Inhibition of electron transfer within a b-cyclodextrin cavity?:
A spectroscopic study

Electron transfer processes are ubiquitous to fundamental chemical and biological transformations. Electron transfer plays a role in processes as diverse as formation and repair of mutagenic pyrimidine dimers in DNA,^1-3 photosynthesis,^4,5 and photonucleation of atmospheric water vapor.⁶ Particularly compelling are current implications that DNA might participate in long-range electron transfer (DNA as a molecular wire).⁷ Given the importance of electron transfer, I have initiated an investigation on intramolecular electron transfer (charge-transfer) in species that contain both donor and acceptor moieties on the same molecule. These investigations probe these processes with solution electronic spectroscopy, absorption, fluorescence emission, and Raman spectroscopy. This experimental work is carried out with instrumentation in the chemistry laser spectroscopy laboratory. Stilbene derivatives that include these moieties are of particular interest since intramolecular electron transfer through these systems can be coupled to conformational changes. Important questions to be answered about these systems include the degree of conformational change that may accompany charge redistribution in these systems and along which coordinates these changes may be affected. Some

conformational coordinates (A in figure below) may promote electron transfer (twisted intramolecular charge‑transfer: TICT) while others may quench these processes (trans-cis isomerization, B in figure below).

Other questions exist as to the role of solvent in stabilizing the charge‑separated state that results from electron transfer. These effects have been considered in dimethylaminobenzonitrile (DMABN), which exhibits charge‑transfer in solution, and is considered the prototypical case of twisted intramolecular charge‑transfer.⁸ My initial work has focused on two systems, DMABN and dimethylaminonitrostilbene (DMANS), which exhibits electron transfer in polar solvent. A particularly intriguing initial observation is that when DMABN is in aqueous solution with b-cyclodextrin (10^-3 M) it does not exhibit the signature of electron transfer in contrast to its behavior in aqueous solution alone. One explanation, which is justified based on the literature, is that DMABN is included in the b-cyclodextrin cavity (~9 Å diameter) as a "guest". This interaction may inhibit the twisting motion and certainly provides a different local environment for the "guest" which may destabilize the state that results from electron transfer. This proposal seeks funding to study this intriguing system in detail.^9,10

b-cyclodextrin	DMABN

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2. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry; 6 ed. McGraw-Hill: New York, 1996.

3. Hollas, J. M. Modern Spectroscopy; 3 ed. John Wiley & Sons: New York, 1996.

4. Richard Knochenmuss; Volker Karbach; Claudia Wickleder; Stephan Graf; Samuel Leutwyler Journal of Physical Chemistry A 1998, 102 , 1935-1944.

5. Masato Kodaka Journal of Physical Chemistry A 1998, 102 .

6. Bach, A.; Hewel, J.; Leutwyler, S. Journal of Physical Chemistry A 1998, 102 , 10476-10485.

7. Ruriko Yoshino ; Kenro Hashimoto; Takuichiro Omi; Shun-ichi Ishiuchi; Masaaki Fujii Journal of Physical Chemistry A 1998, 102 , 6227-6233.

8. Lommatzsch, U.; Gerlach, A.; Lahmann, C.; Brutschy, B. Journal of Physical Chemistry A 1998, 102, 6421-6435.

9. Changenet, P.; Plaza, P.; Martin, M. M.; Meyer, Y. H. Journal of Physical Chemistry A 1997, 101, 8186-8194.

10. Gregoire, G.; Dimicoli, I.; Mons, M.; Dedonder-Lardeux, C.; Jouvet, C.; Martrenchard, S.; Solgadi, D. Journal of Physical Chemistry A 1998, 102, 7896-7902.