1.2.1 Si-O-C consisted linear polysiloxanes
Generally, linear polysiloxanes consist of Si-O-C chain segment are nonemissive or faintly emissive due to the low local density of electron-rich atoms. To our delight, the inset of disulfide bond makes it boosted luminescence owing to the chain distortion. Disulfide bond is a kind of dynamic covalent bond with low electronegativity. On the one hand, it shows weak binding capacity to outer shell electrons and beneficial to reduce HOMO-LUMO energy gap. On the other hand, the disulfide bonds can increase the distortion of polymer chains by the small bond angel. Our group has achieved multicolor fluorescence in linear polysiloxane by introducing disulfide bond and carbonyl groups. The synthesised linear polysiloxane can be excited by visible light (λEx=508 nm) and emit multicolor fluorescence under different excitation wavelength.[49] The emission behavior of the linear polysiloxane is further explained by DFT and TD-DFT. In Figure 5A, multiring TSCs with different sizes were found in disulfide bond-containing linear polysiloxane via Si-O and S-S bridges. The multicolor emission can be well deciphered in view of the MIE mechanism. The flexible Si-O-C chain and the disulfide bond can increase the distortion of main chains, thus shorten the distance between electron-rich groups. Then, the multiring TSC is generated owing to the decreased distance between Si-O, C=C, C(O)O, and S-S groups. The disulfide bond also reduces excitation energy, increases the dipole moment of linear polysiloxanes and rigidifies the conformation, thus favoring visible-light excitation and suppressing nonradiative transitions.[49]
By adjusting the distribution of local conjugated chains, our group further designed two linear polysiloxanes by introducing carbonyl (C=O) and vinyl groups (C=C) and proposed a new emission mechanism named “local conjugation enhanced multicolor emission”.[50] In LPSi-1, the longer local conjugated chain segment (C=C-Si-O-C=O) is formed by the Si-O bond linked carbonyl and vinyl groups. The shorter local conjugation (Si-O-C=O) is fabricated in LPSi-2 as comparison. Spectra study demonstrated that the fluorescent intensity of LPSi-1 is much higher than that of LPSi-2, and LPSi-1 exhibited obvious polychromatic luminescence from blue, cyan, green to red. DFT further indicated that the local conjugation plays key roles in their luminescence behavior. As shown in Figure 5D, the longer local conjugation leads to multiple TSC rings with different sizes in LPSi-1, which facilitated the multicolor emission. While the TSC rings formed in LPSi-2 are uniform with decreased through-space electronic communications, therefore only weaker blue and cyan emissions were observed.[50]