J. Phys. Chem. 1981, 85, 1469-1474
1469
ARTICLES Phosphorescence, Optically Detected Magnetic Resonance, and Electron Paramagnetic Resonance Studies of the Lowest Excited Triplet State of the Ant! Isomer of a 1,5-Diazabicyclo[3.3.0loctadienedione (9,1O-Dioxa-anti-(methyl,hydrogen)bimane) Masaakl Baba,+ Noboru Hlrota,"t§ and Edward M. Kosowert5 Department of Chemistry, Faculty of Science, Kyoto Universlty, Kyoto, Japan: Department of Chemistry, Tei-Aviv Universlty, Tel-Aviv, Israei: and Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York (Received: December 17, 1980: In Final Form: February 11, I98 I)
We have studied the T1 state of the anti isomer of a 1,5-diazabicyclo[3.3.O]octadienedioneby means of phosphorescence emission and excitation spectroscopy, optically detected magnetic resonance (ODMR), and EPR. From the excitation spectra the locations of the T2(3n7r*)and Sl(ln?r*)states were determined in crystals and frozen solutions. It was suggested that the molecule is planar in crystals and acetonitrile but is twisted in methyltetrahydrofuran (MTHF). Contrary to the cases of many aromatic carbonyls, the radiative decay rate at the 0-0 band is the largest for the y sublevel, which indicates the importance of the spin-orbit mixing with the lm* state in the radiative mechanism. The radiative activity of the x sublevel at the vibronic band is due to the 37r7r*-3n7r*and 1n7r*-17r7r*vibronic mixings. The radiationless decay takes place predominantly from the x and y sublevels as expected. In the durene mixed crystal there are two differently oriented molecules which give rise to slightly different zfs and phosphorescence spectra: D = 0.1004 cm-l and IEI = 0.0245 cm-l, and D = 0.1000 cm-' and IEl = 0.0235 cm-l, were obtained for these species.
Introduction The recently synthesized new class of compounds, synhave many and anti-l,5-diazabicyclo[3.3.0]octadienediones interesting photophysical properties.14 These molecules are called syn- and anti-bimanes, and brief formulas are written syn-(R1,R2)bimaneand anti-(R1,R2)bimane.The molecular structures are shown in Figure 1. syn-Bimanes are strongly fluorescent and very weakly phosphorescent. This is presumably because the lowest excited singlet, S1('7r7r*),state is located far below the %7r* state and the intersystem crossing is not effective. On the other hand, anti-bimanes are strongly phosphorescent and very weakly fluorescent. A recent picosecond pulse study showed that intersystem crossing is very fast (>lo1' s-').~ It was suggested that this high rate is due to the proximity of the 3n7r*state to the Sl('a7r*) state. The lowest excited triplet (TI)states of bimanes are of 3 ~ 7 r *character. From the relatively long TI lifetimes, the 3n7r*-37rir* energy separations (am) are considered to be large. We have investigated the T1 state of anti-(CH,,H)bimane by means of low-temperature phosphorescence spectroscopy, zero-field ODMR (optically detected magnetic resonance), and high-field EPR for the following reasons. (1) In order to fully understand the photophysical properties of anti-bimanes, it is important to identify the locations of the 3n7r* and h a * states, but such states have not been detected before. Phosphorescence excitation spectroscopy is well suited for the detection of weak abs o r p t i o n ~and ~ * ~was expected to be helpful in identifying the 3n7r* and Inn* states of anti-bimanes. Kyoto University University. f State University of New York a t Stony Brook.
