Nitro aromatics are the principal components of explosives used in acts of terrorism and within improvised explosive devices, among others. Although high sensitivity towards nitro aromatic explosives has been demonstrated, selective detection and discrimination are critical for practical applications. Fluorescence quenching of metal-organic frameworks (MOFs) is sufficiently sensitive to detect any nitro explosives, but discriminative detection with different numbers of -NO 2 groups is rare. Here we report a stable fluorescent MOF, [Zn 2 (NDC) 2 (bpy)]$G x , 1 (where NDC ¼ 2,6-naphthalenedicarboxylic acid, bpy ¼ 4,4 0bipyridine, and G ¼ guest solvent molecules), whose fluorescence is quenched by trace amounts of nitro aromatics introduced from solution or in vapor phase. The steady-state and time-resolved experiments show that the quenching process is dynamic in nature and the interactions (dipole-dipole, p-stacking) between the MOF and nitro explosives play a crucial role in the discriminative detection of nitro aromatics with different numbers of -NO 2 groups.

Generation of multiple triplet excitons from one singlet exciton (singlet fission, SF) has been reported in several organic molecules recently. The overall SF yield in such molecular materials, however, is controlled by polymorphism in organic semiconductors through noncovalent interactions like van der Waals and weak electrostatic interactions. In this article, we demonstrate how SF is strongly perturbed by even small variations in molecular packing for polymorphic crystals of triisopropylsilyethnyl-anthracene derivatives, TIPS-Ant (PI and PII). Based on quantum chemical calculations, SF dynamics have been computed for both PI and PII polymorphs. PI and PII differ in their intermolecular π···π stacking patterns, which eventually control their electronic properties. Using the incoherent hopping model for the crystals, we computed SF rate through the Marcus electron transfer theory. For both PI and PII, the direct two-electron pathway predominates over the charge-transfer (CT) mediated mechanism. PII has higher triplet yield (∼196%) compared to PI (∼178%). Both time-dependent DFT as well as Weller equation reveal that the charge transfer (CT) state is a high energy state, and hence, CT mediated SF barely influences triplet yield. Interplay of the local excitation (LE), multiple excitation (ME), and correlated triplet (T1T1) energy levels controlled the overall exciton dynamics/diffusion in TIPS-Ant polymorphs. Polymorphism is shown to be a key factor for the rational design of optimal SF in polyaromatic hydrocarbons (PAH).

Luminescent copper nanoclusters (Cu NCs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Here, we demonstrate the synthesis of L-cysteinecapped Cu NCs in aqueous medium having a bright cyan emission (489 nm) with a quantum yield of 6.2%. The structure of the Cu NCs (Cu 7 L 3 ) is investigated by using density functional theory (DFT) calculation and mass spectrometric study. Further, time-dependent density functional theory (TD-DFT) calculations provide the insights of electronic transitions, and it is correlated with experimental data. With near-HOMO−LUMO gap excitation, Cu NCs are excited to the S 4 state and subsequently relaxed to the S 1 state through an internal conversion process with a time scale in the ultrafast region (326.8 ± 6.5 fs). Furthermore, the structural relaxation in S 1 takes place at a picosecond time scale, and the radiative relaxation occurs from S 1 to S 0 . Finally, Cu NCs are attached with imidazole-functionalized partially reduced graphene oxide (ImRGO) via electrostatic attraction. A dramatic photoluminescence (PL) quenching and shortening of the decay time of the Cu cluster in the presence of ImRGO indicate the photoinduced electron transfer process, which is confirmed from ultrafast spectroscopic study. The photoinduced electron transfer in a Cu NC−ImRGO nanocomposite should pave the way for potential applications in light harvesting.

Recent toxicological assessments of graphene, graphene oxides, and some other two-dimensional (2D) materials have shown them to be substantially toxic at the nanoscale, where they inhibit and eventually disrupt biological processes. These shortfalls of graphene and analogs have resulted in a quest for novel biocompatible 2D materials with minimum cytotoxicity. In this article, we demonstrate CN (h2D-CN), a newly synthesized 2D porous graphene analog, to be non-nanotoxic toward genetic materials from an "in-silico" point of view through sequence-dependent binding of different polynucleotide single-stranded DNA (ssDNA) onto it. The calculated binding energy of nucleobases and the free energy of binding of polynucleotides follow the common trait, cytosine > guanine > adenine > thymine, and are well within the limits of physisorption. Ab-initio simulations completely exclude the possibility of any chemical reaction, demonstrating purely noncovalent binding of nucleobases with CN through a crucial interplay between hydrogen bonding and π-stacking interactions with the surface. Further, we show that the extent of distortion inflicted upon ssDNA by CN is negligible. Analysis of the density of states of the nucleobase-CN hybrids confirms minimum electronic perturbation of the bases after adsorption. Most importantly, we demonstrate the potency of CN in nucleic acid transportation via reversible binding of ssDNA. The plausible use of CN as a template for DNA repair is illustrated through an example of CN-assisted complementary ssDNA winding.

Recent reports have suggested that an external electric field (EEF) can assist and even control product selectivity. In this work, we have shown that the barrier for the Huisgen reaction between alkyl (aryl) azide and cyclooctyne(biflurocyclooctyne) is reduced by ∼3-4 kcal mol when an oriented EEF is applied along the reaction axis. As a consequence of their inherently polar transition-states (TSs), a parallel orientation of the EEF results in enhancement of the charge transfer (CT) between the fragments and concomitant increase in the dipole moment along the reaction axes. This leads to an increase in the reaction rate for moderate EEFs in the range of 0.3-0.5 V Å. Since highly polar and directional environments are omnipresent in biological environments, metal-free click reactions can be further accelerated for non-invasive imaging of live-cells. Conceptually, electric field control appears to be a novel tool (catalyst) to drive, and possibly even tune, the reactivity of organic molecules.