Mijanur Rahaman Molla

Hydrogen-bonding-mediated J-aggregation and white-light emission from a remarkably simple, single-component, naphthalenediimide chromophore.

Supramolecular assembly of various functional p systems has been investigated extensively in recent decades with the aim to achieve precise control over the mode of their spatial organization, which governs the photophysical properties of the resulting self-assembled material. Such self-assembled materials with hierarchical structure and defined photophysical properties are considered as promising candidates for applications ranging from biology to materials science. Hydrogen-bonding interactions have played a pivotal role in generating various supramolecular nanostructures from suitably designed small molecular building blocks. While many structurally complex hydrogen-bonding motifs have been utilized to fulfill this objective, surprisingly not much is known regarding the utility of simple selfcomplementary hydrogen-bonding functionalities, such as carboxylic acid, in the context of chromophore assembly. Considering structural simplicity and versatile hydrogenbonding motifs of carboxylic acid group as described in the literature related to crystal engineering, we envisaged it would be worth exploring its utility as a structure-directing functionality in self-assembly of organic functional p-systems. In the recent past, we have been engaged in understanding effect of structural variations on hydrogen-bonding mediated self-assembly of 1,4,5,8-naphthalenetetracarboxdiACHTUNGTRENNUNGimide (NDI) derivatives due to the prospect of using this particular chromophore as an n-type semiconductor. NDI derivatives have also been extensively utilized as building blocks for various elegant supramolecular materials, such as synthetic ion channels, organogels, hydrogels, nanotubes, catenanes, rotaxanes, foldamers, supramolecular photosystems, nanoparticles, and other amphiphilic nanostructures. As a part of our continuing interest in self-assembly of NDI chromophore, we synthesized NDI-1 (Figure 1), which contains a carboxylic acid group that is capable of forming a self-complementary hydrogen-bonding network and a dodecyl chain to provide enhanced solubility in relatively nonpolar solvents. Herein we revealed hydrogen-bonding-mediated, spontaneous J-aggregation of NDI-1 and unprecedented aggregation-induced white-light emission. Self-assembly of NDI-1 was primarily investigated by solvent-dependent UV/Vis studies (Figure 1). In CHCl3, a good solvent for solvating extended p systems, NDI-1 shows wellresolved sharp absorption bands in the range of 300–400 nm due to a p–p* transition along the long axis of the chromophore in the monomeric state. Going from CHCl3 to a less polar solvent, methylcyclohexane (MCH), the absorption bands exhibited drastic reduction in intensity with concomitant redshift of 14 nm, which can be considered as strong evidence for J-type p stacking among the NDI-chromophores. 14,21] Note that for the control molecule NDI-2 (Figure 1), in which the carboxylic acid group was replaced by its methyl ester, no such solvent-dependent changes in the absorption spectra were observed (Figure 1), except for a slight blueshift due to a trivial solvatochromic effect. This ascertains that hydrogen-bonding among the free carboxylic acid functionalities is indeed responsible for the observed self-assembly. Existence of hydrogen bonding was further [a] M. R. Molla, Dr. S. Ghosh Polymer Science Unit Indian Association for the Cultivation of Science 2A & 2B Raja S. C. Mullick Rd. Kolkata, 700032 (India) Fax: (+91) 33-2473-2805 E-mail : psusg2@iacs.res.in Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103600. Figure 1. Top: Structure of various NDI chromophores. Bottom: Solventdependent (dashed line-CHCl3, solid line-MCH) UV/Vis absorption spectra of NDI-1 (black line) and NDI-2 (gray line); concentration =0.1 mm, T= 25 8C, path length =1 cm. Inset: Variable-temperature UV/Vis absorption spectra of NDI-1 (concentration=0.1 mm) in MCH/CHCl3 (95:5).

