The Journal of Pbysical Chemistry, Vol. 83, No. 26, 1979 3397
EPR of EDA Triplet In Mixed Crystals
distribution of absolute triplet state energies created in the excitation process, necessarily give rise to narrow ODMR lines. On the other hand, one can expect that, in some cases depending on the particular properties of a given system, line narrowing might be observed under T, Sopumping when compared to S1 So pumping since different sites are excited in the two cases. The example of 1-naphthol is a case in point. As seen in Figure 5 the resolution of the ODMR transition is clearly improved with T1 So excitation. This can be explained by assuming that two components contribute to the emission as is suggested by the narrowed phosphorescent spectra. Since S1 So excitation gives rise to broad phosphorescent lines, both components contribute to the emission and thus to the “broadened” ODMR signal. However, under T1 So pumping the two components can be resolved optically and the ODMR signals that result when only one component is monitored yield “narrowed” lines. Thus the resolution in the ODMR domain can indeed be improved somewhat by T1 So pumping but the extent of such improved resolution must await further study of this and other systems and the development of a quantitative model. Acknowledgment. We thank Dr. Thijs Aartsma for helpful discussions, James van Zee and Douglas Lantrip for contributions to the computer programs used for experimental control, and Sheldon Danielson and Mark Champion for design and fabrication of various interface modules for this instrumentation. This work was sup-
-
+-
+
-
-
-
ported by Grants GM22603 and CA 19101 awarded by the National Institutes of Health, DHEW. References and Notes (1) J. Schmktt and J. H. Van der Waals, Cbem. Pbys. Lett., 2, 640 (1968). (2) M. Gouterman, B. S. Yamanashi, and A. L. Kwiram, J. Cbem. Pbys., 56. 4073 (1972). (3) C. A. Hutctbofhr,, J. V. Nlcholas, and G. W. Scott, J. Cbem. Pbys., 53, 1906 (1970). (4) . , A. L. Kwiram, MTP Int. Rev. Sci., Pbys. Cbem., Ser. 1 , 4, 271 (1972). (5) D.S. McClure, “Electronic Spectra of Molecules and Ions in Crystals”, Academic Press, New York, 1959. (6) E. V. Shpol’skli, Sov. Pbys. Usp., 6, 411 (1963). (7) K. K. Rebane, ”Impurity Spectra of Solis”, Plenum Press, New York, 1970. (8) N. S. Bayliss and E. G. McRae, J . Phys. Cbem., 58, 1002 (1954). (9) M. Sharnoff, J. Cbem. Pbys., 46, 3263 (1967). (10) A. L. Kwiram, Cbem. Pbys. Lett., 1, 272 (1967). (11) M. A. El-Sayed, Annu. Rev. Pbys. Cbem., 26, 235 (1975). (12) J. U. von Schutz, J. Zucllch, and A. H. Makl, J. Am. Cbem. SOC., 96, 714 (1974). (13) A. H. Makl and J. Zuclich, Top. Current Cbem., 54, 115 (1975). (14) J. van Egmond, B. E. Kohler, and I. Y. Chan, Cbem. Pbys. Lett., 34. 423 (1975). (15) A. L. Kwiiarn, J: B. A. Ross, and D. A. Deranleau, Chem. phys. Lett., 54, 506 (1978). (16) A. H. Zewail, J. Cbem. Pbys., 70, 5759 (1979). 117) E. I. Al’shits, B. I. Personov, and B. M. Kharlamov, Cbem. Pbys. Lett., 40, 116 (1976). (18) J. Funfschilling,E. Wasmer, and I. Zschokke, J . Cbem. Phys., 60, 2949 (1978). (19) A Szabo, Pbys. Rev. Lett., 25, 924 (1970). (20) It should be emphasized that a given molecule In a site of energy E#) does not have access to states at a Merent energy E&./) unless the local lattice structure is somehow converted from configuration ktoj.
