J. Phys. Chem. 1992, 96,4162-4165
4162
Dynamics of the Photoexcited Triplet State of Pentacene in an Asymmetric Double-Well Potential of p-Terphenyl Crystals Jui-Lin Ong, David J. Sloop, and Tien-Sung Lin* Department of Chemistry, Washington University, St. Louis, Missouri 63130 (Received: January 17, 1992; In Final Form: March 13, 1992)
The electron paramagnetic resonance spectrum of the photoexcited triplet state of pentacene-d14in pterphenyl crystals exhibits a doubling at temperatures below the phase transition temperature of p-terphenyl. The relative intensitiesand the separation of the two members of the doublets vary with temperature. We ascribe the spectral changes to the accessibility of each pentacene molecule to two conformations with unequal energies. The data are fitted to a two-conformation jumping model. The energy difference between these two conformations (AE)is calculated to be 53 f 4 cm-I. The activation energy (Nf)is calculated to be 71 4 cm-I with the Arrhenius constant A = (7.8 f 0.2) X IO6 SKI.
*
Introduction Recently there has been renewed interest in the photophysics and the electronic properties of pentacene molecules concerning dynamic nuclear polarization via the photoexcited triplet statelJ and the excitation and detection of single pentacene m0lecules.~3~ Here we wish to report a detailed electron paramagnetic resonance study of the pentacene-d,, and -hi4 in pterphenyl systems (PDFT and PHPT) concerning the effect of the phase transition of the host crystal on the triplet population of the guest molecule. From the X-ray and neutron scattering experiments, the pterphenyl (PT) molecule in the crystalline form is planar in its mean conformation at r m m temperature, but the molecule becomes nonplanar with the central ring rotated out of the molecular T twist in opposite ways) plane (the outer and the center rings of F below the phase transition temperature 193 K (Tc).s-7 The phase transition of PT crystal mainly results from the competition of two opposite effects: ?r-electron delocalization and ortho hydrogen repulsion. The crystal structure transforms from monoclinic (P21/a, 2 = 2) to triclinic (PI, Z = 4). There are now four inequivalent sites in the unit cell. Indeed optical experiments observed four transition origins in the So SIabsorption in the PHPT system at 1.2 K.* However, only two of the four sites are observable in the phosphorescence due to the rate of intersystem crossing and the relative energy disposition of the four sites. However, in our low-temperature electron spin echo (ESE) experiments, we have not observed splittings due to these two inequivalent sites or other spectral changes when the external fields were applied along an arbitrary orientation except the spectral doubling of the canonical x and y directions. This is also in contrast to earlier low-field electron paramagnetic resonance (EPR) studies of chrysene triplet in PT single crystal where multiple sites were observed? We ascribe the observed doubling of the x and y orientations to the accessibility of each pentacene molecule to two conformations with unequal energies. The assignment is based on the following spectral observations: (1) one member of the doublet decreases in intensity with decreasing temperature and becomes unobservable below 20 K, and (2) the separation between the members of the doublets increases with decreasing temperatures. We will term this dynamic behavior as the tweconformation jumping (TCJ) which is associated with the motion of the guest andfor host molecules. The twisting of the central ring of the p-terphenyl molecule along the long molecular axis produces a symmetrical double-well potential with an activation energy of about 200 ~ m - ' .However, ~ when the PT sites are substituted by pentacene molecules, pentacene molecules appear to be subjected to an asymmetric potential field which gives rise to the TCJ phenomenon (vide infra). We have adopted the steady-state Bloch equations to analyze the observed spectral line shape. From spectral simulations, we obtain the jumping rate constants (kABand k,,), the energy difference between two conformations (AE), and the activation energy (AH). These results allow us to examine the lattice dy-
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0022-3654f 92f 2096-4762$03.00/0
namics below the phase transition temperature of p-terphenyl crystals when they are doped with pentacene molecules.
