G. E. Johnson
1512
A Spectroscopic Study of Carbazole by Photoselectionl E. Johnson Wehster Research Center, Xerox Corporation, Webster, New York 14580
(Received September 73, 1973)
Publication costs assisted by the Xerox Corporation
Carbazole and four of its nitrogen-substituted derivatives have been investigated using photoselection techniques. The polarization of the lowest energy transition in absorption as well as both fluorescence and phosphorescence emissions is found to be wavelength dependent in all cases. This indicates the presence of forbidden character in these transitions. Transitions to higher lying states are seen to be uniformly polarized. Introduction of an unsaturated vinyl or phenyl group on the nitrogen atom gives new spectral features at wavelengths underlying and to the blue of the second excited electronic state of carbazole and its N-alkyl derivatives. Assignments of the first four excited singlet states and the lowest triplet state of these carbazoles are made on the basis of these results.
I. Introduction Carbazole and carbazole related systems have in recent years attracted much attention. Poly(N-vinylcarbazole) (PVK) in particular has been the subject of numerous studies, many of which were undoubtedly stimulated by the finding that PVK is one of the most sensitive photoconductive organic polymers.2a Included among these efforts are investigations of the formation and emission of excimer states in PVK film@ and in s ~ l u t i o n and , ~ how these states relate to the process of energy transfer in solid in the generation of charge carriers5 and f i l ~ n s .Interest ~ their transport6 in PVK films has been high. The investigation concerning the transfer of electronic excitation was extended to include doped crystals of N-isopropylcarbazole. h o t her carbazole derivative, 1,3-bis(N-carbazolyl)propane, which serves as a useful model for PVK, has been investigated with regard to the formation of intramolecular excimer states,',* These investigations are representative of the types of photophysical processes which have been of concern in the carbazole system and by no means include all the efforts applied in this direction. A necessary starting point in any attempt to understand the excited state interactions which strongly influence photophysical processes such as excimer formation and emission, energy transfer, and charge carrier generation in the solid state is knowledge concerning the nature of the electronic states of tho monomeric chromophore units themselves. En this regard two significant spectroscopic studies of carbazole have been made. The first vaporphase spectrum of carbazole was measured by Pinkham and Waitg and assignments of the two lowest lying electronic transitions made on the basis of molecular orbital calculations. These results were in agreement with earlier theoretical predictions of Mataga, et a2.I0 Bree and Zwarichll in a very detailed study of carbazole combined polarized infra7ed and Raman spectra of carbazole single crystals with polarized absorption and fluorescence spectra of carbazole dissolved in a fluorene crystal to assign fundamental vibrational modes and the lowest lying electronic state. Another polarized absorption study on neat crystals of carbazole extended the polarization results to include the second electronic transition.12 The (0,O)transitions to the first an second excited singlet states were found to he polarized along the short in-plane (2)and The Journal of Physical Chemistry. Vol. 78. No. 75. 1974
long in-plane (Y) molecular axes respectively (see Figure 1). These results are in agreement with the calculations and thus assignment of these two states as IA1 and in Causymmetry appears firmly established. It is the purpose of this paper to report the results of an investigation concerning the spectroscopic behavior of the lower lying states of carbazole and four of its nitrogensubstituted derivatives using the technique of photoselection,13 The polarization of transitions in absorption is measured relative to both fluorescence and phosphorescence transition moments. Whereas the spectroscopic studies on neat or mixed carbazole crystals are limited to the two lowest lying transitions due to difficulties in achieving sufficiently thin crystals or the overlapping of guest and host absorptions it is possible in rigid glass solutions to extend measurements to higher lying states. The relative orientation of absorbing and emitting transiton moments obtained from photoselection when combined with the results of the crystal studies enables all transitions to be assigned on an absolute basis. The polarization throughout the lowest absorption band is found to be mixed and the results interpreted in terins of the presence of a significant amount of so-called forbidden character in this electronically allowed transition. Details concerning the vibronic coupling mechanism operative in fluorescence and phosphorescence emission are similarly probed and assignment of the lowest triplet state made. 11. Experimental Section
A. Materials. 1. Solutes. N-Isopropylcarbazole (NIPC) (supplied by Dr. H. Hoegl of the Battelle Memorial Institute, Geneva, Switzerland) was obtained in the form of large, optically clear, single crystals grown from the melt following repeated zone refining of the original vacuumsublimed starting material. A num er of experiments were repeated using NICP (Eastman Kodak White Label) purified by multiple recrystallization from isopropyl alcohol (IPA). The experimental results obtained for both materials were in strict agreement. N-Ethylcarbazole (Southern Dyestuff Company, Charlotte, N. C.) was purified by multiple recrystallization from IPA. N-Phenylcarbazole (Aldrich Chemical Company, Inc.,
Spectroscopic Study of Carbazole by Photoselection Milwaukee, FVisc., Research Grade) was purified by multiple recrystallization from IPA. ,V- Vinylcarba sche Aniline and Soda Fabrik A@, distributed r and Wolff and Co., New Uork, 3,.Y,) was pu mu!tiple recrystallization from ~ e t h a n oin~amber glassware to yield thin colorless platekts. Vacuum sublimed NVK yielded identical experimental results. Carbazole (James ~ ~ ~ n t was o n ) zone refined material used as received 2. Solvents. ~ ~ ~ e t ~ y ~ p (3MP) e n ~ a(Phillips n e Petroleum Co., BarIlesviHe, Okla., Pure Grade) was purified by chromatography on an activated alumina column. The resupking materia: had 90% transmittance at 2200 A with zero absorbance at 2500 P\, the shortest wavelength at which it was possible to obtain polarization data. lsopentane (‘isoP) (Hartman-Leddon Co., Philadelphia, Pa., Phorometric Grade) was used as received. EPA (diethyl ether, isoP, ethyl alcohol, in a volume t a ~ n e dfrom American Instrument Co.) was used as rece wed. El Polarization 01 Absorption and Emission ut 77°K. The pbotoselectio measurements were made using wellestablished, and y now familiar, techniques14,I5 and need not be described in detail here. Monochromatic exciting light was provided by a 1-kW General Electric A-H6 ~ i g h - ~ r