Energy transfer within perylene-terrylene dendrimers evidenced by polychromatic transient absorption measurements

Synthesis, Ensemble, and Single Molecule Characterization of a Diphenyl-Acetylene Linked Terrylenediimide Dimer

The synthesis and the photophysical characterization at the ensemble and single molecule level of a terrylenediimide (TDI) dimer are reported. The spectroscopic experimental data are compared with those obtained for the corresponding model compound TDI. Steady-state and ps time-correlated single photon counting have shown that both chromophores in the TDI dimer are in the weak coupling regime allowing their interaction by Förster resonance energy transfer. Femtosecond transient absorption experiments showed an excitation power dependence of the fluorescence decay, which could indicate the occurrence of singlet-singlet annihilation. Single molecule intensity traces were investigated for the TDI dimer and suggested two intensity levels. For both intensity levels several parameters among which emission maximum, fluorescence decay times, antibunching, blinking off-times and rate of dark state formation were compared. The blinking analysis revealed that the yield of dark state formation is an order of magnitude higher when the two chromophores are still active compared to the case where one is photobleached. The off-times remain however similar.

Separating annihilation and excitation energy transfer dynamics in light harvesting systems

The dependence of excitation energy transfer kinetics on the electronic state of the acceptor (ground vs excited) has been resolved with a novel multipulse prePump-Pump-Probe spectroscopy. The primary energy transfer and annihilation dynamics in two model light-harvesting systems were explored: an artificially synthesized carotenoid-zinc-phthalocyanine dyad and a naturally occurring light-harvesting peridinin-chlorophyll protein complex from Amphidinium carterae. Both systems use carotenoid as the primary excitation energy donor with porphyrin chromophores as the acceptor molecules. The prePump-Pump-Probe transient signals were analyzed with Monte Carlo modeling to explicitly address the underlying step-by-step kinetics involved in both excitation migration and annihilation processes. Both energy transfer and annihilation dynamics were demonstrated to occur with approximately the same rate in both systems, regardless of the excitation status of the acceptor pigments. The possible reasons for these observations are discussed in the framework of the Förster energy transfer model.

Origin of Simultaneous Donor−Acceptor Emission in Single Molecules of Peryleneimide−Terrylenediimide Labeled Polyphenylene Dendrimers

Förster type resonance energy transfer (FRET) in donor-acceptor peryleneimide-terrylenediimide dendrimers has been examined at the single molecule level. Very efficient energy transfer between the donor and the acceptor prevent the detection of donor emission before photobleaching of the acceptor. Indeed, in solution, on exciting the donor, only acceptor emission is detected. However, at the single molecule level, an important fraction of the investigated individual molecules (about 10-15%) show simultaneous emission from both donor and acceptor chromophores. The effect becomes apparent mostly after photobleaching of the majority of donors. Single molecule photon flux correlation measurements in combination with computer simulations and a variety of excitation conditions were used to determine the contribution of an exciton blockade to this two-color emission. Two-color defocused wide-field imaging showed that the two-color emission goes hand in hand with an unfavorable orientation between one of the donors and the acceptor chromophore.

A Photophysical Study of Terphenyl Core Oligosulfonimide Dendrimers Exhibiting High Steady-State Anisotropy

The photophysical properties of three dendrimers containing a p-terphenyl core with appended sulfonimide branches of different size and n-octyl chains have been investigated in dichloromethane solution. In the dendrimer absorption spectra contributions from both the branches and the core are clearly identified. The fluorescence spectra show only the characteristic fluorescence of the terphenyl unit. Energy transfer from the appended chromophoric groups to the core does not occur. In the dendrimers, the terphenyl core exhibits a very high fluorescence quantum yield (ca. 0.75) and a short emission lifetime (0.8 ns). These properties allowed investigations of the fluorescence depolarization caused by rotation of the dendrimers. The dendrimers show a very high steady-state anisotropy in dichloromethane solution at room temperature (0.24 for the largest one), compared to that of the parent terphenyl under the same experimental conditions (<0.01) and in rigid matrix (0.33). Both the n-octyl chains and the sulfonimide branches play important roles to slow down the molecular rotation.

Host−Guest Complexes between an Aromatic Molecular Tweezer and Symmetric and Unsymmetric Dendrimers with a 4,4‘-Bipyridinium Core

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host-guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and (1)H NMR titration plots, (i) are of the order of 10(4) M(-1), (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent (1)H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (DeltaG < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causes a displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host-guest complexes between the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.

