Thermodynamic and hydrodynamic fluorescence emission behaviors of polydiphenylacetylenes with different size of substituents

Abstract

Four polydiphenylacetylenes (PDPAs: C3, C6, C9, C12) containing different size of trialkylsilyl groups (C3: trimethyl; C6: triethyl; C9: trin-propyl; C12: trin-butyl) were prepared and their thermodynamic and hydrodynamic fluorescence (FL) emission behaviors were investigated and discussed in relation to physicochemical properties such as fractional free volumes (FFVs) and solubility parameters. These polymers existed in glassy state at room temperature to provide quite tough films. Also, the polymers were amorphous and the intermolecular distance increased with the molecular size of substituent. As the substituent became larger, the FFV of the polymer decreased and the solubility parameter increased. At the same time, the FL quantum efficiency (FLQE) increased both in the toluene solution and film. Exceptionally, however, C3 exhibited a quite high FLQE in solution despite the smallest substituent because of the lower affinity of the polymer with the solvent. When the polymers were heated both in solution and film, the FL change according to temperature became less significant as the substituent became larger. Even in this event, C3 did not follow the rule. C3 exhibited the least significant FL change in film despite the smallest substituent owing to the dense space occupied by the polymer main chains as known from the largest FFV and the shortest intermolecular distance. On the other hand, when the polymers were dissolved in hydrocarbons or swollen in alcohols, the FL change according to viscosity of the solvents became less significant as the substituent became larger. However, as with the thermodynamic FL emission behavior, C3 film showed an exceptionally less remarkable FL change despite the smallest substituent owing to the much larger difference in solubility parameter between the polymer and alcohol.