J . Phys. Chem. 1990, 94, 1800-1806
1800
Electronic States of Chrysene. Linear and Magnetic Circular Dichroism and Quantum Chemical Calculations Jens Spanget-Larsen,**'qLJacek Waluk,f?"and Erik W. Thulstrups Chemistry Department, Danish Engineering Academy, Building 375, DK-2800 Lyngby, Denmark, Center f o r Structure and Reactivity, Chemistry Department, University of Texas, Austin, Texas 7871 2-1 167, and Department of Chemistry, Royal Danish School of Educational Studies, Emdrupvej 115B, DK-2400 Copenhagen NV, Denmark (Received: June 9, 1989; In Final Form: August 30, 1989)
The electronic transitions of chrysene are investigated by linear dichroism spectroscopy in stretched polyethylene and by magnetic circular dichroism spectroscopy, leading to the assignment of seven excited singlet states below 47 000 cm-I. One of these has not previously been observed. The linear dichroism data,combined with previously published fluorescence polarization results, lead to precise determination of transition moment directions for five transitions. The experimental results are compared with theoretical predictions with the PPP, LCOAO, CNDO/S, and INDO/S quantum chemical models.
Introduction Polycyclic benzenoid hydrocarbons and their derivatives are the subject of particular attention because of the carcinogenic activity of many of these compounds. In this connection, a knowledge of the polarization directions of their electronic transitions is of considerable importance in the study of their interaction with DNA.' The electronic spectra of the carcinogen 1,2-benzanthracene and three of its aza derivatives were recently investigated by experimental and theoretical methods,24 including linear dichroism in stretched polyethylene in the UV and IR regions, fluorescence polarization, and magnetic circular dichroism, as well as calculations using several semiempirical quantum chemical models. The result was the assignment of several electronic transitions and a precise determination of their transition moment directions. In the present investigation, we study the electronic spectrum of the tetracyclic polyphene chrysene (CH). Along with many other members of the large family of aromatic hydrocarbons, C H has been the subject of detailed spectroscopic investigations for several decades5-' Singlet-singlet transitions in C H and its six monomethyl derivatives were studied by Becker et a L 7 leading to the recognition of seven a-n* transitions in the region down to 190 nm. Polarized fluorescence studies were carried out by Zimmermann and JOOP,~ who determined approximate relative transition moment directions for four transitions and compared them with the results of early n-electron calculation^.^ More recently, the photoelectron spectrum was reportedlo and correlated with the ultraviolet electronic absorption data.ll Among other significant work, the observation of delayed fluorescence from upper excited states12 and the vibrational analysis based in part on infrared linear dichroism spectra of oriented polycrystalline sampIesI3 seem of particular relevance. As a source for detailed information on the electronic structure of benzenoid hydrocarbons, linear dichroism spectroscopy14-16has turned out to be extremely ~ s e f u l . l ~ The - ~ ~method produces information on molecular alignment, e g , in stretched polymer sheets or liquid crystals, as well as information on transition moment directions. It also often allows detection of otherwise "hidden" transitions. Linear dichroism studies now cover a wide field, from small molecules to macromolecules.20 In the following, we shall see how the method, combined with fluorescence polarization results,* magnetic circular dichroism measurements, and quantum mechanical calculations, gives a detailed picture of the electronic structure of chrysene.
'Danish Engineering Academy.
'University of Texas at Austin. Danish School of Educational Studies. Present address: Department of Life Sciences and Chemistry, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark. 11 Permanent address: institute of Physical Chemistry, Polish Academy of Sciences. 01-224 Warsaw, Kasprzaka 44, Poland. 5 Royal
SCHEME I
/
Chrysene (CH)
Experimental Section Samples of C H were obtained from commercial sources (Fluka, Ega Chemie) and purified by column chromatography, with use (1) Norden, B.; Kubista, M. In Polarized Spectroscopy of Ordered Systems; Samori, B., Thulstrup, E. W., Eds.; Nato AS1 Series C; Kluwer Academic: Dordrecht, The Netherlands, 1988; Vol. 242, pp 133-165. (2) Waluk, J.; Thulstrup, E. W. Chem. Phys. Lett. 1987, 135, 515. (3) Waluk, J.; Mordzibski, A.; Spanget-Larsen, J.; Thulstrup, E. W. Chem. Phys. 1987, 116, 41 1. (4) Waluk, J.; Mordzibski, A.; Spanget-Larsen, J.; Thulstrup, E. W. Chem.
