Theoretical analysis of excited states and energy transfer mechanism in conjugated dendrimers

The excited states of the phenylene ethynylene dendrimer are investigated comprehensively by various electronic‐structure methods. Several computational methods, including SCS‐ADC(2), TDHF, TDDFT with different functionals (B3LYP, BH&HLYP, CAM‐B3LYP), and DFT/MRCI, are applied in systematic calculations. The theoretical approach based on the one‐electron transition density matrix is used to understand the electronic characters of excited states, particularly the contributions of local excitations and charge‐transfer excitations within all interacting conjugated branches. Furthermore, the potential energy curves of low‐lying electronic states as the functions of ethynylene bonds are constructed at different theoretical levels. This work provides us theoretical insights on the intramolecular excited‐state energy transfer mechanism of the dendrimers at the state‐of‐the‐art electronic‐structure theories. © 2014 Wiley Periodicals, Inc.     

Energy transfer in new D-pi-A conjugated dendrimers: their synthesis and photophysical properties

A series of new D-pi-A conjugated dendrimers based on benzothiadiazole and triphenylamine were developed via facile synthetic approaches. By changing the types of bridges between the different functional moieties of these dendrimers, their photophysical properties, especially the intramolecular energy transfer process, were effectively modulated.     

Photo-induced processes in dendrimers

This article deals with different aspects of photo-induced processes in dendrimers, such as site isolation, energy and electron transfer processes, and photo-isomerisation of peripheral groups. By discussing several examples, from our own work, we intend to give some more insight with respect to these topics. To cite this article: A. Dirksen, L. De Cola, C. R. Chimie 6 (2003).     

Charge transfer interactions between conjugated block copolymers and reduced graphene oxides

The charge transfer interactions between reduced graphene oxides and conjugated block copolymers were confirmed by various spectroscopic methods, giving rise to manipulation of the electrical properties of the former.     

Charge transfer processes in the course of metal-ion electrochemical intercalation

Charge transfer in the course of the electrochemical ion intercalation is typically understood as the transfer of an alkali metal ion across the intercalating material/electrolyte interface. The activation energy of this step determines the rate capability of intercalation-based energy storage devices, which calls for the investigation of the origin of the charge transfer limitations in various intercalation systems. The major focus of the experimental studies in this area is on the experimental determination of the charge transfer rates under different experimental conditions, while molecular modeling approaches allow to unveil the mechanistic aspects of the intercalation processes.     

Charge density analysis of some processes involving intramolecular hydrogen transfer

The variations in topology of the electron density along the reaction paths of the keto-enol tautomerism of acetaldehyde, the pinacol rearrangement of protonated 1,2-ethanediol, and the unimolecular decomposition of methanediol, were studied as examples of hydrogen transfer between, respectively, C?O, C?C, and O?O atoms. The evolution of atomic charges, determined using two different atomic partitionings (AIM and Hirshfeld), indicates that the main electronic charge transfer in keto-enol tautomerism takes place between the migrating hydrogen and the carbon of the carbonyl group. The topology of the electron density demonstrates that a hydrogen-bridge structure is never formed along the reaction path of the pinacol rearrangement. The methanediol decomposition follows a concerted mechanism with very small variations in the atomic charges.     

Heterogeneous electron transfer processes in self-assembled monolayers of amine terminated conjugated molecular wires

A versatile synthesis of triarylamine and phenothiazine end-capped oligo(phenyleneacetylene) molecular wires which are terminated by thiol functions is described. The repetitive synthesis allows the preparation of molecular wires with different chain length and different substituents attached to the wire backbone. These molecular wires were used to form dense self-assembled monolayers (SAM) on gold substrates as proved by cyclic voltammetry and quartz crystal microbalance measurements. The heterogeneous electron transfer rate constant of these SAMs was measured by impedance spectroscopy between 1 MHz and 0.1 Hz. The rate constants are somewhat larger for the triarylamine terminated systems than for the phenothiazine compound, due to the higher reorganization energy in the latter. While the molecular wires with electron withdrawing substituents display an electron transfer which is slow enough to be measurable with our impedance setup, we were unable to determine the rate of molecular wires with electron donating substituents.     

