3586
J . Phys. Chem. 1989, 93, 3586-3594
As noted earlier, the equilibrium between FeDTPA2- and FeDTPA-OH3- is slow and, therefore, it is unlikely that normal ligand exchange is involved. It is known that Fe"' can expand its coordination sphere from 6 to 7 and we might speculate that OH- occupies a coordination site that is otherwise available for attack by the DTPA radical. As pointed out previously, on the time scale of the experiment the DTPA radical has not quantitatively converted to its final form (generally considered as a methylene radical of an acetate group). However, it seems likely that we are dealing with a carbon-centered radical as the final form of the transient metal adduct, LFeII'DTPAf'. Our data suggests a mechanism that involves an association, K6 = 2.2 X lo4 M-', followed by a fast rearrangement, k7 = 2.1 X 104 s-l, to a more stable form of the intermediate. The rate of reaction 7 excludes substitution processes but may be consistent with isomerization or tautomerization of the bound radical to its more stable form, albeit at a slightly higher rate than that of the corresponding process of the free DTPA radical (;=lo3 s-1).
Decay of this species, LFeII'DTPAf', under all conditions that were experimentally accessible, involved a bimolecular reaction with excess FeDTPA*-, indicating that the radical form liganded to the iron is insufficiently reducing to decay by intramolecular oxidation-reduction. The catalytic effect of a second FeDTPA2on ligand oxidation may be analogous to that observed in other systems such as Cu"NTA. The reactions between Fe(II1) complexes and carbon-centered radicals have been studied previously in aqueous solution, however, only with ligands such as bipyridyl and phenanthr~line.~'While, in the latter systems, carbon-centered radical reactions were found to proceed either by addition of the radical to the ligand or by an outer-sphere electron transfer, analogous studies of Cu(II), Cr(III), and Co(I1) complexes of DTPA, EDTA, IDA, and NTA have been shown to proceed by an inner-sphere mechanism involving a direct addition of the radical to the metal ion
Reaction 20 results in an increase in the oxidation state of the metal ion, which is apparently partially stabilized by a carbonion like R- adduct. This mechanism was proven by direct comparison of M("+')+-R- transients with stable synthetic complexes in which the metal was in the desired higher oxidation state.48 Furthermore, it was demonstrated by Meyerstein et aL4 that Cd'NTA reacts with carbon-centered radicals to form a transient formally written as NTA-Cu"'-R-, which subsequently reduces an additional Cu"NTA molecule via an outer-sphere mechanism. Intermediates containing a carbon-centered radical bound to a metal ion are conventionally written with the negative charge formally on the alkyl group as a carbanion and the metal in a higher oxidation ~ t a t e , ~e.g., , ~ ~DTPA-Fe"-DTPAand (DTPA-Fe'V-DTPA-)f. It is unlikely, however, that such assignment of the formal charge represents the true distribution of the electrons in this complex. As the spectrum of the transient resembles those of the FeII'DTPA complexes, we have chosen to formally assign the iron bound by a carbon-centered radical the oxidation state +3. Acknowledgment. We thank Drs. Norman Sutin, Carol Creutz, and John Melton for helpful discussions. This research was carried out at Brookhaven National Laboratory under contract DEAC02-76CH00016 with the U S . Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Registry No. DTPA, 67-43-6; DTPA', 119295-96-4;IDA', 3127733-5; NTA', 31277-34-6; Fe'I'DTPA, 105832-27-7; C O ~ ~3812-32-6; -, Br;, 12595-70-9;'CH,CO,, 19513-45-2;'CH2C(CH3),0H, 5723-74-0; 'CHIOH, 2597-43-5;OH, 3352-57-6. (48) Grcdkowski, J.; Neta, P.; Schleseuer, C. J.; Kochi, J. K. J . Phys. Chem. 1985, 89, 4373. (49) Rotman, A.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1985, 24,4158.
Magnetic Field Effects on the Intra- and Intermolecular Excipiex Fluorescence of Phenanthrene and Dimethylaniline Yoshifumi Tanimoto,* Kiyoshi Hasegawa, Natsuo Okada, Michiya Itoh, Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan
Kaoru Iwai, Kayoko Sugioka, Fukuo Takemura, Department of Chemistry, Faculty of Science, Nara Women's University, Kitauoyanishi-machi, Nara 630, Japan
Ryoichi Nakagaki, and Saburo Nagakura Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: July 15, 1988) The magnetic field effects on the exciplex fluorescence of bifunctional methylene chain molecules Ph-n-DMA ( n = 3-10) containing phenanthrene (Ph) and dimethylaniline (DMA) generated by 'Ph* in acetonitrile are interpreted in terms of the radical pair model. The singlet-triplet degeneracy of the radical ion pair (RIP) occurs around n = 10. The solvent effect on the magnetic field modulated exciplex fluorescence intensity of Ph-10-DMA indicates that the RIP is formed in a solvent of dielectric constant e = 8 and the yield increases by several tens by increasing e from 8 to 37. DMA concentration dependence of the BIl2for the exciplex fluorescence generated from the intermolecular Ph/DMA is discussed in connection with those of the intramolecular analogue. Magnetic field effects on the exciplex fluorescence in copolymers containing Ph and DMA reveal that the exciplexes are partly formed by the electron transfer between non-nearest-neighbor moieties. Introduction Since the first observation of the magnetic field effect (MFE) on the photosensitized decomposition of dibenzoyl peroxide in 1976,l we have been deeply concerned with the MFE on various
chemical reactions in order to clarify its mechanism and to develop a new technique of controlling chemical reactions. In condensed phases, the MFE can be interpreted in terms of two mechanisms, i.e., the radical pair mechanism2 and the magnetohydrodynamics
(1) Tanimoto, Y . ;Hayashi, H.; Nagakura, S . ; Sakuragi, H.; Tokumaru, K. Chem. Zhys. Lett. 1976, 41, 261.
