Organic compounds for solid state luminescence enhancement/aggregation induced emission: a theoretical perspective.

Organic luminophores displaying one or more forms of luminescence enhancement in solid state are extremely promising for the development and performance optimization of functional materials essential to many modern key technologies. Yet, the effort to harness their huge potential is riddled with hurdles that ultimately come down to a limited understanding of the interactions that result in the diverse molecular environments responsible for the macroscopic response. In this context, the benefits of a theoretical framework able to provide mechanistic explanations to observations, supported by quantitative predictions of the phenomenon, are rather apparent. In this perspective, we review some of the established facts and recent developments about the current theoretical understanding of solid-state luminescence enhancement (SLE) with an accent on aggregation-induced emission (AIE). A description of the macroscopic phenomenon and the questions it raises is accompanied by a discussion of the approaches and quantum chemistry methods that are more apt to model these molecular systems with the inclusion of an accurate yet efficient simulation of the local environment. A sketch of a general framework, building from the current available knowledge, is then attempted via the analysis of a few varied SLE/AIE molecular systems from literature. A number of fundamental elements are identified offering the basis for outlining design rules for molecular architectures exhibiting SLE that involve specific structural features with the double role of modulating the optical response of the luminophores and defining the environment they experience in solid state.

Theoretical insights on acceptor-donor dyads for organic photovoltaics.

The field of organic photovoltaics has witnessed a steady growth in the last few decades and a recent renewal with the blossoming of single-material organic solar cells (SMOSCs). However, due to the intrinsic complexity of these devices (both in terms of their size and of the condensed phases involved), computational approaches to accurately predict their geometrical and electronic structure and to link their microscopic properties to the observed macroscopic behaviour are still lacking. In this work, we have focused on the rationalization of transport dynamics and we have set up a computational approach that makes a combined use of classical simulations and Density Functional Theory with the aim of disclosing the most relevant electronic and structural features of dyads used for SMOSC applications. As a prototype dyad, we have considered a molecule that consists in a dithiafulvalene-functionalized diketopyrrolopyrrole (DPP), acting as an electron donor, covalently linked to a fulleropyrrolidine (Ful), the electron acceptor. Our results, beside a quantitative agreement with experiments, show that the overall observed mobilities result from the competing packing mechanisms of the constituting units within the dyad both in the case of crystalline and amorphous phases. As a consequence, not all stable polymorphs have the same efficiency in transporting holes or electrons which often results in a highly directional carrier transport that is not, in general, a desirable feature for polycrystalline thin-films. The present work, linking microscopic packing to observed transport, thus opens the route for the in silico design of new dyads with enhanced and controlled structural and electronic features.

Microscopic calculation of absorption spectra of macromolecules: An analytic approach.

We develop a cross-disciplinary approach to analytically compute optical response functions of open macromolecular systems by exploiting the mathematical formalism of quantum field theory (QFT). Indeed, the entries of the density matrix for the electronic excitations interacting with their open dissipative environment are mapped into vacuum-to-vacuum Green's functions in a fictitious relativistic closed quantum system. We show that by re-summing appropriate self-energy diagrams in this dual QFT, it is possible to obtain analytic expressions for the response functions in Mukamel's theory. This yields physical insight into the structure and dynamics of vibronic resonances, since their frequency and width is related to fundamental physical constants and microscopic model parameters. For illustration, we apply this scheme to compute the linear absorption spectrum of the Fenna-Matthews-Olson light harvesting complex, comparing analytic calculations, numerical Monte Carlo simulations, and experimental data.