Chrysene‐Based Blue Emitters

Abstract Chrysene and its bisbenzannulated homologue, naphtho[2,3‐c]tetraphene, were synthesized through a PtCl2‐catalyzed cyclization of alkynes, which also furnished corresponding biaryls subsequent to a Glaser coupling reaction of the starting alkynes. The optoelectronic properties of 5,5′‐bichrysenyl and 6,6′‐binaphtho[2,3‐c]tetraphene were compared to their chrysene‐based “monomers”. Oxidative cyclodehydrogenations of bichrysenyl and its higher homologue towards large nanographenes were also investigated.

The development of new blue-emitting materials based on polycyclic aromatic hydrocarbons( PAHs) continues to be an importantc hallengef or organicc hemistry and material science. [1] During the last decades PAHs such as anthracene, phenanthrene, pyrene and corresponding biarylsw ith blue emission have been employed in optoelectronic devicess uch as OLEDs. [2] Am ajor advantage of PAHe mitters is their increaseds tability. [3] Chrysene is aw ell-known PAHw ith blue fluorescence, but its derivatives are sparselye mployed as emitting materiali no ptoelectronic devices due to their low solubility. [4] One possibility to address this issue is the substitution with solubility mediating groupss uch as aryl and alkynyl as well as (electron-rich)a mino substituents, reducing p-p stacking and aggregation. [5] Along this line, severalc hrysene derivatives with high quantum yields were obtained. [5] However,t he HOMO-LUMO gap is affected by extension of the p-system and/or by electron donating groups leadingt or ed-shifted emissionb ands.S imilart oc hrysene,d ibenzochrysenes [6] have attracted interest as they provide blue to blue-greene mission with remarkably high externale fficiencies up to 2.0 %i n OLEDs. [7,8] However,a mong the different isomers, the synthesis of naphtho [2,3-c]tetraphene is only reportedi no ne article [9] and due to its low solubility it remained poorly investigated. In this contribution,wereport an ew synthetic access to chrysene and its bisbenzannulated homologuea sw ella st otheir biaryl congeners to investigate their optoelectronic properties in terms of their applicability as blue emitters. Biaryl formation from strongly twisted subunits holds promiset or etain the optical gaps without the use of functional groups. Further,t he torsionb etween the two naphtho [2,3-c]tetraphene or chrysene moieties increases solubility of the compounds by reducing pp stacking.
Compared to its monomer 11 (3 mg mL À1 in THF), the solubility of 13 nearly doubled (5 mg mL À1 in THF), for which unfortunately only amorphous solids were obtained after various crystallization attempts. Thus, as expected, formal homocoupling resultedi naremarkable solubility increase, even without the introduction of solubility mediating groups,w hich was rationalized in terms of the twisted nature of the biaryls.
We analyzed the absorption and photoluminescence of the target compounds in n-hexane and in thin films ( Figure 2).
While the normalized absorption spectra of chrysenea nd 5,5'bichrysenyl were nearly identical in solution with absorption of the latter tailing into the visible region,t he emission signal of 5,5'-bichrysenyl was red-shifted by 58 nm compared to purpleblue emitting chrysene, leading to as ky-blue emission. As imilar shift was observed for 11 and 13,t he emission maxima changed from 421 nm to 472 nm furnishingablue-greenish emission. The quantum yield QY in solution (determined with an Ulbricht sphere) increased noticeably between chrysene and naphtho [2,3-c]tetraphene from 11 %t o4 6% and was slightly improved in the biaryls (16 %f or 5,5'-bichrysenyl and 49 %f or 6,6'-binaphtho [2,3-c]tetraphene).
Stability measurements involving 5 and 13 revealed no change of absorption spectra over ap eriod of 5d ays under continuousi rradiation with a3 65 nm hand-held UV lamp under aerobic conditions in solution (see Supporting Information, FiguresS9a nd S10), underlining the high photostability of the compounds.
