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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 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 , 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 . Extremely stable fully-benzenoid PAHs with high optical gaps are encountered on the one side and the very reactive with low optical gaps on the other side. A huge range of molecular sizes is covered from the simplest case 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 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: ; dyes; photophysics; polycyclic aromatic hydrocarbons; UV/vis

INTRODUCTION dramatically different optical and chemical properties are observed. As an example, 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 , 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 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 –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 , triphenylene, , , 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 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. 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 or 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 , 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 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 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. J. Phys. Org. Chem. 2010, 23 315–325 PAHs FOR STRUCTURAL AND OPTICAL STUDIES details about the electronic structure of the molecules.[38] Clar identified three main types of transitions with different intensities and designated them the a, b, and para-bands, which originate from empirically determined correlations between the so-called ‘ortho’ and ‘para’ reactivity and the absorption spectra.[33] Corresponding nomenclatures derived from group theory have been developed.[39] The electronic situation for benzene, as the formal monomer for the class of PAHs, is well understood. The HOMO and the LUMO form both a pair of degenerate molecular orbitals (Fig. 4a). The four possible transitions from the p2/3 into p*4/5 lead, as can 1 be derived theoretically, to transitions from the A1g ground state 1 1 1 into the excited singlet states B2u, B1u, and the degenerate E1u. Figure 5. Absorption and fluorescence spectra of pyrene, measured at The electron correlation gives rise to three electronic transitions 105 M in , the fluorescence is excited at 320 nm, its intensity (a, b, and p) with different energies. is normalized to the absorption maximum In the corresponding UV–vis spectrum of benzene, the structure rich a- and the p-bands correspond to symmetry- forbidden transitions. The p-bands ‘borrow’ intensity from the Figure 5 shows the spectrum of pyrene, demonstrating the neighboring b-bands. Substituents on the benzene ring lower the typical features of optical spectra of PAHs. The high degree of fine symmetry and therefore these two transitions gain intensity. structure both in the absorption and the emission spectra can be In the spectra of extended, condensed PAHs, one notices seen impressively. The b-bands are observed at shortest interesting similarities. In contrast to benzene, the HOMOs pn1 wavelengths, the p-bands at about 335 nm and the low intensity * * and pn and the LUMOs p nþ1 and p nþ2 are not necessarily a-bands at 370 nm. Due to the expressed intensity differences, it degenerate anymore. There are theoretically four transitions is generally sensible to plot the optical spectra of PAHs on a between these orbitals possible (Fig. 4b). logarithmic scale as in Fig. 5. Configuration interactions cause a splitting of degenerate In the following sections, we want to discuss examples of PAHs to configuration states. The energy levels are now found above and highlight important aspects of their optical absorption and below the original value for S1 (compare right side of Fig. 4). As a emission behavior. To better point out differences, classes of PAHs consequence, optical transitions are observed at wavelengths are formed with similar properties but different sizes. We start from longer than expected for the S0 ! S1 transition. The shape and the most stable, least reactive ones and go all the way through the topology of the PAH determines the position of the a-bands different classes with decreasing stability and increasing reactivity. at longest wavelengths with respect to the p-transitions due to different extents of configuration interactions. Annelation of benzene units shifts the band to longer wavelengths. In all cases, FULLY BENZENOID PAH the transition from the HOMO into the LUMO corresponds to the p-bands independent of which band is at the longest According to Clar’s sextet rule, particularly high stabilization wavelength.[40,41] energy and consequently low reactivity is observed if the p- electrons can all be grouped into sextets. One can imagine these PAHs as a set of benzene rings connected by six single bonds to (a) neighboring rings or hydrogen atoms. They are thus called fully E benzenoid. Benzene itself is the smallest representative of this

* group, triphenylene the next bigger one. The high stabilization π 6 1 E1u energy gives rise to a high optical gap. Figure 6 gives an overview π* π* 1 4 5 B1u of those PAHs which have been characterized by means of 1B 2u absorption and emission spectroscopy. The preparation of the π π 2 3 α p β larger ones, i.e. those bigger than 9, represents synthetic 1A breakthroughs and was only possible by a careful design of the π 1g 1 precursors and an optimization of the dehydrogenation reac- tions. The synthetic details, however, shall not be discussed in this review. Going to even larger disks is an important goal of current (b) research in order to synthetically make graphene of defined size E π π)2 π* and periphery to gain a better understanding of this new S4( n-1 n n+2 π* material. n+2 π2 π)π* S3( n-1 n n+2 Among the bigger fully benzenoid PAHs, isomers exist with the π* π π)2 π* n+1 S2( n-1 n n+1 } same number of electron sextets. The optical gap, however, is π2 π)π* almost unaffected by the different shape of these isomers. π S1( n-1 n n+1 n Figure 7 depicts the size dependence of the optical gap, derived from the p-bands, in relation to the number of electron sextets. A π αβ’p β n-1 π2 π)2 S0( n-1 n decrease from 3.9 eV of triphenylene to a level at about 2.2 eV for the very large disks is found. It can be clearly seen that the two Figure 4. Energy levels, configurations, and electronic transitions con- isomers 10 and 11 exhibit almost the identical value despite their sidering configuration interactions in (a) benzene and (b) PAHs different symmetry. 317

J. Phys. Org. Chem. 2010, 23 315–325 Copyright ß 2010 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/poc R. RIEGER AND K. MU¨ LLEN

