V. T. Ramakrishnan

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.

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.

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.

Solvent effects and photophysical studies of a new class of acridine(1,8)dione dyes.

Spectroscopic properties of a new family of acridinedione dyes are reported. The absorption and fluorescence spectra of the different substituted acridinediones have been recorded in different solvents and the difference in the dipole moment between ground and excited state has been obtained by solvatochromic shift method. The value of the Onsager cavity radius was calculated from the total surface area using software PCMODEL. Fluorescence quantum yield and fluorescence lifetime were determined. Radiative and non-radiative constants have been calculated. The triplet-triplet absorption maxima and triplet lifetime show variation depending on the substitution.

A FACILE ONE POT SYNTHESIS OF m-TERPHENYL AND BIARYL DERIVATIVES

An efficient synthesis of m-terphenyl was achieved by treatment of chalcone with malononitrile using either piperidine or morpholine and biaryl derivatives were obtained from α,β unsaturated ketones with malononitrile using pyrrolidine as a base.

Fluorescence Spectroscopic Evidence for Hydrogen Bonding and Deprotonation Equilibrium between Fluoride and a Thiourea Derivative

Interaction of anions with thiourea-linked acridinedione fluorophore was studied by absorption, (1)H NMR, steady-state and time-resolved fluorescence techniques. Addition of AcO(-) and H(2)PO(4)(-) shows a genuine H-bonded complex with thiourea receptor; whereas, F(-) shows stepwise H-bonding and deprotonation of thiourea NH as confirmed by (1)H NMR titration. Free receptor 1 shows emission maximum at 418 nm; whereas, H-bonded complex of 1·F(-) shows a new redshifted emission maximum at 473 nm and the deprotonated 1 exhibits an emission peak at 502 nm. Presence of these three different emitting species was probed by 3D emission spectroscopic studies. Equilibrium between the free receptor 1, 1·F(-) H-bonded complex and deprotonated 1 was confirmed by time-resolved fluorescence studies. Time-resolved area normalised emission spectra (TRANES) of 1 in the presence of F(-) shows two isoemissive points at 456 and 479 nm between time delays of 0-0.5 ns and 1-20 ns, respectively, due to the existence of three emitting species in equilibrium. Observation of such an equilibrium based on fluorescence spectroscopic studies further proves the earlier reported absorption and (1)H NMR spectroscopic studies of H-bonding and deprotonation processes and also illustrates the dynamics of anion-receptor interactions.

Solvent-controlled metal ion binding selectivity and anion interaction of the acridinedione-based heteroditopic host.

Acridinedione-based heteroditopic hosts 1a-c, which contain a flexible oxyethylene moiety as a metal ion binding site and amino hydrogen as an anion receptor unit, were synthesized and characterized. Metal ion and anion binding studies were carried out in different solvents to understand the solvent-induced selectivity of the metal ion binding and anion interactions. In acetonitrile, Ca(2+) alone shows a binding with the oxyethylene unit and -OCH(3) group, which results in a fluorescence enhancement without any spectral shift due to the suppression of the photoinduced electron transfer (PET) process. In chloroform, in addition to Ca(2+), Na(+) also shows a fluorescence enhancement with a 14 nm red shift in the emission maximum. (1)H NMR titration studies confirm that the addition of Na(+) does not involve binding at the oxyethylene moiety, while it shows a binding at the acridinedione carbonyl group and -OCH(3) group, which results in the red shift and fluorescence enhancement, respectively. In the anion interaction studies, F(-) shows a neat deprotonation in polar aprotic solvents, whereas in chloroform, it shows a H-bond complex due to the lower anion desolvation energy. The present study clearly signifies the role of solvent in metal ion selectivity, which is an often unnoticed parameter in metal ion binding.

Ground and excited states acid-base properties of acridine-1,8-dione dyes

Abstract The acid-base properties of an interesting and important bifunctional group of compounds, acridinedione dyes, were studied in a 95:5 water-methanol mixture. The dyes exist in neutral, deprotonated and protonated forms in equilibrium at appropriate acid-base conditions. The neutral form of the dyes absorbs around 380–400 nm and emits around 440–460 nm in the water-methanol mixture. The deprotonated form of the dyes absorbs around 450–470 nm and emits around 500–520 nm. The protonated form of the dyes absorbs around 410–430 nm and emits around 470–500 nm. The p K a ∗ values are calculated using Forsters and fluorimetric titration methods. The variation of the enhancement in the fluorescence intensity and lifetime of the protonated and deprotonated form of the dyes with substituent are discussed. The influence of substituent on the pKa and p K a ∗ values are discussed. The dyes are more acidic state when compared to the ground state. The fluorescence lifetime of the neutral form is around 6.0–9.0 ns and that of the protonated and deprotonated forms of the dye is 3.0–6.0 ns.

A Facile one pot Synthesis of Highly Substituted 1,6 Naphteyridines

Abstract The synthesis of 1,6-naphthyridiner 2a-f by the reaction of chalcones la-f with malononitrile in the presence of pyrrolidine, involving the Michael addition followed by Thorpe-Ziegler cyclization is described.