  A quantum chemistry study of the first singlet (S1) and triplet (T1) excited
states of phenylsulfonyl-carbazole compounds, proposed as useful thermally
activated delayed fluorescence (TADF) emitters for organic light emitting diode
(OLED) applications, was performed with the quantum Equation-Of-Motion
Variational Quantum Eigensolver (qEOM-VQE) and Variational Quantum Deflation
(VQD) algorithms on quantum simulators and devices. These quantum simulations
were performed with double zeta quality basis sets on an active space
comprising the highest occupied and lowest unoccupied molecular orbitals (HOMO,
LUMO) of the TADF molecules. The differences in energy separations between S1
and T1 ($\Delta E_{st}$) predicted by calculations on quantum simulators were
found to be in excellent agreement with experimental data. Differences of 16
and 88 mHa with respect to exact energies were found for excited states by
using the qEOM-VQE and VQD algorithms, respectively, to perform simulations on
quantum devices without error mitigation. By utilizing error mitigation by
state tomography to purify the quantum states and correct energy values, the
large errors found for unmitigated results could be improved to differences of,
at most, 3 mHa with respect to exact values. Consequently, excellent agreement
could be found between values of $\Delta E_{st}$ predicted by quantum
simulations and those found in experiments.
