Theoretical Analysis of Optical Absorption and Emission in Mixed Noble Metal Nanoclusters.

In this work, we studied theoretically two hybrid gold silver clusters, which were reported to have dual-band emission, using density functional theory (DFT) and linear and quadratic response time-dependent DFT (TDDFT). Hybrid functionals were found to successfully predict absorption and emission, although explanation of the NIR emission from the larger cluster (cluster 1) requires significant vibrational excitation in the final state. For the smaller cluster (cluster 2), the Delta H(0-0) value calculated for the T1 -> S0 transition, using the PBE0 functional, is in good agreement with the measured NIR emission, and the calculated T2 -> S0 value is in fair agreement with the measured visible emission. The calculated T1 -> S0 phosphorescence Delta H(0-0) for cluster 1 is close to the measured visible emission energy. In order for the calculated phosphorescence for cluster 1 to agree with the intense NIR emission reported experimentally, the vibrational energy of the final state (S0) is required to be about 0.7 eV greater than the zero-point vibrational energy.