Soumyaditya Mula

Design and development of a new pyrromethene dye with improved photostability and lasing efficiency: theoretical rationalization of photophysical and photochemical properties.

In an attempt to develop a photostable and efficient pyrromethene compound for use in liquid dye lasers, three congeners of the commercially available pyrromethene 567 (PM567) laser dye were synthesized and their photophysical properties, lasing efficiencies, and photochemical stabilities were studied. In general the presence of an aryl group at C-8 of the pyrromethene chromophore increased the photostability. One of the congeners possessing a C-8 trimethoxyphenyl moiety showed significantly improved lasing parameters than PM567. Compared to PM567, the photochemical stability of the new dye was 2-fold, while it showed an equivalent lasing efficiency to that of PM567 at a significantly lower concentration. The increased photostability of these new dye molecules could be explained by theoretical calculation on their capacity to generate singlet oxygen ((1)O(2)) and probability of reaction with (1)O(2). Our calculations were in agreement with the experimental results and indicated that a systematic design of new derivatives of pyrromethene chromophore might lead to improved laser dye molecules.

An ethanol extract of Piper betle Linn. mediates its anti-inflammatory activity via down-regulation of nitric oxide

The leaves of Piper betle (locally known as Paan) have long been in use in the Indian indigenous system of medicine for the relief of pain; however, the underlying molecular mechanisms of this effect have not been elucidated. The anti‐inflammatory and immunomodulatory effects of an ethanolic extract of the leaves of P. betle (100 mg kg−1; PB) were demonstrated in a complete Freunds adjuvant‐induced model of arthritis in rats with dexamethasone (0.1 mg kg−1) as the positive control. At non‐toxic concentrations of PB (5–25 μg mL−1), a dose‐dependent decrease in extracellular production of nitric oxide in murine peritoneal macrophages was measured by the Griess assay and corroborated by flow cytometry using the nitric oxide specific probe, 4,5‐diaminofluorescein‐2 diacetate. This decreased generation of reactive nitrogen species was mediated by PB progressively down‐regulating transcription of inducible nitric oxide synthase in macrophages, and concomitantly causing a dose‐dependent decrease in the expression of interleukin‐12 p40, indicating the ability of PB to down‐regulate T‐helper 1 pro‐inflammatory responses. Taken together, the anti‐inflammatory and anti‐arthrotic activity of PB is attributable to its ability to down‐regulate the generation of reactive nitrogen species, thus meriting further pharmacological investigation.

Steric strain release-directed regioselective functionalization of meso-methyl Bodipy dyes.

Starting from some meso-methyl Bodipy dyes, the corresponding meso-styryl derivatives were synthesized by regioselective Knoevenagel-type condensation with different aromatic aldehydes. The reactions were driven by the alleviation of the structural strain of the alkyl substituted Bodipys that could override the differential acidities of the methyl protons at the pyrrole ring of the Bodipy moiety.

Change of boron substitution improves the lasing performance of Bodipy dyes: a mechanistic rationalisation.

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen ((1)O(2)). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the (1)O(2) generation capacity of these dyes as well as their rate of reaction with (1)O(2), thereby enhancing their lifetimes even under lasing conditions.

Energy Transfer by Way of an Exciplex Intermediate in Flexible Boron Dipyrromethene-Based Allosteric Architectures

We have designed and synthesized a series of modular, dual-color dyes comprising a conventional boron dipyrromethene (Bodipy) dye, as a yellow emitter, and a Bodipy dye possessing extended conjugation that functions as a red emitter. A flexible tether of variable length, built from ethylene glycol residues, connects the terminal dyes. A critical design element of this type of dyad relates to a secondary amine linkage interposed between the conventional Bodipy and the tether. Cyclic voltammetry shows both Bodipy dyes to be electroactive and indicates that the secondary amine is quite easily oxidized. The ensuing fluorescence quenching is best explained in terms of the rapid formation of an intermediate charge-transfer state. In fact, exciplex-type emission is observed in weakly polar solvents and over a critical temperature range. In the dual-color dyes, direct excitation of the yellow emitter results in the appearance of red fluorescence, indicating that the exciplex is likely involved in the energy-transfer event, and provides for a virtual Stokes shift of 5000 cm(-1). Replacing the red emitter with a higher energy absorber (namely, pyrene) facilitates the collection of near-UV light and extends the virtual Stokes shift to 8000 cm(-1). Modulation of the efficacy of intramolecular energy transfer is achieved by preorganization of the connector in the presence of certain cations. This latter behavior, which is fully reversible, corresponds to an artificial allosteric effect.

