Triplets with a Twist: Ultrafast Intersystem Crossing in a Series of Electron Acceptor Materials Driven by Conformational Disorder

Abstract

Control over the populations of singlet and triplet excitons is key to organic semiconductor technologies. In different contexts, triplets can represent an energy loss pathway that must be managed (i.e., solar cells, light-emitting diodes, and lasers) or provide avenues to improve energy conversion (i.e., photon upconversion and multiplication systems). A key consideration in the interplay of singlet and triplet exciton populations in these systems is the rate of intersystem crossing (ISC). In this work, we design, measure, and model a series of new electron acceptor molecules and analyze them using a combination of ultrafast transient absorption and ultrafast broadband photoluminescence spectroscopies. We demonstrate that intramolecular triplet formation occurs within several hundred picoseconds in solution and is accelerated considerably in the solid state. Importantly, ISC occurs with sufficient rapidity to compete with charge formation in modern organic solar cells, implicating triplets in intrinsic exciton loss channels in addition to charge recombination. Density functional theory calculations reveal that ISC occurs in triplet excited states characterized by local deviations from orbital π-symmetry associated with rotationally flexible thiophene rings. In disordered films, structural distortions, therefore, result in significant increases in spin−orbit coupling, enabling rapid ISC. We demonstrate the generality of this proposal in an oligothiophene model system where ISC is symmetry-forbidden and show that conformational disorder introduced by the formation of a solvent glass accelerates ISC, outweighing the lower temperature and increased viscosity. This proposal sheds light on the factors responsible for facile ISC and provides a simple framework for molecular control over spin states.

Mechanochemistry toward Organic “Salt” via Integer‐Charge‐Transfer Cocrystal Strategy for Rapid, Efficient, and Scalable Near‐Infrared Photothermal Conversion

“…The evolution channels and weight factor of the lifetimes for dominant nonradiative processes of excited states in “salt” and “ionic crystal” of both D289‐F4TCNQ (Figure 4) and D2m‐F4TCNQ (Figure S18) match well with each other, which is consistent with their comparable PTC efficiency. Besides, compared to “ionic crystal” of D289‐F4TCNQ or D2m‐F4TCNQ with ISC process of A 2 ∼13.5 % or 16.2 %, the relatively higher weight factor of ISC process with A 2 ∼17.2 % or 21.5 % in the “salt” may be explained by its amorphous stacking with structural distortions facilitating the increases of spin‐orbital coupling and the resulting ISC process [30] …”

“…Besides, compared to "ionic crystal" of D289-F4TCNQ or D2m-F4TCNQ with ISC process of A 2 ~13.5 % or 16.2 %, the relatively higher weight factor of ISC process with A 2 ~17.2 % or 21.5 % in the "salt" may be explained by its amorphous stacking with structural distortions facilitating the increases of spin-orbital coupling and the resulting ISC process. [30] Two application demonstration cases, e. g., water evaporation and deicing measurements, are shown in Figures 5, S20-S22 and Table S5 to display the great potential of the "salt". Homogeneous "salt" loaded PU foams were employed for water evaporation test (Figure 5a).…”

Tuning the Absorption, Fluorescence, Intramolecular Charge Transfer, and Intersystem Crossing in Spiro[fluorene]acridinone

“…The unique double role of aromatic ketones, that is, its electron-accepting and its (n,π*) nature, renders them promising to uncover the complex interplay between different excited-state decay channels following photoexcitation. [19][20][21][22][23][24][25][26][27] Notably, aromatic ketones are well known for their rapid and efficient intersystem crossing (ISC). [11,21,22,[28][29][30][31][32] The lone pair orbital of the carbonyl group contributes to the norbital.…”

“…Charge transfer, [1–8] multiply intersystem crossing, [9–14] and fluorescence [15–18] are all characteristic and fundamental photophysical processes that have drawn great attention. The unique double role of aromatic ketones, that is, its electron‐accepting and its (n,π*) nature, renders them promising to uncover the complex interplay between different excited‐state decay channels following photoexcitation [19–27] …”

Tuning the Absorption, Fluorescence, Intramolecular Charge Transfer, and Intersystem Crossing in Spiro[fluorene]acridinone

“…Charge transfer, [1–8] multiply intersystem crossing, [9–14] and fluorescence [15–18] are all characteristic and fundamental photophysical processes that have drawn great attention. The unique double role of aromatic ketones, that is, its electron‐accepting and its (n,π*) nature, renders them promising to uncover the complex interplay between different excited‐state decay channels following photoexcitation [19–27] …”