e s ~ umercury re arc dispersed through a Bauseh and Lomb FOB-mm focal length monochromator with a reeiprocril h e a t dispersion of 3.3 nm/mm. Linearly polarized exciting light was provided by an ultraviolet transmitting dichroic osited on a fused quartz substrate (Polacoat Znc. Ash, Ohio, formula 105uv film). The rigid solutio e immersed in bubble-free Ibquid nitrogen maintained in a fused quartz, optical dewar The polarized emissiion emanating from the sample was ion system placed at right angles to am. The emission is collected with rtz lens and passed through a second polarizer (Polatoat, Inc.) mounted on the entrance slit of a viewing monocbsorn ator. The viewing monochromator consisted of two Jarrell-Ash, 0.25-m Ebert monochromators (f/3.5, reciprocal linear dispersion o f 3.2 nm/mm) em in a double monochromator configuration. Emissior is dejected with either an EM1 9558 or a RCA l[P 28 p h ~ t o ~ ~ i ~ ltube t ~ ~placed l ~ e r at the exit slit side of the second JarrePI-Ash monochromator. The output current from. the p ~ o t ~ ~ ~ u ~ tubes t ~ p lisi eread r out using a Keithley Model GIOA e’iectrometer. all Absorption and Emission Spectra. All absorption spectra were recorded on a Cary Model 14 spectrophotometer. The emission spectra were recorded using the same experimental a ~ ~ a r I ~ eused ~ ~ eton measure t polarization of emission and erniwion excitation spectra. The phosphorescence ~ i f e ~ i were ~ ~ e determined s by following the outthley 6lOA on an Esterline-Angus Model er. Fluorescence lifetimes were measured iising the time comre ated single photon counting technique The n a ~ o $ ~ ~ fluorometer ~ o ~ ~ d was assembled with ORTEC electronics
Presentation and discussion of the results will proceed in the following manner. First a general, qualitative description of the absorption and emission spectra of the
1513
R R = H
CARBAZOLE
R =-CHZCH, R-
N-ETHYLCARBAZOLE
CH&-CH~
N -ISOPROPYLCARBAZOLE
I
H R = - C H = C H 2 N-VINYLCARBAZOLE R=Q
N-PHENYLCARBAZOLE
Figure 1. Structure and coordinate system for carbazole
different carbazoles is given. Next the polarization of transitions in absorption as determined by polarized emission excitation spectra are discussed. Assignments of the first four electronic states are made. It will be seen that the polarization of the lowest lying absorption band is mixed, signifying the presence of a significant amount of socalled “forbidden” character16 in the symmetry allowed electronic transition. Next the polarized fluorescence spectra as determined by exciting the ((4,O) transitions of the two lowest lying states are discussed. It will be shown bow these results complement the polarization of the ‘Lb ‘A band system and in particular show the lack of any significant out-of-plane polarized intensity. Finally polarization of phosphorescence and polarization of phosphorescence excitation spectra are discussed. These results lead to an unequivocal assignment of the lowest triplet state of these carbazoles, and give insight into the higher order mechanisms leading to dipole allowed character in this transition. A. Absorption and Emission Spectra. Absorption spectra of carbazole, N-isopropylcarbazole (NIPC), N-ethylcarbazole (NEC), N-phenylcarbazole (NPK), and N-vinylcarbazole (NVK) were each measured in nonpolar 7:3 3MP:isoP and polar EPA solvents at room temperature and 77°K. The spectra of carbazole, NEC, and NIPC are, as expected, nearly identical. Those carbazoles containing an unsaturated group on the nitrogen are themselves similar but distinctly different from carbazole and its N-alkyl derivatives. The absorption spectrum of NIPC in 7:3 3MP:isoP mixture at 77°K is shown in Figure 2. Four distinct bands are noted with the respective (0,O) transitions located at 345, 295, 264 and 246 nm, respectively, The spectrum of NPK under the same experimentat conditions is shown in Figure 3. The first two bands, which henceforth will be designatIA and ILa IA, respectively, in the Platt ed ILb notation,17 show considerable vibrational structure. Both bands are characterized by a rather sharp (0,Q) which is the most intense transition in the band and the lack of a significant progression in any vibrational mode. This is characteristic of electronically allowed transitions where there is no appreciable change in the equilibrium nuclear configuration between ground and excited states. These two transitions are essentially unchanged irrespective of whether an unsaturated group or a saturated alkyl group is on the nitrogen. Distinct differences between the two classes of carbazoles do exist however at wavelengths to band. The band the blue of and underlying the lLa
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The Journal of Physical Chemistry, Vol. 78 No. 7 . 5 1974
G. E. Johnson
1514
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I
.-c
I
M
a IO
1':
-2* e
I
W
0
c
I
:6 I-
v)
W
-t
z
w 4
t I
U
z
w
0
$ 2
(I
I
W
U
2 0 26 LL
Figure 2. Absorption spectrum of NlPC in 7:3 3MP:isoP at 77°K.
r
I
I N- PHENYLCARBAZOLE IN 713 3MP-ISOP
250
300
-
350
WAVELENGTH nm
Figure 3. Absorption spectrum of NPK in 7:3 3MP:isoP at 77'K.
in the region of 264 nm which is clearly evident in the spectrum of carbazole, NEC, and NIPC no longer appears in the spectrum of NPK or NVK. Discussion of these differences will be deferred until the polarization of absorption results are presented. The spectroscopic behavior of all the carbazole derivatives was, with the exception of frequency shifts, the same in both the nonpolar hydrocarbon and polar EPA solvents. Due to its limited solubility in 7:3 3MP:isoP it was not possible to study carbazole in this solvent at 77°K. Cooling to 77°K apparently led to the formation of microcrystallites since the rigid solutions had a slightly turbid appearance. Consistent with microcrystal formation was the depolarized nature of fluorescence, and the lack of phosphorescence from these samples as well as agreement of IA band the position of the (0,O) transition of the E b with that reported for the single crystal.12 Emission spectra of each compound were also measured in both solvent mixtures at 77°K. In low temperature glasses the frequency of the (0,O) transition of the ILt, I A fluorescence coincides with that of the corresponding absorption when solutions of approximately 2 x M or less were studied. At concentrations higher than this the effects of reabsorption of emission were particularly severe. The fluorescence spectrum of a dilute solution of N P C in rigid 7:3 3MP:isoP at 77°K is shown in Figure 4. The fluorescence spectra of the other derivatives were nearly identical as indeed the similarity of the ILb lA absorption band in all compounds would suggest. Rather good mirror image symmetry between the fluorescence band and the lowest energy absorption band is evident. +-
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The Journal of Physical Chemistry. Vol. 78. No. 15. 7974
28
29
WAVENUMBER %
30 31 x (0-3CM-I
32
33
Figure 4. Absorption and fluorescence spectra of NIPC in 7:3 3MP:isoP at 77°K.