Phys. 1988, 124, 103. (5) Clar, E. Polvcvclic Aromatic Hydrocarbons; Academic Press: New York; 1964; Vol. 1-and 11. (6) Klevens, H.; Platt, J. J . Chem. Phys. 1949, 17, 470. (7) Becker, R. S.; Singh, I. S.; Jackson, E. A. J . Chem. Phys. 1963, 38,
2144, and references cited therein. (8) Zimmermann, H.; Joop, N. Z . Elektrochem. 1961, 65,66. Gallivan, J. B.; Brinen, J. S. J . Chem. Phys. 1969, 50, 1590. (9) Ham, N. S.; Ruedenberg, K. J . Chem. Phys. 1956,25, 1,13. See also: Favini, G.;Gamba, A,; Simonetta, M. Theor. Chim. Acta 1969, 13, 175, and literature cited therein. (10) Brogli, F.; Heilbronner, E. Angew. Chem. 1972,84, 551. Boschi, R.; Murrell, J. N.; Schmidt, W. Discuss. Faraday SOC.1972, 54, 116. (11) Schmidt, W. J . Chem. Phys. 1977, 66, 828. (12) Nickel, B. Helu. Chim. Acta 1978, 61, 198. (13) Cyvin, B. N.; Klaeboe, P.; Whitmer, J. C.; Cyvin, S. J. Z . Naturforsch. 1982, 37A, 251. (14) Thulstrup, E. W.; Eggers, J. H. Chem. Phys. Lert. 1968, I , 690. Thulstrup, E. W.; Michl, J.; Eggers, J. H. J . Phys. Chem. 1970, 74, 3868. Michl, J.; Thulstrup, E. W.; Eggers, J. H. J . Phys. Chem. 1970, 74, 3878. (15) Thulstrup, E. W. Aspects of the Linear and Magnetic Circular Dichroism of Planar Organic Molecules; Springer-Verlag: Heidelberg, 1980. (16) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light. Solute Alignment by Photoselection in Liquid Crystals, Polymers, and Membranes; VCH: Deerfield Beach, FL, 1986. (17) Michl, J.; Thulstrup, E. W. Acc. Chem. Res. 1987, 20, 192. (18) Thulstrup, E. W.; Michl, J. Elementary Polarization Spectroscopy;
VCH: Deerfield Beach, FL, 1989. (19) Thulstrup, E. W.; Michl, J. Spectrochim. Acta 1988, 4 4 4 767. (20) Samod, B., Thulstrup, E. W., Eds. Polarized Spectroscopy of Ordered Systems; Nato AS1 Series C: Kluwer Academic: Dordrecht, The Netherlands, 1988; Vol. 242.
0022-3654/90/2094- 1800$02.50/0 0 1990 American Chemical Society
Electronic States of Chrysene
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 1801 25 1
30 "
'
~
1
"
40
35 "
1
'
'
"
1
'
"
1
45 "
~
~
1
'
5D ~
LCOAO
-B
~
'?(1@CM-')
5
30
35
48
45
103cm-1
Figure 1. Top: Linear dichroic absorption curves for chrysene (CH) oriented in stretched polyethylene at room temperature. The figure shows E,(?) (full line) and Ed?) (broken line) obtained with the electric vector of the polarized light parallel and perpendicular, respectively, to the stretching direction of the sample. Bottom: Linear combinations (1 K)EZ(?) - 2KEy(t) with K ranging from 0 to 1 in steps of 0.1,
of a silica gel column (Merck, Art. 10402) and dichloromethane as solvent (Merck, Art. 6044). Linear dichroism (LD) spectra were measured in uniaxially stretched (400%)polyethylene sheets. The polyethylene (PE) used was either 100-Fm film material, specially prepared without any additives (Panther Plast A/S, Vordingborg, Denmark), or material cut from the walls of a laboratory bottle made of ca. 2-mm-thick pure PE (Kartell, FRG). To improve the optical quality, the thick pieces cut from the bottle were heated in an oven to 150 OC and then quenched in cold water. C H was introduced into the PE sheets from solutions in chloroform (Merck, spectral quality). Several samples with different C H concentrations were prepared. The weak absorption in the infrared and in the 25 000-30000-~m-~ region was recorded with a thick, stretched PE sample nearly saturated with CH. This sample was prepared by immersing a stretched piece of PE in a concentrated solution of C H in chloroform for 24 h at 35 OC. The sample was