PMRSTUDY OF TRIPLET STATESOF SOMEAROMATIC NITROGEN HETEROCYCLES
3215
Paramagnetic Resonance Study of the Triplet States of Various Aromatic Nitrogen Heterocycles, Biphenyl, and Acenaphthene by Y asuhiko Gondo and August H. Maki Department of Chemistry, University of California, Riverside, California 96606
(Received March 14, 1068)
Electron paramagnetic resonance spectra have been observed on the phosphorescent 8 ( ~ 1 ~ *states ) of 5,6:7,8dibenzoquinoxaline, 1,2:314-dibenzophenazine,l,lO-phenanthroline, 5,6-benzoquinoline1 7,%benzoquinoline, biphenyl] 2,2’-bipyridyl14,4’-bipyridyl1 and acenaphthene. Magnetophotoselection measurements have also been made on some of these compounds in order to aid in the assignments of the principal axes of the zerofield splitting tensors. Among the determined zero-field splitting parameters] the ID1 and IEl values of 0.1068 and 0.000 cm-l, respectively, of the dibenzoquinoxaline are interesting ((Dl= 0.134 cm-l for triphenylene). Polarization directions of the electronic absorption bands are discussed. Lifetimes of the triplet states and their temperature dependences were observed.
Introduction Paramagnetic resonance has proven a powerful method for the investigation of phosphorescent triplet states of organic molecules since the initial observation by Hutchison and Mangum’ of electron paramagnetic resonance signals from triplet naphthalene in a durene single crystall. I n addition, it has been shown that observation of triplet-state epr is possible even in gla~ses,2-~ and this has led to a variety of applications for these systems. Among them, the technique of magnetophoselection5-7 has proven especially useful in relating the principal axis system of the zero-field splitting tenelor to the optical-polarization axes of a molec~le.~-~ I n this palper, the phosphorescent triplet states of various aromatic nitrogen heterocyclic molecules, biphenyl] and acenaphthene have been investigated by epr spectroscopy in rigid glasses of ethanol and of diethyl ether. Magnetophotoselection studies have been carried out on many of these molecules whose triplet states produce sufficiently intense Am = 1 spectra. Phosphorescence lifetimes have been measured. Replacement of an aromatic CH group by a nitrogen atom in a molecule frequently results in an enhancement of the phosphorescence intensity.1° Although the phosphorescence intensity is not necessarily related to the ease of observing the Am = 1 transitions, most of the nitrogen heterocyclic compounds investigated in this paper have produced remarkably intense spectra, thuci facilitating the magnetophotoselection studies. The compounds which we have investigated are given in Figure 1. Experimental Section Epr spectra were measured using a Varian X-band epr spectrometer (V-4502) having a modulation frequency of 100 kHz. Spectra were recorded with the radiofrequency magnetic field perpendicular to the
static field. The static magnetic field was calibrated by means of a proton resonance gauss meter, the frequency of which was monitored by a Hewlett-Packard 5245L electronic counter. The microwave frequency was measured using a Hewlett-Packard 25908 transfer oscillator, 5253B frequency converter] and the same electronic counter. The sample solutions were sealed in quartz tubes of 4 mm 0.d. after degassing on a vacuum line. The sample was irradiated in a slotted-face Varian V-4531 cavity. Light from a 200-W (PEK or Osram) superpressure mercury arc lamp was focused through a quartz optical system. The sample was cooled by a flow of cold nitrogen gas, which was passed through a heat-exchange coil submerged