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J . Phys. Chem. 1990, 94, 6544-6549
van der Waals Complex and Exciplex Formations between Methyl-Substituted 1-Cyanonaphthalene and Triethylamine in a Supersonic Free Jet Michiya Itoh* and Mamoru Sasaki Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: August 21. 1989; In Final Form: March 13, 1990)
The van der Waals (vdW) complex and exciplex formation were investigated in 2-, 4-, and 6-methyl-substituted l-cyanonaphthalenes (2-, 4-, and 6-MCNN) and triethylamine (TEA) in the supersonic expansion. The vdW complex of 6-MCNN and TEA transforms to the exciplex in the excitation of vibronic bands of the complex, while no exciplex formation is detected in the origin band excitation of the complex. Two types of vdW complexes between 4-MCNN and TEA were observed. One of these vdW complexes exhibits sharp satellite bands blue-shifted (1 3 cm-I) from the origin and vibronic bands. Another complex of 4-MCNN/TEA reveals dispersed and red-shifted excitation spectra attributable to the intermolecular vibration. The former complex shows the excess energy dependence of the exciplex formation similar to that in 6-MCNN/TEA, while the latter complex exhibits the exciplex fluorescence even in the excitation of the origin band region. The former and latter vdW complexes seem to have different geometrical structures between 4-MCNN and TEA, though almost identical exciplex may be generated from these complexes. The excitation of the diffused origin band region of the vdW complex between 2-MCNN and TEA affords the exciplex fluorescence. The transformation of vdW complexes between methyl-substituted I-cyanonaphthalenesand TEA to the exciplex will be discussed in terms of the role of methyl substitution on I-cyanonaphthalene to vdW complex formation with TEA.
Introduction The jet-cooled exciplex formation from the van der Waals (vdW) complex as a weak ground-state complex generated in the supersonic free jet is distinguished from the collisional dynamics of the exciplex formation both in the liquid and static vapor phase, which is generally agreed to be a consequence of collision between the excited-state and ground-state molecules through an encounter complex formation.' We have reported the first observation of exciplex formation in a jet-cooled vdW complex in I-cyanonaphthalene ( 1-CNN) and triethylamine (TEA) and remarkable vibrational excess energy dependence of the transformation of the vdW complex to the exciplex.2 Further, Saigusa et al. have suggested the importance of intermolecular vibration within the vdW complex to the transformation dynamics by a nanosecond time-resolved fluorescence study.3 On the other hand, Castella et aL4+ and Anner and H a a ~ have ~ - ~reported several jet-cooled vdW complexes of aromatic hydrocarbons such as anthracene and perylene and amines leading to exciplex formation in the singlet excited state. In some cases, it has been found that an excess energy of excitation is required for transition from the excited vdW to the exciplex ~ t a t e . ~In. the ~ ~ exciplex formation of the jet-cooled molecules, excited-state transformation of the vdW complex to the exciplex is the major problem of this research field. I n the transformation of the vdW complex to the exciplex, several crucial questions arise: How does the initial inter- and intramolecular energy in the vdW complex flow to the active and inactive modes leading to the exciplex formation.? How does charge transfer, accompanied with the geometrical rearrangement, take place from the active mode of the locally excited state of the vdW complex to the exciplex? Are these dynamics processes are really mode specific or are they tati is tical?^ On the basis of these ( I ) (a) Klopffer, W. In Orgunir Molecular Phorophysics; Birks, J. B., Ed.: Wiley-Interscience: New York, 1973; Vol. I . (b) Mataga, N. In The E x ciplex; Gordon, M.,Ware, W. R., Eds.; Academic Press, New York, 1975. ( c ) Ware, W. R.; Watt, E.; Holmes, J . D. J . Am. Chem. Sor. 1974, 96, 7853. (d) Itoh, M.; Mimura, T. Chem. Phys. Letr. 1974, 24, 551. (e) Itoh, M. J . Am. Chem. SOC.1974, 96, 7390. (2) Saigusa, H.: Itoh. M. J. Chem. Phys. 1984,81,5692; Chem. Phys. Leu. 1984, 106, 391 (3) Saigusa. H.; Itoh. M.; Baba, M.; Hanazaki, I . J. Chem. Phys. 1987, 86, 2588. (4) Castella, M.; Prochorow, J.; Tramer, A. J. Chem. Phys. 1984.81.251 I . ( 5 ) Castella, M.; Tramer, A.; Piuzzi, F. Chem. Phys. Leu. 1986, 129, 105. (6) Castella, M.; Tramer. A.; Piuzzi, F. Chem. Phys. Len. 1986, 129, 112. ( 7 ) (a) Anner, 0.;Haas, Y. Chem. Phys. Lerr. 1985, 119, 199. (b) Anner, 0.:Zaura, E.; Haas, Y. Chem. Phys. Letr. 1987, 137, 121. (8) Anner, 0.;Haas, T. J . Phys. Chem. 1986, 90, 4298. ( 9 ) Anner, 0.: Haas, Y. J. Am. Chem. SOC.1988, 110. 1416.
