Deciphering carbon–sulfur rotational distribution in a crystalline host for enhanced red persistent organic phosphorescence

Abstract

Persistent room-temperature phosphorescence (pRTP) from organic host–guest solids has been used for autofluorescence-free high-resolution imaging. However, from the lowest triplet excited state, the radiative transition rate (k_p) decreases and the non-radiative transition rate (k_nr) increases with increasing wavelength. Therefore, the pRTP yield (Φ_p) at long wavelengths is still low. Here we report an enhanced red pRTP from dibenzo[g,p]chrysene substituted with a phenylthio group (PhSDBC) as a guest in a crystalline benzophenone host. Host–guest crystals composed of deuterated PhSDBC and benzophenone showed red pRTP with Φ_p = 28.8% and a lifetime of 1.47 s. Investigation of the correlation between experimental and calculated values of k_p and k_nr revealed that the distribution of C–S rotational conformations of PhSDBC was constrained to a narrow range in the crystalline host. In the constrained distribution, the phenylthio substituents enhanced the cooperation of spin–orbit couplings and the transition dipole moment associated with the higher singlet excited state, which resulted in the selective increase of k_p and in turn the enhanced Φ_p.