- Email: [email protected]
Direct measurement of optical spin orientation in the excited triplet state of aromatic hydrocarbons at room temperature
Volume 119, number 1
CHEMICAL PHYSICS LEITER.9
22 August 1985
DIRECT MEASUREMENT OF OPTICAL SPIN ORIENTATION IN THE EXCITED TRIPLET STATE OF AROMATIC HYDROCARBONS AT ROOM TEMPERATURE Yoshihiro
TAKAGI
Imwirurefor MoIecufar Scrence. Okozoki 444. Japan
Racived
12
June 1985
Optically induced magnelizallon has been observed in polycrys~als and solulions or various aromatic carbonyls, quinones. and aza-aromalics at room lemperalure using a simple pickupxoil detector. The magnetic-Lcld dependence of the mngmtude or Ihe induced magncGadon provided clear evidence of the creation of spin polarizalion in the phom-exiled uiplel stale The dependence of the relaxation raie or Ihe magnelzalion on the magnelic field and the lcmperalure has been measured.
l_ Inlroduction The structure and dynamics of the photo-excited triplet spur state of aromatic molecules have been extensively studied for over two decades by means of various spectroscopic techniques. However, an investigation at room temperature, which might be particularly important regarding a study of spin kinetics Juring the reaction process via the excited triplet state, is highly restricted due to fast spin-lattice relaxation Recently, the combined techniques [l-3] of flash photolysis (or pulse radiolysrs) and the time-resolved electron-spur resonance spectroscopy have been successfully used in studying transient free radicals in solutions and have led to an elucidation of the mechanism for chemically induced dynamic electron polarization (CIDEP). It is the purpose of this article to present a simple lughly time-resolved technique which uses a pickup coil ic detect the spin onentation in the photoexcited triplet state. Such a detection method was previously used for observations of the optical spin orientation in the ground state of paramagnetic solids [4] and liquids [S] _For organic molecules, Levanon et al. [6] were the first to observe optically induced magnetization in trrphenylene and coronene in polymerized methylrnethacrylate at room temperature using a pickup-coil detector with a microsecond time resolution_ However, a detailed analysis of the experimental results was not given. We have applied the same technique to various aromatic compounds. The photo-excitation of aromatic carbonyls, quinones, and aza-aromatics (the lowest excited singlet state (Sr) 1s nsT* wth a strong spin-orbit coupling with the triplet mr* state) shows a very high inter-system crossmg (XC) rate. If the spin-lattice relaxation rate and the radiative or non-radiative decay rate in the lowest triplet state (TJ) is lower than (or comparable to) the JSC rate, a spin POlarization can be produced due to the selectivity in the spin-orbit coupling between SJ and three spin states of T, _Eigenvectors of the triplet spur state under the influence of an external magnetic field H (along the r-axis) are grven by l+)=cos0
lz)+isin0
lx>,
lO)=lr>,
I-_)=isin0
lz)+cosB
Ix),
(1)
where tan 28 = 2gv PHI@,
- 0,)
(I e I < 7r/4) -
lx), ly>, and lz) are eigenfunctions 0 009-2614/85/S (North-Holland
of the dipole-dipole
(21 interaction
0330 @ Elsevier Science Publishers B.V. Physics Publishing Division)
(H, =S.D.S),
where D is the zero-field split5
“ulume
11Y,
nunlDcr
PHYSICS LETTERS
CHbMlCAL
1
23 August 1985
tmg tensor. 0, and D, are the zero-field splitting constants and g,, is they-component of the g-tensor. For W IIx and N 11 z, eqs. (1) and (2), and all the following equations can be rewntten using simultaneous cyclic exchanges ofx. y, z and 4 b, c. Let us assume for the moment that the fast 1SC leads to a superposition of the zero-field spin states e(0)
=01x> + b lu> + clz)
(3)
.
