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Ultraviolet spectra of coumarins and psoralens
JOURNAL
OF
MOLECULAR
Ultraviolet
SPECTROSCOPY
Spectra
4~0,
144-157 (1971)
of Coumarins
and Psoralens’
THOMAS A. MOORE,~ MARIAN L. HARTER~ AND PILL-SOON SONGS Department
of Chemistry,
Texas Tech University, Lubbock, Texas 79409
The P + K* transitions of coumarins in the ultraviolet region have been described on the basis of fluorescence polarization, dichroic spectra, and SCF MO CI results. The two lowest * + a* transitions in coumarin, 4-hydroxycoumarin, and psoralen were found to be polarized nearly parallel to each other, probably along the long molecular axis. The two lowest ?r -+ ?r* transition moments in 8-methoxypsoralen were found to be oriented with a substantial angle between them, in contrast to the case of coumarin and psoralen. It was not possible to identify any n + ?r* bands as they are probably obscured by the strong ?r-+ ?r*bands. INTRODUCTION
Interestingly, the GO phosphorescence bands of coumarin and psoralens (fuocoumarins) are located at exactly the same frequency (1, d), although the absorption and fluorescence bands of psoralens are substantially red shifted with respect to coumarin. Furthermore, fluorescence polarization with respect to the two long-wavelength bands is only slightly different in the case of coumarin (3). In an attempt to elucidate the spectroscopic characteristics of these molecules, the present paper describes assignments of the ultraviolet bands in terms of the polarized fluorescence excitation spectra under photoselective conditions, linear dichroism in a stretched polyethylene film, and SCF MO CI calculations within the Pariser-Parr-Pople (PPP) approximation. MATERIALS
AND METHODS
Materials Coumarin (1,2_benzopyrone), 4-hydroxycoumarin, and psoralen (7H-furo[3,2-g][l]benzopyran-7-one) were used as obtained in our previous work (1, 3). S-Methoxypsoralen was a gift from Professor L. Musajo. All of these compounds were cryst#alline and were essentially free from impurities. The absence of im1This work was supported in part by the Robert A. Welch Foundation (Grant D-182) and the National Science Foundation (Grant GB-21266). 2 An NSF predoctoral trainee (lQ6&1972). 8A Robert A. Welch Foundation Predoctoral Fellow (1971-1972). * To whom reprint requests may be addressed. 144
ULTRAVIOLET
SPECTRA OF COUMARINS AND PSORALENS
145
purities in these samples was assured by determing the independence of excitation spectra upon the emitting frequency and vice versa. Solvents were of spectroscopic quality, and results obtained using ethanol (U.S. Indust,rial Co.) as the solvent are presented in this paper. Polyethylene films were obtained from Sargent-Welch.
Fluorescence and polarization spectra were obtained at 77”K, as described previously (3). For linear dichroic measurements, the compounds of interest were dissolved until saturation in chlorobenzene at 343°K which cont.ained a strip of polyethylene 1 X 3 cm cut from a polyethylene film (thickness 4.4 X low3 cm). After approximately 24 h, the polyethylene was removed, dried and washed with ethanol. The film was then stretched to approximately 6 times its original length (thickness - 2 X 10m3 cm) and was fitted to a special holder which fits into the cell compartment of a Bausch & Lomb Spectronic 600 spectrophotometer. The spectrophotometer has been fitted with Polacoat 105 uv polarizers in both the sample and reference beams. The reference was a stretched film prepared without solutes added. Dichroic ratios were then measured by scanning the absorption spectrum with t,he polarizers either vertical or horizontal. Following the development of Fraser (4, 5), Beer (6’), and Yogev et al. (7), we have used the following expression in analysing dichroic spectra of t.he stretched polyethylene film containing appropriate solute where A Ii and =1, are the absorbances parallel
c&!L= A,
f cos2 a! + 0.33( 1 - f) 0.5 f sin2 (L + O-33(1 - f)
(1)
and perpendicular to the direction of stretching, respectively, f is the fraction of aligned molecules and a! is the angle between the transition moment vector and the effective orientation axis of the molecule. The details of the use of the above equation will follow in the discussion section. Molecular
Orbital Calculations
The PPP SCF MO CI calculations were performed as described previously (3). More than 25 singly excited configurations were built into the CI matrices, and the effect of CI on the orientation of the transition moments was taken into consideration. RESULTS AND DISCUSSION Figure 1 shows the absorption and polarized fluorescence excitation spectra of coumarin with respect to fluorescence emission at 384 nm. The fluorescent state has been assigned (r, 7r*) symmetry (3). The polarization spectrum of coumarin was readily reproduced with only a small difference from one run to another.
146
MOORE,
HARTER,
AND SONG
0.8
1
0.5
0.7
0.6 +
-0.1 -0
0.5 Q) E ; 0.4 8 cn s
P
0.3
f 0.5
a
0.2
0.4
A__L 0.3
0.1
0
0.2
0
400
350
300
250
fVn
FIG. 1. Absorption (-) and polarized fluorescence excitation (-O-O-) spectra of coumarin in ethanol at 77°K. The polarized fluorescence excitation spectrum refers to the ordinate of polarization degree P, while the theoretical oscillator strengths indicated by the vertical lines refer to the f ordinate.
