Electronic spectra of dithieno analogues of phenanthrene

Chemical Physics 40 (1979) 397-404 0 North-Holland Publishing Company

ELECTRONICSPECTRAOFDITHIENOANALOCUESOFPHENANTHRENE T. DAHLGREN,J.GLANS,S.GRONOWITZ Department A.

ofOrganic Chemistry.

University of Lund, Chemical Center, S-220 07 Lund. Sweden

DAVIDSSON,B.NORDiN

Department of Inorganic Chemistry, University of Lund, Chemical Center, S-22007 Lund, Swederz

and P-B. PEDERSEN and E.W. THULSTRUP Deparrment of Chemistry, Aarhus University. 8000 .&hus, C. Denmark

Received 12 February 1979

The magnetic circular dichroism (MCD) spectra of some dithieno analogues of phenantbrene: benzo [2,1+;3,4+‘] dithiophene (I), benzo[1,2-b;4,3-b’] dithiophene (II), benzo[l,Z-c;3,4_c’]dithiophene (III) and beazo[l,2-b;3,4-b’] dithiophene (IV) are reported. For I-III the spectra corresponding to two different transition moment directions could be obtained from lowtemperature linear dichroism spectra. The results compare well with theoretical energies, oscillator strengths, moment directions and MCDB-terms which were obtained from semi-empirical quantum mechanical calculations in the tilectron approximation.

1. Introduction

Y

t The chemical and spectroscopic properties of the polycyclic heteroaromatic systems obtained by the fusion of thiophene rings at the b- or c-sides with other aromatic rings have been extensively studied by Gronowitz and co-workers [l-3]. In tricyclic systems theoretically six isomers can be obtained; three [b,bj-fused, two [b,c]-fused, and one [c,c] isomer. In the previous 0 studies the effect of the mode of fusion on the acidity of thiophene analogues of fluorene has been investiof which four were stable, namely the three [b,b]-fused gated [l], as well as the effect on stabilities and electrosystems benzo [2,1 -b 3,4-b’] dithiophene (I), philic deuteriation of dithionotropylium ions [2] and AY their W spectra [3]. Theoretical spectra calculated within the PPP approximation were in good agreement I with experiment. Recently, we have synthesized all six isomers of the benzodithiophenes which are thiophene analogues of phenanthrene (0) [4,5],

I

T. Dahlgren et al./MCD spectra

398

benzo[ 1,2-h;4,3-b’] dithiophene (II) Y

I/‘r

-I/

,‘yz

%

T II

and benzo [ 1,2-b;3,4-b’] dithiophene (IV) and the [cc] -fused system benzo [ 1,2-c;3,4-c’] dithiophene (III),

-I -da -,.I \ Y

2. Experimental The MCD spectra were recorded at room temperature on a JASCO J-41 spectrometer interfaced with arotatable permanent magnet [6], absorption spectra on a Cary 118-C spectrophotometer; cyclohexane solutions were used with an optical pathlength of 0.10 cm. The light was propagated in the direction of the magnetic field and the field was calibrated using a 1M aqueous CoS04 solution (ep - er = -1.88 X 10B2 M-l cm-l T-1 at 20000 cm-l). Linear dichroism (LD) spectra were recorded in stretched polyethylene matrices in a liquid nitrogen cryostat on a JASCO J-40 spectrometer converted to LD mode according to a method described elsewhere [7]. From the LD and the absorbance,4 , recorded on the same orientated sample with unpolarized light, absorption components Ay and A, were calculated according to standard methods [8,9] _

s-z 3. Calculations

III

WI-

2f43 -

-/I--\

of dithieno analogues of phenanthrene

-

IV.

Their electrophllic substitution, lH and 13C NMR spectra and UV spectra have been studied [4]. In order to obtain more information on the effect of the mode of annelation on the electronic transitions in these systems and their aromatic nature and for a comparison with the iso-electronic n-electron system in phenantbrene, we have undertaken an investigation of the MCD spectra and low temperature LD spectra of these compounds.

