Special Issue: Review Commentary Received: 24 August 2009, Revised: 2 October 2009, Accepted: 13 October 2009, Published online in Wiley InterScience: 3 February 2010 (www.interscience.wiley.com) DOI 10.1002/poc.1644 Forever young: polycyclic aromatic hydrocarbons as model cases for structural and optical studies R. Riegera and K. Mu¨ llena* Polycyclic aromatic hydrocarbons (PAHs) are popular research subjects due to their high stability, their rigid planar structure, and their characteristic optical spectra. The recent discovery of graphene, which can be regarded as giant PAH, has further stimulated the interest in this area. For this reason, the relationship between the geometric and electronic structure and the optical spectra of PAHs are reviewed, pointing out the versatile properties of this class of molecules. Extremely stable fully-benzenoid PAHs with high optical gaps are encountered on the one side and the very reactive acenes with low optical gaps on the other side. A huge range of molecular sizes is covered from the simplest case benzene with its six carbon atoms up to disks containing as much as 96 carbon atoms. Furthermore, the impact of non-planarity is discussed as model cases for the highly important fullerenes and carbon nanotubes. The detailed analysis of the electronic structure of PAHs is very important with regard to their application as fluorescent dyes or organic semiconductors. The presented research results shall encourage developments of new PAH structures to exploit novel materials properties. Copyright ß 2010 John Wiley & Sons, Ltd. Keywords: aromaticity; dyes; photophysics; polycyclic aromatic hydrocarbons; UV/vis INTRODUCTION dramatically different optical and chemical properties are observed. As an example, triphenylene is extremely stable against The recent discovery of graphene – a single layer out of graphite – oxidation and possesses a relatively high optical gap. The isomer has raised enormous interest in the scientific community due to tetracene, in contrast, is easily oxidized and shows absorption at the unusual structure and the physical properties of graphene much longer wavelengths. such as ballistic charge transport or the quantum hall effect.[1–3] This review aims to shed light on the relationship between the Polycyclic aromatic hydrocarbons (PAHs) are well-defined cutouts geometric and electronic structure and the optical spectra of of graphene; the larger representatives are therefore sometimes PAHs. For this reason, the absorption and emission spectra of called nanographenes.[4–6] They can be found naturally in oil, different classes of PAHs shall be discussed to point out similari- coal, and tar, or are produced in combustion processes.[7] Due to ties among and differences between these classes. The synthesis the mutagenic and carcinogenic properties of some PAHs they of the presented molecules is not covered as some excellent are of major concern as pollutants.[8] reviews about this topic have been published.[9,11,13,22–24] For material sciences, however, PAHs are a great benefit. When chemically substituted with aliphatic chains, discotic liquid crystals are formed.[9–11] Their high stability and outstanding NOMENCLATURE structural order in the bulk phase make them very promising In order to discuss the optical properties of PAHs, a common materials for various applications.[12,13] Surface scientists, further- identification system has to be established. As for most other more, profit from the stiff and flat shape which facilitates the chemical substances, several systems exist. In this text, the IUPAC study of molecule–substrate interactions.[14–16] nomenclature is followed as long as possible. According to this The optical absorption and emission behavior is of particular nomenclature, the name of a PAH is derived from a set of PAHs interestforresearchersworkingwithPAHs. Thehighlycharacteristic which possess trivial names.[25] This set contains molecules of absorption spectra can be seen as their fingerprints, making it easy special importance like pyrene, triphenylene, perylene, coronene, to unambiguously identify a compound. This enabled scientists to and a few more. Other PAHs are derived from these by detect PAHs even in the interstellar medium.[17,18] A lot about annellation of further benzene rings. Figure 1 gives an example. aromaticity can be learned when dealing with PAHs.[19] The Perylene is the basis molecule to which three benzene rings are correlation of their geometric structure, i.e. how the rings are attached to obtain tribenzoperylene. If confusion among isomers annellated, to the energy levels of the electrons reveals details about the stabilization of p-bonds through aromaticity. Theoretical models profit from comparison to experimentally measured optical * Correspondence to: K. Mu¨llen, Max Planck Institute for Polymer Research spectra; therefore, the synthesis of the hitherto unknown PAHsis an Mainz, Ackermannweg 10, 55128 Mainz, Germany. urgent challenge for chemists.