1 2 3 4 5 6

9 11 7 8 10

12 13 14

Figure 6. All-benzoid PAHs which have been synthesized

Triphenylene (1) is the most intensively studied fully benzenoid (from D3h for triphenylene to D2h for dibenzopyrene). Also p- and PAH. Its good synthetic accessibility and use as a discotic liquid b-bands are at longer wavelength (329 and 289 nm) than for crystal have made it very important for material science.[42,43] Its triphenylene. is observed at a wavelength of a-bands at 320 nm are very weak due to the high symmetry and 383 nm, maximum phosphorescence at 491 nm.[40,52] thus low transition probabilities. Its strongest absorption peak is Two isomers of fully benzenoid PAHs containing five elec- found at 260 nm with an extinction coefficient of 17 000 m2/mol, tron sextets exist: tetrabenzoanthracene (3) and tribenzo- which is a relatively high value for PAHs, meaning that high values perylene (4).[53,54] Tetrabenzoanthracene shows its a-bands at for the Frank–Condon factors are present in triphenylene.[44,45] shorter wavelengths than tribenzoperylene (383 nm for 3 and The absorption does not reach the visible region, so the 401 nm for 4). The position of the p-bands differs even more, i.e. compound is colorless in all solvents. 336 nm for 3 against 377 for 4. The higher symmetry of Triphenylene shows essentially two fluorescence bands at 355 tetrabenzoanthracene (3)(D2h) over tribenzoperylene (4)(C2v) and 370 nm.[46] Its fluorescence quantum yield is very low (6.6%) leads to a stronger stabilization and therefore to a higher optical due to efficient intersystem crossing.[47] Phophorescence is gap. The b-bands, however, are almost identical for the two detected at 460 nm with a lifetime of 16 s at 77 K.[48,49] isomers (295 nm for 3 and 300 nm for 4). Dibenzopyrene (2) is the next bigger fully benzenoid PAH. Its a- Six electron sextets give rise to four fully benzenoid PAHs (5–8). bands are found at 376 nm, so 56 nm red-shifted with regard to All of them have been synthesized and characterized with optical triphenylene.[50,51] It is much more intense in comparison to absorption spectroscopy.[53–57] The poor solubility due to the big triphenylene which is a consequence of the lowered symmetry size makes it very difficult to record weak absorption bands of these molecules. This is why only the most intense p-bands can be compared in this series. The longest wavelength is found for tribenzoanthanthrene (7, 404 nm), the shortest for tetrabenzo- (8, 344 nm). The triangular tribenzocoronene (5) exhibits the highest symmetry, but its p-bands are found at 363 nm, so at a longer wavelength than tetrabenzopentacene (8). The absorption bands reach the visible region. That is why the compounds beginning from this size are light yellow. The bigger PAHs are so sparingly soluble that they have to be synthesized with solubilizing alkyl chains, which generally have a negligible influence on the electronic structure. There is a huge number of isomers for the bigger PAHs, so typically only those of higher symmetry have been synthesized. The most intensively studied large PAH is hexabenzocoronene (9, HBC), due to its easy accessibility by cyclodehydrogenating hexaphenylbenzene which allows the preparation of various [58–61] Figure 7. Optical band gap derived from the para band in the absorp- derivatives. The a-bands of HBC at 450 nm are found to be tion spectrum versus the size of the PAH very weak because they are symmetry forbidden. The more 318

www.interscience.wiley.com/journal/poc Copyright ß 2010 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2010, 23 315–325 PAHs FOR STRUCTURAL AND OPTICAL STUDIES intense p-bands appear at 390 nm which is an even shorter K-REGION PAHs wavelength than for the smaller tribenzoanthanthrene (7). The highest intensity is measured for the b-bands at 360 nm with a If fully benzenoid PAHs are annellated with another ring in a bay very high molar extinction coefficient of 18 000 m2/mol.[62] The region, the added two p-electrons cannot be included in a sextet. fluorescence spectrum shows several peaks, the band of shortest In fact, this outer bond is less stabilized and possesses partial wavelength is measured at 464 nm, the highest intensity is found olefinic character albeit still being aromatic. These outer p-bonds at 484 nm.[63] HBC phosphoresces most intensively at 565 and are referred to as K-region. Increased reactivity is observed which 575 nm at a temperature of 77 K.[62] is particularly important with regard to their toxicity. The K-region The linearly extended homolog of HBC containing ten electron of those PAHs which are small enough to possess residual sextets (10) has been prepared and characterized.[64] Solubilizing solubility in water is metabolically oxidized to the epoxide which alkyl chains are necessary to dissolve the molecule to a sufficient causes cancer by reacting with DNA.[70–72] Chemically, this K- extent. The strong intermolecular interactions, however, lead to a region can, for example, be used for selective hydrogenations or line broadening of the absorption spectra. Weak bands around conversion to a-diketones.[73–75] 480 nm are observed, the strongest absorption is found at A very important PAH of this class is pyrene (15). It can be 410 nm. One fluorescence band is observed at a wavelength of isolated from and is thus easily accessible. The effect of 530 nm.[65] the K-regions on the UV absorption behavior can be well The huge triangular PAH 11 represents the isomer of 10, both observed in this molecule.[76] The a-bands of pyrene are found at hosting ten electron sextets. It has been investigated as its 372 nm, almost as long as for the much bigger tribenzocoronene trialkylphenyl derivative.[66] The spectrum is very similar to that of (5) (pyrene consists of 16 carbon atoms, tribenzocoronene of 36). triphenylene and the smaller triangle 5. Weak absorption is Another unique feature is the large separation of the p-bands recorded around 500 nm, the most intensive band is found at (334 nm) and the b-bands (272 nm), which stands in contrast to 405 nm. the spectra of fully benzenoid PAH. Molecule 12 resembles the shape of naphthalene and is thus The long lifetime (68 ns) of the excited state in pyrene called supernaphthalene in the literature.[67] By the aid of an enhances excimer formation which can easily be detected by a aggregation suppressing tert-butyl group, a well resolved UV–vis characteristic fluorescence band at 480 nm.[77] Monomeric spectrum has been recorded. The a-bands are found at 602 nm, pyrene emits between 370 and 400 nm with a fluorescence the p-bands at 488 nm, and the b-bands at 445 nm. Most intense quantum yield of 72%.[78,79] The kinetics of the excimer formation fluorescence is observed at 611 nm. are intensively studied and well understood.[47] Many researchers The spectra of the two molecules 13 and 14 (superphenalene) make use of this excimer signal to detect spatial proximity of two are substantially broadened due to aggregation.[68,69] Their pyrene molecules when attached to a macromolecule. If two absorption edge is at 600 nm, the plateau value for giant fully can come in close contact, excimer fluorescence is benzenoid PAHs (Fig. 7). Even bigger PAHs are expected to not detected. In this way, polymer chain dynamics and protein significantly exceed this value. structures have been analyzed.[80–83] The optical properties of fully benzeneoid PAHs as they are Coronene (16) is a PAH with three K-region double bonds reported in the literature are summarized in Table 1. It can be (Fig. 8). Its a-bands at 375 nm are very weak due to the high [84] clearly seen that the optical transitions in this class of PAHs are symmetry (D6h). This position is almost identical to the a- dominated by the number of p-electrons rather than the shape of bands of pyrene, so the optical gap with increasing size does not the disk. As we will observe later, this does not apply for PAHs of follow a comparable trend as for the fully benzenoid PAHs. other classes. Differences of the stabilization energies play a much more

Table 1. Summary of photophysical properties of fully benzenooid PAHs

Absorption Fluorescence Phosphorescence 2 Compound a-bands (nm) p-bands (nm) b-bands (nm) emax (m /mol) (nm) (nm)

1 320 285 260 17 000 355; 370 460 2 376 329 289 5000 383 491 3 383 336 295 10 000 — 530 4 401 377 300 13 800 — 535 5 — 363 323 — — — 6 430 358 329 7800 — 544 7 — 404 324 7700 — — 8 — 344 — 6500 — 555 9 450 390 360 18 000 464; 484 565; 575 10 480 410 — — 530 — 11 500 405 — — — — 12 602 488 445 6500 611 — 13 — 600 — — — — 14 — 600 — — — — 319

