Organic nanoparticles of an extended π-conjugated styryl dye: Modulation of fluorescence peak energy and intensity in the near-infrared (NIR) region

Tuning the near-infrared (NIR) emission of organic nanomaterials is of great interest due to possible use in biosensing and imaging based on the biological window ranged in 650-900 nm. In this study, we report NIR fluorescent organic nanoparticles prepared on the basis of ion association between an extended π-conjugated Styryl-9 cation and tetrakis(4-fluorophenyl)borate (TFPB) anion. Binding of Styryl-9 with TFPB results in the prominent appearance of an absorption band that can be assigned to a non-fluorescent H-aggregate, which is revealed by simultaneous deconvolution of both UV-vis absorption and fluorescence excitation spectra, and supported by quantum chemical calculations. Emission is observed in the NIR region from the nanoparticles, but is reduced by the presence of the H-aggregates. To mitigate the H-aggregation occurred inside the nanoparticle, photochemically-inert, bulky tetraphenylphosphonium cation (co-cation) is concurrently incorporated (co-doped), resulting in the disappearance of the relevant H-aggregates and thereby yielding controllable fluorescence peak energy and intensity in the NIR region. We believe that such methodology for synthesizing ion-based organic nanoparticles with tunable emission will open an avenue in the development of new types of NIR fluorescent nanomaterials.