pH-tunable phosphorescence and light harvesting in cucurbit[8]uril host-guest assemblies.

Host-guest assemblies of halo-phenyl pyridine derivatives and cucurbit[8]uril (CB[8]) exhibited pH-responsive room temperature phosphorescence (RTP) in aqueous media. Moreover, they acted as efficient light-harvesting systems demonstrating triplet-singlet energy transfer to various acceptor dyes.

Efficient light harvesting in self-assembled organic luminescent nanotubes.

Luminescent organic nanotubes derived from the co-assembly of cyanostilbene (CS) based cationic supramolecular polymers and bio-polyanion heparin, a known anticoagulant, have been utilized as highly efficient FRET (fluorescence resonance energy transfer) donors in aqueous media resulting in amplified acceptor emission in the orange-red and near-infrared (NIR). Energy transfer efficiencies higher than 80% and an ultra-high antenna effect of 150 were achieved even at high donor/acceptor ratios (500 : 1-100 : 1) translating to emission quenching of several hundred donors by one acceptor. Utilizing the temperature responsiveness of the FRET process, these systems were employed as ratiometric emission thermometers in the temperature range 20-90 °C. Moreover, the energy transfer was very effective in solid and polymer films. This allowed us to generate multi-color emissions ranging from blue to red including white light in solution as well as in solid and polymer films.

Contrasting luminescence in heparin and DNA-templated co-assemblies of dimeric cyanostilbenes: efficient energy transfer in heparin-based co-assemblies.

Dimeric cationic cyanostilbenes with peripheral alkyl chains demonstrated aggregation in aqueous media depending on the length of the hydrophobic segment and produced luminescent spherical nano-assemblies in the case of long alkyl chain derivatives. In the presence of heparin, a bio-polyanion that is routinely used as an anticoagulant, the self-assembled structures obtained from the amphiphilic dimers showed the formation of higher-order structures whereas the non-assembling dimers exhibited heparin-induced supramolecular structure formation. In both cases, a significant enhancement in the emission was observed. This led to the detection of heparin in aqueous buffer, serum and plasma with a "turn-on" fluorescence response. Interestingly, these derivatives also exhibited luminescence variation in the presence of ctDNA. However, the response towards DNA was opposite to that observed in the case of heparin i.e., "turn-off'' fluorescence response. Notably, depending on the length of the alkyl segment, divergent DNA binding modes of these derivatives were observed. Due to their enhanced luminescence, the heparin-based co-assemblies were further explored as artificial light-harvesting systems exhibiting an efficient energy transfer process to embedded acceptor dyes with a high antenna effect.

Multicolor-Luminescence Including White Light by Photomodulation of Supramolecular Assemblies in Aqueous Media.

Photo-responsive supramolecular systems offer intriguing functional aspects which have led to their applications in diverse fields such as optoelectronics and biomedicine. However, the modulation of the luminescence output in a spatiotemporal fashion by photo-controlled transformation still remains a challenging task. Herein, we report the controlled regulation of the emission color of supramolecular assemblies of amphiphilic cyanostilbenes (CSs) in water through in situ photomodulation employing UV and sunlight. Due to their aggregation-induced emission (AIE) features, the CS chromophores in the supramolecular assemblies exhibited bright greenish-yellow emission. Photoirradiation predominantly led to the formation of a cyclized product exhibiting aggregation-caused quenching (ACQ) features and having efficient cyan-blue emission in water but severely quenched emission in the solid state. Hence, starting from a unicomponent scaffold, photomodulation provided tunable emission ranging from greenish-yellow to cyan-blue including white light in water. Furthermore, using the contrasting AIE and ACQ behavior of the components in the photoirradiated mixtures, we were able to design rewritable fluorescent inks and encryption in solid films indicating the practical utility of these systems.

Cucurbituril Based Luminescent Materials in Aqueous Media and Solid State.

Cucurbit[n]urils, the pumpkin shaped macrocyclic host molecules possessing a hydrophobic cavity and two identical carbonyl portals, have drawn a lot of attention in recent years due to their high-affinity yet dynamic molecular recognition properties in water. The reversible and stimuli-responsive nature of their host-guest complexes imparts "smart" features leading to materials with intriguing optical, mechanical and morphological properties. In this review, we focus on the design of cucurbituril based luminescent materials in aqueous media as well in solid or film state. The design principles of fluorescent complexes, small assemblies as well as supramolecular polymers along with their stimuli-responsive properties and applications in diverse areas such as optoelectronic devices, light harvesting, anti-counterfeiting and information technology, cell imaging, etc are highlighted with selected examples from recent literature. We also discuss examples of room temperature phosphorescent materials derived from purely organic luminogens in the presence of cucurbiturils.

Highly sensitive and ratiometric luminescence sensing of heparin through templated cyanostilbene assemblies.

The assembly of organic dyes on bio-molecular templates is an attractive strategy for the creation of bio-materials with intriguing optical properties. This principle is exploited here for the detection of polyanion heparin, a known anticoagulant, by employing di-cationic cyanostilbene derivatives with inherent aggregation induced emission (AIE) features. The cyanostilbene derivatives exhibited weak cyan-blue monomeric emissions in solutions but upon electrostatic co-assembly with heparin, formed highly luminescent clusters on the polyanion surface. The cyanostilbene chromophores in the clusters exhibited greenish-yellow excimer emissions with remarkably longer life-times (up to 70-fold) and higher quantum yields (up to 85-fold) compared to their aqueous solutions. This led to heparin detection in aqueous buffer in low nanomolar concentrations. Additionally, and more importantly, a ratiometric detection of heparin was achieved in highly competitive media such as 50% human serum and 60% human plasma in medically relevant concentrations.

Selective recognition of ATP by multivalent nano-assemblies of bisimidazolium amphiphiles through "turn-on" fluorescence response.

Bisimidazolium receptors, tagged with chromophoric pyrene at one end and linked to an n-alkyl chain at the other, underwent self-assembly in aqueous media depending on the length of the alkyl segment. The amphiphilic derivatives having n-decyl or longer chains, formed nano-assemblies with cyanic-green emission resulting from the stacked pyrene chromophores in the aggregates. The presence of positive surface charges on the multivalent aggregates led to ATP binding which was accompanied by a significant increase in the excimeric emission intensity. This provided a convenient way of monitoring ATP binding in a "turn-on" mode and an efficient detection of ATP was achieved in aqueous buffer and also in buffer containing 150 mM NaCl at physiological pH value. Furthermore, the multivalent aggregates demonstrated a significant selectivity in ATP detection over ADP, AMP and pyrophosphate.

High affinity heparin detection by multivalent supramolecular polymers through aggregation induced emission.

Amphiphilic di-cationic cyanostilbene derivatives with aggregation induced emission (AIE) features formed luminescent supramolecular polymers in aqueous media. They bind bio-polyanion heparin, a known anticoagulant, in a multivalent fashion through the formation of co-assemblies with highly enhanced greenish-yellow emission. Heparin detection in nanomolar concentrations in buffer and in medically relevant concentrations in human serum and plasma was achieved.

Tunable multi-color luminescence from a self-assembled cyanostilbene and cucurbit[7]uril in aqueous media.

Amphiphilic cyanostilbene 12CS12 underwent self-assembly in aqueous media and exhibited bright greenish-yellow emission in its aggregated state. Macrocyclic host cucurbit[7]uril (CB[7]) was found to encapsulate 12CS12 and the host-guest complex was an efficient bluish-cyan emitter. Variation of the 12CS12 and CB[7] ratio in their mixture generated multiple emission colors including near white light through the color mixing of these two emitting species.