A major part of contemporary nanomaterials research is focused on metal and semiconductor nanoparticles, constituted of extended lattices of atoms or ions. Molecular nanoparticles assembled from small molecules through non-covalent interactions are relatively less explored but equally fascinating materials. Their unique and versatile characteristics have attracted considerable attention in recent years, establishing their identity and status as a novel class of nanomaterials. Optical characteristics of molecular nanoparticles capture the essence of their nanoscale features and form the basis of a variety of applications. This review describes the advances made in the field of fabrication of molecular nanoparticles, the wide spectrum of their optical and nonlinear optical characteristics and explorations of the potential applications that exploit their unique optical attributes.

The classical model for crystallization visualizes the formation of a metastable crystalline nucleus that through density fluctuations reaches a critical size and grows into a stable crystal. The two-step nucleation theory invoking a liquid-like cluster intermediate developed to explain protein crystallization, [1,2] has been shown to be of more general validity, and applicable to macromolecule as well as small molecule crystals. [3] The amorphous-to-crystalline transformation (ACT) familiar in protein-mineral composites, [4][5][6] pharamaceuticals [7,8] and inorganics [9][10][11][12][13][14][15] including phase-change materials, [16] is of potential interest in the context of molecular crystals and nanocrystals as well. Heterogeneous nucleation and growth in confined environments [17] add a further dimension to the crystallization process.The coexistence of amorphous/crystalline regions and the impact of crystallinity on properties in macromolecules are well-known. Targeted fabrication of the amorphous state, realization of the ACT and the crystallinity-property correlation in small-molecule-based materials have been explored little, but are significant from the conceptual as well as application perspectives. Thermal annealing induces such changes, for example, in rubrene [18] and methanofullerene [19] thin films. Crystalline and amorphous forms of (4-biphenylyl)phenyldibenzofulvene were interconverted by solvent vapor fuming, heating and cooling, the former exhibiting enhanced fluorescence. [20] The photosensitivity of dye aggregates increased when crystallinity was induced through solvent vapor fuming. [21] The crystallinity of materials critically affects the charge mobilities in organic light-emitting diodes (OLEDs). [22] To explore the ACT in small-moleculebased materials, it is imperative to develop a molecular design and protocol for the fabrication of amorphous particles followed by their crystallization, and monitor the transformation together with some materials response that evolves concomitantly.Crystals of 7,7-diamino-8,8-dicyanoquinodimethanes (DADQs) exhibit strong fluorescence enhancement relative to their solutions, because of the inhibition of excited-state geometry relaxation in the former case. [23] The emission color can be tuned by structural variations [24][25][26] and nanocrystals with strong emission have been fabricated. [24,27] The zwitterionic DADQs crystallize easily as observed with NLOactive [28] and fluorescent [23][24][25][26][27] derivatives (NLO = nonlinear optical). Present explorations show that specific structural motifs promote the formation of amorphous particles, when solutions are drop-cast onto suitable substrates. As traditional methods failed to induce an ACT, we developed a novel protocol involving fuming of particles partially confined by fixing in a polymer thin film. The transformation is monitored by microscopy along with the accompanying fluorescence switching and enhancement.DADQs bearing aromatic rings linked through conformationally labile bonds [29] are foun...

A 7‐pyrrolidino‐7‐benzylamino‐8,8‐dicyanoquinodimethane, PBEDQ, (1), donor–acceptor–modified electrode yields, in the presence of hydroquinone, (2), an anodic photocurrent with quantum efficiency of 1.5%. The PBEDQ‐functionalized electrode yields, in the presence of the electron acceptor diquat, (3), a cathodic photocurrent with a quantum efficiency corresponding to 2.1%. The electron transfer cascades leading to the anodic or cathodic photocurrents in the different systems are discussed. It is further demonstrated that the integration of 1,4‐dihydronicotinamide adenine dinucleotide, NADH, as electron donor, with the PBEDQ‐modified electrode leads to an anodic photocurrent. This allowed the assembly of a photobioelectrochemical integrated electrode composed of the photoactive PBEDQ donor–acceptor compound, NAD+ as cofactor, and the NAD+‐dependent glucose dehydrogenase, GDH. Irradiation of the integrated electrode in the presence of glucose results in the GDH–biocatalyzed oxidation of glucose to gluconic acid with the concomitant generation of NADH that acts as electron donor for the photoactive donor–acceptor PBEDQ units, leading to the generation of steady‐state anodic photocurrent. The photocurrent intensities are controlled by the concentrations of glucose. The integrated PBEDQ/NAD+/GDH electrodes introduces a functional photobioelectrochemical electrode for the detection of glucose, and demonstrates the assembly of a functional photo‐biofuel cell that uses light and a biomass product (glucose) for the generation of electric power.

Fabrication of an amorphous phase followed by a hierarchy of forms leading to the crystalline state offers a new dimension in the assembly of small molecule based materials. The morphology and extent of crystallinity of the nanostructures in drop-cast thin films of a diaminodicyanoquinodimethane molecule are shown to be tunable through the variation in the composition of the solvent mixture used for dropcasting. The different states of assembly starting from the solvated molecule through amorphous spherical particles, crystalline nanofibers and nano/microcrystals to bulk crystals are shown to be accompanied by a smooth variation of the fluorescence emission, the color evolving from green to blue, and the efficiency increasing steadily with the overall enhancement from the solution to the bulk crystalline state being $400 times. Quantum chemical computations on the molecule and its H-bonded dimer and p-dimers provide insight into the impact of intermolecular interactions in the crystalline assemblies on the electronic structure. The current study illustrates a significant departure from the conventional molecules-to-materials transition, opening up new hierarchical pathways of assembly of molecular materials.

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