J . Phys. Chem. 1991, 95, 766-770
766
= 5 for CIPTZ,' it follow^ that K S D S ~ e (=~ (1.5 ~~) * The 0.3) x 103 M-I (cc = 0.10 MI. electrostatic contribution to the micellar binding can be
KS*e+/KSDSB
estimated from eq 12, where z is the ionic charge and 4 the surface
PIQ= exp(-z4/25.69)
(12)
potential of the micelle; in this equation the variation of 4 with the ionic strength is not considered, but at p = 0.10 M,4 can be estimated to be -75 to -85 mVI9 and the Fe3+/Fe2+ratio for the electrostatic contribution to the binding constant should be 0.04-0.06, in good agreement with the experimental result; at p = 0.02 M, if 4 is assumed in the range -100 to -120 mV, the ratio should be 0.01-0.02 (seebelow). In addition, from eq 11, the ratios of the different intercepts (and slopes) for the different C,,PTZs &e (KSDs~/KSDsB)Clprz/(KSDSB+/KSDSB)-~C4~rPrzc4prz. These ratios are ca. 1.2-1.3 for CIPTZ over C2PTZ and ca. 1.5-2 for CIPTZ over C,PTZ. Although the data are affected by experimental uncertainty, the trend for KS*B+/KSDSB is in the order C I P T Z > C2PTZ > C4FTZ. This suggests that the contribution of the positive charge to the binding constant is decreasing as the overall hydrophobicity is increasing. This fact seems to be confirmed by the results obtained for C12PTZ. At p = 0.10 M and in the range [SDS] = 0.05-0.20 M, KS*l,up = 0.34 f 0.05 M-I. If the value of K", is assumed to be equal to that of C4FTZ (Le., 6.5 M-I), one obtains (KSDSlsexp/P ) = 0.05. Because analogous values for (KSDs~ 1 K S " " ~ ~ have been estimated (see above), it follows that (fYd~+/daB)c,2m,is ca. 1. This conclusion is not pro-
foundly changed if the value of PIfor CI2PTZis chosen in the range 3-15 (in the most unfavorable case, Le., PI = 3, (KSDSB+/KSDSB)c,,prz should be ca. 2). Measurements at p = 0.02 M allowed the estimation of KSDslhxp = 0.12 f 0.03 M-I in the range 0.10-0.15 M SDS. PIresults are slightly depressed at p = 0.02 M for CIPTZ (from 2.4 to 1.9);' the same is assumed for C4PTZ, and with KSDS~e(~~)/KS*~ec(~~~) in the range 0.01-0.02 (see above), it follows again that the ratio KsDsB+/KsDSB for CI2PTZis close to 1 also at this lower ionic strength. Conclusions
In the present study the variation of the hydrophobicity of one reactant and the charge of the micellar aggregates have been shown to greatly affect the location of the reaction and the rate of the electron-transfer process. An attempt has been devoted to evaluate the contribution of electrostatic and hydrophobic effects and their mutual dependencies. The data can also give some useful informations on the interactions of hydrophobic ions with charged membra ne^.^'-'^ Acknowledgment. Support from Eniricerche and MURST is kindly acknowledged. (22) Honig, B. H.; Hubbell, W. L.; Flewelling. R. F. Annu. Reu. Biophys. Biophys. Chem. 1986,15, 163. (23) McLaughin, S.Annu. Reo. Biophys. Biophys. Chem. 1989,18,113. (24) Matsumura, H.; Furusawa, K. Ado. Colloid Interface Sei. 1989,30, 71.