The polarization was corrected for instrumental bias by the Azumi-McGlynn procedure (8). Since the polarization degree (P) across the first absorption band the theoretical maximum of 0.5 for a classical (&l,X = 313 nm) approaches Vavilov-Levshin linear oscillator (9) and the second absorption band (X,,, = 275 nm) shows only slightly lower P value, it can be readily concluded that the transition moments of the two lowest a + ?r* bands are close to being mutually parallel. However, the exact angle 0 between the two transition moment vectors
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
147
cannot be estimated because of the depolarization from P = 0.5 due to what may be described as “randomization factor” (10).j Additionally, the P value (-0.4) for the first ?r t P* band may have been slightly lowered by the ?l+ r* int#ensity hidden underneath the strong n -+ ?r* band. In any case, it is qualit’atively possible to estimate a range of /3 from 10” f 5” using the Levshin-Perrin equation (11, 12) P = (3 cos* e -
l)/(cos” 0 + 3)
(2)
depending upon whether the wavelength-independent “randomization factor” (10) has been neglected or applied. There is a tendency toward lower polarization values in the fluorescence excitation spectrum of coumarin in t,he 240-250 nm region. However, a further measurement beyond 240 nm was not possible due to the reduced transmittancy of the Glan-Thompson polarizer and t’he lack of intensity from the xenon source in this region. Figure 2 shows the dichroic spectra of coumarin in a stretched polyet,hylene film. Values of d are 1.8 and 2.3 at 313 and 270 nm, respectively. Since these values are rather small, a quantit,ative deduction of the transition moment directions cannot be made. In other words, the value of d ‘V 2 can be accounted for in terms of a small orientation factor f or in berms of a large angle cy. An inspection of the dependence of the dichroic ratio on parameters f and cy shows that, the cl value of about 2 can result from a wide range of f (0.2 - 0.6) and a! (0 - 40”) (6). Thus it is consistent with the dichroism in Fig. 2 that the t,wo transition moments are far from being perpendicular, and that t,he second 7r -+ 7r* kansition moment has a somewhat less angle QIt,han the first, one. Since the cl values of the two lowest transitions are only slightly different, result’s extracted from Fig. 2 are consistent with the polarized fluorescence excitation spect’rum (Fig. 1). Figure 3 shows t,he predicted polarization of t,he three low-energy K -+ a* transitions in coumarin. Results after the CI treabment are consistent’ with the polarized fluorescence excitat.ion and dichroic spectra. The predicted angle 8 bet,ween bands a and b is 16”. The third transition c is polarized along the short axis and is therefore perpendicular to the two lowest transit’ions a and b. This seems to be borne out qualitatively by the lower fluorescence polarization and lower dichroism beyond 250 nm. The predicted energies are in excellent agreement with the absorption bands, as shown in Fig. 1. In view of the consistency obtainable from three different sets of data, namely, the polarized fluorescence excitation, linear dichroism, and SCF MO CI calculations, our 5The nature of the “randomization factor” is not well understood, although optical defects due to strain in the sample in the rigid organic glasses may be the cause (10). It is, however, interesting to note that a P value less than 0.5, usually 0.4 - 0.42, is also obtained in a polymethylmethacrylate plastic which was slowly polymerized along with solutes. In the case of all-trans retinol, the P value was found to be about 0.4 in et,hanol at 77X, while it frequently reaches 0.5 in a near Shpol’skii matrix (13).
148
MOORE,
HARTER,
AND SONG
0.5
0.4
P 2
0.3
‘0 2 Q
0.2
0. I
0
I
400 FIG.
I
,rc;; I
350
I
300
,w
1
250
1
2. Dichroic
spectra of coumarin in a stretched polyethylene fdm at 298°K. 11and I refer to the absorbance with polarizer parallel and perpendicular with respect to the axis of stretching, respectively.
0
FIG. 3. Polarization directions of three ?r + ?r* transitions PPP SCF MO with (II) and without (I) CI.
in coumarin
predicted
by the
of the uv bands of coumarin in terms of three ?r -+ ?r* transitions can be considered satisfactory. The first n + r* transition is possibly of pyrone origin (.!I), while the second transition is of benzenoid origin. However, configuration interactions among the various excited species are so extensive that the origin of each of the uv bands has only diagnostic meaning in classifying the electronic bands of the coumarin series.
assignments
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
149
Figure 4 shows the observed and predicted spectra for 4-hydroxycoumarin. Some vibrational structures have appeared upon substitution with the 4-hydroxy group. A strong O-O absorption band in the red edge of the absorption spectrum reflects a negligible change in the location of potential minima. In general, polarization characteristics of the bands a and b are similar to the case of coumarin. Xamely, the angle 0 between the two lowest a + a* transition moments is in the range of 14’ f 5” in agreement wit’h the predicted polarization (0 = 17”) shown in Fig. 5(U). Band c is predict,ed to be nearly perpendicularly polarized with respect to the lowest, transition a. A qualitative confirmation of this prediction is seen, as the polarization degree declines steadily below 250 nm.
I I I I 40 I I I Icm I x10-3 0.8 I30 I I 1 35
0.6
i0
300
0 250 nm
FIG. 4. Absorption (-) and polarized fluorescence excitation (-O-O-) spectra of 4-hydroxycoumarin in ethanol at 77°K. The theoretical results are also indicated. See the legend for Fig. 1 for further explanations.
150
MOORE,
HARTER,
AND
FIG. 5. Polarization directions of four T + T* transitions by the PPP SCF MO with (II) and without (I) CI.
SONG
in 4-hydroxycoumarin
predicted
Figure 6 shows the observed and predicted spectra for psoralen. First,, agreement between the observed absorption maxima and calculated transition energies is satisfactory. The relative extinctions of the uv bands are also in good agreement with the MO calculations. It can be seen that the difference in P across bands a and b is somewhat larger than in coumarin (Fig. l), indicating that B is slightly larger for psoralen. It can be seen t.hat these transitions are far from being perpendicularly polarized. The theory also predicts a relat,ively small angle 0, as shown in Fig. 7(11). The predicted angle is 32”, while the est,imated angle from the polarization spectrum in Fig. 6 is in the range of 20 f 5”. The strong band in the 250-nm region is predicted to consist of two transit,ions c and cl. A refined polarization in the uv region is desirable to resolve this band. There is a relatively strong red shift of the absorption maximum in going from coumarin to psoralen as is expected theoretically from the extended conjugation of the parent system. Figure 8 shows spectra for S-methoxypsoralen. The first absorption band is now obscured by the second band b. The theory also predicts considerable weakening of band a, as is indicated by a small oscillator strength. At the first glance, of the apparent it is easy to misassign it to an n ---f T* band. The frequency maximum of band a remains essentially unchanged upon substitution of a methoxy group at position 8 of psoralen. However, a slight red shift is seen for the second band b, which shifted from 275 nm in coumarin and 290 nm in psoralen t,o 300 nm in 8-methoxypsoralen. Thus, in terms of the intensity weakening of the first band a and the red shift of the second band b with respect to coumarin, 8-methoxypsoralen seems to be a unique model for a detailed analysis of the substituent effect on the electronic spectra of coumarin series. Figure 9 further demonstrates the suggestion that the 8-methoxy group provides a strong perturbation of the a-electron system. Transition a is now nearly short-axis polarized in contrast to all previous coumarin derivatives, although the second transition b remains polarized nearly along the long-axis containing the carbonyl bond. The angle 8 between the transition moment, vee-
ULTRAVIOLET
SPECTRA
25
i.or-
OF COUMARINS
AND PSORALENS
151
40 cm% 1oe3
30
f - I.2 i
C
- 0.8
d
-0.4
400
350
300
250nm
FIG. 6. Absorption (-) and polarized fluorescence excitation (-O-O-) spectra of psoralen in ethanol at 77°K. The theoretical results are also indicated. See the legend for Fig. 1 for further explanations.
