2370
J . Phys. Chem. 1986, 90, 2370-2374
Sensitized Phosphorescence Excitation Spectra of Complexes of Glyoxal, Pyrazlne, and Phenol. Great Enhancement of Phosphorescence Yield by Complexation Akira Goto, Masaaki Fujii, Naohiko Mikami, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: December 9, 1985)
The sensitized phosphorescence excitation spectra of various complexes of glyoxal (small molecule limit), pyrazine (intermediate case), and phenol (statistical limit) prepared by supersonic expansion were observed simultaneously with the fluorescence excitation spectra. A great enhancement of the phosphorescence yield by the complexation was observed for the first time. The enhancement was found to be due to the vibrational predissociation of the complex within its triplet state.
Introduction The phosphorescence of organic molecules had been extensively studied in the solid matrix at low temperature. When a molecule in the solid matrix is excited to its SI state, the intersystem crossing to the isoenergetic triplet level occurs. The vibrationally hot triplet-state molecule dissipates its vibrational energy to the surrounding media, and the triplet molecule in its vibrationless state is produced. Then, the vibrationless triplet molecule emits phosphorescence with a long lifetime. This is the simplified explanation for the phosphorescence mechanism of a molecule in the condensed phase. Since in the solid matrix the phosphorescence always comes from the vibrationless triplet state, the dissipation process of the vibrational energy to the surrounding media must be very efficient. However, our understanding of this process on the molecular level is too vague. The ultracold van der Waals complex or hydrogen-bonded complex formed between a solute molecule and solvent molecules in an isolated condition which is prepared by supersonic expansion may be regarded as “the solute molecule embedded in minisolid at low temperature”. Therefore, the study of the phosphorescence mechanism of such a system is expected to provide a clue for understanding the dissipation process in the solid matrix. To clarify the general mechanism, it is essentially important to study various molecules having the singlet-triplet energy separation (AST) in a wide range. In the present study, we selected three solute molecules: glyoxal as a typical small molecule limit (AST = 2776 cm-I), pyrazine as an intermediate case (AST = 4050 cm-’), and phenol as a statistical limit (AST = 7764 cm-I). The complexes of each molecule with various solvent molecules prepared by supersonic expansion were subject to study. The phosphorescence excitation spectra of the complexes due to the SI So transition were measured by the sensitized phosphorescence technique which we have recently The fluorescence excitation spectra of the complexes were also observed simultaneously with the phosphorescence excitation spectra. We observed for the first time a great enhancement of the phosphorescence yield of the solute molecule by the complexation. The great enhancement was found to be due to the vibrational predissociation of the complex within its triplet state. +-
Experimental Section Three solute molecules were selected: glyoxal (AST = 2776 cm-I) as a small molecule limit, pyrazine (AST = 4050 cm-I) as an intermediate case, and phenol (AST = 7764 cm-’) as a statistical limit. The complexes studied were glyoxal-Ar, glyoxal.C02, glyoxal dimer, pyrazine dimer, pyrazine.(Ar),, pyrazineCO,, pyrazine-CH,CN, pyrazine-biacetyl, phenoleAr, phenolCO,, phenol-(dioxane),, and phenol-(H,O),. The last two are hydrogen-bonded complexes. ( 1 ) Abe, H.: Kamei, S.: Mikami, N.; Ito, M. Chem. Phys. Lerr. 1984, 109, 164.
(2) Kamei, S.; Okuyama, K.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1986. 90, 9 3 .