then removed, and the chloroform was allowed to evaporate from the PE for several hours at room temperature. The much stronger absorption above 30 000 cm-I was measured from thin PE samples soaked with chloroform solutions of C H for 30-120 s. These samples were stretched after the doping with CH. The observed degree of LD was found to be essentially independent of the brand of PE used and the concentration of C H and of whether the samples were stretched before or after the introduction of C H into the PE. The near-ultraviolet LD spectra were measured on a Shimadzu Model MPS-2000 spectrophotometer interfaced with an Apple IIe computer. The spectra were obtained at room temperature with a spectral resolution of 0.5 nm. Rotatable Glan prism polarizers were placed in both sample and reference beams as previously described.21 To reduce random noise, the absorbance was recorded as an average of the results of several (n) mea(21) Myrvold, B. 0.;Spanget-Larsen, J.; Thulstrup, E. W. Chem. Phys.
1986, 104, 305.
Figure 2. Top: Graphical representation of the spectrum of chrysene (CH) calculated by the LCOAO method (Table 11). The thickness and length of the bars correspond to different values of oscillator strengths fand MCD B terms: f< 0.1,O.l 0.6 0.31 f 0.01 0.3 f 0.1 0.53 f 0.01 0.48 f 0.01 0.50 f 0.02 0.50 f 0.03 0.70 f 0.01 0.70 f 0.01 0.70 f 0.05 0.3 f 0.1 0.3 f 0.1 0.50 f 0.02 0.70 f 0.02 0.70 f 0.02
73 f 2 73 f 6 f cp B band 3 rp B
P
(Lb) (La)
1 IB;
2 2 3 4 3 4 5 5 6 6 7 7
IB,'
28.6 30.3 35.2 38.2 38.4 38.5 41.4 43.3 45.6 45.7 45.2 46.5 47.4
IAi 'A, lAgC IB,+ (Bb) 'B,' (B,) lB; IA,
)A,+ IB;
'B,' !A,+
0.59
-34
2.19 0.66
+lo -71
0.36
+4
29.2 31.6 35.0 38.7 39.7 38.3 40.7 43.8 44.6 46.4 45.7 47.8 49.3
0.41
+83' -36
+0.11 +0.05
1 2
27.6 3l.v
0.3
+73 -36
+0.38 +0.53
2.33 0.69 0.01
-1
+83 +79
+3.07 -1.11 -2.27
3 4 5
36.9' 37.641.1
1.1 0.3 0.1
0 f65* f40
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0.18 0.49
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-1.07 +2.80
6 7
44'4) 45.5
0.5
0
-1
+4
'For calculational details, see text. bSee Table I. c ( p is defined in Scheme I. dRoughlyestimated oscillator strengths. 'This angle is sensitive to calculational details, see text. fOnset. BPreviously unobserved feature, see discussion in text. *The experimental error limit is particularly large for this angle (Table I). TABLE 111: S,
+-
SoTransitions in CH Predicted by Standard NDO-Based All-Valence-ElectronsProcedures'
CNDO/S
INDO/S
term
3
f
cp
3
f
(P
3
f
rp
(Ld 2 'B; (L,)
30.1 32.0 35.7 38.6 40.9 39.7 42.3 43.1 45.0 45.3 44.8 48.9 46.8
0.003 0.30
+39 -30
0.0003 0.33
+43 -29
+I6 -36
+5 -8 5 +4
1.02 0.34 0.01
+IO
2.16 0.58 0.04
+5 -8 1 +83
0.05 0.24
+5 +1
0.04 0.31
+24 -1
27.6 30.3 33.9 36.9 39.3 37.7 40.3 41.7 43.6 45.1 42.8 47.9 46.4
0.0005 0.46
1.06 0.36 0.01
29.2 31.2 34.8 35.8 39.6 39.0 41.2 41.7 42.2 45.3 44.0 47.6 45.4
0.12 0.46
+I2
1 'B,,
2 3 4 3 4
IA, IA, IA,
IBu (Bb) 'Bu (Ba) 5 'B, 5 'A, 6 IA, 6 IB, I 'B, 7 !A,
'
CNDO-SDCI
-6 7 +88
-5
'For calculational details, see text. have led to the assignment of observed features close to 45 000 cm-' or 51 000 cm-I to the B, t r a n ~ i t i o nbut , ~ ~these assignments are in disagreement with the theoretical predictions that indicate that the separation of the Bb and B, states is less than 3000 cm-I. We propose that the previously unobserved band 4 with origin at 37 400 cm-I should be assigned to the B, state. This assignment leads to better agreement with the predicted energy separation of the Bb and B, states, and