J. Phys. Org. Chem. 2010, 23 315–325 Copyright ß 2010 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/poc R. RIEGER AND K. MU¨ LLEN

annellation of oligo-paraphenylenes. Every second ring is benzene-like due to an electron sextet, the rest forms K-regions. This is the reason for the high carcinogenity of the molecules in this group.[90] Phenanthrene (21) as the smallest representative demon- 15 16 strates the low aromatic stabilization of the central ring. Upon 17 reaction with bromine, it loses aromaticity at the 9,10 position.[91] In its absorption spectrum, the a-bands are located between 310 and 345 nm, the p-bands between 290 and 330 nm, and the b- bands between 273 and 293 nm.[92] These values are very similar to pyrene which has one ring more. The fluorescence is observed at 350 nm, about 20 nm shorter than pyrene. (22) takes its name from its golden-yellow 18 19 20 appearance (chrysos (greek) ¼ gold). Its a-bands are detected Figure 8. Examples for PAHs with K-region which is highlighted in the between 345 and 360 nm, the p-bands between 285 and 320 nm, structures and the b-bands at 259 and 267 nm, the latter with the highest extinction coefficient (16 000 m2/mol).[93] The molecule emits in the blue region with two characteristic peaks at 374 and 388 nm important role than the number of p-electrons. Also, the p-bands with a fluorescence quantum yield of 23%.[94,95] Chrysene of coronene at 340 nm are about the same as for pyrene, only the phosphoresces with a quantum yield of 5% at 77 K, showing three b-bands are observed at a longer wavelength (305 nm). Coronene characteristic peak groups, around 500, 540, and 590 nm. fluoresces between 420 and 500 nm, significantly longer The bands in the absorption spectrum of (23), the next wavelength than pyrene.[85] The fluorescence quantum yield is bigger homolog in the series, are red shifted by about 15 nm in very low (2.8%) due to a high intersystem crossing rate.[86,87] comparison to chrysene, otherwise the spectra are very similar. Phosphorescence is observed most intensively at a wavelength of The a-bands are found between 358 and 386 nm, the p-bands 578 nm.[88] between 303 and 330 nm, and the b-bands between 257 and A series of K-region functionalized HBC derivatives (17–20) 287 nm. The extinction coefficients are very similar to chrysene. have been synthesized with solubilizing alkyl chains and Fluorescence is observed at 380 nm, and phosphorescence at investigated in terms of electronic spectra.[89] The subtle 500 nm.[40,52] structural differences between these molecules induce a strong Another shift of 15 nm in the absorption spectrum is observed effect on the absorption spectra. The a-bands and the b-bands for fulminene (24).[96] The relative intensities are very similar to follow a linear trend with the number of carbon atoms. The a- chrysene and picene. Two fluorescence peaks at 395 and 409 nm bands of 17 are determined to be 484 nm and that of 20 to be are recorded.[97] 530 nm. The b-bands go up from 380 nm for 17 to 400 nm for 20. While the up to fulminene can be isolated from The p-bands, however, show a different trend. 18 (473 nm) and coal tar, the higher homolog [7] (25) has to be 19 (448 nm) show p-bands at longer wavelengths than 20 synthesized. This has been achieved by photocyclodehydrogena- (425 nm). The symmetry gives rise to this trend as revealed by tion of an -bridged diphenanthrene.[98] Its poor solubility quantum mechanical calculations. Molecule 20 shows D3h makes the analysis very difficult, only the p-bands have therefore symmetry, as a consequence the HOMO/HOMO1 and the been reported. They are measured at 340 nm, again red-shifted in LUMO/LUMO1 are degenerate leading to a transition at shorter comparison to the smaller molecules. wavelengths. The symmetry also has an influence on the fine Table 2 summarizes the photophysical properties of K-region structure of the spectrum. The higher the symmetry, the more PAHs including the phenacene series. The strict size dependence symmetry-forbidden transitions exist, leading to a high degree of as found for the fully benzenoid PAHs does not apply for these fine structure. Lower symmetry as for 17–19 (D2h) broadens the molecules. Differences in aromatic stabilization become more peaks in the spectrum. The intensity of the a-bands in this series important than the number of p-electrons. also depends on the symmetry: the higher the symmetry the lower the intensity. RYLENES

PHENACENES The group of PAHs which are built up by peri-annellation of naphthylenes is called the rylene series (Fig. 10). In naphthalene, A series of zigzag fused PAHs as depicted in Fig. 9 is called the one ring with an electron sextet can be formed, the other ring phenacene series. The molecules can be built up by benzo remains with four p-electrons; an intermediate stabilization energy is the result. The same applies for the higher homologs in the rylene series. The partial olefinic character of the p-electrons leads to an increased reactivity, especially for Diels–Alder reactions in the bay.[99] Substituted with dicarboxylic imides in the two peri-positions, the rylenes form dyes of outstanding [100–102] 21 22 23 24 25 stability. The lower optical gap in comparison to the fully benzenoid Figure 9. The phenacene series from phenanthrene (21) to [7]phena- PAHs is obvious. Perylene (26) shows absorption bands up to cene (25) 440 nm,[103] although having only two p-electrons more than 320

www.interscience.wiley.com/journal/poc Copyright ß 2010 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2010, 23 315–325 PAHs FOR STRUCTURAL AND OPTICAL STUDIES

Table 2. Summary of photophysical properties of PAHs with K-region including the phenacene series

Absorption Fluorescence Phosphorescence 2 Compound a-bands (nm) p-bands (nm) b-bands (nm) emax (m /mol) (nm) (nm)

15 372 334 272 8800 370; 400 593 16 375 340 305 12 500 420; 500 578 17 484 473 380 22 000 490 — 18 510 473 397 21 000 520 — 19 490 448 387 17 000 495 — 20 532 425 399 25 000 530 — 21 345 330 293 6500 350 462 22 360 320 267 16 000 374; 388 500; 540; 590 23 386 330 287 16 000 380 500 24 400 345 295 15 000 395; 409 — 25 — 340 — — — —

