4028
J . Phys. Chem. 1984, 88, 4028-4031
Conservation of Spin Polarization during Triplet-Triplet Energy Transfer in Low-Temperature Matrices Takashi Imamura, Qsamu Onitsuka, Hisao Murai? and Kinichi Obi* Department of Chemistry, Tokyo Institute of Technology, 0-okayama, Meguroku, Tokyo, Japan 152 (Received: February 23, 1984; In Final Form: April 24, 1984)
Transient spin-polarized ESR spectra of triplet states resulting from triplet-triplet energy transfer are studied in organic matrices at 77 K. The spin-polarized ESR spectra of the acceptor triplet produced by the photosensitization were completely different from those of direct photoexcitation. With the aid of the computer simulation, it is concluded that the spin polarization of the donor is transferred to the acceptor by triplet-triplet energy transfer and the Zeeman quantum number is conserved, which is interpreted by the exchange mechanism.
Introduction Since the phosphorescence of naphthalene was observed by energy transfer from photoexcited benzophenone in a low-temperature glass,' triplet-triplet (T-T) energy-transfer processes have been studied by many groups.* Several characters of the triplet state have been determined by using T-T energy transfer: the triplet energy level,' the quantum yields of the intersystem crossing and the phosphorescence: and so on. For T-T energy transfer, Dexters proposed the exchange mechanism, where T-T energy transfer is a spin-allowed process contrary to the dipole-dipole mechanism. The rate constant for T-T energy transfer was extensively studied as a function of the distance between the donor and the acceptor! Farmer et al.' first reported the ESR detection of T-T energy transfer in the benzophenone-naphthalene system. Only a few studies have been reported on T-T energy transfer from the standpoint of spin sublevels. El-Sayed et a1.* carried out a PMDR study of quinoxaline (donor) in naphthalene crystal at 1.6 K and concluded that the spin direction of the donor was preserved in T-T energy transfer. In the course of studies on the energy transfer from the photoexcited phenazine triplet to anthracene in diphenyl single crystals at 1.7 K, Brenner9 demonstrated that the Tx sublevels whose principal axes are identical with the long molecular axes were mostly populated in both phenazine and anthracene, and concluded that the spin polarization was conserved through the energy transfer. Sharnoff and Iturbe'O reported the conservation of spin orientation in the T-T energy transfer from the free triplet excitons to the triplet exciton traps in pure benzophenone crystals at 4.2 K where the slightly displaced benzophenone molecules acted as traps. All the experiments were carried out in single crystals, under regularly oriented conditions. No study is, however, reported about the spin polarization in T-T energy transfer under the randomly oriented condition to the best of our knowledge. Recently we have started studies of photoexcited triplet states by using the time-resolved ESR technique.",'* This technique enables us to observe the spin-polarized spectra of the triplet state prior to the spin-lattice relaxation and allows us to get information concerning the populating rates for each spin sublevel through anisotropic intersystem crossing. The observation of the spin polarization produced by chemical reactions and energy transfer may clarify the peculiarity for each spin sublevel in these processes. In this paper, we will show the time-resolved ESR spectra of triplet states produced by T-T energy transfer in glassy matrices and discuss the conservation of the spin quantum number during T-T energy transfer. Experimental Section A conventional X-band ESR spectrometer, Varian E-1 12, was used for time-resolved measurements with minor modification. t Present address: Department of Materials Science and Laboratory of Magneto-Electron Physics, The University of Electro-Communications, Chofu-shi, Tokyo 182, Japan.
0022-3654/84/2088-4028$01.50/0
The field modulation and the phase-sensitive detection system were removed, and the signals detected by the diode were transferred to a boxcar integrator, NF BX-53 1, after amplification with a wide-band amplifier. The spin-polarized ESR spectra were observed at a fixed delay time, 300 ns. A nitrogen laser (- 1 mJ/pulse, - 8 ns) was used as an excitation light source. The detailed experimental technique was described previously.' Benzophenone (Tokyo Kasei) was recrystallized several times from methanol and hexane. Acetophenone (Tokyo Kasei) and biacetyl (2,3-butanedione) (Kanto Chemicals) were used after vacuum distillation. Zone-refined naphthalene (Tokyo Kasei) was used without further purification. EPA (diethyl ether/isopentane/ethanol = 5:5:2) and ethanol were used as solvents. Spectrograde ether (Merck) and ethanol (Nakarai Chemicals) were used as received and isopentane (Tokyo Kasei) was distilled over sodium. The samples in a quartz ESR tube with a diameter of 5 mm were thoroughly degassed by multiple freeze-pumpthaw cycles. All the experiments were carried out at 77 K.
