5,7,12,14‐Tetrafunctionalized 6,13‐Diazapentacenes

Abstract The synthesis, property evaluation, and single crystal X‐ray structures of four 5,7,12,14‐tetrafunctionalized diazapentacenes are presented. The synthesis of these compounds either starts from tetrabromo‐N,N‐dihydrodiazapentacene or from a diazapentacene tetraketone. Pd‐catalyzed coupling or addition of a lithium acetylide gave the precursors that furnish, after further redox reactions, the diazapentacenes as stable crystalline materials. The performance of the tetraphenyl‐substituted compound as n‐channel semiconductor was evaluated in organic field effect transistors.

Herein, we describet he synthesis of tetrasubstituted 6,13-diazapentacenes by using two different precursors. Azaacenes [1] have aroused great interest, startingw ith the synthesis of the superb n-channel semiconductor TIPS-TAP. [2][3][4] This interest was furthers toked by new synthesest oc onstruct azapentacenes [5] to azaheptacenes, [6] by using Pd-catalyzedf ormation of embedded N,N'-dihydropyrazines, [6,7] and the availability of several privileged, bis(tri-iso-propylsilylethynyl)-substituted aromatic ortho-diamines. [5] These approaches lead to disubstitutedazaacenes. The synthesis of higher substituted azaacenes( tetrasubstituted, hexasubstituted, etc.) is not common, although for their hydrocarbon analogues, [8][9][10] some derivatives have recentlyb een explored, including per-substituted species furnishingt wistacenes. [11] Herein, we decorate the diazapentacene framework either by fourfold Suzuki-Miyaurac oupling [12] or by fourfold addition of al ithium acetylide. Reaction of the literature known tetrabromide 1 [13] with different boronic acids unders tandard palladium catalysis conditions gave the crude N,N'-dihydro-intermediates 2a-c,w hichw ere not furtherc haracterized but immediately oxidized by MnO 2 into the target compounds 3a-c (53-79% overall yield). The dihydro-species 1 is much more soluble( and does not re-oxidize the intermediately formed Pd 0 species) than its oxidized heteroacene counterpart and was employed in our coupling reactions. Because 1 did not undergo Sonogashira reaction directly (see SchemeS1i nthe SupportingI nformation for conditions), we obtained tetrayne 3d by reacting tetraone 4 with an excesso f the lithium salt of TIPS acetylene and treatment of the intermediate with tin dichloride. [14] Compound 5 was isolatedi n 26 %y ield. Oxidation with MnO 2 in acetonitrile then gave 3d in 95 %y ield. Note that the electron-withdrawing pyrazine units enable fourfold nucleophilic addition-TIPS acetylide only adds twice to the corresponding hydrocarbon tetraketone analogue. [15,16] Figure 1d isplays the normalized absorption spectra of nonfluorescent 3a-d (see also Ta ble1). We note that 3a-c display almost identical UV/Vis spectra despite the significante lectronic differences in the substituents of 3a-c.T he substituents only exert an inductive effect but do not increase the conjugation-notu nexpected, becauset he arene groups are heavily twisted with respectt ot he diazapentacene backbone. Compound 3d with the four alkyne groups displays a7 0-80 nm redshifteda bsorption at 743 nm, ac onsequence of the strong conjugation of the four alkyne groups with the diazapentacene nucleus(Scheme 1). [17] Compounds 3a-d were investigated by cyclic voltammetry (Table 1). They can be both oxidizeda nd reduced, suggesting ambipolar behavior. [20] As was expected, 3d and c display the highest oxidation potential. The effect is particularly strong for 3c,f eaturing four CF 3 groups.T he same trend was observed for the reduction potentials, which are À1.14 Vf or 3c and À0.96 Vf or 3d.T he electron affinity for 3c and d are estimated to be À3.7a nd À3.8 eV,r espectively.A lthought he alkyne substituents influence HOMO and LUMO position differently and lead to ad ecreasede lectrochemical and optical gap, electron withdrawing substituents on the aryl groups in 3c stabilize both frontier molecular orbitals (FMOs)s imilarly.I nc omparisont o5 ,7,12,14-tetraphenylpentacene, [9] nitrogen substitution leads to decreased FMO energy levels,asw as expected.
Compounds 3a-d form suitable specimens usefulf or X-ray single crystal analysis ( Figure 2a nd the Supporting Information). In compounds 3a-c,t he diazapentaceneb ackbone is planar,a nd the four aryl groups are oriented parallel to each Table 1. Photophysical and electrochemical properties of 3a-d.
Scheme1.Synthesis of substituted diazapentacenes 3a-d. other and considerably twisted with respect to the diazapentacene (dihedral angles:6 3 8 and 658 for 3a;5 7 8 and 618 for 3b and 668 and 718 for 3c). The molecules of 3a and c pack in a herringbone pattern with no p-p overlap between the molecules. The molecules of 3b pack in p-p stacked dimers with an interplanar distance of 3.60 ,w hich are arrangedi no ne-dimensional slipped stacks.I nt he case of 3d,t he four TIPS-ethynyl groups crowd each other. This leads to at wist of the diazapentacene nucleus with an end-to-end torsion angle of 208. The steric crowding of the four TIPS groups also enforces a bend in the alkynes away from each other,e ven though direct peri interactions are not present due to the pyrazineu nit interspersed between the alkyne-carrying rings. Compound 3d also packs in ah erringbone motif;h ere also, as was expected, [21] pp overlap is absent.T he observed packing suggests that larger acenes,f or example, diazaheptacenes, [22] might be stabilized in the solid state with the currents ubstituent pattern and at the same time display attractive solid-state ordering that would allow their use in ambipolar transistors.
Next issue to address was stability of the diazapentacenes compared to their hydrocarbon analogues. The stabilityo f 5,7,12,14-tetraphenylpentacene was assessed through UV/Vis measurements in dilute solution-it photooxidized in toluene [10] or dichloromethane [9] under ambient conditions (light and air) in less than 20 minutes via endo-peroxide formation. Nitrogens ubstitution protected the system. The absorption profile of alkynylated 3d remains unchanged for 24 hours, photooxidation of 3a-c depends on the electronic demando f the aryl substituents. Electron-deficient trifluoromethyl groups stabilizet he system most (14 %a bsorption loss after 24 h), but even electron-rich,d imethoxy-substituted 3a was still fairly stable (50 %l oss after 24 h).
To initially evaluate the potential of the newly synthesized tetrasubstituted diazapentacenes as n-channel organic semiconductors, organic field-effect transistors (OFETs) were fabricated by physical vapor deposition of 3b (for details regarding the device fabrication, see the Supporting Information). The compound showedn -type charge transport behavior with a maximum electron mobility of 3.2 10 À3 cm 2 V À1 s À1 ,t hreshold voltage 30 Va nd on/offr atio on the level of 10 4 .T he average charge carrierm obility calculated for twelve transistors was 1.76 10 À3 AE 0.51 cm 2 V À1 s À1 .I ncontrast, for parent unsubstituted 6,13-diazapentacene hole mobilities in ar ange of 10 À5 were reported. [3d] This finding clearly highlights the beneficial impact of the 5,7,12,14-substitution pattern on the n-channel device performance andc onstitutes an asset for our future efforts in this area.