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Discrimination between 3ππ* and 3nπ* states in organic molecules by circular polarization of phosphorescence
Volume 161, number 2
CHEMICAL PHYSICS LETTERS
8 September 1989
DISCRIMINATION BETWEEN %t* AND 3nn* STATES IN ORGANIC MOLECULES BY CIRCULAR POLARIZATION OF PHOSPHORESCENCE ****
Peter M.L. BLOK and Harry P.J.M. DEKKERS Department
of Chemistry,Gorlaeus Laboratoties, Leiden University, P.O. Box 9SO2. 2300 RA Leiden, The Netherlands
Received 30 June 1989
The degree of circular polarization gin electronic transitions of chiral molecules is a measure of the ratio of the magnetic and electric dipole transition moments. From theory an upper bound for lgl as a function of dipole strength D can be found. This offers the possibility to distinguish %x* from ‘nn* states in organic molecules. For molecules of approximate Clv symmetry it can be shown that large absolute values of g,,, (order 10-l ) can arise only in the case of Jmr* phosphorescence, whereas emission from %m* states yields low values of g,,, (order lo-‘), This conclusion is substantiated with experimental data on three examples: a-santonin, ( IS)-( - )-I-bromo-a-fenchocamphoronequinone and ( + )-thiocamphor.
1. Introduction
In molecules in which 3nn* and 3~~*states are expected to be proximate, it is of spectroscopic and photochemical interest to know which one is lowest in energy. Several criteria have been developed to discriminate between 3nn* and 3rcrc*phosphorescence. These relate to the magnitude of the phosphorescence lifetime, emission bandshape, the magnitude of the zero-field splitting parameters from triplet ESR measurements and linear polarization of the emission [ 11. We considered it worthwhile to investigate whether in chiral substrates measurement of the circular polarization of the phosphorescence (CPP) can be used to establish the origin of the phosphorescence. In this paper we first discuss the relationship between the magnitude of the degree of circular polarization and whether or not the electronic transitions involved are magnetic dipole allowed. Theory is then illustrated with experimental data on three examples: a-santonin, ( lS)-( - )-l-
*
Dedicated to Professor J.H. van der Waals on the occasion of his retirement from the chair of Physics at L&den University. ** Part of this work has been presented at the FECS Conference on Circular Dichroism, Budapest ( 1987).
188
bromo-a-fenchocamphoronequinone thiocamphor.
and
( + )-
2. Experimental Materials. a-santonin was purchased from Janssen Chimica and recrystallized twice from methanol. ( + )-thiocamphor was prepared [2] from ( + )camphor by reaction with P&, and purified by chromatography in hexane over silica (m.p. 136°C lit. 121: 138-139°C). (IS)-( -)-l-bromo-u-fenchocamphoronequinone was available from stock. Solvents were purified according to conventional methods [3]. Methods. Circular dichroism spectra were measured with a Jobin-Yvon M III dichrograph and absorption spectra with a Cary 14 spectrophotometer. CPP and linear polarization data were obtained using a home-built spectrometer [4], equipped with a chopper [ 5 1. Emission and excitation spectra were recorded on a Spex Fluorolog II spectrofluorimeter with phosphorescence accessory. Low-temperature experiments were performed using a cryostat (Oxford Instruments Ltd., type DN704, modified) or a simple suprasil dewar. When measuring CPP in glasses at low temperature, care must be taken to avoid artifacts that may
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CHEMICAL PHYSICSLETTERS
arise from strain birefringence in the glassy solvent, in the windows of the cryostat and/or of the cuvette. Such artifacts are small if the linear polarization of the luminescence is minimized [ 61. This can be done [ 5,7] by choosing specific configurations of the CPP spectrometer.
3. Theory The rotational strength R in an electronic transition i-j is defined by R = Im _Ulj *mji
(1)
and the dipole strength D by ~=IP~j12+l~,12.
(2)
Here p represents the electric, and I)I the magnetic dipole moment of the electronic transition i-j. The dissymmetry factor g is given by g=4R/D.
(3)
Eqs. ( l)-( 3) hold not only for absorption but also for emission (fluorescence or phosphorescence) provided it is understood that i pertains to the state of lower energy [S,9]. In absorption R(D) can be found from experiment by integrating the circular dichroism (the absorption) over all vibrational fine structure, R=c D=4c
(At/v) dv,
(4)
(E/v) dv,
(5)
s
where c=23x 10m40if R and D are in cgs units and 66 and e in dm3 mol-’ cm-‘. In luminescence the rotational and dipole strength are given by Rlum =c
s
D,,, =4c
(AI/u4) du,
J
(I/v4) dv ,
(6) (7)
where integration is over the entire emission band. Generally an emission experiment is performed in such a way that the constant c cannot be evaluated (but Dlumcan be found from the radiative lifetime of the emission), However, the dissymmetry factor in the luminescence, gl,, - being the ratio of 4R,,,
8 September1989
and D,,,,,, (cf. (3)) - is an absolute quantity. Using ( I ) and (2), and omitting the indices i and .i, g and gl,, are found to be g=4mpcos
e (m2+p2)
--I
(8)
in which 8 is the angle between m and p. From the definitions ofg, Ac( =c,_-tR) and E ( =$(E~++)), it follows that the maximum value of lgl is 2, a situation which is only realized if in a transition m and ,u are equal and (anti)parallel (cf. (8) ). A similar argument holds for g,,,. This maximum value is never attained for organic molecules, because the observed value of D always appears to be 2-3 orders of magnitude larger than the maximum contribution due to the magnetic term (which is w 10m40cgs for m= 1 Bohr magneton, the value for a p-p transition). Consequently, most gvalues will be considerably lower than 2. Fig. 1 shows a semi-logarithmic plot of g as a function of D according to (8). In constructing the figure we have used a value of 1 Bohr magneton (8) for m and the maximum value of cos e. In fact, this figure is a quantitative elaboration of an early suggestion by Mason [lo]. From the figure it appears that in the So+lnn* transition of saturated ketones, where the dipole strength is typically 10-38-10-37 cgs, at most a gvalue of about 1O- ’ can be obtained. For the So-+‘Jo* transition of a&unsaturated ketones, D typically amounts to 2.3X lO-35 cgs (value obtained for testosterone) and so a maximum g-value of 8 x 1O-3 is predicted. The actual values which are observed are much smaller (e.g. + 1x 10e3 in the case of testosterone), obviously because cos 8 is less than 1. So the magnitude of g is connected with the amount of magnetic dipole character in the transition: high gvalues (order 1O- ’ ) are only expected for m-allowed and p-forbidden transitions, whereas for m-forbidden and p-allowed transitions small values (order 10m3) are predicted. With pure spin states the phosphorescent transition is rigorously spin-forbidden and the phosphorescence intensity entirely derives from the initial triplet state and the final singlet state being spin-orbit coupled to other states:
‘b(Qq)-
‘$(Qq) + C ai j
‘$ji(Qq) 7
(9)
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CHEMICALPHYSICSLETTERS
8 September 1989
2.00 t
1.5-
0-I a1
1
I 10
0 10'
1 10'
I Id
Id -
0l10-%gs)
Fig. 1.Semi-logarithmicplot of g as a function of D.
