J. Phys. Chem. 1988, 92, 5693-5696
5693
TABLE IV (Continued)
atom
orbital
-elm
-Hi,
ref
ref
C,
BP
' / 2 V ) ?
Ir
6s 7.584 11.36 41 9.06 27 5.299 8.64 5.73' 4.006 4.115 4.50 6P 5d 12.429 12.17 8.65' 10.734 11.12f Pt 6s 7.693 9.077 14 8.728 43 5.403 5.112 6P 4.129 5.475 5.209 4.062 3.892 5d 13.526 12.59 12.0 11.246 8.522 Au 6s 7.791 10.92 42 9.59 27 5.498 8.65 6P 4.142 5.55 4.76 4.112 6.59 5d 14.613 15.07 12.36 1 1.744 12.od Hg 6s 9.218 9.25 27 5.850 9.50 6P 4.930 5.55 4.503 6.82 5d 19.013 13.33 12.549 12.2d "All quantities are in electronvolts. bThesevalues were taken from the same reference given for the parameter C,. CForthese elements there are two different sets of values reported for H,.dThese values were extrapolated from neighboring atoms in the periodic table. .dr2sp. fdr's. *dx. TABLE V: Parameters for EHM and IEHM Calculations Obtained from a Least-Squares Fit to c , versus ~ 4 Data in the Form of Eq 2"vb
atom
orbital
CP
-Hli
7.655 7.6 (9.10) 7.10 4.079 3.8 (5.32) 3.71 10.820 9.2 (12.6) 9.39 Ru 5s 7.274 7.73 (10.4) 7.03 5P 4.000 4.44 (6.87) 4.05 4d 11.506 11.23 (14.9) 10.44 "All quantities are in electronvolts. bSee footnote c in Table IV. cThis work. Fe
4s 4P 3d
c,
sition-state KS calculation, one can also evaluate the parameters by calculating eim for several values of q and performing a least-squares fit to these data in the form of eq 2. We have found that this procedure does not change significantly the values for C,and 8, that result from eq 22 and 23; however, it improves the estimates of A,, as may be seen in Table V, where we ieport the results for Fe and Ru. Finally, it should be noted that although we have included in the tables the heavy elements for reasons of completeness, in those cases one should make use of the relativistic KS method5' to generate parameters for extended Huckel calculations. We have (57) Liberman, D.; Waber, J. T.; Cromer, D. T. Phys. Reu. 1965, 137, A27-A34.
BP
Bl
AP
A1
6.470 4.494 12.293 6.093 4.302 10.276
7.916 6.30 12.60 6.50 4.23 9.78
0.394 0.522 2.187 0.449 0.460 1.401
0.91 1 0.910 1.710 1.140 1.560 1.400
already initiated such calculations to obtain a complete set of parameters for the lanthanides and the actinides. It is our hope that a systematic set of parameters from a single theoretical source will be very useful for molecular calculations.
Acknowledgment. It is a great pleasure to thank Marcel0 Gglvan for many valuable discussions. We also thank Jost A. Chamizo and J. Mendieta for providing us the list of the usual values of the EH parameters and Prof. P. Pyykko for sending manuscripts prior to publication. This work was supported in part by the Secretaria de Educaci6n Wblica (PRONAES) and by CONACYT through the Programa de Fortalecimiento del Pas-grado Nacional.
Transformatlon of van der Waals Complexes to Exclmer and Exclplex in Jet-Cooled Fluorene and 9-Ethylfluorene Michiya Itoh* and Yasuo Morita Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: September 14, 1987; In Final Form: April 14, 1988)
Upon excitation of the van der Waals (vdW) dimer and heterodimer of fluorene (FR) and 9-ethylfluorene (EFR) generated in a supersonicexpansion, the excimer and exciplex fluorescence spectra and decay times were obtained. The spectral maximum and decay time of the exciplex (FR.EFR)* are almost intermediate between those of the two excimers of (RF),* and (EFR),*. The transformation of the vdW heterodimer to the exciplex takes place from the excited state of the heterodimer corresponding to the locally excited state of EFR, FR.EFR*.
