Preparation and Spectroscopic Characterization of Cyclopenta[l,2-a: 3,4,5-b'c']dicoronene M. Zander* Rütgerswerke AG, D-W-4620 Castrop-Rauxel W. Friedrichsen* Institut für Organische Chemie, Universität Kiel, Olshausenstraße 40/60, D-W-2300 Kiel Z. Naturforsch. 47b, 1314-1318(1992); received April 6, 1992 Dicoronylene Isomers, UV/VIS Absorption Spectra, Fluorescence Spectra The non-alternant hydrocarbon cyclopenta[l,2-a:3,4,5-b'c']dicoronene (3) has been pre­ pared by aluminium chloride catalyzed dimerization of coronene (1). The UV/VIS absorption spectrum, fluorescence spectrum and fluorescence quantum yield of 3 is reported and com­ pared with that of the alternant isomer benzo[l,2,3-bc:4,5,6-b'c']dicoronene (2). Relationships observed in the UV/VIS absorption spectra of alternant and non-alternant polycyclic aromatic hydrocarbons and preliminary theoretical investigations (PMO calculations) on two model systems (C, E) are also reported.

The aluminium chloride catalyzed dimerization Scholl reaction product of 1) was first crystallized (intermolecular Scholl reaction) of coronene (1) from boiling pyrene. Due to its slightly better solu­ [1-3] leads to a mixture of the alternant hydrocar­ bility compared to 2 the isomer 3 is enriched in the bon 2 (benzo[l,2,3-bc:4,5,6-b'c']dicoronene) and product obtained from the mother liquor. This its non alternant isomer 3 (cyclopenta[l,2-a: 3,4,5- product was extracted with boiling 1-methyl-naph- b'c']dicoronene). It has been estimated that the thalene. 3 crystallized in red needles (melting point crude Scholl product consists of approximately >470 °C) from the methylnaphthalene solution on 80% 2 and 20% 3 [3]. However, due to the very cooling to room temperature. As estimated by close similarity of 2 and 3 with regard to solubility UV/VIS absorption spectroscopy the sample con­ in organic solvents and volatility only the main tained « 1 % of the isomer 2 but was found to be product 2 could previously be obtained in pure sufficiently pure for determining reliable spectro­ form. Its structure was unambiguously proven by scopic data. The structure of 3 follows unambigu­ UV/VIS absorption and photoelectron spectro­ ously from the route of formation and the high-re- scopy and comparison of the experimental data solved mass spectrum (chemical ionization, reac­ with results obtained by quantum chemical calcu­ tant gas: methane) which confirmed the chemical lations [3]. In continuation of our work on the syn­ composition C48H2o. thesis and spectroscopic characterization of very large polycyclic aromatic hydrocarbons (PAH) [4] - a subject, which is of current interest in such dif­ ferent fields as astrophysics [5], carbon science [6], and theoretical chemistry [7] - we now succeeded AlCU in the preparation of pure samples of 3 and deter­ mination of some of its spectroscopic properties (UV/VIS absorption spectrum, fluorescence spec­ trum, fluorescence quantum yield). For the preparation of 3 the 2/3-mixture (ob­ tained after vacuum sublimation of the crude In Fig. 1 the UV/VIS absorption spectra (in 1,2,4-trichlorobenzene at 120 °C) of 2 and 3 are * Reprint requests to Prof. Dr. M. Zander or Prof. Dr. W. Friedrichsen. given. The para -band ('La band) of 3 (494 nm) lies at somewhat shorter wavelengths than that of 2 Verlag der Zeitschrift für Naturforschung, D-W-7400 Tübingen (504 nm) while the reverse relation applies to the 0932-0776/92/0900-1314/$ 01.00/0 /?-band (’Bb band) (3: 403 nm, 2: 358 nm). Further, M. Zander-W. Friedrichsen • Cyclopentatl^-arS^^-b'c'Jdicoronene 1315

