Long-Lived Intramolecular Charge Transfer in Persubstituted Perylenediimide

Abstract

Photoinduced charge transfer (CT) states play a pivotal role in increasing the power conversion efficiency of molecular systems used in artificial photosynthesis, photocatalysis, and optronic devices. The absence of intrinsic CT states is one of the main reasons for the poor photoconversion efficiencies of organic chromophores like perylenediimide (PDI). Herein, we explore the excited state dynamics of a persubstituted PDI (AP) with amino groups at the ortho positions and bromine atoms at the bay positions. Due to the influence of bromine atoms and amino groups on the PDI core, nonradiative pathways are accessed on photoexcitation in AP. Femtosecond and nanosecond transient absorption measurements in weakly polar and polar solvents showed the relaxation of the higher singlet excited state in picoseconds time scale, paving the way to an intramolecular charge transfer (ICT) state having a lifetime in the nanoseconds time scale. As the dielectric medium changed from the weakly polar solvent (toluene, ε = 2.38) to a polar solvent (ethyl acetate, ε = 6.02), the lifetime of the solvent stabilized CT state decreased from τ = 69.1 ± 1.7 ns to τ = 47.1 ± 0.5 ns, which confirms the solvent dependency of the ICT state. Theoretical investigations employing surface hopping dynamics suggest that the rate of internal conversion (k_IC = 1.13 × 10¹¹ s^–1) competes with the intersystem crossing (k_ISC = 0.85 × 10¹¹ s^–1) in AP. Amination on the ortho position induces the CT characteristics to the core of the PDI, as evident from hole–electron surface analysis of the S₁ state. Presented results on persubstituted PDI with long-lived relaxed CT states may improve the designing strategies of organic optoelectronic devices.