1134
J. Phys. Chem. 1995, 99, 1134- 1142
Polarized Absorption and Phosphorescence Spectra and Magnetic Circular Dichroism of Dithioimides: Assignment of the Lower 'nz* and 3m*States Stefan C. J. Meskers,t Tadeusz Po#onski,*$fand Harry P. J. M. Dekkers**tgg Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300RA Leiden, The Netherlands, and Department of Organic Chemistry, Technical University, 80-952 Gdansk, Poland Received: July 14, 1994; In Final Form: October 31, 1994@
Electronic absorption, magnetic circular dichroism, and (polarized) phosphorescence and phosphorescence excitation spectra are reported for dithiosuccinimide, dithioglutarimide, and dithiocamphorimide and their N-methyl derivatives. The theory of Engelbrecht et al. (Spectrochim. Acta 1975, 31A, 507) for magnetic circular dichroism (MCD) in singlet-triplet transitions is extended to show that not only the shape but also the magnitude of the MCD is of help in characterizing such transitions. The weak shoulder at -500 nm in the absorption spectra of the dithioimides is, using Czb symmetry labels, identified as the SO 3Bl(nn*) transition. Its polarization is almost exclusively (-99%) parallel to the line C(S)-C(S) in the dithioimide chromophore. Presumably the responsible triplet sublevel derives its radiative character from spin-orbit coupling to the third excited singlet state (S3), 'Bz(nn*). The radiation from the other active sublevel is polarized along the CZaxis of the chromophore. From the absorption spectra and the excitation polarization data, which were obtained via the photoselection method, the lowest excited singlet state is assigned as 'B1(nn*), SZ as 'Az(nn*). The experimental findings are consistent with Czl. symmetry for dithiosuccinimide, and C, symmetry for the chromophore in N-methyldithioglutarimide and N-methyldithiocamphorimide. The data indicate that at 77 K the phosphorescent state, 3BB(nx*),is not appreciably out-of-plane distorted with respect to the ground state.
-
Introduction Thiocarbonyl compounds have proved to be useful models for the study of the photophysics and photochemistry of polyatomic molecules in solution. Their lower excited states, Tl(n,z*) and Sl(nx*), and one at a considerably higher energy, S2(nn*),have been well characterized. For the dithioimide chromophore, which contains two coupled C=S moieties potentially giving rise to a multiplication of the number of excited states, only absorption and circular dichroism (CD) spectra4 have been reported. The order of the two lowest singlet nz* states was proposed4 to be the same as in the parent imide chromophore. We here report the results of a spectroscopic investigation of six aliphatic N-methyldithioimides, which all exhibit a strong phosphorescence. Three electronic transitions appear to be present in the visible part of the absorption spectrum: a singlettriplet and two singlet-singlet n-r* transitions. To locate and assign them we have studied the polarized phosphorescence excitation spectra of samples in which anisotropy was introduced by photoselection, and we have measured the magnetic circular dichroism (MCD) spectra. The photoselection experiments provide us with the relative direction of linear polarization of the transitions. The absolute direction is obtained by refemng to the polarization direction of the strong n-n* transition (SO S3), which is known from calculations.' The MCD spectra appear to be of help in distinguishing the two SO 'nx* transitions and in identifying the longest wavelength absorption band as a singlet-triplet transition. With regard to the latter aspect, we show from theory that not only the characteristic
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' Leiden University.
* Technical University. 5 @
E-mail address:
[email protected]. Abstract published in Advance ACS Absrracts, January 1. 1995
0022-3654/95/2099-1134$09.00/0
shape5 but also the magnitude of the MCD provides valuable information on singlet-triplet transitions.
Experimental Section The dithioimides were obtained from the corresponding imides by reaction with Lawesson's reagent6 and purified by column chromatography on silica gel. Solvents were of spectrograde quality and used without further purification. Measurements at low temperatures were done in the glassy solvent mixtures EPA (diethyl ether/isopentane/ethanol 2:2: 1 by volume) or methanol/ethanol (1: 1). Absorption spectra were recorded on a Beckman 3600 spectrophotometer, emission and excitation spectra on a Spex Fluorolog I1 instrument. Circular and linear polarization-ofluminescence data and emission life times were measured on a home-built spectrometer.' Circular dichroism spectra were recorded on a Jobin-Yvon MI11 instrument, MCD spectra on a JASCO 5-600 dichrograph equipped with a 1.5 T electromagnet. Low temperature measurements were carried out by immersing the sample cell in a suprasil Dewar flask filled with liquid nitrogen or by cooling the sample in a cryostat (DN-704, Oxford Instruments). All luminescence experiments were done using the right-angle mode of the spectrophosphorimeters. For the excitation spectra samples were used with an absorbance smaller than 0.05 in the relevant spectral region. Excitation spectra have been corrected for the wavelength dependence of the intensity of the excitation light, emission spectra for the wavelength-dependent sensitivity of the analyzing monochromator and the photodetector. The linear polarization experiments were done using EPA as a solvent, which yields a practically strain-birefringence-free glass at 77 K, as evidenced by inspection between crossed polarizers. Linear polarization data obtained with horizontally polarized excitation light were found to be zero, as they should be.
