J . Phys. Chem. 1985,89, 5705-5709 groups. These isomers will all have similar electronic spectra. This is not quite true for the binuclear species, since the possible conformations of a specific molecule will be influenced by which of the possible 36 geometric isomers is concerned. In the case of EtMeO(5), for example, the cofacial conformer is apparently not formed with equal ease by all geometric isomers. While we have not found it possible to separate these various geometric isomers, chromatography will yield fractions containing varying mixtures of isomers. The electronic absorption and emission spectra reported in this paper are for representative mixtures of isomers. The reader should be aware that their preparation may have a different mixture and give rise to somewhat different spectroscopic characteristics. The differences will be reflected in a different degree of "average" coupling. We expect such differences to be fairly small except perhaps in the case of EtMeO(5) where we have observed rather larger variations from one sample to another. Note finally that there is only one geometric isomer with respect to the bridging link, say 3,3- but that rotation of one phthalocyanine unit about the other will generate a 3,4- conformational isomer. If rotation is hindered, these two conformers may both be present and may have different spectra. Summary and Conclusions. The EtMeO(5), Cat(4), and tBuCat(4) binuclears show equilibria between cofacial and noncofacial conformations, with the first, a t least in some of its isomers, having the highest proportion of cofacial. Pc-Pc(0) and O( 1) are quite similar in behavior, with no possibility for intramolecular cofacial arrangements, but showing significant inter-ring coupling. C(4) is largely uncoupled with little tendency to form
5705
cofacial conformers, and with a spectrum, under the various conditions, not very different from the mononuclear. This is rather surprising. Molecular models do not show any obvious reasons for this behavior; it would appear sterically possible to form cofacial conformers. The C(2) species has behavior intermediate between C(4) and O( 1) as expected. The electronic spectra of Co(I), Co(II), and Co(II1) derivatives of most of these species have been recorded;38in general the degree of coupling between the two halves of the metallated species parallels the behavior discussed here. Of considerable importance is the observation of a rough correlation between the degree of electronic coupling between the two halves of the cobalt species, and the ability of the cobalt species to electrocatalytically reduce molecular oxygen. Future studies will extend to iron complexes and will seek to understand the mechanism causing this important correlation.
Acknowledgment. We are indebted to the Natural Sciences and Engineering Research Council (Ottawa) and the Office of Naval Research (Washington) for financial support, to Shafi Greenberg for synthesis of the neopentoxyphthalocyanine species, and to Liu Wei for recording some of the data. Registry No. PcH2, 99128-80-0; Pc-Pc(O), 99147-58-7; 0 ( 1 ) , 99128-81-1; C(2), 99128-82-2; C(4), 99128-83-3; Cat(4), 99128-84-4; t-BuCat(4), 99128-85-5; EtMeO(S), 99128-86-6. (38) Liu, W.; Nevin, A,; Hempstead, M. R.; Melnik, M. M.; Lever, A. B.
P.; Leznoff, C. C., submitted for publication in Inorg. Chem.
Pressure- and Temperature-Induced Phase Transitions in Crystalline Tetracene Monitored via Fluorescence Resonances in a Magnetic Field R. Jankowiak,+fH. Bassler,*+and A. Kutoglul Fachbereich Physikalische Chemie und Fachbereich Geowissenschaften, Philipps- Universitat. 0-3550Marburg, FRG (Received: June 11, 1985)
The resonance pattern of the fluorescence intensity of crystalline tetracene in a magnetic field has been analyzed to yield information on the molecular motions accompanying the temperature- or pressure-induced phase transition. The hypothesis has been confirmed that formation of low-temperature and high-pressure phases involves a rotation of the molecule at 1/2,1/2:0 by 15' about the N axis. In addition, molecules at O,O,O and 1/2,1/2,0 rotate by -6 and -9', respectively, about the L axis. Part of the reorientation occurs continuously upon cooling the high-temperature phase and explains the increase of the Davydov splitting.
