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

Nanotubes Self-Assembled from Amphiphilic Molecules via Helical Intermediates

https://www.cell.com/biophysj/pdf/S0006-3495(98)77916-0.pdf

Depolarized Resonance Light Scattering by Porphyrin and Chlorophyll a Aggregates

A dipeptide of the formula Fmoc-Leu-Asp and some other related dipeptides were synthesized in solution by standard methods. When such peptides are dissolved in water at concentrations below 1% at 100 degrees C and cooled below 60 degrees C they form turbid solutions and eventually viscoelastic gels at lower temperatures. Such gels are thermoreversible and can also be disrupted by mechanical agitation. At a concentration of 2 mg/ml the peptide Fmoc-Leu-Asp forms an aqueous gel at 60 degrees C with a shear modulus of 80 Pa measured at a frequency of 1 rad/s. Peptide solutions undergo an abrupt increase in light scattering between 1 and 1.5 mg/ml at both 23 and 60 degrees C. By analogy with previous observations of other systems, this increase appears to be due to the formation of filamentous micelles and the aggregation of filaments into a three-dimensional network. When low molecular weight adamantanamine derivatives, which are inherently non-antigenic antiviral drugs, were incorporated into the Fmoc-Leu-Asp gel and injected into rabbits, high titre specific antibodies were efficiently produced without the need for additional adjuvant. Both the physical properties of the gel and its effect on the antigenicity of low molecular weight compounds suggest a number of practical applications.

Use of a gel‐forming dipeptide derivative as a carrier for antigen presentation

The excited-state dynamics of two conjugated bis[(porphinato)zinc(II)] (bis[PZn]) species, bis[(5,5‘-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD) and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5‘,-15‘-ethynyl-10‘,20‘-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bis[PZn] compounds display intense near-infrared (NIR) transient S1→Sn absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S1→S0 oscillator strength on a 2−20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity;...

Ultrafast singlet excited-state polarization in electronically asymmetric ethyne-bridged bis[(porphinato)zinc(II)] complexes.

Micellar rods and vesicular tubules made of 14''',16'''-diaminoporphyrins

Micellar fibers made of porphyrin and metalloporphyrin amides

Some water-soluble porphyrins aggregate in aqueous media to form fibrous assemblies. When polynucleotides are used as matrices, porphyrin monomers are aligned at the periphery or intercalated. Porphyrin-porphyrin interactions hardly occur. Metal-metal or metal-ligand-metal interactions have been observed in oxo-oligomers, pyrazine-bridged dimers and radical dimers. β-Substituted metallo-porphyrins, however, form no polymers. The only linear polymerization, which has so far been observed in molecular asemblies, occurs with amphiphilic porphyrin amides, amines and carboxylates and their metal complexes in water and of bacteriochlorophyll in DMF/water. These compounds form long-lived high molecular weight micellar, inverse micellar and vesicular fibers spontaneously in aqueous media. The porphyrin fibers in water are about 4-6 nm thick and up to several of μm long.

Polymeric porphyrin and metalloporphyrin assemblies in bulk solution

Non-covalent porphyrin and metalloporphyrin fibers of bimolecular thinness in bulk aqueous media are compared with the well-known H- and J-aggregates of cyanines. The J-aggregates of cyanines fluoresce and are useful as photographic sensitizers. The J-aggregates of porphyrins show light-induced charge separation and the corresponding metal complexes produce stable radical dimers. The distance between the metalloporphyrin centers is calculated from circular dichroism spectra to be 8 A in the J-aggregates and about 4 A in the H-aggregates. Multi-electron reactions of the fibers in the ground and excited states can therefore occur in the fibrous porphyrin assemblies. In amphiphilic tetraphenylporphyrins (“octopus porphyrin”), on the other hand, the porphyrin–porphyrin distance is much larger and the fiber dissolves electron-accepting compounds, e.g. quinones, which also allow for multiple charge separation within such a fiber.

Multi‐electron transfer reactions and exciton interactions in fibrous porphyrin and metalloporphyrin assemblies

Micellar fibers and monolayers made of amphiphilic, cationic porphyrins are stabilized in bulk water by poly(allylamine) at pH values close to the pkaof the amines. Anionic polyelectrolytes, e.g., poly(vinyl sulfate), can also be used, but in order to prevent precipitation one must use a large polymer excess and start with small assemblies. The polyelectrolyte additions not only prevent precipitation of the large molecular assemblies with molecular weights far above 106Da, but also draw back any monomers into the assemblies. Small absorption changes in 3 ns flash photolysis experiments became detectable only in the polymer-stabilized solutions.

Optimized Polyelectrolyte Stabilization Procedures for Polymeric Porphyrin Assemblies in Water Allow the Analysis of Minute Photoreactivities

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