Photogenerated Quartet State Formation in a Compact Ring-Fused Perylene-Nitroxide

Abstract

We report on a novel small organic molecule comprising a perylene chromophore fused to a six-membered ring containing a persistent nitroxide radical to give a perylene-nitroxide, or PerNO(•). This molecule is a robust, compact molecule in which the radical is closely bound to the chromophore but separated by saturated carbon atoms, thus limiting the electronic coupling between the chromophore and radical. We present both ultrafast transient absorption experiments and time-resolved EPR (TREPR) studies to probe the spin dynamics of photoexcited PerNO(•) and utilize X-ray crystallography to probe the molecular structure and stacking motifs of PerNO(•) in the solid state. The ability to control both the structure and electronic properties of molecules having multiple spins as well as the possibility of assembling ordered solid state materials from them is important for implementing effective molecule-based spintronics.

Efficient Radical‐Enhanced Intersystem Crossing in an NDI‐TEMPO Dyad: Photophysics, Electron Spin Polarization, and Application in Photodynamic Therapy

“…The short distance and semi‐rigid linker between the nitroxide spin carrier and the NDI moiety are beneficial for the formation of the eigenstates of the D 1 and Q states, as a result of the strong electron spin–spin exchange interaction. On the other hand, the distance is longer than that in a perylene with a fused TEMPO moiety, which showed a shorter triplet state lifetime (0.1 μs) . Compounds NDI‐Br and NDI were prepared as references for the photophysical studies.…”

“…That is to say, the electronic state of the entire molecule could be a doublet or a quartet state, and the chromophore could be in its singlet or triplet state. One of the most interesting aspects of these systems is that the originally electron‐spin‐forbidden S 1 →T 1 ISC process of the chromophore becomes an electron‐spin‐allowed D 2 →D 1 internal conversion process of the entire molecule, because of the same overall spin multiplicity . As a result, the ISC of the organic chromophore is enhanced; this is the so‐called radical‐enhanced intersystem crossing (EISC).…”

“…Moreover, owing to the mixing of D 1 and Q states due to zero‐field splitting perturbation, the long‐lived Q state ( 4 [T, R ]) can be populated. In other words, it is possible to attain efficient ISC in radical–chromophore hybrids, yet at the same time the triplet state of the chromophore can have a long lifetime …”

“…Importantly, the triplet state quantum yields reported for EISC molecules are usually low and triplet lifetimes are short. For instance, previously, the TEMPO radical was fused to the perylene moiety and the resulting triplet state lifetime was very short ( τ T =0.1 μs), which is not satisfactory for any intermolecular electron or energy transfer. During the preparation of this paper, a TEMPO‐labeled heptamethine aminocyanine was reported; however, the singlet oxygen quantum yield of the TEMPO–heptamethine aminocyanine dyad ( Φ Δ =0.2) is much lower than that of our previously reported TEMPO–BODIPY dyad ( Φ Δ =0.56) .…”

Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

“…Recently the electron spin converter strategy was also reported . Radical‐enhanced ISC based on electron‐spin exchange interaction was also studied . However, various drawbacks exist for these approaches, for example, the synthesis of the molecules is demanding, or ISC kinetics is slow, and the ISC efficiency is unsatisfactory.…”