Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules

Observation of Intramolecular Interaction in Fluorescent Star-Shaped Polymers: Evidence for Energy Hopping between Branch Chains

Time-resolved fluorescence anisotropy measurements were performed on three-branched star-shaped polymers, based on precisely synthesized poly(9,9-di-n-octyl-fluorene vinylene)s containing C₆F₅ end groups. The star-shaped polymers showed identical fluorescence spectra, fluorescence lifetimes, and quantum yields to those of the reference single-chain oligomer. However, a rapid fluorescence anisotropy decay was observed in two kinds of star-shaped polymers, while such decay was not seen in the corresponding single-chain oligomer. On the basis of the analysis using an incoherent hopping model, the observed rapid anisotropy decay is attributable to energy hopping processes between branches within a single polymer species, and its rate was deduced to be ca.100 ps depending upon the core part.

Excitation/detection energy controlled anisotropy dynamics in asymmetrically cyano substituted tri-podal molecules

In the present work, the photophysical properties of two series of asymmetrical tri-podal molecules are studied, in order to determine the dependence of energy localization/delocalization phenomena on excitation and detection wavelength, by means of steady state, femtosecond time-resolved fluorescence and anisotropy spectroscopy. The molecules bear triphenylamine as an electron donating core and an acetylenic or olefinic π-conjugated bridge. At the periphery, they are substituted by no, one, two or three -CN groups used as electron acceptors. Thus, the compounds with only one or two -CN groups are asymmetrically substituted. As a comparison, the photophysics of their dipolar and quadrupolar analogues is also presented. The steady state absorption spectra of the asymmetrical tri-podal compounds exhibit a broadening and a low energy shoulder due to the splitting of the excited states. The fluorescence spectra are more red-shifted in the tri-podal molecules with a single -CN group, providing the first evidence of its mostly dipolar nature. Time-resolved anisotropy measurements by using different excitation and detection wavelengths provide clear evidence that the asymmetrical tri-podal molecules with one or two -CN groups behave like octupolar molecules upon high-energy excitation (the initial anisotropy is found 0.1-0.15), while upon low-energy excitation they reveal a behavior expected for linear dipolar or V-shaped quadrupolar molecules (the initial anisotropy is very close to 0.4 and 0.17, respectively). The symmetrical tri-podal compounds with no or three cyano groups, exhibit an anisotropy depolarization time of 2.5 ps attributed to energy hopping. The amplitude of this energy hopping component is wavelength dependent and increases as the excitation is shifted towards the long wavelength edge.

Design of Linear and Star-Shaped Macromolecular Organic Semiconductors for Photonic Applications

One of the most desirable and advantageous attributes of organic materials chemistry is the ability to tune the molecular structure to achieve targeted physical properties. This can be performed to achieve specific values for the ionization potential or electron affinity of the material, the absorption and emission characteristics, charge transport properties, phase behavior, solubility, processability, and many other properties, which in turn can help push the limits of performance in organic semiconductor devices. A striking example is the ability to make subtle structural changes to a conjugated macromolecule to vary the absorption and emission properties of a generic chemical structure. In this Account, we demonstrate that target properties for specific photonic applications can be achieved from different types of semiconductor structures, namely, monodisperse star-shaped molecules, complex linear macromolecules, and conjugated polymers. The most appropriate material for any single application inevitably demands consideration of a trade-off of various properties; in this Account, we focus on applications such as organic lasers, electrogenerated chemiluminescence, hybrid light emitting diodes, and visible light communications. In terms of synthesis, atom and step economies are also important. The star-shaped structures consist of a core unit with 3 or 4 functional connection points, to which can be attached conjugated oligomers of varying length and composition. This strategy follows a convergent synthetic pathway and allows the isolation of target macromolecules in good yield, high purity, and absolute reproducibility. It is a versatile approach, providing a wide choice of constituent molecular units and therefore varying properties, while the products share many of the desirable attributes of polymers. Constructing linear conjugated macromolecules with multifunctionality can lead to complex synthetic routes and lower atom and step economies, inferior processability, and lower thermal or chemical stability, but these materials can be designed to provide a range of different targeted physical properties. Conventional conjugated polymers, as the third type of structure, often feature so-called "champion" properties. The synthetic challenge is mainly concerned with monomer synthesis, but the final polymerization sequence can be hard to control, leading to variable molecular weights and polydispersities and some degree of inconsistency in the properties of the same material between different synthetic batches. If a champion characteristic persists between samples, then the variation of other properties between batches can be tolerable, depending on the target application. In the case of polymers, we have chosen to study PPV-type polymers with bulky side groups that provide protection of their conjugated backbone from π-π stacking interactions. These polymers exhibit high photoluminescence quantum yields (PLQYs) in films and short radiative lifetimes and are an important benchmark to monodisperse star-shaped systems in terms of different absorption/emission regions. This Account therefore outlines the advantages and special features of monodisperse star-shaped macromolecules for photonic applications but also considers the two alternative classes of materials and highlights the pros and cons of each class of conjugated structure.

