The phenomenon of electric field-induced emission quenching is important in organic light-emitting diodes which require the application of large electric fields for their function. Past experimental work on light-emitting polymers and oligomers showed that field-induced quenching (FIQ) efficiencies are higher in non-rigid molecules such as poly(p-phenylene vinylene) or PPV, than in similar, more planar molecules.(1) Based on this relationship, we proposed that the applied field enhances internal conversion decay channels.(2,3) The current study builds on this idea by examining FIQ in PPV oligomers of varying length using computational methods. Calculations performed at the INDO/S-CI level show the presence of free electron-hole pair (FEHP) states which are stabilized by the uniform external electric field. These FEHP states undergo an avoided crossing with the fluorescent 1Bu¬ bound exciton state at sufficiently high field magnitudes. The magnitude of the electronic coupling between the FEHP and 1Bu state, determined from these avoided crossings, is found to be a function of the field at which these states cross. This function is universal in that it applies across different length oligomers and across different FEHP states on the same oligomer. A model for FIQ is developed by combining this electronic-coupling function with Marcus theory and a simple model for the surrounding dielectric medium. To better reproduce the gradual increase of quenching with field seen in experiment,(3) inhomogeneous broadening of state energies is also included. The resulting model yields reasonable quantitative agreement with FIQ magnitudes, dependence on oligomer length, and threshold field strengths at which quenching is observed.
(1) Moscatelli, A.; Livingston, K.; So, W. Y.; Lee, S. J.; Scherf, U.; Wildeman, J.; Peteanu, L. A. Electric-Field-Induced Fluorescence Quenching in Polyfluorene, Ladder-Type Polymers, and MEH-PPV: Evidence for Field Effects on Internal Conversion Rates in the Low Concentration Limit. J. Phys. Chem. B 2010, 114, 14430–14439.
(2) Smith, T. M.; Hazelton, N.; Peteanu, L. A.; Wildeman, J. Electrofluorescence of MEHPPV and Its Oligomers: Evidence for Field-Induced Fluorescence Quenching of Single Chains. J. Phys. Chem. B 2006, 110, 7732–42.
(3) Smith, T.; Kim, J.; Peteanu, L.; Wildeman, J. Electric Field Effects on Internal Conversion: An Alternative Mechanism for Field-Induced Fluorescence Quenching of MEHPPV and Its Oligomers in the Low Concentration Limit. J. Phys. Chem. C 2007, 111, 10119–10129.