Heavy-element-free triplet accessibility in pyrene-core compounds at room temperature by microcrystal engineering

Abstract

The three possible emissive states of pyrene and its aggregates can be effectively deployed by engineering the molecular structures through the synthesis of three different donor–acceptor (D–A) pyrene derivatives. The donor units were designed to produce different emissive states based on the planarity of the resultant molecule and the local electron density. The molecular designs imparted control over the excited state dynamics to access the triplet states, which was achieved without using any heavy element or component in the system. Spectroscopic and single crystal structure analysis revealed that J aggregation of the associated D–A architecture, as well as reduction of the π–π interaction, play important roles in generating long-lived room temperature phosphorescence (RTP) in crystals. A micelle-assisted microcrystallization strategy was developed to achieve pure organic RTP in water. The phosphorescence lifetimes at RT from the macro- and microcrystals of CzPyCHO are 190 and 160 μs, and those of PhPyCHO are 173 and 161 μs, respectively. These can be enhanced, at 78 K, to 1.82 and 1.74 ms, respectively (CzPyCHO) and 1.36 and 1.76 ms, respectively (PhPyCHO). Formation of hexagonal and rectangular microcrystals could be the reason behind the improved photoluminescence quantum yield (40 and 20% for the macrocrystals and 61 and 83% for the microcrystals of CzPyCHO and PhPyCHO, respectively) as well as the generation of RTP in water.