Introduction The aim of this work is to gain further insight into the molecular motions accompanying the phase transition in crystalline tetracene (TC). Recall that TC undergoes a transition from a high-temperature (HT) to a low-temperature (LT) phase somewhere below 200 K. The exact value of the transition temperature T, depends on the experimental procedure the crystal is subjected to.'-4 Application of hydrostatic pressure can alter the crystal structure as we1L5 A recent X-ray diffraction study6 established the triclinic structure of the LT phase and yielded its unit cell parameters. It also indicated that the transition occurs randomly within individual crystal domains which are either preexisting or defined by external forces acting nonuniformly on the crystal and establishing locally different thermodynamic conditions. Unfort Fachbereich Physikalische Chemie.
*
Fachbereich Geowissenschaften. *Present address: Ames Laboratory, U S . DOE,Iowa State University, Ames. IA 5001 1.
0022-3654/85/2089-5705$01.50/0
tunately, the number of reflections was insufficient for carrying out a complete structural analysis. To nevertheless gain some idea of the type of molecular motions involved in the transition, the reliability index6 ("R value") was calculated as a function of molecular position. It suggested a rotation of the face-centered molecule at 1/2,1/2,0approximately by 15' about the N axis which is normal to the molecular plane. The difficulty connected with the acquisition of a low-temperature diffractogram of T C which is adequate for endeavoring a complete structural analysis prompted us to search for an al( I ) Prikhotko, A. F.; Skorobogatko, A. F. Opt. Spectrosr. (Engl. Transl.)' . .
1966, 10, 33.
(2) Vaubel, G.; Bassler, H. Mol. Crysf. Liq. Cryst. 1970, 12, 39. (3) Turlet. J. M.: Philmtt, M. R. J . Chem. Phvs. 1973. 62, 4260. (4) Kolendritskii, D. 6.;Kurik, M. V.; Piryatikkii, Yu. P. Phys. Status Solidi B 1979, 91, 741. ( 5 ) Kalinowski, J.; Jankowiak, R. Chem. Phys. Lett. 1978, 53, 56. (6) Sondermann, U.; Kutoglu, A,; Bassler, H. J . Phys. Chem. 1985, 89, 1735.
0 1985 American Chemical Society
Jankowiak et al.
5706 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
ternative route to elucidate structural changes associated with the phase transition. In this work we examine structure-sensitive properties of crystalline TC such as the position of resonances of the fluorescence intensity in a magnetic field and the Davydov splitting. Admittedly, these quantities depend on the molecular positions in a complex fashion which prevents disentangling the molecular coordinates in an unambiguous way. On the other hand, a given set of coordinates produces a uniquely defined resonance pattern. Comparing the resonance patterns calculated on the basis of a hypothetical crystal structure with the experimental result is therefore a sensitive test for the validty of the model structure. Adopting this procedure, we shall be able to confirm the earlier hypothesis of a molecular rotation about the N axis yet also conclude on the simultaneous occurrence of molecular rotations about the long molecular ( L ) axes. A pretransitional effect involving only the latter kind of motion accompanies the HT LT transition.
9"
- !