4 Tel-Aviv
0022-3654/81/2085-1469$01.25/0
(2) In the previous study' we noted that coumarin (a molecule with a large am) has sublevel decay properties which are quite different from those of other aromatic carbonyl compounds with small am values (e.g., such as substituted benzaldehydesa1"), It seemed worthwhile to examine whether the T1states of anti-bimanes have similar decay properties. (3) In bimanes, two a,@-unsaturatedcarbonyl systems are connected by two nitrogens. Magnetic properties such as zero-field splittings (zfss) may provide information about the interaction between the two conjugated systems. We discuss the locations and the characters of the T2 and S1 states, zfs, the decay properties of the T1state and the mechanisms of radiative and radiationless transitions. Experimental Section Phosphorescence, ODMR, and EPR experiments were made with durene mixed crystals containing anti(CH3,H)bimane at liquid-helium temperature. Phosphorescence experiments were also made by using a neat crystal and methanol solutions at 4.2 K and 3-methylpentane, acetonitrile, and methyltetrahydrofuran (MTHF) solutions at 77 K. ~~
(1) E. M. Kosower, B. Pazhenchevsky, and E. Hershkowitz, J. Am.
Chem. Soc.. 100.6515 (1978). (2)E.MI Kosower, J, Bernstein, I. Goldberg, B. Pazhenchevsky, and E. Goldstein, J . Am. Chem. SOC.,101, 1620 (1979). (3)D. Huppert, H. Dodiuk, H. Kanety, and E. M. Kosower, Chem. Phys. Lett., 65,164 (1979). (4)E.M.Kosower, H. Kanety, and H. Dodiuk, submitted for publication. (5)W.Rothman, A. Case, and D. Kearns, J. Chem. Phys., 43, 1067 (1965). (6)N.Hirota, J. Chem. Phys., 44, 2199 (1966). (7)E.T.Harrigan, A. Chakrabarti, and H. Hirota, J. Am. Chem. Soc., 98, 3460 (1976). (8) T. H. Cheng and N. Hirota, Mol. Phys., 27,281 (1974). (9)E. T.Harrigan and N. Hirota, Mol. Phys., 31, 631,681 (1976). (10)Y.Hirata and N. Hirota, Mol. Phys., 39, 129 (1980).
0 1981 American Chemical Society
1470
The Journal of Physlcal Chemistry, Vol. 85, No. 11, 1981
Baba et ai. >I
s&R1,
R z )birnane
~ & i - ( R l , R z ) bimane
(a). polycrystalline
_, (b).in durene crystal
Flgure 1. Molecular structure of syn- and anti-(CH3,H)bimane.
anti-(CH3,H)bimane was prepared by following the procedure described in the previous paper.' Durene crystals containing small amounts of bimane were grown from melts by the standard Bridgman method. The solubility of bimane into durene crystal is very poor, and it is considered that the concentration of bimane in the mixed crystal is very low. Durene was carefully purified to remove duraldehyde impurity by the extensive zone refining after recrystallization from ethanol. The phosphorescence and ODMR experiments were made by using a setup and procedures similar to those described previously." The sample, immersed in a liquid-helium cryostat, was irradiated by light from a xenon arc lamp (Osram 900 W) filtered through a NiS04 solution and a glass filter (Toshiba UVD33S). The phosphorescence emissions were dispersed by a 1-m scanning spectrometer (Spex 1704) and detected with a photomultiplier (EM1 9502B). The emission signals chopped by a sector rotating at 300 Hz were amplified by a lock-in amplifier (PAR HR-8). In taking the phosphorescence excitation spectra, the output of the 900-W xenon arc lamp was monochromatized by the Spex spectrometer and focused on the sample. The total phosphorescence was chopped by a sector and was lock-in detected. Durene mixed crystals of -5-mm diameter and 1-cm length were used for taking the excitation spectra. Spectra of the neat crystal were measured by using a crystal grown in quartz tubing of 2-mm i.d. by the Bridgman method. Acetonitrile, 3-methylpentane, methanol, and MTHF solutions contained in quartz tubing of 6-mm i.d. were slowly cooled and frozen before measurement of spectra. The zero-field ODMR signals were obtained by sweeping the microwave sweeper (HP 8690B) and detecting the changes in the phosphorescence intensities. The decay rates from the individual sublevels were determined by the standard MIDP (microwave induced delayed phosphorescence) technique described in the literature." To obtain the microwave modulated spectra, we modulated the phosphorescence chopped at 300 Hz, by amplitude modulation (AM) of the microwave radiation at 1 Hz, and detected the modulated signals with two lock-in amplifiers operating at 300 and 1 Hz. The high-field EPR experiments were made at 4.2 K by using a JEOL ESCXA microwave unit equipped with a cylindrical cavity (TEoll mode) with an irradiation slit. A sample crystal of 1 X 5 X 7 mm was mounted on a wedge made according to the description given by Hutchison and Mangum13 and rotated in a finger tip Dewar which was inserted inside the cavity.