triphenylene (1) which does not absorb at wavelengths longer the p-band goes up with the size of the rylene; in quarterrylene it than 300 nm. The molar extinction coefficients of perylene are reaches 13 800 cm2/Vs. The most intense fluorescence band is low; the most intense peak at 255 nm exhibits a value of not more detected at 680 nm.[111] In contrast to perylene and terrylene, than 4250 m2/mol.[104] This indicates that the Frank–Condon quarterrylene shows a low fluorescence quantum yield of factors are low for the transition from the electronic ground state only 5%. to the excited states. The molecule fluoresces at 435 and 465 nm, The higher rylenes cannot be handled any more due to their and is thus a blue emitter.[52] The fluorescence quantum yield of insolubility. Substituted with tert-butyl groups, however, a soluble 94% is one of the highest among the PAHs.[105] When substituted pentarylene (29) derivative has been obtained.[112] Alkyl groups with tert-butyl groups to reduce self-quenching of aggregated typically have a negligible effect on the electronic structure, so molecules, perylene serves as a blue-emitting dopant material in these results can be well compared to the parent compounds. organic light-emitting diodes.[106] The p-bands are shifted to 750 nm, making pentarylene a blue Terrylene (27), the next bigger representative of the rylene compound. series, is a deep red compound.[107] Its p-bands are found at Table 3 summarized the optical transitions for the rylene series. 560 nm, 100 nm more bathochromic than perylene. This The strong bathochromic shift of the p-bands and the absorption closely matches the output of a rhodamine 6G fluorescence wavelength with the size can be appreciated. In dye laser, making terrylene a preferred subject for single addition, one can notice the smaller red-shift on the b-bands in molecule fluorescence spectroscopy.[108] Two fluorescence bands the series. are observed at 575 and 612 nm, respectively.[109] Its fluorescence quantum yield is close to unity with no notable transition to the triplet state.[110] An extremely high photostability renders ACENES terrylene an excellent fluorescence dye. Quaterrylene (28) is a dark-green substance of extremely low The series constitute a class of linearly fused PAHs as solubility. Only 1-methylnaphthalene dissolves quarterrylene depicted in Fig. 11. Only one electron sextet can be assigned to a sufficiently at 150 8C to measure an absorption spectrum.[107] The ring of the molecule, the lowest aromatic stabilization is the result p-bands are measured at 670 nm. The extinction coefficient of according to Clar’s sextet rule. In fact, acenes possess the lowest

Table 3. Summary of the photophysical properties of the rylene series

Absorption Fluorescence p-bands b-bands emax Compound (nm) (nm) (m2/mol) (nm)

26 440 255 4250 435; 465 27 560 270 6500 575; 612 26 27 28 29 28 670 300 13 800 680 29 750 380 — — Figure 10. The rylene series from perylene (26) to pentarylene (29) 321

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610 nm with a fluorescence quantum yield of 8%.[124] The internal conversion rate from the S1 to the T1 state is found to depend 30 31 32 strongly on the environment, spanning a range of three orders of magnitude.[125,126] This effect is explained by a small energetic difference of the singlet and the triplet state as well as differences of out-of-plane distortions of the guest molecule in different sites 33 34 of the host. Despite its instability in ambient conditions, pentacene is Figure 11. The acene series from to among the most successful materials with charge-carrier mobilities up to 2.2 cm2/Vs in a field-effect transistor.[127] Also, substituted are highly efficient optical gaps among their PAH isomers. Also, their chemical semiconductors.[128] reactivity is much higher than for other PAHs. Pentacene (32) can (33) can hardly be handled, it quickly oxidizes with hardly be handled in air, it quickly oxidizes to the quinone.[113,114] traces of oxygen.[129] Its p-bands are detected at 750 nm. The The acenes can react as in Diels–Alder reactions with molecule fluoresces at 760 nm with a fluorescence quantum yield reactive dienophiles, underlining the low aromatic stabilization of only 1%.[124] Inter system crossing to the triplet state is which gives the acenes an almost olefinic character.[115] Acenes extremely effective; an ISC efficiency of more than 94% has been and their derivatives are important materials for organic determined.[124] electronic devices such as field-effect transistors and light- Heptacene (34) has only been prepared in a PMMA matrix in emitting diodes.[116,117] which it can be detected by UV, but quickly decomposes.[130] For Figure 12 gives an overview of the optical gap in relation to the solution measurements, it is too reactive and too sparingly number of rings. A much stronger decrease with the size is soluble. However, substituted with trialkylsilylethynyl groups, it is observed in comparison to the fully benzenoid PAHs (Fig. 7). The much more stable and soluble, facilitating the photophysical small anthracene (30) with only three rings already shows an investigations.[131] Its absorption reaches the NIR region, the p- optical gap comparable to tribenzocoronene (5) with its ten rings, bands are located at 850 nm. The spectrum of the parent showing its p-bands at 380 nm.[118] Anthracene emits at 400– compound measured in the polymer matrix is consistent with 500 nm and is thus a blue emitter which finds applications in these values. The overview of the acene series is given in Table 4. organic light-emitting diodes.[119,120] Data for the alpha-band are not available as the p-band is Tetracene (31), the isomer of triphenylene (1), is an orange stronger shifted bathochromically, hiding the a-band. solid with its p-bands at 480 nm, so largely in the visible region.[121] In the acene series, the p-bands shift much more [33] bathochromically than the a- and b-bands. This is why the a- Table 4. Summary of the photophysical properties of the bands are hidden under the p-bands in tetracene and the higher acene series acenes. In contrast to most other PAHs, the p-bands are much weaker in the acene series than the b-bands. In the fluorescence Absorption spectra, three characteristic bands are observed at 475, 513, and 555 nm.[122] Fluorescence p-bands b-bands emax The p-bands in pentacene (32) are shifted to about 600 nm 2 giving rise to its purple color. This color can only be observed in Compound (nm) (nm) (m /mol) (nm) freshly prepared samples; upon exposure to air it becomes green [123] 30 380 255 16 000 400; 500 due to oxidation to the quinone. The much more intense b- 31 480 285 20 000 475; 513; 555 bands are found at 300 nm which underlines the above- 32 600 300 16 000 610 mentioned strongly different shifts of these two bands when 33 750 330 — 760 increasing the size of the acene. Fluorescence can be observed at 34 850 350 — —

NON-PLANAR PAHs

Two strategies are common to obtain non-planar PAHs: (1) introduction of five-membered rings which force the disk out of planarity to retain the typical C—C bond distances; (2) steric strain by atom crowding which forces the disk out of planarity. The imperfect overlap of adjacent p-orbitals reduces the stabilization in the molecule. In order to make non-planar PAH, an energy price has to be paid; therefore unusual synthetic techniques like flash-vacuum pyrolysis are required.[132,133] Non- planar PAHs show much reduced intermolecular stacking forces in comparison to their planar counterparts, rendering materials of much higher solubility. (35) is one of the most famous and most Figure 12. Optical gap in the acene series intensively studied non-planar PAHs. It consists of a five- 322