If the spin axes do not coincide with the molecuIar axes, or if two or all of the spin states belong to a common irreducible representation (seen in low-symmetrical molecules), the superposed state in eq (3) holds for such a short time interval that neither the spin-spin interaction. the Zeeman interaction, nor the radiative or non-radiative decay processes can affect the triplet spin state and no stationary state 1s reached. Indeed, the formatlon of the triplet state mto a coherent superposition of spin states at a zero magnetic field was directly demonstrated by Nonhof et al [7] using a microwave free-induction decay slgnal following a flash excitation of a mixed crystal of tetrarnethylpyrazine41~ (TMP) in durene Using eqs. (1) and (2), the time-dependent wavefunction at a time r is 9(r)
= [(u sin’0
+ [(-io
+ ic sin B cos 0) exp(iH+r/fi)+
sin 0 cos Q + c cos28) exp(iE+r/fi)+
( 0 cosZO - ic sin B cos 0) exp(iE_t/11)] ( 10 sin 8 cos B + c de)
exp(iE_@)]
lx> + b exp(iE&fi)ly Iz) ,
where iT+. E_ , and Eo are the eigenenergies of I+>, I -_), and IO>. respectively. Using eq_ (4), of the spin angular momentum are given as CSx) = (o’b + ob*) sin 0 cos 8 [cos(wO_t)
+ i(bc* - b’c)[cos%
cos(~+~r)]
COs(W,~ r) + sin’@ cos(oO_t)]
- ln12)sm(20) oc*)[c0s(2e)lz
tc*)sin
-
e COST
cOs(2e)p
-cos(w+_t)]
cos(L+_t)-(UC*
+a*~)
[co~(u,_~)-COS(U+&]
+ i(ab* - a* b) [sin20 cos(0 +Of)+ c0s2e COS(W~_~)]
) (4)
the expectation
values
+ i(a*b - nb*) sin ~9cos 8 [sir~(w+~t) + sin(wo_f)] +(bc*
+ i(u*c-
+ b*C)[COS’0 Sill(GJ+gf) -
Sill20 Sill(Wo_t)]
,
~c*)[si.0(2e)]2
[cos(20)]‘sin(w+_r), + i(bc* +(ob*
-
b*c)su-~ e case
+a*b)[c0s2e
[sin(w+&+sin(wo_~)l
sinl:o,_t)-sin2e
sin(w+ot)l , (5)
where w+~ = (E, - E,,Yfi, a+_ = (E+ - E_)/fiand w 0_ = (E,-, - E_)/fi are the splitting angular frequencies. G_,, > includes two tune-independent terms and Gs,> and consist of only oscillatory terms originating from the superposed form in eq. (3)_ At a zero field (0 = 0), two terms in each spin component remain non-zero. These terms in Gs,) are responsible for the formation of a free-inductlon decay reported in ref. [7] (n = 0 for TMP)_ If there is no definite phase relation between n, b, and c in eq. (3), the ensemble average of eq. (5) vanishes, except for the first term in (SJ, ). This term appears if lc2 1# la’ 1, i.e. if the ISC causes a population difference between Ix> and Is>. The magnetization that arises from this term is not in a thermal-equilibrium state but is due to un)I2 - I W(f)l-1)12, where equal degrees of state mkings of I+1 > and I-1) in *r(f) It is proportional to I W(t)l+l I +I 1 and I-1 ) are elgenstates for quantization along the applied magnetic field_ The difference in the degree of state mixings increases with an increasing magnetic field, reaches a peak, and gradually tends to zero upon any further increase_ If lc12 - I al2 = 21, i e. a perfect spin selectivity in the KC process is realized, each molecule will have a magnetic moment equal to one Bohr magneton (S = l/2) for a magnetic fieldH = ID, - D,l/2@_
2. Experimental procedure Amongst the various aromatic compounds in which optically induced magnetization has been observed, we will describe benzophenone and‘benzil (in polycrystals) since the magnitudes of their signals were relatively large. The samples were recrystallized usmg an ethanol solution and were ground into powders. The excitation source was an excimer laser (308 nm) pumped dye Iaser which generates 0.3 mJ (14 ns) pulsed radiation of 345 nm. The excita6
Volume 119, number
1
CHEMICAL
PHYSICS
LEITERS
23 August
1985
tion beam was semr-focused on the sample contained in a cylmdrical cell 15 mm-long with quartz windows 5 mm in diameter. A pickup coil with 5 turns was wound around the cell along the direction of the beam. An external magnetic field was applied along this dnection using an electromagnet with small holes in the pole pieces to enable beam propagation. The time-derivative signal of the magnetization was amplified using a wide-band amplifier (Hewlett-Packard 84474) with a bandwidth of 700 MHz and was displayed on a fast oscilloscope (Tektronix 7104). In order to detect the signal with a good signal-to-noise ratio, a home-built transient digitizer system [8] was constructed consisting of a TV camera attached to an oscilloscope screen. A microcomputer-based image processor was also used. This system locks a single transrent signal on the oscilloscope screen as a digitized picture. It serves as a digital signal averager with a time resolution determined only by the choice of the oscilloscope. In our study, the experimental response time of the detector was limited by the duration of the excitation pulse.
3. Results and discussion Fig. 1 shows
the
signals for optically
induced magnetization
in polycrystals
of benzophenone
Fig. 1. Timc-denvative signals of optic3 tion (a) Excitation pulse from an excb laser. (b) Bcruophenonc. H = 1 kG. (c) kG. (d) Benzil H = 0.85 kG. (e) Uenzil
flY lx1 EC
and benzil at
induced
ne:ti-
laser pulrnpe’d dye nzophen
.H =ZkG.