Design and synthesis of sulfobetainic diketopyrrolopyrrole (DPP) laser dyes

We have devised new diketopyrrolopyrrole dyes soluble in polar solvents. The colors of the dyes have been changed by substituting the 3,6-phenyl residues for thiophene groups. The solubility in polar solvents has been improved by alkylation of a dimethylaminopropyne fragment with a 1,3-sultone giving rise to sulfobetaine moieties. These dyes exhibit strong and broad absorption in the visible range (488 nm for the phenyl-DPP and 578 nm for the thiophene-DPP) and intense fluorescence at, respectively, 554 nm (ΦF 0.36) and 609 nm (ΦF 0.19). The phenyl-DPP dye exhibits a maximum broad band lasing efficiency of 14.6% and a maximum narrow band lasing efficiency of 9.5% with a wide tunable range (554 nm to 616 nm) on excitation with a second harmonic of a Q-switched Nd:YAG (532 nm) laser in MeOH/H2O. The lasing efficiency remains unaffected for hours. The thiophene-DPP dye does not show any lasing action.

Rational Design of Boradiazaindacene (BODIPY)‐Based Functional Molecules

Three boradiazaindacene (BODIPY) dyes with different-coloured (greenish-yellow, orange and red) fluorescence and good Stokes shifts were synthesised starting from the greenish-yellow BODIPY dye PM546. The high Stokes shifts of the dyes are due to the release of the steric strain in their excited states relative to that in the highly twisted ground states. One of these compounds might be a useful water-soluble fluorophore, whereas the other two are promising H(+) sensors.

Photophysical properties of the 8-phenyl analogue of PM567: A theoretical rationalization

The absorption and emission spectroscopic investigations of the C-8-phenyl substituted analogue of the pyrromethene dye PM567 in various polar, non-polar as well as protic and aprotic solvents are reported. The solvatochromic shifts of the spectral bands were studied in a multitude of polar, non-polar and protic, aprotic solvents followed by a multilinear regression in which several solvent parameters were simultaneously analysed. Comparison of the experimental results with those obtained by gas phase ab initio computation with CIS, TD-HF and TD-DFT theories using 6-31G* basis set reveal an overestimation of the experimentally measured excitation parameters by all these theoretical models. However, the trends in the experimental results agree with those calculated theoretically.

Metal‐Dependent Reaction Tuning with Cyclopentylmetal Reagents: Application to the Asymmetric Synthesis of (+)‐α‐Conhydrine and (S)‐2‐Cyclopentyl‐2‐phenylglycolic Acid

The reaction profile of the cyclopentyl organometallic reagents with the aliphatic ketones can be tuned to reduction or addition by changing the metal atom. Cyclopentylmagnesium bromide (CPMB) reduces aromatic and aliphatic aldehydes and ketones to the corresponding alcohols without any C-C bond formation and shows good diastereoselectivity in the reduction of the substituted cyclic and polycyclic ketones as well as chiral alpha-oxygenated aliphatic ketones. However, in the presence of 10 mol % of ZnCl(2), the cyclopentylmagnesium halides follow a normal Grignard addition to the ketones to give tertiary alcohols with complete diastereoselectivity. The reductive as well as the addition protocols were used for the asymmetric synthesis of two medicinally important compounds, (+)-alpha-conhydrine and (S)-2-cyclopentyl-2-phenylglycolic acid.

Inhibitory property of the Piper betel phenolics against photosensitization-induced biological damages

The Piper betel phenolics, allylpyrocatechol (APC) and chavibetol (CHV), were found to protect photosensitization-mediated lipid peroxidation (LPO) of rat liver mitochondria effectively, APC being significantly more potent. The better activity of APC vis-à-vis CHV could be attributed to its higher reactivity with (1)O(2), as revealed from the rate constant values of (1)O(2) quenching by the respective phenolics. APC also prevented the detrimental effects of the Type II photosensitization-induced toxicity to mouse fibroblast L929 cells. The results suggest that APC may play an important role in protecting biological systems against damage, by eliminating (1)O(2) generated from certain endogenous photosensitizers.