J. Phys. Chem. 1986, 90, 5146-5149
5146
After the integration and algebra are done, this reduces to
This function is calculated and plotted in Figure 5 with the limits chosen to be symmetric about 6 = 0’. If eq A4 is modified by a Gaussian probability distribution centered about 6 = Oo, then it becomes
Both denominator and numerator are evaluated numerically for
convenience. This is the fast transfer rate relative to reorientation rate limit, where Bo is -90’ and 6, to 90°, chosen so that all space is spanned. The orientation restriction is achieved by varying u to make the Gaussian narrow or broad. The final limiting case is for motion in a plane with a Gaussian angular distribution where the transfer rate is slow relative to the reorientational rate. For this case (ET(t)) becomes
(A8) For convenience these integrals were numerically evaluated. Again, the curves in Figure 6 were calculated for a symmetric interval, Bo = -90’ and 6, = 90°, with u varying to give orientation restrictions.
Fluorescence Emission from the Anthracene Surface by Collision of N, Molecules in Singlet Metastable States Hiroshi Kume, Tamotsu Kondow,* and Kozo Kuchitsu Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo. Bunkyo-ku, Tokyo 113. Japan (Received: April 7 , 1986)
Metastable species in a supersonic beam of N, produced by electron impact were allowed to collide with an anthracene film at 77 K. Fluorescence from the surface was observed, and the excitation function of the fluorescence was obtained. These results showed that the origin of the fluorescence was the electronic energy transfer to surface anthracene from the N2 molecules in the singlet metastable states, a’IZ,,- and alII,, in spite of its low number density in the beam, which was estimated to be only about of the N2(A’Z,+) state. The detection efficiency was estimated to be lom2.The fluorescence spectrum agreed with that from a crystal film of anthracene by laser excitation (365 nm). This observation implied that anthracene molecules on or near the surface were well oriented.
1. Introduction When N2 is excited by electron impact above 9 eV, singlet metastable states, aIII, and a’lZu-, are readily pr0duced.l Emission from the alIIg state, known as the Lyman-BirgeHopfield band, has been studied both theoretically and experimentally, particularly because of its geophysical importance in electron-excited aurorae.2 The forbidden transitional a”, XIZg+,has been found to proceed by a combination of magnetic dipole and electric quadrupole p r o c e ~ s e s . ~The lowest excited singlet state of N2, a’,&-, has also been characterized by highresolution vacuum UV absorption ~pectroscopy.~It is also known that allowed radiative transitions and collision-induced radiationless transitions occur between the a and a’ state^.^,^ However, kinetic experiments on the a and a’ states have led to contradictory results.’ This is because the N, molecule has other excited states, such as A3Z,+, B311,, and B’3Z;,8 that are considered to take part in reactions of these experiments, and it is difficult to estimate their concentrations. Therefore, it is necessary for such kinetic
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(1) Cartwright, D. C.; Trajmar, S.; Chutjian, A.; Williams W. Phys. Rev. 1977., Al6.~1041.~ ~ ~ , (2) Cartwright, D. C. J. Geophys. Res. 1978, 83, 517. (3) Vanderslice, J. T.; Wilkinson, P. G.; Tilford, S. G.J. Chem. Phys. 1965, 42, 2681. (4) Tilford, S . G.; Wilkinson, P. G.; Vanderslice, J. T . Astrophys. J. 1965, ~~
~~
experiments to detect the singlet metastable states selectively. However, the currently available technique for detection of the a state by use of an Auger detector9 is neither direct nor selective because this detector is also sensitive to the triplet metastable states, such as A3Z,+ and E3Zg+. Our recent paper has shown that Nzin the A’&+ state can be detected by observing phosphorescence from solid biacetyl in collision with metastable N, molecules.’0 The present study is a report of an application of this method to a direct detection of the singlet metastable states of N2 by using anthracene instead of biacetyl. We observed fluorescence emission from an anthracene surface as a result of singlet-singlet energy transfer. It was concluded that the signals originated dominantly from the singlet metastable species, whose number density in the beam was about lo-’ of that of the triplet metastable species.” Therefore, it was possible to obtain the excitation function for the singlet states directly from the signals without the need for subtraction of the contribution from the triplet states. The detection efficiency was found to be comparable with that for an Auger detector. It was also shown that only the molecules on the surface were excited by metastable impact.
