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Electronic structure of non-alternant hydrocarbons: Their analogues and derivatives
JOURNAL
OF MOLECULAR
Electronic
SP&ETROSCOPY
Structure Their
XVIII.
The Electronic
30,
66-76 (1969)
of Non-Alternant
Analogues Spectrum
and
Hydrocarbons:
Derivatives
and Electron Affinity
of Fluoranthenel
JOSEFMICHL’ DepartnLent of Chemistry,
University
of Houston,
Houston,
Texas
77004
A comparison of the fine structure of the absorption spectrum of fluoranthene and seven of its simple derivatives recorded at 77°K reveals the presence of seven separate electronic transitions below 43 000 cm-l. Evidence for the existence of three additional transitions in the 43 OO(t50 000 cm-i region is also presented. Gas phase electron affinity is reported. The results are in good agreement with semiempirical T-electron calculations. I. INTRODUCTION
In recent years it has become increasingly apparent that the traditional interpretation (1-S) of the electronic spectrum of fluoranthene (I) is oversimplified. The first indication that the spectrum may contain many more electronic transitions than previously suspected came from semiempirical calcu-
lations by the Pariser-Parr-Pople (PPP) method (4). Corroborating experimental evidence was provided shortly thereafter by measurements of polarized emission (5) and of dichroism of molecules adsorbed in a stretched polyethylene 1Part XVII.: R. Fleischer, K. Hafner, P. Hochmann, J. Wildgruber, R. Zahradnik, Tetrahedron 24, 5943 (1968). 2 On leave from Institute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia. Present address: Department of Chemistry, Aarhus University, Aarhus, Denmark. 66
ELECTRONIC
SPECTRUM
OF FLUORANTHENE
67
foil (6). At the same time, it became clear that it is a very poor approximation to regard fluoranthene as a loose combination of almost noninteracting naphthalene and benzene subunits, certainly in its excited states (5) and probably even in the ground state [see Ref. (7) for references]. Quite to the contrary, the molecule seems to represent a rather interesting non-alternant unit. The present paper reports details of low-temperature absorption measurements on fluoranthene and seven of its simple derivatives. The purpose is to achieve a better understanding of the number and location of the low-lying excited states of fluoranthene. The technique of discerning separate electronic transitions by an investigation of suitable substituent effects is well known [e.g., Refs. (8,9)]. It is based on the observation that vibrational structure of a band is little perturbed by weakly interacting substituents, such as methyl, Jvhile band origins shift measurably. In addition to methyl substitution we have also investigated the effect of 3-aza and 3-methoxy substitutions. Some of the present results were used in a previous discussion on the excited states of fluoranthene (5). II. EXPERIMENTAL
METHODS
Muterids.
Commercial fluoranthene was purified by zone-melting, mp llO.;i”. Small samples of 3-, 7-, and &methylfluoranthenes were kindly donated by Prof. A. Streitwieser, Jr., Chemistry Department, University of California, Berkeley. 3-Azafluoranthene was a kind gift of Prof. C. I?. Koelsch, School of Chemistry, Institute of Technology, University of Minnesota, Minneapolis. Small samples of 1,3- and 2,3_dimethylfluoranthenes and 3-methoxyfluoranthene were obtained by courtesy of Prof. G. Eglinton, Chemistry Department, The University, Glasgow, from collections of Prof. S. H. Tucker. 3-Methylpentane (Phillips Petroleum Co.) was dried and distilled before use. Measurements. Absorption spectra were taken on Cary 15 and Beckman DK-1 instruments using a quartz Delvar vessel with flat windows and a quartz sample cell. Each measurement n-as repeated several times at several concentrations. This is important if as many fine features as possible are to be discerned. Room temperature spectra of the samples were always run for comparison. Emission spectrum of fluoranthene was recorded using a Bausch and Lomb monochromator for exciting light and an Aminco scanning monochromator (slits 0.04 mm). The front surface arrangement was used to minimize reabsorption. The spectrum was corrected for photomultiplier response. It was independent of the wavelengthof the exciting light. Gas-phase electron affinity was measured by the electroncapture method described elsewhere [Ref. (10) and references therein]. III.
RESULTS
ANI) DISCUSSION
ELECTRONIC SPECTRUM The room-temperature spectrum of fluoranthene is shown in Fig. 1 which also shows the O-O components and vibrat,ional structure of electronic transitions as
6s
MICHL
1
ok-_ 50 000
I
I
45 000
II
I
II,
I
40 000
I
I ‘I 35 000
I
I
I
B(cm7
3ocQo
FIG. 1. Room-temperature absorption spectrum of fluoranthene in 3-methylpentane. Below, suggested separation into individual transitions. On top, calculated frequencies of electronic transitions (calculated oscillator strength larger than 0.1 for thick lines, smaller than 0.1 for thin lines).
