Study of Across-Space Intramolecular Charge-Transfer Interaction by Absorption and Fluorescence Spectroscopy

Abstract

Abstract Across-space (through-space) intramolecular charge-transfer (CT) interaction has been observed in homologous series of p-O2NC6H4(CH2)nNHAr (I) and p-O2NC6H4(CH2)nNRAr (II). Introduction of an alkyl group to the amino nitrogen of I, thus producing II, generally increases the intensity of the CT absorption band. It also induces the red-shift of the band position in most cases owing to the increase of the basicity of the donor, while the blue-shift is observed in ortho substituted derivatives. The latter effect is explained by the steric inhibition of the delocalization of nitrogen lone-pair electrons. The CT fluorescence has been observed at room temperature in the lower homologs (n=1,2, and rarely 3) of I and II. Higher intensity of the fluorescence is generally observed in n=1. The increase of intensity is observed when a small amount of benzene is added to a cyclohexane solution. The excitation spectrum shows two peaks; one at the 1Lb transition wavelength of the amine moiety and the other at the CT absorption. The former suggests the possibility that an excited donor collides intramolecularly with a ground state acceptor part (p-nitrophenyl group) to form an excited CT state, and the latter presents a strong evidence for the origin of the fluorescence being the excited CT state.