Results and Discussion Phosphorescence Emission and Excitation Spectra. The phosphorescence emission and excitation spectra of anti-(CH,,H)bimane obtained in a neat crystal and durene (11)T. H. Cheng and N. Hirota, J. Chern. Phys., 56, 5019 (1972). (12)J. Schmidt, D.A. Anthemis, and J. H. van der Waals, Mol. Phys., 22, 1 (1972). (13)C.A.Hutchison, Jr., and B. W. Mangum, J. Chern. Phys., 34,908 (1961).
A
\
I
480 440 400 360 nm Figwe 2. Phosphorescence emission (left) and excitation (right) spectra of anti-(CH3,H)bimane at 4.2 K. (a) Neat crystal. (b) Durene mixed crystal. The spectrum shown in the upper corner was taken under high resolution.
mixed crystals at 4.2 K are shown in Figure 2. Each vibronic band of the phosphorescence spectrum of the durene crystals consists of three peaks, one broad and two sharp ones, which are due to the two different sites as shown from the ODMR and EPR experiments. The origin of the site I spectrum (broad and high-energy one) is located at 439.7 nm (22 743 cm-'), and the origin of the site I1 spectrum is at 438.5 nm. The main vibrational bands are 0-374,O-599,O-980,O-1315,O-1591, and 0-1729 for the site I spectrum. Similar vibrations were observed for the site I1 spectrum. The peaks of the site I1 spectrum are accompanied by a 23-cm-' sideband which is presumably due to lattice vibration. The phosphorescence spectra obtained in acetonitrile, 3-methylpentane, and MTHF are similar to that obtained in the durene crystal, though the resolution of the spectra is poorer. The spectrum obtained in methanol is very broad and structureless even at 4.2 K. The phosphorescence excitation spectrum of the neat crystal shows at least three origins for the absorption bands. The very weak peak at 439 nm (22 770 cm-') is close to the 0-0 band of the phosphorescence spectrum and is assigned to the origin of the So TI absorption of the crystal. The next broad feature (XI),starting at -412 nm (24270 cm-') is assigned to the vibronic bands of the So T1 absorption. The higher intensity of this feature is probably due to the enhanced 3nr* character of the vibrational states of the T1 state. It is known in conjugated enones14and xanthone15that the vibronic bands of the So T1(%r*) absorptions gain intensities as they approach the origins of the 3nr* state. The next band (X&,starting at 397 nm (25 190 cm-l), has a considerable intensity and is assigned to the So T2(3nr*)absorption. The strong band, starting at 377 nm (26510 cm-l), is then assigned to the So Sl('nr*) absorption. The energy separation between the 3nr* and 'nr* states thus determined is 1320 cm-', which is quite consistent with the known 3nr* and 'nr* separations of the aromatic carbonyl compoundssJe and conjugated enones.14 An alternative assignment is possible for the origin of the So T2absorption by taking the X1band as the origin. However, we see the following difficulties for this assignment. In this assignment the X2 band is the vibronic band of the So T2(3nr*)absorption. The main vibronic bands of the So 3nr* absorption spectra are likely due to the C=O stretching vibration in the 3nr* state whose frequency is -1200 cm-'.17 This frequency is considerably
-
-
-
-
-
-+
--
(14)C.R.Jones, D. R. Kearns, and A. H. Maki, J. Chern. Phys., 59, 873 (1973). (15)A. Chakrabarti and N. Hirota, J. Phys. Chern., 80,2966 (1976). (16)H. Hayashi and S. Nagakura, Mol. Phys., 24,801(1974).