[20,21] E-mail: [email protected] At first glance, one may suggest that PAHs constitute a uniform a R. Rieger, K. Mu¨llen 2 classofvery similarmolecules,all builtupofsolelysp carbonsanda Max Planck Institute for Polymer Research Mainz, Ackermannweg, Mainz, few hydrogens. However, depending on the size and geometry, Germany 315 J. Phys. Org. Chem. 2010, 23 315–325 Copyright ß 2010 John Wiley & Sons, Ltd. R. RIEGER AND K. MU¨ LLEN a b r c o p q d n e m h g f l i k j tribenzo[b,n,pqr]perylene perylene Figure 1. Example for an IUPAC nomenclature is to be avoided, the sides to which the benzo groups are gaps and higher reactivity. Larger acenes with exclusively zigzag attached are indicated in square brackets. For the lettering of the periphery cannot be handled in air without oxidation to the side, the PAH is drawn such that the maximum number of rings in quinones. Three types of edges are distinguished: the bay, the cove, a row are oriented horizontally. If more than one orientation is and the fjord region (Fig. 2). A bay region is part of the arm-chair possible, that one is chosen for which the maximum number of periphery. Cove and fjord regions are structural features inducing rings are located in the top right quadrant. The sides are lettered non-planarity of the PAH as the attached hydrogen bonds sterically alphabetically clockwise starting with the leftmost side in the interfere. Helicenes are the extreme example for fjord regions in upper-right quadrant. In the example of Fig. 1, the complete which the interference is so strong that at room temperature stable name of the depicted molecule is thus tribenzo[b,n,pqr]perylene. enantiomers are formed.[32] For more complex structures, units other than benzene can be E. Clar has developed an easy system to estimate the stability of a attached to a basis PAH, such as naphthalene or phenanthrene PAH which is known as Clar’s sextet rule.[33] When drawing the with the prefixes naphtho- and phenanthro-, respectively. structure of a PAH, the p-electrons can be grouped into sextets If the compounds become very big, the names are often difficult within a ring. Sometimes, people draw a circle in the ring to indicate tomanage.Intheliterature,alternativenamesareoccasionallyused this electron sextet as in Fig. 3. In tetracene, only one sextet can be which are derived from the shape of the PAH. A name like triangle assigned to one of the rings, the remaining 12 p-electrons remain (e.g. for 5 or 11 in Fig. 6) alludes to the D3h symmetry of certain ungrouped. According to Clar’s sextet rule, the electron sextets representatives. ‘Supernaphthalene’ or ‘superphenalene’ (12 and possess particularly strong aromatic stabilization; those bonds not 14 in Fig. 6) have also been used to describe a shape which included in a sextet, in contrast, are less stabilized and are more resembles naphthalene or phenalene, but is much bigger in size. susceptible to chemical reactions. In tetracene’s isomer tripheny- lene, all 18 electrons can be grouped into sextets and assigned to one ring each. As a consequence, very high aromatic stabilization is GEOMETRICAL AND ELECTRONIC gained. In fact, triphenylene is very stable even under drastic STRUCTURE OF PAHs conditions. PAHs for which all p-electrons can be grouped into sextets are sometimes called Clar PAHs. To establish a relationship between the geometric and electronic This simple rule is amazingly effective for qualitative esti- structure and the optical properties of PAHs, we first need to look mations of the stability of a PAH. Sophisticated quantum into how PAHs are built up and make clear certain structural mechanical calculations have been performed to explain Clar’s features. Then we can divide them into classes of similar sextet rule in detail and enable quantitative predictions.[21,34–37] properties and discuss the optical properties shared among For the every-day use, however, the simple rule remains an representatives of these classes. important tool for qualitative predictions. PAHs are built up by six-membered rings of sp2-hybridized carbon atoms. Two neighboring rings share two carbon atoms such OPTICAL ABSORPTION SPECTRA that a fully planar and conjugated system is formed. Benzene is regarded as the smallest PAH, naphthalene the next bigger one The absorption spectra of PAHs are quite different from those of composed of two rings. The more rings are annellated, the more most other substances. They are highly resolved, revealing a lot of possible isomers exist. These can be distinguished by their periphery which roughly correlates to the resonance stabilization energy.[26–31] The highest stabilization is gained with an arm-chair periphery (Fig. 2). A zigzag periphery, in contrast, leads to much reduced resonance stabilization and consequently to lower band cove region arm-chair bay region fjord region zig-zag Figure 3. Clar’s sextet rule applied to tetracene (top) and triphenylene (bottom). The rings in the formulae on the right side indicate the sextets, Figure 2. The periphery of PAHs the electrons not in a sextet remain as double bond 316 www.interscience.wiley.com/journal/poc Copyright ß 2010 John Wiley & Sons, Ltd.