Encapsulation of electron donor-acceptor dyads in β-cyclodextrin cavity: unusual planarization and enhancement in rate of electron-transfer reaction

Hariharan, Mahesh ; Neelakandan, Prakash P. ; Ramaiah, Danaboyina (2007) Encapsulation of electron donor-acceptor dyads in β-cyclodextrin cavity: unusual planarization and enhancement in rate of electron-transfer reaction The Journal of Physical Chemistry B, 111 (41). pp. 11940-11947. ISSN 1089-5647

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Official URL: http://pubs.acs.org/doi/abs/10.1021/jp074643d

Related URL: http://dx.doi.org/10.1021/jp074643d

Abstract

Interaction of β-cyclodextrin (β-CD) with a few novel electron donor acceptor dyads 1a-c and 2a-c, having aryl and flexible methylene spacer groups, has been investigated through photophysical, chiroptical, electrochemical, NMR, and microscopic techniques. Dyads 1a and 1c, with p-tolyl and biphenyl spacer groups, respectively, exhibited significantly decreased fluorescence quantum yields and lifetimes in the presence of β-CD, while negligible changes were observed for dyad 1b with an o-tolyl spacer. In contrast, spacer-length-dependent significant enhancement in fluorescence quantum yields and lifetimes was observed for dyads 2a-c, with flexible polymethylene (n = 1, 3, 11) spacer groups. Association constants of β-CD encapsulated complexes have been determined and the contrast behavior observed in these systems is explained through an electron transfer (kET) mechanism based on calculated favorable change in free energy (ΔGET = −1.27 eV) and the redox species characterized through laser flash photolysis studies. Rates of kET have been estimated and are found to increase ca. 2-fold in the case of dyads 1a and 1c when encapsulated in β-CD, while significantly decreased kET values were observed for the dyads 2a-c with flexible spacer (ca. 9-fold for 2c). As characterized through cyclic voltammetry, 2D NMR [correlated (COSY) and nuclear Overhauser enhancement (NOESY) spectroscopy], and laser flash photolysis studies, theβ-CD encapsulation of dyads with aliphatic spacer groups leads to the conformational unfolding of a sandwich type of structure, whereas dyads with rigid aryl spacer groups undergo unusual planarization as compared to the uncomplexed dyads, resulting in enhanced electron-transfer reaction between the donor and acceptor moieties.

Item Type:Article
Source:Copyright of this article belongs to American Chemical Society.
ID Code:63629
Deposited On:29 Sep 2011 06:34
Last Modified:29 Sep 2011 06:34

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