Juno Kim

Radial π‐conjugation provides a unique platform for extending electronic delocalization in cyclic molecular architectures. Although radial π‐conjugation is relatively robust against static disorders in the ground state, both theoretical and experimental observations indicate that excited states undergo localization through exciton self‐trapping once nanorings exceed a critical size. However, direct observation of the ultrafast structural dynamics underlying exciton localization remains a major challenge. Here, we employ ultrafast time‐resolved impulsive stimulated Raman spectroscopy (TR‐ISRS) to track the radial π‐conjugation during excited‐state structural relaxation. Using [n]cycloparaphenylene ([n]CPP) as a model nanoring system, we show that intrinsic structural metrics—ring strain and curvature—dictate the ultrafast nuclear dynamics. Comparative TR‐ISRS analysis along two distinct nonadiabatic transition pathways provides direct vibrational evidence for exciton self‐trapping, a process previously inferred only by ultrafast electronic spectroscopies. Furthermore, we demonstrate that the electron‐transfer (ET) dynamics of photoexcited [n]CPP are governed by the character of the resulting excitonic state: delocalized excitons promote ultrafast, near‐quantitative ET, whereas self‐trapped excitons substantially suppress the ET rate. These findings establish a direct mechanistic link among radial π‐conjugation, ultrafast structural dynamics, and electron‐transfer processes, and offer design principles of functional cyclic π‐conjugated materials.

Radial π‐conjugation provides a unique platform for extending electronic delocalization in cyclic molecular architectures. Although radial π‐conjugation is relatively robust against static disorders in the ground state, both theoretical and experimental observations indicate that excited states undergo localization through exciton self‐trapping once nanorings exceed a critical size. However, direct observation of the ultrafast structural dynamics underlying exciton localization remains a major challenge. Here, we employ ultrafast time‐resolved impulsive stimulated Raman spectroscopy (TR‐ISRS) to track the radial π‐conjugation during excited‐state structural relaxation. Using [n]cycloparaphenylene ([n]CPP) as a model nanoring system, we show that intrinsic structural metrics—ring strain and curvature—dictate the ultrafast nuclear dynamics. Comparative TR‐ISRS analysis along two distinct nonadiabatic transition pathways provides direct vibrational evidence for exciton self‐trapping, a process previously inferred only by ultrafast electronic spectroscopies. Furthermore, we demonstrate that the electron‐transfer (ET) dynamics of photoexcited [n]CPP are governed by the character of the resulting excitonic state: delocalized excitons promote ultrafast, near‐quantitative ET, whereas self‐trapped excitons substantially suppress the ET rate. These findings establish a direct mechanistic link among radial π‐conjugation, ultrafast structural dynamics, and electron‐transfer processes, and offer design principles of functional cyclic π‐conjugated materials.

Precise control over the couplings between molecules that govern excited-state relaxation processes remains a challenge in organic materials. A primary example of this is in singlet fission, which has the potential to boost photovoltaic efficiencies through exciton multiplication. While detailed solution-phase mechanistic studies of this phenomenon now exist, bridging the gap to practical devices has yet to be realized. We address this issue by showcasing several different ways to steer the excited-state relaxation of new fission-active pentacene and fission-inactive anthracene oligomers, highlighting how the balance of inter- and intramolecular coupling plays a crucial role. We achieve this by engineering specific interactions via supramolecular self-assembly. These interactions promote the uncommon occurrence of strong J-aggregation in acenes. Within the aggregates, intermolecular couplings result in the formation of a strongly bound ¹TT state, which outcompetes the parasitic charge transfer process that dominates in isolated molecules. These findings highlight that balancing intra- and intermolecular interactions offers an effective strategy for regulating the singlet fission process, a critical requirement for advancing future solid-state devices and applications.

