Abiodun O. Eseola

Syntheses, Structures, and Fluorescent Properties of 2-(1H-Imidazol-2-yl)phenols and Their Neutral Zn(II) Complexes

A series of 2-(imidazole-2-yl)phenol ligands L1-L6 with the general composition 4-R(4)-5-R(3)-6-R(2)-2-(4,5-R(1),R(1)-1H-imidazole-2-yl)phenol (L1: R(1) = C(2)H(5), R(2) = R(3) = R(4) = H; L2: R(1) = C(6)H(5), R(2) = R(3) = R(4) = H; L3: R(1) = C(6)H(5), R(3) = OCH(3), R(2) = R(4) = H; L4: R(1) = C(6)H(5), R(4) = OCH(3), R(2) = R(3) = H; L5: R(1) = C(6)H(5), R(3) = H, R(2) = R(4) = CH(3); L6: R(1) = C(6)H(5), R(3) = H, R(2) = R(4) = t-Bu) and L7 (2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol) and their neutral Zn(II) complexes (Z1-Z7) were synthesized and characterized by spectroscopic and elemental analyses. Molecular structures of L1, L5, Z1, and Z2 were confirmed by single-crystal X-ray diffraction. L1 crystallized in the monoclinic Cc space group, while L5, Z1, and Z2 all crystallized in the triclinic P1 space group. One-dimensional arrays based on continuous pi-pi stacking interactions and hydrogen bonding were observed for L1 and Z1, while L5 existed as discrete dimeric stack units. Z2 formed hydrogen-bonded 1D network structures but was completely devoid of pi-pi stacking interactions. Emission processes were found to be more dependent on the substituents on phenol as well as condensed media. In contrast to general conclusions on closely related systems in the literature, significant photorelaxation from the excited enol state was observed in the cases of L1 in methanol and L4 in both THF and methanol. Therefore, there exists a certain unusual hindering factor to keto-enol phototautomerism in the ligand-solvent systems. The sensing property of zinc(II) complexes was explored regarding the effects of substituents in their ligands. It was observed that coordination to the zinc(II) ion led to emission quenching for L1 and L2 while causing an enhancement of fluorescent intensity for L3, L4, L5, and L6. A linear relationship was observed between the emission intensity and the concentration of the zinc ion at the 10(-8) M level. Compared to other zinc compounds in this work, fluorescence enhancement in Z3 and Z4 showed that the methoxyl substituent is favorable for fluorescent enhancement.

Electronic/substituents influence on imidazole ring donor–acceptor capacities using 1H-imidazo[4,5-f][1,10]phenanthroline frameworks

Eight imidazole-based compounds 4-methyl-2,6-bis(4,5-diphenyl-1H-imidazol-2-yl)phenol ( A-dp), 2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol ( B-2H), 5-methoxy-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol ( B-2H4M), 3-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol ( C-3H), 2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-4-methoxyphenol ( C-2H5M), 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-2-methoxyphenol ( C-4H3M), 2-(2-methoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline ( C-2M) and 2-(2,4-dimethoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline ( C-2,4M) were synthesized and characterized by elemental, spectroscopic and X-ray single crystal analyses. Two different crystals of A-dp were grown from ethanol and THF, which revealed that A-dp crystallizes in a monoclinic (P2(1)/c) space group while A-dp·2THF crystallizes in a triclinic (P) space group. A-dp·2THF was devoid of any kind of networks whereas the absence of solvent adducts in the ethanol sample produces 1-dimensional single-stranded helices. Contrary to the reported literature conclusion that 1H-imidazole alkylation should increase the donor strength of the imidazole N-base, protonation–deprotonation equilibrium studies on the compounds suggest that push/pull of electron density on the 2-carbon of the imidazole ring by electron-rich/electron-withdrawing substituents is necessary to influence donor capacity of the N-base electrons. Furthermore, the notable increase in pKa,N: values due to ortho/para-directing methoxy substituents supports the conclusion that electron density push towards the 2-position of the imidazole ring is important for improving N-base donor strengths. DFT calculation results using the B3LYP/6-311+G level of theory were conducted to explore possible theoretical explanations.

