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Mechanismen Licht-induzierter Triplett-Aktivierung

Fachliche Zuordnung Theoretische Chemie: Elektronenstruktur, Dynamik, Simulation
Förderung Förderung von 2011 bis 2015
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 194304672
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

The relaxation processes following the electronic excitation of molecules by light are often very involved: The stored excess energy can be released by emission of radiation or by a cascade of nonradiative dissipation processes. The rate constants of the competing processes determine whether the molecule luminesces or is completely dark. If the non-radiative transition is accompanied by a change of spin multiplicity, one speaks of intersystem crossing (ISC). ISC of photoexcited organic molecules typically leads to long-lived triplet states that may undergo photochemical reactions or can transfer the stored energy to other molecules such as molecular oxygen. In other cases, no radiation is emitted at all and the molecule returns quickly to the electronic ground state. For a long time it was believed that ISC was slow in comparison to internal conversion (IC) processes in which the spin multiplicity is preserved. At the beginning of this project, there was expermental evidence that ISC can compete with IC even in molecules composed of light elements only, but the mechanisms and relaxation pathways were not understood. We developed computational methods and tools that enabled us to calculate rate constants for ISC transitions in large molecules at ambient temperatures. Meanwhile, they are used by several theory groups world wide. Within the framework of the current project, the methods were applied to investigate the photophysics of two classes of compounds for which ultrafast ISC had been observed, namely aromatic carbonyl compounds and nitro aromatic compounds. Their strong solvent dependence was particularly intriguing. Our work on these molecules has established the importance of theory for gaining better insight and understanding of the excited-state processes occuring in a photoexcited species. Xanthone, for example, is fluorescent in water but is dark in apolar solvents such as cyclohexane. One major result of our studies is the finding that for each molecule of the xanthone family of dyes a solvent can be found that tunes the optically bright singlet state and the dark triplet state into resonance so that excited state population is transferred back from the triplet to the singlet state and delayed fluorescence (DF) may be observed in addition to prompt fluorescence. We could explain the solvent dependence of DF in xanthone and successfully predict the occurrence of DF in thioxanthone in methanol solution. Our recent prediction that the photoexcited acridone may also show DF in acetonitrile solution still awaits experimental confirmation. Our planned studies on the photophysics of the nitraromatic compounds were delayed by the fact that the DFT/MRCI method produced artifacts when applied to nitrobenzene, the smallest of the nitroaromatic compounds. This came as a surprise because DFT/MRCI is a well-established semi-empirical quantum chemical method for efficiently and reliably computing excited-state properties of organic molecules. As a first step, we had to search for alternative electronic structure methods to describe the photophysics of the nitroarenes. To this end, we joined efforts with the group of Prof. Andreas Dreuw in Heidelberg. Nitrobenzene turned out to be a challenge also for well-established ab initio quantum chemical methods (CC2, SCS-CC2, EOM-CCSD). Because of the multiconfigurational character of the wave function and the unbalanced treatment of double excitations with four open shells by the DFT/MRCI Hamiltonian, we opted for ADC3 as our method of choice for computing the potential energy surfaces while DFT/MRCI wave functions were employed for computing the spin– orbit interaction. Eventually, we were successful in explaining the molecular mechanisms of the fast non-radiative decay of photoexcited nitrobenzene. With regard to the photorelaxation mechanism, the main result of this joint study is the identification of the in-plane(!) ONO bending angle as the decisive coordinate for the ultrafast decay of the excited singlet population. To study polycyclic nitroaromatic compounds, less resource intensive electronic structure methods than ADC3 were required. Their performance with regard to electronic excitation energies and spin–orbit matrix elements was assessed on eight diatomics and fifteen polyatomic molecules. A multi-reference Møller-Plesset perturbation theory (MR-MP2) approach and a redesigned and reparameterized DFT/MRCI approach, simultaneously developed in the framework of another DFG project, turned out to be good alternatives for the description of the electronic structure of this type of molecules.

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