A substantially red-shifted fluorescence emission in 3-hydroxyl-2-naphthanilide in acetonitrile was developed and drastically enhanced upon addition of anions such as F-, AcO-, and H2PO4-, with the enhancement depending on anion basicity. Excited-state intermolecular proton transfer in the sensor-anion hydrogen-bonding complex was suggested to be the signaling mechanism.

p-Dimethylaminobenzanilides with a para or meta substituent at the amido anilino moiety were designed to generate a series of dual fluorescent molecules of variable electron acceptors. Ab initio calculations indicated that the anilino substitution did not lead to an obvious change in the ground-state structures of the fluorophores, and the 1H NMR signal of the amido -NH proton was found to experience a linear downfield shift with increasing σ of the substitutent, supporting the fact that the amido anilino moiety was indeed varied comparably by the substitution. The intramolecular charge transfer dual fluorescence was indeed observed in solvents over a large polarity range from the nonpolar cyclohexane (CHX) through diethyl ether (DEE) and tetrahydrofuran (THF) to highly polar acetonitrile (ACN). It was found that the CT emission shifted to the blue with increasing electron-withdrawing ability of the amido anilino substituent up to a Hammett onstant ó of ca. +0.39 and to the red at higher σ. The solvent polarity variation did not change the ó value at which the CT emission shift direction reverses. Similar variation profiles were also observed with the CT to LE emission intensity ratio and the total fluorescence quantum yield. It was concluded that the CT direction in p-dimethylaminobenzanilides was reversed by the amido anilino substitution, and the CT occurs from amido anilino to benzoyl moiety at low σ, whereas at high σ, the CT switches to that from dimethylamino to the benzanilide moiety, which was also supported from the Hartree-Fock calculations. This finding provides an alternative method based on substitutent effect for identifying the charge-transfer direction in multiple chargetransfer systems. The results suggested that the anilino group could be a much stronger electron donor than an aliphatic amino group, which would be of use in designing electron donor substituted molecules. In both cases the dependences of the CT emission energy against σ of the substituent at the amido aniline phenyl ring were found to be much stronger than that in the ester counterparts of p-dimethylaminobenzanilides. This interesting σ dependence in the CT emission would be of ignificance in developing new fluorescent sensors based on electron donor/acceptor variations in p-dimethylaminobenzanilides.

A series of benzoyl para- and meta-substituted benzanilides (BAs) and N-methylbenzanilides (MBAs) were synthesized and their absorption and fluorescence spectra in nonpolar solvent cyclohexane were investigated. Quantum mechanical calculations indicated that the ground state BAs existed preferentially in the trans configuration, whereas MBAs existed in the cis configuration, and benzoyl substitution hardly changed the ground-state structure in the same series. It was observed that all of the synthesized compounds displayed dual fluorescence in cyclohexane, i.e., a normal emission at ca. 330 nm and an abnormal long-wavelength fluorescence at around 500nm. Although the normal emission of both BAs and MBAs did not show obvious variation, the long-wavelength emission shifted strongly to the red with increasing electron-withdrawing ability of the substituent at the benzoyl moiety. The emission energies of the long-wavelength fluorescence of BAs and MBAs in cyclohexane were found to vary linearly with the Hammett substituent coefficient

The fluorescence emission of 4-(dimethy1amino)chalcone (DMAC) in ionic micellar solutions is examined and discussed in a three-state (E*, A*, and P*) dynamic scheme in an attempt to investigate the formation of the photoinduced biradical state P* in micelles. The emission of DMAC in micelles is mainly from state A* with some from state E*. An extraordinarily high enhancement of the DMAC emission in micelles is observed when in comparison with that of a TICT (twisted intramolecular charge transfer) fluorophore whose excited state consists of two states, LE (locally excited state) and TICT, which correspond to states E* and A*, respectively, while the absorption of DMAC is only slightly strengthened in micelles. The formation of state P* is shown to be restricted in micelles through the blocking of the twisting of the double bond by the high viscosity in the micellar phase. The medium polarity appears to play a relatively minor role in the formation of state P*. The micellar interfacial electric field also exerts an effect, a stronger electric field, favoring the formation of state P*. The dramatic enhancement of the DMAC emission in micelles makes DMAC a potential and feasible fluorescent probe for micelle formation.

A series of neutral N-(substituted-benzamido)-N-phenylthioureas (substituent=p-OC2H5, p-CH3, m-CH3, H, p-Cl, p-Br, m-Cl, and p-NO2) were designed as anion receptors, in which the thiourea binding site was attached to the benzamido moiety via an N-N bond. The absorption spectra of these N-benzamidothioureas in acetonitrile peaked at ca. 270nm were found to show unprecedented red shifts by 7373 to 14325cm-1 in the presence of anions such as AcO-, F-, and H2PO4-. Under the same conditions, the classic neutral thiourea receptors, N-(substituted-phenyl)-N-phenylthioureas, showed absorption spectral shifts in most cases of less than 800 cm-1 with one exception of 6501cm-1. Control experiments, effects of protic solvent, and 1H NMR titration confirmed the formation of hydrogen-bonding complexes between the new N-benzamidothiourea receptors and anions. The binding constants with AcO-, for example, are at 105-107mol-1 L order of magnitude, which are 13 to 590 times those of the corresponding classic N-phenylthioureas in the same solvent. It was found that, whereas the absorption of the N-benzamidothiourea receptors showed essentially no dependence on the substituent, the substantially red-shifted new absorption band of the N-benzamidothiourea-anion binding complex was sensitively subject to the substituent. A linear relationship was found between the absorption energies of the N-benzamidothiourea-acetate binding complexes and the Hammett constants of the substituents with a negative slope of -0.34 eV. This led to the assignment that the substantially red-shifted absorption band was the groundstate intramolecular charge-transfer absorption with the substituent locating in the electron acceptor moiety. It was concluded that anion binding to the thiourea moiety of the N-benzamidothiourea receptors switched on their ground-state charge transfer. An anion-binding induced structural change was suggested to occur around the N-N bond in N-benzamidothioureas, which resulted in a substantially increased electron donating ability of the electron donor in the receptor molecules. As a consequence, the ground-state charge transfer takes place in the N-benzamidothiourea-anion binding complexes, leading to unprecedented red shifts in the absorption spectra and substantially enhanced anion binding affinities than those of the corresponding N-phenylthiourea receptors. N-Benzamido-N-phenylthioureas represent a new generation of neutral thiourea-based anion receptors that show substantially improved anion binding performance important for anion sensing and recognition.