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Moment directions of the electronic transitions of fluoranthene
CHEKKAL
PHY3ICS
LETTERS
1 (1968)
690 - 692.
NORTH-HOLLAND
PUBLISHING
COMPANY,
AMSTERDAM
MOMENT DIRECTIONS OF THE ELECTRONIC TRANSITIONS OF’ FLUORANTHENE Erik W. THULSTRUP Department
of Chemistry. Received
and J. H. EGGERS
Unz’vcvsity of Aadus, 4 March
The moment directions of rhe electronic transitions found by means of the stretched film technique,
Denmark
1968
of fluoranthene
in the range 3’700 & - 2100 A are
1. INTRODUCTION Albrecht [l] and others have often pointed out the great importance of getting information about the polarization of electronic bands. So far, however, no broadly applicable method has been found for obtaining such information. The methods mostly used are the photoselection techniques, .where light is used to select an anisotropic set of molecules from an isotropic sample, and the crystal methods, in which the orientation of mo1ecuPes in tine crystaliine state is used. In some cases the crystal methods, which have been described by McClure [2] and Wolf [S], give very beautiful results. However, the experimental techniques are often complicated and in many cases an assignment of the crystal bands is impossible. The photoselection methods are more applicable but have the drawback of giving information of relative polarizations only. Surveys of these and other methods have been given by Albrecht [l] and Dtlrr {4]. A rarely used method is the film technique in which the orientation of molecules in a stretched film is used. This method has many advantages as pointed out before [5,6]. The experimental technique is simple. Information about absolute moment directions is derived and the method can be used for many molecules. A discussion on the experimental details, the information one can get, and a number of film spectra of different molecules will soon be published [7]. We give only a very short description of the method here. The studied molecule is fluorantherie (fig. l), which has symmetry C2, i.e. that singlet n-rtransitions only occur polarized along the y-axis or along the z-axis. April
1968
(bl
Fig.
1. (a) Fluoranthene. (b) Solution spectrum anthene [S].
of fluor-
The polarized spectrum of fluoranthene has been discussed by us [5] using the film method and by Heilbronner et al. 181 using a photoselection method. The new measurements presented here are in agreement with those in 151 and 183 and give some additional information on the overlapping electronic bands. 2. EXPERIMENTAL The compound was purified by chromatography and dissolved in xylene. A polyethylene film was stretched about 400% and coated with the solution. The xylene having vaporized the electronic absorption spectrum with polarized light was obtained for two different positions of the light vector, i.e. parallel (II) and perpendicular(l) to the direction of stretching. In both cases the beam was perpendicular to the plane of the film. Afterwards the compound was washed out of the film with xylene and two absorption spectra with polarized light were obtained for the pure film using the same positions of the light vector as before. 690
ELECTRONIC
TRANSITIONS OF FLUORANTHENE
The measurements were carried out on a Car-y 14 spectrophotometer at room temperature and the polarizators were calcite Glan prisms. 3. DISCUSSION OF RESULTS The differences between the corresponding absorbance from film + compound and pure film are called E,,(X) and EL(X) for the two orientations of the light vector. These differences are shown in fig. 2 and are assumed to represent the absorption from fluoranthene molecules. From fig. 2 is seen that E,, and EA. are not proportional. This means that the molecules are oriented, but the orientation is not perfect. Nevertheless it is evident that the transitions B, D and H are polarized in a direction different from that of the transition G. If the result of the stretching is assumed to be that the long axes of the molecules tend to be near the direction of stretching (for a discussion of this, see [7]), it is clear that B, D and H are z-axis polarized, while G is y-axis polarized. The spectra contain further information, which can be extracted in different ways [ 1,5,6]. Here we use the following method. Let us call the absorbance curves for perfect orientation A&) and A],(X). We then have the relations E,,(X) = “@#I
+ +$,,(X)
E,(X)
+ fi4A,,(V
= “$Z(X)
(1)
where the constants kl, k2, kg and k4 depend on the orientation. The equations (1) give
A,(A) a E,,(A) - d,,E,(N Ay(A)
(2)
a E,(A)- rl,E,,(A)
with the constants d,, and dl > 0.
691
The problem is now the determination of d,: and dl which will be discussed in detail in [7]. We have already seen that the strong transitions D and G are polarized along the z-axis and’the y-axis respectively. If it is assumed that the variations of E,,(h) and E,(A) in these regions are merely due to the strong transitions, we have from eqs. (2)
(qg!),
a
(=+), - d$gqD =O.(3)
From these equations d,, and dL can be found The result is d,, = 0.67 and d, = 0.48. From eqs. (2) can be found curves proportiona to AJT and AZ, representing transitions polarized parallel to the y-axis and the z-axis respectively. The curves are shown in fig. 3 and are used Eor a discussion on the spectrum. The transition A (fig. 1) is too weak to be seen in the film spectrum. However, Heilh’ronner et al. [8] have used the fluorescence polarization method to show that A must be polarized perpendicular to B. From fig. 3 it is clear that B, D, E and H are polarized parallel to the z-axis. This means that A has a transition moment parallel to the y-axis. From fig. 3 it is also seen that the transitions C, F and G are polarized parallel to the y-r@. The weak y-polarized absorption in the region of B may be due to vibrational components of A which have gained intensity through coupling with C. A similar situation is seen more clearly in the case of F, which seems to be an electronic transition gaining intensity from G. Because of the high intensity it does not seem likely that F is due to vibronic components of D or E (through a non-totally symmetric vibration). This assignment is in agreement with the calculations of Heilbronner et al. [8].
n absorbance
0
H
cl6 -
a3 .