'
Results and Discussion Figure 1 shows the time-resolved ESR spectra of the IAmJ = 2 transition of benzophenone (-0.05 M), biacetyl (-0.5 M), and benzophenone/biacetyl (0.05 M/0.5 M) in ethanol. Under our experimental conditions, the optical density of benzophenone was about 10 times larger than that of biacetyl at excitation wavelength, 337.1 nm, because the extinction coefficient of benzophenone is 2 orders larger than that of biacetyl. The spectra were obtained at a microwave power of 20 mW by opening the gate of the boxcar integrator between 0.3 and 0.8 ps after the laser excitation. In the cases of direct photoexcitation of benzophenone (Figure la) and biacetyl (Figure lb), the signals of the (Am,l = 2 transitions were emissive for benzophenone and absorptive for (1) A. N. Terenin and V. L. Ermolaev, Dokl. Akad. Nauk SSSR, 85, 547 (1952); Trans. Faraday SOC.,52, 1042 (1956). (2) J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, London, 1970; N. J. Turro, "Modern Molecular Photochemistry", Benjamin, Menlo Park, CA, 1978. (3) W. G.Herkstroeter, J . Am. Chem. SOC.,97, 4161 (1975). (4) R. Klein, I. Tatischeff, M. Basin, and R. Santus, J . Phys. Chem., 85, 670 (1981); S. Tobita, M. Arakawa, and I. Tanaka, ibid., in press. (5) D. L. Dexter, J . Chem. Phys., 21, 836 (1953). (6) M. Inokuti and F. Hirayama, J . Chem. Phys., 43, 1978 (1965); M. Tomura, E. Ishiguro, and N. Mataga, J . Phys. SOC.Jpn., 22, 1117, (1967); 25, 1439 (1968). ( 7 ) J. B. Farmer, C. L. Gardner, and C. A. McDowell, J . Chem. Phys., 34, 1058 (1961). (8) M. A. El-Sayed, D. S. Tinti, and E. M. Yee, J . Chem. Phys., 51,5721 (1969). (9) H. C. Brenner, J . Chem. Phys., 59, 6362 (1973).
(IO) M. Sharnoff and E. B. Iturbe, IZD.Akad. Nauk SSSR, Ser. Fiz.,37, 522 (1973). (11) H. Murai, T. Imamura, and K. Obi, Chem. Phys. Lett., 87, 295 (1982). (12) H. Murai, T. Imamura, and K. Obi, J . Phys. Chem., 86, 3279 (1982).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4029
Conservation of Spin Polarization
a)
f
Em
b)
Abs
Benzophenone I
10
15
H (kG) Figure 1. Time-resolved ESR spectra of IAm,l = 2 transition observed in ethanol a t 77 K: (a) benzophenone, (b) biacetyl, and (c) benzophenone/biacetyl. A nitrogen laser ( A = 337.1 nm) was used as an excitation light source.
