Vibronic Model for the Quantum Dynamical Study of the Competition between Bright and Charge-Transfer Excited States in Single-Strand Polynucleotides: The Adenine Dimer Case

A simple vibronic model aimed at investigating the interplay between bright excitonic states and dark charge-transfer (CT) states in stacked adenine (Ade) nucleobases is presented. Two orbitals (the HOMO and the LUMO) for each Ade site have been included in the electronic Hamiltonian, whose parameters have been fitted to reproduce the main features of the absorption spectra of two stacked 9-methyladenine (9Me-A) molecules, computed in aqueous solution at the PCM/TD-PBEO level. Three modes for each adenine unit have been included in the Hamiltonian, to describe the main structural changes among the different excited state minima of the adenine stacked dimer, as described at the TD-DFT level. The developed vibronic Hamiltonian (four electronic states and six nuclear coordinates) has been adopted to perform quantum dynamical calculations of a photoexcited Ade stacked dimer, utilizing the multiconfigurational time-dependent Hartree method. The obtained results indicate that the transfer between the bright excitonic state and the CT state is fast and effective.