real space analysis of excitonic interactions and coherence激子相互作用和连贯性的实空间分析.pdfVIP

3524 J. Phys. Chem. A 2002, 106, Real Space Analysis of Excitonic Interactions and Coherence Length in Helical Aggregates† Andrei Piryatinski,‡ Sanford A. Asher,§ and Shaul Mukamel*,‡ Department of Chemistry, Uni ersity of Rochester, Rochester, New York 14627 and Department of Chemistry, Uniersity of Pittsburgh, Pittsburgh, Pennsylania 15260 Recei ed: July 24, 2001; In Final Form: January 15, 2002 The effective exciton coherence length in molecular aggregates is calculated using a nonlocal, real-space representation of the linear optical response. Simulations of polarized absorption and linear dichroism demonstrate that the spectra depend on local properties of the molecular aggregate segment where the exciton coherence is delocalized. This allows us to limit the calculations to small segments of a large aggregate, avoiding the complexity of computing long-range coherences. I. Introduction (resonance) energy and polarization associated with the orienta- tions of the exciton transition dipoles. For instance, absorption The exciton states in a perfect crystal represent standing spectra from a molecular crystal or ideal aggregate with two waves of electronic excitation delocalized over the entire crystal1,2 with infinitely long coherence lengths. However, this identical nonparallel chromophores in a RU show two perpen- dicularly pola