 or ccl4 at room temperature after 4 min of room temperature irradiationat 320 nm. The amounts ofPC1 and chloromethane in the samples are 0.35 and 0.169 pmol, respectively.
