Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

Light-Harvesting in Carbonyl-Terminated Phenylacetylene Dendrimers: The Role of Delocalized Excited States and the Scaling of Light-Harvesting Efficiency with Dendrimer Size

The photophysics of a family of conjugated phenylacetylene (PA) light-harvesting dendrimers are studied using steady-state and time-resolved optical spectroscopy. The dendrimers consist of a substituted PA core surrounded by meta-branched PA arms. The total number of PA moieties ranges from 3 (first generation) to 63 (fifth generation). By using an alcohol/ketone substituent at the dendrimer core, we avoid through-space Forster transfer from the peripheral PA donors to the core acceptor (in this case, the carbonyl group), which simplifies the analysis of these molecules relative to the perylene-terminated molecules studied previously. The delocalized excited states previously identified in smaller dendrons are seen in these larger dendrimers as well, and their influence on the intersite electronic energy transfer (EET) is analyzed in terms of a point-dipole Forster model. We find that these new delocalized states can both enhance EET (by decreasing the spatial separation between donor and acceptor) and degrade it (by lowering the emission cross section and shifting the energy, resulting in poorer spectral overlap between donor and acceptor). The combination of these two effects leads to a calculated intersite transfer time of 6 ps, in reasonable agreement with the 5-17 ps range obtained from experiment. In addition to characterizing the electronic states and intersite energy transfer times, we also examine how the overall light-harvesting efficiency scales with dendrimer size. After taking the size dependence of other nonradiative processes, such as excimer formation, into account, the overall dendrimer quenching rate k(Q) is found to decrease exponentially with dendrimer size over the first four generations. This exponential decrease is predicted by simple theoretical considerations and by kinetic models, but the dependence on generation is steeper than expected based on those models, probably due to increased disorder in the larger dendrimers. We discuss the implications of these results for dendrimeric light-harvesting structures based on PA and other chemical motifs.

Singlet Energy Migration along an Alternating Block Copolymer of Oligothiophene and Oligosilylene in Solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the Förster theory, the energy migration process was attributed to an incoherent hopping mechanism.

Coherent exciton transport in dendrimers and continuous-time quantum walks

We model coherent exciton transport in dendrimers by continuous-time quantum walks. For dendrimers up to the second generation the coherent transport shows perfect recurrences when the initial excitation starts at the central node. For larger dendrimers, the recurrence ceases to be perfect, a fact which resembles results for discrete quantum carpets. Moreover, depending on the initial excitation site, we find that the coherent transport to certain nodes of the dendrimer has a very low probability. When the initial excitation starts from the central node, the problem can be mapped onto a line which simplifies the computational effort. Furthermore, the long time average of the quantum mechanical transition probabilities between pairs of nodes shows characteristic patterns and allows us to classify the nodes into clusters with identical limiting probabilities. For the (space) average of the quantum mechanical probability to be still or to be again at the initial site, we obtain, based on the Cauchy-Schwarz inequality, a simple lower bound which depends only on the eigenvalue spectrum of the Hamiltonian.

Theoretical studies of electron transfer through dendrimeric architecture

We have analyzed the steady-state electron transfer rate through a bridge of dendrimeric architecture. The difference between the linear chain and the dendrimeric architecture has also been demonstrated with steady-state rate as a main observable in the coherent and incoherent regimes of interactions. It is shown that generally the rate of electron transfer in dendrimeric architecture is faster than the rate associated with their linear chain counterpart with similar kind of bonding connectivities. The rate depends upon the size of the molecule, core branching, and the nature of the coupling among the different nodes on the dendrimer molecule. Depending upon the nature of the donor and acceptor, phenomenological dephasing coefficient due to environment and the geometry of the dendrimeric architecture, the modification of electron transfer rate has been studied. In the regime of fully coherent interactions where all quantum effects are considered the rate shows a multiple inversion due to the dendrimer architecture which is neither available in the regime of incoherent interaction nor in the linear chain case in similar condition. We have discussed about the applicability of our model in metal-molecule-metal junction, photoinduced electron transfer process, and molecular conductor.

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.