3$tQq)+3#(Qq) + C a,(Q) .i
'@j)i(Qq)*
(10)
The amount of mixing is governed by the magnitude of the coefficients aj which, in terms of perturbation theory, contain energy denominators such as E( 3#j)-E( ‘$) and matrix elements (3$j,IL*S( I$>, where L and 5 are the operators for orbital and spin angular momentum, respectively. The variables Q and q in (9) and ( 10) denote the coordinates of nuclei and electrons, respectively. Generally, the admixing of triplet character to the singlet ground state will be small because of the large energy difference involved and thus the intensity of the phosphorescence originates primarily from singlet --$ singlet transitions (cf. (10)). Consider the case that the phosphorescent state 3@is contaminated with one singlet state only, e.g. ‘&. The radiative properties of the triplet state are then determined by the character of the admixed singlet state and the strength of the coupling: R(T+S)=
~u,~*R(S,*‘&) >
O(T~S)=lUjl’O(S,~‘g).
(11) (12)
However, the dissymmetry factor in the phosphorescence equals that of the admixed SO-+‘@jtransition since it does not contain the coefficient a. We consider a chiral molecule with approximate 190
Czy symmetry where the two lowest excited states in the singlet and triplet manifold are nx* and xn*. Then 3~x* phosphorescence derives intensity predominantly from the ‘nr? contamination by first-order spin-orbit coupling because the matrix element ( ‘nff*(L.S131t7c*) is much larger than ( ‘XX*1L*S( 3~~*} [ 11,12 1. Analogously, phosphorescence from the 3nff* level results primarily from a ‘KER* admixture. In this approximation the dissymmetry factor in the two types of phosphorescence is equal to g( S,+‘nx*) and g(SO+‘xx* ), respectively. In practice an exact equality will be illusory. First, this would require identical geometries in ground and phosphorescent state. Further the deviation from CzV symmetry of the chiral molecule will lead to a more complex spin-orbit coupling behaviour. In considering for instance the ?tn* state, one must expect that, apart from ‘nx*, other singlet states are also admixed to a certain extent. If such additional couplings OG cur with other sublevels k of the triplet state, gl,, equals 4CkRk/CkD,, which destroys the correspondence between gl,, and the dissymmetry factor of a particular singlet-singlet transition. However, the observation of a large absolute value of gl,, is still proof of a magnetic-dipole-allowed phosphorescent transition. If, on the other hand, extra couplings occur with the same triplet sublevel, interference effects may even lead to a breakdown of the rule that
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CHEMICAL PHYSICS LETTERS
8 September 1989
high glum values indicate 3mt* emission. For instance, the electric dipole transition moments due to two admixed electric dipole-allowed transitions might nearly cancel, which can give rise to a very small value of C, 1ah 1‘Dk and, occasionally, to a high value of &m- The fact that the phosphorescence is magnetic dipole allowed can no longer be associated with the x7c*character of the spatial part of the triplet wavefunction. Such cases may be recognized by using other experimental criteria for distinguishing 331;n* and ‘no* emission, e.g. phosphorescence band shape. However, if deviation from CZVsymmetry is small, major interference effects are not probable, Lastly we mention that one does not expect spin to contribute appreciably to R (T+ S), that is; the effect due to the operator 2s in the expression for m ( = (e/2mc) (LS 2s) ) is small. The contribution of S to the magnetic dipole transition moment is given by the matrix element ( ‘&o-tIiil:$i12SI ‘@ t C$, ‘@j}, cf. (9) and (10). Nonvanishing terms arise from { I@,,i-1 3@I2SI 3@), i.e. from the phosphorescent triplet state being spin-orbit coupled to the ground state. Since these states are far apart, the effect of the magnetic moment due to spin is expected to be relatively small, the more so if the phosphorescent transition is m allowed due to orbital momentum ( 3nn*-+S,,,for instance). Although probably difficult to identify, inherent spin effects on rotational strength might be best studied in L-forbidden transitions (e.g. ‘mt*-+!!&).
