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Abstract Large Stokes shift (LSS) red fluorescent proteins (RFPs) are highly desirable for bioimaging advances. The RFP mKeima, with coexistingcis‐andtrans‐isomers, holds significance as an archetypal system for LSS emission due to excited‐state proton transfer (ESPT), yet the mechanisms remain elusive. We implemented femtosecond stimulated Raman spectroscopy (FSRS) and various time‐resolved electronic spectroscopies, aided by quantum calculations, to dissect thecis‐ andtrans‐mKeima photocycle from ESPT, isomerization, to ground‐state proton transfer in solution. This work manifests the power of FSRS with global analysis to resolve Raman fingerprints of intermediate states. Importantly, the deprotonatedtrans‐isomer governs LSS emission at 620 nm, while the deprotonatedcis‐isomer's 520 nm emission is weak due to an ultrafastcis‐to‐transisomerization. Complementary spectroscopic techniques as a table‐top toolset are thus essential to study photochemistry in physiological environments. 

Abstract Control of BO₆octahedral rotations at the heterointerfaces of dissimilar ABO₃perovskites has emerged as a powerful route for engineering novel physical properties. However, its impact length scale is constrained at 2–6 unit cells close to the interface and the octahedral rotations relax quickly into bulk tilt angles away from interface. Here, a long‐range (up to 12 unit cells) suppression of MnO₆octahedral rotations in La_0.9Ba_0.1MnO₃through the formation of superlattices with SrTiO₃can be achieved. The suppressed MnO₆octahedral rotations strongly modify the magnetic and electronic properties of La_0.9Ba_0.1MnO₃and hence create a new ferromagnetic insulating state with enhanced Curie temperature of 235 K. The emergent properties in La_0.9Ba_0.1MnO₃arise from a preferential occupation of the out‐of‐plane Mnd_3z²_−r²orbital and a reduced Mn e_gbandwidth, induced by the suppressed octahedral rotations. The realization of long‐range tuning of BO₆octahedra via superlattices can be applicable to other strongly correlated perovskites for exploring new emergent quantum phenomena.