J. Phys. Chem. 1983, 87, 2317-2322
Conclusion Our Raman studies, together with the depression of absorbances and the development of a shoulder at 320 nm in the UV, are good evidence for intercalation of proflavin to DNA in methanol. The Raman studies, in particular, suggest that the interaction is localized on the outer rings. Refinement of the band assignments and acquisition of
2317
excitation profiles should c o n f i i this hypothesis and allow an extremely detailed picture of the binding of proflavin to DNA. The latter experiments awaits completion of an improved experimental system which is currently under construction‘ Acknowledgment. This work was supported by the National Science Foundation Grant CHE 79-15185.
Crystal Structure and Electronic States of Dlthieno[3,2-& :2’,3‘-dlthiophene F. Bertlnelll, P. Palmlerl, C. Stremmenos, Istituto di Chlmlca Flsica e Spettroscopla, 40 136 Bologna, Italy
0. Pellzrl, Istltuto di Chlmica Generale, 43 100 Parma, Italy
and C. Tallanl’ Istltuto di Speffroscopla MoleColere del C.N.R., 40126 Bologna, Italy (Received: August 17, 1982; In Final Form: November 23, 1982)
A structural, spectroscopic, and theoretical study on the electron donor dithieno[3,2-d:2’,3’-d]thiophene(DTT) crystal is presented. The crystals are monoclinic (space group E 1 / n )with four molecules per unit cell; atomic coordinates, bond lengths, angles, and the orientation of the optical indicatrix with respect to crystallographic axes are reported. The polarized UV absorption spectra of the single crystal, the electronic structure, and the symmetries of the first excited molecular singlet states are discussed, the observed transitions being compared with the results of an extended ab initio SCF/CI computation using the ST0/3G expansion of atomic orbitals. The assignments are supported by the analysis of the vibronic structure of the UV spectra and by IR and preresonance Raman spectra where the strong overtone of the C=C stretching implies that this mode couples with the short axis transition moment of the molecule. Evidence of a strong 3lA1-l3Bz spin-orbit interaction leading to an efficient intersystem crossing to the first excited triplet state is deduced from the emission spectra.
Introduction Dithieno[3,2-b:2’,3’-d]thiophene(DTT) is a polycondensed heteroatomic aromatic molecule, consisting of three condensed thiophene rings. There is new interest in compounds of this type because of the possibility that they could be new electron donors to be used as partners in the formation of conducting donol-acceptor ( S A ) complexes.’ In a preliminary investigation we found that in fact with TCNQ a blue D-A complex is formed. We have previously studied the electronic structure and spectra of the two-ring homologous thieno[3,2-b]thiophene (TT).2 In the present work we have studied (i) the crystal structure of DTT, (ii) the polarized absorption spectra of the single crystal in the range of 30 00045 0oO cm-’ (3.7-5.6 eV) together with the vapor and solution spectra, (iii) the emission spectra from Shpolskii matrix and crystal powders, and (iv) the preresonance Raman spectrum of solid DTT. The interpretation of the spectra is supported by the Raman and IR spectra. The assignment of the observed electronic transitions is discussed by making comparisons (1)‘Proceedings of the International Conference on Low-Dimensional Conductors, Boulder, CO, Aug 1981”,1982,Mol. Cryst. Li9. Cryst., Vol. 79. (2) F. Bertinelli, A. Brillante, P. Palmieri, and C. Taliani, J. Chem. Phys., 66,51 (1977).
with the theoretically computed transition energies obtained by ab initio CI methods.
Experimental Section The product was synthesized by following the De Jong and Jassen methods3 By vacuum sublimation, at low temperature (40-45 “C), 3-D prisms suitable for X-ray analysis were grown. They belong to the monoclinic system with well-developed (110) planes. Prisms of the same morphology and symmetry are obtained from benzene and methyl alcohol solutions. The isoorientated polycrystalline sections grown from the melt were optically identified as (201) planes. The orientation of the optical indicatrix in the crystal was established with a five-axis universal stage (Leitz Model UT/5). The space group and the initial cell parameters of the pure crystal were determined from oscillation and zeroand first-layer Weissenberg photographs. Intensity data for 2319 reflections were collected by the 8-28 scan technique on a single-crystal Siemens AED diffractometer for 28 I 58.0°, using Mo K a radiation (A = 0.71069 A). The UV polarized spectra of (201) sections, grown between silica disks, were recorded down to 15 K on a Cary 15 spectrophotometer, using a variable-temperature cryostat (Air Liquide), and at 4.2 K on a Spex 1402 double (3) F. de Jong and M. J. Janssen, J. Org. Chem., 36,1645,1988(1971).
