Theoretical Study of Novel Azo-Tetraphenylporphyrins: Potential Photovoltaic Materials

A density functional theory study was performed to analyze the electron donor–acceptor properties of the cis and trans isomers of a novel azobenzene-containing tetraphenylporphyrin (TPPN₂PhC₁₄H₂₉) with different substituents (Br or TMS). In general, the trans isomers are better electron acceptors than the correspondent cis homologues. Their UV–vis spectra were also obtained and a comparison with available experimental results is included. According to these results, the azo compounds reported here are promising materials for the elaboration of dye-sensitized solar cells because their HOMO–LUMO gaps are close to 2 eV. Moreover, the energy of the high intensity absorption bands also fulfills the requirements needed for the operation of a solar cell built with TiO₂ and the I^–/I₃^– pair

Fluorescence Resonance Energy Transfer in Partially and Fully Labeled Pyrene Dendronized Porphyrins Studied with Model Free Analysis

A series of dendronized porphyrins were synthesized and their photophysical properties were determined by UV–vis absorption, steady-state fluorescence, and time-resolved fluorescence. The constructs contained a porphyrin core connected to a first generation Fréchet-type dendron (Py₂G1) with or without a C₄-butanoate linker, and to a second generation dendron (Py₄G2) with a C₄-linker. Pyrene and porphyrin were selected as donor and acceptor, respectively, for fluorescence resonance energy transfer or FRET. FRET occurred efficiently within the dendronized porphyrins as determined from the extremely weak fluorescence of pyrene. The number of pyrene groups present in the constructs was varied from two to eight, but was found to have little effect on FRET as FRET took place efficiently from an excited pyrene to a ground-state porphyrin. The parameter that was found to affect FRET the most was the distance separating pyrene from porphyrin within a construct. This effect was probed successfully by fitting the pyrene and porphyrin fluorescence decays according to the model free analysis (MFA) which yielded the average rate constant ⟨<i>k</i>_ET⟩ for FRET. ⟨<i>k</i>_ET⟩ increased continuously with decreasing distance separating porphyrin and pyrene as determined by conducting molecular mechanics optimizations on the constructs. The ⟨<i>k</i>_ET⟩ values were used to determine the through-space distance <i>d</i>_Por–Py^TS separating porphyrin from pyrene. <i>d</i>_Por–Py^TS was found to scale as (<i>d</i>_Por–Py^EXT)^0.5, where <i>d</i>_Por–Py^EXT represents the distance separating porphyrin and pyrene when the construct adopts its most extended conformation. This relationship suggests that FRET occurs intramolecularly inside the constructs between pyrene and porphyrin where both dyes are linked by a string of freely jointed Kuhn segments of length <i>l</i>_K = 9 Å

Unexpected Absorbance Enhancement upon Clustering Dyes in a Polymer Matrix

PE films grafted with poly­(methyl acrylate) and labeled with pyrene groups were obtained by irradiation with γ-rays in the presence of acryloyl chloride and further reacting them with 1-pyrenebutanol or 1-pyrenemethylamine. Characterization of the polymer films benefited from the dual use of the pyrene probe as an indicator of, first, polymer chain dynamics by monitoring pyrene excimer formation by fluorescence and, second, polymer morphology by staining the pyrene-rich domains of the films with RuO₄ for scanning electron microscopy (SEM). The grafted polymers labeled with 1-pyrenemethylamine showed much stronger absorbance than those labeled with 1-pyrenebutanol despite having similar pyrene contents. The fluorescence spectra of the grafted polymers labeled with 1-pyrenebutanol exhibited monomer emission, whereas those labeled with 1-pyrenemethylamine exhibited exclusively excimer emission. These dramatic differences could be accounted for by noting that labeling of the grafted poly­(acryloyl chloride) with 1-pyrenemethylamine results in cross-linking of the polymer matrix, with an associated enhancement of the concentration of pyrene in the cross-linked domains, which was confirmed by SEM. Formation of discrete domains in the polymer film can induce multiple scattering at the domain boundaries which lengthens the path of light in the film and increases absorption of the light by the tightly packed pyrene-rich domains. Implementation of this effect for fabrication of plastic color filters should generate more efficient filters which should find numerous practical applications

Synthesis and Characterization of Novel Pyrene-Dendronized Porphyrins Exhibiting Efficient Fluorescence Resonance Energy Transfer: Optical and Photophysical Properties

A novel series of pyrene dendronized porphyrins bearing two and four pyrenyl groups (Py₂-TMEG1 and Py₄-TMEG2) were successfully synthesized. First and second generation Fréchet type dendrons (Py₂-G1OH and Py₄-G2OH) were prepared from 1-pyrenylbutanol and 3,5-dihydroxybenzyl alcohol. These compounds were further linked to a trimesitylphenylporphyrin containing a butyric acid spacer via an esterification reaction to obtain the desired products. Dendrons and dendronized porphyrins were fully characterized by FTIR and ¹H NMR spectroscopy and their molecular weights were determined by matrix-assisted laser desorption ionization time of flight mass spectrometry. Their optical and photophysical properties were studied by absorption and fluorescence spectroscopies. The formation of dynamic excimers was detected in the pyrene-labeled dendrons, with more excimer being produced in the higher generation dendron. The fluorescence spectra of the pyrene dendronized porphyrins exhibited a significant decrease in the amount of pyrene monomer and excimer emission, jointly with the appearance of a new emission band at 661 nm characteristic of porphyrin emission, an indication that fluorescence resonance energy transfer (FRET) occurred from one of the excited pyrene species to the porphyrin. The FRET efficiency was found to be almost quantitative ranging between 97% and 99% depending on the construct. Model Free analysis of the fluorescence decays acquired with the pyrene monomer, excimer, and porphyrin core established that only residual pyrene excimer formation in the dendrons could occur before FRET from the excited pyrene monomer to the ground-state porphyrin core

Studying Pyrene-Labeled Macromolecules with the Model-Free Analysis

The model-free (MF) analysis was applied to the fluorescence decays of 32 pyrene-labeled macromolecules to probe their internal dynamics. Depending on whether a pyrene derivative was attached to the chain ends of a linear chain, randomly along a polymer backbone, or at the chain terminals of dendrimers, the MF analysis was applied to probe the dynamics of polymer ring closure, backbone flexibility, or chain terminal mobility, respectively. For those polymeric constructs whose decays could be fitted according to Birks’ scheme or the fluorescence blob model (FBM), good agreement was obtained between the rate constant for excimer formation retrieved from the MF analysis ⟨<i>k</i>^MF⟩ and those obtained according to the Birks’ scheme or FBM analyses. The MF analysis was also applied to conduct the first successful direct comparison of the chain terminal dynamics of two types of pyrene end-labeled dendrons. Finally, the MF analysis was employed to build a calibration curve against which the internal dynamics of any pyrene-labeled macromolecule can now be benchmarked. This study further confirms the versatility and robustness of the MF analysis to study any type of pyrene-labeled macromolecule