FLUORESCENCE SPECTRUM OF ACRIDINE
Nov., 1963
ganometallic compound. Whereas this comparison is not possible when the ligand is cyclopentadienyl, it is possible when the ligand is benzene (such as dibeiizenechromium). The electronic spectra of other organometallic com-
ACRIDINE : A
Lon'
2481
pounds and their complexes with aluminum halides ndl be the subject of another communication. Acknowledgment.-The author is grateful to Drs. D. C. Lincoln and H. G. Tennent of these Laboratories for stimulating discussions.
TEMPERATURE IXT'ESTIGATION OF ITS ENIGMATIC SPECTRAL CHARACTERISTICS'" BY S. JULES LADNER AND ~ ~RALPHS. BECKER
Department of Chemistry, L-niversity of Houston, Houston, Texas Received M a y 11, 1963
The fluorescence spectrum of acridine, the K-heterocyclic analog of anthracene, is presented. The positions of the band maxima do not agree with those previously reported. Justification of the assignment of this emission as the intrinsic r + T * fluorescence of acridine is given. This shows that acridine does not exhibit a lowest lying singlet excited state of n, T * character. Evidence is presented which suggests the possibility that a three component complex, ethanol-acridine-solvent impurity, exists involving two different kinds of weak interactions. The complexes are dirjcussed with regard to their bearing on the problem of acridine's intrinsic fluorescence
Introduction The existence of an intrinsic fluorescence for acridine, the S-heterocyclic analog of anthracene, has always been a subject of some uacertainty. Bertrand2* reported fluorescence from both acridine and its N-hydrochloride salt. Later, HarrellZbpresented the fluorescence of acridine obtained from a solution in methylcyclohexane-ether, frozen a t 77 OK., presumably verifying the r,r* nature of the lowest singlet excited state of acridine. Xevertheless, it has been suggested that acridine may not exhibit an intrinsic fluorescence. 14ccording to Bowen and Sahu13acridine is nonfluorescent in most organic!liquids, and any fluorescencewhich is observed is due to water impurity. Recently, McGlynn and c o - w o r k e r ~in , ~ a~ ~study of photoconductivity, observed no fluorescence for crystalline acridine or for its solutions in benzene. They suggested that acridine may not possess a T,T* lowest singlet excited state but rather an n,7r" lowest singlet excited state. The fluorescence obtained by Harrellzb was attributed to the reversal of the n , r * and next lowest n-,r* singlet excited state by the presence of water as an impurity, Presumably, this wou1.d be similar to the situation in chlorophyll reported by Fer:nandez and Becker.6 The quantum yield of fluorescence of -lop3 for acridine, reported by Sangster and Irvine,' was also attributed to the presence of impurities. The present Atudy involves a careful investigation of the electronic spectral properties of acridine. This work consists primarily of low temperature (77 OK.) (1) (a) Partially supported b y a grant from the Department of Health, Education, and Welfare, No. 3133 BBC, held by Ralph 9. Beaker; (b) National Science Foun'iation Cooperative Graduate Fellow, June, 1960, t o June, 1963. P a r t of a thesis t o be submitted for partial fulfillment of the r~riuirenientsfor the P h . D . degree, University of Houston. (2) (a) D. Bertrand, Bull. S a c . Chim., 1 2 , 1019 (1946): (b) R. W. Harrell, Dissertation Abstr., 2.L, 2476 (1961). ( 3 ) E. J. B o a e n and J. Sahu, J . Chem. Soc., 3716 (1958); E. J. Bowen, N. J. Holder, and G. B. Woodger, J . Phus. Chem., 66, 2491 (1962). (4) S. P. McGlynn, J . Chem. Phys., 37, 1825 (1962). ( 6 ) M. Kleinerman, L. Azarraga, a n d S. P. RTcGlynn, "The Photoconductive a n d Emission Spectroscopic Properties of Organic R/Ioleoular Materials" i n "Luminescence of Organic and Inorganio Materials," ed. b y H. P. Kallman a n d G. M. Spruch, John Wiley a n d Sons. Ino., New York, N. Y . , 1962, p. 196. (6) J. Fernandez and R. S. Beaker, J. Chem. Phys., 3 1 , 467 (1959). (7) R. C. Sangster a n d J. W. Irvine, Jr., i b i d . , 24, 670 (1956).
