Structural Fine-tuning to Achieve Highly Fluorescent Organic and Water-soluble Thiazolo[5,4-d]thiazole Chromophores.

Fluorescent molecular systems are important for various applications such as sensing of analytes, probes for biologically relevant processes and as optoelectronic materials. Achieving high fluorescence quantum yield across the spectrum of solvent polarity and in solid-state is challenging in molecular materials. Herein, we present a strategy to achieve strongly fluorescent molecular materials based on weak intramolecular charge-transfer (ICT) in a family of unsymmetrical donor-thiazolo[5,4-d]thiazoles-acceptor systems (both neutral and cationic). Detailed photophysical studies reveal that the delicate balance between the donor and acceptor result in high solution-state fluorescence quantum yield (> 80%) in both polar protic and apolar solvents. Quantum chemical computations uncover a hitherto unappreciated insight that the extent of ICT is aptly represented by the change in Mulliken charges between the ground and excited-state for different fragments rather than the classical approach of monitoring the change in dipole moment for the entire molecule. This insight rationalizes the observed photophysical properties and can have implications in the design of tuneable donor-π-acceptor systems.

Hierarchical Self-Assembly and Aggregation-Induced EmissionEnhancement in Tetrabenzofluorene-based Red Emitting Molecules.

The newly synthesized chiral active [5]helicene-like tetrabenzofluorene (TBF) based highly red-emitting molecules exhibit flower-like self-assembly. These molecules display photophysical and structural properties such as intramolecular charge transfer, dual state emission, large fluorescence  quantum yield, and solvatochromism. In TBFID, the indandione functional group attached on both sides as the terminal group offers an A-D-A push-pull effect and acts as a strong acceptor to cause more redshift in solution as well as in solid state as compared to TBFPA (TBF with benzaldehyde functional group in terminal position). The self-assembly studies of TBFID demonstrate the aggregation-induced emission enhancement (AIEE) attributed to the restriction of intramolecular rotation at the aggregated state. Furthermore, TBFID shows high quantum yield and intense red emission, making the molecule fit for organic light-emitting diodes (OLED) and bioimaging applications.

13C NMR Investigations of Hairy-Rod-Like π-Conjugated Mesogens.

Two hairy-rod π-conjugated mesogens comprising dihexylfluorene and didecyloxyterphenyl are synthesized, and their mesophase properties are examined by hot-stage optical polarizing microscope and differential scanning calorimetry techniques. Both mesogens exhibit enantiotropic nematic mesophase with broad mesophase range. A detailed 13C NMR study is carried out in solution as well as in nematic mesophase to understand changes in the orientation of the aliphatic chains. Accordingly, the unusual NMR chemical shift value of one of the methylene carbons of the hexyl chain of fluorene moiety in solution is ascribed to ring current effect changes in nematic mesophase owing to variation in the orientation of the hexyl chain. The change in conformation of lateral and alkyl chains in the nematic phase is clearly noticed compared to isotropic solution as the molecular orientation in mesophase is governed by the orientation of the long molecular axis. Furthermore, the 13C-1H dipolar couplings obtained from 2D separated local field experiments in nematic phase aided the assignments of all of the carbons of the molecules besides offering the local order. To comprehend the orientation of the fluorene unit, three order parameters are necessary whereas for the phenyl rings, two order parameters are found to be sufficient. The molecular biaxiality ( S xx - S yy) for fluorene-based mesogen is found to be higher due to the fused nature of the moiety and the presence of dihexyl chains. The orientational ordering of π-conjugated mesogens is crucial as the optoelectronic properties of them critically depend on the orientation of the constituent chromophores. The complete mapping of order parameters of fluorene, the phenyl rings and the alkyl chains showed that despite both molecules exhibiting nematic mesophase, the orientational constraints are governed by the molecular structure as well as the rigidity of the core unit, i.e., fused versus linearly connected rings.

"Bottom-up" self-assembly and "cold crystallization" of butterfly shaped tetrabenzofluorene molecules.

Butterfly-shaped tetrabenzo[a,c,g,i]fluorene (TBF)-based molecules (1 and 2) were designed, synthesized and well characterized using various spectroscopic techniques. The single crystal X-ray structure of 1 shows the presence of intermolecular 3-D π-π stacking interaction and unprecedented "cold crystallization" in polycyclic aromatic molecules. We report for the first time, the "bottom-up" self-assembly of TBF based organic molecules. The supramolecular studies reveal the formation of vesicles and cuboid-shaped nanocrystals in THF-water and toluene solution, respectively.