We report unique and spontaneous formation of hydrogels of perylene derivatives with melamine. The luminescent gel network is formed by H-type aggregation of the perylene core, supramolecularly cross-linked by melamine units. As a result of controlled aggregation in the extended nanofibers, strong exciton fluorescence emission is observed.

A new chelating ligand [4-methyl-2,6-bis-(pyridin-2-yl-hydrazonomethyl)-phenol] (1) was prepared by the condensation of 2-hydrazinylpyridine with 2,6-diformyl-p-cresol. Compound 1 exhibits weak fluorescence due to intramolecular photoinduced electron transfer (PET). The sensor (1) demonstrates Zn²⁺-specific emission enhancement due to the “PET off” process through a 1 : 1 binding mode with the metal ion. The fluorescence quantum yield of chemosensor 1 is only 0.020, and it increases more than 14-fold (0.280) in the presence of one equivalent of the zinc ion. Interestingly, the introduction of other metal ions causes the fluorescence intensity to remain either unchanged or weakened except for Cd²⁺. The new sensor showed ‘naked-eye’ detection of Zn²⁺ ions: a color change of the solution from colorless to yellow. Ratiometric displacement of Cd²⁺ ions from the complex by Zn²⁺ ions supports the formation of a more stable sensor–Zn²⁺ complex over the sensor–Cd²⁺ complex. The experimental findings have been correlated with theoretical results using the B3LYP functional and 6-31G (d, p), LANL2DZ basis set for Cd²⁺ (2) and Zn²⁺ (3) complexes, respectively, by the Density Functional Theory (DFT) method. Moreover, the ability of probe 1 to sense Zn²⁺ within human melanoma cancer cells has been explored, and the Zn²⁺-probing process in living cells was found to be reversible with zinc chelator solution of N,N,N,N-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) or EDTA.

Seven dinuclear and one dinuclear based dicyanamide bridged polymeric Ni^II complexes of phenol based compartmental ligands (HL¹–HL⁴) have been synthesized with the aim to investigate their catecholase-like activity and to evaluate the most probable mechanistic pathway involved in this process. The complexes have been characterized by routine physicochemical studies as well as by X-ray single crystal structure analyses namely [Ni₂(L²)(SCN)₃(H₂O)(CH₃OH)] (1), [Ni₂(L⁴)(SCN)₃(CH₃OH)₂] (2), [Ni₂(L²)(SCN)₂(AcO)(H₂O)] (3), [Ni₂(L⁴)(SCN)(AcO)₂] (4), [Ni₂(L²)(N₃)₃(H₂O)₂] (5), [Ni₂(L⁴)(N₃)₃(H₂O)₂] (6), [Ni₂(L¹)(AcO)₂(N(CN)₂)]_n (7) and [Ni₂(L³)(AcO)₂(N(CN)₂)] (8), [SCN = isothiocyanate, AcO = acetate, N₃ = azide, and N(CN)₂ = dicyanamide anion; L^1–4 = 2,6-bis(R₂-iminomethyl)-4-R₁-phenolato, where R₁ = methyl and tert-butyl, R₂ = N,N-dimethyl ethylene for L^1–2 and R₁ = methyl and tert-butyl, R₂ = 2-(N-ethyl) pyridine for L^3–4]. A UV-vis spectrophotometric study using 3,5-di-tert butylcatechol (3,5-DTBC) reveals that all the complexes are highly active in catalyzing the aerobic oxidation of (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) in methanol medium with the formation of hydrogen peroxide. An EPR study confirms the generation of radicals during the catalysis. Cyclic voltammetric studies of the complexes in the presence and absence of 3,5-DTBC have been performed. Reduction of Ni^II to Ni^I and that of the imine bond of the ligand system have been detected at ∼−1.0 V and ∼−1.5 V, respectively. Coulometric separation of the species at −1.5 V followed by the EPR study at 77 K confirms the species as an organic radical and thus most probably reduced imine species. Spectroelectrochemical analysis at −1.5 V clearly indicates the oxidation of 3,5-DTBC and thus suggests that the radical pathway is supposed to be responsible for the catecholase-like activity exhibited by the nickel complexes. The ligand centred radical generation has further been verified by density functional theory calculation.

With the aim of controlling helicity of self-assembled fibers, a pair of water soluble perylene derivatives has been designed and synthesized. Intermolecular hydrogen bonding interactions and π–π stacking, combined with the effect of molecular chirality, have been found to guide the aggregation that eventually led to gelation in water upon decreasing pH. This work has demonstrated a new paradigm of simple and non-toxic perylene derivatives for biological applications.