Photophysical properties and DFT analysis of no-σ* hyperconjugated polysilole siloxane for TNT sensing

Polysilole and polysilole siloxane with distinct backbone conjugation modes were investigated to elucidate the relationship between polymer structure, aggregation-induced emission enhancement (AIEE), and fluorescence sensing of 2,4,6-trinitrotoluene (TNT). Polysilole contains a Si–Si-linked backbone exhibiting σ-σ* conjugation, whereas polysilole siloxane contains Si–O–Si linkages associated with n o -σ* hyperconjugation. Both polymers exhibited weak fluorescence in dilute THF solution but pronounced emission enhancement upon aggregation in THF/water mixtures. At a water fraction of 99%, the photoluminescence quantum yields increased from 2.7 to 66% for polysilole and from 0.2 to 10.2% for polysilole siloxane, corresponding to approximately 24- and 51-fold enhancements, respectively. Dynamic light scattering measurements further confirmed the formation of polysilole siloxane nanoaggregates, with the average hydrodynamic diameter decreasing from approximately 270 nm at f w = 90% to 45 nm at f w = 99%. TNT induced concentration-dependent fluorescence quenching in both solution and aggregate states. The Stern–Volmer constant of polysilole siloxane increased from 2.0 × 10 3 M −1 in THF to 1.0 × 10 4 M −1 in the nanoaggregated state, demonstrating an approximately fivefold enhancement in sensing response upon aggregation. The estimated limit of detection (LOD) for TNT decreased from 4.04 μM in THF solution to 0.685 μM in the nanoaggregated state, corresponding to an approximately 5.9-fold improvement upon aggregation. Selectivity measurements showed a weaker fluorescence-quenching response toward DNT and negligible quenching by benzoquinone, indicating a preferential response toward TNT among the analytes examined. Time-resolved fluorescence measurements showed no systematic concentration-dependent shortening of the fluorescence lifetime upon TNT addition, supporting a predominantly static contribution to the quenching process. Density functional theory calculations revealed distinct frontier-orbital distributions for the two polymer backbones, with more pronounced electronic delocalization along the Si–Si-linked polysilole framework than along the Si–O–Si-linked polysilole siloxane. GPC analysis gave M n / M w values of 4.89/5.42 kDa (Đ = 1.11) for polysilole and 5.28/6.35 kDa (Đ = 1.20) for polysilole siloxane, corresponding to average degrees of polymerization of approximately 13. These results establish a direct relationship between polymer backbone linkage, electronic delocalization, aggregation behavior, and fluorescence-quenching response in tetraphenylsilole-based polymers.