Somen Mondal

Revival, enhancement and tuning of fluorescence from Coumarin 6: combination of host–guest chemistry, viscosity and collisional quenching

Fluorescence from Coumarin 6 (C6), a laser dye, decreases considerably due to microcrystal formation in aqueous environments. The fluorescence yield can be effectively enhanced by applying β-cyclodextrin that revives the molecular entity of C6 through host–guest chemistry. C6-β-CD capsules form nanocubes in solution with surface projected hydroxyl groups. Increase in solvent viscosity brings the nanocubes closer following agglomeration, while keeping the molecular entity of C6 intact. This enhances the fluorescence from C6 encapsulated in β-CD nanocubes by 40%. Moreover, this emission can be tuned quantitatively by applying nanoparticles (silver in the present case) at each environmental viscosity level.

Incorporation of Coumarin 6 in cyclodextrins: microcrystals to lamellar composites

Coumarin 6 precipitates in water as microcrystals resulting in a considerable loss in fluorescence yield. Differential interaction of the microcrystals with cyclodextrins of different cavity size enhances the fluorescence yield of the dye by 160 fold in some cases. Highly fluorescent ultrathin lamellar entities form through hydrogen bonding.

Cyclodextrin cavity size induced formation of superstructures with embedded gold nanoclusters

L-cysteine double layer protected gold nanoclusters (Au NCs) have projected thiol groups that induce hydrophobicity around the NCs attracting the relatively hydrophobic cavities of cyclodextrins (CDs) to accumulate around. The different sizes of the CDs result into different accumulating patterns to form spherical to cuboid aggregates with embedded Au NCs.

Modulation of energy/electron transfer in gold nanoclusters by single walled carbon nanotubes and further consequences.

Semiconductor or metallic character in single-walled carbon nanotubes (SWCNTs) is developed because of their chirality and diameter. Depending upon the extent of these characters in a particular sample of SWCNT, various electronic and mechanical applications are formulated. In this work we used protein protected red emitting gold nanoclusters (AuNCs) to enhance the metallic character in SWCNTs through electron transfer induced by photonic excitation. The AuNCs have been synthesized following a known protocol that generates Au(+) protected Au(0) clusters. Normal and carboxylic acid functionalized SWCNTs were obtained commercially for usage in the experiments. The non-functionalized SWCNTs facilitate intersystem electron transfer while the functionalized ones defer the phenomenon, which, in turn, affects the metallic character in the nanotubes. Steady state and time resolved fluorescence spectroscopy prove the dynamics and electrochemistry supports the intersystem electron transfer process.

[2,2′-Bipyridyl]-3,3′-diol in lipid vesicles: slowed down dynamics of proton transfer

(2,2′-Bipyridyl)-3-3′-diol [BP(OH)2] is a well-known compound that undergoes double proton transfer (DPT) that has been previously described as an intramolecular and excited state event in neat solvents and certain constrained environments. Herein, we have explored the interaction of BP(OH)2 with small unilamellar vesicles synthesized using two kinds of lipids, dimyristoyl-L-α-phosphatidylcholine (DMPC) and dimyristoyl-L-α-phosphatidylglycerol (DMPG), which are zwitterionic and anionic, respectively. BP(OH)2 is observed to undergo the formation of a monoketo and a diketo species in sequence along with the creation of dianions aided by the lipid membranes. Interestingly, in the present work, we could witness the formation of the dianionic species under neutral conditions in the presence of lipid vesicles. Moreover, the dynamics of the monoketo species could also be observed as the lipid vesicles supposedly slowed down the overall rate of proton transfer in BP(OH)2. In spite of monitoring in different emitting regions of the spectrum of BP(OH)2 in liposomes, no growth component was observed in the time-resolved fluorescence data. This indicates that in the presence of lipid vesicles all the subsequent species from the parent neutral probe develop in the ground electronic state.