(I) FIG. 7. Polarization direcaions of four ?r -+ r* transitions PPP SCF MO with (II) and without (I) CI.
(II) in psoralen
predicted
by the
152
MOORE, HARTER,
AND SONG
2.
0.7
0.6
- 0.6
de
400 350
300
-
0.4
f
250 nm
FIG. 8. Absorption
(-) and polarized fluorescence excitation (-O-O-) spectra of 8methoxypsoralen in ethanol at 77°K. The theoretical results are also indicated. See the legend for Fig. 1 for further explanations.
tors of a and b is 89”. A range of 40” f 10” can be estimated from the fluorescence excitation polarization spectrum, shown in Fig. 8, and Eq. (2). While quantitative agreement has not been achieved, the theory is consistent with the experiment in that a larger angle 0 is found between transition moments a and b in 8-methoxypsoralen than in other coumarins examined, as was predicted from the MO calculations. In order to strengthen the polarization assignments in S-methoxypsoralen, linear dichroic measurements were carried out. It is, for example, anticipated that band a would show a lower cl value than band 6, since the molecule will preferentially orient itself with its long molecular axis (possibly cont,aining the carbonyl bond) along the axis of stretching of the polyethylene film. From Fig. 2, it can be assumed that the transition moment of band b is oriented more
ULTRAVIOLET
SPECTRA OF COUMARINS AND PSORALENS
a FIG. 9. Polarization directions of four ?r + ?r* transitions by the PPP SCF MO with (XI) and without (I) CI.
153
(I) in %methoxypsoralen
predicted
closely along the carbonyl axis than that of band a, since the former has a slightly higher dichroic ratio and stretching of the film may very well force the molecule to align its carbonyl axis along the stretching axis. Note that band b is predicted to be polarized more closely along the carbonyl axis than band a in coumarin (Fig. 3). In other words, the effective orientation axis6 of an asymmetric molecule will lie very near the preferred molecular orientation axis which is estimated as the axis in the direction perpendicular to the smallest cross section on a molecular model. The effective orientation axis is parallel to the axis of stretching of the film. From Fig. 10, it can be deduced that the preferred molecular orientation axis lies very nearly along the direction of the calculated moment of the transition b at 300 nm. Taking the angle a! between 300-nm transition moment and the effective orientation axis to be zero and using the observed dichroic ratio of d = 7.3, Eq. (1) gives f = 0.68. Although assuming larger values of CLand thus calculating larger values of f would in principle satisfy Eq. (1) for d = 7.3, it is unlikely as the observed f values are generally not greater than the 0.68 calculated here. This is partly because of the presence of microscopically iso6That axis such that if the molecule had a transition polarized along that particular direction it would have a higher dichroic ratio than that of a transition polarized in any other direction. See Ref. (24) for further details.
MOORE, HARTER,
154
30
5
+
35
AND SONG
401
45I cm-‘.10S3
0.6
0” s 0.5 n 5 $ 0.4 0.3
0
400
350
1 I I,0 300 250nm
FIG. 10. Dichroic spectra of 8-methoxypsoralen in a stretched polyethylene film at 298°K. Note that two components (11and I) of the spectra refer to separate absorbance ordinates.