0022-3654/86/2090-2370$01.50/0
The experimental apparatus has been described elsewhere.’ The gaseous mixture of the solute vapor and solvent vapor seeded in He gas (the total pressure of the gaseous mixture is 4.0 atm) was expanded into a vacuum chamber at Torr through a pulsed nozzle of a 0.8-mm-diameter orifice. The second harmonic or the fundamental of a tunable dye laser (Lambda Physik FD-2002) pumped by an Xe-C1 excimer laser (Lambda Physik EMG-103) was used as an exciting source. The laser beam crossed the jet 15 mm downstream. The molecule in the jet was excited to its SI state. The fluorescence excitation spectrum due to the SI So transition was obtained by monitoring the total fluorescence originating from the crossing region. The fluorescence was detected by a photomultiplier (HTV R-562), and the photocurrent was averaged by a boxcar integrator (Brookdeal 9415/9425). On the other hand, the phosphorescenceexcitation spectrum was measured by the sensitized phosphorescence method which we have recently developed.’ The triplet-state molecule produced from the SI state by intersystem crossing travels further downstream for 40 ,us after the laser fire and collides against a liquid-nitrogen-cooled copper surface which is installed 85 mm downstream from the nozzle. The cold surface is covered by solid biacetyl which was supplied from its vapor through another nozzle. The solid biacetyl serves as a phosphor. When the triplet-state molecule in the jet collides with the phosphor, the sensitized phosphorescence resulting from energy transfer is emitted. The phosphorescence was detected by a photomultiplier (HTV R585), and the signal was counted by a photon-counting system (Ortic 9302,9315,9325) in the gated mode with a 30-ms gate width after 40-,us delay from the laser fire. In this method, the sensitized phosphorescence intensity reflects the number of triplet-state molecules in existence at about 40 ,us after the excitation to the SI state. We observed simultaneously the fluorescence and phosphorescence excitation spectra under the same jet condition. Glyoxal was prepared from glyoxal trimer dihydride, purchased from Aldrich, by heating under vacuum over P20,. The gaseous monomer was collected in a sample tube at 195 K. Phenol (Wako, >99%), pyrazine (Tokyo Kasei), and H,O were purified by vacuum sublimation or vacuum distillation. Ar, COz,biacetyl, CH,CN, and dioxane were used without purification.
-
Results and Discussion We shall begin with the complexes of glyoxal, which belongs to a small molecule limit and has a singlet-triplet separation (AST) of 2776 cm-’. Figure 1 shows the fluorescence excitation spectrum and the simultaneously measured phosphorescence excitation spectrum of glyoxal seeded in He + Ar. The spectral region shown is near the 0,O band of the Sl(n,a*) So transition of glyoxal. Comparing the spectra with those seeded in pure He, the bands due to the glyoxal.Ar complex are readily identified and they are indicated by black circles in the figure. It is seen that the bands of the complex appear more strongly in the phosphorescence excitation spectrum than in the fluorescence excitation spectrum. The complex bands with a similar feature are also found around
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0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2371
Complexes of Glyoxal, Pyrazine, and Phenol
Phos. ex.
I I
1 .
I I
22000 Wavenumber I cm-'
I
22ioo
Figure 1. Fluorescence and phosphorescence excitation spectra of glyoxal (1 Torr) seeded in Ar (1 atm) + He (3 atm) in a jet. Bands due to glyoxal-Ar complex are indicated by black circles. D denotes band due to glyoxal dimer. the main vibronic bands 7:, 5;, and 8; of free glyoxal in the fluorescence excitation spectrum as reported by Jouvet et at.3 However, the corresponding bands are completely missing in the phosphorescence excitation spectrum. Jouvet et al. also reported a broad band due to the glyoxal-Ar complex near the 0,O band of free glyoxal in their phosphorescence excitation spectrum which was taken by monitoring the phosphorescence emitted at the barrel shock. In our sensitized phosphorescence excitation method, the broad bands are clearly resolved into several sharp bands as seen in Figure 1 , indicating the advantage of our method. It is noticed from Figure 1 that the phosphorescence yield of the complex is much larger than that of free glyoxal. For example, the complex bands on the higher frequency side of the free 0,O band have intensities comparable with that of the free 0,O band in the phosphorescence excitation spectrum, while their intensities are small compared with that of the free 0,O band in the fluorescence excitation spectrum. The intensity ratio of the complex band in the phosphorescence excitation spectrum to that in the fluorescence excitation spectrum (Zp/ZF)is (2-3) X lo3times larger than the ratio for the 0,O band of free glyoxal, showing a great enhancement of the phosphorescence yield by the complexation. Figure 2 shows the fluorescence and phosphorescence excitation spectra of a gaseous mixture of glyoxal and COz seeded in He. The bands indicated by black circles are those due to the complex formed between glyoxal and C 0 2 . It is seen that a similar structural unit repeatedly appears around the Ot, 7;, and 5: bands of free glycol. The structural unit due to the complex may be explained with low-frequency intermolecular modes and/or with the different kinds of the complex, but we did not try to analyze the structure. An interesting point to be emphasized is the strong appearance of the complex bands in the region near the 8; band of the free molecule in the fluorescence excitation spectrum compared with the structural units associated with the other vibronic bands. Abe et al. have found a similar increase in the fluorescence yield for higher vibronic levels of the pyrimidine-Ar complex, and they showed that the vibrational predissociation of the complex in its higher vibronic state is responsible for the apparent increase of the fluorescence yields4 In the case of the glyoxalCOzcomplex, the dispersed fluorescence spectrum obtained after exciting the complex band associated with the 8; band was found to coincide with that of the 0,O band excitation of free glyoxal. This clearly shows that glyoxalC0, dissociates at the 8' vibronic level in the SI state and produces the glyoxal fragment having no vibrational energy. The dissociation of the complex is also apparent in the phosphorescence excitation spectrum, where the complex bands associated with the Ot, 7;, and 5; bands appear (3) Jouvet, C.; Soap, B. J . Chem. Phys. 1981, 75, 1661. (4) Abe, H.; Ohyanagi, Y.;Ichijo, M.; Mikami, N.; Ito, M. J . Phys. Chem. 1985, 89, 3512.
