Spectrochimica Acts, Vol. 27A.. pp. 621 to 630. Peqtamon Press 1971. PrIntad in Northern Imlmd

Electronic spectra of dibenzothiophene*

A. BREE and R. ZWARICE~ Chemistry Department, University of Bntieh Columbia,

Vancouver 8, B&ah Columbia, Canada

(Received 28 August 1970)

&&a&-The polarized absorption spectrum of dibenzothiophene near 330 nm haa been measured in the pure crystal and m host lattices of fluorene, hexamethylbenzene, blphenyl and (polycrystallme) n-heptane. The assignment lA, + IA, IS confhmed for thm electromc tran- sitlon. The polarized fluorescence spectrum, although rather weak, was recorded and analyzed in terms of the known ground state frequencies. Phosphorescence, havmg an enhanced quantum efficiency due to the presence of the relatively heavy sulphur atom, was emfiy observed. The tnplet state was tentatively assigned A, spatial symmetry. Vibrations of symmetry A,, RI and Es were all promment m phosphorescence, the non-totally symmetric modes appearmg through second-order vibromc spm--orbit perturbations.

INTRODUCTION

THE lowest energy electronic transitions of carbazole and fluorene have been assigned [l, 21 by snalysing the polarized absorption and fluorescence spectra of these mole- cules in single crystals of a suitable host or of the pure material. The longest wave- length absorption band of carbszole is polarized along the short in-plsne molecular axis while the corresponding absorption of fluorene is long-axis polarized. A report of a similar study carried out on dibenzothiophene, another three-ring aromatic molecule, is presented here.

In the complex dibenzothiophene absorption spectrum in solution, the two systems at 326 nm (f = 0.0299) and 287 nm (f = 0.0665) almost certainly represent separste transitions, but each of the more complicated regions beginning st about 268 nm (f = 0.222) and 245 nm (f = 0.646) may represent more than a single electronic transition. PLATT [3-j has suggested that the assignments of these bands, in order of decreasing wavelength, are short, long, long and short-axis polarized, respectively, by comparison with the assignments for phenanthrene. These assign- ments of the 326 and 287 nm systems agree with recent results obtained from broad- band magnetophotoselection experiments on dibenzothiophene in diethylether solution at 77’K [4], and are further supported by photoselection experiments carried out by DORR [5].

In this paper, the work on dibenzothiophene is confined to assigning the symmetry of the lowest-energy singlet electronic state, to investigating the vibrational structure of this excited state from the absorption spectrum, and to studying the ground state vibrational structure from fluorescence and phosphorescence spectra.

* This work was supported by a grant from the National Research Council of Canada. t Present address: Chemtry Department, The State Umversity of New York at Albany,

Albany, New York, U.S.A.

[l] A. BREE and R. ZWARICH, J. Ohem. Phy8.49, 3356 (1968). [2] A. BREE and R. ZWARIOH, J. Ohem. Phy8.51,903 (1969). [3] J. R. PLAY, J. Ohem. Phye. 19, 101 (1961). [4] S. SIEUEL and H. S. JUDEIIW, J. Phya. Ohm. 79,2206 (1966). [S] F. DORR, Angew. Ohem. 6,478 (1966).

621

622 A. B~EE and R. ZWAIUCH

EXPERIMENTAL

Eastman Kodak white label dibenzothiophene was used after undergoing 204 passes on a zone refiner. The procedure followed m purrfymg the fluorene, used as a host material in thus work, has been described elsewhere [S] as have the techmques employed in recordmg the spectra

111. Fluorene forms orthorhombrc crystals [7] of space group Pnam(D$) m whrch the four

molecules in each umt cell occupy specral sites. The molecular plane of symmetry contammg the short m-plane ax18 and molecular normal comcldes with the ab crystal mrrror plane so that the long molecular axis is exactly parallel to c. Thus, transitions active along the long and short molecular axes are completely resolved m polarized spectra from 8 bc section.

THE ABSORPTION SPECTRUM

Microdensitometer tracings of dibenzothiophene absorption spectra in It-heptane (polycrystalline) and fluorene (single crystal) matrices are reproduced in Fig. 1 and an analysis of the vibrational structure in n-heptane and hexamethylbenzene host materials is set out in Table 1. The vibrational structure in the fluorene matrix is broad even at the lowest temperatures attainable so that vibronic symmetry assign- ments are not possible. Although the absorption spectrum in hexamethylbenzene was completely depolarized, the bands were quite sharp and are included in Table 1

for comparison with the a-heptane analysis. In n-heptane, the most intense lines in the leading SWOLSKII multiplet [S] were located as resonance lines at 30,205;

30,341; 30,364 and 30,423 cm-l. For simplicity and clarity, the analysis in Table 1 is based on those intervals associated with the principal origin at 30,423 cm-l.

