Acenes, as ac lass of interesting polycyclic aromatic hydrocarbons (PAHs) made up of linearly fused benzene rings, have attracted attentiond ue to their semiconductor properties. [1] Higher acenes,i nc ontrast,a re increasingly less stable and insoluble, which severelyl imits their utility. [2] Therefore, new methods to prepare acenes or acene-based PAHs with increaseds olubilitya nd stability are desired. [3] Pentacene, consisting of five linearly fused benzene rings, and its derivatives have been widely investigated and applied in electronic devices. [4] Severals trategies to make stabilizedp entacened erivatives or pentacene-based PAHs have been reported. [5] Sterically blockingt he most reactive positions of pentacene is one general method. [6] For example, Anthony and co-workersr eported that introductiono fb ulky silylethyneg roups at the 6,13-positions results in silylethynyl-functionalized pentacenesw ith good stability and solubility (Figure1a). Thoughp entannulation (Figure 1b) [7] or benzannulation [8] (Figure 1c)o ft he central ring can also stabilizeP AHs, the resulting compounds lose acene character due to electronicmodulation. The introduction of additional Clar sextetsa tt he end of acenes is another method to stabilize these compounds, as demonstrated recentlyb yB unz and co-workers, who isolated (triisopropylsilyl)ethynyl (TIPS) substituted tetrabenzopentacene with four Clar sextets(Figure 1d). [9] All the methods mentioned above indicatet hat functionalizing the central ring is necessary (Figure 1a-d) to stabilize the obtained PAHs, even after incorporation of additional Clar sextets (Figure 1d). To the best of our knowledge,t he synthesis of stabilized p-extended pentacenebased PAHs with non-functionalized central rings through the combination of both peri-a nd cata-benzannulation reactions has never been reported. Herein, we describen ew peri-a nd cata-fused pentacened erivative, benzodipyrene (BDP) (Figure 1e), through an InCl 3 -AgNTf 2 -catalyzed four-fold alkyne benzannulation reaction. Additionally,t hese BDPs exhibited good stabilitye ven with an on-functionalized centralr ing.

Multi-fold alkyne benzannulation reactions have been widely used in our group to make functionalized nanographenes as well as graphene nanoribbons. [10] Herein, the stabilized BDPs, 4a and 4b,w ere obtained through an InCl 3 -AgNTf 2 -catalyzed four-fold alkyne benzannulation reaction (Scheme 1). The key precursors 3a,b were isolated in modest yields through a double Suzuki cross-couplingr eactiono f1 with 2,6-bis(trifluoromethanesulfonate) anthracene 2.U nlike our previous work on the synthesis of peropyrenes and teropyrenes through an InCl 3 -catalyzed multifold alkyne benzannulation, [10d] InCl 3 itself failed to fully cyclize 3 to 4.R ecently,w er eported ad omino benzannulation reaction of buta-1,3-diynes to construct irregular nanographenes, in which the InCl 3 -AgNTf 2 catalyst system was used. [10f] Upon treating compound 3 with this system in toluene at 100 8Cf or 24 hours, the BDP products 4a and 4b were isolatedi n6 6a nd 61 %y ields, respectively.

As ingle crystal of 4a suitable for X-ray crystallographic analysis was grown by slowly diffusing methanolinto the CDCl 3 solution. The structure of 4a is nearly flat with the aryl substituents twisted out of conjugation (Figure 2a). The distance between the centralc arbon atom and the aryl substituent is around3 .49 (Figure 2a). Alternating short and long bondlength character near the terminal rings of the BDP core was not observed, as compared to silylethynyl-functionalized pentacene( Figure 2b). [6c] The smallest bond alternation is observed within the central ring, and the bond length at the central ring fusion (1.433 )i ss horter than that in the TIPS-functionalized pentacene (1.449 ). Due to the perpendicularity between the backbone and side aryl chains, 4a packs in as lipstacked arrangement( Figure 2c)w ith am inimum interplanar spacingo f4 .28 (Figure 2d). Deep-red single crystals of 4a were grown under ambient light along with small amountso f colorless single crystalst hat were attributedt ot he endoperoxide product 4a-O 2 .T he structure of the endoperoxide adduct unambiguously confirmed the [4+ +2] cycloaddition reaction of 4a with singlet oxygen ( 1 O 2 )t hat occurred on the central ring (Figure 2e).