0 1903 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 13, 1983
tz
Bertinelli et al.
TABLE I: Final Atomic Parametersa S, S, S3
c, C, c3 c4
c, c, C6
C8 H, H, H3 H 4
Flgure 1. Schematic drawing of the molecular structure of dithieno[3,2-b:3',3'4]thiophene, illustrating the atomic numbering scheme and glving bond distances (in angstroms) and angles (in degrees). The ranges of the esd's are 0.002-0.004 A and 0.1-0.2O,respectively.
monochromator and an immersion cryostat (Pope Scientific Instruments). A couple of lock-in (Par Model 5101) and ratiometer Par Model 188) were used for signal detection of the normalized intensity. The preresonance Raman spectrum of the polycrystalline DTT at 77 K was excited by a nitrogen laser (Laser Elettronica Model A-500) and analyzed with the Spex 1402 and photoelectric detection on a cooled RCA 31034 PMT. The resolution of the spectrum is limited by the natural bandwidth of the nitrogen laser (fwhm = 6 cm-'). Emission spectra from Shpolskii matrices and crystalline powder were excited a t 3122-3128 A (mercury line) and at 3010 A by doubling the Rhodamine 6 G emission of a dye laser (Hiinsch design). Shpolskii matrices were grown from to M degassed solutions in n-C5 to n-C9 hydrocarbons, which were sealed in a thin-walled glass tube and dipped into liquid nitrogen at variable speed (5-0.05 cm/min). The frozen matrices used for emission measurements were transferred or grown directly into quartz cells placed at the window of the immersion cryostat.
X-ray Analysis and Structure Determination The cell parameters, determined from Weissenberg photographs, were refined by least-squares methods for 20 reflections. The crystallographic data are as follows: a = 12.746 (5), b = 10.614 (4),c = 6.005(3)A, @ = 97.53 (4)O V = 805.4 (6)A3, p(Mo Ka) = 8.07 cm-'; monoclinic; space group R 1 / n ;Z = 4 with equivalent positions: x,y,z; Q,i;1/2-x,1/z+y,'/z-z; 1/z+x,'/2-y,'/2+z. By averaging the symmetry-related reflections and applying the observation criterion I L 36(1),we retained 1745 unique data for use in the analysis. A correction to the intensity data was introduced to take into account the effect of a slight decomposition which was noted during the data collection. The intensities were further corrected for Lorentz and polarization effect, but not for absorption or extinction. The structure was solved by direct methods using the computer program MULTAN,4 which showed all non-hydrogen atoms in the E-map, calculated from the most probable phase set. In subsequent cycles of full-matrix least-squares refinement, the thermal parameters were allowed to vary isotropically initially and anisotropically in later stages. Inclusion of the hydrogen atoms, located in a different Fourier map, as isotropic contributions to the refinement led to the final value of R = 0.0427. (4)
G.Germain,P. Main, and M. M. Woolfson, Acta Crystallogr.,Sect.
A, 27, 368 (1971).
x/a
Y/b
z/c
48726 ( 4 ) 67569 ( 4 ) 76265 ( 5 ) 4192 ( 2 ) 4729 ( 2 ) 5734 ( 2 ) 7 4 5 3 (1) 8428 (2) 8609 ( 2 ) 6930 ( 2 ) 5929 ( 2 ) 359 (3) 453 ( 2 ) 882 (2) 912 ( 3 )
86736 ( 6 ) 83477 ( 5 ) 103765 (6) 7809 ( 3 ) 7616 ( 2 ) 8187 ( 2 ) 9258 ( 2 ) 9916 ( 2 ) 10555 (2) 9416 ( 2 ) 8801 ( 2 ) 755 (3) 714 ( 3 ) 997 ( 2 ) 1110 ( 3 )
28417 ( 9 ) -22758 ( 9 ) 3 6 0 8 9 (10) 701 (5) -1053 ( 4 ) -668 ( 4 ) -197 ( 3 ) -26 ( 4 ) 1915 (5) 1656 ( 3 ) 1368 (3) 84 ( 6 ) -238 ( 5 ) -106 ( 5 ) 244 ( 6 )
X105 for S, X104 for C, and X103 for H.
