J. Phys. Chem. 1988, 92, 1807-1813
1807
to those for Euo,oo2Cao 998(NH3)6. In particular, the simple envelope of I5lEu2+hyperfine components observed at the lowest temperature is consistent with the bcc structure of Yb(NH3)6and the absence of any structural transitions at low temperaturesS6 The Eu2+ EPR parameters for EuooozYbl998(NH3)6 are summarized in Table I1 and are very close to those for the other dilute europous compounds investigated in this work and also to the parameters for E u ~ , ~ ~ Y ~ ~studied , ~ ~ previously. ( N H ~ ) The ~ Eu2+ parameters in Table I1 are so close to one another that any detailed discussion of their slight differences is unwarranted.
Conclusions This study has further elucidated the nature of dilute M-NH3 solutions and metallic metal-hexaammine compounds by using low concentrations of lS3Eu2+as the primary paramagnetic probe. In dilute solutions the solvated electron experiences a rather strong magnetic interaction with the Eu2+ moments, whereas in the hexaammine compounds both the Eu2+-conductionelectron and Eu2+-Eu2+ interactions are very weak. Also, the low-temperature EPR spectrum of Eu2+in S T ( N H ~is) suggestive ~ of a structural phase transition. In an analogous fashion, the EPR of Eu2+ as a paramagnetic dopant could be used to study other phenomena in M-NH3 systems, such as the nonmetal-metal transition. Acknowledgment. High-purity metals were kindly supplied by Dr. D. T. Peterson of Ames Laboratory at Iowa State University. The EPR experiments were performed in the Magnetism and Magnetic resonance Facility associated with the Center for Solid temperature, so that it was impossible to obtain reproducible State Science and the Departments of Chemistry and Physics at results. This research was supported by N S F Finally, a sequence of EPR spectra for E u ~ , ~ ~ Y ~ ~ , ~ Arizona ~ ~ ( State N H University. ~ ) ~ Grant DMR-8215315. between 4 and 60 K are shown in Figure 6 . Only the Is1Euz+ raonance is observed, since it has not been possible to detect any Registry No. 1 5 3 E 13982-02-0; ~, NH,, 7664-41-7; Ca(NH,):+, CEPR signals in Yb(NH3)6,even at the highest spectrometer 60086-59-1;Sr(NH,)$+, 20955-09-3;Yb(NH,),'+, 38640-74-3;Eu2+, sensitivities. The 151Eu2+EPR spectra of this alloy are very similar 16910-54-6. Figure 6. EPR spectra of 151E~0.002Yb0.998(NH3)6 at 4 (a), 11 (b), 32 40 (d), and 60 K (e).
(4,
Fluorescence from Diphenylpolyene Vapors Takao Itoh and Bryan E. Kohler* Department of Chemistry, University of California, Riverside, California 92521 (Received: September 23, 1987)
The dependence of fluorescence and fluorescence excitation spectra on buffer gas pressure (perfluorohexane 0-250 Torr) and excited-state decay times has been measured for diphenylbutadiene,diphenylhexatriene,and diphenyloctatetraenevapors. In the case of diphenylbutadiene vapor, fluorescence from l'B, (S,) was observed in addition to the fluorescence from 21Ag (SI),while for diphenylbutadiene seeded in a free jet expansion only the 2IA, fluorescence is observed. The occurrence of the I'B, fluorescence at high total pressure and high vibrational temperature comes from the collisional transfer of population from the 2'Ag state to the I1B, state, while at low pressure and low vibrational temperature there is no mechanism for l'B, repopulation and the fluorescence comes from the primarily 2IA, state. At high buffer gas pressure the I'B, fluorescence yield does not depend significantly on excitation energy. With diphenylhexatriene and diphenyloctatetraene vapors, which have a larger 1'B,-2'Ag separation, only the 2IAg fluorescence was seen. All three molecules showed an increase in fluorescence yield as buffer gas pressure was increased. At the limit of zero buffer gas pressure the fluorescence yield for I'B, excitation decreased strongly with increasing excitation energy for diphenylbutadiene but only weakly for diphenylhexatriene and diphenyloctatraene.
Introduction Diphenylpolyenes have been the subject of a number of spectroscopic investigations,'-I0 not only because these molecules are Hudson, B. S.; Kohler, B. E. Chem. Phys. Lett. 1972, 14, 299. Hudson, B. S.; Kohler, B. E. J . Chem. Phys. 1973, 59, 4984. Hudson, B. S.; Kohler, B. E. Annu. Rev. Phys. Chem. 1974, 25, 437. Heimbrook, L. A.; Kohler, B. E.; Spiglanin, T. A. Proc. Nut/. Acad. Sci: U. S. A . 1983, 80, 4580. (5) Shepanski, J. F.; Keelan, B. W.; Zewail, A. H. Chem. Phys. Letr. 1983, 103. 9. (6) Horwitz, J. S . ; Kohler, B. E.; Spiglanin, T. A . J . Chem. Phys. 1985, 89. 1572.