the observed polarization (essentially short-axis) and MCD B term (negative) for band 4 are consistent with the theoretical predictions for the B, transition (Table 11). Alternatively, band 5 at 41 000 cm-I could be assigned to the B, transition, but combined application of all available criteria (energy, intensity, polarization, MCD) favors the assignment of band 5 to the minus state 5 IB; (see below). Similar application of all four assignment criteria indicates that the assignment of band 7 to the last of the four allowed transitions in the PPP model (7 'B,+) is unproblematic. The remaining three of the seven B, states predicted below 47 000 cm-I correspond to minus states. They are predicted at 28 600, 43 300, and 45 200 cm-l in the PPP model. The most important of these is the first, which corresponds to the Lb state in the perimeter model; it can be described as 46% 12 -1 ) 46% 11 -2) in the PPP model. The two minus states at higher energies can be described as 27% 12 -5) - 27% 15 -2) + 12% 14 - 3 ) - 12% 13 -4) (5 IB;) and 44% 11 -5) - 44% 15 - 1 ) (6 IB,-). Transitions to these states are strictly forbidden in the PPP model, but in the all-valence-electrons theories, the pairing symmetry is more or less strongly perturbed and the transitions gain intensity. The perturbation of the pairing symmetry is strongly dependent on the type of all-valence-electronstheory. For example, in the LCOAO model the Lb state can be described as 46% 12 -1 ) - 48% 11 -2), indicating near-perfect pairing symmetry
--
-
-
-
-
--
-
-
--
--
in this model, while in the CNDO/S model the same state is predicted as 40% (2 - 1 ) - 52% 11 -2), which indicates a significant breakdown of the pairing properties. In general, the wave functions predicted by the CNDO/S and other NDO (neglect of differential overlap) based theories do not even approximate the degeneracies required by the pairing theorem.3*26*29~36~37 This breakdown is largely due to the inadequate treatment of orbital overlap effects and has been found to affect unfavorably the electronic transition moment directions3and MCD B terms37 computed in the CNDO/S model. For some states, the pairing symmetry breaks down completely, even in the LCOAO model, as a result of strong interaction between minus and plus states that are close in energy. In the case of CH, this happens for the states that are labeled as 6 IB; and 7 IB,+ in Table 11. The states predicted in the LCOAO model can be written as 80% 11 -5) - 2% 15 -1 ) 11% 13 - 3 ) (6 IB,) and 1% 11 -5) + 49% 15 - 1 ) - 46% 13 -3) (7 IB,). Obviously, the notion of plus and minus states is inapplicable. As a consequence of this scrambling, the intrinsically forbidden 6 IB,- state borrows optical intensity from 7 IBU+,and the transitions to the two resulting states are both predicted to be intense in the LCOAO model (Table 11). The assignment of the very weak band 1 to the Lbminus state is obvious, and the assignment of band 5 to the second minus state 5 IB; has been mentioned above. The assignment of the 6 'B, state is less straightforward. It is predicted to be optically quite intense (although it correlates with a minus state in the PPP model as just discussed), but it is not clearly observed in the experimental spectrum. It is probably overlapped by the strong long-axis-polarized band around 45 000 cm-l, which was assigned to the 7 IB,
+
--
(36) Ellis, R. L.; J a m , H.H. J . Mol. Spectrosc. 1974, 50, 474. (37) Obbink,J. H.; Hezemans, A. M. F. Chem. Phys. Lett. 1977, 50, 133.
