|Tipo di tesi||Tesi di dottorato di ricerca|
|Titolo||STUDIO A PRINCIPI PRIMI SULLE PROPRIETÀ OTTICHE IN CRISTALLI MOLECOLARI DI TIPO PUSH-PULL J-AGGREGATE|
|Titolo in inglese||FIRST PRINCIPLES STUDY OF THE OPTICAL PROPERTIES IN PUSH-PULL MOLECULAR J-AGGREGATES|
|Settore scientifico disciplinare||FIS/03 - FISICA DELLA MATERIA|
|Corso di studi||PHYSICS AND NANO SCIENCES|
|Data inizio appello||2019-02-18|
|Disponibilità||Accessibile via web (tutti i file della tesi sono accessibili)|
I cristalli molecolari sono sistemi fisici risultanti dall’aggregazione di molecule in varie configurazioni spaziali. Di conseguenza, quando eccitate, le molecole mostrano un comportamento collettivo che è un fenomeno emergente dovuto alle forti interazioni elettroniche di natura inter molecolare. Quando ciò accade, il cristallo molecolare può essere considerato come un’unica macro-molecola con proprietà ottiche completamente diverse da quelle delle singole molecole costituenti. Quest’ultimo è un comportamento tipico dei J-aggregates, cristalli molecolari che manifestano proprietà ottiche particolari come un’intensa e molto stretta banda di assorbimento, chiamata J-band, che compare a più basse energie rispetto alla banda di assorbimento della singola molecola. Tale comportamento non può essere spiegato a partire dalle proprietà ottiche delle singole molecole in quanto ha una natura intrinsecamente collettiva.
Molecular crystals are physical systems formed by aggregation of molecules in different arrangements. As a result, when excited, they typically manifest a collective behavior which is an emergent phenomenon due to the strong inter-molecular electronic interactions. When this happens the molecular aggregate can be treated as one big molecule which manifests optical properties completely different from those of the single molecular units. The latter behavior is a typical feature of J-aggregate molecular crystals which manifest peculiar optical properties such as an intense and narrow absorption band, called J-band, which appears at lower energy with respect to the single molecule absorption band. This behavior cannot be explained in term of the optical properties of the single molecules due to its collective nature. Although the optical properties of J-aggregates have been studied for a long time since their discovery in the 1930s, it is still unclear what kind of microscopic physical mechanisms give origin to the J-band and to what extent solid-state effects play a role in the optical properties of these molecular crystals. Most of the attempted first-principles descriptions do not take thoroughly into account many-body solid-state effects (e.g., long range interactions, screening effects) and are based on strong approximations that are not sufficient to correctly predict their optical properties. On the other hand, the need to correctly describe the optical properties of these materials is also driven by the recent trend to use J-aggregates in strong coupling experiments to study hybrid excitations due to strong matter-light interaction. The principal aim and original contribution of this thesis is to give a comprehensive first-principles description, based on state-of-the-art methods such as Time-Dependent-Density-Functional-Theory (TDDFT) and Many-Body Perturbation Theory (MBPT), on the optical properties of J-aggregates. In particular, we investigate a specific molecular aggregate composed of organic push-pull dyes and focus the analysis on its red-shifted J-band. We adopt two kinds of numerical approaches. In the first one we use TDDFT through which we assess the role of the molecular packing in the enhancement and red-shift of the J-band, along with the effects of the quantum confinement in the optical absorption when the crystal is confined in one or two dimensions. By analyzing the induced charge density associated to the J-band, we conclude that: (i) it is displaced along the crystal principal axis along parallel molecular chains; (ii) the overall red-shift of the J-band results from: (1) competing coupling mechanisms such as interactions between monomers in the same chain giving red-shift and weaker interactions between monomers of different chains giving a minor blue shift; (2) renormalization of the single particle energy levels due to molecular aggregation. In the second approach we shall adopt MBPT framework with GW and Bethe-Salpeter equation (BSE) numerical methods where the former introduces quasi-particle energy corrections and the latter explicitly describes the electron-hole screened interaction (i.e., excitonic effects). With this approach we demonstrate that excitonic effects are present and must be included for a correct description of the optical response of the molecular aggregate. In addition, through exciton wavefunctions analysis of the principal excited states we show that in general there is a certain degree of inter-molecular delocalization of excitons and a predominant intra-molecular charge-transfer behavior. In the final part we shall quantify to what extent plasmonic collective mechanisms are involved in the J-band and will show that this band has essentially no plasmonic behavior.