Steric ploy for alternating donor–acceptor co-assembly and cooperative supramolecular polymerization

The presence of a bulky peripheral wedge destabilizes the homo-assembly of an amide functionalized acceptor monomer and thereby enables alternating supramolecular copolymerization with an amide appended donor monomer via the synergistic effect of H-bonding and the charge-transfer interaction.

.15 mmol ) were taken together with 50 ml dry DMF and the reaction mixture was stirred at 100 C for 48 h under N 2 atmosphere. The reaction was stopped, cooled to rt and poured in 100 mL water and the product was extracted with ethyl acetate (3 x 30 mL). The combined organic layer was washed with water (3 x 10 mL) followed by brine (1 x10 mL) and dried over anhydrous Na 2 SO 4. Excess solvent was evaporated to get the crude product as brown oil which was further purified by column chromatography using basic alumina as a stationary phase and 2% and washed with HCl (1N) solution (3 x 10 mL) followed by brine (1 x 10 mL) and dried over anhydrous Na 2 SO 4 . Then excess solvent was evaporated to get the crude product. It was further purified by column chromatography using silica gel as the stationary phase and 30% ethyl acetatehexane as the eluent to get the pure product (4)

NDI-2-EH:
Compound 5 (350 mg, 0.63 mmol) and 1,4,5,8-napthalenetetracarboxylic-bisanhydride (86 mg, 0.31 mmol) were dissolved in dry DMF (10 mL) and refluxed at 140 °C for 24 h under N 2 atmosphere. After that the reaction mixture was allowed to cool to rt and placed in the refrigerator for 2 h while the crude product came out as an orange solid; which was filtered, and the obtained solid was washed with MeOH several times. The product was further purified by column chromatography by using silica gel as stationary phase and 1% MeOH in CHCl 3 as eluent to obtain the pure product as a light yellow solid. Yield: 280 mg (66%). 1

Description on physical studies
Gelation Study: Stock solutions of all components were made in CHCl 3 at 10 mM concentration. Measured volume of aliquot was transferred to a screw capped vial and the solvent were evaporated by air blowing. Measured amount of methylcyclohexane was added to the vial to make the concentration of solute 10 mM and then the solutions were heated to make homogenous solution and allowed to rest at rt. Approximately after 5-10 min, NDI-2 and DAN-4 formed gel as tested by stableto-inversion method whereas NDI-2-EH or DAN-4 + NDI-2-EH (1:1) remained as free flowing solution.
Alternating supramolecular copolymerization: Equal volume of an aliquot of DAN-4 and NDI-2-EH in CHCl 3 were mixed in a screw capped vial, solvent was evaporated and the red solid obtained was redissolved in measured volume of methylcyclohexane by heating which upon cooling to rt produced a red solution. The solution was allowed to equilibrate at rt for 2 h prior to carry out to any experiment. For estimation of association constant, the red solution was gradually diluted with a known volume of methylcyclohexane and CT-band (λ max = 510 nm) was monitored as a function of concentration (10-5.5 mM). After addition of solvent, the solution was made homogenous and settled for 5 min before recording the absorption spectrum was recorded. K a was estimated by fitting the experimental data to the following equation. 5 1 Where c, A, l and Ԑ indicates concentration, absorbance, optical path length and extinction coefficient, respectively. FT-IR Study: Stock solutions (5 mM) of NDI-2, NDI-2-EH, DAN-4 and NDI-2-EH + DAN-4 were prepared in both chloroform and methylcyclohexane solvents and spectra were recorded at room temperature.
Variable temperature UV-Vis studies: Solution of a given self-assembled chromophore(s) in methylcyclohexane (0.1 mM) was heated from 20 °C to 90 °C using a peltier attached to the UV/Vis machine and spectra were recorded at regular interval. From the temperature-dependent absorption spectra the mole fraction of aggregate at a given temperature T [α Agg (T)] was estimated using following equation.

Mon Agg
Mon Agg Mon , A(T), and A Agg are the absorbance at particular wavelength (326 nm for DAN-4, 377 nm for NDI-2 or NDI-2-EH and 505 nm for DAN-4 + NDI-2-EH) for the monomer (the value was taken from the absorption spectrum of the solution in CHCl 3 ), the solution in methylcyclohexane at temperature T and the fully aggregated stage (lowest temperature spectrum), respectively. α Agg (T) was plotted as a function of T in each case to generate the melting curves shown in Figure 2c. 3 were mixed with appropriate ratio so that the NDI-2-EH concentration remained fixed while that for DAN-4 varied from 7-15% w. r. t. to the NDI-2-EH. Solvent was evaporated and to the solid measured amount of n-decane was added so that [NDI-2-EH] = 0.1 mM in each case. Absorption at a single wavelength (395 nm) was monitored as function of temperature (368 K to 290 K at 1 K/ min cooling rate) by Perkin-Templab software connected to the UV/Vis machine. The cooling curves were attempted to fit either the isodesmic or cooperative 5 . In isodesmic model, we used boltzman function, growth sigmodial in origin 8.0 software. After fitting the data of NDI-2-EH, we obtained correlation coefficient 0.9998. Whereas, the cooling curve of NDI-2-EH in presence of different percentage of DAN-4 failed to fit in isodesmic model. Thus we attempted to fit our data in cooperative model using gnu-software. First we separated elongation and nucleation regime and by using two known equations data was fitted as per reported literature. [6][7] Following a similar procedure, growth of supramolecular polymer was also monitored by temperature dependent (363-293 K, 5 K/ min) DLS studies which showed gradual increase in particle size. From these data diffusion coefficient (D) at each temperature was calculated using Stokes Einstein formula (shown below).

Supramolecular polymerization by nucleation-elongation pathway: Stock solution of NDI-2-EH and DAN-4 in CHCl
Where D, k, T, п, ɳ, R stand for diffusion coefficient, Boltzmann constant, temperature, pi constant, specific viscosity of the solvent and hydrodynamic radius of the aggregates (measured by DLS). 1/D 3 was plotted against α agg at each temperature (estimated from cooling curve in UV/Vis experiment).