Cooperative supramolecular polymerization of an amine-substituted naphthalene-diimide and its impact on excited state photophysical properties

A donor–acceptor–donor (D–A–D) type naphthalene-diimide (NDI-H) chromophore exhibits highly cooperative J-aggregation leading to nanotubular self-assembly and gelation in n-decane.

Materials and methods: Napthalene anhydride, dodecyl amine, 1-bromodecane and 6-choloro-1,3,5 triazine-2,6 diamine were purchased from the Sigma Aldrich Chemical Co. and used without further purification. Solvents were purchased from Merck-India and purified by reported protocol. 1 For physical studies spectroscopic graded solvents have been used. 1

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Compound 2: Mono alkylation was accomplished using a previously reported synthetic procedure. 2 On this basis, dodecyl amine (2.51 gm, 13.56 mmol) and 1-bromodecane (1.5 gm, 6.78 mmol) were mixed in methanol (25 ml) and refluxed for 24h under argon atmosphere. After that, the excess of methanol was removed under reduced pressure to afford a white semi-solid compound which was further purified by column chromatography using silica gel as stationary phase and pet-ether/ ethyl acetate (90/10) as eluent.
An off white waxy material was obtained. Yield = 0.95 gm, 43%. 1  NDI-H: Naphthalene anhydride to dibromo naphthalene anhydride was prepared by following previous literature method. 3 Dibromo naphthalene anhydride was insoluble in most solvents and thus purification was not possible at this stage. The crude product (0.30 gm, 0.70 mmol) was treated with NH 4 OAC (1.08 gm, 14.0 mmol) in acetic acid (15 ml) at 100˚C for 3h under argon atmosphere. After that, the reaction mixture was allowed to settle at rt and products were precipitated out in water. This product also showed poor solubility and thus did not allow purifying at this stage. The crude product (0.15 gm, 0.35 mmol) was treated with freshly prepared N-decyldodecan-1-amine (0.93 gm, 2.82mol) in DMF (8 ml) at 130˚C for 12 h. A bluish colored solution was obtained. The product was extracted with CHCl 3 (20 ml) and washed with acidic (1N HCl) water (40 ml). The organic layer was passed through anhydrous Na 2 SO 4 and the solvent was removed under reduced pressure to obtain a bluish waxy compound. Further, the compound was purified by column chromatography using silica gel as stationary phase and 5% ethyl acetate in petroleum ether as eluent to afford a bluish waxy product. Amount obtained = 0.17 gm, Overall yield = 19 %. 1  M-1: 6-choloro-1, 3, 5 triazine-2, 6 diamine (0.20 gm, 1.37 mmol) was dissolved in dry DMF (7 ml). To this solution compound 2 (0.53 gm, 1.65 mmol), NaHCO 3 (0.14 gm, 1.65 mmol) were added and heated at 140˚C under argon atmosphere for 16 h. After that the reaction mixture was allowed to cool to rt. The crude reaction mixture was diluted with ethyl acetate (20 ml) and washed with brine solution (2 X 30 ml) and water (2 x 30 ml) and dried over anhydrous sodium sulphate. The excess solvent was evaporated to isolate the crude produce as a light yellow waxy material which was purified by column chromatography

Supramolecular polymerization model:
Nucleation-Elongation is one of the model which helps to understand supramolecular polymerization, developed by Ten Eikelder, Markvoort and Meijer 4 .This model describes the equilibrium between the monomer pool and supramolecular polymers where the cooperative growth takes place latter. It extends nucleation-elongation based equilibrium models for growth of supramolecular homopolymers to the case of two monomer and aggregate types and can be applied to symmetric supramolecular copolymerizations, as well as to the more general case of non-symmetric supramolecular copolymerizations.

Symmetric supramolecular polymerization (dimerization):
Due to the fact that supramolecular polymerization occurs via a cooperative mechanism, 4  where ΔH e is the enthalpy corresponding to the aggregation (elongation) process, T the absolute temperature, T e the elongation temperature, R the ideal gas constant. α sat is a parameter introduced to ensure that α agg /α sat does not exceed unity. By using equation 4 at temperatures below the elongation temperature (T<T e ) it is possible to accurately fit the experimental data to the elongation regime. At the

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given concentration the enthalpy release during the self-assembly of the compound in the elongation process (ΔH e ) and the elongation temperature (T e ) can be calculated.
On the other hand in the nucleation regime the fraction of aggregated species (α agg ) can be defined by: Where K a is the dimensionless equilibrium constant of the activation step at the elongation temperature, which gives a measure of the cooperativity of the system.
In the elongation regime the number-averaged degree of polymerization, averaged over all active species <N n > can be described by the following expression: By introducing the value of K a obtained in the nucleation and the parameters α sat and α agg from the elongation regimes, respectively, the number-averaged degree of polymerization ˂N n ˃ can be calculated and plotted at different temperatures, according to equation 6.
The average length of the stack <N n > averaged over the nucleated species at the T e is given by The substitution of K a in equation 7 enables the calculation of the number of aggregated molecules at the elongation temperature.

Estimation of  agg :
As the values for the monomeric ( mon ) and aggregate ( agg ) species can be extracted from the temperature-dependent UV/Vis experiments, the fraction of aggregated species ( agg ) can be calculated by substituting the respective values at each temperature in the following equation:

Transient absorption (TA) measurements:
TA measurements were performed with a home-built pump-probe setup. To measure in the time range of 1-4 ns with a resolution of ~100 fs, the output of a commercial titanium:sapphire amplifier (Coherent LIBRA-HE, 3.5 mJ, 1 kHz, 100 fs) was split into two beams that pumped two independent commercial         Table S1: Results of gelation test in few organic solvents (c= 4mM).