WAVELENGTH.nm
r
0 27
The phosphorescence spectra of NIPC in rigid 7:3 3MP:isoP and EPA solvents a t 77°K are shown in Figure 9. This again is typical of the other derivatives with the following exceptions: phosphorescence from carbazole in the nonpolar hydrocarbon matrix was not observed, as mentioned earlier, due to the formation of microcrystals. NVK did not phosphoresce in either the mixed hydrocarbon or EPA solvent a t 77"K, the result apparently of efficient, nonradiative deactivation of the lowest triplet state. The position of the (0,O) transitions in absorption and phosphorescence along with fluorescence and phosphorescence lifetimes are tabulated in Tables I and 11. The spectroscopic behavior of these carbazoles when going from nonpolar to polar solvents is typical of what is generally observed for transitions to singlet (T,T*)states. It is interesting to note, however, that the phosphorescence (0,O) blue shifts slightly in polar solvent as opposed to the general red shift noted for transitions to singlet (T,T*)states; this rather atypical solvent shift noted for the phosphorescence emission is consistent with other results (vide infra) concerning the lowest triplet state. B. Polarization of Absorption. The relative polarizations of transitions in absorption are determined by measuring a polarized emission excitation spectrum. The low temperature sample of rigidly held, randomly oriented solute molecules is excited at various wavelengths throughout the different absorption bands while emission at a fixed wavelength is detected a t right angles to the exciting light. With the available intensities it was possible to use 1- and 3.2-nm bandwidth light in excitation and emission, respectively. The emission intensity at each excitation frequency is measured using the four possible orientations of the polarizers in the excitation and emission light paths. From these four intensity values the polarization /1 I - ! - ~ v ( I I(~ I l /l /IL~ ) v~ H , ratio N is calculated where N = (11 is the ratio of the intensities of vertically to horizontally polarized emission when excited with vertically polarized the ratio of the intensities of parallel light and (I,~/L)H, and perpendicularly polarized emission with horizontally polarized excitation. The polarization ratio, N, is related to the relative orientation of transition moments in absorption and emission. The theoretical value of N = 3 indicates absorption polarized along a single molecular axis followed by emission polarized along the same axis. The value of N = 0.5 indicates single axis absorption followed by emission along a perpendicular molecular axis.13 Experimentally these theoretical limits are never achieved even for those cases where the molecular symmetry demands that intrinsic molecular transition moments are parallel or perpendicular to one another. It has been cus-
1515
Spectroscopic Study of Carbazole by Photoselection
TABLE I: Results of Absorption and Emission Measurements in Nonpolar 7:3 3MP:isoP Solvent Carbazole
NEC
30,260 34,420 39,250
29,110 34,080 38,020
29,390 34,050
28,950 33,950 37,820 24,260 6.6
INIPC
1
NVK
NPK
29,130 34,070 37,990
29,520 34,310
29,500 34,260
29 000 33,920 37,900 24,320 7.1
29,360 34,270
29,350 34,190
Room Temperature
77'K
~
24,410 6 .O
TABLE TI: Results of AbsorDtion a n d Emission Measurements in Polar EPA Solvent Carbazole
NEC
NIPC
NVK
NPK
29,540 34,330
29,500 34,250
10.1
11 .1
29,260 34,300
29,300 34,220
___I__-
'Lb(O,O), cm-I lLa(OIO),cm-1 lB,(O,O), cm-I 711,
Room Temperature 29,050 29,090 34,040 34,040 38,100 37,950 16.4 77 "K 28,900 28,800 33,920 33,900 37 900 37,900 24,380 24,370 8.2 8.1 16.2
29,720 34,130 39,520 15.7
nsec
29,520 34,020 38,760 24,600 7.7 15 .o
'Lb(O,O), cm-' %,(O,O), cm-' lBa(O,O),em-' ~ L ~ ( O ~em-' O),
see T C ~nsec ,
TRhOB7
tomary, at least in those cases where photoselection has been used in a more quantitative way, in an attempt to detail the nature of the wavefunctions of the combining electronic states, to introduce a "randomization factor" to correct for these departures from ideal behavior.14 This is not deemed necessary here in this more qualitative study. The polarization of emission excitation spectrum of NIPC in 7:3 3MP:isoP at 77°K is shown in Figure 5. Excil.4 band yieldtation at the (0,O) component of the 'Lb ed a value of N = 2.3 when viewing the (0,O) - 700-cm-l vibronic band in fluorescence. Ideally of course one wishes to view the (lQ,O) transition in fluorescence but this is not possible due to interference from scattered exciting light. The value N = 2.3 is, a considerably greater departure from the theoretical limit of N = 3 than might reasonably be expected for the case of parallel transition moments. In this regard many molecules of the same or lower symmetry and with the lowest energy transitions of significantly greater intensity were found to yield values as great as N =: 2.80 on this experimental system when viewing fluoresescence and exciting at wavelengths within the lowest energy absorlption bands. The value of N = 2.80 is thus about the maximum that can be achieved experimentally with this set-up. The value of N = 2.3 achieved with N P C when exciting into the (0,O) of the ILb lA band results from the fact that the (0,O) - 700 cm-l vibronic band in fluorescence is already mixed polarized. As excitation proceeds toward higher frequencies within the 'Eb 6 1i4absorption band the polarization ratio is observed to decrease with alternations occurring at particularly intense vibronic bands. A minimum value of N approaching the theoretical limit of 0.5 is reached at the (0,O) transition of the 'E, I A band indicating that this transition is along a molecular axis very nearly perpendicular to the emitting axis. The polarization ratio remains IA absorption band. The constant throughout the ILa polarization ratio starts to increase as excitation proceeds to higher lying vibrational levels of the 'Ba state and most
24 670 7,1
11.7
11.3
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WAVELENGTH, nm
Polarization of emission excitation spectrum of NlPC in 7:3 3MP:isoP at 77°K: (-) polarization ratio viewing the 700-cm-' vibronic transition in fluorescence; (-0-0-) polarization ratio viewing the (0,O) transition in phosphorescence; (- - - -) absorption spectrum,arbitrary units.