EPR Studies of the Phosphorescent State of EDA Complexes in Mixed Crystal Systems Chong-lao Yu and lien-Sung Lin“ Department of Chemistty, Washington University, St. Louis, Missouri 63 130 (Received August 13, 1979)
The EPR spectra of the phosphorescent state of the following electron donor-acceptor (EDA) complexes imbedded in naphthalene-tetrachlorophthalic anhydride (N-TCPA) crystals have been measured: phenanthrene-TCPA, anthracene-TCPA, and phenazine-TCPA. We have observed triplet signals from both the guest and the host complexes with different spin polarization patterns which indicate the energy transfer from the host to the guest complex is ineffective. The charge-transfer (CT) character, measured from the change in the zero-field splittings and hyperfine splittings upon complexation,for each of the above complexes is 45 f 5% for phenanthreneTCPA, 25 f 10% for anthraceneTCPA, and 14 f 7% for phenazine-TCPA. The orientational studies show that the in-plane axes of the guest donor are distorted with respect to those of the host donor. The distortion is explained in terms of the stacking patterns of donors and acceptors (overlapping principle) in the solid.
Introduction Previously we have reported a completed EPR study of the phosphorescent state of 1:l naphthalene-tetrachlorophthalic anhydride (N-TCPA) complex crystals.’ The study enabled us to obtain the degree of charge-transfer (CT) character, to measure the population and decay rates, to probe the spin polarization, and to determine the structural and dynamical effects in the photoexcitation of the lowest triplet state of N-TCPA crystals. Here we report further EPR studies on the following electron donor-acceptor (EDA) complexes imbedded in N-TCPA crystals: phenanthrene-TCPA, anthracene-TCPA, and phenazine-TCPA. The objectives of the mixed crystal studies are to investigate the energy transfer process between the host complex and the guest complex, and to 0022-365417912083-3397$0 1.OOlO
examine the CT character of the guest complexes (with the same number of 7r electrons) in the same host complex crystal. It has been shown that the hyperfine structure (hfs) data of EDA complex provide a more direct probe than the fine structure (fs) data for estimating the degree of the CT character.2 However, the hfs in neat crystal system is washed out by the exciton motion. Thus one can obtain the hfs data only in mixed crystal systems. This is clearly brought out in our mixed crystal studies. Experimental Section All of the chemicals used in this experiment were purchased from the Aldrich Chemical Co. and purified extensively by the zone-refining method. The 1:l mixture of N-TCPA was further zone-refined before the final 0 1979 American Chemical Society
The Journal of Physical Chemlstty, Vol. 83, No. 26, 1979
3398
3500
3000 Or-%,
'
'
'
'
'
'
'
'
'
Yu and Lln
TABLE I: Zfs Parameters and Degree of Charge Transfer ( c z ) of the Phenanthrene-d,,-TCPA Complex at 77 K
'
zfs. cm-' param- P(d,,)-TCPA/ phenanthrene/ eter N-TCPA biphenyla X i0.0448(1) Y ~0.0117(1) 2 -~0.0334(1) D +0.0500(2) +0.10043(1) E ?0.0283(1) ~0.046576(9) D* 0.0700(3) = 0.12882(2)
D* 60.
0.0699(2)b
a Reference 5.
c2,
45
%
*
5
0.1284(3)b
Obtained from AM = 2 signal.
120
180-
S 2500
'
"
"
3000
'
'
0.5% Phen ( d l o ) in N a p h / T C P A NT absorption P T absorption
'
35b0'
'
'
%e
77'K
o emission
Flgure 1. Angular dependence of EPR signals of phenanthrene-d,,,TCPA/N-TCPA mixed crystals at 77 K. (Upper) The & crystallographic axis was the rotating axis. (Lower) The magnetic field was in the xz plane of the guest molecule.
crystallization from benzene solution. The concentration of guest complex was about 0.1 mol % in mixed crystals. The crystals were grown from benzene solutions by a slow evaporation. Experimental details were given in a previous pub1ication.l The EPR experiments were performed at 77 K.