0 1995 American Chemical Society
The Lower inn* and 3n7c* States of Dithioimides
J. Phys. Chem., Vol. 99, No. 4, 1995 1135
Magnetic Circular Dichroism of Singlet-Triplet Transitions General. In magnetic circular dichroism spectroscopy one measures the difference in absorption of left and right circularly polarized light which occurs if the sample is placed in a magnetic field that is parallel to the direction of propagation of the light beam (laboratory Z-axis). In the electric dipole approximation the MCD associated with a spin-allowed transition from a nondegenerate ground state 0 to an excited state F, measured in an isotropic solution, is given
AE(v)= B(O-F)
afiv-vo) 1 +-A(O-F)] av h
(1)
In this formula a equals 8n3N~/2303hand Bz denotes the magnetic field. The expressions for the MCD parameters A(O-F) and B(O-F) read
In these equations p(P,Q) denotes a matrix element of the electric dipole operator (Le., (PlplQ)), and M(P,Q) that of the magnetic dipole operator which is given in (4). In writing (4)
M = p&-' (L
+ gS)
(4)
we have assumed that the g tensor is isotropic and we have denoted the bohr magneton by ,UB. All wave functions involved in (2) and (3) are eigenfunctions in a zero magnetic field; when evaluating (3) for a degenerate state F with substates FJ one has to use the representation of FJ that diagonalizes the perturbation hamiltonian B-M. The MCD parameters reflect the two ways a magnetic field can influence the light absorption of a molecular system with a nondegenerate ground state. First, Bz can lift the degeneracy of the final state. Such a Zeeman effect gives rise to an A term in the MCD; in singlet-singlet transitions it can occur only if the upper state F carries a permanent magnetic dipole moment due to L. The effect of a magnetic moment due to spin, which is present in singlet-triplet transitions, will be discussed later on. Second, the magnetic field affects the wavefunctions of the system, thereby changing the transition probability. In terms of perturbation theory this effect is described by (2), where the first summation describes the mixing of the ground state with excited states and the second the mixing of the excited state. The shape function for the B term, fiv-YO), is often taken equal to that describing the form of the unpolarized absorption band. An A term in the MCD spectrum has an S-type shape. For samples in solution the wave functions 0, F, and I refer in the first place to vibronic states and, when in the spectrum all vibronic transitions are resolved, their MCD can be described by (1)-(3) wherefiv-Yo) represents the shape of the vibronic absorption band. Often; however, one observes in solution broad absorption bands which are a composite of many unresolved vibronic bands. Then it can be a r g ~ e d ~that . ' ~ for
allowed electronic transitions (1)-(3) again may be used, but with 0, F, and I now referring to electronic states andf(v-vo) to the shape of the overall electronic absorption band peaked at Y O (but, particularly for B terms, this is an approximation)." In a simple view there is a clear distinction between A and B terms. Consider a molecule where 0 F represents an (x,y)allowed electronic transition to a doubly degenerate excited singlet state (E symmetry) which has a nonvanishing magnetic moment M,. In the magnetic field the E state splits into two Zeeman components, one transforming as x iy, the other as x - iy. One Zeeman transition is allowed with left and forbidden with right circularly polarized light, the other behaves the other way around. If the Zeeman splitting is small compared to spectral bandwidth, the typical S-shaped MCD is observed. Consider, on the other hand, the MCD in a molecule with two with energy separation AE. excited singlet states, IF, and In the magnetic field both states are coupled giving rise to the new wavefunctions 'r, i t l r y and 'r, i t l r x at the original energies. Transitions to these states are allowed in left and right elliptically polarized light, which gives rise to two distinct B terms of opposite sign, if AE is large as compared to the spectral bandwidth. If, however, AE is small, the lobes overlap and one observes an S-shaped MCD effect, like in the case of a genuine A term. In practice the distinction between genuine and pseudo A terms is not always easy to make. Singlet-Triplet Transitions. We start this section with a short discussion of the radiative properties of triplet states. We consider the transition from the singlet electronic ground state SO (wave function denoted by 0) to a particular triplet state T in a chromophore of Czv symmetry. This symmetry is sufficiently high for the Cartesian position vectors to belong to different irreducible representations but low enough to prevent the occurrence of orbitally degenerate states. It also implies that the spatial part of the electronic wavefunctions can be chosen to be real. The spin degeneracy of the triplet state T is lifted by Ps,the dipolar interaction of the electron spins, yielding three substates r, (r denotes the relevant irreducible representations of the molecular point group) which differ in energy by the zero field splitting (ZFS). Spin-orbit coupling also may contribute to the ZFS. Each substate is characterized by an antisymmetrized product of a spatial wave function t and one of the spin functions t,, ty, and t,,which, for a two-electron system, are related to the familiar spin functions tl,to,and ~ - 1 . ' ~For convenience we shall assume that t has symmetry A2 in CZ, but the essence of our argument is not invalidated when t should have another symmetry. Singlet-triplet transition probability derives from spin-orbit coupling of singlet and triplet states. If the coupling is weak it can be described by using first-order perturbation theory. Thus the wave function of a triplet sublevel Trcan be written as:
-
+
'rY,
+
+
Here the subscript 0 denotes the electronic wavefunctions (including spin) in the absence of the spin-orbit coupling €Po. The coefficients c, equal
where the denominator equals the energy separation between the triplet state and the admixed singlet state 'Ti with symmetry
1136 J. Phys. Chem., Vol. 99, No. 4, 1995
Meskers et al.
