From molecules to materials: the crucial role of supramolecular interactions Anna Painelli Francesca Terenziani INSTM-Parma Parma University

D-  -A chromophores polar and polarizablepolar and polarizable molecules electron transfermodel systems for electron transfer DCM  < 0.5 Q-3CNQ  > 0.5

a model for interacting D-  -A chromophores layout of the work the same molecule behaves in qualitatively different ways depending on supramolecular architecture collective and cooperative phenomena bottom-up approach to molecular materials new physics with no counterpart at the molecular level molecular properties from solution to supramolecular architectures a case study Q-3CNQ from solution to LB film

Mulliken model two orthogonal basis states two orthogonal eigenstates   measures the molecular polarity

A model for interacting polar and polarizable molecules two-state on-site Mulliken Hamiltonian electrostatic intersite interactions For a cluster of N molecules: 2 N electronic basis functions exact diagonalization for up to sites

cooperative behavior is the ground state of an effective self-consistent Mulliken Hamiltonian mean-field approximation to the ground state Madelung energy the interaction with surrounding molecules affects the properties of each molecule

mean-field excitation energy collective behavior excitonic correction to the k=0 state creating an excitation exciton migration

D+A-D+A- isolated molecules D A squeezing a repulsive clusters, M>0 the polarity varies with supramolecular arrangement the optical frequency is governed by a subtle interplay of mean-field, excitonic and ultraexcitonic effects  inverse intermolecular distance Phys.Rev.B 68, (2003)

modeling solution spectra: JCPA 106, 6286 (2003) Chem.Phys. 295, 35 (2003) bottom-up strategy a case study: Q-3CNQ, the molecular rectifier 0.16  0.14  0.17  sp z0z0 estimated ground state polarity: 0.88, 0.89, 0.92 in collaboration with Robert Metzger LB monolayer fluorescence a.u. absorbance fluorescence a.u. absorbance LB monolayer molecular parameters (eV)

molecular parameters from the analysis of solution spectra But at small tilt angles: neutral Q-3CNQ ?inverse rectification in SA? we can model experimental spectra of LB-monolayers of Q-3CNQ with the same parameters extracted from solution spectra In the LB layer Q-3CNQ is more polar than in solution; the observed frequency is governed by mf effects Experimental frequency in LB film tilt angle 7 Ǻ

multielectron transfer as the primary photoexcitation event upon absorption of a single photon several electrons are transferred on nearby molecular sites Extreme manifestation of cooperative behavior Discontinuous charge crossovers new phenomena appear at the supramolecular level with no counterpart at the molecular level number of D + A - species created upon photoexcitation excitonic regime inverse intermolecular distance J.Am.Chem.Soc.125, 5624 (2003)

conclusions supramolecular interactions in molecular materials cooperative and collective phenomena discontinuous charge crossovers multielectron transfer evolution of the molecular properties from solution to condensed phases supramolecular structure – properties relationships new physics new understanding

thanks Geoff Ashwell (Univ. Cranfield) Alberto Girlando (Univ. Parma) Robert Metzger (Univ. Alabama) Zoltan Soos (Univ. Princeton ) financial support MIUR – COFIN2001 INSTM