electronic absorption and emission measurements on acridine, the acridine-ethanol hydrogen bonded systern, and the acridine hydrochloride salt. Special attention has been given to the elimination of water and other impurities from the solvents and chemicals employed in this work. Data will be presented which will establish the fact that acridine has an inherent fluorescence. This wid1 be ascertained by consideration of the new experimentd data coupled with correlative considerations among the different solvent systems noted in the previous paragraph. 41~0,comparisons of the results of our work with those of other investigations will be considered. In addition, evidence is presented for the formation of unusual complexes at low temperature. Though this phenomenon is interesting in its own regard, the precicie nature of the complex has not been established. Hovrever, the phenomenon has been studied to the extent of establishing the exiistence of these complexes and the circumstances under which they arise. Comment will be made concerning their nature and also their bearing on the confusion which has existed with regard to the acridine fluorescence. Experimental The emission and most of the absorption spectra were obtained a t liquid nitrogen temperature (77'K.) in either of two solvent systems which form clear glasses a t this temperature: (1) EPA or (2) EI.8 The isopentane (Phillips instrument grade) and tile ether (Baker's A.R. grade), for the majority of the work, were purified by fractional distillation over calcium hydride. In addition, the isopentane was chromatographed by paseing it through an activated silica gel column after distillation. Specially purified ether was prepared in the following manner. -42 5 7 , by volume middle-cut from the distillation described above was rcLdistilled over sodium shavings. From the second distillatiorl, only a 2 5 7 , by volume middle portion was taken and it was used immediately. The pure, absolute ethanol (U.S.1.) was used without further purification. The low temperature absorption spectra were recorded on a Baush and Lomb Spectronic 505 or on a Beckman Model DK-1 spectrophotometer. The technique employed the use of 5 Pyrex dewar with an unsilvered portion to admit the light beam. A Beckman Model DU was used to perform a Beer's law study of acridine. Emission spectra were photographed on Kodak 103a-B and ( 8 ) D R. Scott a n d J. B illison, J Phys. Chenz
, 66, 561 (1962).
S. JULES LADNER AND RALPHS. BECKER
2482 7
27 WAVENUMBER x 10-3
28
29
Fig. 1.-Absorption
26
spectrum of acridine in E1 (doubly distilled fresh ether) at 77°K. PEAK
t
/
A
I
I
WAVENUMBER x 1 0 - 3 .
Fig. 2.-Absorption spectra of acridine in E1 as a function of solvent age: A, room temperature, no dependence on solvent age; B, 77'K., ether 40 hr. old; C, 77"K., ether 96 hr. old; all concentrations are equal.
WAVENUMBER x 10-3
Fig. 3.-Absorption spectra of acridine hydrochloride in EPA (old solvent): A, room temperature; B, 77°K.; concentrations are equal.
Vol. 67
103a-F spectroscopic plates employing a Hilger medium glass single prism spectrograph. A slit width of 0.1 mm. was used for all measurements. Excit>ing light was supplied by a Osram HBO, 100-watt, high pressure mercury lamp. Wave length selection was controlled by the use of a Baush and Lomb grating monochromator, and, in all cases, a band pass of 20 mp was used. Spectral curves were obtained from the photographic plates by tracing them on an Applied Research Laboratories niicrophotometer. Acridine was obtained from Matheson Coleman and Bell. It was purified by double sublimation under vacuum a t temperatures of approximately 80". The sublimations were carried out without the use of a cold finger to ensure that there would be no recondensation of any water which might have been present in the acridine. A sample of purified acridine was subjected to a gas chromatographic analysis which resulted in only one peak, inferring a very high degree of purity. This is further substantiated by the fact that a sample of zone refined anthracene gave one small impurity peak. All solutions of acridine were prepared immediately before use. Concentrations of approximately 10-4 M were employed for all absorption measurements, and concentrations ranging from to M were used for the emission work. Acridine hydrochloride was prepared by passing anhydrous hydrogen chloride gas (Matheson Coleman and Bell) through a freshly prepared solution of acridine in EPA for a few seconds.