(2) For example, see: Molin, Yu.N., Ed. Spin Polarization and Magnetic Field Effects in Radical Reactions; Elsevier: Amsterdam, 1984.
0022-3654/89/2093-3586$01.50/0
0 1989 American Chemical Society
Magnetic Effects on the Exciplex Fluorescence (MHD) m e c h a n i ~ m . The ~ former is the interaction between a magnetic field and a radical pair, in which singlet-triplet intersystem crossing (isc) of the pair is affected by a magnetic field. In the latter, a flow of electrolyte is affected by a M H D force F with the relation F = j X (pH), where j , p , and H are the electrolytic current density, the magnetic permeability, and the magnetic field, respectively. For instance, with the aid of several spectroscopic techniques, we have demonstrated that the photoinduced hydrogen abstraction reaction of a great number of aromatic carbonyls exhibits significant MFE in aqueous micellar solution^.^ On the other hand, an external magnetic field has been shown to affect the product yield on electrochemical oxidation of phenyl acetate ion by the M H D m e c h a n i ~ m . ~ The MFE on photoinduced electron-transfer (ET) reaction has been studied by a few research groups5-' Recently, we started to investigate the MFE on this type of reaction,* taking into account its significant role in organic photo~hemistry.~In the present paper,I0 we have undertaken a study of the MFE on exciplex fluorescence of bifunctional chain molecules having phenanthrene (Ph) and dimethylaniline (DMA), with the aid of photostationary, time-resolved, magnetic field modulated fluorescence as well as transient absorption measurements. In addition, the results on the exciplex fluorescence in intermolecular Ph/DMA systems and in copolymers containing Ph and DMA are discussed in relation to those on the methylene chain, model, molecules.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3587 Motor
- - - - - - - - - -. He-Ne Laser
Detector M C , Monochromator P M , Phot 0 mu1t ipl i er i4 Magnet
. ~
5 Sample Recorder Figure 1. Experimental setup for the magnetic field modulated (MFM) fluorescence.
CHART I
Ph-n-DMA Experimental Section Reagents. The method of synthesis of oc-(4-(dimethylamino)phenyl)-w-(9-phenanthryl)alkanes (Ph-rr-DMA, n = 3-10) (3) (a) Watanabe, T.; Tanimoto, Y . ;Sakata, T.; Nakagaki, R.; Hiramatsu, M.; Nagakura, S. Bull. Chem. Sac. Jpn. 1985,58, 1251. (b) Watanabe, T.; Tanimoto, Y . ;Nakagaki, R.; Hiramatsu, M.; Sakata, T.; Nagakura, S. Bull. Chem. Soc. Jpn. 1987,60,4163. (c) Watanabe, T.; Tanimoto, Y . ;Nakagaki, R.; Hiramatsu, M.; Nagakura, S. Bull. Chem. Sac. Jpn. 1987, 60, 4166. (4) (a) Tanimoto, Y . ; Itoh, M. Chem. Phys. Lett. 1981, 83, 626. (b) Tanimoto, Y . ; Udagawa, H.; Itoh, M. J . Phys. Chem. 1983, 87, 724. (c) Tanimoto, Y . ;Udagawa, H.; Katsuda, Y.;Itoh, M. J . Phys. Chem. 1983, 87, 3976. (d) Tanimoto, Y.;Shimizu, K.; Itoh, M. J . Am. Chem. Sac. 1984, 106, 7257. (e) Tanimoto, Y . ;Takashima, M.; Itoh, M. J . Phys. Chem. 1984,88, 6053. (f) Tanimoto, Y . ; Itoh, M. In Physical Organic Chemistry 1986; Kobayashi, M., Ed. Elsevier: Amsterdam, 1987; p 257, and references therein. (5) (a) Schulten, K.; Staerk, H.; Weller, A,; Werner, H.-J.; Nickel, B. Z . Phys. Chem. (Munich) 1976, 101, 371. (b) Werner, H.