~
-141.-( 5.427. - ) Brinkmann, R. T.; Trajmar, S. Ann. Geophys. 1970, 26, 201,
(6) Freund, R. S. J. Chem. Phys. 1972, 56, 4344. (7) van Veen, N.; Brewer, P.; Das, P.; Bersohn, R. J. Chem. Phys. 1982, 77, 4326. (8) Lofthus, A; Krupenie, P. H. J. Phys. Chem. ReJ Data 1977, 6, 113.
2. Experiments and Results 2.1. Emission by Metastable N2 Impact. The apparatus and experimental details have been reported previously. l o The main features of the apparatus are shown in Figure 1. Nitrogen (9) Borst, W. L. Phys. Reu. 1972, AS, 648. (10) Kume, H.; Kondow, T.; Kuchitsu, K. J. Chem. Phys. 1986,84,4031.
0022-3654/86/2090-5 146$01.50/0 0 1986 American Chemical Societv
Fluorescence Emission from the Anthracene Surface
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5147
cn,
(I)
h
,Electron gun
$
r i g h t baffle
a
Deflector II
ij
-
I
Quartz plate Copper block
"
I
h a g n e t i c shield
b Figure 1. Schematicdiagram of the apparatus:1° (I) nozzle beam source: (11) detection chamber. The metastable species of N2,marked N2*,are allowed to collide with the anthracene surface deposited on a copper block. M denotes a multichannel slit for anthracene spray. Charged particles are removed by a deflector. An arrow marked A denotes the direction in which the fluorescence is observed.
molecules expanded through a 50-pm nozzle at a stagnation pressure of 500 Torr were sampled by a skimmer. The N2beam thus formed was introduced into a differentially pumped detection chamber and was bombarded by electrons in an electron impact region; a beam of metastable N2was produced by filtering charged particles. The electron current was typically 60 PA at an impact energy of 15 eV with an energy spread of 0.8 eV (fwhm). A typical pressure of the detection chamber, where the electron gun and the detector were placed, was about 1 X lV5Torr during the measurement. Anthracene (Wako Pure Chemical Industries, blue fluorescein grade) was used without further purification and was deposited onto a copper block cooled to 77 K so as to prevent evaporation of the sample (film A). In the measurements of the emission spectra from the anthracene surface, the metastable N2 beam was prepared by continuous bombardment of electrons on the N, beam and then was allowed to collide with the surface. The emission from the surface was viewed through a quartz window by a Nikon P-250 monochromator equipped with a Hamamatsu R585 photomultiplier; the resolution of the monochromator was set to be 10 nm. The emission, mainly the second positive system of Nz, was produced concurrently at the electron impact region and was also detected by the photomultiplier. The contribution of this stray light to the emission spectrum was estimated as follows: The monochromator was set at a given wavelength and the intensity of the emission, I , , was measured by inserting a quartz plate having 96% transmittance in the 300-700-nm range in the path of the metastable beam. Then the plate was removed from the path, and the intensity, 12, was obtained. The intensity difference, ( I , - 11),was measured at different wavelengths. The measurement was repeated typically 200 times Figure 2a shows the emission spectrum thus obtained at an electron impact energy of 15 eV. A typical statistical error of this spectrum was estimated to be 10%. The emission in the 400-480-nm range vanished when the impact energy was decreased below 8.5 eV. The emission spectrum was measured on repeated deposition of anthracene, but the shape of the spectrum did not change. 2.2. Emission by Laser Excitation. The fluorescence spectra of anthracene deposited on the copper block were also measured by laser excitation. The laser used was a dye laser (Lambda Physik FL2002) pumped by a XeC1-excimer laser (Lambda Physik EMG102) and tuned to emit 365-nm radiation. As shown in Figure 2, the emission spectrum by metastable N2 impact was almost identical with the laser-excited spectrum at room temperature; this spectrum resembles the fluorescence spectrum of an anthracene single crystal." 2.3. Time Dependence of the Fluorescence Emission. The N2 beam was excited by a pulsed electron beam with a pulse width of 100 ps and was allowed to collide with film A. The time (1 1) Ganz, S . Z . Naturforsch., A: Phys., Phys. Chem., Kosmophys. 1978, 33A, 612.
hv 4- An (300 K )
J ,
I
I
L
Ih\....,
h u f An (77 K )
I
I
1
400
450
500
/nm
Wavelength
Figure 2. Fluorescence spectra of the anthracene film (a) at 77 K by the impact of N, metastables, (b) at room temperature by the laser excitation (365 nm), and (c) at 77 K by the laser excitation (365 nm).