0022-3654/88/2092-1807$01.50/0
strongly fluorescent and commercially available but also because such studies advance our understanding of polyene electronic structure and the connection between that structure and fundamental photophysical processes. It is well established that for diphenylbutadiene and diphenylhexatriene in the gas phase the one-photon forbidden 2IA, state is S I ,the lowest energy excited singlet state, and that the ( 7 ) Kohler, B. E.; Spiglanin, T.A. J . Chem. Phys. 1984, 80, 5465. (8) Amirav, A.; Sonnenschein, M.; Jortner, J. Chem. Phys. 1986,102, 305. (9) Horwitz, J. S.; Kohler, B. E.; Spiglanin, T. A . J . Phys. (Les Ulis, Fr.) 1985, 46, 10, C7-381.
(IO) Itoh, T.; Kohler, B. E. J . Phys. Chem. 1987, 91, 1760. 0 1988 American Chemical Society
1808 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
one-photon allowed l'B, state is S2.5-7Even though they have the same ordering of excited states, these two molecules have different photophysical properties. For example, in a free jet expansion where molecules have low vibrational and rotational temperatures and there are no collisions, the fluorescence yield and excited-state lifetime of diphenylbutadiene decrease sharply with increasing excitation energy, indicating the onset of a nonradiative channel at about 1100 cm-I above the 2lA, zero-point l e ~ e I . ~ In - ~ contrast, the excited-state lifetime for diphenylhexatriene under the same conditions depends only weakly on excitation energy up to 3400 cm-' above the 2'A, zero-point level.' Although the nonradiative channel in diphenylbutadiene has been assumed to be trans to cis i s ~ m e r i z a t i o n , ~ this , ~ has not been experimentally verified. The absence of an analogous channel in diphenylhexatriene is not easily accounted for under the isomerization hypothesis. For both molecules the only emission observed in a free jet expansion is the 2'A, l'A, fluorescence. In solution at temperatures near room temperature diphenylhexatriene and diphenyloctatetraene fluorescence comes from both the 2'A, and l'B, states.'" To gain deeper insight into the basis for these differences in isolated molecule and solution fluorescence properties, we have determined the vapor-phase fluorescence of diphenylpolyenes and the dependence of that fluorescence on buffer gas pressure.
Itoh and Kohler
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Experimental Section Sample Preparation. Diphenylpolyenes (Aldrich Chemical Co.) were purified by repeated recrystallization and vacuum sublimation. Special attention was given to the detection of impurities in the case of diphenylbutadiene. None could be found by gas chromatography (Hewlett-Packard 5880 with flame ionization detection) either before or after irradiation. Further, no impurity emission was detected when the irradiated vapor sample was redissolved in methanol and measured with a fluorimeter (Spex). The perfluorohexane buffer gas (Alfa Products) was used without purification after we confirmed that it contained no impurities that emitted under the conditions of our experiments. Samples were prepared on an all-glass vacuum system. Perfluorohexane sealed in a side arm was degassed by repeated freeze-pumpthaw cycles. A small sample crystal in a nonfluorescent 10-mm square quartz cell sealed to the vacuum system was heated to 100 "C at a background pressure of less than lo4 Torr in order to remove volatile impurities. Buffer gas was admitted into the sample cell after degassing. The pressure of the buffer gas was determined by the temperature of the side arm (varied from -20 to 10 "C). The sample cell with buffer gas was then isolated from the buffer reservoir, the contents were trapped by liquid N,, and the cell was sealed off. By measuring the pressure and volume of the buffer gas before trapping and measuring the volume of the cell at the end of the experiment, we determined the buffer gas pressure to within 5%. With the diphenylhexatriene and diphenyloctatetraene samples the pressure of the buffer gas was measured directly by a H g gauge after it was established that the Hg vapor had no detectable effect. To avoid interference from fluorescence from residual sample crystals on the cell walls, we kept the amount of diphenylpolyene sealed into the cell as small as possible. The background pressure of the vacuum system was below Torr. Apparatus. Fluorescence and fluorescence excitation spectra of static vapor samples were measured with a Spex D M l B spectrofluorimeter, and the digital data were transferred to a microcomputer (Hewlett-Packard 98 16) for analysis. Absorption spectra were measured with a Varian DMS 100 spectrophotometer. Temperature of the sample cells was controlled by a home-built thermostated cell holder. For diphenylbutadiene and diphenylhexatriene samples the lower portion of the cell was kept at 95 "C and the upper portion at 75 "C; for diphenyloctatetraene the lower portion of the cell was kept at 105 "C and the upper portion at 85 "C. The emission spectra were corrected for the spectral response of the photomultiplier and monochromator, and the excitation spectra were normalized to the intensity of the exciting light.
