J. Phys. Chem. 1987, 91, 327-331
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Triplet Energy Transport and Microwave-Controlled Triplet Excitation Density in Doped Dlbenzofuran Single Crystals C. von Borczyskowski,* F. Seiff, and D. Stehlik Institut fur Atom und Festkorperphysik, Fachbereich Physik, Freie Universitat Berlin, D - 1000 Berlin 33, FRG (Received: June 17, 1986)
Triplet exciton density and hence triplet energy transport in a dibenzofuran single crystal containing X-traps and doped phenazine (2000 ppm) and anthracene (550 ppm) molecules can be controlled via resonant ODMR transitions of phenazine. Excitation spectroscopy reveals that different emission centers are populated by essentially independent excitation pathways. Nevertheless, communication between these centers is established via the dibenzofuran triplet exciton band. Experimental results for the microwave-controlled energy transport are in qualitative agreement with model calculations. From this we conclude that due to high-power laser excitation more than 50% of the phenazine molecules within the spatially inhomogeneous excitation region are in the excited triplet state.
1. Introduction In two recent publications,’g2 we reported long-range singlet energy transport in dibenzofuran single crystals via so-called perturbed X-traps, which can be said to form a kind of “guest” exciton band. We were able to show that population of these X-traps in their triplet state slows down this energy transport by more than 25%. The reason is that any appreciable steady-state population of long-lived dibenzofuran (DBF) triplet X-traps reduces the number of DBF molecules in the ground state and therefore increases the average separation of DBF X-traps. As we have shown explicitly: the mechanism for the energy transport is the distance-dependent electric dipole-dipole interaction, which is usually the dominant mechanism in singlet energy transport.3 This has been established in a variety of other examples in mixed molecular crystals via sensitized fluorescence both for transport via exciton bands4 and guest exciton bands.5 One conclusion from the energy transport via the perturbed X-trap singlet exciton band was that the concentration of DBF X-traps is rather high, of the order of 1-10%,2,6 in surprising agreement with a recent X-ray structure study,’ which determines about 8% of the DBF molecules to be in a orientational disorder. Our present investigation was intended to check whether the correspondent concentration of DBF triplet X-traps is able to establish an effective energy transport via a triplet guest exciton band as well. However, the situation is quite different for triplet energy transport compared to singlet energy transport: The electric dipole-dipole mechanism should be negligible because it involves spin-forbidden S-T mixing. Instead, exchanges or s~perexchange~ processes could be operative. Moreover, at high concentrations of guest molecules, the transport could proceed via percolation, which is known to provide long-distance transport in disordered molecular crystals.1° Instead of varying the concentration of traps or supertraps by doping-the usual procedurethe concentration of excited triplet states can also be controlled via resonant microwave transitions between triplet spin sublevels provided they have different pop(1) Fischer, U.; von Borczyskowski, C.; Seiff, F.; Stehlik, D. Chem. Phys. Lett. 1983, 97, 476.
(2) von Borczyskowski,C.; Fischer, U.; Stehlik, D. Chem. Phys. Lett. 1984, 112, 150.
(3) Farster, T. In Modern Quantum Chemistry; Sinanoglu, T., Ed.; Academic: New York, 1965; Vol. 3. (4) Wolf, H. C. Adu. A t . Mol. Phys. 1967, 3, 119. ( 5 ) Port, H.; Vogel, D.; Wolf, H. C. Chem. Phys. Lett. 1975, 34, 23. (6) Schweitzer, D.; Zimmermann, H. Z . Naturforsch., A 1979, 34a, 1185. (7) Reppart, W. J.; Galucci, J. C.; Lundstedt, A. P.; Gerkin, R. E. Acta Crystallogr., Sect C Cryst. Struct. Commun. 1984, C40, 1572. (8) Dexter, D. C. J. Chem. Phys. 1953, 21, 836. (9) Robinson, G. W.; Frosch, R. P. J. Chem. Phys. 1962, 37, 1962. (IO) Kopelman, R. In Spectroscopy and Excitation Dynamics of Condensed Molecular Systems; Agranovich, V. M., Hochstrasser, R. M., Eds.; North Holland: Amsterdam, 1983.
