The optical properties of gold nanoclusters of size 2 - 20 have been investigated using time-dependent density functional
theory (TDDFT) to simulate their linear absorption spectra. Relativistic effects have been included by using
pseudopotentials, with the Douglas-Kroll (DK) approximation, and with the zero-order regular approximation (ZORA).
The improved model core potential with scaled relativistic effects (iMCP-SR2) used in combination with either the
BP86, BLYP, or B3LYP exchange-correlation density functional was found to fairly accurately model the spectra of
clusters for which measured spectra are available, although the all-electron ZORA method was best both for accuracy
and computational efficiency. The effects on the optical properties of organic chromophores from coordination with
small gold clusters were preliminarily studied. The extent of enhancement of the absorption properties is seen to depend
on the size and structure of the gold cluster.
We present calculations of one-photon absorption (OPA) spectra for (CdSe)<sub>n</sub> (up to n = 34) quantum dots, carried out
using time-dependent density functional theory. The effects of cluster size on OPA spectra are discussed and compared
with experiment to provide preliminary insights into the formation of stable clusters.
An accurate prediction of two-photon absorption (TPA) spectra, especially the position and magnitude of the peak cross-section, as well as the line width, would be of value in the design of TPA molecules for specific applications. The spectrum can be calculated using the sum-over-states (SOS) formalism, which requires accurate values for the energies of the relevant electronic states and for the transition dipole moments between these states. In addition, the form of the line-width function, such as Gaussian or Lorentzian, as well as its value, must be determined. Time-Dependent Density Functional Theory (TDDFT) has been shown to give accurate excitation energies and ground-state transition dipole moments for a wide variety of molecules. It is now possible to use this method to calculate excited-state transition dipole moments and to subsequently calculate TPA cross-sections using the SOS formalism. We report TDDFT calculations of the TPA spectra for trans-octatetraene and trans-stilbene. Gaussian functions have been used to describe the TPA line-broadening, with the width determined from experiment or previous convention. The energy levels, transition dipole moments, and TPA cross-sections calculated by this and other methods are compared to experiment and to other theoretical methods.
Porphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and alkylation either at the pyrrole position or the meso-aryl groups. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, excitation dynamics. The significant factors influencing limiting behavior appear to be the heavy atom effect, electron donating and withdrawing substituents conformation distortion and changes in conjugation. Detailed understanding will be gained from measurements of photophysical parameters underlying limiting behavior.