Buttarazzi, Edoardo (2025) Development of Theoretical-Computational Protocols for Photophysical and Photochemical Processes in Dyes for Technological Applications. [Tesi di dottorato]

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Abstract

A detailed understanding of the molecular basis of the electron-nuclear interplay and its influence on the charge dynamics is still an open question and is crucial for the rational design of more efficient photo-active materials. From a theoretical-computational point of view several challenges are presents to accurate model such phenomena, taking into account also the ultra-fast time scales in play. This Doctoral work presents the development and application of an unified theoretical-computational framework, the Electron-Nuclear Coupling Mapping Protocol (ENCMP), conceived to connect molecular vibrations, electronic excitations, and photophysical response in dyes and coordination compounds relevant to technological applications. The protocol is based on density functional theory (DFT) and quantifies how selected nuclear motions influence the electronic states, the electronic manifold, and the resulting charge ultra-fast dynamics, thereby linking spectroscopic observables to the underlying molecular mechanisms. The first part of the research concerns squaraine dyes, representative organic chromophores with intense red and near-infrared absorption. Combined experimental-computational analyses clarify how conformation and protonation states control their electronic structure and spectral features. Overall, these results clarify the interplay between molecular conformation and environment in squaraine photophysics and the identification of a conical intersection mediating ultrafast nonradiative decay, providing a valuable guidance for the rational design of efficient squaraine-based optoelectronic and sensing devices. The second part focuses on a Ru(II)-based complex [Ru(dcbpy)_2(NCS)_2]^4- (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), a prototypical sensitizer in dye-sensitized solar cells. Using ENCMP and monitoring the explicit evolution in time of the excited state density, via real-time time-dependent DFT simulations, it is demonstrated that specific vibrational motions within the Ru-NCS and dcbpy fragments actively influence the evolution of the electronic density on the sub-femtosecond timescale. Finally, the proposed protocol is applied to the CoFe-based Prussian Blue Analogue, a computationally challenging mixed-valence coordination compound. The analysis establishes that the bridged cyano ligand stretching modes can be used as probes to unveil the nature of the optical excitations of the transition-metal complex. Overall, the presented Doctoral Thesis establishes a general theoretical-computational protocol for exploring the interplay between molecular vibrations and electronic response in both organic and coordination compounds. The protocol advances the theoretical understanding of light-induced processes and offers predictive insight for the rational design of next-generation materials for energy conversion and photochemical applications.

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