Multiple fluorescence emission from hexyl p-N,N-dihexylaminobenzoate aggregates in water-dioxane binary mixtures: tuning of excited-state photophysics in self-assembly
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The fluorescence and absorption spectra of three-long-alkyl-chain containing ester, hexyl p-N,N-dihexylaminobenzoate (HDHAB), a molecule capable of excited state intramolecular charge transfer (ICT), in water-dioxane binary solvent mixtures were recorded as a function of water volume fraction (Phi). It is clearly shown by the spectral variations that HDHAB molecules start to aggregate in the binary solvent at ca. Phi = 0.5. Absorption spectra indicate that the aggregates are in J-form. It is observed that before aggregation HDHAB emits dual fluorescence typical of the presence of excited state ICT reaction, with ICT emission band red-shifting in position and decreasing in intensity with increasing Phi, while upon aggregation the low-energy emission shows a sudden blue-shift and the fluorescence spectrum is dominated by a band peaked at ca. 435 nm, together with two shoulders at ca. 350 and 400 nm, respectively. The 435 nm band is assigned to the emission from ICT state, while the 400 nm shoulder to excimer, and 350 nm shoulder to locally excited (LE) state. The results show that the ICT reaction in HDHAB aggregates is the dominant excited state reaction, differing from what is observed in cetyl p-N,N-dimethylaminobenzoate aggregates in similar solvents in which the excimer formation was a dominant excited state process. A tuning of the excited-state photophysics in self-assembly is thus shown. (C) 1998 Elsevier Science B.V. All rights reserved.