Navendu Mondal

Effect of the Alkyl Chain Length on the Rotational Dynamics of Nonpolar and Dipolar Solutes in a Series of N-Alkyl-N-Methylmorpholinium Ionic Liquids

Rotational dynamics of two dipolar solutes, 4-aminophthalimide (AP) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, have been studied in N-alkyl-N-methylmorpholinium (alkyl = ethyl, butyl, hexyl, and octyl) bis(trifluoromethansulfonyl)imide (Tf2N) ionic liquids as a function of temperature and excitation wavelength to probe the microheterogeneous nature of these ionic liquids, which are recently reported to be more structured than the imidazolium ionic liquids (Khara and Samanta, J. Phys. Chem. B2012, 116, 13430-13438). Analysis of the measured rotational time constants of the solutes in terms of the Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that with increase in the alkyl chain length attached to the cationic component of the ionic liquids, AP shows stick to superstick behavior, PRODAN rotation lies between stick and slip boundary conditions, whereas anthracene exhibits slip to sub slip behavior. The contrasting rotational dynamics of these probe molecules is a reflection of their location in distinct environments of the ionic liquids thus demonstrating the heterogeneity of these ionic liquids. The microheterogeneity of these media, in particular, those with the long alkyl chain, is further evidence from the excitation wavelength dependence study of the rotational diffusion of the dipolar probe molecules.

Fluorescence Blinking and Photoactivation of All-Inorganic Perovskite Nanocrystals CsPbBr3 and CsPbBr2I

Study of the emission behavior of all-inorganic perovskite nanocrystals CsPbBr3 and CsPbBr2I as a function of the excitation power employing fluorescence correlation spectroscopy and conventional techniques reveals fluorescence blinking in the microsecond time scale and photoinduced emission enhancement. The observation provides insight into the radiative and nonradiative deactivation pathways of these promising substances. Because both blinking and photoactivation processes are intimately linked to the charge separation efficiency and dynamics of the nanocrystals, these key findings are likely to be helpful in realizing the true potential of these substances in photovoltaic and optoelectronic applications.

Biexciton Generation and Dissociation Dynamics in Formamidinium- and Chloride-Doped Cesium Lead Iodide Perovskite Nanocrystals

Recent studies show that perovskites (ABX3-type) comprising mixed A or B cation and/or mixed halide (X) are more stable and efficient materials for photovoltaic applications than their respective pure forms. Herein we report how doping of a small quantity of formamidinium and/or chloride ion influences the single and multiexciton dynamics of CsPbI3 nanocrystals (NCs). With the help of ultrafast pump-probe spectroscopic measurements, we show that chloride doping can enhance the biexciton lifetime of the system significantly by slowing down the Auger recombination (AR) process. The measured biexciton AR time scale (∼195-205 ps) in some of these NCs is the longest among those reported to date for any similar size perovskites. We further demonstrate that suppression of the AR rate and consequent lengthening of biexciton lifetime allow harvest of these species for their utilization through rapid (18-45 ps) electron transfer to fullerene. The insights obtained from this study are expected to help design more efficient doped perovskites for energy conversion purposes.

New carbazole appended subporphyrin displaying intramolecular charge transfer and solid state fluorescence

A new subporphyrin namely, hydroxo-5,10,15-tri(N-propyl-3-carbazolyl)subporphyri natoboron(III) 6 has been synthesized via the reported litreture procedure by the condensation of pyridine-tri-N-pyrrolylborane with 9-propyl-9H-carbazole-3-carbaldehyde. Its porphyrin analog i.e. 5,10,15,20-tetra(N-propyl-3-carbazolyl)porphyrin has also been isolated from the reaction mixture, formed due to scrambling process and complexed with Zn(II). Photophysical properties of both subporphyrin and Zn(II)porphyrin have been studied in detail to confirm the intramolecular charge transfer process in subporphyrin analog. In addition, the ring contracted congener 6 also displays solid state fluorescence.