I
I
2 2000
I
I Wavenumber
I
I
I cm-'
I 2 2 500
I
I
I
I
Figure 2. Fluorescence and phosphorescence excitation spectra of a mixture of glyoxal (3.5 Torr) and C02 (30 Torr) seeded in He. Main bands due to glyoxal-CO2complex are indicated by black circles. Structural units of the complex bands associated with Ot, 7;, and 5; are also indicated. The broken curve indicates the laser power.
but those of the Sf,band are missing. The absence of the 8; bands can be explained by the vibrational predissociation in the SI state which is much faster than intersystem crossing. From the above results, the upper limit of the dissociation energy of glyoxal-CO, in the S, state is determined to be 735 cm-I (which is the vibrational frequency of S1 mode of glyoxal in the SI state). Similarly, the absence of the 7; and higher frequency vibronic bands of glyoxal-Ar in the phosphorescence excitation spectrum mentioned before sets the upper limit of the dissociation energy of this complex at 450 cm-I (which is the SI state vibrational frequency of 72). The value is consistent with more accurate upper limit of 233 cm-I reported by Halberstadt et aL5 Similar to the case of glyoxal-Ar, the bands due to glyoxalC02 in the 0,O region are weak in the fluorescence excitation spectrum but fairly strong in the phosphorescence excitation spectrum. For example, the band at 22 040 cm-' is very weak in the former, but it appears with the strongest intensity in the latter. The relative phosphorescence yield of the band measured by (Ip/IF),,,,l,,/ (IP/IF)freeO,O is estimated to be 1 X lo4, which is about 1 order larger than the yield of glyoxal.Ar ((2-3) X lo3). Since the dissociation energy of glyoxalC02 is larger than that of glyoxal-Ar, the result suggests that the phosphorescence yield of the complex increases with the increase of the dissociation energy. The statement will be supported later with other complexes. It may be worthy to mention here about the phosphorescence yield of glyoxal dimer. In Figure 1, there is the band at 22 042 cm-I denoted by D in the phosphorescence excitation spectrum. This band also appears with pure He carrier and was tentatively assigned by Kamei et al. to glyoxal dimer.2 As seen from Figure 1, the band is again very weak in the fluorescence excitation spectrum and strong in the phosphorescence excitation spectrum. The relative phosphorescence yield of the band is 1 X lo5. It is concluded from the above results that glyoxal as a typical small molecule limit exhibits a great enhancement of the phosphorescence yield by the complexation in the order of 103-105. However, the phosphorescence yield decreases for the complex in the vibronic level above the dissociation limit as a result of the vibrational dissociation in the SI state. Now, we shall take an intermediate case molecule, pyrazine (AST = 4050 cm-I). In Figures 3-7 are shown the fluorescence and phosphorescence excitation spectra of pyrazine and of the mixture of pyrazine with Ar, C 0 2 ,CH3CN, and biacetyl. The spectral region shown is near the 0,O band of the S,(n,r*) So transition of pyrazine. In Figure 3, the bands indicated by black circles have been assigned to pyrazine dimers by means of the time-of-flight MPI spectroscopy by Bernstein.6 It is seen that the dimer bands appear more strongly in the phosphorescence excitation spectrum than in the fluorescence excitation spectrum. The dimer band at -15 cm-' from the 0,O band of free pyrazine
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( 5 ) Halberstadt, N.; Soap, B. J . Chem. Phys. 1984, 80, 2340. (6) Bernstein, E. R., Colorado State University, private communication.