“n
IF=B
_---_-
___
3500
Fig. 2. E,, (A) (solid line) and EL(h) (broken line).
Fig. 3. A&)
C ---______
30(
(solid line) and A,,(X) (broken line)_
E.W.THULSTRUP
692
It shall be mentioned that from these spectra nothing can be said about the number of z-polarized electronic transitions in the region between E and H. We hope to be able to obtain low-temperatslre film spectra, and with these it might be possible to solve this and other of the remaining problems. 4. CONCLUDING
REMARKS
It has been demonstrated that by the film technique it is possible to get detailed information about the polarization of electronic transitions. Also, it has been shown how to extract more information than usual from the experimental film spectra. We have used the film method for a large number of molecules, especially aromatic hydrocarbons with a a-fold axis in the molecular plane, and the technique described here has in
and J.H.EGGERS
nearly all cases been successful. It is our hope that our results will encourage experimentalists to use the film technique, which we think has been overlooked for many years. REFERENCES [l] A.C.Albrecht, J.Mol.Spectroscopy 6 (1961) 84. [2] D.S. McClure, Solid State Phys. 8 (1959) I; 9 (1959) 399. [3] H. C. Wolf, Solid State Phys. 9 (1959) 1. [4] F.DBrr, Angew.Chemie 9 (1966) 457. [SJ J. H. Eggers and E. W. Thulstrup, Papers given at the 8th Eurooean Consress on Molecular Soectroscopy (1965): 161 - _ Y. Tanizaki and S. Kubodera. J. Mol. Spectroscopy 24 (1967) 1. 1’71 L.H. Hansen. J. H. Escrers. B. P.&we and E. W. - - Thulstrup, to be published. [S] W. Heilbronner, J. Michl, J.-P. Weber and R. Zahradnik, Theoret. Chim. Acta 6 (1966) 141.
PHY3ICS
LETTERS
1 (1968)
690 - 692.
NORTH-HOLLAND
PUBLISHING
COMPANY,
AMSTERDAM
MOMENT DIRECTIONS OF THE ELECTRONIC TRANSITIONS OF’ FLUORANTHENE Erik W. THULSTRUP Department
of Chemistry. Received
and J. H. EGGERS
Unz’vcvsity of Aadus, 4 March
The moment directions of rhe electronic transitions found by means of the stretched film technique,
Denmark
1968
of fluoranthene
in the range 3’700 & - 2100 A are
1. INTRODUCTION Albrecht [l] and others have often pointed out the great importance of getting information about the polarization of electronic bands. So far, however, no broadly applicable method has been found for obtaining such information. The methods mostly used are the photoselection techniques, .where light is used to select an anisotropic set of molecules from an isotropic sample, and the crystal methods, in which the orientation of mo1ecuPes in tine crystaliine state is used. In some cases the crystal methods, which have been described by McClure [2] and Wolf [S], give very beautiful results. However, the experimental techniques are often complicated and in many cases an assignment of the crystal bands is impossible. The photoselection methods are more applicable but have the drawback of giving information of relative polarizations only. Surveys of these and other methods have been given by Albrecht [l] and Dtlrr {4]. A rarely used method is the film technique in which the orientation of molecules in a stretched film is used. This method has many advantages as pointed out before [5,6]. The experimental technique is simple. Information about absolute moment directions is derived and the method can be used for many molecules. A discussion on the experimental details, the information one can get, and a number of film spectra of different molecules will soon be published [7]. We give only a very short description of the method here. The studied molecule is fluorantherie (fig. l), which has symmetry C2, i.e. that singlet n-rtransitions only occur polarized along the y-axis or along the z-axis. April
1968
(bl
Fig.
