5486
J. Phys. Chem. 1985, 89, 5486-5488
cis-2-butene cation does not take place, but the 1-butene cation becomes the cis-2-butene cation exclusively. Isobutene cations react with the neutral isobutene molecule to produce both the 2-methylallyl radical and the propagating cation radicals in the polymerization. The ratio of the two products depends upon the
matrix and the temperature. Registry No. I-Butene cation, 34467-39-5; cis-2-butene cation, 34526-43-7; isobutene cation, 34526-44-8; trans-2-butene, 34526-42-6; cis-1-methylallyl radical, 98705-00-1; trans-I-methylally1 radical, 17787-86-9; 2-methylallyl radical, 15157-95-6.
Dimer-Excimer Transformatlon of Fluorene in a Supersonic Expansion Hiroyuki Saigusa* and Michiya Itoh* Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: June 26, 1985)
Upon excitation of a ground-state dimer of fluorene generated in a supersonicexpansion, we have observed excimer fluorescence. The transformation of the dimer to the excimer occurs at the dimer 0; band. The excimer emission, peaking at 360 nm, is strongly red-shifted with respect to fluorene-type emissions. Excimer formation for a van der Waals complex of the fluorene dimer with p-xylene is also reported.
Introduction Many of aromatic molecules are known to form excimers in concentrated solutions,' these excimers being observed as strongly red-shifted bands in the fluorescence spectra. Excimers have been described as dimers which are unbound in their ground state but rather tightly bound in their excited electronic state. Although numerous investigations on the kinetic and thermodynamic properties of these states have been performed in solutions, no excimer emission has been observed in the vapor phase. Studies of concentration quenching have only suggested2 that the quenching occurs via an excimer, for which there is no direct evidence. Recent progress in supersonic jet spectroscopy has been applied to the study of electronically excited aromatic dimers or van der Waals (vdW) clusters. Levy, Haynam, and Brumbaugh have studied tetrazine,3 dimethyltetra~ine,~ and tetrazine-benzene5 dimers by the technique of laser-induced fluorescence spectroscopy and obtained spectroscopic data on the structure of these dimers. Two-color time-of-flight mass spectroscopy studies have been reported for benzene6q7and benzene-toluene7 dimers. By this method, the SI lifetime of the benzene dimer has appeared to be very short as compared to that of the monomer. The shortening of the lifetime has been attributed to excimer formation upon e x c i t a t i ~ n . ~In. ~addition, recent reports from our laboratory have demonstrated* that excitations of ground-state vdW complexes of cyanonaphthalenes with alkylamines lead to the formation of exciplexes. These open up the possibility that an excimer fluorescence is observed upon excitation of a ground-state dimer. However, no excimer fluorescence has been observed by excitation of the benzene dimer owing to the abnormally low fluorescence quantum ~ i e l d . ~ . ~ (1) J. B. Birks, "Photophysics of Aromatic Molecules", Wiley, New York, 1970. (2) (a) A. Davis, M. J. Pilling, and M. J. Westby, Chem. Phys., 63, 209 (1981). (b) B. Stevens, M. S. Walker, and E. Hutton, Proc. Chem. Soc., 62 (1963). (c) W. R. Ware and P. T. Cunningham, J . Chem. Phys., 43, 3826 (1965). (3) C. A. Haynam, D. V. Brumbaugh, and D. H. Levy, J. Chem. Phys., 79, 1581 (1983). (4) C. A. Haynam, D. V. Brumbaugh, and D. H. Levy, J. Chem. Phys., 81, 2282 (1984). (5) D. H. Levy, C. A. Haynam, and D. V. Brumbaugh, Faraday Discuss. Chem. Soc., 73, 137 (1982). (6) J. B. Hopkins, D. E. Powers, and R. E. Smalley, J . Phys. Chem., 85, 3739 (1981). (7) (a) K. S. Law, M. Schauer, and E. R. Bernstein, J. Chem. Phys., 81, 4871 (1984). (b) M. Schauer and E. R. Bernstein, Ibid., 82, 3722 (1985). (8) (a) H. Saigusa and M. Itoh, Chem. Phys. Leu., 106, 391 (1984). (b) H. Saigusa and M. Itoh, J . Chem. Phys., 81, 5692 (1984).