Results Absorption Measurements.-Absorption measurements were made a t various wave lengths within the first electronic absorption band system of acridine in isooctane with concentrations ranging from to 10" M . Within this concentration range, it was found that Beer's law was obeyed. The electronic absorption spectrum of acridine in E1 (doubly distilled ether) frozen a t 77"K., covering the spectral region from 25,000 to 30,000em. -I, is presented in Fig. 1. This spectrum agrees regarding relative intensities and frequencies of absorption bands with that obtained by Wittwer and Zankerg except where these authors omitted the spectrum on the long wave length side of the lowest energy absorption band. The positions of the first few absorption band maxima are compiled in the first row of Table I. Figure 2 shows the variation of the absorption spectrum of acridine in E1 with respect to the age of the solvent. The positions of the absorption band maxima of curve C are presented in the second row of Table I. It is important to note that : (1) the room temperature absorption spectrum is unaffected by the age of the solvent and ( 2 ) the new bands which appear in the low temperature spectrum with increasing solvent age are completely temperature dependent. They appear a t low temperature with old ether and disappear as the temperature is raised or when highly purified fresh ether is used. To determine whether or not the presence of water could be responsible for these new bands, a small amount was added to a solution which had previously exhibited an intermediate intensity of the new absorption bands. This did not result in any change in the absorption spectrum. The electronic absorption spectrum of the acridineethanol hydrogen bonded system in EPA (ether 96 hr. old) in the spectral region 25,000 to 29,400 cm.-l was obtained a t 77°K. The positions of the band maxima are shown in the third row of Table I. The electronic absorption spectrum of acridine hydrochloride in EPA (old) over the spectral region 22,000 to 27,800 cm.-l is shown in Fig. 3. Curve A represents (9) A. Wittwer and V. Zanker, 2. physik. Chem. (Frankfurt), 22, 417 (1959).
FLUORESCENCE SPECTRUM OF ACRIDINE
Nov., 1963
2483
TABLE I POSITIONS O F
ABSORPTION BASD 'MAXIMA O F ACRID IN^, THE ETHANOL HYDROGEN-BONDED SYSTEM, OBTAINED FROM GLASSY SOLUTIONS AT 77°K.
___-_
AND
ACRIDINEHYDROCHLORIDE
Absorption band position, cm.-1
_ _ _ _ _ _ _ I -
Solvent system
1
1'
....
E1 (freuhly purified) E1 (96 hr. old)
3
EPA (old)
25 ,686 25,667
26,281 26,267 26,082
27,078 27 ,056
27,693 27 ,675 27,465
EPA (old)
23,142
23,400
Acridine 26 ,680
5 (max.)
4
3'
2
28,193 28,225 28 ,249
Acridine.HC1
POSITIONS OF THE
FLUORESCENT BAXDMAXIMA CHLORIDE
Solvent sy&tem
Excitation wave length, mp
...
Methylcyclo hexane-ether" E1 (freshly purified) E1 (old) E1 (old) EPA (old) EPA (old)
300 300 395 300 395
EPA (old) EPA (old)
400 435
a
24,533
25,700
TABLE I1 ACRIDINE,THE ETHANOL HYDROGEN-BONDED SYSTEM, OBTaINED FROM GLASSY SOLUTIONS AT 77°K.