Results The degree of CT character reported below is calculated from the measured zero-field splittings (zfs) by use of the following e q ~ a t i o n : ~ D(1) - D(expt) c2 = (1) D(1) - D(D+A-) where D(1) is the zfs value of the locally excited triplet state of the donor (or acceptor), whichever has a lower triplet energy, and D(D+A-) is the value for the ion pair (completely ionized) state. The ion pair term is often assumed to be negligibly small which can introduce a large error in the estimation of CT character. Furthermore any changes in the sign of zfs values due to the different nature of electron distribution and wave function could also foul up .the calculation. Alternately, one can use hfs to calculate the CT character as follows:2 Aii(l) - Aii(expt) c2 = (2) Aii(1) - Aii(DtA-) The Aii(D+A-)value can be safely assumed to be half the value of the Aii(l),i.e. Aii(DfA-) = Aii(1)/2 thus we have 2[Aii(l) - Aii(ex~t)l c2 = (3) Aii(1) The degree of the CT character calculated from eq 3 should reflect a better picture of the EDA system. We will use the known crystal structure of N(d8)-TCPA4 and the results of our EPR studies of N-TCPA crystals1 to analyze the data obtained in the mixed crystal studies. We will designate the out-of-plane axis as the z axis, the
O.l%Anth/TCPA in N a p h / T C P A NT absorption
o emission
A T absorptiin
11
77'K
emission
Flgure 2. Angular dependence of EPR signals of anthracene-TCPA/ N-TCPA mixed crystals at 77 K. (Upper) The &axis was the rotating axis. (Lower) The magnetic field was in the xz plane of the guest molecule.
in-plane long axis as the x axis, and the in-plane short axis as the y axis. Phenanthrene-dlo-TCPA. The orientational dependence of Am = f l resonance fields for phenanthrened,,-TCPA in the N-TCPA host crystal is given in Figure 1. We note the following: (1) Only one pair of signals (from either the guest or the host) appears in the spectra. (2) The out-of-plane principal axis of the fs tensor of the guest coincides with that of the host, but the in-plane axes of the guest shift an angle of 12 f 2' from those of the host (naphthalene). (3) The patterns of steady signals of the guest differ from those of the host, i.e., both low field and high field resonance lines are absorptive for the guest while the low field is emissive and the high field absorptive for the host. We have not observed any resolvable hsf. The calculated zfs parameters and the degree of CT character of the phenanthrene-TCPA complex is given in Table I. The CT character calculated from D* (= ( D 2 + 3E2)'/2)is as much as 45 f 5%. Anthracene-TCPA. The orientational dependence of EPR signals for anthracene-TCPA in N-TCPA crystals is given in Figure 2. A few special features different from phenanthrene-TCPA/N-TCPA are noted as follows: (1) The in-plane axes of anthracene shift an angle of 25 f 2' with respect to the corresponding axes of naphthalene. (2)
The Journal of Physical Chemistty, Vol. 83,No, 26, 1979 3399
EPR of EDA Triplet in Mixed Crystals A-TCPA/N-TCPA
H//X
7 7°K
Phenazine(0.4%) in
N - T C P A 77°K
,
H l l X Low F i e l d
Flgure 3. Hyperfine spectrum of anthracene-TCPA triplet state in N-TCPA crystals at 77 K.
5G
I
H//Z Low F i e l d
,10G,
.
o.s%Phenazine-TCPA i n N - T C P A
N-TCPA
77'K
Phenazine-TCPA
Flgure 4. Angular dependence of EPR signals of phenazine-TCPA/ N-TCPA mixed crystals at 77 K. The magnetic fiekl was in the xy plane.
TABLE 11: Zfs Parameters, Hyperfine Splittings, and Degree of Charge Transfer ( c ' ) of the Anthracene-TCPA Complex at 7 7 K zfs, cm-' parameter
X Y
z
D E
A-TCPAI N-TCPA
anthracene1 biphenyla
i0.0268(2) i 0.0123(2) T 0.0391(1) *0.0586(3) F 0.0073(2)
+0.03223(5) +0.01535(10) i.0.04771(5 ) +0.07156(5) T 0.00844(10)
Flgure 5. Hyperfine spectra of phenazine-TCPA triplet state in N-TCPA crystals at 77 K. The modulation amplitude were as follows: 1,6 G CHollx); 5.0 G (Hollz).