3 ii
j
,
-90
,
,
-60
-30
Orientation of B
brryst
30
0 in
,
,
,
j
60
90
ab plane(degrees1
-
Analysis of the Resonances of the Crystal Fluorescence in a Magnetic Field One of the peculiar features of crystalline TC is that the room-temperature fluorescence is efficiently quenched because the SI singlet exciton undergoes fmion into a pair of triplet excitons via an intermediate triplet state,' following the scheme
s1+ So * (TT) + T + T The reaction is thermally activated, the activation energy being about 0.16 eV. The rate constant for the forward reaction depends on the singlet character of the intermediate tripkt pair (TT) state which can be modulated by a magnetic field B. Since-the amplitude of the modulation varies with the orientation of B relative to the molecular axes, the fluorescence intensity exhibits resonances if the crystal is rotated in a magnetic field. They occur at orientations where the 100) spin state is degenerate with the I+-) and I-+) states. Two different formalisms8 exist for interpreting the resonance pattern. One method involves calculation of the zero-field splitting E*) and the orientation of the principal axes (x*, constants (D*, y*, z*) of the zero-field-splitting (ZFS) tensor for the crystal triplet exciton within the framework of the Sternlicht-McConnell theory9 using the method developed by Tedder.Io It predicts that in the limit of high magnetic fields resonances occur for orientations of B that satisfy the equation D*(cos~y* -
X) + E*(cos'
LY*-
cos2 @*)= 0
(1)
The a*,@*,and y * denote the angles between 3 and the principal axes of the fine structure tensor. In case that D* and E * as well CIS the orientations of the ZFS tensor components are not known, one profitably adopts the second formalism which uses the molecular parameters D and E and the crystallographic orientations of the molecular fine structure tensor of the two inequivalent molecules (xl,y , , zl and x2,y2, z2, respectively) _as input parameters. If L Y ~ ( ~ ) , y1(2) are the angles between B and x ~ ( ~ y1(2), ), z1(21,one arrives at the resonance condition _D(cos2 yI
+ cos2 y2 - 73) + E(COS'a1
+ COS'
L Y ~- COS'
81 - a s 2 p2) = 0 (2)
If a temperature- or pressure-driven process affects the molecular orientations, the resonance condition changes are manifested in a change of the resonance pattern. In the present work the second approach was adopted using the following matrices" for locating (7) For reviews see: (a) Swenberg, C. E.; Geacintov, N. E. In "Organic Molecular Photophysics"; Birks, J. B., Ed.; Wiley: New York, 1973; Vol. 1, p 489. (b) Pope, M.; Swenberg, C. E. In "Electronic Processes in Molecular Crystals"; Clarendon Press: Oxford, 1982; p 139 ff. (8) Groff, R. P.; Avakian, P.; Merrifield, R. E. J . Lumin. 1970, 12, 218; Phys. Reu. B 1970, 1 , 815. (9) Sternlicht, H.; McConnell, H. M. J . Chem. Phys. 1961, 35, 1793. (10) Tedder, S . H. Chem. Phys. 1976, 14, 455.
0
-15
45
Orientotion o f B n ab pianeidegrees)
Figure 1. (a) Resonance pattern of the fluorescence intensity of a tetracene crystal in a magnetic field of 0.35 T at 296 K and at hydrostatic pressures of 0.1 and 320 MPa (from ref l7). Orientation of the magnetic field is relative to the crystallographic b axis. (b) Calculated energies of the 100) and-the I+-) spin states, resp-wtively, as a function of the angle between B and the crystallographicb axis. Full curves are for the HT crystal structure (196 K and 0.1 MPa). Dashed curves are calculated on the assumption of a rotation of molecule I1 at 1/2,'/2,0 by 15" about the N axis and a simultaneous rotation of molecules I (at O,O,O) and I1 about the L axis by -6 and -go, respectively.
the molecular axes L, M , and N relative to the axes of the room-temperature unit cell. molecule I a t (O,O,O)
L (-0.2723) -0.2672 (-0.2672) 0.9239 (0.9 2 39)
M
N
0.3551 0.8942 (0.3861) (0.8813) 0.8652 -0.4258 (0.8498) (-0.4557) 0.3551) 0.1409 0.3598) (0.1284)
molecule I1 at
('/2,'/2,0)
L
M
-0.4289 (-0.2664) (-0.3523) 0.8988 (- 0.2 3 4 8) (0.9284) 0.0872 (0.1165) (0.9 34 8)
'V 0.8607 (0.8948) 0.3778 (0.2982) 0.3404 (0.3315)
Matrix elements represent directional cosines." Values quoted in parentheses are the result of this work and refer to the hightemperature (HT) structure at 187 K, before the H T LT transition occurs. The D and E values used are those of Yarmus et al.I2 (D = 0.052 cm-' and E = -0.0052 cm-I). Choosing different parameter sets reported in the literature (D = 0.0573 cm-' and E = -0.0043 cm-' (ref 13); D = 0.063 cm-I and E = -0.0046 cm-' (ref 14)) would cause only small changes of the resonance pattern. Some error in the absolute values of the zero-field-splitting parameters may, in fact, be the reason for some minor differences between experimental and calculated 295 K resonance patterns. These ambiguities are unimportant in the present context since all conclusions derive from the relative changes of the resonance pattern with temperature or pressure.