tropic regions in the stretched film. The present conclusion regarding parallel or nearly parallel orientation of the b transition moment and the stretching axis thus depends on the fact that the observed d value of 7.3 is exceptionally large (7, 14). Information deduced from Fig. 10 is consistent with the polarized fluorescence excitation spectrum shown in Fig. 8. Namely, the band b should be minimum and maximum in the polarized fluorescence excitation and linear dichroic spectra, respectively, in order for the above deduction concerning parallel orientation of the band b transition moment and stretching axis to be true. Both sets of experiments shown in Figs. 8 and 10 confirm the theoretical predictions. Using the calculated value off and the observed dichroic ratio of 2.5 for the band a at 387 nm, a! is calculated to be 36” for the lowest energy transition, It should be noted that angle LYcould be up to 10 N 15’ for the 300-nm transition
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
15.5
b without requiring a large increase in j’. Therefore, the angle f3between transition moment vectors a and b could be as large as 50”.’ Thus, the range of 36 - 50’ agrees satisfactorily with the polarized fluorescence excitation (Fig. 8) and only qualitatively with the SCF MO CI results (Fig. 9). It is interesting to note, from Figs. 7 and 9, that the direction of the a transit’ion moment in S-methoxypsoralen is essent#ially along the short axis and is not as significantly affected by the configuration interaction as in the case of psoralen. Although the form of the highest-filled MO’s of psoralen (8) and Smethoxypsoralen (9) is similar and that, of bhe lowest-vacant MO’s of psoralen (9) and S-methoxypsoralen (10) is also similar in the region of the common molecular framework, the highest-filled MO of 8-methoxypsoralen is additionallq characterized by a significant, cont,ribution from the methoxy oxygen orbital (BpJ with an expansion coefficient. of -0.30.” However, the coefficient in bhe MO (10) is only -0.07. This yields a transition monopole of 0.015 on the methox) oxygen. As a result, the a transition contains a significant intramolecular charge t,ransfer character along the C-OCHa axis. The short-axis polarizatsion of the a t#ransition in &methoxypsoralen can then be visualized as having a charge transfer character of the I--+ a, type. The t,hird g --t ?T* transition c shows a d value of 5.0 which is smaller and larger than the dichroic ratios of bands b and (b, respectively. This indicates that angle (Y for the third transit,ion moment is far from being zero, although the angle for this transition is smaller than that of the first r + ?r* transition moment (a). These deduct,ions are consistBent with the polarized fluorescence excitat’ion in the region of band c (Fig. 8). CONCLUSIONS
Three uv absorption bands of coumarin series have been assigned to three separate 7r ---f a* transit’ions. The absorption bands below 240 nm have not been characterized owing to difficulties in the measurements of the polarized fluorescence excitat,ion and linear dichroism. The lowest r --+ rr* t’ransition has its origin in the pyrone moiety, and shows a relatively strong red shift upon extension of the m-electron framework of coumarin. With the exception of %methoxypsoralen, the first two low energy g --+ T* transitions are polarized nearly parallel. In one case, 8-methoxypsoralen, the orientations of the transition moment vectors have been deduced from linear dichroism measurements. Polarization charact.eristics of the r * ?r* transitions in the region of 250-400 nm have been described in terms of relative polarization measurements, and the results are con&ent, wibh the predicted orientations of t,he uv transition moments. En7 Angle 0 could be as small as 15” also, but this possibility is unlikely as it is in poor agreement with the polarized fluorescence excitation spectrum and the calculated polarization. 8 Two electrons of the ZP, orbital on the methoxy oxygen were included in the PPP calcrllations.
156
MOORE,
HARTER,
AND
SONG
couragingly, the PPP SCF MO CI calculations have yielded transition energies and polarization directions in good agreement with the observed spectroscopic data. In some cases (e.g., S-methoxypsoralen), the MO calculation correctly reproduced the relative intensity of the first two or three bands. It can also be noted here that without the CI treatment the polarization directions in Fig. 9(I) are completely inconsistent with the experimental data as band a is predicted to be more perpendicular t’o band c than to the band b moment, when the opposite result is observed and predicted after CL In general, CI improves the calculated polarization directions as can be seen from comparing the directions predicted without the use of CI with t’he fluorescence excitation spectra. TableI. TheP-P-PSCF MO CIResultsfor l7+ lT*Transitions in the“VRegion Wa”efunctionb
Energy.
-1
cm
f
Relative Absorbance
a:
7-t8
0.35
0.6
b:
618
0.27
1.d
c:
7-9
0.27
-0.5
d:
7+10
0.10
-1.1
a:
8-rg
0.12
0.5
la:
7+9
0.61
l.Od
c:
7110
0.77
2.5
d:
8+10
0.47
a:
9+10
0.04
0.2
b:
8*1”
0.64
l.Od
c:
s.11
0.92
1.2
d:
8+11
0.23
2.0
e:
771”
0.14
-2.0
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
157
In conclusion, we wish to point out that assignments and determination of the polarization characteristics of uv and visible electronic bands of heterocyclic and asymmetric molecules cannot be readily deduced from theories, group theoretical or otherwise (e.g., free-electron model). Although results obtained in this work are qualit,ative in nature, combinations of different experimental and theoretical methods have proven to be useful in satisfactorily describing t,he electronic spectra of t.he coumarin series. No definite identit,y or location of an 11+ T* band has been determined. It can be suggested that it is hidden under t’he more inbense ?r --+ r* bands. Since coumarins fluoresce relatively strongly and the polarization data are consistent with t’he T ---f r* assignment’, it can be safely concluded that an n + P* transition is not a lowest singlet transition in coumarins under the condition of the present experimenbs. Finally, the Appendix lists the pert,inent data derived from YIO calculat’ions. APPENDIX
Table I lists the results of the PPP SCF MO CI calculations of ?r -+ K* transitions in the uv region. Observed transition energies and relative absorbance are also indicated for comparison where possible. RECEIVED: June 3, 1971 REFERENCES 1. P. S. SONO, M. L. HARTER, T. A. MOORE, BND W. C. HERNDON, Photochem. Photobiol.,
to appear. 2. M. L. HURTER AND P. S. SONG, Abstracts of the 26th Molecular Spectroscopy Symposium, June 14-18, 1971, Columbus, Ohio. S. P. 8. SONG AND W. H. GORDON, J. Phys. Ch.em. 74,4234 (1970). 4. K. II. B. FRASER, J. Chem. Phys. al,1511 (1953). 5. R. I). B. FRBSER, J. Chem. Phys. 24, 89 (1956). 6. M.
BEER, Proc. Royal Sot. London A 236, 136 (1956). 7. A. YOGEV, L. MARGULIES, lick Y. MAZUR, J. Amer. Chem. Sot. 92, 6059 C(1970), references
and
therein.
Chem. Phys. 37,2413 (1962). 9. S. I. V~VILOV AND 1’. L. LEVSHIN, 2. Phys. 16,136 (1923). 10. A. H. K.~L_~NTAR AND A. C. ALBRECHT, Ber. Buns. Phys. Chern. 68,361 (1964). 11. V. L. LEVSHIN, 2. Phys. 32,307 (1925). 18. F. PERRIN, Ann. Phys. 12, 169 (1929). IS. P. S. SONG, T. A. MOORE, W. H. GORDON, M. SUN, AND C.-N. Ou, “Organic Scintillators” (D. L. Horroeks and C. T. Peng, Eds.), p. 521, Academic Press, New York, 1971. 8. T. AZUMI .IND S. P. MCGLYNN, J.