Figure 5.
likely represents the overlap of an oppositely polarized electronic transition at still higher energy. Exactly the same behavior was observed for carbazole and NIPC in EPA solvent at 77°K and for NEC in both rigid hydrocarbon and EPA solvent at 77°K. These results are in accordance with an earlier photoselection study of carbazole in ethanol .I8 As mentioned in the Introduction the polarization of the pure electronic transition moment linking the ground and 1bstates has been shown to be along the short, in-plane (2) molecular axis.11J2 This knowledge alone, combined with the fact that these transitions are unquestionably of a A* + K nature and thus necessarily polarized in the molecular plane, fixes the direction of the transition to the second and third excited states of carbazole, NEC, and NIF'C as along the long in-plane (Y) molecular axis. The transition to the fourth electronic state is apparently poThe J o u r n a l of Physical Chemistry. Vol. 78. No. 75. 1974
G. E, Johnson larized along the Z axis. However, because of low light intensity in this wavelength region the experimental results are only suggestive. This ordering of states agrees with the theoretical assignments of Mataga, et The most distinctive feature of the polarization of fluorescence excitation spectrum is of course the changing polarization ratio as excitation proceeds from the origin to I A band. The pohigher lying vibronic levels of the ILb larization ratio is a maximum a t the origin but decreases sharply at excitation frequencies slightly to the blue. It was noted that for every derivative investigated a distinct shoulder exists immediately to the blue of the (0,O) of the ILb IA band; the same shoulder is also present in the fluorescence and phosphorescence emission bands at energies slightly lower than their respective origins. This shoulder is apparently due to a vibronic transition at approximately 200 can-I to high frequency of the electronic origin. The polarization ratio continues to show an overall decrease as excitation progresses to higher vibronic levels with oscillations present at the more intense vibronic peaks. This behavior clearly signals the presence of vibronic coupling leading to a significant amount of “forbidden” character within this band. The oscillator strength of the lowest electronic transition, as obtained by integration I A band, is 0.056 & 0.002 for all these carbaof the ‘Lb zoles. By separating this band into allowed and “forbidden” components, ~n the manner of Kalantar and Albrecht,Ig one finds approximately 40% of the total intensity is “forbidden.” vibrational analysis of the ‘Lb I A and IL, IA ds based on the photoselection results is presented in Table IIT. The basis-of this analysis is the fact that vibronic theory to first order predicts that the “forbidden” intensity in an allowed electronic transition should appear as separate components each based on a different normal mode. Each “forbidden” component has the same FranckCondon envelope as the allowed part only separated from the true origin by the frequency of the vibronically active normal niode.19 Local minima in the value of N are ob-200 cm-l, (0,O) + served particularly clearly a t (0,O) -450 cm-‘, and (0,O) -1000 cm-I. These vibrational modes are considered to be bz vibronic origins; this is con’ firmed by polarization of fluorescence measurements. Figures 6 and 9 present the polarization of fluorescence excitation spectra of NPK and NVK in 7:3 3MP:isoP at 77°K. The spectra are unchanged in polar EPA solvent. lA and ILa I A bands, inThe polarization of the ‘Lb cluding the vibrovric details, is the same as for carbazole and its N-alkyl der .vatives. Essential differences exist, I A band however, at wavelengths to the blue of the ILa and particularly for NPK, at wavelengths underlying this transition. The distinct long-axis in-plane polarized transition to a 1B2(fBa) state which existed for carbazole, NEC, and N I X is not observed for NPK and NVK. Instead an oppositely polarized transition occurs, which in IA band. the case of NPK strongly overlaps the ‘La Consideration of the absorption spectrum of NPK in the light of the photoselection results indicates this band to be rather broad, unstructured, and of low maximum extinction coefficient. The situation for NVK i s similar althe overlap of the predominantly Z pothe ILL, ‘A band is less severe and the intensity or the “new” band i s apparently greater than for NPK. The polarization ratio for both NPK and NVK again decreases at still higher energies indicating the
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TABLE 111: Vibrational Structure of the l L b +- 1A and ‘La c ‘A Bands of NIPC i , cm-1
A:,
cm-1 Polarization
29,000 -29,200 29,450 29,700 30,000 30,280 30,450 30,680
0 -200 450 700 1000 1280 1450 1680
31,000 31,250 31,450 31,900
2000 2250 2450 2900
33,920 34,630 34,900 35 310 35,590 35,980 36,700 36,970
0 710 980 1390 1670 2060 2780 3050
‘L b z Y Y z Y
z z Y
4-
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+
+ -
+
+
+
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+ -
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The J o u n a l of Physicei Chemistry. Voi. 78. No. 15. 1974
Analysis
z Y Y z
(0,Q) AI 0,200 (bz?) 0,450 (b2) 0,700 (al) 0,1000 (b2) 0,1280 (al) 0,1450 (a1) 0,200 (b2) 1450 (al) 0,450 (bz) 1280 (al) 0,1000 (b2) 700 (al) 0,900 (al) 3- 1280 (al) 0,1000 (bz) 1280 (al) 8,1000 (bp) 3- 1450 (ax) 0 , 2 X 1450 (a,)
-++
+ -+
‘L a Y Y Y Y Y Y
Y Y
(0,O) Bz 0,710 (ad 0,980 (ad 0,1390 (al) 0,710 3- 980 0,710 1390 Q , 2 X 1390 0,710 4 980
+-
+ 1390
presence of a 2 axis polarized transition. The electronic state symmetries of the first four excited states of NPK and NVK are AI, Bz, AI, and Bz, respectively, while those of carbazole, NEC, and NIPC are AI, Bz, Bz, and AI. The similarity of the spectroscopy of carbazole, NEC, and NIPC is not surprising since alkyl groups are in general spectrally inert. Still there are rather significant spectral shifts with N-alkylation and these have been in their theoretical nicely interpreted by Mataga, et description of the electronic structure of carbazole. There an independent systems approach was used to arrive a t the electronic wave functions of carbazole. The electronic wave functions are linear combinations of those of planar biphenyl with those involving the transfer of an electron from >N-H to a vacant orbital on biphenyl. The first (%b) and third (IB,) electronic states of carbazole were shown to be mixed with a considerable amount of CT character while the second (IL,) state was almost totally that of locally excited states of biphenyl. Replacement