P. Ramamurthy

Kinetics of oxidation of glycine and related substrates by diperiodatoargentate (III)

Abstract The reactions of diperiodatoargentate(III) with glycine and related compounds have been examined. The monoperiodatosilver(III) species acts as an active oxidant in comparison to that of diperiodatosilver(III) species. These reactions consist of three kinetically distinguishable steps-induction period, complexation and oxidation. Complexation of these substrates takes place with a second order rate constant of (0.2–1.6)×10 4 dm 3 mol −1 s −1 whereas the redox process occurs at a rate of (0.3–6.0)×10 2 dm 3 mol −1 s − 1 except in case of cysteine with which these processes occurred by an order of magnitude faster. The rate of electron transfer from carboxylic acids to the silver(III) complex is observed to be several order of magnitude smaller in comparison to that of amino acids. Both the rate of complexation and electron transfer are influenced by the structure of the substrates. The aquated silver(III) species is found to be more reactive in comparison to the hydroxylated silver(III) species.

Spin—orbit-coupling-induced triplet formation of triphenylpyrylium ion: a flash photolysis study

Abstract The excited singlet of the 2,4,6-triphenylpyrylium ion (TPP + ) reacts with iodobenzene (PhI) by electron transfer leading to radicals and the triplet state of TPP + . The induced formation of 3 TPP + by spin—orbit coupling has been observed. An encounter complex intermediate is proposed for the quenching of 3 TPP +* by PhI. Homogeneous recombination of radicals is also observed in the millisecond time domain. Flash photolysis has been used to study the excited state reactions of TPP + .

Photophysical studies of acridine(1,8)dione dyes: a new class of laser dyes

Abstract The acridinedione dyes have been known as efficient laser dyes. A study of the photophysical processes is required to understand the charactersitics and identify better lasing dyes in the class. The absorption and fluorescence characteristics of the dyes have been studied. The fluorescence quantum yields are high due to the fairly rigid structure and the substitution on the nitrogen and in the 9-position are found to alter the quantum yields. The non-radiative rate constants are higher when substituted by aryl groups in the 9-position and on the nitrogen. The triplet–triplet absorption maxima and the triplet lifetime show variation depending on the substitution.

Kinetics and mechanism of oxidation of ethylenediamine and related compounds by diperiodatoargentate (III) ion

The oxidation of ethylenediamine by diperiodatoargentate (III) ion has been studied by stopped-flow spectrophotometry. Kinetics of this reaction involves two steps. The first step is the complexation of silver (III) with the substrate and is over in about 10 ms. This is followed by a redox reaction in the second step that occurs intramolecularly from the substrate to the silver (III) center. The rate of reduction of silver (III) species by ethylenediamine, ethanolamine, and 1,2-ethanediol were observed to be 1.2 × 104, 1.1 × 102, and 0.14 dm3 mol−1 s−1, respectively, at 20°C. The reaction rate shows an inverse dependence on [IO] and [OH−] in the low concentration range (≤1 × 10-3 mol dm−3). At higher [OH−] (>1 × 10−3 mol dm−3) the rate of reaction starts increasing and attains a limiting value at very high [OH−]. The rate of deamination of ethylenediamine is enhanced by its complexation with silver (III). The involvement of [AgIII(H2IO6) (H2O)2] and [AgIII(H2IO6) (OH)2]2− are suggested as the reactive silver (III) species kinetically in mild basic and basic conditions, respectively.

Photoinduced Electron Transfer (PET) Based Zn2+ Fluorescent Probe: Transformation of Turn-On Sensors into Ratiometric Ones with Dual Emission in Acetonitrile

Ratiometric sensors for the detection of metal ions have gained increasing attention due to its self-calibration tendency for the environmental effects. In this context, we have synthesized and characterized a dual emitting ratiometric Zn(2+) probe (1) having acridinedione as a fluorophore and N,N-bis(2-pyridylmethyl)amine (BPA) as a receptor unit. Existence of two different conformation of the molecule with photoinduced electron transfer (PET) from amine moiety to the acridinedione fluorophore leads to dual emission, namely locally excited (425 nm) and anomalous charge transfer emission (560 nm) in aprotic solvents. In the presence of one equivalent of Zn(2+), a 15-fold fluorescence enhancement in the locally excited state together with the quenching of charge transfer emission is observed. The intensity changes at the two emission peaks allow a ratiometric detection of Zn(2+) under PET signaling mechanism. The utilization of PET process for the ratiometric fluorescence change will further signify the importance of PET mechanism in sensing action. Addition of Zn(2+) to 1 in acetonitrile/water mixtures shows a single emission peak with fluorescence enhancement.