Time-Resolved EPR Studies on the Photochemlcai Hydrogen Abstraction Reactions and the Excited Triplet States of 4-Substituted Pyridines Shozo Tero-Kubota,* Kimio Akiyama, Tadaaki Ikoma, and Yusaku Ikegami Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira 2-1 -I, Sendai 980, Japan (Received: May 29, 1990; In Final Form: July 10, 1990)
Photochemical hydrogen abstraction reactions and the lowest excited triplet states taking part in the initial process have been investigated for several pyridine derivatives by using the time-resolved EPR method. The emissive CIDEP spectrum obtained from the laser photolysis of Cacetylpyridine (1) in 2-propanol was assigned to the corresponding ketyl radical, proving initial hydrogen abstraction by the carbonyl group. In the cases of 4-cyano-(2) and 4-methoxycarbonyl-(3) pyridines, and 4-pyridinecarboxamide (4), enhanced absorptive CIDEP spectra due to the corresponding 1-hydropyridinyl radicals were observed, suggesting the preferential population to the lowest sublevel in the intersystem crossing. The triplet EPR and phosphorescence spectra observed at 77 K indicate that TIof 1 is mainly the carbonyl nr* state with a small contribution from the m* state. The TI states of 2, 3, and 4 are considered to be of mixed character between the pyridine 3Bl(nr*) and 3Al(rr*)states, though the interaction is smaller than that of unsubstituted pyridine because of raising of the m*state by electron-withdrawinggroups. It is proposed that the TIstates of 2-4 have smaller deviation from the planar conformation than the TI state of unsubstituted pyridine.
Introduction
The time-resolved EPR (TREPR) technique has been successfully employed to elucidate photochemical reaction mechanisms, since the CIDEP spectra observed give information about not only the radical intermediate with a short lifetime but also the character of the excited states taking part in the reaction.'+ ( 1 ) Wong, S. K. J . Am. Chem. Soc. 1978, 100, 5488. (2) (a) Dcpew, M. C.; Liu. Z.; Wan, J. K. S.J. Am. Chem. Soc. 1983.105, 2480. (b) Depew, M. C.; Wan, J. K. S.J . Phys. Chem. 1986, 90, 6597. (3) (a) Akiyama. K.; Two-Kubota, S.;Ikenoue, T.; Ikegami, Y. Chem. Left. 1984. 906. (b) Ikegami, Y. Reo. Chem. Inrermed. 1986, 7, 91. (c) Shimoishi, H.: Akiyama, K.; Tero-Kubota. S.;Ikegami, Y. Chem. Leu. 1988, 251. (4) (a) Yamauchi, S.; Hirota, N. J . Phys. Chem. 1984, 88, 4631. (b) Tominaga, K.; Yamauchi, S.;Hirota, N. J . Phys. Chem. 1988, 92, 5160.
0022-36S4/91/2095-0766$02.50/0
The method is also a powerful tool to measure short-lived excited triplet states in some matrices at low temperatures.',* Although (5) (a) Basu, S.;McLauchlan, K. A.; Ritchie. A. J. D. Chem. Phys. 1983, 79,95. (b) Buckley, C. D.; McLauchlan, K. A. Chem. Phys. 1984,86,323. (c) Buckley, C. D.; Grant, A. I.; McLauchlan, K. A.; Ritchie, A. J. Faraday Discuss., Chem. Soc. 1984,78,250. (d) Buckley. C. D.; McLauchlan, K. A. J . Phofochem. 1984,27,31 1 . (e) Buckley, C. D.; McLauchlan, K. A. Mol. Phys. 1985, 54, 1 . (6) (a) Sakaguchi, Y.; Hayashi. H.; Murai, H.; I'Haya, J.; Mochida, K. Chem. Phys. Lctt. 1985, 120,401. (b) Sakaguchi. Y.; Hayashi, H.: Murai, H.; I'Haya, J. J . Am. Chem. Soc. 1988, 110, 7575. (7) (a) Tero-Kubota, S.; Migita, K.; Akiyama, K.; Ikegami, Y. J . Chem. SOC.,Chem. Commun. 1988, 1067. (b) Akiyama, K.; Ikegami, Y.; TeroKubota, S.J . Am. Chem. Soc. 1987, 109, 2538. (8) (a) Hirota, N.; Yamauchi, S.;Terazima, M. Reu. Chem. Inrermed. 1987,8, 189. (b) Yamauchi, S.;Hirota, N. J . Am. Chem. Soc. 1988, 110, 1346.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 767
