Large Stokes-Shifted Photoluminescence of Sulfur-Containing Imide Compounds and Polyimides Induced by Transient Conformational Changes upon Excitation

The optical absorption and photoluminescence (PL) properties of a series of sulfur-containing semialicyclic polyimides (S-PIs) were examined to develop thermally stable functional polymers with high glass transition temperatures, high optical transparencies, and large Stokes-shifted PL. These S-PIs were synthesized using tetracarboxylic dianhydrides, including thioether (-S-) linkages, such as 4,4-thiodiphthalic anhydride (SDPA), 4,4'-[p-phenylenebis(thio)]diphthalic anhydride (2SDEA), 4,4'-[p-thiobis(phenylene sulfanyl)] diphthalic anhydride (3SDEA), and <i>trans</i>-1,4-cyclohexanediamine (1,4-<i>t</i>DACH). In addition, the optical and PL properties of three types of imide model compounds (MCs) were investigated. All the S-PI films exhibited blue or green fluorescence (FL) under ambient conditions (in air at 295 K), weak room temperature phosphorescence (RTP) under vacuum, and bright green phosphorescence (PH) at lower temperatures (∼77 K). The intersystem crossing (ISC) rates at 77 K were significantly higher than those of the polyimides containing diphenyl-ether (-O-) linkages, primarily due to the enhancement of ISC by the heavy-atom effect of the sulfur atoms. MCs with multiple thioether (-S-) linkages exhibited a peculiar green or greenish-yellow FL with very large Stokes shifts in the CHCl₃ solution. This behavior was due to the transient conformational change from the kinked structure in the ground state to a skewed structure in the excited singlet state. In addition, all the MCs dispersed in a poly(methyl methacrylate) matrix exhibited distinctive RTP owing to the restricted conformational changes in the rigid matrix. These results provide effective guidelines for designing polymers exhibiting large Stokes-shifted FL and PH at longer wavelengths in the visible region.