biacetyl. The polarizations observed were caused by anisotropic intersystem crossing which yield the largest spin population on the TZ sublevel.11~12 In the case of the benzophenone/biacetyl system (Figure IC), the signal of biacetyl was inverted compared to that of the biacetyl system. The triplet energies of benzophenone and biacetyl in polar solventI3 are 24 200 and 20 000 cm-I, respectively. These facts imply that the spin-polarized triplet state of biacetyl is mainly produced by T-T energy transfer from the spin-polarized benzophenone triplet state. Figure 2 shows the time-resolved ESR spectra of benzophenone M) and benzophenone/naphthalene (5 X M/2 X (5 X lo-' M) in EPA at a microwave power of 20 mW. In Figure 2b, it is apparently recognized that a sharp emissive signal appears at ca. 1600 G. This is assigned to the transition at H,,, of the naphthalene triplet. This emissive signal was weakly observed even in a matrix of lower concentration of naphthalene (-2 X lo-* M) but was not detected for the sample containing only naphthalene (-2 X lo-' M). As the energy of the lowest excited singlet of naphthalene, 31 000 cm-I, is higher than the photon energy of the nitrogen laser, 29 700 cm-', the naphthalene cannot be directly photoexcited. The spin polarization of the naphthalene triplet state is, therefore, concluded to be brought about through energy transfer from the benzophenone triplet state whose polarization is caused by anisotropic intersystem crossing. In Figure 2b, besides the IAm,l = 2 transitions in the lower field region, the spectrum of the benzophenone triplet was also distorted by the presence of naphthalene in the field region between 2000 and 4500 G; when the signal intensity in this field region was compared with (1 3) S. L. Murov, "Handbook of Photochemistry", Marcell Dekker, New York, 1973.
H (kG)
Figure 2. Time-resolved ESR spectra observed in EPA at 77 K: (a) benzophenone, (b) benzophenone/naphthalene, and (c) the spectrum of naphthalene obtained by subtracting the spin-polarized benzophenone triplet.
that at Hmln of benzophenone, the emissive signals at fields lower than g = 2 were enhanced, but the absorptive ones on the higher field side were slightly diminished. This result suggests that the spin-polarized signals of naphthalene are superimposed on those of the IAmsl = 1 transition of benzophenone. Figure 2c shows a spectrum obtained by subtracting the spectrum in Figure 2a from that in Figure 2b after the normalization of the peak height for the signals at Hmlnof benzophenone. The spectrum shown in Figure 2c is emissive in all magnetic fields and is attributed to that of the spin-polarized naphthalene triplet state produced by T-T energy transfer. The time-resolved ESR spectrum of acetophenone/naphthalene (lo-] M / 2 X lo-' M) in EPA is shown in Figure 3a. Acetophenone in EPA glass gave no distinct spectrum like benzophenone in EPA glass. The reported zero-field splitting constants of a c e t o p h e n ~ n e 'in~ ~single ~ ~ crystals were vastly dispersed by changing the host molecules, for instance, D = -0.54 cm-I in durene, -0.1964 cm-I in pure acetophenone crystal, and so on. The failure of the time-resolved ESR spectrum detection in EPA glass may be due to the broadening of the spectrum caused by the large D value of acetophenone and countless varieties of D and E values in the inhomogeneous circumstances of glassy matrix.I6 The spectrum shown in Figure 3a is attributed to that of the naphthalene triplet state formed by the energy transfer from the acetophenone triplet state, because naphthalene has no absorption at 337.1 nm and its triplet energy, 21 300 cm-', is lower than that of acetophenone, 25 900 cm-'. The resonance fields at each canonical orientation obtained from the reported valuesl' (14) T. H. Cheng and N. Hirota, Chem. Phys. Lett., 14, 415 (1972). (15) T. H. Cheng and N. Hirota, Mol. Phys., 27, 281 (1974). (16) H. Murai, T. Hayashi, and Y . J. I'Haya, Chem. Phys. Left., 106, 139 (1984). (17) B. S. Yamanshi and K. W. Bowers, J . Magn. Reson., 5, 109 (1971).
Imamura et al.
The Journal of Physical Chemistry, V d 88, No. 18, 1984
4030
T - T Energy Transfer
a)
T
YQ
Em.
H= 0
HfO
Acetophenone
LInfiniteFieldCharacterJ
H*O
Naphthalene
Figure 5. Model proposed to explain the spin polarization transfer in T-T energy transfer. The size of the circles expresses the probability of finding each spin sublevel. The length of the sticks represents the contribution of the infinite-field spin sublevel to each finite-field sublevel.
bl
Simulated I
I
I
20
30
40
I
5.0 *
H(kG)
Figure 3. (a) Time-resolved ESR spectrum observed in the acetophenone/naphthalenesystem in EPA at 77 K. (b) Simulated spectrum obtained by using D = -0.400 cm-', E = 0.050 cm-I, and Px:Py:Pz = 0:0.3:0.7for the donor and D = 0.099 cm-I, E = -0.015 cm-l for the acceptor.
