Naphtho[2,3-b]thiophene diimide (NTI): a mono-functionalisable core-extended naphthalene diimide for electron-deficient architectures

Naphthalene diimide (NDI, Fig. 1), one of the imide-based p-extended structures, has been widely utilized in chromophores, supramolecular architectures, and electronic materials. One of the effective ways to control the opto/electronic properties of NDI is to extend the p-system with fused aromatic or heteroaromatic ring(s), which affords core-extended NDIs, often referred as cNDIs. Whereas most of such cNDIs have two fused-(hetero)aromatic rings on the naphthalene moiety, only a limited number of mono-(hetero)aromatic fused cNDIs have been reported, f, j,k and their utility as building blocks for opto/electronic materials has not been well explored. This is due to the difficulty of selective mono-functionalisation on the NDI core, which is in sharp contrast to perylene diimide (PDI) that has been extensively incorporated into various molecular architectures. For the elaboration of cNDI-based materials, it is desirable to develop mono-functionalisable cNDI-based building blocks. To this end, we have focused on mono-thiophene-fused NDI, namely naphtho[2,3-b]thiophene diimide (NTI, Fig. 1), since the thiophene a-position is a versatile handle for structural modifications. In fact, we have recently reported a cNDI with two fused-thiophene rings, i.e., naphtho[2,3-b:6.7-b0]dithiophene diimide (NDTI, Fig. 1), which has been proved useful in developing NDTI-based conjugated oligomers and polymers. We here first report the synthesis and characterization of NTI, and then its utilization to develop a range of NTI-terminated molecular architectures, such as the dimer (2), NTI-based triads (3, 4), and a dendritic trimer with a benzene core (5), which acted as n-type organic semiconductors (Fig. 1). For the synthesis of 1, we first prepared N,N0-dioctyl-2-(trimethylsilyl)ethynyl-NDI (7a) from the corresponding 2-bromo-NDI (6a, see the ESI†). As depicted in Scheme 1, 7a was effectively converted into the NTI structure with a trimethylsilyl (TMS) group at the thiophene a-position (8a) in 65% isolated yield via a sodium sulphide-promoted thiophene annulation reaction. Fig. 1 Chemical structures of naphthalene diimide (NDI), naphtho[2,3-b]thiophene diimide (NTI, 1), naphtho[2,3-b:6,7-b0]dithiophene diimide (NDTI), and NTI-terminated molecules (2–5).