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cells.[144–146] It also serves as n-type semiconductor in field-effect transistors with high electron mobility.[147] The electronic structure of C60- has been intensively studied and is well understood.[148,149] The strongest absorption bands are measured at 257 and 329 nm which are very short wavelengths for a PAH of 60 carbon atoms. Weak and broad bands, however, can be detected between 450 and 600 nm at which symmetry- forbidden transitions take place.[150] Fluorescence is observed between 650 and 720 nm. Fluorescence quantum yields are very low (0.02%), indicating very rapid non-radiative depopulation of [151] the excited state. In organic solar cells, C60-fullerene hardly contributes to light harvesting due to the low extinction coefficients at wavelengths above 450 nm. The higher homolog C70-fullerene absorbs more strongly up to 600 nm, therefore collects more light and increases the power-conversion efficiency of the solar cell.[152] Hexabenzotriphenylene (38) can formally be regarded as a partially hydrogenated HBC (9) with three opened bonds. It is non-planar, consisting exclusively of six-membered rings. The non-planarity is caused by steric strain of the hydrogen atoms of Figure 13. Examples of non-planar PAHs with structural models the fjord regions. The molecule exists as two atropisomers: one of C2 symmetry (38a) and the other of D3 symmetry (38b). They do not interconvert at room temperature, but only at temperatures membered ring surrounded by five six-membered rings. The above 250 8C.[153] The compound is synthesized by catalytic circumference is too low to render a flat molecule; therefore the trimerization of the highly reactive 9,10-bisdehydrophenan- molecule forms a bowl shape (Fig. 13).[134,135] It is, however, threne.[154] UV irradiation shorter than 380 nm is absorbed, which surprisingly flexible with regard to inversion. By asymmetrical is about the same value as for the p-bands of HBC (9), having the substitution, an activation barrier for the inversion of about 45 kJ/ same number of p-electrons.[155] Also, the extinction coefficients mol has been determined by dynamic NMR measurements, are on the same order, i.e. 16 000 m2/mol. Differences between which is low enough to allow inversion at room temperature.[136] the atropisomers are not reported. The five-membered rings can accommodate negative charge Hexabenzo[a,d,g,j,m,p]coronene (39) is structurally related to which makes corannulene a reasonable electron acceptor.[137] HBC 9 in which the benzo rings are attached ortho to coronene Its UV–vis absorption spectrum shows broader peaks than instead of peri as for 9. The steric congestion of the hydrogen those of flat PAHs.[138] This is mainly a result of the reduced atoms in the cove regions gives rise to a non-planar disk. By symmetry lacking a mirror plane. A strong absorption band is attaching alkyl chains, a has been prepared which found at 300 nm, which is at shorter wavelength than the shows remarkable charge-carrier mobilities.[156] Its UV–vis structurally similar, but planar coronene (16). The peak, however, absorption spectrum is very similar to HBC 9, but the bands extends up to 350 nm. The a-, b-, and p-band assignment is not are red-shifted by 15 nm.[157] This is rather a consequence of less applicable for the non-planar PAHs due to the different symmetry. aromatic stabilization than of the non-planarity. In contrast to Fluorescence is observed at 420 nm, almost identical to coronene, HBC 9, 39 is not fully benzenoid, leading to a smaller optical gap. but also broadened.[139] Also, the fluorescence quantum yield is The extinction coefficients of both HBC molecules are almost comparably low (2.4%) due to efficient intersystem crossing; identical. Molecule 39 shows phosphorescence in the red (bands phosphorescence can be observed at 490 nm.[140] 616 nm and 627 nm) similar to pyrene but with a much longer (36) gives another example of a non-planar PAH phosphorescence lifetime. induced by five-membered rings. Sumanene is a triphenylene in An impressive example of PAH contortion has recently been which the bays are bridged by methylene groups. These do not reported: a teropyrene has been connected to a short alkyl chain contribute to the conjugation as the bridging carbon atoms are to form a cyclophane (40) such that the aromatic disk is bent sp3 hybridized. It has been synthesized in a metathesis reaction significantly.[158] The spectrum shows a surprisingly high degree which is unusual as no particularly high temperatures are of fine structure with the longest absorption wavelength at needed.[141] Its absorption spectrum resembles that of triphe- 500 nm. Fluorescence bands are measured at 520 and 540 nm. nylene with the strongest band at 278 nm.[142] As for corannu- lene, the peaks are broadened, much less fine structure can be observed. A shoulder at 300 nm can be seen which corresponds CONCLUSION to the weak a-bands in triphenylene, but is much more intense. The fluorescence band at 376 nm is almost identical to the planar In this review, it has been shown that PAHs are very diverse with triphenylene.[142] respect to their structural, chemical, and photophysical proper- C60-Fullerene (37) is the ultimate extension of corannulene, ties. Different groups of PAH with similar properties can be forming a sphere of solely carbon atoms, thus being a carbon distinguished. Extremely high stability and high optical gaps are allotrope. It is made in a plasma arc between two graphite found in fully benzenoid PAHs. The size dominates the optical electrodes which produces fullerenes of other sizes as well.[143] gap; isomers of different shapes show practically identical gaps. A Its 12 five-membered rings render it a strong electron K-region introduces a position of increased reactivity, giving rise acceptor which is successfully used in bulk-heterojunction solar to lower optical gaps. The rylene series contains molecules of 323

