J. Phys. Chem. 1900, 84, 3437-3440
3437
Formation of the Charge-Transfer and Constrained Compl@xesof Cobalt( 11) Tetraphemylparphyrin in Rngid Solution S. IKonlshl," M. Hoshlno, and M. Imamura The Instifute of Physical and Chemical Research, Wako, Saitama 35 1, Japan (Receive? June 18, 1980)
The electronic absorption spectrum of solutions in 2-methyltetralhydrofuran (MTHF) of equimolar amounts of cobalt(I1) tetraphenylporphyrin (CoIITPP) and tetracyanoquinodimethane (TCNQ) is very different from the sum of the spectra of solutions of the separate components. In addition, the ESR spectrum of the equimolar solution of the two solutes exhibited only one narrow signal at 77 K with a g value of 2.004 and no signal due to Co"TPP. The absorption spectrum of the TCNQ anion and another new absorption band appear in the spelctrum of the solution of the two solutes after y irradiation at 7'7 K, and the intensities of absorption of the doublet at 553 and 592 nm of the original solution decrease. The ESR spectrum of the irradiated solution of the two components yielded an anisotropic signal with a hyperfine structure attributable to the cobalt nucleus but different from that of Co"TPP. After being warmed up and then brought back to 77 K, the irradiated solution gave absorption and ESR spectra identical with those of Co"TPP in the absence of TCNQ. These spectroscopicdata have been interpreted in terms of the formation of a charge-transfer complex (CT complex) between Co"TPP and TCNQ followed by the formation through electron capture during irradiation of a con,strained complex in which Co"TPP is forced to retain the TCNQ anion in the axial position due to the rigidity of the solvent matrix.
Introduction Considerable attention has been paid to metalloporphyrins in recent decades by researchers in diverse fields, not only bec,ause of their importance in biological systems but also because of the dependence of many of their physicdl and chemical properties on the nature of the central metal ion. In addition to the experimental work by different spectroscopic techniques' to elucidate details of the interaction between central metal ions and porphyrin rings, there have been an increasing number of reports concerned with the molecullar complexes of metalloporphyriins with other types of molecule^.^-^ Co(I1) porphyrins are unique in that they form molecular complexes with such diverse types of compounds as simple molecules, e.g., oxygen and carbon monoxide,6 aromatic compounds, e.g., trinitroben~ene,~ and amines, e.g., piperidine.8 Since CIo(I1) porphyrins have an unpaired electron in the d,z orbital, ESR parameters are very sensitive to the axial perturbation of the metal. For this reason, there has been much ESR work on molecular complexes of Co(1I) porphyrins."12 However, the nature of the interactions among all of the components of such complexes is not fullly understood. In the present paper, we report ESR and optical evidence for formation of a strong charge-transfer complex between Co"TPP and TCNQ in MTHF at 77 K in which most of the negative charge is localized on the TCNQ molecule and the cobalt metal appears to have the valence state of 3+. y irradiation of the sample solution at 77 K converts the charge-transfer compllex through electron capture to a constrained complex iin which CoI'TPP is forced to retain the TCNQ anion iin the axial position owing to the rigidity of the solvent matrix. We have recently reported the observation of such constrained complexes of CoIrTPP.l3 Experimental Section Well-purified Co"TPP, Cu"TPP, and tetraphenylporphyrin (H,'l'PP) were kindly supplied by Dr. K. Yamamoto of the Organic Synthesis Laboratory of our Institute. TCNQ was purchased from Wako Pure Chemical Industries Ltd. and purified twice by sublimation under 0022-3654/80/2084-3437$0 1.OO/O
vacuum. Freshly distilled MTHF was degassed by repeated freeze-pump-thaw cycles and stored over sodiumpotassium alloy under vacuum, Sample solutions were prepared on a vacuum line by distilling MTHF from the storage vessel into a glass apparatus containing the solid sample. The concentration of the sample solution was of the order of M for optical absorption measurements M for ISSR measurements. The co-60 y irradiand ation of the sample solutions was performed at 77 K at a dose rate of ca. 5 X lo4 R/min. Optical absorption spectra were measured with a Cary Model 14 spectrometer. ESR spectra were obtained with a JEOL JES-FE3AX spectrometer operating in the X band with 100-IrHz modulation. Field and frequency calibrations were made by using DPPH powder, Mn2+in MgO powder, and a Takeda Riken TR-5501 frequency counter. ESR and optical absorption measurements at 77 K were done by placing the sample cells in specially built Dewars filled with liquid nitrogen. Irradiated sample solutions were pho tobleached before optical measurements with light from a tungsten lamp incorporated in the spectrometer in order to eliminate absorption by the trapped electrons in the solvent matrix. Results Electronic Ab8;orption Spectra. Figure 1 shows electronic absorption spectra of an MTHF solution containing an equimolar mixture of Co"TPP and TCNQ. The spectrum at room temperature has a single peak at 527 nm and is identical with that of Co"TPP in the absence of TCNQ. When the temperature is lowered to 77 K, the single peak undergoes a red shift and splits into two peaks, one at 553 nm and the other at 592 nm, and a new broad absorption band, with peaks at around 690,740, and 840 nm, appears. In the absence of TCNQ, no such spectral change with templerature was observed. The spectrum at 77 K is similar to those of the cobaltic complexes of TPP in the region between 500 and 620 nm. For example, the MTHF solution alf C1Co"'TPP shows two peaks at 77 K, one at 554 nm and the other at 594 nm.13 The new broad band in the 77 K spectrum which extends to near 900 nm resembles that of the TCNQ anion but without the de@ 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 25, 1980
3438
Konishl et al.