24. E. W.
THULSTRUP, J. MICHL, .IND J. H. EGGERS,
J. Phys. Chem. 74, 3868 (1970).
OF
MOLECULAR
Ultraviolet
SPECTROSCOPY
Spectra
4~0,
144-157 (1971)
of Coumarins
and Psoralens’
THOMAS A. MOORE,~ MARIAN L. HARTER~ AND PILL-SOON SONGS Department
of Chemistry,
Texas Tech University, Lubbock, Texas 79409
The P + K* transitions of coumarins in the ultraviolet region have been described on the basis of fluorescence polarization, dichroic spectra, and SCF MO CI results. The two lowest * + a* transitions in coumarin, 4-hydroxycoumarin, and psoralen were found to be polarized nearly parallel to each other, probably along the long molecular axis. The two lowest ?r -+ ?r* transition moments in 8-methoxypsoralen were found to be oriented with a substantial angle between them, in contrast to the case of coumarin and psoralen. It was not possible to identify any n + ?r* bands as they are probably obscured by the strong ?r-+ ?r*bands. INTRODUCTION
Interestingly, the GO phosphorescence bands of coumarin and psoralens (fuocoumarins) are located at exactly the same frequency (1, d), although the absorption and fluorescence bands of psoralens are substantially red shifted with respect to coumarin. Furthermore, fluorescence polarization with respect to the two long-wavelength bands is only slightly different in the case of coumarin (3). In an attempt to elucidate the spectroscopic characteristics of these molecules, the present paper describes assignments of the ultraviolet bands in terms of the polarized fluorescence excitation spectra under photoselective conditions, linear dichroism in a stretched polyethylene film, and SCF MO CI calculations within the Pariser-Parr-Pople (PPP) approximation. MATERIALS
AND METHODS
Materials Coumarin (1,2_benzopyrone), 4-hydroxycoumarin, and psoralen (7H-furo[3,2-g][l]benzopyran-7-one) were used as obtained in our previous work (1, 3). S-Methoxypsoralen was a gift from Professor L. Musajo. All of these compounds were cryst#alline and were essentially free from impurities. The absence of im1This work was supported in part by the Robert A. Welch Foundation (Grant D-182) and the National Science Foundation (Grant GB-21266). 2 An NSF predoctoral trainee (lQ6&1972). 8A Robert A. Welch Foundation Predoctoral Fellow (1971-1972). * To whom reprint requests may be addressed. 144
ULTRAVIOLET
SPECTRA OF COUMARINS AND PSORALENS
145
purities in these samples was assured by determing the independence of excitation spectra upon the emitting frequency and vice versa. Solvents were of spectroscopic quality, and results obtained using ethanol (U.S. Indust,rial Co.) as the solvent are presented in this paper. Polyethylene films were obtained from Sargent-Welch.
Fluorescence and polarization spectra were obtained at 77”K, as described previously (3). For linear dichroic measurements, the compounds of interest were dissolved until saturation in chlorobenzene at 343°K which cont.ained a strip of polyethylene 1 X 3 cm cut from a polyethylene film (thickness 4.4 X low3 cm). After approximately 24 h, the polyethylene was removed, dried and washed with ethanol. The film was then stretched to approximately 6 times its original length (thickness - 2 X 10m3 cm) and was fitted to a special holder which fits into the cell compartment of a Bausch & Lomb Spectronic 600 spectrophotometer. The spectrophotometer has been fitted with Polacoat 105 uv polarizers in both the sample and reference beams. The reference was a stretched film prepared without solutes added. Dichroic ratios were then measured by scanning the absorption spectrum with t,he polarizers either vertical or horizontal. Following the development of Fraser (4, 5), Beer (6’), and Yogev et al. (7), we have used the following expression in analysing dichroic spectra of t.he stretched polyethylene film containing appropriate solute where A Ii and =1, are the absorbances parallel
c&!L= A,
f cos2 a! + 0.33( 1 - f) 0.5 f sin2 (L + O-33(1 - f)
(1)
and perpendicular to the direction of stretching, respectively, f is the fraction of aligned molecules and a! is the angle between the transition moment vector and the effective orientation axis of the molecule. The details of the use of the above equation will follow in the discussion section. Molecular
Orbital Calculations
The PPP SCF MO CI calculations were performed as described previously (3). More than 25 singly excited configurations were built into the CI matrices, and the effect of CI on the orientation of the transition moments was taken into consideration. RESULTS AND DISCUSSION Figure 1 shows the absorption and polarized fluorescence excitation spectra of coumarin with respect to fluorescence emission at 384 nm. The fluorescent state has been assigned (r, 7r*) symmetry (3). The polarization spectrum of coumarin was readily reproduced with only a small difference from one run to another.
146
MOORE,
HARTER,
AND SONG
0.8
1
0.5
0.7
0.6 +
-0.1 -0
0.5 Q) E ; 0.4 8 cn s
P
0.3
f 0.5
a
0.2
0.4
A__L 0.3
0.1
0
0.2
0
400
350
300
250
fVn
FIG. 1. Absorption (-) and polarized fluorescence excitation (-O-O-) spectra of coumarin in ethanol at 77°K. The polarized fluorescence excitation spectrum refers to the ordinate of polarization degree P, while the theoretical oscillator strengths indicated by the vertical lines refer to the f ordinate.