in liquid nitrogen. The technique of magnetophotoselection was used to aid in the assignment of Am = 1 epr transitions and electronic absorption spectra.’Jl The polarized light was obtained using either a polarizing Polaroid sheet or a glan prism located on a quasi-cylindrical part of the light beam. Either a water sample having 3-cm path or an aqueous solution of nickel sulfate having a 2-cm path was used as an infrared filter. A monochromator was not used. Both flattened and cylindrical quartz (1) C. A. Hutchison, Jr., and B. W. Mangum, J. Chem. Phys., 29, 952 (1958); 34, 908 (1961). (2) J. H. van der Waals and M. S. de Groot, Mol. Phys., 2 , 333 (1959). (3) M. S. de Groot and J, H. van der Waals, ibid., 3, 190 (1960). (4) W. A. Yager, E. Wasserman, and R. M. R. Cramer, J. Chem. Phys., 37, 1148 (1962). (5) Ph. Kottis and R. Lefebvre, ibid., 41, 3660 (1964). (6) J. M.Lohste, A. Haug, and M. Ptak, ibid., 44, 648,654 (1966). (7) M. A. El-Sayed and 8. Siegel, ibid., 44, 1416 (1966). (8) S. Siegel and L. Goldstein, ibid., 43, 3354 (1965). (9) G. P. Rabold and L. H. Piette, Photochem. Photobiol., 5 , 733 (1966). (10) M.A. El-Bayed, J . Chem. Phys., 38, 2834 (1963). (11) 8. Siegel imd H. S. Judeikis, J. Phys. Chem., 70, 2206 (1966). Volume 7.2, Number 9
September 1988
3216
YASUHIKO GONDO AND AUGUST H. MAKI three times. The final purification step was vacuum sublimation. The same procedure was applied for 7,8-benzoquinoline. Biphenyl and 2,2'-bipyridyl were recrystallized three times from ethanol. 4,4'-Bipyridyl dihydrochloride was treated with sodium hydroxide in water, and the 4,4'-bipyridyl precipitated was recrystallized twice from hot water. The product 4,4'bipyridyl dihydrate was dried over phosphorus pentoxide under vacuum. Acenaphthene was recrystallized three times from ethanol. The following solvents were used without further purification: ethanol (U. S. Industries Chemical Co., reagent grade) and diethyl ether (Mallinckrodt, analytical reagent).
Figure 1. The compounds studied in this work, with coordinate axes: (a) 5,6: 7,8-dibenzoquinoxaline, (b) 1,2: 3,4-dibenzophenazine, (c) 1,lO-phenanthroline, (d) 5,6-benzoquinoline, (e) 7,8-benzoquinoline, (f) biphenyl, (9) 2,2'-bipyridyl, (h) 4,4'-bipyridyl, (i) acenaphthene. A right-handed coordinate system is adopted.
tubes (4 mm 0.d.) were used, although a flattened tube was used by El-Sayed and Siegel.' Our results were not noticeably affected by flattening the tube. The arrangement of the optical system was essentially the same as that described by Rabold and Piette.9 Phosphorescence and its excitation spectrum were measured using a conventional phosphoroscope by exciting the samples with light, from a 450-W xenon arc lamp, which was monochromatized by passing through a Bausch and Lomb monochromator as described previously.12 The samples, in quartz tubes (7 mm o.d.), were immersed in liquid nitrogen. Decay constants of phosphorescence were measured by setting the monochromator at an appropriate wavelength and then photographically recording the phosphorescence decay subsequent to excitation by a short pulse of light. The output of a photomultiplier was coupled directly to an oscilloscope. Decay constants of Am = 2 epr transition were measured by setting the static magnetic field at a fixed value which gave a maximum of first-derivative signal and by recording the decay of the maximum after mechanical cessation of excitation. 1,lo-Phenanthroline and biphenyl were obtained from Matheson Coleman and Bell. All the other aromatic compounds used in this work were obtained from Aldrich Chemical Go. 5,6 :7,8-Dibenzoquinoxaline and 1,2 :3,bdibenzophenazine were recrystallized three and two times from benzene, respectively. 1,lO-Phenanthroline monohydrate was recrystallized twice from water and then was dried by heating over phosphorus pentoxide under vacuum. The anhydrous phenanthroline obtained was sublimed under vacuum. 5,6Benzoquinoline was treated with sulfuric acid in ethanol, and the precipitate obtained was treated with sodium hydroxide in water. This process was repeated The Journal of Physical Chemistry