r in the absence of the perturbation. When the electronic wave functions are chosen to be real, the coefficients en are purely imaginary. Likewise, spin-orbit interaction leads to the admixing of triplet character to the electronic ground state, but generally this perturbation will be smaller because of the larger energy denominators i n v ~ l v e d , ' ~ and ~ ' ~in this work we shall neglect it. We further assume in this work that a triplet sublevel mixes with one singlet state (denoted by S) only, so that we can drop in ( 5 ) and (6) the subscripts i. Consider for a moment the case that only one of the three triplet sublevels couples to the singlet manifold. The dipole strength of the SO T transition then equals
-
On the right-hand side of ( 7 ) we have abbreviated the (real) electric dipole transition moment by p,. In this example the SO T transition derives all of its intensity, its linear polarization, and with chiral molecules, its degree of circular polarization (vide infra) from the admixed SO 'Tr transition. Likewise the triplet state T derives its radiative properties from the same singlet-singlet transition. As we shall see, the above model does not give rise to the characteristic MCD of singlet-triplet transitions. That only results if (at least) two triplet sublevels are electric dipole coupled to the ground state. We therefore discuss the MCD for the example of 2 radiative triplet sublevels, one transforming as x, the other as y in Cz,, symmetry.
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Using (l), (lo), and (11) the differential absorption of circularly polarized light in the singlet triplet transition can be written as
-
When expanding the functions f in (13) in a Taylor series around Y - Y O and retaining in the expansion only the first two terms, we obtain (14). This equation shows that in our model the MCD
AE(v)=
in a singlet-triplet absorption band has the frequency dependence of an A term. Therefore (14) can be rewritten in the format of the second term of (1) where A(0- F) now equals
-
In our description of the MCD of singlet-triplet transitions we formally consider the situation that the strength of B is such that the Zeeman interaction energies B*M are negligibly small compared to the triplet's zero field splittings. We then can start out directly from (2). In this equation it is the term involving and 3r,that is foreseen to give a the magnetic coupling of 3rx major contribution to the MCD because of the extreme smallness of the energy denominator involved (which is a ZFS) and, as to be discussed, the large magnitude of the matrix element of M:. Thus the prominent terms in the singlet-triplet MCD are
In writing these equations we have utilized the abbreviations for the electric dipole transition moments already mentioned in ( 7 ) . That equation also gives the dipole strengths of the SO 3r.r and SO 3r? transitions located at the frequencies Y, and vy,respectively. When the singlet-triplet absorption band is broad and structureless it is described by
-
-
In writing (12) we have considered the ZFS to be very small as compared to the half-width off(v-vo), so that it may be taken that the two absorption bands, which we assume to have identical shapes, merge into a single one located at frequency VO
=
(Yr
+ YJ2.