-
(1 1) Robertson, J. M.; Sinclair, V. C.; Trotter, J. Acto C r y s t o b g r . 1961, 14, 691; 1962, IS, 289.
(12) Yarmus, L.; Rosenthal, J.; Chopp, M. Chem. Phys. Lett. 1972, 16, 477. (13) Brinen, J. S.; Orloff, M. K. Chern. Phys. Lett. 1967, 2, 276. (14) Clarke, R. H.; Frank, H. A. J . Chem. Phys. 1976, 65, 39.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5707
Phase Transitions in Crystalline Tetracene
-
P [MPal
100
3 9
500
300
i
pc
0 I I
c
r.7 m D
m I
296 K
a,
1
a c
Y
v)
P
I87 K
2-90
30
-60 -30
60
90
in
F
,
,
1,
bopt
Orientation of B
2 300
ab ploneldegreesl
220
1LO
1 (K) Figure 3. Separation ({) between the magnetic field resonances of the fluorescence intensity. Full curves are measured at 296 K and variable hydrostatic pressure (from ref 17); the dash-dotted curve is the variation measured upon cooling the H T phase to a temperature close to the H T LT transition temperature T,(- 187 K). Crosses are calculated on the basis of the molecular reorientations described in the text.
V
- 45
0
+45
Orienlation of B in ab plane(degrees1
Figure 2. (a) Resonance pattern of the fluorescence intensity of a TC crystal at 296 and 187 K and atmospheric pressure (from ref 18). (b) Calculated energies of the spin states for the H T phase at 296 K (full LT phase transition curve) and for the H T phase close to the H T temperature assuming rotation of molecules I and I1 about the L axes by -2 and -So, respectively (dashed curve).
-
The present analysis does, however, imply that the D and E values are insensitive to both temperature and pressure. In view of the results obtained with C T crystal^,'^-'^ this appears to be a realistic approximation. Figure l a reproduces literature results” for the resonance pattern of the room-temperature TC fluorescence intensity at atmospheric pressure and at 320 MPa. A phase transition occurs at -300 MPa and causes a discontinuous positive shift of the resonance peaks by 4 and go, respectively, giving an increase of the angular difference between the resonance positions, called {, by 5 O . Comparing data obtained With different samples and measured at different laboratories reveals a scatter of the absolute position of the 296 K, 0.1 MPa resonance patterns by about f2O without affecting {values. The reason may & minor misalignment of the (0,0,1) crystal plane with respect to B or some differences in the mosaic structure of the crystals. Since we are only interested in temperature- or pressure-induced relative shifts of the resonances, we eliminate this ambiguity by offsetting all 296 K, 0.1 MPa spectra for coincidence. Crystal cooling causes a continuous shift of the resonance pattern and a continuous increase of the peak separation. We display in Figure 2a only two curves taken from the family of resonance patterns reported in ref 18. Being temperature activated, singlet exciton fission becomes very inefficient below the phase transition temperature and the modulating effect of a magnetic field gradually vanishes. Nevertheless, Swenberg and Geacintov in their review a r t i ~ l e ’rdfer ~ to unpublished results of Burgos, later confirmed by J a n k ~ w i a k , indicating ’~ an increase in the separation of the resonance peaks up to 5 8 O upon crystal cooling. In addition, a hysteresis-like temperature (and cooling rate) dependence was noted which appears to be a characteristic L T phase transitions6 feature of the thermally driven HT Although the low-temperature value of {agrees with the {value of the HP phase, suggesting identity of HP and LT phases, there
-
(15) Haarer, D.; Karl, N. Chem. Phys. Lett. 1975, 21, 49. (16) Mohwald, H.; Erdle, E.; Thaer, A. Chem. Phys. 1978, 27, 79. (17) Kalinowski, J.; Godlewski, J.; Jankowiak, R. Chem. Phys. Lett. 1976, 43. 127. (18) Jankowiak, R.; Kalinowski, J.; Konys, M.; Buchert, J. Chem. Phys. Lett. 1979. 65. 549. (19) Jankowiak, R., unpublished results.
