TICT excited states of o‐ and p‐N,N‐dimethylamino analogues of GFP chromophore

Even though all the p‐N,N‐dimethylaminobenzonitrile (p‐DMABN), cis‐o‐DMABDI, and cis‐p‐DMABDI (the N,N‐dimethylamino analogues of green fluorescence protein chromophore) have the same electron‐donating N,N‐dimethylamino group, unlike the dual fluorescence of p‐DMABN, both cis‐o‐DMABDI and cis‐p‐DMABDI display single fluorescence. To figure out the interesting phenomena, the CAM‐TD‐B3LYP method and the cc‐pVDZ basis set were used to explore geometries, molecular orbitals, electronic transition, dipole moment, and potential energy surfaces of the S₁ excited states of cis‐o‐DMABDI and cis‐p‐DMABDI. We found that the S₁ excited states of cis‐o‐DMABDI and cis‐p‐DMABDI are ¹(π, π*) charge transfer excited states with twisted structures, where the N,N‐dimethylaminobenzene moiety functions as an electron donor, the methyleneimidazolone moiety serves as an electron acceptor, and the electron donor is linked with the electron acceptor by the C─C single bond (P‐bond). The fluorescent emissions of cis‐o‐DMABDI and cis‐p‐DMABDI predicted by the CAM‐TD‐B3LYP/cc‐pVDZ level are quite consistent with the experimental results. For the cis‐o‐DMABDI and cis‐p‐DMABDI, the S₁ locally excited state is less stable than the S₁ twisted intramolecular charge transfer state, and the S₁ LE state is not a stationary point (global minimum). That is why both cis‐o‐DMABDI and cis‐p‐DMABDI display single fluorescence.