of the hydrogen by alkyl groups should lead to a. lowering of the energy of the CT configurations and thus to lowering of the ‘Lb and IB, states of carbazole to an extent greater than the ‘La state. The experimental results are in accordance with this prediction. The ‘Lb and l83, states of carbazoles are red shifted considerably more than the ‘La state upon N-alkylation. (See Tables I and 11.1 Replacement of the hydrogen by unsaturated groups such as vinyl or phenyl has been seen to lead to significant spectroscopic differences from those of carbazole and its N-alkyl derivatives. Most notable is the appearance of a new Z axis polarized transition in both NV the 270-nm region. In spite of this similarity which emerges from the photoselection results the absorption spectra of these two compounds are quite different. At wavelengths corresponding to the distinct IB, IPa band observed in carbazole, NEC, and NIPC, NPK displays rather weak, unstructured absorption while in NVK there is considerable absorption but the transitions are apparently strongly overlapped. The origin of these spectral differences can be rationalized in a qualitative but general manner in terms 4-
Spectroscopic Study of Carbazole by Photoselection
250
270
290
310
330
1517
350
WAVELENGTH , nm
Polarization of emission excitation spectrum of N-PK in 7:3 3MP:isoP at 77'K: (-) polarization ratio viewing the - 7 O O - ~ r n - ~ vibronic band in fluorescence: (-0-0-) polarization ratio viewing t h e (0,Q)band in phosphorescence; ( - - - - - -) Figure 5.
ahsorption spectrum, arbitrary units.
I
250
I
I
270
-
I
290 JK) WAVELENGTH , RRI
I
330
J 350
Figure 7. Polarization of fluorescence excitation spectrum of NVK in 7 3 3MP SOP at 77°K. (--) polarization ratio viewing the (0,O)band in fluorescence, ( - - - - - - ) absorption spectrum,
of the interaction of transition dipoles localized on the
arbitrary units
In the case of both phenyl and vinyl derivatives the orientation of the substituent with respect to the carbazole H transitions localized plane is necessarily such that H on the substituent have a sizeable component along the carbazole fixed Z molecular axis. In the case of the phenyl group for example transitions corresponding to the IBIU lA, transition of benzene would lie almost totally along the 2 direction. The photoselection results along with the absorption spectrum indicate the presence of a 2 axis polatized transition of high intensity at wavelengths beyond 250 nm for carbazole. A splitting due to the interaction of these two 2 polarized transition dipoles leads to two states; the one of lower energy results from the in-phase or head-to-tail arrangement of the transition moments.20 The transition to this state is allowed while the transition to the higher lying state, resulting from the out-of-phase arrangement of transition dipoles, is forbidden. On this basis the existence of a Z polarized transition at wavelengths overlapping with ILa IA and IB, IA bands of carbazole appears reasonable. A similar origin for the 2 polarized transition in NVK appears likely. Here the most probable orientation of the vinyl group with respect to carbazole would be such as to give a sizeable component of a perturbed ethylene transition dipole along the 2 direction. PL dipole-dipole interaction of the high lying 2 polarized carbazole transition with this component of the only accessible ethylene transition also leads to an allowed Z axis polarized transition at lower energy than that of either group alone. In both NPK and NVK the polarization ratio is observed to decrease at still higher energy indicative of a Y or long-axis in-plane polarized transition. In the case of NVK this i s probably due to a 'Ba IA transition like that observed in carbazole, NEC, and NIPC. This transition should remain relatively unperturbed in NVK since there can be very little interaction between the carbazole 'Ba I A transition dipole and that due to the ethylene lo-
calized transition. Energetically these transitions are rather far apart and sterically it would seem that the component of the ethylene a* a transition which lies in the XY plane would be predominantly perpendicular to the Y polarized carbazole transition. Furthermore even if energetically and geometrically some interaction were possible, the state to which a transition is allowed would be higher in energy than either of the two interacting statesnZ0The absorption spectrum of NVK (see Figure 7) is crowded in this region perhaps reflecting the presence of this relatively unperturbed IB, IA carbazole Localized transition. The absorption spectrum of NPK (see Figure 6) reflects IA carbazole tranconsiderable perturbation of the IB, sition; the transition apparently being blue shifted into a region of overlapping bands. Perhaps this reflects on an I A carbazole localized interaction between the IB, transition dipole and a perturbed benzene transition of lBzu IA, parentage which is present at this energy. The allowed transition to the two states arising from this interaction would lie at higher energy than either transition alone. If this interaction is the origin for the rather distinct spectral changes in NPK, it further implies that the phenyl ring undergoes rather free rotation about the nitrogen-carbon bond. For steric reasons alone it would seem that the most probable orientation of the phenyl group would be perpendicular to the plane of the carbazole moiety. However, perpendicular orientation of the phenyl and carbazole planes would give no interaction between Y axis polarized carbazole transition dipoles and transitions to lBzu like states of a phenyl ring. In this case the IB, 4- 'A transition of carbazole should remain relatively unperturbed. Maximum interaction will occur for those molecules in which the planes of the phenyl and carbazoyl groups are parallel. In the rigid glass at 77°K a distribution of orientations is undoubtedly frozen in. This would result in an apparent loss in intensity of the carbazole 'Ba IA transition due to transfer of intensity to the allowed
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The Journal
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G. E. Johnson
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i
NlPC IN EPA AT 7 7 ° K
w NlPC IN 7:3 3MP-IS0
0.5 L..--340
P
AT 77’K
I
I
380
360
I
WAVELENGTH, nm
400
420
I
440
I
460
480
WAVELENGTH, nm
Figure 8. Polarization of fluorescence spectrum of NlPC in 7:3 3MP:isoP at 77°K: upper curve, excited at the (0,O)of the ’Lb ’ A band; lower curve, excited at the (0,O)of the ’La ’A
Figure 9. Phosphorescence spectra of NEPC in 7:3 3MP:isoP and EPA at 77’K.