assigned in the present work. Figure 2 shows the low-temperature emission spectrum of fluoranthene and the onset of its low-temperature absorption. Positions of peaks and shoulders in the low-temperature absorption spectra of the eight compounds measured are presented schematically in Fig. 3 along with proposed simple vibrational analysis. Miniature spectral absorption curves for 1,3-dimethylfluoranthene and 3-methoxyfluoranthene are shown as examples. Figure 4 gives similar data for the 40 000-50 000 cm-’ region recorded at room temperature, background absorption at these wavelengths being too strong at 77°K. In the case of 2,3-dimethylfluoranthene where low-temperature measurement gave a substantially better resolution both results are shown. Miniature spectra of 3-methylfluoranthene and 7-methylfluoranthene are also presented to give a rough idea of the actual spectral curves. Information on the excited states of fluoranthene is summarized in Table I. In the following the individual electronic transitions are discussed starting from that of lowest energy. They are labeled as in Fig. 1. Transition I. The room-temperature spectrum of fluoranthene in nonpolar solvents (Fig. 1) starts with several indistinct shoulders near 25 000 cm-’ just on the onset of a seemingly simple absorption band located between 25 000 and
ELECTRONIC
SPECTRUM
OF FLUORANTHENE
69
1
25 000
G km”
20000
FIG. 2. Part of the absorption (left) and fluorescence emission (right) spectra of fluoranthene in 3-methylpentane glass at 77°K. Concentration m10v4 IV. Front surface excitation (X,X, = 356 mp). Corrected for phot,omultiplier response. 35 000 cm-‘. Little attention had been paid to the shoulders until PPP-type calculations predicted (4) that there should be three electronic transitions in the region of the apparently simple band. It was then suggested that the shoulders are due to a separate weak electronic transition into the lowest excited singlet state. This tentative assignment was supported by the large difference between the wavelength of the first well-developed maximum in the absorption spectrum (27 800 cm-‘) and the first maximum in the fluorescence spectrum [24 900 cm-’ (4)]. No conclusive evidence concerning the existence of a separate transition around 25 000 cm-l was obtained from experiments using stretched polyethylene foils for measurements of dichroism (6) because of the low intensity of absorption in this region. Polarized emission studies (6) supported such an assignment but it was felt that a more unambiguous proof was needed. It is proposed that the present results (Figs. 2, 3) provide a firm basis for the claim that the shoulder corresponds to a separate electronic transition. At 77”K, the fine structure of the shoulder is partially resolved and a simple vibrational
70
MICHL
5 (cm’)
FIG. 3. Positions of peaks (full lines) and shoulders (dotted lines) in the absorption spectra of fluoranthene and its derivatives in 3-methylpentane glass at 77°K. Proposed simple vibrational analysis and positions of 0 components of transitions l-7 (heavy dots) are indicated. Full spectral curves of 3-methoxyfluoranthene and 1,3-dimethylfluoranthene are shown in arbitrary optical density units. For abbreviations see Table I.
3-MeO-FL 3-N-FL &Me-FL T-Me-FL 3-Me-FL 23-Me2-Fi 1,3-Me-F-FL FL
50 000
45 000
ii (cm” 1
40 a?0
FIG. 4. Positions of peaks (full lines) and shoulders (dotted lines) in the absorption spectra of fluoranthene and its derivatives in 3-methylpentane at room temperature. Proposed simple vibrational analysis and positions of 0 components of transitions 7-9 (heavy dots) are indicated. Full spectra of 3-methylfluoranthene and ‘I-methylfluoranthene are shown in arbitrary optical density units. For abbreviations see Table I. The three short lines in the region of transitions 8 and 9 for the 2,3-dimethyl derivative correspond to peaks in a spectrum taken at 77°K.
for fluor
Fluorantheneb Fluoranthene 1,3-Dimethyl2,3-Dimethyl3-Met,hyl7-Methyl&Methyl3-Aza 3-Methoxy-
lZ<’
Ej
-
ei -
ej -
Jij + 2 k’ii”
Configuration
(wt.)
Transition energy Oscillator strength Polarixationd Predominant configuration in excited state (wt.) Other important configurations
Fl Fl 1,3-Me,2,3-Me%3-Me7-MeS-Me3-N3-MeO-
-__
TABLE
I
-1
28 640 52 TGO
24
(15c/;#)
(83% ) 3+ -1
2G 790 0.014 _L 2--, -1
24 670 24 720 -330 -510 - 270 -210 - 540 -880 -620
1
-1
29 GlO 49 620
l+
cosi;.)
II -1 I
-1
-1
axis.
34 930 58 740
3+
(55%;) I+ -3 (“8:: j
3+
31 710 0.070
28 320 0.52 I-
30 910 31 010 -330 -390 -340 0 -100 l-10
3
27 8-10 27 820 -870 -500 -270 -G50 -1GO 470 -350
2
40 340 G2 120
44
-1
-3 43 810 G7 130
2+
Ii
-2 42 030 G3 140
l+
(904; )
l--t
40 GGO 63 980 41 390 GG 020
-3
1+ -3 (66”; ) 3-t -1 (277:))
I
43 490 1.28
2--t -2
-1 (4OYl>) 2+ -2 (36:;,)
i+
I II 2+ -3 (GO:: ) Many
II 4+ -1 (489;, ) 2+ -2 (33% )
l--1 -2
42 -140 0.00
(1460) -700 -Ll 390 0.053
xi
7 42 460 42 190 -950 - 780 -350 -260 -170
-10 990 0.010
6
l-10 0.22
5
~~~ ~~ 39 530 39 370 -80 -4GO -150 -80 - 70 950 - 540
34 840 34 780 -420 -710 -420 -180 -300 500 -1110
4 38 170 38 100 - 220 -790 -430 -510 -430 210 -790
.
Excited state
TR.+WSJU~NS AND SUDS.I'ITUI:NTSHIFTS, AND C.\I,CULATI;D TR.\NSITION ENERGIES
1)77X, 3.methylpentane (cm-l). b Room t,emp (Fig. 1). c Energies (cm-l). Parameter set D of Ref. (5). d Perpendicular (I) or parallel (11) to the molecular t,wofold symmetry e Energy of the configuration given immediately above. f Orbit,al energy difference for the same configuration as ill footnote e.
