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.
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