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intermediate stability, the optical transitions take place at much [18] P. Ehrenfreund, S. B. Charnley, Annu. Rev. Astron. Astrophys. 2000, 38, longer wavelength than for fully benzenoid PAHs of the same 427. number of p-electrons. A very high fluorescence quantum yield [19] M. Randic, Chem. Rev. 2003, 103, 3449. [20] M. Dierksen, S. Grimme, J. Chem. Phys. 2004, 120, 3544. due to low intersystem crossing rates is a common feature in the [21] Z. B. Maksic, D. Baric, T. Mu¨ller, J. Phys. Chem. A 2006, 110, 10135. rylene series. Extremely low optical gaps are encountered in the [22] M. Zander, Polycycl. Aromat. Compd. 1994, 5,1. acene series. The aromatic stabilization of the molecules in this [23] M. D. Watson, A. Fechtenko¨tter, K. Mu¨llen, Chem. Rev. 2001, 101, series is so low that the larger representatives can hardly be 1267. handled in air. Their absorption covers the whole visible [24] A. C. Grimsdale, K. Mu¨llen, Angew. Chem. Int. Ed. 2005, 44, 5592. [25] IUPAC. Nomenclature of Organic : Blue Book (IUPAC Pub- spectrum, reaching the NIR region in the case of heptacene. lications), Pergamon Press, Oxford, 1979. Non-planarity broadens the spectra of PAHs, but leaves the [26] J. R. Dias, Polycycl. Aromat. Compd. 2005, 25, 113. position of the bands almost unaffected with regard to planar [27] J. R. Dias, J. Chem. Inf. Model. 2005, 45, 562. analogous. [28] B. A. Hess, L. J. Schaad, J. Am. Chem. Soc. 1971, 93, 305. [29] J. R. Dias, Acc. Chem. Res. 1985, 18, 241. Clar’s sextet rule is a very easy but surprisingly effective [30] S. E. Stein, R. L. Brown, J. Am. Chem. Soc. 1987, 109, 3721. method to qualitatively predict the properties of a PAH. The more [31] K. Nakada, M. Fujita, G. Dresselhaus, M. S. Dresselhaus, Phys. Rev. B electron sextets can be formed, the more stable is the PAH. Those 1996, 54, 17954. p-electrons not being member of a sextet are much less [32] S. Grimme, J. Harren, A. Sobanski, F. Vogtle, Eur. J. Org. Chem. 1998, stabilized. Many PAHs have found various applications as discotic 1491. [33] E. Clar, The Aromatic Sextet, Wiley, London, 1972. liquid crystals, organic semiconductors, or fluorescence dyes. The [34] A. Misra, D. J. Klein, T. Morikawa, J. Phys. Chem. A 2009, 113, 1151. optical properties reveal a lot of details about the electronic [35] H. Hosoya, Top. Curr. Chem. 1990, 153, 255. structure of the PAHs and also support the understanding of [36] C. H. Suresh, S. R. Gadre, J. Org. Chem. 1999, 64, 2505. aromaticity. Further work in this area is thus very advantageous [37] G. Portella, J. Poater, M. Sola, J. Phys. Org. Chem. 2005, 18, 785. [38] Y. Ruiz-Morales, J. Phys. Chem. A 2002, 106, 11283. both for fundamental research and applied sciences. The [39] H. B. Klevens, J. R. Platt, J. Chem. Phys. 1949, 17, 470. synthesis of PAHs of hitherto unknown structure and new [40] E. Clar, M. Zander, Chem. Ber. 1956, 89, 749. functionalizations of known PAHs may render new materials with [41] E. Clar, O. Kuhn, Liebigs Ann. 1956, 601, 181. improved performance and potentially new applications. [42] S. Kumar, Liq. Cryst. 2005, 32, 1089. [43] D. Adam, P. Schuhmacher, J. Simmerer, L. Haussling, K. Siemens- meyer, K. H. Etzbach, H. Ringsdorf, D. Haarer, 1994, 371, 141. [44] E. Condon, Phys. Rev. 1926, 28, 1182. Acknowledgements [45] P. F. Bernath, Spectra of Atoms and Molecules (Topics in Physical Chemistry), Oxford University Press, Oxford, 2005. The EU financial support through the ONE-P large-scale project, [46] S. M. Meyerhoffer, L. B. McGown, Anal. Chem. 1991, 63, 2082. No. 212311, is gratefully acknowledged. The authors also [47] J. B. Birks, Photophysics of Aromatic Molecules, Wiley and Sons, acknowledge the financial support from the Deutsche For- Hoboken, 1970. [48] H. W. Offen, D. E. Hein, J. Chem. Phys. 1969, 50, 5274. schungsgemeinschaft (transregio SFB TR49). [49] D. Baunsgaard, M. Larsen, N. Harrit, J. Frederiksen, R. Wilbrandt, H. Stapelfeldt, J. Chem. Soc. Faraday Trans. 1997, 93, 1893. [50] E. Clar, Chem. Ber. 1943, 76, 611. [51] W. V. Mayneord, E. M. F. Roe, Proc. Roy. Soc. A 1935, 152, 299. REFERENCES [52] R. Schoental, E. J. Y. Scott, J. Chem. Soc. 1949, 1683. [53] M. Zander, Chem. Ber. 1959, 92, 2744. [1] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183. [54] E. Clar, C. T. Ironside, M. Zander, J. Chem. Soc. 1959, 142. [2] J. R. Williams, L. DiCarlo, C. M. Marcus, Science 2007, 317, 638. [55] Y. Fujioka, Bull. Chem. Soc. Jpn 1984, 57, 3494. [3] J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, T. J. Booth, [56] Y. Fujioka, Bull. Chem. Soc. Jpn 1985, 58, 481. S. Roth, Nature 2007, 446, 60. [57] E. Clar, A. McCallum, Tetrahedron 1964, 20, 507. [4] J. C. Fetzer, Large (C ¼ 24) Polycyclic Aromatic Hydrocarbons: Chem- [58] A. Stabel, P. Herwig, K. Mu¨llen, J. P. Rabe, Angew. Chem. Int. Ed. 1995, istry and Analysis, Wiley and Sons, New York, 2000. 34, 1609. [5] R. G. Harvey, Polycyclic Aromatic Hydrocarbons, Wiley and Sons, New [59] L. Schmidt-Mende, A. Fechtenko¨tter, K. Mu¨llen, E. Moons, R. H. York, 1997. Friend, J. D. MacKenzie, Science 2001, 293, 1119. [6] A. Bjorseth, T. Ramdahl, Handbook of Polycyclic Aromatic Hydro- [60] J. P. Hill, W. S. Jin, A. Kosaka, T. Fukushima, H. Ichihara, T. Shimomura, carbons: Emission Sources and Recent Progress in Analytical Chem- K. Ito, T. Hashizume, N. Ishii, T. Aida, Science 2004, 304, 1481. istry, Marcel Dekker Inc., New York, 1985. [61] F. Ja¨ckel, M. D. Watson, K. Mu¨llen, J. P. Rabe, Phys. Rev. Lett. 2004, 92, [7] N. M. Marinov, W. J. Pitz, C. K. Westbrook, M. J. Castaldi, S. M. Senkan, 188303. Combust. Sci. Technol. 1996, 116, 211. [62] W. Hendel, Z. H. Khan, W. Schmidt, Tetrahedron 1986, 42, 1127. [8] A. Luch, The Carcinogenic Effects of Polycyclic Aromatic Hydrocar- [63] Z. H. Wang, Z. Tomovic, M. Kastler, R. Pretsch, F. Negri, V. Enkelmann, bons, Imperial College Pr, London 2005. K. Mu¨llen, J. Am. Chem. Soc. 2004, 126, 7794. [9] S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hagele, G. Scalia, [64] V. S. Iyer, K. Yoshimura, V. Enkelmann, R. Epsch, J. P. Rabe, K. Mu¨llen, R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel, M. Tosoni, Angew. Angew. Chem. Int. Ed. 1998, 37, 2696. Chem. Int. Ed. 2007, 46, 4832. [65] T. Bo¨hme, C. D. Simpson, K. Mu¨llen, J. P. Rabe, Chem. Eur. J. 2007, 13, [10] R. J. Bushby, O. R. Lozman, Curr. Opin. Colloid Interface Sci. 2002, 7, 7349. 343. [66] X. L. Feng, J. S. Wu, M. Ai, W. Pisula, L. J. Zhi, J. P. Rabe, K. Mu¨llen, [11] S. Kumar, Chem. Soc. Rev. 2006, 35, 83. Angew. Chem. Int. Ed. 2007, 46, 3033. [12] X. L. Feng, V. Marcon, W. Pisula, M. R. Hansen, J. Kirkpatrick, [67] D. Wasserfallen, M. Kastler, W. Pisula, W. A. Hofer, Y. Fogel, Z. H. Wang, F. Grozema, D. Andrienko, K. Kremer, K. Mu¨llen, Nat. Mater. 2009, K. Mu¨llen, J. Am. Chem. Soc. 2006, 128, 1334. 8, 421. [68] F. Do¨tz, J. D. Brand, S. Ito, L. Gherghel, K. Mu¨llen, J. Am. Chem. Soc. [13] J. S. Wu, W. Pisula, K. Mu¨llen, Chem. Rev. 2007, 107, 718. 2000, 122, 7707. [14] K. Mu¨llen, J. P. Rabe, Acc. Chem. Res. 2008, 41, 511. [69] Z. Tomovic, M. D. Watson, K. Mu¨llen, Angew. Chem. Int. Ed. 2004, 43, [15] G. Rapenne, Org. Biomol. Chem. 2005, 3, 1165. 755. [16] P.Ruffieux, O. Gro¨ning, R. Fasel, M. Kastler, D. Wasserfallen, K. Mu¨llen, [70] J. A. Miller, Cancer Res. 1970, 30, 559. P. Gro¨ning, J. Phys. Chem. B 2006, 110, 11253. [71] J. K. Selkirk, R. G. Croy, H. V. Gelboin, Science 1974, 184, 169. [17] B. T. Draine, Annu. Rev. Astron. Astrophys. 2003, 41, 241. 324