Calculated” (CI SCF) anthene
Experimental”
Compound
FI.TOR.~~TII~:N~;:EXPCRIM~:X.I..~I.ENCRGIISS0~0-0
72
MICHL
analysis is possible. The vibrational group moves together but differently from other bands in the spectrum when a substituent is introduced into the molecule and the fine structure remains virtually unchanged. Moreover the first (O-O) vibrational component coincides almost exactly with the first (O-O) vibrational component of fluorescence. The fine structure of the fluorescence bears an approximate mirror-image relationship to that of the “shoulders” up to the point where a more intense absorption band sets in. On the other hand, it is not related to the vibrational structure of the intense band. Transition 1 resembles the La transition in benzenoid hydrocarbons by its weak intensity (log E A 2), increasing somewhat on methyl substitution and strongly on methoxy and aza substitution. In low-temperature spectra, the O-O component of band 1 is a very sharp peak except in 3methylfluoranthene and 2,3_dimethylfluoranthene where it is a poorly developed shoulder 2W-360 cm-r below the first peak. To check whether the shoulder was not due to an impurity we have compared the low-temperature absorption and emission spectra (Aexo = 356 mp). In both cases the emission started with a shoulder coinciding almost exactly with the absorption shoulder assigned as the M) transition. The occurrence of a low-frequency vibration in the fine structure of band 1 is not unique. A very similar observation made on band 2 in the case of other derivatives is mentioned below.
Transitions 2 and 3. When the PPP calculations on fluoranthene were interpreted (4) there still remained two electronic transitions predicted for the region of the apparently simple first absorption band. The peak at 27 800 cm-’ was taken for the O-O vibrational component of one of these and it was noted that the fourth peak in the room temperature absorption spectrum might belong to an independent transition rather than being a part of the vibrational structure of the band starting at 27 800 cm-‘. This possibility was suggested on the basis of a comparison of the solvent shifts of the individual peaks in the spectrum and by a comparison of the shape of the spectrum of fluoranthene with that of some fluoranthenecarboxylic acids. Measurements of dichroism on stretched polyethylene foils (6) indeed showed that the intense absorption band is a composite and consists of two transitions of opposite polarization. The study of polarized emission led to similar conclusions (5). The present results (Fig. 3) provide additional strong evidence for the composite nature of what was originally thought to be the first absorptionbandof fluoranthene. The first two prominent peaks are vibrational components of transition 2, and the second two belong to transition 3. The positions and intensities of the two groups of peaks are affected differently by substituents. In most derivatives, but not in fluoranthene itself, both major peaks in transition 2 are much broader than those of other transitions. In some compounds (e.g., 1,3-dimethylfluoranthene, Fig. 3), each is actually partially split into two components of
ELECTRONIC
SPECTRUM
73
OF FLUORANTHENE
comparable intensity.3 Apparently, the low-frequency vibrational band (-500 cm-‘) which appears as the weak shoulder between the two major peaks of transition 2 and as the barely visible shoulder at about 30 000 cm-l in the room-temperature spectrum of fluoranthene (Fig. 1) can appear with greater intensity in the spectra of the derivatives. The diffuse nature and varying details of shape then decrease the accuracy with which energy of the O-O band can be read. We believe that this feature of transition 2 and the existence of transition 3 also account for our earlier observation (11) that the first four major peaks in the spectra of the five ammonio derivatives of fluoranthene are all shifted with respect to the parent, but each independently of the other three. Protonated aminofluoranthenes were not suitable for a detailed study of the present type since the shifts were very small and the polar solvent which had to be used obliterated fine structure. Because of the broadening of peaks of transition 2, the assignment of the SO band in 3-methoxyfluoranthene and in 7-methylfluoranthene as shown in Fig. 3 is only tentative. In the former compound, the intense shoulder at 27 000 cm-’ could be the origin of transition 2 rather than belonging to the fine structure of transition 1. In the latter, the shoulder assigned as the band origin in Fig. 3 could be a part of the fine structure of transition 1. The assignment shown in Fig. 3 is based only on intensity ronsiderations.
Tmnsition 4. This intense band has the best developed vibrational structure of all the bands and it fits a very regular vibrational sequence (Fig. 3). It has always been recognized as a separate transition; it is traditionally called the p band (I> 5). Transitions 5 and 6. The fine structure in this region of low absorption (37 000 41 000 cm-‘) is rather poorly developed. We were unable to understand it in terms of one electronic transition (assuming that the frequency of the vibrations involved is less than about 1700 cm-‘). Therefore, we assigned tentatively two transitions 5 and 6 in this region. Such an assignment would be in agreement with results obtained using the stretched foil technique (6) [cf. Ref. (5)]. Transition 7. This very intense band rises sharply from the preceding region of low absorption and its long-wavelength tail is responsible for much of the blurring of the fine structure of band 6. Transitions 8, 9, and 10. At room temperature our measurements could be extended slightly above 50 000 cm-’ (Figs. 1, 4). Lack of resolution makes an analysis much more difficult. Still, the existence of at least three additional transitions in this region seems beyond doubt. We are puzzled by the insensitivity of the transition energy of band 10 to substitution but a check of baselines as well as agreement between the two spectrophotometers used suggest that this is real. 3 In view of the high intensity of absorption is due to an impurity
such as an isomeric
seen on the other absorption and doubling t,hree isomers.
peaks. Moreover,
in room-temperature
in this region it is unlikely that the doubling
derivative,
observed
since no such effect is a similar broadening
spectra of l- and 7-fluorofluoranthenes
In this case, we had enough material
tored by vapor-phase
particularly
we recently
chromatography.