H Abstraction Reactions of Pyridines several azaaromatic systems have been in~estigated,~.~.~' there have been only a few studies on the photochemistry and the TI states of pyridine derivatives using the TREPR? It has been reported that steady-state irradiation of pyridine derivatives in alcohols generates the corresponding 1-hydropyridinyl radicals and the yield is significantly enhanced by the addition of sensitizers such as acetone and benzophenone.1b15 Bencze et a1.I6 obtained a pinacol derivative from the photolysis of 4-acetylpyridine (1) in 2-propanol in high yield and deduced the generation of the ketyl radical of 1 as an intermediate. From the studies of reactivityI7and phosphorescence ~pectra,~*.'~ )nr* character was suggested for several pyridyl ketones. In contrast with the proposal, Dohrmann et a1.I2 detected only the l-hydro4-acetylpyridinyl radical by simultaneous in situ EPR and optical absorption spectroscopy. They proposed that the pyridinyl radical is the precursor of the pinacol derivative since secondsrder decay of the radical was measured. We then applied the TREPR method to clarify the initial photochemical process of the pinacol formation and also the excited state associated with the reaction. Photochemical investigations on 4-cyano-(2) and 4-methoxycarbonyl (3) pyridines and 4pyridinecarboxamide (4) in alcohols were also carried out. Much attention has been paid to study the excited triplet (TI) states of pyridine and its derivatives because of their unique character.20 It has been known that pyridine emits very weak phosphorescence and the emission exhibits notable substituent e f f e c ~ ~ l -These ~ ~ are considered to be due to small energy separation between the T I and T2 states. Recently Schmidt and his co-workers9 succeeded in observing the T I state of pyridine by using spin-echo spectroscopy at 2 K. According to their study, the TI state has a mixed character of )Bl(m*) and )Al(7rr*) states as the result of a strong vibronic coupling between the TI and T2 states. Large deviation from the planar structure in the T I state was clarified by detailed analysis of the hyperfine splitting (hfs) tensors of nitrogen and deuterium. Although the T I state of 2 was detected by use of the ODMR method, uite different zfs parameters have been reported in these papers2 3 The existence
9
~
(9)(a) k,F. C.; Buma, W. J.; Schmidt, J. Chem. Phys. krr.1985,117, 203. (b) Buma, W. J.; Groenen, E. J. J.; Schmidt, J. Chem. Phys. krr. 1986, 127, 189. (c) Buma, W. J.; Groenen, E. J. J.; Schmidt, J.; De Beer, R. J . Chem. Phys. 1989,91,6549. (IO) (a) Zeldes, H.; Livingston, R. J . Phys. Chem. 1972,76,3348;1973, 77,2076. (b) Zcldes, H.; Livingston, R. Rudiur. Res. 1974,58. 338; 1975, 62,37. (c) Zelda, H.; Livingston, R.; Bemstein, J. S.J. Magn. Reson. 1976, 21,9;1977,25. 67;1977,26, 103;1979,34, 543. (1 1) (a) Rakowsky, Th.;Dohrmann. J. K. Eer. Bunsen-Ges. Phys. Chem. 1975,70, 18. (b) Dohrmann, J. K.; Becker, R. J . Mugn. Reson. 1977,27, 371. (c) Dohrmann, J. K.; Kieslich, W. J . Mugn. Reson. 1978.31.69;1978, 32,353. (d) Sander, U.;Dohrmann, J. K. Eer. Bunsen-Ges. Phys. Chem. 1979,83,1258.(e) Rakowsky, T.; Dohrmann. J. K. Ber. Bunsen-Ges. Phys. Chem. 1979,83,495. (12)(a) Krohn, H.; Leuschner, R.; Dohrmann, J. K. Eer. Bunsen-Ges. Phys. Chem. 1981,85, 139. (b) Leuschner, R.; Dohrmann, J. K. Eer. Eunsen-Ges. Phys. Chem. 1984,88, 50. (13) Alberti, A.; Guerra, M.; Pedulli, G. F. J. Mugn. Reson. 1979,34,233. (14)Sano, Y.; Tero-Kubota, S.;Ito, 0.; Ikegami, Y. Chem. k r r . 1982,