0022-3654/87/2091-0327$01.50/0
dation and decay rates. Although this method does not offer such a wide variation range in concentration as doping molecular crystals, it is-as will be shown-of advantage in systems where the concentration of traps or defect centers cannot be varied at will because it is a constant of the system itself. Finally, a model for an energy transport via the iriplet exciton band will be presented and compared with the experimental results. 2. Experimental Section
Experiments have been performed in a helium bath cryostat at temperatures of 1.6 K. Optical excitation has been achieved with the UV lines of an argon ion laser (Spectra Physics 171/09) at 333.6-363.8 nm. Emission has been detected via a 0.5-m monochromator (Jarrel-Ash) and a photomultiplier (EM1 9658QB) followed by a photon counting unit (PAR 1182). In the case of ODMR spectroscopy, signal averaging has been performed by using a multichannel analyzer (Tracor N S 570). The ODMR setup has been described elsewhere.” Microwave amplitude modulation has not been used due to the long lifetime of the X-traps and in order to obtain always the proper sign of the ODMR signal amplitude. Excitation spectroscopy has been performed with a standard fluorescence spectrometer (Shimadzu R F 540) which has been equipped with a helium glass cryostat with an optical tail made of Suprasil. For phosphorescence detection, a rotating cylinder was used with two windows a t 180’. Proper optical filters have been inserted in the emission and excitation path to get rid of scattered light. Single crystals have been obtained after extensive zone refining with standard Bridgeman techniques. Nevertheless all crystals contained anthracene (A) as an impurity. Crystals have been doped either by phenazine (P) or acridine (Ac) in concentrations of about 2000 ppm.
3. Results After excitation with the UV lines of an argon ion laser, the emission spectra of DBF doped with 2000 ppm P or Ac show strong phosphorescence of the doped molecules with the emission origin at 639.9 and 625.9 nm, respectively. Additionally, the phosphorescence of DBF X-traps with the origin at 407.3 nm is observed. The existence of these X-traps has already been reported in the literature,6J2 and is probably due to structural imperfections. The concentration of X is expected to be on the order of a t least 1%.* In all our crystals, the fluorescence of A was present despite different syntheses of the host matrix followed by extensive zone refining. The concentration is estimated to be on the order of or less than 50 ppm. (1 1) von Borczyskowski, C.; Fallmer, E. Chem. Phys. Lett. 1983,102,433. (12) Goldacker, W.; Schweitzer, D.; Zimmermann, H. Chem. Phys. 1979,
36, 15.
0 1987 American Chemical Society
328 The Journal of Physical Chemistry, Vol. 91, No. 2, 198 7 A I,/
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von Borczyskowski et al.
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Figure 1. Relative DBF phosphorescence intensity change AI, due to a P-h, ODMR transition as a function of the optical excitation rate koP. The solid lines have been calculated with kZP>> klP, k3P = 200 SKI. Cpo/Cx = 0.16 has been adjusted for the experimental result indicated by A . The experimental points have been included by normalizing koP = 200 s-l to an incident excitation laser power of 50 mW. The figure also shows the measured phenazine phosphorescence intensity Ipph,as a function of the exciting laser power using the same koPabscissa. The broken initial straight slope is expected in case of linear dependence on
the excitation intensity, whereas the solid line has been calculated according to ( 6 ) . These three emission centers could be further characterized by the observation of the 2(EI zero-field transitions by ODMR. The transition frequencies are 664 (P-hs),666 (P-d8),531 (Ac), 592 (X-DBF), and 506 MHz (A). The latter transition (A) has been detected via fluorescence because the low A concentration phosphorescence was unobservable. For P-hs and X-DBF, the triplet lifetimes have been determined to be 4.8 f 0.2 ms and depending on the crystal and excitation intensity 3.1-4.8 s, respectively. These results suggest that the triplet spin sublevels are still isolated at our temperatures of 1.6 K, because the lifetimes correspond to the known decay rates of the most strongly radiative sublevel of PI3 and X.I2 All the above results are consistent with those reported in the literature for P, Ac, and A in various host^.'