the molar absorption coefficient of the para -band is lower for 3 than for 2. In Table I all pairs (or groups of three) of isomeric alternant and non­ alternant PAH’s, respectively, for which spectral data were available, and where the isomers are topologically related in the same way as in the case of 2 and 3, have been collected. As can be seen the spectral relationships observed for 2 and 3 hold (in the qualitative sense) for all other groups of iso­ mers. The rule that the /?ara-band (HOMO- LUMO transition) of the alternant isomer (AH) always appears at longer wavelengths than that of the non alternant isomer (NAH) could also be re­ produced by HMO calculations. In Table II some HMO data (Je(HOMO-LUMO)) for compounds Fig. 1. UV/VIS absorption spectra (1,2,4-trichloroben- 2 -1 9 are given. In all cases studied z/fi(HOMO- zene, 120 °C) of benzo[l,2,3-bc:4,5,6-b'c']dicoronene (2) LUMO)[AH] < zf£(HOMO-LUMO)[NAH] (see ( ------) (position o f band maxima (nm) and molar ab­ Appendix). As can be seen from Fig. 3 there is a sorption coefficients (in brackets): 504 (83,200), 470 (54,050), 440 (24,880), 370 (37,750), 358 (95,500), 342 fairly good linear relationship between the para- (61,800)), and cyclopenta[l,2-a:3,4,5-b'c']dicoronene (3) band of the AH’s (2,4, 7,10,12,13,15,17) and Ae (------) (494 nm (19,552), 462 (14,454), 437 (15,488), 403 (in ß units). The same holds for the NAH’s of (97,727), 381 (44,668), 359 (33,884), 330 (45,709)). Table I (Fig. 4). The calculated value of 3 differs

Table I. Position (nm) and log e (in brackets) o f the para- and /?-band of isomeric alternant and non alternant poly- cyclic aromatic hydrocarbons. Each pair (or group of three) of isomers is made up from identical acene subunits. One exception from the rules discussed has been observed and is marked by an asterisk.

^20^12 ^24^14 ^28^16 ^28^16 ^30^16 ^34 ^18 ^4 8 ^ 2 0 1316______M. Zander-W. Friedrichsen • Cyclopenta[l,2-a:3,4,5-b'c'1dicoronene

X [nm] — ► Fig. 2. Fluorescence spectra (1,2,4-trichlorobenzene, room temperature) of benzo[l,2,3-bc:4,5,6-b'c']dicoro- Ae (in ß ) nene (2) (------) (position of band maxima (nm): 514, 554, 597) and cyclopenta[l,2-a: 3,4,5-b'c']dicoronene (3) Fig. 3. 'La band of the AH’s of Table I vs. zffi(HOMO- (------) (532 nm, 570, 616, 665). LUM O) (in /?-units, HMO values).

Table II. HMO data for 2 -1 9 (Jfi(HOM O-LUM O)) in /?-units.

Ae Ae

2 0.665 11 0.533 3 0.765 12 0.427 4 0.695 13 0.418 5 0.837 14 0.578 6 0.860 15 0.482 7 0.530 16 0.723 8 0.679 17 0.365 9 0.648 18 0.563 10 0.387 19 0.669 considerably from the expected one; obviously this simple model is not sufficient to account for the HOMO-LUMO differences of all of these PAH’s. Ac (in ß) To the best of our knowledge the relationships ob­ Fig. 4. 'La band of the NAH’s of Table I vs. ^ (H O M O - served in the UV/VIS absorption spectra of alter­ LUM O) in /?-units, HMO values). nant and non alternant PAH’s are reported here for the first time. Experimental In Fig. 2 the fluorescence spectra (in 1,2,4-tri- chlorobenzene at room temperature) of 2 and 3 are Preparation of cyclopenta[ 1,2-a: 3,4,5-b'c']di- coronene: The Scholl reaction of coronene (1) was given. As with some other non-alternant PAH’s [ 8] performed as described in [2]. The crude Scholl the hydrocarbon 3 exhibits a rather large Stokes reaction product was sublimed in vacuo (500 °C, shift (1450 cm“1, 2: 390 cm“1). The fluorescence 1 mPa) and the sublimate obtained resublimed quantum yield (trichlorobenzene, room tempera­ under the same conditions. 350 mg sublimate were ture) of the alternant isomer 2 (0.81) is significant­ dissolved in 75 g boiling pyrene and the crystalliz­ ly larger than that of the non alternant isomer 3 ing 2/3-mixture (270 mg) separated by filtration at (0.13). 250 °C. The mother liquor of crystallization was M. Zander-W. Friedrichsen • Cyclopenta[l,2-a:3,4,5-b'c']dicoronene 1317

then diluted with 1-methyl-naphthalene at 120 °C Perkin-Elmer MPF 44 E luminescence spectrom­ and the residue obtained (40 mg) extracted with eter, while fluorescence quantum yields were deter­ 150 ml boiling 1-methyl-naphthalene for 20 min. mined as described in [9] (fluorescence standard: After removing the undissolved material (30 mg) perylene, quantum yield: 0.98). by filtration at 220 °C 1.2 mg 3 (approx. 2%, rela­ tive to the amount of 3 present in the crude Scholl We thank Dr. H. Münster, Finnigan MAT reaction product) crystallized from the 1 -methyl- GmbH, Bremen for measurement of mass spectra naphthalene solution at room temperature. 3 and D. Kampf as well as K. Bullik, Rütgerswerke forms red needles, m.p. >470 °C. Chemical ioniza­ AG, for useful experimental assistance. tion mass spectrometry (reactant gas: methane) revealed: Calcd 597.1644 ,3C,12C47H20 Calcd 597.1599 Found 597.1625 Appendix l2C4gH22 Calcd 598.1721 As has been shown above all alternant hydro­ ,3C,12C47H2I Calcd 598.1677 Found 598.1682 carbons (AH’s) of Table I show a para -band ('La) Measurements: The mass spectrum of 3 was at longer wavelengths than their isomeric non­ measured using a Finnigan MAT 95 mass spec­ alternant (NAH) counterparts. Whether this phe­ trometer. UV/VIS absorption spectra were meas­ nomenon is a general one is open to question, but ured with a Perkin-Elmer 556 UV/VIS spectrome­ it can be shown for a special - admittedly - sim­ ter, quantum-corrected fluorescence spectra with a ple class of AH/NAH isomers that even simple