The Journal of Physical Chemistry, Vol. 85, No. 11, 1981
Lowest Excited Triplet State of an anti-Bimane (a) in acetonitrile
(cl. in MTH F
(b). ~n 3-methylpentane
(d) in methanol
1471
I
polycrystalline
Figure 3. Phosphorescence excitation spectra obtained in different solvents and at dlfferent concentrations: (a) acetonitrile, (b) 3methylpentane, (c) MTHF, (d) methanol.
larger than the energy separation between the X1and Xz bands (920 cm-'). The intensities of the 0 4 and the C=O stretching bands are usually comparablela contrary to the large intensity difference found for the X1 and X2 bands. Furthermore, in this assignment the energy separation between the h a * and 3n7r* states becomes 2240 cm-l, which seems to be somewhat large. The excitation spectrum of the durene mixed crystal does not show the absorptions due to the So T1and So T, transitions, since the concentration of bimane in durene is very low. The first band starting at 378 nm (26460 cm-') is then likely due to the So Sl(lnr*) absorption. The origin of this absorption band is in good agreement with the origin of the So Sl('nr*) absorption of the neat crystal. A strong absorption starts at 360 nm (27 800 cm-', which is considered to be the origin of the So Sz(laa*) absorption. The above assignments are further substantiated by the concentration dependences of the excitation spectra shown in Figure 3. In the excitation spectrum of a thick sample of high concentration, even a weak absorption gives a band of considerable intensity, because a long path length for light compensates for a weak absorption. In such a sample, the path length for a strong absorption becomes very short and the intensities of the weak and strong bands become comparable. The concentration dependence of the excitation spectra in acetonitrile and 3-methylpentane shows that the extinction coefficient of the So S1 absorption is more than 2 orders of magnitude smaller than that of the So S z ( ' m * ) absorption which has extinction coefficients of the order of 10OO0.4 Therefore, the extinction coefficient of the So S1absorption should be less than 100, a value which is reasonable for a So lnr* absorpt i ~ n The . ~ locations of the S1and Sz states in acetonitrile and 3-methylpentane are similar to those in the neat and durene mixed crystals. In methanol the So Sl('nn*) absorption is blue shifted by 1070 cm-l compared to that in the durene crystal, but the So Sl(lnr*) absorption is still detectable as a shoulder in the excitation spectra of concentrated solutions. On the other hand, the excitation spectrum obtained in MTHF is remarkably different. This spectrum does not show the band due to the So 'nr* absorption at any concentration, indicating that the So lnr* absorption is buried under the strong absorption around 340 nm. Then the lnr* state must be very close to or higher than the lrr* state. The locations of the origins of the T1, Tz, S1, and Sz states determined from
-
-
-
-
-
-
-
-
-
-
-
--
(17) S.Dym,R. M. Hochstrasser, and M. Schafer, J. Chem. Phys., 48, 646 (1968). (18) S.Yamauchi and D. W. Pratt, Mol. Phys., 37, 541 (1979).
durene
acetonitrile
3-rrethylpentane methanol
MTHF
Figure 4. Energy-level schemes for anti-(CH,,H)blmane in different solvents.