657. (IS) Hansen, P. Ado. Hererocycl. Chem. 1979,25,205. (16)Bencze, W.L.; Burckhardt, C. A.; Yost, W. L. J. Org. Chem. 1962. 27,2865. (17)Wagner, P. J.; Gerry, C. Mol. Photochem. 1969,1. 173. (18) Arnold, D. R. Ado. Photochem. 1968,6,301. (19)Koyanagi, M.; Goodman, L. Chem. Phys. krr.1973,21, 1. (20)Innes, K. K.; Ross, I. G.; Moomaw, W. R. J. Mol. Specrrosc. 1988, 132,492. (21) Hoover, R. J.; Kasha, M. J . Am. Chem. Soc. 1969,91,6508. (22)(a) Sushida, K.; Fujita, M.; Takemura, T.; Baba. H. J. Chem. Phys. 1983, 78,588. (b) Sushida, K.; Fujita, M.; Takemura, T.; Baba, H. Chem. Phys. 1984,88,221. (23)(a) Weisman, R. B.; Holt, P. L.; Selco, J. 1. J . Chem. Phys. 1982,77, 1600. (b) Selco, J. 1.; Holt, P. L.; Weisman, R. B. J . Chem. Phys. 1983, 79, 3269. (24)N a b h i m a , K.; Matsui, K.; Koyanagi, M.; Kanda, Y. Mem. Fa.Sci. Kyushu Uniu. Ser. C. 1979,I I , 28 1. (25)Letas, K. J.; Power, R. K.; Nishimura, A. M. Chcm. Phys. Lerr. 1979, 65.272.
b
A
1 mT
w
Figure 1. Conventional EPR (a), CIDEP (b), and simulated CIDEP (c) spectra obtained from the photolysis of 4-acetylpyridine in 2-propanol at 0 OC. CIDEP spectrum was taken a t 1 ps after the laser pulse.
of some mixing of the 3Al(7r~*)state with the nearby )Bl(nr*) state has been suggested in 2 from the vibrational structure of the ph~sphorescence~~ and from the zfs parameter^?^ though it was assigned to the n r * state from another ODMR measurement.26 The TI state of 4-phenylpyridine was reported to be m* from the conventional EPR and the phosphorescence spectra.27 This paper investigates on the TI states of a series of 4substituted pyridines (1-4) at 77 K by using the TREPR technique.
Experimental Section Commercial pyridine derivatives were purified by distillation under vacuum or recrystallization from ethanol. Measurements of TREPR were carried out by using an X-band EPR spectrometer (Varian Model E-1WE) without field modulation. The transient EPR signal was taken into a boxcar integrator (NF Model BX531) at arbitrary times after the laser pulse. An excimer laser (Lumonics Model HE-420, XeCl308 nm) was used as the source of light pulse. For continuous irradiation, a high-pressure Hg lamp (Ushio 500 W) was used. The sample solutions for the CIDEP experiments were constantly purged with argon to remove oxygen and flowed into a quartz tube within the EPR cavity. For the measurements of excited triplet EPR spectra, the samples were degassed by using three freezethaw cycles. All sample solutions for TREPR measurements were prepared at a concentration of 0.05 M. Phosphorescence spectra and triplet lifetime were observed at a concentration of lo4 M at 77 K by using a Hitachi Model 850 spectrophotometer. Solvents were purified by distillation after dehydration by the methods reported previously.B Results CIDEP Measurements. Figure la shows the conventional EPR spectrum obtained during the continuous irradiation of 1 (50 mM) (26)Motten, A. G.;Kwiram, A. L. J . Chem. Phys. 1981, 75, 2608. (27)Yagi, M.; Matsunaga, M.; Higuchi, J. Chem. Phys. Lrrr. 1982,86, 219. (28)Riddiclt, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solurnrs; Wiley: New York, 1986.