0
Figure 4. Qualitative explanation of the time-resolved ESR spectra of naphthalene triplet produced by the T-T energy transfer; see text. are indicated in Figure 3a by the dashed arrows. The spectrum embodies the distinctive feature of the spin polarization; all the transitions of both lAmsl = 1 and 2 are emissive. It resembles the subtracted spectrum shown in Figure 2c. These results indicate that the higher Zeeman level in the microwave resonance is predominantly populated by the energy transfer in all the magnetic fields. The emissive polarization of the naphthalene triplet state is qualitatively illustrated in Figure 4 by the solid arrows. The emissive polarization of naphthalene in the magnetic fields is entirely different from the spin polarization caused by the in-
tersystem crossing which yields both absorptive and emissive polarizations such as the spectrum of benzophenone shown in Figure 2a. According to Dexter's exchange mechanism, the rate constant for T-T energy transfer is dependent on a specific orbital interaction and the distance between a donor and an acceptor molecule. In addition, the rate constant is also dependent on the spin functions of both the donor and the acceptor molecules; the rate is expected to reflect the conservation of the spin orientations in T-T energy transfer. Concerning the spin part, two mechanisms are examined; the conservation of the spin orientations in the molecular frame and in the infinite magnetic field. If the spin orientations in the molecular frame are transferred from the donor to the acceptor in T-T energy transfer, the acceptor is expected to show spin-polarized spectra with both absorptive and emissive signals. An example is illustrated in Figure 4 by dashed arrows for the selective population of the Z sublevel; the polarizations at H 11 2 canonical orientation are expected to be absorptive and emissive at the lower and higher resonant fields, respectively. The spin polarization obtained in the acetophenone/naphthalene system cannot be explained by the transfer of the spin orientation in the molecular frame. Therefore, this mechanism is safely excluded under the random configuration of the donor and acceptor molecules. Another possible T-T energy-transfer mechanism is the conservation of spin orientation in the infinite magnetic field. In the infinite magnetic field, the spin angular momenta of the donor and the acceptor have been quantized in the direction of the applied magnetic field. The donor with the quantum number m, produces the acceptor triplet with the same quantum number m, because of the orthogonality of the spin functions. Therefore, the spin orientation in the direction of the applied magnetic field is conserved in T-T energy transfer. The spin function at the finite field does not coincide with the infinite-field spin function but can be expanded in terms of the infinite-field spin functions, so that the populating rate for each sublevel of the acceptor is determined by the contributions of the infinite-field spin sublevels to each finite-field sublevel and the spin polarization of donor triplet. The spin polarization in the acceptor results from the different populating rate for each sublevel as mentioned above. The intersystem crossing of acetophenone in single crystals is '~ known to populate selectively the highest 2 s ~ b l e v e l . ' ~If~the 2 sublevel of the acetophenone triplet has also the fastest populating rate in our system as in the single crystals, before spin relaxation occurs, the highest T+I sublevel has the largest spin population in randomly oriented samples at the finite magnetic field because of the large D value. Since the infinite-field spin sublevel T+l has the largest contribution in the T+Isublevel, the probability of finding the T+' sublevel in T+, is larger than those of other sublevels. The situation is illustrated in Figure 5 , where the size of the circles expresses the probability of finding each spin sublevel. On the other hand, the triplet state of naphthalene has a relatively small D value, and hence the nature of the finite-field sublevel is very similar to that of the infinite-field sublevel.