WAVELENGTH I n v i
Flgure 1. Absorption spectra of the MTHF solution containing 4 X lo4 M Co"TPP and TCNQ: (-) at room temperature; (...) at 77 K.
10
I
1 @
I
A
, 500
I
I
I
1 2 500
1
I
I
I
I I 3000
I
Ill
(G]
I
1 3500
I
I
I
Flgure 3. ESR spectra of the MTHF solution of 4 X M Co"TPP at 77 K (---) containing no TCNQ; (-) containing 4 X I O 3 M TCNQ.
;i----.-,! 1 j
600
700
-.~ ,-./ ,800
\
900
1000
WAVELENGTH I n m l
Flgure 2. Absorption spectra of the MTHF solution containing 4 X lo4 M Co"TPP and TCNQ at 77 K: (-) before y irradiation; (- - -) after 10-min y irradiation, followed by 5-min photobleach; 0 s . ) after 30-min y irradiation, followed by 5-min photobleach.
tailed vibrational structure typical of the TCNQ anion. The change of the absorption spectra of the sample solution caused by y irradiation at 77 K is shown in Figure 2. In the region below 620 nm, the intensities of the two absorption bands at 553 and 592 nm decrease, and a new absorption band at 516 nm appears. In the longer-wavelength region, the absorption spectra exhibit detailed vibrational structure and are identical with that of the TCNQ anion prodiiced by y irradiation in MTHF at 77 K. After the irradiated solution was once warmed to room temperature and brought back to 77 K, it ave an absorption spectrum identical with that of CO' TPP in the absence of TCNQ. ESR Spectra. Figure 3 shows the ESR spectra of Co"TPP in the presence and the absence of TCNQ in MTHF at 77 K. In the presence of TCNQ, the spectrum consists of only one single narrow line with a g value of 2.004, and the signal of Co"TPP disappears almost completely. The sample solution containing Co"TPP and TCNQ showed the ESR spectrum shown in Figure 4 after y irradiation at 77 K. The strong signal between 3150 and 3400 G is due to the solvent radical produced by y irradiation. This strong signal of the solvent radical usually prevents identification of other organic radicals, if any, produced by y irradiation. The remaining signal has a large anisotropy, and, though some of the central lines are masked by the signal of the solvent radical, eight equally spaced lines are resolved as indicated in Figure 4. These lines can be attributed to the hyperfine structure due to 59C0with a nuclear spin of "/> The MTHF solution of Co'ITPP in the absence of TCNQ yielded an ESR spectrum after y irradiation at 77 K showing only the signal due to the solvent radical. After being warmed to room
F
Figure 4. ESR spectra of the MTHF solution containing 4 X Co"TPP and TCNQ at 77 K after 70-min y irradiation.
M
temperature and brought back to 77 K again, the sample solution which exhibited the ESR spectrum shown in Figure 4 gave the spectrum of Co"TPP in the absence of TCNQ plus a weak single narrow line with a g value of 2.004.