4. Experimental results and discussion 4.1. a-santonin The low-temperature absorption and emission spectra and the linear polarization of a-santonin are given in fig. 2. They essentially agree with the data reported by Marsh et al. [ 131 who assigned these features as follows. The weak band system between 300 and 380 nm ( cmax= 30 dm3 mol-’ cm-‘) in the absorption spectrum is due to the &+‘nrc* transition. Its intensity mainly arises from vibronic coupling, presumably with the %t* state since its linear polarization equals that of the SO-+%n* transition. The very weak band observed by Marsh et al. [ 131 at = 393 nm in the phosphorescence excitation spec-
Fig. 2. a-santonin. Top: absorption spectrum, phosphorescepce spectrum and linear polarization data (p). Bottom: g,, in the !&Inn* band (-) and g,,, values (_._).The absorption data pertain to measurements in a methylcyclohexane/isopentane/2methyltetrahydrofuran glass at 9 80 K, the luminescence data to an ethanol/isopentane/diethyl ether glass at 77 K; standard error in the g,,, values is 2 x IO-‘.
trum (t = 0.3 dm3 mol- ’ cm- ’ ) was ascribed to the S0+3nn* absorption. The virtually structureless luminescence (robs= 79 ms/77 K, 2-MTHF) [ 131 was assigned to 31cx*phosphorescence. We have also measured the circular dichroism and the circular polarization of the phosphorescence of cc-santonin (fig. 2) + The CD in the So+ ‘nlc* transition is negative and of medium intensity. The value of the dissymmetry factor appears to depend strongly on wavelength, ranging from = 5 x 1OA2near the origin transition to zero near 3 10 nm. The CD in the weak transition at 393 nm is zero within experimental error. Taking into account the sensitivity of the CD spectrometer this implies that the absolute value of the dissymmetry factor here is less than 4~ 10m4.The degree of circular polarization of the phosphorescence is high and shows a moderate wavelength dependence: -65 to -50X 10-3. According to the theoretical considerations outlined above, the large magnitude of 1g,,, I is strong evidence that the phosphorescence is m allowed and occurs from a 3nn* state. Furthermore, the smallness of the dissymmetry factor in the 393 nm band is entirely consistent with the S0-3nx* assignment of Marsh et al. [ 131 and with their conclusion that its intensity derives from spin-orbit coupling with the (strongly electric dipole allowed) So+ ‘xx* transition. The SD-+‘nit* CD of a-santonin exhibits a pro191
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CHEMICAL PHYSICS LETTERS
gression in a 1150 cm-’ vibration which can be identified with the carbonyl stretching frequency in the ‘nx* state observed in the CD of many a&enones [ 141. It appears that the shape of the CD band corresponds to a nice Franck-Condon distribution of intensities [ 51, which suggests that the CD speo trum arises from electronic rotational strength modulated by Franck-Condon factors. Whereas in CD the vibronically induced component is much smaller than the electronically allowed one, the opposite is true for the unpolarized absorption as evidenced by the strong wavelength dependence of g. This situation is as expected. In local CzV symmetry the So+ ‘nrt* transition is electric dipole forbidden. The chiral molecular framework in which the dienone chromophore is embedded introduces a small electric dipole transition moment which generates all of the CD, but scarcely contributes to the dipole strength. If in emission vibronic coupling effects were absent, both D,,, and RI,, should be modulated by Franck-Condon factors, which would make gl,, independent of wavelength, This apparently is not the case (fig. 2). So the first-order spin-orbit coupling scheme is not sufficient and we have to envisage spinvibronic effects which are expected to introduce So-+‘XX*character into the phosphorescence [ 151. The large value of 1gl,, 1 suggests, however, that the effect is relatively unimportant. If phosphorescence intensity derives primarily from spin-orbit coupling with the ‘mc* state, the equivalence of g and gl,, near the (0,O) transitions of the S+‘nrr* absorption and the phosphorescence indicates that distortion of the triplet state is small. An important distortion has been suggested for asantonin [ 121 and for a$-enones [ 15 ] (later reconsidered for a, Benones [ 16 ] ) .
orescence ( -35x 10-j at 480 nm (bandwidth 10 nm), 77 K) and points to ‘nrr* type phosphorescence. The circular polarization of luminescence of a related compound, camphorquinone, has been reported by Steinberg et al, [ 171. In the long wavelength tail of the luminescence (where the authors supposed fluorescence to be absent) they found small values of 1g,,, 1. According to the present gl,, criterion this would indicate emission from a 3n~*level. 4.3. (+)-thiocamphor The absorption and emission characteristics of thiocamphor in MIP (methylcyclohexane/isopentane glass 1: 3) at low temperatures are depicted in fig. 3. The absorption in the 360-560 nm region is due to the SO+‘mc* transition, except for the low intensity band near 550 nm. The emission can be assigned to phosphorescence because of its lifetime (0.14 ms) [ 181, The CD in the &+‘nx* absorption band turns out to be bisignate. At large wavelengths (~520 nm) the value of g is 1x 10-r (fig. 3). In contrast, the value of g in the 550 nm band appears to be much lower (23x low4 on the average) [ 191. As noted before [ 21, the vast difference between the magnitude of g in this band and that in the &,+‘nx* absorption suggests that the band does not belong to the regular SO+‘nx* system. In fact, the
4.2. (IS)-(-)-l-bromo-CCfenchocamphoronequinone This species contains the a-dione chromophore, shows both fluorescence and phosphorescence at low temperatures. By using a chopper the latter luminescence (A,,, = 549, 573 nm (sh) ) can be isolated. In the phosphorescence band g,,, is small and constant as a function of frequency: + (3f2) x 10e3. The smallness of g,,, contrasts in sign and order of magnitude with the gl,, value in the ‘na*-&, flu192
I
, LOO
I
500
I
600
I
I
Alnml 7W
Fig. 3. Thiocamphor in methylcyclohexane/isopentane glass, 77 K. Top: absorption spectrum, phosphorescence spectrum and
linear polarization data (p). The latter are given with respectto M (emission) and X (excitation) wavelength. Bottom: g,,. in the S,,+%A* region (-) ing,,, is 1 X lo-‘.