There is no long progression in any vibration (this is true also of the fluorescence spectrum) and the electronic origin band is the strongest in the system, although, in the n-heptane matrix where the lines are considerably sharpened, re-emission has apparently weakened the resonance multiplet. It is clear that the molecule undergoes only a very small change of shape following the transition and this is to be expected for a molecule with such a rigid skeleton. Fundamental vibrations of the excited electronic state have been tentatively assigned in Table 1 essentially on the basis that the Franck-Condon factors are small for all vibrations so that overtones and combinations will be weak; a search for a more “well-behaved” matrix must be undertaken before possible perturbations may be identified.

The pure electronic origin in the fluorene matrix is completely absent from the c-polarized spectrum (see Fig. 1). The proper conclusion to draw from this result is that at least one in-plane molecular axis lies normal to c. However it is extremely difficult to imagine how a dibenzothiophene molecule could occupy any other than a substitutional site in the fluorene matrix, even allowing for the removal of more than one host for each guest molecule. This assumption is further supported by the similarities in the physical properties of the two compounds, such as melting- point, unit cell dimensions and even the molecular arrangement within the lattice; for the location of an approximate orthorhombic unit cell in the monoclinic

[6] A. BREE and R. ZWARICEC, Spectrochim. dcta &%A, 713 (1969). [7] D. M. BUZTNS tmd J. IBALL, Proc. Roy. Sot. -7, 200 (1956). [S] E. V. SHPOLSKII, Usp. 2%. Nauk 80, 266 (1963); Sovie8 Phys. Uap. 6, 411 (1963).

Electronic spectra of dlbenzothiophene 628

I- ax6

d.2 c-axis

Fig. 1. Absorption spectra of &benzot~ophene U-I n-heptane (above) and 111 ffuorene (below) at about 15’K. The energy scale refers only to the lower spectra.

dibenzothiophene crystal [9] see the preceding pttper [IO]. Thus, this absorption system m&s an IA, c lA, electronic transition in agreement with PLATT’S predic- tion [3] snd SIEG)EL and JUDEIKIS’ experimental result [4]. However, ss & check the spectrum of dibenzothiophene in s biphenyl matrix wss messured using s (20i) section. The origin band w&s almost completely b polarized again consistent with the above short-&s assignment. As with the fluorene matrix, the absorption and fluorescence spectra, in biphenyl were diffise and little vibrstionsl detail w&s resolved.

[S] R. M. SCHAIWRIN and J. TRO~ER, J. Chem. Sot. 197OA,1661. [lo] A BREE and R. ZWARICH, Spectrochim. Acta WA, 609 (1971).

7

624 A. BXEE and R. ZWARICH

Table 1. Absorption speotra of dibeuzothlophene m n-heptane and hexamethylbenzene (HMB) at about lS’K*

J=Bt n-heptane Remarks

30,160 .¶ 215 m 267 VW 396 w 405 mw

477 m 510 mw

689 m 810 m

1000 mw 1042 m 1083 w 1102 VW 1215 VW 1251 VW 1320 mw

1439 w

30,423 s 213 B 261 w

402 8 420 m 485 B

681 w 693 m 881 mw 891 mw

1001 ms

1081 m

1208 m 1253 w 1308 m 1411 w 1432 w 1486 mw 1521 mw 1569 mw 1648 w 1129w 1190 w 1998 w

O”,gm

213, Ft 261. F? 396, F? 402, F 2 X 213 - 6 485, F 570, F?

693, F 402 + 485 - 6 8B1, F

1001, F 411+ 570 - 5

1087, F 405 + 689 + 8

1208, F 510+ 689-221

1308, F 213 + 1208 - 4

1432, F? 1486, F?; 485 + 1001 213 + 1308

1569, F?; 485 + 1087 - 3 213 + 1432 + 31

2 X 213 + 1308 - 5? 485 + 1308 - 3 693 + 1308 - 3

t A 20 om-’ lattloe mode and woondary ongms, probably markmg .mveral d&m& mtee m the host mat=, 184 (w) and 130 om-’ (VW) to the red of the mam origin are not b&d III the table.