TABLE 111: Zfs Parameters, Hyperfine Splittings, and Degree of Charge Transfer (cz) of the Phenazine-TCPA Triplet State in N-TCPA Host Crystal at 7 7 K zfs, cm-I parameter
cz, %
X Y Z
D
18
E
phenazineTCPAI N-TCPA
phenazine / biphenyla
*0.0330(2) +0.0128(2) 70.0457(2) +0.0686(4) sO.OlOl(2)
+0.0358(1) +0.0135(5) ~0.0496(1) +0.0744(1) i.O.OllO(5)
3.5 i. 0.2 meso 8 . 5 + 0.2 Reference 6.
a
4.0 (Hllx) 8.7 (Hllx)
8
hf, G
hf. G a
cz, %
25
*
protons 4 . 0 i. 0.2 nitrogen 10.0i. 0.4 OL
10
The patterns of steady state signals for the guest is almost the same as those for the host in all orientations except in the neighborhood of Holly where the low-field signal is absorptive and the high-field emissive for the guest. (3) The hfs of anthracene-TCPA is observed for the low-field signal with Hollx (Figure 3). The hfs can be fitted with two hf constants of 3.5 and 8.5 G with an uncertainty of 0.2 G. The splittings with 3.5 G correspond to the protons a t a (1,4,5, and 8) positions, and those at 8.5 G to protons a t meso (9 and 10) positions. The zfs, hfs, and the degree of the CT character of anthracene-TCPA complex are summarized in Table 11. The degree of CT character calculated from zfs is 18% which is about the same as calculated from hfs within experimental uncertainty. We note that the configuration of anthracene differs from that of phenanthrene in the same host crystal. This variation in guest configuration prompts us to undertake a similar study on phenazineTCPA system. Phenazine-TCPA. The orientational dependence of EPR signals is given in Figure 4. We note there is a close resemblance of the guest configuration of phenazineTCPA and anthracene-TCPA in N-TCPA host crystals; the in-plane axes of phenazine rotate an angle of 26 f 2 O away from the corresponding axes of naphthalene. The patterns of steady state signals of phenazine-TCPA are
a
4.3 ( H i k ) 10.2 (Hllz)
1 4 i: 7
Reference 7.
the same as those of anthracene-TCPA. The hfs values with Ho parallel to x and z axes of phenazine-TCPA are given in Figure 5. These measurements were made with mixed crystals of 3 mm X 2.5 mm X 3 mm in size obtained from a Bridgman ingot. The zfs, hfs, and the degree of CT character are given in Table 111. The degree of CT character for phenazine-TCPA is only 8% which is a factor of 2 smaller than that for the anthracene-TCPA system.
Discussion Here we would like to discuss two important aspects of our mixed complex crystal studies: (A) energy transfer and spin polarization, and (B) guest configuration and stacking pattern. ( A )Energy Transfer and Spin Polarization. The energy transfer from the host matrix to a guest site in most organic systems is very efficient, especially if the energy level of the guest is lower than that of the host matrix. Here guest molecules lead to a scavenging process in which almost all the excitation to host molecules finds its way into guest molecules acting as traps. One often observes an emission from guest molecules even though the radiation is absorbed by host molecule^.^ One would further observe transfer of spin polarization (conservation of spin angular momentum) from host molecules to guests in the triplet energy transfer process.s Thus one should observe EPR
3400
The Journal of Physical Chemistry, Vol. 83, No. 26, 7979
triplet signal from the guest molecule chracterized with the host spin polarization if the energy transfer from the host to the guest is effective. In the above mixed complex crystals studies, we observed that (1) EPR signals arise from both the guest and the host and (2) the spin polarization for the guest differs from that for the host. Furthermore, we observed signals only from the guest if a N-TCPA solution was used as a filter to remove the host excitation. Therefore, it seems that triplet energy transfer is not an effective process in these systems. The triplet excitation of the guest could take place via a direct singlet-triplet absorption due to the presence of heavy atoms in the acceptor (TCPA)g or an excitation to the CT singlet state and then intersystem crossing over to the triplet state. (B)Guest Configuration and Stacking Pattern. We will assume that the observed magnetic principal axes (corresponding to the resonance extrema) coincide with the symmetry axes of the donor in the