for a very thorough
but not the other purification
moni-
74
MICHL
In 2,3_dimethylfluoranthene the peak at 44 000 cm-’ could belong to transition 7 or maybe it is the &O band of transition 8. Table I presents a summary of energies of GO vibrational components of all bands assigned in this paper and results of calculations by the semiempirical CI SCF method. These were repeated several times using different approximations for the electron repylsion integrals [other parameters as in Ref. (4), all bond lengths equal to 1.4 A]. All lead to the same order of the lower 4-5 excited states, identical with that suggested earlier (4,5). The order of the more closely spaced higher excited states is more sensitive to changes in parameters. The MatagaNishimoto approximation (12) gives the best quantitative agreement and the results are shown in Fig. 1 and Table I. Since our results confirmed the previous assignment of electronic transitions in fluoranthene which was shown to be in good agreement with semiempirical calculations, we shall not repeat the discussion here. We only wish to point out that the tabulated experimental energies correspond to O-O bands while the calculated ones should correspond to vertical transitions. It is perhaps interesting to note that the results of calculations contradict the often accepted notion that the order of excited states is given by orbital energy differences unless strong configuration interaction is involved as in benzenoid hydrocarbons where first order configuration interaction often pushes the Lb state below the L, state. In fluoranthene, the lowest excited state 1 corresponds to an almost pure 2 -+ - 1 configuration and the state 2 described by an almost pure 1 -+ - 1 configuration has higher energy, although the orbital energy difference for 1 -+ - 1 is of course smaller. This can happen because in the SCF approximation the excitation energy AEi+j depends not only on the energies of the molecular orbitals involved E; , ej , but also on Jij , the Coulomb integral between the ith and jth orbitals, and Kij , the corresponding exchange integral: AE = cj - pi - Jij + 2Kij e These integrals are by no means negligible and their value may vary for different choices of i, j within one molecule (Table I) and from one molecule to another (18. We were wrong in our earlier expectation (11) that the shift of the 1 -+ -1 band of fluoranthene (transition 2) on aza substitution could be predicted by a straightforward application of first order perturbation theory. A bathochromic shift is predicted for 3-aza substitution, a hypsochromic one is found. According to preliminary calculations, the error is due to the neglect of configuration interaction and shall be discussed elsewhere.
ELECTRONAFFINITY We are interested in electron affinities (EA) of nonalternant hydrocarbons particulary in connection with investigations of relations of orbital energy differ-
ELECTRONIC
SPECTRUM
OF FLUORANTHENE
75
ences to excitation energies and of the applicability of the simple Hiickel molecular orbital (HMO) method to spectral predictions (15). We have therefore measured the gas phase EA of fluoranthene by the method of Wentworth and Becker [common intercept, for references see Ref. (IO)] and found a rather high value of 0.63 eV, similar to that of azulene (0.66 eV; cf. nsphthalene, 0.15 eV; phenttnthrene, 0.31 eV). According to HMO calculations (14) high EA should be typical for fluoranthene-like hydrocarbons. In the case of fluoranthene, Sffy” = - 0.376, and use of the regression line of Ref. (10) gives an EA prediction of 0.70 eV. Our semiempirical SCF calculations gave energies for the lowest free molecular orbital (LF-\IO) of fluoranthene which were almost exactly equal to those of the IFNO of azulene. Depending on the approximation for electron-repulsion in tegrals used, the calculated electron affinity of fluoranthene \\-as from 0.07 eV lower to 0.02 eV higher than that calculated for azulene with the same parameters. The experimental value is 0.03 eV lower than that of azulene so that the agreement is quite satisfactory. Recently :\ value of 0.92 eV has been predicted (15) for the gas phase EA of fluoranthene from potentiometric measurements on radical ion solutions. The prediction was made assuming a unit slope for the plot of EA in gas phase against those in solution although a line of unit slope clearly was not the line of the best fit for the data available. Using a line of less than unit slope, giving a better fit for the existing data, one also predicts more correctly the value of gas phase EA foi fluoranthene. A part of the discrepancy may be due to our use of the common intercept approximation in the determination of gas phase EA required by inst,ru mental limitations. Nevertheless, it seems to us that a line of less than :L unit slope is required for correlating solution with gas phase EA. ACKNOWLEDGMENT This work was made possible by the kindness of Professors A. Streitwieser, Jr., C. F. Koelsch, and G. Eglinton; I wish to express my sincere thanks to them for generous gifts of samples. I am further obliged to Professor R. X. Becker of the University of Houston for permitting me to use the facilities of his laboratory and both to him and Dr. E. Chen of the University of Houston for valuable discussions. RECEIVED:
July
22,
196s REFERENCES
1. H. W. D. SJVBBS AND S. H. TUCKER, J. Chem. Sec. 1964, 227. 2. H. H. JAFF$, M. ORCHIN, “Theory and Applications of Ultraviolet Spectroscopy,” p. 343. Wiley, New York, 1962. 3. E. CLAR, “Polycyclic Hydrocarbons,” Vol. 1, p. 94. Academic Press, New York, 1964. 4. J. KOUTECS+, P. HOCHMANN,zi~~ J. MICHL, J. Chem. Phys. &I,2439 (1964). 5. E. HEILBRONNXR, J.-P. WEBXR, J. MICHL, .~ND R. ZAHRAnNfK, Theoret. Chim. Llcta 6, 141 (1966). 6. E. W. THULSTRUP AND J. H. EG~FXZS, Chem. Phys. Letters 1, 690 (1968); cf. lecture at 8th European Congr. Mol. Spectry., Copenhagen (1965).
76
MICHL
7. J. MICHL AND R. ZAHRADNfK, Collection Czech. Chem. Commun. a,3478 (1966). 8. J. L. PATENAUDE, P. SAUVAGEAU, AND C. SANDORFY, Spectrochim. Acta 18,241 (1962) and references therein. 3. R. S. BECKER, I. S. SINGH, AND E. A. JACKSON, J. Chem. Phys. 38,2144 (1963). 10. R. S. BECKER, AND E. CHEN, J. Chem. Phys. 46,2403 (1966). 11. J. MICHL, Spectrochim. Acta 21.2146 (1965). 1.2. N. MATAGA AND K. NISHIMOTO, 2. Physik. Chem. (Frankjurt) 13,140 (1957). 13. J. MICHL AND R. S. BECKER, J. Chem. Phys. 46.3889 (1967). 1.j. R. ZAHRmNfK AND J. MICHL, Collection Czech. Chem. Commun. 31.3442 (1966). 16. J. CHATJDHURI,J. JAGUR-GRODZINSKI, AND M. SZWARC, J. Phys. Chem. 7l, 3063 (1967).