1. (a) Fluoranthene. (b) Solution spectrum anthene [S].
of fluor-
The polarized spectrum of fluoranthene has been discussed by us [5] using the film method and by Heilbronner et al. 181 using a photoselection method. The new measurements presented here are in agreement with those in 151 and 183 and give some additional information on the overlapping electronic bands. 2. EXPERIMENTAL The compound was purified by chromatography and dissolved in xylene. A polyethylene film was stretched about 400% and coated with the solution. The xylene having vaporized the electronic absorption spectrum with polarized light was obtained for two different positions of the light vector, i.e. parallel (II) and perpendicular(l) to the direction of stretching. In both cases the beam was perpendicular to the plane of the film. Afterwards the compound was washed out of the film with xylene and two absorption spectra with polarized light were obtained for the pure film using the same positions of the light vector as before. 690
ELECTRONIC
TRANSITIONS OF FLUORANTHENE
The measurements were carried out on a Car-y 14 spectrophotometer at room temperature and the polarizators were calcite Glan prisms. 3. DISCUSSION OF RESULTS The differences between the corresponding absorbance from film + compound and pure film are called E,,(X) and EL(X) for the two orientations of the light vector. These differences are shown in fig. 2 and are assumed to represent the absorption from fluoranthene molecules. From fig. 2 is seen that E,, and EA. are not proportional. This means that the molecules are oriented, but the orientation is not perfect. Nevertheless it is evident that the transitions B, D and H are polarized in a direction different from that of the transition G. If the result of the stretching is assumed to be that the long axes of the molecules tend to be near the direction of stretching (for a discussion of this, see [7]), it is clear that B, D and H are z-axis polarized, while G is y-axis polarized. The spectra contain further information, which can be extracted in different ways [ 1,5,6]. Here we use the following method. Let us call the absorbance curves for perfect orientation A&) and A],(X). We then have the relations E,,(X) = “@#I
+ +$,,(X)
E,(X)
+ fi4A,,(V
= “$Z(X)
(1)
where the constants kl, k2, kg and k4 depend on the orientation. The equations (1) give
A,(A) a E,,(A) - d,,E,(N Ay(A)
(2)
a E,(A)- rl,E,,(A)
with the constants d,, and dl > 0.
691
The problem is now the determination of d,: and dl which will be discussed in detail in [7]. We have already seen that the strong transitions D and G are polarized along the z-axis and’the y-axis respectively. If it is assumed that the variations of E,,(h) and E,(A) in these regions are merely due to the strong transitions, we have from eqs. (2)
(qg!),
a
(=+), - d$gqD =O.(3)
From these equations d,, and dL can be found The result is d,, = 0.67 and d, = 0.48. From eqs. (2) can be found curves proportiona to AJT and AZ, representing transitions polarized parallel to the y-axis and the z-axis respectively. The curves are shown in fig. 3 and are used Eor a discussion on the spectrum. The transition A (fig. 1) is too weak to be seen in the film spectrum. However, Heilh’ronner et al. [8] have used the fluorescence polarization method to show that A must be polarized perpendicular to B. From fig. 3 it is clear that B, D, E and H are polarized parallel to the z-axis. This means that A has a transition moment parallel to the y-axis. From fig. 3 it is also seen that the transitions C, F and G are polarized parallel to the y-r@. The weak y-polarized absorption in the region of B may be due to vibrational components of A which have gained intensity through coupling with C. A similar situation is seen more clearly in the case of F, which seems to be an electronic transition gaining intensity from G. Because of the high intensity it does not seem likely that F is due to vibronic components of D or E (through a non-totally symmetric vibration). This assignment is in agreement with the calculations of Heilbronner et al. [8].
n absorbance
0
H
cl6 -
a3 .
“n
IF=B
_---_-
___
3500
Fig. 2. E,, (A) (solid line) and EL(h) (broken line).
Fig. 3. A&)
C ---______
30(
(solid line) and A,,(X) (broken line)_
E.W.THULSTRUP
692
It shall be mentioned that from these spectra nothing can be said about the number of z-polarized electronic transitions in the region between E and H. We hope to be able to obtain low-temperatslre film spectra, and with these it might be possible to solve this and other of the remaining problems. 4. CONCLUDING
REMARKS
It has been demonstrated that by the film technique it is possible to get detailed information about the polarization of electronic transitions. Also, it has been shown how to extract more information than usual from the experimental film spectra. We have used the film method for a large number of molecules, especially aromatic hydrocarbons with a a-fold axis in the molecular plane, and the technique described here has in
and J.H.EGGERS
nearly all cases been successful. It is our hope that our results will encourage experimentalists to use the film technique, which we think has been overlooked for many years. REFERENCES [l] A.C.Albrecht, J.Mol.Spectroscopy 6 (1961) 84. [2] D.S. McClure, Solid State Phys. 8 (1959) I; 9 (1959) 399. [3] H. C. Wolf, Solid State Phys. 9 (1959) 1. [4] F.DBrr, Angew.Chemie 9 (1966) 457. [SJ J. H. Eggers and E. W. Thulstrup, Papers given at the 8th Eurooean Consress on Molecular Soectroscopy (1965): 161 - _ Y. Tanizaki and S. Kubodera. J. Mol. Spectroscopy 24 (1967) 1. 1’71 L.H. Hansen. J. H. Escrers. B. P.&we and E. W. - - Thulstrup, to be published. [S] W. Heilbronner, J. Michl, J.-P. Weber and R. Zahradnik, Theoret. Chim. Acta 6 (1966) 141.