----1
25,126 25,680 25,570
OF
2
Fluorescent band poaition, cm. -1 3 3' 4
Acridine 24,631 23,818 24,225 25,290 25 ,070 24,155 23 ,755 25,000 24,025 23,835 Acridine.HC1 22,340 21,350 22,225 21 ,230
23 ,390 (23,510)
22,386 22,880 22,830 22,473 22,620 22,500
AND
ACRIDINEHYDIIJJ-
.-. (5)
6
20 ,939 22,050
20,030 19,710
(21 ,560) 21,100 21,380 21,250 19,055 19,070
Taken from R. W. Harrell.2b
the room temperaturie absorption, and curve I3 represents the spectrum obtained from the rigid glass a t 77 OK. Curve B is very similar to the low temperature spectrum of the acridine cation reported by Wittwer and Z a n k e ~except ,~ for the long wave length ma,ximum at 23,142 cm.-l. The positions of the absorption band maxima are collected in the fourth row of Table I. Emission Measurements.-The fluorescent emission spectrum obtained from a M solution of acridine in E1 (doubly distilled ether) mixture, frozen a t 77"K., is represented in Fig. 4. This measurement was made immediately following the purification of the solvents and corresponds to the sample whose absorption spectrum is presented in Fig. 1. The lorn temperature emission is considered to be moderately strong in view of the short time required to photograph the emission a t narrow d i t widths. It is interesting to note that the intensity of the fluorescence was decreased 50to 100-fold at room temperature. The positions of the principal band maxima are given in the second row of Table 11. Figiure 5 exhibits the variation of the fluorescence spectrum of acridine in E1 (old ether) with respect to two different excitation conditions. The upper curve was obtained by exciting a t 300 mp, while the lower curve employed 395 mp exciting light. Both spectra were recorded on the same photographic plate, using an approximately 2 X lo-* M solution. The positions of the band maxima are compiled in the third and fourth rows of Table 11. The fluorescence spectra of the acridine-ethanol hydrogen bonded system were obtained from a 10-3 M solution in EPA (old) a t 77°K. by exciting a t 300 and 395 mp. Both spectra were taken on the same photographic plate. The positions of the band maxima are shown in the fifth and sixth rows of Table 11. Lastly, the fluorescence spectra of acridine hydrochloride in
EPA (old solvent) .were obtained a t 77°K. from a
M solution by exciting a t 400 and 435 mp. A single photographic plate was employed for both spectra. The positions of the band maxima are shown in the seventh and eighth rows of Table 11. I n all the cases mentioned above, the extreme lorig wave length excitation condition was such that the bandpass overlapped the long wave length half of the leading absorption band of the corresponding low temperature absorption spectrum. We wish to point out that even though the differences involved between the emission spectra obtained a t different excitation conditions are small in some cases, they are always reproducible. This fact establishes their existence. Discussion Zanker'O assigned the first two long wave length transitions of acridine in alcohol, following Platt's nomenclature, 11-12as corresponding to excitation to the ILa and the 'Lb states. I n addition, the fluorescence spectra were given. The absorption maximum at 26,281 cm.-l (Fig. 1) was assigned as the 0-0 lL, transition and the one a t 28,193 cm.-l as the 0-0 of the lLb transition. The lowest energy transtion, *La,tends t o undergo more or less strong red shifts when the molecule is hydrogen bonded, protonated, or the temperature lowered. This feature will form the basis for the majority of the discussion which follows. From a comparison between the spectral curves in Fig. 2, it is obvious that the modification of the absorption spectrum is due to the formation of a new specie at low temperature whose concentration is dependent on (10) V. Zanker, 2.physik. Chsm. (Frankfurt), 2, 52 (1954). (11) J. R. Platt, J . Chem. Phys., 17, 484 (1949). (12) H. B. Klevens and J. R. Platt, ibid., 17, 470 (1949).
2484
S.JULES LADXERAND RALPHS.BECKER
WAVENUMBER
x
Fig. 4.--Fluorescent emission spectrum of acridine in E1 (doubly distilled fresh ether) at 77°K.
>.
c
v,
z W
n
w
+ a a
I I
26
25
Fig. 5.-Fluorescent
23 22 WAVENUMBER x 10-3
24
21
emission spectra of acridine in E1 (old ether) at 77°K.