OF MOLECULAR
Electronic
SP&ETROSCOPY
Structure Their
XVIII.
The Electronic
30,
66-76 (1969)
of Non-Alternant
Analogues Spectrum
and
Hydrocarbons:
Derivatives
and Electron Affinity
of Fluoranthenel
JOSEFMICHL’ DepartnLent of Chemistry,
University
of Houston,
Houston,
Texas
77004
A comparison of the fine structure of the absorption spectrum of fluoranthene and seven of its simple derivatives recorded at 77°K reveals the presence of seven separate electronic transitions below 43 000 cm-l. Evidence for the existence of three additional transitions in the 43 OO(t50 000 cm-i region is also presented. Gas phase electron affinity is reported. The results are in good agreement with semiempirical T-electron calculations. I. INTRODUCTION
In recent years it has become increasingly apparent that the traditional interpretation (1-S) of the electronic spectrum of fluoranthene (I) is oversimplified. The first indication that the spectrum may contain many more electronic transitions than previously suspected came from semiempirical calcu-
lations by the Pariser-Parr-Pople (PPP) method (4). Corroborating experimental evidence was provided shortly thereafter by measurements of polarized emission (5) and of dichroism of molecules adsorbed in a stretched polyethylene 1Part XVII.: R. Fleischer, K. Hafner, P. Hochmann, J. Wildgruber, R. Zahradnik, Tetrahedron 24, 5943 (1968). 2 On leave from Institute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia. Present address: Department of Chemistry, Aarhus University, Aarhus, Denmark. 66
ELECTRONIC
SPECTRUM
OF FLUORANTHENE
67
foil (6). At the same time, it became clear that it is a very poor approximation to regard fluoranthene as a loose combination of almost noninteracting naphthalene and benzene subunits, certainly in its excited states (5) and probably even in the ground state [see Ref. (7) for references]. Quite to the contrary, the molecule seems to represent a rather interesting non-alternant unit. The present paper reports details of low-temperature absorption measurements on fluoranthene and seven of its simple derivatives. The purpose is to achieve a better understanding of the number and location of the low-lying excited states of fluoranthene. The technique of discerning separate electronic transitions by an investigation of suitable substituent effects is well known [e.g., Refs. (8,9)]. It is based on the observation that vibrational structure of a band is little perturbed by weakly interacting substituents, such as methyl, Jvhile band origins shift measurably. In addition to methyl substitution we have also investigated the effect of 3-aza and 3-methoxy substitutions. Some of the present results were used in a previous discussion on the excited states of fluoranthene (5). II. EXPERIMENTAL
METHODS
Muterids.
Commercial fluoranthene was purified by zone-melting, mp llO.;i”. Small samples of 3-, 7-, and &methylfluoranthenes were kindly donated by Prof. A. Streitwieser, Jr., Chemistry Department, University of California, Berkeley. 3-Azafluoranthene was a kind gift of Prof. C. I?. Koelsch, School of Chemistry, Institute of Technology, University of Minnesota, Minneapolis. Small samples of 1,3- and 2,3_dimethylfluoranthenes and 3-methoxyfluoranthene were obtained by courtesy of Prof. G. Eglinton, Chemistry Department, The University, Glasgow, from collections of Prof. S. H. Tucker. 3-Methylpentane (Phillips Petroleum Co.) was dried and distilled before use. Measurements. Absorption spectra were taken on Cary 15 and Beckman DK-1 instruments using a quartz Delvar vessel with flat windows and a quartz sample cell. Each measurement n-as repeated several times at several concentrations. This is important if as many fine features as possible are to be discerned. Room temperature spectra of the samples were always run for comparison. Emission spectrum of fluoranthene was recorded using a Bausch and Lomb monochromator for exciting light and an Aminco scanning monochromator (slits 0.04 mm). The front surface arrangement was used to minimize reabsorption. The spectrum was corrected for photomultiplier response. It was independent of the wavelengthof the exciting light. Gas-phase electron affinity was measured by the electroncapture method described elsewhere [Ref. (10) and references therein]. III.
RESULTS
ANI) DISCUSSION
ELECTRONIC SPECTRUM The room-temperature spectrum of fluoranthene is shown in Fig. 1 which also shows the O-O components and vibrat,ional structure of electronic transitions as
6s
MICHL
1
ok-_ 50 000
I
I
45 000
II
I
II,
I
40 000
I
I ‘I 35 000
I
I
I
B(cm7
3ocQo
FIG. 1. Room-temperature absorption spectrum of fluoranthene in 3-methylpentane. Below, suggested separation into individual transitions. On top, calculated frequencies of electronic transitions (calculated oscillator strength larger than 0.1 for thick lines, smaller than 0.1 for thin lines).
