Direct Readout of Excited-State Lifetimes in Chlorin Chromophores under Electronic Strong Coupling

The mechanisms governing molecular photophysics under electronic strong coupling (ESC) remain elusive to date. Here, we use ultrafast pump–probe spectroscopy to study the excited-state relaxation dynamics of chlorin e6 trimethyl ester (Ce6T) under strong coupling of its transition from the electronic ground state to the Q_y band. Ce6T is a compelling testbed with which to address open questions about excited-state lifetimes under ESC given prior reports of cavity-altered behavior in chlorins. We use dichroic Fabry-Pérot cavities to provide a transparent spectral window in which we can directly track the excited-state population following the optical pumping of either the strongly-coupled Q_y band or the higher-lying B band. This scheme circumvents many of the optical artifacts inherent in ultrafast cavity measurements and allows for facile comparison of strongly-coupled measurements with extracavity controls. We observe no significant changes in excited-state lifetimes for any optical pumping schemes or cavity-coupling conditions considered herein. These results suggest that Ce6T exhibits identical photophysics under ESC and in free space, presenting a new data point for benchmarking emerging theories for cavity photochemistry.