-J.; Schulten, K.; Weller, A. Biochim. Biophys. Acta 1978, 502, 255. (c) Weller, A. 2.Phys. Chem. (Munich) 1982, 130, 129. (d) Treichel, R.; Staerk, H.; Weller, A. Appl. Phys. 1983, B31, 15. (e) Weller, A,; Nolting, F.; Staerk, H. Chem. Phys. Left. 1983, 96, 24. (f) Staerk, H.; Treichel, R.; Weller, A. Chem. Phys. Lett. 1983, 96, 28. (g) Weller, A,; Staerk, H.; Treichel, R. Faraday Discuss. Chem. SOC.1984, 78, 271. (h) Staerk, H.; Kuhnle, W.; Treichel, R.; Weller, A. Chem. Phys. Lett. 1985, 118, 19. ' (6) (a) Michel-Beyerle, M. E.; Haberkorn, R.; Bube, W.; Steffens, E.; SchrGder, H.; N e w e r , H. J.; Schlag, E. W.; Seidlitz, H. Chem. Phys. 1976, 17, 139. (b) Bube, W.; Michel-Beyerle, M. E.; Haberkorn, R.; Steffens, E. Chem. Phys. L e f f .1977,50,389. (c) Michel-Beyerle, M. E.; Kriiger, H. W.; Haberkorn, R.; Seidlitz, H. Chem. Phys. 1979,42,441. (d) Kriiger, H. W.; Michel-Beyerle, M. E.; Seidlitz, H. Chem. Phys. Lett. 1982, 87, 79. (e) Kriiger, H.W.; Michel-Beyerle, M. E.; Knapp, E. W. Chem. Phys. 1983,74, 205. (7) (a) Steiner, U. Z . Naturforsch. 1979, 34A, 1093. (b) Steiner, U. Chem. Phys. Left. 1980, 74, 108. (c) Steiner, U. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 228. (d) Schlenker, W.; Ulrich, T.; Steiner, U. E. Chem. Phys. Lett. 1983, 103, 118. (e) Ulrich, T.; Steiner, U. E.; Fbll, R. E. J . Phys. Chem. 1983,87, 1873. (f) Ulrich, T.; Steiner, U. E. Chem. Phys. Letl. 1984, 112, 365. (g) Schlenker, W.; Steiner, U. E. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1041. (h) Ulrich, T.; Steiner, U. E.; Schlenker, W. Tetrahedron 1986,42,6131. (i) Baumann, D.; Ulrich, T.; Steiner, U. E. Chem. Phys. Lett. 1987, 137, 113. (8) (a) Tanimoto, Y.; Shimizu, K.; Udagawa, H.; Itoh, M. Chem. Lett. 1983, 353. (b) Tanimoto, Y.; Watanabe, T.; Nakagaki, R.; Hiramatsu, M.; Nagakura, S. Chem. Phys. Lett. 1985, 116, 341. (c) Tanimoto, Y . ;Takayama, M.; Itoh, M.; Nakagaki, R.; Nagakura, S. Chem. Phys. Lett. 1986, 129, 414. (d) Nakagaki, R.; Hiramatsu, M.; Mutai, K.; Tanimoto, Y.; Nagakura, S . Chem. Phys. Left. 1987, 134, 171. (9) For a review, see: Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986,86,
401. (10) A preliminary report has appeared: Tanimoto, Y.; Okada, N.; Itoh, M.; Iwai, K.; Sugioka, K.;Takemura, F.; Nakagaki, R.; Nagakura, S . Chem. Phys. Left. 1987, 136, 42.
C 4 - h
Po 1 y ( PhMMA-co-DMAMMA 1
Pol y (VPh-co-OMAMMA) will be given elsewhere." Zone-refined phenanthrene (Ph) was used as supplied. Pyrene (Py) was recrystallized from ethanol, followed by vacuum sublimation. N,N-Dimethylaniline (DMA) and N,N-dimethyl-p-toluidinewere distilled under reduced pressure. Copolymers containing phenanthrene (Ph) and dimethylaniline (DMA)'chromophores, poly(VPh-co-DMAS) (59), poly(VPh-co-DMAMMA) (67), poly(PhMMA-co-DMAS) (50), and poly(PhMMA-co-DMAMMA) (60), were available from previous studies (Chart 1).l2 The percentage of the monomer unit containing the Ph chromophore is given in parentheses. Spectrograde solvents, acetonitrile (MeCN), dimethylformamide (DMF), methanol, 2-propanol, 1,2-dichloroethane, and tetrahydrofuran (THF), were used as received. The concentration of Ph-n-DMA was about lo-" M. The concentrations of Ph and Py in intermolecular systems were about and IO4 M, respectively. In copolymer systems, the concentrations of Ph and DMA chromophores were about lo4 M in monomer units. All solutions were deaerated by repeated freeze-pump-thaw cycles. Appnratus. Photostationary fluorescence spectra were measured with a Hitachi MPF-4 fluorescence spectrometer. Time-resolved ( 1 1) Iwai, K.; Sugioka,
K.; Takemura, F., to be submitted for publication.
(12) (a) Iwai, K.; Takemura, F.; Furue, M.; Nozakura, S. Bull. Chem. Sac. Jpn. 1984,57, 763. (b) Iwai, K.; Yamamoto, K.; Takemura, F.; Furue, M.; Nozakura, S. Macromolecules 1985, 18, 1021.
