Theoretical Study of Intramolecular Charge Transfer Turn-off Switching of N-(4-Dimethylaminobenzoyl) Thiourea Induced by Anion Recognition

Abstract

The development of colorimetric and fluorescence chemical sensors that have a function of in situ selective sensing of biologically important anions is one of the significant issues in host-guest chemistry, and a large number of studies have been carried out in the field. The fluorescent chemical sensor, N-(4-Dimethylaminobenzoyl)thiourea (DMABTU; Chart 1) that has been reported by Wu et al. [1], has two emission wavelengths at ca. 500 nm (stronger) and ca. 350 nm (weaker)) in the absence of anions in chloroform solution. By the addition of anions such as acetate, the emission band at 500 nm decreases while the emission band at 350 nm increases. The broad emission band at around 500 nm was assigned to an intramolecular charge transfer (ICT) transition, and the narrow emission band at around 350 nm to a locally excited (LE) transition. However, a detailed theoretical analysis has not been performed yet on the turn-off switching mechanism of ICT induced by the anion recognition. Therefore, in this work, to clarify the ICT turn-off switching mechanism, we carried out a density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on DMABTU in the ground (S0) and excited (S1) states. We obtained optimized structures of DMABTU-X−(X− = free, acetate) in S0 and S1, and also obtained potential-energy curves (PEC) with respect to a dihedral angle δ between the dimethylamino-group and phenyl-group planes. From the calculations, it was proved that the PEC of DMABTU is not quadratic but nearly flat between δ = 0∼10 degrees, and the potential minimums are located at δ = ca. 8∼9 degrees both in S0 and S1, while the PEC of DMABTU-AcO- is quadratic, and DMABTU-AcO- takes twisted structures (δ = ca. 12 degree in S0 and δ = ca. 21 degree in S1). These results suggest that ICT character of DMABTU is not a twisted ICT (TICT) but a planar ICT (PICT) state.