Systems, mechanisms and unique phenomena associated with fluorescence enhancement from molecules to materials are reviewed, highlighting the critical role of molecular assembly.
Development of a coherent picture of enhanced fluorescence in the aggregated/solid state of molecular materials requires an exploration of the concomitant inhibition of intra and intermolecular non-radiative energy loss pathways. This necessitates a fluorophore that exhibits a systematic variation of the emission enhancement (solid over solution) upon subtle structural tuning at the molecular and supramolecular levels. Diaminodicyanoquinodimethanes with an imidazolidine moiety (1a), reported in 1962 but never structurally characterized, is shown to be ideally suited for this. 1a and its N-ethyl (1b) and N,N 0 -dimethyl (1c) derivatives are synthesized by a modified route and structurally characterized. Systematic change in the molecular structure (a crucial torsion angle varying from B31 to 501) and hence assembly in crystals, increases the fluorescence enhancement from B30 (1a) to B900 (1c). A methodology based on ab initio and lattice energy calculations and analysis of the organization of molecules and their transition dipoles in crystals is developed, to quantitatively assess the inhibition of excited state relaxation and relative energy transfer rates in solids. This approach provides insight into the contribution of intra and intermolecular pathways to the structural tuning of the emission enhancement in 1a-c, and a rational basis to tailor highly emissive molecular solids.
Functional phase-change materials (PCMs) are conspicuously absent among molecular materials in which the various attributes of inorganic solids have been realized. While organic PCMs are primarily limited to thermal storage systems, the amorphous-crystalline transformation of materials like Ge-Sb-Te find use in advanced applications such as information storage. Reversible amorphous-crystalline transformations in molecular solids require a subtle balance between robust supramolecular assembly and flexible structural elements. We report novel diaminodicyanoquinodimethanes that achieve this transformation by interlinked helical assemblies coupled with conformationally flexible alkoxyalkyl chains. They exhibit highly reversible thermal transformations between bistable (crystalline/amorphous) forms, along with a prominent switching of the fluorescence emission energy and intensity.
Heterocyclic building blocks possessing ethylene spacer and amine functionality such as 1-(2-aminoethyl)piperidine (1,2-AEPi), 2-(2-aminoethyl)pyridine (2,2-AEPy) and 1-(2-aminoethyl)pyrrolidine (1,2-AEPr) were reacted with tetracyanoquinodimethane (TCNQ) to give disubstituted compounds namely bis-(1-(2-aminoethyl)piperidino)dicyanoquinodimethane (1), bis-(2-(2-aminoethyl)pyridino)dicyanoquinodimethane (2) and bis-(1-(2-aminoethyl)pyrrolidino)dicyanoquinodimethane (3). Utilization of 1,2-AEPi, 2,2-AEPy and 1,2-AEPr as disubstituents on TCNQ has resulted in interesting crystal structures. Inter- and intramolecular hydrogen-bond mediated and expanded supramolecular structures were observed in the lattices of the crystals. Strong fluorescence was observed in solids and solutions. (2) showed a strong second harmonic generation (SHG) whereas (1) and (3) were found to be SHG inactive. All compounds possess good thermal stabilities.
The general occurrence of fluorescence emission quenching in molecular aggregates is circumvented in select classes of molecules. This has largely been attributed to the rigidification of the molecule and its environment, which hinders non-radiative excited state energy loss through structural relaxation; since such an effect should in principle apply to most aggregates and crystals, there must clearly be other critical factors that make the select molecules exceptional. Discovery of three crystalline structures of a new push-pull molecule in its enantiomorphic and racemic forms, exhibiting not only very high, but distinctly different solid state fluorescence enhancements, has now allowed a systematic investigation of the role of intramolecular and intermolecular excited state energy loss pathways. Crystallographic, spectroscopic and computational investigations provide a detailed appraisal of the assembly patterns in the crystals, and rigorous establishment of an inverse correlation between intermolecular energy transfer and solid state fluorescence. The study provides a clear visualization of the critical role of oriented molecular aggregation in solid state fluorescence efficiency enhancement.
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