The molecular exciton model has received its most extensive development and application in the field of molecular crystals1'2. More recently, numerous applications to non-crystalline molecular composite systems have been made, including van der Waals and hydrogen-bonded dimers, trimers, and higher order aggregates. Another type of composite system has also been investigated, namely the composite molecule consisting of covalently bonded molecular units, with intrinsic individual unsaturated electronic systems so isolated by single bonds that but little or insignificant electronic overlap between units may occur. It is now well established that in molecular aggregates and in composite molecules, exciton effects may be observed if sufficiently strong electronic transitions exist in the component sub-units. The result of exciton splitting of excited states in the composite molecule may be the appearance of strong spectral shifts or splittings (which may be of the order of 2000 cm—1) of the absorption bands for the component molecules. At the same time, as a consequence of the exciton splitting of the excited state manifold, an enhancement of triplet state excitation may result. The purpose of this paper is to present a summary of the various type cases for molecular dimers, trimers and double and triple molecules in the description of the molecular exciton strong-coupling model. Then it will be shown by new experimental examples that, even in those cases where no significant exciton effect is observable in the singlet—singlet absorption spectrum for the composite molecule (intermediate and weak coupling cases), the enhancement of lowest triplet state excitation may still be conspicuous and significant. The ideas which are summarized in this paper have a curious history. Long ago, Kautsky and Merkel3 demonstrated experimentally that aggregation of dyes facilitated their action as photophysical sensitizers in photochemical reactions, at the same time diminishing their fluorescence efficiency. Kautsky attributed these easily demonstrated effects to enhancement of metastable state excitation in the aggregate dye. There is no doubt today that the metastable state he described is the lowest triplet state of the molecules studied. However, he did not distinguish between intrinsic and enhanced metastable (triplet) state excitation, so his interpretations were largely overlooked. Forster in l946 used the quasi-classical vector model to

/pdf/the-exciton-model-in-molecular-spectroscopy-2vwkowivjh.pdf

The exciton model in molecular spectroscopy

The condensation reaction between p-tert-butylphenol and formaldehyde leads in a single step to good yields of cyclic oligomers in which, depending on the reaction conditions, either four, six, or eight phenol units are joined by methylene bridges. The beakerlike shape of the most stable conformation of the tetramer has led to their being given the name “calixarenes” (calix = chalice). Resorcinol can undergo condensation in a similar manner with a variety of aldehydes to afford cyclic tetramers with the same basic structure (the resorcarenes). In both cases the reaction does not require the use of dilution techniques, so that large quantities of product can be readily obtained. In addition, the parent compounds can be modified in various ways, in particular at the phenolic hydroxy groups or the phenyl residues; these approaches can be used separately or in combination. Calixarenes are thus ideal starting materials for the synthesis of various types of host molecules and can also act as building blocks for the construction of larger molecular systems with defined structures and functions. Their potential applications range from use as highly specific ligands for analytical chemistry, sensor techniques and medical diagnostics to their use in the decontamination of waste water and the construction of artificial enzymes and the synthesis of new materials for non-linear optics or for ultrathin layers and sieve membranes with molecular pores.

Calixarenes, Macrocycles with (Almost) Unlimited Possibilities

The challenge of cyclic and acyclic schiff bases and related derivatives

This paper deals with recent and important developments in the field of organic materials for second-order nonlinear optics. Attention is drawn to current trends in chromophore design with a discussion of current progress and problems in this field. A number of important classes of chromophores, such as one-dimensional charge-transfer molecules, octopolar compounds, ionic materials, multichromophore systems and organometallics, are discussed.

Second-order nonlinear optical materials: recent advances in chromophore design

Weak intermolecular forces like dipolar interactions and hydrogen-bonding lead to a variety of different packing arrangements of molecules in crystals and self-assemblies. Such differences in the arrangements change the extent of excitonic splitting and excitation spectra in the multichromophore aggregates. In this tutorial review, the role of such interactions in fine tuning the linear and 1st non-linear optical (NLO) responses in molecular aggregates are discussed. The non-additivity of these optical properties arise specifically due to such cooperative interactions. Calculations performed on dimers, trimers and higher aggregates for model systems provide insights into the interaction mechanisms and strategies to enhance the 1st hyperpolarizabilities of π-conjugated molecular assemblies. Flexible dipole orientations in the alkane bridged chromophores show odd-even variations in their second-harmonic responses that are explained through their dipolar interactions in different conformations. Parameters for the optical applications of molecules arranged in constrained geometry, like in Calix[n]arene, have been elucidated. We also highlight the recent developments in this field of research together with their future prospects.

Dipolar interactions and hydrogen bonding in supramolecular aggregates: understanding cooperative phenomena for 1st hyperpolarizability

The frequency-doubling of 820 nm laser light is o­ne of the properties of a series of nitrocalix-[4]arenes making them suitable, in principle, for the construction of a blue laser. Derivative 1 is the most promising, since it displays the greatest hyperpolarizability and the largest dipole moment and is stable as a film in a polymer matrix for long periods.

/pdf/nitrocalix-4-arenes-as-molecules-for-second-order-nonlinear-3lbq1f6uc3.pdf

Nitrocalix [4]arenes as Molecules for Second-Order Nonlinear Optics

In substituted calix[4]-arenes-cyclophanes consisting of four phenol moieties connected by methylene bridges-the number and also the relative positions of the donor and/or acceptor pi- conjugated (D-pi-A) units can be varied by changing the configuration. The Figure illustrates the cone configuration. The molecular and thin-film NLO properties of three tetra-D-pi- A-substituted calix[4]arenes, including their orientation in thin films and the stability of their NLO properties, are described.

/pdf/highly-ordered-films-of-neat-calix-4-arenes-for-second-order-1bzluwrpdg.pdf

Highly ordered films of neat calix[4]arenes for second order nonlinear optics

Calix[4]arenes are a novel type of molecules for nonlinear optics. In a single molecule four pi-conjugated systems are combined. Corona poled guest-host polymer systems [calix[4]arenes-poly(methyl-methacrylate)] with transparency into the UV and loading up to 100% show a high degree of orientation. The maximum resonant d33 nonlinear coefficient measured at a fundamental wavelength of 590 nm is 51 pm/V. The d33 values of the thin films relax after poling to 65% of their maximum value. Results indicate self-organization of the molecules upon poling.

/pdf/novel-calixarenes-in-thin-films-for-efficient-second-3vqirsq3d7.pdf

Novel calixarenes in thin films for efficient second harmonic generation

Zur Frequenzverdopplung von 820-nm-laser-Licht und damit prinzipiell zum Aufbau eines Blaulicht-Lasers geeignet ist eine Reihe von Nitrocalix[4]arenen. Am aussichtsreichsten erscheint das Derivat 1, das die groste Hyperpolarisierbarkeit und das groste Dipolmoment aufweist und das als Film in einer Polymermatrix uber langere Zeitraume stabil ist.

/pdf/nitrocalix-4-arene-als-verbindungen-fur-die-nichtlineare-3di39ylbkv.pdf

Nitrocalix[4]arene als Verbindungen für die nichtlineare Optik zweiter Ordnung

/pdf/triphenylcarbinol-derivatives-as-molecules-for-second-order-3ddmyddsl5.pdf

Triphenylcarbinol derivatives as molecules for second-order nonlinear optics

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