www.interscience.wiley.com/journal/poc Copyright ß 2010 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2010, 23 315–325 PAHs FOR STRUCTURAL AND OPTICAL STUDIES

[72] P. G. Shields, E. D. Bowman, A. M. Harrington, V. T. Doan, A. Weston, [118] R. N. Jones, Chem. Rev. 1947, 41, 353. Cancer Res. 1993, 53, 3486. [119] X. Y. Jiang, Z. L. Zhang, X. Y. Zheng, Y. Z. Wu, S. H. Xu, Thin Solid Films [73] P. P. Fu, H. M. Lee, R. G. Harvey, J. Org. Chem. 1980, 45, 2797. 2001, 401, 251. [74] J. Hu, D. Zhang, F. W. Harris, J. Org. Chem. 2005, 70, 707. [120] Y. Kan, L. D. Wang, Y. D. Gao, L. Duan, G. S. Wu, Y. Qiu, Synth. Met. [75] R. Rieger, M. Kastler, V. Enkelmann, K. Mu¨llen, Chem. Eur. J. 2008, 14, 2004, 141, 245. 6322. [121] D. R. Maulding, B. G. Roberts, J. Org. Chem. 1969, 34, 1734. [76] E. Clar, W. Schmidt, Tetrahedron 1979, 35, 1027. [122] M. M. Moodie, C. Reid, J. Chem. Phys. 1952, 20, 1510. [77] J. B. Birks, I. H. Munro, D. J. Dyson, Proc. Roy. Soc. Ldn A 1963, 275, [123] R. S. Becker, L. V. Natarajan, J. Phys. Chem. 1993, 97, 344. 575. [124] N. Nijegorodov, V. Ramachandran, D. P. Winkoun, Spectroc. Acta Pt. A [78] C. Bohne, E. B. Abuin, J. C. Scaiano, J. Am. Chem. Soc. 1990, 112, 4226. 1997, 53, 1813. [79] F. Hauke, A. Hirsch, S. Atalick, D. Guldi, Eur. J. Org. Chem. 2005, 1741. [125] H. D. Vries, D. A. Wiersma, J. Chem. Phys. 1979, 70, 5807. [80] J. Duhamel, Acc. Chem. Res. 2006, 39, 953. [126] F. G. Patterson, H. W. H. Lee, W. L. Wilson, M. D. Fayer, Chem. Phys. [81] A. K. Mathew, H. Siu, J. Duhamel, Macromolecule 1999, 32, 7100. 1984, 84, 51. [82] G. S. Jackson, R. A. Staniforth, D. J. Halsall, T. Atkinson, J. J. Holbrook, [127] L. B. Roberson, J. Kowalik, L. M. Tolbert, C. Kloc, R. Zeis, X. L. Chi, R. A. R. Clarke, S. G. Burston, Biochemistry 1993, 32, 2554. Fleming, C. Wilkins, J. Am. Chem. Soc. 2005, 127, 3069. [83] K. Jung, H. Jung, J. H. Wu, G. G. Prive, H. R. Kaback, Biochemistry 1993, [128] C. D. Sheraw, T. N. Jackson, D. L. Eaton, J. E. Anthony, Adv. Mater. 32, 12273. 2003, 15, 2009. [84] J. W. Patterson, J. Am. Chem. Soc. 1942, 64, 1485. [129] R. Mondal, R. M. Adhikari, B. K. Shah, D. C. Neckers, Org. Lett. 2007, 9, [85] E. J. Bowen, B. Brocklehurst, J. Chem. Soc. 1955, 4320. 2505. [86] A. R. Horrocks, F. Wilkinso, Proc. R. Soc. London, Ser. A 1968, 306, 257. [130] R. Mondal, B. K. Shah, D. C. Neckers, J. Am. Chem. Soc. 2006, 128, [87] M. Mitsui, Y. Kobori, A. Kawai, K. Obi, J. Phys. Chem. A 2004, 108, 524. 9612. [88] B. T. Jones, N. J. Mullins, J. D. Winefordner, Anal. Chem. 1989, 61, [131] M. M. Payne, S. R. Parkin, J. E. Anthony, J. Am. Chem. Soc. 2005, 127, 1182. 8028. [89] M. Kastler, J. Schmidt, W. Pisula, D. Sebastiani, K. Mu¨llen, J. Am. Chem. [132] V. M. Tsefrikas, L. T. Scott, Chem. Rev. 2006, 106, 4868. Soc. 2006, 128, 9526. [133] P. W. Rabideau, A. Sygula, Acc. Chem. Res. 1996, 29, 235. [90] T. Shimada, C. L. Hayes, H. Yamazaki, S. Amin, S. S. Hecht, [134] W. E. Barth, R. G. Lawton, J. Am. Chem. Soc. 1971, 93, 1730. F. P. Guengerich, T. R. Sutter, Cancer Res. 1996, 56, 2979. [135] L. T. Scott, M. M. Hashemi, D. T. Meyer, H. B. Warren, J. Am. Chem. Soc. [91] H. Henstock, J. Chem. Soc. 1923, 123, 3097. 1991, 113, 7082. [92] E. Clar, Spectrochim. Acta 1950, 4, 116. [136] L. T. Scott, M. M. Hashemi, M. S. Bratcher, J. Am. Chem. Soc. 1992, 114, [93] E. Clar, J. F. Stephen, J. F. Guyevuil, Tetrahedron 1964, 20, 2107. 1920. [94] T. Vodinh, R. B. Gammage, P. R. Martinez, Anal. Chem. 1981, 53, 253. [137] M. Baumgarten, L. Gherghel, M. Wagner, A. Weitz, M. Rabinovitz, P.C. [95] N. Kanamaru, H. R. Bhattach, E. C. Lim, Chem. Phys. Lett. 1974, 26, Cheng, L. T. Scott, J. Am. Chem. Soc. 1995, 117, 6254. 