Femtosecond Fluorescence Studies of Self-Assembled Helical Aggregates in Solution

For the diamino-bipyridine based C(3)-symmetrical disk molecule, TAB, (sub)picosecond fluorescence transients have been observed by means of femtosecond fluorescence upconversion and picosecond time-correlated photon counting techniques. The dodecyl peripheral side chains of the synthetic compound are large enough to allow, in apolar solvents, self-assembling of the discotic molecules to helical aggregates. In polar solvents, the hydrogen bonding and pi-pi interactions pertaining to the chiral aggregation are compensated by solvation and self-assembling of the disklike molecules is disrupted. For comparison, time-resolved fluorescence measurements have been performed for the subgroup molecule, DAC, which is the (asymmetric) building block for TAB. It is concluded that, after pulsed photoexcitation, TAB and DAC exhibit excited-state intramolecular double proton transfer (ESIDPT) with a typical time of approximately 200-300 fs, irrespective of the degree of aggregation. Picosecond components in the fluorescence of TAB and DAC, ranging from 3 to 25 ps, are representative of vibrational cooling effects in the excited product state. Only aggregated TAB shows a rapid ( approximately 1 ps) decay of its fluorescence anisotropy. This component is attributed to excited-state energy transfer within the aggregate. Finally, the excited-state lifetime of TAB in the aggregated form is found to be an order of magnitude longer than that for TAB in its nonaggregated form. It is inferred that aggregation diminishes the influence of low-frequency twisting motions in the radiationless decay of the excited state.

Ultrafast Dynamics in Multibranched Structures with Enhanced Two-Photon Absorption

The understanding of the mechanism of the enhanced two-photon absorption (TPA) in multibranched chromophore systems is of importance to the design of materials with the large TPA cross-sections and for future applications. In this communication, the mechanism of enhanced TPA properties is investigated. For a dendritic model system, the excited-state dynamics for both population (T1-process) and phase relaxation (T2-process) processes involved are investigated by a combination of time-resolved spectroscopic techniques. The results of time-resolved fluorescence anisotropy are compared with previous results obtained from other branched chromophore systems. It is found that the PRL-701 trimer system, which possesses the large enhancement of two-photon absorption cross-section, gives a faster anisotropy decay (fluorescence upconversion and transient absorption), a longer population relaxation time (fluorescence lifetime), and a weaker coupling to the solvent (a larger photon echo peak shift initial value). New strategies for rational design of large TPA materials can be achieved based on a better understanding of the mechanism of the enhancement.

Synthesis of Triphenylamine-Cored Dendritic Two-Photon Absorbing Chromophores

[structure: see text]. A new series of dendritic two-photon absorbing chromophores containing triphenylamine moiety as a core or branching points have been synthesized through a convergent synthetic strategy. One-photon and two-photon optical properties of these molecules were characterized. In the nanosecond time domain, these molecules exhibited large two-photon absorption (TPA) cross sections up to 7.56-12.2 x 10(-44) s cm(4) at 800 nm, indicating that these molecular structures were viable candidates for various two-photon related applications.

Polaron Delocalization in Ladder Macromolecular Systems

Organic macromolecules with conjugated building blocks have been the focus of extensive research that is motivated, in part, by the potential to create optical and electronic devices. We have shown that palladium-catalyzed amination can assemble triarylamine ladder materials with extended structures. Two ladder macromolecules have been prepared in high yields by a series of twelve or sixteen C-N coupling reactions. Studies of the electronic and optical properties of neutral and oxidized forms of the ladder structures were conducted. The optical and electronic properties of the ladder systems are compared to those of the linear tetra-phenyl-p-phenylenediamine as well as the tetra-p-anisyl-p-tetraazacyclophane. The electrochemistry of the ladder systems consists of a multiwave voltammogram with a relatively low first oxidation potential. Electron paramagnetic resonance spectroscopy of the ladder systems suggests the presence of a large density of delocalized polarons. Linear absorption measurements of the chemically oxidized ladders revealed both polaron and intervalence absorption bands. Steady-state and time-resolved fluorescence measurements were also carried out to characterize the dynamics in these novel systems.

TIME-RESOLVED SPECTROSCOPY OF ORGANIC DENDRIMERS AND BRANCHED CHROMOPHORES

Organic dendrimers have been considered for a number of optical applications and are now of great interest for the purpose of enhanced nonlinear optical effects. In order to understand the mechanism of the enhanced effects in branched structures it is important to probe the fundamental excitations and the degree of intramolecular interactions utilizing various spectroscopic techniques. In this review, the nonlinear optical and excited state dynamics of different dendritic and other branching chromophore structures are discussed. The methods of two-photon absorption, time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift are discussed in regards to the degree of intramolecular coupling in the macromolecular systems. These techniques are also used for a comparison of the dynamics in the linear molecular analog systems as well. Thus, this review focuses on the aspect of intramolecular interactions in a branched system and its importance to enhanced nonlinear optical effects useful for modern optical devices.

The Ultrafast Dynamics and Nonlinear Optical Properties of Tribranched Styryl Derivatives Based on 1,3,5-Triazine

By using the femtosecond laser spectroscopic techniques, we have studied the ultrafast response and the nonlinear optical properties of three molecules with donor-acceptor structure (denoted as T01, T02, and T03). Two-photon absorption (2PA) cross sections measured by the open aperture Z-scan technique were determined to be 77, 90, and 410 GM for T01, T02, and T03, respectively. The relaxation dynamics of the excited states were measured by two-color femtosecond pump-probe and time-resolved photoluminescence (PL) experiments. By changing the solvent from chloroform (CHCl3) to dimethyl sulfoxide (DMSO), the transient dynamics was found changed significantly and the decay time of PL emission decreased dramatically because DMSO with large dipole moment accelerates the cross-transfer process and the nonradiative process in the molecules.

