The role of ring-type structures in nonconventional luminescent polyurethane derivatives†

Nonconventional luminescent materials have been extensively studied in recent years. However, the ambiguity of their luminescence mechanism seriously hinders their development and the establishment of detailed structure–activity relationships. Based on this background, polyurethane derivatives (PUs) with different photophysical behaviors were synthesized by introducing into the backbone either a spirobicyclic structure with an orthogonal conformation of two acetal rings (PU1) or a monocyclic acetal structure giving a stretched linear conformation (PU2 and PU3). Detailed experimental and theoretical calculations show that the spirocyclic acetal in PU1 imparts an orthogonal conformation, reducing the intermolecular contacts, making molecules more inclined to form intramolecular interactions, which are detrimental to the intermolecular interactions and the aggregation behavior. The acetal ring in PU2/PU3 gives stretched conformations, enabling more intermolecular interactions. Meanwhile, high-temperature synthesis facilitates molecular aggregation, which indirectly red-shifts the emission. The proof-of-concept applications in cells and as luminescent inks are reported. Overall, this work contributes new understanding of the significance of ring-type structures without π electrons and gives a new perspective for the design of nonconjugated luminescent polymers.