The polarization was corrected for instrumental bias by the Azumi-McGlynn procedure (8). Since the polarization degree (P) across the first absorption band the theoretical maximum of 0.5 for a classical (&l,X = 313 nm) approaches Vavilov-Levshin linear oscillator (9) and the second absorption band (X,,, = 275 nm) shows only slightly lower P value, it can be readily concluded that the transition moments of the two lowest a + ?r* bands are close to being mutually parallel. However, the exact angle 0 between the two transition moment vectors
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
147
cannot be estimated because of the depolarization from P = 0.5 due to what may be described as “randomization factor” (10).j Additionally, the P value (-0.4) for the first ?r t P* band may have been slightly lowered by the ?l+ r* int#ensity hidden underneath the strong n -+ ?r* band. In any case, it is qualit’atively possible to estimate a range of /3 from 10” f 5” using the Levshin-Perrin equation (11, 12) P = (3 cos* e -
l)/(cos” 0 + 3)
(2)
depending upon whether the wavelength-independent “randomization factor” (10) has been neglected or applied. There is a tendency toward lower polarization values in the fluorescence excitation spectrum of coumarin in t,he 240-250 nm region. However, a further measurement beyond 240 nm was not possible due to the reduced transmittancy of the Glan-Thompson polarizer and t’he lack of intensity from the xenon source in this region. Figure 2 shows the dichroic spectra of coumarin in a stretched polyet,hylene film. Values of d are 1.8 and 2.3 at 313 and 270 nm, respectively. Since these values are rather small, a quantit,ative deduction of the transition moment directions cannot be made. In other words, the value of d ‘V 2 can be accounted for in terms of a small orientation factor f or in berms of a large angle cy. An inspection of the dependence of the dichroic ratio on parameters f and cy shows that, the cl value of about 2 can result from a wide range of f (0.2 - 0.6) and a! (0 - 40”) (6). Thus it is consistent with the dichroism in Fig. 2 that the t,wo transition moments are far from being perpendicular, and that t,he second 7r -+ 7r* kansition moment has a somewhat less angle QIt,han the first, one. Since the cl values of the two lowest transitions are only slightly different, result’s extracted from Fig. 2 are consistent with the polarized fluorescence excitation spect’rum (Fig. 1). Figure 3 shows t,he predicted polarization of t,he three low-energy K -+ a* transitions in coumarin. Results after the CI treabment are consistent’ with the polarized fluorescence excitat.ion and dichroic spectra. The predicted angle 8 bet,ween bands a and b is 16”. The third transition c is polarized along the short axis and is therefore perpendicular to the two lowest transit’ions a and b. This seems to be borne out qualitatively by the lower fluorescence polarization and lower dichroism beyond 250 nm. The predicted energies are in excellent agreement with the absorption bands, as shown in Fig. 1. In view of the consistency obtainable from three different sets of data, namely, the polarized fluorescence excitation, linear dichroism, and SCF MO CI calculations, our 5The nature of the “randomization factor” is not well understood, although optical defects due to strain in the sample in the rigid organic glasses may be the cause (10). It is, however, interesting to note that a P value less than 0.5, usually 0.4 - 0.42, is also obtained in a polymethylmethacrylate plastic which was slowly polymerized along with solutes. In the case of all-trans retinol, the P value was found to be about 0.4 in et,hanol at 77X, while it frequently reaches 0.5 in a near Shpol’skii matrix (13).
148
MOORE,
HARTER,
AND SONG
0.5
0.4
P 2
0.3
‘0 2 Q
0.2
0. I
0
I
400 FIG.
I
,rc;; I
350
I
300
,w
1
250
1
2. Dichroic
spectra of coumarin in a stretched polyethylene fdm at 298°K. 11and I refer to the absorbance with polarizer parallel and perpendicular with respect to the axis of stretching, respectively.
0
FIG. 3. Polarization directions of three ?r + ?r* transitions PPP SCF MO with (II) and without (I) CI.
in coumarin
predicted
by the
of the uv bands of coumarin in terms of three ?r -+ ?r* transitions can be considered satisfactory. The first n + r* transition is possibly of pyrone origin (.!I), while the second transition is of benzenoid origin. However, configuration interactions among the various excited species are so extensive that the origin of each of the uv bands has only diagnostic meaning in classifying the electronic bands of the coumarin series.
assignments
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
149
Figure 4 shows the observed and predicted spectra for 4-hydroxycoumarin. Some vibrational structures have appeared upon substitution with the 4-hydroxy group. A strong O-O absorption band in the red edge of the absorption spectrum reflects a negligible change in the location of potential minima. In general, polarization characteristics of the bands a and b are similar to the case of coumarin. Xamely, the angle 0 between the two lowest a + a* transition moments is in the range of 14’ f 5” in agreement wit’h the predicted polarization (0 = 17”) shown in Fig. 5(U). Band c is predict,ed to be nearly perpendicularly polarized with respect to the lowest, transition a. A qualitative confirmation of this prediction is seen, as the polarization degree declines steadily below 250 nm.
I I I I 40 I I I Icm I x10-3 0.8 I30 I I 1 35
0.6
i0
300
0 250 nm
FIG. 4. Absorption (-) and polarized fluorescence excitation (-O-O-) spectra of 4-hydroxycoumarin in ethanol at 77°K. The theoretical results are also indicated. See the legend for Fig. 1 for further explanations.
150
MOORE,
HARTER,
AND
FIG. 5. Polarization directions of four T + T* transitions by the PPP SCF MO with (II) and without (I) CI.
SONG
in 4-hydroxycoumarin
predicted
Figure 6 shows the observed and predicted spectra for psoralen. First,, agreement between the observed absorption maxima and calculated transition energies is satisfactory. The relative extinctions of the uv bands are also in good agreement with the MO calculations. It can be seen that the difference in P across bands a and b is somewhat larger than in coumarin (Fig. l), indicating that B is slightly larger for psoralen. It can be seen t.hat these transitions are far from being perpendicularly polarized. The theory also predicts a relat,ively small angle 0, as shown in Fig. 7(11). The predicted angle is 32”, while the est,imated angle from the polarization spectrum in Fig. 6 is in the range of 20 f 5”. The strong band in the 250-nm region is predicted to consist of two transit,ions c and cl. A refined polarization in the uv region is desirable to resolve this band. There is a relatively strong red shift of the absorption maximum in going from coumarin to psoralen as is expected theoretically from the extended conjugation of the parent system. Figure 8 shows spectra for S-methoxypsoralen. The first absorption band is now obscured by the second band b. The theory also predicts considerable weakening of band a, as is indicated by a small oscillator strength. At the first glance, of the apparent it is easy to misassign it to an n ---f T* band. The frequency maximum of band a remains essentially unchanged upon substitution of a methoxy group at position 8 of psoralen. However, a slight red shift is seen for the second band b, which shifted from 275 nm in coumarin and 290 nm in psoralen t,o 300 nm in 8-methoxypsoralen. Thus, in terms of the intensity weakening of the first band a and the red shift of the second band b with respect to coumarin, 8-methoxypsoralen seems to be a unique model for a detailed analysis of the substituent effect on the electronic spectra of coumarin series. Figure 9 further demonstrates the suggestion that the 8-methoxy group provides a strong perturbation of the a-electron system. Transition a is now nearly short-axis polarized in contrast to all previous coumarin derivatives, although the second transition b remains polarized nearly along the long-axis containing the carbonyl bond. The angle 8 between the transition moment, vee-
ULTRAVIOLET
SPECTRA
25
i.or-
OF COUMARINS
AND PSORALENS
151
40 cm% 1oe3
30
f - I.2 i
C
- 0.8
d
-0.4
400
350
300
250nm
FIG. 6. Absorption (-) and polarized fluorescence excitation (-O-O-) spectra of psoralen in ethanol at 77°K. The theoretical results are also indicated. See the legend for Fig. 1 for further explanations.