3588
The Journal of Physical Chemistry, Vol. 93, No.9, 1989
A / nm
Figure 2. Fluorescence and absorption spectra of Ph-10-DMA in acetonitrile. Curves 1 and 2 are fluorescence spectra obtained by 300-and 330-nm excitation, respectively. Curve 3 is an absorption spectrum.
fluorescence spectra were measured with an excimer laser (Lambda Physik EMG-SOE, 308 nm) as an exciting light source and a combination of a monochromator (Ritsu N-lO)/photomultiplier (HTV R-666 or lP28)/storage scope (Iwatsu TS-8123) as a detection system. The raw data stored in the scope were fed into a microcomputer (Fujitsu FM-11BS) for analysis. This apparatus was also applied to determine the photostationary fluorescence intensities by replacing a storage scope with a high-impedance chart recorder. Fluorescence decay curves in the nanosecond region were measured by either the above apparatus or time-correlated single photon counting systems (Ortec or Horiba NAES- 1 100) and analyzed by nonlinear least-squares fittings, unless otherwise noted. Picosecond fluorescence measurements were made on the apparatus reported previou~ly,'~ with use of the fourth (266 nm, 15 ps) harmonic of a mode-locked Nd3+YAG laser as a light source. Magnetic field modulated (MFM) fluorescence was measured with a combination of a xenon arc lamp (Ushio UXL-500-0)/ monochromator (Ritsu N-lO)/glass filter (Toshiba UV-D33S) as the exciting light source and a combination of a monochromator (Nikon G-250 or Ritsu N-20)/photomultiplier (HTV R-928)/ phase-sensitive detector (Brookdeal 9053 or N F Electronic Instruments LI-570) as the detection system (Figure 1). Pulsed magnetic fields (2-75 mT, 80 Hz) were generated by rotating a pair of disks, on which four sets of permanent magnets were fixed. Magnetic fields were varied by changing the distance of two disks. Transient absorption spectra were measured with an excimer laser (308 nm) and a xenon arc lamp as the exciting and probe light sources. The transient signal was detected by the detection system used in the time-resolved fluorescence measurements mentioned above. Magnetic fields were applied with conventional permanent magnets (Tokin LM-22), a homemade Helmholtz coil, or an electromagnet (Tokin SEE-9). The residual magnetic field of the electromagnet was canceled by applying a low electric current. The magnetic field was determined by a gaussmeter (Kenis Type 111).
Results and Discussion 1 . Magnetic Field Effects on the Intramolecular Exciplex of Bifunctional Chain Molecules. 1 . I . Assignment of Fluorescence Spectra. In Figure 2 are shown the photostationary fluorescence and absorption spectra of Ph- 10-DMA in acetonitrile. Fluorescence spectra (curves 1 and 2) were obtained by 300- and 330-nm excitation, respectively. The spectrum is influenced by the excitation wavelength. The absorption spectrum (curve 3) was found to be attributable to the sum of those of the two chromophores; a sharp band at 300 nm and a broad one around 330 nm are chiefly due to the Ph and DMA chromophores, respectively. Therefore, the fluorescence band around 500 nm is assigned to the exciplex (exciplex I) generated by the excitation of Ph chromophore, while the 460-nm band is assigned to the one (13) Sumitani, M.; Nakashima, N.; Yoshihara, K.; Nagakura, S. Chem Phys. Lett. 1977, 51, 183.
Tanimoto et al.
JI
I .
0
I
2 T l m e l ns
3
Figure 3. Picosecond fluorescence decay curves of Ph-10-DMA in acetonitrile monitored at (a) the monomer (350-400-nm)and (b) the exciplex (>430-nm)bands, obtained by 266-nmlaser excitation.
(exciplex 11) generated by the DMA excitation. On the other hand, the bands in the 350-400-nm region are attributable to the sum of the fluorescence from 'Ph* and IDMA*. The 500-nm fluorescence exhibits MFE, while the 460-nm fluorescence is insensitive to the magnetic field, as shown later. To make the assignment clear, fluorescence decay in the picosecond region was examined. Figure 3 shows the fluorescence decay curves monitored at the monomer band (350-400 nm) and the long-wavelength band (>430 nm) by 266-nm excitation. The fluorescence of the monomer band shows double-exponential decay (0.3 and 1.8 ns). The fluorescence of the longer wavelength band shows the rise of the signal, giving the rise times of 0.3 and 1.1 ns, and no fast rise within the excitation pulse width ( 1 5 ps) was observed. At the 266-nm excitation, the fluorescence decay curves of the longer wavelength bands observed in the nanosecond time scale are composed of the 50-11s (attributable to the 500-nm fluorescence) and the IO-ns decay component (the 460-nm fluorescence). Therefore, it is concluded that the fluorescence bands that peaked around 500 and 460 nm are not attributable to the ground-state complex but to the exciplexes, since the fluorescence from the ground-state complex should give the rise of the signal within the pulse width of the excitation. The rise times of 1.1-1.8 and 0.3 ns may be attributable to those of exciplexes I and 11, taking into account the fluorescence lifetimes of phenanthrene (50ns) and dimethylaniline (3 ns) in the absence of the partner. The rise time of intramolecular exciplex fluorescence of pyrer~e-(CH~),~-DMA (Py-16-DMA) generated from IPy* in acetonitrile was reported to be 2.1 ns,s which is comparable with the fluorescence rise time of exciplex I. Exciplex formation from either partner of a given electron donor-acceptor were reported on biphenyl/diethylaniline,naphthalene/diethylaniline, and l-~yanonaphthalene/naphthalene.'~~~ In the present intramolecular system, we may detect the exciplex fluorescence generated by the DMA excitation, since the absorbance of DMA is comparable with that of Ph in the 300-350-nm region, and the close proximity of the two chromophores due to the methylene chain is suitable for a fast reaction within the 'DMA* lifetime. A working scheme of the present photoreaction is given in Scheme I. Electron transfer from the ground-state DMA to the excited singlet state 'Ph* results in the formation of an intramolecular radical ion pair (RIP) of phenanthrene anion (2Ph-) and DMA cation radical (2DMA+) in the singlet state, which generates the intramolecular exciplex I. On the other hand, the excitation of the DMA chromophore results in the formation of (14) (a) Knibbe, H.; Rehm, D.; Weller, A. Z . Phys. Chem. (Munich) 1967, 56, 99. (b) Weller, A. Pure Appl. Chem. 1968, 16, 115. (15) (a) Knight, A. E. W.; Selinger, B. K. Chem. Phys. Lett. 1971, 10, 43. (b) McDonald, R. J.; Selinger, B. K . Aust J . Chem. 1971, 24, 1797.