220 1 (K)
300
140
Figure 4. Rotational displacements At9 of molecules I (at O,O,O)and I1 (at 1/2,1/2,0)about the molecular L axis upon cooling the HT phase from 300 K to a temperature slightly above T,. The dashed curve gives the angle C between the planes of molecules I and 11.
is still some experimental ambiguity left, since the H T / H P resonance patterns of the sample used in that work are not available. Therefore, the total relative change of { during the HT LT transition is subject to an uncertainty of about 2O. A summary of existing data on the temperature/pressure-induced variation of { is presented in Figure 3. The 296 K, 0.1 MPa resonance spectrum of Figure 2a can with reasonable accuracy be interpreted in terms of the Merrifield theory.8 This is documented by plotting in Figure l b the variations of the energies W+-of the I+-) pair state a_nd W,, of the 100) pair state as a function of the angle between B and b. If molecule I1 is allowed to rotate by 15’ about the N axis in accord with the proposed LT crystal structure, the increase of {in the high-pressure (HP) phase is approximately recovered, yet without reproducing the overall shift of the resonance pattern. However, a perfect agreement between the computed and experimental spectrum is obtained by assuming a simultaneous rotation of molecule I1 by 15O about the N axis and of molecules I and I1 by -6 and - 9 O about the L axis, respectively (see Figure lb). Negative angle means that a viewer looking along the crystallographic [OiO] direction sees a clockwise rotation of the molecules. A variety of different structure variations was tested of which only the simultaneous rotation of both molecules about their L axis without invoking rotation about the N axis gave reasonable agreement with the experiment. However, we reject this possibility on the basis of lattice energy calculations which predict a repulsive ground state for this type of lattice deformation. We therefore end up with the following matrices describing the orientations of in the HP phase: molecules I ( W , ) and I1 (V,)
-
L
M
N
0.44666 0.85218 0.81594 -0.51399 0.36796 0.10297
L
M
N
-0.14781 -0.45974 0.8804
-0.34284 0.8565 0.3754
0.92574 0.2468 0.28519
)
5708
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
Jankowiak et al. provide, in fact, independent evidence for molecular reorientations in crystalline a n t h r a ~ e n e . ~ ~ Since N rotation of molecule I1 has no significant effect on the resonance interaction term, the discontinuous increase of A at the LT transition is an indication that an additional translaHT LT tional displacement of molecule I1 occurs at the H T transition as suggested previously.6 Being insensitive to translational motions, the magnetic field modulation of the fluorescence is principally unable to test this hypothesis.
-
300
219
180
TEMPERATURE IK)
Figure 5. Relative variation of the Davydov splitting of the lowest singlet transition in the HT phase of tetracene. Curves 1 and 2 are taken from ref 3 and 4, respectively. Curve 4 is the variation of the dipolar contribution to the resonance interaction expected on the basis of the lattice contraction alone, while curve 3 considers the effect of molecular reorientation and lattice contraction.
The continuous temperature-induced variation of { measured in the temperature interval 300 K > T > 180 K can be quantitatively accounted for by assuming a continuous rotation of molecules I and I1 about the L axis. Figures 1b and 3 document the perfect agreement between the { values computed on the basis of the temperature-induced angular displacements indicated in Figure 4.