H=3
Volume 119, number 1
CHEMICAL PHYSICS LEITERS
23 Augtst1985
room temperature The time-derivative signal arises following the rise of the excitation pulse and reverses and then gradually tends to zero with a lime constant dependent on the intensity of the magnetic field. The polarity of the srgnal of benzophenone was opposite to that of benzil. This result directly reflects the difference in the sign of D, - 0, in eq. (2). The inversion of the field direction also causes a change in the sign of 8 in eq. (2), which is consistent with the experiment The dependence of the signal intensity on the magnetic field for benzophenone is shown in fig_ 2a. The solid curve represents the time-independent term in eq. (5) as a function of the field strength_ For benzophenone molecule, the z-axis was taken along the C=O bond and the x-axis normal to the molecular plane. We used 0, = 0.0738 cm-I andD, = -0.1054 cm-l [9] In eq. (2). The sample used in this experiment was a polycrystal in which molecules were randomly or-rented. Accordingly, the magnetic field was also randomly oriented on the molecular frame. Nevertheless, only the calculated values for H liy were in good agreement with the experimental results. Calculations for H fix and H IIz clearly gave different curves, shifted towards the Iower fieId. We can eliminate the H Iix and H IIz cases for the following reason taking rnto account the magnetophotoselection and the spur-state selectivity during the ISC process. In most aromatic carbonyls, the transition moment for the optical excitation to SI is induced along the outof-plane axis of the molecule due to a nm* character As the polarization of the excitation beam is perpendicular to the external magnetic field, there exists no molecule for which the x-axis (normal to the molecular plane) is oriented along the field direction. AccordingIy, we can exclude the case of H IIx. Also, the populating rate of lz> is probably much higher than for lx> and 1~) [lo] *_ Thus, we can roughly take CI=r b = 0, so that the time-independent term in U,) (obtamed from fl,, ) in eq_ (5) by exchanging x, y. z, and a, b, c withy, z, x, and b, c, 4 respectively) vanishes forH IIz. The signal strength at the input of the amplifier was several hundred microvolts at H= 1.0 kG where the signal reached a peak. This magnitude can be compared with that due to a magnetizatron arizlng from approximately 2 X 1013 Bohr magneton (from our experience regarding measurements of ‘&e optical spin orientation in ruby [ 121). However, the number of incident photons absorbed by molecules whose y-axes are parallel to the magnetic field is estunated to be 3 X 1014. In Fact, this value must be reduced because of a scattering loss, a depolarization of the incident light at the diffusing sample surface, and a contamination by the mr* transition, and probably due to the fact that lc12 - [al2 < 1. * Actually.
it is not in a pure crystal
but in a mixed
crystal
that
the preferend
populating
of the 12) state was observed. However,
Lhc small spin-state selectivity found in a measurament on a pure crystal for a fatly long time scale [ 111, is most likely to be due to the fact that the intermedliitc triplet exciton states are initially occupied before they are trapped. Considering the time scale of our mcasurcment, the overpopulation of the lz) state may have been reasonable for the initial spin state at f = 0.
Benzophenone
Benrophenone 9
0 0 -D
I FI& 2. (a) Magnetic-field field and temperature.
8
2
34 Magneiic
dependence
5 67 freld/kG
of signal amplitudes
6I
0
295K
.
77K
b
1234567
in bcnzophenone.
Magnetic
field/kG
(p) Dcpendencc
of signal de-y
rates 0x1the magnetic
Volume 119, number 1
CHEMICAL
PHYSICS
LEITERS
23 August 1985
The tails of the signals indicate a decay of the magnetization. We found field dependences of the decay rates as shown in fig 2b. Decay rates decreased from 1.0 X lo8 to O-2 X 108 for benzophenone and 0.8 X 108 to 0.1 X 108 for benzil for a fiild strength between 0.5 and 4 kG. Regarding decay mechanisms, we now consider the radiative and/ or non-radiative decay ofthe triplet state and the spin-lattice relaxation The former process is likely not to be responsible for the decay rates of these ranges. At high temperatures, the spin-lattice relaxation in paramagnetic materials is predominantly determined by a field-independent two-phonon Raman process. However, cross-relaxation 14,131 may cause a field dependence. A tentative explanation of the observed field dependence is as follows. In the vicinity of a zero field, all molecules have nearly the same energy splitting, i.e. ZFS frequencies. In a strong field, however, molecules have different splitting frequencies which depend on the field direction relative to the molecular axes Therefore, the mutual spin flip-flop between molecules oriented in different directions cannot occur without violating energy conservation. That is, the cross-relaxation decreases with an increasing field. Other molecules found m the polycrystals and liquids listed in table 1 have been investigated. The signals of these compounds were much smaller and the decay rates were one to two orders of magnitude greater than for benzophenone and benzil. It is not clear why only benzophenone and benzil exhibit such exceptionally low decay rates. The excitation light source for detecting a fast signal was a mode-locked Nd 3+: YAG laser. Frequency-tripled Table 1 List of compounds in which optically induced magnetization Compound
has been detected
Solvent or polyuystal
Signal polarity a)
Decay rate b) (108 fr)
benzene
+ + +
910 310
aldehydes and ketones benzophenone
methanol polycrymal methanol PMMA c) polycrystal polycrysti benzene benzene polycrystal polycrystal polycrystal PolYws~ polycrystal polyayti poly=-Ystal PofYcrystal PolYaYstd
benzil benzoin acctophenone bcnzaldehyde n-naphthaldehyde p-naphthaldehyde p-bcnzoquinonc a-naphthoquinone p-naphthoqtione xanthonc enthrone anthraqurnone piperonal heterocyclic
POlYcrystal POlYcrvti POlYcrYstal PolYcrystal PolYcrysteJ POlYWd
phenazine means
that
the
4+ -I-
2 10
l-2 1
S-10 S-10 5 0.7
compounds
PYrazine I-S-naphthyridrne 1--B-naphthyridine phthalazhte quinoxahne
a) potsign b)H= 1.3 kG.
