Rational Design of a Triplet Afterglow Sensitizer Allowing for Bright Long-Wavelength Afterglow Room-Temperature Emission

An efficient and bright red afterglow emission with a lifetime of more than 100 ms is essential for the detection of emission signals independent of autofluorescence from targets in deep locations under in vivo conditions. However, the afterglow room-temperature (RT) emission yield at wavelengths over 600 nm is less than 5.5%. Here, we report a rational design for triplet sensitizers to enable high-efficiency red afterglow RT emission. Two triphenylene derivatives with amino-conjugated substituents were synthesized in the form of triplet sensitizers to induce red afterglow RT emission from a fluorescent acceptor. The material that contained the sensitizer with the longer conjugated substituent and the acceptor produced red afterglow RT emission with a yield of 8.5% for wavelengths exceeding 600 nm. The long conjugated amino-substituent hardly enhanced the fluorescence resonance energy transfer from the sensitizer to the acceptor, causing the red afterglow RT emission yield to decrease. However, the long conjugated substituent greatly enhanced the phosphorescence resonance energy transfer (PRET) from the sensitizer to the acceptor. A large increase in the transition dipole moment between the high-order singlet excited states and the ground state was caused by the long conjugated amino-substituent and was a key factor in enabling the sensitizer to produce selective large-scale enhancement of the PRET for enhanced red afterglow RT emission.