assigned in the present work. Figure 2 shows the low-temperature emission spectrum of fluoranthene and the onset of its low-temperature absorption. Positions of peaks and shoulders in the low-temperature absorption spectra of the eight compounds measured are presented schematically in Fig. 3 along with proposed simple vibrational analysis. Miniature spectral absorption curves for 1,3-dimethylfluoranthene and 3-methoxyfluoranthene are shown as examples. Figure 4 gives similar data for the 40 000-50 000 cm-’ region recorded at room temperature, background absorption at these wavelengths being too strong at 77°K. In the case of 2,3-dimethylfluoranthene where low-temperature measurement gave a substantially better resolution both results are shown. Miniature spectra of 3-methylfluoranthene and 7-methylfluoranthene are also presented to give a rough idea of the actual spectral curves. Information on the excited states of fluoranthene is summarized in Table I. In the following the individual electronic transitions are discussed starting from that of lowest energy. They are labeled as in Fig. 1. Transition I. The room-temperature spectrum of fluoranthene in nonpolar solvents (Fig. 1) starts with several indistinct shoulders near 25 000 cm-’ just on the onset of a seemingly simple absorption band located between 25 000 and
ELECTRONIC
SPECTRUM
OF FLUORANTHENE
69
1
25 000
G km”
20000
FIG. 2. Part of the absorption (left) and fluorescence emission (right) spectra of fluoranthene in 3-methylpentane glass at 77°K. Concentration m10v4 IV. Front surface excitation (X,X, = 356 mp). Corrected for phot,omultiplier response. 35 000 cm-‘. Little attention had been paid to the shoulders until PPP-type calculations predicted (4) that there should be three electronic transitions in the region of the apparently simple band. It was then suggested that the shoulders are due to a separate weak electronic transition into the lowest excited singlet state. This tentative assignment was supported by the large difference between the wavelength of the first well-developed maximum in the absorption spectrum (27 800 cm-‘) and the first maximum in the fluorescence spectrum [24 900 cm-’ (4)]. No conclusive evidence concerning the existence of a separate transition around 25 000 cm-l was obtained from experiments using stretched polyethylene foils for measurements of dichroism (6) because of the low intensity of absorption in this region. Polarized emission studies (6) supported such an assignment but it was felt that a more unambiguous proof was needed. It is proposed that the present results (Figs. 2, 3) provide a firm basis for the claim that the shoulder corresponds to a separate electronic transition. At 77”K, the fine structure of the shoulder is partially resolved and a simple vibrational
70
MICHL
5 (cm’)
FIG. 3. Positions of peaks (full lines) and shoulders (dotted lines) in the absorption spectra of fluoranthene and its derivatives in 3-methylpentane glass at 77°K. Proposed simple vibrational analysis and positions of 0 components of transitions l-7 (heavy dots) are indicated. Full spectral curves of 3-methoxyfluoranthene and 1,3-dimethylfluoranthene are shown in arbitrary optical density units. For abbreviations see Table I.
3-MeO-FL 3-N-FL &Me-FL T-Me-FL 3-Me-FL 23-Me2-Fi 1,3-Me-F-FL FL
50 000
45 000
ii (cm” 1
40 a?0
FIG. 4. Positions of peaks (full lines) and shoulders (dotted lines) in the absorption spectra of fluoranthene and its derivatives in 3-methylpentane at room temperature. Proposed simple vibrational analysis and positions of 0 components of transitions 7-9 (heavy dots) are indicated. Full spectra of 3-methylfluoranthene and ‘I-methylfluoranthene are shown in arbitrary optical density units. For abbreviations see Table I. The three short lines in the region of transitions 8 and 9 for the 2,3-dimethyl derivative correspond to peaks in a spectrum taken at 77°K.
for fluor
Fluorantheneb Fluoranthene 1,3-Dimethyl2,3-Dimethyl3-Met,hyl7-Methyl&Methyl3-Aza 3-Methoxy-
lZ<’
Ej
-
ei -
ej -
Jij + 2 k’ii”
Configuration
(wt.)
Transition energy Oscillator strength Polarixationd Predominant configuration in excited state (wt.) Other important configurations
Fl Fl 1,3-Me,2,3-Me%3-Me7-MeS-Me3-N3-MeO-
-__
TABLE
I
-1
28 640 52 TGO
24
(15c/;#)
(83% ) 3+ -1
2G 790 0.014 _L 2--, -1
24 670 24 720 -330 -510 - 270 -210 - 540 -880 -620
1
-1
29 GlO 49 620
l+
cosi;.)
II -1 I
-1
-1
axis.
34 930 58 740
3+
(55%;) I+ -3 (“8:: j
3+
31 710 0.070
28 320 0.52 I-
30 910 31 010 -330 -390 -340 0 -100 l-10
3
27 8-10 27 820 -870 -500 -270 -G50 -1GO 470 -350
2
40 340 G2 120
44
-1
-3 43 810 G7 130
2+
Ii
-2 42 030 G3 140
l+
(904; )
l--t
40 GGO 63 980 41 390 GG 020
-3
1+ -3 (66”; ) 3-t -1 (277:))
I
43 490 1.28
2--t -2
-1 (4OYl>) 2+ -2 (36:;,)
i+
I II 2+ -3 (GO:: ) Many
II 4+ -1 (489;, ) 2+ -2 (33% )
l--1 -2
42 -140 0.00
(1460) -700 -Ll 390 0.053
xi
7 42 460 42 190 -950 - 780 -350 -260 -170
-10 990 0.010
6
l-10 0.22
5
~~~ ~~ 39 530 39 370 -80 -4GO -150 -80 - 70 950 - 540
34 840 34 780 -420 -710 -420 -180 -300 500 -1110
4 38 170 38 100 - 220 -790 -430 -510 -430 210 -790
.
Excited state
TR.+WSJU~NS AND SUDS.I'ITUI:NTSHIFTS, AND C.\I,CULATI;D TR.\NSITION ENERGIES
1)77X, 3.methylpentane (cm-l). b Room t,emp (Fig. 1). c Energies (cm-l). Parameter set D of Ref. (5). d Perpendicular (I) or parallel (11) to the molecular t,wofold symmetry e Energy of the configuration given immediately above. f Orbit,al energy difference for the same configuration as ill footnote e.