* + + + + + + + +
0.6 10 1 05 3 a10 alo 1-2
mngnetization
C) Polymethylmethacrylate.
was
+ + --++d) + +
5-10 1 5 2 2 1
indud anti-parallel to the field direction d) Polarity changes at H = 0.6 kG. 9
Volume
119. number
1
CHEMICAL
--a---V 2 ns
PHYSICS
LJ?l-l-ERS
Fig. 3. Tm~e-dcrivatie
23 Auwt
signal of optically
iioninpymzinepoIyaystalatH=
indued
1985
m~etizx-
7.8kG.
radiation of a 0.3 rnJ single pulse (20 ps, 355 run) was semi-focused on the sample. A one-turn pickup coil was used. To eliminate high-frequency noise and spurious signals, data were accumulated during fifty shots with the laser irradiation and a difference was taken from the following ftity shots with the laser-off. The time-resolution of the detection system was approximately 1 ns. Fig. 3 shows a signal in a pyrazine polycrystal. Due to the fast magnetization decay (a few nanoseconds) a backshoot with nearly the same amplitude followed the first peak. A comparison of the various signal patterns found for the compounds given in table 1, suggests that the ratio of the first and the following peak bights is a measure of the decay rate. This represents a simple way of determining the relative decay rate around the resolutron limit. We have observed optrcally induced magnetization in benzophenone and benziI in a methanol solution. The observed signals showed very short decay times (less than IO-g s) in contrast to those for polycrys~s. The signal amplitude increased almost in proportion to the field intensrty over a range up to 10 kG Details of the results will be published elsewhere_ In conclusion, we have observed optically induced spin orientations in various aromatic compounds at room temperature using a pickup coil detector, and the decay of the spin orientation was measured directly_ This method represents a simple way of detecting spin orientation, particularly for a non-phosphorescent triplet state. It is also appropriate for measuring the fast spin--lattice relaxation rate independently of the temperature and the magnetic field. Aclarowledgement I wish to express my appreciatron to Professor K. Yoshrhara for his interest in this work and many encouraging suggestions_ I further thank Professor N Hirota for helpful drscussions.
References [l] [2] [3] [4]
A D. Trirunac, J.R. Non-is and R.G. Lawler, J. Chem. Phys. 71 (1979) 4380. 5. Basu, K.A McLauchlan and G R. Scaly, MoL Phys. 52 (1984) 431. S. Vdmauchi and N. HIrota, J. Phys. Chem. 88 (1984) 4631. G.F. Hull, Jr., J-T. Smith and A-F. Quesada, Appl. Opt. 4 (1965) 1117; J P. van der Ziel and N. Bloemhergen, Phys. Rev. 138 (1965) A1287; Y. Takagi, Y. Fukuda, K. Yamada and T_ Hashi, J. Phys. Sot Japan 50 (1981) 2672; Y. Takagi, K. Yamada, Y. Fukuda and T. Hashi, Phys. Letters 98A (1983) 306. [S] Y. Takagi, in: Laser spectroscopy, Vol. 6 (Sprmgcr, Berlin, 1983) p_ 85. [6] H. Levanon, C.L. Kwan and S.I. Weissman, Chem. Phys. Letters 6 (1970) 19. [7] C-J. Nonhof. F.L. Plantenga, J. Schmidt, C.kG.0. Varma and J-H. yan der WaaJs. Chem. 183 Y. TakagJ, Rev. Sci Jnstr. 53 (1982) 1677. [9] K J. Lams, R.K Power and A.M. Nishimur& Chem. Phys. Lettess 65 (1979) 272. [lo] S. Yamauchr and D W. Pratt. MoL Phyr 37 (1979) 541. [11] C-J. Wmsccm and A.H. Maki, Chem. Phys. Lettms 12 (1971) 264[12] Y. Takagi, Y. Fukuda, and T. Haahi, Opt Commtm.. to be published 1131 W-S. Veeman and J.H. van dcr Waals, Chem. Phys. Letters 7 (1970) 65-
10
Phys
Letters
60 (1979)
353.
