A TDDFT investigation of bay substituted perylenediimides: Absorption and intersystem crossing

Thio and Seleno-Psoralens as Efficient Triplet Harvesting Photosensitizers for Photodynamic Therapy

The Psoralen (Pso) molecule finds extensive applications in photo-chemotherapy, courtesy of its triplet state forming ability. Sulfur and selenium replacement of exocyclic carbonyl oxygen of organic chromophores foster efficient triplet harvesting with near unity triplet quantum yield. These triplet-forming photosensitizers are useful in Photodynamic Therapy (PDT) applications for selective apoptosis of cancer cells. In this work, we have critically assessed the effect of the sulfur and selenium substitution at the exocyclic carbonyl (TPso and SePso, respectively) and endocyclic oxygen positions of Psoralen. It resulted in a significant redshifted absorption spectrum to access the PDT therapeutic window with increased oscillator strength. The reduction in singlet-triplet energy gap and enhancement in the spin-orbit coupling values increase the number of intersystem crossing (ISC) pathways to the triplet manifold, which shortens the ISC lifetime from 10-5 s for Pso to 10-8 s for TPso and 10-9 s for SePso. The intramolecular photo-induced electron transfer process, a competitive pathway to ISC, is also considerably curbed by exocyclic functionalizations. In addition, a maximum of 115 GM of two-photon absorption (2PA) with IR absorption (660-1050 nm) confirms that the Psoralen skeleton can be effectively tweaked via single chalcogen atom replacement to design a suitable PDT photosensitizer.

Incorporating spin–orbit effects into surface hopping dynamics using the diagonal representation: a linear-response time-dependent density functional theory implementation with applications to 2-thiouracil

Evaluation of SOC values employs Casida's wave functions and the Breit–Pauli spin–orbit Hamiltonian with effective charge approximation.

Intermolecular Vibrational Modes Speed Up Singlet Fission in Perylenediimide Crystals

We report numerical simulations based on a non-Markovian density matrix propagation scheme of singlet fission (SF) in molecular crystals. Ab initio electronic structure calculations were used to parametrize the exciton and phonon Hamiltonian as well as the interactions between the exciton and the intramolecular and intermolecular vibrational modes. We demonstrate that the interactions of the exciton with intermolecular vibrational modes are highly sensitive to the stacking geometry of the crystal and can, in certain cases, significantly accelerate SF. This result may help in understanding the fast SF experimentally observed in a broad range of molecular crystals and offers a new direction for the engineering of efficient SF sensitizers.

Mapping the Relation between Stacking Geometries and Singlet Fission Yield in a Class of Organic Crystals

By generating two free charge carriers from a single high-energy photon, singlet fission (SF) promises to significantly improve the efficiency of a class of organic photovoltaics (OPVs). However, SF is generally a very inefficient process with only a small number of absorbed photons successfully converting into triplet states. In this Letter, we map the relation between stacking geometry and SF yield in crystals based on perylenediimide (PDI) derivatives. This structure-function analysis provides a potential explanation for the SF yield discrepancies observed among similar molecular crystals and may help to identify favorable geometries that lead to an optimal SF yield. Exploring the subtle relationship between stacking geometry and SF yield, this Letter suggests using crystal structure engineering to improve the design of SF-based OPVs.