Calculated” (CI SCF) anthene
Experimental”
Compound
FI.TOR.~~TII~:N~;:EXPCRIM~:X.I..~I.ENCRGIISS0~0-0
72
MICHL
analysis is possible. The vibrational group moves together but differently from other bands in the spectrum when a substituent is introduced into the molecule and the fine structure remains virtually unchanged. Moreover the first (O-O) vibrational component coincides almost exactly with the first (O-O) vibrational component of fluorescence. The fine structure of the fluorescence bears an approximate mirror-image relationship to that of the “shoulders” up to the point where a more intense absorption band sets in. On the other hand, it is not related to the vibrational structure of the intense band. Transition 1 resembles the La transition in benzenoid hydrocarbons by its weak intensity (log E A 2), increasing somewhat on methyl substitution and strongly on methoxy and aza substitution. In low-temperature spectra, the O-O component of band 1 is a very sharp peak except in 3methylfluoranthene and 2,3_dimethylfluoranthene where it is a poorly developed shoulder 2W-360 cm-r below the first peak. To check whether the shoulder was not due to an impurity we have compared the low-temperature absorption and emission spectra (Aexo = 356 mp). In both cases the emission started with a shoulder coinciding almost exactly with the absorption shoulder assigned as the M) transition. The occurrence of a low-frequency vibration in the fine structure of band 1 is not unique. A very similar observation made on band 2 in the case of other derivatives is mentioned below.
Transitions 2 and 3. When the PPP calculations on fluoranthene were interpreted (4) there still remained two electronic transitions predicted for the region of the apparently simple first absorption band. The peak at 27 800 cm-’ was taken for the O-O vibrational component of one of these and it was noted that the fourth peak in the room temperature absorption spectrum might belong to an independent transition rather than being a part of the vibrational structure of the band starting at 27 800 cm-‘. This possibility was suggested on the basis of a comparison of the solvent shifts of the individual peaks in the spectrum and by a comparison of the shape of the spectrum of fluoranthene with that of some fluoranthenecarboxylic acids. Measurements of dichroism on stretched polyethylene foils (6) indeed showed that the intense absorption band is a composite and consists of two transitions of opposite polarization. The study of polarized emission led to similar conclusions (5). The present results (Fig. 3) provide additional strong evidence for the composite nature of what was originally thought to be the first absorptionbandof fluoranthene. The first two prominent peaks are vibrational components of transition 2, and the second two belong to transition 3. The positions and intensities of the two groups of peaks are affected differently by substituents. In most derivatives, but not in fluoranthene itself, both major peaks in transition 2 are much broader than those of other transitions. In some compounds (e.g., 1,3-dimethylfluoranthene, Fig. 3), each is actually partially split into two components of
ELECTRONIC
SPECTRUM
73
OF FLUORANTHENE
comparable intensity.3 Apparently, the low-frequency vibrational band (-500 cm-‘) which appears as the weak shoulder between the two major peaks of transition 2 and as the barely visible shoulder at about 30 000 cm-l in the room-temperature spectrum of fluoranthene (Fig. 1) can appear with greater intensity in the spectra of the derivatives. The diffuse nature and varying details of shape then decrease the accuracy with which energy of the O-O band can be read. We believe that this feature of transition 2 and the existence of transition 3 also account for our earlier observation (11) that the first four major peaks in the spectra of the five ammonio derivatives of fluoranthene are all shifted with respect to the parent, but each independently of the other three. Protonated aminofluoranthenes were not suitable for a detailed study of the present type since the shifts were very small and the polar solvent which had to be used obliterated fine structure. Because of the broadening of peaks of transition 2, the assignment of the SO band in 3-methoxyfluoranthene and in 7-methylfluoranthene as shown in Fig. 3 is only tentative. In the former compound, the intense shoulder at 27 000 cm-’ could be the origin of transition 2 rather than belonging to the fine structure of transition 1. In the latter, the shoulder assigned as the band origin in Fig. 3 could be a part of the fine structure of transition 1. The assignment shown in Fig. 3 is based only on intensity ronsiderations.
Tmnsition 4. This intense band has the best developed vibrational structure of all the bands and it fits a very regular vibrational sequence (Fig. 3). It has always been recognized as a separate transition; it is traditionally called the p band (I> 5). Transitions 5 and 6. The fine structure in this region of low absorption (37 000 41 000 cm-‘) is rather poorly developed. We were unable to understand it in terms of one electronic transition (assuming that the frequency of the vibrations involved is less than about 1700 cm-‘). Therefore, we assigned tentatively two transitions 5 and 6 in this region. Such an assignment would be in agreement with results obtained using the stretched foil technique (6) [cf. Ref. (5)]. Transition 7. This very intense band rises sharply from the preceding region of low absorption and its long-wavelength tail is responsible for much of the blurring of the fine structure of band 6. Transitions 8, 9, and 10. At room temperature our measurements could be extended slightly above 50 000 cm-’ (Figs. 1, 4). Lack of resolution makes an analysis much more difficult. Still, the existence of at least three additional transitions in this region seems beyond doubt. We are puzzled by the insensitivity of the transition energy of band 10 to substitution but a check of baselines as well as agreement between the two spectrophotometers used suggest that this is real. 3 In view of the high intensity of absorption is due to an impurity
such as an isomeric
seen on the other absorption and doubling t,hree isomers.
peaks. Moreover,
in room-temperature
in this region it is unlikely that the doubling
derivative,
observed
since no such effect is a similar broadening
spectra of l- and 7-fluorofluoranthenes
In this case, we had enough material
tored by vapor-phase
particularly
we recently
chromatography.