Numerous investigations of steady-state and time-resolved fluorescence in the solution and static vapor phase have demonstrated that the excimer and exciplex, distinguished from the excited state of the charge-transfer complex formed in the ground state, are generated by the collision between the excited molecule and the ground-state species.' However, Saigusa and Itoh have the first Observation Of transformation Of the excited state of the van der Waals (vdW) complex to the exciplex, and the
remarkable vibrational energy dependence of the exciplex formation in the supersonic jet-cooled 1-cyanonaphthalene (14") and triethylamine (TEA).z3 Further, Saigusa et a1.4 have reported (1) (a) Kldpffer, W. In Organic Molecular Phorophysfcs;Birks, J . B.,Ed.; Wiley-Interscience: New York, 1973; Vol. 1. (b) Mataga, N. In The Exciplex; Gordon, M.. Ware, W. R., Eds.;Acadamic: New York, 1975. (c) Ware, W. R.; Watt, E.; Holmes, J. D.J. Am. Chem. Soc. 1974,96,7853. (d) Itoh, M.; Mimura, T.G e m . Phys. Lett. 1974, 24, 551.
0022-3654/88/2092-5693$01.50/0 0 1988 American Chemical Society
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the time-resolved fluorescence study of the transformation dynamics of the exciplex formation, which suggests the importance of the intermolecular vibration within the vdW complex to the exciplex formation. These investigations provide us with a new concept of the exciplex formation different from that of encounter collison dynamics in solution and static vapor phase. Castella et aLSv6and also Anner and H a a ~ ' .have ~ reported similar exciplex formation from the excited vdW complex of aromatic hydrocarbons such as anthracene and perylene and amines in the supersonic expansion. Recent supersonic jet spectroscopies have demonstrated a number of homo- and heterodimers of aromatic molecules such as benzene?,10toluene? and tetrazine." Two-color time of flight mass spectroscopy and laser-induced fluorescence indicate that the lifetimes of these dimers are considerably shorter than those of monomers. Hopkins et a1.I0 and also Law et al.9 have suggested that the shortening of fluorescence lifetimes in the dimer may be attributable to the excimer formation upon excitation of the dimer, though no excimer fluorescence was observed." Saigusa and Itoh have reported the excimer formation of fluorene from the vdW dimer of this compound formed in the supersonic expansion.I2 The direct transformation to the excimer has been suggested upon excitation of an origin band of the dimer. This paper describes the excimer and exciplex formations between 9-ethylfluorene (EFR) and fluorene (FR) in jet-cooled conditions. The fluoresence spectra and decay times of the excimer and exciplex were observed upon excitation of the vdW dimer and heterodimer. It is noteworthy that the exciplex was generated from the excited state of the heterodimer corresponding to the locally excited state of EFR, FR-EFR*. However, no significant evidence of the exciplex formation from the locally excited state of FR, FR*-EFR, can be presented at the present stage.
Itoh and Morita
a FR
LL 2'
29,
i1 A/,,,,,
2%
EFR
296
A
d
290
300
Figure 1. (a) Fluorescence excitation spectra of jet-cooled FR at -80 O C (a reservoir temperature) monitored at >320 nm and (b) monitored at 350-400 nm at 100 OC. (c) Fluorescence excitation spectra of jetcooled EFR at -80 O C (a reservoir temperature) monitored at >320 nm (a band at 296.06 nm shows a trace amount of FR contaminated) and (d) monitored at 350-400 nm at -100 "C of reservoir temperature.
Experimental Section Fluorene (Tokyo Kasei, zone refined) and EFR (Aldrich) were used without further purification. The supersonic jet apparatus and procedures employed in the present study have been described in previous papers.24 FR and EFR were contained in a stainless steel container placed in a nozzle chamber and heated (To= 50-100 "C). The dimer and heterodimer of these compounds were prepared in supersonic expansion of He carrier gas at 4 atm. The frequency-doubled output of an excimer laser (Lambda Phys EMG 53 MSC, XeCI) pumped dye laser (Molectron DL 14) crossed the jet expansion at 7-10 mm downstream from the nozzle hole. The fluorescence from the vibronic levels of SI was collected through a monochromator (Nikon G250) or appropriate filter combination (Toshiba D36B and UV32). The excimer and exciplex fluorescence was also collected through a monochromator or various filter combinations, as will be mentioned later. The output of a photomultiplier (Hamamstsu R928) was processed by a boxcar integrater (PAR M162/164) and a personal computer. The fluorescence decay curves were obtained by using a digital oscilloscope (Tektronix 2430) and analyzed by a computer-simulated convolution.