Experimental Section 1. Sample. The mixed crystals, 0.2 mol % (nominal) pentacened14in pterphenyl (PDPT) and pentacene-h,, in pterphenyl (PHFT), used in these experiments were grown by H.-L. Yu using the Bridgman method. The pterphenyl had been extensively zone refined. A polarizing microscope was used to identify the crystallographic axes. The sample, mounted on a KEL-F wedge, was aligned to achieve the maximum magnetic field separation between the high-field and low-field canonical resonance lines on the x (in the xy plane) and z axes, or the minimum field separation on t h e y orientation. The design of the KEL-F wedge is based on the orientations of the molecular axes (x, y, z ) related to the crystallographic axes (a, b, c) obtained in X-ray studies.1° The crystals of p-terphenyl (CI8Hl4)at room temperature have two molecules in a monoclinic unit cell of the dimensions: a. = 8.08 A, bo = 5.60 A, co = 13.59 A, and B = 91' 55'. The space group is C2h5(P21/a).The space group changes to PT below T,with Z = 4-67 2. Equipment. A home-built homodyne pulsed ESE spectrometer was employed in our experiment. The triggering pulses to drive the laser, microwave n/2 and n pulses, boxcar, phase shifter, and lock-in amplifier were generated by programmable pulse generator cards (PC/Pulser, Sytron Corp.) embedded in the expansion slots of an IBM PC/XT computer. Each card can be programmed independently in oscillator, delay, burst, count, and divide modes with a resolution of 5 ns and jitter of less than 100 ps." The pulse sequences used in our ESE field scan experiments are schematically displayed in Figure 1. The microwave pulses were repeated at a 500-Hz rate. The sample was irradiated by a pulsed nitrogen laser (Avco C950, wavelength 337 nm and power 1 mJ/pulse) at a 100-Hz rate. The phase of the first (?r/2) microwave pulse was alternately shifted 90' at a 250-Hz rate. The boxcar window was opened at a 500-Hz rate. The photoexcited echo amplitude changes sign due to the alternating 90' phase shift and is repetitive at a 50-Hz rate. This 50-Hz signal from the boxcar is further processed using a lock-in amplifier with a 50-Hz reference signal. The applied pulse sequences allow us to eliminate much of the background noise associated with the laser, cavity ringing, and receiver recovery. The interval 7 between two microwave pulses is 1.25 gs which is the optimal value to avoid any effect of cavity ringing, receiver recovery, and ESE envelope modulation (ESEEM) due to deuterium hyperfine interaction. The spectrometer operating frequency can be adjusted from 9.20 to 10.05 GHz by tuning the klystron frequency source (Varian V-58 klystron with microwave power output 500 mW) and the cylindrical microwave cavity (TEollmode) where H Iis parallel to the cylinder axis. The frequency is measured with a digital frequency counter. The magnetic field is calibrated and measured
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0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4763
Spectra of Pentacene in p-Terphenyl Crystals
1 3 . 2 8 3 5 MHz 1 3 . 2 9 7 5 MHz'
n
(C)
n
n
n
294K
n
I/
i
n
n
n
n
h
1 3 . 2 7 8 1 MHz
I 13.2797 MHZ
Figure 1. Schematic diagrams of computer-controlled pulse sequences and timing of ESE experiments. All pulses are synchronized to the same 40-MHz crystal-controlled clock. (a) Laser pulses (100 Hz repetition rate). (b) ?7/2-r?rrecho EPR echo simulation pulses (500 Hz). (c) Boxcar gate pulses (500Hz). (d) 90' phase shift pulses (250 Hz). Note that, in this experiment, only the u/2 microwave pulse is phase shifted 90' with respect to the spectrometer reference oscillator when the phase shift pulse is positive. (e) Lock-in-amplifier reference signal (50 Hz). ( f ) Boxcar output signal to lock-in amplifier. This signal is proportional to the echo amplitude.
3043.6
3067.3
3093.6
Figure 3. The 50-Gscan spectra of the low-field transition of pentacene-d14in p-terphenyl crystals at different temperatures for Ho 11 y. (* indicates the proton frequency from proton gaussimeter.)
3030.6
Gauss
Figure 2. The 50-Gscan spectra of the low-field transition of pentacene-d14in p-terphenyl crystals at different temperatures for Ho 11 x.
using a Robinson oscillator (showing the proton frequency) as a magnetometer which has an experimental uncertainty of 0.12 G. The sample temperature is adjusted from 5 K to room temperature by the use of an Oxford ESRlO cryostat system. A thermocouple junction was placed just below the sample to measure the cryogenic helium gas stream temperature for ESRlO system (the setting temperature). We also placed another thermocouple (chromel-gold) just above the sample to calibrate the sample temperature (the reading temperature). In normal operation, it takes about 20 min to reach a stable temperature. The uncertainty of the temperature at sample is l0-2O for the range of 300-60 K, but it increases to 3O-4' for 6&5 K. Other experimental details are given in ref 12.