All calculations were performed in the n-electron approximation, which according to previous experience on tbiophene as well as on nonslternant conjugated hydrocarbons usually gives satisfactory results for energies, transition moments and MCD for most low-lying transitions with at least medium intensity. The present molecules may be expected to show a “hard chromophore” behaviour [lo], and a comparison between the results of simple Pariser, Parr and Pople (PPP) theory [l I] and those of calculations including overlap in various approximations between the basis 2p-orbitals [12] did not show any significant deviations. Neither did inclusion of multiply excited configurations seem to be of great importance, and the calculations presented here are of standard PPP-type with singly excited CL Parameters for the sulfur atoms were similar to those given in ref. [13] _Transition moments were calculated using the dipole length formula and the MCDB-terms were obtained from the perturbation expression [14] : B(O

+F)

=

c
l

X WMII)

4 -J%

C

W-iWII~-~OIMF~

IfF

EI--EF

X
(1)

399

T. Dahlgren et ai.fMCD spectra of dithieizo analogues of phenanthrene 200

250

I

300

I

“In

200

1

LO

ty ’

250

1

tY

_ 300 I

350 I

30

2ll

IO

IO 3.5 3 D.5 1.0

n

2a

I

50

I

45

i0

3;

I

30

j

103cl-d

Fig. 1. Benzo[2,1-b: 3,4_b’]dithiophene (species I). UV absorption (top) obtained in cyclohexane at room temperature and in polyethylene at 83 K (dotted) and MCD spectra (center) obtained in cyclohexane at room temperature. Reduced spectra (below center): AZ (full line) and A,, (dashed line) corresponding to transitions parallel to the molecular axes z andy, respectively, obtained from LD spectra in stretched polyethylene at 83 K. Orientation constants (&, KY) = (0.51,0.34) have been used. Calculated (XI-PPP) results ke shown at the bottom of the feure; oscillator strengths are represented by the areas of the bars, their heights are proportional to -B. Full and dotted bars indicate z- end y-polarized transitions, respectively.

-20

Fii. 2. Benzo[l,2-b: 3A-c’ldithiophene (species II). UV absorption and MCD spectra, reduced spectra obtained from (Kz,Ky) = (0.53,0.29) and calculated transitions, similarly as in fii. 1.

where 9?i and M are the maenetic and electric dbole moment operators, respectively, and EJ the energy of state

II).

T. Dahlgren et aI./MCD spectra of dithieno analogues of phenanthrene

400

r

250

ml

r

350 nm

300

,

I

I

I

0.5

A

0 iol, --05 - -1.0

I

L -E

5

50

-s-

I

50

Lkl

I

LO

I

35

I

30

10&m-’

Fig. 3. Benzot1,Z-c: 3,4_c’]dithiophene (species III). W absorption and MCD spectra, reduced spectra obtained from (&. KY) = Kl.64.0.25) and calculated transitions, similarly as infg_ 1.

4. Results UV and MCD spectra of the benzodithlophenes are shown in figs_ l-4_ Linear dichroism spectra were measured on the C2v symmetric molecules (species I-III). Useful LD information might also be obtained on IV, but unfortunately a too small amount of the compound prevented this. For the molecules I-III all rr-r* transitions must have their transition moments

I

I

50

h5

I

4b

35

3b

I

lO%ti’

Fig. 4. W absorption (top) and MCD spectra (in cyclohexane at room temperature) and calculated results for benzo[l/b: 3,4_b’]dithiophene (species IV).

directed along either they or the z axis andthe arrisotropic absorption can easily be interpreted in terms of two component reduced spectra A,,(A) and A,(A) [S]. The orientation factors necessary for a determination of A,(h) and A,,(A) may be obtained from purely polarized peaks in the spectrum corresponding to two perpendicular directions of the transition moment. The existence of such two transitions seems fairly obvious in the spectra of II where the O-O peak of the first transition near 3 1000 cm-r clearly is polarized along y and where the strong band system around 35000 cm-t can be expected to be almost purely z-polarized. The result shown in fig. 2 has been obtained from: (K,, KY)