^-'^ The surprising observation, however, is a phosphorescence emission of DBF, although the excitation is well below the So-+ SIabsorption origin at 306.3 nm15 which implies that the normally very effective intersystem crossing from SI to TI can not be active in our situation. We also studied the emission intensity as a function of the laser power up to 0.6 W. In all crystals we observed a linear dependence for the phosphorescence of X, but in most of the crystals we observed for P and A a deviation from linearity at higher laser power which is shown for P-hs phosphorescence in Figure 1. 3.1. Phenazine ODMR Effect on Different Emission Centers. In addition to the above properties of the isolated molecules, we also observed interactions between the different emission centers. This is most clearly demonstrated when selectively observing the emission of X, A, and P or Ac and sweeping over the 214 zero-field transition of P or Ac. Saturation of the 2)EJtransition of P-d8 (see Figure 2) results in an increase of the phosphorescence of X and a decrease of the fluorescence of A at the same 21EI transition frequency as for P. Even the hyperfine structure due to the nitrogen quadrupole interactionI6 is observed on all the emission centers. The decrease of the P phosphorescence is consistent with the depopulation and population kinetics of the three triplet spin sublevels x, y , and z of P doped in bi~heny1.l~ The correspondent total depopulation rates for P-hs are k, = 200 s-l and ky = 14 s-l, the relative radiative depopulation rates are kEfel = 1 and k$, = 0.02, and the relative population rates are K,,,,, = 1 and K,,,,, = 0.01 1. The combination of rates results (13) Antheunis, D. A.; Schmidt, J.; van der Waals, J. H. Mol. Phys. 1974, 27, 1521. (14) von Borczyskowski, C. Chem. Phys. Lett. 1982, 85, 293. ( 1 5 ) Karl, N.; Heym, H.; Stezowski, J. J. Mol. Cryst. Liq. Cryst. 1985, 131, 163. (16) Dinse, K. P.; Winscom, C. J. J . Chem. Phys. 1978, 68, 1337.
1 650
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650
3 ' 6
1
710
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Figure 2. 21EI ODMR transition of P-d8 detected on the different characteristic emission wavelengths of DBF triplet X-traps (top), A fluorescence (middle), and P phosphorescence (bottom).
in relative spin sublevel steady-state populations of N , = 1 and N y = 0.16.12 Similar results are observed for Ac; for further analysis we will concentrate only on P-hs in the forthcoming discussion. The exact dependence of the ODMR effect on laser power is difficult to determine because the microwave saturation behavior is very much dependent on the laser power itself. The reason for this is that the crystal is locally heated up at high excitation levels which will result in a faster spin-lattice relaxation rate.I3 For the absolute determination of the ODMR effect, the microwave power should in principle be varied and should be kept as low as possible to avoid saturation effects. This is, however, not possible at very low optical excitation rates, where due to signal-to-noise problems we always had to use a rather high microwave power and hence we are probably dealing with partly saturated ODMR transitions. Under these conditions the P ODMR effects are almost constant as a function of the laser power as is shown in Figure 1 for the P transition detected on the X-trap phosphorescence. The maximum absolute ODMR effect for the P transition is -13%, +3%, and -1% when detecting the emission of P, X, and A, respectively. It should be mentioned that the ODMR transition for A detected via fluorescence" of A is also about -l%, whereas the X-trap ODMR detected on the phosphorescence of X is about -1.5%. 3.2. Excitation Spectroscopy. To get more detailed information on the population pathways of X-trap triplet states, we have performed excitation spectroscopy. (For A fluorescence the already known absorption origin at 385 nm is 0 b ~ e r v e d . l ~ ) Excitation spectra for X-trap and phenazine phosphorescence have been obtained by observing the long-lived phosphorescence. The result is shown in Figure 3. For P phosphorescence we observed a weak excitation origin So SI at 447 nm and a strong origin So S2at 390 nm, which is consistent with results reported for P in biphenyl.I8 It can be clearly seen that even at energies below the DBF singlet absorption, X-trap phosphorescence can be excited in a different way than P phosphorescence. The two pronounced absorption lines indicated could be identified by delayed fluorescence at 77 K as being due to an unknown impurity which was not present in all of the crystals. To make sure that an excitation below SI,e.g., at 380 nm, really results in X-trap phosphorescence, the emission of a crystal without P is also shown in Figure 3 for an increased excitation intensity. The emission can be clearly indentified via the emission origin and the corre-
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( 1 7 ) Clarke, R. H.; Hayes, J. M.; Hofeldt, R. H. J . Mugn. Reson. 1974, 13, 68.