768 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Tero-Kubota et al.
in 2-propanol with a high-pressure Hg lamp (A 1 300 nm) at 0 O C . This spectrum is easily assigned to the neutral l-hydro-4acetylpyridinyl radical from the hfs constants.llC*dOn the other hand, transient EPR measurement on the same solution exhibited the CIDEP spectrum due to a different species. In Figure l b is shown the CIDEP spectrum observed at a delay time of 1 p after the laser pulse. The septets of doublet are due to the (CH,),C‘OH radical (aH(CHJ = 1.97 mT, aH(OH) = 0.05 mT, and g = 2.0031). The hyperfine structure of the complicated spectrum was analyzed with five splitting constants corresponding to the ketyl radical of 1. The assignments were made according to the type of splitting and by comparing them with the calculated spin densities by the McLachlan procedure. The observed spectrum was well reproduced by the computer simulation, in which both T M (E) and RPM (E/A) polarizations were taken into account (Figure IC), where E is an emission and A is an absorption of microwave radiation. The CIDEP signal at the high field of the acetylpyridine ketyl radical is very weak because of the compensation between the two polarization mechanisms. The hfs constants for the 1-hydro-4-acetylpyridine and 4-acetylpyridine ketyl radicals determined were assigned as follows.
+
E b
C
Figure 2. CIDEP spectra obtained from the photochemical hydrogen abstraction from ethanol by 4-cyano- (a) and Cmethoxycarbonylpyridine (b) at -20 O C , and 4-pyridinecarboxamide (c) at -40 OC. The observed spectra were taken at 1 ps after the laser pulse.
On the other hand, laser photolyses of 2-4 gave all A-CIDEP spectra. In Figure 2 is shown the transient EPR spectra of these pyridine derivatives observed at 1 p after the laser pulse in ethanol. Complicated signals at the central part of the spectra are easily assigned to the corresponding 1-hydropyridinyl radicals.”4” Since the photolysis of 4 gave a very weak CIDEP signal, the spectrum was observed at -40 OC inducing severe broadening. The total width of the spectrum well agreed with the steady-state EPR. Significant solvent effects on the hfs constants were observed for the pyridinyl radicals of 2 and 3.
d
.
d’, 0.1
I
8 = 2.0029
12.0033
The quartets of doublet are due to the CH3C‘HOH radical with the EPR parameters of aH(CH3)= 2.27 mT, aH(CH) = 1.55 mT, aH(OH) = 0.05 mT, and g = 2.0032. Similar CIDEP spectra were obtained in 2-propanol solution, in which septet lines due to (CH,),C’OH were observed instead of the ethanol radical. Triplet EPR and Phosphorescence Spectra. In order to clarify the character of TI states taking part in the initial process of photochemical reactions, triplet EPR spectra were measured at 77 K in a glassy matrix. The TREPR spectrum due to the triplet state of 1 in an absolute ethanol matrix observed at 1 p s after the laser pulse is shown in Figure 3a. The emissive signal at 0.137 T corresponds to the IAMsl = 2 transition. The lAMsI = 1 transitions show E at the low-field half and A at the high-field half indicating preferential population to the highest sublevel. The 0 1 = 0.150 and IEI = 0.006 cm-’ were deterzfs parameters of 1 mined by computer simulation (Figure 3a’). No conventional EPR spectrum due to the TI state was obtained under steady-state irradiation of the same solution. Solvent effects were observed on the triplet EPR spectrum for 1. In 2-methyltetrahydrofuran (MTHF) matrix, zfs parameters
0.1
0.2
0.3
0.4
B I T
0.5
0.2
0.3
Ok
0.5
B I T
Figure 3. Observed (a-d) and simulated (a’+’) TREPR spectra for the T , states of 4-acetyl- (a), 4-cyano- (b), 4-methoxycarbonylpyridine(c) and 4-pyridinecarboxamide (d) in ethanol matrix. The spectra were taken at 1 ps after the laser irradiation at 77 K.
