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.