Time-Resolved EPR and Theoretical Investigations of Naphthalene Diimide Spin Dynamics in the Excited State

Abstract

Naphthalene diimides (NDIs) are a promising material for n-type polymer acceptors in thin-film devices such as organic light-emitting diodes. The intersystem crossing (ISC) pathway of NDIs has been extensively studied, relying heavily on the accuracy of quantum chemical calculations. However, the theoretical results are inconsistent with the experimental findings. In this work, time-resolved electron paramagnetic resonance (tr-EPR) was applied to NDIs for the first time in order to elucidate the ISC pathway and assign their electronic structures. With the help of quantum chemical calculations, tr-EPR clearly demonstrated that ISC occurs via an upper excited triplet state (T₄) after photoexcitation. The zero-field splitting (ZFS) constants of the NDIs, which strongly correlate with their spin density distributions, were experimentally determined by tr-EPR. We demonstrate that an ab initio complete active space self-consistent field is required to interpret the ZFS constants of the NDIs. From the theoretical analyses of the constants and phosphorescence measurements, the spin state observed in the tr-EPR spectra was identified as the lowest excited triplet state, illustrating that quick internal conversion occurs after the ISC process.