(010) ll
4I -
3/2 .y I 1/2+2
It
Flgure 2. Crystal structure of dithieno[3,2-b:2',3'-d]thiophene; (010) projection of symmetry-related molecules.
The molecular structure is illustrated, together with the numbering scheme, in Figure 1, which also gives bond distances and angles. The positional parameters are given in Table I. Tables of observed and calculated structure factors and of thermal parameters are available as supplementary material. (See paragraph at end of text regarding supplementary material.) Although each of the three five-membered rings is planar to within experimental error, the molecule as a whole is slightly folded, the diedral angles between the central ring and the terminal rings being 3.0" and 3.1'. Bond distances and angles appear to be normal and are in accord with those in related molecules. Of more interest are the structural parameters involving the sulfur atoms. In particular, the S-C bonds, which are all indicative of some I delocalization, being intermediate between single and double bonds, are not all equal and can be classified into three groups: the longer ones (1.729,1.730 A) involving the central sulfur, the medium (1.720,1.724A) and shorter (1.710,1.712A) ones both involving the terminal sulfurs, with the first related to the outer carbons. These bond lengths are close to those in thiophene (1.717A).5 The three C-S-C bond angles are similar within experimental errors (90.3O,90.6', 90.7O)and are in the range usually found for thiophene derivatives. There are no intermolecular distances shorter than the sum of the normally accepted van der Waals radii. Schematic pictures of the molecules in the unit cell are reported in Figure 2.
Optical Properties At the polarizing microscope, the (110)crystal sections show an oblique extinction direction, and in conoscopic light an emergence of an optic axis at the margin of the fieId is seen. Comparing the optical behavior of (110) ( 5 ) W. R. Harahbarger and S. H. Bauer, Acta Crystallogr. Sect B , 26, 1010 (1970).
The Journal of Physical Chemistry, Vol. 87,No. 13, 1983 2319
Structure of DRhieno[3,2-b :2',3'-d]thiophene
A
TABLE 11: Observed Frequencies (cm-') of DTT Vapor,
Solutions, and Single Crystal vapor
40
35
45
IO'
B
Flgure 3. (A) Polarized electronic crystal spectra at 15 K: (-) ac component; (---), b component. (B) UV. vapor spectrum (-); solution spectra: (- - -) CH,OH (- -) diethyl ether; (-) n-pentane.
-
-
planes with that of (110) planes of the same crystal, we deduce that the optic axes of the indicatrix are symmetrical with respect to the ac plane. The orientations of the principal refractive indexes are as follows: (Y (refractive index) parallel to b, 0 (refractive index) on the ac plane at -45O.1 from a axis, y (refractive index) at + 44O.9 from a axis. The 2V angle is 87'. The crystal sign is positive. The above data were tested with the Biot and Fresnel law6 applied to (110) planes which shown an extinction direction of 32O from the c axis. The isooriented polycrystalline samples grown from melts were identified as (201). They show right extinction and the fast direction is parallel to b.
Electronic Spectra Vapor. The vapor absorption spectrum in the region of 30000-45000 cm-' of the DTT, at 59 OC, is shown in Figure 3B. A strong absorption system with a progression of broad bands separated by about 1250 cm-' originates at 34110 cm-' and has the appearance of an allowed electronic transition. At lower energy of the origin, a single band of much sharper bandwidth is observed at 33 710 cm-'; its low intensity and the remarkable difference in the bandwidth suggest that it belongs to a different electronic transition. Solution. The spectra in ethanol, ethyl ether, and npentane are shown, together with the vapor spectrum, in Figure 3B. All have the same overall appearance, but the bands are broader than in the vapor. A series of bands (6) N. H. Hartshorne and A. Stuart in 'Crystals and the Polarizing Microscope", 4th ed., Arnold, London, 1970.
crystal
diethyl ether
methanol
b
ac
30650 31120 31590 32060 33400 34630 35860
30620 31090 31560 32030 33075 34380 35670
38880 40120 42460
38810 40110 42600
33710
32690 33160
32730 33200
32650 33110
34110 35330 36570
33625a 34560 35800 37040 38280
33665a 34570 35810 37050
33570a 34660 35890 37130 38370
38080 30
n-pentane
a This band is possibly a superposition of the third quantum of system I and also the origin of system 11.
