Identification of Lithocholic Acid as a Molecular Glass Host for Room‐Temperature Phosphorescent Materials

Lithocholic acid was identified as a molecular glass host material for room temperature phosphorescent (RTP) chromophores. Differential scanning calorimetry (DSC) was performed on a series of structurally similar, biologically sourced molecules, including lithocholic acid, beta-estradiol, cholesterol, and beta-sitosterol, in an effort to identify new amorphous molecular glasses independent of plasticizing additives. DSC analysis revealed lithocholic acid and beta-estradiol form stable molecular glasses post thermal processing unlike neat cholesterol and beta-sitosterol. The ability of lithocholic acid and beta-estradiol to stabilize high wt. % loadings of d(10)-pyrene and a mixture of d(10)-pyrene and an iridium chromophore, bis(2,4-difluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III) (FIr6), was also investigated. All beta-estradiol formulations show beta-estradiol cold crystallization. Optical microscopy and wide angle X-ray scattering measurements suggest spherulite beta-estradiol crystals form during this process. Finally, time-resolved luminescence and phosphorescence quantum yield experiments show that the d(10)-pyrene RTP lifetime is longer and the d(10)-pyrene phosphorescence quantum yield is higher in lithocholic acid molecular glasses than in beta-estradiol molecular glasses. The discrepancy in lifetime and quantum yield values is explained by quantitatively smaller rates of non-radiative decay from the triplet state of d(10)-pyrene in lithocholic acid.