2372 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Goto et al.
Pyraz ine Flu. e x .
116b; ~
Pyrazine + CO2 Flu ex
'
I
' ! /I
16a' I
103
Phos ex
18 18 18
.I
. L A
31000
30800
30600 Wavenumber I cm-I Figure 3. Fluorescence and phosphorescence excitation spectra of pyrazine (vapor pressure at roam temperature) seeded in He. The bands due to pyrazine dimer are indicated by black circles.
I
30600
I
I
I
Figure 5. Fluorescence and phosphorescence excitation spectra of pyrazine (vapor pressure at room temperature) + C02 (30 Torr) seeded in He. Bands due to the pyrazine.C02 complex are indicated by black circles.
h r a z i n e + Ar
Pyrazine + CH3CN
Flu. ex
Flu ex 16al
16 bi
.
Phos ex. &
3OAOO
I
31 000
30800 Wavenumber I cm-l
I
I
30800
I
Wavenumber / cm-'
&L
I
31000
Figure 4. Fluorescence and phosphorescence excitation spectra of pyrazine (vapor pressure at room temperature) + Ar (1 atm) seeded in He. The band indicated by a black circle is due to the pyrazineAr complex, and the broad band indicated by an open circle is due to the pyrazine. (Ar), complex. D is the band due to pyrazine dimer.
30kiOO
I
I
30800
31600
Wavenumber I cm-l Figure 6. Fluorescence and phosphorescence excitation spectra of pyrazine (vapor pressure at room temperature) CH3CN (vapor pressure at 263 K) seeded in He. Bands due to the pyrazine.CH$N complex are indicated by black circles.
+
Pyrazine + Biacetyl
is always observed in the other spectra of pyrazine. The phosFlu. ex. phorescence yield of the band measured from (IP/I~)complex/ (Z,/Z,),,,,,,, is 2 X IO4. In the case of the mixture of pyrazine 16bl and Ar (Figure 4), the band shifted by -30 cm-l from the 0,O band I of pyrazine in the fluorescence excitation spectrum is assigned to the origin of the SI So transition of the pyrazine.Ar complex. The corresponding band is missing in the phosphorescence excitation spectrum. However, a broad band shown by an open circle Phos. ex. 2-in the figure can be seen at lower frequency only in the phos, 78 I phorescence excitation spectrum, and it is probably due to py* .I* razine.(Ar), ( n 2 2). Although the phosphorescence yield could not be determined, it is apparent that the yield is very small for pyrazine.Ar and large for pyrazine.(Ar),. The result also suggests that the phosphorescence yield of the complex is larger for the 3 0600 ' 30800 31000 complex having a larger dissociation energy because the dissoWavenumber I cm-' ciation energy for the complete fragmentation of the complex into Figure 7. Fluorescence and phosphorescence excitation spectra of pyrathe constituent molecule and atom(s) is apparently larger in pyzine (vapor pressure at room temperature) + biacetyl (vapor pressure at razine.(Ar), than in pyrazine-Ar. 273 K) seeded in He. Bands due to the pyrazine.biacety1 complex are In Figure 5, the bands due to pyrazineC0, appear in both the indicated by black circles. fluorescence excitation spectrum and the phosphorescence excitation spectrum but more strongly in the latter. The band located of pyrazine.C02 thus obtained is 4 X IO4. at -103 cm-' from the 0,O band of pyrazine can be assigned to The bands due to pyrazine.CH3CN again appear strongly in the origin of the complex. The progression of 18 cm-' develops the phosphorescence excitation spectrum as seen in Figure 6. The band shifted by +41 cm-I from the free 0,O band is the origin of from the origin, the frequency being assigned to an intermolecular mode of the complex. Because of the absence of the 0,O band of the complex, and its phosphorescence yield is estimated to be 2 X lo4 by referring to the dimer bands. The band shifted by +46 free pyrazine in the phosphorescence excitation spectrum, the cm-I from the 16 band of free pyrazine is assigned to the 16f band (I,/I,),,,,,,,/(I,/Z,),,, o,o cannot be obtained from the observed spectra. However, the yield can be determined by referring to of the complex. It is interesting that the intensity of the 16; band the yield of the dimer band ( 1 X IO4) at -15 cm-l from the 0,O of the complex relative to that of the origin band in the phosband of free pyrazine. The phosphorescence yield of the origin phorescence excitation spectrum is anomalously strong considering
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il
.
