A pi-orbital model to study substituent effects in organic room-temperature phosphorescent materials

The substituent effect is a ubiquitous tool to improve the room temperature phosphorescence (RTP) of organic materials, but it remains unclear as to how to rationally manipulate the RTP performance. However, we establish a pi-orbital model to unravel the substituent effect on the organic RTP materials, by using the hybrid quantum mechanics/molecular mechanics (QM/MM) model and N-Electron Valence State Perturbation Theory (NEVPT2). With the substitution of electron-donating groups (EDGs), the increase of the pi-orbital energy level separates the hybrid or adjacent n/pi-orbitals, converting S-1 from (n, pi*) to (pi, pi*) that produces the T-n-dependent intersystem crossing process, accounting for the diverse RTP efficiency caused by EDGs. Moreover, this change also transforms T-1 (n, pi*) to T-1 (pi, pi*), suppressing the nonradiative relaxation of triplet excitons, thereby prolonging the RTP lifetime. Experimental validation was further performed for the predicted organic RTP molecule based on the pi-orbital model. This model provides new guidelines to manipulate the RTP performance in organic materials.