A fluorescence study of 1-p-aminophenyl-4-phenylbuta-1E,3E-diene in organic solvents, 1,4-dioxane-water binary mixtures and micelles

Abstract

1-p-Aminophenyl-4-phenylbuta-1E,3E-diene (1) has been synthesized and its uv-vis absorption and fluorescence emission and excitation spectral properties in a variety of media including organic solvents and 1,4-dioxane-water binary mixtures of varying relative permittivity, and microheterogeneous media of SDS, CTAB and Triton-X-100 micelles have been examined. In contrast to a largely solvent polarity insensitive nature of the uv-vis absorption and fluorescence excitation spectra, the fluorescence emission maximum (λ f m a x ) and the fluorescence quantum yield (Φ f ) of 1 are significantly influenced by polarity of the medium. The aminodiene I in non-polar n-heptane shows λ f m a x at 422 nm, but in relatively more polar solvents it shows two bands at 426-438 and 467-492 nm, depending on the relative permittivity of the medium. In general, as the polarity of medium is increased, the λ f m a x of I undergoes gradual red-shift with decreased fluorescence intensity at the shorter wavelength fluorescence band and enhanced fluorescence intensity at the longer wavelength fluorescence band. Further, as the polarity of the medium is increased, the Φ f decreases. The solvatochromic fluorescence of I has also been discussed in terms of the solvent polarity parameter, Δf. In the micellar medium also, I exhibits two fluorescence bands, which appear more prominently in SDS micelles as compared to in CTAB or in Triton-X-100 micelles. The positions of the fluorescence bands are found to be dependent on the electronic charges of the micelles. Micropolarity of the solubilization site of d in various micelles has also been discussed and it has been suggested that 1 is intercalated in the interfacial domains of the micelles. In general, 1 fluoresces more efficiently in neutral micelles of Triton-X-100 than in ionic micelles of SDS or CTAB. The fluorescence properties of 1 have been discussed in terms of the involvement of apolar, initially prepared locally excited state (in non-polar medium) and dipolar, intramolecular charge transfer excited state (in polar medium) along with the possible influence of the solvent polarity induced energy level re-ordering of the lowest singlet excited states of the aminodiene. This study has brought out interesting features of the excited state structure and dynamics of donor-acceptor diphenylpolyenes and showed the importance of charge transfer excited states in the photoprocesses of linear polyenes in general. Additionally, it provides new directions for designing fluorescence probes as sensors and reporters of the microenvironment of organized assemblies.