Articles you may be interested inElectric field effects on photoinduced electron transfer processes of methylene-linked compounds of pyrene and N,N -dimethylaniline in a polymer film

In this paper, we report the solvent effect of excited state intermolecular proton transfer (ESIPT) reactions of urea compounds in the presence of tetrabutylammonium acetate (TBAAc). We prepared anthracene-urea compounds (9An and 2An), a pyrene-urea compound (Py) and an anthracene-diurea compound (9,10An), which have alkylsulfonyl groups to improve their solubility in various organic solvents. We investigated the solvent effects of the ESIPT reaction using absorption, fluorescence, and 1H NMR spectroscopy along with fluorescence decay measurements in dimethyl sulfoxide (DMSO), acetonitrile (MeCN), tetrahydrofuran (THF) and toluene. The tautomer fluorescence of 9An showed remarkable solvent dependence on the spectral red-shift compared with 2An, Py and 9,10An. As a result of the detailed spectroscopic investigations with regard to the solvent including kinetic analysis of the ESIPT for 9AnAcO-, we revealed that the energy gap between the normal and tautomer forms in the excited state depended on the hydrogen bond acceptor basicity (β), which is one of the Kamlet-Taft solvent parameters. Finally, we discovered that the tautomer structures of aromatic-urea compounds were stabilized by hydrogen bond interactions.

We examined intermolecular hydrogen-bonding interactions of pyrene derivatives bearing a urea unit, referred to as 1PUP (one pyrenyl group) and 1DPU (two pyrenyl groups). 1DPU showed a new emission band around 530 nm along with a locally excited emission (LE) attributed to the pyrene moiety, even at 10−7 M, in contrast to 1PUP, which only had an LE, indicating that the symmetrical configuration allowed stable stacking of 1DPU. Time-resolved emission spectroscopy revealed that the intensity of the new band increased relative to that of the LE with the concentration of 1DPU. Furthermore, addition of tetrabutylammonium acetate (TBAAc) resulted in the disappearance of the new band, due to the formation of hydrogen bonds with 1DPU. These results indicate that the new band may originate from intermolecular hydrogen-bonding interactions between 1DPU molecules through the urea unit, even at very low concentrations, and can be attributed to an excimer emission. This implies that hydrogen bond formation by the urea units of 1DPU favors a conformation that results in the formation of the excimer in the absence of anions, and the strength of the hydrogen-bonding interaction between 1DPU molecules is weaker than the interaction between a 1DPU molecule and an anion.

We report anthracene-diurea compounds which behave as anion sensors based on the fluorescence emission regulated by the substitution position on the anthracene ring. Anthracene-diurea compounds exhibit different excited-state intermolecular proton transfer (ESIPT) reactions depending on the pattern of the substituents. Three new anthracene-diurea compounds that have two phenylurea groups substituted at different positions on anthracene were synthesized. These compounds formed complexes with acetate ions through intermolecular hydrogen bonding between N-H and C[double bond, length as m-dash]O moieties in the ground state. The positions of the substituents greatly affected the excited-state intermolecular proton transfer. 1,5BPUA with urea groups at the 1 and 5 positions exhibited ESIPT reaction, which is energetically favorable for tautomer formation, in the presence of TBAAc. In contrast, 2,6BPUA with urea groups at low-electron-density positions (2 and 6 positions) showed no ESIPT reaction due to the inversion of the lowest unoccupied molecular orbital (LUMO) energy levels of the normal and tautomer states. Detailed spectroscopic measurements showed that the LUMO energy level of the normal form was lowered because the urea group acted as an electron-withdrawing group. In addition, 9,10BPUA exhibited strong electronic interactions between the two phenylurea moieties at the 9 and 10 positions, resulting in an ON-OFF-ON response for acetate ions. Our findings offer guidelines for the molecular design of materials with anthracene moieties based on the substitution patterns of anthracene derivatives.

The photodimerization reaction of anthracene derivatives was performed by capitalizing on intermolecular hydrogen bonds. Anthracene derivatives that can control the dimerization reaction depending on the substitution site were designed by using two anthryl moieties and one urea group, referred to as N,N'-dianthracen-n-ylurea, nDAU (n = 1, 2 and 9), which are symmetrically substituted by 1-anthryl, 2-anthryl and 9-anthryl groups, respectively. We investigated the excimer emission and photodimerization reaction of these anthracene-urea derivatives using absorption, emission, and (1)H NMR spectroscopy along with fluorescence decay measurements. All derivatives showed a concentration dependence of their fluorescence spectra and multiple fluorescence lifetime components even at 10(-6) M. Significantly, 9DAU resulted in an intermolecular photodimerization reaction. These differences in photoreactivity of nDAU may depend on variations in the overlap of the intermolecularly associated anthracene rings of nDAU by hydrogen bonding between intermolecular urea moieties. Furthermore, the dimerization quantum yield of 9DAU was reduced by the addition of tetrabutylammonium acetate (TBAAc). Consequently, we revealed that the substitution site and the addition of TBAAc affected the dimerization reaction of anthracene-urea derivatives.