Photoluminescence and electroluminescence of oligoacetylenic silanes and germanes

A series of light-emitting oligoacetylenic silanes and germanes containing fluorene units was investigated. It was found that the higher oligomers display lower band gaps and that the CSi(Ge)C bond permits π-electron delocalization. The π-d_Si or π-d_Ge interactions aid the π-electrons in passing through the Si (Ge) atoms. The observed differences in fluorescence spectra may also result from a substituent effect of the silyl groups. The SiSi σ bond also permits π-electron delocalization and thus there is a through-bond interaction between π orbitals and the SiSi bond. The conjugation-interrupting Ge sp³ link aids π-electron delocalization along the oligomer backbone less efficiently than does a Si link, thus leading to relatively shorter effective conjugation lengths and higher band gaps. In the higher oligomers, multiple π-d_Si or π-d _Ge interactions permit the vibrational levels of the excited state to split to form sublevels observable as new transitions in the respective absorption spectra. FWHM values of the PL spectra increase with increase of the oligomer backbone length which relates to the increase of the intermolecular interaction between the chromophore units. With increasing oligomer backbone length, the EL efficiency decreases, which can be attributed to the enhanced probability of interchain exciton annihilation in LEDs using higher oligomers.