Two excited state structures of Donor-Acceptor substituted "proton sponge"

Photoexcitation of "proton sponge" 1,8-bis (dimethylamino) naphthalene (DMAN) leads to charge transfer (CT) emission. This work demonstrates that substitution of DMAN with a strong electron acceptor group (CN) results in CN-DMAN and leads to a stronger orbital decoupling between the Donor and Acceptor groups and to a more effective CT process (lambda(F) = 630 nm). The theoretical absorption spectrum calculated for CN-DMAN using the ZINDO method on its ground state ab initio [HF/6-31 G(d)] optimized geometry reproduces a better experimental spectrum than that calculated using the RCIS method. It also shows that AMI excited state (L-1(a)) optimization reveals two quasi-degenerated states with anti-quinoidal (A) and quinoidal (B) structures and the CT nature. Both structures may contribute more or less equally to the fluorescence of CN-DMAN in a solution.