(18) Nava, D. L.; McClure, D. S . Chem. Phys. 1981, 56, 167.
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 329
Microwave-Controlled Triplet Excitation Density LO 000
30000
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I
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Figure 4. Energy level scheme for the three identified emission centers. The singlet SI state of DBF is not included because in all cases the excitation energy was well below S, (DBF). The various rate constants are defined in the text.
I
200
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Figure 3. Excitation spectra of DBF triplet X-traps for two different crystals detected with 2-nm resolution. The detection wavelength has been fixed to the phosphorescence origin of the X-traps at 407.3 nm in a crystal doped with 2000 ppm P-h8.The excitation spectrum fixed at the most prominent vibrational peak at 437 nm has been performed on a crystal which has not been doped with P. For comparison the highenergy part of the X-trap phosphorescence spectrum with the excitation wavelength fixed a t 380 nm is included for a “puren crystal. Also included is the phosphorescence excitation spectrum for P-hs. Arrows below the wavelength scale mark the various UV lines of the argon laser used in the O D M R experiments. The small absorption peaks (*) in the excitation spectrum are assigned to a yet unknown impurity. All crystals contained A in concentrations of less than 50 ppm.
spondent vibrational progressions belonging to X-trap phosphorescence. Almost identical X-trap excitation spectra have been obtained for crystals which have not been doped with P. To conclude, excitation spectroscopy shows that DBF X-traps are directly populated in the triplet state via singlet-triplet absorption most probably into the triplet exciton band of DBF. Contributions from intermolecular excitation energy transport via the guest singlet states of P and A are certainly less than 5%, as concluded from the absence of corresponding contributions of Sz absorption of P in the excitation spectrum even in crystals doped with P. Phosphorescence of P, on the other hand, is predominantly populated via intramolecular pathways. 4. Discussion
The excitation spectra for X-traps and P revealed that these emission centers are predominantly populated via their corresponding intramolecular pathways. This implies also that any direct excitation energy transfer between SI or S2of P and Tl of X-traps is not observable in the purely optical spectroscopic information. This is contrary to the situation reported by various author^'^-^^ where excitation energy transfer from donor to acceptor occurs in the case when the lowest donor singlet state lies below the host singlet but above the host triplet band. Due to this transfer it was possible to observe sensitized phosphorescence of a low-concentration acceptor molecule. In our case, however, SI of P is below T1 of X-traps, and excitation spectra show that the dominant pathway for host triplet excitation is either direct S-T absorption or a population pathway via unknown impurities (19) (a) Hirota, N.; Hutchison, C. A., Jr. J . Chem. Phys. 1964,42,2869. (b) Brenner, H. C.; Hutchison, C. A,, Jr. J . Chem. Phys. 1973, 58, 1328. (20) Brenner, H. C. J . Chem. Phys. 1973, 59, 6362. (21) (a) Zimmermann, H.; Stehlik, D.; Hausser, K. H. Chem. Phys. Left. 1971, J J , 496. (b) Stehlik, D.; Haas, H.; Zimmermann, H. Proc. Mol. Cryst. Symp., 6th 1973, 1 .
