J. Phys. Chem. 1982, 86, 4287-4290
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Electron Spin Echo Modulation of the Photoexcited Triplets of Anthracene in p-Terphenyl Crystals Hslang-Lln Yu,+ Davld J. Stoop,t S. I. Welssman,' Tlen-Sung Lln,' Departments of Chemistry and Physics, Washington Universiv, St. Louis, Missouri 63 130
James R. Noiris, and Michael K. Bowman Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: August 2, 1982)
Electron spin echo envelope modulations (ESEEM) of photoexcited anthracene in p-terphenyl crystals at room temperature are reported. Sources of contribution to the observed ESEEM have been established. The hypefie tensor elements of the anthracene triplet have been determined and compared with the results of the previous ENDOR study.
Introduction Recently we have reported an electron spin echo (ESE) study of the pentacene triplet in p-terphenyl crystals a t room temperature.' There we presented the theoretical ground to explain the observed electron spin echo envelope modulations (ESEEM). We also described the photoexcitation dynamics of the pentacene triplet deduced from the study of two-pulse echo decay vs. delay from laser pulses. The study showed that the ESE technique can be utilized to investigate the paramagnetic properties and spin dynamics of very short-lived triplet states even a t noncryogenic temperatures, e.g., the triplet state of pentacene has a lifetime of 80 ps at room temperature. The limiting times of ESE technique are the phase memory time (TM) relative to cavity ringing deadtime of 200 ns in two-pulse experiments, and the spin-lattice relaxation time (T,) in three-pulse experiments. Here we present some preliminary ESE studies of photoexcited anthracene in p-terphenyl crystals a t room temperature (a lifetime of 40 ps) to further demonstrate the powerful utility of the ESE technique. By means of fast Fourier transform (FFT),we are able to obtain electron-nuclear double resonance (ENDOR) frequencies from ESEEM2 which allow us to determine the hyperfine (hf) tensor elements of the photoexcited molecule. An ENDOR study of anthracene triplets in a phenazine crystal has been reported previ~usly.~ This will enable us to make a comparative test of the capability of the ESE technique to map ENDOR frequencies. Experimental Section All the compounds used in the experiment were purchased from the Aldrich Chemical Co. Both anthracene and p-terphenyl were extensively zone refined. The descriptions of sample preparations, experimental details, and the ESE spectrometer were given previou~ly.'~~ The excitation source was the third harmonic of a Nd:YAG laser (A = 355 nm) with 30-mJ maximum energy. The FFT program is a Nicolet NMR software package. Electron Spin Echo Envelope Modulations and ENDOR Frequencies Below we shall present only the results of two-pulse ( a / 2 - 7 - T )experiments. For a one nucleus (I = 1/2) system, Department of Chemistry. *Departmentof Physics. * Address correspondence t o this author at the Department of Chemistry. Scientist-in-Residence, Argonne National Laboratory, 19%-I. 0022-365418212086-4287$0 1.2510
the ENDOR frequencies of M , = +1and M , = -1 manifolds are given as follows in the high-field limit: w+1 - [B2+ (wI- A)']'/' (1)
+ (wI +
o-' = [B2
(2) where A and B are from hAS,I, + hBS,I, terms of the hyperfine interaction in laboratory frame Hamiltonian, and wI is the "free" nuclear frequency (equal to the nuclear Zeeman splitting). We note that the ENDOR frequency of the M , = 0 manifold is equal to wI. We also note that A is the diagonal hf tensor element and B is the off-diagonal element. The echo envelope modulation amplitude as a function of pulse interval 7 for the transition between M , = -1 and M , = 0 is given in the following expression: a-17
WIT
E(mod,~= ) 1 - 2K sin2 -sin2 2 2
(3)
where
where c and d are the branching parameters (see ref 1for details). We note that there should appear no ESEEM if B = 0 which could happen when the hf principal axes coincide with the electron spin dipolar principal axes and the external field is applied parallel to one of the principal axes (a canonical orientation). In principle, we therefore do not expect to observe ESEEM arising from a particular set of protons when the external field is parallel to the canonical orientation if the hf principal axes of protons are the bisector of
..c..
C"c and along the C-H bond, normal to the molecular plane, and the third mutually perpendicular axis. The designations of the dipolar principal axes of anthracene are the same as those of pentacene: X is for the (1)D.J. Sloop, H.-L. Yu, T.-S. Lin, and S. I. Weissman, J. Chem. Phys. 7 5 , 3746 (1981).
(2)T.-S. Lin, M.K. Bowman, J. R. Norris, and G. L. Closs, Chem. Phys. Lett., 78, 283 (1981). (3)R. H. Clarke and C. A. Hutchison, Jr., J. Chem. Phys., 54, 2962 (1971). (4)J. R.Norris, M. C. Thurnauer, and M. K. Bowman in "Advances in Biological and Medical Physics", J. H. Lawrence, T. W. Gofman, T. L. Hayes, Ed., Vol. 17,Academic Press, New York, 1980,pp 365-415.
