J. Phys. Chem. 1082, 86, 3279-3281
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ducing the severity of wear for moving metal parts under high loads. The superior antiwear action of DBDS over DBS on steels is generally rationalized by assuming the disulfide more readily forms a mercaptide intermediate which later undergoes C-S bond cleavage to produce a sulfide film.12 Our results on silver, however, show that the di- and monosulfides form adsorbed mercaptides with comparable ease. Furthermore, starting from the disulfide, silver sulfide is probably formed before the mercaptide rather than after. We hope that SERS studies on silver in conjunction with classic wear tests will reveal the relationship between surface adsorbates and effective antiwear action. Such tests are feasible under realistic engineering conditions since SERS permits the observation of adsorbates even in the presence of a bulk liquid overlayer of lubricating hydrocarbons.2 The ready observation of SERS signals from decomposed DPhDS and DBDS even in the likely presence of Aga is also pertinent to studies of catalysis. Silver sulfide is believed to catalyze the formation of S2N2from S4N4on Ag gauze a t quite modest temperature^.'^ Formation of S4N4is the key step in producing the conducting polymer (SN),, a “nonmetallic metal”. Our results suggest SERS observations of this process should be feasible.
stretches since the latter are relatively insensitive to the sulfur environment. Indeed, in the family of compounds, DBDS (1602 cm-’), DBS (1598 cm-l), benzyl mercaptan (1602 cm-l), and benzyl mercaptide adsorbed on silver (1600 cm-’), scarcely any variation in ring-stretching frequency occurs. A distinctly different frequency behavior prevails for compounds containing the benzene thio group: DPhDS (1578 cm-’), DPhS (1582 cm-’), phenyl mercaptan (1581 cm-’), and phenyl mercaptide adsorbed on silver (1572 cm-l). Clearly, the ring-stretching frequency in the phenyl mercaptide is significantly affected by adsorption onto the silver surface, in sharp contrast to the adsorbed benzyl mercaptide. These data are consistent with the phenyl mercaptide “lying flat”on the silver surface and the benzyl mercaptide “sticking up” from the surface, with both compounds strongly bound to the silver surface through their sulfur atoms. It seems that the molecular geometry of the phenyl mercaptide (most likely planar, in contrast to the benzyl mercaptide) strongly influences adsorbate configuration. The observation of adsorbed organosulfur monolayers by SERS offers a powerful means to elucidate the mechanism of action of antiwear additives and other boundary lubricants.’0 DBDS, for example, is a prototype “extreme pressure” additive.lOJ1 Most theories of antiwear action presume DBDS decomposes on metal surfaces to form a protective, easily sheared metal sulfide layer thereby re-
Acknowledgment. We thank David A. Weitz for very helpful conversations and advice concerning SERS from island films.
(10)H.Czichos, ‘Tribology, A System Approach to the Science and Technology of Friction Lubrication and Wear”, Elsevier, New York,1978. (11)T. Sakurai, J.Lubr. Technol., 103,473(1981).
(12)B. Dacre and C. H. Bovington, ASLE Trans., 25, 272 (1982). (13)M.M.Labeg, P. Love, and L. F. Nichols, Chem. Rev., 79,1(1978).
Direct Detection of Spin-Polarized ESR Spectra of Biacetyi n7r* Triplet States in Organic Matrices at 77 K Hlsao Mural, Takashl Imamura, and Klnlchl Obl’ Department of Chemistry, Tokyo Institute of Technobgy, Ohokayam, Msguro, Tokyo, Japan (Received: June 9, 1982)
Transient spin-polarized ESR signals of the phosphorescent state of biacetyl have been studied in organic matrices. From analysis of the spectra, it is concluded that the D and E values are positive and negative, respectively, and the initial population to the 2 sublevel whose principal axis lies along the C=O direction is predominant. The increase of the Y sublevel populating rate in n-hexane suggests that the population of the spin sublevels through intersystem crossing is sensitive to minute distortions of the molecular frame. Introduction The photophysical and photochemical properties of the lowest triplet states of biacetyl have been a target of recent spectroscopic research in conjunction with the electronic structure. Chan and his co-workers investigated small a-dimbonyh extensively using the ODMR technique and gained a lot of valuable spectroscopic and kinetic inforThey concluded mation concerning the triplet that the biacetyl triplet sublevel having the fastest populating rate by S1-T1 intersystem crossing was 2 whose principal axis lies approximately along the C=O direction and the most emissive level was also 2. The signs of the D and E values they suggested were negative and positive, (1)I. Y.Chan and R. H. Clarke, Chem. Phys. Lett., 19, 53 (1973). (2)I. Y.Chan and K. R. Walron, Mol. Phys., 34,65 (1977). (3)I. Y.Chan and S. Hsi, Mol. Phys., 34, 85 (1977). 0022-3654/82/2086-3279$01.25/0
re~pectively.~Recently we started time-resolved ESR studies of phosphorescent states and have already reported spin-polarized spectra of the 3 n ~ state * of benzophenone in glasses: In t h i s Lef% the application ofthis technique to biacetyl in o r g ~ matrices c and an important conclusion about the signs of D and E which are different from those be shown* reported by Chan and Hsi3 Experimental Section An X-band ESR spectrometer, Varian E-112, was modified for the time-resolved experiment. The experimental technique was the same as the one reported previously,4 which was introduced by the pioneering work of A MR-50E transient memory and a Weissman et (4) H. Murai, T. Imamura, and K. Obi, Chem. Phys. Lett., 87, 295 (1982).
