Alexandre Haefele

Boron Dipyrromethene Chromophores: Next Generation Triplet Acceptors/Annihilators for Low Power Upconversion Schemes

In the present study, the red-light absorbing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) was used as a triplet sensitizer in conjunction with two distinct iodophenyl-bearing BODIPY derivatives independently serving as triplet acceptors/annihilators poised for photon upconversion based on triplet-triplet annihilation. In deaerated benzene solutions, extremely stable and high quantum efficiency green (Phi(UC) = 0.0313 +/- 0.0005) and yellow (Phi(UC) = 0.0753 +/- 0.0036) upconverted emissions were observed from selective red excitation of the PtTPBP sensitizer at 635 +/- 5 nm. The current systems represent the first examples of photon upconversion where aromatic hydrocarbons do not serve the role of triplet acceptor/annihilator. Notably, the nature of the current chromophore compositions permitted highly reproducible upconversion quantum efficiency determinations while permitting the evaluation of the triplet-triplet annihilation quantum yields in both instances.

Length Dependence for Intramolecular Energy Transfer in Three- and Four-Color Donor-Spacer-Acceptor Arrays

A series of donor-spacer-acceptor triads has been synthesized and fully characterized. Both donor and acceptor units are built from boron dipyrromethene (BODIPY) dyes but they differ in their respective conjugation lengths, and thereby offer quite disparate optical properties. The spacer units comprise an oligomer of 1,4-phenylene-diethynylene repeat units and allow the boron-boron separation distance to be varied progressively from 18 to 38 A. A notable feature of this series is that each subunit can be selectively excited with monochromatic light. Highly efficacious electronic energy transfer (EET) occurs from the first-excited singlet state localized on the conventional BODIPY dye to its counterpart resident on the expanded BODIPY-based nucleus, but the rate constant follows a nonlinear evolution with separation distance. Overall, the rate of EET falls by only a factor of 4-fold on moving from the shortest to the longest spacer. This shallow length dependence is a consequence of the energy gap between donor and spacer units becoming smaller as the molecular length increases. Interestingly, a simple relationship exists between the measured electronic resistance of the spacer unit and the Huang-Rhys factor determined by emission spectroscopy. Both parameters relate to the effective conjugation length. Direct illumination of the spacer unit leads to EET to both terminals, followed by EET from conventional BODIPY to the expanded version. In each case, EET to the expanded dye involves initial population of the second-singlet excited state, whereas transfer from spacer to the conventional BODIPY dye populates the S(2) state for shorter lengths but the S(1) state for the longer analogues. The rate of EET from spacer to conventional BODIPY dye, as measured for the corresponding molecular dyads, is extremely fast (>10(11) s(-1)) and scales with the spectral overlap integral. The relative partitioning of EET from the spacer to each terminal is somewhat sensitive to the molecular length, with the propensity to populate the conventional BODIPY dye changing from 65% for N = 0 to 45% for N = 2. The most likely explanation for this behavior can be traced to the disparate spectral overlap integrals for the two dyes. These systems have been complemented by a molecular tetrad in which pyrene residues replace the fluorine atoms present on the conventional BODIPY-based dye. Here, rapid EET occurs from pyrene to the BODIPY dye and is followed by slower, long-range EET to the opposite terminal. Such materials are seen as highly attractive solar concentrators when dispersed in transparent plastic media and used under conditions where both inter- and intramolecular EET operate.