which we have proved to be present in the crystal. Important for the following discussion is that the triplet state of DBF is not populated significantly via P excitation. The ODMR results, however, prove that nevertheless some communication must exist among the various emission ‘centers in the crystal. In the following we want to outline a model which includes on one hand independent intramolecular excitation pathways but on the other hand a communication among the emission centers via the triplet exciton band of DBF. 4.1. Kinetic Model for Trap-to-Trap Communication via Energy Transport in the Triplet Exciton Band. Our model will make use only of the most simple assumptions possible because, as will be shown in the forthcoming discussion, several additional effects, like inhomogeneous excitation, triplet-triplet annihilation, and triplet-triplet absorption, may quantitatively influence the experimental results but cannot be controlled experimentally all at once and thus are impossible to be treated quantitatively. For the triplet state of P, we assume two completely different population pathways: the predominant intramolecular excitation route as revealed by excitation spectroscopy and a postulated indirect route via the DBF triplet exciton band. The same is true for A. The ratio of intra- to intermolecular population rates will strongly depend on the absolute concentrations of P and A. The schematic energy level diagram in Figure 4 shows the individual rates considered in our model. koP,koX,and koAare the population rates via absorption of optical photons by P, DBF excitons, and A, and they are independent from each other as long as a homogeneous optical absorption applies. klPand k l Aare the respective singlet decay rates, kzP and kzA are the intersystem crossing rates, and k3P,k3X,and k3Aare the triplet decay rates, Le., at low temperature essentially the decay rate of the fastest decaying sublevel. Additional to the intramolecular excitation, energy transport is possible via the DBF triplet exciton band to the various localized guest triplet states with overall energy transport rates kp, kx, and kk These energy transport rates are assumed to be proportional to the ground-state concentration C of the correspondent acceptor molecules and can therefore be described as k p = ypCp,kx = yxCx, and kA = YACA,respectively. y,takes into account that the transport rate might well be dependent on the molecule itself, but it will be assumed to be equal in our model for the various acceptors. The phosphorescence intensity Zx of the X-traps thus can be expressed as
In order to calculate the effect of a P ODMR transition on Ix, we will have to insert the corresponding variation of C p as a function of the microwave power used to saturate the ODMR transition. The singlet ground-state concentration Cpof phenazine is Cp = Cpo- CpT- Cps N Cpo - CpT,where CpT >> Cps are the concentrations of the excited triplet and singlet states and Cpois the total concentration of phenazine. CpTcan be calculated in a simple way2,22taking into account only the dominant spin
von Borczyskowski et al.
330 The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 sublevel rates k, = k3 and K , = k2 because only the x level is predominantly populated and depop~1ated.I~ Omitting the index P for the rates in ( 2 ) and considering only the dominant intramolecular population route, we get
where the assumption k l , kz >> ko, k313has been applied. From (1) and ( 2 ) we thus get
The concentration of excited triplet states can be controlled by resonant microwave transitions. This can be most easily incorporated for the 214 transition, because a fully saturated transition results in an averaged triplet decay rateZk3 = l/2(kX k,) which can be approximated ( k , >> k,)I3 by k3 = I/Zk,. Thus, k3 has to be substituted by ‘/zk3 in ( 2 ) and (3) when the 21E1 transition is saturated. We now express the relative intensity change of IX with and without P O D M R transitions by incorporating the respective phenazine triplet concentration. The subscripts m and o stand for microwave on and off, respectively, and we get after simple algebra
+
The same relative change of excited triplet-state concentration is obtained for A. With the realistic assumption Cx> (CFm- CTo) >> CA, (4) in this case reduces to
However, for A only fluorescence is observable. But as shown an increased excited triplet-state concentration results in a decrease of the fluorescence intensity due to the associated ground-state depletion. For the same reason it was, e.g., possible to detect the microwave transition of A via changes of the fluorescence intensity. With the same argument given before for AZx, the change of P triplet-state concentration also results in a relative change AZA of A triplet states, which is at the same time accompanied by a change of A ground-state concentration and thus necessarily a change of A fluorescence intensity AIA’ = -AZA. 4.2. Comparison with Experimental Results. From the kinetic datal3 summarized in section 3.1, we expect and observe for P a decrease of the phosphorescence intensity when the 214 transition is saturated; see Figure 2 . This is accompanied by an increase of excited P triplet-state concentration, because the almost empty but long-lived y-spin sublevel is now effectively populated. Inserting the kinetic data for P (k2 >> k I 2 )into ( 2 ) , we can calculate AZx = AZA by inserting (2) into (4). The result contains the parameters Cpo/Cx and k3’/koP. In principle the concentrations are known as well as the decay rate k3 (k3 k, = 200 s-l for P-h8I3). Hence, AZx may be calculated as a function of koP,Le., essentially the optical pumping rate. A plot of AI, vs. koPis shown in Figure 1. In fact, the concentration ratio used, CpO/Cx= 0.16, has been adjusted to render the maximum observed AZx = 2.75%. In order to compare the theoretical with the experimental results, the koPscale has to be adjusted. Determination of the absolute optical pumping rate is difficult and unreliable. As a practical approach we arbitrarily adjusted the experimentally determined exciting light intensity scale to the koPscale in Figure 1 such that the best overall agreement was reached between theory and experiment. Also experimentally observed is the phosphorescence intensity of P as a function of the exciting light intensity. By use of the same parameter set, it can be expressed as
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(22) Nakamizo, M.; Matsueda, T. J . Mol. Spectiox 1968, 27, 450. (23) van Dorp, W. G.; Shocmaker, W. H.; Soma, M.; van der Waals, J. H.Mol. Phys. 1975, 30, 1701.