and g,,, values (.~.). Standard error
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CHEMICAL PHYSICS LETTERS
similarity of the values of the dissymmetry factor in the 550 nm band and in the phosphorescence strongly suggests that the 550 nm shoulder is the absorption analogue of the phosphorescence transition, i.e. it is due to the S,,-+T transition [ 2,181. Further evidence is provided by the high degree of linear polarization of this band with respect to the phosphorescence (fig. 3), which implies parallel polarizations of the 550 nm band and the T-+S,, transition. Moreover, this indicates that a distortion of the triplet state, which has been found for thioformaldehyde [ 191, if it occurs at all, does not appreciably affect the chiroptical properties. From the large spectral overlap of the 550 nm band and the phosphorescence, and the equality of g and gl,, in this region, we conclude that the emitting triplet state is not severely distorted. So far we have not discussed the electronic character of the triplet state. From theory it follows [ 201 that in aliphatic thiones the lowest triplet state is of 3nrc*type, and a similar conclusion has been reached by Blackwell et al. [ 2 1 ] on the grounds of the chemical behaviour of that state in quenching experiments. The small value of g’,, we observe is entirely compatible with emission from a 3nrt* level, The absence of tine structure in the circular and linear polarization of the phosphorescence indicates that vibronic coupling effects are of minor importance in the phosphorescence process. This is to be expected if phosphorescence intensity derives predominantly from spin-orbit coupling with a strongly electric dipole allowed Sod ‘xx* transition. In our simple model, the So+3nn* transition derives its radiative properties from the SO-+‘xx*transition, i.e. the latter should have a positive dissymmetry factor. The absorption and CD spectra of thiocamphor in the UV region were reported by Emeis [ 21, who found that the second absorption band (230 nm, cmex=lo4 dm3 mol-’ cm-‘) has a negative CD, and the third (2 15 nm, emax=5 x lo3 dm3 mol-’ cm-‘) a positive CD (magnitude not reported). Whether this implies that in thiocamphor the third and not the second transition is the S&,+‘nn* transition - in contrast with the findings of Lightner et al. [22] from ab initio calculations on thioacetone - or that our model is too simple, we do not yet know. In this respect we mention that it appears from magnetic circular dichroism experiments that more than one triplet sublevel carries intensity [ 18,231.
8 September 1989
5. Conclusion The magnitude of g probes the ratio of the magnetic and electric dipole transition moment; g can be large only for transitions which are magnetically allowed and electric dipole forbidden. In chromophores with approximately CZv symmetry a triplet state of XX*type couples predominantly with mc* singlet states, which leads to relatively large values of g’“rn,if the chromophore is contained in a molecular structure that is sufficiently dissymmetric. In the case of %rr* phosphorescences the absolute values of are always small. This leads to the prediction &urn that the observation of a large (g,,, I indicates a phosphorescent level of predominantly 3rt1t* character. We have illustrated this with the example of a-santonin. a-santonin is not a special case, and we have also established the above for a series of a,B-enones [ 51 with approximate C, symmetry. If the chromophore in the 3~~* state strongly deviates from planarity, the phosphorescence may contain So+ ‘XK* along with S,-+ ‘nx* intensity resulting in substantial deviations of g,,, from g( So+‘nrt*). This may provide a way to study 3Tcrc*-3n~*coupling quantitatively, e.g. in cc,$-enones where the T-T gap is small and varies with solvent. The CPP technique as a diagnostic tool for determining the spatial character of the wavefunction of the triplet state complements the already existing arsenal of criteria, which includes bandshape of the emission, phosphorescence lifetime, triplet ESR measurements and linear polarization of phosphorescence. Like the degree of linear polarization, gl,, is an intensive property of the phosphorescence. Whereas linear polarization is best defined for electric-dipole-allowed phosphorescences (since vibronic coupling effects are then relatively low), g,,, is most informative if the phosphorescence is mag netic dipole allowed.
References [I I S.P. McGlynn, T. Azumi and M. Kinoshita, Molecular spectroscopy of the triplet state (Prentice-Hall, Englewood Cliffs, New Jersey, 1969). 121C.A. Emeis, Ph.D. Thesis, Leiden University ( 1968).
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[3] M. Pestemer, Anleitung zur Messung von Absorptionsspektren im Ultraviolett und Sichtbaren) (Thieme, Stuttgart, 1964) p. 43. [ 41 P.H. Schippers, A. van den Beukel and H.P.J.M. Dekkers, I. Phys. E. 15 (1982) 945. [ 51 P.M.L. Blok, Ph.D. Thesis, Leiden University (1989). [6] H.P.J.M. Dekkers, P.F. Moraal, J.M. Timpcrand J.P. Riehl, Appl. Spectry. 39 ( 1985) 8 18. [7] P.M.L. Blok, P. Schakel and H.P.J.M. Dekkers, submitted for publication. [ 81 CA. Emeis and L.J. Oosterhoff, J. Chem. Phys. 54 (1971) 4809. [ 91 F.S. Richardson and J.P. Riehl, Chem. Rev. 77 (1977) 773. [ 1O]SF. Mason, Proc. Chem. Sot. ( 1962) 137. [ II] M.A. El-Sayed, J. Chem. Phys. 38 (1963) 2834. [ 121 EC. Lim, Excited states, Vol. 3 (Academic Press, New York, 1977) p. 305. [ 131 G. Marsh, D.R. Kearns and M. Fish, J. Am. Chem. Sot. 92 (1970) 2252.
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[ 141 A.F. Beecham and D.J. Collins, Australian J. Chem. 33 (1980) 2189. [ 15] G. Marsh, D.R. Keams and K. Schaffner, Helv. Chim. Acta 51 (1968) 1890;J.Am.Chem.Soc.93 (1971) 3129. [ 161CR. Jones and D.R. Keams, J. Am. Chem. Sot. 99 (1977) 344. [ 171 N. Steinberg, A. Gafni and I.Z. Steinberg, J. Am. Chem. Sot. 103 (1981) 1636. [ 181 H.P.J.M. Dekkers, Ph.D. Thesis, Leiden University ( 1975). [ 191 P.J. Bruna, SD. Peyerimhoff and R.J. Buenker, Chem. Phys. 3 (1974) 35. [ZO] R.H. Judge and G.W. King, Can. J. Phys. 53 (1975) 1927. [21] D.S.L. Blackwell, CC. Liao, R.O. Loutfy, P. de Mayo and S. Paszyc, Mol. Photochem. 4 ( 1972) 17 1. [22] D.A. Lightner, T.D. Bouman, W.M.D. Wijekoon and A.E. Hansen, J. Am. Chem. Sot. 106 (1984) 934. [23] J.P. Engelbrecht, G.D. Anderson, R.E. Linder, G. Barth. E. Bunnenberg and C. Djerassi, Spectrochim. Acta 3 1A ( 1975) 507.
