TRIPLET STATEZERO-FIELD SPLITTINGS OF HETEROCYCLIC MOLECULES
advantage of using a model for the addition reaction based upon the transition state appears to be lost in the process of calculation. We conclude that the model based upon localization energy seems in its simplicity to offer the greatest promise for the calculation of an index of reactivity of a molecule toward the addition of free radicals.
2201
Acknowledgments. We wish to thank Dr. P. N. Daykin of the British Columbia Research Council and Miss M. R. O'Donnell and Mr. D. Lee of the Mond Division of Imperial Chemical Industries Limited for performing part of the computation, and the National Research Council of Canada for financial assistance and a studentship to A. C. R. B.
Triplet State Zero-Field Splittings of Some Structurally Related Aromatic Hydrocarbon and Heterocyclic Molecules1
by Seymour Siege1 and Henry S. Judeikis Aeroepace Corporation, El Segundo, California (Received December 21, 1966)
The triplet state zero-field splitting parameters D and E have been determined from the Am = f l epr spectra of the photoexcited triplet states of several structurally related aromatic hydrocarbons and aromatic heterocyclic molecules, using diethyl ether glasses at 77OK. The values of the splitting parameters in cm-l for the various molecules are as follows: biphenyl, D = 0.1092, E = 0.0036; fluorene, D = 0.1075, E = 0.0033; carbazole, D = 0.1022, E = 0.0066; dibenzofuran, D = 0.1071, E = 0.0092; dibenzothiophen, D = 0.1130, E = 0.0021. The uncertainties in these numbers are f0.2 mK for D and 50.1 mK for E. Lifetimes of the triplet states were also determined. A discussion of the results in terms of conjugation of the heteroatom with the aromatic rings is given. Also, the results are compared to the published values of D and E for phenanthrene. It is found that biphenyl is a better hydrocarbon model than phenanthrene for the heterocyclics investigated in this paper.
I. Introduction The utility of electron paramagnetic resonance (epr) spectroscopy for studying triplet state molecules was considerably increased by the observation that the epr spectra of randomly oriented triplet state molecules gave special significance to those molecules where the principal axes of the electron dipolar spin-spin interaction tensor were parallel to the direction of the applied magnetic field.2 It was no longer necessary to have single crystal hosts to determine the separation of the magnetic sublevels of the triplet state experimentally. The data obtained when using noncrystal-
line rigid media are not as accurate as that obtained when using appropriate single crystals; however, the number of suitable host single crystals available is small, and the accuracy of the glass or plastic technique is sufficient for many purposes. The major drawback of using noncrystalline host media is that it is not usually possible to determine the hyperfine interaction energies between the triplet state electrons and the (1) This work was supported by the U. S. Air Force under Contract No. AF 04(695)-669. (2) W. A. Yager, E. Wasserman, and R. M. R. Cramer, J. Chem. Phys., 37, 1148 (1962).
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SEYMOUR SIEGELAND HENRYS. JUDEIKIS
2202
various nuclear magnetic moments present in the molemolecule cule. The effective in a magnetic spin Hamiltonian field may be written for a triplet as state X =
+
+
PR*g*S DSS2 E(Sz2- Sv2)-
(1)
where the constants D and E can be obtained directly from the Am = =k1 epr spectra and x, y, and z are a principal axes coordinate system. The constants are the zero-field splitting parameters and are related to the spatial distribution of the two triplet electrons over the molecule. For two electrons, when we consider mutual dipolar interactions and ignore spinorbit interactions, the zero-field parameters can be expressed as
R
MOLECULE
8’
.
;
i
‘\
- HH - BIPHENYL -CH2FLUORENE - NH - CARBAZOLE - 0 - DIEENZOFURAN - S - DIBENZOTHIOPHEN
Figure 1. Molecules studied and principal axes system used in analysis.