0022-3654/85/2089-5486$01.50/0
This paper reports the direct observation of excimer fluorescence of fluorene (FR) in a supersonic expansion. The FR molecule was chosen for our initial experiments on excimer formation because (a) FR is known to form excimers in concentrated solutions;IoJ' (b) a model for the excimer configuration has been proposed;" and (c) extensive studies on the solvation of FR by aromatic molecules have been reported in supersonic jets by Even and Jortner.I2 The present results confirm that the transformation of the ground-state FR dimer to the excimer takes place upon excitation of the origin band of the dimer. A preliminary study on the solvation of FR and FR dimer by p-xylene (PX) is also reported. Solvation studies on the FR dimer will provide a unique opportunity to pursue the relationship between excimer formation in fluid solutions and dimer-excimer transformation in a supersonic expansion. Experimental Section The supersonic jet apparatus employed in this work has been described in detail previously.* Information specific to the present study only will be presented. FR dimers were prepared in supersonic expansions of FR and helium. Zone-refined FR crystals were contained in a stainless steel sample container, placed in a nozzle chamber, and heated (To = 5C-105 "C). For the solvation studies, liquid PX was placed in metal tubing, which was attached to the front of the nozzle chamber. The carrier gas at 4 atm was passed through the tubing. The concentration of solvent vapor was varied by changing the temperature T, of the tubing. The resulting mixture was expanded through a 100-pm pinhole. Extreme care was taken to ensure that no impurities or thermal reaction products were observed in the FR sample. The doubled output of a N 2 laser pumped dye laser crossed the jet expansion at a distance of 7 mm downstream from the pinhole. Results FR Dimer Spectra. Figure 1 shows the fluorescence excitation spectrum of FR. The spectrum was taken with the FR sample held at To = 80 O C and a total stagnation pressure of 4 atm. The spectral features have been attributed to the SI vibronic bands of the FR m0n0mer.l~ The strongest feature at 33 777 cm-' is (9) P. R. R. Langridge-Smith, D. V. Brumbaugh, C. A. Haynam, and D. H. Levy, J . Phys. Chem., 85, 3742 (1981). (IO) D. L. Horrocks and W. G. Brown, Chem. Phys. Lett., 5 , 117 (1970). (1 1) F. L. Minn, J. P. Pinion, and N. Filipescu, J . Phys. Chem., 75, 1794 (1 97 1). (12) U. Even and J. Jortner, J . Chem. Phys., 78, 3445 (1983)
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5481
Dimer-Excimer Transformation of Fluorene
I
O,"
1
288
290
292
WA VELENGTH/nm
294
296
298
Figure 1. Fluorescence excitation spectrum of the fluorene monomer in helium at To= 80 OC,where To is the temperature of the fluorene sample
Figure 3. Dispersed fluorescence spectrum obtained by exciting the dimer To= 100 OC. The sharp feature marked with an asterisk contains large contributions from scattered laser light. The spectrum was
container.
taken with a monochromator resolution of SO cm-' (at 330 nm).