Magnetic Effects on the Exciplex Fluorescence
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3589
$
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Figure 4. (a) Magnetic field effect on the photostationary fluorescence of Ph-IO-DMA in acetonitrile, obtained by 300-nm excitation. (b) MFM fluorescence spectrum of Ph-IO-DMA. Magnetic field is 75 mT. SCHEME I
Figure 5. (a) Time-resolved fluorescence spectra of Ph-IO-DMA in acetonitrile, obtained by 308-nm laser excitation. (b) Magnetic field effect on the fluorescence at a 60-ns delay after the laser excitation.
I
I1
555 nm
I
I Exciplex 1
I
--0
I
'
100
50
Time / ns
Figure 6. Magnetic field effect on the fluorescence decay curve of PhIO-DMA in acetonitrile monitored at 555 nm,obtained by 308-nm ex-
l(2A- 2D')
c/
U
'(A- D')*
Exciplex I I
exciplex 11, having a structure slightly different from that of exciplex I as discussed later. Interconversion between exciplex I and exciplex I1 is not included in the scheme, though this possibility is not excluded at the present stage. All the data indicate that it has no primary importance to the reaction, if any. I .2. Magnetic Field Effects on the Fluorescence. We examined the effects of a magnetic field on the fluorescence of Ph-10-DMA in acetonitrile with the aid of three techniques. Figure 4a shows the MFE on the photostationary fluorescence of Ph-IO-DMA. In contrast to the monomer bands around 370 nm, the exciplex fluorescence around 500 nm increases in intensity in the presence of a magnetic field (65 mT). To elucidate the exciplex fluorescence exhibiting MFE, a magnetic field modulated (MFM) fluorescence spectrum was measured, An advantage of this technique is to pick up the fluorescence signal that is magnetic field sensitive from the rest. The result is given in Figure 4b. From the figure, it is concluded that the exciplex I peaking around 500 nm exhibits MFE, while the 460-nm fluorescence from exciplex I1 is magnetic field insensitive. We further analyzed the Ph- 10-DMA fluorescence by timeresolved fluorescence measurements as shown in Figure Sa. The fluorescence of the 350-400-nm band disappears in the initial stage after the laser excitation. The fluorescence of exciplex I1 around 460 nm decays at the middle stage of the delay time, while the fluorescence of exciplex I around 500 nm is predominant in the spectrum at a 60-11s delay after the excitation. As shown in Figure 5b, the exciplex fluorescence of I around 500 nm, observed at a 60-11s delay, increases by about 70% in intensity in a magnetic field (330 mT). On the other hand, the spectra at 6- and 20-ns delay times are insensitive to the magnetic field. These results present unequivocal evidence that the fluorescence of the exciplex I is magnetic field sensitive, which is in good agreement with the results from the MFM fluorescence measurements (Figure 4b).
citation. TABLE I: Magnetic Field Effects on the Exciplex Fluorescence Lifetime of Ph-n-DMA in Acetonitrile" H, mT 0 56 65 108 212 810
T
Ph-10-DMA ns, ~ ns 50 10 ~
67
10
80 93 92
9 11 9
Ph-8-DMA ns T ~ , : ns
T,,~
46
4
38
3
a Experimental error is 2~10%.bLifetimeof exciplex I. CLifetimeof exciplex 11.
The MFE on the fluorescence decay at 555 nm is shown in Figure 6. It is significantly affected by a magnetic field (330 mT). The MFE on the exciplex fluorescence lifetimes analyzed by nonlinear least-squares fittings is listed in Table I. The lifetime of the exciplex I changes from 50 ns at zero field to 92 ns at 810 mT. In contrast, the fluorescence of exciplex I1 has a lifetime of 10 ns and is insensitive to a magnetic field. The magnetic field dependence of the lifetime of exciplex I is in good agreement with the intensity change shown in Figure 4b. No MFE on the fluorescence of exciplex I1 may be explained by its structure. As mentioned above, since the lifetime of IDMA* is about 3 ns in the absence of Ph, ET is limited only in the conformers in which the Ph and 'DMA* are in close proximity, leading to the RIP of short interradical distance. Under the circumstances, a considerable S-T energy gap due to the electron-exchange interaction results in the diminishing of the S-T isc rate. For this reason, no MFE may be observed for the fluorescence of exciplex 11. Further, the fluorescence lifetimes of exciplex I in the absence and presence of a magnetic field are almost constant in the temperatures between +40 and -45 O C (50 f 5 ns at zero field and 90 f 5 ns at 0.58 T). Presumably, the barrier between the exciplex and the RIP is very small compared with the thermal energy at room temperature. 1.3. Magnetic Field Effects on the Transient Absorption Spectra. Figure 7 shows transient absorption spectra of Ph-lO-
Tanimoto et al.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
3590
, 1
-
-e--
1
I
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.6
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50 ns delay
2,
-. *. . *. -
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10.5
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Figure 7. Transient absorption spectra of Ph- IO-DMA in acetonitrile, obtained by 308-nm excitation.
