Unexpected full-color luminescence produced from the aggregation of unconventional chromophores in novel polyborosilazane dendrimers.

Non-conjugated fluorescent polymers (NCPLs) are of interest due to their remarkable biocompatibility, processability and biodegradability. However, the realization of multicolor emitting NCPLs through structure modulation remains a great challenge. In this work, a series of novel yttrium-branched polyborosilazane (PBSZ) structures (PY1-PY3) were prepared. PBSZ exhibits a blue emission peaked at 450 nm, and the introduction of an yttrium-branched-chain generates a new long-wavelength emission center. As the degree of yttrium branching increases, the emerged emission peak shifts from 532 to 646 nm, and its intensity gradually increases to 1.4 times that of the blue emission. CIE chromaticity coordinates indicate that yttrium-branching modulates the emission color from blue (0.19, 0.21) to near white (0.34, 0.40) and red (0.43, 0.36). Particularly, the PY3 sample exhibits an ultra-broad emission spectrum; covering the range of 400-750 nm. Theoretical calculation indicates that the yttrium-branched-chains promote heteroatom delocalization to form "cluster chromophores", generating new orbitals with lower gaps. In addition, experimental results prove that the yttrium-branched-chains balance the flexibility of the molecular backbone and generate stable fluorescent clusters, which intensifies the non-conjugated linking and through-space-conjugation (TSC) effects, thus generating long-wavelength emission. This work proves that yttrium "end-grafting" is a feasible strategy for equilibrium flexibility and to realize full-color emission in non-conjugated polymers.