I
The Journal of Physical Chemistry, Vol. 90, No. 11 1986 2373
Complexes of Glyoxal, Pyrazine, and Phenol
~
Flu. ex.
L
Phos. ex.
0-
0; n
0
n
0
Phos, ex
"
--
35000
Phenol + H20
Flu. e x .
0 0 0
0
* I
36000
u
35900 36000 36300 Wavenumber I cm-'
36400
Figure 8. Fluorescence and phosphorescence excitation spectra of a mixture of phenol (vapor pressure at 325 K) and dioxane (vapor pressure at 263 K) seeded in He. Bands due to free phenol, the phenol.dioxane complex, and the phenol.(dioxane)2complex are indicated.
the corresponding intensity ratio of free pyrazine in the fluores-
cence excitation spectrum. The strong appearance of the hot band suggests a high vibrational temperature of pyrazineC02compared with that of free pyrazine. A similar phenomenon was also found for the pyrazinebiacetyl complex. In Figure 7, the origin and 161 bands of pyrazineebiacetyl appear a t +71 cm-I from the free 0,O band and at +78 cm-1 from the free 16; band, respectively, with anomalously strong intensity of the latter. The phosphorescence yield of the origin band was determined to be 3 X 104. The results obtained for the pyrazine complexes again confirm a great enhancement of the phosphorescence yield of pyrazine by the complexation. The enhancement is on the order of 103-104 except for pyraziheAr whose enhancement is very small. Considering the small dissociation energy of pyrazine-Ar, the statement that the phosphorescence yield is very high for the complex having a large dissociation energy seems to be still valid in the case of pyrazine complexes. We also observed the fluorescence and phosphorescence excitation spectra (Sl(a,a*) So) of the complexes of phenol,' which is a typical statistical limit molecule (AST = 7764 cm-I). It was found that the origin bands due to phenol-Ar (36 316 cm-I) and to phenolC0, (36 384 cm-l) appear strongly in the fluorescence excitation spectra, but they are absent in the phosphorescence excitation spectra, indicating small phosphorescence yield for these van der Waals complexes. It will be interesting to see the results for the phenol complexes having larger dissociation energies. Therefore, we selected the hydrogen-bonded complexes as examples. Figure 8 shows the fluorescence and phosphorescence excitation spectra of the mixture of phenol and dioxane. The band at 36 347 cm-' in the fluorescence excitation spectrum is the origin of free phenol, and it is absent in the phosphorescence excitation spectrum. The bands located in the region 35 940-36 020 cm-I in the fluorescence excitation spectrum are due to phenol-dioxane, and the bands in the region 35 83035 910 cm-' appearing in both the fluorescence and phosphorescence excitation spectra are due to phenol.(dioxane)2. It is quite interesting that free phenol and the 1:l complex are nonphosphorescent but the 1:2 complex is phosphorescent. A similar result was also obtained in the complexes with HzO. Figure 9 shows the fluorescence and phosphorescence excitation spectra of the mixture of phenol and H 2 0 . The band at 35 997 cm-' in the fluorescence excitation spectrum is the origin of phenol-H,O. The corresponding band is missing in the phosphorescence excitation spectrum. The bands indicated by open circles in the figure are due to phenol.(H,0)2 It is Seen that they appear clearly in the phosphorescence excitation spectrum.
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(7) Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1982,86, 1768. Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chern. 1983,87,5083. Gonohe, N.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1985, 89, 3643.
A
I
I
36100
I
36300
36200
Wavenumber I crn-1
Figure 9. Fluorescence and phosphorescence excitation spectra of a mixture of phenol (vapor pressure at 325 K) and water (vapor pressure at 273 K) seeded in He. The band due to the phenol.H20 complex is indicated by a black circle, and those due to the phenol.(H,O), complex are indicated by open circles. FREE FR N -T
, COMPLEX
Dissocia energy t io?