separated by 1250 cm-' dominate the high-energy side of the spectrum, while a short progression of about 430 cm-l is present in the low-energy side. These two progressions evidently belong to two distinct electronic transitions, the lowest of which is apparently more red shifted than the second transition in the vapor. Spectra taken in different solvents of different polarity do not show any remarkable shift (see Table 11). The oscillator strength corresponding to the first absorption system for a cyclohexane solution is 0.6. Crystal. The CaUmolecular point group has A,, B2,and B1 allowed electronic transitions, with polarization parallel to the short and long molecular axes and perpendicular to the molecular plane, respectively. In the crystal, each molecular transition splits into A, and B, allowed factor group components, polarized along the b axis and in the ac plane. The UV spectrum of the (201) crystal plane is shown in Figure 3A. The overall absorption is strongly QC polarized. Four different absorption regions are distinguished the first with origin at 30640 cm-' (hereafter referred to as system I); a second with origin at 33 200 cm-l (11); a third one that originates at 38 850 cm-' (111); and finally an intense absorption system beginning at 42 400 cm-' (IV). All the observed frequencies are collected in Table 11. The first very weak absorption system (system I) consists of a short progression of weak shoulders, separated by about 470 cm-'; its low intensity and superposition with the tail of the main absorption prevent an estimate of the polarization ratios. The following strong system (II), mainly ac polarized, has the factor-group components of the origin at 33 075 cm-' in the ac direction, and 33 400 cm-' in the b direction. A progression of three quanta of about 1250 cm-l, having a rapidly decreasing intensity, is the main feature of this system. A medium-intensity absorption at 38 900 cm-' (111)on the high-energy side of the first strong system (II), with an equal partition of intensity in both polarizations, reveals the nature of a new electronic transition. The edge of a strong absorption (IV) is shown at higher energy and its intensity is presumably higher than system (11).
Vibrational Spectra With the aim of assigning the vibrations active in the absorption and emission spectra, we recorded the Raman spectrum of DTT in CC14and benzene solutions and the IR spectrum of a KBr pellet. In the C2"point group the vibrations of the free molecule are classified according to the following symmetry species: A, (14), B2 (61, B, (13), and A2 (6). Vibrations of species A,, BO,and B, are both
2320
Bertinelli et al.
The Journal of Physlcal Chemistry, Vol. 87, No. 13, 1983
TABLE 111: Raman and IR Modes of DTT in the 400-500and 1300-1500-~m-~ Rangea
I
I
I
I
Raman (solution ) infrared (KBr pellet)
~
cm-l
polarization
482m
pol.
443m 412m 1311m
pol. pol. pol.
1445vs 1474 s
pol. pol.
v,
~
V,
cm-I
480w 460 s (436)w 1302m 1359 vs 1384 m 1411 m 1437m 1465 s, 1471-1473
remarks
A, B, or B, A, A, A , , C-C stretching
A , , C = C stretching A , , C=C stretching
Discrepancies among the frequencies of the Raman and IR spectra may be attributed to the different phases of the samples (i.e.,solution vs. solid).
J " 1
1 Y
inn
IIm
iim
IUY
3 (em") Figure 5. Phosphorescence spectrum in a fast-frozen n-CS matrix at 77 K.
and may be regarded as a lower homologue of DTT. TT shows three strong totally symmetric modes at 1331,1421, and 1486 cm-'; the first is assigned as a C-C stretching and the last two are assigned as C=C stretchings. The preresonance Raman spectrum shown in Figure 4 is obtained by exciting a polycrystalline sample of DTT on the red edge of the absorption contour with light of 337.1 nm. Many more bands are present in this spectrum compared to the ordinary off-resonanceRaman. If we limit our attention to the main features, we notice that four totally symmetric modes, 482,1311,1445, and 1474 cm-l, dominate the spectrum (for convenience we use the solution frequencies). At the same time, combinations and overtones of some of these frequencies show up remarkably; in particular, the overtone of the C=C stretching vibration at 2 X 1445 cm-' is considerably intense. Flguro 4. Preresonance Raman spectrum of solid DTT at 77 K.
