title: Strong Two-Photon Circular Dichroism in Helicenes:  A Theoretical Investigation.
abstract: Using a recently derived origin-invariant quadratic response approach combined with time-dependent density functional theory, four representative helicenes are shown to present a very strong two-photon circular dichroism (TPCD) response, which makes them candidates for the first experimental observation of a TPCD effect. The large response is attributed to the unique combination of chirality and electron delocalization. Comparison with electronic circular dichroism and two-photon absorption (TPA) shows that the three effects exhibit complementary features for unravelling the molecular structures. In particular, for the four (M)-helicenes studied here, the first, i.e., low-energy, dominant Cotton band is always negative, whereas for TPCD it is positive. From an analysis of the frontier orbitals describing most of the one-electron excitation vectors, the largest TPCD response of tetramethoxy-bisquinone-dithia-[7]-helicene has been attributed to the charge-transfer character of the excited state, like for the parent TPA effect. Moreover, the TPCD intensities are found to be mostly governed by the electric and magnetic dipole contributions, while the electric quadrupole terms are, on a relative basis, less important.
authors.affiliation: Teoretisk Kemi, Kemisk Institut, Aarhus Universitet, Langelandsgade 140, 8000 Aarhus C, Denmark.|Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche (IPCF-CNR), Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.|Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, SE-10691 Stockholm, Sweden.|Laboratoire de Chimie Théorique Appliquée, FUNDP, Rue de Bruxelles 61, B-5000 Namur, Belgium.
authors.forename: Branislav
authors.initials: B
authors.lastname: Jansík
authors.affiliation: Teoretisk Kemi, Kemisk Institut, Aarhus Universitet, Langelandsgade 140, 8000 Aarhus C, Denmark.|Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche (IPCF-CNR), Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.|Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, SE-10691 Stockholm, Sweden.|Laboratoire de Chimie Théorique Appliquée, FUNDP, Rue de Bruxelles 61, B-5000 Namur, Belgium.
authors.forename: Antonio
authors.initials: A
authors.lastname: Rizzo
authors.affiliation: Teoretisk Kemi, Kemisk Institut, Aarhus Universitet, Langelandsgade 140, 8000 Aarhus C, Denmark.|Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche (IPCF-CNR), Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.|Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, SE-10691 Stockholm, Sweden.|Laboratoire de Chimie Théorique Appliquée, FUNDP, Rue de Bruxelles 61, B-5000 Namur, Belgium.
authors.forename: Hans
authors.initials: H
authors.lastname: Ågren
authors.affiliation: Teoretisk Kemi, Kemisk Institut, Aarhus Universitet, Langelandsgade 140, 8000 Aarhus C, Denmark.|Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche (IPCF-CNR), Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.|Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, SE-10691 Stockholm, Sweden.|Laboratoire de Chimie Théorique Appliquée, FUNDP, Rue de Bruxelles 61, B-5000 Namur, Belgium.
authors.forename: Benoit
authors.initials: B
authors.lastname: Champagne