0 1982 American Chemical Society
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The Journal of Physical Chemistty, Vol. 86, No. 17, 1982
Letters
b) D N
't
R O
I
0
20
4.0
' *H 6.0 1 kG)
z
A
: 0 '1 - II
2000
4000
6000
H /gauss Flgure 1. ESR spectra of the spin-polarized triplet state of biacetyl observed at 77 K (a) in ethanol, (b) in cyclohexane, and ( c ) in nhexane. The gate of the boxcar integrator was opened between 0.3 and 0.8 ws after laser excitation.
TMC-400 signal averager provided by Kawasaki Electronica were used to record the time profiles of the transient signals. A boxcar integrator, NF Model BX-531, was used to record the spin-polarized ESR spectra at a fixed gate timing. The time-resolved ESR spectra were observed as a direct absorption and emission of microwave radiation. A nitrogen gas laser (A = 337.1 nm) was used as a pulsed excitation light source. G. R. grade biacetyl (2,3-butanedione), Kanto Chemicals, previously distilled in a vacuum line was used. Spectrograde ethanol, cyclohexane, and n-hexane, Nakarai Chemicals, were used as solvents. The high concentration, 1-2 M, of biacetyl was used to gain high optical density because of the small extinction coefficient of biacetyl, E -2, at 337.1 nm. The samples were degassed under vacuum and sealed in a quartz ESR tube with a diameter of 5 mm. The matrices were prepared by quenching these samples at 77 K. All the experiments were carried out at 77 K. Results and Discussion A strong transient microwave absorption was observed at a magnetic field of ca. 1000 G. I t builds up quickly (within 0.3 ps) after laser excitation and decays for a few (5) S. S. Kim and S. I. Weissman, J. Magn. Reson., 24,167(1976);Reu. Chem. Intermed. 3,107 (1979);G.Kothe, S.S. Kim, and S. I. Weissman, Chem. Phys. Lett., 71, 445 (1980).
Flgure 2. An explanation of the spin-polarized ESR spectra of the biacetyl triplet state: (a) D < 0,E > 0 and (b) D > 0,E < 0. The arrows indicate the initial populating levels. See text.
microseconds at a microwave power of 30 mW. The polarized ESR spectrum of the lowest excited triplet state of biacetyl in ethanol glass is given in Figure la. From the spectrum, it is obvious that the signals at fields lower than g = 2 are all absorptive (A) where 2-canonical point (HzJ is superimposed on a strong lh,l = 2 transition, and those on the higher field side are emissive (E) except HY2 which is obscure. Chan and Hsi reported that the 2-principal axis of the biacetyl triplet state in an X-trap was parallel to C=O direction and Y is perpendicular to the molecular plane. If we asume that the principal axes, the signs of the D and E values, and the initial populating level proposed by Chan and Hsi are applicable to our system, the expected time-resolved ESR spectrum should have a polarization reversed compared to our result. The four low-field canonical points and three high-field ones are predicted to be emissive and absorptive, respectively, similar to the time-resolved ESR spectra of benzophenone triplet state reported previously.4 This situation is illustrated in Figure 2a by the solid lines. This serious discrepancy may be explained in the following two ways: (1)In glassy matrices, the rates of the initial population to the spin sublevels are completely different from those in X traps in a single crystal. If the negative value of D suggested by Chan and Hsi3 is correct, the initial population to X and Y sublevels should take place in comparable magnitude, and the populating rate to the 2 sublevel is very slow. This situation is illustrated in Figure 2a by the broken lines.