The matrix element of M: is easy to evaluate. First, since it involves two different spin states orbital angular momentum does not contribute. The contribution from spin equals f i h g p e where the sign depends on the symmetry of the spatial wave function t. If that symmetry is A?, the plus sign is obtained and we get for the ratio AID
From (16) it can be seen that AID vanishes if only one triplet sublevel is electric dipole coupled to the ground state. The = Icv*,uu.j,the equation also shows that, provided IC,& magnitude of the singlet-triplet MCD is identical to that in a singlet-singlet transition to a state having an orbital angular momentum of &B. For this specific situation there is a close analogy between singlet-triplet and singlet-singlet MCD, but generally there are important differences. The principal use of AID values of singlet-singlet transitions is the determination of orbital angular momentum. With singlet-triplet transitions, assuming the spin-orbit coupling to be small, spin is a good quantum number and the spin magnetic moment is known a priori. What can be found here from AID are the sign and the magnitude of the ratio c&cy*pu?, Le., the relative phase and the ratio of the admixed dipole transition moments. When this ratio is 1, the value of AID is maximal; upon decreasing it, AID decreases but rather slowly. Therefore MCD is a sensitive probe to detect small spin orbit couplings next to large ones. Naturally the situation will often be more complicated than assumed in our model. First, we have confined ourselves to the MCD which originates in the coupling of the sublevels of one triplet state by B.S. The Zeeman Hamiltonian B*L which couples T to other states in the triplet manifold may also contribute. In molecules lacking orbitally degenerate states, however, such MCD is of B type and can be separated from the A term because of its different spectral shape. Second, we have assumed that the singlet-triplet transition contains two perpendicular polarizations only (x and y). In general one has also to allow for the presence of z-polarized intensity. This can be done by adding to the c ~ ? * p u y ,term in (16) the extra terms c?c;*p~,u;and c~c,*pu;u, and to the denominator /c;u,I2. We here have derived (16) for the situation that the Zeeman energy B*M
t'
-
28
3 0
v
u- e
>
* m
Z
+
- 4
W
z
C2
i r.
I
4CO
"
'
450
. -_
c
-20
3c
503 "%+&o
WAVELENGTH (nm)
o?.
........
a 025 t
Figure 3. Absorption and MCD spectra of 1 (cyclohexane, -293 K).
.-LN 0
-
the phosphorescence in band 6 (-330 nm) which was assigned to SO 'Bz(nn*). The degree of polarization in the excitation band at -250 nm is drastically lower. We tentatively assign this band, which we do not discuss further, to the z-polarized SO 'Al(mc*) transition. The excitation region corresponding to bands 4 and 5 also provides high p values. In the band system 2-3, the degree of polarization is not a constant but decreases upon increasing wavelength. Near band 2 it approaches zero (in some other compounds, negative values are obtained; see Figure 4 and Table 2). Lastly, the linear polarization of band 1 again is high. In chromophoric symmetry C2,) one expects two types of intensity in the SO IBl(nn*) absorption band: (allowed) x-polarized intensity in the origin band and progressions thereon and, at higher frequencies, vibronically induced intensity having another polarization direction; the latter presumably being y because of the presence of the strong and nearby SO 'Bz(nn*) absorption band. For p one therefore expects a negative value near the origin changing to strongly positive values at higher energies. Such a pattern is observed in the band system 2-3 if it is assumed that the expected p values near the origin band become less negative due to overlap with the positively polarized band 1. The polarization data are therefore compatible with the assignment of the bands 2-3 to the transition to the ~ Bstate. I If, in contrast, lA2 were the lowest excited state, an explanation of the observed change of p in bands 2-3 must invoke two different types of vibronic coupling: one again leading to p-polarized intensity and the other providing x- (or z-) polarized
-
-
3c3
3c3
6%C
500
400
::
WAVELENGTH (nm)
-
Figure 4. Spectra of 1 in EPA at 77 K. (top) Phosphorescence spectrum (C), phosphorescence excitation spectrum in the S O 'nn* region (B), and on another scale, the SO Inn* region (A). (bottom) Linear polarization of the phosphorescence upon excitation at 395 nm (filled symbols) and excitation polarization with respect to 520 nm emission detection (open symbols).
-
-
intensity with about equal efficiency. However, an efficient source for the latter intensity is lacking. The SO lAl(nz*) transition may act as source, but that transition is much weaker than the SO 'B2(nn*) band, and it lies at higher frequencies. This makes the assignment Sl(A2) less likely. A second argument for 'B1 being the first and 'A2 the second excited singlet state is provided by the MCD spectra. The MCD spectrum of 1 is depicted in Figure 3. It exhibits typical A-term behavior in the region of band 1, negative effects in bands 2-3, and small MCD in bands 4-5. Similar features are found with the dithioimides 2 , 3 and (racemic) 5 (4 and 6 were not studied). Postponing a discussion of the effect in band 1 to the next section we conclude that in 1 and the other compounds studied, the value of B is much larger in SO S I than in SO S2. This holds a fortiori for the value of BID (Le., the ratio of MCD and absorbance). Also on an absolute scale, the value of B1D in the SO S1 transition is very large. In the SO IB1 transition one
-
-
-
-
-
The Lower 'm*and 3nn* States of Dithioimides
