Organobase triggered controlled supramolecular ring opening polymerization and 2D assembly.

A carboxylic acid appended naphthalene-diimide (NDI) derivative spontaneously aggregates in decane to generate a kinetically controlled product with irregular fibrillar morphology. By fine-tuning the sample preparation conditions, the carboxylic acid group can be trapped by intra-molecular H-bonds with the adjacent imide carbonyl, which retards the spontaneous aggregation. In the presence of a catalytic amount of a non-nucleophilic organic base (DBU or DMAP), the meta-stable monomer exhibits supramolecular polymerization through a thermodynamically controlled pathway involving simultaneous H-bonding and π-stacking and generates ultra-thin 2D nano-sheets. DMAP/DBU helps in ring-opening of the intra-molecularly H-bonded monomer and in situ breeds the free acid, which, beyond a critical concentration, initiates controlled supramolecular ring opening polymerization (SROP) via the chain-growth mechanism. The 2D polymer acts as a macro-initiator for subsequent two cycles of SROP and produces laterally extended ultra-thin nano-sheets.

Pathway Diversity Leads to 2D-Nanostructure in Photo-triggered Supramolecular Assembly.

This Communication reports photo-triggered supramolecular assembly of a naphthalene-diimide (NDI) derivative, appended with a photo-labile ortho-nitrobenzyl (ONB)-ester protected carboxylic acid. Photo-irradiation produces the free COOH group which facilitates H-bonding-driven face-to-face stacking of the NDI chromophores producing an ultra-thin (height <2.0 nm) two-dimensional (2D) nanosheet. In contrast, spontaneous supramolecular assembly of the same active monomer exhibits entirely different features such as uncontrolled growth, J-aggregation and fibrillar morphology. A completely different pathway for photo-triggered assembly is attributed to the dual function of the photo-caged pro-monomer in i) producing the carboxylic acid in controlled manner and ii) simultaneously inhibiting the spontaneous J-aggregation of the photo-generated monomers by ester-carboxylic acid H-bonding and in turn directing a distinct growth mechanism.

Controllable supramolecular polymerization via a chain-growth mechanism.

A naphthalene-diimide appended carboxylic acid either spontaneously self-assembles (P) by an open-chain H-bonding or can be arrested in an intra-molecularly H-bonded monomeric state (M) depending on the sample preparation method. Sonication of P produces a seed (S) that triggers controllable supramolecular polymerization (SP) of M via a chain growth route with first order kinetics, a tunable mesoscopic structure and a living nature as demonstrated by the chain-extension up to three cycles. SP of an achiral M, initiated by a chiral S, illustrates chiral amplification by the "sergeant and soldiers" effect.

Cooperative supramolecular polymerization of a perylene diimide derivative and its impact on electron-transporting properties.

H-bonding-promoted supramolecular polymerization of a perylene diimide (PDI) building block and its impact on charge carrier mobility were studied. PDI-1, containing a carboxylic acid group, exhibits H-aggregation in a non-polar solvent decalin while in THF or chloroform it remains in the monomeric form. In contrast, the control molecule PDI-2, containing an ester group does not show aggregation even in decalin, indicating that H-bonding among the carboxylic acid is primarily responsible for H-aggregation, which is further verified by the FT-IR study. A variable temperature UV/Vis study establishes a cooperative pathway for the supramolecular polymerization of PDI-1. Microscopy images show a short fibrillar morphology. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements reveal significantly higher electrical conductivity for the PDI-1 film prepared from decalin compared with that prepared from THF/MeOH or the film of non-aggregated PDI-2 prepared from decalin. By combining the transient absorption spectroscopy data (that estimate the charge carrier generation efficiency) and the TRMC evaluated conductivity, the 1D charge carrier mobility of PDI-1 (µe,1D) is estimated to be 0.24 cm2 V-1 s-1, which is among the top values reported for any PDI derivative measured using the same technique.

Phototriggered Supramolecular Polymerization.

Photo-initiated supramolecular polymerization of a naphthalenediimide (NDI-1) derivative containing an ortho-nitrobenzyl (ONB)-protected amide group is demonstrated. In a hydrocarbon solvent (methylcyclohexane), it remains as monomer. Upon photo-irradiation, deprotection of the ONB group produces NDI-2 with a free amide group, which drives supramolecular polymerization by self-complementary H-bonding between the amide groups, leading to gelation. The polymerization rate can be controlled by tuning the energy of the light source. During photopolymerization, a gradual increase in hydrodynamic radius and viscosity is noticed. More interestingly, the morphology of the supramolecular polymer of NDI-2, produced by photo-irradiation, was a spherulite, which is in sharp contrast with the fibrillar morphology of NDI-2 polymer, when assembled spontaneously without a phototrigger. This is ascribed to the ability of the ONB-caged pro-monomer (NDI-1) to act as a chain-stopper by forming a H-bonded complex with the active monomer during the growth of the supramolecular polymer under photo-irradiation.