Naphthalenediimides with High Fluorescence Quantum Yield: Bright-Red, Stable, and Responsive Fluorescent Dyes.

The naphthalenediimide (NDI) scaffold in contrast to its higher congeners possess low-fluorescence. In spite of elegant synthetic developments, a highly emissive NDI is quite rare to find, as well as, a green-light-emitting NDI is yet to be explored. Herein, we report a novel class of symmetric and asymmetric NH2 -substituted core-NDIs (1-5) with tunable fluorescence in the visible region and extending to the NIR frontier. Importantly, the bis-NH2 -substituted NDI 2 revealed quantum yield, Φ f of ≈81 and ≈68 % in toluene and DMSO, respectively, suggesting versatility of the fluorophore in a wide range of solvent polarity. The dye 1 is shown to be the first NDI-based green-light emitter. The donor piperidine group in 5 diminish the Φ f by 40-fold providing a lever to modulate the excited-state intramolecular proton transfer (ESIPT) process. Our synthetic protocol applies a Pd catalyst and a benign hydride source simplifying the non-trivial -NH2 group integration at the NDI-core. TD-DFT calculations predicted strong intramolecular hydrogen bonds in the excited state in the bulk nonpolar medium and responsiveness to solvent polarity. The maximization of the NDI emission outlined here would further boost the burgeoning repertoire of applications of the NDI scaffold.

Long-Range Order in Supramolecular π Assemblies in Discrete Multidecker Naphthalenediimides.

We report herein the solution and solid-state studies of conformationally flexible multidecker naphthalenediimides (NDIs) in which the chromophoric NDI units intramolecularly assemble into a series of discrete π-stacks. The X-ray crystallography reveals the existence of exclusively all-syn NDIs orientations in lower congeners while all-anti in a higher congener, suggesting short- to long-range π···π interactions throughout the slipped πNDI chromophoric array. The UV/vis and fluorescence spectra evaluate the discrete π-stacks by remarkable optical changes upon cooling in solution. Furthermore, we carried out a systematic electrochemical investigation to gain an insight into redox properties of the long-range π-stacked structures. The higher congener (5NDI) shows a ten-electron reversible reduction process in a small working potential window (∼0.8 V). To our knowledge, this is an unusual observation in an organic molecular system to undergo up to ten-electron reduction. These results pave the way to design multidecker π-stacks in which structural control with specific electronic properties would be engineered.

Discrete π Stack of a Tweezer-Shaped Naphthalenediimide-Anthracene Conjugate.

The design and synthesis of a tweezer-shaped naphthalenediimide (NDI)-anthracene conjugate (2NDI) are reported. In the structure of the closed form (πNDI ⋅⋅⋅πNDI stack) of 2NDI, which was elucidated by single-crystal XRD, the existence of C-H⋅⋅⋅O hydrogen bonding involving the nearest carbonyl oxygen atom of an NDI unit was suggested. The tunability of πNDI ⋅⋅⋅πNDI interactions was studied by means of UV/Vis absorption, fluorescence and NMR spectroscopy and molecular modelling. This revealed that the πNDI ⋅⋅⋅πNDI interactions in 2NDI affect the absorption and emission properties depending on the temperature. Furthermore, in polar solvents, 2NDI prefers the stronger πNDI ⋅⋅⋅πNDI stack, whereas the πNDI ⋅⋅⋅πNDI interaction is diminished in nonpolar solvents. Importantly, the conformational variations of 2NDI can be reversibly switched by variation in temperature, and this suggests potential application for fluorogenic molecular switches upon temperature changes.

Conformational Dynamics of Monomer- versus Dimer-like Features in a Naphthalenediimide-Based Conjugated Cyclophane.

The design and synthesis of an enantiomeric pair of 1,8-diethynylanthracene-bridged naphthalenediimide (NDI)-based cyclophanes (Cyclo-NDIs) are reported. Each enantiomer of Cyclo-NDI exhibits a circularly polarized luminescence signal with a relatively large luminescence dissymmetry factor (glum =±8×10-3 ). We have further investigated the modulation of through-space electronic communication between co-facially oriented NDIs in a discrete Cyclo-NDI with changes in the temperature. Tuning of the electronic communication results from the conformational transformation of monomer- versus dimer-like features of Cyclo-NDI, as confirmed by UV/Vis, fluorescence, circular dichroic, and NMR spectroscopic analysis. The temperature-dependent optical response in the Cyclo-NDI through the conformational transformation could be utilized as a highly sensitive and reversible optical thermometer in a wide temperature range (100 to -80 °C).

