Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

Krumkacheva, Olesya A.; Gorelik, V. R.; Bagryanskaya, Elena G.; Лебедева, Н. В.; Forbes, Malcolm D. E.

doi:10.1021/la904788t

Cited by 16 publications

(10 citation statements)

References 46 publications

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“…4−9 The HAT reaction in supramolecular host−guest complexes has been preponderantly investigated using ensemble spectroscopic techniques such as transient absorption spectroscopy, 4,5 timeresolved electron paramagnetic resonance, 6−9 and nuclear magnetic resonance. 9 Due to the exponential dependence of HAT on the reaction distance (r) with a large exponential decay factor of β (i.e., β = 25−40 Å −1 ), 10,11 HAT is extremely sensitive to r and occurs over an extremely narrow distance range. In CD complexes, guest molecules are loosely bound inside the CD cavity via noncovalent host−guest interactions.…”

Section: Introductionmentioning

confidence: 99%

Single-Molecule Fluorescence Spectroscopy of Perylene Diimide Dyes in a γ-Cyclodextrin Film: Manifestation of Photoinduced H-Atom Transfer via Higher Triplet (n, π*) Excited States

et al. 2017

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Supramolecular complexation of γ-cyclodextrin (γ-CD) with N,N'-bis(2,6-dimethylphenyl)perylene-3,4,9,10-tetracarboxylic diimide (DMP-PDI) or N,N'-bis(2,6-dioctyl)perylene-3,4,9,10-tetracarboxylic diimide (C8-PDI) dye in an aqueous solution and in a γ-CD solid film were investigated via ensemble and single-molecule fluorescence spectroscopy. These two perylene diimide derivatives possess almost the same electronic structure but have different terminal functional groups. This structural difference leads to formation of an inclusion complex of γ-CD with DMP-PDI but not with C8-PDI in aqueous solution. In a γ-CD solid film, the distributions of the wavelengths of emission maximum (λ) are strikingly different between these two dyes; a much narrower and blue-shifted λ distribution was observed for C8-PDI relative to DMP-PDI. This difference is attributed to the fact that the C8-PDI molecules are bound at the γ-CD/glass interface as a result of spin-coating of the sample solution, whereas the DMP-PDI molecules form 1:1 and 1:2 inclusion complexes with conformational heterogeneities in the film. In comparison to the case for C8-PDI, more frequent on-off blinking events were observed for DMP-PDI. The blinking statistics of DMP-PDI in the γ-CD film exhibit both single-exponential and nonexponential (i.e., dispersive) kinetics, revealed by robust statistical analysis. Energetic consideration with the aid of theoretical calculations suggests that the underlying photophysics most probably involves hydrogen atom transfer (HAT) between the DMP-PDI guest and γ-CD host via higher excited (n, π*) triplet states. The hypothesis of HAT in the inclusion complex reasonably explains the experimental results; however, a charge transfer hypothesis cannot explain the results. The dispersive kinetics is attributable to the effect of thermal fluctuation in the forward and backward HAT reactions.

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Section: Introductionmentioning

confidence: 99%

Single-Molecule Fluorescence Spectroscopy of Perylene Diimide Dyes in a γ-Cyclodextrin Film: Manifestation of Photoinduced H-Atom Transfer via Higher Triplet (n, π*) Excited States

et al. 2017

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“…Time-resolved (TR) EPR is the method of choice for studying spin-polarized intermediates in solutions. [35][36][37][38][39][40] So far, only one paper was 3 published reporting TR EPR study in ILs. 41 Using photoexcited triplet molecule of Zntetraphenylporphyrin (ZnTPP) authors clearly demonstrated that high viscosity of ILs suppresses electron spin relaxation caused by diffusional rotation of ZnTPP.…”