Triplet Decay Constants Phosphorescence and its excitation spectra were measured for every compound studied in this work and compared with published data.l3-'7 These spectra confirmed that the observed phosphorescent triplet states were all genuine. The observed decay constants of the Am = 2 epr transition and the phosphorescence are given in Table I. All the decays were exponential within experimental error. Table I1 shows the temperature effects on the decay constants of phosphorescence observed for 5,6benzoquinoline, biphenyl, and acenaphthene. Since appreciable differences were not found between the decay constants of the Am = 2 epr transition and those of the phosphorescence, the measurements of temperature effects on the decay constants were not carried out for the other compounds. If the fluctuations in the decay constants of the Am = 2 epr transition are compared with the temperature dependences of phosphorescence decay constants of these three compounds, it is concluded that the temperature was controlled, as expected, a t -175" within a fluctuation of * 5 " in the epr experiments. For all the compounds studied, we find a good correspondence between the decay constants of the epr transition and those of phosphorescence. This is another confirmation that the genuine phosphorescent states of these molecules were observed in the epr experiments. Incidentally, similar temperature effects have been observed in isopropyl alcohol for the phosphorescences of several aromatic compounds, such as naphthalene, phenanthrene, and biphenyl. l8 The (12) D. R. Kearns and W. A. Case, J . Amer. Chem. Soc., 88, 5087 (1966). (13) For 6,6 :7,8-dibenzoquinoxaline and 1,2:3,4dibenzophenaaine, see F. Dorr and H. Gropper, Ber. Bunsenges. Phys. Chem., 67, 193 (1963). (14) For azaphenanthrenes, see: Y . Kanda and R. Shimada, Spectrochim. Acta, 15, 211 (1959); H. Gropper and F. Dorr, Ber. Bunsenges. Phys. Chem., 67, 46 (1963). (15) For biphenyl, see Y . Kanda, R. Shimada, and Y. Sakai, Spectrocham. Acta, 17, 1 (1961). (16) For bipyridyls, see Y . Gondo and Y . Kanda, Bull. Chem. Soc. Jup., 38, 1187 (1965). (17) For aoenaphthene, see A. P. Marchetti and D. R. Kearns, J . Amer. Chem. Soc., 89,768 (1967). (18) G. v. Foerster, 2.Nuturforsch., A, 18, 620 (1963).
PMRSTUDYOF TRIPLET STATES OF SOME AROMATIC NITROGENHETEROCYCLES Table I : The Decay Constants of the Lowest Excited Triplet States" YMethodBPh05
Compd 5,6 :7,8-dibenzoquinoxaline 1,2 :3,4-Dibeneophenazine 1,lO-Phenanthroline 5,6-Benzoquinoline 7,8-Benzoquinoline 2,2'-Bipyridyl 4,4'-Bipyridyl Biphenyl Acenaphthene
Concn,
Epr Ana = 2 transition at
phorescence at
X 10-2
-175 zt 5 ' ,
mol./l.
sec
-196', Bec
0.472
0.94
0.95
0.0286
0.27
0.27
0,900 0.743 0.919 1.40 0.500 0.942 1.37
1.36 2.28-2.57b 1.96 0.89 0.50 2.61-3.29* 1.69-1.83b
1.46 2.97 2.02 0.95 0.53 4.21 2.64
Ethanol a The solvent was diethyl ether for 4,4'-dipyridyl. was used for all the other compounds. These variations were due to temperature fluctuations. No appreciable fluctuations were observed in the decay constants of the epr signals for any compounds other than these three.
