Achieving highly efficient phosphorescence in purely organic luminophors at room temperature remains a major challenge due to slow intersystem crossing (ISC) rates in combination with effective non‐radiative processes in those systems. Most room temperature phosphorescent (RTP) organic materials have O‐ or N‐lone pairs leading to low lying (n, π*) and (π, π*) excited states which accelerate kisc through El‐Sayed's rule. Herein, we report the first persistent RTP with lifetimes up to 0.5 s from simple triarylboranes which have no lone pairs. RTP is only observed in the crystalline state and in highly doped PMMA films which are indicative of aggregation induced emission (AIE). Detailed crystal structure analysis suggested that intermolecular interactions are important for efficient RTP. Furthermore, photophysical studies of the isolated molecules in a frozen glass, in combination with DFT/MRCI calculations, show that (σ, B p)→(π, B p) transitions accelerate the ISC process. This work provides a new approach for the design of RTP materials without (n, π*) transitions.

Heptazine derivatives are promising dopants for electroluminescent devices. Recent studies raised the question whether heptazines exhibit a small regular or an inverted singlet–triplet (IST) gap. It was argued that the S1 ← T1 reverse intersystem crossing (RISC) is a downhill process in IST emitters and therefore does not require thermal activation, thus enabling efficient harvesting of triplet excitons. Rate constants were not determined in these studies. Modeling the excited-state properties of heptazine proves challenging because fluorescence and intersystem crossing (ISC) are symmetry-forbidden in first order. In this work, we present a comprehensive theoretical study of the photophysics of heptazine and its derivative HAP-3MF. The calculations of electronic excitation energies and vibronic coupling matrix elements have been conducted at the density functional theory/multireference configuration interaction (DFT/MRCI) level of theory. We have employed a finite difference approach to determine nonadiabatic couplings and derivatives of spin–orbit coupling and electric dipole transition matrix elements with respect to normal coordinate displacements. Kinetic constants for fluorescence, phosphorescence, internal conversion (IC), ISC, and RISC have been computed in the framework of a static approach. Radiative S1 ↔ S0 transitions borrow intensity mainly from optically bright E′ π → π* states, while S1 ↔ T1 (R)­ISC is mediated by E″ states of n → π* character. Test calculations show that IST gaps as large as those reported in the literature are counterproductive and slow down the S1 ← T1 RISC process. Using the adiabatic DFT/MRCI singlet–triplet splitting of −0.02 eV, we find vibronically enhanced ISC and RISC to be fast in the heptazine core compound. Nevertheless, its photo- and electroluminescence quantum yields are predicted to be very low because S1 → S0 IC efficiently quenches the luminescence. In contrast, fluorescence, IC, ISC, and RISC proceed at similar time scales in HAP-3MF.

10-Methylisoalloxazine (MIA) and its fluorinated derivatives (6-9F-MIA) were investigated by means of quantum chemistry, looking into the influence of fluorination on fluorescence, absorption and inter-system crossing (ISC) in vacuum and in aqueous solution.

Multireference quantum chemical calculations were performed in order to investigate the (reverse) intersystem crossing ((R)ISC) mechanisms of 4,5-di(9H-carbazol-9-yl)-phthalonitrile (2CzPN).

In this work, we report benchmark spin-orbit calculations for a representative set of electronic states including π → π*, n → π*, and π → σ* and Rydberg states of organic molecules. Auxiliary many-electron wave functions (AMEWs) have been generated from left and/or right eigenvectors of Casida's non-Hermitian time-dependent density functional theory (TDDFT) equation. The newly developed Spoiler program has been used to evaluate spin-orbit matrix elements (SOMEs) from full linear response TDDFT and TDDFT calculations in Tamm-Dancoff approximation (TDA) in conjunction with the well-known B3-LYP and PBE0 hybrid functionals. The data thus obtained have been benchmarked against SOMEs from multireference configuration interaction calculations recently performed in our group. It turns out that the TDDFT SOMEs are rather insensitive with regard to the choice of eigenvectors (left, right, or mixed) as long as the AMEWs are normalized. To avoid problematic excitation energies of low-lying triplet excited states, the use of the TDA is recommended. With regard to SOMEs, a slight preference is found for the PBE0 functional.

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three...

Designing highly efficient purely organic phosphors at room temperature remains a challenge because of fast non‐radiative processes and slow intersystem crossing (ISC) rates. The majority of them emit only single component phosphorescence. Herein, we have prepared 3 isomers (o, m, p‐bromophenyl)‐bis(2,6‐dimethylphenyl)boranes. Among the 3 isomers (o‐, m‐ and p‐BrTAB) synthesized, the ortho‐one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. Based on theoretical calculations and crystal structure analysis of o‐BrTAB, the short lifetime component is ascribed to the T1M state of the monomer which emits the higher energy phosphorescence. The long‐lived, lower energy phosphorescence emission is attributed to the T1A state of an aggregate, with multiple intermolecular interactions existing in crystalline o‐BrTAB inhibiting nonradiative decay and stabilizing the triplet states efficiently.

Derivatives of dipole transition moments between spin-orbit coupled (SOC) multireference configuration interaction wave functions have been used in conjunction with vibrational frequencies from density functional theories to compute vibronic S 1 ←S 0 (1 1 B 3u ←1 1 A g) and T 1 ←S 0 (1 3 B 3u ← 1 1 A g) absorption spectra in Herzberg-Teller approximation. The experimentally known spectra are well reproduced. The calculations reveal unexpectedly small spin-orbit couplings between the 1 3 B 3u (3 nπ*) state and nearby optically bright 1 B 2u (1 ππ*) states, thus explaining the absence of the 1 1g0 1b (1 10a0 ν) fundamental in the vibrational fine-structure of the T 1 ←S 0 transition. Adiabatically, two triplet states are found below the S 1 state. The out-of-plane distorted T 2 minimum results from a pseudo Jahn-Teller interaction between two 3 ππ* states of B 1u and B 2u symmetry. At the D 2h-symmetric S 0 and S 1 minimum geometries, the latter states are located well above S 1. The S 1 and T 2 potentials intersect at geometries far away from the Franck-Condon region. This explains the apparently contradictory results that the linewidth in the higher energy regime above the T 1 ←S 0 origin suddenly broadens while no trace of a second triplet state, located energetically below the S 1 origin, could be identified in phosphorescence excitation spectra of the ultracold isolated pyrazine molecule.