A ratiometric fluorescent molecular probe with enhanced two-photon response upon Zn2+ binding for in vitro and in vivo bioimaging

A bipyridine centered donor–acceptor–donor (D–π–A–π–D) type ratiometric fluorescent molecular probe exhibited an unprecedented enhancement in the two-photon absorption (2PA) cross section upon Zn2+ binding. Moreover, owing to the excited state charge-transfer of the fluorophore π-backbone, a significant enhancement in the two-photon (2P) excited fluorescence intensity was observed upon Zn2+ binding, resulting in a 13-fold enhancement in the 2PA cross section and a 9-fold enhancement in fluorescence brightness at 620 nm when compared to the cation-free fluorophore. The large 2PA cross section of 1433 GM and 2P action cross section (860 GM), with an excellent 2P excited fluorescence variation from 517 to 620 nm upon Zn2+ binding, facilitated the ratiometric monitoring of free zinc ions in cells. The low cytotoxicity and good photostability of the fluorophore allowed two-photon Zn2+ imaging of HeLa cells. In addition, in vivo two-photon imaging of Zn2+ ions in hepatocytes of live rats illustrated the viability of the probe in tissue imaging and monitoring of free zinc ions in live cells.

Excited state reactions of acridinedione dyes with onium salts: mechanistic details ☆

Abstract Diaryliodonium and triarylsulphonium salts are thermally stable UV photoinitiators for cationic polymerization. Diaryliodonium salts (DPI-1–7) and triarylsulphonium (type-I, type-II and type-III) salts with different substituent have been prepared. Excited state reactions of these onium salts with decahydroacridinedione dyes (ADD-1–6) have been studied. ADD acts as an efficient sensitizer for the decomposition of these onium salts. Photosensitization occurs through electron transfer, which is confirmed by the observation of enol radical cation of ADD and diarylsulphinium radical cation in the laser flash photolysis. The mechanism of photosensitization and the mechanism of photoacid release have been discussed. Both singlet and triplet state of the ADD are involved in the photosensitization of diaryliodonium salts, whereas singlet state of ADD is alone involved in the photosensitization of triarylsulphonium salts. The quantum yield of photoacid generated in direct and the sensitized process is determined and this allows to gauge the practical efficiency of onium salts/ADD combinations as the photoinitiator for the cationic polymerization applications.

Synthesis of nicotinonitrile derivatives as a new class of NLO materials

Abstract 4,6-Diaryl-2-(pyrrolidin-1-yl)-nicotinonitriles 2a–k and 3-amino-2,4-dicyano-5-aryl-biphenyls 3a–c were synthesized from 1,3-diaryl-prop-2-en-1-ones 1a–k and malononitrile by a convenient one-pot method. Likewise, the reaction of aromatic aldehydes with malononitrile afforded 6-amino-4-aryl-2-(pyrrolidin-1-yl)-pyridine-3,5-dicarbonitriles 6a–f . The reaction of mesityl oxide with malononitrile gave 5-amino-7-(pyrrolidin-1-yl)-2,4,4-trimethyl-1,4-dihydro-1,6-naphthyridine-8-carbonitrile 8 . The NLO studies of the pyridinedinitrile derivatives 6a, b, f showed a high value while that of nicotinonitrile 2b was weak.

Solvent effects on the absorption and fluorescence spectra of some acridinedione dyes: determination of ground and excited state dipole moments

Abstract The acridinediones have been reported earlier as a new class of the laser dyes. The absorption and fluorescence spectra of the different substituted acridinediones have been recorded in different solvents and the difference in the dipole moment between the ground and excited state has been obtained. The value of the Onsager cavity radius was calculated from the radius obtained from the surface area using the software PCMODEL. The ground state dipole moment has been obtained using Guggenheims method. The results were used to obtain the first excited singlet state dipole moment of the acridinedione dyes by the solvatochromic shift method.

Excited singlet state reaction of phenosafranine with electron donors Role of the heavy-atom effect in triplet induction

Phenosafranine fluorescence is quenched effectively by halogenated benzene derivatives in the negative and positive regions of the Gibbs energy change, ΔGet. The observed quenching constants are compared with the kq values calculated using ΔG from the Rehm–Weller expression and kd=2.8×1010 d mol-1 s-1 from the Smoluchowski expression. Good correlation between observed and calculated kq values in the exothermic region of ΔGet reveals that the reaction proceeds here via electron transfer. For positive ΔGet, the difference between experimental and calculated kq values shows that quenching occurs via the heavy-atom effect and not via electron transfer. The induced intersystem crossing efficiency in the presence of heavy-atom-substituted quenchers is evaluated using laser flash photolysis.