Molecular functional materials: molecules and aggregates towards the thermodynamic limit

Molecular functional materials are in the spotlight of scientific research as the most promising path towards innovative applications in sustainable and recyclable optoelectronic devices, in bioimaging, sensing, photonics, teleommunications etc. The virtually infinite possibilities offered by organic synthesis allow for an extremely fine tuning of the molecular properties for improved performances. In this context, theoretical modeling is an essential tool to guide the synthesis, providing clear design strategies for optimized systems. Quantum chemical calculations, available to the scientific community in a large number of software packages, represent one of the most successful and widely exploited approaches to address molecular systems. However, the collective properties of a material are not simply the sum of molecular properties and therefore are often beyond the scope of standard computational tools. Two issues in this respect must be faced. The first and most generally recognized issue falls under the broad definition of environmental effects. As aforementioned, the properties of a molecular system are determined not just by the properties of the molecules themselves, as calculated, e.g., via gas phase ab initio calculations, but are crucially affected by intermolecular interactions. Wildly different environments define the most rich and diverse array of phenomena. The variability is substantial, ranging, for example, from liquid solution to a molecular dispersion in a solid matrix, from a completely disordered amorphous phase to ordered aggregates and crystals. Polar solvation dynamics, static and dynamical disorder, excitonic effects are just some examples of the many possible phenomena that determine the photophysics of the material and its properties like, e. g., charge transport and mobility and energy transfer. A second, more subtle issue arises from the fact that materials are macroscopic systems, so that a reliable and complete description of their behavior and properties cannot be obtained focusing on the microscopic domain of molecular quantum mechanics, but requires entering the fascinating world of open quantum systems, as to effectively connect the microscopic quantum realm of a single (or a few) molecule and the macroscopic world. Energy dissipation is crucial to bridge the gap between quantum mechanics and thermodynamics. An explicit quantum mechanical treatment of a system large enough to ensure a realistic evolution of energy fluxes is clearly unfeasible. Open quantum system approaches offer an elegant strategy to overcame this limit. In few words, out of the macroscopic object a system is identified, corresponding to the portion of the object or to the subset of degrees of freedom of interest. This system interacts with the rest of the world, the bath, that acts as a thermal reservoir. In this thesis, both issues are faced with reference to a specific family of molecular systems, the so-called charge transfer (CT) dyes. CT dyes are a wide class of chromophores whose low energy physics is successfully modeled by essential state models (ESMs), where few electronic states are coupled to few effective vibrational coordinates in a non-adiabatic picture. Specifically, environmental effects on the photophysics of CT dyes in different environments are addressed in the next chapters. The journey is divided in two main parts. In the first part the world of open quantum systems is entered to describe the relaxation of a single photoexcited chromophore in gas phase, in solution or in solid matrices. The second part is then dedicated to molecular aggregates and crystals. In chapter 1, the well known and widely applied ESM for a polar (donor-acceptor) dye is extended to describe the relaxation dynamics following coherent photoexcitation. To this aim the Redfield approach to open quantum systems is adopted. Subtleties and technicalities of the coupling with the bath are carefully analyzed setting a solid basis for subsequent work. The approach is validated via simulations of time-resolved emission spectra. In chapter 2 the model is extended to account for polar solvation. The polar environment is treated classically as an overdamped coordinate whose dynamics is governed by the Smoluchowski equation. The model is applied to investigate the static and dynamical dielectric properties of organic amorphous matrices commonly used as dispersion media for the emitters in organic light emitting diods (OLEDs). The simulation of time-resolved emission spectra of a solvatochromic polar dye (a microscopic polarity sensor) in solvents and solid amorphous matrices of different polarity validates the model and allows to extract for the first time significant information about the relaxation times of these materials, opening a novel perspective on solid state solvation. Chapter 3 initiates the second part of the thesis, introducing the world of molecular aggregates and presenting the celebrated Frenkel-Holstein (FH) Hamiltonian for aggregates of molecules interacting via electrostatic forces. Special focus is put on the interplay between nearest neighbor and next-nearest neighbor interactions in defining the vibronic bandshapes in absorption spectra. The FH Hamiltonian is then used to simulate polarized absorbance spectra of crystalline thin films of SQIB, a squaraine dye suited for photovoltaic applications. In chapter 4 molecular aggregates are addressed in the framework of essential state models, making a clear connection with the standard exciton model discussed in chapter 3. The Redfield relaxation model set up in chapter 1 is applied to dimers of dipolar chromophores in a unified and comprehensive picture able to describe both exciton delocalization in homodimers and resonance energy transfer in heterodimers. In chapter 5, bigger and chiral aggregates are addressed, tackling the problem of chiral proline-derived squaraine aggregates. Squaraines represent a remarkable family of quadrupolar dyes whose aggregates show diverse and interesting properties, including the panchromatic absorption spectrum of nanoparticles and thin films. The general picture is far from clear, with the origin of these peculiar features being alternatively attributed to either disorder or to intermolecular charge transfer (ICT) interactions. A new set of experimental data comprising absorption and circular dichroism spectra of a series of chiral proline-derived squaraine aggregates adds a further piece to the puzzle. In the attempt to elucidate the role of disorder and intermolecular charge transfer, an essential state model extended to account for ICT is used to address chiroptical properties of the aggregates at hand. While a firm conclusion is not yet reached, significant insights are obtained about the physics of these intriguing and fascinating systems.