320
340
360
380
400
Wave'eigtn
17
420
44a
nrr
Figure 1. Fluorescence spectra (corrected) of diphenylbutadienevapor at 368 K with 260 Torr of perfluorohexane buffer gas (upper curve) and with no buffer gas (lower curve). The apparatus used for the free jet experiment was similar to that described p r e v i o ~ s l y .A~ ~Nd:YAG ~ laser pumped dye laser (Quantel YG581-30 and TDLSO) was used as the excitation source. The wavelength-dispersed emission was measured with a monochromator (Jobin Yvon HR320) followed by an OMA (PAR 1421B-1024-G). Fluorescence decays were excited with a pulsed laser (either the Nd:YAG laser pumped dye laser system or a Molectron UV24 N2 laser), detected by a fast-response photomultiplier (Amperex XP1002), and recorded on a transient digitizer (Tektronix 7912AD).
Results Diphenylbutadiene. Diphenylbutadiene fluorescence excited at 290 nm ( l l B u region) with and without buffer gas is shown in Figure 1. Without buffer gas the fluorescence is diffuse and structureless. As buffer gas pressure is increased, the spectrum begins to show some structure and its intensity increases by a factor of 40. In all of the spectra, the maximum intensity is at approximately 350 nm. Although the shape of the fluorescence spectrum at high total pressure is almost independent of the excitation energy, this is not the case when there is no buffer gas. Figure 2 shows the fluorescence spectra of pure diphenylbutadiene vapor excited at different wavelengths. As the excitation energy decreases, the fluorescence spectrum becomes somewhat structured and the wavelength for maximum intensity shifts from 350 to 390 nm. The shape of the fluorescence spectrum obtained by exciting at 320 nm (near the l'B, origin) is similar to that obtained when diphenylbutadiene seeded in a supersonic He expansion is excited at approximately 320 nm.4 Excitation spectra for fluorescence at 400 nm for different buffer gas pressures are shown in Figure 3. At high buffer gas pressure the excitation spectrum corresponds closely to the absorption spectrum. As the buffer gas pressure decreases, the relative intensity of the high-energy side of the excitation spectrum decreases. There is a significant increase in the relative intensity in the 2'A, region as the pressure is decreased. The shape of the excitation spectrum at low pressure without buffer gas is essentially the same as that observed in a supersonic jet: although each band is broadened because of the broader thermal distribution and the broader excitation bandwidth. Fluorescence excitation spectra of pure diphenylbutadiene vapor for detection at different wavelengths are shown in Figure 4. Relative excitation intensity increases as the monitoring wavelength decreases, indicating that the relative contribution of the blue portion of the fluorescence spectrum increases with increasing excitation energy.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
Fluorescence from Diphenylpolyene Vapors
LLL+
1809
L.Ll_L
350
4E0
450
davelength
in
260
Figure 2. Fluorescence spectra (corrected) of pure diphenylbutadiene vapor at 368 K excited at different wavelengths (290,300, 310, and 320 nm).
288
320
300
idavelength
tim
340
in nm
Figure 4. Excitation spectra (corrected) for fluorescence of diphenylbutadiene vapor detected at different wavelengths and different pressures of perfluorohexane buffer gas. The upper pair of spectra are for fluorescence at 400 nm and the lower pair of curves are for fluorescence at 350 nm.
260
280
300
320
Wavelength
340
in nm
Figure 3. Excitation spectra of diphenylbutadiene vapor at 368 K for different buffer gas pressures (260, 82, and 0 Torr).
Fluorescence spectra (uncorrected) measured for diphenylbutadiene seeded into a supersonic jet expansion and excited at two different energies in the 1'B, state are shown in Figure 5. As the excitation energy increases, the fluorescence spectrum becomes diffuse and structureless and the wavelength for maximum intensity shifts slightly to lower energy. Fluorescence intensity at wavelengths shorter than 340 nm is negligible. Since the 2IA, origin is located a t 29 653 cm-' (337.2 this is a clear indication that in a supersonic jet diphenylbutadiene fluorescence comes mostly from the 2IA, state. Excited-state decay times for isolated diphenylbutadiene have been reported p r e v i o ~ s l y . ~At , ~ high buffer gas pressure the emission lifetime is too short to be measured with our equipment (
