Multistable switchable molecular materials are extremely promising forinnovative applications. Multistability represents an extrememanifestation of cooperativity that, as thoroughly discussed in thisthesis, occurs in molecular materials as a result of the subtleinterplay between charge and/or spin degrees of freedom, electrostaticinteractions and phonons. Here we present a theoretical approach tocooperativity in different families of molecular materials, includingCT crystals, crystals of valence tautomeric molecules and spincrossover (SC) systems. Different microscopic models are developed andexploited to describe the variegated physics of the different familiesof materials. The neutral to ionic phase transition (NIT) of mixed stack CT crystalsoffers an intriguing example of cooperativity. A coherent andcomparatively simple picture of the rich and complex phenomenology ofNIT is emerging, based on a modified Hubbard model withelectron-phonon coupling, including molecular and lattice vibrations. Here we report the calculation of the lattice phonon dispersion in thepresence of electron-phonon coupling, that proved the development of aKohn-like anomaly in the optical phonon branch upon approaching NIT. This result quantitatively explains the observation of a sharp andintense diffuse X-ray signal observed in the pretransitional regimefor a couple of systems, ruling out previous interpretations.Moreover, an original implementation of the time correlation functionapproach to spectroscopy was developed that, exploiting the moderntheory of polarization and polarizability, applies to the calculationof infrared and Raman spectra of mixed stack CT crystals.The proposed method fully accounts for anharmonicity, and reliablydescribes vibrational spectra in the proximity of NIT.Bistability induced by electrostatic interactions in clusters ofvalence tautomeric (or donor-acceptor) molecules was predicted a fewyears ago in the guest laboratory. Here we demonstrate that thetemperature dependent tautomerism, detected by Mossbauer spectroscopyin ferrocene-perchlorotriphenylmetyl (Fc-PTM) crystals, offers thefirst experimental example of bistability in crystals of valencetautomeric molecules. The analysis follows a bottom up modelingstrategy: information on the molecular unit are retrieved from adetailed analysis of optical spectra of Fc-PTM in solution.Then, an original implementation of quantum chemical calculations ispresented to get reliable models for intermolecular electrostaticinteractions, responsible for bistability. The approach and theresulting model is general enough to provide guidelines for thesynthesis of bistable crystals based on valence tautomeric molecules. SC transition metal complexes represents one of the most extensivelyinvestigated examples of bistable molecular materials. Cooperativityoccurs in condensed phases from elastic interactions related to thelarge 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 modeldemonstrate the failure of the adiabatic approximation for SC systems.Moreover, preliminary results on crystals of SC complexes demonstratethe possibility to describe cooperative intermolecular interactions,while fully accounting for the non-adiabatic coupling of molecularvibrations to spin degrees of freedom. From the study of multistability in different families of molecularmaterials we got a thorough understanding of the complex physicsunderlying cooperativity, identifying unifying features as well as more specific characteristics.
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