(2) Saigusa, H.; Itoh, M. Chem. Phys. Lett. 1984, 106, 391. (3) Saigusa, H.; Itoh, M. J . Chem. Phys. 1984, 81, 5682. (4) Saigusa, H.; Itoh, M.; Baba, M.; Hanazaki, I. J . Chem. Phys. 1987, 86, 2588. (5) Castella, M.; Prcchorow, J.; Tramer, A. J. Chem. Phys. 1984,81,2511. (6) Castella, M.; Tramer, A,; Piuzzi, F. Chem. Phys. Lett. 1986, 129, 105, 112. (7) Anner, 0.;Haas, Y . Chem. Phys. Lett. 1985, 119, 199. (8) Anner, 0.;Zarura, E.; Haas, Y . Chem. Phys. Lett. 1987, 137, 121. (9) (a) Law, K.S.;Schauer, M.; Bernstein, E. R. J . Chem. Phys. 1985, 81,4871. (b) Schauer, M.; Bernstein, E. R.J. Chem. Phys. 1985,82, 3722. (10) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Phys. Chem. 1981, 85, 3739. (11) Haymann, C. A,; Brumbaugh, D. V.; Levy, D. H. J . Chem. Phys. 1983, 79, 1581. (12) Saigusa, H.; Itoh, M. J . Phys. Chem. 1985, 89, 5486. (13) Horrocks, D. L.; Brown, W. D. Chem. Phys. Lett. 1970, 5, 117. (14) Minn,F. L.; Pinion, J. P.; Filipescue, N. J . Phys. Chem. 1971, 75, 1794.
300
h/nm
350
4 00
Figure 2. Dispersed fluorescence spectra of the EFR and FR mixed free jet obtained by the excitations of the EFR dimer (a) and heterodimer (b).
Results and Discussion Excimer Formations in Fluorene and 9-Ethyljluorene. Figure 1 shows fluorescence excitation spectra of EFR and FR in supersonic expansion. The origin and main vibronic absorption bands of EFR are shifted approximately 2 nm to the red compared with those of FR,I2 as seen in Figure 1. These excitation spectra are attributable to the S1vibronic bands of EFR and FR monomers. The rather complicated bands of EFR in addition to the main bands may be ascribed to vibration of the 9-ethyl group. As mentioned in the Introduction, the previous paper reported the dispersed fluorescence (330-380 nm) due to the excimer of FR, which was observed at the sample reservoir temperature of =lo0 'C. When the concentrations of EFR are increased in the pulsed nozzle by raising the temperature of the sample reservoir, the dispersed fluorescence was also observed at & = 360 nm. Since the dispersed fluorescence is similar to the excimer fluorescence observed in the jet-cooled FR, the spectrum shown in Figure 2 may be attributed to the excimer of EFR. The excitation spectra of the excimer fluorescence monitored at 350-400 nm are vibrationally resolved and red-shifted only = 2 nm from the corresponding monomer bands, as seen in Figure 1 ( b and d). Therefore, it seems that the excimer is not generated from the excited monomer but from a weak ground-state van der Waals complex. Since the intensity ratios of these vibronic bands in the excitation spectra of the complex are not very dependent on the reservoir temperature, the vibronic bands red-shifted from the monomer bands may be attributable to the dimer, not to the polymer or cluster, as mentioned in the previous paper.I2 Therefore, the vibronic band of (EFR)* a t 300.12 nm (33 321 cm-l) in the longest wavelength may be ascribed to the 0; band of the dimer (EFR)2.
Jet-Cooled Fluorene and 9-Ethylfluorene Exciplex
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5695
R 100
la) IO
IJ I
1
0
I
200
I
t/ch
400
R 1oc 299.5
A Inm
300.0
3005
Figure 4. Fluorescence excitation spectra of the FR and EFR ( 5 1 ) mixed free jet monitored at 350-400 nm: (a) at 100 OC, (b) 90 OC, and (c) 80 O C (a reservoir temperature).