component of the two states arising from the interaction. The absorption spectrum of NPK is in qualitative accord with this interpretation. 6. Polarization of Fluorescence. Polarized emission spectra are measured the same way as described earlier for polarization of emission excitation spectra except of course the excitation frequency is fixed while the viewing frequency is varied. Polarization of fluorescence data for NIPC in 7:3 3MP:iSOPa t 77°K are shown in Figure 8. The excitation band width of 6.6 nm was rather broad while the fluorescence band width was 0.32 nm. The upper curve in Figure 8 corI A band. responds to excitation at the (0,O) of the ILb When the fluorescence is viewed in the region of its (0,O) the polarization ratio N approaches the value of 3, characteristic of single axis absorption followed by emission along the same molecular axis. This, of course, is what is expected. The polarization ratio changes throughout the fluorescence band just as was observed for the lowest energy absorption band. The changing polarization ratio observed here is also indicative of a significant amount of “forbidden” character in this allowed transition. ‘The lower curve OF Figure 8 shows the polarization of fluorescence results with excitation into the (0,O) of the IL, IA band. Here the polarization ratio when viewing the (0,O) of the fluorescence band approaches the value of 0.5, a result of course, expected in the light of the measurements d the polarization of absorption. Again the polarization ratio is seen to be wavelength dependent with this dependence being complementary to the changes obsewed with excitation to the origin of the ‘Lb IA band. The mirror-like symmetry which exists between the polarization of fluorescence data under excitation in the (0,O) of the ILb and IL, IA bands is a key obser-
vation. As noted earlier, the results obtained from polarization of emission excitation spectra are not able to distinguish between vibronically active bl or b2 modes. The polarization of fluorescence results do make this distinction. It is seen that for those vibronic transitions in fluorescence where the polarization ratio achieves a minimum IA excitation the ratio is a local maximum with ‘Lb IA excitation. These results exclude the preswith lL, ence of any significant out-of-plane polarized intensity ifi fluorescence. The results described for NIPC in the nonpolar hydrocarbon glass are equivalent for the other derivatives investigated. D. Polarization of Phosphorescence and Phosphorescence Excitation Spectra. The phosphorescence spectra of NIPC in 7:3 3MP:isoP and EPA solvents at 77°K are shown in Figure 9. Again this is typical of the other carbazoles investigated here with the exception of NVK which does not phosphoresce. The spectra are characterized by the appearance of a strong (0,O) followed by considerable vibrational structure which is somewhat better resolved in the nonpolar hydrocarbon glasses. Energies and lifetimes of the lowest triplet state of each compound can be found in Tables I and 11. The appearance of the strong (0,O) transition in phosphorescence indicates that the predominant mechanism whereby the spin forbidden transition linking the lowest triplet and ground states acquires some allowed character is by direct spin orbit coupling of the triplet state to some singlet state(s) having a dipole allowed transition to the ground state. The lowest triplet state, simply by virtue of its long lifetime alone, can be identified as a (a,n*) statez1 and thus must belong to either the A1 and Bz irreducible representation of the CzU point group. Simple group theoretical considerations, knowing that the orbital part of the spin orbit coupling operator transforms as a
-
-
band.
+ -
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+
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The Journal of Physical Chemistry. Vol. 78. No. 75. 1974
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1519
SpectroscopicStudy of Carbazole by Photoselection
TABLE IV: Mechanisms for Bringing Dipole Allowed Singlet Character into the Lowest Triplet State
-
_ I _ -
I. Direct spin orbit coupling Lowest triplet Spin orbit coupling operator
x\ I
R,sAZ
Singlet character, le,, in the lowest triplet state and polarixation of Q r n 6 08"
Ry-BI
I
=3La
I
B&)
Adforbidden) B&)
At
R,=BZ
I
/I
Bl \\&
I
f
B,(x) &(forbidden) &(z)
II. Spin orbit coupling to an intermediate singlet with vibronic coupling in the singlet manifold L,owest triplet
Spin orbit operator Intermediate singlet Active vibrations Singlet state 111. Vibronic coupling in the triplet manifold followed by spin orbit coupling Lowest triplet
Active vibrations Intermediate triplet Spin orbit operator
Singlet state predict that phosphorescence will be polarized
X and Y if the lowest triplet is 3A1(3Lb) and X and Z if it is 3H2(3L,). (See Table IV.) These possibilities are examined by measuring the polarization of the emission excitation spectrum while viewing the electronic origin of the phosphorescence. The results of these emission excitation measurements are displayed in Figures 5 and 6. It has been shown that the transition moment between the ground state and the l A and 'La lA bands is polarized origin of the ILb short- and long-axis in-plane, respectively. The phosphorescence emission appears to be polarized perpendicular to both in-plane axes, since the polarization ratio N approaches 0.5 under both excitations. However, it will be noted that the theoretical limit of 0.5 is approached more closely as excitation proceeds toward the purely Y or longlA transition. This indiaxis in-plane polarixecl IL, cates that the phosphorescence emission contains a small amount of Z axis polarized intensity in addition to the predominant out-of-plane polarized intensity. The lowest triplet state of these carbazoles can be assigned unequivocally on this basis as SIda. The polarizatlon of phosphorescence spectrum of NIPC in EPA at 77°K is shown in Figure 10. The upper curve IA corresponds to excitation at the origin of the ILb band while the lower curve was obtained with excitation to the origin of the lLa 'A band. Of course at the origin OF the phosphorescence emission band this is simply a confirmation of the polarization of phosphorescence excitation results. The changing polarization of phosphorescence. just as m absorption and fluorescence, indicates the presence of vibronrc activity. In addition to the firstorder, direct spin orbit coupling one or more of the nuclear Coordinate dependent mechanisms for mixing dipole allowed character into the lowest triplet state are active. These vibronic mechanisms were first discussed by Albrecht in a study of the benzene problem.22 Second-order
-
+ -
+-
0.5 400
420
440
460
WAVELENGTH P n m
480
-
Figure 10. Polarization of phosphorescence spectra of NIPC in EPA at 77°K: upper curve, excited at the (0,O)of the l L b 'A band: lower curve, excited at the (0,O) of t h e ' l a 'A band.
