Tunable Emission from Fluorescent Organic Nanoparticles in Water: Insight into the Nature of Self‐Assembly and Photoswitching

Abstract

Excitation-dependent tuning of the emission behavior of fluorescent organic nanoparticles (FONs) with two simple luminescent pyrenyl-pyridyl conjugates as model systems is demonstrated. In the case of the compound with a flexible bis-picolyl moiety, the simultaneous presence of multiple ground-state species with distinct absorption and emission characteristics can be observed. The relative ratios of these species can easily be modulated, and it is possible to selectively stimulate any one of them individually by choosing an appropriate excitation channel. Moreover, at high concentration, a drastic change in the nature of the self-assembly is observed, which shifts from donor-acceptor-type self-assembly to exciplex-type self-agglomeration. On the contrary, the compound containing a rigid terpyridine unit has only a single ground state and shows no such tunable emission. However, it can exhibit multiple emission bands in water, whereby the positions of their emission maxima depend on the extent of aggregation-induced planarization of the probe molecules. Overall, this work demonstrates multimodal modulation of FON emission and a gives insight into how molecular order can translate into complete switching of nanoparticle self-assembly and photophysics.

Hydrogen Bonding-Induced Unique Charge-Transfer Emission from Multichromophoric Polypyridyl Ligands: Ratiometric Probing of Methanol Impurity in Commercial Biofuels

“…Thus, we can assume better electronic communication between the aryl moiety and terpyridine unit in case of 1 than that of 2 . Furthermore, we also believe that the excited-state hydrogen bonding interaction in alcoholic solvents, especially in methanol, could planarize the molecules and enhance the CT interaction. − …”

Synthetic Supramolecular Host for D‐(−)‐Ribose: Ratiometric Fluorescence Response via Multivalent Lectin‐Carbohydrate Interactions

“…On the other hand, pH variation studies indicate an off‐on‐off‐type FL response with maximum intensity in the pH range 5.5–8.0 (Figure S4b). Addition of conventional denaturing agents, like urea showed a diminution in aggregate FL with a ∼10 nm blue shift, indicating disruption of long‐range aggregated structures via dissociation of intermolecular hydrogen bonds (Figure S5) [22] …”

“…Addition of conventional denaturing agents, like urea showed a diminution in aggregate FL with a ~10 nm blue shift, indicating disruption of long-range aggregated structures via dissociation of intermolecular hydrogen bonds (Figure S5). [22] To explain the mechanism of interchromophoric association, the FL spectra of 1 was recorded in 60 % (v/v) glycerol-water mixture (Figure 2b). In the presence of glycerol, the FL intensity at aggregate band enhanced.…”

Fine Tuning of the HOMO–LUMO Gap of 6‐(Thiophen‐2‐yl) indolizino[3,2‐c]quinolines and their Self‐Assembly to Form Fluorescent Organic Nanoparticles: Rational Design and Theoretical Calculations

“…In contrast to the large number of studies reported for nanomaterials based on metals, polymers, quantum dots, and carbon dots, [16,18–22] nanoparticles (NPs) based on organic fluorophores have been rarely reported due to aggregation‐caused quenching (ACQ) in the condensed phase, which has been observed for most conventional fluorophores possessing an aromatic system with high planarity [23–24] . For biomedical imaging, the development of FONs with high brightness in water is critical [25–28] . Up to now, FONs with a controlled morphology and tunable emission have been rarely developed [29–31] …”

“…[23][24] For biomedical imaging, the development of FONs with high brightness in water is critical. [25][26][27][28] Up to now, FONs with a controlled morphology and tunable emission have been rarely developed. [29][30][31] Recently, weak intermolecular interactions of organic fluorophores such as π-π stacking, van der Waals forces, and hydrogen bonds, which lead to different morphologies and optical properties in assembled states, have drawn considerable attention for the development of novel functional molecules in optoelectronics, biosensing, bioimaging, etc.…”