T. Dahlgren et al.fMCD spectra of dithieno analogues of phenanthrene = (0~~0.29) or (Szz, Svv) = (0.30, -0.06), where Ki(Sii) are the orientation factors [8], Ki= (cos2(Zi)l and Sii=~ [3ko~~(Zi)) - l)], where Zi is the angle between axis i and the stretching direction Z. In the case of I the assumption of a purelyypolarized long wavelength side of the O-O peak of the first transition near 32000 cm-i seems reasonable. However, the LD curve has a shape different from that of the absorption curve and a complete reduction of the z-polarized component to zero over the whole peak is not possible, probably due to z-polarized vibronic components corresponding to non-totally symmetric modes. If the strong band system around 35000cm-l, similarly as in II, is assumed to be zpolarized, the following values are obtained: (f&K,,) = (0.51,0.34) or (Szz, S,,) = (O-26,0.02). These values are quite close to those obtained for II, which means that the orientation of the molecules is only slightly changed by the change in the position of the sulfur atoms. A similar determination of the orientation factors is not possible in the case of III, since the first transition near 30000 cm-I is too weak to be observed in the stretched sheet. The second transition at 32000 cm-l may be assumed to be mainly z-polarized with only smally-polarized contributions. This assumption leads to: Kz = 0.64 or S,, = 0.46. This is considerably higher. than the values found for I and II, but may be seen as a result of the sulfur atoms being near the “ends” of 111,‘whichdue to the long C-S bonds will increase the length of the molecule. An upper limit K,, < 0.46 (S,,,,< 0.19) can be obtained from the region around 43000 cm-l, where the lowest LD values are found, but it is unlikely that this region is purelyy-polarized. A lower limit for K,, is given by the value for a rod-like orientation [8] :KY > 1 - Kz/2 = 0.18 (,Su,, > -0.23). Clearly values of K,,(Srv) near the above mentioned upper limit are unacceptable, since K,, < 1 -K, = 0.36 (5& < 0.03). Considering the large gap between the upper and lower limits and the position of the possible points in the “orientation triangle” [15], it seems reasonable to adopt a value of KY slightly lower than those obtained for I and II: K,,= 0.25(S,,,, = -0.08). A comparison was made between LD spectra obtained at liquid nitrogen and room temperature. As expected, the vibrational structure was considerably better resolved at low temperature and also the orientation in the polyethylene matrix was significantly

401

higher at low temperature (e.g. for I, K, increases more than 10%). Some spectral shifts were also observed: The first transition in I was blue-shifted by 0.5 nm while the second stronger transition was slightly shifted towards the red at low temperature. For the first transition an S-shaped LD curve Indicated different positions of the absorption maximum for the two orthogonal light polarizations. Such effects have been observed before in polarized spectra [S]. A possible explanation may be that the “solvent” shifts in the sheet for the molecules well aligned with the stretching direction and those less well aligned are different. A comparison between the spectra of I-IV and 0 shows clearly that they are closely related (for MCD, the resemblance is only obvious for the first few transitions, as one might expect). One striking difference, however, is the disappearance of the third transition in 0 in the spectra of I-IV. The combination of LD and MCD results makes an identification of a large number of electronic states possible. The results are shown in table 1.

5. Discussion The benzodithiophenes discussed here are geometrically analogous to and isoelectronic with phenanthrene, and it has been noted [4] that their UV spectra resemble that of phenanthrene [8,16]. However, the unpolarized spectra are obviously very complex and it is clearly difficult to make even a rough identification of the electronic transitions on the basis of only intensities and energies of the different UV bands. MCD and LD, however, provide valuable additional criteria [17] enabling a satisfactory assignment of the most predominant absorption bands. Fig. 1 shows the reduced stretched sheet spectra, the observed unpolarized absorption and MCD, and the results of quantum mechanical calculation for I. The spectra are in several ways similar to those of phenanthrene [8,16] ; the first transition appears as some medium intense peaks near 32000 cm-l _As in 0 it is short-axis polarized (alongy) but with considerably lower intensity, in agreement with the fact that the transition in 0 is forbidden due to the alternant pairing symmetry. The MCD is positive (negative B-term) and also considerably stronger than that of 0.Thestronger second band is z-polarized with negative MCD as in 0.

T. Dahlgren et al./MCD spectra

402

ofdithieno unalogrtes ofphenanthrene

Table 1 p--n* tiansitions of the benzodithiophenes. E transition energies in lo3 cm -’ for absorption maxima-f: oscillator strengths (w = weak). pal.: pokuisation directions (for IV see defiition of angle in 0.4); B: MCD B-terms in 1F3 Debye*&&m-’ ; experimental valueshavebeen obtained according to B = -33.51-‘1 [S]Mdv/u -_Compound

pol.

