Site Effect of Electron Acceptors on Ultralong Organic Room-Temperature Phosphorescence

Herein, we successfully observe the site effect of electron acceptors on ultralong organic room-temperature phosphorescence (UORTP) in the case of 7H-benzo[c]carbazole (BCz) derivatives: cyanophenyl on the nitrogen site can promote intersystem crossing (ISC) efficiency and enhance phosphorescence intensity by facilitating n-π* transitions but make a slight change to the phosphorescence wavelength; cyanophenyl on the naphthalene site can cause a remarkable red shift of phosphorescence wavelength by reducing the T₁ energy level of BCz derivatives and also enhance phosphorescence intensity by promoting ISC but weaken phosphorescence intensity by lowering the molecular symmetry. Three BCz derivatives (1-BCzPhCN, 2-BCzPhCN, and 3-BCzPhCN) with the electron acceptor cyanophenyl at different sites (nitrogen site and naphthalene site) were synthesized through a combination of the nucleophilic substitution reaction and the Suzuki coupling reaction. The phosphorescence properties of 1-BCzPhCN, 2-BCzPhCN, and 3-BCzPhCN in toluene solution, in a copolymerized MMA film, and in a PVA film were measured and analyzed. 1-BCzPhCN emits intrinsic green ultralong phosphorescence at ∼500, ∼536, and ∼580 nm, while 2-BCzPhCN and 3-BCzPhCN give out intrinsic yellow ultralong phosphorescence with a red shift of 27 and 40 nm, showing that cyanophenyl on the naphthalene site leads to a remarkable red shift of the intrinsic phosphorescence wavelength, but cyanophenyl on the nitrogen site makes a slight difference to the intrinsic phosphorescence wavelength. Under the same condition, the phosphorescence intensity is usually ranked as 1-BCzPhCN/3-BCzPhCN > 2-BCzPhCN, demonstrating that cyanophenyl on the nitrogen site promotes ISC and enhances phosphorescence intensity, but cyanophenyl on the naphthalene site reduces molecular symmetry and accelerates nonradiative dissipation. Time-dependent density functional theory calculations verify that cyanophenyl on the naphthalene site shifts the phosphorescence wavelength by reducing the T₁ energy level, and cyanophenyl on the nitrogen site facilitates n-π* transitions to strengthen the phosphorescence intensity. Moreover, three BCz derivatives were doped into DMAP and BBP, separately. The BCz derivatives exhibited different phosphorescence colors and shifts due to interactions with the host materials. We believe this work will give an insight into the structure-property relationship of organic phosphorescence molecules and pave a way for design of colorful UORTP materials.