CHEMICAL PHYSICS LEITER.9
22 August 1985
DIRECT MEASUREMENT OF OPTICAL SPIN ORIENTATION IN THE EXCITED TRIPLET STATE OF AROMATIC HYDROCARBONS AT ROOM TEMPERATURE Yoshihiro
TAKAGI
Imwirurefor MoIecufar Scrence. Okozoki 444. Japan
Racived
12
June 1985
Optically induced magnelizallon has been observed in polycrys~als and solulions or various aromatic carbonyls, quinones. and aza-aromalics at room lemperalure using a simple pickupxoil detector. The magnetic-Lcld dependence of the mngmtude or Ihe induced magncGadon provided clear evidence of the creation of spin polarizalion in the phom-exiled uiplel stale The dependence of the relaxation raie or Ihe magnelzalion on the magnelic field and the lcmperalure has been measured.
l_ Inlroduction The structure and dynamics of the photo-excited triplet spur state of aromatic molecules have been extensively studied for over two decades by means of various spectroscopic techniques. However, an investigation at room temperature, which might be particularly important regarding a study of spin kinetics Juring the reaction process via the excited triplet state, is highly restricted due to fast spin-lattice relaxation Recently, the combined techniques [l-3] of flash photolysis (or pulse radiolysrs) and the time-resolved electron-spur resonance spectroscopy have been successfully used in studying transient free radicals in solutions and have led to an elucidation of the mechanism for chemically induced dynamic electron polarization (CIDEP). It is the purpose of this article to present a simple lughly time-resolved technique which uses a pickup coil ic detect the spin onentation in the photoexcited triplet state. Such a detection method was previously used for observations of the optical spin orientation in the ground state of paramagnetic solids [4] and liquids [S] _For organic molecules, Levanon et al. [6] were the first to observe optically induced magnetization in trrphenylene and coronene in polymerized methylrnethacrylate at room temperature using a pickup-coil detector with a microsecond time resolution_ However, a detailed analysis of the experimental results was not given. We have applied the same technique to various aromatic compounds. The photo-excitation of aromatic carbonyls, quinones, and aza-aromatics (the lowest excited singlet state (Sr) 1s nsT* wth a strong spin-orbit coupling with the triplet mr* state) shows a very high inter-system crossmg (XC) rate. If the spin-lattice relaxation rate and the radiative or non-radiative decay rate in the lowest triplet state (TJ) is lower than (or comparable to) the JSC rate, a spin POlarization can be produced due to the selectivity in the spin-orbit coupling between SJ and three spin states of T, _Eigenvectors of the triplet spur state under the influence of an external magnetic field H (along the r-axis) are grven by l+)=cos0
lz)+isin0
lx>,
lO)=lr>,
I-_)=isin0
lz)+cosB
Ix),
(1)
where tan 28 = 2gv PHI@,
- 0,)
(I e I < 7r/4) -
lx), ly>, and lz) are eigenfunctions 0 009-2614/85/S (North-Holland
of the dipole-dipole
(21 interaction
0330 @ Elsevier Science Publishers B.V. Physics Publishing Division)
(H, =S.D.S),
where D is the zero-field split5
“ulume
11Y,
nunlDcr
PHYSICS LETTERS
CHbMlCAL
1
23 August 1985
tmg tensor. 0, and D, are the zero-field splitting constants and g,, is they-component of the g-tensor. For W IIx and N 11 z, eqs. (1) and (2), and all the following equations can be rewntten using simultaneous cyclic exchanges ofx. y, z and 4 b, c. Let us assume for the moment that the fast 1SC leads to a superposition of the zero-field spin states e(0)
=01x> + b lu> + clz)
(3)
.