J. Phys. Chem., Vol. 99, No. 4, 1995 1139 in cisoid a-diketones'* and as calculated for anhydride~'~-~' and imides.21 Molecular Geometry. For 1 the C2, symmetry labels are adequate according to recent X-ray data on 1F2 For the other compounds the intensity of the SO 'A2 absorption band, particularly with 4 and 6, points to distortions from C2, symmetry in solution at room temperature. It is likely that such distortion (provided it is small) is toward a geometry of C,, and not C2, symmetry because in C, the S2 state and the S3(nn*) state have the same symmetries and can interact, not so in C2 symmetry. Actually, in the latter case the 'B2(nn*) and 'Bl(m*) states can mix, but the linear polarization data show that such mixing does not occur to a significant extent. The X-ray structure23 of 6 supports this: in the solid state the chromophore in 6 has -C, symmetry with small torsional angles (Table 3). Except for 1 and 6, the molecular structures of the dithioimides have not been directly investigated experimentally. We therefore studied them by using semiempirical MNDO calculat i o n ~ ? All ~ the geometrical parameters were fully optimized at the SCF level by minimizing energy, using the Davidon Fletcher Powell Table 3 summarizes the values of selected torsional angles within the dithioimide group. The calculations predict that 1 and 2 adopt planar conformations (C2, symmetry), in accord with the X-ray results on 1. The MNDO method predicts two energy minima for 3 and 4: one corresponding to a sofa conformer (chromophoric symmetry C2J and the second to the quasi-chair (flattened chair) conformer with the chromophore slightly distorted from planarity (C, symmetry). However, the calculated energy difference between these two conformers does not exceed 0.2 kcal/mol. Similarly, the dithiocamphorimides 5 and 6 are predicted to exist in two forms, where the six-membered ring adopts a sofa or a quasichair conformation. In the case of the N-methyl derivative 6 the second form is favored on the basis of a slightly lower heat of formation. The crystal structure of 6 shows a similar molecular geometry as that calculated, but the distortion of the chromophore from planarity is smaller (cf. Table 3). The calculations predict double minimum potential wells for 3-6, where the conformer with chromophoric symmetry C2, is lowest in energy. The energy of the conformer with local C, symmetry is, however, only slightly higher, and for instance, solvation effects may easily affect the relative stabilities of both types of species. Occasionally this might lead to a conformational equilibrium at 300 K, and the dominating conformer could then be probed in low temperature absorption, CD, and excitation spectra. For a solution of 5 in EPA, we have found a marked temperature dependence of the absorption and CD spectra (cf. Figure 7). While the extinction coefficients of bands 2-3 remain constant when taking into account solvent contraction, those of bands 4-5 are reduced by about 25% upon lowering the temperature to 115 K (at lower temperatures the substrate starts to precipitate). The major change in the CD spectrum is the increase of the Cotton effect in band 2 by a factor of -2 (the changes in the spectral region of band 1 will be discussed in the next section). These changes might be the result of a shift of a conformational equilibrium, but the situation is probably much more complex because 1, which undoubtedly is conformationally pure, exhibits similar effects. In EPA at 80 K the absorbance of 1 is unchanged in bands 2-3 but has decreased by -40% in bands 4-5. The intensity pattern in the phosphorescence excitation spectra, taken on dilute samples, should resemble transmission spectra, provided the quantum yield of phosphorescence, Qp, is independent of excitation
-
0.0
I
300
400
500
600
700
WAVELENGTH (nm)
Figure 5. Spectra of 6 in EPA at 77 K. (top) Legend as in Figure 4. The broken curve in the top half is the excitation spectrum obtained with a more concentrated sample solution. (bottom) Phosphorescence polarization upon excitation at 420 nm (filled symbols) and excitation polarization with respect to 590 nm emission detection (open symbols).
expects a substantial B-term MCD from coupling of the 'B1 state to the S3(B2) state (cf. eq 2). First because the matrix element ('B1 IMzI 'B2) (-('Bl lLzl 'Bz)), presumably involving a coupling of a m* and a nn* state which are both built on a configuration containing the same n* orbital, is large. Furthermore the energy separation between the coupled states is relatively small (lo4 cm-' if 'B1 is SI,7 x lo3 cm-' if it were S2).
-
Obviously, in this picture the MCD B term in the SO 'Bz(nn*) transition will be equally large but of opposite sign. The value of \BID(,however, is much larger for SO 'Bl(m*) because the electric dipole moment in the transition to the 'B2 state is large, that to the 'B1 state small (cf. eq 17). This again illustrates the power of MCD spectroscopy to trace weak absorption bands nearby strong (and perpendicularly polarized) ones. In contrast to SO 'Bl(nn*) one expects a small value for BID in the forbidden SO 'A2 transition which has to derive intensity from vibronic coupling or by structural perturbations. It has been shown17 that the MCD in electric dipole forbidden transitions is of second or higher order in the vibronic (or structural) perturbation, as is the dipole strength. Therefore the degree of the magnetically induced circular polarization in the forbidden transition will be of the same order of magnitude as that of the allowed transition from which intensity is stolen. This occurs most efficiently with strongly allowed transitions but precisely these are likely to have small values of BID. We can assign 'B1 to the first and 'A2 to the second excited singlet state (see Figure 6 ) . This order is the same as observed
-
--
1140 J. Phys. Chem., Vol. 99, No. 4, 1995
Meskers et al.