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 22, 7982
i 67
m
;
10
5
Letters Low Field 1
)
High Fieid
1
(
4
Figure 1. Designation of the dipolar principal axes of photoexcited anthracene. Fieid
Low
-b
High Field
H, II X
H,II Z
W
, 0
1
2
3
4
5
(
MHz)
Flgure 3. Fast Fourier transform (FFT) spectra of ESE decay envelopes displayed in Figure 2. L o w Field
High Field
H, II X
r',
0
,
1
,
2
,
,
3
*
,
0
1
2
3
4
5
T(US) Figure 2. The observed two-pulse electron spin echo decay envelopes of photoexcited anthracene in p-terphenyi crystals at room temperature. All of the low-field resonance peaks give emissive echoes, while the high-fiekl peaks give absorptive echoes.
long in-plane axis, Y the short in-plane axis, and 2 the out-of-plane axis (Figure 1). We thus expect the following modulation patterns to appear in anthracene triplets: (a) When HollZ, no modulation should appear from any protons on the ESE decay envelope if the photoexcited state is planar, i.e., B = 0 for all protons. (b) When HollX or Holly, no modulation should appear from the meso protons (9, lo), and the cy protons (1,4, 5, 8) in anthracene (see Figure 1 for numbering). However, if the hf principal axes of these protons deviate from the above conventional axes, modulation will occur. In fact, the ESE technique is a sensitive method to detect such discrepancies. The modulations due to the p protons may appear in both in-plane canonical orientations. The room temperature ESE decay envelopes of anthracene triplet in p-terphenyl crystals in three canonical orientations are given in Figure 2. We observed that pronounced ESEEM appear only in the low-field resonance peak of the X orientation (designated as Xl), and low-field Y orientation (Yl). We note that the above ESEEM arise from the transitions between M,= -1 and M,= 0. Weak modulations were observed in other resonance peaks, namely, X2, Y2, Z1, and 22. The corresponding FFT spectra of these time domain ESEEM are shown in Figure 3. The observed frequencies for X1 (2758
H, 1 IY
l c b , , 2" . 3 . 4 . 3 T(UJs)
Figure 4. Computer-simulated ESEEM of photoexcited anthracene in p-terphenyi crystals: H,(lXand H,1/ Y . An artificial exponential decay function (matched with the observed one) is included in the simulation.
G) are 1.73 (& 0.05), 10.00, and 11.71 MHz. The fundamental ENDOR frequencies are 1.73 and 11.71 MHz, where 11.71 MHz is the "free" proton frequency at this resonance field. Based on the computer simulation of ESEEM (Figure 4), we assign 1.73 MHz to the cy protons which could arise only if the protons interact with the electron spin on nonadjacent carbons, e.g., proton at position 1 interacts with the carbon at position 9 through the dipolar interaction. This clearly indicates that the hf principal axes of the a protons do not coincide with the electron spin dipolar axes. The peak at 10.00 MHz is the
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No. 22, 1982 4289
The Journal of Physical Chemistry, Vol. 86,
TABLE I: Best-Fit Hyperfine Tensor Elements for the Anthracene Triplet (MHzp this work proton position i meso(9,lO)
a(1,4,5,8)
Clarke and Hutchinsonb
p(2,3,6,7)
meso(9,lO)
a( 1,4,5,8)‘
p(2,3,6,7)’
-26.02 -10.22 t 1.34 - 26.249 -10.512 -7.85 -4.29 -5.14 - 7.208 -3.250 -18.03 -8.69 -4.65 -17.696 -8.643 -4.689 A kzz +2.05 0.000 -0.354 +0.504 0.000 Akxy Akxz 0 0 0 -0.211 -0.209 0 0 0.000 0.000 Akyz 0 The uncertainty in A h ’ s is k0.05 MHz. Reference 3. Average value of the inequivalent protons, such as a ( 1 , 5 ) and Akxx Akyy
a
(~(44).
