Does the wavelength dependent photoisomerization process of the p‑coumaric acid come out from the electronic state dependent pathways?

Similar to the anion photoactive yellow protein (PYP) chromophore, the neutral form of the PYP chromophore was also found to exhibit a the wavelength-dependent photoisomerization quantum yield. The isomerization quantum yield increases with the increasing excitation energy on the S1 state, while decreases when being excited to the S2 state. Does this wavelength dependent product yield come out from the specific reaction pathways of the S1 and S2 states? This would mean that, the relaxation pathway of the S2 state is distinct from that of the S1 state and does not involve twisting motion. Does it break Kasha's rule by exhibiting a direct transition from the S2 state to the ground state? The underlying mechanism needs further in. In this article, we employed the on-the-fly dynamics simulations and static electronic structure calculations to reveal the deactivation mechanism of the neutral form of the PYP chromophore. Our results indicated that the CC twisting motion dominates the S1 state decay process. In contrast, for the decay process of the S2 state, an ultrafast transition from the S2 to the S1 state through a planar conical intersection is observed, and the excess energy activates a new reaction channel to the ground state characterized by a puckering distortion of the ring. This pathway competes with the photoisomerization channel. No direct transition from S2 to S0 is observed, hence Kasha's rule is valid for this process. Our calcualtions can provide a reasonable explanation of the wavelength-dependent isomerization quantum yield of neutral PYP chromophore, and we hope it can provide theoretical foundations for comparing the effect of protonation state on the dynamcal behaviors of PYP chromophore.