TABLE 2: Phosphorescence and Phosphorescence Excitation Spectra and Dearee of Linear Polarization D (EPA, 77 KP band 1 2 3 4 5 6 phosphorescence I(4, A, I, I(4, I (4, I (4, 44, cmpd nm P nm P nm P nm P nm P n m P n m P 1 464(0.11) +0.34 444 (1) +0.01 427 (1.1) +0.20 408 (0.9) +0.31 326 +0.40 472 +0.30 2 467(0.07) +0.34 448 (1) -0.09 428(1.0) $0.14 406(1.6) +0.36 389 (1.5) +0.37 332 $0.40 476 $0.35 3 506 (0.18) $0.36 478 (1) -I-0.14 455 (1.5) +0.24 432 (1.8) 4-0.34 418 (1.3) 347 $0.38 511 +0.36 4 537 (0.10) +0.30 512 (1) -0.08 480(1.1) 4-0.12 442 (6.3) +0.34 419 (4.9) +0.34 334 $0.35 548 +0.32 5 502(0.14) +0.34 473 (1) +0.02 452(1.2) +0.23 431 (1.4) +0.30 325 +0.30 511 +0.32 6 531 (0.07) +0.31 500 (1) -0.11 477 (0.9) -0.01 436 (4.5) +0.34 414 (3.1) +0.34 329 +0.34 544 +0.32 a Intensities I are relative to band 2. Wavelength of emission detection in excitation experiments: 520 nm (1,2), 575 nm (3), 600 nm (4), 575 nm (5), and 590 nm (6). Wavelength of excitation in emission measurements: 395 nm (1,2), 410 nm (3), 425 nm (4), and 420 nm (5,6).
I D
m
h
D
20 K
I
3
' f r'
1
I
0
n
w Z W
0
SO
400
Figure 6. Assignment of the lower excited states of dithioimides. The energies chosen are that of 6. The order of the triplet sublevels is arbitrary (see text). TABLE 3: Torsion Angles (deg) and Heats of Formation calculated with MNDO ql(4-3ql(2-3ql(1-2ql(2-3Hr, compd" 2-6) kcal/mol 4-7) 4-5) 3-4) 1 179.9 180.1 0.0 0.2 19.87 180.0 180.0 1(X-ray) 0.0 0.0 179.9 179.7 2 0.0 0.2 22.46 3-s 15.87 179.8 180.0 0.1 0.0 172.2 - 172.2 3-c -8.3 9.6 15.93 179.1 -178.7 0.4 4-s 25.21 0.4 8.6 25.03 4-c 171.4 -172.1 -8.6 5-s -178.5 0.1 2.3 33.27 -178.4 172.8 -171.5 -9.4 10.2 5-c 33.12 -179.1 -178.2 0.3 1.4 6-s 41.87 6-c 169.6 -168.9 -11.5 11.1 41.44 6 (X-ray) 175.8 -177.5 -6.7 4.7 s, sofa conformer; c, quasi-chair conformex.
-
wavelength. Whereas in the absorption spectrum of 1 at 80 K the SO 'A2 band remains the most intense n-Jt* band, in the excitation spectrum (Figure 4), band 4 has a lower intensity than bands 2 and 3. Also with 5 the relative intensity of the SO IAz band in the excitation spectra is smaller than in the low temperature absorption spectra. These observations suggest that upon excitation in the 'Az(nx*) manifold, @p is lowered in comparison to excitation in the SO lv3B1transitions. This points to additional decay channels in the 'A2 state which do not lead to the phosphorescent triplet state. Incidentally, the discrepancies seem to be smaller in 2, 4, and 6; possibly the extra decay channel is more effective with chromophores containing the N-H instead of the N-Me group. The Lowest 3& State. Phosphorescence and Singlet Triplet Transition. All dithioimides studied exhibit an intense
-
-
-
450
500
WAVELENGTH (nm)
Figure 7. Spectra of 5 in EPA. CD spectrum at room temperature (-) and at 80 K (x). Absorption spectrum at 115 K (- - -). luminescence at low temperatures. For 1 and 6 the luminescence spectra are depicted in the top of Figures 4 and 5 , respectively. The other substrates exhibit similar emission bands with maxima as given in Table 2. The quantum yields of luminescence are high: we estimate them to be on the order of 0.5 f 0.2 (determined for 3, excitation wavelength 475 nm; value of 0.97, used as a standard). We find that the emissions from 1-6 all decay strictly exponentially with lifetimes 0.10, 0.12, 0.10, 0.17, 0.1 1, and 0.18 ms (fO.O1 ms), respectively (EPA, 77 K). In organic molecules such long lifetimes are not compatible with fluorescent processes, so we can assign the emissions as phosphorescences. The transition associated with band 1 must involve the same pair of states as the phosphorescence because of (i) the large spectral overlap of band 1 with the emission band and (ii) the high polarization in the red edge of the excitation spectrum (see Figures 4 and 5 and Table 2) which shows that the transition dipole moment in band 1 is to a good extent directed parallel to that in the phoshorescence. In the region of band 1 the MCD spectra of 1 (Figure 3) and 2, 3, and 5 show a bisignate effect due to an A term superimposed on a B term. As discussed, the observation of such MCD is p r i m f a c i e evidence for the singlet-triplet nature of the transition involved. A rough estimate of the A-type contribution can be obtained by subtracting the extrema of the bisignate curve. The resulting values of N D are given in Table 1. They contain a relatively large uncertainty because (i) the B contribution in the MCD must be subtracted, (ii) the singlettriplet absorption band is present only as a shoulder in the absorption spectra, and (iii) a band shape functionfiv--Yo) has to be estimated (dealing with severely overlapping bands, the