If the spin axes do not coincide with the molecuIar axes, or if two or all of the spin states belong to a common irreducible representation (seen in low-symmetrical molecules), the superposed state in eq (3) holds for such a short time interval that neither the spin-spin interaction. the Zeeman interaction, nor the radiative or non-radiative decay processes can affect the triplet spin state and no stationary state 1s reached. Indeed, the formatlon of the triplet state mto a coherent superposition of spin states at a zero magnetic field was directly demonstrated by Nonhof et al [7] using a microwave free-induction decay slgnal following a flash excitation of a mixed crystal of tetrarnethylpyrazine41~ (TMP) in durene Using eqs. (1) and (2), the time-dependent wavefunction at a time r is 9(r)
= [(u sin’0
+ [(-io
+ ic sin B cos 0) exp(iH+r/fi)+
sin 0 cos Q + c cos28) exp(iE+r/fi)+
( 0 cosZO - ic sin B cos 0) exp(iE_t/11)] ( 10 sin 8 cos B + c de)
exp(iE_@)]
lx> + b exp(iE&fi)ly Iz) ,
where iT+. E_ , and Eo are the eigenenergies of I+>, I -_), and IO>. respectively. Using eq_ (4), of the spin angular momentum are given as CSx) = (o’b + ob*) sin 0 cos 8 [cos(wO_t)
+ i(bc* - b’c)[cos%
cos(~+~r)]
COs(W,~ r) + sin’@ cos(oO_t)]
- ln12)sm(20) oc*)[c0s(2e)lz
tc*)sin
-
e COST
cOs(2e)p
-cos(w+_t)]
cos(L+_t)-(UC*
+a*~)
[co~(u,_~)-COS(U+&]
+ i(ab* - a* b) [sin20 cos(0 +Of)+ c0s2e COS(W~_~)]
) (4)
the expectation
values
+ i(a*b - nb*) sin ~9cos 8 [sir~(w+~t) + sin(wo_f)] +(bc*
+ i(u*c-
+ b*C)[COS’0 Sill(GJ+gf) -
Sill20 Sill(Wo_t)]
,
~c*)[si.0(2e)]2
[cos(20)]‘sin(w+_r), + i(bc* +(ob*
-
b*c)su-~ e case
+a*b)[c0s2e
[sin(w+&+sin(wo_~)l
sinl:o,_t)-sin2e
sin(w+ot)l , (5)
where w+~ = (E, - E,,Yfi, a+_ = (E+ - E_)/fiand w 0_ = (E,-, - E_)/fi are the splitting angular frequencies. G_,, > includes two tune-independent terms and Gs,> and
2. Experimental procedure Amongst the various aromatic compounds in which optically induced magnetization has been observed, we will describe benzophenone and‘benzil (in polycrystals) since the magnitudes of their signals were relatively large. The samples were recrystallized usmg an ethanol solution and were ground into powders. The excitation source was an excimer laser (308 nm) pumped dye Iaser which generates 0.3 mJ (14 ns) pulsed radiation of 345 nm. The excita6
Volume 119, number
1
CHEMICAL
PHYSICS
LEITERS
23 August
1985
tion beam was semr-focused on the sample contained in a cylmdrical cell 15 mm-long with quartz windows 5 mm in diameter. A pickup coil with 5 turns was wound around the cell along the direction of the beam. An external magnetic field was applied along this dnection using an electromagnet with small holes in the pole pieces to enable beam propagation. The time-derivative signal of the magnetization was amplified using a wide-band amplifier (Hewlett-Packard 84474) with a bandwidth of 700 MHz and was displayed on a fast oscilloscope (Tektronix 7104). In order to detect the signal with a good signal-to-noise ratio, a home-built transient digitizer system [8] was constructed consisting of a TV camera attached to an oscilloscope screen. A microcomputer-based image processor was also used. This system locks a single transrent signal on the oscilloscope screen as a digitized picture. It serves as a digital signal averager with a time resolution determined only by the choice of the oscilloscope. In our study, the experimental response time of the detector was limited by the duration of the excitation pulse.
3. Results and discussion Fig. 1 shows
the
signals for optically
induced magnetization
in polycrystals
of benzophenone
Fig. 1. Timc-denvative signals of optic3 tion (a) Excitation pulse from an excb laser. (b) Bcruophenonc. H = 1 kG. (c) kG. (d) Benzil H = 0.85 kG. (e) Uenzil
flY lx1 EC
and benzil at
induced
ne:ti-
laser pulrnpe’d dye nzophen
.H =ZkG.
