Femtosecond fluorescence upconversion study of rigid dendrimers containing peryleneimide chromophores at the rim

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

Recent advances in sustainable optoelectronics including photovoltaics, light-emitting diodes, transistors, and semiconductors have been enabled by π-conjugated organic molecules. A fundamental understanding of light-matter interactions involving these materials can be realized by time-resolved electronic and vibrational spectroscopies. In this Minireview, the photoinduced mechanisms including charge/energy transfer, electronic (de)localization, and excited-state proton transfer are correlated with functional properties encompassing optical absorption, fluorescence quantum yield, conductivity, and photostability. Four naturally derived molecules (xylindein, dimethylxylindein, alizarin, indigo) with ultrafast spectral insights showcase efficient energy dissipation involving H-bonding networks and proton motions, which yield high photostability. Rational design principles derived from such investigations could increase the efficiency for light harvesting, triplet formation, and photosensitivity for improved and versatile optoelectronic performance.

Using a Modified Polyamidoamine Fluorescent Dendrimer for Capturing Environment Polluting Metal Ions Zn2+, Cd2+, and Hg2+: Synthesis and Characterizations

One of the most pressing global concerns is how to provide a clean environment for future generations given the exacerbation of urban, agricultural, industrial, and economic activities due to the escalating size of the global population. A polyamidoamine (PAMAM) dendrimer peripherally modified with 4-N,N′-dimethylethylenediamine-1,8-naphthalmide as a chromophore was synthesized and utilized to capture hazardous heavy metal ions. This modified fluorescent dendrimer (FCD) was complexed with Group 12 metal ions (Zn2+, Cd2+, and Hg2+) at a 2:1 (metal: FCD) ratio. Electronic absorption, fluorescence emission, Infra-red (IR), and nuclear magnetic resonance (1H NMR) spectroscopies, conductivity, CHN elemental, thermogravimetry, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses were used to characterize the resulting metal complexes. These assays revealed that the synthesized complexes were yellow-colored, thermally stable, nanoscale-sized, and composed of [M2FCD]·4Cl2. Considerable spectral shifts were observed in the emission and absorption spectra of the FCD molecule after binding the Zn2+ ions, which can be used to differentiate the Zn2+ complex from the other two complexes. This work provides basic data to facilitate the detection, quantification, and removal of environmentally hazardous heavy metal ions through complexation with a fluorescent dendrimer.

Photophysics of perylene monoimide-labelled organocatalysts

A fluorophore-tagged organocatalyst undergoes electron transfer in polar solvents allowing to sense the presence of its free quinuclidine catalytic site.

Structural Induced Control of Energy Transfer within Zn(II)−Porphyrin Dendrimers

We report on a study of singlet-singlet annihilation kinetics in a series of Zn(II)-porphyrin-appended dendrimers, where the energy transfer efficiency is significantly improved by extending the molecular chain that connects the light-harvesting chromophores to the dendrimeric backbone with one additional carbon. For the largest dendrimer having 64 Zn(II)-porphyrins, only approximately 10% of the excitation intensity is needed in order to observe the same extent of annihilation in the dendrimers with the additional carbon in the connecting chain as compared to those without. Complete annihilation, until only one chromophore remains excited, now occurs within subunits of seven chromophores, when half of the chromophores are excited. The improvement of the annihilation efficiency in the largest dendrimer with 64 porphyrins can be explained by the presence of a the two-step delayed annihilation process, involving energy hopping from excited to nonexcited chromophores prior to annihilation. In the smallest dendrimer with only four chromophores, delayed annihilation is not present, since the direct annihilation process is more efficient than the two-step delayed annihilation process. As the dendrimer size increases and the chances of originally exciting two neighboring chromophores decreases, the delayed annihilation process becomes more visible. The additional carbon, added to the connecting chain, results in more favorable chromophore distances and orientations for energy hopping. Hence, the improved energy transfer properties makes the Zn(II)-porphyrin-appended dendrimers with the additional carbon promising candidates as light-harvesting antennas for artificial photosynthesis.

Multichromophoric Polyphenylene Dendrimers: Toward Brilliant Light Emitters with an Increased Number of Fluorophores

Two routes for the introduction of highly fluorescent peryleneimide chromophores into the scaffolding of polyphenylene dendrimers via iterative Diels-Alder cycloadditions are presented. The key intermediates for the divergent dendrimer buildup were two cyclopentadienone branching units carrying two peryleneimides and two masked terminal alkynes. The difference between the two reagents is the mode of incorporation of the chromophores. In the first case, the chromophores were attached to the alpha-position of the tetraphenylcyclopentadienones. In the second case, peryleneimides are used as a "spacer" in the beta-position of the cyclopentadienones giving rise to dendrimers with extended molecular diameters (up to 12 nm) and 24 chromophores within their scaffold. Absorption and emission characteristics of the new multichromophoric nanoparticles were investigated and compared to those of the parent dyes. Additionally, an asymmetrically substituted first-generation dendrimer with six perylene diimide chromophores and one ester functionality is reported. The ester serves as a potential anchor group, and this nanoemitter paves the way to a multichromophoric fluorescence label. All dendrimers have good solubility in common organic solvents, high fluorescence quantum yields, and defined distances between the chromophores, making them attractive candidates for single-molecule spectroscopy.

