Electronic Spectra of the Nanostar Dendrimer: Theory and Experiment

Abstract

We present a sequential molecular dynamics/quantum mechanics (MD/QM) study and steady-state spectroscopy measurements of the nanostar dendrimer (a phenylene-ethynylene dendrimer attached to a ethynylperylene chromophore) to determine the temperature dependence of the electronic absorption process. We studied the nanostar as separate units and performed MD simulations for each chromophore at 10 and 300 K to study the effects of the temperature on the structures. The absorption spectrum of the nanostar, at 10 and 300 K, was computed using an ensemble of 8000 structures for each chromophore. Quantum mechanical (QM) ZINDO/S calculations were performed for each conformation in the ensemble, including 16 excited states for a total of 128 000 excitation energies, and the intensity was scaled linearly with the number of conjugated units. Our calculations and experimental spectra measured for the individual chromophores and the nanostar are in good agreement. We found that for each system, the spectral features are narrow at 10 K because the transitions are localized in wavelength and the absorption energy depends primarily on the length of the chromophore, while at 300 K, the spectra features are quite broad and blue-shifted due to conformational changes on the systems. We explain in detail the effects of temperature and their consequence for the absorption process.

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

“…The existence of such phenylene node results in the localization of molecular orbitals (MOs) at individual linear PE unit [8,9] instead of spreading over the whole conjugated systems. [12][13][14][15] Such exciton transfers (energy transfers between different LE excitations) in p-conjugated dendrimers are responsible for the photo-energy collection that can further induce a series of complicated photochemical and photophysical processes. As a result, the absorption spectrum of the whole system simply becomes the summation over the spectrum of each individual chromosphere.…”

Nonadiabatic Molecular Dynamics Simulations of the Energy Transfer between Building Blocks in a Phenylene Ethynylene Dendrimer

“…27,28 These results indicate that the nanostar can be studied as the sum of separate linear PPE units: two-ring, three-ring, and four-ring para-substituted phenylene ethynylene. The three main peaks in the absorption spectrum correspond to the absorption of each one of these photoactive units.…”

Mechanisms of Light Energy Harvesting in Dendrimers and Hyperbranched Polymers

“…A key aim has been, and remains, the accomplishment of similar levels of efficiency in synthetically less demanding materials, achieved by emulating the photobiological principles of photon capture [4][5][6][7][8]. In the whole field of molecular science, multi-chromophore dendrimers represent the single type of material that has most significantly fulfilled this promise [4,5,[9][10][11][12][13][14][15][16][17][18][19]. Typically, these materials are built in a quasi-fractal geometry from chemically similar, strongly absorbing chromophores organized around a core that acts as an energy trap [10,18].…”

“…In the whole field of molecular science, multi-chromophore dendrimers represent the single type of material that has most significantly fulfilled this promise [4,5,[9][10][11][12][13][14][15][16][17][18][19]. Typically, these materials are built in a quasi-fractal geometry from chemically similar, strongly absorbing chromophores organized around a core that acts as an energy trap [10,18]. Solar energy capture in such materials is not in general aimed at accomplishing biochemical synthesis, but rather as a means of harnessing the energy of light in the cause of effecting molecular-level charge separation.…”