the excitation spectra are summarized in Figure 4. The shift of the lnr* state energy in different environments does not follow the usual shift due to the solvent polarity. By far the largest shift was found in MTHF. Contrary to the normal solvent dependence the lnr* state in 3-methylpentane is blue shifted compared to that in acetonitrile. This unusual solvent dependence of the h a * energy may reflect a structural change of the h a * state. An X-ray structural investigation of anti-(CH,,H)bimane showed that the molecule was planar in the crystal.2 The results of the present ODMR and EPR studies on the compound in a durene host indicate that the molecule is likely to be planar in the Tl state in durene. The energies of the 3nr* and 'nr* states of the bimane in the neat crystal and in a durene host crystal probably represent those of planar excited states. Similarly, the bimane is probably planar within the frozen acetonitrile solution. On the other hand, evidence and arguments that the antibimane ring system is structurely flexible in solution have been p r e ~ e n t e dthe , ~ idea being that avoidance of the nonbonding electron pairs of the two nitrogens for each other leads to a twisted structure in the absence of the constraints imposed by the crystal or host or frozen solvent arrangement. An attempt to detect the So h a * absorption in solution was not successful, suggesting that the lnr* state in solution is close to or higher than the h a * state. Since MTHF forms a glass at 77 K, the anti-bimane may retain the twisted structure with a higher lnr* state at 77 K. Nevertheless, the phosphorescence spectrum in MTHF is very similar to those in durene and acetonitrile, so that it is probable that the anti-bimane has a planar 3 r ~T1 * state in MTHF. Zero-Field ODMR and MIDP. Strong ODMR transitions of anti-bimane in durene host were found at 2.271 and 1.460 GHz by monitoring the phosphorescence at the 439-nm peak and at 2.294 and 1.407 GHz by using the 441-nm peak, indicating that these phosphorescence peaks come from different sites. When we monitor the 0-374(379) vibronic bands, the third transitions (3.731 GHz for site I and 3.701 for site 11) were observed. Since the zfss for the two sites are slightly different, phosphorescence coming from different sites is most conveniently separated by microwave modulated phosphorescence spectra. They are shown in Figure 5. It is considered that the bottom sublevel is the radiatively inactive sublevel. The sublevel energy scheme thus obtained is shown in Figure 6. The values of the zero-field splitting (zfs), total decay rate constants (hi),relative radiative decay rates (h:), the relative steady-state population (N?),and populating rates (Pi) of the spin sublevels of the anti-bimane in durene, obtained by the zero-field ODMR and MIDP experiments,
-
1472
The Journal of Physical Chemistry, Vol. 85, No. 71, 1981
Baba et ai.
TABLE I: Decay and Magnetic Properties of the T, State at Two Sites site I
site I1
k jr
hi, S-' iX ) IY ) i 2)
1.83 2.99 0.144
ID I IEl
0-374,ern-' 0.04 1.27 1 1 0.017 0.14 0.1004ern-' 0.0245 cm-' 0-0
k jr Pi
Ni
hi, S-'
1 0.89 0.09
0.92 0.50 1
2.06 2.64 0.192
a) total Phosphorescence at 1 5 ° K
0-374,cm-' 1.6 1 1 0.026 0.15 0.1000 cm-' 0.0235cm-' 0-0
SITE I
Pi
Ni
1 0.9 0.1
0.9 0.6 1
SITE I I
2.27lGHz
'
2.294GHz
6
Flgure 8. Sublevel scheme: total d )and relative radiative (d) decay of the rates, steady-state populations ( ), and populatlng rates (I) three spin sublevels of the T, state in two sites. ci 1 460GHz (X-YY)
d12.294GHz (Y-
2)
e) 1 407GHz (X-
Yi
2, 'i
are D = 0.11 cm-' and IEl = 0.10 cm-l in the present axis system. The D of bimane is only slightly smaller, but the IEI of bimane is less than 1/4 of the E for a,@-unsaturated ketones. This large difference in IEI probably implies that the excitation is delocalized over the entire bimane molecule. Although the total decay rates from the x and y sublevels are not very different, the relative radiative decay rates are very dependent on the sublevel and vibronic bands. The 0-0 emission comes almost exclusively from the y sublevel in spite of the low molecular symmetry. On the other hand, at the 0-374 band the x sublevel emission is stronger than the y sublevel emission. This is clearly seen from the microwave modulated spectra given in Figure 5. In the total emission the 