Discussion The change of the absorption spectra of the MTHF solution of Co"TPP with temperature in the presence of TCNQ indicates increasing interaction between Co"TPP and TCNQ at low temperature. The values of the two absorption peaks at 553 and 592 nm close to those of the cobaltic porphyrins and the appearance of a new board band in the longer-wavelengthregion which resembles that of the TCNQ anion suggest the formation of a strong CT complex between CoIITPP and TCNQ in which the cobalt ion takes the valence state close to 3+ and the TCNQ is close to an anionic state. The ESR spectra of the corresponding solution give further strong support to the assignment of the species formed at 77 K. The disappearP the appearance ance of the ESR signal due to C O ~ T Pand of a single narrow line with a g value of 2.004 are consistent with the formation of a diamagnetic cobaltic species and an organic radical. Electron transfer has been suggested from the cobalt into the oxygen molecule in dioxygen adducts of cobalt(I1) ~omp1exes.l~ There has been, however, dispute1&17over the mechanism and extent of electron
The Journal of Physical Chemistry, Vol. 84, No. 25, 1980 3439
Complexes of Cobalt(I1) Tetraphenylporphyrin
TABLE I : ISSR Parameters of the Constrained Complexes of CoIITPP in MTHF gll 10-41Ail i, cm-'
T C N Q - . C ~ I % ~ 1.982' 2.14b B ~ -C.~ I I T P P ~ IRTI
(77K)
(77K)
Figure 5. Schematic representationof the fcrmation of chargetransfer and constrained compl~~xes of Co"TPP with TCNQ. The letter S in circles denotes a solvent molecule.
transfer, and at present the nature of the Co(II)-02 bond of these molecular complexes appears to be best expressed in terms of partical electron transfer from the cobalt into the bound dioxygen. Compared with these dioxygen complexes, electron transfer is almost complete in the present system. The absence of the detailed vibrational structure of the TCNQ anion in the absorption spectrum at 77 K could be the result of the strong ion pairing between CoIIITPP and TCNQ-. I t hasl been suggested that a acceptors such as trinitrobenzene interact strongly not with the cobalt but rather with the porphyrin T system.1° This appears, however, not to be applicable to the present case, The absorptioii and ESR spectra of MTHF solutions of TCNQ containing HzTPP or CuPTPP, in which the unpaired electron occupies the 3d,~_~z orbital, showed no indication for the formation of CT complexes. This suggssts that the unpaired electron in ithe 3d,z orbital plays an essential role in ithe formation of the CT complex between CoIITI'P and TCNQ. After y irradiation at 77 K, the solution containing Co"TPP and TCNQ gives absorption spectra which provide clear evidence for formation of the TCNQ anion. Furthermore, observation of the decrease in intensity of, the 553- and 592-nni peaks with the growth of a new absorption band at 516 nm is thought to be due to the recovery of a cobaltous porphyrin. This Co"TPP, however, is considered to undergo strong perturbation from the TCNQ anion in the axial position. 'The appearance of a highly anisotropic ESR spectrum with a hyperfine structure attributable to a cobalt nucleus after y irradiation provides further support to the recovery of the Co(I1) porphyrin. Since the one-electron reduction of solute molecules in alcoholic or ethereal solution by y irradiation at 77 K is a well-estalblished phenomenon,le-mthe recovery of Co(I1) porphyrin and the formation of the TCNQ anion can be ascribed to electron capture by the CT complex. Such a reaction was also observed for other cobaltic porp h y r i n ~ .Foir ~ ~example, two absorption peaks at 554 and 594 nm of C1CoI"TE'P in MTHF at 77 K decrease in intensity and a new absorption band appears at 517 nm upon y irradiation. The corresponding solution yields an anisotropic ESR spectrum with a hyperfine structure due to a cobalt nucleus. Thie whole process described so far can be summarized in the scheme illustrated in Figure 5. The formula on the left in Figure 5 depicts Co'ITPP coordinated weakly by solvent molecules in the axial positions, and there is little interaction between ConTPP and TCNQ. When the temperature is lowered, one of the solvent molecules in the axial positions is replaced by a TCNQ molecule, as shown in the central formula, and a strong CT complex is formled in which most of the unpaired electron is localized on the TCNQ molecule. Upon y irradiation, the CT complex is converted to the constrained complex of Co"TPP as shown in the right-hand formula in which the TCNQ anion strongly perturbs CoI'TPP. ESR parameters of constrained complexes including those previously reported13 as well as the present system
C1-*Co1'TPPe a k0.002. f0.02. e Reference .13.