The optical properties of BDP 4a and 4b were studied by absorption (Figure S3 and S4) and emission spectroscopy (Figure 3). It should be noted that BDP exhibited fine structure in the absorption spectra that is significantly differentf rom other reported pentacene-based PAHs with peri-fusions [7a,b, 8a] along the acene backbone. BDP displays a l max that is blueshiftedb yc a. 40 nm relative to pentacene, even after extending p-conjugation along the short axis of the molecules. No significant changes were observed in the absorption spectra of 4a when going from the solution to the solid state (Figure S5), which indicated that there is no aggregation in the solid state due to the bulky aryl substituents. The emission maximumo f the compound is in the yellow region of the visible spectrum (~560 nm), and the emissionband shows vibrational fine structure analogoust ot he excitation spectrum (Figures 3a nd S6). The observed emission spectrumi se ssentially the mirror image of the excitation spectrum, which indicates that the same states are involved in excitation and emission.T he fluorescence quantumy ields of both BDP derivatives ( % 60 %) are comparable with compounds used in visible light communications technology, [11] making them promisingc andidates for applicationsi np hotonics. As summarized in Table 1, the emission lifetimes for 4a and 4b are ~6ns, which correspond to an emissive rate of ~0.154 ns À1 and are independento ft he lateral substituents in the core ring or the solventu sed. All decay curves were fit to am ono exponential( Figures S9 andS10) indicatingt hat emission occurs from one excited state. TheX -ray structureo ft he endoperoxide adduct 4a-O 2 indicates that BDP acts as as inglet oxygen ( 1 O 2 )s ensitizer,a ss een by the presenceo ft he characteristic phosphorescence band at ~1272 nm (Figure S7). The quantum yields of 1 O 2 generation (F1 O 2 )a re 27 AE 1, 70 AE 2, and 4.9 AE 1.6 %i nt oluene, CHCl 3 ,a nd THF,respectively.

The utility of acene-basedP AHs is strongly limitedd ue to their photochemical decomposition, particularly in the presence of O 2 and light irradiation. Therefore, the key prerequisite for using these newly synthesized BDP as sensitizers is improving their photostabilities. To test the photostability of BDP 4a, we first lookeda tt he photochemical reaction process in airequilibrated CDCl 3 through 1 HNMR spectroscopy (Figure 4). In the aerated CDCl 3 solution and under ambientl ight, the sample decomposed slowly,a nd only at race amount of 4a could be detected after 48 h( Figure4). The appearance of a new signal at 5.95 ppm further indicated the formation of the [nm] [a] l onset,abs [nm] l max,em [nm] [b] Stokes shift

t

[ns] [d] t 1/2 [min] [e] [a] Longest wavelengtha bsorption maximum peak. [b] Emission maximum. [c]Optical gap determinedf rom the onset of the absorptionband. [d] Emission lifetimes (t). [e] Half-life. S14). From Figure 5w ef ind that both BDP molecules are more stable in THF than in tolueneo rC HCl 3 ,w hich is consistent with decreasing yields of 1 O 2 generation in these solvents. Though the lateral aryl substituents have only as mall effect on the photophysical properties of these compounds,t hey significantly impactt he photostabilities. Compound 4a showeda decomposition half-life (t 1/2 )o f1 894 mini nT HF.A fter modification of the lateral aryl groups to increase their steric shielding, the photostability of BDP 4b increased 2.5-fold to 4612 min in THF,w hich is remarkably more stable than pentacene-based PAHs [7a,d-g] and commercial chlorin-based compounds used in photodynamic therapy. [12] In conclusion, we have presented the synthesis of benzodipyrene (BDP) derivatives through InCl 3 -AgNTf 2 -catalyzed fourfold alkyne benzannulation reactions.T he BDP scaffold consists of al ess common and difficult to synthesize benzo[a]pyrene [13] substructure as compared to the more ubiquitous benzo [e]pyrene. [13c] Both BDP molecules exhibitedablueshift in absorbance by ca. 40 nm relative to pentacene. The structure of BDP 4a and its corresponding endoperoxide product 4a-O 2 were unambiguously confirmed by X-ray crystallography.I ncreasing the size of the lateral aryl group dramatically improves the photostability (t 1/2 of 4612 min for 4b)i nT HF.W eb elieve that the described combination of cata-a nd peri-benzannulation strategyi sa ne ffective way to access larger acene-based PAHs with enhanced solubility and stability.