I
L
1
0
Time
2 /ms
Figure 3. Time dependence of the fluorescence from the anthracene surface at 15-eV impact energy.
evolution of the emission at the wavelength of 400 nm (see Figure 2a) was observed by a photomultiplier (R585) through the quartz window and through a 400-nm interference filter with a band-pass of 12 nm (Toshiba KL-40). The time dependence of the emission for a 15-eV impact energy has two peaks, as shown in Figure 3; 0 ms corresponds to the emission of the the sharp peak at t N2(C-B) transition, and the broad peak at about 400 ps corresponds to the anthracene fluorescence by metastable N, impact. This broad peak disappeared completely by inserting the quartz plate on the path of the metastable N2 beam. The shape of the broad peak is mainly determined by the velocity distribution of the metastable N 2 molecules, because the lifetime of the fluorescence of an anthracene crystal at 77 K (- 10 ns)12 is much shorter than the transit time of the metastable N2 molecules.
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(12) Birks, J. B. Mol. Cryst. Liq. Cryst. 1973, 28, 117
5148
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 I
I
I
I
I
, I
5
1
10 Electron
I
15 energy
1
I
20
25
/eV
Figure 4. Excitation function for the fluorescence measured by using the anthracene surface (solid line). The broken curve represents the excitation function measured by the Auger electron emission from a CuB e 4 surface. The threshold energy is 8.5 f 1.0 eV. Error bars indicate estimated uncertainties.
2.4. Excitation Function. The integrated intensity of the broad peak was measured as a function of the electron impact energy in the range of 8-25 eV. The intensity at each impact energy was always normalized to that obtained at 12 eV. The excitation function thus derived is shown in Figure 4 by a solid line. The threshold energy of the excitation function was 8.5 f 1.0 eV, which agrees with the excitation energy of the a’n, (8.5 eV) or the a”Z; state (8.4 eV) of N2.’ The experimental errors in this excitation function were estimated to be less than 10%. For comparison, the excitation function for N,(a) measured by Borst using the Auger electron emission from a Cu-Be-0 metal is also shown (broken line).9 The threshold energy and the peak position of the present excitation function agree well with those given by Borst. However, an apparent discrepancy is observed in the region above 15 eV, as discussed in section 3.1.
3. Discussion 3.1. Fluorescence from the Anthracene Film. When a metastable N2 molecule collides with the anthracene surface, the electronic energy of the metastable molecule is transferred to one of the anthracene molecules on the surface by an electron-exchange intera~ti0n.I~When this exchange mechanism operates, energy transfer from the singlet metastable species of N, to anthracene produces an excited singlet state (S,) of anthracene, and S, cascades promptly to the lowest excited singlet state (SI),from which fluorescence is emitted. On the other hand, anthracene is excited to one of the triplet states, T,, directly by collision of the triplet metastable species of N,, and this state cascades to the lowest triplet state, T,, by rapid internal conversion. In addition, SI transition is negligible ( 10-8),14 the quantum yield of the T, because the spin-orbit coupling in anthracene is very small and all the electronic excited states concerned have a,?r*configurations. Consequently, the contribution of the triplet metastable states to the fluorescence emission can be ruled out. Furthermore, molecules in the lowest triplet state (A3&’) are the only metastable species in the N2beam in the range of impact energy from 6 to 8.5 eV.Io As shown in Figure 4, no fluorescence from the anthracene surface is observed in this energy range. Therefore, it is concluded that the triplet metastable states do not contribute to the fluorescence and that only the singlet metastable states of N 2 are detected by observing the fluorescence of anthracene. Note that the relative concentration of the A state on the anthracene surface is estimated from the excitation cross
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(1 3) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978. (14) Birks, J. B. Photophysics ofAromatic Molecules; Wiley-Interscience: London. 1970