— > Y Y C

2 X ♦ 4- = m ; 2 Y + 4- = n 1318 M. Zander-W. Friedrichsen • Cyclopenta[l,2-a:3,4,5-b'c']dicoronene

HMO considerations are in accord with these ob­ If, e.g., n - 1, v = 3, a = 3, r = 1 (as in C) and servations. It is well known that eigen values and H — 1, v = 3, a = 2, t = 1 (as in E) then vectors of linear polyenes with n centers can be x = z/e(HOMO-LUMO)[E]/ expressed in analytical form (eq. ( 1 ), (2 )) [1 0 ]. zfe(HOMO-LUMO)[C] (5) Ej = 2 cos(nJ/(n + 1)), J = 1 to n (1) can be evaluated analytically. With öß = 0.25 one £ j h = V (2 f ( n + l))sin (7iJnKn + 1 )) (2 ) obtains for n = 6 , 1 0 , and 10 0 the following values Two polyenic systems (A, m atoms; B, n atoms) for x: 1.162, 1.202, and 1.23307. The limiting value can be connected to give either an AH (C, D) or an for x with n —»■ °o is evaluated as x = (n - 0.50)/ isomeric NAH (E, F). According to simple PMO (71 ~ 1 ). In the general case this limiting value of course arguments [ 1 1 ] for m = n the variation of Ej (öej) is given by eq. (3): depends on öß; one obtains = CJuCjvößMV + CJaCJzößax (3) lim(zf£(HOMO-LUMO)[E]/ zfe(HOMO-LUMO)[C] = If for simplicity ößßV = ößm = öß, one obtains for (Ti ~2öß)/(n - 4Sß), (6 ) HOMO (J = n/2) i.e. in the framework of the HMO method one gets <5e(HOMO) = 2/(n + \)(sin(nn^i/ for the model systems C and E A e[E] > A e[C]: 2 (n + l))sin( 7rwv/2 (« + 1)) + sin(nnaf NAH’s of type E absorb at sh orter wavelengths 2(n + l))sin (nnr/(2(n + \)))öß (4) than the isomeric AH’s of type C.

[1] L. Boente, BrennstofF-Chemie 36, 210 (1955). [7] I. Gutman and S. J. Cyvin, Introduction to the [2] M. Zander and W. Franke, Chem. Ber. 91, 2749 Theory of Benzenoid Hydrocarbons, Springer, Hei­ (1958). delberg (1989); J. R. Dias, Handbook of Polycyclic [3] H. J.Lempka, S. Obenland, and W. Schmidt, Chem. Hydrocarbons, Part A, Elsevier, Amsterdam (1987). Phys. 96, 349(1985). [8] W. Karcher (ed.), Spectral Atlas of Polycyclic [4] M. Zander and W. Friedrichsen, Chemiker-Ztg. 115, Aromatic Compounds, D. Reidel Publ. Comp., 360(1991). Dordrecht (1985). [5] A. Leger, L. d’Hendecourt, and N. Boccara (eds): [9] R. Li and R. C. Lim, J.Chem. Phys. 57, 605 (1972). Polycyclic Aromatic Hydrocarbons and Astrophys­ [10] E. Heilbronner and H. Bock, Das HMO-Modell und ics, D. Reidel Publ. Comp., Dordrecht (1987); L. J. seine Anwendung, Grundlagen und Handhabung, Allamandola in I. Gutman and S. J. Cyvin (eds): 2. Aufl., Verlag Chemie, Weinheim (1978). Advances in the Theory of Benzenoid Hydrocar­ [11] M. J. S. Dewar and R. C. Dougherty, The PMO bons (Topics in Current Chemistry, Vol. 153), p. 3ff., Theory of Organic Chemistry, Plenum Press, New Springer, Heidelberg (1990). York (1975). [6] For a review see: M. Zander, Erdöl und Kohle • Erd­ gas ■ Petrochem. 38,496 (1985).