Energy transfer and charge separation dynamics in photoexcited pyrene-bodipy molecular dyads

The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2,6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2-dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the pyrene (Py) subunit leads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by Förster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5-5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with ability to undergo CS.

Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad

Understanding electronic energy transfer (EET) is an important ingredient in the development of artificial photosynthetic systems and photovoltaic technologies. Although EET is at the heart of these applications and crucially influences their light-harvesting efficiency, the nature of EET over short distances for covalently bound donor and acceptor units is often not well understood. Here we investigate EET in an orthogonal molecular dyad (BODT4), in which simple models fail to explain the very origin of EET. On the basis of nonadiabatic ab initio molecular dynamics calculations and ultrafast fluorescence experiments, we gain detailed microscopic insights into the ultrafast electrovibrational dynamics following photoexcitation. Our analysis offers molecular-level insights into these processes and reveals that it takes place on time scales ≲100 fs and occurs through an intermediate charge-transfer state.

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.

Light Harvesting for Organic Photovoltaics

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.

General Force-Field Parametrization Scheme for Molecular Dynamics Simulations of Conjugated Materials in Solution

We describe a general scheme to obtain force-field parameters for classical molecular dynamics simulations of conjugated polymers. We identify a computationally inexpensive methodology for calculation of accurate intermonomer dihedral potentials and partial charges. Our findings indicate that the use of a two-step methodology of geometry optimization and single-point energy calculations using DFT methods produces potentials which compare favorably to high level theory calculation. We also report the effects of varying the conjugated backbone length and alkyl side-chain lengths on the dihedral profiles and partial charge distributions and determine the existence of converged lengths above which convergence is achieved in the force-field parameter sets. We thus determine which calculations are required for accurate parametrization and the scope of a given parameter set for variations to a given molecule. We perform simulations of long oligomers of dioctylfluorene and hexylthiophene in explicit solvent and find peristence lengths and end-length distributions consistent with experimental values.

Self-trapping and excited state absorption in fluorene homo-polymer and copolymers with benzothiadiazole and tri-phenylamine

Excited state absorption (ESA) is studied using time-dependent density functional theory and compared with experiments performed in dilute solutions.

Very fast singlet and triplet energy transfers in a tri-chromophoric porphyrin dyad aided by the truxene platform

A trichromophoric dyad composed of an octa-β-alkyl-palladium(II)porphyrin (donor) and two tri-meso-aryl-zinc(II)porphyrins (acceptors) held by a truxene spacer exhibits very fast rates for triplet energy transfers at 77 (kET(T1) = 1.63 × 108 s-1) and 298 K (kET(T1) = 3.44 × 108 s-1), whereas the corresponding singlet energy transfer rates, kET(S1) = 3.9 × 1010 s-1 (77 K) and kET(S1) = 6.0 × 1010 s-1 (298 K), are also considered fast. The interpretation for these results is that the energy transfer processes proceed via a through bond Dexter mechanism (i.e. double electron exchange) supported by comparison with literature data and evidence for a moderate MO coupling between the donor and acceptor chromophores in the frontier MOs.

Structure-Directed Exciton Dynamics in Templated Molecular Nanorings

Conjugated polymers with cyclic structures are interesting because their symmetry leads to unique electronic properties. Recent advances in Vernier templating now allow large shape-persistent fully conjugated porphyrin nanorings to be synthesized, exhibiting unique electronic properties. We examine the impact of different conformations on exciton delocalization and emission depolarization in a range of different porphyrin nanoring topologies with comparable spatial extent. Low photoluminescence anisotropy values are found to occur within the first few hundred femtoseconds after pulsed excitation, suggesting ultrafast delocalization of excitons across the nanoring structures. Molecular dynamics simulations show that further polarization memory loss is caused by out-of-plane distortions associated with twisting and bending of the templated nanoring topologies.

Truxene: a promising scaffold for future materials

Truxene (10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene), which is a heptacyclic polyarene structure, has attracted a great deal of interest due to its exceptional solubility, high thermal stability and ease with which it may be modified.