are given elsewhere.s The lifetimes of the triplet state molecules were measured by observing the dark decay of the phosphorescent optical emission and the Am = h 2 epr transition intensities. All measurements were made a t 77”K, using M solutions of the aromatic molecules dissolved in Baker Analyzed reagent D = 3/49e~2(~(1,2)!r122 -3 4 ~ ) ) grade diethyl ether; the solutes were either Eastman (2) White Label or Chemical Procurement Laboratories E = 3/4geP2(~(1,2)(~i22 - ~12~/(~(1,2)) research quality chemicals. hfagnetic field measurewhere ge is the free electron “g” value and the quantities ments were made with a Numar hlodel h4-2 nmr prein brackets are averages of the interelectronic distances, cision gaussmeter, an H P Model 524C electronic and corresponding vector components in the principal counter, and an H P h4odel 525A frequency converter axes system, taken over the antisymmetric spatial plug-in unit. Approximate measurement of the epr portion of the two-electron wave f u n ~ t i o n . ~The klystron frequency was obtained with an H P Model particular labeling of the molecular axes is somewhat X532B frequency meter; an H P Alodel 540B transfer arbitrary. However, for molecules of sufficient symoscillator and the electronic counter with an H P metry, a natural choice usually presents itself. For Model 525B plug-in unit were used for accurate klyplanar molecules, z is usually chosen parallel to the stron frequency determinations. normal to the molecular plane. The values of the zero-field splitting constants D This paper presents the results of the experimental and E , as well as the values of gzz, guy, gPZwhere gct determination of the values of zero-field parameters is the component of the g tensor in eq 1 along the ith for the photoexcited, lowest triplet states of the strucprincipal axis, were determined using the equations turally related molecules shown in Figure 1. The developed by Wasserman, Snyder, and Yager.’ Departicular assignment of a molecular axes system used termination of gzz, guy, and gzz gave values in the in the analysis is also given in Figure 1. Assignment range 2.0023 to 2.0035 with an experimental uncerof the epr spectral transitions to the correct molecular tainty of =k0.0015 in each g value. Therefore, within axes was made with the aid of the optical polarization the experimental uncertainty, the g tensor is isotropic, effects discussed in the following paper. Similarities and there was no evidence of contributions from spinbetween the ultraviolet absorption spectra of the heteroorbital interactions to the g values for these molecules. cyclics in Figure 1 and that of phenanthrene (R = The experimentally determined values of D and E are -HC=CH-) has been discussed in several places.*~S given in Table I ; the experimental precision indicated The equivalence of the heteroatom to a -HC=CHin the table arises mainly from the large line widths group instead of CH2 group has been emphasized.5 of the epr lines which preclude the determination of The results of the D and E determinations are discussed in terms of this postulated equivalence; it will be shown (3) J. H. van der Waals and M. S. de Groot, ~ o l Phys., . 2, 333 that, for triplet states, biphenyl is a better hydrocarbon (1959); 3, 190 (1960). than phenathrene for the (4) G. M.Badger and B. J. Christie, J . Chem. Soc., 3438 (1956).
11. Experimental Details and Results A Varian X-band e m sDectrometer was used to observed the Am = i=1 and =!=2transitions of the photoOf the excited state spectrometer and optical excitation configuration L
A
The Journal of Physical Chemistry
(5) H.H.Jaff6 and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley and Sons, Inc., New York, N. Y., 1962,PP 353-361. (6) (a) S. Siegel and H. S. Judeikis, J . Chem. Phys., 41, 648 (1964): (b) S. Siegel and K. B. Eisenthal, ibid., 42, 2494 (1965). (7) E. Wasserman, L. C. Snyder, and W. A. Yager, ibid., 41, 1763 (1964).
TRIPLET STATEZERO-FIELD SPLITTINGS OF HETEROCYCLIC MOLECULES
specific line shape positions to better than h0.5 gauss. Also included in Table I are the results of the lifetime measurements. Both the phosphorescent emission and epr (Am = *2 transitions) dark decay curves were found to be exponential, and the lifetimes derived from both techniques were equivalent (within experimental error). Most of the lifetimes have been reported elsewhere8v9and are in good agreement with the present results. Table I : Zero-Field Splitting Parameters and Lifetimes
Molecule
Phenanthrene Biphenyl Fluorene Carbazole Dibenzofuran Dibenzothiophen
D, cm-1
( = t 0 . 2mK)”
E, cm-1 ( = t O . l mK)a
I, sec (f0.2 sec)b
0.10044 0.1092 0.1075 0.1022 0.1071 0.1130
0.04657 0.0036 0.0033 0.0066 0.0092 0.0021
3.7 4.4 6.3 7.7 5.9 1.3
a The phenanthrene data were taken from the literature (an certainty hO.02 mK is given) and were obtained using a biphenyl single crystal as a host. (See R. W. Brandon, R. E. Gerkin, and C. A. Hutchison, Jr., J. Chem. Phys., 37, 447 (1962); 41, 3717 (1964).) With the axis labeling in Figure 1, D and E for phenanthrene are both positive since D is positive and E negative if z and y are interchanged. (See Brandon, et al.) The other values of D and E are absolute values because the sign of the parameters cannot be obtained from glass data at 77°K; however, there is no reason to assume any change from phenanthrene; therefore, D and E are most likely both positive in this axis system for all the molecules studied here. The phosphorescent emission lifetimes are given; however, the lifetimes derived from the epr data are identical, within experimental error.
From the Am = h 2 transitions, only a combination of the zero-field parameters* can be determined directly for these molecules, Le., D* = ( D 2 3EZ)”’. The values of D*, determined directly from the Am = 1 2 transitions, are given in Table 11; values of D*, determined from the values of D and E in Table I, are also given in Table 11. There seems to be a systematic discrepancy in the results obtained by the two methods, with the values obtained from the Am = *2 transitions being uniformly larger by 1.0 to 1.5 mK. This discrepancy could arise, in part, from experimental measurement error; however, the uniformity of the trend indicates that the specific choice of the maximum of the derivative line shape of the Am = k 2 transit,ion or that another more fundamental cause may also be involved.
