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 Qy 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 Qy 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.