Luminescence and Electrochemical Activity of New Unsymmetrical 3-Imino-1,8-naphthalimide Derivatives

Abstract

A new series of 1,8-naphtalimides containing an imine bond at the 3-position of the naphthalene ring was synthesized using ¹H, ¹³C NMR, FTIR, and elementary analysis. The impact of the substituent in the imine linkage on the selected properties and bioimaging of the synthesized compounds was studied. They showed a melting temperature in the range of 120-164 °C and underwent thermal decomposition above 280 °C. Based on cyclic and differential pulse voltammetry, the electrochemical behavior of 1,8-naphtalimide derivatives was evaluated. The electrochemical reduction and oxidation processes were observed. The compounds were characterized by a low energy band gap (below 2.60 eV). Their photoluminescence activities were investigated in solution considering the solvent effect, in the aggregated and thin film, and a mixture of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) (50:50 wt.%). They demonstrated low emissions due to photoinduced electron transport (PET) occurring in the solution and aggregation, which caused photoluminescence quenching. Some of them exhibited light emission as thin films. They emitted light in the range of 495 to 535 nm, with photoluminescence quantum yield at 4%. Despite the significant overlapping of its absorption range with emission of the PVK:PBD, incomplete Förster energy transfer from the matrix to the luminophore was found. Moreover, its luminescence ability induced by external voltage was tested in the diode with guest-host configuration. The possibility of compound hydrolysis due to the presence of the imine bond was also discussed, which could be of importance in biological studies that evaluate 3-imino-1,8-naphatalimides as imaging tools and fluorescent materials for diagnostic applications and molecular bioimaging.

Keywords: 1,8-naphthalimides; cell imaging; electrochemistry; imines; luminescence.

Scheme 1

(a) The synthetic route of the 3-imino-1,8-naphthalimide derivatives: I—C₆H₁₅N,rfx,EtOH,6h, II—N₂H₄, Pd/C, rfx, EtOH, 24 h, III—MeOH, 24 h, aldehyde with (b) 1,8-naphthalimides derivatives chemical structure, photos of powder under daylight and for AzNI-6 under UV light (λ_ex = 366 nm).

Figure 1

The differential scanning calorimetry (DSC) thermograms of (a) AzNI-2 and (b) AzNI-4.

Figure 2

(a) Cyclic voltammetry (CV) and (b) differential pulse voltammetry (DPV) scans of AzNI-2 (scan rate 0.10 V/s, electrolyte 0.1 M Bu₄NPF₆ in dichloromethane).

Figure 3

The (a) absorption, (b) excitation, and (c) emission spectra of AzNI-2 in various solvents.

Figure 4

The (a) absorption, (b) excitation, and (c) 3D fluorescence spectra of AzNI-2 in chloroform solution by different concentration (c = 100, 80, 40, 20, 10, 1 μM).

Figure 5

PL properties of AzNI-2 in a binary mixture of MeOH/H₂O: (a) with an increasing water content (fw) at excitation of 340 and 430 nm; (b) λ_em intensity vs. the water content (fw) in the solvents mixture. Photographs were taken under λ = 366 nm UV irradiation from a hand-held UV lamp.

Figure 6

Effect of the TFA on photoluminescence (PL) properties of AzNI-2 in chloroform, acetonitrile, and methanol: (a) the proposed mechanism; (b) superimposed emission spectra at excitation 340 or 430 nm; (c) λ_em intensity versus equivalent of TFA. Photographs were taken under λ = 366 nm UV irradiation from a hand-held UV lamp.

Figure 7

(a) The energy level diagram and EL spectra recorded from the working devices with (b) AzNI-1, (c) AzNI-2, and (d) AzNI-6 with their photo under external voltage.

Figure 7

(a) The energy level diagram and EL spectra recorded from the working devices with (b) AzNI-1, (c) AzNI-2, and (d) AzNI-6 with their photo under external voltage.

Figure 8

The fluorescence images of MCF-7 cells that were stained with the tested 1,8-napthalimide compounds (25 µM) for 2 h. The images acquired under 365 and 470 nm LED illumination. Scale bars indicate 25 µm.

Figure 9

Co-localization fluorescence images of AzNI-3–6 at a concentration of 25 µM and MitoTracker Orange dye in MCF-7 cells. The images acquired under 470 nm (for AzNIs) and 550 nm (for mitochondria-dye) LED illumination. Scale bars indicate 25 µm.