(b) 10 H
c
C
8 1
nm) and EFR (300.12 nm) dimers. Since the fluorescence was considerably weak in intensity, the appropriate filter combinations were used for detecting fluorescence time profiles. The obtained decay and the computer-simulated curves based on the double exponential are shown in Figure 3. Decay curves of two monitoring wavelength regions at 330-390 nm (a half-width) and at >350 nm upon excitation of the F R dimer afford almost same decay times of 12 and 70 ns. The shorter decay (12 ns) component amounts to 0.65 in the former detection wavelength, while that at the longer wavelength is 0.35. Then the short and long decay times are attributable to the vdW dimer and the excimer of FR, respectively. Figure 3 also exhibits the fluorescence decays and the simulated curves of EFR dimer. The fluorescence decay times of the dimer and excimer of EFR were obtained to be 9.8 and 95-98 ns, respectively, though the short decay times have less accuracy because of the limitation of the nanosecond spectroscopy. The decay curve in Figure 3b monitored at 322-333 nm (fwhm, 10 nm) exhibits a shorter decay component of 71% attributable to the vdW dimer of EFR. Exciplex Formation between FR and EFR. Taking account of the excimer formation from the vdW dimer of F R and EFR in supersonic expansion, the exciplex fluorescence was expected from the heterodimer of FR and EFR. The fluorescence excitation spectrum of a low concentration in the free jet of the FR and EFR mixed system consists of superimposed spectra of each component molecule. When the reservoir temperature containing the two compounds was increased to 100 OC,rather complicated excitation spectra were observed by monitoring the long-wavelength fluorescence (350-400 nm). The main feature of the excitation spectra consists of the F R and EFR dimers. The near-300-nm region of the spectrum corresponds to the dimer band of EFR, while the 298-nm region to that of FR dimer. Figure 4 exhibits the excitation spectra of the EFR dimer band region monitored at 350-400 nm in the mixed system. When the reservoir temperature was decreased gradually, the EFR dimer band somewhat decreased in intensity, while a strong band peak appeared at 299.94 nm (33 340 cm-') and increased in intensity as shown in Figure 4c, which is blue-shifted 21 cm-' from the origin band of EFR dimer. Since vapor pressure of FR is considerably higher than that of EFR, relative vapor pressure of FR to EFR increased at the reduced reservoir temperature. Therefore, the relative concentration of FR increased compared with that of EFR at the reduced temperature. As a result, the EFR dimer band decreases in intensity, while the heterodimer band may probably increase. Since the blue-shifted band at 299.93 nm (33341 cm-l) was observed in the mixed system, the band may be assigned to the heterodimer of these compounds (Table I). This blue-shifted band of the excitation spectrum of the heterodimer seems to correspond to the locally excited state of EFR, EFR*.FR. Taking account of these arguments, the red-shifted band of the heterodimer at-
-
Ill
0
I
I
200
I
t/ch,
I
400
Figure 3. Fluorescence decay curves of the FR and EFR mixed free jet and computer simulated on@ based on the double exponential with decay times described in the figure. Excitation wavelengths correspond to the origin bands of (a) the FR (298.02 nm) and (b) EFR (300.12 nm) dimers, and (c) the heterodimer (299.93 nm). The detection wavelengths described in th figure were obtained by using the following filter combinations: Toshiba 36B and Corning 0-52 (350-380 nm); Toshiba D33S and Corning 7-51 (330-385 5nm); Andover 3261: (322-333 nm); Corning 0-52 (350 nm). The errors of obtained decay times are approximately f 2 ns.
The dispersed fluorescence spectrum of the jet-cooled EFR by the origin band excitation of the EFR dimer is shown in Figure 2. The excitation of this band might afford a fluoresence spectrum due to the dimer at 300-350 nm. However, a large Stokes-shifted fluorescence was observed in addition to a weak short wavelength fluorescence. If the former intense fluorescence at 350-400 nm and the latter weak one a t 300-350 nm are attributable to the excimer and the locally excited dimer of EFR, respectively, the excimer may be generated from the excited state of the dimer (vdW complex). If this is the case, the fact implies that the transformation rate of the dimer to the excimer is faster than the fluorescent decay of the dimer or comparable, as mentioned previous1y.l2 The fluorescence decays of FR and EFR were observed by the excitation of the respective origin bands of the monomer and dimer. Fluorescence decay curves of monomer exhibit single-exponential decays with decay times of 14.3 f 0.5 ns for FR (excitation wavelength 296.06 nm) and 14.7 f 0.5 ns for EFR (298.06 nm). The fluorescence decay curves of the excimer and the vdW dimer were observed upon excitations of the origin bands of FR (297.49
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The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
TABLE I: Excitation Energies (cm-') of the Origin Bands of FR, EFR, and Their Dimers/Heterodimer, and the Fluorescence Lifetimes (ns) of the Excimer/Exciplex'
monomer dimer excimer/exciplex lifetime, 7,ns
FR 33777 33555 70
EFR 33550 33320 98
EFReFR 33341 78