THI FLUORESCENCE SPECTRUM

The fluorescence spectra of dibenzothiophene as a pure crystal and in fluorene and n-heptane matrices are shown in Fig. 2 and the vibrational intervals are given in Table 2. In n-heptane the intensity distribution amongst the members of each multiplet is different in fluorescence and absorption suggesting that some transfer of energy is possible between the different sites in the matrix. The two strong multiplet components readily seen in Fig. 2 are built on origins at 30,423 and 30,341 cm-l, and the analysis shown on Fig. 2 is the one associated with the former Origin.

The fluorescence origin in the fluorene matrix is well polarized along the b axis, but the vibronio bands in the rest of the spectrum are essentially depolarized and are too broad to be used in an accurate vibrational analysis. The crystal fluorescence is quite sharp with an average band width somewhat less than 10 cm-l and has most of its intensity along the s axis; for a definition of the crystal axis frame b, r, 8 used here see the preceding paper [lo]. These results are again consistent with the short-axis assignment for the electronic transition, since the long axis of the molecule is aligned very nearly parallel to the crystal axis r.

Electronic spectre of dlbenzothlophene 026

c-ax6

W

r-axis

WAVENUMBER (cm-‘)

Fig. 2. Dibenzothophene fluorescence spectra m (a) IS fluorene m&ix, (b) the pure crystal, and (c) an n-heptane matrix at &out 16’K.

It is probable that the emission in the pure crystal arises fn>m a defect site, although this could not be demonstrated since the samples were too thick to be penetrated in absorption experiment& In any event the phonon structure associated with the origin band need not coincide in energy with the Raman data [lo] since

A. BREE and R. ZWUICH

Table 2. Fluorescence spectra of dlbenzothlophene aa a pure crystal and m an mheptme rnatruc at about 16’K

crystal* IIs Ilr n-heptme AImlysuJt

29,923 16m 42s 63w 19, 91 m

101 w 138~ 219 s 310 VW 364 VW 409 w 499 m 706 w

1027 xnw

1132 w 1164 “xv 1174 VW 1206 VW 1237 w

1316 “8

1477 mw 1628 mw

1802 *

1816 w 2093 VW 2297 VW 2336 VW 2626 w 2909 w

1079 w

1669 VW 1688 VW

30.423

214 m

406 VW 499 8 706 m 717 w 902 VW 997 VW

1023 m 1079 w 1132 mw 1169 w 1178 “w 1199 VW 1226 VW 1291 VW 1310 YB 1416 VW 1479 w 1624 m

1691 VW 1603 B 1713 VW 1739”w 1810 m 2102 w 2309 VW 2331 VW 2620 w 2912 w

Ortpn, reabsorbed lattme 1attm 1sttme lattwe 1attme 1att1ce 138, ISI? 214, A, 219 + 91 219+ 138 - 3 406, A, 499, A, 706, A, 214 + 499 + 4 406 + 499 - 2 2 + 499 - 1 1023, A, 1079. B, 1132, A,

499 + 706 - 6 1226, A, 214 + 1079 - 2 1310, A, 2 x 706 + 3 1479, A, 214 + 1310 499 + 1079 - 9 1691, B, 1603, A, 406 + 1310 - 2 706 + 1023 + 10 499+ 1310f 1 499 + 1603 706 + 1603 1023 + 1310 - 2 2 x 1310 1310 + 1603 - 2

* The crystal axes a and c have been defined tn the precedmg paper [IO]. t Where apphcable the rmalym refers to the spectrum m n-heptane.

the level with k = 0 may not lie at the energy minimum of the exoited electronic state. The 138 cm-l interval is associated with the B, fundamental of the ground state [lo]; this mode is active not because of a breakdown in the molecular selection rules but because the distinction between inter- and intramolecular motion is lost when the energies of the vibrations are so similar.

The vibrational structure is understood in terms of the known fundamentals [lo] and the details of the analysis are shown in Table 2. Two weak lines at 1159 and 1178 cm-l are not accounted for end are attributed to combination bsnds arising from fundamentals not themsehes ective in fluorescence, although it is possible that the 1156 cm-l line represents an A, fundamental [lo]. These lines are surprisingly far removed from the nearest intense lines at 10.23 aud 1309 cm-1 with which they presumably interact.