Hydrogen‐Bonding‐Mediated Vesicular Assembly of Functionalized Naphthalene–Diimide‐Based Bolaamphiphile and Guest‐Induced Gelation in Water

This paper describes the spontaneous vesicular assembly of a naphthalene-diimide (NDI)-based non-ionic bolaamphiphile in aqueous medium by using the synergistic effects of π-stacking and hydrogen bonding. Site isolation of the hydrogen-bonding functionality (hydrazide), a strategy that has been adopted so elegantly in nature, has been executed in this system to protect these moieties from the bulk water so that the distinct role of hydrogen bonding in the self-assembly of hydrazide-functionalized NDI building blocks could be realized, even in aqueous solution. Furthermore, the electron-deficient NDI-based bolaamphiphile could engage in donor-acceptor (D-A) charge-transfer (CT) interactions with a water-insoluble electron-rich pyrene donor by virtue of intercalation of the latter chromophore in between two NDI building blocks. Remarkably, even when pyrene was located between two NDI blocks, intermolecular hydrogen-bonding networks between the NDI-linked hydrazide groups could be retained. However, time-dependent AFM studies revealed that the radius of curvature of the alternately stacked D-A assembly increased significantly, thereby leading to intervesicular fusion, which eventually resulted in rupturing of the membrane to form 1D fibers. Such 2D-to-1D morphological transition produced CT-mediated hydrogels at relatively higher concentrations. Instead of pyrene, when a water-soluble carboxylate-functionalized pyrene derivative was used as the intercalator, non-covalent tunable in-situ surface-functionalization could be achieved, as evidenced by the zeta-potential measurements.

Self‐Sorted Assembly in a Mixture of Donor and Acceptor Chromophores

A simple and novel supramolecular approach for orthogonal self-assembly of donor and acceptor chromophores has been demonstrated. Suitably designed 1,5-dialkoxynaphthalene (DAN) and naphthalene tetracarboxylic acid diimide (NDI) derivatives were used as the donor and acceptor systems, respectively. The molecular design for self-sorting relies upon the precise control over the placement of the self-complementary hydrogen-bonding units (amide functionality) with respect to the individual chromophore. By design, the distances between the two amide groups in the donor and acceptor chromophores are not identical, and consequently the effect of the hydrogen-bonding interaction cannot be maximised in the case of alternate donor-acceptor-type pi-stacking. Thus a relatively weak charge-transfer interaction is expected to be sacrificed, and segregated assembly among the individual chromophores should be enforced by virtue of the much stronger effects of hydrogen bonding and pi-pi stacking. Detailed spectroscopic studies were carried out to probe the mode of self-assembly in various derivatives of the DAN-NDI donor-acceptor pairs to establish the utility of the molecular design as a generalised one for orthogonal self-assembly.

Hydrogen‐Bonding Induced Alternate Stacking of Donor (D) and Acceptor (A) Chromophores and their Supramolecular Switching to Segregated States

This paper reports comprehensive studies on the mixed assembly of bis-(trialkoxybenzamide)-functionalized dialkoxynaphthalene (DAN) donors and naphthalene-diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge-transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure-dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self-assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self-assembly propensity, the mode of co-assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen-bonding interaction instead of a weak CT interaction. Consequently, in a highly non-polar solvent like methylcyclohexane (MCH), although kinetically controlled CT-gelation was initially noticed, within a few hours the system sacrificed the CT-interaction and switched over to the more stable self-sorted gel to maximize the gain in enthalpy from the hydrogen-bonding interaction. In contrast, in a relatively less non-polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT-state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D-A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co-assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co-assembly, morphology, and the unprecedented supramolecular-switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.

