Chiral cocrystals with circularly polarized persistent phosphorescence and large second harmonic generation

Abstract

Despite cocrystal strategy is an effective method for preparing room-temperature phosphorescence (RTP) materials, chiral cocrystals with high phosphorescent quantum yields and large asymmetry factors are still limited. Herein, three pairs of chiral cocrystals, namely R/S-IPA, R/S-TPA and R/S-PMA, were prepared by the assembly of IPA / TPA / PMA (IPA = isophthalic acid, TPA = terephthalic acid, PMA = pyromellitic acid) with R/S-THNA (R/S-THNA = R/S-1,2,3,4-tetrahydro-1-naphthylamine). All the cocrystals crystallize in the chiral space group and the IPA / TPA / PMA and THNA molecules are connected to each other by hydrogen bonds and π···π or / and CH···π interactions. Their chiral nature was further confirmed by second harmonic generation (SHG) measurements and R/S-IPA show strong SHG response of about 1.91 / 1.88 times that of KDP. All the compounds exhibit afterglow when the UV lamp was turned off and their lifetimes vary from 479.5 ms to 36.1 ms. The phosphorescent quantum yield of R-PMA is up to 30.5 %, which is higher than that of most cocrystals with RTP. Both circularly polarized luminescence (CPL) and circularly polarized phosphorescence (CPP) were observed in the CPL spectra of all the compounds and their luminescence asymmetry factors (g_lum) reach the order of 10^–2. Based on the different afterglow lifetimes of these products, anticounterfeiting and information encryption were realized. Transparent thin films of the cocrystals were prepared by blending them with polyvinyl alcohol (PVA) and their flexibility and workability were assessed by processing them into different shapes.