PINO Gustavo Ariel
congresos y reuniones científicas
Electronic quantum dynamics approach sheds light on the photophysics of DNA silver quantum dots
Encuentro; 9th International Meeting on Photodynamics and Related Aspects; 2016
One interesting application of the incorporation of metal mediated base paring to DNA, is the production of highly fluorescent and tunable hybrid DNA-Agn clusters.1 A distinctive feature of these is the fact that they have two intense absorption bands, one in the visible which is tunable with DNA bases sequence and/or the number of Ag atoms and the other inthe UV spectral region, near to the DNA absorption, which is common to all of them. Interestingly, the excitation of the common UV band leads to the same fluorescence spectrum as in the case of excitation of the tunable visible band. This suggessthat there must exist a strong and ultra-fast electronic coupling between the DNA excited state and the rod-like Ag n cluster.Recently, a charge transfer (CT) process from cytosine to Ag+ was observed upon UV excitation of the simplest Cytosine-Ag+ (C-Ag+) complex in the gas phase,2 this process could be the ground of the electronic coupling process between both moieties. Nevertheless the detailed mechanism responsible for this coupling is still elusive. In this context, in the present work we explore the spectroscopy and photophysics of DNA-Ag considering twocytosine based model systems, with (dpC6)2Ag6 and without [C2Ag]6 the DNA backbone.The methodology applied is based on the time propagation of the one electron density matrix under the influence of external time-varying electric fields. It is a real time-dependent simulation, where the electronic structure is obtained from a density functional theory based tight-binding (DFTB) Hamiltonian (DFTB+ code). Optical absorption spectra are obtained byintroducing an initial perturbation in the shape of a Dirac delta pulse, the complex polarizability can be obtained from the Fourier transformation of the dipole moment. A deeper understanding of the nature of the absorption bands can be obtained from the time dependent molecular orbital populations following a laser excitation in tune with the excitation energy.The electronic structures of both structures considered here where studied through analysis of their DOS and PDOS. Remarkably similar features were observed for [C2Ag]6 and (dpC6)2Ag6 among which, the most important reveals the presence of common feature zones, where the contribution of states from both moieties (Ag and DNA) is reproduced, and the existence of a gap of states at the Fermi level. The simulation of the absorption spectra of both structures reproduce the features observed in experimental reports, i.e. a series ofbands are observed, one in the UV spectral region near the absorption band of the DNA moiety and new bands in the visible region. Remarkably, the irradiation of the UV band ofboth structures considered here show that in this transition, electrons are promoted from localized π states of the DNA, to delocalized states of the Ag moiety, leading to a net negative charge transfer during light irradiation, as shown in Figure 1. To the best of our knowledge, this is the first report of this photo-induced CT in a ?complete? DNA-Agn structure. Furthermore, this results may set the grounds to understand the common fluorescence obtained when the UV and visible bands are excited. This can be explained considering that the orbitals populated during UV irradiation, responsible of the charge transfer process, are common to those populated during visible irradiation.