CHEMICAL PHYSICS LETTERS
8 September 1989
DISCRIMINATION BETWEEN %t* AND 3nn* STATES IN ORGANIC MOLECULES BY CIRCULAR POLARIZATION OF PHOSPHORESCENCE ****
Peter M.L. BLOK and Harry P.J.M. DEKKERS Department
of Chemistry,Gorlaeus Laboratoties, Leiden University, P.O. Box 9SO2. 2300 RA Leiden, The Netherlands
Received 30 June 1989
The degree of circular polarization gin electronic transitions of chiral molecules is a measure of the ratio of the magnetic and electric dipole transition moments. From theory an upper bound for lgl as a function of dipole strength D can be found. This offers the possibility to distinguish %x* from ‘nn* states in organic molecules. For molecules of approximate Clv symmetry it can be shown that large absolute values of g,,, (order 10-l ) can arise only in the case of Jmr* phosphorescence, whereas emission from %m* states yields low values of g,,, (order lo-‘), This conclusion is substantiated with experimental data on three examples: a-santonin, ( IS)-( - )-I-bromo-a-fenchocamphoronequinone and ( + )-thiocamphor.
1. Introduction
In molecules in which 3nn* and 3~~*states are expected to be proximate, it is of spectroscopic and photochemical interest to know which one is lowest in energy. Several criteria have been developed to discriminate between 3nn* and 3rcrc*phosphorescence. These relate to the magnitude of the phosphorescence lifetime, emission bandshape, the magnitude of the zero-field splitting parameters from triplet ESR measurements and linear polarization of the emission [ 11. We considered it worthwhile to investigate whether in chiral substrates measurement of the circular polarization of the phosphorescence (CPP) can be used to establish the origin of the phosphorescence. In this paper we first discuss the relationship between the magnitude of the degree of circular polarization and whether or not the electronic transitions involved are magnetic dipole allowed. Theory is then illustrated with experimental data on three examples: a-santonin, ( lS)-( - )-l-
*
Dedicated to Professor J.H. van der Waals on the occasion of his retirement from the chair of Physics at L&den University. ** Part of this work has been presented at the FECS Conference on Circular Dichroism, Budapest ( 1987).
188
bromo-a-fenchocamphoronequinone thiocamphor.
and
( + )-
2. Experimental Materials. a-santonin was purchased from Janssen Chimica and recrystallized twice from methanol. ( + )-thiocamphor was prepared [2] from ( + )camphor by reaction with P&, and purified by chromatography in hexane over silica (m.p. 136°C lit. 121: 138-139°C). (IS)-( -)-l-bromo-u-fenchocamphoronequinone was available from stock. Solvents were purified according to conventional methods [3]. Methods. Circular dichroism spectra were measured with a Jobin-Yvon M III dichrograph and absorption spectra with a Cary 14 spectrophotometer. CPP and linear polarization data were obtained using a home-built spectrometer [4], equipped with a chopper [ 5 1. Emission and excitation spectra were recorded on a Spex Fluorolog II spectrofluorimeter with phosphorescence accessory. Low-temperature experiments were performed using a cryostat (Oxford Instruments Ltd., type DN704, modified) or a simple suprasil dewar. When measuring CPP in glasses at low temperature, care must be taken to avoid artifacts that may
Volume161,number2
CHEMICAL PHYSICSLETTERS
arise from strain birefringence in the glassy solvent, in the windows of the cryostat and/or of the cuvette. Such artifacts are small if the linear polarization of the luminescence is minimized [ 61. This can be done [ 5,7] by choosing specific configurations of the CPP spectrometer.
3. Theory The rotational strength R in an electronic transition i-j is defined by R = Im _Ulj *mji
(1)
and the dipole strength D by ~=IP~j12+l~,12.
(2)
Here p represents the electric, and I)I the magnetic dipole moment of the electronic transition i-j. The dissymmetry factor g is given by g=4R/D.
(3)
Eqs. ( l)-( 3) hold not only for absorption but also for emission (fluorescence or phosphorescence) provided it is understood that i pertains to the state of lower energy [S,9]. In absorption R(D) can be found from experiment by integrating the circular dichroism (the absorption) over all vibrational fine structure, R=c D=4c
(At/v) dv,
(4)
(E/v) dv,
(5)
s
where c=23x 10m40if R and D are in cgs units and 66 and e in dm3 mol-’ cm-‘. In luminescence the rotational and dipole strength are given by Rlum =c
s
D,,, =4c
(AI/u4) du,
J
(I/v4) dv ,
(6) (7)
where integration is over the entire emission band. Generally an emission experiment is performed in such a way that the constant c cannot be evaluated (but Dlumcan be found from the radiative lifetime of the emission), However, the dissymmetry factor in the luminescence, gl,, - being the ratio of 4R,,,
8 September1989
and D,,,,,, (cf. (3)) - is an absolute quantity. Using ( I ) and (2), and omitting the indices i and .i, g and gl,, are found to be g=4mpcos
e (m2+p2)
--I
(8)
in which 8 is the angle between m and p. From the definitions ofg, Ac( =c,_-tR) and E ( =$(E~++)), it follows that the maximum value of lgl is 2, a situation which is only realized if in a transition m and ,u are equal and (anti)parallel (cf. (8) ). A similar argument holds for g,,,. This maximum value is never attained for organic molecules, because the observed value of D always appears to be 2-3 orders of magnitude larger than the maximum contribution due to the magnetic term (which is w 10m40cgs for m= 1 Bohr magneton, the value for a p-p transition). Consequently, most gvalues will be considerably lower than 2. Fig. 1 shows a semi-logarithmic plot of g as a function of D according to (8). In constructing the figure we have used a value of 1 Bohr magneton (8) for m and the maximum value of cos e. In fact, this figure is a quantitative elaboration of an early suggestion by Mason [lo]. From the figure it appears that in the So+lnn* transition of saturated ketones, where the dipole strength is typically 10-38-10-37 cgs, at most a gvalue of about 1O- ’ can be obtained. For the So-+‘Jo* transition of a&unsaturated ketones, D typically amounts to 2.3X lO-35 cgs (value obtained for testosterone) and so a maximum g-value of 8 x 1O-3 is predicted. The actual values which are observed are much smaller (e.g. + 1x 10e3 in the case of testosterone), obviously because cos 8 is less than 1. So the magnitude of g is connected with the amount of magnetic dipole character in the transition: high gvalues (order 1O- ’ ) are only expected for m-allowed and p-forbidden transitions, whereas for m-forbidden and p-allowed transitions small values (order 10m3) are predicted. With pure spin states the phosphorescent transition is rigorously spin-forbidden and the phosphorescence intensity entirely derives from the initial triplet state and the final singlet state being spin-orbit coupled to other states:
‘b(Qq)-
‘$(Qq) + C ai j
‘$ji(Qq) 7
(9)
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2.00 t
1.5-
0-I a1
1
I 10
0 10'
1 10'
I Id
Id -
0l10-%gs)
Fig. 1.Semi-logarithmicplot of g as a function of D.