'
Table I1 : Temperature Effect on the Lifetimes of the Phosphorescence of 5,6-Benzoquinoline, Biphenyl, and Acenaphthene in Ethanol" Compd 5,6-Benzoquinoline
Temp, OC
- 196 - 184 -177.5
- 174
- 170
Biphenyl
- 166 - 196 - 185 - 176
- 172 - 171 Acenaphthene
- 196
- 187
- 183 -176.5 - 172 171 167
-
Lifetime, 8ec 2.97 2.85 2.45 1.63 1.39 0.082 4.21 3.83 2.98 3.19 2.69 2.64 2.64 2.64 2.28 1.01 0.58 0.14
For a The concentications are the same as given in Table I. these variable-temperature measurements, the sample tube was attached to a large slowly warming copper block whose temperature was monitored by a thermocouple.
cause is a softening of the glass and the onset of diffusion. This effect will be noticeable first in long-lived triplet states. It may be noted that the decay constants of phosphorescence of the aromatic nitrogen heterocyclic compounds all are shorter than those of
3217
the parent hydrocarbons. For comparison, decay constants of 16, 3.8, and 4.2 sec were reported for triphenylene, phenanthrene, and biphenyl, respectively,19 in EPA a t 77°K. This shortening may be ascribed to increased s pin-orbit coupling caused by nitrogen substitution.1°
Epr Spectra The epr spectra observed of triplet molecules in frozen glassy solvents can be classified roughly as Am = 2 and Am = 1 transitions, where m is the strong-field quantum number of the electron spin. This classification is not rigorous, since a t X-band microwave frequencies and for " T , T * ) states of aromatics which have zero-field splittings of -0.1 cm-l, the strong-field spin states are mixed together. I n glasses, the Am = 2 transitions, or "half-field" transition, is found to occur at about half the magnetic field of the free electron resonance. The Am = 1 transitions, which are allowed only in perpendicular polarization (the static magnetic field perpendicular to the microwave magnetic field) occur as rather sharp peaks, roughly symmetrically distributed about the normal g = 2 field. The fields a t which the sharp peaks are observed are usually called the stationary resonance fields (srf) and occur for those triplets whose principal x, y, and z axes are closely aligned with the static magnetic field. Each orientation gives rise to a pair of transitions, so that six of these transitions may be observed in all. I n triplets whose zero-field splitting tensor20-22has axial symmetry, the z and y fields are the same, and only four Am = 1 transitions are observed. A numerical analysis of the line shape of the Am = 2 transition and its relationship to the zero-field splitting tensor has been given by Kottis and L e f e b ~ r e . ~The ~ Am = 1 transitions have been analyzed by Wasserman, Snyder, and Yager24and by Kottis and Lefebvre.26 Our discussion of the epr spectra refers to these treatments. The degeneracy of the triplet state is lifted even in zero magnetic field as a result of dipolar spin-spin interactions between unpaired electrons and, to some extent, by spin-orbit coupling. Spin-orbit contributions are expected to be relatively more important in aromatic nitrogen heterocyclic molecules than in their parent hydrocarbons. The spin Hamiltonian for the zero-field interaction may be expressed as X = DSz2
+ E(Sz2- 8u )
(1)
(19) R. E. Kellogg and R. P. Schwenker, J. Chem. Phys., 41, 2860 (1964). (20) M. Gouterman and W. Moffitt, ibid., 30, 1107 (1959). (21) R. McWeeny and Y. Mizuno, Proc. Roy. Soc., A259, 554 (1961). (22) A. D.McLachlan, Mol. Phys., 6 , 441 (1963). (23) Ph. Kottis and R. Lefebvre, J. Chem. Phys., 39, 393 (1963). (24) E. Wasserman, L. Snyder, and W. A. Yager, ibid., 41, 1763 (1964). (25) Ph. Kottis and R. Lefebvre, ibid., 41, 379 (1964). Volume 72,Number 9