0-374 band has an intensity stronger than that of the 0-0 band at 1.2 K, but in the x y microwave modulated spectrum the 0-374 peak is extremely weak, because 12,' and 12,' are similar. These results clearly show that the direct spin-orbit mixing is mainly responsible for the y sublevel emission, but the vibronic mixing is necessary for the x sublevel emission. Assuming that the one center integral at the oxygen atom is predominant in the relevant spin-orbit matrix elements as in the cases of many other aromatic carbonyl compound~,'-~~ we discuss the radiative properties on the basis of CZulocal symmetry of the carbonyl moiety. The radiative decay rate of the sublevel i at the 0-0band is given by
-
12: = v3CI(S11~solT1i)12/ [E(SJ - E(Tl)II(S&Yo)12 1
when the contribution due to the mixing in the triplet manifold is negligible. Here S I is the singlet state which mixes with the ith sublevel by spin-orbit coupling. E ( S J and E(Tl) are the energies of the Si and T1 states. Since the T1 state is a 37r7r* state, the relevant mixing singlet states ( S I ) are h a * and laa* (laa*) states. In the Czv symmetry, the y sublevel mixes with the lua* state and the x sublevel mixes with the Inn-* state. Which of the two, k,'(O-O) or k,'(O-0), is larger depends on the energy denominator and l(Sller'lSo)12. In the aromatic carbonyl compounds with relatively small E(lna*) - E(Tl) such as substituted benzaldehydes, k,'(O-0) > k,'(O-0) was generally As E(lna*) - E(TJ becomes large, it;(0-0)
The Journal of Physical Chemistry, Vol. 85,
Lowest Excited Triplet State of an anti-Bimane
TABLE 11: Mechanisms for Radiative and Radiationless Transitions Radiative Transition
(TI
-
e
site
I
No. 11, 198 1 1473
I
site 11
So) Y -
Radiationless Transition (TI
-
So)
Y-
I
-90"
is expected to become larger than rZ:(O-O), since I(lcra*leflSo)12>> I(l n ~ * l e q S ~ ) This l ~ . ~situation ~ was previously found in coumarin.' In the anti-bimane, we also found k,T(O-0) > rZxr(0-O). It may be concluded that this order is generally the case for the carbonyl compounds with large ln7~*-~7r7r*energy separation. The emission from the x sublevel involves vibronic mixing via a low-frequency, out-of-plane vibration. Vibronic couplings in both singlet and triplet manifolds probably contribute to the emission, but the coupling in the triplet manifold may be more important because of smaller energy denominator. The z sublevel emission is expected to be minor when the molecule is planar. The X-ray analysis has shown that anti-bimane is planar in the ground state.2 The small decay rate from the z sublevel shows that the molecule is also nearly planar in the Tl state. The mechanisms of the transition are summarized in Table 11. The quantum yield of the phosphorescence is reported to be 0.45.4 Hence the total decay rates include substantial contributions from radiationless decay. The radiationless decay also takes place from both x and y sublevels. The radiationless decay rate of the sublevel is determined by the matrix element I( 1\konlR13\klo) I where R is the level shift operator. 3\k10and are the wave functions for the 0th vibrational level of the T1 state and the nth vibrational level of the ground state, respectively. In the pure spin Born-Oppenheimer approximation the matrix element is written as20 (l*o,IRl3*1o)
=
(l*On17fso13q10)
C(1*0n17fso13*~u) Ul
(l*OnlTNll*mu') um '
+
(3q~ulTN13*10) /f3E10 -
3Elul +
( 1 * m u ~ ~ 7 f s o ~ 3 ~ 1 0 ) / [ 3 E 10
'Ern"?
where TNis the kinetic energy operator for nuclei. Here QlO is a 37r7r* state and the first term is small for a planar molecule at the equilibrium position. However, a Herzberg-Teller expansion of this term produces the terms involving vibronic interaction which have forms similar to the second and third terms. The main mixing states are 3n7r*,37r7r*, ln7r*, and 'm* states. The states which couple with the x and y sublevels in the CZusymmetry and the mechanisms of radiationless transitions are summarized in Table 11. In contrast to the radiative transition, the radiationless transition from both x and y sublevels are allowed, as observed experimentally. Because of larger energy separations between T1 and 3n7r* and h a * states, (19) Y. Tanimoto, H. Kobayashi, S. Nagakura, and T. Azumi, Chem. Phys. Lett., 16, 10 (1972). (20) N. Kanamaru and E.C. Lim, J. Chem. Phys., 65, 4055 (1976).