H=3
Volume 119, number 1
CHEMICAL PHYSICS LEITERS
23 Augtst1985
room temperature The time-derivative signal arises following the rise of the excitation pulse and reverses and then gradually tends to zero with a lime constant dependent on the intensity of the magnetic field. The polarity of the srgnal of benzophenone was opposite to that of benzil. This result directly reflects the difference in the sign of D, - 0, in eq. (2). The inversion of the field direction also causes a change in the sign of 8 in eq. (2), which is consistent with the experiment The dependence of the signal intensity on the magnetic field for benzophenone is shown in fig_ 2a. The solid curve represents the time-independent term in eq. (5) as a function of the field strength_ For benzophenone molecule, the z-axis was taken along the C=O bond and the x-axis normal to the molecular plane. We used 0, = 0.0738 cm-I andD, = -0.1054 cm-l [9] In eq. (2). The sample used in this experiment was a polycrystal in which molecules were randomly or-rented. Accordingly, the magnetic field was also randomly oriented on the molecular frame. Nevertheless, only the calculated values for H liy were in good agreement with the experimental results. Calculations for H fix and H IIz clearly gave different curves, shifted towards the Iower fieId. We can eliminate the H Iix and H IIz cases for the following reason taking rnto account the magnetophotoselection and the spur-state selectivity during the ISC process. In most aromatic carbonyls, the transition moment for the optical excitation to SI is induced along the outof-plane axis of the molecule due to a nm* character As the polarization of the excitation beam is perpendicular to the external magnetic field, there exists no molecule for which the x-axis (normal to the molecular plane) is oriented along the field direction. AccordingIy, we can exclude the case of H IIx. Also, the populating rate of lz> is probably much higher than for lx> and 1~) [lo] *_ Thus, we can roughly take CI=r b = 0, so that the time-independent term in U,) (obtamed from fl,, ) in eq_ (5) by exchanging x, y. z, and a, b, c withy, z, x, and b, c, 4 respectively) vanishes forH IIz. The signal strength at the input of the amplifier was several hundred microvolts at H= 1.0 kG where the signal reached a peak. This magnitude can be compared with that due to a magnetizatron arizlng from approximately 2 X 1013 Bohr magneton (from our experience regarding measurements of ‘&e optical spin orientation in ruby [ 121). However, the number of incident photons absorbed by molecules whose y-axes are parallel to the magnetic field is estunated to be 3 X 1014. In Fact, this value must be reduced because of a scattering loss, a depolarization of the incident light at the diffusing sample surface, and a contamination by the mr* transition, and probably due to the fact that lc12 - [al2 < 1. * Actually.
it is not in a pure crystal
but in a mixed
crystal
that
the preferend
populating
of the 12) state was observed. However,
Lhc small spin-state selectivity found in a measurament on a pure crystal for a fatly long time scale [ 111, is most likely to be due to the fact that the intermedliitc triplet exciton states are initially occupied before they are trapped. Considering the time scale of our mcasurcment, the overpopulation of the lz) state may have been reasonable for the initial spin state at f = 0.
Benzophenone
Benrophenone 9
0 0 -D
I FI& 2. (a) Magnetic-field field and temperature.
8
2
34 Magneiic
dependence
5 67 freld/kG
of signal amplitudes
6I
0
295K
.
77K
b
1234567
in bcnzophenone.
Magnetic
field/kG
(p) Dcpendencc
of signal de-y
rates 0x1the magnetic
Volume 119, number 1
CHEMICAL
PHYSICS
LEITERS
23 August 1985
The tails of the signals indicate a decay of the magnetization. We found field dependences of the decay rates as shown in fig 2b. Decay rates decreased from 1.0 X lo8 to O-2 X 108 for benzophenone and 0.8 X 108 to 0.1 X 108 for benzil for a fiild strength between 0.5 and 4 kG. Regarding decay mechanisms, we now consider the radiative and/ or non-radiative decay ofthe triplet state and the spin-lattice relaxation The former process is likely not to be responsible for the decay rates of these ranges. At high temperatures, the spin-lattice relaxation in paramagnetic materials is predominantly determined by a field-independent two-phonon Raman process. However, cross-relaxation 14,131 may cause a field dependence. A tentative explanation of the observed field dependence is as follows. In the vicinity of a zero field, all molecules have nearly the same energy splitting, i.e. ZFS frequencies. In a strong field, however, molecules have different splitting frequencies which depend on the field direction relative to the molecular axes Therefore, the mutual spin flip-flop between molecules oriented in different directions cannot occur without violating energy conservation. That is, the cross-relaxation decreases with an increasing field. Other molecules found m the polycrystals and liquids listed in table 1 have been investigated. The signals of these compounds were much smaller and the decay rates were one to two orders of magnitude greater than for benzophenone and benzil. It is not clear why only benzophenone and benzil exhibit such exceptionally low decay rates. The excitation light source for detecting a fast signal was a mode-locked Nd 3+: YAG laser. Frequency-tripled Table 1 List of compounds in which optically induced magnetization Compound
has been detected
Solvent or polyuystal
Signal polarity a)
Decay rate b) (108 fr)
benzene
+ + +
910 310
aldehydes and ketones benzophenone
methanol polycrymal methanol PMMA c) polycrystal polycrysti benzene benzene polycrystal polycrystal polycrystal PolYws~ polycrystal polyayti poly=-Ystal PofYcrystal PolYaYstd
benzil benzoin acctophenone bcnzaldehyde n-naphthaldehyde p-naphthaldehyde p-bcnzoquinonc a-naphthoquinone p-naphthoqtione xanthonc enthrone anthraqurnone piperonal heterocyclic
POlYcrystal POlYcrvti POlYcrYstal PolYcrystal PolYcrysteJ POlYWd
phenazine means
that
the
4+ -I-
2 10
l-2 1
S-10 S-10 5 0.7
compounds
PYrazine I-S-naphthyridrne 1--B-naphthyridine phthalazhte quinoxahne
a) potsign b)H= 1.3 kG.
