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]