46532-GB4
Synthesis and Characterization of Novel Donor/Acceptor Molecules Based on Propellanes: A Probe of Nonclassical Conjugation
Novel donor/acceptor molecules (I) based on the 1,3-diethynylbicyclo[1.1.1]pentane (diethynylpropellane) spacer are being investigated as a probe of nonclassical conjugation through the propellane unit.� The work represents the first experimental study of charge transfer from donor (D) to acceptor (A) chromophores linked by diethynylpropellanes.� The propellane unit should lead to substantial charge transfer between the donor and acceptor moieties via through-bond interaction.1 �Molecules based on diethynylpropellane spacers bearing different donor and acceptor combinations will be prepared and then characterized by UV-vis and fluorescence spectroscopy to determine the extent of interaction through the propellane spacer.
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We have prepared four of the target D/A molecules using sequential Sonogashira coupling2 of the appropriate donor or acceptor halobenzenes to the parent diethynylpropellane, which is available in four steps from 1,1-dibromo-2,2-bis(chloromethyl)cyclopropane.3� Syntheses of additional targets bearing the SMe donor are in progress.� A series of corresponding p-phenylacetylenes were also prepared, and their UV-vis spectra compared to those for the D/A molecules.� In general, the target molecules absorb at longer wavelength than the corresponding p-phenylacetylenes or the parent diethynylpropellane in the UV-vis, and this is attributed to through-bond interaction mediated by the propellane spacer.� The UV-vis absorption spectra are collected in Fig. 1.� As the strength of the D/A combination increases, the longest wavelength absorption shifts to higher wavelength for the series in dioxane:� Ia (OMe/CN at 284nm) <� Ib (NMe2/CN at 288 nm) < �Ic (NMe2/NO2 at 297 nm).� Such sensitivity to the combination of D/A groups suggests communication through the propellane spacer.� The absorption spectra exhibit some solvent dependence, but the behavior is complex.� Surprisingly, the CN/OMe derivative Ia is slightly blue shifted ( < 5 nm) upon going from nonpolar cyclohexane to polar acetonitrile.� Further study of the UV-vis spectra of these molecules is needed to explain this solvent dependent behavior and is currently underway.
Figure 1.� UV-vis Absorption spectra for the parent diethynylpropellane, Ia, Ib and Ic in dioxane (ca. ~ 10-5M).� Inset:� Fluorescence emission spectrum for Ia in pentane, excitation at 285 nm (blue), 280 nm (orange).
Preliminary fluorescence studies indicate significant charge transfer fluorescence for the CN/OMe derivative Ia.� The molecule exhibits a single, broad emission at 618 nm (excitation at 285 nm) that does not correspond to the expected fluorescence of the individual aromatic chromophores.� (See inset in Fig. 1.)� The emission most likely arises from an intramolecular charge transfer state, and we plan to study the effect of solvent polarity on the emission wavelength.� If the emission is highly solvatochromic, it will support this prediction.� We will carry out similar studies on other D/A molecules as they are prepared.
Support from the ACS-Petroleum Research Fund has made a significant impact on the principle investigator's (PI) research program and has allowed the PI to pursue research in a new area of study.� Most importantly, funding has provided research opportunities for five undergraduate students.� The project is challenging and has exposed students to a variety of synthetic and analytical laboratory techniques.� Three students presented their work on this project at an on-campus symposium highlighting undergraduate research; one student will present at the regional Argonne Undergraduate Symposium in November 2008.�
The impact of the funded work is further demonstrated by the fundamental questions it will address regarding nonclassical conjugation through the propellane spacer.� The target molecules appear to exhibit significant polarization, and the effects of the propellane unit on the electronic and optical behavior of these systems will be interesting.� Although beyond the present scope of this work, significant charge transfer interaction may lead to applications of these units as linkers for electron or energy transfer processes.
(1) �Gleiter, R.; Pfeifer, K.-H.; Szeimies, G.; Bunz, U. Angew. Chem., Int. Ed. Engl. 1990, 29, 413-415.
(2)� (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627-630.� (b) Hundertmark, T.; Littke, A.F.; Buchwald, S.L.; Fu, G.C. Org. Lett. 2000, 2, 1729-1731.
(3)� (a) Semmler, K.; Szeimies, G.; Belzner, J. J. Am. Chem. Soc. 1985, 107, 6410-6411. (b) Lynch, K. M.; ��� Dailey, W. P. Org. Synth. 1997, 75, 98-105 (c)� Kaszynski, P.; Michl, J. J. Org. Chem. 1988, 53, 4593-4594.� (d)Levin, M. D.; Kaszynski, P.; Michl, J. Org. Synth. 2000, 77, 249-253.�
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