IR and Raman active, while Az species are solely Raman active. Results for the two regions of interest ( 4 0 0 0 and 1300-1500 cm-') are summarized in Table III. A complete vibrational assignment will be published elsewhere. In the low-frequency region of the solution Raman spectrum, a group of weak bands were measured a t 412, 443, and 482 cm-'; in the same region, the IR spectrum of the solid shows three bands at 436,460, and 480 cm-'.We identify the 482-cm-' mode (480 cm-' in the IR spectrum; the discrepancy is due to the different states of aggregation) as a totally symmetric mode from the strongly polarized behavior. We tentatively attribute a totally symmetric character to the 443 cm-' (436 cm-' IR), while the strong 460-cm-' IR band which does not have a Raman counterpart is classified as either Bzor B1. In the range of 1300-1500 cm-', the Raman spectrum shows three strong bands at 1311,1445, and 1474 cm-'; all vibrations are assigned as totally symmetric modes. This is confirmed by comparison with the assignment of the vibrations of the TT molecule.' In fact, TT consists of two condensed thiophene rings with the sulfur atoms at opposite corners of the molecule
Emission Spectra Phosphorescence spectra are obtained from Shpolskii matrices and from crystalline powders. Several solvents ranging from n-C5 to n-C9 were used in the preparation of matrices, but only in n-C5 can the main features of these well-known spectra8 be observed. A phosphorescence spectrum taken at 77 K from an n-C5 matrix, grown at high cooling rates, is shown in Figure 5. Each vibronic band splits into typical narrow-line multiplets related to various insertion modes of the solute: while the sharpness is due to weak coupling of the aromatic solute with the lattice of the matrix.1° The 0-0band has the most detailed structure, some peaks appearing as doublets only a few cm-' apart. At this temperature, the spectrum shows about 50 lines and is dominated by a long progression of the totally symmetric vibration of 482 cm-' plus local phonon modes. (A Table giving a vibrational analysis of the phosphorescence emission is available as supplementary material.) The increased intensity observed after the third quantum could be explained by a possible overlapping with the 1434-cm-' Raman mode, but the observed peaks fit better with overtones of the 482-cm-' mode. (8)E.V. Shpolskii, Sou. Phys.-Usp. (Engl. Trawl.), 3,372 (1960);5, ,522 IlQ62): _ _ _6. _411 , , _flQ63). ,__-,-___ _ _ _ I
(7)(a) Ya. M. Kimelfeld, M. A. Moscalyova, G. N. Zhizhin, V. P. Litvinov, S. A. Ozalin and Ya. L. Goldfarb, Opt. Spectrosc. (Engl. Trawl.), 28, 599 (1970);(b) Y. Cozien and P. Saumagne, C. R. Hebd. Seances Acad. Sci. Ser. E , 276,375 (1973);(c) Ya. M. Kimelfeld, M. A. Moscaleva, L. M. Mostovaya, G. N. Zhizhin, V. P. Litvinov, and 1. P. Konyaeva, Opt. Spectrosc. (Engl.Trawl.),39,276 (1975).
(9)(a) C. Pfister, Chem. Phys., 2, 171 (1973);(b) T.Vo Dinh, U. T. Kreibich, and U. P. Wild, Chem. Phys. Lett., 24, 356 (1974);(c) T.B. T a " and P. M. Saari, Opt. Spectrosc. (Engl. Traml.),38,594(1975). (10)(a) K. K. Rebane and V. V. Khizhnvakov. Oot. Spectrosc. (Enpl. Trawi.), 14,193 (1963);(b) K. K. Rebane, %np&iiy Spectra of Solids", Plenum Press, New York, 1970; (c) J. L. Richards and S. A. Rice, J. Chem. Phys., 54, 2014 (1971).
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983 2321
Structure of Dithieno[S,P-6 :2',3'-dlthiophene
TABLE IV: Transition Energies to Singlet and Triplet States Computed with the Restricted and More Extended CI Procedures and Transition Moments Computed with the Restricted CI Procedure
(e.v.)
15
t
I w
~~
a4 Singlets B, (3az95bi) 0.251 513 Ai (4bi95bi) 0.271 180 A, (3a,,4a2),(3b,,5b,), 0 . 3 1 3 9 3 2 (3a,3a,,5b15b,1 B, (2a,,5bi) 0.315 896
4ms
---=
I
I
, B, (3a,,5bi) A , (3az,4a,),( 3 b 1 , 5 b , ) Ai (4b,,f'bi) B, (2a,,5bI),(3az,6b,)
Triplets 0.124 194 0.170 551 0.197 593 0.212 505
Restricted CI procedure. dure.