Spectroscopic study of 2-, 4- and 5-substituents on pKa values of imidazole heterocycles prone to intramolecular proton-electrons transfer

New 2-(1H-imidazol-2-yl)phenols (L1Et-L8tBuPt) bearing a phenolic proton in the vicinity of the imidazole base were prepared and characterized. Experimental studies of the dependence of their protonation/deprotonation equilibrium on substituent identities and intramolecular hydrogen bonding tendencies were carried out using electronic absorption spectroscopy at varying pH values. In order to make comparison, 2-(anthracen-10-yl)-4,5-diphenyl-1H-imidazole (L9Anthr) bearing no phenolic proton and 4,5-diphenyl-2-(4,5-diphenyl-1H-imidazol-2-yl)-1H-imidazole (L10BisIm) bearing two symmetrical imidazole base fragments were also prepared and experimentally investigated. DFT calculations were carried out to study frontier orbitals of the investigated molecules. While electron-releasing substituents produced increase in protonation-deprotonation pK(a)s for the hydroxyl group, values for the imidazole base were mainly affected by polarization of the imidazole ring aromaticity across the 2-imidazole carbon and the 4,5-imidazole carbons axis of the imidazole ring. It was concluded that electron-releasing substituents on the phenol ring and/or electron-withdrawing substituents on 4,5-imidazole carbons negatively affects donor strengths/coordination chemistries of 2-(1H-imidazol-2-yl)phenols, and vice versa. Change of substituents on the phenol ring significantly altered the donor strength of the imidazole base. The understanding of pK(a) variation on account of electronic effects of substituents in this work should aid the understanding of biochemical properties and substituent environments of imidazole-containing biomacromolecules.

ESIPT-capable 2,6-di(1H-imidazol-2-yl)phenols with very strong fluorescent sensing signals towards Cr(III), Zn(II) and Cd(II): molecular variation effects on turn-on efficiency

A series of structurally and electronically varied 2,6-di(1H-imidazol-2-yl)phenols that are ESIPT-capable (1–12) as well as the ESIPT-incapable 4,5-diphenyl-2-(3-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)-1H-imidazole (13) were designed and comparatively studied for their molecular effects on their sensitivity and selectivity characteristics as fluorescent chemosensors of Cr3+, Zn2+, and Cd2+. Their single-crystal structures revealed a desired chain of intramolecular hydrogen bonding (ligand 12) as well as possible coordination modes. Probes 1–4 demonstrated very high turn-on sensitivity and selectivity as double fluorescent sensors for Cr3+ and Zn2+ at their different emission wavelengths (blue-shifted in the case of Zn2+). A remarkable 106-fold emission turn-on by Cr3+ was recorded for molecule 2, which, to the best of our knowledge, is an unheard of magnitude for Cr3+ sensitivity. The results suggested that a Cd2+ sensor could be developed by further derivatization of these probes. The possession of symmetrical substitution on both imidazole rings, multiple active protons involved in hydrogen intramolecular bonding relays, and the consequent ESIPT capability of the studied molecules were found to be very beneficial to their successful outcomes as high fluorescent turn-on chemosensors. Modification of the sensor properties, such as sensitivity and selectivity, was achieved through substituent manipulations at certain peripheral positions. Thus, deliberate molecular derivatization was found to be a tool for manipulating the interference and selectivity profiles. The Job plot, single-crystal results, and sustained large Stokes shift in the presence of Cr3+ suggest a one-pocket N^O coordination in a 1 : 1 stoichiometry rather than a binuclear N^O^N two-pocket coordination. Quantum mechanical calculations on the model structures suggested that the successful turn-on results may be associated with the coplanarity settings of the imidazole and phenol rings.