Kume et al. sections and lifetimes to be 10, times as much as that of the singlet metastable stateslo Possible candidates of the singlet metastable states are a’n, and a’’B,-. Under our experimental conditions, the transit time of the metastable N, molecules is about 400 p s , as mentioned in section 2.3. In this time scale, 10-20% of the molecules excited intially to the a state are cascaded to the a’ state and the rest of the metastables decay almost completely to the ground state because of the short lifetime of the a state (1 15 F S ) . ~ However, the metastables in the a’ state, which has a radiative lifetime of 0.5 s4, can reach the surface without significant decrease in the number density due to the decay. By taking into account the excitation cross sections’ and the radiative lifetimes of these two states, the a and a’ states are estimated to contribute to the excitation function in the ratio of 3:7. On the other hand, both the a and a’ states contribute almost equally to the excitation function reported by Borst: because the transit time of metastable N, was as short as 100 p s in his experimental setup. This explains why our excitation function is different from that obtained by Borst in the region above 15 eV. cm2 for the a’ state’ By use of the cross section of 1.O X and the procedure described in a previous paper,I0 the detection efficiency was estimated to be approximately lo-,. This detection efficiency is comparable with the secondary electron yield of a typical Cu-Be-0 surface by collision with N2(a).15 3.2. Fluorescence Spectra. As shown in Figure 2c, the laser-excited fluorescence spectrum of film A appears to be a broad excimer-type emission, whose intensity maximum is located around 450 nm,I6 while there are three peaks in the 390-440-nm region, which can be assigned to the emission of an anthracene monomer.” The observed spectral features can be explained by assuming that film A is almost noncrystalline and only a small pait of it is in a crystalline state. On the other hand, the vibrational structures are clearly resolved in the fluorescence spectrum of film B by laser excitation (Figure 2b). This indicates that the structure of film A is rearranged to crystalline when film A is annealed. The dependence of the structure of an anthracene film on the substrate temperature is consistent with that reported by Maruyama et a1.16 As shown in Figure 2a, similar vibrational structures appear in the fluorescence spectrum when film A is excited by metastable N, impact. These observed spectral features can be interpreted in terms of the molecular orientation near the surface layer. As mentioned in section 3.1, anthracene molecules on the surface accept the energy of metastable N, and are excited to the SI state. Because the diffusion length of the singlet exciton of anthracene is 50-100 nm,17,18the fluorescence emission is expected to occur from molecules located in the region from the top surface to at most 100 nm inside. This is also supported by the following argument: A comparison of the fluorescence spectra in Figure 2, a and b, shows that the relative intensity of the fluorescence peak at 400 nm by N2 metastable impact, which can be assigned to the 0-0 band, is more enhanced than that by laser excitation. This indicates that the effect of reabsorption on the 0-0 band of the fluorescence spectrum is smaller than that on the fluorescence caused by metastable N2 impact. For a thin film (5100 Km) prepared in this experiment, the influence of reabsorption on the 0-0 band intensity is almost independent of the film temperature from 4 to 300 K,Iz but it depends mainly on the region where the fluorescence is emitted. Reabsorption increases as the molecule that emits fluorescence is distant from the surface. When film A is excited by laser photons, the fluorescence is emitted from the molecules located in the region extending to at least 400 nm from the top surface. Thus it is concluded that the fluorescence by metastable impact is emitted from the molecules located on the surface layer or in layers very close to the surface. Its clear vibrational structure demonstrates that anthracene molecules located on or near the surface are well oriented when the film is (15) Borst, W.L.Reu. Sci. Instrum. 1971, 42, 1543. (16)Maruyama, Y.;Takamiya-Ichikawa, K. In?.J . Quantum Chem. 1980, 18, 587. (17) Simpson, 0. Proc. R. SOC.London, A 1957, ,4238, 402. (18) Steketee, J. W.; de Jonge, J. Philips. Res. Rep. 1962, 17, 363.
J. Phys. Chem. 1986, 90, 5149-5153 as thick as 100 pm, even if the film is deposited on the substrate at 77 K. This molecular orientation can be explained as follows: At a low substrate temperature, the impinging anthracene molecules lose their kinetic energy so rapidly that they are trapped, forming a noncrystalline s01id.I~ However, as the film thickness increases the impinging molecules have sufficient surface mobility to form crystallites because the rate of thermal energy dissipation (19)
Lee, K. 0.; Gan, T. T. Chem. Phys. Lett.
1977,41, 120.
5149
in the anthracene film is expected to be smaller as the thickness is increased.
Acknowledgment. We are grateful to Professor Yusei Maruyama for a helpful discussion. We also thank Kiyohiko Someda and Megumi Amatsu for their technical assistance. The present study has been supported by a Grant-in-Aid for Scientific Research by the Ministry of Education, Science and Culture of Japan. Registry No. N,, 7727-37-9; anthracene, 120-12-7.
