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

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