Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems

Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems. IV. Aromaticity and Anti-Aromaticity in Electronic Ground and Excited States Fritz Dietz*,a, Henryk Vogel3, Anna Schleitzer3, Nikolai Tyutyulkovab, Mordecai Rabinovitzc 3 Universität Leipzig, Institut für Physikalische und Theoretische Chemie, Augustusplatz 10/11, D-04109 Leipzig, Germany b University of Sofia, Faculty of Chemistry, BG-1126 Sofia, Bulgaria c University of Jerusalem, Department of Organic Chemistry, Jerusalem 91904, Israel Z. Naturforsch. 52 b, 1072-1076 (1997); received May 13, 1997 Energy Spectra, Anti-Aromatic Ring Systems, Excited States Using various criteria to characterize the terms aromaticity and anti-aromaticity it is shown that the non-benzoid aromatic azulene and the benzoid aromatic naphthalene should have anti-aromatic properties in the first excited singlet state.

Introduction [8 ] that the excitation of molecules which are built up from odd-membered cyclic conjugated ring sys The concept of aromaticity of monocyclic pla tems by annelation or by connection through dou nar conjugated (4 n +2) 7r-electron systems and of ble bonds results in a charge transfer from the for anti-aromaticity of the corresponding 4 n 7r-electron mal negatively charged (4 n +2 ) 7r-electron frag systems has so far been restricted to the ground state ment to the formal positively charged (4 n +2) 7r- [1], From this arises the question: is it possible to fragment. This electron transfer upon excitation cre extend (generalize) the terms aromaticity and anti- ates species in the excited state whose electronic aromaticity to electronic excited states? The aim of structure can be described by anti-aromatic 4 n 7r- the present study is to answer this question. fragments, e.g. Recently, it was shown [2] that the antiaro- matic cyclobutadiene and cyclopentadienyl cation, respectively, show aromatic character in some low- lying excited singlet and triplet states. The aromatic “benzene either loses or reduces the aromatic char acter upon electronic excitation” [ 2 ]. t> - 0 The transition states of pericyclic reactions in ground and excited states were termed to be aro CHD- 0 - © lv - 0 - 0 ) matic or anti-aromatic [3-7]. Special rules for the characterization of the transition state of pericyclic Another example is the s-indacene which shows thermal and photochemical reactions as aromatic all the features of an anti-aromatic compound (small or anti-aromatic depending on the number of atoms excitation energy, bond alternation) [9]. Relatively and on the charge of even- and odd-membered cyclic large excitation energies for the Si -^S 2 and S2—>S3 conjugated ring systems have been proposed [5, 6 ]. transitions and a (quantum-chemically calculated) There are some indications that the aromatic char maximum equalization of the bond lengths in the acter of some compounds in the ground state is excited singlet states are typical criteria for aromatic changed to a more or less anti-aromatic character properties [ 10]. upon electronic excitation of the molecules. Re Various criteria have been used to characterize the sults of quantum-chemical calculations have shown term aromaticity (and anti-aromaticity). Generally, these criteria are connected with typical physical and chemical properties: Reprint requests to Prof. Dr. F. Dietz. (i) geometrical structure,

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. F. Dietz et al. • Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems 1073