(I) FIG. 7. Polarization direcaions of four ?r -+ r* transitions PPP SCF MO with (II) and without (I) CI.
(II) in psoralen
predicted
by the
152
MOORE, HARTER,
AND SONG
2.
0.7
0.6
- 0.6
de
400 350
300
-
0.4
f
250 nm
FIG. 8. Absorption
(-) and polarized fluorescence excitation (-O-O-) spectra of 8methoxypsoralen in ethanol at 77°K. The theoretical results are also indicated. See the legend for Fig. 1 for further explanations.
tors of a and b is 89”. A range of 40” f 10” can be estimated from the fluorescence excitation polarization spectrum, shown in Fig. 8, and Eq. (2). While quantitative agreement has not been achieved, the theory is consistent with the experiment in that a larger angle 0 is found between transition moments a and b in 8-methoxypsoralen than in other coumarins examined, as was predicted from the MO calculations. In order to strengthen the polarization assignments in S-methoxypsoralen, linear dichroic measurements were carried out. It is, for example, anticipated that band a would show a lower cl value than band 6, since the molecule will preferentially orient itself with its long molecular axis (possibly cont,aining the carbonyl bond) along the axis of stretching of the polyethylene film. From Fig. 2, it can be assumed that the transition moment of band b is oriented more
ULTRAVIOLET
SPECTRA OF COUMARINS AND PSORALENS
a FIG. 9. Polarization directions of four ?r + ?r* transitions by the PPP SCF MO with (XI) and without (I) CI.
153
(I) in %methoxypsoralen
predicted
closely along the carbonyl axis than that of band a, since the former has a slightly higher dichroic ratio and stretching of the film may very well force the molecule to align its carbonyl axis along the stretching axis. Note that band b is predicted to be polarized more closely along the carbonyl axis than band a in coumarin (Fig. 3). In other words, the effective orientation axis6 of an asymmetric molecule will lie very near the preferred molecular orientation axis which is estimated as the axis in the direction perpendicular to the smallest cross section on a molecular model. The effective orientation axis is parallel to the axis of stretching of the film. From Fig. 10, it can be deduced that the preferred molecular orientation axis lies very nearly along the direction of the calculated moment of the transition b at 300 nm. Taking the angle a! between 300-nm transition moment and the effective orientation axis to be zero and using the observed dichroic ratio of d = 7.3, Eq. (1) gives f = 0.68. Although assuming larger values of CLand thus calculating larger values of f would in principle satisfy Eq. (1) for d = 7.3, it is unlikely as the observed f values are generally not greater than the 0.68 calculated here. This is partly because of the presence of microscopically iso6That axis such that if the molecule had a transition polarized along that particular direction it would have a higher dichroic ratio than that of a transition polarized in any other direction. See Ref. (24) for further details.
MOORE, HARTER,
154
30
5
+
35
AND SONG
401
45I cm-‘.10S3
0.6
0” s 0.5 n 5 $ 0.4 0.3
0
400
350
1 I I,0 300 250nm
FIG. 10. Dichroic spectra of 8-methoxypsoralen in a stretched polyethylene film at 298°K. Note that two components (11and I) of the spectra refer to separate absorbance ordinates.
tropic regions in the stretched film. The present conclusion regarding parallel or nearly parallel orientation of the b transition moment and the stretching axis thus depends on the fact that the observed d value of 7.3 is exceptionally large (7, 14). Information deduced from Fig. 10 is consistent with the polarized fluorescence excitation spectrum shown in Fig. 8. Namely, the band b should be minimum and maximum in the polarized fluorescence excitation and linear dichroic spectra, respectively, in order for the above deduction concerning parallel orientation of the band b transition moment and stretching axis to be true. Both sets of experiments shown in Figs. 8 and 10 confirm the theoretical predictions. Using the calculated value off and the observed dichroic ratio of 2.5 for the band a at 387 nm, a! is calculated to be 36” for the lowest energy transition, It should be noted that angle LYcould be up to 10 N 15’ for the 300-nm transition
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
15.5
b without requiring a large increase in j’. Therefore, the angle f3between transition moment vectors a and b could be as large as 50”.’ Thus, the range of 36 - 50’ agrees satisfactorily with the polarized fluorescence excitation (Fig. 8) and only qualitatively with the SCF MO CI results (Fig. 9). It is interesting to note, from Figs. 7 and 9, that the direction of the a transit’ion moment in S-methoxypsoralen is essent#ially along the short axis and is not as significantly affected by the configuration interaction as in the case of psoralen. Although the form of the highest-filled MO’s of psoralen (8) and Smethoxypsoralen (9) is similar and that, of bhe lowest-vacant MO’s of psoralen (9) and S-methoxypsoralen (10) is also similar in the region of the common molecular framework, the highest-filled MO of 8-methoxypsoralen is additionallq characterized by a significant, cont,ribution from the methoxy oxygen orbital (BpJ with an expansion coefficient. of -0.30.” However, the coefficient in bhe MO (10) is only -0.07. This yields a transition monopole of 0.015 on the methox) oxygen. As a result, the a transition contains a significant intramolecular charge t,ransfer character along the C-OCHa axis. The short-axis polarizatsion of the a t#ransition in &methoxypsoralen can then be visualized as having a charge transfer character of the I--+ a, type. The t,hird g --t ?T* transition c shows a d value of 5.0 which is smaller and larger than the dichroic ratios of bands b and (b, respectively. This indicates that angle (Y for the third transit,ion moment is far from being zero, although the angle for this transition is smaller than that of the first r + ?r* transition moment (a). These deduct,ions are consistBent with the polarized fluorescence excitat’ion in the region of band c (Fig. 8). CONCLUSIONS
Three uv absorption bands of coumarin series have been assigned to three separate 7r ---f a* transit’ions. The absorption bands below 240 nm have not been characterized owing to difficulties in the measurements of the polarized fluorescence excitat,ion and linear dichroism. The lowest r --+ rr* t’ransition has its origin in the pyrone moiety, and shows a relatively strong red shift upon extension of the m-electron framework of coumarin. With the exception of %methoxypsoralen, the first two low energy g --+ T* transitions are polarized nearly parallel. In one case, 8-methoxypsoralen, the orientations of the transition moment vectors have been deduced from linear dichroism measurements. Polarization charact.eristics of the r * ?r* transitions in the region of 250-400 nm have been described in terms of relative polarization measurements, and the results are con&ent, wibh the predicted orientations of t,he uv transition moments. En7 Angle 0 could be as small as 15” also, but this possibility is unlikely as it is in poor agreement with the polarized fluorescence excitation spectrum and the calculated polarization. 8 Two electrons of the ZP, orbital on the methoxy oxygen were included in the PPP calcrllations.