0.001 0.1 0.1 0.1 1.0 0.1 >02

Y z z

31.6 35.0 39.0 40.5 44 48

0.03 0.2 0.09 0.6 -

Y *

31.3 34.4 31 40.0 42.5 45

0.02 0.25 0.05 0.1 0.1 0.5

29.8 32.0 33.9 36.4 425 44.5 48.3 51.3

W

(Y)

0.07

L

0.05

Y

29.4 33.9 =31 38.1 39.2 45.0 47.0

lII

Iv

-~

f

E

cd

Calculated

Observed

31.2 35.0 38 39 43 44.8 50

0.5 0.01 0.01 0.89

___-

Y

z Y z

Y z

Y 2

Y I

Y z

Y z

z Y

Z Y+Z

0.01 0.1 0.56 0.1 0.1 0.1

E

f

pol.

B

30.3 34.2 38.8 40.7 41.2 42.2 46.5

0.009 0.36 0.19 0.48 1.31 0.10 0.04

Y

-1

-0.7 2.9 0.3 1.5 >o >o

32.9 35.3 43.0 43.6 46.1 455

0.05 0.55 0.68 0.51 0.17 0.43

-2.1 4.1 -0 >O 0.5 -1.3

33.3 34.0 41.9 44.7 42.9 49.1

0.03 0.78 0.08 0.35 059 0.31

Y

-0.07 0.09 -0.2 3.0 >O X -0.7 0.7

31.1 32.2 35.5 38.0 42.7 43.3 46.9 47:3 48.3 49.1 51.3

0.03 0.84 0.26 0.39 0.03 0.01 0.01 0.16 0.00 0.01 0.88

1

32.9 36.8 40.0 42.7 45.8 47.4 48.0 49.5 515

0.03 0.28 1.2 0.3a 0.10 0.09 0.12 0.05 0.14

B

xi

(2

1 o (?)

-0.1 0.15 -0.3 1.4 1.7 -0.7

Z 7

Y 2

Y I

Y 2

Y Z

Y Z

z

Y

Z Y z

z

Y

Z Y z 2

Z

Y Y z

-6” -43” 17” -68” -101” -3” 49” 75O -37”

3 -7 32 -37 14 0 - 5.8 7.7 - 9.6 3.2 12.8 - 8.9 -21.6 22.0 0.4 8.5 - 7;2 - 5.3 - 5.5 5.2 - 2.6 3.0 0.76 - 0.09 - 0.1 0.6 0.004 - 0.21 - 6.3 -1.3 2.2 -5.3 3.3 1.2 4.1 -3.4 -0.7 1.2

a) All results for 0 have been taken from table 1 of ref. [17]; the calculations include overlap explicitly.

The third transition starting near 39000 cm-1 seems to be of comparable intensity; it isy-polarized, and

has negative MCD, similar to the fourth transition in 0 [16]. The strong z-polarized absorption with maximum

T. Dahlgren et aI./MCD

spectra of dithieno analogues of phenanthrene

near 40000 cm-1 seems to be due to two transitions, the first very intense with positive MCD, the second weaker, and with negative MCD. At 45-50000 cm-l the MCD curve shows first a positive, then a negative, -st.rong peak. The calculated results for the first two transitions are in excellent agreement with experiment, .but for the 3-4 higher close-lying transitions the predicted MCD signs do not always agree with the observations. This is not surprising in view of the large density of states - the calculations predict 10 singlet states between 43000 and 56000 cm-l above the ground state. Fig. 2 shows the results for II. Both absorption and MCD spectra are considerably different from those of 0 and I, but are well described by the calculations. The first transition around 32000 cm-l is weak, y-polarized and has a strong positive MCD. The second transition is strong, z-polarized with negative MCD. It is followed by a series of peaks in the curve describingy-polarized absorption starting at 36000 cm-l with a maximum near 39000 cm-l. These are likely to be due to the fairly strongy-polarized transition with weak MCD predicted near 42000 cm-l. The z-polarized peak near 40000 cm-l is considerably weaker than the corresponding one in 0, I or III; it has a weak negative MCD as predicted by the calculations. This transition is overlapped by a y-polarized absorption with a maximum near 43000 cm-l and with negative MCD. Thii transition in turn is overlapped by a stronger z-polarized band around 46000 cm-l with positive MCD. Note that the -unpolarized absorption spectrum does not make a distinction between the two latter transitions possible. In general the agreement with the calculations is excellent in spite of the fact that also in this case the density of states around 50000 cm-l is very high. Fig. 3 shows the results for III. As mentioned earlier the first transition in III near 30000 cm-t is very weak in the absorption, but it is easy to observe in MCD, where it appears as a positive peak as predicted by the calculations. We expect it to be y-polarized. The second transition at 32000 cm-l is strong, z-polarized, and has a weak negative MCD in agreement with the calculated results. The third transition is near 34000 cm-t, it is y-polarized, with a weak positive MCD. The strong z-polarized transition is observed near 36000 cm-l, i.e. considerably lower than in I, II and 0. This transition shows a relatively strong negative MCD. They-