The Lower inn* and 3nn*States of Dithioimides
J. Phys. Chem., Vol. 99, No. 4, 1995 1141
method of moments27 does not work). Due to the relatively large error, we judge that the differences between the AID data of 1 and 2 and also of 3 and 5 are not relevant, but the values of the two pairs differ significantly, cf. next section. We assign the upper state in the singlet-triplet transition, and the lowest triplet state, to 3Bl(nn*). This is consistent with the fact that upon N-methyl substitution of 3 and 5 , band 1 and the low temperature phosphorescence band both exhibit a similar red shift as the SO 'Bl(nx*) transition. In chiral compounds the absolute value of the degree of circular polarization g (Le., the ratio of circular dichroism and absorbance) in a transition to a 3nx*state has an absolute upper bound28of due to the fact that in -C2,, symmetry a 3nn* state couples with Inn* states, and transitions to the latter states intrinsically have small lgl values. For 6 the value of g in band 1 satisfies this criterion; with 5 this is not so. Precisely in the region where the singlet-triplet absorption is located, 5 displays a negative CD lobe of appreciable magnitude4 implying a dissymmetry factor of about -1.4 x lo-* which is clearly at variance with the criterion. However, upon lowering the temperature we find that the amplitude of the CD lobe decreases and at 120 K the signal has virtually vanished (Figure 7). Probably with 5 , the -500 nm CD, which appears to be solvent dependent4 as well, is not due to the singlet-triplet transition but to a singlet-singlet transition of another chemical species (conformer or solvated species). Singlet Contamination and Geometry of the Lowest Triplet State. The excitation polarization data of all dithioimides show that the phosphorescence, and the singlet-triplet transition, are polarized parallel to the strong and relatively nearby SO 'nn* band at -30 000 cm-'. It is therefore highly probable that the 3Bl(nn*) state derives a major part of its radiative properties fom coupling with the 'B2(nnY) state, in accordance with the expectation that the major spin-orbit couplings are those between nn* and nn* states of different multiplicity, not between 3nn* and inn* or 3nn*and 1nn*.29 An estimate of the strength of the coupling can be made using the equation
-
-
where it has been assumed that the extinction coefficients E are a measure for the dipole strengths. For 1 the two extinction coefficients are -7 and 36 x lo3M-' cm-' ,respectively (Table l), so the absolute value of the coefficient c with which 'Bz(nn*)is admixed to the 3Bl(nn*) state is of the order 0.014. Upon inserting in (6) the value 11 x lo3 cm-' for the energy separation of both states, the magnitude of the matrix element (1B2(nn*)I~013Bl(n;t*)) is obtained as -150 cm-'. The values for the dithioimides 3 and 5 are some 30% larger, those of 2, 4, and 6 -20% lower. The large value of the matrix elements, which undoubtedly is related to an internal heavy atom effect of sulfur, may well lead to the situation that spin-orbit coupling dominates the spin-spin splitting, B2 therefore being the lowest triplet sublevel. The presence of an A term in the MCD spectrum of the singlet-triplet band shows that spin-orbit coupling of 3B'(m*) is not restricted to the lB2 state but involves another, perpendicularly polarized, singlet state as well. In Czv symmetry this necessarily is a 'A1 state, possibly the 'Al(nn*)state which is evidenced in the excitation spectra at about 250 nm. The ratio of the two transition dipole moments can be found from the AID values and the generalized eq 16. For 1 and 2 we find the ratio to be -4-0.1, for the pair 3, 5 it is about 2.5 times as large. The observation of identical sign is in line with chromophoric similarity in the dithioimides, the variation in magnitude with structural and/or environmental differences.
