Timescales of N-H bond dissociation in pyrrole: a nonadiabatic dynamics study

The excitation wavelength dependent photodynamics of pyrrole are investigated by nonadiabatic trajectory-surface-hopping dynamics simulations based on time dependent density functional theory (TDDFT) and the algebraic diagrammatic construction method to the second order (ADC(2)). The ADC(2) results confirm that the N-H bond dissociation occurring upon excitation at the origin of the first excited state, S-1(pi sigma*), is driven by tunnelling [Roberts et al., Faraday Discuss., 2013, 163, 95] as a barrier of DE = 1780 cm(-1) traps the population in a quasi-bound minimum. Upon excitation to S-1(pi sigma*) in the wavelength range of 236-240 nm, direct dissociation of the N-H bond takes place with a time constant of 28 fs. The computed time constant is in very good agreement with recently reported measurements. Excitation to the lowest B-2 state in the 198-202 nm range returns a time constant for N-H fission of 48 fs at the B3LYP/def2-TZVPD level, in perfect agreement with the experiment [Roberts et al. Faraday Discuss., 2013, 163, 95]. For the same wavelength range the ADC(2)/aug-cc-pVDZ decay constant is more than three times longer than the experimentally reported one. The accuracy of the B3LYP/def2-TZVPD dynamics is checked against reference complete-active-space second-order perturbation theory (CASPT2) calculations and explained in terms of correct topography of the pi pi* surface and the lack of mixing between the pi pi* and the 3p(x) Rydberg states which occurs in the ADC(2) method.

Quantifying Electronic Phenomena in Organic Chromophores

Antonio Prlj

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.

EPFL2017

Quantifying Electronic Phenomena in Organic Chromophores

Antonio Prlj

EPFL2017