Enhanced Charge-Carrier Mobility Derived from Cyclization of a Silanylene Unit on Dithienosiloles: Syntheses, Photophysical Properties, and Device Fabrication of Dithieno-spiro-siloles

A series of trimethylsilyl-substituted dithieno-spiro-siloles (3), with a four- to six-membered silacycloalkyl substituent at the 1,1-position, were prepared by reacting 3,3′-dilithio-5,5′-bis(trimethylsilyl)-2,2′-dithiophene (2) with the corresponding silacycloalkyl dichlorosilane precursors (1). Arylamino-substituted bis(diarylamino)dithieno-spiro-siloles (6) were also prepared using the same synthetic protocol by reacting 3,3′-dilithio-5,5′-bis(diarylamino)-2,2′-dithiophene (5) with 1. A structural study of the five-membered dithieno-spiro-silole, 1,1-(silacyclopentenyl)dithieno-spiro-silole (3b), was undertaken and showed a reduced intermolecular distance in the solid state. This resulted in enhanced charge-carrier mobility, which was confirmed by a time-of-flight (TOF) measurement of 6b. The electronic properties of dithienosiloles (6) were studied for device fabrication by applying them as emitting materials in multilayer devices. Thus, I−V characteristics of multilayer devices, comprising N,N′-bis(1-naphthyl)-N,N′-diphenylbenzidine (NPB) as the hole-transport layer, 6 as the emitting layer, 2,9-dimethyl-4,7-diphenylphenanthroline (BCP) as the hole-blocking layer, and tris(8-quinolinato)aluminum (Alq3) as the electron-transporting layer, showed that the dithieno-spiro-siloles (6b−d) exhibited turn-on voltages (2.2−2.8 V) lower than those of optimized dithienosilole (2Ph-NPB) (4.3 V). Furthermore, systematic blue shifts in both UV and PL spectra were observed in the bis(trimethylsilyl)dithieno-spiro-silole series (3) as the size of the exocyclic silanylene ring was increased, whereas bis(diarylamino)dithieno-spiro-siloles (6) exhibited only a small variation along the series due to extensive conjugation from the peripheral diarylamine to the dithienosilole core. This blue shift is attributed to elevation of the LUMO level on the basis of a DFT calculation on 3, along with UV spectral data and cyclic voltammograms.