403

polarized absorption near 42000 cm-l is not clearly seen in the MCD, whereas the z-polarized weak peak near 45000 cm-1 corresponds to a negative MCD-peak. The z-polarized band with a maximum near 49000 cm-l seems to consist of two transitions with opposite MCD, but also here the high density of states precludes unambiguous assignment. However, the general agreement between theory and experiment for III is remarkably good. Although no LD results are available for IV, the results shown in fig. 4 allow an identification of a large number of transitions_ The first of these, near 31000 cm-I, is weak with positive MCD, while the stronger second transition near 37000 cm-1 has negative MCD. The strong band system centered around 40000 cm-l seems to be composed of at least 2 transitions: a weaker one at 38000 cm-l with a strong positive MCD and a very strong transition at 40000 cm-1 with negative MCD. The negative and positive peaks in the MCD and the medium intense absorption above 45000 cm-l are reasonably weU accounted for by the calculated transitions in this region. In general, the agreement between theory and experiment for IV is also c:i
T. Dohlgren et al.fMCD spectra of dithieno analogues of phenanthrene

404

of I-IV as aromatic satisfactory_

14 774ectron

systems seems

Acknowledgement

The authors are grateful to the Knut and Alice Wallenberg Foundation for a grant which made pos-

sible the purchase of the MCD equipment. B.N. and E.W.T. are grateful to Professor J. Michl for valuable discussions. Grants from the Swedish Natural Science Research Council to S.G. and B.N., and from NATO to E-W-T., are gratefully acknowledged_

[Sl S. Gronowitz [6] [7] 181 [9] [lo] [ll]

[12] 1131 [14]

References 1151 [l] A. Wiersema and S. Gronowitz. Acta Chem.Stand. 24 (1970) 2593. (21 S. Gronowitz, B. Yom-Tov and U. Michael, Acta Chem. Stand. 27 (1973) 2257. [3] T. Lihefors, U. Michael, B. Yom-Tov and S. Cronowitz, Acta Chem. Stand. 27 (1973) 2485.

141T. Dal&en, Ph. D. Thesis, Lund, Sweden (1977).

[16] [17J [la]

and T. Dahlgren, Chem. Scripta, to be published. B. Norden, R. H%kansson and S. Daniekson, Chem. Scripta 11 (1977) 52. A. Davfdsson and B. Nordin, Chem. Scripta 9 (1976) 49. E.W. Thulstrup. J. Michl and J.H. Eggers, J. Phys. Chem. 74 (1970) 3868. A Dwidsson and B. Nordin, Chem. Scripta 11 (1977)

?MichJ, J. Am. Chem. Sot. 100 (1978) 6801. J.A. Pople, Trans. Faraday Sot. 49 (1953) 1375; R- Pariser and R.G. Parr, J. Chem. Phys. 21 (1953) 466, 167. N.H. Jqkgensen, P.B. Pedersen. E.W. Thulstrup and J. Micbl, Intern. J. QuantumChem. S 12 (1978) 419. J. Fabian, A. Mehlhorn and R. Zahtadnik, J. Phys. Chem. 72 (1968) 3975. P.NI Schatz, and A.J. McCaffery, Quart. Rev. 23 (1969) 552. E-W. Thulstrup and J. Michl. J. Am. Chem. Sot. 98 (1976) 4533; J. Michl, end E-W. Thulstrup, Spectry. Letters 10 (1977) 401. E.W. Thulsttup, I. Mol. Sttuct. 47 (1978) 359. E.W. Thulstr~~p, Intern. J. Quantum Chem. 12 Sl (1977) 325. M. V&k, M-R. Whipple and J. Michl, J. Am. Chem. Sot.

100 (1978) 6867.