-
Temperature-DependentVariations of the Davydov Splitting At the H T LT phase transition the Davydov splitting A of the lowest singlet exciton state increases abruptly. However, reflection3q4as well as photocurrent excitation spectra2also indicate a pretransitional increase of A in the temperature range 300 K > T > T,. Again, there is some difference between the results of different laboratories with regard to absolute values; yet the temperature coefficients, d(A( T)/A300 K)/BT, differ only marginally (0.707 cm-I/K (ref 4) and 0.71 l cm-'/K (ref 3); see Figure 5). Having at hand the information about the temperature-dependent continuous change of the mutual orientation of the T C molecules of the unit cell and, concomitantly, the change of the angle between the molecular planes (Figure 4), it is straightforward to calculate the relative variation of the dipolar contribution to the resonance interaction energy from A cos [6zl(7'),AII(7')]/Rmn3(T), where m, and mll are the transition moments of the inequivalent molecules I and 11. To obtain the result plotted in Figure 4, we assumed that the thermal expansion coefficients a, and ab for the HT phase are the same as those reported for crystalline anthracene.20 Also shown is the change in A expected if the lattice would contract, the molecular orientations remaining invariant. It is obvious that molecular repositioning contributes significantly to the resonance energy. We are aware of Schlosser and Philpott'sZ1conclusion that the simple dipole model is inappropriate for calculating the Davydov splitting in the higher membered acene crystals because it ignores mixing with higher transitions and the contributions of nondipolar interactions. We nevertheless consider it remarkable that about half of the total variation of A can be explained in terms of this approximation. On the other hand, failure of the attempt to rationalize pressureor temperature-induced variations of A in terms of a lattice compression model without considering molecular reorientation explicitly cannot be taken as evidence for breakdown of the dipole approach. For instance, Otto et a1.22as well as Kalinowski and J a n k ~ w i a kfound ~ ~ that in crystalline anthracene A changes at a rate of 0.18 cm-'/MPa (up to 800 MPa) and 0.13 cm-'/MPa, respectively, while volume compression alone can only account ~ ~ difference may well be attributed to for 0.04 ~ m - l / M P a . The pressure-induced molecular rotations. Variations of the resonances of the triplet exciton fusion yield in a magnetic field under pressure
-
(20) (21) (22) (23) Crystal
Mason, R. Acta Crystallogr. 1964, 17, 541. Schlosser, D. W.; Philpott, M. R. Chem. Phys. 1980, 49, 181. Otto, A.; Keller, R.; Rahman, A. Chem. Phys. L e f t . 1977, 49, 145. Kalinowski, J.; Jankowiak, R. "Abstracts of Papers", 10th Molecular Symposium, St. Jovite; 1982; p 144.
-
Comparison with Structure Calculations In continuation of earlier work6 lattice energy calculations were performed based on the atom-atom potential method25assuming nonbonded potentials of the form U,J = Ar,,* + B exp(-0,). In the present work the parameter set of Williams26 was chosen, instead of the Huler-Warshe12' set. While affecting the absolute values of the lattice energies per molecule considerably, this choice has no influence on the earlier result, that the lattice energies of molecules I and I1 differ by approximately 1 kJ/mol, the position of molecule I1 being the less stable one. Previous reasoning LT regarding the molecular motion accompanying the H T phase transition rested on the variation of the reliability index ( R values). The new lattice energy calculations confirm those conclusions by demonstrating that a 15' rotation of molecule I1 about the N axis generates a local minimum in the potential surface. However, simultaneous rotation of both molecules about their L axes does not improve the stability, contrary to what the variation of the fluorescence resonances in a magnetic field suggests. We attribute this failure to the neglect of temperature-dependent dynamic effects in lattice energy calculations of the above sort. This is an illustration of the limitation of this method if used for analyzing subtle structural changes at temperatures where molecular motions have a significant amplitude.