1
3000
4000G
H
I
Flgure 7. Angular dependence of the EPR signals. Dots represent experimental points and solid curves represent the calculated angular dependence.
-+ Flgure 8. Probable orientations of bimane molecules in durene crystals.
TABLE 111: ESR Resonance Fields of onti-(CH,, H )Bimanea site I
&Xb
&Yb
i3 /Izc
site I1
2315 (2334) 3066 (3090)
4191 (4180) 3358 (3370)
2333 4172 (2352) (4165) 3052 3374 (3072) (3388)
2 21 2 (2227)
4367 (4363)
2217 (2227)
4364 (4363)
Calculated values (experimental values in parentheses) in gauss. 9.196 GHz. 9.253 GHz.
the radiationless decay rate from the x sublevel is much smaller than in the case of other aromatic carbonyls with small aEm.9J0 High-Field EPR Experiments. Durene crystal has two molecules per unit cell.13 We have made EPR measuremnts by moFting the durene mixed crystals so that the applied field (H)rotated in the molecular plane of one type of durene molecule. The observed angular dependences of the EPR signals are shown in Figure 7. T_he axes of the molecules are shown in Figure 8. When H is nearly parallel to the L axis of one type of durene molecule, H becomes almost parallel to the N axis of the other type of durene. At this orientation we obtained the z stationary point. The field strengths of the stationary points are in good agreement with those calculated by solving the spin Hamiltonian
eS= gPfi.3 + DSZ2+ E ( S X 2- S,") with g = 2.003 and D and E determined from the ODMR experiments. This indicates that anti-bimane substitutes durene with its molecular plane parallel to that of durene and the T1 state is planar. It is y e n that there are two anti-bimane molecules for which H rotates in the x-y plane of the molecule. The stationary points of the two molecules are separated by
J. Phys. Chem. 1981, 85, 1474-1479
7474
-BOo with slightly different D and E values. The field strengths of ;he stationary points are tabulated in Table 111. Since H is parallel to the L axis of the durene at a dial reading of nearly Oo in Figure 7, the y axes of the bimane molecules are rotated by 40' in the opposite sense. Since the x direction is parallel to the C=O direction, the probable orientations of two bimane molecules with respect to durene are shown in Figure 8. The angular dependences obtained by solving the above spin Hamiltonian are given by the solid lines in Figure 7. Although the agreement between the experimental values and the calculated ones is not perfect, possibly because of errors in mounting, the
agreement is considered to be sufficient to conclude that these two differently oriented bimanes give rise to the site I and site I1 phosphorescence and ODMR spectra. The bimanes in these two orientations have nonequivalent environments which produce slightly different zfs and phosphorescence spectra. We have attempted to observe hyperfine splittings, but so far we have not succeeded.
Acknowledgment. We thank Hanna Dodiuk, Hanna Kanety, and Joshua Hermolin (Tel-Aviv University) for their contributions to this study.
Disproportionation of Semimethylene Blue and Oxidation of Leucomethylene Blue by Methylene Blue and by Fe(II1). Kinetics, Equilibria, and Medium Effects David W. Hay, Stephen A. Martln, Sugata Ray,+ and Norman N. Lichtin" Department of Chemistry, Boston Universw, Boston, Massachusetts 022 15 (Received: March 3, 1980; In Final Form: January 29, 198 1)
The dependence on reaction medium of the kinetics of three ground-state elementary reactions occurring in the iron-methylene blue photoredox system has been investigated by studying the relaxation of the photostationary state and by flash photolysis. The rate constants which have been evaluated include 2k6, for disproportionation of semimethylene blue (S),k,, for syn proportionation (the oxidation of leucomethylene blue (L) by methylene blue (MB)),and klo, for the oxidation of leucomethylene blue by ferric ion. Variations of media include nature of the solvent and anions, ionic strength, and concentration of acid. Values of the equilibrium constant & = k4/2k6 = [S]2/[L][MB]have been derived from the kinetic data and used in conjunction with potentiometric data to determine values of the one-electron standard reduction potentials, cotMBlS and eo'sIL in several media. As in the iron-thionine photoredox system, the half-reduced dye, S, is a minor component of the photostationary state and oxidation of leuco dye by ferric ion appears to proceed via a metastable association complex of the reactants. Mechanistic interpretations of some of the medium effects are suggested.