Emission of thin films of 5, 11 and 13,s pin-castf rom THF, was dramatically red-shifted compared to that observed in solution:5 ,5'-bichrysenyl (5)d isplayed ay ellow photoluminescence with ab road band around l max,film = 546 nm, bathochromically shifted by 104 nm, and naphtho [2,3-c]tetraphene (11)a nd 6,6'-binaphtho [2,3-c]tetraphene ( 13)w ere yelloworange fluorescent in films, with red-shifted maximab y 149 nm and 121 nm, respectively.O nly the thin film emission spectrum of chrysene itself resembled its solution spectrum, althoughaslight change in the intensity distribution of its vibronic structure waso bserved. The otherwise red-shifted emission bands indicated significant p-p interactions of the chromophores of 5, 11 and 13 in thin films. Interestingly,c rystalline 5,5'-bichrysenyl (5)( l max,cryst = 415 nm) was blue emissive (see   Figure S15) yields ap otential with am inimum at 788,i na ccordance with the value obtained via X-ray structurea nalysis. Between 308 and 1508, 5 rotates at room temperature as the potentiale nergies do not exceed 60 kJ mol À1 until the aforementioned torsional angles are reached. [14] As ac onsequence,d ifferent conformations and morphologies in kinetically trapped thin films compared to the single crystals are expected even at ambient temperatures. The frontiermolecular orbitals were, even in the biaryl systems, located on the entire p-system with an odal plane on their single bonds. Biaryls 5 and 13 displayed only slightly increased gaps (E g,calc = 3.73 eV/2.67 eV) compared to that of 3 and 11 (E g,calc = 3.90 eV/2.75 eV), which are in good agreement with the opticalg aps determined from their absorption onsets (see Ta ble 1), due to the twisted structures. Cyclovoltammetry supports the calculated LUMO levels (see Supporting Information, Ta ble S1). The dramatically red-shifted photoluminescence in thin films was thus explicable by the wide range of torsional angleso ft he biaryl systems resulting in partial planarization and intermolecular (p-p)interactions. This feature also explains the red-shifted emission of 5 in solution compared to its crystalline state as well as the red-shifted emissions of biaryls 5 and 13 comparedt ot heir monoaryls 3 and 11 in solution.T o retain blue emission and to reduce red-shifts of these biaryls, both in thin films andi ns olution, partialp lanarization needs to be prohibited by steepening the torsional potentialc urves. This could either be achieved through ortho-functionalization, for example, with methyls ubstituents, resulting in ar eduction of the bandwidtho fa doptable angles at room temperature (508 to 1308)o rby going to terchrysene-based systems. Additionally,a lternative ways of film formation, other than spincoating, allowing for more time for the biaryl rotamers to equilibrate and adopt their energeticallyp referred (twisted) conformation shouldf urnish blue-emitting thin films for use in OLEDs.