3$tQq)+3#(Qq) + C a,(Q) .i
'@j)i(Qq)*
(10)
The amount of mixing is governed by the magnitude of the coefficients aj which, in terms of perturbation theory, contain energy denominators such as E( 3#j)-E( ‘$) and matrix elements (3$j,IL*S( I$>, where L and 5 are the operators for orbital and spin angular momentum, respectively. The variables Q and q in (9) and ( 10) denote the coordinates of nuclei and electrons, respectively. Generally, the admixing of triplet character to the singlet ground state will be small because of the large energy difference involved and thus the intensity of the phosphorescence originates primarily from singlet --$ singlet transitions (cf. (10)). Consider the case that the phosphorescent state 3@is contaminated with one singlet state only, e.g. ‘&. The radiative properties of the triplet state are then determined by the character of the admixed singlet state and the strength of the coupling: R(T+S)=
~u,~*R(S,*‘&) >
O(T~S)=lUjl’O(S,~‘g).
(11) (12)
However, the dissymmetry factor in the phosphorescence equals that of the admixed SO-+‘@jtransition since it does not contain the coefficient a. We consider a chiral molecule with approximate 190
Czy symmetry where the two lowest excited states in the singlet and triplet manifold are nx* and xn*. Then 3~x* phosphorescence derives intensity predominantly from the ‘nr? contamination by first-order spin-orbit coupling because the matrix element ( ‘nff*(L.S131t7c*) is much larger than ( ‘XX*1L*S( 3~~*} [ 11,12 1. Analogously, phosphorescence from the 3nff* level results primarily from a ‘KER* admixture. In this approximation the dissymmetry factor in the two types of phosphorescence is equal to g( S,+‘nx*) and g(SO+‘xx* ), respectively. In practice an exact equality will be illusory. First, this would require identical geometries in ground and phosphorescent state. Further the deviation from CzV symmetry of the chiral molecule will lead to a more complex spin-orbit coupling behaviour. In considering for instance the ?tn* state, one must expect that, apart from ‘nx*, other singlet states are also admixed to a certain extent. If such additional couplings OG cur with other sublevels k of the triplet state, gl,, equals 4CkRk/CkD,, which destroys the correspondence between gl,, and the dissymmetry factor of a particular singlet-singlet transition. However, the observation of a large absolute value of gl,, is still proof of a magnetic-dipole-allowed phosphorescent transition. If, on the other hand, extra couplings occur with the same triplet sublevel, interference effects may even lead to a breakdown of the rule that
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high glum values indicate 3mt* emission. For instance, the electric dipole transition moments due to two admixed electric dipole-allowed transitions might nearly cancel, which can give rise to a very small value of C, 1ah 1‘Dk and, occasionally, to a high value of &m- The fact that the phosphorescence is magnetic dipole allowed can no longer be associated with the x7c*character of the spatial part of the triplet wavefunction. Such cases may be recognized by using other experimental criteria for distinguishing 331;n* and ‘no* emission, e.g. phosphorescence band shape. However, if deviation from CZVsymmetry is small, major interference effects are not probable, Lastly we mention that one does not expect spin to contribute appreciably to R (T+ S), that is; the effect due to the operator 2s in the expression for m ( = (e/2mc) (LS 2s) ) is small. The contribution of S to the magnetic dipole transition moment is given by the matrix element ( ‘&o-tIiil:$i12SI ‘@ t C$, ‘@j}, cf. (9) and (10). Nonvanishing terms arise from { I@,,i-1 3@I2SI 3@), i.e. from the phosphorescent triplet state being spin-orbit coupled to the ground state. Since these states are far apart, the effect of the magnetic moment due to spin is expected to be relatively small, the more so if the phosphorescent transition is m allowed due to orbital momentum ( 3nn*-+S,,,for instance). Although probably difficult to identify, inherent spin effects on rotational strength might be best studied in L-forbidden transitions (e.g. ‘mt*-+!!&).