* + + + + + + + +
0.6 10 1 05 3 a10 alo 1-2
mngnetization
C) Polymethylmethacrylate.
was
+ + --++d) + +
5-10 1 5 2 2 1
indud anti-parallel to the field direction d) Polarity changes at H = 0.6 kG. 9
Volume
119. number
1
CHEMICAL
--a---V 2 ns
PHYSICS
LJ?l-l-ERS
Fig. 3. Tm~e-dcrivatie
23 Auwt
signal of optically
iioninpymzinepoIyaystalatH=
indued
1985
m~etizx-
7.8kG.
radiation of a 0.3 rnJ single pulse (20 ps, 355 run) was semi-focused on the sample. A one-turn pickup coil was used. To eliminate high-frequency noise and spurious signals, data were accumulated during fifty shots with the laser irradiation and a difference was taken from the following ftity shots with the laser-off. The time-resolution of the detection system was approximately 1 ns. Fig. 3 shows a signal in a pyrazine polycrystal. Due to the fast magnetization decay (a few nanoseconds) a backshoot with nearly the same amplitude followed the first peak. A comparison of the various signal patterns found for the compounds given in table 1, suggests that the ratio of the first and the following peak bights is a measure of the decay rate. This represents a simple way of determining the relative decay rate around the resolutron limit. We have observed optrcally induced magnetization in benzophenone and benziI in a methanol solution. The observed signals showed very short decay times (less than IO-g s) in contrast to those for polycrys~s. The signal amplitude increased almost in proportion to the field intensrty over a range up to 10 kG Details of the results will be published elsewhere_ In conclusion, we have observed optically induced spin orientations in various aromatic compounds at room temperature using a pickup coil detector, and the decay of the spin orientation was measured directly_ This method represents a simple way of detecting spin orientation, particularly for a non-phosphorescent triplet state. It is also appropriate for measuring the fast spin--lattice relaxation rate independently of the temperature and the magnetic field. Aclarowledgement I wish to express my appreciatron to Professor K. Yoshrhara for his interest in this work and many encouraging suggestions_ I further thank Professor N Hirota for helpful drscussions.
References [l] [2] [3] [4]
A D. Trirunac, J.R. Non-is and R.G. Lawler, J. Chem. Phys. 71 (1979) 4380. 5. Basu, K.A McLauchlan and G R. Scaly, MoL Phys. 52 (1984) 431. S. Vdmauchi and N. HIrota, J. Phys. Chem. 88 (1984) 4631. G.F. Hull, Jr., J-T. Smith and A-F. Quesada, Appl. Opt. 4 (1965) 1117; J P. van der Ziel and N. Bloemhergen, Phys. Rev. 138 (1965) A1287; Y. Takagi, Y. Fukuda, K. Yamada and T_ Hashi, J. Phys. Sot Japan 50 (1981) 2672; Y. Takagi, K. Yamada, Y. Fukuda and T. Hashi, Phys. Letters 98A (1983) 306. [S] Y. Takagi, in: Laser spectroscopy, Vol. 6 (Sprmgcr, Berlin, 1983) p_ 85. [6] H. Levanon, C.L. Kwan and S.I. Weissman, Chem. Phys. Letters 6 (1970) 19. [7] C-J. Nonhof. F.L. Plantenga, J. Schmidt, C.kG.0. Varma and J-H. yan der WaaJs. Chem. 183 Y. TakagJ, Rev. Sci Jnstr. 53 (1982) 1677. [9] K J. Lams, R.K Power and A.M. Nishimur& Chem. Phys. Lettess 65 (1979) 272. [lo] S. Yamauchr and D W. Pratt. MoL Phyr 37 (1979) 541. [11] C-J. Wmsccm and A.H. Maki, Chem. Phys. Lettms 12 (1971) 264[12] Y. Takagi, Y. Fukuda, and T. Haahi, Opt Commtm.. to be published 1131 W-S. Veeman and J.H. van dcr Waals, Chem. Phys. Letters 7 (1970) 65-
10
Phys
Letters
60 (1979)
353.