Energy Migration in Dendritic Oligothiophene-Perylene Bisimides

Efficient Energy Transfer in a Rylene Imide-Based Heterodimer: The Role of Intramolecular Electronic Coupling

The development of antenna molecules with simplified structures can effectively avoid the complex exciton dynamics resulting from conformational mobility. Two distinct heterodimers TP and TBP comprising a perylenediimide (PDI) donor and terrylenediimide (TDI) acting as an energy sink were investigated. Tuned by varying functionalization positions, the bay-to-bay-linked TP offers a strong chromophore coupling, while the bay-to-N-linked TBP exhibits a weak chromophore coupling. Using transient absorption spectroscopy, we found that TP underwent ultrafast vibrational relaxation (τ_VR < 400 fs) from upper vibrational energy levels of the singlet states after pumping at 490 nm, and followed by electron transfer (ET, τ_ET = 2.5 ps) from TDI to PDI. TBP exhibited ultrafast excitation energy transfer (EET, τ_EET = 0.48 ± 0.1 ps) from the excited PDI donor to TDI acceptor, and the subsequent charge transfer (CT) process was almost quenched. This result provides insight into designing novel small molecules capable of efficient energy transfer.

Chemical Tuning of Exciton versus Charge-Transfer Excited States in Conformationally Restricted Arylene Cages

Covalent assemblies of conjugated organic chromophores provide the opportunity to engineer new excited states with novel properties. In this work, a newly developed triple-stranded cage architecture, in which meta-substituted aromatic caps serve as covalent linking groups that attach to both top and bottom of the conjugated molecule walls, is used to tune the properties of thiophene oligomer assemblies. Benzene-capped and triazine-capped 5,5'-(2,2-bithiophene)-containing arylene cages are synthesized and characterized using steady-state and time-resolved spectroscopic methods. The conformational freedom and electronic states are analyzed using time-dependent density functional theory. The benzene cap acts as a passive spacer whose electronic states do not mix with those of the chromophore walls. The excited state properties are dominated by through-space interactions between the chromophore subunits, generating a neutral Frenkel H-type exciton state. This excitonic state undergoes intersystem crossing on a 200 ps time scale while the fluorescence output is suppressed by a factor of 2 due to a decreased radiative rate. Switching to a triazine cap enables electron transfer from the chromophore-linker after the initial excitation to the exciton state, leading to the formation of a charge-transfer state within 10 ps. This state can avoid intersystem crossing and exhibits red-shifted fluorescence with enhanced quantum yield. The ability to interchange structural modules with different electronic properties while retaining the overall cage morphology provides a new approach for tuning the properties of discrete chromophore assemblies.

Delocalized Excitation or Intramolecular Energy Transfer in Pyrene Core Dendrimers

Light-harvesting and then intramolecular energy transfer are the crucial steps in natural photosynthesis. Dendrimers are one of the most promising artificial light-harvesting antennas. Insight into the relationship between molecular structure and energy transfer (or delocalized excitation) in dendrimers would help in understanding and mimicking photosynthesis. Here, a series of dendrimers T1-T4 based on pyrene as a core and fluorene/carbazole as the dendrons have been studied with time-resolved fluorescence and femtosecond transient absorption spectroscopies, revealing that the large planar structure of T1 and T2 has led to strong coupling of pyrene and fluorene units, enabling delocalized excitation over the entire molecules. But for T3 and T4, the carbazole units linking the first- and second-generation branches have broken the planar structure and suppressed the π-electron delocalization, enabling the Förster resonance energy transfer. The efficient intramolecular energy transfer from peripheral branches to the core occurs within 2 ps.

Synthesis and fluorescence properties of two dendritic molecules based on naphthalimide and triphenylamine

Artificial light-harvesting systems have attracted great interest in biological photosynthesis and photo-voltaic devices areas due to their unique structures, easy purification, low-cost, and convenient processing abilities. Here, two dendritic molecules based on triphenylamine and naphthalimide have been designed and synthesized, their structures were confirmed by ¹ H NMR, ESI-MS, and high resolution mass spectrometry. In these molecules, triphenylamine units perform as the electron donor moiety, and naphthalimide units perform as the electron acceptor. The obvious quenched fluorescence intensity and considerably shortened lifetime of the dendritic molecules combined with the molecular frontier orbital energy levels proved that the dendritic molecules not only are good candidates as hole-transporting materials but also are two excellent photo-induced electron transfer materials. Therefore, it is believed that these dendritic molecules have potential application value in photo-voltaic devices.

