MULTIPLE FLUORESCENCE EMISSION FROM HEXYL P-N,N-DIHEXYLAMINOBENZOATE AGGREGATES IN WATER-DIOXANE BINARY-MIXTURES - TUNING OF EXCITED-STATE PHOTOPHYSICS IN SELF-ASSEMBLY

The fluorescence and absorption spectra of three-long-alkyl-chain cont aining ester, hexyl p-N,N-dihexylaminobenzoate (HDHAB), a molecule cap able of excited state intramolecular charge transfer (ICT), in water-d ioxane binary solvent mixtures were recorded as a function of water vo lume fraction (Phi). It is clearly shown by the spectral variations th at 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 emissi on band red-shifting in position and decreasing in intensity with incr easing Phi, while upon aggregation the low-energy emission shows a sud den blue-shift and the fluorescence spectrum is dominated by a band pe aked 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 st ate, while the 400 nm shoulder to excimer, and 350 nm shoulder to loca lly excited (LE) state. The results show that the ICT reaction in HDHA B aggregates is the dominant excited state reaction, differing from wh at is observed in cetyl p-N,N-dimethylaminobenzoate aggregates in simi lar solvents in which the excimer formation was a dominant excited sta te process. A tuning of the excited-state photophysics in self-assembl y is thus shown. (C) 1998 Elsevier Science B.V. All rights reserved.