Synthesis and photophysical properties of phenothiazine-labeled conjugated dendrimers

A novel class of conjugated dendrimers bearing phenothiazines as peripheral groups and phenylenevinylene-group as a core has been synthesized through the Wittig-Horner reaction in moderate to good yield.     

Excited-state processes in first-generation phenyl-cored thiophene dendrimers

First generation dendrimers with three oligothiophene arms (meta-arranged, 3G1-nS) and four arms (ortho- and para-arranged, 4G1-nS) connected to a central phenyl core were investigated spectroscopically in solution. In all dendrimers, on an ultrafast time scale (<10 ps), two "cooling" processes convert the initially generated, "hot" exciton into the geometrically relaxed, "cold" exciton. A decrease in the triplet yield, particularly evident for the 4-arm dendrimers; intersystem crossing rate; and nonradiative triplet decay time with increasing number of bridging thiophene units n all meet with expectations from prior studies on linear oligothiophenes. A relatively fast internal conversion process (>0.6 ns(-1)) is observed in both dendrimer series, possibly due to increased twisting about the phenyl core that reduces the triplet yields considerably with respect to oligothiophenes. An anomalous shifting of the triplet-triplet absorption spectra characterizes the 4G1-nS dendrimers as unique from the 3G1-nS series in terms of the hindrance of torsional motion and confinement of excited states enforced by the arrangement of dendrons.     

Energy transfer in unsymmetrical phenylene ethynylene dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.     

Charge transfer processes and environmental degrees of freedom: cooperativity and non-linearity

To investigate supramolecular effects in samples with high concentration of push-pull chromophores, we propose a model for interacting polar and polarizable molecules. Each molecule is described in terms of the same two-state picture successfully adopted to model solvated chromophores and electrostatic interactions among different chromophores are introduced. Important supramolecular effects are observed even at the lowest mean-field level, showing up the possibility of tuning molecular polarity from the neutral to the zwitterionic regime or vice versa. Supramolecular effects in excitation spectra are more complex. Here we demonstrate large supramolecular effects beyond mean-field in static optical responses.     

Optically nonlinear energy transfer in light-harvesting dendrimers

Dendrimeric polymers are the subject of intense research activity geared towards their implementation in nanodevice applications such as energy harvesting systems, organic light-emitting diodes, photosensitizers, low-threshold lasers, and quantum logic elements, etc. A recent development in this area has been the construction of dendrimers specifically designed to exhibit novel forms of optical nonlinearity, exploiting the unique properties of these materials at high levels of photon flux. Starting from a thorough treatment of the underlying theory based on the principles of molecular quantum electrodynamics, it is possible to identify and characterize several optically nonlinear mechanisms for directed energy transfer and energy pooling in multichromophore dendrimers. Such mechanisms fall into two classes: first, those where two-photon absorption by individual donors is followed by transfer of the net energy to an acceptor; second, those where the excitation of two electronically distinct but neighboring donor groups is followed by a collective migration of their energy to a suitable acceptor. Each transfer process is subject to minor dissipative losses. In this paper we describe in detail the balance of factors and the constraints that determines the favored mechanism, which include the excitation statistics, structure of the energy levels, laser coherence factors, chromophore selection rules and architecture, possibilities for the formation of delocalized excitons, spectral overlap, and the overall distribution of donors and acceptors. Furthermore, it transpires that quantum interference between different mechanisms can play an important role. Thus, as the relative importance of each mechanism determines the relevant nanophotonic characteristics, the results reported here afford the means for optimizing highly efficient light-harvesting dendrimer devices.     