The Journal of Physical Chemistry, Vol. 86, No. 17, 7982 3281
Letters
TABLE I: Zero-Field Splitting Constants, Polarization of ESR Signal, and AH,,^ of Hmin of Biacetyl 'nn* polarizationa solvent ethanol cyclohexane n-hexane
zfs constant/cm-' ID1 = 0.214 * 0.004 IEl = 0.019 f ID1 = 0.208 f IE I = 0.016 ID1 = 0.205 f IE I = 0.018f
*
0.004 0.005 0.005 0.003
Hxi A
HY, A
HY, 0
HXZ
E
E
A
A
A
(A)
E
(E)
A
A
E
A
E
(E)
Hmin A
HzI
H z ~
0.003
AH,/,/' -150
-100
-
90
A and B denote absorptive and emissive, respectively. 0 denotes obscure polarization and brackets show weak signals.
(2) If we assume the exclusive population to 2 sublevel as proposed by Chan and Hsi, the observed spin-polarized ESR spectrum leads to the positive value of D as illustrated in Figure 2b by the solid lines. It is hard to accept the former possibility because the initial population to the 2 sublevel is predominant in X traps and the circumstance of a glassy matrix does not seem to change the populating sublevels from 2 to both X and Y drastically. In the case of benzophenone reported in the previous paper: the whole situation in glassy matrices and the single crystal is qualitatively identical. We propose therefore that the absolute signs of D and E values are positive and negative, respectively, and the initially populating sublevel by intersystem crossing is mainly 2 in the ethanol matrix. This conclusion is similar to the case of the triplet state of benzil reported by Chan and Nelson! They showed that the D value was positive and the lowest zerefield spin sublevel almost totally dominated the population and depopulation activities. They also proposed a trans (near) planar configuration in the lowest triplet state, similar to the case of the biacetyl triplet state. Concerning the initial populating sublevel and the signs of the D and E values of the biacetyl triplet state, however, there still remains some ambiguity, because little information about the principal axes is available other than the one reported by Chan and Hsi. The obscure polarization of the high-field Y-canonical point (Hm) in ethanol suggests that the populating rate to the Y sublevel is not negligible. This is because the populating rate to the highest level (see H / / Y in Figure 2b), which is determined by the mixing of IX)and IZ), becomes close to that of the middle level IY) at the field of HE. We obtained interesting experimental results concerning the minute molecular distortions using cyclohexane and n-hexane as solvents. The spectrum observed in the cyclohexane matrix shows polarization similar to that observed in ethanol, but tlie intensity of the Y canonical point at the low field ( H n ) is somewhat weak and the signal at the high field (HY2)is absorptive as shown in Figure lb. The polarized ESR spectrum observed in n-hexane showed reverse polarization at the Y-canonical points compared (6)I. Y. Chan and B. N. Nelson, J. Chem. Phys., 62, 4080 (1975).
to that in ethanol as shown in Figure IC. A little increase of the populating rate to the Y sublevel in cyclohexane and much increase in n-hexane can explain the changes of the polarization of Y-canonical points consistently as illustrated in Figure 2b by the broken lines. Chan and Hsi3 reported the change of populating rates to the biacetyl spin sublevels under different circumstance. Their result showed that the populating ratio of the Y sublevel to the 2 sublevel increased in strained biacetyl crystals, but the populating rate to the 2 sublevels was still faster than the others. They suggested that the populating intersystem crossing process was hypersensitive to minute molecular distortion from the planar structure because of the activation of the population at the Y sublevel through vibronically induced one-center spin-orbit interactiona2Our results in cyclohexane and n-hexane resemble those of Chan and Hsi. In fact, cyclohexane and n-hexane matrices are not perfect glasses, rather they seem to be polycrystalline. This suggests that the biacetyl sample may suffer a molecular distortion to some extent in the matrices. The zero-field splitting constants and the full line width at half-height (AHlI2)of lAm,l = 2 transition obtained in these three matrices are tabulated in Table I. There seems to be no significant solvent effect on the zfs parameters. The solvent effect on AHll2of lAmJ = 2 transition may be explained simply by the different width of the distribution of the molecular environment in each matrix. Especially in cyclohexane and n-hexane, the distribution may be much narrower than that in ethanol because of their polycrystalline phase. The broadness of the triplet signal is characteristic of 3n7r* which has already been reported4 in the case of benzophenone 3 n ~ * ,The line width of biacetyl 3 n ~ is * narrower than that of benzophenone 3na* where inhomogeneous distortion of the phenyl ring in glasses may be one of the causes of the broadening. The coupling with 3 ~ state ~ *having higher energy as discussed in the case of benzophenone is not applicable in the biacetyl system, since the separation of %IT* and higher 3 ~ states ~ * is very large: more than 22 OOO cm-'. Acknowledgment. The authors are grateful to Professor Ikuzo Tanaka for his interest in this work. (7)T-K. Ha,Chem. Phys. Lett., 57,64 (1978).