-
Discussion Analysis of the fluorescence resonance pattern confirms the earlier conclusion that both the HT LT and H T H P phase transition of TC are associated by a 15' rotation of molecule I1 about its N axis. In addition, it reveals a simultaneous rotation of molecules I and I1 about the L axis by an amount A291,11which is the sum of a term AO' and a term AO(T) varying continuously with temperature. Upon cooling from 300 to 180 K, which is a representative value for the HT LT phase transition temperature, molecules I and I1 rotate by -2 and -5', respectively, followed by an additional and discontinuous angular displacement of each molecule by -4'. The AO(T) term causes a continuous decrease of the angle between the planes of both molecules from 51.0 to 47.8' (see Figure 4) manifested in the increase of the resonance interaction of both molecules in the excited state (Davydov splitting). The continuous character of the angular displacement is an indication for the asymmetry of the intermolecular potential governing libration of the molecule about the L axis which, by virtue of a moment of inertia argument, is the dominant libration to consider. Since the parallel conformation of a pair of TC molecules has a repulsive ground state, the libration potential will rise faster/slower at angles that are smaller/larger than the equilibrium interplanar angle. As librations become progressively excited, the interplanar angle will therefore increase. On the other hand, no continuous molecular motion is observed prior to the pressure-induced phase transition at a given temperature. This demonstrates that pressure does not noticeably affect the asymmetry of the libration potential. Therefore, the molecular orientations remain unaltered under hydrostatic pressure until at a given reduction of the intermolecular separations transition to the new phase occurs in a single step involving L and
-
-
-
(24) Jankowiak, R.; Kalinowski, J., unpublished results. (25) Kitaigorodskii, A. I. Chem. SOC.Reu. 1978, 7, 133. (26) Williams, D. E. In "Topics In Current Physics"; Metzger, R. M., Ed.; Springer-Verlag: West Berlin, 1981; Vol. 26, p 9. (27) Huler, E.; Warshel, A. QCPE 247, 325.
J. Phys. Chem. 1985,89, 5709-5713 N rotations in the course of a concerted action. The above picture implies that HT and HP structures be, in fact, identical. Since lattice energy calculations do not provide any hint on a second metastable molecular arrangement, this appears to be a plausible conclusion. In fact, all experimental evidence available so far is compatible with this interpretation except fluorescence spectra. Kalinowski et a1.28found that the fluorescence of the HP phase is red-shifted compared to that of the L T phase. However, for the following reason we disregard this observation as evidence against the identity of both phases: (i) TC fluorescence spectra show large inhomogeneous broadening, possibly due to defects. In addition, in the temperature range investigated the 0-0 emission band is subject to reabsorption effects.28 Therefore, the peak of the 0-0 band is not a good indicator for the energetic location of the emitting state. (ii) Pressure may well create additional structural traps for singlet excitons. Fluorescence spectra nevertheless indicate that there is a Stokes shift of order 200 cm-’ between absorption and emission. Occurrence of the asymmetric rotational displacement of molecule I1 provides a straightforward interpretation of this effect in terms of site relaxation of molecule 11. The driving force for the motion is the gradient of the intermolecular potential of an excited pair of molecules which favors a coplanar molecular arrangement. This is equivalent to self-trapping of a singlet exciton by coupling to a librational mode of molecule 11. Since the ground state of the trapping conformation is repulsive, the binding energy of the exciton is only a fraction of the energy loss between absorption and emission and the high mobility of singlet excitons (28) Kalinowski, J.; Jankowiak, R.; Bassler, H. J . Lumin. 1981, 22, 397.
5709
in the HT phasez9 is not in contradiction to the localized concept.
Concluding Remarks This work demonstrates that the analysis of fluorescence resonances in a magnetic field can profitably be used for structural analysis in cases where a full crystallographic study is inaccasible. Bearing in mind that delayed fluoresence resulting from the mutual annihilation of two triplet excitons can also be modulated by a magnetic field,’ it is by no means restricted to crystalline tetracene where one can monitor fission of the singlet exciton. A promising application of this method is the determination of the orientation of guest molecules doped into molecular crystals. Even if the dopant enters the host matrix substantially, it need not and, in fact, does not normally have the same orientation as the molecule it replaces. Examples are tetracene doped into a n t h r a ~ e n e ~ ~ , ~ ’ or pentacene in t e t r a ~ e n e . Analyzing ~~ the resonance pattern of heterofission of singlet excitons or of heterofission of triplet excitons appears to be a relatively simple tool for unravelling the packing characteristics of the guest molecule. The disadvantage of the method is that it is insensitive to translational molecular displacements.
