A neutral receptor with a rigid hydrazine spacer, N-p-(dimethylamino) benzamido-N

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.