Spiro-silacycloalkyl Tetraphenylsiloles with a Tunable Exocyclic Ring:  Preparation, Characterization, and Device Application of 1,1‘-Silacycloalkyl-2,3,4,5-tetraphenylsiloles

A series of 2,3,4,5-tetraphenylsiloles (3), with a four- to six-membered silacyclo alkyl substituent at the 1,1‘-position, have been prepared by a one-pot synthesis of dilithium diyne (2) with the corresponding silacycloalkyl dichlorosilane precursors (1). The structures of the resulting 1,1‘-(silacyclopentenyl)silole (3b) and 1,1‘-(silacyclopentyl)silole (3c) species were studied using X-ray crystallography to obtain geometrical information on exocyclic siloles. Due to the formation of silacyclopentenyl and -pentyl rings, the phenyl substituents on the silole adopted a paddle-wheel conformation to reduce steric hindrance between substituents. The photophysical properties of the silacycloalkyl siloles (3) were examined to elucidate the structure−photophysical property relationship arising from variation of the exocyclic ring size. Indeed, the size of the exocyclic silacycloalkyl ring at the 1,1‘-position affected the maximum peaks in the absorption and emission spectra, with systematic blue shifts being observed with increasing exocyclic ring size. A sequential elevation of the LUMO levels was monitored by observing increases in the reduction potential, as seen in the cyclic voltammograms (CVs), with increasing exocyclic ring size. In addition, due to the formation of exocyclic rings, an enhanced thermal stability was observed on the basis of DSC measurements, showing that silacyclopentylsilole (3c) exhibits the highest T_g value in the series. Indeed, a three-layer device comprising N,N‘-bis(1-naphthyl)-N,N‘-diphenylbenzidine (NPB) as the hole-transport layer, 3c as the emitting layer, and Alq₃ as the electron-transport layer displayed a brightness of 11 000 cd/m² at 11 V with a current efficiency of 2.71 cd/A.