Influence of microheterogeneity on the solution phase photophysics of a newly synthesised, environment sensitive fluorophore 2-((7,8-dimethyl-1-oxo-2,3,4,9-tetrahydro-1H-carbazol-6-yl)oxy)acetic acid and its tagged derivative

• Stokes’s shift of more than 100 nm is observed in the case of DOTHCA when the solvent is changed from a polar to aqueous environment.
• Excited state properties of DOTHCA change radically with variations in its immediate H-bonding environment.
• Abrupt changes are observed around the CMC of each micellar environment in the case of DOTHCA.
• A quantitative assessment of the emission intensity data on Benesi–Hildebrand equation reveals a 1:1 stoichiometry for DOTHCA:β-CD complex.
• Benzocaine tagged derivative of DOTHCA, EDOTHCAB shows different emission behavior in microheterogeneous systems.

Excited state spectral properties like fluorescence quantum yield, emission maxima and lifetime of 2-((7,8-dimethyl-1-oxo-2,3,4,9-tetrahydro-1H-carbazol-6-yl)oxy) acetic acid (DOTHCA) are markedly sensitive to polarity and the H-bonding ability of its immediate local environment. Inspired by the excited state properties of the molecule in homogeneous solvents, attempt has been made to investigate its sensitivity of heterogeneity at the “microscopic” level in bio-mimicking environments like micelles (neutral, cationic, anionic) and β-cyclodextrins. DOTHCA interacts differently as the nature of aqueous micelles alters and prominent deviations are encountered only around critical micellar concentrations (CMCs) of each concerned medium. A quantitative assessment of the emission intensity data yields the binding constant (K) and free energy change (ΔG) for the interaction of DOTHCA with the micelles and β-CD. Use of Benesi–Hildebrand equation reveals a 1:1 stoichiometry for DOTHCA:β-CD complex. Micropolarities of the binding sites of the probe in those above mentioned systems have been determined for all micellar systems from a comparison of the variation of fluorescence properties in water–1,4-dioxane solvent mixtures and with the aid of that information probable location of the probe inside the micelles has also been proposed. DOTHCA has the COOH functionality that has been modified to tag an amino acid derivative of biological significance, benzocaine by forming ethyl 4-(2-((7,8-dimethyl-1-oxo-2,3,4,9-tetrahydro-1H-carbazol-6-yl)oxy)acetamido)benzoate (EDOTHCAB). Spectroscopic properties of EDOTHCAB in microheterogeneous environment behave differently than that of DOTHCA.

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