I
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ot
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.
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- a5-
*
6
0
i 'E 1.5,
105
4 0
10
30
20
40
Die!ectr:c c o n s t a n t , c
Figure 9. (a) Dependence of MFM fluorescence intensity of Ph-IO-DMA on dielectric constant c of solvent. MFM fluorescence intensities were
determined under the same photostationary fluorescence intensity and the magnetic field (75 mT): 1, acetonitrile; 2, DMF; 3, methanol; 4, 2-propanol; 5, dichloroethane;6, THF. Closed circles are the mixtures of THF/acetonitrile. (b) Dependence of fluorescence intensity ratio Iex/I,,, of exciplex and monomer bands of Ph-IO-DMA on dielectric constant c. I,,, is the intensity at 380 nm, and I,, is the one at the peak of the exciplex band (450-490 nm). TABLE 11: Exciplex Fluorescence Lifetimes of Ph-10-DMA in the Absence and Presence of a Maenetic Field"
-
OT
c1
.i , 2
.'I
. 1 .5 . 6 . 7 ,8 T i m e / us
Figure 8. Magnetic field effect on the decay of transient absorption of Ph-10-DMA at 400 nm in acetonitrile.
DMA in acetonitrile. The spectra in the 400-50OVnm region are complicated. An apparent rise of the absorption intensity in the 450-500-nm region is due to the contamination of the fluorescence. Making comparison with the triplet-triplet (T-T) absorption spectrum of phenanthrene observed in acetonitri!e, the absorption bands around 450 and 480 nm are chiefly attributable to the T-T band of the phenanthrene chromophore. The assignment of the absorption bands of ZPh- and 2DMA+ is not apparent because of the intense T-T band of phenanthrene, though they were expected to appear in the 400-440-nm'6-18 and the 450-470-nm region^,'^-^ respectively. However, the transient around 400 nm has a short decay component, which may be attributable to the absorption of ZPh-. In fact, upon application of a magnetic field (0.78 T), the transient decay around 410 nm is significantly affected (Figure 8). The lifetime of ZPh-was found to be 60 ns at zero field and 100 ns at 0.87 T, values in good agreement with those determined from the fluorescence decay of exciplex I, indicating that RIP of ZPh- and 2DMA+ are involved in the reaction. Furthermore, the absorption of the phenanthrene T-T band around 490 nm monitored at a 2004s delay after the laser excitation was found to decrease by about 10% in intensity in the presence of a magnetic field (0.33 T). This result strongly suggests that excited triplet phenanthrene is partly generated from the recombination of the triplet RIP, as reported for the ET reaction (16) Balk, P.; Hoijtink, G. J.; Schreurs, J. W. H. Recl. Trau. Chim. Pays-Bas 1957, 76, 813. (17) DeBoer, E.; Weissman, S. I. Recl. Trau. Chim.Pays-Bas 1957, 76, 824. (18) Christodouleas, N.; Hamill, W. H. J . Am. Chem. Soc. 1964,86,5413. (19) Land, E. J.; Porter, G . Trans. Faraday SOC.1963, 59, 2016. (20) Shida, T.; Hamill, W. H. J . Chem. Phys. 1966, 44, 2369.
solvent (c) THF (7.6) 2-propanol (19.9) MeCN/THF (20) MeCN/THF (25) MeCN/THF (30) MeOH (32.7) DMF (36.7) MeCN (37.4)
T
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1.21
1.36 1.53 1.29 1.86 1.86
"Experimental error is &lo%. bLifetimeof exciplex I. CLifetimeof exciplex 11. dFluoreseencedecay is approximately expressed as a single exponential. of pyrene/DMA5 and anthracene/DMA.6 I .4. Solvent Effects. The yield of RIP is considered a function of solvent polarity. To evaluate the relative yield of RIP in the present system, the relative M F M fluorescence intensity of the exciplex I was measured in various solvents, by keeping a photostationary fluorescence intensity constant, as well as the relative yield of photostationary exciplex fluorescence (Figure 9). A very weak MFM fluorescence was observed in T H F solution, and with increasing dielectric constant t of solvent, the MFM fluorescence intensity increases in contrast with a decrease in the relative yield of exciplex fluorescence. The MFM fluorescence intensities in methanol and 2-propanol deviate from the curves given by those in the THF/acetonitrile mixed solvents, indicating that a specific solventsolute interaction such as hydrogen bonding determines the MFM fluorescence intensity in alcohols. We may, as a first approximation, take the MFM fluorescence intensity as the measure of the RIP yield, though it is a complex function of the rate constants of the reaction processes shown in Scheme I, some of which are considered to depend on the solvent polarity t, and the fluorescence intensity of exciplex I1 contaminates the photostationary fluorescence. Therefore, as shown in Figure 9a, observation of the MFM fluorescence in THF presents unequivocal evidence that the RIP is formed in the solvent whose dielectric constant is as small as 8 and the relative yield increases by more than 50 times with increasing t from 8 to 37. Furthermore, the effect of solvent polarity was also observed in the fluorescence lifetimes (Table 11). Although the lifetimes of the exciplex I are 50-60 ns at zero field in the solvents of e = 8-37, those at 0.36 T disperse from 50 to 90 ns, by increasing the value from 8 to 37. In agreement with the MFM fluorescence study mentioned above, the lifetime ratio ~ ~ ( 0 . 3T)/~1(0 6 T) in
Magnetic Effects on the Exciplex Fluorescence
I
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3591
1~x-D
'ex 'rip A - D Distance
Figure 10. Potential curves of Ph-IO-DMA.
