Modeling Excited States of Molecular Organic Aggregates for Optoelectronics

Abstract

Light-driven phenomena in organic molecular aggregates underpin several mechanisms relevant to optoelectronic applications. Modeling these processes is essential for aiding the design of new materials and optimizing optoelectronic devices. In this review, we cover the use of different atomistic models, excited-state dynamics, and transport approaches for understanding light-activated phenomena in molecular aggregates, including radiative and nonradiative decay pathways. We consider both intra- and intermolecular mechanisms and focus on the role of conical intersections as facilitators of internal conversion. We explore the use of the exciton models for Frenkel and charge transfer states and the electronic structure methods and algorithms commonly applied for excited-state dynamics. Throughout the review, we analyze the approximations employed for the simulation of internal conversion, intersystem crossing, and reverse intersystem crossing rates and analyze the molecular processes behind single fission, triplet-triplet annihilation, Dexter energy transfer, and Förster energy transfer. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 74 is April 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Indolocarbazole as a Platform for Concatenated Crystalline Rotors

“…The calculated fluorescence rates exhibit similar order of magnitude for the four crystals, indicating that the discrepancy in radiative mechanisms is not likely to determine the QY (Table ). The internal conversion rate constant ( k IC ), computed at the FC region within the weak coupling limit, is small for ICZ and rotor I (1.00 × 10 2 and 6.00 × 10 2 s –1 , respectively). However, the predicted rates for k IC increase by a couple of orders of magnitude in Cy-ICZ (1.50 × 10 4 s –1 ) and rotor III (8.68 × 10 6 s –1 ).…”

“…We modeled the transition rates for these deactivation pathways using Fermi's Golden Rule approach, as described in the (Supporting Information eqs S1−S3). 46 The models used here rely on the harmonic approximation; thus, the results should be considered qualitative. The predicted radiative and nonradiative rates are reported in Table 2.…”

“…47 This is expected because all of the states (S 1 , T 1 , and T 2 ) have a ππ* character localized on the same subunit, which remains the same across all the studied cases (Figures S21−S23). The energy gap ΔE Sd 1 Td 1 , modeled in the strong coupling limit ΔE Sd 1 Td 1 = (ΔE Sd 1 T1d 1 ad + λ) 2 /4λ, 46 is large for all of the systems (Table 2), making ISC from S 1 to T 1 improbable. In contrast, the ΔE Sd 1 Td 2 gap is smaller and decreases when going from ICZ, rotor I, Cy-ICZ to rotor III, explaining the increase in the ISC transition rate constant in this series (Table 2).…”

Organic compounds for solid state luminescence enhancement/aggregation induced emission: a theoretical perspective

“…50,67,68 When the CI is the most plausible route for the non-radiative decay in solution, then the enhanced solid-state luminescence is said to be due to a Restricted Access to the Conical Intersection (RACI) that happens in the aggregated phase. If the deexcitation is mediated by a coupling involving the nuclear kinetic energy term 49,[69][70][71] then the mechanism is more likely of type RIM and it can be treated with standard perturbation theory, i.e., Fermi's golden rule (FGR). In summary, the pronounced geometrical reorganization typical of most AIEgens causes the system to access regions of the excited PES far from the minimum where these processes are particularly effective in dissipating the excitation energy and in bringing the system back to the electronic ground state.…”

“…50,67,68 When the CI is the most plausible route for the non-radiative decay in solution, then the enhanced solid-state luminescence is said to be due to a Restricted Access to the Conical Intersection (RACI) that happens in the aggregated phase. If the deexcitation is mediated by a coupling involving the nuclear kinetic energy term 49,69–71 then the mechanism is more likely of type RIM and it can be treated with standard perturbation theory, i.e. , Fermi's golden rule (FGR).…”