.O
08
0.6
0.4
02
0.0 500 600 WAVELENGTH (nm)
400
700
Figure 8. Spectra of 6 in methanoYethanol(1:l) at room temperature. (A) Phosphorescenceexcitation spectrum, (B) excitation spectrum taken with a more concentrated sample solution, (C) phosphorescence band and values of glum.The length of the bars drawn with the glumdata
represents the magnitude of the standard error. With -C2" thioketones similar results were ~ b t a i n e d ; ~interest,'~ ingly there the sign of AID is negative showing that the relative phase of the two singlet admixtures to the triplet is of opposite sign. The constancy of p across the phosphorescence band indicates that vibronic coupling effects are not important in providing dipole strength to the emissive transition, which is consistent with the strong SO 'B2(nn*) transition providing the major part of the intensity. If substantial vibronic coupling effects are lacking, the strong band at the high energy side of the low temperature phosphorescence spectrum is the origin transition. Its high intensity, along with the smallness of the Stokes shift between the singlet-triplet absorption and the phosphorescence (-400 cm-'), shows that, at low temperatures, the dithioimide chromophore distorts very little in the triplet state. Aliphatic thioketones, which also display a singlet-triplet absorption band of n-n* type at ca. 550 nm, are reported to be nonplanar in the triplet state due to an out-of-plane distortion in the C ( S ) m ~ i e t y . ~ O The - ~ ~ circumstance that in dithioimides the electronic excitation energy is delocalized over two C=S groupings may contribute to the smaller distortion. Room Temperature Luminescence. Upon raising the temperature, the intensity of the luminescence of the dithioimides decreases rapidly, and at room temperature it is only a small fraction of the value at 77 K. The luminescence band at 293 K appears to be broader and has less fine structure (cf. Figure 8). Lifetimes are much shorter (on the order of lov5 s) but still too long for fluorescences. The emissions are not due to an impurity, for the excitation spectra again follow qualitatively the absorption spectra. The chiral dithioimides 5 and 6 exhibit circular polarization of the luminescence (CPL). The degree of circular polarization of the luminescence of 6 at -293 K is shown in Figure 8. It appears to be very small at the low energy As mentioned side of the emission band: lgluml < 1 x this is compatible with 3nn* emission.28 Interestingly, at the onset of the emission band the magnitude of glumincreases dramatically, which is not due to an artifact from circular dichroic reabsorption of emission light. There it approaches the value of the degree of circular polarization of the absorption band 2 which can be deduced from the reported CD data4 and the absorption spectra. The effect goes along with the presence of a weak shoulder at the short wavelength side of the phosphorescence band (at -520 nm, cf. Figure 8). This feature,
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1142 J . Phys. Chem., Vol. 99, No. 4, 1995 which is also present with 5, disappears upon cooling. We atribute this intruiging effect to emission from SI. It should be investigated further whether it is normal fluorescence or delayed fluorescence. For some aromatic thiones thermally activated delayed fluorescence has been reported.33 Conclusion Linear polarization spectra using photoselection provide directly the relative direction of transition moments in absorption and luminescence, but the method is not sensitive enough to detect weak bands buried in strong ones. MCD spectra, on the other hand, are particularly informative about weak bands next to strong ones. This feature is particularly important for the study of singlet-triplet transitions, where the mere presence of an A term betrays the spin-forbidden nature, and the magnitude of AID provides information about the relative magnitude and the phase of the transition moments to the triplet sublevels. By combining both techniques the lower excited states of the dithioimides, in order of increasing energy, are found to be 3B1(nn*), 'Bl(nx*), 'A2(nn*), 'B*(xx*), and possibly, lA{(nn*). The 3B1(nx*) state of the dithioimides has two important radiative decay channels; the major one involves spin-orbit coupling to the 'B2(mr*) state, the minor one probably to the 'A1 (nn*) state. The 'Bz(mr*) state also plays a prominent role in the provision of intensity to the SO 'Az(nn*) transition which is forbidden in CzVsymmetry. The large intensity of this transition in N-methyldithioglutarimide and N-methyldithiocamphorimide is taken as an indication for a C, distortion of the chromophore. Perhaps such a criterion can also be used with imides. It would be of interest to further investigate the occurrence of delayed fluoresence in dithioimides. A similar phenomenon has been established with thioketones and is subject to extensive study. In two respects the dithioimides have an advantage over aliphatic thioketones: no less than five excited states are in an easy accessible spectral region and they are (photo-)chemically much more stable. The former advantage also holds with respect to imides, and this suggests the use of the dithioimide chromophore as a structural probe in CD studies.
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Acknowledgment. We wish to thank Professor D. A. Lightner, University of Nevada, for use of his MCD instrument. References and Notes (1) For a recent review, see: Maciejewski. A,; Steer, R. P. Chem. Rev. 1993. 93, 67.
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