0; transition at
1
295
296
WA VELENGTH/nm
297
298
Figure 2. Fluorescence excitation spectra of the fluorene dimer at (a) To= 80 "C and (b) To = 102 OC. These spectra were obtained when emissions were detected at 380 nm with a monochromator resolution of S nm. Even at this detection wavelength, the monomer 0; peak can be readily observed since the emission arising from the 0; band is very strong. The upper and lower traces are normalized to produce the same intensity at the monomer 0; band.
the SI electronic origin band of the monomer. When the emission was dispersed with a filter or a monochromator and only a longer wavelength emission was detected, we have observed, to the red of the monomer 0; band, complicated spectral features. With increasing temperature To of FR, the intensities of these spectral features grew more rapidly than those of the monomer features. Figure 2 compares the fluorescence excitation spectra taken with different concentrations of FR, the spectra being obtained when emissions were detected at 380 nm. For the lower trace the FR sample was held at 80 O C and for the upper trace the sample was at 102 O C . The upper and lower traces were normalized to produce the same intensity at the monomer 0; band. The relative intensity of these red-shifted features to the monomer 0; band greatly depends on the FR concentration, and these features are assigned to a FR dimer. The fact that the intensity distribution of the red-shifted features is less dependent on the FR concentration indicates that these are not associated with mixtures of the dimer and higher polymers of FR. The lowest energy peak of the dimer displaced 227 cm-I to the red of the monomer 0; band is tentatively assigned to the electronic origin of the dimer. The rich and complicated spectrum of the dimer is presumably due to intermolecular vibrational progressions built on several conformational isomers of the dimer. The complexity of the spectrum precludes any definitive analysis of the intermolecular vibrational modes. Another group of spectral features in the proximity of the monomer 0; band can be analyzed as the a l vibronic band of the FR dimer. (13) A. Amirav, U. Even, and J. Jortner, Chem. Phys., 67, 1 (1982).
236
297 298 WA VELENGTH/nm
299
Figure 4. Fluorescence excitation spectrum of a mixture of fluorene and p-xylene at .To= 80 OC and T, = 0 OC,where T, is the temperature of the p-xylene reservoir. The monomer 0; band at -296 nm is severely truncated. The actual intensity ratio of the 0; band of the F R / P X complex to that of the monomer is 1/4.
-
The most important observation in the dimer spectra is that excitations of these dimer peaks produce a broad and large Stokes-shifted emission. In Figure 3 we show the dispersed fluorescence spectrum obtained by exciting the lowest energy feature of the dimer. The emission observed has been attributed to an excimer emission. The intensity maximum is at 360 nm, while the FR excimer in solution exhibits an emission peaking at 370 nm.I0 Therefore, we conclude that an effective transformation of the dimer to the excimer takes place at the 0; band. F R I P X Complex Spectra. The fluorescence excitation spectrum of a mixture of FR and PX is shown in Figure 4. The temperature of FR, To, was kept at 80 "C, while that of PX, T,, was at 0 O C . To the red of the FR 0; band we observe weaker features due to a vdW complex of FR bound to PX. Furthermore, these peaks have a doublet structure with a splitting of -2.0 cm-'. The origin of the doublet cannot presently be accounted for. The lowest energy features are red-shifted with respect to the origin of the FR monomer by 345 and 343 cm-' and assigned to the electronic origin of the fluorenelp-xylene (FR/PX) 1:1 complex. Much weaker features due to FR/(PX), complexes are observed to the red of the FR/PX 1:l complex. The lowest energy peak is displaced 852 cm-I to the red of the monomer 0; band.I4 Increasing the concentration of FR, that of PX being kept constant, produces mixed complexes of the FR dimer with PX. In Figure 5 we show the fluorescence excitation spectrum of the (FR),/PX mixed complex obtained while excimer emissions are monitored at 380 nm. The FR sample was kept at To= 102 O C while the PX sample was at T, = 0 O C . When FR type emissions were detected or when no PX was added, these features were not (14) H. Saigusa and M. Itoh, to be submitted for publication.