-
+ -
-
perturbation theory revealed a very weak first-order term due to the nuclear coordinate dependence of the spin orbit coupling operator and two second-order terms: (1) spin orbit coupling to an intermediate singlet followed by vibronic coupling in the singlet manifold and (2) vibronic coupling in the triplet manifold followed by spin orbit coupling of this intermediate triplet to a singlet state. The majority of the phosphorescence intensity is due to the direct spin orbit coupling of the lowest triplet state with high lying singlet ( g , r * ) , (T,u*) or Rydberg states since it is these states which are dipole coupled to the The Journal o f Physical Chemistry. Vol. 76. No. 15 1974
G. E. Johnson
152Q
ground state with out-of-plane polarized transition moments. A smaller but discernible amount of 2 axis polarized intensity is also present due to direct spin orbit coupling the most likely candidate being the 'Lb state since it lies closest in energy. The polarization ratio observed at the electronic origin of the phosphorescence emission should be observed a t all vibronic transitions involving excitation of totally symmetric a1 vibrations. This is clearly 1640 cm-I), and the case a t 2140, 1640, 2800 (1140 3225 cm-I (2 x 1640 cm-I) to low frequency of the origin. The vibronic transitions at (0,O) - 570 cm-1 and (0,O) 2200 cm-I also appear to be polarized similarly although the ratio is not identical with that of the (0,O).These transitions are weak however, and obviously are strongly overlapped. The polarization ratio is observed to increase rather sharply a t wavelengths immediately to the red of the origin and between particularly strong a1 vibrational modes independent of whether excitation is to the (0,O) of the ILb IA or I&, 6- IA transitions. This signifies the presence of intensity polarized parallel to both 2 and Y molecular axes. Because of the weakness of these vibrations and the considerable overlap which exists no clear cut distinction can be made between those vibronic transitions which are Z or Y polarized. One mode in particular, however, appears do contain more 2 axis character than does the (0,O) or any vibronic transition involving excitation of a1 modes. iPt about (0,O) - 720 cm-I there appears to be more intensity due to coupling with the Z axis polarized ( ; K , T * )IA1 state than occurs uia direct spin orbit coupling. The 72O-crr,r1 mode thus appears to be a vibronic origin in the phosphorescence spectrum and symmetry arguments (see Table XV) tell us that al, a2, or bl modes can all be active i n mixing the 3L, state with IA1 character. Both nontotally symmetric a2 or bl modes of about the right frequency exist However, in both absorption and fluorescence an ax mode with a frequency of about 700 cm-l was observed and it is of some interest to a t least speculate om time possibility of this a1 mode being vibronically active. The existence of vibronically active, totally symmetric modes clearly appears in the Nerzberg-Teller formalism. However, photoselection, a t least in the singlet manifold, cannot identify such activity since both allowed and forbidden parts of the band are polarized alike. High-resolution, single crystal spectroscopy can make this distinction and indeed vibronically active a1 modes have been identified in this manner for ~ h e n a n t h r e n e If . ~the ~ only source of intensity in, the phosphorescence emission was direct spin orbit coupling, then simultaneous excitation of totally symmetric modes should not alter the ratio of X to Z polarized intensity observed at the origin. In first-order spin vibronic or either of the second-order mechanisms the vibronic activity of an a1 mode would appear as a change in the X to Z intensity ratio. This increased amount of 2 polarized intensity is perhaps a manifestation of this. The long-axis in-plane (Y) polarized intensity in phosphorescence can only be mixed in by nontotally symmetric vibrations. (See Table IV.) Here the most accessible route would appear to be spin orbit coupling of the 3L, state to the ILb state which in turn is vibronically coupled to the Ik, state. Spin orbit coupling of the 3L, state with the state undoubtedly exists since 2 polarized intensity appears at the origin. The results of the polarized fluo-
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The Journal of Physical Chemistry. Vol. 78.
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rescence and fluorescence excitation measurements have shown the state to be strongly vibronically coupled to the IL, state. The vibronic transition which appears as a weak shoulder to the red of the phosphorescence origin evidently plays a key role here just as in absorption and fluorescence. In this regard it should be stated that all of the photoselection results would be consistent with interpretation of this distinct shoulder as a vibronic transition involving excitation of a nontotally symmetric bz mode at -200 cm-l. Other experimental evidence for such a mode, however, does not exist. Bree and Zwarich assigned a strong transition at 220 cm-I as being due to an a1 vibration, the lowest frequency bz mode being 505 cm-l.ll In view of their results, it would appear that the photoselection data must reflect something other than a vibronically active b2 mode with a frequency of about 200 cm-l. Perhaps it is not unreasonable to think in terms of excitation from the ground state in which one quantum of an a1 mode with a frequency of 220 cm-I is excited to an upper state in which one quantum of a b:! mode of the appropriate frequency is excited. This point remains unresolved. Previous discussion has stressed the point that the results are independent of the surrounding solvent. Because of the extreme forbiddenness of the 'A transition the possibility that the results might reflect on the perturbing influence of the solvent must be considered and was in fact the primary reason for using both nonpolar hydrocarbon and polar EPA solvent. The wavelength dependence of the polarization ratio observed throughout the phosphorescence spectrum is the same in both solvents and thus the vibronic activity reflects on true molecular parameters. The polarization of phosphorescence excitation spectra, however, revealed a greater amount of short-axis inplane (2)intensity in the nonpolar solvent. This observation does not reflect a certain amount of rotational depolarization of the long-lived phosphorescence in the less viscous 7:3 3MP:isoP matrix since excitation into the IL, IA band gave the same value for N in both solvents. The increased amount of Z-polarized p in nonpolar matrices signifies stronger direct spin orbit coupling to ( T , T * ) singlet states. 'This result IS consistent with the shorter phosphorescence lifetime observed in nonpolar glasses. The greater amount of dipole allowed (a,a*)singlet character in the lowest triplet in addition to the still predominant (a,;~*) or (T,D.*) singlet character shortens the phosphorescence lifetime due to the greater H transitions. intensity of the H* +