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considerations, this study presents vdW complex formation and its transformation to the exciplex in the 2-, 4-, and 6-methylsubstituted I-cyanonaphthalenes (2-MCNN, 4-MCNN, and 6-MCNN) with triethylamine (TEA) in a supersonic free jet. In these methyl-substituted I-cyanonaphthalene/TEA systems, both vdW complex and exciplex fluorescence were observed. Sharp origin and vibronic bands due to the vdW complex between 6MCNN and TEA were observed. The excess vibrational energy dependence of the exciplex formation in the 6-MCNN/TEA complex was observed, which is similar to that in the 1-CNN/TEA system reported previously, while the 2-MCNN and TEA system shows no excess energy dependence of the exciplex formation. In the 4-MCNN/TEA system, two types of vdW complexes were observed.I0 One of these complexes exhibits sharp satellite bands red-shifted from the origin and vibronic bands of 4-MCNN. The exciplex formation from this complex exhibits excess energy dependence and is similar to those in the 6-MCNN/TEA and l CNN/TEA systems. I n another vdW complex in 4-MCNN/ TEA, however, the fluorescence excitation spectra with many vibrational structures were observed, and excitation of the origin band region of this complex leads to the exciplex fluorescence, indicating no significant excess vibrational energy dependence of the exciplex formation. These results on the transformation of the vdW complex to the exciplex in three MCNN/TEA systems will be discussed in terms of the role of methyl substitution in naphthalene on the structure of the vdW complex and on the vibrational energy redistribution, which seems to affect the level crossing of the locally excited states to the charge-transfer state (exciplex).
Experimental Section Fluorescence excitation and dispersed fluorescence spectra in a supersonic free jet were measured with use of the same method '-I2 The gas and procedures as those described mixture of 3-7 Torr of TEA and 1.5-2 atm of He in a 3-L stainless steel bomb was introduced into a MCNN reservoir leading to a pulse nozzle. The fluorescence excitation spectra were measured ( I O ) In the previous paper (Itoh, M.; Sasaki, M. Chem. Phys. Left. 1988, 149. 40). the weak band red-shifted 78 cm-I from the origin was assigned as
arising from the different conformer o f 4-MCNN. However, the band was suggested to be attributable to the dimer of this compound by the vapor pressure dependence (4-MCNN) of intensity ratio of this band to the origin (T. Ebata. M. Ito. and M. Itoh, Japanese Symposium of Molecular Structure and Spectra. Sapporo, Sept 1989, J. Phys. Chem., submitted for publication. ( 1 I ) (a) Itoh, M.; Morita. Y . J . Phys. Chem. 1988, 92, 5693. (b) Saigusa, H . ; Itoh, M. J . Phys. Chem. 1985, 89, 6486. (12) Itoh, M.; Hayashi, A. J . Phys. Chem. 1989, 93, 7789.
0 1990 American Chemical Society
vdW Complex and Exciplex in a Supersonic Free Jet
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The Journal of Physical Chemistry, Vol. 94, No. 17, I990 6545
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Figure 1. (a) Fluorescence excitation spectrum of bare 6 - M C N N in a supersonic free jet monitored at 340-380 nm. (b) Fluorescence excitation spectra of the 6-MCNN and TEA (approximately 6 Torr) mixed system monitored a t 340-380 nm and (c) monitored at >420 nm. Bands indicated by 1 in b and in c are the origin and main vibronic bands of the vdW complex, respectively.