Tailoring Photophysical Processes of Perylene-Based Light Harvesting Antenna Systems with Molecular Structure and Solvent Polarity

The excited-state dynamics of perylene-based bichromophoric light harvesting antenna systems has been tailored by systematic modification of the molecular structure and by using solvents of increasing polarity in the series toluene, chloroform, and benzonitrile. The antenna systems consist of blue light absorbing naphthalene monoimide (NMI) energy donors (D1, D2, and D3) and the perylene derived green light absorbing energy acceptor moieties, 1,7-perylene-3,4,9,10-tetracarboxylic tetrabutylester (A1), 1,7-perylene-3,4,9,10-tetracarboxylic monoimide dibutylester (A2), and 1,7-perylene-3,4,9,10-tetracarboxylic bisimide (A3). The design of these antenna systems is such that all exhibit ultrafast excitation energy transfer (EET) from the excited donor to the acceptor, due to the effective matching of optical properties of the constituent chromophores. At the same time, electron transfer from the donor to the excited acceptor unit has been limited by the use of a rigid and nonconjugated phenoxy bridge to link the donor and acceptor components. The antenna molecules D1A1, D1A2, and D1A3, which bear the least electron-rich energy donor, isopentylthio-substituted NMI D1, exhibited ultrafast EET (τ_EET ∼ 1 ps) but no charge transfer and, resultantly, emitted a strong yellow-orange acceptor fluorescence upon excitation of the donor. The other antenna molecules D2A2, D2A3, and D3A3, which bear electron-rich energy donors, the amino-substituted NMIs D2 and D3, exhibited ultrafast energy transfer that was followed by a slower (ca. 20-2000 ps) electron transfer from the donor to the excited acceptor. This charge transfer quenched the acceptor fluorescence to an extent determined by molecular structure and solvent polarity. These antenna systems mimic the primary events occurring in the natural photosynthesis, i.e., energy capture, efficient energy funneling toward the central chromophore, and finally charge separation, and are suitable building blocks for achieving artificial photosynthesis, because of their robustness and favorable and tunable photophysical properties.

Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Perylenetetracarboxy-3,4:9,10-diimides with large TPA cross-sections.

Tuning the optical and electrochemical properties of conjugated all-thiophene dendrimers via core functionalization with a benzothiadiazole unit

The optoelectronic properties of conjugated thiophene dendrimers can be tuned by core functionalization with a benzothiadiazole unit.

Femtosecond to nanosecond studies of octupolar molecules and their quadrupolar and dipolar analogues

The fs–ps anisotropy dynamics of octupolar, quadrupolar and dipolar molecules with different π-bridges.

Application of PAMAM dendrimers in optical sensing

In this review, recent advances have been reported in those PAMAM dendrimer-based optical sensors that are used for the detection of pH, cations, and other analyte.

BODIPY functionalized o-carborane dyads for low-energy photosensitization

Bodipy-o-carborane dyads undergo a photo-induced electron transfer (PET) process under low energy visible light sensitization and their PET efficiencies are in the 63–71% range.

Intriguing emission properties of triphenylamine–carborane systems

Triphenylamine–carborane systems show three kinds of intriguing emissions that originate from the local excited (LE) state, the twisted intramolecular charge-transfer (TICT) state, and the aggregation-induced emission (AIE) state.

Advances in Photofunctional Dendrimers for Solar Energy Conversion

Dendrimers are regularly and hierarchically branched synthetic macromolecules with numerous chain ends all emanating from a single core, which makes them attractive candidates for energy conversion applications. During photosynthesis and photocatalysis, photoinduced electron transfer and energy transfer are the main processes involved. Studies on these processes in dendritic systems are critical for the future applications of dendrimers in photochemical energy conversion and other optoelectronic devices. In this Perspective, the recent advances of photofunctional dendrimers in energy conversion based on light-harvesting systems, solar cells, and photochemical production of hydrogen will be discussed. The electron-transfer and energy-transfer characteristics in light-harvesting photofunctional dendrimers and the regulation of the electron-transfer process and the stabilization of the charge separation state in hydrogen photoproduction are emphasized.

Conjugated anthracene dendrimers with monomer-like fluorescence

A series of highly fluorescent conjugated anthracene dendrimers having monomeric emission profile, but still exhibiting fast exciton depolarisation, are here presented.