The Chemistry of Organic Nanomaterials

The development of nanotechnology using organic materials is one of the most intellectually and commercially exciting stories of our times. Advances in synthetic chemistry and in methods for the investigation and manipulation of individual molecules and small ensembles of molecules have produced major advances in the field of organic nanomaterials. The new insights into the optical and electronic properties of molecules obtained by means of single-molecule spectroscopy and scanning probe microscopy have spurred chemists to conceive and make novel molecular and supramolecular designs. Methods have also been sought to exploit the properties of these materials in optoelectronic devices, and prototypes and models for new nanoscale devices have been demonstrated. This Review aims to show how the interaction between synthetic chemistry and spectroscopy has driven the field of organic nanomaterials forward towards the ultimate goal of new technology.

A Simple Construction of a Bile Acid Based Dendritic Light Harvesting System

[structure: see text] A facile synthesis of a cholic acid derived dendritic structure labeled with nine naproxens and a single anthracene is reported. This multichromophoric, novel dendritic construct acts as an efficient molecular light harvester.

Synthesis of fluorescent oligonucleotide--EYFP conjugate: towards supramolecular construction of semisynthetic biomolecular antennae

A novel species of DNA--protein conjugate was synthesized by chemically linking DNA oligonucleotides to Aequorea victoria green fluorescent protein mutant EYFP. An additional cysteine was added to the C-terminus of the EYFP by genetic engineering and used to covalently attach amino-modified oligonucleotide with the aid of the heterobifunctional crosslinker sSMCC. EYFP maintained its fluorescence upon conjugation. The oligonucleotide provides an additional binding site to the fluorescent protein, and hence, the EYFP conjugate could be specifically hybridized with both complementary DNA-protein conjugates in-solution as well as immobilized at capture oligonucleotides attached to a solid substrate. These studies are paving the way for future applications in the self-assembly of photoactive supramolecular complexes, such as artificial light-harvesting systems.

Forward (singlet–singlet) and backward (triplet–triplet) energy transfer in a dendrimer with peripheral naphthalene units and a benzophenone core

The photochemical and photophysical behaviour of two dendrimers consisting of a benzophenone core and branches that contain four (4) and eight (5) naphthalene units at the periphery has been investigated in CH(2)Cl(2) solution (298 K) and in CH(2)Cl(2)/CHCl(3) 1:1 v/v rigid matrix (77 K). For comparison purposes, the photophysical properties of dimethoxybenzophenone (1), 2-methylnaphthalene (2) and of a dendron containing four naphthalene units (3) have also been studied. In both dendrimers 4 and 5, excitation of the peripheral naphthalene units is followed by fast (1.1 x 10(9) s(-1) at 298 K, > 2.5 x 10(9) s(-1) at 77 K for 5; 2.9 x 10(8) s(-1) at 298 K, 7 x 10(5) s(-1) at 77 K for 5) singlet-singlet energy transfer to the benzophenone core. On a longer time scale (>1 x 10(6) s(-1) at 298 K, >6 x 10(3) s(-1) at 77 K for 4; 3.1 x 10(7) s(-1) at 298 K, ca. 3 x 10(2) s(-1) at 77 K for 5) a back energy transfer process takes place from the triplet state of the benzophenone core to the triplet state of the peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet-triplet energy transfer to the peripheral naphthalene units. In hydrogen donating solvents, the benzophenone core is protected from degradation by the presence of the naphthalene units. In solutions containing Tb(CF(3)SO(3))(3), sensitization of the green Tb(3+) luminescence is observed on excitation of both the peripheral naphthalene units and the benzophenone core of 5. Upon excitation of the naphthalene absorption band (266 nm) with a laser source, intradendrimer triplet-triplet annihilation of naphthalene excited states leads to delayed naphthalene fluorescence (lambda(max)= 335 nm), that can also be obtained upon excitation at 355 nm (benzophenone absorption band). The results obtained show that preorganization of photoactive units in a dendritic structure can be exploited for a variety of useful functions, including photosensitized emission, protection from undesired photoreactions, and energy up-conversion.

Generation dependent singlet–singlet annihilation within multichromophoric dendrimers studied by polychromatic transient absorption

Intramolecular kinetic processes in a series of shape-persistent meta- and para-substituted polyphenylene dendrimers bearing different peryleneimide chromophores at the rim have been investigated using time-resolved polychromatic transient absorption measurements. The influence of the generation number and different substitution patterns upon these processes was revealed by comparing different compounds. In particular, in multichromophoric systems a singlet-singlet annihilation process was detected. The corresponding time constant was dependent on the generation number.