Controlling molecular conformations of alkyl bridged bis-aminostyrene: Investigation of inter-chromophore interaction

Abstract A series of bis-aminostyrenes bridged by different aliphatic linkers have been designed and synthesized. Because of tuable molecular conformations, the interactions between two aminostyrene moieties were governed to produce various photophysical and electrochemical properties. When the linkers were ethyl and butyl groups, the behavior of bis-aminostyrene was similar with monomeric aminostyrene. After replacing the linker to be a propyl group, the absorption band became broader and the emission profile displayed significant bathochromic shift. The ethyl and butyl linkers might provide an anti-conformation between two aminostyrenes, and the propyl group might tend to set two aminostyrenes in syn-conformation. Therefore, the relative distance between two pendants might be shorter with propyl group as linker, and thus, inter-chromophore interactions such as exciton coupling take place. These results can be further confirmed by temperature-dependent emission spectroscopy. When the temperature was gradually lowered, the emission spectra of bis-aminostyrenes bridged by ethyl and butyl groups showed bathochromic shift, whereas the propyl substituted one revealed slightly hypsochromic shift. Interestingly, the emission spectrum of propyl substituted bis-aminostyrene was same as that of a monomeric aminostyrene at 150 K, and it could be concluded as destruction of inter-chromophore interaction because of a freeze on molecular motion. On the other hand, a cyclic substituent, piperazine, was chosen to bridge two aminostyrenes, and the chair form of six-member ring forbad inter-chromphore interaction between two aminostyrene. Although, the charge transfer process in aminostyrene was perturbed by piperazine, obvious inter-chromophore interaction would not be observed. These four linkers successfully give the understanding of conformation governing intra-molecular interactions.