4. Experimental results and discussion 4.1. a-santonin The low-temperature absorption and emission spectra and the linear polarization of a-santonin are given in fig. 2. They essentially agree with the data reported by Marsh et al. [ 131 who assigned these features as follows. The weak band system between 300 and 380 nm ( cmax= 30 dm3 mol-’ cm-‘) in the absorption spectrum is due to the &+‘nrc* transition. Its intensity mainly arises from vibronic coupling, presumably with the %t* state since its linear polarization equals that of the SO-+%n* transition. The very weak band observed by Marsh et al. [ 131 at = 393 nm in the phosphorescence excitation spec-
Fig. 2. a-santonin. Top: absorption spectrum, phosphorescepce spectrum and linear polarization data (p). Bottom: g,, in the !&Inn* band (-) and g,,, values (_._).The absorption data pertain to measurements in a methylcyclohexane/isopentane/2methyltetrahydrofuran glass at 9 80 K, the luminescence data to an ethanol/isopentane/diethyl ether glass at 77 K; standard error in the g,,, values is 2 x IO-‘.
trum (t = 0.3 dm3 mol- ’ cm- ’ ) was ascribed to the S0+3nn* absorption. The virtually structureless luminescence (robs= 79 ms/77 K, 2-MTHF) [ 131 was assigned to 31cx*phosphorescence. We have also measured the circular dichroism and the circular polarization of the phosphorescence of cc-santonin (fig. 2) + The CD in the So+ ‘nlc* transition is negative and of medium intensity. The value of the dissymmetry factor appears to depend strongly on wavelength, ranging from = 5 x 1OA2near the origin transition to zero near 3 10 nm. The CD in the weak transition at 393 nm is zero within experimental error. Taking into account the sensitivity of the CD spectrometer this implies that the absolute value of the dissymmetry factor here is less than 4~ 10m4.The degree of circular polarization of the phosphorescence is high and shows a moderate wavelength dependence: -65 to -50X 10-3. According to the theoretical considerations outlined above, the large magnitude of 1g,,, I is strong evidence that the phosphorescence is m allowed and occurs from a 3nn* state. Furthermore, the smallness of the dissymmetry factor in the 393 nm band is entirely consistent with the S0-3nx* assignment of Marsh et al. [ 131 and with their conclusion that its intensity derives from spin-orbit coupling with the (strongly electric dipole allowed) So+ ‘xx* transition. The SD-+‘nit* CD of a-santonin exhibits a pro191
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gression in a 1150 cm-’ vibration which can be identified with the carbonyl stretching frequency in the ‘nx* state observed in the CD of many a&enones [ 141. It appears that the shape of the CD band corresponds to a nice Franck-Condon distribution of intensities [ 51, which suggests that the CD speo trum arises from electronic rotational strength modulated by Franck-Condon factors. Whereas in CD the vibronically induced component is much smaller than the electronically allowed one, the opposite is true for the unpolarized absorption as evidenced by the strong wavelength dependence of g. This situation is as expected. In local CzV symmetry the So+ ‘nrt* transition is electric dipole forbidden. The chiral molecular framework in which the dienone chromophore is embedded introduces a small electric dipole transition moment which generates all of the CD, but scarcely contributes to the dipole strength. If in emission vibronic coupling effects were absent, both D,,, and RI,, should be modulated by Franck-Condon factors, which would make gl,, independent of wavelength, This apparently is not the case (fig. 2). So the first-order spin-orbit coupling scheme is not sufficient and we have to envisage spinvibronic effects which are expected to introduce So-+‘XX*character into the phosphorescence [ 151. The large value of 1gl,, 1 suggests, however, that the effect is relatively unimportant. If phosphorescence intensity derives primarily from spin-orbit coupling with the ‘mc* state, the equivalence of g and gl,, near the (0,O) transitions of the S+‘nrr* absorption and the phosphorescence indicates that distortion of the triplet state is small. An important distortion has been suggested for asantonin [ 121 and for a$-enones [ 15 ] (later reconsidered for a, Benones [ 16 ] ) .
orescence ( -35x 10-j at 480 nm (bandwidth 10 nm), 77 K) and points to ‘nrr* type phosphorescence. The circular polarization of luminescence of a related compound, camphorquinone, has been reported by Steinberg et al, [ 171. In the long wavelength tail of the luminescence (where the authors supposed fluorescence to be absent) they found small values of 1g,,, 1. According to the present gl,, criterion this would indicate emission from a 3n~*level. 4.3. (+)-thiocamphor The absorption and emission characteristics of thiocamphor in MIP (methylcyclohexane/isopentane glass 1: 3) at low temperatures are depicted in fig. 3. The absorption in the 360-560 nm region is due to the SO+‘mc* transition, except for the low intensity band near 550 nm. The emission can be assigned to phosphorescence because of its lifetime (0.14 ms) [ 181, The CD in the &+‘nx* absorption band turns out to be bisignate. At large wavelengths (~520 nm) the value of g is 1x 10-r (fig. 3). In contrast, the value of g in the 550 nm band appears to be much lower (23x low4 on the average) [ 191. As noted before [ 21, the vast difference between the magnitude of g in this band and that in the &,+‘nx* absorption suggests that the band does not belong to the regular SO+‘nx* system. In fact, the
4.2. (IS)-(-)-l-bromo-CCfenchocamphoronequinone This species contains the a-dione chromophore, shows both fluorescence and phosphorescence at low temperatures. By using a chopper the latter luminescence (A,,, = 549, 573 nm (sh) ) can be isolated. In the phosphorescence band g,,, is small and constant as a function of frequency: + (3f2) x 10e3. The smallness of g,,, contrasts in sign and order of magnitude with the gl,, value in the ‘na*-&, flu192
I
, LOO
I
500
I
600
I
I
Alnml 7W
Fig. 3. Thiocamphor in methylcyclohexane/isopentane glass, 77 K. Top: absorption spectrum, phosphorescence spectrum and
linear polarization data (p). The latter are given with respectto M (emission) and X (excitation) wavelength. Bottom: g,,. in the S,,+%A* region (-) ing,,, is 1 X lo-‘.