Errors of excitation energy and fluorescence lifetime are approximately f 2 cm-' and f 2 ns, respectively. tributable to the local excitation of FR, EFR-FR*, might be observed in the FR dimer band region. However, no significant red-shifted band corresponding to EFR.FR* was observed at 298.10-298.48 nm. It is probably because the rather strong vibronic band of the EFR dimer was observed in this region, as seen in Figure Id. In order to detect the EFR.FR* heterodimer band in the (FR),* band region, a mixed jet composed of highconcentration EFR and low-concentration FR is required. However, it is very difficult to obtain this experimental condition, since the former has a lower vapor pressure than the latter. Figure 2 shows the fluorescence spectra of the FR and EFR mixed expansion by the excitations of the origin bands of the EFR dimer (300.12 nm, 33 321 cm-I) and of the heterodimer (299.93 nm, 33 341 cm-I). The latter spectrum tentatively ascribed to the exciplex fluorescence between EFR and F R is so similar to the former EFR excimer that both spectra are almost indistinguishable from each other. However, the exciplex fluorescence decay time was obtained to be 78 ns from the deconvolution analysis of the double-exponential decay curves shown in Figure 3. On analogy of the argument on the fluorescence decays of FR and EFR dimer, the short decay components of approximately 8.1 ns in the shorter wavelength region (330-350 nm) were ascribed to the heterodimer (EFR*.FR). In the previous paper, we proposed a simple model for the exciplex formation reaction induced by excitation of a weak vdW complex in the 1-CNN and TEA system. Upon excitation of the complex, conformational changes occur to form the exciplex. The model includes various cases depending on the magnitudes of the transformation rate constatns between the vdW complex and the exciplex and their relaxation rate constants. In the case of the FR and EFR dimers or heterodimer which exhibit no significant excess energy dependence of the exciplex/excimer formation, the temporal behavior of the vdW dimer and exciplex/excimer fluorescence may be expressed as follows IvdW(t)= Ae-'Irl
+ Be-'Ir2
where A , B, and C are preexponential factors and 7,and r 2 are decay times. If the short (Q = -10 ns) and long (7,= 70-100 ns) decay times are normally ascribed to the vdW complex and excimer/exciplex, respectively, the excimer/exciplex fluorescence
Itoh and Morita may exhibit a fluorescence rise in the longest wavelength region. However, no significant rise of the excimer/exciplex was detected in the EFR/FR system, though the fluorescence rise of the exciplex was observed in the 1-CNN/TEA system, as reported in the previous paper.4 Since the energy difference of fluorescence maxima between the vdW complex and the exciplex is much smaller in EFR/FR (4000 cm-I) than in 1-CNN/TEA (8000 cm-I), the excimer/exciplex fluorescence is considerably overlapped with that of the complex, as seen in Figure 2. Therefore, the rise of excimer/exciplex fluorescence cannot be detected in the time profiles of their fluorescence. As mentioned in the introductory section, we have reported the exciplex formation from vdW complex between 1-CNN and TEA and a remarkable excitation energy dependence of the transformation." The origin band excitation of the complex yields only resonance fluorescence of the complex, while the higher vibronic band excitation, especially the intermolecular vibrational one, gives predominantly exciplex fluorescence. The transformation of the vdW complex to the exciplex exhibiting a mode specificity and excess energy dependence suggests that the transformation takes place through a specific vibrational mode and/or the randomization of excess energy among intermolecular modes assessible to the geometrical rearrangement. This paper has demonstrated the excimer/exciplex formation from the vdW dimer in the jetcooled EFR and F R system. The striking difference of the EFR/FR exciplex from 1-CNN/TEA is that there is no significant excess energy dependence of the exciplex formation. As seen in Figures 1 and 4, the vdW dimer bands of EFR and FR (probably also the heterodimer bands) exhibit complicated vibrational structures (6-10 cm-I). If these are attributable to the intermolecular vibrations coupled with the intramolecular ones, it is likely that the intermolecular vibrations in the vdW dimer/ heterodimer play an important role in the transformation to the excimer/exciplex. The band shift of the origin band of the dimer from that of the monomer is 222 cm-' for FR2 and 230 cm-I for EFRz, while that of the heterodimer (EFR-FR) is 209 cm-I from that of EFR monomer. These band shifts suggest a similar electronic and geometrical structures of the heterodimer to the others. Law, Schauer, and Bernstein suggested two distinct conformations of the benzene dimer and benzene-toluene complex: parallel displaced and perpendicular geometries. The molecules in the former were suggested to rearrange into the excimer/exciplex configuration, though no excimer fluorescence was observed. In the F R or EFR dimer and heterodimer reported here, a parallel displaced conformation seems most accessible to the excimer and exciplex, if the structure of exciplex is assumed to be parallel sandwich conformation between aromatic molecules.
Acknowledgment. This work was supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
