Electronic spectra of the nanostar dendrimer: Theory and experiment

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.