1 0 1 = 0.185 and IEI = 0.008cm-’ were obtained. On the other hand, a very broad triplet EPR spectrum with D* (=(# + 31?)1/2) = 0.201 cm-I was observed in a methylcyclohexane (MCH) glassy matrix. Inhomogeneous distributions of D and E values should be taken into consideration to simulate the spectrum in hydrocarbon matrices as observed for several q u i n o n e ~ . ~ ~The , ’ ~ ob0 1 values are smaller than those of acetophenone (0.54 served 1 cm-I in durene31 and S0.25 cm-I in ethanol3*). (29) (a) Murai, H.; Hayashi, T.; I’Haya, Y. J. Chsm. Phys. Lea. 1984, 106, 139. (b) Murai, H.; Minami, M.; Hayashi, Y.; I’Haya, Y. J. Chsm. Phvs. 1985. 93. 333. 130) Shimoishi, H.; Tero-Kubota, S.;Akiyama, K.; Ikegami, Y. J . Phys. Chem. 1989, 93, 5410. (31) Cheng, T.; Hirota, N. Chem. Phys. Lett. 1972, 14, 415; Mol. Phys. 1974, 27, 28 1.
H Abstraction Reactions of Pyridines
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 769 TABLE I: Zfs Parameters and Spin Polarization of the TIStates of 4-Substituted Pvridines in Absolute Ethanol Metrix zero-field sublevel, cm-I 0.100
-0.044 -0.056 0.078
0.014
b h
-0.092
0.080 0.016 -0.097 I
0.l
I
I
0.2
0.3
I
Ok
0.072 0.012
0.5
B I T Figure 4. TREPR spectra of the TI state of 4-cyanopyridine in a 2meth ltetrahydrofuran glassy matrix excited with polarized light: (a) E 11 l a n d (b) l? I 8. The arrow indicates the canonical points.
The phosphorescence spectrum of 1 in an ethanol glassy matrix exhibits a characteristic carbonyl progression (- 1600 cm-l) and shows very short lifetime ( T < 1 ms). These results suggest that the TI state has mainly nu P character, while the EPR spectrum showed relatively small ID1 value. Similar phosphorescence spectrum and short T~ were observed in the MCH matrix except the red shift (Xo,o = 405 nm in ethanol and 413 nm in MCH). On the other hand, 2-4 showed the A AAA/EEE polarized EPR spectra in absolute ethanol matrix (Figure 3b-d). This implies that the anisotropic intersystem crossing (isc) occurred preferentially to the lowest sublevel. The zfs parameters were estimated by iteration of the simulation to get the simulated spectra of Figures 3b'-d'. Line shape used for the spectral component was Gaussian type with the width of 10 mT. However, the observed spectra could not be well reproduced by simulation. Especially, the relative peak height between the lAMsI = 2 and 1 signals does not fit, even if a broader width is used in the spectral calculation. Inhomogeneous distribution of the D and E values may be responsible for the spectral feature. Presumably, this is caused by the deformation of molecular structure in the matrix. Magnetophotoselection experiments were carried out to obtain information about the order of triplet sublevels using GlanThompson prism. Figure 4 shows the typical spectra for 2 4 an MTHF glassy matrix at 77 K. When the electric vectorJE) of the laser light was parallel to the static magnetic fie@ (B), the innermost paiLincreased their intensities and, when E was perpendicular to B, the intermediate pair become relatively strong. Similar results were obtained for 3 and 4. The same polarization patterns and similar triplet EPR spectra observed for these pyridines ( 2 4 ) suggest the localized character of the TI state on the pyridine ring. Small solvent effects on the triplet EPR spectra were observed in these pyridine derivatives. This is considered to be due to a negligible contribution of the spin-orbit coupling term to the zfs parameter^.^' The A-polarized CIDEP spectra (Figure 2) observed in the photolyses of these pyridines are reasonably explained by the polarization pattern of these triplet EPR spectra, if the radicals are generated from the Ti state. The phosphorescence lifetime (3 ms) observed for 2 in ethanol matrix agrees well with the previously reported value.21 Similar lifetime was obtained for 3 (6.0 ms) in ethanol matrix at 77 K. For 4, phosphorescence having very short T~ (
