Rigid-Flexible Skeletons and Methyl Steric Hindrance Mediate Narrow-Range Spectral Tuning of Clusteroluminescence in Nonconjugated Polyurethanes

Abstract

A set of nonconjugated polyurethanes with precisely controllable clusteroluminescence were synthesized via tuning backbone rigidity and steric hindrance introduced by methyl substituents. Rigid-flexible molecular skeletons and steric repulsion work together to modify chain conformation, intermolecular hydrogen bonds, aggregation morphologies and photophysical properties. Incorporation of alicyclic segments strengthens through-space electronic coupling, reduces critical aggregation concentration, and shifts the emission from blue to green, with obvious excitation-dependent luminescence observed. Combined two-dimensional (2D) correlation infrared spectra and molecular dynamics simulations prove that dynamic hydrogen-bond networks and conformational changes control luminescence variations originating from distinct molecular structures. Theoretical computations show that mixed local excitation and charge-transfer transitions, coupled with σ–π* interactions and hydrogen bonds, jointly regulate emission wavelength and photoluminescence quantum yield. Featuring one blue emitter and three green emitters with slight spectral discrepancies, these polyurethane samples act as functional anticounterfeiting units to build a multistage encryption platform combining QR codes, ASCII binary codes, Morse code and Caesar cipher, which supports the encryption and decryption of concealed information. This study elaborates the structure–property relationship of nonconjugated clusteroluminescent polyurethanes and puts forward a practical route to realize narrow-band fine spectral regulation of luminescent polymers.