Considering that 1 has three successive single bonds in the diketone core, identifying the RTP-emitting conformation was important. We reported previously that a 1b crystal had a skew conformation and emitted green RTP, which was different from the solution RTP. 15a Gratifyingly, crystals of 1c exhibited yellow RTP with a 50% p in air (Figure ). The RTP as a single exponential with a p of 79 s, and kp was 6000 s -1 (Figure ). Most importantly, the PL spectrum was almost identical to that of 1c in solution, indicating emission from the same conformer. Single-crystal Xray structure analysis revealed a trans-planar (TP) conformation of 1c, with the thienyl diketone core spatially separated from neighboring molecules by tributylsilyl substituents (Figures and). 21 Thus, the RTP from the 1c crystal, as well as 1 in solution, was unambiguously assigned to the monomer emission from its TP conformer.