Introduction The photophysics, photochemistry, and subsequent ground-state chemistry in solutions containing methylene blue, MB+, and Fe"(H,O):+ are similar to the corresponding phenomena in solutions containing thionine and hexaaquoferrous.1-9 Thus, semimethylene blue, which is produced with high efficiency in the quenching of protonated triplet dye, 3MBH2+,in acid solution by hexaaquoferrous,loundergoes reversible disproportionation, eq 6,11and the resulting leucomethylene blue, MBHQP+ at the pH values of this research (pK,, = 4.5, pKa2 = 5.9 in water),12 is oxidized by Fe"'(H20):+. Knowledge of photostationary state composition, of the kinetics of bulk back-reaction of MBHQP' with Fe(III), of standard oneelectron reduction potentials of methylene blue and semimethylene blue, and of the dependence of these quantities on the medium are necessary for a quantitative understanding of photogalvanic cells employing the ironmethylene blue photoredox system. We have measured specific rates of disproportionation of semimethylene blue, 2k6, in various media by means of conventional or laser flash photolysis-kinetic spectrophotometry and specific rates of oxidation of leuco dye, MBHQ2+,by methylene blue, i.e., syn proportionation, k,, and apparent specific rates of oxidation of MBH3'+ by Fe1rr(H20)63'or related +Deceased Sept 25, 1979. 0022-365418112085-1474$0 1.2510
labile complexes, klo, in various media by a photochemical perturbation technique.13 Values of the equilibrium constant K6 = k+/2k6 and of standard one-electron reduction potentials of MB+ and semimethylene blue have been calculated. Results are compared with the corresponding values for thionine and its reduction products.13J4 Equations 1-10 represent established and assumed elementary steps in the acidified iron-methylene blue (1)Parker, C. A. J. Phys. Chem. 1959, 63, 26. (2) Kato, S.; Morita, M.; Koizumi, M. Bull Chem. SOC. Jpn. 1964, 37, 117. (3) Danziger, R. M.; Bar Eli, K. M.; Weiss, K. J . Phys. Chem. 1967, 71, 2633. (4) Faure, J.; Bonneau, B.; Joussot-Dubien, J. Photochem. Photobiol. 1967, 6, 6 , 331. (5) Bonneau, R.; Fornier de Violet, P.; Joussot-Dubien, J. Photochem. Photobiol. 1974, 19, 129. (6) Wildes, P. D.; Lichtin, N. N.; Hoffman, M. Z.; Andrews, L.; Linschitz, H. Photochem. Photobiol. 1977, 25, 21. (7) Ohno, T.: Osif, T. L.: Lichtin, N. N. Photochem. Photobiol. 1979, 30, 541. (8) Osif, T. L.; Lichtin, N. N. Photochem. Photobiol. 1980, 31, 403. (9) Osif, T. L.; Lichtin, N. N.; Hoffman, M. Z.; Ray, S. J.Phys. Chem. 1980, 84,410. (10) Ohno, T.; Lichtin, N. N. J. Am. Chem. SOC. 1980, 102, 4636. (11) Equations are numbered to correspond to numbering of corresponding reactions in the iron-thionine system in ref 15. (12) Obata. H. Bull. Chem. SOC.J D ~1961. . 34. 1057. (13) Wildes, P. D.; Lichtin, N. N. 3. Phys. 'Chem. 1978, 82, 981. (14) Wildes, P. D.; Lichtin, N. N.; Hoffman, M. Z. J.Am. Chem. SOC. 1975, 97, 2288.
0 1981 American Chemical Society