the presence and absence of a magnetic field (0.36 T) increases with increasing e value. The solvent effect on the fluorescence of exciplex I mentioned above may be qualitatively explained as follows (see Figure 10): Dependence of the stabilization energy of the RIP on solvent polarity may be different from that of the exciplex. In a polar solvent, the RIP is more stable than the exciplex and two states are in dynamic equilibrium with a small barrier. Therefore, the exciplex can deactivate through the S-T isc of the RIP, which is a magnetic field dependent process. For this reason, significant MFE was observed in the fluorescence of exciplex I in polar solvent. On the other hand, in less polar solvent, destabilization of the RIP shifts the equilibrium toward the exciplex, leading to the diminishing MFE on the exciplex fluorescence. Weller and Zachariasse have extensively studied solvent dependence of intermolecular exciplex and RIP formation enthalpies in connection with chemiluminescence reactions.21 Upon their calculation, typical intermolecular exciplexes are likely to dissociate into radical ion pairs in solvents with e 3 7. This conclusion is in good agreement with the present results for the intramolecular system. Petrov et a1.22reported on the solvent effect on M F M intermolecular exciplex fluorescence of Py/DMA. The relative MFM fluorescence intensity has a sharp maximum around e = 26 in contrast with the present results (Figure 9a). The maximum may have appeared because methanol, showing strong solvent-solute interaction as discussed above, was used as a component of mixed solvents having different dielectric constants. On the other hand, their theoretical curve for the dependence of free-radical yield on solvent polarity e qualitatively agrees with the present results though the present reaction is an intramolecular one. 1.5. Chain-Length Dependence of the Magnetic Field Effects. The bifunctional chain compounds Ph-n-DMA with n = 3,6, and 8 exhibit the exciplex fluorescence in almost the same wavelength region (490-500 nm) as Ph-IO-DMA in acetonitrile. The magnetic field dependence of the exciplex fluorescence intensity in acetonitrile was examined for these compounds by photostationary fluorescence. Figure 1 l a shows the intensity ratio ZH/Zo of the photostationary exciplex fluorescence in the presence and absence of a magnetic field H.No significant effect was observed for the compounds with a shorter chain length ( n = 3, 6, 8), although a considerable effect was observed for Ph-8-DMA. Similar magnetic field effects were observed for the M F M fluorescence intensities of Ph-8-DMA and Ph-IO-DMA in acetonitrile and methanol as shown in Figure 1 lb. According to the radical-pair model, the S-T isc of the pair occurs via electron-nuclear hyperfine (hf) interaction at a certain interradical distance R, where the two states are nearly degenerate. The energy gap of the two states is governed by exchange interaction, whose magnitude falls off as exp(-aR). In the case of the radical pair generated by an intermolecular reaction as shown later (section 2), component radicals can separate to attain the S-T degeneracy. Therefore, isc from the singlet to all triplet sublevels (T+, To, and T-) takes place at zero field. In the presence (21) (a) Weller, A,; Zachariasse, K. Chem. Phys. Lett. 1971.10, 590. (b) Weller, A. Z.Phys. Chem. (Munich) 1982, 133, 93. (22) Petrov, N. Kh.; Shushin, A. I.; Frankevich, E. L. Chem. Phys. Lett. 1981, 82, 339.'
tl
0
02
06
04
08
Ii / T Figure 11. (a) Magnetic field effect on the photostationary fluorescence intensities of Ph-n-DMA at 540 nm in acetonitrile. (b) Magnetic field effect on the MFM fluorescence intensities of Ph-n-DMA at 490 nm: 1, Ph-10-DMAin acetonitrile;2, Ph-10-DMA in methanol; 3, Ph-8-DMA in acetonitrile; 4, Ph-8-DMA in methanol.
I; /
2J
Magnetic Field Strength
Figure 12. Energy diagram of the intramolecular radical ion pair.