-
T~~
-
E Tic
II Figure 10. Schematic energy level diagram of the complex and the free molecule produced by the dissociation of the complex.
The results obtained for the phenol complexes may be summarized as follows: the enhancement of the phosphorescence yield of phenol by the complexation is nothing or very small for the van der Waals complexes having small dissociation energies. The enhancement does not even occur in the 1:1 hydrogen-bonded complexes which have larger dissociation energies than the van der Waals complexes. However, for the 1:2 complex, whose dissociation energy for the complete fragmentation into the constituent molecules is much larger, the enhancement does occur. In this sense, the conclusion previously obtained that the phosphorescence yield is higher for the complex having a larger dissociation energy is also valid in the case of phenol. Now, we try to explain the characteristic behaviors of the phosphorescence yield of the complexes. Figure 10 shows the schematic energy level diagram of a complex and the solute fragment produced by the dissociation. When the complex is excited to the SIlevel above the dissociation limit as in the case of the 8l level of glyoxalCO,, vibrational predissociation occurs in the SIstate and produces the free solute molecule in the SI state. The vibrational dissociation will compete with the intersystem crossing from the excited SI level of the complex to the isoenergetic triplet level of the complex. However, since the intersystem crossing is a rather slow process compared with the vibrational predissociation, the phosphorescence yield is expected to be very small. The SIstate fragment molecule produced by the vibrational predissociation may give the phosphorescence via the intersystem crossing to the triplet manifold of the molecule. However, as seen in all the cases studied here, the phosphorescence yield is generally very small for the SIstate free molecule. The above explains, for example, the absence of the 8; band of glyoxal.C02 in the phosphorescence excitation spectrum. On the other hand, when the complex is excited to the SI level below the dissociation limit, the vibrational dissociation within the S, state does not occur. Therefore, in this case, the resonant fluorescence from the level excited and the intersystem crossing to the isoenergetic TI level of the complex are the main relaxation processes. The complex in the TI state p r o d u d by the intersystem crossing gains an excess vibrational energy nearly equal to the singlet-triplet energy separation (AST), which is generally larger than the dissociation energy of the complex. Therefore, the
2374
J . Phys. Chem. 1986, 90, 2314-2379
dissociation of the complex occurs in the triplet state and the free energy of pyrazine.Ar. However, in the other pyrazine complexes, molecule in its triplet state is generated. The triplet free molecule the dissociation energy is fairly large because of large dipole moments or quadrupole moments of the solvent molecules in these thus produced has the vibrational energy nearly equal to AST minus the dissociation energy because the part of the vibrational complexes. Therefore, we observed the high phosphorescence energy of the TI complex is consumed for the d i s ~ o c i a t i o n . ~ ~ ~ yields for the pyrazine complexes other than pyrazine-Ar. Finally, Therefore, the smaller the AST and the larger the dissociation in a statistical limit molecule, the AST is largest, so we need a energy, the vibrationally cooler is the triplet-state molecule larger dissociation energy for the complex to detect the phosproduced. On the other hand, it is known that the lifetime of an phorescence. This is the case of the phenol complexes. In the van der Waals complexes like phenol-Ar and phenolC02, the small isolated triplet-state molecule decreases exponentially with the dissociation energy is not enough to produce the vibrationally increase of the excess vibrational energy in the triplet state.9 cooled free molecule. Even in the hydrogen-bonded complexes Therefore, the triplet-state molecule produced by the dissociation like phenol.dioxane and phenol-H20the large dissociation energy has a longer lifetime for the complex of smaller AST and larger dissociation energy. In the sensitized phosphorescence method for breaking the hydrogen bond is still much smaller than AST. we used, we are measuring the triplet-state molecules in existence However, when the hydrogen-bonded complex having more than two solvent molecules like phenol.(dioxane), or phenol.