difference of the two fundamental frequencies. For Y1 (3018 G), the observed frequencies are 4.87, 7.96, 12.83, and 20.81 MHz. The fundamental frequencies are 7.96 and 12.83 MHz. The peak at 12.83 MHz is due to the “free” proton precession. From the computer simulation of ESEEM (Figure 4) we assign 7.96 MHz to the /3 protons. The peaks at 4.87 and 20.81 MHz are the differences and the sum of the above fundamental frequencies. The weak modulation appearing in the 2 orientation is due to free protons which could arise from an interaction between the guest triplet and nuclei in the host matrix. We note that p-terphenyls crystallize with a simple monoclinic lattice. When anthracene molecules are substitutionally imbedded in the p-terphenyl lattice sites, the local site symmetry of anthracene will be lower than D2,,, and only the inversion symmetry may be retained in the crystal. We therefore would expect the anthracene triplet to show some deviation from planarity due to the environmental effect. However, our experimental conditions (low signal/noise ratio) does not allow us to detect a small deviation from planarity (less than 5 O ) . The observed frequencies are related to the hf tensor elements by the following equation? Wk
= (((S),Akxx+ (S)yAkxy + (S)zAkxz - la1)2+
((s)xAkzy + (S)yAkyy + (s)zAkyz - mod2 + ((S),Ak,, + ( S ) d k y z + (S)zAkZz - no1)2)”2 (5) where I , m,n are the direction cosines of the field Howith respect to the molecular axes x , y , and z, Ak’s are the hf tensor elements of the kth nucleus in MHz unit, and (S) is the expectation value of the electron spin operator S in the electron spin states that diagonalize both the Zeeman and the dipolar Hamiltonians. On the other hand, the observed modulation amplitudes are related to branching parameters c and d as given in eq 4 which can be further related to the hf tensor elements as follows:
A = w-1(2c2 - 1) -
(6)
B = *2W_l.lCl.lCll
(7)
We note that the observed frequencies and modulation amplitudes are nonlinearly related to the hf tensor elements. Thus we have adapted a nonlinear least-squares fitting program to determine the hf tensor elements. The program is of a finite difference method, LevenbergMarquaedt routine, from the IMSL Library. So that the hf elements can be fit uniquely, the fitting requires a greater number of data points than the number of unknowns. We thus have performed many experiments along off-canonical orientations to ensure that we have enough data points (frequencies and modulation amplitude) for the fitting. We further assigned all elements of A,, and A,, to be zero by assuming that the molecule is planar in the triplet state which is justified in view of weak modu-
lation observed in both Z1 and 22. The best fit tensor elements for various protons in the anthracene triplet derived from the above assumption and data treatment are listed in Table I. We have also included the results from the previous ENDOR study3 in the table. We note the following apparent differences: (a) The ENDOR study gave a value of zero for the A,, elements of the CY and the meso protons, whereas we have a value of +0.504 for the CY and -0.354 for the meso protons. The magnitude of A,(cY) relative to the difference between A,,(a) and the free proton frequency is significantly large, and thus gives rise to a large K value which determines the modulation depth (see eq 4). This is not quite so for the meso protons. Physically, we do expect A J a ) to be nonzero due to the fact that the spin density at the meso carbons is much greater than that at the /3 carbons? These differences in the spin density would induce differences in their dipolar interactions onto the CY protons. We should emphasize that it is the off-diagonal element A,,(a) that gives rise to the observed ESEEM in the X1 peak. (b) The ENDOR study gave only the A,, tensor element for the p protons, whereas we have determined a complete set of elements. Our A,, value is in good agreement with the ENDOR value. We should mention that the ENDOR study cannot observe any signals from the p protons except when the external field was parallel to the 2 axis, whereas we have observed modulations due to the /3 protons for Hall Y and for other off canonical orientations. (c) The ENDOR study reported that elements for the CY protons at the 1and 5 positions differ from those at the 4 and 8 positions, and /3(2,6)differs from /3(3,7), while our study cannot detect such differences. This may be due to a difference in the host matrix, phenazine vs. p-terphenyl, and the nature of two-pulse ESE experiments where the FFT (magnitude calculations) often introduce artificial line broadening and thus obscure the finite difference in the appearance of spectral lines. We can then use the best-fit hf tensor elements to simulate the ESEEM. The simulated ESEEM for the two in-plane canonical axes are displayed in Figure 4. The simulated ESEEM match exceedingly well with the observed ones given in Figure 2 with respect to frequencies and amplitudes. We should mention that we did not include any ESEEM arising from the interaction between the guest triplet and host nuclei. Finally, we would like to comment on whether our observed anthracene triplet signal can be related to any of the multiple sites observed in optical experiment at liquid helium temperature^.^ Our ESE field-scan experiments give only two pairs of resonance peaks due to two translational inequivalent sites at an arbitrary orientation. The zero-field splittings calculated from the resonant fields along three canonical axes are Dlhc = 0.0700 f 0.0002, and E l h c = -0.0082 f 0.0002 cm-’ which agree well with the (5) G. J. Small, J. Chem. Phys., 52, 656 (1970).
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reported values within our experimental e r r ~ r . ~Fur,~ thermore we observed well-resolved hyperfine structure for HollZ. Our hf pattern and splittings agree well with the reported values observed in EPR experimenk6 The field-scan experiments indicate that the observed ESE signals arise from anthracenes located at two translational inequivalent sites. No other lattice site can be identified. At least, on our ESE time scale, we cannot differentiate whether our observed signals arise from a single site or an average of multiple sites. However, the formation of multiple sites observed in optical work at low temperatures (6) J. Grivet, Chem. Phys. Lett. 4, 104 (1969).
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could arise from the phase transition a t -190 K of the p-terphenyl crystal.' Thus multiple sites may not exist at room temperature.
Acknowledgment. We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, the NSF-Solid State Chemistry-DMR8205422(TL),NSF-CHE-8120006(SIW), the NIH (BRSG program), and the Division of Chemical Sciences Office of Basic Energy Sciences of the US.Department of Energy for support of this research. (7)
R. W. Olson and M. D.Fayer, J . Phys. Chem., 84, 2001 (1980).