(ii) molecular spectra of a suitable (iso-7r-electronic) reference compound (NMR spectra, energy spectra), which is not cyclically delocalized”. (iii) magnetic properties, The aim of this paper is to characterize the first (iv) chemical reactivity. excited singlet state of azulene 1 and naphthalene 2 as aromatic or anti-aromatic using these criteria. Recently, Schleyer [11] has proposed the follow ing definition: “Compounds which exhibit signifi cantly exalted diamagnetic susceptibility are aro matic. Cyclic delocalization may also result in equalization of the bond length, abnormal chemical 1 2 shifts and magnetic anisotropies, as well as chem ical and physical properties which reflect energetic Results and Discussion stabilization”. The opposite features should be valid for anti-aromatic character. Geometry criterion. Azulene is a typical non- Besides these criteria other typical features can benzoide aromatic compound which can be thought be used to distinguish between aromatic and anti- to be built up from an aromatic cyclopentadienide aromatic character. In an earlier paper [10] we have anion and an aromatic cycloheptatrienyl cation. proposed criteria to characterize the anti-aromatic character of molecules: 1. Geometry criterion. While aromatic monocy cles {e.g. benzene, cyclopropenide anion, tropylium 1a 1b 1c 1d cation) show a maximum bond length equalization, the geometric configurations of anti-aromatic struc The molecular structure of azulene was discussed tures are characterized by an alternation of the bond controversially in the literature [13]. All the ob distances and anomalously long bond distancesR served properties of the molecule correspond to the (in typical casesR > 1.5 A). structurelc (Id) of C2V symmetry with equalized C-C bonds, and not to the valence-tautomeric struc 2. Energy criterion. Aromatic (4n +2) 7r-electron tures la, b of Cs symmetry with alternation of the systems are characterized by a high excitation en bond lengths. The structure lc (Id) was recently ergy and a red shift of the longest-wavelength ab confirmed by high-levelab initio calculations (with sorption if the conjugated 7r-electron system is exbasis sets up to TZP+f quality) [13]. The results tended. Contrary to this behaviour, the relatively(see Fig. 1) are in good agreement with the ex stable Jahn-Teller forms of anti-aromatic species perimentally determined (X-ray analysis assuming have small exitation energies (wide infra). If the C2v symmetry) structure [14]. The structure shows anti-aromatic 4n 7r-electron system is perturbed (exa maximum equalization of the C-C bond lengths tended), the HOMO-LUMO gap is increased and a typical for molecules with aromatic character. blue shift of the longest-wavelength absorption re sults. Upon excitation of azulene to the first excited singlet state it can be expected that the structure 3. Charge distribution criterion. For more com is distorted. One criterion of the anti-aromaticity plex compounds with an odd-membered antiaro-of the 4/7 (7r-electron containing) annulenes is the matic structural element {e.g. a cyclopentadienyl unusually strong alternation of the bond lengths [10] cation fragment), the sum of the7r-net charges (Q) as a result of a second order Jahn-Teller effect [16], of the atoms of the anti-aromatic monocyclic fragFrom the energy spectra of the aromatic molecules ment can be compared with Q of the unsubstituted {e.g. naphthalene or azulene) it follows that for these anti-aromatic monocycle. molecules a Jahn-Teller effect can also be expected An additional criterion is the validity of Breslow'sin their first excited singlet state. The energy of the stability criterion [12]. Breslow [12] has defined electron-vibronic interaction of a molecule in an an anti-aromatic compound as “a cyclic conjugatedelectronic statep (second order perturbation theory) system ... if its 7r-electron energy is higher than that is given by the expression [17] 1074 F. Pietz et al. • Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems

du 'P: du SQ. a £ '2, = E ( 1) En - Er annulene. s, Energy criterion. The experimental excitation en ergies [21] So—>Si and Si —>S2 of azulene are 1.79 Fig. 1. Bond lengths (in A) of azulene in the singlet groundeV and 1.31 eV, respectively. In this case the aro state: abinitio SCF optimized values[13] (outside the matic character of the ground state is indicated rings), and experimental data [14] (inside the rings), andby a relatively high So—>Si energy difference in in the first excited singlet state:ab initio-CASSCF (3- 21G basis set) optimized[15] (outside the rings), and comparison with the lower Si —>S2 energy differ “experimentally” determined[15] (see text, inside the ence which could be a feature of the anti-aromatic rings). character of S i. F. Pietz et al. ■ Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems 1075

Fig. 4. Experimental Sq->Si and Si-»S2 excitation energies of (a) azulene [21], 2-phenylazulene [ 22] and 1,2-benzazulene [22], and (b) naphthalene [22], phenan- threne [23] and 2-phenyl-3-methyl- a naphthalene [23].

