H.C. Joshi

Excited state intramolecular proton transfer in some tautomeric azo dyes and schiff bases containing an intramolecular hydrogen bond

Photophysical properties of several basically important aromatic azodyes (1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol) and Schiff bases (N-(2-hydroxy-1-naphthylmethylidene) aniline and N-(1-hydroxy-2-naphthylmethylidene) aniline) all containing an intramolecular hydrogen bond were studied by both steady-state and time-resolved fluorescence spectroscopy with temperatures down to 98 K. It was found that the fluorescence results from the quinone form (H-form) only. The enol form (A-form) undergoes rapid excited state intramolecular proton transfer (ESIPT) resulting in the excited H-form. The compounds have relatively low quantum yields at room temperature, which increase considerably at low temperatures. Lifetime data at the different temperatures indicate that a substitution by both acceptor or donor groups on the para position in the phenyl ring decreases the deactivation rate and hence results in increased lifetime.

Natively fluorescent isoflavones exhibiting anomalous Stokes’ shifts

The fluorescence behaviour of 19 flavonoids was studied. Three isoflavones (formononetin (F), ononin (FG) and daidzein (D)) were found to exhibit large Stokes’ shifts, possibly due to a change of the structure of the molecule from non-planar in the S0 state to planar in the S1 state. Lifetime measurements were carried out using time-correlated single photon counting spectroscopy to further characterise the mechanism. These large shifts provide a high selectivity, so that fluorescent isoflavones can be readily detected in plant samples by means of reversed-phase LC with fluorescence detection. Attention has to be paid to possibly fluorescent impurities in flavonoid standards, as was observed for daidzin (DG), which is not fluorescent itself, but has a fluorescent isomer. To distinguish between the two compounds, LC with fluorescence and MS detection was used to separate and identify the impurity.

Temperature dependent absorption spectroscopy of some tautomeric azo dyes and Schiff bases

The spectral properties of several aromatic azo dyes of fundamental importance (4-phenylazo-1-naphthol, 1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol) and Schiff bases (N-(2-hydroxy-1-naphthylmethylidene)aniline and N-(1-hydroxy-2-naphthylmethylidene)aniline) are investigated at temperatures down to 100 K. The observed spectral changes are interpreted in terms of the existing tautomeric equilibrium and hydrogen-bonding (H-bonding) with the solvent. In non-polar solvents the decrease in temperature leads, in all compounds except 4-phenylazo-1-naphthol, to a full transformation of the enol tautomeric form (A) to the quinone form (H). The temperature dependence of the equilibrium A ⇌ H exhibits a shift around 240 K. In ethanol, due to intermolecular H-bonding, in the case of the azo dyes full transformation was not observed, while the behavior of the Schiff bases is the same as in non-polar solvents. The thermodynamic parameters of the equilibrium A ⇌ H were estimated in both non-polar solvent and ethanol for all investigated compounds.

Substitution effects on the photophysical characteristics of the salicylic anion

Spectral and photophysical properties of substituted salicylic anions are investigated by steady-state and time-resolved fluorescence spectroscopy for a variety of electron donating substituents at the positions para to the hydroxyl and carboxylic groups. Next to the usual excited-state intramolecular proton transfer in aqueous solution, an excited-state intermolecular proton transfer is found to be responsible for the dual emission observed in the case of 5-aminosalicylic anions.

Fluorescence characteristics of 5-amino salicylic acid: An iodide recognition study

In this paper we report the effect of iodide on the fluorescence of 5-amino salicylic acid (5-ASA). In the absence of iodide, prominent blue green (BG) emission band at ∼465nm (broad) is observed in aprotic solvents whereas violet (V) emission at ∼408nm, blue green (BG) at ∼480nm and green (G) at ∼500nm are observed in case of protic solvents. On the addition of iodide ion (I(-)), the intensity of BG fluorescence is enhanced in case of aprotic solvents. On the other hand the G band is enhanced in protic solvents and decrease in the intensity of the V band is observed. The effect of hydrogen bonding as well as the interplay of neutral and ionic species is invoked to explain the observed results. The study projects the application of this system in iodide recognition in protic/aprotic environments.

Photochemistry of 5-aminoquinoline in protic and aprotic solvents

Photophysical properties of 5-aminoquinoline (5AQ) have been investigated in various non-polar and polar (protic and aprotic) solvents using steady state and time resolved fluorescence. In aprotic solvents, the spectral maxima depend on the polarity. However, in protic solvents both the fluorescence intensity as well decay time show decrease depending on the hydrogen bonding ability of the solvent. The results suggest that photochemistry 5AQ is quite sensitive towards the polarity as well as protic character of the solvent.

Steady state and time-resolved fluorescence study of isoquinoline: reinvestigation of excited state proton transfer.

In the present work we report some hitherto unnoticed features in the steady state and time-resolved measurements of isoquinoline in water and trifluoroethanol (TFE). Absorption spectra reveal that in water, neutrals as well cationic species are present. Emission spectrum shows structured features at shorter wavelengths accompanied with a broad band around 375 nm, which correspond to neutrals and cations respectively. However, time-resolved data indicate that protonation does not take place in the excited state in water. On the contrary, in stronger hydrogen bonding solvent TFE, distribution of decay components is observed and at longer wavelengths a small rise time is present. This is ascribed to neutral and cation-like species present in the ground as well as in the excited state. The difference in the results is explained in terms of different excited state potential energy surfaces for water and TFE; particularly, the presence of a rather small barrier for protonation in case of TFE.

Photochemistry and excited state prototropic behaviour of 8-amino 2-naphthol

In the present work, photophysical behaviour of 8-amino 2-naphthol in various solvents as well as at various pH values has been investigated. Various ground and excited state species are noticed. It exhibits a completely different photo prototropic behaviour as compared to its parent molecule 2-naphthol. The spectral and transient studies indicate that at lower pH values, equilibrium between cation and neutral exists in the excited state. At neutral pH some peculiar behaviour in its spectral properties is noticed.