Results and Discussion The molecular axes of pentacene are designated as follows: the long in-plane axis as the x-axis, the short axis as the y-axis, and the out-of-plane axis as the z-axis. 1. "be Doublet in Field Scan ESE spectra. The field Scan ESE spectra of the PDPT system at different temperatures are given
3067.5
3100.6
Gauss
Figure 4. The 70-Gscan spectra of the low-field transition of pentacene-h14 in p-terphenyl crystals at different temperatures for Ho 11 x.
in Figure 2 (Ho 11 x) and Figure 3 (Ho 11 y). A single spectral line becomes a doublet when the temperature is lowered below T, of FT.We observed the "doublet" splitting in both Ho 11 x and Ho 11 y. We attribute this doublet to conformation splitting. We assign the main peak to the A conformation and the side peak to the B conformation. The ratio of the echo amplitude (PA/PB) and the field separation (Ao)of the doublet increase with decreasing temperature. This behavior can be described by a TCJ model in the xy-plane as a result of the phase transition of PT crystal affecting the distribution of triplet population of the pentacene molecule. The observed spectral behavior as a function of temperature is reversibly reproducible;i.e., there is no apparent hysteresis We observed a similar effect in the protonated PHPT system. For Ho 11 x, the hyperfine pattern is somewhat skewed and asymmetric at room temperature and 6 K (see Figure 4). This may be due to some imperfection (defect) in the PHPT crystal. This is a static behavior. On the other hand, the hyperfine line shape changes as a function of temperature is a dynamic behavior. For Ho 11 y, the line width is only slightly broadened at low
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4764 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 v
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Ong et al.
t
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’ k, Figure 5. Potential energy curve for the two-conformation jumping model of pentacene-d,, in p-terphenyl crystals. The coordinatef ( 8 )is a function of the twist angle of the p-terphenyl central ring.
temperatures. These spectral changes (shape and width) occur only below T, of PT, which is consistent with what we observed in the PDPT system even though we could not resolve the doublet in the PHPT system. Qualitatively, the spectral behavior of PHPT system indicates a TCJ phenomenon even though a quantitative analysis is not possible. The spectral differences between PDPT and PHPT result from the following two factors: (a) The line width of the protonated pentacene is larger than that of the deuterated compound by a factor of 2.5. (b) The protonated pentacene shows barely resolved hyperfine peaks in the x orientation. Below T,, the line shape of the hyperfine peaks undergoes some asymmetric changes, which further obscure the spectral resolution. Thus, we cannot resolve the doublet due to the spectral overlapping. 2. The Jumping Rate Constants k m and k B A . The classical problem of jumping spin between two equally populated sites has been treated by means of Bloch equations previously.” The jumping spin in an unequally populated two-conformation system is schematically shown in Figure 5. The intensity of the resonance absorption is given byI3
WI) Y H ~ ~ A B ~ -B WAB() ~’ / AK ~ A B+ ~ B A X) [(u- uA)’(W - WB)’ + (a- WA)’kBA’ + (u - wB)’kAB’]) (1) Q
where wA and uBare resonance frequencies for conformations A and B; u is the angular frequency of the radiation field HI; kAB is the rate constant from conformation A to B, and kgA for the reverse jumping; y is the gyromagnetic ratio. Note that the system is not at thermal equilibrium in the ESE detection period. The observed spectral intensity arises from strong spin polarization upon laser excitation. The maximum separation of resonance fields Aw, found by seeking the minima of the denominator of M ( 0 , is given by the following expression for 0 < 8 < 7r/2 (the detailed derivation is given in ref 12): pw = 21alqcos (e/3 - T/6)1
(2)
where a
I/Z(kAB + kBA)’ -
1/4(uA
- @BIZ
= 1/4(kABZ- kBA2)(uA cos 8 = 3b/(41~1~/3)’/’
In this report, we will focus our calculations on the PDPT system for Ho 11 x only. There are four parameters in eq 2: kAB, kBA, uA- OB, and Am. These parameters were evaluated in the following steps. (a) A@. We obtain Au by the spectral deconvolution of the 50-G field scan ESE spectra at different temperatures. We have assumed the line-shape function to be Gaussian for both A and
xi
40
60
80 100 120 Temperature (K)
140
160
180
Figure 6. Field separation Au between conformation A and conformation B vs experimental temperature for H,, 11 x in pentacme& in pterphenyl
crystals.
