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

Structure of poly(propyl ether imine) (PETIM) dendrimer from fully atomistic molecular Dynamics Simulation and by Small Angle X-ray scattering

We study the structure of carboxylic acid terminated neutral poly (propyl ether imine) (PETIM) dendrimer from generation 1 through 6 (G1-G6) in a good solvent (water) by fully atomistic molecular dynamics (MD) simulations. We determine as a function of generation such structural properties as: radius of gyration, shape tensor, asphericity, fractal dimension, monomer density distribution, and end-group distribution functions. The sizes obtained from the MD simulations have been validated by Small Angle X-Ray Scattering (SAXS) experiment on dendrimer of generation 2 to 4 (G2-G4). A good agreement between the experimental and theoretical value of radius of gyration has been observed. We find a linear increase in radius of gyration with the generation. In contrast, Rg scales as ~ N^x with the number of monomers. We find two distinct exponents depending on the generations: x = 0.47 for G1-G3 and x = 0.28 for G3-G6 which reveals their non-space filling nature. In comparison with the amine terminated PAMAM dendrimer, we find Rg of G-th generation PETIM dendrimer is nearly equal to that of (G+1)-th generation of PAMAM dendrimer as observed by Maiti et. al. [Macromolecules,38, 979 2005]. We find substantial back folding of the outer sub generations into the interior of the dendrimer. Due to their highly flexible nature of the repeating branch units, the shape of the PETIM dendrimer deviates significantly from the spherical shape and the molecules become more and more spherical as the generation increases. The interior of the dendrimer is quite open with internal cavities available for accommodating guest molecules suggesting using PETIM dendrimer for guest-host applications. We also give a quantitative measure of the number of water molecules present inside the dendrimer.Comment: 33 page

Resonance energy transfer: The unified theory revisited

Resonanceenergy transfer (RET) is the principal mechanism for the intermolecular or intramolecular redistribution of electronic energy following molecular excitation. In terms of fundamental quantum interactions, the process is properly described in terms of a virtual photon transit between the pre-excited donor and a lower energy (usually ground-state) acceptor. The detailed quantum amplitude for RET is calculated by molecular quantum electrodynamical techniques with the observable, the transfer rate, derived via application of the Fermi golden rule. In the treatment reported here, recently devised state-sequence techniques and a novel calculational protocol is applied to RET and shown to circumvent problems associated with the usual method. The second-rank tensor describing virtual photon behavior evolves from a Green’s function solution to the Helmholtz equation, and special functions are employed to realize the coupling tensor. The method is used to derive a new result for energy transfer systems sensitive to both magnetic- and electric-dipole transitions. The ensuing result is compared to that of pure electric-dipole–electric-dipole coupling and is analyzed with regard to acceptable transfer separations. Systems are proposed where the electric-dipole–magnetic-dipole term is the leading contribution to the overall rate

Transparent, Stretchable, and Conductive SWNT Films Using Supramolecular Functionalization and Layer-by-Layer Self-Assembly

We demonstrate films of single-walled carbon nanotubes (SWNTs) on the elastomer polydimethylsiloxane (PDMS) that are stretchable, conductive, and transparent. Our fabrication method uses the supramolecular functionalization of SWNTs with conjugated polyelectrolytes to generate aqueous dispersions of positively- and negatively-charged SWNTs, followed by layer-by-layer self-assembly onto a PDMS substrate. Adding bilayers of positively- and negatively-charged SWNTs to the surface causes the sheet resistance and the % transmittance of the film to both progressively decrease. The sheet resistance decreases sharply in the first five bilayers as the layer-by-layer process efficiently establishes the percolation network, whereas the % transmittance declines more gradually. Films with 25 bilayers are transparent (75% at 550 nm) and conductive (560 ± 90 ohms/sq). The combination of electrostatic and pi-stacking forces very effectively bind the SWNTs within the film, producing smooth film surfaces (root-mean-square roughness of 18 nm) and enabling the films to remain conductive up to 80% elongation. We demonstrate the use of the SWNT films as transparent conductive electrodes in light-emitting devices and as soft strain sensors that are both wearable and transparent

Dynamics of the dispersion interaction in an energy transfer system

On the propagation of resonant radiation through an optically dense system, photon capture is commonly followed by one or more near-field transfers of the resulting optical excitation. The process invokes secondary changes to the local electronic environment, shifting the electromagnetic interactions between participant chromophores and producing modified intermolecular forces. From the theory it emerges that energy transfer, when it occurs between chromophores with electronically dissimilar properties, can itself generate significant changes in the intermolecular potentials. This report highlights specific effects that can be anticipated when laser light propagates across an interface between differentially absorbing components in a model energy transfer system

Origin of spectral broadening in pi-conjugated amorphous semiconductors

We present a study of the picosecond fluorescence dynamics of pi-conjugated semiconducting organic dendrimers in the solid state. By varying the degree of branching within the dendrons, referred to as the dendrimer generation, a control of intermolecular spacing of the emissive core and therefore of the lattice parameter for Forster-type energy transfer is achieved. This allows a distinction between spectral diffusion and excimer formation as the two main sources of spectral broadening in organic semiconductors. Whereas Forster-type dispersive spectral relaxation is independent of temperature but strongly dependent on the interchromophore distance, excimer formation is also strongly thermally activated due to temperature-dependent conformational changes and the influence of thermally activated dynamic disorder. The rapid spectral diffusion allows a determination of the excimer rise in the emission, which is shown to have a profound impact on the steady state luminescence properties of dendrimer films. We show that the dendrimer generation not only allows a microscopic control of intermolecular interactions but also a direct control of the rate of spectral diffusion. Implications for the design of novel materials for optoelectronic devices are discussed