for a very thorough
but not the other purification
moni-
74
MICHL
In 2,3_dimethylfluoranthene the peak at 44 000 cm-’ could belong to transition 7 or maybe it is the &O band of transition 8. Table I presents a summary of energies of GO vibrational components of all bands assigned in this paper and results of calculations by the semiempirical CI SCF method. These were repeated several times using different approximations for the electron repylsion integrals [other parameters as in Ref. (4), all bond lengths equal to 1.4 A]. All lead to the same order of the lower 4-5 excited states, identical with that suggested earlier (4,5). The order of the more closely spaced higher excited states is more sensitive to changes in parameters. The MatagaNishimoto approximation (12) gives the best quantitative agreement and the results are shown in Fig. 1 and Table I. Since our results confirmed the previous assignment of electronic transitions in fluoranthene which was shown to be in good agreement with semiempirical calculations, we shall not repeat the discussion here. We only wish to point out that the tabulated experimental energies correspond to O-O bands while the calculated ones should correspond to vertical transitions. It is perhaps interesting to note that the results of calculations contradict the often accepted notion that the order of excited states is given by orbital energy differences unless strong configuration interaction is involved as in benzenoid hydrocarbons where first order configuration interaction often pushes the Lb state below the L, state. In fluoranthene, the lowest excited state 1 corresponds to an almost pure 2 -+ - 1 configuration and the state 2 described by an almost pure 1 -+ - 1 configuration has higher energy, although the orbital energy difference for 1 -+ - 1 is of course smaller. This can happen because in the SCF approximation the excitation energy AEi+j depends not only on the energies of the molecular orbitals involved E; , ej , but also on Jij , the Coulomb integral between the ith and jth orbitals, and Kij , the corresponding exchange integral: AE = cj - pi - Jij + 2Kij e These integrals are by no means negligible and their value may vary for different choices of i, j within one molecule (Table I) and from one molecule to another (18. We were wrong in our earlier expectation (11) that the shift of the 1 -+ -1 band of fluoranthene (transition 2) on aza substitution could be predicted by a straightforward application of first order perturbation theory. A bathochromic shift is predicted for 3-aza substitution, a hypsochromic one is found. According to preliminary calculations, the error is due to the neglect of configuration interaction and shall be discussed elsewhere.
ELECTRONAFFINITY We are interested in electron affinities (EA) of nonalternant hydrocarbons particulary in connection with investigations of relations of orbital energy differ-
ELECTRONIC
SPECTRUM
OF FLUORANTHENE
75
ences to excitation energies and of the applicability of the simple Hiickel molecular orbital (HMO) method to spectral predictions (15). We have therefore measured the gas phase EA of fluoranthene by the method of Wentworth and Becker [common intercept, for references see Ref. (IO)] and found a rather high value of 0.63 eV, similar to that of azulene (0.66 eV; cf. nsphthalene, 0.15 eV; phenttnthrene, 0.31 eV). According to HMO calculations (14) high EA should be typical for fluoranthene-like hydrocarbons. In the case of fluoranthene, Sffy” = - 0.376, and use of the regression line of Ref. (10) gives an EA prediction of 0.70 eV. Our semiempirical SCF calculations gave energies for the lowest free molecular orbital (LF-\IO) of fluoranthene which were almost exactly equal to those of the IFNO of azulene. Depending on the approximation for electron-repulsion in tegrals used, the calculated electron affinity of fluoranthene \\-as from 0.07 eV lower to 0.02 eV higher than that calculated for azulene with the same parameters. The experimental value is 0.03 eV lower than that of azulene so that the agreement is quite satisfactory. Recently :\ value of 0.92 eV has been predicted (15) for the gas phase EA of fluoranthene from potentiometric measurements on radical ion solutions. The prediction was made assuming a unit slope for the plot of EA in gas phase against those in solution although a line of unit slope clearly was not the line of the best fit for the data available. Using a line of less than unit slope, giving a better fit for the existing data, one also predicts more correctly the value of gas phase EA foi fluoranthene. A part of the discrepancy may be due to our use of the common intercept approximation in the determination of gas phase EA required by inst,ru mental limitations. Nevertheless, it seems to us that a line of less than :L unit slope is required for correlating solution with gas phase EA. ACKNOWLEDGMENT This work was made possible by the kindness of Professors A. Streitwieser, Jr., C. F. Koelsch, and G. Eglinton; I wish to express my sincere thanks to them for generous gifts of samples. I am further obliged to Professor R. X. Becker of the University of Houston for permitting me to use the facilities of his laboratory and both to him and Dr. E. Chen of the University of Houston for valuable discussions. RECEIVED:
July
22,
196s REFERENCES
1. H. W. D. SJVBBS AND S. H. TUCKER, J. Chem. Sec. 1964, 227. 2. H. H. JAFF$, M. ORCHIN, “Theory and Applications of Ultraviolet Spectroscopy,” p. 343. Wiley, New York, 1962. 3. E. CLAR, “Polycyclic Hydrocarbons,” Vol. 1, p. 94. Academic Press, New York, 1964. 4. J. KOUTECS+, P. HOCHMANN,zi~~ J. MICHL, J. Chem. Phys. &I,2439 (1964). 5. E. HEILBRONNXR, J.-P. WEBXR, J. MICHL, .~ND R. ZAHRAnNfK, Theoret. Chim. Llcta 6, 141 (1966). 6. E. W. THULSTRUP AND J. H. EG~FXZS, Chem. Phys. Letters 1, 690 (1968); cf. lecture at 8th European Congr. Mol. Spectry., Copenhagen (1965).
76
MICHL
7. J. MICHL AND R. ZAHRADNfK, Collection Czech. Chem. Commun. a,3478 (1966). 8. J. L. PATENAUDE, P. SAUVAGEAU, AND C. SANDORFY, Spectrochim. Acta 18,241 (1962) and references therein. 3. R. S. BECKER, I. S. SINGH, AND E. A. JACKSON, J. Chem. Phys. 38,2144 (1963). 10. R. S. BECKER, AND E. CHEN, J. Chem. Phys. 46,2403 (1966). 11. J. MICHL, Spectrochim. Acta 21.2146 (1965). 1.2. N. MATAGA AND K. NISHIMOTO, 2. Physik. Chem. (Frankjurt) 13,140 (1957). 13. J. MICHL AND R. S. BECKER, J. Chem. Phys. 46.3889 (1967). 1.j. R. ZAHRmNfK AND J. MICHL, Collection Czech. Chem. Commun. 31.3442 (1966). 16. J. CHATJDHURI,J. JAGUR-GRODZINSKI, AND M. SZWARC, J. Phys. Chem. 7l, 3063 (1967).