Building on the strengths of a double-hybrid density functional for excitation energies and inverted singlet-triplet energy gaps

Abstract

It is demonstrated that a double hybrid density functional approximation (DH-DFA), ωB88PTPSS, that incorporates equipartition of density functional theory (DFT) and non-local correlation however with a meta-GGA correlation functional as well as with the range-separated exchange of ωB2PLYP provides accurate excitation energies for conventional systems as well as correct prescription of negative singlet-triplet gaps for non-conventional systems with inverted gaps, without any necessity for parametric scaling of the same-spin and opposite-spin non-local correlation energies. Examined over " safe" excitations of the QUESTDB set, ωB88PTPSS performs quite well for open-shell systems, correctly and fairly accurately (relative to EOM-CCSD reference) predicts negative gaps for 50 systems with inverted singlet-triplet gaps, is one of the leading performers for intramolecular charge-transfer excitations and achieves near-CC2 and ADC(2) quality for the Q1 and Q2 subsets. Subsequently, we tested ωB88PTPSS on two sets of real-life examples from recent computational chemistry literature; the low energy bands of chlorophyll a (Chl a) and a set of thermally activated delayed fluorescence (TADF) systems. For Chl a, ωB88PTPSS quantitatively and quantitatively achieves DLPNO-STEOM-CCSD-level performance and provides excellent agreement with experiment. For TADF systems, ωB88PTPSS agrees quite well with SCS-CC2 excitation energies as well as experimental values for the gaps between the S1 and T1 excited states.

Heptazine, Cyclazine, and Related Compounds: Chemically-Accurate Estimates of the Inverted Singlet–Triplet Gap

“…Of course, the state-of-the-art methods discussed above are not readily applicable to the design of novel compounds with enhanced properties, a goal typically achieved through the addition of side substituents or the extension of the conjugated core. ,,,,, A wide panel of strategies has been explored in the literature to design new compounds, but the challenge lies in the absence of definitive benchmark values. This lack of a clear reference is evident, for instance, in the varying choices of reference methods made by different research groups when benchmarking double-hybrid functionals, with some using SCS-CC2 and others opting for CCSD. , …”

“…This lack of a clear reference is evident, for instance, in the varying choices of reference methods made by different research groups when benchmarking double-hybrid functionals, with some using SCS-CC2 29 and others opting for CCSD. 30,31 Hence, we decided to perform a benchmark of lower-order approaches for a set of 10 triangulenes, including heptazine, cyclazine, and eight additional molecules displayed in Figure 3, for which we obtained accurate TBE/aug-cc-pVTZ values using the same procedure as that in Tables 1 and 2. The selection of these molecules was inspired by previous literature.…”

Reference Energies for Double Excitations: Improvement and Extension

“…Doubly excited states are ubiquitous in many photochemical mechanisms, such as the photochemistry of polyenes, − in singlet fission where they can play a major role in the formation of the triplet-pair state, in thermally activated delayed fluorescence (TADF) − where their predominant interaction with singlet states can produce molecules with an inverted singlet–triplet gap, − i.e., where the lowest singlet excited state is lower in energy than the lowest triplet state.…”

Computational Design of Multiple Resonance-Type BN Molecules for Inverted Singlet and Triplet Excited States

“…Later on, researchers at RIKEN demonstrated experimentally that HzTFEX 2 (Figure S1), a heptazine derivative, exhibits a negative Δ E ST and, consequently, an rISC rate faster than the ISC rate as estimated by temperature-dependent transient photoluminescence spectroscopy . As the INVEST has been mostly driven by the computational chemistry community, there have been many reports on the theoretical and computational design of molecules showing a negative Δ E ST , but the molecular structures have been largely limited to the boron- and/or nitrogen-doped phenalene-type structures such as cyclazine and heptazine derivatives, − except for a few reports of polyaromatic hydrocarbon core (PAH) compounds such as pentalene, COLDEM azulene-type alternant hydrocarbon molecule (compound 3 in Figure S1) is predicted to show a negative Δ E ST . − …”

“…Double-hybrid functionals within time-dependent density functional theory (TD–DFT) are used in the calculations to estimate Δ E ST and oscillator strength. This class of methods represents an excellent tradeoff between accuracy and computational cost. − ,− Note that wavefunction-based methods, such as the equation of motion coupled-cluster singles and doubles (EOM-CCSD) and the spin-component scaling second-order approximate coupled-cluster (SCS-CC2), are also considered for benchmarking Δ E ST for smaller size systems.…”