  Recently developed electronic pre-resonance stimulated Raman scattering
(epr-SRS) microscopy, in which the Raman signal of a dye is significantly
boosted by setting the incident laser frequency near the electronic excitation
energy, has pushed the sensitivity of SRS microscopy close to that offered by
confocal fluorescence microscopy. Prominently, the maintained narrow line-width
of epr-SRS also offers high multiplexity that breaks the "color barrier" in
optical microscopy. However, detailed understandings of the fundamental
mechanism in these epr-SRS dyes still remain elusive. Here, we combine
experiments with theoretical modeling to investigate the structure-signal
relationship, aiming to facilitate the design of new probes and expanding
epr-SRS palettes. Our ab initio approach employing the displaced harmonic
oscillator (DHO) model provides a consistent agreement between simulated and
experimental SRS intensities of various triple-bond bearing epr-SRS probes with
distinct scaffolds. We further review two popular approximate expressions for
epr-SRS, namely the short-time and Albrecht A-term equations, and compare them
to the DHO model. Overall, the theory allows us to illustrate how the observed
intensity differences between molecular scaffolds stem from the coupling
strength between the electronic excitation and the targeted vibrational mode,
leading to a general design strategy for highly sensitive next-generation
vibrational imaging probes.