following discussion. This should be a valid assumption except for a complex with a high degree of CT character.’O The assumption is substantiated by the fact that the hfs was observed only when the external field was parallel to the dipolar principal axes of the guest donor (see Figures 3 and 5 ) . From the orientational studies (see Figures 1 , 2 , and 4), we note that the principal axes of the guest donor (phenanthrene and anthracene) do not coincide with those of the host donor (naphthalene). The degree of distortion of the guest complex configuration in N-TCPA host crystals depends on the kind of the guest donor present in the system. When the size of the guest molecule is larger than that of the host molecule, one would expect the guest molecule to experience some steric hindrance. However, if one places anthracene or phenanthrene molecules on the top of TCPA in the host lattice, one notes there is plenty of room for anthracene or phenanthrene to undergo substitutional crystallization, i.e., the axes of the guest donor parallel to those of the host donor. Thus the observed shift in the in-plane axes of the guest donor with respect to those of naphthalene may be associated with the degree and/or the nature of CT interactions. We may rule out the degree of CT interaction as the cause of guest distortion based on the following observatons: (1)the shift of the donor long axis in anthracene is twice greater than that in phenanthrene, yet the degree of CT in anthracene complexes is almost three times smaller than that in phenanthrene, and (2) the distortion is the same for anthracene and phenazine, even though they have different degrees of CT character. Thus we may conclude that the degree of guest distortion does not depend on the degree of CT character. The degree of guest distortion may therefore most likely arise from the specificity of CT interaction. The specificity of CT interaction can be easily recognized if one examines the nature of stacking pattern of donors and acceptors in EDA solids. In surveying overlap diagrams (diagrams of stacking pattern) of many T-T* EDA complex crystals,” we found that the “overlapping” of donors and acceptors occur mostly at aromatic sextet sites. An aromatic sextet is represented by a circle inside a hexagon to indicate there are six r electrons in the hexagon.12 Thus there is only one aromatic sextet in all of linear polyacenes in their ground state (naphthalene, anthracene, ...) and two or more in cyclic polyacenes (two in phenanthrene, three in triphenylene,...). The aromatic sextet for TCPA is in the benzenoid ring. If one places one of the side rings of phenanthrene or anthracene (location of the aromatic sextet) on the top of the benzenoid ring of TCPA,
Yu and Lln
I
I c sin d
Flgure 6. The proposed guest phenanthrene-TCPA complex conflguration in the N-TCPA host lattice.
c sin d Flgure 7. The proposed guest anthracene-TCPA complex configuration in the N-TCPA host lattice.
the guest donor would experience a steric hindrance if the long axis of the guest donor is to keep aligned with the long axis of naphthalene. To alleviate this steric effect and to gain the maximum “overlapping” with TCPA, the guest donor would have to position itself in an optimum configuration, namely, a shift in the long axis of the guest donor with respect to that of naphthalene. We may conclude that the guest distortion is a result of balancing the minimum steric hindrance and the maximum donor-acceptor overlapping. The proposed configurations for phenanthrene-TCPA in N-TCPA and anthracene-TCPA in N-TCPA host crystals are given in Figures 6 and 7, respectively. In the survey of overlap diagrams, we further note that the donor and acceptor often stack in two different modes: (a) linear and (b) zigzag (Figure 8). The linear stacking usually occurs in one-centered complex systems in which the donor contains only one aromatic sextet, e.g., in naphthalene. In the phenanthrene case, the two aromatic sextets may be shared by two nearest acceptors along the
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979 3401
EPR of EDA Triplet in Mixed Crystals
(A)
Figure 8. Charge transfer stacking modes in weak EDA complex crystals: (A) linear, (B) zigzag. Flgure 9. Guest substitution modes in weak EDA complex host crystals.
