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Photophysical studies were carried out for simple amino acids like alanine and valine with resorcinol-based aqueous acridinedione (ADDR) dyes. ADDR dyes exhibit interesting excited-state characteristics on altering the substituents at the 9th and 10th sites (Scheme 1). The longest-wavelength absorption maxima remain the same on adding the amino acids to the fluorophore, whereas the excited-state behavior varies significantly mostly based on the nature of the substituent at the 9th position. The absence of fluorescence enhancement was observed with addition of β-alanine, l-alanine, and l-valine to ADDR1 dye (photoinduced electron transfer, PET), whereas addition of glycine exhibits enhancement accompanied with a shift toward a longer-wavelength region. Interestingly, the addition of amino acids to non-PET dyes results in a fluorescence quenching accompanied with a larger shift toward the shorter-wavelength region. The properties of fluorophore and nonfluorophore dyes in the presence of alanine or valine are found to be entirely different from those of glycine. The interaction of alanine with ADDR dyes is predominantly through H-bonding, but the structural aspects of H-bonding interactions of alanine and water are completely different from those of glycine and water. The time-correlated single-photon counting method portrays the existence of fluorophore in two distinguishable microenvironments in the presence of amino acids. The fluorescence spectral technique used as a tool in elucidating the mode of interaction of dye with neutral amino acids in aqueous solution is illustrated in the present study.

Photophysical and electrochemical studies of DDP dye with Formamide and alkyl substituted amides were carried out in water. Addition of Formamide (F), Acetamide (ACM), N,N-Dimethylformamide (DMF), Dimethylacetamide (DMAC) to DDP dye result in an isosbestic point. A fluorescence enhancement of DDP dye is observed on the addition of amides. Apart from the fluorescence enhancement, the addition of formamide result in no significant shift in the position of emission maxima of DDP dye whereas addition of ACM and DMF result in a shift towards the blue and red region respectively. DDP dye exhibits three lifetime components which are unique in lifetime and amplitude. The fluorescence lifetime and relative amplitude of DDP dye varies significantly by addition of amides in aqueous solution which are influenced by amidewater hydrogen-bonding network and hydrophobic influences of the alkyl substituted amides. The nature of interaction between dye and amide be predominantly through hydrogen-bonding wherein the carbonyl oxygen (C=O) of amides are bonded to N-H hydrogen of DDP dye through water molecule. The existence of more than one microenvironment of DDP dye in aqueous phase is elucidated by Electrochemical Impedence Spectroscopy (EIS) through Nyquist plots wherein it signifies that there exist at least three different micro environments which support the existence of different fluorescence lifetimes. Fluorescence spectral technique is used as an efficient tool to elucidate the nature of interaction of water soluble probe with hydrogen-bonding solutes is established in our studies.

Photophysical investigation on the fluorescence decay characteristics of L-tryptophan and a derivative N-acetyl-L-tryptophanamide (NATA) with alkyl amides were carried out in water. L-tryptophan exists in the zwitterionic form and exhibits a biexponential lifetime which is correlated to the existence of rotamer structures. Addition of formamide (F) and dimethylformamide (DMF) results in a decrease in the fluorescence lifetime and its proportion of the most stable structure of L-tryptophan wherein acetamide (ACM) results in an increase of the same. Interestingly, all the amides result in the formation of the lifetime of the rotamer whose lifetime doesn't exist initially and the lifetime and its distribution increases irrespective of the nature of amide. The interaction between L-tryptophan and amide is attributed to hydrogen-bonding such that these interactions influence the relative proportion of the existence of individual rotamers in the presence of amides.Strikingly, in the case of NATA that does not exhibit rotamer structures; the fluorescence lifetime is quenched in the presence of F, whereas ACM and DMF result in a larger fold of enhancement resulting in two different lifetimes. The variation in the fluorescence lifetime and amplitude of the various conformers of L-tryptophan and of NATA is completely governed by the concentration of the amides in solution such that the microenvironment surrounding the fluorophores are completely reorganised. The hydrogen-bonding functional groups in amides that are responsible for the coexistence of rotamers are elucidated and well supported by quantum mechanical (QM) studies. Time-correlated single-photon counting(TCSPC) technique is used as a probe as well as marker in establishing the variation in the lifetime properties of L-tryptophan and NATA with non-fluorescent hydrogen-bonding solutes in water which promotes this as fascinating field of research in the context of fluorescence properties of a complicated amino acid-like tryptophan.

Interaction of guanidine hydrochloride (GuHCl) with 4-dicyanomethylene-2,6-dimethyl-4H-pyran (DDP) dye were investigated by photophysical and electrochemical techniques. An enhancement in the fluorescence intensity and formation of an isosbestic point revels that the ground state and excited state properties of DDP dye. The nature of interaction between DDP dye and GuHCl is predominantly through hydrogen-bonding even thoughelectrostatic interaction in aqueous phase operate. The existence of electrostatic interaction by current response function describes that the shift in anodic peak potential towards less positive value indicate electrostatic binding. The existence of more than one microenvironment of DDP dye in aqueous phase is elucidated by Electrochemical Impedence Spectroscopy (EIS) studies through Nyquist plots. These plots signify that there exist at least three different micro environments or dye existing in different fluorescence lifetimes with varying of amplitude. Fluorescence spectral techniques along with electrochemical studies are used as efficient tools to elucidate the nature of interaction of a water soluble probe with hydrogen-bonding solutes is elucidated in our study.