  Establishing the fundamental chemical principles that govern molecular
electronic quantum decoherence has remained an outstanding challenge.
Fundamental questions such as how solvent and intramolecular vibrations or
chemical functionalization contribute to the decoherence remain unanswered and
are beyond the reach of state-of-the-art theoretical and experimental
approaches. Here we address this challenge by developing a strategy to isolate
electronic decoherence pathways for molecular chromophores immersed in
condensed phase environments that enables elucidating how electronic quantum
coherence is lost. For this, we first identify resonance Raman spectroscopy as
a general experimental method to reconstruct molecular spectral densities with
full chemical complexity at room temperature, in solvent, and for fluorescent
and non-fluorescent molecules. We then show how to quantitatively capture the
decoherence dynamics from the spectral density and identify decoherence
pathways by decomposing the overall coherence loss into contributions due to
individual molecular vibrations and solvent modes. We illustrate the utility of
the strategy by analyzing the electronic decoherence pathways of the DNA base
thymine in water. Its electronic coherences decay in ~ 30 fs. The early-time
decoherence is determined by intramolecular vibrations while the overall decay
by solvent. Chemical substitution of thymine modulates the decoherence with
hydrogen-bond interactions of the thymine ring with water leading to the
fastest decoherence. Increasing temperature leads to faster decoherence as it
enhances the importance of solvent contributions but leaves the early-time
decoherence dynamics intact. The developed strategy opens key opportunities to
establish the connection between molecular structure and quantum decoherence as
needed to develop chemical strategies to rationally modulate it.
