Quantum dynamical approach for the calculation of vibrationally resolved ECD spectra induced by exciton couplings

Quantum dynamical approach for the calculation of vibrationally resolved ECD spectra induced by exciton couplings Daniele Padula, David Picconi, Alessandro Lami, Gennaro Pescitelli, Lorenzo Di Bari, Fabrizio Santoro In the last years we developed effective time-independent methods for the simulation of the spectral lineshape of one- and two-photon absorption and circular dichroism spectra, based on Born Oppenheimer (BO) approximation and quadratic expansions of the initial and final potential energy surfaces (PESs) [1-4]. Multichromoforic systems made up by achiral subunits can exhibit an chiroptical response as a result of exciton coupling between quasi-degenerate local electronic states and, being intrinsically nonadiabatic, cannot be properly described by BO models. The simplest physically meaningful models for those systems can be built from harmonic PESs for each local state and their couplings. This latter might be constant (exciton coupling) or also exhibit a linear or quadratic dependence on the normal coordinates. Quantum dynamics (QD) provides the most convenient approach to compute vibrationally-resolved electronic circular dichorism spectra (ECD) for such coupled-PESs models [5]. Unfortunately QD simulations are usually very time-consuming and this often imposes a somewhat arbitrary reduction of the dimensionality of the system which makes the simulation not straightforward and prone to inaccuracies. Very recently we developed a rigorous approach to face with this problem, proposing a hierarchical transformation of the quadratic vibronic coupling Hamiltonian (including therefore frequency changes and Dushinsky mixings of the normal coordinates) in blocks, so that few blocks can describe the exact nonadiabatic dynamics of the whole system on a short timescale (what is needed for ECD simulations) [6]. Here we present our methodology and its application to a number of dimeric systems, including a dimer of anthracene. [1] F. Santoro, R. Improta, A. Lami, J. Bloino and V. Barone, J. Chem. Phys., , 126, 084509, (2007) [2] F. Santoro, V. Barone, Int. J. Quantum Chem. 110, 476, (2010) [3] J. Bloino, M. Biczysko, F. Santoro, and V. Barone, J. Chem. Theory and Comp. 6, 1256, (2010) [4] N. Lin, F. Santoro, A. Rizzo, Y. Luo, X. Zhao and V. Barone, J. Phys. Chem. A, 113, 4198 (2009) [5] J. Seibt and V. Engel J. Chem, Phys., , 126, 74110, (2007) [6] D. Picconi, A. Lami and F. Santoro, J. Chem. Phys. 136, 244104 (2012)