+-
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IV. Summary and conclusions The lower lying electronic states of carbazole and four of its nitrogen-substituted derivatives have been investigated using photoselection techniques. The relative polarizations of transitions linking the ground and first four excited electronic states were determined. These relative polarizations are then placed on an absolute basis knowing the absolute direction of a single transition available from mixed crystal studies. Assignment of these transitions based on electronic state symmetries is then made. The polarization of transitions throughout the lowest energy absorption band is seen to be wavelength dependent. Wavelength dependent polarization ratios are also observed throughout the fluorescence and phosphorescence emission bands. The existence of mixed polarization in both fluorescence and the lowest lying absorption band signals the presence of a significant amount of "forbid-
Esr of Hydroxycyclchexadienyl Radicals
1521
den" character in the lowest electronic state. Polarization of phosphorescence excitation spectra enables the lowest triplet state to be assigned as 3L,. The observation of wavelength dependent polarization of phosphorescence spectra indicates that in addition to direct spin orbit coupling the lowest triplet state acquires allowed singlet character by certain, higher order, nuclear coordinate dependent mechanisms. P'iausible routes have been suggested on the basis ofthe photoselection results. The spectral features mentioned above are characteristic of all the carbazoles investigated here. Replacement of a hydrogen or alkyl group by an unsaturated vinyl or phenyl group leads to new spectral features at higher energies. These changes have been interpreted in terms of an interaction between transition dipoles localized on carbazole and the ~ n s a ~ ~ r asubstituent. ted Acknowledgment. T e author is indebted to Dr. J. H. Sharp for a number of useful discussions and for his interest throiighout the course of this investigation. The efforts of both referees are gratefully acknowledged. Their careful reading of the original manuscript lead to many critical comments which were indeed valuable. References and Notes ('I) Presented in part at The Twenty-eighth Symposium on Molecular
Structure and Spectroscopy, The Ohio State University, Columbus, (1965); (b) W. Klopffer, 69).
\._.-,. W. Klopffer, J. Chem. Phys., 50,2337 (1969). H. Bauser and W. Klopffer, Chem. Phys. Lett., 7 , 137 (1970). D. M. Pai, J. Chem. Phys., 52, 2285 (1970). W. Klopffer, Chem. Phys. Lett., 4, 193 (1969). W. Klopffer and W. Liptay, Z.Naturforsch. A, 25, 1091 (1970). C. A. Pinkham and S. C. Wait, Jr., J. Mol Spectrosc., 27, 326 (1968). (IO) N. Mataga, Y. Torihashi, and K. Ezurni, Theor. Chim. Acta, 2, 158 (1 964). (11) A. Bree and R. Zwarich, J. Chem. Phys., 49, 3344, 3355 (1968). (12) S. C. Chakravorty and S. C. Ganguly, J. Chem. Phys.. 52, 2760 (1970). (13) A. C. Albrecht, J. Mol. Spectrosc., 6, 84 (1961); Progr. React. Kinet., 5,301 (1970). (14) A. M. Kalantar and A. C. Albrecht, Be?. Bunsenges. Phys. Chem., 68,361 (1964). (15) T. Azurni andS. P. McGlynn, J , Chem. Phys., 37, 2413 (19'62). (16) A. C. Albrecht,J. Chem. Phys., 33, 156 (1960). (17) J. R. Platt, J. Chem. Phys., 17, 484 (1949): see also N. S. Ham and K. Ruedenberg, J. Chem. Phys., 25, 13 (1956). (18) H. Schutt and M . Zirnrnerrnann, fief. Bunsenges. Phys. Chem., 67, 54 (1963). (19) A. H. Kaiantar and A. C. Albrecht, fief. Bunsenges. Phys. Chem., 68, 361, 377 (1964). See, for example, E. G. McRae and M. Kasha, "Physical Processes in Radiation Biology," Academic Press, New York, N. Y., 1964. See, for example, S. K. Lower and M . A. El-Sayed. Chem. Rev., 66, 199 (1966). A. 6. Albrecht, J. Chem. Phys., 38, 354 (1963). D. P. Craig and R . D. Gordon, Proc. Roy. Soc.. Ser. A , 288, 69 (1965). (4) (5) (6) (7) (8) (9)
Electron Spin Resonance Spectra of Some Hydroxycyclohexadienyl Radicals Derived ic Carboxylic Acid Anions Gordon Filby* and Kirsten Gunther Institut fur Radiochem/e, Kernforschungszentrum, Karlsruhe, West Germany (Received October I , 1973, Revised Manuscript Pece,ved February 27, 1974) Publicalioi? costs assisted by Kernforschungszentrum
The addition of hydroxyl radicals to a series of aromatic carboxylic acid anions has been studied by mixing titanous chloride, hydrogen peroxide, and the acid in an aqueous continuous flow system. In the maj3rity of cases (exceptions were phthalic and isophthalic acids) spectra could be interpreted in terms of radicals arising from attack at all possible free ring positions with little or no specificity. Investigation of the influence of pII on the spectra of the tetra- and pentacarboxylic acids indicates that the acid-base properties of the adduct hydroxycyclohexadienyl radicals are not essentially different from those of the parent substance. The geometry and hyperfine splitting constants are briefly discussed.
The electron sDin resonance sDectra of a large - number of short-lived organic radicals in aqueous solution have been observed by rapid-flow mixing of inorganic ions (generally Ti(II1) or Fe(l1)) and hydrogen peroxide in the presence of an organic compound shortly before their passage through the cavity of an esr spectr0meter.l The radical producing reactions may be represented by the simplified scheme Ti(II1) W,O, -+Ti(1V) OHOH. (1)
+
OM 4- RM
--+
+ R. +
+
HZO
(2)
OH
+
RH
-+
RH(OH).
(3)
Numerous investigations have been concerned with the analysis of intermediates,2 reaction kinetic^,^ and comparisons of radicals generated by the Fenton (Fe(III)), titanous ~ h l o r i d e or , ~ photolytic systems.5 In most cases the identities of the radicals formed from a particular substrate are independent of the mode of initiation though the ratios of the radical concentrations (when it is possible to form more than one) often show significant variations. Such variations can however arise from a single substrate The Journal of Physical Chemistry. Vol. 78. No. 15. 1974