2.192
7 7 i. 3 35i. 2 23 f 1
k0.02, remeasured.
gl
2.34b 2.36; 2.34 This work.
are given in Table I. Since the perpendicular components of the hyperfine itensor, IAJ, are not resolved, they are not presented in the table. Although the values of g, for the three complexes listed do not show appreciable change with changes in the anionic axial ligand, the change of the values of glland lAlllare easily understood as follows. The smaller the distance between the unpaired electron in the d,z orbital of Co(I1) and the anionic ligand in the axial position and the more localized the charge in the ligand, the stronger is the perturbation that the unpaired electron of Co(I1) undergoes. Theoretical treatment21s22predicts that gl will increalse and IAll!decrease with increase of axial perturbation, andl the expermental results reported to date on the stable complexes of Co(1I) porphyrins with a variety of ligands in the axial position show this trend.1° Since Cl-Co'ITPP and Br-CoIITPP are produced from the corresponding cobaltic porphyrins, C1Co"'TPP and BrCo"'TPP in which the halogens and cobalt are covalently bonded, the distance between the halogen anions and the Co(1I) in the constrained complexes is thought to be smaller than that of TCNQ-CoIITPP produced from the CT complex. Thjs large distance between the TCNQ anion and Co(I1) and the delocalization of an unpaired electron within the TCNQ anion appear responsible for the absence of the triplet, ESR spectrum of the TCNQ-CoIITPP system. The observatbon of the recovery of Co"TPP in the electronic absorption and ESR spectra at 77 K after the irradiated solution containing ConTPP and TCNQ is once warmed up assures that the porphyrin structure remains unchanged throughout the experimental procedure. The absorption spectra of the MTHF solution originally containing only 'I'CNQ showed the disappearance of both the TCNQ anion and the neutral TCNQ after the irradiated solution was warmed up. This indicates a reaction of the TCNQ anion in the presence of solvent radicals when the temperature is raised. The weak ESR signal with a g value of 2.004 which is observed after the irradiated solution is warmed up is likely to be due to residual TCNQ anion or some other organic radical. In conclusion, lit has been confirmed by electronic and ESR spectra that a strong CT complex is formed between Co"TPP and TCNQ in MTHF at low temperature and that this CT complex is transformed to a constrained complex by y irradiation at 77 K.
Acknowledgment. We thank Dr. K. Yamamoto of this Institute for providing metalloporphyrins and Professor N. N. Lichtin of I3oston University for critical reading of the manuscript. References and Notes (1) D. Dorphin, Ed., "The Porphyrins", Vol. 111 and IV, Academlc Press, New York, 1979. (2) H.A. 0. Hill, A. J. MacFarkne, and R. J. P. Williams, J. Chem. Soc. A , 1704 (1909). (3) C. D. Barry, H. A. 0. Hill, B. E. Mann, R. J. Sadler, and R. J. P. Willlams, J . Am. Chem. Soc., 95, 4545 (1973). (4) 0. Leal, D. L. Anderson, R. 0. Bowman, F. Basolo, and R. L. Burwell, J . Am. Chem. Soc., 97, 5125 (1975). (5) C. J. Weschler, E). M. Hoffman, and F. Basolo, J. Am. Chem. Soc., 97, 5278 (1975).
3440
J. Phys. Chem. 1980, 84, 3440-3443
(6) 6.B. Wayland, J. V. Minkiewicz, and M. E. Abd-Elmageed, J. Am. Chem. Soc., 96, 2795 (1974). (7) H. A. 0. Hill, P. J. Salder, and R. J. P. Wllllams, J. Chem. Soc., Dalton Trans., 1663 (1973). (6) F. A. Walker, J. Am. Chem. Soc., 92, 4235 (1970). (9) H. A. 0. Hill, P. J. Sadler, R. J. P. Williams, and C. D. Barry, Ann. N . Y . Acad. Scl., 208, 247 (1973). (10) F. A. Walker, J. M g n . Reson., 15, 201 (1974). (1 1) B. E. Wayiand and M. E. Abd-Elmageed, J . Am. Chem. SOC.,96, 4809 (1974). (12) L. C.Dicklnson and J. C.W. Chlen, Inofg. Chem., 15, 1111 (1976). (13) S. Konishi, M. Hoshino, K. Yamamoto, and M. Imamura, Chem. Phys. Lett., 72, 459 (1980).
(14) B. M. Hoffman, D. L. Diemente, and F. Basolo, J. Am. Chem. Soc., 92, 61 (1970). (15) D. Getz, E. Melamud, B. L. Silver, and 2. Dori, J. Am. Chem. Soc., 97, 3846 (1975). (16) B. S. Tovrog, D. J., Kitko, and R. S. Drago, J . Am. Chem. SOC., 98, 5144 (1976). (17) R. S. Drago, Inorg. Chem., 18, 1408 (1979). (18) T. Shlda and W. H. Hamill, J. Am. Chem. Soc., 88, 3689 (1966). (19) W. H. Hamill In “Radical Ions”, E. T. Kaiser and L. Kevan, Eds., Interscience, New York, 1968, pp 321-32. (20) M. Nakamura and S. Fujiwara, J. Coord. Chem., 1, 221 (1971). (21) B. R. McGarvey, Can. J. Chem., 53, 2498 (1975). (22) W. C. Lin, Inorg. Chem., 15, 1114 (1976).
