Cooperativity from charge transfer, phonons and electrostatic interactions in functional molecular materials

Abstract

Multistable switchable molecular materials are extremely promising for innovative applications. Multistability represents an extreme manifestation of cooperativity that, as thoroughly discussed in this thesis, occurs in molecular materials as a result of the subtle interplay between charge and/or spin degrees of freedom, electrostatic interactions and phonons. Here we present a theoretical approach to cooperativity in different families of molecular materials, including CT crystals, crystals of valence tautomeric molecules and spin crossover (SC) systems. Different microscopic models are developed and exploited to describe the variegated physics of the different families of materials.

The neutral to ionic phase transition (NIT) of mixed stack CT crystals offers an intriguing example of cooperativity. A coherent and comparatively simple picture of the rich and complex phenomenology of NIT is emerging, based on a modified Hubbard model with electron-phonon coupling, including molecular and lattice vibrations. Here we report the calculation of the lattice phonon dispersion in the presence of electron-phonon coupling, that proved the development of a Kohn-like anomaly in the optical phonon branch upon approaching NIT. This result quantitatively explains the observation of a sharp and intense diffuse X-ray signal observed in the pretransitional regime for a couple of systems, ruling out previous interpretations. Moreover, an original implementation of the time correlation function approach to spectroscopy was developed that, exploiting the modern theory of polarization and polarizability, applies to the calculation of infrared and Raman spectra of mixed stack CT crystals. The proposed method fully accounts for anharmonicity, and reliably describes vibrational spectra in the proximity of NIT.

Bistability induced by electrostatic interactions in clusters of valence tautomeric (or donor-acceptor) molecules was predicted a few years ago in the guest laboratory. Here we demonstrate that the temperature dependent tautomerism, detected by Mossbauer spectroscopy in ferrocene-perchlorotriphenylmetyl (Fc-PTM) crystals, offers the first experimental example of bistability in crystals of valence tautomeric molecules. The analysis follows a bottom up modeling strategy: information on the molecular unit are retrieved from a detailed analysis of optical spectra of Fc-PTM in solution. Then, an original implementation of quantum chemical calculations is presented to get reliable models for intermolecular electrostatic interactions, responsible for bistability. The approach and the resulting model is general enough to provide guidelines for the synthesis of bistable crystals based on valence tautomeric molecules.

SC transition metal complexes represents one of the most extensively investigated examples of bistable molecular materials. Cooperativity occurs in condensed phases from elastic interactions related to the large variation of the molecular size with the spin state. We developed a microscopic vibronic models for SC molecules. The numerically exact non-adiabatic solutions of this model demonstrate the failure of the adiabatic approximation for SC systems. Moreover, preliminary results on crystals of SC complexes demonstrate the possibility to describe cooperative intermolecular interactions, while fully accounting for the non-adiabatic coupling of molecular vibrations to spin degrees of freedom.

From the study of multistability in different families of molecular materials we got a thorough understanding of the complex physics underlying cooperativity, identifying unifying features as well as more specific characteristics.