Appending Diverse π-Extended Acceptors with Tetrathiafulvalene/Dithiafulvalene Donors: Multistate Redox Properties, Radical Ion Generation, and Mid-IR-Absorbing Mixed-Valence States.

Three classes of donor-acceptor (D-A) π-extended chromophores (1-12) were synthesized through a phosphite-mediated cross-coupling reaction, in which the anhydride- or imide-based π-As and number of tetrathiafulvalene (TTF)/dithiafulvalene (DTF) Ds were systematically changed. Large π rings, such as benzoperylene and coronene, were integrated into the TTF/DTF unit, for the first time, to overcome their high insolubility. The anhydride and imide groups in the π acceptors can significantly alter the frontier orbitals and influence the optoelectronic properties. The D moieties allow the formation of radical cations (D.+ ) and the π-extended A moieties aid the formation of radical anions (A.- ) by oxidation/reduction under ambient conditions. The molecules revealed UV/Vis/near-IR absorption, fluorescence extending into the near-IR region, and amphoteric electrochemical properties. Chromophores 10 and 12 show solvatochromism in a wide range of solvents. The π-As with anhydride functionality allow easier electron uptake, relative to the imide groups, whereas the increasing number of D TTF/DTF units make them easy to oxidize. Interestingly, the trans-TTF-fused molecules (1, 6, and 11) exhibited a mixed-valence state in the mid-IR region (ν˜ =5130-4000 cm-1 ). Moderate electron coupling between the redox centers is inferred to the compounds being of Robin-Day class II. The multistate redox activity along with panchromism and near-/mid-IR optical absorption of these systems can be attractive towards advanced switchable materials.

Ambient Water-Stable Dianionic Electron Donors: Intramolecular Noncovalent Conduits Assist Charge Delocalization.

Electron-rich π-conjugated dianions are known to be ambient unstable and their stabilization in ambient water is yet to be realized. We report the first example of an exceptionally stable naphthalenediimide-based dianion in ambient and hot water, forming one of the most stable redox-active dianion. The half-life (t1/2 ) of dianion (1 a2- ) is more than four months in ambient water. The dianionic state was confirmed by X-ray crystallography and by various spectroscopic methods. The noncovalent electronic conduits introduced for the first time in dianions, embrace nO →π*C≡N interactions and aid in delocalizing the dianionic charge as validated from theoretical studies. The dianions harness strong NIR absorption and electron donor ability to organic acceptors and metal ions, which make them suitable for potential green energy applications.

Design, Synthesis and Evaluation of Bifunctional Acridinine-Naphthalenediimide Redox-Active Conjugates as Antimalarials.

A novel class of bifunctional molecules was synthesized integrating acridine (Ac) and redox-active naphthalenediimide (NDI) scaffolds directly and through a flexible linker (en). We evaluated in vitro antiplasmodial activity, physicochemical properties, and a possible mode of action. Theoretical studies suggested electronic segmentation between the electron-rich Ac and electron-deficient NDI scaffolds. Orthogonal Ac-NDI molecules showed activities in the micromolar to submicromolar range against a chloroquine (CQ)-sensitive strain of human malaria pathogen Plasmodium falciparum (maximum activity, IC50: 0.419 µM). The flexible Ac-en-NDI molecules were most potent and showed activity in the nanomolar range against both CQ-sensitive (with most effective compounds, IC50: 3.65 and 4.33 nM) as well as CQ-resistant (with most effective compounds, IC50: 52.20 and 28.53 nM) strains of P. falciparum. Significantly, with CQ-resistant strains, the activity of the most effective compounds was 1 order of magnitude better than that of standard drug CQ. Ac-en-NDI-conjugated molecules were significantly more potent than the individual NDI and Ac-based molecules. The structure-activity relationship (SAR) suggests that the flexible spacer (en) linking the Ac and NDI scaffolds plays a vital role in exhibiting improved potency. None of the molecules triggered hemolysis in culture, and the most potent compounds did not show cytotoxicity in vitro against mammalian fibroblast NIH3T3 cells at their respective IC50 values. The other significant outcome of this work is that some of the investigated molecules have the potential to affect multiple processes in the parasite including the hemozoin formation in digestive vacuoles (DVs), mitochondrial membrane potential, and the redox homeostasis of the parasite.