The reductive functionalization of inert substrates such as chloroarenes is a critical yet challenging transformation relevant to both environmental remediation and organic synthesis. Combining electricity and light is an emerging strategy to access the deeply reducing potentials required for single electron transfer to chloroarenes, yet this approach is limited by poor stability and mechanistic ambiguity. Here we demonstrate heterogeneous electrophotocatalysis using redox-active rylene diimide polymers for the reduction of chloroarenes. We find that the electrophotocatalytic activity varies dramatically as a function of the rylene diimide and the redox-inactive polymer backbone. In particular, a flexible, non-conjugated perylenediimide polymer outperforms all other tested electrophotocatalysts. Transient absorption spectroscopy reveals that precomplexation between the doubly reduced perylenediimide and the haloarene substrate is key to productive catalysis. Overall, this work highlights heterogeneous electrophotocatalysis using insoluble redox-active organic materials and provides critical structure-property insights into solid-state electrophotocatalytic activity, informing the development of next-generation materials for sustainable synthesis.

The reductive functionalization of inert substrates like chloroarenes is a critical yet challenging transformation relevant to both environmental remediation and organic synthesis. Combining electricity and light is an emerging approach to access the deeply reducing potentials required for single electron transfer to chloroarenes, yet this approach is held back by the poor stability and mechanistic ambiguity of current homogeneous systems. Incorporating redox-active moieties into insoluble organic materials represents a promising strategy to unlock new heterogeneous catalytic activity while improving catalyst stability. Herein, we demonstrate the first example of heterogeneous electrophotocatalysis using redox-active rylene diimide polymers for the reduction of chloroarenes. In particular, we find that the electrophotocatalytic activity varies significantly not just as a function of the rylene diimide but also of the redox-inactive polymer backbone. In particular, PTCDA-en, a flexible, non-conjugated perylenediimide polymer, outperforms all other tested materials as an electrophotocatalyst. Using transient absorption spectroscopy, we reveal that precomplexation between the closed-shell PTCDA-en2− and the haloarene substrate is key to productive catalysis. Overall, our work represents the first example of heterogeneous electrophotocatalysis using an insoluble redox-active organic material and provides critical insights into how polymer structure dictates electrophotocatalytic activity in the solid state, guiding the development of next-generation heterogeneous (electro)photocatalysts for sustainable synthesis.

The functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S₁→¹TT). The dark state ¹TT is typically generated from bright S₁ after optical excitation; however, the mechanistic details are hotly debated. Here we report a ¹TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S₁. Using 1TT is directly generated from the ground state. Applying this method to a range of pentacene dimers and thin films of various aggregation types, we determine the critical material properties that enable this forbidden pathway. Through a strikingly simple technique, this result opens the door for new mechanistic insights into ¹TT and other dark states in organic materials.

Multiexciton in singlet exciton fission represents a critical quantum state with significant implications for both solar cell applications and quantum information science. Two distinct fields of interest explore contrasting phenomena associated with the geminate triplet pair: one focusing on the persistence of long-lived correlation and the other emphasizing efficient decorrelation. Despite the pivotal nature of multiexciton processes, a comprehensive understanding of their dependence on the structural and spin properties of materials is currently lacking in experimental realizations. To address this gap in knowledge, molecular engineering was employed to modify the TIPS-tetracene structures, enabling an investigation of the structure–property relationships in spin-related multiexciton processes. In lieu of the time-resolved electron paramagnetic resonance technique, two time-resolved magneto-optical spectroscopies were implemented for quantitative analysis of spin-dependent multiexciton dynamics. The utilization of absorption and fluorescence signals as complementary optical readouts, in the presence of a magnetic field, provided crucial insights into geminate triplet pair dynamics. These insights encompassed the duration of multiexciton correlation and the involvement of the spin state in multiexciton decorrelation. Furthermore, simulations based on our kinetic models suggested a role for quintet dilution in multiexciton dynamics, surpassing the singlet dilution principle established by the Merrifield model. The integration of intricate model structures and time-resolved magneto-optical spectroscopies served to explicitly elucidate the interplay between structural and spin properties in multiexciton processes. This comprehensive approach not only contributes to the fundamental understanding of these processes but also aligns with and reinforces previous experimental studies of solid states and theoretical assessments.

Functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S₁→¹TT). The dark state ¹TT is typically generated from bright S₁ after optical excitation, however the mechanistic details are hotly debated. We report a new ¹TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S₁. Using 1TT is directly generated from the ground state. Applying this method to a range of pentacene dimers and thin films of various aggregation types, we determine the critical material properties that enable this forbidden pathway. Through a strikingly simple technique, this result opens the door for new mechanistic insights into ¹TT and other dark states in organic materials.