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.

Fluorescence and absorption spectra of 1-dimethylaminonaphthalene DMAN.in aqueous-organic binary solvents were recorded as a function of the content of organic components methanol, ethanol, n- and i-propanol, and acetonitrile.for the purpose of understanding the photophysics of DMAN. Whereas the absorption spectra underwent minor change with increasing organic component, the fluorescence spectra experienced a continuous blue-shift in the band position and the fluorescence quantum yield increased reaching a maximum before continuous decrease. It was found that, in a series of aqueous-organic binary solvents, the variations of the quantum yield versus solvent polarity turned at nearly the same solvent polarity equivalent to an ET (30). value of ca. 57 kcalmol-1. It is hence suggested that while the emissive state is intramolecular charge transfer in nature, the radiationless decay channel is solvent-polarity dependent.

The intramolecular charge transfer (ICT) dual fluorescence of p-dimethylaminobenzamide (DMABA) in acetonitrile was found to show highly sensitive response to HSO-4 over several other anions such as H2PO-4, AcO- and ClO-4. In the presence of bisulfate anion the dual fluorescence intensity ratio and the total intensity of DMABA decreased while the dual emission band positions remained unchanged. Absorption titration indicated that a 1:1 hydrogen bonding complex was formed between bisulfate anion and DMABA, which gave a binding constant of 2: 02

A series of substituted-phenyl p-dimethylaminobenzoates with para- or meta-substitution at the ester phenyl ring (p-OH, p-CH3, H, p-Br, m-Cl, and p-NO2) were synthesized and their fluorescence spectra were recorded. Intramolecular charge transfer (CT) dual fluorescence was observed in polar solvents. The CT emission in the same solvent was found to shift to lower energy with increasing electron-withdrawing ability of the substituent, whereas the locally excited (LE) emission hardly showed any change. A linear correlation was found between the CT emission energies of the four of the six esters and the Hammett constants of the substituents in diethyl ether, tetrahydrofuran and ethyl acetate and the linear slopes were -0.190, -0.169 and -0.138 eV, respectively, that decreased with increasing solvent polarity. The implications of the present findings were discussed in terms of structural relaxation during the excited-state CT reaction and of designing new fluorescence chemosensors.

4-(N-phenylamino)benzoic acid (PhABA) was synthesized and its fluorescence spectra were recorded. In aprotic polar solvents, PhABA emitted strongly Stokes-shifted single-banded fluorescence with practically the same wavelength as that of 4-(N,N-diphenylamino)benzoic acid (DPhABA), indicative of the ICT character of the emissive state. 4-(Nisopropylamino) benzoic acid (iPrABA) showed single band emission however, sluggish response to the solvent polarity. In alkanols dual fluorescence of PhABA and DPhABA was observed but with different dependencies on alkanol structures. We concluded that ICT occurring with PhABA was due to the N-phenyl/amino conjugation effect. The work could be of help in understanding speciality of anilino moiety as electron donor compared to aliphatic amino group.