the age of the solvent. The absorption spectrum of this specie has the following features relative to acridine : (1) the IL, transition has undergone a red shift; (2) the 'Lb transition has undergone little or no shift; (3) the transition probability for the I L b transition has increased by at least a factor of two, with a notable increase in the 0-0 band in the 28,200 cm.-l region. These features suggest that some of the acridine molecules have formed a complex with a solvent impurity at low
Vol. 67
temperature. The concentration of the impurity must increase as the solvent ages, since the intensity of the new absorption bands increases in that manner. As was noted in the Results section, this low-temperature complex does not involve the water molecule. In addition, a room temperature Beer's law study eliminated the possibility of dimer formation. Even though this was not done at lorn temperature, the €act that the spectral modifications are completely dependent upon the age of the solvent eliminates dimer formation as the cause of the change (see Fig. l and 2). Although we are still uncertain of the exact nature of this complex, the most likely possibility is an acridine-peroxide complex. The peroxide is formed from the ether simply by time and/or photoxidation. The absorption spectrum of the acridine-ethanol hydrogen bonded system suggests that this system also undergoes additional complex formation at low temperature. We belie\ve that the second and fourth band maxima at 26,082 and 27,466 crn.-I, respectively, correspond to the hydrogen bonded specie, mhile the first and third bands at 26,667 and 27,056 cm.-l, respectively, result from an additional complex specie (Table I). In this case the additional complex also depends on an impurity and may or may not involve the hydrogen bonded acridine molecule. Our analysis of the spectrum is consistent with Rammensee and Zanker,la who list 26,000 cm.-l for the position of the leading absorption band of acridine in ethanol at -180". Similarly, the spectrum of the hydrochloride salt, Fig. 3, shows a leading absorption maximum at 23,142 cm.-l which is attributed as before to a complex specie which forms at low temperature. We feel that in this case it is certain that the new specie involves the hydrochloride salt and an impurity rather than acridine itself. This becomes obvious when one considers that the band occurs on the long wave length side of a band shifted approximately 3000 cm.-l to the red, which shift !esults specifically upon salt formation. The fluorescence spectra appropriate to each case were also recorded. Acridine in E1 (old) is shown in Fig. 5 . Band positions for the other cases are given in Table I1 and Fig. 6. In each of the three cases, the first fluorescence probably corresponds to acridine, acridine-ethanol, or the hydrochloride. The second fluorescences, resulting from selective excitation into the leading absorption bands, we attribute to the complex species. The differences between each pair of fluorescence spectra as well as the relationships between those of the different cases are shown in Fig. 6. As we have stated previously, eren though the differences are small between some cases, they are completely reproducible. The case of the acridine-ethanol hydrogen bonded system is complicated in that it is difficult to decide what specie is responsible for the shifted absorption and emission. The fact that the leading absorption band a t 25,667 cm.-' coincides with that for acridine in EI, 25,686 cm.-l, suggests that the acridine-impurity complex, rather than a three-component acridine-ethanolimpurity specie, is responsible. However, the fact that a three-component system does exist in the case of acridine hydrochloride certainly insinuates the possibility of the latter. Xevertheless, we can say that in (13) Von H. Rammensee and V. Zanker, Z. Anoew. Phua., l a , 237 (1060).
Nov., 1963
28
2485
FLUORESCEKCE SPECTRUM OF L4CRIDINE
27
7
26
25
I
I
WAVENUMBER x 10-3 24 23 I
I
22
21
20
I
I
1
19
EXCITING 308 mu
395 mu
1
300 mu
I
395 mu
(
C
/
(435 rnui ABSORPTION MAXIMA MAXIMA OF COMPLEX SPECIES FLUORESCENCE MAXIMA
.:, ABSORPTION I
400mu
Sk
A ACRIDINE IN E 1 B ACRIDINE IN E PA C ACRIDINE*HCI IN E PA
SHOULDER Fig. B.---Correlation diagram of the lowtemperature absorption and eniission spectra of acridine and complexes
the case of the hydrochloride salt the interaction with the solvent impurity does not occur via a hydrogen bonding mechanism, since the nitrogen nonbonding electrons of acridine are already involved in the bonding between acridine and the HC1 molecule. This fact implies the possibility that the interaction between the solvent impurity with acridine and, possibly, with the acridine-ethanol hydrogen bonded system is also some nonhydrogen-bonding type interaction. This could, of course, lead to some interesting considerations. We would comment that a three-component system involving two different interactions does not seem unusual in the light of recent work by 811is0ii.~~He reported that “acridine acetate” (acridine in the presence of acetic acid) forms complexes with certain donor-type compounds. He suggests that acridine’s electron affinity is greatly altered by its interaction via the nitrogen atom with acetic acid. The over-all picture of the relationships between the absorption spectra and fluorescence spectra for the different cases can be seen more clearly by referring to the correlation diagram given in Fig. 6. We believe that the moderately strong emission spectrum presented in Fig. 4 represents the inherent fluorescent emission of the acridine molecule. The fluorescence obtained differs from that given by Harrel12b in that it lies approximately 500 cm.-’ farther to the blue (see Table 11). However, the general features of both emission spectra are the same. The difference in solvents could hardly be responsible for the energy separation, since we employed ether-isopentane and Harrell used mt:thylcyclohexane-ether. We will make no further comment on this difference except to say that of the two emissions, our origin lies closest to the corresponding true low temperature absorption origin. To justify the premise that the emission we have obtained is truly intrinsic to acridine, we first wish to mention the high degree of purity of the sample and solvents as earlier described. The low temperature absorption spectrum employing the purified solvents (Fig. 1) was recorded immediately after purification. (14) A. C. Alliaon, Nature, 195, 994 (1962).