\
-15
t
lbr
/
Figure 6. Correlation diagram for DTT.
Electronic Structure of DTT In order to classify the lowest excited electronic states of the DTT molecule, one must understand the highest occupied and lowest virtual T MO's. Formally these orbitals may be thought of as resulting from the interaction of the T MO's of the all-cis octatetraene molecule and the ?r orbitals of the three sulfur atoms. The correlation diagram is shown in detail in Figure 6. The actual expression of the MO's of DTT was obtained by using the ST0/3G1' expansion of the atomic orbitals and a modified version12aof the Gaussian 70 program.12b It is found that lb,, la2, 2bl, and 3az correlate with the MO's of the polyene moiety and the 4b1, 2a2, and 3bl are mainly sulfur 3pr orbitals. Therefore, in addition to transitions typical of polyene hydrocarbons (3a2 5b1;3a2 4az), charge-transfer excitations from 3p sulfur atomic orbitals to the lowest antibonding MO's are expected to occur in the low-energy region of the spectrum. This is confirmed by CI computations, restricted to single and selected double excitations (column b in Table IV), which indicate that of the lowest states 21A1 and 21B2 have the highest charge-transfer character. From the CI expression it is noticed that the 21A1 state has a large contribution from doubly excited configuration (3a2,3a2 5bl, 5bl). The existence of similar states, whose wave functions (wf's) in the VB language13have main contributions from covalent structures, was first predicted theoretically for polyene hydrocarbons, and only later confirmed experimentally by spectroscopic measurements, due to the forbidden character of these states. In DTT, charge-transfer excitations in the low energy of the spectrum give a mixed covalent-ionic and partial allowed character to the state. The sequence of covalent
-
~~
transition energy, au
-
-
(11) W.J. Hehre, R. F. Stewart, and J. A. Pople, J. Chem. Phys., 51, 2657 (1969). (12) (a) P. Cremaschi, G. Morosi, and P. Palmieri (unpublished); (b) W. J. Hehre, W. A. Lathan, R. Ditchtield, M. D. Newton, and J. A. Pople, 'Gaussian 70. Ab initio SCF-MO calculations on organic molecules", Quantum Chemistry Program Exchange 236, Indiana University, Bloomington, IN. (13) K. Schulten, I. Ohmine, and M. Karplus, J. Chem. Phys., 64,4422 (1976).
bb
u. au
0.280 37 0.243 64 0.27297
1.802 1.333 0.154
0.301 82
0.202
0.154 14 0.200 86
More extended CI proce-
and ionic states is known to be very sensitive to the details of the theoretical description such as the basis used for the computation. This is confirmed by the results of a more extended CI treatment (column b in Table IV) where all single and double excitations from the main components of the CI w f s, within a given orbital subspace, were included. With the more extended CI, the Al states are predicted to have lower excitation energy compared to the two B, states, contrary to the results of the restricted CI. Within the triplet manifold, the sequence of A, and B2 lowest states is inverted compared to singlets. All B1 and A2 singlet and triplet states are predicted to occour at higher energy compared to the A, and B2 states and they have not been included in Table IV. Discussion We can observe four distinct transitions. Within the first absorption region two electronic states (I and 11) are identified on the basis of the two distinct vibronic progressions which are identified in this spectral region (i.e., the progression of 470 cm-' and a progression of 1250 cm-' at 34 450 cm-'). In the oriented gas model (OGM), the first-order calculated polarization ratios for a (201) face are L : M N = 1.7:6.1:0.01. If we assume that the strong transition at higher energy (system 11)is L polarized as the calculations suggest (vide infra), a transfer of intensity from this state into the second transition leads to a reduced second-order polarization ratio with respect to the OGM. The polarization ratio of the second transition (11)is Iac/Ib= 3.8 and is consistent with an assignment to a transition polarized along the medium axis (M) of the molecule. Given the polarization of the transition, we assign this intense absorption to the 21A1 llAl transition. The transition density for this electronic excitation, which is obtained from the MO and CI expressions in Table IV, suggests for the active vibration a large component of the double-bond stretching vibration of the polyene moiety. Further evidence of the assignment of the first strong transition (system 11) is derived from the preresonance Raman spectrum. The frequency of excitation (29 665 cm-l) of the nitrogen laser lies within the low-energy slope of the first absorption. If one neglects the terms that describe the Herzberg-Teller mixing between the excited and ground states (terms B and C of Albrecht's notation,lk +-
(14) (a) A. C. Albrecht, J. Chem. Phys., 34, 1476 (1961); (b) B. B. Johnson and W. L. Peticolas, Annu. Reu. Phys. Chem., 27,465 (1976), and references therein.