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4031
Conservation of Spin Polarization The largest probability of finding the T+l sublevel of the acetophenone triplet implies the fastest populating rate to the T+J sublevel of naphthalene as discussed above. Consequently, the emissive signals may be observable at any resonant fields. The discussion evolved above is similar to that of the triplet mechanism in CIDEP.~* To make sure the mechanism discussed above, the simulation of the spin-polarized spectrum was carried out by the method after Wong et a1.I8 Assuming the isotropic g value, the spin Hamiltonian Hs, under the applied magnetic field, Ho, is written as follows
Hs = g@H&,
+ D(Szz - 2/3) + E(S,yz - S:)
(1)
where X , Y, and Z are principal axes of the zero-field splitting tensor. According to the treatment reported by Wong et a1.,’8 the spin polarization of triplet states under the applied field in randomly oriented samples is calculated in the following way. When one uses the zero-field spin functions, IT,) (j = X , Y, or Z ) , the eigenfunctions, Ix),of eq 1 are represented as
Ixi) = Fa,,lT,)
( i = +I, N , Qr -0
(2)
In a similar manner, using the infinite spin eigenfunctions, ITk) ( k = +1,0, or -l), the eigenfunctions, Ix),are also expressed by 1%) = CbkrlTk) k
(3)
As is well-known, each zero-field spin sublevel T, is-populated with a different rate for the anisotropic intersystem crossing. The probability P,’ of finding the state xi just after a pulse excitation is given by P,’ = ClU,ilZP,
(4)
where P, is the populating probability for the sublevel T,, providing x,P, = 1. The probability PUA of populating the eigenstate xuA (D = +J, N’, or -J) of the acceptor is given by PuA= I(XUAl\kD)IZ
(5)
where A and D indicate the acceptor and the donor, respectively, and
I*D)
= C(P,?1’21XID)
(6)
I
If the quantum number m, is conserved in T-T energy transfer as described above, the following equation is obtained: (TkDITyA)= 8k.k‘
( k , k’ = + I , 0, Or -1)
(7)
In the exchange mechanism an overlap between electron clouds of the acceptor and the donor is required for the energy transfer (18) S.K. Wong, D. A. Hutchinson, and J. K. S. Wan, J. Chem. Phys., 58, 985 (1973).
and the electron clouds are thought to be anisotropic. Therefore, we have to take into account the dependence of energy-transfer efficiency on the orientation between the acceptor and the donor molecules in the glassy matrix. As the dependence of the exchange integral on the orientation was not easily estimated, the exchange interaction was assumed to be isotropic under the randomly oriented conditions. This assumption is not peculiar, because the magnetophotoselection studiedg of T-T energy transfer showed no strong orientation effects. Considering the random orientation of donor molecules, the probability PuAfor an acceptor with the polar angles 0 and 4 is estimated by the formula PuA(Ho,0,4)=
f0“d 4 ’ f T0
d0’ P,’*(Ho,0,4;0’,+’) sin 0’
(8)
where 0 and 9 are the polar angles of the applied field with respect to the principal molecular axes of the acceptor and 0’ and 4‘ represent the donor system. Figure 3b shows the simulated spectrum of naphthalene produced by T-T energy transfer. The simulation was carried out by considering the reported relative transition probabilities.20 The zero-field splitting constants and the populating probabilities used in the simulation were as follows: the values of D and E were -0.400 and 0.050 cm-’ for the donor and 0.099 and -0.01 5 cm-’ for the acceptor, respectively, and relative intersystem crossing rates of the donor were P+Py:Pz = 0:0.3:0.7. The simulated spectrum was not appreciably altered by the change of D value between -0.54 and -0.20 cm-’ reported in crystal. The E value for acetophenone and the D and E values for naphthalene were reported value^.^^-^^ The relative intersystem crossing rates of the donor were adjustable parameters for the simulation. The ratio obtained seems to be reasonable because Pz is reported to be larger than Px and Py in ~rysta1s.l~Although the simulation did not give a quantitatively satisfactory result, it showed the same characteristic polarization pattern observed in Figures 2c and 3a, exclusively emissive at any resonant fields. These results indicate that the conservation of the quantum number m,is confirmed to be valid during T-T energy transfer. In conclusion, T-T energy transfer from the spin-polarized triplet state brings about spin polarization in the acceptor. The spin polarization of the donor is transferred to the acceptor by the exchange mechanism in which the quantum number m, is conserved between the randomly oriented donor and acceptor molecules. The simulation also supports this conclusion.
Acknowledgment. We are grateful to Professor Ikuzo Tanaka for his interest in this work. Registry No. Benzophenone, 119-61-9; biacetyl, 431-03-8. (19) G. P. Rabold and L. H. Piette, Photochem. Photobiol., 5, 733 (1966); S. Siege1 and L. Goldstein, J . Chem. Phys., 43, 4185 (1965). (20) P. Kottis and R. Lefebvre, J . Chem. Phys., 39, 393 (1963).
