Dynamics of Excited States for Fluorescent Emitters with Hybridized Local and Charge-Transfer Excited State in Solid Phase: A QM/MM Study

Electric-Field Effects on the Internal Charge Reorganization Energies of Crystalline Organic Semiconductors

The synergistic effects of molecular packing and external electric fields (EEFs, including axial and nonaxial fields) on the internal charge reorganization energies (λ) of typical p-type SMOS have been investigated. Combined quantum and molecular mechanics calculations show that, for all-ring-fused rigid molecules single-molecule approximation and neglect of EEFs are adequate for computing λ, while for nonrigid molecules with inter-ring carbon-carbon (IRCC) linkers, the above simplifications may cause a significant deviation from the actual λ. For nonrigid molecules, solid-state packing can prevent "bad" EEFs (Fz and Fyz) from enhancing λ (adverse to charge transfer), while it allows λ to be greatly reduced (in favor of charge transfer) if "good" EEFs (Fx, Fxy, Fxz and Fxyz) are imposed. Last, a simple strategy that can divide λ into each subring contribution for IRCC-linked molecules has been proposed.

Overcoming the Limitation of Spin Statistics in Organic Light Emitting Diodes (OLEDs): Hot Exciton Mechanism and Its Characterization

Triplet harvesting processes are essential for enhancing efficiencies of fluorescent organic light-emitting diodes. Besides more conventional thermally activated delayed fluorescence and triplet-triplet annihilation, the hot exciton mechanism has been recently noticed because it helps reduce the efficiency roll-off and improve device stability. Hot exciton materials enable the conversion of triplet excitons to singlet ones via reverse inter-system crossing from high-lying triplet states and thereby the depopulation of long-lived triplet excitons that are prone to chemical and/or efficiency degradation. Although their anti-Kasha characteristics have not been clearly explained, numerous molecules with behaviors assigned to the hot exciton mechanism have been reported. Indeed, the related developments appear to have just passed the stage of infancy now, and there will likely be more roles that computational elucidations can play. With this perspective in mind, we review some selected experimental studies on the mechanism and the related designs and then on computational studies. On the computational side, we examine what has been found and what is still missing with regard to properly understanding this interesting mechanism. We further discuss potential future points of computational interests toward aiming for eventually presenting in silico design guides.

Is a Single Molecule Sufficient to Determine the Internal Charge Trapping Energy in Crystalline Organic Semiconductors?

In silico diagnoses of charge-transfer efficiencies in organic semiconductors require the accurate computations of transport parameters. We show here that ignoring the molecular packing effects when computing the internal charge trapping energy may cause a severe deviation to the result deduced from the periodic crystal structure. This deviation can reach up to 100 meV in common organic materials. According to the semiclassical Marcus theory, this energy difference can lead to orders of magnitude change in the charge transfer rate. Studying from a total of 45 organic crystals, we find that single-molecule approximation though is adequate for rigid planar molecules yet it fails to describe the trapping energy for molecules that have inter-ring single bond(s) or are subjected to planarity changes during the transition from the isolated state to the embedded state. These results and conclusions may shed light on the removal of theory-experiment discrepancies on charge mobilities and lay the basis for the future fast and precise screening of high-performance organic semiconductors.

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory

Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(p-phenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (Φ_PL-solution ∼ 1) sky blue emissive 4',4‴-((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1'-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D'-Ar-π-D-π-Ar-D' molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (Φ_{PL-solid state} ∼ 0.8) was achieved as a result of suppressed exciton-phonon/vibronic couplings (no π-π interactions and multiple (14 per dimeric form) strong C-H···π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge·nm). Employing the Lippert-Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design-electronic structure-exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2-6 ns) processes were evident following the prompt decays (∼0.4-0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

Substitution effect on luminescent property of thermally activated delayed fluorescence molecule with aggregation induced emission: A QM/MM study

