Organic chromophores for advanced applications: models and spectroscopy

Abstract

This thesis presents an extensive study of charge-transfer (CT) dyes, an interesting class of organic chromophores for advanced applications in the fields of nonlinear optics, energy harvesting, LEDs and solar cells, just to cite a few. The study combines theoretical and experimental work to offer a reliable description of optical properties and spectra of CT chromophores in different environments. CT dyes constitute excellent model systems for investigating charge- and energy-transfer processes, that represent key phenomena in physics, chemistry and biology.

The work starts with the description of essential-state models that are at the heart of the theoretical approach that we have developed for CT dyes. The low-energy physics of CT chromophores is governed by charge resonance between electron-donor (D) and electron-acceptor (A) groups and their optical spectra and properties are dominated by CT transitions. The behavior of CT dyes can be described in terms of comparatively simple models that just account for the few relevant degrees of freedom. These essential-state models are general and allow for the rationalization of optical spectra and properties of families of CT chromophores, and to set up reliable structure-properties relationships as required to guide the chemical synthesis.

A thorough understanding of the physics of CT chromophores requires their detailed spectroscopic characterization. Essential-state models allow for the calculation of linear and nonlinear spectra of polar and multipolar chromophores accounting for molecular vibrations and for polar solvation, making possible a detailed analysis of the spectral position, intensities and bandshapes. In this thesis, we develop the calculation of electroabsorption spectra, and we discuss, both at the experimental and theoretical level, fluorescence thermochromism and low-temperature fluorescence anisotropy spectra of different families of CT dyes.

The models developed for CT dyes in solution naturally lent themselves to be extended to the description of the same dyes in more complex and challenging environments. Specifically, collective and cooperative phenomena are expected in aggregates, films, crystals and more generally in clusters where different chromophores interact via electrostatic interactions. Two families of systems are studied: the first one composed by a zwitterionic DA-chromophore, its covalent dimer and its self-assembled film on gold; the second one composed by a quadrupolar CT chromophore and its covalent dimers. These examples allow to demonstrate the power of the bottom-up approach in this context, and to fully exploit the advantages of essential-state models.

The same electrostatic interactions that are responsible for cooperative and collective phenomena in multichromophoric assemblies drive the phenomenon of energy transfer in heterochromophoric macromolecules. We are able to prove that the extended-dipole approximation for interchromophore interactions leads to qualitatively different results than the more commonly adopted point-dipole approximation. Specifically, the extended-dipole approximation opens new channels for energy transfer through optically dark states, that, in the point-dipole approximation are strictly forbidden.