Synthesis and optical properties of conjugated dendrimers with unsymmetrical branching

An improved synthetic approach to conjugated monodendrons with unsymmetrical branching structures is reported. Dendrimers containing two or three such conjugated monodendrons are synthesized and their optical properties are studied. Such dendrimers exhibit broad absorptions and very high fluorescence quantum yields, making them promising candidates for applications in molecular-based photonics.     

Theoretical studies on conjugated phenyl-cored thiophene dendrimers for photovoltaic applications

Pi-conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of pi-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the core and the arms that are detrimental to pi-electron system delocalization over the structure.     

Evaluation of energy transfer in perylene-cored anthracene dendrimers

Quantitative evaluation of F?rster-type fluorescence resonance energy transfer (FRET) was undertaken by statistical investigations on perylene-cored anthracene dendrimers.     

Energy and electron transfer in bifunctional non-conjugated dendrimers

Nonconjugated dendrimers, which are capable of funneling energy from the periphery to the core followed by a charge-transfer process from the core to the periphery, have been synthesized. The energy and electron donors involve a diarylaminopyrene unit and are incorporated at the periphery of these dendrimers. The energy and electron acceptor is at the core of the dendrimer, which involves a chromophore based on a benzthiadiazole moiety. The backbone of the dendrimers is benzyl ether based. A direct electron-transfer quenching of the excited state of the periphery or a sequential energy transfer-electron-transfer pathway are the two limiting mechanisms of the observed photophysical properties. We find that the latter mechanism is prevalent in these dendrimers. The energy transfer occurs on a picosecond time scale, while the charge-transfer process occurs on a nanosecond time scale. The lifetime of the charge separated species was found to be in the range of microseconds. Energy transfer efficiencies ranging from 80% to 90% were determined using both steady-state and time-resolved measurements, while charge-transfer efficiencies ranging from 70% to 80% were deduced from fluorescence quenching of the core chromophore. The dependence of the energy and charge-transfer processes on dendrimer generation is analyzed in terms of the backfolding of the flexible benzyl ether backbone, which leads to a weaker dependence of the energy and charge-transfer efficiencies on dendrimer size than would be expected for a rigid system.     

Charge transfer in systems of conjugated bonds in cyanobiphenyl molecules: Quantum-chemical calculations of the structure and vibrational spectra

Quantum-chemical calculations of the IR absorption spectra and geometric and electronic structure of cyanobiphenyl molecules

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(I) were performed for various angles between benzene ring planes by the B3LYP/6-31+G** method. It was shown that the stablest conformation of I (X=OCH₃, OC₃H₇) should be the twist conformation with α= 37°, which was in agreement with the gas-phase experimental data. Rotation of benzene rings with respect to each other changed the relative orientation of the interacting π orbitals of the bridge ring carbon atoms and caused charge redistribution over molecule atoms, in particular, over terminal X and CN group atoms. The calculated period of charge oscillations on the alkyl and nitro groups coincided with the period of reversible charge transfer (～5–10 fs) between the conjugated subsystems (benzene ring + substituent) observed as the α angle changed. The rate of charge transfer between the electron donor and electron acceptor groups was calculated to be (3–6)×10⁵ m/s. Charge oscillations on benzene ring carbon atoms and donor and acceptor groups did not cause similar dipole moment oscillations and vibrations in the IR spectrum. The dipole moment of the molecule decreases as the angle between benzene ring planes increases, and the passage to the “perpendicular” conformation should increase the C≡N stretching vibration frequency by ～5 cm^?1 and decrease the intensity of the IR band by ～2 times. The elongation of the aliphatic chain in the X group did not cause noticeable changes in the geometric and electronic structure of the molecule.

     

Synthesis and photophysical properties of tetraphenylethylene-based conjugated dendrimers with triphenylamine core

Two new conjugated dendrimers bearing a tetraphenylethylene moiety as dendrons and triphenylamine as a core have been synthesized through a convergent synthetic strategy using threefold Heck/threefold Sonogashira coupling reaction. These dendrimers showed excellent solubility in common organic solvents and emit light in the blue and violet regions.