September 2968
YASUHIKO GONDOAND AUGUSTH. MAKI
3218 or by
~~
~~
x=
-(XS,Z
+ YS,2 + 2&2)
Table I11 : Root-Mean-Square Zero-Field
(2)
Splitting Parameters, D* (cm-l)a
The parameters in these expressions are related by D = -3/zZ and E = l / z ( Y - X). X, Y, and 2 are the principal values of the zero-field splitting tensor,20-2z and S, S,, and 8, are the spin angular-momentum operators for t,he triplet state. The quantity D* = (Dz 3E2)1’2may be obtained from the position of the A m = 2 transition by the relation
--Am
+
D* = & [ l / d ( h ~ ) ~- ( g @ H r n ~ n ) ~ ] ~ / ~(3) in which v is the microwave frequency, g is the spectroscopic splitting constant of the triplet state (assumed isotropic in this work), and @ is the Bohr magneton. HWZr7Z is the minimum for the Am = transitions Of the polycrystalline sample, where the “piling UP” of intensity occurs, leading to the observed epr peakn2J Hmin was originally thought to coincide with Ho, the low-field derivative maximum of the transition, but the detailed analysis of the line shapez3 has yielded a relationship which can be expressed as
Hmin = HO
+ O.45AHo
(4)
where AH0 is the distance between the low-field derivative maximum and the first zero. The srf of the Am = 1 spectra were used to obtain absolute values of the zero-field splitting parameters by means of the
1x1 = (sP>z(Hz22 - H,I2)/6hV
(5) and the similar expressions for IY[ and 121. H,, and H,, are the magnetic fields of the high- and low-field x-axis peaks, respectively, and g is assumed to be isotropic and equal to the free-electron value. The rootmean-square zero-field splitting parameter, D*, is given by D* = [”/z(X2 Y z Z2)]1/z (6)
+ +
I n order to establish the validity of eq 4, we have calculated D* for the triplet states studied: (a) using eq 3 with Ho in place of Hmtn, (b) using eq 3 with Hmln calculated from eq 4, and (c) using values of X , Y , and 2 obtained from the A m = 1 spectra in eq 6. The results are presented in Table 111. Except for the triplet state of 5,6 : 7,8-dibenzoquinoxaline, Hmdn is seen to give superior agreement with the Am = 1 spectra than is Ho. The zero-field splitting parameters which were obtained from the Am = 1 transitions are presented in Table IV. Values of about 0.1 cm-l were obtained for D in all cases, which is consistent with 3 ( ~ , n *phos) phorescent states.27~28 Relative signs of the zero-field splitting parameters were obtained from the requirement that the trace of the tensor should vanish, i.e. X+Y+Z=O The Journal of Physical Chemistry
~~
-
Am = 1
2 transition*HmWI
transitions
0.1064
0.1049
0.1068
0.0930
0.0909
0.0905
0.1344 0.1323 0.1310 0.1113 0.1129 0.1222 0,1012
0.1331 0.1311 0.1293 0.1092 0.1115 0.1202 0.0988
0.1334 0.1316 0.1296 0.1096 0.1118 0,1199 0.0995
Compd
Ho
5 6 :7,8-Dibenzoquinoxaline 1,2: 3,4Dibenzophenazine 1,lO-Phenanthroline 5,6-Benzoquiholine 7,8-Benzoquinoline BiDhenvl 2,i’-bipyridyl 4,4,-Bipyridyl Acenaphthene
a Solvents and concentrations are given in Table I. An isotropic value of 2.0023 was assumed. See also Table IV, For definitions of Ho and Hmin,see the text and eq 4.
‘
Absolute signs were assigned in some cases by comparison with the hydrocarbon analogs for which they are known. The fundamental epr data are shown in Table V. Quantitative results of magnetophotoselection measurements are given in Table VI for those triplets which gave sufficiently intense A m = 1 spectra for quantitative study. The results of magnetophotoselection studies and details of the assignment of zero-field splitting parameters are given in the following sections, in which we discuss the individual triplet states.
