Reverse Intersystem Crossing of Single Deuterated Perylene Molecules in a Dibenzothiophene Matrix

Intersystem crossing to the long‐lived metastable triplet state is often a strong limitation on fluorescence brightness of single molecules, particularly for perylene in various matrices. In this paper, we report on a strong excitation‐induced reverse intersystem crossing (rISC), a process where single perylene molecules in a dibenzothiophene matrix recover faster from the triplet state, turning into bright emitters at saturated excitation powers. With a detailed study of single‐molecule fluorescence autocorrelations, we quantify the effect of rISC. The intrinsic lifetimes found for the two effective triplet states (8.5±0.4 ms and 64±12 ms) become significantly shorter, into the sub‐millisecond range, as the excitation power increases and fluorescence brightness is ultimately enhanced at least fourfold. Our results are relevant for the understanding of triplet state manipulation of single‐molecule quantum emitters and for markedly improving their brightness. Enhanced brightness of single molecules: Single perylene molecules immobilized in a dibenzothiophene host crystal give rise to narrow optical resonances below 4 K. The single emitters show clear blinking due to long‐lived triplet states. However, the blinking times shorten considerably as the probe laser's intensity increases. The observations are explained by a power‐dependent depopulation of the triplet states via reverse intersystem crossing (rISC), which significantly improves the brightness of the molecules at saturated excitation intensities.