Optical excitations in organic dendrimers investigated by time-resolved and nonlinear optical spectroscopy

Our research is concerned with the optical properties of covalently bound branched multichromophore systems. The presence of strong intramolecular interactions in dendrimers and other branched macromolecules has stimulated new approaches toward improved energy transfer and light-emitting and enhanced nonlinear optical materials, as well as the possibility of delocalized (exciton) excitations in molecular aggregates. This Account summarizes some of our investigations, which combine the use of different time-resolved techniques to examine the dynamics in organic (conjugated) branched structures and provide important structure-function correlations necessary for applications.

Trapping time statistics and efficiency of transport of optical excitations in dendrimers

We theoretically study the trapping time distribution and the efficiency of the excitation energy transport in dendritic systems. Trapping of excitations, created at the periphery of the dendrimer, on a trap located at its core, is used as a probe of the efficiency of the energy transport across the dendrimer. The transport process is treated as incoherent hopping of excitations between nearest-neighbor dendrimer units and is described using a rate equation. We account for radiative and nonradiative decay of the excitations while diffusing across the dendrimer. We derive exact expressions for the Laplace transform of the trapping time distribution and the efficiency of trapping, and analyze those for various realizations of the energy bias, number of dendrimer generations, and relative rates for decay and hopping. We show that the essential parameter that governs the trapping efficiency is the product of the on-site excitation decay rate and the trapping time (mean first passage time) in the absence of decay.

Multichromophoric Förster resonance energy transfer

The theory of Förster resonance energy transfer is generalized for multichromophoric (MC) and nonequilibrium situations. For the first time, it is clarified that the far-field linear spectroscopic information is insufficient for the determination of the reaction rate and that distance dependence of the rate can vary with the disorder and temperature. Application to a light harvesting complex LH2 reveals the important consequences of a MC structure.

Electronic interactions in a branched chromophore investigated by nonlinear optical and time-resolved spectroscopy

The third order nonlinear optical properties of a trimer branched chromophore system and its linear molecule analog are investigated. Two-photon absorption and degenerate four wave mixing measurements were carried out on both systems. An enhancement in the nonlinear optical effect is observed for the branched trimer molecule in comparison to the linear chromophore system. Ultrafast time-resolved measurements were carried out to probe the excited state dynamics in the branched structures. The time-resolved measurements suggest that the two important processes affecting the nonlinear optical properties in the trimer system, charge transfer stabilization and initial electronic delocalization, occur on two different time scales.

Ultrafast fluorescence investigation of excitation energy transfer in different dendritic core branched structures

The mechanism of energy transport in branching structures is suggestively related to the geometry of the multichromophore architecture. In organic conjugated dendrimers, both incoherent (hopping) and coherent energy transfer processes have been observed from different dendritic architectures with different building blocks. In this communication, we report the investigation of three fundamental dendritic architectures (G0) with the same attached chromophores, but with different core atoms, C, N, and P. The synthesis of a phosphorus-containing G0 system with distyrylbenzene chromophores is provided. These three systems provide a comparison by which the relative interaction of branching chromophores can be compared on the basis of their different branching centers. Ultrafast fluorescence anisotropy measurements provide a dual measure of the geometry of the chromophores around the different central units as well as the strength of the interactions among chromophores. The nitrogen-cored system appeared to have both the strongest coupling of chromophore excitation as well as the most planar geometry of the three. Interestingly, the phosphorus system appeared to have the least planar geometry, and its interaction strength was found to be stronger than that observed for the carbon system. These results provide a comparison of the energy migration dynamics of the most common and new dendritic architectures with applications for light emission and light harvesting.

Excitation energy transfer in branched dendritic macromolecules at low (4 k) temperatures

To understand the mode of energy transport in branched dendritic macromolecules, the optical excitation of a dendritic core (A-DSB) at low temperature (4.2 K) was investigated. Fluorescence depolarization measurements were utilized to probe the energy-transfer processes in the branching center at several different temperatures. We found that the anisotropy decay shows an interesting trend at low temperature where depolarization times decreased and the residual anisotropy value also decreased with decreasing temperature. The very fast anisotropy decay suggests a coherent mechanism of energy transport in these systems at low temperature. The contribution of inhomogeneous broadening is suggested as an important factor in the temperature dependence of the anisotropy decay and residual value. The change in inhomogeneous linewidth is responsible for this type of anisotropy behavior.