In Figure 1 PPhm is plotted as a function of koPand compared with the experimental results by adjusting the proportionality constant. Without any change in the koP scale, a satisfying agreement is obtained between experiment and calculation. Also in agreement with the predictions is that -AZAF AZx, comparing the experimentally observed values of -1% and 3%, respectively. Despite our crude model, the agreement between experiment and calculation is qualitatively good. We now have to discuss the parameters kopand Cpo/Cxin some more detail. It is known that and 8%,2-7whereas the concentration of X-traps is between 1%z%6 the concentration of doped P in the melt is 0.2%. From this one would expect a concentration ratio in the range 0.025 < Cpo/Cx < 0.2, which is in rough agreement with the fitted concentration ratio of 0.16. It should be noted, however, that the real P concentration in a grown crystal is probably lowered to 50.1% due to partial zone-refining effects. Another uncertainty concerns 7 , which for the lack of other information we assumed to be equal for P and X in our model calculations. If, e.g., yp > yx, this will show up as an apparently higher concentration of P. W e have not taken triplet-triplet annihilation into account, but an annihilation rate krr would decrease AZx obtained from (1). Neglecting kn would therefore imply that Cpo/Cx has been determined to be too high, which would be in agreement with the above conclusions. The high level of excitation rates, koP,concluded needs some comment. Even in the most optimistic approximations, the excitation rate is estimated to be not more than 10 s-] at a given laser power, which corresponds at maximum to 10’’ photons/s, and P concentration of 2000 ppm. The excitation rates of up to lo3 s-l inferred in Figure 1 can be explained only by the assumption of a strongly inhomogeneous excitation of the crystal surface which is very probable due to the high excitation density of the laser beam. The kind of “saturation” effect of the P phosphorescence intensity (see Figure 1) shows that we are dealing with excitation rates sufficiently high that a considerable amount of the molecules in the excited region of the crystal are in the excited triplet state. At the same time the X-trap phosphorescence does not show such saturation behavior, which is expected because of the much lower absorption probability for direct So T1 excitation. Up to now we assumed an inhomogeneous excitation, but we assumed a homogeneous absorption throughout the excitation profile. On the other hand, an inhomogeneous absorption of P according to Beer’s law would result in the effect that the so far independent population rates koP,koX,and koAwill become dependent on the P ground-state concentration Cp. Thus, a decrease of Cp due to microwave irradiation would result in an increase of the X-trap phosphorescence and at the same time in an increase of the fluorescence of A without the necessity of an energy transfer via the DBF exciton band. However, this possibility can be ruled out because experimentally a decrease of A fluorescence is observed which can be explained by the model outlined in the preceding section.
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5. Conclusions
In conclusion, the distribution of host triplet exciton excitation energy can be controlled via resonant microwave transitions in the excited P triplet state even if Cpo> Cx,which seems possible to obtain in other systems, AZx is approximated by kok2 Arx = k3(k, + kz) even in the limit kokz