The synthesis of large PAHs withouts olubility mediating groups is ac hallenge fors olution-based chemistry.S oluble biaryls, such as 5 and 13,may serve as precursors for large, unsubstituted PAHs-this approachc ould thus complement the hitherto explored one via exhaustive cyclodehydrogenations of polyphenylene dendrimers which are planarized under Scholl conditions. [15] With compounds 5 and 13 in hand, we further investigated the Scholl cyclodehydrogenations to obtain 14 and 15,r espectively,w hich,a samember of the larger PAHs might be utilized as IR emitters and/or be utilized for stimulated emission. [16] Only one synthetic approacht o14 was reported so far but separation from an aphthoindenozethrene [17] side product was only achieved in minusculea mounts by HPLC due to the low solubility,therefore aselective access is of interest. Bichrysenyl derivatives wereemployed by us to obtain several large PAHs ystems, such as graphene nanoribbons and (dibenzo)ovalenes. [10a-c] Based on our experiencew ith Scholl reactions we tried three literature reported reaction conditions: DDQ/CF 3 SO 3 H, FeCl 3 and DDQ/ScOTf 3 . [10c, 18] Onlyt he combination of DDQ with trifluoromethanesulfonic acid furnished 14 as ad eep red double [4]helicene (Scheme 2) with am aximum   Figure S4). Its low solubility prohibited analysis by NMR spectroscopy,e ven at high temperature. The planarization caused ar ed-shift of both absorption (l abs = 495 nm) and emission (l em = 516 nm) maxima-14 is yellow fluorescenti nnhexane. The observed small Stokes shift illustrated the rigidity of the compound. This is the first selectivea ccess to 14 in reasonable yields. Cyclodehydrogenation of 6,6'-binaphtho [2,3-c]tetraphene (13)u nder similar conditions only lead to an insoluble and inseparablem ixture of twofold cyclodehydrogenated product of unknown identitya nd, in traces, to fully oxidized 15 (see Supporting Information, FigureS5f or mass spectra). Higher temperatures and prolonged reactiont imes up to 7days did not drive the reactiont oc ompletion.W hile the solubility of 11 as well as that of its monocyclodehydrogenated reaction intermediate is sufficiently high for ac omplete fusion to yield 14, the limit of this approachi sq uicklym et in terms of solubility of the reactioni ntermediates when attempting to synthesize 15.Assuch, cyclodehydrogenation cannotcompete with oligophenylene planarization in terms of PAHs ize, [15a, b] but offers an access to smaller derivatives with cove edges inaccessible via the latter strategy.W hether 11 really poses the limit to PAH structuresv ia the biaryl approach in terms of size or only its bisbenzannelated derivative, for example, 13,13'-bibenzo[c]tetraphene, may also be fully dehydrogenated, remains to be investigated.
In conclusion, we reported asimple PtCl 2 -mediateds ynthetic strategyt owards chrysene ( 3), 5,5'-bichrysenyl (5), naphtho [2,3c]tetraphene (11)a nd 6,6'-binaphtho [2,3-c]tetraphene (13)v ia 6-endo-dig cyclizations of 1-alkynylbiaryls and their Glaser coupled bialkynes. Although biaryl formation does only slightly influence the calculated frontier molecular orbital energy levels, it results in an increased solubility withouta dditional solubility-mediating groups.T he shallow torsional potential, not greatly hindering rotationa bout the connecting single bond, leads to ap ronounced red-shift in the thin film emissiond ue to partial planarization and intermolecular interactions. In the crystalline state, however,b lue emission is retainedd ue to the herringbone motif of the twisted biaryls. Key to soluble, all sp 2 hybridized, blue fluorescentd erivatives forO LED applications is thus to rigidify the system even further, for example, as in 5,11-polychrysenylene,w hicha lso serveda sagraphene nanoribbon precursor. [10a] An ew approach through polymerization of dihalogenated bialkyne 4 hast he potential to exceed the previously reported oligomers in size due to reduced steric hindrance upon CÀCb ondf ormation. Whereas polyphenylene dendrimersa re conventionally employed as precursors to large unsubstituted PAHs, as they planarize under Scholl conditions, we explored an ew strategyt hrough cyclodehydrogenation of (soluble) biaryl systems in this work, which could furnish nanographenes with hitherto unexplorede dge type combinations. These types of sparselyi nvestigated PAHs are of interest for lasing applications [19] and NIR emission. [16] We investigated oxidative Scholl coupling of 11 furnishing 14,b ut full cyclodehydrogenation of its higherh omologue 13 was only observed in trace amounts: Most likely,s olubility limits the full conversion of the twofold cyclodehydrogenated intermediate, which will be circumventedb yu tilizing surface-assisted fusion [20] in the future.

Experimental Section
Crystallographic data Deposition numbers 1996744, 1996745, 1996746, 1996747, 1996748, and 1996749 contains the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.