Conjugated electrons are responsible for low-energy electronic excitations, large (hyper)polarizabilities, extreme sensitivity to environmental perturbations, making pi-conjugated molecules interesting for a variety of applications. Donor (D) and/or acceptor (A) groups embedded in pi-conjugated molecules offer a way to control molecular properties, as well as the possibility to introduce new phenomena, including multistability. Pi-conjugated molecules, with their flat structures and exposed frontiers orbitals represent interesting building blocks for self-organized materials (aggregates and crystals) where intermolecular and intramolecular charge transfer degrees of freedom come into play, leading to a complex landscape of phenomena and to the appearance of new functionalities.
Intramolecular CT in pi-conjugated molecules is well captured by a family of parametric Hamiltonians, the essential state models, extensively validated and parametrized in the last years against experimental data and quantum chemical calculations. Steady-state and time-resolved linear and non-linear spectra are quantitatively reproduced, including 2D spectra fully, accounting for vibrational coupling and polar solvation and its dynamics. Electrostatic interchromophore interactions lead to exciton models well beyond the standard picture and account for multiexciton generation and multistability.
Pi-conjugated molecules are also the building blocks of so called CT-crystals, a large family of molecular functional materials, intensively studied for their exotic properties spanning the full range from superconductivity to ferroelectricity going through multistability and photoinduced phase transitions. The model of choice for this family of strongly correlated electron systems is the Hubbard model, extended to account for the coupling to molecular and lattice vibrations.
Based on the thorough understanding of the physics of pi-conjugated chromophores and of CT crystals, we propose a new family of parametric Hamiltonians to describe systems where intra and intermolecular CT play a role. We address (low-energy) spectral properties of dyes, aggregates and crystals and address the complex phase diagrams of materials governed by a subtle interplay of intra and intermolecular CT, electrostatic interactions and vibrational degrees of freedom, shedding light on the emergence of new functionalities that further enrich the already wide catalogue of potential applications of pi-conjugated functional materials.
The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement n°607721.