Photoluminescence and Photoreduction of V205 Supported on Porous Vycor Glass Masakazu Anpo, * Ichiro Tanahashi, and Yutaka Kubokawa” Depertment of ApplW Chemistry, College of Englneerlng, University of Osaka Prefecture, Sakal, Osaka 59 1, Japan (Recelved: June 24, 1980)
The studies of the fluorescence and the phosphorescence of V206supported on porous Vycor glass (PVG) have led to determination of the energies of the charge-transferexcited singlet and triplet states on it. The added CO quenches the phosphorescence alone, suggesting the interaction of CO with the excited triplet states. The decay of the phosphorescence in the presence of CO is characterized by superposition of two time constants (one of which is longer than the time constant in the absence of CO). From the comparison of the excitation spectra, it is concluded that the photoformation of CO,, as well as the photouptake of CO observed with Vz05/PVG,is closely associated with the excited triplet states. The vibrational structure of the phosphorescence suggests that the nuclear distance between vanadium and oxygen ions will become longer in the excited states, in agreement with the ease of the photoreduction of VzOs,Le., its photoremoval of lattice oxygen. The quantum yield for photoformation of COz has been determined as -0.04.
Introduction The studies of photocatalysis on metal oxides have received considerable attention in connection with the utilization of solar energy.’ However, there seem to be few studies concerned with the primary process in photocatalysis, e.g., photoformation of hole and electron pairs, the charge separation, and the capture of hole and electrons.2 In view of the significant contribution of the studies of phosphorescence, as well as fluorescence, to the development of photochemistry in the gas phase and in solution? studies of the photoluminescence of metaI oxides4 relating to the photoreaction on them appear to be very useful for clarifying the mechanism of the photocatalysis. A paper about this has already been p ~ b l i s h e d . ~ Although Kazansky et al. have investigated the excited states formed on metal oxides supported on silica gel by spectroscopic technique and found that they exhibit a high reactivity toward the hydrogen abstraction reactions: information on the structure and reactivity of the excited states is still unsufficient. In the present work, by means of emission measurements, we have investigated the photoluminescence of VZO5 supported on porous Vycor glass (PVG) and its photoreduction with CO, since the use of PVG makes it possible to determine the quantum yields of photoreactions on oxides because of its high transparency and large surface area. There have been few studies made of such a determination on oxide surfaces. Experimental Section Materials. The V205/PVG(0.003-0.187 vanadium w t %) was prepared by impregnation of PVG (Corning No. 746685-7930; 160 m2/g, 9.0 X 30 X 1.0 mm) with an 0022-3654/80/2084-3440$01 .OO/O
aqueous solution of NH4V03. The V205/PVG was dried at 350 K and calcined at 850 K in air. The vanadium content was determined by atomic absorption spectrometry. CO (Takachiho Kogyo Co.) was of extrapure grade and used without further purification. Apparatus and Procedures. Details of the apparatus were described previ0usly.~7~ The reactant was introduced to a quartz cell at 298 K on the VZO5/PVGcatalysts which had been evacuated at 623 K (standard treatment) for 2 h. Then UV irradiation was carried out at 298 K by using a high-pressure mercury lamp with a water filter. The dependence of excitation wavelength on the yields of the reactions was measured by using a monochromator equipped with a 500-W Xe lamp. The absorption spectra of the V205/PVGwere determined by measuring transmission through the sample with a Hitachi EPS 3T spectrophotometer at 300 K. A pure PVG specimen pretreated under the same condition was used as a reference. The amount of photoformed COz and the photoinduced uptake of CO were measured with a Pirani vacuum gauge and a Shimazu quadrupole mass spectrometer. The photoluminescence spectra were measured by using a Shimazu RF-501 spectrofluorophotometer with filters to eliminate scattered light in the temperature range of 300-77 K. Details were described previ~usly.~ In the case of the decay curves of photoluminescence, the V205/PVG catalyst was excited with a N2 laser with a nanosecond pulse width at 300 K. Its excitation wavelength of 331.7 nm agreed with the peak of the absorption band of the catalyst. The number of incident photons to the cell was determined by means of potassium ferrioxalate actinometry. The difference between the number of photons passing through the V205/PVG and the corresponding 0 1980 American Chemical Soclety