1806 The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
state. Both transitions, 6 IB, and 7 ’Bu,are predicted to be long-axis polarized, which easily explains why the two overlapping contributions are not resolved in the LD spectra. However, negative and positive MCD B terms are predicted for the 6 ‘B, and 7 IB, transitions, respectively, which indicates that the two transitions should be resolved in the MCD spectrum. This does not seem to be the case; the 45 000-cm-’ band is associated almost exclusively with negative MCD, corresponding to positive B terms. It seems most likely that the absorption due to the 6 ‘B, transition is located close to the onset of the 45000 cm-’ band and that positive MCD due to this transition partly cancels with the strong negative MCD of the 7 ‘B, transition (band 7) and partly contributes to the broad positive MCD band due to the 5 ’B,transition (band 5). On the other hand, it is of course also quite possible that the predicted B terms are in error, particularly in the high-energy region with a relatively high density of states. The transition moment directions predicted by the LCOAO method for those transitions for which precise experimental directions could be derived from the observed spectra are visualized in Figure 3. The agreement with the experimental moment directions is very satisfactory. The performance of the CNDO- and INDO-based methods seems to be inferior to that of the LCOAO method, particularly with respect to the moment direction of the weak Lb transition. For example, the difference between observed and predicted directions is 55-60’ in the case of the INDO calculation. As previously d i s c ~ s s e d ,the ~ , ~failure seems to be associated with a too strong breakdown of the pairing symmetry in the NDO procedures. On the other hand, the satisfactory prediction in the case of the LCOAO calculation is to some extent fortuitous, since the computed Lb moment direction was found to depend significantly on the CI expansion (in view of the extreme weakness of the computed transition, this is not surprising). The result given in Table I1 and shown in Figure 3 was obtained with a limited CI basis of 36 singly excited configurations, with use of the same atomic parameters as in our previous investigation~.3~26,30JI,38 The MCD B terms computed by the LCOAO method are shown in a diagram in Figure 2. The correspondence between observed and calculated MCD spectra is gratifying and invites confidence in the suggested assignments. The observed signs of (38) Spanget-Larsen, J. Manuscript in preparation.
Spanget-Larsen et al. the B terms for the Lb, La, Bb,and B, transitions, namely +,+,+, and -, respectively, are perfectly consistent with those derived by M i ~ h for l ~ an ~ odd-soft MCD chromophore of C,, symmetry, considering magnetic coupling only between the four HOMOLUMO-based transitions in the perimeter model. The LCOAO model reproduces the observed signs, but analysis of the results shows that magnetic coupling with higher excited states is essential for the signs predicted for the low-energy transitions. For example, the B term for the La transition is correctly predicted to be positive, but the leading positive term in the perturbation expansion is due to coupling with the 6 ‘B, state. This can probably be considered as evidence for the distortion of the pure perimeter states in CH.
Concluding Remarks The application of LD and MCD spectroscopy has led to the assignment of seven S,, So transitions in the near-ultraviolet region (Table 11). The short-axis-polarized absorption with onset at 37 400 cm-’ and tentatively assigned by us to the B, state of C H has not previously been observed. This amply illustrates the power of LD and MCD spectroscopy to uncover otherwise “hidden” transitions. The combined information from LD and fluorescence polarization8 spectroscopy allowed precise determination of transition moment directions for five transitions. The experimental results are in good agreement with the moment directions and MCD B terms calculated by the all-valence-electrons LCOAO method. The predicted moment direction for the very weak Lb transition is sensitive to calculational details, such as the extension of the CI expansion. This aspect will be studied in detail in connection with a spectroscopic and theoretical investigation of benzo [a ]~ y r e n e . ) ~
-
Acknowledgment. We are grateful to Arne Colding for purification of samples of chrysene, to Rolf Gleiter for providing computer time at the Universitatsrechenzentrum in Heidelberg and to Janusz Gilewski for help with the construction of the MCD instrument. We also thank Panther Plast A/S, DK-4760 Vordingborg, for a gift of pure polyethylene film and the Danish Natural Science Research Council for financial support. Roskilde University is acknowledged for the loan of an FTIR instrument. Registry No. Chrysene, 21 8-01-9. (39) Spanget-Larsen, J.; Waluk, J.; Thulstrup, E. W. To be published.