I n this spectrum the absorption bands belonging to the acridine-impurity complex did not appear. Immediately following the absorption measurement, the fluorescence spectrum shown in Fig. 4 was recorded. Though more clearly resolved, the intensity of this fluorescence was observed to be equivalent to that obtained from solutions of equal concentration in old solvents when exciting at 300 m l . The two leading bands at 25,680 and 25,290 cm.-l in the fluorescence spectrum obtained using fresh solvents are shifted slightly blue of those in the spectrum obtained with old solvents. This is easily explained by the fact that the leading absorption band of the complex specie, which forms in the solutions made with old solvents, lies 011 top of the 25,680 cm.-l leading fluorescent band (Fig, 6 or compare curve C of Fig. 2 with Fig. 4). The result is strong re-absorption and consequential modification of the two leading fluorescent bands. Other strong evildence in favor of identifying the fluorescence of Fig. 4 as the intrinsic fluorescence of acridine results from a comparison of shifts of the absorption bands with the shifts of the emission bands a13 the solvent is changed from E1 to EPA. The fluorescent emission of the acridine-ethanol hydrogen bonded specie has band maxima at 24,025 and 22,620 which agree with those given by Kochemirovskii and Reznik~va.’~The general features of the fluorescencc match those of the fluorescence spectrum shown in Fig. 4 except for two points: (1) the first band seems to bt: missing in the case of the hydrogen-bonded acridine and (2) the remaining bands are red shifted by approxi-. mately 200 cm.-l (see Fig. 6). The missing band is probably due to re-absorption by the leading absorptiorii band of the impurity complex specie as mentioned above. I n absorption, the two leading bands a t 26,281 and 27,693 cm.-l of acridine are also red shifted ap-, proximately 200 crn.-l to 26,082 and 27,465 ern.-’ for the acridine-ethanol hydrogen-bonded system. This equality of the absorption band shifts with the shifts of (19) A. 8. Koohemirovsku and I. I. Reznikova, Opt. and Spectry., 8, 906 (1960).
KOTES
2486
the emission bands certainly implies that the fluorescence spectrum (Fig. 4) belongs to acridine (Fig. 1). Moreover, the above argument also implies that the acridine mas free from hydrogen bonded water, since, if that were not the case, the addition of ethanol would not be expected to result in further red shifts in the absorption and emission spectra. Finally, we wish to note the mirror image relationship which exists between the fluorescence (Fig. 4) and the portion of the absorption spectrum (Fig. 1) corresponding to the longest wave length transition. This relationship is more easily seen by referring to the absorption spectrum for acridine hydrochloride (Fig. 3) in which more of this transition is exposed. The mirror image relationship would not be expected to exist if emission occurred from a lower lying n,n* state. Thus, the possibility of fluorescence from an n,n* state to the red of the 'La state can be eliminated. The only basis for expecting acridine not to exhibit a ffuorescent emission would be that conversion of the excited molecule to a triplet state or internal conversion were so highly probable that the normal fluorescence could not compete. Under the circumstance, one might expect t o see a reasonable intensity of phosphorescence. However, such is not the case, since experiment has shown the acridine phosphorescence to be very weak. 2b Nevertheless, the argument could be kept intact if a reason for high conversion to a triplet state could be provided, accompanied by a strong internal conversion mode out of the triplet. Such a reason would exist if acridine exhibited a lowest singlet excited state of n, n* character. This usually results in complete conversion to the triplet state. However, recent experimental work by Coppens, Gillet, Nasielski, and DoncktlO places the n,x* state a t 360 mp, %ell above the 0-0 vibrational level of the 1L, state of acridine. Also, this investigation points to the fact that no n,n* state is below a n,n*one. In addition, Bennett1' performed lifetime measurements on the emission of acridine in a water solution and (16) G. Coppens, C. Gillet, J. Nasielaki, and E. V. Donckt, Spectrochim. Acta, 18, 1441 (1962). (17) R. G . Bennett, Rev. Sci. Instr., 81, 1275 (1960).
