A MAGNETOPHOTOSELECTION STUDYOF POLARIZATIONS OF ABSORPTION BANDS
2205
A Magnetophotoselection Study of the Polarizations of the Absorption Bands of
Some Structurally Related Hydrocarbons and Heterocyclic Molecules1
by Seymour Siegel and Henry S. Judeikis Aerospace Corporation, El Segundo, California
(Received December 21, 1966)
The polarizations of the long wavelength electronic absorption bands of the molecules phenanthrene, biphenyl, fluorene, carbazole, dibenzofuran, and dibenzothiophen have been determined by the method of magnetophotoselection. Also, the polarizations of the next higher absorption bands have been estimated for phenanthrene, carbazole, and dibenzothiophen. I n general, the results agree with theory. The bands belonging to the heterocyclic molecules have been assigned to ‘La and lLb designations on the basis of the polarization results. The results are discussed briefly.
I. Introduction The electronic absorption and emission spectra of many molecules containing one or more benzene rings fused to a five-membered heterocyclic system have been rep~rted.Z-~ However, the interpretation and correlation of the heterocyclic spectra have been quite difficult. In general, only qualitative trends have been obtained, and assignments of the spectra to X , T * transitions have been made by analogy with the spectra of structurally similar aromatic hydrocarbon molecules. In this paper, the method of magnetophot~selection~-~ is used to determine the polarization of the long wavelength electronic absorption bands of several structurally related aromatic hydrocarbons and heterocyclic molecules. The molecules studied are given in Figure 1 along with the axis system used in the analysis. For some of the molecules, estimates are also given of the polarizations of the next highest transitions. The molecules studied are the same ones for which the zero-field splitting parameters of the triplet states were determined in the preceding paper.8 The advantage of using the magnetophotoselection method is that the Am = i l electron paramagnetic resonance (epr) spectrum of triplet-state molecules dissolved in a rigid glass gives overwhelming prominence to the molecules oriented so that one of the principal axes of the electron spin-spin dipolar coupling tensor is parallel to the applied magnetic field.s*10 For the experiments in this study, the triplet states are populated by the intramolecular sequence
So h’, S*
T (1) If polarized light is used to excite the populating So + S* transition, the relative intensities of the epr transitions will be directly related to the fraction of the polarization of the optical transition carried along each molecular axis if certain conditions are satisfied.’~’l The necessary conditions which concern intra- and intermolecular processes, molecular symmetry, and molecular motions are given in a previous paper: and these conditions were met to a satisfactory approximation in the experiments being described here. When the polarization of the electric vector E of the exciting light is parallel to the direction of the external magnetic (1) This work was supported by the U. S. Air Force under Contract NO. AF 04(695)-669. (2) R. N. Nurmukhametov and G. V. Gobov, Opt. i Spectroskopia, 18, 227 (1965). (3) G. M.Badger and B. J. Christie, J. Chem. Soc., 3438 (1956). (4) H.H.Jaff6 and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley and Sons, Inc., New York, N. Y., 1962,pp 347-361. (5) M.A. El-Sayed and S. Siegel, J. Chem. Phys., 44, 1416 (1966). (6) S. Siegel and L. Goldstein, ibid., 43, 4185 (1965). (7) S. Siegel and L. Goldstein, “A Study of Triplet-Triplet Transfer by the Method of Magnetophotoselection: 11. Concentration Depolarization,” Aerospace Corp. Report No. TDR-669(6250-20)-2, Dec 1965; also J. Chem. Phys., in press. (8) S. Siegel and H. S. Judeikis, J . Phys. Chem., 70, 2201 (1966). (9) E.Wasserman, L. Snyder, and W. A. Yager, J. Chem. Phys., 41, 1763 (1964). (10) P.Kottis and R. Lefebvre, {bid., 41, 379 (1964). (11) P.Kottis and R. Lefebvre, ibid., 41, 3660 (1964).
Volume 70, Number 7 J u l y 1966
2206
SEYMOUR SIEGELAND HENRYS. JUDEIKIS
M
R
MOLECULE
= CH-HH - CHZ -
-CH
PHENANTHRENE BIPHENYL FLUORENE
-NH-
CARBAZOLE
-0-
DIBENZOFURAN
-sD l BE NZOTHIOPHEN Figure 1. Molecules studied and principal axes system used in analysis.
field H,the intensity I,of the Am = i l epr transition corresponding to any one of the three canonical orientations is proportional to the number of excited triplet state molecules N:([ which have their ith molecular axis at the correct observational angle O f H 5 6. Because the ith axis must be essentially parallels to H for its corresponding epr transition to be observed in the usual derivative epr spectrum, the magnitude of 6 must be very small (Le., OfH = 0). When the assumption that the probability of observation is unity for OtH _< 6 and zero for all other values of 6 is used, expressions are derived in the Appendix for N;(lI) and N , ' ( 1 ) (where the latter term is the number of excited'molecules with OtH 2 6 and H IE ) . If the magnitudes of the incident light intensity are equal in the two polarization directions, then the use of eq A5 and A6 yields an expression for the usual polarization ratio P,as follows
I)
where M , N , and L are the molecular axes defined in Figure 1, r1 is the component of the polarization of the optical populating transition carried along the ith molecular axis, and k is a constant introduced to include the percentage of depolarized (Le., isotropic) light present or other depolarizing effects. The data will be analyzed in terms of eq 2. 11. Experimental Section The epr spectrometer and optical excitation configuration used in this study have been described elsewhere.60'~ The sample container consisted of a 4-mm 0.d. quartz tube that had been flattened at one end to an The Journal of Physical Chemistry
approximate rectangular shape. This crudely fashioned cell was found to be adequate for this study. The tube was placed in a quartz liquid nitrogen dewar with an unsilvered tip directly in the epr cavity. All measurements were made at 77°K. Solutions were prepared from Baker Analyzed reagent grade diethyl ether, and the solutes were either Eastman White Label or Chemical Procurement Laboratories research quality chemicals. The concentrations used were all 0.003 M (roomtemperature concentrations). This concentration was sufficiently high, because of the long triplet state lifetimes involved,* to permit accurate measurements of the Am = *l epr Iines; it was also high enough to avoid uncertainties arising from photochemical effects related to sensitized solvent decomposition.l 3 Yet, the concentration used was sufficiently low to avoid complications from intermolecular triplet-triplet energy transer.' The transmission characteristics of the filter combinations used between the exciting lamp (PEK-500 highpressure mercury arc) and the sample were determined with the use of a Cary 15 spectrophotometer and are given in Figure 2. The absorption spectra of the solutes in ether solution at room temperature were determined on the spectrophotometer and are given by the solid curves in Figure 3. Also given in Figure 2 is the relative spectral output (manufacturer's specifications) of the PEK-500 lamp. I t was found that the broad output of the lamp in the 230-250-my region was of minor importance in populating the triplet states under the experimental conditions used in this study. This conclusion is based on a series of measurements on the triplet state epr intensity of a solution of dibenzofuran, using several sharp-cut filters (some of which are given in Figure 2). The results of these measurements indicate an appreciable dependence of the triplet state population (monitored by means of the Am = zk2 epr transitions) on the cutoff wavelength of the filter in the wavelength region X >260 mp, but a minor or negligible dependence in the region X