2322
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983
the Raman cross section in the adiabatic approximation is14b
where the indexes g and e identify the ground and excited electronic states, and f and v the ground- and the excited-state vibrations, with wave functions xgf and xeu, respectively. Mklnis a component of the transition moment and w1 the exciting frequency. The prime on the sum indicates that from the summation the term corresponding to e = i and e = f is excluded. Assuming that only the lower strong system (11) gives resonance contribution, we drop the sum over all states. Moreover, if we neglect the antiresonance term and consider that the Raman spectrum is at low temperature (77 K), eq 1 simplifies to
This matrix element is different from zero only for totally symmetric (TS) vibration since the initial vibrational level, the ground state, is TS. Then TS vibrations serve as intermediate states, since (TSITS)(TSITS) # 0 In conclusion, the intensity of the preresonance Raman lines, which is proportional to the square of the element cygfd, is governed by the square of the overlap integrals. A consequence of this is that overtones and combination acquire intensity. This is indeed the case for the 1445-cm-‘ frequency, where the first overtone and a combination (1445 + 1474 cm-’) are rather intense and dominates the high-frequency side of the spectrum. This implies that the equilibrium position of the nuclei changes in going from the ground to the excited states, and the change is mostly represented by the vibrational mode of 1445 cm-’ which was previously assigned to a C=C stretching as expected for a transition polarized along the medium axis (M) of the DTT molecules. No firm conclusion regarding the nature of the first transition can be drawn from the crystal spectrum, since any direct observation of the polarization ratio is prevented by its low intensity and the superposition with the second transition (11). On the basis of the orbital and level diagram in Figure 6 and Table IV, it is assigned to the 3’A,
Bertinelii et ai.
state. This is consistent with the nearly forbidden character of the state. The active vibrational coordinate is likely to be a ring deformation with displacement of the S atoms able to modify the charge-transfer character of the state and the intensity of the transition via vibronic coupling among the two excited (2’A1, 3’AJ states. The corresponding ground-state frequency is strongly active in the Raman spectrum and assigned to a totally symmetric vibration. The vibronic structure of the phosphorescence emission from the lowest triplet state, which we identify as a 13Bz, may be similarly explained. The intensity of the emission, which is related to spin-orbit interactions, is likely to be strongly modified by vibronic mixing of the orbital levels which increases the contribution of sulfur 3pa orbitals to the ground state 3az MO. The vibronic mixing of the 2a2, 3a2 MO’s could be a possible mechanism to enhance the intensity of the emission. The 3’A1, 13B1spin-orbit interaction is estimated to be large (heavy-atom effect), which could explain the high efficiency of the phosphorescence compared to the fluorescence. Finally, the two B, states are assigned as follows: the intermediate absorption, system 111, to 21B2;the intense absorption (system IV), to 1’B2. The sequence of these two singlet states has been inverted compared to the results of the CI computations. This could be due either to a limitation of the theoretical description or to a solid-state and/or solvent effect, with stabilization of the state with higher charge-transfer character, and subsequent inversion of the two states.
Acknowledgment. This work has been made possible by support from the Consiglio Nazionale delle Ricerche, Progetto Finalizzato “Chimica Fine e Secondaria”. Helpful discussions on the crystal structure with Prof. M. Nardelli are gratefully acknowledged. We thank Dr. B. Lunelli for the recording of the IR spectra, Dr. P. Zanirato and Prof. G. Pagani for kindly supplying a sample, and Mr. G. Morelli for recording the solution Raman spectrum. We also thank Mr. A. Martiniello and Mr. R. Pezzoli for technical assistance. P.P. acknowledges financial support from the Consiglio Nazionale delle Ricerche (Roma). Registry No. Dithieno[3,2-b:2’,3’-d]thiophene, 3593-75-7. Supplementary Material Available: Tables of observed and calculated structure factors and of thermal parameters and a table giving vibrational analysis of the phosphorescence emission (11 pages). Ordering information is given on any current masthead page.
