Design and synthesis of thiophene-engineered phenanthroimidazoles with vinyl functionality for polymer integration

Abstract

We report the first systematic design of thiophene-engineered phenanthro[9,10-d]imidazoles (PPIs) that combine C2 sulfur-containing π-aryl substitution with N1 styryl functionality, enabling direct polymer integration. Thiophene, benzo[b]thiophene, and thieno[3,2-b]thiophene fragments were introduced at the C2 position to control the electronic structure of the phenanthroimidazole chromophore, while a vinyl group at the N1-aryl fragment provides a polymerizable handle for covalent incorporation into polymer matrices. The target compounds were synthesized via a modular strategy based on Debus-Radziszewski condensation, followed by Suzuki-Miyaura cross-coupling and were comprehensively characterized by spectroscopic, structural, and thermal methods. Density functional theory (DFT) calculations were performed to elucidate the frontier molecular orbital distribution and support the experimentally observed optical properties. Photophysical investigations reveal that C2 sulfur-containing substitution provides an efficient handle for tuning emission properties, leading to gradual bathochromic shifts and controlled modulation of the optical band gap while preserving structured blue to blue-green fluorescence with photoluminescence quantum yields reaching up to 86%. In contrast, the N1 styryl functionality remains electronically decoupled from the emissive core, allowing polymerization without significantly perturbing the intrinsic optical properties of the chromophore. Radical copolymerization with N-vinylcarbazole affords homogeneous luminescent copolymers whose solid-state emission closely mirrors the behavior observed in solutions. Overall, this work establishes a molecular design strategy in which electronic tuning of the phenanthroimidazole chromophore and polymer processability are effectively decoupled within a single scaffold, providing access to processable luminescent polymers with predictable and tunable optical properties.