Mechanistic Insight into the Thermal "Blueing" of Cyanine Dyes.

In recent work to develop cyanine dyes with especially large Stokes shifts, we encountered a "blueing" reaction, in which the heptamethine cyanine dye Cy7 (IUPAC: 1,3,3-trimethyl-2-((1E,3E,5E)-7-((E)-1,3,3-trimethylindolin-2-ylidene)hepta-1,3,5-trien-1-yl)-3H-indol-1-ium) undergoes shortening in two-carbon steps to form the pentamethine (Cy5) and trimethine (Cy3) analogs. Each step blue-shifts the resulting absorbance wavelength by ca. 100 nm. Though photochemical and oxidative chain-shortening reactions had been noted previously, it is simple heating alone or with amine bases that effects this unexpected net C2H2 excision. Explicit acetylene loss would be too endothermic to merit consideration. Our mechanistic studies using 2H labeling, mass spectrometric and NMR spectroscopic analyses, and quantum chemical modeling point instead to electrocyclic closure and aromatization of the heptamethine chain in Cy7 forming Fischer's base FB (1,3,3-trimethyl-2-methyleneindoline), a reactive carbon nucleophile that initiates chain shortening of the cyanine dyes by attack on their polymethine backbones. The byproduct is the cationic indolium species TMP (IUPAC: 1,3,3 trimethyl-2-phenyl indolium).

Enhanced Lifetime of Cyanine Salts in Dilute Matrix Luminescent Solar Concentrators via Counterion Tuning.

Organic luminophores offer great potential for energy harvesting and light emission due to tunable spectral properties, strong luminescence, high solubility, and excellent wavelength-selectivity. To realize their full potential, the lifetimes of luminophores must extend to many years under illumination. Many organic luminophores, however, have a tendency to degrade and undergo rapid photobleaching, leading to the perception of intrinsic instability of organic molecules. In this work we demonstrate that by exchanging the counterion of a heptamethine cyanine salt the photostability and corresponding lifetime of dilute cyanine salts can be enhanced by orders of magnitude from 10 hours to an extrapolated lifetime of greater than 65,000 hours under illumination. To help correlate and comprehend the underlying mechanism behind this phenomenon, the water contact angle and binding energy of each pairing were measured and calculated. We find that increased water contact angle, and therefore increasing hydrophobicity, generally correlate to improved lifetimes. Similarly, a lower absolute binding energy between cation and anion correlates to increased lifetimes. Utilizing the binding energy formalism, we predict the stability of a new anion and experimentally verify with good consistency. Moving forward, these factors could be used to rapidly screen and identify highly photostable organic luminophore salt systems for a range of energy harvesting and light emitting applications.

Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent.

Substituted fluorene structures have demonstrated unusual photochemical properties. Previous reports on the substituted fluorene Schiff base FR0-SB demonstrated super photobase behavior with a ΔpKb of ∼14 upon photoexcitation. In an effort to understand the basis for this unusual behavior, we have examined the electronic structure and relaxation dynamics of the structural precursor of FR0-SB, the aldehyde FR0, in protic and aprotic solvents using time-resolved fluorescence spectroscopy and quantum chemical calculations. The calculations show three excited singlet states in relatively close energetic proximity. The spectroscopic data are consistent with relaxation dynamics from these electronic states that depend on the presence and concentration of solvent hydroxyl functionality. These results underscore the central role of solvent hydrogen bonding to the FR0 aldehyde oxygen in mediating the relaxation dynamics within this molecule.

Intramolecular Relaxation Dynamics Mediated by Solvent-Solute Interactions of Substituted Fluorene Derivatives. Solute Structural Dependence.

Several fluorene derivatives exhibit excited-state reactivity and relaxation dynamics that remain to be understood fully. We report here the spectral relaxation dynamics of two fluorene derivatives to evaluate the role of structural modification in the intramolecular relaxation dynamics and intermolecular interactions that characterize this family of chromophores. We have examined the time-resolved spectral relaxation dynamics of two compounds, NCy-FR0 and MK-FR0, in protic and aprotic solvents using steady-state and time-resolved emission spectroscopy and quantum chemical computations. Both compounds exhibit spectral relaxation characteristics similar to those seen in FR0, indicating that hydrogen bonding interactions between the chromophore and solvent protons play a significant role in determining the relaxation pathways available to three excited electronic states.