Laser Photolysis Studies of Chlororhodium(I I I ) Tetraphenylporphyrin in Ethanol Solutions. Photoinduced Electron Transfer and Ligand Ejection in the Excited Triplet State Mikio Hoshino,* Hiroshi Seki, Katsutoshi Yasufuku, The Institute of Physical and Chemical Research, Wako, Saitama 351 -01, Japan
and Haruo Shizuka Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: April 7, 1986)
Chlororhodium(II1) tetraphenylporphyrin, ClRh(III)TPP, in ethanol solutions reacts with pyridine, Py, to produce the monopyridine adduct, CIRh(III)TPP(Py), in which Py is located in the axial position. The laser photolysis studies of the adduct revealed that the axial pyridine molecule is dissociated via the porphyrin excited triplet state of CIRh(III)TPP(Py): the quantum yield for the dissociation of pyridine is determined as 0.85 A 0.05. The triplet state is found to undergo facile electron transfer to methylviologen, MVZ+,resulting in the formation of the cation radical, MV". The quantum yield for the formation of MV'+ was obtained as 0.5 A 0.1. This value is much larger than the quantum yield (0.20 h 0.02) for the formation of MV'+ measured for the ethanol solution of CIRh(1II)TPP in the presence of MVZ+. Based on these results, (1) mechanisms for the photoinduced ligand ejection and (2) the effects of the axial pyridine on the electron-transfer reaction are discussed.
Introduction Physical and chemical properties of synthetic metalloporphyrins have been subjected to numerous studies owing to their importance as model compounds of natural porphyrins that dominate the redox reactions in vivo.' In photochemistry, studies on photoinduced charge separation of synthetic metalloporphyrins having central metals, Mg and Zn,have been the active area in understanding the role of chlorophylls in photosynthesis.2" In comparison with these porphyrins, metalloporphyrins that have central metals other than Mg and Zn have received less attention. Recently, we became interested in the role of the central metal and the axial ligands for photoinduced charge separation of metalloporphyrins. The previous studies' have shown that (1) the triplet state of indium(II1) tetraphenylporphyrin and methylviologen, MV*+, establishes a triplet exciplex which partly undergoes ionic dissociation to yield the methylviologen cation radical, MV'+, and (2) the triplet state of indium(II1) tetraphenylporphyrin having two triethanolamine molecules in the axial positions undergoes facile electron transfer toward MV2+without forming the stable triplet exciplex. (1) Dolphin, D.,
IV.
Ed. The Porphyrins; Academic: New York,
The present paper reports the electron-transfer reaction from the excited triplet state of chlororhodium(II1) tetraphenylporphyrin, ClRh(III)TPP, to MV2+. The effects of the ligand in the electron-transfer reaction were examined with the use of pyridine as an axial ligand. During the course of this study, we found that the axial pyridine in the monopyridine adduct of ClRh(III)TPP, ClRh(III)TPP(Py), was dissociated upon laser excitation. Since photoinduced ligand ejection is one of the current subjects in photochemistry of metalloporphyrins,*-'' the mechanisms for the ligand ejection were also investigated in detail.
Experimental Section Chlororhodium(II1) tetraphenylporphyrin was synthesized and purified according to the literature.I2 Reagent grade ethanol and pyridine were used as supplied. Optical absorption spectra were recorded on a Hitachi 330 spectrophotometer. The laser photolysis was carried out by using the second harmonic of a Nd:YAG laser (532 nm) from J. K. Lasers Ltd.: the duration and the energy of a laser pulse were 20 ns and 100 mJ, respectively. The detection system of the transient spectra was described e1~ewhere.l~
1979; Vol.
(2) Seely, G. R. Photorhem. Photobiol. 1978, 27, 639-654. (3) Harriman, A,; Porter, G.; Searle, N. J . Chem. Sor., Faraday Trans. 2 1979, 75, 1515-1521. (4) Harriman, A,; Porter, G.; Richoux, M. C . J . Chem. Sor., Faraday Trans. 2 1981, 77, 833-844. ( 5 ) Matsuo, T.; Ito, K.; Takama, K. Chem. Lett. 1980, 1009-1012. (6) Okura, I.; Thuan, N. K. J . Chem. SOC., Faraday Trans. 1 1980, 76, 2209-221 1. (7) Hoshino, M.; Seki, H.; Shizuka, H. J . Phys. Chem. 1985,89,470-474.
(8) Lavalette, D.; Tetreau, C.; Momenteau, M. J . Am. Chem. SOC.1979, 101, 5395-5401. (9) Kim. D.; Kirmaier, C.: Holten, D. Chem. Phvs. 1983, 75, 305-322. (10) Tait, C. D.; Holten, D.; Gouterman, M. J . Am. Chem. Soc. 1984, 106, 6653-6659. (1 1) Hoshino, M. Chem. Phys Lett. 1985, 120, 50-52. (12) Sadasivan, M.; Fleischer, E. J . Inorg. Nucl. Chem. 1968,30,591-601. (13) Hoshino, M.; Imamura, M.; Watanabe, S.; Hama, Y .J . Phys. Chem. 1984,88, 45-49.
0022-3654/86/2090-5 149%01.50/0 0 1986 American Chemical Society
