Self-assembling, structure and nonlinear optical properties of fluorescent organic nanoparticles in water

Owing to their intense emission, low toxicity and solubility in aqueous medium, fluorescent organic nanoparticles (FONs) have emerged as promising alternatives to inorganic ones for the realization of exogenous probes for bioimaging applications. However, the intimate structure of FONs in solution, as well as the role played by intermolecular interactions on their optical properties, remain challenging to study. Following a recent Second-Harmonic Scattering (SHS) investigation led by two of us [Daniel \textit{et al., ACS Photonics}, 2015, \textbf{2}, 1209], we report herein a computational study of the structural organization and second-order nonlinear optical (NLO) properties of FONs based on dipolar chromophores incorporating a hydrophobic triphenylamine electron-donating unit and a slightly hydrophilic aldehyde electron-withdrawing unit at their extremities. Molecular dynamics simulations of the FONs formation in water are associated to quantum chemical calculations, to provide insight on the molecular aggregation process, the molecular orientation of the dipolar dyes within the nanoparticles, as well as the dynamical behavior of their NLO properties. Moreover, the impact of intermolecular interactions on the NLO responses of the FONs is investigated by employing the tight-binding version of the recently developed simplified Time-Dependent Density Functional Theory (sTD-DFT) approach, allowing the all-atom quantum mechanics treatment of nanoparticles.