CIDEP from a Polarized Ketone Triplet State Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution

Abstract

b S Supporting Information S table free radicals, such as nitroxides, are known to efficiently quench photoexcited triplet states of organic molecules. 1 Recently, we have shown that this quenching also occurs, although at a slower rate, if the photoexcited ketone is incarcerated inside of an organic container (octa acid) that separates the excited-state ketone and nitroxide by a molecular wall. 2 For this study, we used the cavitand termed "octa acid" (OA, Chart 1), which is known to incarcerate nonpolar molecules by forming a complex in aqueous solutions at basic pH. 3 It was established that relatively nonpolar ketones, such as 4,4 0 -dimethyl benzil (1, Chart 1), form a 2:1 complex with OA, where two OA molecules encapsulate one molecule of 1 (1@OA 2 ). 4 The ketone 1 shows room-temperature phosphorescence with a lifetime of 596 μs when encapsulated inside of OA. 2 The phosphorescence of 1@OA 2 was quenched efficiently by a positively charged nitroxide (Tx, Chart 1) with a rate constant of 1.5 Â 10 9 M À1 s À1 . We rationalized this high rate constant with the involvement of electrostatic attractions of the positively charged quencher (Tx) and the negatively charged external surface of the OA capsule. No quenching of the phosphorescence of 1@OA 2 was observed by the negatively charged nitroxide (TQ, Chart 1) under our experimental conditions, which was rationalized by electrostatic repulsion between incarcerated 1 and TQ. The upper limit for the rate constant of this quenching was estimated to be ∼10 5 M À1 s À1 , which is 4 orders of magnitude lower than the rate constant for quenching by Tx. 2 The quenching of triplet states of ketones by nitroxides often generates spin-polarized nitroxides, which had been observed by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. 5À7 In these studies, the triplet quenching and spin polarization occurred during collisions of freely diffusing ketones and nitroxides or with the nitroxide that was covalently linked to the ketone. In this Letter, we investigate whether spin polarization of nitroxides is also observable in quenching processes, where the triplet ketone and the nitroxide are separated by a molecular wall (OA).As ketones, we selected 4,4 0 -dimethyl benzil (1) and the thioxanthone derivative 2 (Chart 1). Both ketones have been shown to form 2:1 complexes with OA. 2,8 Efficient triplet quenching of 1@OA 2 by Tx has been demonstrated by phosphorescence measurements. 2 Because 2@OA 2 did not show measurable phosphoresce at room temperature, laser flash photolysis experiments were performed to establish the quenching efficiency of 2@OA 2 triplet states by Tx. Excitation of 2@OA 2 with laser pulses (355 nm, 5 ns pulse width) generated the transient absorption spectrum shown in Figure 1 (left). This transient absorption with a maximum at 650 nm was assigned to the triplet state of 2 based on previous experiments in acetonitrile solution 8 and similarities with unsubstituted thioxanthone. 9 However, the triplet lifetime of 2 is strongly reduced (91 n...

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

“…Molecular design rational for the radical-NDI dyad TEMPOw as selected as the electron spin carrier,d ue to its excellent stability.P reviously it has been used for the preparation of high spin systems for the study of electron spin dynamics. [38,40,43,44,[46][47][48] We used NDI as the visible-light-harvesting moiety in the dyad (Scheme 1). NDI hasb een widely used in fluorescent molecular probesa nd supramolecular chemistry, as well as in photochemistry studies.…”

“…TEMPO was selected as the electron spin carrier, due to its excellent stability. Previously it has been used for the preparation of high spin systems for the study of electron spin dynamics . We used NDI as the visible‐light‐harvesting moiety in the dyad (Scheme ).…”

“…[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.…”

“…On the other hand, controlling the spin dynamics in organic molecules is pivotal for energy and charge transport . Concerning this aspect, stable nitroxide free radicals, for example, 2,2,6,6‐tetramethylpiperidinyloxy (TEMPO) and nitronyl nitroxide, have been used to connect to organic chromophores . In the case of strong spin–spin exchange interactions, the eigenstates of such systems include 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 ]).…”

Water-Soluble Cavitands ☆

“…67 Analogous studies utilizing thioxanthone guests 47 ( Figure 10) revealed that encapsulation and irradiation resulted in excitation of the thioxanthone moiety, which transferred its energy to cationic solutes in bulk solution. 68 Further examination of incarcerated benzil and thioxanthone derivatives in the triplet excited state revealed two quenching processes: dynamic quenching via through-the-wall processes and a static relaxation process. 69 Subsequent studies with encapsulated coumarin dyes and rhodamine 6G have also demonstrated, in addition to short-range (Dexter-type) process, longer-range Fö rster resonance energy transfer-type processes.…”