TABLE I V : D* Values a n d Degree of C T of 1:l EDA Complexes in Toulene R i g i d Glass a t 77 K
donor
db
D*(donor), acceDtor em-! TCPA
0.1284(3)
TCNB
0.1285(4) ~,
PMDA
0.1286(5)
D*(CT), cm-' 0.0699(2) 0.0553(4) 0.0647(4) o.o367(4j 0.0738(5) 0.0137(3)
re1
int S
W W
s
W S
cz, %
45 57 50 71 43 89
same stack and form -.A3(DA)- (exciplex trimer) in a zigzag mode, The possible zigzag configuration of phenanthrene-TCPA complexes could give rise to a greater CT interaction and thereby greater CT character. This is indeed what we have observed in the 1:l phenanthreneTCPA (P-TCPA) crystals where the CT character is greater than that observed in the N-TCPA host ~rysta1.l~ The notion of the greater CT character for two-centered overlapping in phenanthrene-TCPA complexes leads us to perform rigid glass studies. We have observed two Am = 2 CT signals for the complex in toluene rigid glasses at 77 K: 1576 G (strong) and 1598 G (weak). Similar behaviors have also been observed when TCNB (tetracyanobenzene) and PMDA (pyromellitic acid dianhydride) were used as acceptors. The values of D* (calculated from Am = 2 transitions) are given in Table IV. The degree of CT character calculated from the strong signal a t 1576 G is the same as that obtained in the mixed crystal studies (see Table I). Thus the weak signal which gives higher CT character could very well be attributed to the trimer formation .-A3(DA).- as proposed earlier. To substantiate our argument further, we have examined the CT character of phenanthrene-TCNB complexes. It has been reported that these P-TCNB complexes have different CT character in the following host complex crystals: 59% in N-TCNB, 72% in biphenyl-TCNB (BTCNB), and 70% in durene-TCNB (D-TCNB).14 Our rigid glass studies also showed two types of complexation, one with 50% CT character, and the other one with 71 % . The different CT character observed in mixed crystal studies could be explained in terms of the mode of stacking pattern as mentioned earlier. The stacking pattern of P-TCNB in N-TCNB16 should be the same as that of P-TCPA in N-TCPA, a linear mode (see Figure 9A). On
the other hand, the single crystal of 1:l B-TCNB exhibits a zigzag stacking.16 We note the biphenyl also possesses two aromatic sextets which would allow phenanthrene to fit into the host lattice with negligible distortion (see Figure 9B). The zigzag stacking allows the two sextets of phenanthrene to be shared by two nearest TCNB and thereby enhances the CT interaction and gives greater CT character. Finally, the stacking of D-TCNB may be similar to that of hexamethylbenzene-TCNB in a tilt linear mode.14 Phenanthrene molecules may fit into the host lattice with some distortion to gain proper overlapping which gives rise to a higher CT character (Figure 9C). Of course, we could not rule out the energy denominator arguments of Mohwald and Sackmann14 as the possible source of perturbation of the CT character, Detailed crystal structure determinations of these systems could probably help establish the nature of stacking patterns and CT interactions.
Acknowledgment. We are indebted to Professor Sam Weissman for his generosity to let us use his laboratory facilities, and his continuing inspiration and numerous technical assistances. This work was supported by the National Science Foundation.
References and Notes C.-T. Yu and T . 4 . Lin, Chem. Phys., 39, 293 (1979). N. S. Dalal, D. Haarer, J. Bargon, and H. Mohwaid, Chem. Phys. Lett., 40, 326 (1976). S.Nagakura in "Excited States", Vol. 2, E. C. Lim, Ed., Academic Press, New York, 1975, pp 322-378. D. P. Cralg and S. H. Walmsley, "Excitons in Molecular Crystals", W. A. Benjamin, New York, 1968, Chapter 6. R. W. Brandon, R. E. Gerkin, and C. A. Hutchison, Jr., J . Cbm. phys., 41, 3717 (1964). J. Ph. Grlvet, Chem. Phys. Letf., 4, 104 (1969). J. Ph. Grivet and J. M. Lhoste, Chem. Phys. Lett., 3, 445 (1969). R. H. Clarke, Chem. Phys. Lett., 6, 413 (1970). C.-T. Yu and T.-S. Lin, Chem. Phys. Lett., 60, 122 (1976). D. Haarer, private communication. F. H. Herbstein in "Perspectives in Structural Chemistry", Vol. IV, J. D. Dunk and J. A. Ibers, Ed., Wiley, New York, 1971, pp 166-396. E. Cbr, "The Aromatic Sextet", Wiley, London, 1972, and references therein. C.-T. Yu and T . 4 . Lln, unpublished results. H. Mohwald and E. Sackmann, Z.Naturforsch. A, 20, 1216 (1974). S. Kumakura, F. Iwasaki, and Y. Salto, M/.Chem. Soc. Jpn., 40, 1826 (1967). H.Mohwald and E. Sackmann, Chem. Phys. Lett., 21, 43 (1973).
