Ultrafast Singlet Fission and Intersystem Crossing in Halogenated Tetraazaperopyrenes

Abstract

Charge carrier multiplication via singlet fission into two triplet states has the potential to increase efficiencies of photovoltaics by one-third due to the reduction of thermalization losses. In the present work, we investigate tetraazaperopyrenes, a class of N-heteropolycyles, as suitable singlet fission candidates. Using a combined experimental and theoretical approach, fundamentally different mechanisms for triplet formation in solution and thin film are identified. In solution, an ultrafast intersystem crossing process is observed, which is accelerated for heavier halide substituents not only due to enhanced spin−orbit coupling but also due to the energy tuning between the S 1 and T 2 states. In thin films, a correlated triplet pair is formed coherently upon photoexcitation. Subsequently, an excimer formation is observed, which competes with the electronic decorrelation of the triplet pair. The comparison with peropyrene shows that aza-substitutions within the aromatic core can be a powerful strategy for tuning the energy levels of the states important to singlet fission.

Efficient Intramolecular Singlet Fission in Spiro-Linked Heterodimers

“…The agreement between computed and experimental rate constants (SI Table S8) supports a direct mechanism. (iii) Excimers , can be excluded since the distance between monomeric subunits increases in the equilibrium structures of the S 1 state, and experimental trends of relative iSF rates disagree with calculated trends for relaxed S 1 geometries. Notably, thorough analysis (SI section 6.3) of the experimental data also excludes involvement of excimers.…”

Permanent Dipole Moments Enhance Electronic Coupling and Singlet Fission in Pentacene

“…This has led to increased interest in the engineering of the properties of single molecules and molecular aggregates via chemical modification. In the context of a rational design of SF chromophores, several key parameters have been identified, such as electronic coupling strength, molecular arrangement, and the energetic difference of singlet and triplet states. − Current approaches toward an optimization of SF chromophores employ, for example, the addition of bulky side groups to modify the molecular packing in aggregates ,− or the use of dimers of SF chromophores for a controlled variation of the relative molecular arrangement and, thus, of the electronic coupling strength. − Alternatively, the energy difference between singlet and triplet states has been modified by the substitution of carbon atoms of the SF chromophore backbone with heteroatoms, such as nitrogen or sulfur, − or by fluorination of the SF chromophore. − The latter is in the general context of the optimization of organic semiconductors for optoelectronic devices, a well-established approach to increase stability against oxidation or to control energy levels, molecular packing, and charge transport. − …”

Charge-Transfer States at Metal–Organic Interface Limit Singlet Fission Yields: A Photonically Enhanced Pump–Probe Study

“…This number is significantly larger than the highest values achieved in silicon-based solar cells (∼27%) today, and nearly twice as large compared to that of existing organic solar cells (∼19%), , motivating research and industrial activities toward next-generation SF-based photovoltaic devices. The optimization of the SF efficiency has led to the investigation of a vast range of systems, e.g., single molecules, , dimers and trimers, , bridges, chains, aggregates, nanoparticles, crystallites, and crystals . The yield of triplet excitons in SF materials has also been investigated with O 2 catalysts, fullerenes, and Fabry-Pérot resonators …”