Parallel triplet formation pathways in a singlet fission material

Harvesting long-lived free triplets in high yields by utilizing organic singlet fission materials can be the cornerstone for increasing photovoltaic efficiencies potentially. However, except for polyacenes, which are the most studied systems in the singlet fission field, spin-entangled correlated triplet pairs and free triplets born through singlet fission are relatively poorly characterized. By utilizing transient absorption and photoluminescence spectroscopy in supramolecular aggregate thin films consisting of Hamilton-receptor-substituted diketopyrrolopyrrole derivatives, we show that photoexcitation gives rise to the formation of spin-0 correlated triplet pair 1(TT) from the lower Frenkel exciton state. The existence of 1(TT) is proved through faint Herzberg-Teller emission that is enabled by vibronic coupling and correlated with an artifact-free triplet-state photoinduced absorption in the near-infrared. Surprisingly, transient electron paramagnetic resonance reveals that long-lived triplets are produced through classical intersystem crossing instead of 1(TT) dissociation, with the two pathways in competition. Moreover, comparison of the triplet-formation dynamics in J-like and H-like thin films with the same energetics reveals that spin-orbit coupling mediated intersystem crossing persists in both. However, 1(TT) only forms in the J-like film, pinpointing the huge impact of intermolecular coupling geometry on singlet fission dynamics

Modeling vibronic spectra of linear aggregates in MATLAB

Molecular aggregates display a rich array of photophysical properties quite different from the isolated molecules that have applications in photovoltaics and display technology. Hence, understanding their absorption and emission spectra gives clues about packing and coherence properties, essential for energy transport. We attempt to calculate the steady-state spectra of linear aggregates. We approached this problem by following the vibronic excitonic framework discussed by Spano and co-workers [Hestand & Spano, Chem. Rev. 2018, 118, 7069-7163]. The framework allows us to incorporate the influence of nuclear relaxation energy through the one-particle and two-particle states. We also study the effects of local static disorder, finite temperature, and periodic boundary conditions. We implement the calculation of the photoluminescence and absorption spectra in the user-friendly language MATLAB in a manner that allows sequentially increasing the complexity of the model by considering more effects as needed. The calculated spectra have been validated by matching them with spectra produced by Spano et al. Hence, we have implemented a robust code that can model spectra from linear aggregates of any size and orientation, subject to computational limitations. We are currently using these codes to assign the steady state spectra and speculate the morphology of several types of thin films made of donor-acceptor dyes obtained by an experimental collaborator at SSCU; these dyes are being put forward for use for increasing the efficiency of photovoltaic cells through the multi-exciton generation mechanism of singlet fission. The high-level language implementation allows usage without a programming background as long as the theoretical model is understood