Section: Introductionmentioning

confidence: 99%

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

et al. 2015

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Unusual physicochemical properties of ionic liquids (ILs) open vistas for a variety of new applications. Herewith, we investigate the influence of microviscosity and nanostructuring of ILs on spin dynamics of the dissolved photoexcited molecules. We use two most common ILs [Bmim]PF6 and [Bmim]BF4 (with its close analogue [C10mim]BF4) as solvents and photoexcited Zn tetraphenylporphyrin (ZnTPP) as a probe. Time-resolved electron paramagnetic resonance (TR EPR) is employed to investigate spectra and kinetics of spin-polarized triplet ZnTPP in the temperature range 100-270 K. TR EPR data clearly indicate the presence of two microenvironments of ZnTPP in frozen ILs at 100-200 K, being manifested in different spectral shapes and different spin relaxation rates. For one of these microenvironments TR EPR data is quite similar to those obtained in common frozen organic solvents (toluene, glycerol, N-methyl-2-pyrrolidone). However, the second one favors the remarkably slow relaxation of spin polarization, being much longer than in the case of common solvents. Additional experiments using continuous wave EPR and stable nitroxide as a probe confirmed the formation of heterogeneities upon freezing of ILs and complemented TR EPR results. Thus, TR EPR of photoexcited triplets can be effectively used for probing heterogeneities and nanostructuring in frozen ILs. In addition, the increase of polarization lifetime in frozen ILs is an interesting finding that might allow investigation of short-lived intermediates inaccessible otherwise.

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“…Note, however, that A/E or E/A SCRP patterns refer to the rather long inter-radical distances and, as a result, rather small mean exchange values (e.g., comparable to hyperfine interactions). Contrary, if separation of radicals is restricted and they reside close to each other, e.g., diffuse or tumble at radical–radical distances <1 nm, the radical pair S–T – mechanism leads to an observation of net emissive TR EPR spectra, both in liquids and frozen solutions. − Net emission due to SCRP is manifested especially clearly when one of the radical partners is rapidly rotating, as was theoretically considered in ref for the porphyrin–quinone SCRP coupled by strong-exchange interaction. Note that exactly this situation is expected for the SCRP of main-chain and oxo-acyl radicals formed in the bulk of the polymer.…”

Section: Resultsmentioning

confidence: 97%

Continuous Wave and Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Radicals in Fluoroacrylic Porous Polymer Films

et al. 2016

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Fluoroacrylic polymers with inherent micro/nanoporosity are promising media for incorporation of fluorescent molecules and following application as pressure-sensitive paints (PSPs), and UV photostability of PSPs is critically important for their long-term performance. Although photodegradation mechanisms of fluoroacrylic polymers have been studied previously in solutions, they have never been addressed in practically relevant for PSPs solid-state porous films. In this work we combined continuous wave (CW) and time-resolved (TR) electron paramagnetic resonance (EPR) to study UV photodegradation of thin porous films of a few representative fluoroacrylic polymers. Different types of spectra were detected using CW and TR EPR and assigned to the species formed on the inner surface of the pores and in the bulk of the polymer, respectively. The radical pairs formed in the bulk are short-lived, as is evidenced by TR EPR, and most likely recombine back to the initial polymer. On the contrary, the radicals formed on the surface of the pores are metastable in the absence of oxygen; they can be studied by CW EPR and clearly assigned to the radicals of type •C(CH 3 )CH 2 − (so-called propagating radicals) formed via the cleavage of the C−C bond of the ester side chains and consecutive β-scission. Remarkably, their CW EPR spectra closely resemble solution-state spectra, indicating that these radicals are localized in the pores where the mobility of methyl and methylene protons is not suppressed. Thus, based on complementary results of CW and TR EPR, we conclude that UV photodegradation of porous fluoroacrylic polymer films mainly occurs on the inner surface of the pores, which needs to be considered in future development of this type PSPs.

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Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

Cited by 16 publications

References 46 publications

Single-Molecule Fluorescence Spectroscopy of Perylene Diimide Dyes in a γ-Cyclodextrin Film: Manifestation of Photoinduced H-Atom Transfer via Higher Triplet (n, π*) Excited States

Single-Molecule Fluorescence Spectroscopy of Perylene Diimide Dyes in a γ-Cyclodextrin Film: Manifestation of Photoinduced H-Atom Transfer via Higher Triplet (n, π*) Excited States

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

Continuous Wave and Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Radicals in Fluoroacrylic Porous Polymer Films

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