Wave Function Control of Charge-Separated Excited-State Lifetimes

Abstract

Control of excited-state processes is crucial to an increasing number of important device technologies that include displays, photocatalysts, solar energy conversion devices, photovoltaics, and photonics. However, the manipulation and control of electronic excited-state lifetimes and properties continue to be a challenge for molecular scientists. Herein, we present the results of ground-state and transient absorption spectroscopies as they relate to magnetic exchange control of excited-state lifetimes. We describe a novel mechanism for controlling these excited-state lifetimes that involves varying the magnetic exchange interaction between a stable organic radical and the unpaired electrons present in the open-shell configuration of a charge-separated excited state. Specifically, we show that the excited-state lifetime can be controlled in a predictable manner based on an a priori knowledge of the pairwise magnetic exchange interactions between excited-state spins. These magnetic exchange couplings affect the excited-state electronic structure in a manner that introduces variable degrees of spin forbiddenness into the nonradiative decay channel between the excited state and the electronic ground state.

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…The impact of the π‐radical spin on the charge‐separated excited‐state dynamics was also investigated in metal complexes containing π‐radical ligands . As shown in Figure a, Shultz, Kirk, and co‐workers synthesized radical complexes of the ( t Bu 2 bpy)Pt(Cat‐R) type, where Cat, bpy, and R stand for 3‐ tert ‐butyl‐ ortho ‐catecholate, bipyridine, and nitronyl‐nitroxide radical (NN), respectively, and investigated their photoexcited states by magnetic circular dichroism (MCD) spectroscopy.…”

“…(1 a–f)). Equation (2) is an approximation as the mixing between the sing‐doublet and trip‐doublet actually depends on the ( J SQ‐NN − J bpy‐NN )/(2 J SQ‐bpy − J SQ‐NN ) ratio, however J bpy‐NN can be neglected in this case as it is expected to be very small as explained in their own work . Their exchanges were evaluated by using experimental data.…”

“…Thus, the π‐radical spin can impact the excited‐state dynamics. The control of the lifetime of a charge‐separated excited state was demonstrated by varying the exchange interaction between the radical and charge‐separated excited state …”

“…The impact of the p-radical spin on the charge-separated excited-state dynamics was also investigated in metal complexes containing p-radical ligands. [108][109] As shown in Figure 11 a, Shultz,K irk, and co-workerss ynthesized radical complexes of the (tBu 2 bpy)Pt(Cat-R) type, where Cat, bpy,a nd Rs tand for 3tert-butyl-ortho-catecholate,b ipyridine, and nitronyl-nitroxide radical( NN), respectively,a nd investigated their photoexcited states by magnetic circular dichroism (MCD) spectroscopy.I n these complexes, upon excitation of the ligand-to-ligand charget ransfer (LLCT) band, ap hotoinduced charget ransfer occurs from Cat to bpy,l eadingt oa( bpyC)Pt(SQCNNC)t hree-spin system.F or three-spin systems, the spin wavefunctions of the sing-doublet (j S 1 ,1/2 >)a nd trip-doublet (j T 1 ,1/2 >)u ndergo mixing depending on the l,whichisg iven by Equation(2)…”

“…Equation (2) is an approximation as the mixingb etween the sing- doublet and trip-doubleta ctually depends on the (J SQ-NN ÀJ bpy-NN )/(2J SQ-bpy ÀJ SQ-NN )r atio, however J bpy-NN can be neglected in this case as it is expected to be very small as explained in their own work. [109] Their exchanges were evaluated by using experimental data. Thus, the J SQ-NN value was estimated from the temperature dependence of the magnetic susceptibility of the relevant ground-state biradicals.…”