through appropriate filter combinations. The dispersed fluorescence spectra were measured on a Ritsu MC-ION and/or a Nikon P250 grating monochromator. Triethylamine (Nacalai Tesque) was distilled before use, and triethyl-d15-aminewas obtained from MSD Isotopes. 1 -Cyano-2-methyl- and 1 -cyano-4-methylnaphthaleneswere prepared by cyanogenation of 1-bromo-2-methyl- and 1 -bromo4-methylnaphthalenes (both Aldrich Chemicals), respectively. The prepared compounds were each purified by silica gel chromatography, recrystallization from petroleum ether, and repeated sublimation in vacuo. 1 -Cyano-2-methylnaphthalene,mp 86 OC. Anal. Calcd for CI2H9N:C, 86.20; H, 5.43; N, 8.38. Found: C, 86.44; H, 5.39; N, 8.25. 1 -Cyano-4-methylnaphthalene, mp 87 OC. Anal. Found: C, 86.31; H, 5.34; N, 8.17. The structure and purity of these compounds were confirmed by IR, NMR (100 and 400 MHz), and mass spectroscopies and by thin layer chromatography. 1-Cyano-6-methylnaphthalene was prepared from 6-methylI-naphthoic acid, which was obtained from toluene and furoic acid and purified by column chromatography, repeated recrystallization, and sublimation, mp 70-72 OC. Anal. Found: C, 86.40; H, 5.30; N, 8.43. The structure and purity of these compounds were also checked by IR, N M R (100 and 400 MHz), and mass spectroscopies.
Results Fluorescence Excitation Spectra of Methyl-Substituted 1Cyanonaphthalenes. Figure 1a shows the fluorescence excitation spectra of the S I So region of bare 6-MCNN in supersonic expansion. The spectrum exhibits a similar vibrational structure to that of I-CNN reported previously, though the former shifts to the red approximately 3 nm compared with the latter. The low frequency vibration attributable to the 6-methyl torsional motion is not significant in intensity. Further, the fluorescence excitation spectrum of bare 4-MCNN is shown in Figure 2a. The observed bands are also shifted approximately 2 nm to the red compared with those of I-CNN, and rather complicated vibronic bands were observed. The methyl substitution at the 4 positions seems to be responsible for shift and complication of the vibronic bands, though no vibrational analysis of these MCNN can be obtained at the present stage. At a rather high temperature of the sample reservoir of the nozzle, however, a small band redshifted from the origin band of 4-MCNN by 87 cm-I was observed, and a series of vibronic bands red-shifted from each main vibronic band of 4-MCNN was also observed. Since these bands decreased
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31400 31800 Figure 2. U V fluorescence excitation spectra of the jet-cooled 4M C N N / T E A system. (a) Spectrum of bare 4-MCNN was monitored at >340 nm. (b) Spectra of 4-MCNN and TEA (6 Torr) mixed system were monitored a t 350-380 nm. Weak band indicated by x may be a dimer band of 4-MCNN,Io which disappears a t reduced temperature of a sample reservoir.
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31000 31400 31800 32000 Figure 3. Fluorescence excitation spectra of the jet-cooled 2 - M C N N I TEA system. Spectrum a of bare 2-MCNN was monitored at >340 nm. The band indicated by x may be a dimer band of 2-MCNN. Spectra b and c of 2 - M C N N / T E A ( 5 Torr) were monitored a t >420 nm.
in intensity with decreasing reservoir temperature and He stagnation pressure compared with origin and vibronic band intensities of 4-MCNN, the band may be attributable to the dimer or cluster of this compound.1° The fluorescence excitation spectra of jetcooled 2-MCNN is shown in Figure 3a. The spectra exhibit an origin band at 321.35 nm (31 118 cm-I) with 4.1 cm-' vibrational spacing and considerably strong vibrational structures in the blue region (< 150 cm-I). Okuyama et aLI3 and ItoI4 have reported the low frequency transition due to the internal rotation of methyl groups in the jet-cooled 0-,m-, and p-fluorotoluene. They reported (13) Okuyama, K.; Mikami, N.; Ito, M. J . Phys. Chem. 1985,89, 5617 and references therein. (14) Ito. M. J . Phys. Chem. 1987, 91, 517 and references therein.
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TABLE I: Fluorescence Lifetimes of the 6-MCNN Monomer and the vdW Complex with TEA 6-MCNN/TEA bare 6-MCNN X/nm' Aulcm-' 7/nsc X/nma Au1cm-I 350-380 nm 32 I .69 0 58.5 322.05 0 53 3 18.06 355.5 43.0 3 16.59 502.0 33 316.91 504.3 3.7 3 14.90 673.6 31 3 15.22 673.9 1.4 3 13.66 797 30 3 14.00 791