156
MOORE,
HARTER,
AND
SONG
couragingly, the PPP SCF MO CI calculations have yielded transition energies and polarization directions in good agreement with the observed spectroscopic data. In some cases (e.g., S-methoxypsoralen), the MO calculation correctly reproduced the relative intensity of the first two or three bands. It can also be noted here that without the CI treatment the polarization directions in Fig. 9(I) are completely inconsistent with the experimental data as band a is predicted to be more perpendicular t’o band c than to the band b moment, when the opposite result is observed and predicted after CL In general, CI improves the calculated polarization directions as can be seen from comparing the directions predicted without the use of CI with t’he fluorescence excitation spectra. TableI. TheP-P-PSCF MO CIResultsfor l7+ lT*Transitions in the“VRegion Wa”efunctionb
Energy.
-1
cm
f
Relative Absorbance
a:
7-t8
0.35
0.6
b:
618
0.27
1.d
c:
7-9
0.27
-0.5
d:
7+10
0.10
-1.1
a:
8-rg
0.12
0.5
la:
7+9
0.61
l.Od
c:
7110
0.77
2.5
d:
8+10
0.47
a:
9+10
0.04
0.2
b:
8*1”
0.64
l.Od
c:
s.11
0.92
1.2
d:
8+11
0.23
2.0
e:
771”
0.14
-2.0
ULTRAVIOLET
SPECTRA
OF COUMARINS
AND
PSORALENS
157
In conclusion, we wish to point out that assignments and determination of the polarization characteristics of uv and visible electronic bands of heterocyclic and asymmetric molecules cannot be readily deduced from theories, group theoretical or otherwise (e.g., free-electron model). Although results obtained in this work are qualit,ative in nature, combinations of different experimental and theoretical methods have proven to be useful in satisfactorily describing t,he electronic spectra of t.he coumarin series. No definite identit,y or location of an 11+ T* band has been determined. It can be suggested that it is hidden under t’he more inbense ?r --+ r* bands. Since coumarins fluoresce relatively strongly and the polarization data are consistent with t’he T ---f r* assignment’, it can be safely concluded that an n + P* transition is not a lowest singlet transition in coumarins under the condition of the present experimenbs. Finally, the Appendix lists the pert,inent data derived from YIO calculat’ions. APPENDIX
Table I lists the results of the PPP SCF MO CI calculations of ?r -+ K* transitions in the uv region. Observed transition energies and relative absorbance are also indicated for comparison where possible. RECEIVED: June 3, 1971 REFERENCES 1. P. S. SONO, M. L. HARTER, T. A. MOORE, BND W. C. HERNDON, Photochem. Photobiol.,
to appear. 2. M. L. HURTER AND P. S. SONG, Abstracts of the 26th Molecular Spectroscopy Symposium, June 14-18, 1971, Columbus, Ohio. S. P. 8. SONG AND W. H. GORDON, J. Phys. Ch.em. 74,4234 (1970). 4. K. II. B. FRASER, J. Chem. Phys. al,1511 (1953). 5. R. I). B. FRBSER, J. Chem. Phys. 24, 89 (1956). 6. M.
BEER, Proc. Royal Sot. London A 236, 136 (1956). 7. A. YOGEV, L. MARGULIES, lick Y. MAZUR, J. Amer. Chem. Sot. 92, 6059 C(1970), references
and
therein.
Chem. Phys. 37,2413 (1962). 9. S. I. V~VILOV AND 1’. L. LEVSHIN, 2. Phys. 16,136 (1923). 10. A. H. K.~L_~NTAR AND A. C. ALBRECHT, Ber. Buns. Phys. Chern. 68,361 (1964). 11. V. L. LEVSHIN, 2. Phys. 32,307 (1925). 18. F. PERRIN, Ann. Phys. 12, 169 (1929). IS. P. S. SONG, T. A. MOORE, W. H. GORDON, M. SUN, AND C.-N. Ou, “Organic Scintillators” (D. L. Horroeks and C. T. Peng, Eds.), p. 521, Academic Press, New York, 1971. 8. T. AZUMI .IND S. P. MCGLYNN, J.
24. E. W.
THULSTRUP, J. MICHL, .IND J. H. EGGERS,
J. Phys. Chem. 74, 3868 (1970).