Energy Transfer within Zn-Porphyrin Dendrimers: Study of the Singlet−Singlet Annihilation Kinetics

In this article, we explore energy transfer processes within a series of Zn-porphyrin-appended dendrimers by means of excitation intensity dependent transient absorption measurements. We report singlet-singlet annihilation on two distinct time scales of 18 +/- 5 ps and 130 +/- 10 ps in the dimer and the dendrimers. The two distinct processes reflect the presence of two structural conformer distributions. Analysis of the singlet-singlet annihilation transient kinetics shows that sequential annihilation occurs within subunits up to four Zn-porphyrins in the dendrimers. The onset of the singlet-singlet annihilation process depending on the size of the molecule reveals a difference in the number of communicating Zn-porphyrins. We further report a full characterization of the transient absorption kinetics of the monomer over a spectral range from 450 to 730 nm.

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.

Photophysical study of photoinduced electron transfer in a bis-thiophene substituted peryleneimide

Based on femtosecond time-resolved spectroscopy and single photon timing experiments, intramolecular photoinduced charge transfer has been investigated in two systems containing a peryleneimide chromophore (P) and thiophene (T) groups. The first compound bearing a single thiophene ring (PT1) is used as model and shows a behavior similar to P, studied previously, while in the compound with two thiophene rings attached (PT2) electron transfer from the thiophene donor to the peryleneimide acceptor is observed in benzonitrile. Femtosecond fluorescence upconversion and femtosecond transient absorption experiments in benzonitrile indicate that this ion-pair state formation occurs in 19 ps. This ion-pair state then decays with two time constants of 1400 and 820 ps, probably corresponding to different conformations of the thiophene rings.

Single Photon Emission from a Dendrimer Containing Eight Perylene Diimide Chromophores

A novel dendrimer containing eight perylene diimide chromophores has been synthesized and studied by ensemble and single-molecule spectroscopic techniques. Photon anti-bunching (coincidence) measurements on single molecules embedded in zeonex polymer films show that the dendrimer behaves as a deterministic (triggered) single photon source with only one fluorescence photon being emitted following pulsed laser excitation, even when more than one chromophore is excited. This behaviour is due to efficient singlet–singlet annihilation being operative in this dendrimer. Preliminary results indicate that the triplet lifetime and yield for this molecule are similar to the values for a molecule containing a single perylene diimide chromophore.

Direct proof of electron transfer in a rigid first generation triphenyl amine core dendrimer substituted with a peryleneimide acceptor

The combination of nanosecond transient absorption experiments and single photon timing experiments proved the occurrence of an electron transfer process in the triphenyl amine core dendrimer, N1P1, by demonstrating the presence of an ion-pair absorption for N1P1 in solvents of medium polarity. By means of femtosecond transient absorption measurements the rise time of this ion-pair absorption dominated by the radical anion absorption could be determined, resulting in a value of 180 ps in MeTHF and 138 ps in THF. Furthermore, in femtosecond fluorescence upconversion as well as in monochromatic femtosecond transient absorption, a few ps component was resolved which was assigned to a vibrational and solvent relaxation process of the locally excited singlet state of the peryleneimide.

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

The time dependent spectral properties of a first and a second generation dendrimer with peryleneimide chromophores at the rim and a terrylenediimide chromophore in the core were investigated by time resolved polychromatic transient absorption measurements. The obtained results of the dendritic structures were compared with those of three model compounds. In the perylene-terrylene dendrimers a very fast energy transfer process was observed. Besides energy transfer singlet-singlet annihilation could be observed. It could be concluded that competition between the rates of energy transfer and of singlet-singlet annihilation is dependent on the dendrimer generation.

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.

Antibunching in the emission of a single tetrachromophoric dendritic system

The photophysics of a dendrimer containing four chromophores are investigated at the single-molecule level. First, the multichromophoric character of single dendrimers' absorption is probed by modulating the linear polarization of the excitation beam. Subsequently, using circular polarization, the same dendrimers are excited, and their fluorescence transients are recorded. Using pulsed excitation in combination with the classical Hanbury-Brown and Twiss coincidence setup the presented data demonstrate that efficient singlet-singlet annihilation ensures that always only one photon is emitted even when several excitations are generated in an individual multichromophoric molecule.