A quantitative analysis of excitonic superhyperchromism in porphyrin J-/H-aggregates

Aggregation of Carbocyanine Dyes in Choline Chloride-Based Deep Eutectic Solvents in the Presence of an Aqueous Base

Deep eutectic solvents (DESs) have shown potential as novel media to support molecular aggregation. The self-aggregation behavior of two common and popular carbocyanine dyes, 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazole carbocyanine (TDBC) and 5,5'-dichloro-3,3'-di(3-sulfopropyl)-9-methyl-benzothiacarbo cyanine (DMTC), is investigated within DES-based systems under ambient conditions. Although TDBC is known to form J-aggregates in basic aqueous solution, DMTC forms H-aggregates under similar conditions. The DESs used, glyceline and reline, are composed of salt choline chloride and two vastly different H-bond donors, glycerol and urea, respectively, in 1:2 mol ratios. Both DESs in the presence of base are found to support J-aggregates of TDBC. These fluorescent J-aggregates are characterized by small Stokes' shifts and subnanosecond fluorescence lifetimes. Under similar conditions, DMTC forms fluorescent H-aggregates along with J-aggregates within the two DES-based systems. The addition of cationic surfactant cetyltrimethylammonium bromide (CTAB) below its critical micelle concentration (cmc) to a TDBC solution of aqueous base-added glyceline shows the prominent presence of J-aggregates, and increasing the CTAB concentration to above cmc results in the disruption of J-aggregates and the formation of unprecedented H-aggregates. DMTC exclusively forms H-aggregates within a CTAB solution of aqueous base-added glyceline irrespective of the surfactant concentration. Anionic surfactant, sodium dodecylsulfate (SDS), present below its cmc within aqueous base-added DESs supports J-aggregation by TDBC; for similar SDS addition, DMTC forms H-aggregates within the glyceline-based system whereas both H- and J-aggregates exist within the reline-based system. A comparison of the carbocyanine dye behavior in various aqueous base-added DES systems to that in aqueous basic media reveals contrasting aggregation tendencies and/or efficiencies. Surfactants as additives are demonstrated to control and modulate carbocyanine dye self-aggregation within DES-based media. The unique nature of DESs as alternate media toward affecting cyanine dye aggregation is highlighted.

Size variability of the unit building block of peripheral light-harvesting antennas as a strategy for effective functioning of antennas of variable size that is controlled in vivo by light intensity

This work continuous a series of studies devoted to discovering principles of organization of natural antennas in photosynthetic microorganisms that generate in vivo large and highly effective light-harvesting structures. The largest antenna is observed in green photosynthesizing bacteria, which are able to grow over a wide range of light intensities and adapt to low intensities by increasing of size of peripheral BChl c/d/e antenna. However, increasing antenna size must inevitably cause structural changes needed to maintain high efficiency of its functioning. Our model calculations have demonstrated that aggregation of the light-harvesting antenna pigments represents one of the universal structural factors that optimize functioning of any antenna and manage antenna efficiency. If the degree of aggregation of antenna pigments is a variable parameter, then efficiency of the antenna increases with increasing size of a single aggregate of the antenna. This means that change in degree of pigment aggregation controlled by light-harvesting antenna size is biologically expedient. We showed in our previous work on the oligomeric chlorosomal BChl c superantenna of green bacteria of the Chloroflexaceae family that this principle of optimization of variable antenna structure, whose size is controlled by light intensity during growth of bacteria, is actually realized in vivo. Studies of this phenomenon are continued in the present work, expanding the number of studied biological materials and investigating optical linear and nonlinear spectra of chlorosomes having different structures. We show for oligomeric chlorosomal superantennas of green bacteria (from two different families, Chloroflexaceae and Oscillochloridaceae) that a single BChl c aggregate is of small size, and the degree of BChl c aggregation is a variable parameter, which is controlled by the size of the entire BChl c superantenna, and the latter, in turn, is controlled by light intensity in the course of cell culture growth.