Vol. 67
observed two exponential decays of 3.8 X see. and 4.3 X sec. He attributed, via chemical evidence, the longer lifetime to the acridine cation and the shorter one to the acridine molecule. Thus, both the negative arguments and the positive results of Bennett give strong indication that an intrinsic fluorescence should exist for acridine. This is, of course, in agreement with the results of this investigation. Conclusion The acridine molecule has a moderately strong inherent fluorescence. We feel that substantial evidence has been provided to justify its identification as such. It has been shown that the intrinsic fluorescence of acridine can be associated with the longest wave length (T+ n*) electronic transition. This and more specifically the mirror image relationship allows us to conclude that acridine does not exhibit a lowest lying n,n* electronic excited state. The extreme sensitivity of acridine to solvent impurities, mentioned above, has been clearly illustrated in this study by the observed formation of complexes a t low temperature between acridine and a solvent impurity other than water. Both the shifted absorption spectra and fluorescence spectra of these complexes have been presented. Such data in the case of acridine hydrochloride strongly suggest that these complexes are not formed via hydrogen bonding type interaction. The acridine hydrochloride case further suggests that for the solutions of acridine in EPA at 77'K., the observed complex specie involves the acridine-ethanol hydrogen-bonded system. This would mean that the three-component complex (acridine, ethanol, and solvent impurity) results from two different weak interactions. Parallel type complexes have been reported earlier. The complexes appear to involve peroxides. Acknowledgments.-We wish to thank Dr. R. W. Harrell of E. I. Dupont for first suggesting that a further study of acridine would be interesting and for helpful discussions concerning it.
NOTES CONFORMATIONS OF ACETANILIDE AND N-METHYLACETANILIDE1 BY H. BRADPORD THOMPSON AXD KARBN M. HALLBERG
relative position of the carbonyl and the aromatic ring should be indicated readily. Results of this study and of EL similar investigation for methylacetanilide are reported in Table I.
Alfred Nobel Science Laboratories, Gustavus Adolphus College, 9t. Peter, Minnesota
TABLE I ELECTRIC DIPOLEMOMENTS
Received April 16, 1968
The question of the stable conformation(s) in acetanilides has interested several investigators.2.B There is evidence that one planar or nearly planar structure predominates.* By comparison of the electric dipole moment of acetanilide with p-bromoacetanilide, the (1) T h e aid of the National Science Foundarion through a basio research grant a n d through a n undergraduate research participation program supporting K. M. H. i s gratefully acknowledged. We wish to thank Dr. Coluroba Curran, who, as referee for this paper, made a pumber of helpful suggestions. (2) J. W. Smith, J. Chen. Soc., 4700 (1961). (3) R. A. Russell a n d H. W. Thompson, Spectrochim. Acta, 8,138 (1956).
Compound
lI.p.,OC.
Aoetanilide p-Bromoaoetanilide h'-Methylacetanilide p-Bromo-N-methylaoetadide Bromobenzene a
Slope for (E
See ref. 7 and 8.
- $)/(e
114' 167.5-167.8 104.0-104 4 97.5- 97.8
+
2 ) (712 See ref. 2.
Y E l e o t r i o momentsSo This study Smithb 102.5 136.2 87.2
36.8 15.0
3.88 4.47 3.57 2.32 1.48
3.65 4.36
+ 2) us. molar concentration.
The acetanilide case has been studied using electric dipole moments' by Smith,2 whose data are included