Pure organic molecules with blue emission have attracted much attention due to its important application in organic light emitting diodes (OLEDs), especially for which with aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties. Theoretical study to reveal the inner luminescent mechanisms can promote its development. In this work, four kinds of molecules with perfluorobiphenyl (PFBP) unit as acceptor, non-substituted and tert-butyl substituted 9,9-dimethyl-9,10-dihydro-acridine (DMAC) unit as donors, are selected and their photophysical properties are studied in detail. The surrounding environment effects in toluene and solid phase are taken into consideration by the polarized continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. Results show that geometric changes between the first singlet excited state (S₁) and ground state (S₀) are restricted in solid phase with decreased root-mean squared displacement (RMSD). Moreover, the Huang-Rhys factors and reorganization energies we calculated are all decreased in solid phase, which indicates that the non-radiative energy consumption process of S₁ is hindered by enhanced intermolecular interactions in rigid environment, and it brings aggregation induced emission phenomenon. Furthermore, the substitution effect of tert-butyl in donor unit can efficiently decrease the energy gap and increase the spin-orbit coupling (SOC) constant, further promotes the intersystem crossing (ISC) and reverse intersystem crossing (RISC) rates. Meanwhile, molecules with donor-acceptor-donor (D-A-D) configuration have more efficient luminous performance than D-A type molecules due to the enhanced ISC and RISC processes. Thus, tert-butyl substituted D-A-D type molecules have outstanding TADF features. Our investigations provide a theoretical perspective for AIE and TADF mechanisms and propose a design strategy for efficient TADF molecules, which could promote the development of OLEDs.

Thermally activated delayed fluorescence emitters with dual conformations for white organic light-emitting diodes: mechanism and molecular design

Thermally activated delayed fluorescence (TADF) molecules with dual emission have great potential for use as single emitters in white organic light-emitting diodes (WOLEDs).

Effect of intermolecular interaction on excited-state properties of thermally activated delayed fluorescence molecules in solid phase: A QM/MM study

Recently, thermally activated delayed fluorescence (TADF) molecules have attracted great attention since nearly 100% exciton usage efficiency was obtained in TADF molecules. Most TADF molecules used in organic light-emitting diodes are in aggregation state, so it is necessary to make out the intermolecular interaction on their photophysical properties. In this work, the excited-state properties of the molecule AI-Cz in solid phase are theoretically studied by the combined quantum mechanics and molecular mechanics (QM/MM) method. Our results show that geometry changes between the ground state (S₀) and the first singlet excited state (S₁) are limited due to the intermolecular π-π and CH-π interactions. The energy gap between S₁ and the first triplet excited state is broadened and the transition properties of excited states are changed. Moreover, the Huang-Rhys factors and the reorganization energy between S₀ and S₁ are decreased in solid phase, because the vibration modes and rotations are hindered by intermolecular interaction. The non-radiative rate has a large decrease in solid phase which improves the light-emitting performance of the molecule. Our calculation provides a reasonable explanation for experimental measurements and highlights the effect of intermolecular interaction on excited-states properties of TADF molecules.

Theoretical study on the light-emitting mechanism of circularly polarized luminescence molecules with both thermally activated delayed fluorescence and aggregation-induced emission

The light-emitting mechanism of circularly polarized luminescence molecules with both TADF and AIE.

How Inter- and Intramolecular Processes Dictate Aggregation-Induced Emission in Crystals Undergoing Excited-State Proton Transfer

Aggregation-induced emission (AIE) offers a route for the development of luminescent technologies with high quantum efficiencies. Excited-state intramolecular proton transfer (ESIPT) coupled to AIE can produce devices with emission across the visible spectrum. We use a combination of theoretical models to determine the factors that mediate fluorescence in molecular crystals undergoing ESIPT. Using two materials based on 2'-hydroxychalcone as exemplar cases, we analyze how inter- and intramolecular processes determine the emissive properties in the crystal environment. This systematic investigation extends the current interpretation of AIE to polar chromophores with multiple decay pathways. We find that population of nonradiative pathways is dictated by the electronic effects of the substituents and the degree of distortion allowed in the crystal environment. Localization of the electron density is crucial to maximize fluorescence via ESIPT. Our conclusions offer design strategies for the development of luminescent molecular crystals.

Understanding the polymorphism-dependent emission properties of molecular crystals using a refined QM/MM approach

A refined QM/MM approach demonstrated that a monomer model is suitable for describing the emission spectra of crystals without the ππ stacking interaction. Whereas for the crystals with notable intermolecular ππ stacking interaction, the most stable trimer model (or at least a dimer model) should be used for accurately describing the corresponding emission spectra. This approach is applied to understand the emission properties of two kinds of organic polymorphs.

Theoretical perspective of the excited state intramolecular proton transfer for a compound with aggregation induced emission in the solid phase

The ESIPT process and AIE mechanism are theoretically investigated by a QM/MM method.

Molecular stacking effect on photoluminescence quantum yield and charge mobility of organic semiconductors

The molecular stacking effect on photoluminescence quantum yield and charge mobility is theoretically investigated by the QM/MM method and Monte Carlo simulation, respectively.

Excited state dynamics for hybridized local and charge transfer state fluorescent emitters with aggregation-induced emission in the solid phase: a QM/MM study

Investigation on the excited state dynamics to reveal the AIE and HLCT mechanisms by a QM/MM method.