Structure-fluorescence relationship: interplay of non-covalent interactions in homologous 1,3,5-triaryl-2-pyrazolines

Fluorescence intensity depends on the synergy of multiple non-covalent interactions in solutions. Twelve new homologous 1,3,5-triaryl-2-pyrazolines (1-12)center dot Br have been synthesized and characterized on the basis of their spectral (IR, H-1 and C-13 NMR and MS) and microanalytical data to investigate the interplay of non-covalent interactions and their effect on absorption and fluorescence properties by UV-vis and emission spectroscopies. All the compounds showed fluorescence in the blue region of the visible spectrum, but a strong influence of alkyloxy chain length was observed on the emission intensity without causing any major blue-or red-shifts in the emission wavelengths. The absorption and emission maxima (lambda(abs)(max) and lambda(em)(max)) for all the compounds were observed in the range of 404-414 nm and 467-479 nm, respectively. Compound 12 center dot Br showed the maximum emission intensity, indicating the dominant role of weak van der Waals forces in driving the solution state self-assembly in comparison to other relatively strong intermolecular interactions. The influence of different halogen substituents present on the conjugated backbone of the 1,3,5-triaryl-2-pyrazoline skeleton in relation to the increasing alkyloxy chain length and their ultimate role in driving the solution state self-assembly and fluorescence properties are also discussed.