Ultrafast Intersystem Crossing and Spin Dynamics of Photoexcited Perylene-3,4:9,10-bis(dicarboximide) Covalently Linked to a Nitroxide Radical at Fixed Distances

Abstract

Time-resolved transient optical absorption and EPR (TREPR) spectroscopies are used to probe the interaction of the lowest excited singlet state of perylene-3,4:9,10-bis(dicarboximide) ((1*)PDI) with a stable tert-butylphenylnitroxide radical ((2)BPNO(*)) at specific distances and orientations. The (2)BPNO(*) radical is connected to the PDI with the nitroxide and imide nitrogen atoms either para (1) or meta (3) to one another, as well as through a second intervening p-phenylene spacer (2). Transient absorption experiments on 1-3 reveal that (1*)PDI undergoes ultrafast enhanced intersystem crossing and internal conversion with tau approximately = 2 ps to give structurally dependent 8-31% yields of (3*)PDI. Energy- and electron-transfer quenching of (1*)PDI by (2)BPNO(*) are excluded on energetic and spectroscopic grounds. TREPR experiments at high magnetic fields (3.4 T, 94 GHz) show that the photogenerated three-spin system consists of the strongly coupled unpaired electrons confined to (3*)PDI, which are each weakly coupled to the unpaired electron on (2)BPNO(*) to form excited doublet (D(1)) and quartet (Q) states, which are both spectrally resolved from the (2)BPNO(*) (D(0)) ground state. The initial spin polarizations of D(1) and Q are emissive for 1 and 2 and absorptive for 3, which evolve over time to the opposite spin polarization. The subsequent decays of D(1) and Q to ground-state spin polarize D(0). The rates of polarization transfer depend on the molecular connectivity between PDI and (2)BPNO(*) and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.

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

“…The effective generation of high‐spin photoexcited states as well as the role of EISC were supported from the kinetic point of view by the studies carried out on systems constituted of perylene‐3,4:9,10‐bis(dicarboximide) (PDI) and nitroxide radicals (Figure ) . In this study, Wasielewski et al.…”

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

“…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).…”

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

“…Furthermore, the CE mechanism can involve a photoexcited triplet or optically pumped electron spin order that is non-Boltzmann (much greater than thermo-equilibrated spin polarization), and the new polarization methods may be performed by linking a stable radical to a dye moiety (such as 14 in Fig. 1) that absorbs specific photons [150; 151; 152]. Because the electron spin polarization gradient between the excited dye and the stable radical is obtainable without microwaves, DNP without microwave irradiation is speculated.…”