Contrasting behavior of classical salts versus ionic liquids toward aqueous phase J-aggregate dissociation of a cyanine dye

The effect of addition of ionic liquids (ILs) on the aggregation behavior of a cyanine dye, 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH ≥ 10), TDBC preferably exists in its J-aggregated form. Addition of hydrophilic ILs > 5 wt % is observed to disrupt the TDBC J-aggregates, converting them to monomer form most likely because of the interaction between bulky IL cation and the J-aggregates in a time-dependent fashion. This is evidenced by the observed increase in monomer band absorbance at the expense of the absorbance band due to J-aggregates over time. Inorganic salts at similar molar concentrations do not cause this phenomenon but instead induce TDBC precipitation. At low concentrations (<5 wt %), the added IL acts similarly to the inorganic salts, reducing the overall absorbance of TDBC in the solution most likely due to cation exchange causing TDBC precipitation. Addition of a molecular solvent, ethanol, at 15 wt % results in an initial increase in monomer absorbance, albeit to a much lesser extent than for the corresponding molar fraction of IL, which then decreases over time with recovery of J-aggregate absorbance--quite opposite the time-dependent behavior seen for TDBC in PB at pH 12.0 with >5 wt % IL. The unique and dual behavior of ILs as an additive toward affecting cyanine dye aggregation is demonstrated.

Shape of the Q band in the absorption spectra of porphyrin nanotubes: Vibronic coupling or exciton effects?

Absorption and linear dichroism spectra of self-assembled tubular aggregates of TPPS(4) porphyrin are studied theoretically with special emphasis on the low energy part of the spectra (the Q band region) where the coupling with intramolecular vibrations is pronounced. The model Hamiltonian includes both the excitonic coupling between four molecular electronic excited states contributing to the porphyrin Q and B bands as well as the intermediate-strength linear exciton-phonon coupling to one effective high-frequency molecular vibrational mode. Good agreement between the calculated and experimental spectra is obtained. The results allow us to identify the nature of the peaks observed in the Q band region of the aggregate's absorption spectrum; we show that the two most prominent peaks within the Q band originate from two different excitonic subbands. It is shown that the coupling between the Q and B bands plays an important role and the vibronic coupling affects the details of the absorption lineshape.

Significance of the excitonic intensity borrowing in the J-/H-aggregates of bacteriochlorophylls/chlorophylls

A quantitative analysis of the excitonic intensity borrowing for the J-/H-aggregates of the bacteriochlorophylls/chlorophylls (BChls/Chls) in specific, and of porphyrins in general, is presented. The analysis is based on the argument that the mixing between the two energetically well-separated bands, such as the Q and B bands of BChls/Chls, should be considered important if the aggregated system possesses an excitonic superstate. A remarkably simple explanation of the significance of the excitonic intensity borrowing is given: superhyperchromism is manifested by the mediation of interband coupling between the superstates in the two well-separated bands of such aggregates. A comprehensive discussion on the significance of superhyperchromism and on its size-dependence is provided in connection with its effects on the absorption spectra of the BChl/Chl J- and H-aggregates.

The synthesis and properties of bis-1,1′-(porphyrinyl)ferrocenes

Ferrocene-bridged bisporphyrins have been synthesized by the condensation of corresponding dipyrromethane-derived diols with a bisdipyrromethane. Purification of the final compounds has been achieved without chromatography. The specific geometry of these bisporphyrins makes them valuable starting points for building complex molecular and supramolecular structures. In particular it provides a core to which multiple sites of attractive intermolecular interactions can be attached thereby creating compounds predisposed to form complex networks by association. We have studied the structure of bis-1,1'-(porphyrinyl)ferrocenes by 1H NMR, UV-Vis and electrochemistry. Results have shown that complex dynamic processes occur in these molecules (which may involve conformers, formation of H-aggregates and tautomers) and that they have non-typical electrochemical behaviour.