Electromc spectra of dlbenzothlophene 627

THE PHOSPHORESCENCIX SPIWXIWM

~~%n~~rn%~r tra*ings of the phosphorescence spectra of ~~nz~t~ophene in ?&-heptaae and hex~methylbe~ene matrices and in the pure cryst& at about 16°K are reproduoed in Fig. 3, and the ~br&tion&l intwvals together with their wignments based on the crystal spectrum are given in Table 3. For brevity, the

FI& 3. ~hoepho~~n~ spectra of dl~nzot~oFhene m (a) the pure crystal, (b) a hexametbylbenzene matam, and (a) an n-hepiane mebtmx at about l@‘K.

628 A. BREE and R. ZWARICH

analysis in Table 3 covers only the fundamental region since the rest of the spectrum shows normal Franck-Condon combinations and no CH stretches could be identified. Only the weak lines, 1118, 1163 and 1171 cm -l to the red of the origin are not

Table 3. Phosphorescence spectra of dlbenzothiophene m n-heptane and hexemethylbenzene (HMB) matrices and the crystal at about 15’K

n-heptane HMB Crystal Anelysw

24,417 B

,

210 m 221 m

417 ma 429 w 49s ma

660 ms

632 VW 707 me

740 ma 768 m

861 vs

934 8

1024 ma

1061 mw 1072 mw

1134m

1204 w 1227 w 1267 w 1293 VW 1316 w

1349 m 1416 w 1431 VW

1476 w

1666 mw 1682 mw 1602 B

24,096 s 20m

106 VW 141 VW

223 m

413 m

495 m

666 ma

628 VW 707 ms

734 m 767 w 77s w 849 vs 870 m

931 ms

1028 w 1063 VW

1072 m 1093 “Iv 1118 VW 1136 w

1200 w

1261 w 1290 VW

1346 m 1412 VW 1441 VW

1467 w 1491 w

1664 mw 1686 VW 1612 8

1694 VW

29,292 v* 16 m8 40m

11OW 146 w 180 w 216 ms 237 w 264 VW 426 w 438 w 498 ms 614 w 643 VW 664 m 679 VW 643 VW 706 ma 726 w 746 mw 771 w 788 YW 866 8 816 w 894 920 VW 937 m 964 VW

1026 m 1038 w

1070 m 1087 VW 1118 VW 1133 m 1163 w 1171 w 1201 w 1234 w 1268 VW 1286 VW 1316 ms 1333 w 1362 m 1412 VW 1429 VW 1460 w 1476 mw 1494 VW 1627 w 1669 mw 1681 VW 1601 s 1618 mw 1640 w 1693 VW

on@n lattice 1att1ce lsttlce 138. B.? 171; A;? 216. A, 237, B,?, 216 + 16 _t 6 216 + -40 - 2 426, B, 2 X 216 + 6 498, A, 498 + 16 498 + 40 f 6

664. B, 664 + 16 - 1 426 + 216 - 2 706, A, 706+ IS+ 3 746. Bl 771, A, 664+216-t_ 8 866, B, 866 + 16 f 4 866 + 40 - 1 216 + 706 - 2 937. Bl 937 + 16 f 1 1026, A, 1026 + 16 - 4 861+ 212 - 2 1070, A, 1070+ 16+ 1

1133, A, 1133 + 16 + 4? 1133-f- 40-2’ 1201, A,; 498 + 706 - 3 1234, A, 664 + 706 - 2, 1268, B,’ 216+ 1070- 1 1316, A, 1316+ l6+ 1 1362, B,, 498 + 866 - 1 2 x 706 1429, A, 216 + 1234 1476, A, 1476 + 16 f 2, 426 + 1070 - 498 + 1026 + 3 1669, A,: 866 + 706 - 2 1681, B, 1601, A, 1601+ 16 f 1 1601 + 40 - 1 216 + 1476 + 1

1

Electronic spectra of dlbenzothiophene 629

readily understood and these probably represent 8, overtone or combination bands. The analysis has made use of the vibrational intervals from the phosphorescence spectrum itself rather than from the infrared and Raman spectra [lo] ; the two sets of frequencies may not be the same since the electronic transition need not terminate on vibrational levels (1) near the origin of the Brillouin zone or (2) belonging to those irreducible representations of the factor group which are active in the purely vibrational spectra. The measured energies of the vibrations are sufficient to show that A,, B, and B, fundamentals are all active in phosphoresoence. The frequencies 146 and 180 cm-l are not to be included in this list since such low energy molecular vibrations would presumably mix with intermolecular motions and they may be more properly treated as lattice modes.