At extension of the conjugated 7r-electron sys within the five- Q3Ö5) and seven-membered (J2Q1) tem, a red shift of the experimental So—>Si ex rings in So and Si of azulene. While the five- citation energies can be seen (Fig. 4a) for azu- membered ring is partially negatively charged and lene and 2-phenylazulene [22] and azulene and 1,2- the seven-membered ring is partially positively benzazulene [22], respectively. For the correspond charged in the ground state, in the first excited state ing Si —^S 2 excitation energies a blue shift is ob the partial charges of the rings are exchanged. This served at extension of the conjugated 7r-system. can also be expressed by the magnitude and direc Conclusions as to the anti-aromatic character tion of the calculated dipole moment in ground and of the excited state of naphthalene can be drawn first excited singlet states (Fig. 2): in the ground state from the Si—>S2 excitation energy in relation to the dipole moment is directed from the seven- to the the Sq—>S i energy difference. The large experimen five-membered ring (experimental value /.t = 1.7 D tal value AE(S0—>Sj) = 3.97 eV [22] and the very [24]), upon excitation the direction is reversed. The small AE(Si—>S2) = 0.32 eV [22] are an indication dipole moment of the first excited singlet state Si of that the aromatic molecule in the ground state has azulene was experimentally estimated to be ß(Si) = a more or less anti-aromatic character in the first 0.42 D [25, 26]. excited singlet state. As in the case of azulene, the longest-wavelength transition So—>Si shows a red shift if the 7r-system Acknowledgements is extended (Fig. 4b), and a blue shift for the S i —>S 2 transition. The authors thank the Deutsche Forschungsgemein Charge distribution criterion. An additional indi schaft and the Fonds der Chemischen Industrie for finan cation for this is the distribution of the 7r-net charges cial support. 1076 F. Dietz et al. • Structure and Energy Spectra of Molecules Containing Anti-Aromatic Ring Systems

[1] (a) G. M. Badgor, Aromatic Character and Aro- [14] J. M. Robertson, H. M. M. Shearer, G. A. Sim, G. maticity; Cambridge University Press, London G. Watson, Acta Crystallogr. 15, 1 (1962). (1969); [15] F. Negri, M. Z. Zgierski, J. Chem. Phys. 99, 4318 b) J. P. Garrat, Aromaticity, Wiley, New York (1993). (1986); [16] E. R. Davidson, W. T. Borden, J. Chem. Phys.67, c) V. I. Minkin, M. N. Glukhovtsev, B. Y. Simkm, 2191 (1977). Aromaticity and Anti-Aromaticity, Wiley, New [17] A. D. Liehr, Can. J. Phys. 35,1123 (1957); 36,1588 York (1994). (1958);C. A. Coulson, A. Golebiewsky, Mol. Phys. [2] E. J. Padma Malar, F. Neumann, K. Jug, J. Mol. 5, 71 (1962). Struct. (Theochem) 336, 81 (1995). [18] M. J. Bearpark, F. Bernardi, S. Clifford, M. Olivucci, [3] M. G. Evans, Trans. Faraday Soc. 35, 824 (1939). M.A. Robb, B. R. Smith, T. Vreven, J. Am. Chem. [4] M. J. S. Dewar, Molecular Orbital Theory of Or Soc. 118, 169(1996). ganic Chemistry, McGraw-Hill, New York (1969). [19] C. P. Brock, J. D. Dunitz, Acta Crystallogr. B38, [5] R. C. Dougherty, J. Am. Chem. Soc. 93, 7187 2218 (1982). (1971). [20] J. Wolf, G. Hohlneicher, Poster at the 29th Sym [6] M. J. S. Dewar, Angew. Chem. 83, 859 (1971). posium of Theoretical Chemistry, Oberwiesen- [7] F. Bemardi, P. Celani, M. Olivucci, M. A. Robb, thal/Erzgebirge (1993). G. J. Suzzi-Valli, J. Am, Chem. Soc. 117, 10531 [21] D. E. Mann, J. R. Platt, H. B. Klevens, J. Chem. (1995). Phys. 17,481 (1949). [8] N. Tyutyulkov, F. Fratev, M. Ivanova, Theoret. [22] H. B. Klevens, J. Chem. Phys. 18, 1063 (1950). Chim. Acta 20, 385(1971). [23] L. Lang (ed.), Absorption Spectra in the UV and [9] C. Gellini, L. Angeloni, P. R. Salvi, G. Marconi, J. Visible Region, Vol. V, p. 201, Akademiai Kiado, Phys. Chem. 99, 85 (1995). Budapest (1965). [10] F. Dietz, N. Tyutyulkov, M. Rabinovitz, J. Chem. [24] G. W. Wheland, D. E. Mann, J. Chem. Phys. 17,264 Soc. Perkin 2 1993, 157. (1949). [11] P. von R. Schleyer, Pure Appl. Chem. 68, 209 [25] R. M. Hochstrasser, L. J. Noe, J. Chem. Phys. 50, (1996). 1684(1969). [12] R. Breslow, Chem. News 43,90 (1965); Acc. Chem. [26] J. W. Barker, L. J. Noe, A. P. Marchetti, J. Chem. Res. 6, 393(1973). Phys. 59, 1304(1973). [13] S. Grimme, Chem. Phys. Lett. 201, 67 (1993).