+
111. Discussion The assignment of the ultraviolet absorption bands
2203
of the heterocyclic molecules discussed here to T,T* transitions have been made in various place~*~~O (including ref 11) by analogy to the spectra of the aromatic hydrocarbons. Similar arguments have been used to assign the phosphorescent triplet-singlet emission for the heterocyclics to T ,T* transitions.10 The data in Table I agree with the T , T * assignments for the triplet state of these molecules, considering the general similarity of the values of D and E for the hydrocarbons and heterocyclics. The magnetic z direction is shown to be perpendicular to the molecular plane, as is expected for T , T * triplet states. The long lifetimes shown in Table I are also consistent with T,T* triplet state assignments. The most striking aspect of the values of D and E is the large differences between the symmetry of the electron spin distribution in phenanthrene and the symmetry in the heterocyclic aromatics, as reflected by the large differences in the E values. Alternatively, the electronic distributions in the heterocyclic aromatics are very similar to those in biphenyl and fluorene. From the conclusion that the triplet states of the heterocyclics are very similar to the triplet state of biphenyl and quite different from that of phenanthrene, as far as the electronic spatial distribution is concerned, it can be further concluded that the heteroatoms do not enter into any appreciable degree of conjugation with the aromatic rings. In fact, the small value of E observed for dibenzothiophen indicates that sulfur is least effective in introducing added conjugation. The latter conclusion is opposite to that which one would predict on the basis of the relative electronegativities of the heteroatoms. Sulfur has essentially the same electronegativity as carbon and has d orbitals available for partial double bonding; therefore, sulfur might be expected to have the most conjugation. However, sulfur is a large atom and the resultant strain on the molecule may change the geometry sufficiently so that conjugation is minimized. Also, sulfur does not form double bonds easily. The trend in the values of D given in Table I also indicates that there is the least conjugation in the thiophen because it has the largest value of D. The large value of D reflects a greater concentration of electron density in the two rings, thereby decreasing the average value of r12 and increasing the value of the dipolar interaction. Therefore, the geometry of the ~
~~
(8) D. S. McClure, J. Chem. Phys., 17, 905 (1949). (9) A. N. Terenin and V. L. Ermolaev, Izv. Akad. Nauk S S S R Ser.
Fiz., 26, 21 (1962). (10) R. N. Nurmukhametov and G. V. Gobov, Opt. i Spectroskopia, 18, 227 (1965). (11) S. Siege1 and H. S. Judeikis, J. Phys. Chem., 70, 2205 (1966).
Volume 70, Number 7
July 1966
SEYMOURSIEGELAND HENRYS. JUDEIKIS
2204
Table 11: Magnitude of D*
=
(Da
+ 3Ea)'Ia DI*, om-'"
J
Molecule
This work (f0.2 mK)
Phenanthrene Biphenyl Fluorene Carbazole Dibenzofuran Dibenzothiophen
0.1094 0.1076 0.1028 0.1082 0.1131
...
BrandonC (+.04 mK)
This work ( f 0 . 1 mK)
0.12882
...
... ... ...
0.1107 0.1092 0,1043 0.1092 0.1144
...
...
Da*, om-lb Smauerd ( 1 1 . 5 mK)
Thomaone (10.4mK)
0.1335 0.1130 0.1096 0.1044
0.1336 0.1111 0,1088 0.1063
...
... ...
...
Da* - DI*, mK This work ( 1 0 . 3 mK)
... 1.3 1.6 1.5 1.0 1.3
-
a D1*calculated using D and E values in Table I. D2* calculated from Da* = ( l / h ~ ) [ ( ~ / a ) ( h v )3(gpH,)z]'/2 ~ where Y is the klystron frequency and H, is the field position of the low-field maximum of the Am = f 2 epr transition derivative spectrum (see ref 3). See Brandon, et al., ref a in Table I. Methanol or EPA a t 77'K used as the host medium. See B. Smaller, J. Chem. Phys., 37, 1578 (1962). Poly(methy1 methacrylate) used as the host medium. See C. Thomson, ibid., 41, 1 (1964).
triplet state of the dibenzothiophen must be such that the two rings are somewhat nonplanar or the C-C bond between rings is somewhat bent compared to that in biphenyl and the other heterocyclics. Ordering of the molecules in Figure 1 according to the increasing degree of conjugation, as reflected by the magnitude of D,can be given as dibenzothiophen, biphenyl, fluorene, dibeneofuran, carbazole, and phenanthrene. It must be emphasized that this discussion is based on a simple model of a pure T,T*electronic distribution. More subtle effects such as configuration interactions are, by necessity, completely neglected. Also, more
The Journal of Physical Chemistry
obvious effects such as geometry and symmetry changes in the triplet states of these molecules as the nature of the R group changes have not been considered since these geometries are not available. However, within the limitations of the simple model, the data presented here indicate that biphenyl is a better hydrocarbon model molecule than phenanthrene for the discussion of the absorption spectra of the heterocyclic aromatics. This conclusion is in agreement with that made by Nurmukhametov and Gobov on the basis of their recent detailed studies of the fluorescence and phosphorescence spectral0of these molecules.
