A simple intramolecular charge transfer (ICT) compound, 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid methyl ester (DPDAME), has been documented to be a potential molecular reporter for probing microheterogeneous environments of a model transport protein bovine serum albumin (BSA) using spectroscopic techniques. Meteoric modifications to the emission profile of DPDAME upon addition of BSA come out to be a result of its binding to hydrophobic subdomain IIA. The highly polarity-sensitive ICT emission of DPDAME is found to be a proficient extrinsic molecular reporter for efficient mapping of native, intermediate, unfolded, and refolded states of the protein. Experimental data coupled with a reinforcing support from theoretical simulation using CHARMM22 software confirm the binding site of the probe to be the subdomain IIA of BSA, while FRET study reveals a remarkably close approach of our extrinsic molecular reporter to Trp-212 (in domain IIA): the distance between DPDAME and Trp-212 is 1.437 nm. The caliber of DPDAME as an external fluorescence marker also extends to the depiction of protein-surfactant (BSA-SDS) interaction to commendable fruition. Additionally, the protective action of small amounts of SDS on urea-denatured protein is documented by polarity-sensitive ICT emission of the probe. The present study also reflects the enhancement of the stability of BSA with respect to chemically induced denaturation by urea as a result of binding to the probe DPDAME.

Three-component systems, 1a-c and 2a,b, comprising 1,8-naphthalimide and 4-methoxy-1,8-naphthalimide as fluorophore, a dimethylamino moiety as guest binding site and a polymethylene group as spacer, have been synthesized and the fluorescence behavior of these systems has been studied in the absence and in the presence of the salts of several transition metal ions. The systems are found to be very weakly fluorescent compared to their constituent fluorophores (3 and 4) and this observation has been ascribed to photoinduced intramolecular electron transfer (PIET) between the electron rich amino moiety (donor) and relatively electron deficient fluorophore component (acceptor). Spectral and electrochemical data indicate the thermodynamic feasibility of PIET (exergonic free energy changes) in these multicomponent systems and PIET is found to be most efficient in systems where the fluorophore and the amino moiety are separated by two methylene groups. Fluorescence decay behavior of the systems suggest that PIET occurs by a through-space mechanism. In the presence of the transition metal ions, well-known for their fluorescence quenching abilities, the present systems exhibit significant fluorescence enhancement (FE). Moreover, it has been observed that guest-induced FE can even be severalfold higher than that expected from consideration of PIET in the system. It is suggested that a system can exhibit unusually high FE when the guest is capable of inducing FE by more than one means. In the present case, it is shown that preferential solvation of the fluorophore by the water molecules of the hydrated metal salts could be partially responsible for the high FE values.

The excited-state intramolecular proton transfer (ESIPT) reaction of 1-hydroxy-2-naphthaldehyde (HN12) has been studied within the interior of the supramolecular assemblies of alpha-, beta-, and gamma-cyclodextrins (CD) and biomimicking environments of ionic (SDS) and non-ionic (TW-20) micelles. Fluorescence measurements are used to investigate the effect of various supramolecular assemblies on the ESIPT reaction by monitoring the large Stokes-shifted tautomer emission of HN12. Enhanced tautomer emission in the microencapsulated state predicts favorable ESIPT reaction in the supramoleuclar assemblies. Benesi-Hildebrand plots have been employed to ascertain that the stoichiometric ratios of the complexes formed between HN12 and CDs are 1:2, 1:1, and 1:1 for alpha-, beta-, and gamma-CD, respectively. The binding constants (K(1)) and free-energy change (DeltaG) for inclusion complexation are also determined from the linearized Benesi-Hildebrand plots. Steady-state fluorescence anisotropy, REES, excitation anisotropy, and fluorescence lifetime measurements are in line with other experimental findings. Differential action of urea on SDS and TW-20-bound probe has also been investigated.

The photophysical properties of 11-(4-N,N-dimethylaminophthalimido)undecanoic acid (DAPL), a surfactant covalently labeled with a fluorophore, in homogeneous and micellar media are reported. The remarkable sensitivity of the fluorescence properties of DAPL to the polarity of the media is attributed to the existence of a low-lying nonfluorescent twisted intramolecular charge transfer (TICT) state. DAPL is found to be an excellent fluorescence sensor for following the micellar aggregation process. Although DAPL shows enhanced binding with the micelles, the quenching experiments and the fluorescence spectral and intensity data clearly indicate that the fluorescing moiety does not penetrate into the nonpolar core region of the micelles even though the fluorophore is covalently attached to the nonpolar end group of the fatty acid. It is unambiguously established from the fluorescence data that both of the terminal groups of DAPL are located in the interfacial region of the micelles by folding of the long polymethylene chain.

A detailed photophysical study of a pharmaceutically important chlorine-substituted derivative of salicylic acid viz., 5-chlorosalicylic acid (5ClSA), has been carried out by steady-state absorption, emission and time-resolved fluorescence spectroscopy and compared with its parent molecule salicylic acid (SA). A large Stokes-shifted emission band with negligible solvent polarity dependency indicates the spectroscopic signature of excited-state intramolecular proton transfer reaction. Presence of various ground-state species has been confirmed by a thorough investigation in different solvents and by pH variation experiments. Quantum chemical calculation by ab initio Hartree-Fock (HF) and Density Functional Theory (DFT) methods yields results consistent with experimental findings. Particularly, evaluation of potential energy surfaces for the S(0) and S(1) states across the proton transfer coordinate reveals distinct theoretical support for the inoperativeness of the GSIPT reaction as well as the occurrence of ESIPT process. The less favourable ESIPT reaction in 5ClSA is due to reduction of ground-state intramolecular hydrogen bond strength and excited-state negative charge density at the acceptor (-COOH) oxygen atom and increase of the excited-state barrier with respect to SA.

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Supramolecular interaction between an intramolecular charge transfer (ICT) probe, N,N-dimethylaminonaphthyl-(acrylo)-nitrile (DMANAN), and two well-recognized macrocyclic hosts, cucurbit[7]uril (CB7) and β-cyclodextrin (β-CD), has been studied in aqueous medium by absorption, emission, time-resolved measurements, and (1)H NMR spectroscopic methods. The changes in the profiles of the fluorescence spectra illustrate significant modifications in fluorescence intensity, decay time, and quantum yield upon confinement of probe within the hydrophobic cavity of the hosts. Using the Benesi-Hildebrand relationship, the stoichiometric ratio as well as the binding constant of the host-guest complexation has been estimated. The stable inclusion complexes of the probe with different hosts have been supported by DFT and ONIOM based quantum chemical calculations. These methods of measurement establish that the acceptor group of the probe resides inside the hydrophobic cavity of the macrocycle. The competitive binding of metal ions and cationic surfactants to CB7 has been excellently mapped with this guest fluorosensor.