and g,,, values (.~.). Standard error
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similarity of the values of the dissymmetry factor in the 550 nm band and in the phosphorescence strongly suggests that the 550 nm shoulder is the absorption analogue of the phosphorescence transition, i.e. it is due to the S,,-+T transition [ 2,181. Further evidence is provided by the high degree of linear polarization of this band with respect to the phosphorescence (fig. 3), which implies parallel polarizations of the 550 nm band and the T-+S,, transition. Moreover, this indicates that a distortion of the triplet state, which has been found for thioformaldehyde [ 191, if it occurs at all, does not appreciably affect the chiroptical properties. From the large spectral overlap of the 550 nm band and the phosphorescence, and the equality of g and gl,, in this region, we conclude that the emitting triplet state is not severely distorted. So far we have not discussed the electronic character of the triplet state. From theory it follows [ 201 that in aliphatic thiones the lowest triplet state is of 3nrc*type, and a similar conclusion has been reached by Blackwell et al. [ 2 1 ] on the grounds of the chemical behaviour of that state in quenching experiments. The small value of g’,, we observe is entirely compatible with emission from a 3nrt* level, The absence of tine structure in the circular and linear polarization of the phosphorescence indicates that vibronic coupling effects are of minor importance in the phosphorescence process. This is to be expected if phosphorescence intensity derives predominantly from spin-orbit coupling with a strongly electric dipole allowed Sod ‘xx* transition. In our simple model, the So+3nn* transition derives its radiative properties from the SO-+‘xx*transition, i.e. the latter should have a positive dissymmetry factor. The absorption and CD spectra of thiocamphor in the UV region were reported by Emeis [ 21, who found that the second absorption band (230 nm, cmex=lo4 dm3 mol-’ cm-‘) has a negative CD, and the third (2 15 nm, emax=5 x lo3 dm3 mol-’ cm-‘) a positive CD (magnitude not reported). Whether this implies that in thiocamphor the third and not the second transition is the S&,+‘nn* transition - in contrast with the findings of Lightner et al. [22] from ab initio calculations on thioacetone - or that our model is too simple, we do not yet know. In this respect we mention that it appears from magnetic circular dichroism experiments that more than one triplet sublevel carries intensity [ 18,231.
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5. Conclusion The magnitude of g probes the ratio of the magnetic and electric dipole transition moment; g can be large only for transitions which are magnetically allowed and electric dipole forbidden. In chromophores with approximately CZv symmetry a triplet state of XX*type couples predominantly with mc* singlet states, which leads to relatively large values of g’“rn,if the chromophore is contained in a molecular structure that is sufficiently dissymmetric. In the case of %rr* phosphorescences the absolute values of are always small. This leads to the prediction &urn that the observation of a large (g,,, I indicates a phosphorescent level of predominantly 3rt1t* character. We have illustrated this with the example of a-santonin. a-santonin is not a special case, and we have also established the above for a series of a,B-enones [ 51 with approximate C, symmetry. If the chromophore in the 3~~* state strongly deviates from planarity, the phosphorescence may contain So+ ‘XK* along with S,-+ ‘nx* intensity resulting in substantial deviations of g,,, from g( So+‘nrt*). This may provide a way to study 3Tcrc*-3n~*coupling quantitatively, e.g. in cc,$-enones where the T-T gap is small and varies with solvent. The CPP technique as a diagnostic tool for determining the spatial character of the wavefunction of the triplet state complements the already existing arsenal of criteria, which includes bandshape of the emission, phosphorescence lifetime, triplet ESR measurements and linear polarization of phosphorescence. Like the degree of linear polarization, gl,, is an intensive property of the phosphorescence. Whereas linear polarization is best defined for electric-dipole-allowed phosphorescences (since vibronic coupling effects are then relatively low), g,,, is most informative if the phosphorescence is mag netic dipole allowed.
References [I I S.P. McGlynn, T. Azumi and M. Kinoshita, Molecular spectroscopy of the triplet state (Prentice-Hall, Englewood Cliffs, New Jersey, 1969). 121C.A. Emeis, Ph.D. Thesis, Leiden University ( 1968).
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[3] M. Pestemer, Anleitung zur Messung von Absorptionsspektren im Ultraviolett und Sichtbaren) (Thieme, Stuttgart, 1964) p. 43. [ 41 P.H. Schippers, A. van den Beukel and H.P.J.M. Dekkers, I. Phys. E. 15 (1982) 945. [ 51 P.M.L. Blok, Ph.D. Thesis, Leiden University (1989). [6] H.P.J.M. Dekkers, P.F. Moraal, J.M. Timpcrand J.P. Riehl, Appl. Spectry. 39 ( 1985) 8 18. [7] P.M.L. Blok, P. Schakel and H.P.J.M. Dekkers, submitted for publication. [ 81 CA. Emeis and L.J. Oosterhoff, J. Chem. Phys. 54 (1971) 4809. [ 91 F.S. Richardson and J.P. Riehl, Chem. Rev. 77 (1977) 773. [ 1O]SF. Mason, Proc. Chem. Sot. ( 1962) 137. [ II] M.A. El-Sayed, J. Chem. Phys. 38 (1963) 2834. [ 121 EC. Lim, Excited states, Vol. 3 (Academic Press, New York, 1977) p. 305. [ 131 G. Marsh, D.R. Kearns and M. Fish, J. Am. Chem. Sot. 92 (1970) 2252.
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[ 141 A.F. Beecham and D.J. Collins, Australian J. Chem. 33 (1980) 2189. [ 15] G. Marsh, D.R. Keams and K. Schaffner, Helv. Chim. Acta 51 (1968) 1890;J.Am.Chem.Soc.93 (1971) 3129. [ 161CR. Jones and D.R. Keams, J. Am. Chem. Sot. 99 (1977) 344. [ 171 N. Steinberg, A. Gafni and I.Z. Steinberg, J. Am. Chem. Sot. 103 (1981) 1636. [ 181 H.P.J.M. Dekkers, Ph.D. Thesis, Leiden University ( 1975). [ 191 P.J. Bruna, SD. Peyerimhoff and R.J. Buenker, Chem. Phys. 3 (1974) 35. [ZO] R.H. Judge and G.W. King, Can. J. Phys. 53 (1975) 1927. [21] D.S.L. Blackwell, CC. Liao, R.O. Loutfy, P. de Mayo and S. Paszyc, Mol. Photochem. 4 ( 1972) 17 1. [22] D.A. Lightner, T.D. Bouman, W.M.D. Wijekoon and A.E. Hansen, J. Am. Chem. Sot. 106 (1984) 934. [23] J.P. Engelbrecht, G.D. Anderson, R.E. Linder, G. Barth. E. Bunnenberg and C. Djerassi, Spectrochim. Acta 3 1A ( 1975) 507.