Molecular engineering of fluorescent bichromophore 1,3,5-triaryl-Δ2 -pyrazoline and 4-amino-1,8-naphthalimide molecular logic gates

Abstract

Three hybrid 1,3,5-triaryl-D2 -pyrazoline and 4-amino-1,8-naphthalimide molecular logic gates, differentiated by phenyl, para-N,N-dimethylaniline and ferrocene substituents, were designed and synthesised according to fluorophore–receptor, receptor1–spacer–fluorophore–receptor2 and electrondonor–spacer–fluorophore–receptor formats. The electron-rich para-N,N-dimethylaniline and ferrocene substituents, receptor1 and electron-donor, respectively, participate in photoinduced electron transfer (PET). The thermodynamics for PET on oxidation of these two substituents are identical. However, experimental evidence observed in the solid state suggests that PET is faster from ferrocene than N,N-dimethylaniline. Computational calculations using the four-point method indicate that the activation barrier for PET is greater for N,N-dimethylaniline than ferrocene due to a larger inner reorganisation energy. In solution the compounds are endowed with a polarity-tunable internal charge transfer (ICT) mechanism and colourful solvatochromatism. Lippert–Mataga plots, based on data in 15 organic solvents, reveal a significant dipole moment difference between the ground and excited state. In tetrahydrofuran (THF) the steady-state fluorescence emission switches from 11-fold to 34-fold in the presence of H+ or/and Fe3+. The molecules are demonstrated as H+-driven NOT, H+-driven off-on-off (sequential YES and NOT), and H+, Fe3+-driven INHIBIT logic gates, respectively. DFT calculations at the B3LYP/ 6-31+g(d,p) level provide insight into the frontier molecular orbitals and delineate the PET and ICT mechanisms.