An Artificial Light-Harvesting Array Constructed from Multiple Bodipy Dyes

An artificial light-harvesting array, comprising 21 discrete chromophores arranged in a rational manner, has been synthesized and characterized fully. The design strategy follows a convergent approach that leads to a molecular-scale funnel, having an effective chromophore concentration of 0.6 M condensed into ca. 55 nm(3), able to direct the excitation energy to a focal point. A cascade of electronic energy-transfer steps occurs from the rim to the focal point, with the rate slowing down as the exciton moves toward its ultimate target. Situated midway along each branch of the V-shaped array, two chromophoric relays differ only slightly in terms of their excitation energies, and this situation facilitates reverse energy transfer. Thus, the excitation energy becomes spread around the array, a situation reminiscent of a giant holding pattern for the photon that can sample many different chromophores before being trapped by the terminal acceptor. At high photon flux under conditions of relatively slow off-load to a device, such as a solar cell, electronic energy transfer encounters one or more barriers that hinder forward progress of the exciton and thereby delays arrival of the second photon. Preliminary studies have addressed the ability of the array to function as a sensitizer for amorphous silicon solar cells.

Boron Asymmetry in a BODIPY Derivative

A boradiazaindacene (BODIPY) fluorophore with a chirality held on the central boron has been synthesized and the racemate resolved. Dissymetrization of the BODIPY core was obtained by oxidation of the 3-methyl group to the corresponding carboxaldehyde. A hydrogen bond between the aldehyde proton and the fluorine on the boron atom was evidenced by both (1)H NMR and X-ray diffraction. Chiral high-performance liquid chromatography as well as circular dichroism confirm the persistence of both enantiomers.

Borondipyrromethene Dyes with Pentane-2,4-dione Anchors

New, acetylacetone-linked borondipyrromethene (BODIPY) dyes were readily obtained from BODIPY cores by various protocols involving direct grafting with acetylacetone or cross-coupling from a preorganized phenylacacH derivative bearing either an iodo or an ethynyl function. Facile anchoring on TiO(2) powder is obtained and scrutinized by FT-IR spectroscopy.

A General Synthetic Route to 3,5-Substituted Boron Dipyrromethenes: Applications and Properties

An efficient protocol for the direct synthesis of 3-substituted and 3,5-disubstituted BODIPY derivatives via electrophilic attack with NBS was developed. Various substituents like ethers, sugar, hydroxyl, thiophene, sulfur, azide, tertiary amines, alkyne, vinyl, or phosphonate groups were obtained in moderate to excellent yields. The amine-substituted derivatives display unusual spectroscopic and electrochemical properties which were analyzed in solution in the presence of HCl. The diethylamino-substituted derivative has a proton association constant of log β = 4.7, and the disubstituted derivative has two association constants of log β = 6.2 and 12.1 in ethanol. In both cases, the quenching of the fluorescence is explained by photoinduced electron transfer from the tertiary amine to the Bodipy excited state.

Carbonyl derivatives of boradiazaindacene via catalytic CO insertion.

A methodological study is presented dealing with carbopalladation reactions on BODIPY dyes bearing aryl-halogen functions. Using this technique, several ester and amide groups were efficiently introduced on the dyes. These changes do not affect the optical properties of the dyes and thus allow the construction of new BODIPY-based functional dyes with carboxylic anchoring groups or peptide links.

Photochemical Bleaching of an Elaborate Artificial Light-Harvesting Antenna

The target artificial light-harvesting antenna, comprising 21 discrete chromophores arranged in a logical order, undergoes photochemical bleaching when dispersed in a thin plastic film. The lowest-energy component, which has an absorption maximum at 660 nm, bleaches through first-order kinetics at a relatively fast rate. The other components bleach more slowly, in part, because their excited-state lifetimes are rendered relatively short by virtue of fast intramolecular electronic energy transfer to the terminal acceptor. Two of the dyes, these being close to the terminal acceptor but interconnected through a reversible energy-transfer step, bleach by way of an autocatalytic step. Loss of the terminal acceptor, thereby switching off the energy-transfer route, escalates the rate of bleaching of these ancillary dyes. The opposite terminal, formed by a series of eight pyrene-based chromophores, does not bleach to any significant degree. Confirmation of the various bleaching steps is obtained by examination of an antenna lacking the terminal acceptor, where the autocatalytic route does not exist and bleaching is very slow.