5488 The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
ii
297
298
P
299
300
WA VELENG TH/nn
301
302
303
Figure 5. Fluorescence excitation spectrum of a mixture of fluorene and p-xylene at To= 80 OC and T, = 0 OC. The spectrum was obtained when emissions were detected at 380 nm with a monochromator resolution of 5 nm. Features marked D, PI, and P2 are assigned to the fluorene dimer, the fluorene/p-xylene 1:1 complex, and the fluorenelp-xylene 1 :2 complex, respectively. TABLE I: Relative Energies of the 0; and a1 Transitions of Fluorene Clusters‘ 0:
FR clusters FR (FRh (FR)I/(PX)l (FR),/(PX)I (FR),/(PX),
a1
transition energy
shifts
33 777 33550 33552 33432 33434 32926 33 137
0 -227 -225 -345 -343 -852 -641
(O:)*
transition energy
shifts
33983 33759
(al)* 206 -19
33635 33637
-143 -141
shiftsC 206 209 207 202 202
33340
-437
204
“All values are in cm-’. bShifts relative to the FR monomer 0; band at 33 777 cm-’. CShiftsrelative to the 0: bands: shift (al) - shift (0;).
observed. In addition to sharp spectral features, there are also broad underlying background features. The spectral red shift of the lowest energy peak is -640 cm-’ with respect to the FR monomer 0; band and is -410 cm-I with respect to the FR dimer 0.; The relative energies of the 0; and a l bands of the FR/PX complexes together with those of the FR dimer are listed in Table I. The most significant observation is that the excitation of the ground-state (FR),/PX vdW complex also results in the formation of an excimer. The fluorescence maximum (-360 nm) of the excimer is not sensitive to solvation by PX. More red-shifted weaker but sharp features with respect to the origin of the (FR),/PX complex, which can be found in Figure 5 , are readily assigned as the FR/(PX), complex described above.
Discussion The lifetime of the electronic origin of the benzene, dimer has been found to be -40 n ~ while ~ 9that~of the ~ monomer is 103 ns.I5 The shortening of the lifetime of the dimer has been explained as due to the rearrangement of the dimer to an excimer upon e x ~ i t a t i o n . ~ .However, ’~ owing to the abnormally low quantum yield for the dimer, previous laser-induced fluorescence experiments have failed to observe an excimer fluorescence of benzene. We have obtained, upon excitation of the ground state FR dimer, an intense excimer emission of FR. In addition, FR-type emissions arising from the locally excited dimer state are found to be very weak, implying that the transformation rate of the dimer to the excimer is much faster than the fluorescence rate from the dimer state. Benzene is known to form a sandwiched excimer, (15) S. M. Beck, M. G . Liverman, D. L. Monts, and R. E. Smalley, J . Chem. Phys., 70, 232 (1979).
Saigusa and Itoh one molecule being placed above the other in a parallel configuration in which both principal axes coincide.16 Thus, the transition from the excimer state to the ground state is symmetry forbidden and the radiative rate is expected to be quite low. For the excimer of FR, a parallel displaced configuration, two FR molecules being atop one another but displaced from perfect superposition, has been assumed.” The calculated oscillator strength cf < predicts that the excimer emission should be weak. The proposed model cannot account for the observation of the radiative excimer and has to be reexamined. A second striking difference between the benzene and FR dimers is the appearance of vdW modes. The absorption spectrum of the benzene dimer obtained by the mass-selected two-color photoionization technique exhibits only one ~ e a k ~ while . ’ ~ the fluorescence excitation spectrum of the FR dimer (shown in Figure 2) consists of many progressions due to vdW vibrational modes. The strong activity of vdW progression in the FR spectra suggests that the geometry of the SI state of the dimer is somewhat different from that of the ground-state dimer. After absorption to a single vdW state of the dimer, the system relaxes to a dense manifold of isoenergetic excimer states and the dimer emits as an excimer. The sharp absorption features of the dimer with large FranckCondon overlap for the low-lying vdW modes are not characteristic of direct absorption to high-lying vibrational states of the excimer potential surface. To gain more information concerning the suggested dimerexcimer transition of FR, we have carried out a time-resolved study of the excimer fluorescence.14 A preliminary result reveals that the lifetime of the excimer fluorescence is -60 ns which is much longer than that of the monomer 0; band (