174. [138] A. L. Lafleur, J. B. Howard, K. Taghizadeh, E. F. Plummer, L. T. Scott, A. [96] K. F. Lang, J. Kalowy, H. Buffleb, Chem. Ber. 1964, 97, 494. Necula, K. C. Swallow, J. Phys. Chem. 1996, 100, 17421. [97] J. F. McKay, D. R. Latham, Anal. Chem. 1972, 44, 2132. [139] T. J. Seiders, E. L. Elliott, G. H. Grube, J. S. Siegel, J. Am. Chem. Soc. [98] F. B. Mallory, K. E. Butler, A. C. Evans, E. J. Brondyke, C. W. Mallory, 1999, 121, 7804. C. Q. Yang, A. Ellenstein, J. Am. Chem. Soc. 1997, 119, 2119. [140] M. Yamaji, K. Takehira, T. Mikoshiba, S. Tojo, Y. Okada, M. Fujitsuka, [99] S. Alibert-Fouet, I. Seguy, J. F. Bobo, P. Destruel, H. Bock, Chem. Eur. J. T. Majima, S. Tobita, J. Nishimura, Chem. Phys. Lett. 2006, 425, 53. 2007, 13, 1746. [141] H. Sakurai, T. Daiko, T. Hirao, Science 2003, 301, 1878. [100] A. Herrmann, K. Mu¨llen, Chem. Lett. 2006, 35, 978. [142] T. Amaya, K. Mori, H. L. Wu, S. Ishida, J. I. Nakamura, K. Murata, [101] H. Langhals, Heterocycles 1995, 40, 477. T. Hirao, Chem. Comm. 2007, 1902. [102] Z. J. Chen, V. Stepanenko, V. Dehm, P.Prins, L. D. A. Siebbeles, J. Seibt, [143] H. W. Kroto, A. W. Allaf, S. P. Balm, Chem. Rev. 1991, 91, 1213. P. Marquetand, V. Engel, F. Wurthner, Chem. Eur. J. 2007, 13, 436. [144] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, [103] D. Radulescu, G. Ostrogovich, F. Barbulescu, Chem. Ber. 1931, 64, 1323. 2240. [145] B. C. Thompson, J. M. J. Frechet, Angew. Chem. Int. Ed. 2008, 47, 58. [104] Y. Kono, H. Miyamoto, Y. Aso, T. Otsubo, F. Ogura, T. Tanaka, M. [146] C. J. Brabec, J. R. Durrant, MRS Bull. 2008, 33, 670. Sawada, Angew. Chem. Int. Ed. 1989, 28, 1222. [147] K. Itaka, M. Yamashiro, J. Yamaguchi, M. Haemori, S. Yaginuma, [105] I. B. Berlman, Energy Transfer Parameters of Aromatic Compounds, Y. Matsumoto, M. Kondo, H. Koinuma, Adv. Mater. 2006, 18, 1713. Academic Press, London, 1973. [148] P. D. Hale, J. Am. Chem. Soc. 1986, 108, 6087. [106] B. X. Mi, Z. Q. Gao, C. S. Lee, S. T. Lee, H. L. Kwong, N. B. Wong, Appl. [149] S. Leach, M. Vervloet, A. Despres, E. Breheret, J. P. Hare, T. J. Dennis, Phys. Lett. 1999, 75, 4055. H. W. Kroto, R. Taylor, D. R. M. Walton, Chem. Phys. 1992, 160, 451. [107] E. Clar, W. Kelly, R. M. Laird, Monatsh. Chem. 1956, 87, 391. [150] J. P. Hare, H. W. Kroto, R. Taylor, Chem. Phys. Lett. 1991, 177, 394. [108] W. E. Moerner, T. Basche´, Angew. Chem. Int. Ed. 1993, 32, 457. [151] Y. Wang, J. Phys. Chem. 1992, 96, 764. [109] Y. Avlasevich, C. Kohl, K. Mu¨llen, J. Mater. Chem. 2006, 16, 1053. [152] S. Pfuetzner, J. Meiss, A. Petrich, M. Riede, K. Leo, Appl. Phys. Lett. [110] I. Deperasinska, B. Kozankiewicz, I. Biktchantaev, J. Sepiol, J. Phys. 2009, 94, 223307. Chem. A 2001, 105, 810. [153] L. Barnett, D. M. Ho, K. K. Baldridge, R. A. Pascal, J. Am. Chem. Soc. [111] S. A. Tucker, H. Darmodjo, W. E. Acree, J. C. Fetzer, M. Zander, Appl. 1999, 121, 727. Spectrosc. 1992, 46, 1260. [154] D. Pena, D. Perez, E. Guitian, L. Castedo, Org. Lett. 1999, 1, 1555. [112] K. H. Koch, K. Mu¨llen, Chem. Ber. 1991, 124, 2091. [155] N. P. Hacker, J. F. W. McOmie, J. Meunierpiret, M. Vanmeerssche, [113] A. Maliakal, K. Raghavachari, H. Katz, E. Chandross, T. Siegrist, Chem. J. Chem. Soc. Perkin Trans. 1982, 1, 19. Mater. 2004, 16, 4980. [156] S. X. Xiao, M. Myers, Q. Miao, S. Sanaur, K. L. Pang, M. L. Steigerwald, [114] C. R. Kagan, A. Afzali, T. O. Graham, Appl. Phys. Lett. 2005, 86, 193505. C. Nuckolls, Angew. Chem. Int. Ed. 2005, 44, 7390. [115] G. Wittig, Org. Synth. 1959, 39, 75. [157] E. Clar, J. F. Stephen, Tetrahedron 1965, 21, 467. [116] J. E. Anthony, Chem. Rev. 2006, 106, 5028. [158] B. L. Merner, L. N. Dawe, G. J. Bodwell, Angew. Chem. Int. Ed. 2009, 48, [117] J. E. Anthony, Angew. Chem. Int. Ed. 2008, 47, 452. 5487. 325

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