Impact of molecular structure on singlet and triplet exciton diffusion in phenanthroline derivatives

We demonstrate the impact of subtle changes in molecular structure on the singlet and triplet exciton diffusion lengths (L-D) for derivatives of the archetypical electron-transport material 4,7-diphenyl-1,10-phenanthroline (BPhen). Specifically, this work offers a systematic characterization of singlet and triplet exciton transport in identically prepared thin films, highlighting the differing dependence on molecular photophysics and intermolecular spacing. For luminescent singlet excitons, photoluminescence quenching measurements yield an L-D from <1 nm for BPhen, increasing to (5.4 +/- 1.2) nm for 2,9-dichloro-4,7-diphenyl-1,10-phenanthroline (BPhen-Cl-2) and (3.9 +/- 1.1) nm for 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The diffusion of dark triplet excitons is probed using a phosphorescent sensitizer-based method where triplets are selectively injected into the material of interest, with those migrating through the material detected via energy transfer to an adjacent, phosphorescent sensitizer. Interestingly, the triplet exciton L-D decreases from (15.4 +/- 0.4) nm for BPhen to (8.0 +/- 0.7) nm for BPhen-Cl-2 and (4.0 +/- 0.5) nm for BCP. The stark difference in behavior observed for singlets and triplets with functionalization is explicitly understood using long-range Forster and short-range Dexter energy transfer mechanisms, respectively.