Quantifying Electronic Phenomena in Organic Chromophores

Challenging ground and excited state problems in the chemistry of common organic chromophores are investigated with state-of-the-art quantum chemical methods. We present a comprehensive excited state molecular dynamics analysis of (a) fundamental building blocks in organic electronics (thiophene and its derivatives), (b) aggregation-induced emission systems (tetraphenylethylene), and (c) organic fluorophores used for imaging and sensing applications (BODIPY and its derivatives). We identify the efficient excited state deactivation pathways which are essential to understanding the photochemical stability and emissive properties of these compounds. The internal conversion mechanisms of theoretically challenging thiophene and bithiophene molecules are investigated with a trajectory surface hopping approach utilizing reliable electronic structure methods. We gain new insights into the photochemistry and photophysics of these systems, including a new mechanism in thiophene excited state decay and the increased photostability of bithiophene, thereby complementing earlier theoretical and experimental literature. The origin of the non-emissive behavior of tetraphenylethylene in the gas phase is explained by identifying energetically accessible conical intersections which promote radiationless decay. It is implied that restricted access to the conical intersection induces strong emission upon aggregation - a phenomenon that attracted significant research attention recently. Finally, the concept of conical intersection accessibility is utilized to explain the fluorescence quenching in certain meso-substituted BODIPY derivatives. We deliver a full mechanistic picture of the nonradiative decay of these molecules, invoking the role of excited state charge transfer and weak intramolecular interactions. Understanding the failures of quantum chemical electronic structure methods is crucial for subsequent improvements of commonly applied theoretical approximations. To elucidate the origins behind the unbalanced description of the low-lying pipi* excited states of heteroaromatic molecules (including thiophene and its derivatives, or in general fused heteroaromatics), we employ a range of quantum chemical approaches, from more approximate time-dependent density functional theory (TDDFT) to highly accurate wavefunction-based methods. The drawbacks of standard TDDFT were ascribed to the ubiquitous adiabatic approximation, rather than different functional approximations. On the other hand, the performance of wavefunction-based methods is found to be largely dependent on the treatment of electron correlation, which is key for a balanced description of excited states with disparate electronic character. Non-covalent molecular interactions are at the origin of many chemical and physical phenomena. While quantifying intermolecular interactions has become a routine task, intramolecular interactions are still considered particularly difficult to treat by theoretical methods. Here, we develop an original wavefuction-based method for quantifying intermolecular and intramolecular interactions on equal footing. The method, intra-SAPT, makes use of a single Slater determinant wavefunction, subject to perturbational corrections. The scheme decomposes interaction energies into physically meaningful components: electrostatics-exchange, induction and dispersion.

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