(H,O), at 40 ps after the laser fire. The triplet-state molecule having dissociates completely into the constituent molecules, a very large a lifetime longer than 40 ps can be detected, but that having a dissociation energy is required and the vibrationally cooled tripshorter lifetime escaped detection. It is expected from the above let-state fragment may be produced. Thus, the simple mechanism consideration that the complex having smaller AST and larger of the dissociation of the complex in its triplet state proposed here dissociation energy has a larger phosphorescence yield. The qualitatively explains well all the observed features of the phosexpectation is qualitatively fulfilled by the observed results of phorescence of the complexes. various complexes studied here. In a small molecule limit case In conclusion, we observed for the first time the great enwhere AST is small, even if the dissociation energy is small, a large hancement of the phosphorescence yield by the complexation for enhancement of the phosphorescence yield by the complexation molecules ranging from a small molecule limit to a statistical limit. is anticipated. This is the cases for the glyoxal complexes. In The great enhancement was found to be due to the vibrational the case of an intermediate case molecule, AST is larger, so larger predissociation of the complex in its triplet state. dissociation energy will be required for high phosphorescence yield. This is illustrated by the cases of pyrazine-Ar and the other Acknowledgment. We thank H. Abe and S . Kamei for stimpyrazine complexes. In the former, no enhancement of the ulating discussions. phosphorescence yield occurs because of the small dissociation Registry No. CH3CN, 75-05-8;Ar, 7440-37-1;C 0 2 , 124-38-9;H,O, 7732- 18-5; glyoxal, 107-22-2; pyrazine, 290-37-9; pyrazine dimer, (8) Knee, J. L.; Johnson, P. M. J . Chem. Phys. 1984, 80, 13. 101419-87-8;phenol, 108-95-2;biacetyl, 431-03-8; dioxane, 123-91-1; (9) Dietz, T. G.; Duncan, M. A,; Puiu, A. C.; Smalley, R. E. J . Phys. glyoxal dimer, 81313-23-7. Chem. 1982, 86, 4026.
Fluorescence Depolarization of Rhodamine 6G in Glycerol: A Photon-Counting Test of Three-Dimensional Excitation Transport Theory Philip A. Anfinrud, David E. Hart, John F. Hedstrom, and Walter S. Struve* Department of Chemistry and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 5001 1 (Received: December 26, 1985)
Time-correlated photon counting has been used to measure fluorescence concentration depolarizaton for rhodamine 6G in glycerol. The excitation transport theory developed by Gochanour, Andersen, and Fayer yields good approximations to the to 2.4 X experimental decay profiles over the concentration range 1.7 X M. Although the differences between optimized theoretical and experimental profiles are fractionally small, they are readily characterized under present counting statistics. They prove to be dominated by experimental artifacts, arising from excitation trapping by rhodamine 6G aggregates and from self-absorption in solution cells thicker than I O gm.
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I. Introduction It has long been recognized that accurate modeling of fluorescence concentration depolarization in solution is a formidable theoretical problem. Early attempts to describe the influence of Forster dipole-dipole excitation transport' on fluorescence depolarization frequently assumed that transfer was limited to one or two excitation hops from the initially excited m ~ l e c u l eor , ~that ~ ~ excitation was exchanged only between nearestand next-nearest-neighbor molecules in s ~ l u t i o n . ~ - ~ To our knowledge, the first realistic calculations of the probability G S ( t ) (1) (2) (3) (4) (5) (6)
T. Forster, Discuss. Faraday SOC.21, 7 (1959). M. D. Galanin, Tr. Fir.Insf.5 , 339 (1950). S. I. Vavilov, Zh. Eksp. Teor. Fiz. 13, 13 (1943). F. W. Craver and R. S. Knox, Mol. Phys. 22, 385 (1971). F. W. Craver. Mol. Phys. 22, 403 (1971). A. Ore, J . Chem. Phys. 31, 442 (1959).
that excitation is found on the initially excited molecule at time t were provided by Gochanour, Andersen, and Fayer,' who worked out diagrammatic Green's function expansions of solutions to the excitation transport master equation* and obtained successive self-consistent approximations to c"(t). The latter Green's function is related to the fluorescence depolarization in solution by9
where Ill(?) and I L ( t ) are the fluorescence intensity components polarized parallel and normal to the excitation polarization. (This (7) C. R. Gochanour, H. C. Andersen, and M . D. Fayer, J . Chem. Phys. 70, 4254 (1979). ( 8 ) T. Forster, Ann. Phys. 6 , 5 5 (1948). (9) C. R. Gochanour and M. D.Fayer, J . Phys. Chem. 85, 1989 (1981).
0022-3654/86/2090-2374$0 1.5010 0 1986 American Chemical Society
