We introduce a simple quantum-mechanical model for thermally activated delayed fluorescence (TADF).The Hamiltonian is represented in the basis of four spin-mixed diabatic states representing pure charge transfer (CT) and local excitations (LE). The model predicts that it is possible to realize lowest-lying adiabatic singlet (S1) and triplet (T1) states with a small singlet-triplet gap, differing CT/LE contributions, and appreciable LE component in the S1 state. These characteristics can explain the coexistence of fast T1→S1 reverse intersystem crossing and S1→S0 radiative decay in some chromophores. Through the sampling of the parameter space and statistical analysis of the data, we show which parameter combinations contribute the most to the TADF efficiency. We also show that conformational fluctuations of a single model donor-acceptor system sample a significant region of the parameter space and can enhance the TADF rate by almost three orders of magnitude. This study provides new guidelines for optimization of TADF emitters by means of electronic structure and conformation engineering.

<p>We introduce a simple quantum-mechanical model for thermally activated delayed fluorescence (TADF). The Hamiltonian is represented in the basis of four spin-mixed diabatic states representing pure charge transfer (CT) and local excitations (LE). The model predicts that it is possible to realize lowest-lying adiabatic singlet (S1) and triplet (T1) states with a small singlet triplet gap, differing CT/LE contributions, and appreciable LE component in the S1 state. These characteristics can explain the coexistence of fast T1→S1 reverse intersystem crossing and S1→S0 radiative decay in some chromophores. Through the sampling of the parameter space and statistical analysis of the data, we show which parameter combinations contribute the most to the TADF efficiency. We also show that conformational fluctuations of a single model donor-acceptor system sample a significant region of the parameter space and can enhance the TADF rate by almost three orders of magnitude. This study provides new guidelines for optimization of TADF emitters by means of electronic structure and conformation engineering. <br></p>

Triplet-fusion-based photon upconversion holds promise for a wide range of applications, from photovoltaics to bioimaging. The efficiency of triplet fusion, however, is fundamentally limited in conventional molecular and polymeric systems by its spin dependence. Here, we show that the inherent tailorability of metal–organic frameworks (MOFs), combined with their highly porous but ordered structure, minimizes intertriplet exchange coupling and engineers effective spin mixing between singlet and quintet triplet–triplet pair states. We demonstrate singlet–quintet coupling in a pyrene-based MOF, NU-1000. An anomalous magnetic field effect is observed from NU-1000 corresponding to an induced resonance between singlet and quintet states that yields an increased fusion rate at room temperature under a relatively low applied magnetic field of 0.14 T. Our results suggest that MOFs offer particular promise for engineering the spin dynamics of multiexcitonic processes and improving their upconversion performance.

We investigate a new strategy to enhance thermally activated delayed fluorescence (TADF) in organic light-emitting diodes (OLEDs). Given that the TADF rate of a molecule depends on its conformation, we hypothesize that there exists a conformation that maximizes the TADF rate. In order to test this idea, we use time-dependent density functional theory (TDDFT) to simulate the TADF rates of several TADF emitters, while shifting their geometries towards higher TADF rates in a select subspace of internal coordinates. We find that geometric changes in this subspace can increase the TADF rate up to three orders of magnitude with respect to the minimum energy conformation, and the simulated TADF rate can even be brought into the submicrosecond timescales under the right conditions. Furthermore, the TADF rate enhancement can be maintained with a conformational energy that might be within the reach of modern synthetic chemistry. Analyzing the maximum TADF conformation, we extract a number of structural motifs that might provide a useful handle on the TADF rate of a donor-acceptor (DA) system. The incorporation of conformational engineering into the TADF technology could usher in a new paradigm of OLEDs.

We propose a novel method to simulate the chemical kinetics of methanol oxidation on the rutile TiO2(110) surface. Such a method must be able to capture the effects of static disorder (site-to-site variations in the rate constants), as well as dynamic correlation (interdependent probabilities of finding reactants next to each other). Combining the intuitions of the mean-field steady state (MFSS) method and the pair approximation (PA), we consider representative pairs of sites in a self-consistent bath of the average pairwise correlation. Preaveraging over the static disorder in one site of each pair makes this half heterogeneous pair approximation (HHPA) efficient enough to simulate systems of several species and calibrate rate constants. According to the simulated kinetics, a static disorder in the hole transfer steps suffices to reproduce the stretched exponentials in the observed kinetics. The dominant hole scavengers are found to be temperature-dependent: the methoxy anion at 80 K and the methanol molecule at 180 K. Moreover, two distinct subpopulations of 5-coordinate titanium (Ti5c) sites emerge, a high-activity group and a low-activity group, even though no such division exists in the rate constants. Since the division is quite insensitive to the details of static disorder, the emergence of the two groups might play a significant role in a variety of photocatalytic processes on TiO2.