Acknowledgment. Financial support by the Fonds der Chemischen Industrie is gratefully acknowledged. Registry No. TC, 92-24-0. (29) Campillo, A. J.; Hyer, R.; Shapiro, S . C.; Swenberg, C. E. Chem. Phys. Lett. 1917, 48, 495. (30) Ramdas, S . Chem. Phys. Lett. 1979, 60, 320. (31) Dorner, H.; Hundhausen, R.; Schmid, D. Chem. Phys. Lett. 1978,53, 101. (32) Jankowiak, R.; Kalinowski, J. Mol. Cryst. Liq. Cryst. 1978,48, 187.
Backward Electron Transfers wlthin a Geminate Pair Formed in the Quenching of the Phosphorescent States of Rhodium(I I I ) Compounds Takeshi Ohno Chemistry Department, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan (Received: April 12, 1985)
Electron transfer quenching of the phosphorescent states of rhodium(II1) compounds was studied by nanosecond laser photolysis-kineticspectroscopy. Six aromatic amines and three methoxybenzenesin the mixed solvent of water and acetonitrile were oxidized by both the T-T* triplet excited states of rhodium(II1) tris(4,7-diphenyl-l,IO-phenanthroline) and rhodium(II1) tris( 1,lO-phenanthroline)and the ligand field triplet excited states of dichlororhodium(II1)bis(4,7-diphenyl- 1,IO-phenanthroline) and dichlororhodium(II1) bis( 1,lO-phenanthroline). The rates of spin-invertedbackward electron transfers within the geminate radical pair were derived from the efficiencies of the electron transfer product formation in the quenching. The backward electron transfer rates, which are not limited by the diffusion-controlled rate, gradually increased with the exothermicity but any “inverted behavior” did not appear in the highly exothermic region (-1.73 eV). Nonadiabaticity in the backward electron transfer is reflected in the weak dependence of the rate on the exothermicity.
Introduction Electron transfer is one of the most fundamental of chemical reactions. Reaction rates for bimolecular electron transfers in condensed media have been studied theoretically and experimentally. Adiabatic predicted that the reaction rates of electron transfer increase with exothermicity to a maximum and then decrease with increasing exothermicity in the highly exothermic region. A number of experimental studies of bimolecular electron transfer in solution3-’ have found that a collisional (1) Marcus, R. A. J. Chem. Phys. 1964, 43, 679-701. Annu. Rev. Phys. Chem. 1964, 15, 155-96. ( 2 ) Siders, P.; Marcus, R. A. J . Am. Chem. Sot. 1981, 103, 741-7. Marcus, R. A.; Siders, P. J . Phys. Chem. 1982, 86, 622-30. (3) Rehm, D.; Weller, A. Ber. Bunsenges. Phys. Chem. 1969, 73, 834-9.
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process limits the reaction rate before it reaches a proper maximum in the highly exothermic region. Fast electron transfer processes compared to the diffusion process in liquid solution have been measured in the backward reaction within a geminate radical pair which was generated in a bimolecular electron transfer of excited molecules.8-’z The rates of the fast backward electron transfer ~~~
(4) Kikuchi, K.; Tamura, S.; Iwanaga, C.; Kokubun, H.; Usui, Y. Z . Phys. Chem. (Frankfurt am Main) 1917, 106, 7-18. ( 5 ) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J . Am. Chem. SOC.1978, 100, 7219-23. (6) Nagle, J. K.; Dressick, W. J.; Meyer, T. J. J. Am. Chem. SOC.1979, 101, 3993-5. (7) Indelli, M. T.; Ballardini, R.; Scandola, F. J . Phys. Chem. 1984, 88, 2547-5 1 . ( 8 ) Masuhara, H.; Mataga, N. Acc. Chem. Rev. 1981, 14, 312-8.
0 1985 American Chemical Society