of a magnetic field, T+ and T- split away from each other, because of the electronic Zeeman effects, leading to the reduction of the isc rate. In the case of the radical pair generated by the intramolecular ET reaction of Ph-n-DMA, the situation is slightly different from the one mentioned above (see Figure 12). Because of the steric restriction imposed by the methylene chain, the radical pair at the ends of the chain cannot separate freely so that its S-T states are nondegenerate even a t zero field, though the magnitude of the energy gap depends on the interradical distance R. For this reason, no magnetic field effect on the intramolecular exciplex fluorescence of the molecules with n = 3 and 6 shown in Figure 1 l a is attributable to a large S-T separation, leading to vanishing isc of the pair. Similarly, no effect on the fluorescence of exciplex I1 in Ph-10-DMA may be attributable to the large S-T separation, since conformers, in which Ph and IDMA* are in close proximity, may only lead to the exciplex formation because of the short lifetime of IDMA*. For Ph-8-DMA, the S-T separation is still large at zero field and the S-T- level crossing occurs in relatively low field. This level crossing enhances the deactivation of the exciplex via S-T- isc, which is observed as a dip around 70 mT in the ZH/Zo ratio in Figure 1 1. This level crossing is also observed as a decrease in the fluorescence lifetime, as shown in Table I. In the case of Ph-IO-DMA, S and T states are nearly degenerate, though a small dip due to the level crossing still appears at 17 mT. Further increase of the IH/Ioratio in the higher fields can thus be attributed to the Zeeman splitting of T+ and
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
-I
ZDMAl 006 M
5 1010 1
m
v
20
0
40
80
60 H /mT
Figure 14. Magnetic field effect on the MFM fluorescence intensities of intermolecular exciplex of Ph/DMA in methanol. MFM fluorescence 300
500
io0
600
SCHEME I1
h/nm Figure 13. (a) Fluorescence spectrum of a methanol solution of phenanthrene (1.4 X lo-' M) and DMA (0.00s M), obtained by 345-nm excitation. (b) MFM fluorescence spectrum of Ph and DMA in methanol. The magnetic field is 75 mT.
T-, resulting in the reduction of the S-T isc rate. Therefore, the present results indicate that the degeneracy of the S and T states occurs at a methylene chain length of 10 or so. The distance r (A) between the carbon atoms of Ph and DMA, at which the respective chromophores are linked with the chain, was estimated to be 11.3 (n = 8), 12.5 (n = 9), and 13.8 8, (n = lo), in fully stretched conformation, Le., the all-trans form, by the use of the equation r = 1.54{0.665n(n 2) 1) n = even
+ +
r = 1.25(n
+ 1)
intensities were determined under the same photostationary fluorescence intensity.
n = odd
(1)
Here, the distance of the C-C bond and the angle of C-C-C of the methylene chain were assumed to be 1.54 A and 109.3', respectively. Therefore, the degeneracy of the singlet and triplet of the radical pair seems to occur at an approximate interradical distance of 10 A or so, since the mean distance must be shorter than that of the fully stretched form. Magnetic field effects on the exciplex fluorescence intensity and the triplet yield of Py-n-DMA, n = 6-16, were studied by Weller and co-workers.' In contrast to the Ph-n-DMA systems, no exciplex fluorescence attributable to the excitation of the DMA chromophore was reported. However, the reported nonexponential decay of the Py-16-DMA fluorescence suggests that the exciplex fluorescence generated from IDMA* may be also involved in it. Magnetic field and chain length dependences of the Py-n-DMA exciplex fluorescence are essentially comparable with those of the present ones, though the magnitude of the MFE on their systems is relatively small. For example, the intensity ratio I H / I oat H = 0.1 T is about 1.5 for Ph-IO-DMA, while it is 1.2 for Py-lODMA. The hf-induced isc rates of the ZPh--n-2DMA+radical ion pair are comparable with those of 2Py--n-2DMA+, since the weighted average of hf coupling constants Bl12(calc)for the former pairs is almost the same as that for the latter (5.5 mT).5323-25The Py- 16-DMA exciplex fluorescence appears in the longer wavelength region (580 nm) in comparison with the band of Ph-lODMA exciplex (500 nm), and its lifetime (22 ns) is shorter than that of Ph-IO-DMA (50 ns). These facts indicate that a fast nonradiative deactivation process from the Py-n-DMA exciplex to the ground state, due to a large Franck-Condon factor, might somewhat reduce the MFE on the Py-n-DMA exciplex fluorescence. Furthermore, we have reached the same conclusion for the S-T degeneracy of the radical pair in the studies of M F E on the (23) Mobius, K. 2.Nuturforsch. 1965, 20A, 1102. (24) Canters, G. W.; Hendriks, B. M. P.;DeBoer, .I.W. M.; DeBoer, E. Mol. Phys. 1973, 25, 1135. (25) Werner, H.-J.; Schulten, 2.; Schulten, K. J . Chem. Phys. 1977, 67, 646.
A
+
D mhV ' A * *
D
i. +
2 D * ) s '(2A-
i
?D')
e )(A-
1
1 'A*
+
D')*
A t 0
D (A=Ph
D=DMA)
photoredox reaction of N - [w-@-nitrophen~xy)alkyl]anilines~~ and on the biradicals generated from a-[ (2-oxoxanthylcarbony1)oxy]-w-[(xanthyl-2-carbonyl)oxy]alkanes.27 Similar results were reported on the MFE on the isc rate of biradicals generated from 2-phenyl~ycloalkanes.~~ Thus, it can be generally confirmed that the S-T degeneracy of the radical pair connected by a methylene chain takes place at a methylene chain length of about 10. 2. Magnetic Field Effects on the Intermolecular Exciplex. The MFE was further studied on the intermolecular exciplex fluorescence of phenanthrene and dimethylaniline by MFM fluorescence technique. Figure 13a shows the fluorescence spectrum of phenanthrene and dimethylaniline in methanol, the longest wavelength band of phenanthrene at 345 nm being excited to avoid the excitation of the DMA band. Thus, the broad band in the 450-550-11111 region is assigned to the exciplex generated by the phenanthrene excitation, while the one in 350-400 nm to the phenanthrene monomer. From the fluorescence intensity and lifetimes in various DMA concentrations (
