Identification via Virtual Screening of Emissive Molecules with a Small Exciton-Vibration Coupling for High Color Purity and Potential Large Exciton Delocalization

Abstract

A sequence of quantum chemical computations of increasing accuracy was used in this work to identify molecules with small exciton reorganization energy (exciton-vibration coupling), of interest for light emitting devices and coherent exciton transport, starting from a set of ∼4500 known molecules. We validated an approximate computational approach based on single-point calculations of the force in the excited state, which was shown to be very efficient in identifying the most promising candidates. We showed that a simple descriptor based on the bond order could be used to find molecules with potentially small exciton reorganization energies without performing excited state calculations. A small set of chemically diverse molecules with a small exciton reorganization energy was analyzed in greater detail to identify common features leading to this property. Many such molecules display an A-B-A structure where the bonding/antibonding patterns in the fragments A are similar in HOMO and LUMO. Another group of molecules with small reorganization energy displays instead HOMO and LUMO with a strong nonbonding character.

Figure 1

Illustration for calculating reorganization energy (λ) using 1D potential energy surfaces. Under the harmonic and Condon approximation, and F = ℏωΔq. Equations in the manuscript are for potential energy surfaces of higher dimensions.

Figure 2

Comparison of total reorganization energy between the force approach (λ_force) to the common four-point approach (λ_4p). The subfigure shows zoom-in results of small λ (the Pearson’s R = 0.88 for this subset).

Figure 3

(a) Distribution of reorganization energy λ_force in the energy region [0, 0.5] eV (layer i calculation) with red bars presenting the systems with small λ_force for layer ii. (b) Comparison of reorganization energies between eq 1 (λ_4p) and eq 3 (λ_force). The red dashed line indicates the λ_force = λ_4p condition.

Figure 4

Relationship between BOD and reorganization energy for results in layer i. (a) All 4476 molecules (dashed line: λ_force = 0.1 eV) with data point color labeled according to the HOMO–LUMO excitation weight. (b) Box plot that shows the range of expected λ_force for intervals of BOD for molecules with HOMO–LUMO excitation weight larger than 0.9.

Figure 5

Analysis for A1–B–A2 systems for (a) WEPGUU04, (b) VIFSEI, (c) QALLOH, and (d) the model system. The 2nd/3rd columns show HOMOs and LUMOs plots with the definition of blocks for each system. The last column shows block overlaps among MOs (“H” for HOMO and “L” for LUMO).

Figure 6

Model D–conjugate-A systems. (a) The geometries of model systems M1–M3. (b) Plots of HOMO/LUMO. (c) Orbital energies of HOMO and LUMO and excited energies of S₁ state. (d) Relationship between reorganization energy and BOD. The calculation level is the same as in layer iii.