Control of Luminescence Properties in Naphthalene Diimide-Based Gel with Azodibenzoic Acid by Charge Transfer Interaction

Experimental

Materials and Methods. 1H and 13C NMR spectra were measured on a Bruker ARX 300 apparatus. IR spectra were obtained for KBr pellets, in the range 400-4000 cm−1, with a Shimadzu FT-IR 8400S, and Mass spectra were obtained by a JEOL JMS-700 mass spectrometer. The optical absorption spectra of the samples were obtained at 298 K using a UV– vis spectrophotometer (Hitachi U-2900). All fluorescence spectra were recorded in RF-5301PC spectrophotometer.

Fluorescence Lifetime Measurements. Emission lifetime measurements were performed using a conventional laser system. The excitation source used was the 420 nm output of a Spectra-Physics Quanta-Ray Q-switched GCR-150-10 pulsed Nd:YAG lager. Luminescence decay signals were detected by Hamamatsu R928 PMT, recorded on a Tektronix model TDS-620A (500 MHz, 2 GS/s) digital oscilloscope and analyzed using a program for exponential fits.

Rheological Measurements. These were carried out on freshly prepared gels using a controlled stress rheometer (AR-1000N, TA Instruments Ltd., New Castle, DE, USA). Parallel plate geometry of 40 mm diameter and 1.5 mm gap was employed throughout. Following loading, the exposed edges of samples were covered with a silicone fluid from BDH (100 cs) to prevent water loss. Dynamic oscillatory work kept a frequency of 1.0 rad s−1. The following tests were performed: increasing amplitude of oscillation up to 100% apparent strain on shear, time and frequency sweeps at 25 ℃ (60 min and from 0.1–100 rad s−1, respectively). Unidirectional shear routines were performed at 25 ℃ covering a shear-rate regime between 10−1 and 103 s−1. Mechanical spectroscopy routines were completed with transient measurements. In doing so, the desired stress was applied instantaneously to the sample and the angular displacement was monitored for 60 min (retardation curve). After completion of the run, the imposed stress was withdrawn and the extent of structure recovery was recorded for another 60 min (relaxation curve). Dynamic and steady shear measurements were conducted in triplicate and creep (transient) measurements in duplicate.

SEM Observation. Scanning electron micrographs of the samples were taken with a field emission scanning electron microscope (FE-SEM, Philips XL30 S FEG). The accelerating voltage of SEM was 5–15 kV and the emission current was 10 μA.

1H NMR Spectroscopy. The hydrogel (1+2) was made in D2O at 5.0 and 1.3 mg, respectively. For the temperature variable experiment, sample solutions were heated from 25 ℃ to higher temperature with an external temperature controller and the spectral measurements were carried out at different temperatures. On reaching the desired temperature, 10 min equilibrium time was provided before each measurement. The NMR experiment was performed in 300 and 500 MHz spectrometers. For each reading, 100 scans were taken with 1 sec delay time.

Preparation of Binary Gels with 2. In a vial, compound 1 (5 mg) was dissolved in water (200 μL). Compound 2 (0-0.8 equivalent) was added to the soltion 1, then was heated until solution. The solution was maintained at room temperature, and was formed the gel in ambient temperature.

Compound 1. A mixture of 1,4,5,8-naphthalene-tetra-carboxylic dianhydride (NDA) (0.8 g, 3 mmol) and 4-aminopyridine (6 mmol) in DMF (20 mL) was heated under reflux for 8 h. When the reaction mixture reached room temperature, a crystalline solid precipitated out, which was collected by filtration. The crude product was purified by recrystallization from DMF to obtain 1. 1H NMR (300 MHz, DMSO-d6) δ 8.81 (dd, J = 1.6 Hz and 3 Hz, 4Hc), 8.75 (s, 4Ha), and 7.58 (dd, J = 1.6 Hz and 4.5 Hz, 4Hb) ppm. 13C NMR (75 MHz, DMSO-d6) δ 162.80, 151.23, 144.01, 131.01, 127.43, 127.21 and 124.96 ppm. MS (HR-ESI, +ve) m/z: Observed 421.0942 [M+H]+, [M+H]+ calcd = 421.0937. FT IR: 3069.97, 1712.27, 1661.71, 1574.39, 1491.67 cm–1.

Compound 2. Compound 2 was purchased from Aldrich, and used without further purification.