Exciton Dynamics of a Diketo-Pyrrolopyrrole Core for All Low-Lying Electronic Excited States Using Density Functional Theory-Based Methods

Ab initio treatments of interexcited state internal conversion (IC) are more often than not missing from exciton dynamic descriptions, because of their inherent complexity. Here, we define interexcited state IC as a same-spin nonradiative transition between states i and j, where i not equal j not equal 0. Competing directly with multiexciton processes such as singlet fission or triplet photoupconversion, inclusion of this mechanism in the narrative of molecular photophysics would allow for strategic synthesis of chromophores for more efficient photon-harvesting applications. Herein, we present a robust formalism which can model these rates using density functional theory (DFT)-based methods within the Franck-Condon and Herzberg-Teller regime. Using an unsubstituted diketo-pyrrolopyrrole (DPP) core as a case study, we illustrate the exciton dynamics along the first four excited states for both singlet and triplet manifolds, showing ultrafast same-spin transfer mechanisms due to all excited states, excluding the first triplet level, being in close energetic proximity (within 0.8 eV of each other). The resulting electron same-spin rates outcompete the electron spin-flipping intersystem crossing (ISC) rates, with excitons firmly obeying Kasha's rule as they cascade down from the high-lying excited states toward the lower states. Furthermore, we calculated that only the first singlet excited state displayed a reasonable probability of triplet exciton generation, of similar to 40%, with a near-zero chance of the exciton reverting to the singlet manifold once the electron-hole pair are of parallel spin.

Automated summary

This article highlights the importance of ab initio treatments of interexcited state internal conversion (IC) and their potential applications in molecular photophysics. By including this mechanism, more efficient synthesis of chromophores for photon-harvesting applications can be achieved. Using density functional theory (DFT), the authors studied the exciton dynamics and found that same-spin transfer mechanisms dominate over spin-flipping intersystem crossing (ISC) rates, with only the first singlet excited state showing a reasonable probability of triplet exciton generation.