Detailed Results and Discussion A. 6,6:7,8-Dibenzoquinoxaline. The phosphorescence of 5,6 :7,8-dibenzoquinoxaline is extraordinarily intense,13 and it was also easy to observe the A m = 1 spectrum, which is shown in Figure 2. The strong resonance at g = 2 from free-radical photodecomposition products can be observed in Figure 2. Photodecomposition was extensive, so two separate samples had to be used to obtain accurate field measurements. The spectrum is quite similar to that of the parent hydrocarbon, triphenylene, although its 1 D 1 value of 0.1068 cm-’ is quite a bit less than the value of 0.134 cm-l reported3s26for triphenylene. Replacement of a CH group by a nitrogen atom is not expected to affect seriously the zero-field splitting parameters, as can be seen from a comparison of quino~aline,~g quinoline, and i s o q u i n ~ l i n e *with ~ ~ ~ ~naphthalene. The molecular symmetry of 5,6 :7,8-dibenzoquinoxaline does not re(26) J. B. Farmer, C. L. Gardner, and C. A. McDowell, J . Phys. Chem., 69, 953 (1965). (27) H. Sternlicht, J . Chem. Phys., 38, 2316 (1963). (28) J. Higuchi, ibid., 39, 1847 (1963). (29) J. S. Vincent and A. H. Maki, ibid., 39, 3088 (1963). (30) J. S. Vincent and A. H. Maki, ibid., 42, 865 (1966). (31) B. Smaller, (bid., 37, 1578 (1962).
PMRSTUDYOF TRIPLET STATESOF SOMEAROMATICNITROGEN HETEROCYCLES Table IV : Zero-Field Splitting Parameters (cm-l) from Am
=
1Transitions"
X
Compd
f O . 0361b
5,6 :7,8-Dibenzoquinoxaline 1,2 : 3,4-Dibenzophenazine 1,lO-Phenanthroline 5,6-Benzoquinoline" 7,8-Benzoquinolinec Biphenyl 2,2'-Bipyridylc 4,4'-Bipyridyl Acenaphtheine
k0.0447 0.0829 0.0819 0.0805 k0.0400 k0.0488 k0.0439 0.0462
3219
Y
Z
Y = X k0.0126 -0.0140 -0.0136 -0.0131 f0,0329 k0.0243 rt0.0359 0.0181
'F0.0712 T O . 0574 - 0.0692 - 0.0682 -0.0674 T O . 072gd T O . 0733 T0.0798d - 0.0644
D (=-'/zZ)
E (='/z(Y
f0,1068 k0.0862 0.1038 0.1023 0.1011 rt 0.1094 f0.1099 f0.1197 0.0966
- X))
0.000 T O . 0161 - 0.0485 -0.0477 -0.0468 ' F O . 0036 T O . 0122 T O . 0040 -0.0140
Coordinate system is given in Figure 1. An isotropic g value of 2.0023 is asa Solvents and concentrations are given in Table I. sumed. Absolut,e signs for the zero-field splitting parameters of the three azaphenanthrenes and acenaphthene were obtained from Obtained with eq4. See the text. The principal axes of the zero-field splitting a comparison with those of the parent hydrocarbons. tensors should be different from the coordinate axes defined in Figure 1. For convenience, the zero-field splitting parameters are listed The z-axis peaks were not observed. These Z's were obtained from X and Y by using under the column headings of XI Y , and 2. the relation X $- Y 2 = 0.
