Exploring the Concept of Dimerization-Induced Intersystem Crossing: At the Origins of Spin–Orbit Coupling Selection Rules

Abstract

Singlet-Triplet interconversions and transitions (intersystem crossing, ISC) in organic molecules are at the basis of many important processes in cutting-edge photonic applications (Organic Light Emitting Devices, photodynamic therapy…). Selection rules for these transitions are mainly governed by the Spin-Orbit Coupling (SOC) phenomenon. Although the latter relies on complex relativistic phenomena, theoreticians have, with time, developed increasingly sophisticated and efficient approaches to gain access to a satisfactory evaluation of its magnitude. In this work, we bring a coupled experimental and theoretical analysis of the origin of the unusually large ISC efficiency on a series of dimers that differ by their nature (covalent or supramolecular) as well as the distance and relative orientation of their individual chromophoric components. The dynamical nature of the SOC, sometimes overlooked in literature, is discussed in deep.

Origins of Molecular-Twist-Triggered Intersystem Crossing in Functional Perylenediimides: Singlet–Triplet Gap versus Spin–Orbit Coupling

“…As is expected, the energetic cost for twisting the PDI gradually increases with φ and reaches up to 10.4 kcal mol –1 for φ = 40 0 (see S 0 evolution from Figure ). Here, we point out that the dynamics at finite temperature may yield such twisted PDI structures and thereby can modulate the photophysics and related energetics as discussed elsewhere . The S 1 and T n energies, SOC, and λ S 1 – T n between S 1 and T n are calculated to understand the effect of the core-twist on these key energetic parameters that govern the ISC rate.…”

“…Eco-friendly, heavy-atom-free small organic molecules are promising candidates because of their easily tunable physiochemical properties and flexibility. However, negligibly small SOC in these systems leads to inefficient ISC, restricting their practical use as triplet photosensitizers. − Conventionally, a large SOC is achieved by an internal heavy-atom effect. , In addition, strategies such as substitutional doping with light hetero-elements (for example, “N”), defects, dielectric screening, and also introducing geometrical strains were shown to improve the ISC/RISC rates. − Significantly enhanced ISC was reported for heavy-atom-free perylenediimide (PDI) bay-substituted with electron-donor units. , A considerable ISC rate was also reported for a geometrically twisted phenanthrene-fused PDI, whereas the corresponding planar analogue produced a negligibly small ISC rate . PDI functionalized with anthryl and carbazole at the bay positions exhibits long lived triplet with substantially high triplet quantum yield (Φ T ) .…”

Thiochromenocarbazole imide: a new organic dye with first utility in large area flexible electroluminescent devices

“…This dimerization induced ISC has been rationalized in the past for a variety of compounds, 36,37 including in particular bodipy derivatives, 38–40 and already observed and explained by some of us in a previous report on BTI dimers. 41 Here again, TD-DFT calculations revealed that a strong dynamic excitonic coupling between the two TCI subunits in the TCI 2 dimer result in enhanced spin–orbit coupling constant between the excited singlet and lower triplet states (see Fig. 6b and Fig.…”

Impact of Charge-Resonance Excitations on CT-Mediated J-Type Aggregation in Singlet and Triplet Exciton States of Perylene Di-Imide Aggregates: A TDDFT Investigation

“…These results suggest that radiative and nonradiative decays from the lowest triplet exciton states of molecular aggregates, intimately related to the nature and symmetry of the lowest energy state, may be modulated by the intermolecular organization, a concept that could be exploited for systems displaying dimerization-induced triplet state populations [57].…”

“…Thus, triplet formation by charge recombination can be a detrimental, unwanted side effect but can also be exploited as a useful triplet-generation method, for instance in photocatalysis and photodynamic therapy [54,55]. Examples are, among others, triplet-state generation in molecular dyads [56] and molecular dimers in which dimerization-induced triplet population has been invoked [57].…”