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.