Photophysical studies on the 4'-N,N-dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ∼100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (<1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.

There is a demand for the development of reliable, cost-effective, and environment friendly protocols for the synthesis of metal NPs with finely tuned structures for technological applications. In the present work, we have exploited the antioxidant activity of the aqueous extract of dried and pulverized leaves of Murraya koenigii Spreng. as a green multifunctional agent in the synthesis of Au NPs with finely tuned morphologies, and our method is a promising green tool for the synthesis of Au NPs with tailor-made structural properties. Moreover, the uniqueness of the present biogenic synthetic route is that simple experimental parameters like control of the concentration of the multifunctional agent and precursor metal ion and also pH and temperature can be effective in the synthesis of Au NPs with finely tuned structures. We have also proposed a plausible mechanistic pathway for our green synthetic route, which may aid in fine-tuning green chemical methods for the controlled synthesis of various metal NPs. At the same time, our study on the interaction between Au NPs and surface-adsorbed fluorescent groups may also be helpful for physicochemical investigations on energy transfer and near-field optical enhancement. Thus, this type of study may be explored in the future for fabricating novel photonic devices.

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Excited state intramolecular proton transfer (ESIPT) time-constants of 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in high n-alcohols--1-butanol, 1-hexanol and 1-decanol--were measured to be 90 ps, 130 ps and 190 ps, respectively, which are unusually slow. At the same time, the solvation time-constants of the DMA3HF enol in the same set of solvents were measured as 100 ps, 150 ps and >300 ps, respectively. Thus, both the ESIPT and enol solvation time-constants in high n-alcohols increase monotonically with the alkyl chain-length of the solvent, although the increase is not strictly proportional. It appears that the H-bonding capacity of the solvent is the single major factor influencing both processes, causing them to become closely correlated. Solvation causes a drastic change in the solvent molecular configuration around the excited enol, E*, inducing the breakage of DMA3HF···solvent inter-molecular H-bonding, which in turn promotes ESIPT. Following previously reported theoretical work on ESIPT, a qualitative description of the S1 potential energy surface can be formulated, where the involvement of solvent relaxation with the ESIPT process is explained.

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