Oligofluorene-based nanoparticles in aqueous medium: Hydrogen bond assisted modulation of functional properties and color tunable FRET emission

Fluorene-based linear π-conjugated oligomers with different end functional groups having zero- (OF1), one- (OF2), two- (OF3) and three-point (OF4) hydrogen bonding sites were synthesized and characterized. By using a reprecipitation method, self-assembled nanoparticles were prepared in aqueous medium. The spherical shape and amorphous nature of nanoparticles were established by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Zeta potential measurements showed that nanoparticles of OF2-4 have good colloidal stability, whereas those of OF1 have only moderate stability indicating that the hydrogen bonding groups in OF2-4 interact with the polar water molecules providing stability to the assembly. However, the interior of the nanoparticles remained non-polar, thus providing a conducive medium for hydrogen bonding between the oligofluorene molecules. This leads to varying interchromophore interactions in OF1-4 in the nanoparticle state depending on the H-bonding strength of the end groups. Dynamic light scattering (DLS) studies revealed that under identical conditions, the size of the nanoparticles decreased with increasing number of hydrogen bonding sites in the molecule. The interchromophore interactions were evident from the UV-Vis absorption and fluorescence studies. Bright blue fluorescence of the molecules in solution undergoes quenching in the nanoparticle state. The fluorescence quenching significantly increases from OF1 to OF4 indicating enhanced interaction between chromophores with increasing number of hydrogen bonding sites in the molecules. The nanoparticles were used as a donor scaffold for fluorescence resonance energy transfer (FRET) by encapsulating varying amounts of an orange red emitting neutral dye (D1) thereby achieving colour tunable emission including white. FRET studies were also conducted with a cationic dye (D2) adsorbed on the negatively charged nanoparticle surface. The FRET efficiency with both dyes showed direct correlation with the number of hydrogen bonding sites in the molecules.