Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state

Sulfur oxidation state is used to tune organic room temperature phosphorescence (RTP) of symmetric sulfur-bridged carbazole dimers. The sulfide-bridged compound exhibits a factor of 3 enhancement of the phosphorescence efficiency, compared to the sulfoxide and sulfone-bridged analogs, despite sulfone bridges being commonly used in RTP materials. In order to investigate the origin of this enhancement, temperature dependent spectroscopy measurements and theoretical calculations are used. The RTP lifetimes are similar due to similar crystal packing modes. Computational studies reveal that the lone pairs on the sulfur atom have a profound impact on enhancing intersystem crossing rate through orbital mixing and screening, which we hypothesize is the dominant factor responsible for increasing the phosphorescence efficiency. The ability to tune the electronic state without altering crystal packing modes allows the isolation of these effects. This work provides a new perspective on the design principles of organic phosphorescent materials, going beyond the rules established for conjugated ketone/sulfone-based organic molecules.

S6 Figure S4. Powder X-ray diffraction (pXRD) patterns of CBZ-SO crystalline powder (black) compared to their single crystal simulated patterns (red).

CBZ-S
This compound has been previously synthesized. 4 Bis (

Figure S18. Crystal molecular dimers with the shortest intermolecular distance. Eclipsed (left)
and T-shape (right) molecular pairs of CBZ-SO in the crystal. CBZ-SO and CBZ-SO 2 crystals exhibit equivalent molecular pairs. Table S7. Vertical excitation energies (in eV), oscillator strengths (f in parenthesis) to the lowest singlet and triplet states of computed for the eclipsed and T-shape molecular pairs of CBZ-SO n in the crystal ( Figure S17) Table S8. Relative energies (in meV) and SOCs (in cm -1 ) between the lowest excited singlet (S 1 ) and energetically close triplets of the CBZ-SO n dimers computed at the B97X-D/6-31+G(d) level in the crystal molecular structure. The last column qualitatively indicates the weight of the sulfur and oxygen lone pairs participation in the electronic structure of T n .

Phosphorescent emission probability
We evaluate the oscillator strength of triplet emission through perturbation theory by expanding the triplet state wave function as: . (S1) Then, the oscillator strength (f) can be expressed as: (S2) Table S9.
Oscillator strengths (f) for the emission from the two lowest excited triplet states of

CBZ-SO n computed with equation S2 and with the sum over excited singlets in equation S1
running over the 20 lowest states. Computations have been done for the molecular crystal structures at the B97X-D/6-31+G(d) level.