Dyes and Nanoparticles for 2PA Applications: Models and Computations

Molecular functional materials are actively investigated for several applications as lightweight and environmental-friendly materials with highly customizable properties. In this thesis we focus attention on materials for non-linear optical applications, and more specifically for 2-photon absorption. This work is performed in the framework of a collaborative project Nanochemistry of molecular materials for 2-photon functional application (Nano2Fun), funded by the EU commission. The main aim of Nano2Fun is to develop optimized materials and methods for specific applications of 2-photon absorption towards bioimaging and nanofabrication. Within this framework, the contribution of this thesis is to help the experimentalists to rationalize linear and non-linear spectral properties of organic dyes and of their aggregates, offering a sound theoretical analysis. This thesis mainly concentrates on the study of π-conjugated organic dyes, for their exciting non-linear optical properties. Conjugated electrons are responsible for low-energy electronic excitations, large (hyper)polarizabilities, extreme sensitivity to environmental perturbations and these have been captured for studying intramolecular CT in π-conjugated systems embedding electron-donor (D) and/or acceptor (A) groups, by a family of parametric Hamiltonians, the essential state models (ESM), extensively validated and parametrized in the last years against experimental data, quantum chemical calculations and Density Matrix Renormalization Group (DMRG) theory. In order to understand the spectral properties of molecular aggregates and organic nanoparticles of interest for two-photon absorption and NLO application, we have undertaken an extensive series of quantum chemical calculations on dimers of quadrupolar (D-A-D) dyes belonging to the curcumin and squaraine families. Time Dependent Density Functional Theory (TDDFT) has been used for validating the model against a standard known approach. For the squaraine family, TDDFT results present a wide disagreement with the experimental results, while it holds reasonably well for the curcumin family of dyes. Squaraines have also been studied by exploiting finite size symmetrized DMRG calculations to solve the corresponding Pariser-Parr-Pople Hamiltonian. Preliminary results fully support the biradicaloid character of the squaraine ground state. A linearly aligned dipolar chromophore, DANS, has been studied for the collective amplification in second order NLO response, starting with making use of a bottom-up modeling strategy within the realm of ESM to model DANS, and further exploited to build aggregation models, working in close cooperation with DFT to extract intermolecular interactions. Large cooperative and collective effects have been proved in the aggregation of CT dyes, thereby leading to a superlinear dependence of the β-response on the number of aligned molecules inside the aggregate aligned having a neutral ground state and considering that these dyes interact with each other.