Hydrogen migration quickly kills DPP dimer’s excited states.
In brief:
- Ultrafast Nonradiative Decay: Revealed a near-barrierless hydrogen migration pathway in DPP dimers leading to internal conversion within ~400 fs, quenching fluorescence.
- Mechanistic Insights: Identified proton-coupled electron transfer (PCET) as the dominant decay mechanism, driven by an S1/S0 intersection.
- Pathway to Improvement: Proposed molecular functionalization strategies, such as pyrrolo group methylation, to suppress nonradiative decay and enhance photovoltaic performance.
Our latest work, published in the Journal of Computational Chemistry in a special issue commemorating the 80th birthday of Hans Lischka, explores the excited-state dynamics of diketopyrrolopyrrole (DPP) dimers—promising candidates for organic photovoltaics (OPVs). While DPP-based materials have been lauded for their strong absorption, thermal stability, and efficient charge transport, their potential in OPVs is hampered by photophysical limitations, particularly fluorescence quenching and rapid nonradiative decay.
In this study, executed by Ali Al-Jaaidi during his master’s internship in our group, we used time-dependent density functional theory (TDDFT) and surface hopping dynamics to uncover the molecular mechanisms behind these limitations. Our simulations revealed that DPP dimers undergo ultrafast internal conversion within approximately 400 femtoseconds. A near-barrierless hydrogen migration drives this process at the intersection of the first excited state (S1) and ground state (S0), a mechanism that quenches fluorescence and limits excited-state lifetimes.
The excited state dynamics start with a shearing rotation between monomers, bringing the dimer near the minimum of the S1 state. Then, the hydrogen migration occurs between the two monomers, forming a hydrogen-transfer mechanism classified as proton-coupled electron transfer (PCET). This pathway is remarkably efficient due to the absence of significant energy barriers, as confirmed by our nudged elastic band calculations. Once triggered, the hydrogen migration leads to rapid relaxation to the ground state, predicting that these dimers should have short-lived excited states.
While these findings highlight a significant challenge for non-functionalized DPP systems, they also provide a roadmap for improvement. By functionalizing the pyrrolo groups to block hydrogen migration, it may be possible to suppress nonradiative decay and enhance the photophysical properties of DPP-based materials. For instance, our preliminary investigations showed that methylation of the pyrrolo groups could substantially inhibit this ultrafast decay pathway.
Beyond photovoltaics, our study underscores the importance of understanding excited-state dynamics in designing high-performance organic semiconductors. The insights gained here extend to other molecular systems where ultrafast nonradiative decay pathways may limit functionality.
For a detailed discussion about the methods, dynamics, and implications of this research, you can access our full paper in the Journal of Computational Chemistry. By addressing fundamental challenges in DPP-based materials, this work opens exciting avenues for enhancing the efficiency of organic photovoltaics and other optoelectronic devices.
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Reference
[1] A. Al-Jaaidi, J. M. Toldo, M. Barbatti, Ultrafast Dynamics of Diketopyrrolopyrrole Dimers, J. Comput. Chem. 46, e27547 (2025). 10.1002/jcc.27547