+
Table V : Srf and Klystron Frequencies for Triplet State Epr Lines" Am = 2-7
Triplet
5,6 :7,8-Diben:aoquinoxalined 1,2 :3,4-Dibenzophenazine 1,lO-Phenanthroline 5,6-Benzoquinoline' 7,8-Benzoquinoline' Biphenyl 2,2'-Bipyridylf 4,4'-Bipyridyl" Acenaphthed
-
I
Hob
Hminb
YO
1483.4 1519.5 1393.7 1402.7 1407.8 1469.8 1464.8 1436.2 1499.2
1487.5 1524.4 1398.2 1406.9 1413.7 1475.8 1468.8 1442.6 1505.3
9093 8 9105.1 9091.7 9099.4 9101.6 9095.4 9093.4 9094.0 9104.2 I
HZI'
Hzzb
2606 2500 1893 1911 1934 2542 2405 2465 2465
3782 3946 4567 4550 4529 3847 3992 3902 3961
~
_Am------1_ =
7
Hylb
Huab
HsIb
Hzzb
YC
2606 2997 2909 2920 2932 2650 2782 2583 2895
3782 3407 3373 3369 3367 3725 3580 3761 3485
2102 2322 2075 2091 2108
4387 4168 4326 4311 4300
9095.3" 9106.5 9096.0 9096 9099.4 9095.4 9093h 9094, 8i 9093.5
9
g
2066
4419
9
9
2210
4277
" I n ethanol glass at -175 i Bo, unless otherwise specified. The concentrations are given in Table I. Assignment of principal axes from magnetophotoselection, unless otherwise indicated. The magnetic field is expressed in gauss. E The klystron frequency is expressed in megacycles per second. The zero-field splitting tensor has axial symmetry. Equation 4 was used to obtain H,,,, The frequency quoted is for the X and and Hz,u2. The principal axes were assigned by comparison with the parent hydrocarbon. Y transitions. The frequency for the 2 transitions was 9092.6 MHz. See footnote c to Table IV. The z-axis peaks were too weak The value quoted is for X and 2 transitions. The frequency of Y transitions was 9107 MHz, to observe. Run in diethyl ether glass. The principal axes were assigned by comparison with biphenyl triplet. The value quoted is for X transitions. The frequency Assignment of principal axes by comparison with naphthalene triplet state. of Y transitions was 9095.5 MHz.
'
'
Figure 2. Epr Am = 1 spectrum of the phosphorescent triplet state of 5,6: 7,8-.dibenaoquinoxalinein ethanol glass. See Table V.
quire an axially symmetric zero-field splitting tensor, so the result E = 0.000 cm-l is surprising, as is the
departure of D from its value in the triphenylene triplet. These discrepancies may arise partly from enhanced spin-orbit coupling effects in the aza aromatic molecule. 32 B . 1,W :S,.&Dibenzophenazine. The phosphorescence of 1,2:3,4-dibenzophenazine is very intense, as reported in the l i t e r a t ~ r e , and ' ~ it was not difficult to observe the Am = 1 transitions in ethanol glass a t concentrations as low as low4mol/l. The Am = 1 spectrum is shown in Figure 3. A concentration higher mol/l. could not be obtained because of than 3 X limited solubility in ethanol. The assignment of the peaks was aided by a magnetophotoselection experiment on the low-field half of the spectrum. From the electronic absorption spectrum reported by Dorr and GroppeP and the phosphorescence excitation spectrum (32) J. W. MoIver, Jr., and H. F. Hameka, J. Chem. Phys., 45, 767 (1966).
Volume 72, Number 9
September 1068
3220
YASUHIKO GONDOAND AUGUSTH. MAKI
Table VI : Magnetophotoselection Studies. Relative Intensities of X , Y , and 2 Transitions"
Triplet
Srf (assignment), Gauss
------First Isotropic~
Excitation absorption bandb7 , E H E I H Isotropicd
)I
X ,250
m # -
IIH
E IH
. , *
... ... ...
... ... ...
... ...
E
2322 (5) 2500 (X) 2977 ( Y )
0.14 0.33 0.53
0.07 0.56 0.37
1893 (X) 2075 (5) 2909 ( Y )
... ...
... ...
...
... ...
0.23 0.29 0.48
0.39 0.13 0.49
0.12 0.35 0.53
5,6-Benzoquinolinee
1911 (X) 2091 (5) 2920 ( Y )
0.23 0.25 0.52
0.45 0.19 0.36
