π-Conjugation effects on excited-state intermolecular proton-transfer reactions of anthracene-urea derivatives in the presence of acetate anions

This study investigated emissive urea compounds with an anthryl moiety on one side and a substituent group (biphenyl, naphthyl, benzyl, or cyclohexyl) on the other side across from the urea group. This was performed to determine the contribution of π-conjugation on a substituent group to excited-state intermolecular proton-transfer (ESPT) reactions in the presence of acetate anions. Fluorescence lifetime measurements revealed that the rate constant of the ESPT reaction from the normal form to the tautomer form increased with the length of the π-conjugation. Considering that there were a few differences among the wavelengths of the fluorescence maxima for the anthracene-urea derivatives in the presence of acetate anions, we observed that the extension of π-conjugation promoted tautomer formation. This maintained the energy levels of the normal and tautomer forms in the excited state. Furthermore, an anthracene-urea derivative without π-conjugation did not undergo a reverse ESPT reaction, implying that π-conjugation is considerably involved in the reverse ESPT reaction from the tautomer form to the normal form.

Synthesis of natural product hybrids by the Ugi reaction in complex media containing plant extracts

Plant extracts are rich in a wide variety of molecules with diverse biological activities. Chemical engineering of plant extracts has provided a straightforward and simultaneous synthetic route for artificial molecules derived from plant products. This study achieved the synthesis of 13 natural product-like molecules by the Ugi multicomponent reaction using plant extracts as substrates. In particular, the engineering of a mixture of plant extracts demonstrated a unique synthetic route to a series of natural product hybrids, whereby otherwise unencountered naturally occurring molecules of different origins were chemically hybridized in complex media. Even though these reactions took place in complex media containing plant extracts, the well-designed process achieved a good conversion efficiency (~ 60%), chemoselectivity, and reproducibility. Additionally, some of the Ugi adducts exhibited promising inhibitory activity toward protease.

Electronic state of a fluoranthene–urea compound and the kinetics of its emissive tautomer state in the presence of acetate anions

The fluorescence spectrum of 3FU–Ac around 600 nm agrees well with that of 3FU–DBU, indicating that the electronic state of tautomer has a proton-abstracted structure.

LogP determination for highly lipophilic hydrogen-bonding anion receptor molecules

Lipophilicity, usually expressed as octanol-water partition coefficient (logP_o/w), is an important property in biomedical research, drug design and technology. However, high logP_o/w values of complex hydrogen-bonding molecules are not easy to measure or calculate. Exemplary problematic molecules are prospective active components (ionophores) of polymeric sensor membranes - the working elements of ion-selective electrodes. High lipophilicities of the membrane components are crucial for the sensor lifetime. In this work, lipophilicities of a wide range of urea-, carbazole- and indolocarbazole-based anion receptor molecules (some newly synthesized) and two common plasticizers were determined using a chromatography-based approach and/or the COSMO-RS method. Very high logP_o/w values, up to around 20, i.e. far beyond directly experimentally accessible range, were obtained. The agreement between the two approaches ranged from very good to satisfactory. Based on these results, simple fragment-based equations were developed for quick lipophilicity estimation without any specialized software. Membrane-water partition coefficients for the studied compounds were modeled. Limitations and biases of the used methods are discussed.

A Dual Emissive Coumarin-urea Derivative with an Electron-withdrawing Substituent in the Presence of Acetate Anion

We investigated the fluorescent properties, including the excited-state intermolecular proton transfer, of urea derivatives comprising a coumarin ring, which is a widely used fluorophore. We prepared two different coumarin-urea derivatives, 6CU and 7CU, which bear a urea-based substituent at the 6 and 7 positions of a coumarin ring, respectively. In the presence of the acetate ion, 7CU showed additional tautomer fluorescence emission with respect to 6CU, indicating that tautomer formation depends on the positions of the urea-based substituent on the coumarin ring. Thus, the resonance structures of urea derivatives might play an important role in the behavior of dual fluorescence, which is an important phenomenon applicable to photochemical anion sensing. Moreover, in order to further improve the fluorescence properties of the mentioned derivatives, a CF₃ group was introduced in a phenyl ring opposite to a coumarin ring. The fluorescence quantum yield of 7CUCF₃ thus synthesized was 65 times as large as that of 7CU, an observation that renders 7CUCF₃ a suitable anion sensor candidate. The results of this study will contribute to the development of new molecular designs for highly fluorescent sensing.

Kinetic analysis of tautomer forms of aromatic-urea compounds with acetate ions: solvent effect of excited state intermolecular proton transfer

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.

Multi-molecular emission of a cationic Pt(ii) complex through hydrogen bonding interactions

The cationic Pt complexes with amide groups have been found to show dimer emission through hydrogen bonding interactions with counter anions even at low concentration. In order to investigate further details of dimer emission, we prepared three Pt complexes, Pt·B(C₆F₅)₄, Pt·Cl, and Pt·PF₆, whose counter anions possess different strengths of a hydrogen bonding acceptor. Hydrogen bonding interactions in the ground state and excited-state dynamics of the Pt complexes were evaluated by NMR analysis, temperature dependence, and kinetics of dimer emission. These studies revealed that the hydrogen bonding interaction in the ground state is essential for dimer emission, but too strong hydrogen bonding prevents dimer emission due to the inhibition of a stacked dimer formation in the excited state. Owing to this trade-off, the Pt complex with a moderate hydrogen bonding acceptor, PF₆^-, most effectively shows dimer emission. In general, a strong supramolecular interaction efficiently provides a desired assembled structure showing multi-molecular emission. We revealed a unique phenomenon that a moderate interaction is beneficial to effective multi-molecular emission.

Excited-state intermolecular proton transfer dependent on the substitution pattern of anthracene–diurea compounds involved in fluorescent ON1–OFF–ON2 response by the addition of acetate ions

Anthracene–diurea compounds exhibit different excited-state intermolecular proton transfer (ESIPT) reactions depending on the pattern of the substituents.

Control of the intermolecular photodimerization of anthracene derivatives by hydrogen bonding of urea groups in dilute solution

The control of the photodimerization reaction of anthracene–urea compounds was achieved by (1) aggregate conformation depending on the substitution site and (2) inhibition of aggregation by the addition of TBAAc.