The spectrum in n-heptane is particularly complex. The components of the leading multiplet are located at 24,476; 24,417 ; 24,433 ; 24,395 ; 24,367 ; 24,344; 24,327 and 24,297 cm-l. Only the bands associated with the principal origin at 24,417 cm-l are included in Table 3 and although the analysis of such a composite spectrum is complex it reproduces the features of a spectrum at 77’K reported by NUFMJKEAMETOV and GOBOV [ll]. A second weak origin band appears 143 cm-l to the red of the main origin in the hexamethylbenzene spectrum (see Fig. 3) and a second weak dibenzothiophene phosphorescence spectrum is built on it; the two spectra must arise from solute molecules in very different sites in the host lattice. The importance of these spectra lies in the fact that the relative intensities of the B, and B, vibrations increase with respect to the A, modes as the matrix changes from the pure crystal to hexamethylbenzene and n-heptane. In this way the 425 cm-l interval was identified with the B, fundamental and the 1070 cm-l interval with the A, fundamental. The intervals 564, 746, 855,937,1362 and possibly 1581 cm-l also mark non-totally symmetric vibrations ; the 856 cm-l mode, the strongest in the spectrum, does not have an overtone (unlike, for example, the weaker 1601 cm-l vibration).

The pure electronic origin has been assigned from the analysis given in Table 3 since following this assumption essentially all the intervals correspond to known ground state vibrations, many of them non-totally symmetric giving rise to secondary false origins. Thus the transition is spatially allowed although spin forbidden. Further, the spin-orbit coupling interaction is surprisingly sensitive to certain displacements of the nuclear framework and the subsequent appearance of the B, and B, modes in single quanta could, in principle, allow an experimental determin- ation to be made of the orbital symmetry of the triplet state. However, information concerning the polarizations of the lines in the spectrum is incomplete, only an ac section (or rs section) being examined. All bands, irrespective of their vibronic symmetry were somewhat stronger in 8 than r polarization. Since the long molecular axis is directed close to r while both the short and normal molecular axes project on s, this result suggests that the spatial symmetry of the triplet state is A1 and that this state is perturbed by singlet states of symmetry B, and B,; it has been assumed here, in agreement with the results of work on solvent shifts [ll], that

[ll] R. N. NUFLMUKHAMETOV and G. V. GOBOV, Opt. Spechy 18,126 (1965).

630 A. BREE and R. ZWARICE

phosphorescence represents a ~-7r.* and not an n-m* transition. The polarization of the vibronic bands can be interpreted only in terms of further second-order vibronic perturbations.

The brief argument above is tentative since the emission most probably occurs at defect sites in the dibenzothiophene crystal lattice. This is, of course, consistent with the observation that phosphorescence is seen only at 77’K or lower when the efficiency of triplet-triplet rtnnihilrttion is reduced. Attempts to observe T, + S, absorption were not successful even with a crystal nearly 2 cm thick.

CONCLUSION

The pure electronic origin of the weak absorption system at 326 nm of dibenzo- thiophene in a fluorene m&ix is polarized along the b axis end so marks en lA, c lA, transition. Because the origin bsnd is the strongest in the spectrum and there are no prominent progressions in any vibration, the molecule has very nearly the same geometry in the two electronic states. If the potential fields in which the nuclei are constrained are alm similar for the ground and excited electronic state, then it should be possible to correlate vibrations which have .a corresponding activity in the absorption and fluorescence spectra. These correlations are set out below taking the data from the spectra in the n-heptane matrix and enclosing the ground state frequencies in parentheses : 213 (214, A,); 402 (406, A,); 486 (499, A,); 693 (706, a,); 1001 (1023, A,); 1087 (1132, A,); 1208 (1226, A,); 1308 (1310, A,) and 14321 (1479, A,). In this way, most of the possible fundamentals observed in the absorption spectrum are seen to be totally symmetric. Although somewhat tentative, the correlation shows that this system derives little intensity through vibronic coupling with electronic states at higher energy.

The triplet state also has A, spatial symmetry. This assignment is very tentative beccrruse (1) emission (both fluorescence and phosphorescence) for reasons that are not fully understood always tends to become depolarized, presumably because it often arises from imperfect regions of the crystal and (2) the nature and geometry of the particular defect site involved in this phosphorescence are not known. The symmetry assignment of the triplet would be best made from T, + S, absorption.