TABLE I: Field Seonration (AWLFraction of the &bo AmDlitude (Pa and PB), and the Calc&ted’Jumping Rate Constants and k n A (h lo6 SI)) temp, K Aw, G PA PB (H) (A0.18) (AO.01) (hO.01) krn kni 164 4.39 0.59 0.41 4.6 6.7 150 4.89 0.61 0.39 4.1 6.6 131 5.03 0.63 0.37 3.9 6.7 5.81 0.66 0.34 115 3.0 5.9 5.99 0.69 0.31 2.6 100 5.8 6.28 0.72 0.28 2.2 5.5 85 70 6.53 0.75 0.25 1.7 5.2 55 6.53 0.81 0.19 1.3 5.5 0.86 6.96 0.14 0.73 4.5 40 7.31 0.95 0.05 0.14 2.7 26
B conformations in our spectral simulation. (b) uA- uB. Assuming there is no jumping between conformations A and B at absolute zero temperature, we may set uA - uB= ho at T = 0 K. Thus by extrapolating Au vs 7‘ (see Figure 6), we obtain Au = uA WB = 7.46 G at T = 0 K. ( c ) kBA/km.The ratio of the rate constants is equal to the ratio of the echo amplitude PA (on conformation A) and PB (on conformation B) which is also obtained by spectral simulation (part a). The relationship between PA, PB,kAB, and kaA is given by ~ B A / ~ A BP A / P B=
ex~(u/kT)
(3)
where AE is the energy difference between conformations A and B, and k is the Boltzmann constant. Thus by introducing these three values into eq 2, we obtain the jumping rate constants kAB and kBA at different temperatures: 4.6 X IO6 and 6.6 X lo6 s-’ at 164 K, respectively; 1.4 X lo5 and 2.7 X IO6 s-l at 26 K, respectively. The values of Aw, PA, and PB,and the calculated kAB and kgA at various temperatures are summarized in Table I. 3. AE aad AH. Furthermore, from the Arrhenius theory, we have the following equations:
kAB = A exp(-AH/kT) kgA = A exp[-(M - AE)/kT]
(4) (5)
where A is the Arrhenius constant, AH is the activation energy, and AI% is the energy difference between two conformations. The Arrhenius plots of In kAB and In kBA vs 1/ T are given in Figure 7, which yield AH = 71 f 4 cm-I, A E = 53 f 4 cm-’, and A = (7.8 f 0.2) X lo6 s-l. 4. The Nahw of Two-Confonnatioa Jumping. The temperature behavior of the observed doublets indicates a TCJ phenomenon exists for pterphenyl crystals doped with pentacene. The origin of the TCJ may arise from the following. ( a ) Asymmetric H.-H Repulsion between Pentacene and the Adjacent p Terphenyl. From the molecular packing calculations of naphthalene neat crystal^,'^ it has been shown that the equilibrium positions of the naphthalene molecules in the lattice with
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4165
Spectra of Pentacene in p-Terphenyl Crystals
I
I
k
13.5
13 12.51
'4
11.5
111 0
0.005
0.01
0015
0.02 0.025 1/T (K-')
0.03
0.035
I
0.04
Figure 7. Arrhenius plots of In kABand In k g A vs 1/T.
respect to the rotational displacement about the principal molecular axes are determined largely by the hydrogen-hydrogen (H-H) intermolecular repulsion force. Furthermore, semiempirical calculations using the repulsive part of the exp6 function on the equilibrium positions of a mixed organic crystal, phenanthrene in biphenyl, indicated that the intermolecular repulsive energy of phenanthrene gives rise to a double minimum along the long-axis rotational displacement as a consequence of larger amplitude of librational motions along the long axis of the host molecules.15 We may apply the same argument to our system. First let us examine the nature of neat PT crystals where the central ring of pterphenyl molecule twists out-of-plane with respect to the outer rings along the long axis with an amplitude of f23' at the phase transition temperature. When a pentacene molecule replaces a p-terphenyl, protons on the end rings of the guest pentacene will make closer contact with the neighboring protons on the outer rings of the host p-terphenyl molecules which are translationally inequivalent with respect to the substituted sites (Figure 8). The flanking p-terphenyl molecules (translationally equivalent but in different stacks) could have their outer ring rotated in the same sense or in the opposite sense. The protons of the outer rings of PT could then cause H-H repulsion with the protons of the pentacene in two possible configurations and give rise to an asymmetric double-well potential. Even at 113 K, the outer rings of PT molecule still undergoes a rotational displacement of f3O about the long axis.6 Therefore, similar to phenanthrene in biphenyl mixed crystals, we expect inequivalent conformation to exist in the PDPT and PHPT systems. (b) Phonon-Assisted Tunneling in a Two-Level System. The jumping may result from discrete librational motion of the central ring of p-terphenyl driven by phonon-assisted tunneling that has become allowed due to local disorder such as domain ~ a l l . ~InJ ~ this picture, the pentacene molecule is surrounded by a set of two-level systems where the instantaneous pentacene resonance frequency is determined by the complete set of quantum numbers describing the state of each of these two-level systems. These arguments have been applied to interpret the temperature-dependent spectral diffusion rate of single pentacene molecules in a pterphenyl crystal! The theory has also been applied to explain the dynamic behavior of the benzoic crystal system in an asymmetric double-well potential.''