Understanding the Role of H-Bonding in Aqueous Self-Assembly of Two Naphthalene Diimide (NDI)-Conjugated Amphiphiles

Supramolecular architectures with the synchronized combination of various directional noncovalent forces are ubiquitous in biological systems. However, reports of such abiotic synthetic systems involving H-bonding in aqueous medium are rare due to the challenge faced in the formation of such structures by overcoming the competition from the water molecules. In this paper we have studied self-assembly of two structurally related naphthalene-diimide (NDI) conjugated bola-amphiphiles (NDI-1 and NDI-2) in water with an aim to realize the specific role of H-bonding among the hydrazide units present in one of the two building blocks (NDI-2) on the self-assembly. Both chromophores showed vesicular assembly in aqueous solution driven primarily by π-stacking among the NDI chromophores, which could be probed by UV-vis absorption spectra. Contrary to common belief, the lack of an H-bonding group in NDI-1 was found to be a boon in disguise in terms of the stability of the aggregates. Whereas NDI-2 aggregates showed LCST around 65-70 °C owing to the breaking of the H-bonds with increased temperature, the NDI-1 aggregates were found to be structurally intact until 90 °C, which may be attributed to the increased hydrophobicity introduced by the absence of the polar hydrazide group. Further concentration- and solvent-dependent UV-vis studies showed that NDI-1 formed assembled structure at greatly dilute solution and also in a solvent such as THF, confirming greater propensity for its self-assembly. As both bola-amphiphiles contain an electron-deficient NDI chromophore, interaction of their vesicles was studied with an externally added electron-rich pyrene derivative. Surprisingly, NDI-1 did not show any charge-transfer interaction with the donor, whereas NDI-2 could effectively intercalate, leading to a functional membrane with tunable surface functionalities. This was attributed to the additional stability of the intercalated state by H-bonding among the hydrazide units.

How does PEDOT combine with PSS? Insights from structural studies

The severely intractable polymer poly(3,4-ethylenediethoxythiophene) (PEDOT) is brought to stable aqueous solution after in situ combination with polystyrene sulfonic acid (PSS). This solution (PEDOT–PSS) is stable and successfully combines the optical and electrical properties of PEDOT with the water solubility of PSS. This paper explores the physical properties of PEDOT–PSS from morphological and structural aspects and electronic structure studies are employed to understand the interaction between the two polymers. The solution consists of triangular/rectangular shaped nanoparticles and UV-vis spectroscopy is used to establish the highly doped and conductive nature of the sample with bipolarons present as carrier. Theoretical studies reveal the structure of the interpolymer complex and that the PEDOT chain is bent towards the PSS moiety, and thus, PEDOT–PSS is likely to form a partially coiled or helical structure. The unique stability of this system as well as its highly conductive nature is also consistent with the molecular model.

Effects of conducting polymer poly(3, 4-ethylenedioxythiophene) nanotubes on the electro-optical and dielectric properties of a nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl host

We report the results of the optical transmission and the capacitance behavior as a function dc electric field of a pristine liquid crystal and conducting polymer nanotube-liquid crystal composite measured in twisted nematic cells. The threshold and driving voltages have been determined from transmission-voltage curve. There is remarkable reduction in the threshold and driving voltage in the polymer nanotube doped liquid crystal cell which is good from application point of view. The residual dc is also reduced significantly in the doped cell and the reduction is even more than that observed in the carbon nanotube doped same liquid crystal system.

A Comparative Study of Poly (3,4-ethylenedioxythiophene) (PEDOT) Nanotubes Doped Nematic Liquid Crystal (NLC) System and Carbon Nanotubes (CNT) Doped NLC System for Greater Modification of Electro-Optical Properties of the Host NLC 1770-2

The aim of this present paper is to compare the effects of conducting polymer, poly (3,4-ethylenedioxythiophene) nanotubes and carbon nanotubes, on the electro-optical properties of a nematic liquid crystal mixture (1770-2). From the optical transmission and capacitance behavior it has been proved that the conducting polymer – nematic liquid crystal system possesses much lower threshold voltage and driving voltage than the carbon nanotube – nematic liquid crystal system which might be very useful in display applications. The effective elastic constant has been also significantly reduced in the conducting polymer – nematic liquid crystal system.