Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO−Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical−Triplet Interaction

Abstract

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3*)PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1*)PDI --> TEMPO(+*)-PDI(-*) occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3*)PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized (3*)PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3*)PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to (3*)PDI having a spin polarization pattern characteristic of overpopulation of its T(0) sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3*)PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives

“…Fig. 4 shows that the loss of stimulated emission and the rise of PDI •− , features that match previously reported spectra for PDI derivatives (29,34,35), are associated with τ CS ¼ 3.6 ps, while the recovery of the ground state bleach and decay of PDI •− are associated with both of the longer time scales (τ CR ¼ 14 ps and 75 ps), implying that two different ionpair populations are generated upon charge separation and recombine with different rates. Similar results are obtained from SVD and global fitting of the data in toluene (Fig.…”

“…3 and Fig. S2), which is the absorption of PDI •− as has been observed in numerous donoracceptor systems containing 1,7-diphenoxy-PDI derivatives (34,35). Charge separation, as also evidenced by the fast disappearance of stimulated emission at 620 nm, occurs with τ ¼ 1.8 ps and 3.6 ps in toluene and CH 2 Cl 2 , respectively.…”

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.…”

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

“…[37][38][39][40][41][42] Concerning this aspect, stable nitroxide free radicals,f or example, 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)a nd nitronyl nitroxide, have been used to connect to organic chromophores. [38,40,[43][44][45][46][47][48] In the case of strong spin-spin exchange interactions,t he eigenstates of such systemsi nclude doublet states (D 0 state, namely 2 [S 0 , R]; D 2 state, namely 2 [S 1 , R]; D 1 state, namely 2 [T 1 , R]) and quartet state (Q state, namely 4 [T 1 , R]). That is to say, the electronic state of the entire molecule could be a doublet or aq uartets tate, and the chromophorec ould be in its singlet or triplet state.…”

“…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 al ong lifetime. [50] Challenges still remaini nt he above area, for instance, the chromophores used for EISC are limited to anthracene, [41,[51][52][53] perylene, [50] perylene bisimide (PBI), [38,44] BODIPY (boron-dipyrromethene, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), [54] and cyanine. [55] The triplet state properties of molecular systems based on EISC have not usually been studied in detail.I mportantly,t he triplet state quantum yields reported for EISC molecules are usually low and triplet lifetimes are short.…”