Conclusion We interpret the observed doublet splitting as a function of temperature in the PDPT system in terms of an unequally populated TCJ model. This model assumes an asymmetric doublewell potential at the site of the guest pentacene molecule in a host p-terphenyl. The asymmetric double-well potential may be caused by (a) the asymmetric intermolecularhydrogen-hydrogen repulsion force between the pentacene guest and the inequivalent adjacent pterphenyl host molecules along the long molecular axis under
Figure 8. Disposition of the pentacene molecule in the p-terphenyl crystal. a-h are distances between designated hydrogens in angstroms. The two values listed under the phase transition temperature arise from two possible ring orientations of host molecules.
rotational displacement; the two inequivalent conformations are thermally interconvertible; or (b) phonon-assisted tunneling in twdevel system due to local disorder arising from the librational motions of the central ring of p-terphenyl. We used the Bloch equations under the steady-state condition to analyze the spectral line shape of this TCJ spin system. From computer-assisted spectral simulation studies, we have successfully calculated the kinetic parameters and activation energy which allow us to examine the lattice dynamics below the phase transition temperature of p-terphenyl crystals doped with pentacene.
Acknowledgment. We thank Professor S . I. Weissman for his valuable comments. Acknowledgment is made to the National Science Foundation (CHE-9106499) and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Registry No. Pentacene, 135-48-8.
References and Notes (1) Henstra, A.; Lin, T.-S.; Schmidt, J.; Wenckebach, W. Th. Chem. Phys. Lett. 1990, 165, 6. (2) Henstra, A.; Lin, T.-S.; Wenckebach, W. Th. Phys. Lett. A 1991,157, 431. (3) Orrit, M.; Bernard, J. Phys. Reu. Lett. 1990, 65, 2716. (4) Ambrose, W. P.; Basche, Th.; Moerner, W. E. J . Chem. Phys. 1991, 95, 7150. (5) Reitveld, H. M.; Maslen, E. N.; Clews, C. J. B. Acta Crystallogr. 1970, 826, 693. ( 6 ) Baudour, J. L.; Delugeard, Y.; Cailleau, H. Acta Crystallogr. 1976, 832, 150. (7) Baudour, J. L.; Cailleau, H.; Yelon, W. B. Acta Crystallogr. 1977, 833. 1773. (8) Patterson, F. G.; Lee,H. W. H.; Wilson, W. L.; Fayer, M.D. Chem. Phys. 1984, 84, 51. (9) Gerkins, R. E.; Winer, A. M. J . Chem. Phys. 1973, 58, 1360. (10) Wyckoff, R. W. G. Crystal Structures Vol. 6,2nd ed.; Wiley: New York. 1969. (11) Sloop, D. J.; Lin, T.-S. J. Magn. Reson. 1990, 86, 156. (12) Ong, J.-L.Ph.D. Thesis, Washington University, 1991. (13) Carrington, A.; McLachlan, A. D. Introduction To Magnetic Resonance; Harper and Row: New York, 1969; Chapter 12. (14) Craig, D. P.; Mason, R.; Pauling, P.; Santry, D. P. Proc. R. Soc. (London), 1965. --.A286.98.(15) Hochstrasser, R. M.; Small, G. J. J . Chem. Phys. 1968, 48, 3612. (16) Sussmann, J. A. Ann. Phys. 1971,6, 135. (17) Skinner, J. L.; Trommsdorff, H. P. J. Chem. Phys. 1988, 89, 897.
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