Oxygen-Insensitive Delayed Fluorescence Based on Singlet Manifold

Delayed fluorescence molecules have important applications in time-resolved luminescence imaging (including lifetime imaging) systems. However, existing approaches of generating delayed fluorescence in organic molecules rely on triplets as long-lived intermediate states. As such, this triplet-based delayed fluorescence can be severely plagued by its oxygen sensitivity. As the presence of oxygen or ambient air can hardly be avoided in living biosystems, oxygen-insensitive delayed fluorescence is highly desirable. Herein, a new pathway is reported for oxygen-insensitive delayed fluorescence which is produced only within the singlet manifold. Such an example is found in a cyto-compatible and water-soluble trimethine cyanine dye C3T-H, which bears a very high molar extinction coefficient and a narrow-band red emission. It is demonstrated that this dye exhibits notable delayed fluorescence which cannot be diminished by the presence of oxygen or ambient air. Time-resolved imaging of living cells in the presence of oxygen is successfully achieved. Through quantum chemical calculations, a nonemissive dark-state structure in the S-1 state is found to be thermodynamically viable (energy barrier approximate to 8 kcal mol(-1)) through the molecular twisting around the trimethine bridge of C3T-H, which serves as a reservoir of the excitation energy to enable the oxygen-insensitive delayed fluorescence.

Automated summary

Delayed fluorescence molecules have important applications in time-resolved luminescence imaging systems. However, current approaches using triplets as intermediate states are oxygen-sensitive. In this study, a new pathway for oxygen-insensitive delayed fluorescence, produced within the singlet manifold, is reported. The study demonstrates successful time-resolved imaging of living cells in the presence of oxygen.